Evaluating the trade-off between mechanical and electrochemical performance of separators for lithium-ion batteries: Methodology and application

• Methods for mechanical properties of Li-ion battery separators critically analyzed.
• A simple and reliable method for measuring effective conductivity (MacMullin number) devised.
• Major trade-off differences of mechanical and electrochemical properties of separators shown.

Lithium-ion batteries are in widespread use in electric vehicles and hybrid vehicles. Besides features like energy density, cost, lifetime, and recyclability the safety of a battery system is of prime importance. The separator material impacts all these properties and requires therefore an informed selection. The interplay between the mechanical and electrochemical properties as key selection criteria is investigated. Mechanical properties were investigated using tensile and puncture penetration tests at abuse relevant conditions. To investigate the electrochemical performance in terms of effective conductivity a method based on impedance spectroscopy was introduced. This methodology is applied to evaluate ten commercial separators which allows for a trade-off analysis of mechanical versus electrochemical performance. Based on the results, and in combination with other factors, this offers an effective approach to select suitable separators for automotive applications.