Two-dimensional conductive mesopore engineering of ultrahigh content covalent sulfur-doped carbon for superior sodium storage

Abstract

Ultrahigh content (>20 wt%) covalent sulfur-doped carbon (USC) materials exhibit significant potential as cathode materials for sodium–sulfur batteries. Despite sulfur-based redox reactions offering high capacity, their reaction kinetics is limited by poor activity of robust covalent C–S bonds and slow ion and electron transport in carbon matrices. Here, we report a novel 2D mesoporous USC vertically grown on Ti₃C₂ nanosheet (MesoUSC@Ti₃C₂) heterostructure, which shows a 2D hexagonal structure, a high surface area (∼256 m² g⁻¹) and abundant covalent sulfur content (∼26.2 wt% in USC). Spectroscopic characterization studies and kinetics analysis reveal that the 2D conductive mesopore engineering strategy efficiently activates C–S bonds, and accelerates ion and electron transfer, endowing USC with high capacity sodium ion storage and pseudocapacitive-like behavior. As a result, the obtained MesoUSC@Ti₃C₂ delivers a high capacity of 950 mA h g⁻¹ at 0.1 A g⁻¹ and a superior rate performance of 463 mA h g⁻¹ at 5 A g⁻¹. This work opens up new ways to promote the practical applications of sulfur-doped carbon-based materials for advanced energy storage systems.

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Fanxing Bu

Fanxing Bu is an associate professor in the School of Cultural Heritage and Information Management at Shanghai University, which he joined in 2021. He received his PhD from the East China Normal University in 2016. He then worked as a postdoctoral fellow at Fudan University in the Yuxi Xu group and Dongyuan Zhao group. His research interests include tailored design of mesoporous MOFs, COFs, 2D materials and cementitious materials for energy and heritage conservation applications. He has authored more than 50 scientific papers in J. Am. Chem. Soc., Angew. Chem. Int. Ed., Adv. Funct. Mater., ACS Nano, etc.

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Introduction

Sodium-ion batteries have been recognized as some of the most promising candidates for next-generation energy storage systems due to abundant sodium resources and their similar fabrication procedures to those of commercial lithium-ion batteries.^1,2 Among them, sodium–sulfur batteries are prospective candidates for high energy and large-scale applications,^3–5 which can deliver high theoretical energy density of 1274 W h kg⁻¹ corresponding to the final discharge product of Na₂S. However, sodium–sulfur batteries generally suffer from low reversible capacity and poor cycling performance. The unsatisfactory electronic conductivity and sluggish reaction kinetics lead to the insufficient utilization of sulfur and limited capacity. The poor cycling stability is caused by the dissolution and shuttle effect of polysulfide intermediates. The most common strategy to address the above issues is encapsulating sulfur into carbon matrices,^6,7 which can improve the electrochemical performance to a certain degree by constructing a conductive network. However, dissolution and shuttling of the polysulfide have not been averted.

Recently, novel ultrahigh level (>20 wt%) sulfur-doped carbon (USC) materials have been developed to improve cycling stability. After low-temperature sulfidation treatment, sulfur can be incorporated into the carbon matrix network through covalent bonding.⁸ For example, 1,4,5,8-naphthalenetetracarboxylic dianhydride⁹ and 3-aminophenolformaldehyde (3-AF) resin¹⁰ are pyrolyzed at low-temperature (<500 °C) in the presence of sulfur to obtain USC with a high sulfur content of 26.9 wt% and 32 wt%, respectively. The covalent C–S structure ensures that sulfur atoms can be tightly connected with carbon atoms and converted to short-chain sulfide during redox reactions, which effectively alleviates the shuttle effect. However, the low-temperature sulfidation treatment often produces disordered carbons, degrading conductivity. Meanwhile, the strong covalent bonds lower the reactivity of C–S species. Additionally, the low surface to volume ratio decreases the accessibility of active sites. These factors synergistically deteriorate reaction kinetics. In this regard, elaborate structural design is one potential solution to address these issues. Among various advantageous architectures, two-dimensional (2D) porous nanosheets,^11,12 especially 2D conductive mesoporous heterostructures, are prominent due to their merits in synergistically resolving charge and ion transport problems.^13–17 Nevertheless, integrating USC into 2D conductive mesoporous heterostructures has been rarely reported, and the underlying structure–performance relationships urgently need to be revealed.

Here, we develop a novel 2D mesoporous USC vertically grown on Ti₃C₂ nanosheet (MesoUSC@Ti₃C₂) heterostructure for superior sodium ion storage, fabricated through an inorganic salt-mediated interfacial self-assembly and sulfidation strategy. The obtained MesoUSC@Ti₃C₂ heterostructure shows a 2D hexagonal structure, a high surface area (∼256 m² g⁻¹) and substantial sulfur doping (∼26.2 wt% in USC). The 2D conductive mesoporous heterostructure enables the fast ion and electron transportation, which promotes the activation of covalent sulfur species, offering exceptional capacity. Meanwhile, it greatly accelerates the redox reactions and endows USC with pseudocapacitive-like sodium ion storage behavior. As a result, the obtained MesoUSC@Ti₃C₂ delivers an ultrahigh capacity of 950 mA h g⁻¹ at 0.1 A g⁻¹ and an outstanding rate performance of 463 mA h g⁻¹ at 5 A g⁻¹.

Results and discussion

The synthesis of the MesoUSC@Ti₃C₂ heterostructure is schematically illustrated in Fig. 1a. First, amphiphilic Pluronic F127, dopamine (DA) and ammonium hydroxide were dissolved in the TMB/water system to form the TMB/F127/polydopamine (PDA) composite micelles.¹⁸ Subsequently, Ti₃C₂ nanosheets (Fig. S1†) and KCl were introduced to induce the interfacial self-assembly of the composite micelles. We fabricated MesoPDA@Ti₃C₂ in the absence of KCl, as shown in Fig. S2,† and the obtained sample exhibits non-uniform mesopore sizes and structures, along with a low degree of ordering, confirming that the introduction of KCl enhances the structural ordering of MesoPDA@Ti₃C₂. Thus, KCl plays a critical role in improving the ordering degree. After KCl is added to the above reaction system, the first-layered TMB/F127/PDA composite micelles on Ti₃C₂ tend to close-pack into a 2D hexagonal structure. In the following process, highly ordered mesoporous polydopamine@Ti₃C₂ (MesoPDA@Ti₃C₂) with vertical mesopores is formed by the epitaxial self-assembly (Fig. S3–S5†). Finally, the MesoUSC@Ti₃C₂ heterostructure is obtained through sulfidation treatment (600 °C, N₂ atmosphere).

Fig. 1 Synthesis and microstructural characterization studies of MesoUSC@Ti₃C₂. (a) Schematic illustration of the KCl-mediated interfacial self-assembly and the following sulfidation strategy for synthesizing MesoUSC@Ti₃C₂. (b) SEM and (c and d) TEM images, (e) HAADF-STEM image and corresponding elemental mapping results, (f) SAXS, (g) N₂ sorption isotherm and (h) pore size distribution curve of MesoUSC@Ti₃C₂.

Field-emission scanning electron microscopy (FESEM) images (Fig. 1b) show that the MesoUSC@Ti₃C₂ nanosheets inherit the 2D sheet-like morphology of the parent Ti₃C₂ MXene. A large number of open mesopores with a diameter of about 6 nm are clearly observed on the surface of these nanosheets in transmission electron microscopy (TEM) images (Fig. 1c and d). Remarkably, the channels of mesopores with 2D hexagonal structures are vertically arranged on both sides of Ti₃C₂, forming mesoporous USC shells (30 nm, Fig. 1d). In addition, elemental mapping results reveal a homogeneous distribution of elements (including Ti, S, C, and N) in the whole heterostructure, as displayed in Fig. 1e. The small-angle X-ray scattering (SAXS) pattern confirms the highly ordered mesoporous structure of MesoUSC@Ti₃C₂, showing three peaks of a 2D hexagonal structure (Fig. 1f). N₂ sorption results (Fig. 1g) manifest that MesoUSC@Ti₃C₂ possesses a Brunauer–Emmett–Teller (BET) specific surface area of 256 m² g⁻¹. Meanwhile, the pore size distribution obtained by the Barrett–Joyner–Halenda (BJH) model centers at 5.8 nm, which is in agreement with the SEM and TEM results (Fig. 1h).

The X-ray diffraction (XRD) pattern of MesoUSC@Ti₃C₂ reveals the absence of elemental sulfur, indicating that sulfur mainly bonds covalently with the carbon network (Fig. 2a). Moreover, titanium sulfide is not observed (Fig. S6–S11†). Confirmatory evidence was obtained through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The high-resolution XPS spectrum of S 2p demonstrates that the main forms of sulfur are C–S and C–SO_x (x = 2–4) groups, verifying that the interactions between sulfur and carbon are covalent bonds (Fig. 2b).^19,20 Furthermore, Raman spectroscopy displays one broad peak corresponding to the C–S bond located at 375 cm⁻¹, and no obvious peaks corresponding to S–S bonds are observed.^21,22 The high value of the I_D/I_G ratio (1.1) also proves the existence of large amounts of defective sp³ carbon in MesoUSC@Ti₃C₂. The time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to investigate the detailed bonding information of sulfur between carbon networks. USC shows intense CNS⁻ (m/z = 58), C₄S⁻ (m/z = 80), C₃NS⁻ (m/z = 82), and C₅NS⁻ (m/z = 106) signals and weak S₂⁻ (m/z = 64) and S₃⁻ (m/z = 96) peaks (Fig. 2d). These results indicate that S mainly bonds to carbon atoms connected with N and O atoms and a small fraction sulfur atoms form short-chain C–S_x–C (x = 2–3).^23,24 Based on the above results, the schematic illustration of molecular structure transformation after sulfidation is shown in Fig. 2e. The sulfur content in USC is 26.2 wt% according to elemental analysis (Table S1†).

Fig. 2 Molecular structural characterization studies of MesoUSC: (a) XRD pattern, (b) S 2p XPS spectrum, and (c) Raman spectrum of MesoUSC@Ti₃C₂. (d) ToF-SIMS spectrum of pure USC. (e) Schematic illustration of the molecular structure transformation after sulfidation.

We next evaluated the sodium ion storage performance of MesoUSC@Ti₃C₂ by using a 2016-type coin cell within a potential window between 0.01 V and 3 V. At the same time, pure Ti₃C₂ MXene (Fig. S1†) and spherical MesoUSC (Fig. S12†) are employed as the counterparts. At a current density of 0.1 A g⁻¹, MesoUSC@Ti₃C₂ delivers a high initial discharge capacity of 950 mA h g⁻¹ with a Coulombic efficiency of 78%, which is comparable to that of MesoUSC (957 mA h g⁻¹ and 74%) and higher than that of pure Ti₃C₂ MXene (249 mA h g⁻¹ and 43%, see Fig. 3a and S13†). The charge–discharge curves (Fig. 3b) and CV curves (Fig. S14†) reveal that the multi-step redox reactions between the covalent sulfur with sodium contribute to its high capacity. Interestingly, the capacities of USC-based electrodes exhibit slight fading in the first 5 cycles and then gradually increase, which originates from the activation processes of covalently bonded sulfur.^25–27 After 40 cycles, the capacity of USC@Ti₃C₂ increases to 943 mA h g⁻¹, much higher than that of MesoUSC (699 mA h g⁻¹), indicating that the 2D conductive mesoporous heterostructure greatly facilitates the activation process. Meanwhile, MesoUSC@Ti₃C₂ also exhibits excellent rate performance (Fig. 3c). At a high current density of 5 A g⁻¹, MesoUSC@Ti₃C₂ still retains a high capacity of 463 mA h g⁻¹, which is much higher than that of MesoUSC (278 mA h g⁻¹) and pure Ti₃C₂ MXene (Fig. S15†) and superior to those of previously reported sulfur-doped carbon-based materials (Fig. 3d). Moreover, MesoUSC@Ti₃C₂ also displays long-term cycling stability with a high discharge capacity of 476 mA h g⁻¹ after 250 cycles at 5 A g⁻¹ (Fig. 3e).

Fig. 3 Sodium ion storage performance of MesoUSC@Ti₃C₂. (a) Cycling performance at 0.1 A g⁻¹, (b) charge/discharge curves at 0.1 A g⁻¹, and (c) rate performance of MesoUSC@Ti₃C₂ and MesoUSC. (d) Comparison of capacity vs. current density to other sulfur-doped carbon-based materials. (e) Cycling performance of MesoUSC@Ti₃C₂ and MesoUSC at 5 A g⁻¹.

To gain further insight into the ultrahigh capacity of MesoUSC@Ti₃C₂, we investigated its electrochemical reaction mechanism by XPS analysis of the fully discharged/charged products in the first cycle. At the fully discharged state of 0.01 V (Fig. 4a), the S 2p peaks assigned to C–S bonds largely decrease while several new peaks related to Na₂S (162.5 and 168.9 eV), sulfates and thiosulfate species (166.7 and 170 eV) appear.^25,28 When recharged to 3 V, most C–S bonds recovered, suggesting a reversible sulfur-involved redox reaction. At the same time, new peaks corresponding to C=O (286.8 eV), O–C=O (288.5 eV) and CO₃²⁻ (289.5 eV) appeared in C 1s spectra (Fig. 4b) during the discharge process, indicating the formation of a solid electrolyte interface (SEI).²⁹ The above results indicate that the as-prepared USC stores sodium ions by sulfur-involved redox reactions (Fig. 4c) in addition to the adsorption–insertion mechanism of traditional carbonaceous materials, thus delivering the ultrahigh capacity.

Fig. 4 Sodium ion storage mechanism and kinetics. (a) S 2p and (b) C 1s XPS spectra of MesoUSC@Ti₃C₂ before and after the first cycle. (c) Schematic illustration of the Na⁺ storage mechanism. (d) EIS plots. (e) CV curves of MesoUSC@Ti₃C₂ at different scan rates. (f) Contribution ratio of the surface-controlled and diffusion-controlled charge versus scan rate. (g) Schematic model of ion/electron transport processes in MesoUSC@Ti₃C₂ and MesoUSC.

To understand the outstanding rate performance of MesoUSC@Ti₃C₂, its reaction kinetics was evaluated by electrochemical impedance spectroscopy (EIS) analysis and the CV test. MesoUSC@Ti₃C₂ demonstrates a charge-transfer resistance of 10.9 Ω (Fig. 4d), which is comparable to that of Ti₃C₂ (6.2 Ω, Fig. S16†), and six times lower than that of MesoUSC (66.5 Ω). This suggests that the combination of Ti₃C₂ with MesoUSC efficiently enhances the electron transfer. Furthermore, the specific charge storage behaviour is qualitatively analyzed by separating the current response into diffusion-controlled and capacitive contributions using the equation: i(V) = k₁ν + k₂ν^1/2 (Fig. 4e). The normalized contribution ratio of surface-controlled capacity (the red zone) increases from 74% to 92%, along with the increase of the scan rate from 0.2 to 5 mV s⁻¹ (Fig. 4f). The high capacitive contribution demonstrates that the charge storage in MesoUSC@Ti₃C₂ is dominated by the capacitive behaviour.³⁰ Evidently, the 2D conductive mesoporous heterostructure endows MesoUSC@Ti₃C₂ with fast reaction kinetics (Fig. 4g). Initially, the incorporation of Ti₃C₂ MXene improves the conductivity and enables rapid electron transfer. Subsequently, the vertical mesopores serve as unconstrained ion channels and offer fast ion transport. They collectively facilitate the sufficient activation of covalent sulfur and fast sodium ion storage behaviour.

Conclusions

In summary, we report the construction of a novel MesoUSC@Ti₃C₂ heterostructure via an inorganic salt-mediated interfacial self-assembly and sulfidation strategy for superior sodium storage. The obtained MesoUSC@Ti₃C₂ heterostructure with vertical mesoporous channels shows a 2D hexagonal structure, a high surface area (∼256 m² g⁻¹) and abundant covalent sulfur content (∼26.2 wt% in USC). Spectroscopic characterization studies and kinetic analysis demonstrate that the 2D conductive mesoporous heterostructure offers rapid electron and ion transport channels, greatly accelerating the reaction kinetics. This not only promotes the activation of covalent sulfur to offer ultrahigh capacity but also endows MesoUSC@Ti₃C₂ with pseudocapacitive-like sodium storage behavior. Consequently, the newly designed MesoUSC@Ti₃C₂ delivers a high reversible capacity of 950 mA h g⁻¹ at 0.1 A g⁻¹ and good rate performance (463 mA h g⁻¹ at 5 A g⁻¹). This work drives the practical development of high-performance sodium–sulfur batteries.

Data availability

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the Project of Key Laboratory of Silicate Cultural Relics Conservation (Shanghai University), Ministry of Education (No. SCRC2023ZZ03ZD), the National Natural Science Foundation of China (52225204), the Innovation Program of Shanghai Municipal Education Commission (2021-01-07-00-03-E00109), the Natural Science Foundation of Shanghai (23ZR1479200), the “Shuguang Program” supported by the Shanghai Education Development Foundation and Shanghai Municipal Education Commission (20SG33), the Fundamental Research Funds for the Central Universities (2232024Y-01), and the DHU Distinguished Young Professor Program (LZA2022001).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01526j

‡ Jie He, Zhihao Sun and Lei Huang contributed equally to this work.

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