JP7706243B2 - Nonaqueous electrolyte, lithium secondary battery precursor, lithium secondary battery, and method for manufacturing lithium secondary battery - Google Patents

Description

The present disclosure relates to a non-aqueous electrolyte, a lithium secondary battery precursor, a lithium secondary battery, and a method for manufacturing a lithium secondary battery.

Conventionally, from the viewpoint of improving the performance of a battery, the inclusion of various additives in a non-aqueous electrolyte has been investigated.
For example, Patent Document 1 discloses a nonaqueous electrolyte capable of reducing the battery resistance after storage, which contains an electrolyte containing _LiPF6 and a specific compound (1) (e.g., lithium bis(trifluoromethylsulfonyl)imide), an additive A which is a specific compound (A) (e.g., vinylene carbonate), an additive B which is lithium monofluorophosphate and/or lithium difluorophosphate, and an additive C which is a specific compound (C1) (e.g., 1,3-propene sultone) and/or a specific compound (C2) (i.e., a specific cyclic sulfate).
Furthermore, Patent Document 2 discloses a nonaqueous electrolyte solution that contains a monofluorophosphate and/or a difluorophosphate and further contains a compound represented by a specific general formula (1) (for example, 1,3-propene sultone), as a nonaqueous electrolyte solution that can realize a secondary battery that is excellent in both low-temperature discharge characteristics and cycle characteristics.

JP 2019-175577 A JP 2011-49153 A

However, there is a demand for a nonaqueous electrolyte for a battery that can maintain a high capacity retention rate after storage of the battery, reduce the resistance of the battery after storage, and reduce the rate of increase in resistance during storage of the battery.
An object of one aspect of the present disclosure is to provide a nonaqueous electrolyte solution that, when used as a nonaqueous electrolyte solution for a battery, can maintain a high capacity retention rate after storage of the battery, reduce the resistance of the battery after storage, and reduce the rate of increase in resistance during storage of the battery, as well as a lithium secondary battery precursor, a lithium secondary battery, and a method for producing a lithium secondary battery that use the nonaqueous electrolyte solution.

Means for solving the above problems include the following aspects.
<1> At least one compound (A) selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate;
At least one compound (B) selected from the group consisting of compounds represented by the following formula (B):
At least one compound (C) selected from the group consisting of compounds represented by the following formula (C):
A non-aqueous electrolyte solution containing

In formula (B), R ^b1 to R ^b4 each independently represent a hydrogen atom, a fluorine atom, a hydrocarbon group having 1 to 6 carbon atoms, or a fluorohydrocarbon group having 1 to 6 carbon atoms.
In formula (C), R ^c1 to R ^c4 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 6 carbon atoms, a group represented by formula (a), or a group represented by formula (b). In formulas (a) and (b), * represents a bonding position.

<2> The nonaqueous electrolyte solution according to <1>, wherein the compound (B) is the following compound (B-1):

<3> The nonaqueous electrolyte solution according to <1> or <2>, wherein the compound (C) is the following compound (C-1):

<4> The nonaqueous electrolyte solution according to any one of <1> to <3>, wherein the content of the compound (A) is 0.001% by mass to 10% by mass based on the total amount of the nonaqueous electrolyte solution.
<5> The nonaqueous electrolyte solution according to any one of <1> to <4>, wherein the content of the compound (B) is 0.001% by mass to 10% by mass with respect to the total amount of the nonaqueous electrolyte solution.
<6> The nonaqueous electrolyte solution according to any one of <1> to <5>, wherein the content of the compound (B) is 0.001% by mass to 10% by mass based on the total amount of the nonaqueous electrolyte solution.
<7> A positive electrode,
a negative electrode including a negative electrode active material capable of absorbing and releasing lithium ions;
<1> to <6>, and a nonaqueous electrolyte solution according to any one of <1> to <6>.
A lithium secondary battery precursor comprising:
<8> A lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor according to <7>.
<9> A step of preparing a lithium secondary battery precursor according to <7>;
Charging and discharging the lithium secondary battery precursor;
A method for producing a lithium secondary battery, comprising:

According to one aspect of the present disclosure, there is provided a nonaqueous electrolyte that, when used as a nonaqueous electrolyte for a battery, can maintain a high capacity retention rate after storage of the battery, reduce the resistance of the battery after storage, and reduce the rate of increase in resistance during storage of the battery, as well as a lithium secondary battery precursor, a lithium secondary battery, and a method for producing a lithium secondary battery that use the nonaqueous electrolyte.

FIG. 1 is a schematic perspective view showing an example of a laminated battery, which is an example of a lithium secondary battery precursor of the present disclosure or a lithium secondary battery of the present disclosure. 2 is a schematic cross-sectional view in the thickness direction of a laminated electrode body housed in the laminated battery shown in FIG. 1 . FIG. 2 is a schematic cross-sectional view showing an example of a coin-type battery, which is another example of the lithium secondary battery precursor of the present disclosure or the lithium secondary battery of the present disclosure.

In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
In this specification, the amount of each component in a composition means, when a plurality of substances corresponding to each component are present in the composition, the total amount of the plurality of substances present in the composition, unless otherwise specified.

[Non-aqueous electrolyte]
The nonaqueous electrolyte of the present disclosure is
Compound (A) which is at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate;
At least one compound (B) selected from the group consisting of compounds represented by the following formula (B):
At least one compound (C) selected from the group consisting of compounds represented by the following formula (C):
Contains:

In formula (B), R ^b1 to R ^b4 each independently represent a hydrogen atom, a fluorine atom, a hydrocarbon group having 1 to 6 carbon atoms, or a fluorohydrocarbon group having 1 to 6 carbon atoms.
In formula (C), R ^c1 to R ^c4 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 6 carbon atoms, a group represented by formula (a), or a group represented by formula (b). In formulas (a) and (b), * represents a bonding position.

The nonaqueous electrolyte of the present disclosure is a nonaqueous electrolyte that, when used as a nonaqueous electrolyte for a battery, can maintain a high capacity retention rate after storage of the battery, reduce the resistance of the battery after storage, and reduce the rate of increase in resistance during storage of the battery.
That is, the nonaqueous electrolyte solution of the present disclosure can improve the balance of the above-mentioned battery performance (specifically, the capacity retention rate after storage, the resistance after storage, and the rate of increase in resistance during storage).

Below, we will explain each component that may be contained in the nonaqueous electrolyte of this disclosure.

<Compound (A)>
The nonaqueous electrolyte solution of the present disclosure contains compound (A), which is at least one type selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate.
Compound (A) is either or both of lithium monofluorophosphate and lithium difluorophosphate.
The compound (A) preferably contains lithium difluorophosphate.

<Compound (B)>
The nonaqueous electrolyte solution of the present disclosure contains at least one compound (B) selected from the group consisting of compounds represented by the following formula (B):

In formula (B), R ^b1 to R ^b4 each independently represent a hydrogen atom, a fluorine atom, a hydrocarbon group having 1 to 6 carbon atoms, or a fluorohydrocarbon group having 1 to 6 carbon atoms.

The hydrocarbon group having 1 to 6 carbon atoms represented by R ^b1 to R ^b4 is preferably an alkyl group, an alkenyl group, or an alkynyl group, more preferably an alkyl group or an alkenyl group, and particularly preferably an alkyl group.
The hydrocarbon group having 1 to 6 carbon atoms, represented by R ^b1 to R ^b4 , preferably has 1 to 3 carbon atoms, more preferably 1 or 2 carbon atoms, and particularly preferably 1 carbon atom.

The fluorinated hydrocarbon group having 1 to 6 carbon atoms represented by R ^b1 to R ^b4 is preferably a fluorinated alkyl group, a fluorinated alkenyl group, or a fluorinated alkynyl group, more preferably a fluorinated alkyl group or a fluorinated alkenyl group, and particularly preferably a fluorinated alkyl group.
The fluorohydrocarbon group having 1 to 6 carbon atoms represented by R ^b1 to R ^b4 preferably has 1 to 3 carbon atoms, more preferably 1 or 2 carbon atoms, and particularly preferably 1 carbon atom.

In the present disclosure,
The term "fluorohydrocarbon group" refers to a group in which at least one hydrogen atom contained in a hydrocarbon group is replaced with a fluorine atom.
The term "fluorinated alkyl group" refers to a group in which at least one hydrogen atom contained in an alkyl group is replaced with a fluorine atom.
The term "fluorinated alkenyl group" refers to a group in which at least one hydrogen atom contained in an alkenyl group is replaced with a fluorine atom.
The term "fluorinated alkynyl group" refers to an alkynyl group in which at least one hydrogen atom has been replaced with a fluorine atom.

R ^b1 to R ^b4 are each independently preferably a hydrogen atom, a fluorine atom, a methyl group, an ethyl group, a trifluoromethyl group, or a pentafluoroethyl group, more preferably a hydrogen atom or a methyl group, and particularly preferably a hydrogen atom.

Specific examples of the compound (B) include the following compounds (B-1) to (B-21), but the compound (B) is not limited to these specific examples.
Of these, the compound (B-1) is particularly preferred.

<Compound (C)>
The nonaqueous electrolyte solution of the present disclosure contains at least one compound (C) selected from the group consisting of compounds represented by the following formula (C):

In formula (C), R ^c1 to R ^c4 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 6 carbon atoms, a group represented by formula (a), or a group represented by formula (b). In formulas (a) and (b), * represents a bonding position.

In compound (C), specific examples and preferred embodiments of the hydrocarbon group having 1 to 6 carbon atoms represented by R ^c1 to R ^c4 are the same as the specific examples and preferred embodiments of the hydrocarbon group having 1 to 6 carbon atoms represented by R ^b1 to R ^b3 in compound (B).

Specific examples of the compound (C) are shown below, but the compound (C) is not limited to the following specific examples.
Among these, compounds (C-1) to (C-3) are preferred, compounds (C-1) and (C-2) are more preferred, and compound (C-1) is particularly preferred.

<Preferable Content of Compound>
The content of compound (A) is preferably 0.001% by mass to 10% by mass, more preferably 0.003% by mass to 5% by mass, even more preferably 0.03% by mass to 3% by mass, even more preferably 0.1% by mass to 3% by mass, even more preferably 0.2% by mass to 3% by mass, and even more preferably 0.3% by mass to 2% by mass, based on the total amount of the nonaqueous electrolyte.

The content of compound (B) is preferably 0.001% by mass to 10% by mass, more preferably 0.003% by mass to 5% by mass, even more preferably 0.003% by mass to 3% by mass, even more preferably 0.03% by mass to 3% by mass, even more preferably 0.1% by mass to 3% by mass, and even more preferably 0.1% by mass to 2% by mass, based on the total amount of the nonaqueous electrolyte.

The content of compound (C) is preferably 0.001% by mass to 10% by mass, more preferably 0.003% by mass to 5% by mass, even more preferably 0.003% by mass to 3% by mass, even more preferably 0.03% by mass to 3% by mass, even more preferably 0.1% by mass to 3% by mass, and even more preferably 0.1% by mass to 2% by mass, based on the total amount of the nonaqueous electrolyte.

In the nonaqueous electrolyte solution of the present disclosure, from the viewpoint of further improving the balance of battery performance (specifically, capacity retention rate after storage, resistance after storage, and rate of increase in resistance during storage), the ratio of the content of compound (A) to the total content of compounds (A) to (C) is preferably 0.3 or more, more preferably 0.5 or more, and even more preferably 0.6 or more.
The ratio of the content of compound (A) to the total content of compounds (A) to (C) may be less than 1.0, but is preferably 0.9 or less, and more preferably 0.8 or less.

In the nonaqueous electrolyte of the present disclosure, from the viewpoint of further improving the balance of battery performance (specifically, the capacity retention rate after storage, the resistance after storage, and the rate of increase in resistance during storage), the ratio of the content of compound (B) to the total content of compound (B) and compound (C) is preferably 0.1 to 0.9, more preferably 0.2 to 0.8, even more preferably 0.3 to 0.7, and even more preferably 0.4 to 0.6.

Next, we will explain the other components of the non-aqueous electrolyte. A non-aqueous electrolyte generally contains an electrolyte and a non-aqueous solvent.

<Electrolyte>
The electrolyte in the nonaqueous electrolyte of the present disclosure preferably contains a lithium salt, and more preferably contains _LiPF6 .
_LiPF6 has the advantages of being relatively small in molecular weight and excellent in dissociation property, capable of suppressing an increase in the resistance of the non-aqueous electrolyte, having high oxidation stability, and being difficult to decompose.
When the electrolyte contains LiPF ₆ , the ratio of LiPF ₆ in the electrolyte is preferably 10% by mass to 100% by mass, more preferably 50% by mass to 100% by mass, even more preferably 70% by mass to 100% by mass, even more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass.

The concentration of the electrolyte in the nonaqueous electrolyte solution of the present disclosure is preferably 0.1 mol/L to 3 mol/L, and more preferably 0.5 mol/L to 2 mol/L.
The concentration of LiPF ₆ in the nonaqueous electrolyte of the present disclosure is preferably 0.1 mol/L to 3 mol/L, and more preferably 0.5 mol/L to 2 mol/L.

When the electrolyte contains LiPF ₆ , the electrolyte may contain compounds other than LiPF ₆ .
Compounds other than _LiPF6 include:
tetraalkylammonium salts such as (C ₂ H ₅ ) ₄ NPF ₆ , (C ₂ H ₅ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NAsF ₆ , (C ₂ H ₅ ) ₄ N ₂ SiF ₆ , (C ₂ H ₅ ) ₄ NOSO ₂ C _k F _(2k+1) (k = an integer from 1 to 8), (C ₂ H ₅ ) ₄ NPF _n [C _k F _(2k+1) ] _(6-n) (n = an integer from 1 to 5, k = an integer from 1 to 8);
LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _(2k+1) (k=integer from 1 to 8), LiPF _n [C _k F _(2k+1) ] _(6-n) (n = integer from 1 to 5, k = integer from 1 to 8), LiC (SO ₂ R ⁷ ) (SO ₂ R ⁸ ) (SO ₂ R ⁹ ), LiN (SO ₂ OR ¹⁰ ) (SO ₂ OR ¹¹ ), LiN (SO ₂ R ¹² ) (SO ₂ R ¹³ ) (where R ⁷ ~R ¹³ may be the same or different and is a fluorine atom or a perfluoroalkyl group having 1 to 8 carbon atoms (more preferably a fluorine atom or a perfluoroalkyl group having 3 to 8 carbon atoms)) or other lithium salts (i.e., lithium salts other than _LiPF6 );
etc.

<Non-aqueous solvent>
The nonaqueous solvent in the nonaqueous electrolyte solution of the present disclosure may be of only one type, or of two or more types.
As the non-aqueous solvent, various known ones can be appropriately selected.
As the non-aqueous solvent, for example, the non-aqueous solvents described in paragraphs 0069 to 0087 of JP-A-2017-45723 can be used.

The non-aqueous solvent preferably contains a cyclic carbonate compound and a chain carbonate compound.
In this case, the non-aqueous solvent may contain only one type of cyclic carbonate compound and two or more types of chain carbonate compounds.

Examples of the cyclic carbonate compound include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate.
Among these, ethylene carbonate and propylene carbonate, which have a high dielectric constant, are preferred. In the case of a battery using a negative electrode active material containing graphite, it is more preferred that the non-aqueous solvent contains ethylene carbonate.

Examples of chain carbonate compounds include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, dipropyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, dibutyl carbonate, methyl pentyl carbonate, ethyl pentyl carbonate, dipentyl carbonate, methyl heptyl carbonate, ethyl heptyl carbonate, diheptyl carbonate, methyl hexyl carbonate, ethyl hexyl carbonate, dihexyl carbonate, methyl octyl carbonate, ethyl octyl carbonate, dioctyl carbonate, etc.

Specific examples of combinations of cyclic carbonates and chain carbonates include ethylene carbonate and dimethyl carbonate, ethylene carbonate and methyl ethyl carbonate, ethylene carbonate and diethyl carbonate, propylene carbonate and dimethyl carbonate, propylene carbonate and methyl ethyl carbonate, propylene carbonate and diethyl carbonate, ethylene carbonate, propylene carbonate and methyl ethyl carbonate, ethylene carbonate, propylene carbonate and diethyl carbonate, ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate, and ethylene carbonate and dimethyl carbonate. and diethyl carbonate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate, ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, ethylene carbonate, propylene carbonate, methyl ethyl carbonate, and diethyl carbonate, ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, and ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate.

The mixing ratio of the cyclic carbonate compound and the chain carbonate compound is, for example, 5:95 to 80:20, preferably 10:90 to 70:30, and more preferably 15:85 to 55:45, expressed as a mass ratio of the cyclic carbonate compound to the chain carbonate compound. By setting such a ratio, the increase in viscosity of the non-aqueous electrolyte can be suppressed and the degree of dissociation of the electrolyte can be increased, so that the conductivity of the non-aqueous electrolyte, which is related to the charge/discharge characteristics of the battery, can be increased. In addition, the solubility of the electrolyte can be further increased. Therefore, a non-aqueous electrolyte with excellent electrical conductivity at room temperature or low temperature can be obtained, and the load characteristics of the battery at room temperature to low temperature can be improved.

The non-aqueous solvent may contain compounds other than the cyclic carbonate compound and the chain carbonate compound.
In this case, the other compound contained in the non-aqueous solvent may be only one type, or may be two or more types.
Other compounds include cyclic carboxylate compounds (e.g., gamma-butyrolactone), cyclic sulfone compounds, cyclic ether compounds, chain carboxylate compounds, chain ether compounds, chain phosphate compounds, amide compounds, chain carbamate compounds, cyclic amide compounds, cyclic urea compounds, boron compounds, polyethylene glycol derivatives, and the like.
For these compounds, the descriptions in paragraphs 0069 to 0087 of JP-A-2017-45723 can be appropriately referred to.

The proportion of the cyclic carbonate compound and the chain carbonate compound in the non-aqueous solvent is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.
The proportion of the cyclic carbonate compound and the chain carbonate compound in the non-aqueous solvent may be 100% by mass.

The proportion of the nonaqueous solvent in the nonaqueous electrolyte is preferably 60% by mass or more, and more preferably 70% by mass or more.
The upper limit of the proportion of the nonaqueous solvent in the nonaqueous electrolyte solution varies depending on the contents of other components (electrolyte, additives, etc.), but the upper limit is, for example, 99 mass %, preferably 97 mass %, and more preferably 90 mass %.

The nonaqueous electrolyte of the present disclosure can be used as a nonaqueous electrolyte for batteries, and is particularly suitable for use as a nonaqueous electrolyte for lithium secondary batteries.

[Lithium secondary battery precursor, lithium secondary battery]
The lithium secondary battery precursor of the present disclosure is
A positive electrode and
a negative electrode including a negative electrode active material capable of absorbing and releasing lithium ions;
The nonaqueous electrolyte solution of the present disclosure described above,
Includes.

The lithium secondary battery of the present disclosure is a lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor of the present disclosure.

In the present disclosure, the lithium secondary battery precursor means a lithium secondary battery after production and before charging and discharging.
In other words, in the present disclosure, a lithium secondary battery refers to a battery obtained by charging and discharging a lithium secondary battery precursor.

The lithium secondary battery precursor of the present disclosure contains the nonaqueous electrolyte solution of the present disclosure described above.
Therefore, the lithium secondary battery precursor and the nonaqueous electrolyte solution of the present disclosure provide the same effects as those provided by the nonaqueous electrolyte solution of the present disclosure described above.

Hereinafter, each of the components that can be included in the lithium secondary battery precursor of the present disclosure will be described.
Each component that can be included in the lithium secondary battery of the present disclosure is basically similar to each component that can be included in the lithium secondary battery precursor of the present disclosure.
In the lithium secondary battery of the present disclosure, a SEI (Solid Electrolyte Interface) film is preferably formed on the surface of the positive electrode (particularly the positive electrode active material) and/or the negative electrode (particularly the negative electrode active material). The SEI film is a coating that can be formed by charging and discharging a lithium secondary battery precursor. The SEI film contains components in the nonaqueous electrolyte and/or products (e.g., decomposition products) derived from the components.

<Negative electrode>
The negative electrode may include a negative electrode active material and a negative electrode current collector.
The negative electrode active material in the negative electrode can be at least one selected from the group consisting of metallic lithium, lithium-containing alloys, metals or alloys capable of being alloyed with lithium, oxides capable of being doped/dedoped with lithium ions, transition metal nitrides capable of being doped/dedoped with lithium ions, and carbon materials capable of being doped/dedoped with lithium ions (which may be used alone or a mixture containing two or more of these may be used).
Examples of metals or alloys that can be alloyed with lithium (or lithium ions) include silicon, silicon alloys, tin, and tin alloys.
Examples of oxides that can be doped and dedoped with lithium ions include lithium titanate and silicon oxide (preferably SiOx (X is 0.5 or more and less than 1.6), more preferably SiO).
Among these, from the viewpoint of further improving the formability of a coating on the negative electrode and further reducing the initial and/or post-storage resistance of the battery, a carbon material capable of doping and dedoping lithium ions is preferred. Examples of such carbon materials include carbon black, activated carbon, graphite materials (artificial graphite, natural graphite), amorphous carbon materials, etc. The form of the carbon material may be any of fibrous, spherical, potato-like, and flake-like forms.

Specific examples of the amorphous carbon material include hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500° C. or less, and mesophase pitch carbon fiber (MCF).
Examples of the graphite material include natural graphite and artificial graphite. Examples of the artificial graphite include graphitized MCMB and graphitized MCF. Examples of the graphite material that can be used include those containing boron. Examples of the graphite material that can be used include those coated with metals such as gold, platinum, silver, copper, and tin, those coated with amorphous carbon, and those mixed with amorphous carbon and graphite.

These carbon materials may be used alone or in combination of two or more.
As the carbon material, a carbon material having a lattice spacing d(002) of 0.340 nm or less as measured by X-ray analysis is particularly preferred. In addition, as the carbon material, graphite having a true density of 1.70 g/ ^cm3 or more or a highly crystalline carbon material having properties similar thereto is also preferred. By using such a carbon material, the energy density of the battery can be further increased.

There is no particular limitation on the material of the negative electrode current collector in the negative electrode, and any known material can be used.
Specific examples of the negative electrode current collector include metal materials such as copper, nickel, stainless steel, nickel-plated steel, etc. Among these, copper is particularly preferred from the viewpoint of ease of processing.

The negative electrode may include a negative electrode current collector and a negative electrode active material layer provided on at least a portion of the surface of the negative electrode current collector.
The negative electrode active material layer contains at least one negative electrode active material. The negative electrode active material in the negative electrode active material layer preferably contains the above-mentioned carbon material.
From the viewpoints of further improving the formability of a coating on the negative electrode and further reducing the initial resistance and/or resistance after storage of the battery, the content of the carbon material in the negative electrode active material layer is preferably 70 mass % or more, more preferably 80 mass % or more, and even more preferably 90 mass % or more, relative to the total amount of the negative electrode active material layer.
The negative electrode active material layer may further contain at least one binder.
The binder is preferably at least one selected from the group consisting of styrene butadiene (SBR) rubber (e.g., SBR latex), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetyl cellulose. The binder preferably contains SBR latex and carboxymethyl cellulose.
The content of the binder in the negative electrode active material layer is preferably 1 mass % to 20 mass %, more preferably 1 mass % to 10 mass %, and even more preferably 1 mass % to 5 mass %, based on the total amount of the negative electrode active material layer.

The Si content in the entire negative electrode is preferably 5 mass % or less.
When the Si content relative to the entire negative electrode is 5 mass % or less, the formability of a coating on the negative electrode is improved, and the initial resistance and/or the resistance after storage of the battery is reduced.

<Positive electrode>
The positive electrode may include a positive electrode active material and a positive electrode current collector.
Positive electrode active materials in the positive electrode include transition metal oxides or transition metal sulfides such as MoS ₂ , TiS ₂ , MnO ₂ , and V ₂ O ₅ , LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Co _(1-X) O ₂ [0<x<1], Li 1+α Me 1-α O ₂ (Me is a transition metal element including Mn, Ni, and Co, _1.0 ≦(1+α)/(1-α)≦1.6) having an _α-NaFeO 2 type crystal structure, and LiNi _x Co _y Mn _{z O 2} [x+y+z=1, 0<x<1, 0<y<1, 0<z<1] (for example, LiNi _0.33 Examples of _the conductive material include a composite oxide of lithium and _a transition metal, such as LiFePO ₄ and LiMnPO ₄ , polyaniline _, polythiophene, polypyrrole _, polyacetylene, polyacene, dimercaptothiadiazole _, and polyaniline composite. Among _these , a composite oxide of lithium and _a transition metal is particularly preferred. When the negative electrode is lithium metal or a lithium alloy, a carbon material can also be used as the positive electrode. In addition, a mixture of a composite oxide of lithium and a transition metal and a carbon material can also be used as the positive electrode.
The positive electrode active material may be used alone or in combination of two or more. When the positive electrode active material has insufficient conductivity, it may be used together with a conductive assistant to form a positive electrode. Examples of the conductive assistant include carbon materials such as carbon black, amorphous whiskers, and graphite.

There is no particular limitation on the material of the positive electrode current collector in the positive electrode, and any known material can be used.
Specific examples of the positive electrode current collector include metal materials such as aluminum, aluminum alloys, stainless steel, nickel, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; and the like.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer provided on at least a portion of the surface of the positive electrode current collector.
The positive electrode active material layer contains at least one positive electrode active material. The negative electrode active material in the positive electrode active material layer preferably contains a composite oxide containing lithium and a transition metal.
The content of the composite oxide in the positive electrode active material layer is preferably 70% by mass or more, and more preferably 80% by mass or more, based on the total amount of the positive electrode active material layer.
The positive electrode active material layer may further contain at least one of the above-mentioned conductive assistants.
The positive electrode active material layer may further contain at least one binder.
Examples of the binder include polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluororesin, and rubber particles. Examples of the fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of the rubber particles include styrene-butadiene rubber particles and acrylonitrile rubber particles. Among these, fluororesin is preferred from the viewpoint of improving the oxidation resistance of the positive electrode active material layer.
The content of the binder in the positive electrode active material layer is preferably 1% by mass to 20% by mass, and more preferably 1% by mass to 10% by mass, based on the total amount of the positive electrode active material layer.

<Separator>
The lithium secondary battery precursor of the present disclosure preferably includes a separator between the negative electrode and the positive electrode.
The separator is a membrane that electrically insulates the positive electrode from the negative electrode and is permeable to lithium ions, and examples of the separator include a porous membrane and a polymer electrolyte.
As the porous membrane, a microporous polymer film is preferably used, and examples of the material include polyolefin, polyimide, polyvinylidene fluoride, polyester, and the like.
In particular, porous polyolefin is preferred, specifically, a porous polyethylene film, a porous polypropylene film, or a multi-layer film of a porous polyethylene film and a porous polypropylene film can be exemplified. The porous polyolefin film may be coated with another resin having excellent thermal stability.
Examples of the polymer electrolyte include a polymer in which a lithium salt is dissolved, and a polymer swollen with an electrolytic solution.
The nonaqueous electrolyte solution of the present disclosure may be used for the purpose of swelling a polymer to obtain a polymer electrolyte.

<Constitution of lithium secondary battery precursor>
The lithium secondary battery precursor of the present disclosure can have various known shapes, and can be formed into any shape such as a cylinder, a coin, a square, a laminate, a film, etc. However, the basic structure of the lithium secondary battery precursor is the same regardless of the shape, and the design can be modified according to the purpose.
With regard to the basic structure of the lithium secondary battery of the present disclosure, the basic structure of the lithium secondary battery precursor of the present disclosure can also be referred to.

An example of the lithium secondary battery precursor of the present disclosure or the lithium secondary battery of the present disclosure is a laminate type battery.
FIG. 1 is a schematic perspective view showing an example of a laminated battery, which is an example of a lithium secondary battery precursor or a lithium secondary battery of the present disclosure, and FIG. 2 is a schematic cross-sectional view in the thickness direction of a stacked electrode body accommodated in the laminated battery shown in FIG.
The laminated battery shown in Fig. 1 contains a nonaqueous electrolyte (not shown in Fig. 1) and a laminated electrode assembly (not shown in Fig. 1) and includes a laminated exterior body 1 whose periphery is sealed to seal the interior. As the laminated exterior body 1, for example, an aluminum laminated exterior body is used.
2, the laminated electrode body housed in the laminate exterior body 1 includes a laminate in which positive electrode plates 5 and negative electrode plates 6 are alternately stacked with separators 7 interposed therebetween, and a separator 8 surrounding the periphery of the laminate. The positive electrode plates 5, the negative electrode plates 6, the separators 7, and the separators 8 are impregnated with the nonaqueous electrolyte solution of the present disclosure.
Each of the multiple positive electrode plates 5 in the laminated electrode body is electrically connected to a positive electrode terminal 2 via a positive electrode tab (not shown), and a part of this positive electrode terminal 2 protrudes outward from the peripheral edge of the laminated exterior body 1 ( FIG. 1 ). The portion of the peripheral edge of the laminated exterior body 1 from which the positive electrode terminal 2 protrudes is sealed with an insulating seal 4.
Similarly, each of the negative electrode plates 6 in the laminated electrode body is electrically connected to a negative electrode terminal 3 via a negative electrode tab (not shown), and a part of this negative electrode terminal 3 protrudes outward from the peripheral edge of the laminated exterior body 1 ( FIG. 1 ). The portion of the peripheral edge of the laminated exterior body 1 from which the negative electrode terminal 3 protrudes is sealed with an insulating seal 4.
In the laminated battery according to the above example, the number of positive plates 5 is five and the number of negative plates 6 is six, and the positive plates 5 and the negative plates 6 are laminated with separators 7 interposed between them in an arrangement in which the outermost layers on both sides are negative plates 6. However, it goes without saying that the number and arrangement of positive and negative plates in the laminated battery are not limited to this example, and various modifications may be made.

Another example of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure includes a coin-type battery.
FIG. 3 is a schematic perspective view showing an example of a coin-type battery, which is another example of the lithium secondary battery precursor of the present disclosure or the lithium secondary battery of the present disclosure.
3, a disk-shaped negative electrode 12, a separator 15 filled with a nonaqueous electrolyte, a disk-shaped positive electrode 11, and, if necessary, spacer plates 17, 18 made of stainless steel, aluminum, or the like are stacked in this order and housed between a positive electrode can 13 (hereinafter also referred to as a "battery can") and a sealing plate 14 (hereinafter also referred to as a "battery can lid"). The positive electrode can 13 and the sealing plate 14 are crimped and sealed with a gasket 16 interposed therebetween.
In this example, the nonaqueous electrolyte of the present disclosure is used as the nonaqueous electrolyte injected into separator 15 .

The use of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure is not particularly limited, and the battery can be used for various known uses. For example, the battery can be widely used in notebook computers, mobile computers, mobile phones, headphone stereos, video movie players, liquid crystal televisions, handy cleaners, electronic organizers, calculators, radios, backup power supplies, motors, automobiles, electric automobiles, motorcycles, electric motorcycles, bicycles, electric bicycles, lighting equipment, game consoles, watches, power tools, cameras, and other small portable devices and large devices alike.

[Method for manufacturing lithium secondary battery]
The method for producing a lithium secondary battery according to the present disclosure includes:
A step of preparing the lithium secondary battery precursor of the present disclosure described above (hereinafter also referred to as a "preparation step");
charging and discharging the lithium secondary battery precursor;
Includes.

The preparation step may be a step of simply preparing a previously manufactured lithium secondary battery precursor of the present disclosure for a step of charging and discharging, or may be a step of manufacturing a lithium secondary battery precursor of the present disclosure.
The lithium secondary battery precursor is as described above.

In the step of charging and discharging, the lithium secondary battery precursor can be charged and discharged according to a known method.
In this step, the lithium secondary battery precursor may be subjected to a cycle of charging and discharging multiple times.
As described above, by this charging and discharging, an SEI film is preferably formed on the surface of the positive electrode (particularly the positive electrode active material) and/or the negative electrode (particularly the negative electrode active material) in the lithium secondary battery precursor.

Examples of the present disclosure will be described below, but the present disclosure is not limited to the following examples.
In the following examples, "addition amount" means the content relative to the total amount of the finally obtained nonaqueous electrolyte, and "wt %" means mass %.

Example 1
<Preparation of non-aqueous electrolyte>
Ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) were mixed in a volume ratio of 30:35:35 to obtain a mixed solvent as a non-aqueous solvent.
The resulting mixed solvent is
As an electrolyte, _LiPF6 was dissolved so that the concentration in the finally obtained non-aqueous electrolyte was 1.2 mol/L.
Furthermore, as additives, compound (A), compound (B), and compound (C) were each added so that their contents (mass%) in the finally obtained nonaqueous electrolyte solution were 1.0 mass%, 0.2 mass%, and 0.2 mass%, respectively, to obtain a nonaqueous electrolyte solution.
Where:
Lithium difluorophosphate (LiDFP) was used as compound (A),
The compound (B) used was the following compound (B-1):
As the compound (C), the following compound (C-1) was used.

<Preparation of Lithium Secondary Battery Precursor>
A coin-type battery having the configuration shown in FIG. 3 was fabricated as a lithium secondary battery precursor by the following procedure.

(Preparation of negative electrode)
Amorphous coated natural graphite (98 parts by mass) as a negative electrode active material, sodium carboxymethylcellulose dispersed in pure water (1 part by mass as solid content) as a thickener, and styrene-butadiene rubber (SBR) dispersed in pure water (1 part by mass) as a binder were added and mixed to obtain a paste-like negative electrode mixture slurry.
Next, the obtained negative electrode mixture slurry was applied to a strip-shaped copper foil (negative electrode current collector) having a thickness of 10 μm, dried, and then rolled with a roll press to obtain a sheet-shaped negative electrode consisting of the negative electrode current collector and the negative electrode active material layer. The coating density of the negative electrode active material layer at this time was 10 mg/ ^cm2 , and the filling density was 1.5 g/mL.

(Preparation of Positive Electrode)
_{A mixture} of _{LiNi0.5Mn0.3Co0.2O2} (90 parts by mass) as a positive _electrode active material, acetylene black (5 parts by mass) as a conductive additive, and polyvinylidene fluoride (5 parts by mass) as a binder was dispersed in N-methylpyrrolidinone as a solvent to prepare a paste-like positive electrode mixture slurry.
Next, this positive electrode mixture slurry was applied to a strip-shaped aluminum foil (positive electrode current collector) having a thickness of 20 μm, dried, and then rolled with a roll press to obtain a sheet-shaped positive electrode consisting of the positive electrode current collector and the positive electrode active material layer. The coating density of the positive electrode active material layer at this time was 30 mg/ ^cm2 , and the filling density was 2.5 g/mL.

(Preparation of separator)
A microporous polyethylene film having a thickness of 20 μm was punched out into a disk having a diameter of 17 mm to obtain a coin-shaped separator.

(Preparation of coin cell battery)
The above-mentioned negative electrode was punched out into a disk shape having a diameter of 14 mm, and the above-mentioned positive electrode was punched out into a disk shape having a diameter of 13 mm, to obtain a coin-shaped negative electrode and a coin-shaped positive electrode, respectively.
The obtained coin-shaped negative electrode, separator, and coin-shaped positive electrode were stacked in this order in a stainless steel battery can (size 2032), and then 20 μL of the above-mentioned nonaqueous electrolyte was poured into the battery can to impregnate the separator, positive electrode, and negative electrode.
Next, an aluminum plate (thickness 1.2 mm, diameter 16 mm) and a spring were placed on the positive electrode, and the battery can lid was crimped via a polypropylene gasket to seal the battery.
As a result of the above, a coin-type battery having a diameter of 20 mm and a height of 3.2 mm as shown in FIG. 3 was obtained as a lithium secondary battery precursor.

<Evaluation>
The obtained coin-type batteries were subjected to the following evaluations.
The evaluation results are shown in Table 1.
In Table 1, the capacity retention rate after storage, the resistance after storage, and the rate of increase in resistance during storage in each Example are shown as relative values when the value in Comparative Example 1 described below is taken as 100.

(conditioning)
The coin-type battery as the lithium secondary battery precursor was subjected to three cycles of charging and discharging between 2.5 V and 4.2 V at 25° C. in a thermostatic chamber to obtain a first battery as a lithium secondary battery.

(Initial capacity)
The discharge capacity of the first battery was defined as the "initial capacity."

(Initial resistance)
The SOC (State of Charge) of the first battery was adjusted to 50%.
The first battery adjusted to an SOC of 50% was subjected to CC10s discharge at a discharge rate of 0.2 C at −20° C., and the direct current internal resistance (Ω) of the first battery was measured.
Here, CC10s discharge means discharging for 10 seconds at a constant current.
The DCIR was measured based on the current value in the above "CC10s discharge at a discharge rate of 0.2C" (i.e., the current value corresponding to a discharge rate of 0.2C) and the voltage drop (=voltage before the start of discharge-voltage 10 seconds after the start of discharge). The obtained DCIR (Ω) was defined as the "initial resistance (Ω)".
Hereinafter, the first battery after the above-mentioned measurement of the initial resistance is referred to as the second battery.

(keep)
The second battery was charged to 4.2 V and then stored in a thermostatic chamber at 60° C. for 9 days to obtain a third battery.

(Capacity after saving)
The third battery was discharged to 2.5V.
Next, the battery was CC-CV charged to 4.2 V at a charge rate of 0.2 C at 25° C. in a thermostatic chamber, and then discharged to 2.5 V to obtain a fourth battery. The discharge capacity at this time was defined as the “capacity after storage”.
Here, CC-CV charging means charging at a constant current and constant voltage.

(Resistance after storage)
The SOC of the fourth battery was adjusted to 50%.
For the fourth battery adjusted to an SOC of 50%, the DCIR (Ω) was measured at −20° C. by the same method as in the measurement of the initial resistance described above, and the obtained value was taken as the “resistance after storage (Ω)”.
Hereinafter, the fourth battery after the resistance after storage was measured is referred to as the fifth battery.

(Capacity retention rate after storage)
The capacity retention rate (%) after storage was calculated according to the following formula.
Capacity retention rate after storage (%)=(capacity after storage/initial capacity)×100

(Calculation of resistance increase rate during storage)
The resistance increase rate (%) during storage was calculated using the following formula.
Resistance increase rate during storage (%)=(resistance after storage/initial resistance)×100

[Example 2 and Comparative Examples 1 to 3]
The same operations as in Example 1 were carried out except that the types and contents of additives in the nonaqueous electrolyte were changed as shown in Table 1.
The results are shown in Table 1.
In Table 1, "-" means that the corresponding component is not contained.
In Table 1, the number in parentheses under the name of the additive indicates the content (mass %) of that additive.

As shown in Table 1, the batteries of Examples 1 and 2 had a high capacity retention rate after storage, and the resistance after storage and the rate of increase in resistance during storage were reduced. Compared with the batteries of Comparative Examples 1 to 3, the batteries of Examples 1 and 2 had a superior balance of battery performance.
More specifically, the batteries of Examples 1 and 2 using nonaqueous electrolyte solutions containing compound (A), compound (B), and compound (C) as additives showed improved capacity retention after storage and reduced resistance increase rate during storage compared to the battery of Comparative Example 1 using a nonaqueous electrolyte solution not containing compound (C).
Moreover, the battery of Example 1 had an improved capacity retention rate after storage and a reduced resistance after storage, compared to the battery of Comparative Example 2, which did not contain compound (A) among the three additives.
The battery of Example 2 had an improved capacity retention rate after storage and a reduced resistance after storage, compared to the battery of Comparative Example 2 which did not contain compound (A) among the three additives.
Furthermore, the battery of Example 1 had an improved capacity retention rate after storage and a reduced rate of increase in resistance during storage, compared to the battery of Comparative Example 3, which did not contain compound (B) among the three additives.
Furthermore, the battery of Example 2 had an improved capacity retention rate after storage, a reduced resistance after storage, and a reduced rate of increase in resistance during storage, compared to the battery of Comparative Example 3, which did not contain compound (B) among the three additives.

REFERENCE SIGNS LIST 1 Laminate exterior body 2 Positive electrode terminal 3 Negative electrode terminal 4 Insulation seal 5 Positive electrode plate 6 Negative electrode plate 7, 8 Separator 11 Positive electrode 12 Negative electrode 13 Positive electrode can 14 Sealing plate 15 Separator 16 Gasket 17, 18 Spacer plate

Claims (6)

Compound (A) which is at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate;
At least one compound (B) selected from the group consisting of compounds represented by the following formula (B):
At least one compound (C) selected from the group consisting of compounds represented by the following formula (C):
Contains
The content of the compound (A) is 0.1% by mass to 3% by mass based on the total amount of the nonaqueous electrolyte for a battery ,
The content of the compound (B) is 0.03% by mass to 2% by mass based on the total amount of the nonaqueous electrolyte for a battery ,
The content of the compound (C) is 0.03% by mass to 2% by mass based on the total amount of the nonaqueous electrolyte for a battery ,
a ratio of the content of the compound (A) to the total content of the compound (A), the compound (B), and the compound (C) is 0.5 or more and 0.9 or less,
The ratio of the content of the compound (B) to the total content of the compound (B) and the compound (C) is 0.3 to 0.7.
Non-aqueous electrolyte for batteries .

In formula (B), R ^b1 to R ^b4 each independently represent a hydrogen atom, a fluorine atom, a hydrocarbon group having 1 to 6 carbon atoms, or a fluorohydrocarbon group having 1 to 6 carbon atoms.
In formula (C), R ^c1 to R ^c4 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 6 carbon atoms, a group represented by formula (a), or a group represented by formula (b). In formulas (a) and (b), * represents a bonding position. The nonaqueous electrolyte for a battery according to claim 1, wherein the compound (B) is the following compound (B-1):
The nonaqueous electrolyte for a battery according to claim 1 or 2, wherein the compound (C) is the following compound (C-1):
A positive electrode and
a negative electrode including a negative electrode active material capable of absorbing and releasing lithium ions;
The nonaqueous electrolyte for a battery according to any one of claims 1 to 3,
A lithium secondary battery precursor comprising: A lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor according to claim 4. A step of preparing a lithium secondary battery precursor according to claim 4;
Charging and discharging the lithium secondary battery precursor;
A method for producing a lithium secondary battery, comprising: