Lithium insertion in interstitial space was investigated for electrolytic manganese dioxide (EMD), gamma-MnO2, as a new cathode reaction in lithium battery. Its structure and proton dehydration mechanism were characterized by X-ray and neutron diffraction-pattern simulation. The heat-treatment process activated the EMD as insertion electrodes and caused dehydration with structural changes, which were described by modifications in stacking sequence of the ramsdellite (+R) to pyrolusite (+P), and their twinning types (-R and -P). By the heat-treatment of EMD, the -R ratio decreased and the -P increased with increasing temperature, and finally the structure transformed to +P above 400 degrees C. The void size was determined by small-angle neutron scattering and decreased with the treatment temperature. The total neutron diffraction indicated two types of protons with H-O length of 1.0 and 1.9 angstrom, which corresponds to the "Ruetschi" and "Coleman" protons, respectively. The former proton situated in the Mn4+ vacant site and the latter in the center of the tunnel. Lithium insertion properties depended on both the structures and the amount of proton. Dehydration of the Ruetschi proton without destroying interstitial space improved the lithium insertion capacities as reversible cathodes. (c) 2005 The Electrochemical Society. All rights reserved.