The pursuit of sustainable and large-scale energy storage solutions has intensified the search for viable alternatives to lithium-ion batteries (LIBs). Among the contenders, sodium-ion batteries (SIBs) have emerged as a particularly promising candidate due to the natural abundance, low cost, and wide geographical distribution of sodium resources. However, the practical deployment of sodium-ion battery technology is hindered by the lack of suitable anode materials that can offer high capacity, excellent rate capability, and long-term cycling stability, as the graphite anode widely used in LIBs shows inadequate performance for sodium storage.
Transition metal sulfides (TMSs) have attracted considerable attention as potential anode materials for sodium-ion battery applications owing to their high theoretical specific capacities based on conversion reactions. Within this family, bimetallic sulfides like NiCo2S4 present significant advantages over their monometallic counterparts, including richer redox chemistry, higher electronic conductivity, and superior electrochemical activity. The sodium storage mechanism in NiCo2S4 typically involves a conversion reaction:
$$ \text{NiCo}_2\text{S}_4 + 8\text{Na}^+ + 8e^- \leftrightarrow \text{Ni} + 2\text{Co} + 4\text{Na}_2\text{S} $$
This multi-electron process underpins its high theoretical capacity. Despite this promise, the practical application of NiCo2S4 in sodium-ion batteries faces critical challenges. The substantial volume expansion and contraction during the repetitive sodiation/desodiation processes often lead to particle pulverization, loss of electrical contact, and eventual electrode degradation. Furthermore, the intrinsically poor electrical conductivity of sulfides results in sluggish reaction kinetics, severely limiting rate performance and energy efficiency.
To overcome these intrinsic limitations for sodium-ion battery anodes, a sophisticated material design strategy is imperative. This involves constructing hierarchical architectures that can accommodate mechanical strain and compositing with conductive matrices to enhance charge transfer. In this work, we report the rational design and fabrication of a unique ternary composite—NiCo2S4/MXene/Nitrogen-doped Carbon (NiCo2S4/MX/NC) hollow microspheres—as a high-performance anode for sodium-ion batteries. Our approach synergistically integrates several key concepts: (1) Employing MXene, a highly conductive two-dimensional transition metal carbide, to construct an internal conductive network; (2) Utilizing a zeolitic imidazolate framework (ZIF-67) as a self-sacrificial template to derive a nitrogen-doped carbon (NC) scaffold that provides structural integrity and additional active sites; (3) Engineering a hollow spherical morphology to alleviate volume stress, increase the electrode-electrolyte contact area, and shorten ion diffusion paths. The concerted action of these components is engineered to unlock the full potential of NiCo2S4 for efficient and durable sodium-ion storage.
Material Design and Synthesis Rationale
The synthesis of the NiCo2S4/MX/NC hollow microspheres is a multi-step, template-assisted process designed for precise structural control. The selection of each component and the fabrication sequence are critical for achieving the desired architecture and properties essential for a superior sodium-ion battery anode.
1. The Conductive Skeleton: MXene. MXenes, with a general formula of Mn+1XnTx, are chosen for their metallic conductivity, hydrophilic surfaces, and mechanical flexibility. In this composite, MXene nanosheets serve as the primary conductive highway, ensuring rapid electron transport throughout the electrode, which is vital for achieving high rate capability in sodium-ion batteries. Their two-dimensional nature allows them to conformally coat templates and interconnect active material particles.
2. The Structural Template and Carbon Source: Polystyrene (PS) Spheres and ZIF-67. We employ a dual-template strategy. Monodisperse PS microspheres act as the first sacrificial template to define the spherical macro-structure. ZIF-67, a cobalt-based MOF, serves a dual purpose: (i) as the second template guiding the formation of a bimetallic (Ni, Co) hydroxide intermediate, and (ii) as the precursor for in-situ nitrogen-doped carbon. The pyrolysis of ZIF-67 generates a porous, conductive NC matrix that encapsulates the active material, preventing aggregation and buffering volume changes during cycling in the sodium-ion battery.
3. The Active Material: NiCo2S4. The bimetallic sulfide is selected for its high capacity. Its formation is achieved through a facile ion-exchange and subsequent sulfidation process, ensuring strong interfacial contact with both the MXene and the NC matrix.
The stepwise synthesis protocol can be summarized as follows:
| Step | Process | Key Reaction/Outcome |
|---|---|---|
| 1 | MXene Synthesis | Selective etching of Al from Ti3AlC2 (MAX) using LiF/HCl. |
| 2 | MXene/PS Assembly | MXene nanosheets are wrapped onto PS microspheres via electrostatic interactions. |
| 3 | ZIF-67 Growth | Controlled growth of ZIF-67 polyhedra on the MXene/PS surface. |
| 4 | Formation of NiCo-LDH | Ion exchange between Co2+ in ZIF-67 and external Ni2+, leading to a hollow NiCo-layered double hydroxide (LDH) structure via the Kirkendall effect. |
| 5 | Sulfidation & Carbonization | Simultaneous sulfidation of the LDH and carbonization of ZIF-67/PS under N2 atmosphere, yielding the final NiCo2S4/MX/NC hollow microspheres. |
Structural and Compositional Characterization
The successful construction of the hierarchical architecture was confirmed through comprehensive microscopic and spectroscopic analyses. The morphology evolution from solid PS spheres to hollow microspheres is clearly observed. The final NiCo2S4/MX/NC product exhibits well-defined spherical shells with a diameter of approximately 2.5 μm. The shell is composed of interconnected nanoparticles, creating a highly porous surface. Crucially, cross-sectional analysis reveals the hollow interior, a direct result of the dual-template removal. This hollow structure is paramount for the sodium-ion battery anode as it provides void space to accommodate the volume expansion of NiCo2S4 during sodiation.
Elemental mapping via Energy-Dispersive X-ray Spectroscopy (EDS) demonstrates the homogeneous distribution of Ni, Co, and S (from NiCo2S4), Ti (from MXene), C, and N (from the NC matrix) throughout the microsphere architecture. This confirms the intimate integration of all components at the nanoscale. X-ray Diffraction (XRD) patterns of the composite show characteristic diffraction peaks corresponding to the cubic phase of NiCo2S4 (JCPDS No. 20-0782). The peaks are broadened, indicating the nanocrystalline nature of the sulfide particles. The characteristic (002) peak of MXene is not prominent, likely due to its low content, good dispersion, and possible restacking or shielding within the composite.
Further insights into the chemical states and surface composition were gained from X-ray Photoelectron Spectroscopy (XPS). The survey spectrum confirms the presence of all expected elements. High-resolution spectra of Ni 2p and Co 2p show doublets accompanied by satellite peaks, indicative of Ni2+/Ni3+ and Co2+/Co3+ oxidation states, which are typical for NiCo2S4 and beneficial for rich redox activity in sodium-ion battery electrodes. The S 2p spectrum can be deconvoluted into metal-sulfur bonds and surface sulfate/sulfite species. The C 1s spectrum reveals contributions from C-C/C=C, C-N, and C-O bonds, confirming the graphitic nature and nitrogen doping of the carbon derived from ZIF-67. The N 1s spectrum confirms the presence of pyridinic, pyrrolic, and graphitic N, which can enhance the electronic conductivity and provide additional active sites for sodium ion adsorption.
| Feature | Description | Benefit for Sodium-Ion Battery Anode |
|---|---|---|
| Morphology | Hollow Microspheres (~2.5 μm diameter) | Accommodates volume change, provides large surface area. |
| Shell Composition | Nanoparticulate NiCo2S4 embedded in NC matrix | Shortens ion diffusion path, prevents active material aggregation. |
| Internal Conductor | Conformal MXene layer on the inner shell surface | Establishes a highly conductive network for fast electron transfer. |
| Porosity | Mesoporous structure from ZIF-67/PS pyrolysis | Facilitates electrolyte infiltration and rapid ion transport. |
| Chemical State | Mixed-valence Ni/Co, N-doped carbon | Enhances redox activity and overall electrical conductivity. |
Electrochemical Performance as a Sodium-Ion Battery Anode
The sodium storage performance of the NiCo2S4/MX/NC hollow microspheres was systematically evaluated in half-cell configurations versus Na/Na+. The results unequivocally demonstrate the superiority of this composite architecture.
Cyclic Voltammetry (CV) and Reaction Mechanism. Initial CV cycles reveal the complex redox behavior associated with the conversion and alloying/dealloying processes typical of advanced sulfide anodes for sodium-ion batteries. In the first cathodic scan, a broad peak around 0.7 V corresponds to the irreversible formation of a solid electrolyte interphase (SEI) layer and the initial reduction of NiCo2S4 to metallic Ni, Co, and Na2S. In subsequent cycles, cathodic peaks at ~0.45 V, ~0.94 V, and ~1.44 V emerge, indicating multi-step sodium insertion and conversion reactions. The corresponding anodic peaks at ~1.68 V, ~1.91 V, and ~2.01 V are ascribed to the reversible oxidation of metals back to sulfides. The excellent overlap of CV curves from the second cycle onward signifies high reversibility and structural stability of the NiCo2S4/MX/NC electrode, a critical attribute for a long-life sodium-ion battery.
Galvanostatic Charge-Discharge and Cycling Stability. The composite anode delivers exceptional capacity. At a current density of 0.2 A g-1, it exhibits a high initial reversible (charge) capacity of 780 mAh g-1, with a remarkable initial Coulombic efficiency (ICE) of 86.4%. The high ICE is advantageous for practical full-cell assembly in sodium-ion batteries, minimizing active sodium loss. Even more impressive is the long-term cycling stability. After 200 cycles at 0.2 A g-1, the capacity retention is outstanding. When subjected to an ultra-high current density of 10 A g-1, the electrode maintains a stable capacity over 4000 cycles, with an extremely low average capacity decay rate of approximately 0.0034% per cycle. This exceptional cyclability underscores the effectiveness of the hollow structure and conductive matrices in mitigating mechanical degradation and maintaining electrical integrity.
Rate Capability. The rate performance of the NiCo2S4/MX/NC anode was tested at progressively increasing current densities from 0.05 to 10 A g-1. The electrode delivers high capacities across the entire range. A comparative analysis with control samples (NiCo2S4/NC, NiCo2S4/MX, and pure MXene) clearly shows that the ternary composite outperforms them all, especially at high rates. This superior rate capability is a direct benefit of the synergistic design: the MXene network ensures fast electron conduction, the porous hollow structure facilitates rapid ion diffusion, and the nanostructured active material reduces the solid-state diffusion length for sodium ions.
| Material | Capacity @ 0.2 A g-1 (mAh g-1) | Capacity @ 5.0 A g-1 (mAh g-1) | Cycling Stability |
|---|---|---|---|
| NiCo2S4/MX/NC | ~780 | ~580 | ~452 mAh g-1 after 4000 cycles @ 10 A g-1 |
| NiCo2S4/NC | ~628 | ~98 (Rapid decay) | Poor high-rate stability |
| NiCo2S4/MX | ~576 | ~508 | Good rate performance, lower overall capacity |
| MXene | ~152 | ~71 | Stable but very low capacity |
Kinetic Analysis and Charge Storage Mechanism
To gain deeper insight into the reasons behind the excellent performance of this sodium-ion battery anode, detailed kinetic analyses were performed.
Electrochemical Impedance Spectroscopy (EIS). The Nyquist plot of the NiCo2S4/MX/NC electrode exhibits a small semicircle in the high-medium frequency region and a sloping line in the low-frequency region. Fitting with an equivalent circuit model yields a very low charge-transfer resistance (Rct) of 2.06 Ω. This minimal resistance at the electrode/electrolyte interface is attributed to the highly conductive MXene/NC network and the favorable electrode architecture, enabling fast reaction kinetics for sodium-ion storage.
Diffusion Coefficient. The sodium-ion diffusion coefficient (DNa+) was calculated from the low-frequency Warburg region of the EIS data using the following relationship:
$$ Z’ = R_s + R_{ct} + \sigma_\omega \omega^{-1/2} $$
where $\sigma_\omega$ is the Warburg coefficient. The diffusion coefficient is then given by:
$$ D_{Na^+} = \frac{R^2T^2}{2A^2n^4F^4C^2\sigma_\omega^2} $$
Here, R is the gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons transferred per molecule, F is Faraday’s constant, and C is the molar concentration of Na+. The calculated DNa+ for the NiCo2S4/MX/NC electrode is on the order of 10-10 cm2 s-1, which is comparatively high for conversion-type anode materials, confirming facilitated ion transport within the porous hollow structure.
Capacitive Contribution Analysis. The charge storage behavior was further deconvoluted into diffusion-controlled (battery-type) and surface-controlled (capacitive) contributions using CV data at various scan rates. The current (i) obeys a power-law relationship with the scan rate (v):
$$ i = a v^b $$
A b-value of 0.5 indicates a semi-infinite diffusion-controlled process, while a b-value of 1.0 indicates a surface-controlled capacitive process. The calculated b-values for the primary redox peaks of the NiCo2S4/MX/NC electrode are all close to 1, suggesting that capacitive effects dominate the charge storage. This is a highly desirable characteristic for a sodium-ion battery anode, especially for high-rate performance. The quantitative capacitive contribution can be determined by:
$$ i(V) = k_1 v + k_2 v^{1/2} $$
where $k_1 v$ represents the capacitive current and $k_2 v^{1/2}$ represents the diffusion-controlled current. At a scan rate of 1.0 mV s-1, the capacitive contribution accounts for approximately 83% of the total charge storage. This dominant pseudocapacitive behavior originates from the abundant surface atoms of the nanocrystalline NiCo2S4, the faradaic reactions on the nitrogen-doped carbon surfaces, and the large accessible surface area of the hollow microspheres. It explains the electrode’s ability to store a large amount of charge rapidly with minimal structural penalty, a key factor for its excellent rate performance in sodium-ion batteries.
| Parameter | Value / Observation | Implication |
|---|---|---|
| Charge Transfer Resistance (Rct) | 2.06 Ω | Fast reaction kinetics at the interface. |
| Na+ Diffusion Coefficient (DNa+) | ~4.91 × 10-10 cm2 s-1 | Facilitated solid-state ion diffusion. |
| b-value (avg. for peaks) | ~0.88 – 0.99 | Charge storage is predominantly surface-controlled (pseudocapacitive). |
| Capacitive Contribution @ 1.0 mV s-1 | ~83% | Enables high power density and excellent rate capability. |
Conclusion and Perspective
In summary, we have successfully engineered and synthesized a novel ternary composite of NiCo2S4/MXene/N-doped carbon hollow microspheres through a rational dual-template strategy. When evaluated as an anode material for sodium-ion batteries, this composite demonstrates a compelling combination of high specific capacity, exceptional rate capability, and ultra-long cycling stability. The superior electrochemical performance is a direct consequence of the synergistic interplay between its architectural and compositional elements:
- The Hollow Spherical Architecture: Acts as a robust scaffold to effectively buffer the substantial volume changes of NiCo2S4 during repeated sodiation/desodiation, which is the primary enabler of the outstanding cycling stability, even at extremely high current densities in the sodium-ion battery.
- The Conductive MXene Network: Provides a “highway” for rapid electron transport throughout the electrode, drastically reducing internal resistance and polarization, thereby unlocking the high-rate potential of the active material.
- The N-doped Carbon Matrix: Serves as a conductive and flexible buffer that prevents the aggregation of NiCo2S4 nanoparticles, contributes additional capacitive storage sites, and enhances the overall structural integrity of the composite.
- Dominant Pseudocapacitive Behavior: The nanoscale design and porous structure shift the charge storage mechanism towards surface-controlled processes, allowing for fast kinetics and high power delivery, which is crucial for the practical application of sodium-ion batteries in scenarios requiring rapid charging.
This work provides a general and effective design blueprint for overcoming the intrinsic limitations of high-capacity conversion-type electrode materials, not only for sodium-ion batteries but potentially for other energy storage systems such as potassium-ion batteries. Future research may focus on further optimizing the ratio of components, exploring other MXene or MOF derivatives, and scaling up the synthesis process. The integration of such advanced anode materials with compatible high-voltage cathodes will be the critical next step toward realizing high-energy-density, durable, and cost-effective sodium-ion battery technology for large-scale grid storage and beyond.