Key Takeaways & Executive Findings
- •• A novel cationic hydrogel electrolyte (PAPTMA) resolves the tripartite trade-off among ionic conductivity, Zn2+ mobility, and electrochemical stability. • Cationic express pathways enable fast and selective Zn2+ transport via ionic repulsion, achieving high ionic conductivity (28.7 mS cm−1) and Zn2+ transference number (0.79). • The hydrogel exhibits exceptional cycling stability across −15 to 60 °C, with symmetric cells lasting over 6000 h at 1 mA cm−2. • Pouch cells with MnO2 cathode demonstrate remarkable operational stability and mechanical robustness over 150 cycles, highlighting practical flexible battery potential.
Abstract
The development of flexible zinc-ion batteries (ZIBs) faces a three-way trade-off among the ionic conductivity, Zn2+ mobility, and the electrochemical stability of hydrogel electrolytes. To address this challenge, we designed a cationic hydrogel named PAPTMA to holistically improve the reversibility of ZIBs. The long cationic branch chains in the polymeric matrix construct express pathways for rapid Zn2+ transport through an ionic repulsion mechanism, achieving simultaneously high Zn2+ transference number (0.79) and high ionic conductivity (28.7 mS cm−1). Additionally, the reactivity of water in the PAPTMA hydrogels is significantly inhibited, thus possessing a strong resistance to parasitic reactions. Mechanical characterization further reveals the superior tensile and adhesion strength of PAPTMA. Leveraging these properties, symmetric batteries employing PAPTMA hydrogel deliver exceeding 6000 h of reversible cycling at 1 mA cm−2 and maintain stable operation for 1000 h with a discharge of depth of 71%. When applied in 4 × 4 cm2 pouch cells with MnO2 as the cathode material, the device demonstrates remarkable operational stability and mechanical robustness through 150 cycles. This work presents an eclectic strategy for designing advanced hydrogels that combine high ionic conductivity, enhanced Zn2+ mobility, and strong resistance to parasitic reactions, paving the way for long-lasting flexible ZIBs.
1. Introduction
Rechargeable zinc-ion batteries (ZIBs) have been considered a promising solution for stationary energy storage and power supply of flexible devices due to their high theoretical capacity (820 mAh mg−1 and 5855 mAh cm−3), low cost (approximately $2~4 per kg for zinc), and the safety of aqueous electrolytes [1–3]. However, despite these advantages, the development of ZIBs is impeded by several critical issues. In particular, long-lasting parasitic reactions occur between the Zn anode and the aqueous environment, including acidic corrosion, hydrogen evolution reaction (HER), and dendritic growth, which are ascribed to the chemical instability of Zn in water [4–6]. These parasitic reactions, induced by highly reactive water, would severely degrade the operating lifespan of ZIBs.
In comparison with liquid electrolytes, hydrogel electrolytes, with their reduced free-state water content, are expected to effectively suppress these parasitic reactions. Additionally, the unique properties of hydrogels, such as deformability and self-healing, expand their applications in flexible energy storage devices [7–11]. Therefore, quasi-solid-state ZIBs employing hydrogel electrolytes become a promising alternative.
Various synthetic and natural polymeric matrices, e.g., polyacrylamide (PAM) [12], polyvinyl alcohol [13], xanthan gum [14], and carrageenan [15], have been developed to enhance the electrochemical performance of ZIBs [16]. Hydrogels could be modified to further improve their performance, mainly targeting three objectives. (1) Improving ionic conductivity: Enhancing ionic conductivity is crucial for fast interfacial reaction kinetics [17–21]. For example, Yang et al. developed a supramolecular zwitterionic hydrogel electrolyte with a record-high ionic conductivity (σ) of 48 mS cm−1 through molecular engineering [20]. (2) Increasing Zn2+ mobility: Achieving high Zn2+ mobility is essential to prevent the formation of dendrite and loose zinc hydroxide [22–24]. Sun et al. built cationic channels in hydrogels by incorporating ring-shaped α-cyclodextrins with liner polymers, achieving an exceptional Zn2+ transference number (tZn2+) of 0.92 [24]. 3) Reducing water content: Minimizing water content in hydrogel further inhibits parasitic reactions [25]. A lean-water hydrogel with only 20% water content has been fabricated by removing excess free-state water, demonstrating an expanded electrochemical stability window and high resistance to parasitic reactions [26].
Despite these advancements toward a single objective, complex trade-offs among these three targets persist.
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Dewu Lin, Yushuang Lin, Ruihong Pan, Jiapei Li, Anquan Zhu, Tian Zhang, Kai Liu, Dongyu Feng, Kunlun Liu, Yin Zhou, Chengkai Yang, Guo Hong, Wenjun Zhang (2025). Water-Restrained Hydrogel Electrolytes with Repulsion-Driven Cationic Express Pathways for Durable Zinc-Ion Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01704-5
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Frequently Asked Questions
What is the main challenge addressed in this paper?
The paper addresses the three-way trade-off among ionic conductivity, Zn2+ mobility, and electrochemical stability in hydrogel electrolytes for flexible zinc-ion batteries.
How does the PAPTMA hydrogel achieve high Zn2+ transport?
The PAPTMA hydrogel uses long cationic branch chains that create express pathways for rapid Zn2+ transport via an ionic repulsion mechanism, resulting in a high Zn2+ transference number of 0.79 and ionic conductivity of 28.7 mS cm−1.
What are the key performance metrics of the PAPTMA hydrogel?
The hydrogel demonstrates over 6000 hours of reversible cycling at 1 mA cm−2 in symmetric cells, stable operation for 1000 hours at 71% depth of discharge, and excellent performance in pouch cells over 150 cycles.
What is the significance of the water-restrained property?
The water-restrained property inhibits the reactivity of water, reducing parasitic reactions such as corrosion and hydrogen evolution, thereby enhancing the durability and lifespan of zinc-ion batteries.
What practical applications does this hydrogel enable?
The hydrogel enables durable flexible zinc-ion batteries suitable for wearable and portable electronics, as demonstrated by stable operation in 4×4 cm2 pouch cells with MnO2 cathodes.
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