In this paper, tris(trimethylsilyl)borate (TMSB) is evaluated as an electrolyte additive for the self-discharge suppression of 4.9 V LiNi0.5Mn1.5O4 cathode for lithium ion battery. The effect of TMSB on the surface properties of LiNi0.5Mn1.5O4 is investigated via linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry (CA), charge–discharge test, electrochemical impedance spectra (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometer (ICP-AES) and Fourier transform infrared spectroscopy (FTIR). It is found that the LiNi0.5Mn1.5O4 cathode charged to 4.9 V (vs. Li/Li+) suffers a serious self-discharge in 1 mol L−1 LiPF6–EC/DMC (1:2, in weight), which can be suppressed effectively by adding 1 wt.% TMSB into the electrolyte. After storage for 20 days, the voltage of the charged cathode decreases from 4.7 to 0.5 V (vs. Li/Li+) in the additive-free electrolyte, while that remains almost unchanged in the TMSB-containing electrolyte. The self-discharge suppression of the charged LiNi0.5Mn1.5O4 cathode results from the preferential oxidation of TMSB and the subsequent formation of a protective solid electrolyte interphase film, which prevents electrolyte decomposition and protects LiNi0.5Mn1.5O4 from destruction.