Interphase tailoring via fluorophenyl-boron integration for high-voltage LiCoO2 operation

Almost all commercial lithium-ion batteries (LIBs) with LiCoO 2 (LCO) as cathode material are cycled from 3.0 to 4.2 V and their actual specific capacity just ranges from 140 to 160 mA h g −1 , which is much lower than the theoretical specific capacity of LCO of 274 mA h g −1 . To further improve the actual specific capacity of LCO, elevating the upper limit of its working voltage is necessary. However, as the upper limit of its working voltage is elevated to 4.5 V or higher, the LCO crystal will undergo severe irreversible phase transition, and the oxidization decomposition of electrolyte on the cathode’s surface will exacerbate, which will severely reduce the cycling lifespan of batteries, hindering the actual application of high voltage LCO. In this work, we find that 3,5-difluorophenylboronic acid pinacol ester (35-DAPE) is an effective cathode electrolyte interphase (CEI)-forming additive, which can form a robust and stable CEI layer rich in fluorophenyl-groups and B–F/B–O bonds on the surface of LCO cathode, inhibiting the dissolution of cobalt ions and maintain the structural stability of LCO crystal over cycles. The graphite||LCO pouch cells in a voltage range of 3.0–4.5 V with 35-DAPE display a capacity retention rate of 91.1% after 150 cycles at room temperature, compared to that of 2.4% in baseline electrolyte. Besides, rate performance at room temperature and discharging performance at low temperatures of graphite||LCO pouch cells can also be observed with an improvement after the introduction of 35-DAPE. In addition, this work has explained the decomposition mechanism of 35-DAPE and how its products improve the electrochemical performance of graphite||LCO pouch cells in detail, which not only advances the actual application of fluorophenylboronic acid pinacol ester additive in high voltage LIBs but also provides valuable insights for the design of functional electrolyte additives.