Application of layered nickel cobalt manganese oxide as cathode under higher potential than conventional 4.2V yields a significant improvement in energy density of lithium ion battery. However, the cathode fully charged under high potential suffers serious self-discharge, in which the interaction between the cathode and electrolyte proceeds without potential limitation. In this work, we use tris(trimethylsilyl)phosphate (TMSP) as an electrolyte additive to solve this problem. A representative layered nickel cobalt manganese oxide, LiNi1/3Co1/3Mn1/3O2, is considered. The effect of TMSP on self-discharge behavior of LiNi1/3Co1/3Mn1/3O2 is evaluated by physical and electrochemical methods. It is found that the self-discharge of charged LiNi1/3Co1/3Mn1/3O2 can be suppressed significantly by using TMSP. TMSP is oxidized preferentially in comparison with the standard electrolyte during initial charging process forming a protective cathode interface film, which avoids the interaction between cathode and electrolyte at any potential and thus prevents electrolyte decomposition and protects LiNi1/3Co1/3Mn1/3O2 from structure destruction.