Trimethyl Borate as Film-Forming Electrolyte Additive To Improve High-Voltage Performances.

Enhancing the stability of the interface between the electrode and electrolyte at high voltages is crucial concerning the development of Li-ion batteries with high energy densities. Application of some additives in the electrolyte is not only the simplest but also the most effective way to form a protection layer against the electrolyte decomposition and the electrolyte corrosion to the electrode. Herein, we introduce trimethyl borate (TMB) as an additive of the commercial electrolyte to ameliorate the performance of a LiCoO2 cell charged to 4.5 V because its addition lowers the oxidation potential of the baseline electrolyte (3.75 V vs 4.25 V). By being oxidized preferentially and thus forming a compact protection layer of about 25 nm thick on the cathode surface, the additive suppresses the electrolyte decomposition and protects the LiCoO2 cathode against the structural degradation. The capacity retention of the cell after 100 cycles between 2.5 and 4.5 V at 0.1 C increases from 64 to 81% when 2.0 wt % TMB is added into the baseline electrolyte. The X-ray photoelectron spectroscopic results demonstrate the oxidation of TMB on the cathode and therefore the suppressed decomposition of the electrolyte. The results of the X-ray diffraction and Raman spectroscopy show the better structural maintenance of the LiCoO2 material in the TMB-containing electrolyte. The protection mechanism of the TMB additive was comprehensively studied.