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 LiNi_xCo_yMn_1-x-yO₂ (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 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.