Preparation And Performance Study Of Sodium Vanadium Phosphate Fil

As a polyanionic material,Na₃V₂（PO₄）₃ has inferior electronic conductivity,making it poor cycling stability.Research work such as particle nanosizing and morphology control has been done to improve the electrochemical properties.However,the research is based on the modification of materials,and the prepared electrode materials usually need to be mixed with conductive agents and binders to prepare electrode.The production process is cumbersome,and due to the addition of other substances,the electrode layer structure is not dense,resulting in large contact impedance.Thin film electrode has the advantages of binder-free,the thinner and more compact electrode layer,and flexibility,etc.,and the thin film electrode made of Na₃V₂（PO₄）₃ material will fully utilize the properties of sodium superionic conductor.But most of the preparation methods of thin film electrode are costly and complicated.So,it is of great meaning to explore a low-cost and maneuverable method on the preparation of film electrode.In this paper,Na₃V₂（PO₄）₃ film electrode was prepared for the first time by Sol-gel and spin-coating method.The effects of metal foil,silicon slice,stainless steel sheet,iron sheet,titanium sheet and graphite disc on the morphological structure of Na₃V₂（PO₄）₃ films were also investigated,and it is found that the film prepared on graphite disc has intact structure and optimal morphology.By exploring different solidifying and annealing processes,it is shown that different solidification temperatures will affect the purity and crystallinity of the films.Annealing temperature affects the crystallinity of the material.Based on the experimental results,the optimal film morphology structure can be obtained by solidifying at 80°C,annealing at 350°C and 750°C for one hour each.Na₃V₂（PO₄）₃ film electrodes with different thicknesses were also prepared by Sol-gel and spin-coating method,and porous thin film electrodes were obtained.Electrochemical tests show that the specific capacity of the thinnest electrode（NVP/C-1）is as high as 167 m A h g^-1 at 1 C,which exceeds the theoretical capacity of Na₃V₂（PO₄）₃(117 m A h g^-1).It has been experimentally verified that the specific capacity of the electrode is the superposition of the capacity contributed by the graphite substrate.The capacity retention is 87.9%for 5000 cycles at 10 C,but the specific capacity of the charge/discharge platform decreases from 51 m A h g^-1 to 21 m A h g^-1with a capacity retention of only 41.2%.The thickest electrode（NVP/C-4）achieves93.8%capacity retention in the same test,and the specific capacity of the platform shows almost no degradation.Kinetic analysis reveals that the sodium ion diffusion coefficient increases by an order of magnitude with increasing film thickness,which implies better cycling performance in the electrochemical performance tests.