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Lithium air battery

Benjamin PearceFrederik Kjær Larsen

Jasper ChuaMartin SchmidtMaxime Dupont

June 25, 2014

Supervisors:Ali Rinaldi, Poul Norby, Rolf Møller-Nielsen

Kristian Mølhave, Tim Boothwith the help of Mie Møller Storm

NTU-RPI-DTU Innovation Workshop 2014Dept. of Nanotechnology

Abstract

Growing energy consumption and global warming issues are pushing humans towardsfinding better and cleaner energy solutions. Currently, gasoline is one of the most widelyused energy storage mechanisms and it is highly polluting. A possible replacement for gaso-line can be seen in the development of lithium-air (Li-air) batteries. Lithium-air batteriespotentially have a high enough energy density to be a viable replacement for gasoline inautomobiles. However, current Li-air batteries face many significant challenges before theycan be commercialized. This report identifies and investigates challenges and characteriza-tion techniques associated with the cathodic part of aprotic Li-air batteries. Two differentaprotic battery cells were assembled with two different carbon based cathodes, carbon paperand graphite, and discharged at the same current rate per unit area. The cells were disas-sembled, the cathodes were removed and examined by use of scanning electron microscopy.To assess the effects of atmospheric exposure on SEM imaging, the samples were exposed toatmospheric conditions for varying amounts of time. A new technique of encapsulation hasbeen investigated to minimize the influence of atmospheric exposure on the sample betweenremoval from the battery and imaging. The discharge curves produced during experimen-tation show successful discharge, however, the SEM images were insufficient to draw anysignificant conclusions in regards to lithium peroxide deposition, the effects of exposure toatmosphere on cathode imaging or the success of the encapsulation technique employedherein. These results may be anomalous due to an abbreviated experimentation scheduleand possible errors that occured during cathode extraction.

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Contents

1 Introduction 41.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Basic battery function 6

3 Four types of lithium-air batteries 7

4 The aprotic battery 8

5 Challenges facing the aprotic Li-air battery 95.1 Electrolyte challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.2 Cathode challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.3 Air filtration challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Characterization methods 126.1 Ex-situ characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6.1.1 Scanning and transmission electron microscopy . . . . . . . . . . . . . . . 136.1.2 X-ray diffraction and infrared spectroscopy . . . . . . . . . . . . . . . . . 136.1.3 Electrochemical impedance spectroscopy . . . . . . . . . . . . . . . . . . . 14

6.2 In-situ characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2.1 Charge-discharge analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2.2 In-situ based characterization techniques . . . . . . . . . . . . . . . . . . . 16

7 Experimental plan 177.1 Observation of lithium oxide growth on cathodes during discharge . . . . . . . . 177.2 Degradation of the sample due to atmospheric exposure . . . . . . . . . . . . . . 18

8 Results 19

9 Future considerations 22

10 Conclusion 22

A Battery cell assemblies 24A.1 Experimental cell assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24A.2 EL-cell assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

B Discharge procedure for both batteries 26

C Transfer of samples 26

D SEM imaging of the samples 26

E Testing the effects of atmosphere contaminations on samples 26

F Who did what 27

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List of Figures

1 Theoretical energy densities of common energy storage devices . . . . . . . . . . 52 Illustration of a standard lithium-air battery . . . . . . . . . . . . . . . . . . . . 63 Illustration of the four types of li-air batteries . . . . . . . . . . . . . . . . . . . . 74 Illustration of the pores in a porous cathode . . . . . . . . . . . . . . . . . . . . . 105 Illustration of Li2O2 buildup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 FTIR and XRD spectra of cathodes . . . . . . . . . . . . . . . . . . . . . . . . . 137 Bode plot and Nyquist plot after EIS measurement . . . . . . . . . . . . . . . . . 158 Discharge-charge cycle for an aprotic Li-air cell . . . . . . . . . . . . . . . . . . . 169 Picture of the customized sample holders and foil bag . . . . . . . . . . . . . . . 1810 Experimental discharge curve of the two cells . . . . . . . . . . . . . . . . . . . . 1911 SEM pictures of the carbon cathodes before and after discharge . . . . . . . . . . 2012 SEM pictures of the discharged carbon cathodes after atmospheric exposure . . . 2113 EL-cell assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

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1 Introduction

The lithium-air (Li-air) battery is potentially a game changing development in the world ofenergy consumption and storage. Li-air batteries can potentially be a viable replacement forgasoline due to its high energy storage density. The combustion of gasoline produces a largequantity of CO2-emissions. Replacing combustion engines with lithium-air batteries wouldallow for the opportunity to use cleaner energy sources to power the automotive industry [1].To replace all combustion engines in america with lithium-air batteries, an energy productionof 2.7 billion kWh per day would be required [2]. Although the technology has not yet come farenough to allow for a rechargeable commercial lithium-air battery that will remain functionalover a long time span, Li-air batteries have been produced in laboratory environments. Singleuse, non-rechargeable zinc-oxide batteries are used today in products such as hearing aids,which makes it conceivable that Li-air can one day be used for these functions [3]. In fact,Li-air is a misnomer as most batteries in development are actually more accurately described aslithium-oxygen batteries because those are the active reactants [1]. One of the challenges beingresearched with respect to Li-air batteries is to solve the problems associated with non-oxygenelements contained in air, for example nitrogen, water or CO2 [4]. However, in accordance withconvention, this paper will refer generally to these batteries as Li-air.

Li-air batteries are commercially significant largely due to their potential specific energy andenergy density [4]. The high energy density of Li-air batteries is in part due to the low weight oflithium compared to other metals used in batteries such as lead or zinc [1]. This is why lithium-ion batteries have a relatively higher energy density compared to other rechargeable batteriesutilizing heavier metals, such as lead-acid or nickel-cadmium [1]. In addition, the cathode inLi-air batteries also reduces the weight of the battery as compared to Li-ion because the activereactant comes from air and does not need to be contained in a closed cell [1].

The fact that lithium-air batteries have an estimated gravimetric energy density with respectto the anode (thereby excluding the mass of the carbon and oxygen cathode) of 13,000 Wh/kgas compared to gasoline’s 13,200 Wh/kg is significant [1]. This significance can be realizedin any device that currently runs on lithium-ion batteries, however, their true potential canbe readily seen in transportation applications, particularly, automotive applications. A chartcomparing energy densities of various energy storage technologies can be seen in figure 1.

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Figure 1: Theoretical energy densities of common energy storage devices (from [1])

A study from 2010 provided similar comparison estimating useable energy density of gasolinefor automotive applications, based on a 12.6 % tank-to wheel efficiency for US vehicles (ascompared to 18 % for European vehicles [5]), to be 1,700 Wh/kg; to have an equivalent usableenergy density for Li-air batteries would only require a 14.5 % utilization of the theoreticalenergy density [6]. This is significant in light of existing zinc-air batteries, which utilize 40-50 %of their theoretical energy density [6]. Thus, it seems clear that Li-air batteries, if commerciallysuccessful, would be a viable replacement for gasoline. This could reduce fuel emissions andpollution that results from the combustion of gasoline, depending on what energy source isused to fuel the batteries. Currently, oil is considered a finite resource and any energy deliverymechanism that can reduce the global dependence on a finite resource would be highly significant[7]. In addition, with growing concern over the effects of greenhouse gases and carbon-dioxideemissions on the environment resulting from combustion of fossil fuels including but not limitedto gasoline, the development of a clean energy storage technology, such as Li-air batteries, wouldreduce such concerns [6]. Thus, the commercialization of a Li-air battery would not only bea juggernaut in the world of energy storage, but an access point for utilization of alternativegreen energy solutions.

1.1 Scope

The main focus points of the experiments are the imaging of lithium peroxide buildup on thecathodes during discharge using a scanning electron microscope (SEM), the effects of exposingthe cathodes to the atmosphere, and the shielding of the cathodes from the atmosphere usinga copper tape sealing.

Two cells will be constructed for the tests; an experimental cell using a 10 mm diametercarbon paper cathode, and a commercial EL-cell using a 14 mm diameter graphite cathode.

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Both cells will be discharged at a rate of 125 µA.cm−2. The cathodes will then be investigatedin the SEM.

2 Basic battery function

Batteries are used throughout the world as energy storage mechanisms, in devices such ascars, laptops, and mobile phones. This report will focus on batteries, which all function in asimilar manner to the aprotic lithium-air battery as seen in figure 2.

Batteries consist of a negative electrode called an anode, and a positive electrode called acathode. An electrolyte is used to convey ions between the anode and the cathode [8].

When discharging a battery, the material at the anode is oxidized, which produces ions thattravel through the electrolyte to the cathode, where a reduction process occurs. This creates anelectrical potential between the anode and the cathode. The oxidization process at the anodeproduces electrons, which travel through an external circuit to the cathode and participate inthe reduction process. The movement of the electrons is the definition of electricity.

To charge a rechargeable battery, the process is run in reverse, by application of an externalpotential difference. Some batteries are non-rechargeable due to their design, which generallyhave inadequate mechanisms for releasing the gases formed during discharge [3].

Figure 2: Illustration of a standard lithium-air battery. The anode is the lithium metal and thecathode is oxygen, supported by a carbon skeleton (from [1])

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3 Four types of lithium-air batteries

Figure 3: Four different lithium-air technologies. All technologies use lithium metal as theanode, and oxygen as the cathode. The main differences are in the choice of electrolyte (from[6])

Currently, there are four different designs for Li-air batteries being explored and developed;aprotic, aqueous, solid state and combination aqueous and aprotic. Figure 3 shows a basicillustration of each. Each design has its own drawbacks and advantages and each present aunique set of challenges. This report focuses on the aprotic Li-air battery design, which willbe discussed in more detail herein; however, a brief discussion of the other Li-air designs beingexplored is useful.

All four Li-air architectures involve a lithium metal anode and usually a carbon basedcathode [6, 9]. The cathodes are generally porous structures; however, studies have been doneusing different types of cathodes, for example, a common cathode used is made of Super Pcarbon black and a polyvinylidene fluoride (PVDF) binder, while a less common design hasbeen tried using a cathode made of a three dimensional graphene structure [4, 10]. Otherdesigns being studied incorporate carbon nanotube electrodes [11]. Each has an electrolyteseparating the two electrodes to convey lithium ions between the anode and cathode [6, 9].The differences lie in the type of electrolyte and the various barriers used to ensure the properreactions are taking place.

The most commonly researched design is the aprotic (sometimes referred to as non-aqueous)Li-air battery. Though there may be small variations in different aprotic designs all have thesame basic components, a lithium anode, an aprotic electrolyte and a generally carbon basedcathode. This design implements an aprotic electrolyte and organic solvent to convey lithiumions between the electrodes [6]. This is desirable because lithium can react violently with waterand a non-aqueous electrolyte will reduce the risk of such a reaction [6]. Most non-aqueouselectrolytes will form a protective barrier called a solid electrolyte interface (SEI) around the

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lithium on initial exposure [4]. Other systems may employ a protective barrier that will conveylithium ions to the electrolyte in order to ensure the protection of the lithium [4]. Differentnon-aqueous electrolytes are being investigated and used in order to convey the lithium ionsto the cathode; a typical example is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) indimethoxyethane (DME) [4].

A second type of Li-air battery is commonly referred to as an aqueous design. This designlike the non-aqueous design can have variants but the major components are generally similar,incorporating an aqueous electrolyte to conduct the lithium ions to the cathode. In this design,the lithium anode is protected by a lithium conducting electrolyte barrier in order to preventexposure to the aqueous electrolyte, which would result in a caustic reaction [9]. A problemwith this arrangement is that over time the lithium can form dendrites that move throughthe barrier and eventually make contact with the aqueous electrolyte resulting in a similarcatastrophic failure of the battery [4]. The main difference in this design is the use of anaqueous electrolyte to convey lithium to the cathode, which results in different fundamentalreactions than with the non-aqueous electrolyte design, though they will not be explored in thispaper [6]. This is significant because the advantage is that the reaction products at the cathodeare soluble as opposed to the insoluble cathode reaction products that result in the non-aqueousdesigns [1]. In addition, the reaction at the cathode produces a three phase (gas-liquid-solid)relationship, whereas the reaction at the cathode for the non-aqueous design is only two phase(solid-liquid) [1]. The various cathodes used in these designs are generally carbon based as well[4].

A third type of Li-air battery is a combination of the aqueous and aprotic design. Thisalleviates the risk of the lithium coming into contact an aqueous solution but also complicatesthe batteries by adding more components. This design has a non-aqueous electrolyte at theanode and an aqueous electrolyte at the cathode thereby trying to achieve the benefits of bothdesigns [1]. In this design the anodic reactions including the formation of an SEI on the anodeis the same as in the non-aqueous design and the reactions at the cathode are similar to theaqueous design [12]. As seen in Figure 3, a lithium conducting membrane separates the twoelectrolytes [12].

The last type of Li-air battery being studied is the solid-state design. This can be seen infigure 3. This design utilizes a solid electrolyte to convey the lithium ions between the anodeand the cathode [13]. For example, in one study, the anode and cathode are separated by athree-layer lithium conducting membrane laminate [13]. This membrane laminate that servesas the lithium-conducting electrolyte is composed of lithium aluminum germanium phosphate(LAGP) sandwiched between two different specially designed polymer ceramic materials [13].This prevents any possible interaction between an aqueous electrolyte and the lithium anodebecause there is no aqueous electrolyte, however, there are not as many studies available on thistype of battery as there are for the other three designs [1].

4 The aprotic battery

During the discharge process the lithium will oxidize, resulting in a separation of lithiuminto the positive ion Li+ and an electron [6].

Li→ Li+ + e− (1)

The electrons will move through an external circuit while the lithium ions will move through theelectrolyte. This allows for recombination of lithium-ions, electrons and oxygen at the cathode,

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resulting in an electric potential through the following two reactions [4, 1].

2Li+ + 2e− + O2 → Li2O2 (2.96V) (2)

4Li+ + 4e− + O2 → 2Li2O (2.91V) (3)

Both reactions are possible and would result in a successful discharge of the battery. However,due to the strength of the O-O bond, the reverse reaction for Li2O requires greater energy andthus reduces cyclability [4]. Research suggests that the first reaction yielding lithium peroxide(Li2O2) is dominant [1]. Due to a lack of properly revealing in situ characterization techniques,the ratio and particularly the progression of the reactions is unclear and may vary dependingon the components [1].

Furthermore, the reaction between lithium and oxygen can generate the following irreversiblereduction of O2 via a one electron process [4]:

O2 + Li+ + e− → LiO2 (4)

The reaction products Li2O2 and Li2O are insoluble in the aprotic electrolyte resulting in abuildup of the product on the cathode surface as the battery discharges. These precipitates actas electrical insulators, decreasing the electrons ability to access the reduction sites and therebydecrease battery efficiency. When the Li2O2 precipitation buildup reaches a height of 4-5nm,the cell will cease to function. Thus, the precipitate buildup can hinder proper battery function.In addition, the precipitate buildup can also cause blockage of the porous cathode, restrictingoxygen inflow, further inhibiting battery function.

By applying a greater potential the process is thought to be directly reversed, therebycharging the battery. However, details of the inverse reactions are still unclear [6]. In fact, therechargeability or cyclability of the aprotic Li-air batteries is a challenge in itself. Some studieshave been able to cycle the battery a few times but long term cyclability has been a problemdue to degradation of battery components over time [14].

The current state of the art shows that much of the research being done currently is onaprotic batteries [4]. Aprotic batteries are a bit further along as a result than some of theother Li-air battery types [6]. Aqueous battery designs have a greater level of risk due tothe adverse reactions that would occur if the lithium were to make contact with the aqueouselectrolyte. Thus until the dendritic nature of lithium can be overcome with a suitable coatingaround the anode, this risk will not abate [4]. This risk is severe enough to merit a focuson aprotic batteries solely to avoid this problem. Both types of batteries still have significantchallenges associated with them that must be resolved prior to commercialization. Thus, froma commercial perspective, it makes more economic sense to focus on aprotic battery design toavoid potentially damaging litigation resulting from an inflammatory battery failure.

5 Challenges facing the aprotic Li-air battery

The Li-air battery is a promising energy storage technology but there are a number of issuesfacing the battery before it can be commercialized. One of the most important aspects of anybattery is that it should have a high cyclability over long periods of time. This is a majorchallenge when producing a commercial lithium-air battery. However, cyclability involves bothdischarge and charge, and there are enough challenges facing the discharge process alone thatit is hard to assess any problems concerning cyclability at this juncture.

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Three of the biggest challenges facing the discharge process of the Li-air battery concernthe electrolyte, the cathode and the use of air instead of pure oxygen. In addition, a barrierto commercialization can be found in the use of a pure lithium metal anode, which brings upserious safety concerns due to the violent reaction between lithium and water and the tendencyto propagate through a barrier as a result of the dendritic nature [4].

5.1 Electrolyte challenges

The electrolyte is a crucial part of any battery. Ensuring a stable and consistent mediumbetween the anode and the cathode through which lithium ions can travel is very important tohaving an effective battery with a long battery life [14].

Figure 4: Illustration of two possible capacity limitations during discharge. Li2O2 depositedon the carbon matrix causes electric passivation of the cathode and blocks pores from feed ofreactants (from [4])

A good electrolyte requires a strong ion conductivity and low electron conductivity. Whenselecting an electrolyte for the aprotic battery, it is important to keep in mind that the carbonis typically immersed in or wetted by the electrolyte. In other words, the oxygen has to diffusethrough the electrolyte in order to reach the cathode. Therefore the electrolyte needs to have ahigh oxygen solubility. A higher oxygen solubility allows for a greater discharge rate but doesnot affect battery capacity [1, 15].

The most significant challenge facing the electrolyte selection is preventing degradation overtime. To complicate matters currently it is not well understood why electrolytes being testedare degrading over time. Furthermore the electrolyte has to be resistant to oxidation when ina high potential environment [6].

Currently, the reactions produce insoluble by-products which is good in that lithium peroxidewill stay concentrated near the cathode and not dissolve in the electrolyte, but bad in thatlithium peroxide buildup could block access of dissolved oxygen to the cathode, as depictedin figure 4. Specifically, when using porous cathodes with high surface areas which allow forgreater specific capacity, those surface areas can be negated by lithium peroxide buildup thatblocks the pores [16].

All of these challenges combined make choosing a suitable electrolyte a complicated matter.As research progresses even more problems might arise, further complicating selection.

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5.2 Cathode challenges

Similarly to the electrolyte, the cathode presents numerous significant challenges, some ofwhich are closely related to the challenges facing the electrolyte selection.

Many cathodes are being tested with high porosities and high surface areas in order tomaximize the specific capacity of the battery. The difficulty lies in the fact that lithium peroxideprecipitates during the reduction reaction and blocks the pores as well as insulates the cathode,preventing electrons from reaching the reaction site [16]. When the insulating layer of Li2O2

precipitation reaches a thickness of 4-5 nm, the electrons will no longer be able to reach thereaction site, as depicted in figure 5.

Figure 5: Illustration of Li2O2 buildup. GC refers to the graphite cathode (from [17])

Studying lithium build-up on differently designed cathodes is one method researchers areusing to solve these problems. It is not currently understood exactly what composition theprecipitate takes during the reaction and how the precipitate naturally deposits on the cathode.Furthermore, it is unclear the progression of the reactions during charge and discharge. This islargely due to the inability to use SEM and TEM in-situ as a method of characterization. Theexperiments contemplated by this report attempt to address this issue [18].

Another problem very similar to the electrolyte is preventing degradation of the cathodeover time. It is not presently clear exactly why this degradation occurs. However, researchersare currently experimenting with different cathode configurations and materials in order to solvethis problem while continuing to maximize capacity [19, 20].

Additionally, unwanted byproducts produced during the reduction reaction could increasethe overpotential of the lithium-air battery. The buildup of typical byproducts (e.g. Li2CO3)will require greater energy to reverse the reaction during the charge process due to its insulatingproperties, which will lead to an increase in the overpotential, decreasing the energy efficiencyof the battery [4].

5.3 Air filtration challenges

As previously mentioned, one of the reasons the lithium-air battery could potentially havesuch a high energy density is its use of ambient air as the main cathode reactant. However, mostexperiments currently run today utilize pure oxygen rather than air to prevent any unwantedreactions by other molecules than oxygen. It would be particularly damaging if water moleculeswere to infiltrate the battery and make contact with the lithium anode. In addition, if oxygenwere to make contact with the lithium anode as a result of lithium’s dendritic tendencies, it

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would oxidize the lithium, creating an insulating layer that would prevent proper anodic function[4].

For example the ingress of CO2 creates the following side reactions that can disrupt orinsulate the transfer of electrons [4]:

4Li + O2 + 2CO2 → 2Li2CO3

Li2O + CO2 → Li2CO3

Li2O2 + CO2 → 1/2O2 + Li2CO3

2LiOH + CO2 → Li2CO3 + H2O

(5)

When the anode is exposed to H2O the following reactions occur [4]:

2Li + 2H2O→ 2LiOH + H2

LiOH + H2O→ LiOH •H2O(6)

When the anode is exposed to N2 the following reactions occur[4]:

6Li + N2 → 2Li3N

Li3N + 3H2O→ 3LiOH + NH3(7)

To find a commercially viable solution, a membrane or electrolyte must be developed thatwill only allow the diffusion of oxygen from air. Currently a solution to this problem is notreadily available.

6 Characterization methods

The electrochemical reactions happening inside the lithium-air cell, especially at the carboncathode, are not fully understood yet. Since the complete reactions are more complicatedthan the basic reactions, as seen in equations 2 and 3, gaining a thorough understanding ofthe formation of discharge products (e.g. Li2O2, Li2O, LiO2, Li2CO3...) is quite challenging[15, 21]. The techniques described in this section are commonly used to observe and characterizeparts of the processes involved in the lithium-air battery.

There are many characterization techniques available to observe a sample. They can beseparated into two categories: those that are used during experimentation (during charge anddischarge of the battery), called in-situ measurements and those performed when the experimentis not running (when the battery is stopped), called ex-situ measurements.

The ex-situ characterization techniques are more commonly used today since they are morepractical; the relevant parts of the batteries can be taken out of the cell and observed. Howeverthis comes with a set of disadvantages. It is difficult to determine the progression of thereactions, because the processes cannot be witnessed while they are occurring.

On the other hand, application of in-situ methods to a running battery is difficult, as thereactions are happening in a closed environment normally encased in stainless steel, makingthem much more difficult to observe. However, it is conceivable that some ex-situ tools couldbe adapted to be used in-situ to get more accurate information about the reactions.

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6.1 Ex-situ characterization techniques

6.1.1 Scanning and transmission electron microscopy

Scanning and transmission electron microscopy (SEM and TEM) is primarily used to imagethe surface (features and morphology) of the samples and also determine the composition of thesample using energy dispersion spectroscopy (EDS). Porous carbon cathodes are used to achievea high-performance Li-air battery by increasing the reaction surface area. The cathode can bedissected in order to facilitate observations of the pores. This can be done both by breakingthe cathode outside of the SEM or by removing layer by layer with a focused ion beam (FIB)inside the SEM. However, one cannot accurately explain the chemical processes that lead to theformation of the products solely by looking at SEM images or X-ray maps ex-situ.

6.1.2 X-ray diffraction and infrared spectroscopy

X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) might be usedseparately or in combination to investigate the nature of the precipitates formed during dis-charge. XRD is based on the X-ray diffraction in the crystal structure of a crystalline material.The FTIR spectroscopy is based on interferometry. A beam of light is split into two separatebeams, one part is shone on the sample while the other travels a different path, when the beamsare then recombined, an interference pattern can be analyzed. The pattern of diffraction isused to get the intensity for each wavelength (or wavenumber) used in the IR spectrum. Eachelement has a well known IR spectrum (IR bond footprint for each element) with peaks atparticular wavenumbers and a well known XRD pattern with peaks at particular diffractionangles. In figure 6, both the FTIR and the XRD spectra are showing the presence of Li2CO3

at the carbon cathode.

Figure 6: (a) FTIR spectrum of a cathode with Li2CO3 and FTIR spectra of the main impurities,(b) XRD spectra of a charged and a polluted cathode with the reference Li2CO3 XRD peaks.α−MnO2 is a catalyst used and it is investigated as responsible for Li2CO3 formation (figureis partly reproduced from the supplementary information of [22])

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6.1.3 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is another in-situ technique [23]. In anelectrochemical system, each interface of the battery is associated with an impedance. Thisimpedance is measured by applying an alternating voltage at different frequencies and record-ing the resulting alternating currents. The impedance is calculated using Ohm’s law; this ispossible if the reactions are in a steady state, which means that the response to the alternatingvoltage is highly linear. The basis behind EIS is to theoretically model the battery as an RC-circuit (resistor/capacitor circuit). In an RC-circuit, the impedance depends on the frequencyof the applied voltage.

There are two ways of representing the data. The first one is to plot both the real partand imaginary part of the impedance against the frequency. This plot is called a Bode plot(see figure 7-(a)). The other method is to plot the imaginary part against the real part of theimpedance; this is called a Nyquist plot (see figure 7-(b)) [23].

For a Li-air battery, these two plots are used to obtain information about the reactionshappening at the anode (eq. 1) and the reactions happening at the cathode (eq. 2 and eq. 3).Particularly useful information that can be seen is the activation energy of the reactions, thetime constants of the reaction and the values of the resistor-capacitor pairs for an equivalentRC-circuit [23].

In the Bode plot (fig. 7-(a)), one interface is associated with a decay of the real part ofthe impedance and a valley of the imaginary part of the impedance, which is associated with asemi-circle in the Nyquist plot (fig. 7-(a)).

To have a better understanding of the EIS measurement, an equivalent RC-circuit canbe used [23]. In this case, each interface of the battery (the anode-electrolyte and the carbon-electrolyte) can be associated with a resistor and a capacitor in parallel. Each resistor-capacitorpair is then added in series. In this kind of circuit, each resistor-capacitor pair have a frequencydomain where they have a dominant influence on the circuit. The same phenomena happens ina battery. This can be seen in figure 7.

The decays, valleys and semi-circles are linked to the interfaces by varying parameters, suchas oxygen flow rate, lithium salt concentration in the electrolyte and discharge current [23]. Ifone parameter is only influencing one of the reactions happening at one of the interfaces, thenonly one decay, valley and semi-circle is expected to change. For example, if the oxygen flowrate is decreased, the overall redox reaction in the cell is expected to decrease due to the limitingreaction happening at the cathode. Thus, the cathode-electrolyte interface will be associatedwith the decay, valley and semi-circle that changes when oxygen flow rate is decreased. It isimportant to note that a similar analysis can be done in a RC-circuit.

By sequentially varying parameters, as seen in figure 7, the following can be found:

- The real-decay R2, the associated imaginary-valley in the Bode plot and the semi-circleof diameter R2 are associated with the oxygen reduction at the cathode.

- The real-decay R1, the associated imaginary-valley in the Bode plot and the semi-circleof diameter R1 are associated with the lithium oxidation at the anode.

This measurement method is useful for determining the site of the limiting reaction. Thelimiting reactions will be the one with the highest impedance. However, in reality, making thisdetermination can be quite challenging.

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Figure 7: (a) Bode plot and (b) Nyquist plot obtained after EIS measurement of a li-air battery(from [23])

6.2 In-situ characterization techniques

There are not many types of in situ characterization techniques available today. The mainones used in electrochemical experiments do not actually show the reactions as they occur, forexample cyclic voltammetry.

6.2.1 Charge-discharge analysis

The charge-discharge analysis is used to determine the potential between the two electrodesand the capacity of a battery (e.g. the current that can be delivered in a period of time). It isalso used to determine the power density of a battery and whether there is a degradation of thebattery over repeated charge and discharge cycling.

The battery is charged or discharged at a constant current. The potential between the twoelectrodes is recorded. A typical charge-discharge curve for a Li-air battery is shown in fig. 8.

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Figure 8: A single measured discharge-charge cycle for an aprotic li-air cell (based on Super-Pcarbon) operated at 0.1 mA.cm−2 current density (from [6])

The standard potential for the discharge reaction is found through an analysis of the ther-modynamics of the reaction (U0 = 2.96 V [6]). The average difference between potential duringcharge or discharge and the standard potential U0 are respectively called charge and dischargeoverpotentials (ηchg and ηdis). Higher overpotentials result in lower battery efficiency becauseof the high demand of energy during charge and relatively low energy release during discharge.

The open-circuit potential (OCV) is used to get an idea of the potential between the elec-trodes before discharge.

6.2.2 In-situ based characterization techniques

Surface-enhanced Raman spectroscopy (SERS) has been used to observe the reactions of abattery [24]. However, the precision of the methods does not give a sufficient understanding ofthe reactions taking place.

An ideal method would be to image the reactions while they are happening using a TEM withsub-nanometer resolution. It does not appear that this type of imaging has been successfullyperformed. This is likely due to the fact that commercial batteries being researched are on theorder of the centimeter scale and the maximum thickness allowable in a TEM is in the order of200 nm.

Any TEM sample analysis done in-situ should also be closed to prevent electrolyte fromevaporating under the vacuum. In addition, a suitable electric circuit would be required todischarge the battery, which would add complications to TEM imaging.

To conclude, characterization techniques, both in-situ and ex-situ, are numerous and havespecific applications in the determination of some of the properties of the batteries. However, the

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available tools today do not allow for a reliable determination of the elementary mechanisms ofthe oxygen reduction (i.e. lithium oxide formation). As described in this section, the ideal toolthat would allow for such an understanding of the progression of the reactions happening at thecathode would be the use of an in-situ-TEM, with sub-nanometer resolution. The observationof a cell in a TEM has not been described in this section. In order to observe a running batteryunder a TEM, a slice of the battery, including all the components of the battery, would beneeded. Two ways to image a running battery would be to run an ”open battery” with anelectrolyte of sufficient stability to resist a low vacuum in an environmental TEM (ETEM)or to run a ”closed battery”. In both conditions, each battery should not exceed 200 nm inthickness.

However, such an investigation was not feasible in the time frame allotted for this report.

7 Experimental plan

7.1 Observation of lithium oxide growth on cathodes during discharge

Two Li-air battery cells were constructed and discharged at the same rate per unit area.The cathodes chosen for each cell were selected based on the standard sizings associated withthose cells. The diameter of the cathode is important because it determines the surface areaassociated with the rate of discharge. This is due to the fact that these are non-porous thincircular cathodes and their geometry directly correlates to reaction surface area. One was anexperimental cell and the other was a commercial cell from the company EL-cell will be referredto herein as the EL-cell. The experimental cell utilized a thin 10 mm diameter circular carbonpaper cathode. The commercial EL cell was constructed with a thin 14 mm diameter circularexpanded graphite cathode. The components and assembly procedures of the experimentalbattery and the EL-cell battery as well as the discharge procedures can be found in appendicesA and B.

The experimental cell was discharged for 4 hours and 40 minutes at a rate of 125 mA. andthe EL cell was discharged for 3 hours and 20 minutes. Both cells were then deconstructed andthe cathodes were removed in an argon filled glove box. The cathodes were cut into multiplesample pieces, which were subsequently affixed to custom scanning electron microscope (SEM)holders by means of carbon tape. The custom holders were machined from aluminum stock.An image of the custom designed holders can be seen in Figure 9 below. After affixing thecathode samples within the holders a seal was applied using copper tape and the holders werepackaged in thermally sealed foil bags.

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Figure 9: Picture of the customized sample holders: (a) empty, (b) open, (c) sealed and (d) aclosed argon filled foil bag with a sample

The samples were transported to an SEM facility for analysis. This process was conceived asa method to avoid exposing the samples to atmosphere, which risks a reaction with any lithiumperoxide formed on the cathode during discharge. The sealed sample holders were removed fromthe foil bags in a box containing liquid nitrogen vapor and the sample holders were affixed tothe SEM sample holder and placed quickly in the SEM antechamber. The copper tape was thenremoved using tools contained inside the SEM and the samples were analyzed. In this way, itcould be ensured that the sample analysis could be relied upon without contamination resultingfrom atmospheric exposure. An itemized description of the tranfer and imaging procedures canbe found in appendices C and D.

To assess the effects of atmospheric exposure on the samples, one sample was imaged withno exposure, after 6 minutes of exposure to air and then again after 30 minutes of exposure toair and differences in the imaging was analyzed. More details of this process can be found inappendix E.

7.2 Degradation of the sample due to atmospheric exposure

The cathode from the experimental cell was split into sample sections. One sample sectionwas analyzed through SEM imaging and in order to determine the effect on imaging when thesample is exposed to air, the sample was taken out of the SEM and exposed to air for twodifferent periods of time and then re-imaged in the SEM. The first exposure was for 6 minutesand the second was for 45 minutes. An area on the sample was identified and that same areawas imaged after each atmospheric exposure period so that the differences could be analyzedand accurately assessed. In this way, the impact of atmospheric exposure on the sample analysisand correspondingly the importance of protecting the samples from atmospheric exposure couldbe determined.

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8 Results

The goals of the experiments discussed herein were threefold. First, was to compare twodifferent battery cells utilizing different types of carbon based cathodes. Second, was to analyzethe effects, with respect to SEM imaging, of exposure of the cathode samples to atmosphere.Third, was to assess the effectiveness of the custom designed sample holders that were intendedto prevent atmospheric exposure of the samples by sealing them inside the holders with coppertape.

As far as comparing the two different cells and their cathodes, the results did not produceanything of great significance. The discharge curves for the two tested cells are contained inFigure 10. The two discharge curves show two results. The curve corresponding to the ELcell is as expected, with a smooth curve for discharge similar to those found in literature [4].This curve shows a relatively high overpotential that is expected for Li-air batteries but it alsoshows a significant dip at the outset of the discharge process. What this dip means is unclearbut it may be associated with an activation energy needed to start the reaction process. Theexperimental cell, however, showed a different result. It showed a large amount of instabilityin the discharge curve, as can be seen by the jagged oscillation of the curve. It is unclearwhat causes this instability at this time. An approximate value of the overpotential ηdis for theEL-cell is 0.36 V and for the experimental cell is 0.61 V. This is necessarily an approximationbecause full discharge of the batteries were not performed, and the average potential calculatedis not based on the full discharge curve, which tends to have a decreasing average value overtime [4]. The lower discharge overpotential for the EL-cell implies that this cell performed moreefficiently than the experimental cell. This difference in overpotential can be explain by one oftwo reasons. First, it could be that the graphite cathode of the EL cell has more surface areathan the carbon paper cathode of the experimental cell and so there are more reaction sites forthe lithium peroxide to deposit. Second, it could be the result of the two different materials,the graphite cathode could have more active reaction sites than the carbon paper as a result ofits structure.

Figure 10: Experimental discharge of the battery: EL-cell (dashed); experimental cell (solid),discharged at 125 µA.cm−2

(Note: this is a geaometric area)

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SEM images of the two discharged cathodes are contained in figure 11 alongside SEMimages of undischarged cathodes of the same type. It is unclear whether any significant buildupof lithium peroxide occurred during the short discharge times represented in this experiment.This is particularly true for the graphite cathode of the EL-cell. The lack of results for theEL-cell might have been the result of improper sample preparation in that the cathode wasscraped at the surface in order to divide up the samples. This may have clouded the resultsand made it difficult to identify lithium peroxide build up as compared to the undischargedgraphite cathode. Thus, a more appropriate course of action would have been to image thegraphite cathode without dividing it up into separate samples so as to make the comparisonmore clear and lithium peroxide build up more noticeable. However, it is also possible that thebattery was not discharged for long enough to produce any recognizable results because generallyexperimental battery discharge times can be in excess of 35 hours [4]. However, because therewas a successful discharge of the battery there must have been lithium peroxide buildup, eventhough it may not have been noticeable in the imagery.

Figure 11: SEM pictures of (top left corner): undischarged experimental cathode; (top rightcorner): discharged experimental cathode; (bottom left corner): undischarged EL-cell cathode;(bottom right corner): discharged EL-cell cathode

The discharged carbon paper cathode produced clearer imagery than that of the dischargedgraphite cathode. As can be seen in figure 11 a difference is apparent between the dischargedand undischarged cathode. There is clearly some buildup of precipitate and EDS would havebeen able to determine if it was lithium peroxide, but because the EDS was not available duringthe experiment it made identification of the deposits very difficult. In addition, when compared

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to other SEM images of lithium peroxide deposits, it is hard to say the differences seen in theseimages are actually lithium peroxide. It is very possible that the relatively short discharge timeused in this experiment was again the cause of the lack of clarity in the results. In addition, thebest resolution of the images that were obtained was 100 nm, which also could have obscuredobservations of lithium peroxide because a higher resolution may have revealed smaller depositsthat were not as clear at the lower resolution. For example, if the lithium peroxide depositswere less than 5-10 nm thick then they would not be observable by the SEM at its highestresolution.

The second goal of the experiments was to assess the effects of atmospheric exposure ofthe samples on SEM imaging. Images comparing the same sample and sample region beforeexposure to air and after exposure at 6 minutes and 45 minutes can be found in figure 12. Fromthese images there seems to be no appreciable effect of air exposure as the images are verysimilar. However, that is not truly representative of the understanding supported by currentresearch [17]. Thus, there are a few explanations for why no difference was noticed in thisexperiment. First, it could be that because the discharge times were so short, no significantlithium peroxide buildup occurred and so there was nothing to react with air resulting fromexposure. Second, it could be that the length of air exposure was insufficient to produce anoticeable change. Or, third, that the mechanism of preserving the samples from exposure wasunsuccessful, the samples had already degraded before observation.

Figure 12: SEM pictures of the carbon cathodes after (left): 0 min; (middle): 6 mins; (right):45 mins exposure to air atmosphere

The third goal was to assess the effectiveness of the custom designed sample holders andtransportation mechanism. Images of the custom sample holders before addition of the sampleand after application of the copper tape are shown in figure 9. The samples were sealed in theirholders in an argon filled glovebox with copper tape. The sample holders were then thermallysealed in foil bags inside the glovebox. The sample holders were extracted from the bags andthe copper tape was removed or punctured inside the SEM. This should have provided sufficientprotection for the samples. However, if indeed lithium peroxide was produced, then it is possiblethat the mechanism of sealing with copper tape failed to protect the samples from atmosphericexposure during the transfer from the foil bags to the SEM machine. This transfer should havebeen even more risk free in that the unpackaging of the sample holders was done in a nitrogenfilled box that was intended to reduce the risk of exposure to air. Based on the unnoticeableeffects of the exposure to air seen in figure 12, it is possible that this transport mechanismfailed. However, further experimentation would need to be done with this transport vessel toaccurately assess its efficacy. The copper tape seal was very strong, which is why one of thesample holders had to be punctured in order to access the samples with the SEM. This suggests

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that the seal was working to some degree and it is very possible that this transport mechanismwas indeed effective even though further testing would be required to make such a statementwith sufficient confidence.

9 Future considerations

Given that there are serious challenges facing the commercialization of the lithium-air bat-tery, extensive research and testing needs to be done before any product can be sent to market.Much of that testing is well beyond the scope of the experiments discussed in this report. Assuch, there are a few focus points pertaining to resolving the experimental challenges addressedherein.

It is clear that the experiments discussed in this report fell short of expectations. As such,a discussion of future considerations is merited.

One key component of the experiment was the transferring of the cathode samples fromthe argon-filled glovebox to the SEM without exposure to atmosphere. This is a challenge thatmay be faced by many researchers in the future and a viable solution would be valuable. Away to accurately test the viability of the custom designed sample holders would be to place amaterial that reacts quickly but non-violently with air, such as zinc [25], in the sample holdersand expose the holders to atmospheric conditions for an appropriate amount of time to producea reaction. The holders could then be opened in an argon-filled glovebox to determine if theseals held, effectively protecting the sample from the atmospheric exposure.

In order to improve upon the results of these experiments, two steps should be taken. First,the batteries were not fully discharged, resulting in lithium peroxide deposits that were difficultto observe in the SEM. In subsequent experiments, discharging the batteries for a longer periodof time should result in a more robust deposit of lithium peroxide that would be more readilyobservable in the SEM. Second, assuming the viability of the sample holders, atmosphericexposure of the samples produced little to no recognizable difference in SEM imaging. Thoughthis could have resulted from the paucity of lithium buildup due to the short time of discharge,it also could have resulted from insufficient exposure to atmospheric conditions. This could becured by exposing samples for longer periods of time. This theoretically should produce a morenoticable difference in the SEM images.

The reason two different cathodes were used in this experiment was due to the unavailabilityof two identical experimental cells. Ideally, this experiment would have been performed usingidentical cathodes and cells, discharged at the same rate for different periods of time to observethe temporal changes in lithium peroxide precipitation. This would provide a more apt com-parison from which to draw conclusions about the manner of lithium peroxide precipitation.

10 Conclusion

Lithium-air batteries could be a viable replacement for gasoline when they become commer-cially available due to their high energy density. Although significant challenges still remain formany facets of the lithium-air battery designs, it is still a relatively unexplored area of research.This report focused on cathodic irreversibilities resulting from the insulating effect of reductionproduct, lithium peroxide, the effect of air exposure on SEM imaging of discharged cathodesamples, and the viability of a novel method of preventing atmospheric exposure on discharged

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cathode samples.

The cathode alone presents numerous challenges to the proper functioning of a Li-air battery.The main concern affecting efficient discharge and long life cycling is discovering and analyzingthe manner and composition of lithium peroxide deposits on the cathode. The insulating effectof lithium peroxide deposits can retard the reaction by insulating the reaction site from theelectrons travelling through the cathode as well as block pores in porous cathodes that wouldin effect reduce the reactive surface area. If other precipitates are contained in the cathodicbuildup, such as Li2O and Li2CO3, then that too could an to the problems associated withinsulation of the cathode by precipitates. One impediment to a reliable determination of thisprocess is the current unavailability of more informative in-situ characterization techniques,such as in-situ ETEM. Unfortunately, the experiments contained herein were not successful inshedding any light on this process. Further experimentation employing longer discharge timeand possibly different rates of discharge would be necessary.

Another challenge facing the analysis of discharged cathode samples concerns the effectsof exposure to atmosphere. Sample cathodes from discharged Li-air batteries are generallyremoved from the batteries in a glovebox and then transported to an SEM or TEM machinefor imaging. Keeping those samples from atmospheric exposure is crucial to ensuring a properand reliable analysis. The custom designed sample holder and transfer mechanism createdfor the experiments reported herein could be a viable method for ensuring samples remainunadulterated for analysis. Though further experimentation will be necessary to ensure thismethod works, the successful removal of the copper tape within the chamber of the SEM suggeststhat the sample holders could alleviate concerns of atmospheric contamination.

In addition, although this experiment did not reveal a significant effect of exposure to air,it can be assumed that the results were anomalous and further experimentation to determinethe actual extent of the effects of air exposure is merited.

Although the results presented in this report are ultimately inconclusive, they provide astarting point for further experimentation.

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Appendix

A Battery cell assemblies

A.1 Experimental cell assembly

The experimental cell assembly consisted of:

1. Attach o-rings to bottom half of steel battery casing

2. Add 3 drops of DME to first o-ring to ensure air tight seal

3. Attach glass cylinder to one side of the battery module so that battery layer can be stackedwithin

4. Add one 10 mm diameter circular disk of lithium to be used as the anode

5. Add 25 µL of 1M lithium bis(trifloromethanesufonyl)limide (LiTFSI) mixed with Dimethoxyethane(DME) to serve as the electrolyte

6. Add 2 10 mm diameter disks of Celgard paper to be used as a separator between thelithium anode and the carbon paper cathode to prevent short circuits

7. Add 10 mm diameter circular carbon paper cathode of 3.2 mg in weight

8. top off with 30 µL of electrolyte LiTFSI and DME

9. Add 3 drops of DME to second o-ring to ensure air tight seal

10. attach top half of battery casing to complete assembly

A.2 EL-cell assembly

The EL-cell assembly is shown in fig. 13.

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Figure 13: EL-cell assembly (from [26])

1. Screw polyflorotetraethylene (PTFE) plug into hole for the reference electrode, which wasnot used in this experiment so that hole is sealed off

2. Insert Polyether Ether Ketone (PEEK) sleeve into cell module to act as an inert barrierbetween the steel cell casing and the active battery components

3. Add metal locking washer around the sleeve as part of the assembly

4. Add polypropylene insulator

5. Insert 14 mm diameter circular graphite cathode

6. Place 18 mm diameter and 1.5 mm thick circular glass fiber separator on top of the cathodeto separate the anode and cathode to prevent short circuits

7. Place circular perforated disk on top of the glass fiber separator

8. Add 150 µL of 1M LiTFSI mixed with DME to serve as the electrolyte

9. Cut and insert 16 mm diameter circular disk of lithium to act as the anode

10. Insert steel plunger on top of battery layers for assembly purposes

11. place gold plated spring insider plunger to ensure sufficient pressure on the battery layers.Pressure must be maintained to ensure proper battery function

12. Place polymer cylinder called a syphon inside spring to give support and structure tospring system

13. Add ring of polyethylene (PE) to act as a seal for the cell components to ensure it is airtight and act as an insulator from the steel casing

14. Affix top of battery casing

15. Attach tranfer lines for adding oxygen to complete assembly

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B Discharge procedure for both batteries

1. Flush battery with O2 and build up an O2 reservoir

2. Place the experimental cell in a temperature controlled container at 25 degrees celsius.The EL-cell was left outside because the container was designed only to accommodate theexperimental cell

3. Attach electrical leads to the cells

4. Wait for one hour in order for the oxygen to diffuse through the electrolyte around thecathode. When the potential between the two electrodes is stabilized, the oxygen hasdiffused sufficiently

5. Discharge for aprroximately 4 hours, using a current rate of 100 µA for the first batteryand 196 µA for the second one in order to have a curent rate per area of cathode identicalfor both batteries.

C Transfer of samples

1. Disassemble battery in glovebox and extract carbon cathode

2. Split carbon cathodes into pieces

3. Fasten carbon cathode pieces in SEM sample holders

4. Seal sample holders with copper tape on top, to stop interaction with the atmosphere

5. Place sample containers in sealed plastic containers

6. Place plastic containers in thermally protected foil bags and seal by melting open sides

D SEM imaging of the samples

1. Extract sample holders from foil bags in a box containing liquid nitrogen vapor as amanner of further isolating the sample holders from the atmosphere

2. Attach sample holders to SEM sample holder container

3. quickly move the sample into the SEM chamber

4. Remove copper tape in vacuum using tools contained in the SEM

5. Image the sample

E Testing the effects of atmosphere contaminations on samples

1. Image the sample in SEM

2. Expose sample to amosphere for 6 minutes

3. Re-image the sample in SEM

4. Expose sample to atmosphere for 45 minutes

5. Re-image the sample in SEM

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F Who did what

The contributors of the report can be found in the table below.

What was written Who it was started by

Abstract Everyone

Introduction Ben

Motivation, scope and objective Jasper

Basic battery function Frederik

Four types of batteries Ben

The aprotic battery Martin

Challenges facing Li-air battery Everyone

Characterization methods Maxime

Experimental plan Frederik, Ben and Martin

Results Ben, Martin and Maxime

Further research Jasper, Ben, Martin

Conclusion Ben

Note: The report was divided in sections and outlined individually. Afterwards most of thereport was heavily edited by all parties.

What experiment was performed Who did it

Assembly of first cell Mie

Assembly of second cell Ali

Discharge of batteries Mie and Ali

Disassembly of batteries Poul

SEM Imaging Kristian (and Ali for the bot-tom left picture of figure 11)

During all steps of the experimentation at Risø campus, all the members of the groupparticipated at different times in the glovebox work and helped the supervisors to assemble,discharge and dissassemble the two cells in order to have a better understanding of the lithiumair battery technology. However, due to time constraints and material limitations, everythingcouldn’t be fully done by students and much of the work was performed by the supervisors witha detailed explanation given to the students. Frederik spent the longest time in the gloveboxassisting, everyone else spent approximately the same time in the glovebox assisting.

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