The escalating global energy crisis has intensified the pursuit of advanced energy storage technologies. While significant progress has been made in harnessing renewable sources like wind, geothermal, and tidal energy, the efficient storage of this intermittent power remains a critical challenge. Among the various secondary battery systems, sodium-ion batteries have emerged as a particularly promising complement to the ubiquitous lithium-ion technology due to the natural abundance of sodium, lower cost, and enhanced safety profile. These attributes position sodium-ion batteries for wide application in energy storage systems, low-speed electric vehicles, and as a potential component in the broader electric mobility sector. The core components of a sodium-ion battery include the cathode, anode, electrolyte, and separator, with the anode material being a primary focus for performance enhancement. Conventional electrode fabrication involves mixing active materials with conductive additives and polymer binders, followed by coating onto a metallic current collector. This process not only adds inactive weight but can also lead to detachment issues and increased interfacial resistance.
To circumvent these limitations, the strategy of direct in-situ growth of active materials onto conductive substrates has gained considerable traction. This binder-free approach simplifies electrode manufacturing by eliminating the slurry casting and calendaring steps. More importantly, it establishes direct electrical and mechanical contact between the active material and the current collector, facilitating efficient electron transport and accommodating volume changes during cycling. Carbon cloth (CC), a fabric woven from carbon fibers, serves as an excellent candidate for such a substrate. Its three-dimensional porous network, high electrical conductivity, chemical stability, and flexibility make it ideal for constructing integrated electrodes. However, pristine commercial carbon cloth often exhibits chemical inertness and hydrophobicity, which hinders the uniform nucleation and growth of active materials. Therefore, a pre-treatment step is essential to functionalize its surface.
Transition metal oxides, particularly spinel-type NiCo2O4, have attracted attention as potential anode materials for sodium-ion batteries due to their high theoretical capacity based on conversion reactions. The electrochemical reaction can be conceptually represented as:
$$ \text{NiCo}_2\text{O}_4 + 8\text{Na}^+ + 8e^- \leftrightarrow \text{Ni} + 2\text{Co} + 4\text{Na}_2\text{O} $$
However, NiCo2O4 suffers from poor intrinsic electronic conductivity and substantial volume expansion during sodiation/desodiation, leading to rapid capacity fading. Integrating NiCo2O4> with a conductive carbon matrix like carbon cloth is a rational design to address these drawbacks. In this work, we present a facile and scalable method to fabricate a hierarchical NiCo2O4@carbon cloth (NCO@CC) composite via an in-situ hydrothermal growth followed by annealing. The resulting binder-free electrode demonstrates significantly improved electrochemical performance as an anode for sodium-ion batteries compared to its pure powder counterpart.
Experimental Synthesis and Characterization
The synthesis of the NCO@CC composite involved two main steps: substrate pre-treatment and active material growth. A summary of the key synthesis parameters is provided in Table 1.
| Step | Reagent / Condition | Specification / Value |
|---|---|---|
| CC Pre-treatment | Oxidizing Agent | KMnO4 Solution |
| Concentration & Time | 2 mol/L, 24 h immersion | |
| Hydrothermal Growth | Ni Source | Ni(NO3)2·6H2O (1 mmol) |
| Co Source | Co(NO3)2·6H2O (2 mmol) | |
| Structure Directors | NH4F (6 mmol), Urea (15 mmol) | |
| Temperature & Time | 140 °C, 3 h | |
| Annealing | Atmosphere & Condition | Air, 350 °C for 2 h |
Commercial carbon cloth was first cut into pieces and treated with a concentrated potassium permanganate (KMnO4) solution. This oxidative treatment creates hydrophilic functional groups (e.g., -OH, -COOH) on the carbon fiber surface, providing abundant anchoring sites for subsequent crystal growth. For the hydrothermal synthesis, stoichiometric amounts of nickel nitrate, cobalt nitrate, ammonium fluoride (NH4F), and urea were dissolved in deionized water. NH4F and urea act as coordination agents and hydrolysis regulators, crucial for controlling the morphology. The pre-treated CC was immersed in this homogeneous solution and subjected to hydrothermal reaction. The obtained precursor-coated CC was washed, dried, and finally annealed in air to crystallize the spinel NiCo2O4 phase. For comparison, pure NiCo2O4 powder (labeled NCO) was synthesized under identical conditions without the carbon cloth substrate.
The crystal structure was examined using X-ray diffraction (XRD). The morphology and elemental distribution were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The electrochemical evaluation was performed by assembling CR2032 coin cells in an argon-filled glovebox, using the NCO@CC or NCO (mixed with conductive carbon and binder) as the working electrode, sodium metal as the counter/reference electrode, and a glass fiber separator soaked in an electrolyte of 1 M NaClO4 in a mixture of propylene carbonate (PC), ethylene carbonate (EC), and fluoroethylene carbonate (FEC). Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were conducted to assess the electrochemical behavior, rate capability, and cycling stability within a voltage window of 0.01-3.00 V vs. Na+/Na.
Structural and Morphological Analysis
The XRD patterns of both the NCO powder and the NCO@CC composite confirmed the successful formation of the spinel NiCo2O4 phase (JCPDS No. 20-0781). No impurity peaks were detected, indicating high phase purity. For the NCO@CC composite, a broad diffraction hump centered around 25° was observed, corresponding to the (002) graphitic plane of the carbon cloth substrate.
Morphological analysis revealed a striking difference between the two samples. The pure NCO powder showed severe agglomeration of nanoparticles, forming large, irregular micro-sized clusters with limited porosity. In stark contrast, the NCO@CC composite exhibited a well-defined hierarchical architecture. As shown in the SEM images, the interconnected carbon fibers (with diameters of ~10 μm) served as a robust three-dimensional scaffold. On the surface of each fiber, a dense and uniform layer of vertically aligned NiCo2O4 nanoflakes was grown. These nanoflakes, with thicknesses below 100 nm, were interconnected, creating abundant open spaces and channels. This hierarchical structure, integrating 1D conductive micro-fibers and 2D active nanoflakes, offers multiple advantages for a sodium-ion battery anode: i) The carbon fiber core provides a highway for rapid electron transport. ii) The ultrathin nanoflake morphology drastically shortens the diffusion path for Na+ ions. iii) The ample void space between nanoflakes facilitates electrolyte infiltration and accommodates volume changes during cycling.
EDS elemental mapping of the NCO@CC composite further confirmed the uniform distribution of Ni, Co, and O elements across the carbon fiber surface, while the C signal was predominant in the fiber core, visually outlining the successful coating.
Electrochemical Performance in Sodium-Ion Battery
Cyclic Voltammetry and Reaction Mechanism
The initial three cyclic voltammetry (CV) cycles for both electrodes at 0.1 mV/s are shown in Figure 1. During the first cathodic scan, both electrodes exhibited irreversible reduction peaks associated with the reduction of NiCo2O4 to metallic Ni and Co, the formation of Na2O, and the concomitant formation of a solid electrolyte interphase (SEI) layer. For the pure NCO electrode, peaks were observed at ~1.11 V (reduction of Co3+ to Co2+) and ~0.50 V (further reduction to metallic Ni and Co). Notably, these peaks shifted to ~0.98 V and ~0.42 V for the NCO@CC composite, indicating altered reaction kinetics due to the conductive substrate. The anodic scans showed multiple oxidation peaks corresponding to the stepwise re-oxidation of metals to their respective oxides. In subsequent cycles, the CV curves stabilized, and the NCO@CC composite displayed a more rectangular shape at higher potentials (2.0-3.0 V), suggesting a significant contribution from capacitive charge storage, which is beneficial for rate performance.
The general conversion reaction for a spinel oxide M3O4 (where M = Ni, Co) in a sodium-ion battery can be described by:
$$ \text{M}_3\text{O}_4 + 8\text{Na}^+ + 8e^- \rightleftharpoons 3\text{M} + 4\text{Na}_2\text{O} $$
The actual process is more complex, involving intermediate steps and possibly the formation of ternary sodium metal oxides.
Rate Capability and Galvanostatic Profiles
The rate performance, a critical metric for high-power applications, was systematically evaluated. The NCO@CC composite electrode demonstrated a clear superiority over the pure NCO electrode across all tested current densities, as summarized in Table 2.
| Current Density (mA g-1) | NCO Discharge Capacity (mAh g-1) | NCO@CC Discharge Capacity (mAh g-1) | Performance Advantage of NCO@CC |
|---|---|---|---|
| 100 | ~240 (at 4th cycle) | ~270 (at 4th cycle) | ~12.5% higher |
| 200 | ~140 | ~220 | ~57% higher |
| 500 | ~65 | ~170 | ~162% higher |
| 1000 | ~19 | ~135 | >600% higher |
| Recovery at 100 | ~90 | ~250 | ~178% higher |
While the pure NCO electrode suffered from drastic capacity decay as the current increased, the NCO@CC composite maintained appreciable capacity even at 1 A g-1. When the current density was switched back to 100 mA g-1, the NCO@CC electrode recovered most of its capacity, demonstrating excellent structural and electrochemical reversibility. This remarkable rate capability is directly attributed to the integrated architecture of the composite. The carbon cloth backbone ensures rapid electron collection from the entire active material layer, while the nanoflake morphology minimizes the solid-state ion diffusion length. The contribution of surface-controlled capacitive processes can be quantified using the power-law relationship between peak current (i) and scan rate (v) in CV:
$$ i = a v^b $$
where b = 0.5 indicates a diffusion-controlled process and b = 1.0 indicates a capacitive process. The NCO@CC electrode typically exhibits a higher b-value, confirming the larger capacitive contribution.
Long-Term Cycling Stability
The long-term cycling stability of the two electrodes was assessed at a constant current density of 100 mA g-1. The results are graphically summarized in Figure 2. The pure NCO electrode, despite showing a high initial discharge capacity of ~760 mAh g-1, suffered from rapid and continuous capacity decay, retaining only about 9% of its reversible capacity after 50 cycles (from ~400 mAh g-1 at the 2nd cycle to ~37 mAh g-1). In contrast, the NCO@CC composite electrode demonstrated vastly improved stability. It delivered a stable reversible capacity of approximately 300 mAh g-1 after the initial activation cycles and maintained a capacity of 141 mAh g-1 after 50 cycles, corresponding to a capacity retention of 44% relative to its stabilized capacity. This represents more than a 30% improvement in absolute capacity retention compared to the pure NCO sample at the 50th cycle.
The galvanostatic charge-discharge profiles provide further insight. The pure NCO electrode’s voltage plateaus, indicative of the conversion reaction, became less distinct after only 10 cycles, implying a loss of active material and increased polarization. The NCO@CC electrode, however, maintained well-defined plateaus for a much longer duration, confirming the robustness of its hierarchical structure. The enhanced cycling stability can be ascribed to: i) The mechanical flexibility of the carbon cloth substrate, which buffers the volume strain of NiCo2O4. ii) The strong adhesion between the in-situ grown nanoflakes and the carbon fiber, preventing active material detachment. iii) The stable SEI layer formed on the composite surface.
Discussion and Performance Enhancement Mechanism
The superior electrochemical performance of the NCO@CC composite as a sodium-ion battery anode stems from the synergistic effects of its multi-scale hierarchical design. The key factors and their quantitative or qualitative impacts are analyzed below.
1. Enhanced Electronic Conductivity: The core challenge of NiCo2O4 is its low electronic conductivity ($\sigma_{e}$). By directly growing it on highly conductive carbon cloth ($\sigma_{CC} >> \sigma_{NCO}$), the composite’s effective electronic conductivity ($\sigma_{eff}$) is dramatically increased. This reduces the internal resistance (Rint) of the electrode, lowering polarization ($\eta = I \cdot R_{int}$) and improving rate performance. The electronic conductivity enhancement can be modeled as a percolation network where the carbon fibers provide continuous pathways.
2. Accelerated Ionic Transport: The Na+ ion diffusion coefficient (DNa+) is a limiting factor in battery kinetics. According to the simplified relationship for diffusion time ($\tau$) in a particle of characteristic length (L):
$$ \tau \approx \frac{L^2}{D_{Na+}} $$
By reducing the active material’s dimension from micron-sized agglomerates (NCO) to sub-100 nm thick nanoflakes (NCO@CC), the diffusion time is shortened by several orders of magnitude, enabling faster charge/discharge. Furthermore, the porous structure ensures full electrolyte access, maintaining a high effective DNa+.
3. Mechanical Stability and Capacitive Contribution: The integrated structure mitigates pulverization. The volume change strain ($\epsilon$) is accommodated by the void space and the flexible CC substrate, reducing stress. The high surface area of the nanoflakes also promotes capacitive Na+ storage via surface adsorption and near-surface reactions, which is non-diffusion-limited and highly reversible. The total stored charge (Qtotal) can be expressed as the sum of diffusion-controlled (Qdiff) and capacitive (Qcap) contributions:
$$ Q_{total} = Q_{diff} + Q_{cap} = k_1 v^{-1/2} + k_2 v^{-1} $$
where v is the scan rate, and k1, k2 are constants. For NCO@CC, Qcap constitutes a larger fraction, especially at higher rates.
Conclusion and Future Perspectives
In this work, a hierarchical NiCo2O4@carbon cloth composite was successfully fabricated through a simple, scalable, and binder-free in-situ hydrothermal-annealing route for use as an anode in sodium-ion batteries. The composite features vertically aligned NiCo2O4 nanoflakes firmly anchored on a 3D conductive carbon cloth scaffold. This unique architecture addresses the intrinsic limitations of NiCo2O4 by providing efficient electron/ion transport pathways and buffering volume changes.
Electrochemical tests confirmed the composite’s superiority over the pure oxide. The NCO@CC electrode delivered significantly enhanced rate capability, maintaining a capacity of 135 mAh g-1 at a high current of 1 A g-1, where the pure NCO electrode became virtually inactive. More importantly, it exhibited markedly improved cycling stability, retaining 141 mAh g-1 after 50 cycles at 100 mA g-1, which is over 30% higher in capacity retention compared to the control sample. This performance underscores the effectiveness of constructing integrated, binder-free electrodes via in-situ growth on carbon substrates.
Future work will focus on further optimizing this system for the sodium-ion battery platform. This includes: i) Precise control over the nanoflake morphology (e.g., thickness, porosity) and crystallinity to maximize the active surface area and intrinsic activity. ii) Exploring doping strategies (e.g., with Mn, Fe) to improve the intrinsic electronic conductivity and specific capacity of the oxide layer. iii) Investigating pre-sodiation or electrolyte optimization to improve the initial Coulombic efficiency. iv) Pairing the optimized NCO@CC anode with a high-voltage cathode to assemble and test full sodium-ion cells, evaluating practical energy and power density. The strategy demonstrated here is versatile and can be extended to other conversion-type electrode materials, paving the way for developing high-performance, low-cost energy storage devices.