According to the present invention, an electrochemical device comprises: (a) a composite separator comprising a porous substrate having pores, a first porous coating layer formed with a mixture of inorganic particles and a binder polymer, coated on one surface of the porous substrate, and a second porous coating layer formed with a mixture or inorganic particles and a binder polymer, coated on the other surface of the porous substrate, in which the first porous coating layer is thicker than the second porous coating layer; (b) an anode facing the first porous coating layer; and (c) a cathode facing the second porous coating layer. The electrochemical device of the present invention has improved cycle characteristics while having excellent thermal stability.