Abstract
Owing to the development of aqueous rechargeable zinc-ion batteries (ZIBs), flexible ZIBs are deemed as potential candidates to power wearable electronics. ZIBs with solid-state polymer electrolytes can not only maintain additional load-bearing properties, but exhibit enhanced electrochemical properties by preventing dendrite formation and inhibiting cathode dissolution. Substantial efforts have been applied to polymer electrolytes by developing solid polymer electrolytes, hydrogel polymer electrolytes, and hybrid polymer electrolytes; however, the research of polymer electrolytes for ZIBs is still immature. Herein, the recent progress in polymer electrolytes is summarized by category for flexible ZIBs, especially hydrogel electrolytes, including their synthesis and characterization. Aiming to provide an insight from lab research to commercialization, the relevant challenges, device configurations, and life cycle analysis are consolidated. As flexible batteries, the majority of polymer electrolytes exploited so far only emphasizes the electrochemical performance but the mechanical behavior and interactions with the electrode materials have hardly been considered. Hence, strategies of combining softness and strength and the integration with electrodes are discussed for flexible ZIBs. A ranking index, combining both electrochemical and mechanical properties, is introduced. Future research directions are also covered to guide research toward the commercialization of flexible ZIBs.
Highlights
Aqueous zinc ion batteries (AZIBs) currently suffer from unfavorable water-induced side reactions that result in zinc dendrite formation, dissolution of cathode mate-With the growth in big data, there is an increasing demand rials and the formation of byproducts on cathodes, causing in electronic devices for capturing real-time data in various a fast capacity fade
Before the foundation of mild acid Zn/MnO2 batteries,[56] polymer electrolytes for acidic ZIBs have been developed for nearly two decades, as shown in Figure 2a; tremendous breakthroughs are owed to the exploitation of AZIBs by Kang and co-workers in 2012.[17,18,57] Similar to the development of LIBs, organic solid polymers were pioneering to be utilized in ZIBs as the electrolyte host materials such as poly(ethylene oxide) (PEO),[54] poly(vinylidene fluoride) (PVDF),[52] and polyacrylonitrile (PAN),[58] exhibiting ionic conductivities in the
Inspired by the solid polymer electrolyte developed for LIBs, polymer electrolytes for ZIBs are synthesized by dissolving zinc salts into polymer frameworks, such as PEO, PAN and PVDF forming the polymer-salt complexes
Summary
Aqueous zinc ion batteries (AZIBs) currently suffer from unfavorable water-induced side reactions that result in zinc dendrite formation, dissolution of cathode mate-. The energy storage mechanism for AZIBs is elusive, a common agreement for the Zn2+ storage in cathode materials is as shown in Figure 1b–d and can be briefly summarized as i) Zn2+ intercalation/deintercalation, ii) Zn2+ and H+ co-insertion, iii) chemical conversion reaction, and iv) coordination/incoordination reaction. The alleviation of side reactions, such as the zinc dendrite formation and the cathode dissolution, additives in electrolyte,[36] water-in-salt solutions,[37] polymer electrolytes,[38] and electrode coating[39] methods have been explored. The strategies to avoid current issues for AZIBs are still facing challenges, polymer electrolytes applied into ZIBs are essential to tackle these barriers and to exhibit additional mechanical properties
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