Preparation And Properties Of VO₂ Cathode Material For Waterborne Zinc Ion Battery

Aqueous zinc-ion batteries have become the best choice to replace traditional lithium batteries due to their high safety,simple preparation process,environmental friendliness and excellent theoretical capacity.However,the lack of suitable cathode materials with excellent performance is the fundamental reason that limits their development.In this thesis,taking the application of vanadium dioxide（VO₂）in aqueous zinc-ion batteries as the clue,in view of its poor rate performance,structural collapse and pulverization caused by ion intercalation during charging and discharging,by constructing different crystal forms and composite structures,we advanced their electrochemical properties as cathode materials for aqueous zinc-ion batteries and explain the energy storage mechanism.First,β-VO₂ nano-rods were synthesized by a hydrothermal method,and the material can achieve a specific discharge capacity of 325 mAh g^-1at a current density of 0.05 Ag^-1.However,the capacity decays to 0 mAh g^-1 after only 300 cycles at a current density of 1 Ag^-1.In addition,the rate performance is poor,the specific capacity decays rapidly at low rates,and the capacity is low at high current density.In order to improve the electrochemical performance of VO₂,we prepared R-VO₂ microspheres with dense structure by spray drying method,and successfully constructed R-VO₂/CNTs with composite structure by adding carbon nanotubes（CNTs）to the precursor.R-VO₂/CNTs composite microspheres.The construction of the composite structure opens a large number of channels through the microspheres on the R-VO₂ microspheres,which effectively shortens the ion diffusion path and improves the Zn²⁺ion diffusivity.The recombination of CNTs simultaneously increases a large number of energy storage sites and improves the electrochemical performance.The discharge specific capacity of the R-VO₂/CNTs composite microspheres is as high as 335 mAh g^-1 at 0.05 Ag^-1,which is much higher than that of the R-VO₂ microspheres(280 mAh g^-1).The rate performance is also greatly improved,and when the current density is increased to 3 Ag^-1,it still remains at 100 mAh g^-1.In addition,the composite structure provides an additional C-O bonding bond,which increases the stability of the structure and thus still retains 30%of the capacity after 900 cycles at a current density of 1Ag^-1.Finally,we used the spray drying method to prepareβ-VO₂/CNTs composite microspheres andβ-VO₂/CNTs core-shell microspheres,respectively,by adjusting the ratio of ammonium metavanadate and CNTs in the precursor.The results show that the specific capacity ofβ-VO₂/CNTs core-shell microspheres can reach 380 mAh g^-1 at a current density of 0.05Ag^-1.During the repeated charge-discharge process,the elastic contraction and expansion of the CNTs shell prevented the structural collapse of theβ-VO₂ core,thereby significantly reducing the dissolution ofβ-VO₂ and improving the cycling stability of theβ-VO₂/CNTs tube-core-shell microspheres.