MOFs Derived Vanadium Oxide As A Positive Electrode Material For Aqueous Zinc Ion Batteries

With the ever-growing demand for consumer electronics,the steady development of electric vehicles and the grid connection of renewable energy,the development of energy storage technology has been greatly promoted.Among various energy storage systems,lithium ion batteries?LIBs?dominate the commercial secondary battery market due to their high energy density and long cycle life.However,lithium ion battery has some inherent problems such as inflammability of organic electrolyte and scarcity of lithium resources,which seriously affect its large-scale application.Therefore,it is imperative to develop secondary batteries with safe water-based electrolytes,low cost and high energy density.Aqueous zinc ion battery?ZIBs?not only possess the above characteristics,but also with a high theoretical capacity up to 820 mAh g^-1 and a low redox potential of Zn2+/Zn?-0.76 V vs.SHE?,is a potential electrochemical energy storage device.However,it is still in the primary stage because of the limited selection of cathode materials with high rate and long-life cycling stability.In addition,the energy storage mechanism of ZIBs has not been well established.In this paper,we focus on the development of new cathode materials with high-performance,the optimization of the existing cathode materials and in-depth study of the energy storage mechanism of zinc ion battery.Based on the metal organic framework,combined with the strategies of porous materials,composite materials and micro-nano structured design,the main innovative results are as follows:?1?V-MOF was used as the precursor to obtain porous V₂O₅ nano plates?P-V₂O₅?by thermal oxidation treatment,and then it was adopted as the intercalation cathode material for aqueous zinc ion battery.Compared with the commercial V₂O₅?C-V₂O₅?,the effects of specific surface area,pore size distribution and mixed value on battery performance were studied.When the current density is 100 mA g^-1,the obtained P-V₂O₅ electrode has a high capacity of 300 mAh g^-1,while the capacity of C-V₂O₅ is only 60 mAh g^-1.The energy density of this zinc-ion battery is about 230 Wh kg^-1,it is much higher than that of commercial lead-acid batteries.Even at a high current density of 2000 mA g^-1,the capacity of P-V₂O₅ electrode remains at 120 mA h g^-1,which is much higher than that of C-V₂O₅.Moreover,the composite structure of V₂O₅nanometer sheet and carbon based material can also improve its cyclic stability.?2?For the first time,we introduced highly conductive V₂O₃@C as a high-rate,high-stability positive electrode material into the aqueous zinc ion battery and studied the mechanism of its energy storage.Vanadium trioxide?V₂O₃?is a typical corundum structure with tunnel-like three-dimensional structure and metal behavior.This material has great potential in aqueous zinc-ion batteries.We used two V-MOF as precursors to obtain spherical and blocky V₂O₃ and porous carbon composites by pyrolysis in nitrogen environment,and studied the effects of material morphology and pore size distribution on battery rate performance and the effect of porous carbon skeleton on battery cycle stability in detail.The unique channel and proper pore size distribution of corundum V₂O₃ are favorable for rapid intercalation and remove of zinc ions,and result in high rates performance.The carbon skeleton structure has high cyclic stability.The porous spherical V₂O₃@C positive electrode shows a high capacity of 350 mAh g^-1 at 100 mA g^-1,excellent rate performance?250 mAh g^-1 at 2 A g^-1?and impressive long life cycle stability with a 90%capacity retention over 4000 cycles at 5 A g^-1.The storage mechanism of zinc ion in Zn/V₂O₃ system was studied by using several analytical methods and first-principles calculation.?3?Although the synthesis of transition metal oxide hollow structure is well established,the size distribution,shape uniformity,cavity volume,shell clarity and functional groups of products are still difficult to control.We propose a new strategy summarized as "oxidization-sedimentation-recrystallization-formation",which use metal organic framework?MOFs?as the precursor to obtain corresponding metal oxides and carbon composite microspheres with 0,1,2 and 3 shelled hollow spheres.We designed and synthesized spherical Co-MOF and Ni-MOF,the Co₃O₄@C and NiO@C with uniform morphology were obtained by regulating the thermal oxidation conditions,and then we studied the performance of their lithium-ion batteries.The multiple shells assembled from nanocrystalline grains and the large interior cavities between them can benefit the electrolyte permeation,providing abundant active sites for lithium ion storage.The solid eggshell structure and porous carbon distribution between nanocrystals can adapt to the large volume expansion and contraction caused by the repeated Li+insertion/extraction process in the cycling process,thus improving the cycling stability.The thin porous shell can shorten the diffusion distance of Li+and improve the charge transfer kinetics.The three-layered Co₃O₄@carbon hollow microspheres have a high reversible capacity of 1200 mAh g^-1 at 100 mA g^-1,and gradually increase to 1701 mAh g^-1 after 60 cycles.At a current density of 1000 mA g^-1,the capacity can be maintained at 601 mA g^-1 after 500 cycles.?4?The currently developed positive electrode materials of zinc ion battery have poor reversibility in water,and the diffusion kinetics of divalent zinc ion is slow.We used defect engineering strategy to introduce oxygen vacancy into V₂O₅ to improve electrochemical kinetic retardation and cyclic stability of zinc ion battery.The adsorption energy of Zn²+in oxygen-deficient V₂O₅ decreased close to thermal neutrality and the diffusion barrier also decreased significantly.In addition,in oxygen-deficient V₂O₅,the mixed vanadium valence generated by the transfer of the charge around the oxygen atom to the vanadium atom can greatly improve its electrochemical activity and reduce the polarizability.Oxygen defects reduce the band gap of V₂O₅,which leads to higher conductivity and electrochemical activity,thus obtaining better rate performance.Therefore,the oxygen deficient V₂O₅ zinc ion battery has a high reversible capacity of 400 mAh g^-1 and a capacity of 200 mAh g^-1 at high current density of up to 5 A g^-1,showing excellent rate performance.In addition,the long-term cycling performance is good,the capacity after 2000 cycles almost no decline.Compared with V₂O₅ with fewer defects,the capacity of V₂O₅ electrode with more defects increased by 424%at current density of 5 A g^-1,and the remaining capacity increased by 89%after 2000 cycles.