Dual Role of Bis(borate) Additive in Electrode/Electrolyte Interface Layer Construction for High-Voltage NCM 523 Cathode

Increasing the working voltage of a layered oxide cathode is an efficient way to lift the energy density of lithium-ion batteries. However, uncontrollable side reactions and excessive electrolyte decomposition take place at high voltage. Bulk cathode structure degradation and formation of a high working voltage induce excessive growth of the solid-electrolyte interface (SEI) layer and overconsumption of the electrolyte. Construction of a high-voltage stable SEI layer is crucial to enhance the electrochemical performance. In this work, trimethylene borate (TMEB) as a borate ester additive is used to improve the cycled stability of NCM523 cells at 4.5 V charging-cutoff voltage. On one hand, TMEB can be preferentially decomposed during cycling and participate in the film formation process on the surfaces of both anode and cathode. On the other hand, TMEB can tune the solvation sheath by expelling the carbonate solvent molecules out of the solvation sheath to suppress decomposition of the electrolyte. The cells cycled with the 0.5 wt % TMEB-containing electrolyte exhibit improved cycling stability after 200 cycles with the capacity retention of 90.9% indicating TMEB to be an effective additive to improve the lithium-ion batteries at higher charging-cutoff voltage.