Hydrogel Electrolytes for Flexible Aqueous Energy Storage Devices

Abstract

Hydrogel materials are receiving increasing research interest due to their intriguing structures that consist of a crosslinked network of polymer chains with interstitial spaces filled with solvent water. This feature endows the materials with the characteristics of being both wet and soft, making them ideal candidates for electrolyte materials for flexible energy storage devices, such as supercapacitors and rechargeable batteries that are under intensive studies nowadays. More importantly, the highly abundant and tunable chemistries of these hydrogels allow the introduction of novel functionalities into the existing hydrogels so that it is possible to fabricate unprecedented energy storage devices with additional functions. Here, the state-of-the-art advances of the hydrogel materials for flexible energy storage devices including supercapacitors and rechargeable batteries are reviewed. In addition, devices with various kinds of functions, such as self-healing, shape memory, and stretchability, are also included to stress the critical role of hydrogel materials. Furthermore, the challenges embedded in the current technologies are also highlighted and discussed with the hope to continually boost future research for the fast-developing field.

A Self‐Healing Integrated All‐in‐One Zinc‐Ion Battery

“…Poly(vinyl alcohol) (PVA)‐based hydrogel prepared under appropriate conditions can autonomously self‐heal once damaged owing to a large amount of hydroxy side groups in PVA chain segments and the O‐H⋅⋅⋅O hydrogen bonds . Furthermore, PVA is a common polymer that is often used in gel electrolytes ,. Therefore, if Zn ionic species could be introduced into PVA hydrogel matrix by a suitable strategy, self‐healing gel electrolytes of ZIBs would be achieved.…”

“…Among various aqueous metal‐ion batteries, rechargeable Zn‐ion batteries (ZIBs) have attracted increasing attention owing to the merits of Zn, such as excellent theoretical capacity (820 mAh g −1 ), high safety, and low cost . Recently, flexible aqueous ZIBs have been developed based on the polymer‐based gel electrolytes, which endow the ZIBs with integrated configuration and are capable of avoiding relative displacement or detachment between their neighboring components under bending status …”

A Flexible Rechargeable Zinc–Air Battery with Excellent Low‐Temperature Adaptability

“…By contrast, considering the concentrated alkaline solute used in ZABs, the intrinsic colligative property of the solutions can be utilized to mitigate the problems caused by low‐temperatures. Nevertheless, when concentrated alkaline solutions are added to hydrogel electrolytes, the structural stability of polymer backbones can be significantly compromised . The anti‐freezing property of such hydrogel electrolytes depends on interactions among terminal groups on polymer backbones, electrolyte ions, and water .…”

Tough and recoverable triple‐network hydrogels based on multiple pairs of toughing mechanisms with excellent ionic conductivity as stable strain sensors

“…Ionic conductive hydrogels with polymer networks swollen with electrolyte solutions have been explored for applications in flexible devices, including ionic skin sensors and hydrogel‐based electrolytes for energy storage devices due to their excellent biocompatibility, intrinsic flexibility, and interesting electrochemical properties . In particular, the resistance change of ionic gels is much lower than traditional electronic conductors under large strains which makes it very advantageous to be used as strain sensors .…”

“…The chemical and physical hybrid crosslinked double‐network (DN) gels or single‐network gels demonstrate not only tough but also self‐recovery abilities from fatigue damage . With the development of tough hydrogels, ionic conductive hydrogels with enhanced mechanical properties and other healable functions were developed by doping salt (e.g., NaCl and LiCl) in the current tough hydrogels or the salt ions also participate in physical crosslinking . Another approach is using the polyelectrolyte which contain a lot of charge groups to facilitate the ion conduction .…”