Preparation Of TiC@graphene@AVO Composite Nanofibers And Their Electrochemical Zinc Ion Storage Properties

Aqueous zinc ion batteries are important electrochemical energy storage devices,and the properties of their cathode materials are one of the key factors determining the energy storage performance of zinc ion batteries.Amorphous vanadium oxide（AVO）is an excellent cathode material for zinc ion batteries because of its amorphous structure framework,which can provide transportation channels for ion diffusion and transfer.However,nanosized AVO materials have the problems of low conductivity and easy agglomeration,which limits the improvement of their electrochemical energy storage performance.The amorphous vanadium oxide（AVO）is uniformly dispersed in the carbon nano-conductive network structure to form a composite material,which can not only realize the dispersion and efficient utilization of the active material,but also improve the charge transport performance through the carbon conductive network.It is considered to be an effective way to improve the electrochemical energy storage performance of electrode materials.In this paper,Ti C@graphene electrospun nanofibers were used as precursors to construct a hierarchical conductive network structure by combining high-temperature carbothermal and magnesiothermic reduction.Ti C@graphene@AVO composite nanofibers were further prepared on this basis.The composite of Ti C and carbon nanofibers is used to improve the one-dimensional charge transport performance,and the surface graphene hierarchical structure is used to improve the contact area and charge transfer performance with AVO,which significantly improves the zinc ion energy storage performance of AVO.In the research work,the effects of preparation conditions of Ti C@graphene on the morphology,structure and electrochemical properties of the materials were systematically studied,and the effects of different conductive substrates on the obtained AVO zinc ion battery cathode materials were further compared.The main research contents are as follows:（1）Polyvinylpyrrolidone/tetrabutyl titanate organic-inorganic composite nanofibers were prepared by electrospinning,and CNFs-Ti O₂ composite carbon nanofibers were obtained by pre-oxidation and carbonization.Ti C@graphene composite nanofibers with Ti C as the core and hierarchical graphene nanosheets as the shell were synthesized by high temperature carbothermal and magnesiothermic reduction process.The experimental results show that the hierarchical structure of graphene nanosheets will be formed on the surface of the material when the reaction temperature reaches above 800°C.With the increase of reaction temperature and keep the heat time（from 700°C to 1000°C,from 0 h to 6 h）,the crystallinity of Ti C and graphene in the composite nanofibers increased,resulting in a gradual increase in the conductivity of the material and a gradual decrease in the electrochemical impedance.Electrochemical experiments showed that the specific capacity of Ti C@graphene composite nanofibers synthesized at 1000°C and held for 6 hours was 21.4 m A·h/g at a current density of 0.5 A/g,and the capacity retention rate was 71.9%when the current density was increased to 8 A/g.The comparison test results show that the surface charge storage is mainly derived from the surface graphene nanosheet structure,and its excellent rate performance is closely related to the excellent multi-level conductive network structure of the material.（2）Based on the hierarchical conductive network structure of Ti C@graphene electrospun nanofibers,amorphous vanadium oxide（AVO）precursors were loaded on the surface of Ti C@graphene nanofibers by solvothermal method（using glycerol as solvent and ammonium metavanadate as vanadium source）.The precursor was placed in a tube furnace and annealed at 300°C in a nitrogen atmosphere to successfully prepare Ti C@graphene@AVO cathode material.The same method was used and the solvent after the reaction was freeze-dried to obtain CNFs@AVO and AVO powder cathode materials.Electrochemical tests show that Ti C@graphene@AVO cathode material has the best electrochemical performance.It exhibits a reversible specific capacity of 314.9 m A·h/g at a current density of 1 A/g（2.2 times and 1.1times that of AVO and CNFs@AVO,respectively）.When the current density increases from1 A/g to 50 A/g,the capacity retention rate reaches 51.7%（2.8 times and 1.9 times of AVO and CNFs@AVO,respectively）.The capacity is stable at about 263 m A·h/g after 3000 cycles at a current density of 10 A/g.The results of CV curve show that the charge storage process of Ti C@graphene@AVO zinc ion battery is controlled by surface process and diffusion process,in which surface process plays a dominant role.In addition,the reaction mechanism of battery charge and discharge process was discussed by STEM and Element mapping.The reversible insertion/extraction of Zn²⁺and the regular appearance/disappearance of S and F elements were observed.The results show that the energy storage mechanism is the co-insertion/extraction of H⁺and Zn²⁺.