WO2024190050A1

WO2024190050A1 - Secondary battery electrolyte and secondary battery

Info

Publication number: WO2024190050A1
Application number: PCT/JP2024/000259
Authority: WO
Inventors: 将之井原
Original assignee: 株式会社村田製作所
Priority date: 2023-03-16
Filing date: 2024-01-10
Publication date: 2024-09-19

Abstract

Provided is a secondary battery with which it is possible to attain excellent battery characteristics. This secondary battery comprises: a positive electrode; a negative electrode; and an electrolyte including an electrolytic salt and chloride ions. The electrolyte salt contains imide anions. The imide anions include at least one of first imide anions represented by formula (1), second imide anions represented by formula (2), third imide anions represented by formula (3), or fourth imide anions represented by formula (4). The content of chlorine ions in the electrolyte is 5000 ppm by weight or less.

Description

Electrolyte for secondary battery and secondary battery

This technology relates to electrolytes for secondary batteries and secondary batteries.

　With the widespread use of a wide variety of electronic devices such as mobile phones, secondary batteries are being developed as a power source that is small, lightweight, and has a high energy density. These secondary batteries contain a positive electrode, a negative electrode, and an electrolyte (secondary battery electrolyte), and various studies are being conducted on the configuration of these secondary batteries.

Specifically, the electrolyte contains an imide compound represented by R _F ¹ -S(=O) ₂ -NH-S(=O) ₂ -NH-S(=O) ₂ -R _F ² (see, for example, Patent Document 1). Also, the electrolyte salt of the electrolyte contains an imide anion represented by F-S(=O) ₂ -N ^- -C(=O)-N ^- -S(=O) ₂ -F or F-S(=O) ₂ -N ^- -S(=O) ₂ -C ₆ H ₄ -S(=O) ₂ -N ^- -S(=O) ₂ -F (see, for example, Non-Patent Documents 1 and 2).

Chinese Patent No. 102786443

Various studies have been conducted on the configuration of secondary batteries, but the battery characteristics of these batteries are still insufficient, leaving room for improvement.

There is a demand for secondary battery electrolytes and secondary batteries that can provide excellent battery characteristics.

The electrolyte for a secondary battery according to one embodiment of the present technology contains an electrolyte salt and chloride ions. The electrolyte salt contains imide anions, and the imide anions include at least one of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4). The chloride ion content is 5,000 ppm by weight or less.

(Each of R1 and R2 is either a fluorine group or a fluorinated alkyl group. Each of W1, W2 and W3 is either a carbonyl group (>C=O), a sulfinyl group (>S=O) or a sulfonyl group (>S(=O) ₂ ).)

(Each of R3 and R4 is either a fluorine group or a fluorinated alkyl group. Each of X1, X2, X3 and X4 is either a carbonyl group, a sulfinyl group or a sulfonyl group.)

(R5 is a fluorinated alkylene group. Each of Y1, Y2, and Y3 is any one of a carbonyl group, a sulfinyl group, and a sulfonyl group.)

(Each of R6 and R7 is either a fluorine group or a fluorinated alkyl group. R8 is either an alkylene group, a phenylene group, a fluorinated alkylene group, or a fluorinated phenylene group. Each of Z1, Z2, Z3, and Z4 is either a carbonyl group, a sulfinyl group, or a sulfonyl group.)

The secondary battery of one embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte, and the electrolyte has a configuration similar to that of the electrolyte for a secondary battery of one embodiment of the present technology described above.

According to one embodiment of the secondary battery electrolyte or secondary battery of the present technology, the secondary battery electrolyte contains an electrolyte salt and chloride ions, the electrolyte salt contains imide anions, and the imide anions contain one or more of the first imide anion, the second imide anion, the third imide anion, and the fourth imide anion, and the chloride ion content is 5,000 ppm by weight or less, so that excellent battery characteristics can be obtained.

Note that the effects of this technology are not necessarily limited to the effects described here, but may be any of a series of effects related to this technology described below.

FIG. 1 is a perspective view illustrating a configuration of a secondary battery according to an embodiment of the present technology. FIG. 2 is a cross-sectional view showing the configuration of the battery element shown in FIG. FIG. 3 is a block diagram showing a configuration of an application example of a secondary battery.

Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be given in the following order.

1. Electrolyte for secondary battery 1-1. Configuration 1-2. Manufacturing method 1-3. Action and effect 2. Secondary battery 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Use of secondary battery

<1. Electrolyte for secondary batteries>
First, an electrolyte for a secondary battery (hereinafter simply referred to as an "electrolyte") according to an embodiment of the present technology will be described.

<1-1. Configuration>
The electrolytic solution is a liquid electrolyte used in a secondary battery, which is an electrochemical device. However, the electrolytic solution may be used in other electrochemical devices. The type of the other electrochemical device is not particularly limited, but specifically, it is a capacitor or the like.

The electrolyte contains an electrolyte salt and chloride ions (Cl ⁻ ). More specifically, the electrolyte further contains a solvent that disperses (ionizes) the electrolyte salt.

[Electrolyte salt]
The electrolyte salt is a compound that ionizes in a solvent and contains an anion and a cation.

(anion)
The anion includes an imide anion. Specifically, the imide anion includes one or more of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4). That is, the electrolyte salt includes one imide anion as an anion.

However, the type of the first imide anion may be one type or two or more types. The same applies to each of the second imide anion, the third imide anion, and the fourth imide anion.

(Each of R1 and R2 is either a fluorine group or a fluorinated alkyl group. Each of W1, W2, and W3 is either a carbonyl group, a sulfinyl group, or a sulfonyl group.)

The reason why the anion contains an imide anion is as follows. First, when a secondary battery using an electrolyte is charged and discharged, a good quality coating derived from the electrolyte salt is formed on the surface of each of the positive and negative electrodes, suppressing the decomposition reaction of the electrolyte on the surface of each of the positive and negative electrodes. In this case, the decomposition reaction of the solvent is particularly suppressed. Second, the above-mentioned coating is used to improve the migration speed of cations near the surfaces of each of the positive and negative electrodes. Third, the migration speed of cations is improved even in the electrolyte liquid.

(First imide anion)
As shown in formula (1), the first imide anion is a chain anion (divalent negative ion) containing two nitrogen atoms (N) and three functional groups (W1 to W3).

R1 and R2 are not particularly limited as long as they are either a fluorine group (-F) or a fluorinated alkyl group. Thus, R1 and R2 are not, for example, a hydrogen group (-H) or an alkyl group. R1 and R2 may be the same group or different groups.

A fluorinated alkyl group is an alkyl group in which one or more hydrogen groups (-H) have been replaced with a fluorine group. However, the fluorinated alkyl group may be linear or branched with one or more side chains.

The number of carbon atoms in the fluorinated alkyl group is not particularly limited, but is specifically 1 to 10. This is because the solubility and ionization properties of the electrolyte salt containing the first imide anion are improved.

Specific examples of fluorinated alkyl groups include a perfluoromethyl group (--CF ₃ ) and a perfluoroethyl group (--C ₂ F ₅ ).

W1 to W3 are not particularly limited as long as they are either a carbonyl group, a sulfinyl group, or a sulfonyl group. In other words, W1 to W3 may be the same group or different groups. Of course, any two of W1 to W3 may be the same group.

(Second Imide Anion)
As shown in formula (2), the second imide anion is a chain anion (trivalent negative ion) containing three nitrogen atoms and four functional groups (X1 to X4).

The details for R3 and R4 are similar to those for R1 and R2.

X1 to X4 are not particularly limited as long as they are either a carbonyl group, a sulfinyl group, or a sulfonyl group. In other words, X1 to X4 may be the same group or different groups. Of course, any two of X1 to X4 may be the same group, or any three of X1 to X4 may be the same group.

(Tertiary imide anion)
As shown in formula (3), the third imide anion is a cyclic anion (divalent negative ion) containing two nitrogen atoms, three functional groups (Y1 to Y3), and one connecting group (R5).

The fluorinated alkylene group R5 is an alkylene group in which one or more hydrogen groups have been replaced with fluorine groups. However, the fluorinated alkylene group may be linear or branched with one or more side chains.

The number of carbon atoms in the fluorinated alkylene group is not particularly limited, but is specifically 1 to 10. This is because it improves the solubility and ionization properties of the electrolyte salt containing the third imide anion.

Specific examples of fluorinated alkylene groups include a perfluoromethylene group (--CF ₂ --) and a perfluoroethylene group (--C ₂ F ₄ --).

Y1 to Y3 are not particularly limited as long as they are either a carbonyl group, a sulfinyl group, or a sulfonyl group. In other words, Y1 to Y3 may be the same group or different groups. Of course, any two of Y1 to Y3 may be the same group.

(Quaternary imide anion)
As shown in formula (4), the quaternary imide anion is a chain anion (divalent negative ion) containing two nitrogen atoms, four functional groups (Z1 to Z4), and one connecting group (R8).

The details for R6 and R7 are similar to those for R1 and R2.

R8 is not particularly limited as long as it is any one of an alkylene group, a phenylene group, a fluorinated alkylene group, and a fluorinated phenylene group.

The alkylene group may be linear or branched having one or more side chains. The number of carbon atoms in the alkylene group is not particularly limited, but specifically, it is 1 to 10. This is because the solubility and ionization property of the electrolyte salt containing the quaternary imide anion are improved. Specific examples of the alkylene group include a methylene group ( _-CH2- ), _an _{ethylene group (-C2H4-} ₎ , and a propylene group ( _-C3H6- ).

Details regarding the fluorinated alkylene group R8 are the same as those regarding the fluorinated alkylene group R5.

A fluorinated phenylene group is a phenylene group in which one or more hydrogen groups have been substituted with fluorine groups. A specific example of a fluorinated phenylene group is a monofluorophenylene group (-C ₆ H ₃ F-).

Z1 to Z4 are not particularly limited as long as they are either a carbonyl group, a sulfinyl group, or a sulfonyl group. In other words, Z1 to Z4 may be the same group or different groups. Of course, any two of Z1 to Z4 may be the same group, or any three of Z1 to Z4 may be the same group.

(Specific examples of anions)
Specific examples of the first imide anion include anions represented by each of formulas (1-1) to (1-30).

Specific examples of the second imide anion include the anions represented by formulas (2-1) to (2-22).

Specific examples of the third imide anion include the anions represented by formulas (3-1) to (3-15).

Specific examples of the fourth imide anion include the anions represented by formulas (4-1) to (4-65).

(Cation)
The type of cation is not particularly limited. Specifically, the cation contains one or more types of light metal ions. That is, the electrolyte salt contains light metal ions as cations. This is because a high voltage can be obtained.

The type of light metal ion is not particularly limited, but specific examples include alkali metal ions and alkaline earth metal ions. Specific examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. Specific examples of alkaline earth metal ions include beryllium ions, magnesium ions, and calcium ions. In addition, the light metal ion may be an aluminum ion, etc.

Among them, it is preferable that the light metal ions contain lithium ions, because this allows a sufficiently high voltage to be obtained.

(Content)
The content of the electrolyte salt in the electrolytic solution is not particularly limited and can be set arbitrarily. In particular, the content of the electrolyte salt in the electrolytic solution is preferably 0.2 mol/kg to 2 mol/kg. This is because high ionic conductivity can be obtained. The "content of the electrolyte salt" described here refers to the content of the electrolyte salt relative to the solvent.

When measuring the content of electrolyte salt in an electrolyte, the electrolyte is analyzed using one or more of the following analytical methods: inductively coupled plasma (ICP) atomic emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and gas chromatography-mass spectrometry (GC-MS). This allows the weight of the solvent and the weight of the electrolyte salt to be determined, and the content of the electrolyte salt can be calculated.

[Chloride ion]
Chloride ions are included in the electrolyte primarily for reasons that will be explained below.

First, chloride ions are generated during the synthesis of electrolyte salts containing imide anions, and the chloride ions are mixed into the electrolyte solution. For example, when a dehydration condensation reaction occurs during the synthesis of electrolyte salts containing imide anions, chloride ions derived from thionyl chloride (SOCl ₂ ) are generated.

Secondly, electrolyte salts containing imide anions originally contain chloride ions, and these chloride ions are mixed into the electrolyte solution. In this case, chloride ions are generated because electrolyte salts containing imide anions contain chlorine as a constituent element.

Thirdly, when a secondary battery using an electrolyte is charged and discharged, chloride ions are generated due to the decomposition reaction of the electrolyte, etc., and these chloride ions are mixed into the electrolyte. In this case, as described above, electrolyte salts containing imide anions contain chlorine as a constituent element, so chloride ions are generated, and chloride ions are also generated because they are mixed into the electrolyte.

However, chloride ions may be present in the electrolyte for reasons other than those mentioned above.

Here, the content of chloride ions in the electrolyte is 5,000 ppm by weight or less, more specifically, 0 ppm by weight to 5,000 ppm by weight. This is because the chemical stability of the electrolyte is improved, and the decomposition reaction of the electrolyte is suppressed.

In more detail, chloride ions affect the chemical stability of the electrolyte. In this case, if the chloride ion content is more than 5,000 ppm by weight, the chemical stability of the electrolyte decreases, making the electrolyte more susceptible to decomposition when a secondary battery using that electrolyte is charged and discharged. In contrast, if the chloride ion content is 5,000 ppm by weight or less, the chemical stability of the electrolyte improves, making the electrolyte less susceptible to decomposition when a secondary battery using that electrolyte is charged and discharged.

Among these, it is more preferable that the content of chloride ions in the electrolyte is 100 ppm by weight or less, more specifically 0 ppm by weight to 100 ppm by weight, even more preferably 50 ppm by weight or less, more specifically 0 ppm by weight to 50 ppm by weight, and particularly preferably 30 ppm by weight or less, more specifically 0 ppm by weight to 30 ppm by weight. This is because the chemical stability of the electrolyte is further improved.

When measuring the amount of chloride ions in an electrolyte, the electrolyte is analyzed using an analytical method such as ion chromatography. This separates the chloride ions and measures their amount.

[solvent]
The solvent contains one or more of non-aqueous solvents (organic solvents), and the electrolyte solution containing the non-aqueous solvent is a so-called non-aqueous electrolyte solution. The non-aqueous solvent is an ester, an ether, or the like, more specifically, a carbonate ester compound, a carboxylate ester compound, a lactone compound, or the like.

Carbonate compounds include cyclic carbonates and chain carbonates. Examples of cyclic carbonates are ethylene carbonate and propylene carbonate. Examples of chain carbonates are dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

Carboxylic acid ester compounds include chain carboxylates. Specific examples of chain carboxylates include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, ethyl trimethylacetate, methyl butyrate, and ethyl butyrate.

Lactone compounds include lactones. Specific examples of lactones include gamma-butyrolactone and gamma-valerolactone.

The ethers may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc.

[Other electrolyte salts]
The electrolyte may further contain one or more of the other electrolyte salts. This is because the cation migration speed is improved in the vicinity of the surfaces of the positive electrode and the negative electrode, and the cation migration speed is also improved in the electrolyte. The content of the other electrolyte salt in the electrolyte is not particularly limited and can be set arbitrarily.

The type of other electrolyte salt is not particularly limited, but specifically, it is a light metal salt such as a lithium salt. However, the above electrolyte salts are excluded from the lithium salts described here.

Specific examples of lithium salts include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium bis(fluorosulfonyl)imide (LiN(FSO ₂ ) ₂ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ₃ SO ₂ ) ₂ ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF ₃ SO ₂ ) ₃ ), lithium bis(oxalato)borate (LiB(C ₂ O ₄ ) ₂ ), lithium difluorooxalatoborate (LiBF ₂ (C ₂ O ₄ )), and lithium difluorodi(oxalato)borate (LiPF ₂ (C ₂ O ₄ ) _{2 )} . ), lithium tetrafluorooxalatophosphate (LiPF ₄ (C ₂ O ₄ )), lithium monofluorophosphate (Li ₂ PFO ₃ ), and lithium difluorophosphate (LiPF ₂ O ₂ ).

Among them, it is preferable that the other electrolyte salt contains one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, and lithium difluorophosphate. This is because the cation migration speed is sufficiently improved near the surfaces of the positive electrode and negative electrode, and the cation migration speed is also sufficiently improved in the electrolyte solution.

[Other Solvents]
The electrolyte may further contain one or more of the other solvents. During charging and discharging of the secondary battery using the electrolyte, a coating derived from the other solvent is formed on the surface of each of the positive and negative electrodes, so that the decomposition reaction of the electrolyte is suppressed. The content of the other solvent in the electrolyte is not particularly limited and can be set arbitrarily.

The types of other solvents are not particularly limited, but specific examples include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonic acid esters, dicarboxylic acid anhydrides, disulfonic acid anhydrides, sulfates, nitrile compounds, and isocyanate compounds.

(Unsaturated cyclic carbonate)
The unsaturated cyclic carbonate is a cyclic carbonate containing an unsaturated carbon bond (carbon-carbon double bond). The number of unsaturated carbon bonds is not particularly limited, and may be one or more. Specific examples of the unsaturated cyclic carbonate include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate.

(Fluorinated cyclic carbonate)
A fluorinated cyclic carbonate is a cyclic carbonate containing fluorine as a constituent element. That is, a fluorinated cyclic carbonate is a compound in which one or more hydrogen groups of a cyclic carbonate are substituted with fluorine groups. Specific examples of the fluorinated cyclic carbonate include monofluoroethylene carbonate and difluoroethylene carbonate.

(Sulfonic acid ester)
The sulfonate esters include cyclic monosulfonate esters, cyclic disulfonate esters, chain monosulfonate esters, chain disulfonate esters, etc. Specific examples of cyclic monosulfonate esters include 1,3-propane sultone, 1-propene-1,3-sultone, 1,4-butane sultone, 2,4-butane sultone, methanesulfonic acid propargyl ester, etc. Specific examples of cyclic disulfonate esters include cyclodisone, etc.

(Dicarboxylic acid anhydride)
Specific examples of dicarboxylic acid anhydrides include succinic anhydride, glutaric anhydride, and maleic anhydride.

(Disulfonic acid anhydride)
Specific examples of disulfonic anhydrides include ethanedisulfonic anhydride and propanedisulfonic anhydride.

(Sulfuric acid ester)
A specific example of a sulfate ester is ethylene sulfate (1,3,2-dioxathiolane 2,2-dioxide).

(Nitrile compounds)
The nitrile compound is a compound having one or more cyano groups (-CN). Specific examples of the nitrile compound include octanenitrile, benzonitrile, phthalonitrile, succinonitrile, glutaronitrile, adiponitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, 3,3'-oxydipropionitrile, 3-butoxypropionitrile, ethylene glycol bispropionitrile ether, 1,2,2,3-tetracyanopropane, tetracyanopropane, fumaronitrile, 7,7,8,8-tetracyanoquinodimethane, cyclopentanecarbonitrile, 1,3,5-cyclohexanetricarbonitrile, and 1,3-bis(dicyanomethylidene)indane.

(Isocyanate Compounds)
The isocyanate compound is a compound having one or more isocyanate groups (-NCO). A specific example of the isocyanate compound is hexamethylene diisocyanate.

<1-2. Manufacturing method>
When producing an electrolytic solution, an electrolyte salt is added to a solvent. In this case, another electrolyte salt may be added to the solvent, or an additive may be added to the solvent. The electrolyte salt and the like are dispersed or dissolved in the solvent by the above-mentioned process, and thus an electrolyte solution is prepared. This electrolyte solution contains chloride ions for the above-mentioned reasons.

<1-3. Actions and Effects>
According to this electrolyte, the electrolyte contains an electrolyte salt and chloride ions, the electrolyte salt contains imide anions, and the content of chloride ions in the electrolyte is 5000 ppm by weight or less.

In this case, since the electrolyte salt contains imide anions, as described above, when a secondary battery using an electrolyte is charged and discharged, the decomposition reaction of the electrolyte is suppressed by utilizing the high-quality coating derived from the electrolyte salt, and the migration rate of cations is improved.

In addition, because the content of chloride ions in the electrolyte is 5,000 ppm by weight or less, the chemical stability of the electrolyte is improved, as described above. This further suppresses the decomposition reaction of the electrolyte when a secondary battery using the electrolyte is charged and discharged.

As a result, when a secondary battery using an electrolyte is charged and discharged, the decomposition reaction of the electrolyte is significantly suppressed while the cation migration rate is guaranteed. Therefore, a secondary battery with excellent battery characteristics can be realized by using an electrolyte.

In particular, if the electrolyte salt contains light metal ions as cations, a higher voltage can be obtained, and therefore a greater effect can be achieved. In this case, if the light metal ions contain lithium ions, a higher voltage can be obtained, and therefore an even greater effect can be achieved.

In addition, if the content of electrolyte salt in the electrolyte solution is 0.2 mol/kg to 2 mol/kg, high ionic conductivity can be obtained, resulting in even greater effects.

In addition, if the electrolyte further contains one or more of the following electrolyte salts as other electrolyte salts: lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, and lithium difluorophosphate, the cation migration rate is further improved, and thus a greater effect can be obtained.

In addition, if the electrolyte further contains one or more of the following solvents as other solvents: unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonic acid esters, dicarboxylic acid anhydrides, disulfonic acid anhydrides, sulfate esters, nitrile compounds, and isocyanate compounds, the decomposition reaction of the electrolyte is suppressed, and thus a greater effect can be obtained.

2. Secondary battery
Next, a secondary battery using the above-mentioned electrolyte will be described.

The secondary battery described here is a secondary battery that obtains battery capacity by utilizing the absorption and release of electrode reactants, and is equipped with a positive electrode, a negative electrode, and an electrolyte.

The charge capacity of the negative electrode is preferably greater than the discharge capacity of the positive electrode. In other words, the electrochemical capacity per unit area of the negative electrode is preferably greater than the electrochemical capacity per unit area of the positive electrode. This is to prevent electrode reactants from depositing on the surface of the negative electrode during charging.

The type of electrode reactant is not particularly limited, but specifically includes light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium, while specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

Below, we will use an example where the electrode reactant is lithium. A secondary battery that obtains battery capacity by utilizing the absorption and release of lithium is known as a lithium-ion secondary battery. In this lithium-ion secondary battery, lithium is absorbed and released in an ionic state.

<2-1. Configuration>
Fig. 1 shows a perspective view of a secondary battery, and Fig. 2 shows a cross-sectional view of a battery element 20 shown in Fig. 1.

However, in FIG. 1, the exterior film 10 and the battery element 20 are shown in a state where they are separated from each other, and a cross section of the battery element 20 along the XZ plane is shown by a dashed line. In FIG. 2, only a part of the battery element 20 is shown.

As shown in Figures 1 and 2, this secondary battery includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing

films

41 and 42.

As described above, the secondary battery described here uses a flexible or pliable exterior film 10 as an exterior member for housing the battery element 20 inside. Therefore, the secondary battery shown in Figures 1 and 2 is a so-called laminate film type secondary battery.

[Exterior film]
1, the exterior film 10 has a bag-like structure that is sealed with the battery element 20 housed therein. As a result, the exterior film 10 houses a positive electrode 21, a negative electrode 22, and a separator 23, which will be described later.

Here, the exterior film 10 is a single film-like member that is folded in the folding direction F. This exterior film 10 is provided with a recessed portion 10U (a so-called deep drawn portion) for accommodating the battery element 20.

Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

However, the configuration (number of layers) of the exterior film 10 is not particularly limited, so it may be one or two layers, or four or more layers.

[Battery element]
The battery element 20 is housed inside the exterior film 10. The battery element 20 is a so-called power generating element, and includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), as shown in Figures 1 and 2 .

Here, the battery element 20 is a so-called wound electrode body, so that the positive electrode 21 and the negative electrode 22 are wound around the winding axis P while facing each other via the separator 23. This winding axis P is a virtual axis extending in the Y-axis direction, as shown in FIG. 1.

The three-dimensional shape of the battery element 20 is not particularly limited. Here, the battery element 20 has a flat three-dimensional shape, so that the shape of the cross section (cross section along the XZ plane) of the battery element 20 intersecting the winding axis P is a flat shape defined by the major axis J1 and the minor axis J2.

The long axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than that of the short axis J2. The short axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and has a length less than that of the long axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylinder, and therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

(Positive electrode)
As shown in FIG. 2, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

The positive electrode collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. This positive electrode collector 21A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum.

The positive electrode active material layer 21B contains one or more types of positive electrode active materials that absorb and release lithium. However, the positive electrode active material layer 21B may further contain one or more types of other materials such as a positive electrode binder and a positive electrode conductor. The method of forming the positive electrode active material layer 21B is not particularly limited, but specifically includes a coating method.

Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode collector 21A. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode collector 21A on the side where the positive electrode 21 faces the negative electrode 22.

The type of positive electrode active material is not particularly limited, but specifically includes lithium-containing compounds. This lithium-containing compound is a compound that contains one or more transition metal elements as constituent elements along with lithium, and may further contain one or more other elements as constituent elements. The type of other element is not particularly limited, so long as it is an element other than lithium and transition metal elements, but specifically includes elements belonging to groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specifically includes oxides, phosphate compounds, silicate compounds, and borate compounds.

_Specific _examples _of _oxides _include _LiNiO2 _, _LiCoO2 _, _{LiCo0.98Al0.01Mg0.01O2} _, _{LiNi0.5Co0.2Mn0.3O2} _, _{LiNi0.8Co0.15Al0.05O2} _, _{LiNi0.33Co0.33Mn0.33O2} _, _{Li1.2Mn0.52Co0.175Ni0.1O2} _, _Li1.15 ₍ _{Mn0.65Ni0.22Co0.13} ₎ _O2 _, _and _LiMn2O4 _. Specific examples of _phosphate compounds include _LiFePO4 , _LiMnPO4 _, _{LiFe0.5Mn0.5PO4} _, _and _{LiFe0.3Mn0.7PO4} .

The positive electrode binder contains one or more of the following materials: synthetic rubber and polymeric compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluororubber, and ethylene-propylene-diene. Specific examples of polymeric compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

The positive electrode conductive agent contains one or more conductive materials such as carbon materials, metal materials, and conductive polymer compounds. Specific examples of carbon materials include graphite, carbon black, acetylene black, and ketjen black.

(Negative electrode)
As shown in FIG. 2, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. This negative electrode current collector 22A contains a conductive material such as a metal material, and a specific example of the conductive material is copper.

The negative electrode active material layer 22B contains one or more types of negative electrode active materials that absorb and release lithium. However, the negative electrode active material layer 22B may further contain one or more types of other materials such as a negative electrode binder and a negative electrode conductor. The method of forming the negative electrode active material layer 22B is not particularly limited, but specifically includes one or more types of a coating method, a gas phase method, a liquid phase method, a thermal spraying method, and a baking method (sintering method).

Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode collector 22A. However, the negative electrode active material layer 22B may be provided on only one side of the negative electrode collector 22A on the side where the negative electrode 22 faces the positive electrode 21.

The type of negative electrode active material is not particularly limited, but specific examples include carbon materials and metal-based materials, because they provide high energy density.

Specific examples of carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite. The graphite may be natural graphite or artificial graphite.

Metallic materials are a general term for materials that contain one or more of metallic elements and semi-metallic elements that can form an alloy with lithium as constituent elements, and specific examples of the metallic elements and semi-metallic elements include silicon and tin. The metallic materials may be a single element, an alloy, a compound, a mixture of two or more of these, or a material that contains two or more of these phases. Specific examples of metallic materials include _TiSi2 and _SiOx (0<x≦2 or 0.2<x<1.4).

Details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and details regarding the negative electrode conductor are the same as those regarding the positive electrode conductor.

(Separator)
2, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium to pass through in an ion state while preventing the occurrence of a short circuit due to contact between the positive electrode 21 and the negative electrode 22. This separator 23 contains a polymer compound such as polyethylene.

(Electrolyte)
The electrolyte is impregnated into each of the positive electrode 21, the negative electrode 22, and the separator 23, and has the above-mentioned configuration. That is, the electrolyte contains an electrolyte salt and chloride ions, the electrolyte salt contains imide anions, and the content of chloride ions in the electrolyte is within the above-mentioned range.

[Positive lead]
1 and 2, the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The shape of the positive electrode lead 31 is either a thin plate shape or a mesh shape.

[Negative lead]
As shown in Fig. 1 and Fig. 2, the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode current collector 22A of the negative electrode 22, and is led out of the exterior film 10. Here, the lead-out direction of the negative electrode lead 32 is the same as the lead-out direction of the positive electrode lead 31. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. The details of the shape of the negative electrode lead 32 are the same as the details of the shape of the positive electrode lead 31.

[Sealing film]
The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing

films

41 and 42 may be omitted.

The sealing film 41 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesion to the positive electrode lead 31, and a specific example of the polymer compound is polypropylene.

The configuration of the sealing film 42 is the same as that of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesion to the negative electrode lead 32. In other words, the sealing film 42 contains a polymer compound such as polyolefin that has adhesion to the negative electrode lead 32.

<2-2. Operation>
This secondary battery operates in the battery element 20 as follows.

When charging, lithium is released from the positive electrode 21 and is absorbed into the negative electrode 22 via the electrolyte. When discharging, lithium is released from the negative electrode 22 and is absorbed into the positive electrode 21 via the electrolyte. When discharging and charging, lithium is absorbed and released in an ionic state.

<2-3. Manufacturing method>
In the case of manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are each produced and an electrolyte solution is prepared according to the procedure described below. Then, the positive electrode 21, the negative electrode 22, and the electrolyte solution are used to manufacture the secondary battery. A secondary battery is assembled and a stabilization process is performed on the secondary battery after assembly.

Because the method for producing the electrolyte has already been explained, the explanation of the method for producing the electrolyte will be omitted below.

[Preparation of Positive Electrode]
First, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed together to prepare a positive electrode mixture. Then, the positive electrode mixture is poured into a solvent to prepare a paste-like positive electrode mixture slurry. The solvent may be an aqueous solvent or an organic solvent.

Then, the positive electrode active material layer 21B is formed by applying the positive electrode mixture slurry to both sides of the positive electrode current collector 21A. Finally, the positive electrode active material layer 21B is compression molded using a roll press or the like. In this case, the positive electrode active material layer 21B may be heated, or the compression molding of the positive electrode active material layer 21B may be repeated multiple times. In this way, the positive electrode active material layer 21B is formed on both sides of the positive electrode current collector 21A, and the positive electrode 21 is produced.

[Preparation of negative electrode]
The negative electrode 22 is formed by the same procedure as the procedure for producing the positive electrode 21 described above. Specifically, first, a mixture (negative electrode mixture) in which the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent are mixed together is put into a solvent to prepare a paste-like negative electrode mixture slurry. Details regarding the solvent are as described above. Next, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A to form the negative electrode active material layer 22B. Finally, the negative electrode active material layer 22B is compression molded. As a result, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, and the negative electrode 22 is produced.

[Assembly of secondary battery]
First, the positive electrode lead 31 is connected to the positive electrode collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode collector 22A of the negative electrode 22 using a joining method such as welding.

Then, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with the separator 23 interposed therebetween to form a laminate (not shown). The laminate is then wound to produce a wound body (not shown), which is then pressed using a press or the like to form the wound body into a flat shape. The wound body after this formation has a configuration similar to that of the battery element 20, except that it is not impregnated with the electrolyte.

Then, after the roll is placed inside the recess 10U, the exterior film 10 (adhesive layer/metal layer/surface protection layer) is folded so that the exterior films 10 face each other. Next, the outer edges of two of the opposing adhesive layers are joined to each other using an adhesive method such as heat fusion, thereby placing the roll inside the bag-shaped exterior film 10.

Finally, after injecting an electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined to each other by using an adhesive method such as a heat fusion method. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

As a result, the wound body is impregnated with the electrolyte, producing the battery element 20. The battery element 20 is then enclosed inside the bag-shaped exterior film 10, and a secondary battery is assembled.

[Stabilization treatment of secondary battery after assembly]
The assembled secondary battery is charged and discharged. Stabilization conditions such as the environmental temperature, the number of charge/discharge cycles (number of cycles), and charge/discharge conditions can be set arbitrarily. As a result, a coating is formed on the surface of each of the positive electrode 21 and the negative electrode 22, and the state of the battery element 20 is electrochemically stabilized. Thus, the secondary battery is completed.

<2-4. Actions and Effects>
According to this secondary battery, the secondary battery includes an electrolyte, and the electrolyte has the above-mentioned configuration, and therefore, for the above-mentioned reasons, the decomposition reaction of the electrolyte is significantly suppressed while the lithium ion migration speed is guaranteed, and therefore excellent battery characteristics can be obtained.

In particular, if the secondary battery is a lithium-ion secondary battery, sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium, resulting in even greater effects.

Other functions and effects of this secondary battery are similar to those of the electrolyte described above.

3. Modifications
The configuration of the secondary battery can be modified as appropriate, as described below, although the series of modifications described below may be combined with each other.

[Modification 1]
As described above, the electrolyte solution may contain other electrolyte salts in addition to the electrolyte salt containing the imide anion.

In particular, it is preferable that the electrolyte solution contains lithium hexafluorophosphate as another electrolyte salt, and that the content of the electrolyte salt in the electrolyte solution is optimized in relation to the content of the other electrolyte salt in the electrolyte solution.

Specifically, the electrolyte salt contains a cation and an imide anion. Also, the hexafluorophosphate ion contains a lithium ion and a hexafluorophosphate ion.

In this case, the sum T (mol/kg) of the cation content C1 in the electrolyte and the lithium ion content C2 in the electrolyte is preferably 0.7 mol/kg to 2.2 mol/kg. In addition, the ratio R (mol%) of the number of moles M2 of hexafluorophosphate ions in the electrolyte to the number of moles M1 of imide anions in the electrolyte is preferably 13 mol% to 6000 mol%. This is because the migration speeds of the cations and lithium ions in the vicinity of the surfaces of the positive electrode 21 and the negative electrode 22 are sufficiently improved, and the migration speeds of the cations and lithium ions in the electrolyte are also sufficiently improved.

The "content of cations in the electrolyte" described here is the content of electrolyte salt of cations relative to the solvent, and the "content of lithium ions in the electrolyte" is the content of lithium ions relative to the solvent. The sum T is calculated based on the formula T = C1 + C2, and the ratio R is calculated based on the formula R = (M2/M1) x 100.

When calculating the sum T and the ratio R, the secondary battery is disassembled to recover the electrolyte, and the electrolyte is then analyzed using ICP atomic emission spectrometry. This allows the contents C1, C2 and the mole numbers M1, M2 to be determined, and the sum T and the ratio R to be calculated.

In this case, the electrolyte solution contains electrolyte salt, and therefore the same effect can be obtained. In this case, particularly when the electrolyte salt is used in combination with another electrolyte salt (lithium hexafluorophosphate), the total amount of both (sum T) is optimized, and the mixing ratio (ratio R) of both is also optimized. This further improves the migration speed of cations and lithium ions near the surfaces of the positive electrode 21 and the negative electrode 22, and also further improves the migration speed of cations and lithium ions in the electrolyte solution. Therefore, a greater effect can be obtained.

[Modification 2]
In the first modification, the electrolyte solution contains lithium hexafluorophosphate as another electrolyte salt. However, the electrolyte solution may contain lithium bis(fluorosulfonyl)imide as another electrolyte salt instead of lithium hexafluorophosphate. Even in this case, it is preferable that the content of the electrolyte salt in the electrolyte solution is optimized in relation to the content of the other electrolyte salt in the electrolyte solution.

Specifically, the bis(fluorosulfonyl)imide lithium contains lithium ions and bis(fluorosulfonyl)imide ions. In this case, the sum T (mol/kg) of the cation content C1 in the electrolyte and the lithium ion content C2 in the electrolyte is preferably 0.7 mol/kg to 2.2 mol/kg. In addition, the ratio R (mol%) of the number of moles M3 of bis(fluorosulfonyl)imide ions in the electrolyte to the number of moles M1 of imide anions in the electrolyte is preferably 13 mol% to 6000 mol%. This is because the migration speeds of the cations and lithium ions are sufficiently improved near the surfaces of the positive electrode 21 and the negative electrode 22, and the migration speeds of the cations and lithium ions are also sufficiently improved in the electrolyte.

The ratio R is calculated based on the formula R = (M3/M1) x 100. The procedure for calculating the ratio R is as described above, except that the number of moles M3 is specified instead of the number of moles M2.

In this case, the electrolyte solution contains an electrolyte salt, and therefore the same effect can be obtained. In this case, particularly when the electrolyte salt is used in combination with another electrolyte salt (lithium bis(fluorosulfonyl)imide), the total amount (sum T) of the two is optimized, and the mixing ratio (ratio R) of the two is also optimized. This further improves the migration speeds of the cations and lithium ions near the surfaces of the positive electrode 21 and the negative electrode 22, and also further improves the migration speeds of the cations and lithium ions in the electrolyte solution. Therefore, a greater effect can be obtained.

[Modification 3]
A porous membrane separator 23 was used. However, although not specifically shown here, a laminated separator including a polymer compound layer may be used instead of the porous membrane separator 23.

Specifically, the laminated separator includes a porous membrane having a pair of surfaces, and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the adhesion of the separator to each of the positive electrode 21 and the negative electrode 22 is improved, thereby suppressing misalignment of the battery element 20. This suppresses miswinding of the positive electrode 21, the negative electrode 22, and the separator, thereby suppressing swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes polyvinylidene fluoride, etc. This is because polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

In addition, one or both of the porous film and the polymer compound layer may contain one or more types of insulating particles. This is because the insulating particles dissipate heat when the secondary battery generates heat, improving the safety (heat resistance) of the secondary battery. The insulating particles contain one or more types of insulating materials such as inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

When making a laminated separator, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is applied to one or both sides of a porous film. In this case, the precursor solution may contain multiple insulating particles.

Even when this laminated separator is used, the lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22, so the same effect can be obtained. In this case, as described above, swelling of the secondary battery is particularly suppressed, so a greater effect can be obtained.

[Modification 4]
An electrolyte solution that is a liquid electrolyte is used, but an electrolyte layer that is a gel electrolyte may also be used, although this is not specifically shown.

In a battery element 20 using an electrolyte layer, a positive electrode 21 and a negative electrode 22 are wound facing each other with a separator 23 and an electrolyte layer interposed between them. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

Specifically, the electrolyte layer contains a polymer compound together with an electrolyte solution, and the electrolyte solution is held by the polymer compound. This is because leakage of the electrolyte solution is prevented. The composition of the electrolyte solution is as described above. The polymer compound contains polyvinylidene fluoride and the like. When forming the electrolyte layer, a precursor solution containing an electrolyte solution, a polymer compound, a solvent, and the like is prepared, and then the precursor solution is applied to one or both sides of each of the positive electrode 21 and the negative electrode 22.

Even when this electrolyte layer is used, the lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, so the same effect can be obtained. In this case, leakage of the electrolyte is particularly prevented as described above, so a greater effect can be obtained.

<4. Uses of secondary batteries>
Finally, uses (application examples) of the secondary battery will be described.

The use of the secondary battery is not particularly limited. A secondary battery used as a power source may be a main power source or an auxiliary power source in electronic devices and electric vehicles. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the main power source or a power source that can be switched from the main power source.

Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, notebook computers, headphone stereos, portable radios, and portable information terminals. Storage devices such as backup power sources and memory cards. Power tools such as electric drills and power saws. Battery packs installed in electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars (including hybrid cars). Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these applications, one secondary battery may be used, or multiple secondary batteries may be used.

The battery pack may include a single cell or a battery pack. The electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may be a hybrid vehicle that also includes a driving source other than the secondary battery. In a home power storage system, it is possible to use home electrical appliances and the like by utilizing the power stored in the secondary battery, which is a power storage source.

Here, we will specifically explain an example of a use of a secondary battery. The configuration described below is merely an example and can be modified as appropriate.

Figure 3 shows the block diagram of a battery pack, which is an example of an application of a secondary battery. The battery pack described here is a battery pack (a so-called soft pack) that uses one secondary battery, and is installed in electronic devices such as smartphones.

As shown in FIG. 3, this battery pack includes a power source 51 and a circuit board 52. This circuit board 52 is connected to the power source 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

The power source 51 includes one secondary battery. In this secondary battery, the positive electrode lead is connected to the positive electrode terminal 53, and the negative electrode lead is connected to the negative electrode terminal 54. This power source 51 is connected to the outside via the positive electrode terminal 53 and the negative electrode terminal 54, and is therefore capable of charging and discharging. The circuit board 52 includes a control unit 56, a switch 57, a PTC element 58 which is a thermosensitive resistor, and a temperature detection unit 59. However, the PTC element 58 may be omitted.

The control unit 56 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. This control unit 56 detects and controls the usage status of the power source 51.

When the voltage of the power source 51 (secondary battery) reaches the overcharge detection voltage or overdischarge detection voltage, the control unit 56 turns off the switch 57 to prevent charging current from flowing through the current path of the power source 51. The overcharge detection voltage is not particularly limited, but is specifically 4.20V±0.05V, and the overdischarge detection voltage is not particularly limited, but is specifically 2.40V±0.10V.

Switch 57 includes a charge control switch, a discharge control switch, a charge diode, and a discharge diode, and switches between the presence and absence of a connection between power source 51 and an external device in response to an instruction from control unit 56. Switch 57 includes a field effect transistor (MOSFET) that uses a metal oxide semiconductor, and the charge current and discharge current are each detected based on the ON resistance of switch 57.

The temperature detection unit 59 includes a temperature detection element such as a thermistor. This temperature detection unit 59 measures the temperature of the power supply 51 using the temperature detection terminal 55, and outputs the temperature measurement result to the control unit 56. The temperature measurement result measured by the temperature detection unit 59 is used when the control unit 56 performs charge/discharge control in the event of abnormal heat generation, and when the control unit 56 performs correction processing when calculating the remaining capacity.

We will explain an example of this technology.

<Examples 1 to 18 and Comparative Examples 1 to 3>
As described below, after the secondary batteries were manufactured, the battery characteristics of the secondary batteries were evaluated.

[Preparation of secondary battery]
A laminate film type secondary battery (lithium ion secondary battery) shown in FIGS. 1 and 2 was fabricated by the following procedure.

(Preparation of Positive Electrode)
First, 91 parts by mass of a positive electrode active material (LiNi _0.82 Co _0.14 Al _0.04 O ₂ which is a lithium-containing compound (oxide), 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (carbon black) were mixed together to prepare a positive electrode mixture. Next, the positive electrode mixture was put into a solvent (N-methyl-2-pyrrolidone which is an organic solvent), and the organic solvent was stirred to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector 21A (a strip-shaped aluminum foil having a thickness of 12 μm) using a coating device, and then the positive electrode mixture slurry was dried to form a positive electrode active material layer 21B. Finally, the positive electrode active material layer 21B was compression-molded using a roll press machine. As a result, the positive electrode 21 was produced.

(Preparation of negative electrode)
First, 93 parts by mass of the negative electrode active material (artificial graphite, which is a carbon material) and 7 parts by mass of the negative electrode binder (polyvinylidene fluoride) were mixed together to prepare a negative electrode mixture. Next, the negative electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, which is an organic solvent), and the organic solvent was stirred to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of the negative electrode current collector 22A (a strip-shaped copper foil having a thickness of 15 μm) using a coating device, and then the negative electrode mixture slurry was dried to form the negative electrode active material layer 22B. Finally, the negative electrode active material layer 22B was compression-molded using a roll press machine. This resulted in the negative electrode 22 being produced.

(Preparation of Electrolyte)
The electrolyte salt containing the imide anion was added to the solvent, and the solvent was stirred to prepare the electrolyte solution.

The solvent used was a mixture of ethylene carbonate, a cyclic carbonate ester, and gamma-butyrolactone, a lactone. In this case, the solvent mixing ratio (weight ratio) was ethylene carbonate:gamma-butyrolactone = 30:70.

Lithium ions (Li ⁺ ) were used as the cations of the electrolyte salt. As the anions (imide anions) of the electrolyte salt, the first imide anions shown in formulas (1-5), (1-6), (1-21) and (1-22), the second imide anion shown in formula (2-5), the third imide anion shown in formula (3-5) and the fourth imide anion shown in formula (4-37) were used. The contents (mol/kg) of the electrolyte salts in the electrolytic solution were as shown in Tables 1 and 2.

This electrolyte contains chloride ions, and the chloride ion content (ppm by weight) in the electrolyte is as shown in Tables 1 and 2. To change the chloride ion content in the electrolyte, the number of recrystallizations during purification of the electrolyte salt containing imide anions was changed.

For comparison, an electrolyte solution was prepared in the same manner, except that an electrolyte salt not containing an imide anion (lithium hexafluorophosphate (LiPF ₆ )) was used instead of the electrolyte salt containing the imide anion.

(Assembly of secondary batteries)
First, the positive electrode lead 31 (aluminum foil) was welded to the positive electrode current collector 21 A of the positive electrode 21 , and the negative electrode lead 32 (copper foil) was welded to the negative electrode current collector 22 A of the negative electrode 22 .

Then, the positive electrode 21 and the negative electrode 22 were laminated on each other with a separator 23 (a microporous polyethylene film having a thickness of 25 μm) interposed therebetween to produce a laminate. The laminate was then wound to produce a wound body, which was then pressed using a press machine to form the wound body into a flat shape.

Then, the exterior film 10 (adhesive layer/metal layer/surface protection layer) was folded so as to sandwich the roll housed in the recess 10U, and the outer peripheral edges of two sides of the adhesive layer were heat-sealed to each other to house the roll inside the bag-shaped exterior film 10. The exterior film 10 used was an aluminum laminate film in which the adhesive layer (polypropylene film with a thickness of 30 μm), the metal layer (aluminum foil with a thickness of 40 μm), and the surface protection layer (nylon film with a thickness of 25 μm) were laminated in this order from the inside.

Finally, after injecting electrolyte into the bag-shaped exterior film 10, the outer edges of the remaining side of the fusion layer were heat-sealed to each other in a reduced pressure environment. In this case, a sealing film 41 (a polypropylene film with a thickness of 5 μm) was inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 (a polypropylene film with a thickness of 5 μm) was inserted between the exterior film 10 and the negative electrode lead 32. As a result, the electrolyte was impregnated into the wound body, and the battery element 20 was produced.

As a result, the battery element 20 is enclosed inside the exterior film 10, and a secondary battery is assembled.

(Stabilization of secondary batteries after assembly)
The assembled secondary battery was charged and discharged for one cycle in a room temperature environment (temperature = 23 ° C.). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C. During discharging, the battery was discharged at a constant current of 0.1 C until the voltage reached 2.5 V. 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.05 C is the current value at which the battery capacity is fully discharged in 20 hours.

As a result, a coating was formed on the surface of each of the positive electrode 21 and the negative electrode 22, electrochemically stabilizing the state of the battery element 20. Thus, the secondary battery was completed.

[Evaluation of Battery Characteristics]
The battery characteristics (cycle characteristics, storage characteristics, and load characteristics) were evaluated according to the procedures described below, and the results shown in Tables 1 and 2 were obtained.

(Cycle characteristics)
First, the discharge capacity (discharge capacity at the first cycle) was measured by charging and discharging the secondary battery in a high-temperature environment (temperature = 60 ° C.) The charge and discharge conditions were the same as those during the stabilization treatment described above.

　Then, in the same environment, the secondary battery was repeatedly charged and discharged until the total number of cycles reached 100, and the discharge capacity (discharge capacity at the 100th cycle) was measured. The charge and discharge conditions were the same as those during the stabilization treatment described above.

Finally, the cycle retention rate, which is an index for evaluating cycle characteristics, was calculated based on the formula: cycle retention rate (%) = (discharge capacity at 100th cycle/discharge capacity at 1st cycle) x 100.

(Storage characteristics)
First, the discharge capacity (discharge capacity before storage) was measured by charging and discharging the secondary battery one cycle in a room temperature environment (temperature = 23 ° C.) The charge and discharge conditions were the same as the charge and discharge conditions during the stabilization treatment described above.

Subsequently, the secondary battery was charged in the same environment, and the charged secondary battery was stored (storage time = 10 days) in a high-temperature environment (temperature = 80°C), and then the secondary battery was discharged in a room-temperature environment to measure the discharge capacity (discharge capacity after storage). The charge and discharge conditions were the same as those during the stabilization treatment described above.

Finally, the storage retention rate, which is an index for evaluating storage characteristics, was calculated based on the formula: storage retention rate (%) = (discharge capacity after storage/discharge capacity before storage) x 100.

(Load characteristics)
First, the discharge capacity (discharge capacity at the first cycle) was measured by charging and discharging the secondary battery one cycle in a room temperature environment (temperature = 23 ° C.) The charge and discharge conditions were the same as those during the stabilization treatment described above.

　Then, the secondary battery was repeatedly charged and discharged in a low-temperature environment (temperature = -10°C) until the total number of cycles reached 100, and the discharge capacity (discharge capacity at the 100th cycle) was measured. The charge and discharge conditions were the same as those during the stabilization process described above, except that the discharge current was changed to 1C. 1C is the current value at which the battery capacity is fully discharged in 1 hour.

Finally, the load retention rate, which is an index for evaluating load characteristics, was calculated based on the formula: Load retention rate (%) = (discharge capacity at 100th cycle/discharge capacity at 1st cycle) x 100.

[Discussion]
As shown in Tables 1 and 2, the cycle retention rate, storage retention rate, and load retention rate each varied greatly depending on the composition of the electrolyte.

Specifically, when the electrolyte salt did not contain imide anions and the content of chloride ions in the electrolyte was greater than 5,000 ppm by weight (Comparative Example 1), the cycle retention rate, storage retention rate, and load retention rate all decreased.

Furthermore, even though the electrolyte salt contains imide anions, when the content of chloride ions in the electrolyte solution was greater than 5,000 ppm by weight (Comparative Example 2), the cycle retention rate, storage retention rate, and load retention rate all decreased.

Furthermore, even when the content of chloride ions in the electrolyte was 5,000 ppm by weight or less, but the electrolyte salt did not contain imide anions (Comparative Example 3), the cycle retention rate, storage retention rate, and load retention rate all decreased.

In contrast, when the electrolyte salt contained imide anions and the content of chloride ions in the electrolyte was 5,000 ppm by weight or less (Examples 1 to 18), the cycle retention rate, storage retention rate, and load retention rate all increased.

In this case (Examples 1 to 18), the following trends were observed.

First, the cycle retention rate, storage retention rate, and load retention rate were all sufficiently high, regardless of the type of imide anion. Second, when the content of chloride ions in the electrolyte was 50 ppm by weight or less, the cycle retention rate, storage retention rate, and load retention rate all increased more. Third, when the electrolyte salt contained light metal ions (lithium ions) as cations, the cycle retention rate, storage retention rate, and load retention rate all increased sufficiently. Fourth, when the content of electrolyte salt in the electrolyte was 0.2 mol/kg to 2 mol/kg, the cycle retention rate, storage retention rate, and load retention rate all increased more.

<Examples 19 to 36>
Secondary batteries were produced and their battery characteristics were evaluated in the same manner as in Example 3, except that other solvents or other electrolyte salts were added to the electrolyte solution as shown in Tables 3 and 4. In this case, the other solvents or other electrolyte salts were added to the electrolyte solution, and then the electrolyte solution was stirred.

Details of other solvents are as follows. As unsaturated cyclic carbonates, vinylene carbonate (VC), vinylethylene carbonate (VEC), and methyleneethylene carbonate (MEC) were used. As fluorinated cyclic carbonates, monofluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) were used. As sulfonates, cyclic monosulfonates, propane sultone (PS) and propene sultone (PRS), and cyclic disulfonates, cyclodisone (CD), were used. As dicarboxylic anhydrides, succinic anhydride (SA) was used. As disulfonic anhydrides, propane disulfonic anhydride (PSAH) was used. As sulfates, ethylene sulfate (DTD) was used. As nitrile compounds, succinonitrile (SN) was used. As isocyanate compounds, hexamethylene diisocyanate (HMI) was used.

Other electrolyte salts used were lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB) and lithium difluorophosphate (LiPF ₂ O ₂ ).

The contents (wt%) of other solvents in the electrolyte and the contents (wt%) of other electrolyte salts in the electrolyte are as shown in Tables 3 and 4.

As shown in Table 3, when the electrolyte contained other solvents (Examples 19 to 31), two or more of the cycle retention rate, storage retention rate, and load retention rate were increased more than when the electrolyte did not contain other solvents (Example 3).

Also, as shown in Table 4, when the electrolyte solution contained other electrolyte salts (Examples 32 to 36), two or more of the cycle retention rate, storage retention rate, and load retention rate were increased more than when the electrolyte solution did not contain other electrolyte salts (Example 3).

<Examples 37 to 112>
As shown in Tables 5 to 10, secondary batteries were prepared in a manner similar to that of Example 3, except that the electrolyte solution contained lithium hexafluorophosphate (LiPF ₆ ) or lithium bis(fluorosulfonyl)imide (LiFSI) as another electrolyte salt, and the battery characteristics were then evaluated.

In this case, other electrolyte salts were added to the solvent, and the solvent was then stirred. The electrolyte salt content (mol/kg) in the electrolyte solution, the content (mol/kg) of other electrolyte salts in the electrolyte solution, the sum T (mol/kg), and the ratio R (mol%) were as shown in Tables 5 to 10.

When lithium hexafluorophosphate was used as another electrolyte salt, the results shown in Tables 5 to 7 were obtained. That is, when the two conditions of the sum T being 0.7 mol/kg to 2.2 mol/kg and the ratio R being 13 mol% to 6000 mol% were met (Examples 41, 42, 46-54, 56-60, 62-65, 69-74), two or more of the cycle retention rate, storage retention rate, and load retention rate increased more than when these two conditions were not met (Examples 37-40, 43-45, 55, 61, 66-68).

The trends regarding superiority and inferiority described here were also observed when lithium bis(fluorosulfonyl)imide was used as another electrolyte salt (Examples 75 to 112), as shown in Tables 8 to 10.

[summary]
From the results shown in Tables 1 to 10, when the electrolyte solution contains an electrolyte salt and a chloride ion, the electrolyte salt contains an imide anion, and the content of the chloride ion in the electrolyte solution is 5000 ppm by weight or less, a high cycle retention rate, a high storage retention rate, and a high load retention rate were obtained. Therefore, since each of the cycle characteristics, storage characteristics, and load characteristics was improved, excellent battery characteristics were obtained in the secondary battery.

The present technology has been described above with reference to one embodiment and examples, but the configuration of the present technology is not limited to the configuration described in the embodiment and examples, and can be modified in various ways.

Specifically, the battery structure of the secondary battery has been described as being of a laminate film type. However, the battery structure of the secondary battery is not particularly limited, and may be of a cylindrical type, a square type, a coin type, a button type, etc.

Also, the battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked type or a zigzag type. In the stacked type, the positive and negative electrodes are alternately stacked with a separator between them, while in the zigzag type, the positive and negative electrodes are folded in a zigzag pattern while facing each other with the separator between them.

Although the electrode reactant is lithium in the above description, the electrode reactant is not particularly limited. Specifically, as described above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

The effects described in this specification are merely examples, and the effects of this technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to this technology.

The present technology can also be configured as follows.
＜1＞
A positive electrode and
A negative electrode;
an electrolyte solution containing an electrolyte salt and chloride ions;
The electrolyte salt contains an imide anion,
The imide anion includes at least one of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4),
The content of the chloride ions in the electrolytic solution is 5000 ppm by weight or less.
Secondary battery.

(Each of R6 and R7 is either a fluorine group or a fluorinated alkyl group. R8 is either an alkylene group, a phenylene group, a fluorinated alkylene group, or a fluorinated phenylene group. Each of Z1, Z2, Z3, and Z4 is either a carbonyl group, a sulfinyl group, or a sulfonyl group.)
＜2＞
The content of the chloride ions in the electrolytic solution is 50 ppm by weight or less.
The secondary battery according to <1>.
＜3＞
The electrolyte contains light metal ions as cations.
The secondary battery according to <1> or <2>.
＜4＞
The light metal ions include lithium ions.
The secondary battery according to <3>.
＜5＞
The content of the electrolyte salt in the electrolytic solution is 0.2 mol/kg or more and 2 mol/kg or less.
<4> The secondary battery according to any one of <1> to <4>.
＜6＞
The electrolyte solution further contains at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, and lithium difluorophosphate;
The secondary battery according to <5>.
＜７＞
The electrolyte solution further contains lithium hexafluorophosphate or lithium bis(fluorosulfonyl)imide,
the electrolyte salt comprises a cation and the imide anion,
The lithium hexafluorophosphate contains lithium ions and hexafluorophosphate ions,
The lithium bis(fluorosulfonyl)imide contains a lithium ion and a bis(fluorosulfonyl)imide ion,
the sum of the content of the cation in the electrolyte solution and the content of the lithium ion in the electrolyte solution is 0.7 mol/kg or more and 2.2 mol/kg or less;
a ratio of the number of moles of the hexafluorophosphate ion or the bis(fluorosulfonyl)imide ion in the electrolyte solution to the number of moles of the imide anion in the electrolyte solution is 13 mol% or more and 6000 mol% or less;
<4> The secondary battery according to any one of <1> to <4>.
＜8＞
The electrolytic solution further contains at least one of an unsaturated cyclic carbonate, a fluorinated cyclic carbonate, a sulfonic acid ester, a dicarboxylic acid anhydride, a disulfonic acid anhydride, a sulfuric acid ester, a nitrile compound, and an isocyanate compound.
<7> The secondary battery according to any one of <1> to <7>.
＜9＞
It is a lithium-ion secondary battery.
<8> The secondary battery according to any one of <1> to <8>.
＜10＞
Contains an electrolyte salt and chloride ions,
The electrolyte salt contains an imide anion,
The imide anion includes at least one of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4),
The content of the chloride ions is 5000 ppm by weight or less.
Electrolyte for secondary batteries.

Claims

A positive electrode and
A negative electrode;
an electrolyte solution containing an electrolyte salt and chloride ions;
The electrolyte salt contains an imide anion,
The imide anion includes at least one of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4),
The content of the chloride ions in the electrolytic solution is 5000 ppm by weight or less.
Secondary battery.

(Each of R1 and R2 is either a fluorine group or a fluorinated alkyl group. Each of W1, W2 and W3 is either a carbonyl group (>C=O), a sulfinyl group (>S=O) or a sulfonyl group (>S(=O) 2 ).)

(Each of R3 and R4 is either a fluorine group or a fluorinated alkyl group. Each of X1, X2, X3 and X4 is either a carbonyl group, a sulfinyl group or a sulfonyl group.)

(R5 is a fluorinated alkylene group. Each of Y1, Y2, and Y3 is any one of a carbonyl group, a sulfinyl group, and a sulfonyl group.)

(Each of R6 and R7 is either a fluorine group or a fluorinated alkyl group. R8 is either an alkylene group, a phenylene group, a fluorinated alkylene group, or a fluorinated phenylene group. Each of Z1, Z2, Z3, and Z4 is either a carbonyl group, a sulfinyl group, or a sulfonyl group.)
The content of the chloride ions in the electrolytic solution is 50 ppm by weight or less.
The secondary battery according to claim 1 .
The electrolyte contains light metal ions as cations.
The secondary battery according to claim 1 or 2.
The light metal ions include lithium ions.
The secondary battery according to claim 3 .
The content of the electrolyte salt in the electrolytic solution is 0.2 mol/kg or more and 2 mol/kg or less.
The secondary battery according to claim 1 .
The electrolyte solution further contains at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, and lithium difluorophosphate;
The secondary battery according to claim 5 .
The electrolyte solution further contains lithium hexafluorophosphate or lithium bis(fluorosulfonyl)imide,
the electrolyte salt comprises a cation and the imide anion,
The lithium hexafluorophosphate contains lithium ions and hexafluorophosphate ions,
The lithium bis(fluorosulfonyl)imide contains a lithium ion and a bis(fluorosulfonyl)imide ion,
the sum of the content of the cation in the electrolyte solution and the content of the lithium ion in the electrolyte solution is 0.7 mol/kg or more and 2.2 mol/kg or less;
a ratio of the number of moles of the hexafluorophosphate ion or the bis(fluorosulfonyl)imide ion in the electrolyte solution to the number of moles of the imide anion in the electrolyte solution is 13 mol% or more and 6000 mol% or less;
The secondary battery according to claim 1 .
The electrolytic solution further contains at least one of an unsaturated cyclic carbonate, a fluorinated cyclic carbonate, a sulfonic acid ester, a dicarboxylic acid anhydride, a disulfonic acid anhydride, a sulfuric acid ester, a nitrile compound, and an isocyanate compound.
The secondary battery according to claim 1 .
It is a lithium-ion secondary battery.
The secondary battery according to claim 1 .
Contains an electrolyte salt and chloride ions,
The electrolyte salt contains an imide anion,
The imide anion includes at least one of a first imide anion represented by formula (1), a second imide anion represented by formula (2), a third imide anion represented by formula (3), and a fourth imide anion represented by formula (4),
The content of the chloride ions is 5000 ppm by weight or less.
Electrolyte for secondary batteries.

(Each of R1 and R2 is either a fluorine group or a fluorinated alkyl group. Each of W1, W2 and W3 is either a carbonyl group (>C=O), a sulfinyl group (>S=O) or a sulfonyl group (>S(=O) 2 ).)

(Each of R3 and R4 is either a fluorine group or a fluorinated alkyl group. Each of X1, X2, X3 and X4 is either a carbonyl group, a sulfinyl group or a sulfonyl group.)

(R5 is a fluorinated alkylene group. Each of Y1, Y2, and Y3 is any one of a carbonyl group, a sulfinyl group, and a sulfonyl group.)

(Each of R6 and R7 is either a fluorine group or a fluorinated alkyl group. R8 is either an alkylene group, a phenylene group, a fluorinated alkylene group, or a fluorinated phenylene group. Each of Z1, Z2, Z3, and Z4 is either a carbonyl group, a sulfinyl group, or a sulfonyl group.)