Electrode assembly having high flexibility and battery cell including the same

1. An electrode assembly having a one directionally spiral-wound structure with a separator sheet interposed between a positive electrode sheet and a negative electrode sheet, the positive electrode sheet and the negative electrode sheet being wound about a winding axis extending in first and second opposite vertical directions,

wherein the spiral-wound structure of the electrode assembly has a length (x) in a horizontal direction perpendicular to the winding axis and a height (y) in the first vertical direction, and the spiral-wound structure of the electrode assembly has first and second opposite ends that are separated by the length (x) in the horizontal direction, and a center that is equidistant between the first and second opposite ends,

wherein the spiral-wound structure of the electrode assembly is configured to be bent about a bending axis extending in the first and second opposite vertical directions, the bending axis being parallel to the winding axis, the spiral-wound structure having a naturally curved shape having a curvature radius (R) naturally formed without additional external force, the curvature radius (R) relative to the bending axis satisfying the following Equation 1:

S[{ 1/ln( x/y )}* t]=R   1

wherein t is an average thickness (mm) of the spiral-wound electrode assembly in radial directions extending perpendicularly from the bending axis toward the spiral-wound structure and S is a constant of 8 or more, and

wherein the spiral-wound structure of the electrode assembly is configured to be repeatedly movable between two positions including a position in which opposing sides of the electrode assembly have a planar shape parallel to one another and in which the first and second opposite ends of the spiral-wound structure of the electrode assembly are spaced apart from the bending axis by a first distance that is greater than the curvature radius (R) and the center is spaced apart from the bending axis by a second distance that is equal to the curvature radius (R), and a position having the naturally curved shape in which the spiral-wound structure of the electrode assembly is bent about the bending axis according to the Equation 1 and in which the first and second opposite ends and the center of the spiral-wound structure of the electrode assembly are spaced apart from the bending axis by a second distance that is equal to the curvature radius (R).

2. The electrode assembly of claim 1 , wherein the curvature radius (R) is 5 mm to 20 mm.

3. The electrode assembly of claim 1 , wherein the length (x) in the horizontal direction perpendicular to the winding axis is 50 mm to 500 mm.

4. The electrode assembly of claim 1 , wherein a positive electrode lead and a negative electrode lead protrude side by side from the spiral-wound structure of the electrode assembly in the first vertical direction.

5. The electrode assembly of claim 1 , wherein S is a constant of 9 to 12.

6. The electrode assembly of claim 1 , wherein the electrode assembly has a structure having relatively increased flexibility for deformation into a curved shape by dispersing stress in a direction of the relatively long horizontal length.

7. The electrode assembly of claim 1 , wherein the spiral-wound structure of the electrode assembly includes winding of the positive electrode sheet and the negative electrode sheet about the winding axis five times or less.

8. A battery cell having a structure in which an electrode assembly is stored in a flexible battery case together with an electrolyte solution,

wherein the electrode assembly has a one directionally spiral-wound structure with a separator sheet interposed between a positive electrode sheet and a negative electrode sheet, the positive electrode sheet and the negative electrode sheet being wound about a winding axis extending in first and second opposite vertical directions,

wherein the spiral-wound structure of the electrode assembly has a length (x) in a horizontal direction perpendicular to the winding axis and a height (y) in the first vertical direction, and the spiral-wound structure of the electrode assembly has first and second opposite ends that are separated by the length (x) in the horizontal direction, and a center that is equidistant between the first and second opposite ends,

wherein the spiral-wound structure of the electrode assembly is configured to be bent about a bending axis extending in the first and second opposite vertical directions, the bending axis being parallel to the winding axis, the spiral-wound structure having a naturally curved shape having a curvature radius (R) naturally formed without additional external force, the curvature radius (R) relative to the bending axis satisfying the following Equation 1:

S[{ 1/ln( x/y )}* t]=R   1

wherein t is an average thickness (mm) of the spiral-wound electrode assembly in radial directions extending perpendicularly from the bending axis toward the spiral-wound structure and S is a constant of 8 or more,

wherein the spiral-wound structure of the electrode assembly is configured to be repeatedly movable between two positions including a position in which opposing sides of the electrode assembly have a planar shape parallel to one another and in which the first and second opposite ends of the spiral-wound structure of the electrode assembly are spaced apart from the bending axis by a first distance that is greater than the curvature radius (R) and the center is spaced apart from the bending axis by a second distance that is equal to the curvature radius (R), and a position having the naturally curved shape in which the spiral-wound structure of the electrode assembly is bent about the bending axis according to the Equation 1 and in which the first and second opposite ends and the center of the spiral-wound structure of the electrode assembly are spaced apart from the bending axis by a second distance that is equal to the curvature radius (R), and

wherein a curved surface is formed on an outer surface of the flexible battery case when the spiral-wound structure is in the naturally curved shape.

9. A method of forming a flexible battery cell comprising an electrode assembly stored in a flexible battery case, the method comprising:

forming a spiral-wound structure of the electrode assembly having a one directionally spiral-wound structure with a separator sheet interposed between a positive electrode sheet and a negative electrode sheet, the spiral-wound structure having a length (x) in a horizontal direction perpendicular to a winding axis and a height (y) in the first vertical direction, the spiral-wound structure being configured to be repeatedly moved between two positions including a first position and a second position having a naturally curved shape in which the spiral-wound structure is bent about a bending axis extending in the first and second opposite vertical directions, the bending axis being parallel to the winding axis;

before the forming of the spiral-wound structure, selecting the length (x) and the height (y) such that the spiral-wound structure in the stably curved position has a curvature radius (R) relative to the bending axis satisfying the following Equation 1:

S[{ 1/ln( x/y )}* t]=R   1

wherein t is an average thickness (mm) of the spiral-wound electrode assembly in radial directions extending perpendicularly from the bending axis toward the spiral-wound structure and S is a constant of 8 or more; and

inserting the spiral-wound structure into the flexible battery case together with an electrolyte solution, the flexible battery case being configured such that an outer surface of the flexible battery cell has a curved shape when the spiral-wound structure is in the naturally curved shape.