Polymer Modified Electrode And Its Application In Zinc Ion Battery

Aqueous zinc ion batteries（ZIBs）use metal zinc as the negative electrode,and use near-neutral zinc salt aqueous solution as the electrolyte.They have the advantages of low cost,safety,environmental protection,and high energy density.They are a type of aqueous-based secondary battery device that has received much attention in recent years.During the charging and discharging of zinc ion batteries,repeated zinc ion deposition/stripping reactions occur on the surface of the metal zinc anode.Affected by the"tip effect",the electric field distribution on the surface of the zinc anode is not uniform,and zinc ion aggregation and zinc dendrite growth are prone to occur at the tip where the local electric field is strong.The growing zinc dendrites may pierce the separator,causing a short circuit in the battery,and easily fall off from the surface of the current collector to form"dead zinc",causing the battery capacity to decay.Constructing a protective layer on the surface of the negative electrode can help to delay the growth rate of zinc dendrites and reduce the occurrence probability of"dead zinc",thereby improving the cycle stability and safety of zinc ion batteries.Among all zinc ion battery cathode materials,manganese dioxide（MnO₂）has the advantages of abundant resources,high theoretical specific capacity,high equilibrium potential,low cost and environmental protection,and is one of the most promising cathode materials.However,the conductivity of MnO₂ is very low,and the structural stability of the material during cycling is poor.It is prone to capacity attenuation due to the problem of manganese dissolution and loss,which affects the rate performance of the battery and the improvement of cycling stability.By combining MnO₂ with a highly conductive material,the conductivity of the MnO₂ material can be improved,thereby improving the electrochemical performance of the MnO₂ electrode material.In this paper,the following research work on the Zn anode and manganese dioxide cathode in the above ZIBs was carried out using different polymers:（1）A coating of polystyrene（PS）with a uniform cracking structure was prepared on the surface of the zinc negative electrode by a simple scraping method.By adjusting the coating process to change the cracking structure of the coating,the effect of these PS coatings on dendrite growth on the surface of the zinc anode was systematically studied.Using scanning electron microscope（SEM）observed on the surface of zinc anode morphology,bare without PS coating of zinc anode in charge and discharge cycle,appear a large number of large size zinc dendrite,and part of the dendrite is away from the collection of fluid,become do not have zinc electrochemical reactivity of death,and thus cause the negative reaction activity and the capacity of fast attenuation.However,due to the special structure of PS coating and its scoping effect on the growth of zinc deposition,the size of zinc dendrite deposition decreased and the density of zinc deposition increased significantly after the zinc cathode modified by PS coating was cycled.In a symmetrical battery test(deposition current density:0.5m A cm^-2),the PS-coated Zn anode can cycle for more than 700 h with a very low initial deposition overpotential（80.7 m V）,and the cycle stability is much higher than that of untreated Bare zinc anode（99.5 m V,46 h）.In a full battery with activated carbon as the positive electrode(current density:1 A g^-1),the capacity retention rate of the PS-coated Zn anode is higher than that of the untreated bare zinc anode（2000 cycles,77.1%vs.44.9%）.（2）Using polyacrylonitrile（PAN）as the carbon source,MnO/C_（PAN）self-supporting composite fiber membranes were prepared by electrospinning and carbonization treatment,and the carbonation temperature on the zinc storage performance of the composite fibers was studied influences.In order to further improve the valence state and content of manganese element in the fiber,the MnO/C_（PAN）composite fiber was subjected to secondary treatment by potassium permanganate solution oxidation method,and the fiber was successfully transformed into MnO₂/C_（PAN）@MnO₂ composite carbon fiber.In this composite material,the one-dimensional fiber structure can not only reduce the migration distance of reactive ions in the electrode material,but also help to improve the conductivity and effective utilization of the material.Electrochemical tests show that at a high current density of 0.5 A g^-1,the specific discharge capacity of MnO₂/C_（PAN）@MnO₂ composite carbon fiber for the first time is not as good as that of pure phase MnO₂(113 vs.143.1 m Ah g^-1),but the cyclic stability has been greatly improved（400 cycles,capacity retention rate:91.1%vs.25.6%）.The impedance test after cycling also showed that the MnO₂/C_（PAN）@MnO₂composite carbon fiber has a high ion diffusion capacity,and the internal resistance is also significantly lower than the pure phase MnO₂ sample（61.1 vs.125Ω）.