Provided herein is a lithium battery including: a cathode including a cathode active material; an anode including an anode active material; an electrolyte between the cathode and the anode; and a separator impregnated with the electrolyte, wherein the separator includes cellulose nanofibers, and wherein a differential scanning calorimetry (DSC) thermogram of the separator evinces an exothermic reaction peak, represented by a differential value (dH/dT), at a temperature in a range of about 150° C. to about 200° C.