Enhancing the Electrochemical Performance of a High-Voltage LiCoO₂ Cathode with a Bifunctional Electrolyte Additive

Increasing the cutoff charge voltage is a promising strategy to enhance the actual discharge capacity of a LiCoO2 cathode. However, the oxidative decomposition of a carbonate electrolyte and the structural destruction of the cathode are intensified under a high operating voltage of 4.5 V, resulting in the poor cycling stability of a LiCoO2-based battery. Herein, a bifunctional electrolyte additive, tris(2-cyanoethyl) borate (TCEB), is proposed to improve the cyclability of LiCoO2∥Li cells at elevated voltages. Due to the synergistic effect of boron–oxygen bonds (−B–O−) and nitrile groups (−C≡N), the TCEB additive can be preferentially decomposed on the surface of the LiCoO2 cathode, which generates a uniform and stable cathode electrolyte interphase (CEI) film to stabilize the cathode/electrolyte interface. As a result, a LiCoO2∥Li battery using a TCEB-containing electrolyte shows excellent cycling performance with a capacity retention of 78.2% after 200 cycles at 1C and 4.5 V. In contrast, a battery cycled in a base electrolyte experiences a rapid capacity decay with a relatively low capacity retention of 40.4% at the same conditions. This work sheds light on an effective strategy for attaining stable cycling of the LiCoO2 cathode at high voltages via interfacial modification using an electrolyte additive, achieving high-energy-density lithium-ion batteries.