Lithium ion batteries (LIB) are rapidly becoming the most common source of stored energy for everything from personal electronic devices to electric vehicles and long-term energy storage. A diagram of a battery is shown in Figure 1.
One of the key components of the battery is the porous separator which prevents contact between the anode and cathode and allows transport of the lithium ions during charging and discharging cycles. Some of the requirements for a battery separator include: good electronic insulator, minimal electrolyte resistance, mechanical and dimensional stability, chemical resistance to the electrolyte, ability to prevent migration of colloidal or soluble species between the electrodes, readily wetted by electrolyte, and uniformity in thickness and properties (2). Polyolefin separators made from polypropylene (PP), polyethylene (PE), or laminations of PE and PP are often used for lithium ion batteries with organic electrolytes.
Polyolefin separators are made by wet or dry processes all of which result in forming micropores in the film and in the case of uniaxially stretched films, imparting high tensile strength in the machine direction (MD) and relatively weak properties in the transverse direction (TD). Biaxially stretched films made from β-nucleated isotactic PP and the wet process result in films with comparable properties in both directions. The advantages and disadvantages of the processes are extensively discussed in the literature (2) (3) (4).
The purpose for this note is to detail the basic thermal analysis and mechanical techniques used to characterize a typical separator made from PP.
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