Hydrogel Electrolytes-Based Rechargeable Zinc-Ion Batteries under Harsh Conditions

Abstract

Rechargeable zinc (Zn)-ion batteries (RZIBs) with hydrogel electrolytes (HEs) have gained significant attention in the last decade owing to their high safety, low cost, sufficient material abundance, and superb environmental friendliness, which is extremely important for wearable energy storage applications. Given that HEs play a critical role in building flexible RZIBs, it is urgent to summarize the recent advances in this field and elucidate the design principles of HEs for practical applications. This review systematically presents the development history, recent advances in the material fundamentals, functional designs, challenges, and prospects of the HEs-based RZIBs. Firstly, the fundamentals, species, and flexible mechanisms of HEs are discussed, along with their compatibility with Zn anodes and various cathodes. Then, the functional designs of hydrogel electrolytes in harsh conditions are comprehensively discussed, including high/low/wide-temperature windows, mechanical deformations (e.g., bending, twisting, and straining), and damages (e.g., cutting, burning, and soaking). Finally, the remaining challenges and future perspectives for advancing HEs-based RZIBs are outlined.

1. Introduction

The flourishing development of flexible and wearable electronics has spawned a huge demand for efficient energy storage devices [1, 2]. Commercial lithium-ion batteries (LIBs) are suffering from flammable organic electrolytes despite their sophisticated technologies and dominant market in power resources [3]. Recently, rechargeable zinc-ion batteries (RZIBs) have attracted fast-growing interest in various energy storage applications, benefiting from the high safety, low cost, and eco-friendliness of water systems [3–5]. Moreover, the Zn metal anode possesses natural abundance, facile manufacturing, nonflammability in water, and a desirable theoretical volume capacity (820 mAh g−1 or 5855 mAh cm−3) [6]. Nevertheless, the Zn anode suffers from poor reversibility because of the undesired Zn dendrite growth and water-induced side reactions (e.g., H2 evolution and Zn corrosion) [7]. Moreover, the traditional oxide-based cathode materials (e.g., V2O5 and MnO2) are easily dissolved in aqueous electrolytes, leading to inferior electrochemical performance of batteries [8–10]. Besides, the aqueous electrolytes endure a narrow voltage window along with hydrogen/oxygen evolution reaction (HER and OER), resulting in continuous consumption of electrolytes and battery failure [11–13].

Hydrogel electrolytes (HEs) are pivotal in developing high-performance RZIBs, as they effectively address the limitations of aqueous electrolytes [14–18]. Firstly, HEs inhibit Zn dendrite formation by modifying the anode surface [4]. Secondly, their quasi-solid nature reduces fluidity, thereby mitigating cathode material dissolution. Furthermore, water-saturated HEs reduce the reactivity of free water, expanding the voltage window and minimizing water-induced side reactions [19, 20]. Notably, hydrogels can serve multiple roles in RZIBs, acting as electrolytes, separators, protective layers, active materials, and binders, thanks to their mechanical strength, self-healing ability, hydrophilicity, and wide-temperature adaptability [21].

To date, massive efforts have been made to develop HEs for RZIBs to adapt to different scenarios [22–31]. For instance, cellulose, polyvinyl alcohol (PVA), and polyhydric additives have been employed to improve the electrochemical performance of HEs-based RZIBs in a wide temperature range of −70 to 100 °C [22–24]; polyacrylamide (PAM), cooperating with gelatin, cellulose, and chitosan, has served as HEs with good mechanical strength to resist diverse deformations and damages [25–27]; the ultra-hyperelastic and highly entangled hydrogel species have been designed for relieving swelling effect [28, 29]; and the additives of flame retardants have been introduced into HEs to prevent combustion of batteries [30, 31]. However, the harsh conditions of extreme temperatures, mechanical deformations, and damages will bring out the negative effects on the electrode–electrolyte interface.