BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to an electrode, a power storage device, an electronic device, and a method for fabricating the electrode.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.

2. Background Art

In recent years, a variety of power storage devices, for example, nonaqueous secondary batteries such as lithium-ion batteries (LIBs), lithium-ion capacitors (LICs), and air cells have been actively developed. In particular, demand for lithium-ion batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as cell phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion batteries are essential as rechargeable energy supply sources for today's information society.

A negative electrode for power storage devices such as lithium-ion batteries and lithium-ion capacitors is a structure body including at least a current collector (hereinafter referred to as a negative electrode current collector) and an active material layer (hereinafter referred to as a negative electrode active material layer) provided over a surface of the negative electrode current collector. The negative electrode active material layer contains an active material (hereinafter referred to as a negative electrode active material) which can receive and release lithium ions serving as carrier ions, such as carbon or silicon.

At present, to form a negative electrode of a lithium-ion battery which contains a general graphite-based carbon material, for example, graphite, acetylene black (AB), and polyvinylidene fluoride (PVdF) are used as a negative electrode active material, a conductive additive, and a binder, respectively. These materials are mixed with a solvent to make a paste. The paste is applied to a current collector and then the solvent is evaporated. In this manner, the negative electrode is made.

It is known that such a negative electrode of a lithium-ion battery and a lithium-ion capacitor has an extremely low electrode potential and a high reducing ability, and thus an electrolytic solution using an organic solvent is reductively decomposed. The range of potentials in which the electrolysis of an electrolytic solution does not occur is referred to as a potential window. The negative electrode potentials of a lithium-ion battery and a lithium-ion capacitor are out of the potential windows of an electrolytic solution in many cases. Therefore, the electrolytic solution is reductively decomposed, and a decomposition product called a passivating film (also referred to as a solid electrolyte film) is formed on the surface of the negative electrode. The passivating film prevents further reductive decomposition of the electrolytic solution; therefore, lithium ions can be inserted into the negative electrode with the use of a low electrode potential below the potential window of the electrolytic solution (for example, see Non-Patent Document 1).

Patent Document 1 discloses a charging method for a lithium-ion battery by which a passivating film on a negative electrode is prevented from being damaged and the negative electrode and the battery are inhibited from deteriorating.

REFERENCE

Patent Document

[Patent Document] Japanese Published Patent Application No. 2013-149609

Non-Patent Document

[Non-Patent Document 1] Zempachi Ogumi, “Lithium Secondary Battery”, Ohmsha, Ltd., the first impression of the first edition published on Mar. 20, 2008, pp. 116-118

SUMMARY OF THE INVENTION

A passivating film is a reductive decomposition product of a reductive decomposition reaction of an electrolytic solution or a product of a reaction between a reductive decomposition product and an electrolytic solution. For example, in the case where a negative electrode active material is graphite with a layered structure, a passivating film is formed between layers in an edge surface of the graphite and on a surface (basal surface) of the graphite.

However, when carrier ions are intercalated into the graphite and thus the volume of the graphite increases, part of the passivating film is separated from the edge surface or surface of the graphite and part of the graphite is exposed in some cases.

In addition, the thickness of the passivating film gradually increases by repeated charge and discharge. The passivating film having an increased thickness is susceptible to the volume expansion of a negative electrode active material, and part of the passivating film is easily separated.

When a surface of the negative electrode active material is exposed by the separation of the passivating film, another passivating film is formed on the surface.

A passivating film of a negative electrode is formed by a battery reaction in charging, and electric charge used for formation of the passivating film cannot be released. Thus, the passivating film results in irreversible capacity, reducing the discharge capacity of a battery.

In addition, separation of the passivating film from the negative electrode active material and formation of other passivating films on the surface of the negative electrode active material from which the previous passivating film has been separated by repeated charge and discharge further reduce the discharge capacity of the battery.

For example, the amount of lithium ions that are carriers is decreased in accordance with the number of electrons used in the electrochemical decomposition reaction of an electrolytic solution in a lithium-ion battery. Therefore, repeated generation of passivating films by charge and discharge of the lithium-ion battery, the capacity of the lithium-ion battery is decreased. In addition, the higher the temperature is, the faster the electrochemical reaction such as reductive decomposition of the passivating film proceeds. Thus, the capacity of the lithium-ion battery decreases more easily when charge and discharge are repeated at high temperatures.

When the negative electrode is soaked in the electrolytic solution, a binder that is part of a negative electrode active material layer absorbs a liquid ingredient and is expanded or deformed in some cases. If adhesion between the negative electrode material and the binder is weak, a material in the active material layer is separated due to expansion or deformation of the binder, which leads to decrease of electric conductivity in the active material layer. This also makes it easier to decrease the capacity of the lithium-ion battery.

Not only lithium-ion secondary batteries but also other power storage devices such as lithium-ion capacitors have the above problems.

An object of one embodiment of the present invention is to provide an active material with a film having an excellent insulating property. Another object is to make it difficult to form a passivating film by inhibiting the electrochemical decomposition of an electrolytic solution on an electrode as much as possible. Another object is to inhibit decrease of the initial discharge capacity of a power storage device.

Another object of one embodiment of the present invention is to provide an active material with a film having high flexibility and high adhesion to the active material. Another object is to inhibit separation of the film and generation of another passivating film in an electrode. Another object of one embodiment of the present invention is to provide a film that increases adhesion between an active material and a binder. Another object is to prevent decrease of electric conductivity in an active material layer due to expansion or the like of an active material or a binder. Another object is to improve the long-term cycle performance of a power storage device.

Another object of one embodiment of the present invention is to provide an active material with a film having high heat resistance. Another object is to inhibit a reductive decomposition reaction of an electrolytic solution at high temperatures. Another object is to provide a power storage device where discharge capacity is less likely to be decreased even when charge and discharge are repeated at high temperatures.

Another object of one embodiment of the present invention is to provide a formation method by which a uniform thin film is efficiently formed on a surface of an active material. Another object of one embodiment of the present invention is to provide a manufacturing method for an electrode where the decomposition reaction of an electrolytic solution is inhibited as much as possible and a passivating film is less likely to be formed. Another object of one embodiment of the present invention is to provide a manufacturing method for an electrode where expansion of an active material and deterioration due to use at high temperatures are inhibited.

Another object of one embodiment of the present invention is to provide a novel electrode, a novel power storage device, or the like. Note that the descriptions of these objects do not disturb the existence of other objects. Note that one embodiment of the present invention does not necessarily achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is an electrode including a current collector and an active material layer. The active material layer includes an active material, a film including silicone, a conductive additive, and a binder. The active material is in the form of particles. The film including silicone covers at least part of the active material.

One embodiment of the present invention is a power storage device including a first electrode and a second electrode. The first electrode is configured to operate as one of a positive electrode and a negative electrode. The second electrode is configured to operate as the other of the positive electrode and the negative electrode. The first electrode includes a current collector and an active material layer. The active material layer includes an active material, a film including silicone, a conductive additive, and a binder. The active material is in the form of particles. The film including silicone covers at least part of the active material.

One embodiment of the present invention is an electronic device including the power storage device with the above structure and a display panel, a light source, an operation key, a speaker, or a microphone.

One embodiment of the present invention is a manufacturing method for an electrode including the following steps: a step of forming a mixed liquid of an active material, silicone, and a first solvent; a step of spraying the mixed liquid from a nozzle and evaporating the first solvent to form an active material with a film thereon; a step of forming a paste including the active material with the film thereon, a conductive additive, a binder, and a second solvent; and a step of applying the paste to a current collector and evaporating the second solvent to form an active material layer.

One embodiment of the present invention is a manufacturing method for an electrode including the following steps: a step of forming a mixed liquid of an active material, silicone, a conductive additive, and a first solvent; a step of spraying the mixed liquid and evaporating the first solvent with a spray dryer to form a mixture with a film thereon; a step of forming a paste including the mixture with the film thereon, a binder, and a second solvent; and a step of applying the paste to a current collector and evaporating the second solvent to form an active material layer.

In any of the above structures, the active material preferably includes at least one of graphite, silicon, and silicon monoxide. In any of the above structures, the silicone preferably includes a hydrophobic functional group. In any of the above structure, the silicone preferably includes a hydrophobic functional group including at least one of a phenyl group, an alkyl group, an alkoxy group, a carbonyl group, and an ester group. In any of the above structure, the silicone preferably includes a hydrophilic functional group.

One embodiment of the present invention can provide an active material with a film having a high insulating property. In addition, one embodiment of the present invention makes it difficult to form a passivating film by inhibiting the electrochemical decomposition of an electrolytic solution in an electrode as much as possible. The above effects can inhibit decrease of initial discharge capacity of a power storage device.

One embodiment of the present invention can provide an active material with a film that has high flexibility and high adhesion to the active material. Separation of the film and regeneration of a passivating film in an electrode can be suppressed. One embodiment of the present invention can provide a film that increases adhesion between an active material and a binder. An electron conduction path can be prevented from being blocked in an active material layer due to expansion or the like of an active material or a binder because the film functions as a binder. The above effects can improve long-term cycle performance of a power storage device.

One embodiment of the present invention can provide an active material coated with a film having high heat resistance. This can inhibit a reductive decomposition reaction of an electrolytic solution at high temperatures. Because of achievement of the above objects, a power storage device where discharge capacity is less likely to be decreased even when charge and discharge are repeated at high temperatures can be provided.

One embodiment of the present invention can provide a formation method by which a uniform thin film is efficiently formed on a surface of an active material. One embodiment of the present invention can provide a manufacturing method for an electrode where a decomposition reaction of an electrolytic solution is inhibited as much as possible and a passivating film is less likely to be formed. One embodiment of the present invention can provide a manufacturing method for an electrode where expansion of an active material or deterioration due to use at high temperatures are inhibited.

One embodiment of the present invention can provide a novel electrode, a novel power storage device, or the like. 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.

