TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to an electronic device and its operating system.

Note that electronic devices in this specification mean all devices including secondary batteries, and electro-optical devices including secondary batteries, information terminal devices including secondary batteries, and the like are all electronic devices.

BACKGROUND ART

Portable electronic devices and wearable electronic devices are under active development. For example, a thin portable electronic book is disclosed in Patent Document 1.

Portable electronic devices and wearable electronic devices operate using batteries as power sources. Portable electronic devices need to withstand extended use and thus can incorporate high-capacity secondary batteries. In that case, there occurs a problem that the size and weight of the electronic devices are large. In view of this problem, a small and thin high-capacity secondary battery that can be incorporated in a portable electronic device is under development.

Metal cans are used as exterior bodies of secondary batteries, and electrolytes and the like are contained in the metal cans.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. S63-15796

DISCLOSURE OF INVENTION

A metal can used as an exterior body has a problem of increasing the weight of a secondary battery. Moreover, it is difficult to manufacture a thin metal can by molding and fabricate a secondary battery using a thin metal can, in order to obtain a thin secondary battery.

The use of a film (also referred to as a laminate film) including a stack of metal foil (e.g., aluminum foil or stainless steel foil) and a resin (heat-seal resin) as an exterior body allows fabrication of a secondary battery that is thinner and more lightweight than a secondary battery using a metal can. An object of one embodiment of the present invention is to provide a novel power storage device, a novel secondary battery, or the like. Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

In the case where a film, which has lower strength than a metal can, is used as an exterior body of a secondary battery, a current collector provided in a region surrounded by the exterior body, an active material layer provided on a surface of the current collector, or the like might be damaged when force is externally applied to the secondary battery. The current collector includes a projection (also referred to as an electrode tab portion) for connection to a lead electrode. When the secondary battery is bent by external force, part of the current collector suffers damage such as a crack in the vicinity of the projection (electrode tab portion), leading to breakage of the secondary battery.

A cushioning material is provided in a region surrounded by an exterior body of a secondary battery. Specifically, a cushioning material is provided on the periphery of a current collector such that a sealing portion of an exterior body (film) is located outside the cushioning material. To increase capacity, a plurality of units each including at least a positive electrode current collector, a separator, and a negative electrode current collector are stacked in a region surrounded by an exterior body. Note that a cushioning material is provided between the exterior body and the outermost current collector. The cushioning material has a thickness larger than that of the separator and that of the current collector. Furthermore, the cushioning material may be formed using the same material as that for the separator so as to have a rolled-sheet shape.

Examples of the shape of the cushioning material include a flat plate shape, a bar-like shape, a spherical shape (e.g., the shape of a plastic bead or a glass bead), and a rectangular solid shape. For example, a sheet-like plastic film with slits may be employed. Furthermore, a plurality of cushioning materials may be provided in a region surrounded by an exterior body. The materials may have different sizes and shapes. For example, an aggregate including bound multiple fiber threads (glass fiber) may be used as the cushioning material. Alternatively, an aggregate (weave) including organic resin threads woven like a cloth may be used. Furthermore, a sheet-like material that is rolled or folded may be used. Specifically, the cushioning material (plastic film) with a thin plate-like shape the area of which is larger than that of the current collector is provided in the region surrounded by the exterior body, so as to overlap with the current collector. One embodiment disclosed in this specification is a secondary battery including a film, and a first current collector, an active material layer, a second current collector, and a cushioning material in a region surrounded by the film. The cushioning material is a plastic film whose area is larger than the area of a region where the first current collector and the second current collector overlap with each other.

A frame-like cushioning material is provided in a region surrounded by an exterior body such that a planar shape of a combination of a current collector and the cushioning material that fits the outer shape of the current collector is almost a rectangle. In this case, even when the current collector and the cushioning material are not in contact with each other in a region surrounded by the exterior body, sealing the outer edge of the exterior body by thermocompression bonding makes the boundary between the current collector and the cushioning material unclear in appearance. In the appearance of a secondary battery disclosed in this specification, the outer edge is thin because of thermocompression bonding. A center portion where a stack (e.g., a first current collector, a separator, and a second current collector) exist has a larger thickness than the outer edge. A cushioning material is provided between the center portion and the outer edge, and a portion between the center portion and the outer edge is thicker than the outer edge. Furthermore, a first lead electrode that is electrically connected to the first current collector in the stack projects and is exposed, and a second lead electrode that is electrically connected to the second current collector projects and is exposed.

A material of the cushioning material is preferably an insulator (e.g., plastic, rubber (natural rubber or synthetic rubber), glass, nonwoven fabric, or paper). In particular, a material of the cushioning material is preferably an elastic material (e.g., synthetic rubber such as silicone rubber, fluorine rubber, chloroprene rubber, nitrile-butadiene rubber, ethylene-propylene rubber, or styrene-butadiene rubber). Specifically, a material having a larger elastic modulus than the separator is used as a material of the cushioning material. Alternatively, a porous material including air bubbles (e.g., sheet-like styrofoam or sponge rubber formed using any of the synthetic rubber materials listed above) may be used as a material of the cushioning material. A gelled material may be used as a material of the cushioning material.

Alternatively, a conductive material that has an insulating surface can be used as a material of the cushioning material. Examples of a material of the cushioning material include carbon fiber whose surface is coated with an organic resin; metal foil (e.g., aluminum foil, copper foil, or stainless steel foil) having a surface over which an inorganic insulating film such as a silicon oxide film is formed; and metal foil whose surface is coated with an organic resin.

The cushioning material provided in the region surrounded by the exterior body in a secondary battery allows the current collector and the like to be stably positioned. When the secondary battery is bent to have a desired form, the cushioning material can also be bent so that the secondary battery can have the desired form, contributing to maintaining of the bent form of the secondary battery. Furthermore, a restricting function of preventing the secondary battery from being bent more than necessary may be provided. The cushioning material can also serve as a framework of the secondary battery.

The cushioning material is not necessarily provided in the region surrounded by the exterior body, and may be provided so as to be partly exposed. In that case, the cushioning material itself serves as part of the exterior body, that is, a sealing material.

Depending on the mode of an electronic device, a secondary battery provided in the electronic device is bent, and it is preferred that a cushioning material be also bendable. Thus, a material of the cushioning material is preferably flexible. Furthermore, even when the volume of an electrolytic solution in the secondary battery is reduced because of deterioration over time, generation of wrinkles and a change in appearance of the secondary battery by bending can be suppressed. Moreover, the cushioning material, which is located in a region surrounded by an exterior body, relieves concentration of an impulse that is applied to the secondary battery, protecting the secondary battery from being locally bent and damaged.

Another embodiment of the present invention is a power storage unit including a plurality of first current collectors for positive electrodes and second current collectors for negative electrodes in a region surrounded by an exterior body.

The power storage unit of one embodiment of the present invention can change its form with a curvature radius of greater than or equal to 10 mm, preferably greater than or equal to 30 mm. One or two films are used as the exterior body of the power storage unit. In the case where the power storage unit has a layered structure, the power storage unit has a cross section sandwiched by two curves of the film(s) when bent.

Description is given of the curvature radius of a surface with reference toFIGS. 7A to 7C. InFIG. 7A, on a plane1701along which a curved surface1700is cut, part of a curve1702forming the curved surface1700is approximate to an arc of a circle, and the radius of the circle is referred to as a radius1703of curvature and the center of the circle is referred to as a center1704of curvature.FIG. 7Bis a top view of the curved surface1700.FIG. 7Cis a cross-sectional view of the curved surface1700taken along the plane1701. When a curved surface is cut along a plane, the radius of curvature of a curve in a cross section differs depending on the angle between the curved surface and the plane or on the cut position, and the smallest radius of curvature is defined as the radius of curvature of a surface in this specification and the like.

In the case of bending a power storage unit in which a component1805including electrodes and an electrolytic solution is sandwiched between two films as exterior bodies, a radius1802of curvature of a film1801close to a center1800of curvature of the power storage unit is smaller than a radius1804of curvature of a film1803far from the center1800of curvature (FIG. 8A). When the power storage unit is curved and has an arc-shaped cross section, compressive stress is applied to a surface of the film on the side closer to the center1800of curvature and tensile stress is applied to a surface of the film on the side farther from the center1800of curvature (FIG. 8B). However, by forming a pattern including projections or depressions on surfaces of the exterior bodies, the influence of a strain can be reduced to be acceptable even when compressive stress and tensile stress are applied. For this reason, the power storage unit can change its form such that the exterior body on the side closer to the center of curvature has a curvature radius greater than or equal to 10 mm, preferably greater than or equal to 30 mm.

Note that the cross-sectional shape of the power storage unit is not limited to a simple arc shape, and the cross section can be partly arc-shaped; for example, a shape illustrated inFIG. 8C, a wavy shape illustrated inFIG. 8D, or an S shape can be used. When the curved surface of the power storage unit has a shape with a plurality of centers of curvature, the power storage unit can change its form such that a curved surface with the smallest radius of curvature among radii of curvature with respect to the plurality of centers of curvature, which is a surface of the exterior body on the side closer to the center of curvature, has a curvature radius greater than or equal to 10 mm, preferably greater than or equal to 30 mm.

One embodiment of the present invention can be used for various power storage devices. Examples of such power storage devices include a battery, a primary battery, a secondary battery, a lithium-ion secondary battery (including a lithium-ion polymer secondary battery), and a lithium air battery. In addition, a capacitor is given as another example of the power storage devices. For example, with a combination of the negative electrode of one embodiment of the present invention and an electric double layer positive electrode, a capacitor such as a lithium-ion capacitor can be fabricated.

The degree of change in form of a secondary battery due to external force, that is, a change in part of an internal structure of the secondary battery due to external force can be adjusted by a material or the position of a cushioning material. For example, the cushioning material can suppress sharp bending so that the internal structure of the secondary battery is not broken. Thus, the cushioning material can protect the internal structure from being damaged by external bending force. Furthermore, a novel power storage device, a novel secondary battery, or the like can be provided. Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

