The Structure Design Of LiFePO₄ Cathode Materials With Fe₂P Introduced For Li-ion Battery And The High-rate Capability

On the basis of an overall review on the research advances of the LiFePO4 cathode materials, and for the disadvantage of the low high-rate capability of the LiFePO4 materials and the shortcomings of the reduction in tap density and commonly hence the decease in volume capacity caused by the commonly used methods of carbon coating and particle size reduction of LiFePO4 particles, the present work puts forward to improve the high-rate capability of LiFePO4 almost without carbon or with low carbon content by means of in suit introducing a high electron conductive phase of Fe2P. The LiFePO4 particle size is designed to be sub-micron. Compared with the LiFePO4 materials with carbon coating and nano particle size, the structure of the present LiFePO4 materials possesses higher tap density, and also favors the operation of the cathode fabrication. A solid state method is used for the synthesis of the LiFePO4 materials. This method is simple and provide high yield.LiaCO3, FeC2O4 and NH4H2PO4 were used as the raw materials for the synthesis of LiFePO4 materials. The effect of the synthesis parameters. including the calcination temperature, time, atmosphere, on the formation of Fe2P phase, the particle size and the crystallinity of LiFePO4 was studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, etc. The electrochemical properties of the LiFePO4 materials were tested by galvanostatical cycle, electrochemical impedance spectra and cyclic voltammetry, etc. The key factors which influence the electrochemical performane of LiFePO4 materials, especially high-rate capability, have been investigated. The investigation also aims to reveal the role of Fe2P to the capacity of LiFePO4 at different discharge rates, to obtain the optimized content of Fe2P in improving the high-rate capability of the LiFePO4 materials and the effect of the structure factors of the LiFePO4 materials on the electronic kinetics as well as the effect mechanism. Besides, Carbon coating and Fe2P phase were simultaneously introduced in order to further improve the high-rate capability.In the case of no carbon source was added, the particles of the product grew and the content of Fe2P was increased with the temperature increasing from 600℃to 800℃. The product calcined at 700℃for 10 hours in the atmosphere of N2+5vol%H2 has sub-miscron particle size (ca.500 nm), and contains 3.7 wt.% Fe2P and trace carbon, providing the best high-rate capability compared with the others. The capacities are 110,100 and 85 mAh/g at the discharge rates of 5,10 and 15C, respectively. It is further found that, the calcination atmospheres of N2+H2 and Ar+H2 with the same 5 vol%H2 performed different influence on the formation of Fe2P. The content of Fe2P in the LiFePO4 materials calcined in N2+H2 is somewhat higher than that of those calcined in Ar+H2, while the particle size and the crystallinity are very close. This slight higher content of Fe2P caused a higher high-rate capability for the LiFePO4 material calcined in the N2+5vol%H2 atmosphere. However, Fe2P is electrochemical inert, and the formation of Fe2P usually caused the formation of other impurities, such as Li3PO4, which result in the decrease of the LiFePO4 content in the product, and consequently lower the theoretical capacity. In the present work, the LiFePO4 materials with different amount of Fe2P were synthesized by varying the calcination time further. Electrochemical study shows that the LiFePO4 material containing 3-4 wt.%Fe2P possesses a better high-rate capability than those with either higher or lower Fe2P contents. Moreover, carbon coated LiFePO4/C materials was synthesized by using PVDF (polyvinylidene fluoride) as the carbon source, and Fe2P was simultaneously introduced in order to further improve the high-rate capability. It is found that the coexistence of the carbon coating and Fe2P can improve strongly the high-rate capability of the LiFePO4 material, even the amount of each is low. The LiFePO4 material with 2.0 wt.% carbon and 2.2 wt.% Fe2P achieves specific discharge capacities of 127,113 and 104 mAh/g at the discharge rates of 5.10 and 15C. respectively, which are all higher than the LiFePO4 materials with comparable single carbon or Fe2P, and even higher Fe2P contents.The investigation of the electrochemical properties of the LiFePO4 materials with different amount of Fe2P, different particle size and crystallinity show that, the higher purity, the better crystallinity and the smaller particle size of LiFePO4 is important for the LiFePO4 material getting high capacity at the low discharge rate, and the action of Fe2P is less important. However, at the high discharge rate, Fe2P is a key factor for the LiFePO4 material providing superior capacity, especially in the case that the LiFePO4 material has submicron particle size and almost without carbon. For the LiFePO4 material with comparable particle size and without or with insufficient Fe2P, the high-rate capability is considerably reduced. The present work provides a new route in fabricating LiFePO4 materials with superior high-rate capacity, high tap density and high volume capacity.