Synergetic Effect of Electrolyte Coadditives for a High-Voltage LiCoO2Cathode

Currently widely used carbonate-based electrolytes face difficulty in ensuring that the lithium-ion batteries work beyond 4.2 V for the purpose of energy density improvement. Herein, we report a novel electrolyte that emphasizes the synergistic effect of fluoroethylene carbonate (FEC) and 2-(trifluoromethyl) phenyl boric acid (2-TP) as coadditives, enabling the LiCoO 2 cathode to operate stably under high voltages. With the addition of 1% 2-TP and 10% FEC into a carbonate-based electrolyte, LiCoO 2 shows a significantly improved cyclic stability. Spectral characterizations, electrochemical measurements, and theoretical calculations demonstrate that the improved cyclic stability can be attributed to the cathode–electrolyte interphase (CEI) derived from FEC and 2-TP. These two additives are preferentially oxidized on the LiCoO 2 electrode into their oxidation decomposition products to construct a robust and low-impedance CEI with inorganic LiF uniformly dispersed in the organic B-containing matrix. This unique CEI construction provides a facile solution to the challenges in developing high-energy-density lithium-ion batteries based on high-voltage cathodes, not limited to LiCoO 2 .