The escalating global energy consumption and concomitant environmental challenges associated with fossil fuels have intensified the search for sustainable and efficient energy storage solutions. Among various contenders, rechargeable batteries, particularly lithium-ion batteries (LIBs), have dominated the portable electronics market. However, concerns regarding the limited geographical distribution and rising cost of lithium resources have renewed significant interest in sodium-ion batteries (NIBs). Sodium shares similar chemical properties with lithium, allowing for the adoption of analogous cathode materials. Nevertheless, the larger ionic radius (Na⁺: 1.02 Å vs. Li⁺: 0.76 Å) and higher molar mass of sodium pose distinct challenges for anode materials, rendering some successful LIB anodes, like graphite, unsuitable for NIBs. Consequently, identifying high-performance, cost-effective anode materials is crucial for advancing NIB technology.
Titanium dioxide (TiO₂), particularly its anatase polymorph, has emerged as a promising anode candidate for sodium-ion battery systems due to its natural abundance, low cost, structural stability, and relatively safe operating potential. It operates primarily on an intercalation mechanism for sodium storage. However, its practical application is severely hampered by intrinsically low electronic conductivity and sluggish Na⁺ diffusion kinetics, leading to poor rate capability and limited accessible capacity. Furthermore, the repeated insertion and extraction of Na⁺ ions can induce local structural strain and particle pulverization, causing capacity fade over cycles.
To overcome these limitations, nanostructuring and carbon compositing are widely adopted strategies. Designing TiO₂ with nanoscale dimensions shortens the ion diffusion path. Embedding these nanoparticles within a conductive carbon matrix significantly enhances the overall electronic conductivity of the electrode, buffers volume changes during cycling, and can prevent nanoparticle aggregation. Porous carbon architectures offer the additional benefit of a high surface area, facilitating electrolyte penetration and providing more active sites for sodium storage reactions.
In this work, we present a facile and scalable synthesis of hierarchical porous TiO₂/C hybrids via a biopolymer self-crosslinking route followed by controlled carbonization. The unique synthesis leverages the ionic cross-linking property of sodium alginate with Ti⁴⁺ ions, forming a well-defined “egg-box” gel structure. Subsequent carbonization yields a composite where ultrafine anatase TiO₂ nanoparticles are uniformly confined within a three-dimensional, interconnected porous carbon network. This distinctive architecture addresses the key challenges of TiO₂ anodes for sodium-ion battery applications: the carbon network ensures efficient electron transport, the porosity guarantees rapid ion access, and the nanoconfinement mitigates particle growth and aggregation. We systematically investigate the effect of carbonization temperature on the material’s physicochemical properties and its corresponding electrochemical performance as an anode in a sodium-ion battery. The optimized hybrid exhibits remarkable cycling stability and excellent rate capability, demonstrating great potential for practical energy storage devices.
Synthesis and Structural Characterization
The synthesis pathway is based on the spontaneous cross-linking reaction between alginate biopolymers and multivalent metal ions. Alginate, a natural polysaccharide, contains guluronic acid (G) blocks that selectively bind cations, leading to the formation of ordered junction zones often described as an “egg-box” structure.
Synthesis Procedure: A sodium alginate solution (1.5 wt%) was prepared. A 0.15 M aqueous solution of titanium(IV) sulfate (Ti(SO₄)₂) was slowly added to the alginate sol under stirring, with a volumetric ratio of 5:2 (Ti-solution: alginate sol). The Ti⁴⁺ ions immediately cross-linked the alginate chains, forming a stable hydrogel. This gel was thoroughly washed with deionized water to remove unbound ions and freeze-dried to preserve its nano-architecture. The resulting dried Ti-alginate monolith was then carbonized in a tubular furnace under a flowing nitrogen atmosphere. The temperature was ramped at 5 °C/min to various target temperatures (500°C, 600°C, 700°C, 800°C, 900°C), held for 1 hour, and then cooled naturally. The final black products were collected as TiO₂/C-T, where T denotes the carbonization temperature.
| Sample | Carbonization Temp. (°C) | TiO₂ Content (wt%)* | BET Surface Area (m²/g) | Primary Pore Size (nm) |
|---|---|---|---|---|
| TiO₂/C-500 | 500 | ~55 | 201.5 | Micro/Meso |
| TiO₂/C-600 | 600 | 60.3 | 175.4 | Meso (∼7) |
| TiO₂/C-700 | 700 | ~62 | 141.2 | Meso |
| TiO₂/C-800 | 800 | ~65 | 98.7 | Meso/Macro |
| TiO₂/C-900 | 900 | ~68 | 45.3 | Macro |
*Estimated from TGA residue in air.
The morphology evolution with carbonization temperature was striking. At 500°C, the carbon scaffold appeared as a smooth, interconnected network with very few visible nanoparticles. At 600°C, the carbon backbone became more defined, and a uniform distribution of ultra-fine nanoparticles (<5 nm) was observed on its surface. This homogeneity is a direct consequence of the confined “egg-box” precursors. At 700°C, the nanoparticles remained well-dispersed but showed slight growth. At 800°C and above, significant nanoparticle coalescence and growth occurred, leading to larger, irregular TiO₂ aggregates on the carbon matrix, which is detrimental for electrochemical activity.
X-ray diffraction (XRD) analysis confirmed the crystallographic structure. Samples carbonized at 500°C and 600°C exhibited broad diffraction peaks matching the anatase phase of TiO₂ (JCPDS #21-1272). The increasing sharpness of the peaks from 500°C to 600°C indicated improved crystallinity. At temperatures ≥700°C, additional peaks belonging to the rutile phase began to appear, and their intensity grew with temperature, indicating a phase transformation. The sample carbonized at 900°C was predominantly rutile. The crystallite size (D) can be estimated using the Scherrer equation:
$$ D = \frac{K \lambda}{\beta \cos \theta} $$
where \(K\) is the shape factor (~0.9), \(\lambda\) is the X-ray wavelength, \(\beta\) is the full width at half maximum (FWHM) in radians, and \(\theta\) is the Bragg angle. Applying this to the (101) peak of anatase showed a clear increase in crystallite size from ~4 nm at 600°C to over 15 nm at 800°C.
Raman spectroscopy provided insights into the carbon structure. All composites displayed characteristic D band (~1340 cm⁻¹, disorder-induced) and G band (~1590 cm⁻¹, graphitic sp² carbon) features. The intensity ratio \(I_G/I_D\) is a semi-quantitative indicator of graphitization. The ratio for TiO₂/C-600 was approximately 1.21, suggesting a moderately ordered carbon structure facilitated by the catalytic effect of TiO₂ nanoparticles during carbonization. This ratio did not increase significantly at higher temperatures, likely because excessive TiO₂ particle growth impeded the ordered reorganization of carbon atoms.
Thermogravimetric analysis (TGA) in air was used to determine the composite’s composition. The major weight loss between 350°C and 500°C corresponds to the combustion of the carbon matrix. The remaining stable weight represents the TiO₂ content, as listed in Table 1.
N₂ adsorption-desorption isotherms revealed the porous nature of the hybrids. TiO₂/C-600 exhibited a type-IV isotherm with a distinct H3 hysteresis loop, indicative of mesoporous slit-shaped pores. The pore size distribution (PSD) derived from the DFT method showed a hierarchical structure with a prominent peak around 7 nm and a wide distribution extending into the macropore region. This hierarchical porosity is essential for a high-performance sodium-ion battery anode, as mesopores provide high surface area for charge storage, while macropores serve as ion-buffering reservoirs and facilitate rapid electrolyte transport.
| Property / Technique | TiO₂/C-600 | TiO₂/C-800 | Remarks |
|---|---|---|---|
| Dominant TiO₂ Phase | Anatase | Anatase + Rutile | XRD |
| Avg. TiO₂ Size (nm) | ~4-5 | >15 | HʀTEM, Scherrer |
| Carbon Graphitization (IG/ID) | ~1.21 | ~1.25 | Raman |
| Surface Area (m²/g) | 175.4 | 98.7 | BET |
| Pore Volume (cm³/g) | 0.31 | 0.22 | BJH adsorption |
Electrochemical Performance in Sodium-Ion Battery
The electrochemical properties of the TiO₂/C hybrids were evaluated in a half-cell configuration versus Na/Na⁺. Cyclic voltammetry (CV) of the TiO₂/C-600 electrode at 0.1 mV/s revealed redox peaks centered around 0.74 V (cathodic) and 0.87 V (anodic), corresponding to the reversible Ti⁴⁺/Ti³⁺ redox couple during Na⁺ insertion/extraction into/from the anatase structure. The reaction can be represented as:
$$ \text{TiO}_2 + x\text{Na}^+ + x\text{e}^- \leftrightarrow \text{Na}_x\text{TiO}_2 \quad (0 < x \lesssim 0.5) $$
The small polarization (~0.13 V) indicates good electrochemical reversibility. The broad cathodic hump below 0.5 V in the first cycle is attributed to solid electrolyte interphase (SEI) formation and irreversible reactions with the carbon surface.
Galvanostatic charge-discharge (GCD) profiles at 100 mA/g aligned with the CV data. The first discharge capacity was significantly higher than subsequent cycles due to irreversible processes. From the second cycle onward, the profiles exhibited clear sloping plateaus corresponding to the Ti⁴⁺/Ti³⁺ redox, with excellent overlap indicating high cyclability.
| Current Density (mA/g) | Discharge Capacity (mAh/g)* | Cycle Number | Capacity Retention |
|---|---|---|---|
| 100 | 269 (2nd cycle) | 300 | 180.4 mAh/g (67%) |
| 500 | 144 | 50 | Stable |
| 1000 | 125 | 1000 | 102.3 mAh/g (~82%) |
| 5000 | 107 | 10 | Stable |
*Reversible capacity based on total composite mass.
The long-term cycling performance was exceptional. At a moderate current of 100 mA/g, the TiO₂/C-600 electrode delivered a reversible capacity of 180.4 mAh/g after 300 cycles, demonstrating excellent stability. More impressively, at a high current density of 1000 mA/g, it retained a capacity of 102.3 mAh/g after 1000 cycles, with a near-unity Coulombic efficiency throughout. This outstanding performance underscores the structural robustness imparted by the carbon matrix in this sodium-ion battery anode.
The rate capability test further highlighted the kinetic advantages of the hierarchical porous structure. The electrode delivered average capacities of 220, 144, 125, and 107 mAh/g at current densities of 100, 500, 1000, and 5000 mA/g, respectively. When the current was switched back to 100 mA/g, the capacity recovered to 201 mAh/g, confirming the high reversibility and resilience of the structure against rapid cycling.
Electrochemical impedance spectroscopy (EIS) provided quantitative insights into the charge transfer kinetics. The Nyquist plots consisted of a depressed semicircle in the mid-frequency region (charge transfer resistance, \(R_{ct}\)) and a sloping line in the low-frequency region (Warburg impedance, related to Na⁺ diffusion). The TiO₂/C-600 electrode exhibited a significantly lower \(R_{ct}\) (1127 Ω) compared to the TiO₂/C-800 electrode (1430 Ω) after several cycles, indicating more facile charge transfer at the electrolyte/electrode interface.
The sodium-ion diffusion coefficient (\(D_{Na^+}\)) can be estimated from the low-frequency Warburg region using the following equations:
$$ Z’ = R_s + R_{ct} + \sigma \omega^{-1/2} $$
$$ D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where \(\sigma\) is the Warburg factor obtained from the slope of \(Z’\) vs. \(\omega^{-1/2}\), \(R\) is the gas constant, \(T\) is temperature, \(A\) is electrode area, \(n\) is electrons transferred per molecule, \(F\) is Faraday’s constant, and \(C\) is the molar concentration of Na⁺ in the electrode. The calculated \(D_{Na^+}\) for TiO₂/C-600 was on the order of \(10^{-12}\) cm²/s, which is about an order of magnitude higher than that for TiO₂/C-800 (\(10^{-13}\) cm²/s). This confirms that the nano-sized particles and open porous framework in the 600°C hybrid drastically enhance ion diffusion kinetics, a critical factor for the rate performance of a sodium-ion battery.
To deconvolute the capacity contributions, we analyzed the current response at different scan rates (\(v\)). The current (\(i\)) obeys a power-law relationship with scan rate: \(i = a v^b\). A \(b\)-value of 0.5 indicates a diffusion-controlled intercalation process, while a value of 1.0 signifies a surface-controlled capacitive process. For the TiO₂/C-600 electrode, the \(b\)-value for both anodic and cathodic peaks was determined to be approximately 0.75-0.85, suggesting a mixed storage mechanism with a significant contribution from surface or near-surface processes (pseudocapacitance). This pseudocapacitive behavior, facilitated by the large surface area and nanoscale TiO₂, is highly desirable as it leads to faster kinetics and better rate performance. The quantitative capacitive contribution (\(k_1 v\)) can be separated from the diffusion-controlled contribution (\(k_2 v^{1/2}\)) using the equation:
$$ i(V) = k_1 v + k_2 v^{1/2} $$
At a scan rate of 1.0 mV/s, the capacitive contribution accounted for over 60% of the total charge storage, explaining the excellent rate capability.
| Material | Reversible Capacity (mAh/g) | Current Density (mA/g) | Cycle Number | Key Feature |
|---|---|---|---|---|
| Anatase Nanoparticles | ~92 | 1845 | 1000 | Pure nano-TiO₂ |
| Carbon-coated TiO₂ | ~150 | 20 | 100 | Core-shell sphere |
| TiO₂/Graphene | ~90 | 12000 | 4000 | High-rate graphene composite |
| N-doped TiO₂ | ~129 | 2000 | 2000 | Anion doping |
| Black TiO₂−x | ~186 | 33.6 | 1000 | Oxygen vacancies |
| This work: TiO₂/C-600 | 180.4 | 100 | 300 | Hierarchical porous hybrid |
| This work: TiO₂/C-600 | 102.3 | 1000 | 1000 | Excellent long-term cycling |
Conclusion
In summary, we have successfully synthesized a hierarchical porous TiO₂/C hybrid material through a simple and scalable biopolymer gel route followed by optimized carbonization. The process capitalizes on the self-crosslinking of alginate with Ti⁴⁺ ions to create a precursor that transforms into a composite with uniformly dispersed, ultrafine anatase TiO₂ nanoparticles embedded within a 3D interconnected, porous carbon matrix. The carbonization temperature is a critical parameter, with 600°C yielding the optimal structure that balances high TiO₂ content, good crystallinity, nanoscale particle size, sufficient carbon graphitization, and a hierarchical pore network.
When evaluated as an anode material for sodium-ion battery, the TiO₂/C-600 hybrid demonstrates a synergy of its constituent properties. The conductive carbon network facilitates electron transport, the porous structure ensures rapid electrolyte access and Na⁺ diffusion, and the nanoconfinement of TiO₂ buffers volume changes and promotes surface-driven pseudocapacitive storage. This synergy translates into remarkable electrochemical performance: high reversible capacity, outstanding long-term cycling stability at both moderate and high rates, and superior rate capability. The detailed kinetic analysis confirms enhanced Na⁺ diffusion and a significant capacitive contribution to charge storage. This work underscores the effectiveness of biomolecule-assisted design in creating advanced nanostructured composites for energy storage, presenting a promising, cost-effective anode candidate for the next generation of sodium-ion battery technology.