Provided are separators for use in batteries and capacitors comprising (a) at least 50% by weight of an aluminum oxide and (b) an organic polymer, wherein the aluminum oxide is surface modified by treatment with an organic acid to form a modified aluminum oxide, and wherein the treatment provides dispersibility of the aluminum oxide in aprotic solvents such as N-methyl pyrrolidone. Preferably, the organic acid is a sulfonic acid, such as p-toluenesulfonic acid. Also preferably, the organic polymer is a fluorinated polymer, such as polyvinylidene fluoride. Also provided are electrochemical cells and capacitors comprising such separators.