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Highly Concentrated Electrolytes for Rechargeable Lithium Batteries
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry. Chalmers University of Technology.ORCID iD: 0000-0002-3966-6219
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 [en]
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: urn:nbn:se:uu:diva-406483OAI: oai:DiVA.org:uu-406483DiVA, id: diva2:1413015
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
List of papers
1. Critical evaluation of the stability of highly concentrated LiTFSI - Acetonitrile electrolytes vs. graphite, lithium metal and LiFePO4 electrodes
Open this publication in new window or tab >>Critical evaluation of the stability of highly concentrated LiTFSI - Acetonitrile electrolytes vs. graphite, lithium metal and LiFePO4 electrodes
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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.

Keywords
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:nbn:se:uu:diva-351302 (URN)10.1016/j.jpowsour.2018.03.019 (DOI)000430897700041 ()
Funder
Swedish Energy Agency, 39042-1
Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2020-03-09Bibliographically approved
2. Highly Concentrated LiTFSI-EC Electrolytes for Lithium Metal Batteries
Open this publication in new window or tab >>Highly Concentrated LiTFSI-EC Electrolytes for Lithium Metal Batteries
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2020 (English)In: ACS Applied Energy Materials, ISSN 2574-0962, Vol. 3, no 1, p. 200-207Article in journal (Refereed) Published
Abstract [en]

Concentrated electrolytes have the potential to increase the stability for batteries with lithium metal anodes. In this study, liquid electrolytes were created by mixing ethylene carbonate (EC), a solid at room temperature, with a high concentration of LiTFSI salt. The binary LiTFSI–EC highly concentrated electrolytes have the benefit of extremely low volatility as compared to conventional organic electrolytes and also allow for cycling vs Li metal anodes. Using a LiTFSI–EC electrolyte with molar ratio 1:6, the Coulombic efficiency for Li plating/stripping on Cu is 97% at a current density of 1 mA cm–2 with a 2 mAh cm–2 capacity, pointing to a practically useful performance. In a full cell setup using a commercial LiFePO4 (LFP) cathode, the efficiency is maintained, proving compatibility. In comparison to other carbonate-based electrolytes, there is less accumulation of decomposition products on the surface of a cycled Li film, which in part explains the improved cycle life. In all, this electrolyte system shows promise in terms of electrochemical stability and may allow for safe Li metal batteries due to the inherent physical stability.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-406478 (URN)10.1021/acsaem.9b01203 (DOI)000510104700026 ()
Funder
Swedish Energy Agency, 39042-1StandUp
Available from: 2020-03-09 Created: 2020-03-09 Last updated: 2020-03-23Bibliographically approved
3. Interactions and Transport in LiTFSI-based Highly Concentrated Electrolytes
Open this publication in new window or tab >>Interactions and Transport in LiTFSI-based Highly Concentrated Electrolytes
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(English)Manuscript (preprint) (Other academic)
Keywords
highly concentrated electrolyte, ionicity, rate capability, transference
National Category
Physical Chemistry Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-406480 (URN)
Available from: 2020-03-09 Created: 2020-03-09 Last updated: 2020-03-09
4. Additives and Separators for LiTFSI – EC Electrolytes vs. Lithium metal, Graphite and NMC622 Electrodes
Open this publication in new window or tab >>Additives and Separators for LiTFSI – EC Electrolytes vs. Lithium metal, Graphite and NMC622 Electrodes
(English)Manuscript (preprint) (Other academic)
Keywords
Concentrated electrolyte; non-volatile electrolyte; Al passivation; additives; separators; Li-ion battery
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-406481 (URN)
Available from: 2020-03-09 Created: 2020-03-09 Last updated: 2020-03-09

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5678910118 of 16
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