Lithium ion insertion and extraction reactions with Hollandite-type manganese dioxide free from any stabilizing cations in its tunnel cavity

Lithium ion insertion and extraction reactions with a hollandite-type alpha-MnO2 specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a Li+-inserted alpha-MnO2 specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the alpha-MnO2, 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 alpha-MnO2 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 alpha-MnO2 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) LiPF6) lithium cell. An initial discharge with a capacity of approximately 230mAh/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 alpha-MnO, specimen showed a poor discharge property although no cationic residues or residual H2O molecules remained in the tunnel space. Rietveld refinement from X-ray powder diffraction data for a Li+-inserted specimen of (Li2O)(0.12)MnO2 showed it to have the hollandite-type structure (tetragonal; space group I4/m; a = 9.993(11) and c = 2.853(3) angstrom; 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 (Li2O)(0.12)MnO2 showed that Li2O 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. (c) 2005 Elsevier Inc. All rights reserved.