Preparation And Modification Of Sodium Vanadium Phosphates As Cathode Material For Sodium-ion Batteries

As a high potential cathode material for commercial sodium-ion batteries,Na₃V₂?PO₄?₃ has a theoretical specific capacity of 117 mAh g^-1 with a voltage plateau at 3.4 V.The material has good thermal stability and three-dimensional Na⁺ion diffusion channels,but its electronic conductivity is low due to the inherent characteristics of its structure,which results in poor rate performance and long-term cycling performance at large current densities.This paper prepared Na₃V₂?PO₄?₃ with different morphology and carbon layer properties through different preparation methods with the optimized conditions and obtained excellent electrochemical performance.Na₃V₂?PO₄?₃ was prepared by a solid state method and solvent evaporation method,respectively in which electrolytes were optimized and compared.Moreover,the electrochemical performance at room temperature and high temperature were tested and analysed.The sintering temperature time was optimized,Na₃V₂?PO₄?₃ with a complete carbon layer was obtained.The material exhibited excellent electrochemical performance with the capacity of 68 mAh g^-1 at a current of 30 C and the initial capacity of 83%after 1000 times at 5 C.Ex-situ XRD tests were carried out during charge/discharge and the two-phase reaction was proved.The diffusion coefficient of Na⁺ions was studied by PITT method and the results showed that the diffusion coefficient exhibits a"V"type with the deintercalation of Na⁺during the charge process.Porous Na₃V₂?PO₄?₃ materials were prepared by a freeze-drying method on the basis of solvent evaporation method.The effects of different precursor concentrations on the morphology and properties of the materials were investigated.The porous structure was formed during frezee-drying process because of sublimation of solvent.It can accommodate the volume change of the electrode material during the charge/discharge process,shorten the diffusion path of Na⁺ions and increase the contact area between the electrolyte and the electrode material,thereby improving the electrochemical performance of the material under high current.The porous material has a capacity retention of 85%after 1000 cycles at a current of 5 C.The influence of precursors were studied.The variable oxidation states of vanadium were studied and it was found that the increase of V^?content in the precursor plays a role in improving electrochemical performance.Carbon network between particles was constructed and the dispersability of Na₃V₂?PO₄?₃ particles was improved by the introduction of the dispersant PVP.The addition of the dispersant allows the Na₃V₂?PO₄?₃ particles to connect each other through the carbon network,improving the electronic conductivity and thus improving electrochemical performance of the material.Within a certain range,when the amount of dispersant increases,the electrochemical performance of the material gradually enhances.However,when excess dispersant is added,carbon is left in the product,inhibiting the electrochemical performance of the material.When the mass ratio between PVP and Na₃V₂?PO₄?₃ is 0.6,the material has the best electrochemical performance.It has a capacity of 81 mAh g^-1 at a current of 30 C,and reaches a specific capacity of 84 mAh g^-1 after 2000 cycles at 10 C with a capacity retention of88%.The carbon layer of Na₃V₂?PO₄?₃ material was N-doped through different N sources,additon method and contents.N-doping can increase the number of active sites,surface defects in the material and provide electron to carbon material,thereby improving its performance.It was found that the introduction of the N source after the formation of the precursor and before sintering can effectively improve electrochemical performance.Comparing the two N sources,better performance was obtained when urea is used as N source.Through optimizing the N doping amount,the material exhibits excellent electrochemical performance can be obtained.At a high current of 40 C,it still delivered a capacity of 72 mAh g^-1.The influence of C-N-S coating on the material was also studied.The introduction of S can further increase defects on the carbon surface and improve electronic conductivity of the material.The N-S doping of the carbon layer was carried out using thiourea,and the doping amount was optimized.The influence of the material was analyzed by combining ionic and electronic conductivities.The N-S doped material has a specific capacity of 78 mAh g^-1 at 50 C.Furthermore,full batteries using Na₃V₂?PO₄?₃ and Na₂Ti₂O₅ as anode materials were assemblied.The two full batteries have different attenuation mechanism:when Na₃V₂?PO₄?₃ is used as anode,the charge/discharge voltage platform is basically unchanged,but the specific capacity attenuation is faster;when Na₂Ti₂O₅ is used as anode,the polarization of NVP//NTO increases faster but the specific capacity is slowly attenuated.