| As one of the main cathode materials for the lithium-ion power battery, the lithium iron phosphate (LiFePO4) has been a focus for researchers since it was reported. However, its low electronic and ionic conductivities are always the obstacle to the large-scale application. To solve this problem, this paper proposes a new method to modify LiFePO4. That is coating a layer of Li3V2(PO4)3cathode material on the surface of LiFePO4particles and using the fast ion conductor feature of Li3V2(PO4)3to optimize the surface properties of LiFePO4, which can result in the improvement of its electrochemical performances. XRD, SEM, TEM, EDS, galvanostatic current charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to investigate the microstructure and electrochemical performances, and the effects of coating layer on the physical properties and electrochemical performances of LiFePO4are also studied.Firstly, the monoclinic Li3V2(PO4)3/C is successfully synthesized by the sol-gel method, and the electrochemical performances of the sample under the optimal synthesis conditions (calcination temperature750℃, calcination time4h) is excellent. When the charge-discharge voltage range is2.5-4.5V, the initial discharge capacities of the sample at0.1C,5C, and10C rates are141.0,117.1and110.3mAh·g-1. The capacity retentions at5C and10C are88.0%and77.8%after140cycles.The Li3V2(PO4)3/C is also prepared by spray drying. Compared with that synthesized by sol-gel, the sample possesses better performance. It consists of regular, spherical particles with sub-micron or micron size. Its tap density is up to1.61g·cm-3, which is much higher than that by sol-gel (0.98g·cm-3), and the carbon coating layer on the particle surface is more uniform. Between2.5V and4.5V, the first discharge capacities of Li3V2(PO4)3/C at0.1C,5C, and10Care161.3,127.6and111.6mAh·g-1. The capacity retentions are90.5%and88.1%at5C and10C after200cycles. Though the voltage platform of the third Li+in Li3V2(PO4)3is about4.55V (vs. Li+/Li), it is confirmed by the charge-discharge tests and cyclic voltammetry that some can be reversibly extracted/inserted in the4.3-4.5V rang. In addition, the diffusion coefficient of Li3V2(PO4)3is in the order of10-10-10-11cm2·s-1measured using the CV method. The EIS tests show that the charge transfer impedance Rct decreases with the increase of the external voltage.On the basis of the previous study, the core-shell structure xLiFePO4/C@Li3V2(PO4)3/C is successfully synthesized by sol-gel method. In the optimized synthetic conditions (pH value of the sol is4, the calcination temperature is750℃, calcination time is4h, the LiFePO4and Li3V2(PO4)3molar ratio is9:1, the mole of citric acid is equivalent to that of vanadium), the composite9LiFePO4/C@Li3V2(PO4)3/C has much better electrochemical performances than the pristine LiFePO4/At0.1C,5C and10C, the first discharge capacities of the pristine LiFePO4/C are160.5,100.6and40.6mAh·g-1, while, the coated LiFePO4/C discharge capacities are up to162.2,134.8and105.0mAh·g-1. After190cycles, the capacity retentions of the pristine LiFePO4/C are88.3%and72.5%at1C and5C, whereas, the coated LiFePO4/C has no capacity fade at1C and capacity retention of93.8%at5C, and its discharge capacity at190th cycle can still reach99.3mAh·g-1at10C. In addition, the surface of LiFePO4/C core is actually covered by two homogeneous layers——the inner of fast ion conductor Li3V2(PO4)3and the outer of amorphous carbon, and some V3+and Fe2+ions enter into LiFePO4and Li3V2(PO4)3, respectively. The cyclic voltammetry analysis shows that the potential difference between redox peaks of the coated LiFePO4(0.220V) is distinctly less than that of the pristine one (0.664V). AC impedance measurements indicate that the lithium ion diffusion coefficient D and exchange current density i0of the modified LiFePO4are both increased by one order of magnitude.In addition, we also prepare9LiFePO4/C@Li3V2(PO4)3/C using spray drying and mechanical activation method to coat Li3V2(PO4)3/C on LiFePO4/C surface, and analyze the impacts of the coating layer on LiFePO4/C structure and electrochemical behavior. Consistent with the sol-gel method results, the electrochemical performances of the composites synthesized by these two methods are significantly superior to those of uncoated LiFePO4/C. |
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