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environments, [7-12] where Zn deposition even during shelf time is continuously interfered by competitive hydrogen evolution through H 2 O decomposition (Zn + 2H 2 O → Zn(OH) 2 + H 2) and by consuming both the electrolyte and active Zn metal. This long-neglected problem will remarkably affect calendar life of batteries, which is equivalently important with the intensively studied cycling life of batteries. As revealed in our preliminary quantitative study shown in Figure 1a, immersing Zn in 2 m ZnSO 4 electrolyte will induce an ≈0.25 mmol h-1 cm-2 hydrogen flux. The continuous hydrogen evolution will cause local pH changes, which further induce the formation of loose and brittle Zn 4 SO 4 (OH) 6 •xH 2 O (3Zn(OH) 2 + ZnSO 4 •xH 2 O → Zn 4 SO 4 (OH) 6 •xH 2 O), as revealed in Figure 1b. [13-18] It is generally assumed that these by-products augment the tortuosity and irregularity at the electrode/electrolyte interface with physical contact surface being increased (Figure 1c-h), which will further accelerate hydrogen evolution reaction. The above issues necessitate an effective method to detect hydrogen evolution. Nevertheless, for a Zn-based battery, these assumption and hypotheses are only based on oversimplified observations of battery swelling, gas bubbles, or conducting polarization curve in the absence of Zn 2+ condition, not reflecting physical truth of battery. [19-21] Hydrogen production during electrochemical procedure and even shelf time has not been precisely quantified. Consequently, efforts to make Zn metal a valid anode material may be misdirected. Quantifying the hydrogen production on the electrode during Zn deposition is key to understanding the mechanisms leading to capacity loss and battery failure. On the other hand, these Zn protrusions caused by hydrogen evolution reaction will attract more Zn 2+ flux ("tip" effect) [22] under concentrated electric field during electrochemical cycling, thus accelerating the vertical growth of Zn dendrites instead of planar growth and hydrogen production further flourish (Figure 1g). Up to now, various strategies have been evolved to prohibit the Zn dendrite growth, such as electrolyte optimization, [8,23-25] Zn surface coating, [9,21,26-29] and alloying. [30] To some extent, these strategies stabilize Zn metal, but they do The hydrogen evolution in Zn metal battery is accurately quantified by in situ battery-gas chromatography-mass analysis. The hydrogen fluxes reach 3.76 mmol h −1 cm −2 in a Zn//Zn symmetric cell in each segment, and 7.70 mmol h −1 cm −2 in a Zn//MnO 2 full cell. Then, a highly electronically insulating (0.11 mS cm −1) but highly Zn 2+ ion conductive (80.2 mS cm −1) ZnF 2 solid ion conductor with high Zn 2+ transfer number (0.65) is constructed to isolate Zn metal from liquid electrolyte, which not only prohibits over 99.2% parasitic hydrogen evolution but also guides uniform Zn electrodeposition. Precisely quantitated, the Zn@ZnF 2 //Zn@ZnF 2 cell only produces 0.02 mmol h −1 cm −2 of hydrogen (0.53% of the Zn//Zn cell). Encouragingly, a hig...

An ionic‐liquid‐based Zn salt electrolyte is demonstrated to be an effective route to solve both the side‐reaction of the hydrogen evolution reaction (HER) and Zn‐dendrite growth in Zn‐ion batteries. The developed electrolyte enables hydrogen‐free, dendrite‐free Zn plating/stripping over 1500 h cycle (3000 cycles) at 2 mA cm–2 with nearly 100% coulombic efficiency. Meanwhile, the oxygen‐induced corrosion and passivation are also effectively suppressed. These features bring Zn‐ion batteries an unprecedented long lifespan over 40 000 cycles at 4 A g–1 and high voltage of 2.05 V with a cobalt hexacyanoferrate cathode. Furthermore, a 28.6 µm thick solid polymer electrolyte of a poly(vinylidene fluoride‐hexafluoropropylene) film filled with poly(ethylene oxide)/ionic‐liquid‐based Zn salt is constructed to build an all‐solid‐state Zn‐ion battery. The all‐solid‐state Zn‐ion batteries show excellent cycling performance of 30 000 cycles at 2 A g–1 at room temperature and withstand high temperature up to 70 °C, low temperature to –20 °C, as well as abuse test of bending deformation up to 150° for 100 cycles and eight times cutting. This is the first demonstration of an all‐solid‐state Zn‐ion battery based on a newly developed electrolyte, which meanwhile solves the deep‐seated hydrogen evolution and dendrite growth problem in traditional Zn‐ion batteries.

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.

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