Off-Stoichiometry Induced Few-Nanometer Surface Layer for High-Performance Layered Cathode in Nonaqueous and Aqueous Electrolytes

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.