Preparation And Properties Of Mn₃O₄@MWCNT Composites Materials In Aqueous Zinc-ion Batteries

Aqueous rechargeable zinc ion batteries are widely recognized for their high safety performance,environmental protection,abundant resources and easy preparation.At present,the development of aqueous rechargeable zinc ion batteries is mainly limited by the positive electrode materials.Manganese oxides are widely used in aqueous rechargeable zinc ion cathode materials due to their high specific capacity,low cost,and environmental friendliness.However,it has problems such as poor conductivity,resulting in poor cycle stability and rate performance.In addition,the preparation method of manganese oxide is often complicated,which limits its practical application in aqueous rechargeable zinc ion batteries.In order to improve these problems,in this paper,trimanganese tetraoxide was used as the research object,combined with conductive multi-walled carbon nanotubes,and a simple preparation method was used to synthesize a complex of trimanganese tetraoxide supported on the surface of multi-walled carbon nanotubes?Mn₃O₄@MWCNT composite?,and then its electrochemical properties and mechanism were studied.In addition,the electrochemical properties of different valence metal ion electrolytes in aqueous Zn//Mn₃O₄@MWCNT battery systems were investigated,which is useful for improving the electrochemical performance of aqueous rechargeable zinc ion batteries.The specific research contents are as follows:1.The Mn₃O₄@MWCNT composites were prepared by low temperature coprecipitation method.The effects of different synthesis temperatures and concentrations on the structure and electrochemical properties of the composites were discussed.At 60°C,the synthesis of trimanganese tetraoxide nanoparticles was uniformly distributed on multi-walled carbon nanotubes at a concentration of 0.3 mol L^-1 and 0.15 mol L^-1,And the average particle size of trimanganese tetraoxide was 15 nm and 30 nm respectively.It is found that the specific capacity of 300 mAh g^-1 at a current density of 100 mA g^-1,which is twice compared with pure trimanganese tetraoxide.It has an initial specific capacity of 220 mAh g^-11 at a current density of 500 mA g^-1 and a stable cycle of 300 cycles.The results showed that the multi-walled carbon nanotubes can effectively improve the conductivity of trimanganese tetraoxide,thereby improving the cycle stability,rate performance and specific capacity of Zn//Mn₃O₄ batteries.2.During the charging/discharging process,the electrochemical process mechanism of Mn₃O₄@MWCNT electrode material in the neutral electrolyte was investigated.The results show that Zn//Mn₃O₄@MWCNT battery realizes energy storage through the mutual transformation between Mn²⁺,Mn³⁺and Mn⁴⁺,and the deposition and dissolution of zinc ions on the surface of positive electrode material in the form of ZnSO₄[Zn?OH?₂]₃·xH2O.At the same time,reversible conversion of trimanganese tetraoxide from nanoparticle to nanosheet structure during charging was observed.The formation of the trimanganese tetraoxide nanosheet after charging helps to increase the contact area between the electrolyte and the electrode material,thereby accelerating the transfer speed of electrons and ions,and improving the rate performance and specific capacity of the battery.3.In order to further improve the electrochemical performance of Zn//Mn₃O₄@MWCNT battery,the electrochemical performance of different valence metal ions(such as K⁺,Mg²⁺,Al³⁺)aqueous electrolyte in Zn//Mn₃O₄@MWCNT battery system was investigated.The results show that the Zn//Mn₃O₄@MWCNT battery exhibits the initial specific capacity of 350 mAh g^-1,325mAh g^-1,220 mAh g^-1 at a current density of 100 mA g^-1 with potassium sulfate,magnesium sulfate and aluminum sulfate as electrolytes,respectively,which shows potential application prospects.The electrochemical performance of Zn//Mn₃O₄@MWCNT battery with K₂SO₄+ZnSO₄+MnSO₄ mixed aqueous solution as electrolyte was carried out.