Mechanical Properties of Lithium-Ion Battery Separators

Abstract

The lithium-ion battery has become an important energy storage technology to support the rapid growth of renewable energy production and hybrid/electric vehicle usage. However, these batteries degrade over time as charging and discharging them cause the electrodes to expand and contract, creating large cyclical mechanical stresses on the battery components. This thesis project seeks to understand the effects of mechanical stress on the polymer separator, which allows lithium ion to diffuse through while electrically insulating the electrodes. Specifically, the separators have been tested under compression and tension, at various strain rates, and in dry and wet environments. Results from this project have shown that compression tests yield data that are difficult to compare with those taken from tension tests. The separators tested in liquid environments have shown that, under fast compression rates, fluid inside the separator pores increases the Young’s modulus and yield stress of the separators. At slow compression rates, the chemistries of the liquid and the polymer separator come into play. Dimethyl carbonate (DMC) and diethyl carbonate (DEC), both of which are organic solvents commonly used as lithium-ion battery electrolytes, have been observed to cause significant mechanical softening in the separators. Other solvents like acetone have also been shown to soften the separator material, while liquids like water and methanol do not show significant mechanical alteration of the separator.