Highly Concentrated Electrolytes for Rechargeable Lithium Batteries
Doktorsavhandling, 2020

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 cannot 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.

Li-ion battery

Highly concentrated electrolyte

SEI

Al dissolution

ion transport

Li metal battery

PJ-salen, Fysikgården 2B, Göteborg
Opponent: Serena Corr, The University of Sheffield, Storbritannien

Författare

Viktor Nilsson

Chalmers, Fysik, Kondenserade materiens fysik

Highly Concentrated LiTFSI-EC Electrolytes for Lithium Metal Batteries

ACS Applied Energy Materials,; Vol. 3(2020)p. 200-207

Artikel i vetenskaplig tidskrift

V. Nilsson, D. Bernin, D. Brandell, K. Edström and P. Johansson – Interactions and Transport in LiTFSI-based Highly Concentrated Electrolytes

V. Nilsson, P. Johansson, K. Edström and R. Younesi – Additives and Separators for LiTFSI–EC Electrolytes vs. Lithium metal, Graphite and NMC622 Electrodes

En övergång till eldrivna vägfordon, fartyg och flygplan är nödvändig inte bara för att minska våra CO2-utsläpp, men också för att förbättra luftkvalitén i närmiljön och för att inte uttömma jordens oljereserver. Litiumbatterier spelar en viktig roll i den pågående elektrifieringen av alla dessa fordon.

Dagens litiumjonbatterier och nästa generations batterier är kemiskt komplicerade system. Att de alls fungerar beror på att kemiska nedbrytningsprodukter inuti batteriet sätter sig på elektrodytorna och passiverar dessa.

För att öka batterisäkerheten utan att tappa livslängd och energitäthet har nya batterielektrolyter testats. Dessa är baserade på mindre skadliga kemikalier och använder en extremt hög saltkoncentration som säkerställer att elektrolyterna är mindre lättflyktiga och reaktiva. Elektrolyterna kan användas för att skapa säkrare litiumbatterier för olika tillämpningar.

Electric cars, ships, and airplanes are essential not only to reduce our CO2 emissions but to improve the local air quality and to avoid depletion of our oil reserves. Lithium batteries play a major role in the rapidly ongoing electrification of transport.

The lithium-ion batteries of today and the next generation of batteries are chemically complicated systems. The fact that they even work is thanks to fortunate chemical degradation, which forms passivating decomposition products on the electrode surfaces.

To increase the safety of the batteries while maintaining or improving the energy density and lifetime, a new kind of electrolytes were tested for application in lithium batteries. These electrolytes are based on less harmful chemicals and feature an extremely high salt concentration, which ensures that they are less volatile and reactive. The electrolytes can be used to design safer lithium batteries for various applications.

Ämneskategorier

Oorganisk kemi

Materialkemi

Annan kemiteknik

Drivkrafter

Hållbar utveckling

Styrkeområden

Transport

Energi

Materialvetenskap

ISBN

978-91-7905-260-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4727

Utgivare

Chalmers tekniska högskola

PJ-salen, Fysikgården 2B, Göteborg

Online

Opponent: Serena Corr, The University of Sheffield, Storbritannien

Mer information

Senast uppdaterat

2020-04-09