Quantifying Sodium Metal SEI Dissolution and Relation to Corrosion Using Quartz Crystal Microbalance

Sodium-metal batteries present a more abundant alternative to lithium-based batteries, with high theoretical capacity and comparable reduction potential. Ether electrolytes perform well in sodium-metal batteries under continuous cycling and are presumed to have a stable solid-electrolyte interphase (SEI). However, the solubility of sodium SEI components is a cause for concern, and SEI evolution during calendar aging is not commonly studied for the ether sodium-metal system. In this work, SEI mass formation and dissolution are quantitatively measured for NaPF6 in diglyme. For varying potential hold levels, significant SEI dissolution is observed, from 55 to 76% of the originally formed SEI. Characterization shows that in addition to organic component dissolution, inorganic salts such as NaF and NaxPOyFz are unstable in SEI during open circuit rest. This instability results in continuous corrosion of sodium metal during calendar aging, highlighting an important understanding and concern of SEI stability in sodium-metal batteries.