Inorganic surface coatings such as Al2O3 are commonly applied on positive electrode materials to improve the cycling stability and lifetime of lithium-ion cells. The beneficial effects are typically attributed to the chemical scavenging of corrosive HF and the physical blockage of electrolyte components from reaching the electrode surface. The present work combines published thermochemistry data with new density functional theory calculations to propose a new mechanism of action: the spontaneous reaction of the LiPF6 electrolyte salt with Al2O3-based surface coatings. Using 19F and 31P solution nuclear magnetic resonance spectroscopy, it is demonstrated that the storage of LiPF6-containing electrolyte solution with Al2O3 produces LiPO2F2, a well-known electrolyte additive. The production of LiPO2F2 is also observed for electrolyte solutions that were stored for 14 days at 40 °C with Al2O3-coated LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Co0.15Al0.05O2 (NCA) materials. Given the beneficial nature of this species for the lifetime and stability of lithium-ion cells, this reaction is here proposed to similarly benefit the performance of cells that use Al2O3-coated cathode materials.
Hall et al. (Wed,) studied this question.