Superionic Conductivity Facilitated by Correlated Migration in Sodium Silicate Solid Electrolytes

Sodium solid‐state batteries are attractive for next‐generation energy storage due to their high energy density and inherent safety. However, their practical application is hindered by the scarcity of solid electrolytes that simultaneously exhibit high ionic conductivity and favorable electrochemical stability. Here we synthesized a series of Na‐rich silicates, Na 5 MSi 4 O 12 (M = In, Yb, Er, Y, Gd), and systematically evaluated their ion transport properties and electrochemical stability. The optimized composition achieves a room‐temperature ionic conductivity of 2.4 mS cm −1 . Molecular dynamics simulations reveal that the high ionic conductivity is mainly attributed to correlated Na + ‐ion migration. Beyond fast ion transport, Na 5 MSi 4 O 12 demonstrates good compatibility with sodium metal through the formation of a kinetically stabilized interface. To further improve interfacial Na + ‐ion transport kinetics, a Na─Sn─F composite anode is introduced to promote the formation of a NaF‐rich interphase, which enhances interfacial stability and leads to significantly increased critical current density. Benefiting from its high ionic conductivity and good electrochemical stability, Na 5 MSi 4 O 12 enables solid‐state cells to achieve long‐term stability over 1500 cycles with minimal capacity fading. Our findings highlight the critical role of correlated migration in endowing superionic conduction in solid electrolytes and provide guidelines for the design of fast ion conductors.