Aging of Lithium Ion Battery Electrolytes – Novel Analysis Techniques for Elucidation of Potentially Hazardous Decomposition Products

Abstract

In present time, energy and consequently energy storage are topics of utmost importance. With regard to the latter, the lithium ion battery (LIB) is the most promising candidate to fulfill the demanding requirements of energy storage in terms of energy density, cycling stability and safety. But, electrolytes of state-of-the-art LIBs exhibit diverse decomposition mechanisms and thus decomposition products. Hence, identification of compounds and as a result mechanistic conclusions were in focus in recent years.[1] Additionally, quantitative data of the potential hazardousness of those compounds is crucial. This importance is even more emphasized since some decomposition products show structural similarities to phosphorus-based pesticides or even worse, chemical warfare agents. For structural elucidation of compounds, high-resolution organic mass spectrometric techniques were used as they deliver excellent sensitivity and low limits of detection. Concerning quantification, inductively coupled plasma-mass spectrometry (ICP-MS) was used due to the high structural variety of compounds and lack of commercially available analytical standards. For this purpose, a sector-field mass spectrometer (SF-MS) delivered reliable quantitative data in high resolution and in a broad linear range while still offering excellent detection limits. Thus, quantities could be assigned to the respective compounds that were identified parallel. Prior to detection, the species of interest were separated depending on their chemical characteristics. Since a large variety of compounds with different chemical properties was found, comprehensive analysis using different chromatographic techniques was adjusted accordingly for specific separation of compounds. The results were interpreted with regard to total decomposition products in electric vehicles and toxicological aspects, eventually, delivering a risk estimate of potentially hazardous compounds of LIB electrolytes. In this study, investigation of LIB electrolytes from lab-scale and field-tested EVs are interrelated regarding variety and especially quantity of phosphorous compounds of the decomposition route of the conducting salt LiPF6.[2] Simulated abusive treatment through use of elevated temperature outside of the stability window of electrolyte components and voltages exceeding the nominal cycling range are analyzed and interrelated with extracted electrolytes of field-tested cells. [1] S. Nowak and M. Winter, Journal of The Electrochemical Society, 2015, 162 (14), A2500-A2508. [2] V. Kraft, W.Weber et al., RSC Advances, 2016; 6 (1), 8-17.