Na₃Zr₂Si₂PO₁₂: A Stable Na⁺-Ion Solid Electrolyte for Solid-State Batteries

Solid electrolytes (SEs) offer great potential as the basis for safer rechargeable batteries with high energy density. Aside from excellent ion conductivity, the stability of SEs against the highly reactive metal anode is also a prerequisite to achieve good performance in solid-state batteries (SSBs). Yet, most SEs are found to have limited thermodynamic stability and are unstable against Li/Na metal. With the combination of AC impedance spectroscopy, first-principles calculations, and in situ X-ray photoelectron spectroscopy, we unequivocally reveal that a NaSICON-structured Na3Zr2Si2PO12 electrolyte forms a kinetically stable interface against sodium metal. Prolonged galvanostatic cycling of symmetric Na|Na3Zr2Si2PO12|Na cells shows stable plating/stripping behavior of sodium metal at a current density of 0.1 mA cm-2 and an areal capacity of 0.5 mA h cm-2 at room temperature. Evaluation of Na3Zr2Si2PO12 as an electrolyte in SSBs further demonstrates its good cycling stability for over 120 cycles with very limited capacity degradation. This work provides strong evidence that Na3Zr2Si2PO12 is one of the few electrolytes that simultaneously achieve superionic conductivity and excellent chemical/electrochemical stability, making it a very promising alternative to liquid electrolytes. Our findings open up a fertile avenue of exploration for SSBs based on Na3Zr2Si2PO12 and related SEs.