Na₂Ti₃O₇-Assisted Grain-Boundary Tailoring of NASICON Electrolytes for Room-to-Subzero-Temperature All-Solid-State...

NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) is a promising solid electrolyte for all-solid-state sodium metal batteries, yet its practical performance is restricted by poor ceramic densification, sluggish grain-boundary Na⁺ transport, and unstable sodium-metal interfaces. Herein, we report a reaction-derived heterointerface engineering strategy guided by an adhesion-migration dual criterion. Theoretical screening identifies TiO₂ as an optimal rigid oxide heterophase that combines favorable interfacial adhesion with a low Na⁺ migration barrier at the NZSP interface. Instead of directly adding TiO₂, Na₂Ti₃O₇ is introduced as a reactive sintering aid to generate TiO₂-derived nanocrystals and Na-containing species in situ during sintering, enabling simultaneous grain-boundary reconstruction, sodium-loss compensation, and ceramic densification. NZSP-4 wt.% NTO (NZSP-Ti4) shows 94.20% relative density, 1.65 mS cm^-1 room-temperature conductivity, and a 0.27 eV activation energy. It also enables a 1.55 mA cm^-2 critical current density and over 1600 h of reversible Na cycling at 0.2 mA cm^-2. Coupled with an integrated NZSP/Na₃V₂(PO₄)₃ cathode architecture, liquid-free and pressure-free full cells deliver durable cycling and stable subzero operation. This work establishes a rational route for reactive grain-boundary engineering in ceramic solid electrolytes.