Construction of a Free-Standing Bismuth Carbon Nanofiber-Based Composite Anode Integrated with Molybdenum Disulfide for High-Performance Sodium-Ion Batteries

Abstract

Developing free-standing electrodes without the need of metal current collectors, binders, and conductive additives are essential for promoting the development of sodium-ion batteries (SIBs) to attain higher energy density. In this study, we developed and effectively synthesized a novel three-dimensional free-standing sodium-ion battery anode material with the composition of Bi@MoS₂@C carbon nanofibers by cleverly utilizing the energy storage advantages of each material. By growing MoS₂ nanospheres on Bi carbon nanofibers and coating them with a carbon layer, this free-standing system achieves both structural optimization and synergistic performance enhancement. Experimental results show that this composite electrode has a remarkably high initial specific capacity of 275.31 mA h g⁻¹ at a current density of 0.5 A g⁻¹, significantly exceeding that of Bi carbon nanofibers (150.6 mA h g⁻¹). Furthermore, it retains a capacity retention of 96.07% after 800 cycles, which significantly exceeds that of pristine MoS₂ (72.33 mA h g⁻¹) as a sodium-ion battery anode. The significant performance improvement originates from the free-standing structural design and synergistic effects of Bi carbon nanofibers, MoS₂ nanospheres and carbon layer, which not only provide 3D electron transport pathways and improved conductivity but also effectively accommodate volume changes during the charging and discharging processes. This work offers a promising and practical strategy for designing high-performance free-standing energy storage electrodes through hybrid mechanisms and synergistic effects.

1. Introduction

In recent years, lithium-ion batteries have emerged as the dominant technology in the fields of energy storage and electric vehicles, owing to their high energy density and long cycle life [1,2,3]. Nevertheless, at the same time, issues such as limited lithium reserves, high costs and uneven global distribution have spurred researchers and engineers to continually seek more cost-effective alternatives [4,5,6]. By contrast, sodium resources are abundant in reserves, widely distributed and readily accessible [7,8,9,10]. This has made sodium-ion batteries an increasingly prominent energy storage system, combining sound economic viability with sustainable potential [11,12,13,14]. It is worth noting that, compared to lithium, sodium exhibits a lower chemical reactivity [15,16]. However, the sodium ion radius (1.02 Å) is significantly larger than that of the lithium ion (0.76 Å) [17,18,19,20,21,22]. The graphite anodes commonly used in lithium-ion batteries are difficult to apply directly to sodium-ion batteries [23,24,25]. Therefore, the development of high-capacity, low-cost anode materials has become crucial to advancing the next generation of high-energy-density sodium-ion batteries.

Currently, research on electrode materials predominantly focuses on powder-based forms [26]. These typically require the incorporation of heavy metal-based current collectors, binders, and conductive additives. Therefore, developing self-supporting integrated electrode structures facilitates the construction of continuous conductive networks and robust mechanical support systems. This enables efficient utilization of active materials and functional integration of electrode structures, holding significant value for advancing high-performance energy storage systems. Hard carbon ranks as one of the most feasible anode materials, benefiting from economical synthesis and a sodium storage capacity usually falling within 280–320 mA h g⁻¹ [27,28,29,30]. Bismuth, representative of alloy-type anodes, possesses a remarkable theoretical specific capacity of 386 mA h g⁻¹ together with an exceptional volumetric capacity of 3800 mA h L⁻¹ [6,27,31,32]. However, during (de)alloying reactions, they experience a volume expansion of nearly 250% [33,34], leading to structural degradation and rapid capacity decay [35,36]. Molybdenum disulfide (MoS₂) is recognized as a compelling conversion-type negative electrode material on account of its signature layered configuration and substantial layer gap (0.62 nm along the (002) facet) [33,37]. Nonetheless, the practical deployment of this material encounters obstacles due to its low intrinsic electronic conductivity and pronounced volume expansion during electrochemical cycling, which impair rate capability and long-term cycling stability [38]. In comparison, carbon-coated materials can improve electronic conduction and serve as a buffering matrix to accommodate volume variation, thereby aiding in the design of electrode systems with stable structure and manageable cost [39,40].

This study fabricated a free-standing Bi@MoS₂@C CNFs composite via a combined electrospinning-hydrothermal method for use as a sodium-ion battery anode [41,42,43,44,45,46]. In this integrated architecture, the synergistic combination of Bi and MoS₂ contributes to increased volumetric capacity. Meanwhile, the uniformly dispersed Bi nanoparticles within the CNFs not only enhance structural stability but also serve as anchoring sites for the growth of MoS₂ nanospheres. The glucose-derived carbon coating further enhances overall electronic conductivity while effectively accommodating volume changes during cycling, thus improving the structural integrity and long-term cycling stability of the electrode [44,45,46]. By adopting this free-standing design, the typically required inactive components in conventional electrode architectures are eliminated. This structural innovation establishes an integrated conductive network that not only enhances electron transport pathways but also optimizes the distribution and utilization of active materials within the electrode. Experimental data indicate that the electrode exhibits a specific capacity of 275.31 mA h g⁻¹ under a current density of 0.5 A g⁻¹, along with maintaining a capacity retention of 96.07% following 800 cycles, demonstrating outstanding cycling stability and promising cost-effectiveness [47,48].

2. Materials and Methods

2.1. Materials

Bi(NO₃)₃·5H₂O (Macklin for Shanghai), polyvinylpyrrolidone (PVP, Sigma-Aldrich for USA, average Mw = 58,000), ethylene glycol, PAN (polyacrylonitrile, Sigma-Aldrich for USA, average Mw = 150,000), HNO₃, thiourea (Aladdin for Shanghai), ammonium molybdate tetrahydrate (Aladdin for Shanghai), anhydrous glucose (Aladdin for Shanghai), and DMF (N,N-dimethylformamide, 99% purity, Macklin for Shanghai) were purchased from the company without any purification process.

2.2. Synthesis of Bi Nanopowder and Bi CNFs

To prepare Bi nanopowder, 1.5 g of Bi(NO₃)₃·5H₂O and 0.06 mmol of polyvinylpyrrolidone (PVP, Mw = 58,000) was solubilized in 50 mL of ethylene glycol, with continuous magnetic stirring at room temperature until homogeneity was achieved. Subsequently, we added 10 mL of 1 M HNO₃ solution to the mixture dropwise, which was stirred until clarity was achieved. The resulting clear solution was shifted into a polytetrafluoroethylene-lined stainless steel autoclave and maintained at 150 °C over a period of 12 h. Finally, centrifugation was used to collect the bismuth nanopowder.

Thereafter, the synthesized Bi nanopowder (0.15 g) was mixed into 4.5 mL of DMF (dimethylformamide) under vigorous mixing for a duration of 12 h. The obtained spinning solution was placed into a 10 mL syringe via fitting a 24 G needle. Electrospinning was performed with 15 kV, 1 mL h⁻¹ flow rate, and 15 cm needle-collector distance. After spinning, the as-formed precursor fiber membrane was detached from the collector and carefully peeled off. The membrane was then vacuum-dried at 60 °C for 18 h, succeeded by a pre-oxidation step in a muffle furnace. The temperature was raised to 250 °C at a heating rate of 2 °C min⁻¹ under air, held for 2 h, and then allowed to cool slowly. Afterwards, the sample was moved into a tube furnace. In this furnace, it was subjected to heating in an argon environment to 600 °C at the same ramp rate, held for 2 h, and finally carbonized to yield the desired Bi CNFs.

2.3. Synthesis of Bi@MoS₂ CNFs and Bi@MoS₂@C CNFs

In detail, a blend of 0.2 g thiourea, 0.2 g ammonium molybdate tetrahydrate (Aladdin), and 0.2 g PVP (molecular weight 58,000) was solubilized in 60 mL deionized water with continuous agitation until the mixture became clear. Subsequently, 50 mg of Bi CNFs were fully immersed in the prepared solution. Both the mixture and the submerged sample were sealed in a 100 mL Teflon-lined autoclave, then heated at 200 °C for 24 h. After completion of the reaction, we collected the product, rinsed it repeatedly with deionized water and absolute ethanol to remove impurities, and dried it overnight in a vacuum oven at 60 °C. To further improve the crystallinity of MoS₂, the obtained sample was treated by annealing in a tube furnace under an argon environment. The temperature was elevated to 600 °C at 2 °C min⁻¹ and kept for 2 h. Subsequently, it was gradually cooled to room temperature, yielding the final product Bi@MoS₂ CNFs. The pure MoS₂ nanospheres (NS) sample was prepared without adding Bi CNFs.

To improve the electrical transport properties and mechanical stability of the composite, the surface of Bi@MoS₂ CNFs was coated with glucose-derived carbon. In detail, 50 mg glucose was solubilized in 60 mL deionized water with continuous stirring. Then, 50 mg of the as-prepared Bi@MoS₂ CNFs were submerged in the solution. The mixture was moved into a 100 mL PTFE-lined autoclave (PTFE = polytetrafluoroethylene) and underwent hydrothermal treatment at 180 °C for 12 h. Once the reaction had completed, the product was collected, cleaned by rinsing thoroughly with deionized water and ethanol, and dried under vacuum in an oven at 60 °C for 12 h. Finally, the dried sample underwent heat-treatment in a tube furnace at 600 °C for 2 h (2 °C min⁻¹ heating rate) under an inert atmosphere, producing the target product Bi@MoS₂@C CNFs.

2.4. Characterization

The crystal structures of all samples were analyzed by X-ray diffraction (XRD, DX-2700BH, Haoyuan, Dandong, for China). XPS analysis was carried out using an X-ray photoelectron spectrometer (XPS, ESCALAB 250XI, Thermo Fisher Scientific, USA). Raman spectroscopy measurements were performed using a Raman spectrometer (inVia, Renishaw, UK). The morphology, lattice structure, and interfacial bonding features of the anode heterojunction was analyzed using field-emission scanning electron microscopy (FE-SEM, ZEISS Gemini SEM 300, for Germany) and field emission transmission electron microscopy (TEM, Thermo Fisher Talos F200 SG2, for USA). Thermal gravimetric analysis (TGA) was performed using a thermogravimetric analyzer (TG209F3, Netzsch, Germany).

2.5. Electrochemical Measurements

All battery assembly procedures were performed within an argon-filled glove box. For the pure MoS₂ powder electrodes, we mixed the active material, polyvinylidene fluoride (PVDF) binder, and conductive carbon black in an N-methyl-2-pyrrolidone (NMP) solution at a weight ratio of 8:1:1. The slurry was coated uniformly onto copper foil and vacuum-dried at 60 °C for 15 h to obtain the pure MoS₂ electrodes. With the exception of the pure MoS₂, all other samples were used directly as working electrodes without the need for additional current collectors, binders, or conductive additives. The free-standing samples, with a mass loading of 1.5–2 mg, were employed directly as anodes in CR2032 coin-type half-cells. Note that the method for calculating specific capacity in our work refers to the entire free-standing electrode. Sodium metal foil was utilized as both the counter and reference electrode, and a glass fiber membrane was adopted as the separator. The electrolyte contained 1 M NaClO₄ in propylene carbonate (PC) with 5 wt% fluoroethylene carbonate (FEC) added. Electrochemical charge–discharge tests were performed using a Neware BTS-4000 battery test system (Shenzhen, China). Cyclic voltammetry (CV) was conducted at scan rates ranging from 0.3 to 10 mV s⁻¹. Using a Solartron electrochemical workstation (Farnborough, UK), electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 10⁵ to 10⁻² Hz [49].

3. Results and Discussion

Figure 1 outlines the stepwise synthesis of the Bi@MoS₂@C CNFs. Initially, Bi powder was synthesized by a straightforward hydrothermal route, subsequently fabricated into Bi CNFs via electrospinning. Subsequently, MoS₂ nanospheres were uniformly grown onto the CNFs’ surface by means of a secondary hydrothermal treatment. Finally, a conductive carbon layer derived from glucose was coated onto the sample to improve its electrical conductivity and mitigate structural degradation during cycling. By combining successive hydrothermal and annealing steps, the targeted composite was successfully obtained.

The crystal structures of the synthesized samples were confirmed by XRD analysis. As shown in Figure 2a, the XRD pattern of Bi CNFs displays distinct peaks at 27.1°, 37.9° and 39.6°, which can be attributed to the (012), (104), and (110) diffraction planes of hexagonal bismuth (JCPDS No. 98 000 0118). For pure MoS₂, characteristic reflections appear at 14.1°, 32.9°, 39.5° and 58.7°, corresponding to the (002), (100), (103) and (110) planes of the 2H-MoS₂ phase (JCPDS 75-1539). In the diffraction profile of Bi@MoS₂ CNFs, peaks from both Bi and MoS₂ are clearly present, confirming the successful fabrication of the composite. The relative attenuation of the Bi peaks in the composite is likely attributed to the coverage by MoS₂ nanosphere. Moreover, the XRD pattern of Bi@MoS₂@C CNFs indicates an additional slight drop in peak intensity after the glucose-derived carbon coating.

At approximately 1350 and 1580 cm⁻¹ two characteristic peaks are displayed in the Raman spectrum (Figure 2b), assigned to the disorder-induced D band and graphitic G band of carbon, respectively. The intensity ratio I_D/I_G reflects the degree of graphitization and defect density, with a higher value indicating more structural disorder. The calculated I_D/I_G ratios were approximately 1.336 for Bi CNFs, 1.256 for Bi@MoS₂ CNFs, and 1.270 for Bi@MoS₂@C CNFs. The decrease in this ratio after carbon coating suggests a moderate increase in graphitic ordering, which contributes to improved electrical conductivity and enhanced cycling stability.

The surface elemental composition was determined using X-ray photoelectron spectroscopy (XPS). Figure 3a shows the full X-ray photoelectron spectroscopy (XPS) spectrum, which contains information on the elements Bi, Mo, S, and C. In the high-resolution C 1s spectrum (Figure 3b), peaks positioned at 284.8 eV, 286.3 eV, and 287.5 eV correspond to C–C (sp² hybridized carbon), C–O, and C=O bonds, respectively. The C–C peak was also used as a reference for charge correction. The corresponding peak-fitting results of the Bi 4f spectrum are presented in Figure 3d. As reported in the previous literature, the high-resolution Bi 4f spectrum exhibits fitted peaks at 159.6 eV and 164.9 eV, corresponding to Bi 4f_7/2 and Bi 4f_5/2 species [44,50]. The deconvoluted peaks at 162.5 eV and 163.7 eV are assigned to S 2p_3/2 and S 2p_1/2 species, respectively. Thus, analysis confirms that both Bi and S elements coexist on the material surface. Figure 3c shows the distinct peaks at 229.6 eV and 232.8 eV, assigned to Mo 3d_5/2 and Mo 3d_3/2, and indicate the presence of Mo⁴⁺ ions in the material. Moreover, a low-intensity peak which appears at 226.7 eV in the Mo 3d spectral region was identified as the S 2s orbital signal [51,52]. According to the quantitative results from XPS, the carbon signal is the strongest, and the atomic ratio of Mo to S is close to 2:1, which matches the stoichiometric ratio of MoS₂, confirming the successful growth of the outer-layer MoS₂. Clearly, the synthesis of the anticipated sample has been successfully demonstrated through XRD patterns, Raman spectroscopy, and XPS spectra.

Figure 4a displays an SEM image of bismuth-carbon nanofibers (Bi CNFs), showing a networked nanofiber structure with diameters of approximately 200 nm. The SEM image in the upper right corner shows bismuth (Bi) nanopowder, which exhibits a spherical structure with sizes around 100 nm. The pure MoS₂ sample, presented in Figure 4d, consists of nanospheres ranging from 100 to 200 nm in size. Following MoS₂ modification, Figure 4b illustrates that MoS₂ grows uniformly along the one-dimensional axis of the carbon nanofibers, accompanied by a slight increase in diameter. As shown in Figure 4c, the overall morphology of the Bi@MoS₂@C CNFs is preserved after the carbon-coating step.

Using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS), the microstructure and elemental distribution of the Bi@MoS₂@C CNFs were further examined. In Figure 5a,d, the images clearly show the integrated structure composed of carbon nanofibers, bismuth, MoS₂, and the carbon coating, which aligns with the SEM results. TEM images obtained from different regions (Figure 5b,c) reveal a lattice spacing of 0.62 nm, corresponding to the (002) plane of MoS₂, while a spacing of 0.33 nm observed outside the carbon nanofibers matches the (012) plane of bismuth. In addition, EDS elemental mapping (Figure 5a) shows that bismuth mainly exists in the form of nanoparticles embedded within the carbon nanofibers.

Electrochemical performance was evaluated using sodium-ion half-cells assembled with the working electrodes. Rate-capability tests were performed at various current densities ranging from 0.1 to 10 A g⁻¹, as illustrated in Figure 6a. The galvanostatic charge–discharge (GCD) profiles of the Bi@MoS₂@C CNFs electrode during the initial five cycles at 0.1 A g⁻¹ are shown in Figure 6b. In these rate capability tests, all electrodes exhibited a greater specific capacity in the initial cycle than in the following cycles. In the initial cycle, a discharge specific capacity of 534.30 mA h g⁻¹ and a charge specific capacity of 365.96 mA h g⁻¹ were observed for the Bi@MoS₂@C CNFs electrode, yielding an initial Coulombic efficiency of 68.49%. The irreversible capacity loss of approximately 31.51% is mainly associated with the following factors: (1) irreversibly generating a stable solid electrolyte interphase (SEI) on the electrode surface, where sodium ions and electrolyte are consumed; (2) trapping of some sodium ions in structural defects or voids within the carbon coating or composite; and (3) possible parasitic reactions or electrolyte decomposition occurring at low potentials. As shown in Figure 6a, the specific capacities of Bi CNFs at different current densities (0.1, 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹) are 276.0, 186.2, 146.7, 119.0, 95.4, 66.5, and 48.3 mA h g⁻¹, respectively. The specific capacities of Bi@MoS₂ CNFs under the same current densities are 316.8, 284.0, 241.7, 213.9, 174.4, 90.3, and 28.9 mA h g⁻¹, respectively. For Bi@MoS₂@C CNFs, the corresponding values are 360.8, 321.0, 272.3, 240.8, 190.8, 114.0, and 47.8 mA h g⁻¹, respectively. Meanwhile, the specific capacities of pure MoS₂ are 450.6, 411.2, 348.5, 309.9, 269.0, 182.7, and 117.5 mA h g⁻¹, respectively. We can see under a current density of 0.1 A g⁻¹, the specific capacity increased from 276.0 mA h g⁻¹ for Bi CNFs to 316.8 mA h g⁻¹ for Bi@MoS₂ CNFs following the incorporation of MoS₂. Further modification with a carbon coating promoted graphitization, leading to a higher capacity of 360.8 mA h g⁻¹ for Bi@MoS₂@C CNFs. At 0.5 A g⁻¹, the Bi@MoS₂ CNFs electrode delivered an initial capacity of 272.3 mA h g⁻¹. Notably, after cycling at a high rate of 10 A g⁻¹ and reverting to 0.5 A g⁻¹, the capacity kept a constant level of 262.3 mA h g⁻¹, highlighting excellent reversibility. These results confirm that the added carbon layer effectively enhances the electrochemical performance of the electrode material.

Figure 6c presents the electrochemical impedance spectroscopy (EIS) Nyquist plots of the Bi@MoS₂ CNFs and Bi@MoS₂@C CNFs electrodes. The spectra consist of a semi-circular arc in the high-frequency region and a linear tail in the low-frequency region. This relationship shows that the semicircle diameter corresponds to the charge transfer resistance (Rct) at the electrode, and the line slope reflects sodium-ion diffusion resistance. These EIS features show that a smaller semicircle diameter and a steeper slope indicate lower charge transfer resistance (Rct) and faster ion diffusion, respectively. Clearly, the Bi@MoS₂@C CNFs electrode shows a reduced semicircle diameter and a sharper slope, demonstrating improved charge-transfer kinetics and enhanced ion transport. We also performed equivalent circuit modeling on the impedance data. The R_(SEI+CT) values for Bi@MoS₂ CNFs and Bi@MoS₂@C CNFs were determined to be 259.3 Ω and 206.8 Ω, respectively. A smaller R_(SEI+CT) indicates that the electrode reaction can proceed more rapidly, which enhances the charge–discharge efficiency (reducing energy loss) and cycling stability of the battery.

Figure 6d reveals the comparative cycling performance of the four activated electrodes under a current density of 0.5 A g⁻¹. It is noteworthy that only the curves for Bi@MoS₂ CNFs and Bi@MoS₂@C CNFs exhibit significant noise in the figure. Since all samples were tested under identical experimental conditions, it is reasonable to suggest that the noise in the cycling curves likely originates from the intrinsic heterogeneous multiphase structure and multistep reaction kinetics inherent to the composite materials. Figure 6e displays the galvanostatic charge–discharge (GCD) profiles of the Bi@MoS₂@C CNFs electrode recorded at selected cycle numbers (the 1st, 200th, 400th, 600th, and 800th cycles). Although an exceptionally high initial discharge specific capacity of 366.77 mA h g⁻¹ was observed for pure MoS₂, its capacity deteriorates rapidly during cycling owing to structural degradation and volume expansion. After 800 cycles, the capacity declines to only 73.98 mA h g⁻¹, corresponding to a low retention rate of about 20%. By comparison, the Bi CNFs electrode showed high cycling stability, still retaining 96.07% of its initial capacity at the 800th cycle. By integrating the respective merits of Bi, MoS₂, and a carbon layer, the Bi@MoS₂@C CNFs electrode exhibited an initial specific capacity of 279.56 mA h g⁻¹ and maintained 264.51 mA h g⁻¹ (96.07% retention) after 800 cycles. In contrast, the uncoated Bi@MoS₂ CNFs electrode retained only 29% of its capacity (70.83 mA h g⁻¹) under identical experimental parameters. The comprehensive sodium-storage performance of the Bi@MoS₂@C CNFs electrode is further highlighted in the radar chart presented in Figure 6f.

Figure 6g presents the thermogravimetric (TG) curve of the Bi CNFs. The analysis indicates a relatively low bismuth content of approximately 30.94 wt%, which helps explain the moderate energy density observed for the Bi CNFs alone. This result also underscores the potential for further enhancing the energy density by compositing with molybdenum disulfide and applying a carbon coating.

Figure 6h shows that, despite undergoing long-term volume changes, the overall fibrous structural framework of the composite material was retained, with no large-scale fracture or collapse observed. However, in high-magnification images, the fiber surfaces appear rougher, which is consistent with the expected volume expansion/contraction of the active material under carbon layer confinement, yet this did not lead to overall structural failure.

To clearly and objectively demonstrate the performance advantages of this work,

Table 1. Electrochemical energy storage performance of reported Bi/MoS₂-based materials in sodium-ion battery applications.

Materials	Specific Capacity (mA h g−1)/Current Density (A g−1)	Cycle Stability (Capacity Retention Rate/Cycle Count)	Electrode Structure/Load Capacity
Bi@MoS2@C CNFs	367.26/0.1	96.07%/800 (0.5 A g−1)	Self-supporting, without adhesive
Bi/3DPG [53]	270/0.1	93.3%/500 (0.1 A g−1)	Slurry coating, including binder
Bi/CFC [54]	346/0.05	93.2%/300 (0.05 A g−1)	Self-supporting, without adhesive
Bi@C [55]	319/0.2	95.6%/1500 (1 A g−1)	Slurry coating, including binder
Bi@SnSb [56]	345.4/0.1	85.2%/500 (1 A g−1)	Slurry coating, including binder
Bi@N-C [57]	208.0/0.1	95.3%/4000 (10 A g−1)	Slurry coating, including binder
TiO2-Bi/CNFs [45]	276/0.1	-/2000 (2 A g−1)	Slurry coating, including binder
MoS2 CNFs [58]	381.7/0.1	74.8%/600 (0.1 A g−1)	Self-supporting, without adhesive
Bi/C [59]	302.27/0.1	90%/500 (0.1 A g−1)	Self-supporting, without adhesive

presents a systematic performance comparison. This comparative analysis will confirm that through precise material selection (leveraging the complementary capacity and kinetic characteristics of bismuth and molybdenum disulfide) and innovative structural design (featuring a freestanding dual-carbon protective layer), this study achieves an outstanding balance among specific capacity, cycling lifespan, and rate capability. This highlights its potential as a high-performance anode material for sodium-ion batteries.

Cyclic voltammetry (CV) measurements were conducted to investigate the electrochemical behavior of the electrode samples. Typical CV curves obtained from the first five cycles (0.01–3.0 V vs. Na⁺/Na) are presented in Figure 7a–c. For the Bi CNFs, Bi@MoS₂ CNFs, and Bi@MoS₂@C CNFs electrodes, the first cycle exhibits a distinct profile associated with solid electrolyte interphase (SEI) formation. During the interval between cycles 2–5, the curves exhibit high overlap in all cases, demonstrating highly reversible Na⁺ insertion/extraction reactions. The typical second-cycle CV curves for all samples are presented in Figure 7d. In agreement with earlier reports, bismuth—a representative alloying-type anode material—shows redox peaks around 0.73 V (vs. Na⁺/Na), which correspond to the stepwise alloying/de-alloying reactions with sodium as expressed by the following equations [59]:

$$\begin{gathered} \mathrm{B}\mathrm{i}+\mathrm{x}{\mathrm{N}\mathrm{a}}^{+}+\mathrm{x}{\mathrm{e}}^{-}\leftrightarrow {\mathrm{N}\mathrm{a}}_{\mathrm{x}}\mathrm{B}\mathrm{i} \\ {\mathrm{N}\mathrm{a}}_{\mathrm{x}}\mathrm{B}\mathrm{i}+\left(3-\mathrm{x}\right){\mathrm{N}\mathrm{a}}^{+}+\left(3-\mathrm{x}\right){\mathrm{e}}^{-}\leftrightarrow {\mathrm{N}\mathrm{a}}_3\mathrm{B}\mathrm{i}. \end{gathered}$$

MoS₂ follows a conversion-type sodium storage mechanism: a distinct reduction peak appears at around 0.57 V (vs. Na⁺/Na), corresponding to its sodiation process, whereas a broad oxidation peak at approximately 1.95 V characterizes the desodiation behavior of MoS₂. The CV profile of the Bi@MoS₂ CNFs electrode essentially represents a superposition of the features from pure Bi CNFs and pure MoS₂, confirming the coexistence of both alloying and conversion mechanisms.

To further explore the electrochemical kinetics, the charge storage mechanism was analyzed by means of the power-law dependence ($i=a{V}^b$) relating peak current (i) to scan rate (v) for quantitative assessment. By taking logarithms of both sides, the expression becomes $\log \left(i\right)=\log \left(a\right)+b\mathrm{l}\mathrm{o}\mathrm{g}\left(v\right)$, where a and b are fitting parameters [49]. Acting as a key indicator of the storage mechanism, the exponent b shows that values near 0.5 imply diffusion control, whereas values approaching 1.0 reflect surface-dominated capacitive behavior. CV experiments were conducted under different scan rates, and the derived curves are shown in Figure 7e. Therefore, Figure 7f is a logarithmic plot of the peak current versus scan rate for the Bi@MoS₂@C CNFs electrode over the range of 0.3–10.0 mV s⁻¹. For the Bi@MoS₂@C CNFs electrode samples, the b-values of the currents corresponding to the anodic and cathodic peaks were determined to be 0.37 and 0.53, respectively, based on the slope analysis of the log(i)–log(v) curves.

The obtained b-values offer insight into the dominant kinetic processes. With an anodic b of 0.37 and a cathodic b of 0.53, the electrochemical reactions are governed by a mixed mechanism. The kinetics are predominantly diffusion-controlled, consistent with Faradaic charge storage involving sodium-ion insertion/extraction and alloying/conversion reactions. The minor departure of the b-values from the ideal 0.5 indicates a partial contribution from surface-controlled capacitive processes. Benefiting from the abundant active sites provided by its high specific surface area, as well as its open porous nanostructured morphology, the system achieves efficient charge transport at the electrode/electrolyte interface, thereby significantly enhancing its electrochemical performance. Bi@MoS₂@C CNFs electrode exhibits excellent rate performance, which stems from the synergistic effect of its diffusion-limited and capacitive charge storage mechanisms.

4. Conclusions

In summary, a free-standing Bi@MoS₂@C CNFs composite anode with a three-dimensional architecture was successfully fabricated through a straightforward and cost-effective combination of electrospinning and hydrothermal processing. This integrated electrode can be used directly without conventional metallic current collectors, binders, or conductive additives, which substantially simplifies cell assembly, lowers production costs, and offers a feasible route toward higher battery energy density. Moreover, by combining different sodium-storage mechanisms, the electrode delivers well-balanced overall performance. At a current density of 0.5 A g⁻¹, it retains a specific capacity of 275.31 mA h g⁻¹ after 800 cycles, corresponding to a capacity retention of 96.07%, which reflects high capacity, good rate capability, and long-term cycling stability. This study presents a practical synthesis strategy and provides design principles for developing easily processable, low-cost, and high-performance free-standing anodes for sodium-ion batteries.