Engineering Stable Decomposition Products on Cathode Surfaces to Enable High Voltage All‐Solid‐State Batteries

Sulfide solid electrolytes such as Li6PS5Cl hold high promise for solid state batteries due to their high ionic conductivity; however, their oxidation potential of ~2.5 V is not compatible with high voltage Ni‐rich cathodes such as LiNixCoyMn1−x−yO2 (x≥0.8). Here, we devise an effective, conformal and thin coating on the cathode active material guided by density functional theory, which suppresses the oxidative decomposition of Li6PS5Cl as shown by experiment. The nanometric coating on nickel‐rich NMC85 enabled capacity retention of 82% after 200 cycles (2.8‐4.3 V vs Li+/Li) using Li6PS5Cl as the solid electrolyte. In comparison, cells with an uncoated CAM only displayed 56% capacity retention. The coated‐NCM85 cells also demonstrate much better rate performance and higher capacity. The enhanced performance is due to the formation of a stable amorphous cathode‐electrolyte interphase accruing from the decomposition products of the LiPO2F2 precursor (as predicted by DFT), which protect the sulfide electrolyte from oxidation. The coating fabricated in this cost‐effective process showed superior performance to state‐of‐the‐art coatings such as LiNbO3. This work highlights the importance of rationally designing stable coating materials based on their potential decomposition products and confirms the suitability of a low‐cost and conformal coating to enable sulfide electrolyte‐based all‐solid‐state batteries.