Separator Technology for Liquid Lithium - Ion Batteries

　　Separators in lithium - ion batteries serve a dual role: preventing short circuits between electrodes and allowing lithium ion passage. They are thin (10–25 μm), porous films (pore size: 100–500 nm, porosity: 30–50%) made of polymers or ceramic - polymer composites. Key separator technologies include:

　　1. Polymeric Separators: The Industry Standard

　　Polyolefin - based separators (polyethylene [PE], polypropylene [PP]) dominate the market due to their balance of cost, mechanical strength, and chemical resistance.

　　Single - Layer PE/PP: PE separators (melting point: ~130°C) offer high ionic conductivity but poor thermal stability. PP separators (melting point: ~160°C) provide better heat resistance but lower conductivity.

　　Multilayer PE/PP Composites: e.g., PP/PE/PP trilayers, where the middle PE layer ensures ionic transport, and the outer PP layers prevent thermal runaway by melting at higher temperatures (shutdown temperature: ~135°C), blocking ion flow.

　　2. Ceramic - Coated Separators: Enhancing Thermal Stability

　　To address the thermal limitations of polyolefins (e.g., shrinkage above 150°C in PE), ceramic coatings (e.g., Al₂O₃, SiO₂, ZrO₂) are applied to the separator surface:

　　Mechanism: The ceramic layer acts as a thermal barrier, maintaining dimensional stability up to 200°C. It also absorbs electrolyte, improving wettability and reducing interfacial resistance.

　　Applications: High - energy - density batteries (e.g., NCM811/graphite systems) and fast - charging batteries, where thermal runaway risks are higher.

　　3. Polymeric Blends and Composite Separators

　　Polyvinylidene Fluoride (PVDF): Offers chemical resistance and higher thermal stability than polyolefins but is more expensive. Often blended with polyolefins or ceramics for niche applications.

　　Electrospun Nanofiber Separators: Ultra - thin (~5 μm) mats with high porosity, enabling faster ion transport. However, their low mechanical strength limits large - scale adoption.

　　4. Key Performance Metrics

　　Ionic Conductivity: Dependent on porosity and electrolyte absorption. Ceramic - coated separators typically have 10–15% higher conductivity than pure polyolefins.

　　Mechanical Strength: Measured by puncture resistance (e.g., PE separators: ~200 N/cm). Ceramic coatings can double this value.

　　Thermal Stability: Assessed via dimensional change at high temperatures. Polyolefin separators may shrink by 10–15% above 150°C, while ceramic - coated variants show <5% shrinkage.

　　5. Challenges and Innovations

　　Thinning for Higher Energy Density: Separators are being thinned from 16 μm to 9 μm or lower, but this increases the risk of punctures. Nanocomposite coatings (e.g., Al₂O₃/PVDF) are being developed to maintain strength at reduced thickness.

　　Solid - State Transition: In semi - solid batteries, separators may be replaced by polymer - electrolyte composites, but liquid electrolytes still require traditional separators for now.

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