Preparation And Properties Of Na₃V₂（PO₄）₃ Cathode Material For Sodium-ion Batteries

The development and utilization of new energy sources and the construction of large-scale power grids have increased the demand for lithium-ion batteries,one of the most important energy storage devices.The limited lithium resources are difficult to meet the long-term growth of demand.Sodium and lithium belong to the same main group,and the sodium element is rich in reserves and low in price.The sodium ion battery that is composed of sodium is one of the most possible substitutes for lithium ion battery.The NASICON structure Na₃V₂（PO₄）₃ with an open three-dimensional framework structure has attracted much attention as a cathode material for sodium ion batteries.but its low electronic conductivity has limited its application.In this study,carbon-coated Na₃V₂（PO₄）₃ materials are prepared by normal temperature reduction-mechanical activation method and pre-reduction-electrospinning method,and the electronic conductivities of the materials are improved while lowering the sintering temperature.The xNa₃V₂（PO₄）₃·yNa₃V₂（PO₄）₂F₃/C composites are prepared by mechanical activation method to further increase the ionic conductivity of Na₃V₂（PO₄）₃ and optimize the electrochemical performance.Na₃V₂（PO₄）₃/C is prepared by normal temperature reduction-mechanical activation method.Oxalic acid is used as reducing agent and carbon source.The vanadium in the raw material is reduced to low-valence state in the process of mechanical activation at normal temperature and pressure,which promotes the subsequent crystallization process and reduces the sintering temperature.The influences of different kinds of raw materials,mechanical activation time and sintering temperature on the phase structure,microstructure and electrochemical properties of the materials are studied.The optimal synthesis conditions are as follows:NaH₂PO₄+NH₄VO₃ as raw material,mechanical activation time 8 h,sintering temperature 700°C.Na₃V₂（PO₄）₃/C samples prepared under this condition consist of particles coated with an amorphous carbon layer（⁵ nm）.Electrochemical tests show that a pair of obvious charge and discharge platforms appeared around 3.4 V（Na/Na⁺）.The initial discharge specific capacities of the samples at 0.1 C and 2 C rates are 110.1 mAh·g^-1and77.5 mAh·g^-1,respectively,capacity retention after 90 cycles at 0.1 C rate is 90.6%.Carbon-coated Na₃V₂（PO₄）₃ one-dimensional nanofibers were prepared by pre-reduction-electrospinning for the first time,and the fiber diameter is around 500 nm,which reduced the size of the material particles.The chelating agent oxalic acid added to the spinning solution can reduce V⁵⁺to V³⁺to a certain extent,adjusting the viscosity and electrical conductivity of the spinning solution and ensuring that the precursor and the final product are uniform and smooth.Spindle-shaped protrusions are found in samples without oxalic acid,and budding grains are left on the surface of nanofibers with too short a sintering time.If the sintering temperature is too high or the heating rate is too low,nanofibers will be broken.Na₃V₂（PO₄）₃/C with excellent electrochemical performance could not be obtained under the above conditions.When the temperature is raised to 800°C at a heating rate of2.5°C·min^-1and the temperature is maintained for 10 h,the product sample is a well-crystallized Na₃V₂（PO₄）₃ nanofiber with a diameter of 400 nm coated with an amorphous carbon layer of about 10 nm.The initial discharge specific capacity of the sample reaches115.6 mAh·g^-1at 0.05 C,and remains 77.9 mAh·g^-1at 10 C.The capacity retention after 100 cycles at 0.05 C is still 97.0%.Continuous,uniform,and smooth Na₃V₂（PO₄）₃/C nanofibers have a continuous network of high-efficiency electron and ion channels,resulting in better electrochemical performance.xNa₃V₂（PO₄）₃·yNa₃V₂（PO₄）₂F₃/C are synthesized by mechanical activation method.Studies have shown that the increase in the mutual doping of the two phases in the composite and the increase of the phase interface can provide more active sites for the transport of sodium ions and increase the ionic conductivity of Na₃V₂（PO₄）₃.The effect of the composite ratio on the electrochemical performances of the materials are studied,and the optimal compositeratiois1:1.Theinitialdischargespecificcapacityof Na₃V₂（PO₄）₃·Na₃V₂（PO₄）₂F₃/C at 0.1 C,1 C,2 C rates are 120.2,104.9 and 89.2 mAh·g^-1.The capacity retention after cycling 50 times at a 0.1 C rate is 93.3%.Its electrochemical performance is better than Na₃V₂（PO₄）₃/C sample prepared under the same conditions.Cyclic voltammetry test of this sample shows that The composite modification of Na₃V₂（PO₄）₂F₃ as the second phase can reduce the potential difference between redox peaks of Na₃V₂（PO₄）₃ by 0.075 V and reduce the redox peak potential difference of Na₃V₂（PO₄）₂F₃ to 0.094 V and 0.078 V.It is proved that the composite Na₃V₂（PO₄）₂F₃ can effectively increase the discharge specific capacity of Na₃V₂（PO₄）₃/C,improve the cycle performance,and reduce the polarization during charge and discharge.