Development Of A Semi-Solid Separator/Electrolyte Layer With High Ionic Conductivity And Mechanical Strength For Flexible Zinc Ion Batteries

With the rapid advancement of flexible wearable electronic devices,there has been an increasing focus on developing flexible energy storage devices.Zinc ion batteries in aqueous systems are emerging as a promising candidate for flexible energy storage,owing to their high specific capacity,low cost,superior safety,and environmental friendliness.However,there are serious problems of zinc dendrites on the surface of the electrodes,which not only affect ion transport but also lead to a decline in battery cycle life and even short circuits and failure.Meanwhile,flexible zinc ion batteries need to withstand external forces such as bending,folding,and impact in practical applications,so the electrolyte applied to the battery needs to have high mechanical performance.This thesis explores the design and preparation of two semi-solid separator/electrolyte layers,which are characterized by high ion transport performance and mechanical strength.The effectiveness of these layers in preventing the formation of zinc dendrites is examined,as well as their applicability in flexible zinc ion batteries.The specific research contents are as follows:（1）In this study Kevlar fibers are used because it possesses high mechanical strength but low ion conductivity.To enhance their ion conductivity,deprotonation treatment is performed to produce negatively charged aramid nanofibers.They are then self-assembled with positively charged Zn²⁺via electrostatic interaction,resulting in the creation of aramid nanofiber functionalized separators.These separators demonstrate high ion transport performance and excellent mechanical performance,and are prepared using vacuum filtration.The results show that the surface of this functionalized separator has abundant polar groups with good affinity for zinc,which can homogenize zinc ion flux,enable uniform nucleation and growth of zinc ions,and it can enhance the mobility of zinc ions and inhibit the growth of zinc dendrites by accelerating the dynamics of zinc ion transport.Electrochemical performance tests show that a symmetric battery assembled with this functionalized separator obtain a high zinc ion transfer number of 0.897 and a low zinc ion deposition activation energy of 10.10 KJ·mol^-1,which can achieve a reversible deposition/stripping process of more than 180 h.Moreover,when applied to the full battery,it achieves a high initial specific capacity of 220 mAh·g^-1 and sustained charging and discharging cycles for more than 1,200 cycles.The assembled flexible zinc-ion batteries also show good electrochemical stability.However,the rigidity of the separators is too high while the flexibility and extensibility are inadequate,which cannot better meet the needs of flexible zinc ion batteries for various deformations during practical applications.（2）Gelatin hydrogels with inherently high ionic conductivity and good flexibility are used.Based on the salting out property of Hofmeister effect,a high mechanical strength and electrochemical stability hydrogel electrolyte is prepared by introducing SO₄^2-into the pre-formed gelatin hydrogel by a one-step impregnation method.And the effects of different anions on the mechanical and electrochemical performance of the hydrogel electrolyte are investigated.The salting out property of SO₄^2-can enhance the hydrophobicity of the polymer chain,strengthen the hydrogen bond interaction and crosslinking density between molecular chains,and improve the mechanical strength of the hydrogel.The prepared hydrogel electrolyte has a high tensile strength of 1.5 MPa and a high ionic conductivity of 2.35×10^-1 S·cm^-1.When applied to flexible zinc ion batteries,it sustains charging and discharging cycles for more than 9,300 cycles,and withstand various external forces.The polar groups such as carboxyl and amino groups on the surface of the gelatin also have good affinity for zinc,which can homogenize zinc ion flux,enable uniform nucleation and growth of zinc ions,and inhibit the growth of zinc dendrites.This design strategy provides a simple method for improve the mechanical performance of hydrogel electrolytes and overcoming the interface problems of electrodes.