| The shortage of lithium resources and the huge hidden danger of lithium ion battery in safety and environmental protection make it increasingly urgent to develop a new efficient energy storage system.Aqueous zinc-ion battery is expected to become one of the new generation of energy storage systems to replace lithium ion battery due to its advantages of low redox potential,safety,environmental protection and simple operation process.However,the slow reaction kinetics and structural instability of cathode material for zinc-ion battery seriously limit its further development.Therefore,exploring suitable cathode materials is the key to promote the development and commercialization of aqueous zinc ion battery.Vanadium-based materials are one of the most promising cathode materials because of their advantages of multivalence state,high theoretical capacity,open frame structure and abundant resources.In this paper,three different crystalline vanadium oxy compounds of HNaV6O16·4H2O,(NH4)2V4O9·H2O@MWCNT(NVOH@MWCNT)and Na1.1V3O7.9 were prepared by a simple synthetic method.The effects of structure and morphology on electrochemical properties and dynamic behavior during charge and discharge were studied,and the zinc storage mechanism was revealed.(1)Proton-substituted HNaV6O16·4H2O nanosheets were synthesized by a simple one-step hydrothermal method,which exhibited the morphology of stacked irregular nanosheets with a thickness of about 20-30 nm.The unique crystal structure of V6O16 as well as interlaminar proton and sodium hydrate ions provide rapid ionic diffusion kinetics and structural stability.The HNaV6O16·4H2O electrode exhibits a specific capacity as high as 185.6 mA h g-1 at a small current density of 0.1 A g-1 and achieves a capacity retention of 63%after 2000 cycles at a large current density of 5 A g-1,demonstrating excellent zinc storage performance and long cycle stability.It also has an energy density of 364.1 Wh kg-1 and a power density of 5225.4 W kg-1.Kinetic analysis shows that the redox reaction under high current density is mainly controlled by pseudocapacitance.The energy storage mechanism of HNaV6O16·4H2O electrode is the reversible Zn2+ion intercalation transition,which is accompanied by the change of the valence state of the element during the charging and discharging process,showing a high degree of reversibility of cycling.The unique crystal structure of V6O16 and the presence of protons and sodium hydrate ions bring rapid ionic diffusion kinetics and structural stability.(2)NVOH@MWCNT nanoplates with thickness of about 200 nm and full size in the range of 1-2 μm were prepared by ultrasonic combined with hydrothermal method,in which MWCNT interlaced with each other.The NVOH@MWCNT electrode exhibits a high specific capacity of 353.7 mA h g-1 at a current density of 0.1 A g-1.Even at a large current density of 20 A g-1,the specific capacity still reaches 126.2 mA h g-1,and the capacity retention rate is as high as 99%after 2000 cycles at a current density of 5 A g-1.Kinetic analysis indicates that the electrode is characterized by a highly reversible zinc ion storage mechanism during the intercalation transformation.The layered V4O9 framework and the N-H/O hydrogen bonds formed by NH4+intercalation make NVOH@MWCNT flexibly expand/contract during charging and discharging,maintaining the structural stability,while the recombination of MWCNT and the presence of crystal water accelerate the diffusion of ions.(3)The stacked ultrathin Na1.1V3O7.9 nanobelt cathode material was obtained by calcination method,which exhibits a specific capacity of 209.4 mA h g-1 at a current density of 0.1 A g-1,and reaches a capacity retention rate of 63%after 20000 cycles at a high current density of 5 A g-1.It also shows a high energy density of 161.2 Wh kg-1 and a power density of 10503.6 W kg-1.In addition,the kinetic analysis of Na1.1V3O7.9 electrode material reveals that the redox reaction is mainly dominated by pseudocapacitance process. |
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