Structural and Electrochemical Characterizations on Li₂MnO₃-LiCoO₂-LiCrO₂System as Positive Electrode Materials for Rechargeable Lithium Batteries

Cobalt and chromium substituted lithium-excess manganese oxides, Li[Li 0.2 Co x Cr 0.4- x Mn 0.4 ]O 2 ( x = 0, 0.2, and 0.4), are synthesized by a simple combustion method. Crystal structures of the samples are examined by an X-ray diffraction (XRD) method. The samples crystallize into a rhombohedral layered phase with in-plane cation ordering between lithium and transition metal ions. Li[Li 0.2 Co 0.2 Cr 0.2 Mn 0.4 ]O 2 can deliver more than 200 mAh g −1 of discharge capacity above 3.0 V vs. Li at a rate of 10 mA g −1 . Discharge capacity further increases to 300 mAh g −1 when the Li cell is discharged to 1.5 V with appearance of a voltage plateau at 1.7 V. A combined study of synchrotron XRD and X-ray absorption spectroscopy (XAS) reveals that originally trivalent chromium ions are oxidized to hexavalent state, resulting in the migration of chromium ions into face-shared tetrahedral sites in lithium layers. Although a part of hexavalent chromium ions are immobilized at the tetrahedral sites, the reduction process to 1.5 V allows the chromium ions to migrate to octahedral sites associated with the reduction process to trivalent state. The chromium substitution has a beneficial effect on stabilization of oxide ion framework structures of the lithium-excess manganese system by suppressing oxygen loss upon charge to a high voltage region, leading to good cyclability as electrode materials.