Lithium ion secondary battery and lithium ion secondary battery module

Abstract

To provide a lithium ion secondary battery with improved cycle characteristics.SOLUTION: A 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, and 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 difluorophosphate; and the concentration of the lithium difluorophosphate in the electrolyte measured by a predetermined method is 0.01 mass% or more and 1.10 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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 comprising: 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 comprising a carbon-based material and a silicon-based compound; and an electrolyte solution containing a lithium salt and a non-aqueous solvent, the electrolyte solution comprising a cyclic carbonate and / or a linear solvent.

[0004] Patent Document 2 aims to provide a lithium ion secondary battery with high capacity and excellent charge / discharge cycle characteristics, and describes a lithium ion secondary battery having the general composition formula Li a Ni (1-b-c) Co b M cA positive electrode containing a lithium-containing composite oxide represented by O2 (where M is at least one element of Mn and Al, 0.9 ≦ a ≦ 1.3, 0 < b, 0 < c, b + c ≦ 0.5), 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 negative electrode containing, as a negative electrode active material, 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, and a lithium ion secondary battery in which the relationship between the ratio of the composite in the negative electrode active material before and after the first charge and discharge and the content of each cyclic carbonate in the non-aqueous electrolyte is specified are disclosed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

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

Means for Solving the Problems

[0007] The inventors of the present invention have intensively studied to achieve the above problems. As a result, the inventors of the present invention have found that the cycle characteristics of a lithium ion secondary battery can be improved by setting the concentration of lithium difluorophosphate in the electrolyte in a lithium ion secondary battery within a specific range, and have completed the present invention.

[0008] [1] A lithium ion secondary battery including a positive electrode including a positive electrode active material layer, a negative electrode including a negative electrode active material layer, and an electrolyte, the negative electrode active material contained in the negative electrode active material layer has an SEI film on at least a part of its surface, the electrolyte solution contains lithium difluorophosphate, A lithium ion secondary battery according to the following <Method 1>, wherein the concentration of the lithium difluorophosphate in the electrolyte solution is 0.01% by mass or more and 1.10% by mass or less. <Method 1> The lithium ion secondary battery was disassembled under an inert gas atmosphere, and then 0.2 g of the electrolyte solution collected from the disassembled 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 substance to prepare a second sample. Next, 0.5 g of the second sample was used to 19 The amount of lithium difluorophosphate in the second sample is measured by F-NMR, and then the concentration of lithium difluorophosphate in the first sample is calculated from the amount of lithium difluorophosphate in the second sample, and the concentration of lithium difluorophosphate in the electrolytic solution is calculated using the following formula (1): Equation (1): Concentration of the lithium difluorophosphate in the electrolyte solution=(Amount of the lithium difluorophosphate in the first sample) / (Amount of the electrolyte solution collected from the disassembled lithium ion secondary battery)×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 50 is 3.0 μm or more and 30.0 μm or less, 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 [2], wherein the average particle size is 1.0 μm or more and 20.0 μm or less. [4] The lithium ion secondary battery according to [2] or [3] above, 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 50 and a graphite powder (A2) different from the graphite powder (A1), The median diameter D of the graphite powder (A1) 50 is the median diameter D of the graphite powder (A2) 50 The lithium ion secondary battery according to [4], [6] The lithium ion secondary battery according to [5] above, 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 whose surfaces do not contain amorphous carbon. [8] The median diameter D of the graphite powder (A1) 50 The lithium ion secondary battery according to any one of [5] to [7], wherein the average particle size 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 [5] to [8], wherein the average particle size is 1.0 μm or more and 20.0 μm or less.

[10] The median diameter D of the graphite powder (A1) 50 is D1, and the median diameter D of the graphite powder (A2) 50 The lithium ion secondary battery according to any one of [5] to [9] above, wherein when D2 is defined as D2, the ratio of D2 to D1, D2 / D1, is 0.40 or more and less than 1.0.

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

[11] , wherein the negative electrode active material (B) comprises one or more kinds 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] , wherein the Si-C composite particles contain a porous carbon material as the 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 laser diffraction scattering method 50 The lithium ion secondary battery according to

[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 [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 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 [2] to

[16] above, 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 [1] to

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

[19] The lithium ion secondary battery according to

[18] above, 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 the respective contents of nickel, cobalt, and manganese in the particles (C) are such that, when the total content of nickel, cobalt, and manganese in the particles (C) is taken as 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. [twenty one] Capacity retention rate R at 45℃ according to the following <Method 2> 45 The lithium ion secondary battery according to any one of [1] to

[20] above, wherein the % of the charge / discharge current is 82.0% or more. <Method 2> The lithium ion secondary battery was placed in a thermostatic chamber at 45°C, and then the lithium ion secondary battery was charged and discharged according to the following <charge and discharge cycle>, and the first discharge capacity was measured. Next, the lithium ion secondary battery was repeatedly charged and discharged 499 times in total according to the following <charge and discharge cycle>, and then the lithium ion secondary battery was charged and discharged according to the following <charge and discharge cycle>, and the 500th discharge capacity was measured. Next, the capacity retention rate R 45 Calculate. Formula (2): The capacity retention rate R 45 = (500th discharge capacity) / (1st 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. [twenty two] 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, when the total 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 taken as 100 parts by mass. [twenty three] A lithium ion secondary battery module comprising the lithium ion secondary battery according to any one of [1] to

[22] above. [Effects of the Invention]

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

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

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. To avoid complexity, when there are multiple identical components in the same drawing, only one of them may be labeled with a symbol, and not all of them. The drawings are for illustrative purposes only, and the shapes and dimensional ratios of the components in the drawings do not necessarily correspond to the actual products. In this specification, the term "poly(meth)acrylic acid" represents a concept that includes both polyacrylic acid and polymethacrylic acid. In this embodiment, each component may be used alone or in combination of two or more. In addition, the symbol "to" indicating a numerical range means "at least" or "not more than," and includes both the upper and lower limits. In this specification, SOC stands for "State Of Charge." The current C in this specification represents the 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 the lithium ion secondary battery of this embodiment, 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 the lithium ion secondary battery of this embodiment, the electrolyte includes lithium difluorophosphate, and the concentration of lithium difluorophosphate in the electrolyte is 0.01 mass% or more and 1.10 mass% or less according to the following <Method 1>. <Method 1> The lithium-ion secondary battery is disassembled under an inert gas atmosphere. Then, 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 19 The amount of lithium difluorophosphate in the second sample is measured by F-NMR. Then, the concentration of lithium difluorophosphate in the first sample is calculated from the amount of lithium difluorophosphate in the second sample. Then, the concentration of lithium difluorophosphate in the electrolyte is calculated using the following formula (1). Equation (1): Concentration of lithium difluorophosphate in the electrolyte = (amount of lithium difluorophosphate in the first sample) / (amount of electrolyte collected from the disassembled lithium-ion secondary battery) × 100 The lithium ion secondary battery of this embodiment has the above-described configuration, and thus can improve 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. Figure 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, so 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 poured into the container and sealed therein. The container may also house an electrode group in which multiple electrode pairs are stacked.

[0016] <Method of manufacturing lithium-ion secondary batteries> In this embodiment, the lithium ion secondary battery can be fabricated according to a known method. The electrode can be, for example, a laminate or a wound body. The exterior can be a metal exterior or an aluminum laminate exterior. The battery may have any shape, such as a laminate, coin, button, sheet, cylindrical, rectangular, or flat shape, but a laminate 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 electrolytic solution contains an electrolyte and an organic solvent. In the lithium ion secondary battery of this embodiment, the electrolytic solution contains lithium difluorophosphate (hereinafter also referred to as LiPO2F2) as the electrolyte.

[0019] In the lithium ion secondary battery of this embodiment, the electrolyte other than LiPO2F2 may be, for example, lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorobis(oxalato)phosphate (LiDODFP), LiClO4, LiBF4, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiB 10 Cl 10 , LiAlCl4, LiCl, LiBr, LiB(C2H5)4, CH3SO3Li, LiC4F9SO3, Li(CF3SO2)2N, lithium salts of lower fatty acids, and lithium salts of lower carboxylic acids, preferably containing one or more selected from the group consisting of LiPF6, LiFSI, and LiCl, and more preferably containing LiPF6.

[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. the solvent may comprise one or more selected from the group consisting of nitrogen-containing compounds such as nitromethane, formamide, and dimethylformamide; organic acid esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, and ethyl propionate; phosphate esters such as phosphate triesters; diglymes; triglymes; sulfolanes such as sulfolane and methylsulfolane; oxazolidinones such as 3-methyl-2-oxazolidinone; and sultones such as 1,3-propane sultone, 1,4-butane sultone, and naphtha sultone, and preferably comprises a carbonate, and more preferably comprises one 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 LiPO2F2 in the electrolyte when the lithium ion secondary battery is fabricated (hereinafter also referred to as LiPO2F2 concentration (at fabrication)) is preferably 0.1 mass % or more and 1.2 mass % or less, more preferably 0.2 mass % or more and 1.1 mass % or less, and even more preferably 0.3 mass % or more and 1.0 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, the concentration of LiPF in the electrolyte when the lithium ion secondary battery is fabricated (hereinafter also referred to as LiPF concentration (at fabrication)) 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, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[0024] In the lithium ion secondary battery of this embodiment, the LiPF concentration (at the time of manufacture) 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, from the viewpoint of reducing corrosion of battery components.

[0025] In the lithium ion secondary battery of this embodiment, the LiPF concentration (at the time of manufacture) 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, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery and reducing corrosion of the battery components.

[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 part of its surface, and preferably has an SEI film 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 its additives. 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 part of the surface of the negative electrode (negative electrode active material) by the method described in the "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 part 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 the 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 this 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 More than 3.00g / cm 3 or less, more preferably 1.00 g / cm 3 More than 2.50g / cm 3 or less, more preferably 1.30 g / cm 3 More than 2.00g / cm 3 The following is the result.

[0033] <Negative electrode active material> The negative electrode active material of the present embodiment preferably contains 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 contains one or more materials selected from the group consisting of a carbon material and a Si-based material, and even more preferably contains both a carbon material and a Si-based material. Examples of the carbon material of the present embodiment include graphite powder, hard carbon, soft carbon, and any mixture thereof, and graphite powder is preferred. 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 contains Si-C composite particles from the viewpoint of further improving the cycle characteristics of the 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 even 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 this embodiment, the negative electrode active material of this embodiment preferably contains 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 thickness 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). More preferably, the graphite powder of this embodiment 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 measured by a laser diffraction scattering method. 50and graphite powder (A2) different from graphite powder (A1), and 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 that do not contain amorphous carbon on the surface, and more preferably contains artificial graphite particles that do not contain amorphous carbon on the surface.

[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 50From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, the thickness 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 50 is D1, and the median diameter of graphite powder (A2) is D 50 and D2, the ratio of D2 to D1, D2 / D1, 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 even more preferably 0.60 or more and 0.70 or less, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[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, even more preferably 80 parts by mass or more and 120 parts by mass or less, and even 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, even more preferably 68 parts by mass or more and 85 parts by mass or less, and even 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 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 a 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 kinds 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 thickness 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, and even more preferably contains Si-C composite particles, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[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 make up 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 that involve heat treating 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 the Si-C composite particles is not particularly limited, but for example, the following method can be adopted. The porous carbon material is placed in a tubular furnace, and the atmosphere inside the furnace is replaced with argon gas. A mixed gas of silane gas (1-3 mol %) and nitrogen gas is then passed through the furnace at a flow rate of 250-350 sccm. The porous carbon material is treated at 450-550°C, 700-800 Torr, and for 90-150 minutes to obtain a product. The product is then cooled to room temperature to obtain Si-C composite particles. More specifically, the method for producing the Si-C composite particles can be the method described in the Examples.

[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 thickness 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, even more preferably 10 parts by mass or more and 28 parts by mass or less, and even 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 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 B From 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 even 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 this 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.

[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 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, 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 additive in negative electrode active material layer> The conductive additive 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 additive 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 includes, for example, one or more selected from the group consisting of copper, stainless steel, nickel, titanium, and alloys thereof. The negative electrode current collector may be in the form of, for example, a foil, a flat plate, or a mesh. The thickness of the negative electrode current collector is not particularly limited, but is, for example, 1 μm to 50 μm.

[0062] <Method of manufacturing the 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.

[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] <Cathode active material layer> From the viewpoint of further improving the cycle characteristics of the 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 this 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 even 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 More than 6.0g / cm 3 or less, more preferably 2.0 g / cm 3 More than 5.0g / cm 3 More preferably 2.5 g / cm or less3 More than 4.5g / cm 3 The following is the result.

[0067] <Cathode active material> The positive electrode active material of this embodiment is preferably a material with high electronic conductivity, which can reversibly release or absorb lithium ions and facilitate electron transport. From the viewpoint of high electronic conductivity, the positive electrode active material preferably includes one or more materials selected from the group consisting of lithium and transition metal composite oxides, transition metal sulfides, transition metal oxides, and olivine-type lithium phosphates. Examples of lithium and transition metal composite oxides 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 TiS2, FeS, and MoS2. Examples of transition metal oxides include MnO, VO, and VO. 13 , TiO2, 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 even 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] <Particle (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) composed of a single crystal of a lithium-nickel-cobalt-manganese composite oxide and particles (C2) composed of 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, 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 taken as 100 mol, from the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery.

[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 taken as 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 the positive electrode active material layer> The binder in the positive electrode active material layer of this 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 additive in positive electrode active material layer> The conductive additive 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 additive 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 even 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 to 50 μm.

[0080] <Positive electrode manufacturing method> 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.

[0081] <Other components of lithium-ion secondary batteries> 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> The separator preferably includes a substrate and a ceramic layer provided on at least one surface of the substrate, from the viewpoints of improving heat resistance and reducing thermal shrinkage of the separator. The substrate may be, for example, a porous polyolefin film made of polyethylene, polypropylene, or a laminate of these. The ceramic layer can be formed, for example, by applying a ceramic layer-forming material to a substrate and drying it. The ceramic layer-forming material can be, for example, a material obtained by dispersing or dissolving an inorganic filler and a binder in a solvent. The inorganic filler and binder 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 batteries> The characteristics of the lithium ion secondary battery of this embodiment will be described below.

[0084] [LiPO2F2 concentration in lithium-ion secondary batteries] In the lithium ion secondary battery of this embodiment, the concentration of LiPO2F2 in the electrolyte by <Method 1> is, from the viewpoint of improving the cycle characteristics of the lithium ion secondary battery, 0.01 mass % or more and 1.10 mass % or less, preferably 0.05 mass % or more and 1.05 mass % or less, more preferably 0.10 mass % or more and 1.00 mass % or less, even more preferably 0.15 mass % or more and 0.95 mass % or less, even more preferably 0.17 mass % or more and 0.90 mass % or less, even more preferably 0.19 mass % or more and 0.85 mass % or less, and even more preferably 0.20 mass % or more and 0.80 mass % or less. The LiPO2F2 concentration in the electrolyte solution according to <Method 1> can be adjusted by, for example, adjusting the LiPO2F2 concentration in the electrolyte solution (LiPO2F2 concentration (at the time of preparation)) injected when preparing a lithium ion secondary battery, the amount of the electrolyte solution, the amount of additives, the type and blending ratio of the negative electrode active material in the negative electrode active material layer, the type and blending ratio of the positive electrode active material in the positive electrode active material layer, and / or the conditions of initial charge and discharge. More specifically, the method for measuring the LiPO2F2 concentration in the electrolyte solution can be the method described in the Examples.

[0085] [Capacity retention rate at 45℃ R 45 ] In the lithium ion secondary battery of this embodiment, the capacity retention rate R at 45°C according to the following <Method 2> 45 From the viewpoint of further improving the cycle characteristics of the lithium ion secondary battery, is preferably 82.0% or more, more preferably 83.0% or more, even more preferably 84.0% or more, even more preferably 85.0% or more, even more preferably 86.0% or more, even more preferably 87.0% or more, even more preferably 88.0% or more, even more preferably 89.0% or more, even more preferably 90.0% or more, even more preferably 91.0% or more, even more preferably 92.0% or more, even more preferably 93.0% or more, even more preferably 94.0% or more, and even more preferably 95.0% or more. Capacity retention rate at 25℃ R 25 The upper limit of is not particularly limited, but is, for example, less than 100%, and may be 99.0% or less, or 98.0% or less.

[0086] <Method 2> The lithium ion secondary battery of this embodiment is placed in a thermostatic chamber at 45°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 is calculated from the following formula (2): 45 Calculate. Formula (2): Capacity retention rate R 45 = (500th discharge capacity) / (1st discharge capacity) × 100

[0087] <Charge / discharge cycle> The lithium-ion secondary battery is charged at 30 mA until it reaches an upper limit voltage of 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 it reaches a lower limit voltage of 2.5 V.

[0088] <Applications of lithium-ion secondary batteries> The lithium ion secondary battery of the present embodiment can improve the cycle characteristics of the lithium ion secondary battery and can be used in various applications, such as, but not limited to, industrial applications, consumer applications, automotive applications, and residential applications.

[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 components 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 can also be adopted. Furthermore, the present invention is not limited to the above-described embodiment, and modifications and improvements within the scope that do not impair the effects of the present invention are included in the present invention. [Example]

[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 materials (hereinafter also referred to as negative electrode active material 1) were used as the negative electrode active material. For the negative electrode active material 1, the following negative electrode active material (A) and negative electrode active material (B) were used in combination. 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) to the negative electrode active material (B) was 80:20 (mass ratio).

[0095] [Negative electrode active material (A)] For the negative electrode active material (A), the following graphite powder (A1) and graphite powder (A2) were used in combination. The graphite powder (A1) and the graphite powder (A2) were blended so that the ratio of the graphite powder (A1) to the graphite powder (A2) was 50:50 (mass ratio).

[0096] [Graphite powder (A1)] As the graphite powder (A1), artificial graphite particles containing amorphous carbon on the surface were used. The following graphite powder 1 was used for the graphite powder (A1). · 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. The following graphite powder 2 was used for the graphite powder (A2). · 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 for the Si-C composite particles. · Si / C particles 1 (median diameter D 50 : 4.8 μm, particles produced according to the following <Production of Si / C particles 1>)

[0099] [Production of Si / C particles 1] Porous carbon material 1 (median diameter D 50The porous carbon material 1 was treated at 500°C, 760 Torr, and 120 minutes under the conditions of a mixed gas of 2 mol% silane gas and 98 mol% nitrogen gas at a flow rate of 300 sccm. The product was then cooled to room temperature to obtain Si / C particles 1. EDS mapping was performed using an energy dispersive X-ray analyzer on the cross section of the obtained Si / C particle 1. The results of the EDS mapping confirmed that the Si / C particle 1 contains silicon and that silicon is present in at least some of the pores of the porous carbon material 1 in the Si / C particle 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 :3.5μm)

[0101] The following materials were used for the electrolyte. Solvent 1: Ethylene carbonate (hereinafter referred to as EC) Solvent 2: Ethyl methyl carbonate (hereinafter referred to as EMC) Electrolyte: Lithium hexafluorophosphate (LiPF6) Electrolyte: Lithium difluorophosphate (LiPO2F2) The following materials were used for the separator. Separator: 10 μm thick microporous polyethylene film with ceramic coating 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 referred to as PAA) Conductive additive: Single-walled carbon nanotubes (hereinafter referred to as CNTs) 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 referred to as PVDF) Conductive additive: Single-walled carbon nanotubes (hereinafter referred to as CNTs) Solvent: N-methyl-2-pyrrolidone (hereinafter referred to as NMP)

[0104] <Method for measuring particle size of positive electrode active material and negative electrode active material> Using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, model number: SALD-2300), the volume-based median diameter D 50 Here, the median diameter D of the positive electrode active material 50 The median diameter D of the negative electrode active material was measured after suspending the positive electrode active material in a sodium hexametaphosphate aqueous solution as a dispersion medium and ultrasonically dispersing it. 50 The negative electrode active material was suspended in a dispersion medium, ultrasonically dispersed, and then the measurement was performed. The measurement was performed five times, and the average value was calculated as the median diameter D 50 It was decided.

[0105] (Examples 1 to 3, Comparative Examples 1 and 2) <Fabrication of lithium-ion secondary batteries> A lithium ion secondary battery was fabricated through the following steps, which will be described in detail below.

[0106] First, the electrolyte solutions to be used in the lithium ion secondary batteries of each example were 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 (LiPF6) was dissolved in the obtained mixed solvent so that its content was 12 mass%. LiPO2F2 was also dissolved in the mixed solvent so that the LiPO2F2 content was the LiPO2F2 concentration (at the time of preparation) listed in Table 1, to prepare the electrolyte solutions for each example.

[0107] Next, the positive electrodes to be used in the lithium ion secondary batteries of each example were fabricated in the following manner. The positive electrode active material, binder (PVDF), and conductive additive (CNT) shown in Table 1 were dispersed in a solvent (NMP) at a mass ratio of positive electrode active material:PVDF:CNT=97.5:1.5:1.0 to prepare positive electrode slurries. Next, the positive electrode slurry was applied to a 12 μm thick aluminum foil positive electrode current collector so that the initial charge capacity per unit area was 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 press the positive electrode slurry to a density of 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 to be used in the lithium ion secondary batteries of each example were fabricated in the following manner. The negative electrode active material, binder (PAA), and conductive additive (CNT) shown in Table 1 were dispersed in a solvent (pure water) at 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 slurry was applied to a copper foil with a thickness of 8 μm, which was used as a negative electrode current collector, so that the initial charge capacity per unit area was 4.3 mAh / cm 2 The applied 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 in the following manner. One double-sided coated positive electrode and two single-sided coated negative electrodes were placed opposite each other with the coated surfaces facing each other through a separator, and stacked in the following order: negative electrode, separator, positive electrode, separator, negative electrode. The stack thus fabricated was then wrapped in a laminated outer casing made by processing a film primarily composed of aluminum. The aluminum 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 produce 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, and the laminated cell was then sealed under reduced pressure to produce a laminated lithium ion secondary battery. The amount of the electrolyte solution injected 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 batteries> 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 the lithium ion secondary battery of each example after initial charge and discharge. Note that, in the following, the charge and discharge of the lithium ion secondary battery was performed using a charge and discharge device in an environment at a temperature of 25°C. The lithium ion secondary battery of each example was charged at a charging current of 0.05 C up to a battery voltage of 3.2 V, and then left to stand for 12 hours for pre-charging. Next, the battery was charged at a constant current of 0.05C until the battery voltage reached 4.25V. After the battery voltage reached 4.25V, constant voltage charging was continued until the current value dropped to 0.015C. The battery was then left to rest for 10 minutes. After that, constant current discharging was performed at a discharge current of 0.33C until the battery voltage reached 2.5V. After the battery voltage reached 2.5V, the battery was left to rest 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 dropped to 0.05 C. The charged lithium-ion secondary battery was then stored for 48 hours in an environment at a temperature of 45°C while the battery voltage was monitored. After storage, the lithium-ion secondary battery was returned to an environment at a temperature of 25°C and constant current discharge was performed at a discharge current of 1 C until the battery voltage reached 2.5 V. Next, constant current discharge was performed at a discharge current of 0.33 C until the battery voltage reached 2.5 V. After the battery voltage reached 2.5 V, it was left to stand 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 dropped to 0.05 C. Next, 10 minutes after the end of charging, constant current discharging was performed at a discharge current of 1 C until the battery voltage reached 2.5 V. Next, constant current discharging was performed at a discharge current of 0.33 C until the battery voltage reached 2.5 V. After the battery voltage reached 2.5 V, the battery was left to stand for 10 minutes. Next, the battery was charged at a constant current of 0.33 C until the battery voltage reached 4.25 V. After the battery voltage reached 4.25 V, the battery was charged at a constant voltage until the current dropped to 0.05 C. The battery was then left for 10 minutes. After that, the battery was discharged at a constant 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 the initial charge and discharge were measured by the following method. The measurement results are shown in Table 1.

[0112] [LiPO2F2 concentration after initial charge / discharge] For the lithium ion secondary battery of each example after the initial charge and discharge, the LiPO2F2 concentration after the initial charge and discharge was measured by the following method. The lithium ion secondary battery after the initial charge and discharge of each example was decomposed in an inert gas atmosphere at a temperature of 25°C and a relative humidity of 3% 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 a second sample. 19 The amount of LiPO2F2 in the second sample was measured by F-NMR. The amount of LiPO2F2 in the first sample was then calculated from the amount of LiPO2F2 in the second sample. The concentration of LiPO2F2 in the electrolyte (LiPO2F2 concentration after initial charge / discharge) was then calculated using the following formula (1): Equation (1): Concentration of LiPO2F2 in the electrolyte = (amount of LiPO2F2 in the first sample) / (amount of electrolyte collected from the disassembled lithium-ion secondary battery) × 100

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

[0114] [Capacity retention rate at 45℃ R 45 ] For each example of the lithium-ion secondary battery after the initial charge and discharge, the capacity retention rate R at 45°C 45 was measured by the following method. The lithium ion secondary battery after the initial charge and discharge in each example was placed in a thermostatic chamber at 45°C. Then, the lithium ion secondary battery was charged and discharged according to the following <charge and discharge cycle>, and the first discharge capacity was measured. Then, the lithium ion secondary battery was charged and discharged 499 times in total according to the following <charge and discharge cycle>. Then, the lithium ion secondary battery was charged and discharged according to the following <charge and discharge cycle>, and the 500th discharge capacity was measured. Next, the capacity retention rate R 45 was calculated. Formula (2): Capacity retention rate R 45 = (500th discharge capacity) / (1st discharge capacity) × 100

[0115] <Charge / discharge cycle> The lithium-ion secondary battery was charged at 30 mA until it reached an upper limit voltage of 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 it reached a lower limit voltage of 2.5 V.

[0116] [Table 1] [Explanation of symbols]

[0117] 1 Cathode 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 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 solution, the negative electrode active material contained in the negative electrode active material layer has an SEI film on at least a part of its surface, the electrolyte solution contains lithium difluorophosphate, A lithium ion secondary battery according to the following <Method 1>, wherein the concentration of the lithium difluorophosphate in the electrolyte solution is 0.01% by mass or more and 1.10% by mass or less. <Method 1> The lithium ion secondary battery was disassembled under an inert gas atmosphere, and then 0.2 g of the electrolyte solution collected from the disassembled 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 substance to prepare a second sample. Next, 0.5 g of the second sample was used to １９ The amount of lithium difluorophosphate in the second sample is measured by an F-NMR method, and then the concentration of lithium difluorophosphate in the first sample is calculated from the amount of lithium difluorophosphate in the second sample, and the concentration of lithium difluorophosphate in the electrolytic solution is calculated using the following formula (1): Equation (1): Concentration of the lithium difluorophosphate in the electrolyte solution=(Amount of the lithium difluorophosphate in the first sample) / (Amount of the electrolyte solution collected from the disassembled lithium ion secondary battery)×100

2. 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, 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 ５０ and a graphite powder (A2) different from the graphite powder (A1), The median diameter D of the graphite powder (A1) 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. 7. The lithium ion secondary battery according to claim 5, wherein the graphite powder (A2) contains graphite particles whose surfaces do not contain amorphous carbon.

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. 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 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 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 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 includes 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 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 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. 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. 20. The lithium ion secondary battery according to claim 18 or 19, wherein, when the total content of nickel, cobalt, and manganese in the particles (C) is 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. Capacity retention rate R at 45°C according to the following <Method 2> ４５ The lithium ion secondary battery according to any one of claims 1 to 20, wherein the % charge / discharge ratio is 82.0% or more. <Method 2> The lithium ion secondary battery was placed in a thermostatic chamber at 45°C, and then the lithium ion secondary battery was charged and discharged in accordance with the following <Charge and Discharge Cycle>, and the first discharge capacity was measured. Next, the lithium ion secondary battery was repeatedly charged and discharged 499 times in total in accordance with the following <Charge and Discharge Cycle>, and then the lithium ion secondary battery was charged and discharged in accordance with the following <Charge and Discharge Cycle>, and the 500th discharge capacity was measured. Next, the capacity retention rate R ４５ Calculate. Formula (2): The capacity retention rate R ４５ = (500th discharge capacity) / (1st 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 claims 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, when the total 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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