Low-activation-energy bipolar organic nanostructures for high-capacity and ultralong-life aqueous calcium-ion batteries

Abstract

Rechargeable aqueous calcium-ion batteries (CIBs) provide a promising solution to problems of large-scale energy storage due to their divalent-electron transfer, resource abundance, and high capacity. However, their advancement is challenged by suboptimal anode materials with low exposure of redox-active motifs in densely stacked and disorganized structures due to high spatial energy barriers, resulting in limited capacity and durability. We designed low-activation-energy bipolar organic nanostructures (BONs) through integrating dual-electron benzoquinone and 4,4′-azodianiline units into extended π-conjugated polymeric skeletons through multi-intermolecular H-bonds (N–H⋯O) and π–π interactions. The well-organized rod geometries of BONs delivered consecutive electron delocalization pathways to fully expose built-in multi-redox carbonyl/azo/amine motifs and strengthen the anti-dissolution ability in aqueous electrolytes. Consequently, stable 4 e⁻ Ca²⁺/H⁺/OTF⁻ storage was initiated in the BONs anode with an ultralow activation energy (0.22 eV), thereby liberating a state-of-the-art capacity (302 mAh g⁻¹) and lifespan (100 000 cycles) among all reported organics in CIBs. Besides, the BONs anode could be leveraged to design an advanced BONs‖KCoFe(CN)₆ full battery with superior capacity (210 mAh g⁻¹), high energy density (221 Wh kg⁻¹ anode) and long-lasting cycling stability (20 000 cycles). This work constitutes a major advance in designing multi-redox organic nanostructures for better CIBs.

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New concepts

Aqueous calcium-ion batteries (CIBs) are emerging energy-storage devices owing to the low redox potential of Ca²⁺/Ca and the natural abundance of calcium resources. However, the low exposure of redox-active motifs in the densely stacked and disorganized structures of suboptimal anode materials due to high spatial energy barriers poses significant challenges for their advancement, resulting in limited capacity and durability. We designed bipolar organic nanostructures (BONs) with consecutive electron delocalization pathways to fully expose built-in multi-redox motifs and strengthen the anti-dissolution ability in aqueous electrolytes. Consequently, anion–cation fused 4 e⁻ charge storage was initiated in the BONs anode with an ultralow activation energy (0.22 eV), liberating the state-of-the-art capacity (302 mAh g⁻¹) and lifespan (100 000 cycles) among all reported organics in CIBs. Besides, the structural and performance merits of the BONs anode were further highlighted in a BONs‖KCoFe(CN)₆ full cell, which delivered superior capacity (210 mAh g⁻¹), high energy density (221 Wh kg_anode⁻¹) and long-lasting cycling stability (20 000 cycles). This work constitutes a major advance in designing multi-redox organic nanostructures for high-performance aqueous CIBs.

Introduction

Aqueous calcium-ion batteries (CIBs) have attracted the attention of researchers as ideal candidates for large-scale energy storage due to abundant resources, environmental friendliness, and the dual-electron redox properties of calcium.^1–10 Massive effort has been made to explore high-performance electrode materials for Ca²⁺ storage, which is considered a crucial factor in boosting CIBs.^11–18 Inorganic materials such as graphite and Si/Sn/Se-based alloys have been investigated as anodes for CIBs.^19,20 However, their applications suffer from significant volume changes caused by repeated Ca²⁺ (de)insertion, which leads to rapid capacity degradation (<1000 cycles).^21,22 Recently, aromatic small-molecule organic anodes have been used because of their abundant resources, flexible structures, and functional designability. These features allow for systematic modulation of electrochemical activity and redox kinetics towards efficient energy storage.^23–30 Besides, the rapid diffusion and storage of Ca²⁺ can be achieved by the weak intermolecular interactions of organics.³¹ Such advantages make organic anodes more important for promoting CIBs.

Designing multiple redox-active sites in organic anode materials that permit more electron transfer is key to improving the capacity of CIBs. In this regard, organic small molecules with a high proportion of active sites, such as 5,7,12,14-pentaerythritone, 3,4,9,10-perylenetetracarboxylic dianhydride and perylene tetra-carboxylic diimide, have been developed as high-capacity anodes for advancing CIBs.^32–35 However, a notable problem with the vast majority of organic molecules lies in their high solubility in aqueous electrolytes, which induces rapid capacity loss and a short lifespan (<3000 cycles).^36,37 To overcome the dissolution barrier, there have been a few preliminary studies on polymerizing soluble redox-active molecules into π-conjugated polymer structures for stable CIBs.^38–42 For example, Lv and colleagues developed a pyrazine-pyridinamine coupled polymer anode to demonstrate strong anti-dissolution ability for affording excellent durability (10 000 cycles) and capacity (152 mAh g⁻¹).⁴¹ In addition, Shi and colleagues designed nitrogen-rich π-conjugated covalent organic structures with multiple carbonyl groups as an anode, which initiated stable carbonyl redox reactions for high-capacity CIBs (253 mAh g⁻¹), together with good stability (3000 cycles).³⁹ Despite promising progress in broadening the design horizon of organic materials for CIBs, their development is in the infancy. Reported polymers are often trapped by limited Ca²⁺-accessible capacity (generally <250 mAh g⁻¹) due to low exposure of redox-active units and high spatial energy barriers in densely stacked and disorganized polymeric structures (e.g., large particles and blocks).^43–49 Thus, more attempts are needed for further success in unlocking the electrochemical limitations of CIBs. This can be achieved by strategically designing stable organic anode materials with well-arranged structures and readily accessible redox sites, but challenges remain.

In this work, we designed multi-site-active and ultrastable bipolar organics nanostructures (BONs) by fusing dual-electron-redox benzoquinone (BQ) and 4,4′-azodianiline (PD) units into extended π-conjugated polymeric skeletons through rich intermolecular hydrogen-bonding interaction (N–H⋯O) and π–π stacking. BONs with well-ordered rod geometries afford successive electron delocalization pathways to promise high accessibility of pre-designed multi-active carbonyl/azo/amine sites with a very low energy barrier (0.22 eV). Moreover, the extended interconnected rod nanostructures strengthened the anti-dissolution ability of the BONs anode in aqueous electrolytes. Consequently, the BONs anode delivered a state-of-the-art capacity and lifespan among all reported organics in CIBs. Anion–cation fused 4 e⁻ charge storage was revealed in the BONs anode, involving multi-step redox reactions of CF₃SO₃⁻, Ca²⁺, and H⁺ with –NH–, C=O, and N=N sites, respectively. Besides, the structural and performance merits of the BONs anode were further highlighted in a BONs‖KCoFe(CN)₆ full cell, which delivered superior capacity, energy density and cycling durability. Hence, we elucidated the electrochemical potential of BONs for better CIBs.

Results and discussion

Owing to the electron-pushing ability of amine groups and the electron-pulling effect of carbonyl motifs, two-electron-donor PD and two-electron-acceptor BQ could polymerize to form linear organics with lamellar geometries and bipolar multi-redox sites bridged by –NH– bonds. They could then self-assemble into BONs with well-arranged rod-like nanostructures, driven by (N–H⋯O) and π–π interactions among neighbouring π-conjugated networks (Fig. 1a and Fig. S1, S2).

Fig. 1 Structural characterization of BONs. (a) Molecular structure. (b) FT-IR spectra. (c) Scatter plots of RDG against sign (λ₂)ρ and gradient isosurface. (d) SEM images of BONs at various polymerization times (scale bar: 500 nm). (e) Frontier molecular orbitals and energy levels. (f) Calculated E_g values. (g) UV/Vis spectra of Ca(OTF)₂/H₂O solution soaked with PD, BQ, or BONs for 3 months.

Fourier-transform infrared (FT-IR) spectroscopy showed the characteristic peaks of H-bonds (N–H⋯O) at 3232 cm⁻¹ (Fig. S2). Meanwhile, FT-IR spectra displayed a shift of the C=O stretching vibration of BQ to a higher frequency for BONs (Fig. 1b), suggesting the formation of intramolecular H-bonds (N–H⋯O). XRD patterns displayed a strong diffraction signal at 27.5° (Fig. S3), which could be attributed to the typical π–π stacking of organic modules, thus improving structural stability. There were obvious blue signals (−0.03 to −0.02 a.u.) and green spikes (−0.01 to 0.01 a.u.) of the sign (λ₂)ρ in the reduced density gradient (RDG) simulation, which were signatures of H-bonds and π–π stacking forces between contiguous organic skeletons (Fig. 1c).

Scanning electron microscopy (SEM) images further demonstrated the gradual assembly of polymeric blocks into well-arranged rod-like geometries at increasing reaction times (Fig. 1d and Fig. S4, S5), together with uniform atomic distribution, porous structure, and high thermal stability (Fig. S6–S8). In general, a highly conjugated structure triggers a reduction in the energy levels of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO).^50–59 A LUMO–HOMO gap (ΔE) of 2.14 eV was achieved for BONs (Fig. 1e), which is advantageous for achieving superior electron migration efficiency. Thanks to the π–conjugated nanostructure configuration, the optical energy gap (E_g) of BONs was 2.05 eV (Fig. 1f), thereby liberating high inherent electronical conductivity (Fig. S9) to the propel redox reaction with low energy barriers. Compared with soluble BQ and PD (Fig. 1g), BONs did not have UV/Vis absorption signals after soaking in Ca(OTF)₂/H₂O solution, confirming their structural robustness and excellent anti-dissolution ability. Overall, BONs integrated multi-accessible redox-bipolar sites, low-energy-barrier electron migration pathways, and superior structural stability, making BONs promising candidates for advanced CIBs.

The electrochemical performances of the BONs anode were studied in a three-electrode system with BONs as the working electrode, Ag/AgCl as the reference electrode, platinum sheet as the counter electrode, and 5 M Ca(OTF)₂/H₂O as the electrolyte (OTF⁻ = CF₃SO₃⁻) (Fig. S10). Linear sweep voltammetry (LSV) of the 5 M Ca(OTF)₂/H₂O electrolyte (Fig. S11) demonstrated a wide electrochemical window. A high discharge capacity of 302 mAh g⁻¹ at 0.2 A g⁻¹ with an average discharge voltage of −0.31 V (Fig. 2a) were obtained for the optimized BONs anode (polymerization time: 60 min) (Fig. S12) based on the galvanostatic charge–discharge (GCD) test, which surpassed the capacities of PD (151 mAh g⁻¹) and BQ (195 mAh g⁻¹) anodes (Fig. S13). Even at 100 A g⁻¹, the BONs anode held an impressive capacity of 101 mAh g⁻¹, revealing excellent redox vitality at large currents. Differential capacity curves of the BONs anode showed three pairs of redox signals (Fig. 2b), which agreed with the cyclic voltammetry (CV) results of the first three cycles at 0.1 mV s⁻¹ (Fig. 2c), indicating three-step charge storage. Moreover, the BONs anode demonstrated highly reversible and stable capacities at current densities of 0.2–100 A g⁻¹ (Fig. 2d), together with high energy efficiency of 55–60% (Fig. S14), unveiling efficient ion diffusion and storage. When the current density was reverted to 40 A g⁻¹, the capacity of the BONs anode recovered to 140 mAh g⁻¹, highlighting its excellent electrochemical reversibility. Notably, the rate capacity of the BONs anode outperformed that of all reported materials in CIBs (Fig. 2e and Table S1).^{30,31,37–39,42,54} Impressively, the BONs anode delivered high cycling stability after 1000 times with 100% capacity retention at 1 A g⁻¹ (Fig. 2f) and 94% capacity retention at 5 A g⁻¹ (Fig. S15). Importantly, it exhibited a record lifespan of 100 000 cycles with 80% residual capacity at 10 A g⁻¹ (Fig. 2g, h and Table S1).^{18,30,31,37–39,54} The BONs anode with a high mass loading of 10.1 mg cm⁻² exhibited high-capacity retention of 80% after 1000 cycles at 5 A g⁻¹ (Fig. S16), demonstrating a practical perspective. The BONs electrode was also applicable to aqueous Zn(OTF)₂ and Mg(OTF)₂ electrolytes (Fig. S17), demonstrating good electrochemical performances. The excellent all-round electrochemical metrics of the BONs anode make it a highly competitive anode for advanced aqueous CIBs. Various spectral characterizations were conducted to delve into the structural variation of the BONs anode at seven marked electrochemical states (Fig. 3a). In the FT-IR spectra (Fig. 3b), the vibration signals of –NH– species at 796 cm⁻¹ exhibited an upward trend during discharging (state A → B), together with a decrease in S=O species of OTF⁻ at 1057 cm⁻¹, validating the removal of OTF⁻ at –NH– sites. In contrast, the vibrational intensity of C=O groups at 1630 cm⁻¹ gradually decreased during discharging (state B → C), accompanied by the emergence of C–O peaks at 1425 cm⁻¹. Meanwhile, an O–H signal could not be detected at 3200 cm⁻¹, which suggested the coordination interaction of C=O groups with Ca²⁺ rather than H⁺. Subsequently, the vibration signals of N=N bonds at 1580 cm⁻¹ decreased during discharging (state C → D), meanwhile N–H groups at 3230 cm⁻¹ were generated. This result indicated that N=N groups, as redox-active sites, coordinated with H⁺. After charging (state D → E → F → G), all signals returned to initial levels. Overall, the –NH–, C=O, and N=N groups of BONs are highly reversible electroactive sites to propel anion–cation hybrid redox reactions. The high-resolution XPS spectra of N 1s and O 1s signals were analysed to further reveal the redox behavior of the BONs anode during the electrochemical process. N 1s XPS spectra (Fig. 3c) in state A could be deconvoluted into three peaks, corresponding to =NH⁺– (403.0 eV), –NH– (400.5 eV) and N=N (399.0 eV). During discharging (state A → B), the peak intensity of =NH⁺– gradually decreased while that of –NH– increased, indicating the removal of OTF⁻ from BONs. With proceeding of the discharge, the peak intensity of N=N groups were almost unchanged (state B → C) and gradually decreased (state C → D), meanwhile a new peak appeared at 402.0 eV (state D), which could be credited to the coordination reaction between N=N sites and H⁺ to form N–H groups. During the subsequent charge process (state D → E → F → G), these redox-active signals restored to initial states. The structural change in N=N moieties in BONs agreed with PD (Fig. S18). Furthermore, O 1s XPS spectra consisted of C=O at 532.3 eV and C–O species at 533.3 eV (Fig. 3d). The intensity of the C=O peak remained unchanged during discharging (state A → B) and decreased (state B → C), together with an increase in C–O peaks, indicating coordination between Ca²⁺ and the C=O groups of the BONs anode. The change details of ionic carriers in the BONs anode were also monitored by 2D contour maps of Ca 2p and S 2p XPS spectra (Fig. 3e and f). The Ca 2p signal increased during discharge (state B → C) because of Ca²⁺ coordination, and then weakened during recharging (state E → F) through Ca²⁺ removal (Fig. 3e). Conversely, S 2p signals decreased upon discharge (state A → B) and increased in the following charge course (state F → G, Fig. 3f), providing proof of OTF⁻ uptake/release within BONs.

Fig. 2 Electrochemical performances of the BONs anode in 5 M Ca(OTF)₂ aqueous electrolyte. (a) GCD curves at various current densities. (b) Differential capacity profile at 0.2 A g⁻¹. (c) CV profiles at 0.1 mV s⁻¹. (d) Rate metrics. (e) Capacity comparison of BONs with reported anodes. (f) Cyclic stability at 1 A g⁻¹. (g) Long-term cycling performance at 10 A g⁻¹. (h) Lifetime comparison of BONs with reported anode materials.

Fig. 3 Structural changes in the BONs anode during charge/discharge. (a) A potential-capacity profile. (b) Overview of FT-IR spectra. XPS spectra of (c) N 1s, (d) O 1s, (e) Ca 2p, and (f) S 2p at various electrochemical states. (g) UV/Vis spectra and photographs of Ca(OTF)₂/H₂O electrolyte after soaking the BONs anode during (dis)charging.

Besides, no UV/Vis signals and colorless aqueous Ca(OTF)₂ electrolytes soaked with the BONs anode at different (dis)charged states confirmed structural robustness and anti-dissolution (Fig. 3g). Considering the weak acidity of Ca(OTF)₂/H₂O electrolyte (pH ≈ 4),⁵⁴ the contribution of H⁺ to the whole energy storage of the BONs anode was further studied in HOTF/H₂O electrolyte (containing H⁺, pH ≈ 4). In HOTF/H₂O electrolyte, one pair of redox signals overlapped with CV of BONs in Ca(OTF)₂/H₂O electrolyte (Fig. S19a), indicating proton storage at −0.60/−0.69 V. Meanwhile, the capacity of the BONs anode (Fig. S19b) in HOTF/H₂O electrolyte (140 mAh g⁻¹) approached one-half of Ca(OTF)₂/H₂O electrolyte (302 mAh g⁻¹, ≈ 4e⁻ redox reactions), corresponding to 2H⁺ charge storage. These results emphasized an anion–cation co-storage mechanism of the BONs anode during (dis)charge: OTF⁻ removal proceeds in discharge (state A → B) and uptake in charge (state F → G); Ca²⁺ and H⁺ coordination occurs in discharge (state B → C, state C → D) and de-coordination in charge (state E → F, state D → E), respectively.

The redox kinetics of the BONs anode was unraveled via CV curves based on Dunn's method.^49,60–62 CV profiles held similar shapes within a scan rate of 2–10 mV s⁻¹ (Fig. 4a), substantiating the high electrochemical reversibility of the BONs anode. There were three pairs of redox signals in CV curves (labelled as P_R1, P_R2, P_R3, P_O1, P_O2, and P_O3), revealing a three-step redox reaction process. According to the power-law relationship between the peak current (i) and sweep speed (v),^63–66 the evaluated b-values for six cathodic/anodic peaks were all close to 1 (Fig. 4b), which untangled the rapid surface-dominant charge storage behavior. At 6 mV s⁻¹, 91.1% of the total stored charge originated from surface redox reactions (Fig. 4c), with a minor diffusion-limited contribution of 8.9%. As the scan rate increased, the capacitive contribution exceeded the diffusion-limited contribution, gradually rising from 86.9% to 96.4% (Fig. 4d).

Fig. 4 Redox mechanism of the BONs anode. (a) CV profiles. (b) Calculated b values. (c) CV curves with pseudocapacitive-dominated and diffusion-controlled contribution at 6 mV s⁻¹. (d) Capacitive contributions at various scan rates. (e) EIS spectra. (f) E_a values. (g) Amphoteric redox behaviors of BONs during the electrochemical process. Inset in (e) is a typical equivalent circuit, which includes the equivalent series resistance (R_s), charge transfer resistance (R_ct), Warburg impedance (Z_w) and constant phase angle element (CPE).

Besides, the interfacial activation energy (E_a) of BONs was estimated via electrochemical impedance spectroscopy (EIS) based on the relationship between charge transfer resistance (R_ct) and temperature (T).^67–70E_a for the charge storage process of BONs was 0.22 eV (Fig. 4e, f and Fig. S20), which was lower than that for PD (0.34 eV) and BQ (0.45 eV). This result demonstrated its high-kinetics interfacial ion migration process with low energy barriers due to favorable cation–anion Coulomb interactions. Overall, experimental results indicated the rapid and stable three-step 4e⁻ reaction of the BONs anode (Fig. 4g), which sequentially entailed mainly 1 e⁻ amine reactions with one OTF⁻, whereupon 1 e⁻ carbonyl redox with 0.5Ca²⁺, followed by 2 e⁻ azo coordination with 2H⁺. Overall, BONs have conquered the challenges of limited accessible redox-active sites and structural instability in recently reported organic materials, achieving state-of-the-art CIBs in comprehensive metrics (Table S1).

To elucidate the redox behavior of the BONs anode, density functional theory (DFT) calculations were performed. Using the simulated molecular electrostatic potential (ESP) distribution from an optimized structural model (Fig. 5a),^71,72 the carbonyl, azo, and amine groups were identified as redox-active centres of BONs. The π-electron localization function map revealed the extended π-conjugated structure of BONs (Fig. 5b),⁷³ suggesting high aromaticity and efficient π-electron localization throughout the molecular skeleton. In the DFT-optimized reaction pathway (Fig. 5c),⁷⁴ BONs undergo a three-step 4 e⁻ coordination process during (dis)charging, involving multi-step (de)coordination of OTF⁻, Ca²⁺, and H⁺ with –NH–, C=O, and N=N sites. During discharge (step 1), a negative binding energy (ΔE) of −3.14 eV is required for OTF⁻ coordination of one –NH– site to form initial BONs (initial state). During further discharging, the ΔE₁ of step 2-1 exceeds that of step 2 (Fig. S21). Thus, 0.5Ca²⁺ preferentially coordinates with the carbonyl motifs of BONs in step 2, and then two H⁺ continue to react with an azo group (step 3). The ΔE required for step 2 (−9.60 eV) is less than that for step 3 (−6.36 eV), ensuring sequential progression. This efficient binding environment enables rapid and stable multielectron redox in BONs for CIB operation.

Fig. 5 Theoretical simulations of redox behaviors of BONs at different electrochemical states. (a) MEP distribution. (b) ELF–π map. (c) Optimized geometries and corresponding ΔE values after ion coordination. (d) Differential charge isosurfaces at state I, II, and III.

The differential charge isosurfaces^75,76 were simulated to obtain insights into the binding properties of anion/cation-coordinated BONs (Fig. 5d). Based on Bader charge analyses, there were obvious charge accumulation and depletion regions between OTF⁻, Ca²⁺ and H⁺ sites of BONs, thereby enhancing the electrochemical activity and durability of aqueous CIBs.

To evaluate the electrochemical potential of the BONs anode in practical applications, an aqueous CIBs full cell was established (Fig. 6a), involving a Prussian blue analog (KCoFe(CN)₆) cathode (Fig. S22) and 5 M Ca(OTF)₂/H₂O electrolyte. The CV curves of the full battery at 1 mV s⁻¹ showed three pairs of reduction/oxidation peaks (Fig. 6b). In addition, the capacity of the BONs‖KCoFe(CN)₆ battery was 210, 185, 143, 117, 100, 80, and 72 mAh g⁻¹ at a current density of 1, 2, 5, 10, 20, 40, and 60 A g⁻¹, respectively (Fig. 6d–f and Table S2). Meanwhile, the BONs‖KCoFe(CN)₆ battery delivered high energy efficiency of ∼74.5% (Fig. S23). Consequently, the BONs‖KCoFe(CN)₆ battery exhibited an energy density of 221 Wh kg⁻¹ (based on the mass loading of the BONs anode, Table S3). The excellent capacity, energy density, and rate performance of the BONs‖KCoFe(CN)₆ battery were superior to those of recently reported aqueous calcium-ion cells (Tables S2 and S3).^{30–32,37,38,76} For comparison, based on the total mass loading of BONs and the KCoFe(CN)₆ cathode, the BONs‖KCoFe(CN)₆ battery achieved an energy density of 57.3 Wh kg⁻¹ cell. This is highly competitive with that of recently reported CIBs, including TB-COFs‖Ca_xCuHCF (31 Wh kg⁻¹),³⁹ PTCDI‖CuHCF (46.8 Wh kg⁻¹),³² PNDIE‖Ca_0.3CuHCF (54 Wh kg⁻¹),⁴² PT‖KCoFe(CN)₆·xH₂O (57.3 Wh kg⁻¹),³¹ PTCDI‖Cu-PBA (58.3 Wh kg⁻¹),³⁰ and PTHAT-COF‖Mn-PBA (59.2 Wh kg⁻¹)³⁸ batteries. Importantly, the full battery liberated an unprecedented cycling stability with 78% capacity retention after charge carriers and BONs, with notable charge shifts (state I: 0.46 e; state II: 0.81 e; state III: 0.39 e). These strong chemical interactions drove the redox reactions of opposite-charge carriers (Ca²⁺, H⁺ and OTF⁻), which fully utilized the multiple redox 20 000 cycles at 10 A g⁻¹ (Fig. 6g and h), representing a new starting point in reported CIBs.^{31,37–39,42,75,77} The comprehensive electrochemical metrics in terms of superior capacity, high-rate performance and durable lifespan suggest the great application potentials of the BONs anode in CIBs towards advanced energy storage.

Fig. 6 Electrochemical performances of an aqueous BONs‖KCoFe(CN)₆ battery. (a) Battery configuration (schematic). (b) CV curves for a BONs‖KCoFe(CN)₆ full cell at 1 mV s⁻¹. (c) GCD curves and (d) rate capacities at different current densities. (e) Specific capacity comparison with reported calcium-ion batteries. (f) Energy density comparison with reported calcium-ion batteries based on mass loading of the anode. (g) Cycling performances. (h) Lifespan comparison with reported calcium-ion batteries.

Conclusions

Low-activation-energy, multi-redox-active and ultrastable BONs were designed by integrating dual-electron-redox benzoquinone and 4,4′-azodianiline units into extended π-conjugated polymeric skeletons through rich intermolecular H-bonds (N–H⋯O) and π–π interactions. The well-arranged rod morphologies of BONs afforded consecutive electron delocalization pathways to allow high accessibility of redox-active carbonyl/azo/amine sites with ultralow activation energy (0.22 eV), affording a high capacity (302 mAh g⁻¹) in CIBs. Moreover, the BONs anode with extended interconnected rod nanostructures achieved anti-dissolution in aqueous electrolytes to liberate a record cycling life (100 000 cycles). Experimental and theoretical studies revealed the anion–cation co-storage mechanism of the BONs anode, which involved multi-step (de)coordination of OTF⁻, Ca²⁺, and H⁺ with –NH–, C=O, and N=N sites, respectively. Besides, the assembled BONs‖KCoFe(CN)₆ full cell delivered high capacity (210 mAh g⁻¹), energy density (221 Wh kg_anode⁻¹), and long cycle life (20 000 cycles). This work broadens the design horizons of organic nanostructures for advanced CIBs.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

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

Supplementary information (SI) is available. All data is available in the main text or the SI. The SI file contains detailed materials, experimental methods, additional data that support the findings presented in the main text of the manuscript. It includes the synthesis, calculation methods, characterizations, and electrochemical tests, together with the supplementary figures and tables. See DOI: https://doi.org/10.1039/d5mh01474c.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (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.

Notes and references

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