Vanadium-based Nanomaterials:Structural Design,Synthesis,Electrochemical Energy Storage And Mechanism

In this dissertation,we focus on the strategies to improve the performance of vanadium based nanomaterials in the applications of electrochemical energy storage devices,including lithium ion battery,sodium ion battery and hybrid supercapacitor,including material design,synthesis,characterization to electrochemical performance and their storage mechanism,aiming to obtain high capacity,high power,long life and low cost electrochemical energy storage devices.The details are summarized briefly as follows:（1）In order to overcome issues of vanadium oxide structure degradation and poor cycling stability for lithium ion batteries,we design and synthesize the vanadium oxide nanoscrolls to enhance their electrochemical performance.By controlling the surfactants and reaction conditions,novel polygonal vanadium oxide nanoscrolls are synthesized in solution for the first time.The unique polygonal nanoscroll structure is favorable for improving the cyclability and rate capability in energy storage applications.The"interlayer spacing self-adjust buffering effect"is proposed and elucidated,which is confirmed by Finite element method simulations.Based on hydrothermal method,we use the synergistic effect of two surfactants to synthesize the nanoscroll buffered VO₂（B）hybrid nanomaterial.As cathode for lithium ion battery,it could accommodate the volumetric changes during rapid ion insertion/deinsertion,resulting in long-life（over 1000 cycles）and high-rate performance.（2）To address the sluggish electrochemical reaction kinetics of vanadium oxides,we synthesize the 3D vanadium oxide hydrogels which composed of interconnected ultrathin nanobelts and self-rolled nanoscrolls through one-pot hydrothermal liquid exfoliation strategy.Further in-situ combining with multi-wall carbon nanotubes（MWCNTs）,the flexible self-standing composite films with controllable thickness is successfully fabricated.Combining the advantages of ultrathin nanobelts,porous structure,and highly conductive MWCNTs,the flexible self-standing composite films realize the electron/ion bi-continuous transport and achieve remarkable rate performance.A high capacity of 145 mAh g^-1 is still obtained at a high current density of 12.0 A g^-1.Meanwhile,the composite film exhibits excellent cycling stability(capacity retention of 83.5%after 1000 cycles at 4.0 A g^-1).The optimized composite film displays both high power density and high energy density.A high energy density of 735 Wh kg^-1is obtained at power density of 0.26 kW kg^-1.Meanwhile,at high power density of 26 kW kg^-1,the energy density still maintains 304 Wh kg^-1.（3）In order to overcome the poor sodium storage performance of vanadium oxide,we develop a facile hydrothermal method to synthesize iron pre-intercalated vanadium oxide ultrathin nanobelts（Fe-VO_x）with constricted interlayer spacing.Applying the Fe-VO_x as cathode for SIB,the lattice breathing phenomenon during sodiation/desodiation is largely inhibited and the interlayer spacing is stabilized due to the"pillar effect"of pre-intercalated iron ion for reversible and rapid Na⁺insertion/extraction.The doping iron ion enhances the electronic conductivity,resulting in enhanced cycling and rate performance compared to that of common vanadium oxide nanobelts.Further combination the merits of iron pre-intercalation and rGO,the Fe-VO_x/rGO nanocomposites display improved electrochemical performance.At the current density of 0.1 A g^-1,the nanocomposites deliver a high capacity of 215 mAh g^-1 and excellent cycling stability.Meanwhile,it displays a high-rate capacity of 101 mAh g^-1 at the current density of 2.0 A g^-1.This work presents a new strategy to inhibite the lattice breathing for enhanced sodium storage performance through interlayer spacing engineering.（4）Through"top-down"approach,we design and synthesize the porous VN nanosheets consisted by interconnected nanoparticles with rich pores and oxygen doping.Through adjusting the annealing conditions,the particle size,surface area,porous structure,and oxygen doping amount are well tuned.The as-synthesized porous VN nanosheets exhibit both remarkable lithium storage and sodium storage performance.The energy storage mechanism is also investigated systematically and it is demonstrated that the capacitive capacity contribution dominates the total capacity,which is beneficial for high-rate capacity.In addition,the smaller particle size,larger surface area,more oxide doping is beneficial for realizing better electrochemical performance.The excellent cycling performance is due to the highly reversible VN crystal and the robust porous nanosheet morphology during repeated cycles.The porous VN nanosheets with excellent high-rate and long-life performance show potential applications for hybrid supercapacitor.