(Invited) Effects of Crystalline Structure on the Electrochemical Properties of High Voltage Spinel Cathode Materials

LiNi_0.5Mn_1.5O₄ powders were prepared via a spraying-dry method followed by heating at temperatures between 600 and 1000^oC for 8 hours in air with oven cooling. In order to eliminate the particle size effects, 900^oC samples were annealed at 700^oC under air for various durations. The crystalline structure of the prepared samples were investigated from their high resolution neutron powder diffraction data collected with Echidna at ANSTO and refined with Rietica Ver. 1.77. The results of Rietveld refinement for the NPD patterns manifest that the 700^oC prepared sample contains 85% of ordered P4₃32 and 15% of disordered Fd m phases whereas the powders prepared at temperatures higher than 800^oC prevailed with disordered Fd m phase (72�`75%) with amount of rock-salt Li_xNi_1-xO increases from 1 to 4% with heating temperature. For the annealed samples, the molar fraction of the ordered phase increases with increasing duration of annealing and converts completely into ordered phase after annealing for 60 hours. It suggests that the phase conversion and Li_xNi_1-xO evolution/dissolution are reversible [1]. From the results of capacity retention study of the cells comprised with the prepared powders, it was found that sample prepared at 900^oC with oven cooling exhibits the most promising performance among the prepared powders. These results are different from those reported by Fang et al. [2], however, they are similar to those reported previously that the disordered phase has better electrochemical properties than the ordered phase [3-5]. Though various reasons had been suggested [3, 6-7], the effects of crystalline structure on the cycling performance of high voltage spinel are still not clear. In this study, the effects are hope to reveal from the results of dissolution study for transition metals upon storage and cycling, linear sweep voltammetry for electrolyte oxidation on high voltage spinel, cyclic voltammetry for Li⁺ ion diffusivity, and the phase evolution detected by high resolution TEM for the fresh and cycled electrodes and in-situ NPD study.

Reference:

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