In recent years, the demand for efficient and cost-effective energy storage systems has surged, driven by the rapid growth of renewable energy and electric vehicles. Lithium-ion batteries (LIBs) have dominated the market, but concerns over lithium scarcity and geopolitical constraints have prompted a search for alternatives. Sodium-ion batteries (SIBs) emerge as a promising candidate due to the abundance of sodium resources and similar electrochemical mechanisms to LIBs. However, the larger ionic radius and molar mass of sodium ions often lead to poor kinetics and structural degradation in electrode materials, hindering their practical application. Therefore, developing advanced anode materials with high capacity, stability, and fast charge transfer is crucial for advancing sodium-ion battery technology.
Transition metal selenides, such as NiSe2 and MoSe2, have garnered attention as anode materials for sodium-ion batteries due to their high theoretical capacities and good conductivity. For instance, MoSe2 offers a theoretical capacity of around 500 mAh g−1, while NiSe2 can reach up to 600 mAh g−1. Despite these advantages, single-component selenides suffer from severe volume changes during sodium ion insertion/extraction, leading to capacity fading and poor cyclability. To address this, constructing heterojunctions between different metal selenides can enhance charge transfer and structural stability through synergistic effects. Additionally, designing self-supporting electrodes with nanostructured arrays, like nanosheets on conductive substrates, can mitigate pulverization and improve electrochemical performance. In this work, we report the synthesis of a carbon cloth-supported NiSe2/MoSe2 heterojunction nanosheet array (NiSe2/MoSe2@CC) via a solvothermal-calcination method. This material demonstrates exceptional performance as an anode for sodium-ion batteries, leveraging its unique architecture and heterojunction interface.
The preparation of NiSe2/MoSe2@CC involved a two-step process. First, carbon cloth (CC) was functionalized by sequential cleaning in acetone, distilled water, and 5 M nitric acid to remove impurities and enhance surface activity. Then, a precursor solution was prepared by dissolving ammonium molybdate (0.43 mmol), nickel acetate (3 mmol), and polyvinylpyrrolidone (PVP, 0.25 g) in 80 mL of ethanol under vigorous stirring. The cleaned carbon cloth was immersed in this solution and transferred to a Teflon-lined autoclave for solvothermal reaction at 180°C for 10 hours. After cooling, the precursor-coated carbon cloth was washed and dried. In the second step, selenization was performed by placing the precursor and selenium powder in a dual-zone tube furnace under a H2/Ar atmosphere, heating at 3°C min−1 to 450°C for 2 hours. For comparison, single-component NiSe2@CC and MoSe2@CC were synthesized under similar conditions without the other metal source. The materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) to analyze their morphology, structure, and composition.
The morphological features of the synthesized materials were examined by SEM. The NiSe2/MoSe2@CC sample exhibited a uniform array of interconnected nanosheets grown directly on carbon cloth fibers, forming a three-dimensional network. This structure provides a large surface area and facilitates electrolyte penetration, which is beneficial for sodium-ion battery applications. In contrast, NiSe2@CC showed agglomerated nanoflowers, while MoSe2@CC displayed loosely packed nanosheets prone to collapse. The structural integrity of NiSe2/MoSe2@CC can be attributed to the PVP-assisted growth and heterojunction formation, which stabilizes the nanosheet array. The XRD patterns confirmed the coexistence of NiSe2 (PDF#41-1495) and MoSe2 (PDF#29-0914) phases in NiSe2/MoSe2@CC, with distinct peaks corresponding to their crystal planes. For instance, the diffraction peaks at 29.9° and 33.6° are assigned to NiSe2, while those at 13.7° and 37.6° belong to MoSe2. The XPS analysis further validated the chemical states, showing Ni 2p, Mo 3d, and Se 3d signals consistent with Ni2+/Ni3+, Mo4+, and Se2− species, respectively. Table 1 summarizes the key structural parameters derived from XRD and XPS.
| Material | Crystal Phase | Lattice Parameters (Å) | Binding Energy (eV) for Key Elements |
|---|---|---|---|
| NiSe2/MoSe2@CC | NiSe2 (cubic), MoSe2 (hexagonal) | a=5.96 (NiSe2), a=3.28, c=12.9 (MoSe2) | Ni 2p3/2: 853.7; Mo 3d5/2: 229.2; Se 3d5/2: 55.7 |
| NiSe2@CC | NiSe2 (cubic) | a=5.96 | Ni 2p3/2: 853.5 |
| MoSe2@CC | MoSe2 (hexagonal) | a=3.28, c=12.9 | Mo 3d5/2: 229.0 |
The electrochemical performance of NiSe2/MoSe2@CC as an anode for sodium-ion batteries was evaluated in half-cell configurations. Cyclic voltammetry (CV) curves were recorded between 0.01 and 3.0 V vs. Na/Na+ at various scan rates. The initial CV cycle showed reduction peaks at 1.15 V and 0.65 V, corresponding to sodium insertion and conversion reactions:
$$\text{NiSe}_2 + x\text{Na}^+ + x e^- \rightarrow \text{Na}_x\text{NiSe}_2$$
$$\text{MoSe}_2 + y\text{Na}^+ + y e^- \rightarrow \text{Na}_y\text{MoSe}_2$$
followed by further reduction to metallic Ni and Mo with Na2Se formation. The oxidation peaks at 1.85 V and 2.0 V indicate the reversible conversion. In subsequent cycles, the CV curves overlapped, suggesting good reversibility. Galvanostatic charge-discharge (GCD) tests at 0.1 A g−1 revealed a high initial discharge capacity of 1412.8 mAh g−1 and a charge capacity of 803.5 mAh g−1, with an initial coulombic efficiency of 56.9%. The capacity loss is typical for sodium-ion battery anodes due to solid electrolyte interface (SEI) formation. After 100 cycles, the reversible capacity stabilized at 648.6 mAh g−1, demonstrating excellent cycling stability. The rate capability was assessed at current densities from 0.1 to 5.0 A g−1, as summarized in Table 2. The capacity retention at high rates underscores the material’s robust kinetics, attributed to the heterojunction and nanosheet array.
| Current Density (A g−1) | Discharge Capacity (mAh g−1) | Charge Capacity (mAh g−1) | Coulombic Efficiency (%) |
|---|---|---|---|
| 0.1 | 661.8 | 630.7 | 95.3 |
| 0.2 | 606.3 | 587.1 | 96.8 |
| 0.5 | 543.4 | 525.9 | 96.8 |
| 1.0 | 499.4 | 487.2 | 97.6 |
| 2.0 | 427.3 | 418.5 | 97.9 |
| 5.0 | 397.8 | 392.4 | 98.6 |
To delve deeper into the charge storage mechanism, kinetic analysis was performed using CV data at scan rates from 0.2 to 1.0 mV s−1. The relationship between peak current (i) and scan rate (v) follows a power law:
$$i = a v^b$$
where a and b are constants. A b-value of 0.5 indicates diffusion-controlled behavior, while 1.0 signifies capacitive processes. For NiSe2/MoSe2@CC, the b-values for redox peaks were calculated as 0.70, 0.66, 0.63, and 0.60, suggesting a mix of diffusion and capacitive contributions, with the latter dominating. The capacitive contribution can be quantified by the equation:
$$i = k_1 v + k_2 v^{0.5}$$
where k1v represents capacitive current and k2v0.5 is diffusion-controlled current. At a scan rate of 0.8 mV s−1, the capacitive contribution accounted for 83.7% of the total charge storage. This high pseudocapacitive behavior is advantageous for sodium-ion battery anodes, as it enables fast charge transfer and reduces stress from volume changes. The enhanced kinetics can be linked to the heterojunction interface, which facilitates electron transport, and the nanosheet array, which shortens ion diffusion paths. Table 3 lists the b-values and capacitive contributions at different scan rates.
| Scan Rate (mV s−1) | b-value (Reduction Peak) | b-value (Oxidation Peak) | Capacitive Contribution (%) |
|---|---|---|---|
| 0.2 | 0.70 | 0.66 | 57.7 |
| 0.4 | 0.68 | 0.64 | 69.1 |
| 0.6 | 0.65 | 0.62 | 77.9 |
| 0.8 | 0.63 | 0.60 | 83.7 |
| 1.0 | 0.62 | 0.59 | 89.9 |
The superior performance of NiSe2/MoSe2@CC in sodium-ion batteries stems from multiple factors. First, the in-situ growth of nanosheets on carbon cloth creates a conductive, self-supporting framework that prevents active material detachment and maintains structural integrity during cycling. Second, the NiSe2/MoSe2 heterojunction generates built-in electric fields at interfaces, accelerating charge transfer and enhancing reaction kinetics. Third, the porous nanosheet array offers abundant active sites for sodium ion storage and allows efficient electrolyte infiltration. These attributes collectively contribute to high capacity, excellent rate capability, and long-term stability. Compared to single-component selenides, the heterojunction material shows a 65% improvement in capacity retention after 100 cycles, highlighting the synergy between Ni and Mo species. Furthermore, the material’s pseudocapacitive nature reduces reliance on slow diffusion processes, making it suitable for high-power sodium-ion battery applications.
In conclusion, we have successfully synthesized a carbon cloth-supported NiSe2/MoSe2 heterojunction nanosheet array as an advanced anode for sodium-ion batteries. The material exhibits a reversible capacity of 648.6 mAh g−1 at 0.1 A g−1 after 100 cycles, along with outstanding rate performance and kinetic properties. The design principles—combining heterojunction engineering with nanostructured self-supporting electrodes—provide a viable strategy for developing high-performance sodium-ion battery anodes. Future work could explore other transition metal selenide combinations or optimize the synthesis parameters to further enhance energy density and cyclability. This research contributes to the ongoing efforts to make sodium-ion batteries a competitive alternative for large-scale energy storage.
The development of such materials is pivotal for advancing sodium-ion battery technology, which holds promise for grid storage and portable electronics. By addressing key challenges like volume change and slow kinetics, heterojunction-based anodes like NiSe2/MoSe2@CC pave the way for more efficient and durable energy storage systems. As research progresses, we anticipate further innovations in material design that will unlock the full potential of sodium-ion batteries in the global energy landscape.