Two-dimensional ion chromatography for the separation of ionic organophosphates generated in thermally decomposed lithium hexafluorophosphate-based lithium ion battery electrolytes

• Development of a two-dimensional IC method for the analysis of lithium ion battery electrolytes.
• Application different stationary phases and eluents on the 2nd dimension to overcome co-elution.
• Investigation of the chemical stability of several ionic organophosphates in water and acetonitrile.
• Hyphenation to an ICP-MS system for latter quantification.

A two-dimensional ion chromatography (IC/IC) technique with heart-cutting mode for the separation of ionic organophosphates was developed. These analytes are generated during thermal degradation of three different commercially available Selectilyte™ lithium ion battery electrolytes. The composition of the investigated electrolytes is based on 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate/dimethyl carbonate (50:50 wt%, LP30), ethylene carbonate/diethyl carbonate (50:50 wt%, LP40) and ethylene carbonate/ethyl methyl carbonate (50:50 wt%, LP50). The organophosphates were pre-separated from PF6− anion on the low capacity A Supp 4 column, which was eluted with a gradient step containing acetonitrile. The fraction containing analytes was retarded on a pre-concentration column and after that transferred to the high capacity columns, where the separation was performed isocratically. Different stationary phases and eluents were applied on the 2nd dimension for the investigation of retention times, whereas the highly promising results were obtained with a high capacitive A Supp 10 column. The organophosphates generated in LP30 and LP40 electrolytes could be separated by application of an aqueous NaOH eluent providing fast analysis time within 35 min. For the separation of the organophosphates of LP50 electrolyte due to its complexity a NaOH eluent containing a mixture of methanol/H2O was necessary. In addition, the developed two dimensional IC method was hyphenated to an inductively coupled plasma mass spectrometer (ICP-MS) using aqueous NaOH without organic modifiers. This proof of principle measurement was carried out for future quantitative investigation regarding the concentration of the ionic organophosphates. Furthermore, the chemical stability of several ionic organophosphates in water and acetonitrile at room temperature over a period of 10 h was investigated. In both solvents no decomposition of the investigated analytes was observed and therefore water as solvent for dilution of samples was proved as suitable.