Papers by Author: Jun Ming Guo

Abstract: LiMn₂O₄ was synthesized by flameless solution combustion at 600°C for 3 hours (h). The influence of HNO₃ on the morphologies, crystal structure and electrochemical performances of the material was investigated. The results show that the main phase of all synthesized products is LiMn₂O₄, and the impurities are Mn₂O₃ or Mn₃O₄ depending on the concentration of HNO₃ (C_HNO3). While C_HNO3=0 and 15 mol L^-1, the impurity was Mn₂O₃, when the concentrations of HNO₃ from 3 to 9 mol L^-1, the impurity was Mn₃O₄; at C_HNO3=12 mol L^-1, the synthesized product was single phase; C_HNO3≤12 mol L^-1, with the increase of C_HNO3, particles size grew from 70-130 nm to 140-500 nm, however, C_HNO3 is up to 15 mol L^-1, particles become small (70-140 nm); the single phase of LiMn₂O₄ obtained the maximum first discharge capacity (119.7 mAh g^-1) at C_HNO3=12 mol L^-1, but its retention rate was undesirable, while C_HNO3=15 mol L^-1, the cycling performance of the product was the optimum with first discharge capacity of 118.5 mAh g^-1 and capacity retention of 90.9 % after 40 cycles at 0.2 C.

Abstract: Spinel LiMn₂O₄ was prepared by a molten-salt combustion synthesis using eutectic acetate salts as starting materials without any additional molten-salt at 400°C, 500°C, 600°C and 700°C for 3h. The experimental results show that the main phase of the produts is spinel LiMn₂O₄, and the impurities are Mn₂O₃ or Mn₃O₄. It has been found that elevated temperature was easy to generate Mn₃O₄, and low temperature was easy to generate Mn₂O₃. The product prepared at 600 °C is single phase LiMn₂O₄ and has good crystallinity. With increasing combustion reaction temperature, the particle sizes of the products were decreased. The product prepared at 600 °C has the highest initial specific capacity of 116.5 mAh•g^-1 at 0.2C, the capacity retention was only 77.2% after 50 cycles.

Abstract: The LiMg_0.06Mn_1.94O₄ calcined at 500 °C for 3 hours then sintered at 600 °C, 700 °C and 800 °C for 3 hours by flameless liquid-phase combustion synthesis are prepared. Particle properties were characterized by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. Besides, galvanostatic tests and electrochemical impedance spectroscopy were performed to investigate the cycling performance. Particle properties analyses showed that LiMg_0.06Mn_1.94O₄ sintered at 600 °C has a single phase and the average grain size is about 80-200 nm with a little agglomeration, it also displays the highest initial capacity of 114.2 mAh/g and still remains 82.7% after 40 cycles. Results showed that sintered temperature by nitric acid assisted liquid-phase combustion method should be 600 °C.

Abstract: LiMn₂O_4-yF_y were synthesized by a novel method named liquid phase flameless combustion reaction with LiNO₃, MnAc₂.4H₂O and LiF as raw materials calcined at 600 °C for 3 h with HNO₃ as aided oxidant. All samples were investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and electrochemical performance. The results show that: all samples have main phase of LiMn₂O₄ with impurity of Mn₃O₄ and the vibrational bands of Mn-O are a little red shift by doping F, which indicated that the F- enter the host structure of LiMn₂O₄ successfully. The electrochemical performance show that the initial discharge capacities of F-doped samples are lower than pristine LiMn₂O₄, which is 117.7 mAh•g^-1. However, the capacity retention of LiMn₂O_3.96F_0.04 and LiMn₂O_3.90F_0.10 are 73.6% and 74.5%, respectively, which are higher than pristine LiMn₂O₄, which is only 69.0% after 40 cycles.

Abstract: The LiCrxMn2-xO4 material used for lithium ion battery cathode material was successfully prepared by a molten-salt flameless combustion synthesis method which used the LiNO3, LiAc•2H₂O, Mn(NO₃)₂, MnAc₂•4H₂O as raw materials and Cr(NO₃)₃•9H₂O as dopant. The results indicated that, the high-purity spinel LiCr_xMn_2-xO₄ was successfully prepared although including a little Mn3O4. The impurity peak intensity of Mn₃O₄ gradually weakened until it disappeared with the increase of the amount of Cr-doped. The product was monophasic LiCr_xMn_2-xO₄ when x (Cr) ≥ 0.1. It can be found that with Cr doped, the aggregation of the sample decreased. The structural stability, electrochemical activity and reversibility of LiCr_xMn_2-xO₄ was better than that of pristine LiMn₂O₄. The discharge capacity and capacity retention had been improved. The LiCr_0.02Mn_1.98O₄ had the highest initial capacity of 120.7 mAh•g^-1 and the capacity retention was 78.5% after 40 cycles.

Abstract: Spinel LiMn2O4 was prepared by solution combustion synthesis. The effect of fuel content and calcination procedure on phase composition and microscopic structure of LiMn2O4 was studied. X-ray diffraction patterns showed that fuel content had no obvious influence on the grain size and phase purity of LiMn2O4. Higher calcination temperature led to higher phase purity, lager grain size, and better crystallization of resultant LiMn2O4. Below 600°C the effect of calcination time was inconspicuous, which became notable above 700°C. Scanning electron microscope images showed that nanocrystalline LiMn2O4 was obtained when the calcination temperature was lower than 600°C and the grain size increased at higher temperatures.

Abstract: Spinel LiMn2O4 powders were prepared by solution combustion synthesis using nitrate and acetate salts as raw materials and urea as fuel. The phase composition of as-synthesized powders was identified by XRD and the microscopic structure was examined by SEM. Single-phase spinel LiMn2O4 was prepared when acetate salts were used, and the incorporation of nitrate salts resulted in the formation of Mn2O3. The products consisted of slight agglomerations of fine particles with the size of 50-200nm. It was found that the addition of nitrate salts increased the reaction rate and the yield of LiMn2O4 was depressed when more nitrate salts were used as a reactant.