A dual-template synergistic assembly strategy towards the synthesis of extra-small nitrogen-doped mesoporous carbon nanospheres with large pores†

Caicheng Song‡ ^ab, Yiwen Guo‡ ^c, Tianwei Wang ^a, Kun Liu ^bc, Pin-Yi Zhao ^bde, Ying Liu ^b, He Huang ^a, Rongwen Lu *^a and Shufen Zhang ^a
aState Key Laboratory of Fine Chemicals, Frontiers Science Center for Smart Materials, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China. E-mail: lurw@dlut.edu.cn
^bSINOPEC, Dalian Res Inst Petr & Petrochem Co. Ltd, 96 Nankai St, Dalian 116045, P. R. China
^cInstitute of Materials and Technology, Dalian Maritime University, Dalian 116026, China
^dInstitute for Materials Discovery, University College London, WC1E 7JE, UK
^eDepartment of Chemistry, University College London, WC1H 0AJ, UK

First published on 27th June 2024

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1. Introduction

Mesoporous nanomaterials characterized by high specific surface area, large pore volume, abundant mesostructures, nanoscale skeleton, and nanoconfined effects have attracted widespread attention.^1–3 In particular, mesoporous carbonaceous nanomaterials have fascinating features, including low density, lightness, conductivity, biocompatibility, chemical inertness and thermal stability, making them ideal candidates for a wide range of applications in catalyst supports, electrode materials, drug carriers, and gas/liquid adsorbents.^4–7 In addition, introducing heteroatoms, such as nitrogen, into carbon frameworks can effectively provide or enhance the electronic and chemical properties of carbon nanomaterials, thereby improving their application performance.^8–10

So far, various advanced approaches have jointly driven the vigorous development of the morphology and pore structure of mesoporous carbon materials. In particular, their nanoscale spherical morphology can reduce viscosity effects and shorten the mass transfer path for mass transport.^11,12 Large mesopores (3–20 nm) can facilitate diffusion, transport, and loading of large guest molecules.¹³ At present, the large particle size colloidal silica-hard template method, the high-molecular-weight amphiphilic block copolymer-soft template method and the emulsion-induced interface assembly approach have been applied to obtain large-pore mesoporous carbon nanospheres.^14–16 Although small particle sizes are also considered, research has mainly concentrated on particles larger than 100 nm. Research on mesoporous carbon nanospheres with smaller particle sizes, such as 50 nm, and large pore sizes is scarce, which unquestionably limits the understanding of the physicochemical properties and performance of ultra-small particle size mesoporous carbon nanospheres.

Melamine, a long-standing industrial product with abundant nitrogen content, can be employed as nitrogen and carbon sources for synthesizing nitrogen-doped mesoporous carbon materials. Currently, the widely utilized melamine is melamine–formaldehyde resin (MF) synthesized by continuous condensation of melamine and formaldehyde.^17–19 The condensation process has high reactivity and is carried out quickly in random directions, thus reducing the controllability of product morphology. Besides, MF lacks a hydrogen bonding site, and its self-assembly capability is poor, which greatly limits its application in synthesizing nitrogen-doped mesoporous carbon via the assembly strategy.^12,20 Faced with this dilemma, innovating the polymerization route for melamine utilization is undoubtedly a smart choice, but it is hard to execute.

Herein, mesoporous carbon nanospheres with ultra-small particle size, large pore size and abundant nitrogen content were facilely synthesized by the dual-template synergistic assembly strategy. Based on the synergistic effect of the combination of surfactants, commercial sodium oleate (anionic surfactant) and triblock copolymer-P123 (nonionic surfactant) were used as double templates to construct nanomicelles. A novel melamine-based polymer precursor with self-assembly capability was prepared through the constructed Schiff base chemical reaction path. The nanomicelles were co-assembled with the melamine-based oligomers to synthesize nitrogen-doped mesoporous carbon nanospheres (NMeCS). The particle size of NMeCS fluctuates between 55 and 60 nm with a pore size of ∼5.5 nm and a nitrogen content of 15.7 wt%, showing great potential for lithium-ion batteries.

2. Experimental section

2.1 Chemicals

Melamine (AR, 99%) and sodium dodecylbenzenesulfonate (95%, SDBS) were purchased from Damao Chemical Reagent Factory (Tianjin, China). 3,5-Diaminobenzoic acid (98%, 3,5-DA) was supplied by Energy Chemical (Shanghai, China). 2,4-Diaminobenzenesulfonic acid (98%, 2,4-DA), sodium oleate (CP, SO), sodium laurate (98%, SL), sodium laurylsulfonate (AR, 98%, SLS) and sodium stearate (96%, SS) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Formaldehyde solution (37–40 wt%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Pluronic P123 and Pluronic F127 were purchased from Sigma-Aldrich. All reagents were used directly without additional purification, and deionized water was used in all the experiments.

2.2 Synthesis of nitrogen-doped mesoporous polymer nanospheres

0.020 g (0.066 mmol) of sodium oleate (SO) and 0.060 g (0.010 mmol) of P123 were dissolved in 20 mL of deionized water and stirred vigorously for at least 3 h, and the obtained solution was denoted as solution A. 0.504 g (4.000 mmol) of melamine and 0.091 g (0.600 mmol) of 3,5-diaminobenzoic acid were dissolved in 180 mL of deionized water at 30 °C, and the obtained solution was denoted as solution B. Subsequently, solution A was completely transferred to solution B and stabilized for 60 min at 30 °C and 500 rpm. Then, the above conditions were kept unchanged, 4 mL of formaldehyde solution was added to trigger the polymerization reaction, and then the reaction was continued for 2 h to obtain a purplish-grey turbid liquid. The obtained liquid was separated for 20 min at a rate of 10000 rpm and washed three times to obtain a purplish-grey solid. The obtained solids were dried in an oven at 65 °C for 12 h to obtain nitrogen-doped mesoporous polymer nanospheres, which were denoted as NMePS.

2.3 Synthesis of nitrogen-doped mesoporous carbon nanospheres

The NMePS were ground into a solid powder and spread evenly in a long crucible. Then, the crucible filled with samples was transferred to a tubular furnace, and the temperature of the tubular furnace was raised to 100 °C at a rate of 10 °C min⁻¹ under a nitrogen atmosphere. Subsequently, the temperature was increased to 350 °C at a rate of 1 °C min⁻¹ and the pyrolysis was continued under these conditions for 2 h to remove the unstable species and residual templates. Next, the temperature was raised to 700 °C and further carbonized at this temperature for 2 h. After natural cooling to room temperature, a black carbon powder of nitrogen-doped mesoporous carbon nanospheres was collected and denoted as NMeCS.

3. Results and discussion

Based on the synergistic effect of surfactant compounding, extra-small mesoporous carbon nanospheres with a large pore size and high nitrogen content were synthesized efficiently by the dual-template synergistic assembly strategy, as illustrated in Scheme 1. It is well known that the combination of surfactants can significantly reduce the critical micelle concentration (cmc) and regulate the template function of micelles.^21,22 As a strong base and weak acid type long-chain fatty acid salt anionic surfactant, SO can get partially hydrolysed into oleic acid in aqueous solution with pH ∼ 4–8.²³ The constructed system is acidic with a pH of ∼5.73 due to the presence of 3,5-diaminobenzoic acid. The usage of SO is only ∼0.33 mM (relatively dilute); under these conditions, oleic acid (C₁₇H₃₃COOH) and oleate ions (C₁₇H₃₃COO⁻) coexist at the air/water interface and the aqueous solution of the system.²⁴ After adding P123, the hydrophobic groups of oleic acid and oleate ions were embedded into the hydrophobic groups of P123 based on hydrophobic interactions, forming nanomicelles (nanoemulsions) with oleic acid as the core (SO–P123).^21,25 With the addition of formaldehyde solution, the formation of melamine-based oligomers (MP) was triggered, and stable MP–SO–P123 composite micelles were formed based on the Coulomb force and hydrogen bonding interactions with nanomicelles. As MP continued to undergo co-polymerization, MP–SO–P123 continued to aggregate and self-assemble into polymer nanospheres due to the decrease of surface energy, thereby obtaining nitrogen-doped mesoporous polymer nanospheres (NMePS). Subsequently, nitrogen-doped mesoporous carbon nanospheres (NMeCS) were obtained after carbonization and pyrolysis under a nitrogen atmosphere.

The NMePS obtained exhibit a typical spherical morphology with a rough surface and particle sizes below 100 nm (Fig. S1a and b†). The corresponding particle size distribution is shown in Fig. S1c,† with an average particle size mainly distributed at around 65 nm. Large pores in NMePS can be observed from TEM images, indicating that they are a typical mesoporous material (Fig. 1a–c). According to statistics, the mesoporous pore size of NMePS is mostly distributed between 6 and 7 nm (as illustrated in Fig. 1a). The relatively large mesoporous size can provide sufficient shrinkage margin for subsequent pyrolysis carbonization to prepare nitrogen-doped mesoporous carbon nanospheres. The STEM image of NMePS further reveals their mesoporous structural features (Fig. 1d). In addition, the elemental distribution image shows that the distribution densities of C and N atoms in NMePS are basically the same (Fig. 1e and f), indicating that N atoms are evenly distributed in the sample framework. Furthermore, the elemental analysis results showed that the carbon and nitrogen contents of NMePS were around 46.3 wt% and 30.9 wt%, respectively, further proving the high nitrogen content doping characteristics of NMePS.

Several typical absorption peaks of functional groups can be found in the FTIR spectra of NMePS (Fig. S2a†). The absorption peaks at 1548 cm⁻¹ and 811 cm⁻¹ are attributed to the CN stretching vibration and bending vibration peaks in the triazine ring, indicating the presence of melamine in the polymer framework. The absorption peaks near 1506 cm⁻¹ and 1341 cm⁻¹ are the characteristic bands of the aromatic skeleton structure and the C–N stretching vibration peaks of the aromatic carbon, respectively, corresponding to 3,5-DA in the polymer framework. The absorption peaks at 3330 cm⁻¹ and 1105 cm⁻¹ belong to the N–H stretching vibration peak of a newly generated secondary amine and the (alkyl) C–N stretching vibration peak of a newly formed chemical bond, respectively. The absorption peaks near 2926 cm⁻¹ and 2855 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of methylene, respectively, and they still exist in the polymer (NPS) prepared without using SO-P123, indicating the formation of methylene (–CH₂–) in the polymer framework structure (Fig. S2b†). These newly generated functional groups may be generated by adding the primary amine group of melamine to the imine bond of Schiff base intermediates formed between 3,5-DA and formaldehyde.

¹³C (CP-MAS) NMR was used to further obtain information about the C atom in the NMePS framework to prove the polymerization mechanism. As shown in Fig. S3a,† the strong signal peak at 166 ppm in the ¹³C (CP-MAS) NMR spectrum of NMePS belongs to the resonance signal of the carbon atom in the triazine ring of melamine. The resonance peaks with chemical shifts at 158 ppm, 147 ppm, 108 ppm, and 104 ppm are attributed to the resonance signals of different carbon atoms in the aromatic ring of the 3,5-DA molecular structure, respectively. The peak with a chemical shift at around 174 ppm belongs to the resonance signal of the carboxyl carbon atom of 3,5-DA. The resonance signal with a chemical shift at around 75 ppm may belong to the newly generated methylene carbon atom (–CH₂–). Besides, the resonance signals between 0 and 50 ppm come from the SO-P123 used, as only when SO-P123 is used, signal peaks appear in this spectral region (Fig. S3b†). Based on the functional group information revealed by the FTIR spectrum and the carbon atom information provided by the ¹³C (CP-MAS) NMR spectrum, the polymerization reaction mechanism and polymer framework structure were speculated (Fig. S3a and S4†). Specifically, formaldehyde solution is introduced into the precursor system of the polymerization reaction and it preferentially reacts with highly reactive 3,5-DA to form Schiff base intermediates rich in imine (–CN–). Subsequently, the primary amine of melamine is added to the imine bond (–CN–) of the Schiff base intermediate and further crosslinked into a polymer framework. This result is consistent with the polymerization mechanism proposed in our previous report.^26,27

The key parameters involved in the synthesis process were investigated to fully understand the strategy. Firstly, the role of SO and P123 in the synthesis was studied. As shown in Fig. S5a and b,† when no surfactant was used in the synthesis, the prepared material appeared as typical polymer nanospheres (NPS), and no obvious mesoporous or other structures were observed in the TEM images, which proved that the SO–P123 composite surfactant was the template for the construction of mesoporous structures. When using SO alone as a template, the resulting material is an unevenly sized ellipsoid, and its TEM image does not show a clear mesoporous structure (Fig. S5c and d†). This result indicates an interaction between SO and the precursor in the system, namely the Coulomb interaction, which can affect the morphology of the material. In addition, it can be observed from Fig. S5c† that a small portion of the samples have hollow structures, which further illustrates that under acidic conditions, sodium oleate will partially hydrolyse into oleic acid and form micelles to guide the formation of this hollow structure.²⁸ As shown in Fig. S5e and f,† when only P123 is introduced as a template into the system, the obtained material has an imperfect spherical morphology, and the size of the related samples is much smaller than that of the samples, as shown in Fig. S5a–d.† This result indicates that P123 also has a controlling effect on the morphology of the samples. Besides, the internal structure of the material is uneven, indicating that P123 may play a decisive role in the pore structure of the material. There is also an interaction with the precursor, which may be based on the hydrogen bonding between the PEO end of P123 and melamine and 3,5-DA. These results further prove that the synthesis of NMePS is based on the synergistic effect between SO and P123.

Further investigation was conducted on the influence of the ratio of SO to P123 on the samples. When a small amount of P123 is introduced (SO:P123-2:1), sporadic mesoporous structures appear in the sample, indicating that the interaction between P123 and SO has begun (Fig. S6a†). However, fewer nanomicelles have been formed at this time, and the sample is still a chaotic ellipsoid dominated by SO as a template. When the mass of P123 continues to increase to the same as that of SO, the shape of the sample changes from ellipsoidal to nearly spherical (Fig. S6b†). Compared with the TEM images of Fig. S6a,† it was found that the particle size of the sample decreased, and the mesoporous structure of each polymer nanosphere increased. The results showed that the micelles formed by P123 in the system increased, and more oleic acid and oleate ions were embedded into the hydrophobic groups of P123 micelles, thus creating more composite nanomicelles, in which case the effect of the single SO template was weakened. When the mass ratio of SO and P123 was further adjusted to 1:2, the mesoporous structure in the sample increased. The particle shape completely changed to spherical, and the particle size further decreased (Fig. S6c†), indicating that the reaction system had taken composite nanomicelles as the dominant template. As shown in Fig. S6d–f,† when the usage of P123 is further increased to 3–5 times the mass of SO, the obtained sample is a typical mesoporous polymer nanosphere. The particle size and mesoporous structure of the sample showed no significant changes, meaning that the composite nanomicelles completely dominated the reaction system. This result further confirms that the nanomicelles constructed by the synergistic effect between SO and P123 act as the template for synthesizing mesoporous structures and indicates that the mass ratio of SO to P123 should not be less than 1:3.

The effect of 3,5-diaminobenzoic acid (3,5-DA) on the structure of the sample was also investigated. As shown in Fig. S7,† with an increase in the amount of 3,5-DA, the prepared mesoporous polymer gradually changed from spherical to irregular shape. The particle size of the sample gradually increased. However, the mesoporous structure of the sample did not change drastically, which may be due to the fact that an increase in the amount of 3,5-DA increased the polymerization rate. The root cause of this situation is the destruction of the balance of cooperative self-assembly between melamine-based oligomers and nanomicelles, which makes the nucleation and growth of the formed composite polymer uncontrollable, resulting in rapid accumulation of the polymer in a random direction. Besides, when 3,5-DA was replaced by 2,4-diaminobenzenesulfonic acid (2,4-DA) (pH ∼ 5.71), the particle size of the polymer nanospheres increased, and there were sparse mesoporous pores in the polymer nanospheres (Fig. S8†). The reason may be that the spatial structures of the melamine-based oligomers formed by 2,4-DA and 3,5-DA are different, and the polymerization rates of the two reactions are also different, resulting in a difference in the composite micelle self-assembly polymer. Finally, the types of dual-template agents were expanded. As shown in Fig. S9a,† when SO-F127 is used, the prepared material appears as polymer nanospheres containing mesoporous structures. As shown in Fig. S9b,† when SL-P123 is used as the template agent, the prepared material appears to be typical mesoporous polymer nanospheres, consistent with NMePS. When SS-P123 was used as the template, there were abundant mesoporous structures in the prepared polymer nanospheres (Fig. S9c and d†). Compared with NMePS, the particle size of the samples increased, and the mesoporous structures were more obvious. When SLS-P123 and SDBS-P123 were used as template agents, respectively, the obtained materials were polymer nanospheres, but only a few mesoporous structures existed in the polymers (Fig. S9e and f†). The results showed that amphiphilic block copolymer surfactants had a major influence on the mesoporous structure and morphology of the target products. The fatty acid salt-type anionic surfactant favours the designed mesoporous structure.

The thermogravimetric analysis (TGA) curve of NMePS in the 25–800 °C temperature range shows three distinct weight loss intervals (Fig. S10†). Below 250 °C, it is in a slow weight loss range with a weight loss rate of about 15%, which may correspond to the evaporated moisture in the polymer and the release of ammonia.¹² The weight loss rate accelerates at 250–420 °C, and the overall loss rate reaches ∼52%. Within this interval, chemical bond cleavage, sublimation of melamine and 3,5-DA, and evaporation of hydrogen cyanide, ammonia, and formaldehyde occur in the polymer.²⁹ In the interval above 420 °C, the weight loss rate of the sample slows down, and the frameworks further condensate with the decomposition of unstable species.³⁰ Finally, the overall mass retention rate is ∼25% at 800 °C. The TGA curves of polymers (NPS) prepared without surfactants are consistent with those of NMePS, but the weight loss rate is significantly lower than that of NMePS. This may be due to the improved mass and heat transfer ability of the mesoporous structure, which accelerates the pyrolysis rate and diffusion rate of unstable species.

SEM images of nitrogen-doped mesoporous carbon nanospheres (NMeCS) show that their excellent spherical morphology is still maintained without obvious breakage after the pyrolysis treatment of the corresponding NMePS (Fig. S11a and b†). The particle size distribution of NMeCS is 55–60 nm, smaller than that of the main particle size distribution of NMePS (65–70 nm), which is caused by the shrinkage of the polymer network skeleton during carbonization and pyrolysis (Fig. S11c†). TEM images revealed that NMeCS maintained abundant mesopores, and there was no significant change in mesoporous structure characteristics compared with NMePS (Fig. 2a–c). The STEM image more intuitively shows that NMeCS are typical mesoporous carbon nanospheres without obvious sintering break (Fig. 2d). In addition, the elemental (C and N) mapping images show that NMeCS maintain excellent nitrogen doping characteristics (Fig. 2e and f).

Nitrogen absorption–desorption measurements further revealed the pore properties of NMeCS at −196 °C. According to the IUPAC classification criteria, the nitrogen adsorption–desorption isotherms of NMeCS exhibit typical I/IV class characteristics (Fig. 3a). There is significant adsorption in the relatively low-pressure region (P/P₀ < 0.01), indicating the presence of a certain amount of micropores in the material structure (Fig. S12†), which is attributed to the decomposition of unstable species in the polymer components and the shrinkage of the polymer framework during the carbonization and pyrolysis processes. The hysteresis loop in the medium pressure section with a relative pressure of 0.40 < P/P₀ < 0.70 reflects the mesoporous structural characteristics of NMeCS. The hysteresis loop in the relatively high-pressure region (P/P₀ > 0.83) indicates the presence of large mesopores, which may be ascribed to interparticle stacking between the nanoparticles. According to the pore size distribution of the Barrett-Joyner-Halenda (BJH) method based on the isotherm of the adsorption branch of NMeCS, there is a narrow pore size distribution at ∼5.5 nm, which corresponds to the mesoporous structure of NMeCS (Fig. 3b). In addition, there is a wide pore size distribution at ∼30 nm, which belongs to the large mesopores generated by particle stacking. The Brunauer-Emmett-Teller (BET) specific surface area (S_BET) of NMeCS is 531 m² g⁻¹, the total pore volume (V_total) is 0.60 cm³ g⁻¹, and the mesoporous volume (V_Me) is 0.44 cm³ g⁻¹, with V_Me accounting for approximately 73% of V_total. The excellent mesoporous properties will endow the material with a high charge transfer rate and mass transfer ability. In addition, nitrogen absorption–desorption tests were performed on the carbon nanospheres (NCS) synthesized without using the SO–P123 template agent. As shown in Fig. 3c, the nitrogen absorption–desorption isotherms of NCS can be classified as typical type I, which indicates that there is a certain amount of adsorption in the relatively low-pressure section (P/P₀ < 0.01) corresponding to the micropores in the material. Only in the relatively high-pressure region (P/P₀ > 0.85), a hysteresis loop corresponding to the characteristics of stacked pores exists. In the BJH pore size distribution of NCS (Fig. 3d), there is only a distribution range belonging to stacked pores at ∼37 nm, further confirming the template effect of the SO–P123 dual-template on the synthesis of NMeCS.

XPS characterization is used to reveal the chemical information of NMeCS. The XPS spectrum of NMeCS shows three distinct peaks corresponding to the binding energies of C 1s at ∼285 eV, N 1s at ∼398 eV and O 1s at ∼532 eV,³¹ where the atomic percentages of C, N and O are 82.8 at%, 15 at% and 2.2 at%, respectively (Fig. S13a†). The asymmetric C 1s spectrum of NMeCS can be highly resolved as C–C/CC at 284.6 eV, C–N/C–O at 285.5 eV, CN at 286.9 eV, and O–CO at 289.8 eV, respectively (Fig. 4a).^32,33 The high-resolution N 1s spectrum can be deconvoluted into three peaks, including pyridinic N (398.2 eV), pyrrolic N (400.6 eV) and oxidized N (403.1 eV),^34,35 with corresponding percentage contents of ∼40.3%, ∼48.0% and ∼11.7%, respectively (Fig. 4b). It is revealed that the material has a high edge nitrogen content (pyridinic N/pyrrolic N, 88.3%). It has been established that pyridinic N and pyrrolic N can promote charge accumulation and ion diffusion and also have a strong affinity for lithium ions, providing deposition defect sites for lithium ions.^36–38 The high-resolution O 1s spectrum contains two peaks belonging to adsorbed oxygen (531.8 eV) and C–OH (533.1 eV), respectively (Fig. S13b†).³⁹ Moreover, elemental analysis shows that the mass percentage content of nitrogen elements in NMeCS is about 15.7 wt%, consistent with the XPS results. The XRD pattern of NMeCS exhibits two broad diffraction peaks near 2θ at 23.2° and 43.7°, corresponding to the (002) and (101) characteristic diffractions of amorphous carbon, respectively (Fig. 4c).⁴⁰ The Raman spectrum of NMeCS shows two broad peaks at chemical shifts at around 1350 cm⁻¹ and 1380 cm⁻¹, corresponding to the D band representing disordered carbon and the G band representing graphite carbon, respectively (Fig. 4b). The peak intensity ratio I_D/I_G of the D and G bands is 1.02, indicating a low degree of graphitization, mainly characterized by amorphous carbon.⁴¹

Nowadays, nitrogen-doped porous carbon materials as carbonaceous electrodes have been proved to have a gain effect in energy storage devices, such as lithium-ion batteries (LIBs), due to their excellent electrical conductivity and abundant active sites.^42–44 Therefore, NMeCS, which have unique physicochemical and structural properties, were applied to LIBs to investigate their application performance. Served as anode materials in LIBs, the NMeCS obtained the first discharge/charge capacities of 732.3 and 470.7 mA h g⁻¹, respectively, corresponding to a coulombic efficiency of 64.3% (Fig. 5a). The voltage plateau can be observed at around 0.95 V in the discharge curve, which is attributed to the formation of the solid electrolyte interfacial (SEI) layer. XPS analysis revealed that the main components of the SEI layer may include LiF, Li₂CO₃, LiOH, ROCOOLi, Li_xPF_y and Li_xPO_yF_z (Fig. S14†). After 100 cycles at 100 mA g⁻¹ (Fig. 5b), the reversible discharge specific capacity of the NMeCS electrode reaches 383.7 mA h g⁻¹ with almost 100% coulombic efficiency. The low initial coulombic efficiency may be attributed to the fact that the formation process of the SEI consumes more electrolytes. The rate performance is an essential index for evaluating the electrochemical performance of batteries. In Fig. 5c and d, the initial discharge capacities of the NMeCS electrode are 851.3, 432.8, 354.7, 299.0, 269.7, and 247.6 mA h g⁻¹ at current densities of 100, 200, 400, 600, 800, and 1000 mA g⁻¹, respectively. When the current density returns to 100 mA g⁻¹, the capacity can be restored to the original capacity. This indicates that the electrode possesses excellent rate capability and high reversibility. The Nyquist plot consists of a semicircle in the high-medium frequency region corresponding to the electrolyte/electrode resistance (R_e) and charge transfer resistance (R_ct) and a straight line tilted in the low frequency region corresponding to the Warburg resistance (Fig. S15†). This reveals that the fitted R_e and R_ct are about 3.1 and 98.6 Ω, respectively. In addition, the morphology and pore structure of NMeCS are basically maintained during the cycling process with good stability (Fig. S16†). As expected, like several reported carbon-based materials, NMeCS are also a potential candidate for LIBs (Table S1†).

4. Conclusion

In summary, nitrogen-doped mesoporous carbon nanospheres with a particle size of 55–60 nm, a pore size of 5.5 nm, and a nitrogen content of 15.7 wt% were efficiently synthesized through a dual-template synergistic assembly strategy. The synthetic approach is based on the synergistic effect of the combination of surfactants. Sodium oleate, one of the surfactants used, can be partially hydrolysed to oleate ions and oleic acid under acidic conditions and construct nanomicelles with P123 through hydrophobic interaction. Subsequently, nitrogen-doped mesoporous polymer nanospheres were prepared by co-assembly between the constructed nanomicelles and melamine-based oligomers. Finally, the target nitrogen-doped mesoporous carbon nanospheres were obtained after pyrolysis. The unique physical properties of NMeCS, such as a high specific surface area (531 m² g⁻¹), a large pore volume (0.60 cm³ g⁻¹), and large pores, endow them with efficient mass transport, coupled with their abundant nitrogen content, showing excellent performance in LIBs. With 100 cycles at a current density of 100 mA g⁻¹, NMeCS maintained a specific discharge capacity of 383.7 mA h g⁻¹ and a coulomb efficiency of almost 100%. This work not only broadens the particle size distribution range of mesoporous carbon nanomaterials, but also provides a reference for applying extra-small particle size and large-pore mesoporous carbon nanospheres in LIBs.

Data availability

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

Author contributions

Caicheng Song: investigation, data curation, formal analysis, methodology, writing – original draft, and writing – review and editing. Yiwen Guo: data curation, methodology and validation. Tianwei Wang: methodology and data curation. Kun Liu: data curation and formal analysis. Pin-Yi Zhao: formal analysis. Ying Liu: data curation. He Huang: data curation. Rongwen Lu: funding acquisition, conceptualization, resources, and writing – review and editing. Shufen Zhang: funding acquisition and resources.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22078044 and 22238002), the Fundamental Research Funds for the Central Universities (DUT22LAB610), the National Key Research and Development Program of China (2022YFB3806500), and the Dalian University of Technology (Dalian University of Technology Innovative Research Team, DUT2022TB10).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr01072h

‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2024