Ca²⁺/Zn²⁺ alginate hydrogel electrolyte for high-performance zinc–ion batteries

Abstract

The growing energy crisis has intensified the focus on green energy, sparking widespread interest in aqueous zinc-ion batteries. However, their development has been hindered by issues in the zinc anode. Here, Ca²⁺/Zn²⁺ alginate hydrogel electrolyte was designed to effectively suppress dendritic growth and parasitic side reactions. The Ca²⁺ primary cross-linking provides a regular “egg-box” network framework for fast ion transport, whereas secondary cross-linking with Zn²⁺ creates a denser, interpenetrating network with calcium, thereby enhancing the hydrogel's mechanical strength. Furthermore, the abundant –OH and –COO⁻ groups on the alginate chains formed hydrogen bonds with H₂O, which reduced water activity. Meanwhile, the abundant –OH and –COOH groups on the alginate chains formed hydrogen bonds/coordination with H₂O/Zn²⁺, reducing the activity of H₂O and strengthening the ion confinement effect. Therefore, the Zn/SCZ/Zn symmetric cell achieved stable cycling for over 900 hours at 2 mA cm⁻² and 2 mAh cm⁻², while the Zn/SCZ/MnO₂ battery retained 62.03% of its capacity after 700 cycles. This Ca²⁺/Zn²⁺ dual-ion crosslinking strategy for the alginate hydrogel electrolyte offers a novel approach to address the limitations of conventional aqueous electrolytes.

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Introduction

Aqueous zinc-ion batteries (AZIBs) have attracted extensive attention due to their inherent safety and low cost. Zinc metal is particularly attractive for use as an anode in AZIBs owing to its high theoretical specific capacity (gravimetric: 820 mAh g⁻¹, volumetric: 5855 mAh cm⁻³), abundant natural reserves of Zn metal, and low electrochemical potential (−0.76 V vs. SHE).^1–3 However, critical challenges such as electrolyte leakage, rampant Zn dendrite growth, hydrogen evolution reaction (HER) triggered by active water molecules, and poor low-temperature tolerance of aqueous electrolytes severely hinder the commercialization of AZIBs.⁴ Hydrogel electrolytes represent a unique class of quasi-solid soft materials, distinct from both conventional liquid and solid electrolytes. Composed primarily of a 3D crosslinked polymer network and trapped aqueous phase, hydrogel electrolytes exhibit shape retention while enabling solute diffusion and ion transport through the porous matrix.⁵ These characteristics make them a promising solution to the key challenges facing AZIBs.^6–8 As demonstrated by Wang et al., a flexible antifreeze hydrogel electrolyte was synthesized using guar gum/sodium alginate/ethylene glycol (GG/SA/EG), which enabled assembled batteries to maintain high specific capacity and exceptional cycling performance even below −20 °C.⁹ Chen et al. developed a sorbitol-modified wheat straw cellulose hydrogel electrolyte, achieving highly reversible zinc stripping/plating across a broad temperature range (20 to −40 °C).¹⁰

Sodium alginate (SA) is a natural polysaccharide composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G) units linked by β-1,4-glycosidic bonds. Known for its excellent stability, solubility, and biocompatibility, SA can rapidly form hydrogels through simple ionotropic gelation at room temperature, offering low-cost, eco-friendly, and scalable processing advantages.^11,12 SA-based hydrogel electrolytes represent a promising platform for AZIBs. The abundant –COO⁻ and –OH functional groups on SA polymer chains endow it with intrinsic hydrophilicity and zincophilicity, making it an ideal candidate for stabilizing Zn anodes. The uniformly distributed –COO⁻ groups not only facilitate optimized Zn²⁺ transport but also mitigate water-induced side reactions through hydrogen bond regulation, demonstrating significant potential for high-performance AZIBs.^13–15 The mechanical strength of hydrogels is one of the key factors in suppressing Zn dendrite growth. Higher hydrogel strength can mechanically inhibit Zn dendrite formation. However, Zn²⁺-crosslinked SA hydrogels exhibit poor mechanical properties due to the monodentate coordination between SA chains and Zn²⁺—where each Zn²⁺ ion interacts with only one oxygen atom of the carboxyl group, leading to hydrogel formation. This binding mode results in a loose and fragile structure in the zinc alginate gel (SZn), with inferior mechanical strength, rendering it ineffective in suppressing dendrite growth on Zn anodes. SZn hydrogels are prone to rupture due to dendrite and byproduct accumulation, causing battery short-circuit failure. Additionally, SZn hydrogels suffer from low ionic conductivity. Therefore, enhancing the overall performance of SZn hydrogels is essential.^16–18 The calcium alginate hydrogel (SCa) is similar to SZn in that during hydrogel crosslinking, Ca²⁺ interacts with carboxyl groups in the G-blocks to form an ‘egg-box’ structure. However, not all Ca²⁺ ions act as crosslinkers—some adsorb onto SA chains as CaCl₂-associated complexes. Compared to SZn, SCa exhibits a larger cellular network structure, enabling greater liquid electrolyte uptake and higher water retention capacity, thereby achieving enhanced ionic conductivity and improved electrochemical performance. Furthermore, the stronger interaction between Ca²⁺ and carboxyl groups in G-blocks endows the gel network with superior structural stability.^17,19,20

Inspired by abovementioned approaches, a Ca²⁺/Zn²⁺ dual-ion crosslinking strategy was developed to prepare the Ca²⁺/Zn²⁺ alginate hydrogel (SCZ). Ca²⁺ crosslinking enhances the gel network's stability, improving both mechanical and electrochemical properties, while the Zn²⁺ secondary crosslinking supplies Zn²⁺ ions and improves gel flexibility. The optimized SCZ hydrogel exhibits high ionic conductivity (18.01 mS cm⁻¹) and excellent mechanical strength (tensile strength: 0.73 MPa). When used in Zn‖Zn symmetric cells, it enables an ultralong cycling lifespan exceeding 1000 h. Moreover, full cells assembled with SCZ retain 62.03% capacity after 700 cycles.

Results and discussion

Fig. 1a illustrates the preparation of the SCZ hydrogel electrolyte (details in the SI). Initially, primary crosslinking with Ca²⁺ was performed to form a regular “egg-box” network framework, followed by secondary crosslinking and ion exchange using a 2 mol L⁻¹ ZnSO₄ solution to obtain the SCZ hydrogel electrolyte. The secondary crosslinking with Zn²⁺ improved the crosslinking density of the SCZ hydrogel, thereby enhancing the mechanical strength of the hydrogel (Fig. 1b).²¹ As shown in Fig. 1c and d, the SCZ hydrogel features a larger pore structure and higher porosity (67.78%, S4 in the SI) than the SZn hydrogel, promoting higher water content (64.01%, S4 in the SI) and enhanced ionic conductivity, which is beneficial to the rate capability.²²Fig. 1e demonstrates the torsional and tensile tests of SCZ, highlighting its exceptional flexibility. Fig. 1f and g present the conductivity tests of the SCZ hydrogel, where it successfully functioned as a wire to illuminate a red LED, showcasing its outstanding electrical conductivity.

Fig. 1 (a) Diagram illustrating the preparation process of sodium alginate and (b) schematic representation of dual-ion crosslinking. The cross-sectional SEM images of (c) SZn and (d) SCZ. (e) Torsion experiment of the SCZ hydrogel. (f) and (g) conductivity evaluation of the SCZ hydrogel.

Fig. 2a shows the FTIR spectra of SA powder, SZn and SCZ hydrogels, indicating that both SA and the resulting hydrogels contain abundant hydroxyl and carboxyl groups (evidenced by the broad absorption band at ∼3400 cm⁻¹ and the sharp peak at ∼1600 cm⁻¹). Compared to pure SA, both types of crosslinked hydrogels exhibit varying degrees of blue shift at ∼1600 cm⁻¹. This spectral change confirms successful coordination between metal ions (Ca²⁺/Zn²⁺) and carboxyl groups on the polymer chains.^23,24 Furthermore, to validate the practicality of SCZ, the ionic conductivity and tensile strength of SZn and SCZ hydrogels were measured. Analysis of the Nyquist plots and stress–strain curves (Fig. 2c and Fig. S1) confirms that the SCZ hydrogel possesses superior ionic conductivity, tensile strength, and fracture toughness compared to SZn (Fig. 2b and Fig. S2), demonstrating that the modification successfully enhances both mechanical strength and ionic conductivity. The poor mechanical strength and low ionic conductivity of the SZn hydrogel may be attributed to microstructural inhomogeneity, which is caused by the excessively rapid kinetics.^17,20,25 Here, the secondary cross-linking strategy was adapted to form a uniform and robust scaffold. Therefore, the SCZ hydrogel electrolyte exhibits excellent integrated mechanical and electrochemical properties. These results demonstrate that dual-crosslinking can moderately improve the performance of SA-based hydrogels.

Fig. 2 (a) FTIR spectra of SA powder, SZn, and SCZ. (b) Ionic conductivity and (c) tensile strength of SZn and SCZ. (d) Electrochemical stability windows (ESW) and (e) Tafel polarization curves of ZnSO₄, SZn, and SCZ electrolytes at 25 °C. (f) CV curves of Zn‖Cu asymmetric cells with ZnSO₄, SZn, and SCZ electrolytes.

Furthermore, to evaluate the feasibility of SZn and SCZ hydrogel electrolytes for zinc-ion battery applications and characterize their electrochemical properties, we measured the electrochemical stability window (ESW) of both hydrogels and a 2 M ZnSO₄ aqueous solution using Zn‖SS cells (SS: stainless steel electrode). The ESW test is a standard method for assessing the electrochemical stability of electrolytes.²⁶ As shown in Fig. 2d, the ESW values for ZnSO₄, SZn, and SCZ electrolytes are 2.07 V, 2.03 V, and 2.19 V, respectively. The aforementioned data indicate that secondary cross-linking contributes to the expansion of the electrochemical stability window of the SCZ hydrogel electrolyte. This enhancement may be attributed to the following: (1) the improved structural stability from secondary crosslinking; and (2) the abundant hydrophilic groups on SA chains that form hydrogen bonds with water molecules, thereby reducing water activity and suppressing the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which postpones water decomposition.²⁷ Understanding the corrosion resistance of Zn foil in different electrolytes is crucial for long-term battery cycling. Therefore, we conducted Tafel polarization tests for the three electrolytes (Fig. 2e). From the Tafel curve fitting, the corrosion potential (E_corr) and corrosion current (I_corr) can be derived. A more positive E_corr and a lower I_corr indicate reduced corrosion tendency and slower corrosion rates.²⁸ The corrosion potential and corrosion current for the three electrolytes ZnSO₄, SZn, and SCZ were measured as E_corr = −0.8565 V, −0.8513 V, and −0.8525 V, and I_corr = 11.12 µA, 7.43 µA, and 6.85 µA, respectively. The results demonstrate that hydrogel electrolytes significantly reduce both the corrosion potential and current of Zn foil, indicating a weakened corrosion tendency. This confirms that the SCZ hydrogel electrolyte can effectively suppress Zn foil corrosion, thereby protecting the Zn anode.

As shown in Fig. S4, we characterized the deposition behavior of Zn²⁺ in three different electrolytes. The results indicate that at a fixed overpotential of −150 mV, the Zn symmetric cell using the SCZ electrolyte entered stable 3D diffusion within approximately 210 s.²⁹ Stable 3D diffusion effectively suppresses dendrite formation. In contrast, the symmetric cell with the SZn electrolyte required over 420 s to reach stability.³⁰ This stands in sharp contrast to the ZnSO₄ electrolyte, where the current continuously increased over time, exhibiting typical 2D diffusion. Continuous 2D diffusion leads to severe Zn dendrite growth, ultimately impairing the battery's cycling performance.³¹ As shown in Fig. 2f, we further measured the cyclic voltammetry (CV) curves of Zn‖Cu asymmetric cells with different electrolytes to investigate Zn plating/stripping behavior. The Zn nucleation overpotentials in ZnSO₄, SZn, and SCZ electrolytes were determined to be 62, 47, and 45 mV, respectively. These results demonstrate that both hydrogel electrolytes enable lower Zn nucleation overpotentials compared to liquid electrolyte, with SCZ showing the smallest overpotential; the regular and uniform pore structure of SCZ facilitates faster Zn²⁺ transport; the abundant zincophilic functional groups (–OH and –COO⁻) on SA chains modify the Zn²⁺ solvation structure by reducing coordinated water molecules in the solvation sheath, which decreases the desolvation energy barrier and accelerates the desolvation process. These combined effects promote favorable Zn deposition kinetics, leading to more uniform Zn plating and effective dendrite suppression.³²

The influence of the electrolytes on Zn anode cycling stability was assessed by investigating the Zn plating/stripping behavior in Zn‖Zn symmetric and Zn‖Cu asymmetric cells employing different electrolytes. As shown in Fig. 3a, under 1 mA cm⁻² current and 1 mAh cm⁻² capacity, symmetric cells using SZn and ZnSO₄ electrolytes displayed abnormal voltage polarization at 55 h and short-circuit failure at 81 h, respectively. This failure is likely attributed to Zn dendrite penetration, which damaged the glass fiber separator.¹⁵ The abnormal polarization in batteries using SZn electrolyte is attributed to its low ionic conductivity and poor mechanical properties from its heterogeneous microstructure.³³ Based on both enhanced mechanical properties and fast ionic conductivity of the SCZ hydrogel electrolyte, the Zn‖Zn symmetric cell employing the SCZ exhibits significantly lower voltage polarization compared to its SZn counterpart, maintaining stable operation for over 1000 h. The Zn‖Zn cell with SCZ electrolyte demonstrates the lowest charge transfer resistance (Fig. S3), further confirming the fast ionic conductivity of the SCZ hydrogel electrolyte. Compared to batteries using glass fiber separators, the abundant –OH and –COO⁻ groups on SA chains in SCZ hydrogel electrolytes simultaneously suppress water-induced side reactions and optimize Zn²⁺ deposition kinetics. As a result, symmetric cells equipped with SCZ electrolytes demonstrate exceptional cycling stability.³⁴ Furthermore, symmetric cells employing the SCZ electrolyte demonstrated stable operation for over 900 h at high current density (2 mA cm⁻² current and 2 mAh cm⁻² capacity), while cells with SZn and aqueous electrolytes exhibited abnormal polarization and shortened cycle life, further highlighting the enhanced mechanical strength and superior ionic conductivity from SCZ (Fig. 3b). As shown in Fig. 3c, the Zn‖Zn symmetric cell with SCZ also displayed outstanding rate capability, maintaining stable performance at 1, 2, 3, 5, and 10 mA cm⁻² with polarization voltages consistently below 200 mV. In contrast, cells with the SZn electrolyte showed the aforementioned voltage polarization issues, and those with the ZnSO₄ electrolyte became unstable at 3 mA cm⁻², with voltage curves rising sharply. These results conclusively demonstrate the superior performance of the SCZ hydrogel electrolyte. Compared with the relevant reports in the past two years, symmetrical batteries using the SCZ electrolyte also show comparable cycle stability (Table S1). A similar trend was observed in asymmetric cells: Zn‖Cu cells with SCZ achieved the highest average coulombic efficiency (Fig. 3d).

Fig. 3 Electrochemical performance of Zn‖Zn symmetric cells with different electrolytes at 25 °C: (a) cycling performance at 1 mA cm⁻², 1 mAh cm⁻², (b) cycling performance at 2 mA cm⁻², 2 mAh cm⁻², (c) rate capability under varying current densities. (d) Coulombic efficiency of Zn‖Cu asymmetric cells at 1 mA cm⁻², 1 mAh cm⁻².

Fig. 4a–c show SEM images of Zn anodes cycled 20 times at 1 mA cm⁻² current and 1 mAh cm⁻² capacity using three different electrolytes. The Zn anode with the SCZ electrolyte exhibits a finer, denser, and more uniform surface morphology, indicating excellent deposition homogeneity and structural stability. In contrast, the Zn anode with the ZnSO₄ electrolyte shows poor surface morphology, where significant dendrite growth driven by tip effects can easily puncture the separator, leading to battery short-circuit failure. Moreover, its loose zinc deposits are prone to detaching and forming “dead zinc”, considerably reducing the battery's cycle life.^35,36 The Zn anode cycled in the SZn electrolyte presents a macroscopically flat but structurally porous surface, likely resulting from sluggish deposition kinetics associated with limited ion transport. A limited number of dendritic structures were also identified on the surface.

Fig. 4 (a)–(c) SEM images of zinc anodes after 20 cycles at a plating state in Zn‖Zn symmetric cells using different electrolytes, tested at 1 mA cm⁻² and 1 mAh cm⁻². (d) XRD patterns of the pristine Zn foil and the cycled Zn anodes after 20 cycles at a plating state in Zn‖Zn symmetric cells with different electrolytes.

To verify the side-reaction suppression effect of SCZ electrolyte and determine the preferred Zn deposition orientation, X-ray diffraction (XRD) was employed to analyze the deposited phases after 20 stripping/plating cycles with three different electrolytes (Fig. 4d). The XRD patterns revealed that both hydrogel electrolytes and ZnSO₄ electrolyte exhibited higher relative intensity for the (101) crystal plane, which may be attributed to the initial crystal structure of the Zn foil. However, by comparing the relative peak intensities of the (002) and (101) planes (denoted as I_(002)/(101)), it was found that the symmetric cell with SCZ electrolyte showed I_(002)/(101) = 0.14; cells with SZn and ZnSO₄ electrolytes exhibited ratios of 0.08 and 0.09, respectively. These results demonstrate that the SCZ hydrogel electrolyte can promote preferential Zn deposition on the (002) plane, thereby exhibiting dendrite suppression capability. This beneficial effect likely originates from its high ionic conductivity, which ensures rapid Zn²⁺ diffusion.^15,37 Meanwhile, XRD analysis revealed that the abundant –OH and –COO⁻ groups in both hydrogel electrolytes effectively resist byproduct formation. In contrast, the Zn anode cycled in ZnSO₄ electrolyte – lacking such protective functional groups – generated insulating byproducts, compromising electrochemical performance.

To further evaluate the practical applicability of SCZ electrolyte, Zn–MnO₂ full cells were assembled using three electrolytes (ZnSO₄, SZn, and SCZ) for comparative analysis. Fig. 5a presents the cyclic voltammetry (CV) curves of Zn–MnO₂ full cells at a scan rate of 0.5 mV s⁻¹. The full cell assembled with SCZ electrolyte demonstrates significantly higher current density, which can be attributed to (1) the abundant polar functional groups in SCZ that effectively modify the electrode/electrolyte interfacial properties, and (2) the optimized Zn²⁺ insertion kinetics, collectively contributing to enhanced full-cell performance. Therefore, we measured the electrochemical impedance spectra (EIS) of the three full cells. The results show that the modified SCZ exhibits lower diffusion and charge transfer resistances, demonstrating its ability to optimize ion transport and enhance charge transfer at the anode/electrolyte interface, thereby confirming its superior performance (Fig. 5b). Due to its exceptional performance, the SCZ-assembled full cell demonstrates remarkable rate capability and high reversibility. In contrast, Zn–MnO₂ cells with ZnSO₄ and SZn electrolytes suffer from poor reversibility and high-current-induced failure, respectively (Fig. 5c). In self-discharge tests, SCZ demonstrated outstanding performance. The full cell assembled with SCZ maintained a high coulombic efficiency of 93.52% after 48 h of rest, surpassing that of both ZnSO₄ and SZn systems, which confirms its exceptional ability to suppress parasitic side reactions (Fig. 5d–f). Consequently, SCZ also exhibited superior long-cycle stability, exhibiting 62.03% capacity retention after 700 cycles at 0.2 A g⁻¹, whereas cells with ZnSO₄ and SZn electrolytes degraded to 50% retention within 200 cycles (Fig. 5g). In summary, the modified SCZ hydrogel electrolyte effectively protects the Zn anode, optimizes Zn²⁺ transport, and enhances interfacial charge transfer, collectively leading to significantly improved battery cycling performance.

Fig. 5 Electrochemical performance of Zn‖MnO₂ full cells with three different electrolytes at 25 °C: (a) CV curves at a scan rate of 0.5 mV s⁻¹, (b) electrochemical impedance spectra (EIS), (c) rate capability under varying current densities, (d)–(f) self-discharge evaluation tests, (g) long-term cycling performance at 0.2 A g⁻¹.

Conclusions

In summary, the Ca²⁺/Zn²⁺ alginate hydrogel (SCZ) electrolyte was developed, presenting an effective solution to address both dendrite growth and hydrogen evolution reactions for aqueous zinc-ion batteries. By combining the regular 3D network framework formed by Ca²⁺ with the enhanced cross-linking density provided by Zn²⁺, the SCZ hydrogel achieves a synergistic improvement in both mechanical properties and ionic conductivity. Furthermore, the abundant –OH and –COO⁻ groups within the SCZ hydrogel regulate the hydrogen-bond network and suppress H₂O activity, enabling a highly reversible Zn stripping/plating process. The SCZ-assembled symmetric cell achieves stable operation for over 1000 h, while the full cell maintains 62.03% capacity retention after 700 cycles, demonstrating exceptional cycling stability. With its simple fabrication process and outstanding performance, SCZ provides a novel strategy for developing high-performance zinc-ion batteries.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

All the data supporting this article have been included in the main text and the supplementary information (SI). Supplementary information: materials; preparation of hydrogel electrolytes; characterization; water content and porosity; electrochemical impedance spectra (EIS); strength and toughness; EIS of Zn‖Zn symmetric cells; performance comparison. See DOI: https://doi.org/10.1039/d5sm00906e.

Acknowledgements

This work is supported by Shandong Provincial Natural Science Foundation (No. ZR2022ME181) and the National Natural Science Foundation of China (No. 51702123). Shuhua Yang is grateful for the start-up research funding from University of Jinan.

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