Lithium-ion secondary battery and method of manufacture thereof

1. A lithium-ion secondary battery comprising:

a positive electrode having a positive electrode collector, and a positive electrode mixture layer formed on a surface of the positive electrode collector;

a negative electrode having a negative electrode collector, and a negative electrode mixture layer formed on a surface of the negative electrode collector;

a separator disposed between the positive electrode and the negative electrode;

at least the positive electrode mixture layer includes a pattern of a high-density portion of high mixture density and a low-density portion having a lower mixture density than the high-density portion and being in contact with the high-density portion, the low-density portion having a smaller area than an area of the high-density portion when viewed in plan and the pattern of the low-density portion and the high-density portion being repeated in an alternating manner along a lengthwise or a widthwise direction in the positive electrode mixture layer; and

a density gradient region at a boundary between the high-density portion and the low-density portion, and in at least a part of regions in a vicinity of the boundary, the density gradient region being a region where a density shows gradual changes from place to place.

2. The lithium-ion secondary battery according to claim 1 , wherein the low-density portion has an area that is 1/10 of the area of the high-density portion, or smaller.

3. The lithium-ion secondary battery according to claim 2 , wherein the low-density portion has an area that is between 1/20 to 1/10 of the area of the high-density portion.

4. The lithium-ion secondary battery according to claim 1 , further comprising an electricity-generating unit that includes the positive electrode, the negative electrode, and the separator in a wound type configuration.

5. The lithium-ion secondary battery according to claim 1 , wherein a planar width of the density gradient region is at least 1/5 of a planar width of the low-density portion.

6. The lithium-ion secondary battery according to claim 1 , wherein the positive electrode collector is an aluminum foil.

7. The lithium-ion secondary battery according to claim 1 , wherein the negative electrode collector is a copper foil.

8. The lithium-ion secondary battery according to claim 1 , wherein the high-density portion and the low-density portion have substantially a same thickness.

9. A method for manufacturing a lithium-ion secondary battery, the method comprising:

producing a positive electrode that includes a positive electrode collector, and a positive electrode mixture layer formed on a surface of the positive electrode collector;

producing a negative electrode that includes a negative electrode collector, and a negative electrode mixture layer formed on a surface of the negative electrode collector;

disposing the positive electrode, a separator, and the negative electrode so as to interpose the separator between the positive electrode and the negative electrode;

at least the positive electrode mixture layer includes a pattern of a high-density portion of high mixture density and a low-density portion having a lower mixture density than the high-density portion and being in contact with the high-density portion, the low-density portion having a smaller area than an area of the high-density portion when viewed in plan and the pattern of the low-density portion and the high-density portion being repeated in an alternating manner along a lengthwise or a widthwise direction in the positive electrode mixture layer; and

producing a density gradient region at a boundary between the high-density portion and the low-density portion, and in at least a part of regions in a vicinity of the boundary, the density gradient region being a region where a density shows gradual changes from place to place.