The global pursuit of carbon neutrality has placed unprecedented demands on advanced, cost-effective, and safe energy storage technologies. While lithium-ion batteries (LIBs) have dominated the portable electronics and electric vehicle markets, concerns regarding the limited geographical distribution and rising cost of lithium resources necessitate the development of complementary technologies. Sodium-ion batteries (SIBs) emerge as a compelling alternative due to the natural abundance of sodium, lower cost, and similar intercalation chemistry to LIBs. However, the larger ionic radius of Na+ (1.02 Å) compared to Li+ (0.76 Å) poses significant challenges for electrode materials, often leading to sluggish kinetics, severe structural strain, and poor cycling stability during the insertion and extraction processes. Therefore, the exploration and design of novel anode materials with high capacity, superior rate capability, and long-term cyclability are critical for the advancement of sodium-ion battery technology.
Among various anode candidates for sodium-ion batteries, alloy-type materials based on group IV and V elements (e.g., Si, Ge, Sn, P) offer high theoretical capacities due to multi-electron transfer reactions forming Na-rich alloys. Germanium (Ge) is particularly attractive because of its high theoretical capacity (369 mAh g-1 for NaGe), good electronic conductivity, and faster Na+ diffusion compared to silicon. Ternary germanates, such as Zn2GeO4, combine the benefits of Ge with other electrochemically active or inactive elements, often presenting improved structural stability. The sodium storage mechanism in Zn2GeO4 typically involves a conversion-alloying reaction:
$$ \text{Zn}_2\text{GeO}_4 + 8\text{Na}^+ + 8e^- \rightarrow 2\text{Zn} + \text{Ge} + 4\text{Na}_2\text{O} $$
followed by the alloying reaction:
$$ \text{Ge} + x\text{Na}^+ + xe^- \leftrightarrow \text{Na}_x\text{Ge} \quad (x \leq 3.75) $$
Despite this high-capacity promise, bare Zn2GeO4 suffers from poor electronic conductivity, large volume expansion during sodiation/desodiation, and severe particle agglomeration, leading to rapid capacity fading and limiting its practical application in sodium-ion batteries.
To overcome these intrinsic limitations, constructing nanocomposites with conductive matrices is a widely adopted and effective strategy. Two-dimensional (2D) materials, with their large specific surface area, excellent mechanical flexibility, and high electrical conductivity, are ideal substrates for anchoring active nanoparticles. MXenes, a rapidly growing family of 2D transition metal carbides, nitrides, and carbonitrides, have shown exceptional promise in energy storage. Their general formula is Mn+1XnTx, where M is an early transition metal (e.g., Ti, V, Nb), X is carbon and/or nitrogen, and Tx represents surface functional groups (-O, -OH, -F). Ti3C2Tx, the most studied MXene, exhibits metallic conductivity, hydrophilic surfaces, and tunable interlayer spacing, making it an excellent conductive scaffold and buffer material for accommodating volume changes in sodium-ion battery anodes. The integration of Zn2GeO4 with MXene is expected to yield synergistic effects: the MXene sheets prevent the aggregation of Zn2GeO4 nanoparticles, facilitate electron transport, buffer mechanical stress, and potentially contribute additional capacity through Na+ intercalation between its layers. This paper details the synthesis, characterization, and comprehensive electrochemical evaluation of a Zn2GeO4/MXene nanocomposite as a high-performance anode for sodium-ion batteries.
Experimental Methodology: Synthesis and Characterization
The fabrication of the Zn2GeO4/MXene composite involves a two-step process: the synthesis of fluorine-free MXene followed by an in-situ hydrothermal growth of Zn2GeO4.
1. Synthesis of Ti3C2Tx MXene
A fluorine-free alkaline etching method was employed to prepare Ti3C2Tx MXene. Briefly, 0.1 g of Ti3AlC2 MAX phase was added to 25 mL of a concentrated NaOH solution (27.5 mol L-1). The mixture was stirred and then transferred to a Teflon-lined stainless-steel autoclave, which was purged with argon and sealed. The hydrothermal reaction was conducted at 270 °C for 12 hours. After cooling, the product was washed repeatedly with deionized water via centrifugation until the supernatant reached near-neutral pH. The final sediment was collected and dried under vacuum at 65 °C for 12 hours to obtain multilayer Ti3C2Tx powder.
2. Synthesis of Zn2GeO4/MXene Nanocomposite
First, sodium germanate (Na2GeO3) precursor was prepared by solid-state reaction. Equimolar amounts of Na2CO3 and GeO2 were ground and calcined at 900 °C for 12 hours. For the composite synthesis, a stoichiometric amount of the as-prepared Ti3C2Tx MXene (corresponding to a 1:1 molar ratio of Zn2GeO4 to MXene) was dispersed in 10 mL deionized water via ultrasonication for 2 hours. Subsequently, zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was added to the MXene suspension and further sonicated. Separately, the Na2GeO3 precursor was dissolved in 10 mL deionized water. The sodium germanate solution was then added dropwise into the MXene/zinc acetate mixture under vigorous stirring. The combined solution was maintained at 100 °C under continuous stirring for 3 hours to facilitate the hydrothermal reaction. The final product was collected by filtration, washed thoroughly, and vacuum-dried at 80 °C. For comparison, pure Zn2GeO4 was synthesized via an identical procedure but without the addition of MXene.
| Step | Reactants | Conditions | Product |
|---|---|---|---|
| MXene Synthesis | Ti3AlC2, NaOH (27.5M) | 270°C, 12 h, Hydrothermal | Ti3C2Tx Multilayers |
| Precursor Prep | Na2CO3, GeO2 | 900°C, 12 h, Solid-state | Na2GeO3 |
| Composite Formation | Ti3C2Tx, Zn(Ac)2·2H2O, Na2GeO3 | 100°C, 3 h, Hydrothermal | Zn2GeO4/MXene |
3. Material Characterization
The crystal structures of all samples were analyzed using X-ray diffraction (XRD, Bruker D8 Focus) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5° to 90°. The morphological features and microstructure were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi SU8010).
4. Electrochemical Measurements
Electrochemical tests were performed using CR2032 coin-type cells assembled in an argon-filled glovebox. The working electrode was prepared by mixing the active material (Zn2GeO4/MXene or pure Zn2GeO4), conductive carbon black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 7:2:1 with N-methyl-2-pyrrolidone (NMP) as the solvent. The slurry was coated onto a copper foil current collector and dried at 120 °C under vacuum for 12 hours. Sodium metal foil was used as the counter/reference electrode, and a glass fiber membrane served as the separator. The electrolyte was 1.0 M NaClO4 (or NaPF6) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume). Galvanostatic charge-discharge cycling tests were conducted on a LAND battery test system within a voltage window of 0.01–3.00 V versus Na+/Na. Cyclic voltammetry (CV) measurements were performed on a CHI660E electrochemical workstation at a scan rate of 0.2 mV s-1. Electrochemical impedance spectroscopy (EIS) was carried out on the same workstation over a frequency range from 100 kHz to 0.01 Hz with an amplitude of 10 mV.
Results and Discussion: Structural, Morphological, and Electrochemical Analysis
1. Phase and Structural Analysis
The XRD patterns in Figure 1(a) confirm the successful synthesis of materials. The pattern for the pristine Ti3AlC2 MAX phase matches the standard PDF card (#52-0875). After alkaline etching, the characteristic (104) peak of Ti3AlC2 at around 39° significantly diminishes, while a new, broad (002) diffraction peak appears at a lower angle (≈7.3°). This shift to a lower angle indicates an increase in the c-lattice parameter, which is direct evidence for the successful removal of the Al layers and the formation of layered Ti3C2Tx MXene. The presence of residual, weak Ti3AlC2 peaks suggests the etching was not 100% complete. The XRD pattern of the pure Zn2GeO4 sample, shown in Figure 1(b), can be perfectly indexed to the rhombohedral Zn2GeO4 phase (PDF#11-0687). For the Zn2GeO4/MXene composite, all the distinctive diffraction peaks of Zn2GeO4 are present without peak shifting, confirming the preservation of its crystal structure after compositing. Additionally, the characteristic (002) peak of MXene is observable at ≈9.5° in the composite pattern, providing clear evidence for the successful integration of both components.
2. Morphological Evolution
SEM imaging reveals the dramatic morphological benefits of compositing. The pristine MXene exhibits a typical accordion-like, multi-layered structure resulting from the etching process, though the layers appear somewhat stacked and agglomerated due to hydrogen bonding and van der Waals forces. The pure Zn2GeO4 synthesized via the same hydrothermal route predominantly consists of nanorods with lengths of about 200 nm and widths of about 120 nm. However, these nanorods suffer from severe aggregation into large clusters due to their high surface energy, which would detrimentally reduce the electrochemically active surface area and hinder electrolyte penetration in a sodium-ion battery.
In striking contrast, the Zn2GeO4/MXene composite displays a radically different and advantageous architecture. The Zn2GeO4 nanorods are uniformly distributed and anchored onto the surface and between the layers of the MXene sheets. This configuration effectively prevents the agglomeration of Zn2GeO4, leading to a much higher exposed surface area. The MXene acts as a conductive, flexible, and mechanically robust scaffold. This intimate contact ensures efficient electron transfer from the active material to the current collector and provides a buffer space to accommodate the volume changes of Zn2GeO4 during repeated sodiation/desodiation cycles in the sodium-ion battery. Furthermore, the layered MXene itself offers additional active sites for Na+ storage via intercalation or surface redox reactions, contributing to a synergistic enhancement in capacity.
3. Electrochemical Performance in Sodium-Ion Batteries
The electrochemical performance was systematically evaluated to demonstrate the superiority of the Zn2GeO4/MXene composite as a sodium-ion battery anode.
Cyclic Voltammetry (CV)
The initial five CV cycles of the Zn2GeO4/MXene electrode at 0.2 mV s-1 are shown in Figure 3(a). In the first cathodic scan, a broad reduction peak around 0.54 V corresponds to the irreversible reduction of Zn2GeO4 to metallic Zn and Ge, the formation of Na2O, and the inevitable formation of a solid-electrolyte interphase (SEI) layer. The sharp peak near 0.01 V is attributed to Na+ intercalation into the MXene layers. In the first anodic scan, two broad oxidation peaks at approximately 0.20 V and 0.85 V are observed, corresponding to the de-alloying of NaxGe and the re-oxidation of metallic Zn. In subsequent cycles, the cathodic peak shifts to around 0.30 V and remains stable, indicating an activation process and the establishment of a reversible electrochemical reaction. The overlapping CV curves from the 2nd to the 5th cycle suggest good reversibility and structural stability of the composite electrode in the sodium-ion battery.
Galvanostatic Charge-Discharge and Cycling Stability
The long-term cycling performance at a current density of 100 mA g-1 is the most critical metric. The results are summarized and compared in the table below.
| Material | 1st Discharge Capacity (mAh g-1) | 1st Charge Capacity (mAh g-1) | Initial Coulombic Efficiency (%) | Capacity after 100 cycles (mAh g-1) | Coulombic Efficiency at 100th cycle (%) |
|---|---|---|---|---|---|
| Pure Zn2GeO4 | ~450 | ~180 | ~40 | 35.5 | ~98 |
| Zn2GeO4/MXene | 768.4 | 339.1 | 44.1 | 75.0 | >99 |
The Zn2GeO4/MXene composite delivers a remarkably high initial discharge capacity of 768.4 mAh g-1. The significant capacity loss between the first discharge and charge (resulting in an initial Coulombic efficiency of ~44%) is common for conversion/alloying anodes and is primarily due to SEI formation and irreversible side reactions. However, from the second cycle onward, the Coulombic efficiency rapidly increases and stabilizes above 99%, indicating highly reversible sodium storage. After 100 cycles, the composite retains a stable reversible capacity of 75 mAh g-1. In stark contrast, the pure Zn2GeO4 anode suffers from rapid degradation, retaining only 35.5 mAh g-1 after 100 cycles. This means the Zn2GeO4/MXene composite demonstrates more than a 110% improvement in capacity retention compared to the bare material. The enhanced cycling stability is a direct consequence of the MXene matrix, which mitigates pulverization, maintains electrical connectivity, and suppresses the continuous decomposition of the active material.
Kinetics and Impedance Analysis
Electrochemical impedance spectroscopy (EIS) provides insights into the interfacial properties and charge transfer kinetics. The Nyquist plots for both electrodes after several cycles consist of a semicircle in the high-medium frequency region (associated with the charge-transfer resistance, Rct, at the electrode/electrolyte interface) and a sloping line in the low-frequency region (related to Na+ diffusion within the electrode, Warburg impedance).
The fitted equivalent circuit yields quantitative values. The Rct for the Zn2GeO4/MXene composite is 26.12 Ω, which is less than half of the value for pure Zn2GeO4 (59 Ω). This lower charge-transfer resistance is pivotal for the performance of the sodium-ion battery, as it indicates faster reaction kinetics at the interface. The improvement stems from the highly conductive MXene network that facilitates electron transport to and from every Zn2GeO4 nanorod, and the stable composite structure that minimizes the breakdown and reconstruction of the SEI layer.
The Na+ diffusion coefficient (DNa+) can be estimated from the low-frequency Warburg region using the following equation:
$$ D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
Where R is the gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons transferred, F is Faraday’s constant, C is the molar concentration of Na+ in the electrode, and σ is the Warburg coefficient obtained from the slope of Z’ vs. ω-1/2. The composite electrode typically exhibits a smaller σ value, implying a higher DNa+, due to the shortened diffusion paths and improved interfacial contact enabled by the MXene scaffold.
Synergistic Storage Mechanism and Performance Enhancement
The outstanding electrochemical performance of the Zn2GeO4/MXene composite in a sodium-ion battery can be attributed to a multifaceted synergistic mechanism:
1. Conductivity Enhancement: The metallic conductivity of MXene creates a “highway” for electrons throughout the electrode, overcoming the poor intrinsic conductivity of Zn2GeO4. This ensures that all active material participates in the redox reactions, improving capacity utilization and rate capability.
2. Morphological Control and Buffering: MXene sheets physically separate Zn2GeO4 nanorods, preventing agglomeration and maximizing the electrode-electrolyte contact area. More importantly, the flexible and strong MXene layers act as a buffer, absorbing the mechanical stress generated by the large volume expansion (≈200-300%) during the Ge alloying process (NaxGe formation). This preserves the structural integrity of the electrode over many cycles.
3. Additional Capacity Contribution: MXene itself is an active material for sodium-ion storage. Na+ can intercalate between its layers or interact with its surface functional groups via pseudocapacitive reactions. The total capacity is thus a sum of contributions from both components:
$$ Q_{total} = Q_{Zn_2GeO_4 (Conversion+Alloying)} + Q_{MXene (Intercalation/Pseudocapacitive)} $$
4. Stable Interface Formation: The robust composite structure reduces the continuous fracture and exposure of fresh surfaces of Zn2GeO4, leading to the formation of a more stable and thinner SEI layer. This minimizes irreversible electrolyte consumption and capacity fade, which is a common failure mode in sodium-ion battery anodes.
Conclusion and Perspectives
In summary, a Zn2GeO4/MXene nanocomposite was successfully synthesized via a fluorine-free MXene preparation followed by an in-situ hydrothermal method. When evaluated as an anode material for sodium-ion batteries, the composite demonstrated significantly enhanced electrochemical performance compared to its pure Zn2GeO4 counterpart. The key improvements include a higher reversible specific capacity (75 mAh g-1 vs. 35.5 mAh g-1 after 100 cycles at 100 mA g-1) and excellent cycling stability with Coulombic efficiency exceeding 99%. These gains are directly attributable to the strategic integration with MXene, which provides superior electrical conductivity, acts as a mechanical buffer against volume changes, prevents active material aggregation, and contributes additional sodium storage capacity.
This work underscores the great potential of designing MXene-based nanocomposites to overcome the intrinsic limitations of high-capacity alloying/conversion anodes for sodium-ion batteries. Future research could focus on: (i) optimizing the mass ratio between Zn2GeO4 and MXene for even better performance; (ii) exploring other types of MXenes (e.g., V2C, Nb2C) with different work functions and interlayer spacings; (iii) engineering the surface chemistry of MXene to further enhance Na+ adsorption and kinetics; and (iv) scaling up the synthesis process for practical applications. The findings presented here contribute a valuable strategy toward the development of high-performance, durable, and cost-effective anode materials for the next generation of grid-scale and portable sodium-ion battery systems.