Solid polymer electrolyte degradation products

characterized by chromatographic techniques

Jaroslav Miná_¹, Yi-Hsuan Chen², Lukas Herbers¹, Martin Winter^1,2, Gunther Brunklaus², Simon-Wiemers Meyer¹, Sascha Nowak¹

1 University of Münster, MEET Battery Research Center Corrensstraße 46, 48149 Münster

2 Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich Corrensstraße 46, 48149 Münster

The degradation of lithium-ion battery components is one of the most limiting factors for the succesfull long-term application of batteries in any area. To enable the development of various strategies to mitigate the degradation reactions, the knowledge of the degradation mechanisms is of primary importance. In case of liquid electrolytes, chromatography-based techniques have proved to be an indispensable tool for this task, since they can separate and identify particular degradation products[1]. For instance, gas chromatography-mass spectrometry (GC-MS) measurements using isotope labelling suggested that alkyl carbonate decomposition products may arise from the series of reactions between dimethyl carbonate and ethylene carbonate[2]. On the other hand, high-performance liquid chromatography-mass spectrometry (HPLC-MS) was used to clarify the formation mechanism of oligo-carbonates and glycols[3] or to suggest various organofluoro phosphate structures[4] in ethylene carbonate-based electrolytes. Despite the large amount of literature dedicated to the application of chromatographic techniques to study liquid electrolytes, these techniques have not been widely used in case of solid polymer electrolytes (SPE).

In this work, the SPE degradation was studied by means of GC-MS and HPLC-MS. The experiments were carried out with coin cells containing state-of-the-art polyethylene oxide (PEO) or polycaprolactone (PCLO) SPE, Li metal anode and NMC622 cathode. The cells were cycled at 4.3 and 5.0 V at 60°C (PEO) or 40°C (PCLO). After disassembling the coin cells in a dry room, the NMC cathode and SPE were put into separate headspace vials. The degradation products were subsequently extracted by solid-phase microextraction fiber (SPME) and determined using low- and high-resolution GC-MS. The amount of extracted degradation products was largely dependent on the type of extraction fiber (polyacrylate, divinylbenzene- and carboxen-polydimethylsiloxane). In addition to SPME experiments, cathode material and polymer electrolyte were subjected to acetonitrile extraction, the extracts being characterized by HPLC-MS and ion chromatography.

[1] Y. Stenzel, F. Horsthemke, M. Winter, and S. Nowak, ‘Chromatographic Techniques in the Research Area of Lithium Ion Batteries: Current State-of-the-Art’, Separations, vol. 6, no. 2, p. 26, May 2019, doi: 10.3390/separations6020026.

[2] C. Peschel et al., ‘Analysis of Carbonate Decomposition During Solid Electrolyte Interphase Formation in Isotope_Labeled Lithium Ion Battery Electrolytes: Extending the Knowledge about Electrolyte Soluble Species’, Batteries & Supercaps, vol. 3, no. 11, pp. 1183–1192, Nov. 2020, doi: 10.1002/batt.202000170.

[3] J. Henschel, C. Peschel, S. Klein, F. Horsthemke, M. Winter, and S. Nowak, ‘Clarification of Decomposition Pathways in a State_of_the_Art Lithium Ion Battery Electrolyte through ¹³ C_Labeling of Electrolyte Components’, Angew. Chem. Int. Ed., vol. 59, no. 15, pp. 6128–6137, Apr. 2020, doi: 10.1002/anie.202000727.

[4] J. Henschel, J. L. Schwarz, F. Glorius, M. Winter, and S. Nowak, ‘Further Insights into Structural Diversity of Phosphorus-Based Decomposition Products in Lithium Ion Battery Electrolytes via Liquid Chromatographic Techniques Hyphenated to Ion Trap-Time-of-Flight Mass Spectrometry’, Anal. Chem., vol. 91, no. 6, pp. 3980–3988, Mar. 2019, doi: 10.1021/acs.analchem.8b05229.