Bifunctional Al Dopant for Enhancing Bulk and Grain Boundary Conductivities in Sodium Ion Conducting NASICON Ceramics

Compared to Li⁺ and Na⁺ ion-conducting sulfides with conductivities of ≈10⁻² S cm⁻¹ at room temperature, oxide ceramics typically exhibit conductivities an order of magnitude lower, rendering them less attractive as electrolytes for all-solid-state batteries (ASSBs). This study presents NASICON-based electrolyte, achieving a remarkable conductivity of 6.0 × 10⁻³ S cm⁻¹ at room temperature, rivaling that of sulfide-based electrolytes. This is accomplished by optimizing the composition in the range Na_3+x+yZr_2−yAl_ySi_2+xP_1−xO₁₂ (0.10 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.1). Despite the issue of reduced sinterability with increasing Si/P ratio due to the formation of a viscous SiO₂-rich grain boundary interphase, the addition of Al₂O₃ effectively reduces the viscosity and improves the sinterability. In other words, a strategy of engineering the liquid phase that is in situ generated from the host phase is viable. The enhanced conductivity is attributed not only to the lowered grain boundary resistivity but also to lattice expansion from modified Na occupations in the crystal structure. Furthermore, this material demonstrates a wide electrochemical window, suppressed partial electronic conductivity, low polarization voltage in direct contact with a Na anode, and charge-discharge cycles with minimal polarization when directly interfaced with a NASICON-type cathode, repositioning it as a promising electrolyte for ceramic ASSBs.