Research On Modified Spinel Li-rich Lithium Manganese Oxides With High Performance

With the rapid development of portable electronic products,the demand for cheaper lithium ion batteries than past ones is more urgent.Previous research results indicate that the cathode materials in lithium ion battery system determine the battery performance.Spinel Li4Mn5O12 are considered to be the most promising cathode materials on 3 V voltage range for their rich material sources,low price,simple preparation process and high inserted lithium quality.This dissertation aims to investigate which is the determined effect on the electrochemical performance of LiMn5O12 among sintering temperature,lithium source,manganese source and doping ion.Many advanced experimental techniques,such as X-ray diffraction,charge-discharge cycling experiment,cyclic voltammogram,electrochemical impedance spectroscopy,Fourier transform infrared spectroscopy and scanning electron microscopy,were employed to investigate the relationship among the samples' structure,surface morphology and electrochemical performance.The main results showed as follows:(1)The samples with the theoretical composition of Li4Mn5O12 were prepared using a solid state sintering method by changing sintering temperature,lithium sources and manganese sources.The sample prepared at 550 ? by using Li2C2O4 and MnC2O4·2H2O as starting materials exhibits the best electrochemical performance in as-prepared samples.It exhibits the 1st cycle capacity of 29.0 mAh·g-1(-22?),159.9 mAh·g-1(30?)and 165.1 mAh·g-1(55?)while it was cycled at the current of 200 mA·g-1.The lithium-ion diffusion coefficient of the sample is 4.31×10-7 cm2·s-1.(2)The trivalent metallic ions doped samples were prepared using a solid state sintering method by adding trivalent metallic ions into the precursors during the preparation process.The sample with the theoretical composition Li4Mn4.94Fe0.06O11.97 exhibits the best electrochemical performance in all doped samples.It exhibits the 1st cycle capacity of 113.6 mAh·g-1(-22?),178.9 mAh·g-1(30?)and 185.3 mAh·g-1(55?)while it was cycled at the current of 200 mA·g-1.It exhibits the 1st cycle capacity of 155.0 mAh·g-1(400 mA·g-1),147.9 mAh-g-1l(800 mA·g-1)and 131.7 mAh·g-1(1000 mA·g-1)while it was cycled at 30?.The lithium-ion diffusion coefficient of Li4Mn4.94Fe0.06O11.97 is 4.88×10-7 cm2·s-1.(3)The bivalent metallic ions doped samples were prepared using a solid state sintering method by adding bivalent metallic ions into the precursors during the preparation process.The sample with the theoretical composition Li4Mn4.95Mg0.05O11.95 exhibits the best electrochemical performance in all doped samples.It exhibits the 1st cycle capacity of 72.7 mAh-g-1(-22?),180.9 mAh·g-1(30?)and 181.7 mAh·g-1(55?)while it was cycled at the current of 200 mA·g-1.It exhibits the 1st cycle capacity of 146.7 mAh·g-1(400 mA·g-1),153.5 mAh·g-1(800 mA·g-1)and 140.7 mAh·g-1(1000 mA·g-1)while it was cycled at 30?.The lithium-ion diffusion coefficient of Li4Mn4.95Mg0.05O11.95 is 4.57× 10-7 cm2·s-1.(4)The rare earth metal ions doped samples were prepared using a solid state sintering method by adding rare earth metal ions into the precursors during the preparation process.The sample with the theoretical composition Li4Mn4.98La0.02O11.99 exhibits the best electrochemical performance in all doped samples.It exhibits the 1st cycle capacity of 105.3 mAh-g-1(-22?),160.5 mAh·g-1(30?)and 176.7 mAh·g-1(55?)while it was cycled at the current of 200 mA·g-1.It exhibits the 1st cycle capacity of 152.1mAh·g-1(400 mA·g-1),143.0 mAh·g-1(800 mA·g-1)and 137.1 mAh·g-1(1000 mA·g-1)while it was cycled at 30?.The lithium-ion diffusion coefficient of the Li4Mn4.98La0.02011.99 is 5.61×10-7 cm2·s-1.