Ultrafine alpha-Al2O3 particles with a high purity of 99.997% and an average size of 300-400 nm were successfully prepared via hydrothermal and jet-milling processes. The jet-milled Al2O3 powders were coated on a conventional polyethylene (PE) separator, and the effects of the Al2O3 particle size on the coating properties of the Al2O3-coated PE separator were investigated. When compared with both the Al2O3-coated PE separator containing unratified particles (D-50 = 3.75 mu m) and the PE separator, the Al2O3-coated PE separator incorporating the jet milled particles (D-50 = 037 mu m) exhibited a high porosity (60-63%) and a high electrolyte uptake (243-245%). The cell with the Al2O3-coated PE separator showed a similar rate capability and cycling performance at room temperature compared with the PE separator. Regarding the thermal behavior, the Al2O3-coated PE separator has a lower enthalpy, lower thermal shrinkage, and improved melt integrity compared with the pristine PE separator. (C) 2017 Elsevier B.V. All rights reserved.