Study On Electrocrystallization Process Of Zinc Metal Anode In Aqueous Electrolyte And Application Of Polyacrylonitrile As Protective Layer

Aqueous zinc-ion batteries are safe,economical,environment-friendly,which have high capacity and energy density,and zinc metal resources are abundant.They are considered to be the next generation of new batteries with great potential,especially when Zinc metal is used as anode.However,the zinc anode faces the following problems in the cycle:(1)The zinc ion is easy to form dendrite in the cycle of deposition and stripping,and the longer the cycle time is,the more serious the dendrite problem will be.It may even puncture the membrane and cause a short circuit inside the battery,causing safety problems;(2)Hydrogen evolution and by-products.In neutral or weak acid electrolyte,H⁺and Zn²⁺have a competitive relationship in the reduction process,and the generated hydrogen will increase the battery internal pressure,causing safety risks.At the same time,the change of p H environment in the local anode will lead to the generation of by-products,which hinder ion transport and increase the internal resistance of the battery.(3)Metal zinc infiltration in aqueous electrolyte for a long time will lead to the passivation problem.This will make the redox reaction of the anode more and more difficult to carry out.In addition,the mismatch between the capacity of anode and cathode leads to the difficulty in achieving the theoretical energy density of the aqueous zinc ion battery,which is also one of the main reasons for the limitation of the commercialization of the zinc metal based battery.Aiming at the above problems,this paper systematically studied the deposition process of zinc anode.Combined with the relationship between over-potential,energy and nucleation size in the field of electrodeposition,explored the influence of current density and deposition substrate on nucleation size in the process of zinc electrodeposition,and finally obtained the relationship between nucleation size and zinc deposition morphology.In the experiment,we tested the first deposition overpotential of zinc ions under three different current densities.Specifically,the larger the current density was,the larger the overpotential was,and the smaller the size of zinc nucleus was.Then we tested the nucleation status on different deposition substrates,and found that the deposition substrate not only affected the size of zinc nucleus,but also affected the growth direction of zinc nucleation.Compared with(100)_Zn and(101)_Zn directions,the nucleation direction of(002)_Zn is more conducive to the nucleation of zinc parallel to the substrate direction,and such growth direction will make the deposition morphology of zinc more smooth.Finally,the size of zinc nuclei was regulated by electrochemical means to make the zinc nuclei grow under the same conditions.It was found that the final deposition morphology of anode with small size of zinc nuclei was more delicate and smooth,which was of great significance for improving the problem of zinc dendrite.In addition,the improvement of zinc anode with modified polyacrylonitrile(PAN)coating was also investigated.To be specific,polyacrylonitrile(PAN)is calcined in the air to get the modified PAN.The polar functional group N-H of the modified PAN has the characteristic of guiding the uniform deposition of zinc ions.Therefore,it is mixed with the binder PVDF in a certain proportion to make an organic coating on the zinc anode,which can prevent the appearance of dendrite.The protective effect of the coating also reduces the degree of polarization of the negative electrode,avoiding the direct contact between the zinc anode and the electrolyte,so the zinc anode is not easy to be corroded,delaying the passivation process of the metal anode,and the amount of by-products generated is also greatly reduced.Therefore,compared with the uncoated battery,the long cycle performance of the modified PAN@Zn battery reaches 600h,and can maintain 99%of the coulomb efficiency to 1000 cycles without attenuation.This proves that the strategy of using organic coating to improve the anode of zinc metal is effective.