Failure Mechanisms of Lithium-ion Battery Electrolytes: Detection and Mitigation

Doctoral thesis, 2014

The state-of-the-art electrolyte of the lithium-ion batteries (LIBs) present in most portable electronics contains the salt LiPF6 , organic solvents such as ethylene carbonate and dimethyl carbonate and a number of additives. This is true regardless of the exact active materials i.e. electrodes or cell design e.g. prismatic or pouch chosen. Despite LIB commercial success, these electrolytes suffer from poor chemical and thermal stability. As an example, elevated temperatures cause a decrease in battery performance attributed to electrolyte decomposition.The aim of this thesis was both detailed studies of the degradation of electrolytes, model systems as well as commercial, and discussions and verifications of suggested mitigation strategies - all based on a proper understanding at a molecular level. Various routes to electrolyte decomposition have been explored and elevated temperatures, impurities, and the electrochemical stability toward redox reactions at the electrolyte/electrode interfaces were suggested as main parameters affecting the electrolyte functionality. Using several different detection techniques, the electrolytes have been characterized with respect to their physical and safety related properties and analyzed by vibrational spectroscopy to track changes at the molecular level. Novel NMR and combined ab initio/TGA-FTIR strategies were employed to understand thermal decomposition. Mitigation strategies included the application of additives, in particular flame retardants, or a change in chemistry toward intrinsically more stable components, e.g. ionic liquids. Each additional component needed careful optimization in the electrolyte and trade-offs with performance decline were evaluated. The main conclusion was that if LIBs continue to use LiPF6 in organic solvents - material purity and thermal stability must be enhanced.