Structure transformations and ionic conductivity in germanides of sodium and potassium

Abstract

In this study, structural transformations and ionic conductivity of sodium and potassium germanides were investigated using density functional theory and molecular dynamics simulations with machine learning interatomic potentials. Thermodynamically stable and metastable phases of Na–Ge and K–Ge systems were identified, confirming previously predicted NaGe, Na₂Ge, and Na₉Ge₄ as stable in the Na–Ge system, and K₄Ge₂₃, K₃Ge₁₇, and KGe in the K–Ge system. Thermal stability and ionic conductivity were analyzed, revealing that several metastable Na–Ge structures remain kinetically stable up to 600 K. Most Na–Ge phases have high ionic conductivity up to 10⁻² S cm⁻¹ at room temperature, due to low diffusion activation barriers and interconnected diffusion paths. In contrast, K–Ge phases exhibit limited structural diversity and diffusion, primarily vacancy-driven, with ionic conductivity an order of magnitude lower than Na–Ge compounds. The use of machine learning potentials allowed us to study large systems (several thousands atoms), and long (several nanoseconds) molecular dynamics runs with ab initio accuracy. Our findings suggest that Na–Ge and K–Ge compounds hold potential as anode materials due to their favorable ionic conductivity and stability at moderate and elevated temperatures.