LiAl x Mn 1Ϫx O 2 and LiCr x Mn 1Ϫx O 2 compounds (x Յ 0.10) with an ␣-NaFeO 2 layer-type crystal structure have been prepared by a high temperature solid-state route. The compounds are well ordered with Al or Cr sharing the Mn sites and no detectable cation mixing between Mn and Li layers. Both Al-and Cr-substituted materials cycled between 4.4 and 2.0 V in Li cells at 55ЊC show high stability of capacity. However the Al-substituted materials undergo a structural transformation to a spinel-type phase, accompanied by evolution of the discharge voltage curve to a two-plateau profile. Materials modified with small amounts of Cr show a different evolution of the discharge voltage. X-ray diffraction data for cycled LiCr x Mn 1Ϫx O 2 cathodes show that these compounds do not undergo significant transformation to spinel on cycling, but retain a hexagonal structure established on first charge.

Lithium manganese oxides, and in particular the spinel-structured LiMn 2 O 4 , have been investigated as potential active cathode materials for lithium ion batteries. Recently both orthorhombic and monoclinic LiMnO 2 have attracted considerable attention. It has been reported that Al doping allows the preparation of monoclinic LiAl x Mn 1−x O 2 under suitable reaction conditions, and furthermore improves the capacity retention of both o-LiAl x Mn 1−x O 2 and m-LiAl x Mn 1−x O 2 . The aim of this study was to elucidate the structural effects of Al doping with particular attention to the surface properties of the material. X-ray diffraction data reveal that Al induces monoclinic stacking faults in orthorhombic LiAl x Mn 1−x O 2 and at Al contents of ~5% the preferred cation ordering becomes that of monoclinic LiAl x Mn 1−x O 2 . X-ray photoelectron spectroscopy measurements show that the Al is homogeneously incorporated throughout the grains up to its solubility limit, and no surface enrichment of Al is observed. The XPS data indicate that Mn in the near-surface region of the material is predominantly present in its Y3 oxidation state, even when annealed to temperatures of up to 250 °C.

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The crystal structures, microstructures and electrochemical properties of Al-doped lithium manganese oxide materials LiAlxMn1−xO2 (0 ≤ x ≤ 0.1) prepared by solid state reactions have been investigated. A1 doping results in increased cation disorder in the orthorhombic polymorph of LiMnO2, and produces layered monoclinic LiMnO2 with an α-NaFeO2 type crystal structure. The formation of monoclinic LiAlxMn1-xO2 confirms earlier observations by Chiang et al. [1,2]. A mechanism is proposed for the orthorhombic-monoclinic transformation, based on Li-Mn inversion in the orthorhombic structure. Al ions substitute in Mn sites in the monoclinic phase and give rise to microstrain in the [2 0 -l] planes. Microstructural analysis by scanning electron microscopy has revealed Al-deficient striations which may represent residual zones of orthorhombic phase. In cycling tests in Li button cells, increasing the amount of Al dopant extends the number of cycles required for the capacity to evolve to its maximum value, but results in increased stability of the capacity at 55 °C. The layered structure of the monoclinic materials is retained on the first cycle, but transforms to a spinel-type structure on extended cycling.