Synergistic Effect of Anatase/Rutile Nanoparticle and Nitrogen Doping on NH₂-MIL-125(Ti)-Derived Porous Carbon as High Performance Lithium-Sulfur Battery Cathodes

Lithium-sulfur (Li-S) batteries have garnered significant attention as a future energy storage systems owing to their high theoretical capacity, energy density, and abundance of sulfur in nature. However, their development has been hindered by the polysulfide shuttle effect, which adversely affects the cycling stability of Li-S batteries. Various approaches – such as designing the pore structure of porous carbon, or introducing heteroatoms and/or metal oxides into the porous carbon – have been applied to address these issues. For instance, Yang et al. controlled the polysulfide adsorption on the porous carbon cathodes which depends on the phase of titanium dioxide (TiO₂) particles (<i>i.e.</i>, anatase or rutile) introduced in the electrodes^[1]. Furthermore, Kim et al. improved the retention of Li-S battery via coupled effects of nitrogen (N) vacancies and the rutile phase of TiO_2-x^[2]. However, the synergistic effects from the correlation among these factors (<i>e.g.</i>, heteroatom doping and phase type of TiO₂ particles) have not yet been systematically studied.<br/>In this work, we present a micro-sized NH₂-MIL-125(Ti) metal organic framework (MOF)-derived porous carbon (<i>micro</i>-cNMT) as a host material of sulfur for Li-S battery cathodes. <i>micro</i>-cNMT possesses a hierarchical micro-/meso-porous structure, N heteroatoms, and mixed phases of TiO₂ nanoparticles (<i>i.e.</i>, both anatase and rutile). These parameters collectively enhanced the performance of Li-S batteries through their synergistic effect. Specifically, <i>micro</i>-cNMT enhanced the prevention of polysulfide shuttle effects by the cooperation of physical and chemical adsorptions, facilitated by its hierarchical porous structure and N/TiO₂ nanoparticle doping, respectively. Porous structure of <i>micro</i>-cNMT not only accommodated active sulfur and its volume transition, but also facilitated Li-ion diffusion, which is confirmed by both cyclic voltammetry (CV) at various scan rates and Randles-Sevcik equation. Anatase and rutile phases of TiO₂ nanoparticles preferentially adsorbed short-chain polysulfide and long-chain polysulfide, respectively, preventing the loss of active sulfur throughout the entire discharging process. The long-chain polysulfide adsorption was confirmed by direct visualization of Li₂S₆ migrating from electrolyte to the <i>micro</i>-cNMT. Furthermore, scanning electron microscope (SEM) images of cathodes after 100 cycles of discharge-charge and operando X-ray diffraction (XRD) measurement during the first discharge demonstrated that the <i>micro</i>-cNMT enhances short-chain polysulfide adsorption, thereby facilitating sulfur reduction reactions from polysulfide to Li₂S. As a result, the <i>micro</i>-cNMT cathode improved cycling stability owing to a synergistic effect from N and mixed phases of TiO₂ (0.39% decay at 0.1 C for 100 cycles), compared to the control group without N or with a single phase of TiO₂ nanoparticles (either anatase or rutile only).<br/><br/>References<br/>Ziyi Yang, Chengxin Peng, Ruijin Meng, Lianhai Zu, Yutong Feng, Bingjie Chen,† Yongli Mi, Chi Zhang, & Jinhu Yang., <i>ACS Cent. Sci.</i>, 5, 1876-1883, <b>(2019)</b><br/>Kim, H., Yang, J., Gim, H., Hwang, B., Byeon, A., Lee, K. H., & Lee, J. W., <i>Electrochim Acta.</i>, 408, 139924, <b>(2022)</b>