Understanding Ion Transport in Hydrogel Electrolyte and Charge Transfer on Electrode-Hydrogel Interfaces for Wearable Zinc-Ion Battery

Description

Wearable energy storage is essential for smart clothing integrated with energy harvesters for powering onboard electronics and thus forms an inspiring subject with emerging high-quality scientific reports in the past decade. Flexible supercapacitors, metal-ion and metal-air batteries have been explored as candidates. The zinc-ion battery (ZIB) is promising, owing to the relatively high energy density and abundance of zinc. Meanwhile, safety of aqueous electrolytes in ZIB satisfies the primary criterion for wearable application, in addition to other merits of low cost and high conductivity. Recently, hydrogel electrolyte was introduced in ZIB, combining an insulative framework and ion-conductive solution as separator and electrolyte, respectively, simultaneously preventing liquid leakage and enhancing flexibility.Since corrosion of zinc anode is severe in strong alkaline and acidic environment, neutral and mild acidic electrolytes, such as zinc sulfate and triflate are preferred in ZIB. The reduced desolvation energy of triflate enhances the reaction kinetics of the zinc electrodeposition on anode and intercalation in cathodes. However, the high cost of zinc triflate limits its adoption in practical ZIB.Thus, we propose a low-cost alternative, zinc methanesulfonate electrolyte, which is expected to show the same effect on the desolvation step toward fast reaction kinetics, owing to approximate anion structure as triflate.Furthermore, the zinc ion transport in hydrogel, which determines the inner resistance of ZIB, is affected by counter ions, as reflected by the different ionic conductivities of sulfate- and triflate-based hydrogels. Therefore, the reaction kinetics and ion transport in zinc methanesulfonate-based hydrogel will be studied, which have never been investigated previously. Further, the measured ionic conductivities of hydrogels result from the movement of charge carriers through electrostatic pressure and to a lesser extent diffusion. In operational chargedischarge conditions, however, zinc ion diffusion by concentration gradient caused by anodic and cathodic reactions contributes to a greater extent. Hence, studying the diffusivity without the influence of electric field will delineate the ion transport processes, as well as the impact of charge carriers and hydrogel microstructures.Thus, we propose to investigate the diffusivity and ionic conductivity of hydrogel electrolytes to reveal the impacts of hydrogel microstructure and counter ions on ion transport.Since the distribution of zinc and counter ions cannot be separately studied by experiments, and the fabrication of hydrogels with different microstructures is laborious, a combination of mathematical modeling and experiments of diffusivity and ionic conductivity will be carried out.Evolution of solid electrolyte interface (SEI) and SEI layer significantly affect reaction kinetics and electrode stability. The limited research in flexible ZIB also demonstrated the implication of quasi-SEI or artificial SEI layer of hydrogel electrolytes. In addition, combining the stress of polymer framework and high diffusivity of aqueous solutions, the effect of hybrid electrolyte on the suppression of zinc dendrite growth has been recently simulated, highlighting the importance of understanding such interfacial reactions. However, the reaction mechanisms and kinetics of the interfacial reactions involving hydrogels are not well-studied.Thus, a microscopic visualization study of zinc dendrite on the anode hydrogel interface (AHI) will be carried out to illustrate the shape evolution and growth of the dendrite. Subsequently, an in-situ forming AHI layer is proposed to suppress dendrite formation, and the impact of hydrogel on charge transfer through the cathode hydrogel interface (CHI) will be investigated by impedance spectroscopy.The gained knowledge of ion transport and interfacial reactions will illustrate the role of hydrogel electrolyte in ZIB, guiding the promotion and design of wearable energy storage systems.