Synthesis,crystal Structure And Electrochemical Performances Of Li₃V_1-xP_xO₄ And Na₂VFe₂?PO₄?₃/C

Lithium-ion batteries?LIBs?have been permeated in portable electronics and play important roles in electric vehicles and stationary grid storage.Extensive studies have been carried out to develop new high-capacity materials for higher-energe density LIBs.As the widespread use of LIBs,the geographically constrained Li mineral reserves will drive up the prices of LIBs.Therefore,sodium-ion batteries become the alternative technology where the Na resources are nearly infinite and low cost.Thus,developing a suitable system for SIBs has been concerned.The main content of this thesis has two parts.Fisrtly,carbon-coated Li₃V₁-_xP_xO₄?0?x?1?solid solution powders were synthesized by a solid-state method.Their phase relationship and electrochemical performances as anode materials for LIBs were investigated.Secondly,a new alluaudite compound Na₂VFe₂?PO₄?₃/C was obtained via a facile sol-gel method.Its electrochemical properties were evaluated as a cathode material for SIBs.Carbon-coated Li₃V₁-_xP_xO₄?0?x?1?solid solutions were synthesized by a solid-state method.From the XRD patterns,we found that between 0?x?0.25 or 0.4?x?1 area,Li₃V₁-_xP_xO₄ present a single phase of?-or?-Li₃PO₄ structure,respectively.Moreover,mixed phase compounds were obtain when 0.25<x<0.4.Electrochemical test results indicated that Li₃V_0.95P_0.05O₄/C anode deliver⁴00 mAh g^-1 capacities after 100 cycles,which is better than that of Li₃VO₄/C.A new alluaudite-type Na₂VFe₂?PO₄?₃ with carbon composite has been prepared for the first time via a simple sol-gel method with citric acid as an assistant agent and carbon source.The crystal structure and morphology of Na₂VFe₂?PO₄?₃/C composite are well characterized by X-ray diffraction,scan electron microscope and energy-dispersive X-ray analysis.Our experimental results show that a three-dimensional[VFe₂?PO₃?₃]^2-framework in this alluaudite structure provides the large tunnels to enable the sodium-ion transporting readily which contributes to the electrochemical performance greatly.When evaluated as a cathode for sodium-ion battery,the Na₂VFe₂?PO₄?₃/C composite delivers a first specific charge capacity of66.7 mAh g^-1 and a discharge capacity of 65.8 mAh g^-1 at 5 mA g^-1 current density.After 50cycles,the charge-discharge capacity(⁶0.6 mAh g^-1)shows a good capacitance reverse?⁹2%?compared to the first cycle which exhibits a stable cycling performance.Cyclic voltammetric profiles confirm the multi-electron reactions including the Fe²⁺/Fe³⁺,V²⁺/V³⁺and V³⁺/V⁴⁺redox couples.