Lithium ion insertion and extraction reactions with Hollandite-type manganese dioxide free from any stabilizing cations in its tunnel cavity
- National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
Lithium ion insertion and extraction reactions with a hollandite-type {alpha}-MnO{sub 2} specimen free from any stabilizing cations in its tunnel cavity were investigated, and the crystal structure of a Li{sup +}-inserted {alpha}-MnO{sub 2} specimen was analyzed by Rietveld refinement and whole-pattern fitting based on the maximum-entropy method (MEM). The pH titration curve of the {alpha}-MnO{sub 2} specimen displayed a monobasic acid behavior toward Li{sup +}, and an ion-exchange capacity of 3.25meq/g was achieved at pH>11. The Li/Mn molar ratio of the Li{sup +}-inserted {alpha}-MnO{sub 2} specimen showed that about two Li{sup +} 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{sup +}-inserted {alpha}-MnO{sub 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, 1mol/dm{sup 3} LiPF{sub 6}) 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{sup +} ions could be electrochemically inserted into one unit cell of the hollandite-type structure. By contrast, the parent {alpha}-MnO{sub 2} specimen showed a poor discharge property although no cationic residues or residual H{sub 2}O molecules remained in the tunnel space. Rietveld refinement from X-ray powder diffraction data for a Li{sup +}-inserted specimen of (Li{sub 2}O){sub 0.12}MnO{sub 2} showed it to have the hollandite-type structure (tetragonal; space group I4/m; a=9.993(11) and c=2.853(3)A; Z=8; R{sub wp}=6.12%, R{sub p}=4.51%, R{sub B}=1.41%, and R{sub F}=0.79%; S=1.69). The electron-density distribution images in (Li{sub 2}O){sub 0.12}MnO{sub 2} showed that Li{sub 2}O 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{sup +} ions during cycling.
- OSTI ID:
- 20729081
- Journal Information:
- Journal of Solid State Chemistry, Vol. 178, Issue 9; Other Information: DOI: 10.1016/j.jssc.2005.06.023; PII: S0022-4596(05)00247-1; Copyright (c) 2005 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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