Unraveling propylene oxide formation in alkali metal batteries.

Abstract

The increasing need for electrochemical energy storage drives the development of post-lithium battery systems. Among the most promising battery types are sodium-based battery systems. However, like its lithium predecessor, sodium batteries suffer from various issues like parasitic side reactions, which lead to a loss of active sodium inventory, thus reducing the capacity over time. Some problems in sodium batteries arise from an unstable solid electrolyte interphase (SEI) reducing its protective power. While it is known that the electrolyte affects the SEI structure, the exact formation mechanism of the SEI is not yet fully understood. Here we follow the initial SEI formation on sodium metal submerged in propylene carbonate with and without the electrolyte salt sodium perchlorate. We combine X-ray photoelectron spectroscopy, gas chromatography, and density functional theory to unravel the sudden emergence of propylene oxide after adding sodium perchlorate to the solvent. We identify the formation of a sodium chloride layer as a crucial step in forming propylene oxide by enabling precursors formed from propylene carbonate on the sodium metal surface to undergo a ring-closing reaction. We identify changes in the electrolyte decomposition process, propose a reaction mechanism to form propylene oxide and discuss alternatives based on known synthesis routes.