Preparation And Electrochemical Performance Of Na₃V₂?PO₄?₃/C Cathode Materials For Sodium Ion Batteries

Na₃V₂?PO₄?₃?NVP?,a NASICON-structure with open three dimensional framework,high charge-discharge specific capacity and good thermal stability,all these advantages make NVP a promising cathode material for SIBs.However,due to its polyanionic structure,it suffered from poor electronic conductivity,which limit its commercial production and application.Several approaches have been developed to relieve this issue,such as controlling the particle size,coating the Na₃V₂?PO₄?₃ with conductive materials,or doping with metal ions.Among them,the carbon coating is an effective way for enhancing the capability and cycle stability of NVP.Therefore,here we present several feasible method to improve the electronic conductivity of cathodes and get insight into its sodium storage performances and mechanisms,and the detail research are summarized as follows:Firstly,Na₃V₂?PO₄?₃ was synthesized by a simple solid-state reaction with PVA as carbon source.The results showed that the sample with 15 wt.%PVA content?NVP-15?,exhibited better rate performance and cycling stability.Based on the microstructural and morphological characterization,it can be seen that the NVP-15 nanoparticles??200nm?had an irregular morphology and uniform particle size distribution.For Na-storage,Dischare capacity of NVP-15 are 101,99,96,85,73,35 mAh g^-1,at 0.1,0.5,1,2,5,10 C,and the capacity retentions reaches up to 92%at 0.1 C after 50 cycles.Therefore,the electrochemical performance of Na₃V₂?PO₄?₃/C need to be improved.Secondly,Na₃V₂?PO₄?₃?NVP?grown on carbon nanofiber?CNF?have been prepared by a modified electrospinning method,the use of PVA promotes the growth of Na₃V₂?PO₄?₃crystals on the surface of the conductive carbon fibers.We studied the effect of spinning parameters and the PVA concentration in the electrospinning solution for the fiber structure and electrochemical properties.The results show that the flow rate of the electrospinning solution was 0.4 ml h^-1 used for the PVA concentration of 10 wt.%,setting the high voltage power 25 kV,the collector was made of an aluminum foil and keep the collector-to-tip the distance was 18cm,the roller speed of 800 r min^-1,the conductive network built up by the obtained nanofiber after thermal peocess,it played a vital role in improving electrochemical performance of the Na₃V₂?PO₄?₃/C nanofibers?NVP/CNFs?.The NVP/CNFs cathode exhibited an initial specific capacity of 106 mAh g^-1 at a current density of 0.1 C,the discharge capacity is up to 103,79,63,32 mAh g^-1 at the current density of 1,5,10,20 C,and a capacity retention of 94%was maintained after 100 cycles at 0.1 C.Thirdly,this work focuses on the preparation of a 3D flexible Na₃V₂?PO₄?₃/C fiber membrane,as self-standing cathode for Na-ion batteries,via a facile and simple electrospinning method,is followed by a hot-pressing process.The physical and electrochemical properties of the prepared cathode materials are examined by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,and electrochemical tests.The temperature of the thermal process is a key parameter for controlling the structural organization of the material,as well as the size and dispersion of Na₃V₂?PO₄?₃ nanoparticles on the carbon surface.Hence,Na₃V₂?PO₄?₃nanoparticles,with a size of 40 nm and highly disperse on the carbon nanofibers,are obtained after calcination at 800 ^oC.In addition,this sample Na₃V₂?PO₄?₃/CNF-800 exhibits the best electrochemical performances among all the samples.For instance,it displays a considerably high initial discharge capacity of 109,84,77,and 71 mAh g^-1 at a current density of 0.1,10,20,and 30 C,respectively.Moreover,the Na₃V₂?PO₄?₃/CNF-800 shows notable cycle stability with about 95.3%capacity retention of its initial capacity after 1000 cycles at 2 C.These high performances may be attributed to the unique 3D conductive network of the carbon fibers,which hinder the aggregation of Na₃V₂?PO₄?₃ nanoparticles,improve their conductivity,and act as a frame scaffold to buffer large volume strains during repeated charge/discharge.