Thermal stability of NMC442 cathode material studied by accelerating rate calorimetry

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. The thermal stability of LiNi0.40Mn0.37Co0.23O2 cathode material from a Kokam 8 Ah pouch cell with 1 M LiPF6 in EC/DMC (1:1) was studied by accelerating rate calorimetry (ARC). Currently, there are no standard procedures for ARC tests on battery materials, and it is unknown how changes in the ARC setup would affect the outcome of the tests. Four different calorimetry test setups were tested in order to understand the influences of the setup on the result: A side plug setup (where the sample was placed in a sealed tube attached to the calorimeter lid), a side branch setup (where the sample in the lid was connected to an external gas system), a highvolume setup (with a 500 mL volume expansion of the gas system) and a throne setup (where the sample was placed in an insulated throne on the calorimeter floor). 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. It was proposed that a drop in temperature rate between the two stages of heating could be due to endothermic processes. During the initial stage of selfheating (175–240°C), the setup was not found to impact the results, and variations at this stage was attributed to personal errors in the sample preparation procedure. 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. Thus, it was concluded that the test setup is very important for the outcome of the test, and the reactivity of the cathode material is dependent on pressure. The onset temperature for exothermic activity was consistently 175±5°C for all the material tests. Higher thermal reactivity was measured for the full cell, which had an onset temperature of 92°C and reached thermal runaway at 210°C.