A comparative study of commercial automotive prismatic Li-ion cells using nail penetration test, differential scanning calorimetry and thermogravimetric analysis

A critical penetration depth defined from a nail penetration test is an index to demonstrate, how robust a LIB cell is against internal short circuit (ISC) and thermal runaway (TR). Six different types of automotive prismatic cells are studied using nail penetration test and their resilience to the induced ISC is evaluated. Aim of this work is to comprehensively answer the question "what is(are) the key parameter(s), which define the resilience of a LIB safety against ISC on a cell level?". Several candidates as key parameter are inspected such as thicknesses of components, mass fraction of electrolyte and capacity. Among all properties, a strong correlation between safety and Ni molar fraction in NMC and NCA is found; a LIB cell consisting of cathode active material with higher Ni molar fraction tolerates less ISC and experiences TR with a smaller number of short-circuited electrodes. To understand this dependence of the safety on the cathode active material, the positive electrodes harvested from SOC 100 % LIB cells are analyzed using differential scanning calorimetry and thermogravimetric analysis. The findings suggest that the thermal stability of a positive electrode with the related electrolyte is the main factor, which determines safety on a cell level.