Nanoporous composite separators are disclosed for use in batteries and capacitors comprising a nanoporous inorganic material and an organic polymer material. The inorganic material may comprise Al2O3, AlO(OH) or boehmite, AlN, BN, SiN, ZnO, ZrO2, SiO2, or combinations thereof. The nanoporous composite separator may have a porosity of between 35-50%. The average pore size of the nanoporous composite separator may be between 10-90 nm. The separator may be formed by coating a substrate with a dispersion including the inorganic material, organic material, and a solvent. Once dried, the coating may be removed from the substrate, thus forming the nanoporous composite separator. A nanoporous composite separator may provide increased thermal conductivity and dimensional stability at temperatures above 200° C. compared to polyolefin separators.