Lithium-ion secondary battery

Abstract

This lithium ion secondary battery comprises a positive electrode, a negative electrode, and an electrolytic solution, wherein: the negative electrode has a current collector and a negative electrode active material layer provided on at least one side of the current collector; the negative electrode active material layer contains a negative electrode active material containing graphite particles and amorphous carbon fine particles; the electrolytic solution contains at least a non-aqueous solvent, an electrolyte salt, and an additive; and the additive contains fluorosulfonate and methoxysulfonate. Consequently, it is possible to achieve an improvement in output characteristics and an improvement in high-temperature storage characteristics in a low-SOC region of the lithium ion secondary battery.

Description

[Technical Field]

[0001] The present disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] In the automotive industry, fuel economy and environmental regulations are becoming stricter in each country and region. To comply with these regulations, attention is being paid to the development of battery-powered electric vehicles (EVs) and fuel cell vehicles (FCVs), which do not emit carbon dioxide. However, EVs face challenges, such as insufficient charging infrastructure and the long charging time compared to refueling. Fuel cell vehicles face challenges, such as the high cost of building hydrogen station infrastructure and the high cost of fuel cells. Therefore, plug-in hybrid electric vehicles (PHEVs) and hybrid electric vehicles (HEVs), which are powered by both an internal combustion engine and a battery and emit low carbon dioxide, are becoming leading candidates for complying with fuel economy and environmental regulations.

[0003] Lithium-ion secondary batteries are generally used in PHEVs and HEVs. Patent Document 1 describes a lithium-ion secondary battery in which a non-aqueous electrolyte solution contains, together with an electrolyte and a non-aqueous solvent, at least one compound selected from the group consisting of lithium fluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium difluorophosphate, and the negative electrode contains a negative electrode active material containing Si or Si metal oxide and graphite particles. Patent Document 2 describes a lithium-ion secondary battery in which a non-aqueous electrolyte solution contains lithium hexafluorophosphate (LiPF6) as the electrolyte and further contains vinylene carbonate (VC), lithium bis(oxalato)borate (LiBOB), and lithium difluoro(oxolato)borate (LiDFOB) as additives. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-106174 [Patent Document 2] Japanese Patent Publication No. 2020-167054 Summary of the Invention [Problem to be solved by the invention]

[0005] PHEVs and HEVs require lithium-ion secondary batteries that have a small increase in resistance even in the low state of charge (SOC) range, provide stable output, and can suppress capacity loss and resistance increase even in long-term high-temperature storage environments.

[0006] The techniques described in Patent Documents 1 and 2 do not aim to improve both the output characteristics in a low SOC region and the high-temperature storage characteristics, and a technique for achieving both is needed.

[0007] A primary object of the present disclosure is to provide a lithium ion secondary battery that can achieve both improved output characteristics in a low SOC region and improved high-temperature storage characteristics. [Means for solving the problem]

[0008] The lithium ion secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte solution. The negative electrode has a current collector and a negative electrode active material layer provided on at least one surface of the current collector. The negative electrode active material layer includes a negative electrode active material containing graphite particles and amorphous carbon fine particles. The electrolyte solution includes at least a non-aqueous solvent, an electrolyte salt, and an additive, and the additive contains a fluorosulfonate and a methoxysulfonate. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a lithium ion secondary battery that can achieve both improved output characteristics in a low SOC region and improved high-temperature storage characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an external perspective view of a lithium ion secondary battery according to an embodiment; [Figure 2] 1 is an exploded perspective view of a lithium-ion secondary battery according to an embodiment; [Figure 3] FIG. 2 is an exploded perspective view showing a state in which a part of the wound group according to the embodiment is unfolded. [Figure 4] FIG. 2 is a conceptual diagram showing a mixed state of graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles in a negative electrode active material layer according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments will be described with reference to the drawings as appropriate. The following description illustrates specific examples of the contents of the present disclosure, and the present disclosure is not limited to these descriptions. Various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed herein. Furthermore, in all drawings used to explain the present disclosure, components having the same function are designated by the same reference numerals, and repeated description thereof may be omitted. Furthermore, for the sake of convenience, the dimensional ratios of the drawings may differ from the actual ratios, and some components may be omitted from the drawings. Furthermore, in this application, a numerical range expressed using the symbol "~" includes the numerical values ​​before and after the symbol "~" as the lower and upper limits, respectively.

[0012] 1 and 2, a lithium-ion secondary battery 100 according to an embodiment includes a battery can 1 and a battery lid 6. The battery can 1 has a rectangular bottom surface 1d, side surfaces including a pair of opposing wide side surfaces 1b with relatively large areas and a pair of opposing narrow side surfaces 1c with relatively small areas, rising from the bottom surface 1d, and an opening 1a that opens upward at the upper ends of the wide side surfaces 1b and the narrow side surfaces 1c. Here, upward refers to the Z direction in FIGS. 1 and 2.

[0013] The opening 1a of the battery can 1 is sealed by a battery lid 6. The battery lid 6 has a substantially rectangular flat plate shape, and is welded to close the opening 1a of the battery can 1, thereby sealing the battery can 1.

[0014] A gas release valve 10 is integrally provided on the battery lid 6. When the pressure inside the battery can 1 increases, the gas release valve 10 opens, allowing gas to be released from inside the battery can 1, thereby reducing the pressure inside the battery can 1. This ensures the safety of the lithium-ion secondary battery 100.

[0015] A liquid filling port 9 is formed in the battery lid 6 for filling the electrolyte into the battery can 1. The liquid filling port 9 is sealed by a liquid filling plug 11 after the electrolyte has been filled into the battery can 1. The liquid filling plug 11 is joined to the battery lid 6 by laser welding to seal the liquid filling port 9 and hermetically seal the lithium ion secondary battery 100.

[0016] The battery lid 6 is further provided with a positive electrode side through-hole 46 and a negative electrode side through-hole 26 .

[0017] A positive electrode external terminal 14 and a negative electrode external terminal 12 are provided above the battery lid 6. A positive electrode current collector plate 44 and a negative electrode current collector plate 24 are provided below the battery lid 6 and inside the battery can 1.

[0018] The positive external terminal 14 and the positive current collector plate 44 may be made of, for example, an aluminum alloy, and the negative external terminal 12 and the negative current collector plate 24 may be made of, for example, a copper alloy.

[0019] The positive electrode external terminal 14 and the negative electrode external terminal 12 each have a welded joint to which a bus bar or the like is welded. The welded joint has a rectangular block shape that protrudes upward from the battery lid 6. The lower surface of the welded joint faces the surface of the battery lid 6, and the upper surface of the welded joint is located at a predetermined height and is approximately parallel to the battery lid 6.

[0020] The positive current collector 44 has a rectangular plate-shaped positive current collector base 41 facing the underside of the battery lid 6, and a positive electrode side connection end 42 extending from a side end of the positive current collector base 41 along the wide side surface 1b of the battery can 1 toward the bottom surface 1d. Similarly, the negative current collector 24 has a rectangular plate-shaped negative current collector base 21 facing the underside of the battery lid 6, and a negative electrode side connection end 22 extending from a side end of the negative current collector base 21 along the wide side surface 1b of the battery can 1 toward the bottom surface 1d. A positive electrode side opening hole 43 and a negative electrode side opening hole 23 are formed in the positive current collector base 41 and the negative current collector base 21, respectively.

[0021] A positive electrode connecting portion 14a and a negative electrode connecting portion 12a are provided so as to protrude from the lower surfaces of the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively. The positive electrode connecting portion 14a and the negative electrode connecting portion 12a are formed integrally with the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively.

[0022] The positive electrode connection portion 14a has a cylindrical shape that can be inserted into the positive electrode side through-hole 46 of the battery lid 6 and the positive electrode side opening hole 43 of the positive electrode current collector base 41. Similarly, the negative electrode connection portion 12a has a cylindrical shape that can be inserted into the negative electrode side through-hole 26 of the battery lid 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21. The positive electrode connection portion 14a passes through the positive electrode side through-hole 46 of the battery lid 6 and the positive electrode side opening hole 43 of the positive electrode current collector base 41, penetrating the battery lid 6 and the positive electrode current collector base 41. The positive electrode external terminal 14 and the positive electrode current collector 44 are electrically connected and fixed to the battery lid 6 via the positive electrode connection portion 14a. Similarly, the negative electrode connection portion 12a passes through the negative electrode side through-hole 26 of the battery lid 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21, penetrating the battery lid 6 and the negative electrode current collector base 21. The negative electrode external terminal 12 and the negative electrode current collector plate 24 are electrically connected via a negative electrode connecting portion 12 a and are fixed to the battery lid 6 .

[0023] The positive electrode external terminal 14 is electrically connected to the wound pack 3 (described later) via the positive electrode connection portion 14a and the positive electrode current collector plate 44. Similarly, the negative electrode external terminal 12 is electrically connected to the wound pack 3 via the negative electrode connection portion 12a and the negative electrode current collector plate 24. When the lithium ion secondary battery 100 is being charged, electricity is supplied from an external power source to the wound pack 3 via the positive electrode external terminal 14, the positive electrode connection portion 14a, and the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12a, and the negative electrode current collector plate 24. When the lithium ion secondary battery 100 is being discharged, electricity is supplied from the wound pack 3 to an external load via the positive electrode external terminal 14, the positive electrode connection portion 14a, and the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12a, and the negative electrode current collector plate 24.

[0024] In order to electrically insulate the positive electrode current collector 44, the negative electrode current collector 24, the positive electrode external terminal 14, and the negative electrode external terminal 12 from the battery lid 6, a gasket 5 is provided between each of the positive electrode external terminal 14 and the negative electrode external terminal 12 and the battery lid 6, and an insulating plate 7 is provided between each of the positive electrode current collector 44 and the negative electrode current collector 24 and the battery lid 6. Examples of materials for the insulating plate 7 and the gasket 5 include insulating resin materials such as polybutylene terephthalate, polyphenylene sulfide, and perfluoroalkoxy fluororesin.

[0025] An electrolyte solution EL and a wound group 3 are housed in a battery can 1. The electrolyte solution EL is injected into the battery can 1 through a liquid inlet 9. As shown in FIG. 3 , the wound group 3 has a negative electrode 32, a positive electrode 34, and two separators 33 and 35. The separator 35, the negative electrode 32, the separator 33, and the positive electrode 34 are stacked in this order and wound in a flat shape. The separator 35 is located at the outermost periphery of the wound group 3, and the negative electrode 32 is located inside it. The two separators 33 and 35 electrically insulate the positive electrode 34 and the negative electrode 32.

[0026] The wound group 3 has a pair of opposing end faces 3a, 3b perpendicular to the winding axis and a side face 3c between the pair of end faces 3a, 3b. The side face 3c has a pair of curved portions with a semicircular cross section that face each other and a flat portion formed continuously between the pair of curved portions. The wound group 3 is placed inside the battery can 1 so that the flat portion of the side face 3c and the wide side face 1b of the battery can 1 are approximately parallel.

[0027] Hereinafter, the main parts of the lithium-ion secondary battery 100 according to the embodiment will be described, focusing on the positive electrode 34, the negative electrode 32, and the electrolyte solution EL. Furthermore, the "positive electrode mixture layer" corresponds to the "positive electrode active material layer" according to the present disclosure, and the "negative electrode mixture layer" corresponds to the "negative electrode active material layer" according to the present disclosure.

[0028] <Positive electrode> The positive electrode 34 has a positive electrode current collector 34a and a positive electrode mixture layer 34b (positive electrode active material layer) provided on at least one surface, preferably both surfaces, of the positive electrode current collector 34a. In the description of the positive electrode, the "positive electrode current collector" may also be simply referred to as the "current collector."

[0029] The positive electrode current collector 34a is formed from any material that is highly conductive and does not alloy with lithium ions. The positive electrode current collector 34a may have a plate (sheet) shape. For example, a positive electrode foil, specifically, a metal foil such as aluminum foil, can be used as the positive electrode current collector 34a. A portion 34c (hereinafter referred to as the "positive electrode current collector exposed portion") that is not covered with the positive electrode mixture layer 34b is provided at one end in the width direction of the positive electrode current collector 34a. The positive electrode current collector exposed portion 34c is provided on and in the vicinity of the end face 3a of the winding group 3. The positive electrode current collector exposed portion 34c faces the positive electrode side connection end 42 of the positive electrode current collector plate 44 and is electrically connected thereto.

[0030] The positive electrode mixture layer 34b contains a positive electrode active material. The positive electrode active material may contain, for example, a ternary material containing at least nickel, cobalt, and manganese. Specifically, a composite oxide of nickel, cobalt, and manganese may be used. In one preferred embodiment, the positive electrode active material is a ternary lithium-containing composite oxide represented by the following general composition formula (1):

[0031] Li 1+X M A O2(1) (wherein X satisfies -0.15≦X≦0.15, and M A represents an element group including at least one selected from the group consisting of Mn and Al, Ni, and Co. The ternary lithium-containing composite oxide represented by the above general composition formula (1) has high thermal stability and stability in a high potential state, and by using this oxide, the safety and various battery characteristics of a lithium ion secondary battery can be improved.

[0032] The positive electrode mixture layer 34b may further include at least one of a conductive additive and a binder, and preferably both.

[0033] The conductive additive is not particularly limited, and may be, for example, a carbon-based material. The carbon-based material may be crystalline carbon, amorphous carbon, or a mixture thereof. Examples of crystalline carbon include artificial graphite, natural graphite (e.g., flake graphite), or a mixture thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or a mixture thereof).

[0034] The binder is not particularly limited, but examples that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polypropylene fluoride, polychloroprene fluoride, butyl rubber, nitrile rubber, styrene butadiene rubber (SBR), polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylic resins, and mixtures thereof.

[0035] The positive electrode 34 can be formed, for example, as follows. A positive electrode active material, and optionally at least one of a conductive additive and a binder, are dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP), water) to prepare a paste or slurry positive electrode mixture composition. This positive electrode mixture composition is applied to the surface (one or both sides) of the positive electrode current collector 34a, dried, and optionally subjected to a calendar treatment to form the positive electrode mixture layer 34b. This results in the positive electrode 34. However, the positive electrode is not limited to being formed by the above method, and may be formed by other methods.

[0036] <Negative electrode> The negative electrode 32 has a negative electrode current collector 32a and a negative electrode mixture layer 32b (negative electrode active material layer) provided on at least one side, preferably both sides, of the negative electrode current collector 32a. In the description of the negative electrode, the "negative electrode current collector" may also be simply referred to as the "current collector."

[0037] The negative electrode current collector 32a is formed from any material that is highly conductive and does not alloy with lithium ions. One widthwise end of the negative electrode current collector 32a is provided with a portion 32c (hereinafter referred to as the "negative electrode current collector exposed portion") that is not covered with the negative electrode mixture layer 32b. The negative electrode current collector exposed portion 32c is provided on the end face 3b of the wound group 3 and in its vicinity. The negative electrode current collector exposed portion 32c faces the negative electrode side connection end 22 of the negative electrode current collector plate 24 and is electrically connected thereto.

[0038] The portion of the negative electrode 32 coated with the negative electrode mixture layer 32b is preferably wider in the width direction than the portion of the positive electrode 34 coated with the positive electrode mixture layer 34b, such that the portion coated with the positive electrode mixture layer 34b is sandwiched between the portions coated with the negative electrode mixture layer 32b. In one preferred embodiment, the positive electrode current collector exposed portion 34c and the negative electrode current collector exposed portion 32c are bundled together at their flat portions and connected by welding or the like. Note that although the separators 33 and 35 are wider in the width direction than the portion coated with the negative electrode mixture layer 32b, they are wound around the ends of the positive electrode current collector exposed portion 34c and the negative electrode current collector exposed portion 32c in positions where the current collectors are exposed, so this does not hinder bundling and welding.

[0039] The negative electrode mixture layer 32b includes a negative electrode active material containing graphite particles and amorphous carbon fine particles. The negative electrode active material is not particularly limited as long as it contains graphite particles and amorphous carbon fine particles, but the amorphous carbon fine particles are preferably supported on the graphite particles, and particularly, the graphite particles are preferably a mixture of graphite particles (A) and graphite particles (B) supporting the amorphous carbon fine particles.

[0040] The negative electrode mixture layer 32b may further include at least one of a negative electrode additive and a binder, preferably both, in addition to the negative electrode active material. In one aspect of the embodiment, the negative electrode mixture layer 32b includes a negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, and the graphite particles (B) carrying amorphous carbon fine particles have a density of amorphous carbon fine particles per unit area of ​​0.4 particles / μm 2 In another aspect of the embodiment, the anode mixture layer 32b includes anode active material and a binder that holds the anode active material, and the anode active material includes graphite particles and amorphous carbon fine particles. The anode mixture layer 32b may further include anode additives containing copper oxide.

[0041] The graphite particles are not particularly limited, but examples thereof include natural graphite particles and artificial graphite particles, with natural graphite particles being preferred. Examples of natural graphite include flake graphite, lump graphite, and amorphous graphite. The graphite particles (A) and the graphite particles constituting the graphite particles (B) carrying amorphous carbon microparticles may be the same type of graphite particles or different types of graphite particles.

[0042] Examples of negative electrode active materials other than graphite include carbon-based materials such as non-graphitizable carbon (hard carbon) and easily graphitizable carbon (soft carbon).

[0043] Regarding graphite, graphite whose surface is coated with amorphous carbon may be used. By coating with amorphous carbon, it is possible to prevent reaction with excess electrolyte. Examples of amorphous carbon include pitch. That is, the graphite particles constituting the graphite particles (A) and the graphite particles (B) carrying amorphous carbon microparticles are preferably graphite particles coated with amorphous carbon, more preferably pitch-coated graphite particles. Furthermore, the graphite particles constituting the graphite particles (A) and the graphite particles (B) carrying amorphous carbon microparticles are particularly preferably natural graphite particles coated with amorphous carbon.

[0044] The supported amorphous carbon fine particles constituting the graphite particles (B) carrying amorphous carbon fine particles are not particularly limited, and examples thereof include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. The supported amorphous carbon fine particles refer to amorphous carbon particles scattered on the surface of graphite particles, and are different from the amorphous carbon coating graphite particles described above. The amorphous carbon coating graphite particles refers to amorphous carbon covering the entire or partial surface of graphite particles.

[0045] The graphite particles (B) carrying amorphous carbon particles have an amorphous carbon particle count of 0.4 particles / μm 2 or more, and preferably the number of amorphous carbon particles per unit area is 0.4 / μm 2~2.4 pieces / μm 2 and more preferably, the number of amorphous carbon particles per unit area is 0.8 / μm 2 ~2.0 pieces / μm 2 In addition, when the graphite particles support a small amount of amorphous carbon particles, that is, when the number of amorphous carbon particles per unit area is 0.4 particles / μm 2 Graphite particles carrying amorphous carbon particles with a particle size of less than 0.4 particles / μm do not qualify as graphite particles (B). 2 Graphite particles carrying amorphous carbon fine particles at a molecular weight of less than 1000 may be used as the graphite particles (A).

[0046] The average particle size of the graphite particles (A) is preferably 4 μm or more and 12 μm or less, and more preferably 5 μm or more and 10 μm or less.

[0047] The average particle size of the graphite particles (B) carrying amorphous carbon fine particles is preferably 4 μm or more and 12 μm or less, more preferably 5 μm or more and 10 μm or less. The average particle size of the graphite particles (A) and the average particle size of the graphite particles (B) carrying amorphous carbon fine particles may be the same or different.

[0048] The average particle size of the supported amorphous carbon particles constituting the graphite particles (B) on which amorphous carbon particles are supported is preferably smaller than the average particle size of the graphite particles (A) and the average particle size of the graphite particles constituting the graphite particles (B) on which amorphous carbon particles are supported. The average particle size of the amorphous carbon particles is preferably 0.05 μm or more and 0.5 μm or less, more preferably 0.1 μm or more and 0.4 μm or less.

[0049] As a negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, for example, as shown in Fig. 4, graphite particles (A) 50 not carrying amorphous carbon fine particles and graphite particles (B) 52 carrying amorphous carbon fine particles (amorphous carbon fine particles 56 carried by graphite particles 54) are preferably present in a mixed state in the negative electrode active material layer. Note that in Fig. 4, components constituting the negative electrode active material layer other than the graphite particles (A) and the graphite particles (B) carrying amorphous carbon fine particles are omitted.

[0050] When the negative electrode active material contains graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, the mass ratio thereof (graphite particles (A) / graphite particles (B) carrying amorphous carbon fine particles) is preferably 0.25 or more and 5 or less, more preferably 0.5 or more and 2 or less.

[0051] The negative electrode active material may be, for example, a material obtained by mixing a graphite material with carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black as a conductive additive, a composite material obtained by mixing such a conductive additive with a graphite material and then coating it with amorphous carbon, or a material obtained by mixing a graphite material with non-graphitizable carbon (hard carbon) or easily graphitizable carbon (soft carbon). The shape of the negative electrode active material is not particularly limited and may be, for example, spherical, flaky, fibrous, or pulverized forms of these.

[0052] As a result of extensive research, the present inventors have found that by using a negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon microparticles, a lithium ion secondary battery can reduce internal resistance over a wide range from a low SOC region to a high SOC region while maintaining battery capacity, thereby providing high output and good storage characteristics.

[0053] Furthermore, as a result of extensive research, the present inventors have found that a lithium-ion secondary battery having a high output and good storage characteristics can be achieved by including a negative electrode active material containing graphite particles and amorphous carbon fine particles, and a negative electrode active material layer having a negative electrode active material and a binder that holds the negative electrode active material, and thereby reducing internal resistance over a wide range from a low SOC region to a high SOC region while ensuring battery capacity.

[0054] The negative electrode active material layer preferably contains copper oxide as a negative electrode additive. When the negative electrode active material layer contains 0.5% by mass to 15% by mass of copper oxide relative to 100% by mass of the total of the negative electrode active material and copper oxide, the lithium ion secondary battery can more preferably have high output and good storage characteristics because the internal resistance can be reduced over a wide range from a low SOC region to a high SOC region while ensuring the battery capacity.

[0055] Copper oxide is included as a negative electrode additive. The copper oxide may be copper(I) oxide (CuO), copper(II) oxide (CuO), or a mixture thereof. That is, the copper oxide may be at least one copper oxide selected from CuO and CuO. The negative electrode additive may be particulate. The shape of the particulate negative electrode additive is not particularly limited and may be, for example, spherical, flaky, fibrous, or a pulverized form thereof. The particulate negative electrode additive may be particles containing copper(I) oxide (CuO), copper(II) oxide (CuO), or a mixture thereof, or may be particles consisting essentially of copper(I) oxide (CuO), copper(II) oxide (CuO), or a mixture thereof. These various particulate negative electrode additives may be used alone or in combination.

[0056] In the negative electrode mixture layer 32b, the negative electrode active material and the negative electrode additive may exist as separate particles that are not composited with each other. This allows the negative electrode active materials to be electrically connected to each other without being hindered by the negative electrode additive having a high electrical resistance, thereby suppressing an increase in the internal resistance of the lithium ion secondary battery. The negative electrode additive may have an average particle size smaller than the average particle size of the negative electrode active material, and may have an average particle size of 1 μm to 10 μm. This allows the negative electrode active materials to be electrically connected to each other without being hindered by the negative electrode additive having a high electrical resistance, thereby suppressing an increase in the internal resistance of the lithium ion secondary battery. The average particle sizes of the negative electrode active material (e.g., graphite particles, amorphous carbon fine particles, etc.) and the negative electrode additive are determined based on SEM observation images.

[0057] The binder for the negative electrode mixture layer 32b may be the same as the materials exemplified as the binder for the positive electrode mixture layer 34b.

[0058] The negative electrode mixture layer 32b may further contain a dispersant, such as carboxymethyl cellulose (CMC).

[0059] The negative electrode 32 can be formed, for example, as follows. First, a negative electrode active material, a negative electrode additive containing copper oxide, a binder, and optionally a dispersant are prepared. The negative electrode active material and the negative electrode additive may be in particulate form. The negative electrode active material and the negative electrode additive may be separate particles that are not composited with each other. The negative electrode active material, the negative electrode additive, the binder, and optionally a dispersant are dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) or water) to prepare a paste or slurry negative electrode mixture composition. This negative electrode mixture composition is applied to the surface (one or both sides) of the negative electrode current collector 32a, dried, and optionally calendered to form the negative electrode mixture layer 32b. This results in the negative electrode 32. However, the negative electrode is not limited to being formed by the above method and may be formed by other methods.

[0060] <Electrolyte> The electrolyte (non-aqueous electrolyte) EL of the lithium-ion secondary battery 100 contains at least a non-aqueous solvent, an electrolyte salt, and an additive, the additive containing fluorosulfonate and methoxysulfonate. The electrolyte contains the additives fluorosulfonate and methoxysulfonate, in addition to the non-aqueous solvent and electrolyte salt. This strengthens the protective coating called the solid electrolyte interface (SEI) formed on the surface of the negative electrode active material of the negative electrode, thereby suppressing side reactions that occur at the interface between the electrolyte and the negative electrode active material, even under high-temperature storage conditions. Therefore, by using this electrolyte in combination with the above-described negative electrode, it is possible to achieve both improved output characteristics in the low SOC region and improved high-temperature storage characteristics.

[0061] The additive, the electrolyte salt, and the non-aqueous solvent according to the embodiment will be specifically described below.

[0062] 1. Additives The additive contains a fluorosulfonate and a methoxysulfonate. Here, the fluorosulfonate and the methoxysulfonate will be described.

[0063] Fluorosulfonates The fluorosulfonate is represented by the following general composition formula (2).

[0064] X1(FSO3) n (2) (In the formula, X1 represents a counter cation of the fluorosulfonate, and n represents the valence of the counter cation.)

[0065] The counter cation of the fluorosulfonate is not particularly limited, but examples thereof include lithium, sodium, and potassium.

[0066] Specific examples of fluorosulfonates include lithium fluorosulfonate (FSO3Li), sodium fluorosulfonate, and potassium fluorosulfonate, with lithium fluorosulfonate being particularly preferred. Lithium fluorosulfonate is represented by the following structural formula (2-1):

[0067] [ka]

[0068] The fluorosulfonate may be used alone or in any combination of two or more in any ratio. The content of the fluorosulfonate in the electrolyte is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.3% by mass or more. The content of the fluorosulfonate in the electrolyte is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1.5% by mass or less. Within this range, the effect of improving the high-temperature storage characteristics of the lithium-ion secondary battery can be easily achieved, and battery swelling due to increased gas generation can be avoided.

[0069] b.Methoxysulfonate Methoxysulfonates are represented by the following general composition formula (3).

[0070] X2(CH3OSO3) m (3) (In the formula, X2 represents a counter cation of methoxysulfonate, and m represents the valence of the counter cation.)

[0071] The counter cation of the methoxysulfonate is not particularly limited, but examples thereof include lithium, sodium, and potassium.

[0072] Specific types of methoxysulfonates include lithium methoxysulfonate (CH3OSO3Li), sodium methoxysulfonate, potassium methoxysulfonate, etc., and lithium methoxysulfonate is particularly preferred. Lithium methoxysulfonate is represented by the following structural formula (3-1).

[0073] [ka]

[0074] The methoxysulfonate may be used alone or in any combination of two or more kinds at any ratio. The content of the methoxysulfonate (e.g., lithium methoxysulfonate) contained in the electrolyte solution is, for example, 0.001% by mass or more, preferably 0.1% by mass or more. The content of the methoxysulfonate (e.g., lithium methoxysulfonate) contained in the electrolyte solution is, for example, 1% by mass or less, preferably 0.8% by mass or less, and more preferably 0.5% by mass or less. Within such a range, the effect of improving the high-temperature storage characteristics of the lithium-ion secondary battery can be easily exhibited, and the methoxysulfonate can be uniformly dissolved in the electrolyte solution.

[0075] c.Other The additive is not particularly limited as long as it contains a fluorosulfonate and a methoxysulfonate, but it is preferable that it further contains a lithium borate and a difluorophosphate, and among these, it is preferable that it further contains a vinylene carbonate compound.

[0076] (a) Lithium borate Specific examples of lithium borates include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxolato)borate (LiDFOB), with LiBOB being particularly preferred. Lithium bis(oxalato)borate (LiBOB) is represented by the following structural formula (4-1). Lithium difluoro(oxolato)borate (LiDFOB) is represented by the following structural formula (4-2).

[0077] [ka]

[0078] [ka]

[0079] The lithium borate may be used alone or in any combination of two or more kinds at any ratio. The content of the lithium borate contained in the electrolyte solution is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.4% by mass or more. The content of the lithium borate contained in the electrolyte solution is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within this range, the effect of improving the high-temperature storage characteristics of the lithium ion secondary battery can be easily achieved, and an increase in negative electrode resistance due to excessive addition can be avoided.

[0080] (b) Difluorophosphate Difluorophosphates are represented by the following general composition formula (5).

[0081] X3(F2PO2) k (5) (In the formula, X3 represents a counter cation of the fluorophosphate, and k represents the valence of the counter cation.)

[0082] The counter cation of the difluorophosphate is not particularly limited, but examples thereof include lithium, sodium, potassium, and the like.

[0083] Specific types of difluorophosphates include lithium difluorophosphate (LiPO2F2), sodium difluorophosphate, potassium difluorophosphate, etc., and lithium difluorophosphate is particularly preferred. Lithium difluorophosphate is represented by the following structural formula (5-1).

[0084] [ka]

[0085] The difluorophosphate may be used singly or in any combination of two or more in any ratio. The content of the difluorophosphate contained in the electrolyte is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.5% by mass or more. The content of the difluorophosphate contained in the electrolyte is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within this range, the effect of improving the high-temperature storage characteristics of the lithium-ion secondary battery is easily achieved.

[0086] (c) Vinylene carbonate compounds Specific examples of vinylene carbonate compounds include vinylene carbonate (1,3-dioxol-2-one), methyl vinylene carbonate (4-methyl-1,3 dioxol-2-one), and ethyl vinylene carbonate (4-ethyl-1,3 dioxol-2-one).

[0087] The vinylene carbonate compound may be used alone or in a combination of two or more in any ratio. Among them, vinylene carbonate (C3H2O3) is particularly preferred because it provides excellent effects. Vinylene carbonate is represented by the following structural formula (6-1):

[0088] [ka]

[0089] The content of the vinylene carbonate compound contained in the electrolytic solution is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.4% by mass or more. The content of the vinylene carbonate compound contained in the electrolytic solution is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within this range, the effect of improving the high-temperature storage characteristics of the lithium-ion secondary battery can be easily achieved, and battery swelling due to increased gas generation can be avoided.

[0090] 2. Electrolyte salts In the electrolytic solution, at least one type of electrolyte salt is dissolved in a non-aqueous solvent. The electrolyte salt is a lithium salt, and the lithium salt is preferably, for example, a lithium salt containing fluorine (fluorine-containing lithium salt). Specific types of fluorine-containing lithium salts include inorganic anion salts such as lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4), and organic anion salts such as lithium trifluoromethanesulfonate (LiCF3SO3), and among these, LiPF6 is particularly preferred.

[0091] The lithium salt may be used alone or in any combination of two or more kinds in any ratio. The content of the lithium salt contained in the electrolyte solution is arbitrary as long as it does not significantly impair the effects of the present disclosure, but is, for example, 0.01% by mass or more, preferably 0.1% by mass or more. The content of the lithium salt contained in the electrolyte solution is, for example, 30% by mass or less, preferably 20% by mass or less.

[0092] 3. Non-aqueous solvent The electrolyte solution contains at least one non-aqueous solvent, examples of which include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates.

[0093] Specific examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Specific examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC), etc. Specific examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc.

[0094] The nonaqueous solvent may be used alone or in any combination of two or more kinds at any ratio. From the viewpoint of the balance between the dielectric constant and viscosity of the electrolyte, the nonaqueous solvent preferably contains at least a cyclic carbonate and a chain carbonate. Specifically, the nonaqueous solvent preferably contains at least one cyclic carbonate solvent and at least one chain carbonate solvent. In this case, the content ratio of the cyclic carbonate and the chain carbonate contained in the nonaqueous solvent is preferably 10:90 to 50:50 by volume. Furthermore, the nonaqueous solvent preferably contains ethylene carbonate as the cyclic carbonate and ethyl methyl carbonate and dimethyl carbonate as the chain carbonate.

[0095] <Other> The separators 33 and 35 have an insulating function to prevent a short circuit between the positive electrode 34 and the negative electrode 32, and a function to retain the electrolyte. The separators 33 and 35 are not particularly limited, but may be, for example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide, or a laminated sheet thereof (for example, a sheet having a three-layer structure of PP / PE / PP).

[0096] A layer containing an inorganic material (e.g., alumina particles) and a binder may be provided on one or both surfaces of the separators 33, 35. This prevents the separators 33, 35 from melting and maintains their insulating function even when the lithium-ion secondary battery 100 is used under abnormal conditions (e.g., when the temperature of the lithium-ion secondary battery rises to 160°C or higher due to overcharging or crushing). This improves the safety of the lithium-ion secondary battery 100.

[0097] If necessary, a core (not shown) may be disposed on the innermost periphery of the wound pack 3. As the core, a resin sheet or the like having higher bending rigidity than any of the positive electrode current collector, the negative electrode current collector, and the separators 33 and 35 can be used.

[0098] An insulating protective film (not shown) may be optionally wrapped around the wound group 3. The insulating protective film is not particularly limited, but may be made of, for example, a single sheet or multiple film members made of a synthetic resin such as PP (polypropylene), and has a length that allows it to be wound around the winding center in a direction parallel to the flat surface of the wound group 3 and perpendicular to the winding axis direction. [Example]

[0099] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0100] [Example 1] First, Li as the positive electrode active material 1.0 Ni 0.33 Co 0.33 Mn 0.33 O2 powder, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared.

[0101] Next, the positive electrode active material, the conductive additive, and the binder were mixed in a mass ratio of 90:5:5. N-methyl-2-pyrrolidone (NMP) was added to the obtained mixture to adjust the viscosity, thereby obtaining a positive electrode slurry (a slurry-like positive electrode mixture composition).

[0102] Next, a 15 μm thick aluminum foil was prepared as the positive electrode current collector 34a. Next, an uncoated portion to be the welded portion (positive electrode current collector exposed portion 34c) was left on both sides of the positive electrode current collector 34a, and the positive electrode slurry was simultaneously applied in two layers by slot die coating. The positive electrode slurry layer was then dried and pressed to form a positive electrode mixture layer 34b, thereby producing the positive electrode 34 shown in FIG. 3.

[0103] Next, as negative electrode active materials, natural graphite particles coated with amorphous carbon (negative electrode active material A), and natural graphite particles coated with amorphous carbon and carrying amorphous carbon microparticles (negative electrode active material B), styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a dispersant were prepared.

[0104] Next, negative electrode active material A, negative electrode active material B, binder, and dispersant were mixed so that the mass ratio of the total of negative electrode active material A and negative electrode active material B:binder:dispersant was 98:1:1. The contents (total 98 mass%) of negative electrode active material A and negative electrode active material B and their specifications are shown in Table 1 below. Ion-exchanged water was added to the obtained mixture to adjust the viscosity, and a negative electrode slurry (slurry-like negative electrode mixture composition) was obtained.

[0105] Next, a copper foil with a thickness of 10 μm was prepared as the negative electrode current collector 32a. Next, an uncoated portion to be the welded portion (negative electrode current collector exposed portion 32c) was left on both sides of the negative electrode current collector 32a, and the negative electrode slurry was simultaneously applied in two layers by slot die coating. Next, the negative electrode slurry layer was dried and pressed to form the negative electrode mixture layer 32b, and the negative electrode 32 shown in FIG. 3 was produced.

[0106] Next, separators 33 and 35 were sandwiched between the prepared positive electrode 34 and negative electrode 32 to prepare a wound pack 3 having the configuration shown in Fig. 3. The positive electrode side connection end 42 and the negative electrode side connection end 22 of the positive electrode current collector 44 and the negative electrode current collector 24 connected to the battery lid 6 were welded to the uncoated portions of the wound pack 3 (positive electrode current collector exposed portion 34c, negative electrode current collector exposed portion 32c), the wound pack 3 was covered with an insulating protective film (not shown), the wound pack 3 was sealed in a battery can 1, and the battery lid 6 and the battery can 1 were welded together (see Figs. 1 and 2).

[0107] Next, a non-aqueous solvent containing a cyclic carbonate (ethylene carbonate (EC)) and a chain carbonate (ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC)) with a volume ratio of 30:70 was prepared. Next, an electrolyte solution was prepared by dissolving LiPF as an electrolyte salt and additives FSO3Li (fluorosulfonate), CH3OSO3Li (methoxysulfonate), LiBOB (lithium borate), LiPO2F2 (difluorophosphate), and VC (vinylene carbonate (C3H2O3)) in this non-aqueous solvent. In this case, the contents of LiPF6 (electrolyte salt) contained in the electrolyte were 1.2 mol / L, the contents of FSO3Li contained in the electrolyte were 1.0 mass%, the contents of CH3OSO3Li contained in the electrolyte were 0.5 mass%, the contents of LiBOB contained in the electrolyte were 0.4 mass%, the contents of LiPO2F2 contained in the electrolyte were 1.0 mass%, and the contents of VC contained in the electrolyte were 0.4 mass%, as shown in Table 1. Next, the prepared electrolyte was poured into the battery can 1 through the pouring port 9 of the battery lid 6, and the pouring port 9 was then sealed with the pouring plug 11, thereby producing a lithium-ion secondary battery 100.

[0108] [Examples 2 and 3 and Comparative Examples 1 and 2] A lithium ion secondary battery 100 was fabricated in the same manner as in Example 1, except that the content of each additive contained in the electrolyte solution was changed as shown in Table 1 below.

[0109] [evaluation] For the lithium-ion secondary batteries prepared in Examples 1 to 3 and Comparative Examples 1 and 2, after initialization by charging and discharging, each characteristic was evaluated according to the following procedure. The evaluation results are shown in Table 2 below.

[0110] <Obtaining the SOC-open circuit voltage (OCV) curve> In a lithium-ion secondary battery placed in a thermostatic chamber at 25°C, the battery capacity was discharged at a discharge current of 1C in 5% increments of the battery capacity from 4.2V, and after a 1-hour pause, the voltage was taken as OCV to obtain the SOC-open circuit voltage (OCV) curve.

[0111] <Evaluation of output characteristics in the low SOC region at low temperature> First, in a lithium-ion secondary battery placed in a thermostatic chamber at 25°C, constant voltage-constant current charging (CC-CV charging) was performed at a charging current of 1C from SOC 0% to SOC 25% according to the relationship of the SOC-OCV curve. Next, the lithium-ion secondary battery was left standing in a thermostatic chamber at -10°C for 5 hours. Then, the lithium-ion secondary battery was discharged at a constant current of 9C for 10 seconds, and the voltage drop value due to discharge was measured. And from the measured voltage drop value, the discharge direct current resistance (DCR) at SOC 25% was obtained. The DCR (relative value) at SOC 25% of each lithium-ion secondary battery of each Example and each Comparative Example, with the DCR at SOC 25% of the lithium-ion secondary battery of Comparative Example 1 taken as 100, is shown in Table 2 below. Note that the smaller the DCR value, the better. The smaller the DCR value at SOC 25%, the lower the internal resistance of the lithium-ion secondary battery in the low SOC region is reduced.

[0112] <Evaluation of characteristics before and after high-temperature storage> First, the lithium-ion secondary battery placed in a 25°C thermostatic chamber was subjected to CC-CV charging at a charging current of 1C for 2.5 hours until the battery voltage reached 4.2V. After a 30-minute pause, constant-current discharge (CC discharge) was performed at a discharge current of 1C until the battery voltage reached 2.8V, and the capacity before high-temperature storage (initial capacity) was determined. The capacities (relative values) before high-temperature storage of the lithium-ion secondary batteries of each Example and Comparative Example, normalized to the capacity (initial capacity) of the lithium-ion secondary battery of Comparative Example 1 before high-temperature storage (100), are shown in Table 2 below. Furthermore, the lithium-ion secondary battery placed in a 25°C thermostatic chamber was subjected to CC-CV charging at a charging current of 1C from SOC 0% to SOC 50% in accordance with the SOC-OCV curve. After a 30-minute pause, the lithium-ion secondary battery was discharged at a constant current of 30C for 10 seconds, and the voltage drop due to discharge was measured. The DCR at SOC 50% before high-temperature storage was determined from the measured voltage drop. The DCR (relative value) at SOC 50% before high-temperature storage of the lithium ion secondary batteries of each Example and each Comparative Example, normalized to the DCR at SOC 50% before high-temperature storage of the lithium ion secondary battery of Comparative Example 1 being 100, is shown in Table 2 below.

[0113] Next, the lithium-ion secondary battery placed in a 25°C thermostatic chamber was subjected to CC-CV charging at a charging current of 1C from SOC 0% to SOC 80% based on the SOC-OCV curve. The lithium-ion secondary battery was then transferred to a 70°C thermostatic chamber and held there for 100 days. After that, the lithium-ion secondary battery was removed from the 70°C thermostatic chamber and transferred to a 25°C thermostatic chamber. It was then subjected to constant-current discharge (CC discharge) at a discharge current of 1C until the battery voltage reached 2.8V. The lithium-ion secondary battery was then placed in a 25°C thermostatic chamber and subjected to CC-CV charging at a charging current of 1C for 2.5 hours until the battery voltage reached 4.2V. After a 30-minute pause, it was subjected to constant-current discharge (CC discharge) at a discharge current of 1C until the battery voltage reached 2.8V, and the capacity after high-temperature storage was determined. The capacities (relative values) after high-temperature storage of the lithium-ion secondary batteries of each Example and each Comparative Example, normalized with the capacity of the lithium-ion secondary battery of Comparative Example 1 after high-temperature storage set to 100, are shown in Table 2 below. Furthermore, the lithium-ion secondary batteries placed in a thermostatic chamber at 25°C were subjected to CC-CV charging at a charging current of 1C from SOC 0% to SOC 50% in accordance with the SOC-OCV curve. After a 30-minute rest, the lithium-ion secondary batteries were discharged at a constant current of 30C for 10 seconds to measure the voltage drop due to discharge, and the DCR at SOC 50% after high-temperature storage was calculated from the measured voltage drop. The DCR (relative values) after high-temperature storage of the lithium-ion secondary batteries of each Example and each Comparative Example, normalized with the DCR at SOC 50% after high-temperature storage of Comparative Example 1 set to 100, are shown in Table 2 below.

[0114] [Table 1]

[0115] [Table 2]

[0116] <Consideration> The compositions of the electrolytes and the evaluation results of the lithium-ion secondary batteries of each Example and Comparative Example are shown in Tables 1 and 2. As shown in Tables 1 and 2, it is suggested that Examples 1 and 2 can achieve both a reduction in DCR in the low SOC region and an improvement in high-temperature storage characteristics (capacity retention rate and DCR retention rate). On the other hand, Comparative Examples 1 and 2 show a decrease in high-temperature storage characteristics, and it is thought that it is not possible to achieve both a reduction in DCR in the low SOC region and an improvement in high-temperature storage characteristics.

[0117] Therefore, by employing the electrolyte solution of the present disclosure as the electrolyte solution for a lithium ion secondary battery, it is possible to provide a lithium ion secondary battery that has high output in a low SOC region and good high-temperature storage characteristics.

[0118] Those skilled in the art can use the above description to make the most of the present disclosure. The claims and embodiments disclosed herein are merely descriptive and exemplary and should not be construed as limiting the scope of the present disclosure in any way. With the aid of this disclosure, changes can be made to the details of the above embodiments without departing from the basic principles of the present disclosure. In other words, various modifications and improvements of the embodiments specifically disclosed in the above specification are within the scope of the present disclosure. [Explanation of symbols]

[0119] 1: battery can, 1a: opening, 1b: wide side surface, 1c: narrow side surface, 1d: bottom surface, 3: winding group, 5: gasket, 6: battery lid, 7: insulating plate, 9: liquid filling port, 10: gas exhaust valve, 11: liquid filling plug, 12: negative electrode external terminal, 12a: negative electrode connection part, 14: positive electrode external terminal, 14a: positive electrode connection part, 21: negative electrode current collector base, 22: negative electrode side connection end, 23: negative electrode side opening hole, 24: negative electrode current collector, 26: negative electrode side through hole, 32: negative electrode, 32a: negative electrode current collector, 32b: negative electrode mixture layer, 32c: exposed portion of negative electrode current collector, 33: separator, 34: positive electrode, 34a: positive electrode current collector, 34b: positive electrode mixture layer, 34c: exposed portion of positive electrode current collector, 35: separator, 41: base portion of positive electrode current collector, 42: positive electrode side connection end portion, 43: positive electrode side opening hole, 44: positive electrode current collector, 46: positive electrode side through hole, 50: graphite particle (A), 52: graphite particle (B) carrying amorphous carbon microparticles, 54: graphite particle, 56: amorphous carbon microparticles, 100: lithium ion secondary battery.

Claims

1. A battery comprising a positive electrode, a negative electrode, and an electrolyte solution, the negative electrode has a current collector and a negative electrode active material layer provided on at least one surface of the current collector, the negative electrode active material layer includes a negative electrode active material containing graphite particles and amorphous carbon fine particles, the electrolytic solution contains at least a non-aqueous solvent, an electrolyte salt, and an additive; The lithium ion secondary battery, wherein the additive contains a fluorosulfonate and a methoxysulfonate.

2. 2. The lithium ion secondary battery according to claim 1, wherein the additive further contains lithium borate and difluorophosphate.

3. 3. The lithium ion secondary battery according to claim 2, wherein the lithium borate is lithium bis(oxalato)borate (LiBOB) and the difluorophosphate is lithium difluorophosphate.

4. The lithium ion secondary battery according to claim 2 or 3, wherein the additive further contains a vinylene carbonate compound.

5. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the non-aqueous solvent contains at least a cyclic carbonate and a chain carbonate, and a content ratio of the cyclic carbonate and the chain carbonate contained in the non-aqueous solvent is 10:90 to 50:50 by volume.

6. 6. The lithium ion secondary battery according to claim 5, wherein the non-aqueous solvent contains ethylene carbonate as the cyclic carbonate and ethyl methyl carbonate and dimethyl carbonate as the chain carbonate.

7. 4. The lithium ion secondary battery according to claim 1, wherein the electrolyte salt is lithium hexafluorophosphate.

8. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the content of the fluorosulfonate contained in the electrolytic solution is 0.3 mass% to 1.5 mass%.

9. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the content of the methoxysulfonate contained in the electrolyte solution is 0.5 mass% or less.

10. 4. The lithium ion secondary battery according to claim 1, wherein the amorphous carbon fine particles are supported on the graphite particles.

11. 11. The lithium ion secondary battery according to claim 10, wherein the negative electrode active material contains, as the graphite particles, graphite particles (A) and graphite particles (B) carrying the amorphous carbon fine particles.

12. 4. The lithium ion secondary battery according to claim 1, wherein the amorphous carbon fine particles have an average particle size of 0.05 μm or more and 0.5 μm or less.

13. the positive electrode has a current collector and a positive electrode active material layer provided on at least one surface of the current collector, 4. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material layer contains a positive electrode active material containing a ternary material containing at least nickel, cobalt, and manganese.

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