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

Polyanionic Na₃V₂?PO₄?₃ is considered to be one of the most promising cathode materials for sodium ion batteries in terms of its 3D open frame structure,excellent structural stability,large theoretical energy density(about 400 Wh kg^-1),stable voltage platform?about 3.4 V vs.Na⁺/Na?,and good thermal stability?up to450°C?.However,the intrinsically low electron conductivity leads to a poor high-rate reversible capacity,reducing the power performance of sodium-ion batteries.Based on the improvement of the electrochemical activity of Na₃V₂?PO₄?₃,this paper explored the effects of the material synthesis conditions,the addition of surfactant in the preparation process and ion doping on the microstructure and electrochemical properties of Na₃V₂?PO₄?₃.The main research contents are as follows:1.The Na₃V₂?PO₄?₃/C composites were synthesized by ball milling and high-temperature solid-state reaction method using gluconic acid lactone as carbon source.The effects of synthesis conditions?carbon source content,ball milling time and calcination temperature?on the crystal structure,microstructure and electrochemical properties of Na₃V₂?PO₄?₃/C were investigated.The ball milling times are 1 h,5 h,10 h and 24 h,respectively.The calcination temperatures are700°C,750°C,800°C and 850°C,respectively.The contents of carbon in the products are about 1 wt%,3 wt%,5 wt%,7 wt%and 10 wt%,respectively.The Na₃V₂?PO₄?₃/C with ball milling time of 10 h displays the best rate and cycling performances in the voltage range of 2.0-4.3 V.The specific discharge capacity is 107mAh g^-11 at 20 C,along with high capacity retention of 81%after 1000 cycles at 10 C.The Na₃V₂?PO₄?₃/C prepared at 800°C possesses small and uniform particle size.It delivers the discharge capacity of 91 mAh g^-1 at 40 C and capacity retention of 88%after 500 cycles at 1 C.The Na₃V₂?PO₄?₃/C with optimized carbon amount of 7 wt%shows outstanding electrochemical performance(112 mAh g^-1 at 0.5 C).Consequently,the material synthesis conditions have significant affects on the microstructure and electrochemical properties of Na₃V₂?PO₄?₃/C composite.2.The Na₃V₂?PO₄?₃/C composites were synthesized via a high-temperature solid-state reaction method using sodium oleate as both sodium source and carbon source.The precursor were prepared by ball milling?BM?,sol-gel?SG?,hydrothermal?HT?,and hydrothermally assisted sol-gel?HT-SG?,respectively.The effect of the precursor preparation process on the crystallinity,particle morphology,particle size distribution,and electrochemical properties of the Na₃V₂?PO₄?₃/C composite of were investigated.The specific discharge capacities of NVP@BM,NVP@HT,NVP@HT-SG and NVP@SG at 1 C in the voltage range of 2.0-4.3 V are 106,81,92and 85 mAh g^-1,respectively.Particularly,NVP@BM displays the best rate capability and cycling performance due to its smaller particle size,better particle dispersion and higher crystallinity compared to that of other contrast samples.3.The Na₃V₂?PO₄?₃/C composites were prepared by surfactant-assisted sol-gel method.The surfactants are polyethylene glycol?PEG?,cetyl trimethyl ammonium bromide?CTAB?,polyethylene pyrrolidone?PVP?and sodium lauryl sulfate?SDS?,respectively.The effect of different surfactants on the morphology,microstructure and electrochemical properties of Na₃V₂?PO₄?₃/C composite was studied.The Na₃V₂?PO₄?₃/C prepared using PEG demonstrates smaller particle size?50-200 nm?compared to that of other contrast samples.The NVP@PEG exhibits high charge/discharge capacities of 109/108 mAh g^-1 at 1 C with coulombic efficiency of99%.The discharge capacity reaches 92 mAh g^-11 at a super high rate of 60 C,and the capacity retention retains 91%after 400 cycles at 1 C.In addition,the influence of PEG molecular weight?M=200,1000,4000,10000,20000?on the electrochemical property of Na₃V₂?PO₄?₃/C was further investigated.The results show that the molecular weight of PEG significantly affects the crystallinity,particle size,particle agglomeration and electrochemical activity of the material.Among them,NVP@C-PEG/4000 displays the superior rate capability,and the discharge capacities of 108,106,106,105,102 and 95 mAh g^-1 can be reached at 1,2,5,10,20 and 40 C,respectively.Moreover,NVP@C-PEG/4000 demonstrates excellent cycling stability with capacity retention of 85%after 1000 cycles at 10 C.4.The influence of K doping on the crystal structure,morphology and electrochemical properties of Na₃V₂?PO₄?₃ was investigated.The XRD results demonstrate that K doping does not change the crystal structure,but enlarge the cell volume of Na₃V₂?PO₄?₃.The SEM images show that K doping can effectively reduce the particle size of Na₃V₂?PO₄?₃.In addition,Raman result shows that K doping does not affect the carbon conductivity.The XPS and EDS results confirm the presence of K in Na₃V₂?PO₄?₃ crystal.The cell tests reveal that the Na_2.95K_0.05V₂?PO₄?₃/C displays a high discharge capacity of 114 mAh g^-1 at 1 C in the voltage range of 2.0-4.3 V and a superior high-rate capability with a discharge capacity of 94 mAh g^-1 at 40 C,around 1.40 times of that of undoped Na₃V₂?PO₄?₃(67 mAh g^-1).The capacity retentions of Na_2.95K_0.05V₂?PO₄?₃/C and Na₃V₂?PO₄?₃/C after 1000 cycles at 1 C are96%and 88%,respectively.The CV and EIS results indicate that K doping can not only significantly reduce electrode reaction polarization and charge transfer impedance,but also facilitate fast Na⁺transportation,resulting in an improvement of the electrochemical performance of Na₃V₂?PO₄?₃.