Disclosed herein is a unit cell for a lithium ion secondary battery, which includes an electrode laminate formed in such a manner that a plurality of unit structures are stacked, each of which includes and least one electrode and at least one separation layer; and at least one conductive sheet layer located between certain layers in the electrode laminate and electrically connected to an electrode lead. The conductive sheet layer of the unit cell for the lithium ion secondary battery rapidly conducts current to the outside or generates heat in quantity smaller than the quantity of heat generated in positive and negative electrodes when short-circuit occurs due to a physical or electrical impact applied to the battery. Accordingly, it is possible to reduce the risk of firing or explosion due to the physical or electrical impact to improve the safety of the lithium ion secondary battery.