Lithium ion batteries are rechargeable batteries that are characterized by very high power densities. Such batteries have become very common place, for example from everyday electronic products such as cell phones to electric vehicles. What is not commonly appreciated is that voids play a very important role in such batteries. There are four components in a lithium ion cell: anode, cathode, separator, and the nonaqueous electrolyte. Different chemistries are used; the anode is graphite, the cathode is an oxide (LiCoO2), and the alternating layers of anode and cathode are separated by a porous polymer separator, which is generally made of polypropylene (PP), polyethylene (PE), or a laminate of PP and PE. In all cases a critical feature of the separator is a controlled amount and uniform size of porosity in the separator.