The urgent global transition towards sustainable energy systems has placed electrochemical energy storage at the forefront of scientific and industrial innovation. While lithium-ion batteries have dominated the landscape for portable electronics and electric vehicles, concerns regarding the geopolitical scarcity, uneven distribution, and rising cost of lithium resources have prompted an intensive search for complementary or alternative technologies. Sodium-ion batteries represent one of the most promising candidates, owing to the natural abundance, low cost, and wide geographical distribution of sodium. However, the practical deployment of sodium-ion batteries is significantly hindered by the lack of high-performance anode materials that offer long-term cycling stability, high rate capability, and large reversible capacity, surpassing the limitations of conventional hard carbon.
Transition metal sulfides, particularly cobalt-based sulfides, have attracted considerable attention as potential anode materials for sodium-ion batteries due to their high theoretical specific capacities based on conversion reaction mechanisms. The general reaction can be represented as:
$$ MS_2 + 4Na^+ + 4e^- \rightleftharpoons M + 2Na_2S $$
where M represents a transition metal like Co or Fe. Despite this promise, several intrinsic drawbacks plague these materials, including poor electronic conductivity, severe volume expansion during sodiation/desodiation, sluggish reaction kinetics, and particle aggregation upon cycling. These issues lead to rapid capacity decay and poor rate performance, limiting their practical application in sodium-ion batteries.
To overcome these challenges, a common and effective strategy involves nanostructuring the active material and confining it within a conductive carbon matrix. The carbon scaffold serves multiple critical functions: it enhances the overall electronic conductivity, buffers the mechanical stress from volume changes, prevents the aggregation of active nanoparticles, and can contribute additional capacity through sodium-ion adsorption. Furthermore, heteroatom doping (e.g., with N, S) into the carbon lattice can introduce defects, modify the electronic structure, and create more active sites for sodium storage, thereby significantly boosting the electrochemical performance of the composite in sodium-ion batteries.
In this work, we report a facile, green, and cost-effective synthetic strategy to fabricate a high-performance anode material for sodium-ion batteries. Our approach centers on the in-situ formation of ultrafine iron-cobalt disulfide (FeCoS2) nanocrystals uniformly confined within an N,S-codoped carbon (NSC) matrix, denoted as FeCoS2⊂NSC. The synthesis leverages a room-temperature solid-state self-assembly reaction to form a molecularly homogeneous precursor, followed by a one-step thermal treatment that simultaneously induces carbonization and sulfidation. This method effectively addresses the common issues of particle aggregation and inhomogeneous compositing. The resulting nanocomposite exhibits exceptional sodium storage properties, including high reversible capacity, outstanding long-term cycling stability, and remarkable rate capability, making it a highly promising anode candidate for advanced sodium-ion batteries.
Innovative Synthesis via Solid-State Self-Assembly
The synthesis of the FeCoS2⊂NSC nanocomposite is elegantly simple and scalable, avoiding the use of solvents or complex procedures. The process involves two key stages, as summarized in the table below:
| Stage | Process | Key Reactions/Events | Outcome |
|---|---|---|---|
| Stage 1: Precursor Formation | Solid-state grinding at room temperature. | Self-assembly reaction between o-vanillin, o-phenylenediamine, and metal acetates (Co2+, Fe2+) to form a bimetallic bis-Schiff base complex dispersed within sulfur powder. | Molecular-level homogeneous mixture of metal ions and organic ligands. |
| Stage 2: Thermal Conversion | Annealing under inert (N2) atmosphere. | Simultaneous carbonization of the organic ligand and sulfidation of the metal ions by surrounding sulfur vapor. | In-situ formation of FeCoS2 nanocrystals embedded in the N,S-codoped carbon matrix. |
Specifically, cobalt(II) acetate tetrahydrate, iron(II) acetate, o-vanillin, o-phenylenediamine, and sulfur powder are mixed in a molar ratio of 1:1:4:2:20 and thoroughly ground in an agate mortar at room temperature for one hour. During this grinding process, a solid-state self-assembly reaction occurs, leading to the formation of a reddish-brown powder precursor. This precursor consists of CoII and FeII complexes with the bis-Schiff base ligand, intimately mixed with elemental sulfur. This molecular-scale homogeneity is crucial for the subsequent step.
The precursor is then annealed in a tube furnace under a continuous nitrogen flow. The thermal treatment is performed at different temperatures (500, 600, and 700 °C) for one hour to optimize the process. During this step, two critical reactions occur concurrently:
1. Carbonization: The organic bis-Schiff base ligand decomposes and transforms into a nitrogen-rich carbonaceous matrix.
2. Sulfidation: The metal ions (Co2+, Fe2+) react with sulfur vapor generated from the surrounding sulfur powder to form sulfide nanocrystals.
The nitrogen from the ligand and sulfur from the atmosphere incorporate into the carbon lattice, creating the N,S-codoped carbon (NSC) matrix. The nanocomposite obtained at 700 °C is labeled FeCoS2⊂NSC-700. For comparison, control samples including single-metal sulfides (CoS⊂NSC-700, FeS⊂NSC-700) and the bare carbon matrix (NSC-700) were also synthesized under identical conditions.
Structural and Compositional Characterization
We employed a suite of characterization techniques to unravel the phase, morphology, and chemical composition of the synthesized materials, which are directly linked to their performance in sodium-ion batteries.
Phase and Crystallinity (XRD): The X-ray diffraction patterns of the composites annealed at different temperatures revealed the evolution of crystallinity. The sample treated at 700 °C (FeCoS2⊂NSC-700) showed distinct diffraction peaks that could be indexed to the hexagonal phase of FeCoS2 (PDF#75-0607). The peak positions were intermediate between those of pure CoS and FeS, confirming the formation of a solid solution where Fe2+ ions uniformly substitute Co2+ ions in the CoS lattice, rather than a simple physical mixture of the two separate sulfides. The crystallinity improved with increasing annealing temperature.
Morphology and Nanostructure (TEM/STEM): Transmission electron microscopy provided direct visualization of the nanocomposite’s architecture. The low-resolution TEM images show a porous, three-dimensional carbon matrix hosting numerous dark, ultrafine nanoparticles. High-resolution TEM and HAADF-STEM images of the FeCoS2⊂NSC-700 composite reveal that the FeCoS2 nanoparticles are exceptionally small and uniformly dispersed. Statistical analysis of multiple images indicates an average particle size of approximately 3.4 nm. The clear lattice fringes with interplanar spacings of 0.291 nm, 0.255 nm, and 0.196 nm correspond to the (100), (101), and (102) planes of FeCoS2, respectively. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirms the homogeneous distribution of Fe, Co, S, C, and N throughout the composite, with the atomic ratio of Fe to Co close to 1:1.
Chemical State and Doping (XPS): X-ray photoelectron spectroscopy was used to probe the surface chemical states and confirm heteroatom doping. The survey spectrum confirms the presence of Fe, Co, S, C, N, and O. The high-resolution spectra provide detailed insights:
The Co 2p spectrum exhibits peaks at 781.8 eV (Co 2p3/2) and 797.8 eV (Co 2p1/2) with a spin-orbit splitting of 16.0 eV, characteristic of Co-S bonds in a sulfide environment. The Fe 2p spectrum shows signatures for both Fe2+-S (peaks at 710.9 and 724.3 eV) and a minor amount of Fe3+-O likely due to surface oxidation. Crucially, the S 2p spectrum deconvolutes into several components. The doublet at 161.5 eV and 162.7 eV is assigned to the S 2p3/2 and S 2p1/2 of metal-sulfur (M-S) bonds in FeCoS2. Another doublet at 163.6 eV and 164.8 eV corresponds to thiophene-type S (C-S-C), providing direct evidence for successful sulfur doping into the carbon framework. The C 1s spectrum confirms the presence of C-C, C-N/C-S, and C=O bonds, while the N 1s spectrum reveals the coexistence of pyridinic N, pyrrolic N, and graphitic N species. This multi-heteroatom doping is pivotal for enhancing the electrochemical activity of the carbon matrix in sodium-ion batteries.
Carbon Quality and Content: Raman spectroscopy of the FeCoS2⊂NSC-700 composite shows the characteristic D band (~1342 cm-1, disorder-induced) and G band (~1568 cm-1, graphitic) of carbon materials. The intensity ratio ID/IG is calculated to be 1.38, which is higher than that of the control samples (e.g., 1.07 for CoS⊂NSC-700). This indicates a higher degree of structural defects and disorder within the carbon matrix of the FeCoS2 composite, which can be beneficial for providing more active sites for sodium ion storage. Thermogravimetric analysis (TGA) in air was used to determine the component mass fractions. Based on the weight loss profile and accounting for the oxidation of sulfides to sulfates and subsequent decomposition, the FeCoS2 content in the FeCoS2⊂NSC-700 composite was estimated to be approximately 45.3 wt%, with the remaining 54.7 wt% being the NSC matrix.
Electrochemical Performance in Sodium-Ion Batteries
The electrochemical sodium storage properties of the FeCoS2⊂NSC nanocomposites were systematically evaluated in CR2032 coin-type half-cells versus Na/Na+. The key performance metrics for sodium-ion batteries—cycling stability, capacity, and rate capability—were investigated.
Cyclic Voltammetry (CV) and Reaction Mechanism: The initial CV cycles of the FeCoS2⊂NSC-700 electrode reveal the complex electrochemical processes. During the first cathodic scan, a sharp reduction peak around 0.68 V is observed, attributed to the irreversible formation of a solid-electrolyte interphase (SEI) film and the initial conversion reaction of FeCoS2 with Na+:
$$ FeCoS_2 + yNa^+ + ye^- \rightarrow Na_yFeCoS_2 $$
$$ Na_yFeCoS_2 + (4-y)Na^+ + (4-y)e^- \rightarrow Fe + Co + 2Na_2S $$
A broad reduction feature below 0.5 V corresponds to further SEI formation and sodium insertion into the defective NSC matrix. In the subsequent anodic scan, a broad oxidation peak near 1.40 V corresponds to the reversible conversion of the metallic Fe/Co nanoparticles back to sulfides. From the second cycle onward, the CV curves almost perfectly overlap, indicating highly reversible electrochemical reactions and excellent structural stability of the composite upon cycling, a critical requirement for durable sodium-ion batteries.
Galvanostatic Charge-Discharge and Cycling Stability: The galvanostatic charge-discharge profiles are consistent with the CV analysis. The FeCoS2⊂NSC-700 electrode delivers a high initial discharge and charge specific capacity of 739.9 and 486.4 mAh g-1, respectively, at a current density of 0.1 A g-1. The initial Coulombic efficiency of 65.7% is associated with the irreversible SEI formation. Most importantly, the electrode demonstrates exceptional long-term cycling stability. After 300 charge-discharge cycles at 0.1 A g-1, a large reversible charge capacity of 310.4 mAh g-1 is retained, corresponding to a capacity retention of 69.3% relative to the second cycle’s charge capacity. This performance significantly outperforms the control samples (FeS⊂NSC-700, CoS⊂NSC-700, NSC-700) and the composites synthesized at lower temperatures, as summarized in the table below.
| Sample | Current Density (A g-1) | Reversible Charge Capacity after 300 cycles (mAh g-1) | Key Observation |
|---|---|---|---|
| FeCoS2⊂NSC-500 | 0.1 | Low capacity, stable | Poor crystallinity, low activity. |
| FeCoS2⊂NSC-600 | 0.1 | Rapid capacity decay | Insufficient carbonization/sulfidation. |
| FeCoS2⊂NSC-700 | 0.1 | 310.4 | Excellent stability & high capacity. |
| FeS⊂NSC-700 | 0.1 | Rapid capacity decay | Poor structural stability. |
| CoS⊂NSC-700 | 0.1 | Low capacity, stable | Hard texture, less defective carbon. |
| NSC-700 | 0.1 | Low capacity, stable | Carbon-only contribution. |
Rate Capability: The rate performance of the FeCoS2⊂NSC-700 anode is outstanding, which is vital for sodium-ion batteries requiring fast charging or high-power delivery. The electrode was tested at progressively increasing current densities from 0.1 to 5 A g-1. It delivered average charge capacities of 415.7, 367.8, 311.1, 262.3, 216.0, and 146.0 mAh g-1 at 0.1, 0.2, 0.5, 1, 2, and 5 A g-1, respectively. Notably, even at a very high current density of 5 A g-1, a substantial capacity of 146.0 mAh g-1 was maintained, representing 35.1% of the capacity at 0.1 A g-1. When the current density was returned to 0.1 A g-1, the capacity recovered to 407.5 mAh g-1, demonstrating remarkable structural resilience and electrochemical reversibility. The rate capability of FeCoS2⊂NSC-700 far exceeds that of all other control samples, especially at high rates.
Mechanistic Insights into the Enhanced Performance
The superior electrochemical performance of the FeCoS2⊂NSC-700 nanocomposite in sodium-ion batteries can be attributed to its unique hierarchical architecture and chemical composition, engineered through our synthetic design:
- Synergistic Bimetallic Sulfide (FeCoS2): The formation of a FeCoS2 solid solution, rather than separate FeS and CoS phases, likely modifies the electronic structure and creates more electrochemically active sites compared to the single-metal sulfides. The iron doping may also enhance the intrinsic conductivity and reactivity of the sulfide phase.
- Ultrafine Nanocrystals and Confinement Effect: The in-situ synthesis yields extremely small FeCoS2 nanoparticles (~3.4 nm) uniformly embedded in the carbon matrix. This nanoconfinement effect has multiple benefits: it drastically shortens the diffusion paths for both Na+ ions and electrons, facilitates rapid reaction kinetics, and most importantly, effectively restricts the volume expansion of the sulfide during cycling and prevents nanoparticle aggregation. This is the key to the excellent cycling stability.
- Conductive and Active Carbon Matrix: The N,S-codoped carbon (NSC) matrix serves as a robust, continuous 3D conductive network, ensuring efficient electron transport throughout the electrode. The high ID/IG ratio from Raman indicates a highly defective carbon structure. The doped heteroatoms (N and S) not only improve wettability with the electrolyte but also create numerous defects and active sites that can reversibly adsorb/desorb sodium ions, contributing extra capacity beyond the conversion reaction of FeCoS2.
- Integrated Nanocomposite Structure: The one-pot synthesis ensures strong interfacial contact between the FeCoS2 nanoparticles and the NSC matrix. This intimate contact guarantees mechanical integrity and maintains electrical connectivity during repeated sodium insertion/extraction processes, which is crucial for maintaining performance over hundreds of cycles in a sodium-ion battery.
The combination of these factors can be quantitatively appreciated in the context of capacity contribution and stability. The total capacity (Qtotal) of the composite anode can be considered as a sum of contributions from the conversion reaction of FeCoS2 (Qconv) and the capacitive storage from the NSC matrix (Qcap), which includes double-layer capacitance and pseudo-capacitance from heteroatom defects:
$$ Q_{total} = Q_{conv} + Q_{cap} $$
Our designed structure maximizes both Qconv (through highly accessible, nano-sized active material) and Qcap (through a highly doped, defective carbon), while the confinement ensures the long-term stability of both mechanisms.
Conclusion and Outlook
In summary, we have successfully developed a novel FeCoS2⊂NSC nanocomposite via a facile and scalable room-temperature solid-state reaction followed by thermal treatment. This strategy enables the in-situ formation of ultrafine FeCoS2 nanocrystals uniformly confined within a highly defective N,S-codoped carbon matrix. When evaluated as an anode material for sodium-ion batteries, the optimized nanocomposite (FeCoS2⊂NSC-700) demonstrates a compelling combination of high reversible capacity, exceptional long-term cycling stability (310.4 mAh g-1 after 300 cycles at 0.1 A g-1), and remarkable rate capability (146.0 mAh g-1 at 5 A g-1).
The performance surpasses that of its single-metal sulfide counterparts and the carbon-only matrix, highlighting the synergistic effects of bimetallic sulfide formation, nanostructural confinement, and multi-heteroatom doping. This work provides a general, green, and cost-effective synthetic paradigm that can potentially be extended to the preparation of other heteroatom-doped carbon-confined polymetallic chalcogenide nanocomposites for advanced energy storage applications. The demonstrated properties position the FeCoS2⊂NSC composite as a highly promising candidate for the next generation of high-performance, durable anodes for practical sodium-ion batteries, contributing to the development of sustainable and large-scale electrochemical energy storage systems.