Investigation On Synthesis And Modification Of Na₃V₂（PO₄）₃ Polyanion Cathode Materials For Sodium-ion Batteries

Polyanionic positive electrode,material vanadium sodium phosphate（Na₃V₂（PO₄）₃,NVP）,has attracted much attention due to its stable structure,long cycle life and good safety.However,the poor electronic conductivity and low sodium ion diffusion coefficient caused by its polyanionic structure limit its commercial production and application.Carbon coating is an efficient and economical modified method for improving electronic conductivity and sodium ion diffusion.Therefore,in this paper,NVP was modified by using low-cost anthracite as carbon source via traditional solid-state method,and investigated the effect of the dosage of anthracite on the performance of NVP,meanwhile the carbon coating layer was doped by differernt additives so as to further improve the electrochemical performance of NVP.The research results show as below:Nanosheet structured NVP/C composite with superior rate performance has been synthesized using anthracite as the carbon source via traditional solid-state reaction and the effect of anthracite dosage on the electrochemical performance of NVP/C has also been investigated.The results show that the introduction of anthracite does not change the crystal structure of NVP.The effective 3D continuously conductive network composed of carbon layer and carbon bump greatly reduces the charge transfer resistance of the NVP/C electrode and also largely reduce the polarization,so that the NVP/C material exhibits excellent electrochemical performance.The NVP/C composite prepared with 10 wt%anthracite（NVP/C-10）exhibits the highest rate capability and cycle stability.Especially the NVP/C-10 electrode still delivers a high specific capacity of 97.5 m Ah g^-1even after800 cycles at 5 C with a very low capacity fading ratio of 0.012%per cycle.The excellent rate capability and cycle stability of NVP/C-10 can be ascribed to the synergistic effects of the nanosheet structure and the effective 3D continuously conductive network.Our results demonstrate that anthracite can be a promising carbon source for the preparation of NVP/C and other polyanion cathode materials as well.In order to further improve the cycle stability and rate performance of NVP,polyaniline was used to further modify NVP based on anthracite as carbon source.Nitrogen doped carbon coated NVP composite（NVP/C-PANI）was successfully synthesized by a traditional solid-phase method with anthracite as carbon source and polyaniline as nitrogen source,and the electrochemical performance was also studied.The results show that the introduction of polyaniline does not change the crystal structure of NVP.Due to the N doping into the carbon layer,the electrical conductivity of NVP/C-PANI has been improved,resulting in the further improved electrochemical properties of NVP.The NVP/C-PANI electrode exhibits an initial capacity as high as 115.8m Ah g^-1at 1 C with a high capacity retention ratio of 95.7%after 50 cycles.A multi-heteroatom（N,S and F）doped carbon coated co-modified NVP（labeled as NVP/C-ILs）was fabricated by using anthracite as carbon source and ionic liquid 1-et hyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（[EMIM]TF2N）as an additive.As a cathode material for SIBs,the NVP/C-ILs electrode exhibits high specific capacity,superior rate performance and excellent cycling stability.For instance,it delivers a high capacity of 87.5 m Ah g^-1with a superior capacity retention ratio of 95%after 1000 cycles at 10 C.The impressive electrochemical performance of NVP/C-ILs can be attributed to effectively conductive networks for electrons and Na⁺ions induced by a joint effect of N,S and F co-doping on carbon.