Manipulating crystallographic growth orientation by cation‐enhanced gel‐polymer electrolytes toward reversible low‐temperature zinc‐ion batteries

Abstract

Aqueous zinc‐ion batteries (AZIBs) have garnered significant research interest as promising next‐generation energy storage technologies owing to their affordability and high level of safety. However, their restricted ionic conductivity at subzero temperatures, along with dendrite formation and subsequent side reactions, unavoidably hinder the implementation of grid‐scale applications. In this study, a novel bimetallic cation‐enhanced gel polymer electrolyte (Ni/Zn‐GPE) was engineered to address these issues. The Ni/Zn‐GPE effectively disrupted the hydrogen‐bonding network of water, resulting in a significant reduction in the freezing point of the electrolyte. Consequently, the designed electrolyte demonstrates an impressive ionic conductivity of 28.70 mS cm⁻¹ at −20°C. In addition, Ni²⁺ creates an electrostatic shielding interphase on the Zn surface, which confines the sequential Zn²⁺ nucleation and deposition to the Zn (002) crystal plane. Moreover, the intrinsically high activation energy of the Zn (002) crystal plane generated a dense and dendrite‐free plating/stripping morphology and resisted side reactions. Consequently, symmetrical batteries can achieve over 2700 hours of reversible cycling at 5 mA cm⁻², while the Zn || V₂O₅ battery retains 85.3% capacity after 1000 cycles at −20°C. This study provides novel insights for the development and design of reversible low‐temperature zinc‐ion batteries.

INTRODUCTION

In recent years, the advancement of renewable energy generation technologies, such as wind, solar, and tidal power, has reduced dependence on fossil fuels. However, owing to the intermittency and uneven distribution of renewable energy resources, the absorption of large-scale electricity production and cross-regional transportation of electricity remain unresolved challenges. Therefore, the development of affordable and dependable energy storage systems is essential to address the current challenges. Lithium-ion and sodium-ion batteries are highly favored because of their ultra-high energy density and exceptional electrochemical cycling behavior.¹ However, the use of combustible and deleterious organic electrolytes raises concerns about their suitability for large-scale energy storage device systems.² Recently, aqueous zinc-ion batteries (AZIBs) have emerged as the top contenders for large-scale energy storage owing to their intrinsic safety, affordability, and eco-friendly nature.^3–8 However, the advancement of AZIBs is mainly hindered by problems such as low reversible capacity and finite cycle stability, making it difficult to promote large-scale use. Currently, three main challenges hinder the application of AZIBs. First, the nonuniform distribution of the electrode/electrolyte interface electric field micro-environment and the chaotic arrangement of the substrate crystal plane orientation can easily lead to the tip effect, which can result in aggressive dendrite growth capable of breaching separators and causing hazardous short circuits.^9–11 Second, highly active metallic Zn is directly exposed to aqueous electrolytes, leading to detrimental side reactions including zinc corrosion and the hydrogen evolution reaction (HER), causing battery polarization and hindering ion transport.^12–14 Furthermore, the commercial viability of AZIBs is severely impacted by harsh conditions, particularly low temperatures. The polarization of AZIBs is exacerbated by cold temperatures, and the aqueous electrolyte may freeze at subzero temperatures.^15,16

Several effective strategies have been proposed to tackle these issues, such as the construction of a solid electrolyte interphase,^17,18 electrolyte optimizations,^19,20 and the utilization of hydrogel electrolytes instead of liquid electrolytes (LEs).²¹ Hydrogel electrolytes combine the advantages of aqueous and solid electrolytes. The water-saturated hydrogel matrix effectively dissolves a variety of salt ions, providing essential ionic conductivity for driving electrochemical reactions. The soft and moist interface of the hydrogel electrolyte can closely adhere to the anode, and its natural water-saturated polymer network promotes ion transfer, resulting in high ion conductivity (usually one to two orders of magnitude higher than the conductivity of LIBs).²² Furthermore, the robust mechanical properties of the hydrogel electrolyte make it an ideal option for serving as a separator, effectively preventing short circuits.^23,24 Benefiting from the merits mentioned above, hydrogel electrolytes exhibit extraordinary compatibility with AZIBs. Recently, extensive research has been dedicated to perfecting the design and manufacturing of hydrogel electrolytes. For example, polyacrylamide (PAM), polyacrylic acid (PAA), poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), gelatin, and polyalginate acid (SA) hydrogel electrolytes and their cross-linked polymers have shown potential as desirable gel-polymer electrolytes owing to their high ion conductivity and mechanical strength.^25–30

Conventional hydrogel electrolytes primarily act as physical frameworks for ion transport and electronic insulation. However, they still encounter the following challenges: First, the lack of effective modulation of the interface electric field environment disrupts the oriented migration and deposition of zinc ions, leading to uncontrollable dendritic growth. Therefore, focusing on the electrostatic shielding effects of metal cations can regulate the behavior of zinc deposition through preferred crystallographic orientation modification, thereby fundamentally solving the problem of dendrite growth.^31–34 Meanwhile, the electrostatic interaction between Zn²⁺ and hydrophilic groups, including hydroxyl, carboxylic, and amino groups, can significantly reduce the number of H₂OH₂O hydrogen bonds, which results in fewer water-induced side reactions and helps achieve a lower freezing point.^35–37 Hence, it is essential to thoroughly examine the intricate interplay between metal ions, water molecules, and macromolecule chains and to understand the significance of hydrogel structures in enhancing the overall anti-freezing electrochemical properties.

Inspired by this, a bimetallic cation-enhanced gel polymer electrolyte (Ni/Zn-GPE) was fabricated to achieve highly (002)-textured Zn deposition without specific substrates. The Ni/Zn-GPE was synthesized through the initial synthesis of a PAM and carboxylated chitosan (CS) crosslinked hydrogel and then immersed in a 2 M ZnSO₄ solution with a specific concentration of Ni²⁺. Based on the experimental results and theoretical calculations, the dominance of the Ni/Zn-GPE can be explained by: (1) the adsorption of Ni²⁺ creates an electrostatic shielding interphase, promoting the selective electrodeposition of horizontally aligned platelet shapes with improved (002) orientations. (2) The strong interaction of H₂O-Ni/Zn-GPE weakens the hydrogen bond strength of H₂OH₂O, minimizing the side reactions of electrolyte decomposition and decreasing the freezing point. Consequently, the Ni/Zn-GPE enabled reversible Zn plating/stripping and cycling stability at extremely low temperatures while maintaining high stability. This innovative strategy introduces a method for designing hydrogel electrolytes with exceptional stability and freezing resistance. Additionally, this underscores the critical role of nickel cations in regulating zinc deposition and modifying the preferred crystallographic orientation.

RESULTS AND DISCUSSION

Design, synthesis, and structure characterizations of Ni/Zn-GPEs

The interaction mechanism between the Zn atom and the nickel adsorption layer at the electrode/electrolyte interface is illustrated in Figure 1. As depicted in Figure 1A, the zinc surface of the pure Zn-GPE displays random large protrusions and an agglomerated electric field at the tips. This uneven distribution of Zn²⁺ flux led to excessive dendrite growth on the Zn anode surface. Additionally, highly active water molecules can cause significant hydrogen evolution, leading to parasitic corrosion. In comparison, as shown in Figure 1B, the in situ-formed nickel hydroxide adsorption layer can drastically equalize the surface electric field and minimize the variation in the diffusion energy barriers of Zn adatoms among different facets, which promotes the 3D diffusion of zinc ions and ultimately achieves compact and strong (002)-textured preferred crystallographic orientation modification. In addition, polymers rich in functional groups boast a strong network of H-bonding and electrostatic interactions, which not only diminish the activity of free water and alleviate side reactions but also notably reduce the freezing point.

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The Ni/Zn-GPE was prepared using a simple one-pot method involving thermal polymerization. In this process, PAM is grafted onto the CS molecular chain, followed by crosslinking to produce a three-dimensional crosslinked hydrogel (CSAM). CS is a modified polysaccharide with hydrophilic functional groups that trap water molecules, helping to prevent H₂O-induced corrosion and the HER. The introduction of PAM further enhanced the mechanical properties and ionic conductivities of the GPEs. In the second step of the process, the pre-gel hydrogel was immersed in a solution containing specific kosmotropic ions, which enhanced its mechanical strength through Hoffmeister's “salting out” phenomenon.³⁸ As illustrated in Figure 2A, the metal cation strongly interacts with the polymer chains, allowing it to penetrate the polymer chains and disrupt the H-bond in CSCS and PAMPAM. This ultimately results in the amelioration of the mechanical properties of the hydrogel. Among various kosmotropic anions, we chose zinc sulfate (ZnSO₄) as the electrolyte salt. Further introduction of nickel salts greatly affects the physical properties and electrochemical behavior of the GPEs. To investigate the role of Ni cations, we prepared Ni/Zn GPEs with varying concentrations of Ni²⁺ and labeled them accordingly. For example, a mixture of 0.05 M Ni²⁺ and 2 M Zn²⁺ GPE was labeled 0.05-Ni/Zn GPE. Simultaneously, the homogeneity of the different GPEs was demonstrated by energy-dispersive spectroscopy (EDS) mapping images with homogeneously distributed C, O, N, Zn, and Ni (Figures S1 and S2).

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The chemical structure of the Ni/Zn-GPEs was analyzed using FTIR spectroscopy (Figure 2B and Figure S3). Compared to CS-GPE, which is simply composed of CS, new crosslinking characteristic peaks appeared when the PAM segments were introduced. The characteristic peaks at 1611 and 1453 cm⁻¹ are separately attributed to β(NH) and β(CH). Furthermore, the peaks observed at 1656, 1551, and 1414 cm⁻¹ are separately attributed to ν(CO), ν(CNH), and ν(CN), present in both CS and AM constituents. Additionally, the characteristic peak at 1067 cm⁻¹, provided by ZnSO₄, is also detected in the spectrum. All these results indicate that a cross-linked structure composed of PAM and CS chains was successfully synthesized, and electrolyte salt molecules also successfully entered the cross-linked network.³⁹

Non-covalent interactions and anti-freezing properties of Ni/Zn-GPEs

Non-covalent interactions in the gel network were analyzed using density functional theory (DFT) and FTIR spectroscopy. As depicted in Figure 2C, the binding energies of PAMH₂O and CSH₂O were much higher than that of H₂OH₂O, confirming that the GPE can disconnect the H-bonds between water molecules. Eventually, this can help alleviate the invasion of H₂O-induced corrosion and the HER. Figure 2D shows that the OH stretching vibration can be separated into three characteristic peaks within the range of 3000 to 3800 cm⁻¹, These peaks correspond to strong (3253 cm⁻¹), medium (3416 cm⁻¹), and weak H-bonds (3550 cm⁻¹), respectively.^40,41 After the introduction of PAM and metal cations, the strong H-bond in water clearly decreased while the intensity of the medium H-bond increased, suggesting that the strong H-bond network in water has been significantly disrupted. In addition, the thermogravimetric analysis (TGA) results show minimal weight loss of free water between 30 and 80°C, indicating that water molecules are firmly confined in the hydrogel network (Figure S4).⁴²

Excellent freezing resistance is essential for the application of hydrogels at extremely low temperatures. Satisfactorily, the solid–liquid transition temperature (T_t) of the Ni/Zn-GPE was much lower than that of the pure Zn-GPE (Figure 2E and Figure S5). Moreover, the T_t of Ni/Zn-GPE decreased as the salt concentration increased, which was attributed to the weakening of the strong H-bonding network between the H₂O molecules, which minimized the possibility of forming ordered molecular arrangements. Consequently, the freezing point of the hydrogel drops.²⁶ The Raman spectra were obtained at various temperatures for the 0.05 Ni/Zn-GPE. Figure 2F shows no evident ice peak at approximately 3140 cm⁻¹, indicating excellent anti-freezing performance even at −60°C. A high ionic conductivity at low temperatures is crucial for achieving good battery performance under such conditions. The results shown in Figure 2G, Figure S6, and Table S1 demonstrate that all GPEs exhibit higher ionic conductivities than the LE. Additionally, the conductivity of the LE decreases sharply to 2.45 mS cm⁻¹ at −20°C owing to the electrolyte freezing. In comparison, the pure Zn-GPE and 0.05 Ni/Zn-GPE maintain ionic conductivities as high as 22.37 and 28.70 mS cm⁻¹, indicating good ion transport performance at low temperatures.

As a crucial component of AZIBs, hydrogel electrolytes should be tough and stretchable to prevent premature damage to batteries during cyclic charging/discharging. As shown in Figure S7, after the introduction of kosmotropic Ni²⁺, the tensile strength and elongation of the GPEs were enhanced. These findings can be attributed to the interactions of metal ions with macromolecular chains and H₂O molecules.⁴³

Investigations of zinc deposition and H₂ evolution-suppressed behavior

Understanding the crystallization that occurs at the interface during the electrochemical deposition process is essential to achieving high reversibility in Zn anodes. In specific electrolyte systems, a compact, flat, and pure Zn electrodeposition layer is feasible. Furthermore, the creation of a smoother and brighter surface necessitates the formation of tiny grains to establish a continuous, dense structure. The process of nucleation and growth during Zn electrodeposition can be analyzed using cyclic voltammetry (CV), chronoamperometry (CA), and dimensionless curves. The peaks at −0.2 V/0.14 V in Figure 3A can be assigned to the reversible redox peaks of the Zn plating/stripping processes, and the interval between the A site (crossover point) and the B site (point potential to reduce) represents the nucleation overpotential (NOP). The NOP of Ni/Zn-GPE is higher than that of pure Zn-GPE, which indicates that a larger quantity of nuclei leads to the formation of finely grained and dense zinc deposits.^44,45 The current-time curves for different GPEs were conducted by symmetrical zinc batteries (Figure S8) at −150 mV. The current–time curves were normalized using the 3D multiple nucleation model developed by Scharifker and Hill.⁴⁶ Figure 3B shows that the initial activation of the crystalline nucleus on the anodes in the Ni/Zn-GPE occurs in just 24.8 s, which is much shorter than that in the pure Zn-GPE. Moreover, after the growth process of nuclei, the anodes in the Ni/Zn-GPE reached the final steady state of surface energy (t₁) in a shorter time, indicating that the anodes in the Ni/Zn-GPE rapidly shifted to 3D diffusion, resulting in a preferred (002) crystalline orientation. The nucleation and growth of anodes in all GPEs followed the mixed 2D/3D growth mode, and the zinc electrodeposition in the Ni/Zn-GPE was closer to 3D instantaneous nucleation.^47,48 More details can be obtained from Figure S9.

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The processes of nucleation and growth have also been studied under galvanostatic conditions with a current density of 20 mA cm⁻² (Figure 3C). Regarding the anodes in the Ni/Zn-GPE, Zn electrodeposition was initiated with a significant NOP spike, followed by stabilization at a consistent crystal nucleus growth overpotential plateau. However, for anodes in pure Zn-GPE, the nucleation and growth progress can be clearly divided into two stages: (1) In the initial phase, the concentration of zinc ions near the electrode surface continuously decreases, and ion diffusion is controlled by Fick's second law. Thus, the overpotential increases continuously after nucleation is completed until the Sand's time is reached (t_sand = 4.35 s, calculation process shown in the Supporting Information). This indicates the duration necessary for the ion concentration near the electroplating electrode to decrease to zero at an adequately high current density. At this stage, owing to the mass-transfer control effect, Zn dendrites did not form. (2) After reaching Sand's time, the occurrence of electroconvection significantly enhances mass transport and reduces overpotentials. Consequently, the electrodeposits at the top have a higher current density, leading to self-accelerated growth and the formation of dendrites.^49–51

Scanning electron microscopy (SEM) was employed to investigate the morphological evolution that occurred during electrodeposition on the Cu substrates. It was found that, owing to the tip-clustered electric field, the Zn electrodeposits manifested a mossy and porous structure in pure Zn-GPE, which made the surface rough (Figure 3D and Figure S10A). In comparison, the Zn electrodeposits in the Ni/Zn-GPE manifested a compact morphology with a hexagonal grain morphology, growing conformally into platelets with a highly exposed (002) surface. The platelets were arranged parallel to the substrate, creating a shiny surface, and were firmly attached to the Cu foil (Figure 3E and Figure S10B–D). The TEM image (inset of Figure 3E) of the Zn electrode after cycling in the Ni/Zn-GPE further demonstrates the dominance of the (002) crystal planes in the Zn deposits. The x-ray diffraction (XRD) patterns of the Zn electrodeposits were used to explore the relationship between the crystallographic orientation and Ni²⁺ concentration (Figure 3F and Figure S11). The XRD results showed that the undesired diffraction peak, which corresponds to the by-products of Zn₄(OH)₆SO₄·3H₂O on the Zn surface, was significantly reduced by the addition of nickel salt. Moreover, the optimal concentration of Ni²⁺ in the GPE was determined to be 0.05 M (0.05 Ni/Zn-GPE). This concentration has a maximum regulation of the peak intensity proportion of 0.44 for I₍₀₀₂₎/I₍₁₀₁₎, highlighting the optimal crystal orientation for Zn (002) plane growth.^52,53

The corrosion resistance of electrodes is a crucial factor in assessing the electrochemical properties of batteries. Owing to its high hydrogen evolution potential, H₂ accumulates on the anode surface, causing serious battery inflation issues. The reaction between the electrode and water can cause a buildup of OH⁻ concentration at the electrode/electrolyte interface, leading to Zn corrosion and the production of by-products.^54,55 To verify the corrosion and HER processes, hydrogen evolution polarization and linear scan voltammetry (LSV) measurements were conducted with different GPEs. According to Figure 3G, the addition of Ni²⁺ to GPE results in a positive shift of the corrosion potential from −0.981 to −0.978 V. Additionally, the corrosion current density was reduced from 3.72 to 2.64 mA cm⁻². These findings indicate that the addition of Ni²⁺ inhibits the corrosion tendency and rate of the Zn anode.^56–58 The protective effect of Ni/Zn-GPE was also supported by LSV measurements, as displayed in Figure 3H and Figure S12. At the beginning of the HER, a negative shift in potential from −0.12 to −0.18 V (vs. Zn²⁺/Zn) was observed, and the upper bound of the potential window increased from 2.86 to 3.02 V, demonstrating that the water splitting overpotential increased with the introduction of Ni²⁺.⁵⁹

Zn plating/stripping reversibility under low-temperature conditions

Encouraged by these benefits, the reversibility of the Zn plating and stripping was evaluated. Notably, the Zn|0.05 Ni/Zn-GPE|Zn cell exhibited a steady voltage profile with minimal voltage hysteresis and an impressive lifespan exceeding 3300 h (Figure S13). Meanwhile, the overpotential of Zn|pure Zn-GPE|Zn rapidly increased from the 20th hour until an internal short circuit occurred at approximately 520 h. Additionally, the stability of reversibility in low-temperature environments at −20°C was also tested. Figure 4A illustrates that the Zn|0.05 Ni/Zn-GPE|Zn cell consistently sustained a stable potential hysteresis, exhibiting no fluctuating potential response at elevated current densities. As depicted in Figure 4B, even with a current density of 10 mA cm⁻² and a charge capacity of 5 mAh cm⁻², the Zn|0.05 Ni/Zn-GPE|Zn cell exhibits exceptional cycling stability for over 870 h, with a low potential hysteresis of approximately 100 mV. As depicted in Figure 4C, the Zn|0.05 Ni/Zn-GPE|Zn symmetrical cell remains stable for over 2700 h at 5 mA cm⁻²/1 mAh cm⁻². However, the Zn|pure Zn-GPE|Zn cell was only able to cycle for less than 600 h before voltage fluctuations were observed owing to severe polarization. The Zn|0.05 Ni/Zn-GPE|Zn cells remained stable over the potential, whereas the Zn|pure Zn-GPE|Zn cells exhibited severe voltage fluctuations during a continuous loop. These results surpass those of most currently reported gel electrolytes at high current densities (Figure 4D and Table S2). The reversibility was also evaluated using cycle measurements in asymmetrical cells. The Zn|0.05 Ni/Zn-GPE|Cu cell demonstrated an average CE of 99.4% over 7500 cycles (Figure 4E). In contrast, the CEs of the Zn|pure Zn-GPE|Cu and Zn|LE|Cu cells significantly decreased after 3000 cycles and showed substantial oscillation after 200 cycles. These issues are attributed to dendrite formation and parasitic reactions.

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Mechanism for optimizing crystal orientation modification

Understanding the intricate relationship between the electrode and electrolyte is vital for optimizing cell performance. This research focused on studying the formation mechanism of planar Zn deposition and the adsorption behavior of Ni²⁺ to gain valuable insights into enhancing battery efficiency. The chemical composition and properties of the electric double layer (EDL) will affect the desolvation and deposition of zinc ions (Figure 5A). Before the desolvation process, the solvated Zn ions and H₂O molecules preferentially entered the HP at the Zn anode surface, leading to an extremely inhomogeneous Zn²⁺ flux distribution. Ni²⁺ can freely move within the polymer network and tends to preferentially adsorb onto Zn, which can significantly reduce the desolvation-activating energy and promote the deposition of solvated zinc ions (Figure S14). Furthermore, direct-current polarization and EIS were used to accurately determine the migration rate of Zn²⁺ (Figure S15). The results indicated that at room temperature, the Zn²⁺ transference number ($t_{{\mathrm{Zn}}^{2+}}$), which was calculated to be 0.465 in the 0.05 Ni/Zn-GPE, was higher than that for the pure Zn-GPE ($t_{{\mathrm{Zn}}^{2+}}$=0.138), verifying that the weakened Zn²⁺ solvation ensured fast Zn²⁺ migration. The 0.05 Ni/Zn-GPE also exhibited a high Zn²⁺ transference number of 0.463 at −20°C. High Zn²⁺ transference numbers at low temperatures can alleviate concentration polarization, thereby promoting the electrochemical process with a competitive reaction dynamic.⁶⁰

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DFT simulations were performed to test this hypothesis. As depicted in Figure 5B, the adsorption energy of the Ni adatom on the corresponding Zn (002) facet (−2.93 kJ mol⁻¹) and Zn (101) facet (−3.99 kJ mol⁻¹) is significantly higher than that of the Zn adatom, indicating the predominant adsorption of the Ni adatom on the Zn anode. Therefore, before the zinc deposition process, nickel cations preferentially form an adsorption layer on the zinc surface. To validate the chemical composition of the adsorption layer on the Zn surface, XPS was performed to characterize the Zn deposits after cycling in pure Zn-GPE and 0.05 Ni/Zn-GPE. As shown in Figure 5C, no distinct signal corresponding to Ni was detected in the pure Zn-GPE, whereas a noticeable nickel signal was observed in the GPE containing the nickel salt. In addition, the characteristic nickel peaks corresponding to Ni(OH)₂ located at 855.7 and 873.3 eV can be observed in the Ni 2p spectra (Figure 5D), suggesting that the nickel element mainly exists in the form of nickel hydroxide owing to the alkaline environment on the electrode surface.^61–63 Moreover, SEM and EDS analyses were conducted on the cross-section of the deposited Zn anode, and nickel elements were found to be uniformly dispersed in the zinc deposits, indicating that the nickel hydroxide adsorption layer was uniformly distributed on the zinc surface and continued to provide protection during the deposition process (Figure S16).

The specific adsorption behavior of nickel hydroxide was validated using impedance methods based on the non-Faradaic EDL capacitance adsorption process (Figure 5E). According to previous reports, owing to the positive value of the zeta potential of the Ni(OH)₂ suspension,⁶⁴ the adsorption of nickel cations will result in a positive shift in the potential of the capacitance minimum.^65–69 As the concentration of nickel salt in GPE increased from 0 to 0.025 to 0.05 M, the potential of the capacitance minimum positively shifted from −0.04 to −0.01 to 0.05 V (vs. Zn²⁺/Zn), indicating the adsorption of nickel cations. The further decrease in capacitance with the addition of nickel salt can be attributed to the strong adsorption of nickel cations on the Zn electrode surface.

Theoretical calculations (Figure 5F,G) suggest that a Zn adatom exhibits a minimum diffusion energy barrier in the Zn (002) facet (0.004 eV) compared with the Zn (101) face. This can be attributed to the smoothness of the close-packed plane. Moreover, crystal nuclei featuring exposed high-index planes demonstrated a higher vertical growth rate than lateral growth rate, leading to uneven growth and the formation of a dendritic deposition morphology. By comparison, the diffusion energy barrier of the Zn adatom on the Ni(OH)₂-001 facet and Ni(OH)₂-101 facet are 0.05 and 0.12 eV, respectively, which are much lower than that of the Zn-101 facet. Moreover, the difference in the diffusion energy barriers of Zn adatoms between different facets of nickel hydroxide was much lower than that of different Zn facets, validating that the migration of Zn adatoms on the Ni(OH)₂ facet was significantly faster than that on the Zn (101) facet. In summary, these results indicate that the uneven growth of differently oriented nuclei can be effectively suppressed.^49,70–73

Electrochemical performances of AZIBs

To demonstrate the potential of the Ni/Zn-GPE for future applications, Zn|pure Zn-GPE|V₂O₅ and Zn|0.05 Ni/Zn-GPE|V₂O₅ full cells were fabricated using V₂O₅·nH₂O as the cathode. The CV test results (Figure S17) illustrate that the cells employing different GPEs displayed two analogous sets of redox peaks assigned to the embedding of H⁺ and Zn²⁺. The cell with the Ni/Zn-GPE exhibited higher peak currents owing to its ameliorative electrochemical kinetics. SEM images revealed that the cycled Zn|0.05 Ni/Zn-GPE|V₂O₅ anode maintained a smooth and horizontal morphology after repeated plating/stripping processes. The hexagonal platelet-shaped grains exhibited distinct microsteps (Figure 6A,B). In contrast, the cycled Zn|pure Zn-GPE|V₂O₅ anode exhibited a porous structure and vertically grown random Zn flakes (Figure 6C,D).

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In addition, the implementation of the Ni/Zn-GPE improved the rate capability of the full cell, as evidenced by the higher capacities at various current densities (Figure 6E). The Zn|0.05 Ni/Zn-GPE|V₂O₅ cell showed remarkably prolonged cycling stability, sustaining 75.6% of its capacity even after 1000 cycles, whereas a significant capacity decay was detected in the Zn|pure Zn-GPE|V₂O₅ cell after only 150 cycles (Figure 6F and Figure S18). Furthermore, their capacity retention at low temperatures was also tested. Even at a frigid temperature of −20°C, 0.05 Ni/Zn-GPE demonstrated a reversible capacity retention above 95% across 500 cycles. Meanwhile, the pure Zn-GPE only retained 20% of its capacity. Subsequently, the Zn|pure Zn-GPE|V₂O₅ cell underwent a short circuit. Resting measurements were performed to detect the attenuation of the open-circuit voltage and self-discharge characteristics. After a rest period of 24 h, the 0.05 Ni/Zn-GPE demonstrated a lower self-discharge tendency with a higher discharge capacity retention of 93.1%, whereas the LE and pure Zn-GPE retained only 66.7% and 81.0%, respectively, owing to side reactions (Figure 6G–I).

CONCLUSION

We successfully engineered a bimetallic cation-enhanced gel polymer electrolyte (Ni/Zn-GPE) to address dendrite formation, side reactions, and low-temperature ionic conductivity issues in AZIBs. Molecular dynamics simulations and spectroscopic analysis clarified that the weakened hydrogen bond strength of H₂OH₂O in the Ni/Zn-GPE enabled the anti-freezing feature of the hydrogel electrolyte. Furthermore, the Ni²⁺ introduced into the Ni/Zn-GPE was transferred in situ into lamellar Ni(OH)₂, which can serve as an electrostatic shielding layer to confine the oriented nucleation and growth of Zn along the (002) crystal plane. The combined advantages of the Ni/Zn-GPE provide a dense and dendrite-free plating/stripping morphology and resist side reactions in zinc-ion batteries. In addition, the ionic conductivity of the Ni/Zn-GPE at subzero temperatures shows good potential. The symmetrical batteries can achieve reversible cycling time for over 2700 h at 5 mA cm⁻², while the Zn || V₂O₅ battery exhibits remarkable long-term cycling stability, sustaining 75.6% of its capacity even after 1000 cycles at a low temperature of −20°C. Overall, this study provides valuable insights that will help facilitate the realization of low-temperature AZIBs through the structural engineering of gel polymers.

EXPERIMENTAL SECTION

The solution was prepared by dissolving 2.000 g of CMCS in 20 mL of deionized water. Next, 0.016 g of ammonium persulfate was added, followed by 4.000 g of AM, and finally, 0.020 g (0.5 wt% of AM) of MBAA. The mixture was vigorously stirred to obtain a perfectly homogeneous precursor solution and then heated at 45°C for 60 min to form a CSAM pre-gel. The CSAM pre-gel was poured into a circular (diameter = 6 cm, depth = 3 mm) PTFE mold and underwent 3 h of thermal polymerization at 60°C. Finally, different GPEs were obtained by soaking the samples in a 2 M ZnSO₄ solution with NiSO₄ at concentrations of 0, 0.01, 0.025, 0.040, 0.050, 0.075, 0.100, and 0.150 M for 24 h.

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China, Grant/Award Number: 22075290; Natural Science Foundation of Shandong Province, Grant/Award Number: ZR2023QB183, ZR2022QB078, ZR2021MB020, and ZR2021MB029.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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