Lithium ion insertion and extraction reactions with Hollandite-type manganese dioxide free from any stabilizing cations in its tunnel cavity |
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Authors: | Norihito Kijima Yasuhiko Takahashi Junji Akimoto Junji Awaka |
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Institution: | National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan |
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Abstract: | 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; Rwp=6.12%, Rp=4.51%, RB=1.41%, and RF=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. |
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Keywords: | _method=retrieve& _eid=1-s2 0-S0022459605002471& _mathId=si84 gif& _pii=S0022459605002471& _issn=00224596& _acct=C000051805& _version=1& _userid=1154080& md5=166c58f2cd3a376017ae89d2a0b627d0')" style="cursor:pointer α-MnO2" target="_blank">" alt="Click to view the MathML source" title="Click to view the MathML source">α-MnO2 Hollandite-type structure X-ray powder diffraction Rietveld analysis Maximum-entropy method Electron-density distribution Ion-exchange property Charge-discharge property |
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