Separator

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
• nonwovens
• 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

Materials

Over the years a number of materials have been used as the separator in various cell chemistries.

• nickel-based batteries:
  • porous cellophane
  • nylon
  • polyolefin film
• sealed lead-acid batteries
  • glass fibre mat
• lithium-ion batteries
  • Polyethylene (PE)
  • polypropylene (PP)
  • Polyethylene / Polypropylene (PE/PP)

Properties

In lithium-ion cells the separators have the following typical properties:

• Thickness 8µm to 25µm
• Pore size <1 µm
• Porosity 40–60%
• Permeability (Gurley) <0.025 s·µm⁻¹
• Dimensional stability No curl up and lay flat
• Thermal stability <5% shrinkage after 60 min at 90 °C

References

1. Li, A.; Yuen, A.C.Y.; Wang, W.; De Cachinho Cordeiro, I.M.; Wang, C.; Chen, T.B.Y.; Zhang, J.; Chan, Q.N.; Yeoh, G.H. A Review on Lithium-Ion Battery Separators towards Enhanced Safety Performances and Modelling Approaches. Molecules 2021, 26, 478.
2. Jang J, Oh J, Jeong H, Kang W, Jo C. A Review of Functional Separators for Lithium Metal Battery Applications. Materials (Basel). 2020 Oct