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An electroactive spinel solid solution, LiFexMn2−xO4 false(0⩽x⩽0.5false), was prepared and investigated. X-ray Rietveld refinements showed a small amount of transition metal ions in tetrahedral 8a sites in Fe-substituted samples. The 57normalFe Mössbauer spectrum of LiFe0.5Mn1.5O4 at 300 K is composed of two doublets and attributed to trivalent Fe. In the voltage range 3.0 and 5.3 V vs. normalLi/Li+, two reversible plateaus appeared at 4.9 and 3.9 V during discharge. Combined data of X-ray diffraction, Mn K-edge X-ray absorption near-edge structure (XANES), and 57normalFe Mössbauer spectroscopy obtained in situ during charge and discharge demonstrate that the plateau around 4 V on charging is associated with the oxidation of Mn3+ to Mn4+ and the plateau around 5 V with the partial oxidation of Fe3+ to Fe4+. © 2001 The Electrochemical Society. All rights reserved.

Lithium cobalt oxide (LiCoO 2 ) with ␣-NaFeO 2 -type (layered rock-salt) structure is now extensively used as a positive electrode material in commercial lithium-ion batteries. In this structure, LiO 6 and CoO 6 octahedra share their corners and stack alternatively along the c axis direction, which allows two-dimensional diffusion of Li ions during electrochemical deintercalation and intercalation. Because the Co in LiCoO 2 is a relatively rare metal, dopants such as Ni, 1 Cr, 2 Mn, 3 B, 4 Al 5 have been used to reduce material costs. Although Fe doping 6,7 is among the effective methods to accomplish this aim, the incorporation of Fe has been limited to 10% per formula unit (LiCo 0.9 Fe 0.1 O 2 ) by solid-state reaction at high temperature (800ЊC 6,7 ). Although Alcantara et al. 7 claimed that the solubility limit of Fe in LiCoO 2 was 20%, a small nonindexable peak near the 101 peak (2 ϭ 36Њ) was observed for LiCo 0.8 Fe 0.2 O 2 . This limitation originates from sample contamination by the formation of cubic ␣-LiFeO 2 with a cation-disordered structure, as for the case of Fe-doped LiNiO 2 , 8 because LiFeO 2 , isostructural with LiCoO 2 (layered-LiFeO 2 ), is a metastable form and can be obtained by a softchemical synthetic route such as ion-exchange and hydrothermal reactions. A soft-chemistry method is necessary to form rhombohedral Fe-doped LiCoO 2 with high Fe content.Charge-discharge voltages and capacities of Li/LiCoO 2 cells are sensitive to the starting materials and LiCoO 2 preparation conditions. 9-12 Some structural models for Li-extracted LiCoO 2 have been reported. 9,10 Concerned about Fe doping effects into LiNiO 2 , Reimers et al. 8 showed that a single-phase layered structure could be obtained up to LiNi 0.8 Fe 0.2 O 2 and that the charge and discharge capacities decreased with increasing Fe content. Recently, Prado et al. 13 reported that a homogeneous solid solution could be obtained up to LiNi 0.7 Fe 0.3 O 2 , a rhombohedral symmetry was maintained up to Li 0.4 Ni 0.9 Fe 0.1 O 2 during electrochemical oxidation, and some of the trivalent iron was oxidized to the tetravalent state after electrochemical charging of a Li/LiNi 0.9 Fe 0.1 O 2 cell. Although Alcantata et al. examined the cation distribution and electrochemical properties of Fe-doped LiCoO 2 , 7 no systematic study of changes of crystal structure and the valence state of 3d metals after charge-discharge cycle tests has been reported for Li/LiCo 1Ϫx Fe x O 2 cells.We have successfully obtained metastable layered LiFeO 2 14 and layered LiMnO 2 15 with an ␣-NaMnO 2 structure similar to ␣-NaFeO 2 using a mixed-alkaline hydrothermal reaction below 300ЊC. Up to 25% Fe-doped LiCoO 2 could be prepared by hydrothermal reaction at 220ЊC. 16 Structural changes and valency of Co and Fe during charge-discharge tests of Li/LiCo 1Ϫx Fe x O 2 cells were studied to understand the Fe doping effect on the LiCoO 2 positive electrode. ExperimentalIron-doped LiCoO 2 samples (LiCo 1Ϫx Fe x O 2 nominal x ϭ 0.00 (sample A), 0.05, 0.10, 0.15, and 0.25) were ...

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