Design of a novel electrolyte additive for high voltage LiCoO2 cathode lithium-ion batteries: Lithium 4-benzonitrile trimethyl borate

Abstract A novel electrolyte additive designed as a cathode interface formation enhancer for high voltage lithium-ion battery (LIB) systems is investigated. Lithium 4-benzonitrile trimethyl borate (LBTB) is synthesized by a two-step reaction, and the results show that the cycling performance of Li/LiCoO2 cell at 4.4 V is significantly improved from 15.08% to 73.21% by adding 2.0 wt% of LBTB to the electrolyte (1.0 M LiPF6 in EC/DEC/EMC = 3/2/5, wt.%). A combination of electrochemical methods, ex-situ spectroscopy characterization techniques and computational calculations are employed to investigate the mechanisms of action for this additive. Cyclic voltammograms indicate the additive is preferably oxidized on the LiCoO2 surface prior to the electrolyte oxidation and results in a more stable and robust passivation layer than the electrolyte alone. Computational calculations support the observed sacrificial oxidation of LBTB prior to the electrolyte solvents decomposition. The CEI formed by LBTB decomposition mitigates the dissolution of transition metal and subsequently improves the cycling stability of the LiCoO2 cathodes at high voltages. Surface characterization techniques reveal the presence of -B-O- products derived from LBTB decomposition on the surface of the LCO cathode.