ET14003
Examensarbete 30 hp
November 2014
Insights into the morphological
changes undergone by the anode
in the lithium sulphur battery system
Anurag Yalamanchili
Masterprogrammet i energiteknik
Master Programme in Energy Technology
To my beloved grandparents: Yalamanchili Satya Lakshmi Devi Yalamanchili Umamaheshwara Rao, R.I.P. Karnati Jhansi Lakshmi, R.I.P. Karnati Venkateshwarlu, R.I.P. Abstract
Insights into the morphological changes undergone by
the anode in the lithium sulphur battery system
Anurag Yalamanchili
Teknisk- naturvetenskaplig fakultet
UTH-enheten
Besöksadress:
Ångströmlaboratoriet
Lägerhyddsvägen 1
Hus 4, Plan 0
Postadress:
Box 536
751 21 Uppsala
Telefon:
018 – 471 30 03
Telefax:
018 – 471 30 00
Hemsida:
http://www.teknat.uu.se/student
In this thesis, the morphological changes of the anode surface in lithium sulphur cell,
during early cycling, were simulated using symmetrical lithium electrode cells with
dissolved polysulphides (PS) in the electrolyte. Electron microscopy (SEM) was used
as the principal investigation technique to study and record the morphological
changes. The resulting images from the SEM were analysed and discussed. The initial
surface structure of the lithium anode largely influenced the ensuing morphological
changes taking place through lithium dissolution (pits) and lithium deposition
(dendrites) during discharge and charge respectively. The rate of lithium dissolution
and deposition was found to be linearly proportional to the current density applied to
the cell and the effect of cycling on the anode was proportional to the total charge of
the cell in general in agreement with the expected reaction. The effect of
self-discharge on the anode was also studied using photoelectron spectroscopy (XPS)
in tandem with SEM. The results indicated that self-discharge, occurring in the form of
corrosion of the anode SEI by PS reduction, was influenced by the altered
morphology of the cell after cycling.
The findings presented in this project can be understood as a preliminary description
for the morphological changes in the anode and their influence in the performance of
lithium sulphur battery, which can be further investigated by more advanced methods.
Handledare: Carl Tengstedt och Matthew Lacey
Ämnesgranskare: Daniel Brandell
Examinator: Roland Mathieu
ET14 003
Tryckt av: Ångström Laboratoriet, Uppsala Universitet
Populärvetenskaplig Sammanfattning Populärvetenskaplig Sammanfattning Petroleumbaserade bränslen används flitigt globalt sett idag och transportsektorns oljekonsumtion står för 19 % av världens totala energikonsumtion, framför allt på grund av vägtransporter. De två karaktäristiska storheterna som avgör en energikällas användbarhet i transporter är energin per massenhet (specifik energi) och energin per volymsenhet (energidensiteten). Bensin (12.2 kWh/kg; 8.9 kWh/L) och diesel (11.9 kWh/kg; 10.1 kWh/L) utgör idag de dominerande oljebaserade flytande bränslena beroende på möjligheterna till transport och lagring. Emellertid bidrar fossila bränslen till växthuseffekten genom emissioner som uppstår under förbränning, framför allt av koldioxid (CO2), vilket bidrar till global uppvärmning. Detta har lett till utvecklingen av alternativa energikällor för transportsektorn. Användningen av biobränslen och nya energilagringsenheter utgör strategier för att ersätta petroleumbaserade bränslen. Kemisk energilagring, där batterier används för att lagra elektrisk energi från förnybara källor och för att driva elektriska fordon, är en löftesrik teknik för att förändra transportsektorn. Litiumbaserade sekundära batterier är idag mycket utnyttjade inom portabel elektronik och undersöks därför för tillämpningar i elfordon. Litiummetallbatterier bestående av metalliskt litium som anod har studerats i ca 40 år. Metalliskt litium har en mycket hög teoretisk kapacitet (3860 mAh/g) och låg densitet (590 kg/m3), vilket gör materialet idealt för mobila och portabla tillämpningar. En rad utmaningar hindrar dock kommersialiseringen av tekniken. De två huvudsakliga problemen har varit säkerhetsproblem som grundas i potentiella inre kortslutningar och den stora ytarean hos den reaktiva litiummetallen, samt litiummetallens oförmåga att behålla sin kapacitet över ett stort antal upp‐ och urladdningscykler (låg cyklingseffektivitet). Båda dessa problem har att göra med ytförändringar hos litiumelektroden. Den tidigare forskningen på litiumbatterier genererade en alternativ teknik, litium‐jonbatteriet (LIB), vilket så småningom attraherade ett större intresse än litiummetallbatteriet. LIB består i allmänhet av litiumjoner som sätts in i en värdstruktur (som t ex grafit). Den reversibla insättningen av litiumjoner (s k interkalation) i anoden under upp‐ och urladdning skapade en stabil elektrokemi över en längre cyklingsperiod, och gav därmed en hög cyklingseffektivitet. Dock är kapaciteten begränsad i ett LIB; den teoretiska specifika energidensiteten i ett typiskt LIB är 387 Wh/kg. Trots den rätt låga energidensiteten har LIB‐tekniken introducerats brett bland elfordon under det senaste decenniet, som i halvkombin i Populärvetenskaplig Sammanfattning Nissan Leaf. LIB har dock begränsats av sin teoretiska kapacitet och dess nuvarande praktiska specifika energi (160 Wh/kg) möter inte de långsiktiga målen för kommersiella elfordon så att transportindustrin kan ersätta fossila bränslen. Nyliga upptäckter inom kemisk energilagring för elfordon har dock förnyat intresset för litiummetallbatterier, vilka har högre kapacitet än LIB. En nästa generations batteriteknologi, litium‐
svavelbatterier (LiSB) med en teoretisk specifik energi på 2500 Wh/kg, betraktas som nära praktisk användning i elfordon. Dock kvarstår problemen associerade med cyklingseffektivitet och säkerhet som identifierats för litiummetallbatterier de senaste 40 åren. Sion Power kunde uppvisa ett prototypsystem för LiSB som gav mellan 350 Wh/kg och 600 Wh/kg, men LiSB‐batterierna klarade inte av att vidhålla kapaciteten som sjönk med 20 % efter 30‐60 cykler till skillnad från målet på 1000 cykler. Dessutom är LiSB behäftat med allvarliga säkerhetsproblem associerade till litiummetallbatterier. Ett unikt problem för LiSB är den komplicerade kemiska process som uppträder i cellen under cykling, och vilken resulterar i reducerad kapacitet och kort livslängd, vilket också delvis har associerats med anoden. Medan många studier har utförts på LiSB under det senaste decenniet har endast en mindre del studerat anoden och dess degradering. Det här projektet syftar till att undersöka olika felmekanismer som uppstår via litiummetallanoden i ett LiSB, och att hitta faktorerna som initierar dem genom att studera morfologin på anodens yta under varierande grad av cykling. Elektronmikroskopi, en enkel men precis teknik, används för att skapa en bild av anodens ytor i ett LiSB. ii Acknowledgements Acknowledgements First and foremost, I would like to thank Kristina Edström, head of the Ångström Advanced Battery Group at Uppsala University and Jarmo Tamminen, head of the Division of Materials Technology for Hybrid Electronics as Scania CV AB for providing me with the opportunity to work with in this collaborative project. I sincerely express my gratitude to my supervisors Matthew Lacey and Carl Tengstedt for their unwavering support and patience for my work throughout this project. Matt, it has been a pleasure to work with you the past few months and without your guidance and clarity, this project would not have been completed. I thank you very much for your confidence in my ability to successfully complete my thesis. Carl, I cannot thank you sufficiently for everything you did for me, from introducing me material studies, to dedicating your time over and above the requirement to help me write up a good report and more importantly make a good presentation. Both of you never gave up on me and your motivations have inspired me greatly. Working with the both of you has been immensely beneficial and a lot fun at the same time. I wish you all the best and hope to stay in touch. I am also indebted to my thesis reviewer, Daniel Brandell for his feedback and direction in this thesis. Your feedback was very useful in refining report and also, thank you very much for translating my popular summary to Swedish. I thank Julia Maibach for introducing me to photoelectron spectroscopy and her help in using the XPS system and studying the self‐discharge experiment. Your help and feedback has really helped me improve my thesis and report. I extend my gratitude to Fabian Jeschull for his general support and guidance with my project and his technical support in the lab as well. Thank you both for your support during my defence as well. A special appreciation for Dan Persson from Swerea KIMAB for his visit to Uppsala and his expertise in corrosion science. I am grateful to Tomas Nyberg, my examiner for his assurance to let me complete my project despite my logistical challenges. I appreciate Karl Bengtsson Bernander, my student opponent for your feedback on my report. I thank Henrik Eriksson for technical support in the lab. Also I acknowledge the help provided by Chao Xu, Pontus Svens, Ali Rafieefar and Paulius Malinovskis. I would like to express my greatest gratitude to my parents, Madhavi and Prasad and my sister, Chetana. Lastly, a shout‐out to my friends, Mujtaba, Mirish, Frank, Sashank, Goverdhan, Rizwan and Anand for making what would have been a very difficult time in Germany very enjoyable. You’re all awesome!
iii Abbreviations and Definitions Abbreviations and Definitions Definitions Anion The negative charge carrier in the cell Anode The electrode from which positive charges are extracted in a cell during discharge Cation The positive charge carrier in the cell Cathode The electrode from which negative charges are extracted in a cell during discharge Current density (J) (A/m2) Amount of current applied per unit area of the electrode surface Cycling Repeated process of charge and discharge of a cell Cycling efficiency Ability to retain initial charge capacity over multiple cycles Dendrites Ramified deposits of lithium found on the lithium metal surface from deposition during charge Disproportionation Simultaneous oxidation and reduction of a species to form two separate products Glyme a group of solvents based on alkyl ethers of ethylene glycol or propylene glycol that are capped by methyl groups Gravimetric energy density or Amount of energy per unit mass Specific energy (kWh/kg) Groove line The imprint of stresses caused by rollers involved in the manufacturing of lithium Half‐cell reaction Part of the overall cell reaction taking place at a particular electrode Oxidation Loss of an electron by an atom, ion or a molecule leading to an increase in the oxidation state Pits Cavities formed on the lithium metal surface from dissolution during discharge Polysulphides A group of sulphur compounds containing multiple chains of sulphur atoms that stay dissolved in the electrolyte at the end of a charge or discharge process iv Abbreviations and Definitions Redox A chemical reaction in which the reactant atoms undergo a change in their oxidation state Reduction Gain of an electron by an atom, ion or a molecule leading to a decrease in the oxidation state Self‐discharge A phenomenon where a cell loses its charge in open circuit conditions Shuttle A phenomenon in lithium sulphur batteries where dissolved polysulphide ions are repetitively reduced at the anode during discharge and oxidised at the cathode during the subsequent charge over multiple cycles Volumetric energy density Amount of energy per unit volume (kWh/m3) Abbreviations General LIB Lithium ion battery LiSB Lithium sulphur battery SEI Solid Electrolyte Interface PS Polysulphide Li‐S Lithium‐sulphur cell Li‐Li Symmetrical lithium electrode cell Eavg Average cell voltage µa/c Ionic mobilities of the anions or cations D Ambipolar diffusion constant tion Ion transport number or transference number of the ion τ Sand’s time (The time by which dendrite growth is delayed Vm Molar volume vtip Velocity of dendrite tip growth propagation F Faraday constant h Planck’s constant v Abbreviations and Definitions Frequency of light SoC State of charge DoD Depth of discharge FIB Focussed ion beam SHES Self‐healing electrostatic shield Chemical PEO Poly(ethylene oxide) PVdF Polyvinylidendifluoride PVP Solid Electrolyte Interface SBR Styrene‐butadiene rubber DME 1‐2 dimethoxyethane DOL 1‐3 Dioxalane TEGDME Tetraethylene glycol dimethyl ether THF
Tetrahydrofuran Li2S Lithium sulphide LiClO4 Lithium perchlorate LiNO3 Lithium nitrate LiBF4 Lithium tetrafluoroborate LiPF6
Lithium hexafluorophosphate LiBOB Lithium bis(oxalato)borate LiTFSI Lithium bis(trifluoromethane sulfonyl) imide LixNOy
Oxidised lithium nitrogen species LixSyOz
Oxidised lithium sulphur species Li2Sx
Lithium sulphide species Analytical and characterization techniques SEM Scanning electron microscopy EDX Energy dispersive X‐ray spectrometry XPS X‐ray photoelectron spectroscopy AFM Atomic force microscopy vi Table of Contents Table of Contents Populärvetenskaplig Sammanfattning ................................................................................................... i Acknowledgements ........................................................................................................................... iii Abbreviations and Definitions ............................................................................................................ iv 1 Introduction ................................................................................................................................. 1 2 Background .................................................................................................................................. 3 2.1 2.1.1 Electrochemical reactions within the Li‐S cell ....................................................................... 4 2.1.2 Undesirable PS redox reactions ............................................................................................ 5 2.1.3 The sulphur cathode ............................................................................................................. 6 2.1.4 Electrolyte ............................................................................................................................. 7 2.2 3 Lithium Sulphur (Li‐S) battery ....................................................................................................... 3 The lithium metal anode ............................................................................................................... 9 2.2.1 Dendrite growth models ..................................................................................................... 11 2.2.2 Solid Electrolyte Interface ................................................................................................... 13 Experimental procedures ........................................................................................................... 17 3.1 Surface characterization ............................................................................................................. 17 3.1.1 Electron microscopy ............................................................................................................ 17 3.1.2 X‐ray photoelectron spectroscopy (XPS) ............................................................................ 18 3.2 Anode sample preparation ......................................................................................................... 20 3.2.1 Materials ............................................................................................................................. 20 3.2.2 Cathode preparation ........................................................................................................... 20 3.2.3 Electrolyte preparation ....................................................................................................... 20 3.2.4 Cell assembly and cycling .................................................................................................... 21 3.2.5 Sample extraction and transfer .......................................................................................... 22 3.3
SEM Image Analysis.................................................................................................................... 22 Table of Contents 4 Experimentation ......................................................................................................................... 25 4.1 Comparison of Li‐S cells with Li‐Li symmetrical cells .................................................................. 25 4.1.1 Results and observations .................................................................................................... 26 4.1.2 Discussion ............................................................................................................................ 30 4.2 Effect of varying sulphur loading on lithium morphology .......................................................... 31 4.2.1 Results, observations and analysis ...................................................................................... 32 4.2.2 Discussion ............................................................................................................................ 35 4.3 Effect of self‐discharge of Li‐S cells on the lithium anode .......................................................... 37 4.3.1 Results ................................................................................................................................. 37 4.3.2 Discussion ............................................................................................................................ 41 4.4 Effect of cycling on the Li anode ................................................................................................. 42 4.4.1 Results ................................................................................................................................. 43 4.4.2 Discussion ............................................................................................................................ 45 5 Conclusions and outlook............................................................................................................. 46 6 References ................................................................................................................................. 48 List of Figures Figure 1: Typical structure of a Li‐S cell and its working during discharge ................................................... 4 Figure 2: Charge‐discharge profiles of a Li–S cell .......................................................................................... 4 Figure 3: The chemical structures of the linear and cyclic ether components used in this project ............. 8 Figure 4: SEM images of anode morphologies of Li‐S cell at 100% depth of discharge (DoD) after increasing number of cycles.......................................................................................................................... 9 Figure 5: SEM image of the cross section of the anode after 250 cycles ................................................... 10 Figure 6: The formation of dead lithium on the lithium anode) ................................................................. 11 Figure 7: The breakdown and repair of the SEI layer during deposition and dissolution .......................... 14 Table of Contents Figure 8: The role of various surface species and the SEI species in morphological changes .................... 15 Figure 9: Schematic of the components inside the XPS chamber .............................................................. 19 Figure 10: Assembly structures for (a) Li‐S cell; (b) Li‐Li cell ....................................................................... 21 Figure 11: Pit identification using Image‐J .................................................................................................. 23 Figure 12: The frequency of approximate intervals of pit sizes in the image calculated. .......................... 24 Figure 13: Area occupied by dendrite formations in the presence of unfilled pits .................................... 24 Figure 14: State of Charge: 0% Lithium metal morphologies after 1 discharge from Li‐S and Li‐Li cells.... 26 Figure 15: State of Charge: 6.25% Lithium metal morphologies after 1 discharge and 6.25% charge from Li‐S and Li‐Li cells......................................................................................................................................... 27 Figure 16: State of Charge: 50% Lithium metal morphologies after 1 discharge and half charge from Li‐S and Li‐Li cells ............................................................................................................................................... 28 Figure 17: State of Charge: 100 % Lithium metal morphologies after 1 discharge and full charge from Li‐S and Li‐Li cells ............................................................................................................................................... 29 Figure 18: Representative macroscopic morphologies of all samples ....................................................... 33 Figure 19: The frequency of approximate intervals of pit sizes for the samples studies. .......................... 34 Figure 20: Current density versus approximate pit sizes ............................................................................ 35 Figure 21: Current density versus dendrite area fraction ........................................................................... 35 Figure 22: Fluorine spectrum F1s ............................................................................................................... 38 Figure 23: Sulphur spectrum S2p ............................................................................................................... 38 Figure 24:Macroscopic image of the morphology of the soaked Li sample ............................................... 38 Figure 25: Macroscopic image of the morphology of the charged Li sample ............................................ 38 Figure 26: Microscopic image of the morphology of the charged Li sample ............................................. 39 Figure 27: Image of the surface of the self‐discharged Li sample. ............................................................. 39 Figure 28: Macroscopic image of the morphology of the self‐discharged Li sample ................................ 39 Figure 29: Microscopic image of the morphology of the self‐discharged Li sample ................................. 39 Table of Contents Figure 30: Representative macroscopic morphologies of all samples after full cycles and after subsequent discharge ................................................................................................................................. 44 List of Tables Table 1: List of cells cycled/simulated to their respective SOCs ................................................................. 25 Table 2: Dimensional statistics of the anode samples from Li‐Li cells and Li‐S cells .................................. 30 Table 3: Li‐Li cells at their respective simulated SOC cycled at varying current densities ......................... 32 Table 4: Dimensional statistics associated with the morphologies of the samples ................................... 34 Table 5: List of samples studied in the experiment .................................................................................... 37 Table 6: List of anode samples and cycling conditions ............................................................................... 42 Introduction 1 Introduction The increased energy consumption and increased role of the automotive and transportation industries have led to an increasing reliance on fossil fuels such as petroleum on a global scale. Oil consumption for the transportation sector accounted for over 19 % of the global primary energy demand in 2013, predominantly due to road transportation [1–4]. The two characteristics of energy sources which determine their usage in transportation are the amount of energy per unit mass (gravimetric energy density) and the amount of energy per unit volume (volumetric energy density). Gasoline (12.2 kWh/kg; 8.9 kWh/L) and diesel (11.9 kWh/kg; 10.1 kWh/L) are the principal petroleum‐based liquid fuels used at present due to their ease of transport and storage [5]. However, emissions from fossil fuels contribute to a large extent to the greenhouse effect through their usage, primarily emission of carbon dioxide (CO2), resulting in global warming [6]. These issues arising from the usage of fossil fuels, has led to the development of alternative energy sources to power the transportation sector. Use of biofuels and energy storage devices are considered as sustainable strategies to replace petroleum based fuels [7,8]. Electrical energy storage devices, due to their ability to transfer energy with low consumption of materials and no emission of CO2, are preferred over other strategies in the long term. Lithium ion batteries (LIBs) seem to be among the better technologies for use in electric energy storage in electric vehicles with a theoretical gravimetric energy density of 387 Wh/kg (calculated using solely the energy obtained per unit mass of the active electrode components) [9]. LIBs, in general, consist of lithium ions inserted into a host structure (such as graphite) for the electrode materials. However their practical capacities including the mass of other cell and battery components are limited to 150 – 230 Wh/kg [10]. The use of lithium metal instead of lithiated graphite as anode material raises the possible theoretical energy capacity from 372 mAh/g [11]to 3860 mAh/g in lithium metal batteries, making them ideal for use in mobile applications [12]. The use of metallic lithium as anode is however not trivial, suffering from severe safety challenges and a major capacity loss during cycling [13,14]. Recent advances in chemical energy storage for electric vehicles has renewed the interest in lithium metal batteries. Lithium sulphur batteries (LiSBs) can be considered as a good candidate technology. Sulphur was first introduced to lithium metal batteries in 1962 [15]. Sulphur had an advantage over conventional cathode compounds used in the lithium metal batteries such as vanadium pentoxide (V2O5) or bismuth trioxide (Bi2O3) due to its abundance, low cost and low toxicity [16–19]. In 2010, Sion Power have shown that prototype LiSB systems can provide higher practical energy densities at 350 Wh/kg at 1 Introduction present and 600 Wh/kg in the near future. However, LiSBs were unable to retain their capacities, showing a 20 % capacity fade over 30 – 60 cycles as compared to the long term goal of 1000 cycles [20,21]. In addition, LiSBs present serious safety concerns associated with the lithium metal batteries of the previous generation. The LiSB system, as with other lithium metal batteries, faces significant challenges associated with the lithium metal electrode, i.e., low cycling efficiency of the battery and the safety issues caused by growth of “dendrites” or “mossy” lithium on charge, especially at higher rates [12,14,22,23]. The dendrites can cause internal short circuits in the battery [12,24,25]. Repeated cycling causes permanent deformations of the electrode surface by anodic corrosion and dendrite growth. In addition, lithium metal, due to its electro‐positivity, reacts with the electrolyte instantly resulting in the formation of a thermodynamically stable passivation layer over the electrode known as the solid‐electrolyte interface (SEI). The continuous deterioration of the lithium anode over multiple cycles causes the depletion of the electrolyte, resulting in increasing interfacial resistance and capacity fade [9,22,26,27]. The Li‐metal surface can also be affected by side reactions involving intermediate polysulphides (PS) that have been dissolved in the electrolyte from the cathode during discharge. These PS species typically also contribute to the formation of the SEI, and participate in the electrochemical charge reactions by diffusing back to the cathode to be re‐oxidised to form higher order PS leading to an internal redox “shuttle” mechanism [9,20,28,29]. This redox of the PS species is suggested as a reason for reduced use of active sulphur and lithium masses leading to low cycling efficiency and self‐discharge mechanism (low shelf life) in the lithium sulphur system [20,30]. A comprehensive study to understand the morphological changes undergone by the lithium metal electrode in the initial cycles, and the factors influencing them, under the similar experimental conditions would be a good start to understanding their contribution to failure mechanisms in the LiSB system. Scope: In this thesis, the morphological changes occurring at the surface of lithium anodes in liquid electrolyte Li‐S cells during the initial cycles were studied extensively using electron microscopy as the chief investigation method. Photoelectron spectroscopy was also used to chemically characterize the passivation layer formed on the surface on the anode. 2 Background 2 Background It is crucial to understand the context of this project in view of the recent advances in study of the LiSB system as well as follow the electrochemical processes taking place inside a Li‐S cell to understand the experiments conducted in this project. This chapter presents a detailed description of the various components and the electrochemistry of the Li‐S cell. 2.1 Lithium Sulphur (Li‐S) battery LiSB systems consist of a sulphur based cathode, a chemically inert separator, a lithium metal anode and an electrolyte to facilitate the electrochemical reactions. As a cathode, sulphur has an electrochemical potential of 2.24 V vs Li/Li+, which is small compared to LIB systems (2.4 – 4.7 V). This is somewhat compensated by the high specific capacity of the Li‐S cell at 1675 mAh/g [14,16]. The most common electrolyte in Li‐S cells is liquid electrolyte consisting of organic solvents and lithium salts held within a porous polymer separator. Solid or polymer based electrolytes can also be used, but are presently considered to have too low ionic conductivity [31–33]. A very simplified explanation of the charge/discharge mechanism: When a Li‐S cell in a charged state is connected to an external load, an oxidation reaction takes place at the lithium anode, from which electrons are transferred to the cathode via the external circuit with dissolution of lithium into the electrolyte as lithium ions. The half‐cell reaction taking place at the anode is given by: →
Simultaneously, reduction takes place at the cathode with the ultimate reduction of elemental sulphur to sulphide. The half‐cell reaction at the cathode is given by: 16
→ 8
Li2S is ultimately formed as a solid precipitate as the final state of the discharge process. The overall discharge reaction of the Li‐S is therefore: 16
→ 8
Eavg = 2.2 V The typical structure of a Li‐S cell during discharge is shown in Figure 1. 3 Background Figure 1: Typical structure of a Li‐S cell and its working during discharge (Reprinted from [9] Copyright 2012, with permission from Nature Materials) The reaction is reversible when the cell is charged by an external circuit by electrical energy. However, the charge/discharge mechanism is more complicated than as explained and involve many different steps. 2.1.1 Electrochemical reactions within the Li‐S cell The charge/discharge mechanisms within the Li‐S cell consist of numerous chemical reactions taking place at the electrodes and in the electrolyte. The reduction of elemental sulphur at the cathode to lithium sulphide (Li2S) involves a number of intermediary reactions. The actual reduction mechanisms of the sulphur cathode are still under discussion. Lacey et. al. suggested a typical model of the fundamental reduction process of the sulphur cathode in a liquid electrolyte system. 1 2
3
Figure 2: Charge‐discharge profiles of a Li–S cell (Reprinted from [34] Copyright 2014, with permission from Elsevier) 4 Background During discharge, three stages of electrochemical reactions take place at the cathode as shown in Figure 2 [34]. During the first stage (~2.3 V), elemental sulphur in solid phase (S8 (S)) is dissolved into the electrolyte (S8 (L) ) and is then reduced to long chain (S82‐) ions. During the second stage, these sulphur ions are reduced further to shorter chain ions by disproportionation reactions, where a particular species is simultaneously oxidised and reduced to form two separate products, as shown below: 1
4
→
This stage typically involves fast reaction kinetics due to the liquid phase reactions witnessing a potential drop from 2.3 – 2.1 V. Further disproportionation of the S62‐ to shorter chain ions like S42‐ takes place during the third stage at an almost stable potential (~2.1 V). During this stage the viscosity of the electrolyte increases due to the generation of anion species. It was suggested that short chain anions like S42‐ typically formed polysulphides (PS) through disproportionation rather than electrochemical reduction given by: 4
→
3
This disproportionation and redox of S42‐ and S62‐ takes place throughout most of this stage eventually resulting in the formation Li2S. In addition, the reduction of various short chain sulphur ions formed from disproportionation results in the precipitation of solid phase Li2S at the electrodes. Towards the end of discharge, precipitates of lower order PS, Li2S deposit on the cathode. The solid PS precipitates tend to agglomerate at the cathode over prolonged cycling of the Li‐S cell. These agglomerates remain electrochemically inaccessible during the most subsequent charge‐discharge processes. During charge, almost all PS are transformed to Sm2‐ ions through facile oxidation kinetics over a stable voltage rise and are further oxidized to form elemental sulphur towards the end of charge. The typical overall reactions taking place in the vicinity of the cathode are given by [16,35]: → 2
8
→ 16
2.1.2 Undesirable PS redox reactions The higher order and intermediary PS ions which remain dissolved in the electrolyte, tend to diffuse towards the lithium anode and react corrosively with the lithium metal to form lower order PS. These 5 Background reactions take place both electrochemically as well as chemically. Zhang [36] provides general equations for these reactions as given below: →
1
1
2
2
→
2
(Electrochemical oxidation) (Chemical reduction) These reduced PS diffuse back from the anode to the cathode on the next subsequent charge cycle to be re‐oxidised and a subsequent diffusion to the anode causing a polysulphide “shuttle” between the electrodes. A significant capacity of the Li‐S cell is lost in driving the shuttle mechanism, resulting in underutilization of capacity. Under idle conditions, this mechanism carries on until the PS are reduced to form solid Li2S which can deposit at the anode or elsewhere [9,20,37]. In addition to the shuttle, the PS reduction reactions also cause three main parasitic mechanisms which challenge the use of LiSBs in practical applications: 
Active consumption of active sulphur and lithium materials to form solid PS through electrochemical reduction, resulting in capacity fade over subsequent cycles (low cycling efficiency) [20,38]. 
Passive consumption of active lithium material by dissolved PS to from reduced PS species through chemical reduction, resulting in capacity fade when idle (self‐discharge)[20,39]. 
Corrosion of the passivation layer (SEI) on the anode thereby aiding in the degradation of the anode [40]. The composition of the SEI in presence of sulphur containing electrolytes is discussed further in the forthcoming sections of this chapter. 2.1.3 The sulphur cathode Elemental sulphur, due to its low conductivity (5 x 10‐30 S/cm at 25 oC), cannot be used as the standalone cathode [17,41]. Materials with high electric conductivity such as carbon are used to form composite cathodes, electrically enhancing their conductivity while simultaneously remaining inert to the chemical reactions. In order to retain the advantages of high specific energy of the LiSB, elemental sulphur must contribute to at least 70 % wt. of a sulphur‐carbon composite cathode in a conventional Li‐S cell [9,42]. Towards the end of a discharge, solid precipitate PS (Li2S) deposit on the cathode. Li2S (1.66 g/cm2) which is less dense than elemental sulphur (2.07 g/cm2), occupies more volume than elemental sulphur. Hence, a high surface area and porosity is required by the carbon composite to accommodate these 6 Background volume changes. Typically one or more binders are added in minor proportions to the sulphur cathode to maintain mechanical stability of the composite electrode. Polymers such as polyethylene oxide (PEO) [43–46], polyvinylidendifluoride (PVdF) [47,48], or styrene‐butadiene rubber (SBR) [46,49] have been preferred as binder material. One of the main strategies to limit or control the effect of the dissolved PS has been to trap the dissolved PS species through sorption on the walls the porous carbon cathode structure, thereby preventing them from diffusing towards the anode and initiate the shuttle, in effect creating PS “reservoirs”. Over the past decade several approaches have been considered in the design of the cathode structure through the use of various kinds of functional carbon materials and metal oxides such as alumina and silica [9,16,50]. The general approach in all these materials has been to expose the maximum possible surface area of the functional material structure to elemental sulphur and PS in order to use their absorption and adsorption properties. While results using varied approaches have yielded positive results, it is important to note that the effectiveness of sorption properties in trapping PS is limited. Considering that in most of the cathode structures the loaded sulphur content was below 70% of the total volume, these approaches are not likely suited for practical applications. For the purpose of studying the anode in the Li‐S cell, the formulation of the cathode structure in this project can be achieved by simpler methods based on amorphous carbon black and water soluble binder materials. 2.1.4 Electrolyte Goodenough and Kim [51] have listed the requirements of an optimal electrolyte in a lithium metal battery. In general, a good electrolyte should have high ionic conductivity, low electronic conductivity, low viscosity, intrinsic chemical and thermal stability with the electrodes, preferably low flammability, a large operating temperature range, low cost and low toxicity. The complicated electrochemistry of LiSBs adds two additional requirements. In light of the high reactivity of the PS anions and their radicals, the electrolyte must also be chemically stable against the PS anions. Since dissolved PS drive most of the charge transfer reactions with the Li‐S cell, the electrolyte must also be able to dissolve large quantities of elemental sulphur and PS. Usually, the electrolyte is a complex mixture of one or more solvents containing lithium salts and other additives. 2.1.4.1 Solvent The selection of a suitable solvent is essential for optimal utilization of the active materials in the Li‐S cell, particularly to accommodate the numerous cell reactions and the changing solubilities of different 7 Background PS in the electrolyte. Considering the given requirements for the electrolyte, conventional solvents for other lithium metal batteries based on carbonates, phosphates and esters have been ruled incompatible for LiSBs [52] since they tend to interact with PS anions through redox, nucleophilic and radical reactions [36,51]. Numerous other compatible solvents have been studied for application of to the Li‐S cell. Linear ether glyme based solvents such as 1‐2 dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) have been preferred for LiSBs due to their ability to dissolve larger amounts of PS, low viscosity and faster reaction kinetics to the PS anions [53–55]. In general, a mixture of linear and cyclic ethers (e.g. 1‐3 Dioxalane (DOL)) with a relatively high solubility for PS are used. Solvents such as DME‐DOL [53,56,57], TEGDME‐DOL [45,58,59], and TEGDME‐THF [60] have shown acceptable performance in balancing initial discharge capacity and cycling efficiency. Wang et. al. reported that a ratio of DME‐DOL in either 1:1 or 2:1 yielded optimum results [54]. The typical chemical structures of DME (linear ether) and DOL (cyclic ether) are shown in Figure 3. DOL
Figure 3: The chemical structures of the linear and cyclic ether components used in this project 2.1.4.2 Electrolyte salts and additives Lithium salts are added to the solvents in order to obtain a high transference number for lithium and ionic conductivity. However it is important the chosen salt(s) is compatible with the PS dissolved in the electrolyte. Conventional lithium salts used in other lithium metal batteries like LiPF6, LiBF4 and LiBOB have been ruled out due to this requirement [36]. Lithium salts have also been used as additives to form a stable passivation layer on the lithium anode. LiClO4 and LiTFSI have been used extensively in the electrolyte due to their low charge transfer resistance, ability to form a stable passivation layer on the anode quickly, thus reducing the effect of PS redox reactions [61,62]. One of the most important discoveries in the recent past has been the suitability of LiNO3 and other nitrate based salts as electrolyte additives for LiSBs by Mikhaylik et. al. [63]. LiNO3 can react with the anode surface and form a stable passivating layer which works actively to oxidize PS during redox reactions, thereby reducing the 8 Background effect of shuttle mechanism in the cell. However, Zhang reported that when cell potential drops below 1.6 V, LiNO3 is consumed by the cathode as well, forming insoluble reduction products that adversely affect the reversibility of the cell [64]. Moreover, the use of LiNO3 is not very effective in solving the self‐
discharge problem caused by the chemical redox. Therefore, it is recommended that to retain the beneficial effects of nitrate salts, they must be used as a co‐salt rather than a sole electrolyte additive, and ensure that the cut‐off potentials are kept above 1.7 V. 2.2 The lithium metal anode The primary challenges in LiSBs have been attributed to the lithium anode. Lithium, due to its nucleophilic nature, reacts immediately with the electrolyte to form the SEI. Over multiple cycles, lithium dissolution during discharge and lithium deposition during charge have been known to cause three major effects on the lithium anode: 
Porous dissolution of lithium on the anode leading to uneven morphology after first discharge

Deposition of lithium in the form of dendrites during charge

Evolution of mossy, powder‐like rough surface morphology exposing greater surface area
These three phenomena have caused problems for the Li‐S cell as summarized by Mikhaylik [20]: 
Electrolyte depletion to repair the SEI over freshly exposed lithium during cycling

Unacceptable cell swelling

Thermal instability due to greater surface reactivity

Possibility of internal cell short circuit due to contact of Li dendrite with the cathode [25]
The typical morphology of a lithium anode in the Li‐S cell is shown in Figure 4 [20]. Figure 4: SEM images of anode morphologies of Li‐S cell at 100% depth of discharge (DoD) after increasing number of cycles. (a) 30 cycles (b) 352 cycles (Reprtinted from [20] Copyright 2010, with permission from The Electrochemical Society) 9 Background Previous studies of morphological changes in lithium metal batteries Lithium metal anodes have been extensively studied for rechargeable batteries from primary lithium batteries. Several studies have been devoted to the study of the morphological changes at a Li anode on repeated cycling with different cathodes and electrolytes. For example, Lopéz et. al. [65] studied the evolution of lithium morphology over 250 cycles and found that the anode swelled over cycling and adopted a structure with several distinct layers as shown in Figure 5 [65]. It was also found that rather than dendrite growth, the evolution of the mossy porous layers and its reaction with the electrolyte caused capacity fade and low cycling efficiency. Yoshimatsu et. al. [66] described growth and dissolution mechanisms of lithium dendrites and proposed a scheme for the isolation and deactivation of pieces of Li dendrites through dendrite thinning in the form of “dead lithium” (chemically active but electrochemically inactive) on the top of the anode surface as shown in Figure 6 [67]. Further investigation by Arakawa et. al. on the same occurrence indicated that isolated Li could have been formed due to heterogeneous dissolution of the needle‐like Li. It was also argued that sizes and shapes of the dendrites formed over multiple cycles was greatly dependent on the applied current density [68]. Top dendritic layer Intermediate porous layer Bottom residual anode substrate Figure 5: SEM image of the cross section of the anode after 250 cycles. The thicknesses of the three layers are Dendritic: 15 µm; Porous: 300 µm; Substrate: 50 µm (Reprinted from [65] Copyright 2003, with permission from The Electrochemical Society) 10 Background Figure 6: The formation of dead lithium on the lithium anode (Reprinted from [67] Copyright 2004, with permission from American Chemical Society) Effect of initial surface state on lithium morphology Gireaud and co‐workers have investigated the effect of initial lithium surface state on the ensuing morphological changes [69]. The use of a “sellotape” method to induce defects on the lithium anode prior to cycling showed that lithium dissolution takes place preferentially along the grain boundaries on the lithium surface and the cracked lines/stress lines. This has been attributed to the higher interfacial energy at these points where the passivation layer breaks down quickly [70]. It was also found that dendrite growth through lithium deposition in the subsequent charge took place preferentially along the surface defects such as dissolution pits due locally enhanced current densities. The study noted that the initial surface state of the anode substrate is also a key parameter that can influence the ensuing morphological changes. 2.2.1 Dendrite growth models Electrochemical deposition on metals such as zinc, copper, tin and lithium have all exhibited fractal deposits on the surface. While most electrochemical deposits involve a one‐time deposition, the rechargeable nature of lithium batteries requires multiple depositions, which becomes a significant 11 Background problem during cycling. Xu and co‐workers have presented a comprehensive summary of the dendrite growth models in rechargeable lithium anodes [71]. Chazalviel first proposed a general model for origins and growth of ramified metallic electrodeposits in dilute salt solutions attributing the growth of these deposits to the development of a space charge region due to depletion of anions at the metal electrodes [72]. This model has been adapted by Brissot et. al. for models based on lithium metal batteries using binary electrolyte with lithium salts and a polymer electrolyte such as PEO [73]. DC polarisation of the cell creates a concentration gradient in the cell which is given by the equation: μ
∂C
∂x
μ
μ
Where J is the localized effective current density, e is the elementary charge, D ambipolar diffusion constant and µa and µLi are the ionic mobilities of the anions and Li+ ions respectively. It was proposed that when >
, where CO is the initial salt concentration and L is the inter‐electrode distance, the anionic concentration at the anode approaches zero. The time taken for the anionic concentration to drop to zero is defined as Sand’s time (τ) and it is given by: 2 1
Where tLi is the ion transport number of lithium ions. It was indicated that at Sand’s time, an excess build‐up of positive charges from Li+ ions takes place resulting in a space charge region forming a large electric field and rise in local current density (Ohm’s law), resulting in the nucleation of dendrites [25,73]. The Sand’s time τ, the time by which onset of dendrite growth is delayed, is inversely proportional to applied current density squared J2. However a sufficiently low current density leads to a minimal ion concentration gradient, thereby negating the formation of dendrites [71]. The limiting current density, beyond which dendrite growth takes place is given as: ∗
2
1
12 Background Experiments conducted to examine the deposition of lithium at various current densities showed that dendrite nucleation and growth in the shape of elongated dendrites occurred even when current densities were lower that Chazalviel’s limiting current density. The growth of these dendrites was attributed inhomogeneities in the local SEI. Also over multiple cycles, it was observed that dendrite nucleation and growth occurred earlier than Sand’s time due to the breakdown and repair of the SEI from the previous cycle [73–76]. The SEI plays a significant role in any initial morphological changes on the lithium anode. Barton and Brockis [77] proposed an alternate model for dendrite growth applicable in liquid electrolytes suggesting that growth of nucleated dendrites was controlled by the spherical diffusion of cations. Monroe and Newman adopted this model with the thermodynamic reference points of the lithium anode and the conditions associated with ion concentrations proposed by Brissot et. al. [78]. They found that, in liquid electrolytes, dendrite growth rate increases across the surface and is greatly dependent on applied current density. The velocity at which the tip of the dendrite propagates is given by: Where Jn is the effective current density normal to the hemispherical lithium dendrite tip, Vm is the molar volume of lithium and F is the faraday constant. Alternatively, the Chazalviel model also predicted velocity of the dendrite tip propagation as a function of the local electric field E, given by [72]: μ
2.2.2 Solid Electrolyte Interface When immersed in an electrolyte, lithium instantly reacts with the solvents and lithium salts forming irreversible reaction products. The product of this reaction is a thermodynamically stable passivation layer which prevents any further reactions between the electrolyte and the electrode known as the solid electrolyte interface (SEI) [79]. This is a layer also formed in LIBs, and it is well known that its thickness and composition is highly dependent on the salts and solvents of the electrolyte. During the electrochemical reactions, the SEI remains ionically conductive but electronically insulating. The SEI also remains chemically impermeable to the molecules of the electrolyte during electrochemical 13 Background reactions [81]. Aurbach et. al. conducted AFM studies of SEI formed on lithium anodes immersed and cycled in various carbonate organic solvents [80]. Nanomorphological studies of the SEI revealed that the SEI structure reflected the uneven heterogeneous nature of the surface film through a mosaic structure. This led to the suggestion that during discharge, regions with presence of an SEI with an inhomogeneous structure served as preferential spots for uneven dissolution of lithium and other electrochemical activity. It was proposed that at low current densities, the SEI is able to withstand the effect of a relatively low volume changes. However at higher current densities, the SEI breaks down exposing fresh pristine lithium to the electrolyte and causing the depletion of the electrolyte solution in a reduction with the lithium (SEI repair). This mechanism is considered as one of primary causes for electrolyte depletion as shown in Figure 7 [80]. Figure 7: The breakdown and repair of the SEI layer during deposition and dissolution (Reprinted from [80] Copyright 2000, with permission from American Chemical Society) Studies of the chemical composition of the SEI revealed the chemical and physical influences on the ability of the SEI to accommodate the volume changes of the anode. Of particular interest to this project are the SEIs formed in DME and DOL. Aurbach et. al. investigated the SEI formed in glyme based solvents and concluded that the ether linkage in linear ethers such as DME and diglyme is actively attacked by the lithium metal resulting in the surface film formed being dominated by ionic species such as LiOH, 14 Background Li2O and organic lithium salts such as lithium alkoxy species: ROLi, RCOLi, RCO2Li and salt anions and contaminants [82]. Surface films dominated by ionic species cannot accommodate the volume changes of the anode due to their inelastic physical properties, eventually breaking down under deposition [83]. A study by Morigaki and Ohta advocated that inorganic lithium compounds such as Li2O tend to form along the ridge lines and grain boundaries of the lithium anode, suggesting an SEI dominated locally by ionic species [84]. The study of the surface films formed in DOL showed that similar surface species dominated the SEI only with the addition of oligomer products of polydioxolane [85–87]. These oligomers are insoluble and stick to the lithium anode thereby leading to the formation of elastomers, which provide flexibility to the SEI allowing it to accommodate the volume changes of the anode during cycling. The effect of DOL and ethers forming alkoxy species such as DME is shown in Figure 8 [83]. Li in alkoxy species producing solutions
Figure 8: The role of various surface species and the SEI species in morphological changes (Reprinted from [83] Copyright 2000, with permission from Elsevier) A major influence on the SEI composition in a Li‐S cell is the presence of dissolved PS in the electrolyte. Aurbach et. al. conducted a study examining the chemical composition of the SEI using on the lithium metal etched in a PS dissolved electrolyte solution with and without LiNO3 additives [26]. The x‐ray photoelectron spectroscopy (XPS) study of the SEI indicated that a lithium strip etched in the electrolyte without the additive exhibited the formation of a small quantity of Li2S formed after 5 hours of etching and a large quantity of Li2S formed after 9 days. This was not observed in the lithium strip etched in the LiNO3 contained solution. Instead, various species of LixSOy compounds were noted in greater quantities than the insoluble Li2S. This has been attributed to the reaction of LixNOy species in the SEI with PS to 15 Background form LixSOy. The presence of LixSOy compounds is implicated in the PS redox suppressing properties of the SEI formed in the presence of LiNO3. A similar study by Xiong and co‐workers reported that a stable SEI was formed on the lithium anode over multiple cycles [88]. The structure of the SEI after multiple cycles was described as of 2 sub‐layers of passivation consisting of oxidised lithium sulphur species at the top and the bottom layer consisting of reduced nitrate species and insoluble PS precipitates. Underneath these was a layer of ionic lithium compounds as formed due to atmospheric contaminants. 16 Experimental procedures 3 Experimental procedures In this project, morphological changes on the lithium anode were studied by surface characterization. The anode samples were extracted from Li‐S cells and symmetrical Li‐Li cells. This chapter provides a brief description of the setup under which these cells were assembled and the samples were extracted and studied in this project. The methods and the working principle of the instruments used to study these samples are also detailed. 3.1 Surface characterization Two methods were used for surface characterization of the lithium metal anodes. Electron microscopy was used as the primary method of investigation and x‐ray photoelectron spectroscopy was used for supplementary study. 3.1.1 Electron microscopy Electron microscopy is a method for obtaining high resolution images at microscopic and sub‐
microscopic scales. The instrument used in this project is a scanning electron microscope (SEM), which uses electrons for imaging sample surfaces. Electrons are generated from a source in a column mounted directly above the sample chamber of the instrument. These electrons, when incident on the surface, interact with the electrons at the specimen surface through elastic and inelastic collisions causing the surface to emit its own electrons that are detected by electron detectors. The electron detectors synchronize the intensity of the detected electron signals with the incident beam to produce an image of the sample morphology displayed on a monitor. The SEM used in this project is a Zeiss ΣIGMA Series Field Emission Scanning Electron Microscope (FE‐
SEM). This project uses images formed from the secondary electrons emitted from the lithium anode specimen placed in the SEM. Secondary electrons are low energy electrons emitted sample surface under a probe current from the incident beam. Secondary electrons are good indicators of the surface morphology due to the relatively shallow depth (10 – 100 nm) from which they are emitted. They are detected using an Everhart‐Thornley detector. An in‐lens detector placed in the SEM column was also used to obtain highly focussed images at very high magnification (< 2 μm). The samples were studied in the SEM under high vacuum (typically 10‐13 bar) with an accelerating voltage of the incident beam varying from 1 kV to 20 kV. 17 Experimental procedures 3.1.1.1 Energy dispersive X‐ray Spectrometry (EDX) X‐rays are also emitted from the sample surface in the SEM. When high energy electrons impact the atoms of the surface, the electrons from the atoms at various states are ejected, creating vacancies. These vacancies are filled by other electrons e.g. valence electrons. As a result, x‐rays are emitted with energy equal to the difference in states, characteristic to the element from which the x‐rays were emitted. The x‐rays are detected by an EDX detector, in this project, an X‐MaxN detector from Oxford Instruments. It must be noted that the EDX detector is an approximate method of elemental analysis and cannot be calibrated for lithium samples as the detection of characteristic x‐rays in the EDX detector is limited to elements with atomic numbers (Z) greater than 4 in principle due to the low energy of the emitted characteristic x‐ray photons. And in practice, the accuracy of the readings for EDX is low for elements with Z < 10. 3.1.1.2 Error possibilities on SEM image formation The atmosphere of the SEM chamber under venting during introduction and removal of the samples was nitrogen gas which ideally does not react with the highly reactive lithium metal. However, given the reactive nature of lithium metal and inevitable presence of atmospheric species that could contaminate the samples during initial transfer of samples, it not possible to get an entirely pristine image of the lithium surfaces as they are when extracted from cycled cells. However precautions and measures have been taken to ensure the images used in this project can be the closest approximation to the possible surface morphologies. Charging can also occur locally in case of insulated samples, which manifests in the SEM images in the form of very high or very low brightness, making it difficult to identify the surface topography of the sample. But this phenomenon was identified easily during studies. 3.1.2 X‐ray photoelectron spectroscopy (XPS) X‐ray photoelectron spectroscopy (XPS) is a method of characterizing the chemical composition XPS is based on the Photoelectric Effect discovered by Hertz in 1887 [89] and described in its present form by Einstein in 1905 [90]. Electrons emitted by the photoelectric effect have a kinetic energy (Ekinetic) equal to the photon energy minus its binding energy (Ebinding) and the work function of the sample (
). In XPS, the electron kinetic energy is measured from which the electron binding energy is calculated given the known photon energy [91,92]. The binding energy of the electron is specific to a particular bonding state of the element in a compound. High resolution XPS can furthermore be used to quantify the chemical environments, giving their influences on binding energies. 18 Experimental procedures The kinetic energy of the emitted photoelectron is given by equation below: Where is the energy of the incident x‐ray. Figure 9: Schematic of the components inside the XPS chamber (Reprinted from [93] Copyright 2014, with permission under Creative Commons) A simplified scheme of an XPS system is shown in Figure 9 [93]. Inside the XPS system, the main components are the x‐ray source, the sample, the electron energy analyser and the electron detector. The XPS system is operated under ultrahigh vacuum conditions, typically around 10‐9 mbar. The XPS system used for this project is the PHS XPS 5600, supplied by Physical Electronics with a probing depth of 10 – 100 Å. In this project, the approximate chemical composition of the SEI formed on the lithium anode samples is studied using the XPS in tandem with the morphological examination of the same samples using the SEM. The data obtained from the XPS studies was analysed using Wavemetrics Igor Pro and was presented as elemental spectral lines in binding energy versus intensity graphs. The binding energy exhibiting a high intensity for each spectral line was correlated with the possible list of compound species from which the electrons were emitted. The compounds exhibiting the binding energies were obtained using the NIST XPS database [94]. The binding energies for all the spectral lines were initially referenced to the Fermi level of the Li samples and were corrected with an offset calculated by using the binding energy for Li (~ 55 eV) as a reference. The approximate spatial spots on the samples where the XPS analysis was used were marked using photographs of the samples and were investigated using the SEM for morphological and EDX studies. 19 Experimental procedures 3.2 Anode sample preparation In this project, two types of cells, Li‐S cells and symmetrical Li‐Li, were assembled and cycled up to various states of charge (SoCs). All the cells were assembled in an argon (Ar) filled glove box (O2: < 1 ppm; H2O: < 8 ppm) to avoid contamination of the cells and electrodes by O2 and H2O present in the air. The constituent cell materials such as electrolytes and the lithium anode strips were prepared and stored in the glove box under a similar atmosphere. 3.2.1 Materials Sulphur powder, Carbon black Super P Li (Timcal Graphite), poly (ethylene oxide) (PEO, MW = 4,000,000; Aldrich), polyvinylpyrrolidone (PVP, MW = 360,000; Aldrich), 1,2‐dimethoxyethane (DME; Novolyte), 1,3‐
dioxolane (DOL; anhydrous; Aldrich) were used as received. Lithium nitrate (LiNO3; Aldrich), lithium perchlorate (LiClO4) and lithium sulphide (Li2S; Aldrich) were dried under vacuum at 120 °C prior to use. 3.2.2 Cathode preparation For the Li‐S cells a carbon‐sulphur composite cathode was prepared by coating cathode ink to an aluminium substrate. 0.5 g of sulphur, 0.4 g of Super P carbon black, 0.08 g of PEO and 0.02 g of PVP were dispersed in 8 ml of water and mixed by ball milling for 2 hours to prepare the cathode ink. The ink was bar‐coated on to an Al foil and left to dry at room temperature for 48 hours. Circular cathodes of a diameter of 20 mm were punched out and dried under vacuum in the glove box at 298 K for a period of 16 hours. 3.2.3 Electrolyte preparation For both, the Li‐S cells and Li‐Li cells, the electrolytes were prepared in the glove box. In case of Li‐S cells, 5 ml of electrolyte consisted of 1 M (0.532 g) of LiClO4, 0.25 M (0.086 g) of LiNO3, and 2.5 ml of DOL and 2.5 ml of DME. In case of Li‐Li cells, 5 ml of electrolyte consisted of 1 M (0.1064 g) of LiClO4, 0.25 M (0.172 g) of LiNO3, and 2.5 ml of DOL and 2.5 ml of DME, 0.1 M (0.046 g) of Li2S and 0.1 M (0.257 g) of S8, to give a polysulphide‐saturated electrolyte with an average PS stoichiometry of Li2S9 (Since the cell did not contain a cathode, the sulphur anions were used in the experiment in a dissolved state). The electrolyte was placed on a magnetic stirrer to dissolve the sulphur compounds over a period of 72 hours. After 72 hours the solution obtained had a deep reddish brown colour due to the dissolved PS in the solution. 20 Experimental procedures 3.2.4 Cell assembly and cycling Vacuum‐sealed pouch cells were assembled under Ar atmosphere in the glove box. A schematic diagram of the cell assembly is given in Figure 10. Multi‐layered PP/Al/PP (aluminium laminate) encapsulation was used as the cell container. Copper (for anode) and aluminium (for cathode in case of Li‐S cells) strips were used as current collectors. Lithium metal, supplied by Cyprus Foote Materials Limited, of thickness 125 µm was cut into approximate dimensions of 40 mm X 40 mm strips for use as anode in the cells. For symmetrical Li‐Li cells, a 20 mm diameter circular piece of Li foil was used as the working electrode. Polyethylene membranes (SOLUPOR, Lydall Performance Materials), of a diameter of 22 mm were used as separators for the cell. The volume of electrolyte was fixed at 35 µL per mg of sulphur in the case of Li‐S cells (determined empirically), and 80 µL in total for Li‐Li cells. The electrolyte was applied onto the cathode before application of the separator in the case of Li‐S cells; for Li‐Li cells, the separator was first added and followed by the electrolyte. Current collector (counter electrode) Current collector (counter electrode)
Current collector (Working electrode)
Current collector (Working electrode) Lithium metal strip (Working electrode) Electrolyte with sulphur Sulphur based Cathode (Working electrode) Electrolyte Separator Separator
Lithium metal strip (counter electrode) Lithium metal strip (counter electrode) Pouch cell Pouch cell (a) Li‐S cell (b) Li‐Li Cell
Figure 10: Assembly structures for (a) Li‐S cell; (b) Li‐Li cell
21 Experimental procedures An open circuit voltage of 3 V vs Li/Li+ was considered typical for the cathode in Li‐S cells and Li‐Li cells had a potential of 0 V. All cells were cycled via galvanostatic cycling equipment under compressive pressure to various states of charge (SoCs) at different current densities depending on the experiment. 3.2.5 Sample extraction and transfer Lithium metal samples from cycled cells were also extracted in the glove box and washed using the neat electrolyte solvents (1:1 DME: DOL). For study using the SEM, the samples were placed in sample vials, vacuum sealed in pouch encapsulation inside the glove box and transferred to the SEM chamber through a glove bag (AtmosBag; Sigma Aldrich). The atmosphere of the glove bag was purged of air by evacuation and refilling of the bag with nitrogen gas at least three times prior to opening the samples. For chemical characterization using XPS, the washed samples were placed in a sample holder inside a portable load‐lock transfer system in the battery assembly glove box. 3.3 SEM Image Analysis SEM images were analysed using Image‐J microscopic image analysis software in order to identify the distributions of pits, their dimensions and the distribution of dendrite formations at various states of charge in some experiments. The method for Image J analysis is as follows: Evenly illuminated images similar to the image shown in Figure 9 (image of a lithium anode at a fully discharged state within the first cycle) were used for this analysis to get an approximate idea of the pit dimensions on the lithium surface. Image‐J allows the user to set a pixel to chosen unit of length ratio by using a “Straight” line function and the “Set scale” function under the “Analyse” dropdown menu. In this case, after measuring the scale, already in the image, it was identified as 2.89 µm per pixel on the image. The image was cropped to obtain a simple legend free image. Unevenly illuminated areas in the same image could also be cropped out using the same function. The image was then smoothened to obtain round up irregular shapes on the image and obtain a slightly noise free image using the “Smooth” function in the “Process” menu. Further smoothening can be achieved by applying the “Bandpass Filter” in the “FFT” option in the “Process” menu. A black and white contrast binary filter then applied to the image, after which the lithium substrate was rendered white and the pit formations were rendered black. To do this a “Threshold” function from the “Adjust” option in the “Image” menu was used. These black regions in the image were identified as circular shapes with irregular circumferences. Using the “Measure particles” function in the image 22 Experimental procedures “Analyse” menu, which also considers the circularity of the identified shapes, the areas of the pits (dark regions in the image) and the total area covered by pits are measured and generated. This image transformation process from image smoothing to pit identification on the image is shown in Figure 11. Figure 11: Pit identification using Image‐J The areas of pits from other compatible images of the same sample were also obtained in the same method. The areas of the pits found in all the images obtained from the same sample were extracted and pasted on an excel spreadsheet. The pit diameters were calculated from the areas on the excel sheet. A histogram of all the diameters was generated in order to identify an approximate interval for the most prominent pit sized found in each sample. These values displayed as a graph in Figure 12 as intervals of diameters against the frequency. Using this information, the mean pit width and standard deviation were calculated. For example, for the sample under consideration it was calculated as 27 µm with a standard deviation of 11 µm. The approximate area fraction, of the pits as compared to the substrate for this sample was calculated as 10 %. Using similar analysis for area covered by dendrites, the area fraction of the regions covered by dendrites within a cycled region were calculated. These values were used to identify the growth of the 23 Experimental procedures dendrites during re‐deposition. The transformation of the SEM image to quantify the area fraction by Image‐J is shown in Figure 13. Figure 12: The frequency of approximate intervals of pit sizes in the image calculated. Figure 13: Area occupied by dendrite formations in the presence of unfilled pits Dendrite widths were measure simply by using the “Line” function and recording the line lengths drawn across the dendrites at various points and noting the line lengths using the “Measure” function. The values were extracted and analysed statistically using Excel spreadsheets. It must be noted that the use of Image‐J for morphology analysis is only an approximate method of obtaining the preliminary statistics of the features on the anode samples as the method is dependent on even illumination of the SEM images.
24 Experimentation 4 Experimentation This chapter presents and discusses the results from various experiments conducted in the project and is divided in four experiments investigating the lithium metal anode morphology under various conditions of cycling. 4.1 Comparison of Li‐S cells with Li‐Li symmetrical cells In this experiment, 4 Li‐S cells and 4 Li‐Li cells were assembled and cycled at various states of charge (SoCs) within 1 cycle. The anode samples were then examined by the SEM. The resulting images were then observed and analysed. The morphologies of Li‐S cells and Li‐Li cells are compared and contrasted. The aim of the experiment was to examine the effects of operating the cells on the lithium metal anodes at a constant current density. The cells were cycled at 200 µA/cm2. The list of Li‐S cells that were cycled and the Li‐Li cells that were used for electrochemical dissolution and deposition of lithium are listed in Table 1. Table 1: List of cells cycled/simulated to their respective SOCs Li‐S cells SoC (after cycling) Active Sulphur mass (mg) Cycling Procedure 0 % 3.545 Discharge 6.25 % 3.515 Discharge + Charge 50 % 3.315 Discharge + Charge 100 % 2.645 Discharge + Charge SoC (after cycling) Duration of charge Cycling procedure 0 % 6 hours Discharge 6.25 % 6 hours + 23 mins Discharge + Charge 50 % 6 hours + 3 hours Discharge + Charge 100 % 6 hours + 6 hours Discharge + Charge Li‐Li cells 25 Experimentation 4.1.1 Results and observations SEM images were captured at high magnification to examine the surface morphology of the lithium metal. Images representative of different anode morphologies (on macroscopic and microscopic scales) are presented below in Figure 14 for SoC 0%; Figure 15 for SoC 6.25 %; Figure 16 for SoC 50 %; and Figure 17 for SoC 100 %. From the examination of the lithium metal samples from the images and under the SEM, a number of observations could be made. These observations are listed below and are referenced to the representative images presented below. Li‐S cell (A)
(B)
Macroscopic Li‐Li cell (C)
(D)
Microscopic Figure 14: State of Charge: 0% Lithium metal morphologies after 1 discharge from Li‐S and Li‐Li cells (Macroscopic: (A) and (B); Microscopic: (C) and (D)) (Li‐S cells: (A) and (C); Li‐Li cells (C) and (D)) 26 Experimentation Li‐S cell (A)
(B)
Macroscopic Li‐Li cell (C)
(D)
Microscopic Figure 15: State of Charge: 6.25% Lithium metal morphologies after 1 discharge and 6.25% charge from Li‐S and Li‐Li cells (Macroscopic: (A) and (B); Microscopic: (C) and (D)) (Li‐S cells: (A) and (C); Li‐Li cells (C) and (D)) As seen from Figure 14 (A) and (B) and Figure 15 (A) and (B), pits are formed during discharge as lithium is electrochemically dissolved in both cases, Li‐S cells and Li‐Li cells. Linear patterns of pitting were observed along marks from production (groove lines) on the lithium substrate (result of manufacturing) in both types of cells. On microscopic scale, the shapes of the pits were roughly circular (Figure 14 and Figure 15 (B), (D)), with the exception of the Li‐S sample cycled to SoC 6.25 % (Figure 15 (A), (C)). This sample had a mix of regular and irregular shaped pits. Images of the initial stages of lithium deposition (Figure 15 (C), (D)) showed that lithium deposition in the form of threads (commonly known as dendrites) nucleated at the edges of the lithium pits. Images of the SoC 0 % Li‐Li sample showed presence of impurities on the sample at some regions as seen in Figure 14 (D) 27 Experimentation Li‐S cell (A)
(B)
Macroscopic Li‐Li cell (C)
(D)
Microscopic Figure 16: State of Charge: 50% Lithium metal morphologies after 1 discharge and half charge from Li‐S and Li‐Li cells (Macroscopic: (A) and (B); Microscopic: (C) and (D)) (Li‐S cells: (A) and (C); Li‐Li cells (C) and (D)) Li deposition was not uniformly distributed across all the regions where lithium dissolution occurred in Li‐S cells (Figure 16 (A), Figure 17 (A)). In all samples, dendritic growth was observed in the vicinity of the pits usually covering the pits or filling them, making it difficult to identify if Li deposition filled out of the pits or grew over them. As seen from Figure 17 (C), Li growth filled out the pits formed during Li dissolution in some regions of Li‐S cells. No such phenomenon was observed in a “fully charged” sample of the Li‐Li cell. Otherwise, Li growth in both cells at full charge was found to be similar. Another observation made during sample extraction was that distribution of the electrolyte on the electrode/separator while sealing the cell during assembly influenced the area of the lithium electrode that participated in the electrochemical reactions 28 Experimentation Li‐S cell (A)
(C)
(B)
Macroscopic Li‐Li cell (D)
Microscopic Figure 17: State of Charge: 100 % Lithium metal morphologies after 1 discharge and full charge from Li‐S and Li‐Li cells (Macroscopic: (A) and (B); Microscopic: (C) and (D)) (Li‐S cells: (A) and (C); Li‐Li cells (C) and (D)) The approximate estimated values of pit sizes for each of the 4 Li‐S samples and the 4 Li‐Li samples were calculated using Image‐J. These values are listed in Table 2. Given that the data has been calculated from the analysis of the images alone, the results can only be considered as an approximate measure of the surface feature statistics. In some spots pits of much higher diameters (70 µm) were found, but only in very low frequency, possibly due to irregular Li dissolution. The pit diameters in Table 2 are an approximate range of the diameters/widths of the pits calculated with a confidence level of about 90 %. 29 Experimentation Table 2: Dimensional statistics of the anode samples from Li‐Li cells and Li‐S cells Range of pit diameters (µm) SOC (after cycling) Li‐S cells Dendrite widths (µm) Li‐Li cells Li‐S cells Mean dendrite area % within cycled area with pits Li‐Li cells Li‐S cells Li‐Li cells 0 % 12 – 45 14 – 35 ‐ ‐ 0 0 6.25 % 16 – 39 17 – 39 1 – 5 2 – 4 1.5 1.5 50 % 25 – 55 18 – 42 3 – 7 4 – 7 40 42 100 % 20 – 50 12 – 32 4 – 8 5 – 10 56 65 4.1.2 Discussion During initial studies, that Li‐S cells used in the project were unable to maintain stable cycling beyond 5 cycles with LiClO4. Also during experimentation, the mass of sulphur varied per sample and was inconsistent due to erosion of the sulphur coating during assembly. Use of symmetrical Li‐Li cells using lithium metal strips for both electrodes was considered as an alternative method for more repeatable and consistent experiments. However, the symmetrical cell approach necessitated prior addition of polysulphides in the electrolyte to maintain the same surface chemistry as in the Li‐S cell. The experiment examined the possibility of using Li‐Li cells in the subsequent experiments without the presence of a composite cathode which might have possible effects on the anode morphologies. As seen from the observations and results of the image analysis, the effect of the electrochemistry of the lithium metal anode is independent of the presence of a composite cathode. The surface feature dimensions in both cells are approximately equal. The distribution of dendrites with respect to the pits was approximately equal in both the cells Li‐S and Li‐Li as seen in Table 2. The irregular shapes of pits in one sample (Li‐S: SOC 6.25 %) (Figure 15 (A), (C)) are considered an exception to the otherwise uniform Li‐dissolution mechanism. A possible explanation for these shapes could due to unintentional deformation of the Li substrate caused while handling the Li electrode during cell assembly. Lithium dissolution occurred mainly along the groove lines on the lithium metal substrate, where the SEI was most likely to be inhomogeneous, a result from the manufacturing process of lithium, consistent with literature observations by Gireaud et. al. [69] and Aurbach et. al. [82,85,95]. A strong localized electric field, due to higher interfacial energy at the grooves, could also facilitate Li dissolution. Similarly, Li deposition in the form of dendrites also occurred in the vicinity of the pits consistent with the 30 Experimentation observations in previous literature [69,74–76], indicating that the growth of dendrites is preferential to regions with inconsistent SEI and surface defects (in this case, edges of pits formed on dissolution). The only occurrence of dendrites filling out the pits was seen in a few regions of the SoC 100 % Li‐S sample (Figure 17 (C)). It is difficult to justify this occurrence and since this did not occur elsewhere, it was not studied further. Apart from these observations, the morphologies of the anode surfaces in the Li‐S and Li‐Li cells at various stages of cycling were very similar, thereby facilitating the use of Li‐Li cells for further experiments in this project. 4.2 Effect of varying sulphur loading on lithium morphology In LiSBs made for practical applications, the total capacity of the battery can increased by increasing the active sulphur mass in the cathode (sulphur loading), while simultaneously maintaining the gravimetric energy density of LiSBs. This project aims to understand the influence of the varying sulphur loading on the lithium metal morphology. The condition of varying sulphur loading in the cell is simulated in Li‐Li cells by varying the current density applied to the cell, without varying the duration of charge or discharge (i.e., rate of charge (C‐
rate)); i.e. cells were discharged at three different current densities within the same amount of time, indicating that the hypothetical Li‐S cell equivalents had different total charge capacities through different amounts of sulphur content. 12 samples were studied for this experiment at various states of charge within 1 cycle. 8 samples were prepared in addition to the Li‐Li samples from the previous experiment. The list of samples and their cycling conditions are listed in Table 3. 31 Experimentation Table 3: Li‐Li cells at their respective simulated SOC cycled at varying current densities Current Density (J) (μA/cm2) Cycling Procedure Duration 1 Discharge 6 hours 200 1 Discharge + 6.25 % Charge 6 hours + 23 mins 1 Discharge + 50 % Charge 6 hours + 3 hours 1 Discharge + 100 % Charge 6 hours + 6 hours 1 Discharge 6 hours 400 1 Discharge + 6.25 % Charge 6 hours + 23 mins 1 Discharge + 50 % Charge 6 hours + 3 hours 1 Discharge + 100 % Charge 6 hours + 6 hours 1 Discharge 6 hours 800 1 Discharge + 6.25 % Charge 6 hours +23 mins 1 Discharge + 50 % Charge 6 hours + 3 hours 1 Discharge + 100 % Charge 6 hours + 6 hours 4.2.1 Results, observations and analysis Representative macroscopic images of the samples studied are presented in Figure 18. Images of the samples were analysed using Image‐J as in the previous experiment. Most of the images were successfully analysed. However, in the case of fully charged samples at 800 µA/cm2, estimated calculations were used for pit dimensions, since the images were not able to provide sufficient pit formations due to the thick dendritic layer on top of the lithium substrate. The dimensional statistics of the Li‐Li cells are listed in Table 4 and presented as histogram in Figure 19. The relationships between current density and morphological measurements are shown in Figures 20 and 21. 32 Experimentation 200 µA/cm2 400 µA/cm2 800 µA/cm2 (A) (B) (C)
(D) (E) (F)
(G) (H) (I)
(J) (K) (L)
State of charge: 100 % State of charge: 50 % State of charge: 6.25 % State of charge: 0 % Figure 18: Representative macroscopic morphologies of all samples (Current density increasing across; State of charge increasing downwards) (Red circles indicate the presence of dendrites on groove lines, as discussed in this section) 33 Experimentation Table 4: Dimensional statistics associated with the morphologies of the samples Current Density (J) (μA/cm2) Pit diameters/widths (µm) Dendrite thickness (µm) Dendrite area fraction 14 – 35 N/A 0 % 17 – 39 2 – 4 1.5 % 18 – 42 4 – 7 42 % 100 12 – 32 5 – 10 65 % 0 35 – 84 N/A 0 % 29 – 78 5 – 10 < 1 % 30 – 70 4 – 12 46 % 100 33 – 74 5 – 13 74 % 0 49 – 160 N/A 0 % 46 – 139 4 – 10 11 % 55 – 155 4 – 10 65 % 49 – 160 (approximated) 7 ‐ 12 90 % SOC % 0 6.25 50 6.25 50 6.25 50 100 200 400 800 Figure 19: The frequency of approximate intervals of pit sizes for the samples studies. 34 Experimentation Figure 20: Current density versus approximate pit sizes Figure 21: Current density versus dendrite area fraction From the examination analysis of the samples, a number of observations were drawn. The observations are listed below, referenced to their images. 
Sizes of the lithium pits, formed during discharge, were directly proportional to the applied current density. The results of Image‐J analysis from Table 4 and Figure 20 also support this observation. 
The shapes and continuities of the pits, with increasing current density were increasing in irregularity (Figures 18 (A) – (F)). The area of some of the pits in the sample at SOC 100% charged at 800 µA/cm2 had to be assumed of rectangular shapes instead of the circular shapes used in other samples, thereby approximating the diagonal measurements as pit widths. 
The dissolution of lithium still occurred along the groove lines for samples of all current densities. (Figure 18 (A) – (C)). 
The growth of the dendrites during charging, seems to have started at the impurities and defects often at the edges of pits evident as noted in the SOC 6.25% and 50% samples (Figures 18 (D) – (G)). 
Dendrite growth at higher current densities was more expansive and covered a larger area on the surface. This was evident in SOC 50% samples cycled at 400 and 800 µA/cm2 (Figure 18 (H), (I), (k) and (L)) and in Figure 21. 
The approximate thickness of the dendrites varied only marginally (Table 4), showing that it is independent of the applied current density. 4.2.2 Discussion As observed, even at higher current densities, the lithium dissolution still occurred along the groove lines, and grew along to form irregular shapes of pitting, as seen in the previous experiment. The almost 35 Experimentation linear proportionality of pit sizes with increased current density is perhaps understandable as the applied current density is directly proportional to the amount of Li dissolved from the surface. While the size of the pits has increased, it is difficult however to observe whether the depth of the pits has increased. A possible method to study the depths of the pits formed would be to examine the cross‐
section of the anode with the SEM. However this was not examined because the cross‐sectional surface of the lithium sample was altered during sample preparation through physical and possibly chemical distortion with a blade used for cutting the sample. Use of advanced methods such as focussed ion beam (FIB) to cut the Li sample was beyond the scope of this project. Nucleation of dendrites, occurred only along groove lines and surface defects due to a higher interfacial energy even in samples cycled at progressively higher current densities. However dendrite growth occurred across the surface of the anode more expansively when the cells were charged at 800 µA/cm2. It can be suggested that dendrite growth followed Monroe and Newman’s model for liquid electrolytes, where the growth of the dendrites was directly proportional to the applied current density (refer chapter 2.2.1). In a deviation to the previous study by Arakawa et. al. [68], the shapes and sizes of the dendrites were independent of the applied current density, at least within the first cycle. Since the electric field, resulting from the applied current density, is strongest at the surface and weakens with increasing distance from the anode, the dendrites begin to grow perpendicular to the surface of the anode. Considering that the abundance of Li+ ions decreases with increasing distance from the anode, the growth direction shifts gradually to grow parallel to the anode after some distance [73]. Depending on the type of the separator membrane used, the dendrites can possibly grow beyond through the pores of the separator and can contact the positive electrode, resulting in a short circuit This experiment can be understood as an indication of the effect of increasing the sulphur loading (i.e., the areal capacity) in the application, thereby increasing the effective current density at the anode. This would result in a growing inhomogeneity in the lithium surface caused by increasing strength in the applied current, which could effectively deteriorate the sample over a number of cycles, also sometimes causing other effects such as short circuits (only happens when the lithium penetrates the separator and comes in contact with the cathode) and electrolyte depletion by passivation over a higher area of the exposed of the lithium surface. Given that practical conditions for batteries involve an applied current density of 2 – 10 mA/cm2, extensive dendrite growth is inevitable. However, at higher current densities, dendrite growth can 36 Experimentation probably be slowed down by increasing tLi, the transference number of lithium ions in the electrolyte, as proposed by the Chazalviel model. This can be increased by increasing the concentration of lithium salts in the electrolyte, thereby mitigating dendrite growth to an extent, if not totally prevent it. The typical values for tLi in dilute electrolytes is approximately 0.3 and studies have shown that this can be increased to 0.6‐0.7 in concentrated electrolytes [96]. 4.3 Effect of self‐discharge of Li‐S cells on the lithium anode This experiment aims to chemically characterize the surface films formed on the lithium anode from three distinct conditions of cycling in order to understand the self‐discharge, caused by chemical PS reduction, on the anode. This experiment used XPS for chemical characterization combined with SEM for study of Li anode morphologies at various cases. The XPS studies were conducted by Julia Maibach at the Ångström Advanced Battery Centre at Uppsala University. A current density of 400 µA/cm2 was applied to Li‐Li cells which were cycled. A pristine Li was studied in the XPS by Dr. Maibach as a reference for XPS studies of other samples which were soaked in PS containing electrolyte. The samples studied in this experiment and their experimental conditions are listed in Table 5. The samples which are underlined were studied by the SEM for morphologies. The self‐
discharge condition in the Li‐Li cell was simulated by storing the cell in idle open circuit conditions for a period of 10 days after the 1st cycle. Table 5: List of samples studied in the experiment Sample Idle duration after assembly Idle duration after 1st cycle Pristine Li ‐ ‐ Soaked Li 12 hours ‐ Charged Li 0 hours 0 hours Self‐discharged Li 0 hours 10 days 4.3.1 Results Both XPS and SEM were used to study mechanisms of self‐discharge of Li‐S cells. The SEM analysis was done on the specific areas of the samples that had been studied by XPS, making it possible to correlate surface texture with element analysis. EDX was used as a complement to the SEM studies, to verify origin the sulphur on a microscopic level and hence correlating SEM and XPS. The XPS spectrum of 37 Experimentation fluorine (F1s) for the pristine, soaked, charged and self‐discharges Li samples is presented in Figure 22 and the spectrum of sulphur (S2p) for the soaked, charged and self‐discharged Li samples is presented in Figure 23. The SEM images for soaked Li (Figure 24), charged Li (Figures 25 and 26) and self‐discharged Li (Figures 27‐29) samples are also presented in the following pages with sample‐wise observations. LiF
LixSyOz Figure 22: Fluorine spectrum F1s: the obtained intensity versus binding energy results for lithium samples Figure 24:Macroscopic image of the morphology of the soaked Li sample Li2Sx
Figure 23: Sulphur spectrum S2p: the obtained intensity versus binding energy results for lithium samples Figure 25: Macroscopic image of the morphology of the charged Li sample 38 Experimentation Figure 26: Microscopic image of the morphology of the charged Li sample Figure 27: Image of the surface of the self‐discharged Li sample. Note: The red circles highlight the rough mossy features found in the vicinity of the pits on the surface Figure 28: Macroscopic image of the morphology of the self‐
discharged Li sample. Note: The red circles highlight the rough mossy features found on the surface of the dendrites Figure 29: Microscopic image of the morphology of the self‐
discharged Li sample. Note: The red circles highlight the rough mossy features found on the surface of the dendrites Pristine Li As seen from the F1s spectrum in Figure 22, the peak centred on 685 eV, attributed to LiF, was noted as a higher emission for the pristine Li sample as compared to the other samples. Since no fluorine based compounds were used in the sample preparation, the presence of fluorine could be attributed to a number of external sources such as adsorption of F‐containing species in the glove box atmosphere on the sample surface or fluorine contamination of the lithium metal during the manufacturing process. Soaked Li XPS studies of the soaked Li sample also showed a peak for LiF of lower intensity as compared to the pristine Li sample. The sulphur spectral line (S2p) (Figure 23) showed a peak at 166 eV – 170 eV, which was attributed to a range of oxidised sulphur species (LixSyOz), where the oxidation state of S is positive. 39 Experimentation A low intensity peak was noted as at 161 – 162 eV, which was indicative of sulphide species such as Li2Sx, where the oxidation state of S is negative. This indicated that LixSyOz compounds are present in higher concentration than Li2Sx at the surface. The sample also exhibited a peak for sulphur compounds of intermediate oxidation states (163 – 165 eV) as well the presence of other forms of sulphur on the SEI. SEM studies of the soaked Li sample showed no presence of pits or dendrites (Figure 24) indicating that no electrochemical reactions took place in the Li‐Li cell without cycling. While the XPS results indicated that some chemical reactions occurred at the anode surface, these were not visible by the SEM due to the spatial resolution of the technique. Charged Li XPS studies of the charged Li sample showed no peaks for F1s. Sulphur spectrum, S2p showed 2 peaks of similar intensities for Li2Sx (161 – 162 eV) and LixSyOz (166 – 170 eV), suggesting that both kinds of sulphur species were formed during cycling. SEM studies of the sample revealed a dendrite and pit formations in the surface. A mossy rough texture, indicating a form of corrosive reaction, was observed on the surface of the dendrites as observed in Figures 25 and 26. Such morphology was not seen in a similar sample studied in the previous experiment. It could be possible that due to storage in the glove box for an extended period of 10 days after the XPS studies has resulted in contamination of the sample. Self‐Discharged Li The sample had been cycled and kept idle in assembled state for 10 days. XPS studies of the sample revealed that for sulphur S2p, the peak for Li2Sx compounds (161 – 162 eV) was significantly higher than the peak for LixSyOz compounds (166 – 170 eV), as compared to other samples. This indicated that a greater quantity of sulphide based species were found on the surface than oxidised sulphur compounds. SEM studies of the sample revealed interesting features in the vicinity of pits and dendrites on the sample substrate. Mossy textures were found at the edges of the pits (as highlighted by the red circles in Figure 27 on the lithium surface. Similarly, distinct rough surface features were encountered frequently on the surface of Li outgrowths (dendrites) from cycling as highlighted in Figures 28 and 29. The EDX study of a cycled region on the sample indicated a presence of sulphur along with oxygen and carbon. However, it must be noted that carbon peaks in the EDX could be exaggerated due to the presence of residual carbon from the conductive carbon tape stuck to the sample in the presence of the electron beam in the SEM and is therefore unreliable. 40 Experimentation 4.3.2 Discussion The XPS study of the samples has provided distinguishing insights about the composition of the SEI on each sample. The decreasing concentrations of fluorine based compounds with the increasing exposure to the electrolyte and the electrochemical reactions can be explained by the formation of over‐layers of other compounds from the electrolyte, resulting in a thicker SEI. Due to the limited probing depth of XPS, it is possible that the signal for fluorine was suppressed in the deeper regions of the SEI. When soaked in the electrolyte, the dissolved PS molecules rapidly react with the lithium metal in the presence of LiNO3 resulting in the formation of oxidised sulphur species on the soaked sample as seen in Figure 22. LiNO3 in the electrolyte prevents the formation of a high concentration of sulphide compounds on the lithium metal surface by readily reacting with the lithium surface to form a passivating layer consisting of stable compounds. When a current is applied to the cell, the constituent compounds in the electrolyte instantly reduce at the lithium surface to form further passivating compounds on the surface of the charged Li sample as seen with oxidised sulphur species (SOx) (Figure 20). The peak for sulphide compounds (Li2Sx) can be explained by the electrochemical redox reaction in the Li‐S cell (refer Chapter 2.1.2) resulting in the deposition of solid PS on the anode. The self‐discharged sample has exhibited a significantly larger quantity of Li2S with respect to LixSyOz species that the charged sample. This indicates that the chemical PS redox reaction takes effect when the cell was left idle for 10 days. The dissolved PS in the electrolyte corroded the lithium anode forming the precipitates Li2Sx on the SEI. Previous literature on the phases of lithium sulphide have pointed out that Li2S is the only thermodynamically compound that could exist in the operating window of the Li‐S cell [97]. While other PS species such as Li2S2 and Li2S4 may have precipitated initially, it is possible that they were most likely to be consumed in reactions with the electrolyte components due to their meta‐
stable state. This experiment thereby suggests that in spite of the addition of LiNO3 to the electrolyte, self‐discharge reactions are not entirely contained, at least for storage periods over 10 days. The SEM study of the self‐discharged sample revealed interesting insights about the effect of self‐
discharge on the Li morphology. While the formation of sulphide and oxidised sulphur species was not visibly distinct, the effect of the PS redox shuttle could be seen manifesting on the lithium anode in the form of mossy or visibly dissimilar surface features as highlighted in Figures 24 – 26. These features were found in the vicinity of the surface defects (in this case, edges of pits and dendrites) of the substrate, suggesting that corrosion of the anode by the chemical PS redox reactions was localised to specific 41 Experimentation regions on the surface. Inhomogeneous SEI and higher interfacial energy of the large surface imperfections may have facilitated preferential corrosion. Based on this observation it can be argued that the surface state of the anode also influences the mechanism by which dissolved PS chemically corrode the anode, indicating the role of anode morphology in self‐discharge. Further experiments must be conducted to confirm this argument based on repeatable results. It must also be noted that EDX study is a very approximate method to chemically characterise the surface of the Li anode and also study lighter elements such as oxygen, and is therefore accurately applicable only to identify sulphur presence on the sample surface. Moreover, the source for higher oxygen presence in the result could be due to atmospheric or other contamination of rough surface features during SEM transfer. 4.4 Effect of cycling on the Li anode In this experiment, 7 Li‐Li cells were assembled and cycled under a uniform current density of 400 µA/cm2 to various number of cycles, extracted and studied using the SEM. The morphologies of the Li anodes were qualitatively studied in order to identify any distinct surface erosions that might have resulted from the repetitive electrochemical reactions. The SoC 100% sample charged at 400 µA/cm2 from Experiment 4.2 was also studied. The cycling conditions for the lithium samples are listed in Table 6. Table 6: List of anode samples and cycling conditions Number of cycles Duration of cycling 1 Cycle 12 hours 1 Cycle + Discharge 18 hours 2 Cycles 24 hours 2 Cycles + Discharge 30 hours 4 Cycles 48 hours 4 Cycles + Discharge 54 hours 8 Cycles 96 hours 8 Cycles + Discharge 102 hours 42 Experimentation 4.4.1 Results The images representative of the anode morphologies at microscopic scale are presented in Figure 28. As observed from Figure 30, Li dissolution after the first cycle left a spiky features on top of the dendrites seen in (B). The initial smooth texture of the dendrites was witnessed less frequently. However as seen from (C), towards the end of the 2nd cycle, the typical dendritic structure of the lithium anode was still maintained. The dendrite surface texture from the observed regions was smooth and at the lithium metal substrate was noticeable. In case of the subsequent discharge sample, Li dissolution from the surface occurred in the form of erosive defects and pits and possible pitting which may have occurred underneath the dendrites. The sample revealed a rough texture material which was on the edges of the dendrites. As seen from image (D), a few features resembled thin spiky material on the dendrites and the surface. As seen from (E) dendrite deposits formed clumps on the surface at the end of the 4th cycle rather than branching out as noted in the first cycle in previous experiments. The rough, spiky features also seemed to agglomerate and deposit on the substrate. The spiky features were encountered more frequently on this sample. The morphology subsequent discharge sample (F), was dominated by spiky lithium features while the presence of relatively fewer dendrites. The substrate of the lithium surface appeared to consist of pits and lithium dissolution features from the previous cycles. It could be suggested from these observations that in the later cycles, lithium dissolution was taking place at the surface of the dendrites rather than the surface of the substrate. At the end of the 8th cycle (G), the surface was dominated by structures that resembled the lithium fragments. Dendritic deposition was not observed on this sample. Instead, agglomerations of dendrites appeared to have formed. After the subsequent discharge (H), the surface was dominated by fragments (highlighted in (H)) and spiky, rough features. Lithium pits and dendrites were observed, possibly as the substrate of the lithium sample was not visually accessible. 43 Experimentation Cycled sample Cycled + discharged sample (A) (B)
(C) (D)
(E) (F)
(G) (H)
8 Cycles 4 Cycles 2 Cycles 1 Cycle Figure 30: Representative macroscopic morphologies of all samples after full cycles and after subsequent discharge (Number of cycles increasing downwards) (Cycled samples: (A), (C), (E) and (G); Cycled + discharged samples: (B), (D), (F) and (H)) (1 Cycle: (A) and (B); 2 Cycles: (C) and (D); 4 Cycles: (E) and (F); 8 Cycles (G) and (H). Red circles indicate agglomerates as discussed in this section 44 Experimentation 4.4.2 Discussion As seen from the observations, lithium morphology transitions over increasing number of cycles from dendrites and pits to sharp, rough deposits, particularly seen after a discharge step. On further increasing the number of cycles, it appeared that the dendrite formations and the sharp deposits were segregated into district agglomerates of their surface texture. Further cycling produced what seemed to be the presence of lithium fragments. Li dissolution, obstructed by the abundant dendrites at the surface, ceases to take place at the substrate of the anode. This could result in possible Li dissolution of the dendrites, which are essentially, extensions of the lithium substrate, during discharge. A similar mechanism was identified by López et. al. [65]. Due to the protruding nature of the dendrites, lithium dissolution can also germinate at the dendrites, leading to possible thinning of dendrites in lithium metal batteries (Figure 4) and rise of needle‐like Li as noted in previous studies [66,68]. Uneven dissolution of the dendrites, based on the homogeneity of the SEI and the interfacial energy on the dendrite surfaces might have caused the formation of sharp and rough dendrites. These features upon further dissolution form the rough surface material which lose electrical contact with the substrate. However, further study needs to be conducted to understand the possible origins of the sharp and rough deposits which appeared on the edges of the dendrites at the after the 5th discharge and also the possible reasons as to what causes the agglomeration of the surface features to form fragments. Extensive quantitative studies of the anode morphologies are also required to support the observations made in this experiment. 45 Conclusions and outlook 5 Conclusions and outlook In this work, the morphological changes undergone by the lithium metal anode in the lithium sulphur battery system under various experimental conditions of cycling have been studied using electron microscopy, a surface sensitive method, as the main technique to image and analyse the lithium anode morphologies. Also, a brief investigation was carried out using photoelectron spectroscopy (XPS) in tandem with the SEM understand the influence of the lithium anode morphology on the self‐discharge phenomenon in the Li‐S cell and vice versa. Through the use of simple symmetrical Li‐Li cells, it was revealed that almost all resultant formations of electrochemical dissolution (pits) and deposition (dendrites) occurred preferentially along groove lines and surface defects from the manufacturing and prior handling of the Li metal anode. This observation was explained by the presence of a higher interfacial energy and the presence of an inhomogeneous solid electrolyte interface (SEI) at these surface defects, suggesting that the initial surface state of the anode significantly affects the ensuing morphological changes, consistent with the literature. Overall, the experiment also showed that the effect of the electrochemistry on the morphologies of Li‐S cells and Li‐Li cells was similar and that Li‐Li cells can be used as an alternative and reliable method to simulate the morphological changes on the anode. The effect of increased sulphur loading on the anode was simulated by varying the current density applied on the Li‐Li cells while cycling. Lithium dissolution was found to be linearly proportional to the applied current density, evident in the increasing sizes of lithium pits with increasing current density consistent with the expected process of lithium dissolution. Lithium deposition across the surface of the anode was found to be directly proportional to current density with the rate of lithium deposition proportional to the current density, consistent with the predictions of the Monroe and Newman model of dendrite growth. Another interesting observation was that the approximate widths of the dendrites was independent of the applied current density, at least within the first cycle. Overall it can be summarized that with increased sulphur loading, the electrochemical effects on the Li anode will be more severe and continuous cycling using higher sulphur content in a Li‐S cell will result in accelerated erosion of the anode. This study could be extended further by SEM examination the cross‐sections of Li anode samples through advanced methods of sample preparation (FIBs). 46 Conclusions and outlook The effect of multiple cycles on the Li anode was examined qualitatively using the SEM. The observations revealed that the Li anode undergoes severe erosion within the first 10 cycles. This experiment can be investigated further by examining the anodes after consecutive cycles to study the source of dendrite agglomerations observed at later cycles. The effect of self‐discharge on the anode was studied using XPS and the results indicated that while LiNO3 was able to partially mitigate the effect of PS reduction on the anode initially, it was unable to prevent the formation of Li2Sx. The effect of self‐discharge was seen in the form of corrosion on the anode by chemical PS redox reactions. When studied in the SEM, it was revealed that corrosion was localised taking place preferentially at the edges of pits and on the surfaces of dendrites. This experiment has shown that the morphology of the anode also influences the mechanism by which PS corrode the anode and it could be suggested that an eroded anode surface is more prone to chemical redox corrosion and thereby, self‐discharge. Further studies using XPS analysis in combination with SEM could help support these arguments. In addition to the recommendations mentioned in the summary, the initial lithium anode morphologies could also be studied using electron microscopy after cycling in the presence of caesium and rubidium salt additives in the electrolyte. Ding and co‐workers found that during lithium deposition, Cs+ and Rb+ cations, which at low concentrations, have a lower reduction potential than Li+ ions, are attracted to the protuberant tip of a lithium dendrite, electrostatically forming a shield around the dendrite, thereby forcing the Li+ ions to deposit in an alternate region due to electrostatic repulsion [98]. This mechanism, known as the self‐healing electrolyte electrostatic shield (SHES), helps in the formation of a relatively smoother morphology on the anode. SHES is considered as one of the most promising strategies for dendrite suppression in lithium metal batteries. 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