Programming the electrostatic landscape for fast Na⁺ transport in NASICON solid electrolytes

Fast ion transport in solid electrolytes is typically optimized through geometric diffusion pathways and carrier concentration, yet the governing role of the electrostatic energy landscape remains underexplored. Here, we demonstrate that ion transport can be systematically programmed via electrostatic landscape engineering. Using aliovalent Tm³⁺-doped Na₃Zr₂Si₂PO₁₂ as a model system, substitution at Zr⁴⁺ sites induces charge-compensation-driven Na⁺ enrichment and redistributes framework electron density, thereby weakening ion-lattice interactions and flattening migration energy barriers. This coupled electronic-structural modulation expands diffusion pathways and promotes densification across multiple length scales. As a result, the optimized electrolyte exhibits a high ionic conductivity of 1.21 mS cm^-1, ultralow electronic conductivity (1.2 × 10^-8 S cm^-1), and improved compatibility with Na metal. Solid-state cells deliver 97.3% capacity retention over 300 cycles at 1C. More broadly, this work establishes electronic-structure-based electrostatic-landscape programming as a general design principle for fast ion conduction and interfacial stability in solid electrolytes.