| The synthesis, modification and electrochemical performance of rechargeable lithium batteries and cathode materials were reviewed in detail.The cathode material LiFePO4 for lithium ion battery was prepared by co-precipitation-carbothermal reduction reaction method. The effect of solution concentration on the FePO4·xH2O precursor and LiFePO4 was studied. The results show that no impurities exist in the FePO4·xH2O synthesized with solution concentration from 0.1M to 1.5M. LiFePO4/C made from FePO4·2H2O precursor under the condition that the solution concentration of 1M show excellent electrochemical performance. The initial discharge capacity of LiFePO4 prepared under the optimized condition was 159 mAh·g-1 at 0.1 C rate and capacity retention remains 99.7 % after 30 cycles.The kinetics behaviors of LiFePO4 material for lithium ion battery were investigated by means of linear sweep voltammetry and chronoamperometry. It is shown that with lithium interaction into LiFePO4 material, the exchange current density was increased gradually. Meanwhile, the level of diffusion coefficient (DLi+) for LiFePO4 material is between 10-15 cm2·s-1 and 10-14 cm2·s-1.And doping has something with the exchange current density and diffusion coefficient (DLi+).The modification of LiFePO4 by Mg and Ni doping was investigated. The crystal structure, surface morphology and electrochemical properties of LiFePO4 were systematically researched. The results indicated that the structure of the material wasn’t changed after Mg and Ni doping. Appropriate amount of Mg and Ni doping can get the particle size of LiFePO4 smaller and uniform distribution. Apparently, the initial charge and discharge capacity of LiFePO4 samples went down but the corresponding coulombic efficiency and cycling performance were enhanced. The discharge capacity of LiFePO4, LiMgo.o2Fe0.98P04, and LiNi0.03Fe0.97PO4 were 159,158 and 156 mAh·g-1, respectively. And, the capacity was respectively kept 114,139 and 134 mAh·g-1 at 0.5 C after 100 cycles and the capacity retained 106,128 and 120 mAh·g-1 cycling at 1 C after 50 cycles. The Mg-doped and Ni-doped LiFePO4 samples were discussed by AC impedance and the Rct value of LiMg0.02Fe0.98PO4 and LiNi0.03Fe0.97PO4 was decreased. The potential difference between the oxidation potential and the reduction potential of LiMgo.o2Fe0.98PO4 and LiNi0.03Fe0.97PO4 were decreased to 0.24V and 0.29V respectively, resulting in the obvious enhancement of the reversibility of electrode reaction.Li2FeSi04 materials doped with Ni and Mn were synthesized via a novel mechanical activation-high temperature solid state method. The effect of Ni doping on physical structure and electrochemical performance of Li2FeSiO4 was investigated. The results were shown that appropriate Ni doping was unable to influence the structure of product. And then, the morphology of product tended to be regular, increasing the charge-discharge capacity and improving the cycle performance. Li2Fe0.7Ni0.3SiO4 shows the highest discharge capacity and the best cyclying stability. The initial discharge capacity was 118.7 mAh·g-1 and capacity retention remains 87.2% after 30 cycles. The effect of Mn doping, sintering temperature, sintering time and Li/Si ratio on Li2Fe1-xMnxSiO4/C’s performance were systematically researched. The cycle performance of Li2Fe1-xMnxSiO4/C prepared at 600℃for 16h and a Li/Si moral ratio of 2.04 as the optimum condition and the electrochemical properties of Li2Fe0.9Mn0.1SiO4/C synthetized using sucrose and glucose as carbon source and reductant. It delivered an initial capacity of 149.8mAh/g between 1.5V and 4.8V at C/16 rate and a capacity retention ratio of 90.1% after 30 cycles. The cycle performance of Li2Fe0.9Mn0.1SiO4/C became worse with increasing the discharge rate and capacity remains 74.9 mAh·g-1 after 30 cycles. Simultaneously, Li2Fe0.9Mn0.1SiO4/C with different content of sucrose and glucose were prepared. The grain size of Li2Fe0.9Mn0.1SiO4/C decreased with the increase of carbon content. Compared with Li2Fe0.9Mn0.1SiO4/C using sucrose as carbon source, Li2Fe0.9Mn0.1SiO4/C using glucose displayed smaller particles and more homogeneous distribution. the sample with 15%(wt,%) glucose exhibited excellent performance, with an initial discharge capacity of 154.7mAh/g and a capacity retention rate of 92.2% after 30 cycles.The lithium deintercalation-intercalation kinetics of modified Li2FeSiO4 materials were investigated by cyclic voltammetry, electrochemical impedance spectroscopy methods, and a fitting equivalent circuit diagram was raised. The results further proved the modified Li2FeSi04 had better electrochemical performance than the pristine one. |
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