Ultra-Stable Aqueous Zinc-Ion Batteries Enabled by Trace Ionic Liquid–Polar Solvent Synergistic Induction of Vertically Oriented (101) Facet Epitaxial Growth

Abstract

Aqueous zinc-ion batteries (AZIBs) are promising for grid-scale storage due to their safety, low cost, and environmental benignity. However, water-dipole enrichment in the inner Helmholtz plane (IHP) of Zn anodes triggers hydrogen evolution, corrosion, and dendrites, limiting cycle life. We report a trace “ionic liquid–polar solvent coupling” strategy: adding only 0.01 M EMIMBF₄ and 0.03 M DMSO to 2 M ZnSO₄ electrolyte. Hydrophobic EMIM⁺ adsorbs on the IHP to expel interfacial water, while BF₄⁻ enters the primary solvation shell and DMSO penetrates both first and second shells of Zn²⁺, forming a water-deficient coordination environment. This interfacial–solvation synergy suppresses parasitic reactions and directs preferentially oriented Zn deposition exclusively along the (101) facet, enabling dense vertical plating and in situ formation of a compact, inorganic-rich SEI (ZnCO₃–ZnSO₃–Zn(OH)₂). Consequently, Zn||Zn cells cycle stably for >5362 h at 1 mA cm⁻²/1 mAh cm⁻²; Zn||Cu cells achieve 1300 cycles with 99.8% average Coulombic efficiency; and Zn||V₂O₅ full cells retain 326.4 mAh g⁻¹ after 500 cycles. This work shows that minimal additive loading can simultaneously engineer the electrode–electrolyte interface and crystallographic deposition pathway, offering a simple yet robust design for ultra-stable AZIBs.

1. Introduction

The excessive consumption of fossil fuels has driven tremendous demand for renewable energy sources—such as solar, wind, and hydropower—that are both environmentally sustainable and economically viable to meet the extensive demands of today’s energy market [1,2,3]. Electrochemical energy storage (EES) has emerged as a promising solution to mitigate the intermittency and variability inherent in these renewable sources, attracting significant research interest [4]. Aqueous zinc-ion batteries (AZIBs) have emerged as strong candidates for large-scale EES due to their low redox potential (−0.76 V vs. SHE), high theoretical specific capacity (819 mAh g⁻¹), and abundant Zn metal resources [5,6,7]. Moreover, AZIBs typically employ non-flammable aqueous electrolytes instead of organic electrolytes, making them safer metal-based batteries compared to those using other types of electrolytes [8]. However, the Zn anode in current aqueous zinc-ion batteries still faces several challenges, such as dendrite growth, hydrogen evolution reaction (HER), and side reactions. Moreover, these challenges are interrelated, forming a vicious cycle: during Zn²⁺ deposition/stripping, the non-uniform deposition of Zn²⁺ leads to the formation of zinc dendrites; the growth of dendrites increases the exposed surface area of the Zn anode, thereby accelerating HER. When HER occurs in the aqueous electrolyte, the local pH near the Zn anode increases, which induces the formation of by-products [9,10,11]. In addition, the strong interaction between Zn²⁺ and solvated H₂O molecules weakens the O–H bonds in water, facilitating the release of H⁺, which further causes severe hydrogen evolution reaction (HER) during Zn²⁺ deposition.

To date, researchers have made numerous efforts to enhance the application prospects of aqueous zinc-ion batteries, such as surface modification of Zn anodes [12] and the introduction of electrolyte additives [13]. Among the aforementioned strategies, the introduction of electrolyte additives has attracted extensive research interest due to their simple processing, precise dosage control, and cost-effectiveness. To date, various additives have been employed to improve the electrochemical performance of aqueous zinc-ion batteries, including inorganic additives, ionic additives, and metallic additives. For instance, ZnCl₂ has been used to construct highly concentrated electrolytes to suppress HER; however, such high-concentration electrolytes may pose potential corrosiveness to battery components. Consequently, organic electrolyte additive systems have gained increasing attention, such as N,N-dimethylformamide (DMF)-based electrolytes [14], carbonate-based electrolytes [15], and dimethyl sulfoxide (DMSO)-based electrolytes. For example, Liu et al. developed a DMF–H₂O hybrid electrolyte in which DMF enters the Zn²⁺ solvation sheath; however, a certain amount of H₂O remains in the solvation sheath, resulting in insufficient suppression of HER [16]. Xu et al. introduced an organic electrolyte additive that enhances the interaction between Zn²⁺ and water molecules while reducing the average coordination number of Zn²⁺, enabling a Zn symmetric cell to achieve a cycling lifespan of 3000 h at 1 mA cm⁻² [17]. Qiu et al. found that adding 50 vol% DMSO together with 0.1 M H₂SO₄ to a 2 M ZnSO₄ electrolyte system widens the operating voltage window to 0.8–1.9 V (vs. Zn²⁺/Zn), suppresses oxygen evolution side reactions at the cathode, and enables ZnMn₂O₄ to deliver a capacity of 207 mAh g⁻¹ at 0.1 A g⁻¹ [18]. Feng et al. reported that introducing 20 vol% DMSO reshapes the (Zn(H₂O)₆)²⁺ solvation sheath into [Zn(H₂O)_m(DMSO)_n]²⁺, effectively suppressing dendrite growth and allowing the Zn||Zn symmetric cell to cycle stably for over 2100 h at 20 °C [19]. Collectively, these studies highlight a fundamental trade-off in current electrolyte engineering strategies: high additive concentrations effectively suppress side reactions but compromise ionic transport, while low concentrations maintain conductivity but fail to completely eliminate water-induced degradation. As a widely studied class of electrolyte additives, organic molecules containing specific functional groups such as –N–, –O–, S=O, and C=O have been demonstrated to form hydrogen bonds with H₂O, thereby modulating the solvation structure of Zn²⁺ [20]. Although polar solvents like dimethyl sulfoxide (DMSO) have been extensively incorporated into aqueous zinc-ion battery electrolytes, current approaches commonly face a dilemma: “high dosage—high viscosity” versus “low dosage—residual water—insufficient suppression of side reactions.” Specifically, when the DMSO volume fraction exceeds 50%, the operating voltage window of the ZnMn₂O₄ cathode can be extended to 0.8–1.9 V, delivering a capacity of 207 mAh g⁻¹; however, this leads to significantly increased electrolyte viscosity. In contrast, when DMSO is limited to below 20 vol%, a large amount of active water remains in the (Zn(H₂O)₆)²⁺ solvation shell, resulting in only partial suppression of hydrogen evolution and dendrite growth, and limiting the symmetric cell cycling lifetime to approximately 2100 h. Although various organic additives have been employed to modulate the solvation structure, most of them struggle to precisely manipulate the crystallographic orientation of Zn deposition.

Moreover, it is worth noting that the crystallographic orientation of zinc directly determines its deposition morphology and cycling stability. Recent work by Liu et al. has demonstrated that, compared to the commonly observed (002) plane, the (101)-oriented zinc exhibits lower lattice mismatch, higher atomic adsorption energy, and a more uniform electric field distribution, which collectively promote vertically aligned, dense epitaxial growth and thereby significantly enhance the cycling lifetime of symmetric cells [21]. Nevertheless, achieving such a preferred (101) orientation while simultaneously constructing a robust solvation shell and shielding the interface usually requires high concentrations or complex formulations. Realizing this dual regulation with a simplified, low-dosage electrolyte system remains a formidable challenge. Therefore, developing a low-dosage, synergistic regulation strategy that simultaneously targets both the solvation structure and the interface chemistry is imperatively desired.

Motivated by the aforementioned challenges and the intrinsic stability of the Zn (101) crystal plane, this work proposes an “ionic liquid–polar solvent coupling” strategy employing trace amounts of 0.01 M EMIMBF₄ and 0.03 M DMSO. Mechanistically, DMSO penetrates both the primary and secondary solvation shells of Zn²⁺, while BF₄⁻ anions occupy the primary shell, collectively reconstructing the solvation environment and significantly attenuating Zn²⁺–H₂O interactions. At the interface, EMIM⁺ cations accumulate within the Helmholtz layer to provide an electrostatic shielding effect that repels interfacial water molecules, while the S=O groups of DMSO form a hydrogen-bonding network with bulk water to effectively suppress the hydrogen evolution reaction (HER). This synergistic modulation of interfacial chemistry and crystallographic orientation orchestrates vertical, dense preferred Zn growth along the Zn(101) plane, thereby eliminating the “tip effect” and inducing a compact SEI rich in ZnCO₃, ZnSO₃, and Zn(OH)₂. Consequently, the Zn||Zn symmetric cells encapsulate an ultralong lifespan of 5362 h, and Zn||Cu cells achieve nearly 1400 cycles with an average Coulombic efficiency of 99.8% (at 1 mA cm⁻², 1 mAh cm⁻²). This study presents a facile, low-dosage, and universally applicable electrolyte engineering protocol for high-performance aqueous zinc-ion batteries.

2. Materials and Methods

Chemicals: Zn foil (thickness: 0.03 mm) was purchased from Dongguan Kelude New Energy Technology Co., Ltd. (Dongguan, China); ZnSO₄·7H₂O was obtained from Chengdu Kelong Chemical Co., Ltd. (Chengdu, China) 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄, >99%) and V₂O₅ cathode powder (>99%) were supplied by Aladdin and Future (Jilin) Materials Technology Co., Ltd. (Jilin, China), respectively. All chemicals were used as received without further purification.

Electrolyte Preparation: A 2 M ZnSO₄ aqueous electrolyte was first prepared by dissolving the requisite amount of ZnSO₄·7H₂O in deionized water. Subsequently, 0.0404 g of EMIMBF₄ was dissolved in 20 mL of the above solution to yield 2 M ZnSO₄ + 0.01 M EMIMBF₄. To the 2 M ZnSO₄ + 0.01 M EMIMBF₄ electrolyte, 0.0469 g of DMSO was added to obtain 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO. The additive-free 2 M ZnSO₄ solution served as the control electrolyte.

A 2 M ZnSO₄ aqueous electrolyte was first prepared by dissolving the requisite amount of ZnSO₄·7H₂O in deionized water. Subsequently, 0.0156 g, 0.0469 g, and 0.0781 g of DMSO were respectively dissolved in 20 mL aliquots of this solution, yielding modified electrolytes with concentrations of 2 M ZnSO₄ + 0.01 M DMSO, 2 M ZnSO₄ + 0.03 M DMSO, and 2 M ZnSO₄ + 0.05 M DMSO.

Material Characterization: Surface morphology and microstructural evolution of the electrodes after cycling were examined using field-emission scanning electron microscopy (SEM, Hitachi SU 8020) (Hitachi High-Tech, Tokyo, Japan). Crystallographic information was obtained by X-ray diffraction (XRD, Bruker D8 Advance) (Bruker, Karlsruhe, Germany)over the 2θ range from 5 to 90° at a scan rate of 2° min⁻¹. Chemical states of the electrode surface were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA). Changes in the hydrogen-bond network and Zn²⁺ solvation structure were probed via Raman spectroscopy (HORIBA Scientific, Longjumeau, France)) and Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet iS20, Waltham, MA, USA). Solvation structures of Zn²⁺ in electrolytes were investigated by liquid-state nuclear magnetic resonance (NMR; Bruker 400 MHz, Bruker, Ettlingen, Germany).

Electrochemical Measurements: All electrochemical tests were conducted in CR2016 coin cells assembled in ambient air using glass microfiber separators (Whatman). Each cell contained 80 µL of electrolyte. Galvanostatic charge–discharge measurements were performed on a Neware battery tester. Zn||Zn symmetric cells were cycled at 1 mA cm⁻² with an areal capacity of 1 mAh cm⁻² to evaluate long-term cycling stability; current densities were systematically varied to assess rate capability. Zn||Cu half-cells were cycled at identical current densities and durations to determine Coulombic efficiency (CE). Zn||V₂O₅ full cells were cycled at 0.5 A g⁻¹ to probe long-term cyclability and capacity retention, and stepwise current densities were applied to investigate rate performance. Deposition overpotentials were recorded during galvanostatic discharge of Zn||Cu cells.

Electrochemical impedance spectroscopy (EIS) was carried out on a CHI 760E electrochemical workstation over 0.01 Hz–100 kHz with an AC amplitude of 5 mV. Linear sweep voltammetry (LSV) was performed using Zn||SS cells by sweeping the potential from −0.3 V to 0 V at 1 mV s⁻¹. Cyclic voltammetry (CV) was conducted on the same workstation: Zn||Cu cells were cycled between −0.2 V and 0.4 V, Zn||V₂O₅ cells between 0.3 V and 1.7 V, and Zn||Zn symmetric cells between −1.2 V and 0.3 V, all at 1 mV s⁻¹. Chronoamperometry (CA) was implemented in a two-electrode configuration with Zn foils as both working and counter electrodes; a −150 mV overpotential was applied for 1000 s to study Zn²⁺ diffusion behavior.

3. Results and Discussion

To investigate the effect of DMSO additive on side reactions at the Zn anode, Tafel curves and linear sweep voltammetry (LSV) curves of the Zn anode in different electrolyte systems were examined. As shown in Figure 1, the corrosion potential of 2 M ZnSO₄ + 0.03 M DMSO is −1.02 V, which is superior to those of the other two compositions. Moreover, 2 M ZnSO₄ + 0.03 M DMSO exhibits a lower onset potential, indicating that it provides the best suppression of the hydrogen evolution reaction (HER). Cyclic voltammetry (CV) reveals that the potential difference for 2 M ZnSO₄ + 0.03 M DMSO is smaller (from 0.416 V to 0.377 V) compared to the other two compositions, suggesting a lower energy barrier and higher reversibility.

From the nucleation overpotentials of the three compositions, it can be seen that the nucleation barrier at 1 mA cm⁻² in the 2 M ZnSO₄ + 0.03 M DMSO electrolyte is 37.3 mV, which is lower than that in 2 M ZnSO₄ + 0.01 M DMSO (40.8 mV) and 2 M ZnSO₄ + 0.05 M DMSO (43.9 mV), demonstrating that the concentration of 0.03 M DMSO is more favorable for uniform Zn²⁺ deposition compared to the other concentrations. At a relatively low DMSO concentration, DMSO can interact with H₂O through the S=O group and partially disrupt the continuous hydrogen-bond network among water molecules. This reduces the activity of free water and facilitates a more stable Zn deposition process. Meanwhile, DMSO can also participate in the local Zn²⁺ solvation/desolvation environment, thereby lowering the Zn nucleation barrier.

However, when the DMSO concentration is further increased, excessive DMSO may increase the viscosity of the electrolyte and weaken Zn²⁺ transport kinetics. It may also lead to stronger Zn²⁺–DMSO interactions, which increase the desolvation difficulty at the electrode surface. As a result, the Zn nucleation process is no longer continuously promoted at higher DMSO concentrations. Therefore, an optimal DMSO concentration exists at which the balance between water-activity suppression and Zn²⁺ transport/desolvation kinetics is achieved.

The chronoamperometry (CA) curves show that all three compositions rapidly enter stable three-dimensional diffusion behavior, indicating that the DMSO additive can quickly form a stable and uniform diffusion boundary layer on the electrode surface. Electrochemical impedance spectroscopy (EIS) further reveals that the solution resistance increases with rising DMSO concentration, which may be attributed to the stronger interaction between DMSO and Zn²⁺ that raises the migration energy barrier.

Figure 2 shows the rate performance of Zn||Zn cells with different DMSO concentrations at current densities ranging from 0.5 mA cm⁻² to 5 mA cm⁻². As can be seen from the figure, 2 M ZnSO₄ + 0.01 M DMSO and 2 M ZnSO₄ + 0.03 M DMSO exhibit similar polarization voltages, both of which are lower than that of 2 M ZnSO₄ + 0.05 M DMSO, indicating that the additive concentrations of 0.01 M and 0.03 M DMSO provide better stability during Zn deposition/stripping.

Combining the above electrochemical characterization results, it is evident that among the three groups with 0.01 M, 0.03 M, and 0.05 M DMSO added to 2 M ZnSO₄, the 0.03 M DMSO concentration offers both the lowest nucleation barrier and effective suppression of hydrogen evolution, and is therefore selected as the DMSO concentration for subsequent experiments.

Once Zn metal comes into contact with an aqueous solution, it spontaneously triggers the hydrogen evolution reaction (HER), thereby exacerbating the self-corrosion of the Zn anode [22]. To demonstrate the protective effect of the EMIMBF₄-DMSO composite additive system, morphological and phase analyses were performed on the Zn anodes of Zn||Zn symmetric cells after 20 cycles in three different electrolyte systems: ZnSO₄ electrolyte, 2 M ZnSO₄ + 0.01 M EMIMBF₄, and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO. Figure 3 shows that in pure ZnSO₄ electrolyte, dense flake-like by-products form on the anode surface, resulting from the inherent surface roughness of metallic Zn and the gradual growth and accumulation of zinc dendrites, indicating severe corrosion of the Zn surface. After 20 cycles in 2 M ZnSO₄ electrolyte with 0.01 M EMIMBF₄, the anode surface retains a relatively flat morphology, with only a small amount of flake-like by-products observed. In contrast, after 20 cycles in the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte system, the deposited zinc is tightly adhered to the substrate, forming a dense and flat deposition layer. Static contact angle measurements were conducted to evaluate the wettability between the Zn electrode and various electrolytes (Figure 4). The contact angle results further show that, compared to the additive-free electrolyte (77°), the Zn surface becomes significantly more hydrophobic in the presence of EMIMBF₄ (89.9°) and EMIMBF₄ + DMSO (98°), indicating the formation of a hydrophobic interfacial layer [23].

To further investigate the composition of the by-products, X-ray diffraction (XRD) tests were conducted on the original pure zinc foil and the Zn anodes cycled in 2M ZnSO₄ electrolyte, 2M ZnSO₄ + 0.01M EMIMBF₄, and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte for 20 cycles. Figure 5a shows that after cycling in a 2M ZnSO₄ electrolyte without the addition of EMIMBF₄, three new sets of diffraction peaks appear at 12.3°, 24.7°, and 32.8° on the recycled zinc foil. By comparison with the Inorganic Crystal Structure Database (ICSD), these by-products are identified as Zn₄SO₄(OH)₆·0.5H₂O (PDF#44-0674). However, when cycled in the electrolyte containing EMIMBF₄, the number of by-product peaks on the zinc foil is significantly reduced, with only one diffraction peak observed at 12.3°. This indicates that the addition of EMIMBF₄ can effectively reduce the formation of by-products during cycling, thereby mitigating the corrosion of the Zn surface and potentially enhancing the stability and efficiency of the battery. This finding highlights the beneficial role of EMIMBF₄ as an additive in electrolytes, which not only improves the morphological stability of the zinc anode but also reduces unwanted side reactions leading to by-product formation. This indicates that EMIMBF₄ possesses a certain degree of anti-corrosion effect. However, in the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO system, almost no diffraction peaks are observed. Furthermore, analysis of the intensity ratios of different crystal planes from XRD (Figure 5b) shows that the ratios I₀₀₂/I₁₀₀ and I₀₀₂/I₁₀₁ for Zn(002) to Zn(101) and Zn(002) to Zn(100), respectively, were examined. In the ZnSO₄ electrolyte, these ratios increased from 1.39 and 0.19 for bare zinc to 3.50 and 0.32, indicating that Zn primarily grows along the (002) plane. With the addition of EMIMBF₄, the I₀₀₂/I₁₀₀ ratio continues to rise, suggesting an increase in the exposure of the (100) plane, leading to its preferential growth [24]. The Zn(100) plane has a higher surface energy compared to the Zn(002) plane [25], and during Zn deposition, rapid growth of the (100) plane without protection can easily trigger the “tip effect,” resulting in the formation of Zn dendrites [26]. Notably, the specific adsorption of EMIM+ on the (100) crystal plane acts as a “terminator,” effectively reducing the interfacial energy of the (100) crystal plane and dynamically suppressing vertical growth rates along that direction; By limiting the deposition rate perpendicular to the (100) plane, this additive promotes lateral diffusion and merger of zinc nuclei, shifting the deposition pattern from tip-driven dendrite growth to planar filling. Protecting the Zn(100) plane and controlling its growth along the (100) direction is an effective way to inhibit Zn dendrites.

Additionally, due to the high surface energy, the Zn(100) surface is prone to severe side reactions, and covering the high-energy surface also helps suppress side reactions. Protecting the Zn(100) plane and controlling its growth along the (100) direction is an effective approach to suppress Zn dendrites. In the EMIMBF₄-DMSO electrolyte, the ratios I₀₀₂/I₁₀₁ and I₀₀₂/I₁₀₀ increased from 0.11 and 1.17 in the 2 M ZnSO₄ + 0.01 M EMIMBF₄ system to 0.17 and 2.4, indicating that the EMIMBF₄-DMSO electrolyte promotes preferential growth of Zn along the (101) plane. Compared to the (002) plane, Zn grown along the (101) plane maintains a stable vertical epitaxial growth pattern with faster mass transfer kinetics, which is conducive to sustained and stable regulation. Moreover, compared to the (100) plane, growth along the (101) plane more effectively releases strain, thereby providing higher stability [27].

To further verify the suppression effect of the EMIMBF₄–DMSO additive system on side reactions, Tafel and linear sweep voltammetry (LSV) tests were performed on metallic Zn in three electrolytes: ZnSO₄, 2 M ZnSO₄ + 0.01 M EMIMBF₄, and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO. As shown in Figure 5c, upon addition of only 0.01 M EMIMBF₄, the corrosion potential of Zn shifts positively from −0.993 V to −0.976 V, and the corrosion current density decreases dramatically from 1.259 mA cm⁻² to 0.027 mA cm⁻², indicating that EMIMBF₄ significantly suppresses corrosion. With the further introduction of 0.03 M DMSO, the corrosion potential shifts further positively to −0.957 V, and the corrosion current density drops to 0.004 mA cm⁻², confirming that the DMSO additive provides even stronger inhibition of corrosion reactions and results in the lowest corrosion tendency among the tested systems. Moreover, linear sweep voltammetry (LSV) tests (Figure 5d) further support the above conclusion: the addition of EMIMBF₄ shifts the onset potential of the hydrogen evolution reaction (HER) to more negative values, indicating that EMIMBF₄ can suppress hydrogen gas evolution and confirming its inhibitory effect on HER. Contact angle tests shown in Figure 4 show that, compared to additive-free electrolytes, the zinc surface becomes significantly hydrophobic in the presence of EMIMBF₄. This result corroborates the significant reduction in corrosion current in the Tafel curve (from 1.259 mA cm⁻² to 0.0267 mA cm⁻²) and the negative shift of the HER starting potential in LSVs. Upon further addition of DMSO, the HER onset potential shifts even further negatively, demonstrating that DMSO can provide additional suppression of HER.

To investigate the regulatory effect of the EMIMBF₄ additive on the anodic electrode interface (AEI) during Zn deposition, Figure 5e presents the cyclic voltammetry (CV) curves of Zn||Cu cells in ZnSO₄, 2 M ZnSO₄ + 0.01 M EMIMBF₄, and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolytes. The CV curves reveal that the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte system exhibits a wider electrochemical window compared to both ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄. Furthermore, the nucleation overpotential of 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO is lower than that of 2 M ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄, indicating that this electrolyte system further promotes nucleation kinetics [28]. In addition, compared with pure ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄, the CV curve of 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO shows a lower peak current, suggesting that Zn²⁺ deposition/stripping proceeds with a higher energy barrier [1]. Figure 5f shows the deposition overpotentials of Zn||Cu cells in ZnSO₄, 2 M ZnSO₄ + 0.01 M EMIMBF₄, and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolytes. In the 2 M ZnSO₄ + 0.01 M EMIMBF₄ electrolyte, the nucleation barrier at 1 mA cm⁻² is 30.6 mV, lower than that in the ZnSO₄ electrolyte (37.3 mV), demonstrating that the addition of EMIMBF₄ increases the number of nucleation sites on the Zn surface, thereby reducing the nucleation barrier and facilitating uniform Zn²⁺ deposition. Moreover, in the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO system, the nucleation barrier at 1 mA cm⁻² is further reduced to only 29.6 mV—lower than that in the EMIMBF₄-only system—further confirming that the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte can increase nucleation sites and lower the energy barrier more effectively, thus significantly promoting uniform Zn²⁺ deposition. In addition, electrochemical impedance spectroscopy (EIS) was employed to investigate interfacial Zn²⁺ transport in Zn||Zn symmetric cells (Figure 6). The addition of EMIMBF₄ reduces the charge transfer resistance, and with the further incorporation of DMSO, the charge transfer resistance is substantially decreased, confirming enhanced reaction kinetics. A large charge transfer resistance impedes the Zn deposition process, which explains the poor cycling performance observed in the ZnSO₄ electrolyte. To further investigate the diffusion effects of different additives, current–time (CA) curves were measured at a constant overpotential of −150 mV. As shown in Figure 7, the CA curves provide insights into the initial nucleation process and surface evolution Clearly, the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte system initially exhibits transient two-dimensional diffusion, which rapidly transitions to nearly stable three-dimensional diffusion behavior. In contrast, the CA curve for the 2 M ZnSO₄ electrolyte shows that the current density of the Zn anode only reaches a stable three-dimensional diffusion after approximately 400 s. This indicates that the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte enhances the planarity, uniformity, and compactness of Zn deposition [29], thereby effectively suppressing dendrite growth.

To comprehensively reflect the regulatory effect of the EMIMBF₄ + DMSO additives on the Zn²⁺ solvation structure, Fourier transform infrared (FTIR) spectroscopy tests were conducted (Figure 8a–c). In the ZnSO₄ solution, the O-H bond stretching peak at 3221.504 cm⁻¹ indicates that SO₄²⁻ forms a stable solvation structure with Zn²⁺. Upon the introduction of EMIMBF₄, EMIM⁺ expels interfacial water molecules to the outer Helmholtz plane and partially replaces water, while BF₄⁻ enters the first solvation shell. This leads to a slight blue shift of the O-H stretching peak to 3224 cm⁻¹ due to the weakening of the hydrogen bonding network. Additionally, BF₄⁻ can act as a hydrogen bond acceptor, interacting with water molecules, reducing the number of hydrogen bonds between water molecules, and thereby lowering the activity of free water in the electrolyte Meanwhile, the asymmetric stretching peak of SO₄²⁻ does not shift, indicating that the direct coordination effect of EMIMBF₄ on SO₄²⁻ is limited. Upon further addition of DMSO, there is a slight blue shift in the O-H stretching peak, indicating that hydrogen bonding occurs between DMSO and H₂O [30]. In the 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO electrolyte, the activity of H₂O is suppressed, and there is a slight blue shift in the stretching vibration peak of SO₄²⁻. This is attributed to the weakening of the electrostatic coupling between Zn²⁺ and SO₄²⁻, suggesting that DMSO participates in the solvation shell of Zn²⁺ [31].

In the Raman spectrum, the O–H stretching vibration of water molecules (3050–3750 cm⁻¹) appears as a broad peak that can be deconvoluted into three components: strongly hydrogen-bonded (HB, ~3230 cm⁻¹), weakly hydrogen-bonded (~3420 cm⁻¹), and non-hydrogen-bonded (~3560 cm⁻¹) species As shown in Figure 8d–f, in 2 M ZnSO₄, the ν(O–H) band is located at 3401.7 cm⁻¹. After the addition of EMIMBF₄, EMIM⁺ displaces interfacial water molecules to the outer Helmholtz plane, while BF₄⁻ replaces part of the water molecules in the first solvation shell. This weakens the hydrogen-bonding network, causing the ν(O–H) peak to blue-shift to 3418.1 cm⁻¹. Meanwhile, the ν(SO₄²⁻) peak remains unchanged at 984.85 cm⁻¹, confirming that EMIMBF₄ has limited direct interaction with SO₄²⁻. Upon further introduction of DMSO (Gutmann donor number DN = 29.8), DMSO penetrates both the first and second solvation shells of Zn²⁺ [32]. As a high-donor-number additive, DMSO preferentially bonds with free water molecules, thereby reconstructing the original water–water hydrogen-bonding network into a DMSO–water hydrogen-bonding network. This disrupts the continuous hydrogen-bonding chains among free water molecules. Moreover, DMSO acts as a strong hydrogen-bond acceptor with high electron-donating ability and preferentially coordinates with Zn²⁺ [33]. Consequently, the ν(O–H) peak further blue-shifts to 3426.3 cm⁻¹, and the ν(SO₄²⁻) peak simultaneously blue-shifts to 986.22 cm⁻¹, indicating a slight increase in the bond order of SO₄²⁻ due to the reconstructed hydrogen-bonding environment.

To comprehensively evaluate the regulatory effect of EMIMBF₄ on the Zn²⁺ solvation structure, NMR spectroscopy was performed. As shown in Figure 9, upon addition of EMIMBF₄, the ¹H NMR peak shifts slightly from 4.73 ppm to 4.72 ppm, indicating that EMIMBF₄ induces the release of several coordinated H₂O molecules from the Zn²⁺ solvation shell and weakens the interaction between Zn²⁺ and H₂O molecules. Upon further addition of DMSO, the ¹H NMR resonance peak shifts upfield to 4.74 ppm. This shift is attributed to DMSO—possessing a high Gutmann donor number (DN = 29.8)-preferentially coordinating into the first solvation shell of Zn²⁺ and displacing some coordinated water molecules. This substitution reduces the electron density around the remaining water protons, resulting in enhanced deshielding and a downfield shift of the chemical signal [34].

After cycling at 1 mA cm⁻² and 1 mAh cm⁻² for 20 cycles, X-ray photoelectron spectroscopy (XPS) tests were conducted on Zn anodes in 2 M ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄ electrolytes to verify the adsorption effect of EMIMBF₄ on the Zn anode (Figure 10, Figure 11 and Figure 12). Quantitative XPS analysis showed a 2.2-fold increase in C-C strength (64,500 vs. 29,186 a.u.), a 1.3-fold increase in C-O strength (20,826 vs. 16,509 a.u.), and a 1.4-fold increase in carbonate strength (17,419 vs. 12,384 a.u.) for the sample containing EMIMBF₄ compared to the control sample containing ZnSO₄ alone. Since EMIMBF₄ is the only organic species in the electrolyte, and its imidazolium cation is rich in C-C bonds, the disproportionate amplification of this C-C signal—especially compared with the moderate increase in C-O and carbonate—strongly indicates that the additional Carbon comes from adsorbed EMIM, rather than uniformly distributed pollutants. From the C 1s spectra, all three samples exhibit high levels of carbon content. This is mainly due to the presence of inorganic carbonate ions from ZnCO₃ and organic components such as C-O (286.2 eV) and C-C (284.8 eV) bonds. These components may originate from partially unreduced products of Zn²⁺-anion (BF₄⁻) complexes [35]. In the 2 M ZnSO₄ + 0.01 M EMIMBF₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO systems, the peak intensity of carbon elements is significantly higher compared to pure ZnSO₄ electrolyte, indicating the formation of a solid electrolyte interphase (SEI) layer rich in ZnCO₃ with a certain thickness Analysis of the S 2p and O 1s spectra confirms the presence of ZnSO₃ and Zn(OH)₂ in the samples. These substances are formed by the decomposition of zinc ion-anion complexes and DMSO. Previous studies have shown that ZnSO₃ can serve as an effective component of the SEI, promoting uniform Zn²⁺ deposition and inhibiting water decomposition reactions Through the above analysis, it can be concluded that the introduction of EMIMBF₄ and DMSO electrolyte additives leads to the formation of a uniform SEI containing abundant ZnSO₃, Zn(OH)₂, and other inorganic species. This stable SEI regulates Zn²⁺ deposition and slows down the further decomposition of DMSO and BF₄⁻, effectively suppressing dendrite growth on the Zn anode, thus ensuring long cycle life for aqueous zinc-ion batteries (AZIBs).

Depth-profiling XPS was further performed to investigate the thickness, compositional gradient, and compactness of the SEI formed on Zn anodes in different electrolytes in Figure 13. For the Zn anode cycled in 2 M ZnSO₄, obvious O- and S-containing species remain detectable after sputtering, indicating the formation of a thick and loose interphase dominated by corrosion by-products and sulfate-derived species. Such an inhomogeneous layer is unable to effectively block water penetration, resulting in continuous HER and corrosion. After introducing DMSO, the intensity of O/S-containing by-products decreases, suggesting that DMSO can partially suppress water-induced side reactions by regulating the hydrogen-bond network and reducing the activity of free water. However, the remaining sulfate/sulfite-related signals after sputtering indicate that the interphase is still not sufficiently compact. In contrast, the Zn anode cycled in the DMSO/EMIMBF₄-containing electrolyte shows a more distinct depth-dependent composition. The outer layer contains additive-derived organic species, while the Zn signal becomes more prominent after sputtering, indicating a relatively thin and compact SEI. Meanwhile, the suppressed O- and S-containing by-product signals suggest that continuous water decomposition and sulfate-related side reactions are effectively inhibited. Therefore, the DMSO/EMIMBF₄ electrolyte enables the formation of a compact SEI with an organic-rich outer layer and a stable inorganic inner layer composed of ZnCO₃, ZnSO₃, and Zn(OH)₂, which contributes to homogeneous Zn²⁺ flux, suppressed HER/corrosion, and highly reversible Zn plating/stripping. The formation of such a compact and compositionally gradient SEI layer is highly favorable for long-lifespan Zn batteries. As highlighted by recent investigations concerning advanced interfacial designs, an ideal SEI should possess both water-blocking capability and high ionic conductivity to physically isolate active Zn from bulk free water while directing uniform Zn²⁺ flux. In our system, the EMIMBF₄/DMSO-induced SEI efficiently fulfills these criteria, thereby synergistically suppressing severe dendritic growth and parasitic reactions.

However, sulfate- or sulfite-related S 2p signals are still observed after sputtering, indicating that the interphase formed with DMSO alone is still not sufficiently compact. Therefore, DMSO can partially reduce side reactions by decreasing free-water activity, but the protective effect remains limited without further interfacial regulation.

Meanwhile, the Zn 2p signal is present, but the persistence of O and S species after sputtering indicates that the surface layer is not a thin and well-defined SEI, but rather a relatively thick, loose, and inhomogeneous accumulation of corrosion products. This type of interphase is generally unable to effectively block water penetration, thus leading to continuous HER, corrosion, and dendritic Zn growth.

To verify the stability of the Zn anode in the 2 M ZnSO₄ + 0.01 M EMIMBF₄ electrolyte system, Zn||Zn symmetric cells were assembled. As clearly shown in Figure 14a, the cell with pure 2 M ZnSO₄ fails within a very short time, whereas the addition of EMIMBF₄ enables the symmetric cell to exhibit a stable Zn plating/stripping process at a current density of 1 mA cm⁻² and an areal capacity of 1 mAh cm⁻², with a lifespan exceeding 2300 h. Moreover, the cell with the EMIMBF₄ + DMSO additive demonstrates an even more remarkable stability, achieving over 5300 h of cycling—indicating an exceptionally stable Zn plating/stripping behavior and confirming that the Zn anode is more stable in the presence of both EMIMBF₄ and DMSO additives.

Figure 14b presents the rate capability of Zn||Zn symmetric cells at various current densities ranging from 0.5 mA cm⁻² to 5 mA cm⁻². It can be observed that the polarization voltage of 2 M ZnSO₄ + 0.01 M EMIMBF₄ is consistently higher than that of 2 M ZnSO₄ across all current densities. This is attributed to the incorporation of EMIMBF₄ into the Zn²⁺ solvation sheath, which increases the energy barrier required for Zn deposition, ultimately resulting in a higher polarization potential for the 2 M ZnSO₄ + 0.01 M EMIMBF₄ electrolyte compared to the baseline 2 M ZnSO₄ [36]. Upon further addition of DMSO, the polarization voltage is significantly reduced. The rate performance of the Zn||Zn symmetric cells thus demonstrates that DMSO markedly enhances the cycling performance of the Zn anode.

In addition, Zn||Cu cells were assembled to evaluate the reversibility and stability of the Zn metal anode. As shown in Figure 14c, the Zn||Cu cell with EMIMBF₄ additive achieves nearly 1000 stable cycles at 1 mA cm⁻² and 1 mAh cm⁻², with an average Coulombic efficiency (CE) of 99.7%. When DMSO is further added to the electrolyte, the electrochemical performance improves even further, enabling stable cycling for over 1300 cycles with a high and stable CE of 99.8%. In contrast, the cell using only ZnSO₄ electrolyte fails after just 59 cycles due to severe dendrite growth and detrimental side reactions.

To evaluate the practicality of the EMIMBF₄–DMSO additive-based electrolyte, Zn||V₂O₅ full cells were assembled, and their rate capability was tested over a current density range of 0.5 to 3 A g⁻¹ (Figure 15a). At the low current density of 0.5 A g⁻¹, the cell employing the EMIMBF₄–DMSO electrolyte delivers a higher charge/discharge specific capacity compared to that using 2 M ZnSO₄. As the current density increases, the full cell with the EMIMBF₄–DMSO additive consistently maintains superior capacity output.

Furthermore, the cycling performance of full cells based on these three electrolyte systems was evaluated. Figure 15b,c present the galvanostatic charge–discharge (GCD) curves of Zn||V₂O₅ cells using 2 M ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO at different current densities. The results show that the EMIMBF₄–DMSO-containing full cell retains a higher specific capacity, consistent with the aforementioned rate performance.

To further investigate the impact of the EMIMBF₄–DMSO additive system on the electrochemical performance of aqueous zinc-ion full cells, Zn||V₂O₅ cells were assembled, and their cyclic voltammetry (CV) curves are shown in Figure 15d. Two distinct pairs of redox peaks are observed in all three electrolytes, corresponding to the two-step Zn²⁺ insertion/extraction reactions in V₂O₅. This confirms that the addition of EMIMBF₄ and DMSO to the ZnSO₄ electrolyte does not alter the fundamental redox behavior of the cathode.

Figure 15e,f compare the long-term cycling performance of full cells using 2 M ZnSO₄ and 2 M ZnSO₄ + 0.01 M EMIMBF₄ + 0.03 M DMSO at a current density of 0.5 A g⁻¹. Clearly, the EMIMBF₄–DMSO-based full cell maintains nearly 100% Coulombic efficiency throughout 500 cycles and exhibits a significantly higher discharge specific capacity—reaching up to 326.4 mAh g⁻¹—compared to the maximum value of 197.13 mAh g⁻¹ achieved with the pure ZnSO₄ electrolyte. It is particularly noteworthy that both the ZnSO₄ electrolyte and the full-cell system containing the EMIMBF₄ additive exhibit a fluctuating discharge capacity pattern characterized by an initial increase followed by a gradual decline during cycling. This phenomenon can be attributed to the typical “activation-degradation” behavior of the V₂O₅ cathode. In early cycles, continuous charge–discharge processes progressively activate previously inaccessible Zn²⁺ storage sites, leading to temporary capacity increases. As cycling progresses, however, trace dissolution of active materials, accumulation of lattice stress, and interface side reactions become more pronounced, ultimately resulting in capacity decay [37].

Table 1. Comparison of the electrochemical performance of Zn-ion full cells with various advanced aqueous electrolytes.

Cathode	Electrolyte Strategy	Current Density	Specific Capacity	Cycle Life (Cycles)/Retention	Reference
V2O5	2 M ZnSO4 + 0.01 M EMIMBF4 + 0.03 M DMSO	0.5 A g−1	326.4 mAhg−1	500 cycles (Stable)	This work
V2O5	2 M ZnSO4 + 20 vol% DMSO	1.0 A g−1	~310 mAhg−1	1000 cycles (~85%)	Adv. Energy Mater. 2022 [2]
V2O5	3 M Zn(CF3SO3)2 (High concentration)	0.5 A g−1	380 mAhg−1	500 cycles (~80%)	Adv. Mater. 2020 [38]
V2O5	2 M ZnSO4 + Glucose additive	0.5 A g−1	350 mAhg−1	500 cycles (88%)	Chem. Eng. J. 2022 [39]
V2O5	2 M ZnSO4 + PEG additive	0.5 A g−1	~300 mAhg−1	300 cycles (~85%)	Adv. Energy Mater. 2020 [40]
V2O5	2 M ZnSO4 + Trace Maltose	0.5 A g−1	340 mAhg−1	600 cycles (90%)	Nat. Commun. 2021 [41]

shows the comparison of the electrochemical performance of Zn-ion full cells with various advanced aqueous electrolytes. It summarizes the electrochemical performance of reported aqueous Zn-ion full cells, including electrolyte composition, cathode material, current density, specific capacity, cycle number, capacity retention, and Coulombic efficiency. In our work, the Zn||V₂O₅ full cell using the optimized EMIMBF₄/DMSO-containing electrolyte delivers a high discharge capacity of 326.4 mAh g⁻¹ at 0.5 A g⁻¹, and maintains nearly 100% Coulombic efficiency after 500 cycles. Compared with previously reported aqueous Zn-ion full cells, the present electrolyte shows competitive cycling stability and high cathode utilization while using only trace additives.

4. Conclusions

This study introduces and validates an innovative ‘ionic liquid–polar solvent coupling’ strategy that addresses key performance bottlenecks in aqueous zinc-ion batteries (AZIBs), including dendrite growth, hydrogen evolution reaction (HER), and side reactions. By adding a minimal combination of 0.01 M EMIMBF₄ and 0.03 M DMSO, this strategy effectively regulates the interfacial chemistry and solvation structure of Zn²⁺, promoting dense, preferentially oriented Zn deposition along the (101) plane. The synergistic interaction between EMIM⁺ and DMSO not only suppresses HER but also stabilizes the formation of a compact solid electrolyte interphase (SEI), rich in ZnCO₃, ZnSO₃, and Zn(OH)₂, enhancing both uniformity and cycle stability. The remarkable efficiency of this trace-additive system stems from the complementary functions of each component, creating a cascade effect that amplifies the interfacial modification beyond what would be expected from simple additive contributions.

As a result, Zn||Cu cells exhibit outstanding cycling stability with over 1300 cycles at 1 mA cm⁻² and 1 mAh cm⁻², achieving an average Coulombic efficiency of 99.8%. Zn||Zn symmetric cells show significant cycle life extension, while Zn||V₂O₅ full cells retain a capacity of 326.4 mAh g⁻¹ after 500 cycles, maintaining nearly 100% Coulombic efficiency. Remarkably, the strategy achieves these performance improvements at low additive concentrations, avoiding issues like increased electrolyte viscosity commonly associated with high-dosage additives.

This work provides a simple yet highly effective approach to designing high-performance electrolytes for AZIBs and offers valuable insights for advancing electrolyte-interface synergistic strategies in other metal-based aqueous batteries. Future research may explore the application of this strategy in other battery systems to enhance their stability and efficiency, paving the way for broader use of aqueous batteries in large-scale energy storage.