Electrochemical Flux Synthesis of Type-I Na₈Si₄₆ Clathrates: Particle Size Control Using a Solid or Molten Na-Sn Mass Transport Mediator

Sodium-filled silicon clathrates have a host of interesting properties for thermoelectric, photovoltaic, and battery applications. However, the metastability of the clathrates has made it difficult to synthesize them with the desired morphology and crystallite size. Herein, we demonstrate an electrochemical method whereby Na₄Si₄ dissolved in a Sn-based flux is converted to the Na₈Si₄₆ type-I clathrate using galvanostatic (constant current) oxidation. The temperature has a large influence on the products, with the reactions at 485 °C resulting in clathrates with small particle sizes (1-2 μm), while larger single crystals are obtained at 538 °C. The difference in microstructure is attributed to the solid vs liquid state of the Na-Sn phase at the reaction temperature, which is supported by the observed voltage profiles. The demonstrated method is promising for the tunable growth of Si clathrates and could be applicable to a broad range of intermetallic compounds.