JP7411476B2 - Nonaqueous electrolyte for lithium ion secondary batteries, method for producing the same, preparation for nonaqueous electrolytes, and lithium ion secondary batteries - Google Patents

Description

The present disclosure relates to a non-aqueous electrolyte for a lithium ion secondary battery, a method for producing the same, a preparation liquid for the non-aqueous electrolyte, and a lithium ion secondary battery.

Lithium difluorophosphate has been known as an additive for nonaqueous electrolytes of lithium ion secondary batteries.
For example, Patent Document 1 describes an electrolytic solution containing an organic solvent and a solute as an excellent nonaqueous electrolytic solution that suppresses self-discharge and improves storage characteristics after charging when storing a lithium ion secondary battery. is disclosed. In this electrolytic solution, the organic solvent contains at least one additive selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate.

Patent No. 3439085

However, with respect to the above-mentioned conventional lithium ion secondary battery using a non-aqueous electrolyte containing lithium difluorophosphate, there is room for further improvement in terms of reducing battery resistance at the initial stage and after charge/discharge cycles.

An object of one aspect of the present disclosure is a nonaqueous electrolyte for a lithium ion secondary battery that can reduce the battery resistance of the lithium ion secondary battery at the initial stage and after the charge/discharge cycle, a method for manufacturing the same, and an initial stage and charge/discharge cycle. It is an object of the present invention to provide a lithium ion secondary battery in which the subsequent battery resistance is reduced, and a preparation liquid for a nonaqueous electrolyte suitable for producing the above-mentioned nonaqueous electrolyte for the lithium ion secondary battery.

Means for solving the above problems include the following aspects.
<1> A nonaqueous electrolyte for a lithium ion secondary battery containing an electrolyte, a nonaqueous solvent, and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate, and EC which is ethylene carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, the LiDFP site, and the EC site in the complex compound are respectively counted as LiPF ₆ , LiDFP, and EC, the volume ratio [EC: (DMC, EMC, and DEC)] is 30. :70, the content of LiDFP is 0.4% by mass based on the total amount of the non-aqueous electrolyte for lithium ion secondary batteries, and when the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L,
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a reference are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm,
Non-aqueous electrolyte for lithium-ion secondary batteries.
<2> In the complex compound,
the molar ratio of the LiPF ₆ site to the LiDFP site is 0.95 to 1.05;
The molar ratio of the EC site to the LiDFP site is 10 or more and less than 18.
The non-aqueous electrolyte for lithium ion secondary batteries according to <1>.
<3> The non-aqueous lithium ion secondary battery according to <1> or <2>, which is used as an electrolyte of a lithium ion secondary battery including a positive electrode containing a positive electrode active material that is a lithium nickel cobalt manganese oxide. Electrolyte.
<4> A non-aqueous electrolyte preparation containing a non-aqueous solvent and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, the LiDFP site, and the EC site in the complex compound are respectively counted as LiPF ₆ , LiDFP, and EC, the volume ratio [EC: (DMC, EMC, and DEC)] is 30. :70, the content of LiDFP is 0.4% by mass based on the total amount of the non-aqueous electrolyte preparation solution, and the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L.
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a reference are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm,
Preparation liquid for non-aqueous electrolyte.
<5> A method for producing a non-aqueous electrolyte for a lithium ion secondary battery, including a step of mixing the non-aqueous electrolyte preparation according to <4> and an electrolyte.
<6> Positive electrode and
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
The non-aqueous electrolyte for lithium ion secondary batteries according to <1> or <2>,
A lithium-ion secondary battery equipped with.
<7> A positive electrode containing a positive electrode active material that is a lithium nickel cobalt manganese oxide;
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
The non-aqueous electrolyte for lithium ion secondary batteries according to <3>,
A lithium-ion secondary battery equipped with.

According to one aspect of the present disclosure, a nonaqueous electrolyte for a lithium ion secondary battery that can reduce the battery resistance of the lithium ion secondary battery at the initial stage and after the charge/discharge cycle, a method for manufacturing the same, and the initial stage and charge/discharge cycle. A lithium ion secondary battery with reduced battery resistance and a nonaqueous electrolyte preparation solution suitable for producing the above-mentioned nonaqueous electrolyte for a lithium ion secondary battery are provided.

1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to an embodiment of the present disclosure. ⁷ Li-NMR analysis results in Example 1 and Comparative Example 1. ^19F -NMR analysis results in Example 1 and Comparative Example 1. ^19F -NMR analysis results in Example 1 and Comparative Example 1.

In the present disclosure, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.
In the present disclosure, if there are multiple substances corresponding to each component in the composition, unless otherwise specified, the amount of each component in the composition refers to the total amount of the multiple substances present in the composition. means.
In this disclosure, the term "step" is used not only to refer to an independent process, but also to include a process that is not clearly distinguishable from other processes as long as the intended purpose of the process is achieved. .

[Prepared liquid for non-aqueous electrolyte]
The non-aqueous electrolyte preparation liquid of the present disclosure includes:
Contains a non-aqueous solvent and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, LiDFP site, and EC site in the complex compound are included as LiPF ₆ , LiDFP, and EC, respectively, the volume ratio [EC: (DMC, EMC, and DEC)] is 30:70, When the composition was adjusted so that the content of LiDFP was 0.4% by mass with respect to the total amount of the non-aqueous electrolyte preparation solution and the concentration of LiPF ₆ was 1 mol/L,
A resonance peak of chemical shift value by ⁷ Li-NMR analysis based on LiCl (lithium chloride) was observed within the range of -0.280 ppm to -0.288 ppm,
The resonance peak of the chemical shift value by ^19F -NMR analysis using NaF (sodium fluoride) as the standard is in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm. Observed within the range.

The nonaqueous electrolyte preparation liquid of the present disclosure is a liquid used for preparing a nonaqueous electrolyte for lithium ion secondary batteries (hereinafter also simply referred to as "nonaqueous electrolyte").
In this disclosure, the term "preparation" is synonymous with making and manufacturing.
According to the nonaqueous electrolyte for a lithium ion secondary battery prepared by the preparation liquid for a nonaqueous electrolyte of the present disclosure, the battery resistance of the lithium ion secondary battery at the initial stage and after a charge/discharge cycle can be reduced.
More specifically, a non-aqueous electrolyte prepared using a preparation containing the above-mentioned complex compound containing a LiDFP moiety is different from a non-aqueous electrolyte prepared by adding single LiDFP, which is a known additive. In comparison, it is more effective in reducing battery resistance at the initial stage and after charge/discharge cycles (for example, see Examples below).
The reason why such an effect is produced is not clear, but in a non-aqueous electrolyte prepared using a preparation solution containing the above-mentioned complex compound containing a LiDFP moiety, the structure of the complex compound in the non-aqueous electrolyte is It is thought that this is because the complex compound is maintained stably, and this complex compound contributes to reducing the battery resistance at the initial stage of the lithium ion secondary battery and after charge/discharge cycles.
The nonaqueous electrolyte preparation of the present disclosure has the effect of reducing the battery resistance of a lithium ion secondary battery at the initial stage and after the charge/discharge cycle, and therefore has the effect of improving the rate characteristics and cycle characteristics of the lithium ion secondary battery. It is also expected to have the following.

(Non-aqueous solvent)
The non-aqueous electrolyte preparation liquid of the present disclosure contains a non-aqueous solvent.
The non-aqueous solvent includes at least one selected from the group consisting of DMC, which is dimethyl carbonate, EMC, which is ethyl methyl carbonate, and DEC, which is diethyl carbonate.
It is particularly preferred that the non-aqueous solvent contains DMC.
The nonaqueous solvent may contain at least one solvent species other than DMC, EMC, and DEC.
Regarding other solvent types, reference can be made to the solvent types that can be contained in the non-aqueous electrolyte for lithium ion secondary batteries, which will be described later.
In the nonaqueous electrolyte preparation liquid of the present disclosure, the ratio of the total amount of DMC, EMC, and DEC to the total amount of the nonaqueous solvent is preferably 50% by mass to 100% by mass, and more preferably 60% by mass. % to 100% by weight, more preferably 80% to 100% by weight.

(complex compound)
The non-aqueous electrolyte preparation liquid of the present disclosure contains a complex compound.
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate.

(Resonance peak by NMR analysis)
The non-aqueous electrolyte preparation liquid of the present disclosure includes:
When the LiPF ₆ site, LiDFP site, and EC site in the complex compound are included as LiPF ₆ , LiDFP, and EC, respectively, the volume ratio [EC: (DMC, EMC, and DEC)] is 30:70, When the composition was adjusted so that the content of LiDFP was 0.4% by mass with respect to the total amount of the non-aqueous electrolyte preparation solution and the concentration of LiPF ₆ was 1 mol/L,
A resonance peak of chemical shift value by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
The resonance peak of the chemical shift value by ¹⁹ F-NMR analysis using NaF as the standard is within the range of -72.605 ppm to -72.620 ppm and within the range of -73.860 ppm to -73.870 ppm. Observed.

The resonance peak observed within the above range of -0.280 ppm to -0.288 ppm is the resonance peak of Li in the complex compound.
The resonance peak observed within the above range of -72.605 ppm to -72.620 ppm and the resonance peak observed within the above range of -73.860 ppm to -73.870 ppm are both due to This is the resonance peak of F.

The nonaqueous electrolyte preparation of the present disclosure has a volume ratio [EC: (DMC, EMC, and DEC)] of 30:70, and the LiDFP content is 0 with respect to the total amount of the nonaqueous electrolyte preparation. .4% by mass, and the concentration of LiPF ₆ is not limited to 1 mol/L.
Specifically, the "volume ratio [EC: (DMC, EMC and DEC)] is 30:70, the content of LiDFP is 0.4% by mass with respect to the total amount of the non-aqueous electrolyte preparation solution, and the LiPF _6. When the composition is adjusted so that the concentration of 30:70, the content of LiDFP is 0.4% by mass based on the total amount of the nonaqueous electrolyte preparation solution (i.e., the liquid sample described below), and the concentration of LiPF ₆ is adjusted to a composition of 1 mol/L. This means that when the above ⁷ Li-NMR analysis and the above ¹⁹ F-NMR analysis are performed on the obtained liquid sample, the above-mentioned resonance peaks are observed.

Generally, nuclear magnetic resonance (NMR) is a phenomenon related to the spin state of atomic nuclei.
An atomic nucleus has a unique nuclear spin quantum number. Such an atomic nucleus has the property of a small magnet that rotates around its axis, and in a magnetic field it splits into several energy levels depending on its spin state (Zeeman effect).
In nuclear magnetic resonance (NMR) measurement, the state of an atom (nucleus) is measured by detecting the energy difference between the split energy levels.
Atoms (nuclei) are shielded by electrons, and the higher the electron density, the greater the shielding effect, which lowers the frequency at which the effective magnetic field that the atoms (nuclei) receives resonates.

⁷ In Li-NMR analysis, the resonance peak of Li in LiPF ₆ is observed within the range of approximately −0.270 ppm to −0.290 ppm, although it varies depending on the interaction of surrounding atoms with the Li atom.
The resonance peak _of Li in LiPF ₆ is, for example, -0. It is observed around 279 ppm (see Comparative Example 1 below).
The following comparative complex (X) is a complex considered to be contained in a general non-aqueous electrolyte for lithium secondary batteries containing LiPF ₆ and EC.

In the non-aqueous electrolyte preparation solution of the present disclosure after adjusting the composition as described above for the solution containing the comparative complex (X), the resonance peak of Li in the ⁷ Li-NMR analysis is from -0.280 ppm to Observed within the range of -0.288 ppm. This resonance peak is shifted to the lower frequency side compared to the case of the solution containing the comparative complex (X) (around -0.279 ppm).

In ¹⁹ F-NMR analysis, the resonance peak of F in LiPF ₆ is observed within the range of approximately -72.000 ppm to -74.000 ppm, although it varies depending on the interaction of surrounding atoms with the F atom.
The resonance peaks of F in LiPF ₆ are observed, for example, at around −72.580 ppm and around −73.830 ppm in a solution containing the comparative complex (X) (see Comparative Example 1 below). The reason why two resonance peaks of F are observed is considered to be that F adjacent to Li and F not adjacent to Li are present in the comparison complex (X).

In the non-aqueous electrolyte preparation solution of the present disclosure after adjusting the composition as described above for the solution containing comparative complex (X), the resonance peak of F in ¹⁹ F-NMR analysis ranges from -72.605 ppm to It is observed in the range of -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm. Each of these resonance peaks is shifted to the lower frequency side compared to the case of the solution containing comparative complex (X) (around -72.580 ppm and around -73.830 ppm).

As described above, in the non-aqueous electrolyte preparation liquid of the present disclosure whose composition has been adjusted, the resonance peaks of Li and F are each shifted to the lower frequency side with respect to the solution containing the comparative complex (X). .
This means that the non-aqueous electrolyte preparation liquid of the present disclosure whose composition has been adjusted has a higher electron density near Li and a higher electron density near F than a solution containing the comparative complex (X). .
The reason for this is that in the complex compound contained in the non-aqueous electrolyte preparation, an excess of EC, which exceeds the theoretical coordination number (i.e. 4; see Comparative Complex (X) mentioned above), coordinates with respect to Li ⁺ . It is assumed that this is due to the fact that
As a result, the structure of the complex compound is stably maintained in the prepared non-aqueous electrolyte, and it is thought that the effect of reducing the battery resistance of the lithium ion secondary battery at the initial stage and after the charge/discharge cycle is exhibited.

(Molar ratio [EC site/LiDFP site])
In the above complex compound, the molar ratio of the EC site to the LiDFP site (molar ratio [EC site/LiDFP site]) is preferably 10 or more and less than 18.
When the molar ratio [EC site/LiDFP site] is 10 or more, the stability of the complex compound is further improved in the prepared non-aqueous electrolyte, and the stability of the complex compound is improved during the initial stage and after the charge/discharge cycle of the lithium ion secondary battery. The effect of reducing battery resistance is more effectively expressed.

When the molar ratio [EC site/LiDFP site] is less than 18, the amount of EC brought into the non-aqueous electrolyte from the complex compound is limited to some extent, so there are fewer restrictions on the solvent composition in the non-aqueous electrolyte. Become.

(Molar ratio [LiPF ₆ sites/LiDFP sites])
In the above complex compound, the molar ratio of LiPF ₆ sites to LiDFP sites (molar ratio [LiPF ₆ sites/LiDFP sites]) is preferably 0.95 to 1.05 from the viewpoint of stability of the complex compound.

In a more preferred embodiment of the above complex compound, the molar ratio [LiPF ₆ sites/LiDFP sites]) is 0.95 to 1.05, and the molar ratio [EC sites/LiDFP sites]) is 10 or more and less than 18. This is an aspect.

(Example of manufacturing method for non-aqueous electrolyte preparation (manufacturing method A))
Next, an example of a manufacturing method (hereinafter referred to as "manufacturing method A") for manufacturing a non-aqueous electrolyte preparation according to the present disclosure will be described.
Production method A includes LiPF ₆ , LiDFP, EC, and a non-aqueous solvent containing at least one selected from the group consisting of DMC, _EMC , and DEC. The method includes a step of obtaining a non-aqueous electrolyte preparation containing a complex compound containing an EC site and a non-aqueous solvent.
Manufacturing method A may include other steps as necessary.

In the step of obtaining a non-aqueous electrolyte preparation, the charging molar ratio of LiPF ₆ to LiDFP (ie, charging molar ratio [LiPF ₆ /LiDFP]) is preferably 0.95 to 1.05.
When the charging molar ratio [LiPF ₆ /LiDFP] is 0.95 to 1.05, a complex compound with excellent stability is formed.

In the step of obtaining a nonaqueous electrolyte preparation, the molar ratio of EC to LiDFP (i.e., molar ratio [EC/LiDFP]) is preferably 10 or more and less than 18.
When the charging molar ratio [EC/LiDFP] is 10 or more and less than 18, a complex compound with excellent stability can be obtained, and there are fewer restrictions on the solvent composition of the non-aqueous electrolyte.

In the step of obtaining a nonaqueous electrolyte preparation, the charging molar ratio [(DMC, EMC, and DEC)/EC] is preferably 0.2 to 1.5.
When the charging molar ratio [(DMC, EMC, and DEC)/EC] is 0.2 or more, EC becomes easier to dissolve, and as a result, the ability to form a complex compound is further improved.
When the charging molar ratio [(DMC, EMC, and DEC)/EC] is 1.5 or less, restrictions on the solvent composition when preparing the nonaqueous electrolyte can be further reduced.

[An example of a method for manufacturing a non-aqueous electrolyte for lithium ion secondary batteries]
An example of the method for manufacturing a non-aqueous electrolyte for a lithium ion secondary battery according to the present disclosure includes a step of mixing the above-described preparation for a non-aqueous electrolyte according to the present disclosure and an electrolyte.
According to the manufacturing method according to this example, it is possible to manufacture a non-aqueous electrolyte for a lithium ion secondary battery that can reduce the battery resistance of the lithium ion secondary battery at the initial stage and after a charge/discharge cycle.

In the mixing step in this example, the above-mentioned nonaqueous electrolyte preparation, electrolyte, and other components may be mixed, if necessary.
Other components include nonaqueous solvents, known additives, and the like.
Regarding the electrolyte and other components, refer to the section on nonaqueous electrolyte for lithium ion secondary batteries described later.

[Nonaqueous electrolyte for lithium ion secondary batteries]
The non-aqueous electrolyte for lithium ion secondary batteries of the present disclosure (hereinafter also referred to as "the non-aqueous electrolyte of the present disclosure") includes:
A nonaqueous electrolyte for a lithium ion secondary battery containing an electrolyte, a nonaqueous solvent, and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate, and EC which is ethylene carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, LiDFP site, and EC site in the complex compound are included as LiPF ₆ , LiDFP, and EC, respectively, the volume ratio [EC: (DMC, EMC, and DEC)] is 30:70, When the composition was adjusted so that the content of LiDFP was 0.4% by mass with respect to the total amount of the nonaqueous electrolyte for lithium ion secondary batteries and the concentration of LiPF ₆ was 1 mol/L,
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a standard are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm.

According to the non-aqueous electrolyte of the present disclosure, it is possible to reduce the battery resistance of a lithium ion secondary battery at the initial stage and after a charge/discharge cycle.
More specifically, the non-aqueous electrolytic solution of the present disclosure containing a complex compound containing a LiDFP moiety has an initial and It has an excellent effect of reducing battery resistance after charge/discharge cycles (for example, see Examples below).
Although the reason for this effect is not clear, in a non-aqueous electrolyte containing the above-mentioned complex compound containing a LiDFP site, the structure of the above-mentioned complex compound is stably maintained even in the non-aqueous electrolyte, and this This is thought to be because the complex compound contributes to reducing the battery resistance of the lithium ion secondary battery at the initial stage and after charge/discharge cycles.
The nonaqueous electrolyte of the present disclosure has the effect of reducing the battery resistance of the lithium ion secondary battery at the initial stage and after the charge/discharge cycle, and therefore has the effect of improving the rate characteristics and cycle characteristics of the lithium ion secondary battery. is also expected.

(Electrolytes)
The non-aqueous electrolyte of the present disclosure contains an electrolyte.
Lithium salts are preferred as the electrolyte.
Examples of lithium salts include;
Lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium hexafluorotantalate (LiTaF ₆ ), Inorganic acid anion salts such as lithium aluminate chloride (LiAlCl ₄ ) and lithium decachlorodecaborate (Li ₂ B ₁₀ Cl ₁₀ );
Lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium bis(trifluoromethanesulfonyl)imide (Li(CF ₃ SO ₂ ) ₂ N), lithium bis(pentafluoroethanesulfonyl)imide (Li(C ₂ F ₅ SO _{2 )} ) ₂ N) and other organic acid anion salts;
etc.
Among these, lithium hexafluorophosphate (LiPF ₆ ) is preferred.
The nonaqueous electrolyte of the present disclosure may contain only one type of lithium salt, or may contain two or more types of lithium salt.

The concentration of electrolyte in the non-aqueous electrolyte of the present disclosure is preferably 0.1 mol/L to 3 mol/L, more preferably 0.5 mol/L to 2 mol/L.
Further, the concentration of LiPF ₆ in the non-aqueous electrolyte of the present disclosure is preferably 0.1 mol/L to 3 mol/L, more preferably 0.5 mol/L to 2 mol/L.
When the concentration of the electrolyte is 0.1 mol/L or more, the performance and life characteristics of the battery are further improved.
When the electrolyte concentration is 3 mol/L or less, the performance of the battery at low temperatures is further improved.

(Non-aqueous solvent)
The non-aqueous electrolyte of the present disclosure contains a non-aqueous solvent.
The non-aqueous solvent includes at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate, and EC which is ethylene carbonate.
In the nonaqueous electrolyte of the present disclosure, the ratio of the total amount of DMC, EMC, DEC, and EC to the total amount of the nonaqueous solvent is preferably 50% by mass to 100% by mass, and more preferably 60% by mass. The amount is preferably 80% to 100% by weight, more preferably 80% to 100% by weight.

The nonaqueous solvent in the nonaqueous electrolyte of the present disclosure may include DMC, EMC, DEC, and other solvent types other than EC.
Other solvent types include, for example, cyclic carbonates (excluding EC), fluorine-containing cyclic carbonates, chain carbonates (excluding DMC, EMC, and DEC), fluorine-containing chain carbonates, From aliphatic carboxylic acid esters, fluorine-containing aliphatic carboxylic esters, γ-lactones, fluorine-containing γ-lactones, cyclic ethers, fluorine-containing cyclic ethers, chain ethers, and fluorine-containing chain ethers At least one selected from the group consisting of:

Examples of the cyclic carbonates (excluding EC) include propylene carbonate (PC) and butylene carbonate (BC).
Examples of the fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC).
Examples of chain carbonates (excluding DMC, EMC, and DEC) include methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), and dipropyl carbonate (DPC).
Examples of aliphatic carboxylic acid esters include methyl formate, methyl acetate, and ethyl propionate.
Examples of γ-lactones include γ-butyrolactone.
Examples of the cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, and 1,4-dioxane.
Examples of chain ethers include 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, 1,2-dimethoxyethane, and 1,2-dibutoxyethane.
Other examples include nitriles such as acetonitrile, and amides such as dimethylformamide.
These solvent types can be used alone or in combination of two or more types.

The proportion of cyclic carbonates (including EC) and chain carbonates (including DMC, EMC, and DEC) in the nonaqueous solvent is preferably 80% by mass or more, more preferably 90% by mass or more. and more preferably 95% by mass or more.
The proportion of cyclic carbonates and chain carbonates in the nonaqueous solvent may be 100% by mass.

The proportion of the nonaqueous solvent in the nonaqueous electrolyte of the present disclosure is preferably 60% by mass or more, more preferably 70% by mass or more.
The upper limit of the proportion of the nonaqueous solvent in the nonaqueous electrolyte of the present disclosure depends on the content of other components (electrolytes, additives, etc.), but the upper limit is, for example, 99% by mass, preferably 97% by mass. It is mass %, more preferably 90 mass %.

(complex compound)
The non-aqueous electrolyte of the present disclosure contains a complex compound.
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate.
The complex compound contained in the non-aqueous electrolyte of the present disclosure is the same complex compound as the complex compound in the non-aqueous electrolyte preparation solution of the present disclosure described above, and the preferred embodiments are also the same.

The content of the complex compound in the nonaqueous electrolyte of the present disclosure can be adjusted as appropriate.
In the nonaqueous electrolyte of the present disclosure, the content of the LiDFP site of the complex compound (that is, the content of the complex compound converted to the amount of LiDFP) is preferably 0.1 mass with respect to the total amount of the nonaqueous electrolyte. % to 2% by weight, more preferably 0.1% to 1% by weight.

(Other ingredients)
The non-aqueous electrolyte of the present disclosure may contain components other than those described above.
Other components include known additives for non-aqueous electrolytes.

(Resonance peak by NMR analysis)
The non-aqueous electrolyte of the present disclosure has _a volume ratio [EC: (DMC _, EMC and DEC)] was 30:70, the content of LiDFP was 0.4% by mass based on the total amount of non-aqueous electrolyte for lithium ion secondary batteries, and the composition was adjusted so that the concentration of LiPF ₆ was 1 mol/L. When adjusting,
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a standard are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm.

The nonaqueous electrolyte of the present disclosure has a volume ratio [EC: (DMC, EMC, and DEC)] of 30:70, and the content of LiDFP is 0.4 mass with respect to the total amount of the nonaqueous electrolyte preparation solution. %, and the concentration of LiPF ₆ is not limited to 1 mol/L.
Specifically, the "volume ratio [EC: (DMC, EMC and DEC)] is 30:70, the content of LiDFP is 0.4% by mass with respect to the total amount of the non-aqueous electrolyte preparation solution, and the LiPF _6. When the composition is adjusted so that the concentration of The LiDFP content was adjusted to 0.4% by mass based on the total amount of the nonaqueous electrolyte preparation solution (i.e., the liquid sample described below), and the LiPF ₆ concentration was 1 mol/L. This means that when the above ⁷ Li-NMR analysis and the above ¹⁹ F-NMR analysis are performed on the obtained liquid sample, the above-mentioned resonance peaks are observed.

Each resonance peak in the nonaqueous electrolyte of the present disclosure is the same as each resonance peak in the nonaqueous electrolyte preparation of the present disclosure described above, and has the same technical meaning.
For each resonance peak in the non-aqueous electrolyte of the present disclosure, the description of each resonance peak in the non-aqueous electrolyte preparation of the present disclosure described above can be referred to.

The above-described non-aqueous electrolyte of the present disclosure can be particularly suitably used as an electrolyte in a lithium ion secondary battery including a positive electrode containing a positive electrode active material that is lithium nickel cobalt manganese oxide (NCM).
The lithium nickel cobalt manganese oxide (NCM) will be described later.
In a lithium ion secondary battery including a positive electrode active material that is lithium nickel cobalt manganese oxide (NCM), Ni is likely to be eluted into the nonaqueous electrolyte due to decomposition of the nonaqueous electrolyte containing EC and the like. Due to the elution of Ni, the crystal structure of the positive electrode active material changes, and the resistance to intercalation and desorption of Li becomes high, and as a result, the battery resistance tends to increase at the initial stage and after charge/discharge cycles.
When the nonaqueous electrolyte of the present disclosure is used as the nonaqueous electrolyte of such a lithium ion secondary battery, a high-quality SEI (Solid Electrolyte Interface) film is formed on the surface of the positive electrode active material by the complex compound. As a result, the above-mentioned problem (increase in battery resistance both initially and after charge/discharge cycles) is improved.

(Method for producing non-aqueous electrolyte)
There is no particular limitation on the method for manufacturing the nonaqueous electrolyte of the present disclosure described above.
The non-aqueous electrolyte of the present disclosure can be manufactured by a known method of mixing the components contained therein.
Moreover, the non-aqueous electrolyte of the present disclosure can also be manufactured by the above-mentioned "Example of the method for manufacturing a non-aqueous electrolyte for a lithium ion secondary battery."

[Lithium ion secondary battery]
The lithium ion secondary battery of the present disclosure includes:
a positive electrode;
a negative electrode;
a separator placed between the positive electrode and the negative electrode;
The above-mentioned non-aqueous electrolyte of the present disclosure,
Equipped with

In the lithium ion secondary battery of the present disclosure, the complex compound in the nonaqueous electrolyte of the present disclosure is one that has been added (i.e., contained) in advance to the nonaqueous electrolyte before manufacturing the lithium ion secondary battery. It may be. That is, the lithium ion secondary battery of the present disclosure may be manufactured using a nonaqueous electrolyte containing the above complex compound.Furthermore, in the lithium ion secondary battery of the present disclosure, the above complex compound in the nonaqueous electrolyte may be added to the nonaqueous electrolyte in the lithium ion secondary battery after manufacturing the lithium ion secondary battery. For example, first, a lithium ion secondary battery is manufactured using a nonaqueous electrolyte that does not contain the above complex compound, and then the above complex compound is added to the nonaqueous electrolyte of the manufactured lithium ion secondary battery. Accordingly, the lithium ion secondary battery of the present disclosure may be manufactured. In addition, first, a lithium ion secondary battery is manufactured using a nonaqueous electrolyte containing the above complex compound, and then the above complex compound is further added to the nonaqueous electrolyte of the manufactured lithium ion secondary battery. By doing so, the lithium ion secondary battery of the present disclosure may be manufactured.

An example of a lithium ion secondary battery according to an embodiment of the present disclosure will be described below with reference to FIG. 1, but the lithium ion secondary battery of the present disclosure is not limited to the following example.

FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery 1, which is an example of a lithium ion secondary battery according to an embodiment of the present disclosure.
The lithium ion secondary battery 1 is an example of a stacked lithium ion secondary battery.
In addition to this stacked lithium ion secondary battery, the lithium ion secondary battery according to the embodiment of the present disclosure may have a structure in which a positive electrode, a separator, a negative electrode, and a separator are layered and wound in this order. Also included is a wound type lithium ion secondary battery having the following.

As shown in FIG. 1, the lithium ion secondary battery 1 includes a battery element 10 to which a positive electrode lead 21 and a negative electrode lead 22 are attached. The lithium ion secondary battery 1 has a structure in which a battery element 10 is enclosed inside an exterior body 30 formed of a laminate film.
In the lithium ion secondary battery 1, the positive electrode lead 21 and the negative electrode lead 22 are led out in opposite directions from the inside of the exterior body 30 to the outside.
The positive electrode lead 21 and the negative electrode lead 22 can be attached to the positive electrode current collector and the negative electrode current collector by, for example, ultrasonic welding or resistance welding.
Although not shown, the positive electrode lead and the negative electrode lead may be led out in the same direction from the inside of the exterior body 30 to the outside.

In this example, the battery element 10 includes a positive electrode 11 in which a positive electrode composite layer 11B is formed on both main surfaces (i.e., front surface and back surface) of a positive electrode current collector 11A on a thin plate, and a separator. 13 and a negative electrode 12 in which a negative electrode composite material layer 12B is formed on both main surfaces of a negative electrode current collector 12A, are stacked (see FIG. 1).
In this structure, a positive electrode composite layer 11B is formed on one main surface of the positive electrode current collector 11A of the positive electrode 11, and a positive electrode composite layer 11B is formed on one main surface of the negative electrode current collector 12A of the negative electrode 12 adjacent to the positive electrode 11. The negative electrode composite material layer 12</b>B and the negative electrode composite material layer 12</b>B facing each other with the separator 13 interposed therebetween.

A non-aqueous electrolyte (not shown) of the present disclosure is injected into the exterior body 30 of the lithium ion secondary battery 1. This nonaqueous electrolyte is impregnated into the positive electrode composite material layer 11B, the separator 13, and the negative electrode composite material layer 12B to form one cell layer 14.
In the lithium ion secondary battery 1, a plurality of unit cell layers 14 are stacked, so that the plurality of unit cell layers 14 are electrically connected in parallel.

Each element other than the non-aqueous electrolyte in the lithium ion secondary battery of the present disclosure will be described below.

(Negative electrode)
The lithium ion secondary battery of the present disclosure includes a negative electrode.
The negative electrode may include a negative electrode current collector and a negative electrode composite material layer.
The negative electrode composite material layer may be provided on only one side of the negative electrode current collector, or may be provided on both sides of the negative electrode current collector.
For example, the negative electrode 12 in the above example has a configuration in which a negative electrode composite material layer 12B is provided on one or both sides of a negative electrode current collector 12A.
The negative electrode composite material layer contains a negative electrode active material and a binder. It is preferable that the negative electrode composite material layer further contains a conductive additive.

The negative electrode current collector may be of any type as long as it has conductivity, and for example, one made of metal or an alloy may be used.
Specifically, examples of the negative electrode current collector include copper, aluminum, nickel, stainless steel (SUS), and the like. Among these, aluminum or copper is preferred from the viewpoint of balance between high conductivity and cost.
Here, aluminum means pure aluminum and aluminum alloys, and "copper" means pure copper or copper alloys.
Especially preferred as the negative electrode current collector is copper foil.
The copper foil is not particularly limited, but includes rolled copper foil, electrolytic copper foil, and the like.

The thickness of the negative electrode composite material layer is preferably 5 μm or more, more preferably 10 μm or more.
Further, the thickness of the negative electrode composite material layer is preferably 200 μm or less, more preferably 100 μm or less, and still more preferably 75 μm or less.
If the thickness of the negative electrode composite layer is within the above range, it is easy to achieve sufficient lithium storage and release functions for charging and discharging at high charging and discharging rates.

The above-mentioned negative electrode composite material layer may include a negative electrode active material, a conductive additive, and a binder.

-Negative electrode active material-
Examples of negative electrode active materials include metallic lithium, lithium-containing alloys, metals or alloys that can be alloyed with lithium, oxides that can dope and dedope lithium ions, and transition metals that can dope and dedope lithium ions. At least one type selected from the group consisting of nitrides and carbon materials capable of doping and dedoping with lithium ions (it may be used alone, or a mixture containing two or more of these types may be used) can be used.
Among these, carbon materials that can be doped and dedoped with lithium ions are preferred.

Examples of the carbon material include carbon black, activated carbon, graphite materials (for example, artificial graphite, natural graphite, etc.), amorphous carbon materials, and the like.
The carbon material may be in any of fibrous, spherical, potato-like, and flake-like forms.
The particle size of the carbon material is not particularly limited, but is, for example, 5 μm to 50 μm, preferably 20 μm to 30 μm.

Specific examples of the amorphous carbon material include hard carbon, coke, mesocarbon microbeads (MCMB) fired at 1500° C. or lower, and mesophase pitch carbon fiber (MCF).

Examples of graphite materials include natural graphite and artificial graphite.
As the artificial graphite, graphitized MCMB, graphitized MCF, etc. are used.
As the graphite material, one containing boron can also be used.
As the graphite material, those coated with metals such as gold, platinum, silver, copper, and tin, those coated with amorphous carbon, and those coated with a mixture of amorphous carbon and graphite can also be used.

These carbon materials may be used alone or in combination of two or more.

-Conductive aid-
It is preferable that the negative electrode composite material layer contains a conductive auxiliary material.
As the conductive aid, a known conductive aid can be used.
Known conductive additives are not particularly limited as long as they are conductive carbon materials, but include graphite, carbon black, conductive carbon fibers (carbon nanotubes, carbon nanofibers, carbon fibers), fullerenes, etc. They can be used alone or in combination of two or more.
Commercially available carbon blacks include, for example, Toka Black (registered trademark) #4300, #4400, #4500, #5500 (manufactured by Tokai Carbon Co., Ltd., Furnace Black), Printex (registered trademark) L, etc. (manufactured by Degussa Corporation, Furnace Black), Raven (registered trademark) 7000, 5750, 5250, 5000ULTRAIII, 5000ULTRA, etc., Conductex (registered trademark) SC ULTRA, Conductex 975ULTRA, etc., PUER BLACK (registered trademark) 100, 115, 205, etc. (Columbi Manufactured by Yan, Furnace black), #2350, #2400B, #2600B, #30050B, #3030B, #3230B, #3350B, #3400B, #5400B, etc. (manufactured by Mitsubishi Chemical Corporation, furnace black), MONARCH (product name) 1400, 1300, 900 , Vulcan (registered trademark) -Li (manufactured by TIMCAL), Ketjen Black (registered trademark) EC-300J, EC-600JD (manufactured by Akzo), Denka Black (registered trademark), Denka Black HS-100, FX-35 (manufactured by Denki Kagaku Kogyo) , acetylene black), etc.
Examples of graphite include, but are not limited to, artificial graphite, natural graphite (for example, scaly graphite, lumpy graphite, earthy graphite, etc.).

-binder-
As the binder, one or two selected from styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, carboxymethyl cellulose (CMC), and hydroxypropyl methyl cellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetyl cellulose. More than one species can be used in combination.
As the binder for the negative electrode composite layer, it is desirable to use an appropriate mixture of styrene-butadiene rubber and carboxymethyl cellulose.

The content of the binder in the negative electrode composite layer is from 0.1% by mass to the negative electrode composite layer, from the viewpoint of achieving both physical properties (electrolyte permeability and peel strength) of the negative electrode composite layer and battery performance. It is preferably 4% by mass.
When the content of the binder is 0.1% by mass or more, the adhesiveness of the negative electrode composite layer to the negative electrode current collector and the binding property between negative electrode active materials are further improved.
When the content of the binder is 4% by mass or less, the amount of the negative electrode active material in the negative electrode composite layer can be increased, so that the battery capacity is further improved.

-Other ingredients-
In addition to the above-mentioned components, the negative electrode composite material layer may contain other components.
Other components include thickeners, surfactants, dispersants, wetting agents, antifoaming agents, and the like.

-Method for forming negative electrode composite layer-
The negative electrode composite material layer can be manufactured, for example, by applying a negative electrode composite material slurry containing a negative electrode active material and a binder onto the surface of a negative electrode current collector and drying the slurry.
It is preferable to use water as the solvent contained in the negative electrode composite material slurry, but if necessary, for example, a liquid medium that is compatible with water may be used to improve the coating properties on the current collector. You can.
Liquid media that are compatible with water include alcohols, glycols, cellosolves, amino alcohols, amines, ketones, carboxylic acid amides, phosphoric acid amides, sulfoxides, carboxylic acid esters, and phosphoric acid esters. ethers, nitriles, etc., and these may be used within the range of being compatible with water.

The method of applying the negative electrode mixture slurry to the negative electrode current collector and the drying method are not particularly limited.
Examples of the coating method include slot die coating, slide coating, curtain coating, and gravure coating.
Examples of the drying method include drying with warm air, hot air, and low humidity air; vacuum drying; and drying with infrared rays (for example, far infrared rays) irradiation.
The drying time and drying temperature are not particularly limited, but the drying time is, for example, 1 minute to 30 minutes, and the drying temperature is, for example, 40°C to 80°C.
The manufacturing method of the negative electrode composite material layer is to apply a negative electrode mixture slurry onto the negative electrode current collector, dry it, and then apply pressure treatment using a mold press, roll press, etc. to reduce the porosity of the negative electrode active material layer. It is preferable to include a step of lowering the .

(positive electrode)
The lithium ion secondary battery of the present disclosure includes a positive electrode.
The positive electrode may include a positive electrode current collector and a positive electrode composite material layer.
The positive electrode composite material layer may be provided on only one side of the positive electrode current collector, or may be provided on both sides of the positive electrode current collector.
For example, the positive electrode 11 in the above example has a configuration in which a positive electrode composite material layer 11B is provided on one or both sides of a positive electrode current collector 11A.
The positive electrode composite material layer includes a positive electrode active material and a binder. It is preferable that the positive electrode composite material layer further contains a conductive additive.

As the positive electrode current collector, various conductive materials can be used, and for example, those made of metal or alloy are used.
Specifically, examples of the positive electrode current collector include aluminum (that is, pure aluminum or aluminum alloy), nickel, SUS, and the like. Among these, aluminum is preferred from the viewpoint of balance between high conductivity and cost.
Particularly preferred as the positive electrode current collector is aluminum foil.
The aluminum foil is not particularly limited, but examples include A1085 material and A3003 material.

The thickness of the positive electrode composite material layer is preferably 5 μm or more, more preferably 10 μm or more.
Further, the thickness of the positive electrode composite material layer is preferably 200 μm or less, more preferably 100 μm or less, and still more preferably 75 μm or less.
If the thickness of the positive electrode composite layer is within the above range, sufficient lithium intercalation and desorption functions can be easily achieved for charging and discharging at high charging and discharging rates.

The positive electrode active material, conductive additive, and binder that may be contained in the positive electrode composite layer will be described below.

-Cathode active material-
The positive electrode active material is not particularly limited as long as it is a material capable of intercalating and deintercalating lithium, and any positive electrode active material commonly used in lithium ion secondary batteries can be used.
The positive electrode active material is preferably an oxide containing lithium (Li) and nickel (Ni) as constituent metal elements.
The oxide may contain at least one metal element (eg, a transition metal element, a typical metal element, etc.) other than lithium and nickel. In this case, it is preferable that the content of metal elements other than lithium and nickel is equivalent to or smaller than nickel in terms of the number of atoms.
Examples of metal elements other than Li and Ni include Co, Mn, Al, Cr, Fe, V, Mg, Ca, Na, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, Examples include one or more metal elements selected from the group consisting of La and Ce.
These positive electrode active materials may be used alone or in combination.

A preferred positive electrode active material includes lithium nickel cobalt aluminum oxide (NCA) represented by the following formula (1).
Li _t Ni _1-x-y Co _x Al _y O ₂ ... Formula (1)
[In formula (1), t is 0.95 or more and 1.15 or less, x is 0 or more and 0.3 or less, y is 0 or more and 0.2 or less, and the sum of x and y is less than 0.5. ]
Specific examples of NCAs include LiNi _0.8 Co _0.15 Al _0.05 O ₂ and the like.

A preferred positive electrode active material also includes lithium nickel cobalt manganese oxide (NCM) represented by the following formula (2).
LiNi _a Co _b Mn _c O ₂ ... Formula (2)
[In formula (2), a, b and c are each independently greater than 0 and less than 1, and the sum of a, b and c is 0.99 to 1.00. ]
NCM has a high energy density per volume and excellent thermal stability.
Specific examples of NCM include LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ and the like.

The content of the positive electrode active material in the positive electrode composite layer is, for example, 10% by mass or more, preferably 30% by mass or more, more preferably 50% by mass or more, and particularly preferably 70% by mass or more.
Further, the content of the positive electrode active material in the positive electrode composite material layer is, for example, 99.9% by mass or less, preferably 99% by mass or less.

-Conductivity aid-
The positive electrode composite material layer preferably contains a conductive additive.
As the conductive aid, a known conductive aid can be used.
Specific examples and preferred embodiments of the conductive additive that may be included in the positive electrode composite material layer are the same as specific examples and preferred embodiments of the conductive additive that may be included in the negative electrode composite material layer.

-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), vinylidene fluoride-hexafluoropropylene copolymer, and the like.
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 composite material layer.
One type of binder can be used alone, or two or more types can be used in combination as necessary.

-Other ingredients-
In addition to the above-mentioned components, the positive electrode composite material layer may contain other components.
Other components include thickeners, surfactants, dispersants, wetting agents, antifoaming agents, and the like.

-Method for forming positive electrode composite layer-
The positive electrode composite material layer can be manufactured, for example, by applying a positive electrode composite material slurry containing a positive electrode active material and a binder onto the surface of a positive electrode current collector and drying the slurry.
The solvent contained in the positive electrode material slurry is preferably an organic solvent such as N-methyl-2-pyrrolidone (NMP).
A preferred embodiment of the method for forming the positive electrode composite material layer is the same as the preferred embodiment of the method for forming the negative electrode composite material layer described above.

(Separator)
The lithium ion secondary battery of the present disclosure includes a separator.
A separator (for example, separator 13 in the above example) is arranged between the positive electrode and the negative electrode.
Examples of the separator include porous flat plates containing resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Moreover, as a separator, the nonwoven fabric containing the said resin is also mentioned.
A preferred example is a porous resin sheet having a single layer or multilayer structure mainly composed of one or more types of polyolefin resin.

The thickness of the separator can be, for example, 15 μm to 30 μm.
One preferred embodiment is a separator having a shutdown function and including a porous resin layer made of a thermoplastic resin such as polyethylene. According to this aspect, when the temperature of the separator reaches the softening point of the thermoplastic resin, the thermoplastic resin melts and the pores are clogged, thereby making it possible to cut off the current.

The lithium ion secondary battery according to the embodiment of the present disclosure preferably includes an SEI coating derived from the complex compound of the present disclosure on at least one of the surface of the positive electrode active material and the surface of the negative electrode active material.
Such an SEI coating can significantly suppress an increase in battery resistance of a lithium ion secondary battery at the initial stage or after a charge/discharge cycle.

Examples of the present disclosure will be shown below, but the present disclosure is not limited to the following examples.
In addition, unless otherwise specified, "room temperature" means 25° C., and "%" means mass %. Moreover, "wt%" is synonymous with "mass %".

[Example 1]
<Preparation of nonaqueous electrolyte preparation solution 1 containing complex compound 1 (EC/LiPF ₆ /LiDFP=0.60/0.04/0.04 (molar ratio)) and DMC>
After purging a 200 mL flask equipped with a stirrer, a thermometer, a gas inlet line, and an exhaust line with dry nitrogen gas, ethylene carbonate (EC) (52.836; 0.60 mol) and a nonaqueous solvent were added. dimethyl carbonate (DMC) (67.560 g; 0.75 mol) was added thereto and stirred at room temperature to dissolve EC and obtain a solution.
Lithium hexafluorophosphate (LiPF ₆ ) (6.076 g; 0.04 mol) and lithium difluorophosphate (LiDFP) (4.316 g; 0.04 mol) were added to the resulting solution, followed by 1 hour at room temperature. By stirring, a non-aqueous electrolyte preparation liquid 1 containing a complex compound 1 (EC/LiPF ₆ /LiDFP=0.60/0.04/0.04 (molar ratio)) and DMC was obtained.

<Preparation of non-aqueous electrolyte 1 (complex compound addition)>
Dimethyl carbonate (DMC) (668.458 g) as a non-aqueous solvent, ethylene carbonate (EC) (332.464 g) as a non-aqueous solvent, and lithium hexafluorophosphate (LiPF ₆ ) (144.595 g) as an electrolyte. ) and stirred at room temperature for 1 hour. The above-mentioned non-aqueous electrolyte preparation liquid 1 was added thereto, and the mixture was further stirred at room temperature for 1 hour to obtain non-aqueous electrolyte 1 (complex compound added).
Here, the addition amount of the non-aqueous electrolyte preparation ₁ is determined by the _volume ratio [ EC:DMC] was 30:70, the content of LiDFP was 0.4% by mass based on the total amount of non-aqueous electrolyte 1, and the concentration of LiPF ₆ was adjusted to be 1 mol/L.

[Comparative example 1]
<Preparation of comparative non-aqueous electrolyte 1 (LiDFP addition)>
DMC, EC, LiDFP, and LiPF ₆ were used, the volume ratio [EC:DMC] was 30:70, the content of LiDFP was 0.4% by mass with respect to the total amount of non-aqueous electrolyte 1, and LiPF Comparative non-aqueous electrolyte 1 was prepared in which the concentration of _{No. 6} was 1 mol/L.

<NMR analysis>
Non-aqueous electrolyte 1 of Example 1 (complex compound added) and comparative non-aqueous electrolyte 1 (LiDFP added) were subjected to NMR analysis (in detail, ⁷ Li-NMR and ¹⁹ F-NMR analysis) under the following conditions. ) was carried out.

-NMR analysis conditions-
Equipment: Bruker AVANCE III600
Nuclides: ⁷ Li, ¹⁹ F
Test tube: 5mmφ
Measurement temperature: Room temperature Measurement solvent: No dilution with NMR measurement solvent. Measures the sample (non-aqueous electrolyte) as it is.
Chemical shift standard: Measure lithium chloride (LiCl) and sodium fluoride (NaF) dissolved in heavy water using a double tube.
Sample preparation: The sample (non-aqueous electrolyte) was opened in a glove box with an argon atmosphere and collected into an NMR sample tube.

FIG. 2 shows the results of ⁷ Li-NMR analysis in Example 1 and Comparative Example 1.
3 and 4 are the results of ¹⁹ F-NMR analysis in Example 1 and Comparative Example 1.

As shown in FIG. 2, in Example 1, the resonance peak of Li was observed within the range of -0.280 to -0.288 ppm (specifically, -0.284 ppm).
On the other hand, in Comparative Example 1, the resonance peak of Li was observed at -0.279 ppm.
As shown in FIG. 3, in Example 1, the resonance peak of F was observed within the range of -72.605 to -72.620 ppm (specifically, -72.612 ppm).
On the other hand, in Comparative Example 1, the resonance peak of F was observed at -72.580 ppm.
As shown in FIG. 4, in Example 1, the resonance peak of F was observed within the range of -73.860 to -73.870 ppm (specifically, -73.865 ppm).
On the other hand, in Comparative Example 1, the resonance peak of F was observed at -73.830 ppm.

From the above results, Li and F in the non-aqueous electrolyte 1 of Example 1 have resonance peaks on the lower frequency side compared to Li and F in the comparative non-aqueous electrolyte 1 of Comparative Example 1. You can see that there is a shift.
That is, it can be seen that the non-aqueous electrolyte 1 of Example 1 has a higher electron density near Li and F than the comparative non-aqueous electrolyte 1 of Comparative Example 1.
The above results show that in Comparative Nonaqueous Electrolyte 1 of Comparative Example 1, EC with the theoretical coordination number (i.e. 4; see Comparative Complex (X) above) coordinates with Li ⁺ . In the non-aqueous electrolyte 1 of Example 1, it is thought that this indicates that an excess of EC, which exceeds the theoretical coordination number, is coordinated with Li ⁺ .

[Example 101]
<Production of lithium ion secondary battery>
A lithium ion secondary battery (hereinafter also simply referred to as "battery") was produced by the following operations.

(Preparation of negative electrode)
1%-CMC aqueous solution (i.e., 1 mass of carboxymethyl cellulose (CMC)) per 960 g of natural graphite as a negative electrode active material and 10 g of Super-P (conductive carbon, BET specific surface area 62 m ² /g) as a conductive additive. % aqueous solution) was added and mixed for 30 minutes.
To the resulting mixture, 300 g of a 1% CMC aqueous solution was added and kneaded for 30 minutes, and then 250 g of a 1% CMC aqueous solution was added and kneaded for 30 minutes.
To the obtained kneaded material, 50 g of styrene butadiene rubber (SBR) (40% emulsion) as a binder was added and mixed for 30 minutes, followed by vacuum defoaming for 30 minutes.
As a result of the above, a negative electrode composite slurry having a solid content concentration of 45% was obtained.

The negative electrode composite slurry was applied to a part of one side of a copper foil (thickness: 10 μm) as a negative electrode current collector and dried so that the applied mass after drying was 11.0 mg/cm ² . Next, the negative electrode composite slurry was applied to a part of the opposite side (uncoated side) of the copper foil so that the applied mass was 11.0 mg/cm ² and dried.
The thus obtained double-sided coated copper foil (22.0 mg/cm ² ) was dried in a vacuum drying oven at 120°C for 12 hours, and then pressed using a small press to a pressing density of 1.45 g/cm ³ . A negative electrode original fabric was obtained by compressing it so that
This negative electrode original fabric includes a copper foil as a negative electrode current collector, and negative electrode composite material layers provided on both sides of the copper foil. Both sides of the copper foil each include a region where the negative electrode composite material layer is formed and a region (ie, a blank space) where the negative electrode composite material layer is not formed.
By slitting the negative electrode original fabric, the front side includes a 58 mm x 372 mm negative electrode composite material layer and a tab welding margin, and the back side includes a 58 mm x 431 mm negative electrode composite material layer, A negative electrode A-1 including a tab welding margin was obtained.

(Preparation of positive electrode)
920 g of NCM523 (i.e., LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) as a positive electrode active material, 20 g of Super-P (conductive carbon manufactured by TIMCAL) as a conductive additive, and After 20 g of KS-6 (scaly graphite manufactured by TIMREX) was mixed for 10 minutes, 100 g of N-methylpyrrolidone (NMP) was added thereto and further mixed for 20 minutes.

Next, 150 g of an 8% PVDF solution (PVDFW #7200 manufactured by Kureha dissolved in NMP) was added and kneaded for 30 minutes, and then 150 g of the above 8% PVDF solution was added and kneaded for 30 minutes. Thereafter, 200 g of the above 8%-PVDF solution was added and kneaded for 30 minutes. Next, 80 g of a solution dissolved in NMP was added and kneaded for 30 minutes. Thereafter, 27 g of NMP was added to adjust the viscosity and mixed for 30 minutes, followed by vacuum defoaming for 30 minutes.
Through the above steps, a positive electrode composite slurry having a solid content concentration of 60% was obtained.

The above positive electrode composite slurry was applied to a part of one side of an aluminum foil (thickness 20 μm, width 200 mm) as a positive electrode current collector and dried so that the applied mass after drying was 19.0 mg/cm ² . . Next, on a part of the opposite side (uncoated side), the positive electrode composite slurry was similarly applied to the aluminum foil so that the applied mass was 19.0 mg/cm ² and dried.
The thus obtained double-sided coated aluminum foil (38.0 mg/cm ² ) was dried in a vacuum drying oven at 130° C. for 12 hours, and then, using a 35-ton press, the pressing density was 2.9 g/cm ³ A positive electrode original fabric was obtained by compressing it so that the following was obtained.
This positive electrode original fabric includes an aluminum foil as a positive electrode current collector, and positive electrode composite material layers provided on both sides of this aluminum foil. Both sides of the aluminum foil each include a region where the positive electrode composite material layer is formed and a region (i.e., a blank space) where the positive electrode composite material layer is not formed.
By slitting the positive electrode original fabric, the front side includes a positive electrode composite material layer of 56 mm x 334 mm and a margin for tab welding, and the back side includes a positive electrode composite material layer of 56 mm x 408 mm, A positive electrode C-1 including a tab welding margin was obtained.

(Battery assembly)
As the separator, a polyethylene porous membrane (60.5 mm x 450 mm) with a porosity of 45% and a thickness of 25 μm was used.
The above negative electrode A-1 and separator, and the above positive electrode C-1 and separator were stacked and wound in this order to obtain a wound body. The obtained wound body was press-molded to obtain a molded body.
Next, an aluminum positive electrode tab is bonded to the blank space of the positive electrode C-1 in the molded body using an ultrasonic bonding machine, and a nickel negative electrode tab is bonded to the blank space of the negative electrode A-1 in the molded body using an ultrasonic bonding machine. It was joined with The molded body in which the positive electrode tab and the negative electrode tab were joined was sandwiched between laminate sheets, and three sides were heat-sealed to obtain a laminate body. At this time, the positive electrode tab and the negative electrode tab were made to protrude from one heat-sealed side of the laminate.

Next, the laminate was dried under reduced pressure at 70° C. for 12 hours using a vacuum dryer while drawing a vacuum. Next, while continuing to vacuum, the non-aqueous electrolyte 1 (complex compound added) was injected into the laminate from the remaining side, and then the remaining side was heat-sealed. The amount of electrolyte injected was 4.7 g.
Through the above steps, a battery (ie, a lithium ion secondary battery) was obtained.

(Battery activation process)
The above battery was held at room temperature under the atmosphere for 24 hours, and then the following activation treatment was performed on this battery.
-Activation processing-
Constant current charging (0.05C-CC) was performed at 0.05C for 4 hours, and then rested for 12 hours.
Next, perform constant current constant voltage charging (0.1C-CCCV) at 0.1C to 4.2V (SOC 100%), rest for 30 minutes, and then constant current discharge (0.1C -CC).
Next, a charge/discharge cycle (ie, charging to 4.2V at 0.1C-CCCV and discharging to 2.8V at 0.1C-CC) was repeated five times.
Next, the battery was fully charged at 4.2V (SOC 100%) and stored at room temperature for 5 days.

<Battery performance evaluation>
For the battery subjected to the above activation treatment, the initial battery resistance value and the battery resistance value after the cycle test were measured as described below.
The results are shown in Table 1.
In Table 1, both the initial battery resistance value and the battery resistance value after the cycle test are expressed as relative values when the value of Comparative Example 1 is taken as 100.

(Measurement of initial battery resistance (DC-IR))
The battery after the activation treatment was charged at room temperature to a constant voltage of 4.2V, then discharged at room temperature at a constant current of 0.1C, and the potential drop was measured for 10 seconds from the start of discharge. Similarly, the battery was charged to a constant voltage of 4.2V at room temperature, and discharged at xC constant current with x=0.2, 0.5, and 1.0, and the potential drop was measured for 10 seconds from the start of discharge. The battery resistance (direct current resistance; DC-IR) at the initial stage of discharge was calculated from the potential drop when discharging at each current rate.

(Measurement of battery resistance (DC-IR) after cycle test)
After the activation process, the battery is subjected to a charge/discharge cycle (in detail, one cycle is charging to 4.2V at 0.1C-CCCV and discharging to 2.8V at 0.1C-CC). 100 cycles (discharge cycle) were performed (cycle test).
Next, the battery was fully charged at 4.2V (SOC 100%) and stored at room temperature for 5 days.
After storage, the battery resistance (DC-IR) after the cycle test was measured in the same manner as the initial battery resistance (DC-IR) measurement.

[Example 102]
The same operation as in Example 101 was performed except that non-aqueous electrolyte 1 (complex compound added) was changed to non-aqueous electrolyte 2 (complex compound added) described below.
The results are shown in Table 1.

Non-aqueous electrolyte 2 (complex compound added) was prepared in the same manner as non-aqueous electrolyte 1 (complex compound added) except for the following points.
In the preparation of non-aqueous electrolyte 2 (complex compound added), when the LiPF ₆ site, LiDFP site, and EC site in complex compound 1 are included as LiPF ₆ , LiDFP, and EC, respectively, the volume ratio [EC: DMC] was 30:70, the content of LiDFP was 0.8% by mass based on the total amount of the non-aqueous electrolyte, and _{the concentration of LiPF 6 was} 1 mol/L. The amount of preparation in step 1 was adjusted.

[Comparative example 101]
The same operation as in Example 101 was performed except that non-aqueous electrolyte 1 (complex compound added) was changed to the following comparative non-aqueous electrolyte (without additives).
The results are shown in Table 1.

A comparative nonaqueous electrolyte (without additives) was prepared in the same manner as comparative nonaqueous electrolyte 1 (with LiDFP added) except that LiDFP (5.190 g) was not added.
In the comparative nonaqueous electrolyte (without additives), the volume ratio [EC:DMC] is 30:70, and the concentration of LiPF ₆ is 1 mol/L.

[Comparative example 102]
The same operation as in Example 101 was performed except that the non-aqueous electrolyte 1 (complex compound added) was changed to the aforementioned comparative non-aqueous electrolyte 1 (LiDFP added).
The results are shown in Table 1.

[Comparative Example 103]
The same operation as in Comparative Example 102 was performed except that Comparative Nonaqueous Electrolyte 1 (LiDFP added) was changed to the following Comparative Nonaqueous Electrolyte (LiDFP added, LiDFP content 0.8% by mass).
The results are shown in Table 1.

The comparison non-aqueous electrolyte (LiDFP addition, LiDFP content 0.8% by mass) was obtained by changing the amount of LiDFP charged so that the content of LiDFP was 0.8% by mass relative to the total amount of the non-aqueous electrolyte. was prepared in the same manner as Comparative Nonaqueous Electrolyte 1 (LiDFP added).

As shown in Table 1, the batteries of Examples 101 and 102 using non-aqueous electrolytes containing complex compound 1, as well as the batteries of Comparative Example 101 using non-aqueous electrolytes containing no additives, It can be seen that the battery resistance values of the batteries of Comparative Example 102 and Comparative Example 103 containing single LiDFP without forming complex compound 1 are also reduced at the initial stage and after the cycle test.

From the NMR results in Example 1 and Comparative Example 1 described above (see FIGS. 2 to 4), the non-aqueous electrolytes in Examples 101 and 102 have a higher Li Contains a large amount of ⁺ -coordinated EC, which allows the structure of the complex compound to be stably maintained in the non-aqueous electrolyte, and this complex compound contributes to reducing the battery resistance value at the initial stage and after charge/discharge cycles. It is thought that

1 Lithium ion secondary battery 10 Battery element 11 Positive electrode 11A Positive electrode current collector 11B Positive electrode composite layer 12 Negative electrode 12A Negative electrode current collector 12B Negative electrode composite layer 13 Separator 14 Cell layer 21 Positive electrode lead 22 Negative electrode lead 30 Exterior body

Claims (7)

A nonaqueous electrolyte for a lithium ion secondary battery containing an electrolyte, a nonaqueous solvent, and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate, and EC which is ethylene carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, the LiDFP site, and the EC site in the complex compound are respectively counted as LiPF ₆ , LiDFP, and EC, the volume ratio [EC: (DMC, EMC, and DEC)] is 30. :70, the content of LiDFP is 0.4% by mass based on the total amount of the non-aqueous electrolyte for lithium ion secondary batteries, and when the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L,
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a reference are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm,
Non-aqueous electrolyte for lithium-ion secondary batteries. In the complex compound,
the molar ratio of the LiPF ₆ site to the LiDFP site is 0.95 to 1.05;
The molar ratio of the EC site to the LiDFP site is 10 or more and less than 18.
The non-aqueous electrolyte for a lithium ion secondary battery according to claim 1. The nonaqueous electrolyte for a lithium ion secondary battery according to claim 1 or 2, which is used as an electrolyte for a lithium ion secondary battery including a positive electrode containing a positive electrode active material that is a lithium nickel cobalt manganese oxide. A preparation liquid for a non-aqueous electrolyte containing a non-aqueous solvent and a complex compound,
The non-aqueous solvent contains at least one selected from the group consisting of DMC which is dimethyl carbonate, EMC which is ethyl methyl carbonate, and DEC which is diethyl carbonate,
The complex compound includes a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate,
When the LiPF ₆ site, the LiDFP site, and the EC site in the complex compound are respectively counted as LiPF ₆ , LiDFP, and EC, the volume ratio [EC: (DMC, EMC, and DEC)] is 30. :70, the content of LiDFP is 0.4% by mass based on the total amount of the non-aqueous electrolyte preparation solution, and the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L.
A resonance peak by ⁷ Li-NMR analysis based on LiCl was observed within the range of -0.280 ppm to -0.288 ppm,
Resonance peaks by ¹⁹ F-NMR analysis using NaF as a reference are observed in the range of -72.605 ppm to -72.620 ppm and in the range of -73.860 ppm to -73.870 ppm,
Preparation liquid for non-aqueous electrolyte. A method for producing a non-aqueous electrolyte for a lithium ion secondary battery, comprising the step of mixing the non-aqueous electrolyte preparation according to claim 4 and an electrolyte. a positive electrode;
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
The non-aqueous electrolyte for a lithium ion secondary battery according to claim 1 or 2,
A lithium-ion secondary battery equipped with. a positive electrode containing a positive electrode active material that is a lithium nickel cobalt manganese-based oxide;
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
The nonaqueous electrolyte for a lithium ion secondary battery according to claim 3,
A lithium-ion secondary battery equipped with.