Understanding structural and electrochemical properties of metal-doped NASICON-type solid polymer electrolytes for all-solid-state sodium-metal batteries

Polymer electrolytes are contemplated as prominent contenders for rechargeable solid-state sodium-metal batteries (RSSBs) due to their notable flexibility, elevated energy density, and strengthened safety features. However, their limited cycling stability, low electrochemical performance, and inadequate ionic conductivity pose significant challenges to their practical applications. Herein, we discovered 2 eco-friendly approaches to develop advanced polymer-based solid composite electrolytes. The approach involved synthesizing sodium zirconium silico phosphate (Na₃Zr₂Si₂PO₁₂ (NZSP)) and its doped variants—Na₃Zr₁.₈Mg₀.₂Si₂PO₁₂ (NZMSP), Na₃Zr₁.₈La₀.₂Si₂PO₁₂ (NZLSP), and Na₃Zr₁.₈Sc₀.₂Si₂PO₁₂ (NZSSP)—via a sol-gel method, followed by the fabrication of the composite electrolytes using a solution casting technique, specifically Stokes’ method. To further enhance the ionic conductivity, Mg²⁺, La³⁺, and Sc³⁺ ions were introduced as dopants, replacing Zr⁴⁺ in the NZSP lattice. Among the doped fillers, the Sc³⁺substituted NZSP (NZSSP) exemplified the good ionic conductivity, reaching 0.38 × 10⁻³ S cm⁻¹. Moreover, the developed electrolytes showed excellent capability in suppressing sodium dendrite formation. The NZSSP-based electrolyte delivered remarkable electrochemical performance in sodium-metal batteries, achieving an excellent coulombic efficiency 99% and a discharge capacity of 80.9 mAh g⁻¹ over 100 cycles. These results highlight the potential of the developed polymer-infused solid composite electrolytes for enabling safe and highly productive RSSBs, paving the way for next-generation energy storage technologies.