A porous membrane placed between electrodes of opposite polarity, permeable to ionic flow but preventing electric contact of the electrodes.
The considerations that are important and influence the selection of the separator include the following:
- Electronic insulator
- Minimal electrolyte (ionic) resistance
- Mechanical and dimensional stability
- Sufficient physical strength to allow easy handling
- Chemical resistance to degradation by electrolyte, impurities, and electrode reactants and products
- Effective in preventing migration of particles or colloidal or soluble species between the two electrodes
- Readily wetted by electrolyte
- Uniform in thickness and other properties
In most batteries, the separators are either made of nonwoven fabrics or microporous polymeric films. Batteries that operate near ambient temperatures usually use organic materials such as cellulosic papers, polymers, and other fabrics, as well as inorganic materials such as asbestos, glass wool, and SiO2. In alkaline batteries, the separators used are either regenerated cellulose or microporous polymer films. Lithium batteries with organic electrolytes mostly use microporous films.
The type of separator can be divided into the following groups:
- microporous films
- ion exchange membranes
- supported liquid membranes
- solid polymer electrolytes
- solid ion conductors
There are a number of things that can cause an internal short circuit within a battery cell. The primary focus has to be on manufacturing and the processes deployed to mitigate or reduce these risks.
- Metallic foreign body in the raw materials
- Introduction of a metallic particle during cell production
- Damage to electrodes
- Particles or damage during tab welding
- Particles from cell case welding
- Dendrite growth during cell use