The electrolyte is a crucial part of any lithium battery, strongly affecting longevity and safety. It has to survive rather severe conditions, not the least at the electrode/electrolyte interfaces. Current commercial electrolytes based on 1 M LiPF 6 in a mixture of organic solvents balance the requirements on conductivity and electrochemical stability, but they are volatile and degrade when operated at temperatures above ca. 70°C. The salt could potentially be replaced with e.g. LiTFSI, but corrosion of the aluminium current collector is an issue. Replacing the graphite negative electrode by Li metal for large gains in energy density challenges the electrolyte further by exposing it to freshly deposited Li, leading to poor coulombic efficiency (CE) and consumption of both Li and electrolyte. Highly concentrated electrolytes (up to > 4 M) have emerged as a possible remedy, by a changed solvation structure such that all solvent molecules are coordinated to cations – leading to a lowered volatility and melting point, an increased charge carrier density and electrochemical stability, but a higher viscosity and a lower ionic conductivity. Here two approaches to highly concentrated electrolytes are evaluated. First, LiTFSI and acetonitrile electrolytes with respect to increased electrochemical stability and in particular the passivating solid electrolyte interphase (SEI) on the anode is studied using electrochemical techniques and X-ray photoelectron spectroscopy. Second, lowering the liquidus temperature by high salt concentration is utilized to create an electrolyte solely of LiTFSI and ethylene carbonate, tested for application in Li metal batteries by characterizing the morphology of plated Li using scanning electron microscopy and the CE by galvanostatic polarization. While the first approach shows dramatic improvements, the inherent weaknesses cannot be completely avoided, the second approach provides some promising cycling results for Li metal based cells. This points towards further investigations of the SEI, and possibly long-term safe cycling of Li metal anodes. / Elektrolyten är en fundamental del av ett litiumbatteri som starkt påverkar livslängden och säkerheten. Den måste utstå svåra förhållanden, inte minst vid gränsytan mot elektroderna. Dagens kommersiella elektrolyter är baserade på 1 M LiPF 6 i en blandning av organiska lösningsmedel. De balanserar kraven på elektrokemisk stabilitet och jonledningsförmåga, men de är lättflyktiga och bryts ned när de används vid temperaturer över ca. 70°C. Saltet skulle kunna bytas ut mot t.ex. LiTFSI, vilket ökar värmetåligheten avsevärt, men istället uppstår problem med korrosion på den strömsamlare av aluminium som används för katoden. Genom att byta ut grafitanoden i ett Li-jonbatteri mot en folie av litiummetall kan man öka energitätheten, men då litium pläteras bildas ständigt nya Li-ytor som kan reagera med elektrolyten. Detta leder till en låg coulombisk effektivitet genom nedbrytning av både Li och elektrolyt. Högkoncentrerade elektrolyter har en mycket hög saltkoncentration, ofta över 4 M, och har lags fram som en möjlig lösning på många av de problem som plågar denna och nästa generations batterier. Dessa elektrolyter har en annorlunda lösningsstruktur, sådan att alla lösningsmedelsmolekyler koordinerar till katjoner – vilket leder till att de blir mindre lättflyktiga, får en ökad täthet av laddningsbärare, och en ökad elektrokemisk stabilitet. Samtidigt får de en högre viskositet och lägre jonledningsförmåga. Här har två angreppssätt för högkoncentrerade elektrolyter utvärderats. I det första har acetonitril, som har begränsad elektrokemisk stabilitet och ett högt ångtryck, blandats med LiTFSI för en uppsättning av elektrolyter med varierande koncentration. Dessa har testats i Li-jonbatterier och i synnerhet den passiverande ytan på grafitelektroder har undersökts med både röntgen-fotoelektronspektroskopi (XPS) och elektrokemiska metoder. En markant förbättring av den elektrokemiska stabiliteten observeras, men de inneboende bristerna hos elektrolyten kan inte kompenseras fullständigt, vilket skapar tvivel på hur väl detta kan fungera i en kommersiell cell. Med det andra angreppssättet har hög saltkoncentration nyttjats för sänka smältpunkten för en elektrolyt baserad på etylenkarbonat, som annars inte kan används som enda lösningsmedel. Dessa elektrolyter har testats för användning i Limetall-batterier genom långtidstest, mätning av den coulombiska effektiviteten och analys av deponerade Li-ytor med svepelektronmikroskop. Resultaten är lovande, med över 250 cykler på 0.5 mAh/cm2 och en effektivitet på över 94%, men framförallt observeras en mycket jämnare deponerad Li-yta, vilket kan möjliggöra säker cykling av Li-metall-batterier. Ett logiskt nästa steg är studier av Liytan med t.ex. XPS för att utröna vad som skiljer den från ytan som bildats i en 1 M referenselektrolyt.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21787号 / 工博第4604号 / 新制||工||1717(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 安部 武志, 教授 作花 哲夫, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Anion intercalation in the graphite positive electrode of a dual-ion battery requires high potential (> 4.3 V vs Li+/Li), which aggravates parasitic reactions involving electrolyte decomposition and Al corrosion, manifesting in poor coulombic efficiency, cycle life, and quick self-discharge. This study aims to investigate the stability of anion-intercalated graphite electrodes in a 4 M solution of lithium bis(fluorosulfonyl)imide (LiFSI) in ethyl methyl carbonate (EMC) using both in-situ and ex-situ Raman spectroscopy. The concentrated electrolyte is essential as it limits parasitic reactions at the cathode-electrolyte interface (CEI) occurring in parallel to anion intercalation. Using electrochemical methods including cyclic voltammetry, and post-mortem electron microscopy it was confirmed that the Al current collector is largely stable at potentials as high as 5.2 V in the electrolyte under consideration; no dissolved Al species were detected using EDX characterization. Results from the cyclic voltammetry study also indicate that parasitic reactions can be mitigated when the cut-off potential is limited to 5.0 V leading to higher coulombic efficiency (CE = 94 %) and more stable discharge capacity (85.17 mAh g-1). However, extending the potential to 5.1 and 5.2 V results in the discharge capacity increasing by almost 20 mAh g-1, though at the expense of the coulombic efficiency, which decreases from 94 to 76 %. Upon raising the cut-off potential to 5.3 V, the CE significantly decreased (20.62 %) as a result of extensive solvent decomposition ultimately leading to much quicker capacity fading.  Based on SEM images taken after 50 cycles, graphite particles did not sustain any structural or morphological change during cycling regardless of the cut-off potentials applied. Further tests were conducted on Li-graphite DIBs using galvanostatic methods in the range from 3 to 5 V, and at different specific currents (20, 50, and 100 mA g-1). Though the cells exhibited good performance in terms of capacity retention, and cycle life at all currents, the coulombic efficiency tended to decrease as the test currents were lowered. This observation confirms the presence of parasitic reactions which are only visible when the experimental timescale is sufficiently long. At 50 and 100 mA g-1, the CE reached > 98 % which further verifies the kinetic aspect of electrolyte decomposition reactions. It is evident that self-discharge sustained in the course of open-circuit potential (OCP) relaxation of the fully charged cell can reveal the stability of the electrolyte and the anion-intercalated graphite. Raman spectroscopy measurements conducted in-situ and ex-situ on graphite electrodes charged and discharged to a series of potential cut-offs reveal the existence of self-discharge leading to extraction of anions from the graphite particles. This was demonstrated through the spectral appearance of E2g2(i) band next to E2g2(b) band at a fully intercalated state, as opposed to the in-situ spectrum, which only showed one intercalated band (E2g2(b)). It can be concluded that concentrated electrolytes (such as 4 M LiFSI in EMC) only provide kinetic stability and are unable to entirely inhibit parasitic reactions at the interface. This further highlights the need for electrolyte additives that can create a more stable interfacial passivation layer on the positive electrode so that more reversible anion intercalation can be attained.

No description available.

高ニッケル三元系材料は商用のLiCoO2正極より高い容量を有するため、EVsで使用するリチウムイオン電池の正極材料の候補として有望である。本論文に、著者は濃厚電解液とフッ素化溶媒を用いた希釈電解液に着目し、高ニッケル三元系LiNi0.8Co0.1Mn0.1O2(NCM811)の充放電サイクル特性を向上させた。電解液中の溶媒化構造の観点から、濃厚電解液の希釈効果がNCM811の充放電特性に及ぼす影響を詳細に検討した。 / Ni-rich ternary materials have higher capacity than the commercial LiCoO2 positive electrode, and therefore they are promising candidates for the positive electrode material of lithium ion batteries for use in EVs. In this thesis, the author focused on highly concentrated electrolytes and their diluted electrolytes with fluorinated solvents to improve the cycling performance of a Ni-rich ternary LiNi0.8Co0.1Mn0.1O2 (NCM811) for practical application. Dilution effects of the concentrated electrolytes on the charge/discharge properties of NCM811 were discussed in detail from the viewpoint of the solvation structure in the electrolyte. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Les travaux de la thèse portent sur l’impact de la structure des électrolytes hautement concentrés (ÉHC) à base de Lithium Bis[trifluorométhane(sulfonyl)]imide (LITFSI) et d’acétonitrile (ACN) dans les réactions de transfert d’électron et les interactions présentes avec les différentes espèces en jeu. Ces électrolytes sont étudiés comme électrolyte dans les dispositifs de stockage d’énergie (batteries, supercapaciteurs), mais la recherche sur les transferts d’électron dans ces ÉHC est presque inexistante. Les travaux sont présentés en deux volets; dans le premier, nous nous sommes concentrés à assurer de la qualité des ÉHC. Nous avons identifié les principales sources d’eau dans ces électrolytes : la présence d’eau varie selon le fournisseur de sel et le taux d’adsorption d’eau de l’électrolyte. Nous avons aussi analysé les impacts de la quantité d’eau sur les propriétés physicochimiques et la fenêtre de stabilité électrochimique. Une teneur d’eau dans les ÉHC sous 1000 ppm n’affecte pas les propriétés physicochimiques. Cependant, la fenêtre de stabilité électrochimique est affectée par une faible présence d’eau (>200 ppm), particulièrement la stabilité en réduction. Le second volet porte sur l’étude du transfert d’électron du couple Fc+/Fc dissout et adsorbé à l’électrode dans les ÉHC LiTFSI : ACN. Nous avons montré que la cinétique du transfert d’électron varie avec la concentration (dilué vs. hautement concentré) et avec l’état d’oxydation du couple rédox (Fc+ vs Fc). La constante de transfert d’électron est plus élevée avec le Fc+ que le Fc dans les milieux dilués, mais la situation est inversée dans les ÉHC. En complément à l’électrochimie, les études Raman couplées à l’électrochimie ont révélé que cette différence provient de l’environnement chimique qui diffère entre les deux espèces, dues à la charge des deux espèces (Fc+ vs. Fc) aux différentes concentrations de sel. Les travaux de cette thèse sont les premiers à montrer l’électrochimie d’une molécule électroactive couplée avec l’utilisation de méthode spectroscopique pour le couple Fc+/Fc dans les ÉHC. Cette recherche ouvre la porte à l’utilisation de ces méthodes d’analyse pour les ÉHC et montre un grand potentiel pour des applications autre que le stockage d’énergie. Les résultats obtenus sont un premier pas vers la formulation d’ÉHC adaptés aux applications d’électrocatalyse : l’utilisation des interactions électrostatiques présentes à haute concentration pourraient ralentir les réaction secondaires formant des cations ou ralentir la diffusion de cations impliqués dans les réactions de transfert d’électron couplées. / The work of this thesis focuses on the impact of the structure of highly concentrated electrolytes (HCE) based on Lithium Bis[trifluoromethane(sulfonyl)]imide (LITFSI) and acetonitrile (ACN) on the electron transfer reactions and the interactions present with the different species involved. These electrolytes are studied as electrolytes in energy storage devices (batteries, supercapacitors), but research on electron transfers in these HCE is almost non-existent. The work is presented in two parts; in the first part, we focused on ensuring the quality of HCE. We identified the main sources of water in these electrolytes: the presence of water varies depending on the salt supplier and the water adsorption rate of the electrolyte. We also analyzed the impacts of the amount of water on the physicochemical properties and the electrochemical stability window. A water content in HCE below 1000 ppm does not affect the physicochemical properties. However, the electrochemical stability window is affected by low water content (>200 ppm), especially the reduction stability. The second part deals with the study of the electron transfer of the dissolved and adsorbed Fc+/Fc couple at the electrode in LiTFSI: ACN HCE. We have shown that the electron transfer kinetics varies with concentration (dilute vs. highly concentrated) and with the oxidation state of the redox couple (Fc+ vs. Fc). The electron transfer constant is higher with Fc+ than Fc in dilute media, but the situation is reversed in HCE. In addition to electrochemistry, Raman studies coupled with electrochemistry revealed that this difference in electron transfer comes from the chemical environment which differs between the two species, due to the charge of the two species (Fc+ vs. Fc) at different salt concentrations. The work of this thesis is the first to show the electrochemistry of an electroactive molecule coupled with the use of spectroscopic methods for the Fc+/Fc couple in HCE. This research opens the door to the use of these analytical methods for HCE and shows a great potential for applications other than energy storage. The results obtained are a first step towards the formulation of HCE adapted to electrocatalysis applications: the use of electrostatic interactions present at high concentration could slow down the secondary reactions forming cations or slow down the diffusion of cations involved in coupled electron transfer reactions.