Despite the long history in commercial lithium-ion batteries, layered lithium cobalt oxide (LiCoO2) suffers from structural degradations that shorten the cycle life when operating at high voltages (i.e., 4.5 V vs Li/Li+) in nonaqueous electrolytes or even at moderate voltages in aqueous electrolytes. This limited performance originates from the O3-to-O1 phase transition involving cobalt dissolution or the leaching of Li2O. Here, we report a one-pot synthesis that yields LiCoO2 bearing a spinel-Co3O4 surface structure with a thickness of 2 nm via lithium-deficient stoichiometry (Li:Co = 0.98:1). The lithium-deficiency induces the spinel structure, a thermodynamically preferred phase at the given stoichiometry, by temperature-specific phase separation. The spinel surface layer mitigates cobalt dissolution and oxygen gas evolution or avoids direct contact with the electrolyte causing the leaching of Li2O, improving the cyclability in nonaqueous and aqueous electrolytes by 35% and 20% after 100 cycles compared to that of LiCoO2 when the upper cutoff voltages were imposed at 4.5 V (vs Li/Li+) and 0.8 V (vs Ag/AgCl), respectively.