Abstract: Provided are separators for use in an electrochemical cell comprising (a) an hydrated aluminum oxide of the formula Al2O3.xH2O, wherein x is less than 1.0 and (b) an organic polymer, wherein the hydrated aluminum oxide comprises organic substituents. Preferably, x of the hydrated aluminum oxide is less than 0.8, or more preferably, x is less than 0.6. Preferably, the organic substituents comprise a reaction product of a multifunctional monomer, such as a divinyl ether of an ethylene glycol, and/or an organic carbonate with an aluminum oxide, such as pseudo-boehmite or a hydrated aluminum oxide.