Synergistic Effects of Suberonitrile-LiBOB Binary Additives on the Electrochemical Performance of High-Voltage LiCoO₂Electrodes

Suberonitrile (SUN) and lithium bis(oxalate)borate (LiBOB) were investigated as binary additives for Li-ion batteries that used LiCoO 2 as a cathode, cycled at high cutoff potentials and utilized LiPF 6 -based electrolyte. Linear sweep voltammetry (LSV) results revealed that LiBOB oxidized prior to the decomposition of the reference electrolyte. By contrast, SUN has a higher oxidization potential, and in the electrolyte it provided better electrolyte stability at higher potentials. In the electrolyte with binary additives, the electrochemical performance of LiCoO 2 was enhanced significantly, and the initial coulombic efficiency increased to 94% compared with 90% in reference electrolyte. The cell also exhibited capacity retention of 62% after 500 cycles, a strong contrast with the 25% measured in reference electrolyte. Preliminary studies using electrochemical impedance spectra (EIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the combination of the two additives had a unique influence on the structure and composition of the cathode-electrolyte interface (CEI) on LiCoO 2 . Ex situ X-ray diffraction (XRD) confirmed that structural change of the LiCoO 2 material was not the main reason causing poor cycling performance when charging to the 4.5 V cutoff potential.