Method of preparing secondary battery

A method of preparing a secondary battery, the method comprising:

• pre-lithiating an electrode assembly comprising an electrode structure comprising a plurality of electrodes and a plurality of separators;
• and a metal substrate,
• wherein the plurality of electrodes and the plurality of separators are alternatingly stacked,
• wherein the metal substrate is present on an outermost surface of the electrode structure in a direction in which the electrodes and the separators are stacked,
• wherein the plurality of electrodes comprises negative electrodes, each comprising negative electrode active material layer, and positive electrodes, each comprising positive electrode active material layer, wherein each positive electrode and negative electrode are spaced apart from each other with one separator of the plurality of separators disposed therebetween,
• wherein the pre-lithiating comprises:
• applying a first current by electrically connecting one of the plurality of positive electrodes one of the plurality of negative electrodes;
• and
• applying a second current by electrically connecting the metal substrate and one of the plurality of positive electrodes, after applying the first current.