Imaging Interfacial Liquid-Solid Phase Transformations in Solid-State Na-Ion Batteries with Liquid Anodes

Batteries will play an essential role in a low-carbon, sustainable and electrified energy landscape. In particular, the rapid growth in adoption of electric vehicles will require terawatt hour capacities of lithium-ion batteries. This massive growth is motivating beyond lithium chemistries (e.g., sodium, zinc, etc.) for growing demands in stationary storage for homes and grid applications. Currently, sodium metal has limited applications in a solid form because of irreversibility that manifest during cycling. One of the challenges with building solid-state sodium metal batteries is the need for continuous solid-solid interfacial contact during cycling.¹ Herein, we examine a room-temperature sodium-potassium (NaK) liquid anode as a potential solution to maintain anode-electrolyte contact. However, it has been generally overlooked that NaK alloys only maintain its liquid form inside a certain composition window. Na metal plating/stripping during battery cycling will likely induce sufficient local compositional changes to the NaK alloy and cause liquid-solid phase transformation, which can result in contact loss.² In this work, room-temperature liquid NaK alloy anodes paired with solid-state Na₃Zr₂Si₂PO₁₂ (NZSP) electrolytes were tested to demonstrate how this liquid-solid phase transformation impacts the electrochemistry of liquid anodes. In-situ 3D X-ray tomography imaging is used to visualize the interfacial phase transformation process and elucidate non-uniform changes in composition upon electrodissolution and plating. The work seeks to examine how composition changes in liquid metal alloys impact interfacial contact and charge/ion transfer at solid interfaces. Long term this can add in the adoption of room temperature liquid metal applications for grid and stationary energy storage applications.<br/>(1) Zhao, C. et al. <i>Adv. Energy Mater.</i> <b>2018</b>, <i>8</i> (17), 1703012.<br/>(2) Leonchuk, S. S. et al. <i>J. Mater. Chem. A</i> <b>2022</b>, <i>10</i> (43), 22955–22976.