Polyphosphates-Based Cathode-Electrolyte Interphase for 4.65 V LiCoO₂.

Abstract

The stable interfacial chemistry of LiCoO₂ (LCO) is the cornerstone for its high-voltage operation. However, interfacial side reactions, originating from the Co/O catalytic sites, lead to ethylene carbonate (EC) oxidation, lithium hexafluorophosphate (LiPF₆) hydrolysis, and LCO surface degradation. Herein, we propose that the formation of functional phosphate derivatives (FPD) in the electrolyte, which are generated from the reaction between tris(trimethylsilyl) phosphate (TMSP) and PF₆ ^- anions, can achieve targeted regulation of the polyphosphates cathode-electrolyte interphase (CEI). During cycling, the FPD spontaneously forms covalent interactions with surface Co/O sites, triggering in situ polymerization that constructs a robust and full-coverage CEI on LCO. The resulting CEI is enriched with polyphosphates and LiF/Li₂O, which ensures high thermodynamic stability and fast Li⁺ transport kinetics simultaneously. Consequently, the LCO with optimized electrolyte (1.0 M LiPF₆ in fluoroethylene carbonate (FEC)-based solvents with 2.0 wt.% TMSP) demonstrates exceptional cell performance with a high capacity retention of 81.0% after 3000 cycles within 3.0-4.6 V (vs. Li/Li⁺), 81.6% after 2000 cycles within 3.0-4.65 V, and shows the feasibility in the LCO||graphite pouch cell with a retention of 80.0% after 500 cycles. This work provides a new insight into the uniform CEI construction for high-voltage LCO cathodes through functional electrolyte engineering.