N-heterocyclic π-conjugated quinone cathodes with multiple chelation for robust sodium batteries

Abstract

Redox-active organic materials have arisen as promising electrode materials for rechargeable batteries owing to their merit of structural diversity and environmental compatibility. However, their practical application in sodium ion batteries (SIBs) has been hindered by the high dissolution in liquid electrolytes and the sluggish redox kinetics. Herein, a series of π-conjugated N-heterocyclic quinones (TTAQ, DPT, DNQ-PQ, and DNQ-PTO) with chelation groups were systematically developed. Among them, DNQ-PTO exhibited exceptional performance, delivering a reversible capacity of ∼275 mA h g⁻¹ at 0.1 A g⁻¹, which referred to a six-electron redox process involving a four-electron transfer for C=O/C–O and a two-electron transfer for the C=N/C–N group. Moreover, the DNQ-PTO cathodes demonstrated remarkable rate capability, superior cycling stability (∼89% capacity retention after 6000 cycles at 5 A g⁻¹), and excellent low-temperature performance (∼91.4% capacity retention over 100 cycles at −40 °C). The outstanding electrochemical performance of DNQ-PTO stemmed from its extended conjugation system and well-dispersed chelation groups, which facilitated efficient charge delocalization and structural stability. This work demonstrated that the critical challenges in capacity, stability, and kinetics of organic electrode materials could be addressed through molecular structure optimization, which established a new paradigm for high-performance organic electrodes.

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Kun Fan

Kun Fan is an Associate Professor at School of Chemistry and Environmental Engineering, Wuhan Institute of Technology. He obtained his PhD degree in 2019 from the Nanjing University and subsequently conducted postdoctoral research at Huazhong University of Science and Technology. His current research focuses on novel conjugated organic materials and conjugated coordination polymers for electrochemical energy storage.

Introduction

Organic redox-active electrodes have arisen as a promising alternative to conventional electrodes for sodium-ion batteries (SIBs) owing to their flexible molecular structure that facilitates rapid Na⁺ ion insertion/extraction with negligible structural changes, thereby enhancing cycling stability and electrochemical efficiency.^1–4 Moreover, the structural adaptability of organic materials also allows for the tailoring of their physical and chemical properties, such as solubility, conjugation and redox activity, making them highly versatile for optimizing battery performance.^5–8 As important representatives of organic electrode materials, p-benzoquinone (BQ) and its derivatives have been extensively studied for their high theoretical capacity owing to two-electron redox processes.^9,10 However, organic quinone electrodes still face critical challenges in SIBs, including the high solubility in electrolytes, poor structural integrity, slow ionic diffusion and sluggish redox kinetics.^11,12 To address these issues, various strategies, such as molecular polymerization, incorporation of conductive additives, and modification of binders and separators, have been extensively explored.^13–17 However, these approaches only partly address the challenges, and some issues are still not fully resolved.

Recent studies have demonstrated that π-conjugated quinones with N-heterocycles represent a promising breakthrough for organic carbonyl electrodes.¹⁸ The intrinsic multi-electron capability of π-conjugated quinones and N-heterocycles makes them particularly attractive for enhancing the energy density of SIBs.^19–21 The extended conjugated system and the electron-donating character of the N-heterocycles synergistically enhance charge delocalization, enabling highly efficient electron transport during redox processes, which is crucial for achieving superior rate capability in SIBs.^22,23 Furthermore, incorporating N-heterocycles into the π-conjugated framework can also increase the effective coordination sites for sodium ions, thereby leading to enhanced cycling stability and capacity retention.^24,25 However, despite these advantages, the evaluation of the stability of N-heterocyclic groups during redox cycling and their cooperative charge storage mechanisms with π-conjugated quinone groups is still lacking, which leads to an ambiguous structure–property relationship.^26,27

Herein, a series of π-conjugated quinones incorporating N-heterocycles, namely TTAQ, DPT, DNQ-PQ and DNQ-PTO, were rationally designed and synthesized.^28–32 Although all four compounds featured multiple redox-active sites, their electrochemical properties showed considerable differences. Benefiting from the high charge delocalization and well-dispersed chelation group, DNQ-PTO exhibited a reversible capacity of ∼275 mA h g⁻¹ at 0.1 A g⁻¹, confirming a six-electron redox process that referred to a four-electron transfer for C=O/C–O groups and two-electron transfer for C=N/C–N groups. DNQ-PTO cathodes also showed excellent rate performance (∼213 mA h g⁻¹ at 10 A g⁻¹) and exceptional stability (∼89% capacity retention after 6000 cycles at 5 A g⁻¹), which outperformed the other three compounds and most reported organic electrode materials. Moreover, DNQ-PTO exhibited 91.4% capacity retention after 100 cycles even at −40 °C, demonstrating good charge-transfer kinetics and structural stability at low-temperature. These results revealed that precisely tailored chelating sites within π-conjugated frameworks could significantly enhance their electrochemical performance, which provided a new paradigm for molecular engineering of high-performance organic electrodes.

Results and discussion

Synthesis and characterization

1,4,5,8-Tetraaza-9,10-anthraquinone (TTAQ) has demonstrated great potential as a cathode material for lithium-ion batteries, due to its multiple redox-active sites and low molecular mass, which contribute to a high theoretical energy density.²⁸ The proximity of carbonyl and nitrogen-containing heterocycles within the structure facilitates the formation of multiple chelating interactions with the inserted cations, thereby promoting the redox reaction (Fig. 1a).¹⁸ However, according to density functional theory (DFT) calculations, the limited π-conjugation system in TTAQ leads to a wide band gap, which would restrict electron mobility and increases cycling polarization (Fig. 1b). As such, three compounds (DPT, DNQ-PQ and DNQ-PTO) with extended conjugation structures were elaborately selected to address this limitation (Scheme S1a). It's worth noting that DNQ-PTO possesses a lower band gap than TTAQ, DPT and DNQ-PQ, which implies a better charge transfer capability to achieve superior rate performance and excellent cycling stability in SIBs (Fig. 1b).^33–35

Fig. 1 (a) The molecular electrostatic potential images and (b) the LUMO–HOMO plots of TTAQ, DPT, DNQ-PQ and DNQ-PTO.

Dibenzo[b,i]-phenazine-5,7,12,14-tetrone (DPT) was prepared by the condensation reaction of 2,3-diaminonaphthalene and 2,3-dihydroxynaphthalene followed by oxidation. The resulting structure features four carbonyl groups symmetrically arranged within a conjugated system containing a phenazine group (Fig. 1a and Scheme S1b).³⁰DNQ-PQ and DNQ-PTO were synthesized by the condensation reaction of 2,3-diamino-1,4-naphthoquinone (DNQ) with 9,10-phenanthraquinone (PQ) or pyrene-4,5,9,10-tetrone (PTO), respectively (Scheme S1).^32,36,37 The successful synthesis was confirmed by high-resolution mass spectrometry (HR-MS) and elemental analysis (EA) (Fig. S1 and Table S1). As shown in scanning electron microscopy (SEM) images, the synthesized TTAQ and DNQ-PQ exhibited a regular rod-like morphology, while the DPT has a sheet structure, which was consistent with previous reports (Fig. 2a–c).^30,36 However, the larger conjugated structure of DNQ-PTO reduced its solubility in the reaction solvents, thus leading to rapid solid precipitation and yielding the as-synthesized samples with smaller particle size (Fig. 2d).³² As shown in Fig. 2e, DNQ-PTO exhibited the best thermal stability with decomposition temperatures up to 550 °C, which was higher than that of the other three compounds. This result suggested that the thermal stability was consistent with the degree of conjugation rather than the particle size. The high crystallinity of the as-synthesized samples was further confirmed by powder X-ray diffraction patterns (PXRD) (Fig. 2f). Moreover, the chemical composition was verified in detail by Fourier transform infrared (FT-IR) spectra (Fig. 2g and S3–S6). Compared with the reactants, the characteristic vibrations of –NH₂ at the 3500–3300 cm⁻¹ region disappeared with a clear signal of C=N stretching vibration appearing in all four compounds, suggesting the successful formation of the pyrazine moiety after condensation reactions (Fig. S3–S6).^20,38 The stretching vibration of C=O bonds was observed at 1706, 1697, 1690 and 1688 cm⁻¹ in TTAQ, DPT, DNQ-PQ and DNQ-PTO, respectively. The red-shift of the C=O stretching vibration in DNQ-PTO compared to the other three compounds implied increased electron delocalization upon extending the conjugation structures (Fig. 2g).^19,25 Benefiting from increased conjugation, DNQ-PTO was almost insoluble in most solvents, including the DME electrolyte, demonstrating superior stability compared to TTAQ, DPT and DNQ-PQ (Fig. 2h and S2). As a result, DNQ-PTO featured a higher activity density and poorer solubility, which was able to maintain a higher theoretical capacity with improved stability and charge transport.

Fig. 2 Structural characterization of TTAQ, DPT, DNQ-PQ and DNQ-PTO. (a–d) SEM images, (e) TGA curves, (f) PXRD patterns and (g) FT-IR spectra, and (h) the solution stability in DME (top: powder, bottom: electrodes).

Electrochemical performance

To assess the redox performance of the four organic electrodes, cyclic voltammetry (CV) profiles were studied in the potential window of 0.8–3.2 V (vs. Na⁺/Na) at 1 mV s⁻¹ (Fig. S7). As shown in Fig. 3a, the TTAQ electrode exhibited two distinct reduction peaks centered at 2.66 and 2.10 V, accompanied by one broad peak in the 0.80–1.42 V range during the initial cathodic scan. The two higher-voltage peaks (>1.5 V) corresponded to the reduction of C=O bonds, while the low-voltage peak originated from the partial reduction of C=N bonds (see below). Additionally, the area integral ratio of these peaks was approximately 2 : 1, suggesting a three-electron redox process, which agreed well with the previously reported work (Fig. S9 and S10).²⁸ However, the rapid fading of the redox peaks in subsequent cycles reflected the poor electrochemical stability of the TTAQ electrode, while the DPT electrode displayed a more complex redox process, which delivered four prominent reduction peaks in the ranges of 2.20–2.85 V and 1.0–1.7 V, along with a less distinct peak at 0.8–1.0 V. The reduction peaks at higher voltages were sharp and well-defined, while the peaks at lower voltages were broad and weak, suggesting different electrochemical behaviors across the voltage range due to the different redox groups (Fig. S11 and S12).^34,39 Similar phenomena were also observed in the case of the DNQ-PQ electrode, in which only two obvious peaks centered at 2.19 and 1.70 V were found in the potential range of 1.5–2.3 V along with an incomplete reduction peak at the low voltage range in the cathodic scan, while three oxidation peaks with the area ratio of 0.5 : 1 : 1 were clearly observed in the anodic scan (Fig. 3a, S13 and S14). When the potential window was extended to 0.01 V, four pairs of redox peaks were recorded, indicating the low reduction potential of C=N bonds (Fig. S8).⁴⁰ However, the multiple redox peaks of DPT and DNQ-PQ electrodes significantly diminished in subsequent scans and became almost indistinguishable after 20 cycles, suggesting the loss of electrochemical stability during the prolonged charge–discharge process (Fig. S7 and S8).

Fig. 3 (a) Comparison of the CV curves of TTAQ, DPT, DNQ-PQ and DNQ-PTO. The GCD curves of (b) TTAQ, (c) DPT, (d) DNQ-PQ and (e) DNQ-PTO at 0.1 A g⁻¹.

Different from the other three organic electrode materials, DNQ-PTO exhibited obvious and stable redox peaks in all the voltage ranges (Fig. S7 and S8). As shown in Fig. 3a, three characteristic redox peaks were observed during the initial cathodic scan of DNQ-PTO electrodes within the potential window of 0.8–3.2 V. The two pairs of redox peaks in the 1.5–3.0 V range could be attributed to the reversible reaction of the C=O/C–O groups, while the broad peak in the 0.8–1.5 V range was associated with the conversion of the C=N to C–N bonds.^19,20 Quantitative integration of these redox peak revealed a strict 1 : 1 : 1 area ratio for the three redox couples, confirming a total six-electron transfer process in the DNQ-PTO electrode (with each redox reaction involving a two-electron transfer, as discussed below) (Fig. S15, S16 and Table S2). Moreover, the peak shapes remained nearly identical and the currents exhibited negligible variation across subsequent cycles, demonstrating the highly reversible redox behavior and excellent cycling stability of the DNQ-PTO electrode.

In addition, the Na⁺ storage performance of the four organic cathodes was further assessed by galvanostatic charge–discharge (GCD) tests. As shown in Fig. 3b, the TTAQ electrode exhibited an initial discharge capacity of ∼328 mA h g⁻¹ at 0.1 A g⁻¹, which was less than the theoretical capacity of three electron transfers, coinciding with the incomplete reduction peak at low voltage in CV profiles, while the initial discharge capacities of DPT and DNQ-PQ electrodes were ∼224 and ∼235 mA h g⁻¹, which were close to their theoretical capacities of 236.4 and 223.3 mA h g⁻¹ for three-electron storage (Fig. 3c, d and Table S2). However, all three organic cathodes exhibited low initial coulombic efficiency (CE) and suffered from rapid capacity fading during cycling (Fig. S17).

In contrast, the DNQ-PTO electrode exhibited exceptional performance, which demonstrated a reversible capacity up to ∼275 mA h g⁻¹ with a high CE of 98.8% after three replication cycles. This observed capacity confirmed a six-electron redox process per molecule (theoretical: 284.0 mA h g⁻¹), which referred to a four-electron transfer for C=O/C–O and two-electron transfer for the C=N/C–N group (vide infra) (Fig. 3e and Table S2). The DNQ-PTO electrode exhibited ∼86.5% capacity retention after 100 cycles, demonstrating superior electrochemical stability to the other three organic cathodes (Fig. 4a and S17).¹⁸

Fig. 4 (a) Cycling performance of the DNQ-PTO electrode at 0.1 A g⁻¹. (b) The rate capability of DNQ-PTO. (c and d) The long-term cycling performance of the DNQ-PTO electrode at 1 A g⁻¹ and 5 A g⁻¹.

The rate performance of the DNQ-PTO electrode was investigated at current densities of 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g⁻¹, which delivered an average discharge capacity of 265, 247, 238, 232, 225, 218 and 213 mA h g⁻¹, respectively (Fig. 4b). Remarkably, the DNQ-PTO electrode retained ∼80% of its capacity when the current density increased from 0.1 to 10 A g⁻¹, displaying excellent rate capability. Upon returning to 0.1 A g⁻¹, the DNQ-PTO cathode regained a high specific capacity of ∼237 mA h g⁻¹, confirming its robust structural stability and good electrochemical reversibility. Moreover, as presented in Fig. 4c, an extraordinary capacity of ∼212 mA h g⁻¹ with CE up to 100% was maintained after 2000 cycles at a current of 1 A g⁻¹, which outperformed many other organic electrodes (Fig. S18 and Table S3). The long-term cyclability of the DNQ-PTO electrode was further assessed at a high current density of 5 A g⁻¹, which exhibited ∼89% capacity retention (∼208 mA h g⁻¹) after 6000 cycles with an ultralow decay rate of 0.0018% per cycle, demonstrating exceptional stability (Fig. 4d).

To further elucidate the electrochemical kinetics of the DNQ-PTO electrode, the CV profiles were measured at various sweeping rates. As depicted in Fig. S19a, all redox peaks remained almost unchanged with increasing scan rate, indicating superior rate performance. Additionally, the capacity contribution from diffusion controlled processes or the capacitive effects were then analyzed from the i = aν^b equation, where i represents the peak current at a given scan rate of ν.^41–43 The average slope b at the redox and oxidation peaks was 0.86 and 0.93, respectively, indicating a capacitive-controlled process during cycling (Fig. S19b). In addition, the capacitive contributions at scan rates of 0.2, 0.5, 1.0, 2.0, and 5.0 mV s⁻¹ were assessed as 88.5%, 89.5%, 91.3%, 93.4%, and 99.1%, respectively (Fig. S20). This suggested that the capacity of DNQ-PTO was primarily contributed by pseudocapacitive processes, which accounted for its good rate capability.⁴⁴ To investigate the sodium-ion diffusion kinetics, the galvanostatic intermittent titration technique (GITT) method was performed (Fig. S21).⁴⁵ The calculated Na⁺ diffusion coefficient (D_Na⁺) was around ∼10⁻⁹ cm² s⁻¹ across various potentials, which was higher than that of other representative organic electrodes. These results further revealed that the extended conjugated structure would facilitate an ordered ionic diffusion pathway, thereby leading to the superior rate performance of the DNQ-PTO electrode.

The combination of excellent reaction kinetics and high ion mobility of the DNQ-PTO electrode would facilitate low-temperature sodium storage, making it highly advantageous for practical application.^46–48 As depicted in Fig. 5a, the CV curves at −40 °C exhibited only a slightly increased polarization between the redox peaks compared to that at 25 °C, indicating good electron transfer efficiency even at low temperatures. This exceptional behavior could be stemmed from the enhanced conjugated structure of DNQ-PTO, which facilitated effective charge delocalization and electron transfer even under low-temperature conditions. Remarkably, the DNQ-PTO electrode maintained excellent electrochemical performance even at −40 °C, which delivered a reversible capacity of up to ∼211 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 5b). The five-electron redox process was consistent with the quantitative integration of the three redox peaks (Fig. S22). The capacity loss at low temperature may be ascribed to the incomplete redox reactions caused by the sluggish kinetics and decreased ion mobility.^49–51 Moreover, the DNQ-PTO electrode demonstrated excellent cycling stability with 91.4% capacity retention (∼192.8 mA h g⁻¹) after 100 cycles (Fig. 5c), which is superior to most of the reported conjugated organic/polymeric materials for low-temperature SIBs (Fig. 5d and Table S4).^46,50–55

Fig. 5 (a) CV curves of the DNQ-PTO electrode at 25 °C and −40 °C. (b) The GCD profiles and (c) cycling performance of the DNQ-PTO electrode at −40 °C. (d) Comparison of the specific capacity of DNQ-PTO and other organic electrode materials at low-temperature.

Charge storage mechanism

To investigate the Na⁺ ion storage mechanisms in the DNQ-PTO electrode, in situ/ex situ analytical techniques were performed to monitor the structural variation during electrochemical cycling.⁵⁶Fig. 6a shows the in situ ATR-FTIR spectra of the DNQ-PTO electrode at different voltage states. Upon electrochemical reduction, the characteristic C=O stretching vibration at 1689 cm⁻¹ progressively decreased in intensity, accompanied by an increase in C–O stretching vibration at 1080 cm⁻¹, demonstrating that the carbonyl groups serve as the primary electrochemical reaction centers (Fig. S23a and S24). Moreover, the growing intensity of aromatic C–N vibration (1375 cm⁻¹) below 1.5 V confirmed the redox activity of C=N double bonds during the reduction process (Fig. 6a and S23b). These results were consistent with the presence of a couple of redox peaks at low voltage (0.8–1.5 V) in CV curves, which further revealed a dual redox-center mechanism involving both C=O and C=N groups in the DNQ-PTO electrode (Fig. 6e).¹⁹ During the charging process, all characteristic vibrational signals recovered to their original states, confirming the excellent electrochemical reversibility of DNQ-PTO electrodes.

Fig. 6 (a) The in situ FTIR spectra of the DNQ-PTO electrode. (b) Ex situ EPR spectra and (c) ex situ O 1s and (d) N 1s XPS spectra of the DNQ-PTO electrode at different potentials. (e) Proposed reaction mechanism for the DNQ-PTO electrode.

Additionally, ex situ electron paramagnetic resonance (EPR) spectra were employed to characterize radical species generation during electrochemical cycling. As shown in Fig. 6b, when the DNQ-PTO electrode was first discharged to 2.0 V, an obvious sharp signal (g = 2.003) was observed, which was characteristic of the C–O˙ radical species formed via single-electron reduction of carbonyl (C=O) groups,⁵⁷ while further discharge to 1.5 V resulted in a decreased EPR signal, indicating a full reduction of C=O double bonds to C–O single bonds. The reversible binding energy shift in O 1s XPS spectra during electrochemical cycling further corroborated the reversible redox conversion between C=O and C–O groups in the DNQ-PTO electrode (Fig. 6c and S25).^58,59 All experimental observations within the 1.5–3.2 V range supported the proposed four-electron transfer mechanism involving all the four quinone groups in the DNQ-PTO electrode (Fig. 6e). Upon further discharge to 0.8 V, the reappearance of the EPR signal confirmed the partial reduction of C=N double bonds and the formation of radical intermediates (C–N˙) with an unpaired electron.⁶⁰ Additionally, the ex situ N 1s XPS spectra also exhibited reversible transformation of C=N and C–N bonds in the low-voltage region (Fig. 6d). Quantitative capacity analysis revealed a two-electron transfer process within the 0.8–1.5 V potential window, which correlated with the partial reduction of four C=N double bonds in each DNQ-PTO unit, further providing direct evidence for the proposed redox mechanism (Fig. 6e and S26–S28). The negligible variation of C=N bonds between 1.5 and 3.2 V indicated that the N-heterocycle served as a chelating site in this range, which was further supported by the appearance of a reversible peak at around ∼403.0 eV.

Conclusion

In summary, we systematically investigated the potential of a series of π-conjugated quinones with N-heterocycles (TTAQ, DPT, DNQ-PQ, and DNQ-PTO) as high-performance organic cathodes for SIBs. Among these, the DNQ-PTO electrode emerged as a standout candidate, delivering a superior rate capability (∼80% capacity retention at 10 A g⁻¹versus 0.1 A g⁻¹), ultralong cycling stability (∼89% capacity retention after 6000 cycles at 5 A g⁻¹) and remarkable low-temperature adaptability (∼91.4% capacity retention after 100 cycles at −40 °C), which were superior to those of the other three compounds with smaller conjugated systems. The excellent cyclability and superior rate performance of DNQ-PTO stemmed from its unique molecular structure, which combined the multiple redox active sites, extended π conjugated structure and well-dispersed chelation group. These results not only advanced our understanding of structure–electrochemistry relationships in π-conjugated N-heterocyclic quinones, but also established crucial molecular design principles for developing high-performance organic electrode materials toward future sustainable energy storage systems.

Experimental section

Synthesis of TTAQ

2,3,5,6-Tetraamino-1,4-benzoquinone (TABQ, 3.00 mmol, 504 mg) and glyoxal (40 wt% in H₂O, 60 mmol, 6.85 mL) were added to glacial acetic acid (100 mL). The mixture was stirred for 3 days at room temperature. The resulting precipitate was collected by vacuum filtration, followed by thorough washing with deionized water. Then the crude product was ultrasonically dispersed in ethanol (100 mL), followed by heating to 80 °C under reflux with stirring for 1 h. After cooling, the mixture was filtered and vacuum-dried at 80 °C for 6 h, yielding the product as a grey solid (63%).

Synthesis of DPT

A two-step synthetic route was used to synthesize DPT. Firstly, a stoichiometric mixture of 2,3-dihydroxynaphthalene (2 mmol, 320 mg) and 2,3-diaminonaphthalene (2 mmol, 316 mg) was thoroughly ground and transferred into a Pyrex tube. The tube was evacuated, flame-sealed, and then heated at 180 °C for 45 min. After cooling, the crude product was treated with acetone. The precipitate was collected by vacuum filtration, sequentially washed with deionized water and acetone, and then vacuum-dried at 80 °C for 6 h to afford DHDBP (6,13-dihydrodibenzo[b,i]phenazine) (yield: 39%). Secondly, the as-prepared DHDBP was suspended in 25 mL of 5 M H₂SO_4, followed by slow addition of K₂Cr₂O₇ (7 mmol, 2 g) as an oxidant. The reaction mixture was heated and stirred at 80 °C for 4 h. After cooling, the precipitates were isolated by vacuum filtration, washed with deionized water, and then vacuum-dried at 80 °C for 6 h to afford DPT as a yellow powder (yield: 60%).

Synthesis of DNQ-PQ

A mixture of DNQ (1 mmol, 188 mg), PQ (1 mmol, 208 mg), and phthalic acid (0.05 mmol, 8 mg) in ethanol/water (10 mL, 7 : 3 v/v) was stirred at 60 °C for 2 h. After cooling, 20 mL pre-cooled deionized water was added, and the resulting mixture was allowed to stand at room temperature overnight. The yellow precipitates were collected by vacuum filtration, washed sequentially with deionized water and ethanol, and then dried under vacuum to yield DNQ-PQ (yield: 86%).

Synthesis of DNQ-PTO

A well-dispersed suspension of DNQ (2 mmol, 376 mg) and PTO (1 mmol, 266 mg) in 150 mL of a 1 : 1 (v/v) ethanol/glacial acetic acid solution was refluxed at 90 °C for 24 h with vigorous stirring. After cooling, the precipitates were collected by filtration and sequentially washed with hot acetic acid, deionized water and ethanol. The product was then vacuum-dried at 80 °C for 6 h to afford a green powder (yield: 84%).

Material characterization

HR-MS analysis was conducted on an Agilent 1100 LC/MSD Trap mass spectrometer. The elemental analysis for C, H, and N was conducted using a Vario Micro Cube analyzer, whereas the O content was measured with a Vario EL Cube analyzer. PXRD was collected on a PANalytical X'Pert3 powder diffractometer with Cu Kα radiation. SEM was performed using a ZEISS Gemini 300 field-emission microscope. FT-IR spectra were recorded using a Thermo Fisher Nicolet iS50 instrument. TGA was conducted on a PerkinElmer Pyris1 analyser under a N₂ atmosphere. XPS analysis was conducted on a Thermo Fisher Escalab 250Xi system. EPR spectra at X-band frequency were collected using a Bruker A300 spectrometer.

Electrochemical measurements

Before electrochemical testing, all active materials were vacuum-dried at 80 °C for 12 h. Electrode slurries were fabricated by homogenizing the active material, Super P and polyvinylidene fluoride (PVDF) binder (6 : 3 : 1 wt%) using N-methyl-2-pyrrolidone (NMP) as the dispersion solvent. The slurry was then uniformly coated onto pre-cleaned Al foil, followed by vacuum drying (80 °C, 6 h) to remove residual solvent. The CR2032 coin cells were fabricated in an Ar-filled glovebox employing Na metal as the counter electrode, polypropylene (Celgard2500) and glass fiber membranes (Whatman GF/B) as separators, and 1 M NaPF₆ in DME as the electrolyte. The CV and GCD profiles were measured at different temperatures using a BioLogic VMP3 workstation and LANHE-CT2001A test system (Wuhan, China), respectively. To evaluate low-temperature performance, the coin cells were first evaluated over 10 cycles at 0.1 A g⁻¹ at 25 °C, followed by long cycling at 0.1 A g⁻¹ at −40 °C.

Computational method

The molecular structure optimization, single point energy and electrostatic potential (ESP) calculation were performed with the Gaussian 16 program using B3LYP-D3 functional and 6-31+g(d) basis set. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions were visualized using Multiwfn program and Visual Molecular Dynamics (VMD) software.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Experimental details and electrochemical performance. See DOI: https://doi.org/10.1039/d5ta04805b.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22301227), the Open and Innovation Fund of Hubei Three Gorges Laboratory (SC240008), the Science Foundation of Wuhan Institute of Technology (No. 23QD01) and Graduate Innovative Fund of Wuhan Institute of Technology (CX2024007).

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Footnote

† These authors contributed equally.

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