Thermal Stability Assessment of Sodium Solid Electrolytes

ABSTRACT All‐solid‐state batteries (ASSBs) are widely regarded as promising nextgeneration energy storage systems due to their high energy density and enhanced safety. Na‐based ASSBs, in particular, offer compelling advantages through the use of earth‐abundant and low‐cost materials. However, a critical knowledge gap remains regarding the thermal stability of sodium solid electrolytes (SEs)—especially their reactivity with electrode materials—hindering reliable safety assessment. Herein, we present a systematic comparison of the thermal behavior of six representative sodium SEs: Na 3 Zr 2 Si 2 PO 12 , Na 3 PS 4 , NaAlCl 4 , Na 2 ZrCl 6 , NaAlCl 2.5 O 0.75, and PEO. While inorganic SEs demonstrate good intrinsic thermal stability, most exhibit significant exothermic reactions with cathode or anode materials upon heating, releasing considerable heat that could trigger thermal runaway. Notably, chloride‐based SEs show markedly different reactivities—NaAlCl 4 reacts violently with Na 15 Sn 4 , whereas Na 2 ZrCl 6 remains remarkably stable, highlighting the crucial role of reaction kinetics and melting point in modulating thermal stability. The findings reveal that electrode–electrolyte compatibility under thermal stress—not just the stability of the SE alone—is a decisive factor for ASSB safety. Our work underscores the critical role of electrode‐electrolyte thermal stability, offering new insights into the safety design of all‐solid‐state batteries.