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 Na₃Zr₂Si₂PO₁₂ (NZSP) yields a dual-phase NaCoPO₄/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.