Highly stable Na₃Zr₂Si₂PO₁₂ composite electrolytes for all-solid-state sodium Na-metal batteries enabled by high entropy and interfacial regulation.

Practical realization of solid-state sodium metal batteries is largely restricted by inadequate electrode-electrolyte interfacial compatibility and uncontrolled dendrite propagation. This study proposes an innovative "bulk-interface" synergistic optimization strategy. By coupling high-entropy design with interface engineering, a composite system comprising monoclinic-phase and rhombohedral-phase crystalline domains and amorphous phases is successfully constructed. A toughening mechanism based on "hard crystalline grains-soft amorphous phases" is established. The rhombohedral and amorphous phases broaden ion migration pathways, reduce the Na+ migration energy barrier, effectively alleviate volume expansion, and optimize ion transport pathways. By constructing a SnI2/Na metal interface layer, SnI₂ exhibits high reactivity owing to its low work function and weak SnI bonds, promoting the in situ formation of an SEI layer. The in situ reaction between SnI₂ and Na metal generates a dense NaI/Na15Sn4 interface layer, effectively suppressing dendrite growth. This interface layer reduces the interfacial resistance to 4.5 Ω·cm2. The symmetric cell achieves stable cycling for over 3000 h at 0.1 mA·cm-2 and delivers a high current density of 1.6 mA·cm-2. A half-cell assembled with a Na3V2(PO4)3 cathode exhibits an initial capacity of 110.51 mAh·g-1 at 0.5C, with a 91.3% capacity retention after 200 cycles. This system also shows good compatibility with the high-voltage Na3V2P2O10F cathode (operating voltage window >4.2 V). Through multiscale structural regulation, this study reveals the structure-activity relationship of high-entropy ionic conductors, providing a new material platform and design strategy for high-performance all-solid-state sodium batteries.