Structural Design And Electrochemical Properties Of Na₄FeV（PO₄）₃ Cathode Material For Sodium-ion Batteries

With the advantages of abundant resources,high safety,and low price,sodium-ion batteries are expected to partially replace lithium-ion batteries in the fields of energy storage and low-speed electric vehicles.At present,the development of cathode materials with high safety,high energy/power density,low cost,and long life is the key to the development of Na-ion batteries.The polyanionic cathode material Na₃V₂（PO₄）₃ has obvious advantages in terms of structural stability and cycle life,and is considered to be very promising for commercialization.However,the expensive and toxic V hinders the commercial application of this material.Therefore,replacing V with a low-valent green and electrochemically active transition metal Fe has important academic and application value.In this context,Fe-substituted sodium vanadium iron phosphate is selected as the research object.Given the key problems of materials such as low intrinsic electronic conductivity,slow electrochemical reaction kinetics,and low operating voltage,we adopt strategies such as surface modification,construction of composite structures,and component control to improve the electrochemical performance of materials.The main findings are as follows:（1）Aiming at the problems of low intrinsic electronic conductivity and low electrochemical reactivity of Na₄FeV（PO₄）₃,using the strategy of surface modification and electrode structure construction,reduced graphene oxide coated Na₄FeV（PO₄）₃@r GO@CNT composites with interwoven carbon nanotubes were prepared by a simple sol-gel method.The reduced graphene oxide layer is uniformly coated on the surface of Na₄FeV（PO₄）₃,and the carbon nanotubes construct a three-dimensional conductive network,which effectively improves the electrical conductivity of the material,thus the reaction kinetics of the electrode is greatly enhanced,and the sodium ion diffusion coefficient D_Na+of the material is10^-9～10^-11 cm²s^-1.The discharge capacity of first discharge is 156 m Ah g^-1at a rate of 0.1 C.（2）For the problems of rate performance and slow electrochemical reaction kinetics of Na₄FeV（PO₄）₃,Na₄FeV（PO₄）₃@Al₂O₃ with different content of alumina layers（1～3 wt%）were synthesized by sol-gel method,and determine the optimum amount of coating.The Na₄FeV（PO₄）₃@2%Al₂O₃ material exhibits a specific discharge capacity of 56.4 m Ah g^-1 at an ultra-high rate of 50 C,and a capacity retention rate of 51.2%after 500 cycles at a rate of 10 C.In addition,the NFVP@2%Al₂O₃ also exhibits good charge-discharge capability under high-rate charging conditions.The GITT calculation results show that the sodium ion diffusion coefficient of NFVP@2%Al₂O₃ is 10^-8～10^-11 cm² s^-1during charge/discharge process.The pseudocapacitance contribution increases with increasing scan rate,and the pseudocapacitance contribution remains as high as 81.5%even at a low scan rate of 0.1 m V s^-1.（3）Given the problems of low specific capacity and poor electrochemical reactivity of Na₄FeV（PO₄）₃,we adopted the strategy of component control and synthesized Na_3.5Fe_0.5V_1.5（PO₄）₃,Na_3.6Fe_0.6V_1.4（PO₄）₃,Na_3.7Fe_0.7V_1.3（PO₄）₃,and Na_3.8Fe_0.8V_1.2（PO₄）₃ cathode materials by sol-gel method,and the effects of the components on the structure,morphology,and electrochemical properties of the materials were systematically analyzed.Na_3.6Fe_0.6V_1.4（PO₄）₃showed two voltage plateaus at 3.4 V and 2.5 V.The reversible specific capacity was 110.1 m Ah g^-1 at a rate of 0.5 C and 72.1 m Ah g^-1 at a rate of 30 C.In addition,we also modified it with alumina coating.The discharge capacity of Na_3.6Fe_0.6V_1.4（PO₄）₃@2%Al₂O₃ is 118.6 m Ah g^-1 at a rate of 0.5 C.The charging/discharging curves showed a plateau at 4.0 V corresponding to the V⁴⁺/V⁵⁺redox reaction,and the CV results also showed a V⁴⁺/V⁵⁺corresponding redox peak and exhibit high reversibility.