MXene‐Derived TinO2n−1 Quantum Dots Distributed on Porous Carbon Nanosheets for Stable and Long‐Life Li–S Batteries: Enhanced Polysulfide Mediation via Defect Engineering

The application of Li-S batteries has been hindered by the shuttling behavior and sluggish reaction kinetics of polysulfides. Here an effective polysulfide immobilizer and catalytic promoter is developed by proposing oxygen-vacancy-rich Ti_n O_{2 n -1} quantum dots (OV-T_n QDs) decorated on porous carbon nanosheets (PCN), which are modulated using Ti₃ C₂ T_x MXene as starting materials. The T_n QDs not only confine polysulfides through strong chemisorption but also promote polysulfide conversion via redox-active catalysis. The introduction of oxygen vacancies further boosts the immobilization and conversion of polysulfides by lowering the adsorption energy and shortening the bond lengths. The PCN provides a physical polysulfide confinement as well as a flexible substrate preventing OV-T_n QDs from aggregation. Moreover, the two building blocks are conductive, thereby effectively improving the electron/charge transfer. Finally, the ultrasmall size of QDs along with the porous structure endows OV-T_n QDs@PCN with large specific surface area and pore volume, affording adequate space for S loading and volume expansion. Therefore, the OV-T_n QDs@PCN/S delivers a high S loading (79.1 wt%), good rate capability (672 mA h g^-1 at 2 C), and excellent long-term cyclability (88% capacity retention over 1000 cycles at 2 C). It also exhibits good Li⁺ storage under high S-mass loading and lean electrolyte.