Hierarchical Porous Cathode for High-Capacity Lithium-Sulfur Batteries

A research team at Shanghai Jiao Tong University developed a hierarchical porous TiO₂@NPC (Nanoporous Carbon) derived from a Metal-Organic Framework (MOF), and subsequently created a TiO₂@NPC@S cathode with a high sulfur loading.

Lithium-ion batteries are widely used in devices like electric cars and portable electronics. However, as the capacity of traditional cathode electrodes nears their theoretical limits, researchers are looking for alternative energy storage options.

Lithium-sulfur batteries (LSBs) are a promising alternative because they offer an energy density of 2500 Wh/kg and a high theoretical capacity of 1675 mAh/g. However, LSBs have some challenges. Sulfur and its by-products, like Li₂S₂ and Li₂S, are poor conductors, which slows down reactions and affects ion transport.

This leads to instability during charging and discharging, as well as low efficiency. The conversion of sulfur (S8) to Li₂S causes a significant volume increase of 80 %, which damages the cathode and reduces the battery’s lifespan. Additionally, the "shuttle effect" of soluble polysulfides (Li₂Sn, 2 < n ≤ 8) reduces the amount of sulfur used and causes self-discharge.

The preparation process involves several steps. First, to create the MOF precursors, phthalic acid and tetrabutyltitanate were stirred at room temperature in a solution of N, N-dimethylformamide and methanol. The mixture was then treated with strong swirling and ultrasonic treatment.

After heating the mixture for 20 hours at 155 °C in a hydrothermal kettle, it was cleaned and dried to produce the MOF precursors. These precursors were then carbonized in a nitrogen environment at 500 °C for 12 hours in a high-temperature tube furnace to create TiO₂@NPC. Finally, to make TiO₂@NPC@S, TiO₂@NPC was mixed with sublimed sulfur at a mass ratio of 3:7, vacuum-sealed, and heated for 12 hours at 160 °C in a muffle furnace.

The materials were characterized using various techniques. TiO₂@NPC showed a regular 3D pill structure with a hierarchical porous architecture, as seen in SEM and TEM images. After sulfur storage, the pores in TiO₂@NPC@S appeared to be filled, indicating effective sulfur immobilization. The faint sulfur-related diffraction peaks in TiO₂@NPC@S suggested efficient sulfur dispersion, and XRD confirmed the anatase structure of TiO₂@NPC.

XPS spectra further confirmed the creation of O-S and Ti-S chemical bonds, which help anchor the sulfur and reduce the shuttle effect. TG analysis showed that TiO₂@NPC@S contained nearly 64.09 % sulfur. Nitrogen adsorption-desorption studies revealed that TiO₂@NPC had a multi-level pore structure with a BET specific surface area of 155.3428 m²/g, which enabled sulfur volume expansion and facilitated electrolyte infiltration.

The performance of the TiO₂@NPC@Sl electrode was impressive in electrochemical experiments. Its initial capacity in galvanostatic charge-discharge tests at 0.5 C was 1327.35 mAh/g. With an average capacity loss of only 0.16 % per cycle, it maintained a capacity of 601.54 mAh/g after 300 cycles, outperforming the commercial Y-50@S material. In rate performance tests, it showed 928 mAh/g at 1 C and 743 mAh/g at 1.5 C.

In contrast, the Y-50@S electrode quickly lost capacity at higher rates. EIS studies showed that TiO₂@NPC@S had significantly reduced charge-transfer resistance, improving conductivity and enabling faster reaction kinetics.

The study presents a new approach to LSB cathode design. The TiO₂@NPC@S cathode has great potential to improve the development of high-performance energy storage devices by addressing the main challenges with LSBs. This could significantly influence the future of sustainable energy applications.

Journal Reference:

Shi, P., et al. (2025) MOF-derived 3D hierarchical porous TiO₂ @ NPC @ S as high-performance cathodes for Li-S batteries. Carbon Future. doi.org/10.26599/CF.2025.9200035