Discharge capacities of the Li 1.01 Mn 1.99 O 4 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 H 2 O to 1.0M LiPF 6 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 LiPF 6 , on the contrary, no capacity fading was observed with H 2 O additive at 80°C storage, while the CF 3 SO 3 H addition resulted in 40% of capacity fading. These results show that plausible acid derived from the reaction of LiPF 6 with H 2 O 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 (CH 3 ) 3 SiNHSi(CH 3 ) 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.