Design Of The Vanadium Oxides Based Electrode And Their Application In Aqueous Zinc-ion Batteries

Considering the energy security and environmental protection,we should intensify the development of new energy.At present,we are in the critical period of new and old energy conversion.How to provide large-scale,safe,reliable,low-cost,and efficient collection,storage and utilization technology for new clean energy is an urgent key issue,which puts forward higher requirements for rechargeable batteries.However,lithium-ion batteries cannot meet the application requirements of large-scale stationary energy storage due to the high cost and limited resources of Li.In addition,batteries that use flammable and toxic organic electrolyte have serious safety risks,which greatly limit their further large-scale application.Therefore,zinc-ion batteries（ZIBs）based on aqueous electrolyte are the best alternative for large-scale energy storage applications due to their inherent safety and overwhelming price advantage.Despite that,ZIBs are in their infancy and have a lot of issues,including disputed reaction mechanism,unsatisfying electrochemical performances,and limited energy density.Therefore,it is urgent to invest more research attentions on theory and materials investigation in order to facilitate its practical application and to benefit the public.As one of the important electrode materials for aqueous ZIBs,vanadium oxide electrode possesses many advantages,such as high specific capacity,good reversibility,and low cost.However,owing to the complex side reactions in water-containing electrolyte system,the reaction mechanism of vanadium oxide electrode in ZIBs is too controversial to reach a consensus.Divalent Zn²⁺exhibits strong electrostatic interaction with the host framework of vanadium oxide,which limits its diffusion kinetics inside the crystal,resulting in poor rate performance and power density.In the process of long-term cycling,vanadium oxide is prone to dissolution and volume expansion,which leads to the rapid performance degradation and poor cycling stability.Besides,the current preparation process of electrode materials is relatively complex,and the mass loading is limited,which hinder their large-scale application.In order to solve the above issues as the starting point,through the design of vanadium oxide electrode and mechanism analysis,high-performance cathode materials for aqueous zinc ion batteries were developed in this paper.The obtained innovative results are as follows:（1）An in situ electrochemical conversion strategy was developed to convert oxygen-doped vanadium nitride（O-VN）into high-valence vanadium oxide（VO_x）during the first charge process,which was used as the high-performance cathode for aqueous ZIBs.The activation process was accompanied by the release of H⁺and the decrease of the electrolyte p H,and part of the cathode material dissolved into the electrolyte in the form of V-ions.Through a series of characterizations of XRD,Raman,XPS,ICP-OES,SEM and p H monitoring,a reversible deposition and dissolution process of vanadium ions was proved to occur in the subsequent discharge/charge process,which can supply extra capacity for the battery.Vanadium-ions deposited on the surface of the electrode during the discharge process forming low-valence vanadium oxide through reduction reaction;and in the charging process,the vanadium oxide reversibly oxidized and dissolved under the electrochemical effect.Owing to the reversible cationic conversion process,combined with the intercalation reaction of Zn²⁺in VO_x,the O-VN based ZIBs exhibit ultra-high capacity of 705m Ah g^-1.（2）The intercalation mechanism of layered vanadium oxide was further studied.Two discharge platforms generally exist in the discharge process of vanadium oxide cathode,but the detailed electrochemical process has not yet got systematically studied.Herein,a two-step intercalation mechanism of Zn²⁺in V₃O₇·H₂O electrode was determined for the first time by combining first-principles calculation and a series of ex-situ characterizations.The reversible intercalation process was well confirmed by the different diffusion barriers and the segmented electrochemical kinetics at different discharge depths.In addition,a novel facile synthesis method for V₃O₇·H₂O nanoarray cathode was developed in this work,which can be controlled to synthesize self-supporting electrodes with different mass-loadings(1.0-12 mg cm^-2).Furthermore,an empirical model was constructed to evaluate the utilization of active materials under different mass-loadings.This binder-free nanoarray electrode was also fabricated as soft-pack batteries with high mass-loading,demonstrating a high-capacity performance comparable to the state-of-the-art pouch-type ZIBs.（3）By modifying the synthesis method for the self-supporting vanadium oxide nanoarray electrodes,we further synthesized a Mg²⁺-intercalated V₂O₅·n H₂O electrode（Mg VOH）at room temperature.The layered cathode material would transform into amorphous structure with the Zn²⁺intercalates into the host,which is conducive to the rapid transport of Zn²⁺because of the more active sites and isotropic diffusion channels in the amorphous materials.More importantly,through the analysis of the discharging/charging process of the battery,the amorphous Mg VOH was found to undergo a tardy phase transition in the repeated cycling process,and gradually generate Zn₃（OH）₂V₂O₇·2H₂O（ZOV）.The product of ZOV has a fixed effect on vanadium species and prevents the dissolution of vanadium in cathode.Moreover,ZOV with open framework is also an active host of Zn²⁺,which can continue to provide capacity.Therefore,the amorphous Mg VOH exhibited a long cycle life of up to 2000 cycles due to the protection of the active product of ZOV.（4）In order to prepare vanadium oxid-based cathode with high capacity,high rate capability and long-term cycle stability,we further develop a strategy of composite materials to build synergistic effects to further improve the electrochemical performance of vanadium oxide electrodes.We designed the reduced graphene oxide（r GO）wrapped polyaniline/vanadium oxide（PVOH）superlattice structure composites material（r G/PVOH）as the high-performance cathode for ZIBs.The polyaniline/vanadium oxide superlattice with a large lattice spacing of 13.8（?）is conducive to the Zn²⁺insertion/extraction and facilitate its diffusion kinetics.The r GO possesses excellent electrical conductivity and good mechanical flexibility,which not only provides a fast electronic transfer path for the electrode,but also supplies a buffer for the volume expansion of the active material in the（dis）charge process to prevent the collapse and spalling of the active material.Therefore,r G/PVOH composite exhibited improved discharge specific capacity and superior rate capability and cycling stability in comparison with pure PVOH and hydrated vanadium oxide（VOH）due to the synergistic effect from the composite.The r G/PVOH could deliver a high capacity of 375 m Ah g^-1,and retain more than 350 m Ah g^-1 after 100 cycles at the small current density of 0.5 A g^-1.It can render a remarkable capacity of 83 m Ah g^-1 at a high current density of 20 A g^-1,and exibite a long-term life span of 2000 cycles at 10 A g^-1.In addition,ex-situ XRD,Raman,XPS,SEM-EDX and in-situ FTIR characterization techniques were utilized to analyze the zinc-ion storage mechanism,which proved the excellent reversibility of the electrochemical reaction for the r G/PVOH composites.