JP7345418B2 - Lithium ion secondary battery - Google Patents

Description

The present disclosure relates to lithium ion secondary batteries.

BACKGROUND ART Lithium ion secondary batteries have been known that contain lithium difluorophosphate as an additive in their non-aqueous electrolyte.
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, even with the above-mentioned conventional lithium ion secondary battery that uses a non-aqueous electrolyte containing a fluorophosphate compound such as lithium difluorophosphate, there is still room for further improvement in rate characteristics and cycle durability. was there.
Furthermore, the structure and state of fluorophosphoric acid compounds in non-aqueous electrolytes have not yet been sufficiently elucidated.

An object of one embodiment of the present disclosure is to provide a lithium ion secondary battery with excellent rate characteristics and cycle durability.

As a result of extensive studies to achieve the above object, the present inventor has developed a complex compound with a specific structure, which is produced by appropriately mixing and using lithium hexafluorophosphate, lithium difluorophosphate, and ethylene carbonate, as an additive. The inventors have discovered that excellent battery characteristics can be achieved and the above objectives can be achieved by using a non-aqueous electrolyte containing a lithium ion secondary battery according to the present disclosure.

That is, the means for solving the above problems include the following aspects.
<1> A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
The additive contains lithium ions, hexafluorophosphate ions, difluorophosphate ions, and ethylene carbonate molecules, and in differential scanning calorimetry, the exothermic peak A indicating the maximum value of heat flow per unit mass is 190. Comprising a complex compound that occurs within a temperature range of 280°C to 300°C, and exhibits an exothermic peak B showing a maximum value of heat flow per unit mass within a temperature range of 280°C to 300°C.
Lithium ion secondary battery.

<2> A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
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 additive is a complex compound containing 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. including,
The nonaqueous electrolyte _has a 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, and the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L. , the electrical conductivity is 10.55 mScm ⁻¹ or less,
Lithium ion secondary battery.

According to one aspect of the present disclosure, a lithium ion secondary battery with excellent rate characteristics and cycle durability is provided.

1 is a cross-sectional view showing an embodiment of a lithium ion secondary battery of the present disclosure. It is a graph showing DSC measurement results of specific additives, etc. It is a graph showing the relationship between the volume ratio of EC and electrical conductivity. It is an XPS analysis spectrum of the positive electrode surface and the negative electrode surface of a lithium ion secondary battery. (a) is a TEM image of the cross section of the positive electrode active material of the lithium ion secondary battery of Example 4, and (b) is a TEM image of the cross section of the positive electrode active material of the lithium ion secondary battery of Comparative Example 7. (a) is a spectrum diagram showing the transmission electron energy loss of oxygen in the positive electrode active material of the lithium ion secondary battery of Example 4, and (b) is a spectrum diagram showing the transmission electron energy loss of oxygen in the positive electrode active material of the lithium ion secondary battery of Comparative Example 7. FIG. 2 is a spectrum diagram showing transmission electron energy loss of oxygen. It is a graph showing cycle characteristics of a battery. It is a graph showing the relationship between the initial C rate and the discharge capacity maintenance rate. It is a graph showing the relationship between C rate and discharge capacity retention rate after a cycle test. It is a graph showing the rate of increase in battery resistance at the initial stage and after the cycle test. This is an early Nyquist plot. This is a Nyquist plot after a cycle test.

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. .

[Lithium ion secondary battery]
The lithium ion secondary battery of the first aspect of the present disclosure includes:
A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
The additives include lithium ions (i.e., Li ⁺ ), hexafluorophosphate ions (i.e., PF ₆ ⁻ ), difluorophosphate ions (i.e., PF ₂ O ₂ ⁻ ), and ethylene carbonate molecules (hereinafter referred to as “EC molecules”). ), and in differential scanning calorimetry, exothermic peak A, which indicates the maximum value of heat flow per unit mass, appears within the temperature range of 190°C to 210°C, and the heat per unit mass Contains a complex compound in which an exothermic peak B indicating a maximum flow rate occurs within a temperature range of 280 ° C. to 300 ° C.
It is a lithium ion secondary battery.

The lithium ion secondary battery according to the second aspect of the present disclosure includes:
A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
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 additives are a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP (i.e. LiPF ₂ O ₂ ) which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate. Contains a complex compound containing
The nonaqueous electrolyte has a volume ratio [EC: (DMC, EMC, and DEC)] when the LiPF ₆ site, LiDFP site, and EC site in the complex compound are included as LiPF ₆ , LiDFP, and EC, respectively. When the composition is adjusted so that the ratio is 30:70, the LiDFP content is 0.4% by mass based on the total amount of the non-aqueous electrolyte, and the LiPF ₆ concentration is 1 mol/L, the electrical conductivity is 10.55 mScm ^-1 or less,
It is a lithium ion secondary battery.

The lithium ion secondary batteries of the first and second aspects of the present disclosure (hereinafter also simply referred to as "lithium ion secondary batteries of the present disclosure") have excellent rate characteristics and cycle durability.
The lithium ion secondary battery of each aspect may satisfy the requirements specified in another aspect.
For example, the lithium ion secondary battery of the first aspect may satisfy the requirements of the lithium ion secondary battery of the second aspect.

Although it is not clear why the above effects are achieved in the lithium ion secondary battery of the present disclosure (i.e., the lithium ion secondary batteries of the first and second aspects), the complex compound containing PF ₂ O ₂ ^- When added to a non-aqueous electrolyte, the structure of such a complex compound is stably maintained even in the non-aqueous electrolyte, and this complex compound increases the battery resistance of the lithium ion secondary battery at the initial stage and after charge/discharge cycles. It is thought that this contributes to the reduction of As a result, it is thought that the rate characteristics and cycle characteristics of the lithium ion secondary battery are improved.

In the lithium ion secondary battery of the present disclosure, the complex compound as an additive in the nonaqueous electrolyte 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, as an additive in the nonaqueous electrolyte. The complex compound may be added to the nonaqueous electrolyte in the lithium ion secondary battery after the lithium ion secondary battery is manufactured. For example, first, a lithium ion secondary battery is manufactured using a nonaqueous electrolyte that does not contain the complex compound of the present disclosure, and then the above complex compound is 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. 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 also include, for example, a positive electrode, a separator, a negative electrode, and a separator stacked in this order and wound in a layered manner. Also included is a wound-type lithium ion secondary battery having a structure in which

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 this lithium ion secondary battery 1, the positive electrode lead 21 and the negative electrode lead 22 are each led out from the inside of the exterior body 30 toward the outside in opposite directions.
The positive electrode lead 21 and the negative electrode lead 22 can be connected to the positive electrode current collector and the negative electrode current collector by welding (for example, ultrasonic welding, resistance welding, etc.).
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 thin plate-shaped positive electrode current collector 11A, and a separator. 13 and a negative electrode 12 in which a negative electrode composite layer 12B is formed on both main surfaces (i.e., front surface and back surface) of a thin plate-shaped negative electrode current collector 12A. (See Figure 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 of the lithium ion secondary battery of the present disclosure will be described below.

<Nonaqueous electrolyte>
The nonaqueous electrolyte includes a nonaqueous solvent, an electrolyte, and an additive.

(Additive)
The additive in the first aspect is a complex compound containing Li ⁺ , PF ₆ ⁻ , PF ₂ O ₂ ⁻ , and EC molecules, and is an exothermic compound that exhibits a maximum value of heat flow per unit mass in differential scanning calorimetry. It includes a complex compound in which peak A occurs within a temperature range of 190°C to 210°C, and exothermic peak B, which indicates the maximum value of heat flow per unit mass, occurs within a temperature range of 280°C to 300°C.

It is thought that the EC molecules in the complex compound in the first aspect contribute to stably maintaining the structure of the complex compound.
An example of the complex compound in the first aspect is a complex compound containing a LiPF ₆ site consisting of Li ⁺ and PF ₆ ^- , a LiDFP site consisting of Li ⁺ and PF ₂ O ₂ ^- , and a plurality of EC molecules (i.e., complex compound in the second embodiment). The preferred amount of EC molecules in this example will be described later as a molar ratio [EC/PF ₂ O ₂ ⁻ ].
The form of the complex compound according to this example may be in the form of a sol rather than in the form of dry powder.

-Exothermic peak A and exothermic peak B-
When performing differential scanning calorimetry (DSC) measurement on the complex compound in the first embodiment,
An exothermic peak A indicating the maximum value of heat flow per unit mass occurs within the range of 190 ° C. to 210 ° C., and
Exothermic peak B, which indicates the maximum value of heat flow per unit mass, appears within the range of 280°C to 300°C.

The DSC measurement in the first embodiment uses a heat flux type differential scanning calorimeter (for example, the differential scanning calorimeter "DSC7000X" manufactured by Hitachi High-Tech Science), with a heating rate of 5°C/min, and a measurement temperature range of 40°C to This refers to differential scanning calorimetry performed at 340°C.
The full width half maximum (FWHM) of the exothermic peak A is, for example, 30° C. or less, although it depends on the measurement resolution.
The FWHM of the exothermic peak B is, for example, 50° C. or lower, and preferably 45° C. or lower, although it depends on the resolution.

It is considered that exothermic peak A is mainly caused by the thermal decomposition of the LiPF ₆ site, and exothermic peak B is due to the thermal decomposition of the LiDFP site and the EC molecule.
That is, the phenomenon in which both the exothermic peak A and the exothermic peak B appear is a phenomenon peculiar to a complex compound containing the LiPF ₆ site, the LiDFP site, and the EC molecule.
Here, the exothermic peak A is not expressed in a complex compound consisting only of LiPF ₆ sites and EC molecules, nor is it expressed in a complex compound consisting only of LiDFP sites and EC molecules (for example, see Figure 2 described below). .

- Molar ratio [PF ₆ ^- /PF ₂ O ₂ ^- ] -
In the complex compound in the first aspect, the molar ratio [PF ₆ ⁻ /PF ₂ O ₂ ⁻ ] (i.e., the molar ratio of hexafluorophosphate ion to difluorophosphate ion) is preferably set from the viewpoint of stability of the complex compound. is 0.95 to 1.05.

-Molar ratio [EC/PF ₂ O ₂ ^- ])-
In the complex compound in the first aspect, the molar ratio [EC/PF ₂ O ₂ ⁻ ] (that is, the molar ratio of ethylene carbonate molecules to difluorophosphate ions) is preferably 10 to 50 from the viewpoint of stability of the complex compound. It is more preferably 18-50, still more preferably 20-40, even more preferably 25-35.
When the molar ratio [EC/PF ₂ O ₂ ^- ] is 10 or more, the stability of the complex compound is further improved in the prepared non-aqueous electrolyte, and as a result, the initial stage and The effect of reducing battery resistance after charge/discharge cycles is more effectively expressed.
When the molar ratio [EC/PF ₂ O ₂ ^- ] is 50 or less, the amount of EC brought into the non-aqueous electrolyte from the complex compound is limited to some extent, so the solvent composition in the non-aqueous electrolyte is restricted. less.

- Molar ratio [Li ⁺ /PF ₂ O ₂ ^- ] -
In the complex compound in the first aspect, the molar ratio [Li ⁺ /PF ₂ O ₂ ⁻ ] (that is, the molar ratio of lithium ions to difluorophosphate ions) is preferably 1.95 from the viewpoint of stability of the complex compound. ~2.05.

Although there are no particular limitations on the method for producing the complex compound in the first embodiment described above, production method A described below is preferred.

- An example of a method for producing a complex compound in the first embodiment (Production method A) -
^Next _{, a complex} ^{compound (} An example of the manufacturing method (hereinafter referred to as "manufacturing method A") of "complex compound A" will be described below.
Complex compound A is not particularly limited other than that it contains Li ⁺ , PF ₆ ^- , PF ₂ O ₂ ^- , and EC molecules.
The range of such complex compound A includes the above-mentioned complex compound of the present disclosure (i.e., in DSC measurement, exothermic peak A occurs within the range of 190°C to 210°C, and exothermic peak B occurs within the range of 280°C to 300°C). (complex compounds expressed within the range) are also included. However, complex compound A is not limited to being the complex compound of the present disclosure described above (that is, it is not limited to expressing exothermic peak A and exothermic peak B).

Production method A is to mix lithium hexafluorophosphate (LiPF ₆ ), lithium difluorophosphate (LiDFP), ethylene carbonate (EC), and organic solvent A with a boiling point of 150°C or lower, and then combine complex compound A and organic obtaining a mixture containing solvent A;
a step of removing organic solvent A from the mixture to obtain complex compound A;
including.
Manufacturing method A may include other steps as necessary.

-Process of obtaining a mixture-
The step of obtaining a mixture is to mix LiPF ₆ , LiDFP, EC, and an organic solvent A having a boiling point of 150° C. or less to form a complex compound A (i.e., Li ⁺ , PF ₆ ⁻ , PF ₂ O ₂ ⁻ This is a step of obtaining a mixture containing a complex compound A) containing an EC molecule and an organic solvent A.

In the step of obtaining the mixture,
The charging molar ratio of LiPF ₆ to LiDFP (i.e., charging molar ratio [LiPF ₆ /LiDFP]) is 0.95 to 1.05,
The charging molar ratio of EC to LiDFP (ie, charging molar ratio [EC/LiDFP]) is 10 to 50.

When the charging molar ratio [LiPF ₆ /LiDFP] is 0.95 to 1.05, complex compound A with excellent stability is formed.

When the charging molar ratio [EC/LiDFP] is 10 to 50, complex compound A with excellent stability can be obtained, and there are fewer restrictions on the solvent composition of the nonaqueous electrolyte.
The charging molar ratio [EC/LiDFP] is preferably 18-50, more preferably 20-40, even more preferably 25-35.

The organic solvent A is not particularly limited as long as it has a boiling point of 150° C. or less, but carbonate is preferred, and chain carbonate is more preferred.

Organic solvent A more preferably contains at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC).
In this case, the proportion of the total amount of DMC, DEC, and EMC in the total amount of organic solvent A is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and Preferably it is 80% by mass to 100% by mass.

In the step of obtaining the mixture, the charged molar ratio of organic solvent A to ethylene carbonate (hereinafter also referred to as "charged molar ratio [organic solvent A/EC]") is preferably 0.2 to 1.5.
When the charging molar ratio [organic solvent A/EC] is 0.2 or more, EC becomes easier to dissolve, and as a result, the formation property of complex compound A is further improved.
When the charging molar ratio [organic solvent A/EC] is 1.5 or less, organic solvent A can be more easily removed in the process of obtaining complex compound A described later, and as a result, organic solvent is present in complex compound A. Remaining of A as an impurity is further suppressed.

-Process of obtaining complex compound A-
The step of obtaining complex compound A is a step of removing organic solvent A from the above mixture to obtain complex compound A.
The method for removing organic solvent A from the above mixture is not particularly limited, and examples thereof include known methods such as drying (freeze drying, natural drying, heat drying, etc.), filtration, and decantation. Among these, freeze-drying is preferred from the viewpoint of quickly and stably removing the complex compound.

The content of the complex compound in the nonaqueous electrolyte of the present disclosure can be adjusted as appropriate.
When the nonaqueous electrolyte of the present disclosure includes the complex compound, the content of the LiDFP site in the complex compound (i.e., the content of the complex compound converted to the amount of LiDFP) is relative to the total amount of the nonaqueous electrolyte. , preferably 0.1% to 2.0% by weight, more preferably 0.2% to 1.0% by weight.
When the content of the LiDFP site is 0.1% by mass or more, the effect of the complex compound is more effectively exhibited.
When the content of the LiDFP site is 2.0% by mass or less, the electrical conductivity of the non-aqueous electrolyte is further reduced.

(Electrolytes)
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 non-aqueous electrolyte in the present disclosure may contain only one type of lithium salt, or may contain two or more types of lithium salt.

The concentration of the electrolyte in the non-aqueous electrolyte in 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)
Examples of the nonaqueous solvent include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, fluorine-containing chain carbonates, aliphatic carboxylic acid esters, fluorine-containing aliphatic carboxylic acid esters, and γ-lactones. At least one nonaqueous solvent selected from the group consisting of , fluorine-containing γ-lactones, cyclic ethers, fluorine-containing cyclic ethers, chain ethers, and fluorine-containing chain ethers can be used.

Examples of the cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate (BC).
Examples of the fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC).
Examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), dipropyl carbonate (DPC), and the like. .
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 and chain carbonates in the nonaqueous solvent is preferably 80% by mass or more, more preferably 90% by mass or more, and still 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 in 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 in 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 %.

The above-mentioned non-aqueous electrolyte of the present disclosure can be suitably used as a non-aqueous 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). can.
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.

(Preparation of non-aqueous electrolyte)
The above nonaqueous electrolyte can be produced, in principle, by mixing each component, that is, a nonaqueous solvent, an electrolyte, and a complex compound (and other components as necessary).
As mentioned above, the above additives can of course be mixed into the non-aqueous electrolyte before filling the battery, but the additives can also be mixed into the non-aqueous electrolyte filled into the battery after the battery is manufactured. It is also possible to mix.

<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.
When the thickness of the positive electrode composite layer is within the above range, sufficient lithium intercalation and desorption functions are likely to be 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.

<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 aluminum, copper, 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.
Copper foil is particularly preferably used as the negative electrode current collector. 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, sufficient lithium intercalation and desorption functions can be easily achieved 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 (eg, 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. Furnace made by Columbian 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. It's okay.
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.

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 .

<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.

<Electrical conductivity (second aspect)>
The lithium ion secondary battery according to the second aspect of the present disclosure includes:
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
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 additives are a LiPF ₆ site consisting of LiPF ₆ which is lithium hexafluorophosphate, a LiDFP site consisting of LiDFP (i.e. LiPF ₂ O ₂ ) which is lithium difluorophosphate, and an EC site consisting of EC which is ethylene carbonate. Contains a complex compound containing
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 of the non-aqueous electrolyte is adjusted so that the content of LiDFP is 0.4% by mass based on the total amount of the non-aqueous electrolyte and the concentration of LiPF ₆ is 1 mol/L, the electrical conductivity is 10.55 mScm ^-1 or less,
It is a lithium ion secondary battery.

The fact that the electrical conductivity is 10.55 mScm ^-1 or less means that the structure of the complex compound described above is stably maintained even in the non-aqueous electrolyte.
In the lithium ion secondary battery of the second aspect, the rate characteristics and cycle characteristics of the lithium ion secondary battery are improved because the electrical conductivity is 10.55 mScm ^-1 or less.

The lower limit of the electrical conductivity is, for example, 10.00 mScm ^-1 , 10.10 mScm ^-1 , 10.10 mScm ^-1 , or 10.30 mScm ^-1 .

The nonaqueous electrolyte in the second embodiment has a volume ratio [EC: (DMC, EMC, and DEC)] of 30:70, and a LiDFP content of 0.4% by mass based on the total amount of the nonaqueous electrolyte. However, the concentration of LiPF ₆ is not limited to 1 mol/L.
Specifically, the non-aqueous electrolyte in the second embodiment has a volume ratio [EC: (DMC, EMC, and DEC)] of 30:70, and the content of LiDFP is 0.5% relative to the total amount of the non-aqueous electrolyte. When the composition is adjusted so that the concentration of LiPF ₆ is 4% by mass and 1 mol/L, the electrical conductivity may be 10.55 mScm ⁻¹ or less.

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 %. Further, "wt%" is synonymous with "mass%".

[Example 1]
<Preparation of complex compound (hereinafter also referred to as "specific additive")>
(EC/LiPF ₆ /LiDFP=1.2/0.04/0.04 (molar ratio))
After purging with dry nitrogen gas, a 100 mL flask equipped with a stirrer, a thermometer, a gas inlet line, and an exhaust line was charged with ethylene carbonate (EC) (105.672 g; 1.20 mol) and organic solvent A. Dimethyl carbonate (DMC) (135.120 g; 1.50 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 reaction solution (ie, a mixture containing the specific additive and DMC) was obtained.
The resulting reaction solution was freeze-dried at -5°C for 12 hours to remove organic solvent A (DMC), resulting in a sol (116.06 g) with EC/LiPF ₆ /LiDFP=1.2/ A specific additive (ie, a complex compound) having a composition of 0.04/0.04 (molar ratio) was obtained.
The obtained specific additive was subjected to differential scanning calorimetry under the conditions described above using a differential scanning calorimeter "DSC7000X" manufactured by Hitachi High-Tech Science.
The results are shown in Figure 2.

[Comparative example 1]
<Preparation of comparative compound 1>
(EC/LiPF ₆ =1.2/0.04 (molar ratio))
The same operation as in the preparation of the specific additive was performed except that lithium difluorophosphate (LiDFP) (4.316 g; 0.04 mol) was not used, and as a solid product, EC/LiPF ₆ = 1.2/0 Comparative Compound 1 having a composition of .04 (molar ratio) was obtained.
The obtained solid product was subjected to differential scanning calorimetry in the same manner as the specific additive. The results are shown in Figure 2.

[Comparative example 2]
<Preparation of comparative compound 2>
(EC/LiDFP=1.2/0.04 (molar ratio))
The same operation as in the preparation of the specific additive was performed except that lithium hexafluorophosphate (LiPF ₆ ) (6.076 g; 0.04 mol) was not used, and as a solid product, EC/LiPF ₆ = 1.2 Comparative Compound 2 having a composition of /0.04 (molar ratio) was obtained.
The obtained solid product was subjected to differential scanning calorimetry in the same manner as the specific additive. The results are shown in Figure 2.

As shown in Figure 2, for the specific additive (EC/LiPF ₆ /LiDFP = 1.2/0.04/0.04), the maximum value of the heat flow rate (mW/g) per unit mass is around 200°C. An exothermic peak (i.e., exothermic peak A) existing at 287° C. and an exothermic peak (i.e., exothermic peak B) at which the maximum value of the heat flow rate per unit mass (mW/g) exists around 287°C were observed. .
The full width at half maximum (FWHM) of exothermic peak A was about 10°C, and the full width at half maximum (FWHM) of exothermic peak B was about 40°C.

Exothermic peak A was not observed in Comparative Compound 1 (EC/LiPF ₆ =1.2/0.04) and Comparative Compound 2 (EC/LiDFP = 1.2/0.04 (molar ratio)).
Exothermic peak B was not observed in comparative compound 2 (EC/LiDFP=1.2/0.04).
In Comparative Compound 1, a peak near 293°C (FWHM is about 30°C) similar to exothermic peak B was observed. Compared to the peak of Comparative Compound 1 near 293°C, the exothermic peak B of the additive was shifted to the lower temperature side by about 5°C and was a broader peak.

From the above results, the specific additive (EC/LiPF ₆ /LiDFP=1.2/0.04/0.04) is a complex compound consisting of EC, LiPF ₆ and LiDFP, and EC, LiPF ₆ and It was suggested that this compound has a different chemical structure from a compound consisting of one or two of LiDFP.

<Preparation of electrolyte>
[Example 2]
After adding the specific additive obtained in Example 1 to dimethyl carbonate (DMC) as a non-aqueous solvent, LiPF _{6 brought in from the specific additive is added so that LiPF 6 as} an electrolyte becomes 1M-LiPF ₆ . Ethylene carbonate (EC), which dissolves the amount of LiPF ₆ except for the amount of ethylene carbonate (EC) brought in from the above-mentioned specific additive, is added to the solvent composition of the final electrolyte solution at 30 vol. %EC [EC amount (ml)/Total solvent amount (ml) x 100 (vol.%)] By dissolving the amount of EC excluding the EC amount, preparation containing an electrolyte and a non-aqueous solvent A solution was obtained.

LiPF ₆ , EC, and DMC were added to the obtained preparation solution to prepare a non-aqueous electrolyte 1 containing 1M LiPF ₆ and 0.4 wt % LiDFP.

[Example 3]
LiPF ₆ , EC, and DMC were added to the above preparation solution to obtain a non-aqueous electrolyte 2 containing 1M LiPF ₆ and 0.8 wt% LiDFP.

A comparative electrolytic solution preparation solution was obtained in the same manner as in Example 2, except that Comparative Compound 1 was used instead of the specific additive.

[Comparative example 3]
LiPF ₆ , EC, DMC, and LiDFP were added to the obtained comparative electrolyte preparation solution to obtain a comparative non-aqueous electrolyte 1 containing 1M of LiPF ₆ and 0.4 wt % of LiDFP.

[Comparative example 4]
LiPF ₆ , EC, DMC, and LiDFP were added to the comparative electrolyte preparation solution to obtain a comparative non-aqueous electrolyte 2 containing 1 M of LiPF6 and 0.8 wt % of LiDFP.

[Comparative example 5]
LiPF ₆ , EC, and DMC were added to the comparative electrolyte preparation solution to obtain a comparative nonaqueous electrolyte 3 containing LiPF ₆ at a ratio of 1M.

For the above non-aqueous electrolytes 1 and 2 and comparative non-aqueous electrolytes 1 to 3, the volume ratio of the non-aqueous solvents EC and DMC was changed, and the non-aqueous The electrolyte was used as an electrolyte and the electrical conductivity of each was measured.
The measurement results are shown in Table 1.
Further, FIG. 3 shows a graph showing the relationship between the volume ratio of EC and the electrical conductivity.

A comparison between Example 2 and Comparative Example 3, and a comparison between Example 3 and Comparative Example 4 shows that even in the non-aqueous electrolyte, the excess EC contained in the complex compound does not dissociate and maintains its structure. I understand that.
It can be seen that the electrical conductivity is improved when the volume ratio of EC:DMC in the non-aqueous solvent is 25:75 to 35:65.

[Example 4]
<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 pressed using a small press to a pressing density of 2.9 g/cm ³ . A positive electrode original fabric was obtained by compressing it so that it became .
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 nonaqueous electrolyte 1 of Example 2 was injected into the interior of 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 in 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.

[Comparative example 6]
A lithium ion secondary battery was obtained in the same manner as in Example 4, except that Comparative Nonaqueous Electrolyte 1 of Comparative Example 3 was used in place of Nonaqueous Electrolyte 1 of Example 2.

[Comparative Example 7]
A lithium ion secondary battery was obtained in the same manner as in Example 4, except that Comparative Nonaqueous Electrolyte 3 of Comparative Example 5 was used in place of Nonaqueous Electrolyte 1 of Example 2.

<Analysis of electrode active material surface>
After the activation treatment, the surfaces of the positive and negative electrodes of the lithium ion secondary batteries of Example 4 and Comparative Example 7 were subjected to elemental analysis by XPS. The XPS analysis spectrum is shown in FIG. 4, and the XPS analysis results are shown in Tables 2 and 3.

From FIG. 4, compared to the lithium ion secondary battery of Comparative Example 7, the lithium ion secondary battery of Example 4 has more fluorine on the low energy side (lower binding energy) on the surfaces of the positive and negative electrodes. It is presumed that the difluorophosphoric acid residue of the complex compound decomposes on the electrode surface to form a film. Further, from Tables 2 and 3, it can be seen that the battery of Example 4 contains more phosphorus than the battery of Comparative Example 7.
These results indicate that the surface of the electrode active material is affected by complex compounds formed using hexafluorophosphoric acid residues, difluorophosphoric acid residues, and ethylene carbonate (EC) molecules as ligands, or their decomposition products. It is assumed that there are.

After the activation treatment, the batteries of Example 4 and Comparative Examples 6 and 7 were subjected to a charge/discharge cycle treatment.
The charge/discharge cycle treatment was performed at room temperature for 100 cycles, each cycle consisting of charging to 4.2V at 0.1C-CCCV and discharging to 2.8V at 0.1C-CC. ) was stored at room temperature for 5 days in a fully charged state.

Cross-sectional TEM images of the positive electrodes of the lithium ion secondary batteries of Example 4 and Comparative Example 7 after the charge/discharge cycle treatment were taken, and the positive electrode active materials were excavated from the surface to the inside while nickel (Ni), manganese (Mn), and cobalt were added. Transmission electron energy loss spectroscopy of (Co) and oxygen (O) was measured.

Figure 5(a) shows a TEM image of the cross section of the positive electrode of the lithium ion secondary battery of Example 4 after the charge/discharge cycle treatment, and Figure 5(b) shows a TEM image of the cross section of the positive electrode of the lithium ion secondary battery of Comparative Example 7. ).
Compared to the positive electrode active material of the lithium ion secondary battery of Comparative Example 7, the positive electrode active material of the lithium ion secondary battery of Example 4 had a surface change due to the charge/discharge cycle treatment (indicated by the dotted line in the TEM image). It can be seen that the difference in contrast) is small.

In addition, FIG. 6(a) shows a spectrum diagram showing the transmission electron energy loss of oxygen in the positive electrode active material of the lithium ion secondary battery of Example 4 after the charge/discharge cycle treatment, and FIG. A spectrum diagram showing the transmission electron energy loss of oxygen in the positive electrode active material of a lithium ion secondary battery is shown in FIG. 6(b).

In the oxygen spectrum diagram shown in FIG. 6, the lower spectrum is the surface side of the active material, and the upper spectrum is the inside side of the active material.
Focusing on the locations indicated by arrows in the spectrum shown in Figure 6, in Example 4 (a), a large change in the shape of the spectrum is observed from the surface (lowermost spectrum) to the interior (upper spectrum). However, in Comparative Example 7 shown in (b), the shape of the spectrum near the surface (lowest and second from the bottom) is significantly different from the shape of the spectrum inside (third from the bottom).

From the results shown in FIGS. 5 and 6, the lithium ion secondary battery of Example 4 after the charge/discharge cycle treatment is characterized by the desorption of oxygen from the crystal structure of the positive electrode active material accompanied by oxidation of the electrolyte, and the loss of Ni, Co, Mn, etc. It is presumed that changes in the crystal structure due to desorption from the crystal structure of the positive electrode active material accompanying the elution of transition metals are suppressed.
On the other hand, in the lithium ion secondary battery of Comparative Example 7 after the charge/discharge cycle treatment, the crystal structure on the surface of the positive electrode active material was changed due to desorption of oxygen in the positive electrode active material, elution of transition metals, etc. It is assumed that this change in crystal structure is the cause of the increase in resistance.

<Performance evaluation>
Regarding the lithium ion secondary batteries of Example 4 and Comparative Examples 6 and 7 after the activation treatment, initial battery resistance values, rate characteristics after cycle tests, and battery resistance values were measured as described below.

(Measurement of discharge capacity maintenance rate)
FIG. 7 shows the relationship between the number of charge/discharge cycles and the discharge capacity retention rate (cycle characteristics) of the lithium ion secondary batteries of Example 4 and Comparative Examples 6 and 7 after the activation treatment.
From FIG. 7, it can be seen that in the lithium ion secondary battery of Example 4, the decrease in discharge capacity after the charge/discharge cycle was suppressed.

(Measurement of rate characteristics)
The batteries of each example were charged to a constant voltage of 4.2 V at room temperature, and then discharged to 2.8 V at a constant current of 0.1 C at room temperature, and the discharge capacity was measured. Similarly, the battery was charged to a constant voltage of 4.2V at room temperature, and discharged at xC rate constant current with x=0.2, 0.5, and 1.0, and the discharge capacity of each C rate was measured. The discharge capacity maintenance rate was calculated from the discharge capacity when discharging at each C rate using the discharge capacity of 0.1 C constant current as a reference.
The relationship between the initial C rate and the discharge capacity retention rate is shown in FIG. 8, and the relationship between the C rate and the discharge capacity retention rate after the cycle test is shown in FIG.
From FIGS. 8 and 9, it can be seen that the lithium ion secondary battery of Example 4 is capable of rapid charging and discharging.

(Measurement of battery resistance (DC-IR))
The batteries of each example were charged at room temperature to a constant voltage of 4.2 V, then discharged at room temperature at a constant current of 0.1 C, and the potential drop was measured for 10 seconds from the start of discharge. Similarly, the battery was charged to a constant voltage of 4.2 V 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.
FIG. 10 shows the rate of increase in battery resistance, with the lithium ion secondary battery of Comparative Example 7 set as 100.
From FIG. 10, it can be seen that in the lithium ion secondary battery of Example 4, the increase in battery resistance was suppressed.

(Measurement of impedance characteristics)
The batteries of each example were charged to a constant voltage of 4.2 V at room temperature, and the impedance at each frequency was measured by the AC impedance method.
The initial Nyquist plot is shown in FIG. 11, and the Nyquist plot after the cycle test is shown in FIG. 12.

The semicircular portion on the left side of the Nyquist plot represents the charge transfer process, and the linear portion on the right side represents the mass transfer process.

From FIG. 11, it can be seen that in the lithium ion secondary battery of Example 4, the semicircular portion representing the charge transfer process is small, and the charge transfer resistance is small.

In addition, as can be seen from the comparison between FIGS. 11 and 12, the lithium ion secondary battery of Example 4 has a small change in the Nyquist plot, and the lithium ion secondary battery of Example 4 suppresses battery deterioration. I understand 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 (2)

A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
The additive contains lithium ions, hexafluorophosphate ions, difluorophosphate ions, and ethylene carbonate molecules, and in differential scanning calorimetry, the exothermic peak A indicating the maximum value of heat flow per unit mass is 190. Comprising a complex compound that occurs within a temperature range of 280°C to 300°C, and exhibits an exothermic peak B showing a maximum value of heat flow per unit mass within a temperature range of 280°C to 300°C.
Lithium ion secondary battery. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte, and an additive,
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 additive is a complex compound containing 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. including,
The nonaqueous electrolyte _has a 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, and the composition is adjusted so that the concentration of LiPF ₆ is 1 mol/L. , the electrical conductivity is 10.55 mScm ⁻¹ or less,
Lithium ion secondary battery.