Flexible Zn‐Ion Batteries: Recent Progresses and Challenges

Abstract

the developing trend of modern electronic technologies, [6] and significant efforts have been devoted to transforming these energy storage systems to their light, flexible, small, and thin counterparts. [7] The flexible battery market is forecast to increase rapidly from $69.6 million in 2015 to $958.4 million in 2022. [8] Lithium-ion batteries (LIBs) historically and presently dominant the markets of rechargeablebattery for portable devices because of the lightness of lithium and high energy density of the battery systems. [9,10] Nonetheless, LIBs are marred by the high cost and the shortage of lithium resources. Moreover, the aprotic electrolytes used in LIBs are generally toxic and flammable. This fact causes great safety issues for LIBs, especially when they are used in wearable/implantable applications in close contact with human body. It is highly challenging to assemble flexible LIBs due to the requirement of a highly reliable protective packaging to avoid the electrolyte leakage and reconcile with the washing need of wearable devices in practical applications. [3] Moreover, due to the high barrier encapsulation, the volumetric performance would be severely restricted, especially when LIBs are miniaturized. In this context, it is highly desirable to prepare flexible "beyond Li-based" batteries with safe, low-cost, and eco-friendly aqueous electrolytes. [11] As alternatives for LIBs, multivalent ion battery technologies (Zn-ion battery, ZIB, Mg-ion battery, and Al-ion battery) are of high interest for electrochemical energy storage. In comparison with LIBs operating with single-electron transfer, multivalent ion batteries employ multielectron transfer during the charge/ discharge processes, thus delivering much higher volumetric energy densities. [12][13][14] Since the first utilization of Zn in batteries in 1799, [15] Zn metal has captured increasing attention as an ideal anode material. In earlier studies, Zn anodes were widely explored in many alkaline battery systems, such as alkaline zinc-MnO 2 batteries, [16] zinc-nickel batteries, [17][18][19][20] zinc-silver batteries, [21] and zinc-air batteries. [22][23][24] Zn metal is able to offering both high gravimetric and high volumetric capacities (820 and 5855 mAh cm −3 ). [25] Moreover, Zn has the merits of low cost, low-toxicity, abundance in earth crust (≈300 times higher than for lithium), environmental benignity, easily recyclable, and intrinsic safety. [14,26] These advantages directly drove the recent renaissance of Zn anode based batteries. [15] However, the use of corrosive alkaline electrolyte leads to the Zn-dendritic (sharp, needle-like metallic protrusions) growth [27,28] and soluble ZnO 2 2− formation on Zn anode, which poison the cathode and result in the rapid capacity To keep pace with the increasing pursuit of portable and wearable electronics, it is urgent to develop advanced flexible power supplies. In this context, Zn-ion batteries (ZIBs) have garnered increasing attention as favorable energy storage devices for flexible electronics, ...

Antifreezing Hydrogel with High Zinc Reversibility for Flexible and Durable Aqueous Batteries by Cooperative Hydrated Cations

“…Dissolved ions in hydrogels substantially modify the structure and dynamics of water molecules due to the hydration. As a result, the addition of commonly used electrolyte, 2 mol L −1 ZnSO 4 , into the PAAm hydrogel (referred to as Z-PAAm hereafter) significantly reduces the contribution from intermolecular hydrogen bonding due to two strongly hydrated ions (Zn 2+ and SO 4 2− ). [30] We identify two main contributions to the water vibration band of the pure PAAm: intermolecular hydrogen bonds at 3220 cm −1 and asymmetric stretching at 3420 cm −1 .…”

“…The structure of the water molecules is revealed by the Raman spectra in Figure 1b. However, the strong ion pairing between Zn 2+ and SO 4 2− that induces the aggregation of ions alleviates the hydration. [31,32] In general, hydrated ions in water diminish the intermolecular hydrogen bonds and elevate the asymmetric stretching.…”

“…As a result, the addition of commonly used electrolyte, 2 mol L −1 ZnSO 4 , into the PAAm hydrogel (referred to as Z-PAAm hereafter) significantly reduces the contribution from intermolecular hydrogen bonding due to two strongly hydrated ions (Zn 2+ and SO 4 2− ). Therefore, LiCl (4 mol L −1 ), a highly soluble salt containing highly hydrated Li + ion, is introduced, forming a mixture solution containing Li + , Zn 2− , Cl − , and SO 4 2− ions. [33][34][35] This is supported by the fact that the reduction of intermolecular hydrogen bonds and enhancement of asymmetric stretching of Z-PAAm is less obvious than PAAm containing 2 mol L −1 ZnCl 2 ( Figure S2a, Supporting Information), even though the hydration number of Cl − is negligible in water.…”

“…However, the strong ion pairing between Zn 2+ and SO 4 2− that induces the aggregation of ions alleviates the hydration. The high solubility of chlorides can effectively prevent ion aggregation, thus ensuring hydration of Li + , Zn 2− , and SO 4 2− even at a high concentration. [36] Therefore, in order to further intensify the hydration of water in hydrogels, adding alternative ions with high hydration number is a more efficient strategy than pushing the concentration of ZnSO 4 .…”

“…The former attribute gives rise to an excellent capacity retention (98%) achieved by the flexible aqueous battery based on LiFePO 4 and Zn upon cooling down to −20 °C from room temperature, as well as the outstanding durability under various mechanical deformations over a wide temperature range. Highly hydrated cations not only lower the freezing temperature of the hydrogel but also improve the reversibility of the zinc anode cycled in the hydrogel.…”