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

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.