Electrospinning Machine | High-performance Zn-I2 Batteries Enabled by Porous Hetero-Carbon Nanofiber Hosts with TiO2 Homojunctions

This work successfully prepared a carbon nanofiber (CNF) host integrated with titanium dioxide (TiO₂) nanoparticles through a simple electrospinning process followed by annealing treatment. By varying the annealing temperature, the phase composition of the integrated TiO₂ could be regulated, consequently leading to differences in the host's degree of iodine confinement. Figure 1 confirms the successful preparation of the doped homojunction-type titanium dioxide (HTO). Raman spectroscopy results indicate that the material possesses a high degree of graphitization and electrical conductivity. Furthermore, HTO@C not only possesses high-crystallinity-quality titanium dioxide-carbon heterojunctions and homojunctions but also has a specific surface area exceeding 600 m²·g⁻¹ and an average pore size of approximately 3 nm. These characteristics contribute to enhancing iodine adsorption capacity, restricting the diffusion of iodine species, and thereby improving the battery's electrochemical performance.

Figure 1 Morphological characterization of HTO nanofibers.

The study showed that the iodine solution containing HTO@C changed from brown to nearly transparent within 1 hour, while solutions containing ATO@C and RTO@C remained light yellow even after 5 hours. This phenomenon indicates that HTO@C has a stronger ability to adsorb iodine, benefiting from its built-in electric field and superior charge separation efficiency. UV-Vis spectroscopy showed a significant reduction in I₃⁻ intensity, further confirming HTO@C's strong iodine adsorption capacity, attributed to its porous structure and good chemical affinity. XPS analysis detected iodine, oxygen, titanium, and carbon elements, indicating the successful infiltration of I₂; meanwhile, the content of I⁻ in HTO@C significantly increased (65.8%), reflecting its strong adsorption capacity and catalytic reduction ability. Elemental mapping of I₂/HTO@C confirmed the uniform loading of I₂. TGA analysis showed that the impregnated iodine content in the composite material was 30%, and the thermal stability of I₂/HTO@C was better than other samples, because its superior structure favors iodine adsorption and confinement-related reactions.

Figure 2 Iodine adsorption and confinement performance tests.

The performance of Zn-I₂ batteries fabricated with different cathodes is shown in Figure 3. The Nyquist plot of the HTO battery shows a lower charge transfer resistance than other batteries, indicating that the energy band structure of HTO facilitates fast electron transport and electrode reactions. The HTO battery maintained a capacity exceeding 217.5 mAh·g⁻¹ after 1000 cycles at 1 A·g⁻¹, with an average coulombic efficiency of 99.5%; simultaneously, it exhibited low charge-discharge polarization. After 50,000 cycles at 4 A·g⁻¹, the battery demonstrated an outstanding capacity retention rate of 98.9%, with performance far surpassing other Zn-I₂ batteries. This is attributed to the synergistic hetero/homojunction structure of HTO@C, which enhances the catalytic activity for the iodine redox reaction and improves iodine utilization.

Figure 3 Full battery performance tests.

Figure 4a shows the CV curves of current density versus potential for the battery in the voltage range of 0.4-1.6 V at a scan rate of 1 mV·s⁻¹. The redox peaks of the HTO battery are located at 1.34 V and 1.24 V, with a polarization of about 0.1 V; whereas the reduction peaks of ATO and RTO batteries are more negative, indicating slower kinetics and larger polarization. The homojunction in HTO can enhance interface polarization, accelerate charge transfer, and promote the rapid conversion between I₂ and I⁻. The Tafel plot in Figure 4b shows that the HTO battery has the smallest slope (20.06 mV·dec⁻¹), indicating the fastest polyiodide conversion kinetics. At scan rates from 0.2 to 1 mV·s⁻¹, the CV curves of the HTO battery show the smallest polarization, reflecting excellent kinetics and consistency, and its pseudocapacitive contribution is significantly higher than other batteries, confirming the positive role of the homojunction. After 24 hours of open-circuit storage, the HTO battery's capacity retention rate was 95.29%, indicating its effective suppression of the polyiodide shuttle effect.

Figure 4 Polyiodide conversion kinetics tests.

In-situ UV-Vis spectroscopy analysis revealed that the absorbance of I₃⁻ in the I₂/HTO@C battery remained consistently low throughout the cycling process, indicating that HTO@C effectively suppresses the formation and shuttling of polyiodides. This is attributed to its ability to promote electron transfer and redox reactions. Density functional theory (DFT) calculation results demonstrated significant charge transfer and more negative binding energies for iodine species (I⁻, I₂, and I₃⁻) on the HTO battery surface. This indicates that the bonding strength between HTO and polyiodide ions is substantially higher than that of traditional I₂-TiO₂ bonds, resulting in excellent thermodynamic binding properties, which are attributed to the built-in electric field effect generated within the homojunction interface layer.In summary, the porous structure of HTO@C facilitates the formation of continuous conductive pathways and provides physical adsorption capacity, while the integration of TiO₂ nanoparticles constructs a heterojunction structure that promotes mass transfer and enhances reaction kinetics. Furthermore, the built-in electric field formed by the optimized HTO structure generates strong interfacial interactions with soluble polyiodide ions, thereby enhancing the catalytic conversion effect on polyiodides.

Figure 5 In-situ tests and computational analysis.

As shown in Figures 6a-b, zinc-iodine pouch cells were assembled in the study using a hydrogel electrolyte. A power supply system composed of two pouch cells connected in series was able to power an LED panel displaying the "JNU" logo. Even when subjected to various states such as side folding, double folding, rolling, or piercing, its power supply performance remained unaffected, demonstrating the battery's excellent mechanical flexibility, foldability, and high safety. At different bending angles ranging from 0° to 180°, the battery's capacity retention rate reached as high as 95.54% (Figure 6c); after 50 charge-discharge cycles, its performance showed no significant decline, exhibiting excellent cycling stability. This series of results indicates that the I₂/HTO@C cathode holds great potential in battery applications, being particularly suitable for integration into wearable textiles.Furthermore, by combining I₂/HTO@C active material with a carbon nanotube array coating, composite yarns were fabricated. These were then integrated with zinc wire anodes and hydrogel electrolyte to construct self-powered storage units. These zinc-iodine yarn batteries can function as polycrystalline silicon solar cells (6V, 150mA), efficiently collecting and storing solar energy during the daytime for powering functional electronic devices. A system composed of four zinc-iodine yarn batteries connected in series could easily illuminate the "JNU" LED logo (Figures 6d-e).

Figure 6 Pouch and yarn battery demonstrations.

In summary, this study developed a novel iodine host material (HTO@C) through interface band engineering. This material features a synergistic heterojunction/homojunction structure specifically designed for high-efficiency cathodes in zinc-iodine batteries. Based on this unique synergistic structure, the HTO@C host effectively suppresses the formation of soluble polyiodides and enhances interfacial reaction kinetics. Experimental results demonstrate that zinc-iodine batteries using I₂/HTO@C as the cathode exhibit outstanding rate performance and ultra-long cycling stability, maintaining a capacity retention rate of 98.9% after over 50,000 cycles. Furthermore, the I₂/HTO@C cathode also possesses excellent mechanical properties, making it suitable for various battery configurations such as pouch cells and yarn batteries. In conclusion, this research not only proposes a feasible method for preparing iodine host cathodes with enhanced redox kinetics through interface band engineering but also opens new pathways for the development of batteries with superior mechanical flexibility.