Capillary Electrophoresis as Analysis Technique for Battery Electrolytes: (i) Monitoring Stability of Anions in Ionic Liquids and (ii) Determination of Organophosphate-Based Decomposition Products in LiPF6-Based Lithium Ion Battery Electrolytes

In this work, a method for capillary electrophoresis (CE) hyphenated to a high-resolution mass spectrometer was presented for monitoring the stability of anions in ionic liquids (ILs) and in commonly used lithium ion battery (LIB) electrolytes. The investigated ILs were 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI) and 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI). The method development was conducted by adjusting the following parameters: buffer compositions, buffer concentrations, and the pH value. Also the temperature and the voltage applied on the capillary were optimized. The ILs were aged at room temperature and at 60 ◦C for 16 months each. At both temperatures, no anionic decomposition products of the FSI− and TFSI− anions were detected. Accordingly, the FSI− and TFSI− anions were thermally stable at these conditions. This method was also applied for the investigation of LIB electrolyte samples, which were aged at 60 ◦C for one month. The LP30 (50/50 wt. % dimethyl carbonate/ethylene carbonate and 1 M lithium hexafluorophosphate) electrolyte was mixed with the additive 1,3-propane sultone (PS) and with one of the following organophosphates (OP): dimethyl phosphate (DMP), diethyl phosphate (DEP), and triethyl phosphate (TEP), to investigate the influence of these compounds on the formation of OPs.