Abstract
Surface modification of a cathode (e.g., lithium layered oxide, NCM) has become ever more important in lithium-ion batteries, particularly for pursuing higher energy densities and safety at high voltage. This is because structural degradation of the cathode can be mitigated significantly. Herein, an organic complex is introduced for metal phosphate (e.g., AlPO4) modification through a new film-forming process in nonaqueous solution. This general strategy overcomes the challenge of nonuniform coating in current precipitation methods and then opens a new avenue toward ultrathin surface modification on a molecular scale. As one example, as-prepared AlPO4-coated NCM exhibits much improved structural and electrochemical stability; meanwhile, thermal runaway can be suppressed significantly in overcharged cells using the modified NCM, demonstrating higher and reliable safety features. The great improvements benefit from the uniform and ultrathin AIPO(4) coating, which inhibits the collapse and conversion of the layered structure to spinel, especially to the rock salt structure at high-voltage conditions, as confirmed by HRTEM and EELS.