Identification and Quantification of the Main Electrolyte Decomposition By-product in Na-Ion Batteries through FEC: Towards an Improvement of Safety and Lifetime

Electrolyte is a crucial point for the safety and the lifetime of sodium-ion batteries (NIBs). The electrolyte decomposition must be avoided in order to guarantee NIB development in the future. In this work, the responsibility of the well-known by-product ethylene glycol bis-(methyl carbonate) (EGMC) in creating overcharging is discussed. A liquid GC/MS technique was used to detect EGMC in 1 M NaPF6 in EC:DMC (1:1) as the electrolyte in a half cell configuration, i.e with sodium metal as a counter electrode. The lack of reproducible electrochemical results due to random overcharges is linked to the presence of EGMC. It is demonstrated that FEC (fluoroethylene carbonate) additive could increase SEI stability on the sodium metal electrode and prevent EGMC formation. The strong impact of this by-product has been observed in the case of two types of positive electrode materials: a layered oxide and a fluorophosphate. Moreover, an optimization of the FEC quantity was studied, depending on the electrode material and the cell design. (C) 2016 The Electrochemical Society. All rights reserved.