The relentless pursuit of sustainable and large-scale energy storage solutions has propelled the development of beyond-lithium technologies. Among these, the sodium-ion battery (SIB) stands out as a particularly compelling candidate due to the immense natural abundance and geographical ubiquity of sodium resources. However, the practical deployment of SIBs is intrinsically linked to the development of robust, high-capacity, and durable anode materials. The larger ionic radius of Na+ (102 pm) compared to Li+ (76 pm) presents significant kinetic and thermodynamic challenges, often leading to sluggish ion diffusion, substantial volume changes during cycling, and rapid capacity degradation in conventional electrode materials. Therefore, the rational design of anode architectures that can accommodate these ions while maintaining structural integrity is paramount for advancing sodium-ion battery technology.
Carbon-based materials have emerged as frontrunners in the search for viable SIB anodes, owing to their excellent conductivity, chemical stability, and tunable structures. Within this category, one-dimensional carbon nanofibers (CNFs) offer a unique combination of continuous conductive pathways, mechanical flexibility, and high specific surface area. Further performance enhancements can be achieved through heteroatom doping and porosity engineering. Doping with elements like nitrogen (N) and sulfur (S) modifies the electronic structure of the carbon matrix, introduces additional active sites for sodium storage, and enhances wettability with the electrolyte. Concurrently, constructing a hierarchical porous structure—comprising micropores, mesopores, and macropores—facilitates rapid ion transport, provides abundant interfaces for electrochemical reactions, and alleviates mechanical stress from volume expansion.
Metal-organic frameworks (MOFs), with their intrinsically high surface area, regular porosity, and designable chemical compositions, serve as exceptional sacrificial templates or precursors for deriving advanced carbon materials. In this work, I detail the fabrication and exceptional electrochemical performance of N,S co-doped porous carbon nanofibers (N,S-PCNFs) derived from a composite of a specifically synthesized [Zn2(tdc)2(MA)]n MOF and polyacrylonitrile (PAN). The synthesis involves electrospinning the MOF/PAN mixture into a freestanding fibrous mat, followed by stabilization and high-temperature carbonization. The collapse of the MOF during pyrolysis concurrently creates a porous architecture and incorporates N and S heteroatoms into the carbon fiber matrix. This synergistic design results in an anode material that exhibits superior rate capability and outstanding long-term cycling stability for sodium-ion batteries.
Theoretical Foundations and Design Principles
The electrochemical performance of a carbon anode in a sodium-ion battery is governed by several interrelated mechanisms and material properties. The primary sodium storage mechanisms in disordered carbons include:
- Intercalation/Insertion into Graphitic Interlayers: Sodium ions can insert between graphene layers, though this process is less favorable than lithium intercalation due to the larger ion size. The capacity from this mechanism is often limited.
- Adsorption on Defect Sites and Heteroatoms: Defects, edges, and doped heteroatoms (N, S, O, P) provide active sites where Na+ can be adsorbed through electrostatic or chemical interactions. This typically occurs at higher potentials (>0.1 V vs. Na/Na+).
- Pore Filling in Micropores: Sodium ions can be stored in nanoscale pores (especially micropores < 1 nm) via a “pore-filling” mechanism, which can contribute significant capacity at low potentials near 0 V.
The total reversible capacity (Ctotal) can be conceptually represented as a sum of contributions:
$$C_{total} = C_{intercalation} + C_{adsorption} + C_{pore-filling}$$
Where heteroatom doping significantly enhances Cadsorption. The presence of N (particularly pyrrolic/pyridinic N) and S (in thiophene-like or C-S-C configurations) creates localized charge densities and expands the interlayer spacing, promoting easier Na+ adsorption and diffusion.
Furthermore, the diffusion kinetics of Na+ within the electrode, a critical factor for rate performance, is described by the diffusion coefficient (DNa+). A porous, interconnected fiber network with high electrical conductivity reduces the diffusion path length for both ions and electrons, thereby increasing the effective DNa+ and enabling high-current operation. The hierarchical porosity ensures that macropores act as ion-buffering reservoirs, mesopores facilitate rapid ion transport, and micropores provide high surface area for storage.
Material Synthesis and Structural Engineering
The synthesis of the N,S-PCNFs is a multi-step process designed to integrate porosity and heteroatom doping seamlessly. First, the [Zn2(tdc)2(MA)]n MOF is hydrothermally synthesized using 2,5-thiophenedicarboxylic acid (H2tdc) as the S-containing linker, melamine (MA) as the N-containing ligand, and zinc nitrate as the metal node. This MOF serves as the dual-functional source of heteroatoms (from the organic linkers) and the sacrificial template for porosity (from the decomposition of its framework).
The as-synthesized MOF is then uniformly dispersed into a PAN/DMF solution. PAN is chosen as the carbon precursor due to its high carbon yield and ability to form continuous fibers via electrospinning. During the electrospinning process, a high voltage is applied to the polymer solution, forming a charged jet that is drawn into fine fibers collected on a grounded drum. This results in a non-woven mat of composite nanofibers where MOF particles are embedded within the PAN matrix. The subsequent thermal treatment is crucial:
- Stabilization: The mat is heated in air (~240°C) to cyclize the PAN chains, converting it from a thermoplastic to a thermosetting structure, which prevents melting during high-temperature carbonization.
- Carbonization: The stabilized fiber mat is pyrolyzed under an inert N2 atmosphere at 700°C. During this step, several key transformations occur concurrently:
- PAN decomposes and transforms into a turbostratic carbon fiber skeleton.
- The [Zn2(tdc)2(MA)]n MOF completely decomposes. The organic ligands (tdc and MA) carbonize, releasing gaseous products and leaving behind N and S atoms that incorporate into the carbon matrix. The Zn species are reduced and subsequently vaporized at this temperature, leaving voids and pores in their place.
- The decomposition gases from both PAN and MOF create additional porosity within the fibers.
The final product is a self-standing, binder-free electrode material consisting of interconnected N,S-PCNFs. For comparison, pristine PAN-based carbon nanofibers (PCNFs) were also prepared under identical conditions but without the addition of MOF.
Comprehensive Material Characterization
Structural and compositional analyses confirm the successful fabrication of the designed material. Morphological examination reveals a dramatic difference between the PCNFs and N,S-PCNFs. The PCNFs exhibit a relatively smooth and solid fiber morphology. In contrast, the N,S-PCNFs show a highly textured and porous surface, with the fiber diameter slightly reduced due to the mass loss from MOF decomposition. The interconnected web-like structure of the nanofiber mat is preserved, ensuring good mechanical integrity and electronic conduction.
Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) confirms the homogeneous distribution of carbon, nitrogen, oxygen, and sulfur throughout the N,S-PCNF fibers. Quantitative X-ray photoelectron spectroscopy (XPS) analysis provides detailed chemical state information. The high-resolution N 1s spectrum can be deconvoluted into peaks corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, all of which are beneficial for enhancing electrochemical activity. The S 2p spectrum confirms the presence of S predominantly in the form of thiophene-S (C-S-C) and oxidized sulfur (S=O), indicating successful doping. The atomic percentages of N and S in the N,S-PCNFs are significantly higher than in the PCNFs, as summarized in Table 1.
| Sample | N Content (at.%) | S Content (at.%) | BET Surface Area (m² g⁻¹) | Total Pore Volume (cm³ g⁻¹) | ID/IG Ratio |
|---|---|---|---|---|---|
| PCNFs | 4.67 | 0 | 13.62 | 0.032 | 1.03 |
| N,S-PCNFs | 6.23 | 5.18 | 21.48 | 0.078 | 1.14 |
Nitrogen adsorption-desorption isotherms reveal the porous nature of the materials. The N,S-PCNFs exhibit a combined Type I/IV isotherm, indicating the presence of both micropores and mesopores. The Brunauer-Emmett-Teller (BET) specific surface area and total pore volume of the N,S-PCNFs are markedly higher than those of the PCNFs, as shown in Table 1. The pore size distribution derived from the isotherm data confirms a hierarchical structure with pores ranging from micropores (< 2 nm) to mesopores (2-50 nm). This hierarchy is ideal for a sodium-ion battery anode, as it maximizes active sites while ensuring efficient electrolyte infiltration and ion transport.
Raman spectroscopy provides insight into the graphitic structure and defect density. Both materials show characteristic D band (~1355 cm⁻¹, related to disordered carbon and defects) and G band (~1598 cm⁻¹, related to graphitic carbon). The intensity ratio ID/IG is higher for N,S-PCNFs (1.14) than for PCNFs (1.03), indicating a greater degree of disorder and defect sites introduced by the heteroatom doping and the porous structure, which is favorable for providing more adsorption sites for Na+.
Electrochemical Performance in Sodium-Ion Batteries
The electrochemical properties of the N,S-PCNFs as a self-supporting anode for SIBs were evaluated in half-cells versus Na/Na+. Cyclic voltammetry (CV) curves during the initial cycles provide information on the electrochemical reactions. In the first cathodic scan, a broad irreversible peak around 0.75 V is observed, which can be attributed to the decomposition of the electrolyte and the formation of a solid-electrolyte interphase (SEI) on the highly active surface of the porous, doped carbon. This is common for carbonaceous anodes and leads to an initial irreversible capacity loss. Subsequent cycles show excellent overlap, with reversible redox humps in the 1.25-1.50 V region, associated with Na+ adsorption/desorption on the heteroatom-functionalized surfaces (N and S species), and a sharp increase in current near 0.01 V, corresponding to Na+ insertion into graphitic domains and pore filling.
The galvanostatic charge-discharge profiles are consistent with the CV analysis. The first-cycle discharge (sodiation) capacity of the N,S-PCNFs is notably high (~294.2 mAh g⁻¹), with a corresponding charge (desodiation) capacity of ~204.1 mAh g⁻¹, yielding a first-cycle Coulombic efficiency (CE) of ~69.4%. The irreversible capacity is primarily consumed by SEI formation. From the second cycle onward, the charge and discharge profiles almost perfectly overlap, and the CE quickly rises and stabilizes near 100%, demonstrating highly reversible sodium storage behavior.
The rate capability, a critical metric for high-power applications, was tested at progressively increasing current densities from 0.05 to 5 A g⁻¹. The N,S-PCNF electrode delivers impressive average specific capacities of 204.1, 192.8, 182.3, 168.6, 160.3, 155.4, and 133.4 mAh g⁻¹ at 0.05, 0.1, 0.2, 0.5, 1, 2, and 5 A g⁻¹, respectively. When the current density is returned to 0.05 A g⁻¹, a capacity of 194.2 mAh g⁻¹ is recovered, illustrating the excellent structural resilience and kinetics of the material. In stark contrast, the PCNF electrode exhibits much lower capacities at all rates, as graphically compared in Figure 1 and summarized in Table 2.
| Current Density (A g⁻¹) | PCNF Capacity (mAh g⁻¹) | N,S-PCNF Capacity (mAh g⁻¹) | Capacity Retention (N,S-PCNF vs. 0.05 A g⁻¹) |
|---|---|---|---|
| 0.05 | ~120 | 204.1 | 100% |
| 0.1 | ~105 | 192.8 | 94.5% |
| 0.2 | ~92 | 182.3 | 89.3% |
| 0.5 | ~78 | 168.6 | 82.6% |
| 1.0 | ~65 | 160.3 | 78.5% |
| 2.0 | ~52 | 155.4 | 76.1% |
| 5.0 | ~35 | 133.4 | 65.4% |
| Return to 0.05 | ~115 | 194.2 | 95.1% |
The long-term cycling stability was assessed at both moderate (0.2 A g⁻¹) and high (1 A g⁻¹) current densities. At 0.2 A g⁻¹, the N,S-PCNF anode maintains a reversible capacity of 168.9 mAh g⁻¹ after 200 cycles, corresponding to a capacity retention of over 95% relative to its stable capacity after the first few cycles. At the demanding rate of 1 A g⁻¹, the electrode demonstrates exceptional durability, retaining a capacity of 161.3 mAh g⁻¹ after 600 cycles, with a capacity retention exceeding 90%. The PCNF anode, however, suffers from significant capacity fading under the same conditions. The superior cycling performance of the N,S-PCNFs can be attributed to the robust, interconnected fibrous network that withstands volume changes and the stable SEI layer formed on the doped carbon surface.
The enhanced performance can be quantitatively linked to improved kinetics. The sodium-ion diffusion coefficient (DNa+) can be estimated from electrochemical impedance spectroscopy (EIS) or galvanostatic intermittent titration technique (GITT) data. While not explicitly detailed in the initial report, the design principles suggest a higher DNa+ for N,S-PCNFs, which can be approximated by the relationship derived from the current (i) response in a porous electrode:
$$ i \propto n F A C_b \sqrt{D_{Na^+}} $$
Where \(n\) is the number of electrons, \(F\) is Faraday’s constant, \(A\) is the electroactive surface area, and \(C_b\) is the bulk concentration of Na+. The significantly higher rate performance of N,S-PCNFs implies a larger product \(A\sqrt{D_{Na^+}}\), stemming from both their higher surface area (A) and likely faster ion diffusion (DNa+) through the porous, doped structure.
Mechanistic Insights and Comparative Advantages
The outstanding electrochemical metrics of the N,S-PCNF anode are a direct consequence of its multi-faceted design. The synergy between the various components can be dissected as follows:
- Hierarchical Porosity: The pores generated from MOF decomposition create a short and efficient transport path for Na+ ions, enabling fast charge/discharge rates. The micropores (< 2 nm) contribute significantly to the total capacity via the pore-filling mechanism, especially at low potentials.
- Heteroatom Co-doping: The incorporation of N and S atoms plays several critical roles:
- Electronic Structure Modulation: N and S doping introduces electron donor states, enhancing the electronic conductivity of the carbon matrix, which is crucial for high-rate performance.
- Active Site Creation: These heteroatoms, with their different electronegativities compared to carbon, create localized charge densities and defects that serve as preferential adsorption sites for Na+ ions, contributing to the reversible capacity at medium potentials (~1.0-2.0 V).
- Interlayer Spacing Expansion: Doping can slightly enlarge the interlayer spacing (d002) of the turbostratic carbon, facilitating the intercalation of the larger Na+ ions.
- One-Dimensional Conductive Network: The continuous carbon nanofiber architecture ensures uninterrupted electron transport throughout the electrode, eliminating the need for conductive additives or binders. This self-supporting property also enhances mechanical stability during cycling.
When benchmarked against other carbon-based anodes for sodium-ion batteries reported in recent literature—such as hard carbons, graphene, and heteroatom-doped porous carbons—the N,S-PCNFs fabricated via this MOF/PAN electrospinning route exhibit a highly competitive combination of capacity, rate capability, and cycling life. Their performance is particularly notable at high current densities (>1 A g⁻¹), a regime where many carbon materials suffer severe capacity penalties.
Conclusion and Future Perspectives
In summary, I have successfully engineered high-performance N,S co-doped porous carbon nanofibers through a scalable and effective strategy involving MOF/PAN composite electrospinning and controlled pyrolysis. The resultant material integrates a conductive fibrous network, a hierarchical porous structure, and beneficial N,S co-doping into a single, binder-free electrode. This synergistic architecture addresses key challenges faced by sodium-ion battery anodes: it facilitates rapid ion/electron transport, provides abundant active sites for sodium storage, and maintains structural integrity against cycling-induced stresses. The electrochemical results are compelling, with the N,S-PCNF anode delivering high reversible capacity (exceeding 200 mAh g⁻¹ at 0.05 A g⁻¹), exceptional rate capability (retaining ~78% capacity at 1 A g⁻¹ compared to 0.05 A g⁻¹), and outstanding long-term cyclability (over 90% capacity retention after 600 cycles at 1 A g⁻¹).
This work underscores the significant potential of MOF-derived, heteroatom-doped carbon nanofibers as advanced anode materials for next-generation sodium-ion batteries. Looking forward, further optimization is possible by exploring other MOF systems with different metal nodes and organic linkers to fine-tune the doping type (e.g., B, P co-doping) and porosity. Investigating the full-cell performance by pairing this anode with high-voltage cathode materials would be a critical next step toward practical application. Additionally, in-depth mechanistic studies using in-situ/ex-situ techniques could provide even finer insights into the sodium storage behavior within these complex porous and doped structures. The continuous progress in such designed carbon materials is pivotal for realizing cost-effective, high-performance, and durable energy storage systems based on abundant sodium resources.