Preparation And Electrochemical Properties Of Anion-substituted Na₃V₂（PO₄）₃-based Cathode Materials For Sodium-ion Batteries

With the continuous development of the electronics industry,sodium ion batteries（SIBs）have attracted widespread attention due to the abundant sodium resources,and are expected to become the next-generation low-cost energy storage systems.However,compared to a variety of anode candidates with high Na-storage capacity,we still need to further explore and design the advanced cathode materials in SIB.In the process of the research and development of high-performance SIBs cathode materials,Na₃V₂（PO₄）₃（NVP）with Na super ionic conductor（NASICON）structure has an open three-dimensional sodium ion transport channel and a stable crystal structure framework,making sodium ion transport fast a during the process of charge and discharge.However,the transition metal MO₆ octahedron is separated by polyanion groups in the structure of the polyanion cathode materials,which causes the decreased intrinsic electronic conductivity and then hinders its electrochemical performances,especially high rate performance.In order to solve the above problems,researchers have proposed strategies for improving the electrochemical performance by carbon coating,morphology optimization,and foreign ion doping or substitution.In this paper,PO₄^3-is replaced by multivalent anionic groups to regulate the crystal structure,ion and electron transport kinetics,and electrochemical performances of NVP-based materials.The specific research contents include the following two aspects:（1）We designed to use the multivalent anion P₂O₇^4-to partially replace PO₄^3-in Na₃V₂（PO₄）₃ to form Na_3+xV₂（PO₄）_3-x（P₂O₇）_x,and successfully synthesized Na_3+xV₂（PO₄）_3-x（P₂O₇）_x（NVPP_x,x=0,0.05,0.1,0.15）four materials using a simple sol-gel method.The P₂O₇^4-substitution improves the electrochemical performances of the NVP-based materials,of which NVPP_0.1shows the best electrochemical performances as the cathode material in SIBs.On the one hand,the substitution of multivalent anion P₂O₇^4-increases the active Na content in NVP-based materials,and we find that the substitution of P₂O₇^4-activates the V⁵⁺/V⁴⁺redox reaction by the charge and discharge curves,which enables NVP-based materials to deliver enhanced specific capacity.On the other hand,the substitution of an appropriate amount P₂O₇^4-will make the materials’ particle size smaller,reduce the distance of Na⁺ transport,and improve the electrode reaction kinetics.In addition,compared with PO₄ tetrahedron,the P₂O₇ group is more stable,when it is introduced into NVP-based materials,the stability of the NVPP_x materials will be improved,so the substitution of P₂O₇^4-will improve the high rate performance and cycle stability of the materials,compared to the NVP material.（2）The multivalent and isostructural anion SiO₄^4-was introduced into NVP to form Na_3+xV₂（PO₄）_3-x（SiO₄）_x（NVPSi_x）,and the regulation of the anion site in NVP material was realized by sol-gel method.A series of NVPSi_x（0≤x≤0.15）materials were synthesized,and the structural evolution,electrochemical properties and kinetic properties of electrode materials were investigated by experimental characterizations and theoretical calculation.NVPSi_x materials were applied in SIBs system,showing excellent electrochemical performances,especially NVPSi_0.1,whose discharge specific capacity can reach 109.4 mAh g^-1at 0.2 C and still deliver specific capacity 82.5 mAh g^-1at 20 C,compared with 99 mAh g^-1at 0.2 C and 49.7 mAh g^-1at 20 C of NVP,suggesting the SiO₄^4-substitution makes the rate performance of NVP-based materials get obvious improvement.Moreover,NVPSi_0.1material shows excellent long cycle stability,and the capacity retention rate is 98% under 1 C after 500 cycles.The excellent electrochemical performances are mainly attributed to the substitution of SiO₄^4-,which changes the crystal structure of NVP-based materials,widens the Na⁺ transport channel,increases the occupancy of active Na（2）sites,reduces the band gap of the materials and thus improves their electronic conductivity.In addition,the improvement of Na⁺ diffusion kinetics is also the key to the improvement of electrochemical performance by the calculation of CV scanned at different rates and GITT tests.Coupled NVPSi_0.1with the hard carbon ande to assemble the sodium-ion full battery,the discharge specific capacity is still 92.1 mAh g^-1available after 100 cycles under 1 C,which proves the practical application value of NVPSi_0.1.This strategy of modifying polyanionic cathode materials by the substitution of multivalent and isostructural anions provides the researchers working on the field of energy storage important reference value to design high-performance polyanion cathode materials in the future.