New Chemical Insights into the Beneficial Role of Al₂O₃ Cathode Coatings in Lithium-ion Cells

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 LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) and LiNi_0.8Co_0.15Al_0.05O₂ (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.