The over-stoichiometric Li 1+x CoO 2 (x = 0.1, 0.2, and 0.3) cathode materials were synthesized from an aqueous solution of lithium nitrate (LiNO 3 ) as an excess lithium precursor and their effects on the electrochemical properties of all-solid-state lithium batteries employing Li 2 S-P 2 S 5 glass-ceramics solid electrolytes are investigated. A combination of X-ray diffraction, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission spectroscopy reveals that the excess lithium forms a residual Li 2 CO 3 layer on the surface of as-prepared Li 1+x CoO 2 particles during the synthesis process. While regarded as an impurity phase in lithium battery systems using liquid electrolytes due to its detrimental effects on electrochemical performance, the Li 2 CO 3 formed on the surface of the over-stoichiometric Li 1+x CoO 2 powders is identified in all-solid-state lithium battery systems using sulfide solid electrolytes to act as an effective coating material to suppress the interfacial side reactions despite its low ionic and electronic conductivity. All-solid-state lithium ion batteries (ASS-LIBs) including inorganic solid electrolytes are consistently at the center of attention in the discussion on safety issues arising from the flammability of liquid electrolytes in conventional LIBs. ASS-LIB technology has come a long way in the last few decades through the development of oxide and sulfide solid electrolytes with practical levels of high ionic conductivities comparable to organic liquid electrolytes.1-4 Therefore, the rate determining step is now no longer Li + ion migration in the solid electrolyte and overall cell resistance is determined mainly at the interface between the cathode active material and solid electrolyte. However, ASS-LIBs employing oxide and sulfide solid electrolytes have some crucial interfacial problems resulting in low reversible capacity and capacity fade for practical applications, due to limited contact area and undesirable interfacial side reactions between the cathode active material and the oxide/sulfide solid electrolyte. 5,6 Many studies are in progress to understand and resolve the interfacial side reaction problem responsible for this capacity loss in order to develop ASS-LIBs with high energy density and long cycle life required for electric vehicles. Several researchers have suggested that the origin of the interfacial degradation is mutual diffusion of chemical species between solid electrolyte and cathode, 5,7 or the formation of a space-charge layer. 8,9 When garnet-structured Li 7 La 3 Zr 2 O 12 (LLZ) is contacted with LiCoO 2 , mutual diffusion of Co and La, Zr occurs at the interface during a thermal treatment to improve adhesion between the cathode material and the oxide solid electrolyte.10 Problems regarding this mutual diffusion of Co and S have been also reported in ASS-LIBs with Li 2 S-P 2 S 5 solid electrolytes, especially during initial charging process. At present, the fundamental mechanism is not clear, but interfacial control via sur...