Improving cycling performance of LiCoPO4 cathode material by adding tris(trimethylsilyl) borate as electrolyte additive

Tris(trimethylsilyl) borate (TMSB) is used as electrolyte additive to overcome the severe capacity fading of LiCoPO4 cathode at high voltage. The presence of TMSB effectively improves the cyclic stability of LiCoPO4 cathode in Ethylene carbonate (EC)-based electrolyte between 3 V and 5 V, when the electrolyte contains with 1 wt% TMSB shows the best cycle performance. For instance, LiCoPO4 cathode delivers an initial discharge capacity of 144 mA h g− 1 at 0.1C and with a capacity retention of 76% after 50 cycles, while it's only 132 mA h g− 1 and 45% for that in normal electrolyte without TMSB. The linear sweep voltammetry (LSV) and cyclic voltammetry (CV) results indicate that TMSB oxidizes preferentially to normal electrolyte and forms a stable SEI film on LiCoPO4 surface at the first cycle which effectively reduces the electrode polarization. The electrochemical impedance spectroscopy (EIS) results reveal that the stable SEI film alleviates the growth of SEI layer on the LiCoPO4 surface and relieves the resistance increase to facilitate Li-ion transfer the interface between electrode and electrolyte. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that a 20 nm film can be observed on the cathode surface after 50 cycles as the electrolyte containing with 1 wt% TMSB. Moreover, X-ray photoelectron spectrometer (XPS) patterns confirm that the oxidation products of TMSB participate in the SEI film formation. The enhancement of electrochemical performances is attributed to the thin and steady protective SEI film that originates from TMSB, which stabilizes the interface of the cathode and electrolyte, suppresses the continuous decomposition of normal electrolyte at 5 V high voltage and relieves the resistance increase to facilitate Li-ion transfer at the interface during the cycling process.