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

Sodium superionic conductors (NaSICONs) with general formula NaM 2 A 3 O 12 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 Na 1+ x M 2 Si x P 3- x O 12 compositions. We unravel and examine independently two key aspects impacting the Na + ion transport in NaSICONs: structural factors determined by introduced M 4+ 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 M 4+ 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.