Hydrated sodium manganese oxide (HSMO) was synthesized by reducing sodium permanganate with ethanol. The discharge and charge properties for mixtures of HSMO and acetylene black (AB) were investigated. AB was added twice, first while synthesizing HSMO and next while preparing the electrode. AB in synthesis resulted in the appearance of an additional redox reaction below 2V versus Li/Li+ and was effective in enlarging the discharge and charge capacity. On the other hand, AB added during electrode fabrication reduced resistance and improved cycle performance. In discharge–charge tests using 10Ag−1 HSMO, 45mAhg−1 composite of specific capacity could be maintained after 10 cycles, if AB was added at levels (in weight ratio to HSMO) of 0.25 during synthesis and 0.75 in electrode fabrication, or 0.5 and 0.5, respectively. This current and capacity corresponded to a power density of 10kWkg−1 and an energy density of 90Whkg−1, assuming an operating voltage of 2.0V.