Research into the safety of lithium-ion batteries, with a focus on thermal runaway, is critical due to the widespread use of these batteries in numerous applications. Safety concerns arise from the potential for thermal runaway, which can lead to fires or explosions. This study focuses on the influence of the electrolyte and its additives on the safety performance of lithium-ion batteries. Both thermal and electrical abuse scenarios were considered in this research.
In comparison to commercial cells with unknown electrolyte and additives, our approach used machine-made dry cells with a capacity of 1 Ah. The negative electrode is graphite, while the positive electrode is NMC622. The control electrolyte is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). We also used the control electrolyte with either 5% vinylene carbonate (VC) or 5% fluoroethylene carbonate (FEC). Analysis of the formation showed a deviation at the beginning of the initial charge sequence. Different shoulders in the voltage curve can be attributed to the decomposition of EC, VC and FEC. An Incremental Capacity Analysis (ICA) of the charge step transforms the shoulders into clearly visible peaks at 2.75 V, 2.65 V and 2.45 V, demonstrating the decomposition of the film-forming additives prior to EC from the electrolyte.
All cells were characterized prior to Accelerating Rate Calorimeter (ARC) testing. Safety tests were performed using the Heat-Wait-Seek (HWS) method with fully charged cells (SoC100) inside an EV-ARC (Thermal Hazard Technologies). The results show a slightly earlier exothermic onset for cells without additives at around 135°C. Thermal runaway temperatures for all compositions were in the range of 195°C to 205°C.
In addition, electrical abuse was conducted with an overcharge test. All cells were fully charged prior to the test. After reaching the cut-off voltage of 4.2 V, the charge was continued at 1 A, corresponding to 1C. The pouch cells used in this study do not have a safety feature that would prevent overcharging. None of the cells went into runaway during the one hour overcharge. Cells without additives started to generate gas earlier than those with VC or FEC. In addition, the voltage limit of 7.4 V was reached earlier due to the faster resistance increase caused by cathode collapse and gas generation.
A thorough understanding of thermal runaway is critical to the safety of lithium-ion batteries. An accurate understanding of the electrolyte composition provides a deeper insight into its safety implications. Future research will further investigate the effects of aged cells with the same electrolyte combinations.