Integrated Self-Supporting Na₂Se/Na₂S@CNF Cathode for Long-Cycle Room-Temperature Sodium–Sulfur Batteries

Abstract

Room-temperature sodium–sulfur batteries have broad prospects, but Na₂S cathodes suffer from poor electrochemical performance and insufficient air stability. By introducing Na₂SeO₃ as a precursor, a Na₂Se/Na₂S@CNF composite cathode is prepared. TEM reveals continuous heterojunction interfaces. Electrochemical performance is significantly enhanced: the initial charge capacity reaches 664 mAh g⁻¹, and the reversible capacities at 0.2, 0.5, 1, 2, 3, and 5 C rates are 774, 722, 636, 574, 494, and 396 mAh g⁻¹ respectively, with a reversible capacity of 600 mAh g⁻¹ after 300 cycles at 1 C.

1. Introduction

The pressing global challenges of the greenhouse effect and energy crisis demand a swift transition away from fossil fuels. Renewable energy sources—solar, wind, tidal, and biomass—offer abundant, clean alternatives, but their intermittent electricity generation requires reliable energy storage systems. In this context, room-temperature sodium–sulfur (RT Na–S) batteries have attracted growing interest due to their high theoretical energy density (1274 Wh kg⁻¹), low cost, and resource abundance. However, the practical application of Na₂S cathodes is hindered by their poor electronic conductivity and limited air stability, which severely compromise electrochemical performance [1,2,3,4,5,6,7,8,9].

To overcome these drawbacks, researchers have explored the use of chalcogen congeners of sulfur—selenium and tellurium—which form sodium chalcogenides (Na₂Se, Na₂Te) with superior conductivity. For instance, Wu Yuping’s group designed a Na₂S/Na₂Te heterostructure embedded in a carbon matrix via a simple carbothermic reaction [10]. This structure provides abundant electron/ion diffusion channels and catalytic surfaces for polysulfide conversion, significantly improving active material utilization and mitigating the shuttle effect. Similar strategies have been applied to organosulfur cathodes. In one study, selenium was introduced as a “eutectic accelerator” into sulfurized polyacrylonitrile (S@pPAN) [11]. The resulting Se_0.08S_0.92@pPAN composite forms Se–S bonds uniformly dispersed within the conductive PAN framework. Selenium doping simultaneously enhances electronic conductivity, increases the sodium-ion diffusion coefficient, and lowers the diffusion energy barrier and reaction overpotential. Consequently, the cathode exhibits excellent cycling stability and rate capability in both ether- and ester-based electrolytes, with selenium itself contributing additional capacity.

Another approach involves the room-temperature sodium–selenium disulfide (Na–SeS₂) system. Substituting elemental sulfur with SeS₂ significantly improves overall conductivity. To suppress polysulfide/polyselenide shuttling, Xu Maowen’s team designed a core–shell Co₉S₈@SiO₂/C host material [12]. The hollow interior accommodates volume expansion, while the outer SiO₂ shell and Co₉S₈ particles synergistically adsorb soluble species, effectively confining them. Combined with in situ/ex situ characterizations, this work elucidated the reaction mechanism and paved the way for the further development of Na–SeS₂ batteries.

Inspired by these advances, we introduce selenium into sodium sulfide cathodes by incorporating sodium selenite (Na₂SeO₃) into the electrospinning precursor, successfully constructing a Na₂Se/Na₂S@CNF composite. In this material, both Na₂Se and Na₂S are uniformly dispersed within the carbon nanofibers and form a closely contacted heterojunction interface. Benefiting from selenium’s stronger metallic character and higher electronic conductivity, this architecture establishes an efficient electron transport network and synergistically enhances the redox kinetics of sulfur species, thereby improving the overall electrochemical performance of the cathode. Electrochemical tests show that the introduction of Na₂Se significantly increases the initial charge capacity from 408.0 mAh g⁻¹ to 664 mAh g⁻¹ at 0.1 C, confirming that it effectively lowers the initial energy barrier for Na2S activation. In rate performance tests, this cathode delivered reversible capacities of 774, 722, 636, 574, 494, and 396 mAh g⁻¹ at 0.2, 0.5, 1, 2, 3, and 5 C, respectively, and when the current was returned to 0.2 C, the capacity quickly rebounded, demonstrating excellent rate tolerance. After 300 cycles at a high rate of 1 C (1 C = 1675 mA g⁻¹), it still maintained a high capacity of 600 mAh g⁻¹, far superior to that of the undoped Na₂S@CNFs (347 mAh g⁻¹).

2. Materials and Methods

2.1. Preparation of Freestanding Na₂Se/Na₂S@CNF Cathodes

Firstly, PVP (K88-K96) (1.200 g) and Na₂SO₄ (0.480 g) were dissolved in a mixed solvent of water, and Na₂SeO₃ (0.073 g) and F127 (0.020 g) were then added to the solution (F127 was added as a surfactant to enhance the dispersion of Na₂SO₄ and Na₂SeO₃ in the precursor solution, preventing particle agglomeration during the solvent evaporation process [13]). The resulting solution was electrospun at a voltage of 21 kV and a flow rate of 0.7 mL h⁻¹ to prepare Na₂SO₄- Na₂SeO₃-PVP nanofibers on an aluminum foil collector. The prepared nanofiber membrane was subsequently preoxidized at 170 °C for 12 h with a heating rate of 2 °C min⁻¹. Finally, it was annealed at 780 °C under an Ar atmosphere for 2 h to obtain a self-supporting Na₂Se/Na₂S@CNF cathode. For comparison, the Na₂S@CNF cathode was prepared following a similar procedure.

The areal loading of the Na₂S is approximately 1 mg cm⁻².

2.2. Characterization

To investigate the nucleation and decomposition behavior of Na₂S, a battery was assembled using the aforementioned active material as the working electrode and a sodium foil as the counter electrode. Then, 50 µL 0.2 M Na₂S₆ in DME was used as the catholyte (sulfur and Na₂S were dissolved in DME solution at a molar ratio of 5:1, and then stirred overnight, resulting in a dark brown solution), with an additional 25 µL 1 M NaFSI DME serving as the anolyte. During nucleation testing, the cell was first discharged to 1.0 V at a constant current of 1/24 C (1 C = 1675 mA g⁻¹), and then the potential was held constant at 0.8 V until the current dropped below 10⁻⁵ A.

2.3. Electrochemical Measurements

XRD patterns were collected on a D8 diffractometer (Bruker, Berlin, Germany) with Cu Kα radiation in the 2θ range of 10–80° to identify the crystalline phases. The morphologies and element distribution were assessed by Scanning Electron Microscopy (SEM) (Regulus 8230, Hitachi, Ltd., Chiyoda, Japan) and Transmission Electron Microscopy (TEM) (Tecnai G2 F20 S-TWIN, Thermo Fisher Scientific, Waltham, MA, USA), with microscopes equipped with an EDS detector. Raman spectroscopy was tested with 532 nm laser excitation (LABRAM, HR). XPS analysis was conducted using an ESCALAB 250Xi spectrometer (VG, Altrincham, UK) with Al Kα radiation. In order to exclude the surface contamination, the XPS was tested after Ar+ etching for 2 min. The samples were first washed using DIGLYME and dried under vacuum, and then transferred into the analysis chamber by a vacuum transfer device.

CR2032 coin cells were assembled in an argon-filled glove box (<1 ppm O₂ and H₂O) using glass fiber (GF/A) separators. The electrolyte was 80 μL localized high-concentration electrolyte (LHCE, NaFSI: DME: TTE = 1:1.2:1, by molar ratio) (a locally high-concentration electrolyte was selected to measure the electrochemical performance since it is suitable for RT Na-S batteries by suppressing the polysulfide shuttle effect and preventing sodium dendrite growth) [14]. The activation number of cycles was five. All cycling and rate performance tests were conducted using a Neware battery test system (NIHW-200-160CH-8, Shenzhen Neware Technology Co. Ltd., Shenzhen, China) at a voltage range of 0.8–2.8 V (vs. Na⁺/Na). The active materials were calculated based on the sulfur-equivalent mass. Electrochemical Impedance Spectroscopy (EIS) was carried out on a VPM300 analyzer (Bio-logic, Seyssinet-Pariset, France). The cyclic voltammetry (CV) measurements were evaluated using the same electrochemical workstation at various scan rates from 0.1 to 0.5 mV s⁻¹ over the potential range from 0.8 V to 2.8 V. The Randles–Sevcik equation was used to identify the Na⁺ diffusion coefficient. First, an initial formation cycle was performed by charging to 3.2 V and then discharging to 0.8 V at 0.1 C. Subsequently, the cells were activated by 5 additional charge/discharge cycles within the normal voltage window of 0.8–2.8 V. All subsequent long-term cycling tests were carried out strictly between 0.8 V and 2.8 V.

Galvanostatic Intermittent Titration Technique (GITT) test: The assembled Na-S batteries were cycled at 0.1 C for 20 cycles to achieve a stable state. Then, GITT tests were performed by applying a 5 min discharge or charge at 0.1 C followed by a 50 min rest period in the potential range from 0.8 V to 2.8 V.

2.4. Calculation Details

In the calculations, I_p is the peak current (mA); ν is the scanning rate (mV s⁻¹); A is the surface area of the electrode; D is the diffusion coefficient of the active substance; and C is the concentration of the active substance. I_p is proportional to v^1/2, and the slope, diffusion coefficient of the active substance D, can be calculated by fitting the data.

Theoretical expressions of the current–time transients of four classic electrochemical deposition models were presented as follows [15]:

$$2\mathrm{DI}:{\displaystyle \frac{i}{i_m}}=\left({\displaystyle \frac{t}{t_m}}\right)\left\{exp\left[{\displaystyle \frac{t_m^2-t^2}{{2t}_m^2}}\right]\right\}$$

(1)

$$2\mathrm{DP}:{\displaystyle \frac{i}{i_m}}={\left({\displaystyle \frac{t}{t_m}}\right)}^2\left\{exp\left[{\displaystyle \frac{-2\left(t^2-t_m^3\right)}{3t_m^2}}\right]\right\}$$

(2)

$$3\mathrm{DI}:{\displaystyle \frac{i}{i_m}}={\left({\displaystyle \frac{1.9542}{t/t_m}}\right)}^{1/2}\left\{1-exp\left[-1.2564\left({\displaystyle \frac{t}{t_m}}\right)\right]\right\}$$

(3)

$$3\mathrm{DP}:{\displaystyle \frac{i}{i_m}}={\left({\displaystyle \frac{1.2254}{t/t_m}}\right)}^{1/2}\left\{1-exp\left[-2.3367{\left({\displaystyle \frac{t}{t_m}}\right)}^2\right]\right\}$$

(4)

3. Results and Discussion

3.1. Material Characterization Analysis

The morphology and internal structural characteristics of the Na₂Se/Na₂S@CNFs were characterized by SEM, TEM, and XRD. Figure 1 shows that nanofibers were successfully prepared, with no obvious particles observed, indicating that the products were all encapsulated within the nanofibers.

Figure 1. (a) SEM image and EDS surface distribution mappings of (b) C, (c) Na, (d) S, and (e) Se of Na₂Se/Na₂S@CNFs.

For comparison, Na₂S@CNF samples without the addition of Na₂SeO₃ were also prepared in the experiment. Both samples exhibited similar nanofiber morphologies, demonstrating that the introduction of Na₂SeO₃ did not alter the carbon structure of the nanofibers. EDS and elemental mapping revealed, in detail, the distribution of C, Se, Na, and S elements within the Na₂Se/Na₂S@CNFs. The results show that C, Se, Na, and S are uniformly distributed throughout the composite, which also indicates the homogeneity of the active materials (Na₂S and Na₂Se) within the nanofibers. Since the electronic conductivity of Na₂Se is superior to that of Na₂S, the uniform distribution of Na₂Se enhances the material’s conductivity, lowers the initial reaction energy barrier of the Na₂S cathode, and improves the battery performance.

As shown in Figure 2, the XRD of the Na₂Se/Na₂S@CNF composite is basically consistent with the standard pattern of Na₂S (PDF#23-0441). No diffraction peaks of Na₂SO₄ are observed, indicating a high conversion rate during the carbothermic reduction process. Compared with the standard pattern of Na₂Se (PDF#47-1669), only one main peak is present for Na₂Se, and this diffraction peak is broadened, which may be attributed to the relatively small content and small particle size of Na₂Se. Apart from Na₂S and Na₂Se, no other impurity peaks are found in the XRD pattern, confirming that the material has been successfully prepared.

Figure 2. XRD pattern of Na₂Se/Na₂S@CNFs.

A microstructural analysis of the material was carried out by TEM. Figure 3a shows that the diameter of the Na₂Se/Na₂S@CNF composite nanofibers is approximately 700 nm. TEM analysis also reveals that the fiber diameters of the two samples differ considerably. As shown in Figure 3b, Na₂S and Na₂Se are dispersed as fine particles within the nanofibers of the Na₂Se/Na₂S@CNFs. Meanwhile, the lattice fringes of Na₂S and Na₂Se are continuous at the interface with no obvious gaps, indicating that Na₂S and Na₂Se exist in the material in the form of heterojunctions.

Figure 3. (a) TEM and (b) HRTEM images of Na₂Se/Na₂S@CNFs.

The lattice fringe spacings observed in the TEM images are 0.341 nm and 0.327 nm, corresponding to the (200) crystal planes of Na₂Se and Na₂S, respectively. These results were verified by fast Fourier-transform (FFT) patterns and inverse FFT analysis (Figure S1).

To further investigate the microcrystalline structure of the Na₂Se/Na₂S@CNF material, a selected area electron diffraction (SAED) analysis was performed (Figure 4a). In the SAED pattern, no sharp and clear diffraction rings were observed; instead, diffuse diffraction halos predominated, indicating that the overall crystallinity of the sample was limited. This is consistent with the amorphous nature of the carbon nanofiber matrix and the small grain size of the active components.

Figure 4. (a) SAED image and (b) Raman spectroscopy of Na₂Se/Na₂S@CNFs.

The Raman analysis (Figure 4b) shows that there are characteristic peaks at 1354 cm⁻¹ and 1594 cm⁻¹, corresponding to the D band (disordered structure) and G band (ordered structure) of graphite, respectively. The ID/IG ratio of Na₂Se/Na₂S@CNFs is lower than that of Na₂S@CNFs, indicating that the introduction of Na₂Se promotes the graphitization of the carbon matrix to a certain extent, which is beneficial to electron conduction.

In the Na₂Se/Na₂S@CNFs (Figure 5), the S 2p spectrum shows doublets at 161.9 eV and 163.2 eV, which can be assigned to S²⁻ originating from Na₂S in the sample, whereas the doublets at 163.8 eV and 165.1 eV correspond to S₂²⁻, attributable to Na₂S₂ (which may be formed by the partial decomposition of Na2S during high-temperature heating). The Se 3d spectrum confirms the presence of Na₂Se in the sample [16]. Compared with Na₂Se/Na₂S@CNFs, in the S 2p spectrum of Na₂S@CNFs, the doublets with binding energies at 161.3 eV and 162.5 eV arise from Na₂S in the sample; the doublets at 163.3 eV and 164.4 eV originate from Na₂S₂. In addition, the doublets at higher binding energies are attributed to the partial oxidation of Na₂S in air or unreduced Na₂SO₄. The Na₂S content in Na₂Se/Na₂S@CNFs was determined by calculating the mass change before and after water washing: the total mass fraction of the active components (Na₂S and Na₂Se) in the composite is approximately 66 wt%. Among them, the mass fraction of Na₂S is about 55 wt%, from which the molar ratio of Na₂S to Na₂Se is estimated to be approximately 8:1 (Table S1).

Figure 5. High-resolution XPS spectra of (a) C 1s, (b) S 2p and (c) Se 3d for Na₂Se/Na₂S@CNFs.

3.2. Electrochemical Performance Analysis

To more objectively evaluate the influence of the intrinsic structural design of the cathode material on electrochemical performance and to eliminate, as much as possible, interference from the electrolyte on the test results, this study employed a superior localized high-concentration electrolyte (LHCE) for cell assembly and testing.

This electrolyte can effectively suppress polysulfide shuttling, thereby more faithfully reflecting the performance advantages of the as-prepared cathode materials. Likewise, prior to cycling, the Na₂Se/Na₂S@CNF cathode was subjected to an activation treatment by charging from the open-circuit voltage to 3.2 V, in order to overcome the high energy barrier required for Na₂S activation, activate part of the difficult-to-activate Na₂S, increase the capacity of the sodium–sulfur battery, and thereby enable better performance during subsequent cycling [17].

As shown in Figure S2, the initial charge–discharge curves of the Na₂Se/Na₂S@CNF cathode at 0.1 C indicate that the initial charge capacity of this cathode is 664 mAh g⁻¹, higher than the 408.0 mAh g⁻¹ of Na₂S@CNFs. This is attributed to the stronger metallic character and better conductivity of selenium; the introduction of Na₂Se improves the electronic conductivity of the material, allowing Na₂S to release a higher capacity during activation. The electrochemical reaction mechanism of the Na₂Se/Na₂S@CNF cathode was investigated by CV. Compared with Na₂S@CNFs, Na₂Se/Na₂S@CNFs exhibit smaller voltage polarization and a sharper oxidation peak, indicating that Na₂Se lowers the reaction energy barrier and accelerates charge transfer. The Na⁺ diffusion kinetics were analyzed from CV curves at different scan rates (Figure 6a,b). At low scan rates, Na₂Se/Na₂S@CNFs and Na₂S@CNFs show similar oxidation and reduction peaks; however, as the scan rate increases, the reduction peak of Na₂Se/Na₂S@CNFs changes. Further comparison of the peak current of oxidation peak I (Figure 6c) shows that Na₂Se/Na₂S@CNFs exhibit higher peak currents at different scan rates, indicating that Na₂Se improves the conversion reaction kinetics of Na₂S.

Figure 6. CV curves of (a) Na₂Se/Na₂S@CNFs and (b) Na₂S@CNFs tested at different scan rates; (c) peak currents of the electrochemical process (I: Na₂S_x → S₈).

As shown in Figure 7a, the Na₂Se/Na₂S@CNF cathode material exhibits excellent rate performance. At current densities of 0.2, 0.5, 1, 2, 3, and 5 C, the reversible specific capacities are 774, 722, 636, 574, 494, and 396 mAh g⁻¹, respectively. When the current density is returned from 5 C to 0.2 C, the specific capacity quickly recovers to its original level, indicating the superior rate capability of the Na₂Se/Na₂S@CNF material and its potential for good adaptability and reliability in practical applications. In addition, the charge–discharge curves of Na₂Se/Na₂S@CNFs at different current densities (Figure 7b) remain essentially stable.

Figure 7. (a) Rate performance of Na₂Se/Na₂S@CNFs at current rate from 0.2 to 5 C; (b) charge–discharge curves of Na₂Se/Na₂S@CNFs at different current rates.

The Na₂Se/Na₂S@CNF sample exhibits outstanding cycling performance. After 300 cycles at 1 C, it still maintains a high reversible capacity of 600 mAh g⁻¹ (Figure 8), far exceeding the capacity retention of Na₂S@CNFs (347 mAh g⁻¹). These results demonstrate that the Na₂Se/Na₂S@CNF cathode material exhibits excellent capacity delivery and remarkable cycling stability.

Figure 8. (a) Charge–discharge curves of Na₂Se/Na₂S@CNFs at different cycle numbers; (b) long-cycle performance of Na₂Se/Na₂S@CNFs at 1 C.

As shown in Figure S3, after 50 cycles at 0.1 C, the fiber morphology of the Na₂Se/Na₂S@CNF cathode remains intact, with no obvious structural collapse or agglomeration. Meanwhile, the elemental mapping results show that C, Se, Na, and S are highly uniformly distributed in the cathode. The battery capacity increases during the cycling process. This phenomenon is primarily attributed to the gradual activation of the electrode material during the initial cycling stages. As the charge–discharge process proceeds, the electrolyte progressively penetrates deeper into the porous carbon nanofiber network, wetting more of the Na₂S/Na₂Se active sites that were initially inaccessible. This also indirectly reflects the homogeneity of the active material distribution within the material. This evidence fully demonstrates the structural stability of the fiber material, and although the introduced Na₂Se is relatively small in amount, its uniform distribution helps accelerate the conversion and redistribution of polysulfides during cycling. Such a uniform and stable electrode structure is the foundation for ensuring the long-term cycling life of the battery.

3.3. Reaction Kinetics Analysis

As mentioned above, Na₂Se reduces electrochemical polarization and facilitates Na₂S activation during the activation process, thereby promoting the oxidative decomposition of Na₂S, improving the utilization of active materials, and releasing more capacity. To further reveal the differences in Na⁺ diffusion kinetics among different host materials, GITT was employed to characterize the electrodes (Figure 9).

Figure 9. GITT curve of (a) Na₂Se/Na₂S@CNFs and (b) Na₂S@CNFs.

The GITT curves clearly show that Na₂Se/Na₂S@CNFs exhibit lower polarization voltage during charge–discharge cycling, indicating faster ion diffusion kinetics and better electrochemical reaction reversibility, thus further confirming the significant promoting effect of Na₂Se introduction on the reaction kinetics.

To further investigate the kinetic performance of the electrode materials, the sodium-ion diffusion coefficient (DNa⁺) was calculated and compared using GITT data combined with Fick’s second law. This diffusion rate is a physical quantity used to measure the speed of sodium-ion movement within the electrode material; a faster rate implies that the battery can withstand higher charge/discharge currents, enabling fast charging or high-power output. As shown in Figure 10, at the initial stage of charging, the DNa⁺ value of Na₂Se/Na₂S@CNFs is significantly higher than that of Na₂S@CNFs. In the subsequent low-voltage region, its DNa⁺ is slightly lower than that of the comparison sample, but immediately afterwards, in a relatively large high-voltage region, its diffusion coefficient is clearly higher than that of Na₂S@CNFs. This result directly demonstrates that the presence of Na₂Se significantly promotes the activation of Na₂S and its conversion kinetics. During the discharge process, the DNa⁺ of Na₂Se/Na₂S@CNFs is also higher than that of Na₂S@CNFs at the initial stage, and when discharged to the low-voltage region of 0.8 V, its diffusion coefficient still maintains an advantage.

Figure 10. Calculated Na⁺ diffusion coefficients for Na₂Se/Na₂S@CNFs||Na and Na₂S@CNFs||Na batteries under (a) charging and (b) discharging.

To further investigate the electrodeposition kinetics of Na₂S, we studied the nucleation efficiency of Na₂S through potentiostatic nucleation analysis. The conversion of short-chain Na₂S₂ to Na₂S is the main source of capacity during discharge. To distinguish the electrodeposition of Na₂S from the reduction of Na₂S₆ in the solution, the battery was first galvanostatically discharged to 1.0 V at C/24 rate, followed by electrodeposition at a fixed potential of 0.8 V.

The nucleation capacity calculated according to Faraday’s law shows that the nucleation capacity of Na₂Se/Na₂S@CNFs is 191.84 mAh g⁻¹, which is greater than the 52.23 mAh g⁻¹ of Na₂S@CNFs. In addition, the Na₂Se/Na₂S@CNF cathode exhibits an earlier nucleation response, fully demonstrating that Na₂Se has excellent electrocatalytic activity toward short-chain sodium polysulfides, effectively accelerating the reduction kinetics of Na₂S (Figure 11a,b). The superior kinetics can promote the conversion of short-chain Na₂S₂ to Na₂S, which can provide more capacity for the battery during discharge. To further explore the regulation mechanism of Na₂Se/Na₂S@CNFs on the deposition pathway of Na₂S, we analyzed the dimensionless current–time transient curves obtained from potentiostatic discharge and fitted them to four classical electrochemical deposition models (Figure 11c,d). Among them, the 2D instantaneous nucleation and 2D progressive nucleation models involve two-dimensional planar growth, while the 3D instantaneous nucleation and 3D progressive nucleation models describe diffusion-controlled three-dimensional hemispherical nucleation and growth. The fitting results indicate that the Na₂S precipitation process on both electrodes does not conform to the 2D nucleation models, suggesting that the nucleation process may not depend on specific crystal planes. The deposition process on Na₂Se/Na₂S@CNFs conforms to the 3D instantaneous nucleation model, whereas the deposition behavior on Na₂S@CNFs lies between 3D instantaneous nucleation and 3D progressive nucleation. This result strongly demonstrates that the generated Na₂Se exerts a significant synergistic regulation effect on Na₂S nucleation, guiding it toward a more uniform and kinetically favorable instantaneous nucleation mode.

Figure 11. Na₂S nucleation tests based on (a) Na₂Se/Na₂S@CNFs and (b) Na₂S@CNFs; dimensionless current–time transient curves for (c) Na₂Se/Na₂S@CNFs and (d) Na₂S@CNFs.

The elemental distribution indicates that the material deposited on the cathode surface is Na₂S. Compared with the irregular, blocky Na₂S growth morphology on the Na₂S@CNF cathode, the Na₂Se/Na₂S@CNF electrode surface forms a flat and uniform Na₂S deposition layer (Figures S4 and S5), indicating that Na₂Se induces a more uniform and denser nucleation process.

To investigate the kinetic issues during the charge–discharge process, in situ EIS measurements were employed. As shown in Figures S6 and S7, the impedance of the Na₂Se/Na₂S@CNFs cathode throughout the entire charge–discharge process is lower than that of Na₂S@CNFs. This is because the introduction of Na₂Se increases the electronic conductivity of the cathode and simultaneously accelerates the conversion of NaPSs, directly reducing the electron transport resistance within the electrode. The impedance of the Na₂Se/Na₂S@CNF cathode gradually decreases during charging, which is attributed to the extraction of sodium ions, leading to the progressive oxidation of poorly conductive Na₂S into polysulfides and, ultimately, to elemental sulfur. During discharge, the impedance gradually increases because insulating Na₂S gradually deposits on the electrode surface and on the nanofibers, covering active sites and physically blocking electron transport pathways. However, the presence of Na₂Se enhances the conductivity of the insulating products throughout the charge–discharge process and promotes the conversion of intermediates, thereby suppressing excessively rapid impedance growth. This is the intrinsic reason why the Na₂Se/Na₂S@CNF electrode exhibits excellent rate performance and cycling stability.

The DRT method was further employed to conduct an in-depth analysis of the impedance evolution during the charge–discharge cycling process (Figure 12). DRT decomposes the total impedance into different time scales, and the four observed characteristic time constants correspond to the cathode charge-transfer impedance (τ₁, τ2), polysulfide diffusion impedance (τ₃), and ionic diffusion impedance (τ₄), respectively [18].

Figure 12. DRT analyses of Na₂Se/Na₂S@CNFs during (a) charging and (b) discharging processes.

At the initial stage of charging (Na₂S → short-chain NaPSs), Na₂Se/Na₂S@CNFs exhibit a relatively high τ₁ value, which is attributed to the fact that the electrode surface is covered with insulating Na₂S, requiring electrons to pass through the Na₂S layer to initiate the desodiation reaction. However, its τ₄ value is lower, because Na₂Se may contribute to improved apparent ion-transport kinetics.

When charged to approximately 1.2 V, all impedance values except τ₄ decrease, as Na₂S is gradually consumed and kinetically more favorable short-chain polysulfides are generated. The recovery of the conductive network makes charge transfer exceptionally facile. As the charging voltage reaches around 2.4 V, the concentration of polysulfides in the cathode region decreases, and the reduction in reactant concentration leads to an increase in the impedance value corresponding to τ₃ in the DRT. For Na₂Se/Na₂S@CNFs, because it can rapidly convert these polysulfides, the increase in τ₃ is much smaller than that of the comparison sample.

When charging approaches 2.8 V, the impedance does not rise sharply compared with Na₂S@CNFs. This is attributed to the electronic conductivity provided by Se, ensuring that even at high potentials, charge can be transferred rapidly to complete the final reaction steps. Therefore, the presence of Se effectively suppresses interface degradation caused by sluggish kinetics through improving electronic conductivity and catalyzing polysulfide conversion.

As the reaction proceeds to the discharge stage (S₈ → NaPSs → Na₂S), NaPSs are gradually reduced, eventually depositing solid, highly insulating Na₂S on the electrode surface. Although the impedance of Na₂Se/Na₂S@CNFs increases during discharge, compared with Na₂S@CNFs, the increases in τ₁, τ2, and τ₃ for Na₂Se/Na₂S@CNFs are smaller and their final values are lower. In Na₂S@CNFs, due to the lack of catalysis from Na₂Se, polysulfide conversion is slower, which may lead to uneven Na₂S deposition and the formation of large agglomerates. In contrast, the slower impedance rise in Na₂Se/Na₂S@CNFs demonstrates that Na₂Se can improve conductivity and catalyze conversion, also playing a critical role in optimizing kinetics and suppressing interface degradation during discharge.

To further elucidate the state of the Na₂Se/Na₂S@CNF material at different potentials, ex situ XPS was employed to analyze the evolution of intermediates during charge–discharge (Figure 13). When the battery is charged to 2.0 V, characteristic doublets corresponding to both Na2S and sodium polysulfides are present in the sample, indicating the onset of oxidation of the solid-state discharge product. When charged to 2.8 V, the characteristic peaks of Na₂S in the Na₂Se/Na₂S@CNF electrode disappear completely and are converted to S₈ peaks, indicating that the reaction is nearly complete. In contrast, the doublets at 160.1 eV and 161.1 eV persist in Na₂S@CNFs, indicating the presence of unreacted Na2S. The introduction of Na₂Se significantly improves the reaction kinetics of this step by enhancing the electronic conductivity of Na₂S, effectively promoting the reaction process and thus improving the utilization of active materials. When discharged to 1.5 V, the S₈ in the Na₂Se/Na₂S@CNF electrode has been completely reduced to short-chain NaPSs, whereas the control Na₂S@CNF sample still shows obvious residual S₈ signals, indicating sluggish reaction kinetics in the absence of Na₂Se. Until the final discharge state of 0.8 V, no S₈ signal is detected in either sample, confirming the completeness of the reaction. The introduction of Na₂Se improves the ionic/electronic conductivity of the cathode and optimizes the redox kinetics of the sulfur cathode, which is key to enhancing the overall reaction efficiency.

Figure 13. (a) S 2p spectra of Na₂Se/Na₂S@CNFs charged to 2.0 V; (b) S 2p spectra of Na₂Se/Na₂S@CNFs charged to 2.8 V; (c) S 2p spectra of Na₂Se/Na₂S@CNFs discharged to 1.5 V; (d) S 2p spectra of Na₂Se/Na₂S@CNFs discharged to 0.8 V.

4. Conclusions

In this study, Na₂Se/Na₂S@CNF composite cathode materials were successfully fabricated, and their structural features and electrochemical performance were systematically investigated. The results reveal that the porous carbon coating and the highly graphitized carbon fiber skeleton synergistically construct an efficient electron/ion transport network while potentially helping mitigate polysulfide loss. More importantly, the Na₂Se component induces Na₂S to grow via a three-dimensional instantaneous nucleation mode, forming a uniform and compact deposition layer, which significantly enhances the active material utilization and cycling stability. Benefiting from these advantages, the Na₂Se/Na₂S@CNFs deliver a specific capacity of 396 mAh g⁻¹ at a high rate of 5 C and retain 600 mAh g⁻¹ after 300 cycles at 1 C, demonstrating outstanding rate capability and long-term cycling stability. In future work, we will systematically optimize the electrode architecture to achieve higher areal loadings and evaluate the electrochemical performance under practical conditions, including lean electrolyte and high-mass-loading configurations. This work provides new insights into the design of cathode materials for room-temperature sodium–sulfur batteries.