We have been developing lithium-ion batteries for electric power storage and have chosen cell chemistries having a high energy density and long life. The cell chemistry consisted of a positive electrode containing a lithium–manganese spinel or a mixture of it with a layered-manganese-based material, and a negative electrode containing a hard carbon. The 8Ah-class cells consisting of these cell chemistries showed that their extrapolated lives were long enough to withstand a cycling load for 10 years of use. A comparison of the cycle life data with the storage life data suggested the possibility to separate the capacity fading caused only by the storage and that only by the cycling, which is expected to be the basis of a prediction method for the calendar life. However, much work still needs to be done to achieve it. We also manufactured two types of 100Ah-class cells as an experiment based on the results for the 8Ah-class cells. They showed specific energies of 100Whkg−1 and 106Whkg−1.