A gradient oxy-thiophosphate-coated Ni-rich layered oxide cathode for stable all-solid-state Li-ion batteries

High-energy Ni-rich layered oxide cathode materials such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) suffer from detrimental side reactions and interfacial structural instability when coupled with sulfide solid-state electrolytes in all-solid-state lithium-based batteries. To circumvent this issue, here we propose a gradient coating of the NMC811 particles with lithium oxy-thiophosphate (Li₃P_1+xO₄S_4x). Via atomic layer deposition of Li₃PO₄ and subsequent in situ formation of a gradient Li₃P_1+xO₄S_4x coating, a precise and conformal covering for NMC811 particles is obtained. The tailored surface structure and chemistry of NMC811 hinder the structural degradation associated with the layered-to-spinel transformation in the grain boundaries and effectively stabilize the cathode|solid electrolyte interface during cycling. Indeed, when tested in combination with an indium metal negative electrode and a Li₁₀GeP₂S₁₂ solid electrolyte, the gradient oxy-thiophosphate-coated NCM811-based positive electrode enables the delivery of a specific discharge capacity of 128 mAh/g after almost 250 cycles at 0.178 mA/cm² and 25 °C.