Comprehensive characterization of propylene carbonate based liquid electrolyte mixtures for sodium-ion cells

Abstract

In this study, 1 M sodium perchlorate (NaClO4) containing binary electrolytes based on propylene carbonate and X (X = dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, ethylene carbonate, 1,2-butylene carbonate, monoglyme, diglyme, tetraglyme, sulfolane) are studied due to their electrochemical and physicochemical properties as well as compatibility with sodium metal. The reactivity towards sodium is compared between mixtures with and without NaClO4 and degradation products are analyzed by gas chromatography. It is shown that NaClO4 plays a crucial role in electrolyte decomposition and gas formation. It could be shown that mixtures of linear and cyclic carbonates form coupling products during storage with Na metal, namely dialkane propane-1,2-diyl dicarbonates, independently of the presence of sodium perchlorate. Additionally, gas analysis of PC electrolyte over Na shows a pronounced formation of CO and propylene oxide during storage if NaClO4 is present in the sample. Overall, the electrolyte “PC+EC+NaClO4” is identified as most favorable system within the examined series with respect to decomposition characteristics (formation of decomposition products) as well as acceptable physicochemical and electrochemical properties, e.g. plating-stripping behavior, cycle tests, conductivity and solubility. A sustainability screening of the electrolyte formulations reveals a high toxic concern in case of glyme-based mixtures as well as sulfolane. From a life cycle perspective, however, glyme-based mixtures have in overall lower environmental footprints.