Modification For Na₃V₂（PO₄）₃ Based Cathode In Sodium Ion Batteries

Sodium ion batteries（SIBs）,with the low cost and rocking-chair design,have been deemed as the most promising substitute for lithium ion batteries.In the current studies,developing high-performance SIBs largely depended on the excellent cathode materials.Of all cathode materials,the Na⁺superionic conductor-type（NASICON）Na₃V₂（PO₄）₃（NVP）shown the great potential for its great thermal stability and 3-dimensional frame structure.However,the weak intrinsic ions and electronic conductivity have greatly limited the further development of NVP.Here,we combine the multiple strategies including ions doping and carbon-based materials decorating to enhance the performance of NVP.Through the above modifying the pathway for rapid ion/electron transfer was constructed,leading to improved kinetic behavior and electrochemical properties.The studied content is shown below:1.The Si/Zr co-doped Na₃V_2-xZr_x（PO₄）_3-x（Si O₄）_x/C samples were successfully prepared to enhance the structure stability,and the effects of doping amounts on the electrochemical performance were explored.Profiting from the expanded cell volume,the optimized sample（x=0.03）exhibits the best rate performance and long cycle stability.According to the cyclic voltammetry test results,there is a new peak appeared at～3.7V,corresponding to the release of third sodium ion of active material.This new higher voltage plateau enable the Na₃V_1.97Zr_0.03（PO₄）_2.97（Si O₄）_0.03/C electrode to obtain an excellent specific energy of403.5Whkg^-1 which higher than that of theoretical value 397.8Wh kg^-1 for NVP.Besides,after 2000 cycles at 12 C,the initial capacity of Na₃V_1.97Zr_0.03（PO₄）_2.97（Si O₄）_0.03/C can still retain 71.4%.2.Performance improvement for Si doped NVP by reduced graphene oxide（rGO）decorating.Due to the large ion radius of Si⁴⁺than V³⁺,Si doping V sites could expand the Na⁺migration channel and effectively promote the ionic conductivity.At the same time,Si doping induces the formation of porous morphology and then increases the reaction sites between the active material and electrolyte.The introduced rGO can inhibit the agglomeration of particles,construct a conductive network,and significantly improve the electronic conductivity.Here,Na_3.1V₂（PO₄）_2.9（SiO₄）_0.1/C@rGO composite is prepared by sol-gel method,and electrochemical results indicate that the performance of the NVP cathode material was significantly improved.Its initial discharge capacity is 113.6m Ah g^-1(close to the theoretical capacity of 117m Ah g^-1).Additionally,the ionic diffusivity of modified Na^3.1V²（PO⁴）^2.9（Si O⁴）^0.1/C@rGO was significantly higher than that of undoped NVP.3.Performance improved by Zr-doped and reduced graphene oxide（rGO）decorating.The doped Zr⁴⁺with larger ion radius is beneficial to accelerate the migration of sodium ions and then increase the ion diffusivity.At the same time,the conductive network constructed by graphene can connect the adjacent particles,which is conducive to ionic and electronic transfer,and then improve the electrochemical performance.Specially,introduced graphene efficiently inhibits the agglomeration of the particles during calcination.These small particles are evenly distributed on the surface of graphene.During charge/discharge processing,sodium ions and electrons would migrate rapidly throughout the carbon network due to the excellent electrical conductivity of graphene.The optimized Na_2.9V_1.9Zr_0.1（PO₄）₃/C@rGO sample exhibits excellent long cycle stability at the ultrahigh current rate.The initial capacity of Na_2.9V_1.9Zr_0.1（PO₄）₃/C@rGO sample still maintain 74.3%after 3000 cycles at 200C rate.