155

CHAPTER- VI

COMBUSTION SYNTHESIS OF NANO Li4Ti5O12 AS HIGH

PERFORMANCE Li- ION BATTERY ANODE

6.1. Introduction

6.1.1. Nanomaterials as electrodes:

Nanomaterials are attractive for developing Li-ion battery electrode materials

with superior properties.1 They offer high surface area, good electron transport and

shorter diffusion paths for Li+ and electron pathway. High surface area of

nanomaterials offer potential benefits of new reactions at nanoscale that are not

possible with bulk particles; thereby increasing the specific capacity. Higher

electrode- electrolyte contact area exerted by nanocrystalline materials increases the

Li+

ion flux at the interface and facilitates faster charge-discharge process. Better

accommodation of strain associated with lithium ion insertion and deinsertion could

also be achieved with nanomaterials based electrodes. Reduced dimension of

nanomaterials permits shorter path length for Li-ion diffusion. As the particle size

decreases, the distance a lithium ion has to travel (diffusion length) in the solid state

of the electrode also decreases.

Time constant for the solid state diffusion of Li+ ion through the bulk of the particle

is given by the equation,

τ =

Where, τ is the diffusion time constant,

L is the diffusion length and

D is the diffusion coefficient.

Diffusion coefficient, ‘D’ being constant for a specific material, diffusion time (τ)

decreases with particle size1 (Diffusion length L refers to distance between surface to

core of the particle). Hence full capacity of the material can be realized at higher

L2

D

156

rates, if the diffusion length is small. The storage and removal of lithium at faster

rates results in increased power density of the electrode.

On the whole, nanomaterials show exceptional properties as electrode in Li-

ion batteries.2 Hence, viable synthesis methods for the preparation of nanomaterials

are crucial for developing high performing Li-ion battery electrodes. Synthesis

involving high temperature and longer reaction time always leads to bigger particles.

Special synthesis techniques such as sol gel synthesis, spray drying and other

solution routes have been used for the preparation of nanoparticles of lithium

intercalation compounds. Here we applied combustion method for the synthesis of

nano materials and studied the effect of nanosizing of electrode material in the rate of

lithium intercalation deintercalation process. Li4Ti5O12 (LTO) crystallizing in cubic

spinel-phase has been synthesized in nanocrystalline form by single-step-solution-

combustion method and its electrochemical properties were compared with the bulk

materials prepared by solid state synthesis.

6.1.2. Li4Ti5O12 as anode:

Titanium based oxides such as, anatase phase of TiO2 and Li4Ti5O12 have been

extensively studied for their application in Li-ion batteries as anode material.

Nanocrystalline Li4Ti5O12 (LTO) has an attractive features as Li-ion battery anode

material3-4

as (a) it is cheaper, (b) it intercalate/de-intercalate lithium with little

change in the unit-cell volume

(c) it experiences zero-strain during lithium

intercalation/deintercalation;5-7

hence it has excellent cycling performance with very

less irreversibility and long cycle life (d) it offers a convenient voltage-plateau at 1.5

V vs. Li+/Li compatible with polymer electrolytes and high-voltage cathodes;

8 the

voltage plateau happens to be much above the reduction potential of most organic

electrolytes mitigating the formation of solid-electrolyte interface (SEI) and

irreversible capacity loss,9-10

(e) it has high enough operating voltage-plateau that

avoids formation of lithium dendrites making the battery safer.7-11

The only

disadvantage of LTO is its poor electronic conductivity that limits its full capacity at

high charge-discharge rates.11

157

Efforts have been expended by various groups in order to increase the electronic

conductivity of LTO (a) by doping,12-13

(b) by its nitridation to form oxynitride

species on its surface,14

and (c) by carbon coating of LTO particles.15-16

The increase

in electronic conductivity of LTO has facilitated higher-rate operability of LTO

anodes so that LiMn2O4/Li4Ti5O12 ion cells are on the verge of being commercialized

(Argonne National Laboratory report). It is also well known that shorter diffusion-

length for Li+-ions and electronic transport in nano particles improve the high-rate

performance of lithium battery anodes. To this end, much effort has been made to

synthesize nano-structured LTO with varying morphologies and particle size, and

their electrochemical performance studied. For example, Li et al.17

have shown

improved performance with one-dimensional (1D) LTO nano-structures, namely

nanotubes/nanowires synthesized by hydrothermal method. Nanotubes of LTO have

also been synthesized using alkali hydrothermal reaction.18

Tang et al.19

have

reported nano-sheets of flower like LTO synthesized by hydrothermal route in the

presence of glycol solution which delivered high capacity even at very high charge-

discharge rates. Sorensen et al.20

have reported 3D-ordered macroporous LTO with

thin wall thickness that could deliver excellent high-rate capacities. Jiang et al.21

have reported the synthesis of hollow spheres of LTO by sol-gel method using

carbon spheres as templates; it exhibited high-rate performance. Shen et al have

synthesized carbon coated LTO microspheres that showed improved rate

performance.22

Furthermore, nano-particles of LTO have also been realized by spray

pyrolysis,23

cellulose assisted combustion24-26

as well as other decomposition

techniques.27-28

6.1.3. Combustion synthesis:

Combustion or solution-combustion synthesis is a well known method for the

preparation of nanocrystalline oxides.29-32

The method, involving redox reaction

between oxidizer and a fuel; is simple, instantaneous and cost-effective. The method

yields nano-sized material almost instantaneously and is used to prepare a variety of

complex oxides in nanocrystalline form.29,30,33,34

The process is fast and takes less

than 10 minutes to obtain the final product. This method gives products which are

pure, homogeneous, crystalline, nanometer sized, with high surface area and narrow

158

size distribution. This method has been successfully used to prepare structured

ceramics, pigments, magnetic materials and catalysts.

Combustion synthesis is different from other synthesis techniques such as pechini,

sol-gel etc which release toxic gases as by-products. Complete combustion reaction

(Oxidizer/Fuel, O/F = 1) is reported to produce CO2, N2 and H2O and this has been

confirmed by mass spectrometry results in literature. Further evolution of large

amount of gases leads to porous materials.

The stoichiometry of oxidizer (nitrates) to fuel (Glycine) ratio used here is 1 meaning

that it is a complete combustion to give CO2, N2 and H2O vapors along with

Li4Ti5O12. The chemical reaction for the stoichiometry used in the synthesis can be

written as

45 TiO(NO3)2 + 36 LiNO3 + 70 C2H5NO2 → 9 Li4Ti5O12 + 140 CO2 + 175 H2O +

98 N2

Indeed in our experiments, we did not observe evolution of NOx fumes. Therefore we

believe CO2, N2, and H2O vapors are the major products formed along with LTO

during the combustion reaction.

Yuan et al.23,25

have reported the synthesis of nano LTO powders by combustion

synthesis with combustion reaction initiated at < 250 C. The combustion product in

this case happens to be partially-formed LTO with anatase and rutile TiO2 impurities

requiring further thermal treatment of the combustion product at temperatures > 800

C for about 5h to obtain single-phased LTO. By contrast, the method reported in

this study is different and helps in realizing single-phased LTO directly from the

combustion reaction of metal nitrates-glycine initiated at about 800 oC; LTO thus

synthesized is nanocrystalline with high porosity and exhibits superior rate-

performance compared to LTO prepared from the methods reported earlier.

6.2. Experimental

LTO nano-powders were prepared by solution-combustion synthesis using titanyl

nitrate [TiO(NO3)2] and LiNO3 as the oxidant precursors and glycine as the fuel. The

aqueous solution of titanyl nitrate [TiO(NO3)2] was obtained apriori from titanium

159

isopropoxide [Ti(i-OPr)4]. Stoichiometric amount of [Ti(i-OPr)4] was added to ice-

cooled isopropanol. A large excess of isopropanol was taken in order to avoid the

rapid hydrolysis of [Ti(i-OPr)4]. The reaction temperature was maintained at <10C

to avoid TiO2 formation during the reaction. [Ti(i-OPr)4] is then slowly hydrolysed to

[Ti(i-OH)4] by the addition of dilute ammonia solution. [TiO-(OH)2] thus formed is

separated out by centrifugation, washed with water and dissolved in 1:1 nitric acid to

form titanyl nitrate [TiO(NO3)2] solution. Ti-ion concentration in the solution was

estimated by the colorimetric method in the presence of H2O2 and H2SO4.35

For this

purpose an acidic solution (1M sulphuric acid) of titanyl nitrate is treated with 3%

H2O2. It develops yellow colouration due to formation of titanium complex species.

As per the Lambertz- Beers law, the intensity of color is directly proportional to the

concentration of titanium present in the solution. The intensity of the solution is

measured using UV probe spectrophotometer (Shimadzu UV probe) and compared

with standards prepared using potassium titanyl oxalate.

In a typical preparation, an aqueous redox mixture containing stoichiometric amount

of titanyl nitrate (0.0362 mole), 2 g of LiNO3 (0.0289 mole) and 4.22 g of glycine

(0.0562 mole) were taken in a 120 ml alumina crucible and placed into a muffle

furnace pre-heated to 800 oC. The stoichiometry of metal nitrate to glycine is

calculated assuming the complete combustion to yield corresponding oxide product

and CO2, N2, H2O as by-products.27

The aqueous redox mixture boils and water

evaporates followed by frothing, and bursts into combustion with white incandescent

light. The reaction completes in a few seconds. The reaction crucible was then

removed from the furnace and allowed to cool. The froth-like voluminous powder

was crushed with mortar and pestle, and stored for further studies. Bulk particles of

LTO have been prepared by solid state synthesis by grinding Li2CO3 and TiO2 and

heating at 800 C with 24/48 h with intermediate grinding.

X-ray diffraction patterns for the LTO samples were recorded using X’pert PRO-

PANalytical Diffractometer using CuKα radiation. The morphology of the powder

samples was studied using Scanning Electron Microscope (JEOL 5600LV).

Transmission Electron microscopy studies of the samples were conducted using

Technai 20 Transmission Electron Microscope operated at 200 kV. Chemical

160

analyses of the samples were carried out using Perkin Elmer Atomic Absorption

Spectrometer. The surface area for powdered samples are measured by BET method

using Surface Area Analyzer (Belsorp Japan). The electrochemical characterizations

were performed using CR2032 coin-type cells. The positive electrode material was

prepared by blending ball milled mixture of 80% active material and 15% SP carbon

with 5 % Polyvinylidene fluoride (dissolved in N-methyl-2-pyrrolidone). The

electrode slurry obtained was coated over stainless steel disc and dried at 80C

overnight under vacuum. The weight of active material taken was roughly 4mg.cm-2

.

The cells were assembled and sealed in an argon-filled glove box with lithium foil as

the anode and 1M LiPF6 dissolved in EC/DMC (1:1 by volume) as the electrolyte.

The cells thus fabricated were cycled galvanostatically in the voltage range between

1V and 2 V versus lithium using VMP3Z (Biologica) Multi-Channel

Potentiostat/Galvanostat.

6.3. Results and Discussion

The redox mixture of metal nitrates and glycine when heated combust with the large

evolution of gases like CO2, H2O, N2 etc. and yield voluminous combustion residue.

Thus a fluffy, froth like product was formed after the combustion of aqueous nitrates

of Li and Ti with Glycine fuel as shown in Fig 6.1.

Figure 6.1, Voluminous Li4Ti5O12 formed after the combustion of corresponding metal

nitrates with glycine at 800 C.

161

It is found that 3.32 g of the formed product occupy about 120 cc volume. The

product is highly voluminous with a tap density measured after grinding is as low as

2.27 g/cc against theoretical density of 3.73 g/cc.

The combustion reaction has been initiated by applying different temperatures and

the as prepared materials have been analyzed using Powder X-ray diffraction pattern.

Fig. 6.2 shows powder X-ray diffraction patterns for the combustion products at

varying temperatures.

Figure 6.2, Powder X-ray diffraction patterns for the products formed by the

combustion of lithium and titanium nitrates with glycine at (a) 500 C , (b) 700 C, (c)

800 C and (d) Li4Ti5O12 prepared by solid-state method.

162

XRD patterns for the phases formed by the combustion of reactant nitrates < 600 C

are shown in Fig. 6.2(a) and 6.2(b). These XRD patterns comprise Li2TiO3 along

with anatase and rutile forms of TiO2. The most intense reflection for anatase at 2 =

25.3o is denoted as () and that for rutile at 2 = 27.5

o is denoted as (*). The

diffraction pattern comprises broad peaks indicating the nanocrystalline nature of the

products. The broad peaks make it difficult to distinguish between Li2TiO3 and

Li4Ti5O12 phases as their diffraction peaks overlap each other. But, the diffraction

peaks at 2 = 20.4o and 22.2

o are specific to Li2TiO3 and are absent for Li4Ti5O12.

However, these peaks are smeared due to the low crystallinity of the material, with

less than 8 % intensity, causing difficulty in phase identification between Li2TiO3

and Li4Ti5O12. Furthermore, the absence of any other peaks corresponding to lithium

salts and the presence of sufficient quantity of TiO2 impurities indicate the product to

be Li2TiO3, a low-temperature phase pertaining to Li-Ti-O system. However, the

XRD pattern for the product formed at 700 C (Fig. 6.2(b)) indicates the formation of

Li4Ti5O12 as the major phase with small amounts of anatase and rutile TiO2 as

impurities. The XRD pattern of the combustion product formed at 800 C (Fig.

6.2(c)) corresponds to pure Li4Ti5O12 without any impurity phases and the pattern

could be indexed in spinel structure with Fd3m space group. The lattice parameter

‘a’ for Li4Ti5O12 calculated using powder cell program is 8.361 Å, which agrees well

with the literature.6 The XRD pattern for the sample prepared by solid-state reaction

is also shown in Fig. 6.2(d) for comparison (lattice parameter a = 8.357 Å).

Fig. 6.3(a-e) depicts the microstructure of combustion-derived LTO obtained by

SEM at different magnifications. As seen from the micrographs, the aggregates are

flaky and highly porous in nature (Fig. 6.3 (a) and (b)). The porous structure is

obtained due to concomitant formation of material with vigorous evolution of large

volumes of gases during combustion reaction. The pore sizes vary between few nano

meters to few microns but typically between 150 nm to 1.5 μm (Fig. 6.3 (c-f)).

Higher magnification-image shown in Fig 6.3 (e) and (f) indicates a unique

architecture containing agglomeration of smaller particles of size < 150 nm with

embedded nano and micro pores.

163

Figure 6.3, Scanning electron micrographs for nanocrystalline Li4Ti5O12 at different

magnifications synthesized by combustion method. Figure shows highly porous

Li4Ti5O12 with pore size ranging between 150 nm to 1.5 μm.

Transmission electron microscopic studies are also performed on LTO synthesized at

800 oC (LTO-800), and the bright field images at two different magnifications are

shown in Fig. 6.4 (a) and (b).

(a) (b)

(d) (c)

(f) (e)

164

Figure 6.4, (a) and (b) transmission electron micrographs for Li4Ti5O12, (c) electron

diffraction pattern, and (d) line profile of electron diffraction in comparison with

standard LTO ‘d’ spacing (blue vertical lines).

The transmission electron micrographs show that the flakes consists of agglomerated

well crystallized LTO nano-particles with sizes ranging between 10 nm and 100 nm.

All the particles have similar contrast indicating the absence of impurities like

lithium salts or carbon. The irregular pore structures created by the fusion of

agglomerates are seen in the micrographs. The selected area electron diffraction

(SAED) pattern for individual crystallite (Fig. 6.4 (b) inset) exhibits single crystal-

like spot pattern. The average-electron-diffraction pattern for Li4Ti5O12 is shown in

Fig. 6.4(c). The electron-diffraction pattern exhibits sharp spots suggesting a highly

crystalline sample. The line profile corresponding to the electron diffraction pattern

is deduced using process diffraction program and is shown in Fig. 6.4(d). The line

0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

(840

) (731

)(4

44)

(533

)

(440

)

(333

)

(331

)

(400

)

Net

Li4Ti

5O

12- Pdf no. 00-049-0207

(311

)

Inte

nsit

y ov

er c

ircl

es (

arb.

unit

)

d-spacing (angstrom)

(a) (b)

(c) (d)

165

profile of electron diffraction matches well with Li4Ti5O12 (JCPDS No:00-049-0207)

and confirms the phase purity at trace levels.

In order to measure the actual surface area and further to know the porosity of

combustion derived samples; the BET surface area measurements are conducted on

the LTO samples. The plots of adsorption-desorption isotherm, BET and BJH

showing porosity distribution are shown in Fig. 6.5.

Figure 6.5, BET measurements of combustion and solid state derived LTO samples,

(a) Typical N2 adsorption-desorption plot (b) BJH plot showing pore size distribution.

166

Adsorption-desorption isotherm of combustion derived LTO belongs to type-

IV that shows a loop in the range of 0.4-0.9 (Fig. 6.5 (a))36-37

and indicates

mesoporous (2- 50 nm) structure of the sample. In contrast, solid state derived LTO

show almost no hysteresis loop (type-III isotherm- Fig. 6.5 (a)) and is not

mesoporous. The specific surface area of combustion derived LTO is 3.9 m2.g

-1 and

is much higher than the solid state synthesized LTO of 0.76 m2.g

-1. Porous structure

of combustion derived sample is further confirmed by the broad pore size

distribution in the mesoporous range in comparison to solid state sample (Fig. 6.5

(b)). The results demonstrated that combustion derived sample is highly porous;

however, micropores as seen in SEM images have not been identified in the BET

measurements due to the limitation range (1-100 nm) of the technique.

Electrochemical analysis:

The cyclic voltammaograms for LTO-800 sample vs. Li at scan rate of 0.1 mV.s-1

in

the voltage region between 1 V and 2 V is shown in Fig. 6.6. It shows a current peak

in the cathodic sweep and is completely reversible in the anodic sweep. Anodic peak

is observed at 1.5 V and the corresponding cathodic peak appeared around 1.64 V.

Figure 6.6, Cyclic voltammogram of combustion derived Li4Ti5O12. A peak at 1.5 V is

appeared during anodic reduction and is completely reversible (1.64 V) in the cathodic

sweep.

167

The peak positions observed here match well with the redox potential for

Ti3+

/Ti4+

in spinel phase. The sharp peaks are attributed to the high crystallinity of

the sample. Difference between the peak potential of cathodic and anodic sweep is

much less (0.14 V) showing the good voltage efficiency of the material. Capacity

value is calculated by integrating the area under the time vs. voltage plot. The

capacity value of 150 mAh.g-1

is completely reversible in the complete charge-

discharge cycle. The coulombic efficiency is ~100% for the sample. Hence, the

prepared LTO shows high voltage and coulombic efficiency which is an important

criteria for better electrode material.

The peak observed is the signature of coexistence of two phases and hence represent

a phase transition with change in lithium content. Existence of phase transition on

intercalation/ deintercalation stages are also demonstrated in the voltage-

composition profile of the constant current experiments as sloping curves represent

solid solution domain and plateau represent two phase mechanism. The voltage vs.

composition curves for LTO/Li cell cycled galvanostatically at C/2 (1 Li in 2h) rate

in the potential window between 2.0 V and 1.2 V are shown in Fig. 6.7(c). The

cycling behavior is typical of Li4Ti5O12 with a flat plateau at an average potential of

1.55 V and is attributed to the coexistence of two phases, namely Li4Ti5O12 and

Li7Ti5O12. The exact mechanism of electrochemical-driven reaction for the inter-

conversion of Li4Ti5O12 [Li4/3Ti5/3O4] into Li7Ti5O12 [Li7/3Ti5/3O4] and vice versa

with their cation distribution in unit cell is well documented in the literature.38-41

Li4/3Ti5/3O4 has the spinel structure with lithium atoms occupies the 8a site and 1/3 of

16d sites. Remaining 2/3 of 16d site is occupied by titanium atoms and is represented

as [Li]8a[Li1/3Ti5/3]16d[O4]32e. During the discharge process (intercalation of lithium)

Li-ions occupy available 16c sites while tetrahedral Li-ions present in 8a sites also

rearrange in 16c sites. At the end of deintercalation, the structure can be represented

as [Li2]16c[Li1/3Ti5/3]16d[O4]32e with rocksalt structure.

[Li]8a[Li1/3Ti5/3]16d[O4]32e+ Li++ e− [Li2]16c[Li1/3Ti5/3]16d[O4]32e

The spinel structure for Li4Ti5O12 and the rocksalt structure for Li7Ti5O12 phases are

closely related with both having the same symmetry.42

As a consequence, extremely

small lattice expansion of about 0.07% takes place. Therefore, it is often called a

168

zero-strain material. The structural models for Li4Ti5O12 and Li7Ti5O12 to visualize

the topotactic insertion/extraction of Li are also included in Fig. 6.7(a) and (b).

Figure 6.7, Structure of (a) spinel Li4Ti5O12 and (b) ordered rocksalt-phase Li7Ti5O12; (c)

voltage-composition curves showing the electrochemical transformation of Li4Ti5O12 –

Li7Ti5O12 during galvanostatic charge-discharge cycles at C/2 rate at 30 ºC.

During the first discharge Li4Ti5O12 takes about 2.92 Li atoms corresponding to a

capacity of 170 mAh.g-1

; it matches well with the expected theoretical capacity for

Li4Ti5O12. Although a small irreversibility of about 5% is observed in the

3Li+ + 3e

-

Li4Ti5O12 Li7Ti5O12

(a) (b)

169

subsequent charge cycles, the overall capacity after the first cycle remains almost

unchanged at 162 mAh.g-1

for several cycles. A small electrode polarization of about

0.02 V is observed between charge-discharge curves which indicates the existence of

good inter-particle electrical contacts and better ion transport due to the high degree

of porosity of the combustion-made powders.

In order to find out the effect of nanosizing on the rate capability of electrode

material, nanocrystalline Li4Ti5O12 is subjected to cycling at different charge-

discharge rates and the results are compared with the bulk sample prepared by solid-

state synthesis. Fig. 6.8 shows the voltage vs. capacity plot for nanocrystalline

Li4Ti5O12 at several charge-discharge rates in the voltage range between 2.0 V and 1

V. At 0.5 C charge-discharge, LTO-800 sample exhibits capacity value of 170

mAh.g-1

, a value close to the theoretical value of 175 mAh.g-1

. The capacity values

at 1C, 5C and 10C rates are found to be 158, 147 and 140 mAh.g-1

, respectively, with

closely placed voltage plateaus below 10C, indicating that the crystallite size has

only a minor effect on the rate capability of LTO below 10C. At charge-discharge

rates > 10 C, there is a considerable drop in capacity values together with a drop in

discharge voltage plateau which cause high electrode polarization. Both of these are

associated to the sluggish Li-ion diffusion kinetics at very high rates. However, a

capacity of 125, 102, 90 and 70 mAh.g-1

is obtained for 25, 50, 80 and 100C rates,

respectively. These capacity values are higher than those reported in the literature

for nanosized LTO at similar rates24,25,43,44

(electrode surface used here is 1cm2; with

loading of 4 mg of active material per cm2). These values are by far quite higher than

those exhibited by bulk samples prepared by solid-state reaction (inset to Fig.6.8). It

is seen from the inset in Fig. 6.8 that the capacity (95 mAh.g-1

) obtained at 5C rate

with bulk LTO is much low in comparison to the capacity (105 mAh.g-1

) obtained

with nanosized LTO at 50C rate. This is most likely nested in the morphology of our

mesoporous powders made of fused nanoparticles with the nanoparticles providing a

short diffusion path for Li and the pores fill of electrolytes ensuring a high flux of Li.

However, too much porosity may result in a low volumetric energy density.

Fig. 6.9 shows plots for capacity as a function of cycle numbers for

combustion derived and solid-state prepared samples (shown as inset) in the voltage

window between 2.0 V and 1.0 V at different charge-discharge rates.

170

Figure 6.8, Capacity-voltage profile for nanocrystalline Li4Ti5O12 synthesized by

combustion method at different rates. Inset shows capacity voltage profile for bulk

Li4Ti5O12 prepared by solid-state method.

Figure 6.9, Capacity vs. cycle number plot for nanocrystalline Li4Ti5O12 synthesized by

combustion method at different discharge rates. Inset shows capacity vs. cycle

number for bulk Li4Ti5O12.

171

As seen from these data, combustion-derived LTO shows excellent capacity retention

irrespective of the rate used. Interestingly, more than 95% capacity is retained even

after 100 cycles. A solid-state synthesized compound also exhibits good capacity-

retention (inset in Fig. 6.9) but with lower rate-capability.

Constant power experiments have been carried out in order to measure the effective

capacity and power handling ability of the material. Charge-discharge processes have

been carried out by applying constant power and the time required for one complete

charge- discharge cycle is calculated. Ragone plots are derived by plotting the

applied power and specific energy (power time). Ragone plot shown in Fig 6.10

compares power handling capability of combustion derived sample and solid state

derived sample.

Figure 6.10, Ragone plot comparing the performance of Li4Ti5O12 prepared by (-▲-)

combustion method and (-●-) solid-state method when paired with metallic lithium; (ж)

DOE benchmark for HEVs.

172

The plot shows higher specific power and specific energy values for the combustion-

derived LTO/ Li half cells in relation to that derived by solid-state route (Fig. 6.11).

Specific energy values for both the samples at 70 W/kg specific powers correspond

to ~270 Wh/kg. At high specific-power of 5000 W/kg, combustion-derived sample

exhibits a specific energy of 182 Wh/kg against only about 3 Wh/kg for the solid-

state sample. Even during 5 min of charge/discharge, the combustion-derived

material yields 167Ah/kg which is about 95% of the theoretical capacity as against

70 Ah/kg for solid-state-derived samples which is only 40% of the theoretical

capacity. It is seen from the data in Fig. 6.10 that the specific-energy values for LTO/

Li systems meet the HEV requirements.

6.4. Conclusions

Combustion synthesis is successfully used for the preparation of nanocrystalline

Li4Ti5O12 with particle size ranging between 20 nm and 50 nm with reaction times on

the order of the minute. A capacity value of 170 mAh.g-1

is achieved at 0.5C rate. At

rates < 10C, the observed capacity is closer to the theoretical value and a value of

140 mAh.g-1

is obtained at 10C rate. Higher rates affect moderate decrease in

capacity with more than 40% of the theoretical capacity retained at 100C.

Irrespective of rates used, nanocrystalline Li4Ti5O12 retain their initial capacity up to

100 cycles. These results suggest that Li4Ti5O12 prepared by combustion technique is

superior in terms of both high rate-capability and capacity retention, which is

attributed to the presence of highly crystalline nano-particles of Li4Ti5O12 which

agglomerate to form highly porous nanostructures having 4 m2.g

-1 BET surface

area. Thus, combustion synthesis is an attractive method for the preparation of

nanocrystalline, high power electrode materials for lithium-ion batteries.

173

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Lists of publications

Publications included in the thesis:

1. M. Sathiya, A.S. Prakash, K. Ramesha and A.K. Shukla, “Nitrate-melt

synthesised HT-LiCoO2 as a superior cathode material for Lithium-ion batteries”

Materials., 2 (2009) 857.

2. M. Sathiya, A.S. Prakash, K. Ramesha and A.K. Shukla, “Rapid synthetic

routes to prepare LiNi1/3Mn1/3Co1/3O2 as a high voltage, high-capacity Li-ion

battery cathode”. Mater. Res. Bull., 44 (10) (2009) 1990.

3. A. S. Prakash, P. Manikandan, K. Ramesha, M. Sathiya, J-M. Tarascon, and A.

K. Shukla, “Solution-Combustion Synthesized Nanocrystalline Li4Ti5O12 As

High-Rate Performance Li-ion Battery Anode” Chemistry of Materials., 22,

(2010) 2857.

4. M. Sathiya, A.S. Prakash, K. Ramesha, J-M. Tarascon and A.K. Shukla, “V2O5-

anchored Carbon Nanotubes for enhanced electrochemical energy storage”

Journal of American Chemical Society., 133(40) (2011) 16291.

5. M. Sathiya, A. S. Prakash, K. Ramesha and A. K. Shukla “Nitrates-melt

synthesized LiNi0.8Co0.2O2 and its performance as cathode in Li-ion cells”

Bulletin Of Material Science., 34 (2011) 7.

6. M. Sathiya, K. Hemalatha, K. Ramesha, J-M. Tarascon and A. S. Prakash,“

Synthesis, structure and electrochemical properties of the layered sodium

insertion cathode material : NaNi⅓Mn⅓Co⅓O2” Submitted to Chemistry of

Materials.

178

Other Publications

7. Ramdas B. Khomane, A. S. Prakash, K. Ramesha, M. Sathiya, “ CTAB-assisted

sol-gel synthesis of Li4Ti5O12 and its performance as anode material for Li-ion

batteries” Material Research Bulletin., 46(7) (2011) 1139.

8. K. Ramesha, A. S. Prakash, M. Sathiya, Girithar Madras, A. K. Shukla

“Synthesis and photocatalytic properties of Ag[Li1/3Ru2/3]O2: A new delafossite

oxide” Material Science and Engineering B., 176 (2011) 141.

9. K. Ramesha, A. S. Prakash, M. Sathiya, Girithar Madras, A. K. Shukla

“Synthesis of new (Bi,La)3MSb2O11 phases (M = Cr, Mn, Fe) with KSbO3-type

structure and their magnetic and photocatalytic properties” Bulletin Of Material

Science., 34 (2011) 271.

10. Rajalakshmi Ramachandran, M. Sathiya, K. Ramesha, A. S. Prakash, Giridhar

Madras, A. K. Shukla, “Photocatalytic properties of KBiO3 and LiBiO3 with

tunnel structures” Journal of Chemical Sciences., 123(4) (2011) 517.