Improved stability of highly concentrated LiBF4/fluorinated ethyl acetate-based electrolyte solutions with a co-solvent for LiNi0.8Co0.1Mn0.1O2 positive electrodes in lithium ion batteries

A nearly saturated 2,2,2-trifluoroethyl acetate (FEA)-based electrolyte solution has a low viscosity and delivers superior charge/discharge performance for LiNi0.8Co0.1Mn0.1O2 (NCM811) positive electrodes for lithium ion batteries. However, FEA has a low flash point, and hence needs to be mixed with an appropriate co-solvent to improve the safety of electrolyte solutions. In this study, two kinds of co-solvent were introduced into LiBF4/FEA electrolyte solutions and the LiBF4 concentration was increased. The mixture ratio of FEA and a co-solvent had a major effect on the charge/discharge performance of NCM811. The resultant nearly saturated electrolyte solution further improved the charge/discharge properties in terms of both cycle- and high rate performance. A very thin protective layer formed on NCM811 to suppress continual decomposition of the electrolyte solution, and did not interfere with interfacial Li+ transfer between the NCM811 electrode and electrolyte solution. In addition, 2,2-difluoroethyl acetate, which has a higher flash point than FEA, was used as a main solvent to study the highly concentrated electrolyte solutions.