Provided is a binder for an electrode mix containing a semi-interpenetrating polymer network (semi-IPN) of polyvinyl alcohol and polyurethane, wherein the polyurethane is mixed to crosslink with the polyvinyl alcohol to form semi-IPN, and a lithium secondary battery comprising the same. The binder has superior adhesion to the electrode, excellent electrolyte resistance and improved elongation percentage, and therefore it is possible to prevent exfoliation or separation of electrode active materials from current collectors which occurs during repeated charge/discharge cycles. As a result, the capacity or power out of secondary batteries can be maintained at a constant level.