Stabilizing LiCoO2 at 4.6 V by regulating anti-oxidative solvents†

Abstract

For LiCoO₂ (LCO) operating at high voltages (>4.5 V vs. Li/Li⁺), the intensive side reactions between LCO and traditional ethylene carbonate (EC)-based electrolytes with LiPF₆ salts can produce plenty of corrosive species (such as HF and HPO₂F₂), causing severe surface degradation. Herein, anti-oxidative fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) were selected as co-solvents to reduce the generation of corrosive species. Besides, PF₆⁻ anions enrich the Helmholtz plane of the LCO/electrolyte interface and promote the formation of a robust cathode/electrolyte interphase (CEI) featuring LiF/Li_xPO_yF_z/Li₃PO₄ inorganics and P-containing organics under the synergy of fluorinated solvents, which significantly inhibits the catalysis of highly oxidative Co⁴⁺/Oⁿ⁻ (0 < n < 2). Benefiting from the reduced corrosive species and reinforced CEI, the layered structure of the LCO surface is well preserved during long-term cycling, with a highly reversible O3/H1-3 phase transition. Consequently, a LCO||graphite pouch cell exhibits a remarkable capacity retention of 85.7% after 500 cycles in 3.0–4.55 V. This work provides a new insight into developing advanced functional electrolytes for high-voltage lithium-ion batteries.