The over-stoichiometric Li1+xCoO2 (x = 0.1, 0.2, and 0.3) cathode materials were synthesized from an aqueous solution of lithium nitrate (LiNO3) as an excess lithium precursor and their effects on the electrochemical properties of all-solid-state lithium batteries. employing Li2S-P2S5 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 Li2CO3 layer on the surface of as-prepared Li1+xCoO2 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 Li2CO3 formed on the surface of the over-stoichiometric Li1+xCoO2 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. (C) The Author(s) 2015. Published by ECS. All rights reserved.