Author Anita Liu
Selecting the right separator for the right cell and application is critical.
Material Defines Character
- PE (Polyethylene): Melting point around 130°C. It features a “thermal shutdown” property, melting to block current and enhance safety.
- PP (Polypropylene): Higher melting point (~160°C), offers better mechanical strength, but has a higher shutdown temperature.
- Ceramic-Coated Separator: Coated with ceramic particles like alumina (Al₂O₃) on a PP/PE base, it boasts high-temperature resistance (>200°C without shrinking), improved mechanical strength, and better electrolyte wettability.
- High-End Options: Aramid separator (heat resistant >300°C), Non-woven separator (high porosity, but lower mechanical strength).
Thickness & Porosity: The Balance of Thinness and Permeability
Thickness: Typically 5~25 μm. Thinner separators facilitate ion transport and increase overall energy density (by saving space and weight), but raise the risk of being pierced by dendrites.
- Consumer electronics (phones/laptops) often use 7~12 μm separators for high energy density.
- Power batteries (e.g., EVs) tend to use slightly thicker ones (16~20 μm) to balance safety and power performance.
Porosity: A range of 40%~60% is optimal:
- Too low porosity restricts ion transport, increasing internal resistance.
- Too high porosity may reduce mechanical strength and increase short-circuit risk.
Thermal Stability & Strength: The Dual Safeguards
- Thermal Shrinkage: Should be low (e.g., <5% at 120°C). Ceramic coatings can reduce it further to <1%.
- Shutdown Temperature: ~130°C for PE, ~160°C for PP. Upon reaching this temperature, resistance spikes sharply, helping to halt thermal runaway.
- Puncture Strength: Should be high (>300 gf; power batteries often require >500 gf) to prevent penetration by metal dendrites.
- Tensile Strength: Needs to be balanced in both transverse (TD) and machine (MD) directions to avoid tearing during winding or stacking.
Process Showdown: Dry Process vs. Wet Process
Separator manufacturing primarily follows two routes: Dry Process and Wet Process, each with its strengths.
- The Dry Process is more of a “practical workhorse”: lower cost, high strength. It typically produces separators with a thickness range of 16-40 μm and porosity around 30%-40%. Its puncture strength is 200-400 gf, with a higher shutdown temperature (~160°C for PP). It’s well-suited for LFP or energy storage batteries where cost and cycle life are priorities.
- The Wet Process is the “precision specialist”: it can produce much thinner separators (5-20 μm) with higher porosity (40%-60%) and more uniform pore size. Its transverse tensile strength and puncture strength (up to 130-150 MPa and 300-550 gf, respectively) are generally superior to dry process, but at a higher cost. Its shutdown temperature is around 130°C (for PE), primarily serving high-nickel NMC batteries that pursue high energy density.
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.
See also: Shutdown Separators