Inorganic particulate film/poly(methyl methacrylate) (PMMA)/inorganic particulate film trilayer separators are prepared by means of simple dip-coating of inorganic particle layers on to both sides of PMMA thin films. The mechanical, thermal and electrochemical characteristics of the trilayer are investigated using scanning electron microscopy, a universal tensile machine, a thermal shrinkage test and a charge–discharge test. As a polymer matrix, PMMA has exceptional compatibility with the carbonate-based liquid electrolyte, which can result in improved battery/cell performance. When inorganic Al2O3 particles are used to coat the PMMA film, drawbacks associated with gel-type membranes, namely, poor dimensional stability and thermal stability are greatly improved. This inorganic trilayer membrane is believed to be an inexpensive, novel separator for application in lithium-ion batteries.