Transforming Interfacial Reactivity Into Stability for Durable High‐Current Solid‐State Sodium Batteries

Interfacial instability remains the key obstacle to reliable oxide‐based solid‐state batteries (SSBs). Here we demonstrate a monolithic, self‐regulating mixed ionic‐electronic conducting (MIEC) interface that transforms interfacial reactivity into long‐term stability in SSBs. Introducing cobalt into NASICON‐type Na 3 Zr 2 Si 2 PO 12 (NZSP) yields a dual‐phase NaCoPO 4 /NZSP composite electrolyte, which evolves during cycling into a nanoporous interphase containing Co nanoparticles embedded in NASICON matrix. This reaction‑derived interphase enlarges the active area, homogenizes ion flux, and guides uniform sodium deposition. Extending this concept to a tri‐layer electrolyte architecture with Co‐modified outer layers and pristine NZSP core enables a self‐limiting reaction stabilizing both interfaces. Optimized cells achieve a critical current density of 7.3 mA cm −2 at 60°C and sustain symmetric‐cell cycling over 3000 h at 1 mA cm −2 . Full cells deliver >99% capacity retention over 1200 cycles at 2 C. This work establishes interfacial chemistry as a tunable design principle for durable, high‐current solid‐state metal batteries.