WO2023002537A1

WO2023002537A1 - Lithium secondary battery

Info

Publication number: WO2023002537A1
Application number: PCT/JP2021/027010
Authority: WO
Inventors: 浩井本; 健緒方
Original assignee: ＴｅｒａＷａｔｔＴｅｃｈｎｏｌｏｇｙ株式会社
Priority date: 2021-07-19
Filing date: 2021-07-19
Publication date: 2023-01-26

Abstract

The present invention provides a lithium secondary battery which has a high energy density and excellent cycle characteristics. A lithium secondary battery according to one embodiment is provided with a positive electrode, a negative electrode comprising a negative electrode collector that does not include a negative electrode active material before initial charging, and a separator disposed between the positive electrode and the negative electrode. The peeling strength between the negative electrode collector and the separator is 0.1 N or more.

Description

Lithium secondary battery

The present invention relates to lithium secondary batteries.

In recent years, technology that converts natural energy such as sunlight or wind power into electrical energy has attracted attention. Along with this, various secondary batteries have been developed as power storage devices that are highly safe and capable of storing a large amount of electrical energy.

Among them, lithium secondary batteries that charge and discharge by moving lithium ions between positive and negative electrodes are known to exhibit high voltage and high energy density. As a typical lithium secondary battery, a positive electrode and a negative electrode have an active material capable of holding lithium elements, and lithium ions are charged and discharged by exchanging lithium ions between the positive electrode active material and the negative electrode active material. Secondary batteries (LIBs) are known.

In addition, for the purpose of realizing high energy density, lithium secondary batteries (lithium metal batteries, LMB) using lithium metal as the negative electrode active material instead of materials capable of inserting lithium ions such as carbon materials. is being developed. For example, US Pat. No. 6,200,000 discloses a rechargeable battery that uses a lithium metal-based electrode as the negative electrode.

In addition, lithium secondary batteries using negative electrodes that do not have negative electrode active materials such as carbon materials and lithium metal are being developed for the purpose of further increasing energy density and improving productivity. For example, Patent Document 2 discloses a lithium secondary battery including a positive electrode, a negative electrode, a separator and an electrolyte interposed therebetween. A lithium secondary battery is disclosed that migrates from the positive electrode to form lithium metal on a negative current collector within the negative electrode. Patent Document 2 discloses that such a lithium secondary battery solves the problems caused by the reactivity of lithium metal and the problems occurring during the assembly process, and provides a lithium secondary battery with improved performance and life. We disclose what we can do.

Japanese Patent Publication No. 2006-500755 Japanese Patent Application Publication No. 2019-505971

However, when the present inventors made a detailed study of conventional batteries including those described in the above patent documents, they found that at least one of energy density and cycle characteristics was insufficient.

For example, it is difficult to sufficiently increase the energy density and capacity of a lithium secondary battery including a negative electrode having a negative electrode active material due to the volume and mass occupied by the negative electrode active material. In addition, with respect to anode-free lithium secondary batteries having a negative electrode that does not have a negative electrode active material, in conventional batteries, dendrite-like lithium metal is likely to be formed on the surface of the negative electrode due to repeated charging and discharging, resulting in a short circuit and a decrease in capacity. Cycle characteristics are not sufficient because deterioration tends to occur.

In addition, in anode-free lithium secondary batteries, a method has been developed in which a large physical pressure is applied to the battery to keep the interface between the negative electrode and the separator at a high pressure, in order to suppress uneven growth during deposition of lithium metal. ing. However, since the application of such a high voltage requires a large mechanical mechanism, the weight and volume of the battery as a whole increase, and the energy density decreases.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a lithium secondary battery with high energy density and excellent cycle characteristics.

A lithium secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode including a negative electrode current collector having no negative electrode active material before initial charge, and a separator disposed between the positive electrode and the negative electrode. In addition, the peel strength between the negative electrode current collector and the separator is 0.1 N or more.

The inventors have found that the cycle characteristics of lithium secondary batteries can be improved by increasing the peel strength between the negative electrode current collector and the separator to a predetermined level or higher. In the lithium secondary battery of the present invention, the peel strength between the negative electrode current collector and the separator is set to 0.1 N or more, thereby improving cycle characteristics.

In the above lithium secondary battery, the separator preferably has a separator coating layer at least on the negative electrode side.

Further, in the above lithium secondary battery, the thickness of the separator coating layer is preferably 0.7 μm or more. In the above lithium secondary battery, the separator preferably has a Gurley air permeability of 180 sec/100 cc or more. In the above lithium secondary battery, the separator preferably has a Gurley air permeability of 600 seconds/100 cc or less.

As described above, the thickness of the separator coating layer or the Gurley air permeability of the separator satisfies predetermined conditions, thereby improving the cycle characteristics of the lithium secondary battery.
Further, a lithium secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode including a negative electrode current collector that does not have a negative electrode active material before initial charge, and a separator disposed between the positive electrode and the negative electrode. , wherein the separator has a separator coating layer at least on the negative electrode side, the separator coating layer has a thickness of 0.7 μm or more, and the separator has a Gurley air permeability of 180 sec/100 cc or more and 600 sec/100 cc or less. be.

The present inventors have found that a lithium secondary battery having a negative electrode including a negative electrode current collector that does not have a negative electrode active material before initial charging has a configuration that satisfies a predetermined thickness of the separator coating layer and the Gurley air permeability of the separator. The inventors have found that by doing so, the cycle characteristics of the lithium secondary battery can be improved. In the lithium secondary battery of the present invention, as described above, the thickness of the separator coating layer is 0.7 μm or more, and the Gurley air permeability of the separator is 180 sec/100 cc or more and 600 sec/100 cc or less. Cycle characteristics can be improved by this.

According to the present invention, it is possible to provide a lithium secondary battery with improved cycle characteristics.

1 is a schematic cross-sectional view of a charged lithium secondary battery according to an embodiment of the present invention; FIG. 1 is a schematic cross-sectional view of a lithium secondary battery before charging or after discharging according to an embodiment of the present invention; FIG. 1 is a schematic cross-sectional view showing how a lithium secondary battery according to an embodiment of the present invention is used; FIG.

Hereinafter, embodiments of the present invention (hereinafter referred to as "present embodiments") will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted. In addition, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of the drawings are not limited to the illustrated ratios.

[Structure of Lithium Secondary Battery]
FIG. 1 is a schematic cross-sectional view of a lithium secondary battery 100 according to an embodiment of the invention. As shown in FIG. 1, the lithium secondary battery 100 includes a positive current collector 110, a positive electrode 120, a negative electrode 130, and a separator 140. As shown in FIG. The negative electrode 130 includes a negative electrode current collector 131 and a lithium metal layer 132 . However, FIG. 1, in which the negative electrode 130 includes the negative electrode current collector 131 and the lithium metal layer 132, shows the configuration in which the lithium secondary battery 100 is charged. , the structure does not have the lithium metal layer 132 . Separator 140 includes separator base material 141 and separator coating layer 142 .

Each configuration of the lithium secondary battery 100 will be described below.

(negative electrode)
The negative electrode 130 does not have a negative electrode active material. As used herein, the term “negative electrode active material” refers to a material that causes an electrode reaction, that is, an oxidation reaction and a reduction reaction, at the negative electrode. Specifically, the negative electrode active material of the present embodiment in the present specification includes lithium metal and a host material of lithium element (lithium ion or lithium metal). A host material for elemental lithium means a material provided to hold lithium ions or lithium metal to the negative electrode. Mechanisms for such retention include, but are not limited to, intercalation, alloying, and occlusion of metal clusters, typically intercalation.

In the lithium secondary battery 100 of the present embodiment, the negative electrode does not have a negative electrode active material before the battery is initially charged. Discharge occurs. Therefore, in the lithium secondary battery 100 of the present embodiment, the volume occupied by the negative electrode active material and the mass of the negative electrode active material are reduced compared to a lithium secondary battery having a negative electrode active material, and the volume and mass of the entire battery are reduced. In principle, the energy density is high because it is small.

In the lithium secondary battery 100 of the present embodiment, the negative electrode does not have a negative electrode active material before the battery is initially charged, lithium metal is deposited on the negative electrode by charging the battery, and the deposited lithium metal is removed by discharging the battery. electrolytically eluted. Therefore, in the lithium secondary battery of this embodiment, the negative electrode does not substantially contain the negative electrode active material even when the battery is discharged. Therefore, in the lithium secondary battery 100 of this embodiment, the negative electrode functions as a negative electrode current collector.

In the present specification, the term "lithium metal is deposited on the negative electrode" means not only the deposition of lithium metal on the surface of the negative electrode, but also the surface of the solid electrolyte interface (SEI) layer formed on the surface of the negative electrode and described later. It also includes the deposition of lithium metal on the For example, when the lithium secondary battery 100 of the present embodiment is charged, lithium metal deposits as a lithium metal layer 132 on the interface between the negative electrode current collector 131 and the separator 140 as shown in FIG.

Comparing the lithium secondary battery of this embodiment with a lithium ion battery (LIB) and a lithium metal battery (LMB), they differ in the following points.

In a lithium ion battery (LIB), the negative electrode has a host material of elemental lithium (lithium ion or lithium metal), and upon charging of the battery, such material is charged with elemental lithium, and the host material releases elemental lithium, thereby forming a battery. is discharged. The LIB is different from the lithium secondary battery 100 of the present embodiment in that the negative electrode has a lithium element host material.

A lithium metal battery (LMB) is manufactured using an electrode with lithium metal on its surface, or using lithium metal alone as a negative electrode. That is, the LMB differs from the lithium secondary battery of the present embodiment in that the negative electrode has lithium metal as the negative electrode active material immediately after the battery is assembled, that is, before the battery is initially charged. The LMB uses an electrode containing lithium metal, which is highly combustible and reactive, in its manufacture, but the lithium secondary battery 100 of the present embodiment uses a negative electrode that does not contain lithium metal, so it is safer and more productive. It is excellent for

In this specification, the phrase "the negative electrode does not have a negative electrode active material" means that the negative electrode does not have or substantially does not have a negative electrode active material. The fact that the negative electrode does not substantially contain a negative electrode active material means that the amount of the negative electrode active material contained in the negative electrode is sufficiently smaller than that of the positive electrode active material in terms of capacity ratio. The content of the negative electrode active material in the negative electrode is, for example, 10% by mass or less with respect to the entire negative electrode. The content of the negative electrode active material in the negative electrode is preferably 5.0% by mass or less, may be 1.0% by mass or less, or may be 0.1% by mass or less with respect to the entire negative electrode. , 0.0% by mass or less. When the negative electrode does not have a negative electrode active material or the content of the negative electrode active material in the negative electrode is within the above range, the lithium secondary battery has a high energy density.

As used herein, the term “lithium secondary battery including a negative electrode that does not have a negative electrode active material” means that the negative electrode does not have a negative electrode active material before the initial charge of the battery or at the end of discharge (state shown in FIG. 2). ). Therefore, the phrase "negative electrode without negative electrode active material" includes "negative electrode without negative electrode active material before the initial charge of the battery or at the end of discharge", "negative electrode active material other than lithium metal regardless of the state of charge of the battery". and does not have lithium metal before initial charge or at the end of discharge", or "negative electrode current collector that does not have lithium metal before initial charge or at the end of discharge". . Alternatively, in the above phrase, "before initial charging or at the end of discharging" may be replaced with the phrase "before initial charging". In addition, the “lithium secondary battery having a negative electrode without negative electrode active material” may also be referred to as an anode-free lithium battery, a zero-anode lithium battery, or an anode-less lithium battery.

In this specification, the battery "before the initial charge" means the state from the time the battery is assembled to the time it is charged for the first time. In addition, when the battery is "at the end of discharging", it means that no discharge occurs even if the voltage of the battery is further lowered. , preferably 1.0 V or more and 3.0 V or less.

In the negative electrode of the present embodiment, the content of the negative electrode active material other than lithium metal is 10% by mass or less, preferably 5.0% by mass or less, relative to the entire negative electrode, regardless of the state of charge of the battery. It may be 1.0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less.
In addition, the negative electrode of the present embodiment has a lithium metal content of 10% by mass or less, preferably 5.0% by mass or less, relative to the entire negative electrode before initial charge or at the end of discharge. It may be 0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less. The negative electrode preferably has a lithium metal content of 10% by mass or less with respect to the entire negative electrode before initial charge and at the end of discharge (among these, the lithium metal content is preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, or may be 0.0% by mass or less.)

In the lithium secondary battery 100 of the present embodiment, when the voltage of the battery is 1.0 V or more and 3.5 V or less, the content of lithium metal may be 10% by mass or less with respect to the entire negative electrode ( It is preferably 5.0% by mass or less, may be 1.0% by mass or less, may be 0.1% by mass or less, or may be 0.0% by mass or less.); Battery is 1.0 V or more and 3.0 V or less, the lithium metal content may be 10% by mass or less (preferably 5.0% by mass or less, and 1 .0% by mass or less, 0.1% by mass or less, or 0.0% by mass or less); or the voltage of the battery is 1.0 V or more and 2.5 V In the case below, the lithium metal content may be 10% by mass or less with respect to the entire negative electrode (preferably 5.0% by mass or less, and may be 1.0% by mass or less. , 0.1% by mass or less, or 0.0% by mass or less.).

Further, in the lithium secondary battery 100 of the present embodiment, when the battery voltage is 4.2 V, the mass M of lithium metal deposited on the negative electrode is _4.2 , and when the battery voltage is 3.0 V, The ratio M _3.0 /M _4.2 of the mass M _3.0 of the lithium metal deposited on the negative electrode is preferably 20% or less, more preferably 15% or less, and still more preferably 10%. It is below. The ratio M3.0/M4.2 may be 8.0% or less, _5.0 % or less, _3.0 % or less, or 1.0% or less. may be

Examples of negative electrode active materials include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, metals that are alloyed with lithium, and alloys containing such metals. Examples of the carbon-based substance include, but are not limited to, graphene, graphite, hard carbon, mesoporous carbon, carbon nanotube, and carbon nanohorn. Examples of the metal oxide include, but are not particularly limited to, titanium oxide-based compounds, tin oxide-based compounds, and cobalt oxide-based compounds. Examples of metals alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

The negative electrode of the present embodiment is not particularly limited as long as it does not have a negative electrode active material and can be used as a current collector. At least one selected from the group consisting of metals, alloys thereof, and stainless steel (SUS). In addition, when using SUS for a negative electrode, as a kind of SUS, conventionally well-known various things can be used. The above negative electrode materials are used individually by 1 type or in combination of 2 or more types. In this specification, the term "metal that does not react with Li" means a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of the lithium secondary battery.

The negative electrode of the present embodiment is preferably made of at least one selected from the group consisting of Cu, Ni, Ti, Fe, alloys thereof, and stainless steel (SUS), and more preferably, It consists of at least one selected from the group consisting of Cu, Ni, alloys thereof, and stainless steel (SUS). The negative electrode is more preferably Cu, Ni, alloys thereof, or stainless steel (SUS). The use of such a negative electrode tends to improve the energy density and productivity of the battery.

The average thickness of the negative electrode of the present embodiment is preferably 4 µm or more and 20 µm or less, more preferably 5 µm or more and 18 µm or less, and still more preferably 6 µm or more and 15 µm or less. According to such an aspect, the volume occupied by the negative electrode in the battery is reduced, so that the energy density of the battery is further improved.

(positive electrode)
The positive electrode 120 is not particularly limited as long as it is generally used in lithium secondary batteries, and a known material can be appropriately selected depending on the application of the lithium secondary battery. From the viewpoint of improving battery stability and output voltage, the positive electrode preferably contains a positive electrode active material.

As long as it has a positive electrode active material, the positive electrode 120 is not particularly limited as long as it is generally used for lithium secondary batteries, and known materials can be appropriately selected depending on the application of the lithium secondary battery. Since the positive electrode 120 includes a positive electrode active material, it has high stability and high output voltage.

As used herein, a "positive electrode active material" is a substance that causes an electrode reaction, that is, an oxidation reaction and a reduction reaction, at the positive electrode. Specifically, the positive electrode active material of the present embodiment includes a host material of lithium element (typically lithium ion).

Examples of such positive electrode active materials include, but are not particularly limited to, metal oxides and metal phosphates. Examples of the metal oxide include, but are not limited to, cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. Examples of the metal phosphate include, but are not particularly limited to, iron phosphate-based compounds and cobalt phosphate-based compounds. Typical positive electrode active materials include LiCoO2, _LiNixCoyMnzO ( _x + _y + _z =1), LiNixMnyO ( _x + _y = ₁ ), _LiNiO2 , LiMn2O4, LiFePO, LiCoPO, _LiFeOF , LiNiOF, and _TiS2 . The above positive electrode active materials are used singly or in combination of two or more.

The positive electrode 120 may contain components other than the positive electrode active material described above. Examples of such components include, but are not particularly limited to, known conductive aids, binders, polymer electrolytes, and inorganic solid electrolytes.

The conductive aid in the positive electrode 120 is not particularly limited, but examples include carbon black, single-wall carbon nanotubes (SWCNT), multi-wall carbon nanotubes (MWCNT), carbon nanofibers (CF), and acetylene black. The binder is not particularly limited, but examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylic resin, and polyimide resin.

The content of the positive electrode active material in the positive electrode 120 may be, for example, 50% by mass or more and 100% by mass or less with respect to the entire positive electrode 120 . The content of the conductive aid may be, for example, 0.5% by mass or 30% by mass or less with respect to the entire positive electrode 120 . The content of the binder may be, for example, 0.5% by mass or 30% by mass or less with respect to the entire positive electrode 120 . The total content of the solid polymer electrolyte and the inorganic solid electrolyte may be, for example, 0.5 mass % or less than 30 mass % with respect to the entire positive electrode 120 .

(Positive electrode current collector)
A positive electrode current collector 110 is arranged on one side of the positive electrode 120 . The positive electrode current collector 110 is not particularly limited as long as it is a conductor that does not react with lithium ions in the battery. Examples of such a positive electrode current collector include aluminum.

The average thickness of the positive electrode current collector 110 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and still more preferably 6 μm or more and 15 μm or less. According to this aspect, the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100 is reduced, so that the energy density of the lithium secondary battery 100 is further improved.

(separator)
The separator 140 is a member for separating the positive electrode 120 and the negative electrode 130 to prevent the battery from short-circuiting and ensuring ionic conductivity of lithium ions serving as charge carriers between the positive electrode 120 and the negative electrode 130 . It is composed of a material that does not have electronic conductivity and does not react with lithium ions. Moreover, the separator 140 also plays a role of retaining the electrolytic solution. Although the material constituting the separator itself does not have ionic conductivity, lithium ions are conducted through the electrolyte by holding the electrolyte in the separator. The separator 140 is not limited as long as it fulfills the above role, but is composed of, for example, a porous polyethylene (PE) film, a polypropylene (PP) film, or a laminate structure thereof.

The separator 140 may be covered with a separator covering layer 142 . The separator coating layer 142 may cover both sides of the separator 140, or may cover only one side. The separator coating layer 142 is not particularly limited as long as it is a member that does not react with lithium ions. Examples of such a separator coating layer include, but are not limited to, polyvinylidene fluoride (PVDF), a mixture of styrene-butadiene rubber and carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), and lithium polyacrylate. (Li-PAA), polyimide (PI), polyamideimide (PAI), and binders such as aramid. In the separator coating layer, inorganic particles such as silica, alumina, titania, zirconia, magnesium oxide, magnesium hydroxide, and lithium nitrate may be added to the binder. The separator 140 may be a separator without the separator coating layer 142 or may be a separator with the separator coating layer 142 .

The average thickness of the separator 140 is preferably 30 µm or less, more preferably 25 µm or less, and even more preferably 20 µm or less. According to this aspect, the volume occupied by the separator 140 in the lithium secondary battery 100 is reduced, so that the energy density of the lithium secondary battery 100 is further improved. Also, the average thickness of the separator 140 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. According to such an aspect, the positive electrode 120 and the negative electrode 130 can be separated more reliably, and the short circuit of the battery can be further suppressed.

(Electrolyte)
The lithium secondary battery 100 preferably has an electrolyte. In the lithium secondary battery 100, the electrolyte may be infiltrated into the separator 140, or may be enclosed in a sealed container together with the laminate of the positive electrode current collector 110, the positive electrode 120, the separator 140, and the negative electrode 130. . The electrolytic solution is a solution that contains an electrolyte and a solvent, has ionic conductivity, and acts as a conductive path for lithium ions. Therefore, according to the embodiment containing the electrolytic solution, the internal resistance of the battery is further reduced, and the energy density, capacity and cycle characteristics are further improved.

The electrolytic solution preferably contains, as a solvent, an alkyl fluoride compound having at least one of a monovalent group represented by the following formula (A) and a monovalent group represented by the following formula (B).

However, in the formula, the wavy line represents the bonding site in the monovalent group.

In general, in an anode-free lithium secondary battery having an electrolyte, a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode, etc. by decomposing the solvent, etc. in the electrolyte. In the lithium secondary battery, the SEI layer suppresses further decomposition of the components in the electrolytic solution, resulting irreversible reduction of lithium ions, generation of gas, and the like. In addition, since the SEI layer has ionic conductivity, the reactivity of the lithium metal deposition reaction becomes uniform in the plane direction of the negative electrode surface on which the SEI layer is formed. In the lithium secondary battery 100, when the above alkyl fluoride compound is used as a solvent, an SEI layer is likely to be formed on the negative electrode surface, and the growth of dendritic lithium metal on the negative electrode is further suppressed. Characteristics tend to be further improved.

In this specification, the compound "contained as a solvent" means that the compound alone or a mixture with other compounds is liquid in the usage environment of the lithium secondary battery. Any material can be used as long as it can produce an electrolytic solution in a solution phase.

Examples of such fluorinated alkyl compounds include compounds having an ether bond (hereinafter referred to as "ether compounds"), compounds having an ester bond, and compounds having a carbonate bond. From the viewpoint of further improving the solubility of the electrolyte in the electrolytic solution and from the viewpoint of facilitating the formation of the SEI layer, the alkyl fluoride compound is preferably an ether compound.

As the ether compound which is a fluorinated alkyl compound, an ether compound having both a monovalent group represented by the formula (A) and a monovalent group represented by the formula (B) (hereinafter referred to as "primary fluorine solvent "Also called.), an ether compound having a monovalent group represented by the formula (A) and not having a monovalent group represented by the formula (B) (hereinafter referred to as a "secondary fluorine solvent" Also referred to as.), and an ether compound having no monovalent group represented by formula (A) and having a monovalent group represented by formula (B) (hereinafter, "third fluorine solvent" Also called.) and the like.

Examples of the first fluorine solvent include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2, 3,3-tetrafluoropropyldiethoxymethane, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyldiethoxypropane, and the like. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is preferable as the first fluorine solvent from the viewpoint of effectively and reliably exhibiting the effects of the above-mentioned fluorinated alkyl compound. .

Examples of the second fluorine solvent include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1 , 1,2,2-tetrafluoroethyl ether, propyl-1,1,2,2-tetrafluoroethyl ether, 1H,1H,5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether. From the viewpoint of effectively and reliably exhibiting the effects of the above-mentioned fluorinated alkyl compounds, the secondary fluorine solvent includes 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl-1 ,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether is preferred.

Examples of the third fluorine solvent include difluoromethyl-2,2,3,3-tetrafluoropropyl ether, trifluoromethyl-2,2,3,3-tetrafluoropropyl ether, fluoromethyl-2,2,3 ,3-tetrafluoropropyl ether, and methyl-2,2,3,3-tetrafluoropropyl ether. Difluoromethyl-2,2,3,3-tetrafluoropropyl ether is preferable as the tertiary fluorine solvent from the viewpoint of effectively and reliably exhibiting the effects of the above alkyl fluoride compounds.

The electrolytic solution may contain a solvent that does not have both the monovalent group represented by formula (A) and the monovalent group represented by formula (B). Examples of such solvents include, but are not limited to, triethylene glycol dimethyl ether, dimethoxyethane (DME), diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, Fluorine-free solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, and triethyl phosphate, and methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, 1,1 , 1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane, methyl-2,2,3,3,3-pentafluoropropyl ether, 1, Fluorine-containing solvents such as 1,2,3,3,3-hexafluoropropyl methyl ether, ethyl-1,1,2,3,3,3-hexafluoropropyl ether, and tetrafluoroethyltetrafluoropropyl ether is mentioned.

The above solvents, including the above fluorinated alkyl compound, can be used singly or in combination of two or more.

The content of the fluorinated alkyl compound in the electrolytic solution is not particularly limited, but is preferably 40% by volume or more, more preferably 50% by volume or more, and still more preferably It is 60% by volume or more, and more preferably 70% by volume or more. When the content of the fluorinated alkyl compound is within the above range, the SEI layer is more likely to be formed, which tends to further improve the cycle characteristics of the battery. The upper limit of the content of the alkyl fluoride compound is not particularly limited. It may be present, may be 90% by volume or less, or may be 80% by volume or less.

The electrolyte contained in the electrolytic solution is not particularly limited as long as it is a salt, and examples thereof include salts of Li, Na, K, Ca, and Mg. A lithium salt is preferably used as the electrolyte. The lithium salt is not particularly limited, but LiI, LiCl, LiBr, LiF, _LiBF4 , _LiPF6 , _LiAsF6 _, _LiSO3CF3 , LiN( _SO2F ) ₂ , LiN _(SO2CF3)2 _, _LiN ₍ _SO2CF2CF3 ) ₂ , _LiB ₍ _C2O4 ) ₂ , _LiBF2 ₍ _C2O4 ), _LiB ₍ _O2C2H4 ) ₂ , _LiB ₍ _O2C2H4 ) _F2 , LiB(OCOCF ₃ ) ₄ , LiNO ₃ , Li ₂ SO ₄ and the like. The above lithium salts are used singly or in combination of two or more.

The concentration of the electrolyte in the electrolytic solution is not particularly limited, but is preferably 0.5 M or higher, more preferably 0.7 M or higher, still more preferably 0.9 M or higher, and even more preferably 1.0 M or higher. be. When the electrolyte concentration is within the above range, the SEI layer is formed more easily and the internal resistance tends to be lower. The upper limit of the electrolyte concentration is not particularly limited, and the electrolyte concentration may be 10.0M or less, 5.0M or less, or 2.0M or less.

(Use of lithium secondary battery)
FIG. 3 shows one mode of use of the lithium secondary battery of this embodiment. A lithium secondary battery 200 has a positive electrode current collector 110 and a negative electrode 130 in the lithium secondary battery 100, and a positive electrode terminal 220 and a negative electrode terminal 210 for connecting the lithium secondary battery to an external circuit are respectively joined. The lithium secondary battery 200 is charged and discharged by connecting the negative terminal 210 to one end of an external circuit and the positive terminal 220 to the other end of the external circuit. External circuits are, for example, resistors, power supplies, devices, or potentiostats.

The lithium secondary battery 200 is charged by applying a voltage between the positive electrode terminal 220 and the negative electrode terminal 210 such that a current flows from the negative electrode terminal 210 through the external circuit to the positive electrode terminal 220 . By charging the lithium secondary battery 200, deposition of lithium metal occurs on the negative electrode.

In the lithium secondary battery 200, a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode 130 (interface between the negative electrode 130 and the separator 140) by the first charge (initial charge) after assembly of the battery. may The SEI layer to be formed is not particularly limited, but may contain, for example, an inorganic compound containing lithium and/or an organic compound containing lithium. A typical average thickness of the SEI layer is 1 nm or more and 10 μm or less.

When the positive electrode terminal 220 and the negative electrode terminal 210 of the charged lithium secondary battery 200 are connected via an external circuit as necessary, the lithium secondary battery 200 is discharged. As a result, lithium metal deposited on the negative electrode is electrolytically eluted.

(Manufacturing method of lithium secondary battery)
The method for manufacturing the lithium secondary battery 100 as shown in FIGS. 1 and 2 is not particularly limited as long as it is a method capable of manufacturing a lithium secondary battery having the above configuration. is mentioned.

First, the positive electrode 120 is prepared by a known manufacturing method or by purchasing a commercially available one. The positive electrode 120 is manufactured, for example, as follows. A positive electrode mixture is obtained by mixing the above-described positive electrode active material, a known conductive aid, and a known binder. The compounding ratio is, for example, 50% by mass or more and 99% by mass or less of the positive electrode active material, 0.5% by mass or less 30% by mass of the conductive aid, and 0.5% by mass or less of the binder with respect to the entire positive electrode mixture. % by mass or less. The obtained positive electrode mixture is applied to one side of a metal foil (for example, Al foil) having a predetermined thickness (for example, 5 μm or more and 1 mm or less) as a positive electrode current collector, and press-molded. The molded body thus obtained is punched into a predetermined size to obtain the positive electrode 120 formed on the positive electrode current collector 110 .

Next, the negative electrode 130 can be prepared by washing the above negative electrode material, for example, a metal foil (eg, electrolytic Cu foil) with a thickness of 1 μm or more and 1 mm or less with a solvent containing sulfamic acid.

Next, the separator 140 having the configuration described above is prepared. The separator 140 may be manufactured by a conventionally known method, or a commercially available product may be used. The electrolytic solution may be prepared by dissolving the above electrolyte (typically lithium salt) in the above solvent.

Next, the positive electrode current collector 110 with the positive electrode 120 formed thereon, the separator 140, and the negative electrode 130 obtained as described above are laminated in this order to obtain a laminate as shown in FIG. The lithium secondary battery 100 can be obtained by enclosing the laminated body obtained as described above in a sealed container together with an electrolytic solution. Examples of the closed container include, but are not particularly limited to, a laminate film.

The inventors have found that the cycle characteristics can be improved by satisfying a predetermined condition in the lithium secondary battery 100 having a negative electrode including a negative electrode current collector that does not have a negative electrode active material before initial charging. . The predetermined conditions are that the thickness of the separator coating layer is set to a predetermined value or more, the Gurley air permeability of the separator is set to a predetermined range, and the peel strength between the negative electrode current collector and the separator is set to a predetermined value or more. be. Hereinafter, using the lithium secondary battery 100 of the present embodiment, the thickness of the separator coating layer and the Gurley air permeability of the separator are changed, and the peel strength between the negative electrode current collector and the separator is measured while maintaining the capacity. An experiment was conducted to determine the number of cycles to reach a rate of 80%.

[Method for measuring peel strength]
In this embodiment, the peel strength between the negative electrode current collector 131 and the separator 140 was measured as follows.

In the following examples and comparative examples, test cells each having a 45 mm x 45 mm negative electrode 130, a 50 mm x 50 mm separator 140, and a 40 mm x 40 mm positive electrode 120 and sealed with a laminate film were used. The negative electrode 130 and the positive electrode 120 were each arranged centrally with respect to the separator 140 .

First, the test cell was charged at a constant current with a current of 0.05 C until the output voltage reached 4.2 V. After resting for 10 minutes, the output voltage reached 3.0 V with a current of 0.05 C. Constant current discharge was performed until After that, the laminate was opened, and a laminate including the negative electrode 130, the separator 140, and the positive electrode 120 was taken out. Next, the positive electrode 120 was peeled off from the laminate and removed. Next, after cutting the negative electrode terminal, the laminate including the negative electrode 130 and the separator 140 was cut in half. As a result, a laminate including the negative electrode 130 of 45 mm×22.5 mm and the separator 140 of 50 mm×25 mm was obtained. All the experiments from opening the laminate to measuring the peel strength below were conducted in an Ar glove box.

The peel strength test was conducted in accordance with "Adhesive - Peel strength test method - Part 2: 180 degree peel" specified in Japanese Industrial Standards (JIS) K 6852-2: 1999. As a measuring device, a digital force gauge FGP-5 manufactured by Nidec-Shimpo Corporation was used.

First, in the laminate, the longitudinal end face of the separator 140 was slightly peeled off from the negative electrode current collector 131 to separate the negative electrode current collector 131 side and the separator 140 side. At this time, the amount of peeling was set to less than 15 mm so that an adhesive portion of 30 mm or more remained between the negative electrode current collector 131 and the separator 140 . The end of the negative electrode current collector 131 of the laminate was attached to the fixed measuring device, the end of the separator 140 of the laminate was attached to the other movable side, and the separator 140 side was manually moved horizontally. The maximum value measured by the above measuring device was read. At this time, the side of the separator 140 that was manually moved was moved at a moving speed of 1±0.2 mm/sec until the negative electrode current collector 131 and the separator 140 were completely separated.

[Method for measuring the number of cycles]
In the cycle test, which measures the number of cycles at which the volume retention rate is 80% or less, constant current charging is performed at a current of 0.1 C to 4.2 V, and after a rest for 10 minutes, a current of 0.1 C is constant to 3.0 V. One cycle of charging and discharging was defined as current discharging, and the number of cycles at which the capacity after charging became 80% or less of the initial capacity was measured.

[Examples 1-4 and Comparative Examples 1-2]
In Examples 1 to 4 and Comparative Examples 1 and 2, the thickness of the separator coating layer 142 coated on the separator base material 141 was changed, and the number of cycles at which the volume retention ratio was 80% or less was measured. As described below, by changing the thickness of the separator coating layer 142 coated on the separator substrate 141, the Gurley air permeability of the separator 140 and the peel strength between the negative electrode current collector 131 and the separator 140 has changed. Accordingly, the number of cycles at which the volume retention rate of the lithium secondary battery 100 becomes 80% or less has changed.

In addition, the Gurley air permeability is measured by the method specified in JIS P8117: 2009 "Paper and paperboard - Air permeability and air resistance test method (intermediate area) - Gurley method", using a B-type testing machine. The Gurley value of the separator 140 was measured using

The separator 140 was produced as follows. First, as the separator base material 141, PE (polyethylene) porous material having a thickness of 12 μm and a Gurley air permeability of 208 sec/100 cc was prepared. An 8% NMP (N-methyl-2-pyrrolidone) solution of PVdF#9300 manufactured by Kureha Corporation was prepared and coated on both sides of the separator base material 141 . Immediately after coating, it was immersed in water to extract NMP from the coating layer to obtain a separator 140 having PVdF porous layers as separator coating layers 142 on both sides of the separator substrate 141 . At this time, the thickness of the separator coating layer 142 was adjusted by adjusting the coating thickness of the NMP solution.

The thickness of the separator coating layer 142, that is, the thickness of the PVdF layer, was calculated by laminating 10 sheets of the separator 140 coated as described above and subtracting the thickness of the PE porous layer from the total thickness. .

The experimental results of Examples 1-4 and Comparative Examples 1-2 conducted under the above conditions are as follows.

In addition, in Table 1, in Comparative Example 1 in which the coating layer thickness is 0, the experiment was conducted without providing the separator coating layer 142 .

In Comparative Example 1 in which the separator coating layer 142 was not provided, the Gurley air permeability was 0.04 sec/100 cc, the peel strength was 140 N, and the number of cycles at which the volume retention rate was 80% or less was 60 cycles. In Comparative Example 2 in which the separator coating layer 142 was 0.5 μm, the Gurley air permeability was 0.08 sec/100 cc, the peel strength was 145 N, and the number of cycles at which the volume retention rate was 80% or less was 79 cycles. .

On the other hand, in Examples 1 to 4 in which the separator coating layer 142 was 1 μm or more, the Gurley air permeability was 230 sec/100 cc or more, the peel strength was 0.15 N or more, and the volume retention rate was 80% or less. was 102 cycles or more, and relatively good results were obtained.

[Examples 5-7 and Comparative Examples 3-4]
In Examples 5-7 and Comparative Examples 3-4, the separator 140 which does not have the separator coating layer 142 and consists only of the separator substrate 141 was used. In this experiment, the peel strength between the negative electrode current collector 131 and the separator 140 changes as the Gurley air permeability changes, and the number of cycles at which the volume retention rate of the lithium secondary battery 100 becomes 80% or less. In order to confirm that the changes, experiments were conducted using various separator substrates 141 having different Gurley air permeability. Moreover, the thickness of the separator base material 141 was also changed.

The experimental results of Examples 5-7 and Comparative Examples 3-4 conducted in this way are as follows.

In Comparative Example 3, in which the separator 140 had a thickness of 9 μm and a Gurley air permeability of 61 seconds/100 cc, the peel strength was 0.03 N, and the number of cycles at which the volume retention ratio was 80% or less was 52 cycles. In addition, in Comparative Example 4 in which the thickness of the separator 140 was 20 μm and the Gurley air permeability was 651 sec/100 cc, the peel strength was 0.08 N, and the number of cycles at which the volume retention rate was 80% or less was 78 cycles. .

On the other hand, in Example 5 in which the separator 140 had a thickness of 15 μm and a Gurley air permeability of 474 seconds/100 cc, the peel strength was 0.13 N, and the number of cycles at which the volume retention ratio was 80% or less was 105 cycles. rice field. In Example 6 in which the separator 140 had a thickness of 21 μm and a Gurley air permeability of 87 seconds/100 cc, the peel strength was 0.23 N, and the number of cycles at which the volume retention ratio was 80% or less was 106 cycles. In Example 6, in which the separator 140 had a thickness of 12.2 μm and a Gurley air permeability of 233 seconds/100 cc, the peel strength was 0.36 N, and the number of cycles at which the volume retention rate was 80% or less was 150 cycles. .

[Summary of experimental results]
As described above, a certain correlation was observed between the peel strength between the negative electrode current collector 131 and the separator 140 and the number of cycles at which the volume retention rate was 80% or less. In particular, in the lithium secondary battery 100 in which the peel strength between the negative electrode current collector 131 and the separator 140 is 0.1 N or more, the number of cycles at which the volume retention ratio is 80% or less exceeds 100 cycles, which is relatively good. good results were obtained.

Also, good cycle characteristics were obtained by setting the peel strength between the negative electrode current collector 131 and the separator 140 to preferably 0.2 N or more, more preferably 0.3 N or more.

The lithium secondary battery of the present invention has high energy density and excellent cycle characteristics, so it has industrial applicability as an electricity storage device used for various purposes.

DESCRIPTION OF

SYMBOLS

100, 200... Lithium secondary battery, 110... Positive electrode collector, 120... Positive electrode, 130... Negative electrode, 131... Negative electrode collector, 132... Lithium metal layer, 140... Separator, 141... Separator base material, 142... Separator Coating layer 210... Negative electrode terminal 220... Positive electrode terminal

Claims

a positive electrode;
a negative electrode including a negative electrode current collector that does not have a negative electrode active material before initial charging;
a separator disposed between the positive electrode and the negative electrode;
The peel strength between the negative electrode current collector and the separator is 0.1 N or more,
Lithium secondary battery.
The separator has a separator coating layer at least on the negative electrode side,
The lithium secondary battery according to claim 1.
The thickness of the separator coating layer is 0.7 μm or more,
The lithium secondary battery according to claim 2.
Gurley air permeability of the separator is 180 sec/100 cc or more,
4. The lithium secondary battery according to claim 2 or 3.
The Gurley air permeability is 600 sec/100 cc or less,
The lithium secondary battery according to claim 4.
a positive electrode;
a negative electrode including a negative electrode current collector that does not have a negative electrode active material before initial charging;
a separator disposed between the positive electrode and the negative electrode;
The separator has a separator coating layer at least on the negative electrode side,
The thickness of the separator coating layer is 0.7 μm or more,
The separator has a Gurley air permeability of 180 sec/100 cc or more and 600 sec/100 cc or less.
Lithium secondary battery.