The escalating global demand for energy, driven by technological advancement and societal development, has necessitated the urgent exploration and innovation of next-generation energy storage systems. While lithium-ion batteries (LIBs) have dominated the landscape due to their high energy density, operating voltage, and cycle life, concerns over lithium resource scarcity, cost volatility, and safety issues have spurred significant interest in alternative chemistries. Sodium-ion batteries (SIBs) emerge as a compelling candidate, leveraging the natural abundance and low cost of sodium, coupled with its chemical similarity to lithium. However, the larger ionic radius of Na+ (1.02 Å vs. 0.76 Å for Li+) often leads to sluggish reaction kinetics and significant volume expansion in electrode materials during cycling, resulting in rapid capacity decay and poor rate capability. Overcoming these intrinsic challenges through innovative material design is paramount for the development of high-performance SIBs.
Among various anode materials explored for sodium-ion battery applications, titanium-based oxides, particularly sodium titanates, have garnered attention for their structural stability, low cost, and suitable operating potential, which helps mitigate sodium dendrite formation. Compounds like Na2Ti3O7 and Na2Ti6O13 offer advantages but are often hampered by low electronic conductivity and modest specific capacity. In contrast, layered NaTiO2 presents a promising alternative with a theoretically favorable structure for Na+ (de)intercalation. However, its practical application is limited by synthetic challenges that often yield morphologically irregular particles with suboptimal electrochemical contact. Concurrently, the family of two-dimensional transition metal carbides/nitrides, known as MXenes, has revolutionized materials science for energy storage. Derived from selective etching of MAX phases, MXenes like Ti3C2Tx (where Tx represents surface terminations like -O, -OH, -F) possess exceptional properties including metallic conductivity, hydrophilic surfaces, tunable interlayer spacing, and high redox activity. These characteristics make Ti3C2 MXene an excellent conductive scaffold or active matrix in composite electrodes for batteries.
This work strategically integrates the merits of both components by designing and synthesizing a novel layered composite, Ti3C2 MXene/NaTiO2. The approach involves the in-situ growth of NaTiO2 nanosheets on the surface of pre-etched, layered Ti3C2 MXene. This architecture aims to synergistically combine the high conductivity and robust mechanical framework of MXene with the high Na+ storage capacity and favorable intercalation potential of NaTiO2. The intimate contact and layered morphology are expected to facilitate rapid ion and electron transport, accommodate volume changes, and expose abundant active sites, thereby addressing key limitations in sodium-ion battery anode materials. A systematic investigation into the synthesis parameters, particularly hydrothermal temperature, was conducted to optimize the structure and, consequently, the electrochemical performance of the composite.
Experimental Methods
Synthesis of Ti3C2 MXene
The Ti3C2 MXene matrix was synthesized via selective etching of aluminum from the MAX phase precursor, Ti3AlC2. In a typical procedure, 2 grams of Ti3AlC2 powder (200 nm – 10 μm) was gradually added to 40 mL of hydrofluoric acid (HF, 40 wt.%) under continuous magnetic stirring in a polypropylene beaker. The etching reaction was allowed to proceed at 35°C for 48 hours. The resulting mixture was then repeatedly washed with deionized water via centrifugation (3500 rpm for 5 min per cycle) until the supernatant pH reached approximately 6. The final sediment, a multi-layered Ti3C2 clay, was collected and freeze-dried for 24 hours to obtain the delaminated MXene matrix.
Synthesis of Ti3C2 MXene/NaTiO2 (Ti3C2/NTO) Composites
The composites were synthesized via a one-pot hydrothermal method. Firstly, the as-prepared Ti3C2 MXene was dispersed in 30 mL of deionized water by ultrasonication for 30 minutes to form a homogeneous colloidal solution. Sodium sulfide nonahydrate (Na2S·9H2O) was used as the sodium source and added to the MXene dispersion under vigorous stirring. The molar ratio of Ti3C2 (considering the Ti content) to Na2S·9H2O was maintained at 1:2. After stirring for an additional 30 minutes, the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The hydrothermal treatment was carried out at different temperatures (120°C, 150°C, 180°C) for 12 hours to study the temperature effect. The resulting products were labeled as Ti3C2/NTO-120, Ti3C2/NTO-150, and Ti3C2/NTO-180, respectively. For comparison, a sample was also prepared by magnetically stirring the same mixture at room temperature for 8 days, labeled Ti3C2/NTO-cw. All final products were washed with deionized water and ethanol several times and dried at 60°C under vacuum overnight.
Material Characterization
The crystal structure and phase composition of the samples were characterized by X-ray diffraction (XRD) using a diffractometer with Cu Kα radiation (λ = 1.5406 Å), scanning from 5° to 80° at a step size of 0.02°. The morphological features and elemental distribution were investigated using field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS).
Electrochemical Measurements
Electrochemical evaluations were performed using CR2032 coin cells assembled in an argon-filled glovebox. The working electrode was fabricated by mixing the active material (Ti3C2/NTO composite), conductive carbon black, and sodium carboxymethyl cellulose (CMC) binder in a weight ratio of 7:2:1. Deionized water was used as the solvent to form a homogeneous slurry, which was then coated onto a copper foil current collector and dried at 60°C under vacuum for 12 hours. Sodium metal foil was used as the counter/reference electrode. The electrolyte was 1 M NaClO4 in a mixture of ethylene carbonate and propylene carbonate (EC:PC, 1:1 by volume) with 5 wt.% fluoroethylene carbonate (FEC) additive. Glass fiber was used as the separator. Cyclic voltammetry (CV) tests were conducted between 0.01 and 3.00 V at a scan rate of 0.5 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range from 100 kHz to 0.01 Hz with an AC amplitude of 5 mV. Galvanostatic charge-discharge (GCD) cycling and rate capability tests were carried out using a battery testing system within the same voltage window.
Results and Discussion
Structural and Morphological Evolution
The successful synthesis of the layered Ti3C2/NTO composite hinges on two critical steps: the effective etching of the MAX phase and the subsequent in-situ growth of NaTiO2. XRD analysis confirms the phase transformation. The precursor Ti3AlC2 shows characteristic peaks of the MAX phase. After HF etching, the most intense (104) peak of Ti3AlC2 near 39° disappears, and the (002) peak shifts from ~9.5° to a lower angle of ~8.9°, indicating the successful removal of the Al layers and an increase in the c-lattice parameter due to the introduction of functional groups and intercalated water, yielding multi-layered Ti3C2 MXene. SEM images of this etched product reveal the characteristic “accordion-like” multilayer morphology.
Following the hydrothermal reaction with Na2S·9H2O, the XRD patterns of the composites exhibit new diffraction peaks. Alongside the retained (002) peak of Ti3C2 (now further shifted to ~7.2°, indicating expanded interlayer spacing), distinct peaks appear at approximately 40.6° and 60.8°, which can be indexed to the (104) and (110) planes of layered NaTiO2 (JCPDS No. 89-0802), confirming the formation of the target composite. Minor impurity phases such as NaTi8O13 are also detected, which may influence the local crystallinity of NaTiO2. Notably, the sample prepared at room temperature (Ti3C2/NTO-cw) shows very weak and broad NaTiO2 peaks, indicating poor crystallinity.
The morphology is profoundly influenced by the hydrothermal temperature. SEM analysis shows that Ti3C2/NTO-120 maintains a well-defined, open layered structure with ample interlayer space, resembling an expanded accordion. This porous architecture is highly beneficial for electrolyte penetration and Na+ diffusion. As the synthesis temperature increases to 150°C and 180°C, the composites (Ti3C2/NTO-150 and Ti3C2/NTO-180) still exhibit a layered form, but the layers appear denser and less separated. The room-temperature sample, Ti3C2/NTO-cw, shows a poorly developed structure with particulate matter coating the surfaces, offering fewer active sites for electrochemical reactions. EDS elemental mapping of Ti3C2/NTO-180 confirms the homogeneous distribution of Na, Ti, O, and C throughout the layered framework, verifying the uniform integration of NaTiO2 with the Ti3C2 matrix. It is proposed that amorphous carbon or functional groups on the MXene surface act as a “glue,” facilitating the nucleation and anchoring of NaTiO2 nanosheets. This intimate coupling is crucial for enhancing electronic conductivity and mitigating the detachment or aggregation of active material during the repeated Na+ insertion/extraction processes in a sodium-ion battery.
Electrochemical Performance Analysis
The electrochemical properties of the composites as anodes for sodium-ion batteries were systematically evaluated. Cyclic voltammetry (CV) curves at 0.5 mV s-1 provide insight into the redox processes. All composites show similar CV shapes but differ significantly in current response and integrated area, which correlates directly with capacity. Ti3C2/NTO-120 exhibits the largest CV area and the most distinct redox peaks. In the first cathodic scan, a small peak around 2.32 V is attributed to the formation of a solid electrolyte interphase (SEI). A prominent pair of redox peaks between 2.09 V and 1.49 V corresponds to the reversible insertion/extraction of Na+ into/from the NaTiO2 lattice, likely following a reaction scheme involving Ti3+/Ti2+ redox couple:
$$ \text{NaTiO}_2 + x\text{Na}^+ + x\text{e}^- \rightleftharpoons \text{Na}_{1+x}\text{TiO}_2 $$
Another broad reduction hump below 1.0 V, more pronounced in the MXene-rich composite, may be associated with Na+ storage on defect sites, functional groups of MXene, or interfacial capacitance. The high overlap of subsequent cycles for Ti3C2/NTO-120 indicates excellent reversibility, a key attribute for a durable sodium-ion battery anode.
Electrochemical impedance spectroscopy (EIS) reveals the kinetics of charge transfer. The Nyquist plots consist of a depressed semicircle in the high-to-medium frequency region, representing the charge-transfer resistance (Rct) at the electrode/electrolyte interface, and a sloping line in the low-frequency region, associated with Na+ diffusion (Warburg impedance). Ti3C2/NTO-120 demonstrates the smallest semicircle diameter, indicating the lowest Rct among the samples. This is attributed to its optimal open structure and the synergistic effect between conductive MXene and well-crystallized NaTiO2, facilitating faster charge transfer—a critical factor for achieving high rate performance in sodium-ion batteries.
The galvanostatic charge-discharge profiles and cycling performance at 200 mA g-1 are summarized below. The superior performance of the Ti3C2/NTO-120 composite is evident.
| Sample | Max. Discharge Capacity (mAh g-1) | Average Discharge Capacity (mAh g-1) | Capacity after 250 cycles (mAh g-1) | Coulombic Efficiency (~%) |
|---|---|---|---|---|
| Ti3C2/NTO-120 | 266.30 | 230.54 | 253.60 | ~100 |
| Ti3C2/NTO-150 | 137.80 | 117.65 | ~120 | ~100 |
| Ti3C2/NTO-180 | 127.00 | 109.62 | ~112 | ~100 |
| Ti3C2/NTO-cw | 85.80 | 66.44 | ~70 | ~100 |
Ti3C2/NTO-120 delivers the highest initial discharge capacity of 266.30 mAh g-1 and maintains a remarkable 253.60 mAh g-1 after 250 cycles, corresponding to a high capacity retention of 95.2%. All composites show stable cycling with Coulombic efficiency near 100% after the first few cycles. The gradual increase in capacity during early cycles, observed especially for Ti3C2/NTO-120, suggests an electrochemical activation process where the layered structure progressively opens up, and more active sites become accessible for Na+ storage.
The rate capability, a crucial metric for high-power sodium-ion battery applications, was evaluated by cycling the cells at progressively increasing current densities from 100 to 2000 mA g-1 and then back to 100 mA g-1. The performance is quantified in the following table:
| Current Density (mA g-1) | Ti3C2/NTO-120 Capacity (mAh g-1) | Ti3C2/NTO-150 Capacity (mAh g-1) | Capacity Retention vs. 100 mA g-1 |
|---|---|---|---|
| 100 | 227.56 | 115.40 | 100% (baseline) |
| 200 | 215.90 | 105.80 | ~95% / ~92% |
| 500 | 198.35 | 98.70 | ~87% / ~86% |
| 1000 | 176.62 | 89.25 | ~78% / ~77% |
| 2000 | 144.74 | 80.20 | ~63.6% / ~69.5% |
| 100 (recovery) | 220.85 | 118.60 | ~97.0% / ~102.8% |
Ti3C2/NTO-120 demonstrates exceptional rate performance. Even at a high current density of 2000 mA g-1, it retains a substantial capacity of 144.74 mAh g-1, which is 63.6% of its capacity at 100 mA g-1. When the current density is returned to 100 mA g-1, the capacity recovers to 220.85 mAh g-1, 97.0% of the initial value at that rate, underscoring the structural robustness and kinetic superiority of this optimized composite anode. The excellent rate capability can be attributed to the combined factors of high electronic conductivity from the MXene network, short Na+ diffusion pathways within the thin NaTiO2 nanosheets and expanded MXene interlayers, and low charge-transfer resistance. The Na+ diffusion coefficient (DNa+) can be qualitatively assessed from the low-frequency Warburg region of the EIS data, and the relationship is given by:
$$ D_{Na^+} = \frac{R^2T^2}{2A^2n^4F^4C^2\sigma_\omega^2} $$
where R is the gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons per molecule, F is Faraday’s constant, C is the molar concentration of Na+, and σω is the Warburg factor derived from the slope of Z’ vs. ω-1/2. The open structure of Ti3C2/NTO-120 facilitates a lower σω, implying a higher DNa+, which is consistent with its outstanding high-rate performance in sodium-ion batteries.
Conclusion
In summary, a novel layered Ti3C2 MXene/NaTiO2 composite was successfully designed and synthesized via a two-step process involving HF etching of Ti3AlC2 followed by an in-situ hydrothermal reaction. The synthesis parameters, specifically the hydrothermal temperature, were found to critically govern the composite’s morphology and crystallinity. The optimized composite (Ti3C2/NTO-120) possesses an ideal architecture characterized by an open, layered framework with expanded interlayer spacing, which provides numerous pathways for rapid ionic and electronic transport while effectively buffering volume changes.
When evaluated as an anode material for sodium-ion batteries, the Ti3C2/NTO-120 composite exhibits exceptional electrochemical properties: a high reversible specific capacity of 253.60 mAh g-1 after 250 cycles at 200 mA g-1, outstanding cycling stability with 95.2% capacity retention, and remarkable rate capability with 144.74 mAh g-1 delivered at a high current density of 2000 mA g-1. This performance surpasses that of many reported titanium-based oxide anodes and highlights the effectiveness of the composite design strategy. The synergistic combination of the conductive and mechanically flexible MXene scaffold with the high-capacity NaTiO2 active material addresses key challenges of slow kinetics and structural degradation in SIB anodes. This work demonstrates the significant potential of MXene-based layered composites as high-performance electrode materials for advanced sodium-ion batteries, paving the way for further exploration of such hybrid architectures in energy storage applications.