Novel NH₄V₄O₁₀-Reduced Graphene Oxide Cathodes for Zinc-Ion Batteries: Theoretical Predictions and Experimental Validation

Abstract

This investigation explores the potential of enhancing aqueous zinc-ion batteries (AZIBs) through the introduction of a novel cathode material, NH₄V₄O₁₀ (NVO), combined with reduced graphene oxide (rGO). Utilizing Density Functional Theory (DFT), it was hypothesized that the incorporation of rGO would increase the interlayer spacing of NVO and diminish the charge transfer interactions, thus promoting enhanced diffusion of Zn²⁺ ions. These theoretical predictions were substantiated by experimental data acquired from hydrothermal synthesis, which indicated a marked increase in interlayer spacing. Significantly, the NVO–rGO composite exhibits remarkable cyclic durability, maintaining 95% of its initial specific capacity of 507 mAh g⁻¹ after 600 cycles at a current density of 5 A g⁻¹. The electrochemical performance of NVO–rGO not only surpasses that of pristine NVO but also outperforms the majority of existing vanadium oxide cathode materials reported in the literature. This study underscores the effective integration of theoretical insights and experimental validation, contributing to the advancement of high-performance energy storage technologies.

1. Introduction

As global fossil fuel reserves continue to deplete, there is an urgent need to transition towards more sustainable energy infrastructures that prioritize the harnessing of renewable energy sources, such as wind and tidal energy [1,2]. A predominant challenge with these renewable sources is their inherent intermittency, which necessitates the development of efficient energy storage solutions. This topic has become a critical focal point in contemporary scientific discourse. Over the past decade, lithium-ion batteries (LIBs) have emerged as a central technology in this field, garnering significant interest due to their high energy density and commendable cycle life [3,4], thereby establishing their dominance in the domain of electrochemical energy storage. However, concerns regarding the limited availability of lithium, escalating costs, and potential safety risks associated with organic electrolytes pose significant challenges for the application of LIBs in large-scale clean energy grids [5,6].

In evaluating diverse battery technologies, aqueous zinc-ion batteries (AZIBs) have attracted significant interest. Their increasing prominence is attributed to several advantageous attributes: the global abundance of zinc, enhanced safety due to the use of aqueous electrolytes, a commendable theoretical specific capacity of 820 mAh g⁻¹, and a favorable redox potential of −0.76 V versus the Standard Hydrogen Electrode (SHE) [7,8]. These characteristics enable AZIBs to be effectively utilized in large-scale grid applications, positioning them as a viable supplement to LIBs.

Despite these advantages, the technology faces significant challenges. A primary issue is the pronounced electrostatic interactions between Zn²⁺ ions and electrode materials [9]. These interactions create substantial potential energy barriers during the intercalation and de-intercalation of Zn²⁺ ions, leading to structural degradation in cathode materials through repeated charge–discharge cycles [10].

The cathode plays a crucial role in determining the electrochemical performance of batteries, particularly in the case of AZIBs. Consequently, developing high-performance cathode materials that facilitate rapid Zn²⁺ migration and exhibit high capacity is essential for the practical implementation of AZIBs. A variety of cathode materials, including manganese (Mn)-based [11,12,13], vanadium (V)-based compounds [14,15], and Prussian blue analogs [16,17], have been extensively investigated. Among these, vanadium-based oxides are distinguished by their range of oxidation states from V²⁺ to V⁵⁺. This variability in oxidation states provides greater structural and coordination flexibility, and the possibility for multi-electron transfer, which can lead to enhanced specific capacities [18]. However, the diversity of these oxidation states also presents challenges, including slow reaction kinetics and complex reaction mechanisms that can hinder performance.

The NH₄⁺ insertion vanadium–oxygen system, specifically Ammonium vanadate (NH₄V₄O₁₀, referred to as NVO), exhibits a high oxidation state contributing to its high specific capacity [19]. The ample ionic radius and minimal mass of NH₄⁺ serve as structural “pillars” between layers, facilitating the insertion and de-insertion of Zn²⁺ ions. A crystal face spacing methodology was employed to optimize the cathode material performance of ammonium vanadate [20]. The crystallographically engineered NVO, NH₄V₃O₈, and (NH₄)₂V₃O₈ exhibit interplanar spacings of 9.8 Å, 7.9 Å, and 5.6 Å, respectively. These spacings correlate directly with ion diffusion rates, with NVO demonstrating the most substantial interlayer diffusion channels, thus showing superior specific capacity and cyclability. At a current of 1 A g⁻¹, NVO starts with an initial capacity of 361.6 mAh g⁻¹, which diminishes to 275.3 mAh g⁻¹ after 100 cycles, maintaining 76% of its initial capacity.

Furthermore, Kim et al. synthesized NVO compounds, adopting both plate-like and ribbon-like hybrid structures, through a microwave-assisted synthesis method [21]. These materials achieved a specific capacity of 417 mAh g⁻¹ at a current density of 0.25 A g⁻¹ and 170 mAh g⁻¹ at 6.4 A g⁻¹, illustrating the potential for reaction time reduction in synthesis processes. Additionally, Tamilselvan et al. synthesized ultrathin NVO nanosheets on pretreated carbon cloth via a hydrothermal method, eschewing traditional organic binders in cathode preparation to enhance performance. This method yielded a specific capacity of 434 mAh g⁻¹ at a current density of 0.5 A g⁻¹.

Despite the promising results observed, the cyclic and rate performances of NVO materials require further optimization to align with the standards necessary for practical applications. Recent studies suggest that compositing with reduced graphene oxide (rGO) is an effective strategy for enhancing these properties [22,23,24]. A hierarchical flower-shaped (NH₄)₂V₃O₈/rGO composite was synthesized using a straightforward one-step hydrothermal method [25]. This composite demonstrated a notable specific capacity of 375.3 mAh g⁻¹ at a current density of 0.2 A g⁻¹, along with exceptional cycling stability, maintaining 265.0 mAh g⁻¹ after 3700 cycles at a current density of 5.0 A g⁻¹.

Motivated by these findings, Density Functional Theory (DFT) calculations were performed, which indicated that the integration of rGO with NVO could increase the interlayer spacing of the material. Additionally, rGO is theorized to act as a protective layer, mitigating the charge transfer interactions between Zn²⁺ ions and the vanadium oxide matrix, thereby facilitating improved Zn²⁺ diffusion. These theoretical predictions prompted further experimental investigation. Following synthesis via hydrothermal methods, the composite material exhibited an increased interlayer spacing and an enhanced initial specific capacity of 507 mAh g⁻¹ at a current density of 0.2 A g⁻¹. Remarkably, the material retained 95% of its initial capacity after 600 cycles at an elevated current density of 5 A g⁻¹, thereby confirming the theoretical predictions and underscoring the potential of rGO-enhanced NVO composites for advanced energy storage applications.

2. Results and Discussion

2.1. DFT Calculations

In order to investigate the effects of the rGO composite on the NVO system, we conducted a series of Density Functional Theory (DFT) calculations. We constructed models for Zn²⁺ embedded in pristine NVO (Zn–NVO) and Zn²⁺ embedded in an NVO system composited with rGO (Zn–NVO@rGO). Both systems were subjected to stringent structural optimizations, the outcomes of which are illustrated in Figure 1a,c, respectively. These figures highlight the expansion in interlayer spacing subsequent to the incorporation of rGO, which facilitates the diffusion of Zn²⁺ ions.

To further examine the specifics of Zn charge transfer, we calculated the charge density difference, as depicted in Figure 1b,d. In these visual representations, blue regions denote areas where charge density around Zn²⁺ has decreased, whereas yellow regions indicate an increase in charge density. It is important to note that in the pristine NVO system, there is a discernible charge transfer between Zn²⁺ and the oxygen atoms in the V–O bonds across two distinct layers. This transfer results in strong electrostatic interactions between Zn²⁺ and the host material, which hinders the diffusion of Zn²⁺ ions.

However, the introduction of rGO significantly alters this dynamic. rGO serves as a shielding layer, reducing the extent of charge transfer to only the oxygen in the V–O bonds of one layer of the NVO system. This reduction in charge transfer weakens the electrostatic interactions between Zn²⁺ and the vanadium oxide host, thereby promoting the diffusion of Zn²⁺ ions and enhancing the electrochemical performance of the Zn–NVO@rGO system. These DFT calculations are instrumental in providing a microscopic view of the electronic interactions and structural changes induced by the addition of rGO.

2.2. Morphological Characterization

To validate the outcomes of the DFT calculations, we synthesized the NVO and NVO@rGO composites via a single-step hydrothermal method. In the synthesized NVO@rGO composite materials, the rGO constitutes approximately 15 wt% of the total mass. The structural compositions of these materials were analyzed using X-ray Powder Diffraction (XRD). As depicted in Figure 2a, the XRD patterns of NVO@rGO align well with the reference pattern of NH₄V₄O₁₀ (JCPDS 31-0075), showcasing characteristic peaks at 7.44°, 25.06°, 30.32°, 45.84°, and 48.6°, which correspond to the standard peaks of NVO. The XRD analysis confirmed that the synthesized NVO closely matched the NH₄V₄O₁₀ standard as well.

Notably, the (001) plane peak of NVO@rGO demonstrated a shift towards lower angles, suggesting an increase in the interlayer spacing. This observation aligns with our DFT predictions and indicates enhanced intercalation capabilities, potentially facilitating improved ion diffusion pathways, crucial for the electrochemical performance of the composite material. However, primarily based on the observed increase in interplanar spacing from DFT predictions and XRD analysis, it is insufficient to conclusively demonstrate graphene’s integration into the NVO crystal structure. Other intercalated species such as water or ammonium ions might also contribute to the altered interplanar spacing. Consequently, we propose that the formation of a composite structure is a more plausible scenario for the observed phenomena.

In addition to the X-ray diffraction, Fourier Transform Infrared Spectroscopy (FTIR) was employed to further investigate the structural composition of NVO and NVO@rGO. As depicted in Figure 2b, both materials exhibited similar FTIR spectra, indicating a preservation of the base structural features upon rGO integration. However, the spectrum for NVO@rGO displayed several additional, albeit weaker, absorption peaks that are characteristic of rGO functional groups. Specifically, the absorption peak at 3436 cm⁻¹ is attributed to the stretching vibrations of –OH groups, indicative of residual hydroxyl functionalities or absorbed water molecules. The peak at 1728 cm⁻¹ corresponds to the C=O-stretching vibrations, typically found in carboxylic acid groups or ketones, while the peak at 1222 cm⁻¹ represents the C–O–C vibrations, which are commonly associated with ethers or esters. The relative weakness of these oxygen-containing functional group peaks suggests incomplete reduction in the graphene oxide to rGO.

A Raman spectroscopic analysis was performed on the NVO@rGO composite, and the results are presented in Figure 3. As detailed in the revised Figure 3, both the NVO and NVO@rGO samples exhibit peaks at 775 cm⁻¹ and 825 cm⁻¹, corresponding to the host material’s characteristics. Importantly, the Raman spectrum of NVO@rGO shows additional peaks at 1350 cm⁻¹ and 1580 cm⁻¹, which are distinctive of rGO. These peaks represent the D band and G band, respectively, which are indicative of the in-plane vibrations of sp²-hybridized carbon atoms and the stretching vibrations of sp²-hybridized carbon atoms coupled with π electrons. The presence of these bands in the NVO@rGO composite and their absence in the pure NVO substantiate the effective incorporation of rGO.

The intensity ratio of the D band to the G band (I_D/I_G), a metric of defect density and graphitization level, was calculated to be 1.01 for the NVO@rGO composite. This I_D/I_G ratio indicates a well-balanced graphitization with a moderate level of defects, which supports the favorable electrochemical properties observed in this study.

Scanning Electron Microscopy (SEM) was utilized to investigate the microstructural morphology and elemental composition of the synthesized NVO@rGO composite. Images presented in Figure 4a,b reveal that NVO@rGO consists of uniform ultrathin nanosheets. The ultrathin structure is advantageous for enhancing the surface area available for electrochemical reactions and facilitates rapid ion transport.

Further analysis using SEM Energy-Dispersive X-ray Spectroscopy (EDS) mapping (shown in Figure 4c) confirmed the homogeneous distribution of carbon (C), oxygen (O), nitrogen (N), and vanadium (V) elements within the composite. The presence of these elements in a uniformly distributed manner indicates effective incorporation of rGO into the NVO matrix. This homogeneous distribution is crucial for ensuring consistent electrochemical behavior across the material, which is essential for the stability and reliability of battery performance.

To gain further insight into the microstructural details of NVO@rGO, Transmission Electron Microscopy (TEM) was employed. The TEM images, as shown in Figure 4b and Figure 5a, distinctly reveal the nanosheet morphology of the composite. These images also clearly demonstrate the uniform coating of rGO on the NVO cathode material. The high-resolution TEM (HRTEM) images, displayed in Figure 5c, distinctly show the lattice fringes of NVO@rGO, with a lattice spacing of 0.198 nm that corresponds to the (−205) plane of NVO. This precise observation of lattice fringes underscores the crystalline nature of the NVO within the composite and supports the structural integrity of the material even after the incorporation of rGO.

Furthermore, the Selected Area Electron Diffraction (SAED) patterns, presented in Figure 5d, exhibit a ring-like diffraction pattern typical of polycrystalline materials. This pattern clearly resolves the diffraction rings corresponding to the (400), (001), and (110) planes of NVO, indicating that the crystalline phases of NVO are well-preserved within the composite. The presence of these distinct diffraction rings also helps confirm the phase purity and crystalline quality of the material, which are essential for predictable and stable electrochemical performance.

In addition, the TEM image of pure NVO is presented in Figure 6, enabling a direct comparison with the TEM images of NVO@rGO. The TEM analysis clearly reveals that pure NVO possesses a nanosheet morphology. Notably, there is no evidence of encapsulation by rGO in the pristine NVO sample. This observation sets a critical baseline for evaluating the morphological changes in the composite material.

A detailed comparison between the TEM images of pure NVO and NVO@rGO indicates a uniform coating of NVO by rGO in the composite. Such a feature is conspicuously absent in the pristine NVO, highlighting the distinctive interaction and morphological integration between NVO and rGO within the composite framework. This integration is indicative of a successful synthesis process, which presumably enhances the electrochemical properties of the composite material.

2.3. Electrochemical Properties Characterization

To evaluate the electrochemical behavior of NVO@rGO in reaction processes, the material was assembled into a coin cell for electrochemical performance testing. Cyclic voltammetry (CV) scans were initially performed at a scan rate of 0.1 mV s⁻¹ (illustrated in Figure 7a), where a good overlap is observed in the CV curves from the second to the fourth cycle, indicating a good cyclic stability. The presence of multiple oxidation and reduction peaks within these curves confirmed that the electrochemical reactions of NVO@rGO are complex and involve multiple steps. Specifically, the oxidation and reduction peaks at 1.5 V and 1.3 V, respectively, are associated with the intercalation and deintercalation of NH₄⁺ ions [26,27], highlighting the active role of ammonium in the electrochemical process.

Subsequently, the cycling performances of both NVO and NVO@rGO were evaluated at a current density of 0.2 A g⁻¹, as depicted in Figure 7b. The initial specific capacity of NVO was measured at 464 mAh g⁻¹, with a capacity retention of 94% after 50 cycles. In contrast, NVO@rGO demonstrated a superior initial capacity of 507 mAh g⁻¹ and exceptional capacity retention, maintaining 100% after the same number of cycles. Furthermore, NVO@rGO exhibited remarkable cycle stability, preserving its performance over 1400 cycles under these conditions. This improvement in both initial capacity and retention for NVO@rGO compared to pristine NVO is a testament to the beneficial effects of rGO integration, which enhances the structural stability and electrochemical reactivity of the material. These results are in line with the predictions from DFT calculations, which suggested that the addition of rGO would facilitate better ion diffusion and charge transfer.

Moreover, the performance of NVO@rGO not only surpasses that of pristine NVO but also exceeds most of the vanadium oxide cathode materials reported in current literature (as shown in

Table 1. Comparison with reported electrochemical properties of vanadium oxidation chemicals.

Cathode Materials	Electrochemical Performance (Capacity Retention, Cycle Numbers)	Reference
NVO@rGO	507 mAh g−1 at 0.2 A g−1 (100%, 50 cycles)	This work
(NH4)0.37V2O5·0.15H2O	398 mAh g−1 at 0.5 A g−1 (90%, 50 cycles)	[28]
KNH4V4O10	405 mAh g−1 at 0.4 A g−1 (92.1%, 100 cycles)	[19]
Cs–V2O5	369 mAh g−1 at 0.1 A g−1 (90%, 200 cycles)	[29]
(NH4)xV2O5·nH2O	372 mAh g−1 at 0.1 A g−1 (63%, 50 cycles)	[26]
Zn3V2MoO8	337 mAh g−1 at 0.2 A g−1 (90%, 70 cycles)	[30]
NaCaV2O5	310 mAh g−1 at 0.5A g−1 (100%, 200 cycles)	[31]
(NH4)2V4O9	493 mAh g−1 at 0.2 A g−1 (83.2%, 100 cycles)	[32]
V3O7/V6O13	415 mAh g−1 at 0.1 A g−1 (73%, 100 cycles)	[33]
Na2V6O16·3H2O	401 mAh g−1 at 0.3 A g−1 (91.8%, 300 cycles)	[34]
Od–HNaVO@rGO	380 mAh g−1 at 0.5 A g−1 (97.4%, 200 cycles)	[35]

). This superior performance can be attributed to the structural and electronic enhancements brought about by the incorporation of rGO into the NVO matrix, which optimize the material’s electrochemical properties.

Figure 8 presents the galvanostatic charge–discharge (GCD) curves for NVO@rGO and NVO at a current density of 0.2 A g⁻¹. The curves exhibit similar shapes, indicating analogous electrochemical processes are underway in both samples. The high degree of overlap in the GCD curves suggests that both materials, NVO@rGO and NVO, exhibit excellent cyclic stability. This similarity in electrochemical behavior can be attributed primarily to the inherent properties of NVO in both cases.

Furthermore, Figure 8a distinctly illustrates the charge and discharge capacities of the NVO@rGO composite, noting a significant observation in the initial cycle where the charge capacity exceeds the discharge capacity. This phenomenon substantiates the premise that the Coulombic efficiency in the first cycle is inherently less than 100%, a conclusion that appears to contradict the cycling performance data presented in Figure 7b. The discrepancy can be attributed to a misinterpretation of the transient phase behavior depicted in Figure 6b as reflective of a stable Coulombic efficiency. In contrast, Figure 8a provides the raw capacity data, emphasizing that the excess in charge capacity over discharge capacity is due to initial material activation and ion-trapping processes. These findings underscore the necessity of considering the dynamic changes in material behavior during initial cycles to accurately assess the electrochemical performance of the NVO@rGO composite.

Figure 9 showcases the rate performance of NVO@rGO compared to NVO, as the current density was increased stepwise from 0.2 A g⁻¹ to 5 A g⁻¹. The specific capacities for NVO@rGO at these current densities were recorded at 495, 494, 491, 470, and 401 mAh g⁻¹, respectively. Notably, when the current density was reverted back to 0.2 A g⁻¹, the specific capacity of NVO@rGO impressively recovered to 518 mAh g⁻¹. This recovery and the overall higher capacities at each rate compared to NVO underscore the superior rate capability and resilience of the composite material.

The enhanced performance of NVO@rGO can be attributed to the incorporation of rGO, which not only improves the electrical conductivity of NVO but also serves as a structural scaffold. This scaffold helps in stabilizing the crystal structure of NVO during electrochemical cycling, mitigating volume expansion, which often leads to rapid capacity fade in such materials. Additionally, the presence of rGO likely facilitates better electron and ion transport within the electrode, enhancing both the rate capability and cycling stability.

The long-term cycling performance, illustrated in Figure 10, further highlights the durability of NVO@rGO. Tested at a high-current density of 5 A g⁻¹, NVO@rGO demonstrated an initial specific capacity of 388 mAh g⁻¹ and maintained a retention rate of 95% after 600 cycles. In contrast, NVO showed a significantly lower capacity retention under the same conditions. This superior performance of NVO@rGO is indicative of its potential in applications requiring high-power outputs and long-term reliability.

To further elucidate the underlying electrochemical reaction mechanisms, CV curves were examined at various scan rates across a potential window of 0.2 to 1.6 V. Figure 11 illustrates the CV profiles for NVO@rGO and NVO, which exhibit similar morphologies, suggesting that analogous electrochemical processes occur within both materials. Notably, as the scan rate increases, both the oxidation and reduction peaks demonstrate shifts of varying magnitudes, a behavior indicative of electrochemical polarization.

In these CV analyses, the relationship between the peak current (i) and the scan rate (v) is described by the following equations:

i = av^b

(1)

log(i) = log(a) + blog(v)

(2)

where a and b are constants determined empirically. The coefficient b is particularly informative, indicating the degree to which the electrochemical reactions are governed by diffusion or capacitive processes. Generally, b values that are close to 0.5 suggest a reaction mechanism dominated by diffusion, whereas values approaching 1 imply predominant capacitive control.

The calculated b values, as presented in Figure 12a,b, for the four distinct peaks of NVO@rGO are 0.81, 0.97, 0.84, and 0.96, and for NVO, they are 0.73, 1, 0.86, and 0.78, respectively. These values, all within the range of 0.5 to 1, confirm that ion transport within these materials is influenced by both diffusion and surface capacitive effects. However, the prevalence of values nearing 1 highlights that capacitive contributions play a significant role in defining their overall capacitance behavior.

Electron paramagnetic resonance (EPR) testing, as depicted in Figure 13a, revealed the presence of oxygen vacancies within the NVO@rGO composite. These defects are crucial as they often act as active sites for electrochemical reactions and can significantly influence the electronic properties and reactivity of the material [26,28].

Further investigation into the electrochemical properties of NVO@rGO was conducted using Electrochemical Impedance Spectroscopy (EIS). EIS analysis was performed to evaluate the charge transfer resistance and the ionic diffusion resistance before and after cycling. The Nyquist plots typically exhibit a semicircular region at high frequencies, corresponding to the charge transfer resistance, and a linear region at low frequencies, indicative of the ionic diffusion resistance. The EIS measurements, conducted over a frequency range from 0.01 Hz to 100 kHz across various cycle numbers, are presented in Figure 13b. Notably, the impedance after 40 cycles is lower than that before cycling, suggesting an enhancement in electrochemical reactivity and conductivity of the composite due to electrochemical activation and possible restructuring of the electrode material during cycling.

The Galvanostatic Intermittent Titration Technique (GITT) was utilized to quantify the diffusion coefficient of Zn²⁺ ions in the NVO@rGO composite, providing a quantitative measure of ion mobility critical for electrochemical applications. The GITT experiments were methodically carried out through a series of pulses followed by a constant current operation and a subsequent relaxation period. Specifically, a current density of 0.1 A g⁻¹ was applied, with a relaxation time set at 30 min, and measurements were taken at 10 s intervals (shown in Figure 13c,d). The calculation is based on the following equation:

$$D^{GITT}={\displaystyle \frac{4}{\pi \tau }}{\left({\displaystyle \frac{m_B{V}_M}{M_{B}S}}\right)}^2{\left({\displaystyle \frac{{\Delta E}_S}{\Delta E_t}}\right)}^2$$

(3)

where τ represents the relaxation time, m_B is the mass of active material, V_M denotes the molar volume, M_B is the molar mass, S is the surface area of the electrode, and ΔE_S and ΔE_t are the steady-state and transient potential changes, respectively. Using this model, we calculated the diffusion coefficient of Zn²⁺ ions to be 10⁻¹¹ cm² s⁻¹. This value indicates the mobility of Zn²⁺ ions within the NVO@rGO matrix, which is crucial for assessing the efficiency of ion transport.

2.4. Storage Mechanism of Zn²⁺

To further investigate the zinc storage mechanism within the NVO@rGO system during electrochemical reactions, analyses were carried out using ex situ XRD and ex situ X-ray photoelectron spectroscopy (XPS). A button cell employing 3 M Zn(CF₃SO₃)₂ as the electrolyte was assembled and subjected to ten cycles of charging and discharging. Subsequently, the electrode was extracted, washed with distilled water, and dried prior to XRD examination. The results, presented in Figure 14, show characteristic peaks at 7.44°, 25.06°, 30.32°, 45.84°, and 48.6°. These peaks correspond to the standard diffraction angles for NH₄V₄O₁₀, aligning with the (001), (110), (–311), (–205), and (020) crystallographic planes, respectively. The persistence of these peaks throughout the cycling process suggests that the incorporation of reduced graphene oxide (rGO) does not modify the fundamental reaction dynamics within the battery. Notably, the emergence and dissolution of new phases were observed across the entire charging and discharging cycles, indicating dynamic changes in the electrode structure.

In Figure 14, the left portion presents an enlarged view of the (001) crystallographic plane of NVO@rGO. Notably, the peak corresponding to the (001) plane shifts toward higher diffraction angles as the electrode transitions from a charged state at 1.6 V to a discharged state at 0.2 V. This shift indicates a reduction in interlayer spacing, which is attributed to the electrostatic interactions between Zn²⁺ ions and the bilayer structure. When the electrode is returned to the fully charged state at 1.6 V, the peak relocates to its original position, affirming the structural integrity of the NVO@rGO electrode. This resilience is ascribed to the mechanical support and enhanced electrical conductivity provided by graphene within the composite.

Additionally, the XRD data reveal the appearance and subsequent disappearance of two distinct sets of diffraction peaks during the discharge to 0.6 V and recharge to the same voltage. The first set of peaks, located at 5.98° and 18.32°, is associated with the intercalation and deintercalation of Zn²⁺ ions within the NVO@rGO electrode matrix. These peaks disappear as the Zn²⁺ ions are extracted at 0.6 V. A second set of peaks, observed at 12.08° and 33.2°, corresponds to the formation of a new phase, Zn₃(OH)₂V₂O₇·2H₂O. This phase is a recognized intermediate in the electrochemical processes of vanadium-based AZIBs [36,37], underscoring the dynamic nature of the phase transformations occurring within these cells.

To confirm the presence of the specified compound, SEM analyses were performed on electrodes in their initial state (Figure 15a), after discharge to 0.2 V (Figure 15b) and following recharge to 1.6 V (Figure 15c). The SEM images of the electrode in its initial state showed no extraneous materials. However, upon discharge to 0.2 V, micrometer-scale flake-like structures emerged on the surface of the active material. These structures are consistent with the morphology of Zn₃(OH)₂V₂O₇·2H₂O, as described in existing literature [38]. These structures dissipated upon recharging to 1.6 V, aligning with changes observed in ex situ XRD analyses and demonstrating the reversible intercalation and deintercalation of Zn²⁺ ions throughout the cycling process. To further verify the identity of the intermediate phase, the battery was discharged to 0.2 V, and EDS mapping of the micrometer-scale flakes on the electrode was conducted (Figure 15d). The EDS analysis indicated a uniform distribution of Zn, V, and O elements within these structures, confirming their composition as Zn₃(OH)₂V₂O₇·2H₂O. Additionally, the presence of these by-products led to the formation of uniform holes on the surface of the cathode plate, a morphological alteration that likely increases the contact area available for electrochemical reactions and potentially enhances the system’s electrochemical performance.

Concurrently, XPS analyses were performed on electrodes in various states: pristine, after discharging to 0.2 V, and after recharging to 1.6 V (Figure 16), to elucidate the chemical state transformations of the cathode material during the electrochemical process. Initially, four prominent peaks were detected: V 2p_1/2 and V 2p_3/2 at 516.39 eV and 523.90 eV, respectively, characteristic of V⁴⁺, and V 2p_1/2 and V 2p_3/2 at 517.72 eV and 525.26 eV, respectively, indicative of V⁵⁺. Upon discharging to 0.2 V, an increase in the intensity of V⁴⁺ peaks was observed, signaling a reduction state favorability. Furthermore, during the discharge to 0.2 V, the vanadium-based cathode material underwent dissolution in the aqueous solution, resulting in the formation of the byproduct Zn₃(OH)₂V₂O₇·2H₂O [39]. Subsequent recharging to 1.6 V led to the predominance of V⁵⁺ over V⁴⁺, reverting to the initial electrochemical state and demonstrating the structural reversibility of the cathode material.

In addition, the zinc states were monitored across different levels of discharge. Initially, no peaks corresponding to Zn²⁺ were discernible. Nevertheless, upon discharging to 0.2 V, peaks at 1022.7 eV and 1045.56 eV, corresponding to Zn 2p_1/2 and Zn 2p_3/2, manifested, suggestive of Zn²⁺ intercalation. Notably, a residual absorption peak for zinc persisted upon recharging to 1.6 V. This occurrence is a typical reaction artifact, arising from incomplete extraction of Zn²⁺, thereby resulting in the formation of residual, or “dead”, zinc.

3. Materials and Methods

3.1. Calculation Method

The DFT calculations were performed using the Vienna Ab Initio Simulation Package (VASP 6.0) [40]. The exchange–correlation interactions were described by the Perdew–Burke–Ernzerhof (PBE) functional [41]. Our computational strategy incorporated a plane–wave basis set, with a chosen energy cutoff of 400 eV, in conjunction with the Projector Augmented Wave (PAW) method [42,43]. We set the convergence thresholds for the self-consistent field iterations at 1 × 10⁻⁵ eV, and for the force convergence at 0.01 eV Å⁻¹. The Brillouin zone was sampled using a Γ-centered k-point mesh of 2 × 4 × 1 for both the relaxation of the geometric structures and the total energy calculations.

3.2. Preparation of Material

The synthesis of NVO and NVO@rGO was accomplished through a one-step hydrothermal method.

NVO Synthesis: Initially, NVO was synthesized by dissolving 8 mmol of ammonium metavanadate in 60 mL of deionized water, which was heated to 60 °C until complete dissolution occurred. The solution was then allowed to cool to room temperature. In parallel, 8 mmol of oxalic acid was dissolved in 10 mL of deionized water and was slowly added to the ammonium metavanadate solution with continuous stirring. After vigorous stirring at room temperature for 0.5 h, the combined solution was transferred to a 100 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 180 °C for 24 h. Upon natural cooling to room temperature, the resultant dark green product was isolated in a centrifuge tube and washed multiple times with alternating cycles of deionized water and anhydrous ethanol, followed by centrifugation. The final product was then freeze-dried for 12 h to yield NVO.

NVO@rGO Synthesis: The NVO@rGO composite was synthesized by initially weighing 50 mg of graphene oxide (GO), which was then dispersed in 50 mL of distilled water. This mixture underwent sonication at a frequency of 45 kHz for 30 min to achieve a uniform dispersion of the GO. The GO utilized was sourced from Nanjing XFNANO Materials Tech Co., Ltd., located in Nanjing, China. The sonication process was meticulously monitored to ensure consistent dispersion throughout the solution. Then, 0.9359 g of NH₄VO₃ was added to this dispersion and dissolved at 60 °C, after which the solution was allowed to cool to room temperature. Separately, 1.386 g of H₂C₂O₄·2H₂O was dissolved in 10 mL of distilled water. Once dissolved, it was slowly added to the graphene oxide solution containing NH₄VO₃ under vigorous stirring for 0.5 h. This mixture was then transferred to a 100 mL Teflon-lined autoclave and subjected to a hydrothermal reaction at 180 °C for 24 h. After cooling to room temperature, the resultant black product was processed in the same manner as NVO, involving multiple washes, centrifugation, and freeze-drying, to yield the final product, designated as NVO@rGO.

3.3. Electrode Fabrication

The electrode composition comprising the active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive agent was added in a 6:3:1 ratio into a mortar. The mixture was ground extensively to ensure uniform blending and achieve a fine, smooth powder. Subsequently, N-methyl-2-pyrrolidone (NMP) was added dropwise, and the mixture was further milled until the resulting paste displayed a shiny surface. Titanium foil was employed as the current collector, onto which the paste was evenly spread. The coated foils were then dried in a vacuum oven at 110 °C for 12 h. After cooling to room temperature, the electrodes were punched into circular disks of 1 cm diameter, with an active material load of approximately 1.3 mg cm⁻².

3.4. Structure and Morphology Characterization

The structural properties of all materials synthesized during the electrochemical process were characterized using XRD with Cu Kα radiation (Smart Lab SE, Tokyo, Japan). Morphological and crystalline features of these materials were systematically investigated using SEM (Hitachi SU8010, Tokyo, Japan) and TEM (FEI Talos F200X, Waltham, MA, USA). The elemental distribution within the samples was analyzed using SEM equipped with EDS. Additionally, XPS (Thermo ESCALAB 250Xi, Waltham, MA, USA) using an Al Kα anode was employed to determine the elemental composition and to monitor changes in the valence states in both powder samples and electrode slices.

Furthermore, using a VERTEX 70 spectrometer (Saarbrücken, Germany) and employing the attenuated total reflectance (ATR) method, FTIR was utilized to elucidate the functional groups present within the materials. This approach facilitated comprehensive comparative analyses. Additionally, the presence of O_d in the materials was confirmed through EPR spectroscopy. These measurements were conducted using an EMX nano spectrometer, specifically designed for the detection of unpaired electrons.

3.5. Electrochemical Measurements

The electrochemical properties of the synthesized materials were extensively assessed through the assembly and evaluation of CR2032-type button cells, designated as the prototype cells for this study. During the assembly phase, zinc foil was employed as the anode, whereas titanium foil served as the current collector. A glass fiber material was utilized as the separator. The chosen electrolyte was a 3 M solution of zinc trifluoromethanesulfonate (Zn(CF₃SO₃)₂). The entire assembly process was conducted under standard atmospheric conditions and concluded with the cells being hermetically sealed using a specialized sealing apparatus.

CV and EIS measurements were performed using a CHI 760E electrochemical workstation. For the CV measurements, a voltage range of 0.2 to 1.6 V was applied at scan rates from 0.1 to 0.5 mV s⁻¹. EIS measurements were conducted across a frequency range of 0.01 Hz to 100 kHz in a potentiostatic mode with an amplitude of 10 mV. Further evaluations, including assessments of specific capacity, GCD curves, and GITT, were conducted using the Battery Test System CT2001A, produced by Wuhan LAND Electric Co. (Wuhan, China), within the same voltage range of 0.2 to 1.6 V. For the GITT measurements, a constant current mode was employed, featuring a pulse duration of 10 min followed by a relaxation period of 1 h.

4. Conclusions

This study presents a significant advancement in the development of AZIBs through the integration of a novel cathode material, NVO, with rGO. The introduction of rGO to the NVO structure, as hypothesized and supported by DFT calculations, effectively increases the interlayer spacing and reduces charge transfer interactions. This modification facilitates a more efficient diffusion of Zn²⁺ ions, addressing a crucial challenge in the performance of vanadium-based cathode materials. The experimental results from hydrothermal synthesis have validated our theoretical predictions, demonstrating a substantial increase in interlayer spacing. The electrochemical tests further highlight the remarkable durability of the NVO–rGO composite, which retains 94.54% of its initial specific capacity of 507 mAh g⁻¹ after 600 cycles at a high current density of 5 A g⁻¹. This performance not only surpasses that of pristine NVO but also exceeds the majority of existing vanadium oxide cathode materials in terms of cyclic stability and capacity retention.

These findings underscore the potential of NVO–rGO composites in enhancing the practical applicability of AZIBs, suggesting a pathway to more sustainable, cost-effective, and safer battery technologies. The integration of theoretical insights and experimental validation in this research contributes to the field of energy storage, supporting the transition towards renewable energy sources by improving the efficiency and reliability of energy storage solutions.