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

Abstract Sulfide solid electrolytes such as Li 6 PS 5 Cl 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 LiNi x Co y Mn 1−x−y O 2 (x≥0.8). Using guidance from density functional theory, we devise an effective, conformal, and thin coating on the cathode active material, which suppresses the oxidative decomposition of Li 6 PS 5 Cl 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 Li 6 PS 5 Cl 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 LiPO 2 F 2 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 LiNbO 3 . 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.