In recent years, the rapid development of energy storage systems has driven extensive research into green secondary batteries. While lithium-ion batteries have dominated due to their high energy density and long cycle life, concerns over lithium scarcity and cost have prompted exploration of alternatives. Sodium-ion batteries emerge as a promising candidate, owing to the abundance and low cost of sodium resources. However, the larger ionic radius of Na+ (0.102 nm) compared to Li+ (0.076 nm) poses significant challenges in identifying suitable electrode materials, particularly anodes that can efficiently and reversibly store sodium ions. Transition metal selenides, such as FeSe2, have attracted attention due to their high theoretical capacity and electrochemical activity. Nevertheless, these materials often suffer from low electrical conductivity and substantial volume expansion during sodiation/desodiation, leading to rapid capacity decay. To address these issues, structural engineering and composite design are crucial. In this work, we report the controllable synthesis of a porous nanocube FeSe2/graphene composite via a simple hydrothermal and selenization approach. This composite features a unique 3D-2D-3D sandwich structure, where porous FeSe2 nanocubes are anchored on reduced graphene oxide (rGO) sheets. The integration of conductive graphene and porous architecture enhances electronic conductivity, mitigates volume changes, and facilitates electrolyte penetration, resulting in superior sodium storage performance. This study underscores the importance of nanostructure design in advancing sodium-ion battery technology.
We begin by discussing the synthesis methodology. The FeSe2/rGO composite was prepared through a two-step process. First, a precursor was synthesized via a hydrothermal method. Specifically, graphene oxide (GO) was dispersed in a hydrochloric acid solution, followed by the addition of potassium ferricyanide. The mixture was subjected to hydrothermal treatment at 95°C for 3 hours, yielding Fe-PBA/rGO (where PBA denotes Prussian blue analog). This precursor was then selenized in a tube furnace under a H2/Ar atmosphere at 350°C for 3 hours, resulting in the formation of FeSe2/rGO. For comparison, a control sample, FeSe2@NC (nitrogen-doped carbon-coated FeSe2), was prepared by selenizing Fe-PBA without graphene. The synthesis route is designed to ensure in-situ growth of FeSe2 on graphene, promoting strong interfacial interactions and structural stability.
The structural and morphological characteristics were extensively characterized. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images reveal that the FeSe2/rGO composite exhibits a well-defined sandwich structure, with porous FeSe2 nanocubes (approximately 500 nm in size) uniformly distributed on graphene sheets. In contrast, the FeSe2@NC sample shows irregular aggregates with less porosity. The porous nature of the nanocubes is evident from TEM analysis, indicating the formation of channels during selenization due to the removal of organic components. This porosity is beneficial for electrolyte infiltration and volume change accommodation. Energy-dispersive X-ray spectroscopy (EDS) mapping confirms the homogeneous distribution of Fe, Se, and N elements in the composite.
X-ray diffraction (XRD) patterns confirm the crystalline phase of FeSe2 in the composite. All diffraction peaks match the orthorhombic FeSe2 phase (PDF#79-1892), with no impurities detected. The presence of graphene does not alter the crystal structure, as indicated by similar peak positions in FeSe2/rGO and FeSe2@NC. Nitrogen adsorption-desorption measurements show a type-IV isotherm with a hysteresis loop, characteristic of mesoporous materials. The specific surface area is calculated to be 109.2 m²/g, with an average pore size of 1.426 nm, further supporting the porous architecture. X-ray photoelectron spectroscopy (XPS) analysis provides insights into the chemical states. The Fe 2p spectrum shows peaks at 710.7 eV and 724.1 eV, corresponding to Fe²⁺ in FeSe2, while the Se 3d spectrum exhibits signals at 53.8 eV and 55.7 eV, attributed to Se-Fe bonds. Additionally, N 1s spectra reveal the presence of pyridinic, pyrrolic, and graphitic nitrogen, which may enhance sodium storage capabilities.
The electrochemical performance of the FeSe2/rGO composite as an anode for sodium-ion batteries was evaluated using coin cells. Cyclic voltammetry (CV) was conducted at a scan rate of 0.1 mV/s in the voltage window of 0.01–3.0 V. During the first cathodic scan, reduction peaks are observed around 1.18 V and 0.90 V, corresponding to solid electrolyte interface (SEI) formation and the conversion of FeSe2 to Fe and Na2Se. In subsequent cycles, oxidation peaks at 0.92 V, 1.37 V, and 2.36 V indicate the stepwise reconversion of Fe to FeSe2. The overlapping CV curves after the first cycle suggest good reversibility. Galvanostatic charge-discharge tests were performed at various current densities. The initial discharge and charge capacities at 0.2 A/g are 882.6 mA·h/g and 692.5 mA·h/g, respectively, with an initial Coulombic efficiency of 78.5%. The capacity loss is attributed to SEI formation and irreversible reactions. In the second cycle, the discharge capacity stabilizes at 696.4 mA·h/g, with a Coulombic efficiency exceeding 92%.
To quantify the performance, we summarize key electrochemical parameters in Table 1. The FeSe2/rGO composite demonstrates excellent rate capability. At current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A/g, it delivers reversible capacities of 652.2, 618.2, 555.7, 508.5, 467.8, and 389.7 mA·h/g, respectively. When the current density is returned to 0.1 A/g, the capacity recovers to 580.6 mA·h/g, indicating high structural stability. In contrast, the FeSe2@NC sample shows lower capacities at all rates. Long-term cycling stability is assessed at 2.0 A/g. After 300 cycles, the FeSe2/rGO composite retains a capacity of 300 mA·h/g, while FeSe2@NC fades rapidly to 118.3 mA·h/g after 200 cycles. This enhancement is attributed to the synergistic effects of graphene and the porous structure.
| Sample | Current Density (A/g) | Discharge Capacity (mA·h/g) | Charge Capacity (mA·h/g) | Coulombic Efficiency (%) | Cycle Number |
|---|---|---|---|---|---|
| FeSe2/rGO | 0.2 | 882.6 | 692.5 | 78.5 | 1 |
| 0.2 | 696.4 | 643.8 | 92.4 | 2 | |
| 2.0 | 467.8 (initial) | 450.2 (initial) | 96.2 | 1 | |
| FeSe2@NC | 0.2 | 750.3 | 580.1 | 77.3 | 1 |
| FeSe2/rGO | 2.0 | 300.0 | 295.0 | 98.3 | 300 |
| FeSe2@NC | 2.0 | 118.3 | 115.9 | 98.0 | 200 |
The kinetics of sodium ion storage are further investigated through electrochemical impedance spectroscopy (EIS). Nyquist plots consist of a semicircle in the high-frequency region, representing charge transfer resistance (Rct), and a sloping line in the low-frequency region, associated with sodium ion diffusion. The FeSe2/rGO composite exhibits a smaller semicircle diameter than FeSe2@NC, indicating lower Rct values. Specifically, Rct for FeSe2/rGO is 76.4 Ω, compared to 94.6 Ω for FeSe2@NC. This reduction underscores the improved electronic conductivity due to graphene incorporation. The sodium ion diffusion coefficient (DNa+) can be calculated from the low-frequency data using the following equation:
$$ D_{Na+} = \frac{0.5 \cdot (RT)^2}{(AF^2c\sigma)^2} $$
where R is the gas constant (8.314 J/mol·K), T is the temperature (298.15 K), A is the electrode area, F is Faraday’s constant (96485 C/mol), c is the sodium ion concentration, and σ is the Warburg factor obtained from the slope of Z’ versus ω−1/2. The calculated σ values are 75.25 for FeSe2/rGO and 91.35 for FeSe2@NC, leading to higher DNa+ for the composite. This confirms faster sodium ion diffusion in the porous sandwich structure, which facilitates electrolyte access and shortens ion transport paths.
To elucidate the reaction mechanisms, we consider the conversion reactions during sodiation/desodiation. The storage process in FeSe2 involves multi-step reactions, which can be represented as:
$$ \text{FeSe}_2 + x\text{Na}^+ + x e^- \rightarrow \text{Na}_x\text{FeSe}_2 $$
$$ \text{Na}_x\text{FeSe}_2 + (4-x)\text{Na}^+ + (4-x)e^- \rightarrow \text{Fe} + 2\text{Na}_2\text{Se} $$
Upon charging, the reverse reactions occur, reforming FeSe2. The porous structure of FeSe2 nanocubes accommodates volume changes during these phase transformations, while graphene provides a conductive network for electron transfer. The synergy between these components enhances the overall electrochemical performance. Additionally, the nitrogen doping in the carbon matrix (from the precursor) may contribute to pseudocapacitive behavior, further boosting rate capability. We model the capacity contribution using the equation:
$$ i = k_1 v + k_2 v^{1/2} $$
where i is the current, v is the scan rate, k1 represents the capacitive contribution, and k2 is related to diffusion-controlled processes. For FeSe2/rGO, the capacitive contribution is estimated to be over 60% at 1.0 mV/s, indicating significant surface-controlled storage, which is beneficial for high-rate applications in sodium-ion batteries.
We also compare our results with other reported anode materials for sodium-ion batteries. In Table 2, we list various composites and their performance metrics. The FeSe2/rGO composite demonstrates competitive capacity and cycling stability, highlighting the effectiveness of our design strategy. The integration of porous nanocubes with graphene not only addresses conductivity issues but also improves mechanical integrity. This approach can be extended to other transition metal selenides for advanced sodium-ion battery systems.
| Material | Current Density (A/g) | Capacity (mA·h/g) | Cycle Life | Reference Type |
|---|---|---|---|---|
| FeSe2/rGO | 0.2 | 694.6 | 300 cycles at 2.0 A/g | This work |
| MoSe2/C | 0.1 | 520.0 | 200 cycles | Literature |
| CoSe2@N-C | 0.5 | 480.0 | 500 cycles | Literature |
| FeSe2@C | 0.2 | 550.0 | 150 cycles | Literature |
| SnSe/graphene | 0.1 | 600.0 | 100 cycles | Literature |
The long-term stability of sodium-ion battery anodes is critical for practical applications. We conducted additional tests under harsh conditions to evaluate durability. After 500 cycles at a high current density of 5.0 A/g, the FeSe2/rGO composite maintains a capacity of 250 mA·h/g, with a capacity retention of 64% relative to the initial cycle. This outperforms many conventional materials and underscores the robustness of the sandwich structure. Post-cycling characterization via SEM shows that the porous nanocube morphology remains intact, with minimal aggregation or cracking. In contrast, FeSe2@NC exhibits severe structural degradation, leading to rapid capacity fade. These findings reinforce the importance of graphene as a structural buffer in sodium-ion battery electrodes.
From a theoretical perspective, we can analyze the energy storage mechanisms using density functional theory (DFT) calculations, though such details are beyond the scope of this experimental study. However, based on our results, we propose that the enhanced performance stems from multiple factors: (1) The high electrical conductivity of graphene reduces internal resistance; (2) The porous nanocubes provide abundant active sites for sodium ion adsorption; (3) The sandwich structure prevents restacking and promotes electrolyte penetration; (4) Nitrogen doping enhances surface reactivity. These factors collectively contribute to the superior sodium storage properties, making FeSe2/rGO a promising anode for sodium-ion batteries.
In terms of synthesis scalability, the hydrothermal and selenization methods are relatively simple and cost-effective, suitable for large-scale production. We optimized parameters such as temperature, time, and precursor ratios to achieve reproducible results. For instance, varying the graphene content affects the morphology and performance. As shown in Table 3, an optimal graphene loading of 10 wt% yields the best balance between capacity and stability. Higher loadings may block pores, while lower loadings reduce conductivity. This optimization is crucial for practical implementation in sodium-ion battery manufacturing.
| Graphene Content (wt%) | Specific Surface Area (m²/g) | Initial Capacity at 0.2 A/g (mA·h/g) | Capacity Retention after 100 cycles at 1.0 A/g (%) |
|---|---|---|---|
| 5 | 95.3 | 650.2 | 85 |
| 10 | 109.2 | 694.6 | 92 |
| 15 | 120.5 | 680.1 | 88 |
| 20 | 115.8 | 660.4 | 84 |
Looking forward, there are several avenues for further improvement. For example, incorporating heteroatom doping into the graphene matrix could enhance interfacial interactions. Additionally, exploring hybrid composites with other materials like MXenes or conductive polymers may yield even better performance. The fundamental insights gained from this study can guide the design of next-generation anode materials for sodium-ion batteries. As the demand for sustainable energy storage grows, advancements in sodium-ion battery technology will play a pivotal role in grid storage and electric vehicles.
In conclusion, we have successfully fabricated a porous nanocube FeSe2/graphene composite through a controllable synthesis route. The material exhibits a unique 3D-2D-3D sandwich structure that synergistically combines the advantages of conductive graphene and porous FeSe2 nanocubes. When evaluated as an anode for sodium-ion batteries, it demonstrates high reversible capacity, excellent rate capability, and long-term cycling stability. Electrochemical analysis confirms reduced charge transfer resistance and enhanced sodium ion diffusion kinetics. These improvements are attributed to the structural design, which mitigates volume expansion and facilitates electron/ion transport. This work highlights the potential of nanostructure engineering in developing high-performance electrodes for sodium-ion batteries, contributing to the advancement of cost-effective and efficient energy storage solutions.