Ab initio modelling of alkali-ion battery electrolyte properties

Abstract

Lithium-ion batteries are omnipresent in modern electronics. They can be found in laptops, mobile phones, and electric vehicles. However, there is room for both improvement, as the thermal instability of the dominant lithium salt used in batteries today, LiPF6 , causes safety concerns, and more fundamental changes, as there is a limited amount of lithium available – resulting in sodium-ion batteries being a nascent field of study.

This thesis looks in detail at some underlying fundamental features affecting properties of electrolytes of both lithium-ion and sodium-ion batteries. These properties include the oxidative stability of the anions of the lithium and sodium salts (important for voltage and safety); the cationanion interaction strength (important for conductivity); the solvation of the lithium and sodium cations in the common carbonate solvents (important for conductivity and the (de-)solvation at the anodes/cathodes); and the thermal stability of the anions and the possible decomposition reactions (important for safety).

The properties are mainly studied for a number of both novel and well established anions. Some of the novel anions involve completely new concepts for anion design for alkali-ion battery electrolytes. The systems are studied with a number of ab initio methods, most based on density functional theory (DFT). These include high level calculations of benchmark quality. The applicability of DFT and the selection of DFT functionals is also studied. Novel calculation strategies were employed to understand thermal decomposition.