Effect of Post-Heating Treatment of Ultrasonically Sprayed and Pyrolysed LiMn2O4 Powders on the Performance of Secondary Battery

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The LiMn2O4 spinel oxides have been successfully synthesized with ultrasonic spray pyrolysis method from an aqueous mixing solution of lithium nitrate and manganese nitrate. The effects of post heat treatment on the physicochemical properties of the LiMn2O4 powders have been carefully examined by means of X-ray diffraction spectra, titration of manganese valence and X-ray absorption spectroscopy. The results indicated that the post heat activation would influence the microstructure and crystallization of the pyrolysed particles. This profile variation in the potential plateau is considered to be caused by an increase in the amount of active Mn(superscript 3+). The more amount of active Mn(superscript 3+), the flatter and longer the plateau is. This result is in agreement with the variation trend of lattice parameters which increase with increasing the heat treatment time. The capacity loss increases with increasing the heat treatment time. From the fitting results of X-ray absorption spectra, the (Mn-O) and (Mn-Mn) bond distances are found to become larger as the heat-treatment time is prolonged. The cell with 4-hour heat-treatment powder as cathode gives best electrochemical performance and exhibited an initial highest capacity of 129mAhg^(-1). The capacity after 30 cycles can still reaches to 121mAhg^(-1). Among the parameters of the valence change, lattice constant and Debye-Waller factor discussed in the study, the short range ordering (Debye-Waller factor, σ) is most critical to the electrochemical performance of the cell. In other words, the Debye-Waller factor can be used as an important index to evaluate the quality of the LiMn2O4 powder as cathode materials in lithium ion secondary cell. The post heat treatment is an effective way to improve the ultrasonically spray-pyrolysed powders in terms of the electrochemical characteristics.