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

Energy crisis and environmental pollution are urgent issues to be solved today.The energy pattern of world is developing toward low-carbon multi-energy integration,and the development and utilization of new energy technologies are becoming more and more important.Sodium ion batteries have received wide attentions due to their abundant sodium resources,good safety performance,excellent rate performance and low temperature performance.The cathode material plays a crucial role in its comprehensive electrochemical performance,in which the NASICON structure Na₃V₂（PO₄）₂F₃ has an open three-dimensional framework and stable crystal structure,showing good thermal and cycling stability.The high voltage plateau,large theoretical capacity and high energy density makes it an attractive cathode material.However,the low electrical conductivity and difficult preparation of pure phase materials limit its development.In this paper,Na₃V₂（PO₄）₂F₃ material is used as the research object and spray drying technology is used as the preparation method to investigate the transformation mechanism of structure and morphology during the preparation process.The cathode Na₃V₂（PO₄）₂F₃/C with excellent rate and long cycle performance is obtained by optimizing the cathode material.The main research results are as follows.The main results of the paper are as follows.（1）The Na₃V₂（PO₄）₂F₃/C cathode material is synthesized by two-step spray drying,in which the effects of the mixing method and ratio of Na F and VPO₄ and carbon nanotubes on the structure,morphology and electrochemical performance are investigated.The obtained Na₃V₂（PO₄）₂F₃/C possesses good electrochemical performance,delivering capacity of 104.5,94.8,78.8 m Ah g^-1 at 0.1,5,and 10 C,respectively.The retention ratio of capacity is 67.1%after 400 cycles at current density of 10 C.（2）The pure Na₃V₂（PO₄）₂F₃/C cathode material is synthesized by one-step spray drying technique,in which fluoropolymer polytetrafluoroethylene（PTFE）and bromine-containing surfactant cetyltrimethyl-ammonium bromide（CTAB）are introduced as hard template and soft template.It is found that PTFE is cracked during high temperature sintering to form the reactive fluorine-containing gas,which reacts with the amorphous sodium salt to form Na F,thus compensating for the loss of fluorine.The Br^-in CTAB can replace the suspended F^-to occupy the lattice and perform anion doping during the high temperature sintering.The synergistic effect of PTFE and CTAB can control the fluorine content well,and the pure Br-doped Na₃V₂（PO₄）₂F₃/C is synthesized.The hard template PTFE can be used as a solid framework in the spray drying process to strengthen the structural stability of spherical particles.The pore channels formed by gas overflow during sintering can accelerate the wetting of electrolyte and the diffusion of Na⁺.Due to the hydrophobic effect,the soft template CTAB adsorbs on the PTFE surface and guides the self-assembly of the precursor,and the synergistic effect of the PTFE and CTAB in terms of morphology forms a solid porous spherical Na₃V₂（PO₄）₂F₃/C.The density functional theory（DFT）calculation shows that the Br doping not only reduces the charge localization and increases the conductivity,but also lowers the diffusion energy barrier of Na⁺.Based on the optimized structure and morphology,the spherical Na₃V₂（PO₄）₂F₃/C exhibits excellent electrochemical performance with high rate capacity of 116.1,105.1 and 95.2 m Ah g^-1at current densities of 1,10 and 30 C.The cycling retention ratio is as high as 98.3%after 1000cycles at 10 C,demonstrating excellent cycle performance and structural stability.