The pursuit of sustainable and cost-effective energy storage solutions has intensified in recent years, driven by the growing demand for portable electronics, electric vehicles, and grid-scale systems. Among various technologies, lithium-ion batteries have dominated the market due to their high energy density and long cycle life. However, concerns regarding lithium scarcity, geopolitical supply chain issues, and rising costs have spurred significant interest in alternative battery chemistries. Sodium-ion batteries represent a compelling alternative, primarily because sodium is abundant, widely distributed, and inexpensive. The fundamental electrochemistry of sodium-ion batteries shares similarities with lithium-ion systems, but the larger ionic radius of Na+ (1.02 Å compared to 0.76 Å for Li+) introduces distinct challenges. This size difference can lead to substantial structural stress during ion insertion/extraction, often resulting in poor cycling stability, slow kinetics, and capacity fading in electrode materials. Consequently, developing robust anode materials that can accommodate Na+ reversibly and efficiently is critical for advancing sodium-ion battery technology.
Carbon-based materials, such as hard carbons, have been extensively explored as anodes for sodium-ion batteries due to their low cost, good electronic conductivity, and reasonable capacity. However, they often suffer from low initial Coulombic efficiency and limited specific capacity, which restrict the overall energy density of sodium-ion batteries. Transition metal dichalcogenides, particularly molybdenum disulfide (MoS2), have emerged as promising high-capacity anode candidates. MoS2 possesses a layered structure with weak van der Waals forces between S-Mo-S layers, which facilitates ion intercalation. Its theoretical capacity for sodium storage is approximately 670 mAh/g, based on a four-electron transfer reaction forming Mo and Na2S. Nonetheless, pristine MoS2 exhibits rapid capacity decay during cycling in sodium-ion batteries, primarily due to its poor intrinsic electronic conductivity, significant volume expansion (up to ~300%) upon sodiation, and aggregation of active material. These issues lead to mechanical degradation, loss of electrical contact, and increased polarization, ultimately impairing battery performance.
To overcome these limitations, nanostructuring and carbon hybridization are effective strategies. Electrospinning is a versatile and scalable technique for producing continuous one-dimensional nanofibers with high surface area, tunable porosity, and excellent mechanical flexibility. By integrating active materials like MoS2 into carbon nanofibers via electrospinning, one can enhance electronic conductivity, buffer volume changes, and prevent nanoparticle aggregation. Moreover, designing hierarchical or multi-layered architectures can further optimize ion and electron transport pathways. In this work, we focus on fabricating a sandwich-structured composite, denoted as C@MoS2/C@C, through a sequential electrospinning approach. This design features a central layer of MoS2-decorated carbon nanofibers sandwiched between two outer layers of pure carbon nanofibers. The outer carbon layers serve as protective buffers and additional conductive networks, while the inner layer provides high capacity from MoS2. This configuration aims to maximize the interfacial contact with the electrolyte, facilitate rapid Na+ diffusion, and alleviate mechanical stress during cycling, thereby improving the stability and rate capability of the anode in sodium-ion batteries.
The electrochemical performance of electrode materials is governed by multiple kinetic processes, including charge transfer at the electrode-electrolyte interface and solid-state diffusion of ions within the active material. Quantitative analysis of these processes is essential for understanding and optimizing battery behavior. Key formulas used in this study include the power-law relationship for current response at various scan rates:
$$i = a v^b$$
where \(i\) is the peak current, \(v\) is the scan rate, and \(a\) and \(b\) are constants. The value of \(b\) indicates the charge storage mechanism: \(b = 0.5\) suggests diffusion-controlled behavior (e.g., intercalation), while \(b = 1\) corresponds to capacitive-dominated processes (e.g., surface adsorption). For mixed mechanisms, the current can be deconvoluted into capacitive and diffusion contributions using the equation:
$$i = k_1 v + k_2 v^{0.5}$$
where \(k_1 v\) represents the capacitive contribution and \(k_2 v^{0.5}\) represents the diffusion contribution. Additionally, the sodium-ion diffusion coefficient (\(D_{Na^+}\)) can be estimated from electrochemical impedance spectroscopy (EIS) data using the formula:
$$D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2}$$
where \(R\) is the gas constant, \(T\) is the temperature, \(A\) is the electrode area, \(n\) is the number of electrons transferred, \(F\) is Faraday’s constant, \(C\) is the sodium-ion concentration, and \(\sigma\) is the Warburg coefficient derived from the low-frequency linear region of the Nyquist plot. These formulas are applied extensively in our analysis to elucidate the kinetic advantages of the sandwich-structured anode.
The synthesis of the C@MoS2/C@C composite involves a multi-step process. First, MoS2 nanoflowers are prepared via a hydrothermal method. Typically, sodium molybdate and thioacetamide are dissolved in deionized water and reacted at 180°C for 12 hours. The product is collected by centrifugation, washed, and freeze-dried to obtain MoS2 powder. For electrospinning, two separate solutions are prepared: Solution A contains polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF), and Solution B is a homogeneous mixture of Solution A with dispersed MoS2 nanoflowers. The electrospinning setup employs a high-voltage power supply, a syringe pump, and a rotating collector. The sequential spinning process begins with Solution A to form a pure carbon fiber layer (5 hours), followed by Solution B to create the MoS2-incorporated middle layer (2 hours), and finally Solution A again to deposit the top carbon layer (5 hours). The as-spun nanofiber mat is vacuum-dried and then stabilized and carbonized in an argon atmosphere at 800°C for 2 hours with a heating rate of 3°C/min. This yields the final sandwich-structured C@MoS2/C@C composite. To investigate the effect of MoS2 loading, three variants are prepared with different amounts of MoS2 added to Solution B: 0.5 g, 0.7 g, and 1.0 g, labeled as C@MoS2/C@C-0.5g, C@MoS2/C@C-0.7g, and C@MoS2/C@C-1g, respectively.
Material characterization is performed using various techniques. X-ray diffraction (XRD) patterns are recorded to identify crystalline phases. Raman spectroscopy provides information on structural order and defects. Morphology and microstructure are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Surface chemical composition and bonding states are analyzed via X-ray photoelectron spectroscopy (XPS). For electrochemical evaluation, the composite is directly used as an anode without additional binders or conductive additives. Electrodes are assembled into coin cells (CR2032) with sodium metal as the counter/reference electrode, a glass fiber separator, and an electrolyte of 1 M NaClO4 in a mixture of ethylene carbonate and diethyl carbonate (1:1 by volume). Galvanostatic charge-discharge tests are conducted within a voltage window of 0.01–3.0 V vs. Na+/Na at various current densities. Cyclic voltammetry (CV) is performed at scan rates from 0.2 to 1.0 mV/s. Electrochemical impedance spectroscopy (EIS) measurements are taken over a frequency range of 0.01 Hz to 100 kHz at open-circuit potential.
The XRD patterns of all three composites confirm the successful incorporation of MoS2. Distinct diffraction peaks are observed at approximately 14.4°, 32.6°, 39.5°, and 58.2°, corresponding to the (002), (100), (103), and (110) planes of hexagonal MoS2 (JCPDS No. 24-0513). No impurity phases are detected, indicating high purity. The broad hump around 25° is attributed to amorphous carbon from the PAN-derived fibers. Raman spectra exhibit characteristic peaks for MoS2 at 375.3 cm−1 (E12g in-plane vibration) and 400.1 cm−1 (A1g out-of-plane vibration). The carbon-related D band (~1330 cm−1) and G band (~1580 cm−1) are also present. The intensity ratio ID/IG is calculated to assess graphitization degree; lower ratios suggest more ordered carbon structures. The values for the composites are summarized in Table 1.
| Sample | ID/IG Ratio | MoS2 Loading (g) | Specific Surface Area (m²/g) |
|---|---|---|---|
| C@MoS2/C@C-0.5g | 1.12 | 0.5 | ~250 |
| C@MoS2/C@C-0.7g | 1.05 | 0.7 | ~280 |
| C@MoS2/C@C-1g | 1.18 | 1.0 | ~300 |
Notably, C@MoS2/C@C-0.7g shows the lowest ID/IG, implying a balance between disorder (beneficial for Na+ storage) and conductivity. The specific surface area, estimated from nitrogen adsorption-desorption isotherms, increases with MoS2 loading due to the nanoflower morphology.
Morphological analysis reveals the successful formation of the sandwich structure. Cross-sectional SEM images clearly show three distinct layers: two outer dense carbon fiber layers and a middle layer with MoS2 nanoflowers anchored on carbon fibers. The MoS2 nanoflowers, with diameters of 200–500 nm, are uniformly distributed and tightly bonded to the fiber surface, preventing detachment during cycling. TEM images further confirm that MoS2 nanosheets are encapsulated by a thin amorphous carbon coating, which enhances electronic connectivity and mitigates volume expansion. High-resolution TEM shows lattice fringes with interplanar spacings of 0.62 nm, matching the (002) plane of MoS2. Elemental mapping via STEM-EDS demonstrates homogeneous distribution of C, N, Mo, and S throughout the fiber matrix, with nitrogen originating from PAN pyrolysis, contributing to additional active sites for sodium storage.
XPS analysis provides insights into surface chemistry. The survey spectrum confirms the presence of C, N, O, Mo, and S elements. High-resolution Mo 3d spectrum displays doublets at 229.5 eV (Mo 3d5/2) and 232.7 eV (Mo 3d3/2), characteristic of Mo4+ in MoS2. A minor peak at 236.1 eV corresponds to Mo6+, likely from surface oxidation. The S 2p spectrum shows peaks at 162.3 eV (S 2p3/2) and 163.4 eV (S 2p1/2), indicative of S2− in MoS2. Additional peaks at 168.6 eV and 169.7 eV are assigned to sulfate species (SOx) from slight surface oxidation. The N 1s spectrum can be deconvoluted into pyridinic N (398.5 eV), graphitic N (401.4 eV), and Mo 3p (395.4 eV). Pyridinic N is known to enhance electrochemical activity by providing defect sites. The C 1s spectrum reveals bonds such as C-C (284.7 eV), C-N (285.8 eV), and C=O (288.8 eV). The presence of heteroatoms (N, O) improves wettability and facilitates Na+ adsorption, which is beneficial for sodium-ion battery performance.
The electrochemical performance of the sandwich-structured anodes is systematically evaluated in sodium-ion batteries. Initial galvanostatic discharge-charge profiles at 0.1 A/g are presented in Figure 1 (not shown here, but described). All composites exhibit typical voltage plateaus corresponding to Na+ insertion into MoS2 and conversion reactions. The initial discharge capacities are 420.1 mAh/g for C@MoS2/C@C-0.5g, 474.5 mAh/g for C@MoS2/C@C-0.7g, and 510.3 mAh/g for C@MoS2/C@C-1g. The initial Coulombic efficiencies are 65.3%, 66.2%, and 64.8%, respectively, with capacity loss attributed to solid electrolyte interphase (SEI) formation and irreversible side reactions. In subsequent cycles, the efficiencies quickly rise above 98%, demonstrating good reversibility. The cycling stability at 0.1 A/g over 100 cycles is summarized in Table 2.
| Sample | Initial Discharge Capacity (mAh/g) | Capacity after 100 cycles (mAh/g) | Capacity Retention (%) | Average Coulombic Efficiency (cycles 2-100) |
|---|---|---|---|---|
| C@MoS2/C@C-0.5g | 420.1 | 235.6 | 56.1 | 98.5 |
| C@MoS2/C@C-0.7g | 474.5 | 270.1 | 56.9 | 98.9 |
| C@MoS2/C@C-1g | 510.3 | 200.4 | 39.3 | 97.8 |
C@MoS2/C@C-0.7g delivers the highest retained capacity and excellent retention, underscoring the optimal MoS2 loading. The sandwich structure effectively buffers volume changes, while the outer carbon layers maintain structural integrity. In contrast, C@MoS2/C@C-1g suffers from severe capacity fading due to excessive MoS2 causing mechanical strain and particle detachment. These results highlight the importance of balanced design in achieving stable sodium-ion battery anodes.
Rate capability tests are conducted at current densities ranging from 0.1 to 2 A/g. The discharge capacities at each rate are listed in Table 3.
| Current Density (A/g) | C@MoS2/C@C-0.5g Capacity (mAh/g) | C@MoS2/C@C-0.7g Capacity (mAh/g) | C@MoS2/C@C-1g Capacity (mAh/g) |
|---|---|---|---|
| 0.1 | 420.1 | 474.5 | 510.3 |
| 0.2 | 380.5 | 430.2 | 450.6 |
| 0.5 | 320.8 | 380.1 | 350.4 |
| 1.0 | 250.3 | 310.7 | 260.9 |
| 2.0 | 160.2 | 180.6 | 150.8 |
| Return to 0.1 | 240.1 | 280.0 | 210.5 |
C@MoS2/C@C-0.7g consistently outperforms the others, maintaining 180.6 mAh/g at 2 A/g and recovering 280.0 mAh/g when the current returns to 0.1 A/g. This superior rate performance is attributed to the enhanced kinetics from the hierarchical structure, which provides short diffusion paths and high conductivity. Such characteristics are crucial for high-power applications of sodium-ion batteries.
Cyclic voltammetry (CV) curves at 0.1 mV/s reveal redox peaks associated with sodium storage mechanisms. During the first cathodic scan, a broad reduction peak around 0.8 V corresponds to Na+ intercalation into MoS2 to form NaxMoS2 and SEI formation. Subsequent reduction peaks at 0.5 V and 0.2 V are related to conversion reactions yielding Mo and Na2S. In the anodic scan, peaks at 1.8 V and 2.3 V represent the reversible oxidation of Mo to MoS2 and desodiation of Na2S. From the second cycle onward, the CV curves overlap well, indicating stable electrochemical reactions. To quantify charge storage kinetics, CV scans at various rates (0.2–1.0 mV/s) are analyzed using the power-law equation \(i = a v^b\). Plotting log(i) versus log(v) yields b-values for anodic and cathodic peaks. For C@MoS2/C@C-0.7g, b-values are approximately 0.89 and 0.99 for reduction and oxidation peaks, respectively, suggesting dominant capacitive behavior. This is further confirmed by separating the current response into capacitive and diffusion-controlled portions using \(i = k_1 v + k_2 v^{0.5}\). The capacitive contribution percentages at different scan rates are calculated and presented in Table 4.
| Scan Rate (mV/s) | Capacitive Contribution (%) for C@MoS2/C@C-0.7g | Diffusion Contribution (%) |
|---|---|---|
| 0.2 | 58.2 | 41.8 |
| 0.4 | 63.1 | 36.9 |
| 0.6 | 68.5 | 31.5 |
| 0.8 | 72.4 | 27.6 |
| 1.0 | 75.8 | 24.2 |
The increasing capacitive contribution at higher scan rates indicates surface-controlled processes, which enable fast charge storage and excellent rate capability. This behavior is highly desirable for sodium-ion batteries targeting high-power demands.
Electrochemical impedance spectroscopy (EIS) measurements provide insights into interfacial resistance and ion transport. Nyquist plots consist of a semicircle in the high-medium frequency region (representing charge transfer resistance, Rct) and a sloping line in the low-frequency region (representing Warburg diffusion impedance). Fitted equivalent circuit parameters are summarized in Table 5.
| Sample | Rs (Ω) | Rct (Ω) | σ (Warburg coefficient, Ω s−0.5) | DNa+ (cm²/s) estimated |
|---|---|---|---|---|
| C@MoS2/C@C-0.5g | 3.2 | 85.6 | 45.3 | 1.8 × 10−12 |
| C@MoS2/C@C-0.7g | 2.9 | 62.4 | 38.7 | 2.5 × 10−12 |
| C@MoS2/C@C-1g | 3.5 | 110.2 | 50.1 | 1.2 × 10−12 |
Here, Rs is the ohmic resistance of the electrolyte and contacts. C@MoS2/C@C-0.7g exhibits the lowest Rct and Warburg coefficient, indicating facilitated charge transfer and faster Na+ diffusion. The sodium-ion diffusion coefficient (DNa+) is estimated using the formula provided earlier. The higher DNa+ for C@MoS2/C@C-0.7g correlates with its superior rate performance. After 50 cycles, the Rct of C@MoS2/C@C-0.7g decreases to 45.1 Ω, suggesting improved interfacial stability due to SEI maturation and better electrolyte infiltration. These results underscore the kinetic advantages imparted by the sandwich architecture.
To further elucidate the stability mechanism, we consider the stress distribution during sodiation/desodiation. The volume change of MoS2 upon full sodiation can be approximated by the formula:
$$\Delta V = \frac{V_{\text{sodiated}} – V_{\text{pristine}}}{V_{\text{pristine}}} \times 100\%$$
where \(V_{\text{pristine}}\) and \(V_{\text{sodiated}}\) are the volumes of MoS2 before and after Na+ insertion, respectively. Theoretical calculations suggest \(\Delta V\) can exceed 300%. In the sandwich structure, the outer carbon layers act as mechanical buffers, absorbing the strain and preventing crack propagation. The effective stress (\(\sigma_{\text{eff}}\)) experienced by the active material can be reduced according to:
$$\sigma_{\text{eff}} = \sigma_0 \cdot \frac{E_c}{E_c + E_m}$$
where \(\sigma_0\) is the stress in bare MoS2, and \(E_c\) and \(E_m\) are the elastic moduli of the carbon shell and MoS2, respectively. This stress reduction contributes to the enhanced cycling stability observed.
Moreover, the capacity contribution from different mechanisms can be modeled. The total capacity (Qtotal) of the composite in a sodium-ion battery comprises contributions from MoS2 conversion (Qconv), carbon intercalation (Qcarbon), and surface capacitance (Qcap):
$$Q_{\text{total}} = Q_{\text{conv}} + Q_{\text{carbon}} + Q_{\text{cap}}$$
Assuming linear superposition, we can express:
$$Q_{\text{conv}} = n_{\text{MoS2}} \cdot F \cdot x$$
where \(n_{\text{MoS2}}\) is the moles of MoS2, \(F\) is Faraday’s constant, and \(x\) is the number of electrons transferred per MoS2 (theoretically up to 4). For carbon, the capacity often follows a pseudo-constant behavior:
$$Q_{\text{carbon}} = k \cdot \sqrt{t}$$
where \(k\) is a rate-dependent constant and \(t\) is time. The capacitive part is directly proportional to scan rate as previously discussed. These models help rationalize the high capacity and stable performance of the sandwich-structured anode.
In summary, the sandwich-structured C@MoS2/C@C composite fabricated via sequential electrospinning demonstrates exceptional performance as an anode material for sodium-ion batteries. The optimal sample, C@MoS2/C@C-0.7g, delivers a high reversible capacity of 270.1 mAh/g after 100 cycles at 0.1 A/g, excellent rate capability (180.6 mAh/g at 2 A/g), and robust cycling stability. The design leverages the synergy between MoS2 (high capacity) and carbon nanofibers (conductivity and buffering). The outer carbon layers protect the active material, while the inner MoS2-rich layer ensures sufficient Na+ storage. Kinetic analyses reveal dominant capacitive charge storage, enabling fast reaction kinetics. This work provides a scalable and effective strategy for developing high-performance anodes for sodium-ion batteries, addressing key challenges such as volume expansion and poor conductivity. Future studies could explore other transition metal sulfides or selenides in similar architectures, optimize layer thicknesses, and investigate full-cell configurations with compatible cathodes to advance practical sodium-ion battery systems.
The promising results underscore the potential of nanostructured composites in overcoming the limitations of traditional electrode materials. As research on sodium-ion batteries continues to evolve, such innovative designs will play a pivotal role in enabling cost-effective, safe, and high-energy-density storage solutions for a sustainable energy future. The integration of advanced characterization, computational modeling, and scalable fabrication methods will further accelerate the development of next-generation sodium-ion batteries.