Understanding the Na+ Conductance and Failure Evolution of Ceramic Electrolytes for Solid‐State Sodium Metal Batteries from Ion Dynamics

Ceramic electrolytes (CEs) offer high ionic conductivity and exceptional stability for improving performance and safety in solid‐state sodium metal batteries (SSMBs). However, inevitable defects and disorders in polycrystalline materials pose challenges in understanding ion transport and failure mechanisms within CEs. Addressing these issues requires decoupling the complex ionic dynamic processes. Herein, Na 3 Zr 2 Si 2 PO 12 (NZSP) CEs are prepared with varying defect concentrations and crystal chemistry by NaF addition, and conducted both qualitative and semi‐quantitative analyses to diagnose their failure evolution upon electrochemical cycling with metallic sodium electrodes. Dynamic characterizations reveal the correlation between relaxation behavior and atomic interactions in the amorphous interphase at grain boundaries. Furthermore, two models are proposed to describe the failure evolution of CEs: one dominated by dendrite growth and the other dominated by solid‐electrolyte interphase (SEI) formation during sodium plating/stripping. In the latter model, the SEI relaxation distribution function (γSEI) derived from the distribution of relaxation times (DRT) analysis, assessed by electrochemical impedance spectroscopy, serves as a criterion for determining and predicting the direction of failure evolution. This work provides new insights into failure evolution mechanisms of CEs and demonstrates the effectiveness of DRT‐based diagnosis for optimizing solid‐state sodium metal batteries.