Strong Donor–Acceptor Effect Enables Efficient Polyiodides Confinement and Fast Redox Kinetics Toward Zinc–Iodine Batteries

ABSTRACT

Aqueous zinc–iodine batteries (ZIBs) based on iodine (I₂) redox conversion suffer from severe polyiodides shuttling and sluggish redox kinetics, leading to poor cycling stability. Herein, we propose a donor–acceptor (D–A) synergetic interaction strategy to enable high‐performance ZIBs by employing thiophene‐rich covalent triazine frameworks (SCTF‐DCT) as an advanced I₂ host catalyst. The strong D–A cooperative effect combined with a rich micro‐mesoporous structure not only optimizes the polyiodides adsorption but also enhances the conversion kinetics of I₂ species. Benefiting from the strong D–A effect, the as‐assembled ZIBs with I₂ loaded SCTF‐DCT (I₂@SCTF‐DCT) cathode demonstrate exceptional cycling life, exceeding 100,000 cycles with a minimal decay rate of only 0.000198% per cycle. In/ex situ characterizations and theoretical calculations reveal the reversible redox mechanism of I₂ species and highlight the significance of the D–A collaborative effect. This work elucidates the mechanisms of adsorption and catalytic conversion of I₂ species via a D–A synergetic interaction strategy, providing a promising approach for designing advanced host catalysts for next‐generation ZIBs and other energy storage systems.

The synergetic D–A interaction strategy in SCTF‐DCT enables effective polyiodides adsorption through strong interaction force, while simultaneously boosting electrocatalytic activity for I₂ reduction reaction and promoting redox kinetics of I₂ species, thereby jointly contributing to high electrochemical performance for I₂@SCTF‐DCT‐based Zinc–iodine batteries.

1. Introduction

The ever‐growing demand for effective and sustainable energy storage systems has propelled tremendous efforts toward advanced battery technologies [1, 2, 3]. Among various battery technologies, aqueous zinc–iodine batteries (ZIBs) have attracted increasing interest due to their intrinsic safety, eco‐friendliness, and cost‐effectiveness [4, 5, 6]. ZIBs are currently being explored for both two‑electron I₂/I⁻ conversion and multi‐electron iodine redox conversion [7]. Although multi‐electron ZIBs offer significantly enhanced energy density, they typically suffer from hydrolysis side reactions of high‐valent iodine intermediates and complex interfacial behaviors [8, 9], posing greater challenges to system stability and reversibility. Even in two‑electron systems, achieving stable operation and long cycle life for ZIBs remains challenging owing to low conductivity of iodine (I₂), serious polyiodides shuttling, and sluggish redox kinetics [10, 11]. To address the above‐mentioned issues of ZIBs, significant efforts, including development of cathode host catalysts [12, 13, 14, 15, 16, 17, 18], electrolyte engineering [19, 20, 21], functional binders [22, 23], separator optimization [24, 25, 26], and anode protection [27, 28, 29], have been devoted. Among these approaches, the development of cathode host catalysts has attracted widespread attention due to their structural designability and diversity.

Encouragingly, various cathode host catalysts have been developed to enhance the electrochemical performance of ZIBs, including traditional carbon‐based nanomaterials (carbon fibers and carbon nanotubes) [30, 31], carbon‐based single‐atom‐catalysts [32, 33, 34, 35], heteroatom‐doped porous carbon [1, 36, 37], metal nitrides‐modified porous carbon [38], and porous aromatic frameworks [10, 39]. Among these materials, porous carbon‐based nanomaterials have been widely used as I₂ host catalysts owing to their high electrical conductivity and structural stability. For example, Ma et al. developed a cobalt nanoparticles‐embedded porous carbon, which significantly enhanced I₂ redox kinetics through the “Trap‐adsorption‐Catalysis” principle [16]. Zhou et al. reported a porous two‐dimensional carbon nanosponges with abundant P‐dopants, which chemically anchored I₂ species and facilitated their redox conversion [1]. However, the preparation of carbon nanomaterials typically requires high‐temperature pyrolysis (800–1100°C), which not only increases the production cost but also causes uncontrolled structural changes accompanied by loss of active sites, hindering the precise preparation of catalysts and their large‐scale commercial application in ZIBs. Therefore, developing advanced I₂ host catalysts with accurately designed active sites is essential and urgent.

Owing to their well‑defined structures, high crystallinity, and extensive porosity, covalent organic frameworks (COFs) are promising cathode candidates for diverse secondary batteries [40]. However, the scalable production of highly crystalline COFs remains a large challenge, hindering their practical applications [41]. Conversely, covalent triazine frameworks (CTFs), featuring triazine rings as connecting units, hold great promise for I₂ adsorption due to their abundant pore structure, high nitrogen content, structural diversity, and straightforward ionothermal synthesis with high yield [42, 43]. Furthermore, the mature ionothermal synthesis methodology of CTFs effectively enhances their electrical conductivity while preserving numerous precise active sites, thereby overcoming the intrinsic limitations of high‐temperature pyrolyzed carbon nanomaterials. Most impressively, the well‐defined active sites of CTFs provide a compelling basis for elucidating structure–activity relationship. Recent studies showed that the thiophene unit offers unique advantages for adsorption and catalysis [44, 45], making it an attractive active site for ZIBs. Significantly, the thiophene unit acts as a strong electron donor, while the triazine ring serves as an effective electron acceptor. The unique donor–acceptor (D–A) effect endows CTFs with exceptional and tunable electronic properties. Specifically, the D–A synergistic effect can not only optimize intermediates adsorption but also facilitate charge transfer, thereby enhancing the overall catalytic activity and selectivity. Therefore, introducing abundant thiophene units into CTFs to construct D–A type CTFs represents a promising approach for developing high‐performance cathode host catalysts toward ZIBs. However, to the best of our knowledge, the D–A type CTFs as I₂ host catalysts for metal‐iodine batteries have not yet been reported, and the D–A synergetic interaction strategy for boosting ZIBs performance remains unexplored.

In this work, we developed a thiophene‐rich D–A type CTF (SCTF‐DCT) as a highly efficient I₂ cathode catalyst for ZIBs. Unlike traditional carbon materials that passively anchor iodine species through physical confinement or weak chemical interactions, the strong D–A effect in SCTF‐DCT enables effective polyiodide adsorption via charge‐transfer interactions while simultaneously boosting the electrocatalytic activity and redox kinetics of I₂ reduction reaction. As a result, the as‐fabricated ZIBs with I₂ loaded SCTF‐DCT (I₂@SCTF‐DCT) cathode exhibit superior long‐term cyclability, with a negligible capacity fading over 100,000 cycles at a high current density of 5 A g⁻¹ (0.000198% per cycle). Experimental analysis and theoretical calculations jointly unveil the redox mechanism of I₂@SCTF‐DCT cathode, underscoring the critical role of the D–A collaborative effect in activating and stabilizing I₂ species during cycling. This work innovatively proposes a D–A synergistic interaction strategy to boost ZIBs performance, paving a new avenue toward next‐generation electrochemical energy storage systems.

2. Results and Discussion

2.1. Preparation and Characterizations of Cathode Host Catalysts

The thiophene‐rich D–A type CTF (SCTF‐DCT) was directly prepared via an anhydrous ZnCl₂‐catalyzed ionothermal reaction of 2,5‐dicyanothiophene (DCT) monomer (Figure 1a and Scheme S1). The thiophene units and triazine rings of SCTF‐DCT function as electron donor and electron acceptor, respectively, creating a D–A collaborative interaction that boosts ZIBs performance (Figure 1a). Furthermore, the introduction of thiophene units into SCTF‐DCT can effectively regulate its electronic structure and create a distinct electrostatic potential gradient (Figure 1b), which is conducive to enhancing the interaction with subsequent I₂ species [5, 13]. For comparison, two CTFs with different donor strengths were prepared using 2,6‐dicyanopyridine (DCP) and 1,3‐dicyanobenzene (DCB) as monomers and were designated as NCTF‐DCP (Scheme S2) and CTF‐DCB (Scheme S3), respectively. The detailed synthesis procedures of CTFs are provided in the experimental section of the Supporting Information. Notably, CTF‐DCB shows a weak D–A structure due to the weak electron‐donating ability of benzene, whereas NCTF‐DCP, with an inherently electron‑deficient pyridine ring, features a dual‑acceptor configuration [46].

FIGURE 1.

a) Schematic illustration for the structure of SCTF‐DCT cathode host catalyst towards ZIBs. b) ESP distribution image of SCTF‐DCT. c) FTIR spectra of SCTF‐DCT and DCT. HR‐XPS d) N 1s and e) S 2p spectra for SCTF‐DCT. f) N₂ adsorption–desorption isotherms of SCTF‐DCT (Inset: the pore size distribution curve of SCTF‐DCT calculated by DFT method). g) HR‐TEM, h) HAADF‐STEM and corresponding EDS mapping images of C, N, and S elements for SCTF‐DCT.

The successful preparation of SCTF‐DCT was the first confirmed by Fourier transform infrared (FT‐IR) spectroscopy. As shown in Figure 1c, FT‐IR spectrum of SCTF‐DCT indicates that the vibrational band for cyano groups (2228 cm⁻¹) of DCT monomer vanished, while the characteristic vibration band of the triazine ring (1496 and 1348 cm⁻¹) appeared [47], demonstrating that the polymerization reaction of DCT monomer proceeded successfully. Additionally, the peak at 804 cm⁻¹ corresponds to the C─S─C stretching vibration, evidencing the successful incorporation of thiophene units [48]. The ¹³C cross‐polarization/magic angle spinning nuclear magnetic resonance (¹³C CP/MAS NMR) spectrum of SCTF‐DCT (Figure S1) exhibits two peaks at 169 and 132 ppm, attributing to the sp² carbon of triazine rings and aromatic carbon of thiophene units, respectively [49]. To further validate the structure of SCTF‐DCT, x‐ray photoelectron spectroscopy (XPS) was performed. As shown in Figure S2a, survey XPS spectra reveal distinct C 1s, N 1s, and O 1s signals for both SCTF‐DCT and NCTF‐DCP, while the S 2p signal is only observed in SCTF‐DCT. From high‐resolution XPS (HR‐XPS) C 1s spectrum of SCTF‐DCT (Figure S2b), three peaks at 284.8, 286.4, and 288.7 eV are identified as C─C, C═N, and C═O species, respectively [5]. The HR‐XPS N 1s spectrum of SCTF‐DCT mainly displays three main peaks (Figure 1d), corresponding to triazine N, pyrrolic N, and graphitic N [50], respectively. Regarding the S 2p spectrum of SCTF‐DCT (Figure 1e), two peaks at 164.1 and 165.3 eV correspond to carbon‐bonded sulfur of thiophene units (C−S−C), while the weak peak at 167.8 eV is associated with oxidized sulfur species [49]. Overall, the D–A structure of SCTF‐DCT was adequately demonstrated through the aforementioned characterizations. Besides, the structures of NCTF‐DCP and CTF‐DCB were also confirmed through FT‐IR and XPS analyses (Figure S3).

The powder x‐ray diffraction (PXRD) patterns of SCTF‐DCT and NCTF‐DCP exhibit broad diffraction peaks rather than distinct sharp peaks (Figure S4), demonstrating their amorphous structures. Additionally, Raman analysis in Figure S5 shows that the peak intensity ratio of D‐band and G‐band (I_D/I_G) for SCTF‐DCT (1.127) is pretty close to that of NCTF‐DCP (1.126), indicating their comparable defect degree [51]. The hierarchically porous structures of SCTF‐DCT and NCTF‐DCP were evaluated through N₂ adsorption–desorption isotherms at 77 K (Figures 1f and S6). The Brunauer–Emmett–Teller (BET) surface areas (S_BET ) of SCTF‐DCT and NCTF‐DCP are 827 and 1260 m² g⁻¹, with corresponding total pore volumes (V_t ) of 0.572 and 0.698 cm³ g⁻¹, respectively (Table S1). The rich micro‐mesoporous structure can provide faster mass transfer ability for the transformation of I₂ species [51]. The microscopic morphological structure of CTFs was characterized by scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HR‐TEM). SEM images in Figure S7 indicate that SCTF‐DCT possesses an irregular flake‐like stacking morphology, similar to that of NCTF‐DCP (Figure S8). Furthermore, the layered structure with uniform distribution of C, N, and S elements for SCTF‐DCT was further confirmed by its HR‐TEM images (Figures 1g and S9), high‐angle annular dark‐field scanning TEM (HAADF‐STEM) and corresponding energy‐dispersive x‐ray spectroscopy (EDS) mapping images (Figure 1h). Thermal gravimetric analysis (TGA) curve of SCTF‐DCT exhibits its extraordinary thermal stability (Figure S10a). Besides, SCTF‐DCT retains its chemical structure with uniform elemental distribution after prolonged soaking in strong acids and strong bases (Figures S10b,c, S11, and S12), indicating its excellent chemical stability. The high physicochemical stability of SCTF‐DCT is attributed to its highly conjugated and rigid structure formed by strong C─C covalent bonds.

2.2. Evaluation of Iodine Confinement Ability of SCTF‐DCT

Traditional porous materials mainly rely on physical interactions and pore confinement effect to adsorb I₂. However, the weak physical force leads to inferior I₂ adsorption performance, accompanied by a tendency for I₂ desorption at high temperatures. In contrast, the strong D–A interaction of SCTF‐DCT generates numerous high‐activity chemisorption sites, making it an ideal I₂ host for ZIBs. To prepare iodine cathode composites, I₂‐loaded SCTF‐DCT (I₂@SCTF‐DCT), I₂‐loaded NCTF‐DCP (I₂@NCTF‐DCP), and I₂‐loaded CTF‐DCB (I₂@CTF‐DCB) were prepared via a solution adsorption method. As displayed in Figure S13a, compared with NCTF‐DCP and CTF‐DCB, the visibly lighter color of I₂ solution after SCTF‐DCT adsorption indicates its better I₂ adsorption ability. The result is attributed to the unique D–A effect of SCTF‐DCT, which provides abundant chemical active sites and enhances the interaction with I₂. XPS survey spectra in Figure 2a reveal distinct I 3d signal in I₂@SCTF‐DCT compared with SCTF‐DCT, confirming the successful incorporation of I₂ species into SCTF‐DCT. Furthermore, porous properties of SCTF‐DCT and NCTF‐DCP before and after I₂ adsorption show obvious changes (Figures 2b and S13b and S14), and the S_BET and V_t values of I₂@SCTF‐DCT and I₂@NCTF‐DCP decreased significantly due to the successful loading of I₂ (Table S1). TEM and SEM images in Figures S15 and S16 confirm that I₂@SCTF‐DCT retains the flake‐like stacking morphology similar to its precursor, and corresponding EDS mapping images reveal a homogeneous distribution of I element throughout the SCTF‐DCT sample (Figures 2c and S16d). Similarly, I₂@NCTF‐DCP also maintains its original morphology with a uniform distribution of I element (Figure S17). According to the combination of TGA and gravimetry method (Figure S18), the I₂ uptake capacities were quantified as ≈ 31%, 28%, and 27% for I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB, respectively.

FIGURE 2.

a) XPS survey spectra and b) N₂ adsorption–desorption isotherms of I₂@SCTF‐DCT and SCTF‐DCT. c) HAADF‐STEM and corresponding elemental mapping distributions for C, N, S, and I elements as well as overlap for I₂@SCTF‐DCT. HR‐XPS d) N 1s and e) S 2p spectra for I₂@SCTF‐DCT and SCTF‐DCT. f) HR‐XPS I 3d spectrum of I₂@SCTF‐DCT. g) Raman spectra of I₂@SCTF‐DCT, SCTF‐DCT, and pure I₂. UV–vis spectra of h) I₂ adsorption and i) I₃ ⁻ adsorption for SCTF‐DCT, NCTF‐DCP, and CTF‐DCB.

To elucidate the adsorption mechanism of I₂ species by the D–A effect of SCTF‐DCT, XPS, electron paramagnetic resonance (EPR), and Raman spectroscopies were conducted. HR‐XPS N 1s spectra of SCTF‐DCT after I₂ adsorption reveal a significant shift in the peak corresponding to triazine units (Figure 2d). Similarly, the binding energy of S 2p for I₂@SCTF‐DCT also moves significantly after I₂ adsorption (Figure 2e). Moreover, EPR spectroscopy of I₂@SCTF‐DCT exhibits higher signal intensity than pristine SCTF‐DCT (Figure S19a), further indicating the presence of electron transfer interaction between iodine species and SCTF‐DCT structure [52]. These results indicate that there is a strong charge‐transfer interaction between D–A structure and I₂ molecules. HR‐XPS I 3d spectrum in Figure 2f confirms the presence of polyiodide species (I₃ ⁻ and I₅ ⁻) within I₂@SCTF‐DCT structure. Furthermore, Raman spectrum of I₂@SCTF‐DCT shows distinct peaks at 106.3 and 162.4 cm⁻¹ (Figure 2g), corresponding to stretching vibration of I₃ ⁻ and I₅ ⁻, respectively. After the adsorption of crystalline I₂, PXRD pattern of I₂@SCTF‐DCT still exhibits amorphous nature (Figure S19b), likely due to the generation and uniform adsorption of polyiodide species [5]. Furthermore, NCTF‐DCP also exhibits a similar I₂ adsorption behavior to SCTF‐DCT, as confirmed by relevant spectral analyses (Figure S20). To validate the D–A collaborative effect on the confinement ability of I₂ species, the relevant immersion and adsorption experiments were conducted. As displayed in Figure S21, I₂@SCTF‐DCT shows a nearly undetectable I₃ ⁻ signal after soaking the materials in electrolyte for three months, in contrast to pure I₂ and I₂@NCTF‐DCP, demonstrating its superior I₂ confinement ability. Besides, whether in I₂ solutions (Figures 2h and S22) or polyiodide solutions (Figures 2i and S23), SCTF‐DCT demonstrates faster and more efficient adsorption performance than NCTF‐DCP and CTF‐DCB, confirming its greater adsorption capability. Apart from the confinement role of rich micro‐mesoporous structure for I₂ species, the excellent adsorption and confinement ability of SCTF‐DCT for I₂ species was mainly contributed to the strong D–A synergistic effect, which provides a large number of high‐activity chemisorption sites and significantly strengthens the interaction with I₂ species.

2.3. Electrochemical Performance of I₂@SCTF‐DCT‐Based ZIBs

In view of the strong D–A effect of SCTF‐DCT and its outstanding polyiodides confinement ability, the electrochemical performance of I₂@SCTF‐DCT cathode was studied. To systematically evaluate the relationship between loading content and cycling performance, I₂@SCTF‐DCT electrodes with varying iodine loadings (22%, 31%, and 39%) were prepared by varying the initial concentration of iodine solutions (Figure S24a–c). As shown in Figure S24d, a loading of 31% was chosen as the optimal iodine content to balance active material mass and long‑term cycling stability, and all subsequent electrochemical tests were conducted under this condition. Cyclic voltammetry (CV) was utilized to explore the redox conversion kinetics of I₂ species on I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB cathodes (Figures 3a and S25). The I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB cathodes exhibit distinct oxidation‐reduction peaks, corresponding to 1.33/1.2, 1.35/1.19, and 1.34/1.16 V, respectively [53]. I₂@SCTF‐DCT‐based ZIBs show a smaller voltage gap (0.13 V) than I₂@NCTF‐DCP (0.16 V) and I₂@CTF‐DCB (0.18 V) cathodes, illustrating its lower polarization yet better reversibility. Tafel slopes of I₂@SCTF‐DCT, I₂@NCTF‐DCP and I₂@CTF‐DCB electrodes obtained from the CV curves are 263.6, 369.8, and 352.7 mV dec⁻¹ (Figure 3b), respectively, indicating the fastest reaction kinetics of I₂@SCTF‐DCT cathode. Additionally, the highly overlapped CV curves over the first five cycles further demonstrate excellent redox reversibility and stable electrochemical behavior of I₂@SCTF‐DCT electrode (Figure 3c). The rate performance of ZIBs with I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB cathodes was studied (Figures 3d and S26). The I₂@SCTF‐DCT electrode exhibits high specific capacities of 197, 180, 166, 154, 148, and 141 mAh g⁻¹ at 0.5, 1, 2, 3, 4, and 5 A g⁻¹, respectively, significantly outperforming I₂@NCTF‐DCP and I₂@CTF‐DCB cathodes (Figure 3d). When the current density is switched back from 5 to 0.5 A g⁻¹, I₂@SCTF‐DCT cathode still shows a high specific capacity of 191 mAh g⁻¹, confirming its good redox reversibility. The potential polarization of I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB was statistically analyzed under different current densities, as displayed in Figures 3e and S27. The smaller potential polarization of I₂@SCTF‐DCT cathode compared with I₂@NCTF‐DCP and I₂@CTF‐DCB cathodes indicates that SCTF‐DCT can effectively reduce voltage polarization and promote I₂ species redox kinetics. Besides, I₂@SCTF‐DCT cathode achieves a reversible specific capacity of 193.2 mAh g⁻¹ with 96.8% capacity retention (CR) and 99.5% average Coulombic efficiency (ACE) at 0.5 A g⁻¹ after 1100 cycles (Figures 3f and S28), superior to I₂@NCTF‐DCP cathode (CR: 84.8%, ACE: 99.1%) and I₂@CTF‐DCB (CR: 85.4%, ACE: 99.02%) cathodes. Furthermore, I₂@SCTF‐DCT electrode demonstrates an ultra‐long cycle life over 100,000 cycles at 5 A g⁻¹, with 80.2% CR and 99.9% ACE (Figures 3g and S29), far exceeding the cyclic performance of I₂@NCTF‐DCP (66.2% CR of 8900 cycles) and I₂@CTF‐DCB (69.2% CR of 6900 cycles) cathodes. Although NCTF‐DCP possesses a higher surface area and larger pore volume than SCTF‐DCT, it shows inferior electrochemical performance due to the absence of a donor–acceptor pair. Instead of structural characteristics, the chemisorption and catalytic active sites derived from the D–A cooperative effect are of paramount importance for polyiodide confinement and redox kinetics. As shown by the galvanostatic charge–discharge (GCD) and CV curves (Figure S30), SCTF‐DCT cathode shows negligible capacity contribution owing to its lack of redox activity, confirming that Zn²⁺ and H⁺ contribute almost no capacity during charge/discharge. This is because SCTF‐DCT lacks reversible redox‑active centers within the operating voltage window of ZIBs [54, 55]. The remarkable electrochemical performance of I₂@SCTF‐DCT cathode, particularly its ultra‐long lifespan at high current density, surpasses that of most previously‐reported ZIBs with advanced I₂ electrodes (Figure 3h; Table S2), including Ni‐Co‐DA/PCNs/I₂ [56], CAC@I₂ [23], and FeCoNi@I₂ [57].

FIGURE 3.

a) CV curves at 0.5 mV s⁻¹ and b) corresponding Tafel plots during reduction processes with I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB cathodes. c) The first five CV curves of ZIBs with I₂@SCTF‐DCT at 0.5 mV s⁻¹. d) Rate capability measured from 0.5 to 5 A g⁻¹, e) polarization voltages at 0.5 to 5 A g⁻¹, f) cycling performance at 0.5 A g⁻¹, and g) long‐term cycling performance at 5 A g⁻¹ for ZIBs with I₂@SCTF‐DCT, I₂@NCTF‐DCP, and I₂@CTF‐DCB cathodes. h) Comparison of electrochemical performance for I₂@SCTF‐DCT with recently reported I₂ cathodes.

To further highlight the practical application potential of the D–A effect for ZIBs, electrochemical performance of both coin cells and pouch cells was assessed under high iodine loading conditions. When the iodine areal loading reaches 10.7 mg cm⁻², I₂@SCTF‐DCT cathode delivers a specific capacity of 152 mAh g⁻¹ at 2 A g⁻¹ and maintains stable cycling for 2000 cycles (Figure S31a). Moreover, the as‐fabricated pouch cell with I₂@SCTF‐DCT cathode exhibits a specific capacity of 122 mAh g⁻¹ at 2 A g⁻¹ and delivers stable cycling for 400 cycles (Figure S31b), demonstrating its good practical application potential. To study the universality of the D–A interaction strategy across different zinc salts, additional cycling performance tests were conducted using 2 M Zn(CF₃SO₃)₂ and 1 M Zn(CH₃COO)₂ as electrolytes. Figure S32 demonstrates that I₂@SCTF‐DCT cathodes exhibit satisfactory cycling stability in both electrolytes after 2000 cycles at 2 A g⁻¹, achieving capacity retentions of 90.3% and 73.4% in Zn(CF₃SO₃)₂ and Zn(CH₃COO)₂ electrolytes, respectively. These results indicate that the D–A interaction strategy possesses universal applicability. The superior electrochemical performance, promising application prospects and broad applicability of I₂@SCTF‐DCT‐based ZIBs are fundamentally attributed to the strong D–A collaborative effect of SCTF‐DCT, rather than solely to chemical adsorption via sulfur atoms or pore structures. Specifically, the triazine ring acts as an anchoring center, chemisorbing polyiodides species through charge transfer interactions. Meanwhile, the incorporation of the electron‐donating thiophene unit generates a robust donor–acceptor cooperative effect, which not only reinforces polyiodide confinement but also enables it to act as a catalytic center. Thus, the strong D–A synergistic effect of SCTF‐DCT provides abundant chemisorption sites for enhanced polyiodide confinement, while simultaneously modulating the electronic structure to lower the kinetic barrier and accelerate the redox kinetics of iodine species.

The self‐discharge rate, a key indicator for evaluating energy storage performance, is primarily influenced by the polyiodides dissolution. As shown in Figure 4a,b, after resting in a fully charged state for 48 h, SCTF‐DCT cathode still maintains a high capacity retention of 87.6%, superior to NCTF‐DCP cathode (81.5%), highlighting the superior polyiodides confinement ability of SCTF‐DCT. Following a prolonged 480 h rest, I₂@SCTF‐DCT cathode exhibits higher voltage stability than I₂@NCTF‐DCP electrode (Figure S33), further confirming its remarkable polyiodides confinement ability. Furthermore, I₂@SCTF‐DCT cathode maintains similar capacity over rest intervals of 1 to 4 days (Figure 4c), demonstrating its exceptional structural and electrochemical stability. After long‐term cycling, the separator, Zn anode, and I₂@SCTF‐DCT cathode from the disassembled I₂@SCTF‐DCT‐based ZIBs were systematically investigated. As shown in Figure S34, the separator of I₂@SCTF‐DCT‐based ZIBs retains its original color compared with that of I₂@NCTF‐DCP‐based ZIBs, suggesting effective suppression of polyiodides shuttling by SCTF‐DCT. Additionally, the Zn anode paired with I₂@SCTF‐DCT cathode exhibits a smoother surface than that coupled with I₂@NCTF‐DCP cathode (Figure S35), indicating that the Zn anode corrosion and Zn dendrites growth for I₂@SCTF‐DCT‐based ZIBs was effectively suppressed. The side reaction of Zn anode mainly derives from the polyiodides shuttling [51], as described below: Zn + I₃ ⁻ ↔ Zn²⁺ + I⁻ + heat. This reaction regenerates I⁻ and Zn²⁺, while producing heat. The outstanding polyiodides confinement ability of SCTF‐DCT is closely related to its rich micro‐mesoporous structure and strong D–A synergetic effect. The abundant micro‐mesoporous structure provides ample physical space for polyiodides confinement, while the strong D–A cooperative effect regulates its electronic structure and creates a large number of chemisorption sites, significantly enhancing the interaction with polyiodides and improving its confinement effect. Furthermore, the microstructure and chemical composition of I₂@SCTF‐DCT cathode remain virtually identical after a long period of cyclic testing (Figures S36 and S37), underscoring its exceptional structural stability, which is crucial for stable electrochemical performance.

FIGURE 4.

Self‐discharge performance of ZIBs with a) I₂@SCTF‐DCT and b) I₂@NCTF‐DCP cathodes. c) Cycling performance after different resting days at 2 A g⁻¹ and d) CV curves at different scan rates for I₂@SCTF‐DCT‐based ZIBs. e) The relationship between log (i) and log (v) obtained from CV curves of I₂@SCTF‐DCT electrode. The capacitive and diffusion contributions for f) I₂@SCTF‐DCT and g) I₂@NCTF‐DCP cathodes. h) EIS curves and i) ionic diffusion coefficient of I₂@SCTF‐DCT and I₂@NCTF‐DCP cathodes.

Considering the critical role of D–A effect of SCTF‐DCT in enhancing the performance of ZIBs, we conducted an in‐depth kinetic study of I₂@SCTF‐DCT‐based battery using electrochemical methods, including CV at different scan rates, electrochemical impedance spectroscopy (EIS), Tafel plots, and galvanostatic intermittent titration technique (GITT) measurements. Figure 4d displays that the CV curves still retain a remarkably stable shape at higher scan rates, demonstrating excellent high‐rate tolerance of I₂@SCTF‐DCT cathode. The electrode reaction process of I₂@SCTF‐DCT‐based ZIBs was analyzed according to the power‐law equation: i = ab^v [5, 10, 58]. The calculated b values of cathodic and anodic peaks for I₂@SCTF‐DCT cathode are 0.813 and 0.816 (Figure 4e), respectively, indicating that its redox process was jointly controlled by both diffusion and capacitive behavior, similar to I₂@NCTF‐DCP cathode (Figure S38). Furthermore, the capacitance contributions of I₂@SCTF‐DCT and I₂@NCTF‐DCP cathodes were further determined according to the following equation: i = k₁v + k₂v^1/2 [4, 17]. The capacitive contributions of I₂@SCTF‐DCT cathode are 78.24%, 80.01%, 82.17%, 85.36%, 88.74, and 92.05% at different scan rates of 0.5, 1, 2, 3, 4, and 5 mV s⁻¹ (Figure 4f), respectively, which are higher than that of I₂@NCTF‐DCP cathode (Figure 4g), indicating its better rate performance and faster redox kinetics. The charge transfer resistance (R _ct) of I₂@SCTF‐DCT cathode is lower than that of I₂@NCTF‐DCP‐based ZIBs (Figure 4h; Table S3), indicating its faster charge transfer kinetics. As shown in Figure S39, the exchange current densities (i₀) derived from Tafel extrapolation for I₂@SCTF‐DCT cathode (0.35 mA cm⁻²) are significantly higher than that of I₂@NCTF‐DCP cathode (0.098 mA cm⁻²), demonstrating that the D–A synergistic effect greatly accelerates intrinsic charge‐transfer kinetics of iodine redox reaction. Besides, compared with I₂@NCTF‐DCP cathode, I₂@SCTF‐DCT cathode exhibits a higher ionic diffusion coefficient during cycling (Figures 4i and S40), further revealing its faster reaction kinetics.

2.4. Mechanism Study of I₂@SCTF‐DCT‐Based ZIBs

To gain deep insights into the underlying adsorption and catalysis mechanisms of I₂@SCTF‐DCT‐based ZIBs, in/ex situ characterizations (in situ Raman, ex situ XPS, ex situ FT‐IR, and in situ UV–vis) were conducted. In situ Raman spectra of I₂@SCTF‐DCT cathode in the initial discharging stage show two peaks at 107 and 166 cm⁻¹ (Figure 5a), assigned to I₃ ⁻ and I₅ ⁻ species [2, 5, 59], respectively. As discharging proceeded, the polyiodide peaks (I₅ ^‒ and I₃ ^‒) gradually attenuated until they disappeared (Figure 5a), confirming their complete conversion (I₅ ⁻→I₃ ⁻→I⁻). As the charging voltage increased to 1.6 V, the characteristic polyiodide peaks reappeared and attained maximum intensity (Figure 5b), corresponding to an efficient reverse oxidation reaction (I^‒→I₃ ⁻→I₅ ^‒). Figure 5c presents the GCD curve of I₂@SCTF‐DCT cathode, with the marked voltage states (A–E) analyzed by ex situ XPS (Figure 5d). During the initial discharge state (A site), the peaks observed at 618.6 and 630.2 eV correspond to the stretching vibration of I₃ ⁻, whereas those at 620.2 and 631.7 eV are ascribed to the stretching vibration of I₅ ⁻ [60]. Upon discharging to 1.1 V (B site), the intensity of I₅ ⁻ signal significantly weakens, as accompanied by the two newly‐emerged peaks for I⁻ species (617.5 and 628.8 eV) [60]. Upon discharging the I₂@SCTF‐DCT cathode to C site, the polyiodides peaks vanish as the I^‒ signals intensify to their maximum, indicating the full reduction of polyiodides to I⁻. During the subsequent charging process (D and E sites), the I⁻ signal gradually disappears with a concomitant increase in the peak intensity of polyiodides, signifying the reversible oxidation of I⁻ to polyiodides.

FIGURE 5.

In situ Raman spectra of I₂@SCTF‐DCT cathode for a) discharge and b) charge states. c) GCD curve with marked voltage states, where the marked points (A–E) were tested by ex situ XPS. d) HR‐XPS I 3d of I₂@SCTF‐DCT cathode at marked points of A–E. In situ UV–vis spectra of electrolyte at different discharge and charge states for e) I₂@SCTF‐DCT and f) I₂@NCTF‐DCP cathodes, respectively.

To validate the reversible changes of thiophene and triazine functional groups, ex situ FT‐IR spectroscopy was conducted on I₂@SCTF‐DCT cathode at various charge/discharge states. As shown in Figure S41a,b, the characteristic peaks of the triazine ring and thiophene unit exhibit a slight redshift with increasing discharge depth, attributed to the progressive conversion of polyiodides (I₃ ⁻, I₅ ⁻) to I⁻. The different affinities of these functional groups toward I⁻ versus polyiodides alter the local chemical environment, leading to decreased vibrational frequencies. Upon charging, as I⁻ is oxidized back to polyiodides, the peaks show a slight blue shift and nearly return to their original positions (Figure S41c,d). This reversible peak shift behavior provides direct evidence that the interactions between the D–A structure and iodine species undergo reversible changes during cycling, thus confirming the dynamic involvement of these functional groups in the iodine conversion process. Furthermore, we employed in situ UV–vis spectroscopy to monitor polyiodides species throughout the charging‐discharging cycles. The characteristic absorption peaks at 287 and 347 nm are assigned to I₃ ⁻ species [13, 61]. Compared with I₂@NCTF‐DCP cathode (Figures 5f and S42b), I₂@SCTF‐DCT electrode displays a weaker I₃ ⁻ absorbance during both charging and discharging process (Figures 5e and S42a), confirming its effective suppression of polyiodides.

To elucidate the critical roles of strong D–A synergetic effect for iodine species adsorption and conversion, DFT calculations were conducted. All DFT calculations were performed based on three optimized catalyst models (SCTF‐DCT, NCTF‐DCP, and CTF‐DCB), as shown in Figure S43. As shown in Figure 6a, the frontier molecular orbitals of SCTF‐DCT, NCTF‐DCP, and CTF‐DCB were calculated. The calculated HOMO energies follow the order: SCTF‐DCT (−6.81 eV) > NCTF‐DCP (−6.92 eV) > CTF‐DCB (−7.13 eV), whereas the LUMO energies follow the reverse order: SCTF‐DCT (−3.09 eV) < NCTF‐DCP (−2.57 eV) < CTF‐DCB (−2.48 eV). Consequently, the corresponding HOMO‐LUMO bandgap (ΔE _gap) follows the order: SCTF‐DCT (3.72 eV) < NCTF‐DCP (4.35 eV) < CTF‐DCB (4.65 eV). For SCTF‐DCT, the thiophene unit (strong donor) elevates the HOMO level, while the triazine moiety (strong acceptor) stabilizes the LUMO level, thereby forming a strong D–A structure. Compared with NCTF‐DCP and CTF‐DCB, the narrowest bandgap of SCTF‐DCT endows it with the most efficient intramolecular charge transfer, characteristic of a pronounced D–A synergistic effect. The Gibbs free energies (G) and transition state energy barriers (E_a ) of the iodine reduction reaction (IRR) pathways were calculated for SCTF‐DCT, NCTF‐DCP, and CTF‐DCB, respectively. The calculated iodine reduction follows I₅ ⁻ → I₃ ⁻ → I⁻ pathway, with the corresponding chemical reaction equations: (1) I₅ ⁻ + 2e⁻ → I₃ ⁻ + 2I⁻ and (2) I₃ ⁻ + 2e⁻ → 3I⁻. Here, the kinetic barrier for each step is defined as the energy difference between the corresponding transition state (TS1, TS2) and the preceding intermediate. Specifically, the E _a for TS1 (I₅ ⁻ → I₃ ⁻ step) on SCTF‐DCT (0.64 eV) is significantly lower than that of NCTF‐DCP (1.06 eV) and CTF‐DCB (0.82 eV), and the E _a for TS2 (I₃ ⁻ → I⁻ step) on SCTF‐DCT (0.45 eV) is also lower than that of NCTF‐DCP (0.80 eV) and CTF‐DCB (0.50 eV). These results prove that the D‐A synergistic effect in SCTF‐DCT reduces the kinetic energy barriers for both key transition states of IRR. Moreover, SCTF‐DCT exhibits a more negative ΔG value than NCTF‐DCP and CTF‐DCB during the IRR process, indicating its greatest thermodynamic spontaneity of the reaction. The lower transition state energy barrier and greater thermodynamic spontaneity on SCTF‐DCT enable iodine species to overcome the reaction bottlenecks more easily, resulting in faster conversion kinetics.

FIGURE 6.

a) HOMO‐LUMO energies for SCTF‐DCT, NCTF‐DCP, and CTF‐DCB. b) Gibbs free energies and transition state energy barriers for the IRR pathways on SCTF‐DCT, NCTF‐DCP, and CTF‐DCB, where *I₅ ⁻, *I₃ ⁻, and *I⁻ represent the iodine intermediates adsorbed on the active sites of catalysts. c) Calculated adsorption energies of iodine species (I⁻, I₂, I₃ ⁻, and I₅ ⁻) on SCTF‐DCT, NCTF‐DCP, and CTF‐DCB. Charge‐density difference diagrams of d) I⁻, e) I₂, f) I₃ ⁻, and g) I₅ ⁻ on SCTF‐DCT. Charge‐density difference diagrams of h) I⁻, i) I₂, j) I₃ ⁻, and k) I₅ ⁻ on NCTF‐DCP.

As summarized in Figure 6c, SCTF‐DCT, NCTF‐DCP, and CTF‐DCB all exhibit negative adsorption energies toward various iodine species (I⁻, I₂, I₃ ⁻, and I₅ ⁻), confirming their spontaneous adsorption behavior. Notably, SCTF‐DCT shows lower adsorption energies for all iodine species than NCTF‐DCP and CTF‐DCB (Figure 6c), indicating that the D–A synergistic effect enhances iodine species immobilization and confinement, thereby effectively suppressing polyiodide shuttling. The corresponding optimized adsorption configurations are provided in Figures S44–S46. The density of states (DOS) near the Fermi level around I atoms was further analyzed. As shown in Figure S47, the partial DOS (PDOS) of I₂ adsorbed on SCTF‐DCT shifts toward lower energies compared with that on NCTF‐DCP [13], indicating its higher stability of the electronic and adsorbed states. Additionally, the charge density difference between I₂ and SCTF‐DCT (Figure 6d–g) caused by the strong D–A cooperative effect is more pronounced than that with NCTF‐DCP (Figure 6h–k) and CTF‐DCB (Figure S48), indicating its stronger interfacial interactions. Overall, neither NCTF‐DCP nor CTF‐DCB possesses a genuine electro‐donor unit, which limits the effective confinement and catalytic conversion of iodine species, thereby resulting in poor electrochemical performance for both materials. By sharp contrast, SCTF‑DCT benefits from a strong D–A synergy between thiophene donor and triazine acceptor (Table S4), achieving optimal performance across all four key descriptors (ΔE_ads , ΔE_gap , ΔG, E_a ). Consequently, its electrochemical performance is far superior to those of the other two materials, suggesting that the D–A synergistic effect is the fundamental origin of the outstanding performance of SCTF‑DCT. These computational findings demonstrate that the strong D–A effect of SCTF‐DCT not only strengthens polyiodides confinement but also accelerates the redox reaction kinetics of I₂ species, underscoring its critical role in improving ZIBs performance.

3. Conclusion

In summary, a synergistic D‐A interaction strategy is proposed to simultaneously address the issues of polyiodide shuttling and sluggish reaction kinetics towards ZIBs. Specifically, SCTF‐DCT with strong D‐A cooperative effect is employed as an advanced I₂ host catalyst to confine polyiodides and accelerate redox dynamics of I₂ species, thereby enabling ZIBs with exceptional cycling life. As expected, I₂@SCTF‐DCT‐based ZIBs stably operate 100,000 cycles with a 80.2% capacity retention and 99.9% average Coulombic efficiency at 5 A g⁻¹, surpassing most previously‐reported advanced ZIBs. Furthermore, in/ex situ characterizations coupled with theoretical calculations collectively reveal the reversible redox mechanism of I⁻/I₃ ⁻/I₅ ⁻ species and the pivotal role of D‐A cooperative effect in enhancing the ZIBs performance. This work presents a novel D‐A collaborative interaction strategy for developing catalytic conversion‐type hosts in ZIBs, underscoring the great potential of the strong D‐A synergistic effect for electrochemical energy storage.

Author Contributions

Yuliang Zhao: methodology, writing‐original draft, investigation, formal analysis. Yiyang Wang: funding acquisition, investigation. Hongliang Huang: investigation, funding acquisition. Chuangang Hu: investigation, funding acquisition. Yanjing He: software, investigation, writing‐review and editing. Dong Liu: conceptualization, writing‐review and editing, formal analysis, investigation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie72762‐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.