Synthesis And Performance Research Of Sodium Vanadium Fluorophosphate Cathode Material For Sodium Ion Batteries

In order to solve the problems of lack of raw material and high cost of lithium ion battery,the researchers constantly trying to find one kind of battery as the ideal substitutes for lithium ion batter y.However,sodium-ion batteries have attracted wide attention due to its abundant raw materials,low price and similar working principle to lithium-ion batteries in recent years.In this paper,we select the cathode material Na₃V₂（PO₄）₂F₃of sodium-ion battery as the research object and explore the comprehensive influence of ideal carbon source,the proportion of Na F and VPO ₄,and modification on it.The modification methods include Na₃V₂（PO₄）₃ coated on Na₃V₂（PO₄）₂F₃/C surface and using plasma enhanced chemical vapor deposition（PECVD）to doped the coated carbon layer with nitrogen,so as to enhance the structural stability of Na₃V₂（PO₄）₂F₃/C sample and improve the electrochemical performanc.The main results are given as follows:（1）The cathode material Na₃V₂（PO₄）₂F₃/C of sodium ion battery was synthesized by sol-gel two-step method.The results show that the synthesized material is pure phase Na₃V₂（PO₄）₂F₃ when PVDF is used as the carbon source,which is because it can inhibit the volatilization of fluorine and supplement the loss of fluorine.However,glucose and sucrose are used as carbon sources,the impurity phase of Na₃V₂（PO₄）₃ appeared in the synthesized Na₃V₂（PO₄）₂F₃ material.In addition,pure phase Na₃V₂（PO₄）₂F₃ can be synthesized by two-step method when Na F in different proportions reacts with precursor VPO₄.When the ratio of Na F to VPO₄ is 1.5:1 and PVDF is used as the carbon source,the material has the excellent electrochemical performance.It s discharge capacity reaches 104.2 m Ah?g^-1 at a high rate of 5 C.And the capacity retention rate of the sample p-NVPF reaching 86.4%after 100 cycles at 1 C,which is also the optimum among three samples.（2）Surface modification of Na₃V₂（PO₄）₂F₃/C coated with Na₃V₂（PO₄）₃.the prepared Na₃V₂（PO₄）₃ by the sol-gel method was weighed and added into the synthesized Na₃V₂（PO₄）₂F₃/C material in a certain proportion.The results show that the Na₃V₂（PO₄）₂F₃ shows the best the electrochemical performance when the coating mass of Na₃V₂（PO₄）₃ is 1%of Na₃V₂（PO₄）₂F₃/C,the discharge capacity of NVPF-1%sample is 107.9 m Ah·g^-1 at a high rate of 5 C.The capacity retention rate of uncoated sample is 86.4%after 100 cycles at 1 C,while the capacity retention rate of the sample after coated with Na₃V₂（PO₄）₃ has been greatly improved.In particular,the capacity retention rate of t he NVPF-1%sample reached 98.5%and the discharge capacity hardly decay after 100 cycles.This is due to the Na₃V₂（PO₄）₃ possess the special fast ion conductor structure and relatively low ion conductivity activation energy,which can improve the transport efficiency of sodium ions,thus enhancing the electrochemical performance of the material.（3）The PECVD method is used to achieve the nitrogen-doped coating carbon layer,thereby improving the structural stability and electrochemical performance of the cathode material Na₃V₂（PO₄）₂F₃/C of the sodium ion battery.Nitrogen gas is used as a gas source and a chemical active plasma is formed locally by microwave or radio frequency ionization,which is easy to react with the coated carbon layer of the sample in the PECVD furnace.The results found that the sample NVPF@N-3 shows the best rate and cycle performance when the working power is 50 W and the gas flow rate is 50 ml·min^-1.And the discharge specific capacity is 102.6 m Ah·g^-1 at a high rate of 10 C.The capacity retention rate after 100 cycles of charge and discharge at 1 C is 96.5%,and the cycle retention rate is 91.4%after 1,000 cycles at10 C.The results show that after the modification of the coated carbon layer of Na₃V₂（PO₄）₂F₃/C with nitrogen-doped by PECVD method,the crystal structure of the material can be stabilized and the electrochemical performance of the material can be improved.