Dual-protection strategy for superior stability and performance of zinc powder-based anodes in aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) are an attractive alternative to lithium-ion batteries due to their safety, cost-effectiveness, and environmental friendliness. However, the commercialization of AZIBs is hindered by issues such as dendrite formation, side reactions, and poor utilization of zinc anodes. To address these challenges, we developed a dual-protection strategy incorporating reduced graphene oxide (rGO)-encapsulated zinc powder and a polyacrylic acid (PAA) binder. The rGO layer acts as a physical barrier, suppressing dendrite growth and minimizing side reactions, while the PAA binder enhances electrolyte affinity and ensures uniform zinc-ion deposition through hydrogen bonding. This synergistic system demonstrated exceptional electrochemical performance, achieving stable cycling with a significantly reduced overpotential. Symmetric cells exhibited prolonged cycle life exceeding 670 h at a high depth of discharge (33%) with minimal degradation. Additionally, full cells paired with ammonium vanadate nanofiber cathodes achieved high capacities and excellent retention, outperforming conventional zinc-powder-based anode configurations. This work provides a scalable and practical approach to improving the stability and performance of zinc powder-based anodes, offering a viable pathway toward next-generation energy storage systems.

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1. Introduction

Lithium-ion batteries (LIBs) have dominated the energy storage market, supporting applications ranging from portable electronics to electric vehicles, and laying a foundation for the sustainable development of next-generation technologies.^1–3 Despite their widespread adoption, LIBs face significant safety and environmental challenges, primarily due to the flammable organic electrolytes used, which increase the risk of fires and explosions. These issues have driven the search for safer and more environmentally friendly alternatives.^4–6 Aqueous zinc-ion batteries (AZIBs) have emerged as a promising solution due to their high theoretical capacity (820 mA h g⁻¹), low standard reduction potential (−0.76 V vs. SHE), excellent atmospheric stability, and cost-effectiveness. Additionally, aqueous electrolytes are safer and more environment-friendly than organic electrolytes.^7,8 However, significant challenges associated with the anode materials—including dendrite growth, side reactions, and limited cycle life—have hindered the commercialization of AZIBs.

Zinc foil, the most commonly used anode material in AZIB research, suffers from rapid dendrite formation due to its two-dimensional planar structure, which promotes vertical zinc-ion deposition and creates areas of high local current density. This leads to the uncontrolled growth of dendrites, which can physically connect the anode and cathode, resulting in short circuits that severely reduce the battery's lifespan.^9,10 Additionally, the thick structure of zinc foil contributes to low utilization rates, as only the surface of the foil is actively involved in the electrochemical reactions, while the inner material remains unused. This results in shallow depths of discharge (DOD < 3%), causing significant resource wastage.^9–11 Surface coating strategies, such as applying carbon or polymeric layers, have been employed to address these issues by improving zinc-ion deposition uniformity and suppressing dendrite growth.^12,13 While these approaches have demonstrated increased zinc utilization in symmetric cells (Table S1†), they often suffer from significantly reduced cycling stability under high DOD conditions. This trade-off arises because deeper zinc stripping accelerates dendrite formation and side reactions, even in symmetric configurations. In full-cell systems, the situation becomes even more challenging because a higher degree of zinc stripping from the anode necessitates a proportional cathodic capacity to fully accommodate the generated Zn²⁺ ions. As a result, the cathode must be substantially thickened, making it difficult to sustain high electrochemical performance at the full-cell level. Therefore, despite high DOD values observed in symmetric cell tests, Zn foil anodes face inherent limitations in realizing long-life and practically scalable full-cell systems. Thinning the zinc foil is a straightforward solution to increase DOD and material utilization. However, producing ultra-thin zinc foil remains both technologically challenging and cost-prohibitive, limiting its practical application.^12,13

Recently, zinc powder-based electrodes have gained attention as potential alternatives to zinc foil. Zinc powder offers better tunability and higher industrial value due to its versatility and cost-effectiveness. These advantages, combined with zinc's intrinsic safety, make it suitable for advanced applications such as flexible 3D-printed batteries.^14–17 However, zinc powder-based anodes face their own challenges that limit their practical application. The high surface area of zinc powder accelerates unwanted side reactions, such as corrosion and passivation, at the zinc-electrolyte interface, leading to the formation of by-products that hinder reversible redox reactions.^18–21 Furthermore, zinc's hydrophobic nature results in poor electrolyte affinity, promoting localized zinc-ion nucleation and exacerbating dendrite growth.^22–24

To improve the affinity of zinc powder-based anodes, hydrophilic polymers have been proposed as binders, as they enhance the interaction between the electrode and the aqueous electrolyte, facilitating uniform ion diffusion. However, these binders are prone to dissolution in aqueous environments, leading to the degradation of electrode integrity over time.^22–24 Consequently, hydrophobic polymers are more commonly employed as binders, as they resist dissolution in aqueous electrolytes. However, their low affinity for the electrolyte hinders uniform zinc-ion deposition and overall electrode performance.^25–27 Additionally, the large surface area of zinc powder amplifies irreversible side reactions at the zinc-electrolyte interface, including corrosion and passivation. These reactions result in the formation of by-products that block reversible redox reactions of Zn²⁺ ions and inhibit uniform zinc deposition.^15,28–30 To mitigate these side reactions, hydrophobic materials such as ethylene vinyl acetate (EVA)–carbon nanotube (CNT) composites, carbon, and graphene have been employed to form protective barriers that prevent direct contact between zinc powder and water.^16,29,31,32 While effective in suppressing side reactions, these strategies often reduce electrode wettability, limiting zinc utilization rates. Moreover, the implementation of these protective layers frequently requires high-temperature or high-pressure processes, which increase production complexity and costs.²⁵

To address these challenges, we developed a dual-protection matrix composed of reduced graphene oxide (rGO)-encapsulated zinc powder and a hydrophilic polyacrylic acid (PAA) binder. The rGO protective layer suppresses dendrite growth and minimizes side reactions by preventing direct contact between the zinc powder and the electrolyte. Meanwhile, the PAA binder enhances electrolyte affinity, which facilitates more uniform and stable zinc-ion deposition. In addition, the residual functional groups of rGO form hydrogen bonds with PAA, reducing binder dissolution and maintaining strong adhesion between electrode components. These synergistic effects significantly improve the electrochemical stability and cycle life of zinc powder-based anodes. Our dual-protection matrix-incorporated anode demonstrated exceptional performance under a DOD 33% condition, maintaining stable voltage profiles for over 670 h at 2 mA cm⁻² and 2 mA h cm⁻² in symmetric cells. The stable voltage profile was maintained even at a high charge/discharge rate of 10C. Furthermore, a full cell paired with a vanadium-based cathode exhibited a high specific capacity of 147 mA h g⁻¹ at 20 A g⁻¹ and retained 80% of its capacity after 2000 cycles at 5 A g⁻¹. These results highlight the practicality and scalability of our approach for advancing AZIB technology.

2. Experimental section

2.1. Preparation of reduced graphene oxide (rGO)-encapsulated zinc powder (rGO-X)

The encapsulation process of reduced graphene oxide (rGO) onto zinc powder is illustrated in Fig. 1a. A 5 mg mL⁻¹ aqueous dispersion of graphene oxide (GO, Angstron Materials) was diluted with deionized (DI) water to obtain a 1 mg mL⁻¹ GO dispersion. The pH of the GO dispersion was adjusted to approximately 4 using dilute hydrochloric acid (HCl, 35–37%, Daejung Chemicals) to enhance the reduction reaction. Subsequently, 2 g of zinc powder (<45 μm, Sigma Aldrich) was mixed with the pH-controlled GO dispersion in different amounts (0.1, 0.5, 1, and 1.5 wt%). The mixture was stirred for 1 min to ensure uniform interaction between the zinc powder and GO. The resulting mixture was washed three times with DI water to remove residues, ensuring the purity of the encapsulation. The remaining DI water solvent was evaporated using a Thinky mixer at 2000 rpm for 15 minutes, followed by vacuum drying at 60 °C for 2 h. The obtained rGO-encapsulated zinc powders were labeled as rGO-X, where X represents the weight percentage of GO (0.1, 0.5, 1, or 1.5 wt%).

Fig. 1 (a) Schematic illustration of the rGO encapsulation process for rGO-encapsulated zinc powder (rGO-X). (b) Digital and SEM images comparing bare Zn powder (left) and rGO-0.5 (right).

2.2. Preparation of ammonium vanadate nanofiber (AVNF)

Ammonium vanadate nanofiber (AVNF, (NH₄)₂V₆O₁₆·1.5H₂O) was synthesized using a simple sonochemical method, as introduced in previous research.³³ A yellow dispersion was prepared by dissolving 0.4 g of vanadium pentoxide (V₂O₅, Sigma Aldrich) and 0.2 g of ammonium persulfate (APS, Sigma Aldrich) in 100 mL of DI water. The dispersion was then subjected to ultrasonic treatment for 30 minutes using an ultrasonic generator equipped with a titanium tip, operating at a frequency of 20 kHz and a power output of 500 W. After sonication, the color of the solution changed from yellow to deep red, indicating the conversion of V₂O₅ to AVNF solutions. The reaction involves the reduction of V⁵⁺ to mixed-valence V⁴⁺/V⁵⁺ species, facilitated by the cavitation effect of ultrasound. This process leads to the formation of ammonium vanadate nanostructures, where NH₄⁺ ions stabilize the framework and promote the formation of hydrated nanofibers with enhanced electrochemical properties. The solid AVNF was obtained through a filtration process using ethanol and DI water, followed by vacuum drying at 60 °C for 12 h to stabilize the crystal structure. The crystalline phase and structure of the product were analyzed via X-ray diffraction (XRD) measurements (Fig. S1a†).

2.3. Characterization

The crystal structures were investigated using X-ray diffraction (XRD, MiniFlex600, Rigaku) with Cu Kα radiation (λ = 0.15406 nm), scanned from 20° to 80° at a rate of 2° min⁻¹ for Zn-based samples and from 5° to 60° at a rate of 5° min⁻¹ for AVNF to capture relevant diffraction peaks. Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS5, Thermo Fisher Scientific) was performed in the spectral range of 4000–500 cm⁻¹ to verify the presence of hydrogen bonding between polyacrylic acid (PAA) and reduced graphene oxide (rGO). Raman spectroscopy (DXR3xi, Thermo Fisher Scientific) was carried out with a 532 nm excitation laser to analyze structural defects and graphitic characteristics by measuring the I_D/I_G ratio. X-ray photoelectron spectroscopy (XPS, Theta Probe, Thermo Fisher Scientific) was used to analyze the chemical bonding states and elemental composition of the samples, utilizing an Al Kα X-ray source (hν = 1486.7 eV). Field-emission scanning electron microscopy (FE-SEM, Apreo S Hivac, and Verios G4 IC, FEI) was employed to examine the surface morphology and cross-sectional structures of bare and rGO-encapsulated zinc powder. Cross-sectional observations were prepared using a polisher (M1061, FISCHIONE). The thickness of the rGO layer was measured using high-resolution transmission electron microscopy (HR-TEM, JEM 2100F, JEOL), providing nanoscale detail of the encapsulation layer. CHNS Elemental analysis (FLASH EA1112, Thermo Fisher Scientific) was conducted to measure the ratio of the rGO layer on the zinc powder. The electrical conductivity of the rGO-encapsulated zinc powder was measured using a 4-point probe (CMT-100M).

2.4. Electrochemical measurements

To fabricate the anodes with hydrophobic and hydrophilic binders, bare zinc powder, carbon black, and poly(vinylidene fluoride) (PVDF) or poly(acrylic acid) (PAA) binder were mixed in a mass ratio of 8 : 1 : 1 in N-methyl pyrrolidone (NMP) solvent. The mixed slurry was cast onto stainless steel foil (current collector) using a doctor blade. The cast electrodes were dried overnight in a vacuum oven at 110 °C for electrodes with PVDF binder and at 80 °C for electrodes with PAA binder. The bare Zn powder electrodes with PVDF and PAA binders were labeled as ZPV and ZPA, respectively. The rGO-encapsulated Zn powder (rGO-X) electrodes with PVDF and PAA binders were labeled as rGO-X-ZPV and rGO-X-ZPA, respectively. After drying, the electrodes were pressed using a roll press and punched into 14 mm disks. The average mass loading of the anodes was approximately 4.55 mg cm⁻². The cathode was fabricated using a process similar to that of the anodes. AVNF, carbon black, and PVDF were mixed in a mass ratio of 7 : 2 : 1 with NMP solvent to form a slurry. The slurry was cast onto stainless steel foil and dried overnight in a vacuum oven at 110 °C. The dried cathodes were punched into 14 mm disks, with an average mass loading of 0.9–1.0 mg cm⁻². All coin-type cells tested were assembled in CR2032 cells with a 2.5 M zinc trifluoromethanesulfonate (Zn(OTf)₂) and glass fiber separators of 19 mm diameter (Whatman GF/A). The assembled cells were aged for 24 h at 25 °C before electrochemical testing. Galvanostatic charge/discharge of symmetric and full cells was carried out on a WBCS3000S battery cycler (WonA-Tech) at 25 °C. The symmetric cell was operated with a sequence of discharging followed by charging, with cut-off voltages of −2.1 V and 2.1 V. In the full cell, the cut-off voltage was set to 0.2 V and 1.8 V. Cycle performance was measured at a current density of 5 A g⁻¹ following three pre-cycles at 0.5, 1, and 3 A g⁻¹ to stabilize the cell. Linear sweep voltammetry (LSV) was performed on a ZIVE SP1 potentiostat analyzer (WonA Tech, Korea) at a scan rate of 1 mV s⁻¹. The LSV curve was analyzed by assembled CR2032 coin cells with stainless steel foil as a working electrode. Cyclic voltammetry (CV) tests were conducted at a scan rate of 0.1 mV s⁻¹ over a voltage range of 0.2–1.8 V versus Zn²⁺/Zn. Electrochemical impedance spectroscopy (EIS) was carried out using a ZIVE SP1 impedance analyzer (WonA Tech, Korea) in the frequency range of 10⁵–10⁻¹ Hz, and the data were analyzed using an equivalent circuit model.

3. Results and discussion

To prevent side reactions such as passivation and dendrite formation, the zinc powder was encapsulated with an rGO protective layer through a simple mixing process using spontaneous reduction.³⁴ The adjusted GO dispersion (pH ≈ 4) has a higher standard reduction potential (−0.4 V vs. SHE) than zinc (−0.76 V vs. SHE).³⁵ Thus, the potential difference is positive, enabling the spontaneous reduction of GO. The reaction proceeds as follows:

(Zn + GO + 2H⁺ → Zn²⁺ + rGO + H₂O) (1)

Fig. 1a illustrates the rGO encapsulation process on the zinc powder. When the zinc powder was immersed in the pH-adjusted GO dispersion and stirred, a spontaneous redox reaction occurred at the interface between the GO sheets and the zinc powder. Subsequently, the GO sheets were reduced to form an rGO protective layer on the zinc powder. As the rGO encapsulated the Zn powders, the Zn particles precipitated out of the dispersion, leading to a change in the dispersion's appearance. Specifically, the initial brown GO dispersion became transparent, indicating that the GO sheets had been fully consumed and deposited as rGO layers on the Zn powder. To determine the actual amount of rGO coated on the Zn powder surface, CHNS elemental analysis was performed. Fig. S2† presents the CHNS results for bare Zn powder and rGO-X samples (X = 0.1, 0.5, 1.0, and 1.5 wt%). As the rGO content increased, the carbon-to-zinc ratio showed a proportional rise, closely matching the designed rGO loading levels. The corresponding carbon increments (ΔCarbon) were 0.11, 0.61, 1.17, and 1.85 wt%, respectively, aligning well with the nominal loading amounts within the measurement error range (±0.1–0.3 wt%). These results indicate that the graphene oxide in the GO dispersion was effectively reduced through a spontaneous redox reaction with the Zn powder and predominantly encapsulated the Zn particle surfaces. The rGO-encapsulated zinc powders were then dried and used directly as the active material. Fig. 1b shows SEM and digital images of the bare Zn powder and rGO-0.5 sample, highlighting the changes in the zinc powder's surface morphology due to the rGO encapsulation layer. The initially gray and rough surface of the bare zinc powder was transformed into a darker and smoother surface, attributed to the formation of a uniform rGO coating. To investigate the effect of rGO content on surface morphology, additional SEM analysis was performed on the rGO-0.1, rGO-1.0, and rGO-1.5 samples. As shown in Fig. S3,† for rGO-0.1 (Fig. S3a†), the Zn surface was not completely coated, and discontinuities in the rGO layer were observed. In contrast, both rGO-1.0 (Fig. S3b†) and rGO-1.5 (Fig. S3c†) exhibited thicker coating layers than rGO-0.5, but the surface coverage became increasingly non-uniform, likely due to rGO aggregation or uneven deposition during coating. Among the tested samples, rGO-0.5 exhibited the most uniform and smooth surface morphology, indicating well-distributed and conformal rGO coverage.

To determine the optimal rGO content for encapsulation, the cycle life and voltage hysteresis of symmetric cells composed of rGO-X, carbon black, and a PVDF binder were analyzed. As shown in Fig. S4,† the cycle life improved from 98 h to 182 h, and the voltage hysteresis decreased from 33.95 mV to 24.35 mV as the rGO content increased from 0 to 0.5 wt%. This improvement is attributed to the rGO-encapsulated layer, which mitigates side reactions between the zinc powder and the electrolyte and suppresses dendrite formation during repeated Zn-ion stripping/plating. Consequently, rGO-encapsulated Zn powder facilitates reversible redox reactions. However, when the rGO content exceeded 0.5 wt%, the cycle life decreased, and the voltage hysteresis increased.

To further validate the optimal rGO content, rate performance tests were conducted on symmetric cells assembled with rGO-X-ZPV electrodes. Fig. S5† shows voltage profiles of symmetric cells with rGO-X-ZPV electrodes cycled at various current densities ranging from 0.5 to 10 mA cm⁻² with a fixed areal capacity of 1 mA h cm⁻². Among the tested compositions, the rGO-0.5-ZPV electrode exhibited the most stable voltage profiles and the lowest polarization across all current densities, demonstrating excellent rate capability. In contrast, the rGO-1.0-ZPV and rGO-1.5-ZPV electrodes showed increased voltage hysteresis at high current densities, which can be attributed to the thicker and less conductive rGO layers that hinder charge transfer. Meanwhile, the rGO-0.1-ZPV electrode exhibited higher overpotentials than rGO-0.5-ZPV even at low current densities, indicating insufficient surface encapsulation and poor interfacial stability. In addition, contact angle measurements were performed to evaluate the electrolyte wettability of the rGO-X-ZPV electrodes. As shown in Fig. S6,† the contact angle progressively decreased with increasing rGO content—60° for rGO-0.1-ZPV, 46° for rGO-0.5-ZPV, 31° for rGO-1.0-ZPV, and 22° for rGO-1.5-ZPV—demonstrating enhanced hydrophilicity of the electrode surface. This improvement is attributed to residual oxygen-containing functional groups in partially reduced rGO, which promote favorable interactions with the aqueous electrolyte. However, despite improved wettability at higher rGO loadings, excessive coating thickness and aggregation negatively impact charge transport. Beyond 0.5 wt% rGO, the disordered stacking of rGO layers introduces additional resistance to both ionic and electronic conduction. This is supported by the EIS results in Fig. S7 and Table S2,† where rGO-0.5-ZPV exhibited the lowest charge transfer resistance (R_ct) of 7.12 Ω, whereas rGO-1.0-ZPV and rGO-1.5-ZPV showed substantially higher R_ct values of 25.58 Ω and 29.44 Ω, respectively. These results demonstrate that the 0.5 wt% rGO provides the optimal balance, ensuring sufficient surface coverage for chemical protection and wettability, while preserving a continuous and conductive network for efficient charge transfer.

To better understand the correlation between electrochemical performance and rGO loading, additional structural and electrical analyses were conducted. As shown in Fig. S8,† TEM analysis revealed a progressive increase in the thickness of the rGO encapsulation layer with increasing rGO content. Thicker rGO coatings are likely to introduce additional interfacial resistance, which can hinder charge transfer during Zn-ion cycling. This observation is consistent with the increased voltage hysteresis and reduced rate performance seen in the rGO-1.0 and rGO-1.5 samples. To quantitatively assess the impact on electrical conductivity, four-point probe measurements were performed for Zn powders with varying rGO loadings. The results, summarized in Table S3,† indicate a decreasing trend in electrical conductivity with increasing rGO content. While bare Zn powder exhibited a conductivity of 109.52 S m⁻¹, this value declined to 65.71 S m⁻¹ for rGO-1.5. This reduction is attributed to both the contact resistance between stacked rGO layers and the inherently lower conductivity of disordered rGO compared to metallic Zn. These findings further confirm that 0.5 wt% rGO encapsulation provides the optimal balance between surface protection and electrical conductivity. While higher rGO contents (1.0 and 1.5 wt%) introduce thicker coatings that impede electron transport and reduce direct contact between Zn and the electrolyte, insufficient rGO (0.1 wt%) results in incomplete encapsulation and poor interfacial stability.³⁵ In contrast, the rGO-0.5 composition enables conformal and uniform coating with minimal resistance, thereby achieving the longest cycle life, the lowest voltage hysteresis, and the most stable electrochemical performance among the tested samples.

XRD analysis was performed to confirm the changes in the crystal structure of zinc powder during the rGO encapsulation process (Fig. 2a). The XRD patterns of rGO-0.5 showed no differences before and after the encapsulation, and no detectable peaks corresponding to ZnO by-products were observed. This indicates that there is no side reaction between Zn and H₂O during the encapsulation process, as the graphene layer effectively inhibits such reactions. Raman spectroscopy and XPS analysis were conducted to verify the spontaneous reduction of GO and its structural state. Fig. 2b shows the Raman spectra of GO and rGO-0.5. The peaks observed at 1347 cm⁻¹ and 1590 cm⁻¹ are assigned to the D band, which represents the vibrations of sp³ hybridized carbon, and the G band, which represents the in-plane vibrations of sp² carbon atoms, respectively.^36,37 The intensity ratio of the two bands (I_D/I_G) was 1.15 for rGO-0.5, which is higher than that for GO. The increase in the I_D/I_G ratio indicates the removal of functional groups during the reduction of GO, resulting in the restoration of the sp² structure and the formation of small-scale graphite domains that are smaller than the original sp² domains in GO.³⁸ These small-scale sp² graphite domains have a higher degree of disorder and more edge planes, causing the D band intensity to surpass that of the G band in rGO-0.5. This observation confirms that the reduction of GO was successfully achieved during the encapsulation process. Fig. 2c shows the XPS C 1s spectra of GO and rGO-0.5. The four peaks observed at 284.5, 286.6, 288.1, and 289.0 eV correspond to C–C/C=C, C–O, C=O, and O–C=O, respectively. Upon the reduction of GO to rGO, the intensities of the C–O, C=O, and O–C=O peaks significantly decrease, indicating the removal of oxygen-containing functional groups during the reduction process.

Fig. 2 (a) XRD patterns and (b) Raman spectra of bare Zn powder and rGO-0.5. (c) XPS C 1s spectra of the GO and rGO-0.5. (d) SEM images of bare Zn powder and rGO-0.5 after immersion. (e) XRD patterns of bare Zn powder and rGO-0.5 before and after immersion in the electrolyte for 7 days. (f–h) Cross-sectional SEM images of ZPV electrodes at the initial state, after the stripping process, and after the following plating process. (i–k) Cross-sectional SEM images of rGO-0.5-ZPV electrodes under the same conditions.

To evaluate the rGO encapsulation layer as a protective layer for zinc powder, we conducted the immersion test and observed cross-sectional SEM images after Zn-ion stripping/plating. In the immersion test, zinc powder and rGO-0.5 were immersed in a 2.5 M zinc trifluoromethanesulfonate electrolyte for 7 days to observe the microstructural and phase changes. As shown in Fig. 2d, the surface of the zinc powder displayed the formation of numerous plate-like structures that were absent prior to immersion. This indicates the formation of ZnO by-products due to reactions with the electrolyte. In contrast, the rGO-0.5 sample maintained its spherical shape with no significant surface changes after immersion, suggesting that the rGO layer effectively inhibited such reactions. XRD analysis further revealed clear distinctions between the two samples after the immersion test. As shown in Fig. 2e, the zinc powder exhibited prominent peaks corresponding to the ZnO phase, indicating side reactions with the electrolyte. However, the rGO-0.5 sample showed no peaks other than those for zinc, and no differences were observed between the pre-immersion and post-immersion tests. These results suggest that the rGO layer effectively protects the zinc powder from the electrolyte and maintains its initial state even after prolonged exposure. Fig. 2f–k present cross-sectional SEM images of the ZPV and rGO-0.5-ZPV electrodes in their initial state, after the stripping process at 1 mA cm⁻², 1 mA h cm⁻², and after the following plating process at 1 mA cm⁻², 1 mA h cm⁻². Initially, a thin film structure was observed on the rGO-0.5-ZPV electrode, indicating the presence of the rGO layer on the zinc powder surface, which was not observed for the bare zinc powder (Fig. 2f and i). After deposition following the stripping process, the bare zinc powder exhibited significant deformation of its spherical shape due to uncontrolled dendrite growth (Fig. 2h). In contrast, the rGO-0.5 sample maintained a spherical morphology with a thin zinc layer formed beneath the graphene layer (Fig. 2k). This layer was not observed during the stripping, suggesting that the graphene layer physically suppressed dendrite growth during zinc ion deposition (Fig. 2i). These findings highlight the effectiveness of the rGO layer in maintaining the integrity of the zinc powder and preventing dendrite formation.

In aqueous zinc-ion battery anodes, the affinity of the anode for electrolytes is an important factor that significantly affects the electrochemical performance. A high affinity of hydrophilic electrodes allows the electrolyte to penetrate the electrode more easily and facilitates ion diffusion and the ion transfer kinetics of Zn²⁺.³⁹ Additionally, it minimizes the localized deposition of Zn²⁺ by reducing the interfacial energy between the anode and electrolyte, therby extending the lifespan of zinc powder-based anodes.^18–21Fig. 3a–d shows the electrolyte affinity of electrodes with different active materials and binders. The PVDF binder and the zinc powder both exhibit low hydrophilicity, resulting in a contact angle of 120° for the ZPV electrode (Fig. 3a), which is higher than that for zinc foil (72°) (Fig. S9†). The rGO-0.5-ZPV electrode showed a significantly reduced contact angle of 46° (Fig. 3b), due to the residual hydrophilic functional groups of rGO. In contrast, the contact angles of the ZPA and rGO-0.5-ZPA electrodes were drastically reduced to 15° and 10°, respectively, following the replacement of the binder with polyacrylic acid (PAA) (Fig. 3c and d). However, hydrophilic binders such as PAA have a critical issue in that they tend to dissolve easily in aqueous electrolytes, which weakens the binding of the electrode. Therefore, while hydrophilic binders can be a promising strategy for improving the lifespan of anode, their application remains challenging. To address this issue, the carboxyl groups in PAA were utilized to form hydrogen bonds with the rGO-encapsulated Zn powder, thereby preventing the dissolution of PAA in the electrolyte and maintaining its adhesion to the electrode components. Since Zn powder lacks functional groups capable of hydrogen bonding, it was encapsulated with reduced graphene oxide (rGO), which not only protects Zn powder from dendrite formation and passivation but also possesses residual functional groups capable of hydrogen bonding. FT-IR spectroscopy was performed to verify the hydrogen bonding between PAA and rGO-encapsulated Zn powder. Fig. 3e shows FT-IR spectra of PAA, rGO-0.5, and rGO-0.5-ZPA. The C=C stretching peak at 1629 cm⁻¹ was consistent for both PAA and rGO-0.5. However, the C=O stretching peak of PAA was observed at 1707 cm⁻¹ in rGO-0.5-ZPA, which is lower than that of PAA (1716 cm⁻¹). This peak shift is due to the formation of hydrogen bonds between PAA and rGO.^40–42 Additionally, immersion tests were conducted to observe changes in the electrode caused by the dissolution of the PAA binder. Fig. 3f and g show digital images of the ZPA and rGO-0.5-ZPA electrodes before and after immersion in electrolyte for one day. In the case of ZPA, the electrode was partly detached from the current collector due to the weakened binding property of PAA (Fig. 3f). However, the rGO-0.5-ZPA electrode remained stably attached to the current collector without any noticeable changes after immersion (Fig. 3g). This demonstrates that while hydrophilic binders are susceptible to dissolution in aqueous electrolytes, the rGO layer prevents this by forming hydrogen bonds with the active material. Consequently, rGO-0.5-ZPA electrodes exhibit improved stability and adhesion, showcasing the potential of hydrophilic polymers as binders for aqueous zinc-ion battery anodes.

Fig. 3 Contact angle measurements of electrolyte on (a) ZPV, (b) rGO-0.5-ZPV, (c) ZPA, and (d) rGO-0.5-ZPA electrodes. (e) FT-IR spectra of PAA, rGO-0.5-ZPA, and rGO-0.5. (f and g) Digital images of ZPA and rGO-0.5-ZPA electrodes before and after immersion tests. (h–j) XPS survey spectrum of the rGO-0.5-ZPA-immersed solution dried on a Si substrate.

To further validate the role of hydrogen bonding between PAA and rGO-encapsulated zinc powder in preventing PAA dissolution, an immersion test was performed on ZPA, rGO-0.5-ZPA, and rGO-0.5-ZPV electrodes. This follows the FT-IR results in the previous section, which demonstrated the formation of hydrogen bonds between PAA and rGO. By comparing the mass variation and chemical composition of the electrodes before and after immersion, the effectiveness of these hydrogen bonds in stabilizing the binder was investigated. For the test, the electrodes were immersed and stored at 25 °C for 1 day. Fig. S10 and Table S4† show the mass variation of the electrodes before and after the immersion test. The weights of the ZPA and rGO-0.5-ZPA electrodes decreased from 31.441 mg to 31.067 mg and from 31.278 mg to 31.230 mg, respectively. Assuming that the mass loss is primarily due to the dissolution of the hydrophilic polymer binder PAA, the PAA-based mass loss rates were calculated as approximately 52.67% for the ZPA electrode and 6.92% for the rGO-0.5-ZPA electrode, respectively. In comparison, the rGO-0.5-ZPV electrode, which used a hydrophobic polymer binder (PVDF), exhibited a minimal mass reduction of 0.14% (31.275 mg to 31.232 mg). As PVDF is insoluble in aqueous electrolytes, this mass loss is attributed to the dissolution of other minor components, such as active material or carbon additives, rather than the binder itself. Using this baseline, the additional mass loss rates for the ZPA and rGO-0.5-ZPA electrodes relative to the rGO-0.5-ZPV electrode were calculated as 46.47% and 0.72%, respectively. These values indicate that the mass loss of the rGO-0.5-ZPA electrode is primarily suppressed by the hydrogen bonding between PAA and rGO, whereas the significant 46.47% mass loss in the ZPA electrode is mainly attributed to the dissolution of the PAA binder, demonstrating the critical limitations of hydrophilic polymer binders like PAA.

Subsequently, to detect the dissolution of PAA from the electrodes, 0.5 mL of the solution used to immerse each electrode was dropped onto a Si substrate and dried in a vacuum environment at 60 °C for XPS analysis. Fig. 3h–j shows the XPS survey spectrum of each Si substrate. The Si substrate dried with the solution from the ZPA electrode immersion exhibited a prominent C 1s peak intensity, suggesting dissolution of PAA. For the Si 2p spectrum, the peak intensity for the Si substrate from the ZPA electrode immersion was significantly lower than that of the other two samples. This reduction is attributed to the dissolved PAA coating the Si substrate, which attenuates the Si signal. Conversely, the strong Si 2p signals observed for the rGO-0.5-ZPA and bare Si samples indicate that the dissolution of PAA was effectively suppressed. These results suggest that the significant mass change observed for the ZPA electrode during the immersion test is primarily attributed to the dissolution of the PAA binder into the solution. In contrast, the hydrogen bonding between the rGO coating layer and PAA effectively prevents PAA dissolution, maintaining the integrity and stability of the rGO-0.5-ZPA electrode.

The galvanostatic test of symmetric cells was conducted to evaluate the cycle and rate capability of the ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes. Fig. 4a and S11† show the rate capability tested at current densities of 0.5–10 mA cm⁻² with a fixed capacity of 1 mA h cm⁻², corresponding to a DOD of 33%. As the current density increased, the voltage profile of ZPV was significantly higher compared to the other electrodes. When the current density reached 10 mA cm⁻², the potential of ZPV exceeded the cut-off voltage, showing an unstable voltage profile. Even after the current density was decreased to 0.5 mA cm⁻², the potential remained unstable and shortly thereafter exceeded the cut-off voltage again. In contrast, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes demonstrated low and stable voltage profiles of 0.19 V, 0.17 V, and 0.14 V at a current density of 10 mA cm⁻², respectively. Their stability was maintained even when the current density was decreased to 0.5 mA cm⁻². Although hydrophilic binders like PAA are generally susceptible to dissolution in aqueous electrolytes, the ZPA electrode demonstrated better cycling performance than the PVDF-based ZPV electrode. This result can be attributed to the enhanced surface wettability and favorable interfacial ion transport properties imparted by the carboxylic groups in PAA. These features facilitate uniform Zn²⁺ flux, reduce interfacial polarization, and lower charge-transfer resistance. These interfacial advantages contribute to the improved plating/stripping reversibility of ZPA, despite the intrinsic solubility concerns associated with hydrophilic binders. This performance is further enhanced in the rGO-0.5-ZPA electrode, where hydrogen bonding between PAA and the rGO coating layer helps retain the binder and maintain structural integrity under prolonged cycling conditions. Fig. 4b presents the cycle capability measured at 1 mA cm⁻² and 1 mA h cm⁻² under the same DOD condition. The rGO-0.5-ZPA electrode exhibited an extended cycle capability with over 330 h cycle life, whereas the voltage of ZPV, ZPA, and rGO-0.5-ZPV gradually increased and exceeded the cut-off voltage at 100 h, 152 h, and 185 h, respectively. Furthermore, the cycling capability of rGO-0.5-ZPA was further enhanced under the DOD 33% condition at a higher capacity density of 2 mA cm⁻² and 2 mA h cm⁻² (Fig. 4c). While rGO-0.5-ZPA maintained an extended cycle capability over 670 h, ZPV, ZPA, and rGO-0.5-ZPV exhibited a distinct tendency of increasing potential and failure at 150 h, 318 h, and 480 h, respectively. To further examine the high-rate capability of the electrodes, symmetric cell cycling tests were conducted at an elevated current density of 3 mA cm⁻² with an areal capacity of 1 mA h cm⁻² (Fig. S12†). Under these demanding conditions, the rGO-0.5-ZPA electrode demonstrated the most stable voltage profile and the longest cycle life among all tested samples. These results suggest that the improved reversibility of Zn-ion stripping/plating is due to the synergistic effects of the rGO protective layer and the PAA binder. The rGO protective layer inhibits passivation and dendrite formation, which are the primary causes of the short cycle capability in zinc-ion battery anodes. Additionally, the hydrophilic binder PAA maintains strong adhesion between the electrode components and improves the affinity of electrodes for electrolytes, facilitating uniform Zn-ion diffusion and deposition.

Fig. 4 (a) Rate performance and (b) cycle performance of symmetric cells with the ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes at 1 mA cm⁻² and 1 mA h cm⁻². (c) Cycle performance at 2 mA cm⁻² and 2 mA h cm⁻².

To further investigate the effects of the rGO encapsulation layer and PAA binder on the cycle performance, the electrodes were examined using XRD and SEM after 25 and 50 cycles. Fig. 5a–d shows the XRD patterns of ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes at the Initial state, 25 cycles, and 50 cycles. By-products resulting from the reaction of the Zn powder and the electrolyte during charge/discharge cycles were observed. The XRD pattern of ZPV shows peaks corresponding to ZnO (32°, 34°, 57°, and 63°) and Zn_xOTf_y(OH)_2x–y·nH₂O (22°, 33°, and 59°), which appear at 25 cycles and become more pronounced at 50 cycles (Fig. 5a).⁴⁷ The reaction equation of the ZnO and Zn_xOTf_y(OH)_2x–y·nH₂O is as follows:

Zn + 4OH⁻ → Zn(OH)₄²⁻ + 2e⁻ → ZnO + 2OH⁻ + H₂O (2)

Zn²⁺ + a_y/2Zn(OTf)₂ + 2OH⁻ + anH₂O ↔ Zn_xOTf_y(OH)_2x–y·nH₂O (a(2x − y) = 2) (3)

Fig. 5 XRD patterns of (a) ZPV, (b) ZPA, (c) rGO-0.5-ZPV, and (d) rGO-0.5-ZPA anodes at the initial, after 25 cycles, and after 50 cycles. (e) LSV curves of asymmetric cells with stainless steel as the cathode. (f and g) EIS spectra of symmetric cells with ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes at the initial state and after 50 cycles.

ZnO is a non-conductive and irreversible by-product that forms in an uncontrolled morphology on the zinc surface, reducing the conductivity of the active material and impeding Zn-ion redox reactions.^28,43 Zn_xOTf_y(OH)_2x–y is a reversible by-product that reverts as the pH decreases, helping to maintain a constant pH.^44–47 However, due to the continuous consumption of H⁺ ions by the hydrogen evolution reaction (HER), the elevated pH makes Zn_xOTf_y(OH)_2x–y an irreversible by-product similar to ZnO, blocking access to zinc ions and impeding redox reactions.⁴⁸ Due to the continuous zinc consumption by the side reactions, it is difficult to achieve high utilization rates and reversible Zn-ion stripping/plating. In the XRD pattern of ZPA, the by-product peaks detected in ZPV were not observed at the initial state and 25 cycles, but they became detectable at 50 cycles (Fig. 5b). The carboxylic group of PAA interacts strongly with the Zn solvation shell, replacing water molecules and promoting zinc ion de-solvation, which delays HER and side reactions.^24,49 However, repeated Zn-ion stripping/plating causes volume changes and PAA dissolution, leading to detachment of PAA from the zinc powder, and eventually allowing side reactions similar to those in ZPV. The XRD pattern of rGO-0.5-ZPV was similar to that of ZPA. While no by-products were detected at 25 cycles, their peaks became distinct at 50 cycles (Fig. 5c). This indicates that the rGO layer acts as a protective barrier, delaying HER and side reactions by preventing direct contact between the zinc powder and the electrolyte. However, localized pH increases still lead to the formation of Zn_xOTf_y(OH)_2x–y·nH₂O over prolonged cycles. In contrast, the XRD pattern of rGO-0.5-ZPA showed no peaks related to side reactions even after 50 cycles, demonstrating the synergistic effect of the rGO-encapsulated zinc powder and PAA binder. The rGO layer protects zinc powder by preventing direct contact with the electrolyte, while the PAA binder remains stable through hydrogen bonding with rGO, effectively suppressing HER and side reactions.

Fig. 5e presents the linear sweep voltammetry (LSV) curve to evaluate HER on the electrodes. At a current of −10 mA, the potential values for ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA were measured as −0.089, −0.106, −0.096, and −0.125 V, respectively. A less negative potential value (closer to zero) at the same current indicates easier HER occurrence, as less overpotential is required to drive the reaction. The ZPV electrode exhibited the least negative potential (−0.089 V), indicating the most favorable conditions for HER due to the absence of a protective layer or functional binder like PAA, allowing water molecules to freely interact with the zinc surface. In contrast, the ZPA electrode showed a more negative potential (−0.106 V), demonstrating that the PAA binder delays HER by forming strong interactions with the Zn solvation shell and reducing the availability of water molecules near the zinc surface. However, the lack of a physical barrier limits its ability to fully suppress HER. The rGO-0.5-ZPV electrode (−0.096 V) showed that the rGO encapsulation layer delays HER by acting as a physical barrier, reducing water adsorption on the zinc surface. However, without a binder capable of promoting zinc ion de-solvation, its HER suppression remains moderate. Among all tested electrodes, the rGO-0.5-ZPA electrode exhibited the most negative potential (−0.125 V), indicating the highest resistance to HER. This improvement results from the synergistic effects of the rGO layer and PAA binder: the rGO layer prevents direct contact with the electrolyte, while the PAA binder promotes zinc ion de-solvation by replacing water molecules, further suppressing HER.

Electrochemical impedance spectroscopy (EIS) was employed to analyze the influence of the rGO protective layer and the PAA binder on the charge transfer resistance (R_ct) of the ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes. Fig. 5f and g present the Nyquist plots of symmetric cells at the initial state and after 50 cycles, with the inset depicting the equivalent circuit model. The semicircle observed in the Nyquist plots corresponds to R_ct, representing charge-transfer reactions at the surface of the active materials. As shown in Fig. S13,† the R_ct of ZPV increased more than 10-fold from its initial value, reaching 133 Ω after 50 cycles. This significant increase is attributed to the formation of a large quantity of irreversible by-products on the zinc powder, which impedes the redox reaction of zinc ions. In comparison, the R_ct of ZPA and rGO-0.5-ZPV increased by 7-fold and 6-fold, respectively, indicating that the PAA binder and the rGO protective layer partially suppressed by-product formation. Notably, the rGO-0.5-ZPA electrode exhibited only a four-fold increase in R_ct, with a value of 21.42 Ω after 50 cycles, about one-sixth that of ZPV. These results indicate that the rGO protective layer and the PAA binder effectively reduced polarization by restraining the formation of by-products on the zinc powder surface. The rGO layer prevents direct contact with the electrolyte, while the PAA binder facilitates zinc ion de-solvation, collectively maintaining lower R_ct and enhancing electrode performance during repeated cycling.

The charge-transfer kinetics and thermal activation behavior of the electrodes were further investigated through temperature-dependent EIS, with activation energies calculated using Arrhenius analysis based on the inverse of the charge-transfer resistance (1/R_ct). As shown in Fig. S14,† the Nyquist plots of ZPV and ZPA revealed a significant increase in semicircle diameter with decreasing temperature, indicating a sharp rise in R_ct. In contrast, rGO-0.5-ZPV and rGO-0.5-ZPA maintained relatively small semicircles across the entire temperature range (30–70 °C), suggesting more efficient and thermally stable charge-transfer behavior. The corresponding Arrhenius plots and calculated activation energies are presented in Fig. S15 and Table S5,† respectively. ZPV and ZPA electrodes exhibited relatively high activation energies of 16.74 and 16.14 kJ mol⁻¹, respectively. By contrast, the rGO-0.5-ZPV and rGO-0.5-ZPA samples showed significantly lower values of 4.30 and 2.51 kJ mol⁻¹, respectively. These results indicate that the synergistic effects of the rGO encapsulation layer and the PAA binder substantially reduce the interfacial energy barrier, thereby facilitating more efficient ion and electron transport even under high-temperature conditions. This thermally robust interfacial behavior plays a critical role in enhancing both the electrochemical reactivity and long-term stability of the zinc powder-based anodes.

To verify the dendrite suppression, the surface morphology of the active materials was examined using SEM. Fig. 6a–h presents the top-view SEM images of the electrodes at 25 and 50 cycles. In the ZPV electrode, plate-like dendrite structures were observed on the zinc powder surface as early as 25 cycles (Fig. 6a), becoming larger and more pronounced at 50 cycles (Fig. 6e). This is attributed to the uneven Zn-ion stripping/plating process and the formation of by-products by side reactions with the electrolyte. In the ZPA electrode, relatively uniform Zn-ion stripping/plating was observed at 25 cycles without dendrite formation (Fig. 6b). However, at 50 cycles, fine plate-like dendrites appeared due to PAA dissolution, which reduced its effectiveness in restraining side reactions (Fig. 6f). For the rGO-0.5-ZPV electrode, no dendrites were observed at 25 cycles (Fig. 6c), as the rGO protective layer physically suppressed dendrite growth. However, at 50 cycles, localized zinc ion deposition led to dendrite formation that penetrated the rGO layer, primarily due to its low affinity for the electrolyte (Fig. 6g). This hydrophobic nature of the rGO layer limited uniform Zn-ion diffusion, resulting in localized deposition. Additionally, repeated cycling may have caused localized pH increases, which exacerbated dendrite growth and eventually led to the penetration of the graphene layer. In contrast, the rGO-0.5-ZPA electrode exhibited no dendrite formation at either 25 or 50 cycles (Fig. 6d and h), attributed to the synergistic effects of the rGO layer and PAA binder. The high affinity of PAA promoted uniform zinc ion deposition, while the rGO layer prevented dendrite growth and maintained the stability of PAA by suppressing its dissolution. Cross-sectional SEM images (Fig. 6i–l and S16†) further confirmed these findings. ZPV and ZPA exhibited numerous dendrites and distorted morphologies (Fig. 6i and j), while rGO-0.5-ZPV showed dendrites penetrating the graphene layer (Fig. 6k). In contrast, rGO-0.5-ZPA maintained a stable rGO layer and preserved the spherical morphology of zinc powder with no dendrites observed (Fig. 6l).

Fig. 6 Top-view SEM images of (a–d) ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes after 25 cycles, and (e–h) after 50 cycles. (i–l) Cross-sectional SEM images of ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA electrodes after 50 cycles.

To verify the dendrite suppression effects of the rGO protective layer and the PAA binder, in situ optical microscopy was conducted. Fig. S17† presents time-lapse optical images of ZPV and rGO-0.5-ZPA electrodes during continuous Zn plating under identical current conditions, captured at 0, 15, 30, and 60 min. In the case of the ZPV electrode, a large number of dendrites were clearly observed as early as 15 min. As Zn plating progressed, the dendrites grew significantly, and by 60 min, the entire surface was covered with dendritic structures, indicating highly uneven Zn deposition (Fig. S17a–d†). In contrast, the rGO-0.5-ZPA electrode maintained a uniform surface without any visible dendrite formation at 15 min. Even after 60 min of plating, the surface remained relatively smooth and did not exhibit noticeable roughening compared to ZPV (Fig. S17e–h†). This difference provides direct visual evidence that the rGO protective layer and hydrophilic polymer binder effectively suppress dendrite growth.

To investigate the potential of the rGO-0.5-ZPA anode for advanced energy storage systems, the electrochemical performance of full cells was evaluated in a voltage range of 0.2–1.8 V versus Zn²⁺/Zn. The full cells were assembled using ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA as the anodes, paired with an ammonium vanadate nanofiber (AVNF) cathode and a 2.5 M zinc trifluoromethane sulfonate electrolyte. Fig. S18† shows the cyclic voltammetry (CV) profiles of ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA full cells at a scan rate of 0.1 mV s⁻¹. The CV curves exhibit two oxidation peaks at 0.72 V and 1.10 V and two reduction peaks at 0.55 V and 0.8 V, corresponding to the V⁴⁺/V³⁺ and V⁵⁺/V⁴⁺ redox couples, respectively. These characteristics are consistent with previously reported cation-intercalated vanadate cathodes.^46,50–52Fig. 7a shows the specific capacities of the full cells at current densities from 0.5 to 20 A g⁻¹. At a current density of 0.5 A g⁻¹, the initial specific capacity of ZPV, ZPA, rGO-0.5-ZPV, and rGO-0.5-ZPA full cells were 255, 294, 305, and 323 mA h g⁻¹, respectively. As the current density increased to 20 A g⁻¹, the specific capacities decreased to 48, 77, 87, and 147 mA h g⁻¹, corresponding to 18.8%, 26.2%, 28.5%, and 45.5% of the initial values. The observed differences in the initial specific capacities among the full cells are primarily attributed to variations in the interfacial properties of the zinc powder-based anodes, particularly in terms of electrolyte wettability, surface uniformity, and Zn²⁺ plating/stripping behavior. As shown in Fig. 3a–d, the PAA-containing electrodes (ZPA and rGO-0.5-ZPA) exhibit significantly lower contact angles than the PVDF-based electrodes (ZPV and rGO-0.5-ZPV), reflecting superior electrolyte affinity due to the hydrophilic carboxylic groups in PAA. Among these, the rGO-0.5-ZPA electrode shows the lowest contact angle, resulting from the combined effects of the PAA binder and the residual oxygen-containing functional groups of rGO. This improved wettability facilitates efficient electrolyte penetration and Zn²⁺ accessibility to active surfaces. Notably, the rGO-0.5-ZPV electrode, despite lacking a hydrophilic binder, exhibits a moderately low contact angle (46°), markedly better than that of the bare ZPV electrode (120°), due to the rGO coating. Furthermore, its initial specific capacity is slightly higher than that of ZPA, suggesting that factors beyond wettability, such as interfacial homogeneity and reaction uniformity, are also critical. The rGO coating layer improves the spatial uniformity of Zn²⁺ flux, suppresses local concentration and potential gradients, and mitigates undesired interfacial reactions like passivation and dendrite formation. These effects lead to more efficient Zn²⁺ utilization and higher coulombic efficiency during the initial cycles. Electrochemical impedance spectroscopy (Fig. 5f and g) supports this interpretation, showing that rGO-0.5-ZPA exhibits the lowest charge-transfer resistance among all samples. The combination of excellent electrolyte wettability, stable Zn²⁺ interfacial behavior, and suppressed side reactions enables the rGO-0.5-ZPA full cell to achieve the highest initial capacity and superior rate performance. This is further supported by EIS results after prolonged cycling (Fig. S13†), which demonstrate the stable interfacial characteristics of the rGO-encapsulated electrodes.

Fig. 7 (a) Rate performance and (b) cycle performance of full cells with AVNF cathodes at 5 A g⁻¹. (c–f) GCD curves of full cells with (c) ZPV, (d) ZPA, (e) rGO-0.5-ZPV, and (f) rGO-0.5-ZPA anodes at 25, 200, 1000, and 2000 cycles. (g) Comparison of electrochemical performance of the rGO-0.5-ZPA symmetric cell with previously reported zinc powder-based symmetric cells at a 1C rate.

To further investigate the influence of cathode loading and N/P ratio on full-cell performance, additional galvanostatic tests were conducted at increased cathode mass loadings of 1.5 and 2.0 mg cm⁻², corresponding to N/P ratios of approximately 5.3 and 4.0, respectively (Fig. S19†). Although these configurations are often pursued to improve practical energy density, the full-cell performance gradually declined in terms of both specific capacity and rate capability with increased cathode loading and reduced N/P ratio. This degradation is attributed to not only increased ion diffusion resistance and lower active material utilization in thicker cathodes, but also to the reduced Zn excess in the anode. A lower N/P ratio limits the zinc reservoir available for reversible stripping/plating, thereby increasing the risk of interfacial instability and dendrite formation under prolonged cycling. Therefore, the condition with a cathode loading of 1.0 mg cm⁻² (N/P = 7.9) provides a well-balanced full-cell configuration that ensures both long-term stability and effective utilization of the anode material.

Fig. 7b–f shows the cycle performance and galvanostatic charge–discharge (GCD) curves of the full cells. The ZPV full cell exhibited rapid capacity degradation, retaining only 4.2% (3.36 mA h g⁻¹) of its initial capacity at 1000 cycles and ultimately failing (Fig. 7c). This poor performance is attributed to high overpotential, side reactions, and dendrite growth, which result in irreversible charge/discharge behavior. In contrast, ZPA and rGO-0.5-ZPV full cells retained 58.4% (83.53 mA h g⁻¹) and 54.01% (89.67 mA h g⁻¹) of their initial capacities after 1000 cycles and 48.29% (69.11 mA h g⁻¹) and 44.8% (74.38 mA h g⁻¹) after 2000 cycles, respectively (Fig. 7d and e). Despite initially higher capacities due to faster Zn-ion stripping/plating kinetics, the rGO-0.5-ZPV full cell experienced rapid capacity fade after 200 cycles. This degradation is caused by the breakdown of the graphene layer and passivation from side reactions. The rGO-0.5-ZPA full cell demonstrated exceptional capacity retention, maintaining 91.78% (181.43 mA h g⁻¹) after 1000 cycles and about 80% (157.97 mA h g⁻¹) after 2000 cycles (Fig. 7f). This superior performance is attributed to the combined effects of the rGO protective layer, which resists penetration and suppresses dendrite growth, and the PAA binder, which facilitates uniform Zn-ion deposition and prevents side reactions. To further examine the long-term electrochemical durability of the electrodes, cyclic voltammetry (CV) measurements were periodically performed every 100 cycles during full-cell operation. As shown in Fig. S20,† the rGO-0.5-ZPA full cell maintained consistent redox peak positions and minimal variations in peak area from the 1st to the 500th cycle, indicating excellent electrochemical reversibility and stable charge storage behavior. These results highlight the robust interfacial stability of the rGO-0.5-ZPA electrode, which is attributed to the synergistic effect of the conductive and protective rGO layer and the hydrophilic PAA binder. This dual-function architecture effectively maintains ion transport pathways and suppresses interfacial degradation, enabling outstanding long-term cycling performance.

To evaluate the Zn²⁺ ion diffusion kinetics in our electrodes, GITT analysis was performed under full-cell configuration. As shown in Fig. S21 and S22,† and summarized in Table S6,† the rGO-0.5-ZPA electrode exhibited the highest Zn²⁺ diffusion coefficient of 3.61 × 10⁻⁵ cm² s⁻¹ during discharge. This improvement is attributed to the combined effect of the PAA binder's hydrophilic character and the conductive rGO shell, which synergistically facilitates ion and electron transport across the electrode–electrolyte interface. These results further validate the efficacy of our dual-protection design in enhancing charge transport dynamics.

Fig. 7g compares the electrochemical performance of the rGO-0.5-ZPA electrode with previously reported zinc powder-based electrodes. Even under high current densities and capacities that led to higher DOD values, the rGO-0.5-ZPA electrode outperformed previously reported systems. This demonstrates the potential of the dual-protection strategy utilizing a hydrophilic PAA binder and rGO-encapsulated zinc powder to enable high-performance aqueous zinc-ion batteries.

4. Conclusion

In this study, we developed a dual-protection matrix combining rGO-encapsulated zinc powder and a hydrophilic PAA binder to address the challenges faced by zinc powder-based anodes for aqueous zinc-ion batteries (AZIBs). The rGO encapsulation layer effectively suppressed dendrite growth and side reactions by preventing direct contact between the zinc powder and the electrolyte. Simultaneously, the PAA binder enhanced the electrolyte affinity, promoting uniform Zn-ion deposition while maintaining strong adhesion through hydrogen bonding with rGO, thus mitigating binder dissolution. The synergistic effects of the rGO layer and PAA binder enabled the anode to maintain its structural integrity and performance over extended cycling. In symmetric cells, the rGO-0.5-ZPA anode exhibited exceptional cycle stability, achieving over 670 h of operation at a high utilization rate of 33% with a stable voltage profile. In full-cell configurations paired with an AVNF cathode, the rGO-0.5-ZPA anode demonstrated outstanding electrochemical performance, including a high specific capacity of 147 mA h g⁻¹ at 20 A g⁻¹ and superior capacity retention of 80% after 2000 cycles at 5 A g⁻¹, outperforming other configurations. This work highlights the scalability and practicality of the dual-protection system for advancing AZIBs, offering a low-cost and efficient strategy to enhance the performance and durability of zinc powder-based anodes. Such advancements are pivotal for the development of safe, sustainable, and high-performance energy storage systems.

Data availability

All supporting data for this study are included in the ESI† file accompanying this article.

Author contributions

Jinhyeong Yoon: validation, formal analysis, investigation, methodology, data curation, writing – original draft. Jihong Kim: validation, formal analysis, investigation, methodology, data curation. Kangmin Lee: validation, formal analysis, investigation, methodology, data curation. Jong Eun Chae: formal analysis, investigation, data curation. Chiho Song: validation, formal analysis, investigation, data curation. Hyeonmin Jo: methodology, data curation. Hee-Dae Lim: methodology, investigation, data curation. Neetu Bansal: formal analysis, investigation, data curation. Rahul R. Salunkhe: methodology, validation, formal analysis, writing – review & editing, supervision. Heejoon Ahn: conceptualization, methodology, validation, formal analysis, visualization, writing – original draft, writing – review & editing, supervision, project administration, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2022R1A2C2010451 and RS-2025-00521491), and partly by the Korea Institute for Advancement of Technology (KIAT) through a grant funded by the Ministry of Trade, Industry, and Energy (MOTIE) (RS-2024-00417730, HRD Program for Industrial Innovation).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta00445d

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