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

Abstract

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 Na 4 Si 4 , but phase selectivity of the product is often difficult to achieve. Herein, we report the selective formation of the type I clathrate Na 8 Si 46 using electrochemical oxidation at 450 °C and 550 °C. A two-electrode cell design inspired by high-temperature sodium-sulfur batteries is employed, using Na 4 Si 4 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 Na 4 Si 4 and Na 8 Si 46 . 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 Na 8 Si 46 is evaluated for the first time, showing similar electrochemical characteristics to those in the type II clathrate Na 24 Si 136 . 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.