| Rapid development of EV or HEV requests high power Li-ion batteries (LIBs) to have higher energy, longer life and better safety than before. Recently, spinel LiNi0.5Mn1.5O4 attracts more attention of many researchers due to its lower cost, higher voltage and rate capability. However, severer electrolyte decomposition on higher voltage leads to unsatisfactory cycling stability at high rate, which limits its use. In order to improve the rate cycling capability of LiNi0.5Mn1.5O4,the effects of annealing temperature on the capacity and rate capability of LiNio.5Mn1.5O4 were studied, and modification of LiNi0.5Mn1.5O4 by F-doping, Cr- and F- co-doping, Co- and F- co-doping were investigated in this paper.The effects of annealing temperatures on capacity and rate capability of LiNio.5Mn1.5O4 were investigated. The results show that the all samples prepared at 800℃～900℃showed better spinel structure. With increasing the temperatures, the crystal particles grew gradually, but the initial discharge capacity at 0.2C decreased. The sample prepared at 800℃showed the highest capacity of 129.1 mAh·g-1. However when discharged at 1C～5C, the sample prepared at 900℃exhibited higher capacity. Its capacity reached 100.8 mAh·g-1 at 2C. Increasing annealing temperatures made crystal particles grow bigger, the electrolyte decomposition reduce and the Mn3+ content increase, which reduced the film impedance and charge transfer impedance, and then improved the rate capability of LiNio.5Mn1.5O4. However, the improvement was limited due to the enlarged Li+ diffusion path and decrease of initial capacity.The studies on the modification of LiNi0.5Mn1.5O4 by F-doping show that all LiNi0.5Mn1.5O4-xFx samples prepared at 800℃～900℃showed better spinel structure. F-doping can reduce the impure phase NiO or LixNi1-xO. The doped F amount and annealing temperatures were two major factors affecting the crystal particles growth, morphology and electrochemical properties of samples. The sample with x=0.05 prepared at 850℃revealed better crystalline morphology, suitable particle size distribution, better reduce of electrolyte decomposition, the smallest film impedance and charge transfer impedance, leading to remarkable improvement of rate capability. Its capacity reached about 125 mAh·g-1 at 0.2C, and retained at about 120 mAh·g-1 after charge-discharged at 0.2C,0.5C,1C and 2C each 10 cycles. Too high annealing temperatures and too more doped F made the sample agglomerate seriously to form too big particles, and then deteriorate its electrochemical performance.The investigation of Cr- and F- co-doped LiNio.5Mn1.5O4 has been carried out first to our knowledge. The results show that all LiNio.5Mn1.5-xCrxO4-xFx samples prepared at 800℃～900℃showed better spinel structure. Impure phase NiO or LixNi1-xO was not observed. The co-doped samples prepared at 800℃and 850℃exhibited effective reducing electrolyte decomposition, leading to remarkable decrease of film impedance and charge transfer impedance, and then improving the rate capability significantly. Among the samples, the sample with x=0.025 prepared at 850℃showed uniform particle size, better crystalline morphology, the smallest electrochemical impedance and the best electrochemical performance. Its capacity reached about 124.1 mAh·g-1 at 0.2C, and retained at about 120 mAh·g-1 after charge-discharged at 0.2C,0.5C,1C and 2C each 10 cycles. Annealing at 900℃made the sample agglomerate seriously to form too big particles, and then deteriorate its electrochemical performance.The effects of Co- and F- co-doping on the electrochemical performance of LiNi0.5Mn1.5O4 also have been investigated first to our knowledge. The results show that all LiNi0.5Mn1.5-xCoxO4-xFx samples prepared at 800℃～900℃displayed better spinel structure. No NiO or LixNi1-xO impure phase was formed. Co-doping had little effect on the particle morphology of samples. Compared with base sample, the co-doped sample with x=0.04 prepareed at 800℃showed a little decreased impedance and smaller improved rate capability. Its discharge capacity reached 132.4mAh·g-1 at 0.2C. However, increasing the co-doped ion amount accelerated the electrolyte decomposition, enhanced the film impedance and charge transfer impedance, leading to deterioration of electrochemical performance.Based on above results, it is concluded that modification of LiNi0.5Mn1.5O4 by F-doping or Cr-and F- co-doping can improve rate capability remarkably. In the cases, optimizing the annealing temperatures and doped ion amounts can make sure to prepare modified higher rate LiNi0.5Mn1.5O4 with well crystalline, uniform particle size distribution, better reduce of electrolyte decomposition and smaller film impedance and charge transfer impedance. |
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