Role of Electrolyte Oxidation and Difluorophosphoric Acid Generation in Crossover and Capacity Fade in Lithium Ion Batteries

Poor cycling performance for many high voltage lithium ion batteries (LIB) has been attributed to damage of the anode solid electrolyte interphase (SEI) resulting from crossover reactions. Transition-metal ion crossover has been proposed as a primary source of SEI damage and capacity loss, especially for high-voltage spinel cathodes. However, deposition of transition metals on the anode SEI may not be the primary source of SEI degradation. This investigation focuses on the oxidative decomposition of LiPF6 in ethylene carbonate (EC)-based carbonate electrolytes to generate acidic species which subsequently cross over to the anode and degrade the anode SEI components. The generation of the strong acid, difluorophosphoric acid (F2PO2H), has been quantified for both graphite || LiNi0.5Mn1.5O4 and graphite || LiMn2O4 cells. There is a correlation between the concentration F2PO2H, SEI degradation, and the capacity loss of the cells.