The relentless advancement of technology and the corresponding surge in global energy demand have precipitated a dual crisis of depleting finite fossil fuel resources and severe environmental degradation, fundamentally challenging sustainable development. In this context, electrochemical energy storage systems, particularly rechargeable batteries, are pivotal. While lithium-ion batteries (LIBs) have dominated the market for decades, concerns regarding the limited and geographically concentrated lithium reserves have spurred intensive research into alternative chemistries. Sodium-ion batteries (SIBs) emerge as a compelling candidate due to the natural abundance and low cost of sodium, coupled with its physicochemical similarities to lithium. However, a significant hurdle for the commercialization of SIBs is their generally lower energy density compared to LIBs, largely dictated by the performance of electrode materials. While substantial progress has been made in developing high-capacity cathode materials, the search for durable, high-energy-density anode materials remains a critical challenge. Graphite, the standard anode in LIBs, exhibits poor sodium storage capability due to thermodynamic instability, necessitating the exploration of new host materials.
Among various alternatives, tin-based materials are highly attractive anodes for sodium-ion battery applications. They offer high theoretical specific capacity (e.g., 847 mAh g-1 for SnS2, 780 mAh g-1 for SnSe2 based on alloying with Na to form Na15Sn4), good electronic conductivity, natural abundance, and environmental benignity. Tin selenides (SnSex) have recently gained attention over their sulfide counterparts due to the higher metallicity and larger interlayer spacing of Se, which can facilitate faster ion/electron transport and accommodate volume changes more effectively. The sodium storage mechanism in SnSe2 typically involves a combination of conversion and alloying reactions:
$$ \text{SnSe}_2 + x\text{Na}^+ + x\text{e}^- \rightarrow \text{Na}_x\text{SnSe}_2 \quad \text{(Intercalation)} $$
$$ \text{Na}_x\text{SnSe}_2 + (4-x)\text{Na}^+ + (4-x)\text{e}^- \rightarrow \text{Sn} + 2\text{Na}_2\text{Se} \quad \text{(Conversion)} $$
$$ \text{Sn} + y\text{Na}^+ + y\text{e}^- \leftrightarrow \text{Na}_y\text{Sn} \quad (0 \le y \le 3.75) \quad \text{(Alloying)} $$
The final alloying reaction with Na can theoretically deliver a high capacity. However, the drastic volume expansion (over 420% for the full alloying process from Sn to Na15Sn4) during repeated sodiation/desodiation cycles leads to severe pulverization of the active material, loss of electrical contact, and continuous electrolyte decomposition. This results in rapid capacity fading and poor cycling stability, hindering practical application.
To mitigate these issues, a widely adopted strategy is to engineer nanostructured materials and composite them with conductive carbon matrices. Nanostructuring (e.g., nanoparticles, nanosheets, nanorods) can shorten ion diffusion paths, provide larger electrode-electrolyte contact area, and better accommodate mechanical strain. Incorporating carbonaceous materials (graphene, carbon nanotubes, carbon cloth) enhances the overall electrical conductivity and acts as a structural buffer to confine volume changes and prevent agglomeration. In particular, constructing self-supporting, binder-free electrodes by directly growing active materials on conductive, flexible substrates like carbon cloth (CC) offers distinct advantages: i) Elimination of inert binders and conductive additives increases the overall energy density. ii) The 3D porous and interconnected architecture of CC ensures efficient electron transport and ion diffusion. iii) The mechanical flexibility and robustness of CC provide a stable scaffold that can endure volume variations, maintaining electrode integrity. iv) The direct growth ensures strong adhesion between the active material and the current collector, reducing interface resistance.
In this work, we present the rational design and synthesis of a self-supporting, flexible anode for sodium-ion battery by directly growing SnSe2 nanorods on a carbon cloth substrate (SnSe2/CC) via a facile two-step hydrothermal-selenization method. The synergistic coupling between the SnSe2 nanorods and the 3D carbon cloth framework addresses the intrinsic challenges of tin-based anodes. The CC substrate not only provides a highly conductive network for rapid electron transfer but also creates ample void space to accommodate the volume expansion of SnSe2 during cycling. The direct growth ensures excellent adhesion and structural stability. Consequently, the as-prepared SnSe2/CC composite anode demonstrates superior sodium storage performance, including high reversible capacity, excellent rate capability, and remarkable cycling stability, showcasing its great potential for application in advanced sodium-ion batteries.
Experimental Synthesis and Material Design
The synthesis of the SnSe2/CC composite involves a sequential process of precursor formation followed by selenization. First, a commercially available carbon cloth is meticulously pretreated to functionalize its surface. The CC is cut, ultrasonically cleaned in acetone to remove impurities, and then immersed in a piranha solution (a 3:1 mixture of concentrated H2SO4 and HNO3) for several hours. This oxidative treatment introduces oxygen-containing functional groups (e.g., -COOH, -OH) on the carbon fibers, significantly enhancing their hydrophilicity and providing abundant nucleation sites for the subsequent growth of tin oxide precursors.
The second step is the hydrothermal deposition of a SnOx precursor. An aqueous solution is prepared by dissolving 0.5 g of SnCl2·2H2O in 70 mL of deionized water under stirring. Subsequently, 0.36 g of urea (CO(NH2)2) and 0.5 g of hexamethylenetetramine (HMTA, C6H12N4) are added. Urea acts as a precipitating agent, slowly hydrolyzing to provide OH– ions, while HMTA serves as a pH buffer and a structural directing agent. The cleaned carbon cloth is immersed in this solution and transferred to a Teflon-lined stainless-steel autoclave, which is sealed and maintained at 120°C for 12 hours. During this hydrothermal process, tin hydroxide/oxide species nucleate and grow on the activated carbon fibers, forming a SnO-SnO2/CC intermediate. The reaction can be summarized as:
$$ \text{Sn}^{2+} + 2\text{OH}^- \rightarrow \text{Sn(OH)}_2 \downarrow $$
$$ \text{Sn(OH)}_2 \xrightarrow{\Delta} \text{SnO} + \text{H}_2\text{O} $$
$$ 4\text{Sn}^{2+} + O_2 + 6\text{H}_2\text{O} \rightarrow 4\text{SnO(OH)} \downarrow + 8\text{H}^+ \quad \text{(partial oxidation)} $$
After cooling, the product is washed and dried.
The final step is the gas-phase selenization. The SnO-SnO2/CC composite and pure selenium powder are placed separately in two zones of a dual-temperature tube furnace. Under a continuous argon flow, the furnace is heated to 400°C at a rate of 2°C min-1 and held for 3 hours. The selenium powder in the upstream zone vaporizes and is carried by the Ar gas to react with the tin oxide precursor, resulting in its complete conversion to crystalline SnSe2. The probable chemical reaction is:
$$ \text{SnO}_x + (2+x)\text{Se}_{(v)} \xrightarrow{\Delta, \text{Ar}} \text{SnSe}_2 + x\text{SeO}_2 \uparrow $$
The obtained material is a flexible, self-supporting electrode ready for use. For comparison, pure SnSe2 powder was also synthesized using an identical procedure without the carbon cloth substrate.
Structural and Morphological Characterization
The crystal structure and phase purity of the synthesized materials were examined by X-ray diffraction (XRD). The XRD pattern of the SnSe2/CC composite shows distinct diffraction peaks at 14.5°, 30.7°, 40.0°, 44.1°, and 47.6°, which can be indexed to the (001), (101), (102), (003), and (110) planes of hexagonal SnSe2 (P$\bar{3}$m1 space group, JCPDS No. 23-0602). The characteristic broad diffraction peak from the carbon cloth substrate is observed around 25°. No impurity peaks from tin oxides or elemental selenium are detected, confirming the complete selenization and high purity of the SnSe2 phase. The XRD pattern of the synthesized SnSe2 powder is identical, showing the same crystalline structure.
The morphology and microstructure were investigated by scanning electron microscopy (SEM). The low-magnification SEM image reveals that the carbon cloth maintains its woven, flexible structure composed of interconnecting microfibers. At higher magnification, it is evident that the surface of each carbon fiber is uniformly and densely covered with vertically aligned nanorod arrays. These SnSe2 nanorods have a length of several hundred nanometers and a width of tens of nanometers, creating a highly porous, interconnected network. This unique 3D hierarchical architecture is crucial for electrochemical performance: the nanorod morphology provides a large active surface area and shortens the diffusion length for sodium ions, while the abundant pores between the nanorods and fibers accommodate volume changes and facilitate electrolyte infiltration. In stark contrast, the SnSe2 powder synthesized without the carbon cloth substrate exhibits severe aggregation of irregular particles and rods, forming large, dense clusters that would hinder ionic and electronic transport.
Further insights into the chemical composition and electronic states were gained through X-ray photoelectron spectroscopy (XPS). The survey spectrum confirms the presence of Sn, Se, C, and O elements. The high-resolution Sn 3d spectrum can be deconvoluted into two pairs of doublets. The primary pair with binding energies at 487.2 eV (Sn 3d5/2) and 495.7 eV (Sn 3d3/sub>) corresponds to Sn4+ in SnSe2. A minor pair at slightly lower energies (486.4 eV and 494.7 eV) is attributed to Sn2+ states, likely associated with surface defects or partial reduction. The Se 3d spectrum is fitted with three components: the main doublet for Se2- in SnSe2 (Se 3d5/2 at 53.68 eV and Se 3d3/2 at 54.58 eV), and a higher energy component at 55.48 eV assigned to Se-C bonds, indicating a strong chemical interaction between the SnSe2 nanorods and the carbon cloth substrate. This strong coupling is essential for enhancing charge transfer and structural integrity. Furthermore, the presence of Sn2+ and Se-C bonds suggests the existence of Se vacancies and defect sites, which can serve as additional active centers for sodium ion adsorption and potentially improve the reaction kinetics.
Electrochemical Performance Evaluation in Sodium-Ion Battery
The sodium storage properties of the SnSe2/CC electrode were evaluated by assembling CR2032 coin-type half-cells versus sodium metal. The flexible SnSe2/CC was directly used as the working electrode without any binder or conductive additive. Cyclic voltammetry (CV) was performed to understand the electrochemical reaction mechanisms. During the initial cathodic scan, a broad reduction peak around 1.5-1.8 V corresponds to the intercalation of Na+ into the SnSe2 interlayers to form NaxSnSe2, accompanied by the inevitable formation of a solid-electrolyte interphase (SEI) layer due to electrolyte decomposition. The sharp peak near 0.5 V is attributed to the subsequent conversion reaction (NaxSnSe2 to Sn and Na2Se) and the beginning of the alloying reaction. In the following anodic scan, the peak at approximately 0.3 V is related to the de-alloying of NaySn, while the broad peaks between 1.2-2.0 V correspond to the reversible conversion back to SnSe2. From the second cycle onward, the CV curves almost overlap, indicating excellent reversibility of the sodiation/desodiation processes.
Galvanostatic charge-discharge (GCD) profiles provide quantitative capacity information. The initial discharge (sodiation) and charge (desodiation) capacities of the SnSe2/CC electrode are 1080.8 mAh g-1 and 696.6 mAh g-1, respectively, yielding a first-cycle Coulombic efficiency (ICE) of 64.5%. The irreversible capacity loss is mainly ascribed to SEI formation and some irreversible phase transitions. In contrast, the SnSe2 powder electrode shows a lower ICE of only 46.7% (950.2/443.6 mAh g-1), highlighting the beneficial role of the carbon cloth in suppressing excessive electrolyte decomposition and improving reversibility. The subsequent GCD curves show stable plateaus consistent with the CV peaks, and the Coulombic efficiency quickly rises and stabilizes above 97%.
The long-term cycling stability, a critical metric for practical sodium-ion battery applications, was tested at a current density of 0.1 A g-1. The SnSe2/CC electrode demonstrates remarkable capacity retention. After 100 cycles, it maintains a high reversible discharge capacity of 541.0 mAh g-1, significantly outperforming the SnSe2 powder electrode, which fades rapidly to 223.4 mAh g-1 after the same number of cycles. The capacity retention of the SnSe2/CC composite over 100 cycles is impressive. The stable cycling performance is a direct consequence of the robust 3D conductive network and the porous, flexible structure of the carbon cloth, which effectively buffers the volume strain and prevents the disintegration of the active SnSe2 nanorods.
| Material | Current Density (A g-1) | Cycle Number | Reversible Capacity (mAh g-1) | Capacity Retention |
|---|---|---|---|---|
| SnSe2/Carbon Cloth (This work) | 0.1 | 100 | 541.0 | ~78% (vs. 2nd cycle) |
| SnSe2 Powder (This work) | 0.1 | 100 | 223.4 | < 50% |
| SnSe2/rGO Composite (Literature) | 0.1 | 100 | ~480 | High |
| Porous SnSe/ZnSe@C (Literature) | 0.1 | 100 | ~520 | Excellent |
The rate capability, which reflects the electrode’s performance under fast charging/discharging conditions, was evaluated by subjecting the cell to progressively increasing current densities from 0.1 to 5 A g-1 and then back to 0.1 A g-1. The SnSe2/CC electrode delivers average discharge capacities of approximately 630, 580, 540, 490, and 430 mAh g-1 at 0.1, 0.2, 0.5, 1, and 2 A g-1, respectively. Even at a very high rate of 5 A g-1, a capacity of about 350 mAh g-1 is retained. When the current density is abruptly returned to 0.1 A g-1, the specific capacity recovers to 503.9 mAh g-1, demonstrating exceptional structural resilience and electrochemical reversibility. The SnSe2 powder electrode, however, suffers from drastic capacity decay at elevated rates and poor recovery, underscoring the kinetic limitations imposed by its aggregated morphology.
Electrochemical impedance spectroscopy (EIS) was conducted to analyze the electrode kinetics. The Nyquist plots consist of a semicircle in the high-to-medium frequency region, representing the charge-transfer resistance (Rct) at the electrode/electrolyte interface, and an inclined line in the low-frequency region, corresponding to Warburg impedance (Zw) related to sodium-ion diffusion. The SnSe2/CC electrode exhibits a significantly lower Rct value (148.9 Ω) compared to the SnSe2 powder electrode (189.6 Ω). This reduced interfacial resistance confirms the superior electrical conductivity and more efficient charge transfer process enabled by the intimate contact between SnSe2 nanorods and the highly conductive carbon cloth network. The enhanced kinetics directly contribute to the improved rate performance.
The superior performance of the SnSe2/CC anode can be quantitatively understood by analyzing the contribution of capacitive and diffusion-controlled processes. The current (i) at a fixed potential (V) follows a power-law relationship with the scan rate (v):
$$ i = a v^b $$
where the b-value can be determined from the slope of log(i) vs. log(v). A b-value of 0.5 indicates a diffusion-controlled process, while 1.0 indicates a capacitive process. For the SnSe2/CC electrode, the b-values calculated for the anodic and cathodic peaks are typically between 0.7 and 0.9, suggesting a significant contribution from surface-controlled capacitive behavior (including pseudo-capacitance from surface redox reactions and double-layer capacitance). This capacitive contribution, which is faster and less destructive to the structure than bulk diffusion, can be quantified by separating the total current at a specific potential:
$$ i(V) = k_1 v + k_2 v^{1/2} $$
where $k_1 v$ represents the capacitive contribution and $k_2 v^{1/2}$ represents the diffusion-controlled contribution. Analysis reveals that the capacitive contribution can exceed 70% at higher scan rates (e.g., 1.0 mV s-1). This dominant capacitive behavior is facilitated by the nanorod morphology providing a large accessible surface area and the conductive carbon cloth enabling rapid charge collection, which explains the excellent rate capability of this sodium-ion battery anode.
Discussion on Synergistic Enhancement Mechanisms
The outstanding sodium-ion battery performance of the SnSe2/CC composite stems from the synergistic interaction between its architectural and compositional components. The design principles are multi-faceted:
1. Structural Buffering and Confinement: The 3D carbon cloth acts as a flexible yet robust scaffold. During the repeated volume expansion/contraction of SnSe2, the interconnected carbon fibers elastically deform to accommodate the strain, preventing the crack propagation and pulverization that plague bulk or powdered electrodes. The porous space between the SnSe2 nanorods and the carbon fibers provides additional “buffer zones”. This effectively maintains the structural integrity of the entire electrode over long-term cycling, as evidenced by post-cycling SEM observations showing the preserved nanorod-on-fiber morphology.
2. Enhanced Conductivity and Charge Transfer: The carbon cloth serves as a “highway” for electrons. The direct growth of SnSe2 on the carbon fibers establishes excellent electrical contact, minimizing interfacial resistance. This integrated conductive network ensures that electrons generated during the electrochemical reactions are rapidly collected and transported to the external circuit. The XPS-detected Se-C bonds indicate a strong chemical linkage, further facilitating charge transfer at the interface and stabilizing the composite structure.
3. Optimized Kinetics and Ion Diffusion: The one-dimensional nanorod morphology of SnSe2 significantly shortens the diffusion path length for sodium ions ($L$). The diffusion time ($\tau$) is related to the diffusion coefficient ($D$) and the diffusion length by the equation $\tau \propto L^2/D$. By reducing $L$ to the nanoscale, $\tau$ is dramatically decreased, enabling faster reaction kinetics. Furthermore, the hierarchical porosity from the macro-pores of the carbon cloth to the meso-pores between nanorods ensures full electrolyte penetration and a large electroactive surface area, maximizing the electrode-electrolyte contact area for ion exchange.
4. Defect Engineering: The presence of Sn2+ states and Se vacancies, as suggested by XPS, introduces active sites that can enhance the adsorption of sodium ions and possibly lower the energy barrier for charge transfer. These defects can also modify the local electronic structure, potentially improving intrinsic conductivity.
The overall sodium storage process in this composite can be visualized as follows: Upon discharge (sodiation), sodium ions from the electrolyte readily infiltrate the porous electrode. They simultaneously undergo intercalation, conversion, and alloying reactions with the SnSe2 nanorods. The generated electrons are efficiently shuttled away through the conductive carbon cloth network. The volume expansion is isotropically absorbed by the surrounding void space and the flexible carbon scaffold. Upon charge (desodiation), the process reverses smoothly due to the maintained electrical and mechanical connections. This harmonious interplay ensures high capacity, fast kinetics, and long-term stability.
Conclusion and Perspectives
In summary, we have successfully fabricated a flexible, self-supporting SnSe2/carbon cloth composite anode through a scalable hydrothermal-selenization method for high-performance sodium-ion batteries. The material features SnSe2 nanorods uniformly anchored on a 3D conductive carbon cloth framework. This ingenious design ingeniously tackles the core issues of tin-based anodes: the carbon cloth substrate provides exceptional electrical conductivity, robust mechanical support, and ample buffer space for volume changes, while the SnSe2 nanorods offer a large reaction surface and short ion diffusion paths.
As a result, the SnSe2/CC anode exhibits exemplary electrochemical properties: a high reversible capacity of 541.0 mAh g-1 after 100 cycles at 0.1 A g-1, outstanding rate capability with a recovery capacity of 503.9 mAh g-1 after high-rate testing, and low charge-transfer resistance. The performance surpasses that of its powder counterpart and compares favorably with other SnSe2-based anodes reported in the literature.
This work not only presents a promising anode candidate for next-generation sodium-ion batteries but also demonstrates a versatile and effective design principle for mitigating volume expansion in alloying/conversion-type electrode materials. The strategy of constructing self-supporting, binder-free electrodes by integrating active nanomaterials with 3D conductive scaffolds can be extended to other metal chalcogenides (e.g., Sb2S3, MoS2, FeS2) or alloying metals (Sn, Sb, P). Future research could focus on further optimizing the SnSe2 morphology (e.g., ultrathin nanosheets, hollow structures), heteroatom-doping of the carbon cloth to enhance its reactivity, or pre-sodiation treatments to improve the initial Coulombic efficiency. Integrating this high-performance anode with suitable high-voltage cathode materials to construct full sodium-ion battery cells will be a crucial step toward practical application. The development of such advanced materials is key to realizing cost-effective, safe, and high-energy-density sodium-ion batteries for large-scale energy storage systems.