In Situ Embedded Catalytic Sites Accelerate Redox Kinetics for High-Performance Sodium-Sulfur Batteries

Sodium-sulfur (Na-S) batteries are hindered by sluggish sulfur conversion kinetics and polysulfides dissolution. Although catalysts are widely introduced to improve kinetics, the crucial question persists in how catalytic sites can be effectively accessible to polysulfides especially when pursuing a highly confined sulfur redox pathway. Here we bring catalyst accessibility into focus and demonstrate that a synthetic strategy based on in situ embedded catalytic sites maximizes catalytic accessibility and efficiency. Through a bottom-up, precursor-integrated host synthesis route, we in situ embedded Nb₂O₅ catalyst into carbon nanotubes in parallel with formation of extensive nanoporosity. Subsequent sulfur impregnation yields a cathode structure (I-Nb₂O₅@C-S) featuring abundant catalyst-pore-sulfur triple-phase interfaces. As a result, I-Nb₂O₅@C-S delivers among the most promising sulfur utilization in literature, achieving 1540 mAh g^-1 at 0.1 C, and 1044 mAh g^-1 at 3 C with a low-capacity decay of 0.027% per cycle over 1500 cycles. In contrast, a counterpart with spatially isolated Nb₂O₅ and sulfur exhibits deficient catalytic accessibility and negligible improvement in sulfur redox kinetics. Kinetic diagnostics, combined with niobium K-edge and operando sulfur K-edge x-ray absorption spectroscopy, reveal that direct accessibility of catalytic sites to sulfur is essential for synergizing polysulfide confinement and kinetic improvement.