A bifunctional biomass-derived additive for constructing a hybrid SEI layer to enhance the cycling stability of aqueous zinc-ion batteries under high current densities

Abstract

Employing biomass-derived additives for electrolyte modulation constitutes an effective and highly promising strategy to address the core challenges of aqueous zinc-ion batteries (AZIBs). Although existing biomass-derived additives combine environmental benignity with cost-effectiveness, they remain constrained by challenges in terms of long-term cycling stability compatible with high current densities. Herein, we proposed the utilization of biomass-derived ferulic acid (FA) as an additive to fabricate a bifunctional aqueous electrolyte, which enhanced the electrochemical performance of AZIBs through coordination regulation and surface adsorption. Theoretical calculations revealed that FA modulated the Zn²⁺ solvation structure, preferentially adsorbed on the Zn (002) plane, weakened the Zn²⁺–H₂O interaction, and inhibited the hydrogen evolution reaction (HER). Electrochemical measurements confirmed that the Zn//Zn symmetric cell exhibited stable cycling for 4000 h at 0.5 mA cm⁻² (0.5 mAh cm⁻²) and 1400 h at 10 mA cm⁻² (10 mAh cm⁻²). The Zn//Cu half-cell achieved a high coulombic efficiency (CE) of 99.53% after 2000 cycles at 5 mA cm⁻², whereas the Zn//P-NVO full cell retained 85.6% of its initial capacity after 1000 cycles at 2 A g⁻¹. This green and low-cost strategy provides a feasible route toward the industrialization of AZIBs.

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1. Introduction

Aqueous zinc-ion batteries (AZIBs) have emerged as a highly promising new energy storage technology amid the global energy transition. Characterized by high safety, low cost, abundant zinc resources, and excellent theoretical capacity,^1–4 they hold broad application prospects in large-scale energy storage and portable electronic devices, which are highly aligned with the carbon neutrality goal.⁵ Nevertheless, the direct contact between the zinc anode and aqueous electrolyte readily triggers a series of side reactions, severely impeding the electrochemical performance and cycling lifespan of AZIBs.^6–8 Owing to the high chemical reactivity of zinc, it tends to undergo spontaneous redox reactions with water molecules in the electrolyte, leading to hydrogen evolution (HER). This process not only consumes active zinc species but also forms a fragile and crack-prone porous passivation layer on the anode surface. The continuous rupture of this passivation layer exposes fresh active sites, thereby exacerbating the occurrence of side reactions.⁹ Moreover, the uneven distribution of Zn²⁺ concentration, local current inhomogeneity, and intrinsic surface roughness of the zinc anode can drive preferential Zn²⁺ deposition at protruded sites, resulting in the formation of zinc dendrites. These dendrites may eventually pierce the separator, leading to internal short circuits of the battery.^10–12

To address the aforementioned issues, researchers have developed various modification strategies including artificial protective layer construction,^13–15 electrode structure optimization,^16–18 and electrolyte engineering regulation.^19–22 Among these, electrolyte engineering is regarded as the most promising approach for industrial application due to its advantages of simple operation, strong universality, high reproducibility, and low cost. In recent years, the introduction of biomass-based additives into electrolytes has evolved into an effective regulation strategy.^23–25 Rich in multiple functional groups, these compounds can modulate the electrode/electrolyte interface behavior through multiple mechanisms such as coordination and adsorption. Meanwhile, they circumvent the problems of environmental pollution and high preparation costs associated with traditional chemically synthesized additives, which is consistent with the development requirements of green energy storage technologies. Nevertheless, existing biomass-based additives still suffer from performance limitations: Zhang et al.²⁶ demonstrated that theaflavin (TF) can reconstruct the solvation structure of Zn²⁺, but the polymerization tendency of polyphenolic macromolecules leads to the accumulation of thick interfacial films, resulting in a voltage hysteresis exceeding 200 mV under high current densities. Zhao et al.²⁷ proposed that malic acid (MA) chelates Zn²⁺ and regulates zinc deposition behavior via the synergistic effect of carboxyl and hydroxyl groups, yet the in situ formed interfacial protective layer lacks sufficient stability under high current densities, and its structural damage easily triggers battery performance degradation. Chen et al.²⁸ verified that phytic acid (PA) optimizes the zinc ion environment through strong coordination, but the excessively thick interfacial film formed hinders ion transport, making it difficult to meet the requirements for stable cycling under high current densities. Although these additives can improve battery performance by regulating the solvation structure or constructing interfacial layers, they generally fail to simultaneously achieve high current density adaptability and long-term cycling stability. Particularly under current density conditions above 10 mA cm⁻², the performance improvement effect is significantly weakened, which cannot support the stable operation of batteries.

To address these drawbacks, this work selected ferulic acid (FA), a bioactive molecule derived from biomass, as a functional additive for the electrolyte, and verified that it endowed the battery with exceptional cycling stability via a dual mechanism of coordination regulation and surface adsorption. Theoretical calculations confirmed that FA regulated the solvation structure of Zn²⁺, weakened their interaction with solvated water molecules, and preferentially adsorbed on the zinc anode surface to induce the formation of a thin, uniform and dense interfacial protective layer, thereby suppressing the hydrogen evolution side reaction and guiding the uniform deposition of Zn²⁺. Experimental tests validated its modification effect. The results showed that the Zn//Zn symmetric cell exhibited a cycling lifespan of over 4000 h at 0.5 mA cm⁻² and 0.5 mAh cm⁻², and the lifespan exceeded 1400 h at 10 mA cm⁻² and 10 mAh cm⁻²; the Zn//Cu half-cell achieved a coulombic efficiency of 99.53% after 2000 cycles at 5 mA cm⁻²; and the full cell delivered a capacity retention of 85.6% after 1000 cycles at a current density of 2 A g⁻¹. These findings collectively highlighted the remarkable advantages of the FA additive in boosting the comprehensive electrochemical performance of Zn²⁺ batteries.

2. Experiments

2.1 Electrolyte preparation

Zinc sulfate heptahydrate (ZnSO₄·7H₂O, AR) was dissolved in deionized (DI) water to prepare a 2 M zinc sulfate (ZnSO₄) electrolyte. Subsequently, different amounts of ferulic acid (C₁₀H₁₀O₄, AR, 97%) were added to the 2 M ZnSO₄ electrolyte, followed by stirring at room temperature for 5 h to ensure complete dissolution of the solute. The resulting solutions were free of crystallization, clear and transparent, yielding ZnSO₄–FA electrolytes with different concentrations. These electrolytes were designated as ZSO + 1 mM FA, ZSO + 1.5 mM FA, ZSO + 2 mM FA, and ZSO + 2.5 mM FA, respectively.

2.2 P-NH₄V₄O₁₀ (P-NVO) preparation

Phosphorus-doped ammonium vanadate (P-NVO) was synthesized via a facile hydrothermal method. Specifically, ammonium metavanadate (NH₄VO₃, AR, 99.5%) was first dispersed in deionized (DI) water. The resulting mixture was heated to 60 °C and stirred until complete dissolution. After cooling to room temperature, oxalic acid dihydrate (H₂C₂O₄·2H₂O, AR, 98.0%) was added as the reducing agent. Ten seconds later, ammonium phosphate was introduced into the mixture, which was then magnetically stirred for 0.5 h. During this process, the solution was observed to gradually transform into a clear brownish-yellow color. Finally, the homogeneous solution was transferred into a 100 mL Teflon-lined autoclave, which was sealed tightly and heated at 180 °C for 6 h. Upon natural cooling of the autoclave to room temperature, the resulting precipitate was collected, rinsed thoroughly with distilled water and anhydrous ethanol several times, and subsequently dried at 60 °C for 24 h.²⁹

2.3 Materials characterization

The surface chemical composition of the samples was characterized by X-ray photoelectron spectroscopy (XPS, Kratos AXIS SUPRA⁺, UK). During the test, XPS sputtering depth profiling was performed using low-current monoatomic Ar⁺ with an energy of 500 eV and a scan size of 2 mm × 1 mm. All XPS spectra were calibrated by adjusting the detected contaminant carbon C 1s peak to 284.8 eV. The infrared spectral characteristics of the samples were obtained via a Fourier transform infrared spectrometer (FT-IR, VERTEX 70, Bruker, Germany). The crystal structure of the samples was analyzed using an X-ray diffractometer (XRD-700, Shimadzu, Japan). The morphology and microstructure of the samples were characterized using a scanning electron microscope (SU-8010, Hitachi, Japan) and a transmission electron microscope (JEOL 2010F, Japan). For Raman spectroscopy measurements, a Labram HR laser confocal Raman spectrometer was employed, with an excitation laser wavelength of 532 nm.

2.4 Electrochemical measurement

CR2032 coin cell cases were employed as the encapsulation carriers, with Whatman glass fiber used as the separator. Commercial zinc foil (100 μm thickness and 12 mm diameter) served as the metal anode (unless otherwise specified, the anode dimensions were maintained at this standard). Copper foils (100 μm thickness and 12 mm diameter) and P-NVO materials were utilized as the cathodes. Zn symmetric cells (Zn//Zn), Zn//Cu half-cells, and Zn//P-NVO full cells were assembled at room temperature under an air atmosphere. For the Zn symmetric cells, electrochemical measurements including Tafel polarization, linear sweep voltammetry (LSV), and chronoamperometry (CA) were conducted using a Bio-Logic VSP-300 electrochemical workstation. The Tafel test was performed with a potential scanning range of −0.5 to 0.5 V (relative to the open-circuit potential, OCP) at a scan rate of 1 mV s⁻¹; the corrosion current density and corrosion potential were obtained by fitting the Tafel curves to evaluate the corrosion stability of the Zn electrode in the electrolyte. The LSV measurements were carried out over the voltage ranges of 0 to 1 V and 0 to −3 V (vs. Zn/Zn²⁺) at a scan rate of 1 mV s⁻¹. The CA test was performed continuously for 600 s at a constant potential of −150 mV. Meanwhile, electrochemical impedance spectroscopy (EIS) measurements were implemented on the Zn symmetric cells at the OCP, with the frequency range set from 106 to 10⁻² Hz and the applied AC signal amplitude of 5 mV. In addition, the rate capability and cycling performance of Zn symmetric cells were investigated during galvanostatic charge–discharge at various current densities. The plating/stripping coulombic efficiency of the Zn//Cu half-cells was determined under the conditions of 5 mA cm⁻² (1 mAh cm⁻²). The electrode preparation process was as follows: the active electrode slurry was prepared by mixing acetylene black, polytetrafluoroethylene (PTFE) emulsion, and active materials at a mass ratio of 2 : 1 : 7. Subsequently, the mixture was rolled into a film with a uniform thickness and pressed onto conductive carbon paper with a diameter of 1 cm.

2.5 Density functional theory (DFT) calculation methods

In molecular dynamics (MD) studies, the initial amorphous structural model was constructed using the Amorphous Cell module of Materials Studio 2023. This module enables the precise generation of target molecular unit cells according to pre-defined system dimensions and molecular counts, laying a solid foundation for subsequent simulations. After model construction, the system was imported into the Forcite module to undergo geometry optimization and molecular dynamics simulations sequentially and molecular dynamics simulations. For geometry optimization, the COMPASS III force field was employed to describe intermolecular interactions, and the conjugate gradient algorithm was utilized to achieve energy minimization until the system energy converged to a stable threshold. Following geometry optimization, MD simulations were conducted under either the NVT or NPT ensemble to investigate the thermodynamic properties and dynamic behaviors of the system. The Berendsen thermostat was adopted for temperature control, with a time step of 1 fs and a total simulation duration of 100 ps to ensure system equilibrium. Periodic boundary conditions (PBCs) were applied along the x, y, and z directions to minimize boundary effects and guarantee the authenticity of simulations. Through the aforementioned workflow, the equilibrium structure and dynamic evolution trajectory of the system were obtained, and the radial distribution function was further analyzed to reveal information about the short-range ordered structure among atoms or molecules.³⁰

Electronic structure calculations were carried out using the Dmol³ module of Materials Studio 2023 based on density functional theory (DFT), aiming to acquire key parameters such as interaction energy, molecular electrostatic potential (ESP), and frontier molecular orbitals (HOMO and LUMO), which provide theoretical support for the analysis of microcosmic interaction mechanisms. Preprocessing before calculations: the initial structures of target molecules and composite systems were constructed using the 3D atomic structure editor, and unreasonable configurations were eliminated by applying the COMPASS III force field with fine convergence accuracy to obtain low-energy initial structures. Core parameter settings for Dmol³: the GGA-PBE functional was used to describe electronic exchange–correlation interactions, and the self-consistent field (SCF) convergence accuracy was set to 1.0 × 10⁻⁵ Ha to ensure computational stability. The calculation of molecular electrostatic potential (ESP) was realized by checking the relevant options in the Properties tab, with a Fine grid density adopted. The ESP distribution was visualized on the van der Waals surface, and electron-enriched/deficient regions as well as potential electrostatic interaction sites were identified based on the positive–negative distribution and numerical magnitude. The calculation of frontier molecular orbitals was automatically completed by checking the “Calculate HOMO–LUMO” option, and the orbital energy values and electron cloud distribution characteristics were subsequently extracted.³¹

3. Results and discussion

Four forms of FA electrolytes with different concentrations were prepared, and a 2 M ZnSO₄ electrolyte (ZSO) was set as the control group. After 24 hours of sealed static storage at ambient temperature, no instability phenomena such as stratification or recrystallization were observed (Fig. S1). This confirmed the excellent chemical compatibility of FA with the electrolyte system and the favorable storage stability of the FA-containing electrolytes, thereby establishing a robust foundation for subsequent performance evaluations. The optimal concentration of FA in the ZSO electrolyte was determined using the Nyquist plots of electrolytes with different FA concentrations as displayed in Fig. S2. The calculated ionic conductivity results indicated that the ZSO + 1.5 mM FA electrolyte achieved the highest ionic conductivity, outperforming the other concentrations (Fig. S3). This phenomenon confirmed that FA can effectively lower the internal ion transport resistance of the electrolyte and optimize the zinc ion migration dynamics at this concentration. To investigate the effect of the FA concentration on the electrochemical performance of the electrolytes and the interfacial behavior of zinc electrodes, characterization and analysis were carried out using Tafel (Fig. S4), LSV (Fig. S5) and CA measurements (Fig. S6). The Tafel measurements showed that the corrosion current density of the ZSO + 1.5 mM FA electrolyte was markedly lower than those of the electrolytes with the other FA concentrations. The LSV results indicated that this system possessed a wider electrochemical window. The CA tests verified that the current response for zinc plating/stripping in this electrolyte was more stable, which allowed the regulation of zinc ion nucleation and growth, suppressed dendrite formation, and further strengthened the interfacial stability of the electrode. Furthermore, in the Zn//Zn symmetric cell measurements, ZSO + 1.5 mM FA demonstrated the best cycling stability across all concentrations studied (Fig. S7). In summary, excessively high FA concentrations impede Zn²⁺ migration, whereas excessively low concentrations are unable to efficiently modulate zinc ion deposition. Consequently, ZSO + 1.5 mM FA was determined as the optimal electrolyte recipe for subsequent measurements.

FA is a naturally occurring phenolic acid with the merits of wide accessibility and low cost. Its molecular structure (Fig. S8) contained one phenolic hydroxyl group, one methoxy group and one carboxyl group simultaneously. Among them, both the phenolic hydroxyl group and the carboxyl group possessed strong polarity and nucleophilicity and could achieve effective coordinative adsorption on the zinc metal surface. Previous studies demonstrated that organic molecules with multiple polar functional groups could form stable hydrogen bonds with water molecules, reduce the reactivity of free water in the system, and further weaken the strong solvation interaction between Zn²⁺ and H₂O.^22,32,33 In addition, the phenolic hydroxyl group in the FA molecule also acted as a radical scavenging site, which could effectively quench the reactive free radicals in the electrolyte, thereby effectively suppressing the occurrence of various side reactions.^34,35 First, the structural characteristics of FA were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). As shown in Fig. S9, the peak at 3435.46 cm⁻¹ was assigned to the stretching vibration of the phenolic hydroxyl group (–OH), while the peak at 2927.94 cm⁻¹ corresponded to the C–H stretching vibration within the methoxy moiety. The absorption peak at 1687.71 cm⁻¹ was attributed to the C=O stretching vibration of the carbonyl group, and the peak at 1278.71 cm⁻¹ corresponded to the C–O stretching vibration of the phenolic ether bond. Fig. 1a and b present the FTIR spectra of the electrolytes containing different concentrations of FA. Specifically, in Fig. 1b, as the FA concentration increased, the O–H stretching vibration exhibited a blue-shift trend, indicating that FA molecules disrupted the hydrogen bond network of O–H containing groups through competitive hydrogen bonding interactions. This regulatory effect is further verified by the Raman spectra (Fig. 1c and Fig. S10–S12): with the FA concentration increasing, the intensity of the characteristic peak associated with the intermolecular HOH–OH₂ interaction among water molecules diminishes, whereas the peak assigned to HOH–OSO₃²⁻ interactions exhibits a gradual enhancement. This spectral evolution revealed that the hydrogen-bonded network of water molecules in the electrolyte system was reconfigured, as partial native strong hydrogen-bonded motifs were disrupted and subsequently converted into weak hydrogen bonds with OSO₃²⁻ anions.³⁶ The combined characterization results from the aforementioned FTIR and Raman spectra collectively confirmed the ability of FA to reconstruct and regulate the hydrogen bond network of the electrolyte.

Fig. 1 (a and b) FTIR spectra of ZnSO₄ electrolytes with different FA contents. (c) Raman spectra of ZnSO₄ electrolytes with different FA contents. (d) ESP maps of FA and H₂O. (e) Binding energies of Zn²⁺–H₂O and Zn²⁺–FA. (f) Comparison of the LUMO and HOMO levels between FA and H₂O. (g) Adsorption energies of H₂O and FA on different Zn crystal surfaces. (h) The configurations of the FA electrolyte system and its solvation structure were simulated via MD simulations. (i) The corresponding RDF curves.

The electrostatic potential (ESP) mapping results of FA molecules (Fig. 1d) clearly demonstrated that their multiple oxygen-containing functional groups endow specific regions of the molecules with high electronegativity, thereby providing abundant active binding sites for Zn²⁺.³⁷ Density functional theory (DFT) calculations were performed to systematically investigate the interaction mechanism between FA and Zn²⁺. As illustrated by the results (Fig. S13), the binding energy between FA and Zn²⁺ (−11.02 eV) was significantly lower than that of H₂O with Zn²⁺ (−4.64 eV), which indicated that Zn²⁺ has a stronger tendency to form coordination bonds with the polar oxygen-containing functional groups in FA molecules rather than to coordinate with water molecules. This coordination interaction enhanced the diffusion kinetics of Zn²⁺ and further helped to regulate the flux of Zn²⁺.³⁸ Furthermore, the binding energy between FA and H₂O was lower than that between water molecules (Fig. 1e), indicating that FA molecules can compete with water molecules for binding through intermolecular hydrogen bonds. This disrupted the inherent hydrogen-bond network in the aqueous electrolyte, thereby effectively suppressing the corrosion and hydrogen evolution side reactions on the Zn anode.^28,39Fig. 1f presents the HOMO and LUMO energy levels of H₂O and FA molecules. The LUMO energy level of H₂O (1.38 eV) was significantly higher than that of FA (−3.03 eV). This energy level difference indicated that FA has a stronger electron-accepting ability, allowing it to preferentially participate in electron transfer on the Zn anode surface and thereby suppress the hydrogen evolution side reaction induced by water molecules gaining electrons. Furthermore, the band gap of FA was much smaller (2.75 eV vs. 8.13 eV for H₂O), indicating that FA has a lower activation energy when participating in charge transfer.

To gain in-depth insights into the working mechanism of FA on the zinc anode surface, we further calculated the adsorption energies of FA on three zinc crystal facets: (002), (100), and (101), and focused on exploring the differences between two adsorption configurations (horizontal adsorption and vertical adsorption). As shown in the calculation results of Fig. 1g, FA exhibited high binding energies on the zinc surface in both horizontal and vertical adsorption configurations. This feature indicated that strong interfacial affinity existed between FA and the zinc substrate in both adsorption types. Such robust interactions promoted the preferential formation of a high-density FA adsorbed protective layer on the zinc anode surface, which effectively blocked the direct contact between H₂O molecules and the zinc substrate, thereby mitigating parasitic reactions such as hydrogen evolution and corrosion. Notably, in the horizontal adsorption configuration, FA exhibited the lowest adsorption energy of −2.68 eV on the (002) plane, which was lower than those on the (100) plane (−2.28 eV) and the (101) plane (−2.39 eV), suggesting the strongest adsorption tendency. In contrast, in the vertical adsorption configuration, the adsorption energies of FA on the different crystal planes displayed a distinct trend: the adsorption energy on the (002) plane was −0.93 eV, which was higher than those on the (100) plane (−1.02 eV) and the (101) plane (−1.05 eV), corresponding to the weakest adsorption tendency. Although the adsorption trends varied between the two configurations, the strong preferential adsorption of FA on the (002) crystal facet in the horizontal adsorption configuration was dominant. According to Bravais’ law and Woolf's construction principle, crystal planes with slower growth rates were ultimately exposed and dominated the crystal morphology.⁴⁰ Thus, the preferential adsorption of FA on the (002) plane slowed down the Zn²⁺ deposition rate on this plane, further guided the preferential growth of zinc along the (002) plane and facilitated the formation of a uniform and flat zinc deposition layer.⁴¹ Furthermore, the analysis of the charge density difference in H₂O–Zn (002) and FA–Zn (002) also revealed that the charge transfer between FA and the zinc crystal facet was more pronounced (Fig. S14). This result further corroborated the existence of strong interfacial interactions between FA and the zinc substrate.

Subsequently, molecular dynamics (MD) simulations were employed to investigate the solvation environment of Zn²⁺. The results of the MD simulations are presented in Fig. S15 and Fig. 1h. Fig. S15 shows that in the ZSO solution, Zn²⁺ is coordinated by six water molecules, forming a typical zinc ion solvation structure, which is consistent with previous reports.^25,38 In contrast, Fig. 1h shows that the addition of FA molecules reduces the number of water molecules in the solvation structure, which was achieved through the competitive coordination between its polar functional groups (carboxyl and phenolic hydroxyl groups) and Zn²⁺. Furthermore, the coordination number analysis in Fig. 1i and Fig. S16 revealed that the coordination number of Zn–(O)H₂O decreased significantly from 4.83 to 4.55, confirming that FA replaced part of the coordinated H₂O molecules. Meanwhile, the coordination number of Zn–(O)SO₄²⁻ increased from 1.14 to 1.35, indicating that more SO₄²⁻ entered the Zn²⁺ solvation sheath. This facilitated the formation of Zn²⁺–SO₄²⁻ coordination intermediates, optimized the migration pathway and charge transfer kinetics of Zn²⁺, and reduced the interface impedance.⁴² We further calculated the solvation energies and electrostatic potential energies of the two solvation structures, Zn²⁺–6H₂O and Zn²⁺–4H₂O–SO₄²⁻–FA (Fig. S17 and S18). The results showed that the solvation energy of Zn²⁺–4H₂O–SO₄²⁻–FA was lower than that of Zn²⁺–6H₂O, suggesting that the FA additive could promote the desolvation of Zn²⁺ during the zinc deposition process. Moreover, the increase in the electrostatic potential energy of the system indicated that the introduction of FA as an electrolyte additive helped form a more stable solvation shell.

The corrosion inhibition performance of the ZSO + 1.5 mM FA electrolyte was systematically investigated via electrochemical methods. As shown in Fig. 2a, the corrosion current density of the zinc anode in the ZSO + 1.5 mM FA electrolyte system was determined to be 1.264 × 10⁻³ mA cm⁻², which is significantly lower than that in the pristine ZSO electrolyte system (3.052 × 10⁻³ mA cm⁻²). Subsequently, the corrosion resistance was verified by immersing the zinc foil in different electrolytes. As shown in the X-ray diffraction (XRD) patterns of Fig. S19, the corrosion by-product (ZnSO₄·Zn(OH)₂·xH₂O) was observed on the zinc sheet immersed in the ZSO electrolyte. However, for the sample immersed in the ZSO + 1.5 mM FA electrolyte, the growth of basic zinc sulfate is effectively inhibited. These results clearly demonstrate that FA molecules interact with the zinc anode surface and subsequently form a protective interfacial layer, which effectively mitigates the corrosion kinetics of the zinc anode. As depicted in Fig. 2b, the LSV curves reveal that the ZSO + 1.5 mM FA electrolyte effectively restrains the HER and OER, thus extending the electrochemical stability window. Fig. S20 shows the thickness changes of the batteries before and after cycling. The results indicate that the Zn symmetric cell assembled with the ZSO + 1.5 mM FA composite electrolyte exhibits a thickness increase of only 2 mm after 200 cycles, which is significantly lower than that of the Zn symmetric cell in the pure ZSO electrolyte system (5 mm). This phenomenon demonstrates that the ZSO + 1.5 mM FA electrolyte can effectively inhibit the hydrogen evolution side reaction of the Zn electrode. Based on the aforementioned analysis, the inhibitory effect of FA on the hydrogen evolution reaction (HER) of the zinc anode stems from two core mechanisms. First, FA molecules exhibit significantly stronger affinity for the zinc surface than H₂O and possess superior adsorption kinetics, enabling them to preferentially occupy the active sites for hydrogen evolution. This forms an adsorbed layer that blocks the direct contact between H₂O and the zinc surface as well as the reduction of H⁻. Second, FA forms stable coordination structures with Zn²⁺, which weakens the interaction between Zn²⁺ and solvated H₂O, destroys the stability of the solvation shell, and reduces the number of H₂O molecules carried by Zn²⁺ during migration, thereby effectively suppressing the generation and evolution of hydrogen gas.

Fig. 2 (a) Tafel curves of Zn//Zn symmetric cells. (b) LSV curves of Zn//Cu half-cells. (c) Contact angles of Zn foils with ZSO and ZSO + 1.5 mM FA electrolytes. (d) CV curves of Zn//Zn symmetric cells. (e) Nucleation overpotential of Zn//Cu half-cells during the first cycle. (f) CA curves of Zn//Zn symmetric cells. (g and h) EIS plots of Zn batteries with ZSO and ZSO + 1.5 mM FA electrolytes at different temperatures. (i) Corresponding activation energies.

We measured the contact angles between the zinc foil and different electrolytes and found that the angle for ZSO + 1.5 mM FA was notably smaller than that for the pristine ZSO electrolyte with a contact angle of 65.5° (Fig. 2c). Such a reduced value demonstrates that FA enables the formation of an interface with enhanced zinc affinity and homogeneous ion transport. The CV curve in Fig. 2d indicated that in the ZSO + 1.5 mM FA electrolyte system, the nucleation overpotential (NOP) of zinc was higher, corresponding to a stronger nucleation driving force, which in turn promoted the formation of smaller zinc nuclei. This phenomenon facilitated the formation of a denser zinc deposition layer and effectively suppressed dendrite propagation, predominantly stemming from the coordination interaction between Zn²⁺ and the polar oxygen-containing functional groups of FA. As widely recognized, a higher NOP leads to a smaller nucleus radius, indicating that FA-aided zinc deposition yields uniform morphologies. Similarly, the increased NOP of Zn//Cu asymmetric cells after FA addition further confirms that FA facilitated dense and uniform zinc electrodeposition (Fig. 2e). Zn²⁺ nucleation and growth behaviors were investigated via chronoamperometric (CA) curves. The pristine ZSO system exhibited a typical two-dimensional (2D) diffusion behavior, with the current continuing to increase within 300 s. This indicated that Zn²⁺ accumulated due to the “tip effect” and triggered dendrite growth. In contrast, after the incorporation of FA, the curve displayed a three-dimensional (3D) diffusion behavior, and the current stabilized rapidly. This indicated that the adsorbed FA provided sufficient and uniform nucleation sites, facilitating the uniform and dense deposition of Zn²⁺, thereby significantly suppressing the heterogeneous growth of zinc starting from the nucleation stage (Fig. 2f).⁴³

Furthermore, Fig. 2g and h present the electrochemical impedance spectroscopy (EIS) profiles of Zn//Zn symmetric cells measured over the temperature range of 30–70 °C. It is evident that the charge transfer resistance (R_ct) of both electrolyte systems decreased with the increase of temperature. Based on the Arrhenius equation, the activation energy (E_a) for Zn²⁺ transport was calculated (Fig. 2i), with a value of 26.91 kJ mol⁻¹ in the ZSO electrolyte and a reduced value of 15.10 kJ mol⁻¹ in the ZSO + 1.5 mM FA electrolyte. The significant reduction in E_a demonstrated that the introduction of FA effectively lowered the migration energy barrier for Zn²⁺ transport across the interfacial protective layer, thereby promoting more rapid Zn²⁺ solvation and reduction kinetics.⁴⁴ As a result, the overall electrochemical efficiency and long-term stability of the reaction system were remarkably enhanced. Furthermore, the R_ct of the FA-containing electrolyte was lower than that of the pristine ZSO electrolyte (Fig. S21), with a corresponding increase in ionic conductivity (Fig. S22). All electrochemical test results indicated that the FA electrolyte system not only exhibited a lower corrosion current density and a higher HER overpotential, but also had a more stable ion transport behavior and a more efficient solvent reduction mechanism. These key performance advantages worked together, significantly enhancing the overall efficiency of the electrochemical reaction and ensuring the sustainability and controllability of the reaction process.

To investigate the regulatory effect of FA on the electrochemical deposition/stripping behavior of the Zn anode, Zn//Zn symmetric cells were fabricated, and cycling stability tests were conducted under various current densities and areal capacities. As presented in Fig. 3a, under the test conditions of 0.5 mA cm⁻² and 0.5 mAh cm⁻², the FA electrolyte system exhibited highly stable deposition/stripping behavior, achieving a long cycle life exceeding 4000 h. Even under such harsh test conditions of 10 mA cm⁻² and 10 mAh cm⁻², this electrolyte system still exhibited excellent performance and could achieve stable cycling for 1400 h (Fig. 3b). Furthermore, when the current density was 20 mA cm⁻², the battery with the FA electrolyte could still maintain excellent long-term cycling stability (Fig. S23). Rest–recovery cycling tests further verified that the FA electrolyte possessed both rapid current response capability and durable interfacial protection efficacy. Even after undergoing a rest period and resuming cycling, the system still sustained stable electrochemical performance (Fig. 3c). Besides, under the condition of a fixed areal capacity of 1 mAh cm⁻², the cycling stability of Zn//Zn symmetric cells was systematically investigated over a current density range of 1 to 20 mA cm⁻² in different electrolyte systems (Fig. 3d). The results demonstrated that the FA-containing electrolyte could significantly enhance the cycling stability of the Zn anode. Even at a high current density of 20 mA cm⁻², no obvious increase in voltage polarization or cycling instability was observed. More notably, when the current density was restored to the initial value, the cells could still maintain stable Zn plating/stripping cycles without any performance degradation. This excellent electrochemical stability was presumably attributed to the robust solid electrolyte interphase (SEI) layer formed by the FA molecules adsorbed on the Zn anode surface. This layer could effectively protect the anode by inhibiting the heterogeneous nucleation, growth, and propagation of Zn dendrites. Fig. S24 presents the scanning electron microscopy (SEM) images of the Zn anode surface after 50 h of cycling in Zn//Zn symmetric cells under different current densities (1 mA cm⁻², 5 mA cm⁻², 10 mA cm⁻², and 20 mA cm⁻²), and the Zn anode cycled in the FA-containing electrolyte exhibited a uniform and dense deposition morphology at low current densities. When the current density increased to 20 mA cm⁻², a small number of dispersed pores appeared on the anode surface. This phenomenon further confirmed that FA could effectively inhibit the growth of zinc dendrites and also improve the uniformity and stability of the zinc deposition process. In sharp contrast, the zinc anode in the ZSO electrolyte system exhibited distinctly different morphological characteristics. As the current density increased, surface features such as dendrite corrosion became increasingly pronounced. Such morphological differences further corroborated the electrochemical test results previously reported in this study.

Fig. 3 (a and b) Galvanostatic cycling performance tests of the Zn//Zn symmetric cell under different current densities and capacities and (c) intermittent galvanostatic cycling performance test. (d) Comparison of rate performance at different current densities. (e) CE curves of Zn//Cu half-cells. (f and g) Corresponding GCD curves for ZSO + 1.5 mM FA and ZSO electrolytes, respectively. (h) Deep charge–discharge tests of different electrolytes. (i) Comparison of the cycling stability of Zn anodes with literature-reported works.

Coulombic efficiency (CE) serves as a crucial indicator for assessing the reversibility of zinc anodes. As shown in Fig. 3e, the Zn//Cu half-cell employing the FA electrolyte achieved an average CE of 99.53% over 2000 cycles at 5 mA cm⁻² and 1 mAh cm⁻². The corresponding galvanostatic charge/discharge profiles further corroborated its superior stability (Fig. 3f and g). In stark contrast, in the ZSO electrolyte system, the zinc anode was prone to parasitic reactions, accompanied by dendrite growth and the formation of irreversible passivation by-products. These issues directly led to significant CE fluctuations and a substantially shortened cycle life. These results collectively demonstrated that the FA electrolyte could effectively suppress side reactions and dendrite growth on the zinc anode by establishing a stable electrode/electrolyte interface. Depth of discharge (DOD) was a critical metric for evaluating the battery lifespan and operational efficiency. In the DOD test, the thickness of the zinc foil was reduced from 100 μm to 50 μm. Even under more stringent test conditions, the battery with the FA electrolyte still exhibited outstanding deep-cycle performance, maintaining stable operation for over 600 hours at a DOD of 60% (1 mA cm⁻²). In contrast, the battery cycled with the pristine ZSO electrolyte underwent rapid short-circuiting merely 75 hours after the initiation of discharge cycles, which is attributed to uncontrolled zinc deposition (Fig. 3h). Compared with various additive-modified electrolyte systems reported recently (Fig. 3i),^{26–28,45–51} the ZSO + 1.5 mM FA electrolyte developed in this work exhibited more remarkable advantages in enhancing the reversibility of zinc anode deposition/dissolution. This result further highlighted the unique interfacial regulation effect and application potential of FA as an electrolyte additive for aqueous AZIBs.

Additionally, surface characterization was conducted on Zn foils cycled at a current density of 10 mA cm⁻², aiming to validate the suppression effect of the FA additive against Zn dendrite growth. From Fig. S25, it can be directly observed that the surface of the zinc foil cycled in the ZSO electrolyte exhibited uneven corrosion pits and dendritic structures. In contrast, the surface of the zinc foil cycled in the FA-added electrolyte remained uniform and dense, with no obvious by-products or dendrite growth observed. The zinc deposition processes in different electrolytes were further investigated via SEM. The surface SEM image (Fig. 4a) of the zinc anode sample cycled 25 times in the ZSO electrolyte exhibited a typical nanofibrous structure with uniform grain size and excellent dispersion. However, after 50 cycles (Fig. 4b), the nanofibrous structure disappeared completely, transforming into large-sized polygonal bulk crystals accompanied by significant grain agglomeration and size coarsening. Such “morphological coarsening” was recognized as one of the core causes for the cycling performance degradation of electrochemical materials, as it led to a reduction in active sites and an increase in ion transport resistance. In contrast, after 25 cycles in the ZSO + 1.5 mM FA electrolyte (Fig. 4c), the surface was covered with uniform and fine aggregates of amorphous small particles. After 50 cycles (Fig. 4d), the zinc anode surface still maintained a dispersed small-particle morphology, with no large crystals formed and no obvious coarsening of grain size observed. This result confirmed that FA can regulate the deposition behavior of the zinc anode and inhibit the growth of zinc dendrites. Cross-sectional SEM images revealed that the surface of the zinc anode cycled in the ZSO electrolyte exhibited distinct rough features, with interfacial delamination and microcracks present in local regions. In contrast, the surface of the zinc anode cycled in the ZSO + 1.5 mM FA electrolyte was dense and flat, and no obvious interfacial delamination or cracks were observed at the interface. The elemental composition of the SEI was analyzed using an energy spectrometer (Fig. S26), which confirmed that elements such as S, Zn, C and O were all involved in the formation of the SEI. EIS measurements were performed on the Zn//Zn symmetric cells both before and after cycling. As shown in Fig. S27, the impedance of the FA electrolyte system was lower than that of the ZSO electrolyte system both before and after cycling, and the magnitude of impedance increase after cycling was smaller than that of the ZSO system. The above findings demonstrated that the FA additive could significantly optimize the ionic transport properties of the electrolyte for aqueous Zn²⁺ batteries, greatly enhanced the interfacial stability of the zinc anode, and inhibited the degradation of the battery interfacial structure during cycling.

Fig. 4 (a, b and e) SEM images of Zn anodes cycled in the ZSO electrolyte after different cycle numbers. (c, d and f) SEM images of Zn anodes cycled in the ZSO + 1.5 mM FA electrolyte after different cycle numbers. (g–j) XPS spectra of C 1s, O 1s, S 2p, and Zn 2p on the surface of the Zn anode after 100 cycles as a function of Ar⁺ sputtering time. (k) HRTEM images of the zinc anode after cycling in the ZSO + 1.5 mM FA system.

X-ray photoelectron spectroscopy (XPS) was employed to probe the chemical composition of the SEI layer on the zinc anode surface after 50 cycles in the ZSO + 1.5 mM FA electrolyte system, with depth-resolved analysis performed at different sputtering depths. Based on the elemental ratios at different sputtering depths (Fig. S28), it was observed that the atomic percentage of C 1s was approximately 47.93% at 0 s of etching and decreased significantly as the etching time increased to 40 s and 80 s. In contrast, the atomic percentages of O 1s and Zn 2p were about 32.82% and 17.38% at 0 s, respectively, and increased to approximately 40% after 40 s, where they remained stable. The atomic percentage of S 2p remained at a relatively low level throughout the measurement. In the C 1s spectrum (Fig. 4g), the presence of O–C=O, C–O, and C–C characteristic peaks at 0 s demonstrated that FA was effectively adsorbed onto the zinc surface via its carboxyl and hydroxyl functional groups, thereby forming a compact organic adsorption layer. As the sputtering time proceeded to 40 s and 80 s, the intensities of all characteristic peaks in the C 1s spectrum exhibited a gradual attenuation trend, which was attributed to the diminished decomposition of FA molecules at greater depths. Furthermore, inorganic zinc oxide (ZnO) and zinc sulfide (ZnS) were separately detected in the O 1s and S 2p spectra (Fig. 4h and i). After 40 s of Ar⁺ sputtering, the intensities of their characteristic peaks increased, indicating that both species were formed in the inner layer of the SEI. Additionally, with further Ar⁺ sputtering, the signal of SO₄²⁻ remained stable, demonstrating that inorganic compounds were mainly distributed in the inner layer of the SEI. The above XPS characterization results fully confirmed that FA could participate in the formation of the SEI layer and stably anchor at the interface area, ultimately forming an organic–inorganic composite SEI layer that combined the flexibility of the organic component with the stability of the inorganic component. High-resolution transmission electron microscopy (HRTEM) further elucidated the nature of the SEI on the cycled zinc anode. As illustrated in Fig. 4k, in the ZSO + 1.5 mM FA electrolyte, the organic components derived from FA decomposition form an amorphous layer on the surface of the zinc anode. Beneath this layer, the intersecting lattice fringes with d-spacings of 0.28, 0.19, 0.30, and 0.27 nm can be indexed to ZnO (100), ZnS (311), ZnSO₃, and ZnCO₃ (104), respectively. This organic–inorganic hybrid SEI played a pivotal role in protecting metallic zinc, effectively facilitating Zn²⁺ migration and suppressing the formation of zinc dendrites.

To assess the practical applicability of the ZSO + 1.5 mM FA electrolyte, a P-NH₄V₄O₁₀ (P-NVO) cathode was fabricated via a hydrothermal method, followed by the assembly of Zn//P-NVO full cells for systematic investigations into their electrochemical performances. Phase characterization and morphological analysis of the as-fabricated cathode were performed using XRD and SEM (Fig. S29 and S30). The results demonstrated that the material exhibited a typical nanosheet-like morphology without obvious bulk agglomeration, featuring excellent dispersibility and uniform overall distribution. The ZSO + 1.5 mM FA electrolyte remarkably improved the reversibility of Zn anode electrodeposition and stripping, a phenomenon directly manifested in the CV curves (Fig. 5a). Compared with the electrolyte without FA, the CV curve of the electrolyte containing FA displays a substantially larger integral area. Notably, the CV curve profile of the ZSO + 1.5 mM FA electrolyte system is consistent with that of the pristine ZSO system. This finding verified that the incorporation of FA merely optimized the electrochemical reaction kinetics of the Zn anode without modifying the intrinsic energy storage mechanism of the P-NVO cathode. EIS characterization based on Nyquist plots revealed that the total impedance of the full cell with the FA electrolyte was substantially lower than that of the ZSO counterpart (Fig. 5b). Notably, the reduced R_ct suggested that the incorporation of FA facilitated interfacial charge transfer in the full cell. Furthermore, the good consistency between Galvanostatic Intermittent Titration Technique (GITT) results and EIS data further verified the regulatory effect of FA incorporation on the ion diffusion coefficient of the system.

Fig. 5 (a) CV curves of Zn//P-NVO batteries with different electrolytes. (b) EIS Nyquist plots. (c) GITT curves. (d) Rate performance of Zn//P-NVO batteries and (e) their cycling performance at a current density of 2 A g⁻¹. (f) Corresponding GCD curves of the full cells after 500 cycles at 2 A g⁻¹. (g) Self-discharge performance of Zn//P-NVO batteries using ZSO and ZSO + 1.5 mM FA electrolytes. (h) LED lighting experiment.

The rate capability test (0.2–2 A g⁻¹) of the full cell demonstrated that the FA electrolyte full cell exhibited higher capacity and better capacity retention at both high and low current densities. This finding was further corroborated by the GCD curves obtained at different current densities (Fig. S31 and S32). The full cell with the FA electrolyte also exhibited excellent long-term cycling stability (Fig. 5d). After 1000 consecutive cycles at a current density of 2 A g⁻¹, it maintained a high specific capacity of 324.24 mAh g⁻¹, corresponding to an outstanding capacity retention rate of 85.6%. In sharp contrast, the Zn//P-NVO full cell using the pristine ZSO electrolyte only delivered a specific capacity of 113.76 mAh g⁻¹ under identical cycling conditions, with a capacity retention rate of merely 45.0%. The GCD curve at the 500th cycle further verified the excellent capacity retention of the FA-modified full cell (Fig. 5e). Additionally, the self-discharge behavior of the Zn//P-NVO battery was further evaluated to assess its long-term energy storage stability. The Zn//P-NVO full cell assembled with the ZSO + 1.5 mM FA electrolyte maintained 85.1% of its initial capacity after resting for 24 h (Fig. 5f). In sharp contrast, the counterpart using the pristine ZSO electrolyte only achieved a capacity retention rate of 50.7% under the same conditions. Notably, two Zn//P-NVO full cells configured with the ZSO + 1.5 mM FA electrolyte connected in series successfully powered an LED panel, demonstrating promising practical application potential (Fig. 5h).

4. Conclusion

In this work, a dual-function aqueous electrolyte system was successfully developed. Using ferulic acid (FA) derived from biomass as a functional additive, highly reversible and stable cycling of the zinc anode was achieved. Density functional theory (DFT) calculations verified that FA preferentially adsorbed on the Zn (002) crystal plane, which could effectively inhibit the hydrogen evolution reaction (HER) and dendrite growth. This electrolyte system exhibited excellent high-current performance, and the Zn//Zn symmetric cells operated stably for 1400 hours at 10 mA cm⁻² and 10 mAh cm⁻² and sustained over 600 hours of cycling at 60% depth of discharge (DOD). The Zn//Cu half-cells delivered a coulombic efficiency of 99.53% after 2000 cycles at 5 mA cm⁻². In addition, combined characterization via XPS and HRTEM further confirmed that the SEI layer formed in this system possessed an organic–inorganic bicomponent structure. This unique composite structure balanced the flexibility and mechanical stability of the interface and provided crucial support for suppressing dendrite growth and hydrogen evolution side reactions on the Zn anode. The Zn//P-NVO full cells retained 85.6% of their initial capacity after 1000 cycles at 2 A g⁻¹. Featuring environmental friendliness and low cost, this electrolyte system provided a feasible strategy for the industrialization of aqueous Zn²⁺ batteries, thereby facilitating the advancement of green energy storage technologies.

Author contributions

Pengcheng Song: writing – original draft, data curation, investigation. Zihan Wang: writing – review, methodology. Heshun Geng and Wenxuan Liu: methodology, data curation. Fang Hu: writing – review and editing, supervision, methodology, investigation, data curation, conceptualization. Yusheng Wu: project administration, formal analysis. Kai Zhu: investigation, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6qi00079g.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51974188) and the Natural Science Foundation of Liaoning Province (No. 2025-MS-116).

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