Preparation And Sodium Storage Performance Of Sodium Vanadium Fluorophosphate/Carbon Composites

Sodium-ion batteries(SIBs)as the most appealing energy storage systems have attracted increasing attention owing to the fact that of sodium resources are abundant,widely distributed,inexpensive,and similar energy storage mechanisms as lithium-ion batteries.Sodium vanadium fluorophosphate(NVPF)is a promising cathode material with many advantages for SIBs.Its open framework structure provides paths for the diffusion of sodium ions,strong covalent bonds inside the crystal make the material structurally stable,thermally stable and safe,and the inductive effects between PO43-and F-ensure the material to achieve a high working voltage.However,NVPF is poorly conductive and with limited charge transport,which do harm to its actual electrochemical performance.In order to solve the above problems,we enhance the performance of sodium storage by constructing sodium vanadium fluorophosphate/graphene aerogel(NVPF/GA)composite with a three-dimensional conductive network.The specific process is as follows:(1)In view of the poor conductivity of NVPF,NVPF/GA with a three-dimensional continuous conductive network was synthesized via a chemical reduction method,while graphene oxide(GO)and NVPF worked as precursors.The interconnected porous structure inside the composite material endows high conductivity,superior structural stability and rapid diffusion channels of ions,thereby improving sodium storage performance.The material was evaluated with coin cell,among which,the specific capacity of NVPF/GA was deeply influenced by the mass ratio of NVPF to GA.It was found that when the mass ratio of NVPF to GA is 4:1,the composite delivered the best performance.At the current density of 1 C(1C=130 mA h g-1),NVPF/GA delivered a specific capacity of 134.3 mA h g-1.After 100 cycles,the specific capacity remained at 127 mA h g-1,and the capacity retention was 94.6%.Even at 5 C,the material still delivered a capacity of 106.2 mA h g-1.The freestanding electrode was obtained after simply compressed,NVPF/GA 400 delivered the best performance.At 0.2 C,the first discharge specific capacity could reach 113.7 mA h g-1,even after 100 cycles,the specific capacity still remained at 105.8 mA h g-1.(2)Current collector is an essential part in sodium ion batteries.Aluminum foil is the most commonly accepted current collector for sodium ion battery cathodes.However,during long-term charging and discharging processes,Aluminum foil is susceptible to localized corrosions,which will result in the increase of resistance and other side effects.Carbon cloth is an attractive current collector with excellent corrosion resistance,high conductivity,flexibility,and chemically stable.The existence of pores provides more channels for Na+transport.Based on the previous work,a third component carbon cloth was introduced to improve the conductivity via constructing dual conductive carbon networks and replace the aluminum foil as a current collector in this experiment.NVPF/GA was used as the electrode material while the aluminum foil and carbon cloth were used as the current collectors.The batteries were assembled and tested.The results showed that NVPF/GA 400/CC1 delivered the best performance.The specific capacity was still maintained at 123.5 mA h g-1 after 200 cycles at 1 C and the capacity retention was 95.4%.Even at 10 C,the material still delivered a specific capacity of 108.4 mA h g-1.What'more,sodium ion full cell was assembled while NVPF/GA 400/CC1 worked as positive electrode,antimony/graphene/carbon nanocomposite(Sb/G/C)worked as negative electrode and 1M NaClO4 EC:PC=1:1 v/v+5%FEC was used as electrolyte.The sodium ion full cell delivered a specific capacity of 82.1 mA h g-1 even after 100 cycles.