Sodium phosphate additives and mixed electrolytes of Li(C2F5SO2)(2)N (LiBETI) and LiPF6 were examined to improve cycle performances of Li1.04Mn1.96O4/graphite cells at high temperatures. The cycle performance of those cells exceeded 80% at 100 cycles at 55 degrees C in an optimum combination of a 0.975 M LiBETI and 0.025 M LiPF6/ethylene carbonate (EC):dimethylcarbonate (DMC) (1:2) electrolyte solution containing 5% of Na4P2O7 or 5% of Na5P3O10 additive. Such a cycle performance is considered to fulfill the requirement for practical use. (c) 2007 The Electrochemical Society.