Influence of boron substitution at Si-site and associated interplay between structure, water-stability, ion-transport and interfacial resistance of NASICON-structured Na3Zr2Si2PO12-based solid electrolyte

Abstract

Boron (B³⁺) substitution for Si⁴⁺ in the tetrahedral site of NASICON-structured Na₃Zr₂Si₂PO₁₂-based polycrystalline solid electrolyte (NZSP), as confirmed via diffraction and solid-state ¹¹B NMR, has been found to improve the sinter-density to ∼98.4 %ρ_th (from ∼93.2 %ρ_th) and enlarge the grain size by ∼73 %. The combination of enhanced grain/bulk Na-ion conductivity due to increment in the Na-content and lowered grain-boundary resistance due to enlarged grain size, as well as accrued sinter-density, results in an overall enhancement of Na-ion conductivity by ∼143 %. Shrinkage of unit cell and reduction in grain boundary area also render B³⁺-substituted NZSP 'water-stable'. Furthermore, B³⁺-substitution improves mechanical properties and lowers Na/NZSP interfacial resistance (by ∼58 %; to ∼194 Ω cm²), which are beneficial towards suppression of Na-dendrite growth/penetration. Overall, this work not only reveals an interesting interplay between the above aspects for inorganic ion conductors, but also showcases the importance of adopting an integrated strategy towards the development of superior solid electrolytes for next-generation solid-state batteries.