English

• Sodium-ion batteries (SIBs) possess considerable promise for future energy storage technologies owing to their abundant resources, superior safety, and exceptional electrochemical stability. Nevertheless, SIBs encounter various obstacles due to the higher radius of Na-ion (1.02 Å) in comparison to Li-ion (0.76 Å). These issues include sluggish ion diffusion rates and substantial fluctuations in electrode volume, leading to inadequate battery rate performance and cycle stability [1]. Continuing efforts are being made to overcome these limitations, including in the development of high-performance anode materials for SIBs [2]. Transition metal sulfides (TMSs) have attracted considerable interest because of their plentiful availability, eco-friendliness, and high theoretical capacity in SIBs [3].

  MoS₂ is a two-dimensional transition metal dichalcogenide consisting of hexagonally organized molybdenum atoms sandwiched between layers of sulfur. The ease of inserting and extracting Na-ions is enhanced by the weak van der Waals forces that exist between the layers. Nevertheless, the actual usability and durability of MoS₂ are now constrained by its low inherent conductivity and the tendency for volume change [4]. In order to tackle these problems, generally used approaches involve combining MoS₂ with conductive carbon materials to improve their overall performance [5]. Graphene and doped carbon have been effectively combined with MoS₂ to enhance its electrochemical performance [6]. For example, a composite of MoS₂ and graphite demonstrated enhanced long-term stability due to the capacity of the graphite sheets to improve conductivity and structural stability [7]. Furthermore, to improve the cyclic stability and electrical conductivity of composites, they can be prepared as bimetallic sulfides, in addition to the inclusion of carbon components [8]. The sodium present in polymetallic sulfides is progressively accumulated through sodium inlays/desodiation processes of the polymetallic components at varying voltages, resulting in a higher frequency of more plentiful redox reactions [9]. This leads to an increased reversible capacity and charge-discharge capacity [10,11]. The polymetallic sulfides and carbon composites improved the speed of the chemical reactions, the transfer of electrical charge, and the ability to conduct electricity at the interface between the solid electrolyte. Zinc sulfide (ZnS) exhibits a greater specific capacity of 926.3 mAh/g [12] compared to MoS₂. Additionally, ZnS has a suitable voltage range, indicating its potential for use in SIB applications.

  Furthermore, metal-organic frameworks are ideal models and initial substances for creating metal sulfide/carbon composites, owing to their uniform structure and precise surface area [13,14]. Zeolitic imidazolate frameworks (ZIFs) undergo pyrolysis, which allows for the transformation of organic ligands into nano-porous carbon frameworks that are doped with nitrogen [15]. These frameworks maintain the structural properties of their predecessors and display remarkable attributes such as significant porosity and a substantial surface area. When coupled with polymetallic sulfides like ZnS and MoS₂, frameworks exhibit enhanced Na-storage properties due to their compositional and structural characteristics [16,17]. Nevertheless, the ZnS and MoS₂ anode exhibits low conductivity and inadequate cycle performance due to the volume changes that take place during the lithiation and delithiation processes [18]. Constructing a hybrid with a conductive matrix is an effective method for mitigating the effects of volume change. The g-C₃N₄ carbon material is commonly used as a conducting matrix because of its high specific surface area, impressive mechanical strength, and great chemical stability [19]. In addition, the construction of a heterojunction enables the combination of the benefits of two materials, which in turn promotes effective movement of electrons and ions and improves total conductivity. Furthermore, the heterojunction decreases changes in volume, which leads to less structural harm during cycling and ultimately enhances cycling stability [20]. Moreover, the heterojunction improves the effectiveness of ion transport pathways, resulting in enhanced ion diffusion and rate performance.

  This study utilizes a rational design strategy to create a multilayer ZnS/MoS₂ heterostructure in order to tackle problems associated with low electrical conductivity and significant volume change during the discharge/charge process. The objective is to attain a combination of superior performance at a high rate and long-lasting stability over multiple cycles. The addition of N-doped carbon to the composite improves its stability, enhances several redox processes, facilitates efficient movement of electrons and ions at different surfaces, and results in exceptional electrochemical performance.

  A strategy was proposed by work function mismatch to achieve a p-type MoS₂/n-type ZnS sandwich structure. The possible formation process of the heterostructure junction is illustrated in Fig. 1a. In the experimental design, the molar ratio of ZnS to MoS₂ must be 1:1 to ensure the formation of a heterojunction between one ZnS molecule and one MoS₂ molecule [16,17,20]. The charge transfer is primarily influenced by the disparity in work function between the p-type MoS₂ and n-type ZnS. A hole depletion layer is formed next to the ZnS area and a hole accumulation layer is formed next to the MoS₂ region as a result of the work function mismatch that causes holes to migrate from the n-type ZnS toward the p-type MoS₂ close to the p-n interface. When a MoS₂/ZnS heterostructure junction forms, an electric field is created, which inhibits further electron-hole pair transport until a unified Fermi Energy is established [21]. The interface exhibits band bending, the presence of an existing electric field, and the formation of depletion regions. The formation of the heterogeneous structure between MoS₂ and ZnS leads to the accumulation of positive and negative charges at the interface of MoS₂ and ZnS, respectively [22]. This charge accumulation generates an internal electric field (E-field) from MoS₂ to ZnS. Due to the potential difference between MoS₂ and ZnS, Na-ions are attracted to the MoS₂ side. As Na-ion concentration increases on the MoS₂ side, the electric interface field eventually dissipates due to electron neutralization (Fig. S1 in Supporting information). To further substantiate our hypothesis, we utilized density functional theory (DFT) analysis on the ZnS/MoS₂ composites (Figs. 1b and c). The top and side views of the constructed heterostructure are illustrated in Figs. S2a and b (Supporting information). The optimized lattice parameters for monolayer black phosphorus are a = b = 3.43 Å, indicating a trigonal crystal system. The relative stabilities of these structures are compared using binding energy, defined as:

  $ E_{\mathrm{b}}=E_{\mathrm{hj}}-E_{\mathrm{MoS}_2}-E_{\mathrm{ZnS}} $

  (1)

  Figure 1

  

  Figure 1.  (a) Energy band structures of the T-MS/C heterostructure (top, isolated materials, bottom, heterostructure junction formation). (b) Charge differential density diagram (yellow indicates electron accumulation and blue indicates electron dissipation). (c) Band gap of T-MS/C heterojunction.

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  In Eq. 1, E_hj, E_MoS2, and E_ZnS represent the energies of the heterojunction monolayer, MoS₂, and ZnS, respectively. Our calculated E_b for the heterojunction monolayer is −2.73 eV. The charge differential density map was computed to investigate the charge transfer within the heterostructure, as shown in Fig. 1b. Upon the approach of the two-dimensional materials to form the heterostructure, electron depletion at the ZnS site indicated electron transfer towards MoS₂, creating a downward internal electric field. This facilitated electron movement towards MoS₂ when Na ions are present in the heterostructure. The formation energy of the heterostructure adsorbing Na ions is given by:

  $ E_{\mathrm{b}}=E_{\mathrm{hj}+\mathrm{Na}}-E_{\mathrm{MoS}_2}-E_{\mathrm{ZnS}}-2 E_{\mathrm{Na}} $

  (2)

  The preparation of the nitrogen (N)-doped carbon-coated ZnS/molybdenum sulfide heterojunction (T-MS/C) composite is illustrated in Fig. 2a. The ZnS/MoS₂ precursor was prepared via hydrothermal synthesis, followed by mechanical grinding with g-C₃N₄ and adding a pyrrole carbon source for polymerization to form PPy using an initiator. X-ray diffraction (XRD) analysis of the material confirms the formation of ZnS/MoS₂ bimetallic sulfide with high purity and crystallinity. The diffraction peaks align well with the standard signatures of ZnS and MoS₂ (Fig. S4 in Supporting information) [23]. Raman spectroscopy results are presented in Fig. S5a (Supporting information). The peaks at 376.5 and 402.8 cm^-1 correspond to the characteristic vibration modes of E_2g and A_1g of MoS₂, respectively. The 1381.3 and 1576.4 cm^-1 peaks represent the D and G peaks of carbon materials, indicating the presence of carbon in the composite material [24]. The I_D/I_G values of T-MS/C, g-C₃N₄-coated ZnS/MoS₂ heterojunction (α-MS/C), and ZnS/MoS₂ heterojunction coated with pyrolyzed polypyrrole (β-MS/C) are 1.19, 1.10, and 0.98, respectively. Thermogravimetric analysis (TGA) in air atmosphere is conducted to determine the carbon content of the T-MS/C composite (Fig. S6 in Supporting information). At 200 ℃, weight loss is due to the evaporation of adsorbed water, whereas between 200 ℃ and 510 ℃, weight loss is attributed to carbon combustion, resulting in a loss of around 32% of carbon weight. At 510 ℃, ZnS transforms into ZnO, and MoS₂ converts to MoO₃, leading to an additional weight loss of approximately 13%. After undergoing the transformations, the residual material percentage by weight amounts to 51%. The porous structures of prepared samples were revealed by Brunauere Emmette Teller (BET). As shown in Fig. S7a (Supporting information), all four samples show type Ⅳ isotherms and H4 hysteresis loops. The SSA of T-MS/C, α-MS/C, β-MS/C and pure ZnS/MoS₂ (MS) are 226.2104, 50.6, 120.4, and 25.0 m²/g, respectively. The specific surface area (SSA) of T-MS/C is larger than that of the other three samples because the pyrolysis of g-C₃N₄ generates many voids and N-doped in T-MS/C, which is conducive to the generation of abundant active sites. According to Fig. S7b (Supporting information), most of the pore sizes of T-MS/C, α-MS/C, and β-MS/C are distributed around 2 nm, and there are abundant microporous and mesoporous structures in all of them. However, the pore size distribution of pure MS is around 4 nm. The high SSA and abundant pore structure of T-MS/C enhance electrolyte penetration and SEI film formation while inhibiting polysulfide dissolution. Consequently, the T-MS/C electrode is expected to exhibit optimal Na-storage performance.

  Figure 2

  

  Figure 2.  (a) Sketch of the T-MS/C preparation process. (b, c) SEM images of T-MS/C. (d) TEM image of T-MS/C. (e, f) HR-TEM image of T-MS/C. (g) SAED pattern of T-MS/C. (h) HAADF-STEM and elemental mappings of T-MS/C.

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  The nanostructure and morphology of T-MS/C are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images reveal that T-MS/C possesses a layered coated polyhedral structure similar to the pure MS precursor (Figs. 2b and c). It illustrates the presence of a nanocube encapsulated by a nanoporous nanosheet. This fact is attributed to nitrogen-containing products generated during g-C₃N₄ decomposition, which create pores and expose part of ZnS/MoS₂ morphology. SEM images of α-MS/C, α-MS/C, and pure MS are shown in Fig. S8 (Supporting information). TEM images further confirm the presence of the polyhedral structure, with MoS₂ nanosheets and porous carbon layers covering the surface (Fig. 2d). The chemical components of the T-MS/C composite were investigated using X-ray photoelectron spectroscopy (XPS) experiments. Fig. S5b (Supporting information) shows the high-resolution XPS spectrum of C 1s, with characteristic peaks at 285.6 and 287.6 eV corresponding to C—C and C—N bonds, respectively, confirming the successful nitrogen doping of the pyrolyzed carbon in the composite. High-resolution TEM (HR-TEM) images show lattice distances corresponding to the (002) plane of MoS₂ and the (200) plane of ZnS, confirming the heterostructures of ZnS/MoS₂ (Figs. 2e and f). Selected area electron diffraction (SAED) patterns provide additional evidence for the existence of ZnS and MoS₂ phases (Fig. 2g) [25]. Finally, high-angle annular dark field (HAADF) imaging and elemental mapping demonstrate the coexistence of Zn, Mo, C, N, and S elements in the T-MS/C sample, with MoS₂ nanosheets wrapped around the ZnS polyhedron (Fig. 2h). Fig. S5c (Supporting information) presents the high-resolution spectrum of N 1s, with strong peaks at 398.1 and 400.9 eV attributed to pyridinic N and pyrrolic N, respectively, further confirming the successful nitrogen doping [26,27]. The nitrogen doping-induced electron cloud bias creates a strong coupling effect at the nitrogen interface, accelerating electron migration and providing more defects/active sites for Na-ions insertion. Moreover, the synergistic effect resulting from the combination of ZnS/MoS₂ and the N-doped carbon matrix contributes to forming a robust electrode structure. This synergistic effect effectively mitigates volume changes and prevents electrode crushing that may occur during the charge/discharge processes, enhancing the electrode structure's stability.

  The electrochemical performances of the different anode compositions are investigated as an anode for SIBs. Fig. 3a presents the cyclic voltammetry (CV) curves for the T-MS/C anode over five cycles, with a scanning rate of 0.5 mV/s and a potential range of 0.01 to 3.0 V. During the initial cathodic sweep, reduction peaks appear at around 0.73 and 0.86 V in the CV curves. These peaks are ascribed to the conversion of MoS₂ to metallic Mo and Na₂S. The conversion and alloying of ZnS, which produces Na_xZn and Na₂S, as well as the production of an irreversible solid electrolyte interphase (SEI) layer, is what causes the broad reduction peak to be seen between 0.01 V and 0.2 V [28]. In the following cathodic sweep, the reduction peaks exhibit changes, indicating irreversible transformations and structural rearrangements within the anode. During the following anodic sweep, a peak at 0.87 V is observed, which is associated with the dealloy reaction of Na_xZn back into ZnS. Additionally, another peak at 1.83 V corresponds to the oxidation of Na₂S and Mo to form MoS₂. The CV curves display a significant overlap on consecutive scans, which is evidence of the excellent reversibility of the T-MS/C electrode. This characteristic indicates the electrode's ability to undergo reversible Na-storage processes effectively [29]. The mechanisms involved in Na-storage are summarized in Supplementary Material [30,31]. The CV curves obtained for the α-MS/C electrode, β-MS/C electrode, and pure MS electrode exhibit similar redox peaks, suggesting comparable mechanisms for the insertion/extraction reactions of Na-ions (Figs. S9a-c in Supporting information).

  Figure 3

  

  Figure 3.  (a) CV curves of T-MS/C electrode at 0.5 mV/s. (b) Cycling performance of T-MS/C, α-MS/C, β-MS/C and pure MS electrodes at 0.2 A/g for 100 cycles. (c) Charge/discharge curves of T-MS/C electrode at 0.2 A/g. (d) Long-term cycling performance of T-MS/C, α-MS/C, β-MS/C and pure ZnS/MoS₂ electrodes at 1.5 A/g up to 1000 cycles. (e) Charge/discharge curves of T-MS/C electrode at 0.1-10 A/g. (f) Rate capability of T-MS/C, α-MS/C, β-MS/C and pure MS electrode at 0.1-10 A/g. (g) Comparison of the rate performance of the ZnS/MoS₂-based Na-ion anodes reported.

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  The cycling stability of the T-MS/C electrode at 0.2 A/g is depicted in Fig. 3b. After 100 cycles, the electrode achieves a specific capacity of 690.8 mAh/g. The galvanostatic charge/discharge (GCD) measurements for the T-MS/C electrode were conducted at current density at 0.2 A/g (Fig. 3c). The discharge potential platforms align with the CV curves. To assess the stability of the anode materials under high-rate conditions, the resulting electrodes were subjected to cycling performance testing at 1.5 A/g (Fig. 3d). The exact synthesis of multilayer coated ZnS/MoS₂ heterostructure has outstanding sodium storage capabilities, as shown by the T-MS/C electrode's ability to retain a specific capacity of 222.6 mAh/g after 1000 cycles. The GCD measurements for the T-MS/C electrode were conducted at current densities ranging of 0.1-10 A/g and are presented in Fig. 3e. As the current densities increase, the charge plateau gradually extends due to increased polarization. Fig. 3f displays the reversible specific capacities of T-MS/C as 797, 773, 697, 644, 529, 462, 365, and 306 mAh/g at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 10 A/g, respectively. When the current density is switched back to 0.1 A/g, T-MS/C delivers a reversible capacity of 656 mAh/g, demonstrating its stable cycling stability and high-rate capabilities in SIBs. The enhanced performance observed in T-MS/C materials, as compared to α-MS/C, β-MS/C, and pure MS counterparts, can be credited to the efficient heterojunction and carbon-coated structure building (Figs. S10-S12 in Supporting information). We compare our results with other ZnS/MoS₂-based Na-ion anodes reported in the literatures (Fig. 3g) [15,19,30,32]. All the reported specific capacities are below 690.8 mAh/g, much lower than our work, showing the state-of-the-art superior rate performance of T-MS/C. And more references are compared in Table S1 (Supporting information).

  To investigate the Na-ion diffusion mechanism of T-MS/C electrode, galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) were performed. Using Fick's second law, GITT is used to understand the diffusion kinetics further and estimate the Na-ion diffusion coefficient of the four electrodes (Depiction S1 in Supporting information) [33]. Figs. 4a and b present the Na-ions diffusion coefficient during the charging process. The average Na-ions diffusion coefficients (D_Na) for the T-MS/C, α-MS/C, β-MS/C, and pure MS anodes are approximately 10^–7.9, 10^–8.3, 10^–8.6, and 10^–8.7 cm²/s, respectively, within the voltage range of 0.05–3 V. The T-MS/C anode exhibits the highest diffusion coefficient of Na-ions with approximately 10^–7.9 cm²/s. CV tests were conducted on the T-MS/C, α-MS/C, β-MS/C, and pure MS anodes at different scan rates, and the contribution rate of capacitance was calculated. At a scanning rate of 0.6 mV/s, the T-MS/C anode shows a capacitance contribution rate of 87.2% (Fig. 4c). In Fig. 4d, the contribution rate of capacitances increases with the scanning rate. The T-MS/C anode demonstrates a high ion diffusion rate and a significant capacitance contribution, which can be attributed to rich phase interfaces and small crystal domains resulting from the ZnS/MoS₂ heterostructure. The energy level difference between these interfaces facilitates the rapid transmission of ions and electrons. It leads to a notable pseudo-capacitance effect, thereby significantly enhancing the electrochemical performance of the T-MS/C anode. The T-MS/C anode's electrode kinetics were investigated using CV at various scanning speeds (v). The findings show that diffusion-limited redox reactions and capacitive behavior are presented in electrochemical reactions [34]. Fig. 4e displays the CV curves of the T-MS/C electrode at scan rates ranging from 0.2 mV/s to 1.0 mV/s. Furthermore, the peak current (i) follows a proportionate connection with the square root of the scanning rate rather than increasing with it. This observation suggests the occurrence of both non-Faradaic and Faradaic charge/discharge processes (Depiction S2 in Supporting information). As shown in Fig. 4f, log(i) and log(v) exhibit a linear connection at each peak potential. This fact indicates the electrochemical reactions in the T-MS/C electrode are primarily governed by a pseudocapacitive process, as indicated by the "b" values of 0.82, 0.99, 0.92, 0.88, 0.79, and 0.85, respectively. This behavior plays a pivotal role in facilitating the swift insertion and extraction of Na-ions [35]. Overall, from the mechanism investigation, we conclude the formation of ZnS/MoS₂ provides a built-internal electric field, enabling efficient carrier transfer and enhancing the kinetics of charge transfer in the presence of NC. Consequently, the rate capability is significantly improved, enabling rapid insertion and extraction of Na-ions from T-MS/C.

  Figure 4

  

  Figure 4.  (a) GITT curves of the T-MS/C, α-MS/C, β-MS/C and pure MS for SIBs at different discharge degrees. (b) Na-ion diffusion coefficient diagram at different discharge states. (c) CV curve at 1.0 mV/s with the pseudo-capacitive portion representation with violet colour. (d) Pseudo-capacitive contribution fraction (%) from 0.2 mV/s to 1.0 mV/s. (e) CV curves of the T-MS/C electrode at scan rates ranging from 0.2 mV/s to 1.0 mV/s. (f) Peak current density plots at scan rates from 0.2 mV/s to 1.0 mV/s. (g) Nyquist plots and randles equivalent circuit of T-MS/C, α-MS/C, β-MS/C and pure MS electrodes. (h) Plot of the Real part of the complex impedance versus ω ^−1/2 at open-circuit potential before cycling. (i) SEM image of T-MS/C after 1000 cycles at 1.5 A/g. (j) SEM image of T-MS/C after rate cycles. (k) TEM image of T-MS/C after 1000 cycles at 1.5 A/g. (l) TEM image of T-MS/C after rate cycles.

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  Electrochemical impedance spectroscopy (EIS) is assessed before and after 1000 cycles to further examine the Na-ion diffusion mechanism of T-MS/C, α-MS/C, β-MS/C, and pure MS anodes. The EIS test results for all electrodes are depicted in Fig. 4g, while the illustration of Fig. 4h presents the corresponding equivalent circuit model employed for simulating the Nyquist plots. The four electrodes' Nyquist diagrams in Fig. 4g exhibit a semicircular form in the high-frequency range and a linear slope in the low-frequency range. This semicircle represents the combined resistance of the electrode and electrolyte, known as the charge transfer resistance (R_ct). Conversely, the low-frequency line reflects the diffusion of Na-ions (W_o) [36]. By employing the Z view program to fit the experimental data, the calculated values of R_ct for the T-MS/C, α-MS/C, β-MS/C, and MS electrodes are determined as 403.3, 634.2, 848.9, and 727.1 Ω, respectively (Table S2 in Supporting information). In contrast to the α-MS/C, β-MS/C, and pure MS electrodes, the T-MS/C electrode shows more excellent electronic conductivity and reduced charge transfer resistance. The Warburg coefficient (σ), which is calculated using the method in Depiction S3 (Supporting information), is used to evaluate the diffusion factor (D) value [37]. The value of σ is inversely proportional to D and represents the slope of the curve in Fig. 4h. Through linear fitting, the calculated values of σ for the T-MS/C, α-MS/C, β-MS/C, and pure MS electrodes are determined as 110.46, 377.11, 584.31, and 5151.12, respectively (Table S3 in Supporting information). To evaluate the cycling performance throughout the charge/discharge process in terms of the morphology of the T-MS/C electrode, SEM and TEM inspections are performed after 1000 cycles at 1.5 A/g. As depicted in Figs. 4i-l, the T-MS/C nanocube structures remain remarkably intact without collapsing, confirming excellent structural stability. These results underscore the practical preservation of the ZnS/MoS₂ heterostructure structure within the layered hetero-carbon, highlighting its remarkable structural integrity during the charge/discharge process.

  In summary, we present an N-doped carbon-coated multilayer T-MS/C material with a ZnS/MoS₂ heterostructure that has excellent Na-storage capabilities. The multilayer construction improves overall electrochemical performance by enabling ion diffusion while reducing structural deterioration. In addition, the N-doped carbon covering works as a protective barrier, limiting particle aggregation and ensuring electrode stability while cycling. The experimental results show that the T-MS/C electrode has outstanding electrochemical characteristics, with a reversible capacity of 690.8 mAh/g after 100 cycles at 0.2 A/g. The electrode retains a constant capacity of 222.6 mAh/g even after 1000 cycles at a higher current density of 1.5 A/g, demonstrating its long-term stability and rate capabilities. Its excellent electrochemical features, including as high capacity, cycling stability, and rate capability, make it a promising choice for creating next-generation energy storage systems.

  Declaration of competing interests

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Hong Yin: Data curation, Funding acquisition, Investigation, Methodology, Writing – original draft. Danyang Han: Formal analysis, Software. Wei Wang: Methodology, Visualization. Zhaohui Hou: Project administration, Resources. Miao Zhou: Conceptualization, Funding acquisition, Resources, Supervision, Writing – review & editing. Ye Han: Data curation, Validation. İhsan Çaha: Methodology, Resources. João Cunha: Data curation, Software. Maryam Karimi: Methodology, Resources. Zhixin Tai: Validation, Visualization. Xinxin Cao: Conceptualization, Methodology, Supervision, Visualization, Writing – review & editing.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 52271211, 52171207), the HORIZON-Marie Skłodowska-Curie Actions-2021-PF (No. 101065098), European Union; Hunan Provincial Natural Science Foundation of China (No. 2022JJ40162), the Science and Technology Innovation Program of Hunan Province (No. 2023RC3185), and the Taishan Industrial Experts Program (No. tscx202211146).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110537.

  
  
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  Figure 1  (a) Energy band structures of the T-MS/C heterostructure (top, isolated materials, bottom, heterostructure junction formation). (b) Charge differential density diagram (yellow indicates electron accumulation and blue indicates electron dissipation). (c) Band gap of T-MS/C heterojunction.

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  Figure 2  (a) Sketch of the T-MS/C preparation process. (b, c) SEM images of T-MS/C. (d) TEM image of T-MS/C. (e, f) HR-TEM image of T-MS/C. (g) SAED pattern of T-MS/C. (h) HAADF-STEM and elemental mappings of T-MS/C.

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  Figure 3  (a) CV curves of T-MS/C electrode at 0.5 mV/s. (b) Cycling performance of T-MS/C, α-MS/C, β-MS/C and pure MS electrodes at 0.2 A/g for 100 cycles. (c) Charge/discharge curves of T-MS/C electrode at 0.2 A/g. (d) Long-term cycling performance of T-MS/C, α-MS/C, β-MS/C and pure ZnS/MoS₂ electrodes at 1.5 A/g up to 1000 cycles. (e) Charge/discharge curves of T-MS/C electrode at 0.1-10 A/g. (f) Rate capability of T-MS/C, α-MS/C, β-MS/C and pure MS electrode at 0.1-10 A/g. (g) Comparison of the rate performance of the ZnS/MoS₂-based Na-ion anodes reported.

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  Figure 4  (a) GITT curves of the T-MS/C, α-MS/C, β-MS/C and pure MS for SIBs at different discharge degrees. (b) Na-ion diffusion coefficient diagram at different discharge states. (c) CV curve at 1.0 mV/s with the pseudo-capacitive portion representation with violet colour. (d) Pseudo-capacitive contribution fraction (%) from 0.2 mV/s to 1.0 mV/s. (e) CV curves of the T-MS/C electrode at scan rates ranging from 0.2 mV/s to 1.0 mV/s. (f) Peak current density plots at scan rates from 0.2 mV/s to 1.0 mV/s. (g) Nyquist plots and randles equivalent circuit of T-MS/C, α-MS/C, β-MS/C and pure MS electrodes. (h) Plot of the Real part of the complex impedance versus ω ^−1/2 at open-circuit potential before cycling. (i) SEM image of T-MS/C after 1000 cycles at 1.5 A/g. (j) SEM image of T-MS/C after rate cycles. (k) TEM image of T-MS/C after 1000 cycles at 1.5 A/g. (l) TEM image of T-MS/C after rate cycles.

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