Research On Vanadium Oxide Cathode Material For Rechargeable Zinc Battery

The efficient storage and conversion of renewable energy is the most direct and effective way to solve energy shortages and environmental pollution.The development of advanced energy storage secondary battery technology is of great significance for the effective use of renewable energy.Lithium-ion batteries have been widely used as high-efficiency energy storage devices in the field of portable electronic equipment and electric vehicles.However,the increasingly scarce and unevenly distributed lithium resources have increased the price of lithium-ion batteries.At the same time,traditional flammable organic electrolytes have caused the battery safety issues limits the further development of lithium-ion batteries in the field of large-scale energy storage.Rechargeable aqueous zinc batteries are considered to be an ideal choice for large-scale energy storage systems due to their abundant zinc resources,cheap and easy access,high safety of aqueous electrolytes and environmental friendliness.The cathode materials is an important functional part of battery chemistry,whose physical and chemical properties and performances directly affects the energy density and cycling performances of batteries.Among them,vanadium oxides have the characteristics of large layered structure,multiple variable valence states and high energy storage capacity,which have been favored by researchers.However,vanadium oxides as cathode materials still have the following three scientific problems that need to be solved urgently:（1）Zn²⁺has a high charge and a large atomic weight,which will cause its slow migration kinetics,restricting the rapid charge and discharge capacity,vanadium oxides in the aqueous electrolyte exist a dissolution phenomenon,which will cause the attenuation of cycling life of batteries;（2）The effect of interlayer structure water content of vanadium oxides on the reaction kinetics of Zn²⁺and electrochemical performance of cathode materials needs to be clarified;（3）In aqueous electrolytes,A variety of cations(such as Zn²⁺,H⁺)have a competitive relationship in the intercalation of layered structure.The reaction mechanism is still unclear,and further research is needed.Based on this,this paper designs an integrated layered V₅O₁₂·6H₂O cathode material.Its large layered structure（¹.18nm）facilitates the reversible de-intercalation of polyvalent cations.By optimizing the electrolyte,the battery cycling stability is significantly improved;revealing the effect of interlayer structure water content of vanadium oxide cathode（taking V₂O₅·nH₂O as an example）on electrochemical zinc storage behavior and battery performance;clarifing the zinc storage mechanism of traditional V₂O₅ cathode in aqueous ZnSO₄ electrolyte.The main results and contents are as follows:1.Rational design and electrochemical performance of integrated layered vanadium oxide cathode.Using in-situ electrochemical deposition method,V₅O₁₂·6H₂O（VOH）nanoribbons can be uniformly deposited on the surface of the stainless steel mesh substrate,which can be directly used as a cathode material without additional treatment.This simple electrochemical deposition method can simplify the electrode preparation process and avoid the use of inactive materials（such as binders and conductive agents）,which is conducive to improve energy density of batteries and reduce cost of electrode assembly.By using the optimal electrolyte（3 M Zn（CF₃SO₃）₂）,the VOH cathode achieves a reversible capacity of346.5 mAh g^-1(0.5 A g^-1)and long-cycle stability(the capacity retention rate is 94%after1000 cycles(2 A g^-1)).Its large layered structure（1.18 nm）and abundant interlayer structure water can significantly improve the kinetics of Zn²⁺diffusion(the diffusion coefficient of Zn²⁺is 10^-10–10^-11 cm² s^-1),which makes the VOH cathode show excellent rate performance(278 mAh g^-1,3.0 A g^-1).We combined the electrochemical analysis methods and various characterization methods（such as XRD,XPS,TEM,etc.）to clarify the electrochemical zinc storage mechanism of VOH cathode material,that is,Zn²⁺is reversibly inserted/extracted between VOH layers during the discharge/charge process,accompaning by a reversible decrease/increase of vanadium valence.In addition,attributing to the integrated flexibility of VOH,we have constructed a flexible quasi-Zn-VOH battery device and developed a flexible quasi-solid gel electrolyte.Electrochemical tests show that the flexible Zn-VOH battery can achieve stable work in different bending states.and the capacity retention rate is as high as96%after 50 cycles.2.Study on the influence of water content in the interlayer structure of V₂O₅·nH₂O on electrochemical zinc storage behavior.The traditional V₂O₅·nH₂O cathode material was synthesized by a simple room temperature synthesis method,and the water content in V₂O₅·nH₂O was adjusted by adjusting calcination temperature.XRD,TGA,FTIR,SEM and other characterization methods were used to determine the effect of different heat treatment temperatures on the water content of interlayer structure and on the morphology and structure of V₂O₅·nH₂O cathodes.At the calcination temperatures of 0,150,200 and 300°C,the water content of V₂O₅·nH₂O is n=1.3,0.8,0.5 and 0,respectively.Through a variety of electrochemical analysis methods,we delved into the effect of V₂O₅·nH₂O interlayer structure water content on the reaction capacity,rate performance,cycle stability,pseudocapacitive behavior and Zn²⁺diffusion coefficient.The results show that the higher the water content in the interlayer structure（n=1.3）,the better electrochemical performance of V₂O₅·nH₂O,such as the reversible capacity of 331 mAh g^-1(0.5 A g^-1),long cycling stability with a retention rate of 92.5%(2 A g^-1)after 1000 cycles and rate performance of 267 mAh g^-1(2.0 A g^-1).Interlayer structure water of vanadium oxide theoretically has a function of charge shielding,which can reduce the effective charge of Zn²⁺and the electrostatic repulsion between Zn²⁺and crystal structure of cathode,improving electrochemical reaction kinetics of Zn²⁺.3.Study on zinc storage mechanism of V₂O₅ cathode in aqueous ZnSO₄ electrolyte.Layered V₂O₅ cathode materials have been studied in rechargeable aqueous zinc batteries,but their charging and discharging mechanisms are still controversial.Based on this,we have revealed the electrochemical reaction mechanism of V₂O₅ cathode in aqueous ZnSO₄electrolyte through characterization methods such as electrochemical test,XRD,XPS,UV-vis,SEM,and TEM.Research shows that during the discharge process,H⁺and Zn²⁺are co-embedded in the V₂O₅ cathode material,and the first and second discharge voltage platforms are mainly contributed by H⁺and Zn²⁺intercalation,respectively.At the same time,by-products of Zn₄SO₄（OH）₆·5H₂O will be gradually produced on the surface of cathode,which attributing to the H⁺insertion will cause the local pH rise of the electrolyte（H₂O→H⁺+OH^-）,which triggers SO₄^2-,Zn²⁺and OH^-precipitation reaction.During the charge process,H⁺and Zn²⁺are gradually released from the V₂O₅ cathode structure,accompaning by the dissolution of Zn₄SO₄（OH）₆·5H₂O.We further studied the influence of electrolyte concentration on the electrochemical performance of V₂O₅ cathode.The results show that high-concentration electrolyte（3 M）can reduce the activity of water molecules,reducing the dissolution of active materials,and improving the cycling stability of the cathode material.Therefore,the V₂O₅ cathode shows excellent cycle stability with a capacity retention rate of92.4%(2 A g^-1)after 1200 cycles.In addition,electrochemical tests show that V₂O₅ cathode has a pseudocapacitive behavior during the charge and discharge process,which is conducive to improving rate performance of battery,and showing a capacity of 407.7 and 304.6 mAh g^-1at 0.5 and 4.0 A g^-1,respectively.