Robust cross-linked Na₃V₂O_1.6(PO₄)₂F_1.4@rGO&MWCNTs as a high-performance cathode for aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) are advantageous for grid-scale energy storage applications due to their high safety, cost-effectiveness, and environmental sustainability. However, developing high-rate and long-life cathodes for AZIBs remains a significant problem. Herein, a composite (Na₃V₂O_1.6(PO₄)₂F_1.4@rGO&MWCNTs) that integrates Na₃V₂O_1.6(PO₄)₂F_1.4 nanoparticles within a 3D conductive matrix comprising 2D reduced graphene oxide (rGO) and 1D multi-walled carbon nanotubes (MWCNTs) has been prepared through a microwave-assisted hydrothermal method followed by calcination, enhancing surface electronic conductivity to improve electrochemical performance. The Na₃V₂O_1.6(PO₄)₂F_1.4@rGO&MWCNTs cathode exhibits a high capacity of 98.4 mAh g⁻¹ at 0.2 A g⁻¹ and maintains a capacity of 52.8 mAh g⁻¹ over 6000 cycles at 5 A g⁻¹. The soft-pack battery demonstrates excellent cycling stability, maintaining stable performance over 1000 cycles at a high current density. Remarkably, it exhibits robust mechanical stability, delivering a stable output voltage even under various mechanical stress conditions. The outstanding performance arises from enhanced conductivity, high pseudocapacitive contribution, improved Zn²⁺ diffusion coefficients, low charge transfer resistance, robust structural framework, and reversible Zn²⁺ insertion/extraction mechanism verified by in situ electrochemical impedance spectroscopy, distribution of relaxation times, cyclic voltammetry, galvanostatic intermittent titration technique, ex situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy.

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Introduction

Lithium-ion batteries (LIBs) have gained extensive popularity owing to excellent energy density, outstanding cycling stability, as well as the mature manufacturing process.^1,2 As the demand for energy storage intensifies, LIBs face challenges in grid-scale energy storage applications, stemming from both the high cost tied to lithium scarcity and concerns about the toxicity, safety risks, and potential environmental contamination posed by organic electrolytes.^3–5 Among numerous rechargeable batteries, aqueous zinc-ion batteries (AZIBs) have emerged as a competitive candidate due to their cost competitiveness, inherent safety, and eco-friendly attributes.^6–8 In particular, with its exceptional theoretical capacity (820 mAh g⁻¹) and feasible redox potential (−0.76 V vs. the standard hydrogen electrode), a zinc metal anode lays a solid foundation for developing battery systems that feature energy density and reliable operational performance.^9–11

At present, Prussian blue analogs, manganese-based materials, vanadium-based materials, and organic compounds have been widely studied as cathode materials for AZIBs.^12–14 Among these, Na₃V₂(PO₄)₃, a NASICON-type vanadium-based material, has garnered significant attention due to its unique three-dimensional ionic conduction paths and high structural stability.^15,16 The charge effect of the PO₄³⁻ group in Na₃V₂(PO₄)₃ can enhance working voltage and alleviate volume change upon repeated cycles, contributing to higher energy density.¹⁷ Huang's group pioneered the successful application of Na₃V₂(PO₄)₃ as a cathode material for AZIBs, demonstrating a reversible capacity of 97.0 mAh g⁻¹ at 0.5C using 0.5 M Zn(CH₃COO)₂ as electrolyte, along with a capacity retention of 74% over 100 cycles.¹⁸ Further improvements were reported by Hou et al., who reported that introducing highly electronegative F⁻ into the phosphate framework significantly boosted working voltage and energy density.¹⁹ Consequently, sodium vanadium fluorophosphate (Na₃V₂O_2x(PO₄)₂F_3−2x, 0 ≤ x≤ 1) has attracted notable attention due to its elevated operational voltage and rapid ion transfer capabilities.^20,21 Moreover, this compound, known for its robust binding affinity of fluorine atoms to the local environment, typically demonstrates greater structural stability in comparison to Na₃V₂(PO₄)₃. Partial substitution of F⁻ with O²⁻ weakens the inductive effect, reducing electrostatic interactions between the main structure and Zn²⁺, thereby promoting swift Zn²⁺ transport with minimized polarization.²² Compared to the single-electron redox process between V⁴⁺ and V⁵⁺ in Na₃V₂(PO₄)₂O₂F, the modified Na₃V₂(PO₄)₂O_1.6F_1.4 with a lower average vanadium oxidation state (+3.8) demonstrates enhanced electron transfer capability. This modification supplies an extra 0.4 electrons per formula unit, resulting in improved reversible capacity.

Despite these advantages, the low electronic conductivity of these materials remains a challenge. To address this limitation, conductive materials are often incorporated to enhance charge transfer. For instance, the Na₃V₂(PO₄)₂F₃@C cathode was first utilized in 2 M Zn(CF₃SO₃)₂ electrolyte by Jiang's team, achieving an operating voltage of 1.6 V with a stable capacity of 46.0 mAh g⁻¹ at 1 A g⁻¹.²³ Similarly, Feng and his team successfully synthesized Na₃(VO)₂(PO₄)₂F@CNT through a hydrothermal method, exhibiting a capacity of 50.0 mAh g⁻¹ after 200 cycles at 2C (1C = 130 mA g⁻¹).²⁴ While single-carbon coating can elevate the electrochemical characteristics to a certain degree, the rate capabilities are still limited. A more effective strategy combines multiple conductive materials with sodium vanadium fluorophosphate to create synergistic effects, significantly enhancing conductivity and electrochemical performance.

Therefore, a method involving microwave-assisted hydrothermal treatment followed by high-temperature calcination has been employed to prepare high-purity Na₃V₂O_1.6(PO₄)₂F_1.4 (N3VOPF). To enhance the conductivity, N3VOPF has been successfully combined with reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs), forming N3VOPF@rGO&MWCNTs composites. The synergistic combination of rGO and MWCNTs enhances surface reaction kinetics and reduces voltage polarization, contributing to improved rate performance along with prolonged cycling durability. In an aqueous electrolyte comprising 4 M lithium triflate (LiCF₃SO₃) and 2 M zinc triflate (Zn(CF₃SO₃)₂), the N3VPOF@rGO&MWCNTs electrode exhibits an impressive specific capacity of 98.4 mAh g⁻¹ at 0.2 A g⁻¹ while maintaining a capacity of 52.8 mAh g⁻¹ over 6000 cycles at 5 A g⁻¹. Furthermore, when configured in the soft-pack battery, it consistently delivers an exceptional reversible capacity of 61.4 mAh g⁻¹ even after 1000 cycles at 1 A g⁻¹, highlighting its high capacity and superior electrochemical stability. The reaction kinetics and reversible Zn²⁺ insertion/extraction mechanism are explored by cyclic voltammetry (CV) at various scan rates, electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT), ex situ X-ray diffraction (XRD), and ex situ X-ray photoelectron spectroscopy (XPS). In situ EIS and distribution of relaxation times (DRT) are further conducted to elucidate the detailed dynamic evolution of charge transfer kinetics. These findings collectively underscore the potential of N3VOPF@rGO&MWCNTs-2 as a competitive high-performance cathode for advanced AZIBs.

Experimental section

Initially, 0.4 mmol vanadium(III) acetylacetonate (V(acac)₃, Aladdin, 97%) and 1.6 mmol vanadium(IV) oxide acetylacetonate (VO(acac)₂, Aladdin, 98%) were dissolved in anhydrous ethanol (24 mL) under continuous magnetic stirring for 20 min to obtain solution A. Solution B was prepared by dissolving 2 mmol NH₄H₂PO₄ (Aladdin, 99%) and 3.34 mmol NaF (Aladdin, 98%) in deionized water (12 mL) with continuous magnetic stirring for 20 min. Solutions A and B were mixed for 10 min to obtain solution C. Finally, MWCNTs and GO (2 mg mL⁻¹) were mixed and subsequently stirred for 10 min to acquire solution D. Solution D was ultrasonicated for 1 h and then mixed with solution C, stirring for an additional 10 min to yield the final solution. The mixture was introduced to a polytetrafluoroethylene reactor and hydrothermally treated from room temperature to 130 °C at 10 °C min⁻¹ and reacted for 1 h. The product underwent three purification cycles with deionized water/ethanol and was dried at 60 °C for 12 h to acquire the precursor material. The dried precursor was calcined in a N₂ environment at 600 °C for 7 h to synthesize N3VOPF@rGO&MWCNTs. By modulating the MWCNT concentration while maintaining a constant 10 wt% rGO coating, four distinct N3VOPF@rGO&MWCNTs samples were prepared: N3VOPF@rGO (0 wt% MWCNTs), N3VOPF@rGO&MWCNTs-1 (5 wt% MWCNTs), N3VOPF@rGO&MWCNTs-2 (10 wt% MWCNTs), and N3VOPF@rGO&MWCNTs-3 (15 wt% MWCNTs). The structure characterization and battery assembly process are detailed in the ESI.†

Results and discussion

Fig. 1a illustrates the synthesis process of N3VOPF@rGO&MWCNTs. V(acac)₃ and VO(acac)₂ were dissolved in ethanol to form solution A, while NH₄H₂PO₄ and NaF were dissolved in deionized water to obtain solution B. The mixed solution containing MWCNTs and GO was subjected to microwave-assisted heating at 130 °C for 1 h. Following purification, the product was calcined under N₂ at 600 °C for 7 h to yield the final samples. The crystal structure features [VO₅F] octahedra and [PO₄] tetrahedra interconnected through shared oxygen atoms within the ab plane and fluorine atoms along the c-axis as displayed in Fig. 1a, forming a stable open-framework structure conducive to ion transport.²⁵ XRD Rietveld refinement patterns (Fig. 1b–e) reveal well-defined crystal structures with excellent reliability factors (R_p, R_wp, and R_exp) for all four samples. The corresponding lattice parameters, detailed in Tables S1–S4† demonstrate systematic variations with MWCNT content. Specifically, N3VOPF@rGO&MWCNTs-2 exhibits refined lattice parameters of a = b = 6.384 Å, c = 10.632 Å, and V = 433.349 ų. The diffraction peaks located at 16.2°, 16.6°, 28.0°, 32.7°, and 40.0° are consistent with the Na₃V₂O₂(PO₄)₂F phase (PDF#01-076-3645, I4/mmm space group), corresponding to the (101), (002), (200), (202), and (220) crystal planes, respectively. The identical peak positions, intensity ratios, as well as peak sharpness confirm the high purity of all four materials.

Fig. 1 (a) Diagram of the preparation process. (b–e) XRD Rietveld refinement patterns of N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3.

Scanning electron microscopy (SEM) was used to examine the morphologies of N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3, revealing that all four compounds consist of homogeneously distributed N3VOPF nanoparticles with sizes ranging from 20 to 50 nm as depicted in Fig. 2. As the MWCNT content increases, N3VOPF nanoparticles can be more evenly dispersed on the rGO layers, leading to a well-integrated ternary structure without observable aggregation. The addition of MWCNTs within the rGO matrix establishes an interconnected 3D conductive network, improving charge transport and electrical conductivity.

Fig. 2 SEM images of (a) N3VOPF@rGO, (b) N3VOPF@rGO&MWCNTs-1, (c) N3VOPF@rGO&MWCNTs-2, and (d) N3VOPF@rGO&MWCNTs-3.

Raman spectra (Fig. 3a) of N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3 reveal the D band (defect) at 1346 cm⁻¹ and G band (graphitization) at 1581 cm⁻¹, respectively.²⁶ The graphitization extent in carbon materials can be reflected by using the intensity ratio (I_D/I_G), where higher values signify increased structural disorder and defects, while lower ratios indicate a more ordered structure.²⁷ N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3 samples exhibit comparable I_D/I_G ratios of 1.03, 1.03, 0.98, and 1.06, respectively, attributed to the consistent annealing process, leading to similar graphitization and defects, thus facilitating rapid charge transfer.²⁸ The performance differences primarily stem from variations in MWCNT content rather than structural defects. The valence information of N3VOPF@rGO&MWCNTs-2 was characterized using XPS. As evidenced by the survey spectrum (Fig. 3b), the existence of characteristic peaks for V 2p, Na 1s, O 1s, F 1s, C 1s, and P 2p, indicates the successful synthesis of the target compound. Hybrid vanadium states are detected in the V 2p spectrum (Fig. 3c), with four distinct peaks at 523.3 (V⁴⁺ 2p_1/2), 516.1 (V⁴⁺ 2p_3/2), 521.8 (V³⁺ 2p_1/2), and 514.6 eV (V³⁺ 2p_3/2).²⁹ The integrated peak area ratio of V³⁺/V⁴⁺ is 1 : 4, further confirming the stoichiometric ratio of the vanadium element in the material. Additionally, the O 1s spectrum is deconvoluted into three components, representing the C=O bond at 532.6 eV, the V–O bond at 530.2 eV, and the P–O bond at 528.9 eV (Fig. 3d).³⁰ As shown in Fig. 3e, the P 2p signal appears at 131.9 eV.³¹ The collective results conclusively validate the successful synthesis of N3VOPF@rGO&MWCNTs-2. Thermogravimetric (TG) curves in Fig. 3f and S1† demonstrate that the carbon content in N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3 is 5.9, 12.6, 18.0, and 23.5 wt%, respectively.

Fig. 3 (a) Raman spectra of N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3. XPS spectra of N3VOPF@rGO&MWCNTs-2, (b) full spectrum, (c) V 2p, (d) O 1s, and (e) P 2p. (f) TG curve of N3VOPF@rGO&MWCNTs-2.

The electrochemical properties of the electrodes were measured in a mixed 2 M Zn(CF₃SO₃)₂/4 M LiCF₃SO₃ aqueous electrolyte. As presented in Fig. 4a, the initial CV curve of the N3VOPF@rGO&MWCNTs-2 electrode shows two oxidation peaks observed at 1.61 and 1.86 V, indicating a stepwise extraction of Na⁺ from the host material during the first charging process, along with vanadium oxidation from low valence to high valence.²⁹ The reduction peak located at 1.50 V is ascribed to Zn²⁺ insertion. These redox peaks coincide with the voltage plateaus observed in the galvanostatic charge–discharge (GCD) curves of N3VOPF@rGO&MWCNTs-2 (Fig. 4b). The nearly overlapping CV curves from the 2nd to 4th cycles demonstrate excellent reversibility and structural stability.

Fig. 4 (a) CV curves at 0.1 mV s⁻¹. (b) Initial GCD profiles at 0.2 A g⁻¹. (c) Cycling performance at 0.2 A g⁻¹. (d) Rate performance. (e) Cycling performance at 5 A g⁻¹.

Pure N3VOPF only delivers a low discharge capacity of 51.3 mAh g⁻¹ at 0.2 A g⁻¹ due to high charge transfer resistance (R_ct, 2073 Ω) as shown in Fig. S2.† The addition of rGO and MWCNTs can significantly improve the performance of this material. N3VOPF@rGO&MWCNTs-2 exhibits superior cycling stability as shown in Fig. 4c and S3.† Its discharge capacity increases from 98.4 mAh g⁻¹ (1st cycle) to 113.9 mAh g⁻¹ (40th cycle), likely due to progressive electrolyte infiltration.³² As shown in Fig. 4d and S4,† the N3VOPF@rGO&MWCNTs-2 electrode delivers reversible capacities of 98.6, 87.8, 80.6, 75.0, 62.2, and 51.4 mAh g⁻¹ at progressively higher current densities ranging from 0.2 to 5 A g⁻¹. Remarkably, when returning to 0.2 A g⁻¹, the capacity is restored to 100.6 mAh g⁻¹, indicating superior rate capability. However, N3VOPF@rGO, N3VOPF@rGO&MWCNTs-1, and N3VOPF@rGO&MWCNTs-3 can only deliver capacities of 19.4, 23.6, and 48.6 mAh g⁻¹ at 5 A g⁻¹, respectively. The enhanced electrochemical performance of N3VOPF@rGO&MWCNTs-2 is due to its optimal carbon composition, which achieves an ideal balance between conductivity enhancement and active material preservation, facilitating efficient Zn²⁺ transfer.³³ Excessive carbon, while enhancing conductivity, reduces the proportion of active substances, and then negatively impacts the electrochemical performance.³⁴ Conversely, deficient carbon fails to enhance conductivity adequately. Fig. 4e confirms the outstanding long-term cycling stability of N3VOPF@rGO&MWCNTs-2, retaining a reversible capacity of 52.8 mAh g⁻¹ after 6000 cycles at a high current density of 5 A g⁻¹, outperforming N3VOPF@rGO (12.5 mAh g⁻¹), N3VOPF@rGO&MWCNTs-1 (27.5 mAh g⁻¹), and N3VOPF@rGO&MWCNTs-3 (45.7 mAh g⁻¹).

The Zn²⁺ storage kinetics of N3VOPF@GO&MWCNTs-2 were systematically studied through CVs, GITT, and EIS. To quantify the migration parameters of zinc ions, the relationship between sweep velocity and current is described by using the following equations:

i_p = av^b (1)

log(i_p) = b log(v) + log(a) (2)

where i_p presents the peak current and v indicates the scanning rate, while a and b correspond to variable fitting parameters. A b value close to 0.5 indicates diffusion-controlled behavior, whereas a value approaching 1 signifies surface-controlled pseudocapacitance behavior.^35,36 As illustrated in Fig. 5a, b and S5–S7,† the b values approach 1, suggesting predominantly pseudocapacitive Zn²⁺ storage behavior. The relative contribution ratios in diffusion-controlled and pseudocapacitive processes are further calculated using eqn (3), as depicted in Fig. 5c, d and S8–S11.†

i = k₁v + k₂v^1/2 (3)

With increasing scan rates, the pseudocapacitive behavior becomes more apparent. The pseudocapacitive contribution rates of N3VOPF@rGO&MWCNTs-2 at 0.1, 0.5, 1, and 1.5 mV s⁻¹ are 65, 77, 83, and 88%, respectively, exceeding those of the other three samples at the same scanning rates. The improvement in electrochemical performance, specifically in rate capability, is typically attributed to the increased pseudocapacitive contributions.³⁷ Furthermore, the GITT was utilized to analyze the reaction kinetics of N3VOPF@rGO and N3VOPF@rGO&MWCNTs-1/2/3. The Zn²⁺ diffusion coefficient is calculated according to eqn (4):where τ corresponds to the pulse duration, m_B presents the mass of the active material, V_M denotes the molar volume, M_B indicates the molecular mass of the active material, A represents the interface area, ΔE_s characterizes the steady-state potential change, and ΔE_τ quantifies the voltage change resulting from pulse duration.³⁸ As displayed in Fig. 5e and Table S5,† the average Zn²⁺ diffusion coefficient of N3VOPF@rGO&MWCNTs-2 is approximately 10⁻¹⁰ cm² s⁻¹, surpassing the values of the other three samples. EIS further elucidates the reasons for the superior performance of N3VOPF@rGO&MWCNTs-2, as illustrated in Fig. 5f. Nyquist plots reveal a semicircle arc in the mid-to-high frequency range and a Warburg impedance feature at low frequencies. Compared to N3VOPF@rGO (1849 Ω), N3VOPF@rGO&MWCNTs-1 (1563 Ω), and N3VPF@rGO&MWCNTs-3 (1014 Ω), N3VOPF@rGO&MWCNTs-2 exhibits the lowest R_ct (844 Ω), which facilitates rapid charge transfer and ion diffusion.³⁹ The combined results from CVs, GITT, and EIS demonstrate the fast ion/electron transport kinetics in the N3VOPF@rGO&MWCNTs-2 cathode, enhancing the Zn²⁺ storage performance.

Fig. 5 (a) CV curves of N3VOPF@GO&MWCNTs-2 at different scan rates. (b) Linear relationship between log(i) and log(v). (c) Capacitive contribution portion at 1 mV s⁻¹ of N3VOPF@GO&MWCNTs-2. (d) The contribution ratio of pseudocapacitance at various scanning rates. (e) GITT curves and the calculated diffusion coefficient. (f) Nyquist plots.

In situ EIS was further employed to quantitatively analyze the charge transfer kinetics during the charging/discharging processes, realizing the reason behind the outstanding electrochemical characteristics of N3VOPF@rGO&MWCNTs-2. In situ EIS curves for the first two cycles demonstrate a gradual reduction in the R_ct values throughout charging and a gradual increase during discharging (Fig. 6a). Through equivalent circuit fitting, the R_ct value decreases from 880 Ω (open-circuit potential) to 148 Ω (charge voltage of 1.9 V) as displayed in Fig. 6b, owing to Na⁺ extraction during the charging process. Then the R_ct value increases to 1311 Ω upon discharging due to Zn²⁺ insertion-induced electrostatic interactions. The second cycle exhibits reduced R_ct values compared to the first cycle, indicating improved charge transfer kinetics after the initial activation. The experimental results demonstrate an inverse proportionality between the R_ct and the cation (Zn²⁺ or Na⁺) concentration. To gain deeper insights into the internal electrochemical processes, the DRT technique was employed to precisely distinguish the specific sections by deconvoluting the impedance data (Fig. 6c–e, S12 and S13†). The time constant (τ) analysis confirms that the contact resistance (R_s) is found at approximately 10⁻⁵ s, the ionic diffusion in the cathode–electrolyte interface (R_CEI) is situated in the range of 10^−4.5–10⁻⁴ s, the R_ct of the electrochemical reaction is in the range of 10⁻⁴–10⁻¹ s, and the diffusion of Zn²⁺ (R_d) is located in the range of 10^−0.5–10^0.5 s, respectively.^40,41 The peak strength of R_CEI demonstrates sustained growth during the first cycle, reaching a steady state in the subsequent cycle, suggesting that a stable CEI layer is established after the initial cycle. Notably, the peak strength of R_ct diminishes at roughly 1.6 V during charging and 1.4 V during discharging, but it remains strong at other potentials during the initial cycle. The observed phenomenon is likely attributed to the oxidation (1.6 V) and reduction (1.4 V) processes of N3VOPF@rGO&MWCNTs-2, accelerating electron transport and reducing the R_ct values.^42,43 Moreover, in the second cycle, the R_ct and R_d values significantly decrease compared to the first cycle due to the structural adaptation and stable CEI layer during the repeated Zn²⁺ (de)intercalation, which enhances charge transfer and Zn²⁺ reaction kinetics.^44,45Ex situ SEM images (Fig. S14†) reveal that the initial tight integration of N3VOPF nanoparticles with rGO and MWCNTs remains intact for the pristine, 1st, 20th, and 50th cycles, demonstrating its excellent morphological stability. Besides, the characteristic peaks of N3VOPF can still be observed after 50 cycles (Fig. S15†), indicating its good structural integrity and facilitating electrochemical performance.

Fig. 6 (a) In situ EIS curves of the N3VOPF@rGO&MWCNTs-2 cathode. (b) R_ct values from in situ EIS fitting results. (c–e) DRT plots calculated from EIS measurements at different potentials. (f) Ex situ XRD patterns of N3VOPF@rGO&MWCNTs-2. Ex situ XPS of (g) V 2p, (h) Zn 2p, and (i) Li 1s.

The Zn²⁺ storage mechanism of N3VOPF@rGO&MWCNTs-2 was investigated through ex situ XRD and XPS analyses. Fig. 6f demonstrates that the diffraction peaks at 16.6°, 27.9°, and 32.7° corresponding to the (002), (200), and (202) lattice planes exhibit angular migration during electrochemical cycling. Upon charging, these peaks shift toward higher angles, suggesting decreased interplanar spacing caused by Na⁺ extraction. Conversely, the discharge process induces a reverse shift to lower angles, which reflects the Zn²⁺ insertion.⁴⁶ Meanwhile, when charged to 1.9 V, characteristic peaks emerge at 524.6 eV (V⁵⁺ 2p_1/2) and 517.6 eV (V⁵⁺ 2p_3/2) as shown in Fig. 6g, corresponding to the oxidation of V³⁺ and V⁴⁺ to V⁵⁺.²⁹ Upon discharging to 0.4 V, the original hybrid V 2p spectrum is regenerated. As depicted in Fig. 6h, distinct Zn²⁺ signals appear in the fully discharged state, confirming its insertion into the host material, while residual traces at 1.9 V likely originate from electrolyte-induced surface adsorption.³⁴ The Na 1s signal exhibits an evident decrease after charging and remains nearly unchanged during the subsequent discharging process, demonstrating the absence of Na⁺ insertion into the cathode material (Fig. S16†). The Li⁺ signal remains undetectable throughout the charging/discharging process, confirming its absence in the electrochemical reactions (Fig. 6i). These observations verify the reversible Zn²⁺ insertion/extraction mechanism in N3VOPF@rGO&MWCNTs-2.

To evaluate the practical application value of N3VOPF@rGO&MWCNTs-2 for energy storage, soft-pack batteries (5.0 × 6.0 cm²) incorporating a N3VOPF positive electrode and zinc negative electrode were assembled. Fig. 7a and b illustrate the initial three GCD curves and differential capacitance curves at 0.1 A g⁻¹, indicating a prominent discharge voltage plateau at approximately 1.5 V with an initial discharge capacity of 89.5 mAh g⁻¹ and Coulombic efficiency of 98.3%. After 45 cycles, the cell maintains a stabilized capacity of 66.7 mAh g⁻¹, representing 74.5% retention of its initial capacity as shown in Fig. 7c. This soft-pack battery also exhibits favorable rate capabilities with values of 106.4, 71.1, 54.0, 42.9, 36.7, and 29.2 mAh g⁻¹ as current densities increase from 0.1 to 1.3 A g⁻¹, as illustrated in Fig. 7d and e. Remarkably, when reverting to 0.1 A g⁻¹, the cell demonstrates excellent capacity restoration to 78.8 mAh g⁻¹, highlighting its exceptional rate capability. This flexible soft-pack battery maintains stable operation under mechanical stress, successfully powering a digital timer at different bending angles and even under cutting conditions (Fig. 7f). Furthermore, two flexible soft-pack batteries can also supply sufficient electrical power for an LED toy as shown in Fig. 7g. Notably, the soft-pack battery retains a reversible capacity of 61.4 mAh g⁻¹ over 1000 cycles at 1 A g⁻¹, proving its long-term cycling life and high stability. These findings validate N3VOPF@rGO&MWCNTs-2 as a viable cathode material for developing reliable and advanced electrochemical storage technologies with excellent cycling stability and mechanical flexibility.

Fig. 7 (a) Initial GCD profiles of the soft-pack battery at 0.1 A g⁻¹. (b) The corresponding dQ/dV plots for the initial three cycles. (c) Cycling performance of the soft-pack battery at 0.1 A g⁻¹. (d) Rate performance of the soft-pack battery. (e) GCD curves of the soft-pack battery at current densities from 0.1 to 1.3 A g⁻¹. (f) A digital timer powered by a soft-pack battery under bending and cutting conditions. (g) Cycling performance of the soft-pack battery at 1 A g⁻¹ (inset: an LED toy lit by two soft-pack batteries in series).

Conclusions

This study demonstrates a microwave-assisted hydrothermal method coupled with post-annealing for the successful synthesis of Na₃V₂O_1.6(PO₄)₂F_1.4@rGO&MWCNTs. This composite features Na₃V₂O_1.6(PO₄)₂F_1.4 nanoparticles enclosed within a three-dimensional conductive network that integrates two-dimensional rGO sheets and one-dimensional MWCNTs. It delivers an initial reversible capacity of 98.4 mAh g⁻¹ at 0.2 A g⁻¹ and retains a remarkable capacity of 52.8 mAh g⁻¹ after 6000 cycles at 5 A g⁻¹. In situ EIS, DRT, CV, and GITT measurements validate the reaction kinetics in detail, revealing low internal resistance and rapid ion/electron transport capabilities. The reversible Zn²⁺ insertion/extraction process is revealed by ex situ XRD and XPS. Furthermore, this composite in a soft-pack battery can maintain stable operation under different mechanical stresses. It exhibits a reversible capacity of 61.4 mAh g⁻¹ after 1000 cycles at 1 A g⁻¹, highlighting the outstanding adaptability and cycling stability in practical applications. These results underscore the N3VOPF@rGO&MWCNTs composite as a competitive cathode material for developing long-life and high-rate AZIBs.

Data availability

All data included in this study are available upon request from the corresponding author.

Author contributions

Rui Jiang: investigation, writing – original draft, data curation, Jiarui Lin: investigation, Xiaoyan Shi: data curation, Botao Zheng: investigation, Qiaofeng Huang: investigation, Junling Xu: data curation, Lianyi Shao: conceiving the research, writing the original draft, writing – review & editing, funding acquisition, validation, supervision, Zhipeng Sun: validation, supervision, funding acquisition, Qingqing Zhang: data curation, validation, supervision, and Lifeng Hang: data curation, validation, supervision.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21905058). We would like to thank the Analysis and Test Center of the Guangdong University of Technology for helping with XPS analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional results. See DOI: https://doi.org/10.1039/d5ta02751a

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