The Preparation Of Vanadium-based Metal-organic Frameworks As Cathode Materials For Advanced Aqueous Zinc-metal Batteries

Commercial lithium-ion batteries have safety hazards due to the use of flammable and explosive organic electrolytes,so it is urgent to development safe and stable new batteries.The rechargeable aqueous zinc metal batteries have the potential to replace lithium-ion batteries in some application areas（such as large-scale energy storage systems）for high-safety（aqueous electrolyte instead of carbonate electrolyte）and low-cost（metal zinc as a low-price anode）.At present,the difficulty and opportunity for developing advanced aqueous zinc metal batteries are the design of the electrode materials and the matching of related electrolytes.In this paper,we focused on the design and development of new aqueous zinc metal battery cathode materials,and synthesized vanadium-based MOF by using metal-organic framework compounds（MOFs）as the starting point.The possibility of directly using MOFs as aqueous zinc battery cathode has been investigated.The electrochemical performance,phase transition mechanism,and characteristic behavior of V-MOF cathode in different electrolytes were studied in detail.The main work is as follows:（1）Preparation,surface engineering and battery performance of V-MOF:Firstly,V-MOF was synthesized by solvothermal method.Then,V-MOF were heat treated at low temperature for removing impurities in pores meanwhile maintaining stabile structure.The heat treatment conditions（temperature and atmosphere）were explored,also.Characterization tests showed that the structure of V-MOF was unchangeable after heat treatment,while the specific surface area of the heat-treated V-MOF was significantly increased to 1116 m² g^-1.Also,the hydrophilicity of V-MOF was also improved.Using 3 M Zn SO₄ as the electrolyte,the Zn//V-MOF battery had high initial discharge specific capacity of 216.3 m Ah g^-1 at 0.2 A g^-1,and could keep high discharge specific capacity of 145.6 m Ah g^-1 after 2000cycles at 5 A g^-1.（2）Dual-cation electrolytes boosting Advanced Zn//V-MOF Aqueous Batteries:Considering the influence of aqueous electrolyte on the cathode and anode,other cationic electrolytes were added to Zn SO₄ electrolyte in order to improve the performance of Zn//V-MOF battery.Also,the reasons why the performances of Zn//V-MOF battery were improved in double cation electrolyte has been explored.In the dual-ion electrolyte（3 M Zn SO₄/1 M Li₂SO₄）,Zn//V-MOF battery had excellent rate performance(318.2,304.6,291.0,269.4,246.6,227.9 m Ah g^-1 specific capacity at 0.5,1,2,5,10,and 15 A g^-1,respectively)and long cycle stability(99.9%capacity retention rate after 1000 cycles at 2 A g^-1and 247.7 m Ah g^-1after3200 cycles at 5 A g^-1).It has been proved that Li⁺could be reversibly inserted/extracted into the V-MOF structure together with Zn²⁺.The ionic diffusion rate of V-MOF was improved in Zn²⁺/Li⁺electrolyte and the contribution of pseudo-capacitance was increased.Besides,Li⁺in electrolyte alleviated the growth of dendrites in zinc anode.（3）Optimization of voltage window based on 3 M Zn SO₄/1 M Li₂SO₄electrolyte:For the purpose of avoiding the reactions of hydrogen evolution at cathode electrode and oxygen evolution at anode electrode,the optimal charge and discharge voltage of the Zn//V-MOF battery in 3 M Zn SO₄/1 M Li₂SO₄ electrolyte were explored.The charge-discharge potentials of 1.2～1.8 V and 0.1～0.4 V were explored respectively.Finally,taking the polarization of electrodes under high current into consideration,the optimal voltage window of Zn//V-MOF aqueous battery in this paper was 0.2-1.6 V.