In order to improve the electrochemical performance of LiFePO4, Li0.99Nb0.01FePO4/C composite materials were synthesized with citric acid coating and Nb2O5 doping. The physical chemistry and electrochemical performances of Li0.99Nb0.01FePO4/C were investigated by X-Ray diffraction (XRD) and Transmission electron microscope (TEM). The results show that Li0.99Nb0.01FePO4/C has smooth charge-discharge voltage platform, with first capacity of 151.6 mAh•g-1 (0.1C) and no obvious capacity fading after 16 cycles. The materials have favorable high rate discharge performances, with the first capacity of 131.6 mAh•g-1 at 0.5C, 119.8 mAh•g-1 at 1.0C and 106.2 mAh•g-1 at 2.0C. Equivalent circuits analysis shows that the impedance, especially electrode reaction resistance of Li0.99Nb0.01FePO4/C are significantly reduced compared with the pure LiFePO4 and this helps to improve the electric conductivity, discharge capacity and cycle performance. The diffusion coefficient of Li+ in Li0.99Nb0.01FePO4/C is 2.51×10-12 cm2•s-1 increased two orders of magnitude in comparison with the pure LiFePO4. Carbon-coating makes a more remarkable contribution to lithium diffusion than Nb5+ ion doping.

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This paper is concerned with the problem of proportional reinsurance control strategy with dividend process in insurance company. The existence of optimal control strategy is proved by variational inequality,and the specific construct of optimal control strategy and the explicit structure of optimal return function are derived.

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