The Low-temperature Synthesis Of Sodium Vanadium Fluorophosphates Based On ?(?) And Investigation Of Their Na-storage Performance

Na3(VO1-xPO4)2F1+2x(0?x?1),cathode material of sodium-ion batteries,has attracted wide attention due to its stable chemical structure,high working voltage and discharge capacity.As it is widely known,the energy consumption and cost in producing a battery is mainly decided by the cathode producing process.However,expensive precursors and high energy consumption are still the problem in synthesis of Na3(VO1-xPO4)2F1-x2x(0?x?1).To decrease the cost and energy consumption in producing Na3(VO1-xPO4)2F1+2x(0?x?1),a concept about integration of precursor separation and synthesis of material is proposed.As for the synthesis of Na3(VO1-xPO4)2F1+2x(0?x?1),it is integration of the extraction of vanadium and the synthesis of Na3(VO1-xPO4)2F1+2x(0?x?1).Based on this concept,we developed a one-pot hydrothermal method to synthesize carbon coated Na3(VOPO4)2F,and a room-temperature method to synthesize Na3(VOPO4)2F.Both of the two methods use NaVO3,industrial product isolated from vanadium slages,as vanadium sources.In order to characterize the valence of vanadium in the as-synthesized Na3(VOPO4)2F,we developed a new method based on UV-Vis absorption spectra to characterize the apparent valence of vanadium.Firstly,a one-pot hydrothermal method to synthesize carbon coated Na3(VOPO4)2F is developed,using NaVO3 as vanadium sources,glucose as carbon sources and reducing agent.The as-synthesized Na3(VOPO4)2F monocrystal has a special Rubik-like morphology with a diameter of 6 ?m.The as-synthesized Na3(VOPO4)2F delivers an excellent sodium-storage property with a discharge capacity of 113 mAh g-1 at 10 C and capacity retention of 76.7%after 1400 cycles at 5 C.In order to further decrease the energy consumption in synthesis of Na3(VOPO4)2F,room-temperature synthesis of Na3(VOPO4)2F is studied.As poor crystalline limited the improvement of the Na-storage performance of the as-synthesized Na3(VOPO4)2F.Methods to improve the crystallinity of material are studied.Doping graphene and carbon coating was also studied to further improve rate performance of Na3(VOPO4)2F.Although the as-synthesized Na3(VOPO4)2F didn't show an excellent Na-storage performance,this study established a foundation for the future research about room-termperature synthesis of polyanionic compounds.Na3(VO1-xPO4)2F1-2x(0?x?1)with different valence of vanadium has different crystal structure and electrochemical behavior.Hence,it is necessary to characterize the valence of vanadium in Na3(VOPO4)2F.Reducing high valence of vanadium to low valence is inevitable if NaVO3 is used as precusor.So it is necessary to characterize the specific valence of vanadium in the as-synthesized Na3(VOPO4)2F.Hence,a new method based on UV-Vis absorption spectra to characterize the apparent valence of vanadium in Na3(VOPO4)2F is developed.Compared with other methods,which use XPS,IR,XANES,solid-state NMR to characterize apparent valence of vanadium,this method is simpler and has a potential application prospects.