| Sodium vanadium phosphate(NVP)is a polyanionic compound with a sodium superionic conductor(NASICON)with an open three-dimensional framework,rapid ion migration,and good structural stability.Sodium vanadium phosphate can also provide a large theoretical capacity of 117 mAh g-1 and a voltage stage of 3.4 V(relative to Na+/Na),making it one of the most promising sodium ion cathode materials.However,the weak intrinsic conductivity of sodium vanadium phosphate and the structural collapse during cycling seriously limit its cycle life and rate performance,and hinder the further development and application of sodium vanadium phosphate cathode.When operating at low temperatures,the decrease in ion diffusion kinetics and the increase in electrochemical polarization will further lead to the deterioration of electrode electrochemical performance,limiting its application in cold climates.In order to improve the ionic and electronic conductivity of the cathode of sodium vanadium phosphate and improve the low-temperature performance of the electrode,this paper mainly adopts the dual modification strategy of ion doping and carbon recombination to replace atoms at different spatial sites(Na,V,PO43-)of sodium vanadium phosphate,improve the ionic conductance of the electrode,and recombine with the conductive matrix to improve the electronic conductivity of the composite.Based on the above structural optimization,the electrochemical activity of the sodium vanadium phosphate cathode at room temperature and low temperature is improved.The specific content includes the following three aspects:1.Preparation and electrochemical performance of nitrogen-doped carbon-coated Zr-doped Na3V2(PO4)3:Zr4+ was successfully replaced by twostep calcination by simple sol-gel method.At the same time,nitrogen-doped carbon was introduced by the self-polymerization of dopamine hydrochloride,and Zr-doped NVP porous cathode material(Na3V2-xZrx(PO4)3/NC)coated with nitrogen-doped carbon was prepared,which improved the migration of electrons/ions inside the electrode and reduced electrochemical polarization.In addition,Zr doping can further improve the stability of NASICON structure.The reversible specific capacity of the optimized Na3V1.9Zr0.1(PO4)3/NC electrode for 100 cycles at a current density of 0.1 A g-1 is 109.8 mAh g-1,and the specific capacity retention rate is 84.4%after 2000 cycles at a high current density of 20 A g-1.In addition,a high reversible specific capacity of 103.7 mAh g-1 can still be obtained after 100 cycles at a current of 0.1 A g-1 at a low temperature of-20℃.The results of cyclic voltammetry,electrochemical impedance,reaction kinetics,and ex situ X-ray diffraction showed that the prepared 0.1Zr-NVP/NC electrode had good electrode stability,fast ion mobility rate and excellent electrochemical stability.2.Preparation and electrochemical performance of carbon nanotube composite K doped Na3V2(PO4)3:Through ball milling and high-temperature calcination,K+partially replaced Na+ at the Nal position,and K+with a large ion radius can induce the columnar effect and effectively improve the structural stability of sodium vanadium phosphate.At the same time,uniformly wound carbon nanotubes(CNTs)form a good conductive network,which establishes favorable interconnection for K-doped NVP particles,thereby significantly improving the electronic conductivity of the sodium vanadium phosphate cathode material.The modified K-NVP/C@CNT electrode showed excellent electrochemical performance,and the reversible specific capacity of 112.3 mAh g-1 remained after 100 cycles at a current density of 0.1 A g-1.Tested at a high current of 20.0 A g-1,a high reversible specific capacity of 70.1 mAh g-1 and excellent rate performance can be obtained after 2000 cycles.In addition,the modified material also showed excellent electrochemical performance at low temperature,and a high reversible specific capacity of 103.7 mAh g-1 could still be obtained after 1000 cycles at a high current density of 20.0 A g-1,and the capacity retention rate was as high as 85.2%.This chapter improves the reaction kinetics of the NVP cathode through a double modification strategy,demonstrating that the synergy between doping and bonding to conductive materials can significantly improve the electrochemical performance of the NVP cathode.3.Preparation and electrochemical performance of three-dimensional conductive carbon crosslinked F doped Na3V2(PO4)3:This chapter proposes a double modification strategy of ball milling method,the introduction of F can partially replace PO43-position,and at the same time induce V vacancy,and further combine with three-dimensional(3D)nitrogen-doped carbonaceous framework(NC)to obtain three-dimensional conductive carbon crosslinked F doped Na3V2(PO4)3.F doping and V vacancies can significantly reduce NVP particle size and enhance Na+ migration,thereby increasing ionic conductivity.At the same time,the introduction of three-dimensional nitrogen-doped carbon can improve the electronic conductivity of the composite,provide effective structural buffer,and enhance the stability of the electrode.The results of sodium storage test show that after 100 cycles at a current density of 0.1 A g1,its reversible capacity is 113.8 mAh g-1,which is close to the theoretical value(117 mAh g-1).The capacity retention rates at 1.0 A g-1 and 20.0 A g-1 current densities were 93.75%(4800 cycles)and 92.7%(1000 cycles),respectively.Even at-20℃,excellent specific capacity was achieved(102.6 mAh g-1 after 100 cycles at a current density of 0.1 A g-1 and a high capacity retention rate(up to 86.6%after 1000 cycles at a high current of 20.0 A g-1).F doping,V vacancies and the construction of three-dimensional conductive network greatly improve the low-temperature reaction kinetics of NVP cathode and ensure its good performance at low temperature. |
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