Thio-LISICON Li₁₀GeP₂S₁₂ equivalent Li₁₀SnP₂S₁₂ (LSPS) is comparable in ionic conductivity yet with a lower cost as an electrolyte for all solid state batteries (ASSBs). ASSBs with Li₁₀SnP₂S₁₂ electrolyte, lithium-indium alloy anode, and LiCoO₂ cathode were fabricated and their electrochemical performance at 60 °C was examined. Atomic layer deposition of Li₃NbO₄ on LiCoO₂ was conducted to improve the interfacial stability. The Li₃NbO₄ coating significantly improves the cycle stability of the ASSB, which retains about 90% initial capacity after 60 cycles at a current density of 0.13 mA/cm², while the ASSB with uncoated LiCoO₂ loses ~ 35% initial capacity after 50 cycles. Electrochemical impedance spectroscopy test indicates a rapid growth of charge transfer resistance upon cycling for the cell with the uncoated LiCoO₂, primarily due to the surface instability and build-up of a space charge layer between LSPS and LiCoO₂. However, the ASSBs with Li₃NbO₄ coated LiCoO₂ show a more stable interface with a negligible impedance increase upon cycling, attributable to the ‘buffering’ and ‘passivating’ roles of the Li₃NbO₄ coating. The interfacial microstructure was analyzed to elucidate the underlying reasons for the impedance increase and the pivotal role of Li₃NbO₄ coating. Our study indicates that surface coating significantly improves the cycle stability of the ASSBs with LSPS as the electrolyte.