Preparation And Modification Of Na₃V₂（PO₄）₃ And Research Of Its Sodium Storage Properties

With the excessive consumption of fossil fuel,as well as the worsened environmental problems,the demand for new energy storage devices is growing.Due to the wide distribution and large reserves of sodium resources,sodium ion batteries（SIBs）are expected to be effective alternative to lithium ion batteries（LIBs）.However,the larger radius of Na⁺results in a slow kinetic process.This problem can be alleviated by designing and synthesizing electrode materials with special structures.Na₃V₂（PO₄）₃（NVP）is considered as a promising cathode material due to its unique structure of NASICON,high ionic conductivity and high operating voltage（3.4 V vs.Na/Na⁺）.However,two major problems of poor electronic conductivity and serious volume expansion during charge/discharge process need to be solved before it can be commercialized.Focusing on these two problems,this thesis modified NVP by carbon coating and ion doping,thus obtained a composite cathode material with superior sodium storage performance.The specific research contents are as follows:（1）N-butanol was employed as an important co-solvent to synthesize carbon-coated NVP（b NVP）with high graphitization degree of the carbon layer,to improve electronic conductivity.In addition,the C layer effectively protects the structure of NVP and improves the mechanical stability of the material.The test of half battery shows that b NVP has better sodium storage performance compared to NVP synthesized without butanol.It exhibits high capacity of 110.3 m Ah·g^-1 at 1 C.And after 3500 cycles at 30 C,the capacity retention is 94%.Even after 3,800 cycles at 50 C,the b NVP can still obtain a highly reversible capacity of 64 m Ah·g^-1 with a capacity retention of 92%.The electrochemical performance of the full cell（b NVP||Ni S₂/Fe S）shows the potential of the as-synthesized b NVP for practical application.（2）In order to further improve the high-rate performance of NVP,foreign atom doping was introduced to the carbon-coated NVP.B-doped NVP@C（NVP@CB-X）with high electrical conductivity was prepared by electrostatic spinning.One-dimensional nanofiber structure can effectively shorten the ion/electron diffusion path and further improve the electrical conductivity of the material.In addition,the mechanisms of different B configurations to improve the performance of sodium storage are identified by DFT calculations:Compared with pure carbon layer,the material doped with B has higher electrical conductivity;and the BC₂O mainly reduces the diffusion barrier of sodium,thus improving the kinetic performance;while the BCO₂ enhances the structural stability of the material to improve the cycling stability.Only with a balanced content of BCO₂ and BC₂O（BC₂O:BCO₂=1.5:1）,the material exhibits the best sodium storage properties with a high-rate performance of 94.2 m Ah·g^-1 at 30 C,a long cycling stability of 103 m Ah·g^-1 at 10 C and 91 m Ah·g^-1 retained after 1500 cycles,with only 0.08‰loss per cycle.