Thermal Stability and Performance of Li-ion Batteries at Elevated Temperatures: Separator Effects

Lithium-ion batteries have been optimized for a limited temperature range (ca. 0-40 °C) and experience rapid capacity fade at elevated temperature (>50 °C). Cycling data and design of experiment (DOE) studies establish that the commonly used polyolefin-based separator is an important factor contributing to poor battery performance at elevated temperatures upwards of 100 oC. Understanding this failure mechanism is key to unlocking solutions that broaden the operating temperature range, enabling new applications in space exploration, drilling equipment, and automotive technologies. Here, an in-depth analysis was undertaken of the high-temperature failure mechanisms for cells containing a trilayer polypropylene/polyethylene/polypropylene (PP/PE/PP) separator with either a LiNi0.33Mn0.33Co0.33O2 (NMC111) or LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode, a graphite anode, and an electrolyte composed of 1.0 M LiPF6 in ethylene carbonate:ethyl methyl carbonate (EC:EMC, 1:1 v/v). Electrochemical methods consisting of coin cell cycling and electrochemical impedance spectroscopy (EIS) revealed capacity loss, reduced coulombic efficiencies, and large resistance increases during prolonged exposure at 100 °C for trilayer separators relative to polyimide separators. Spectroscopic measurements showed inhomogeneities within the heated, trilayer separator with localized deformations, indicating that increasing temperatures cause the porosity to decrease until complete continuity was observed. Tensile and puncture testing revealed that a 100 °C environment was the dominant factor decreasing tensile strength and increasing variability. Overall, this study identifies multiple failure mechanisms associated with polyolefin separators in cells cycling at 100 °C.