A flexible zinc-ion battery based on the optimized concentrated hydrogel electrolyte for enhanced performance at subzero temperature

Electrolyte and Additive Engineering for Zn Anode Interfacial Regulation in Aqueous Zinc Batteries

Aqueous Zn-metal batteries (AZMBs) have gained great interest due to their low cost, eco-friendliness, and inherent safety, which serve as a promising complement to the existing metal-based batteries, e.g., lithium-metal batteries and sodium-metal batteries. Although the utilization of aqueous electrolytes and Zn metal anode in AZMBs ensures their improved safety over other metal batteries meanwhile guaranteeing their decent energy density at the cell level, plenty of challenges involved with metallic Zn anode still await to be addressed, including dendrite growth, hydrogen evolution reaction, and zinc corrosion and passivation. In the past years, several attempts have been adopted to address these problems, among which engineering the aqueous electrolytes and additives is regarded as a facile and promising approach. In this review, a comprehensive summary of aqueous electrolytes and electrolyte additives will be given based on the recent literature, aiming at providing a fundamental understanding of the challenges associated with the metallic Zn anode in aqueous electrolytes, meanwhile offering a guideline for the electrolytes and additives engineering strategies toward stable AZMBs in the future.

High-Fluidity/High-Strength Dual-Layer Gel Electrolytes Enable Ultra-Flexible and Dendrite-Free Fiber-Shaped Aqueous Zinc Metal Battery

Fiber-shaped aqueous zinc-ion batteries (FAZIBs) with intrinsic safety, highcapacity, and superb omnidirectional flexibility hold promise for wearable energy-supply devices. However, the interfacial separation of fiber-shaped electrodes and electrolytes caused by Zinc (Zn) stripping process and severe Zn dendrites occurring at the folded area under bending condition seriously restricts FAZIBs' practical application. Here, an advanced confinement encapsulation strategy is originally reported to construct dual-layer gel electrolyte consisting of high-fluidity polyvinyl alcohol-Zn acetate inner layer and high-strength Zn alginate outer layer for fiber-shaped Zn anode. Benefiting from the synergistic effect of inner-outer gel electrolyte and the formation of solid electrolyte interphase on Zn anode surface by lysine additive, the resulting fiber-shaped Zn-Zn symmetric cell delivers long cycling life over 800 h at 1 mA cm-2 with dynamic bending frequency of 0.1 Hz. The finite element simulation further confirms that dual-layer gel electrolyte can effectively suppress the interfacial separation arising from the Zn stripping and bending process. More importantly, a robust twisted fiber-shaped Zn/zinc hexacyanoferrate battery based on dual-layer gel electrolyte is successfully assembled, achieving a remarkable capacity retention of 97.7% after bending 500 cycles. Therefore, such novel dual-layer gel electrolyte design paves the way for the development of long-life fiber-shaped aqueous metal batteries.

Toward Simultaneous Dense Zinc Deposition and Broken Side-Reaction Loops in the Zn//V2 O5 System

The Zn//V2 O5 system not only faces the incontrollable growth of zinc (Zn) dendrites, but also withstands the cross-talk effect of by-products produced from the cathode side to the Zn anode, inducing interelectrode talk and aggravating battery failure. To tackle these issues, we construct a rapid Zn2+ -conducting hydrogel electrolyte (R-ZSO) to achieve Zn deposition modulation and side reaction inhibition in Zn//V2 O5 full cells. The polymer matrix and BN exhibit a robust anchoring effect on SO4 2- , accelerating Zn2+ migration and enabling dense Zn deposition behavior. Therefore, the Zn//Zn symmetric cells based on the R-ZSO electrolyte can operate stably for more than 1500 h, which is six times higher than that of cells employing the blank electrolyte. More importantly, the R-ZSO hydrogel electrolyte effectively decouples the cross-talk effects, thus breaking the infinite loop of side reactions. As a result, the Zn//V2 O5 cells using this modified hydrogel electrolyte demonstrate stable operation over 1,000 cycles, with a capacity loss rate of only 0.028 % per cycle. Our study provides a promising gel chemistry, which offers a valuable guide for the construction of high-performance and multifunctional aqueous Zn-ion batteries.

Smart Aqueous Zinc Ion Battery: Operation Principles and Design Strategy

The zinc ion battery (ZIB) as a promising energy storage device has attracted great attention due to its high safety, low cost, high capacity, and the integrated smart functions. Herein, the working principles of smart responses, smart self-charging, smart electrochromic as well as smart integration of the battery are summarized. Thus, this review enables to inspire researchers to design the novel functional battery devices for extending their application prospects. In addition, the critical factors associated with the performance of the smart ZIBs are comprehensively collected and discussed from the viewpoint of the intellectualized design. A profound understanding for correlating the design philosophy in cathode materials and electrolytes with the electrode interface is provided. To address the current challenging issues and the development of smart ZIB systems, a wide variety of emerging strategies regarding the integrated battery system is finally prospected.

Tough Hydrogel Electrolytes for Anti-Freezing Zinc-Ion Batteries

As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze-vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co-nonsolvency and "salting-out" that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open-cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6 MPa), freeze-tolerance (< -77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N-isopropylacrylamide) and poly(N-tertbutylacrylamide-co-acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

Highly Stretchable, Self-Healing, and Low Temperature Resistant Double Network Hydrogel Ionic Conductor as Flexible Sensor and Quasi-Solid Electrolyte

With the rapid development of flexible energy storage and wearable strain sensing, conductive hydrogels are attracting attention as electrolyte materials for flexible strain sensors and flexible supercapacitors due to their excellent flexibility and wetting properties. In this work, antifreezing hydrogels with high stretchability, adhesion, and conductivity are designed and prepared by introducing phosphoric acid solutions into polyacrylamide and chitosan systems. The multifunctional hydrogel samples prepared by this method can be used as both quasi-solid electrolytes and wearable strain sensors. The hydrogel-based supercapacitor shows a charge/discharge efficiency of 99.67% and a capacitance retention of 98.85% after 10 000 cycles charge/discharge tests at -30 °C. The tiny characteristic heartbeat wave forms are detected by the hydrogel as a flexible strain sensor. It is foreseeable that PCP multifunctional hydrogel can be a promising flexible material for a new generation of flexible sensors and flexible energy storage devices in a certain range of temperatures.

Comparative Review on the Aqueous Zinc-Ion Batteries (AZIBs) and Flexible Zinc-Ion Batteries (FZIBs)

Lithium-ion batteries (LIBs) have been considered an easily accessible battery technology because of their low weight, cheapness, etc. Unfortunately, they have significant drawbacks, such as flammability and scarcity of lithium. Since the components of zinc-ion batteries are nonflammable, nontoxic, and cheap, AZIBs could be a suitable replacement for LIBs. In this article, the advantages and drawbacks of AZIBs over other energy storage devices are briefly discussed. This review focused on the cathode materials and electrolytes for AZIBs. In addition, we discussed the approaches to improve the electrochemical performance of zinc batteries. Here, we also discussed the polymer gel electrolytes and the electrodes for flexible zinc-ion batteries (FZIBs). Moreover, we have outlined the importance of temperature and additives in a flexible zinc-ion battery. Finally, we have discussed anode materials for both AZIBs and FZIBs. This review has summarized the advantages and disadvantages of AZIBs and FZIBs for future applications in commercial battery technology.

Electrolyte Engineering Enables High Performance Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) feature high safety, low cost, environmental-friendliness, and promising electrochemical performance, and are therefore regarded as a potential technology to be applied in large-scale energy storage devices. However, ZIBs still face some critical challenges and bottlenecks. The electrolyte is an essential component of batteries and its properties affect the mass transport, energy storage mechanisms, reaction kinetics, and side reactions of ZIBs. The adjustment of electrolyte formulas usually has direct and obvious impacts on the overall output and performance. In this review, advanced electrolyte strategies are overviewed for optimizing the compatibility between cathode materials and electrolytes, inhibiting anode corrosion and dendrite growth, extending electrochemical stability windows, enabling wearable applications, and enhancing temperature tolerance. The underlying scientific mechanisms, electrolyte design principles, and recent progress are presented to provide a better understanding and inspiration to readers. In addition, a comprehensive perspective about electrolyte design and engineering for ZIBs is included.

Recent Progress in Aqueous Ammonium-Ion Batteries

Batteries using a water-based electrolyte have the potential to be safer, more durable, less prone to thermal runaways, and less costly than current lithium batteries using an organic solvent. Among the possible aqueous battery options, ammonium-ion batteries (AIBs) are very appealing because the base materials are light, safe, inexpensive, and widely available. This review gives a concise and useful survey of recent progress on emerging AIBs, starting with a brief overview of AIBs, followed by cathode materials, anode materials, electrolytes, and various devices based on ammonium-ion storage. Aside from summarizing the most updated electrodes/electrolytes in AIBs, this review highlights fundamental mechanistic studies in AIBs and state-of-the art applications of ammonium-ion storage. The present work reviews various theoretical efforts and the spectrum studies that have been used to explore ionic transport kinetics, electrolyte structure, solvation behavior of ammonium ions, and the intercalation mechanism in the host structure. Furthermore, diverse applications of ammonium-ion storage apart from aqueous AIBs are discussed, including flexible AIBs, AIBs that can operate across a wide temperature range, ammonium-ion supercapacitors, and battery-supercapacitor hybrid devices. Finally, the review is concluded with perspectives of AIBs, challenges remaining in the field, and possible research directions to address these challenges to boost the performance of AIBs for real-world practical applications.

Optimization Strategies of Electrolytes for Low-temperature Aqueous Batteries

Aqueous rechargeable batteries (ARBs) are considered promising electrochemical energy storage systems for grid-scale applications due to their low cost, high safety, and environmental benignity. With the demand for a wide range of application scenarios, batteries are required to work in various harsh conditions, especially the cold weather. Nevertheless, electrolytes would freeze at extremely low temperatures, resulting in dramatically sluggish kinetics and severe performance degradation. Here, we discuss the behaviors of hydrogen bonds and basic principles of anti-freezing mechanisms in aqueous electrolytes. Then, we present a systematical review of the optimization strategies of electrolytes for low-temperature aqueous batteries. Finally, the challenges and promising routes for further development of aqueous low-temperature electrolytes are provided. This review can serve as a comprehensive reference to boost the further development and practical applications of advanced ARBs operated at low temperatures.

From room temperature to harsh temperature applications: Fundamentals and perspectives on electrolytes in zinc metal batteries

As one of the most competitive candidates for the next-generation energy storage systems, the emerging rechargeable zinc metal battery (ZMB) is inevitably influenced by beyond-room-temperature conditions, resulting in inferior performances. Although much attention has been paid to evaluating the performance of ZMBs under extreme temperatures in recent years, most academic electrolyte research has not provided adequate information about physical properties or practical testing protocols of their electrolytes, making it difficult to assess their true performance. The growing interest in ZMBs is calling for in-depth research on electrolyte behavior under harsh practical conditions, which has not been systematically reviewed yet. Hence, in this review, we first showcase the fundamentals behind the failure of ZMBs in terms of temperature influence and then present a comprehensive understanding of the current electrolyte strategies to improve battery performance at harsh temperatures. Last, we offer perspectives on the advance of ZMB electrolytes toward industrial application.