The Effect of Internal Pressure on Thermal Runaway Temperature

Abstract

Lithium-ion batteries have developed into one of the most popular secondary batteries on the market today due to high voltage, long lifetime and high energy density. However, lithium ion batteries may have safety issues, and several fires are reported. Thermal stability is one of many parameters used to evaluate the safety aspects of lithium ion batteries. Understanding thermal stability at material level is essential for the further development into safer lithium ion batteries. Calorimetric methods are widely used in thermal stability tests for lithium-ion battery components. Accelerating rate calorimetry (ARC) is one of the most popular methods used to analyse full cells and differential scanning calorimetry (DSC) is the most common method for testing thermal stability of materials. DSC is however less sensitive than ARC, and uses smaller sample sizes in a fixed volume. This makes comparison between material samples and full cells difficult. Currently, there are no standard procedures for ARC tests on battery materials, and this work is an effort to standardize ARC lithium-ion material measurements. The thermal stability of LiNi0.40Mn0.37Co0.23O2 fully charged cathode material from a commercial 8 Ah pouch cell with 1 M LiPF6 in EC/DMC (1:1) was studied by ARC. Four different calorimetry test setups were tested in order to understand the influences of the setup on the result. Regardless of the setup, the cathode/electrolyte mixture was found to have two stages of self-heating, where the rate of the first stage influenced the temperature at which thermal runaway (heating rate > 10°C min-1) occurred during the second stage. During the initial stage of self-heating (175–240°C), the setup was not found to impact the results. At temperatures above 240°C, the reactivity was found to be highly dependent on pressure. Samples at low pressure (near 1 bar) did not reach thermal runaway, whereas the samples in the higher-pressure setups all reached thermal runaway at 250–260°C. Figure 1 shows the ARC measurements of two NMC cathode materials mixed with electrolyte at approximately 1.2 bar and 3.9 bar at 315 °C. It was concluded that the test setup is very important for the outcome of thermal stability measurements on fully charged LiNi0.40Mn0.37Co0.23O2 cathode material, and the reactivity of this cathode material is dependent on pressure. This is an important finding because most lithium-ion cells will have ventilated at temperatures above 150–200 °C and experience reduced internal pressure. DSC and ARC material measurements in closed containers with high pressure would not be representative for measuring lithium-ion cathode material reactivity. Figure 1: Heat rates and pressure for two samples of LiNi0.40Mn0.37Co0.23O2 with 1 M LiPF6 in EC/DMC (1:1) in two different setups. Figure 1