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Critical evaluation of the stability of highly concentrated LiTFSI - Acetonitrile electrolytes vs. graphite, lithium metal and LiFePO4 electrodes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry. Department of Physics, Chalmers University of Technology;Alistore-ERI European Research Institute, FR3104 CNRS, Rue Baudelocque, 80039 Amiens Cedex, France.ORCID iD: 0000-0002-3966-6219
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0003-2538-8104
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0002-8019-2801
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry. Alistore-ERI European Research Institute, FR3104 CNRS, Rue Baudelocque, 80039 Amiens Cedex, France.ORCID iD: 0000-0003-4440-2952
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2018 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 384, p. 334-341Article in journal (Refereed) Published
Abstract [en]

Highly concentrated LiTFSI - acetonitrile electrolytes have recently been shown to stabilize graphite electrodes in lithium-ion batteries (LIBs) much better than comparable more dilute systems. Here we revisit this system in order to optimise the salt concentration vs. both graphite and lithium metal electrodes with respect to electrochemical stability. However, we observe an instability regardless of concentration, making lithium metal unsuitable as a counter electrode, and this also affects evaluation of e.g. graphite electrodes. While the highly concentrated electrolytes have much improved electrochemical stabilities, their reductive decomposition below ca. 1.2 V vs. Li+/Li° still makes them less practical vs. graphite electrodes, and the oxidative reaction with Al at ca. 4.1 V vs. Li+/Li° makes them problematic for high voltage LIB cells. The former originates in an insufficiently stable solid electrolyte interphase (SEI) dissolving and continuously reforming – causing self-discharge, as observed by paused galvanostatic cycling, while the latter is likely caused by aluminium current collector corrosion. Yet, we show that medium voltage LiFePO4 positive electrodes can successfully be used as counter and reference electrodes.

Place, publisher, year, edition, pages
2018. Vol. 384, p. 334-341
Keywords [en]
Highly concentrated electrolyte, Li-ion battery, SEI, Al corrosion, Self-discharge
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-351302DOI: 10.1016/j.jpowsour.2018.03.019ISI: 000430897700041OAI: oai:DiVA.org:uu-351302DiVA, id: diva2:1209603
Funder
Swedish Energy Agency, 39042-1Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2020-03-09Bibliographically approved
In thesis
1. Highly Concentrated Electrolytes for Lithium Batteries: From fundamentals to cell tests
Open this publication in new window or tab >>Highly Concentrated Electrolytes for Lithium Batteries: From fundamentals to cell tests
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

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.

Place, publisher, year, edition, pages
Göteborg: Department of Physics, Chalmers University of Technology, 2018. p. 40
Keywords
Li-ion battery, SEI, Highly concentrated electrolyte, Al corrosion, Li metal battery
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-351339 (URN)
Presentation
2018-06-14, Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 15:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 39042-1
Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2018-05-23Bibliographically approved
2. Highly Concentrated Electrolytes for Rechargeable Lithium Batteries
Open this publication in new window or tab >>Highly Concentrated Electrolytes for Rechargeable Lithium Batteries
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 are almost all based on 1 M LiPF6 in a mixture of organic solvents and while these balance the many requirements of the cells, they are volatile and degrade at temperatures above ca. 70°C. The salt could potentially be replaced with e.g. LiTFSI, but dissolution of the Al current collector would be an issue. Replacing the graphite 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 and consumption of both Li and electrolyte. Highly concentrated electrolytes (HCEs) have emerged as a possible remedy to all of the above, by a changed solvation structure where all solvent molecules are coordinated to cations – leading to a lowered volatility, a reduced Al dissolution, and higher electrochemical stability, at the expense of higher viscosity and lower ionic conductivity.

In this thesis both the fundamentals and various approaches to application of HCEs to lithium batteries are studied. First, LiTFSI–acetonitrile electrolytes of different salt concentrations were studied with respect to electrochemical stability, including chemical analysis of the passivating solid electrolyte interphases (SEIs) on the graphite electrodes. However, some problems with solvent reduction remained, why second, LiTFSI–ethylene carbonate (EC) HCEs were employed vs. Li metal electrodes. Safety was improved by avoiding volatile solvents and compatibility with polymer separators was proven, making the HCE practically useful. Third, the transport properties of HCEs were studied with respect to salt solvation, viscosity and conductivity, and related to the rate performance of battery cells. Finally, LiTFSI–EC based electrolytes were tested vs. high voltage NMC622 electrodes.

The overall impressive electrochemical stability improvements shown by HCEs do not generally overcome the inherent properties of the constituent parts, and parasitic reactions ultimately leads to cell failure. Furthermore, improvements in ionic transport can not be expected in most HCEs; on the contrary, the reduced conductivity leads to a lower rate capability. Based on this knowledge, turning to a concept of electrolyte compositions where the inherent drawbacks of HCEs are circumvented leads to surprisingly good electrolytes even for Li metal battery cells, and with additives, Al dissolution can be prevented also when using NMC622 electrodes.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1913
Keywords
Li-ion battery, SEI, Highly concentrated electrolyte, Al dissolution, Li metal battery, ion transport
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-406483 (URN)
Public defence
2020-04-08, PJ-salen, Fysikgården 2B, Göteborg, 13:00 (English)
Opponent
Supervisors
Available from: 2020-03-17 Created: 2020-03-09 Last updated: 2020-03-25Bibliographically approved

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