Safe use of the emerging Li1+xMn2O4/C Li-ion batteries requires an electrolyte that is resistant to oxidation up to 5 V both at room temperature and at 55-degrees-C. Permutations of different solvents and lithium-based salt mixtures were tested for their resistance to oxidation at high voltages on Li1+xMn2O4 electrodes and for their ability to maintain high ionic conductivity over a wide temperature range. This survey allowed us to select new electrolyte compositions that are highly stable against oxidation which consist of mixtures of ethylene carbonate (EC), dimethyl carbonate (DMC) and lithium hexafluorophosphate (LiPF6). This new electrolyte composition can be used in practical Li11+xMn2O4/carbon Li-ion batteries, and shows enhanced safety characteristics and performance in terms of cycle life and self-discharge. The electrochemical stability of this new electrolyte composition against oxidation in contact with other highly oxidizing materials, such as LiCoO2 or LiNiO2 has also been verified.