WO2022210393A1

WO2022210393A1 - Lithium secondary battery and method for producing lithium secondary battery

Info

Publication number: WO2022210393A1
Application number: PCT/JP2022/014547
Authority: WO
Inventors: 滝太郎山口; 慎吾松本
Original assignee: 住友化学株式会社
Priority date: 2021-03-31
Filing date: 2022-03-25
Publication date: 2022-10-06
Also published as: JP2022156172A

Abstract

A lithium secondary battery which is provided with an aluminum negative electrode that is capable of absorbing and desorbing lithium ions, a positive electrode that is capable of absorbing and desorbing lithium ions, and an electrolyte, wherein: the aluminum negative electrode is configured from an aluminum-containing metal; and formula (1) is satisfied. (1): 20 Ω/cm2 ≤ Rs ≤ 100 Ω/cm2 (The lithium secondary battery is discharged, and the voltage after discharge is represented by V0. Subsequently, the lithium secondary battery is charged at 1.0 mA/cm2, and the voltage after 2 seconds is represented by V1. The direct-current resistance value per unit area of the positive electrode as calculated from the difference between V1 and V0 (V1 - V0) is represented by Rs.)

Description

Lithium secondary battery and method for manufacturing lithium secondary battery

The present invention relates to a lithium secondary battery and a method for manufacturing a lithium secondary battery.
This application claims priority based on Japanese Patent Application No. 2021-059729 filed in Japan on March 31, 2021, the content of which is incorporated herein.

　Concerning the negative electrode that constitutes the lithium secondary battery, studies are being conducted to improve battery performance by using a material that has a larger theoretical capacity than graphite, which is the conventional negative electrode material. As such materials, similar to graphite, metallic materials capable of intercalating and deintercalating lithium ions, for example, have attracted attention.

As an example of a negative electrode made of a metal material, for example, Patent Document 1 describes a negative electrode composed of a negative electrode active material for a secondary battery, which is a porous aluminum alloy and contains at least one of silicon and tin. ing.

JP 2011-228058 A

The metal negative electrode described in Patent Document 1 generally has a problem that the cycle retention rate tends to decrease.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a lithium secondary battery in which the cycle retention rate is less likely to decrease, and a method for manufacturing the lithium secondary battery.

The present invention includes the following [1] to [4].
[1] Lithium, comprising an aluminum negative electrode capable of intercalating and deintercalating lithium ions, a positive electrode capable of intercalating and deintercalating lithium ions, and an electrolyte, wherein the aluminum negative electrode is made of an aluminum-containing metal and satisfies the following (1): secondary battery.
20 Ω/cm ² ≤ Rs ≤ 100 Ω/cm ² (1)
(The lithium secondary battery is discharged, and the voltage after discharge is V0. After that, it is charged at 1.0 mA/cm ² and the voltage after 2 seconds is V1. The difference between V1 and V0 (V1-V0 ) is the DC resistance value per unit area of the positive electrode calculated from Rs.)
[2] The lithium secondary battery according to [1], which satisfies the following (2).
10 Ω/cm ² ≤ Ra ≤ 100 Ω/cm ² (2)
(Ra is the AC resistance value of the aluminum negative electrode obtained by discharging the lithium secondary battery, measuring the AC impedance, and analyzing the Cole-Cole plot of the obtained impedance.)
[3] A method for producing a lithium secondary battery comprising an aluminum negative electrode capable of intercalating and deintercalating lithium ions, a positive electrode capable of intercalating and deintercalating lithium ions, and an electrolyte, wherein the aluminum negative electrode is made of an aluminum-containing metal. a step of assembling a pre-battery, a step of charging and discharging the pre-battery for the first time, and an aging step of storing the discharged pre-battery at an ambient temperature of 50° C. or higher for 4 hours or more. .
[4] The method for producing a lithium secondary battery according to [3], wherein an increase rate of Ra of the battery after the aging process to Ra of the pre-battery before the aging process satisfies 0% or more and 20% or less.

According to the present invention, it is possible to provide a lithium secondary battery in which the cycle retention rate is less likely to decrease, and a method for manufacturing the lithium secondary battery.

It is a schematic diagram for demonstrating the action mechanism of this invention. 1 is a schematic diagram showing an example of a lithium secondary battery; FIG.

<Lithium secondary battery>
This embodiment is a lithium secondary battery including an aluminum negative electrode capable of intercalating and deintercalating lithium ions, a positive electrode capable of intercalating and deintercalating lithium ions, and an electrolyte.
An aluminum negative electrode consists of an aluminum containing metal.

The lithium secondary battery of the present embodiment is manufactured by carrying out initial charging and discharging after assembling a pre-battery and going through a predetermined aging process.

In this specification, a lithium secondary battery before being charged and discharged for the first time is referred to as a pre-battery.
As used herein, "initial charging and discharging" means charging and discharging that are performed first after assembling a pre-battery. FIG. 1(a) is a schematic cross-sectional view of an aluminum negative electrode 40 placed in a pre-battery. When the pre-battery is charged for the first time, lithium ions are occluded on the surface of the aluminum negative electrode 40 and a Li—Al alloy layer 41 is formed. The Li—Al alloy layer is an alloy layer of lithium and aluminum. This state is shown in FIG. Since the Li—Al alloy layer 41 contains lithium ions, it can be charged and discharged.

Next, when the first discharge is performed, lithium ions are desorbed, and the Li--Al alloy layer 41 becomes a state in which a plurality of columnar structures 41a are arranged. This state is shown in FIG. The surface of the columnar structure 41a has an unstable crystal structure and is in a fragile state.

When the aging process is carried out in the discharged state, that is, in the state shown in FIG. 1(c), the Li—Al layer 42 having a columnar structure with a stabilized surface is formed. The present invention relates to the technical concept of judging the formation of a Li—Al alloy layer having a columnar structure with a stabilized surface using the value of Rs as an index.

The aluminum negative electrode satisfies the following (1).
20 Ω/cm ² ≤ Rs ≤ 100 Ω/cm ² (1)
(The lithium secondary battery is discharged, and the voltage after discharge is V0. After that, it is charged at 1.0 mA/cm ² and the voltage after 2 seconds is V1. The difference between V1 and V0 (V1-V0 ) is the DC resistance value per unit area of the positive electrode calculated from Rs.)

[Calculation method of Rs]
V0 is the voltage of the lithium secondary battery in the discharged state, and V1 is the voltage of the lithium secondary battery after the aging process. V1 becomes a higher value than V0 because the aging process increases the resistance value. Using a pre-battery assembled by the method described later, V1 and V0 are calculated respectively.

The difference (V1-V0) between V1 and V0 is calculated, and the DC resistance value Rs is calculated by the following formula.
Rs=V/I
(Rs is the DC resistance value, V is V1-V0, and I is the current (mA/cm ² ).)

According to the study of the present inventors, it was found that the resistance value increases when the pre-battery is aged in a discharged state. It is considered that the increase in the resistance value is due to the change in the crystal state of the Li—Al alloy. Specifically, it is thought that as crystallization progressed, it became difficult for lithium ions to detach, and the resistance increased.

It is believed that the crystal structure of the Li-Al alloy stabilizes as crystallization progresses. Therefore, when Rs satisfies (1), it is considered possible to manufacture a lithium secondary battery in which the cycle retention rate is less likely to decrease.

It is believed that if the aging process is not performed in the discharged state, the surface will not stabilize, although the resistance value will not increase. For this reason, it is thought that the crystal structure of the surface remains in a fragile state. This state is shown in FIG.

(1) is preferably any one of the following (1)-1 to (1)-3.
20 Ω/cm ² ≤ Rs ≤ 100 Ω/cm ² (1)-1
20 Ω/cm ² ≤ Rs ≤ 50 Ω/cm ² (1)-2
20 Ω/cm ² ≤ Rs ≤ 30 Ω/cm ² (1)-3

The lithium secondary battery preferably satisfies the following (2).
10 Ω/cm ² ≤ Ra ≤ 100 Ω/cm ² (2)
(Ra is the AC resistance value of the aluminum negative electrode obtained by discharging the lithium secondary battery, measuring the AC impedance, and analyzing the Cole-Cole plot of the obtained impedance.)

[How to calculate Ra]
Complex impedance measurement is performed at room temperature in the frequency band from 10 ⁶ Hz to 10 ⁻¹ Hz, and surface resistance is obtained by approximating the arc of the aluminum negative electrode component with a semicircle from the frequency band from 300 Hz to 10000 Hz.

Using a pre-battery assembled by the method described later, AC impedance measurement is performed using a potentio/galvanostat (SI1287, manufactured by Solartron) and a frequency response analyzer (1255B, manufactured by Solartron) as impedance measuring devices. The pre-battery is placed in a constant temperature bath at 25° C., and the impedance spectrum of the test battery is measured by the AC impedance method at a frequency of 10 ⁶ to 10 ⁻¹ Hz and a voltage amplitude of 10 mV.

According to the studies of the present inventors, it was found that aging a pre-battery in a discharged state increases the resistance value of a battery using an aluminum negative electrode. It is considered that the increase in the resistance value is due to the change in the crystal state of the Li—Al alloy. Since the crystal structure of the Li—Al alloy is believed to be stabilized by the aging process, it is believed that the use of an aluminum negative electrode whose Ra satisfies (2) makes it possible to manufacture a lithium secondary battery in which the cycle retention rate is less likely to decrease.

(2) is preferably any one of the following (2)-1 to (2)-3.
10 Ω/cm ² ≤ Ra ≤ 50 Ω/cm ² (2)-1
10 Ω/cm ² ≤ Ra ≤ 40 Ω/cm ² (2)-2
10 Ω/cm ² ≤ Ra ≤ 30 Ω/cm ² (2)-3

Materials constituting the lithium secondary battery of the present embodiment will be described below.

<<Aluminum negative electrode>>
In this embodiment, the aluminum negative electrode is made of an aluminum-containing metal.
Examples of the aluminum negative electrode include aluminum negative electrodes 1 to 3 described below.

[Aluminum negative electrode 1]
The aluminum negative electrode 1 is made of an aluminum-containing metal. The aluminum-containing metal acts as a negative electrode active material.
The aluminum-containing metal of the aluminum negative electrode 1 has a non-aluminum metal phase dispersed in an aluminum metal phase.
A non-aluminum metallic phase means a metallic phase that does not contain aluminum.
The non-aluminum metal phase preferably comprises a non-aluminum metal compound containing one or more selected from the group consisting of Si, Ge, Sn, Ag, Sb, Bi, In and Mg.

The non-aluminum metal phase is more preferably composed of a non-aluminum metal compound containing one or more selected from the group consisting of Si, Ge, Sn, Ag, Sb, Bi and In.

(Manufacturing method of aluminum negative electrode 1)
The aluminum negative electrode 1 is preferably manufactured by a manufacturing method including an alloy casting process and a rolling process.

Alloy Casting Step When casting is performed, first, a predetermined amount of a metal that constitutes a non-aluminum metal phase is added to aluminum or high-purity aluminum to obtain a mixture 1 . High-purity aluminum can be obtained by the method described below. Next, the mixture 1 is melted at 680° C. or higher and 800° C. or lower to obtain a molten alloy 1 of aluminum and metal.

Aluminum with a purity of 99.9% by mass or more, high-purity aluminum with a purity of 99.99% by mass or more, or the like can be used as aluminum constituting the aluminum phase.

The metal constituting the non-aluminum metal phase is one or more selected from the group consisting of Si, Ge, Sn, Ag, Sb, Bi, In and Mg. High-purity silicon having a purity of 99.999% by mass or more is used as the metal constituting the non-aluminum metal phase, for example.

The molten alloy 1 cleaned by vacuum treatment or the like is cast using a mold to form an ingot.
As the mold, an iron mold heated to 50° C. or more and 200° C. or less or a graphite mold is used. The aluminum negative electrode 1 can be cast by a method of pouring the molten alloy 1 at 680° C. or higher and 800° C. or lower into a mold. Alternatively, an ingot may be obtained by semi-continuous casting.

- Rolling process The ingot of the obtained alloy can be used for the aluminum negative electrode 1 by cutting as it is. The ingot is preferably rolled, extruded, or forged into a plate shape. Moreover, it is more preferable to make it into a plate shape by rolling.

The ingot rolling step is, for example, a step of performing hot rolling and cold rolling to process the ingot into a plate shape.
For example, hot rolling is performed repeatedly until the aluminum ingot reaches the desired thickness under the conditions of a temperature of 350 ° C. or higher and 550 ° C. or lower and a processing rate per rolling of 2% or higher and 30% or lower. . Here, the "rolling rate" means the rate of change in thickness when rolled. For example, when a plate with a thickness of 1 mm is reduced to a thickness of 0.7 mm, the processing rate is 30%.

After hot rolling, if necessary, it is preferable to perform an intermediate annealing treatment before cold rolling. The intermediate annealing treatment is performed, for example, by heating the hot-rolled sheet material to raise the temperature, and then allowing it to cool.

In the temperature raising step in the intermediate annealing treatment, the temperature may be raised to, for example, 350°C or higher and 550°C or lower. Further, in the temperature raising step, for example, a temperature of 350° C. or more and 550° C. or less may be maintained for about 1 hour or more and 5 hours or less.

In the cooling step in the intermediate annealing treatment, cooling may be performed immediately after raising the temperature. In the cooling step, it is preferable to cool down to about 20°C.

Cold rolling is preferably performed at a temperature below the recrystallization temperature of aluminum. Moreover, it is preferable to repeatedly roll the aluminum ingot until it reaches the target thickness under the condition that the rolling rate per rolling is 1% or more and 20% or less. As for the temperature of cold rolling, the temperature of the metal to be rolled may be adjusted from 10° C. to 80° C. or less.

After cold rolling, heat treatment may be further performed. The heat treatment after cold rolling is usually performed in the atmosphere, but may be performed in a nitrogen atmosphere or a vacuum atmosphere. In addition to softening the work-hardened plate material by heat treatment, various physical properties, specifically hardness, electrical conductivity and tensile strength, may be adjusted by controlling the crystal structure.
The heat treatment conditions include, for example, conditions for heat treatment at a temperature of 300° C. or higher and 400° C. or lower for 5 hours or longer and 10 hours or shorter.

The thickness of the aluminum negative electrode 1 is preferably 5 μm or more, more preferably 6 μm or more, and even more preferably 7 μm or more. Moreover, it is preferably 200 μm or less, more preferably 190 μm or less, and even more preferably 180 μm or less.
The above upper limit and lower limit of the thickness of the aluminum negative electrode 1 can be combined arbitrarily. In this embodiment, the thickness of the aluminum negative electrode 1 is preferably 5 μm or more and 200 μm or less.
The thickness of the aluminum negative electrode 1 may be measured using a thickness gauge or vernier caliper.

Method for Purifying Aluminum When high-purity aluminum is used as the material for the aluminum negative electrode or the material for the alloy, examples of refining methods for purifying aluminum include a segregation method and a three-layer electrolysis method.

The segregation method is a purification method that utilizes the segregation phenomenon during the solidification of molten aluminum, and multiple methods have been put into practical use. As one form of the segregation method, there is a method of pouring molten aluminum into a container, heating the upper molten aluminum while rotating the container, and solidifying refined aluminum from the bottom while stirring. High-purity aluminum with a purity of 99.99% by mass or more can be obtained by the segregation method.

The three-layer electrolysis method is an electrolysis method for highly purifying aluminum. As one form of the three-layer electrolysis method, first, relatively low-purity aluminum or the like (for example, JIS-H2102 with a purity of 99.9% by mass or less, grade 1 or so) is added to the Al-Cu alloy layer. do. Thereafter, the molten state is used as an anode, and an electrolytic bath containing, for example, aluminum fluoride and barium fluoride is placed thereon to deposit high-purity aluminum on the cathode.
High-purity aluminum with a purity of 99.999% by mass or more can be obtained by the three-layer electrolysis method.

　The method of purifying aluminum is not limited to the segregation method and the three-layer electrolysis method, and other known methods such as the zone melting refining method and the ultra-high vacuum dissolution method may be used.

[Aluminum negative electrode 2]
As the aluminum negative electrode 2, the following high-purity aluminum or high-purity aluminum alloys (1) to (5) are preferable.
(1) High-purity aluminum-magnesium alloy 1
It is an alloy of aluminum with a purity of 99.999% and magnesium. The content of magnesium in the total amount of aluminum-containing metal is 0.1% by mass or more and 4.0% by mass or less. The average corrosion rate is 0.04 mm/year to 0.06 mm/year.
(2) High-purity aluminum-magnesium alloy 2
It is an alloy of 99.9% pure aluminum and magnesium. The content of magnesium in the total amount of aluminum-containing metal is 0.1% by mass or more and 1.0% by mass or less. The average corrosion rate is 0.1 mm/year to 0.14 mm/year.
(3) High Purity Aluminum-Nickel Alloy An alloy of aluminum with a purity of 99.999% and nickel. The content of nickel in the total amount of aluminum-containing metal is 0.1% by mass or more and 1.0% by mass or less. The average corrosion rate is 0.1 mm/year to 0.14 mm/year.
(4) High-purity aluminum-manganese-magnesium alloy
It is an alloy of 99.99% pure aluminum, manganese and magnesium. The total content of manganese and magnesium in the total amount of aluminum-containing metals is 1.0% by mass or more and 2.0% by mass or less. The average corrosion rate is 0.03 mm/year to 0.05 mm/year.
(5) High Purity Aluminum Aluminum with a purity of 99.999%. The average corrosion rate is 0.05 mm/year.

(Manufacturing method of aluminum negative electrode 2)
The method for manufacturing the aluminum negative electrode 2 will be described separately for the method 1 for manufacturing the aluminum negative electrode 2 in which the aluminum negative electrode 2 is high-purity aluminum and the manufacturing method 2 for the aluminum negative electrode 2 in which the aluminum negative electrode 2 is a high-purity aluminum alloy.

In the manufacturing method 1 of the aluminum negative electrode 2 and the manufacturing method 2 of the aluminum negative electrode 2, aluminum is first highly purified. Examples of the method for highly purifying aluminum include the method for highly purifying aluminum described in the above (Method for producing aluminum negative electrode 1).

- Manufacturing method 1 of aluminum negative electrode 2
The method 1 for producing the aluminum negative electrode 2 preferably includes a step of casting high-purity aluminum and a step of rolling.

..Casting step By casting highly purified aluminum by the above-described method, an aluminum ingot having a shape suitable for rolling can be obtained.
When casting, for example, high-purity aluminum is melted at about 680° C. or higher and 800° C. or lower to obtain molten aluminum.
The molten aluminum is preferably cleaned by removing gas and non-metallic inclusions.

The cleaned molten aluminum is cast using a mold to form an ingot.
As the mold, an iron mold heated to 50° C. or more and 200° C. or less or a graphite mold is used. The aluminum negative electrode 2 can be cast by a method of pouring molten aluminum at 680° C. or higher and 800° C. or lower into a mold. Alternatively, an ingot may be obtained by semi-continuous casting.

..Rolling step The aluminum ingot thus obtained can be used for the aluminum negative electrode 2 by cutting as it is. In this embodiment, the aluminum ingot is preferably rolled, extruded, or forged into a plate material. In this embodiment, rolling is more preferable.
The rolling step can be performed by the same method as the rolling step described in the method for producing the aluminum negative electrode 1 .

- Manufacturing method 2 of aluminum negative electrode 2
The method 2 for manufacturing the aluminum negative electrode 2 preferably includes a step of casting a high-purity aluminum alloy and a step of rolling.

..Casting step When casting is performed, first, a predetermined amount of a metal element is added to high-purity aluminum to obtain a mixture 2 . Next, the mixture 2 is melted at 680° C. or higher and 800° C. or lower to obtain a molten alloy 2 of aluminum and metal.
The metal element to be added is preferably one or more selected from the group consisting of Mg, Ni, Mn, Zn, Cd and Pb. The metal containing these elements to be added preferably has a purity of 99% by mass or more.

A high-purity aluminum alloy ingot is obtained by the same method as the casting process in the manufacturing method of the aluminum negative electrode 1, except that the molten alloy 2 is used.

Rolling step A rolling step is performed by the same method as the manufacturing method 1 of the aluminum negative electrode 2 described above.

The thickness of the aluminum negative electrode 2 is preferably 5 μm or more, more preferably 6 μm or more, and even more preferably 7 μm or more. Moreover, it is preferably 200 μm or less, more preferably 190 μm or less, and even more preferably 180 μm or less.
The above upper limit and lower limit of the thickness of the aluminum negative electrode 2 can be combined arbitrarily. In this embodiment, the thickness of the aluminum negative electrode 2 is preferably 5 μm or more and 200 μm or less.

[Aluminum negative electrode 3]
The aluminum negative electrode 3 is an aluminum-containing metal.
The Vickers hardness of the aluminum negative electrode 3 is preferably 10 HV or more and 70 HV or less, more preferably 20 HV or more and 70 HV or less, even more preferably 30 HV or more and 70 HV or less, and particularly preferably 35 HV or more and 55 HV or less.

When the aluminum negative electrode 3 absorbs lithium, the crystal structure of the metal forming the aluminum negative electrode may be distorted.
If the Vickers hardness is equal to or less than the above upper limit, it is presumed that the distortion of the crystal structure can be relaxed and the crystal structure can be maintained when the aluminum negative electrode 3 absorbs lithium. Therefore, the lithium secondary battery using the aluminum negative electrode 3 is less likely to decrease its cycle retention rate even when charging and discharging are repeated.

Vickers hardness uses the value measured by the following method.

[Measuring method]
As an index of hardness of the aluminum negative electrode 3, Vickers hardness (HV0.05) is measured using a micro Vickers hardness tester.
Vickers hardness is a value measured in accordance with JIS Z2244:2009 "Vickers hardness test - test method". Vickers hardness is calculated from the diagonal length of the indentation left on the surface after pressing a square pyramidal diamond indenter into the surface of the aluminum negative electrode 3 and releasing the test force.
The above standard stipulates that the hardness symbol should be changed according to the test force. In this embodiment, for example, the micro Vickers hardness HV is 0.05 at a test force of 0.05 kgf (=0.4903 N).

(Negative electrode current collector)
When a negative electrode current collector is used for an aluminum negative electrode, the material of the negative electrode current collector can be a strip-shaped member made of a metal material such as Cu, Ni, or stainless steel. Among them, it is preferable to use Cu as a forming material and process it into a thin film because it is difficult to form an alloy with lithium and is easy to process.

Examples of the method of supporting the negative electrode mixture on such a negative electrode current collector include a method of pressure molding, and a method of making a paste using a solvent or the like, coating it on the negative electrode current collector, drying it, and then pressing and crimping it.

When the aluminum negative electrode also serves as a current collector, a separate current collector may not be necessary.

≪Positive electrode≫
The positive electrode has a positive electrode active material.
A lithium-containing compound or other metal compound can be used for the positive electrode active material. Examples of lithium-containing compounds include lithium-cobalt composite oxides having a layered structure, lithium-nickel composite oxides having a layered structure, lithium-manganese composite oxides having a spinel structure, and lithium iron phosphate having an olivine structure. .

Other metal compounds include, for example, oxides such as titanium oxide, vanadium oxide or manganese dioxide, or sulfides such as titanium sulfide or molybdenum sulfide.

(Conductive material)
A carbon material can be used as the conductive material. Examples of carbon materials include graphite powder, carbon black (eg, acetylene black), and fibrous carbon materials. Since carbon black is fine particles and has a large surface area, by adding a small amount of carbon black to the positive electrode mixture, the conductivity inside the positive electrode can be increased, and the charge/discharge efficiency and output characteristics can be improved.

The ratio of the conductive material in the positive electrode mixture is preferably 5 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. If a fibrous carbon material such as graphitized carbon fiber or carbon nanotube is used as the conductive material, this ratio can be lowered.

(binder)
A thermoplastic resin can be used as the binder. Examples of thermoplastic resins include polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), ethylene tetrafluoride/propylene hexafluoride/vinylidene fluoride. fluororesins such as copolymers, propylene hexafluoride/vinylidene fluoride copolymers and tetrafluoroethylene/perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene.

These thermoplastic resins may be used in combination of two or more. A fluororesin and a polyolefin resin are used as a binder, and the ratio of the fluororesin to the entire positive electrode mixture is 1% by mass or more and 10% by mass or less, and the ratio of the polyolefin resin is 0.1% by mass or more and 2% by mass or less. It is possible to obtain a positive electrode mixture having both high adhesion to the current collector and high bonding strength inside the positive electrode mixture.

(Positive electrode current collector)
As the positive electrode current collector, a belt-like member made of a metal material such as Al, Ni, or stainless steel can be used. Among them, it is preferable to use Al as a forming material and process it into a thin film because it is easy to process and inexpensive. The positive electrode current collector may be made of the same material as the negative electrode current collector.

As a method of supporting the positive electrode mixture on the positive electrode current collector, there is a method of pressure-molding the positive electrode mixture on the positive electrode current collector. In addition, the positive electrode mixture is made into a paste using an organic solvent, and the obtained positive electrode mixture paste is applied to at least one side of a positive electrode current collector, dried, and pressed to adhere, thereby forming a positive electrode on the positive electrode current collector. A mixture may be supported.

When the positive electrode mixture is made into a paste, organic solvents that can be used include amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether-based solvents such as tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester solvents such as dimethylacetamide, amide solvents such as N-methyl-2-pyrrolidone;

Examples of methods for applying the positive electrode mixture paste to the positive electrode current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

A positive electrode can be manufactured by the method described above.

(Electrolytes)
The electrolyte that the lithium secondary battery has may be a liquid electrolyte or a solid electrolyte. The liquid electrolyte includes an electrolytic solution containing an electrolyte and an organic solvent.

_-Electrolyte solution Electrolytes contained in the electrolyte solution include LiClO4, _LiPF6 , _LiAsF6 , _LiSbF6 , _LiBF4 , _LiCF3SO3 _, LiN _(SO2CF3)2 _, _and _LiN ₍ _SO2C2F5 ). ₂ , LiN( _SO2CF3 )( _COCF3 ) _, Li ₍ _C4F9SO3 ), _LiC(SO2CF3)3 _, _Li2B10Cl10 _, _LiBOB ₍ where _BOB is bis( oxalato) borate), LiFSI (here, FSI is bis(fluorosulfonyl)imide), lower aliphatic carboxylic acid lithium salt, _LiAlCl4 and other lithium salts, these two Mixtures of the above may also be used. Among them, as the electrolyte, at least one selected from the group consisting of LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(SO ₂ CF ₃ ) ₂ and LiC(SO ₂ CF ₃ ) ₃ containing fluorine It is preferred to use one containing one.

Examples of organic solvents contained in the electrolytic solution include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di( Carbonates such as methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethyl ether, 2,2,3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, 2-methyl ethers such as tetrahydrofuran; esters such as methyl formate, methyl acetate, propyl propionate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, 1,3-propanesultone, or those obtained by further introducing a fluoro group into these organic solvents (hydrogen atoms possessed by organic solvents of which at least one is substituted with a fluorine atom) can be used.

As the organic solvent, it is preferable to use a mixture of two or more of these. Among them, a mixed solvent containing carbonates is preferable, and a mixed solvent of a cyclic carbonate and a non-cyclic carbonate and a mixed solvent of a cyclic carbonate and an ether are more preferable. A mixed solvent containing ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is preferable as the mixed solvent of the cyclic carbonate and the non-cyclic carbonate. The electrolyte solution using such a mixed solvent has a wide operating temperature range, does not easily deteriorate even when charged and discharged at a high current rate, does not easily deteriorate even after long-term use, and uses natural graphite as an active material for the negative electrode. , and has many features of being resistant to decomposition even when graphite materials such as artificial graphite are used.

Further, as the electrolytic solution, it is preferable to use an electrolytic solution containing a fluorine-containing lithium salt such as LiPF ₆ and an organic solvent having a fluorine substituent, since the safety of the obtained lithium secondary battery is increased. Mixed solvents containing fluorine-substituted ethers such as pentafluoropropylmethyl ether and 2,2,3,3-tetrafluoropropyldifluoromethyl ether and dimethyl carbonate do not retain their capacity even when charged and discharged at a high current rate. It is more preferable because of its high retention rate.

The electrolyte may contain additives such as tris (trimethylsilyl) phosphate and tris (trimethylsilyl) borate.

- Solid Electrolyte As the solid electrolyte, for example, an organic polymer electrolyte such as a polymer compound containing at least one of a polyethylene oxide polymer compound, a polyorganosiloxane chain, and a polyoxyalkylene chain can be used. In addition, a so-called gel type in which a non-aqueous electrolyte is held in a polymer compound can also be used. Li ₂ S—SiS ₂ , Li ₂ S—GeS ₂ , Li ₂ SP ₂ S ₅ , Li ₂ S—B ₂ S ₃ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS Inorganic solid electrolytes containing sulfides such as ₂ -Li ₂ SO ₄ and Li ₂ S--GeS ₂ --P ₂ S ₅ may be mentioned, and mixtures of two or more of these may be used. By using these solid electrolytes, the safety of the lithium secondary battery can sometimes be improved.

Also, when a solid electrolyte is used, the solid electrolyte may play the role of a separator, in which case the separator may not be required.

(separator)
When the lithium secondary battery has a separator, the separator may be in the form of a porous film, nonwoven fabric, or woven fabric made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin, or a nitrogen-containing aromatic polymer. can be used. Moreover, the separator may be formed using two or more of these materials, or the separator may be formed by laminating these materials.

In order for the separator to allow the electrolyte to pass satisfactorily when the battery is in use (during charging and discharging), the air permeability resistance according to the Gurley method defined in JIS P 8117 must be 50 seconds/100 cc or more and 300 seconds/100 cc or less. It is preferably 50 seconds/100 cc or more and more preferably 200 seconds/100 cc or less.

In addition, the porosity of the separator is preferably 30% by volume or more and 80% by volume or less, more preferably 40% by volume or more and 70% by volume or less. The separator may be a laminate of separators with different porosities.

<Method for manufacturing lithium secondary battery>
A method for manufacturing the lithium secondary battery of this embodiment will be described.
Hereinafter, a method for manufacturing a lithium secondary battery according to this embodiment will be described with reference to the drawings. In addition, in all the drawings below, the dimensions and ratios of the constituent elements are appropriately changed in order to make the drawings easier to see.

A lithium secondary battery manufactured according to the present embodiment includes an aluminum negative electrode capable of intercalating and deintercalating lithium ions, a positive electrode capable of intercalating and deintercalating lithium ions, and an electrolyte.
Lithium secondary batteries include non-aqueous electrolyte secondary batteries using an electrolytic solution as an electrolyte. Lithium secondary batteries include all-solid-state batteries using a solid electrolyte as the electrolyte.

The manufacturing method of the lithium secondary battery of the present embodiment includes a step of assembling a pre-battery, a step of charging and discharging the pre-battery for the first time, and an aging step. Each step will be explained.

[Process of assembling a pre-battery]

FIG. 2 is a schematic diagram showing an example of a lithium secondary battery. Cylindrical lithium secondary battery 10 is manufactured as follows.

First, as shown in FIG. 2, a pair of strip-shaped separators 1, a strip-shaped positive electrode 2 having a positive electrode lead 21 at one end, and a strip-shaped negative electrode 3 having a negative electrode lead 31 at one end are arranged as follows: 1 and the negative electrode 3 are stacked in this order and wound to form an electrode group 4 .

Next, after housing the electrode group 4 and an insulator (not shown) in the battery can 5, the can bottom is sealed, the electrode group 4 is impregnated with the electrolytic solution 6, and the electrolyte is arranged between the positive electrode 2 and the negative electrode 3. . Further, by sealing the upper portion of the battery can 5 with the top insulator 7 and the sealing member 8, the lithium secondary battery 10 can be manufactured.

The shape of the electrode group 4 is, for example, a columnar shape such that the cross-sectional shape of the electrode group 4 cut in the direction perpendicular to the winding axis is a circle, an ellipse, a rectangle, or a rectangle with rounded corners. can be mentioned.

In addition, as the shape of the lithium secondary battery having such an electrode group 4, a shape defined by IEC60086, which is a standard for batteries defined by the International Electrotechnical Commission (IEC), or JIS C 8500 can be adopted. . For example, a shape such as a cylindrical shape or a rectangular shape can be mentioned.

Further, the lithium secondary battery is not limited to the wound type configuration described above, and may have a layered configuration in which a layered structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of laminated lithium secondary batteries include so-called coin-type batteries, button-type batteries, and paper-type (or sheet-type) batteries.

[Initial charging/discharging step]
After the pre-battery is assembled by the above method, the aluminum negative electrode of the pre-battery is allowed to occlude lithium ions and is charged for the first time. Then discharge.

[Aging process]
After the initial charging and discharging, the pre-battery in the discharged state is stored at an ambient temperature of 50°C or higher for 4 hours or longer.
The lower limit of the storage time is 4.5 hours or more, 5 hours or more, or 6 hours or more.
The upper limit of the storage time is 20 hours or less, 15 hours or less, or 10 hours or less.
The upper limit and lower limit of storage time can be combined arbitrarily. Examples of combinations include storage times of 4 hours to 20 hours, 5 hours to 15 hours, and 6 hours to 10 hours.

From the viewpoint of stabilizing the crystal structure of the Li—Al alloy, the storage temperature is preferably 52° C. or higher, more preferably 55° C. or higher. The upper limit of the storage temperature is, for example, 100° C. or lower and 80° C. or lower.
Storage temperature is, for example, 50° C. or higher and 100° C. or lower, or 55° C. or higher and 80° C. or lower.

Combinations of storage time and storage temperature include the following combinations.
・4 hours or more and 20 hours or less at 50°C or more and 100°C or less.
- 5 hours to 15 hours at 55°C to 80°C.

The rate of increase in Ra of the battery after the aging process with respect to Ra of the pre-battery before the aging process is preferably 0% or more and 20% or less, preferably 8% or more and 15% or less.

The increase rate of Ra is obtained by the following formula.
(Ra of battery after aging process−Ra of pre-battery before aging process)/Ra of pre-battery before aging process×100

It is believed that the above aging process stabilizes the Li-Al alloy layer. Here, "the Li--Al alloy layer is stabilized" means that the crystal structure of the aluminum-lithium alloy is stabilized and the formation of a new alloy layer is suppressed. When the Li—Al alloy layer is stabilized, the discharge capacity tends to be stabilized. Therefore, by including the aging step, it is possible to obtain a lithium secondary battery in which the cycle retention rate is less likely to decrease.

Conventionally, when manufacturing a lithium secondary battery using graphite as a negative electrode material, aging is performed after the lithium secondary battery is assembled.
The purpose of conventional aging is to stably generate an SEI (Solid Electrolyte Interface) film on the electrode surface. Conventional aging is, for example, room temperature aging or high temperature aging.

On the other hand, when aluminum is used as the negative electrode material, there is no noticeable difference between the shape of the initial charge curve and the shape of the charge curve after the second cycle in the results of the charge/discharge test.
Based on this result, it was considered, according to conventional knowledge, that when aluminum is used as the negative electrode material, treatment for stably forming an SEI coating on the electrode surface is unnecessary. For this reason, when aluminum is used as the negative electrode material, it has been conventionally believed that aging for stable formation of an SEI coating on the electrode surface is usually unnecessary.
However, the present inventors confirmed that after the first discharge, the structure of the electrode surface changes, becomes brittle, and furthermore, a new surface is generated, and it is clear that improvement is necessary. became.

According to the studies of the present inventors, it was found that the surface of the aluminum negative electrode becomes brittle after the first charge and discharge, and therefore aging is necessary to improve crystallinity and form SEI on the surface.

In the present specification, "the cycle retention rate is less likely to decrease" means that the value of the cycle retention rate measured by the method below is in the range of 90% or more and 99% or less.

[Method for measuring cycle maintenance rate]
First, a pre-battery of a coin-type lithium secondary battery is allowed to stand at room temperature for 10 hours, so that the separator and the positive electrode mixture layer are sufficiently impregnated with the electrolytic solution.
Next, at room temperature, the battery is charged at a constant current of 1 mA to 4.2 V, then charged at a constant voltage of 4.2 V for 5 hours, and then discharged at a constant current of 1 mA to 3.4 V. Initial charge/discharge of the pre-battery is performed in this manner. The discharge capacity is measured, and the obtained value is defined as "initial discharge capacity" (mAh).
After the initial charging and discharging of the pre-battery, the pre-battery in the discharged state is aged under predetermined conditions, and then charged at 1 mA and discharged at 1 mA in the same manner as the initial charging and discharging conditions.
After that, the discharge capacity (mAh) at the 100th cycle is measured.
From the second discharge capacity after aging (the third cycle from the initial stage) and the discharge capacity at the 100th cycle, the cycle retention rate is calculated by the following formula.
Cycle retention rate (%) = 100th cycle discharge capacity (mAh) / 2nd discharge capacity (mAh) x 100

Next, the present invention will be described in more detail by way of examples.

<Rs calculation method>
Rs was calculated by the method described in [Method for calculating Rs] above.

<How to calculate Ra>
Ra was calculated by the method described in [Method for calculating Ra] above.

<Example 1>
[Preparation of negative electrode]
The silicon-aluminum alloy used in Example 1 was produced by the following method.
By heating and holding high-purity aluminum (purity: 99.99% by mass or more) and high-purity chemical silicon (purity: 99.999% by mass or more) at 760 ° C., the silicon content is 1.0% by mass. A molten aluminum-silicon alloy was obtained.
Next, the molten alloy was held at a temperature of 740° C. and a degree of vacuum of 50 Pa for 2 hours for cleaning.
The molten alloy was cast in a cast iron mold (22 mm×150 mm×200 mm) dried at 150° C. to obtain an ingot.

Rolling was performed under the following conditions. After both surfaces of the ingot were chamfered by 2 mm, cold rolling was performed from a thickness of 18 mm at a reduction ratio of 99.6%. The thickness of the obtained rolled material was 100 μm.
A high-purity aluminum-silicon alloy foil (thickness: 100 μm) having an aluminum purity of 99.999% and a silicon content of 1.0% by mass was cut into a disk shape of φ16 mm to manufacture an aluminum negative electrode 11 .

[Preparation of positive electrode]
90 parts by mass of lithium cobalt oxide (product name: Cellseed, manufactured by Nippon Kagaku Kogyo Co., Ltd., average particle size (D50): 10 μm) as a positive electrode active material, 5 parts by mass of polyvinylidene fluoride (manufactured by Kureha Co., Ltd.) as a binder, and a conductive material 5 parts by mass of acetylene black (product name: Denka Black, manufactured by Denka Co., Ltd.) was mixed as a material, and 70 parts by mass of N-methyl-2-pyrrolidone was further mixed to prepare a positive electrode mixture.

The obtained electrode mixture was applied onto an aluminum foil having a thickness of 15 μm as a current collector by a doctor blade method. The coated electrode mixture was dried at 60° C. for 2 hours and then vacuum dried at 150° C. for 10 hours to volatilize N-methyl-2-pyrrolidone. The coating amount of the positive electrode active material after drying was 21.5 mg/cm ² .

After rolling the obtained laminate of the electrode mixture layer and the current collector, it is cut into a disk shape of φ14 mm to obtain a laminate of the positive electrode mixture layer using lithium cobaltate as a forming material and the current collector. A positive electrode was produced.

[Preparation of electrolytic solution]
An electrolytic solution was prepared by dissolving LiPF ₆ at a ratio of 1 mol/liter in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at EC:DEC=30:70 (volume ratio). did.

≪Manufacturing of lithium secondary batteries≫
[Process of assembling a pre-battery]
A polyethylene porous separator is placed between the negative electrode and the positive electrode, housed in a battery case (standard 2032), the above electrolytic solution is injected, and the battery case is sealed to obtain a diameter of 20 mm and a thickness of 20 mm. A 3.2 mm coin-shaped pre-battery was fabricated.

[Initial charging/discharging step]
A coin-shaped pre-battery was galvanostatically charged to 4.2 V at 1 mA at room temperature.
After that, constant current discharge was performed at 1 mA to 3.4V.

[Aging process]
After that, it was stored at 60° C. for 8 hours in a discharged state. Thus, a lithium secondary battery 1 was manufactured.

The Rs of the lithium secondary battery 1 was 50Ω, the Ra was 35Ω/cm ² , and the increase in Ra before and after aging was 10%.

Regarding the lithium secondary battery 1, the cycle retention rate measured by the method described in [Method for measuring cycle retention rate] above was 90%.

<Comparative Example 1>
A lithium secondary battery 2 was manufactured in the same manner as in Example 1, except that the [aging step] was changed to 8 hours at 25°C.

The Rs of the lithium secondary battery 2 was 10 Ω/cm ² , the Ra was 34 Ω/cm ² , and the increase in Ra before and after aging was 5%.

Regarding the lithium secondary battery 2, the cycle retention rate measured by the method described in [Method for measuring cycle retention rate] above was 70%.

<Comparative Example 2>
A lithium secondary battery 3 was manufactured in the same manner as in Example 1, except that the [aging step] was changed to 45° C. for 4 hours.

The Rs of the lithium secondary battery 3 was 10Ω, the Ra was 30Ω/cm ² , and the increase in Ra before and after aging was 7%.

Regarding the lithium secondary battery 3, the cycle retention rate measured by the method described in [Method for measuring cycle retention rate] above was 80%.

<Comparative Example 3>
A lithium secondary battery 4 was manufactured in the same manner as in Example 1, except that the [aging step] was changed to 45° C. for 8 hours.

The Rs of the lithium secondary battery 4 was 10Ω, the Ra was 32Ω/cm ² , and the increase in Ra before and after aging was 7%.

Regarding the lithium secondary battery 4, the cycle retention rate measured by the method described in [Method for measuring cycle retention rate] above was 60%.

From the above results, it was confirmed that a lithium secondary battery that has undergone a predetermined aging process after the first charge and discharge and whose Rs satisfies (1) is less likely to decrease in cycle retention rate.

In addition, it was confirmed that when the predetermined aging process was not performed after the first charge and discharge, the rate of increase in Ra before and after aging was lower than in the example, in other words, the change in resistance value was smaller than in the example.

DESCRIPTION OF SYMBOLS 1... Separator, 2... Positive electrode, 3... Aluminum negative electrode, 4... Electrode group, 5... Battery can, 6... Electrolyte solution, 7... Top insulator, 8... Sealing body, 10... Lithium secondary battery, 21... Positive electrode lead, 31 Negative electrode lead

Claims

an aluminum negative electrode capable of intercalating and deintercalating lithium ions;
a positive electrode capable of intercalating and deintercalating lithium ions;
comprising an electrolyte and
The aluminum negative electrode is made of an aluminum-containing metal,
A lithium secondary battery that satisfies the following (1).
20 Ω/cm 2 ≤ Rs ≤ 100 Ω/cm 2 (1)
(The lithium secondary battery is discharged, and the voltage after discharge is V0. After that, it is charged at 1.0 mA/cm 2 and the voltage after 2 seconds is V1. The difference between V1 and V0 (V1-V0 ) The DC resistance value per unit area of the positive electrode calculated from ) is defined as Rs.)
2. The lithium secondary battery according to claim 1, which satisfies the following (2).
10 Ω/cm 2 ≤ Ra ≤ 100 Ω/cm 2 (2)
(Ra is the AC resistance value of the aluminum negative electrode obtained by discharging the lithium secondary battery, measuring the AC impedance, and analyzing the Cole-Cole plot of the obtained impedance.)
an aluminum negative electrode capable of intercalating and deintercalating lithium ions;
a positive electrode capable of intercalating and deintercalating lithium ions;
comprising an electrolyte and
A method for producing a lithium secondary battery in which the aluminum negative electrode is made of an aluminum-containing metal,
a step of assembling a pre-battery;
a step of charging and discharging the pre-battery for the first time;
A method for producing a lithium secondary battery, comprising: an aging step of storing the pre-battery in a discharged state at an ambient temperature of 50° C. or higher for 4 hours or longer.
The method for manufacturing a lithium secondary battery according to claim 3, wherein the increase rate of Ra of the pre-battery after the aging process with respect to Ra of the pre-battery before the aging process is 0% or more and 20% or less.