The separator is the thinnest component in a lithium-ion cell but one of the most failure-critical. A polyolefin base film (PE, PP, or trilayer PP/PE/PP) provides mechanical strength and shutdown function, but its low thermal stability limits operation. Ceramic coating — a thin layer of inorganic particles bonded to the polymer substrate — was developed specifically to address this limitation by adding thermal dimensional stability, improved electrolyte wettability, and dendrite puncture resistance.
Ceramic Coating Materials
Alumina (α-Al₂O₃): The dominant filler for ceramic separators. Calcined α-alumina with D50 = 0.5–2.0 μm and high purity (>99.5% Al₂O₃) is preferred. The angular particle morphology creates a tortuous pore structure that resists dendrite penetration. Thermal stability exceeds 1000 °C, well above the polyolefin substrate melt point (130–165 °C), so the ceramic layer provides a skeleton that prevents catastrophic shrinkage during thermal runaway.
Boehmite (γ-AlOOH): An aluminum oxyhydroxide phase with platelet morphology. Boehmite offers better coating smoothness and particle-substrate adhesion than α-alumina because its platelet shape packs more conformally against the fiber structure of PE or PP base film. Boehmite decomposes to γ-alumina above 450 °C, providing secondary thermal protection. BET surface area is typically 5–15 m²/g versus 1–4 m²/g for alumina.
Fumed silica: Used as a functional additive at 2–5 wt% of ceramic solids to improve slurry rheology and coating uniformity. Its high surface area and thixotropic behavior help maintain particle suspension during slot-die coating.
Separator Coating Specification Table
| Parameter | PE Base Film | Alumina-Coated | Boehmite-Coated |
|---|---|---|---|
| Total thickness | 9–12 μm | 14–18 μm | 13–17 μm |
| Ceramic coating thickness | — | 2–4 μm per side | 2–3 μm per side |
| Porosity | 38–42% | 35–40% | 36–41% |
| Gurley value (s/100 mL) | 200–350 | 300–500 | 280–450 |
| Thermal shrinkage (130 °C / 1 h) | 3–8% | < 1% | < 1% |
| Electrolyte uptake (EC:DMC) | 150–180% | 200–250% | 210–260% |
| Puncture strength (gf) | 250–350 | 350–500 | 320–480 |
Coating Formulation
A typical ceramic coating slurry: alumina or boehmite 15–25 wt% in NMP or water, plus PVDF binder at 3–8 wt% of dry ceramic weight. For water-based systems, CMC and styrene-butadiene rubber (SBR) replace PVDF and the pH is maintained at 8–10 to prevent alumina agglomeration.
Coating is applied by slot-die or gravure roll to both sides of the base film, with wet thickness controlled to deposit 2–4 μm dry ceramic per side. Drying temperature must not exceed 80 °C for PE substrates to prevent thermal distortion.