Comparing Different Thermal Runway Triggers for Automotive Lithium-Ion Batteries

Abstract

It is of crucial importance to guarantee safety of enhanced Lithium-ion batteries (LIB) in electric vehicles and consumer electronic products. Failure cases of LIB causing high temperatures and undesirable chemical reactions which can result in exothermic reactions and following thermal runaway (TR) have to be avoided reliably. For certain potential failure cases of batteries, toxic (CO, HF...) and flammable gases (CO, CH4, C2H4...)1,2 and breathable particles are ejected.In order to understand the LIB failing behavior and to prevent failures and their consequences, different LIB safety tests, also named abuse tests, have been developed. Popular examples are: Thermal runway triggered by overtemperature, overcharge, over-discharge, external and internal short circuit, hot spots, crush or the presence of foreign objects, like nail penetration3. The tests are carried out and documented in literature on single cell, module or even battery pack level.This scientific contribution shows the setup and the results of different TR triggers on single cells in a custom-made thermal runaway reactor at VIRTUAL VEHICLE. The cell type is a state-of-the-art automotive pouch cell. Chosen triggers focus on for example:• overtemperature• overcharge• nail penetrationAfter the new or aged cell is positioned inside the thermal runway reactor and the measurement equipment is fixed, the reactor is closed and flushed with nitrogen. Each cell is cycled to 100% SOC. If the setup is prepared, different triggers are tested for the single cell. The overtemperature experiment is designed to increase the cell surface temperature with ~ 2°C/min and the overcharge experiment is conducted with 1 C.The results of differently triggered TR are compared in three main categories: Temperature behavior of the cell, amount of produced vent gas and vent gas composition.Inside the reactor, the temperature behavior of the cell is recorded with thermocouple measurements on both sides of the cell surface, the cell tabs, close to the venting positions of the cell and inside the TR reactor. With pressure sensors inside the reactor, the pressure increase due to gas generation is recorded. With the ideal gas law, the amount of produced not condensed vent gas can be calculated. The vent gas composition is measured with Fourier Transform Infrared (FTIR) spectrometer and gas chromatograph in parallel.In the presentation the high impact of overcharge experiments on produced gas amount and the increased toxicity of the gas components will be shown. With increasing energy density of current state-of-the-art nickel manganese cobalt oxide (NMC) LIBs’ significantly higher impact of failing batteries can be observed in temperature behavior, gas production and gas toxicity. To conclude the presentation an estimated trend of the main categories - maximum reached cell surface temperature, amount of produced vent gas and amount toxic compounds in the vent gas - will be given.1. Golubkov, A. W. et al. Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes – impact of state of charge and overcharge. RSC Adv. 5, (2015).2. Ribière, P. et al. Investigation on the fire-induced hazards of Li-ion battery cells by fire calorimetry. Energy Environ. Sci. 5, 5271–5280 (2012).3. Coman, P. T., Rayman, S. & White, R. E. A lumped model of venting during thermal runaway in a cylindrical Lithium Cobalt Oxide lithium-ion cell. J. Power Sources 307, 56–62 (2016).