High-voltage multi-S-heterocyclic covalent organic frameworks for zinc–organic batteries with high energy density and ultralong life

Abstract

Organic p-type cathodes for Zn–organic batteries (ZOBs) have high voltage (1.0–1.2 V), but exhibit limited redox capacity (generally <250 mAh g⁻¹) due to low-density active sites. Here, we design multi-S-heterocyclic covalent organic frameworks (S-COFs) by integrating two-electron dithiophene and three-electron trithiophene motifs via a condensation reaction, which act as a new p-type cathode for high-performance ZOBs. Electron-rich S-heterocyclic motifs contribute to ultralow-activation-energy electron delocalization paths (0.25 eV) and low molecular orbital energy levels (2.26 eV), thus giving a high redox voltage of 1.3 V for Zn‖S-COF batteries. Furthermore, a stable 30 e⁻ charge storage is accomplished in dithiophene/trithiophene modules of the S-COF cathode by (de)coordination with CF₃SO₃⁻ anions, affording a high capacity of 310 mAh g⁻¹. This remarkable combination of high voltage and capacity propels the energy density of ZOBs to a high level (403 Wh kg⁻¹). Besides, the excellent anti-dissolution ability of the S-COF cathode in aqueous electrolytes extends the battery life to 60 000 cycles with 81.2% capacity retention at 10 A g⁻¹. Our work establishes a new paradigm to design high-voltage-capacity COFs, paving the way for next-generation high-performance ZOBs.

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Broader context

Aqueous zinc batteries are promising candidates for next-generation energy storage applications due to high safety, suitable redox potential and high capacity of Zn anodes. Benefiting from resource richness and structural and functional designability, organic materials are competitive cathodes for advancing Zn–organic batteries (ZOBs). n-Type organics (coupling cations) typically exhibit high capacities but low redox voltage (<1.0 V). In contrast, p-type organics (storing anions) demonstrate elevated redox potentials owing to their low electron energy levels, but are limited by low capacity (<250 mAh g⁻¹) due to the high proportion of redox-inactive parts. Due to their electron-rich structures and low molecular orbital energy levels, high-voltage S-heterocyclic organic molecules have received attention as multielectron high-capacity cathodes for propelling ZOBs. However, their high solubility in electrolytes results in capacity loss after cycling. Here, we design multi-S-heterocyclic covalent organic frameworks (S-COFs) by integrating two-electron dithiophene and three-electron trithiophene motifs via a condensation reaction as a new p-type cathode for high-performance ZOBs. Electron-rich S-heterocyclic motifs contribute to ultralow-activation-energy electron delocalization paths and low molecular orbital energy levels, thus endowing Zn‖S-COF batteries with high redox voltage and high energy density. Our work provides new insights for designing multi-redox heterocyclic organic materials for advanced ZOBs.

Introduction

Aqueous zinc batteries have been deemed to be promising candidates for grid-scale energy storage due to the safety, low cost, suitable reaction potential and high capacity of Zn anodes.^1–4 One key objective is to seek high-performance cathodes to advance Zn batteries.^5–7 Due to their resource richness, structural designability, and functional adjustability, aromatic organic materials are regarded as promising cathodes for advancing Zn–organic batteries (ZOBs).^8–11 According to the energy storage mechanism between redox-active groups and ionic carriers, organic cathodes can be classified as n-type (coupling cations)^12–14 and p-type (storing anions).^15–17 The n-type organics including quinones, nitroaromatics and imines typically exhibit high capacities but low average redox potentials (<1.0 V vs. Zn/Zn²⁺).^18–21 Unlike their n-type counterparts, p-type organics (e.g., triphenylamines, tert-N compounds, and nitroxides) demonstrate elevated redox potentials and rapid charge transfer kinetics owing to their low electron energy levels, but are limited by low capacity (<250 mAh g⁻¹) due to the high proportion of redox-inactive parts.^15,22–25 To maximise the p-type high-voltage advantage, one of the effective strategies is to increase the density of redox-active sites in p-type organics that allow multielectron transfer, thereby endowing ZOBs with superior capacity and energy density, but this is still an ongoing task.

Due to their electron-rich structures and low molecular orbital energy levels, high-voltage S-heterocyclic organic molecules (e.g., thianthrene and tetrathiafulvalene) have received attention as multielectron high-capacity cathodes for advancing ZOBs.²⁶ However, they are generally plagued by high solubility in electrolytes, resulting in irreversible capacity loss after cycling.^27,28 To overcome the dissolution issue, researchers resorted to polymerizing soluble S-heterocyclic molecules for stable ZOBs.²⁹ However, the twisted polymeric chains and random folding characteristics often result in disordered stacking structures, thereby limiting the full utilization of redox-active sites and energy storage.³⁰ Covalent organic frameworks (COFs), as a class of promising organic materials, deliver customizable structures and long-lasting porosities, which can be constructed from the assembly of low-dimensional segments and show great potential in advancing energy storage technologies.^31–35 They can inherit high-density redox sites of their building blocks and obtain extra unconventional merits of more exposed active sites, continuous charge transfer paths, and excellent skeleton robustness.^36–39 Therefore, developing multielectron p-type S-heterocyclic COFs with high-density accessible redox sites and ultrastable well-arranged topographies is urgent and significant for revolutionizing the energy metrics of ZOBs, but still full of challenges.

In this work, multi-S-heterocyclic covalent organic frameworks (S-COFs) are designed as a p-type cathode for activating ZOBs with high energy density and ultralong life. The nanochain-assembled continuous electron delocalization geometries of S-COFs are formed by integating trithiophene and dithiophene motifs via Schiff-base reactions. Electron-rich S-heterocyclic motifs contribute to ultralow-activation-energy electron delocalization paths and low molecular orbital energy levels, thus giving a high redox voltage for Zn‖S-COF batteries. Furthermore, a stable 30 e⁻ charge storage is first accomplished with 12 e⁻ transfer in dithiophene units, followed by 18 e⁻ transfer in trithiophene modules by the (de)coordination with CF₃SO₃⁻ anions. This profitable electrochemical process endows the high-performance Zn‖S-COF cell with high capacity, high redox voltage, ultralong cycle life, and high energy density. Our work provides new insights for the self-assembly design of multi-redox heterocyclic organic materials towards state-of-the-art ZOBs.

Results and discussion

Owing to the electron-withdrawing ability of aldehyde groups and the electron-pushing effect of amino groups, three-electron benzo[1,2-b:3,4-b′:5,6-b″]trithiophene-2,5,8-tricarbaldehyde (BTT) and two-electron thieno[3,2-b]thiophene-2,5-diamine (TD) can polymerize to form imine-linked multi-S-heterocyclic covalent organic frameworks (S-COFs) via Schiff-base reactions, which then self-assemble into well-arranged nanochains driven by multiple π–π stacking interactions (Fig. 1a) among neighboring conjugated structures. Powder X-ray diffraction (PXRD) characterization patterns of S-COFs show reflection peaks at 2.8°, 5.7°, 7.8° and 25.6°, which are assigned to the (100), (110), (200) and (001) planes, respectively. In addition, these experimental PXD patterns agree well with the calculated eclipsed AA stacking models (inset of Fig. 1b and Tables S1 and S2), and the Pawley refined PXRD profiles match well with the experimental data (R_wp = 2.70% and R_p = 1.62%), as evidenced by the negligible pattern differences. Of note, the broad peak at 25.6° for S-COFs is ascribed to the π–π stacking between the interlaminations. Fourier transform-infrared (FT-IR) spectra (Fig. 1c) show the remarkable disappearance of C=O bands and the generation of C=N species, proving the polymerization reaction between BTT and TD through Schiff base reactions. Furthermore, the full-range X-ray photoelectron spectrum (XPS) verifies the disappearance of the O 1s signal (C=O groups) and the appearance of strong S 2s/2p signals (C–S groups) at 177.16/248.49 eV and the N 1s signal at 400.12 eV (Fig. S1), which confirms the polymerization between BTT and TD for the successful fabrication of S-COFs.

Fig. 1 Structural characterization of S-COFs: (a) synthetic route; (b) PXRD patterns; (c) FT-IR spectra; (d) scatter plots of RDG against sign(λ₂)ρ and the gradient isosurface; (e) calculated E_g values; (f) electrical conductivities of three organic cathode materials; (g) LOL–π map; and (h) Frontier molecular orbitals and energy levels.

Compared with irregular BTT and TD molecules (Fig. S2), the scanning electron microscopy (SEM) image of S-COFs displays botryoidal superstructures (Fig. S3) built of nanochain modules (∼50 nm in diameter), along with high thermal stability (Fig. S4). In addition, S-COFs exhibit a substantially high surface area of 1273.96 m² g⁻¹ and a hierarchical porous structure (Fig. S5), which help to improve the diffusion kinetics of charge carriers and the reaction activity of redox groups. Furthermore, the high-resolution transmission electron microscopy (HR-TEM) images show an interlayer distance, which confirms the high crystallinity of S-COFs (Fig. S6 and S7). All of the above results strongly support the successful construction of the high crystallinity imine-linked S-COFs by efficient polycondensation. Reduced density gradient (RDG) simulation was performed, showing obvious green spikes positioned at −0.02 to 0.00 a.u. of the sign(λ₂)ρ. Such a result is a signature of the existence of π–π interactions between adjacent S-heteromacrocylic buliding blocks (Fig. 1d). The extended π-conjugation and ordered crystal covalent scaffold of the S-COF cathode minimize structural defects to provide efficient electron transfer pathways, thereby achieving significantly improved conductivity (54.2 S cm⁻¹, Fig. 1e) compared to organic small molecules of BTT (18.3 S cm⁻¹) and TD (14.3 S cm⁻¹). Such a high electron conductivity demonstrates superior electron transfer efficiency (Fig. S7–S10). In addition, thanks to the π–π stacking interactions, S-COFs deliver a low optical energy gap (E_g) of 2.26 eV, which have desirable high inherent electronical conductivity and swift change transport for stimulating the high-kinetics redox reaction (Fig. 1f). Besides, compared with soluble BTT and TD small molecules, S-COFs have no ultraviolet-visible (UV-Vis) absorption signal after soaking in aqueous solution. This confirms the structural robustness and anti-dissolution ability of S-COFs, which contributes to long-lasting redox activity (Fig. S11).

Density functional theory (DFT) calculations were performed to reveal the electronic configuration of S-COFs, which decides the energy levels and redox characteristics.^40,41 S-COFs exhibit extended π-conjugated aromaticity, as shown by the localized orbital π-electron locatization function (LOL-π) map, indicating strong π-electron delocalization paths throughout the S-heterocyclic backbone (Fig. 1g). In theory, a highly conjugated structure triggers a reduction in the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO).^42–44 The results indicate that S-COFs exhibit a narrow LUMO–HOMO gap (ΔE_HOMO–LUMO) of 2.45 eV (Fig. 1h and Table S3), which is promising for superior electron transfer efficiency. Overall, S-COFs integrate multi-S-heterocyclic sites, high-voltage p-type redox units and ultrastable structures, which are expected to be highly competitive candidates for advanced ZOBs.

The electrochemical performance of the S-COF cathode was investigated in ZOBs using Zn anodes and an aqueous 3 M Zn(OTF)₂ electrolyte (Figs. S12). Galvanostatic charge/discharge (GCD) curves (Fig. 2a) of the Zn‖S-COF cell show an excellent discharge capacity of 310 mAh g⁻¹ at 0.1 A g⁻¹ with an average discharge voltage of 1.3 V, exceeding that of Zn‖BTT (200 mAh g⁻¹) and Zn‖TD (101 mAh g⁻¹) batteries (Fig. S13 and S14). Two distinct voltage platforms of GCD curves indicate the successive two-step charge storage of the Zn‖S-COF cell, which is consistent with cyclic voltammetry (CV) curves of the first three cycles (Fig. 2b). CV profiles of the S-COF cathode show two pairs of redox peaks at 1.67/1.42 V (P_O1/P_R1) and 1.58/1.25 V (P_O2/P_R2) (Fig. 2b), which can be ascribed to the coordination of OTF⁻ ions with dithiophene units first followed by thiophenal units. The S-COF cathode exhibits an impressive capacity of 178 mAh g⁻¹ at 20 A g⁻¹, indicating the large-current tolerance and stability of the Zn‖S-COF battery. Furthermore, the Zn‖S-COF cell exhibits highly reversible capacities at different current densities of 0.1–20 A g⁻¹ (Fig. 2c), suggesting fast ion transport and storage with the S-COF cathode. Significantly, the Zn‖S-COF battery exhibits a high Coulomb efficiency of 99.5% after 1000 cycles at 0.1 A g⁻¹ (Fig. 2d). The average mass loss of the cathode before cycling (41.31 mg) and after 1000 cycles (41.12 mg) was found to be <0.5 wt%, indicating that the active material itself experienced negligible dissolution. More importantly, the S-COF cathode utilizes the advantages of high-capacity-voltage p-type reactions, endowing the Zn‖S-COF battery with an outstanding energy density of 403 Wh kg⁻¹ (based on the mass loading of S-COFs in the cathode, Fig. 2e and Fig. S15, Table S4).^{26,27,32,45–52,55}

Fig. 2 Electrochemical performances of the Zn‖S-COF cell: (a) GCD curves at various current densities; (b) CV profiles at 1 mV s⁻¹; (c) rate metrics; (d) cyclic stability at 0.1 A g⁻¹; (e) contour plots of energy density compared with the reported organic cathodes based on cathode loading; (f) calculated b values; (g) capacitive contributions at various scan rates; and (h) long-term cycling performance at 10 A g⁻¹.

The redox kinetics of the Zn‖S-COF cell was unraveled via CV profiles using Dunn's method with the scan rate from 1 to 5 mV s⁻¹.^7,53–55 CV profiles exhibit similar shapes with slightly shifted redox peaks (Fig. S16), substantiating the high electrochemical reversibility of the Zn‖S-COF battery. There are two pairs of redox signals in CV curves (denoted as P_R1, P_R2, P_O1, and P_O2), revealing a two-step redox reaction process. According to the power law equation i = kv^b between peak current (i) and the scan rate (v), the high-power exponent b-values of 0.84–0.89 are all close to 1, indicating rapid surface-dominant charge storage behavior (Fig. 2f). In contrast to the diffusion-controlled process, surface-capacitive contributions account for 85–95% of total charge storage across all scan rates, indicating that the fast surface capacitive dynamics absolutely dominates the charge storage of S-COF cathodes (Fig. 2g). Moreover, the Zn‖S-COF battery exhibits an unprecedented lifespan of 60 000 cycles with an 81.2% capacity retention at 10 A g⁻¹ (Fig. 2h and Fig. S17, S18 and Tables S4 and S5, based on the mass loading of 2.5 mg cm⁻²), along with relatively low periodic capacity loss per 10 000 cycles, underscoring the excellent electrochemical reversibility. Overall, BTT and TD molecules exhibit low capacity and unsatisfactory cycling stability (Fig. S14) due to their easy dissolution in aqueous electrolytes (Fig. S11) and inherent low conductivity (Fig. S9), resulting in inferior accessibility of redox-active sites. In contrast, S-COFs with a π-conjugated multi-S-heterocyclic superstructure configuration exhibit well-defined nanochain geometries (Fig. S3), high conductivity (Fig. S9), low energy gap (Fig. 1e), and anti-dissolution (Fig. S11), which contribute to high redox activity for superior capacity retention after long-term cycling (Fig. 2h).

SEM images, spectral surveys and electrochemical tests of the S-COF cathode after long-term cycling confirm its superior structure stability and anti-dissolution ability (Fig. S19–S23). The self-discharge behavior of the Zn‖S-COF battery was investigated at the fully charged state of 1.8 V at 0.1 A g⁻¹, displaying a high-capacity retention of 91% after a rest step of 7 days (Fig. S24). This result suggests the good structural stability of the S-COF cathode during the operation of the Zn‖S-COF battery. The extended π-conjugation structure and ordered crystal covalent skeleton of S-COFs prevent structural degradation and irreversible transitions of redox-active motifs, which minimize the self-discharge behavior to afford highly reversible and stable charge storage during the electrochemical cycling process (Fig. 2c). Besides, the battery still achieves a large capacity of 230 mAh g⁻¹ with 10 mg cm⁻² S-COFs in the cathode (Fig. S25–S27), demonstrating bright practical outlooks for advanced energy supply. The excellent all-round electrochemical metrics make S-COFs a promising advanced cathode for ZOBs.

To elucidate the storage mechanism of the Zn‖S-COF cell, various spectroscopic characterization studies were conducted to monitor the structural variation of the S-COF cathode at different charge/discharge states. Two pairs of voltage platforms of the GCD curve profile represent the two-step continuous charge storage of the Zn‖S-COF battery (Fig. 3a). Regarding FT-IR spectra (Fig. 3b), the peak intensity of C–S species of dithiophene units in TD (1028 cm⁻¹) decreases (states A → B) and is almost unchanged (states B → C) during charging, accompanied by the increase of =C–S⁺– (1109 cm⁻¹) species, signifying the high electrochemical activity of C–S motifs. In contrast, the peak intensity of C–S species of trithiophene units in BTT (658 cm⁻¹) is almost unchanged during charging (states A → B) and gradually decreases upon further charging (states B → C), together with the increase of =C–S⁺– (1109 cm⁻¹) species. Of note, the S=O signal of OTF⁻ anions at 1057 cm⁻¹ is gradually increased during charging (states A → C). During discharging (states C → E), all redox signals return to their initial states. Such a result indicates that S-COFs first undergo the redox reaction between dithiophene units and OTF⁻ anions, followed by trithiophene units.

Fig. 3 Structural evolution of the S-COF cathode during the operation of the Zn‖S-COF cell. (a) A voltage–capacity profile. (b) Overview of FT-IR spectra. XPS spectra of (c) S 2p and (d) O 1s at various electrochemical states. (e) E_a values of S-COF, BTT and TD cathodes in ZOBs. (f) Amphoteric redox behaviors of S-COFs during the electrochemical process.

Thus, both p-type C–S motifs of dithiophene/thiophenal modules in S-COFs are highly reversible active groups to propel OTF⁻ redox reactions. High-resolution X-ray photoelectron spectroscopy (XPS) surveys of the S 2p signal were carried out to further verify the charge details of OTF⁻ ions in the S-COF cathode during battery operation (Fig. 3c). Upon charging, the S 2p XPS peaks shift toward higher binding energies, corresponding to sulfur oxidation (S → S⁺), and subsequently revert to lower energies upon discharge. After charging, the two distinct oxidized sulfur species exhibit energies at 165.0 eV and 166.1 eV correspond to oxidized C–S^3/2–C and C–S^1/2–C groups. Upon charging, S-COFs lose electrons and oxidize, absorbing anions (e.g., CF₃SO₃⁻) in the electrolyte to maintain the conservation charge. The concentration of =C–S⁺– moieties increases during charging (states A → C), accompanied by the appearance of a new peak at 168.0 eV corresponding to OTF⁻ anions. During discharging (states C → E), all XPS signals return to their initial levels. These findings indicate the oxidation of C–S groups in S-COFs via OTF⁻ coordination, which is further confirmed by the reversible evolution of OTF⁻ species in the 2D contour maps of O 1s XPS spectra (Fig. 3d). Meanwhile, the F 1s signal in XPS spectra gradually increases during charging due to OTF⁻ coordination (Fig. S28) and then decreases after discharging by OTF⁻ removal, suggesting OTF⁻ (de)coordination at C–S sites in the S-COF cathode. Furthermore, time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was employed to probe the spatial distribution of OTF⁻ anions on the charged S-COF cathode (Fig. S29). The intensity of the characteristic fragment for F⁻ significantly increases in the charged state. Conversely, upon discharging, the signal intensity of OTF⁻ fragments decreases to an almost non-existent level. This reversible change in the OTF⁻ fragment intensity confirms the reversible OTF⁻ uptake/release in the S-COF cathode via mass balance during each round of electrochemical cycling.

No UV/Vis signals and colorless aqueous Zn(OTF)₂ electrolytes soaked with the S-COF cathode confirm its structural robustness and anti-dissolution (Fig. S30–S32). The interfacial activation energy (E_a) of S-COFs was estimated based on the relationship between charge transfer resistances (R_ct) and temperature (T).⁵⁶ The E_a value for the OTF⁻ ion storage process in the S-COF cathode is 0.25 eV (Fig. 3e and Fig. S33) based on the Arrhenius equation, which is much lower than that in the BTT cathode (0.31 eV) or TD cathode (0.34 eV), and exceeds the value recently reported for heterocyclic organic materials (0.30–0.40 eV). To further figure out the reaction process of the S-COF cathode during the electrochemical process, control electrochemical experiments for TD and BTT cathodes were performed in a 3 M Zn(OTF)₂/H₂O electrolyte. A pair of redox peaks (1.41/1.57 V) of the CV profile of the Zn‖TD battery overlaps with that of the Zn‖S-COF battery (Fig. S34). This means that the OTF⁻ reaction occurs in C–S groups of the dithiophene cathode. In addition, a pair of oxidization/reduction signals (1.23/1.67 V) is detected in the Zn‖BTT battery, suggesting that C–S groups of thiophenol can also accommodate OTF⁻ ions. Given that the theoretical capacity of the S-COF cathode is 325 mAh g⁻¹ per S-heterocyclic hexagonal unit, the experimental capacity is 310 mAh g⁻¹ (Fig. 2a). When deducting the capacity contribution of acetylene black (15 mAh g⁻¹, Fig. S35), the real capacity is 295 mAh g⁻¹ with an ultrahigh active-site utilization (98.7%), which corresponds to ≈30 e⁻ charge storage process involving 2 e⁻ transfer at a dithiophene group first followed by 3 e⁻ transfer at a trithiophene unit.^57,58 Comprehensive characterization studies including FT-IR spectroscopy (Fig. 3b), XPS (Fig. 3c and Fig. S28), electrochemical analysis (Fig. S36–S39), and theoretical calculations (Fig. 4) have confirmed the 30 e⁻ redox process of the S-COF cathode involving OTF⁻ anion storage (Fig. 3e). Experimental results elucidate that the multi-site-redox p-type S-COF cathode starts two-step 30 e⁻ anionic charge storage, involving first 12 e⁻ transfer within dithiophene modules, followed by 18 e⁻ transfer within trithiophene units (Fig. 3f). Overall, S-COFs overcome the structural limitations of low-density electron-transfer redox units in previously reported p-type organic materials, achieving state-of-the-art ZOBs.

Fig. 4 Theoretical simulations of redox behaviors of S-COFs at different electrochemical states: (a) MEP distribution, (b) redox behaviors of p-type units, (c) optimized geometries and corresponding ΔG values after ion coordination, and (d) differential charge isosurfaces in states I and II.

Theoretical calculations were further performed to decipher the redox mechanism of the S-COF cathode during the charge storage process. The redox-active sites across the organic scaffolds were deduced based on the molecular electrostatic potential (MEP) simulation.⁵⁹ On the van der Waals surface of S-COFs, the trithiophene unit with positive MEP (red area) represents electron-donating p-type active motifs (Fig. 4a), which can couple anionic carriers (Fig. 4b).³⁸ Moreover, the optimized S-heteromacrocyclic group of S-COFs exhibits a minimum dipole-moment of 0 Debye and delivers a desirable π-conjugated configuration for promoting electron transfer.

During the first charging stage (step 1), twelve OTF⁻ ions couple with three C–S groups of six two-electron thiophene units in S-COFs to form the anion-coupled S-COF-12OTF⁻ complex (state I) by delivering a negative binding energy (ΔE₁) of −17.91 eV (Fig. 4c). During the subsequent charging process (step 2), eighteen OTF⁻ ions coordinate with eighteen C–S sites of trithiophene units by delivering a negative ΔE₂ of −11.85 eV. Step 1 has a lower ΔE₁ value compared to step 2, suggesting that steps 1 and 2 can proceed sequentially. In accordance with the minimal energy theory, a robust two-step 30 e⁻ redox process is triggered in the S-COF cathode, corresponding to two pairs of discharge platforms of the Zn‖S-COF cell (Fig. 2a and Fig. S40). The different charge isosurfaces were applied to investigate the binding properties of OTF⁻ ion coordinated S-COFs. Based on the Bader charge analysis,^7,42,60–62 there are obvious charge accumulation and depletion between OTF⁻ ions and thiophene motifs of S-COFs (Fig. 4d), indicating their strong coordination to form robust ligand configurations with notable charge shifts (2.32 e of state I; 4.53 e of state II). The strong chemical interactions facilitate the alternate redox reactions of opposite charge carriers to maximize the utilization of multi-redox-site S sites of S-COFs, enhancing the electrochemical activity and durability.

Conclusions

In conclusion, multi-S-heterocyclic S-COFs are designed as a p-type cathode for high-performance ZOBs. S-COFs are designed via the Schiff-base reaction between trithiophene units of BTT and dithiophene units of TD to form multi-S-heterocyclic geometries. Electron-rich S-heterocyclic motifs contribute to ultralow-activation-energy electron delocalization paths and low molecular orbital energy levels, thus giving a high redox voltage for the Zn‖S-COF battery. This profitable p-type organic S-COF cathode is redox-accessible for CF₃SO₃⁻ anions, which triggers fast and stable 30 e⁻ transfer within dithiophene units first followed by trithiophene modules. The assembled Zn‖S-COF cell exhibits high capacity, high redox voltage, high energy density and ultralong cycle life. Our work provides new insights for the design of multi-redox heterocyclic organic materials towards advanced ZOBs.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

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

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: Fig. S1–S40, Tables S1 and S6. See DOI: https://doi.org/10.1039/d5ee04802h.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (no. 22272118, 22172111, and 22309134), the Shanghai Rising-Star Program (23YF1449200), the Zhejiang Provincial Science and Technology Project (2022C01182), and the Fundamental Research Funds for the Central Universities. The authors extend their gratitude to Dr Feng Zheng (from the Instrument Analysis Center of Tongji University) for providing valuable assistance with the TOF-SIMS analysis.

Notes and references

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