Materials Today Chemistry
Selection of hydrogel electrolytes for flexible zinc–air batteries
Introduction
With the increase of energy demand, the problem of energy shortage has become increasingly serious. Moreover, consumption of fossil energy aggravates environmental pollution. Green and sustainable clean energy is a promising candidate, driving the technological development of energy storage of batteries [[1], [2], [3], [4], [5], [6]].
Although the traditional battery technologies are relatively mature, the problems of high cost, low efficiency, and serious safety risks remain unresolved [7,8]. Compared with lithium-ion batteries, metal–air batteries have a higher energy density because the oxygen involved in the reaction is not stored in the battery in advance and can be drawn directly from the surrounding air [9]. Among them, zinc–air batteries are preferred, mainly because lithium metal is active and easy to catch flammability and explosion [[10], [11], [12], [13], [14]]. The corrosion of magnesium and aluminum is serious, and the regeneration is difficult. Whereas the stability and electrochemical reversibility of zinc are better than that of lithium, magnesium, and aluminum [15], and the energy density of zinc (7280 Wh/L) is higher than that of lithium (6160 Wh/L). Therefore, the zinc–air battery is a very promising power source and energy storage technology [[16], [17], [18], [19]]. However, the problems of electrolyte leakage and water evaporation in traditional zinc–air batteries not only have great safety risks, but also directly affect lifetime and performance of the batteries, which cannot meet the demands of flexible electronic devices. Hydrogels are three-dimensional macromolecular polymeric substances that can absorb water and swell, which can remain gelatinous in water without dissolution [[20], [21], [22], [23], [24]]. Hydrogel electrolytes have been widely used in flexible zinc–air batteries due to their good operability and application, certain ionic conductivity, mechanical strength, and good interface contact with electrodes [[25], [26], [27]].
Hydrogels overcome the defects of the aqueous electrolytes of the traditional zinc–air batteries and increase ductility and flexibility of the batteries, expanding the application scope of zinc–air batteries [[28], [29], [30], [31]]. In addition, the structure and property of hydrogels play an important role in enhancing the performance of zinc–air batteries, indicating that hydrogels can not only give the battery flexibility, but also have good application value in the flexible zinc-air batteries [32,33]. Developing high quality hydrogel electrolytes for zinc–air batteries has been the goal of many researchers. So far, there have been a lot of reports on the electrodes and catalysts of zinc–air batteries, but there are few reviews on the hydrogel electrolytes that can be applied to flexible zinc–air batteries [[34], [35], [36], [37]]. In this paper, hydrogel electrolytes applicable to zinc–air batteries in recent years are reviewed, and the principle of flexible zinc–air batteries, the advantages and the preparation methods of hydrogels are introduced. The conductivity, mechanical properties, environmental adaptability, and electrolyte–electrode interface compatibility of the hydrogel electrolytes are also summarized. Finally, the prospects for further development and application of hydrogel electrolytes are put forward, providing availability for the future development of hydrogel electrolytes for flexible zinc-air batteries.
Section snippets
Hydrogel electrolytes
Hydrogels are a kind of gel-like substance with water as the dispersion medium [38]. The common hydrogels are usually made by introducing some hydrophobic groups and hydrophilic residues into water-soluble polymers with a network cross-linking structure. The introduced hydrophilic residues combine with the water molecules and connect the water molecules to the interior of the network, while the hydrophobic residues expand when they contact water, a cross-linked polymer produced by such a
Battery structure
As the name implies, flexible battery is a kind of battery which has mechanical flexibility and can withstand repeated folding, stretching, and curling. It has broad application prospects in the field of flexible electronics [65,66]. The concept of flexible batteries is relative to solid or liquid batteries that do not have the ability to deform [[67], [68], [69]]. For all solid or aqueous batteries, the properties of them completely negate the possibility of flexible batteries, and the
Advantages of hydrogels
Hydrogels have attracted more and more researchers' attention because of its unique cross-linked network structure and water absorption ability, and its application fields are more and more widely. In particular, it has both ‘wet’ and ‘soft’ characteristics, which makes it an ideal electrolyte material for flexible electronic devices [[91], [92], [93], [94], [95], [96], [97]]. In recent years, some major energy storage devices, such as supercapacitors and flexible batteries, have already
Crosslinking method
The preparation of hydrogels depends on the formation and cross-linking of polymer network structure. Therefore, the cross-linking method is one of the common measures for preparing hydrogel electrolytes, including physical cross-linking method and chemical cross-linking method [121]. Physical crosslinking mainly refers to the Van der Waals forces and hydrogen bonds between molecular chains and the interaction between hydrophilic or hydrophobic molecules, under which the polymer crosslinking
Conductivity
As the electrolyte of flexible batteries, the conductivity of hydrogel is an important index to measure its performance [[140], [141], [142]]. The conductivity of hydrogel polymer electrolytes is closely related to the number and movement of charged particles inside the electrolytes [[143], [144], [145]]. Hydrogel electrolyte is a polymer with cross-linked structure. Therefore, it has higher water absorption, better flexibility, and ionic conductivity [[146], [147], [148]]. Crosslinking refers
Prospect
With the aggravation of energy problems and the increasingly serious environmental pollution, energy conservation and emission reduction as well as the development of clean energy become the development trend of today. The transition from traditional energy to clean energy has become a general direction, which also promotes the development of new battery storage system. Zinc–air batteries are promising and valuable energy conversion and utilization devices due to technological, cost, high-dense
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Natural Science Foundation of China (21706013) and the State Key Laboratory of Automotive Safety and Energy under Project No. KF2024.
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