New Chemical Insights into the Beneficial Role of Al 2 O 3 Cathode Coatings in Lithium-ion Cells

Inorganic surface coatings such as Al 2 O 3 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 LiPF 6 electrolyte salt with Al 2 O 3 -based surface coatings. Using 19 F and 31 P solution nuclear magnetic resonance spectroscopy, it is demonstrated that the storage of LiPF 6 -containing electrolyte solution with Al 2 O 3 produces LiPO 2 F 2, a well-known electrolyte additive. The production of LiPO 2 F 2 is also observed for electrolyte solutions that were stored for 14 days at 40 °C with Al 2 O 3 -coated LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) and LiNi 0.8 Co 0.15 Al 0.05 O 2 (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 Al 2 O 3 -coated cathode materials.