Preparation And Electrochemical Performance Of Na₃V₂（PO₄）₂O₂F Cathode Materials For Sodium-Ion Batteries

Sodium ion batteries（SIBs）and lithium ion batteries（LIBs）have similar working mechanisms.Compared with scarce lithium resources,sodium resources are more abundant in the earth crust,thus the price of sodium is lower and the cost of SIBs is cheaper.Therefore,SIBs are considered to be one of the competitors with huge development potential for large-scale energy storage systems in the future.However,SIBs still need to overcome many challenges in the future,especially the cathode materials,such as low energy density and power density,poor cycle stability,and poor adhesiveness of the binder,which seriously hinder the further development of SIBs in the field of energy storage.Hence,it is particularly important to design and prepare cathode materials with excellent electrochemical performance for SIBs.Among the extensively studied cathode materials for s SIBs,polyanionic phosphate cathode materials with a sodium superionic conductor（NASICON）structure have the obvious advantages of structural stability and diversity.Its crystal structure is a 3D large tunnel structure frame composed of MO₆（M usually stands for transition metal）octahedron and PO₄ tetrahedron.It has a faster Na⁺migration rate and the larger polyanion frame is favorable to structural stability and safety,which drive widespread concern among scientists.Among them,the typical cathode material with NASICON structure should be Na₃V₂（PO₄）₃（NVP）.Its relatively open 3D ion diffusion tunnel not only facilitates the migration speed of Na⁺,but also improves the stability of the crystal structure,arousing extensive research of the researchers.Though NVP cathode material has obvious advantages,its inherent low electronic conductivity limits the extraction of its full performance.In addition,the relatively low operating voltage（3.3-3.4 V）of NVP results in an energy density of approximately 394 Wh kg^-1,which is much lower than commercial LIBs cathode material(for example,LiFePO₄ cathode material has an energy density of nearly 500-530 Wh kg^-1),which severely limits the practical application of NVP cathode for SIBs.Based on the above shortcomings of NVP,this paper successfully prepared Na₃V₂（PO₄）O₂F（NVPOF）with high voltage,high capacity and high energy density by using F^-anion with strong electronegativity to regulate the anion site of NVP cathode material.Simultaneously,the morphology,electrochemical performance and the binder of prepared NVPOF cathode material were optimized.The specific research content and results are as follows:（1）Regulate the anion sites of NVP materials.By using the F^-anion with strong electronegativity to regulate the anion site of NVP,some of the high ionic V-F bonds replace the covalent V-O bonds.The weakening of the covalentity of the V-O bonds leads to a higher redox potential during the Na⁺de/intercalation process,which improves the working voltage and the discharge specific capacity of the material and enhanced the energy density of the battery.By using a relatively simple hydrothermal synthesis method and precisely adjusting the pH value and hydrothermal temperature during the synthesis process,nano-level carbon-free NVPOF cathode materials were successfully prepared.Then the material was applied to SIBs system,and its electrochemical performance and electrode kinetic reaction mechanism were thoroughly studied.The results show that the optimal synthesis condition is the pH=7,the hydrothermal temperature is 170℃,and the hydrothermal time is 12 hours（h）.The prepared NVPOF is one dimensional rod-shaped nanoblock with a length of about 600 nm and a width of 300 nm and uniform height.The electrochemical properties of NVPOF prepared under different conditions were tested.The results show that the NVPOF prepared under the optimal conditions has the most excellent sodium storage performance.The NVPOF prepared under the optimal conditions as cathode material for sodium ion batteries has a discharge specific capacity of up to123.2 mA h g^-11 and the average voltage of 3.8 V.Meanwhile,the kinetics of NVPOF prepared under different synthesis conditions were deeply explored.The NVPOF cathode material with high voltage,high capacity and high energy density is obtained by F^-anion with strong electronegativity to regulate the anion site of NVP,which lays the foundation for the practical application of the material.The design of regulating the anion site of the compounds can also provide effective ideas for improving the working voltage and energy density of other secondary battery cathode material systems.（2）In order to further optimize the electrochemical performance of the prepared NVPOF cathode material,the choice of binders in the preparation of electrode materials was investigated.By comparing the aqueous binder-sodium alginate（SA）,carboxymethylcellulose sodium（CMC）,acrylonitrile multi-copolymer（LA133）and the organic binder-poly（vinylidene fluoride）（PVDF）,a highly effective aqueous binder SA that is beneficial for the performance of NVPOF cathode materials was obtained,and the Na⁺migration kinetics and mechanism were studied in depth.The results show that the cathode electrolyte interphase interface（CEI）layer formed by SA on the surface of NVPOF cathode is advantageous to stabilize the electrode structure and improve the electrode dynamics.In addition,we also assembled the NVPOF electrode prepared by SA into full battery,and verified the practicality of SA as a binder for cathode material of sodium ion batteries.This can provide a new way of thinking for improving the performance of other cathode materials.