Todorokite type MgMnO2. yH(2)O which possesses 3 x 3 large tunnel structure was prepared through hydrothermal treatment of layer manganese dioxide, buserite, at 120-200 degreesC. In non-aqueous Li and Mg cells, electrochemical insertion of Lif and Mg2+ into the todorokite products occurred utilizing the preferable large tunnel for their diffusion. By heat-treatment up to 300 degreesC, cycle performances in Li cell were improved by removing water molecules from the todorokite tunnel.