Synthesis Of Porous Vanadium Potassium Phosphate/Carbon Cathode Materials For Potassium Ion Battery By Citric Acid Assisted Sol-Gel Method

Potassium-ion batteries are potential low-cost energy storage batteries.K₃V₂（PO₄）₃ with stable and open NASICON structure is a promising candidate cathode material for potassium-ion batteries due to its igh working voltage and good thermal stability.However,K₃V₂（PO₄）₃suffers form its unclear crystal structure and low actual specific capacity.Therefore,various methods（e.g.the first-principle calculation,Rievelt refinement,synchrotron radiation absorption spectroscopy（EXAFS）,etc.）were applied to determine the crystal structure of K₃V₂（PO₄）₃.The results of Rievelt refinement and EXAFS show that K₃V₂（PO₄）₃ is an orthogonal structure with Pnna space group.According to the results of the first-principle calculation,the band gap of K₃V₂（PO₄）₃ is 1.1 e V,and the diffusion barrier of K⁺in K₃V₂（PO₄）₃ is 0.501 e V.On this basis,a citric acid-assisted sol-gel method was developed to prepare K₃V₂（PO₄）₃/C.The effects of calcination temperature and calcination time on the structure and the electrochemical performance of the prepared K₃V₂（PO₄）₃/C were investigated.Fisrtly,the cell volume and grain size of the prepared sample gradually increase with the increase of calcination temperature,while the strain gradually decreases.Secondly,the unit cell volume of the prepared samples gradually increases with the extension of calcination time,while the grain size and strain first decrease and then increase.Thirdly,the specific surface area,specific discharge capacity,charge transfer impedance,and potassium ion diffusion coefficient first increase and then decrease with the increase of calcination temperature and calcination time.K₃V₂（PO₄）₃/C prepared by calcination at 800℃for 12h is a three-dimensional porous strucutre composed of carbon-coated K₃V₂（PO₄）₃ nanoparticles and abundant mesopores.This sample has a higher initial capacity（96 m Ah/g at 20m A/g）and better rate performance（47m Ah/g at 200m A/g and 37 m Ah/g at 400m A/g）.The enhanced performance of K₃V₂（PO₄）₃/C can be attributed to its three-dimensional porous conductive structure,which can improve the conductivity,promotes the penetration of electrolyte,reduce the electrode polarization,accelerate the diffusion kinetics of potassium ion,and enhance the stability of electrode structure.The results on the structure of K₃V₂（PO₄）₃ provied here can provide basic data for further investigateing its structure and performance.This strategy of synthesizing three-dimensional porous conductive structure provides an insight into the synthesis of high-performance K₃V₂（PO₄）₃.