(In-)Stability of LiPF6-based Electrolytes for Lithium-ion Batteries

Licentiatavhandling, 2012

The state of the art lithium-ion batteries present in most portable electronics consist of the salt LiPF6, organic solvents such as ethylene carbonate and dimethyl carbonate (commonly seen as EC/DMC), and finally, a number of additives - all components enclosed by different kinds of electrode chemistries. Despite their commercial success, all LiPF6 based electrolytes suffer from poor chemical and thermal stability. Elevated temperatures cause a decrease in battery performance, attributed to electrolyte decomposition and increased film formation on both electrodes. The work of this thesis ranges from degradation studies on a commercial electrolyte to discussing subsequent mitigation strategies. The aim is to identify the decomposition mechanism, and find suitable countermeasures in order to guarantee stable electrolyte and battery performance at elevated temperatures. The stability of a base electrolyte is studied using TGA-FTIR spectroscopy, where the in the TGA heated electrolyte decomposes and the evolved gases are directly transferred to an IR gas cell for characterization. The gaseous species are properly identifi#28;ed with the help of ab initio calculations and this way a simple picture of the decomposition pathway can be drawn. The same base electrolyte - now stored at 85 C for up to 160 h - is further studied with both NMR and Raman spectroscopy. The results of the techniques combined confirm the formation of highly reactive species after only 30 minutes and that the decomposition mechanism involves the polymerization of the organic solvents. Furthermore, the thermal (in-)stability is found to be unaffected by the addition of the film forming additive vinylene carbonate (VC)and the gas reducing agent propargyl methane sulfonate (PMS).