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

Selecting a Battery Cell Separator

Selecting the right separator for the right cell and application is critical.

• Thickness
• Porosity
• Stability
• Strength

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

Ceramic Coatings

A Comparison between Alumina-coated and Boehmite-coated Separators:

The ceramic particles are held onto the base film with a polymer binder — typically PVDF or the PVDF-HFP copolymer, sometimes PMMA. Binder loading is a trade-off in its own right: too little and the ceramic layer can delaminate from the base film in service; too much thickens the coating and raises tortuosity, pushing up ionic resistance and reducing rate capability. Current separator development is aimed partly at thinner, more scalable ceramic-coating deposition — including binder-free approaches — to capture the thermal-stability benefit without the full mass and impedance penalty of a conventional slurry-coated layer.

Lamination

Lamination bonds the separator directly to both electrode faces using heat and pressure to soften a thin adhesive coating already on the separator before the electrode stack is built. Rather than leaving the separator as a free-floating layer held in place only by winding tension or external pack clamping.

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
  • 13µm CATL 161Ah LFP prismatic
• 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

Shutdown Separators

A shutdown separator is designed to stop ion transport between the anode and cathode once the cell reaches a defined temperature, acting as a passive, intrinsic protection layer before a cell reaches thermal runaway. 

Wet Process vs Dry Process Manufacturing

The polyolefin base film itself is made by one of two routes. The dry process extrudes PP or PE into a sheet, anneals it to build up crystalline lamellae, then stretches the sheet uniaxially (machine direction only) to pull the lamellae apart and open a network of narrow, slit-shaped pores. There is no plasticizer to remove afterwards, which keeps the line simpler and lower-cost, but the pore structure is directional and less uniform than the alternative.

The wet process (extraction process) melt-blends the polymer with a plasticizing oil first, extrudes that blend into a film, then stretches it biaxially, machine and transverse direction both, before a solvent washes the plasticizer back out, leaving a more rounded, interconnected pore network. Wet-process films are generally more uniform and can be taken thinner without losing mechanical integrity, which is why the route dominates high-energy-density automotive cells, though the solvent-recovery equipment makes it more capital-intensive than a dry-process line.

Dry vs Wet Separator Technology

The dry vs wet differentiation is essentially the difference in the way they are produced in the factory. From PolyPropylene (PP) or PolyEthylene (PE) particles that are used to produce the film:

• PP Dry Separator: the separator is produced without solvents being used in the process
• PE Wet Separator: the separator is produced using solvents.

Large-format industrial and automotive cells commonly use these base films in a PP/PE/PP trilayer construction rather than a single layer, giving the separator a built-in thermal shutdown response, see Shutdown Separator for how the PE core and PP skin interact during a thermal event.

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
3. Arora, P., Zhang, Z. “Battery Separators.” Chemical Reviews, 2004, 104(10), 4419-4462. doi.org/10.1021/cr020738u

See also: Electrolyte, Degradation Mechanisms, Shutdown Separator, Power versus Energy Cell Cost