Lithium ion insertion and extraction reactions with a hollandite-type α -MnO 2 specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a Li + -inserted α -MnO 2 specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the α -MnO 2 specimen displayed a monobasic acid behavior toward Li + , and an ion-exchange capacity of 3.25 meq/g was achieved at pH > 11 . The Li/Mn molar ratio of the Li + -inserted α -MnO 2 specimen showed that about two Li + ions can be chemically inserted into one unit cell of the hollandite-type structure. As the amount of Li content was increased, the lattice parameter a increased while c hardly changed. On the other hand, the mean oxidation number of Mn decreased slightly regardless of Li content whenever ions were exchanged. The Li + -inserted α -MnO 2 specimen reduced topotactically in one phase when it was used as an active cathode material in a liquid organic electrolyte (1:1 EC:DMC, 1 mol/dm 3 LiPF 6) lithium cell. An initial discharge with a capacity of approximately 230 mAh/g was achieved, and the reaction was reversible, whereas the capacity fell steadily upon cycling. About six Li + ions could be electrochemically inserted into one unit cell of the hollandite-type structure. By contrast, the parent α -MnO 2 specimen showed a poor discharge property although no cationic residues or residual H 2O molecules remained in the tunnel space. Rietveld refinement from X-ray powder diffraction data for a Li + -inserted specimen of (Li 2O) 0.12MnO 2 showed it to have the hollandite-type structure (tetragonal; space group I 4 / m ; a = 9.993 ( 11 ) and c = 2.853 ( 3 ) Å ; Z = 8 ; R wp = 6.12 % , R p = 4.51 % , R B = 1.41 % , and R F = 0.79 % ; S = 1.69 ). The electron-density distribution images in (Li 2O) 0.12MnO 2 showed that Li 2O molecules almost fill the tunnel space. These findings suggest that the presence of stabilizing atoms or molecules within the tunnel of a hollandite-type structure is necessary to facilitate the diffusion of Li + ions during cycling.