Synthesis and electrochemical properties of nanocrystalline Li[NixLi(1-2x)/3Mn(2-x)/3]O2 prepared by a simple combustion method

Nanocrystalline Li[NixLi(1-2x)/Mn-3((2-x)/3)]O-2 powders were prepared by a simple combustion method and investigated using X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), particle size analysis (PSA), and galvanostatic charge/discharge cycling. According to the XRD analysis, single-phase compounds with a layered structure were obtained for powders with 0 less than or equal to x less than or equal to 0.25, while mixtures were obtained for powders with 0.30 less than or equal to x less than or equal to 0.50. Rietveld analysis revealed that single-phase Li[NixLi(1-2x)/3Mn(2-x)/3]O-2 is basically a layered rock-salt structure in which a small amount of Ni occupies the 3a sites. The initial discharge capacity of a Li/Li[NixLi(1-2x)/3Mn(2-x)/3]O-2 cell with x = 0.20 was about 288 mA h g(-1) corresponding to about 91% of the theoretical value, when it was cycled in the voltage range of 4.8-2.0 V with a specific current of 20 mA g(-1) at 30 degreesC. As far as we know, charge/discharge cycling on an Li/Li[Ni0.20Li0.20Mn0.60]O-2 cell gives the highest discharge capacity of 288 mA h g(-1) among the LiMO2-based (M = Co, Ni, and Mn) cathode materials. A very promising factor for high-rate capability applications was an excellent rate capability in continuous cycling at specific currents ranging from 20 mA g-1 to 900 mA g-1, due to the nanocrystalline particle size of 80-200 nm. The origin of the 4.5 V plateau was investigated by means of weight loss measurement and XAS for the charged/discharged electrodes. The weight loss measurement for the charged electrodes gave indirect evidence that the 4.5 V plateau did not originate from the ejection of oxygen. In XAS, the Mn oxidation state of 4+ did not change during the charge/discharge process, and surprisingly the Ni did not further oxidize beyond about 3+.