Flat secondary battery

1. A flat lithium ion secondary battery comprising:

a laminate-type power generation element in which two or more plate-like positive electrode plates and negative electrode plates are laminated via each of separators; and

a pair of rectangular exterior members when viewed from a lamination direction of the two or more positive electrode plates and negative electrode plates, the rectangular exterior members sealing the laminate-type power generation element and an electrolyte solution, wherein

at least one exterior member of the pair of the rectangular exterior members comprises:

an abutting part including an abutting surface that abuts against an uppermost layer electrode of the two or more positive electrode plats and negative electrode plates;

a sealing part at which the rectangular exterior members overlap each other at an outer circumferential position of the rectangular exterior members; and

an extending part that extends from the abutting part to the sealing part,

the positive electrode plate includes an NMC composite oxide and polyvinylidene fluoride,

the negative electrode plate includes a mixture of graphite and polyvinylidene fluoride or a mixture of graphite and styrene-butadiene copolymer latex,

the electrolyte solution includes ester-based solvent of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate as an organic liquid solvent,

the flat lithium ion secondary battery satisfies

1.04

≤

L

b

2

+

d

2

≤

1.21

and

1.66≤ L/d≤ 1.94,

a value of a ratio of a battery surface area to a rated capacity of the flat lithium ion secondary battery is 5 cm2/Ah or more, the rated capacity is 3 Ah or more, and the battery surface area is a projected area of the flat lithium ion secondary battery including a battery exterior body,

a capacity retention ratio is 80% or more and the capacity retention ratio is a capacity retention ratio to initial capacity retention ratio of the flat lithium ion secondary battery when used 1000 cycles, and

an aspect ratio of the electrode plates is 1 to 3 and the aspect ratio is defined as a ratio of a length of the long sides to a length of the short sides of a lamination surface of the two or more positive electrode plates and negative electrode plates,

wherein, in a cross section of the flat secondary battery when cutting through the flat secondary battery by a plane parallel to sides that form the rectangular exterior members and along the lamination direction,

the L represents a length of the extending part,

the b represents a length from an inter-sealing part electrode of the two or more positive electrode plates and negative electrode plates to the sealing parts, the inter-sealing part electrode is located at height of the sealing part in the lamination direction, and

the d represents a thickness of the laminate-type power generation element from the inter-sealing part electrode to the uppermost layer electrode.

2. The flat lithium ion secondary battery according to claim 1 ,

wherein the extending part includes a parallel plane that is in the same plane with the abutting surface, and the flat secondary battery satisfies

0.25≤ a/d≤ 0.88

wherein, in the cross section, the a represents a length of the parallel plane in a direction perpendicular to the lamination direction.

3. The flat lithium ion secondary battery according to claim 1 ,

wherein the extending part includes a first extending part that extends from an end part of the abutting part and a second extending part that is inclined to the abutting surface between the first extending part and the sealing part, and

a length of the first extending part in the cross section decreases as the d increases due to use of the flat secondary battery.