High-Efficiency Polysulfide Trapping with g-C₃N₄/CNT Hybrids for Superior Lithium-Sulfur Batteries

Abstract

Commercialization of lithium-sulfur (Li-S) batteries is critically hampered by the severe lithium polysulfide shuttle effect. Hence, designing multifunctional materials that synergistically provide physical confinement of polysulfides, chemical entrapment, and catalytic promotion is a viable route for improving Li-S battery performance. Herein, graphitic carbon nitride (g-C₃N₄) with abundant nitrogen atoms was used as the chemical adsorption material to realize a “physical-chemical” dual confinement for polysulfides. Furthermore, the integration of CNTs with g-C₃N₄ is intended to substantially enhance the conductivity of the cathode material. Consequently, the synthesized g-C₃N₄/CNT composite, which functions as an effective polysulfide immobilizer, significantly improved the cycling stability and discharge capacity of Li-S batteries. This enhancement can be attributed to its potent adsorption and catalytic activities. Li-S cells utilizing g-C₃N₄/CNT cathodes exhibit exceptional discharge capacity and notable rate capability. Specifically, after 100 cycles at 0.2 C, the discharge capacity was 701 mAh g⁻¹. Furthermore, even at a high rate of 2 C, a substantial capacity of 457 mAh g⁻¹ was retained.

1. Introduction

Since the utilization of lithium-ion batteries (LIBs) in portable electronic devices was established in the 1990s [1,2], tremendous efforts have been made to improve their energy density. However, its theoretical density is too low to satisfy the growing energy requirements [3]. Thus, research on high-energy-density battery systems has intensified in recent years [4,5].

Lithium-sulfur (Li-S) batteries are promising next-generation energy storage systems that have attracted significant interest due to their high theoretical specific capacity (1675 mAh g⁻¹), natural abundance of sulfur, and eco-friendliness [6,7,8]. Nevertheless, the commercialization of Li-S batteries faces significant barriers, primarily short cycle life and low practical energy density, attributable to three key factors: (i) the poor electrical conductivity of sulfur/Li₂S restricts charge/mass transport, limiting active material utilization; (ii) repeated ~80% volume fluctuations during cycling—caused by the density mismatch between S (2.07 g cm⁻³) and Li₂S (1.66 g cm⁻³)—induce electrode structural degradation and compromise safety; (iii) soluble lithium polysulfides (LiPSs) migrate to the anode, forming irreversible Li₂S₂/Li₂S deposits that reduce Coulombic efficiency [9,10]. While recent breakthroughs in cathode design and electrolyte engineering have shown promising pathways to mitigate these issues [11], challenges in scalable fabrication of high-performance systems persist.

To alleviate these issues, the further development of Li-S batteries has focused on investigating the physical confinement and chemical entrapment of LiPSs, as well as the catalytic acceleration of LiPS conversion, to fabricate long-lasting Li-S batteries. LiPS anchoring materials are regarded as an effective method to inhibit LiPS dissolution [12]. Multiple carbon-based composites—such as carbon nanotubes [13], graphene [14], and porous carbon architectures [15,16]—have been utilized to encapsulate sulfur in Li-S batteries, physically confining LiPSs through Van der Waals interactions. While these materials improve conductivity, their weak adsorption often fails to suppress long-term shuttle effects [17]. To enhance chemisorption, polar materials, including metal oxides (e.g., TiO₂ and MnO₂) [18,19], sulfides (e.g., CoS₂ and WS₂) [20,21], and single-atom catalysts [22], have shown promise. However, these materials typically exhibit limited conductivity or require complex synthesis routes.

However, single materials typically possess some fundamental drawbacks that prevent simultaneous further optimization towards higher conductivity and catalytic ability to satisfy practical applications in sulfur reactions. An alternative approach is to form hybrid materials with different components that combine the advantages of individual materials and compensate for their individual drawbacks to design better Li-S batteries. Wang’s group reported enhanced polysulfide immobilization and catalytic conversion through in situ growth of NiCo₂S₄ nanoparticles on reduced graphene oxide [23]. Lin and co-workers reported that the highly N-doped carbon/graphene hybrid could effectively suppress the cumbersome shuttle effect [24]. Despite recent progress in binary composites (e.g., conductive carbon/polar material hybrids), the simultaneous optimization of conductivity, adsorption capacity, and scalable fabrication remains challenging. This gap necessitates the development of rationally designed multifunctional hosts that integrate complementary properties using simple methods.

Graphitic C₃N₄ (g-C₃N₄) is a polar non-metal material like layered graphite. The dominant pyridinic nitrogen can serve as LiPS adsorption sites to immobilize and interact with LiPSs to accelerate the electrochemical conversion processes [25,26]. In addition, g-C₃N₄ exhibits low density, environmental compatibility, and ease of synthesis. These advantages establish it as a viable cathode material for long-cycle Li-S batteries, as demonstrated by Nazar’s group [27]. However, the intrinsically low electronic conductivity of g-C₃N₄ severely restricts charge transfer kinetics. For instance, Huangfu et al. demonstrated through four-point probe measurements that pristine g-C₃N₄ nanosheets exhibit conductivity values as low as 5.6 × 10⁻⁵ S cm⁻¹ [28], while Song et al. reported similarly suppressed conductivities (~1.3 × 10⁻³ S cm⁻¹) that impair sulfur utilization in Li-S batteries [29].

While g-C₃N₄ was initially proposed as a polysulfide anchor, its practical application remains hindered by its low conductivity and limited pore engineering. In this study, we design a conductive hybrid scaffold by integrating CNTs within a porous g-C₃N₄ matrix. This synergistic structure not only enhances charge transfer but also provides hierarchical pores for sulfur confinement and catalytic sites for accelerated conversion kinetics, addressing key limitations of single-component hosts. To address these limitations, we designed a hierarchical g-C₃N₄/CNT composite using a scalable two-step process. Unlike complex hybrid syntheses that require multi-step modifications [30], our approach directly couples conductive CNTs with nitrogen-rich g-C₃N₄. This leverages: (i) g-C₃N₄’s inherent polysulfide affinity via pyridinic N sites [27], (ii) CNT’s 3D conductive network for charge transfer, and (iii) synergistic catalysis at their interfaces—a feature not exploited in prior g-C₃N₄-based hosts [27]. The sustained capacity of 701 mAh g⁻¹ (g-C₃N₄/CNT) after 100 cycles at 0.2 C outperforms pure g-C₃N₄ cathodes (440 mAh g⁻¹) and competes favorably with contemporary hosts such as ZIF-8-derived MOFs (515 mAh g⁻¹) [31], VO₂-VS binary composites (589 mAh g⁻¹) [32], and NiCo₂S₄/graphene hybrids (650 mAh g⁻¹) [23]. Notably, our capacity retention at 2 C (457 mAh g⁻¹) exceeds that of typical MOF cathodes (300–400 mAh g⁻¹) and MoS₂-based systems (∼400 mAh g⁻¹) under equivalent conditions [6,33], while utilizing a low-cost, eco-friendly carbon/nitrogen architecture. In this composite, g-C₃N₄ provides abundant reaction sites for polysulfide capture and catalytic transformation, while the introduced CNTs form a conductive network that supports fast charge/mass transfer, which is confirmed by the CV results at different scanning rates. Consequently, the g-C₃N₄/CNT composite delivers synergistic enhancement in Li-S batteries, effectively suppressing polysulfide shuttling and boosting electrochemical performance. This synergy-driven design strategy enables practical Li-S batteries and provides a scalable paradigm for related energy storage systems, such as Na/K-S batteries.

2. Materials and Methods

The interconnected macroporous structure with thin, curled g-C₃N₄ nanosheets was synthesized via a two-step protocol: (1) 1.5 g melamine was dispersed in 90 mL deionized water under continuous stirring at 90 °C for 1 h; (2) the resultant mixture underwent hydrothermal treatment in a 150 mL Teflon-lined stainless steel autoclave (Xi’an Changyi Instrument Equipment Co., Ltd., Xi’an, China) at 185 °C for 24 h. The product was collected by filtration, washed, dried, and subsequently calcined at 520 °C for 4 h in an air atmosphere.

The g-C₃N₄/CNT composite was synthesized as follows: (1) 0.2 g g-C₃N₄ was dispersed in 150 mL deionized water via 60-min ultrasonic treatment to form a homogeneous suspension, ultrasonication was performed using a KQ-300DE instrument (Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China) at 40 kHz and 300 W for 60 min; (2) 0.14 g multilayered CNTs (XFNANO Materials Tech Co., Ltd., Nanjing, China, ~1 µm length, ~20 nm diameter) were introduced into the solution; (3) the mixture was magnetically stirred for 4 h followed by drying at 60 °C to yield the composite powder.

Preparation of sulfur cathodes. The S@g-C₃N₄/CNT composite was prepared using a traditional melt-diffusion approach by mixing g-C₃N₄/CNT and pure sulfur powder at a mass ratio of 1:3. Afterwards, the sample was ground and calcined at 155 °C in Ar for 12 h to obtain S@g-C₃N₄/CNT.

Physical characterization was performed as follows: Phase analysis was performed using X-ray diffraction (XRD, Bruker D8 Focus, Bruker Corporation, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å). Morphological and structural characterizations were performed using scanning electron microscopy (SEM, ZEISS GeminiSEM 500, Carl Zeiss AG, Oberkochen, Germany) and transmission electron microscopy (TEM, JEOL JEM-2100F, JEOL Ltd., Akishima, Tokyo). Surface properties, including specific surface area and pore size distribution, were determined through nitrogen adsorption-desorption measurements at 77 K (BET, V-Sorb 2800P system, Gold APP Instruments Corporation China, Beijing, China). Chemical state analysis was conducted via X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe, ULVAC-PHI Incorporated, Kanagawa, Japan), and thermal stability was evaluated by thermogravimetric analysis (TGA, TA Q600 thermobalance, TA Instruments, New Castle, DE, USA).

Electrochemical characterization was conducted using CR2032 coin cells with S@g-C₃N₄/CNT cathodes. The cathode slurry was formulated by homogenizing active material, Super P conductive carbon, and PVDF binder at an 8:1:1 mass ratio in N-methyl-2-pyrrolidone solvent, followed by doctor-blade coating onto carbon-coated aluminum foil. After drying at 60 °C, electrodes were punched into 9.0 mm disks. Cells employed 1.0 M LiTFSI and 0.1 M LiNO₃ in DOL/DME (1:1 v/v) electrolyte. Electrochemical evaluation comprised cyclic voltammetry (CHI-660C workstation, CH Instruments, Inc., Shanghai, China, 0.1 mV s⁻¹ scan rate), electrochemical impedance spectroscopy (Versa STAT4 system, AMETEK Inc., Berwyn, PA, USA, 100 kHz–0.01 Hz), and galvanostatic cycling (Neware BTS4000 tester, Neware Technology Ltd., Shenzhen, China, 1.7–2.8 V voltage window).

Symmetric cells were constructed with identical g-C₃N₄/CNT (or pristine g-C₃N₄) electrodes as both working and counter electrodes. The Li₂S₆ electrolyte (0.1 M) was prepared by stoichiometrically reacting Li₂S and sulfur (1:5 mass ratio) in a DOL/DME (1:1 v/v) solvent. Subsequently, 40 µL of the polysulfide solution was injected into the cell. Specifically, the E/S ratio used in coin cell assembly was ~10 µL mg⁻¹. The areal mass loading of sulfur in the cathodes was ~2 mg cm⁻², unless otherwise stated. For all electrochemical measurements, pure lithium metal foil was used as both counter and reference electrodes, ensuring an excess lithium supply; therefore, the N/P ratio was not applicable in this configuration.

Linear Sweep Voltammetry (LSV): A 9:1 (w/w) mixture of g-C₃N₄/CNT (or pristine g-C₃N₄) and PVDF binder in NMP was homogenized and drop-cast onto a glassy carbon working electrode (0.07 cm²). Measurements employed 0.1 M Li₂S in methanol electrolyte. Reference electrodes were test-specific: Ag/AgCl for LSV in methanol, none for symmetric cells, and Li metal for full-cell CVs; all potentials were referenced to their native scales.

Li₂S Nucleation Testing: Cells configured with g-C₃N₄/CNT (or pristine g-C₃N₄) cathodes and lithium foil anodes used 0.25 M Li₂S₈ in a tetraethylene glycol dimethyl ether (TEGDME) electrolyte. After galvanostatic discharge to 2.06 V, potentiostatic deposition was performed at 2.05 V.

Density functional theory (DFT) calculations employed the projector-augmented wave method and a plane-wave basis set within the Vienna Ab Initio Simulation Package (VASP) to investigate Li₂S₄ adsorption on g-C₃N₄, g-C₃N₄/CNT, and CNT substrates [34]. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation was used to describe the exchange-correlation effects, with Grimme’s D3 dispersion correction applied for adsorption energy (E_ads) calculations [35]. Based on the XRD and TEM results, the C₆N₇ ring served as the structural unit for the g-C₃N₄ lattice, defined by the following parameters: a = 12.52 Å, b = 12.71 Å, c = 24.51 Å, α = 90°, β = 90°, and γ = 120°. Multi-walled carbon nanotubes were used to construct slab models for both g-C₃N₄/CNT and CNT, adopting the same lattice parameters as those of g-C₃N₄. Identical Li₂S₄ molecule configurations were bonded to each of the surface models. All models incorporated a 15 Å vacuum layer that was perpendicular to the surface. Computational settings included a plane-wave cutoff energy of 400 eV and a 2 × 2 × 1 Monkhorst-Pack k-point mesh. Self-consistent field iterations converged at 1 × 10⁻⁴ eV, and ionic relaxation ceased when maximal atomic forces fell below 0.02 eV Å⁻¹. Dipole corrections were applied to all slabs. Adsorption energy was calculated using the following formula:

$$E_{\mathrm{ads}}=E_{\mathrm{LS}}+E_{\mathrm{sur}}-E_{{\mathrm{LS}-}_{\mathrm{sur}}}$$

(1)

where E_LS and Eₛᵤᵣ represent the total energies of the isolated Li₂S₄ molecule and the clean substrate surface (g-C₃N₄, g-C₃N₄/CNT, or CNT), respectively, and $E_{{\mathrm{LS}-}_{\mathrm{sur}}}$ is the total energy of the Li₂S₄-adsorbed system. In this study, a positive E_ads value indicates a stable adsorption configuration relative to the separated adsorbate and substrate.

3. Results and Discussions

XRD spectra in Figure 1 characterize CNT, g-C₃N₄, g-C₃N₄/CNT composite, and S@g-C₃N₄/CNT. The commercial CNT exhibits characteristic (002) and (100) reflections at 26.2° and ~43°, confirming graphitic crystallinity [36]. The two peaks located at 2θ of about 27.8° and 13.1° in g-C₃N₄ correspond to (002) and (100) planes [37]. The XRD profile of g-C₃N₄/CNT composites predominantly reflects g-C₃N₄ characteristics, with CNT diffraction signatures masked by their low loading concentration. The g-C₃N₄ (002) peak shifts from 27.8° to 28.1° in the composite, corresponding to a d-spacing reduction from 0.321 nm to 0.317 nm (Table S3). This compression reflects strong interfacial coupling with CNTs, which facilitates charge transfer between components. Concurrently, crystallite size decreases from 18.2 nm to 15.7 nm, indicating CNT-induced fragmentation increases the number of accessible active sites. Characteristic peaks of orthorhombic sulfur (α-S₈, JCPDS 08-0247) at 2θ = 23.1° (222), 25.9° (026), and 26.8° (206) are clearly identified in the S@g-C₃N₄/CNT pattern, confirming successful sulfur infusion into the composite host [38]. The absence of sulfur peaks in g-C₃N₄/CNT further validates that sulfur loading occurred post-scaffold synthesis.

Figure 2a shows an interconnected macroporous architecture with a thin, curled nanosheet morphology in pristine g-C₃N₄. Upon CNT integration (Figure 2b,c), the composite evolves into a sponge-like matrix in which the g-C₃N₄ and CNT phases interpenetrate homogeneously. This hierarchical porosity accommodates sulfur infiltration while mitigating volume expansion during cycling. Concurrently, the percolating CNT network enhances bulk conductivity, facilitating rapid charge/mass transport. Uniform elemental distribution is further evidenced by C/N mappings (Figure 2e,f) correlating with the composite’s SEM morphology (Figure 2d). Homogeneous sulfur distribution within the S@g-C₃N₄/CNT composite is further confirmed by EDX mapping (Supplementary Materials Figure S1), consistent with its effective infiltration into the porous host.

TEM was used to further explore the properties of the g-C₃N₄/CNT samples. Figure 3a,b displays the TEM results of g-C₃N₄/CNT, which demonstrate that both g-C₃N₄ and CNTs preserve their initial morphology, and CNTs are uniformly distributed throughout the g-C₃N₄ host structure. The interconnected CNT network facilitates rapid charge/mass transfer, while the g-C₃N₄ nanosheets provide abundant reaction sites for polysulfide capture and transformation. The crystalline features of g-C₃N₄/CNT are investigated by HRTEM (Figure 3c,d), which show the typical (002) graphitic plane for CNTs with stripe intervals of 0.340 nm [39,40].

Figure 4a presents the N₂ adsorption-desorption isotherm of the g-C₃N₄/CNT composite, which exhibits a characteristic Type IV profile [41]. The material achieves a high specific surface area of 66.485 m² g⁻¹, confirming its porous architecture. Corresponding pore size distribution (Figure 4b) reveals a dominant mesoporous peak centered at 2.5 nm. These engineered mesopores facilitate polysulfide adsorption and mitigate shuttle effects through physical confinement. The BET surface area (66.485 m²/g) and pore volume (0.203 cm³/g) of g-C₃N₄/CNT exceed those of most reported g-C₃N₄ hosts (typically < 30 m²/g) [24], primarily due to the CNT-induced 3D hierarchical porosity. The dominant mesopores (centered at 2.5 nm) synergize with the visible macroporous framework (Figure 2c) to confine polysulfides while enabling efficient, unhindered ion transport. This architecture contributes to high sulfur utilization (74 wt%) and stable cycling.

XPS was used to study the surface properties of the materials. Only C, N, and O were observed in the survey spectrum shown in Figure 5a, which demonstrates the purity of the synthesized products. The C1s spectrum of g-C₃N₄/CNT (Figure 5b) reveals two dominant peaks at 284.8 eV (C-C) and 289.3 eV (C-N) [42], with an additional C-OH component at 285.8 eV, indicating oxygen-containing functionalities on CNTs [43]. Deconvolution of the N1s region (Figure 5c) identifies three nitrogen configurations: pyridinic N (398.7 eV), pyrrolic N (400.4 eV), and graphitic N (401.2 eV) originating from g-C₃N₄ [44]. Thermogravimetric analysis (Figure 5d) quantifies sulfur loading at 74 wt% in both S@g-C₃N₄ and S@g-C₃N₄/CNT composites. The presence of C-OH species (285.8 eV) on CNTs further augments polysulfide immobilization. Oxygen functional groups engage LiPS via hydrogen bonding and Li⁺ coordination, complementing g-C₃N₄’s nitrogen-mediated chemisorption. This cooperative effect between oxygen (physical-chemical adsorption) and nitrogen (chemical adsorption) sites establishes a multi-modal trapping network, reducing shuttle effects and enhancing cycle stability relative to non-functionalized carbons [45]. To further validate the proposed physical-chemical dual-confinement mechanism, we performed DFT calculations (Figure S2 in the Supplementary Materials). The DFT results quantitatively compare the adsorption energies of Li₂S₄ on g-C₃N₄, g-C₃N₄/CNT, and CNT surfaces, demonstrating that the g-C₃N₄/CNT composite exhibits strong adsorption (4.23 eV), which is beneficial for the polysulfide reduction reaction.

The synthesized composite was designed for robust polysulfide adsorption and accelerated conversion of sulfur species. Linear sweep voltammetry (LSV, Figure 6a) reveals enhanced current response kinetics in g-C₃N₄/CNT compared to pristine g-C₃N₄, validating the superior catalytic conversion of Li₂S to polysulfides. This enhancement can be attributed to the improved electrical conductivity and active material utilization. Complementary cyclic voltammetry (CV) of symmetric cells with Li₂S₆ electrolyte (Figure 6b) demonstrates sharper redox peaks at ±0.36 V for the composite, indicating accelerated polysulfide interconversion. The reduced ΔE (0.36 V vs. 0.51 V for pure g-C₃N₄) quantitatively confirms the superior electrocatalytic activity of g-C₃N₄/CNT, where minimized polarization accelerates LiPS redox cycling. Excellent reaction reversibility is confirmed by overlapping CV profiles during cycling (Figure 6c). The Li₂S deposition capacity (Figure 6d,e) is 233.3 mAh g⁻¹ for g-C₃N₄/CNT, indicating efficient polysulfide-to-Li₂S conversion catalysis. The multifunctional mechanism (Figure 6f) integrates strong chemisorption with catalytic cycling kinetics, enabling rapid sulfur redox reactions.

Cyclic voltammetry of S@g-C₃N₄/CNT cathodes (Figure 7a) exhibits two distinct redox couples. The cathodic peak at 2.30 V (C) signifies reduction of elemental sulfur to long-chain polysulfides (Li₂Sₓ, 4 ≤ x ≤ 8), while the 2.00 V peak (B) corresponds to further reduction to solid Li₂S₂/Li₂S. Anodic activity at 2.40 V (A) reflects sulfur regeneration. Notably, the dual oxidation peaks at ~2.30/2.40 V vs. Li⁺/Li maintain their intensity over cycles, demonstrating rapid Li₂S/Li₂S₂ decomposition kinetics and exceptional electrochemical reversibility. In contrast, S@g-C₃N₄ cathodes (Figure 7b) display a single broad oxidation peak and kinetic hysteresis at elevated scan rates, indicating sluggish sulfur electrochemistry. Minimal increase in peak separation (0.10 → 0.40 mV s⁻¹) further confirms the superior reaction reversibility of S@g-C₃N₄/CNT. The catalytic superiority of g-C₃N₄ is further evidenced by scan-rate-dependent peak shifts (Table S2), where smaller polarization changes indicate stabilized transition states during sulfur reduction. The attenuated peak (C) shift (34 mV vs. 106 mV) in S@g-C₃N₄/CNT confirms g-C₃N₄’s catalytic function in reducing reaction hysteresis, synergistically enhanced by CNT-enabled rapid charge transfer. Comparative linear fits of peak currents (Figure 7c,d) enable quantitative analysis of Li⁺ diffusion kinetics via the Randles–Ševčík equation [46]:

I = 269000 × n^1.5 × A × D^0.5 × C × v^0.5

Within the Randles–Ševčík framework, the peak current (I) correlates with the electron transfer number (n), electrode geometric area (A), Li⁺ diffusion coefficient (D), bulk Li⁺ concentration (C), and scan rate (v).

As shown in the results, the S@g-C₃N₄/CNT cathode demonstrates a higher slope rate than the S@g-C₃N₄ cathode, confirming the accelerated Li⁺ diffusion rate. The minor y-intercepts in peak current plots (Figure 7c,d) originate from pseudocapacitive charge storage at conductive interfaces, with reduced values for S@g-C₃N₄/CNT confirming enhanced electrode kinetics—consistent with its lower ΔE in symmetric-cell CVs (Figure 6b).

Cycling stability at 0.2C (Figure 8a) reveals superior performance of S@g-C₃N₄/CNT over S@g-C₃N₄ cathodes. The composite cathode delivers an initial capacity of 1227 mAh g⁻¹ (vs. 1127 mAh g⁻¹ for pristine), maintaining 701 mAh g⁻¹ after 100 cycles with minimal decay. In contrast, the control cathode retains only 440 mAh g⁻¹. Galvanostatic charge/discharge profiles of S@g-C₃N₄/CNT (Figure 8b) align with CV data: the 2.30 V plateau corresponds to S₈ → Li₂Sₙ (4 ≤ n ≤ 8) reduction, while the extended 2.00 V plateau signifies Li₂Sₙ → Li₂S₂/Li₂S conversion. Oxidation plateaus at ~2.36/2.45 V reflect Li₂S₂/Li₂S → S₈/Li₂S₈ reactions. This stable voltage plateau configuration enhances cycling efficiency by mitigating polarization during sulfur redox transitions.

Rate capability analysis (Figure 8c) reveals superior performance of the S@g-C₃N₄/CNT cathode, delivering specific capacities of 1264 (0.2C), 961 (0.5C), 659 (1C), and 457 mAh g⁻¹ (2C)—surpassing S@g-C₃N₄ at all rates. Upon reverting to 0.2C, the composite cathode recovers 1055 mAh g⁻¹, demonstrating exceptional rate resilience. The superior performance of the S@g-C₃N₄/CNT composite compared to representative cathodes reported in the recent literature is comprehensively summarized in Table S1 (Supplementary Materials). As evident from Table S1, our optimized S@g-C₃N₄/CNT composite achieves a remarkable Cycle performance, significantly exceeding values reported for state-of-the-art cathode materials. This enhancement is primarily attributed to the synergistic effect between g-C₃N₄ and CNTs, which facilitates efficient charge separation and transfer which also proved by the reduced ΔE by g-C₃N₄/CNT in Figure 6b. Electrochemical impedance spectroscopy (Figure 8d) corroborates enhanced reaction kinetics, where reduced charge-transfer resistance in S@g-C₃N₄/CNT facilitates rapid charge transport. Long-term cycling (Figure 8e) shows that the composite maintains 383 mAh g⁻¹ after 500 cycles from an initial 639 mAh g⁻¹. This stability originates from synergistic effects: (1) N-rich sites in g-C₃N₄ and hierarchical porosity enable effective polysulfide confinement, and (2) CNT networks substantially elevate electrical conductivity.

4. Conclusions

This work develops a facile two-step synthesis of hierarchically porous g-C₃N₄/CNT composites with an enhanced specific surface area (66.5 m² g⁻¹). As a multifunctional polysulfide reservoir, the designed architecture implements physicochemical dual confinement through (i) physical entrapment via mesopores (2.5 nm dominant) and (ii) chemical anchoring at N sites. The resulting S@g-C₃N₄/CNT cathodes deliver exceptional rate capability (457 mAh g⁻¹ at 2C) and cycling stability (383 mAh g⁻¹ after 500 cycles @1C). This strategy establishes a cost-effective pathway toward high-performance Li-S batteries via synergistic pore engineering and conductive-network optimization.