Patent ID: 8592082

Claim:
A secondary battery comprising: an electrode assembly comprising: a first electrode plate; a second electrode plate; and a separator between the first electrode plate and the second electrode plate wound together, wherein the first electrode plate comprises a first electrode current collector and a first active material layer on the first electrode current collector, the first active material layer comprising a first active material, a binder and a conductive agent, and wherein a portion of the first active material layer at a central portion of the electrode assembly comprises the first active material at a lower density than a density of the first active material at a portion of the first active material layer at a peripheral portion of the electrode assembly, and wherein the portion of the first active material layer at the central portion of the electrode assembly and the portion of the first active material layer at the peripheral portion of the electrode assembly each have pores, the portion of the first active material layer at the central portion of the electrode assembly having a larger amount of the pores than the portion of the first active material layer at the peripheral portion of the electrode assembly, and wherein the pores are formed by a foaming agent, and wherein the first active material layer has a leading edge positioned at one end of the first electrode plate, corresponding to the central portion of the electrode assembly, a terminal edge positioned at the other end of the first electrode plate, corresponding to the peripheral portion of the electrode assembly, and the density of the first active material of the first active material layer gradually increases from the leading edge to the terminal edge.