Controlling Interfacial Chemistry in Nanoconfined NaZn(BH ₄ ) ₃ Enables Superior Sodium-Ion Conductivity

Sodium-based bimetallic borohydrides are attractive solid-state ion conductors but suffer from intrinsically low room-temperature ionic conductivity. Here, we enhance Na+ transport in NaZn(BH4)3 via nanoconfinement within mesoporous oxides. Using an ultrasound-assisted liquid-phase infiltration method, NaZn(BH4)3 was incorporated into SBA-15 (SiO2) and γ-Al2O3, achieving room-temperature ionic conductivities of 1.16 × 10–5 and 3.85 × 10–4 S cm–1, respectively, 2–3 orders of magnitude higher than that of the pristine material. Pair distribution function analysis revealed local structural rearrangements, including the emergence of Na–O correlations, indicative of altered Na+ coordination and migration topology. X-ray absorption fine structure spectroscopy revealed a modified Zn coordination environment at the oxide–hydride interface, consistent with the formation of a defect-rich, conductive interfacial phase. These structural modifications coincided with a significant reduction in Na+ migration activation energy, suggesting a cooperative effect of interfacial phase formation, space-charge potentials, and lattice disorder. This work establishes nanoconfinement-driven interfacial chemistry engineering as a broadly applicable approach for achieving high ionic conductivity in borohydride electrolytes, offering opportunities for high-performance, safe, and lightweight solid-state sodium batteries.