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

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₊M₂SiP_3-O₁₂ 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.