Enhanced Electrochemical Performance of Carbon-Coated Nano-ZnO as an Anode Material for High-Rate Ni-Zn Batteries

Abstract

Nickel–zinc batteries are promising candidates for safe, cost-effective, and high-power energy storage. However, the poor cycling stability of zinc anodes, mainly caused by dendrite growth and dissolution, remains a major challenge for their practical application. Herein, carbon-coated nano-ZnO (ZnO@C) composites were synthesized via a sol–gel method using polyvinyl alcohol (PVA) and zinc acetate as precursors. By systematically tuning the carbon content, the ZnO@C-6 sample with a carbon-to-ZnO mass ratio of 1:6 exhibited the best structural and electrochemical performance. Characterization confirmed a uniform amorphous carbon layer that enhanced conductivity and inhibited ZnO dissolution. Electrochemical tests demonstrated that ZnO@C-6 exhibited high reversible capacity (500 mAh g⁻¹ at 12 C after 1000 cycles), coulombic efficiency (>80%), and superior rate capability up to 30 C. Post-cycling SEM confirmed that the carbon coating effectively inhibits dendrite formation and preserves electrode morphology. These findings highlight the critical role of carbon coatings in stabilizing ZnO-based anodes and offer a viable pathway toward high-performance Ni-Zn batteries.

1. Introduction

The growing environmental concerns and energy crisis caused by fossil fuel consumption are driving a global transition toward renewable energy. Efficient, safe, and sustainable energy storage systems are essential to integrate intermittent renewable sources into modern power grids [1,2,3]. However, their large-scale deployment in stationary energy storage systems (ESS) is limited by uneven lithium resource distribution, high cost, and safety issues associated with flammable organic electrolytes [4,5]. Consequently, the development of cost-effective, safer, and environmentally friendly aqueous battery systems has emerged as a prominent research direction.

Among various candidates, aqueous zinc-based batteries (AZBs) have garnered significant attention due to their inherent advantages, including zinc abundance, environmental benignity, low cost, and high safety [6,7]. In particular, nickel–zinc secondary batteries (NZBs) stand out for their high output voltage of up to 1.85 V and excellent rate capability, making them especially promising for high-power and fast-charging applications [8]. Moreover, nickel-based cathode materials such as Ni(OH)₂ exhibit good electrochemical stability, further enhancing the overall reversibility of the system [9,10]. Despite these advantages, the zinc anode still suffers from several critical issues that hinder the long-term stable operation of NZBs. These include the uncontrolled deposition of zinc in the electrolyte leading to dendrite growth, volume expansion and structural degradation during cycling, hydrogen evolution side reactions causing gas release, as well as morphological changes and pulverization of ZnO/metallic Zn during charge–discharge processes [11,12,13]. In particular, uneven Zn deposition, the “tip effect,” and surface passivation of ZnO have been identified as major factors that accelerate dendrite formation and reduce the long-term stability of the electrode [14]. Such problems severely reduce the battery’s reversible capacity and coulombic efficiency, while also compromising its cycle life and operational safety.

To address these challenges, various strategies have been proposed, including electrolyte additives, functional separators, and surface coatings [15,16]. Among these approaches, surface coating has been demonstrated as a simple yet effective method for improving interfacial stability and conductivity. Carbon materials, with high electrical conductivity, chemical inertness, and processing versatility, are widely employed as protective layers on zinc anodes [17]. Such carbon coatings not only promote uniform zinc deposition and suppress dendrite formation but also create stable electron/ion transport pathways at the electrode–electrolyte interface, thereby reducing polarization. Most existing studies, however, have concentrated on coating commercial micro-sized ZnO particles with carbon. While this approach improves electrode stability, the limited specific surface area and scarcity of electrochemically active sites in micron-sized ZnO restrict its capacity output in high-performance energy storage devices [18,19,20]. In contrast, nano-sized ZnO offers higher discharge capacity and faster reaction kinetics due to its reduced particle size, increased surface area, and shorter ion/electron diffusion paths [21,22]. Nevertheless, the high surface energy of nanoscale ZnO promotes dissolution in alkaline electrolytes, leading to rapid capacity decay. Therefore, integrating the high reactivity of nano-ZnO with the protective and conductive features of carbon coatings is expected to achieve both high capacity and long-term cycling stability for advanced zinc anodes.

Based on this analysis, we synthesize carbon-coated nano-ZnO (ZnO@C) via a simple sol–gel method, employing PVA and EC as dual carbon sources to regulate both the shell thickness and the overall carbon content. By adjusting the carbon-to-ZnO ratio, we achieve a uniform amorphous carbon shell that enhances electronic conductivity, suppresses ZnO dissolution, and homogenizes Zn²⁺ flux. Characterization results confirm that the carbon coating is uniformly distributed on the surface of the nano-ZnO particles, forming a tightly bound core–shell structure that effectively inhibits ZnO dissolution and dendrite formation. Electrochemical testing identifies ZnO@C-6 (C:ZnO = 1:6) as the optimal composition, delivering superior cycling stability and rate performance compared to both pure ZnO and other ZnO@C variants. This work offers a novel design strategy and technical route for developing high-performance zinc anode materials for nickel–zinc batteries, with broad application potential.

2. Experimental Section

2.1. Materials

Zinc acetate dihydrate (Zn(Ac)₂·2H₂O, ≥99.0%), polyvinyl alcohol (PVA, Mw ≈ 30,000, 99% hydrolyzed), ethyl cellulose (EC, viscosity 10–15 mPa·s), commercial Ni(OH)₂ containing Co (for Ni-Zn battery cathode), and ethanol (CH₃CH₂OH, ≥99.5%) were all purchased from Shanghai McLean Co., Ltd. (Shanghai, China). All chemicals were of analytical grade and used directly without any further purification.

2.2. Fabrication Procedures

2.2.1. Preparation of Zn(Ac)₂-PVA-EC Gel Precursor

In this study, a facile and efficient sol–gel method was employed to synthesize carbon-coated nano-ZnO composite materials (denoted as ZnO@C). The influence of carbon content on the microstructure and electrochemical performance of the composites was systematically investigated. Firstly, 0.1 g of ethyl cellulose (EC, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China, analytical grade) was dissolved in 4 mL of anhydrous ethanol (C₂H₅OH, Sinopharm Group Co., Ltd., Beijing, China, AR grade) under magnetic stirring to form a homogeneous polymeric carbon precursor solution. Meanwhile, 1.7682 g of zinc acetate dihydrate (Zn(Ac)₂·2H₂O, Aladdin, analytical grade) and 0.3 g of polyvinyl alcohol (PVA, Mw ≈ 30,000, Aladdin, AR grade) were dissolved in 6 mL of deionized water under vigorous stirring in a water bath at 90 °C for 30 min until complete dissolution. The solution was then cooled to room temperature. The EC–ethanol solution was slowly dropped into the zinc salt–PVA solution under continuous stirring, followed by an additional 30 min of stirring to obtain a stable Zn(Ac)₂-PVA-EC ternary sol system. The resulting sol was dried at 80 °C in a vacuum oven (DZF-6050, Shanghai Yiheng Technology Co., Ltd., Shanghai, China) for 12 h to yield the dry gel precursor.

2.2.2. Synthesis of Carbon-Coated Nano-ZnO Materials

The dried gel was then transferred into a quartz boat and placed in a tube furnace (OTF-1200X, Hefei Kejing Biotechnology Co., Ltd., Hefei, China). Under a continuous flow of high-purity nitrogen gas (99.999%, ~100 mL min⁻¹), the sample was heated at a ramp rate of 5 °C min⁻¹ to 600 °C, held at this temperature for 2 h, and then allowed to cool naturally to room temperature, resulting in a black carbon-coated nano-ZnO composite. To systematically investigate the effect of carbon content, the ratio of EC to PVA was adjusted to control the carbon-to-ZnO mass ratios at 1:4, 1:5, 1:6, 1:7, and 1:8. The detailed precursor formulations, including the precise amounts of EC, PVA, and Zn(Ac)₂·2H₂O used for each sample, are summarized in Table S1, and corresponding samples were labeled as ZnO@C-4, ZnO@C-5, ZnO@C-6, ZnO@C-7, and ZnO@C-8, respectively.

2.3. Materials Characterization

A series of comprehensive characterization techniques were employed to evaluate the structure and composition of the synthesized ZnO@C composites. X-ray diffraction (XRD) was performed using a Bruker D8 Advance diffractometer (Victoria, Australia) with Cu-Kα radiation (λ = 1.5406 Å), scanning in the 2θ range of 10°–80°, to identify the crystalline phase and phase purity of the ZnO. The morphology of the samples was examined using field-emission scanning electron microscopy (FESEM, ZEISS GeminiSEM 500, New South Wales, Australia) and transmission electron microscopy (TEM, Talos F200X, Thermo Fisher, New South Wales, Australia). Energy-dispersive X-ray spectroscopy (EDS) mapping was conducted to visualize the spatial distribution of Zn, O and C elements, confirming the presence and uniform coverage of the carbon coating layer. Raman spectroscopy (excitation wavelength 532 nm, Renishaw inVia Reflex, Gloucestershire, UK) was employed to analyze the carbon structure. The intensity ratio of the D and G bands (I_D/I_G) was used to assess the degree of disorder and graphitization of the carbon coating. Thermogravimetric analysis (TGA) was carried out using a PerkinElmer TGA4000 (Waltham, MA, USA)thermal analyzer in an air atmosphere, with a heating rate of 10 °C min⁻¹ from room temperature to 800 °C. Inductively coupled plasma–atomic emission spectroscopy (ICP-OES, Thermo Scientific iCAP 7000 Series, New South Wales, Australia) was further employed to quantify the elemental composition. Electrical conductivity with the pressure range of 5–100 MPa was measured using a four-probe method on a Suzhou Jingge ST2742C powder conductivity tester (Suzhou, China).

2.4. Electrochemical Measurements

2.4.1. Half-Cell Three-Electrode System Test

For electrochemical measurements, the ZnO@C active material, Super P conductive carbon, and polyvinylidene fluoride (PVDF) binder were mixed in mass ratio of 7:2:1. The mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to form a uniform slurry, which was then coated onto a nickel foam current collector. The coated electrodes were vacuum-dried at 80 °C for 12 h and subsequently pressed under 10 MPa. The mass loading of active material was controlled in the range of 2.0–2.5 mg cm⁻². Electrochemical performance tests were conducted using a standard three-electrode system on a CHI660E electrochemical workstation (Shanghai Chenhua, Shanghai, China). In this setup, the Hg/HgO electrode and platinum wire served as the reference and counter electrodes, respectively. The electrolyte was a 2.0 mol·L⁻¹ KOH aqueous solution saturated with ZnO. The electrochemical tests included cyclic voltammetry (CV) at 10 mV s⁻¹, galvanostatic charge–discharge (GCD) at different current densities, and electrochemical impedance spectroscopy (EIS) in the frequency range of 0.01 Hz to 100 kHz with a 5 mV AC perturbation. These tests were used to evaluate charge transfer resistance, interfacial behavior, and rate capability of the electrodes.

2.4.2. Full Cell Assembly and Performance Evaluation

To further assess the practical applicability of the ZnO@C composites, full cells were assembled using ZnO@C as the anode and commercial spherical Ni(OH)₂ as the cathode. The cathode electrodes were prepared using the same procedure as the anodes, and active material loading of the cathode was approximately 3.0 mg cm⁻². CR2032-type coin cells were constructed using a Whatman GF/D glass fiber membrane as the separator and 6 mol L⁻¹ KOH solution saturated with zinc oxide as the electrolyte. For all full cells, the capacity ratio of cathode to anode was adjusted to approximately 1.2:1, ensuring that the cathode was slightly in excess so that the anode remained the limiting electrode. This configuration guarantees a fair comparison across different ZnO@C composites.

All electrochemical tests of the full cells were conducted in a thermostatic chamber at 25 ± 1 °C. In the cycling stability test, a constant current density of 12 C was applied with the following procedure: charging for 5 min, resting for 5 min, and then discharging to 1.2 V at the same rate. The charging process was controlled by a capacity cut-off corresponding to the theoretical value of ZnO (≈650 mAh g⁻¹), followed by a 5 min rest, and then discharged to 1.2 V at the same rate. The full cells were subjected to galvanostatic charge–discharge cycling, rate performance evaluation, and long-term cycling tests to comprehensively assess the electrochemical performance and practical potential of the prepared ZnO@C materials in nickel–zinc battery systems.

3. Results and Discussion

Structural Characterization

The synthesis process of carbon-coated nano-ZnO is illustrated in Figure 1. Zinc acetate dihydrate (Zn(Ac)₂·2H₂O) was used as the zinc source, while polyvinyl alcohol (PVA) and ethyl cellulose (EC) served as carbon precursors. Through a simple sol–gel method, a Zn(Ac)₂-PVA-EC gel was first obtained by homogeneous mixing and solvent evaporation. Subsequently, the gel was pyrolyzed at high temperature under nitrogen atmosphere. During this process, Zn(Ac)₂ decomposed to form ZnO nanoparticles, while PVA and EC carbonized to generate an amorphous carbon layer uniformly coated on the particle surface. This in situ carbon coating strategy offers several advantages: (i) the conformal carbon layer effectively prevents direct contact between ZnO particles, thereby reducing particle agglomeration during calcination; (ii) the carbon matrix enhances the overall electrical conductivity of the composite; (iii) the core–shell structure provides structural integrity and contributes to improved electrochemical stability of the electrode material. By tuning the mass ratio of carbon precursors to zinc salt, the carbon content in the final ZnO@C composite could be flexibly controlled.

X-ray diffraction (XRD) analysis was performed to confirm the phase composition and crystallinity of the synthesized ZnO@C composites. As shown in Figure 2a, X-ray diffraction (XRD) patterns of ZnO@C composites synthesized with different carbon-to-ZnO mass ratios all exhibit similar diffraction peaks matching the hexagonal wurtzite ZnO phase (JCPDS No. 36-1451) [23], with no detectable impurity phases, indicating complete thermal decomposition of the precursor. Although carbon was introduced via the pyrolysis of PVA and EC, no distinct carbon peaks were observed, likely due to the low carbon content or the amorphous nature of the carbon matrix [24]. Further insights into the microstructure were obtained through TEM characterization. As shown in Figure 2b,c, the TEM and HRTEM images of the ZnO@C-6 sample, taken after ultrasonic dispersion, clearly reveal that the ZnO nanoparticles are encapsulated by a carbon matrix. The HRTEM image shows a well-resolved lattice fringe with an interplanar spacing of 1.9 Å, which corresponds to the (102) plane of hexagonal ZnO, further confirming the crystalline nature of the ZnO core. In addition, as shown in Figure S1, the carbon distribution can be classified into two distinct states. First, a conformal carbon coating layer with a thickness of approximately 10–15 nm can be observed on the surface of individual ZnO nanoparticles, serving as a protective shell that stabilizes the interface and facilitates electron transport. Second, part of the carbon is dispersed in the inter-particle regions, forming a bridging conductive network that enhances particle connectivity, prevents agglomeration, and improves the mechanical integrity of the composite. Figure 2d presents the elemental mapping of the ZnO@C-6 sample via EDS analysis, clearly demonstrating a uniform distribution of Zn, O and C elements. This confirms the successful formation of a carbon-coated ZnO structure. As shown in the elemental composition histogram (Figure S2), the atomic ratio of Zn to O is close to 1:1, and a distinct carbon signal is also detected, providing further evidence of carbon encapsulation. The homogenous carbon layer is beneficial for enhancing both the electrical conductivity and the structural stability of the material.

To further investigate the morphological features of the carbon-coated ZnO materials, SEM images of pure ZnO and the five ZnO@C composites with varying carbon contents (ZnO@C-8, ZnO@C-7, ZnO@C-6, ZnO@C-5 and ZnO@C-4) are shown in Figure 2e–j. As shown in Figure 2e, pure ZnO exhibits a typical hexagonal prism morphology, with particle diameters ranging from 300 to 500 nm. Upon introducing PVA and EC via the sol–gel method, significant morphological changes were observed. When a small amount of carbon is introduced, as in ZnO@C-8 (Figure 2f), the morphology remains largely blocky, indicating that the relatively thin and discontinuous carbon coating has limited effect on particle shape. With a moderate increase in carbon content, as in ZnO@C-7 (Figure 2g), the particles become noticeably more rounded, suggesting that the presence of amorphous carbon begins to modulate nucleation and growth during thermal decomposition. For ZnO@C-6 and ZnO@C-5 (Figure 2h,i), a modest reduction in ZnO nanoparticle size (50–100 nm) and improved dispersion were observed. With further increases in the carbon ratio, ZnO@C-4 (Figure 2j) exhibited an irregular, blocky morphology with relatively uniform dispersion, while exhibited aggregates of smaller nanoparticles (~50 nm).

The carbon structure of the ZnO@C-6 composite was further investigated using Raman spectroscopy. Typically, carbonaceous materials exhibit two characteristic Raman bands: the D-band around ~1350 cm⁻¹, associated with disordered carbon or structural defects, and the G-band around ~1580 cm⁻¹, corresponding to the in-plane vibration of sp²-hybridized graphitic carbon [25]. The intensity ratio of these bands (I_D/I_G) is commonly used to assess the degree of disorder or defect density in carbon materials. As shown in Figure 3a, the ZnO@C-6 sample displays two distinct peaks located at approximately 1332 cm⁻¹ (D-band) and 1584 cm⁻¹ (G-band), indicating the coexistence of disordered and graphitized carbon domains. The calculated I_D/I_G ratio is approximately 1.003, suggesting a relatively high degree of structural disorder in the carbon matrix. Such a disordered carbon network is beneficial for providing abundant active sites for electrochemical reactions [26]. These results confirm the successful integration of carbon into the ZnO framework. In addition, thermogravimetric analysis (TGA) was conducted in an air atmosphere to quantify the carbon content in the ZnO@C-6 composite. The TGA curve (Figure 3b) reveals two main stages of weight loss. The first weight loss (~2%) occurs between 30 and 300 °C and is attributed to the evaporation of physically adsorbed and chemically bound water, indicating the material possesses low water adsorption [27]. A second major weight loss of approximately 11% is observed in the 300–450 °C range, corresponding to the oxidation of the carbon coating. Based on this, the carbon content in ZnO@C-6 is estimated to be around 11 wt%, further validating the formation of the carbon-coated structure. Moreover, the TGA curves of composites with different carbon contents are shown in Figure S3. The results clearly reveal that under thermal analysis in air, the final weight loss gradually increases with increasing carbon coating, which matches well with the designed precursor ratios and confirms the intended gradient in carbon content across the ZnO@C samples.

To further investigate the elemental composition and chemical states of the ZnO@C-6 composite, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The survey spectrum (Figure 4a) confirms the presence of Zn, O and C elements, indicating the successful formation of the ZnO@C composite structure. The high-resolution Zn 2p spectrum (Figure 4b) shows two distinct peaks centered at binding energies of 1022.3 eV and 1045.5 eV, which correspond to Zn 2p_3/2 and Zn 2p_1/2, respectively [28]. The energy separation of approximately 23 eV between these two peaks is consistent with the typical spin–orbit splitting of Zn²⁺ species, confirming the divalent oxidation state of zinc in ZnO [29]. The O 1s spectrum (Figure 4c) can be deconvoluted into three components centered at 531.0 eV, 532.1 eV and 533.1 eV. These peaks are attributed to lattice oxygen in Zn-O bonds, oxygen in carboxylate groups (O-C=O), and C-O bonds from residual organic species or carbon coating, respectively [30]. The presence of carboxyl and ether groups suggests partial retention of oxygen-containing functionalities from the carbon precursors, which may contribute to surface wettability and electrochemical activity. The C 1s spectrum (Figure 4d) displays two main peaks located at 284.80 eV and 286.50 eV, which are ascribed to C-C/C=C and C-O bonding environments, respectively. These features are characteristic of amorphous carbon derived from the pyrolysis of polyvinyl alcohol (PVA) and ethyl cellulose (EC) [31]. The dominance of sp²-hybridized carbon (C-C/C=C) further supports the formation of a conductive carbon layer that can enhance the electronic transport properties of the composite. These findings are consistent with Raman and TGA results and collectively verify the successful formation of a ZnO@C core–shell structure with tailored surface chemistry.

The electrochemical behavior of pure ZnO and the series of ZnO@C composite electrodes with varying carbon contents was investigated via cyclic voltammetry (CV). As shown in Figure 5a, all samples exhibit a pair of well-defined redox peaks within the potential window of −1.6 V to −0.9 V at a scan rate of 10 mV s⁻¹, indicating reversible Zn²⁺/Zn redox reactions. Among all tested electrodes, ZnO@C-6 demonstrates the highest oxidation peak current and the largest enclosed area under the CV curve, suggesting enhanced electrochemical activity and a greater number of accessible redox-active sites under identical testing conditions. The CV data for all samples are summarized in Table S2. Generally, the electrochemical reversibility of redox reactions can be evaluated by the potential separation (ΔE_a,c) between the anodic and cathodic peaks [32]. The ΔE_a,c values for ZnO@C-4, ZnO@C-5, ZnO@C-6, ZnO@C-7, and ZnO@C-8 are 0.250 V, 0.252 V, 0.246 V, 0.249 V, and 0.273 V, respectively. Notably, ZnO@C-6 exhibits the smallest peak separation (0.246 V), reflecting superior redox reversibility and more favorable reaction kinetics.

The corrosion resistance of the pure ZnO and ZnO@C electrodes was further evaluated using Tafel polarization tests. In general, the corrosion potential (E_corr) indicates the thermodynamic tendency toward corrosion, with more positive values corresponding to a lower tendency for the electrode to dissolve. The corrosion current density (I_corr) reflects the rate of corrosion, where lower values indicate better resistance [32]. Tafel polarization measurements were conducted in a three-electrode system, with the ZnO-based electrode as the working electrode, a platinum wire as the counter electrode, and the Hg/HgO electrode as the reference. The electrolyte was a 2.0 mol·L⁻¹ KOH aqueous solution saturated with ZnO, and the potential was scanned at 0.5 mV·s⁻¹ to obtain the anodic and cathodic branches of the polarization curve. As illustrated in Figure 5b and detailed in Table S3, ZnO@C-6 shows the most positive corrosion potential (−1.372 V) and the relatively low corrosion current density (0.168 × 10⁻² mA cm⁻²) among all carbon-coated samples, indicating outstanding corrosion resistance. This enhanced anti-corrosion performance can be attributed to the uniform and dense carbon coating, which effectively isolates the ZnO surface from direct contact with the electrolyte, thereby mitigating dissolution of the active material [33,34]. Moreover, the carbon layer promotes homogeneous current distribution across the electrode surface, reducing localized corrosion and improving the overall interfacial stability. To further provide direct quantitative evidence of reduced Zn dissolution, inductively coupled plasma atomic emission spectroscopy (ICP-OES) was conducted. Specifically, 100 mg of pure ZnO and ZnO@C-6 powders were separately immersed in 10 mL of 6 M KOH solution for 24 h at room temperature. As shown in Figure S4, the dissolved Zn concentrations were measured as 109.2 mg L⁻¹ for pure ZnO and 64.8 mg L⁻¹ for ZnO@C-6, confirming that the carbon coating markedly reduces Zn leaching under harsh alkaline conditions. Taken together, both electrochemical Tafel analysis and ICP-OES quantification provide consistent evidence that ZnO@C-6 possesses superior corrosion resistance and interfacial stability compared with pure ZnO, which is critical for enhancing long-term performance in nickel–zinc batteries.

To further assess the interfacial charge transfer characteristics of the ZnO@C electrodes, electrochemical impedance spectroscopy (EIS) was conducted. Figure 5c presents the Nyquist plots of the five ZnO@C electrodes, each displaying a semicircle in the high-frequency region (associated with charge transfer resistance, R_ct) and a linear tail in the low-frequency region (related to Warburg ion diffusion behavior). An equivalent circuit model was used for fitting the EIS data via ZView 3.3 software, and the extracted parameters are listed in Table S4. The charge transfer resistance (R_ct) values for pure ZnO and the ZnO@C-8 to ZnO@C-4 electrodes are 12.4 Ω, 23.0 Ω, 17.3 Ω, 10.3 Ω, 17.8 Ω, 24.6 Ω, respectively. Significantly, ZnO@C-6 exhibits the lowest R_ct value of 10.3 Ω. From an electrochemical kinetics perspective, a lower R_ct value implies reduced energy barriers for interfacial electron transfer and minimized electrochemical polarization. As such, ZnO@C-6 not only facilitates more efficient charge transport but also enhances the utilization of active materials during electrochemical cycling. Further insight into the intrinsic electronic transport properties was obtained by measuring the powder resistivity and conductivity of ZnO@C-6 with a standard four-probe method (Figure S5). For the ZnO@C-6 composite, under different applied pressures (5–100 MPa), the resistivity ranges from 4739.3 to 1160.4 Ω·cm, corresponding to a conductivity of 2.1 × 10⁻⁴ to 8.6 × 10⁻⁴ S·cm⁻¹.

The morphology and half-cell electrochemical analysis of carbon-coated ZnO electrodes with various C:ZnO mass ratios reveal that ZnO@C-6 (C:ZnO = 1:6) possesses the most optimal carbon coating. This uniform and appropriate carbon layer not only enhances the electrical conductivity of the ZnO electrode but also effectively inhibits the dissolution of active species into the electrolyte, thereby enabling stable and high-capacity energy storage [35,36]. To further evaluate the practical applicability of the ZnO@C materials, Ni-Zn full cells were assembled using commercial Ni(OH)₂ as the cathode, and pure ZnO as well as the five ZnO@C variants as the anodes. The schematic configuration of the Ni-Zn battery is shown in Figure 6a. In the long-term cycling performance tests, a constant current density of 12 C was applied. The charge/discharge voltage profiles offer further insight into battery performance. As shown in Figure 6b, compared with pristine ZnO, the charge–discharge voltage plateaus of the ZnO@C electrodes exhibit a certain degree of variation after introducing different carbon contents. These differences mainly originate from changes in ZnO particle size and the thickness of the carbon coating. Specifically, smaller ZnO nanoparticles provide a larger specific surface area, which affects the electrochemical reaction and diffusion path of Zn²⁺ ions at the electrode-electrolyte interface, thus slightly changing the discharge platform. Concurrently, the carbon layer thickness directly influences the charge-transfer resistance as well as the kinetics of electron and ion transport. A moderately thick carbon coating can effectively suppress particle agglomeration and form a continuous conductive network, enhancing overall electronic conductivity, but it may also lengthen the ion diffusion pathway, resulting in minor shifts in the charging voltage plateau. In comparison, pure ZnO showed an increasing charging plateau, declining discharging plateau, and significant capacity loss over cycling (Figure S6), reflecting severe polarization and structural degradation. The ZnO@C-6 electrode maintained nearly unchanged voltage profiles over multiple cycles (Figure S7), demonstrating minimal polarization and stable electrochemical behavior. This improvement is attributed to the uniform carbon coating, which enhances electronic conductivity, reduces internal resistance, and maintains interfacial stability. Furthermore, the voltage-time profiles recorded between the 800th and 820th cycles (Figure S8) were nearly identical, confirming the long-term cycling durability and robustness of the ZnO@C-6 electrode.

As shown in Figure 6c, the ZnO@C-6-based full cell achieved the best cycling performance, retaining a high discharge capacity of approximately 500 mAh g⁻¹ after 1000 cycles at a high current density of 12 C, outperforming those assembled with pure ZnO and other ZnO@C samples. In contrast, the capacities of ZnO@C-4, ZnO@C-5, and ZnO@C-7 dropped below 500 mAh g⁻¹ after 800 cycles, indicating a faster fading rate. The ZnO@C-4 electrode exhibited a certain degree of capacity increase during the initial cycles. This behavior is likely due to the gradual activation of additional active sites within the electrode and improved electrolyte wetting, which enhances the electrochemical utilization of ZnO. Meanwhile, batteries using pristine ZnO and ZnO@C-8 as anodes failed earliest and displayed relatively low capacities, indicating that either the absence of a carbon coating or a very low carbon content cannot effectively suppress Zn dissolution or dendrite formation, leading to rapid electrode degradation and limited cycle life. The coulombic efficiency of the ZnO@C-6-based battery remained consistently above 80% throughout 1000 cycles (Figure 6d), indicating excellent cycling stability under demanding high-rate conditions. This stability is attributed to the appropriately engineered carbon coating, which improves electrical conductivity, suppresses Zn dissolution, and ensures uniform current distribution, thereby reducing local current density and polarization. Moreover, as shown in Table S5, compared with other advanced Zn anode materials reported in the literature, such as ZnO@CNT, ZnO@RGO, and MOF-derived carbon-coated ZnO, etc., ZnO@C-6 exhibits superior high-rate performance and long-term cycling stability [18,37,38,39,40]. Notably, while many reported materials are limited to lower current densities (1–2 C) and shorter cycle life (<600 cycles), ZnO@C-6 maintains 500 mAh g⁻¹ at 12 C over 1000 cycles, demonstrating the effectiveness of the sol–gel-derived carbon coating in enhancing both the kinetics and durability of the electrode.

To further investigate the electrochemical behavior of carbon-coated ZnO as an anode for Ni-Zn batteries, rate capability tests were conducted using a battery testing system. Figure S9 compares the rate performances of the pure ZnO and ZnO@C-6 electrodes under various current densities. As shown in Figure S9, both electrodes exhibit an increase in discharge capacity with increasing current density from 1 C to 12 C. This behavior is primarily related to the suppression of hydrogen evolution at higher current densities. According to the Tafel equation [27]:

η H₂ = a + b log(j)

(1)

where ηH₂ is the hydrogen evolution overpotential, j is the current density, and a, b are material-dependent constants. The equation implies that the overpotential for hydrogen evolution increases with current density, which helps inhibit parasitic hydrogen evolution reactions at the Zn electrode. This suppression reduces self-corrosion and self-discharge, thereby enhancing the battery’s overall efficiency. Additionally, at higher current densities, the anode is exposed to strongly reducing the potential for a shorter duration before reaching the cut-off voltage, further limiting the occurrence of irreversible hydrogen evolution reactions. However, at high rates, increased electrode polarization and mass transport limitations typically lead to voltage hysteresis and capacity fading. As seen in Figure S9, the discharge capacity of the pure ZnO electrode rapidly deteriorates when the current density exceeds 12 C. In contrast, the ZnO@C-6 electrode maintains a relatively high and stable capacity even at 30 C (19.5 A g⁻¹), indicating excellent high-rate performance. This superior rate capability of ZnO@C-6 can be attributed to the uniformly distributed carbon coating, which serves multiple functions: (i) it stabilizes the ZnO particle structure during fast charge/discharge cycles; (ii) it suppresses the dissolution of ZnO in the alkaline electrolyte; and (iii) it improves electronic conductivity and mitigates concentration polarization. During the sol–gel synthesis, the choice and amount of the organic precursor (ligand) significantly influence the carbon layer thickness as well as the resulting ZnO particle size, both of which critically affect the electrochemical performance of the composite. Collectively, these advantages enable rapid and reversible Zn²⁺/Zn redox reactions, thereby ensuring outstanding high-rate electrochemical performance. This optimized balance between carbon layer thickness, particle morphology, and electrochemical kinetics enables ZnO@C-6 to deliver high reversible capacity, superior rate capability, consistent with the observed high-rate performance.

Zinc dendrite formation, induced by uneven Zn deposition, remains one of the major obstacles limiting the cycling stability of Ni-Zn batteries. To elucidate the role of the carbon coating in enhancing the cycling durability of ZnO-based electrodes, post-cycling surface morphology was examined using scanning electron microscopy (SEM) for both pure ZnO and ZnO@C-6 electrodes cycled at a high current density of 12 C. As shown in Figure 7a, the surface of the pure ZnO electrode after 500 cycles exhibits a large number of irregular protuberances and flaky deposits. These morphological features suggest the occurrence of inhomogeneous Zn deposition during cycling, where the localized electric field is intensified at sharp edges due to the tip effect. This leads to accelerated dendritic growth and pronounced structural deformation of the electrode. Such uncontrolled Zn deposition not only degrades the electrode’s integrity but also increases the risk of short-circuiting. In contrast, the ZnO@C-6 electrode retains a much smoother and more uniform surface even after 1000 cycles under the same conditions (Figure 7b), with no apparent dendritic structures observed. This result strongly indicates that the introduction of a carbon coating layer on ZnO effectively suppresses dendrite formation and maintains electrode structural stability over prolonged cycling.

The Zn deposition behaviors of pure ZnO and ZnO@C-6 electrodes are schematically illustrated in Figure 7c. In the case of pure ZnO, the Zn²⁺ ions preferentially deposit at electrochemically active sites, particularly at the tips and edges of surface protrusions. This promotes uneven deposition and exacerbates the formation of dendritic structures. In contrast, the carbon-coated ZnO@C-6 electrode provides a more homogeneous surface with enhanced electronic conductivity and uniform current distribution. The carbon layer also acts as a buffer, reducing the local ion concentration gradient and facilitating more uniform Zn²⁺ flux across the electrode interface. Moreover, the carbon coating may serve as a physical barrier that stabilizes the electrode-electrolyte interface, thereby mitigating parasitic side reactions and improving Zn reversibility. The combined effects of ion flux homogenization, interfacial stabilization, and improved conductivity contribute to the remarkable suppression of Zn dendrites and enhanced long-term cycling performance observed in the ZnO@C-6 electrode.

4. Conclusions

In summary, a series of carbon-coated nano-ZnO (ZnO@C) materials were successfully prepared with tunable carbon contents to investigate the effect of carbon coating on the electrochemical performance of ZnO-based anodes. The ZnO@C-6 composite, with an optimized C:ZnO mass ratio of 1:6, exhibited the most desirable properties, including enhanced electronic conductivity, suppressed ZnO dissolution, and improved electrochemical reversibility. The uniform carbon layer not only reduced charge-transfer resistance but also significantly improved corrosion resistance and structural integrity during long-term cycling. When assembled into Ni-Zn full cells, the ZnO@C-6 anode enabled excellent cycling stability and high-rate performance, outperforming both pure ZnO and other ZnO@C variants. Importantly, the carbon coating effectively suppressed dendrite growth by facilitating homogeneous Zn²⁺ ions flux and mitigating electrode polarization. These findings provide valuable insights into the rational design of carbon-coated Zn-based materials and underscore the potential of ZnO@C-6 as a robust and efficient anode material for advanced aqueous rechargeable Zn-based batteries.