Highly Concentrated Electrolytes for Rechargeable Lithium Batteries

Doctoral thesis, 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.