We selected a maleimide-based branched oligomer (BO) as a high-temperature and long-cycle-life additive and studied the influence of different amounts of the BO additive on the electrochemical performance of lithium-ion batteries. The electrochemical performances of the additive-containing electrolytes in combination with a cell composed of a LiCoO2 cathode and a mesocarbon microbead (MCMB) anode were tested in coin cells. The cyclic voltammetry (CV) results showed that the oxidation potential of the BO is about 1.9V versus Li/Li+ and a thin film grown on the surface of the cathode. In the present work, 1.0wt.% BO in 1.0M LiPF6/EC:DEC (4:6 by vol.%) electrolyte was the optimal condition for the improvement of the cycle ability of battery, especially, at high temperature (55°C) condition.