Electrochemical Synthesis of Type I Na8Si46 Clathrate with a Na Β''-Alumina Solid Electrolyte
Authors: Dopilka Andrew , Childs Amanda , Bobev Svilen , Chan Candace K. 
Published: December 2020

Intermetallic clathrates are a class of materials comprising face-sharing polyhedral cages of Tetrel (Si, Ge Sn) elements that encapsulate alkali or alkaline earth metal guest atoms. With their unique structures and tunable cage sizes, clathrates have received much interest for their thermoelectric, superconducting, optoelectronic, and electrochemical properties. Due to the large interest in Tetrel elements for Li-ion anodes, our group has been investigating the electrochemical properties of clathrates for Li-ion battery applications1–4. We recently studied the guest free Type I clathrates (i.e. Si46) with ab initio calculations and found that Li insertion is favorable into the empty cages and the migration barriers are very low (0.15-0.3 eV) suggesting possible applications as anodes. However, obtaining pure phase selection of the Si clathrates from the desodiation of the Na4Si4 Zintl phase has been difficult due to the lack of kinetic control during the reaction. To address this challenge, we demonstrate a proof-of-concept electrochemical cell which desodiates Na4Si4 at 450-550 ºC to phase selectively form the type I Na8Si46 clathrate phase.
The electrochemical cell operates at 450-550 ºC and is comprised of a Na4Si4 working electrode, Na β''-alumina solid electrolyte, and a Na or Sn counter electrode (Figure 1). The electrochemical cell is operated at a constant current using the galvanostatic intermittent titration technique (GITT) which allows for a better understanding of the reaction processes by observing the voltage under open circuit conditions. We find that desodiation of Na4Si4 with a 1-2 mA/cm2 current density at 450 ºC results in the phase selective formation of the type I Na8Si46 clathrate via a two-phase reaction mechanism. By altering the temperature and the Na vapor pressure (by choice of counter electrode), we demonstrate further control over the phase selection and product morphology.
These results demonstrate that the bulk conversion of Na4Si4 via electrochemical oxidation to Na8Si46 is possible and we believe that this system can be applied to other Zintl compound precursors. We preliminary show that type II Ge clathrate can be prepared from Na4Ge4 at 350 ºC using the same approach. Compared to other oxidation pathways to obtain intermetallic clathrates, the electrochemical method has distinct advantages. For instance, the rate of oxidation can be controlled independently of the temperature of reaction through tuning of the current density, which could lead to greater control over the reaction products. Due to the flexible nature of the electrochemical synthesis, we expect these results to have important implications for the solid-state synthesis of clathrates for Li-ion battery applications.

Figure 1 Schematics of the (a) electrochemical cell and (b) proposed electrochemical reactions. Na4Si4 is oxidized to Na8Si46 at the interface with the electrolyte via removal of Na+ and e-; the Na+ ions are reduced at the liquid metal counter electrode to form either Na metal or NaxSn.
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Figure 1