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 image-MnO2 specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a image-inserted image-MnO2 specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the image-MnO2 specimen displayed a monobasic acid behavior toward image, and an ion-exchange capacity of 3.25 meq/g was achieved at image. The Li/Mn molar ratio of the image-inserted image-MnO2 specimen showed that about two image 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 image-inserted image-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/dm3 LiPF6) 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 image ions could be electrochemically inserted into one unit cell of the hollandite-type structure. By contrast, the parent image-MnO2 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 image-inserted specimen of (Li2O)0.12MnO2 showed it to have the hollandite-type structure (tetragonal; space group image; image and image; image; image, image, image, and image; image). The electron-density distribution images in (Li2O)0.12MnO2 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 image ions during cycling.