Iron oxide-based anodes for Li-ion batteries. Synthesis ... · Maria de Fátima Grilo da Costa...
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Iron oxide-based anodes for Li-ion batteries. Synthesis and Characterization Antunes Staffolani Thesis to obtain the Master of Science Degree in Chemistry (Electrochemistry) Supervisors: Prof a . Maria de Fátima Grilo da Costa Montemor Prof. Francesco Nobili Examination Committee Chairperson Profª. Isabel Maria Delgado Jana Marrucho Ferreira Supervisor: Profª. Maria de Fátima Grilo da Costa Montemor Members of the Profa a . Sónia Cristina da Conceição de Matos Eugénio committee: Profa. Maria de Fátima Grilo da Costa Montemor December 2017
Transcript of Iron oxide-based anodes for Li-ion batteries. Synthesis ... · Maria de Fátima Grilo da Costa...
Iron oxide-based anodes for Li-ion batteries.
Synthesis and Characterization
Antunes Staffolani
Thesis to obtain the Master of Science Degree in
Chemistry
(Electrochemistry)
Supervisors: Profa. Maria de Fátima Grilo da Costa Montemor
Prof. Francesco Nobili
Examination Committee
Chairperson Profª. Isabel Maria Delgado Jana Marrucho Ferreira
Supervisor: Profª. Maria de Fátima Grilo da Costa Montemor
Members of the Profaa. Sónia Cristina da Conceição de Matos Eugénio committee: Profa. Maria de Fátima Grilo da Costa Montemor
December 2017
RESUMO O objetivo desta dissertação é o de estudar compostos baseados em óxidos de ferro como novos materiais de
conversão para cátodos de baterias de ião Lítio, com vista a aumentar a densidade de energia e de potência,
bem como o seu tempo de vida. Estas propriedades podem ajudar a desenvolver sistemas de armazenamento
de energia para sistemas portáteis e a sua utilização. Esta dissertação encontra-se dividida em 3 partes:
◦ A primeira parte faz uma revisão do estado da arte dos princípios básicos de baterias recarregáveis,
bem como todos os componentes de baterias de ião lítio, materiais de cátodo, ligantes, eletrólitos e
materiais de ânodo.
◦ A segunda parte apresenta as técnicas utilizadas neste trabalho, nomeadamente para a sua
caracterização físico-química, nomeadamente microscopia eletrónica de varrimento, difração de raios
X, e técnicas eletroquímicas, incluindo espectroscopia de impedância eletroquímica.
◦ A terceira parte evidencia os resultados experimentais e a sua discussão. É feita uma análise detalhada
da caracterização dos materiais e sua resposta eletroquímica.
Foram estudados 3 materiais de conversão: nanopartículas de Fe3O4 (sintetizados pela via de co-precipitação),
FeVan (template de Fe3O4 om vanilina), e Fe3O4@rGO (compósito de Fe3O4 com óxido de grafeno reduzido)
com vista á sua utilização como ânodos de bateriais de ião lítio.
Os elétrodos foram preparados usando ácido poli-acrílico como ligante e etanol como solvente.
Palavras chave. Baterias de ião lítio, ânodos, óxidos de ferro, magnetite, grafeno
ABSTRACT The aim of this thesis is to study Iron oxide-based anode as novel conversion materials for lithium ion batteries.
Lithium-ion batteries (LIBs) feature high energy density, high discharge power, and long service life. These
characteristics facilitated a remarkable advance in portable electronics technology and the spread of information
technology devices throughout society.
The thesis can be divided in three part:
◦ The first part recall the basis of lithium ion batteries with basic definition of rechargeable batteries, as well
as all the components of a LIBs talking about cathodes materials, binders, electrolytes and in particular
of anodes materials.
◦ The second part explain all the experimental technique used for the study which vary from morphological
and structural technique as Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) to
electrochemical technique as Galvanostatic Cycles with Potential Limitation (GCPL), Cyclic
Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS).
◦ In the third part first the result of the structural and morphological analysis are exposed, and then all the
electrochemical performances of the studied materials.
Three conversion materials viz. Fe3O4 nanoparticles (synthesized by base promoted co-precipitation method),
FeVan (Fe3O4 templated with a vanillin template) and Fe3O4@rGO (composite material of Fe3O4 with reduced
graphene oxide) are tested as anodes for Lithium Ions batteries.
Electrodes are prepared using high-molecular weight Poly Acrylic Acid as improved binder and ethanol as low
cost and environmentally friendly solvent.
Key words. Lithium-ion batteries, anodes, iron oxide, magnetite, graphene
INDEX
1. Introduction ..................................................................................................................................... 1
1.1 What is a battery? .................................................................................................................... 2
2. Lithium-ion Batteries ....................................................................................................................... 5
2.1 Anode Materials ....................................................................................................................... 6
2.1.1 Insertion/de-insertion materials ............................................................................................. 7
2.1.1.1 Carbon based materials ................................................................................................. 7
Hard Carbon ............................................................................................................................... 8
Graphene .................................................................................................................................... 8
Intercalation/De-intercalation of various carbons ..................................................................... 10
2.1.1.2 Titanium based oxides ................................................................................................. 11
Spinel Li4Ti5O12 (LTO) .............................................................................................................. 11
Titanium dioxide (TiO2, Titania) ................................................................................................ 12
2.1.2 Alloying / de-alloying materials ............................................................................................ 12
2.1.2.1 Silicon ........................................................................................................................... 13
2.1.2.2 Germanium ................................................................................................................... 14
2.1.3 Conversion materials ........................................................................................................... 14
2.1.3.1 Iron Oxides ................................................................................................................... 16
2.2 Cathode materials .................................................................................................................. 17
2.2.1 Layered oxide cathodes .................................................................................................. 19
2.2.2 Spinel oxide cathodes ..................................................................................................... 20
2.3 Electrolytes ............................................................................................................................. 21
2.4 Binders ................................................................................................................................... 22
2.4.1 PVdF ................................................................................................................................ 23
2.4.2 CMC ................................................................................................................................ 24
2.4.3 PAA ................................................................................................................................. 24
3. Experimental techniques .............................................................................................................. 26
3.1 X-Ray Diffraction (XRD) ......................................................................................................... 26
3.2 Thermogravimetric Analysis (TGA) ........................................................................................ 28
3.3 Scanning Electron Microscope (SEM) ................................................................................... 29
3.4 Galvanostatic cycles ............................................................................................................... 33
3.5 Cyclic Voltammetry ................................................................................................................. 35
3.6 Electrchemical Impedance Spectroscopy (EIS) ..................................................................... 36
4 Synthesis and Characterization of Fe3O4 nanoparticles, Fe3O4 with vanillin template and
Fe3O4@rGO as anodes for LIBs ...................................................................................................... 38
4.1 Synthesis of Fe3O4 nps, FeVan and Fe3O4@rGO ................................................................. 39
4.2 Thermogravimetric analysis ................................................................................................... 41
4.3 SEM analysis .......................................................................................................................... 42
4.4 XRD ........................................................................................................................................ 45
4.5 Electrodes preparation and cell assembling .......................................................................... 47
4.5.2 Cell assembling ............................................................................................................... 48
4.6 Cyclic Voltammetry ................................................................................................................. 49
4.7 Galvanostatic cycles ............................................................................................................... 52
4.7.1 Long Cycling 1C .............................................................................................................. 52
4.7.3 Galvanostatic Profiles and differential analysis ............................................................... 54
4.7.4 Long cycling 2C and 4C .................................................................................................. 56
4.7.5 Rate Capability ................................................................................................................ 58
4.8 EIS .......................................................................................................................................... 60
5. Conclusions .................................................................................................................................. 63
Images Index
Figure 1- Average CO2 emission (g / Km) (1) .................................................................................... 1
Figure 2-Use of electric and hybrid vehicles on Europe from 2012 (1) .............................................. 1
Figure 3-Ragone plot for various electrochemical energy conversion systems (3) ........................... 2
Figure 4-Energy storage capability of most common primary batteries ............................................. 3
Figure 5-Energy storage capability of most common secondary batteries ........................................ 4
Figure 6-Lithium-ions Battery Mechanism .......................................................................................... 6
Figure 7-Illustration of Potential vs. Li/Li+ and specific capacity of anode materials for the next
generations of LIBs .................................................................................................................... 7
Figure 8-Graphic example of non-graphitizing, partially graphitizing and graphitizing carbon .......... 8
Figure 9- Honeycomb structure of Graphene ..................................................................................... 9
Figure 10-Graphite oxide with different oxidized functional groups: a) Epoxy bridges, b) Hydroxyl
groups, c) Carboxyl groups ....................................................................................................... 9
Figure 11-Graphical scheme of Graphene synthesis by Hummer's method and subsequent
reduction .................................................................................................................................. 10
Figure 12-Schematic illustration for the stage structures of GICs .................................................... 11
Figure 13-On the left) SPinel structure of LTO before lithiation; On the right) Rock salt structure of
LTO after lithiation ................................................................................................................... 12
Figure 14-Allotropic forms of Titanium dioxide ................................................................................. 12
Figure 15-Graphic example on how the volume expansion/contraction affect different morphologies
of Silicon .................................................................................................................................. 14
Figure 16-Typical voltage vs. composition profile of the first two and half cycles for an electrode . 15
Figure 17-Theoretical (black bars), first discharge (dark grey), and charge (light grey) specific
gravimetric capacities of different compounds that react with lithium through a conversion
reaction. ................................................................................................................................... 16
Figure 18-Struture model of Fe3O4/graphene composite ............................................................... 17
Figure 19-Plots comparing various classes of high-voltage positive electrode materials (31) ........ 18
Figure 20-Layered structure of LiCoO2 ............................................................................................ 19
Figure 21-Crystal structure of spinel LiMn2O4 .................................................................................. 20
Figure 22-Common solvent used in LIBs. From the left-PC (polycarbonate), DMC (dimethyl
carbonate), EC (ethylene carbonate), DME (dimethyl ethyl ether), DEC (diethyl carbonate) . 21
Figure 23-Polyvinylidene difluoride structure ................................................................................... 23
Figure 24-Carboxymethyl cellulose structure ................................................................................... 24
Figure 25-Polyacrylic acid structure ................................................................................................. 24
Figure 26-graphic example on how PAA can stabilize volume expansion....................................... 25
Figure 27-Graphical example of Bragg's Law .................................................................................. 26
Figure 28-Schematic view of an X-Ray tube with a copper anode .................................................. 27
Figure 29-Schematic view of INEL CPSD 180 (38) ......................................................................... 27
Figure 30-Schematic view of Bragg-Brentano geometry ................................................................. 28
Figure 31-TGA scheme .................................................................................................................... 29
Figure 32-Comparison between an optical microscope and a scanning electron microscope ........ 30
Figure 33-Scheme of a field emission gun. The tungsten tip act as a cathode and the electric field
between cathode and the two anodes extract electrons from the tip with a beam thickness of
100 nm ..................................................................................................................................... 31
Figure 34-Objective lenses in SEM analysis. From the left: pinhole lenses, immersion lenses and
snorkel lenses. ......................................................................................................................... 31
Figure 35-Both cases of positive and negative bias on ET detector. In case a) there is a negative
bias and SE are rejected. In case b) SE are attracted and collected from ALL angles. ......... 32
Figure 36-Schematic view of a snorkel lens with TTL detector and ET detector ............................. 32
Figure 37-Example of GCPL Ewe vs. Time plot ............................................................................... 33
Figure 38-Example of Number of Cycles Vs. Specific Capacity plot. .............................................. 34
Figure 39-Example of Specific Capacity Vs. Ewe plot ..................................................................... 34
Figure 40-Example of Ewe Vs. dQ/dE plot ....................................................................................... 35
Figure 41-CV example ..................................................................................................................... 35
Figure 42-Simulation of an impedance spectrum in Nyquist format. Re = 1 Ω, Rt = 2 Ω, Cdl = 100
µF, Aw = 1 Ω s-0.5, fc = 795.77 Hz ........................................................................................... 37
Figure 43-Randles circuit of an electrochemical systems with a single faradaic reaction with a semi-
infinite linear diffusion .............................................................................................................. 37
Figure 44-TGA of FeVan .................................................................................................................. 41
Figure 45-TGA of Fe3O4@rGO ........................................................................................................ 41
Figure 46-Fe3O4 nps SEM image at 40.54 K X ............................................................................... 42
Figure 47-Fe3O4 nps SEM image at 274.58 K X ............................................................................. 42
Figure 48-FeVan SEM image at 46.56 K X ...................................................................................... 43
Figure 49-FeVan SEM image at 298.40 K X .................................................................................... 43
Figure 50-Fe3O4@rGO SEM image at 40 K X ................................................................................ 44
Figure 51-Fe3O4@rGO SEM image at 275 K X ............................................................................. 44
Figure 52-XRD of Fe3O4 nps ............................................................................................................ 45
Figure 53-XRD of Fe3O4@rGO ........................................................................................................ 46
Figure 54-XRD of FeVan .................................................................................................................. 46
Figure 55-Example of glove box....................................................................................................... 48
Figure 56-T Cell schematics ............................................................................................................. 48
Figure 57-Cyclic voltammetry of Fe3O4 nps ..................................................................................... 49
Figure 58-Cyclic Voltammetry of FeVan .......................................................................................... 50
Figure 59-Cyclic voltammetry of FeVan 9 cycles ............................................................................. 51
Figure 60-Cyclic Voltammetry of Fe3O4@rGO ................................................................................. 51
Figure 61-Long cycling at 1C of Fe3O4 nps ...................................................................................... 52
Figure 62-Long Cycling at 1C of FeVan ........................................................................................... 53
Figure 63-Long Cycling at 1C of Fe3O4@rGO ................................................................................. 54
Figure 64-On the left) Galvanostatic profiles E vs V of Fe3O4 nps; On the right) Differential analysis
of Fe3O4 nps ............................................................................................................................ 54
Figure 65-On the left) Galvanostatic profiles of FeVan; On the right) Differential analysis of FeVan
................................................................................................................................................. 55
Figure 66-On the left) Galvanostatic profiles of Fe3O4@rGO; On the reight) Differential analysis of
Fe3O4@rGO ............................................................................................................................. 55
Figure 67-Long Cycling at 2C and 4C of Fe3O4 nps ........................................................................ 56
Figure 68-Long Cycling at 2C and 4C of FeVan .............................................................................. 57
Figure 69-Long Cycling at 2C and 4C of Fe3O4@rGO ..................................................................... 57
Figure 70-Rate capability and C-rate profile of Fe3O4 nps ............................................................... 58
Figure 71-Rate Capability and C-rate profiles of FeVan .................................................................. 59
Figure 72-Rate capability and C-rate profiles of Fe3O4@rGO ......................................................... 59
Figure 73-EIS spectrum of the three samples .................................................................................. 60
Figure 74-All EIS cycles of Fe3O4 and Fe3O4@rGO ........................................................................ 61
Figure 75-EIS enlarged view of Fe3O4 nps ...................................................................................... 61
Figure 76-EIS enlarged view of Fe3O4@rGO ................................................................................... 62
Figure 77-EIS enlarged view of FeVan ............................................................................................ 62
Tables Index
Table 1-Common primary batteries .................................................................................................... 3
Table 2-Oxidation potentials of common solvents used in LIBs (33) ............................................... 21
Table 3-Reduction potentials of common solvents used in LIBs ..................................................... 22
Table 4-Conductivity of common solvents mixtures used in LIBs .................................................... 22
Table 5-Fe3O4 nps layer composition ............................................................................................... 47
Table 6-Fe3O4@rGO layer composition ........................................................................................... 47
Table 7-FeVan layer composition..................................................................................................... 47
1
1. Introduction
In recent years the exponential growth of smart devices and wearable devices such as smartphones,
laptops, smartwatches, etc. led the industry to look up for high energy density batteries with a reduced
weight and size.
Current concerns about limited energy resources, coupled to the need to decrease greenhouse gas
emissions (Figure 1), has brought about the need to consider renewable energies at a large scale.
Figure 1- Average CO2 emission (g / Km) (1)
Report (1) says that CO2 emission from vehicles start decreasing from 2009 and these results were
achieved by the increase of use of electric vehicles (Figure 2). This is only a poor example on how the
technology is changing and where the research has to focus.
Figure 2-Use of electric and hybrid vehicles on Europe from 2012 (1)
However, due to the intermittent and/or diffuse nature of these renewable sources, efficient energy
storage for mobile systems is a must. Among the various energy conversion/storage systems proposed
over the two last centuries, electrochemical storage and more specifically batteries seem to be very well
positioned to satisfy these needs (2).
2
The energy content of a system like a battery can be visualized by Ragone Plots (Figure 3), which
compares the specific energy (Wh Kg-1) and the specific power (W Kg-1) of different storage devices
such as capacitors, supercapacitors, batteries and fuel cells.
Looking at the Ragone plots, fuel cells are High-Energy systems thanks to their high-energy availability,
while supercapacitor and capacitors are High-Power Systems thanks to their high-rate of energy
deliverance. Batteries have an intermediate power especially due to the high internal resistances (ESR)
that are too large to continue to deliver high-power pulses without increasing the risk of performance
failures (3). They have also an intermediate energy and for this reason are the most common and
versatile electrochemical storage in portable and wearable devices in comparison to capacitors and
supercapacitors (used especially for pulse power application), or fuel cells (used in backup power for
buildings).
With the target of improve this kind of technology the reasearch needs to focus on the chemistry of this
batteries and especially on aspect such as:
Higher energy density;
Lighter and compact batteries;
Higher shelf life;
Cheaper materials.
1.1 What is a battery?
Usually people think battery is that device needed to provide electrical energy to electrical devices, but
from the chemist point of view it is more than that.
In chemistry and electrochemistry a battery is that devices capable to convert chemical energy into
electrical energy and vice-versa (on rechargeable batteries).
How is a battery supposed to do that?
Batteries are made up of one or more cells connected together in series or parallel, whereas each cell
consists of three major components: anode, cathode and electrolyte.
Figure 3-Ragone plot for various electrochemical energy conversion systems (3)
3
They work via redox reactions, where one of electrode is oxidized and gives electrons to the other
electrode that is reduced, creating a current flow.
◦ Anode: the reducing electrode which is oxidized during battery discharge;
◦ Cathode: the oxidizing electrode which is reduced during battery discharge;
◦ Electrolyte: the ionic conductor, which provides the medium for transfer of charge between the
electrodes. Typically it is liquid but can be found also solid or gel-like forms.
During the discharge, the half-reaction of oxidation will take place at the anode, while the half-reaction
of reduction will take place at the cathode. By the oxidation reaction of the anode an electron flow is
generated which, through an external circuit, reaches the cathode where the reduction reaction occurs.
Between the two electrodes there is a potential difference, the anode has always a negative potential (-
) and the cathode has always a positive potential (+). The electric tension created between them is the
driving force that makes the discharge a spontaneous reaction. The electrical circuit is close by the
electrolyte, which means that while electrons flow from the anode to the cathode, also positive ions of
electrolyte flow in the same direction.
Batteries can be classified in two categories: Primary or Secondary batteries, determined by the
possibility of recharge. Primary batteries (Table 1) cannot be recharged once they are discharged and
so they are discarded. The most common primary batteries are also called “dry cells” due to the liquid
electrolyte confined in an absorbent material.
Table 1-Common primary batteries
The main advantages of these kind of batteries are good shelf life, high energy density and ease of use.
In Figure 4 some energy storage capability of the most common primary batteries are reported.
Secondary batteries, also known as accumulators, can be recharged applying an opposite current. They
are used mainly in portable devices (smartphones, laptops, smartwatches etc.), power tools, hybrid and
electrical vehicles.
Figure 4-Energy storage capability of most common primary batteries
Battery Nominal Voltage / V Anode Cathode Electrolyte Energy Density / WhL-1
Leclanche (Zinc-Carbon) 1,5 Zn foil MnO2 aq ZnCl2-NH4 165
Alkaline 1,5 Zn powder MnO2 aq KOH 400
Zn-Air 1,2 Zn powder Carbon aq KOH 1000
Li-MnO2 3 Li foil MnO2 LiCF3SO3 or LiClO4 580
Li-FeS2 1,6 Li foil FeS2 LiCF3SO3 or LiClO4 500
4
In Figure 5 it is possible to observe different kinds of rechargeable batteries and compare their energy
storage capabilities.
Figure 5-Energy storage capability of most common secondary batteries
Lithium ion batteries are characterized by high power density, high discharge rates, flat discharge curves
and good low temperature performances. Their energies are generally lower than those of primary
batteries.
Below some typical physical quantities are listed, which describe the characteristics of a battery:
◦ Theoretical Capacity (Q): it is the amount of electric charge stored in the battery. It is
expressed in Coulomb (C) or Ampere*hours (Ah) and its value is given by:
𝑄 = 𝑥 × 𝑛 × 𝐹
Where n is the number of electrons exchanged in the redox reaction, x is the number of moles
of active material, and F is the Faraday constant (96494 C per mole of electrons). Much more
useful is the specific capacity, i.e. the theoretical capacity based on the weight or the volume of
the electrodic material, expressed in Ah Kg-1 or Ah L-1.
◦ Theoretical Energy (E): expressed in Joule (J) or Watt*Hours (Wh) and depends by the
Theoretical Capacity and Potential.
𝐸 = 𝑄 × 𝑉
◦ Maximum Current (I): Maximum current produced in the discharge phase. It is expressed in
Ampere (A).
◦ Potential (V): Working Potential of the battery. It is expressed in Volts (V).
◦ Power (P): it is the energy produced in time unit. It is a quantity expressed in Watts (W). By
definition 1 W is the power produced by a circuit with a current flow of 1 A and a potential of 1
V.
𝑃 = 𝐼 × 𝑉 =𝑄 × 𝑉
𝑡
◦ Efficiency: Percentage of charge given by the discharge phase respect to the charge given by
the charge phase.
◦ Cyclic Life: Number of cycles charge-discharge that a rechargeable battery can sustain.
Usually a secondary battery is considered exhausted when its actual capacity becomes less
than the 80 % of the theoretical capacity.
5
◦ State of Charge: Amount of charge in a battery, expressed as a percentage fraction of the total
capacity.
◦ State of Health: is the physical condition of a battery.
2. Lithium-ion Batteries
In recent years, the need of portable power has accelerated due to the miniaturization of electronic
appliances, where in some cases the battery system is as much as half of the weight and volume of
powered devices.
Lithium has the lightest weight, highest voltage, and great energy density of all metals. The first
published interest in lithium batteries began with the work of Harris in 1958 (4).
The work led to the development and commercialization of the first primary lithium battery during the
1970s. The most used systems include lithium/sulfurdioxide (Li/SO2), lithium-thionylchloride (Li/SOCl2),
lithium-sulfurylchloride (Li/SO2Cl2), lithium-polycarbon monofluoride (Li/(CFx)n), lithium-manganese
dioxide (Li/MnO2), and lithium-iodine (Li/(poly-2-vinyl pyridine)In) (5).
The 1980s brought many attempts to develop a rechargeable lithium battery; several studies of fast ion
conduction in solids demonstrated that alkali ions could move rapidly in an electronically conducting
lattice containing transition metal atoms in a mixed valence state. When the host structure is fully
populated with alkali metal atoms, the transition metal atom is in the reduced state. As the lithium ions
are removed from the host, the transition metal atom is oxidized.
A good host structure is suitable for lithium-ions battery if:
◦ mixed with ionic-electronic conductor;
◦ the removal of lithium (or other alkali) does not change the structure over a large range of the
solid solution;
◦ the lithiated structure exhibits a suitable potential difference versus lithium;
◦ the volume changes of insertion/removal processes are not too large;
◦ has a working voltage compatible with the electrolyte (5).
This led to the development of rechargeable lithium-ions batteries during late 1970s and 1980s using
lithium insertion compounds as positive electrodes. Common commercialized systems were Li/TiS2 and
Li/MoS2, but the safety problems due especially for the metallic lithium anodes limited the commercial
application.
During this years Steele suggested graphite layers as potential candidate for anode material of a lithium-
ions battery (6).
The next decade saw batteries based with insertion/removal host compounds serving as both
electrodes.
6
Let’s see the mechanism of a common lithium-ion battery (Figure 6).
Figure 6-Lithium-ions Battery Mechanism
Usually a lithium-rich electrode (cathode), a lithium-poor electrode (anode), an electrolyte and a
separator compose these batteries.
During the discharge reaction, the anode is oxidized and the deinsertion of lithium ions from the anodic
structure take place. Then the lithium ions migrate through the electrolyte and the separator until
reaching the cathode where the insertion to the host structure take place. At the same time, through an
external circuit, electrons flow from the anode to the cathode generating an electric current.
The potential of the cell will be given by the difference of the chemical potential between the anode and
the cathode ∆𝐺 = −𝐸𝐹.
Taking in consideration as electrochemical model the cell graphite/LiMO2 (generic lithium metal oxide,
where the metal is usually a transition metal), it can be represented by the following reaction of
charge/discharge:
Anode reaction: 𝐿𝑖𝑥𝐶 ↔ 𝐶 + 𝑥𝐿𝑖+ + 𝑥𝑒−
Cathode reaction: 𝐿𝑖1−𝑥𝑀𝑂2 + 𝑥𝐿𝑖+ + 𝑥𝑒− ↔ 𝐿𝑖𝑀𝑂2
Overall reaction: 6𝐶 + 𝐿𝑖𝑀𝑂2 ↔ 𝐿𝑖𝑥𝐶6 + 𝐿𝑖1−𝑥𝑀𝑂2
The electrolyte can be aqueous, non-aqueous solutions or polymers. The most used are solutions in
which Lithium hexafluorophosphate (LiPF6) salt is dissolved in organic carbonate such as mixture of
ethylene carbonate (EC) and dimethyl carbonate (DMC), propylene carbonate (PC), diethyl carbonate
(DEC) and so on.
The separator is used to separate the two electrodes in order to avoid short-circuit and needs to be
permeable to the electrolyte.
2.1 Anode Materials
Lithium metal is one of the most attractive anode material in rechargeable batteries because of its anode
potential (-3.045 vs. standard hydrogen electrode) and its high specific capacity (3860 mAh g-1) (7), but
its use is limited by the safety concerns coming from the dendrites formation that can cause a short
circuit between cathode and anode.
7
The most used anode material is graphite owing to its flat and low working potential vs. lithium, low cost
and good cycle life. However, graphite allows intercalation of one lithium ion per six carbon atoms
leading to a reversible capacity of 372 mAh g-1 (8).
Therefore, the path leading to LIBs with improved energy and power density has, as major challenge,
the selection of suitable anode materials that can provide high capacity, ease diffusion of Li-ions into
the anode, along with good cycling life, and free from safety concerns (Figure 7).
Figure 7-Illustration of Potential vs. Li/Li+ and specific capacity of anode materials for the next generations of LIBs
Many efforts have been done in the research of innovative carbonaceous or non-carbonaceous
materials for anode materials.
2.1.1 Insertion/de-insertion materials
2.1.1.1 Carbon based materials
Carbon-based materials with different morphologies have been studied as good candidate for anode
materials in LIBs due especially for their proprieties, such as good chemical, thermal and
electrochemical stability and good insertion/de-insertion reversibility.
Furthermore, carbon based materials show an electrochemical activity towards the electrolyte only at
very low potentials and high resistance from corrosion due to HF. In this regard, the use of carbonaceous
materials or carbon coating can provide a way to mitigate the consumption of the active material towards
the formation of Solid Electrolyte Interphase (SEI).
The variety of carbon materials used as anode in LIBs can be divided in two categories (Figure 8),
according to the degree of crystallinity and carbon atoms stacking:
◦ Soft Carbon (graphitizable carbons) where crystallites are stacked almost in the same direction;
8
◦ Hard carbon (non-graphitizable carbons) where crystallites have disordered orientation.
Figure 8-Graphic example of non-graphitizing, partially graphitizing and graphitizing carbon
In particular, the former is quite famous in the battery community, in fact it shows a good capacity (372
mAh g-1), good reversibility and good cycling life.
Despite their massive production and the relative low cost of the industrial processes, these classes of
carbon materials have, as major issue, a low specific capacity (i.e.372 mAh g-1), especially for
applications such as HEV, PHEV or PEV. Hence, the use of graphitic carbon is limited only to low power
devices such as laptops, smartphones and other portable devices.
Now, the research activity is strongly focused on porous carbon, carbon nanotubes (9), nanofibers (10)
and graphene (11), as the most promising carbon based anode materials. The unique shape and the
nanoscale size of these structures can substantially increase the energy storage capacity.
Hard Carbon
Even though soft carbon represents the state of art of anode materials, the limited capacity (372 mAh
g-1) and the high voltage hysteresis hinder its use only to low power systems. Hard carbon represents a
good substitute due to the high reversible capacity (more than 500 mAh g-1) in the potential range of 0-
1.5 V vs Li/Li+. Hard carbon have random alignment of graphene sheets which provides many voids to
accommodate lithium, however the manner in which lithium diffusion occurs inside hard carbons makes
lithium diffusion very slow, namely very poor rate capacity. This carbon material has two drawbacks: low
initial coulombic efficiency and low tap density. To overcome these problems a number of strategies
have been pursued such as surface oxidation (12), fluorination or metal coating.
Graphene
Graphene (Figure 9) consist in a honeycomb structure of sp2 carbons bonded in a two dimensional sheet
with a single-atom thickness. This material has drawn much attention because of its admirable
9
properties and versatility in a number of fields such as chemical, physical, biological and engineering
sciences.
The most important proprieties of graphene are good electrical conductivity, relevant mechanical
strength, high charge mobility and high surface area which make itself a suitable material for LIBs. The
theoretical lithium storage capacity of this material is controversial, two configurations have been
proposed to go beyond the limitation of the graphite LiC6 configuration: one is the double layer
adsorption configuration and another is the covalent molecule configuration.
Figure 9- Honeycomb structure of Graphene
In the former case, both side of graphene sheet adsorb Li ions yielding to Li2C6 and a storage capacity
of ≈ 780 mAh g-1; in the latter case each Li ion is trapped at a covalent site of the benzene ring yielding
to LiC2 and storage capacity of ≈ 1116 mAh g-1 (13).
Graphene can be prepared by different methods such as hydrazine reduction process, low-temperature
pyrolysis and electron beam irradiation. The most used method is the Hummer method and subsequent
exfoliation and reduction of graphite oxide. Hummers' method (14) was developed in 1958 as a safer,
faster and more efficient method of producing graphite oxide (Figure 10).
Figure 10-Graphite oxide with different oxidized functional groups: a) Epoxy bridges, b) Hydroxyl groups, c) Carboxyl
groups
The original procedure starts with 100 g graphite and 50 g of sodium nitrate in sulfuric acid at 66 °C,
which is then cooled to 0 °C. 300 g of potassium permanganate is then added to the solution and stirred.
Water is added in increments until the solution is approximately 32 liters.
After the oxidation of graphite to graphite oxide, there is the micro-mechanical exfoliation of the structure
to obtain graphene oxide sheets. This procedure is commonly made by mechanical exfoliation by
10
sonication with an ultrasonicator. Now the obtained graphene oxide sheets need to be reduced which
partly restores the structure and properties of graphene. The reduction can be taken by different
methods (all chemicals) such as adding hydarizne, NaBH4 or hydrazine vapour. Usually after the
reduction there is an annealing step in which the product is taken at high temperatures (800-1100 °C)
with an argon or argon/hydrogen environment, to restore the sp2 behaviour of the carbons.
The process can be summarized in figure 11.
Figure 11-Graphical scheme of Graphene synthesis by Hummer's method and subsequent reduction
Intercalation/De-intercalation of various carbons
Lithium ion is intercalated within graphite to form lithium-graphite intercalation compounds (Li-GICs).
GICs are layered compounds that atomic or molecular layers of a different chemical species called the
intercalate are inserted between the graphene sheets of host graphite.
The most important and characteristic property of GICs is the staging phenomenon, which is
characterized by intercalate layers that are periodically arranged in a matrix of graphene sheets (15).
The stage structure changes successively from a higher to a lower stage during electrochemical lithium
intercalation and in the opposite direction during lithium de-intercalation.
11
Schematic illustrations for the stage structures of GICs are shown in Figure 12. These stage structures
are designated in terms of stage index n, which denotes the number of graphene sheets between
adjacent intercalate layers (5).
Figure 12-Schematic illustration for the stage structures of GICs
2.1.1.2 Titanium based oxides
Titanium based oxides have drawn significant attention due to their low cost, good insertion/de-insertion
reversibility, good cycling life and because allow the design of system with minor safety concerns.
However, they show also low theoretical capacity, in the range 175-330 mAh g-1, and low electronic
conductivity.
The insertion/de-insertion reversibility of titanium based oxides and the capacity depends a lot on their
structure, shape and size. It was found that nanostructured titanium oxides lead to a better capacity and
longer cycle life than the bulk material.
Titanium dioxide with various allotropic forms and spinel Li4Ti5O12 have been extensively studied for
anode purposes.
Spinel Li4Ti5O12 (LTO)
Spinel Li4Ti5O12 is considered the most appropriate anode material due to its lithium insertion/de-
insertion reversibility and for the high operating voltage of 1.55 V. The insertion/extraction in LTO occurs
from the lithiation of the spinel structure leading to a lithiated rock salt structure Li7Ti5O12 (Figure 13).
The high operating potential guarantees safety conditions, in fact the formation of the solid electrolyte
interphase (SEI) is mitigated and the development of dendrites, typical issue in carbon based anodes,
is avoided.
However, the low capacity (175 mAh g-1) and the low conductivity of LTO limits its use.
To overcome these issues two solutions can be taken: the first is surface coating to increase the
electrical conductivity (e.g. carbon coated LTO (16)) and the second one is to downsize to nanoscale
for the maximum lithium diffusion (e.g. nanocrystalline LTO (17)).
12
Figure 13-On the left) SPinel structure of LTO before lithiation; On the right) Rock salt structure of LTO after lithiation
Titanium dioxide (TiO2, Titania)
Titanium dioxide is a very promising material as anode for lithium ion batteries, in fact it is suitable for
mass production and low cost. Titania has excellent safety and stability characteristic and an operating
potential of 1.5 V vs Li/Li+. Moreover, TiO2 has admirable advantages such as high electro-activity,
strong oxidation capability, good chemical stability, high abundance and structural diversity.
Titanium dioxide can host one mole of lithium per one mole of TiO2 with a maximum theoretical capacity
of 330 mAh g-1. However the exploitation of the insertion/de-insertion mechanism in titania is the real
challenge. The lithium intercalation/de-intercalation strongly depends on its crystallinity, particle size and
structure (18) (19).
Titania ha several allotropic forms (Figure 14), the most known are rutile (tetragonal, P42/mnm), anatase
(tetragonal, I41/amd) and brookite (orthorhombic, Pbca).
Figure 14-Allotropic forms of Titanium dioxide
2.1.2 Alloying / de-alloying materials
The next generation of LIBs is expected to fulfil the power demand of high energy consuming devices,
to power electric vehicles and HEVs and to be used in stationary applications. Hence, the increase of
specific capacity represents an important topic for the research of new materials.
Materials that can satisfy these requirements can be for example silicon, germanium, silicon monoxide
and tin oxide, which react with lithium according to the alloying mechanism.
13
The alloying stands for insertion of lithium within the crystal structure of the host materials, according to
the following reaction:
𝐿𝑖 + 𝑥𝑀 ↔ 𝐿𝑖𝑀𝑥
where the reactant M can be an element or a compound. Depending on whether or not a phase
transformation take place, these reactions can be further divided in two categories:
◦ Solid-solution reaction;
◦ Addition reaction.
In a solid-solution reaction, no phase or structure transformation occurs when lithium enters in the
framework structure of the host material while; in addition reaction the lithiated structure of LiMx is
different from the initial delithiated structure of M.
Lithium insertion/extraction in crystalline Si, Sn, Al and Sb are considered as addition reactions because
of the very limited solubility of lithium in these elements.
The reactions of Li with Mg and amorphous Si are regarded as solid-solution reactions.
Alloying materials are considered the most promising for anodes especially due to their high specific
capacity that range from 783 mAh g-1 for tin oxide and 4211 mAh g-1 for silicon.
Even though these materials exhibit the highest capacity in the LIBs studies, their poor cycling life due
to the high volume expansion/contraction and the larger irreversible capacity at the initial cycles hinder
their commercial use.
These issues can be overcome by downsizing the particle from microscale to nanoscale, and the
fabrication of composites.
2.1.2.1 Silicon
Silicon has the highest specific capacity (i.e. 4200 mAh g-1, Li22Si5) and the discharge potential is almost
close to graphite (i.e. 0.4 V vs Li/Li+). Finally, silicon is the second most abundant element on earth
hence inexpensive and eco-friendly. Many groups have studied the electrochemical lithiation of Si
electrodes, and it has been clarified that the high specific capacity value is due to the formation of
intermetallic Li-Si binary compounds such as Li12Si7, Li7Si3, Li13Si4 and Li22Si5 (20).
However, some issues prevent the employment of silicon in lithium-ion batteries like the high volume
expansion (≈ 400 %) and the irreversible capacity.
Secondly, the formation of Si compounds at the solid electrolyte interface inhibits the alloying / de-
alloying process.
Many studies have been performed and found that the electrical contact of silicon with the conductive
carbon and the current collector undergoes a drop during the volume expansion/contraction.
14
Many efforts were focused on nanostructured silicon to overcome the volume expansion issue, and
especially on the morphology aspect. For example nanowires, nanosphere and nanotubes were
considered for their ability to provide the free volume needed for the expansion/contraction of Si (Figure
15).
Figure 15-Graphic example on how the volume expansion/contraction affect different morphologies of Silicon
Despite the mentioned nanostructures showed superior electrochemical properties, their fabrication is
not cost-effective, and thus this emerging technology is still not applicable at an industrial level.
In order to realize effective anodes for LIBs, research groups are seeking for alternative fabrication
methods such as the hydrothermal and solvothermal techniques.
2.1.2.2 Germanium
Germanium is an extensively studied anode material due to its high reversible capacity (1623 mAh g-1)
with Li22Ge5 as equivalent stoichiometry and reversible alloy/de-alloy reactions (21).
Even though Ge is considerably more expensive and shows lower capacity than silicon, it has desirable
advantages such as high intrinsic electrical conductivity and lithium diffusion 400 times faster than in Si
at room temperature.
However, as for silicon, the practical use of germanium is hindered by its volume expansion/contraction
(≈ 300 %).
Ge nanostructures, such as nanoparticles (22), nanowires (23) and nanotubes (24) can effectively
sustain the volume changes with better efficiency than bulk and microstructures. Noticeably,
improvements have also been observed with hybrid composites of Ge nanoparticles using conductive
matrices.
2.1.3 Conversion materials
The advent of the 21st century brought interest onto a new reactivity concept with the reversible
electrochemical reaction of lithium with transition metal oxides, according to what is conventionally
referred to as “conversion reaction,” generalized as follows:
𝑀𝑎𝑋𝑏 + (𝑏 ∙ 𝑛)𝐿𝑖 ↔ 𝑎𝑀 + 𝑏𝐿𝑖𝑛𝑋
15
Where M = transition metal, X = anion, and n = formal oxidation state of X.
Conversions reactions had already been reported for some oxides and sulfides, and even different
degrees of reversibility had been observed.
The key to the reversibility of the conversion reaction seems to lie in the formation, upon complete
reduction of the metal, of nanoparticles that, owing to the large amount of interfacial surface, are very
active toward the decomposition of the matrix of the lithium binary compound (LinX) in which they are
embedded when a reverse polarization is applied.
The footprint of the reduction process is a representative voltage plateau with length typically equivalent
to the amount of electrons required to fully reduce the compound (Figure 16).
Figure 16-Typical voltage vs. composition profile of the first two and half cycles for an electrode
Transition metal oxide, which react through conversion mechanism, are deserving the most attention.
In figure 17 are reported different TMs oxide studied in literature with their theoretical, first discharge
and first charge specific capacity.
Despite these premise, TMs oxide associated to conversion mechanism usually suffers from a series of
issues:
◦ Remarkable structural change;
◦ Volume expansion.
In order to improve electrochemical behaviour of these compounds several approaches have been
pursued:
◦ Use of nanostructure (nanoparticles (25), nanorods (26), hollow sphere (27));
16
◦ Composite to stabilize the structural changes and improve conductivity.
Figure 17-Theoretical (black bars), first discharge (dark grey), and charge (light grey) specific gravimetric capacities of
different compounds that react with lithium through a conversion reaction.
2.1.3.1 Iron Oxides
The very large capacities associated to the conversion reaction of Fe2O3 (1007 mAh g-1) coupled with
its low toxicity and cost make it a possible candidate as anode material for LIBs.
Fe3O4 was also the object of research in the first reports of conversion reactions in metal oxides. It has
high theoretical specific capacity (927 mAh g-1) and relatively high voltage plateau.
Iron (II,III) oxide suffers as the alloying and conversion materials of a capacity drop during the first 50
cycles due to the volume expansion/contraction of lithium insertion/de-insertion and conversion reaction.
17
Researchers tried to overcome this issue synthesising at nanoscale to improve the lithium diffusion and
increasing the free volume for the volume expansion. Different kind of morphologies have been studied
like nanoparticles (25), nanorods (26), hollow sphere (27) but also composite with graphene (28) to
stabilize the iron oxide during the discharge and charge process (Figure 18).
Figure 18-Struture model of Fe3O4/graphene composite
Finally, a reversible conversion reaction has also been demonstrated for FeO with a gradual loss upon
cycling.
2.2 Cathode materials
Practical lithium-ion batteries are based on insertion/de-insertion cathode and anode material. Following
the discovery of an oxide cathode material LiCoO2 operating at almost 4 V by Goodenough and co-
workers in the 1980s (29), commercialization of the first rechargeable Li-ion cell was realized by Sony
in the early 1990s with a carbonaceous anode material.
To the present day, state-of-art lithium-ion cells maintain the original configuration, employing a
successive generation of LiCoO2 materials.
Cathode materials, in order to be successful materials, need to have some structural features and some
physical proprieties like:
◦ Crystal structure: The compound must have a crystal structure that do not vary during lithium
insertion/extraction and allow a good lithium diffusion into it.
◦ Good electrical conductivity.
◦ High operating voltage.
In recent years anode materials research have made great strides with high specific capacity materials
(i.e. alloying materials such as silicon or conversion materials) but, unfortunately the positive
counterparts has largely lagged behind. Substantial efforts, from the academic and the industrial
research were focused on the design and optimization of novel positive electrode materials with large
capacities (i.e. ≥ 200 mAh g-1) and high operating voltage (i.e ≥ 4 V vs. Li/Li+). Meanwhile major attention
have been focused on design novel electrolyte systems that can accommodate larger operating potential
(30).
18
Notable potential cathode candidates include nickel-rich layered oxides (LiNi1xMxO2, M = Co, Mn and
Al), lithium-rich layered oxides (Li1+xM1-xO2, M = Mn, Ni, Co, etc.), high-voltage spinel oxides
(LiNi0.5Mn1.5O4), and high-voltage polyanionic compounds (phosphates, sulfates, silicates, etc.) (31).
Figure 19 shows proprieties and limitations of these cathode materials.
Figure 19-Plots comparing various classes of high-voltage positive electrode materials (31)
LiNi1-xMxO2 with lower nickel content has already succeeded in commercialization (e.g.,
LiNi1/3Co1/3Mn1/3O2), and Ni-rich compositions ((1-x) ≥ 0.6) are being further optimized and are on track
to reach the 300 Wh kg-1 gravimetric energy density milestone in the near future. LiNi0.5Mn1.5O4 offers
only a moderate energy density, but holds great promise for high-power applications if stable electrolyte
systems to withstand its high operating voltage (4.7 V vs. Li/Li+) could be realized.
The use of lithium-rich manganese based Li1+xM1-xO2 with high capacity (≈ 250 mA h g-1) has prompted
massive interest in recent years to compete with Ni-rich LiNi1-xMxO2 for a larger boost in energy density
(both gravimetric and volumetric) at a lower cost (31).
19
2.2.1 Layered oxide cathodes
Several oxides with general formula LiMO2 (M = V, Cr, Co and Ni) crystallize in a layered structure in
which Li+ and M3+ ion occupy the alternate (111) planes of the rock salt structure to give a layer sequence
of –O-Li-O-M-O- along the c axis as for LiCoO2 shown in figure 20.
Figure 20-Layered structure of LiCoO2
This structure is designated as the O3 layer structure since Li+ ion occupy the octahedral sites and there
are three MO2 sheets unit per cells. The structure with a strongly bonded MO2 layers allow a reversible
extraction/insertion of lithium ions from/into lithium planes. The interconnected lithium ion sites through
the edges-shared LiO6 provide fast lithium-ion diffusion. On the other hand, the edge-shared MO6
octahedral arrangement with a direct M-M boning can provide a good electronic conductivity (depending
on the nature of M) (32).
20
2.2.2 Spinel oxide cathodes
A few oxide with general formula LiM2O4 (M = Ti, V and Mn) crystallize in a spinel structure in which
lithium and the M3+/M4+ ions occupy 8a and 16d octahedral sites of the cubic close-packed oxygen array
(Figure21).
Figure 21-Crystal structure of spinel LiMn2O4
A strong edge-shared octahedral [M2]O4 array permits a reversible extraction of the Li+ ions from the
tetrahedral sites without collapsing the 3-dimensional spinel framework.
An additional lithium can be inserted in the empty 16c octahedral sites giving Li2M2O4. However, the
repulsion between the 8a Li+ and the 16c Li+ (common faces) leads to a displacement of the former to
an empty 16c octahedral site giving the rock-salt structure (Li2)16c(M2)16dO4. Just like the layered oxide,
the M-M interactions due to the edge-shared MO6 octahedral arrangement give a good electronic
conductivity (depending on the nature of M) (32).
21
2.3 Electrolytes
The electrolyte is an ionic conductor that leads the charge transfer between the anode and the cathode.
The basic requirements of a suitable electrolyte for electrochemical devices are high ionic conductivity,
low melting point and high melting point, chemical and electrochemical stability, and safety. The solvent
properties, and dynamics of ions solvent interaction, must be understood in designing new electrolytes.
The most used electrolyte are lithium salt such as LiClO4, LiPF6 and LiBF4 dissolved in organic aprotic
solvents.
Electrolytes based on solvent mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC)
and/or diethyl carbonate (DEC) are commonly used in lithium ion batteries with 4 V cathodes because
of the high oxidation potential of the solvent. PC, DMC, EC, DME, DEC are common solvent used in
LIBs (Figure 22).
Figure 22-Common solvent used in LIBs. From the left-PC (polycarbonate), DMC (dimethyl carbonate), EC (ethylene
carbonate), DME (dimethyl ethyl ether), DEC (diethyl carbonate)
The window of the oxidation/reduction stability is a first requirement for electrolyte systems in lithium-
ion cells.
In table 2 are listed the oxidation potential of common solvent used in LIBs.
Table 2-Oxidation potentials of common solvents used in LIBs (33)
5,7 V
5,5 V
4,9 V
4,7 V
>6,0 V
>6,0 V
>6,0 V
>6,0 V
4,9 V
4,9 V
SolventOxidation Potential vs Li/Li+
1 M LiClO4 1 M LiPF6
5,8 V
5,8 V
Propylene carbonate (PC)
Ethylene Carbonate (EC)
DMC
DEC
1,2-Dimethoxy ethane (DME)
1,2-Diethoxy ethane (DEE)
22
Electrochemical reduction of electrolytes is related to the formation of SEI on the surface of an anode
electrode. In further studies, Zhang et al. (34) investigated the reduction potentials of five organic
carbonates EC, PC, DEC, DMC, and vinylene carbonate (VC) by cyclic voltammetry using inert (Au or
glassy carbon) electrodes in THF/LiClO4 supporting electrolyte (Table 3).
Table 3-Reduction potentials of common solvents used in LIBs
Hayashi et al. (35) also measured the specific conductivity of PC-mixed or EC-mixed solvent electrolyte
(1:1 in volume). The results at 20°C are shown in Table 4. In general, the order of increasing conductivity
was for DME>DEE≈DMC>DEC.
Table 4-Conductivity of common solvents mixtures used in LIBs
Common mixture used in ordinary LIBs are LiPF6 dissolved in EC/DMC or EC/DMC with VC (vinyl
carbonate) in small percentage as an additive.
2.4 Binders
In the design of LIBs, it should be appropriate study not only the active materials, but also all the
materials that lead to the battery realization itself. The binder is definetly the most important among
them. It ensure a cohesion between the active materials and the conductive additives, and the cohesion
between the active materials and the collector, which can be copper (anode material) or aluminium
(cathode material and LTO).
Furthermore, the binder helps to contain the volume expansion/contraction, avoiding the pulverization
of the A.M. and the detachment from the collector (36).
The most used binder is polyvinylidene difluoride (PVdF), a polymer that guarantee good
electrochemical properties but needs N-methyl-2-pyrrolidone (NMP) which is toxic and irritant.
Many efforts have been focused on the research of alternative binders, which can be used in
environmental- and human-friendly solvents such as ethylic alcohol and water.
0,86
0,25
1,36
1,32
1,00-1,60
1,32
1,4
Potential Values for Solvent Reduction (V vs Li/Li+)
Calculated Experimental
1,46
1,33
1,24
Solvent
EC
DEC
PC
DMC
VC
1 M LiClO4 1 M LiPF6 1 M LiClO4 1 M LiPF6
DME 12 14 14 15
DEE 7,5 9,5 8,5 10
DMC 6,5 10 8 10
DEC 4 7 6 7
Co-Solvent
Specific Conductivity (20°C) / mS cm-1
PC mixed solvent electrolyte EC mixed solvent electrolyte
23
Here there are the most used binders in recent years and their corresponding solvent:
◦ Polyvinylidene difluoride (PVdF) with N-methyl-2-Pyrrolidone;
◦ Polyacrylic acid (PAA) with ethylic alcohol;
◦ Carboxymethyl cellulose (CMC) with water.
2.4.1 PVdF
Polyvinylilidene difluoride (Figure 23) is an omopolymer of vynilidene difluoride, one polymer of the
family of fluoropolymers, known for their chemical and thermal stability.
It is an elastomer partially fluorinated with high performances such as:
◦ Good mechanical properties and abrasion resistance;
◦ Resistance to strong and oxidant acids;
◦ High solubility in polar solvents;
◦ Good resistance to temperature between -40 °C and +150 °C.
Figure 23-Polyvinylidene difluoride structure
PVdF is used for both anode and cathode electrodes.
It can be prepared by radicalic polymerization of the monomer vinylidene difluoride.
It has the disadvantages of the use of NMP that as said before is toxic and irritant, but it is also
expensive, and in operation at high temperatures can give, thanks to lithium or the lithiated graphite, LiF
with an exothermic and potentially dangerous reaction.
24
2.4.2 CMC
The carboxymethyl cellulose (Figure 24) is a derivative compound of cellulose with some hydroxyl
group substituted by carboxymethyl group obtained by esterification.
Figure 24-Carboxymethyl cellulose structure
CMC is prepared by pre-treating cellulose with NaOH to smash the crystal structure, and then
chloroacetic acid is added, giving CMC and NaCl. The functional properties of CMC depend entirely
on the substitution grade of the hydroxyl groups. Different kinds of synthesis lead to different grade of
substitution and usually it is between 0.6-0.95 per monomer.
It is also used as a sodium salt (Na-CMC) prepared by reaction of cellulose with chloroacetic acid and
NaOH.
It is a water soluble binder, cheaper than PVdF, usually used for the synthesis of green electrodes.
2.4.3 PAA
Polyacrylic acid (Figure 25) is a synthetic polymer obtained from the monomer acrylic acid.
Figure 25-Polyacrylic acid structure
At neutral pH, it is an anionic polymer and it is a polyelectrolyte.
PAA is hygroscopic, it can absorb an equal quantity in weight of water and can resist huge volume
expansion/contraction making it a suitable polymer for binder purpose.
It is produced by radical polymerization of acrylic acid with a peroxide, nitro-compound or others radical
initiators.
It can absorb huge amounts of water and may compromise the running the electrode. This problem can
be easily overcome by drying the electrode in oven before cell assembling.
25
For the A.M. – Binder interface, PAA is a stronger binder than PVdF due to its ability on making hydrogen
bonds with A.M. particles (e.g. metal oxide). Shinici et Al. also demonstrated that PAA can take huge
volume expansion/contraction and maintain the adhesion avoiding detachment(Figure 26 (37)).
Figure 26-graphic example on how PAA can stabilize volume expansion
26
3. Experimental techniques
3.1 X-Ray Diffraction (XRD)
X-Ray diffraction is a technique used for studying the composition and the crystal structure of an
unknown compound, in which an X-ray beam hits the sample and diffracts in specific angles depending
on the crystal phase.
Since many materials can form crystals (such as salts, metals, minerals, semiconductors, organic,
inorganic and also biological molecules like DNA, nucleic acids and proteins), XRD has become a
fundamental techniques in the development of many scientific fields.
During the XRD analysis, the sample is irradiated by X-ray source and diffract the beam along the crystal
planes, giving destructive and constructive interference depending on the incident angle and the
distance between planes.
The phenomenon behind XRD can be understood by the Bragg’s Law (Figure 27), that explains at which
angles there will be constructive interference.
Figure 27-Graphical example of Bragg's Law
Bragg diffraction occurs only when radiation has a wavelength comparable to atomic spacing.
X-Rays can be produced by several types of sources such as:
◦ X-Ray tubes;
◦ Synchrotron;
◦ Radioisotopes.
In the thesis work were used two different XRPD (X-Ray Powder Diffraction) instruments, both with X-
Ray tube as source but with different geometries and detectors.
X-Ray tubes (Figure 28), also called Coolidge’s tube, are composed by cathode (heated tungsten
filament) in which electron are extracted by an electric field, and an anode that can be W, Cr, Cu, Mo,
Rh. Au, Fe, Co. When the electrons hits the anode, there will be two different interactions:
◦ The electrons are slowed down and the energy is converted into a “braking radiation”
(bremsstrahlung radiation);
27
◦ The electrons have sufficient energy to cause the expulsion of a core electron of the metal
anode. At this point, an outer electron will fill the vacancy and the energy will be released as an
X-Ray radiation.
Figure 28-Schematic view of an X-Ray tube with a copper anode
The main difference of the two different XRD instruments are:
A curved detector INEL CPSD 180 (Curved Position Sensitive Detector);
A Bragg-Brentano geometry with sample and punctual detector undergoing θ / 2 θ scan.
The INEL CPSD 180 (Figure 29) is made up of a curved anode and a segmented anode. In the space
between the anode and the cathode there is an atmosphere of 85 % Argon and 15 % CO2. A Kapton
window lead the photons to enter the detector. Once an X photon enters the detector the gas will be
ionized and the electrons accelerated by the electric field between cathode and anode: this electrons
have sufficient energy to ionize other gas atoms generating a current. The current will be revealed by
the segment of the cathode which is perpendicular to the enter point of the photon.
Figure 29-Schematic view of INEL CPSD 180 (38)
The main advantage of this detection geometry than a Bragg-Brentano geometry detector is short time
of record to obtain a good spectrum, because the needed angles are acquired together at the same
time. The main drawback of these features is a resolution limited by the number of available channels.
28
In the Bragg-Brentano geometry (Figure 30), the sample is fixed in a precise focalized position, which
will be preserved changing simultaneously the incident angle and the detection angle.
Figure 30-Schematic view of Bragg-Brentano geometry
All the angles are collected sequentially so that means a longer time for recording the diffractogram, but
the resolution is limited only by the precision of sample and detector position.
3.2 Thermogravimetric Analysis (TGA)
Thermal analysis techniques involves heat exchange and commonly control or measure temperature.
There exist three different kind of thermal analysis:
◦ Thermogravimetric Analysis (TGA);
◦ Differential Thermal Analysis (DTA);
◦ Scanning Differential Calorimetry (DSC);
The one used to study the synthesized active materials is TGA in which temperature is controlled and
the sample weight is monitored. With this kind of analysis it is possible to take information about physical
transformations (like sublimation or vaporization) or chemical transformations (like oxidation or
decomposition). In practical terms, a temperature ramp is applied to the sample and with a microbalance
the weight is monitored during the analysis.
This instrument can only give information about at which temperature the weight changes, and cannot
indicate which chemical species are formed or are responsible for these variations. In order to do that,
the instrument can be coupled with an FTIR (Fourier Transform Infrared Spectroscopy) or an MS (Mass
Spectrometer).
29
In electrochemical studies, this kind of analysis is usually used after a composite synthesis, in which an
active material is paired with carbon (graphene sheets, nanotubes, nanowire, carbon coating etc.), to
know the percentage of carbonaceous material in the composite.
A common thermogravimetric analyser (Figure 31) is composed by:
◦ A microbalance;
◦ An oven;
◦ A gas-feeding system (for inert or reactive environment);
◦ Automated control system for acquire and elaborate data.
When the sample weight changes upon temperature variation (A), the position of the balance arm (B)
change and the light emitted by the lamp (C) is recorded by a photodiode. A feedback current is applied
to a magnetic coil (below the lamp C) to restore the pristine position and the current is recorded by the
acquisition system (D), elaborated by E and plotted by F.
3.3 Scanning Electron Microscope (SEM)
A Scanning Electron Microscope (SEM) is an electron microscope in which the sample is scanned in
the surface by a focused beam of electrons. The electrons interact with the sample giving information
about the topography and the composition.
The main signal produced by the interaction electron-sample are:
◦ Backscattered electrons (BSE);
◦ Secondary electrons (SE);
◦ X-Rays emitted by the sample.
All these signals are the result of scattering events, which can be divided in elastic scattering (no energy
exchange) and inelastic scattering (energy exchange between the two parts).
Figure 31-TGA scheme
30
Backscattered electrons are the result of elastic scattering: the electrons penetrate the sample and
escape from it; while secondary electrons come from the outer shell of the sample atoms and have a
lower energy than BSE.
BSE and SE are the information carriers of the imaging signal, BSE carries info about the sample
composition, the image will be lighter in heavier elements and darker in lighter elements (higher atomic
weight lead to higher number of elastic scattering event, steep surface will give brighter image than flat
surface); and void or inclusions while SE carries info only about the sample surface.
The instrument structure is quite similar to an optical microscope (Figure 32), in both there is a light
source (lamp in optical and electron gun in SEM), objective and focusing lenses that in the case of SEM
are coils that apply a magnetic field to the electron beam.
Figure 32-Comparison between an optical microscope and a scanning electron microscope
Two types of electron gun exist: thermionic emission gun and field emission gun. In the former electrons
are emitted because of Boltzmann’s distribution, upon heating some electrons have enough energy
exceed the work function. The latter uses an electric field to extract electrons from the metal tip (usually
tungsten, Figure 33).
31
Figure 33-Scheme of a field emission gun. The tungsten tip act as a cathode and the electric field between cathode and the
two anodes extract electrons from the tip with a beam thickness of 100 nm
The main design of objective lenses (focusing lenses) in SEM are pinhole lenses, immersion lenses and
snorkel lenses (Figure 34).
Figure 34-Objective lenses in SEM analysis. From the left: pinhole lenses, immersion lenses and snorkel lenses.
The pinhole lenses has no limit in the sample size but the magnetic field is only inside the lenses (higher
aberrations). In the immersion lenses, the sample is inside the lenses and limit
the aberrations but also the sample size. The snorkel takes the main advantages of both former lenses
by applying a magnetic field that extends directly to the sample. With snorkel lenses two types of
detectors can be used: a TTL detector (Through The lens detector) and ET detector (Everhart-Thornley
detector).
32
The first one is a SE dedicated detector, the electrons spiral up through the lenses and are attracted to
the detector by a bias (Figure 36). The ET detector (Figure 35) can discriminate SE and BSE. Is
composed by a Faraday cage and a scintillator inside it. By applying a negative bias on the Faraday
cage SE have not enough energy and are rejected, BSE have enough energy to hit the scintillator (their
trajectories are not affected). Applying a positive bias the SE trajectories are bent and they are attracted
to the detector.
Figure 35-Both cases of positive and negative bias on ET detector. In case a) there is a negative bias and SE are rejected. In
case b) SE are attracted and collected from ALL angles.
In figure 36 there is a schematic view of the double detector structure with a snorkel lens.
The instrument used for the morphologies studies is a SEM ZEISS Sigma 300 which use a TTL and ET
detector and a snorkel lens as objective lens.
Figure 36-Schematic view of a snorkel lens with TTL detector and ET detector
33
3.4 Galvanostatic cycles
It is a potentiometric technique applied to study how the capacity of a cell change during its cycle life.
The potentiometric techniques measure the equilibrium potential and depends on the Nernst’s equation:
𝐸 = 𝐸0 +𝑅𝑇
𝑛𝐹𝑙𝑛
[𝑂𝑥]
[𝑅𝑒𝑑]
Where: R = Gas constant (8.31 J mol-1 K-1);
T = Temperature in K;
n = Number of transferred electrons;
F = Faraday constant (96494 C).
Galvanostatic cycles (GCPL, Galvanostatic Cycles with Potential Limitation), measure potential
variation of an electrode during the charge and discharge process at constant or controlled amounts.
The current, and so the speed of charge / discharge, can be chosen and is often expressed as C-rate
computed from the cell capacity. The C-rate is a measure of the rate at which a battery is charged or
discharged. For example, a C-rate of 1C means that the necessary current is applied or drained from
the battery to complete discharge or charge in one hour, a C/2 rate 2 hours and so on.
The potential can be measured in intervals of time or potential and the plot Ewe vs. Time (Figure 37)
will furnish info on the capacity and the stability of the cell.
Figure 37-Example of GCPL Ewe vs. Time plot
More important is the elaboration of the collected data, which result into three different plot:
◦ Specific Capacity (mAh / g-1) Vs. Number of Cycles;
◦ Ewe Vs. Specific Capacity (also called profiles);
◦ dQ / dEVs. Ewe (differential analysis).
34
With the former plot (Figure 38) is possible to determine the irreversible capacity that is the capacity lost
at the first cycle (usually due to the SEI formation), and how it is the capacity evolves during the cycle
life. The irreversible capacity can be easily calculated by subtracting the specific capacity of discharge
and charge of the first cycle.
Figure 38-Example of Number of Cycles Vs. Specific Capacity plot.
The second type of elaboration allows to compare the Specific Capacity and the Ewe, analysing how
the Specific Capacity changes upon potential variation.
Figure 39-Example of Specific Capacity Vs. Ewe plot
In figure 39 the capacity loss during the first cycle is clearly visible. The plateau represent a phase
transition that happens during the cycling of the cell (lithium insertion/deinsertion, conversion reaction,
alloying etc.).
The differential analysis (Figure 40) allows to observe in detail what happens within the electrode.
35
Figure 40-Example of Ewe Vs. dQ/dE plot
This kind of plot is quite similar to a cyclic voltammetry plot and each peaks is due to a specific reaction.
3.5 Cyclic Voltammetry
Cyclic voltammetry (CV) is a type of potentiodynamic measurement. In a CV experiment, the working
electrode potential is ramped linearly until it reaches the set potential, then the working potential is
ramped linearly in the opposite direction to return to the initial potential. The cycle can be repeated as
many times as needed.
The rate of voltage change over time is usually known as scan rate. The potential is recorded between
the working electrode and the reference electrode, while the current between the working electrode and
the counter electrode.
Figure 41-CV example
Usually, cyclic voltammetry is used to investigate the behaviour of a redox couple like redox potential or
electrochemical reaction rates, and if the reaction is reversible or non-reversible.
In figure 41 there is an example of cyclic voltammetry. In this case, it can be easily understood which
reactions are reversible or not. The peak at ≈ 0.6 V during the cathodic scan of the first scan is
irreversible and probably due to the reduction reaction of the active material and the irreversible
reductive reaction of the electrolyte (to form the solid electrolyte interphase film, SEI film).
36
Observing the peaks in the further cycles, we can say that there are only reversible reaction of the redox
couple (reduction on the cathodic scan and oxidation on the anodic scan).
The way to discriminate if a reaction is reversible or not is to observe the potential of the cathodic peak
and the anodic peak (empirically, the more the peak get closer, the more the reaction is reversible) or
the current ratio ipa/ipc where ipa and ipc stands for the anodic and cathodic current respectively. The more
the ratio get closer to one (similar height on the peaks), the more the reaction is reversible.
In Li-ion batteries studies, CV is used especially for understand which reactions and how many reaction
happen at the first cycle and how it is their behaviour in the further cycles.
3.6 Electrchemical Impedance Spectroscopy
(EIS)
One of the most important and fundamental laws on Physics is that of Ohm’s law.
This law define the relationships between the potential, the current and the resistance in an ideal
conductor. The Ohm’s law state that a current I through a conductor between two points is proportional
to the potential V across the two points. The constant ratio is the resistance R in that segment.
Mathematically:
𝑅 =𝑉
𝐼
The applied current can be direct (dc) for an ideal resistance. When interfaces are present as in an
active electrochemical system, alternate current is needed to avoid polarization of electrodes.
The proportionality will turn out to be a complex number and is impedance Z.
𝑍 =𝐸𝑡
𝐼𝑡
=𝐸0sin (𝜔𝑡)
𝐼0sin (𝜔𝑡 + 𝜑)
Where Et is the potential at time t, E0 the amplitude of the signal, ω is the radial frequency.
It is the response current shifted in phase (ϕ) and has a different amplitude than the applied current I0.
Expressing the impedance as a complex number, the potential is described as:
𝐸𝑡 = 𝐸0 exp(𝑗𝜔𝑡)
and the current response:
𝐼𝑡 = 𝐼0 exp(𝑗𝜔𝑡 − 𝜑)
The impedance is then represented as a complex number:
𝑍(𝜔) =𝐸
𝐼= 𝑍0 exp(𝑗𝜑) = 𝑍0(𝑐𝑜𝑠𝜑 + 𝑗𝑠𝑖𝑛𝜑)
37
Dara representation
The expression for Z(ω) is composed of a real and an imaginary part. If the real part is plotted on the X-
axis and the imaginary part is plotted on the Y-axis of a chart, giving a "Nyquist Plot" (Figure 42).
Figure 42-Simulation of an impedance spectrum in Nyquist format. Re = 1 Ω, Rt = 2 Ω, Cdl = 100 µF, Aw = 1 Ω s-0.5, fc =
795.77 Hz
The impedance response of the electrochemical system can be modelled by an Equivalent circuit. In
the equivalent circuit, every electrochemical process (e.g. charge transfer, electrolyte resistance etc.)
can be represented by an electrical element of the circuit, such as Capacitor or Resistor. In the case on
figure 42, the equivalent circuit will be:
Figure 43-Randles circuit of an electrochemical systems with a single faradaic reaction with a semi-infinite linear diffusion
Where Re represent the electrolyte or ohmic resistance, Rt is the charge-transfer resistance, Cdl is the
double-layer capacitance and Zw is the Warburg impedance related to diffusion.
38
4 Synthesis and Characterization of
Fe3O4 nanoparticles, Fe3O4 with vanillin
template and Fe3O4@rGO as anodes for
LIBs
The aim of this work is the study of Fe3O4 as an innovative anode material for lithium-ion batteries and
possible solutions for the characteristic issues of conversion materials (voltage hysteresis, volume
expansion and remarkable structural change).
Three materials were studied:
◦ Fe3O4 nanoparticles (Fe3O4 nps);
◦ Fe3O4 with vanillin templates (FeVan);
◦ Fe3O4 composite with reduced graphene oxide (Fe3O4@rGO).
Each material were first morphologically, structurally (SEM, XRD) and thermally studied(TGA, only
FeVan and Fe3O4@rGO). Finally, electrochemical characterization was carried out. The electrochemical
analysis is usually planned as followed:
◦ Cyclic voltammetry to understand the potential window of the material, at which potential take
place reactions, the reactions behaviour (reversible or irreversible).
◦ GCPL long cycling at 1C. Usually used to study the cycling stability of the material, and
extrapolate profiles and differential analysis.
◦ GCPL high rate long cycling at 2C and 4C.
◦ Rate capability. It is a GCPL technique which consist of 5 cycles at C / 10, 5 * C / 5, 5 * C / 2, 5
* 1 C, 5 * 2 C, 5 * 5 C, 5 * 10C and 1C cycles.
◦ Electrochemical Impedance Spectroscopy.
39
4.1 Synthesis of Fe3O4 nps, FeVan and
Fe3O4@rGO
Both Fe3O4 nps and FeVan were synthesized by co-precipitation method, while Fe3O4@rGO were
synthesized by sonication of graphite oxide and subsequent reduction with hydrazine.
The co-precipitation is an easy way to produce nanoparticles, the key factors for the best result are:
◦ The concentration ratio between the two salts;
◦ The base concentration;
◦ The stirring rpm;
Fe3O4 nps were synthesized with the following procedure:
1.093 g of FeCl2 ∙ 4H2O + 3.282 g FeCl3 ∙ 6H2O
+ 40 mL of dist. H2O
+ 100 mL of NH4
After reaching 80 °C
+30 mL of Conc. NH4OH
Reflux for 8h
The precipitate was washed 4 times with H2O, 3 times with Acetone and 2 times with ethanol. After that,
it was dried in vacuum with rotavapor and then dried in oven at 65 °C.
The final result is a brownish fine powder.
FeVan synthesis is quite similar to the Fe3O4 nps synthesis, and differs only with the addition of anillin
following the scheme:
1.0 g of FeCl2 ∙ 4H2O in 20 mL of H2O + 2.7 g FeCl3 ∙ 6H2O in 20 mL of H2O
40
+ 1 g of vanillin in 10 mL in H2O
After reaching 70 °C
+50 mL of Conc. NH4OH until pH 11
Reflux overnight at 70 °C
The precipitate was washed 3 times with H2O, 3 times with Acetone and 3 times with ethanol. After that
was annealed in oven at 800 °C for 8 h with a rate of 2 °C / min.
As for Fe3O4 nps, FeVan is a brownish fine powder.
Fe3O4@rGO was synthesized taking the Fe3O4 nps and following this methodic:
200.95 mg of graphite oxide
Mechanical Exfoliation by
Sonication 1h (power 50 W, pulse 1 s)
+ 499.69 mg of Fe3O4 Nanoparticles
Sonication 1h (power 50 W, pulse 1 s)
+ 10 mL Hydrazine hydrate in ice bath
Sonication 2h (power 50 W, pulse 1 s)
Filtration with Millipore (0.2 µm GTTP) and ethanol.
The solution was firstly dried at room temperature and then dried in oven at 60 °C.
41
4.2 Thermogravimetric analysis
As said before, TGA analysis was conducted only on FeVan and Fe3O4@rGO to verify the amount of
carbon on the active material and to allow calculation of the specific capacity of the composites material.
Both samples were heated with a rate of 10 °C / min in air atmosphere.
Figure 44 shows the TGA of FeVan.
Figure 44-TGA of FeVan
TGA of FeVan sample is shown taking into account pre-annealing and post-annealing powders. The
pre-annealing FeVan shows a weight drop due to transformation of carbon into carbon dioxide. The
post-annealing FeVan shows a sloping region, corresponding to a weight increase probably due to Fe
(II) oxidation into Fe (III).
In figure 45 we can see TGA of Fe3O4@rGO.
The sample shows a high weight drop (about 24 %) at ≈ 450 °C due to the transformation of carbon into
carbon dioxide.
From the analysis Fe3O4@rGO shows a composition of 76,46 % of Fe3O4 and 23.54 % of rGO, and from
that the new specific capacity can be estimated as 881.63 mAh g-1.
Figure 45-TGA of Fe3O4@rGO
42
4.3 SEM analysis
All three samples were analysed by SEM analysis to investigate the morphology and the nanoparticles
size.
All the samples were analysed at ≈ 40 K X magnification and at ≈ 270 K X.
Figure 46 and 47 show Fe3O4 nps at 200 nm and 20 nm scales respectively.
Figure 46-Fe3O4 nps SEM image at 40.54 K X
Figure 47-Fe3O4 nps SEM image at 274.58 K X
As we can see the nanoparticles of Fe3O4 are so tiny (≈4-5 nm) that focusing was difficult, proving the
good outcome of the reaction.
43
Figure 48 and 49 shows FeVan at the two selected scales. As we can see, particles are much
bigger (from ≈20 nm to ≈100 nm) than the previous material due to the templated structure.
Figure 48-FeVan SEM image at 46.56 K X
Figure 49-FeVan SEM image at 298.40 K X
44
Figure 50 and 51 show the SEM images of Fe3O4@rGO.
Figure 50-Fe3O4@rGO SEM image at 40 K X
In both SEM scans, the sample presents graphene sheets embedding nanoparticles bigger than
the pristine oxide, suggesting a probable agglomeration during the composite synthesis.
Looking closer at the graphene sheets, is possible to see in transparency nanoparticles inside it.
Figure 51-Fe3O4@rGO SEM image at 275 K X
45
4.4 XRD
The crystal structure three samples were analysed by XRD technique to check the presence of
impurities and the outcome of the reaction.
The samples Fe3O4 nps and Fe3O4@rGO were analysed with a XRD diffractometer equipped with
curved detector INEL CPSD 180, while FeVan were analysed with a XRD diffractometer equipped with
Bragg-Brentano θ / 2 θ geometry detector.
Figure 52-XRD of Fe3O4 nps
The X-ray diffractogramm of the pristine magnetite powder, presents a series of peaks consistent with
Fe3O4 as indexed in JCPDS, card no. 19-0629.
The X-ray diffractogramm of the magnetite / rGO composite (Figure 53) differ a bit with the previous
sample. The sample still has the Fe3O4 XRD pattern (black labelled (220), (311), (400), (422), (511),
(440)) and other peaks (red labelled (104) and (113)), due to Fe2O3 impurities. Probabily the other peaks
of the Fe2O3 XRD pattern are overlapped with the predominant peaks of Fe3O4.
46
Figure 53-XRD of Fe3O4@rGO
The FeVan sample gaves the most resolute peaks, thanks to the Bragg-Brentano geometry, and present
a series of peaks consistent with Fe3O4 and Fe2O3 XRD pattern (Figure 54).
Figure 54-XRD of FeVan
As for Fe3O4@rGO, the black labelled peaks are assigned to Fe3O4 spinel structure and the red ones
are assigned to Fe2O3 spinel structure. Looking at the XRD spectrum, the hematite pattern is much more
present than in the previous sample making think to an high percentage of Fe2O3 impurities.
47
4.5 Electrodes preparation and cell assembling
The electrodes preparations are quite similar between samples even if the active material is different.
The materials used for this procedure are the polymeric binder (especially used for its mechanical
properties), a conductive material (usually the active material, i.e. metal oxide, has a poor electronic
conductivity) and the active material.
After fixing the ratio of components (common ratio are 70:20:10 or 80:10:10, active material : conductive
material : binder) the binder is weighted and put into a vial with the solvent and a magnetic stirrer. The
binder is the first added component because it takes time to dissolve and the amount of solvent needs
to be adjusted for a good density of the slurry.
After that the active material and the conductive material are weighted and milled in a mortar or with ball
mill until reaching an homogenous powder. The obtained powder is slowly added to the binder solution
and solvent is adjusted for the best density. The slurry is left stirring for a fixed time (depending on
binder) and then casted onto a metal foil (copper for anodes and aluminium for cathodes). For the
casting a special tool called “Doctor Blade” is used, in order to cast the slurry with a fixed thickness (e.g.
200 µm, 100 µm).
Then the obtained layer is left drying (drying condition also depend on binder and solvent).
The polymeric binder used for this work is PAA because it works better with Fe3O4 than the common
PVdF (25), with ethanol as solvent and Super C65 as conductive material.
The layer were prepared with a ratio of 70:20:10 with 200 or 100 mg of active material and once casted
left drying at 70 °C for 2 hours.
In table 5, 6 and 7 the composition of Fe3O4 nps, Fe3O4@rGo and FeVan layers are reported
respectively.
Table 5-Fe3O4 nps layer composition
Table 6-Fe3O4@rGO layer composition
% Needed (mg) Weighted (mg) Real %
FeVan 80% 100 101,3 80,0
Super C65 10% 12,5 12,7 10,0
PAA (MW 450000) 10% 12,5 12,6 10,0 Table 7-FeVan layer composition
% Needed (mg) Weighted (mg) Real %
Fe3O4 nps 70% 200 198,5 69,3
Super C65 20% 57,1 57,5 20,2
PAA (MW 450000) 10% 28,5 30,2 10,5
% Needed (mg) Weighted (mg) Real %
Fe3O4@rGO 70% 200 199,9 69,6
Super C65 20% 57,1 58,7 20,4
PAA (MW 450000) 10% 28,5 28,7 10,0
48
Once the layer is dried the electrodes are cut with a tool called “EL-Cell EL-Cut” with a diameter of 9
mm. The cutted electrodes can either be pressed or not, and then taken in small pocket and dried in
oven at 120 °C under void for 12 hours for removing moisture traces. Now the electrodes are ready to
be assembled in cell on the glove box (Figure 55).
Figure 55-Example of glove box
The cell are assembled in glove box because of lithium that can react with moisture, oxygen and
nitrogen:
2 𝐿𝑖 + 2 𝐻2𝑂 → 2 𝐿𝑖𝑂𝐻 + 𝐻2
6 𝐿𝑖 + 𝑁2 → 𝐿𝑖3𝑁
4 𝐿𝑖 + 𝑂2 → 2 𝐿𝑖2𝑂
So the glove box, which is a closed chamber filled with argon, is the perfect environment for cell
assembling.
4.5.2 Cell assembling
The most common cell used in lithium battery study are the “T cells” (Figure 56).
Figure 56-T Cell schematics
49
They are composed of 3 electrodes:
◦ Working electrode;
◦ Counter electrode;
◦ Reference electrode.
The working electrode is made up of the sample under examination, while the reference and the counter
electrode are made up of metallic lithium and all of them are connected to steel collectors. The central
body is in Polypropilene and is sealed with o-ring and Teflon on thread.
The T cell is used instead of a common cell with 2 electrodes because in order to minimize polarization,
which is principally due by the resistance of the active material and the charge / discharge current ΔV=I
∙ R.
With a reference electrode, the potential is recorded in presence of negligible current and polarization
(OCV, Open Circuit Voltage).
4.6 Cyclic Voltammetry
Cyclic voltammetry is the first electrochemical measurament taken on all the synthesised materials, with
the purpose of understanding the electrochemical behavior of samples under cycling. All the
measurament were taken with a scan rate of 0.1 mV / s within a potential window of 0.001 V – 3.000 V
vs Li+/Li.
Figure 57 shows the Cyclic voltammetry of the first 3 cycles of Fe3O4 nps.
Figure 57-Cyclic voltammetry of Fe3O4 nps
50
During the first cathodic scan three peaks can be observed at 1.56 V (*), 1.02 V (B) and 0.75 V (A). The
peak at 1.56 V (*) has been observed also for other transition metal oxides conversion material (39),
and describes not better identified irreversible interfacial processes only occurring during first discharge.
For the peak at 1.02 V (B), Thackeray (40) proposed a mechanism in which intercalation of Lithium into
the spinel structure of Fe3O4 occurs according to the reaction:
𝐹𝑒3𝑂4 + 2𝐿𝑖+ + 2𝑒− → 𝐿𝑖2𝐹𝑒3𝑂4
The subsequent reduction of Fe to Fe0 (described with the sharp peak at 0.75 V (A)) occurs by the
conversion reaction leading to the composite Fe0 nanoparticles and Li2O:
𝐿𝑖2𝐹𝑒3𝑂4 + 6𝐿𝑖+ + 6𝑒− → 3𝐹𝑒0 + 4𝐿𝑖2𝑂
In this potential reagion also the decomposition of electrolyte towards the carbon surface, giving the
passivation layer, occurs.
After the first cycle, peak (B) disappears and peak (A) shifts to 0.8 V.
During the anodic scan, a couple of broad peaks (C) and (D) are visible at 1.57 V and 1.89 V and can
attributed to the oxidation of Fe to Fe2+ and Fe3+ respectively.
In Figure 58 the cyclic voltammetry of FeVan is reported.
Figure 58-Cyclic Voltammetry of FeVan
As for Fe3O4 nps, there are two anodic peaks (C) and (D) at 1.58 V and 1.89 V respectively, while the
cathodic scan look completely different, showing only one peak at 0.75 V (A) with two tips.
The presence of the peak due to the interalation mechanism of lithium into the Fe3O4 spinel structure is
missing, and probabily overlapped with peak (A), giving a larger peak than the former sample and the
two tips.
51
As for Fe3O4 nps, after the first cycle the peak (A) shifts to 0.98 V (second cyle) and 0.93 V (third cycle).
Looking at the further cycles in figure 59, the peak (A) slowly shift to 0.9 V (A*) upon cycling.
The positive scan shows the two peaks (C) and (D) at 1.58 V and 1.89 V due to the oxidation of Fe to
Fe2+ and Fe3+ respectively.
Similar cyclic voltammetry curve have been previously observed on Fe2O3 (41), which explains the
difference between the Fe3O4 nps scan and the FeVan scan.
Figure 59-Cyclic voltammetry of FeVan 9 cycles
The last sample analysed was Fe3O4@rGO and its Cyclic voltammetry is reported in figure 60.
Figure 60-Cyclic Voltammetry of Fe3O4@rGO
52
The scan seems pretty similar to the scan of Fe3O4 nps, with two cathodic peaks (A) and (B) at 0.76 V
and 1.0 V respectively, and two anodic peaks (C) and (D) at 1.52 V and 1.86 V respectively.
As before, the peak (B) is responsible of the lithium intercalation mechanism proposed by Thackeray
(40) and peak (A) is due to the conversion reaction:
𝐿𝑖2𝐹𝑒3𝑂4 + 6𝐿𝑖+ + 6𝑒− → 3𝐹𝑒0 + 4𝐿𝑖2𝑂
After the first cycle the peak (B) disappears and the peak (A) shifts to 0.86 V.
The peak (C) and (D), as before, are due to the oxidation reaction of Fe to Fe2+ and Fe3+.
4.7 Galvanostatic cycles
4.7.1 Long Cycling 1C
All the samples were first tested with a specific current of 1C (924 mA g-1 for Fe3O4 nps and FeVan, and
881 mA g-1 for Fe3O4@rGO). From the obtained result the cycles profile and the derivative for the
differential analysis were extrapolated. All the cells ran within a potential window of 0.001 V – 3.000 V
vs Li+/Li and LiPF6 1M in EC:DMC 1:1.
The obtained result for Fe3O4 are reported in Figure 61.
Figure 61-Long cycling at 1C of Fe3O4 nps
The graph reports a very high specific capacity at the first discharge of 1810 mAh g-1 and a subsequent
value of 1231 mAh g-1 on the following charge step with an efficiency of 68 %. This high irreversible
capacity can attributed to first cycle irreversible processes such as the formation of the passivation layer
on the very large surface of the nanoparticles of active material making up the electrode. The specific
53
capacity drops until the 30th cycle, with an efficiency ranging from 89 to 97 %, and reaching a value of
527 mAh g-1. After the 30th cycle, the specific capacity goes up reaching the constant value of 570 mAh
g-1 and an efficiency between the charge / discharge close to 99 %.
The observed specific capacity was not as expected of 924 mAh g-1 and this can be regarded to different
reasons:
◦ A possible desegregation of the active material with a detachment of it from the conductive
collector or from other electrode portions.
◦ Iron aggregation with subsequent low conductivity, low lithium diffusion and detachment from
the collector.
◦ As said, too small nanoparticles size which leads to high irreversible capacity.
These issues can be overcome with the vanillin template structure or with graphene sheet that promise
a good conductivity and mechanical resistance to the volume expansion.
The vanillin-derived templated structure gave the results shown in Figure 62.
Figure 62-Long Cycling at 1C of FeVan
FeVan reports a first discharge specific capacity of 1421 mAh g-1 and a subsequent value of 984 mAh
g-1 on the following charge step, with a 1st cycle efficiency of 69 %. As for Fe3O4 nps, this high irreversible
capacity is due to the formation of the passivation layer.
The specific capacity reaches a value of 847 mAh g-1 on the 100th cycle with an efficiency ranging from
97 % to close to 99 %.
As observed, the template structure helped to the mechanical stress leading to a specific capacity close
to 924 mAh g-1 and a good cycle life and stability at 1C.
54
The Fe3O4@rGO composite sample, shown in figure 63, exhibit an higher initial stability than the pristine
magnetite sample, but it seems still unstable on long cycling, with a drop of the specific capacity at 690
mAh g-1 at the 100th cycle.
Figure 63-Long Cycling at 1C of Fe3O4@rGO
The specific capacity of the first discharge reaches a value of 1634 mAh g-1 and the subsequent charge
half-cyle a value of 1186 mAh g-1, with an efficiency of 72 %. The specific capacity remains higher than
1000 mAh g-1 for the first 40 cycles, with an efficiency close to 100 % in the first 20. Capacity values
higher than the theoretical ones have already been reported for conversion materials by several authors,
and could be associated with several mechanisms. Among those, interfacial lithium storage can provide
extra capacity (42).
4.7.3 Galvanostatic Profiles and differential analysis
Looking at the galvanostatic E vs. Q charge / discharge profiles (Figure 64) of Fe3O4 nps, the first
discharge step reveals a plateau at 1.06 V and a longer one at 0.80 V.
Figure 64-On the left) Galvanostatic profiles E vs V of Fe3O4 nps; On the right) Differential analysis of Fe3O4 nps
55
The subsequent charge shows a slopping plateau from 1.5 V to 1.87 V. From the second discharge on,
any sign of the previuos plateaus disappears in favor of a more s plateau at 0.89 V. During the charge,
the slopping plateau is still present from 1.60 V to 1.85 V. The differential analysis shows a sharp peak
at 0.82 V and 1.06 V, which correspond to the plateaus of the galvanostatic profiles. During the charge
there are two peaks (C) and (D) at 1.55 V and 1.83 V due to the oxidation of Fe to Fe2+ and Fe3+. From
the second cycle, the peak (B) disappears and (A) shift at 0.90 V with a minor intensity.
All the obtained data are consistent with the previous cyclic voltammetry result.
The profiles of FeVan (Figure 65) looks quite different than the former sample.
Figure 65-On the left) Galvanostatic profiles of FeVan; On the right) Differential analysis of FeVan
The first discharge cycle reveal a large plateau at 0.75 V instead of two of Fe3O4 nps, while the slopping
plateau on the charge cycle is still present. From the second cycle the discharge plateau shift from 0.75
V to 1.02 V and a small plateau appears at 1.35 V. From the third cycle only one plateau is present
during the discharge, and a slopping plateau is present during the charge as for Fe3O4 nps. On the
differential analysis, the first discharge has a peak at 0.75 V with two tips as in the cyclic voltammetry
and the oxidation peaks during the charge. The discharge of second cycle has overlapped peaks at 1.02
and 1.35 V. Similar peaks and profiles have been observed in Fe2O3 cyclic voltammetry (41).
In Figure 66 the galvanostatic E vs Q profiles and the differential analysis of the composite Fe3O4@rGO
sample are depicted.
Figure 66-On the left) Galvanostatic profiles of Fe3O4@rGO; On the reight) Differential analysis of Fe3O4@rGO
56
The first discharge profile reveals one small plateau at 1.01 V and a larger one at 0.81 V while the
charge step shows a slopping plateau from 1.54 V to 1.83 V. This plateau are consistent with the
differential analysis in which peak (A) at 0.80 V and peak (B) at 1.01 V represent the plateau on the first
discharge, and the peak (C) and (D) at 1.54 V and 1.80 V represent the slopping plateau of oxidation.
As for the previuos samples, the data are consinstent with the cyclic voltammetry.
The large voltage hysteresis, the visible change in the shape of capacity profiles during the first
charge/discharge cycle are typical features of conversion materials. This behavior has been explained
by several authors, one of the most importants involve the increase in surface area undertaken by the
material during the first discharge cycle from the pristine oxide nanoparticles to the Li2O/M composite
(43).
4.7.4 Long cycling 2C and 4C
The sample were tested at 2C and 4C (1848 mA g-1 and 3696 mA g-1 for Fe3O4 nps and FeVan, 1762
mA g-1 and 3524 mA g-1 for Fe3O4@rGO) to investigate their behaviour at high rate and importantly their
stability upon high currents.
Fe3O4 nps, in figure 67, shows a similar curve of the long cycling at 1C in which the capacity drops
rapidly to low value. The first discharge at 2C reveal a capacity of 1397 mAh g-1 and the subsequent
charge step 984 mAh g-1 with a coulombic efficiency of 70 %. The specific capacity decrease
until the 30th cycle in which slowly become stable at ≈ 430 mAh g-1 and an efficiency of 99.5
%.
Figure 67-Long Cycling at 2C and 4C of Fe3O4 nps
At 4C, the first discharge release a capacity of 1367 mAh g-1 and a charge capacity of 942 mAh g-1 with
an efficiency of 68 %. As for 2C test, the cell becomes stable after the 30th cycles with a low capacity of
270 mAh g-1. Note that all the galvanostatics test on Fe3O4 nps have the same behaviour and look
smother with the increasing of the C-rate.
Figure 68 depict the test result of FeVan. The first discharge at 2C shows a specific capacity of 1373
mAh g-1 and 900 mAh g-1 for the charge step, with a coulombic efficiency of 65 %. The cell becomes
57
stable after the 70th cycle with an efficiency ranging from 97 % to 99 % and reaching the specific capacity
of 620 mAh g-1.
At 4C, the first discharge takes a specific capacity of 1311 mAh g-1 and a charge capacity of 900 mAh
g-1, with an efficiency of 69 %. The capacity decreases until the 20th cycle with an efficiency ranging
from 96 % to 98 % reaching the stable value of 740 mAh g-1 and efficiency close to 99 %.
Figure 68-Long Cycling at 2C and 4C of FeVan
The results of the Fe3O4@rGO composite material are in figure 69.
Figure 69-Long Cycling at 2C and 4C of Fe3O4@rGO
Observing the graphs is easily noticeable that Fe3O4@rGO gives best performance at high rates this is
probably due to the graphene and its high electronic conductivity.
The first discharge at 2C gave a capacity of 1529 mAh g-1 and the subsequent charge step 1103 mAh
g-1 with an efficiency of 72 %. The capacity remain quite stable until cycle 55th, giving an average value
of 1050 mAh g-1 and efficiency constant at 97 %.
At 4C, this composite material exhibit the best behaviour compared to the other samples, with an
irreversible capacity of 1323 mAh g-1 and a charge capacity of 993 mAh g-1 and efficiency of 75 %. The
cell remain quite stable during the cycling with an average specific capacity of 980 mAh g-1 and efficiency
close to 99 %, giving evidence that the composite Fe3O4 reduced graphene oxide works better at high
rates.
58
FeVan shows minor irreversible processes than the other sample, probably giving it more stable
performances. On the other hand, its high rate performances are limited probably by its electronic
conductivity, leading Fe3O4@rGO a more suitable material for high rate uses.
4.7.5 Rate Capability
Rate Capability, as said before, is a test in which charge / discharge cycles at different rates are
performed: 5 cycles at C / 10, 5 at C / 5, 5 at C / 2, 5 at 1C, 5 at 2C, 5 at 5C, 5 at 10C, finally 1C cycles
are reported. This kind of test is considered the “stress-test” for lithium ion batteries and is useful to
investigate how a cell reacts at different polarization and how many capacity can be retained after that
stresses.
The galvanostatic E vs Q profiles at different C-rates are reported as well.
Figure 70 depicts the pristine magnetite rate capability.
Figure 70-Rate capability and C-rate profile of Fe3O4 nps
At C / 10 the sample presents an unstable capacity with an irreversible capacity of 2240 mAh g-1 and a
charge capacity of 1238 mAh g-1 and an efficiency of 55%. The capacity at C / 10 range from 1374 to
1039 mAh g-1 and a maximum efficiency of 90 %. At C / 5 the capacity decrease in a range from 1049
to 767 mAh g-1 and higher efficiency of ≈ 95 %. The capacity decrease and the efficiency increase with
the increasing of the speed, at C / 2 ≈ 550 mAh g-1 and efficiency of 94 %, at 1C 380 ≈ mAh g-1 and 96
%, at 2C ≈ 270 mAh g-1 at 97 % of efficiency, at 5C ≈ 170 mAh g-1 with 98 % and 10 C with a specific
capacity of 120 mAh g-1 and efficieny of 98 %. The unstable capacity shown in the low-rate cycles is
especially due because of predominant chemical processes (irreversible), while at high rates this kind
of reactions are reduced.
The final 1C cycles show a full reversible specific capacity of 350 mAh g-1 and an efficiency of 99.2 %.
The C-rate profiles shows that at slow speed the charge and discharge curve are smoothed, in fact in
the first discarge there is only one pleateau instead of two and also the slopping plateau of the charge
looks smoothed. At higher rate the curve become smaller and so the plateau especially due to the low
polarization of the cell.
59
In figure 71 the result of FeVan are shown.
Figure 71-Rate Capability and C-rate profiles of FeVan
The capacity of the first discharge is 2226 mAh g-1 and the charge capacity 1472 mAh g-1 with an
efficiency of 66 %. Capacity value ranged from 1609 mAh g-1 at C / 10 to 195 mAh g-1 at 10 C, with
coulombic efficiencies ranging from 92 % at C / 10 and 96 % at 10 C.
The final 1C cycles reveal an initial specific capacity of 390 mAh g-1 that decrease reaching the stable
value of 310 mAh g-1. The templated structure takes little improvements at high rate (195 mAh g-1 instead
of 120 mAh g-1 at 10C) while the final capacity initially remain the same and then decrease. The role of
low-rate in enhancing irreversible processes is confirmed.
Fe3O4@rGO (Figure 72) shows an irreversible capacity of 1888 mAh g-1 and an initial charge capacity
of 1233 mAh g-1 with a coulomic efficiency of 65 %. The specific capacity range from 1414 mAh g-1 at C
/ 10 to 554 mAh g-1 at 10 C, and efficiencies ranging from 89 % at C / 10 to 99.10 % at 10C.
The final 1C cycles release an average capacity of ≈ 900 mAh g-1.
As for pristine magnetite, the profile curves look smoother and the first discharge does not have two
plateaus but only one.
The incredible result of the composite material, especially at high C-rate can be attributed to the
graphene which increase the electronic conductivity and allow ciclability at high current densities. The
final high specific capacity can be attributed also to graphene, which enhances the mechanical
resistance again stresses due to different C-rate cycles.
Figure 72-Rate capability and C-rate profiles of Fe3O4@rGO
60
The instability at low-rates and the behaviour of Fe3O4@rGO can be attributed to the high surface of
graphene which leads to an higher amount of SEI and irreversible processes.
4.8 EIS
Electrochemical impedance spectroscopy were conducted at the OCV (Open Circuit Voltage), at the
potential in which the conversion reaction takes place in the 1st cycles (0.75 V for all the three samples)
and then every 10 cycles at the potential of the conversion reaction (0.9 V for Fe3O4 nps and
Fe3O4@rGO, 0.95 V for FeVan). The applied frequencies range from 9 mHz to 101 KHz.
For sake of simplicity, the obtained impedance spectrum will be shown only for the first 30 cycles.
The electrode show a typical impedance dispersions reveal common features, that are typical of
electrodes where Li storage take place, with common features such as:
◦ An intercept on the real axis, corresponding to electrolyte resistance;
◦ An high frequency arc due to accumulation of charge and migration through the passivation
layer;
◦ A medium frequency arc, corresponding to interfacial charge-transfer and accumulation of
charge at the electrical double layer;
◦ A 45° dispersion bending toward vertical line due to diffusion to a blocking electrode.
Figure 73-EIS spectrum of the three samples
At the first glance, FeVan shows an higher impedance than the other two samples. This feature can be
attributed to the nanoparticles size. As we saw on SEM, FeVan exhibit a nanoparticles size of ≈ 50 nm
61
which is bigger than the other two samples, leading to a smaller surface exposed to electrolyte and low
lithium ion and electron migration through the nanoparticles.
Fe3O4 nps and Fe3O4@rGO shows almost the same impedance in the first cycle, even if Fe3O4@rGO
seems to be more stable, but in the further cycles (Figure 74) the composite material exhibit a higher
impedance than the pristine nanoparticles.
Figure 74-All EIS cycles of Fe3O4 and Fe3O4@rGO
As for FeVan, the higher impedance shown by Fe3O4@rGO with respect to pristine nanoparticles can
be explained by the Fe3O4 particles size, which are agglomerated by the synthesis of the composite.
Even if Fe3O4@rGO has a higher impedance than the pristine nanoparticles, it seems to be the most
stable material between the all three samples. This can be explained by the embedding rGO matrix,
which stabilize the Fe3O4 morphology upon cycling.
Enlarging on the EIS spectrum of Fe3O4 nps (Figure 75), the OCV and the first cycle show a perfect
semicircle while the further cycle have only a hint of that circle, suggesting a probable degregation of
the active material and detachment from the current collect. The first cycle shows also a little circle after
the high frequency arc probabily due to inductance of the current collector / electrometer.
Figure 75-EIS enlarged view of Fe3O4 nps
62
Fe3O4@rGO (Figure 76) shows perfect arc related to charge transfer for all the first 30 cycles confirming
morphological stability, plus the same arctifact on the first cycle, as well as at OCV as well.
Figure 76-EIS enlarged view of Fe3O4@rGO
FeVan (Figure 77) shows elongated arc for all the initial cycles, probabily due to the nanoparticle size.
Figure 77-EIS enlarged view of FeVan
63
5. Conclusions
The main goal of this study, as discussed in the introduction, is to develop strategies to improve the
electrochemical performances of Fe3O4 electrodes for Lithium-Ion batteries.
Three Iron oxide-based anode materials were developed in the attempt of find the new nanostructure
able to satisfy the market demand, and especially the energy storage need for renewable energies.
First, Fe3O4 nanoparticles were synthesized as a conversion material fo LIBs. A simple base-promoted
coprecipitation method was used for the synthesis of the nanoparticles.
The material structure and morphology were investigated by SEM and XRD, showing nanoparticle size
of 10 nm and perfect spinel structure of Fe3O4 compared to JCPDS, card no. 19-0629. The
electrochemical study revealed a cycle instability at the first cycles which leads to a fast capacity
decrease in the first 30 cycle, reaching the specific capacity of 570 mAh g-1. The instability is probabily
due to the nanoparticle size, that are so tiny to increase the irreversible capacity at the first cycle, and
to detachment from the current collector due to the volume expansion during charge and discharge. The
rate capability tests revealed poor performances especially at high rates (120 mAh g-1 at 10 C), while
the impedance spectroscopy reveals low impedance in the first 30 cycles, due to the particle size which
leads to a better conductivity of lithium ion and electron, and instability of the active material with an
increase of the impedance upon cycling.
As a second attempt, a Fe3O4 were synthesized by coprecipitation method with a templated structure of
vanillin. The sample was then annealed, so that vanillin burnt leaving a carbon template. In SEM analysis
the sample revealed a particle size higher than the pristine conversion material obviuosly due to the
template. In XRD, the obtained structure revealed an high amount of Fe2O3 and the TGA of the annealed
sample shows an increase in weight of the probable oxidation of Fe (II) of Fe3O4 into Fe (III). The
electrochemical test outlined a good stability with a specific capacity of 920 mAh g-1 and good capacity
at high rates (620 mAh g-1 at 2 C and 740 mAh g-1 at 4 C). Poor performances were achieved at higher
rates of 5 C and 10 C were the capacity decrease to 230 and 195 mAh g-1 respectively. The returned 1
C on rate capability shows no improvements with capacity of 310 mAh g-1 and the impedance
spectroscopy revealed high impedance, which can be the cause of the poor high rate performances,
due to higher particles size.
The third attempt was a composite material made of Fe3O4 embedded in a reduced graphene oxide,
produced by the pristine nanoparticles of the first attempt.
The composite was synthesized with sonication method in which graphite oxide were mechanically
exfoliated and then chemically reduced by hydrazine.
In SEM, Fe3O4@rGO revealed an agglomeration of the particles with an increase in size while the TGA
outline a composition of 24 % of graphene and 76 % of Fe3O4.
With the XRD it was possible to discover small Fe2O3 impurities.
64
During cycling, the composite some instability at 1 C, with capacity dropping from 1200 mAh g-1 to 800
mAh g-1. As the rates increase the performances and the stability increase (however less stable than
FeVan)with a specific capacity of 1000 mAh g-1 at 2 C and a constant specific capacity of 993 mAh g-1.
The rate capability outline excellent results with a specific capacity at 10 C of 554 mAh g-1 and a returned
specific capacity at 1 C of ≈ 900 mAh g-1. The impedance spectroscopy shows a low impedance in the
first 30 cycles that increases during cycling, this is in agreement with a probable decomposition of the
electrodes at slow rates due to chemical process that are predominant than the electrochemical process,
indeed as the rate increases the electrochemical process becomes predominant and the performances
of the electrode are more table.
Concluding pristine Fe3O4 reveal poor performances and poor cycle life due to the instability of itself
upon cycling, FeVan shows good stability and good specific capacity especially at slow rate, while
Fe3O4@rGO shows a good specific capacity at slow rates and excellent performances at high rates
which makes it an excellent materials for high energy and power densities purposes.
Future works will include:
◦ Trying to assemble cell with as electrolyte EC : DMC 1 : 1 and a small percentage of Vinyl
Carbonate, which it should decrease the irreversible capacity and lead to a more stable cell.
◦ Trying different morphologies like nanotubes, nanowire and nanorings.
◦ Trying mixed oxide as ZnFe2O4, MnFe2O4 etc.
◦ Trying other composite such as carbon coating or graphite composite.
65
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Ringraziamenti
Eccomi qui finalmente, a scrivere questi ringraziamenti finali della mia laurea magistrale.
Sembra passato così poco tempo da quando ho scritto i ringraziamenti della laurea triennale;
eppure nel frattempo sono successe così tante cose: ho concluso egregiamente due anni di un
bellissimo corso di studi, ho passato sei mesi un una capitale europea come Lisbona, che mi ha
insegnato e vedere la vita da un altro punto di vista, soprattutto mi ha mostrato com’è vivere
autonomamente. C’è stato un terremoto che mi ha tolto una casa piena di ricordi, e mi ha fatto
soffrire vedere i miei affetti in difficoltà da più di 2000 km di distanza, non poter essere lì a
dare un aiuto. Comincio ringraziando la mia famiglia, che spero un giorno creda in me come io
credo in loro. Su tutti mio padre e mia madre, che in un modo o nell’altro mi hanno sempre
spronato. Ringrazio i mie fratelli Reynaldo, Eder e Francesca, fratelli come amici. Tutte le
nonne che mi vogliono bene, a cui non interessano troppo i miei risultati…l’importante è che
io abbia mangiato, ma è giusto così. Grazie di cuore ai miei compagni di viaggio a Lisbona:
Betto, Francesco, Riccardo, Serena, Chiara, Lucia, Riccardo, Luca, Ezio, e le nuove conoscenze
che ho fatto, come Fabrizio. Mille grazie a Lucio e Cinzia, un pensiero anche a Rodolfo e
Gigliola che chiedono sempre di me. Un grazie forse non basta, ma grazie a Professor Nobili
che mi ha permesso di eseguire questo periodo di stage presso il suo laboratorio con il suo
prezioso gruppo di ricerca: grazie mille Gilberto, grazie Marta e grazie Serena, grazie professor
Tossici, ogni vostro consiglio è stato prezioso. Grazie alla fortuna che certe volte sembra si sia
scordata completamente di me e che invece alle volte fa delle sorprese inaspettate!
Ribadisco un concetto della triennale ringraziando Antunes senza il quale non sarei andato da
nessuna, ma proprio nessuna, parte. Perché a me non è mai interessato svegliarmi alle 4 di
mattina per impararmi centinaia di definizioni a memoria, purtroppo o per fortuna a me è
sempre piaciuto capire le cose, sono fatto così.
Grazie a voi tutti che avete avuto la pazienza di leggere tutti questi grazie, e chiedo scusa se ho
dimenticato qualcuno.
No, di te amore mio non mi sono dimenticato, ti lascio per ultima come il dolce. A questo punto
non saprei nemmeno cosa dirti, sei l’unica che quando vedo tutto nero mi fa fare un passo
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indietro e cambiare prospettiva, che sai essere più buona e allo stesso tempo più cattiva di me.
Grazie anche a te per i sei mesi che sono stato a Lisbona,perché hai messo la mia felicità prima
della tua; per non aver mai chiesto troppo o troppo poco, nonostante la distanza fosse davvero
tanta per noi due che siamo un po’ come gli alpaca, che devi tenerne almeno due sennò poi
diventano tristi e vanno in depressione. A te che non mi stai né davanti né dietro ma sempre di
fianco.
Credo di aver ringraziato davvero tutti a questo punto, probabilmente non ci sarà un ‘altra tesi
su cui scriverò dei ringraziamenti, ma mai dire mai.
“Cosa aggiungere potrebbe un narratore
a quanto già narrato dall'attore;
a me non resta altro che sparire,
fare un bell'inchino e poi svanire.
Come Cyrano che confessa e muore a piedi del suo grande eterno amore,
anch'io finito il mio cammino mi accascio e vado verso il mio destino.
Che è quello di chi inizia e già finisce,
sboccia e dopo un attimo appassisce,
di chi vive soltanto un paio d'ore,
sperando in un applauso e dopo muore”
