Rational Design of Crosslinked Polymeric Microsphere Coatings for Thermally Stable and Swelling‐Resistant Lithium‐Ion Battery Separators

The lithium‐ion batteries increasingly require separators that combine high thermal stability, low swelling, and excellent electrolyte wettability, yet conventional ceramic coatings add weight and reduce porosity while acrylate‐based polymer coatings often lack heat resistance. Here, we report a rationally engineered class of highly crosslinked, monodisperse polymeric microspheres synthesized via emulsion polymerization of styrene (St), methyl methacrylate (MMA), acrylonitrile (AN), and dipentaerythritol hexaacrylate (DPHA). The synergistic monomer design confers controlled particle nucleation, high thermal robustness ( T 3 up to 370°C), and exceptional resistance to electrolyte swelling (<11%). When applied as thin (∼1.5 µm) separator coatings, the microspheres pack into uniform, porous, and strongly wettable architectures that significantly enhance separator performance. The coated separators exhibit minimal thermal shrinkage (5.6% at 150°C), more than double the electrolyte infiltration of ceramic‐coated controls, and markedly improved ionic conductivity (0.80 mS cm − 1 ). Pouch cells assembled with these separators deliver superior cycling stability and rate capability. This work establishes a high‐performance, lightweight, and scalable alternative to ceramic coatings for next‐generation lithium‐ion battery separators.