Morphology, structure, and electrochemistry of solution-derived li Mn0.5-x Cr2x Ni0.5-x O2 for lithium-ion cells

In recent years Li Mn0.5 Ni0.5 O2 has emerged as a promising alternative to LiCo O2 because of its lower cost, better high voltage stability, and improved thermal abuse characteristics. In this paper we report on Li [Mn0.5-x Cr2x Ni0.5-x] O2 (0≤2x≤0.2) compounds, which is formed by the partial substitution of nickel and manganese in Li Mn0.5 Ni0.5 O2 by chromium. The Li [Mn0.5-x Cr2x Ni0.5-x] O2 particles, prepared by a chemical solution route, displayed a faceted morphology, with particle size increasing with increasing chromium content. Rietveld refinement of X-ray diffraction data from the oxides showed that the c lattice parameter of the layered α-NaFe O2 -type structure increased, and lithium/nickel intermixing decreased, with increasing chromium content. Electrochemical cycling in the 3-4.3 V voltage window indicated that the discharge capacity was highest for the 2x=0.05 oxide composition. Charge and discharge capacities decreased with increasing chromium content for electrodes cycled in the 3-4.8 V range, and cells containing the higher chromium content oxides showed significant polarization. Peaks corresponding to oxygen loss were observed in dQdV plots of cells containing the 2x=0, 0.05, and 0.1 oxides, but not for the 2x=0.2 oxide. The first cycle "irreversible" capacity observed for cells cycled up to 145 mAhg capacity was recovered by deep-discharging the cells to voltages below 1.5 V. Oxidation state and local structural analysis of X-ray absorption spectroscopy data from the deep-discharged and as-prepared samples showed remarkable similarity, which indicated that the deep-discharge allowed the sample to almost regain its original state. Significant capacity loss was observed during electrochemical cycling, especially for the high chromium containing oxides, which appears to result from rate limitations induced by the coupling of lithium diffusion and chromium migration to and from the tetrahedral sites during the charge-discharge cycles.