MOF-derived TiO2@NPC@S: A high-performance cathode material for lithium-sulfur batteries

In recent years, lithium-ion batteries have been widely applied in portable electronic devices and electric vehicles, playing a crucial role in modern life. However, the capacity of their traditional cathode electrodes is approaching the theoretical limit, which has spurred the scientific community to actively explore alternative energy storage solutions.

Lithium-sulfur batteries (LSBs) stand out as a promising alternative, boasting a high theoretical specific capacity of 1675 mAh/g and an energy density of 2500 Wh/kg. Nevertheless, LSBs face several inherent challenges. Sulfur and its discharged intermediates, like Li₂S₂ and Li₂S, are insulative, resulting in sluggish redox reactions and poor ion transportation. This leads to low electrochemical utilization and cycling instability. During charge-discharge, the cathode experiences an 80% volume expansion when S₈ converts to Li₂S, which damages the electrode structure and shortens the battery's cycling life. Additionally, the "shuttle effect" caused by soluble polysulfides (Li₂Sn, 2 < n ≤ 8) leads to self-discharge and reduced sulfur utilization.

A research team from Shanghai Jiao Tong University has developed a novel solution. They synthesized a MOF (Metal-Organic Framework)-derived hierarchical porous TiO₂@NPC (Nanoporous Carbon) and further fabricated the TiO₂@NPC@S cathode with high sulfur loading.

The preparation process involves multiple steps. First, MOFs precursors were synthesized by stirring phthalic acid and tetrabutyltitanate in a mixture of N, N-dimethylformamide and methanol at room temperature, followed by ultrasonic treatment and intense stirring. The mixture was then heated in a hydrothermal kettle at 155 °C for 20 hours, and after washing and drying, the MOFs precursors were obtained. These precursors were carbonized at 500 °C for 12 hours under a nitrogen atmosphere in a high-temperature tube furnace to form TiO₂@NPC. Finally, TiO₂@NPC was mixed with sublimed sulfur at a mass ratio of 3:7, vacuum-sealed, and heated at 160 °C for 12 hours in a muffle furnace to yield TiO₂@NPC@S.

To comprehensively analyze the materials, the researchers employed various characterization techniques. SEM and TEM images revealed that TiO₂@NPC had a regular 3D pill-structure with a porous hierarchical architecture. After sulfur storage, the pores in TiO₂@NPC@S appeared filled, indicating successful sulfur infiltration and immobilization. XRD analysis confirmed the anatase structure of TiO₂@NPC, and the weak sulfur-related diffraction peaks in TiO₂@NPC@S suggested good sulfur dispersion. XPS spectra further verified the formation of chemical bonds like O-S and Ti-S, demonstrating a strong chemical anchoring effect on sulfur, which helps mitigate the shuttle effect. TG analysis determined that the sulfur content in TiO₂@NPC@S was approximately 64.09%. Nitrogen adsorption-desorption tests showed that TiO₂@NPC had a multi-level pore structure with a BET specific surface area of 155.3428 m²/g, facilitating electrolyte infiltration and accommodating sulfur volume expansion.

Electrochemical measurements demonstrated the excellent performance of the TiO₂@NPC@S electrode. In galvanostatic charge-discharge tests at 0.5 C, it achieved an initial capacity of 1327.35 mAh/g. Even after 300 cycles, the capacity remained at 601.54 mAh/g, with an average capacity decay of only 0.16% per cycle, far surpassing the commercial Y-50@S material. In rate performance tests, at 1 C, the capacity was 928 mAh/g, and at 1.5 C, it reached 743 mAh/g, while the Y-50@S electrode's capacity declined rapidly at higher rates. EIS tests indicated that TiO₂@NPC@S had a significantly lower charge-transfer resistance, enabling faster reaction kinetics and better conductivity.

This research provides an innovative method for designing LSB cathodes. By addressing the key issues of LSBs, the TiO₂@NPC@S cathode holds great potential for promoting the development of high-performance energy storage devices, which may have a far-reaching impact on the future of sustainable energy applications.

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