Severe interfacial degradation occurs in sulfide-based all-solid-state batteries (ASSBs) at high voltages, due to the parasitic reaction between the solid electrolyte (SE) and Ni-rich layered oxide. Herein, we show that a conformal nanoscale coating of Li₆PS₅Cl (LPSCl) on LiNi_0.8Co_0.1Mn_0.1O₂ (NCM) promotes the homogeneous parasitic reaction and improves the high-voltage stability, enabling durable cell operation, even after extended storage at 4.25 V <i>vs.</i> Li/Li⁺.