Gas Analysis of Large-Format Automotive Lithium-Ion Battery Cells during Formation

Abstract

Due to the decision by the European Union to stop sales of combustion-based vehicles after 2035, all car manufacturers are in search to close this gap with alternative solutions. As Lithium-Ion batteries (LIBs) are already an integral part of our everyday life, battery based electric vehicles (BEVs) are expected to be one possible alternative for the individual mobility of tomorrow. Therefore, all original equipment manufacturers (OEMs) who want to produce their own battery cells must deal with many challenges as the LIB is the most expensive single component used in EVs. Reducing costs of LIB production is making EVs more affordable and even more sustainable. One major production-related factor is the dwell-time under dry-room conditions as its disproportionately increases the overall production costs. Dry-Room conditions can´t be avoided as materials like the electrolyte and active material rely on water-free handling conditions [2-3, 5].Battery cells are typically assembled in a dry state. Cells are then filled with electrolyte and a so-called precharge step is performed. During this, the solid-electrolyte interface (SEI) is formed on the negative electrode when the electrolyte is partially reduced. The formation process results in a significant gas evolution, where typical gasses like CO, CO2, H2, O2, C2H4 can be formed [1, 4, 5]. After main gas evolution, the cells can be sealed and transported into normal atmosphere, where further charging and evaluation protocols are performed. In order to understand the voltage-related gas evolution during first charging, new analytical methods are needed. The hyphenation of electrochemical cycling and mass spectrometry to so-called Online-Electrochemical Mass Spectrometry (OEMS) is a powerful technique to investigate the gas evolution inside LIBs.In this project the development of OEMS for large-format prismatic LIBs will be outlined. The feasibility of studying battery cell formation reactions and the influence of typical parameters, like temperature or charging rate onto the gassing rate will be discussed. The obtained experiments are designed to give new insights into the gas formation during first charge and how the results compare in size to already well described model-cell OEMS-experiments [2, 3]. The cells are based on a graphite / NMC811 system with ethylene carbonate (EC) and ethyl methyl carbonate (EMC) 5/7 w% (LP57-electrolyte). The cells are produced by our BMW inhouse prototype production line and provide a clear as simple composition of all components. In contrast, commercially available LIBs differ in many aspects, e.g., containing a complex mixture of different volatile organic carbonates as well as quite often unknown additives, which may complicate the analysis [4]. The inhouse production also allows a unique integration of a commercially available Swagelok fitting in the lid of the LIB.