Solid-State Electrochemical Synthesis of Silicon Clathrates Using a Sodium-Sulfur Battery Inspired Approach

Clathrates of Tetrel elements (Si, Ge, Sn) have attracted interest for their potential use in batteries and other applications. Sodium-filled silicon clathrates are conventionally synthesized through thermal decomposition of the Zintl precursor Na4Si4, but phase selectivity of the product is often difficult to achieve. Herein, we report the selective formation of the type I clathrate Na8Si46 using electrochemical oxidation at 450 °C and 550 °C. A two-electrode cell design inspired by high-temperature sodium-sulfur batteries is employed, using Na4Si4 as working electrode, Na β″-alumina solid electrolyte, and counter electrode consisting of molten Na or Sn. Galvanostatic intermittent titration is implemented to observe the oxidation characteristics and reveals a relatively constant cell potential under quasi-equilibrium conditions, indicating a two-phase reaction between Na4Si4 and Na8Si46. We further demonstrate that the product selection and morphology can be altered by tuning the reaction temperature and Na vapor pressure. Room temperature lithiation of the synthesized Na8Si46 is evaluated for the first time, showing similar electrochemical characteristics to those in the type II clathrate Na24Si136. The results show that solid-state electrochemical oxidation of Zintl phases at high temperatures can lead to opportunities for more controlled crystal growth and a deeper understanding of the formation processes of intermetallic clathrates.