FIELD OF THE INVENTION

The present invention relates to a lithium ion secondary battery and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

As a conventional lithium ion secondary battery, those in which an electrode assembly including a positive electrode and a negative electrode is housed and sealed in an outer package made of a hard container such as a metal can or an outer package made of a flexible laminate film are known. In recent years, as portable electronic devices such as mobile phones have become lighter and smaller, the use of an outer package made of a laminate film has been studied for lithium ion secondary batteries mounted on the portable electronic devices from the viewpoint of a reduction in thickness.

For example, as a portable electronic device, for example, regarding a living body-attached biological information measurement device, Patent Document 1 (WO 2012/140707 A) proposes using a lithium ion secondary battery in which a sheet-shaped electrode assembly is housed in an outer package made of a laminate film as a driving power source.

SUMMARY OF THE INVENTION

As the electrode assembly, a wound-type electrode assembly in which a long positive electrode and a long negative electrode are wound with a separator interposed therebetween, and a sheet-shaped electrode assembly, i.e., a stack-type electrode assembly in which a plate-shaped positive electrode and a plate-shaped negative electrode are stacked in a plane with a separator interposed therebetween have been known. The use of the wound-type electrode assembly is characterized in that, as the long positive electrode and the long negative electrode are wound, a binding force is applied to the entire electrode assembly, and the battery reaction between the positive electrode and the negative electrode occurs uniformly, so that the cycle characteristics are excellent. On the other hand, the use of the stack-type electrode assembly has a problem that the battery reaction between the positive electrode and the negative electrode is not uniform and the cycle characteristics are reduced because a binding force is not applied to the electrode body unlike the wound type.

Moreover, in a lithium ion secondary battery including an outer package made of a metal can, a binding force is applied to the entire electrode assembly by the metal can, and the battery reaction between the positive electrode and the negative electrode occurs uniformly, so that the cycle characteristics are excellent. On the other hand, in a lithium ion secondary battery including an outer package made of a laminate film, because the binding force equal to or more than the atmospheric pressure is not applied to the electrode assembly, there is a problem that the battery reaction between the positive electrode and the negative electrode becomes non-uniform and the cycle characteristics are reduced.

Therefore, in a lithium ion secondary battery in which a stack-type electrode assembly is housed in an outer package made of a laminate film, there is a problem that the cycle characteristics are reduced as compared to a case where the wound-type electrode assembly is housed in a metal can.

Therefore, it is an object of the present inventors to provide a lithium ion secondary battery in which a stack-type electrode assembly is housed in an outer package made of a laminate film, which has excellent cycle characteristics, and a method of manufacturing the same.

In order to solve the above problem, a lithium ion secondary battery according to an aspect of the present invention includes an electrode assembly having a plate-shaped positive electrode and a plate-shaped negative electrode stacked with a separator interposed therebetween; and an outer package made of a laminate film housing the electrode assembly. The lithium ion secondary battery has a polygonal shape having at least one set of a width and a length in a plan view thereof, and C/(A×B) is 0.50 (N/mm2) to 1.20 (N/mm2), where A is a thickness of the lithium ion secondary battery, B is a width of the lithium ion secondary battery, and C is a three-point bending strength of the lithium ion secondary battery.

According to the above, it is possible to provide a lithium ion secondary battery in which a stack-type electrode assembly housed in an outer package made of a laminate film has excellent cycle characteristics. Also, the present invention makes it possible to increase the degree of freedom of designing a portable electronic device on which the lithium ion secondary battery is mounted.

Further, according to another aspect of the present invention, the lithium ion secondary battery has a rectangular shape in a plan view.

Further, according to another aspect of the present invention, the lithium ion secondary battery has an L-shape in a plan view.

Further, according to another aspect of the present invention, the separator has an adhesive layer at least on one surface.

Further, according to another aspect of the present invention, the A is 0.45 mm or more.

Further, another aspect of the present invention is a method of manufacturing the lithium ion secondary battery according to the aspect, the method comprising: a step of producing an electrode assembly having a plate-shaped positive electrode and a plate-shaped negative electrode stacked with a separator interposed therebetween; a step of housing the electrode assembly in an outer package made of a laminate film; an injection step of injecting an electrolyte into the outer package housing the electrode assembly; a sealing step of sealing the outer package under reduced pressure into which the electrolyte has been injected; a heat pressing step of heat-pressing the sealed outer package at 60° C. or higher and lower than 100° C.; and an initial charge step of initial charge at a temperature of 25° C. or higher and lower than 100° C.

According to the manufacture method, it is possible to provide a lithium ion secondary battery in which a stack-type electrode assembly is housed in an outer package made of a laminate film and which has excellent cycle characteristics.

Further, the separator preferably has an adhesive layer at least on one surface.

Further, the electrolyte is preferably injected such that the amount per battery capacitance is 1.3 to 1.7 g/Ah.

According to the present invention, it is possible to provide a lithium ion secondary battery in which a stack-type electrode assembly is housed in an outer package made of a laminate film and which has excellent cycle characteristics.

