Coupled LiPF6 Decomposition and Carbonate Dehydrogenation Enhanced by Highly Covalent Metal Oxides in High-Energy Li-Ion Batteries

The (electro)chemical reactions between positive electrodes and electrolytes are not well understood. We examined the oxidation of a LiPF6-based electrolyte with ethylene carbonate (EC) with layered lithium nickel, manganese, and cobalt oxides (NMC). Density functional theory calculations showed that the driving force for EC dehydrogenation on oxides, yielding surface protic species, increased with greater Ni content in NMC. Ex situ infrared and Raman spectroscopy revealed experimental evidence for EC dehydrogenation on charged NMC surfaces. Protic species on charged NMC surfaces from EC dehydrogenation could further react with LiPF6 to generate less-coordinated F species such as PF3O7-like and lithium nickel oxyfluoride species on charged NMC particles and HF and PF2O2- in the electrolyte. Larger degree of salt decomposition was coupled with increasing EC dehydrogenation on charged NMC with increasing Ni or lithium deintercalation. An oxide-mediated chemical oxidation of electrolytes was proposed, providing new insights in stabilizing high-energy positive electrodes and improving Li-ion battery cycle life.