| Recently,Li-excess and manganese based layered material,a solid solution system with complex integration of the layered Li2MnO3 and LiMO2 components,considered to be the most promising lithium-ion battery materials due to their extraordinarily high theoretical capacity,low cost and high safety in li-ion batteries.In this dissertation,series of Li-rich xLi2MnO3·(1-x)Li[Ni0.4Mn0.45Co0.15]O2 cathode materials with layered structure were prepared by using a "two step drying-sintering"method.The effects of surface treatment of the cathode materials were further to be investigated.Many advanced techniques,such as X-ray diffraction,charge-discharge cycling,cyclic voltammogram,electrochemical impedance spectroscopy,fourier transform infrared spectroscopy and scanning electron microscopy,were employed to investigate the crystal structure,surface morphology and electrochemical performance of the as-prepared samples.The main experimental results are summarized as follows:1.Li-rich and Mn-based cathode materials with a layered ?-NaFeO2 structure were synthesized by using "two step drying-sintering" method and using MnC2O4·2H2O as manganese source.And all as-synthesized samples which were prepared by changing the ratio of Mn and Co in the raw material.The sample with the theoretical composition of 0.2Li2MnO3·0.8Li[Ni0.4Mn0.45Co0.15]O2 shows good electrochemical performance.It exhibits the discharge capacity of 159.1 mAh·g-1 in the initial cycle and 158.3 mAh·g-1 in the 40th cycle while it was cycled at the current of 200 mA·g-1 and 30?.It exhibits the discharge capacity of 181.3 mAh·g-1 in the 1st cycle and 195.5 mAh·g-1 in the 30th cycle when it was cycled at the current of 200 mA·g-1 and 55?.Then samples with different ratio of Li2MnO3 and Li[Ni0.4Mn0.45Co0.15]O2 were prepared.The sample with a theoretical composition of 0.5Li2MnO3·0.5Li[Ni0.4Mn0.45Co0.15]O2 exhibits the best electrochemical performance.It exhibits the initial discharge capacity of 130.2 mAh·g-1 and the 40th discharge capacity of 181.9 mAh·g-1 while it was cycled at the current of 200 mA·g-1.The binding strength of the Ni-O,Mn-O and Co-O in the 0.5Li2MnO3.0.5Li[Ni0.4Mn0.45Co0.15]O2 is higher than that of 0.2Li2MnO3·0.8Li[Ni0.4Mn0.45Co0.15]O2.2.Li-rich and Mn-based cathode materials with a layered ?-NaFeO2 structure were synthesized by using MnO as manganese source.The sample with a theoretical composition of 0.6Li2MnO3·0.4Li[Ni0.4Mn0.45Co0.15]O2 exhibits the best electrochemical performance.It exhibits the capacities of 116.9 mAh·g-1,113.9 mAh·g-1,86.9 mAh·g-1 and 83.9 mAh·g-1 in the first cycle,204.4 mAh·g-1,176.7 mAh·g-1,166.0 mAh·g-1 and 132.9 mAh·g-1 in the 40th cycle while it was at the current of 200 mA·g-1,400 mA·g-1,800 mA·g-1 and 1000 mA·g-1,respectively.It found that the electrochemical reaction of 0.6Li2MnO3·0.4Li[Ni0.4Mn0.45Co0.15]O2 is controlled by the impedance of charge transfer and the impedance of Warburg.3.Solid solution cathode material with the theoretical composition of 0.6Li2MnO3·0.4Li[Ni0.4Mn0.45Co0.15]O2 was synthesized by using MnO as manganese source.In order to improve the first charge/discharge efficiency of the 0.6Li2MnO3·0.4Li[Ni0.4Mn0.45Co0.15]O2 sample,citric acid and triammonium citrate were used to treat the as-prepared sample.The experimental results revealed that the sample which treated by using 20 wt.%of citric acid exhibits the best electrochemical performance in all citric acid treated samples.It exhibits the discharge capacity of 184.5 and 137.4 mAh·g-1 in the 1st cycle,215.9 and 166.9 mAh·g-1 in the 40th cycle when cycled at the current of 200 and 1000 mA·g-1,respectively.And the sample which treated by using 40 wt.%of triammonium citrate exhibits the best electrochemical performance in all the triammonium citrate treated samples.It exhibits the discharge capacity of 191.2 and 125.3 mAh·g-1 in the 1st cycle,215.7 and 155.9 mAh·g-1 in the 40th cycle when cycled at the current of 200 and 1000 mA·g-1,respectively.After the treatment of citric acid or triammonium citrate,the rate performance of the samples was improved. |
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