Insights into Electrolyte Decomposition Phenomena Utilizing Solid Phase Microextraction - Gas Chromatography - Mass Spectrometry in Combination with an in Situ Lithium Ion Cell

Abstract

In the last decades, the interest in lithium ion battery (LIB) technology has been rising steadily and thereby the system became an important field for scientific research. Numerous techniques for post mortem analysis of battery materials have been reported in literature - ranging from surface investigation e.g. scanning electron microscopy (SEM) to the analysis of the emerging passivation layers or the investigation of the liquid electrolytes. The analysis of the LIB electrolytes is of special interest due to their instability at typical electrode potentials. Thereby the aging phenomena in the electrolyte, which is in contact with all parts of the LIB, are a good indicator for the cell status. However, the state-of-the-art sample preparation for electrolyte analysis requires the mechanical opening of the cell and subsequent electrolyte extraction.[1] Thus, the cells are destroyed during the sample preparation and it is not possible to achieve data for multiple different states-of-health (SOHs) of one specific cell. The gas chromatography - mass spectrometry (GC-MS) provides the option to analyze the gases above a heated sample making the typically applied carbonate solvents accessible. However, the sample heating is not compatible with the commonly applied conducting salt (LiPF6) and thereby introducing additional aging phenomena. Therefore, an automated sample pretreatment was considered, which was developed in the work group of Pawliszyn.[2] The Solid phase microextraction (SPME) in combination with GC-MS provides several benefits with regard to the LIB environment - namely the headspace sampling option, the preconcentration of volatile organic compounds at room temperature and the small amount of removed sample.[3] This work reports on the development of a round cell (18650 format) for in situ investigations of the electrolyte. Proof of principle experiments have been executed with different setups relying on passive diffusion or utilizing a peristaltic pump for enhanced analyte uptake. Emerging decomposition products and consumed film forming additives have been analyzed at different SOHs. The cell response to the sampling procedure was investigated by C-rate tests, which did not show significant changes. Furthermore, the cell setup provides the possibility to investigate the electrolyte during charge / discharge operation, which might prove or disprove assumed changes in the electrolytes during cell operation. [1] Mönnighoff, X. et al., Journal of Power Sources 2017, 352, 56-63. [2] C. L. Arthur and J. Pawliszyn, Analytical Chemistry, 1990, 62, 2145-2148. [3] Horsthemke, F. et al., RSC Advances 2017, 7 (74), 46989-46998.