Interstitial or interstitialcy: effect of the cation size on the migration mechanism in NaSICON materials.

Abstract

Sodium superionic conductors (NaSICONs) with general formula NaM2A3O12 have attracted significant attention as solid electrolytes for all solid-state batteries owing to their remarkable room temperature ionic conductivity in the order of 10-3 S cm-1. Their flexible structural framework, which allows the incorporation of various aliovalent cations, affects the Na+ ion transport. However, establishing a straightforward correlation between Na+ mobility and NaSICON composition proves challenging due to competing influences such as framework alteration and stoichiometric changes of the cation substituents and thus the mobile Na+ ions. Therefore, we systematically investigate the NaSICON system across various Na1+xM2SixP3-xO12 compositions. We unravel and examine independently two key aspects impacting the Na+ ion transport in NaSICONs: structural factors determined by introduced M4+ framework cations and the substitution level (x). By employing DFT calculations, we explore the interstitial- and interstitialcy-like migration mechanisms, revealing that these mechanisms and the associated migration energies are primarily influenced by metastable transient states traversed during the Na+ ion migration. The stability of these transient states, in turn, depends on the spatial arrangement of the Na+ ions, the size of the M4+ cations defining the structural framework, and x. This study enhances our fundamental understanding of Na+ ion migration within NaSICONs across a wide range of compositions. The findings offer valuable insights into the microscopic aspects of NaSICON materials and provide essential guidance for prospective studies in this field.