Regulating Zinc Metal Anode Interface Through Mixed Zinc Salt Aqueous Electrolyte

Lithium ion batteries of organic systems have high cost,limited resources and safety problems.Large-scale energy storage systems pay more attention to low cost and high safety.Compared with non-aqueous Lithium ion battery systems,aqueous zinc ion batteries（AZIBs）can accurately meet these requirements,thus becoming a promising candidate for large-scale energy storage.However,the electrochemical performance of AZIBs is far from satisfactory.In mild acidic aqueous electrolytes such as 2 M zinc sulfate（Zn SO₄）and zinc trifluomesylate（Zn（OTF）₂）,there is uneven local current density on the surface of Zn electrode,leading to the nonuniform zinc deposition and dendrite formation.The electric field strength at the dendrite position is strong,and zinc ions will continuously deposit at the dendrite position,resulting in the growth of dendrite,which will eventually puncture the separator and cause short circuit of the battery.In addition,Zn metal is thermodynamic unstable in mild acidic aqueous electrolyte.Direct contact of Zn metal anode with active water molecules leads to hydrogen evolution reaction and the increase of OH^-concentration at the electrode/electrolyte interface,which produce zinc hydroxylated sulfate（ZHS）by-product with poor conductivity.The formation of ZHS seriously affects the ionic conductivity of Zn surface.With the accumulation of by-product,the metal zinc and electrolyte are blocked,and the conductivity of zinc ions decreases,which will eventually break the battery.In conclusion,when the electrode/electrolyte interface p H is unstable or lack of solid electrolyte interphase（SEI）,serious Zn dendrites and side reactions will occur on Zn electrode surface.Therefore,the stability of electrode/electrolyte interface are highly needed for AZIBs.To solve the above problems,a series of new mixed aqueous electrolyte systems are designed in this thesis,which can not only buffer the p H value of the electrode/electrolyte interface,but also induce the in-situ formation of inorganic-organic SEI layer,thus greatly inhibiting the formation of Zn dendrites and side reactions.The contents of this thesis are as follows:1)The mixed electrolyte was prepared by simple mixing the wildly used 2 M Zn SO₄ and 2 M Zn（OTF）₂aqueous solution.The mixed electrolyte can form a uniform Znx（SO₄）_y（OTF）_z（OH）_2x-y-z·n H₂O（ZSF）passivation layer on Zn electrode surface,thus regulating the OH^-concentration at the electrode/electrolyte interface and stabilizing the interface p H at about 4.9.During cycling,the ZSF passivation layer can be transformed into a uniform organic-inorganic SEI layer,which significantly inhibits the formation of Zn dendrites and side reactions.The results show that the anode life of zinc in the mixed electrolyte is increased significantly,which can reach more than 2000h,and a high Coulomb efficiency（CE）of 99.5%is achieved.2)The mixed aqueous electrolyte was prepared by mixing 2 M Zn SO₄ and 2 M Zn（TFSI）₂.The mixed electrolyte can also generate in situ SEI and buffer the p H changes on the electrode surface to a certain extent,thus effectively inhibiting the growth of dendrites and the occurrence of side reactions.The results show that the life of zinc in the mixed electrolyte is increased greatly,which can reach more than 2800 h,and the high CE of 99.52%is achieved.