Lithium ion secondary battery and lithium ion secondary battery module

Abstract

A lithium ion secondary battery (10) provided with a positive electrode that includes a positive electrode active material layer (1), a negative electrode that includes a negative electrode active material layer (2), and an electrolyte, wherein: the negative electrode active material included in the negative electrode active material layer (2) has an SEI film on at least a portion of the surface thereof; the electrolyte contains lithium bis(fluorosulfonyl)imide; and the lithium bis(fluorosulfonyl)imide concentration in the electrolyte determined using a prescribed method is at least 0.6 mass%.

Description

Lithium-ion secondary battery and lithium-ion secondary battery module

[0001] The present invention relates to a lithium ion secondary battery and a lithium ion secondary battery module.

[0002] A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte. Techniques relating to lithium ion secondary batteries are described in Patent Documents 1 and 2.

[0003] Patent Document 1 discloses that lithium secondary batteries have various inherent problems, some of which are related to the manufacturing and operating characteristics of the negative electrode, and discloses that the secondary battery exhibits excellent life characteristics and safety by providing a positive electrode in which a positive electrode mixture containing a positive electrode active material is applied to a current collector; a negative electrode in which a negative electrode mixture containing a negative electrode active material is applied to a current collector, the negative electrode including a carbon-based material and a silicon-based compound; and an electrolyte including a lithium salt and a non-aqueous solvent, the electrolyte including a cyclic carbonate and / or a linear solvent.

[0004] Patent Document 2 aims to provide a lithium ion secondary battery having a high capacity and excellent charge / discharge cycle characteristics, and describes a lithium ion secondary battery having a general composition formula of Li ａ Ni （１－ｂ－ｃ） Co ｂ M ｃ O ２ (where M is at least one element of Mn and Al, and 0.9≦a≦1.3, 0<b, 0<c, b+c≦0.5); a negative electrode containing, as a negative electrode active material, a composite of a material containing Si and O as constituent elements (where the atomic ratio x of O to Si is 0.5≦x≦1.5) and a conductive material, and a carbon material other than the conductive material contained in the composite; and a non-aqueous electrolyte containing a cyclic carbonate having a C═C double bond and a halogen-substituted cyclic carbonate, wherein the relationship between the proportion of the composite in the negative electrode active material and the content of each cyclic carbonate in the non-aqueous electrolyte before and after initial charge and discharge is specified.

[0005] JP 2018-88419 A JP 2011-233368 A

[0006] The present invention provides a lithium ion secondary battery with improved cycle characteristics.

[0007] The present inventors have conducted extensive research to achieve the above object, and as a result, have found that the cycle characteristics of a lithium ion secondary battery can be improved by adjusting the concentration of lithium bis(fluorosulfonyl)imide in an electrolyte solution in the lithium ion secondary battery to a specific range, thereby completing the present invention.

[0008] [1] A lithium ion secondary battery comprising: a positive electrode including a positive electrode active material layer; a negative electrode including a negative electrode active material layer; and an electrolyte solution, wherein the negative electrode active material included in the negative electrode active material layer has an SEI film on at least a portion of its surface; the electrolyte solution contains lithium bis(fluorosulfonyl)imide; and the concentration of the lithium bis(fluorosulfonyl)imide in the electrolyte solution is 0.6 mass% or more according to the following <Method 1>. <Method 1> The lithium ion secondary battery is disassembled under an inert gas atmosphere, and 0.2 g of the electrolyte solution collected from the disassembled lithium ion secondary battery is dissolved in 1.0 g of deuterated acetonitrile to prepare a first sample. Next, 0.002 g of hexafluorobenzene is added to the first sample as a reference material to prepare a second sample. Next, 0.5 g of the second sample is used to １９The amount of lithium bis(fluorosulfonyl)imide in the second sample is measured by F-NMR, and then the concentration of lithium bis(fluorosulfonyl)imide in the first sample is calculated from the amount of lithium bis(fluorosulfonyl)imide in the second sample. The concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution is calculated using the following formula (1): Formula (1): Concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution = (Amount of lithium bis(fluorosulfonyl)imide in the first sample) / (Amount of the electrolyte solution collected from the disassembled lithium ion secondary battery) x 100. [2] The lithium ion secondary battery according to [1], wherein the negative electrode active material includes a negative electrode active material (A) and a negative electrode active material (B) of a type different from the negative electrode active material (A). [3] The volume-based median diameter D of the negative electrode active material (A) measured by a laser diffraction scattering method is ５０ is 3.0 μm or more and 30.0 μm or less, and the volume-based median diameter D of the negative electrode active material (B) measured by a laser diffraction scattering method ５０ [4] The lithium ion secondary battery according to [2] above, wherein the negative electrode active material (A) contains graphite powder. [5] The graphite powder is a graphite powder (A1) and a graphite powder (A2) having a volume-based median diameter D ５０ and a graphite powder (A2) different from the graphite powder (A1), wherein the median diameter D of the graphite powder (A1) is ５０ is the median diameter D of the graphite powder (A2). ５０ [6] The lithium ion secondary battery according to [5], wherein the graphite powder (A1) contains graphite particles containing amorphous carbon on the surface thereof. [7] The lithium ion secondary battery according to [5] or [6], wherein the graphite powder (A2) contains graphite particles not containing amorphous carbon on the surface thereof. [8] The median diameter D of the graphite powder (A1) ５０[9] The lithium ion secondary battery according to any one of [5] to [7], wherein the median diameter D of the graphite powder (A2) is 5.0 μm or more and 30.0 μm or less. ５０

[10] The lithium ion secondary battery according to any one of [5] to [8], wherein the median diameter D of the graphite powder (A1) is 1.0 μm or more and 20.0 μm or less. ５０ D １ , the median diameter D of the graphite powder (A2) ５０ D ２ When we do this, D １ D against ２ Ratio D ２ / D １ is 0.40 or more and less than 1.0.

[11] The lithium ion secondary battery according to any of [5] to [9] above, wherein the content of the graphite powder (A1) in the negative electrode active material (A) is 50 parts by mass or more and 200 parts by mass or less, relative to 100 parts by mass of the content of the graphite powder (A2) in the negative electrode active material (A).

[12] The lithium ion secondary battery according to any of [2] to

[11] above, wherein the negative electrode active material (B) contains one or more particles selected from the group consisting of silicon oxide particles and Si-C composite particles containing silicon and a carbon material.

[13] The lithium ion secondary battery according to

[12] above, wherein the carbon material in the Si-C composite particles contains a porous carbon material, and the silicon is present in at least a portion of the pores of the porous carbon material.

[14] The volume-based median diameter D of the Si-C composite particles measured by a laser diffraction scattering method is ５０

[15] The lithium ion secondary battery according to any one of [2] to

[14] , wherein the content of the negative electrode active material (A) in the negative electrode active material layer is 50 parts by mass or more and 99 parts by mass or less, when the total amount of the negative electrode active material layer is 100 parts by mass.

[16] The lithium ion secondary battery according to any one of [2] to

[15] , wherein the content of the negative electrode active material (B) in the negative electrode active material layer is 1 part by mass or more and 50 parts by mass or less, when the total amount of the negative electrode active material layer is 100 parts by mass.

[17] The lithium ion secondary battery according to any one of [2] to

[15] , wherein the content of the negative electrode active material (A) in the negative electrode active material layer is W Ａ The content of the negative electrode active material (B) in the negative electrode active material layer is W Ｂ When we do this, W Ｂ W against Ａ Ratio of W Ａ / W Ｂ

[18] The lithium ion secondary battery according to any one of [1] to

[17] , wherein the positive electrode active material contained in the positive electrode active material layer comprises particles (C) containing one or more kinds selected from particles (C1) constituted by a single crystal of a lithium-nickel-cobalt-manganese composite oxide and particles (C2) constituted by a polycrystal of a lithium-nickel-cobalt-manganese composite oxide.

[19] The lithium ion secondary battery according to

[18] , wherein the content of nickel in the particles (C) is 80 mol or more when the total content of nickel, cobalt, and manganese is 100 mol.

[20] The lithium ion secondary battery according to

[18] or

[19] , wherein, when the total content of nickel, cobalt, and manganese in the particles (C) is taken as 100 mol, the content of nickel is 80 mol or more and 99 mol or less, the content of cobalt is 0.5 mol or more and 10 mol or less, and the content of manganese is 0.5 mol or more and 10 mol or less.

[21] The capacity retention rate R at 25°C measured by the following <Method 2> ２５The lithium ion secondary battery according to any one of [1] to

[20] , wherein the capacity retention rate R is 87.0% or more. <Method 2> The lithium ion secondary battery is placed in a thermostatic chamber at 25°C, and then the lithium ion secondary battery is charged and discharged in accordance with the following <charge and discharge cycle>, and the first discharge capacity is measured. Next, the lithium ion secondary battery is repeatedly charged and discharged in accordance with the following <charge and discharge cycle> until a total of 499 cycles have been completed. Next, the lithium ion secondary battery is charged and discharged in accordance with the following <charge and discharge cycle>, and the 500th discharge capacity is measured. Next, the capacity retention rate R is calculated from the following formula (2): ２５ The capacity retention rate R is calculated using the formula (2). ２５ = (the 500th discharge capacity) / (the first discharge capacity) × 100 <Charge-discharge cycle> The lithium ion secondary battery is charged at 30 mA until an upper limit voltage of 4.25 V is reached, and then, after the upper limit voltage of 4.25 V is reached, the lithium ion secondary battery is charged at a constant voltage until 2.5 hours have elapsed since the start of charging, and then, the lithium ion secondary battery is discharged at a constant current of 30 mA until a lower limit voltage of 2.5 V is reached.

[22] The lithium ion secondary battery according to any one of [1] to

[21] , wherein the content of the electrolyte solution in the lithium ion secondary battery is 15 parts by mass or more and 60 parts by mass or less, where the sum of the content of the negative electrode active material and the content of the positive electrode active material contained in the positive electrode active material layer is 100 parts by mass.

[23] A lithium ion secondary battery module comprising the lithium ion secondary battery according to any one of [1] to

[22] .

[0009] According to the present invention, a lithium ion secondary battery with improved cycle characteristics can be provided.

[0010] 1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to an embodiment of the present invention.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. To avoid complexity, when multiple identical components are present in the same drawing, only one of the components may be labeled with a reference symbol, and not all of the components may be labeled with a reference symbol. The drawings are for illustrative purposes only. The shapes and dimensional ratios of each component in the drawings do not necessarily correspond to actual products. In this specification, the term "poly(meth)acrylic acid" refers to a concept that encompasses both polyacrylic acid and polymethacrylic acid. Each component in this embodiment may be used alone or in combination with two or more types. In addition, the term "to" indicating a numerical range means "greater than or equal to" or "less than or equal to," and includes both the upper and lower limits. In this specification, SOC refers to "State of Charge." In this specification, current C refers to C rate.

[0012] The present invention will be described below based on embodiments.

[0013] (Lithium-ion secondary battery) In this embodiment, the lithium-ion secondary battery includes a positive electrode including a positive electrode active material layer, a negative electrode including a negative electrode active material layer, and an electrolyte. In this lithium-ion secondary battery, the negative electrode active material included in the negative electrode active material layer has an SEI film on at least a portion of its surface. In this lithium-ion secondary battery, the electrolyte includes lithium bis(fluorosulfonyl)imide, and the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is 0.6 mass% or more according to the following <Method 1>. <Method 1>: The lithium-ion secondary battery is disassembled under an inert gas atmosphere. Next, 0.2 g of the electrolyte collected from the disassembled lithium-ion secondary battery is dissolved in 1.0 g of deuterated acetonitrile to prepare a first sample. Next, 0.002 g of hexafluorobenzene is added to the first sample as a reference material to prepare a second sample. Next, 0.5 g of the second sample is used to １９The amount of lithium bis(fluorosulfonyl)imide in the second sample is measured by F-NMR. Next, the concentration of lithium bis(fluorosulfonyl)imide in the first sample is calculated from the amount of lithium bis(fluorosulfonyl)imide in the second sample. Next, the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution is calculated using the following formula (1): Formula (1): Concentration of lithium bis(fluorosulfonyl)imide in electrolyte solution = (amount of lithium bis(fluorosulfonyl)imide in the first sample) / (amount of electrolyte solution collected from the disassembled lithium ion secondary battery) x 100. By having the above-described configuration, the lithium ion secondary battery of this embodiment can improve its cycle characteristics.

[0014] <Configuration of Lithium-Ion Secondary Battery> Next, the configuration of the lithium-ion secondary battery of this embodiment will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing an example of the lithium-ion secondary battery of this embodiment. In this embodiment, the lithium-ion secondary battery 10 includes a positive electrode including a positive electrode active material layer 1, a negative electrode including a negative electrode active material layer 2, and an electrolyte. A separator 5 can be provided between the positive electrode and the negative electrode. A plurality of electrode pairs of a positive electrode and a negative electrode can be provided.

[0015] The lithium-ion secondary battery 10 has a positive electrode including a positive electrode current collector 3 and a positive electrode active material layer 1 containing a positive electrode active material provided thereon. The lithium-ion secondary battery 10 also has a negative electrode including a negative electrode current collector 4 and a negative electrode active material layer 2 containing a negative electrode active material provided thereon. The positive electrode and negative electrode are stacked, for example, with a separator 5 interposed between them, such that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. An electrode pair consisting of the positive electrode and the negative electrode is housed in a container formed by an outer casing 6 and an outer casing 7. A positive electrode tab 9 is connected to the positive electrode current collector 3, and a negative electrode tab 8 is connected to the negative electrode current collector 4. The positive electrode tab 9 and the negative electrode tab 8 are extended outside the container. An electrolyte solution is injected and sealed within the container. The container may also house an electrode group in which multiple electrode pairs are stacked.

[0016] <Method for manufacturing lithium ion secondary battery> In this embodiment, the lithium ion secondary battery can be manufactured according to a known method. For example, a laminate or a wound body can be used as the electrode. As the exterior body, a metal exterior body or an aluminum laminate exterior body can be appropriately used. The shape of the battery may be any shape such as a laminate type, coin type, button type, sheet type, cylindrical type, square type, or flat type, but a laminate type battery is preferred.

[0017] Next, the lithium ion secondary battery will be described by dividing it into its components.

[0018] <Electrolyte> In the lithium ion secondary battery of this embodiment, the electrolyte contains an electrolyte and an organic solvent. In the lithium ion secondary battery of this embodiment, the electrolyte contains lithium bis(fluorosulfonyl)imide (hereinafter also referred to as LiFSI) as the electrolyte.

[0019] In the lithium ion secondary battery of this embodiment, the electrolyte other than LiFSI is, for example, lithium hexafluorophosphate (LiPF ６ ), lithium difluorobis(oxalato)phosphate (LiDODFP), lithium difluorophosphate (LiPO ２ F ２ ), LiClO ４ , LiBF ４ , LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiB １０ Cl １０ , LiAlCl ４ , LiCl, LiBr, LiB(C ２ H ５ ) ４ , C.H. ３ SO ３ Li, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ２N, lithium salts of lower fatty acids, and lithium salts of lower carboxylic acids, and preferably LiPF ６ and LiCl, more preferably LiPF ６ Includes.

[0020] In the lithium ion secondary battery of this embodiment, the organic solvent is not particularly limited as long as it can dissolve the electrolyte. Examples of the organic solvent include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC); lactones such as γ-butyrolactone and γ-valerolactone; ethers such as trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; sulfoxides such as dimethyl sulfoxide; oxolanes such as 1,3-dioxolane and 4-methyl-1,3-dioxolane; and acetonitrile. nitrogen-containing compounds such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, ethyl propionate, and the like; phosphate esters such as phosphate triesters; diglymes; triglymes; sulfolanes such as sulfolane and methyl sulfolane; oxazolidinones such as 3-methyl-2-oxazolidinone; and sultones such as 1,3-propane sultone, 1,4-butane sultone, and naphtha sultone, and preferably contains carbonates, and more preferably contains one or two or more selected from the group consisting of EC and EMC.

[0021] In the lithium-ion secondary battery of this embodiment, the electrolyte solution may further contain an additive, such as one or more additives selected from the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfite (ES).

[0022] In the lithium ion secondary battery of this embodiment, the concentration of LiFSI in the electrolyte solution during production of the lithium ion secondary battery (hereinafter also referred to as LiFSI concentration (at production)) is preferably 0.7 mass % or more and 4.8 mass % or less, more preferably 0.8 mass % or more and 4.5 mass % or less, even more preferably 0.9 mass % or more and 4.0 mass % or less, still more preferably 1.0 mass % or more and 3.5 mass % or less, and still more preferably 1.0 mass % or more and 3.2 mass % or less, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0023] In the lithium ion secondary battery of this embodiment, LiPF in the electrolyte solution during the production of the lithium ion secondary battery ６ The concentration of (hereinafter referred to as LiPF ６ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the concentration of the lithium ion secondary battery (also referred to as the concentration (at the time of preparation)) is preferably 1.0 mass % or more, more preferably 5.0 mass % or more, even more preferably 10.0 mass % or more, even more preferably 10.5 mass % or more, even more preferably 11.0 mass % or more, and even more preferably 11.5 mass % or more.

[0024] In the lithium ion secondary battery of this embodiment, LiPF ６ From the viewpoint of reducing corrosion of battery components, the concentration (at the time of preparation) is preferably 20.0 mass % or less, more preferably 18.0 mass % or less, even more preferably 15.0 mass % or less, even more preferably 14.0 mass % or less, even more preferably 13.0 mass % or less, and even more preferably 12.5 mass % or less.

[0025] In the lithium ion secondary battery of this embodiment, LiPF ６ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery and reducing corrosion of the battery components, the concentration (at the time of preparation) is preferably 1.0 mass % or more and 20.0 mass % or less, more preferably 5.0 mass % or more and 18.0 mass % or less, even more preferably 10.0 mass % or more and 15.0 mass % or less, even more preferably 10.5 mass % or more and 14.0 mass % or less, even more preferably 11.0 mass % or more and 13.0 mass % or less, and even more preferably 11.5 mass % or more and 12.5 mass % or less.

[0026] In the lithium ion secondary battery of this embodiment, the content of the electrolyte solution in the lithium ion secondary battery is preferably 15 parts by mass or more and 60 parts by mass or less, more preferably 20 parts by mass or more and 55 parts by mass or less, even more preferably 25 parts by mass or more and 50 parts by mass or less, and even more preferably 30 parts by mass or more and 45 parts by mass or less, when the total of the content of the negative electrode active material and the content of the positive electrode active material in the lithium ion secondary battery is taken as 100 parts by mass, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0027] <Negative Electrode> The negative electrode of this embodiment includes a negative electrode active material layer. From the viewpoint of further improving the battery performance of the lithium ion secondary battery, the negative electrode of this embodiment preferably includes a negative electrode current collector and the negative electrode active material layer of this embodiment.

[0028] In the lithium-ion secondary battery of this embodiment, the negative electrode active material contained in the negative electrode active material layer has an SEI (Solid Electrolyte Interphase) film on at least a portion of its surface, preferably on the entire surface. Generally, the SEI film is formed by decomposition of the electrolyte on the surface of the negative electrode during initial charge / discharge. Specifically, the SEI film is a film formed on at least a portion of the surface of the negative electrode active material by decomposition products of the electrolyte and additives to the electrolyte. The SEI film functions to insert lithium ions into or extract them from the negative electrode, while also reducing further decomposition of the electrolyte on the surface of the negative electrode.

[0029] For example, an SEI film can be formed on at least a portion of the surface of the negative electrode (negative electrode active material) by the method described in "Initial charge and discharge of lithium ion secondary battery" in the Examples. That is, the lithium ion secondary battery of this embodiment has an SEI film on at least a portion of the surface of the negative electrode active material contained in the negative electrode active material layer, and is therefore a lithium ion secondary battery after at least initial charge and discharge.

[0030] <Negative electrode active material layer> From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the negative electrode active material layer of the present embodiment preferably contains the negative electrode active material of the present embodiment and a binder, and more preferably contains the negative electrode active material of the present embodiment, a binder, and a conductive additive.

[0031] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the thickness of the negative electrode active material layer of the present embodiment is preferably 10 μm or more and 250 μm or less, more preferably 15 μm or more and 200 μm or less, even more preferably 20 μm or more and 100 μm or less, and still more preferably 25 μm or more and 75 μm or less.

[0032] The density of the negative electrode active material layer of this embodiment is preferably 0.50 g / cm from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery. ３ 3.00g / cm or more ３ or less, more preferably 1.00 g / cm ３ 2.50g / cm or more ３ More preferably, 1.30 g / cm or less ３ 2.00g / cm or more ３ The following is the result.

[0033] <Negative Electrode Active Material> The negative electrode active material of this embodiment preferably includes one or more materials selected from the group consisting of a carbon material, a lithium-based metal material, a Si-based material, and a conductive polymer material, more preferably includes one or more materials selected from the group consisting of a carbon material and a Si-based material, and even more preferably includes both a carbon material and a Si-based material. Examples of the carbon material of this embodiment include graphite powder, hard carbon, soft carbon, or any mixture thereof, and preferably includes graphite powder. Examples of the Si-based material of this embodiment include silicon oxide particles and Si-C composite particles containing silicon and a carbon material (hereinafter also referred to as Si-C composite particles), and preferably includes Si-C composite particles from the viewpoint of further improving the cycle characteristics of a lithium-ion secondary battery.

[0034] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the negative electrode active material in the negative electrode active material layer of this embodiment is preferably 50.0 parts by mass or more and 100.0 parts by mass or less, more preferably 75.0 parts by mass or more and 99.0 parts by mass or less, even more preferably 85.0 parts by mass or more and 98.5 parts by mass or less, even more preferably 90.0 parts by mass or more and 98.0 parts by mass or less, and still more preferably 95.0 parts by mass or more and 97.5 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0035] In the lithium ion secondary battery of the present embodiment, the negative electrode active material of the present embodiment preferably includes a negative electrode active material (A) and a negative electrode active material (B) of a type different from the negative electrode active material (A), from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0036] <Negative Electrode Active Material (A)> In the lithium ion secondary battery of this embodiment, the negative electrode active material (A) preferably contains a carbon material, more preferably contains graphite powder, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0037] In the lithium ion secondary battery of this embodiment, the volume-based median diameter D of the negative electrode active material (A) measured by a laser diffraction scattering method is ５０ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the average particle size is preferably 3.0 μm or more and 30.0 μm or less, more preferably 5.0 μm or more and 27.5 μm or less, even more preferably 7.5 μm or more and 22.5 μm or less, even more preferably 8.0 μm or more and 15.5 μm or less, and even more preferably 9.0 μm or more and 12.0 μm or less.

[0038] From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the graphite powder of this embodiment preferably contains graphite powder (A1) and a type of graphite powder (A1') different from the graphite powder (A1). The graphite powder of this embodiment more preferably contains graphite powder (A1) and a type of graphite powder (A1') different from the graphite powder (A1) that has a volume-based median diameter D ５０ and a graphite powder (A2) different from the graphite powder (A1), wherein the median diameter D ５０ is the median diameter D of the graphite powder (A2)５０ Greater than.

[0039] From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the graphite powder (A1) preferably contains graphite particles containing amorphous carbon on the surface, and more preferably contains artificial graphite particles containing amorphous carbon on the surface.

[0040] From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the graphite powder (A2) preferably contains graphite particles whose surfaces do not contain amorphous carbon, and more preferably contains artificial graphite particles whose surfaces do not contain amorphous carbon.

[0041] From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the graphite powder (A1') preferably contains graphite particles having no amorphous carbon on the surface thereof, and more preferably contains artificial graphite particles having no amorphous carbon on the surface thereof. The graphite powder (A1') may contain the graphite powder (A2).

[0042] In the lithium ion secondary battery of this embodiment, the volume-based median diameter D of the graphite powder (A1) measured by a laser diffraction scattering method ５０ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the average particle size is preferably 5.0 μm or more and 30.0 μm or less, more preferably 6.0 μm or more and 27.5 μm or less, even more preferably 7.5 μm or more and 25.0 μm or less, even more preferably 9.0 μm or more and 20.0 μm or less, even more preferably 11.0 μm or more and 18.0 μm or less, and even more preferably 12.0 μm or more and 16.0 μm or less.

[0043] In the lithium ion secondary battery of this embodiment, the volume-based median diameter D of the graphite powder (A2) measured by a laser diffraction scattering method ５０ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the average particle size is preferably 1.0 μm or more and 20.0 μm or less, more preferably 3.0 μm or more and 18.0 μm or less, even more preferably 5.0 μm or more and 17.0 μm or less, even more preferably 7.0 μm or more and 15.0 μm or less, even more preferably 7.5 μm or more and 13.0 μm or less, and even more preferably 8.0 μm or more and 12.0 μm or less.

[0044] In the lithium ion secondary battery of this embodiment, the median diameter D of the graphite powder (A1) ５０ D １ , the median diameter D of the graphite powder (A2) ５０ D ２ When we do this, D １ D against ２ Ratio D ２ / D １ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the value of is preferably 0.40 or more and less than 1.0, more preferably 0.45 or more and 0.90 or less, even more preferably 0.50 or more and 0.80 or less, even more preferably 0.55 or more and 0.75 or less, and still more preferably 0.60 or more and 0.70 or less.

[0045] In the lithium ion secondary battery of this embodiment, the content of the graphite powder (A1) in the negative electrode active material (A) is preferably 50 parts by mass or more and 200 parts by mass or less, more preferably 60 parts by mass or more and 160 parts by mass or less, even more preferably 70 parts by mass or more and 140 parts by mass or less, still more preferably 80 parts by mass or more and 120 parts by mass or less, and still more preferably 90 parts by mass or more and 110 parts by mass or less, when the content of the graphite powder (A2) in the negative electrode active material (A) is taken as 100 parts by mass, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0046] In the lithium ion secondary battery of this embodiment, the content of the negative electrode active material (A) in the negative electrode active material layer is preferably 50 parts by mass or more and 99 parts by mass or less, more preferably 60 parts by mass or more and 95 parts by mass or less, even more preferably 65 parts by mass or more and 90 parts by mass or less, still more preferably 68 parts by mass or more and 85 parts by mass or less, and still more preferably 70 parts by mass or more and 80 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100 parts by mass, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0047] <Negative Electrode Active Material (B)> In the lithium ion secondary battery of this embodiment, the negative electrode active material (B) preferably contains a Si-based material, more preferably contains Si / C powder, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery. The Si / C powder may contain one or more types selected from the group consisting of silicon oxide particles and Si-C composite particles containing silicon and a carbon material, as described below.

[0048] In the lithium ion secondary battery of this embodiment, the volume-based median diameter D of the negative electrode active material (B) measured by a laser diffraction scattering method ５０ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the average particle size is preferably 1.0 μm or more and 20.0 μm or less, more preferably 1.5 μm or more and 19.0 μm or less, even more preferably 2.5 μm or more and 17.0 μm or less, even more preferably 3.5 μm or more and 11.0 μm or less, and even more preferably 4.0 μm or more and 9.0 μm or less.

[0049] In the lithium ion secondary battery of this embodiment, the negative electrode active material (B) preferably contains a Si-based material, more preferably contains one or more types selected from the group consisting of silicon oxide particles and Si—C composite particles containing silicon and a carbon material, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, and further preferably contains Si—C composite particles.

[0050] The Si—C composite particles are particles in which the carbon material contains a porous carbon material and silicon is present in at least some of the pores of the porous carbon material.

[0051] Examples of porous carbon materials that constitute the Si-C composite particles include activated carbon, aggregates of carbon fibers, aggregates of carbon nanotubes, carbon obtained by heat treating resins or organic materials, hard carbon, etc. Porous carbon materials can be produced by methods for producing activated carbon or known production methods involving heat treatment of polymers, but commercially available products may also be purchased, and are not limited to these, as long as silicon can be produced or incorporated into the pores of the porous carbon.

[0052] In this embodiment, the method for producing Si—C composite particles is not particularly limited, but the following method can be used, for example. A porous carbon material is placed in a tubular furnace, and the atmosphere inside the tubular furnace is replaced with argon gas. Next, a mixed gas of silane gas containing 1 to 3 mol % silane gas and nitrogen gas is flowed into the tubular furnace at a flow rate of 250 to 350 sccm, and the porous carbon material is treated under conditions of 450 to 550°C, 700 to 800 Torr, and 90 to 150 minutes to obtain a product. Next, the product is cooled to room temperature to obtain Si—C composite particles. More specifically, the method described in the Examples can be used for producing Si—C composite particles.

[0053] In the lithium ion secondary battery of this embodiment, the volume-based median diameter D of the Si—C composite particles measured by a laser diffraction scattering method ５０ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the average particle size is preferably 1.0 μm or more and 16.0 μm or less, more preferably 1.5 μm or more and 12.0 μm or less, even more preferably 2.0 μm or more and 10.0 μm or less, even more preferably 2.5 μm or more and 8.0 μm or less, and even more preferably 3.0 μm or more and 7.0 μm or less.

[0054] In the lithium ion secondary battery of this embodiment, the content of the negative electrode active material (B) in the negative electrode active material layer is preferably 1 part by mass or more and 50 parts by mass or less, more preferably 5 parts by mass or more and 40 parts by mass or less, even more preferably 8 parts by mass or more and 30 parts by mass or less, still more preferably 10 parts by mass or more and 28 parts by mass or less, and still more preferably 12 parts by mass or more and 25 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100 parts by mass, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0055] In the lithium ion secondary battery of this embodiment, the content of the negative electrode active material (A) in the negative electrode active material layer is W Ａ , the content of the negative electrode active material (B) in the negative electrode active material layer is W Ｂ When we do this, W Ｂ W against Ａ Ratio of W Ａ / W ＢFrom the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the value of is preferably 1.0 or more and 20.0 or less, more preferably 2.0 or more and 15.0 or less, even more preferably 2.5 or more and 10.0 or less, even more preferably 2.8 or more and 8.0 or less, and still more preferably 3.0 or more and 6.0 or less.

[0056] <Binder in Negative Electrode Active Material Layer> The binder in the negative electrode active material layer of the present embodiment includes, for example, one or more selected from the group consisting of fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyhexafluoropropylene (PHFP), polyvinyl fluoride (PVF), and copolymers of vinylidene fluoride and hexafluoropropylene; polycarboxylic acid-based polymers such as poly(meth)acrylic acid; conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles; synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR); and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin.

[0057] Among these, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the binder in the negative electrode active material layer of the present embodiment preferably contains one or more selected from the group consisting of a fluororesin, a polycarboxylic acid polymer, and a synthetic rubber, more preferably contains one or more selected from the group consisting of PVDF, a polycarboxylic acid polymer, and SBR, even more preferably contains a polycarboxylic acid polymer, and even more preferably contains poly(meth)acrylic acid.

[0058] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the binder in the negative electrode active material layer of this embodiment is preferably 0.1 parts by mass or more and 10.0 parts by mass or less, more preferably 1.0 parts by mass or more and 7.0 parts by mass or less, and even more preferably 2.0 parts by mass or more and 5.0 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0059] <Conductive Aid in Negative Electrode Active Material Layer> The conductive aid in the negative electrode active material layer of this embodiment includes, for example, one or more selected from the group consisting of carbon materials such as carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. Among these, the conductive aid in the negative electrode active material layer of this embodiment preferably includes a carbon material, more preferably includes carbon nanotubes, and even more preferably includes single-walled carbon nanotubes, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0060] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the conductive additive in the negative electrode active material layer of this embodiment is preferably 0.01 parts by mass or more and 5.0 parts by mass or less, more preferably 0.03 parts by mass or more and 1.0 parts by mass or less, even more preferably 0.05 parts by mass or more and 0.5 parts by mass or less, and still more preferably 0.07 parts by mass or more and 0.3 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0061] <Negative electrode current collector> The negative electrode current collector of this embodiment contains, for example, one or more selected from the group consisting of copper, stainless steel, nickel, titanium, and alloys thereof. The shape of the negative electrode current collector may be, for example, a foil, a flat plate, a mesh, or the like. The thickness of the negative electrode current collector is not particularly limited, but is, for example, 1 μm or more and 50 μm or less.

[0062] <Method for Producing Negative Electrode> The method for producing the negative electrode is not particularly limited and can be carried out according to a generally known method. For example, the method described in the Examples can be used as the method for producing the negative electrode.

[0063] <Positive Electrode> The positive electrode of this embodiment includes a positive electrode active material layer. From the viewpoint of further improving the battery performance of the lithium ion secondary battery, the positive electrode of this embodiment preferably includes a positive electrode current collector and the positive electrode active material layer of this embodiment.

[0064] <Positive Electrode Active Material Layer> From the viewpoint of further improving the cycle characteristics of a lithium ion secondary battery, the positive electrode active material layer of the present embodiment preferably contains the positive electrode active material of the present embodiment and a binder, and more preferably contains the positive electrode active material of the present embodiment, a binder, and a conductive additive.

[0065] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the thickness of the positive electrode active material layer of the present embodiment is preferably 10 μm or more and 250 μm or less, more preferably 15 μm or more and 200 μm or less, even more preferably 20 μm or more and 100 μm or less, and still more preferably 25 μm or more and 75 μm or less.

[0066] The density of the positive electrode active material layer of this embodiment is preferably 1.0 g / cm from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery. ３ 6.0g / cm or more ３ or less, more preferably 2.0 g / cm ３ 5.0g / cm or more ３ More preferably 2.5 g / cm or less ３ 4.5g / cm or more ３ The following is the result.

[0067] <Positive Electrode Active Material> The positive electrode active material of this embodiment is preferably a material with high electronic conductivity, from the viewpoint of facilitating electron transport by being able to reversibly release or absorb lithium ions. From the viewpoint of high electronic conductivity, the positive electrode active material preferably contains one or more materials selected from the group consisting of composite oxides of lithium and transition metals, transition metal sulfides, transition metal oxides, and olivine-type lithium phosphates. Examples of composite oxides of lithium and transition metals include lithium-nickel-cobalt-manganese composite oxide, lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-manganese-nickel composite oxide, and lithium-nickel-cobalt-aluminum composite oxide. Examples of transition metal sulfides include TiS ２ , FeS, MoS ２ Examples of transition metal oxides include MnO, V, and the like. ２ O ５ , V ６O １３ , TiO ２ etc.

[0068] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the positive electrode active material in the positive electrode active material layer of this embodiment is preferably 50.0 parts by mass or more and 99.9 parts by mass or less, more preferably 75.0 parts by mass or more and 99.5 parts by mass or less, even more preferably 85.0 parts by mass or more and 99.0 parts by mass or less, even more preferably 90.0 parts by mass or more and 98.5 parts by mass or less, and still more preferably 95.0 parts by mass or more and 98.0 parts by mass or less, when the entire positive electrode active material layer is taken as 100.0 parts by mass.

[0069] <Particles (C)> In the lithium ion secondary battery of this embodiment, the positive electrode active material contained in the positive electrode active material layer preferably contains particles (C) containing one or more kinds selected from particles (C1) constituted by a single crystal of a lithium-nickel-cobalt-manganese composite oxide and particles (C2) constituted by a polycrystal of a lithium-nickel-cobalt-manganese composite oxide.

[0070] In the lithium ion secondary battery of this embodiment, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of nickel in the particles (C) is preferably 80 mol or more, more preferably 80 mol or more and 99 mol or less, even more preferably 82 mol or more and 99 mol or less, even more preferably 84 mol or more and 98 mol or less, even more preferably 85 mol or more and 96 mol or less, even more preferably 86 mol or more and 95 mol or less, and even more preferably 88 mol or more and 94 mol or less, when the total content of nickel, cobalt, and manganese in the particles (C) is taken as 100 mol.

[0071] In the lithium ion secondary battery of this embodiment, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of cobalt in the particles (C) is preferably 0.5 mol or more and 10 mol or less, more preferably 1 mol or more and 10 mol or less, even more preferably 2 mol or more and 10 mol or less, even more preferably 2.5 mol or more and 10 mol or less, even more preferably 3 mol or more and 8 mol or less, and even more preferably 4 mol or more and 6 mol or less, when the total content of nickel, cobalt, and manganese in the particles (C) is 100 mol.

[0072] In the lithium ion secondary battery of this embodiment, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of manganese in the particles (C) is preferably 0.5 mol or more and 10 mol or less, more preferably 1 mol or more and 10 mol or less, even more preferably 2 mol or more and 10 mol or less, even more preferably 2.5 mol or more and 10 mol or less, even more preferably 3 mol or more and 8 mol or less, and even more preferably 4 mol or more and 6 mol or less, when the total content of nickel, cobalt, and manganese in the particles (C) is 100 mol.

[0073] In the lithium ion secondary battery of this embodiment, when the number of moles of nickel in the particles (C) is mN, the number of moles of cobalt in the particles (C) is mC, and the number of moles of manganese in the particles (C) is mM, the ratio mN / (mC+mM) of the number of moles of nickel mN to the sum of the number of moles of cobalt mC and the number of moles of manganese mM is preferably 4 or more and 50 or less, more preferably 4.5 or more and 25 or less, even more preferably 5 or more and 16 or less, even more preferably 7 or more and 12 or less, and even more preferably 8 or more and 11 or less, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0074] <Binder in Positive Electrode Active Material Layer> The binder in the positive electrode active material layer of the present embodiment includes, for example, one or more selected from the group consisting of fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyhexafluoropropylene (PHFP), polyvinyl fluoride (PVF), and copolymers of vinylidene fluoride and hexafluoropropylene; polycarboxylic acid polymers such as poly(meth)acrylic acid; conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles; synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR); and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin.

[0075] Among these, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the binder in the positive electrode active material layer of the present embodiment preferably contains one or more selected from the group consisting of fluororesin, polycarboxylic acid polymer, and synthetic rubber, more preferably contains one or more selected from the group consisting of PVDF, polycarboxylic acid polymer, and SBR, and even more preferably contains PVDF.

[0076] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the binder in the positive electrode active material layer of this embodiment is preferably 0.05 parts by mass or more and 10.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, even more preferably 0.2 parts by mass or more and 2.5 parts by mass or less, and still more preferably 0.5 parts by mass or more and 2.0 parts by mass or less, when the total amount of the positive electrode active material layer is taken as 100.0 parts by mass.

[0077] <Conductive Aid in Positive Electrode Active Material Layer> The conductive aid in the positive electrode active material layer of this embodiment includes, for example, one or more selected from the group consisting of carbon materials such as carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. Among these, the conductive aid in the positive electrode active material layer of this embodiment preferably includes a carbon material, more preferably includes carbon nanotubes, and even more preferably includes single-walled carbon nanotubes, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0078] From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the content of the conductive additive in the positive electrode active material layer of this embodiment is preferably 0.05 parts by mass or more and 10.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, even more preferably 0.2 parts by mass or more and 2.5 parts by mass or less, and still more preferably 0.5 parts by mass or more and 2.0 parts by mass or less, when the total amount of the positive electrode active material layer is taken as 100.0 parts by mass.

[0079] <Positive Electrode Current Collector> The positive electrode current collector of this embodiment includes, for example, one or more selected from the group consisting of aluminum, stainless steel, nickel, titanium, and alloys thereof. The positive electrode current collector may be in the form of, for example, a foil, a flat plate, or a mesh. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 1 μm or more and 50 μm or less.

[0080] <Method for Producing Positive Electrode> The method for producing the positive electrode is not particularly limited and can be carried out according to a generally known method. For example, the method described in the Examples can be used as the method for producing the positive electrode.

[0081] <Other Configurations of Lithium-Ion Secondary Battery> The lithium-ion secondary battery of this embodiment includes an electrolyte, a positive electrode, and a negative electrode, and preferably further includes a separator. The separator is not particularly limited as long as it can be used in lithium-ion secondary batteries, and generally known separators can be used.

[0082] <Separator> From the viewpoint of improving heat resistance and reducing thermal shrinkage of the separator, the separator preferably includes a substrate and a ceramic layer provided on at least one surface of the substrate. Examples of the substrate that can be used include polyethylene, polypropylene, and polyolefin-based porous films laminated with these. The ceramic layer can be formed, for example, by applying a ceramic layer-forming material to the substrate and drying the applied material. Examples of the ceramic layer-forming material that can be used include inorganic fillers, binders, and the like dispersed or dissolved in any solvent. The inorganic fillers and binders are not particularly limited as long as they are known materials used in separators for lithium-ion secondary batteries.

[0083] <Characteristics of Lithium-Ion Secondary Battery> The characteristics of the lithium-ion secondary battery of this embodiment will be described below.

[0084] [LiFSI Concentration in Lithium-Ion Secondary Battery] In the lithium-ion secondary battery of this embodiment, the LiFSI concentration in the electrolyte solution obtained by <Method 1> is 0.6% by mass or more, preferably 0.7% by mass or more, more preferably 0.8% by mass or more, even more preferably 0.9% by mass or more, even more preferably 1.0% by mass or more, even more preferably 1.1% by mass or more, even more preferably 1.2% by mass or more, even more preferably 1.3% by mass or more, and even more preferably 1.4% by mass or more, from the viewpoint of improving the cycle characteristics of the lithium-ion secondary battery. Furthermore, in the lithium-ion secondary battery of this embodiment, the LiFSI concentration in the electrolyte solution obtained by <Method 1> is 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less, and even more preferably 2.7% by mass or less, from the viewpoint of further improving the cycle characteristics of the lithium-ion secondary battery. Furthermore, in the lithium ion secondary battery of this embodiment, the concentration of LiFSI in the electrolyte by <Method 1> is preferably 0.6 mass % or more and 5.0 mass % or less, more preferably 0.7 mass % or more and 4.0 mass % or less, even more preferably 0.8 mass % or more and 3.0 mass % or less, even more preferably 0.9 mass % or more and 3.0 mass % or less, even more preferably 1.0 mass % or more and 3.0 mass % or less, even more preferably 1.1 mass % or more and 3.0 mass % or less, even more preferably 1.2 mass % or more and 3.0 mass % or less, even more preferably 1.3 mass % or more and 3.0 mass % or less, and even more preferably 1.4 mass % or more and 2.7 mass % or less, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery. The LiFSI concentration in the electrolyte solution according to Method 1 can be adjusted by, for example, adjusting the LiFSI concentration in the electrolyte solution (LiFSI concentration (at the time of preparation)) injected during the preparation of the lithium-ion secondary battery, the amount of the electrolyte solution, the amount of additives, the type and proportion of the negative electrode active material in the negative electrode active material layer, the type and proportion of the positive electrode active material in the positive electrode active material layer, and / or the initial charge / discharge conditions. More specifically, the LiFSI concentration in the electrolyte solution can be measured by the method described in the Examples.

[0085] [Capacity retention rate R at 25°C ２５In the lithium ion secondary battery of this embodiment, the capacity retention rate R at 25° C. according to the following <Method 2> ２５ From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the capacity retention rate R at 25°C is preferably 87.0% or more, more preferably 87.5% or more, even more preferably 88.0% or more, even more preferably 88.5% or more, even more preferably 89.0% or more, even more preferably 89.5% or more, even more preferably 90.0% or more, even more preferably 90.5% or more, even more preferably 91.0% or more, even more preferably 91.5% or more, and even more preferably 92.0% or more. ２５ The upper limit of is not particularly limited, but is, for example, less than 100%, and may be 99.0% or less, 98.0% or less, or 95.0% or less.

[0086] <Method 2> The lithium ion secondary battery of this embodiment is placed in a thermostatic chamber at 25°C. Next, the lithium ion secondary battery is charged and discharged according to the following <charge and discharge cycle>, and the first discharge capacity is measured. Next, the lithium ion secondary battery is charged and discharged 499 times in total according to the following <charge and discharge cycle>. Next, the lithium ion secondary battery is charged and discharged according to the following <charge and discharge cycle>, and the 500th discharge capacity is measured. Next, the capacity retention rate R ２５ Formula (2): Capacity maintenance rate R ２５ = (500th discharge capacity) / (1st discharge capacity)×100

[0087] <Charge / Discharge Cycle> The lithium ion secondary battery is charged at 30 mA until the upper limit voltage reaches 4.25 V. After the upper limit voltage of 4.25 V is reached, the lithium ion secondary battery is charged at a constant voltage until 2.5 hours have elapsed since the start of charging. The lithium ion secondary battery is then discharged at a constant current of 30 mA until the lower limit voltage of 2.5 V is reached.

[0088] <Applications of Lithium-Ion Secondary Battery> The lithium-ion secondary battery of the present embodiment can improve the cycle characteristics of the lithium-ion secondary battery, and therefore can be used in a variety of applications. The lithium-ion secondary battery of the present embodiment can be used for, for example, industrial, consumer, automotive, residential, etc., but is not limited thereto.

[0089] <Lithium-ion secondary battery module> The lithium-ion secondary battery module of this embodiment includes the lithium-ion secondary battery of this embodiment. The lithium-ion secondary battery module of this embodiment preferably includes two or more lithium-ion secondary batteries of this embodiment connected in series or parallel. The lithium-ion secondary battery module of this embodiment more preferably further includes a housing capable of accommodating two or more lithium-ion secondary batteries of this embodiment connected in series or parallel. The lithium-ion secondary battery module of this embodiment more preferably further includes one or more devices selected from the group consisting of a protection circuit that protects the lithium-ion secondary battery from overcurrent, a balancing circuit that equalizes the voltage between the electrodes of the lithium-ion secondary battery, a controller that controls the lithium-ion secondary battery, a cooler that can cool the lithium-ion secondary battery, and a heater that can heat the lithium-ion secondary battery.

[0090] The lithium-ion secondary battery module of this embodiment can be used in a battery system including two or more electrically connected lithium-ion secondary battery modules and a battery control system. Examples of battery systems include battery packs, stationary storage battery systems, automotive power storage battery systems, automotive auxiliary storage battery systems, and emergency power storage battery systems.

[0091] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements that do not impair the effects of the present invention are included in the present invention.

[0092] The present embodiment will be described in detail below with reference to examples, etc. However, the present embodiment is not limited to the descriptions of these examples.

[0093] First, the materials used in each example are listed.

[0094] The following material (hereinafter also referred to as negative electrode active material 1) was used as the negative electrode active material. The following negative electrode active material (A) and negative electrode active material (B) were used in combination as the negative electrode active material 1. The negative electrode active material (A) and the negative electrode active material (B) were blended so that the ratio of the negative electrode active material (A): the negative electrode active material (B) was 80:20 (mass ratio).

[0095] [Negative electrode active material (A)] The negative electrode active material (A) was a combination of the following graphite powder (A1) and graphite powder (A2). The negative electrode active material (A1) and the negative electrode active material (A2) were blended in a mass ratio of negative electrode active material (A1):negative electrode active material (A2) = 50:50.

[0096] <Graphite Powder (A1)> As the graphite powder (A1), artificial graphite particles containing amorphous carbon on the surface were used. For the graphite powder (A1), the following graphite powder 1 was used. Graphite powder 1 (artificial graphite particles containing amorphous carbon on the surface, median diameter D ５０ : 14.0 μm)

[0097] <Graphite Powder (A2)> As the graphite powder (A2), artificial graphite particles not containing amorphous carbon on the surface were used. For the graphite powder (A2), the following graphite powder 2 was used. Graphite powder 2 (artificial graphite particles not containing amorphous carbon on the surface, median diameter D ５０ : 9.0 μm)

[0098] [Negative Electrode Active Material (B)] The following Si—C composite particles (hereinafter also referred to as Si / C particles) were used as the negative electrode active material (B). The following Si / C particles 1 were used as the Si—C composite particles. Si / C particles 1 (median diameter D ５０ : 4.8 μm, particles prepared according to the <Preparation of Si / C particles 1> below)

[0099] <Preparation of Si / C particles 1> Porous carbon material 1 (median diameter D ５０A porous carbon material 1 having a particle diameter of 4.8 μm was placed in a tubular furnace, and the atmosphere in the furnace was replaced with argon gas. A mixed gas of 2 mol % silane gas and 98 mol % nitrogen gas was then flowed into the tubular furnace at a flow rate of 300 sccm, and the porous carbon material 1 was treated under conditions of 500°C, 760 Torr, and 120 minutes to obtain a product. The product was then cooled to room temperature to obtain Si / C particles 1. EDS mapping was performed on a cross section of the obtained Si / C particles 1 using an energy dispersive X-ray analyzer. The results of the EDS mapping confirmed that the Si / C particles 1 contained silicon and that silicon was present in at least a portion of the pores of the porous carbon material 1 in the Si / C particles 1.

[0100] The positive electrode active material used was the following positive electrode active material 1. Positive electrode active material 1: Particles (median diameter D ５０ : 3.5 μm)

[0101] The following materials were used for the electrolyte solution: Solvent 1: Ethylene carbonate (hereinafter also referred to as EC) Solvent 2: Ethyl methyl carbonate (hereinafter also referred to as EMC) Electrolyte: Lithium hexafluorophosphate (LiPF ６ Electrolyte: Lithium bis(fluorosulfonyl)imide (LiFSI) The following materials were used for the separator: Separator: Microporous polyethylene film with a thickness of 10 μm, ceramic coated on both sides

[0102] In addition to the negative electrode active material, the following materials were used for the negative electrode: Negative electrode current collector: copper foil (thickness: 8 μm) Binder: polyacrylic acid (hereinafter also referred to as PAA) Conductive additive: single-walled carbon nanotubes (hereinafter also referred to as CNT) Solvent: pure water

[0103] In addition to the positive electrode active material, the following materials were used for the positive electrode: Positive electrode current collector: aluminum foil (thickness: 12 μm) Binder: polyvinylidene fluoride (hereinafter also referred to as PVDF) Conductive additive: single-walled carbon nanotubes (hereinafter also referred to as CNT) Solvent: N-methyl-2-pyrrolidone (hereinafter also referred to as NMP)

[0104] <Method for Measuring Particle Diameters of Positive Electrode Active Material and Negative Electrode Active Material> A laser diffraction particle size distribution analyzer (Shimadzu Corporation, model number: SALD-2300) was used to measure the volume-based median diameter D of each of the positive electrode active material and the negative electrode active material by laser diffraction scattering. ５０ Here, the median diameter D of the positive electrode active material was measured. ５０ The median diameter D of the negative electrode active material was measured after suspending the positive electrode active material in a dispersion medium (0.1 mass % sodium hexametaphosphate aqueous solution) and ultrasonically dispersing the suspension. ５０ The negative electrode active material was suspended in a dispersion medium (0.1% by mass sodium hexametaphosphate aqueous solution), and the measurement was performed after ultrasonic dispersion. The measurement was performed five times, and the average value was taken as the median diameter D ５０ It was decided.

[0105] Examples 1 to 3, Comparative Example 1 <Fabrication of Lithium-ion Secondary Battery> A lithium-ion secondary battery was fabricated in the following steps, which will be described in detail below.

[0106] First, the electrolyte solution used in each lithium-ion secondary battery was prepared according to the following procedure. Ethylene carbonate and ethyl methyl carbonate were mixed in a mass ratio of EC:EMC = 30:70 to obtain a mixed solvent. Lithium hexafluorophosphate (LiPF ６ The electrolyte solution for each example was prepared by dissolving lithium bis(fluorosulfonyl)imide (LiFSI) in the mixed solvent so that the LiFSI content was 12 mass %. The LiFSI content was the LiFSI concentration (at the time of preparation) shown in Table 1.

[0107] Next, the positive electrodes used in the lithium-ion secondary batteries of each example were prepared by the following procedure. The positive electrode active material, binder (PVDF), and conductive additive (CNT) shown in Table 1 were dispersed in a solvent (NMP) in a ratio of positive electrode active material:PVDF:CNT = 97.5:1.5:1.0 (mass ratio) to prepare positive electrode slurries. The positive electrode slurries were then applied to a 12 μm thick aluminum foil positive electrode current collector to prepare a positive electrode current collector having an initial charge capacity per unit area of ​​4.0 mAh / cm. ２ The applied positive electrode slurry was then dried to obtain a positive electrode laminate. Next, a roll press was used to apply a coating amount such that the density of the positive electrode active material layer became 3.5 g / cm. ３ The positive electrode laminate was pressed with a pressure such that the positive electrode of each example was fabricated.

[0108] Next, the negative electrodes used in the lithium-ion secondary batteries of each example were prepared by the following procedure. The negative electrode active material, binder (PAA), and conductive additive (CNT) shown in Table 1 were dispersed in a solvent (pure water) in a mass ratio of negative electrode active material:PAA:CNT = 96.9:3.0:0.1 to prepare negative electrode slurries. Next, the negative electrode slurries were applied to a negative electrode current collector made of copper foil with a thickness of 8 μm, with an initial charge capacity per unit area of ​​4.3 mAh / cm. ２ The negative electrode active material layer was coated in such an amount that the density of the negative electrode active material layer became 1.65 g / cm. The coated negative electrode slurry was then dried to obtain a negative electrode laminate. ３ The negative electrode laminate was pressed with a pressure such that the negative electrode of each example was fabricated.

[0109] Next, the lithium-ion secondary batteries of each example were fabricated according to the following procedure. One double-sided coated positive electrode and two single-sided coated negative electrodes were arranged with their coated surfaces facing each other via a separator, and were stacked in the following order: negative electrode, separator, positive electrode, separator, negative electrode. The resulting laminate was then wrapped in a laminated outer casing formed by processing an aluminum-based film. The aluminum foil and copper foil cut out for current collection protruded from the laminated film. The edge containing the protruding current-collecting foil and the other two edges were then heat-sealed to fabricate a laminated cell with only one edge open. A predetermined amount of the electrolyte solution prepared above was poured into the opening of the laminated cell. The laminated cell was then vacuum-sealed to fabricate a laminated lithium-ion secondary battery. The amount of the electrolyte solution to be injected was set so that the amount of the electrolyte solution was 30 parts by mass when the total amount of the positive electrode active material and the negative electrode active material was 100 parts by mass.

[0110] <Initial Charging and Discharging of Lithium-Ion Secondary Battery> The lithium-ion secondary battery of each example was initially charged and discharged under the following conditions to form an SEI film on the surface of the negative electrode active material, thereby obtaining a lithium-ion secondary battery after initial charging and discharging for each example. Note that, hereinafter, charging and discharging of the lithium-ion secondary battery was performed using a charge / discharge device in an environment at a temperature of 25°C. The lithium-ion secondary battery of each example was charged to a battery voltage of 3.2V at a charging current of 0.05C, and then left for 12 hours to perform pre-charging. Next, constant current charging was performed to a battery voltage of 4.25V at a charging current of 0.05C. After reaching a battery voltage of 4.25V, constant voltage charging was performed until the current value decreased to 0.015C. Next, the battery was left for 10 minutes. Thereafter, constant current discharging was performed to a battery voltage of 2.5V at a discharge current of 0.33C. After reaching a battery voltage of 2.5V, the battery was left for 10 minutes. Next, constant current charging was performed to a battery voltage of 4.25V at a charging current of 0.33C. After the battery voltage reached 4.25V, constant voltage charging was performed until the current value dropped to 0.05C. The charged lithium-ion secondary battery was then stored for 48 hours in a 45°C environment while monitoring the battery voltage. After storage, the lithium-ion secondary battery was returned to a 25°C environment and subjected to constant current discharge at a discharge current of 1C to a battery voltage of 2.5V. Next, constant current discharge was performed at a discharge current of 0.33C to a battery voltage of 2.5V. After the battery voltage reached 2.5V, the battery was left for 10 minutes. Next, constant current charging was performed at a charge current of 0.33C to a battery voltage of 4.25V. After the battery voltage reached 4.25V, constant voltage charging was performed until the current value dropped to 0.05C. Next, 10 minutes after the end of charging, constant current discharge was performed at a discharge current of 1C to a battery voltage of 2.5V. Next, constant current discharge was performed at a discharge current of 0.33C to a battery voltage of 2.5V. After the battery voltage reached 2.5V, the battery was left for 10 minutes. Next, constant current charging was performed at a charging current of 0.33 C until the battery voltage reached 4.25 V. After the battery voltage reached 4.25 V, constant voltage charging was performed until the current value decreased to 0.05 C. The battery was then left for 10 minutes. Thereafter, constant current discharging was performed at a discharging current of 0.33 C until the battery voltage reached 2.5 V.

[0111] <Measurement of Lithium Ion Secondary Battery Characteristics> The characteristics of the lithium ion secondary batteries obtained in each example after initial charge and discharge were measured by the following method. The measurement results are shown in Table 1.

[0112] [LiFSI Concentration After Initial Charge / Discharge] The LiFSI concentration after initial charge / discharge of each lithium ion secondary battery was measured by the following method. The lithium ion secondary battery after initial charge / discharge of each example was decomposed in an inert gas atmosphere at a temperature of 25°C and a relative humidity of 3% RH or less. Next, 0.2 g of the electrolyte solution collected from the decomposed lithium ion secondary battery was dissolved in 1.0 g of deuterated acetonitrile to prepare a first sample. Next, 0.002 g of hexafluorobenzene was added to the first sample as a reference material to prepare a second sample. Next, 0.5 g of the second sample was used to prepare １９ The amount of LiFSI in the second sample was measured by F-NMR. The amount of LiFSI in the first sample was then calculated from the amount of LiFSI in the second sample. The LiFSI concentration in the electrolyte (LiFSI concentration after initial charge / discharge) was then calculated using the following formula (1): Formula (1): LiFSI concentration in electrolyte = (amount of LiFSI in the first sample) / (amount of electrolyte collected from the disassembled lithium-ion secondary battery) x 100

[0113] The above １９ Measurement and analysis by F-NMR method were carried out under the following conditions: Nuclear magnetic resonance apparatus: JNM-ECA400WB (manufactured by JEOL Ltd.) Measurement items: １９ F Solvent: deuterated acetonitrile Observation frequency: 400 MHz Number of accumulations: 128 Chemical shift standard: hexafluorobenzene １９ F-NMR spectrum measurement method: Single pulse method Measurement temperature: 25°C

[0114] [Capacity retention rate R at 25°C ２５ For each example of the lithium ion secondary battery after the initial charge and discharge, the capacity retention rate R ２５was measured by the following method. The lithium ion secondary battery after the initial charge and discharge of each example was placed in a thermostatic chamber at 25°C. Next, the lithium ion secondary battery was charged and discharged according to the <charge and discharge cycle> described below, and the first discharge capacity was measured. Next, the lithium ion secondary battery was charged and discharged 499 times in total according to the <charge and discharge cycle> described below. Next, the lithium ion secondary battery was charged and discharged according to the <charge and discharge cycle> described below, and the 500th discharge capacity was measured. Next, the capacity retention rate R ２５ The capacity retention rate R was calculated using the formula (2). ２５ = (500th discharge capacity) / (1st discharge capacity)×100

[0115] <Charge-Discharge Cycle> The lithium ion secondary battery was charged at 30 mA until the upper limit voltage reached 4.25 V. After the upper limit voltage of 4.25 V was reached, the lithium ion secondary battery was charged at a constant voltage until 2.5 hours had elapsed since the start of charging. The lithium ion secondary battery was then discharged at a constant current of 30 mA until the lower limit voltage of 2.5 V was reached.

[0116]

[0117] This application claims priority based on Japanese Patent Application No. 2024-057082, filed March 29, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0118] REFERENCE SIGNS LIST 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode current collector 4 Negative electrode current collector 5 Separator 6 Exterior body 7 Exterior body 8 Negative electrode tab 9 Positive electrode tab 10 Lithium ion secondary battery

Claims

1. A lithium ion secondary battery comprising a positive electrode including a positive electrode active material layer, a negative electrode including a negative electrode active material layer, and an electrolyte, wherein the negative electrode active material included in the negative electrode active material layer has an SEI film on at least a portion of its surface, and the electrolyte contains lithium bis(fluorosulfonyl)imide, and the concentration of the lithium bis(fluorosulfonyl)imide in the electrolyte is 0.6 mass% or more according to the following <Method 1>. <Method 1> The lithium ion secondary battery is disassembled under an inert gas atmosphere, and 0.2 g of the electrolyte collected from the disassembled lithium ion secondary battery is dissolved in 1.0 g of deuterated acetonitrile to prepare a first sample, and 0.002 g of hexafluorobenzene is added to the first sample as a reference material to prepare a second sample, and 0.5 g of the second sample is used to １９ The amount of lithium bis(fluorosulfonyl)imide in the second sample is measured by F-NMR, and then the concentration of lithium bis(fluorosulfonyl)imide in the first sample is calculated from the amount of lithium bis(fluorosulfonyl)imide in the second sample, and the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution is calculated using the following formula (1): Formula (1): Concentration of lithium bis(fluorosulfonyl)imide in the electrolyte solution = (amount of lithium bis(fluorosulfonyl)imide in the first sample) / (amount of the electrolyte solution collected from the disassembled lithium ion secondary battery) x 100.

2. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material comprises a negative electrode active material (A) and a negative electrode active material (B) of a type different from the negative electrode active material (A).

3. The volume-based median diameter D of the negative electrode active material (A) measured by a laser diffraction scattering method ５０ is 3.0 μm or more and 30.0 μm or less, and the volume-based median diameter D of the negative electrode active material (B) measured by a laser diffraction scattering method ５０ The lithium ion secondary battery according to claim 2, wherein the average particle diameter is 1.0 μm or more and 20.0 μm or less.

4. The lithium ion secondary battery according to claim 2 or 3, wherein the negative electrode active material (A) contains graphite powder.

5. The graphite powder is a graphite powder (A1) having a volume-based median diameter D measured by a laser diffraction scattering method. ５０ and a graphite powder (A2) different from the graphite powder (A1), wherein the median diameter D of the graphite powder (A1) is ５０ is the median diameter D of the graphite powder (A2). ５０ The lithium ion secondary battery according to claim 4 , 6. The lithium ion secondary battery according to claim 5, wherein the graphite powder (A1) contains graphite particles having amorphous carbon on the surface thereof.

7. The lithium ion secondary battery according to claim 5 or 6, wherein the graphite powder (A2) contains graphite particles that do not contain amorphous carbon on the surface.

8. The median diameter D of the graphite powder (A1) ５０ The lithium ion secondary battery according to any one of claims 5 to 7, wherein the average particle diameter is 5.0 μm or more and 30.0 μm or less.

9. The median diameter D of the graphite powder (A2) ５０ The lithium ion secondary battery according to any one of claims 5 to 8, wherein is 1.0 μm or more and 20.0 μm or less.

10. Median diameter D of the graphite powder (A1) ５０ D １ , the median diameter D of the graphite powder (A2) ５０ D ２ When we do this, D １ D against ２ Ratio D ２ / D １ The lithium ion secondary battery according to any one of claims 5 to 9, wherein the value of is 0.40 or more and less than 1.

0.

11. The lithium ion secondary battery according to any one of claims 5 to 10, wherein the content of the graphite powder (A1) in the negative electrode active material (A) is 50 parts by mass or more and 200 parts by mass or less, when the content of the graphite powder (A2) in the negative electrode active material (A) is taken as 100 parts by mass.

12. The lithium ion secondary battery according to any one of claims 2 to 11, wherein the negative electrode active material (B) comprises one or more particles selected from the group consisting of silicon oxide particles and Si-C composite particles containing silicon and a carbon material.

13. The lithium ion secondary battery according to claim 12, wherein the carbon material of the Si-C composite particles comprises a porous carbon material, and the silicon is present in at least some of the pores of the porous carbon material.

14. The volume-based median diameter D of the Si-C composite particles measured by a laser diffraction scattering method ５０ The lithium ion secondary battery according to claim 12 or 13, wherein the average particle diameter is 1.0 μm or more and 16.0 μm or less.

15. The lithium-ion secondary battery according to any one of claims 2 to 14, wherein the content of the negative electrode active material (A) in the negative electrode active material layer is 50 parts by mass or more and 99 parts by mass or less, when the total amount of the negative electrode active material layer is 100 parts by mass.

16. The lithium-ion secondary battery according to any one of claims 2 to 15, wherein the content of the negative electrode active material (B) in the negative electrode active material layer is 1 part by mass or more and 50 parts by mass or less, when the total amount of the negative electrode active material layer is 100 parts by mass.

17. The content of the negative electrode active material (A) in the negative electrode active material layer is W Ａ The content of the negative electrode active material (B) in the negative electrode active material layer is W Ｂ When we do this, W Ｂ W against Ａ Ratio of W Ａ / W Ｂ The lithium ion secondary battery according to any one of claims 2 to 16, wherein the value of is 1.0 or more and 20.0 or less.

18. The lithium-ion secondary battery according to any one of claims 1 to 17, wherein the positive electrode active material contained in the positive electrode active material layer includes particles (C) containing one or more types selected from particles (C1) composed of a single crystal of lithium-nickel-cobalt-manganese composite oxide and particles (C2) composed of a polycrystal of lithium-nickel-cobalt-manganese composite oxide.

19. The lithium ion secondary battery according to claim 18, wherein the content of nickel in the particles (C) is 80 mol or more when the total content of nickel, cobalt, and manganese is 100 mol.

20. A lithium ion secondary battery according to claim 18 or 19, wherein the respective contents of nickel, cobalt, and manganese in said particles (C) are such that, when the total content of nickel, cobalt, and manganese in said particles (C) is 100 mol, the nickel content is 80 mol or more and 99 mol or less, the cobalt content is 0.5 mol or more and 10 mol or less, and the manganese content is 0.5 mol or more and 10 mol or less.

21. Capacity retention rate R at 25°C according to the following <Method 2> ２５ The lithium ion secondary battery according to any one of claims 1 to 20, wherein the capacity retention rate R is 87.0% or more. <Method 2> The lithium ion secondary battery is placed in a thermostatic chamber at 25°C, and then the lithium ion secondary battery is charged and discharged in accordance with the following <charge and discharge cycle>, and the first discharge capacity is measured. Next, the lithium ion secondary battery is repeatedly charged and discharged in accordance with the following <charge and discharge cycle> until a total of 499 cycles have been completed, and then the lithium ion secondary battery is charged and discharged in accordance with the following <charge and discharge cycle>, and the 500th discharge capacity is measured. Next, the capacity retention rate R is calculated from the following formula (2): ２５ The capacity retention rate R is calculated using the formula (2). ２５ = (the 500th discharge capacity) / (the 1st discharge capacity)×100 <Charge / Discharge Cycle> The lithium ion secondary battery was charged at 30 mA until an upper limit voltage of 4.25 V was reached, and then, after the upper limit voltage of 4.25 V was reached, the lithium ion secondary battery was charged at a constant voltage until 2.5 hours had elapsed since the start of charging, and then, the lithium ion secondary battery was discharged at a constant current of 30 mA until a lower limit voltage of 2.5 V was reached.

22. A lithium ion secondary battery according to any one of claims 1 to 21, wherein the content of the electrolyte in the lithium ion secondary battery is 15 parts by mass or more and 60 parts by mass or less, when the sum of the content of the negative electrode active material and the content of the positive electrode active material contained in the positive electrode active material layer is 100 parts by mass.

23. A lithium ion secondary battery module comprising the lithium ion secondary battery according to any one of claims 1 to 22.

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