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 α-MnO₂ specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a Li⁺-inserted α-MnO₂ specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the α-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 α-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 α-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⁺ ions could be electrochemically inserted into one unit cell of the hollandite-type structure. By contrast, the parent α-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⁺-inserted specimen of (Li₂O)_0.12MnO₂ showed it to have the hollandite-type structure (tetragonal; space group I4/m; a=9.993(11) and ; 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)_0.12MnO₂ 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⁺ ions during cycling.