Controllable Preparation Of Sodium Vanadium Fluorophosphate Cathode Materials By Microwave Hydrothermal Method And Studies On The Properties

Sodium-ion batteries（SIBs）have been considered to be one of the energy storage devices most likely to replace the lithium-ion batteries（LIBs）because of the advantages of abundant resources and low cost of sodium.The electrode material has a great influence on the battery performance.The cathode material occupies the most important position.The quality of the cathode material directly determines the performance index of the final sodium ion battery.Therefore,the research on the cathode material is the key to developing high performance SIBs,which is also the focus and hotspot of current research.Recently,Na₃V₂（PO₄）O₂F with NASICON structure has attracted much interest in virtue of stable structure,high average voltage high theoretical capacity and theoretical energy density.Also it has high ion conductivity However,the poor electronic conductivity of Na₃V₂（PO₄）O₂F limits the increase of rate performance.In order to solve these problems,Na₃V₂（PO₄）O₂F micro-nano structures have been synthesized by hydrothermal method followed metal ion doping and carbon coating which alter the intrinsic structure and increase the electronic conductivity of electrode materials respectively.Therefore,it has stablished effective relationship between structure and electrochemical performance.The specific works are as follows:1.Composite materials of pure Na₃V₂（PO₄）O₂F,composite of graphene and Na₃V₂（PO₄）O₂F were prepared by microwave hydrothermal method.Various characterizations showed that Na₃V₂（PO₄）O₂F was successfully prepared by microwave hydrothermal method,and the addition of graphene did not affect the structure of the prepared products.At the current density of 1C,the capacity of pure Na₃V₂（PO₄）O₂F is 68 mAh g^-1,while the capacity of composite graphene can reach 127 mAh g^-1.Even at the high rate of 20C,the reversible capacity of 100.5mAh g^-1 can be maintained.2.On the basis of microwave hydrothermal synthesis of Na₃V₂（PO₄）O₂F,in order to improve the poor electronic conductivity,the bulk doping of V-site in Na₃V₂（PO₄）O₂F with different doses of Mn was studied in this paper.It was found that the electrochemical performance of Na₃V_2-xMn_x（PO₄）O₂F（x=0.02,0.04,0.06）series compounds obtained after doping was the best when x=0.04,which could be maintained at 1C multiple.The reversible cycle capacity of80 mAhg^-1 is significantly higher than that of Na₃V₂（PO₄）O₂F with x=0.02 and 0.06 manganese doping ratios of⁶0 mAh g^-1 and⁴0 mAh g^-1,and the rate performance has been significantly improved.At the current density up to 20C,the discharge specific capacity of 37.3 mAh g^-1 can be maintained.3.NaV₃（PO₄）₃/C negative electrode materials and graphene and Na₃V₂（PO₄）O₂F composites were prepared by sol-gel method.NaV₃（PO₄）₃/C semi-battery undergoes 300 charge-discharge cycles with a specific discharge capacity of 185.2 mAh g^-1 at a current density of 100 mA g^-1.The working voltage of the battery ranges from 1.5 to 3 V,the medium discharge voltage is 2.3V,the recyclable capacity is about 50 mAh g^-1,the cycle stability is better,and the specific capacity is 34 mAh g^-1 at high current density of 1000 mA g^-1.