Research And Modification Of NASICON Type Na₃V₂?PO₄?₃ Cathode Materials

The application of lithium ion batteries is limited that the high cost of lithium resources and the shortage of resources.Sodium have similar electrochemical properities with lithium,in addition,it is cheaper,simple to extract and have a wide range of sources.The rate and dynamic performance of sodium ion battery is not as good as that of Li ion battery,but it has advantages over Ni MH battery and liquid flow battery:high voltage and good stability.Sodium super ion conductor?NASICON?structure material in cathode material of sodium ion battery has good structural stability.NASICON structure Na₃V₂?PO₄?₃?NVP?is a promising cathode material for sodium ion batteries,and electrode materials have been studied more and more widely.In order to solve the shortcomings of low electronic conductivity of NVP materials,we have studied and modified the electronic conductivity of the materials.The precursor was synthesized by sol-gel method and sintered by high-temperature solid-state method to obtain NVP cathode material.Changing the calcination temperature and holding time explores the optimal sintering regime for this material.The material prepared by calcining at 750 ^oC for 8 h under argon atmosphere has a regular appearance and good crystallinity,showing the best performance.It shows that the initial discharge specific capacity reaches 103.9mAh·g^-1 and Coulomb efficiency up to 96.38%.The doping modification can significantly increase the ionic conductivity of the material,so further studies of Mn and Fe doping of NVP have been conducted.The results showed that the morphology and cell volume of NVP materials changed.Replacement portion vanadium ions by low valent metal state position,the occurrence of defects within the crystal shapes a P type semiconductor,Accelerate the ion diffusion rate,improve the performance of the material.When the doping amount of Mn is x=0.6,the capacity retention is 89.88%after 1000^th at 5 C.When the doping amount of Fe is x=0.1,the capacity retention is 98.92%after 1000^th at 5 C.Improving the electrical conductivity of NVP,Na₃V₂?PO₄?₃@CSs?NVP@CSs?composites were constructed by introducing carbon spheres?CSs?into via composite modification.I By characterization of physical structure the material has a good morphology,uniform particle size,and the composite does not change the crystal structure of the material.The material is coated by carbon sphere,and its electronic conductivity is improved.In addition,the amorphous carbon makes the material bond tightly and the cycle performance is improved.The Na-metal cells with NVP@CSs cathode achieved high charge-discharge capacity in the intital.Reduced of internal resistance and polarization voltage,improved obviously of cycle performance.The discharge capacity of 101.5 mAh·g^-1 and 97.51 mAh·g^-1 in the initial and 500^th cycle at 1 C,respectively,with a capacity retention of 96.06%.When cycled at 5 C,the cell with NVP@CSs cathode showed the discharge capacity of 95.6 mAh·g^-1 in the intital,a reversible capacity of 90 mAh·g^-1 over 1000^th with a capacity loss of 5.84%.