Optimization Design Of Vanadium Oxide Cathodes For Aqueous Zinc-Ion Batteries And Their Energy Storage Mechanisms

As the environmental and energy problems become increasingly prominent,the development of economic,safe,environmental friendly and sustainable energy storage devices has become the focus of the international community.Aqueous zinc-ion batteries(AZIBs)based on mildly acidic or near-neutral aqueous electrolytes have attracted extensive attention as next-generation power sources storage devices,with outstanding merits in cost,high security and high theoretical specific capacity.However,they remain in their infancy,due to the limited choice of cathodes.Furthermore,the energy storage mechanisms are not well established yet,which will further restrict the material design.Therefore,it is necessary to find suitable Zn2+ intercalated materials and reveal their true storage mechanism.Among various candidates,vanadium-based oxide cathodes have attracted much attention due to their abundant chemical valence states,high theoretical capacity and natural abundance.However,AZIBs are still limited by sluggish kinetics of Zn2+ intercalation and unstable structure of cathode in aqueous electrolytes.In this paper,we follow the ideas of material structure design,synthesis,characterization and mechanism exploration.Provide experimental support and theoretical basis for the development of high performance vanadium-based oxides,by unraveling the corresponding relationship between material structure and performance,as well as the storage mechanism of Zn2+ in vanadium-based oxide cathodes.(1)Li0.45V2O5·0.89H2O(LVO)was successfully synthesized by wet-mixing of LiOH·H2O and V2O5,with subsequent hydrothermal method for the first time.The hydrated lithium ions as the intercalated guest species were inserted into the V2O5 interlayer.The as-obtained LVO with a stable lamellar structure,enlarged interlayer space,weak interlayer repulsive force and low molecular weight,which shows a specific capacitance of 403 mAh g-1 at 0.1 A g-1,excellent rate capability(a specific capacitance of 189 mAh g-1 at a high current density of 5A g-1)and long lifespan with capacity retention of 86%over 1 000 cycles at 10 A g-1.Furthermore,the LVO-based AZIBs possess a synergistic insertion mechanism of Zn2+ and H+.This work provides an efficient design strategy for synthesizing advanced vanadium-based oxide cathode materials for the high-performance Zn2+ energy storage system.(2)The tunable ion diffusion channel and crystal structure stability play important roles in cathode materials for long-life and high-capacity.In this work,a synergistic intercalated strategy of iron ion and alkylammonium cation is proposed to adjust the layer spacing and stabilize the interlayer crystal structure of vanadium-based oxides.Iron ions as guest species provide strong electrostatic attraction with the negative electricity VOx lattice to stabilize the layered structure.Meanwhile,alkylammonium cation further expands the interlayer spacing and increases the surface hydrophobicity,which is beneficial to increase Zn2+ storage performance and inhibit the dissolution of as-obtained vanadium oxides in aqueous electrolyte.With high structural stability,enhanced layer spacing and surface hydrophobicity,the iron ion and alkylammonium cation co-intercalated vanadium oxide(FeVO-12)offers reversible specific capacity of 408 mAh g-1 at 0.1 A g-1,a specific capacitance of 208 mAh g-1 at 5 A g-1 and an excellent cycling stability with retention of 90%over 1000 cycles at 10 A g-1.Furthermore,the FeVO-12 possesses a co-insertion mechanism of Zn2+ and H+.This co-intercalation strategy provides a new modus operandi for developing stable and highly efficient vanadium oxide cathode toward AZIBs.(3)The intercalation strategy is helpful to improve the electrochemical performance of vanadium-based oxides,but the existing cationic intercalation agents only enlarge the layer spacing and stabilize the layer structure,do not contribute to the electrochemical activity.In this work,methylene blue intercalated vanadium oxide(HVO-MB)was prepared by sol-gel and ion exchange method.Methylene blue not only extended the layer spacing,providing sufficient transport channel for of Zn2+,but also stored Zn2+ by coordination reaction,increasing the Zn2+ storage performance of HVO-MB.As cathode of AZIBs,HVO-MB exhibits a specific capacitance of 418 mAh g-1 at 0.1 A g-1,high rate capability(a specific capacitance of 243 mAh g-1 at a high current density of 5 A g-1)extraordinary stability(88%of capacitance retention after 2 000 cycles at 10 A g-1).The electrochemical kinetics shows that HVO-MB exhibits large pseudocapacitance charge storage behavior due to the fast electron transfer and ion migration provided by the coordination reaction and expanded interlayer distance.Furthermore,the HVO-MB-based AZIBs possess a synergistic Zn2+ insertion and coordination reaction mechanism.(4)The previous works were mainly based on the cationic intercalation strategy.Although breakthroughs were made in specific capacity and rate performance,the cyclic performance has not met the application requirements.In this work,a rod-shaped composite(C@VO2@V2O5)was successfully synthesized using a V-based metal-organic framework(V-MIL-88)as precursor by a carbonization and subsequent oxidation processes.The porous carbon with high conductivity supporting and protecting vanadium oxides enhances the electron transport efficiency and alleviates the dissolution of vanadium oxide during the cycling.Furthermore,synergistic effect of layered structure(V2O5)and channel structure(VO2)energy storage endows the C@VO2@V2O5 high specific capacity and long-life cycling stability.Meanwhile,the heterojunction structure at the two-phase interface not only accelerates intercalation kinetics of Zn2+ ions,but also further consolidates the stability of the layers of V2O5 during the cyclic process.The C@VO2@V2O5 cathode delivers a promising cycle performance(capacity retention of 90.3%over 2 000 cycles at 5 A g-1).The porous carbon supporting and protecting the heterostructure vanadium oxide structure puts forward a new idea for designing advanced cathode materials for high-performance Zn2+ energy storage systems.