As a researcher focused on next-generation energy storage, I have been investigating alternatives to lithium-ion batteries. Sodium-ion batteries present a compelling option due to the greater abundance and lower cost of sodium resources. Among various anode candidates, anatase titanium dioxide (TiO2) stands out for its good structural stability during ion insertion/extraction, low cost, and environmental friendliness. However, its practical application in sodium-ion batteries is significantly hindered by intrinsically low electronic conductivity and sluggish Na+ ion diffusion kinetics, primarily due to the larger ionic radius of Na+ compared to Li+.
To address these limitations, heteroatom doping has been widely explored as an effective strategy. Single-atom doping, such as with S, N, or P, can enhance the electronic conductivity and introduce beneficial defects. Nevertheless, the doping efficiency for single atoms in TiO2 is often limited by low solid solubility, resulting in a relatively low doping concentration. Furthermore, a single dopant typically offers a singular functional improvement. Therefore, the question arises: can a dual-atom doping strategy not only achieve a higher effective doping concentration but also create a synergistic effect to further boost the electrochemical performance for sodium-ion battery anodes?
In this work, we explored a novel S, N co-doping approach on self-supported TiO2 nanotube arrays. We hypothesized that using a precursor containing both S and N atoms could facilitate higher incorporation levels of heteroatoms into the TiO2 surface structure. This, in turn, would create more oxygen vacancies and modify the electronic structure more profoundly than single doping, leading to superior Na+ storage kinetics and cycling stability in sodium-ion batteries.
Experimental Synthesis and Methodology
We began with the fabrication of the TiO2 nanotube (TNT) array substrate. A titanium foil was first mechanically polished with sandpapers to remove the native oxide layer, followed by ultrasonic cleaning in acetone, ethanol, and deionized water. The anodization was performed in a two-electrode setup with the Ti foil as the anode and a graphite rod as the cathode. The electrolyte consisted of 0.56 g NH4F dissolved in a mixture of 95 mL ethylene glycol (EG) and 5 mL deionized H2O. A constant voltage of 60 V was applied at room temperature for 20 min. The initially formed layer was ultrasonically removed in DI water, and a second anodization under identical conditions was conducted to obtain highly ordered, amorphous TiO2 nanotube arrays.
The S, N co-doping was achieved via a simple annealing process. The as-anodized TNT sample and 2 grams of thiourea (CH4N2S) powder were placed in separate ceramic boats upstream and downstream, respectively, within a tube furnace. The furnace was heated to 450 °C at a rate of 5 °C/min under a flowing Ar atmosphere, held for 2 hours, and then cooled naturally. Thiourea decomposes upon heating, releasing H2S and NH3 gases, which act as the S and N sources, respectively. The final product is denoted as TiO2-SN. For comparison, a pristine TiO2 sample (TiO2) was prepared by annealing the TNT array under the same conditions but without the thiourea precursor.
The morphological and structural characteristics of the samples were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical performance as an anode for sodium-ion batteries was evaluated by assembling CR2032 coin cells in an argon-filled glovebox, using sodium metal as the counter/reference electrode, a glass fiber separator, and an electrolyte of 1 M NaCF3SO3 in diglyme with 5% fluoroethylene carbonate (FEC) additive. Galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) tests were conducted on battery testers and electrochemical workstations.
Structural and Compositional Characterization
SEM images reveal that both the TiO2 and TiO2-SN samples maintain a well-aligned nanotubular architecture with an average tube diameter of approximately 120 nm and a wall thickness of about 8 nm. The doping process did not cause obvious structural collapse or significant morphological alteration. TEM analysis provides deeper insight. While the pristine TiO2 nanotube walls show clear lattice fringes corresponding to the anatase (101) plane with a spacing of 0.35 nm, the TiO2-SN sample exhibits large amorphous regions on the surface of the nanotubes alongside crystalline areas with the same lattice spacing. This indicates that the S, N co-doping primarily affects the surface layer of the nanotubes, introducing disorder without changing the bulk crystal lattice parameter.
XRD patterns confirm that both samples crystallize in the anatase phase (JCPDS #21-1272) after annealing. The diffraction peaks for TiO2-SN are slightly broader and less intense compared to those of pristine TiO2, consistent with the presence of surface disorder observed in TEM. Rietveld refinement of the XRD data yielded weighted profile R-factors (Rwp) of 8.08% and 6.53% for TiO2 and TiO2-SN, respectively, indicating reliable fitting results. The refined lattice parameters (a, b, c) showed negligible change, confirming that S and N doping did not incorporate into the bulk crystal lattice but were concentrated on the surface.
XPS analysis was crucial for determining the surface chemical states and doping concentrations. The survey spectra confirm the presence of Ti and O in both samples, with additional N and S signals only in the TiO2-SN sample. High-resolution spectra provide detailed information:
- Ti 2p: The peaks for Ti4+ 2p3/2 and 2p1/2 in TiO2-SN shift to lower binding energies by 0.10 eV and 0.08 eV, respectively, compared to pristine TiO2. This is a clear indication of increased electron density around Ti atoms, likely due to the creation of oxygen vacancies (OV) induced by heteroatom doping.
- O 1s: Deconvolution of the O 1s peak reveals two components: lattice oxygen (Ti-O) and oxygen vacancies (OV). The OV concentration increases dramatically from 15.3% in TiO2 to 25.4% in TiO2-SN, quantitatively proving the defect-enhancing effect of dual doping.
- N 1s: The N 1s spectrum for TiO2-SN shows a single peak centered at 396.5 eV, which is characteristic of N atoms in an interstitial Ti-N-O configuration. This type of doping is known to be effective in improving both ionic and electronic conductivity.
- S 2p: The S 2p spectrum displays two doublets. The peaks at ~163.7 eV correspond to S2- substituting for O2- sites, while the peaks at ~168.3 eV are attributed to S4+ substituting for Ti4+ sites. This confirms that S incorporates via substitutional doping.
Quantitative XPS analysis indicates successful co-doping with high mass fractions of 4.76% for N and 1.53% for S. These values are notably higher than those typically reported for single-atom doping in TiO2, validating our dual-doping strategy. Raman spectroscopy further supports these findings. The characteristic anatase modes (Eg, B1g, A1g, Eg) are present in both samples, but the main Eg peak for TiO2-SN shows a noticeable blue shift and broadening, confirming reduced symmetry of Ti-O bonds and the introduction of defects like oxygen vacancies.
Electrochemical Performance in Sodium-Ion Batteries
The electrochemical properties of the TiO2-SN and pristine TiO2 anodes were systematically evaluated in sodium-ion battery half-cells. All specific capacities are calculated based on the mass of the active TiO2 material (1C = 335 mA g-1).
Rate Capability: The rate performance, a critical metric for high-power sodium-ion batteries, is outstanding for the TiO2-SN electrode. As shown in the table below, it delivers high reversible capacities across a wide range of current densities.
| Current Density | TiO2-SN Capacity (mA h g-1) | TiO2 Capacity (mA h g-1) |
|---|---|---|
| 0.1 C | 296.6 | 165.3 |
| 0.2 C | 268.3 | 151.8 |
| 0.5 C | 247.1 | 138.5 |
| 1.0 C | 236.9 | 128.7 |
| 3.0 C | 225.8 | 105.4 |
| 5.0 C | 211.5 | 88.2 |
| 10.0 C | 197.7 | 68.1 |
| 15.0 C | 175.5 | 55.0 |
| 20.0 C | 166.2 | 45.9 |
| 25.0 C (~8.4 A g-1) | 158.1 | 38.5 |
Remarkably, even at an ultra-high current density of 25.0 C (approximately 8.4 A g-1), the TiO2-SN anode retains a capacity of 158.1 mA h g-1, significantly outperforming the pristine TiO2 anode (38.5 mA h g-1). When the current density is switched back to 1.0 C after this rigorous test, the capacity of TiO2-SN recovers to 287.3 mA h g-1, demonstrating excellent structural robustness and electrochemical reversibility.
Long-Term Cycling Stability: The cycling stability was assessed at both moderate and high rates. At 1.0 C, both electrodes show excellent capacity retention over 100 cycles due to the inherent stability of the TiO2 structure. However, the true advantage of doping is revealed under high-rate cycling conditions. At an extremely high current density of 25.0 C, the TiO2-SN electrode exhibits exceptional long-term stability. After approximately 3,500 cycles, it maintains a reversible capacity of 158.3 mA h g-1, corresponding to a high capacity retention of 94.8% (average decay rate of only 0.0015% per cycle). In stark contrast, the pristine TiO2 electrode suffers from severe capacity fading, retaining only 39.8% of its initial capacity after 3,500 cycles.
The superior performance of our TiO2-SN anode is placed in context with other reported TiO2-based anodes for sodium-ion batteries in the following table.
| Material | Morphology | Rate Performance (Current Density / Capacity) | Long Cycling Performance | Reference |
|---|---|---|---|---|
| TiO2-SN (This work) | Nanotube Array | 8,375 mA g-1 / 158.1 mA h g-1 | ~3,500 cycles, 94.8% retention at 8,375 mA g-1 | – |
| TiO2-S | Nanotube Array | 3,350 mA g-1 / 136.0 mA h g-1 | ~4,400 cycles, 91% retention at 3,350 mA g-1 | [2] |
| TiO2-P | Nanotube Array | 3,350 mA g-1 / 147.0 mA h g-1 | ~1,000 cycles, ~100% retention at 3,350 mA g-1 | [4] |
| TiO2-N | Nanotube Array | 6,700 mA g-1 / 114.0 mA h g-1 | ~5,000 cycles, ~100% retention at 3,350 mA g-1 | [3] |
| TiO2-NC | Nanofiber | 5,000 mA g-1 / 118.1 mA h g-1 | ~2,000 cycles, ~100% retention at 5,000 mA g-1 | [18] |
Kinetic Analysis and Storage Mechanism
To understand the origin of the enhanced performance, we performed detailed kinetic analyses. Cyclic voltammetry (CV) curves at a slow scan rate of 0.1 mV s-1 show a pair of broad redox peaks around 0.77/0.83 V (cathodic/anodic) for TiO2-SN, corresponding to the Ti4+/Ti3+ redox couple coupled with Na+ insertion/extraction. The potential separation (ΔE) between these peaks is only 0.058 V for TiO2-SN, much smaller than the 0.291 V for pristine TiO2, indicating significantly improved reaction kinetics and reversibility in the sodium-ion battery anode.
The current response (i) at a fixed potential during CV scans follows a power-law relationship with the scan rate (v):
$$ i = a v^{b} $$
where a and b are adjustable parameters. The b-value, obtained from the slope of log(i) vs. log(v) plot, indicates the storage mechanism: b = 0.5 suggests a diffusion-controlled process, while b = 1.0 suggests a capacitive-controlled process. We calculated b-values of 0.91 for TiO2-SN and 0.82 for TiO2. The higher b-value for TiO2-SN signifies a greater contribution from surface-controlled capacitive behavior, which is highly beneficial for fast charge/discharge. Quantitatively, the capacitive contribution at a scan rate of 2.0 mV s-1 is 85.10% for TiO2-SN, significantly higher than the 71.47% for TiO2. This dominant capacitive storage explains the exceptional rate capability of the doped sample.
Electrochemical impedance spectroscopy (EIS) revealed a much lower charge-transfer resistance (Rct) for TiO2-SN (43.1 Ω) compared to TiO2 (563.2 Ω). This order-of-magnitude reduction in resistance directly correlates with the enhanced electronic conductivity from S, N co-doping and oxygen vacancies.
The sodium-ion diffusion coefficient (DNa+) was quantitatively determined using the Galvanostatic Intermittent Titration Technique (GITT). The diffusion coefficient is calculated using the following equation:
$$ D = \frac{4}{\pi \tau} \left( \frac{m_B V_M}{M_B S} \right)^2 \left( \frac{\Delta E_s}{\Delta E_\tau} \right)^2 $$
where τ is the current pulse duration, mB, MB, and VM are the mass, molar mass, and molar volume of the active material, S is the electrode/electrolyte contact area, ΔEs is the steady-state voltage change after the pulse, and ΔEτ is the voltage change during the constant current pulse. The calculated DNa+ values for TiO2-SN are nearly one order of magnitude higher than those for pristine TiO2 throughout the charge/discharge process, confirming dramatically accelerated Na+ ion diffusion kinetics in the S, N co-doped electrode material for the sodium-ion battery.
Conclusion
In summary, we have successfully developed a high-performance anode for sodium-ion batteries through a synergistic S, N co-doping strategy applied to self-supported TiO2 nanotube arrays. This dual-doping approach, using thiourea as a single-source precursor, achieved high surface concentrations of both N (4.76%) and S (1.53%), which individually exceed typical levels achieved by single-atom doping. The co-doping introduced a surface amorphous layer, generated a high concentration of oxygen vacancies (25.4%), and enhanced the electronic structure, leading to a synergistic improvement in both electronic and ionic conductivity.
The resultant TiO2-SN anode exhibits extraordinary electrochemical performance in sodium-ion batteries: high specific capacity (296.6 mA h g-1 at 0.1C), outstanding rate capability (158.1 mA h g-1 at an ultra-high rate of 25.0C), and remarkable long-term cycling stability (94.8% capacity retention after ~3,500 cycles at 25.0C). Kinetic analyses confirm that the storage process is predominantly capacitive-controlled, with significantly reduced charge-transfer resistance and accelerated Na+ diffusion. This work demonstrates that rational multi-heteroatom doping is a powerful strategy to overcome the intrinsic limitations of metal oxide anodes, paving a promising way for developing high-power, long-life sodium-ion batteries for large-scale energy storage applications.