If you've ever had a coin cell short out during the first charge cycle, there's a good chance the separator was the culprit — not the electrode chemistry. The separator is the quiet, unglamorous layer sitting between your cathode and anode, and it's just as capable of ruining an experiment as any active material. Choosing the wrong one for your test format, electrolyte, or voltage window is one of the most common (and most avoidable) sources of failed coin cells and irreproducible data in battery R&D labs.

This guide breaks down the major separator membrane families researchers use today — polyethylene (PE), polypropylene (PP), glass fiber, and ceramic-coated separators — so you can match the right membrane to your chemistry, test format, and budget.

What a Battery Separator Actually Does

A separator has one core job: physically keep the cathode and anode apart so they can't short-circuit, while still allowing lithium (or sodium) ions to pass freely through its pores. That means every separator is a balancing act between three competing properties:

• Porosity and pore structure — enough open volume to soak up electrolyte and let ions move without excessive resistance
• Mechanical strength — enough puncture resistance to survive winding, stacking, and cell pressure without tearing
• Thermal stability — the ability to hold its shape (or safely shut down ion transport) if the cell overheats

No single material maximizes all three, which is exactly why different separator types exist for different jobs.

Polyolefin Separators: PE and PP

Polyethylene (PE) and polypropylene (PP) are the polyolefin separators that dominate commercial lithium-ion cells, and they're the default choice for most coin cell and pouch cell R&D work involving standard liquid electrolytes.

Polyethylene (PE) separators are prized for their "shutdown" behavior. PE has a relatively low melting point (around 130–135°C), so if a cell starts to overheat, the PE separator softens and its pores collapse, physically blocking ion flow and stopping the reaction before thermal runaway can accelerate. This makes PE a common safety layer, either alone or as the middle layer in a trilayer PP/PE/PP separator.

Polypropylene (PP) separators have a higher melting point (around 155–165°C) and better mechanical and oxidative stability, which makes them more chemically robust against oxidizing cathode surfaces at high voltage. PP is often used as the outer layer in multilayer separators specifically because it holds its shape at temperatures where PE has already shut down, preserving some mechanical integrity even after a safety event.

For general-purpose lithium-ion coin cell testing with a standard carbonate electrolyte, a single-layer PE separator film or PP microporous separator — commonly sold in thicknesses around 12–25 µm — is usually the right starting point unless you have a specific reason to deviate.

Glass Fiber Separators

Glass fiber (GF) separators are a lab-research staple that you won't typically find in commercial cells, but they're extremely common in early-stage material screening for one simple reason: they're forgiving.

Glass fiber separators are thick (often 260–700 µm), highly porous, and wick up electrolyte readily, which means they tolerate sloppier lab technique and higher electrolyte volumes without drying out mid-test. They're also chemically inert and stable across a wide temperature range, making them a good default when you're validating a new electrode material or electrolyte formulation and don't want the separator itself to be a variable.

The trade-off is that glass fiber's thickness and low mechanical strength make it impractical for anything beyond coin cell-scale testing — it isn't representative of what a commercial-format cell will use, and its high electrolyte uptake can quietly change your effective salt concentration versus a thin polyolefin separator. Use it for fast material screening, not for final performance validation. Common grades used in labs include glass fiber separator (GF-A), glass fiber separator (GF-B), and glass fiber separator (GF-F).

Ceramic-Coated Separators

Ceramic-coated separators start with a base PE or PP film and add a thin layer of ceramic particles — typically alumina (Al₂O₃) or boehmite — bonded to one or both sides. This coating dramatically improves thermal stability (ceramic-coated separators can resist shrinkage well beyond the melting point of the base polymer) and improves electrolyte wettability and puncture resistance.

Ceramic coating matters most in two situations: high-nickel NCM/NCA cathode systems, where thermal stability margins are tighter, and any cell design where you need the separator to physically survive contact with a rougher or higher-pressure electrode stack — for example, silicon-containing anodes that expand during cycling. If your research involves high-energy-density chemistries or elevated safety-testing requirements, a ceramic-coated separator film is worth the added cost over a bare polyolefin film.

How to Choose: A Quick Decision Framework

Situation	Recommended Separator
Standard lithium-ion coin cell screening	PE or PP microporous separator
Early-stage material or electrolyte screening	Glass fiber separator
High-nickel NCM/NCA or silicon anode systems	Ceramic-coated separator
High-temperature or abuse-tolerance testing	Ceramic-coated or trilayer PP/PE/PP separator
Sodium-ion or aqueous system prototyping	Glass fiber or PE, depending on electrolyte chemistry

 

As a general rule: use glass fiber when you want the separator to be the least interesting variable in an early screening test, and move to a polyolefin or ceramic-coated separator once you're validating results that need to translate toward a realistic cell format.

Frequently Asked Questions

Is a thicker separator always safer?

Not necessarily. A thicker separator does add mechanical margin, but it also increases internal resistance and adds dead weight/volume, which lowers energy density. Safety in commercial cells comes more from material choice (ceramic coating, shutdown behavior) than raw thickness.

Can I substitute glass fiber for a polyolefin separator in a pouch cell?

Generally, no. Glass fiber is too thick and mechanically weak for stacked or wound pouch cell formats. It's best reserved for coin cell-scale screening.

Does separator choice affect cycle life data?

Yes. Separator porosity and electrolyte uptake directly affect ionic resistance and can shift capacity retention and rate performance results, so keeping the separator consistent across a comparison study is essential for reproducible data.

What separator thickness is standard for coin cell R&D?

Most polyolefin separators used in coin cell research fall in the 12–25 µm range, while glass fiber separators are typically much thicker, often 260 µm or more. Once you've settled on a separator, pair it with a coin cell case for assembly and validation.

Browse the full experimental materials category at Canrud for cathode, anode, and electrolyte materials to complete your next test cell build.