Synthesis And Characterizations Of Li₃V₂(PO₄)₃ And Na₂FePO₄F Polyanion Cathode Materials

Great efforts have been done on lithium-ion batteries since SONY introduced the first commercial lithium-ion battery in 1991. In the early stage, cathode materials for lithium-ion batteries mostly used transition metal oxides, such as LiCoO2, LiNiO2 and LiMn2O4. In 1997, Goodenough proposed polyanion LiFePO4 as a cathode material. Since then, considerable studies have been performed to varies transition metal polyanion materials, such as LiMnPO4, Li3V2(PO4)3, LiVPO4F, Li2FeSiO4 and so on. Among these materials, Li3V2(PO4)3 has shown great potential because of its high lithium ion mobility, large energy density and high specific capacity. However, the intrinsic low electronic conductivity of Li3V2(PO4)3 is also a big obstacle for its practical application. Therefore, the first part of this work is devoted to improve the electronic conductivity of Li3V2(PO4)3 for better electrochemical performances.Firstly, we successfully prepared Li3V2(PO4)3 using a PVA assisted sol-gel method. The decomposition process of the precursor was discussed via TG analysis, based on which we determined the proper synthesis temperature of Li3V2(PO4)3. We studied the structure properties of the material using varies techniques including XRD, FTIR, Raman, SEM and TEM. Then we studied the electrochemical properties of the material by charge-discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is shown that the material exhibited good cycling performance. In particular, Li3V2(PO4)3 showed much larger lithium ion diffusivities than those of LiFePO4, which was attributed to its special Nasicon type structure and unique phase transition behaviors.Then, we prepared Li3V2(PO4)3 using two kinds of solid-state reaction method, carbothermal reduction (CTR) and H2 reduction. Residual carbon was confirmed for the material prepared by CTR method. This residual carbon did not change the monoclinic structure of Li3V2(PO4)3. SEM and TEM analysis showed that the material prepared by CTR method had smaller particle size and higher electronic conductivity than that prepared by H2 reduction method. The surface carbon layer prevented the formation of SEI film and the resolution of vanadium into electrolyte. Based on these, the carbon coated material exhibited superior electrochemical performance to that of un-coated conterpart.Another attempt to improve the electronic conductivity of Li3V2(PO4)3 was to prepare Li3V2(PO4)3/Cu composite cathode material. XRD and XPS analysis showed that Cu did not change the monoclinic structure of Li3V2(PO4)3. The electronic conductivity of the material was indeed enhancing by Cu adding. The Li3V2(PO4)3/Cu composite cathode material showed improved electrochemical performance with respect to Li3V2(PO4)3.Like LiFePO4, Li3V2(PO4)3 is a typical lithium-based cathode material. Recent studies have shown that some sodium-based materials such as NaVPO4F, Na3V2(PO4)2F3 and Na2FePO4F are also potential cathode materials for lithium ion batteries. This makes it possible to select cathode materials in a much wider field, which will reduce the overlean on lithium resources. Among these sodium-based cathode materials, Na2FePO4F has attracted significant interests due to its good electrochemical performance. However, as other polyanion cathode materials, the poor electronic conductivity of Na2FePO4F also hinders its electrochemical performance. Besides this, there are also many other problems need to be resolved for this material, such as its poor rate performance and the understanding of its electrochemical reaction mechanism. The latter part of this paper was therefore focused on these problems.We successfully prepared carbon coated Na2FePO4F cathode material using a simple solid-state reaction method. Then we studied the structure properties of the material by XRD and Raman scattering. TEM showed that the material has a nano size and core-shell structure, with the core was Na2FePO4F and the shell was a surface carbon layer. The materials had a high electronic conductivity about 1.5×10-3 Scm-1. The electrochemical reaction mechanism was analyzed combining with different techniques including elemental analysis, CV and EIS. It is shown that the electrochemical mechanism of Na2FePO4F is evolved in the transformation from Na+ extraction to Li+/Na+ hybrid ion insertion and then to Li+ insertion. The material showed good rate performance, which exhibited a reversible capacity of 90 mAhg-1 at 1 C rate. Finaly, we studied the electrochemkical kinetics of Na2FePO4F using EIS and PITT. It is observed that the material showed large Li+ diffusion coefficients because of its unique"solid-solution-like"transition behavior and 3D diffusion parthways. This is very helpful for the material to obtain good high rate performance.On all accounts, this work gives us a comprehensive understanding on the preparation of Li3V2(PO4)3 and Na2FePO4F polyanion cathode materials, as well as their structural and electrochemical properties.