The development of advanced electrode materials for sodium-ion batteries (SIBs) is crucial for the progression of energy storage technologies. In this study, we successfully fabricated V5S8/CoS nanoparticles confined within self-supported carbon nanofibers (CNFs) using a facile electrospinning method followed by a sulfidation process. Extensive characterization of the resulting V5S8/CoS-CNFs revealed their unique structural attributes, featuring a one-dimensional nanofiber morphology with enhanced Na+ transport pathways. These V5S8/CoS-CNFs exhibited a remarkable reversible capacity of 201 mAh g−1 even at a high current density of 5 A g−1, along with a stable cycling performance of 165 mA h g−1 after 300 cycles at 2 A g−1. The incorporation of both V5S8 and CoS within the nanofiber structure substantially enhanced the pseudocapacitance effect, thereby improving sodium storage capabilities. The exceptional electrochemical properties of the binder-free V5S8/CoS-CNFs anode can be attributed to its heterogeneous composition embedded within CNFs. This composition effectively boosts the rate of sodium ion diffusion by generating a built-in electric field (BEF) at the V5S8 and CoS interface, alleviating volume stress during charge-discharge processes and enhancing overall conductivity. Our findings underscore the potential of V5S8/CoS-CNFs as high-performance anode materials for SIBs, offering valuable insights into the design and development of advanced electrode materials for future energy storage applications.
Research Article
Synergistic V5S8/CoS Nanoparticles-Embedded Carbon Nanofiber as Binder-Free Flexible Anode for Sodium-Ion Batteries
https://doi.org/10.21203/rs.3.rs-3379644/v1
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V5S8/CoS nanoparticles
nanofiber
heterostructure
kinetic properties
sodium-ion batteries༛
Lithium-ion batteries (LIBs) have captured a significant portion of the market due to their relatively high energy density and long cycle life[1, 2]. However, limited resources severely constrain the development of LIBs[3, 4]. On the other hand, Sodium-ion batteries (SIBs) have garnered attention due to abundant reserves and a similar 'rocking chair' working mechanism model as LIBs[5]. However, SIBs also face the same problems as LIBs. For instance, there is a lack of an excellent performance anode with high capacity, long cycle performance, and high initial coulombic efficiency (ICE) [6].
Based on the Na+ storage mechanism, anode materials can be categorized into three types: intercalation-type, alloy-type, and conversion-type[7]. Conversion-type anode materials offer the advantage of very high capacity, but they face challenges related to slow kinetics, including the formation of amorphous and intermediate phases, as well as side reactions[8]. These issues result in large voltage hysteresis, volume expansion, and low initial coulombic efficiency (ICE). Transition metal sulfides (TMSs) generally exhibit higher ductility compared to their corresponding oxides (TMOs)[9]. Among TMSs, cobalt sulfide stands out as a representative p-type semiconductor, known for its cost-effectiveness, abundant natural reserves, and considerable theoretical capacity[10]. Additionally, vanadium-based materials have garnered significant interest due to their low cost, abundant resources, and rich valence states[11]. To address these challenges, the construction of heterostructures with nanometer-distributed various components has been proposed, taking advantage of the "nano-crystallization effect" and "synergistic effect"[12]. Heterostructures can induce interfacial energy band changes between different components, forming heterojunctions that create a built-in electric field (BEF) in a defined direction, effectively enhancing electron transport at the interface[13]. Furthermore, heterostructures with abundant phase boundaries can accelerate solid-state diffusion and induce significant pseudocapacitance effects[14]. Consequently, heterostructure materials exhibit superior ion diffusion kinetics compared to monomaterials[15], making them an effective approach to achieving superior anode materials[16].
Electrospinning is widely employed in nanomaterial synthesis, particularly for fabricating heterostructures[17]. It allows for precise modulation of the composition, structure, and assembly of nanofibers[18]. CNFs are considered one of the most suitable materials for constructing free-standing electrodes due to their unique structure and transport properties[19–21]. Additionally, CNFs can be used directly as electrodes, eliminating the need for the addition of electrochemically inert materials such as binders[22].
Combining the requirements for both conductivity and stability, we successfully integrated V5S8/CoS heterostructures into polyacrylonitrile carbon nanofibers (V5S8/CoS-CNFs) using a straightforward electrostatic spinning method, followed by a one-step sulfidation process. The resulting V5S8/CoS-CNFs exhibited outstanding performance in terms of both rate capability (201 mAh g− 1 at 5 A g− 1) and cycling stability (165 mAh g− 1 after 300 cycles at 2 A g− 1). The remarkable performance of these materials can be attributed to the synergistic effects of two factors: firstly, the creation of a built-in electric field (BEF) within the heterostructure and secondly, the guided directional transport of ions and electrons facilitated by the electrospun carbon fibers. Additionally, we conducted a systematic investigation into the effects of active material content on the structural stability and electrochemical properties of the carbon fibers. This work lays a solid foundation for the development of high-performance free-standing electrode materials. It's worth noting that our work with V5S8/CoS-CNFs represents a significant contribution, as there have been no prior reports on enhancing the performance of Sodium-ion batteries (SIBs) to this extent.
2.1 Materials
Polyacrylonitrile (PAN, Mw = 150,000) was obtained from Dongguan Haosheng Plastic Raw Materials Co., Ltd. N,N-Dimethylformamide (DMF, HPLC, ≥ 99.9%) was purchased from Shanghai Macklin Biochemical Co., Ltd. C4H6CoO4•4H2O (AR) and VO(acac)2 (99%) were also sourced from Shanghai Macklin Biochemical Co., Ltd. The electrolyte used for Sodium-ion batteries (SIBs) consisted of 1M NaPF6 in DIGLYME (100 vol%).
2.2 Synthesis of Z-V5S8/CoS-CNFs
In the experiments, 0.938 g of polyacrylonitrile (9 wt%) was dissolved in 10 mL of DMF and stirred for 4 h at room temperature to form a homogeneous PAN solution. Then, 0.249 g of C4H6CoO4•4H2O (0.1 mol L− 1) and 0.265 g of VO(acac)2 (0.1 mol L− 1) were added to the above solution and stirred for 6 h to create the spinning solution. This spinning solution was loaded into a 10 mL syringe equipped with a 20 G needle. Each spinning process utilized 20 mL of the spinning solution. For electrostatic spinning, the working voltage was set at 18 kV, the distance between the needle and the collector was 15 cm, the spinning temperature was maintained at 35°C with humidity at 30%, and the pushing speed was 0.0015 mm s− 1. The drum rotation speed was set at 200 r min− 1, allowing the precursor fibers to be collected on the roller. The precursor nanofibers were initially stabilized in an oven at 80°C for 8 h to ensure complete solvent evaporation, after which the yellow fibers were collected from the oven. Subsequently, they underwent pre-oxidation in a tube furnace under an air atmosphere at 260°C (heating rate 2°C/min) for 2 h. Finally, the fibers were vapor-phase sulfided along with sublimated sulfur (mass ratio = 1:10) in a tube furnace under a nitrogen atmosphere at 800°C (heating rate 5°C/min) for 2 h, resulting in the formation of 0.2-V5S8/CoS-CNFs. The metal content in the samples was adjusted by varying the masses of C4H6CoO4•4H2O and VO(acac)2 in the PAN solution while maintaining a constant molar concentration ratio of the two metal ions in the solution, CV:CCo=1:1. The corresponding samples were denoted as Z-V5S8/CoS-CNFs (Z = 0, 0.1, 0.15, 0.2, 0.3), where Z represents the total molar concentration of the two types of metals in the spinning solution. The samples at Z = 0 were referred to as CNFs for shortness.
2.3 Material Characterization
The morphology and microstructure of the samples were examined using a scanning electron microscope (SEM, Vltra55) and a transmission electron microscope (TEM, JEOL JEM-F200). The crystal morphology and Raman spectra of the samples were determined using an X-ray diffractometer (XRD, ARL XTRA) and a Raman spectrometer (Raman, ATR30007), respectively. Additionally, the composition of each component in the samples was analyzed through thermogravimetric analysis (TGA, TG 209 F1 Libra®).
2.4 Electrochemical measurements
The electrochemical performance of the materials was investigated by assembling 2032 coin-type half cells within an Ar-filled glove box (with H2O and O2 levels both < 0.1 ppm). Free-standing electrodes (CNFs, Z-V5S8/CoS-CNFs) were cut into discs with a diameter of 14 mm (as shown in Fig. S1c) and directly employed as working electrodes. Mass loadings on each electrode fell within the range of 2.12–5.18 mg. Glass fibers (Whatman GF/C) and sodium metal were utilized as the separator and counter electrodes, respectively. The electrolyte used was 1 M NaPF6 in diglyme (100 vol%). To ensure proper wetting of the separator, newly installed batteries were left undisturbed for 8–12 h before testing[23]. Various electrochemical tests were conducted, including cyclic voltammetry (CV, 0.1 mV s− 1, in the voltage range of 0.05–2.5 V) and electrochemical impedance spectroscopy (EIS, 5 mV amplitude, in the frequency range of 10− 2 Hz to 105 Hz) using a CHI760E electrochemical workstation (Chenhua, Shanghai, China). Additionally, galvanostatic charge/discharge (GCD, in the voltage range of 0.05–2.5 V) and galvanostatic intermittent titration technique (GITT, at 0.1 A g− 1, with 30-minute discharge followed by a 60-minute relaxation period) were performed using a NEWARE CT-3008W battery test system. It's worth noting that the charge/discharge capacity was calculated based on the entire mass of the disk, and all electrochemical performance tests were conducted at room temperature.
3.1 Characterization of Z-V5S8/CoS-CNFs
Figure 1 illustrates the synthesis process of free-standing Z-V5S8/CoS-CNFs anode materials. The sample was prepared through a pre-oxidation treatment (Fig. S1a) followed by a subsequent one-step sulfidation process (Fig. S1b). The morphology and crystal structure of Z-V5S8/CoS-CNFs were characterized using SEM and TEM. As depicted in Fig. 2a-2c, SEM analysis revealed the smooth surface of the CNFs. However, the surface exhibited roughness after pre-oxidation, likely attributed to the expulsion of small gas molecules from the material during the pre-oxidation process[24].
The structure of Z-V5S8/CoS-CNFs presents a one-dimensional homogeneous nanofiber structure with an average diameter of 200 nm (Fig. S2). This small size is pivotal in shortening the Na+ transport path during charge and discharge processes[25]. As the metal concentration increases, there is a slight trend of increasing fiber diameter. Additionally, there is a noticeable increase in the number of metal particles adhering to the fiber surface. Beyond a metal concentration of 0.2 mol L− 1, spherical and lamellar crystals become distinctly visible on the fiber surface (Fig. S3). These correspond to cobalt sulfides and vanadium sulfides, respectively, indicating that the carbon nanofiber fails to effectively bind the metal sulfide crystals that form during the sulfidation process.
Subsequently, the 0.2-V5S8/CoS-CNFs were further characterized by TEM. In Fig. 2d, it can be observed that V5S8 and CoS are uniformly dispersed within the CNFs, with the darker granular shapes representing the nanoparticles of the two metal sulfides, aligning with the original design. In Fig. 2e, the two distinctive lattice stripes of the substances correspond to the (420) crystal plane of V5S8 and the (002) crystal plane of CoS, with interplanar distances of 0.227 nm and 0.276 nm, respectively. The clearly defined interface between the two crystalline surfaces confirms the presence of a heterostructure. The crystal plane spacing periodogram (Fig. 2e) further supports the accurate determination of V5S8 and CoS lattice spacing. Interfaces and lattice mismatch (m) can be calculated using Eq. (1) [26]:
$$\text{m=}\frac{\left|{\text{d}}_{\text{1}}\text{sin}\text{θ-}{\text{d}}_{\text{2}}\right|}{\text{0.5×(}{\text{d}}_{\text{1}}\text{sin}\text{θ}\text{+}{\text{d}}_{\text{2}}\text{)}} \left(1\right)$$
Where m represents the lattice mismatch, d₁ and d₂ are the interplanar spacings associated with specific crystallographic planes, and θ denotes the angle of incidence, also known as the bragg angle.
For example, lattice fringes corresponding to the (420) plane of V5S8 and the (002) plane of CoS, observed at a 50 ° angle (as shown in Fig. 2e), exhibited a lattice mismatch (m) of 42.25%. Notably, significant lattice distortions at the interfaces of bimetallic sulfides can generate an abundance of crystal defects, thereby enhancing conductivity and providing more active sites for Na+ storage[27]. The selected area electron diffraction (SAED) plot in Fig. 2g reveals multiple concentric diffraction rings associated with V5S8 and CoS, thus confirming the formation of the V5S8/CoS heterostructure. The yellow dashed ring pattern corresponds to the (222) and (-224) crystal planes of V5S8, while the green dashed ring pattern corresponds to the (101) and (110) crystal planes of CoS (Fig. S4). Elemental mapping in Fig. 2h clearly demonstrates the uniform distribution of S, V, and Co within the 0.2-V5S8/CoS-CNFs, consistent with the observations in the TEM images.
To determine the sample composition, both CNFs (as shown in Fig. S5) and 0.2-V5S8/CoS-CNFs (as shown in Fig. 3a) underwent XRD analysis to investigate their physical phases. It is noteworthy that the diffraction peaks of 0.2-V5S8/CoS-CNFs exhibit a strong correspondence with the characteristic peaks of V5S8 (ICSD, Card No. 97-065-1379) and CoS (ICSD, Card No. 97-062-4857), thereby confirming the successful incorporation of transition metal sulfides (TMSs) into the CNFs. Notably, these diffraction peaks are characterized by their sharpness and high intensity, indicating a high degree of crystallinity.
In the Raman spectra, the characteristic D peak at 1351.792 cm− 1 and G peak at 1590.824 cm− 1 are evident. The D peak is associated with the disordered structure of carbon, vacancies, and other defects, while the G peak is related to ordered sp2 hybridized graphitic carbon[28]. Notably, the intensity ratio of the D and G peaks is often indicative of the concentration of defects within the carbon structure[29]. In the Raman spectra of CNFs, the intensity ratio of the D and G peaks was measured as 1.42. Upon the incorporation of metal sulfides, the intensity ratio for 0.2-V5S8/CoS-CNFs increased to 1.92 (as shown in Fig. 3b). This higher intensity ratio of the D and G peaks in 0.2-V5S8/CoS-CNFs indicates a higher concentration of defects, which is beneficial for improving the rate performance and storage capacity of the material. The mass fractions of V5S8 and CoS within the 0.2-V5S8/CoS-CNFs were determined to be 16.78% and 14.93%, respectively, based on the weight loss curves (Fig. S6).
3.2 Electrochemical Performance of Z-V5S8/CoS-CNFs in Sodium-ion Battery
To further elucidate the advantages of sodium storage, a 2032 button cell was assembled using Z-V5S8/CoS-CNFs as the anode and sodium metal as the counter electrode. The results of CV for the first three cycles of 0.2-V5S8/CoS-CNFs are presented in Fig. 4a. For comparison, CVs of Z-V5S8/CoS-CNFs (Z = 0, 0.1, 0.15, 0.3) are shown in Fig. S7. In the initial cathodic scan, two distinct reduction peaks at 1.107 V and 0.803 V are observed. The first reduction peak at 1.107 V is attributed to the insertion of sodium ions into V5S8 and CoS, while the subsequent peak at 0.803 V can be ascribed to the further insertion of Na+ ions, resulting in the formation of V, Co, and amorphous Na2S during the conversion reaction[30]. However, these two peaks disappear in subsequent scans, likely due to the formation of the solid electrolyte interphase (SEI) and irreversible embedding of Na+[31].
In the subsequent anodic scans, three oxidation peaks at 1.416 V, 1.648 V, and 1.924 V are observed, corresponding to the multi-step reversible reactions leading to the formation of V5S8 and CoS. Additionally, a peak at 0.051 V suggests the insertion of Na+ ions into amorphous carbon[32]. Notably, CV curves exhibit good overlap after the first cycle, indicating that the electrochemical reaction of the 0.2-V5S8/CoS-CNFs electrode is reversible, and the material exhibits excellent structural stability. The above reactions can be described by the following equations[30, 33]:
Discharge process：
V5S8+xNa++Xe-＝NaxV5S8 (2)
NaxV5S8+(16-x)Na++(16-x)e-=8Na2S+5V (3)
CoS+xNa++xe-=NaxCoS (4)
NaxCoS+(2-x)Na++(2-x)e-=Na2S+Co (5)
Charge process：
8Na2S+5V= NaxV5S8+(16-x)Na++(16-x)e- (6)
NaxV5S8=V5S8+xNa++Xe- (7)
Na2S+Co= NaxCoS+(2-x)Na++(2-x)e- (8)
NaxCoS=CoS+xNa++xe- (9)
Fig. 4b and Fig. S8 illustrate the discharge-charge curves of 0.2-V5S8/CoS-CNFs and Z-V5S8/CoS-CNFs (Z=0, 0.1, 0.15, 0.3) at a current density of 0.2 A g-1. Notably, the initial discharge-charge capacity of 0.2-V5S8/CoS-CNFs is 398.64/320.93 mAh g-1, showcasing an impressive initial coulombic efficiency (ICE) of 80.5 % during the first discharge-charge cycle. Importantly, the ICE of 0.2-V5S8/CoS-CNFs surpasses that of previously reported electrospun sodium electrode materials based on TMSs, as detailed in Table S1. Furthermore, the discharge-charge curves for the subsequent four cycles exhibit excellent overlap, providing compelling evidence of the outstanding cycling performance exhibited by 0.2-V5S8/CoS-CNFs.
The cycling performance of the Z-V5S8/CoS-CNFs electrode material at 0.2 A g-1 is depicted in Fig. 4c. The 0.2-V5S8/CoS-CNFs samples exhibited an initial specific capacity of 398.64 mAh g-1 during the first discharge and maintained a stable capacity of 233 mAh g-1 after 250 cycles. In contrast, CNFs, 0.1-V5S8/CoS-CNFs, and 0.15-V5S8/CoS-CNFs samples retained capacities of 112.76 mAh g-1, 168.46 mAh g-1, and 187.45 mAh g-1, respectively, after 250 cycles. These results indicate lower sodium storage capacity in CNFs, 0.1-V5S8/CoS-CNFs, and 0.15-V5S8/CoS-CNFs due to their lower V5S8 and CoS content. Additionally, the specific capacity of the 0.3-V5S8/CoS-CNFs sample exhibited a continuous decay during cycling and was lower than that of the 0.2-V5S8/CoS-CNFs sample. This decay may be attributed to the larger-sized V5S8 and CoS agglomerates, which had limited capacity release capabilities[23]. These findings highlight that maintaining moderate amounts (less than 31.71 %) of V5S8 and CoS uniformly dispersed and efficiently confined by CNFs effectively suppresses volume expansion of the active substance and stabilizes the electrode structure[34]. Conversely, when the V5S8 and CoS content in the fibers increased to 50.76 %, significant precipitation and aggregation of V5S8 and CoS occurred (Fig. S2), impeding the CNFs' ability to facilitate volume changes.
The electrolyte gradually decomposes on the surfaces of V5S8 and CoS during cycling, leading to a rapid deterioration of sodium storage capacity[35]. A similar trend is observed in the discharge capacity statistics of Z- V5S8/CoS-CNFs (Fig. S9). Fig. 4d presents the rate performance, where the 0.2- V5S8/CoS-CNFs samples deliver a high specific capacity of 415 mAh g-1 in the first cycle. Subsequently, at current densities of 0.2, 0.5, 1, 2, and 5 A g-1, the specific capacities are 310, 268, 253, 229, and 201 mAh g-1, respectively. Upon resetting the current density to 0.2 A g-1, the capacity rapidly recovers to 264 mAh g-1, showcasing excellent rate performance. This can be attributed to the appropriate carbon content, which ensures a stable carbon skeleton, suppresses volume expansion, and maintains material stability during electrochemical cycling.
Fig. 4e illustrates the relationship between discharged capacitance and voltage at different current densities, where the voltage plateau remains consistent at all current densities. This indicates low electrochemical polarization in the samples, a favorable trait for their application in energy storage devices. This behavior results from the stable heterostructures of the active interface and minimal electrochemical polarization phenomenon[36]. Notably, the comparison of 0.2-V5S8/CoS-CNFs with previously reported anode materials (Fig. 4f) [37-43] underscores its exceptional rate performance.
As expected, 0.2-V5S8/CoS-CNFs exhibit remarkable long-term cycling stability (Fig. 4g) with a Coulombic efficiency of nearly 100 %. To further validate their enduring stability, SEM images of 0.2-V5S8/CoS-CNFs before and after cycling are provided in Fig. S10. Clearly, the electrode fiber structure remains well-preserved after cycling, with no noticeable changes in diameter or metal precipitation or agglomeration. This confirms the efficacy of the fiber structure in suppressing volume changes during cycling, aligning with the experimental design.
To gain further insights into the reaction kinetics of 0.2-V5S8/CoS-CNFs, we conducted CV experiments at various scanning rates, as illustrated in Fig. 5a. The CV curves exhibit consistent oxidation and reduction potentials, which align with the peak position at 0.1 mV s-1, indicating that there are no significant changes in electrochemical reaction kinetics across different scan rates. The peak currents (i) of the redox peaks exhibit variations with scan rate (v), and their relationship conforms to Eq. (10), where the value of b can provide insights into the electrochemical mechanism[44]. A value of b close to 0.5 signifies the dominance of diffusion-controlled behavior, while a value near 1.0 suggests the predominance of capacitive behavior. To determine the value of b, Eq. (10) is logarithmically transformed into Eq. (11). As depicted in Fig. 5b, the b-values for peaks 1 to 5 are 0.80, 0.86, 0.98, 0.65, and 0.82, respectively. All these b values fall within the range of 0.5-1.0, indicating the coexistence of both diffusion-controlled and capacitive behaviors during charging and discharging processes. This suggests that the electrode material exhibits pseudocapacitive properties, where both diffusion and capacitance mechanisms contribute significantly to the electrochemical processes[45].
In the provided equations Eq. (12), k1v represents the contribution of pseudocapacitance, while k2v1/2 represents the contribution of ion diffusion. Fig. 5c illustrates the calculated pseudocapacitance contributions at different scan rates. The observed increase in capacitance contribution with higher scan rates confirms the rapid transport of Na+ ions within the material. As depicted in Fig. 5d, at a scan rate of 1.0 mV s-1, the surface capacitive behavior of the material accounts for 89.84 % of the contribution, with diffusion contributing only 10.16 %. The pseudocapacitance contributions at other scan rates are shown in Fig. S11. The larger capacitance contribution is advantageous for facilitating the charging and discharging processes of the material at high rates[46]. A comparison reveals that 0.2-V5S8/CoS-CNFs exhibit higher pseudocapacitance contributions compared to CNFs at the same scan rate (Fig. S12). This indicates that the inclusion of V5S8 and CoS enhances the pseudocapacitance effect, consequently improving the kinetics of sodium storage.
The reaction kinetics were further characterized through EIS tests. As depicted in Fig. 5e, the Nyquist plot consists of semicircles and slanted lines. The semicircles in the high-frequency region are associated with system impedance (Rs) and charge transfer impedance (Rct), while the slanted straight lines in the low-frequency region indicate diffusion behavior, related to the diffusion impedance of Na+[47, 48]. It is evident that electrodes with higher metal concentrations exhibit smaller Rs and Rct. Table S1 provides specific values of impedance after fitting EIS theory. As the metal concentration increases, the sample demonstrates enhanced electrical conductivity, thereby accelerating the transport of electrons and ions, and consequently, the reaction kinetics during the electrochemical process. Additionally, the EIS analysis was further extended to estimate the diffusion coefficient of Na+ ions (DNa+), based on the Z’-ω(-1/2) (ω=2πf) curves in the low-frequency region. DNa+ is calculated using the following equation[49]:
Where, R, T, A, F, n, and C represent the gas constant, absolute temperature, electrode surface area, Faraday constant, molar electron transfer number, and Na+ molar concentration, respectively. The Warburg coefficient σ is determined from the slope of the straight line. There is an observed trend of decreasing sodium ion diffusion coefficient with increasing metal concentration, which could potentially contribute to the capacity decay observed in concentrations up to 0.3-V5S8/CoS-CNFs.
The diffusion kinetics of 0.2-V5S8/CoS-CNFs were further investigated using GITT. DNa+ coefficient can be calculated by the following equation[50]:
where τ represents the relaxation time, ∆Es stands for the change in steady-state potential after the current pulse, and ∆Et represents the potential change (V) under a stable electric energy pulse without iR drop (Fig. 6b). L corresponds to the length of the Na+ diffusion path, which is simplified to the thickness of the electrode. Fig. 6a illustrates the discharge-charge GITT curve of the 0.2-V5S8/CoS-CNFs electrode. By calculating the diffusion coefficient of sodium ions during discharge charging (Fig. 6d), it can be observed that an increase in the metal sulfide content leads to a decrease in the Na+ diffusion coefficient, consistent with the results of the EIS tests. The diffusion coefficients ranged between 10-8 and 10-7 cm2 s-1, which are significantly higher than those of other materials in the same field (Table. S3). The excellent kinetic properties of the material may be attributed to its unique structural design, which facilitates the migration of sodium ions.
In summary, we have successfully prepared a flexible V5S8/CoS-CNFs membrane through electrostatic spinning followed by a one-step high-temperature sulfidation process. This method results in the formation of heterogeneous V5S8/CoS nanocrystals that are uniformly confined within the CNFs. The membrane serves as a binder-free anode for sodium-ion batteries, and the heterostructure between V5S8 and CoS significantly reduces the sodium-ion migration barrier while enhancing the capacitive contribution of the composite. Our electrode exhibits pseudocapacitive behavior, a high sodium diffusion coefficient, fast electrochemical kinetics, and excellent rate performance. Moreover, the orchestrated structure accommodates the volume expansion of active materials, thereby enhancing the cycling stability of the sodiation-desodiation process. The obtained electrode demonstrates outstanding rate performance, delivering a capacity of 201 mA h g-1 at 5 A g-1, and maintains a high reversible capacity of 165 mA h g-1 at 2 A g-1 after 300 cycles. This work provides valuable insights and guidance for the structural and compositional design of free-standing anodes for sodium-ion storage, advancing the development of high-performance energy storage materials for future applications.
CRediT authorship contribution statement
Junjie Dai: Experimental Design, Method Research, Data Arrangement, Investigation and Analysis, Paper Writing. Fangshun Zhu: Implementation of the experimental process, data collection, data collation, paper revision. Balaji Murugesan: Research Implementation, Paper Design, Paper Revision, Technical Support. Qing Zhang: Experimental assistance, data acquisition. Xiaochong Zhou and Wenbin Ni: Statistical analysis, research and technical support. Yurong Cai：Programme design, project management, guidance support, final review of papers, funding.
Declaration of Competing Interest
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.
Data availability
Data will be made available on request.
Supplementary information
Provided in a separate document.
Compliance with Ethical Standards
Not Applicable.
Acknowledegments
The Zhejiang Provincial Key R&D Program (Grant No. 2023C04045) provided financial assistance for this project.
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