Dual Interlocked Mediators Enable Single-Ion-Conducting Quasi-Solid-State Electrolytes for Ultrafast-Charging Long-Life Sodium Metal Batteries

Abstract Quasi-solid-state electrolytes (QSEs) are critical for ultrafast-charging yet high-safety sodium metal batteries (SMBs), yet their implementation is hindered by sluggish Na + transport in bulk and at interfaces. Here, we propose dual interlocked mediator engineering that transcends conventional independent approaches by coupling cationic Sn 2+ salt with anionic difluoro(oxalato)borate (DFOB⁻) salts to simultaneously regulating bulk ion transport and bilateral interface chemistry. During QSE preparation, Sn 2+ initiates in situ cationic polymerization, while DFOB⁻ acts as a retarding agent to suppress runaway polymerization. The first interlocking effect in the Sn-FB QSE bulk builds a uniform network, enabling near-unity Na + transference number (0.94) and robust puncture strength (8.5 kPa). During cell operation, Sn 2+ is reduced to form a hybrid NaSn alloy-based solid-electrolyte interphase, while DFOB⁻ oxidizes to generate a robust yet thin cathode–electrolyte interphase, respectively. This second interlocking effect creates adaptable bilateral interphases that facilitate Na + diffusion and mitigate interfacial degradation. As a result, the symmetric cells exhibit 6000 h stability, and full cells retain 80.1 mAh g –1 at an ultrafast-charging rate of 15C and retain 90% capacity at 3C over 2000 cycles. Furthermore, high-mass-loading full cells and pressure-free pouch cells are demonstrated, underscoring the potential of dual interlocked mediator engineering for practical SMBs.