Shanghai Semitech New Material Co., Ltd.
1628 Lijing Road, Lingang New Area, 200000, Shanghai, China.
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Separator Ceramic Coating for Lithium Batteries
Lithium BatterySeparatorCeramic Coating
Separator Ceramic Coating
Fumed alumina and fumed silica for thermally stable separators
Ceramic-coated separators are now standard in high-energy NMC, NCA, and high-Ni cells. SEMITECH supplies fumed alumina as the dominant ceramic phase and fumed silica for slurry rheology control.
Contents
Coating Specs
Layer thickness2–6 μm
Heat resistance>180 °C
Shrinkage<5%
Loading3–8 g/m²
Why ceramic-coat the separator?
A bare polyolefin separator (PE or PP) shrinks above 130 °C and loses dimensional integrity well before thermal runaway. Ceramic coating closes that gap — and is now mandatory for high-energy automotive cells.
Ceramic coatings serve four functions simultaneously: thermal shutdown delay (intact ceramic skeleton holds shape past 180 °C), shrinkage suppression (less than 5% at 150 °C versus 30%+ for uncoated PE), electrolyte wettability (high surface area pulls electrolyte into pores), and dendrite suppression (mechanical barrier to lithium plating). Without ceramic coating, modern high-energy cells cannot pass thermal abuse tests like nail penetration or hot box.
How fumed alumina builds the ceramic layer
Fumed alumina (γ-Al₂O₃) is produced by flame hydrolysis of aluminum chloride, yielding aggregates of 13–50 nm primary particles fused into 200–500 nm clusters. The high BET surface area (80–130 m²/g) creates an open, porous coating that holds electrolyte while resisting compaction during calendaring. Fumed silica acts as the slurry stabilizer — its hydrogen-bond-driven gel network keeps alumina particles suspended during coating and prevents settling in the slurry tank.
Why fumed over precipitated alumina? Fumed grade has higher purity (>99.9% Al₂O₃), narrower particle size distribution, and more consistent batch-to-batch BET surface area — all critical for the ±0.5 μm coating thickness control that battery makers demand.
SEMITECH product lineup
Fumed Alumina · SEMIAL series
Primary particle 13–20 nm. BET 100–130 m²/g. >99.9% Al₂O₃. Hydrophilic surface. Three particle-size grades for thin-coat (2 μm) and thick-coat (6 μm) applications.
Fumed Silica · SEMISIL 200
Primary particle ~12 nm. BET 200 m²/g. Hydrophilic. Used at 0.5–1.5 wt % in alumina slurries to control viscosity, prevent settling, and improve coating uniformity.
Coating process integration
Ceramic-coated separators are produced in three main steps:
STEP 1
Slurry Prep
Fumed Silica
Disperse alumina in water/PVDF binder; fumed silica controls rheology.
STEP 2
Coating
Gravure / Slot
Apply 2–6 μm wet film on PE or PP base separator.
STEP 3
Drying
60–80 °C
Evaporate solvent without damaging porosity.
STEP 4
QC
Shrinkage Test
Hot oven test at 150 °C / 1 h verifies dimensional stability.
Selection guide
| Cell Type | Coating Thickness | Recommended Alumina Grade | Silica Loading |
|---|---|---|---|
| LFP (energy) | 2–3 μm | SEMIAL 13 | 0.5 wt% |
| NMC532 / NMC622 | 3–4 μm | SEMIAL 20 | 0.8 wt% |
| NMC811 / NCA | 4–6 μm | SEMIAL 30 | 1.0–1.5 wt% |
| Solid-state pilot | 5–10 μm | Custom | NDA project |
FAQ
Can boehmite (γ-AlOOH) replace fumed alumina entirely?
For mid-energy LFP and NMC532 cells, yes — boehmite is lower cost and adequate. For NMC811, NCA, and high-rate cells, fumed alumina’s higher purity and narrower particle distribution remain preferred. Many manufacturers blend the two: boehmite for cost, fumed alumina for performance margin.
Why use fumed silica in an alumina-based slurry?
Alumina particles tend to settle in water-based slurries within hours. Fumed silica forms a thixotropic gel network at 0.5–1.5 wt% loading that suspends alumina indefinitely while still allowing the slurry to flow when sheared during coating.
What surface treatment do you offer?
Standard SEMIAL grades are hydrophilic for water-based slurries. Hydrophobic (PDMS or HMDS-treated) versions are available for solvent-based PVDF/NMP slurries on demand.
What is the typical alumina-to-binder ratio?
Most water-based recipes use 90–95 parts alumina to 5–10 parts binder (PVDF, CMC, or acrylic latex). Fumed silica is added at 0.5–1.5 parts on top.
Do you supply free samples for separator R&D?
Yes — 100 g and 1 kg trial sizes ship within 2 weeks. NDA-protected custom particle sizes require 4–6 weeks pilot lead time.
Part of the Lithium Battery Materials hub. Datasheets and COA available on request.