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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 27, Number 9
BKCSDE 27(9)
September 20, 2006

Title	Effects of Sulfur Substitution on Chemical Bonding Nature and Electrochemical Performance of Layered LiMn_0.9Cr_0.1O_2-xSx
Author	Seung Tae Lim, Dae Hoon Park, Sun Hee Lee, Seong-Ju Hwang, Young Soo Yoon, Seong-Gu Kang
Keywords	Sulfur substitution, Layered lithium manganate, Structural distortion, Cyclability, Mn migration
Abstract	Sulfur-substituted LiMn_0.9Cr_0.1O_2-xSx (0 ≤ x ≤ 0.1) layered oxides have been prepared by solid state reaction under inert atmosphere. From powder X-ray diffraction analyses, all the present lithium manganates were found to be crystallized with monoclinic-layered structure. Electrochemical measurements clearly demonstrated that, in comparison with the pristine LiMn_0.9Cr_0.1O₂, the sulfur-substituted derivatives exhibit smaller discharge capacities for the entire cycle range but the recovery of discharge capacity after the initial several cycles becomes faster upon sulfur substitution. The effect of the sulfur substitution on the chemical bonding nature of LiMn_0.9Cr_0.1O_2-xSx has been investigated using X-ray absorption spectroscopic (XAS) analyses at Mn and Cr K-edges. According to Mn K-edge XAS results, the trivalent oxidation state of manganese ion remains unchanged before and after the substitution whereas the local structure around manganese ions becomes more distorted with increasing the substitution rate of sulfur. On the other hand, the replacement of oxygen with sulfur has negligible influence on the local atomic arrangement around chromium ions, which is surely due to the high octahedral stabilization energy of Cr^+III ions. Based on the present experimental findings, we have suggested that the decrease of discharge capacity upon sulfur substitution is ascribable to the enhanced structural distortion of MnO₆ octahedra and/or to the formation of covalent Li-S bonds, and the accompanying improvement of cyclability would be related to the depression of Mn migration and/or to the pillaring effect of larger sulfur anion.
Page	1310 - 1314
Full Text	PDF

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