Disclosed is an electrochemical device, which comprises: (A) a binder comprising polymer particles obtained from the polymerization of: (a) 20-70 parts by weight of a (meth)acrylic acid ester monomer; (b) 20-60 parts by weight of a vinyl monomer; and (c) 0.01-30 parts by weight of an unsaturated carboxylic acid monomer, based on 100 parts by weight of a binder polymer; and (B) electrochemical cells stacked multiply by using the binder, wherein the binder allows electrode active material particles in an electrode to be fixed and interconnected among themselves and between the electrode active material and a collector, and the electrode and a separator that is in contact with the electrode are bonded to each other by way of hot fusion. The binder is also disclosed. The binder has excellent adhesion and thermal bonding characteristics, and thus is useful for an electrochemical device comprising multiply stacked electrochemical cells, and can improve the overall quality of a battery.