Layered manganese oxide (K x MnO2) powders were synthesized by simple decomposition of KMnO4 at 300–800°C in air. These manganese oxides were characterized by powder X-ray diffractometry, atomic absorption spectrometry, chemical redox titration, and X-ray photoelectron spectroscopy. Their electrochemical performances for secondary lithium batteries were investigated in a LiClO4–propylene carbonate (PC) solution. After removing side products, which were water soluble, the K x MnO2 (x≤0.484) cathode unerwent reversible electrochemical intercalation/deintercalation of lithium. When the K x MnO2 powders were synthesized at 300°C and then washed with water or acidic solution after the synthesis, they exhibited satisfactory behavior with a specific capacity of 130–140 mAh g−1 in the region of 3 V vs. Li/Li+ for more than 20 cycles.