The escalating demand for large-scale energy storage systems has highlighted the limitations of conventional lithium-ion batteries (LIBs), primarily due to the geographical concentration and finite reserves of lithium resources. In this context, sodium-ion batteries (SIBs) have emerged as a compelling and cost-effective alternative, leveraging the natural abundance and low cost of sodium. However, the larger ionic radius of Na+ compared to Li+ presents significant challenges, including sluggish reaction kinetics, substantial volume expansion during cycling, and rapid capacity degradation in electrode materials. Consequently, the development of high-performance, durable anode materials is a critical frontier in advancing sodium-ion battery technology.
Among various candidates for sodium-ion battery anodes, conversion-type metal sulfides, particularly antimony sulfide (Sb2S3), have attracted considerable attention. Sb2S3 offers a high theoretical specific capacity (approximately 946 mAh g-1) through a combined conversion and alloying reaction mechanism with sodium ions. The storage mechanism can be described by the following sequential reactions:
$$ \text{Sb}_2\text{S}_3 + 6\text{Na}^+ + 6e^- \rightarrow 2\text{Sb} + 3\text{Na}_2\text{S} \quad \text{(Conversion)} $$
$$ 2\text{Sb} + 6\text{Na}^+ + 6e^- \leftrightarrow 2\text{Na}_3\text{Sb} \quad \text{(Alloying/Dealloying)} $$
Despite this high capacity, the practical application of Sb2S3 in sodium-ion batteries is severely hindered by its inherent poor electronic conductivity and the enormous volume fluctuations (often exceeding 300%) associated with these reactions. These issues lead to particle pulverization, loss of electrical contact, and continuous degradation of the solid-electrolyte interphase (SEI), resulting in fast capacity fade and poor rate capability.
A well-established strategy to mitigate these drawbacks involves compositing the active material with conductive carbon matrices. Graphene, with its exceptional electrical conductivity, mechanical flexibility, and high specific surface area, serves as an ideal scaffold. Its two-dimensional structure can encapsulate active particles, enhance electron transport, and accommodate mechanical stress. Further performance enhancement can be achieved through heteroatom doping, such as nitrogen, which modifies the electronic structure of graphene, introduces more active sites, and improves wettability with the electrolyte, thereby facilitating ion diffusion. The synthesis method plays an equally crucial role. The freeze-drying (lyophilization) technique is particularly advantageous for fabricating electrode materials. It involves rapid freezing of an aqueous precursor dispersion, followed by sublimation of the ice under vacuum. This process effectively prevents the agglomeration of nanoparticles and the restacking of graphene sheets, yielding a lightweight, porous, three-dimensional (3D) aerogel-like structure with intimate contact between components. This architecture is highly beneficial for sodium-ion battery electrodes, providing abundant channels for electrolyte infiltration and ample void space to buffer volume changes.
In this study, a composite material consisting of Sb2S3 nanorods uniformly embedded within a 3D nitrogen-doped graphene (NG) network was successfully fabricated via a facile freeze-drying method. The nitrogen-doped graphene was first prepared hydrothermally using ammonia, which acted as both a reducing agent for graphene oxide and a nitrogen source. Sb2S3 was synthesized separately and then integrated with the NG scaffold in an aqueous suspension prior to freeze-drying. This designed Sb2S3@NG composite synergistically combines the high capacity of Sb2S3 with the conductive, flexible, and buffering properties of the N-doped 3D graphene network. When evaluated as an anode for sodium-ion batteries, the composite exhibits significantly improved electrochemical performance, including high reversible capacity, excellent rate capability, and superior cycling stability compared to bare Sb2S3. This work demonstrates a promising strategy for designing high-performance conversion/alloying-type anodes for next-generation sodium-ion batteries.
Experimental Section: Materials and Methods
1. Synthesis of Nitrogen-Doped Graphene (NG)
A homogeneous suspension was prepared by mixing 80 mL of graphene oxide (GO) dispersion (1 mg mL-1) with 20 mL of aqueous ammonia (25-28 wt%). After vigorous stirring, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 180 °C for 24 hours. During this process, GO is reduced to graphene, and nitrogen atoms from ammonia are doped into the graphene lattice. The resulting black hydrogel was collected and washed repeatedly with deionized water and ethanol via centrifugation to remove impurities. The final product, nitrogen-doped graphene (NG), was obtained by freeze-drying the purified hydrogel for 24 hours.
2. Synthesis of Antimony Sulfide (Sb2S3)
Pure Sb2S3 was synthesized as a control sample via a simple hydrothermal method. Typically, 1.5 g of antimony trichloride (SbCl3) and 1.5 g of L-cysteine were dissolved in 120 mL of deionized water under magnetic stirring. The clear solution was then transferred into an autoclave and heated at 180 °C for 12 hours. The resulting black precipitate was collected, washed thoroughly, and dried in an oven at 60 °C overnight.
3. Synthesis of Sb2S3@NG Composite via Freeze-Drying
The composite was fabricated using a freeze-drying assembly process. First, 60 mg of the as-prepared NG was dispersed in 40 mL of deionized water via ultrasonication for 1 hour to form a stable ink. Then, 0.6 g of the pre-synthesized Sb2S3 powder and 0.2 g of cetyltrimethylammonium bromide (CTAB, used as a dispersant) were added to the NG ink. The mixture was vigorously stirred for several hours to ensure uniform adsorption of Sb2S3 particles onto the graphene sheets. The homogeneous suspension was then pre-frozen in a freezer at -20 °C. The completely frozen solid was subsequently transferred to a freeze-dryer and lyophilized at -70 °C under vacuum for approximately 12 hours, yielding the final Sb2S3@NG composite aerogel.
The summary of the synthesis parameters is presented in the table below.
| Material | Precursor 1 | Precursor 2 / Additive | Process | Conditions | Key Function |
|---|---|---|---|---|---|
| Nitrogen-Doped Graphene (NG) | GO Dispersion | Ammonia | Hydrothermal | 180°C, 24 h | Reduction & N-doping |
| Sb2S3 | SbCl3 | L-cysteine | Hydrothermal | 180°C, 12 h | Active material synthesis |
| Sb2S3@NG | NG + Sb2S3 | CTAB | Freeze-Drying | -70°C, 12 h (Vacuum) | 3D Composite assembly |
4. Material Characterization
The morphology and microstructure of the samples were examined using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping. Crystal structure was analyzed by X-ray diffraction (XRD) with Cu Kα radiation. The specific surface area and pore structure were determined from nitrogen adsorption-desorption isotherms using the Brunauer–Emmett–Teller (BET) method. Raman spectroscopy was performed to investigate the graphitic structure and defect density. The surface chemical states and successful nitrogen doping were confirmed by X-ray photoelectron spectroscopy (XPS).
5. Electrochemical Measurements
Electrochemical tests were conducted using CR2032 coin-type half-cells assembled in an argon-filled glovebox. The working electrodes were prepared by mixing the active material (Sb2S3@NG or bare Sb2S3), conductive carbon black, and sodium carboxymethyl cellulose (CMC) binder in a weight ratio of 7:2:1. The slurry was coated onto a copper foil current collector and dried under vacuum at 60°C. Sodium metal foil served as the counter/reference electrode. The electrolyte was 1.0 M sodium perchlorate (NaClO4) in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) (95:5 by volume). Glass fiber was used as the separator. Cyclic voltammetry (CV) was performed between 0.01 and 3.00 V (vs. Na+/Na) at a scan rate of 0.1 mV s-1. Galvanostatic charge-discharge (GCD) tests were carried out within the same voltage window at various current densities to evaluate rate performance and cycling stability. Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range from 100 kHz to 10 mHz.
Results and Discussion
1. Morphological and Structural Characterization
SEM images reveal distinct morphological differences between the samples. The hydrothermally synthesized bare Sb2S3 consists of rod-like particles with diameters of 100-200 nm and lengths of several micrometers, which tend to agglomerate into large clusters. In contrast, the Sb2S3@NG composite exhibits a unique interconnected 3D porous architecture. The Sb2S3 nanorods are homogeneously dispersed and embedded within the thin, wrinkled graphene sheets. This structure is a direct result of the freeze-drying process, which prevents the restacking of graphene and the aggregation of Sb2S3. The graphene sheets act as both a conductive web linking the active particles and a flexible buffer matrix. Elemental mapping via EDS confirms the uniform distribution of carbon (C), nitrogen (N), sulfur (S), and antimony (Sb) throughout the composite, indicating successful integration of Sb2S3 with the N-doped graphene framework.
The XRD patterns of both Sb2S3 and Sb2S3@NG are presented. All diffraction peaks can be indexed to the orthorhombic phase of stibnite Sb2S3 (JCPDS No. 42-1393). No impurity phases are detected. The composite pattern shows broader and less intense peaks for Sb2S3 compared to the pure sample, suggesting smaller crystallite size or lower crystallinity due to confinement by the graphene sheets. A broad hump around 20-30° in the Sb2S3@NG pattern corresponds to the (002) plane of disordered carbon from the graphene component.
Nitrogen adsorption-desorption analysis provides insight into the textural properties. The isotherms for both materials can be classified as Type IV with an H3 hysteresis loop, indicating the presence of mesopores. The calculated BET specific surface area of Sb2S3@NG is 15.88 m2 g-1, which is approximately five times higher than that of bare Sb2S3 (3.16 m2 g-1). This significant increase is attributed to the introduction of the high-surface-area graphene and the porous 3D structure formed during freeze-drying. A larger surface area is highly beneficial for sodium-ion battery electrodes as it offers more active sites for electrochemical reactions, improves contact with the electrolyte, and shortens the diffusion path for Na+ ions.
Raman spectroscopy was used to characterize the carbon structure in the composite. The spectrum displays two prominent peaks at approximately 1345 cm-1 (D band) and 1589 cm-1 (G band). The D band is associated with structural defects and disordered carbon, while the G band corresponds to the in-plane vibration of sp2-bonded carbon atoms. The intensity ratio ID/IG is calculated to be 1.02, indicating a relatively high degree of defects. These defects originate from the reduction process, nitrogen doping, and the intertwined structure with Sb2S3, which can be advantageous by providing more active sites for sodium ion storage and enhancing surface reactivity.
XPS analysis confirms the successful nitrogen doping and the chemical states of elements in the Sb2S3@NG composite. The survey scan clearly shows the presence of C, N, O, S, and Sb. The high-resolution C 1s spectrum can be deconvoluted into four components: C-C/C=C (284.4 eV), C-N (285.6 eV), C-O (286.5 eV), and O-C=O (289.0 eV). The presence of the C-N peak verifies the incorporation of nitrogen into the carbon lattice. The N 1s spectrum is fitted with two major peaks at 398.2 eV and 399.3 eV, corresponding to pyridinic-N and pyrrolic-N, respectively. These nitrogen species are known to enhance the electronic conductivity and electrochemical activity of carbon materials. The S 2p spectrum shows doublets for S 2p3/2 and S 2p1/2 at binding energies of 161.6 and 162.8 eV, characteristic of S2- in Sb2S3. The Sb 3d spectrum overlaps with the O 1s signal; the peak at 530.3 eV is assigned to Sb 3d5/2 of Sb3+ in Sb2S3.
2. Electrochemical Performance for Sodium-Ion Battery
The sodium storage behavior was first investigated by cyclic voltammetry (CV). The initial three CV cycles of the Sb2S3@NG electrode at 0.1 mV s-1 are shown. During the first cathodic scan, three reduction peaks are observed. A broad peak around 1.05 V is attributed to the irreversible formation of the solid-electrolyte interphase (SEI) film. The sharp peak at approximately 0.70 V corresponds to the conversion reaction of Sb2S3 to metallic Sb and Na2S (Equation 1). The peak near 0.30 V is associated with the alloying reaction of Sb with Na to form Na3Sb (Equation 2). In the subsequent anodic scan, two oxidation peaks appear at about 0.76 V and 1.27 V, which are ascribed to the dealloying of Na3Sb and the reversible conversion of Sb and Na2S back to Sb2S3, respectively. From the second cycle onward, the cathodic peaks shift to slightly higher potentials (0.90 V and 0.40 V) and the curves become highly overlapping, indicating reduced polarization and improved reversibility after the initial activation. In comparison, the CV curves of bare Sb2S3 show larger peak separations and more pronounced peak intensity fading, suggesting greater polarization and inferior electrochemical reversibility.
The galvanostatic charge-discharge profiles of Sb2S3@NG at different current densities exhibit the characteristic plateaus corresponding to the redox reactions identified in the CV, confirming the two-step sodium storage mechanism. The initial discharge and charge capacities of the Sb2S3@NG electrode at 0.1 A g-1 are 1079.7 and 931.8 mAh g-1, respectively, yielding a high initial Coulombic efficiency (ICE) of 86.3%. This impressive ICE is attributed to the stable 3D conductive network and the effective SEI formation facilitated by the N-doped graphene surface. In stark contrast, bare Sb2S3 delivers a much lower initial discharge capacity of 643.2 mAh g-1 and an ICE of only 64.3%, primarily due to severe irreversible side reactions and poor electrical contact.
The rate capability of both electrodes was evaluated by cycling at progressively increasing current densities from 0.1 to 2.0 A g-1. The performance is summarized in the table below. The Sb2S3@NG composite demonstrates outstanding rate performance, delivering average reversible capacities of 855.2, 759.1, 701.6, 650.1, and 610.8 mAh g-1 at 0.1, 0.2, 0.5, 1.0, and 2.0 A g-1, respectively. When the current density is switched back to 0.1 A g-1, a high capacity of 790.1 mAh g-1 is recovered, representing a remarkable retention of 92.4%. Conversely, the capacity of bare Sb2S3 plummets rapidly with increasing current density, and its capacity recovery is poor. This superior rate performance of the composite is directly linked to the enhanced kinetics provided by the 3D NG network, which ensures fast electron transfer and ion diffusion.
| Current Density (A g-1) | Average Discharge Capacity – Sb2S3 (mAh g-1) | Average Discharge Capacity – Sb2S3@NG (mAh g-1) | Capacity Retention (Sb2S3@NG vs. 0.1 A g-1 baseline) |
|---|---|---|---|
| 0.1 | 413.6 | 855.2 | 100% |
| 0.2 | 298.5 | 759.1 | 88.8% |
| 0.5 | 185.2 | 701.6 | 82.0% |
| 1.0 | 110.7 | 650.1 | 76.0% |
| 2.0 | 68.4 | 610.8 | 71.4% |
| 0.1 (Return) | 205.1 | 790.1 | 92.4% |
The long-term cycling stability, a critical metric for practical sodium-ion battery applications, was tested at a current density of 0.5 A g-1. The Sb2S3@NG electrode exhibits excellent stability, maintaining a high reversible capacity of 578.8 mAh g-1 after 150 cycles, with a capacity retention of 82.5% relative to the capacity at the 5th cycle. The Coulombic efficiency quickly stabilizes near 99.5% after the first few cycles. In contrast, the capacity of bare Sb2S3 fades rapidly to below 100 mAh g-1 within 50 cycles. The exceptional cycling performance of the composite is attributed to the robust 3D conductive scaffold, which effectively accommodates the volume changes of Sb2S3, prevents particle aggregation and pulverization, maintains structural integrity, and ensures continuous electrical pathways throughout cycling.
Electrochemical impedance spectroscopy (EIS) was conducted to analyze the interfacial charge transfer resistance. The Nyquist plots consist of a semicircle in the high-medium frequency region (related to charge transfer resistance, Rct) and an inclined line in the low-frequency region (related to sodium ion diffusion). The fitted Rct value for the Sb2S3@NG electrode (81.4 Ω) is significantly lower than that of the bare Sb2S3 electrode (132.6 Ω). This reduced charge transfer resistance quantitatively confirms that the N-doped graphene network dramatically enhances the electrical conductivity of the composite electrode and facilitates faster electrochemical reaction kinetics, which is consistent with its superior rate and cycling performance.
Post-cycling SEM examination of the Sb2S3@NG electrode after 150 cycles reveals that the overall 3D porous architecture is well-preserved. The Sb2S3 particles remain in close contact with the graphene sheets without obvious detachment or severe aggregation. This observation provides direct evidence of the structural buffering effect of the graphene network, which is the fundamental reason for the enhanced cycling stability in this sodium-ion battery anode.
Conclusion
In summary, a three-dimensional Sb2S3@nitrogen-doped graphene composite was successfully synthesized via a simple and scalable freeze-drying method for application as a high-performance anode in sodium-ion batteries. The freeze-drying process is crucial for constructing an interconnected, porous architecture where Sb2S3 nanorods are uniformly embedded within a conductive N-doped graphene network. This unique structure addresses the key challenges of Sb2S3-based anodes: it provides a highway for rapid electron transport and ion diffusion, offers abundant void space to buffer the large volume expansion during sodiation/desodiation, and maintains mechanical integrity over prolonged cycling.
As a result, the Sb2S3@NG composite exhibits comprehensively superior electrochemical properties compared to bare Sb2S3. It delivers a high reversible capacity of 1079.7 mAh g-1 at 0.1 A g-1 with an outstanding initial Coulombic efficiency of 86.3%. It demonstrates remarkable rate capability, retaining 610.8 mAh g-1 even at a high current of 2.0 A g-1. Most importantly, it shows excellent long-term cycling stability, maintaining 578.8 mAh g-1 after 150 cycles at 0.5 A g-1 with a capacity retention of 82.5%. The synergistic combination of the high-capacity conversion/alloying material with a tailored 3D conductive carbon host, facilitated by the freeze-drying technique, presents a highly effective design strategy. This work provides valuable insights and a promising pathway for developing durable and high-energy-density anode materials for the next generation of sustainable and cost-effective sodium-ion batteries.