Minimal Transport Units Govern Oxygen-Defect Stabilization and Transport for Lightweight Solid Electrolytes

Solid electrolytes are central to electrochemical energy technologies, including fuel cells, sensors, catalysis, membrane separation, and electrolyser. However, most established oxide-ion solid electrolytes are built around heavy B-site cations embedded in rigid and highly connected coordination frameworks, leading to widespread high-weight and sluggish ionic transport that is strongly coupled to large-amplitude lattice relaxations. This intrinsic challenge hinders further performance optimization and constrains the rational design of lightweight electrolytes. Herein, we propose a minimal transport unit-based design paradigm that combines simplified structural motifs with light-element chemistry, enabled by the exceptional flexibility of B-O polyhedra in coordination, rotation, deformation, and connectivity. As a proof of concept, Sc_{1- x}Zn_xBO_{3- x /2}, constructed from isolated BO₃ units, exhibits high oxide ion conductivity (σ(1000°C) ∼ 1.5 × 10^-2 S/cm), alongside excellent thermo-mechanical stability. Oxygen vacancies are stabilized through the formation of B₂O₅ units rather than isolated BO₂ species. Long-range oxide-ion migration is mediated by dynamic oxygen exchange between minimal BO₃ and B₂O₅ units via continuous breaking and reforming of B₂O₅ units, with transient BO₂ configurations as intermediates. This study demonstrates minimal transport units as a governing principle for defect stabilization and ionic conduction in lightweight solid electrolytes, offering a general design framework for portable and scalable high-temperature energy technologies.