School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, China
* Corresponding author.. E-mail address: wangqh@jsnu.edu.cn (Q. Wang). 1 These authors contributed equally to this work.
Received Date:
09 April 2024 Accepted Date:
13 June 2024 Revised Date:
24 May 2024 Available Online:
15 July 2025
Abstract:
Zinc metal batteries (ZMBs) are considered to be promising energy storage devices in the field of large-scale energy storage due to the advantages of high energy density, good safety and environmental friendliness. However, the commercialization of ZMBs has been hampered because of the problems caused by aqueous electrolytes, such as hydrogen evolution reaction, electrolyte leakage, and water evaporation. Gel polymer electrolytes (GPEs) have attracted extensive attention due to the features of high security and low water content. However, the disadvantages of poor ion transport rate, easily freezing at low temperature and low mechanical strength are not conducive to the rapid development and practical application of ZMBs. The rational design and fabrication of multifunctional polymer-based frameworks are considered to be effective strategy to obtain high-performance GPEs. In this review, the recent advancements of GPEs with various polymers are generalized. The strategies for the improvement of ionic conductivity, low temperature resistance and mechanical strength of these GPEs, such as adding inorganic fillers, building double cross-linked networks and introducing functional groups, are summarized. The effects of the GPEs on the self-healable ability, inhibiting dendrite growth, and cycling stability of the ZMBs are also discussed. Finally, the key problems and development prospects of GPEs are proposed, which will provide possibility for the further development of GPEs.
To solve the increasingly prominent energy crisis and environmental pollution problems, it is necessary to vigorously develop new renewable and clean energy sources, such as solar energy, wind energy and tidal energy. But the efficient use of these new types of energy sources is severely limited by their intermittency and geography [1,2]. Therefore, developing large scale energy storage and conversion systems is a pressing task [3,4]. Lithium-ion batteries (LIBs) possess the advantages of high energy density and highly power density, and have been used in various fields, such as portable energy storage devices and new energy electric vehicles [5-7]. However, the complex assembly process, scarce lithium resource, flammability of electrolyte and high cost make it difficult to be used for large-scale energy storage [8-10]. Therefore, various new types of energy storage systems have been explored, among which aqueous zinc metal batteries (ZMBs) have attracted widespread attention due to their high energy density and simple assembly process [11,12]. Metal Zn is used as anode and possesses the advantages of abundant natural reserve, easy to obtain, high theoretical specific capacity (5855 mAh/cm3, 820 mAh/g) and a low redox potential (−0.76 V vs. SHE) [13,14]. In addition, the aqueous electrolytes possess higher ionic conductivity, higher safety and better environmental friendliness than organic electrolytes, which are more in line with green chemistry requirements [15]. Based on the above advantages, ZMBs are considered to be one of the most potential devices for large-scale energy storage [16].
In 1950, people successfully developed an alkaline zinc-manganese battery using Zn powder as anode, electrolytic manganese dioxide as cathode, and NaOH or KOH as electrolyte, which was called "primary battery". In 1986, based on the traditional alkaline zinc-manganese battery, people replaced the alkaline electrolyte with ZnSO4 electrolyte, and developed an aqueous zinc-ion battery (ZIB) for the first time [17]. It was not until 2012 that Kang et al. proposed the concept of aqueous ZIB and explained the working principle [18], that is, Zn2+ is reversibly embedded and detached between the positive and negative electrodes to achieve energy conversion. For the Zn/MnO2 battery, Zn is peeled off from the negative electrode, converted into Zn2+ and migrates to the positive electrode, embedded in the pore structure of α-MnO2 during the discharge process. Conversely, Zn2+ is released from the pore structure of α-MnO2, and then reduced to Zn on the surface of the Zn anode and deposited during charge process.
Thanks to the clear explanation of the working principle and the continuous developments of cathode materials, research on ZMBs has continued to deepen in recent years [19,20]. However, some insurmountable problems have hindered the commercialization of batteries [21]. Firstly, the Zn anode has poor thermodynamic stability in neutral or weakly acidic electrolytes and hydrogen evolution reaction is prone to happen. The accumulation of escaped H2 will increase the pressure inside the battery and even cause the battery to bulge, reducing the safety and cycling performance of ZMBs [22-24]. Secondly, Zn2+ is preferentially deposited in protuberant locations and cause uncontrollable dendrite growth due to the "tip effect" during the charge process [25,26]. The as-formed dendrites can easily pierce the separator and cause a short circuit, even explosion of the batteries [27]. Thirdly, side reactions easily occur during the reversible charge-discharge process, which will cause the increase of pH value of the local solution around the Zn anode and produce by-products, such as ZnO, Zn(OH)2 and Zn2(OH)2SO4 [28]. These by-products have poor conductivity and reduce the nucleation active sites, leading to the passivation of Zn anode [29-31]. To address the above issues, researchers have proposed a series of targeted modification strategies, mainly including structural design and surface modification of Zn anode [32-34], electrolyte optimization [35,36] and separator modification [37,38]. Among them, electrolyte modification is simple to operate, easy to implement, and has good effect, thus has attracted widespread attention from researchers.
As an ionic conductive medium, electrolyte plays an important role in ZMBs [39]. In order to effectively regulate the plating/stripping behaviour of metal Zn and inhibit the chemical corrosion reaction, researchers have proposed a series of optimization strategies for electrolytes [40], including using functional electrolyte additives [41-43] and developing high-concentration ("water-in-salt") electrolyte [44,45]. The deposition behaviour of Zn can be effectively controlled by changing the solvation structure of Zn2+ or forming an electrostatic shielding layer around Zn2+ using electrolyte additives. However, some other horrible problems, such as cathode material dissolution [35,46], narrow electrochemical window [47] and hydrogen evolution and electrolyte leakage [48] are hard to solve for aqueous electrolytes [49,50]. Therefore, researchers turned their attention to "beyond aqueous" electrolytes [51], mainly including organic electrolytes, all-solid electrolytes and gel polymer electrolytes [52,53].
Researchers found that organic electrolytes, which have wide electrochemical stability windows, work well at high voltages and are compatible with high-voltage cathode materials [54]. At the same time, the problem of side reactions of Zn anode in aqueous electrolyte can be solved. However, organic electrolytes possess higher viscosity and lower ionic conductivity than aqueous electrolytes, which are not conducive to rapid ion transport [55]. Worse still, organic electrolytes are usually toxic and environmentally unfriendly, not in line with the requirements of green chemistry [56].
With the rapid growth of demand for flexible wearable electronic products, all-solid batteries with the advantages of good flexibility, no electrolyte leakage, low cost, and high safety [57] using solid electrolytes appear [58]. All-solid electrolytes are typically prepared using polymers or inorganic compounds and liquids to form cross-linked or mixed solutions [59] with high mechanical strength and excellent chemical stability, which can withstand enormous external pressure and simplify the manufacturing process, and avoid short circuit problems caused by diaphragm piercing [60]. However, the ion transport in solid polymer electrolytes (SPEs) mainly occurs in the amorphous regions above the glass transition temperature (Tg) [61]. Under room temperature (RT), the crystallinity of pure all-solid electrolytes is high, resulting in low ionic conductivity, which cannot meet the requirements of commercial batteries ( > 0.1 mS/cm at RT) [62]. In addition, the unstable interface between electrode and solid electrolyte, and large interface impedance seriously exacerbate the cycling performance and rate capability of ZMBs [63]. Therefore, it is necessary to explore an electrolyte that can integrate the advantages of solid electrolyte and liquid electrolyte.
The quasi-solid-state gel polymer electrolytes (GPEs) composed of liquid electrolytes and polymer matrixes keep the mechanical stability of all-solid-state electrolytes and the fast ion diffusion kinetics of liquid electrolytes, thus have received extensive attention in the field of metal batteries [64,65]. Moreover, the hydrophilic chain of gel can well combine with liquid electrolyte, greatly reducing the risk of electric leakage and short circuit in the use of flexible wearable devices, and effectively improving their safety performance. The abundant functional hydrophilic groups in the polymer chains, such as -OH and -COOH, -SO3, -NH2 [66,67] can interact with Zn2+ to reduce bound water and effectively limit dendrite growth, significantly improving the cycling stability and usage ratio of Zn anode [68,69].
In addition, GPEs with high mechanical strength, toughness, self-healing ability, frost resistance and other special functions can also be prepared by using the characteristics of different functional groups on the polymer chain [70-72]. Mechanical strength is one of the standards for testing the performance of GPEs. ZMBs need to exhibit good flexibility and mechanical stability under external forces. If the GPE cracks when the flexible battery is bent, it will increase the interface resistance and promote the occurrence of side reactions [73]. Although GPEs show better tensile properties than aqueous electrolytes and solid electrolytes, most GPEs will produce large residual deformation after stretching, which may lead to the failure of flexible batteries [74]. Due to the limitation of the freezing point, aqueous electrolytes will freeze at low temperatures, resulting in slower charge transfer and increased electrode/electrolyte interface resistance. The proportion of free water in the quasi-solid batteries assembled by GPEs are reduced, and it will not freeze in the environment above −10 ℃ [75]. However, at ultra-low temperatures (usually < −20 ℃), the free water in the hydrogel will freeze, causing dramatic changes in the properties of the electrolyte, forming ice crystals, and resulting in ductile brittle transition, which ultimately lead to the failure of the battery [76]. Unlike solid electrolytes, ion transport is primarily controlled by the swollen gelled phase in GPEs. The decreases of Zn2+ mobility number will cause the decrease of ionic conductivity of the electrolyte [77-79].
This review focuses on the research progress of multifunctional GPEs for high-performance ZMBs, mainly including physicochemical characteristics, effect mechanism and practicability. The major drawbacks, main challenges and perspectives on the development of GPEs are also discussed.
2.
Polyvinyl alcohol (PVA)-based GPEs
Several types of gel polymers with intrinsic structural stability and water-retention capability have been employed as host material to construct stable quasi-solid-state ZMBs. PVA is one of the most prevalent polymer host due to the semi-crystal property, high hydrophilicity, good film-forming properties and chemical inertness [80,81]. When mixed with electrolytic salts, PVA-based GPEs could deliver high ionic conductivity. When suffering from damage, PVA-based hydrogels obtained by a proper strategy would automatically repair [82,83]. A serious of PVA/KOH hydrogel electrolytes have been developed for flexible Zn-air batteries (ZABs) due to the high conductivity, high oxygen diffusion coefficient and the low viscosity of KOH [84-86]. It is demonstrated that the amount of KOH and water inside the membrane greatly influence the ionic conductivity of the gel polymer electrolyte [87]. Traditional PVA/KOH hydrogel electrolytes possess weak water retention capacity, resulting in rapid decrease of ionic conductivity. Moreover, PVA GPEs tend to lose their structural stability and mechanical robustness under strong alkaline condition, resulting in quick failure of flexible batteries [88-91].
2.1
Designing the matrix structure of PVA to improve the stability
To enhance the alkaline tolerance of PVA/KOH electrolyte, much work has been devoted to modify the polymer host. Structural design of the GPEs is a superior strategy for the improvement of the overall performance [92-94]. Yoon's group designed a hierarchically porous PVA/PAA hybrid GPE embedded with 6 mol/L KOH (Figs. 1a and b). Due to the enhanced chain segmental motion in the polymer network and the numerous ion diffusion channels, the GPE delivered improved ionic conductivity of 212 mS/cm (Fig. 1c). The as-prepared ZABs displayed excellent cycling stability, good rate performance, as well as superior flexibility, and could be used in smart watch [95]. Lee et al. fabricated a class of 3D-interconnected PVA/poly acrylic acid (PAA) nanofiber polymer blend electrolyte membranes with artificially-engineered, bi-continuous anion-conducting/-repelling phases (Fig. 1d). The membrane possessed unique ion conduction behaviour of selective transport of OH− and Zn(OH)42−, which played a viable role in improving the cycling stability of ZABs (Fig. 1e) [96]. Various other double cross-linked networks, such as PVA/PAM (Fig. 1f) [97], Agar-PVA/GO (Fig. 1g) [98], are also constructed for high performance ZMBs.
Figure 1
Figure 1.
(a) Schematic of the fabrication process of the HP-PVA/PAA GPE. (b) PVA/PAA unit cells modeled using all-atom MD simulations. (c) Nyquist plots and ionic conductivities of the pristine PVA, PVA+PAA, and HP-PVA/PAA membranes. Reproduced with permission [95]. Copyright 2023, Elsevier. (d) Conceptual illustration underlying the function of PBE membrane as a selective ion transport channel. (e) Conceptual illustration of Zn(OH)42− crossover through Celgard3501 and PBE membrane. Reproduced with permission [96]. Copyright 2016, The Royal Society of Chemistry. (f) Schematic of the chemical structure and hydroxide conduction mechanism in PVA/N-PAM/KOH GPE. Reproduced with permission [97]. Copyright 2023, American Chemical Society. (g) Configuration of sandwich-type ZABs using Agar-PVA/GO GPE. Reproduced with permission [98]. Copyright 2023, Elsevier.
Interpenetrating two or more polymer networks (IPN) is a unique combination for polymers, in which one of the network polymers is cross-linked in the presence of the other. This is distinct from other polymer combinations such as simple blends or copolymers. IPN exhibits two distinct characteristics: first, the combined IPN has the advantages of both polymers; second, the IPN morphology can be adjusted under specific polymerization condition [99]. For example, Lu's group designed a sulfonated polymer matrix via graft polymerization of sulfonated starch, acrylic acid monomers with potassium persulfate and N,N'-methylene-bis-acrylamide (Fig. 2a) [100]. The strong anionic sulfonate groups of the as-prepared GPEs induced the exposure of preferred Zn (002) plane, thus inhabiting Zn dendrite growth. Moreover, the nano-attapulgite electrolyte additives helped to enhance the ionic conductivity and electrolyte uptake (Fig. 2b). Due to the synergistic effects, the fabricated flexible ZABs displayed ultralong cycling life of 450 h (Fig. 2c). Tafur et al. fabricated an alkaline gel polymer electrolyte by immersing the film composed of PVA and terpolymer composed of butyl acrylate, vinyl acetate, and vinyl neodecanoate (VAVTD) in 12 mol/L KOH. The as-obtained PVA-VAVTD KOH GPE displayed high conductivity of 0.019 S/cm at RT and enabled the ZAB with high specific capacitance of 195 mAh/g at the discharge current of 5 mA/cm2 [101]. Zhong's group optimized the PVA-based GPEs via multiple cross-linking reactions among PVA, PAA, and graphene oxide (Fig. 2d) to enhance the mechanical strength (tensile stress reached 67 kPa), ionic conductivity (155 mS/cm), and water retention capability. Moreover, the followed introduction of KI additive in the traditional alkali ion conductor changed the path of the conventional oxygen evolution reaction and reduced the charging potential of the flexible ZABs to 1.69 V (Fig. 2e) [102].
Figure 2
Figure 2.
(a) Schematic of the structure of the sulfonate functionalized nanocomposite (nano-SFQ) GPE. (b) Mechanistic analysis of the interface between the zinc anode and nano-SFQ GPE. (c) Comparison of the cycling life of the nano-SFQ-based ZAB with previously reported ZABs. Reproduced with permission [100]. Copyright 2023, Wiley-VCH. (d) Schematic of the fabrication process for KI-PVAA-GO GPE. (e) Comparison of the cycle life and charge potential between KI-PVAA-GO-based ZABs and some previously reported ZABs. Reproduced with permission [102]. Copyright 2020, Wiley-VCH. (f) Schematic of the structure of near-neutral poly(vinyl alcohol)-alkalized MXene. Reproduced with permission [105]. Copyright 2022, Elsevier. (g) Zn plating/stripping in the Zn/Zn symmetrical cells with different GPEs at 0.5 mA/cm2. Reproduced with permission [106]. Copyright 2022, Elsevier. (h) Schematic of the flexible ZAB and the inner structure of the porous PVA-based nanocomposite GPE. Reproduced with permission [107]. Copyright 2018, Elsevier.
2.2
Using additives to improve the ionic conductivity and mechanical properties
Some non-volatile organic additives have been added to the hydrogel to prevent the loss of water, however, these additives decrease the ionic conductivity of the hydrogel [103,104]. Therefore, some novel inorganic electrolyte additives are developed to retain water and transport ions. Liang et al. designed a near-neutral GPE composite comprising PVA and hydroxy‑functionalized MXene (Fig. 2f) which served as a water reservoir, and possessed high ionic conductivity of 77.6 mS/cm. The assembled ZABs showed a long cycle lifespan of 160 h at a current density of 2 mA/cm2 and a charging-discharging period of 15 min [105]. Feng et al. developed MXene-derived TiO2 nanosheets/PVA hybrid gel electrolyte, which brought higher ionic conductivity and higher tensile strength with suitable TiO2 additive amount. As a consequence, highly stable Zn plating/stripping was achieved (over 3000 h at 0.5 mA/cm2) (Fig. 2g) [106]. Hu et al. reported a porous PVA-based nanocomposite GPE using SiO2 as fillers for flexible ZABs (Fig. 2h). The obtained porous PVA-based nanocomposite GPE containing 5 wt% SiO2 exhibited a superior ionic conductivity (57.3 mS/cm) and high electrolyte retention capability, as well as good thermal and mechanical properties (The maximum stress of porous PVA-5 wt% SiO2 polymer membranes is 705 kPa) [107].
2.3
Using new types of ionic conductors to improve the ion conductivity
New types of ionic conductors have also been explored to enhance the ionic conductivity and structural ability of PVA based electrolyte [108]. Chen's group fabricated a PVA/Zn(CF3SO3)2 hydrogel electrolyte with stable electrochemical performance via a facile freezing/thawing strategy (Fig. 3a). The PVA/Zn(CF3SO3)2 hydrogel electrolyte possessed high ionic conductivity of 12.6 S/cm due to the unique 3D porous network structure, as well as excellent self-healing property due to a large amount of hydroxyl side groups in PVA chain segments and the O-H···O hydrogen bonds (Fig. 3b). Even after three times of cutting/self-healing during 200 charge/discharge cycles, the self-healing Zn/PANI batteries still delivered a stable specific capacity of 81.4 mAh/g (Fig. 3c), which was almost the same as the original case [109]. Zhong's group used tetraethylammonium hydroxide (TEAOH) as the ionic conductor with PVA as the polymer host in the polymer electrolyte (Fig. 3d). PVA has good hydrophilicity because -OH act as hydrogen-bond acceptors that readily absorb water, and can prevent macro-phase separation. The sufficient amount of free OH− in the TEAOH aqueous solution so it can be used as a OH− conductor in polymer electrolytes. The PVA/TEAOH electrolyte exhibited better water retention capability than PVA/KOH electrolyte, as well as high ionic conductivity of 30 mS/cm even after two weeks (Fig. 3e) [110]. Lu's group used PVA/ZnCl2/MnSO4 gel as electrolyte for Zn-MnO2@poly(3,4-ethylenedioxythiophene) (PEDOT) battery, benefiting from a PEDOT buffer layer, the battery obtained high energy density of 504.9 Wh/kg, together with a peak power density of 8.6 kW/kg, as well as high CE (Fig. 3f) [111]. Zhu's group prepared aramid nanofiber (ANF)-reinforced PVA organo-hydrogels containing dim ethyl sulfoxide (DMSO)/H2O mixed solvents by solution casting and 3D printing methods (Fig. 3g). Due to the high dielectric constant of the added DMSO and the low density of polymer chains per unit volume resulted from the high solvent content in the organo-hydrogels, the ANF-PVA organo-hydrogel electrolyte presented high ionic conductivity ranging from 1.1 S/m to 34.3 S/m at −50~60 ℃ (Fig. 3h). The ZABs not only exhibited high specific capacity (262 mAh/g) with ultra-long cycling life (355 cycles, 118 h) even at −30 ℃ (Fig. 3i) [112].
Figure 3
Figure 3.
(a) Schematic process of fabricating PVA/Zn(CF3SO3)2 electrolyte. (b) Schematic illustrating the structure of self-healing integrated all-in-one Zn/PANI batteries. (c) Cycle performance of the self-healing Zn/PANI batteries at original state and after multiple cutting/self-healing times. Reproduced with permission [109]. Copyright 2018, Wiley-VCH. (d) Synthesis of the TEAOH-PVA electrolyte and schematic of a flexible ZAB. (e) Ionic conductivity of the TEAOH-PVA and KOH-PVA electrolytes after storage for two weeks. Reproduced with permission [110]. Copyright 2019, American Chemical Society. (f) Cycling performance of the Zn-MnO2@PEDOT battery at 1.86 A/g for 300 cycles. Reproduced with permission [111]. Copyright 2017, Wiley-VCH. (g) Schematic of the formation and micro-structure of ANF-PVA organo-hydrogels. (h) Nyquist plot of the ANF-PVA electrolyte containing 6 mol/L KOH at varied temperature. (i) Specific discharge capacity plots of the ANF-PVA organo-hydrogel-based ZABs at 2 mA/cm2. Reproduced with permission [112]. Copyright 2023, Wiley-VCH.
As one of the most widely used polymer matrixes, PVA mixed with KOH solution to form GPE, the ionic conductivity of the electrolyte depends on the contents of KOH and water. However, excessive KOH will lead to poor mechanical strength, and excessive water may affect the low temperature resistance of the battery [113]. We need to find a way to balance the relationship between them. Constructing a multi-layer polymer framework is an efficient method. For example, the composite of electrospun nanofiber membrane and gel polymer can effectively improve the ionic conductivity and mechanical strength of the electrolyte to adapt to the shape change of the battery during operation, which provides a new idea for the application of ZIBs in the field of flexible wearability [114].
3.
Polyacrylamide (PAM)-based GPEs
PAM possesses numerous hydrophilic amide groups (-CONH2), which can form network structure with high stretch-ability by strong hydrogen bonds with water molecules [115-117]. Moreover, metal ions are possibly complexing with the carbonyl oxygen and nitrogen in C=O and -NH2 groups to enable ion mobility in the PAM gel [118]. Therefore, PAM is expected to be a promising host polymer for GPE [119]. However, the mechanical properties and ionic conductivity (~17 mS/cm) of pure PAM cannot satisfy the requirements of high-performance GPEs at the same time [120,121]. To overcome these problems, various approaches including grafting on PAM matrix or using different additives have been reported [122].
3.1
Grafting on PAM matrix to improve the mechanical properties and ionic conductivity
Zhang et al. introduced triblock polymer Pluronic F127 and layered GO into PAM matrix (Fig. 4a), and then designed a dual-surfactant-optimized gel polymer (named PAM-F/G). When the PAM-F/G GPE absorbed 6 mol/L KOH solution, it obtained improved mechanical characteristics (The breaking strength and elongation rate of PAM-F/G gel are significantly greater than those of PAM gel) and a higher ionic conductivity (276 mS/cm at 20 ℃). A flexible ZAB assembled by PAM-F/G electrolyte showed a highly power density (155 mW/cm2) and could even operate reliably ( > 40 h) at −20 ℃ (Fig. 4b) [123]. Liu et al. used 2-acrylamido-2-methylpropane sulfonic acid without adding ZnSO4 to synthesize a PAM based GPE (referred as ZnPAM-AMPS) (Fig. 4c). The GPE exhibited a high ionic conductivity of 10.7 mS/cm, and high electrochemical stability window to 1.6 V. What is more, ZnPAM-AMPS had less free water so that it could reduce the dissolution of ammonium vanadium oxide, the Zn/(NH4)2V4O9 battery demonstrated high stability (175 mAh/g after 80 cycles) with ZnPAM-AMPS GPE [124]. Chen et al. proposed a PAM based GPE which contained graphene oxide (GO) and ethylene glycol (EG). The GO improved the mechanical properties of GPEs with a fracture elongation of ~644%, and accelerated ion transportation by constructing a 3D macro-porous network, and the EG improved frost resistance (Fig. 4d). Because of the synergistic effect, the PAM/GO/EG exhibited high ionic conductivity (14.9 mS/cm at −20 ℃) (Fig. 4e). The flexible Zn-MnO2 battery assembled with PAM/GO/EG could supply high specific capacities (183.2 mAh/g at 0.2 A/g) even at −20 ℃ (Fig. 4f) [125]. Hu et al. synthesized a hydrogel electrolyte formed of hydroxyethyl cellulose (HEC) and PAM, made use of radical polymerization and cross-linking reinforcement methods, and then constructed a 3D network structure (Fig. 4g). Due to the porous network and hydrophilic groups, the HEC/PAM electrolyte exhibited ionic conductivity of 3.17 × 10−2 S/cm2 and high decomposition voltage (2.64 V). The hydrophilic groups enhanced the inter facial compatibility among the electrode and electrolyte. Moreover, the hydrogel electrolyte alleviated the dissolution of the V2O5 cathode obviously. It could also stabilize zinc plating/stripping (Fig. 4h) and inhibited the occurrence of side reactions, which improved the stability of the electrode [126]. Zhou et al. reported a 3-methacry-loxypropyltrimethoxysilane (MPS) modified HNTs (M-HNTs)-cross-linked PAM hydrogel electrolyte (Fig. 4i). Through free radical polymerization, the M-HNTs constructed a 3D network with PAM. Owing to rich polar groups and inter-molecular hydrogen bonding, the M-HNTs/PAM hydrogel electrolyte achieved high ionic conductivity and good mechanical properties (it could be stretched even to 1200% of the original length). Besides, it could also suppress the growth of Zn dendrites [127]. Various other methods are also employed to improve properties of PAM-based GPEs such as acetamide/zinc perchlorate hexahydrate (AA/ZPH) ionic liquid (IL)- PAM polymer electrolytes (defined as IL-PAM) [128], freeze-thawed-PAM-PVA hydrogel [129].
Figure 4
Figure 4.
(a) Structure schematic of PAM, PAM-F127, PAM-GO, and PAM-F/G GPEs. (b) At 2 mA/cm2 discharge/charge curves of the PAM-F/G-based battery at −20, 0, and 20 ℃. Reproduced with permission [123]. Copyright 2022, American Chemical Society. (c) Schematic of the ZnPAM-AMPS hydrogel structure. Reproduced with permission [124]. Copyright 2023, Elsevier. (d) The schematic of PAM/GO/EG gel. (e) Nyquist plots and the ionic conductivity values of PAM/GO/EG GPE at different temperatures. (f) Cycling performances of Zn-MnO2 batteries with PAM/GO/EG at 1 A/g at different temperatures. Reproduced with permission [125]. Copyright 2020, Frontiers Media. (g) Schematic of synthesis and polymer network structure of HEC/PAM hydrogel. (h) Schematic of zinc deposition/stripping process regulated by HEC/PAM hydrogel electrolyte. Reproduced with permission [126]. Copyright 2024, Springer. (i) Formation mechanism of the integrated network within M-HNTs/ PAM hydrogel. Reproduced with permission [127]. Copyright 2021, Elsevier.
3.2
Using different additives to improve the anti-freezing properties of PAM
GPEs enable ZMBs with good flexibility and high safety at room temperature. However, it is hard to avoid the freezing problem at low temperature, which causes sharp decrease of charge transfer and self-healing function, thus deteriorating the electrochemical performance [130]. Generally, the anti-freezing property of GPEs could be realized by the addition of organic solvents (e.g., ethylene glycol (EG), glycerol (Gly) and dim-ethyl sulfoxide (DMSO)) or soluble ions (e.g., sulfuric acid, potassium hydroxide and zinc chloride) with high concentrations [131-133].
Polar organic solvents such as EG and DMSO have been applied to improve the anti-freezing properties of the electrolyte, since they can change the H-bond network of aqueous electrolytes. Moreover, they also can lower the saturated vapor pressure of water, thus decrease its freezing point and inhibit the formation of ice crystallites [134]. Qu et al. prepared a GPE (named as AF-SH—CPAM) by the in-situ polymerization of acrylamide (AM) monomer, methylene-bis-acrylamide (BIS) and ammonium per-sulfate (APS) in a water/EG solution (Fig. 5a). It showed an excellent self-healing property. The assembled Zn/PANI batteries could recover its original capacity though after encountering many times cutting operations (Fig. 5b). It could still attain outstanding specific capacity of 160.3 mAh/g and a highly capacity retention of 87.3% after 600 cycles at −20 ℃ [135]. Lei's group added DMSO into PAM to suppress the hydrogen evolution reaction by reconstructing the solvation sheath structure of Zn2+ (Fig. 5c). The PAM organo-hydrogel electrolyte had high ionic conductivities of 0.40, 0.087 and 475.6 mS/cm at −40, −60 and 60 ℃ respectively. The ZAB displayed excellent cycling stability of over 300 h (at 0.5 mA/cm2) with over 90% capacity retention at −60 ℃ [136]. As a common cryoprotectant and humectant, Gly can provide massive of hydroxyl groups to construct strong hydrogen bonds with water molecules [137]. Consequently, it can avoid the crystallization of GPEs at low temperatures. When they are exposed to ambient conditions, it can also alleviate the evaporation of water. Wang et al. developed an anti-freezing and anti-drying GPE based on PAM and Gly (Fig. 5d). The GPE displayed a high ionic conductivity of 9.65×10−5 S/cm at −40 ℃. Impressively, they exhibited excellent electrochemical stability under different bending states even at low temperatures. The Zn/SWCNTs/PANI ZIBs without encapsulation exhibited 98% capacity retention in ambient condition after 30 days [138]. Hu et al. presented a functional GPE consist of PAM, ZnSO4, Gly, and acetonitrile (AN), which had excellent mechanical properties. The forces between H2O/Zn2+ and oxygen-containing groups lowered the activity of water molecules. It also regulated the bonding interaction and the shell structure of Zn2+ to adjust Zn deposition behaviours (Fig. 5e). The batteries with GPEs showed high cycling stability (Zn/Zn battery steadily cycles over 3000 h) and high reversibility (CE reached 99.5%). Even at −20 ℃, the Zn/Zn battery could cycle over 500 h [139].
Figure 5
Figure 5.
(a) Schematic of the anti-freezing property of the AF-SH—CPAM poly-electrolyte. (b) Cycle performance at 0.5 A/g of the AF-SH-ZIB at original state and after different cutting/self-healing times. Reproduced with permission [135]. Copyright 2022, Elsevier. (c) Zn2+ solvation structure and formed H-bond between DMSO and H2O molecules. Reproduced with permission [136]. Copyright 2022, Nature. (d) Configuration and bonding mechanism of the PAM-H2O-Gly electrolyte. Reproduced with permission [138]. Copyright 2022, Springer. (e) Schematic on a possible mechanism of Zn deposition in ZnSO4/GL/AN hydrogel electrolyte. Reproduced with permission [139]. Copyright 2022, Elsevier. (f) Schematic and mechanism of the Zn(BF4)2-PAM hydrogel electrolytes. (g) Capacity and capacity retention of Zn/PANI batteries after being bent for several times at −70 ℃. Reproduced with permission [140]. Copyright 2023, Wiley-VCH. (h) Schematic of the Zn dendrite formation and water evaporation issues in a GPE for ZABs and MD simulation results of the PAM-SC hydrogel. (i) Schematic of the PAM-SC hydrogel composition. Reproduced with permission [141]. Copyright 2023, Wiley-VCH.
Except for the organic liquids, anionic additives with strong electronegativity were also developed to enhance the anti-freezing ability of the PAM GPEs. Liu et al. designed an anti-freezing hydrogel electrolyte composed of saturated zinc tetrafluoroborate (Zn(BF4)2) and PAM (Fig. 5f). Because of the strong electronegativity of F atoms in BF4− anions, the O—H···O between water molecules is replaced by O—H···F, which inhibited the formation of ice crystal lattice and enabled the Zn(BF4)2-PAM hydrogel electrolyte with ultra-low freezing point of lower than −70 ℃, excellent flexibility (Fig. 5g), and high ionic conductivity (2.38 mS/cm at −70 ℃). As a result, the Zn/PANI@SWCNTs ZIBs exhibited good flexibility and excellent cycling stability at −70 ℃ [140]. Zhou et al. developed a polarized GPE of PAM-sodium citric (PAM-SC) for ZABs (Fig. 5h). The strong hydrogen bonds between the polarized -COO− groups and water molecules (Fig. 5i) prevented water from freezing and evaporating at extreme temperatures. The PAM-SC GPE exhibited a high ionic conductivity of 324.8 mS/cm. Moreover, the polarized -COO− groups also exhibited zincophilic effect on the Zn anode to uniformize the Zn2+ transport flux and inhibit the formation of Zn dendrites. Consequently, the ZAB showed long cycling life of 700 cycles even at ultra-low temperature of −40 ℃ [141].
At present, adding organic solvents to form hydrogen bonds to inhibit the growth of ice crystals is the main method to improve the low temperature resistance of PAM GPE, but this method cannot solve the problem of free water freezing in the gel [73]. It is very important to develop batteries which can be able to work in extremely harsh low-temperature environment so that it will work in wide-temperature ranges [142]. Developing new method such as increasing the water content in the GPE frameworks can improve the anti-freezing performance, which is beneficial to the commercial application of GPEs.
4.
Poly-acrylic acid and sodium polyacrylate-based GPEs
4.1
Poly-acrylic acid (PAA)-based GPEs
PAA is another promising candidate for polymer electrolytes of ZMBs [143], which is normally cross-linked by the chemical cross-linking originated from double bonds through free radical polymerization. Furthermore, the abundant carbonyl and hydroxyl groups on the side chains of PAA endow it with a great potential of gelation by hydrogen bonding and good affinities with aqueous electrolytes [144]. PAA-based GPEs have excellent alkaline electrolyte absorption capability and can provide high ionic conductivity, showing great potential in the application of ZABs [145]. However, owing to high water uptake, PAA-based GPEs often display comparatively bad mechanical strength, resulting in deficient physical support between the battery's electrodes and a remarkable increase in the ohmic polarization of the battery [146].
To solve above problems, Song et al. reported a PAA-based composite GPE with the aluminium oxide (Al2O3) filler (Fig. 6a). After massive investigations, it was found that when 20 wt% Al2O3 was added to the PAA polymer, the gel polymer exhibited improved mechanical strength (The tensile strength of PAA-20 wt% Al2O3 gel polymers is 104.5 kPa). The corresponding GPE showed a high ionic conductivity of 186 mS/cm (Fig. 6b). This optimized GPE enabled the assembled ZAB to display a long cycling life of 384 h (Fig. 6c) and a large power density of 77.7 mW/cm2 [145]. Tang et al. constructed dual-network cross-linked PAA-Fe3+-chitosan (PAA-Fe3+-CS) polymer, and infiltrated it in a mixed aqueous solution consist of NH4Cl and ZnCl2 (Fig. 6d). The hydrogel electrolyte obtained high ionic conductivity but exhibited low corrosiveness to Zn metal anode (ZMA), thus the ZABs had a long cycle life up to 120 h at 5 mA/cm2. What is more, they introduced the CS molecular beams into the PAA framework, when in the near-neutral saturated electrolyte, these CS molecules precipitated and folded due to the Hofmeister effect. Thus, the interfacial adhesion strength of the hydrogel electrolyte on both air cathode and ZMA could be enhanced effectively. The ZABs exhibited a superior tolerance to repeated mechanical deformation, allowing more than 360 continuous bending-recovery cycles [147].
Figure 6
Figure 6.
(a) Schematic of the fabrication process for the PAA-Al2O3 GPE and its application in the ZAB. (b) Ionic conductivities of PAA and PAA-Al2O3 GPEs. (c) Galvanostatic discharge and charge profiles at 2 mA/cm2. Reproduced with permission [145]. Copyright 2021, American Chemical Society. (d) Schematic of the mechanism of PAA-Fe3+-CS hydrogel entry into tough bonding to both the surfaces of air cathode and ZMA under the action of Hofmeister effect. Reproduced with permission [147]. Copyright 2022, Wiley-VCH. (e) Preparation diagram of PAA-Zn(AC)2-ACG GPE. (f) Photographs of PAA-Zn(AC)2-ACG GPE in various deformation. Reproduced with permission [148]. Copyright 2022, The Electrochemical Society. (g) Schematic of the chemical structure of RDC/N-PAA/KOH polyelectrolyte. (h) The stress-strain curves at break for three types of SPEs. (i) Charge-discharge tests of the ZAB in different bending states at 2 mA/cm2. Reproduced with permission [149]. Copyright 2022, Elsevier.
In addition to mechanical strength, researchers have also further improved the ionic conductivity and water retention capacity of PAA-based GPEs. Ma et al. used poly sodium p-styrene sulfonate (PSS) through π-π interaction with graphene to prepare an aquatic colloidal graphene (ACG) and co-cross-linked with PAA-Zn(AC)2 to form an alkaline GPE (recorded as PAA-Zn(AC)2-ACG) (Fig. 6e). The alkaline GPE had a high ionic conductivity of 407.9 mS/cm, an electrochemical stability window of 1.7 V, and a 72.5% water retention ability. Consequently, the assembled ZAB could work steadily with a high capacity of 672 mAh/g at 0.5 mA/cm2, and had a continuous charge-discharge duration of 43.3 h. Even when the battery was bent and deformed (Fig. 6f), there was no obvious voltage attenuation [148]. Zhang et al. utilized cotton cellulose with high crystallinity as main network backbone to improve the alkali-resistance of the electrolyte. Hydrophilic acrylic acid monomers penetrate into regenerated degreasing cotton (RDC), and an in-site polymerization occurred to construct a second backbone. It was beneficial to the OH− conduction and increased the water content of the system (Fig. 6g). The physical cross-linking and the hydrogen bonding formed between cellulose chains and PAA networks could improve the mechanical properties and ionic conductivity (430 mS/cm). Surprisingly, the ZAB assembled using RDC/N-PAA/KOH electrolytes exhibited specific capacity (731.5 mAh/g), mechanical flexibility (the tensile strengths reached 381.43 kPa) (Fig. 6h) and outstanding cycling stability (70 cycles) (Fig. 6i) [149].
4.2
Sodium polyacrylate (PANa) -based GPEs
PANa is an absorbent polymer composed of a 3D cross-linked network, in which the ionic groups are located inside and the hydrogel network is outside [150]. It is considered to be a promising alkaline hydrogel electrolyte due to the advantages of excellent charge-discharge cycling stability and favourable mechanical properties in the strong alkaline corrosive electrolyte [117]. On one hand, arising from an osmotic pressure difference associated with the concentration difference in ionic groups inside and outside the hydrogel network, PANa hydrogel exhibits water super absorbent performance, which endow the PANa hydrogel good water retention ability and high ionic conductivity during the long charging-discharging cycling [151]. On the other hand, PANa hydrogel has a significant large interaction energy with water molecules due to containing rich inorganic ion and oxygen-containing functional group, which can allow to the PANa hydrogel to work well at sub-zero temperature [152]. However, due to the decrease of interaction force between chains of macro-molecules caused by swelling effect, the mechanical strength of PANa hydrogel decreases sharply. Therefore, it was crucial to solve the swelling effect of PANa hydrogel and maintain an integrated characteristic including the good mechanical property, high ionic conductivity and excellent anti-freezing performance [90].
Ma et al. synthesized a dual-network hydrogel electrolyte consist of PANa and cellulose (Fig. 7a). PANa chains contributed to the formation of soft domains, the carboxyl groups neutralized by hydroxyls and cellulose as potassium hydroxide stabilizer, all of this enhanced alkaline tolerance (Fig. 7b). The obtained super-stretchable, flat ZAB exhibited a highly power density of 210.5 mW/cm2 upon being 800% stretched (Fig. 7c). The hydrogel infiltrated by 6 mol/L KOH solution still showed over 1000% stretch ability [117]. Liu et al. incorporated rigid GO and cellulose nanofibers (CNFs) into a PANa-bonded network to improve the elasticity and stretch ability of the GPE (Fig. 7d). The GO nanosheets could provide rich OH- and carboxy, so that to form a well-defined ionic conductive channel. As a result, the PANa/CNF/GO GPE possessed enhanced mechanical properties (the incorporation of the CNF and GO increases the maximum strain of the GPE to 1370%) and high ionic conductivity (178.6 mS/cm) (Fig. 7e). The ZABs assembled operated steadily without energy loss after multiple folds (Fig. 7f) [153]. Chen et al. created a dual-network hydrogel based on PANa and starch (Fig. 7g). Thanks to the strengthening molecular entanglement and hydrogen bonding of starch, the mechanical properties of and ionic conductivity of the electrolyte were improved effectively (the breaking strength is 10.85 kPa and the ionic conductivity is 82 mS/cm). Due to the ionic hydration, the ZAB with PANa-starch (PANa-St/KOH) exhibited an excellent anti-freezing capability, and showed extraordinary electrochemical performance at −20 ℃ (Fig. 7h) [154].
Figure 7
Figure 7.
(a) Synthetic procedure of the PANa-cellulose hydrogel electrolyte using MBAA, acrylate and cellulose. (b) Schematic reflecting structure of PANa-cellulose hydrogel electrolyte entrapped KOH and water via the interactions of hydrogen bonds. (c) Max power density as a function of the tensile strain. The insets are the photographs of the flat-shaped ZAB at a fully released state and 800% strain. Reproduced with permission [117]. Copyright 2019, Wiley-VCH. (d) Schematic and chemical reaction process of the fabrication of the PANa/CNF/GO GPE. (e) Ionic conductivity of PANa, PANa/CNF, and PANa/CNF/GO hydrogels. (f) Galvanostatic discharge and charge curves of the ZABs under different conditions. Reproduced with permission [153]. Copyright, 2023 American Chemical Society. (g) Synthetic procedure of the PANa-St/KOH hydrogel electrolyte obtained via polymerization and swelling, and schematic of the chemical structure of PANa and starch chains in the PANa-St/KOH hydrogel. (h) Long-term cycling tests of the stretchable ZAB at 25 ℃ and −20 ℃. Reproduced with permission [154]. Copyright 2021, Elsevier.
Both PAA and PANa based GPEs have the problem of poor mechanical strength due to high water absorption but weak water retention capacity. At present, the existing scheme is to add inorganic fillers to enhance the flexibility of the skeleton, but this method cannot fundamentally solve the challenge. How to improve the stability of the skeleton and ensure the constant ionic conductivity in the gel still needs further exploration [155].
5.
Carrageenan, guar gum, xanthan gum and gelatin
Gelatin, guar gum, carrageenan, and xanthan gum are collectively referred to as bio-polymers because they are generally extracted from natural organisms or characterized by natural degradation and environmental friendliness. These kinds of polymers inexpensive, hydrophilic, non-toxic and renewable, and contain rich hydrophilic groups. They can combine with water molecules to improve ionic conductivity while limiting the loss of water molecules. They are ideal candidates for good GPEs.
5.1
Carrageenan
Kappa-carrageenan (C24H36O25S2−2) is a natural polymer consisting of 3-linked β-d-galactose-4-sulfate and 4-linked 6-anhydro-α-galactopyranose, which has one negative charge in per disaccharide repeating unit [156]. The structure of kappa-carrageenan consists of hydroxyl groups, which enables the formation of coordinate bonds with cations (Fig. 8a). It is abundant in nature, non-toxic, renewable, bio-compatible, and cost effective compared to synthetic polymers [157]. A few works have been reported in literature to make polymer electrolytes using kappa-carrageenan, such as bio-polymer blend based on kappa-carrageenan and cellulose derivatives, which exhibit ionic conductivity of 3.25 × 10−4 S/cm [158]. Moreover, kappa-carrageenan also has been used as electrolyte for energy storage devices (e.g., supercapacitors), where they achieve electrolyte with mechanical stability or high values of ionic conductivity [92]. Liu et al. dissolved kappa-carrageenan in ZnSO4/MnSO4 solution, and filled the solution in the network of rice paper (denoted as KCR). The rice paper could decrease the possibility of short circuit and improve the mechanical properties of electrolyte. The KCR electrolyte displayed a high ionic conductivity of 3.32 × 10−2 S/cm at RT. The Zn/MnO2 battery with KCR demonstrated a fast charging and discharging capability (120.0 mAh/g at 6.0 A/g), outstanding cycling stability, and high reliability bearing cycles of bending (Fig. 8b) [159].
Figure 8
Figure 8.
(a) The molecular formula of kappa-carrageenan. (b) Discharge curves under normal and bending conditions. Reproduced with permission [159]. Copyright 2019, The Royal Society of Chemistry. (c) Schematic of the structure of the Zn-MnO2 battery. (d) Schematic of the morphology change of the zinc foil in guar gum electrolyte and aqueous electrolyte after cycling. Reproduced with permission [163]. Copyright 2019, Elsevier. (e) A hand-made flower using the xanthan gum electrolyte and the molecular formula of xanthan gum. (f) Photograph of aqueous solutions of 2 mol/L ZnSO4 and 0.1 mol/L MnSO4 after adding 10 wt% different GPEs. Reproduced with permission [165]. Copyright 2013, The Royal Society of Chemistry. (g) Cycling performance under different bending states (0.5 A/g) of the flexible Zn/NaV3O8 battery. Reproduced with permission [169]. Copyright 2018, Nature. (h) Cycling performance of Zn/GHE/LiMn2O4 battery at 25 mA/g. (i) Zn/GHE/Zn/LiMn2O4 batteries under different conditions. Reproduced with permission [170]. Copyright 2018, The Royal Society of Chemistry.
Guar gum (C10H14N5Na2O12P3), a natural polymer that has been widely used as a stabilizer and thickener in various food products. The bio-polymer presents a very ordered structure of α,β−1,4-mannose chain interposed with α−1,6-galactose substituents in almost every second unit [160,161]. The guar gum is abundant in nature, water-soluble, non-toxic, renewable, bio-compatible, and cost effective. Impressively, the guar gum electrolyte fabrication process was simple and efficient, did not require a water and oxygen-free environment. It is soluble in water, salt tolerant, and has high acid resistance, so it has been an ideal candidate for solid state electrolyte [162]. Huang et al. dissolved guar gum in ZnSO4/MnSO4 solution. The eco-friendly electrolyte was highly flexible and conductive (1.07 × 10−2 S/cm). As a result, the flexible Zn/MnO2 battery (Fig. 8c) delivered a high specific capacity (308.2 mAh/g at 0.3 A/g). The guar gum electrolyte could effectively inhibit the growth of Zn dendrites (Fig. 8d), leading to a remarkable cycling cyclability (85% capacity retention after 2000 cycles at 6.0 A/g) and high bending durability (81.3% capacity retention after continuously bending to 180° for 1000 cycles). Moreover, the Zn/MnO2 battery with guar gum electrolyte also functioned in a wide temperature window (5–45 ℃) [163].
5.3
Xanthan gum
Xanthan gum (C8H14Cl2N2O2) is an exopolysaccharide, which is consisted of α,β−1,4-linked glucan backbone along with trisaccharide side chains attaching on the alternating d-glucosyl residues (Fig. 8e). The hydroxyl groups on the side chain of xanthan gum could attract water molecules. The long carbon chain can strengthen the interaction between the xanthan gum and water molecules. Xanthan gum delivers a high ionic conductivity due to its high salt-tolerance and high hydrophilic ability [65]. With these advantages favourable for a high ionic conductivity, high solubility and viscidity, it shows a great potential to act as a GPE [164]. Di et al. developed a sulfate-tolerant bio-electrolyte, it was prepared by dissolving xanthan gum in ZnSO4/MnSO4 solution. The bio-electrolyte was hydrating, adhesive and adaptive. It also had a high ionic conductivity (1.46 × 10−2 S/cm). The xanthan gum electrolyte showed better stability and homogeneity (Fig. 8f). Even at −8 ℃, it also exhibited an ionic conductivity of 2.5 × 10−3 S/cm. Moreover, the xanthan gum electrolyte could suppress Zn dendrite growth, there were less dendrites although cycled after 1000 cycles. The Zn/MnO2 battery assembled using such gum electrolyte showed excellent electrochemical performance including highly rate capability, good cyclability (about 90% capacity retention and about 100% CE over 330 cycles at 1 C) [165].
5.4
Gelatin
Gelatin (C102H151O39N31) is an amphoteric poly-electrolyte composed of three types of chains in a triple helix [166]. Gelatin shows its advantages including environmental friendless, high salt conductivity [167]. The presence of abundant hydrophilic groups (such as amidogens, hydrogens, and carbonyls) can improve the ionic conductivity. Under heating conditions gelatin can be dissolved in aqueous electrolyte. And then during cooling process it can be transformed into helices to form a solid network. Because of this, gelatin can achieve a high flexibility when it was coated on the electrode [168]. For example, Wan et al. added gelatin into ZnSO4 solution at RT, and assembled quasi-solid-state Zn/NaV3O8 batteries. They tested the cycling performance of a representative battery with a length of 9 cm at different bending states (Fig. 8g). When the battery was bent to form a circular column with a diameter of even 2 cm, it was still able to display a steady capacity of 145 mAh/g. Moreover, after the battery recovered from bending state to flat state after 90 cycles, the capacity could be still up to 133 mAh/g [169]. Han et al. proposed a gelatin hydrogel electrolyte (GHE) consisted of 0.5 mol/L Li2SO4 and 0.5 mol/L ZnSO4 thanks to the thermo-reversibility of the gelatin. A quickly cooling technique had been used since the higher cooling rate could obtain more orderly localized gelatin molecules and a more robust electrolyte, which delivered a high ionic conductivity of 6.15 × 10−3 S/cm. They found that the GHE could inhibit the self-corrosion and dendrites growth of Zn foil. Moreover, GHE could also show a high-water conservation ability and decrease the Mn dissolution into electrolyte. Due to these advantages, the Zn/LiMn2O4 battery displayed a highly capacity retention was nearly 90% after 100 cycles at 25 mA/g (Fig. 8h). The battery could output power to an LED regardless of soaking in water, bending, twisting, and crimping resist cutting (Fig. 8i) [170].
Biomass-based GPEs are low-cost, renewable, non-toxic, and have rich functional groups, such as -COOH, -NH2 and so on, which can be used in large-scale electrochemical energy storage equipment. However, there are still limited researches at present, and the problems of low ionic conductivity and poor mechanical strength seriously damage the electrochemical performance or flexibility of batteries. Multiple bones can be constructed by grafting with other traditional polymers to form an interpenetrating polymer network with excellent ionic conductivity and flexibility [171].
6.
Conclusions and perspectives
The application of GPEs could well solve the problems of aqueous electrolytes, such as electrolyte leakage, cathode material dissolution, narrow voltage window and so on. However, some inadequacies are still need to solve to push out of the stage of laboratory exploration.
(1) Although compared with aqueous electrolytes, GPEs greatly reduce the water content, reduce the risk of electrolyte leakage, and improve the cycle stability of batteries. There is still the problem of water evaporation, which leads to slow ion transportation or the gel hardening, causing the battery failure. Therefore, we need to take certain measures to suppress water evaporation. For example, hydrophilic groups are grafted onto polymer chains to form hydrogen bonds with water molecules to improve the water retention capacity of GPEs.
(2) Adding organic anticoagulants or soluble hydrated ions can improve the antifreeze performance of GPEs, but improper addition may lead to lower ionic conductivity and poor cycle performance of the battery. So, it is necessary to explore new effective anti-low temperature modification strategies to reduce the amount of free water in the gel and increase the amount of bound water. For example, to prepare glassy hydrogels and improve the intrinsic frost resistance of the gel.
(3) To be better applied to flexible wearable devices, the mechanical stability of GPEs still needs to be improved, and the construction of multiple cross-linking networks is the most effective and convenient method. The construction of a new second backbone through the cross-linking of two or more polymers can not only effectively improve the mechanical property of the GPEs, but also maintain the ionic conductivity, and even establish an ion channel that is more conducive to ion transport, so as to obtain ZMBs with long cycle life and fast kinetic rate.
(4) Biomass-based GPEs are environmentally friendly, inexpensive and biodegradable, which is conducive to the sustainable development of resources. However, there are few related studies at present. How to improve the mechanical properties and ionic conductivity of biomass-based GPEs is still a challenge that needs in-depth researches. Cross-linking with common polymer matrixes to construct ion transport channels can be attempted.
(5) Compared with aqueous electrolytes, the voltage window of GPEs is wider, which can alleviate the hydrogen evolution reaction caused by water decomposition, but it is still lower than 2.0 V, which is far lower than that of LIBs using organic electrolytes, and is not suitable for high-voltage working environments. To solve this problem, we can imitate the scheme of LIBs, introduce ionic liquids to widen the electrochemical window by using functional groups.
(6) At present, the functions of GPEs have not been fully developed. For example, special functions such as self-healing and thermal reversible need to be further explored, so as to better meet the requirements for flexible wearable devices and future requirements for energy storage devices.
In summary, the exploration of GPEs with high ionic conductivity, high mechanical properties and good temperature compatibility is significant for the commercialization of ZMBs. Although significant progress has been made, it is necessary to further improve the structure stability, thermal reversibility and self-protection ability of GPEs to promote the practical application of ZMBs in harsh external environment.
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.
This work was supported by the National Natural Science Foundation of China (No. 22075115), the Natural Science Foundation of Jiangsu Province (No. BK20211352), the Natural Science Foundation (No. 22KJA430005) of Jiangsu Education Committee of China, Joint Funds of the National Natural Science Foundation of China (No. U2141201) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. 2024XKT0500).
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Figure 1
(a) Schematic of the fabrication process of the HP-PVA/PAA GPE. (b) PVA/PAA unit cells modeled using all-atom MD simulations. (c) Nyquist plots and ionic conductivities of the pristine PVA, PVA+PAA, and HP-PVA/PAA membranes. Reproduced with permission [95]. Copyright 2023, Elsevier. (d) Conceptual illustration underlying the function of PBE membrane as a selective ion transport channel. (e) Conceptual illustration of Zn(OH)42− crossover through Celgard3501 and PBE membrane. Reproduced with permission [96]. Copyright 2016, The Royal Society of Chemistry. (f) Schematic of the chemical structure and hydroxide conduction mechanism in PVA/N-PAM/KOH GPE. Reproduced with permission [97]. Copyright 2023, American Chemical Society. (g) Configuration of sandwich-type ZABs using Agar-PVA/GO GPE. Reproduced with permission [98]. Copyright 2023, Elsevier.
Figure 2
(a) Schematic of the structure of the sulfonate functionalized nanocomposite (nano-SFQ) GPE. (b) Mechanistic analysis of the interface between the zinc anode and nano-SFQ GPE. (c) Comparison of the cycling life of the nano-SFQ-based ZAB with previously reported ZABs. Reproduced with permission [100]. Copyright 2023, Wiley-VCH. (d) Schematic of the fabrication process for KI-PVAA-GO GPE. (e) Comparison of the cycle life and charge potential between KI-PVAA-GO-based ZABs and some previously reported ZABs. Reproduced with permission [102]. Copyright 2020, Wiley-VCH. (f) Schematic of the structure of near-neutral poly(vinyl alcohol)-alkalized MXene. Reproduced with permission [105]. Copyright 2022, Elsevier. (g) Zn plating/stripping in the Zn/Zn symmetrical cells with different GPEs at 0.5 mA/cm2. Reproduced with permission [106]. Copyright 2022, Elsevier. (h) Schematic of the flexible ZAB and the inner structure of the porous PVA-based nanocomposite GPE. Reproduced with permission [107]. Copyright 2018, Elsevier.
Figure 3
(a) Schematic process of fabricating PVA/Zn(CF3SO3)2 electrolyte. (b) Schematic illustrating the structure of self-healing integrated all-in-one Zn/PANI batteries. (c) Cycle performance of the self-healing Zn/PANI batteries at original state and after multiple cutting/self-healing times. Reproduced with permission [109]. Copyright 2018, Wiley-VCH. (d) Synthesis of the TEAOH-PVA electrolyte and schematic of a flexible ZAB. (e) Ionic conductivity of the TEAOH-PVA and KOH-PVA electrolytes after storage for two weeks. Reproduced with permission [110]. Copyright 2019, American Chemical Society. (f) Cycling performance of the Zn-MnO2@PEDOT battery at 1.86 A/g for 300 cycles. Reproduced with permission [111]. Copyright 2017, Wiley-VCH. (g) Schematic of the formation and micro-structure of ANF-PVA organo-hydrogels. (h) Nyquist plot of the ANF-PVA electrolyte containing 6 mol/L KOH at varied temperature. (i) Specific discharge capacity plots of the ANF-PVA organo-hydrogel-based ZABs at 2 mA/cm2. Reproduced with permission [112]. Copyright 2023, Wiley-VCH.
Figure 4
(a) Structure schematic of PAM, PAM-F127, PAM-GO, and PAM-F/G GPEs. (b) At 2 mA/cm2 discharge/charge curves of the PAM-F/G-based battery at −20, 0, and 20 ℃. Reproduced with permission [123]. Copyright 2022, American Chemical Society. (c) Schematic of the ZnPAM-AMPS hydrogel structure. Reproduced with permission [124]. Copyright 2023, Elsevier. (d) The schematic of PAM/GO/EG gel. (e) Nyquist plots and the ionic conductivity values of PAM/GO/EG GPE at different temperatures. (f) Cycling performances of Zn-MnO2 batteries with PAM/GO/EG at 1 A/g at different temperatures. Reproduced with permission [125]. Copyright 2020, Frontiers Media. (g) Schematic of synthesis and polymer network structure of HEC/PAM hydrogel. (h) Schematic of zinc deposition/stripping process regulated by HEC/PAM hydrogel electrolyte. Reproduced with permission [126]. Copyright 2024, Springer. (i) Formation mechanism of the integrated network within M-HNTs/ PAM hydrogel. Reproduced with permission [127]. Copyright 2021, Elsevier.
Figure 5
(a) Schematic of the anti-freezing property of the AF-SH—CPAM poly-electrolyte. (b) Cycle performance at 0.5 A/g of the AF-SH-ZIB at original state and after different cutting/self-healing times. Reproduced with permission [135]. Copyright 2022, Elsevier. (c) Zn2+ solvation structure and formed H-bond between DMSO and H2O molecules. Reproduced with permission [136]. Copyright 2022, Nature. (d) Configuration and bonding mechanism of the PAM-H2O-Gly electrolyte. Reproduced with permission [138]. Copyright 2022, Springer. (e) Schematic on a possible mechanism of Zn deposition in ZnSO4/GL/AN hydrogel electrolyte. Reproduced with permission [139]. Copyright 2022, Elsevier. (f) Schematic and mechanism of the Zn(BF4)2-PAM hydrogel electrolytes. (g) Capacity and capacity retention of Zn/PANI batteries after being bent for several times at −70 ℃. Reproduced with permission [140]. Copyright 2023, Wiley-VCH. (h) Schematic of the Zn dendrite formation and water evaporation issues in a GPE for ZABs and MD simulation results of the PAM-SC hydrogel. (i) Schematic of the PAM-SC hydrogel composition. Reproduced with permission [141]. Copyright 2023, Wiley-VCH.
Figure 6
(a) Schematic of the fabrication process for the PAA-Al2O3 GPE and its application in the ZAB. (b) Ionic conductivities of PAA and PAA-Al2O3 GPEs. (c) Galvanostatic discharge and charge profiles at 2 mA/cm2. Reproduced with permission [145]. Copyright 2021, American Chemical Society. (d) Schematic of the mechanism of PAA-Fe3+-CS hydrogel entry into tough bonding to both the surfaces of air cathode and ZMA under the action of Hofmeister effect. Reproduced with permission [147]. Copyright 2022, Wiley-VCH. (e) Preparation diagram of PAA-Zn(AC)2-ACG GPE. (f) Photographs of PAA-Zn(AC)2-ACG GPE in various deformation. Reproduced with permission [148]. Copyright 2022, The Electrochemical Society. (g) Schematic of the chemical structure of RDC/N-PAA/KOH polyelectrolyte. (h) The stress-strain curves at break for three types of SPEs. (i) Charge-discharge tests of the ZAB in different bending states at 2 mA/cm2. Reproduced with permission [149]. Copyright 2022, Elsevier.
Figure 7
(a) Synthetic procedure of the PANa-cellulose hydrogel electrolyte using MBAA, acrylate and cellulose. (b) Schematic reflecting structure of PANa-cellulose hydrogel electrolyte entrapped KOH and water via the interactions of hydrogen bonds. (c) Max power density as a function of the tensile strain. The insets are the photographs of the flat-shaped ZAB at a fully released state and 800% strain. Reproduced with permission [117]. Copyright 2019, Wiley-VCH. (d) Schematic and chemical reaction process of the fabrication of the PANa/CNF/GO GPE. (e) Ionic conductivity of PANa, PANa/CNF, and PANa/CNF/GO hydrogels. (f) Galvanostatic discharge and charge curves of the ZABs under different conditions. Reproduced with permission [153]. Copyright, 2023 American Chemical Society. (g) Synthetic procedure of the PANa-St/KOH hydrogel electrolyte obtained via polymerization and swelling, and schematic of the chemical structure of PANa and starch chains in the PANa-St/KOH hydrogel. (h) Long-term cycling tests of the stretchable ZAB at 25 ℃ and −20 ℃. Reproduced with permission [154]. Copyright 2021, Elsevier.
Figure 8
(a) The molecular formula of kappa-carrageenan. (b) Discharge curves under normal and bending conditions. Reproduced with permission [159]. Copyright 2019, The Royal Society of Chemistry. (c) Schematic of the structure of the Zn-MnO2 battery. (d) Schematic of the morphology change of the zinc foil in guar gum electrolyte and aqueous electrolyte after cycling. Reproduced with permission [163]. Copyright 2019, Elsevier. (e) A hand-made flower using the xanthan gum electrolyte and the molecular formula of xanthan gum. (f) Photograph of aqueous solutions of 2 mol/L ZnSO4 and 0.1 mol/L MnSO4 after adding 10 wt% different GPEs. Reproduced with permission [165]. Copyright 2013, The Royal Society of Chemistry. (g) Cycling performance under different bending states (0.5 A/g) of the flexible Zn/NaV3O8 battery. Reproduced with permission [169]. Copyright 2018, Nature. (h) Cycling performance of Zn/GHE/LiMn2O4 battery at 25 mA/g. (i) Zn/GHE/Zn/LiMn2O4 batteries under different conditions. Reproduced with permission [170]. Copyright 2018, The Royal Society of Chemistry.
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