Defect‐Engineered Metal–Organic Frameworks via Coordination Competition Induction for Long‐Life Aqueous Zinc–Ion Batteries

ABSTRACT

Metal–organic frameworks (MOFs) with well‐defined crystalline structures provide ideal platforms for elucidating the intrinsic relationship between structure and electrochemical performance in aqueous zinc–ion batteries (AZIBs). However, the limited number of electrochemically active metal sites in MOFs constrains Zn²⁺ storage capacity and reaction kinetics. In this study, a ligand‐competition‐induced defect engineering strategy was adopted, where partial substitution of dicarboxylate ligands with monocarboxylate ligands during the synthesis of Br‐MIL(V)‐47 enables the ordered construction of controllable coordinatively unsaturated V sites. The results indicate that the moderate introduction of unsaturated V sites enhances framework flexibility and spatial buffering, effectively alleviating local structural distortion induced by repeated Zn²⁺ insertion/extraction and suppressing structural collapse and irreversible phase transitions. In/ex situ spectroscopic analyses further confirm the reversible structural evolution. The optimized 0.4‐SSA‐TPA cathode demonstrates excellent cycling stability. Experimental and theoretical analyses collectively indicate that the formation of unsaturated V sites induced local electron density redistribution, thereby facilitating reversible redox reactions. This study provides important insights into the precise design of MOF materials toward next‐generation energy storage applications.

This study presents a ligand‐competition‐induced defect engineering strategy to construct defect‐rich X‐SSA‐TPA frameworks with abundant coordinatively unsaturated V sites and open porosity, enabling rapid Zn²⁺ diffusion and charge transfer. In situ and ex situ analyses reveal reversible structural evolution and electron redistribution at V sites, which enhance redox kinetics and thereby deliver excellent cycling stability.

1. Introduction

With the rapid development of renewable energy technologies and the growing demand for high safety [1] and low‐cost energy storage systems [2], aqueous batteries have attracted widespread attention due to their inherent safety and environmental friendliness [3, 4, 5]. Among them, aqueous zinc–ion batteries (AZIBs) are regarded as highly promising candidates for large‐scale energy storage, benefiting from the high theoretical specific capacity of Zn anodes (820 mAh g⁻¹) [6, 7, 8]. However, the intrinsic trade‐off between electrochemical performance and structural stability of cathode materials remains a central scientific challenge hindering the further development of AZIBs [9]. Cathode materials with highly stable frameworks are typically limited by a scarcity of electrochemically active sites, resulting in low specific capacities and sluggish reaction kinetics [10]. In contrast, highly active cathode materials tend to undergo irreversible structural evolution during repeated Zn²⁺ insertion/extraction processes, resulting in accelerated capacity decay and poor cycling stability [11]. Therefore, developing cathode materials that balance high electrochemical activity and structural stability is essential for revealing structure‐performance relationships in high‐performance AZIBs.

Metal–organic frameworks (MOFs), owing to their highly designable pore structures [12], flexible framework characteristics [13, 14], and rich chemical functionalities [15], provide a unique platform for the precise regulation of material structures and the elucidation of structure‐performance relationships [16]. However, the limited availability of coordinatively unsaturated metal sites in conventional MOFs constrains the desolvation process and transport kinetics of Zn²⁺, thereby restricting their practical application in AZIBs [17, 18, 19]. To address this limitation, introducing defects via the targeted substitution of a portion of dicarboxylate ligands with monocarboxylate ligands is an effective strategy to create coordinatively unsaturated metal sites [20]. These coordination defects can introduce open metal active sites and induce local structural reconstruction without compromising the structural stability of the MOFs backbone framework [21, 22], thereby enabling precise regulation of the structural microenvironment of cathode materials. The defect‐induced modulation of the local microenvironment not only facilitates accelerated charge‐transfer kinetics and ion diffusion, but also alleviates the structural strain generated during repeated Zn²⁺ insertion/extraction processes [23, 24, 25]. Although defect‐tailored MOFs have demonstrated remarkable advantages in electrocatalysis and other electrochemical systems, systematic studies on their application as cathode materials in AZIBs remain scarce.

In this study, we propose a defect engineering strategy to promote the structural evolution of MOFs during the charge‐discharge processes of AZIBs and to significantly enhance their Zn²⁺ storage performance. Defective V‐MOFs (denoted as X‐SSA‐TPA) were constructed by partially substituting 2‐bromoterephthalic acid (Br‐TPA) with sulfosalicylic acid (SSA) within the framework. The coordinatively unsaturated metal active sites and the highly open porous architecture in X‐SSA‐TPA effectively promote Zn²⁺ diffusion kinetics and charge‐transfer processes, thereby endowing the cathode with outstanding rate performance. When the molar ratio of Br‐TPA to SSA during synthesis is 8:2, the resulting 0.4‐SSA‐TPA exhibits superior electrochemical performance and stability compared with other ratios and pristine Br‐MIL(V)‐47. As a result, the optimized 0.4‐SSA‐TPA cathode maintains a high discharge capacity of 111.5 mAh g⁻¹ even over 6000 cycles at 5.0 A g⁻¹. Theoretical calculations further confirm that the defective 0.4‐SSA‐TPA exhibits a lower Zn²⁺ migration energy barrier, providing strong theoretical support for its superior electrochemical performance.

2. Results and Discussion

Defect engineering is regarded as an effective strategy for regulating the metal coordination environment and electrochemical activity of MOFs. Controllable defects were constructed in the Br‐MIL(V)‐47 framework by partially substituting Br‐TPA ligands with SSA ligands during the synthesis (Figure 1). When Br‐TPA is used as the sole ligand (Br‐MIL(V)‐47), each V site in the intact crystal is fully saturated with six V─O bonds connected to either the carboxylate groups of the ligands or the terminal ‐OH groups. However, SSA contains only one carboxylic acid functional group. Upon coordination with V metal sites, the ‐SH group in SSA exhibits strong electron‐donating ability, which enhances the coordination strength of the carboxyl group, stabilizes high‐valent V centers, and modulates their electronic structure. The bromine substituent in Br‐TPA exerts an electron‐withdrawing effect, weakening the coordination ability of the carboxyl group. Consequently, SSA more readily partially replaces Br‐TPA during coordination competition, thereby inducing the formation of coordinatively unsaturated V sites. By tuning the molar ratio of Br‐TPA to SSA during the synthesis (9:1, 8:2, 7:3, 6:4, 5:5, and 4:6), the defect concentration was precisely regulated, and the resulting samples were denoted as 0.2‐SSA‐TPA, 0.4‐SSA‐TPA, 0.6‐SSA‐TPA, 0.8‐SSA‐TPA, 1.0‐SSA‐TPA, and 1.2‐SSA‐TPA. For coordinatively saturated Br‐MIL(V)‐47, the rigid coordination environment is unable to buffer the local stress induced by the desolvation of [Zn(H₂O)₆]²⁺ and the variation in V valence states, thereby triggering structural collapse. In contrast, in X‐SSA‐TPA, the coordinatively unsaturated V sites convert static structural defects into dynamic electrochemical active centers, thereby achieving the synergistic optimization of reaction kinetics, Zn²⁺ storage sites, and structural stability.

FIGURE 1.

Defect engineering in Br‐MIL(V)‐47 via partial substitution of Br‐TPA with SSA, generating coordinatively unsaturated V sites and enhanced electrochemical kinetics.

Defect‐engineered X‐SSA‐TPA was rationally constructed via ligand substitution chemistry (Figure 2a). Scanning electron microscopy (SEM) observations show that both Br‐MIL(V)‐47 and X‐SSA‐TPA possess uniform spindle‐shaped morphologies (Figures 2b,c and S1), with an average particle width of approximately 0.55 µm (Figure S2), suggesting that ligand substitution does not significantly disrupt the overall morphological features of the materials. When the Br‐TPA/SSA molar ratio decreases to 4:6, the spindle‐like structure begin to undergo self‐assembly at higher SSA incorporation levels. Optical photographs further reveal that (Figure S3), with increasing SSA molar content, the sample color gradually deepens from dark gray to black, accompanied by a decrease in yield, suggesting that a high SSA content exerts a pronounced influence on the crystal growth process. The high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) and the corresponding energy‐dispersive spectroscopy (EDS) elemental mapping reveal that C, O, Br, and V are uniformly distributed in both Br‐MIL(V)‐47 and 0.4‐SSA‐TPA, whereas the S element is observed exclusively in 0.4‐SSA‐TPA (Figures 2d,e and S4). This further confirms that SSA successfully partially replaces Br‐TPA and introduces unsaturated metal sites into the framework structure of X‐SSA‐TPA. This observation further confirms that SSA successfully replaces part of the Br‐TPA ligands and is integrated into the framework, thereby introducing coordinatively unsaturated metal sites in the X‐SSA‐TPA structure.

FIGURE 2.

(a) Schematic representation of the synthetic route for the 0.4‐SSA‐TPA cathode. SEM images of (b) Br‐MIL(V)‐47 and (c) 0.4‐SSA‐TPA. HAADF and EDS elemental mapping of (d) Br‐MIL(V)‐47 and (e) 0.4‐SSA‐TPA. (f) Molecular structures and electrostatic potential maps of SSA and Br‐TPA. (g) MESP comparison between SSA and Br‐TPA. (h) HOMO/LUMO energy levels and energy gaps of SSA and Br‐TPA. (i) XRD patterns of Br‐MIL(V)‐47 and X‐SSA‐TPA. (j) FTIR spectra of Br‐MIL(V)‐47 and X‐SSA‐TPA. (k) N₂ adsorption–desorption isotherms of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA.

Molecular electrostatic potential (MESP) analysis provides insight into the electron density distribution within the SSA and Br‐TPA structures (Figure 2f,g) [26]. In SSA, pronounced negative electrostatic potential regions are observed around both the C═O and C─S groups (−33.85 kcal mol⁻¹), which can serve as active coordination sites for V³⁺. In contrast, the negative electrostatic potential of Br‐TPA is mainly localized on the C═O groups (−32.65 kcal mol⁻¹), indicating that SSA exhibits a stronger competitive advantage in coordinating with V³⁺. This facilitates the partial substitution of Br‐TPA and induces the formation of coordination‐unsaturated V active sites. Furthermore, the HOMO‐LUMO energy band gaps (ΔE_H‐L) analysis (Figure 2h) indicates that SSA possesses a narrower band gap (4.58 eV) than Br‐TPA (4.67 eV). The smaller band gap implies improved electronic conductivity and more favorable charge‐transfer behavior [27]. Consequently, the coordination‐competitive strategy enables the construction of X‐SSA‐TPA with unsaturated sites, which is expected to accelerate Zn²⁺ storage kinetics and significantly enhance electrochemical activity.

X‐ray diffraction (XRD) was employed to evaluate the effect of SSA introduction on the crystal structure of Br‐MIL(V)‐47. The diffraction peak positions of Br‐MIL(V)‐47 and X‐SSA‐TPA match well with the simulated pattern [28], and no obvious peak shifts are observed, indicating that the introduction of SSA does not disrupt the topology of the Br‐MIL(V)‐47 framework (Figure 2i). With increasing SSA content, the diffraction peak at approximately 8.65° corresponding to the (011) plane gradually decreases in intensity and exhibits peak broadening, indicating that the substitution of Br‐TPA by SSA, owing to its monocarboxylate coordination nature, weakens the original bridging coordination network, thereby reducing the long‐range structural order of the framework and inducing local lattice strain.

Fourier transform infrared (FTIR) spectroscopy further reveals the structural characteristics of X‐SSA‐TPA (Figure 2j). The absorption peaks at 1482 and 1552 cm⁻¹ can be assigned to the characteristic vibrations of the aromatic ring skeleton, while the peaks observed at 1250 cm⁻¹ (C─O stretching vibration) and 1039 cm⁻¹ (C─S stretching vibration) provide further confirmation of the presence of benzene ring structures as well as oxygen‐ and sulfur‐containing functional groups in X‐SSA‐TPA after coordination [29]. Meanwhile, the peak observed at 572 cm⁻¹ is attributed to the stretching mode of O─V─O bonds, indicating the formation of vanadium‐oxygen coordination units [30, 31]. These results demonstrate that, in X‐SSA‐TPA, VCl₃ forms stable coordination structures with the Br‐TPA and SSA ligands. In addition, N₂ adsorption‐desorption measurements at 77 K (Figure 2k; Figures S5 and S6) show that X‐SSA‐TPA exhibits a significantly higher specific surface area (28.99–74.79 m² g⁻¹) than Br‐MIL(V)‐47 (26.21 m² g⁻¹) and contains abundant mesopores (7.53–15.39 nm), which are expected to provide sufficient space for the rapid diffusion of Zn²⁺ (1.48 Å) and its solvated ion [Zn(H₂O)₆]²⁺ (0.86 nm) in AZIBs (Table S1) [32]. Compared with Br‐MIL(V)‐47, the higher surface area of X‐SSA‐TPA can be attributed to coordination defects induced by the partial substitution of Br‐TPA with the monocarboxylate SSA, thereby promoting the formation of a more porous structure.

X‐ray photoelectron spectroscopy (XPS) was utilized to probe the variations in electronic structures between Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. The XPS survey spectrum confirms that 0.4‐SSA‐TPA contains only C, O, Br, S, and V, with no detectable impurities (Figure S7). The V 2p spectra of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA were deconvoluted into two sets of doublets (Figure 3a), corresponding to V³⁺ (V 2p_3/2 at 515.34 eV and V 2p_1/2 at 522.71 eV) and V⁴⁺ (V 2p_3/2 at 516.67 eV and V 2p_1/2 at 524.08 eV), respectively [33]. Compared with Br‐MIL(V)‐47, the V 2p peaks of 0.4‐SSA‐TPA exhibit an overall shift toward higher binding energies, indicating that the introduction of SSA decreases the local electron density around the V centers, thereby inducing defect formation and generating unsaturated V sites [34]. Meanwhile, the high‐resolution O 1s spectra of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA were fitted into three peaks, assigned to V‐O (≈530.05 eV), O═C (≈531.59 eV), and O‐H (≈532.66 eV) configurations (Figure 3b). The C 1s spectra of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA, originating from the carbon species in the organic ligands, can be deconvoluted into three peaks at 284.62, 285.52, and 288.61 eV, which are assigned to C═C/C─C, C─O, and C═O bonds, respectively (Figure 3c) [35]. In addition, the S 2p spectrum of 0.4‐SSA‐TPA can be deconvoluted into characteristic peaks at 163.39, 164.48, and 166.55 eV, which are assigned to S 2p_3/2, S 2p_1/2, and oxidized S‐O species, respectively, indicating that the sulfur‐containing ligand SSA has been successfully introduced into the framework and partially substituted for Br‐TPA, thereby producing a 0.4‐SSA‐TPA material possessing unsaturated sites (Figure S8).

FIGURE 3.

(a–c) XPS spectra of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (d) V K‐edge XANES of 0.4‐SSA‐TPA, V₂O₃, VO₂, and V foil. (e) Fourier transform of V EXAFS in R‐space of 0.4‐SSA‐TPA, V₂O₃, VO₂, and V foil. (f) FT‐EXAFS fitting curve of V K‐edge. EXAFS wavelet transforms of (g) V foil, (h) V₂O₃, and 0.4‐SSA‐TPA.

Synchrotron‐based x‐ray absorption spectroscopy (XAS) was conducted to elucidate the coordination environment and chemical configuration of the V metal centers in 0.4‐SSA‐TPA and standard samples (including V₂O₃, VO₂, and V foil). From the normalized x‐ray absorption near‐edge structure (XANES) spectra (Figure 3d), the absorption edge of 0.4‐SSA‐TPA lies between those of V₂O₃ and VO₂, indicating that the V centers exhibit a mixed valence state between +3 and +4, consistent with the XPS results [36, 37]. Fourier transform extended x‐ray absorption fine structure (FT‐EXAFS) spectra show that 0.4‐SSA‐TPA exhibits a dominant peak at approximately 1.46 Å, which can be assigned to typical V‐O coordination (Figure 3e). Meanwhile, no scattering peak corresponding to V─V bonds is observed at around 2.32 Å, indicating that the V species in 0.4‐SSA‐TPA are atomically dispersed. To accurately elucidate the coordination environment of the V species in 0.4‐SSA‐TPA, EXAFS fitting analysis was performed (Figures 3f and S9). The optimal fitting results for the first coordination shell indicate that the average coordination number of the V─O bond is 4.73, suggesting that each V center is coordinated with approximately five O atoms. Furthermore, the corresponding wavelet transform (WT) results show that 0.4‐SSA‐TPA exhibits only one intensity maximum at approximately 6.75 Å⁻¹ in k‐space [38]. This feature is clearly distinct from that of V foil (~7.54 Å⁻¹) and close to that of V₂O₃ (~6.78 Å⁻¹), confirming the presence of V‐O coordination in 0.4‐SSA‐TPA (Figure 3g–i).

The electrochemical properties of Br‐MIL(V)‐47 and X‐SSA‐TPA in AZIBs were systematically investigated. The cyclic voltammetry (CV) curves of Br‐MIL(V)‐47 and X‐SSA‐TPA cathodes are presented in Figure S10. All cathode materials exhibit two distinct pairs of redox peaks at 1.05/0.93 V and 0.66/0.54 V, corresponding to the multistep redox reactions of V during the Zn²⁺ and H⁺ insertion/extraction process [39, 40]. Meanwhile, compared with the Br‐MIL(V)‐47 cathode, the 0.4‐SSA‐TPA cathode exhibits lower polarization potential and a broader CV area, indicating that the defect‐induced coordinatively unsaturated sites in 0.4‐SSA‐TPA effectively modulate the local electronic environment of the V metal centers, thereby enabling superior specific capacity (Figure 4a). To verify the involvement of protons in the electrochemical process of the 0.4‐SSA‐TPA cathode, galvanostatic charge/discharge (GCD) tests were conducted in electrolytes with controlled pH values, including Zn(CF₃SO₃)₂ (pH = 4) and CF₃SO₃H solutions (pH = 4, 1.8, and 0.35) (Figure S11). As the H⁺ concentration increases (0.0005, 0.005, and 0.1 M CF₃SO₃H), the discharge capacity increases markedly, indicating the co‐participation of Zn²⁺ and H⁺ in the electrochemical reaction. Meanwhile, the GCD curves of the 0.4‐SSA‐TPA cathode exhibit two well‐defined voltage plateaus (Figure 4b), consistent with the multistep redox behavior observed in CV, further confirming a reversible Zn²⁺ insertion/extraction mechanism. Under the same conditions, 0.4‐SSA‐TPA delivers a discharge specific capacity of 244.8 mAh g⁻¹. Figures 4c and S12 present the rate performance of Br‐MIL(V)‐47 and X‐SSA‐TPA cathodes at various current densities. The 0.4‐SSA‐TPA cathode delivers specific capacities of 236.3, 211.9, 177.2, 166.6, 154.6, 140.7, and 123.9 mAh g⁻¹ at 0.3, 0.5, 1.0, 1.5, 2.0, 3.0, and 5.0 A g⁻¹, respectively, outperforming Br‐MIL(V)‐47 and other X‐SSA‐TPA cathode. When the current density is returned to 0.3 A g⁻¹, a reversible specific capacity of 233.8 mAh g⁻¹ is recovered, with a capacity retention of about 98.9%, indicating excellent electrochemical reversibility [41]. As shown in Figure S13, the GCD curves of the 0.4‐SSA‐TPA cathode at current densities ranging from 0.3 to 5.0 A g⁻¹ exhibit enhanced charge/discharge capacities and low voltage polarization, indicating excellent reaction kinetics. Moreover, comparative tests were conducted in electrolytes with different concentrations of Zn(CF₃SO₃)₂ as well as 3 M ZnSO₄. The results demonstrate that, in the 3 M Zn(CF₃SO₃)₂ electrolyte, the 0.4‐SSA‐TPA (247.8 mAh g⁻¹) cathode exhibits the best cycling stability and capacity retention after 100 cycles at 0.3 A g⁻¹ (Figure 4d; Figures S14 and S15), which are markedly higher than those of 0.2‐SSA‐TPA (193.1 mAh g⁻¹), 0.6‐SSA‐TPA (213.8 mAh g⁻¹), 0.8‐SSA‐TPA (187.9 mAh g⁻¹), 1.0‐SSA‐TPA (190.2 mAh g⁻¹), 1.2‐SSA‐TPA (104.6 mAh g⁻¹), and Br‐MIL(V)‐47 (127.0 mAh g⁻¹).

FIGURE 4.

(a) CV curves of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (b) GCD curves of the Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (c) Rate performance of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (d) Bar plot of cycling performance at 0.3 A g⁻¹. (e) GITT curve of 0.4‐SSA‐TPA. (f) Zn²⁺ diffusion coefficients (Log DZn²⁺). (g) Contour plots of CV curves of 0.4‐SSA‐TPA. (h) The b‐values of 0.4‐SSA‐TPA. (i) Capacitive contributions of 0.4‐SSA‐TPA at various scan rates. (j) Cyclic stability test of 0.4‐SSA‐TPA at 5 A g⁻¹. (k) Comparison of the cycling stability of 0.4‐SSA‐TPA with other reported AZIB cathodes.

To gain deeper insight into the influence of coordinatively unsaturated metal sites on the reaction kinetics of X‐SSA‐TPA cathodes (Figure 4e), the Zn²⁺ diffusion coefficient (D _Zn ²⁺) of 0.4‐SSA‐TPA was evaluated using the galvanostatic intermittent titration technique (GITT). The obtained D _Zn ²⁺ values of 0.4‐SSA‐TPA fall within the range of 10⁻¹²–10⁻¹⁰ cm² s⁻¹ (Figure 4f). To further elucidate the redox behavior and Zn²⁺ storage mechanism (Figures 4g and S16), CV measurements of the Br‐MIL(V)‐47 and X‐SSA‐TPA cathodes were conducted at various scan rates (0.2–1.2 mV s⁻¹). As the scan rate increases, the redox peak positions of the 0.4‐SSA‐TPA cathode exhibit only slight shifts, indicating favorable reaction reversibility and fast electrochemical kinetics. Based on the relationship i  =  av^b provided in the Supporting Information, thecvalues of the Br‐MIL(V)‐47 and X‐SSA‐TPA cathodes were obtained by linear fitting of the log(i) versus log(v) plots (Figures 4h and S17). For the 0.4‐SSA‐TPA cathode, the b values of peaks 1–4 are 0.982, 0.987, 0.976, and 0.965, respectively, indicating that the charge storage behavior is predominantly governed by capacitive processes [42]. The capacitive contributions of the 0.4‐SSA‐TPA cathode at different scan rates are 83.54%, 85.70%, 86.91%, 88.53%, 92.51%, and 95.63% (Figure 4i), respectively, all of which are higher than those of Br‐MIL(V)‐47 and other X‐SSA‐TPA samples, indicating its superior rate performance (Figures S18–S25). Electrochemical impedance spectroscopy (EIS) results (Figures S26 and S27) indicate that, compared with other electrode materials, the 0.4‐SSA‐TPA cathode exhibits the lowest charge transfer resistance (R _ct = 112.2 Ω), with the corresponding equivalent circuit model shown in the insets. This indicates that defect‐induced unsaturated metal sites effectively reduce the interfacial charge‐transfer barrier, thereby enhancing the charge‐transfer kinetics. After 6000 cycles at 5 A g⁻¹, 0.4‐SSA‐TPA retained a reversible specific capacity of 111.5 mAh g⁻¹, further demonstrating that the introduction of an appropriate amount of defects effectively enhances the cycling stability of V‐MOFs (Figure 4j). Notably, the initial rise in specific capacity followed by stabilization can be attributed to the gradual activation of electroactive sites and the improvement of electrode‐electrolyte wettability during the early cycles, which facilitate enhanced Zn²⁺ accessibility and utilization [43, 44]. The post‐cycling SEM image (Figure S28) shows that 0.4‐SSA‐TPA retains its initial spindle‐like morphology, further confirming the improved structural stability. The 0.4‐SSA‐TPA cathode exhibits electrochemical performance in AZIBs superior to that of most previously reported cathodes (Figure 4k and Table S2).

To further clarify the underlying charge storage mechanism and structural evolution of the 0.4‐SSA‐TPA cathode upon cycling (Figure 5a), systematic analyses were carried out using multiple in/ex situ characterization techniques at different electrochemical states. Figures 5b and S29 illustrate the in situ XRD evolution of the 0.4‐SSA‐TPA cathode during the electrochemical process. During discharge, the diffraction peaks located at approximately 8.1°, 12.1°, and 17.4°, corresponding to the (011), (020), and (022) planes, exhibit slight shifts toward lower angles, indicating crystal lattice expansion induced by Zn²⁺/H⁺ insertion. During charging, with the extraction of Zn²⁺/H⁺, these diffraction peaks gradually revert to their original positions, demonstrating that the 0.4‐SSA‐TPA cathode undergoes highly reversible structural evolution during the Zn²⁺/H⁺ insertion/extraction process. Meanwhile, Figure S30 presents the ex situ XPS spectra of the 0.4‐SSA‐TPA cathode at various electrochemical states. The V 2p spectra (Figure 5c) reveal the coexistence of V⁴⁺ and V³⁺ species in the material. Upon discharging to 0.3 V, the V⁴⁺ peak intensity decreases while the V³⁺ peak increases, indicating that the Zn²⁺ insertion process is accompanied by electron transfer, leading to the reduction of V⁴⁺ to V³⁺. After charging to 1.5 V, the V⁴⁺ peak becomes dominant again with a reduced V³⁺ component, further demonstrating that highly reversible redox reactions occur in 0.4‐SSA‐TPA during cycling [45]. Meanwhile, the Zn 2p spectra exhibit pronounced Zn 2p_3/2 and Zn 2p_1/2 characteristic peaks in the discharged state (Figure 5d), whereas the peak intensity significantly decreases after charging, further confirming the reversible insertion/extraction behavior of Zn²⁺.

FIGURE 5.

(a) GCD curve of 0.4‐SSA‐TPA. (b) In situ XRD patterns of 0.4‐SSA‐TPA. (c) Ex situ V 2p XPS spectra. (d) Ex situ Zn 2p XPS spectra. (e) In situ FTIR spectra of 0.4‐SSA‐TPA. (f,g) Band structures of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (h,i) DOS of Br‐MIL(V)‐47 and 0.4‐SSA‐TPA. (j,k) Migration path for Zn²⁺ in Br‐MIL(V)‐47 and 0.4‐SSA‐TPA.

In situ FTIR spectra (Figures 5e and S31) further reveal that, during discharge, the characteristic absorption peaks of O‐H (1397 cm⁻¹) and C‐O (1208 cm⁻¹) groups gradually increase in intensity, indicating reversible coordination between Zn²⁺/H⁺ and oxygen atoms in the ligands, thereby forming Zn‐O‐C or C‐O‐H groups. In contrast, during charging, the C‐O/O‐H absorption peaks display a reverse variation, confirming the reversible transition of the 0.4‐SSA‐TPA cathode from the reduced state to the oxidized state. Meanwhile, the vibration peak of the O‐V‐O framework at 716 cm⁻¹ exhibits reversible intensity variations in accordance with the insertion/extraction of Zn²⁺/H⁺. Moreover, SEM images (Figures S32 and S33) and the corresponding EDS elemental mapping (Figure S34) at different electrochemical states further confirm the structural stability of the 0.4‐SSA‐TPA cathode and the reversible insertion/extraction of Zn²⁺ during cycling.

Density functional theory (DFT) calculations further confirm that the unsaturated metal sites formed by defects introduced through partial substitution of Br‐TPA with SSA play a critical role in modulating Zn²⁺ storage behavior. Compared with Br‐MIL(V)‐47 (1.07 eV), the band gap of 0.4‐SSA‐TPA decreases to 0.82 eV, indicating that the introduction of SSA effectively reconstructs the local electronic structure of the V centers (Figure 5f,g). The higher polarizability of the S atoms enhances the orbital hybridization between S 3p and V 3d orbitals, thereby narrowing the valence‐conduction band gap and lowering the electron‐transition barrier, which facilitates charge transport [46]. The density of states (DOS) results (Figure 5h,i) show that 0.4‐SSA‐TPA exhibits a higher electronic density near the Fermi level, further confirming its enhanced electronic conductivity. The Zn²⁺ diffusion pathways and energy barriers (Figures 5j,k and S35) show that while the migration barrier of Br‐MIL(V)‐47 is 0.51 eV, it drops to 0.28 eV in 0.4‐SSA‐TPA, as the introduction of monocarboxylate ligands induces defects that form coordinatively unsaturated sites, thereby facilitating Zn²⁺ transport [47]. Consequently, the open metal sites generated by the unsaturated coordination environments in 0.4‐SSA‐TPA reduce coordination constraints and promote rapid Zn²⁺ transport, thereby enabling enhanced capacity and cycling performance while maintaining structural stability.

To further evaluate the practical application potential of 0.4‐SSA‐TPA in AZIBs, a flexible Zn/0.4‐SSA‐TPA soft pack battery was assembled, with its structural components schematically illustrated in Figure 6a. The CV curves of the Zn/0.4‐SSA‐TPA soft pack battery at various scan rates exhibit distinct redox peaks (Figures 6b and S36), consistent with those of the coin cell (Figure 4a), confirming the stable electrochemical behavior and reversible reaction mechanism of the 0.4‐SSA‐TPA cathode across different battery configurations. The capacitive contributions of the Zn/0.4‐SSA‐TPA soft pack battery at different scan rates are calculated to be 80.23%, 83.91%, 86.75%, 89.58%, 91.66%, and 94.89%, respectively (Figure S37), thereby endowing it with superior rate performance. The b values of the two pairs of redox peaks are determined to be 0.932, 0.908, 0.928, and 0.905 (Figure S38), indicating that the charge storage behavior is likewise dominated by capacitive processes. The Zn/0.4‐SSA‐TPA soft pack battery was subjected to EIS and cycling performance tests under various bending angles (0°, 90°, 135°, and 180°). As shown in Figure 6c,d, after 200 cycles at 1 A g⁻¹, a high specific capacity of 145.8 mAh g⁻¹ is retained with only slight R_ct fluctuations, indicating that the Zn/0.4‐SSA‐TPA soft pack battery preserves outstanding mechanical and electrochemical stability during bending [48]. Moreover, the Zn/0.4‐SSA‐TPA soft pack battery exhibits excellent rate performance, delivering discharge specific capacities ranging from 177.3 to 101.8 mAh g⁻¹ at current densities from 0.3 to 5 A g⁻¹ (Figure 6e). When the current density is restored to 0.3 A g⁻¹, the capacity retention can still reach 95.19%. Safety tests further reveal that even under bending and partial cutting conditions, the open‐circuit voltage of the Zn/0.4‐SSA‐TPA soft pack battery remains stable, and it can continuously power electronic devices such as mobile phones, LED panels, and wristbands (Figure 6f–o; Figures S39 and S40), thereby confirming its excellent safety and practical application potential under harsh conditions.

FIGURE 6.

(a) Schematic illustration of the assembled Zn/0.4‐SSA‐TPA soft‐pack battery. (b) Contour plots of CV curves. (c,d) EIS spectra and cyclic stability under various bending states. (e) Rate performance of the Zn/0.4‐SSA‐TPA soft‐pack battery. (f) Zn/0.4‐SSA‐TPA soft pack battery size. Zn/0.4‐SSA‐TPA soft‐pack battery powering mobile phone at (g) 0° and (h) 90° bending states. Open‐circuit voltage of the Zn/0.4‐SSA‐TPA soft pack battery under bending angles (i) 0°, (j) 90°, and (k) 180°. (i–o) Powering the LED sign and wristband under various states.

3. Conclusion

In summary, we employ a defect engineering strategy by partially substituting the dicarboxylate Br‐TPA with the monocarboxylate SSA to successfully construct X‐SSA‐TPA featuring abundant coordinatively unsaturated V sites, which is further applied as a cathode material for AZIBs. Compared with Br‐MIL(V)‐47, the defect‐induced unsaturated V sites in X‐SSA‐TPA effectively regulate the local electronic structure of the metal centers, significantly enhancing Zn²⁺ adsorption and interfacial charge‐transfer kinetics, thereby delivering higher specific capacity and faster reaction kinetics. Benefiting from the optimized structural and kinetic advantages, the 0.4‐SSA‐TPA cathode maintains 111.5 mAh g⁻¹ after more than 6000 cycles at 5 A g⁻¹. Moreover, the flexible Zn/0.4‐SSA‐TPA soft‐pack battery further confirms its practical application potential. This study achieves precise modulation of MOF crystal structures via ligand substitution to construct defect‐rich electrodes, providing guidance for rational MOF‐based energy storage design.

Author Contributions

Yanfei Zhang: conceptualization, writing – original draft, investigation, software. Qian Li: conceptualization, software, writing – original draft. Wanchang Feng: conceptualization, software, investigation. Haotian Yue: conceptualization, investigation, software. Yichun Su: conceptualization, investigation, software. Shengjie Gao: conceptualization, investigation, software. Mohsen Shakouri: conceptualization and software. Huan Pang: conceptualization, investigation, software, writing – original draft.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72792‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.