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Original Article
2025
:18;
4072025
doi:
10.25259/AJC_407_2025

Synthesis and characterization of polyzwitterionic hydrogel electrolytes for enhancing the interface stability of a Zn-metal anode

, , , , , , , , ,
aDepartment of School of Materials and Environmental Engineering, Chengdu Technological University, No. 1, Section 2, Zhongxin Avenue, Pidu District, Chengdu City, Sichuan Province, Chengdu, 611730, Sichuan, China
bState Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, No. 8 Xindu Avenue, Xindu District, Chengdu City, Sichuan Province, Chengdu, 610500, China
cDepartment of Chemistry and Chemical Engineering School, Mianyang Teachers’ College, Mianxing West Road No.166, Mianyang, 621000, China

* Corresponding authors: E-mail addresses: guomeiling0913@163.com (M. Guo); gao.sujuan@outlook.com (S. Gao)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Zinc-ion batteries (ZIBs) based on hydrogel electrolytes have broad application prospects in wearable sensors, implantable devices, and soft robotics. However, designing a hydrogel electrolyte with high ionic conductivity and outstanding interface stability remains challenging. Especially, the dendrite and side reactions caused by unstable interfaces limit the application of ZIBs. Herein, the polyzwitterionic hydrogel electrolyte (P(AM-co-AMPS-co-DMC)) with dual-ion migration channels was developed based on the synergistic effect between the polyzwitterionic network and high-concentration saline. The resultant hydrogel electrolyte exhibited excellent ionic conductivity (46.19 mS cm-1), surpassing most reported single-ion hydrogels. The Zn–Zn symmetric cell with hydrogel electrolyte could maintain a long cycling lifespan of 2000 h at a current density of 1 mA cm-2 with a capacity of 1 mA h cm-2. Additionally, the Zn-Cu cell using the hydrogel electrolyte also exhibited more stable Coulomb efficiency with a high average value of 99.1%. This work provides a novel design philosophy for flexible electrolytes with exceptional long-term stability, which will facilitate the adoption of flexible ZIBs in emerging flexible electronics.

Keywords

Conductivity
Interface stability
Hydrogel electrolyte
Zinc ion battery

1. Introduction

In recent years, the successive emergence of flexible electronic devices such as soft sensors [1], human-machine interfaces [2], and soft robotics [3] has attracted significant research attention. There exists a substantial market demand for developing flexible energy storage batteries that can match these devices with characteristics of bendability, implantability, and wearability [4]. Among numerous battery devices, lithium-ion batteries (LIBs) currently predominate the rechargeable battery market due to their unparalleled energy density and cycle retention rate of over 1000 cycles. [5]. However, the further development of LIBs faces notable limitations. On one hand, mechanical deformations like bending or folding can lead to electrolyte leakage, combustion, and explosion. On the other hand, the naturally low abundance, coupled with their recent price surges, has substantially increased production costs [6]. Consequently, developing cost-effective, flexible batteries with enhanced safety has emerged as a critical research priority in advanced battery technologies.

Compared with LIBs, zinc-ion batteries (ZIBs) have shown significant potential for flexible wearable electronic devices due to their unparalleled volumetric energy density, abundant resource, environmental friendliness, and safety [7]. However, ZIBs are faced with many quandaries. ZIBs contain a water-rich anode-electrolyte interface, which can cause a series of harmful phenomena, such as the hydrogen evolution reaction, the oxygen evolution reaction, corrosion, insoluble by-products, and severe zinc dendrite formation. These issues collectively lead to a decrease in the interface stability and the Coulombic efficiency (CE) of the ZIBs [8]. Therefore, interfacial stability of the Zn-metal anode is recognized as a critical determinant for long cycling of ZIBs.

Recently, different strategies have been proposed to stabilize the interface of the Zn-metal anode and electrolyte, including the construction of the protective layer on the zinc anode surface [9], Zn anode structural design [10], modification engineering of separator [11], and the optimization of electrolyte composition [8]. Among these, electrolyte engineering has been considered the most effective approach because of its simple preparation and low cost [12]. For instance, Geng [13] et al. developed a glycol-based eutectic electrolyte with hyper-concentrated ionic clusters (dense ion networks formed via ion-dipole interactions). This eutectic electrolyte had a high concentration of ionic functional groups that can achieve a highly reversible Zn plating/stripping process. Niu [14] et al. prepared an aqueous electrolyte with nanoscale hydrophobic confinement to protect metals from direct water corrosion and prevent dendrite growth. The average CE of the aqueous electrolyte with nanoscale hydrophobic confinement is up to 99.3%. Chen [15] et al. reported an aqueous electrolyte with ammonium hydroxide as an additive to provide excellent electrochemical stability. The Zn–Zn symmetric cell using ammonium hydroxide aqueous electrolyte delivered a lifespan of around 1500 h at a current density of 1 mA cm-2. In summary, the aforementioned liquid electrolyte engineering strategies have improved the interface stability of Zn-metal anode to some extent. However, there is still a risk of solvent leakage from liquid electrolytes after long-term bending [16]. Moreover, high concentrations of salts or other additives not only increase battery production costs but also lead to high viscosity of electrolyte and a significant decrease in ionic conductivity [17].

Compared with the liquid electrolyte, hydrogel electrolytes derived from polymeric networks were considered as promising candidates owing to their abundant functional groups, outstanding flexibility, and superior processability [18]. These materials can simultaneously function as both electrolyte and separator, thereby fundamentally addressing critical safety issues associated with liquid electrolytes, including leakage and flammability [19]. More importantly, the reduced population of free water molecules in hydrogel electrolytes, combined with the hydrophilic polymer matrix that forms a protective coating on the Zn-metal anode surface, effectively suppresses dendritic growth [20]. However, conventional hydrogels suffer from a trade-off between ionic conductivity and interfacial stability: the restricted free water content inevitably lowers ionic conductivity [18]. Although incorporating zwitterionic polymers can enhance conductivity, such improvements yield only modest gains. For instance, Zhanadilov [21] et al. reported a zwitterionic hydrogel electrolyte with 97% CE, yet its conductivity plateaued at 24.32 mS cm⁻1, thus limiting high-power applications. Therefore, designing hydrogel electrolytes that concurrently deliver high ionic conductivity and interfacial durability remains a formidable challenge.

Herein, a polyzwitterionic hydrogel electrolyte was prepared to enhance interface stability and ionic conductivity. The polyzwitterionic hydrogel electrolyte was synthesized by a one-step solution polymerization of acrylamide (AM), 2-acryl-amido-2-methyl-propanesulfonate, and [2-(Methacryloyloxy)ethyl] trimethylammonium chloride monomers (P(AM-co-AMPS-co-DMC)). In the P(AM-co-AMPS-co-DMC) hydrogel system, polyacrylamide (PAM) chain segments and water molecules could form hydrogen bond interactions to reduce the activity of water molecules. The P(AMPS-co-DMC) chain segments as a polyzwitterionic network could establish dual-ion migration channels to regulate zinc-ion transport pathways. Therefore, this structural design could improve the interface stability. In addition, the synergistic effect hydrogel in electrolyte effectively circumvents the significant ionic conductivity reduction typically observed in conventional high-concentration electrolytes. Owing to these advisable structural designs, the hydrogel electrolyte achieved a remarkable ionic conductivity of 46.19 mS cm-1, and the Zn–Zn symmetric cell using hydrogel electrolyte presented outstanding cycling stability over 2,000 h at a current density of 1 mA cm-2. Besides, the Zn-Cu cell using the hydrogel electrolyte also exhibited a high CE value of 99.1%.

2. Materials and Methods

2.1. Materials

AM (99%), [2-(Methacryloyloxy)ethyl] trimethylammonium chloride (DMC, 75 wt% in H2O), 2-acryl-amido-2-methyl-propanesulfonate (AMPS, 98%), Methylene-bis-AM (BIS, 99%), ammonium persulfate ((NH4)2S2O8, 99.5%), Zinc Chloride (ZnCl2, 99%), and tetramethyl ethylenediamine (TEMED, 99%) were purchased from McLean Industrial Corporation. Zinc foils (thickness 100 μm) and Cu foils (thickness 100 μm) were purchased from Kelude New Energy Co., Ltd.

2.2. Synthesis of hydrogel electrolyte

The hydrogel electrolyte was synthesized using one-step solution polymerization. First, AM (0.25 g), DMC (0.2 g), and AMPS (0.05 g) were dissolved in water with ultrasound for 10 min. Then, ZnCl2 (4-10M) was dissolved into the above solution with ultrasonic treatment until it completely dissolved. Subsequently, the initiator (NH4)2S2O8 (0.006 g), crosslinker Methylene-bis-Acrylamide (BIS) (0.001 g), and accelerator tetramethyl ethylenediamine (TEMED) (20 µL) were added to the solution. Afterwards, the solution was transferred to the corresponding mold and the reaction was polymerized at 40°C for 8 h to achieve the desired thickness. The hydrogel electrolyte prepared using the aforementioned methods was named as P(AM-co-AMPS-co-DMC) hydrogel. The thickness of the hydrogel electrolyte was about 100 μm. To characterize the structure of freeze-dried hydrogel, the hydrogel without zinc chloride was synthesized by the same method.

2.3. Physicochemical characterization

The P(AM-co-AMPS-co-DMC) hydrogel was freeze-dried to examine its chemical structure and morphology. The chemical structure of the synthesized hydrogel electrolyte was examined by Fourier transform infrared (FTIR). The FTIR was recorded using a Thermo Fisher Nicolet Is10 spectrometer with the wavenumber ranging from 400 to 4000 cm-1. The morphology of the synthesized hydrogel electrolyte was characterized by a scanning electron microscope (SEM) with a Zeiss EVO MA 15 microscope. The hydrogel sample was sputter-coated with gold before imaging to determine pore size distribution.

2.4. Conductivity measurements

The conductivity of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte was evaluated by impedance tests through an electrochemical workstation (CHI600E). The impedance tests utilized a 5 mV voltage perturbation across a frequency sweep range of 1 Hz‒100 kHz. To facilitate the test, the hydrogel electrolyte was assembled into a blocking battery. As a comparison, the aqueous electrolyte of 2M ZnCl2 was also assembled into a blocking battery. The conductivity of the blocking batteries was determined according to the Eq. (1):

(1)
σ = H R d × S

where H means the thickness of the ionic conductor, S means the area of the ionic conductor, and R d of the electrolyte is the resistance.

2.5. Transference number measurement

The Zn2+ transference numbers (tZn) were measured using the Bruce-Vincent method. DC polarization measurements were conducted with a potential of 10 mV in the Zn||Zn cells until the current reached a steady state, and corresponding electrochemical impedance spectroscopy (EIS) measurements were collected before and after the DC polarization. The tZn was calculated according to the Eq. (2):

(2)
t Z n   = I S ( Δ V − R 0 I 0 ) I 0 ( Δ V − R s I s )

where ΔV is the applied potential, I0 is the initial current, R0 is the initial resistance, Is is the steady-state current, and Rs is the steady-state resistance.

2.6. Cycle stability test

The cycling stability of cells was measured by a Neware battery cycler (CT-4008T-5V10mA-164). The hydrogel with different concentrations of ZnCl2 was assembled into Zn-Zn symmetric cells. As a comparison, ZnCl2 aqueous solution was also used to assemble a liquid Zn-Zn symmetric cell using a glass fiber as the separator. In the galvanostatic electroplating/stripping process, each electroplating or stripping process lasted for 1 h.

Furthermore, the hydrogel electrolyte with the optimal concentration of ZnCl2 was assembled into a Zn-Cu cell. As a comparison, ZnCl2 aqueous solution was also used to assemble a liquid Zn-Cu cell using a glass fiber as the separator. The CE of Zn-Cu cell was systematically evaluated through galvanostatic cycling with 1 h plating/stripping intervals.

3. Results and Discussion

The specific synthetic protocol of the hydrogel electrolyte was delineated in Figure 1. Initially, an amount of AM, AMPS, and DMC was dissolved in the deionized water. Subsequently, ZnCl2, (NH4)2S2O8, BIS, and TEMED were incorporated into the above solution. Finally, the as-prepared solution was transferred to a mold undergoing in situ solution polymerization to yield the P(AM-co-AMPS-co-DMC) hydrogel electrolyte. In the resultant hydrogel electrolyte system, the 3D network architecture was synergistically stabilized through the combined effects of physical crosslinking (hydrogen bonding and ionic bonding) and chemical crosslinking (covalent bonding), ensuring structural integrity and enhanced ionic transport capabilities. The amino groups in AM monomers established a high-density hydrogen-bonding network. A polyzwitterionic network formed by 2-acryl-amido-2-methyl-propanesulfonate (AMPS) and [2-(Methacryloyloxy)ethyl] trimethylammonium chloride (DMC) monomers generated dynamic ionic crosslinking. The introduction of the BIS constructs a stable covalent backbone. This hierarchical bonding strategy could endow the hydrogel electrolyte with synergistically reinforcing structural stability. Furthermore, AM and water molecules could also form hydrogen bond interactions to reduce the activity of water molecules. Concurrently, the polyzwitterionic network established dual-ion migration channels, which could regulate zinc-ion transport pathways. This dual synergistic strategy could suppress Zn dendritic growth and enhance cycling stability. In addition, the synergistic effect between the polyzwitterionic network and high-concentration saline was achieved during a single gel electrolyte system. The highly polar ionic functional groups on the polyzwitterionic network facilitated salt dissociation while simultaneously serving as hopping sites for zinc ion migration. This unique ion transport strategy effectively circumvents the significant conductivity reduction typically observed in conventional high-concentration electrolytes. Consequently, the fabricated P(AM-co-AMPS-co-DMC) hydrogel electrolyte demonstrated exceptional ionic conductivity.

Illustration of the synthesis process for the P(AM-co-AMPS-co-DMC) hydrogel electrolyte.
Figure 1.
Illustration of the synthesis process for the P(AM-co-AMPS-co-DMC) hydrogel electrolyte.

The chemical structure of the P(AM-co-AMPS-co-DMC) hydrogel was investigated by FTIR. As shown in Figure 2(a), the peaks of the hydrogel electrolyte at 3276.26 cm−1 and 601.56 cm−1 were attributed to amino groups (–NH2) in PAM chain segments [1]. Concurrently, the peaks at 1735.80 cm−1 and 1659.90 cm−1 were displayed in the hydrogel electrolyte, which were consistent with the ester bond (O–C=O) and tertiary amide bond (C–N), respectively [22]. This confirmed that the P[2-(Methacryloyloxy)ethyl] trimethylammonium chloride (PDMC) chain segments exist in the P(AM-co-AMPS-co-DMC) hydrogel. Moreover, the presence of P2-acryl-amido-2-methyl-propanesulfonate (PAMPS) polymer chains was verified by the absorption bands at 1259.71 cm−1 and 1065.52 cm−1, which assigned to the asymmetric and symmetric stretching vibrations of sulfonic acid group (-SO3), respectively [23]. In addition, the peak of at 1453.57 cm−1 was designated as methyl group probably [22]. This confirmed the successful incorporation of PDMC and PAMPS chain segments into the cross-linked PAM network.

(a) FTIR spectra of the P(AM-co-AMPS-co-DMC) hydrogel. (b) SEM image of the P(AM-co-AMPS-co-DMC) hydrogel. (c) Pore size distribution of the P(AM-co-AMPS-co-DMC) hydrogel. (d) Stress-strain curve of the P(AM-co-AMPS-co-DMC) hydrogel.
Figure 2.
(a) FTIR spectra of the P(AM-co-AMPS-co-DMC) hydrogel. (b) SEM image of the P(AM-co-AMPS-co-DMC) hydrogel. (c) Pore size distribution of the P(AM-co-AMPS-co-DMC) hydrogel. (d) Stress-strain curve of the P(AM-co-AMPS-co-DMC) hydrogel.

SEM imaging was used to investigate the internal morphology and porosity of the P(AM-co-AMPS-co-DMC) hydrogel. As shown in Figures 2(b) and (c), the macroporous architecture of the P(AM-co-AMPS-co-DMC) hydrogel demonstrated an average pore diameter of 100∼200 µm, which was beneficial for the rapid ion migration. The porous architecture of the P(AM-co-AMPS-co-DMC) hydrogel inherited a rough surface morphology, presumably stemming from hydrogen-bonding interactions between polymeric chains and water molecules [24]. It is particularly noteworthy that the large pore structure incorporated hierarchical microporous substructures, with the smaller pores measuring less than 50 μm in diameter. The formation of these densely packed hierarchical microporous substructures was predominantly attributed to the development of dual-ion migration channels within the polyzwitterionic network [25]. The mechanical properties of the as-prepared P(AM-co-AMPS-co-DMC) hydrogel were evaluated by the typical strain–stress curve, as illustrated in Figure 2(d). It reveals that the P(AM-co-AMPS-co-DMC) hydrogel has a high tensile strength of 55.2 kPa and a large fracture elongation of 451%, which confirms that the P(AM-co-AMPS-co-DMC) hydrogel has excellent mechanical flexibility and broad application prospects in flexible energy storage.

The ionic conductivity of hydrogel electrolytes serves as a critical quantitative parameter dictating battery electrochemical performance, where the ion flux during redox processes fundamentally governs the specific energy density [26]. To systematically evaluate the ionic transport performance, comprehensive zinc-ion conductivity measurements were implemented through symmetrical blocking cell configurations. Electrochemical impedance spectroscopy was systematically employed through the electrochemical workstation to elucidate the salt concentration evolution of ionic conductivity. As shown in Figure 3(a), ZnCl2 aqueous solutions were configured into symmetrical blocking cells for electrochemical impedance spectroscopy characterization. Its test results revealed an inherent resistance of approximately 2 Ω for the ZnCl2 aqueous electrolyte system. Electrochemical impedance spectroscopy of hydrogel electrolytes with different salt concentrations was shown in Figure 3(b). The inherent resistance exhibited a non-monotonic concentration dependence, initially decreasing, then increasing with elevated salt concentration.

(a) EIS of the ZnCl2 aqueous electrolyte. (b) EIS of the P(AM-co-AMPS-co-DMC) hydrogel electrolytes with different salt concentrations. (c) Ionic conductivities of the ZnCl2 aqueous electrolyte and the hydrogel electrolyte. (d) EIS spectra and the current variation with the polarization of Zn/ ZnCl2 aqueous electrolyte /Zn symmetric cell. (e) EIS spectra and the current variation with the polarization of Zn/ hydrogel electrolytes /Zn symmetric cell.
Figure 3.
(a) EIS of the ZnCl2 aqueous electrolyte. (b) EIS of the P(AM-co-AMPS-co-DMC) hydrogel electrolytes with different salt concentrations. (c) Ionic conductivities of the ZnCl2 aqueous electrolyte and the hydrogel electrolyte. (d) EIS spectra and the current variation with the polarization of Zn/ ZnCl2 aqueous electrolyte /Zn symmetric cell. (e) EIS spectra and the current variation with the polarization of Zn/ hydrogel electrolytes /Zn symmetric cell.

The ionic conductivity of ZnCl2 aqueous electrolyte and the P(AM-co-AMPS-co-DMC) hydrogel electrolyte could be calculated by taking the value of inherent resistance into the corresponding formula. As shown in Figure 3(c), the ionic conductivity of ZnCl2 aqueous electrolyte was measured at 10.88 mS cm-1, while the hydrogel electrolyte with 4M ZnCl2 exhibited an 82% enhancement in ionic conductivity (19.80 mS cm-1). Further increasing ZnCl2 to 6M yielded a 108% improvement over the baseline (22.61 mS cm-1). Notably, at the optimized 8M ZnCl₂ concentration, the P(AM-co-AMPS-co-DMC) hydrogel electrolyte achieved a remarkable ionic conductivity of 46.19 mS cm-1, representing a 325% surge versus the aqueous control. The exceptional ionic conductivity primarily originated from the synergistic effect in the hydrogel electrolyte. The polyzwitterionic network could form dual-ion migration channels, serving as coordinated hopping sites for Zn2⁺ migration, thereby achieving outstanding ionic conductivity [27]. To better understand the Zn ion transport, we also measured the transference numbers of both a glass fiber separator with ZnCl2 aqueous solution and the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with 8M ZnCl₂ (Figures 3d and e). The calculated tZn2+ of the hydrogel electrolyte was 0.954, which was much higher than that of the liquid electrolyte (0.317).

To evaluate the advantages of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte, the electrochemical stabilities were inspected via the plating/stripping tests of Zn–Zn symmetric cells. As shown in Figure 4(a), the Zn–Zn symmetric cell using ZnCl2 aqueous electrolyte demonstrated progressive voltage polarization during cycling, culminating in complete failure after only 53 h of continuous operation at a current density of 1 mA cm-2 with a capacity of 1 mA h cm-2, indicative of severe interfacial instability. Impressively, the Zn–Zn symmetric cell using the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with 4M ZnCl2 delivered a lifespan of around 500 h with a stable voltage plateau (Figure 4b). Moreover, the stable cycling time of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with 6M ZnCl2 further increased to 900 h (Figure 4c). Notably, at the optimized 8M ZnCl₂ concentration, the Zn–Zn symmetric cell of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte presented outstanding cycling stability over 2,000 h, demonstrating 38-fold longevity enhancement compared to conventional ZnCl2 aqueous systems (Figure 4d). The enhanced cycling stability of hydrogel electrolytes arises from a dual synergistic strategy. Primarily, the hydrogen bond between AM and water reduces direct contact of Zn-metal anode and electrolytes, effectively mitigating electrochemical side reactions and consequent dendrite formation [28]. Additionally, the polyzwitterionic network established a dual-ion migration channel that could restrict ion diffusion of two-dimensional and thereby suppress dendritic proliferation through interfacial charge homogenization [29]. Furthermore, the hydrogel electrolyte-based symmetric cell with 10M ZnCl2 exhibited moderate cycling stability degradation (Figure 4e), likely attributable to compromised interfacial charge homogenization that induced heterogeneous zinc deposition morphology [30]. Remarkably, at high current density (5 mA cm⁻2) and capacity (5 mAh cm⁻2), the P(AM-co-AMPS-co-DMC) hydrogel electrolyte enabled exceptional cycling stability (280 h), which was much higher than ZnCl2 aqueous systems (Figure 4f).

The cycling performance of Zn-Zn symmetric cells under varied electrolyte conditions: (a) ZnCl₂ aqueous electrolyte at 1 mA cm⁻2; (b-e) P(AM-co-AMPS-co-DMC) hydrogel electrolytes with incrementally increased ZnCl2 concentrations (4M, 6M, 8M, 10M) at 1 mA cm⁻2; (f) High-current (5 mA cm⁻2) comparison between aqueous and hydrogel (optimized concentration) electrolytes.
Figure 4.
The cycling performance of Zn-Zn symmetric cells under varied electrolyte conditions: (a) ZnCl₂ aqueous electrolyte at 1 mA cm⁻2; (b-e) P(AM-co-AMPS-co-DMC) hydrogel electrolytes with incrementally increased ZnCl2 concentrations (4M, 6M, 8M, 10M) at 1 mA cm⁻2; (f) High-current (5 mA cm⁻2) comparison between aqueous and hydrogel (optimized concentration) electrolytes.

SEM images were conducted to explore the surficial structure of Zn-metal anode after cycling at high current density (5 mA cm-2) and capacity (5 mAh cm-2). As shown in Figure 5(a), a quantity of zinc dendrites was deposited on the surface of the Zn-metal anode after the cell cycled for 50 h with ZnCl2 aqueous electrolyte. In contrast to the severe dendrites observed with liquid electrolytes, the hydrogel electrolyte flow system produced a smooth, uniform Zn-metal anode surface morphology after 50 h at 5 mA cm-2 (Figure 5b), which indicated that the P(AM-co-AMPS-co-DMC) hydrogel electrolyte could effectively inhibit the formation of dendritic structures, thereby enhancing the cycle stability of the hydrogel electrolyte [31].

SEM images of the Zn anode surface from Zn–Zn symmetric cell cycled for 50 h with (a) the ZnCl2 aqueous electrolyte and (b) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at high current density and capacity.
Figure 5.
SEM images of the Zn anode surface from Zn–Zn symmetric cell cycled for 50 h with (a) the ZnCl2 aqueous electrolyte and (b) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at high current density and capacity.

The cycle stability of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte was further investigated in Zn–Cu cells under controlled conditions (1 mA cm-2, 1 mAh cm-2, 0.8 V cutoff). As shown in Figure 6(a), the Zn–Cu cell using ZnCl2 aqueous electrolyte delivered a lifespan of around 20 h with an unstable voltage platform. Notably, the Zn–Cu cell of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte presented excellent cycling stability over 700 h, which was about 35 times that of Zn–Cu cell using ZnCl2 aqueous electrolyte (Figure 6b). Moreover, the Zn-Cu cell with ZnCl2 aqueous electrolyte exhibited a disordered and irregular charge/discharge curve (Figure 6c). The observed cycling instability could be principally attributed to the progressive formation of dendritic structures in the Zn-metal anode during electrochemical cycling, which induces localized current density heterogeneity [32]. These zinc dendritic structures exhibited the propensity to penetrate the separator and cause micro short circuits, resulting in abrupt voltage drops or stochastic fluctuations [15]. In contrast, the charge/discharge curves of the P(AM-co-AMPS-co-DMC) hydrogel electrolyte in Zn-Cu cell presented excellent repeatability even after cycling 300 times (Figure 6d). The coulomb efficiency (CE) of Zn–Cu cells containing ZnCl2 aqueous electrolyte and hydrogel electrolyte was evaluated to prove the electroreversibility. As shown in Figure 6(e), the Zn–Cu cell using ZnCl2 aqueous electrolyte suffered from erratic CE (76.8% average, 35 cycles) due to uncontrolled dendrite growth, while the cell with hydrogel electrolyte achieved a record 99.1% CE retention through 370 cycles. All these results demonstrated that the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with high concentration ZnCl2 could achieve uniform plating/stripping process and suppress dendrite growth, thereby improving the interface stability of Zn-metal anode [33]. Harnessing the benefits of P(AM-co-AMPS-co-DMC) hydrogel electrolyte in ZIBs, it underwent a comprehensive comparison with previously hydrogel electrolyte regarding their synthesis complexity, current density, cycle life, CE, and ionic conductivity As it can be seen from Table 1, the P(AM-co-AMPS-co-DMC) hydrogel electrolyte outperformed currently reported hydrogel electrolyte across various performance metrics, underscoring its potential application in flexible energy storage devices.

Zn–Cu cell of (a) ZnCl2 aqueous electrolyte and (b) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with 8 M ZnCl2 cycled at a current density of 1 mA cm-2. The charge/discharge curves of the Zn-Cu cell using (c) ZnCl2 aqueous electrolyte and (d) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at a current density of 1 mA cm-2. (e) The CE of the Zn–Cu cell with ZnCl2 aqueous electrolyte and the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at a current density of 1 mA cm-2.
Figure 6.
Zn–Cu cell of (a) ZnCl2 aqueous electrolyte and (b) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte with 8 M ZnCl2 cycled at a current density of 1 mA cm-2. The charge/discharge curves of the Zn-Cu cell using (c) ZnCl2 aqueous electrolyte and (d) the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at a current density of 1 mA cm-2. (e) The CE of the Zn–Cu cell with ZnCl2 aqueous electrolyte and the P(AM-co-AMPS-co-DMC) hydrogel electrolyte at a current density of 1 mA cm-2.
Table 1. Comparison of synthesis complexity, current density, cycle life, CE, and ionic conductivity of various hydrogel electrolytes.
Materials Synthesis complexity Current density (mA·cm-2, mAh·cm-2) Cycle life (h) Coulombic efficiency (%) Ionic conductivity (mS cm-1) Ref
zwitterionic hydrogel Simple 1, 0.2 4000 97 24.32 [21]
PAM/DMSO hydrogel Complex 1,1 1400 99.1 19.2 [34]
dual-network hydrogel Simple 0.5,0.5 2000 20 32.91 [7]
PVA/PA hydrogel Complex 0.2,0.2 1200 / 19 [35]
SIP-CS hydrogel Simple 2.2 1200 / 23.20 [36]
dual-crosslinked hydrogel Complex 1,1 500 99.1 6.7 [37]
P(AM-co-AMPS-co-DMC) hydrogel Simple 1,1 2000 99.1 46.19 This work

4. Conclusions

In summary, the P(AM-co-AMPS-co-DMC) hydrogel electrolyte was synthesized by one-step solution polymerization. FTIR spectra and SEM micrographs were used to analyses the chemical structure and morphology of the hydrogel electrolyte. The synergistic effect between the polyzwitterionic network and high-concentration saline endowed the hydrogel electrolyte with an exceptional ionic conductivity of 46.19 mS cm-1. The structural design of P(AM-co-AMPS-co-DMC) chain segments with dual-ion migration channels could improve the interface stability. Based on this design, the hydrogel-based Zn–Zn symmetric cell achieved a superior 2000 h cycle life at 1 mA cm-2, outperforming aqueous systems by 38× in longevity. The Zn-Cu cell using the hydrogel electrolyte delivered an excellent cycle performance over 370 cycles with a CE value of 99.1%. This work offers a groundbreaking roadmap for developing electrolytes with industrial-grade durability and high ion transport efficiency.

Acknowledgment

This work was supported by the Natural Science Foundation of Sichuan Provincial (No.2024NSFSC1035) and the Chengdu Technological University Doctoral Foundation (No.2023RC003).

CRediT authorship contribution statement

Meiling Guo: Methodology, Funding acquisition, Writing-original draft. Tao Ren: Investigation, Data curation, Validation. Sujuan Gao: Validation, Writing-Reviewing and Editing. Shuchun Hu: Conceptualization, Supervision. Fengchun Zhang: Investigation, Validation. Yanqi Feng: Investigation, Methodology. Jinde Yu: Formal analysis. Yin Liu: Writing-Reviewing and Editing. Zhe Wang: Visualization. Shishan Xue: Methodology.

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.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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