Investigation On Zinc Storage Performance Of Polyoxovanadate Materials As Cathodes For Aqueous Zinc Ion Batteries

In recent years,researchers have achieved great progress in exploring the development of high capacity,high energy density,low cost and high safety electrode materials for aqueous zinc ion batteries（ZIBs）.However,these materials are focused on manganese-based compounds,vanadium-based compounds,Prussian blue and its analogues,and organic compounds,while new materials are less frequently reported.In addition,most of the cathode materials investigated presently are suffering from structural instability and poor cycling stability,which seriously restrict their further application and development.Therefore,it is particularly important to find suitable cathode materials.As representatives of new energy storage materials,polyoxovanadates（POVs）have gained much attention from researchers for their structural diversity and multi-electron redox properties.Among them,(V₁₀O₂₈)^6-is the most structurally characterized decavanadate anion in POVs and has been investigated extensively.Nevertheless,there is a paucity of anion-related derivatives for aqueous ZIBs.Therefore,in this work,a series of derivatives based on the decavanadate anion(V₁₀O₂₈)^6-were synthesized and their electrochemical properties,kinetic behavior and zinc storage mechanism were investigated and discussed in depth,thus filling the gap in this research area and contributing new ideas for the design and synthesis of cathode materials for aqueous ZIBs.The key contents are as follows.（1）K₂Zn₂V₁₀O₂₈·16H₂O（KZVO-16）materials were prepared using a simple liquid-phase synthesis method.K₂Zn₂V₁₀O₂₈·3.17H₂O（KZVO-3.17）and K₂Zn₂V₁₀O₂₈（KZVO）materials were obtained based on different calcination temperatures,and they were employed as cathodes for aqueous ZIBs,respectively.By comparing the electrochemical properties of the three systems,it can be noticed that KZVO possessed a shorter activation cycle and better long-term cycling stability.This was principally attributed to the fact that water molecules occupied a large number of active sites,and the detachment of water molecules facilitated the embedding and detachment of Zn²⁺.As a result,the KZVO presented a high initial discharge specific capacity of 223.4 m Ah g^-1 at a current density of 0.1 A g^-1 and a voltage window of0.2-1.9 V.Even after 800 long cycles at 2.0 A g^-1,KZVO still maintained a reversible discharge specific capacity of 145 m Ah g^-1.Furthermore,the kinetic analysis indicated the presence of pseudocapacitance effect in the material(71.5%contribution of pseudocapacitance at 0.6 m V s^-1).The ex-situ XRD characterization tests demonstrated that the reaction mechanism of the KZVO material during galvanostatic charging and discharging was the reversible insertion and extraction of Zn²⁺.（2）Na₆V₁₀O₂₈·18H₂O（NVO-18）was prepared and dehydrated to obtain Na₆V₁₀O₂₈（NVO）by a liquid-phase precipitation modulation method,and the electrochemical properties of such derived materials based on(V₁₀O₂₈)^6-clusters and the mechanism of zinc storage were further investigated.Given the related work in the last section,extensive electrochemical performance tests were performed directly on the NVO/Zn cell system.The results demonstrated that the NVO could achieve a discharge specific capacity of up to 279.5 m Ah g^-1 at a current density of 0.1 A g^-1.Additionally,no significant capacity loss was observed even at a high current density of 2.0 A g^-1 for a long cycle of 2000 cycles.According to the pseudocapacitance analysis and GITT test,the high pseudocapacitance effect and the high Zn²⁺diffusion coefficient of the NVO material were confirmed.Also,a series of ex-situ characterization tests reconfirmed the reversible insertion and removal of Zn²⁺in the main framework of NVO during cycling.Notably,the irreversible phase Zn₃（OH）₂V₂O₇·2H₂O（ZVOH）generated during the cycle was also able to store Zn²⁺in concert with the NVO host material,which further confirmed the storage mechanism of Zn²⁺in this type of derived material.（3）To further analyze the reason why the(V₁₀O₂₈)^6-cluster-based derivatives can operate stably during cycling,the same series of K₄Na₂V₁₀O₂₈·10H₂O（KNVO-10）was synthesized by liquid-phase p H adjustment,and K₄Na₂V₁₀O₂₈（KNVO）was prepared to determine the specific kinetic behavior during cycling and the variation of characteristic parameters related to impedance.The electrochemical performance tests revealed that the KNVO/Zn cell can achieve a high reversible discharge specific capacity of 102.5 m Ah g^-1 even at a long cycle of1000 cycles at a current density of 1.0 A g^-1.In addition,the KNVO/Zn cell also presented decent rate performance with high average cycle retention at each current density.Secondly,the pseudocapacitance effect in the system was further quantified based on Trasatti analysis,which also reinforced the fact that the reaction was dominantly controlled by capacitance from another perspective.By analyzing the changes in the characteristic parameters related to the impedance before and after cycling,we can find that the increase of the KNVO phase shortened the diffusion path of Zn²⁺in the material,decreased the electronic resistance,and promoted charge transfer.Not only that,the real/virtual capacitance also increased gradually during the cycling process,which accelerated the rapid response of the material near the surface and thus promoted the subsequent storage of Zn²⁺,which further revealed the reason for the cycling stability.