Layered Li(Ni0.2Mn0.2Co0.6)O2 synthesized by a molten salt method for lithium-ion batteries
Abstract
Sub-micron size Li(Ni0.2Mn0.2Co0.6)O2 was synthesized by the molten salt method at 800 °C and 900 °C using LiOH:LiNO3 eutectic salt for the first time and the phases were characterized by X-ray diffraction (XRD), SEM, density and BET surface area. Rietveld refinement of the XRD data showed 5 and 3% cation-mixing in the compound synthesized at 800 °C and 900 °C, respectively. Galvanostatic charge–discharge cycling at 30 mA g−1 between 2.5 and 4.4 V vs. Li at room temperature showed the second cycle discharge capacities of 119 and 133 mA h g−1 for the phases synthesized at 800 °C and 900 °C, respectively. The capacity retention was 81% and 87%, respectively between 2 and 50 cycles. After reheating the 900 °C sample for another 2 hours at 900 °C, the XRD pattern shifted obviously to low angle, which indicated a reduction of Ni3+ to Ni2+ and was further proved by X-ray photoelectron spectroscopy (XPS) measurement. The re-heated compound showed an improved discharge capacity of 159 mA h g−1 (2nd cycle) and it retained a capacity of 123 mA h g−1 at the end of the 50th cycle, corresponding to a capacity-retention of 77%. Cyclic voltammetry studies on the above compound clearly showed the redox peaks due to Ni2+/4+ and Co3+/4+ and the 4.5 V-structural transition was not suppressed. The cathodic performance of the phases improved upon cycling to the cut-off voltage of 4.3 V.
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