Synthesis And Electrochemical Properties Of Micro-nano Structure LiFePO₄ By Hydrothermal Method For Lithium Ion Batteries

Due to its high specific surface area,hierarchical LiFePO₄ can not only shorten the Li⁺ diffusion distance,but also prevent the particles aggragtion,and it is considered to be the most important cathode material for power industries,energy storage and profession development.Therefore,the preparation and the research on this kind of material have attracted wide attention of the researchers.Some studies have shown that the electrochemical performance of Li Fe PO 4 is closely related to its crystal structure,morphology and grain size.Based on this,a simple solvothermal method combined with carbon coating is attempted to prepared the layer structure and the dumbbell-like micro-nano LiFePO₄ to improve its electrochemical properties in this paper.The main results are as follows:The micro-nano layer structure LiFePO₄ was successfully synthesized by solvothermal method in a short time without any additive,using FeSO₄·7H₂O and Li H₂PO₄ as raw material.Based on the single factor experiment of the time,we proosed the growth mechanism of the micro-nano layer structure LiFePO₄.The results indicate that the layered micro-nano LiFePO₄ with high crystallinity and dispersion were obtained by solvothermal with EG/DMF as the solvent at 225 ℃ for 3.5 h.The average size of these layered LiFePO₄ were about 3-8 μm,and they composed of nanoplates,whose size was about 20-30 nm.The charge-discharge test showed that the initial discharge capacity is 112 m Ah/g at 0.1C rate,and the capacity retention was 94% after 50 cycles.The micro-nano dumbbell-like LiFePO₄ which self-assembled by nanords was successfully synthesized by solvothermal method in a short time with ascorbic acid as reducing agent and EG as a solvent.The nanords exposed（212）face with the diameter was about 50 nm.And the dumbbell-like LiFePO₄ were about 10 μm in length.On this basis the further studies show that the Li3PO4 and Fe3（PO4）2 precipitation will generate at first,and then transform into Li Fe PO 4 in the process of forming the micro-nano dumbbell-like LiFePO₄.The morphology will also undergo three processes: rapid nucleation,reunion and orientated growth.The electrochemical testing results show that the discharge specific capacities are 145.8,136.2,118 and 95.6 m Ah/g at 0.1C,0.2C,0.5C and 1C,respectively.We used glucose as carbon source to coat the layered and the dumbbell-like LiFePO₄ by ex-situ carbon coating.And the carbon coated samples were further characterized.The results showed that the discharge specific capacities of these two samples both increaseed compared with the pure LiFePO₄.However,the increase of the layered LiFePO₄ was not obvious,because the gap between layers ha d been filled carbon,which causing it lost the micro-nano structure.While the discharge specific capacities of the dumbbell-like sample given a large increase.Then we researched the influence of in-situ carbon coating and ex-situ carbon coating on the material.The results indicated that the discharge specific capacity of the LiFePO₄/C sample prepared by in-situ carbon coating decreased to 115.1 m Ah/g due to the Li 3PO4 impurity.While when we the prepared the sample by ex-situ carbon coating,the surface of LiFePO₄ was coated by a carbon layer with the thickness 2 nm,and the discharge specific capacity of this sample reached to 158 m Ah/g at 0.1C.