Citation

Wang Xiaojun, Zhang Yizhou, Guo Linwei, Li Jianwei, Wang Peng, Yang Lei, Liu Zhiming. V₂CT_X MXene-derived ammonium vanadate with robust carbon skeleton for superior rate aqueous zinc-ion batteries[J]. Chin. Chem. Lett., 2025, 36(8): 111231  

Xiaojun Wang^a,*, Yizhou Zhang^a, Linwei Guo^a, Jianwei Li^a, Peng Wang^a, Lei Yang^b, Zhiming Liu^a,*     

The global energy crisis and environmental pollution issues have caused an increased focus on renewable energy sources [1–3]. Development of reliable electrical energy storage (EES) systems is an essential for the effective utilization of renewable energy resources due to its intermittent nature [4–6]. Among them, lithium-ion batteries (LIBs) are the most widespread energy storage system due to their high energy density and long lifetime [7–14]. However, the security issues associated with organic electrolytes and the high cost of raw materials have constrained their further application [15,16]. In this context, aqueous zinc ion batteries (ZIBs) have stand out due to its abundance and air-stable zinc metal anode, high theoretical capacity (820 mAh/g) and low redox potentials (−0.76 V vs. the standard hydrogen electrode) [17–19]. However, the high charge density of Zn²⁺ leads to large charge transfer resistance and delayed diffusion at the electrode surface. Meanwhile, the strong electrostatic interaction between Zn²⁺ and the lattice of the host material leads to the collapse of the crystal structure, which results in inferior cycling life [20–24]. Therefore, finding suitable cathode materials that can overcome the inherent disadvantages of Zn²⁺ storage has become a major priority in realizing the large-scale application of ZIBs [25].

Recently, various layered vanadium-based oxides have proved to be broadly promising cathode materials for aqueous ZIBs because of their tunable layer structure and multivalent states of vanadium ions [26]. Among of them, layered ammonium vanadate has getting more and more attention because of its small relative molecular mass and large ionic size of the ammonium ions, which result in large interlayer spacing and high specific capacity [27–29]. However, layered ammonium vanadate materials often suffer from irreversible delocalization of ammonium ions during charging and discharging process, which leads to poor cycling life [30]. Wang et al. designed Na-doped Na_0.3(NH₄)_0.6V₄O₁₀⋅0.4H₂O (NVO-Na) nanorods, the intercalated Na⁺ act as pillars, which can stabilize the NVO-Na structure together with the structural water molecules as well as the remaining NH₄⁺ [31]. However, using ionic intercalation modified ammonium vanadate tend to have unsatisfactory electrical conductivity [32]. In general, the conventional method is to combine vanadium-based materials with carbon matrix through high-temperature annealing to improve electrical conductivity of electrode [33–35]. Niu et al. developed a composite of amorphous vanadium oxide derived from MOF (MIL-88B(V)) and in-situ residual carbon framework by high-temperature annealing as a cathode with excellent rate capability and long-term cycling stability [36]. Nevertheless, ammonium vanadate are accompanied by the loss of NH₄⁺ during heat treatment in high temperature, which limits the application of above strategies [37,38]. Therefore, designing a strategy to uniformly coat carbon layer on ammonium vanadate surfaces under a relatively mild conditions is a great challenge. V₂CT_x MXene, an important transition metal carbide containing both vanadium and carbon elements, is considered as an ideal precursor to realize the strategy for preparation of ammonium vanadate with simultaneous formation of uniform carbon cladding under mild conditions.

Herein, we designed an ultra-thin carbon layer coated ammonium vanadate (NHVO@C) composite via in-situ derived V₂CT_x MXene strategy in a moderate tempetature, in which the V₂CT_x acts as a bifunctional precursor, on the one hand, as a vanadium source in-situ derived into hydrated ammonium vanadate with a large layer spacing, and on the other hand, the residual carbon element of V₂CT_x MXene forms a robust conductive carbon skeleton in the in-situ derivatization process. Such process eliminates the need for a high-temperature carbonization process, avoids the loss of NH₄⁺ during synthesis, and achieves an interfacial coupling between the carbon material and the host structure, that can enhance electron transport kenitics and achive excellent rate performance. Moreover, the amorphous carbon layer as a solid skeleton inhibits the NH₄⁺ dissolution and structural collapse during the charging and discharging process, which greatly improves the long cycle stability of the battery. As the result, the aqueous ZIBs with NHVO@C flexible cathode exhibit ultrahigh rate capability (158.6 mAh/g at 20 A/g) as well as excellent cycling stability (71.2% capacity retention at 10 A/g after 5000 cycles), and the NHVO@C film cathode with high active material loading of 10.2 mg/cm² based-soft pack batteries also exhibit good electrochemical stability. This work provides insights for the design of carbon composite cathode materials with superior performance.

Fig. 1a shows the preparation process of NHVO composite. Firstly, V₂AlC MAX was selectively etched in a mixed solution of HCl and NaF to obtain layered V₂CT_x MXene. Subsequently, by using NH₄Cl as the ammonia source and V₂CT_x MXene as the vanadium source, the NHVO@C composite, consisted of ammonium vanadate nanoribbons and conductive carbon skeleton, was synthesized by a one-step hydrothermal method. Remarkably, ammonium vanadate was derived from the sheets of V₂CT_x MXene during the hydrothermal process, inheriting the remaining carbon of V₂CT_x MXene to form a homogeneous amorphous carbon coating on the surface. The amorphous carbon as a robust skeleton can enhance the electrical conductivity as well as mitigate the structural collapse of the NHVO@C composite during charging and discharging process. Correspondingly, NHVO without a conducting carbon skeleton were also prepared within the same conditions using commercial V₂O₅.

Figure 1

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Fig. 1. (a) Synthesis process of NHVO@C composite. SEM image of (b) V2CTx MXene and (c, d) NHVO@C composite. (e) TEM image, (f) HRTEM image, (g) HAADF image and elemental mappings of NHVO@C composite.

The morphology of the NHVO@C samples were explored by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. Accordion-like V₂CT_x MXene as precursor for synthesizing NHVO@C was obtained by selective etching in a mixed solution of HCl and NaF (Fig. 1b). Within the subsequent hydrothermal treatment, ammonium vanadate nanoribbons were in-situ derived from the lamellae of V₂CT_x MXene (Fig. 1c). High-magnification SEM image shows that the in-situ derived nanoribbons of NHVO@C composite form a three-dimensional conductive network structure (Fig. 1d). In addition, Figs. S1a and b (Supporting information) show that NHVO derived from commercial V₂O₅ without a carbon backbone has similar cross-linked nanoribbon morphology. The internal structural features of the NHVO@C composite were further explored by TEM. Fig. 1e shows that the NHVO@C are also composed of nanoribbons in the internal crosslinked structure, indicating the complete derivatization of MXene to NHVO@C. After further measurements, the size distribution curve shows that the width of the nanoribbon array is concentrated around 30–70 nm, with an average width of 48 nm (Fig. S2 in Supporting information). The high-resolution TEM image shows distinct lattice stripes with 0.191 nm spacing, corresponding to the (601) crystallographic plane of the (NH₄)₂V₈O₂₀·xH₂O phase (JCPDS No. 45–1363). Moreover, the ultra-thin carbon layer can be observed in Fig. 1f, which demonstrated the existence of carbon skeleton on the surface of NHVO@C [39,40]. The TEM-EDS mapping in Fig. 1g shows the uniform distribution of vanadium (orange), nitrogen (green), and carbon (red) in NHVO@C, which further confirmed the homogeneous formation of amorphous carbon skeleton on the surface of NHVO. Such results demonstrate the successful preparation of NHVO@C composites in-situ derived V₂CT_x MXene. The uniform and robust amorphous carbon skeleton can not only enhance the conductivity and shorten the ionic transmission paths, but also inhibit ammonium ion solubilization and suppress the structural collapse of the material during the charging and discharging process. More importantly, the different ammonia content and morphologies of ammonium vanadate was synthesized by adjusting hydrothermal times for 6 h and 18 h, which named NHVO@C-6h and NHVO@C-18h, respectively. Figs. S3a-c (Supporting information) show the nanoribbon morphology of NHVO@C was gradually derived completely as the hydrothermal time was increased from 6 h to 12 h, but gradually fragmented beyond 12 h. Simultaneously, the ammonia content of the NHVO@C composite gradually decreased (Fig. S3d in Supporting information). Moderate removal of ammonium ions can generate more active sites and provide more diffusion paths, which is favorable for the fast reaction kinetics of Zn²⁺.

The phase transition of V₂CT_x MXene to NHVO is further explored. The X-ray diffraction (XRD) pattern shows that the diffraction peaks of NHVO@C are well corresponded to the standard pattern of (Na, Ca) (V, Fe)₈O₂₀·nH₂O (Space group: C2/m, JCPDS No. 45–1363) (Fig. 2a) [41,42]. Compared to the standard profile, the (001) diffraction peak of NHVO@C is shifted from 8.176° towards a lower angle of 7.934°, which is corresponded to the increase of the lattice spacing from 1.081 nm to 1.113 nm based on the Bragg equation, indicating the intercalation of ammonium radical ions with large ions radius. Benefiting from the large lattice spacing, NHVO@C possessed more active sites and thus a higher specific capacity. Further Rietveld refinement of the powder XRD patterns was performed to determine the crystal structure (Fig. 2b). The refinement results show that NHVO@C is determined to have a monoclinic structure with a space group of C2/m and a lattice parameter of a = 11.6204 Å, b = 3.6149 Å, c = 11.0905 Å, α = γ = 90.0°, and β = 102.4°. The reliability coefficients were found to be R_p = 5.693%, G_of = 2.42 and R_wp = 5.84%, proving the high reliability of the refinement results. Based on the results, the crystal structure of NHVO@C was illustrated in Fig. 2c. In addition, the NHVO@C-6h and NHVO@C-18h composites showed consistent XRD images, demonstrating the same physical phase (Fig. S4).

Figure 2

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Fig. 2. (a) XRD patterns of the NHVO@C composite and V2CTx MXene. (b) The Rietveld refined XRD pattern of NHVO@C composite. (c) Crystal structure model of NHVO@C composite. (d, e) The high-resolution XPS spectra of V 2p and C 1s of NHVO@C composite and V2CTX MXene, respectively. (f) TGA curve of NHVO@C composite. (g) BET curves of NHVO@C composite and V2CTx MXene. (h) Raman spectra of NHVO@C composite and NHVO sample.

In addition, the X-ray photoelectron spectroscopy (XPS) were further performed to analyze the valence changes during the process of in situ derivatization of V₂CT_x MXene to NHVO@C. As shown in Fig. 2d, V 2p_1/2 and V 2p_3/2 of V₂CT_x MXene can be decomposed into six convolution peaks corresponding to V⁴⁺ (524.5 and 516.7 eV), V³⁺ (522.2 and 514.4 eV) and V²⁺ (521.2 and 513.6 eV), respectively [43]. In contrast, in the V 2p spectra of NHVO@C, the convolution peaks corresponding to V³⁺ and V²⁺ disappeared entirely and the convolution peaks corresponding to V⁵⁺ (525.1 and 517.3 eV) appeared, indicating the complete conversion of V₂CT_x MXene to NHVO@C. The signal corresponding to V⁴⁺ is attributed to the insertion of NH₄⁺ leading to partial conversion of V⁵⁺ to V⁴⁺ for maintaining charge neutrality [44]. In the C 1s spectrum of V₂CT_x MXene (Fig. 2e), the fitted peaks located at 288.8, 286.5, 284.9, and 282.8 eV can be corresponded to C-F, C-O, C-C, and C-V bonds, respectively. And the fitted peaks corresponding to C-V bonds disappeared in the C 1s spectra of NHVO@C, further proving the complete derivatization of V₂CT_x [45]. Combining the results of Rietveld refinement, XPS as well as SEM and TEM tests, it is obvious that MXene as a vanadium source has been fully derivatized into ammonium vanadate with surface-coated carbon, and its residual carbon has been fully utilized. The TGA curve of NHVO@C exhibits two distinct phases (Fig. 2f), in the range of 0–150 ℃, physisorbed H₂O molecules, structural water, and some of the NH₄⁺ are detached from the structure of the material, contributing to a weight loss of 4.93%. Subsequently, in the range of 150–350 ℃, amorphous carbon is eliminated from the structure along with the remaining part of NH₄⁺, resulting in a mass loss of 3.05% [44]. In addition, the BET curves showed that the specific surface area of NHVO@C increased from 10.89 m²/g to 24.31 m²/g compared to V₂CT_x MXene, which was attributed to the transition of the morphology from an accordion-like structure to cross-linked nanoribbons, and the adsorption isotherm of NHVO@C showed a type-Ⅳ profile with H3 hysteresis loop, which demonstrated the presence of both microporous and mesopores (Fig. 2g) [39]. The pore size distribution curves show that NHVO@C has a multistage pore structure consisting of microporous mesopores and macropores, which promotes the infiltration of electrolyte and increases its contact with the active substance, thus significantly shortening the ion transport pathway. In addition, the Fourier transform infrared spectroscopy (FTIR) spectra of NHVO@C and NHVO (Fig. S5 in Supporting information) demonstrate the peaks at 1388 and 3150 cm^-1 corresponding the bending and stretching modes of N-H, respectively, which prove the presence of NH₄⁺. Raman spectroscopy was used to further determine the presence of the carbon skeleton and its phase composition. Fig. 2h shows that the NHVO@C sample has two distinct broad peaks at 1337 and 1574 cm^-1, attributed to the D-band of defective carbon and the G-band of graphitic carbon, respectively. In contrast, the NHVO without carbon skeleton was not observed to have significant peak signals at the same locations.

In order to evaluate the effect of the NHVO@C on the storage performance of zinc ions, a coin cell was assembled by using metallic zinc foil as anode, glass fibers as the separator, and 3 mol/L Zn(CF₃SO₃)₂ as the electrolyte. The electrochemical performances of NHVO with different hydrothermal times were also tested, proving NHVO@C has superior capacity and cycling stability due to its more regular morphology and reasonable NH₄⁺ content (Fig. S6 in Supporting information). Fig. 3a illustrates the first three cyclic voltammetry (CV) cycles of NHVO@C at a scan rate of 0.1 mV/s. NHVO@C shows three different couples of redox peaks, where the two cathodic peaks at 0.42 and 0.52 V correspond to the reduction reaction from V⁴⁺ to V³⁺, and the anodic peaks at 0.53 and 0.66 V correspond to the oxidation reaction from V³⁺ to V⁴⁺. While the cathodic peak at 0.95 V corresponds to the V⁵⁺ to V⁴⁺ reduction reaction, the anodic peak at 0.96 V corresponds to the V⁴⁺ to V⁵⁺ oxidation reaction [46,47]. These redox peaks remained overlapped throughout the initial three CV cycles, indicating the good reversibility of NHVO@C. Fig. 3b shows the galvanostatic charge-discharge (GCD) curves of NHVO@C and NHVO for the first cycle at a current density of 0.1 A/g. NHVO@C displays two pairs of charge/discharge plateaus, which match well with the two pairs of redox peaks shown in CV curves. The results show that NHVO@C has an initial specific capacity of 551.8 mAh/g, which is significantly higher than the initial specific capacity of NHVO (509.2 mAh/g). Moreover, NHVO@C has a midpoint voltage difference that is significantly lower than that of NHVO, indicating the reduction of the charge/discharge overpotential of NHVO@C. The rate capability of the NHVO@C electrode was subsequently evaluated (Figs. 3c and d, Fig. S7 in Supporting information). At a current density of 20 A/g, NHVO@C still has a high specific capacity of 158.6 mAh/g, reflecting its excellent rate performance. The superior rate performance due to its external robust carbon skeleton, which establishes a highly conductive network for electron transport. Meanwhile, its porous structure promotes the penetration of electrolyte and accelerates the ion transport rate. Notably, the Zn//NHVO@C cell achieved the highest specific energy of 292.28 Wh/kg at a specific power of 53.42 W/kg and had a specific energy of 87.38 Wh/kg at a high power density of 10.489 kW/kg (Fig 3e). The electrochemical properties were significantly superior to those of other ammonium vanadate and vanadium oxide materials reported in previous studies, including (NH₄)₂V₆O₁₆·1.5H₂O nano belts [48], (NH₄)₂V₄O₉ [49], (NH₄)₂V₆O₁₆·1.5H₂O nano wires [50], PANI-VOH [51], NH₄V₄O₁₀ [52], V₂O₅ [53], (NH₄)₂V₁₀O₂₅·8H₂O [29]. Fig. 3f compares the long-term cycling stability of NHVO@C and NHVO at a current density of 1.0 A/g. Benefiting from the stable crosslinked carbon skeleton structure, the NHVO@C exhibits excellent cycling stability and demonstrates a capacity retention of 91.2% after 100 cycles at 1 A/g. In contrast, NHVO without the carbon skeleton showed a rapid decay of specific capacity, with unstable capacity fluctuations and eventual failure after 80 cycles. In addition, the NHVO@C material still has a capacity retention of 86.1% after 80 cycles at 0.1 A/g, while the NHVO material without carbon skeleton has a capacity retention of only 73.3% (Fig. S8 in Supporting information). The NHVO@C and NHVO electrode were immersed in the electrolyte for two weeks to perform the solubility test (Fig. S9 in Supporting information). It is obvious that the electrolyte immersed with NHVO showed a deeper color of yellow, which proves the dissolution of massive amount of ammonium vanadate, while the electrolyte soaked in NHVO@C showed an inconspicuous color, demonstrating its good stability in the electrolyte. The long-term cycling stability of the battery at high current density was further investigated. The NHVO@C exhibits a high capacity of 230.2 mAh/g at a high current density of 10 A/g and a capacity retention of 71.2% after 5000 cycles, which demonstrates excellent long term cycle stability (Fig. 3g).

Figure 3

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Fig. 3. (a) CV curves of NHVO@C composite in the initial three cycles at 0.1 mV/s. (b) The GCD curves of NHVO@C composite and NHVO at 0.1 A/g in the first cycle. (c) Rate performance of NHVO@C composite and NHVO sample. (d) The corresponding GCD profiles at various current densities of NHVO@C composite. (e) Ragone plot of NHVO@C composite compared with reported vanadium-based cathode materials. (f) Cyclability of NHVO@C composite and NHVO sample at 1 A/g. (g) Long-life cyclability of NHVO@C composite at 10 A/g.

In order to further investigate the effect of conducting carbon skeleton on the reaction kinetics of NHVO@C cathode, CV tests were conducted at different scan rates (ranging from 0.1 mV/s to 1.0 mV/s, Fig. 4a). As the scanning rate increases, the shape of the redox peaks remains basically the same, with the potentials of the oxidation peaks slightly shifted to the positive side and the potentials of the reduction peaks slightly shifted to the negative side, indicating the benefit of the robust carbon skeleton with high electrical conductivity, which maintains a favorable redox reversibility even at high scanning rates. Comparatively, with increasing scan rate, a higher electrochemical polarization and slower reaction kinetics were presented in NHVO cathode (Fig. S10 in Supporting information). The b-values which reflect the storage Zn²⁺ process were calculated according to methods in the previous literature. The peak current and the scan rate are related by the following Eq. 1, where i is the current density, v is the scan rate, a and b are variable parameters. The b-values range from 0.5 to 1, where b-value of 0.5 indicates that the electrochemical process is dominated by diffusion behavior, whereas a b-value of 1 indicates that the surface capacitance controls the electrochemical process. The b-values corresponding to the four peaks of NHVO@C are 0.84, 0.77, 0.98, and 0.74, respectively (Fig. 4b), indicating the capacitive dominance of the NHVO@C cathode in the storage behavior of Zn²⁺. Furthermore, the contribution to the capacitive behavior at different scan rates was calculated according to (1), (2). By using the quantitative relationship between the current densities (i) at fixed potentials (V) and scan rates (v), the contribution percentages of diffusion (k₁v) and capacitance-controlled (k₂v^1/2) processes can be obtained. The capacitive contributions at 0.1, 0.2, 0.3, 0.5, 0.7, 1.0 mV/s sweep speeds are 60.4%, 69.3%, 75.4%, 79.2%, 83.4%, 86.2%, respectively (Figs. 4c and d). The high contribution proportion of the capacitive behavior is the reason for the excellent charge transfer kinetics of the NHVO@C cathode.

Figure 4

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Fig. 4. (a) CV curves of the NHVO@C composite at different scan rates. (b) log(peak current) vs. log(scan rate) plot for each redox peak in NHVO@C composite and their corresponding b values. (c) Capacitive contribution of the NHVO@C composite at various scan rates. (d) Contribution ratios of capacitance and diffusion-controlled of NHVO@C composite at 0.7 mV/s. (e, f) The GITT curves at 0.05 A/g and the corresponding ion-diffusion coefficients of NHVO@C composite. (g) Nyquist plots of the NHVO@C composite and NHVO (inset illustrates the electrical equivalent circuit used for fitting EIS spectra). (h) Plot of the real part of impedance vs. ω−1/2 in the low-frequency region.

In order to further demonstrate the role of conductive carbon skeleton for improving the diffusion kinetics of Zn²⁺, the diffusion coefficients (D_Zn²⁺) of Zn²⁺ in NHVO@C versus NHVO cathode materials were measured by using the galvanostatic intermittent titration technique (GITT, Fig. 4e). The calculations in Fig. 4f shows that NHVO has logD_Zn²⁺ values between −9.60 and −10.52 during charging and discharging based on Eq. 3, whereas the logD_Zn²⁺ values of NHVO@C are between −9.26 and −10.32 (τ, m_B, V_M, M_B and S are the pulse current duration, weight, molar volume and molar mass of the active material, electrode pole piece area, respectively). The higher logD_Zn²⁺ range interval indicates higher ion diffusion coefficients, suggesting that the conductive carbon skeleton enabled the zinc ions to possess faster diffusion rates, which favors charge carrier transport and leads to superior rate performance.

Fig. 4g compares the Nyquist curves of NHVO@C with NHVO. Wherein, the semicircle in the high-frequency part indicates that NHVO@C has a charge transfer resistance (R_ct) of 131.2 Ω, which is significantly lower than that of NHVO (258.3 Ω), highlighting the improved electronic conductivity of the conducting carbon skeleton. In addition, Fig. 4h illustrates the real part of impedance (Z’) as a function of frequency (ω^−1/2) in the low-frequency region, the curve corresponding to NHVO@C shows a smaller slope than NHVO, indicating a more rapid transport ratio of Zn²⁺ within the NHVO. Overall, the in-situ derivatization of NHVO@C by V₂CT_x MXene achieves a tight combination of the conductive carbon skeleton and the active material. The robust conductive carbon skeleton stabilizes the main structure during charging and discharging, mitigating the structural collapse caused by irreversible deamidation. Moreover, the ultra-thin carbon layer enhances the conductivity while forming a porous structure, accelerating the penetration of the electrolyte, thus reduced the ionic transport barriers, facilitated the charge transfer of the material, and accelerated the reaction kinetics of the Zn²⁺ storage.

Subsequently, the crystal structure evolution of the NHVO@C cathode during Zn²⁺ storage was investigated by ex situ XRD tests over a complete charge/discharge cycle (Fig. 5a and Fig. S11 in Supporting information). The results show that the (110) surface located at 34.1° shifted to higher angle during the discharge process and then shifted to lower angle during the charging process and returned to the initial state, corresponding to the intercalation of Zn²⁺ during the discharge process and the deintercalation of Zn²⁺ during the charging process, respectively. The lattice spacing of the NHVO@C cathode material discharged to 0.2 V and charged to 1.6 V was observed using TEM, and the inverse Fourier transform image showed that the (110) crystalline plane spacing of NHVO@C decreased from 0.351 nm to 0.349 nm (Fig. 5b), which corresponded to the results of the ex-situ XRD. In addition, during this cycling process, a new set of diffraction signals can be observed between 20° and 30°, which can be indexed co-product Zn₃(OH)₂V₂O₇·2H₂O (PDF #50–0570) in the vanadium oxide/Zn(CF₃SO₃)₂ system. During the embedding of Zn²⁺ in the cathode material, the simultaneous embedding of H⁺ induced the dissociation of water molecules in the electrolyte, resulting in the generation of -OH groups. The -OH group further reacted with hydrated zinc ions to generate the alkali salt byproducts Zn₃(OH)₂V₂O₇·2H₂O [38,40,41,44]. Moreover, ex-situ XPS tests were conducted to further investigate the elemental valence changes of NHVO@C during Zn²⁺ storage. Fig. 5c shows the V 2p spectra of the NHVO@C cathode in the pristine state versus discharged to 0.2 V and charged to 1.6 V, respectively. Two peaks corresponding to V⁵⁺ (517.4 eV) and V⁴⁺ (516.1 eV) can be observed in the V 2p spectrum of the pristine state. After discharging to 0.2 V, part of V⁵⁺ is reduced due to the intercalation of Zn²⁺ and the intensity of the peak corresponding to V⁵⁺ decreases while the intensity of the peak corresponding to V⁴⁺ increases. After charging to 1.6 V, the V 2p signal was basically recovered to the original state. In the O 1s spectra of the pristine electrode, three peaks located at 530.1 eV (attributed to lattice oxygen), 531.1 eV (attributed to surface adsorbed oxygen), and 532.2 eV (corresponding to the H₂O molecule) can be observed (Fig. 5d). When discharged to 0.2 V, the peaks corresponding to the H₂O molecule as well as the intensity of the -OH peaks increased significantly, demonstrating the co-intercalation of H₂O and Zn²⁺ as well as the generation of Zn₃(OH)₂V₂O₇·2H₂O. Correspondingly, after charging to 1.6 V, the intensities of these two peaks decreased, corresponding to the partial deintercalation of H₂O and the partial reversible decomposition of Zn₃(OH)₂V₂O₇·2H₂O. In addition, the Zn 2p spectra in Fig. 5e shows a strong Zn signal after the NHVO@C electrode fully discharged and weakened after fully charged, confirming the above conclusion of Zn²⁺ intercalation and deintercalation process. Overall, Fig. 5f specifically illustrates the storage mechanism of reversible intercalation by Zn²⁺ in the NHVO@C cathode.

Figure 5

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Fig. 5. (a) A complete discharge-charge procedure of NHVO@C electrode at 0.1 A/g and corresponding ex-situ XRD patterns of NHVO@C electrode at selected states. (b) HRTEM and relevant IFFT images of NHVO@C electrode at dis-0.2 V/cha-1.6 V states. (c) V 2p, (d) O 1s, (e) Zn 2p XPS spectra of NHVO@C electrode at different states. (f) Schematic illustration of crystal structure change of NHVO@C electrode during the discharge/charge process.

Based on the safety advantages of aqueous ZIBs, flexible wearable devices have become one of their main application prospects in recent years. The high energy density and flexibility of NHVO@C cathode enable it to excellently satisfy the requirements of flexible wearable devices. Therefore, the Zn//NHVO@C soft-packed battery was assembled in the order of stainless-steel foil, NHVO@C cathode, separator and electrolyte and Zn foil (Fig. 6a). Based on the excellent flexibility of the NHVO@C cathode (Fig. 6b), the cycling performance of the Zn//NHVO@C soft-packed battery was tested under various degrees of bending, and the results illustrate that even after 90-degree and 180-degree bending, the soft-packed battery still has 80.1% capacity retention after 100 cycles (Fig. 6c). Moreover, to investigate the feasibility of the Zn//NHVO@C soft-packed battery as energy storage device for flexible wearable devices, two identical Zn//NHVO@C soft-packed batteries were connected in series to light up an array of 30 LED bulbs (Fig. S12a in Supporting information). Fig. S12b (Supporting information) shows the GCD curves of the flexible pack battery at two stages before and after bending recovery, respectively. It is impressive that undergoing severe bending tests, the flexible pack battery shows only a small capacity degradation, while the shape of the charging and discharging platform of the curves remains unchanged, which reflects the great potential of the flexible NHVO@C for the application in the field of flexible energy storage devices. To further explore the practical applications potential of NHVO@C composites in ZIBs, NHVO@C composite film with a mass loading of about 8 mg/cm² as the cathode of coin-cells was fabricated. As can be seen from Fig. 6d, the high-loading NHVO@C composite has a high areal capacity of 3.7 mAh/cm² at 0.5 A/g and 84.9% capacity retention after 100 cycles. Significantly, the soft-packed battery assembled with NHVO@C films that high-loading of about 10.2 mg/cm² can drive an electronic thermometer properly, which further proves its practical application potential (Fig. 6e).

Figure 6

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Fig. 6. Practicality testing of flexible NHVO@C composite: (a) Fabrication procedure of the flexible Zn//NHVO@C soft pack batteries. (b) Optical photographs showing a flexible NHVO@C composite film folded twice in half. (c) Cycling performance (current density of 2.0 A/g) of Zn//NHVO@C soft pack batteries in straight and various bending states. (d) Cycling performances of the Zn//NHVO@C batteries with high-loading active material of ~8.0 mg/cm2 at 0.5 A/g. (e) Optical photograph of an electronic thermometer powered by two Zn//NHVO@C soft pack ZIBs in series with high-loading active material of ~10.2 mg/cm2.

In summary, we have synthesized NHVO@C by one-step hydrothermal method that in-situ derived V₂CT_x MXene in a relatively mild temperature. The synthesis method avoids NH₄⁺ loss due to the use of carbonization process in a relatively moderate temperature and achieves a uniform interfacial coupling of carbon and ammonium vanadate. Significantly, the ultra-thin amorphous carbon layer as a conductive framework lowers the migration energy barriers and accelerates ion/electron transport kinetics on the one hand, which lead to an excellent rate performance (551.8 mAh/g at 0.1 A/g and 158.6 mAh/g at 20 A/g). On the other hand, the robust carbon skeleton mitigates the irreversible dissolution of ammonium ions and stabilizes the material structure during charge/discharge process, resulting in a superior long-cycle cycling performance (91.2% capacity retention at 1 A/g after 100 cycles, and 71.2% capacity retention at 10 A/g after 5000 cycles). Moreover, the flexible NHVO@C cathode without collectors and binders greatly enhancing the energy density of the ZIBs (292.28 Wh/kg). Impressively, the assembled Zn//NHVO@C soft-packed battery exhibits outstanding practicality and stable electrochemical performance even under various bending conditions. The excellent performance of NHVO@C provides a new idea for the heat-treatment-free synthesis process of carbon-composite ammonium vanadate electrode materials and paves the way for the commercialization of high-performance zinc-ion batteries.

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.

Xiaojun Wang: Writing – review & editing, Writing – original draft, Visualization, Supervision, Resources, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. Yizhou Zhang: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Linwei Guo: Writing – original draft, Formal analysis. Jianwei Li: Visualization, Funding acquisition, Data curation. Peng Wang: Visualization, Project administration, Funding acquisition, Formal analysis. Lei Yang: Writing – review & editing, Validation, Project administration, Funding acquisition, Data curation. Zhiming Liu: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

This study was financially supported by the National Natural Science Foundation of China (Nos. 52402271, 22005167 and 52302273), the Youth Innovation Team Project for Talent Introduction and Cultivation in Universities of Shandong Province (No. 2024KJH129), the Taishan Scholar Project of Shandong Province of China (Nos. tsqn202211160, tsqn202312199), Shandong Provincial Natural Science Foundation of China (Nos. ZR2022QE003 and ZR2023QE176) and China Postdoctoral Science Foundation (No. 2023M741810).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111231.