This nonprovisional application is based on Japanese Patent Application No. 2018-044215 filed on Mar. 12, 2018 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND

Field

The present disclosure relates to a positive electrode, a lithium-ion secondary battery, and a method of producing a positive electrode.

Description of the Background Art

Japanese Patent Laying-Open No. 2003-157852 discloses forming a film on a surface of an aluminum (Al) foil by heating the Al foil in water.

SUMMARY

When a short circuit is caused in a battery by, for example, an impact exerted from outside the battery (hereinafter, also called “external input”) and if the positive electrode current collector (an Al foil, with a low resistance) comes into direct contact with the negative electrode (counter electrode), the short-circuit current is considered to be increased. The increase in the short-circuit current is considered to lead to an increase in heat generation. The “short circuit caused by an external input” is simulated by a nail penetration test, for example. Hereinafter, the “short circuit caused by an external input” may be simply referred to as “short circuit”.

If a high-resistance film is formed on a surface of the Al foil, heat generation occurring upon a short circuit is expected to be decreased. The reason is considered to be that the high-resistance film inhibits the Al foil from coming into direct contact with the negative electrode.

In Japanese Patent Laying-Open No. 2003-157852, the film formation on a surface of the Al foil is carried out by heating the foil in water. This heat treatment is also called “boehmite treatment”. Usually, a film formed by boehmite treatment tends to be hard. The hard film is considered to be broken or crushed when an electrode (positive electrode) is compressed during battery production. This breakage of the film is considered to lead to an increase in heat generation that occurs upon a short circuit.

An object of the present disclosure is to reduce heat generation that occurs in a battery upon a short circuit caused by, for example, an impact exerted from outside the battery.

In the following, the technical structure and the effects according to the present disclosure are described. It should be noted that part of the action mechanism according to the present disclosure is based on presumption. Therefore, the scope of claims should not be limited by whether or not the action mechanism is correct.

[1] A positive electrode according to the present disclosure is a positive electrode for a lithium-ion secondary battery.

The positive electrode includes at least a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer is formed on a surface of the positive electrode current collector. The positive electrode current collector includes an aluminum foil and a porous film. The porous film covers a surface of the aluminum foil. The porous film contains at least aluminum oxide. The porous film has a thickness not smaller than 10 nm and not greater than 800 nm. The porous film has a dynamic hardness not lower than 5 and not higher than 200.

The porous film according to the present disclosure contains aluminum oxide. In other words, the porous film is a ceramic film. The ceramic film can have a high resistance. It is considered that the porous film can inhibit the Al foil from coming into direct contact with a negative electrode upon a short circuit.

The dynamic hardness is a measure of the hardness of the porous film. The lower the dynamic hardness is, the softer the porous film is considered to be. The dynamic hardness of a film formed by the ordinary boehmite treatment is considered to be higher than 200. By the way, the dynamic hardness of the porous film according to the present disclosure is not higher than 200. Therefore, it is considered that the porous film according to the present disclosure is softer than a film formed by the ordinary boehmite treatment and therefore, upon electrode compression, continues covering a surface of the Al foil without being broken or crushed. Consequently, the positive electrode according to the present disclosure is considered to reduce heat generation that occurs upon a short circuit.

The dynamic hardness is not lower than 5. Forming a porous film having a dynamic hardness lower than 5 is considered to be difficult. The porous film has a thickness not smaller than 10 nm and not greater than 800 nm. In the configuration in which the thickness is smaller than 10 nm, only a small heat-generation-reducing effect can be obtained. In the configuration in which the thickness is greater than 800 nm, battery resistance can increase to a non-negligibly high level.

[2] The dynamic hardness of the porous film may be not higher than the dynamic hardness of the aluminum foil.

When an external input is applied and a foreign body penetrates the positive electrode, the positive electrode is considered to receive an impact. This impact is considered to cause deformation of the positive electrode. Upon deformation of the positive electrode, the porous film (which is softer than the Al foil) is considered to be likely to follow the contour of the Al foil. As a result, exposure of the Al foil is expected to be inhibited.

[3] The positive electrode according to the present disclosure may further include a conductive material. The conductive material may be disposed within the positive electrode active material layer and within pores formed in the porous film.

Normal use of the battery requires electronic conduction be established between the Al foil and the positive electrode active material layer. Because the conductive material is thus disposed within the positive electrode active material layer and within pores formed in the porous film, a desired electronic conduction is expected to be established between the Al foil and the positive electrode active material layer for normal use of the battery.

[4] The porous film may have a thickness not greater than 500 nm.

In the configuration in which the porous film has such a thickness, battery resistance is expected to be decreased.

[5] A lithium-ion secondary battery according to the present disclosure includes at least the positive electrode according to any one of [1] to [4] above and a negative electrode.

In the lithium-ion secondary battery according to the present disclosure, the level of heat generation that occurs upon a short circuit is expected to be small. The reason is considered to be that the porous film inhibits the Al foil from coming into direct contact with the negative electrode upon a short circuit.

[6] A method of producing a positive electrode according to the present disclosure is a method of producing a positive electrode for a lithium-ion secondary battery. The method of producing a positive electrode according to the present disclosure includes at least the following (a) to (d):

(a) preparing an aluminum foil;

(b) forming a porous film on a surface of the aluminum foil by bringing the aluminum foil into contact with superheated steam so as to form a positive electrode current collector;

(c) forming a positive electrode active material layer on a surface of the positive electrode current collector; and

(d) compressing the positive electrode current collector and the positive electrode active material layer together, wherein

the porous film contains at least aluminum oxide,

the porous film after compression has a thickness not smaller than 10 nm and not greater than 800 nm, and

the porous film after compression has a dynamic hardness not lower than 5 and not higher than 200.

By the method of producing a positive electrode according to the present disclosure, the positive electrode according to [1] above may be produced. The method of producing a positive electrode according to the present disclosure uses superheated steam. The “superheated steam” herein refers to water vapor heated to a temperature higher than 100° C. It is considered that in superheated steam, compared to either in water vapor at 100° C. or in hot water, hydrothermal reaction proceeds rapidly and the resulting porous film is likely to have pores with great pore sizes formed in the porous film. Having pores with great pore sizes, the porous film is considered to be soft. More specifically, it is considered that the porous film can have a dynamic hardness not higher than 200.

[7] In the method of producing a positive electrode according to the present disclosure, the positive electrode active material layer may be formed so as to contain a conductive material. A part of the conductive material may be disposed within pores formed in the porous film by compressing the positive electrode current collector and the positive electrode active material layer.

By this production method, the positive electrode according to [3] above may be produced.

The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings.

