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 investigatedforpotential applications as anodes in batteries due to their cage-likestructures that can accommodate electrochemical insertion of guestions. To synthesize type II Ge clathrates (Ge-136), severalexperimental routes use thermal or electrochemical desodiation ofthe Zintl phase compound Na4Ge4. However, themechanism by which Na atoms are removed from the precursor to formclathrates is not well understood. Herein, we use first-principlesdensity functional theory and nudged elastic band calculations tounderstand the reaction mechanism and formation energies of the productstypically observed in the synthesis, namely, Na & delta;Ge136 (0 < & delta; < 24) type II clathrates and hexagonalphase Na1-x Ge3+z . Specifically, we confirm the energetic feasibility of Navacancy formation in Na4Ge4 and find that thebarrier for Na vacancy migration is only 0.37 eV. This relativelylow energy barrier is consistent with the ease with which Na4Ge4 can be desodiated to form the products. We also discussthe energetics, sodium migration pathways, and potential electrochemicalperformance of Ge-136 as anode material for Na-ion batteries.Overall, this study highlights how first-principles calculations canbe used to understand the synthesis mechanism and desodiation processesin clathrate materials and will help guide researchers in the designand evaluation of new open framework compounds as viable materialsfor energy storage applications.