Investigation On Electrochemical Sodium Storage Performance Of Beta-VOPO₄/NaVOPO₄

Sodium-ion batteries?SIBs?show similar working mechanism as lithium-ion batteries?LIBs?,but they are superior to LIBs in low cost,high energy density and sufficient reserves of sodium-based minerals.Therefore,SIBs represent a promising alternative to lithium-ion batteries for the next generation energy storage devices.A critical challenge in SIBs is the poor practical performance of the cathodes.Among various cathode candidates,polyanionic vanadium phosphate?beta-VOPO₄ and beta-NaVOPO₄?has high working voltage?3.4 V vs.Na/Na⁺?,good cyclic stability and thermal stability,but its ionic conductivity and electronic conductivity is problematic,which hinder the practical capacities.Therefore,we have conducted research works to improve the conductivity and hence the comprehensive performance such as the capacities,cycling life and rate capabilities.Firstly,the conductivities of beta-VOPO₄ and beta-NaVOPO₄ were studied.Particle sizes were reduced and conductivities were enhanced by a simple ball-milling strategy.XRD,SEM,XPS and Raman were used to characterize the structure,valence and morphology of the samples after ball-milling and carbon compositing.The particle size was significantly reduced.Then the electrochemical properties of the samples were studied.The capacity,impedance and diffusion coefficient of sodium ions were improved.The optimum conditions were selected as follows:carbon content was 2 wt%or 5 wt%,and ball-milling time was 10 h.The long-term performance and rate performance were studied at room temperature and high temperature.Secondly,conductive polymer coated beta-VOPO₄.XRD confirmed that poly?3,4-ethylene dioxy thiophene??PEDOT?coating had no effect on the structure.Its electrochemical properties were tested.The appropriate PEDOT content was selected to be 10wt%.FTIR and TGA were used to characterize the successful coating and the content of the coating.Then the electrochemical properties were studied at room temperature and high temperature.Finally,the behavior that beta-VOPO₄/C and beta-NaVOPO₄/C electrochemically intercalated with the second Na⁺was studied to improve the theoretical capacity of the electrode.The electrochemical performance of the electrode was tested at 4.3-1.0 V.The actual discharge capacities of the first cycle were 280 mAh g^-11 and 317 mAh g^-1,respectively.The voltage platform of the second sodium ion intercalation was 1.29 V.Through GITT,XRD,XPS,FTIR and elemental maping,we can reach the following conclusion that the reaction of second sodium insertion is a two-phase process.When the discharge voltage reaches 1.0 V,diffraction peak are shifted completely,a large number of V³⁺are produced,V=O disappears,and the chemical formula of the product is Na₂VOPO₄.