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 αα-MnO2 specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a Li+Li+-inserted αα-MnO2 specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the αα-MnO2 specimen displayed a monobasic acid behavior toward Li+Li+, and an ion-exchange capacity of 3.25 meq/g was achieved at pH>11pH>11. The Li/Mn molar ratio of the Li+Li+-inserted αα-MnO2 specimen showed that about two Li+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+Li+-inserted αα-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 Li+Li+ ions could be electrochemically inserted into one unit cell of the hollandite-type structure. By contrast, the parent αα-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 Li+Li+-inserted specimen of (Li2O)0.12MnO2 showed it to have the hollandite-type structure (tetragonal; space group I4/mI4/m; a=9.993(11)a=9.993(11) and c=2.853(3)Å; Z=8Z=8; Rwp=6.12%Rwp=6.12%, Rp=4.51%Rp=4.51%, RB=1.41%RB=1.41%, and RF=0.79%RF=0.79%; S=1.69S=1.69). 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 Li+Li+ ions during cycling.

Three-dimensional EDD images viewed along the (a) [001] and (b) [100] directions for (Li2O)0.12MnO2(Li2O)0.12MnO2. The equi-density level was 0.3/Å30.3/Å3. The solid boxes indicate the unit cell.Figure optionsDownload as PowerPoint slide