The use of orthorhombic NaxMnO2 as a cathode material for alkali metal polymer electrolyte batteries is described for the first time. This sodium manganese bronze has a tunnel structure and can reversibly intercalate up to 0.55 to 0.6 alkali metal ions (Li+ or Na+) per manganese at moderate current densities, corresponding to capacities of 160 to 180 mAh/g. Li/PEO/Na0.2LixMnO2 cells [PEO = poly(ethylene oxide)] to date have been cycled over ninety times at 0.1 mA/cm2, with excellent capacity retention. Na/PEO/NaxMnO2 and NaxMnO2/PEO/NaxMnO2 cells have been cycled over sixty times to date at the same rate, showing moderate capacity fading.