In-situ constructed protective bilayer enabling stable cycling of LiCoO₂ cathode at high-voltage

The practical application of high-voltage lithium cobalt oxide (LCO) has been hampered by the severe degradation of its structural integrity. In this work, a protective bilayer was fabricated on LCO surfaces by means of large-scale and facile electrolyte engineering. The protective bilayer consisting of a LiF-rich cathode-electrolyte interphase (CEI) as the outermost layer and a layer of disordered spinel structure as the inner layer was uniformly fabricated in-situ. The high-resistance CEI layer inhibited the fast transfer of Li ions from LCO to bulk electrolyte during the first few cycles, resulting in the significantly increased local overpotential on the LCO surface. As a consequence, the LCO surface underwent a phase transformation from the layered phase to the spinel phase first, forming the spinel phase inner layer due to the voltage rising beyond 4.55 V (vs. Li/Li+). The CEI and spinel layers effectively blocked the dissolution of transition-metal (TM) ions into the electrolyte during cycling and inhibited the formation of the structurally defective rock-salt phase that would hasten cycling-induced structural degradation. The formation of the protective bilayer effectively prevented the phase transition from the bulk layered LCO structure into spinel and then rock salt, thereby reducing decay of its cycling capacity. Remarkably, the graphite||LCO pouch cell with optimized electrolyte retained 78.9% of its capacity even after 1000 cycles under the operation voltage window of 3.0-4.55 V (vs. Li/Li+). This study provides guidance for the development of effective surface treatment strategies for stable layered cathodes with high capacity and cyclability.