(Digital Presentation) Simultaneous Transforming Residual Lithium Compounds and Stabilizing Surface Lattice Oxygen for Ni-Rich Cathode Via Atomic Layer Deposition and Post-Annealing

Abstract

Unstable surface species such as residual lithium compounds and surface lattice oxygen at charge state are detrimental factors for the battery performance of Ni-rich cathode (LiNixCoyMn1−x−yO2, x ≥ 0.6, NCM), which induce serious side reactions and lead to rapid capacity loss during cycling. With the advantages of sufficient contacts between gas and solid, gas reactants are introduced to reducing residual lithium compounds via gas-solid reactions or building protective coatings to isolate surface lattice oxygen from the electrolyte via gas-phase deposition, which effectively enhances the cycling stability of NCM. Atomic layer deposition (ALD), as a special coating technology, possesses both the nature of the gas-solid reaction and gas-phase deposition. It is significant to investigate the mechanism of ALD in enhancing the cycling stability of NCM.Here, we report a method to improving the rate capacity and cycling stability of single-crystal NCM via Al2O3 and B2O3 ALD by constructing a highly Li-ions conductive and electrochemical stable surface. The ultrathin coatings are uniformly coated on the surface of NCM. Moreover, surface composition characterizations show the reductions of residual lithium compounds and formations of lithium-containing coatings after the ALD process. By adding a post-annealing process, Al3+ cations and BO3- polyanions doping form on the surface of NCM, which robustly enhanced the stability of surface lattice oxygen of NCM via strong Al-O bond and B-O bond. Moreover, as acidic oxides, B2O3 shows better performance than Al2O3 in reducing residual lithium compounds during the ALD process. Different from Al3+ cations doping occupying the transition metal ions position, BO3- polyanions doping occupies the oxygen ions position, which does not deteriorate the capacity of NCM. Transformation of residual lithium compounds and stabilizing surface lattice oxygen on NCM greatly decrease the formation of LiF and degradation of surface structure, which is the key to the improvement of rate capacity and cycling stability.