Type II Germanium Clathrates from Zintl Phase Precursor Na₄Ge₄: Understanding Desodiation Processes and Sodium Migration Using First-Principles Calculations

Type II germanium clathrates have recently been investigated for potential applications as anodes in batteries due to their cage-like structures that can accommodate electrochemical insertion of guest ions. To synthesize type II Ge clathrates (Ge₁₃₆), several experimental routes use thermal or electrochemical desodiation of the Zintl phase compound Na₄Ge₄. However, the mechanism by which Na atoms are removed from the precursor to form clathrates is not well understood. Herein, we use first-principles density functional theory and nudged elastic band calculations to understand the reaction mechanism and formation energies of the products typically observed in the synthesis, namely, Na_δGe₁₃₆ (0 < δ < 24) type II clathrates and hexagonal phase Na_1–xGe_3+z. Specifically, we confirm the energetic feasibility of Na vacancy formation in Na₄Ge₄ and find that the barrier for Na vacancy migration is only 0.37 eV. This relatively low energy barrier is consistent with the ease with which Na₄Ge₄ can be desodiated to form the products. We also discuss the energetics, sodium migration pathways, and potential electrochemical performance of Ge₁₃₆ as anode material for Na-ion batteries. Overall, this study highlights how first-principles calculations can be used to understand the synthesis mechanism and desodiation processes in clathrate materials and will help guide researchers in the design and evaluation of new open framework compounds as viable materials for energy storage applications.