Abstract All-solid-state batteries with inorganic solid electrolytes are ideal to overcome the safety issues related to the flammable organic electrolyte in lithium ion batteries. Sulfide materials are promising inorganic electrolytes due to their high ionic conductivity and good elasticity. Nevertheless, their application is limited by their high reactivity and instability at the cathode/electrolyte (Li[Ni0.8Co0.15Al0.05]O2/75Li2S–22P2S5–3Li2SO4) interface. In this study, LiInO2 and LiInO2–LiI were introduced as new cathode coating materials to suppress such undesirable reactions. The LiInO2–LiI composite coating layer reduced the undesirable interfacial reactions and prevented the diffusion of S and P ions from the sulfide electrolyte to the oxide cathode. Moreover, the electrochemical properties of all-solid-state cells were improved by the cathode coating. The LiInO2–LiI-coated electrode presented better rate capability and lower impedance than the pristine and LiInO2-coated electrodes. Hence, the LiInO2–LiI composite coating was successful at improving the cathode stability while providing superior electrochemical properties.