Synthesis And Electrochemical Performance Study Of The Core-shell M₃V₂（PO₄）₃/C （M=Li,Na） Nanocomposite

In recent years, the increasing consumption of fossil fuel drives people to look for new alternative energy resources at an unprecedented urgency. Lithium ion batteries have attracted widely attentions for its high energy densities and excellent cycle performance. Li3V2(PO4)3has been considered to be an promising cathode material for lithium-ion batteries for its robust structure, high theory capacity density and high, operating volatage, howerer, the low electronic conduction significant limited its electrochemical performance.In this thesis, Li3V2(PO4)3@C core-shell nanoparticles with typical sizes of20-40nm were synthesized by a hydrothermal-assisted sol-gel method. Ascorbic acid and PEG-400were adopted as carbon sources and reductants. The obtained uniform Li3V2(PO4)3@C nanocomposite was composed of a Li3V2(PO4)3core with high phase purity and a graphitized carbon shell, as characterized by XRD, SEM, TEM, and Raman analysis. The nanocomposite exhibited remarkable rate capability and cyclability, delivering a discharge capacity of138mA h g-1at5C in the voltage range of3-4.8V and a capacity retention of86%after1000cycles. The superior performance of Li3V2(PO4)3@C indicates its promising application as a cathode material for advanced rechargeable lithium-ion batteries.Na3V2(PO4)3is an attractive cathode material for sodium ion batteries due to its high theoretical energy density and stable three-dimensional (3D) NACISON structure. As an extension of our hydrothermal assisted sol-gel method, Na3V2(PO4)3@C core-shell nanocomposite has been synthesized through a similar method. Ascorbic acid and PEG-400were synergistically used to limit the particle growth and give the conductive carbon coating. As a result, the as-prepared material was composed of a nanosized Na3V?(PO4)3core with typical size of-37nm and a uniformly coated carbon shell, showing durable cycle life and remarkable rate capability as cathode for sodium ion batteries. The nanocomposite rendered an initial capacity of94.9mAh g-1at5C and a remarkable capacity retention of96.1%after700cycles. Moreover, a full cell using the as-prepared composite as both the cathode and anode active material has been successfully built, showing an average operation voltage of1.7V with considerable cyclability. The excellent electrode performance is attributed to the combination of particle downsizing and carbon coating that can favor the migration of both electrons and ions.The synthetic strategy in this thesis provides a universal method for the synthesis of both Li3V2(PO4)3and Na3V2(PO4)3, and less limited by the material to be prepared, it may be easy to extend to the design of other uniformly nanosized core-shell materials for electrodes with a high-rate requirement.