Influence of Battery Cell Components and Water on the Thermal and Chemical Stability of LiPF6 Based Lithium Ion Battery Electrolytes

Lithium ion battery electrolytes based on LiPF6 and organic solvents are known to degrade at elevated temperatures. The degradation reactions can be caused either chemically e.g. by simple contact with battery cell components and/or electrochemically during cycling. This study is focused on thermally induced chemical reactions of the electrolyte with different battery cell components. These reactions are monitored by means of quantitative NMR spectroscopy. The results allow for categorizing the influences of the components according to their reactivity against HF. Inert materials (graphite, carbon black, polyvinylidene difluoride, polyolefinic and ceramic separator) do not show any observable influence on the thermal stability of the electrolyte. If the materials react with HF but the reaction does not form water in significant amounts (Li metal and LiNi1/3Co1/3Mn1/3O2), there is also no influence observable. In contrast to that, materials, which can form water in contact with HF at significant rates (glass fiber separator, Si and LiFePO4), can lead to a slightly increased or even severe electrolyte degradation. However, if the material neutralizes the acid HF (carboxymethyl cellulose), it stabilizes LiPF6 against water sources. Furthermore, the results of this study show that LiPF6 is stable at temperatures up to 80°C, if no water sources are present. This stability is most likely also given for even higher temperatures.