No abstract

LiFePO4/C composite cathode material prepared by carbothermal reduction method was coated by metal oxide MnO2, Al2O3, CuO, respectively, by a chemical precipitation method. The effects of metal oxide coating on the structure and electrochemical performance of LiFePO4/C composites were systematically investigated. The structure and morphology of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the electrochemical properties were evaluated by constant-current charge/discharge cycling tests. It is found that the coating of metal oxide could greatly improve its high-rate dischargeability and cycling performance. The LiFePO4/C cathode material coated by MnO2 exhibits a specific discharge capacity of 118.5 mAh/g at 3C rate, much higher than the uncoated sample (95.1 mAh/g), with a capacity degradation rate of only 6.3 % after 250 cycles at 3C rate.

Phase pure Li4Ti5O12/C composite was synthesized by sol-gel method using lithium acetate CH3COOLi•2H2O and tetrabutyl titanate [Ti(OC4H9)4] as starting materials, oxalic acid as chelating agent and sucrose as an additional carbon source. The as-prepared samples were characterized by means of TG-DTA, XRD and SEM. The electrochemical properties were investigated in terms of constant-current charge/discharge cycling and high-rate dischargeability. SEM analysis indicated that the prepared Li4Ti5O12/C composite using sucrose and oxalic acid as carbon source showed a spongy nano-particle aggregate structure, with average nano-particle size of 80-100 nm. Electrochemical results showed that the Li4Ti5O12/C composite prepared in the presence of sucrose exhibited better electrochemical performance with specific discharge capacities of 204.7, 171.6, 155.3 and 154.6 mAh/g at 0.2C, 1C, 2C, and 5C rates, respectively. And the discharge capacity could still reach 143.9mAh/g after 80 cycles at 1C rate, exhibiting excellent cycling performance.

Mg2+ doped LiFePO4 was synthesized from Li3PO4, FeSO4 and MgSO4 by a hydrothermal synthesis at 150 °C（Li1-xMgxPO4, x=0.00, 0.01,0.02,0.04,0.06）. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge-discharge test. The results showed that Mg2+ dissolved in the LiFePO4 lattice. When the content is in the range of 0 to 6 mol%, Mg2+ caused the shrinkage of LiFePO4 cell volume. The capacity of doped and undoped samples at low discharging rate was similar, about 145mAhg-1 for 0.2C. But the sample doped with 2-4 mol% Mg2+ has higher capacity and longer cycle lifetime than the undoped one at 5C.

Lithium iron phosphate with varied Fe/P molar ratio was synthesized from LiOH, FeSO4, and H3PO4by hydrothermal route at 180°C for 6 h. The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), chemical analysis, and constant current charge-discharge cycling test. It was found that at the same pH value of reaction, the Fe/P ratio had a major effect on the content of impurity phase, crystal structure and electrochemical performance of the samples. However, it had a minor effect on the morphology of the samples. A single phase structure was obtained for the samples with the Fe/P ratio of 0.97-1.02. The sample with the Fe/P ratio of 0.97 exhibited the best electrochemical behaviors, whose specific discharge capacities could reach 152.7, 144.8 and 133.2mAhg-1at 0.2C, 1C and 5C rate, respectively, with the capacity retention rate close to 100% after 50 cycles at 5C. It is believed that the excellent electrochemical performance of specific discharge capacity, rate capability and cycling stability is attributed to the nonstoichiometry of LiFePO4, which results in the Li-rich defective crystal structure and the decrease of cell parameters, thus facilitating the discharge behaviors at high rates.