English

• Virtually all portable electronic devices, from cell phones and laptops to hybrid vehicles, rely on rechargeable battery technology [1,2]. Nevertheless, the widely used lithium-ion batteries (LIBs) have significant safety and environmental issues due to the formation of lithium dendrites, the high reactivity of lithium metal, and the toxicity of organic electrolytes [3–6]. Against this background, aqueous zinc-ion batteries (ZIBs) are suitable for large-scale energy storage, owing to their unique advantages, such as high theoretical capacity (820 mAh/g and 5855 mAh/cm³), low redox potential (−0.76 V vs. the standard hydrogen electrode), relatively high stability in water, and abundant resources [7,8]. Unfortunately, ZIBs still lack cathode materials which are structurally stable and have high capacity [9]. Currently, host materials for Zn²⁺ ions are predominantly composed of oxygen compounds that feature either a layered or tunnel-like structure. Notable examples include manganese oxide and vanadium oxide, as well as the versatile Prussian blue and its various derivatives [10]. Vanadium-based materials, compared to other cathode material systems, hold great potential as cathode materials for AZIBs, owing to their high theoretical zinc storage capacity, multi-valence of vanadium, and low raw material costs [11,12]. However, they face challenges such as inherently low electronic conductivity, incompatibility between interlayer spacing and Zn²⁺/Zn(H₂O)_n²⁺, and strong electrostatic interactions with zinc ions [13]. These issues lead to slow Zn²⁺ diffusion rates, dissolution of cathode materials in the electrolyte, and collapse of the host structure, severely hindering their utilization in large-scale applications.

  To address the limitation of rate performance and cycling stability, numerous strategies have been proposed, such as the application of nanostructured materials, amorphous phases, and the fabrication of protective coatings [14]. In terms of enhancing electronic conductivity, blending cathode materials with highly conductive carbon materials is the most cost-effective measure [15]. However, these common conductive materials like carbon black and graphene are low-dimensional. Due to the limited conductive contact with active materials, it is difficult to form a continuous and stable conductive network [16]. Suppressing the dissolution/collapse of cathode materials while improving electronic and ionic transport properties poses a significant challenge. In recent years, cathode materials with carbon coatings synthesized through methods such as mechanical ball milling, wet chemical methods, and thermal plasma have received widespread attention in various battery systems. The composite cathode materials prepared exhibit a stable structure which can inhibit adverse reactions of active particles, significantly improve the reaction kinetics, and achieve structural stability along with controllable volume changes [17]. However, complex multi-step processing or the use of toxic raw materials makes these methods environment unfriendly and less cost-effective [18]. Therefore, there is an urgent need for a simple and economical approach to prepare composite vanadium-based cathode materials with high ionic/electronic conductivity and structural stability.

  Herein, we successfully constructed novel carbon-coated V₂O_5-x nanoparticles (V₂O_5-x@C) via a simple plasma-enhanced chemical vapor deposition (PECVD) method, which significantly boosted the superior cycling stability and reaction kinetics as the cathode for AZIBs. As demonstrated through various electrochemical and material characterizations, V₂O_5-x@C cathode combines the superior structural characteristics of V₂O₅ nanoparticles and uniform conductive carbon coating layer, more oxygen vacancies, larger specific surface area, and higher zinc storage capacity, which guarantees fast electron and ion transfer kinetics and structural stability of the electrode. Impressively, a large specific surface area (4.1615 m²/g) and excellent stability of electrode in the aqueous electrolyte can be well achieved, which can be ascribed to the unique composite structure. Benefiting from rapid reaction kinetics and cycling stability, V₂O_5-x@C cathode delivers a high initial specific capacity of 149.2 mAh/g, with an excellent capacity retention of 83.8% at 5 A/g after 1000 cycles. Even at a high current density of 10 A/g, the specific discharge capacity of V₂O_5-x@C cathode still can remain at 80.2 mAh/g after 1000 cycles, with an attenuation of approximately 25.8%. This research suggests that directly constructing carbon-coating layer on cathode materials through PECVD method could vastly motivate the development of the design principle for high-performance cathode materials towards superior electrochemical energy storage systems.

  The synthetic procedure of V₂O_5-x@C cathode is illustrated in Fig. 1a. The V₂O₅ nanoparticles were coated with a uniform amorphous carbon layer via PECVD using methane (CH₄) as the carbon precursor. Meanwhile, the vanadium of the +5 oxidation state was partially downgraded to +4 and +3, concurrently with the emergence of oxygen vacancies which ensured electroneutrality, culminating in the fabrication of the V₂O_5-x@C cathode. This is because the ion bombardment generated by gas ionization in the plasma promotes the reduction of vanadium, thereby reducing their valence. It is worth noting that unlike traditional wet chemical coating procedures, PECVD method can achieve uniform coatings with ideal carbon layer thickness without the need for additional work steps or the generation of by-products. The morphology and microstructure of the V₂O₅ and V₂O_5-x@C were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM) characterizations. As shown in Fig. S1 (Supporting information), the pristine V₂O₅ exhibits irregular particle distribution with sizes accompanied by serious agglomeration. Under the action of high-energy plasma, the irregular shape of V₂O₅ nanoparticles undergoes a transformation, and the V₂O_5-x@C reveals the uniform spherical nanoparticle morphology (Fig. 1b). Noticeably, by strictly controlling the degree of reduction, V₂O_5-x@C can still maintain good dispersibility without significant agglomeration, which can increase the electrochemical active surface area of the cathode material and be beneficial for improving the capability to store zinc.

  Figure 1

  

  Figure 1.  (a) Schematic illustration of the synthesis process of the V₂O_5-x@C cathode material. (b) SEM images of V₂O_5-x@C and (c) EDS mapping of C, V, and O elements. (d–f) TEM images of the V₂O_5-x@C.

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  In addition, the energy dispersive spectroscopy (EDS) elemental mapping images in Fig. 1c confirm the uniform dispersion of the O and V elements among the entire spherical particles, and C element is distributed on the outer surface of nanoparticles, while nearly no C element is found in the pure V₂O₅ (Figs. S2 and S3 in Supporting information), further evidencing the homogeneous coverage of carbon coating on the surface of V₂O_5-x@C. A closer observation of transmission electron microscopy (TEM) clearly shows a thin and uniform coating layer on the surface of V₂O_5-x@C nanoparticles and the average thickness of the carbon coating layer is around 20 nm (Fig. 1d). Combined with the high-resolution TEM images in Figs. 1e and f, the defined lattice fringes can be observed, in which the interplanar spacing about 0.57 nm and 0.34 nm can be indexed to the (001) plane of V₂O₅ and (101) plane of VO₂, respectively [19].

  Consistent results can also be observed in X-ray diffraction (XRD) pattern in Fig. 2a, the pattern of V₂O_5-x@C exhibits the characteristic diffraction peaks of VO₂ (JCPDS No. 76-0677), except the characteristic diffraction peaks of V₂O₅ (JCPDS No. 09-0387). Moreover, unlike the brownish yellow V₂O₅ powder, V₂O_5-x@C powder displays a uniform brownish black color, as shown in the insets of Fig. 2a. Similar result can be further proved by the Raman measurement (Fig. 2b). Raman spectra reveal that V₂O₅ and V₂O_5-x@C display similar stretching and bending vibrations of chemical bonds in the region of 100–1200 cm⁻¹, indicating that the V₂O_5-x@C composite cathode still retains the V=O and V-O-V chemical bonds of V₂O₅ (Fig. 2c). Remarkably, the peak at 877 cm⁻¹ represents the C-O-C band of carbon, which correspond to the carbon layer on the surface of V₂O_5-x@C [20,21]. These results further confirm that during the PECVD process, a portion of V⁵⁺ in V₂O₅ powder is reduced to low valence states and exists on the surface of the nanoparticles.

  Figure 2

  

  Figure 2.  (a) XRD patterns (Insets show photographic images of V₂O_5-x@C and V₂O₅ powders), (b) Raman shift, (c) normalized Raman spectroscopy with an energy range of 650–750 cm⁻¹, (d) EPR spectra, (e) TGA curves, and (f) BET adsorption isotherm, of the synthesized V₂O_5-x@C and pure V₂O₅ powders.

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  We deemed that a large number of oxygen vacancies will be generated during the transition of V element from +5 valence to low valence states, so Raman spectroscopy and electron paramagnetic resonance (EPR) spectra were conducted to prove this conjecture. Combining the results of Figs. 2b and c, it can be found that the Raman spectrum of V₂O_5-x@C has a slight blue shift relative to the pristine V₂O₅, which can be considered as the effect of existent oxygen vacancies [22–24]. In order to determine the content of oxygen vacancies, we obtained the EPR spectra of pristine V₂O₅ and V₂O_5-x@C samples (Fig. 2d). A clear oxygen vacancy signal at a typical g-factor equal to 2.006 is observed in the EPR spectra, demonstrating the content of oxygen vacancies can be significantly enhanced through PECVD treatment. Plentiful oxygen vacancies can stimulate cathode adsorption affinity which would contribute to the improvement of storage capacity and electrochemical performance. Furthermore, the carbon loading in the V₂O_5-x@C was characterized by thermogravimetric analysis (TGA) [25]. The 0.66% weight loss of V₂O_5-x@C between 0 and 350 ℃ could be ascribed to the removal of carbon coating (Fig. 2e). It is noteworthy that although the carbon content is low, the PECVD treatment process not only covers the surface of V₂O₅ with a carbon layer, but also significantly improves the uniformity of nanoparticles and suppresses agglomeration, thereby significantly increasing the surface area (4.1615 m²/g for V₂O_5-x@C, and 2.8695 m²/g for V₂O₅) (Fig. 2f), which is conducive to the diffusion and transport of zinc ions [26–28].

  To determine the valence variation of V states and the generation of carbon layer after reduction during PECVD process, the V₂O₅ and V₂O_5-x@C electrodes were evaluated via X-ray photoelectron spectrometer (XPS) measurement. As depicted in Fig. 3a, V 2p and O 1s peaks are observed in the XPS data of both V₂O₅ and V₂O_5-x@C electrodes, while an obvious C 1s peak can be found in the data of V₂O_5-x@C electrode. Besides, in the C 1s spectrum, the three peaks are concentrated at 284.8, 286.7 and 288.8 eV corresponding to the C-C/C-H, C-O and C=O bonds, respectively, proving that uniform and stable carbon coating is generated on the surface of the cathode materials during the preparation process (Fig. 3b) [29–31]. Only the peaks of V⁵⁺ present at 517.6 and 525.0 eV in the V 2p spectrum of pristine V₂O₅, which is in good consistent with the V₂O₅ crystal structure (Fig. 3c). Conversely, the V 2p spectrum of V₂O_5-x@C electrode displays three spin-orbit peaks, which can be attributed to V⁵⁺ (517.6 and 525.0 eV), V⁴⁺ (516.5 and 523.4 eV), and a low content of V³⁺, indicating that PECVD process can lead to the formation of a mixed valence state in V₂O₅ (Fig. 3d) [19,32]. This facilitates an increase in electrical conductivity through electron hopping among V³⁺, V⁴⁺, and V⁵⁺. Concurrently, the formation of oxygen vacancies helps to preserve electroneutrality, which in turn enhances ionic conductivity and improves charge-transfer kinetics [33,34]. Moreover, benefiting from the carbon layer coated on vanadium-based oxide nanoparticles, the structure and interface stability of the electrode in aqueous electrolyte have been significantly improved [35,36]. After soaking V₂O₅ and V₂O_5-x@C electrodes in the electrolytes for 4 days, it can be observed that the color of the electrolyte within soaked V₂O₅ electrode has significantly turned yellow, which is due to the continuous dissolution of V₂O₅ in the electrolyte (Fig. 3e). The optical image of the V₂O₅ electrode surface also shows a large number of pores and uneven surfaces caused by the dissolution of the cathode material (Fig. 3f). Contrarily, the V₂O_5-x@C electrode immersed in the electrolyte did not show significant dissolution, and there is still a dense distribution of active material on the surface of the electrode.

  Figure 3

  

  Figure 3.  (a) Wide-scan XPS spectra of V₂O_5-x@C and V₂O₅. (b) C 1s XPS spectra of V₂O_5-x@C. V 2p XPS spectra of (c) V₂O₅ and (d) V₂O_5-x@C. (e) Optical photographs of V₂O_5-x@C and V₂O₅ cathodes soaked in the electrolyte. (f) Optical photographs of the surfaces of V₂O_5-x@C and V₂O₅ cathodes after soaking in the electrolyte for 4 days.

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  To investigate the electrochemical capacity retention and cycling stability, the performance and coulombic efficiency of V₂O₅ and V₂O_5-x@C electrodes are tested during extended cycling (Fig. S4 in Supporting information). Firstly, two pairs of redox peaks are found in the CV curves which are shown in Fig. 4A, indicating the Zn²⁺ intercalation/deintercalation reactions. Meanwhile, the reduction of the potential difference between the redox peaks of V₂O_5-x@C electrode demonstrates enhanced electrochemical activity and reversibility. In addition, the promotion of V₂O_5-x@C on electron transfer is characterized by Nyquist impedance spectra (Fig. 4B). The Zn//V₂O_5-x@C full cell delivers a far smaller interface impedance than that of Zn//V₂O₅ cell, indicating the facilitated charge transfer process and good interfacial compatibility at the anode/electrolyte interface [37,38]. The rate performance is given in Figs. 4C–E. The enhanced electrochemical kinetics demonstrates that the V₂O_5-x@C electrode is highly effective in facilitating the Zn²⁺ diffusion process.

  Figure 4

  

  Figure 4.  Electrochemical performance of V₂O_5-x@C and V₂O₅ electrodes. (A) CV curves at a scan rate of 1 mV/s. (B) Nyquist plots. (C) Rate capacity at 0.5–10 A/g. (D, E) Discharge curves at different current densities. Cycle performance and coulombic efficiency at (F) 5 A/g and (G) 10 A/g. (H) Galvanostatic charge/discharge profiles at 10 A/g.

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  Zn//V₂O₅ cell exhibits lower initial specific capacity, and with increasing current density, the specific capacity significantly decreases, mainly due to lower electronic conductivity and limited diffusion of ions in active materials. On the contrary, Zn//V₂O_5-x@C full cell delivers higher specific capacities at various current densities, and remains 277.6 mAh/g when returned to 1 A/g. It is concluded that V₂O_5-x@C composite cathodes exhibit a highly stable structure and faster reaction kinetics during the current fluctuation, which can lead to an extremely stable performance during long-term cycling (Figs. 4F–H) [39]. V₂O_5-x@C composite cathode shows a retained capacity of 83.8% upon 1000 cycles at 5 A/g, even at a higher current density of 10 A/g, the specific discharge capacity of V₂O_5-x@C cathode still can remain at 80.2 mAh/g after 1000 cycles. The cyclic stability of V₂O_5-x@C composite cathode is also much superior to most reported works under various testing conditions (Table S1 in Supporting information). Noticeably, according to the interfacial impedance after cycling (Fig. S5 in Supporting information), the interface stability is better in V₂O_5-x@C composite cathode [40]. Moreover, it is generally approved that the Raman signal at around 980 cm⁻¹ from SO₄²⁻ can reflect the concentration change of zinc ions to some extent [41]. As shown in Fig. S6 (Supporting information), a constant Raman signal is observed at the interface of V₂O_5-x@C cathode during cycling, testifying optimized ion flux and fast transfer dynamics of Zn ions at the interface. Importantly, the Zn//V₂O_5-x@C full cell shows better self-discharge performance, which is evidenced by higher capacity retention over 34 h (Fig. S7 in Supporting information). The electrochemical performance is greatly optimized owing to the improved electrical conductivity, enhanced structural stability, and the increased specific surface area of the V₂O_5-x@C composite cathode.

  It is widely recognized that lattice and phase transformations are crucial during the charge/discharge process [42–44]. To elucidate these structural dynamics, ex situ XRD and XPS analyses were conducted on V₂O_5-x@C cathode, shedding light on its evolution during the intercalation/deintercalation of Zn²⁺ ions. Specifically, the XRD diffraction patterns at varying voltages provide a visual representation of the structural transition (Figs. 5a–c). During the discharge phase, as the voltage decreased from 1.6 V to 0.4 V, a slight shift of the diffraction peak can be observed moving towards lower angles. This deviation is indicative of Zn²⁺ ions inserting into the electrode, leading to an expansion of the interlayer spacing [45]. Conversely, during the charging phase, the peak reverts to its initial angle, corresponding to the desertion process of Zn²⁺ ions. Notably, throughout this charge/discharge process, no phase transition or byproduct formation were detected. This observation confirms that the phase composition of V₂O_5-x@C remains stable and unaltered, underscoring its robustness and reliability as an electrode material in the application of AZIBs [46].

  Figure 5

  

  Figure 5.  (a–c) Ex situ XRD pattern of V₂O_5-x@C electrodes during the charge/discharge process in the first cycle. (d–f) High-resolution V 2p spectra of V₂O_5-x@C electrodes under different voltages. (g) Diagram of the alteration in the lattice structure during the zinc ions intercalation/deintercalation process.

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  Additionally, the variation trend of the V 2p peak during the charge and discharge processes is depicted in Figs. 5d–f. As the results mentioned in Fig. 3d shows, V⁵⁺ is dominant in the pristine V₂O_5-x@C electrode, with a minor presence of V³⁺ and V⁴⁺. Upon discharging to 0.4 V, there is a significant reduction in the proportion of V⁵⁺, coinciding with an increase in the percentages of V³⁺ and V⁴⁺. This shift implies the insertion of Zn²⁺ ions, which leads to a reduction in the valency of vanadium. Instead, when the cell was charged to 1.6 V, the valence state of vanadium reverts to its original state, signifying that the Zn²⁺ ions can be efficiently deserted [47]. The alterations in the lattice structure throughout the Zn²⁺ intercalation and deintercalation processes are clearly presented in Fig. 5g. The V₂O_5-x@C nanoparticles are tightly combined with a carbon coating layer, which remains stable during both the Zn²⁺ insertion and extraction processes. A marked reduction in the content of Zn²⁺ occurs after the charge process. According to the above analysis results, V₂O_5-x@C electrode exhibits the high reversibility of the lattice structure, with no indication of phase transition or the formation of any byproducts, which is in favor of remarkably improving the electrochemical performance.

  In conclusion, a novel V₂O_5-x@C composite cathode composed of oxygen-vacancy-abundant V₂O_5-x nanoparticles and carbon layer with high conductivity was successfully constructed via a simple PECVD method. Uniformly dispersed spherical V₂O_5-x nanoparticles significantly increase the surface area, providing more active sites for electrochemical reactions. The rich oxygen vacancies and the conductive carbon layer coating ensure rapid electron transfer and greatly enhance the diffusion kinetics of zinc ions. Additionally, the composite structure of the cathode guarantees the structural stability of the electrode during the repeating charge/discharge process and the interface reliability in the aqueous electrolyte. Benefiting from the fast electron/ion kinetics, the novel V₂O_5-x@C composite cathode achieves ideal surface area (4.1615 m²/g) and excellent cycling stability, as demonstrated via various material characterizations and electrochemical analyses. Impressively, V₂O_5-x@C electrode exhibits the comprehensive performance of excellent rate and cycle stability, along with minimal structural alteration upon Zn²⁺ intercalation and high reversibility. The Zn//V₂O_5-x@C cell achieves a high initial specific capacity of 149.2 mAh/g at 5 A/g, possessing a capacity retention of 83.8% over 1000 cycles. Such stabilized crystal structure and enhanced ion diffusion coating of V₂O_5-x@C cathode herald promising prospects for the development of high-performance AZIBs.

  Declaration of competing interest

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

  CRediT authorship contribution statement

  Canglong Li: Software, Methodology. Tao Liao: Writing – review & editing. Dongping Chen: Validation. Tiancheng You: Investigation. Xiaozhi Jiang: Investigation. Minghan Xu: Investigation, Formal analysis. Huaming Yu: Writing – original draft, Visualization, Supervision. Gang Zhou: Supervision, Software. Guanghui Li: Project administration. Yuejiao Chen: Writing – review & editing, Supervision, Funding acquisition.

  Acknowledgments

  This research was financially supported by the National Natural Science Foundation of China (No. 52377222) and Natural Science Foundation of Hunan Province (No. 2023JJ20064).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110557.

  
  
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  Figure 1  (a) Schematic illustration of the synthesis process of the V₂O_5-x@C cathode material. (b) SEM images of V₂O_5-x@C and (c) EDS mapping of C, V, and O elements. (d–f) TEM images of the V₂O_5-x@C.

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  Figure 2  (a) XRD patterns (Insets show photographic images of V₂O_5-x@C and V₂O₅ powders), (b) Raman shift, (c) normalized Raman spectroscopy with an energy range of 650–750 cm⁻¹, (d) EPR spectra, (e) TGA curves, and (f) BET adsorption isotherm, of the synthesized V₂O_5-x@C and pure V₂O₅ powders.

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  Figure 3  (a) Wide-scan XPS spectra of V₂O_5-x@C and V₂O₅. (b) C 1s XPS spectra of V₂O_5-x@C. V 2p XPS spectra of (c) V₂O₅ and (d) V₂O_5-x@C. (e) Optical photographs of V₂O_5-x@C and V₂O₅ cathodes soaked in the electrolyte. (f) Optical photographs of the surfaces of V₂O_5-x@C and V₂O₅ cathodes after soaking in the electrolyte for 4 days.

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  Figure 4  Electrochemical performance of V₂O_5-x@C and V₂O₅ electrodes. (A) CV curves at a scan rate of 1 mV/s. (B) Nyquist plots. (C) Rate capacity at 0.5–10 A/g. (D, E) Discharge curves at different current densities. Cycle performance and coulombic efficiency at (F) 5 A/g and (G) 10 A/g. (H) Galvanostatic charge/discharge profiles at 10 A/g.

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  Figure 5  (a–c) Ex situ XRD pattern of V₂O_5-x@C electrodes during the charge/discharge process in the first cycle. (d–f) High-resolution V 2p spectra of V₂O_5-x@C electrodes under different voltages. (g) Diagram of the alteration in the lattice structure during the zinc ions intercalation/deintercalation process.

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