For the most part, battery separators can be grouped into two categories; non-wovens and microporous membranes. The choice of a separator for each battery system must take into consideration such factors as cost, thickness, processibility, formability, pore size, tortuosity of pores, tensile strength, melt point, range of operating temperature stability, puncture resistance, dielectric strength, particle sizes of the anode and cathode materials, and stresses developed during product discharge.
Typically, carbon zinc and alkaline batteries use non-woven separators. In these cases, cost is an important design consideration, and the high conductivity of the electrolyte enables thicker separator materials. However, the thickness of the separator also defines the gap or distance between electrodes, which can impact the cell’s ability to support high current densities. Pores are not well defined and are typically on the order of several microns, while the thicknesses are often several hundreds of microns.
In contrast, lithium cells require thin microporous separators based on the low conductivity of organic electrolytes. These microporous separators more closely resemble membranes with a thickness of 25 microns or less with pore diameters in the hundredths of microns. They most often are polyolefins consisting of polyethylene or polypropylene.
In selecting a separator for a particular battery design, it’s also important to consider its stability in the electrolyte, ability to wet out quickly by the electrolyte, ability to maintain electrolyte within its pore structure throughout the battery’s life, and the impact it has on ionic mobility of the electrolyte salt. Since separator pores are seldom channels straight through the separator, the difference in true ion path versus the separator thickness is referred to as tortuosity. A certain amount of tortuosity is desired to prevent shorting due to particle penetration and soft shorts from dendritic bridging between electrodes, while maintaining low tortuosity is often required to achieve the best high rate performance. Therefore, striking the right balance of pore size and structure is an important design consideration.
The trend today is toward more value-added membranes such as ceramic coatings, ion selectivity, coatings to improve electrolyte absorption and reduce impedance, and dendrite blocking.