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, Sc1- xZnxBO3- 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.