| Energy crisis and environmental pollution promote the development of renewable energy such as wind,tidal and solar energy.The storage and utilization of intermittent sustainable energy has a great demand for the energy storage.Lithium-ion batteries are widely used in electronics and electric vehicles.However,low lithium resources,high production cost and security risks arising from the flammable and toxic organic electrolyte have greatly impeded the further development of lithium-ion batteries.Compared to organic electrolytes,aqueous electrolytes show higher safety,superior ionic conductivity,lower cost,and greater environmental friendliness.Thus,aqueous rechargeable batteriy becomes a competitive candidate for grid-scale energy storage.Rechargeable aqueous zinc ion batteries have aroused great interests,due to the rich natural sources of zinc mental on the crust,simple preparation process,low redox potential(-0.763 V vs.standard hydrogen electrode)and high theoretical capacity(820 mA h g-1).However,aqueous zinc ion batteries show lower operating voltage,limited discharge capacity,complex side reactions,and their energy density and long cycling life need to be further improved.Therefore,the exploration of suitable cathode material is the key to solve these problems.Many materials,such as manganese oxides,vanadium oxides,prussian blue and its analogues,organic compounds,have been reported as cathode materials for aqueous zinc ion batteries.Among them,vanadium oxides exhibit high capacity because of the multiple oxidation states(V3+/V4+/V5+)and multiple electron transfer in the redox reaction.However,the structural collapse of vanadium oxides cause capacity fading upon cycling.Although the intercalation of ions or water molecules in the interlayer structure have confirmed to improve cycle stability of vanadium oxides,there are still some problems need to be solved.For example,the pre-intercalated metal ions may dissolve out from the interlayer structure,which cannot stabilize the crystal structure during the long-term cycling process.The pre-intercalated water molecules and metal ions will reduce the specific weight capacity of vanadium oxides.The influence of different metal ions on electrochemical performance of vanadium oxides is not clear.In addition,the diffusion kinetics of zinc ions is sluggish due to the electrostatic interactions between the intercalated zinc ions and VO skeleton,which leads to capacity decay and phase transformation during cycling.Besides,vanadium oxides exhibit a lower average operating voltage(～0.7 V),which limits the further improvement of energy density.Therefore,it is still a challenge to achieve high energy density and long-term cycling stability of vanadium oxides for aqueous zinc ion storage.Therefore,this paper is conducted in vanadium oxides and aims to realize vanadium oxides cathode materials for zinc ion storage with high stability and high energy density by regulating the structure.The main research contents are as follows:(1)In view of the reduction of specific weight capacity caused by pre-intercalated metal ions,a strategy to improve the long cycling life of vanadium oxides by pinning effect is proposed.A small amount of Ca2+ions was pre-intercalated into V2O5 layers by a simple onestep hydrothermal method.Benefiting from the high Ca-O bond energy(464 kJ mol-1),the structure collapse of vanadium oxides upon cycling was prevented.Thus,the cathode showed no capacity decay after 3000 cycles at the current density of 10 A g-1.Meanwhile,the influence of a large amount of ion doping on weight specific capacity was reduced.The intercalated calcium ions enlarged the interlayer spacing,which provided a large channel for the diffusion of zinc ions.Structural waters effectively reduced the interaction between intercalated zinc ions and VO framework and promoted fast ion migration.In addition,the storage mechanism was analyzed by a series of characterization techniques,and it was found that the irreversible second phase(Zn3(OH)2V2O7·2H2O)was generated during cycling,which also worked as active materials for aqueous zinc ion batteries.(2)In view of the unclear influence of different doped ions on electrochemicalperformance of vanadium oxides,the strategy of regulating the properties of vanadium oxides by dual ions doping was proposed.Na ions and Mn ions co-pre-intercalated into the interlayer of V8O20 by a simple one-step hydrothermal method.The influence of different doped ions on electrochemical performance was analyzed experimentally and theoretically.It was found that the Na+ ions can promote the diffusion of zinc ions,and the Mn2+/Mn3+ ions can improve the electrical conductivity of vanadium oxides.The synergy between different ions was the key to improve the electrochemical performance of cathode materials.Besides,the influence of ion doping amount on the electrochemical performance was investigated.With the increase of manganese ion doping amount,the long-term cycling stability of cathode was further improved.The capacity retation of cathode material was improved to 99%after 1000 cycles at the currentdensity of 4 A g-1.In addition,the method of Na/Mn co-doping can be extended to other combinations of different alkali metal ions and transition metal ions,and a series of double-ion co-doped vanadium oxides with excellent electrochemical performance have been prepared.(3)In view of the sluggish diffusion kinetics and phase transformation of vanadium oxides during cycling,oxygen vacancies and phosphate groups coordinated vanadium oxides are developed through defect engineering to design cathode materials with high capacity and durability.The capacity retation of cathode material was 96.8%after 3000 cycles at the current density of 10 A g-1.The local strain in the structure of vanadium oxides caused by oxygen vacancies and phosphate groups leads to the distortion of the layered structure.A "cavity" like structure is formed between two distorted layers,which provides abundant space for zinc ion storage.Meanwhile,the strong interaction between intercalated zinc ions and VO framework is weakened,and the diffusion kinetics of zinc ion is promoted.In addition,due to the repulsive force between phosphate groups and hydroxyl groups,the phase transformation of cathode during cycling is effectively inhibited and the structure of vanadium oxide is stabilized.(4)In view of the problem of vanadium oxides with low working voltage,the strategy of manganese acetate as electrolyte additives was proposed.Due to the adsorption effect of CH3COO-ions,the barrier of the dissolution process for MnO2 was reduced.Thus,the Mn4+/Mn2+redox reaction was realized in the high voltage region.Meanwhile,the V5+/V4+ redox reaction occurred in the low voltage region.The cathode exhibits an energy density of 169.3 Wh kg-1 at a power density of 846.5 W kg-1.Two ionic redox reaction not only endowed the battery with higher capacity,but also improved the working voltage,which provides a feasible strategy for designing high energy density aqueous zinc ion batteries. |
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