Discharge capacities of the Li1.01Mn1.99O4 cathode and Mn content dissolved from the cathode were examined on the addition of some chemicals, as the storage temperatures and time were changed. On the addition of 1000ppm of H2O to 1.0M LiPF6 solution in EC/DMC (1:2 in volume), 41% of capacity of the cathode was lost after 24h storage at 80°C, in contrast to 5% loss in the case of no additive. Water would be responsible for causing the capacity fading of the cathode under such condition as elevated temperatures. In the EC/DMC solution without LiPF6, on the contrary, no capacity fading was observed with H2O additive at 80°C storage, while the CF3SO3H addition resulted in 40% of capacity fading. These results show that plausible acid derived from the reaction of LiPF6 with H2O causes the capacity fading on the lithium manganese oxide at 80°C, relatively high storage temperature. On the other hand, addition of 500–1000ppm of (CH3)3SiNHSi(CH3)3 resulted in less capacity fading of the cathode and made Mn dissolution decrease drastically. Therefore, dehydration and acid-neutralization in the electrolyte solution would restrain the capacity fading of the lithium manganese oxide cathode during high temperature storage, and we confirmed it in some experiments.