Synthesis And Electrochemical Performance Of Vanadium Based Cathode Materials For Aqueous Zinc Ion Batteries

With the aggravation of energy shortage and environmental deterioration,the demand for advanced energy storage systems is increasing.The increasing cost,safety,and recyclability of lithium-ion battery systems has prompted researchers to turn their attention to other battery systems.Aqueous zinc-ion batteries,of which benefited by high theoretical specific capacity of zinc anode(820 m Ah g^-1),low redox potential（-0.76 V vs.S.H.E.）,rich in crustal reserves,are expected to replace lithium-ion batteries in large-scale energy storage,due to its non-combustion,non-toxic and high safety of aqueous electrolytes.Therefore,the search and improvement of cathode materials for aqueous zinc ion batteries has become a hot spot of research.Based on the layered V₂O₅ cathode material,the electrochemical properties of V₂O₅were improved by interlayer intercalation to form a vanadium bronze material.As a novel cathode material for zinc ion batteries,the zinc storage kinetic and long cycle performance of vanadium bronze were further improved by complexing with conductive carbon.The specific research contents are as follows:V₂O₅ nanorods were synthesized by sol-gel method,and a series of material characterization and electrochemical tests were carried out,the influence of Zn²⁺intercalation on the crystal lattice of V₂O₅ was analyzed by ex-situ XRD and XPS.Fe-V₂O₅ with Fe intercalation was prepared by hydrothermal method.The Fe-V₂O₅ with Fe intercalation exhibited better rate performance.However,the specific capacity of Fe-V₂O₅ electrode is decreased compared to V₂O₅ due to the occupancy of active sites by Fe ions and the large atomic number.The NH₄V₄O₁₀（NVO）cathode material with NH₄⁺intercalation was synthesized by hydrothermal method,and the optimal hydrothermal reaction method of NVO was studied.Benefited by the relatively small atomic mass of NH₄⁺,the NVO positive electrode exhibits high specific capacity(303 m Ah g^-1,0.5 A g^-1)and good rate performance(273.5,216.4,167.1,106.7 m Ah g^-1;0.5,1,2,5 A g^-1).The dilemma between high specific capacity and rate performance is solved.More importantly,the failure mechanism of NVO electrode during charge-discharge process was discussed by comparing TEM images after different charge-discharge cycles.In order to improve the electrochemical performance of the NVO electrode.An ammonium vanadium bronze nanosheets bridged by polypyrrole（NVO@PPy）as cathodes significantly enhance the cycle life and rate performances of aqueous zinc-ion batteries is reported.At a high charge-discharge rate of 28 C(10 A g^-1),the discharge capacity gradually increases with the number of cycles and reaches the maximum value of 195 m Ah g^-1 over 290cycles,and the reversible capacity maintains 58%(108.8 m Ah g^-1)even after 2000 cycles.The fast charging-discharging and stable cycling performances are attributed to the high electronic/ionic conductivity of NVO@PPy and the stabilizing effect of PPy on interlayer ammonium.The zinc ion storage mechanism of the NVO@PPy is jointly contributed by intercalation/deintercalation and pseudocapacitance behaviour,the latter not only provides extra capacity but also facilitates fast charge-discharge capability.