JP6445758B2 - Lithium ion secondary battery and method for manufacturing lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery and a method for producing a lithium ion secondary battery.

  As with conventional lithium-ion rechargeable batteries for notebook PCs, lithium-ion rechargeable batteries for portable devices such as smart phones and smart tablets and for electric vehicles are also becoming more energy-efficient. In order to improve the density, it has been proposed to use a metal-based active material such as a silicon-based active material and a tin-based active material for the negative electrode active material.

  Since the above metal-based active materials have a high specific capacity (mAh / g), the use of these as negative electrode active materials for lithium ion secondary batteries is expected to increase the energy density of lithium ion secondary batteries. The

However, unlike a graphite active material, in particular, a silicon-based active material easily undergoes (decomposition) reaction with LiPF ₆ and ethylene carbonate, which is a main solvent, during a charging process. Furthermore, since the volume change at the time of charging / discharging is larger than that of the graphite active material, the silicon-based active material generates an unreacted surface (new surface) with LiPF ₆ and ethylene carbonate every time charging / discharging occurs. Thus, reacting the newly formed surfaces and LiPF ₆ and ethylene carbonate silicon-based active material in each of the charging and discharging, LiPF ₆ and ethylene carbonate is decomposed. For this reason, in the lithium ion secondary battery using a silicon-type active material, charging / discharging efficiency falls significantly, and thereby, a cycle (cycle) life deteriorates remarkably.

  Patent Documents 1 and 2 disclose techniques aimed at solving the above problems. Specifically, in the technique disclosed in Patent Document 1, the negative electrode active material is composed of an amorphous or microcrystalline silicon thin film, and a hydrogen atom is replaced with a fluorine atom that is easily bonded to a silicon atom. A carbonate compound is used as the electrolyte. In the technique disclosed in Patent Document 2, the negative electrode active material is composed of silicon and a transition metal, and pores are provided inside the negative electrode active material.

JP 2012-38737 A JP 2012-178287 A

  However, the lithium ion secondary batteries disclosed in Patent Documents 1 and 2 still have insufficient practical characteristics such as cycle life and storage capacity. Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to use a lithium-ion secondary battery that uses a negative electrode active material containing silicon and has excellent practical characteristics. And a manufacturing method thereof.

  In order to solve the above-described problems, according to one aspect of the present invention, a negative electrode active material including a silicon-based active material and a carbon-based active material, and a different structure depending on the G / D ratio of the carbon-based active material. And an electrolyte containing an S═O compound. A lithium ion secondary battery is provided.

  According to this aspect, the negative electrode active material includes a silicon-based active material and a carbon-based active material, and the electrolytic solution includes different S═O compounds depending on the G / D ratio of the carbon-based active material. Therefore, the S═O compound decomposes preferentially over other solvents (for example, ethylene carbonate) during charging, for example, and the decomposition product covers the negative electrode active material. Therefore, decomposition of the solvent on the negative electrode active material is suppressed. Furthermore, the decomposition product of the S═O compound does not inhibit the entry and exit of lithium ions into the negative electrode active material. Therefore, the battery characteristics of the lithium ion secondary battery are improved.

  Here, when the G / D ratio of the carbon-based active material is greater than 2, the electrolytic solution is a first S═O compound represented by any one of the following chemical formulas 1 to 3, or a first S═ It may contain a mixture of O compounds.

・・・（化学式１）

... (Chemical formula 1)

・・・（化学式２）

... (Chemical formula 2)

・・・（化学式３）

... (Chemical formula 3)

Here, R ₁ and R ₁ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₁ and R ₁ ′ may be substituted,
R ₃ and R ₃ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms and may be linked to each other, and at least a part of the hydrogen atoms constituting R ₃ and R ₃ ′ are May be replaced,
R ₄ and R ₄ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₄ and R ₄ ′ may be substituted.

  According to this viewpoint, the S═O compound can more effectively suppress the decomposition of the solvent, and as a result, the battery characteristics of the lithium ion secondary battery are improved.

  Further, when the G / D ratio of the carbon-based active material is 2 or less, the electrolytic solution contains a second S═O compound represented by the following chemical formula 4 or 5 or a mixture of the second S═O compounds. You may go out.

・・・（化学式４）

... (Chemical formula 4)

・・・（化学式５）

... (Chemical formula 5)

  According to this viewpoint, the S═O compound can more effectively suppress the decomposition of the solvent, and as a result, the battery characteristics of the lithium ion secondary battery are improved.

Here, one of R ₅ and R ₅ ′ is a hydrocarbon group having 3 or more carbon atoms, and the other is a hydrocarbon group having 1 or 2 carbon atoms, and constitutes R ₅ and R ₅ ′. At least some of the hydrogen atoms may be substituted,
R ₂ is a hydrocarbon group having 1 or more carbon atoms, and at least a part of the hydrogen atoms constituting R ₂ may be substituted.

  Further, the electrolytic solution may contain a hydrofluoroether.

  According to this viewpoint, the discharge capacity of the lithium ion secondary battery is further improved.

  The electrolytic solution may contain at least one of a chain carbonate ester and a fluoro ethylene carbonate.

  According to another aspect of the present invention, a step of producing a negative electrode active material by mixing a silicon-based active material and a carbon-based active material, and a G / D ratio of the carbon-based active material. Thus, there is provided a method for manufacturing a lithium ion secondary battery, comprising: selecting an S═O compound; and producing an electrolytic solution containing the selected S═O compound.

  According to this aspect, a lithium ion secondary battery using a silicon-based active material as the negative electrode active material and having improved battery characteristics is provided.

  As described above, according to the present invention, a lithium ion secondary battery using a silicon-based active material as a negative electrode active material and having improved battery characteristics is provided.

It is sectional drawing which shows the lithium ion secondary battery which concerns on embodiment of this invention.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

[Configuration of lithium ion secondary battery]
First, based on FIG. 1, the structure of the lithium ion secondary battery 10 which concerns on this embodiment is demonstrated.

The lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, and a separator layer 40. The charge ultimate voltage (redox potential) of the lithium ion secondary battery 10 is, for example, 4.3 V (vs. Li ^/ Li ⁺ ) or more and 5.0 V or less, particularly 4.5 V or more and 5.0 V or less. The form of the lithium ion secondary battery 10 is not particularly limited. That is, the lithium ion secondary battery 10 may be any one of a cylindrical shape, a square shape, a laminate shape, a button shape, and the like.

  The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductor as long as it is a conductor, and is made of, for example, aluminum, stainless steel, nickel-coated steel, or the like.

The positive electrode active material layer 22 includes at least a positive electrode active material, and may further include a conductive agent and a binder. The positive electrode active material is, for example, a solid solution oxide containing lithium, but is not particularly limited as long as the material can electrochemically occlude and release lithium ions. The solid solution oxide is, for example, Li _a Mn _x Co _y Ni _z O ₂ (1.150 ≦ a ≦ 1.430, 0.45 ≦ x ≦ 0.6, 0.10 ≦ y ≦ 0.15,. 20 ≦ z ≦ 0.28), LiMn _x Co _y Ni _z O ₂ (0.3 ≦ x ≦ 0.85, 0.10 ≦ y ≦ 0.3, 0.10 ≦ z ≦ 0.3), LiMn _1.5 Ni _0.5 O ₄ . The content of the positive electrode active material is preferably 85% by mass to 96% by mass with respect to the total mass of the positive electrode mixture (positive electrode active material, binder, and conductive agent), and is preferably 88% by mass to 94% by mass. More preferably, it is as follows. When the content of the positive electrode active material is within such a range, battery characteristics (for example, cycle life and storage capacity) are particularly improved.

  The conductive agent is, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, or the like, but is not particularly limited as long as it is intended to increase the conductivity of the positive electrode. The content of the conductive agent is preferably 3% by mass or more and 10% by mass or less, and more preferably 4% by mass or more and 6% by mass or less with respect to the total mass of the positive electrode mixture. When the content of the conductive agent is within such a range, the battery characteristics are particularly improved.

  Examples of the binder include polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, acrylonitrile butadiene rubber, and acrylonitrile butadiene rubber. Fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, nitrocellulose, and the like, and a positive electrode active material and a conductive agent on the current collector 21. Bind There is no particular limitation as long as it can be used. The content of the binder is preferably 3% by mass or more and 7% by mass or less, and more preferably 4% by mass or more and 6% by mass or less with respect to the total mass of the positive electrode mixture. When the content of the binder is in such a range, the battery characteristics are particularly improved.

The density (g / cm ³ ) of the positive electrode active material layer 22 is not particularly limited, but is preferably 2.0 or more and 3.0 or less, and more preferably 2.5 or more and 3.0 or less. . When the density of the positive electrode active material layer 22 is in such a range, the battery characteristics are particularly improved. If the density exceeds 3.0 g / cm ³ , the particles of the positive electrode active material are broken, and the electrical contact between the broken particles is impaired. As a result, the utilization factor of the positive electrode active material decreases, so that the original discharge capacity cannot be obtained, and polarization easily occurs. Furthermore, the positive electrode active material is charged to a potential equal to or higher than a set potential, causing decomposition of the electrolytic solution and elution of the active material transition metal, resulting in deterioration of cycle characteristics. Also from such a viewpoint, the density of the positive electrode active material layer 22 is preferably within the above range. The density of the positive electrode active material layer 22 is obtained by dividing the surface density after rolling of the positive electrode active material layer 22 by the thickness after rolling of the positive electrode active material layer 22.

  The positive electrode active material layer 22 is produced, for example, by the following manufacturing method. That is, first, a positive electrode mixture is prepared by dry-mixing a positive electrode active material, a conductive agent, and a binder. Next, the positive electrode mixture is dispersed in a suitable organic solvent to form a positive electrode mixture slurry (slurry). The positive electrode mixture slurry is applied onto the current collector 21, dried, and rolled to produce a positive electrode active slurry. A material layer is formed.

  The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 may be any conductor as long as it is a conductor, for example, aluminum, stainless steel, nickel-plated steel, or the like.

  The negative electrode active material layer 32 includes at least a negative electrode active material, and may further include a binder. The negative electrode active material includes a silicon-based active material containing silicon and a carbon-based active material. That is, the present inventor has found that when the negative electrode active material is composed only of a silicon-based active material, the battery characteristics are not sufficiently increased. And when this inventor examined further about the negative electrode active material, a carbon type active material was mixed with a silicon type active material, and the S = O compound which has a structure according to G / D ratio of a carbon type active material It has been found that the battery characteristics are improved by adding to the electrolyte. Detailed examination contents will be described later.

The silicon-based active material is a material that contains silicon (atom) and can electrochemically occlude and release lithium ions. Examples of the silicon active material include fine particles of silicon alone, fine particles of silicon oxide, and alloys based on silicon. The oxide of silicon is represented by SiO _x (0 ≦ x ≦ 2). An alloy containing silicon as a basic material is an alloy in which the mass% of silicon relative to the total mass of the alloy is the largest among all the metal elements constituting the alloy, and examples thereof include a Si—Al—Fe alloy.

  On the other hand, the carbon-based active material is a material that contains carbon (atom) and can electrochemically occlude and release lithium ions. Examples of the carbon-based active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, and the like).

The carbon-based active material has a specific G / D ratio. That is, two peaks (G band (band) and D band (band)) are observed in the Raman spectrum of the carbon-based active material. The G band is a peak observed in the wave number range of 1580 ± 20 cm ⁻¹ , and the D band is a peak observed in the wave number range of 1355 ± 20 cm ⁻¹ . The G / D ratio is calculated by dividing the area of the G band by the area of the D band. The area of each peak may be calculated by, for example, a sectional quadrature method. The higher the G / D ratio, the higher the reactivity of the carbon-based active material (for example, the tendency to decompose the solvent of the lithium ion secondary battery). The present inventor pays attention to the G / D ratio of the carbon-based active material, and includes an S═O compound having a different structure depending on the G / D ratio in the electrolytic solution, so that the battery characteristics of the lithium ion secondary battery can be improved. I found out that it would improve dramatically. Details will be described later.

  In addition, the mass ratio of the silicon-based active material and the carbon-based active material in the negative electrode active material is not particularly limited. That is, the negative electrode active material may be silicon-rich, carbon-rich, or the same amount of both. Moreover, it is preferable that content of a negative electrode active material is 90 to 98 mass% with respect to the total mass of a negative electrode mixture (a negative electrode active material and a binder). When the content of the negative electrode active material is in such a range, the battery characteristics are particularly improved.

  The binder is also the same as the binder constituting the positive electrode active material layer 22. When the positive electrode active material layer 22 is applied on the current collector 21, carboxymethyl cellulose (hereinafter referred to as CMC) as a thickener may be used in an amount not less than 1/10 and not more than the mass of the binder. The content of the binder including the thickener is preferably 1% by mass or more and 10% by mass or less with respect to the total mass of the negative electrode mixture. When the content of the binder including the thickener is in such a range, the battery characteristics are particularly improved.

The density (g / cm ³ ) of the negative electrode active material layer 32 is not particularly limited, but is preferably 1.0 or more and 2.0 or less, for example. When the density of the negative electrode active material layer 32 is in such a range, the battery characteristics are particularly improved. The negative electrode active material layer 32 is formed, for example, by dispersing a negative electrode active material and a binder in a suitable solvent (for example, N-methyl-2-pyrrolidone or water). The slurry is applied on the current collector 31 and dried. The density of the negative electrode active material layer 32 is obtained by dividing the surface density after rolling of the negative electrode active material layer 32 by the thickness after rolling of the negative electrode active material layer 32.

  The separator layer 40 includes a separator 40 a and an electrolytic solution 43. The separator 40 a includes a base material 41 and a porous layer 42. The substrate 41 is made of a material selected from polyethylene, polypropylene, and the like, and includes a large number of first pores (pores) 41a. In FIG. 1, the first pores 41a are spherical, but the first pores 41a can take various shapes. The diameter of the first pore 41a is distributed within a range of 0.1 to 0.5 μm, for example. The diameter of the first pore 41a is, for example, a diameter when the first pore 41a is regarded as a sphere, that is, a sphere equivalent diameter. The first pore 41a is measured by, for example, an automatic poloximeter Autopore IV, Shimadzu Corporation. For example, this measuring device measures the pore size distribution of the first pores 41a, and further measures the pore size having the highest distribution as a representative value. In addition, the hole diameter of the pore 41a which exists in the surface layer of the base material 41 is measurable also by the scanning electron microscope JSM-6060 JEOL Ltd., for example. For example, this measuring apparatus measures the hole diameter of each of the first pores 41a in the surface layer.

  The porosity of the base material 41 is, for example, 38 to 44%. When the porosity of the base material 41 is in such a range, the cycle life is particularly improved. The porosity of the base material 41 is obtained by dividing the total volume of the first pores 41a by the total volume of the base material 41 (the total volume of the resin portion of the base material 41 and the first pores 41a). The porosity of the base material 41 is measured by, for example, an automatic poloximeter Autopore IV, Shimadzu Corporation. The thickness of the substrate 41 is preferably 6 to 19 μm. When the thickness of the base material 41 is within such a range, the cycle life is particularly improved.

  The porous layer 42 is made of a material different from the base material 41, for example, a material selected from polyvinylidene fluoride, polyamidoimide, and aramid (aromatic polyamide). A large number of second pores (pores) 42a are included. In FIG. 1, the second pores 42a are spherical, but the second pores 42a can take various shapes.

  The second pore 42a is different from the first pore 41a. Specifically, the hole diameter and porosity of the second pore 42a are larger than the value of the first pore 41a. That is, the pore diameter of the second pores 42a is distributed within a range of, for example, 1 to 2 μm. The diameter of the second pore 42a is, for example, a diameter when the second pore 42a is regarded as a sphere, that is, a sphere equivalent diameter, and is measured by, for example, a scanning electron microscope JSM-6060 JEOL Ltd. This measuring device measures the hole diameter of each of the second pores 42a.

In addition, as a polyvinylidene fluoride applied to the porous layer 42, KF polymer KF polymer # 1700, # 9200, # 9300 etc. which can be considered are considered, for example. The weight average molecular weight of polyvinylidene fluoride is about 500,000 to 1,000,000. The porous layer 42 may be synthesized by itself, or an existing one may be purchased.

  The porosity of the separator 40a is, for example, 39 to 58%. When the porosity of the separator 40a is in such a range, the cycle life is particularly improved. Here, the porosity of the separator 40a is the total volume of the first pore 41a and the second pore 42a, the total volume of the separator 40a (the resin portion of the base material 41 and the first pore 41a, and the porous layer 42). Divided by the total volume of the resin portion and the second pore 42a). The porosity of the separator 40a is measured by, for example, an automatic poloximeter Autopore IV, Shimadzu Corporation. Since the porosity of the separator 40a is larger than the porosity of the base material 41, the porosity of the porous layer 42, ie, the porosity of the second pore 42a, is the porosity of the base material 41, ie, the first pore 41a. It can be said that it is higher than the porosity.

  The thickness of the porous layer 42 is preferably 1 to 5 μm. The total thickness of the separator 40a, that is, the sum of the thickness of the base material 41 and the thickness of the porous layer 42 is preferably 10 to 25 μm. The cycle life is particularly improved when the thickness of the porous layer 42 and the separator 40a falls within these ranges. In FIG. 1, the porous layer 42 is provided on both the front and back surfaces of the base material 41, that is, on both the surface on the positive electrode 20 side and the surface on the negative electrode 30 side. From the viewpoint of improving the cycle life of the lithium ion secondary battery, the porous layer 42 is preferably provided on both the front and back surfaces of the base material 41.

  The air permeability of the base material 41 (the air permeability defined by JIS P8117) is not particularly limited, but is preferably, for example, 250 to 300 sec / 100 cc. The air permeability of the separator 40a is not particularly limited, but is preferably 220 to 340 sec / 100 cc, for example. The cycle life is particularly improved when the air permeability of the base material 41 and the separator 40a falls within these ranges. The air permeability of the base material 41 and the separator 40a is measured by, for example, Gurley type air permeability meter G-B2 Toyo Seiki Co., Ltd.

  The separator 40a is formed by, for example, applying a coating liquid containing a resin that constitutes the porous layer 42 and a water-soluble organic solvent to the substrate 41, and then coagulating the resin and removing the water-soluble organic solvent. It is formed. Thus, although the separator 40a becomes a multilayer structure which has the base material 41 and the porous layer 42, a single layer structure (for example, structure of only the base material 41) may be sufficient.

The electrolytic solution 43 includes a lithium salt, a solvent, and an S═O compound serving as an additive. The lithium salt serves as an electrolyte for the electrolytic solution 43. Such lithium salts, other lithium hexafluorophosphate _{(LiPF 6), LiClO 4,} LiBF 4, LiAsF 6, LiSbF 6, LiSO 3 CF 3, LiN (SO 2 CF 3), LiN (SO 2 CF _{2 CF 3), LiC (SO} 2 CF 2 CF 3) 3, LiC (SO 2 CF 3) 3, LiI, LiCl, LiF, LiPF 5 (SO 2 CF 3), LiPF 4 (SO 2 CF 3) 2 , etc. Is mentioned. Of these lithium _{salts, LiPF 6, LiClO 4, LiBF} 4, LiAsF 6, LiSbF 6 is particularly preferred. When these lithium salts are dissolved in the electrolytic solution 43, the battery characteristics are particularly improved. In the electrolytic solution 43, any one of these lithium salts may be dissolved, and a plurality of types of lithium salts may be dissolved.

  The concentration of the lithium salt (when a plurality of types of lithium salts are dissolved in the electrolytic solution 43, the total concentration of the lithium salts) is preferably 1.15 to 1.5 mol / L, and 1.3 More preferably, it is ˜1.45 mol / L. When the concentration of the lithium salt is in such a range, the battery characteristics are particularly improved.

  The solvent includes various non-aqueous solvents used for lithium ion secondary batteries. More preferably, the solvent contains at least one of hydrofluoroether (HFE), a chain carbonate, and ethylene fluorocarbonate.

Hydrofluoroethers have improved oxidation resistance by substituting some of the ether hydrogens with fluorine. Examples of such hydrofluoroethers include 2,2,2-trifluoroethyl methyl ether (CF ₃ CH ₂ OCH ₃ ), 2,2,2- Trifluoroethyl difluoromethyl ether (CF ₃ CH ₂ OCHF ₂ ), 2,2,3,3,3-pentafluoropropyl methyl ether (CF ₃ CF ₂ CH ₂ OCH ₃ ), _{2, 2, 3} , ₃ , ₃ - pentafluoropropyl difluoromethyl ether _{(CF 3 CF 2 CH 2 OCHF} 2), 2,2,3,3,3- pentafluoro-propyl-1,1,2,2-tetrafluoroethyl ether _(CF 3 CF 2 CH _{2 OCF 2 CF 2 H),} 1,1,2,2- tetrafluoroethyl methyl ether _(HCF 2 _CF 2 OC _3), 1,1,2,2-tetrafluoroethyl ethyl ether _{(HCF 2 CF 2 OCH 2 CH} 3), 1,1,2,2- tetrafluoroethyl propyl ether _{(HCF 2 CF 2 OC 3 H} 7) 1,1,2,2-tetrafluoroethyl butyl ether (HCF ₂ CF ₂ OC ₄ H ₉ ), 1,1,2,2-tetrafluoroethyl isobutyl ether (HCF ₂ CF ₂ OCH ₂ CH (CH ₃ ) ₂ ), 1,1,2,2-tetrafluoroethyl isopentyl ether (HCF ₂ CF ₂ OCH ₂ C (CH ₃ ) ₃ ), 1,1,2,2-tetrafluoroethyl-2,2,2-tri trifluoroethyl ether _{(HCF 2 CF 2 OCH 2 CF} 3), 1,1,2,2- tetrafluoroethyl-2,2,3,3-tetrafluoro-flop Pill ether _{(HCF 2 CF 2 OCH 2 CF} 2 CF 2 H), hexafluoroisopropyl ether _{((CF 3) 2 CHOCH 3} ), 1,1,3,3,3- pentafluoro-2-trifluoromethyl-propyl Methyl ether ((CF ₃ ) ₂ CHCF ₂ OCH ₃ ), 1,1,2,3,3,3-hexafluoropropyl methyl ether (CF ₃ CHFCF ₂ OCH ₃ ), 1,1,2,3,3 Examples include 3-hexafluoropropyl ethyl ether (CF ₃ CHFCF ₂ OCH ₂ CH ₃ ) and 2,2,3,4,4,4-hexafluorobutyl difluoromethyl ether (CF ₃ CHFCF ₂ CH ₂ OCHF ₂ ). . The hydrofluoroether may be composed of any one of these substances, but may be a mixture of these substances. 10-60 volume% is preferable with respect to the total volume of the solvent of the electrolyte solution 43, and, as for the volume ratio of hydrofluoroether, 30-50 volume% is more preferable. When the volume ratio of the hydrofluoroether is in such a range, the battery characteristics are particularly improved.

  The chain carbonate is a carbonate having a chain structure. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. The electrolytic solution 43 preferably contains one or more of these chain carbonates. 5-60 volume% is preferable with respect to the total volume of the solvent of the electrolyte solution 43, and, as for the volume ratio of chain | strand-shaped carbonate ester, 20-50 volume% is more preferable. The battery characteristics are particularly improved when the volume ratio of the chain carbonate is in such a range.

  10-30 volume% is preferable with respect to the total volume of the solvent of the electrolyte solution 43, and, as for the volume ratio of fluoroethylene carbonate, 15-20 volume% is more preferable. When the volume ratio of monofluoroethylene carbonate is in such a range, the cycle life is particularly improved. Moreover, the electrolyte solution 43 may further contain various nonaqueous solvents used for the lithium ion secondary battery.

  An S═O compound is a compound having an S═O structure. S = O compounds have different structures depending on the G / D ratio of the carbon-based active material. That is, the present inventor has found that even if only the silicon-based active material is used as the negative electrode active material, the battery characteristics are not sufficient. Therefore, the present inventor has focused on mixing a carbon-based active material with a silicon-based active material. As a result, the battery characteristics were slightly improved, but the battery characteristics were still insufficient.

Therefore, the present inventor conducted the following experiment in order to find out the reason. That is, the present inventor produced a lithium ion secondary battery using lithium foil as a positive electrode active material, a mixture of a silicon-based active material and a carbon-based active material as a negative electrode active material, and ethylene carbonate as a solvent. The inventor then charged and discharged the lithium ion secondary battery a plurality of times, and the behavior of the lithium ion secondary battery at that time, specifically, the potential of the negative electrode active material (Li / Li ⁺ reference) and the dQ / dV value The correlation with was confirmed. As a result, in the charging process of the lithium ion secondary battery, the dQ / dV value was disturbed when the potential of the negative electrode active material was around 0.4V. In addition, during the second and subsequent charging / discharging, the potential of the negative electrode active material transitioned between about 0.5 V to 10 mV.

  As a result, the present inventor has found that the solvent (ethylene carbonate) is decomposed on the negative electrode active material at least during the charging process, and that the solvent is decomposed every time the battery is charged. Then, the inventor considered that the battery characteristics were not sufficient because the decomposition product of the solvent was deposited on the negative electrode active material, thereby preventing the lithium ions from entering and leaving the negative electrode active material.

Therefore, the present inventor considered adding an additive that satisfies the following three conditions to the electrolyte.
(1) Decomposes preferentially over the solvent during charging (ie, at a higher potential than the solvent).
(2) The decomposition product suppresses decomposition of the solvent. For example, the decomposition product covers the negative electrode active material, thereby suppressing contact between the solvent and the negative electrode active material.
(3) The decomposition product hardly inhibits lithium ions from entering and exiting the negative electrode active material.

The inventor diligently studied additives that satisfy the above conditions. As a result, the present inventor paid attention to S═O compounds (compounds having S═O bonds) and conducted the following experiments. Specifically, the present inventors produced a lithium ion secondary battery using lithium foil as a positive electrode active material, a mixture of a silicon-based active material and a carbon-based active material as a negative electrode active material, and ethylene carbonate as a solvent. In addition, the present inventor added an S═O compound to the electrolytic solution. The inventor then charged and discharged the lithium ion secondary battery a plurality of times, and the behavior of the lithium ion secondary battery at that time, specifically, the potential of the negative electrode active material (Li / Li ⁺ reference) and the dQ / dV value The correlation with was confirmed. As a result, in the charging process of the lithium ion secondary battery, the dQ / dV value was disturbed at a potential higher than 0.4V, and the dQ / dV value was hardly disturbed at a potential around 0.4V. Furthermore, the battery characteristics were clearly improved (see Examples described later). Therefore, it is estimated that the S = O compound decomposes preferentially over the solvent, the decomposition product suppresses the decomposition of the solvent, and the decomposition product hardly inhibits the entry and exit of the lithium ion into the negative electrode active material. . That is, the S═O compound satisfies all the above three conditions.

  Note that there are various types of carbon-based active materials as described above. These carbon-based active materials are selectively used depending on, for example, the energy density required for the lithium ion secondary battery or the use of the lithium ion secondary battery. Therefore, in order for the lithium ion secondary battery 10 according to the present embodiment to be practical, it is necessary that the lithium ion secondary battery 10 can cope with various carbon-based active materials.

  On the other hand, since the carbon-based active material has a specific surface area larger than that of the silicon-based active material, there is a possibility that the solvent (or S═O compound) is decomposed in a larger amount than the silicon-based active material. Therefore, the S═O compound is required to suppress decomposition of the solvent while withstanding consumption (decomposition) on the carbon-based active material.

  Therefore, the present inventor further examined the S═O compound so that the battery characteristics are improved regardless of which carbon-based active material is used as the negative electrode active material. Specifically, since the carbon-based active material may decompose a solvent (or S═O compound) in large quantities, the present inventor uses a parameter (Parameter) that indicates the reactivity of the carbon-based active material. Focusing on a certain G / D ratio, the correlation between the G / D ratio and the S═O compound was examined.

  As a result, the present inventor has found that suitable S═O compounds differ depending on the G / D ratio of the carbon-based active material. More specifically, the present inventor succeeded in classifying the S═O compound into the four quadrants (categories) shown in Table 1 below according to the G / D ratio of the carbon-based active material.

  Here, in Table 1, “Gr” means a carbon-based active material. “Effective” means that the battery characteristics are improved as compared with a lithium ion secondary battery in which the electrolyte does not contain S═O, and “no effect” means that the electrolyte does not contain S═O. It means that the battery characteristics are the same or lower than those of the ion secondary battery. The structural formula in parentheses indicates the structure of S═O bond.

  The first quadrant is a quadrant corresponding to G / D ratio> 2 and “effective”, and the second quadrant is a quadrant corresponding to “effective” or less G / D ratio 2. The third quadrant is a quadrant corresponding to a G / D ratio> 2 and “no effect”, and the fourth quadrant is a quadrant corresponding to a G / D ratio of 2 or less and “no effect”.

  The S═O compound belonging to the first quadrant is represented by any one of the following chemical formulas 1 to 3.

・・・（化学式１）

... (Chemical formula 1)

・・・（化学式２）

... (Chemical formula 2)

・・・（化学式３）

... (Chemical formula 3)

Here, R ₁ and R ₁ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₁ and R ₁ ′ may be substituted. R ₃ and R ₃ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms and may be linked to each other, and at least a part of the hydrogen atoms constituting R ₃ and R ₃ ′ are May be substituted. R ₄ and R ₄ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₄ and R ₄ ′ may be substituted.

  Examples of the S═O compound represented by Chemical Formula 1 include ethyl methyl sulfone (EMS) and diethyl sulfone (DES). Examples of the S═O compound represented by Chemical Formula 2 include dimethyl sulfite (DMSi), diethyl sulfite (DESi), and ethylene sulfite (ES). Examples of the S═O compound represented by Chemical Formula 3 include dimethyl sulfate (DMSa) and diethyl sulfate (DESa).

  The S═O compound belonging to the second quadrant is represented by any one of the following chemical formulas 4 to 5.

・・・（化学式４）

... (Chemical formula 4)

・・・（化学式５）

... (Chemical formula 5)

Here, one of R ₅ and R ₅ ′ is a hydrocarbon group having 3 or more carbon atoms, and the other is a hydrocarbon group having 1 or 2 carbon atoms, and constitutes R ₅ and R ₅ ′. At least a part of the hydrogen atoms may be substituted. R ₂ is a hydrocarbon group having 1 or more carbon atoms, and at least a part of the hydrogen atoms constituting R ₂ may be substituted.

  Examples of the S═O compound represented by Chemical Formula 4 include ethyl isopropyl sulfone (EiPS), ethyl butyl sulfone (EBS), and butyl isopropyl sulfone (BiPS). Examples of the compound represented by Chemical Formula 5 include butane sultone (BS).

  The S═O compound belonging to the third quadrant is, for example, sulfolane (SL), and the S═O compound belonging to the fourth quadrant is, for example, propane sultone (PS).

  Therefore, in this embodiment, when the G / D ratio of the carbon-based active material is larger than 2, the electrolytic solution 43 includes at least one S═O compound belonging to the first quadrant. Further, when the G / D ratio of the carbon-based active material is 2 or less, the electrolytic solution 43 includes at least one S═O compound included in the second quadrant.

  The addition amount of the S═O compound is preferably 0.5 to 5.0% by volume, more preferably 3.0 to 5.0% by volume, based on the total volume of the solvent, 3.0 More preferably, it is volume%. When the addition amount of the S═O compound is less than 0.5% by volume, the effect of the S═O compound is not sufficiently exhibited, and when the addition amount of the S═O compound exceeds 5% by volume, electrolysis is performed. This is because the viscosity of the liquid becomes very high. In this case, the ionic conductivity of the electrolytic solution is lowered, and the electrolytic solution is less likely to permeate each active material, so that the discharge capacity is reduced.

  Various additives may be added to the electrolytic solution 43. Examples of such additives include a negative electrode action additive, a positive electrode action additive, an ester additive, a carbonate ester additive, a sulfate ester additive, a phosphate ester additive, and a borate ester additive. Additive, acid anhydride additive, electrolyte additive and the like. Any one of these may be added to the electrolytic solution 43, or a plurality of types of additives may be added to the electrolytic solution 43. It is preferable that the addition amount of an additive is 0.01-5.0 mass% (outside number) with respect to the total mass of the electrolyte of the electrolyte solution 43, and a solvent (S = O compound is included). When the additive amount is in such a range, the battery characteristics are particularly improved.

[Method for producing lithium ion secondary battery]
Next, a method for manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is produced as follows. First, a slurry is formed by dispersing a mixture of a positive electrode active material, a conductive agent, and a binder in the above ratio in an organic solvent (for example, N-methyl-2-pyrrolidone). Next, the positive electrode active material layer 22 is formed by forming (for example, coating) the slurry on the current collector 21 and drying the slurry. The coating method is not particularly limited. Examples of the coating method include a knife coater method and a gravure coater method. The following coating steps are also performed by the same method. Next, the positive electrode active material layer 22 is pressed by a press machine so as to have a density within the above range. Thereby, the positive electrode 20 is produced.

  The negative electrode 30 is also produced in the same manner as the positive electrode 20. First, a slurry is formed by dispersing a mixture of a negative electrode active material and a binder in the above ratio in an organic solvent (for example, N-methyl-2-pyrrolidone). Next, the negative electrode active material layer 32 is formed by forming (for example, coating) the slurry on the current collector 31 and drying the slurry. Next, the negative electrode active material layer 32 is pressed by a press machine so as to have a density within the above range. Thereby, the negative electrode 30 is produced.

  The separator 40a is produced as follows. First, the coating liquid is prepared by mixing the resin constituting the porous layer 42 and the water-soluble organic solvent at a mass ratio of 5 to 10:90 to 95. Here, examples of the water-soluble organic solvent include N-methyl-2-pyrrolidone, dimethylacetamide (DMAc), and tripropylene glycol (TPG). Next, this coating solution is formed (for example, coated) with a thickness of 1 to 5 μm on both surfaces or one surface of the substrate 41. Next, the resin in the coating liquid is solidified by treating the base material 41 coated with the coating liquid with a coagulating liquid. Here, as a method for treating the base material 41 coated with the coating liquid with the coagulating liquid, for example, a method of impregnating the base material 41 coated with the coating liquid with the coagulating liquid, A method of spraying a coagulating liquid on the processed base material 41 is conceivable. Thereby, the separator 40a is produced. Here, the coagulating liquid is, for example, a mixture of water in the above water-soluble organic solvent. The mixing amount of water is preferably 40 to 80% by volume with respect to the total volume of the coagulating liquid. Next, the separator 40a is washed with water and dried to remove water and the water-soluble organic solvent from the separator 40a.

  Subsequently, the electrode structure is produced by sandwiching the separator 40a between the positive electrode 20 and the negative electrode 30. When the porous layer 42 is formed only on one surface of the base material 41, the negative electrode 30 is opposed to the porous layer 42. Next, the electrode structure is processed into a desired shape (for example, a cylindrical shape, a square shape, a laminate shape, a button shape, etc.) and inserted into a container of the shape. Next, by injecting the electrolytic solution having the above composition into the container, each pore in the separator 40a is impregnated with the electrolytic solution. Here, when the G / D ratio of the carbon-based active material is larger than 2, the S═O compound added to the electrolytic solution is selected from the S═O compounds belonging to the first quadrant, and the G / D of the carbon-based active material. When the D ratio is 2 or less, the compound is selected from S═O compounds belonging to the second quadrant. Thereby, a lithium ion secondary battery is produced.

  Next, examples will be described. In addition, each parameter (for example, hole diameter) in each of the following examples was measured by the apparatus described above.

[Example 1]
[Production of lithium ion secondary battery]
This inventor performed Example 1 demonstrated below in order to confirm the improvement effect of the cycle life by a S = O compound. First, this inventor produced the lithium ion secondary battery 10 as follows. Regarding the positive electrode 20, first, 90% by mass of Li ₂ CoO ₂ , 6% by mass of ketjen black, and 4% by mass of polyvinylidene fluoride were dispersed in N-methyl-2-pyrrolidone to form a slurry. Next, the positive electrode active material layer 22 was formed by coating the slurry on the aluminum current collector foil as the current collector 21 and drying the slurry. Next, the positive electrode active material layer 22 was pressed by a press machine, so that the density of the positive electrode active material layer 22 was 2.3 g / cm ³ . Thereby, the positive electrode 20 was produced.

For the negative electrode 30, a silicon alloy (Si: Al: Fe = 55: 29: 16 (mass ratio)) of 10.2% by mass, a carbon-based active material of 81.8% by mass, and a polyacrylic acid-based binder of 8.0% by mass. Was dispersed in N-methyl-2-pyrrolidone to form a slurry. Next, the negative electrode active material layer 32 was formed by coating the slurry on the aluminum current collector foil as the current collector 31 and drying the slurry. Next, the negative electrode active material layer 32 was pressed by a press machine, so that the density of the negative electrode active material layer 32 was 1.45 g / cm ³ . Thereby, the negative electrode 30 was produced. The mass ratio of the silicon-based active material and the carbon-based active material in the negative electrode 30 is about 10/80.

  About separator 40a, aramid (Sigmadrich Japan Co., Ltd. product Poly [N, N ′-(1,3-phenylene) isophthalamide]) and a water-soluble organic solvent (5.5: 94.5% by mass) The coating liquid was produced by mixing in a ratio. Here, what mixed DMAc and TPG by the mass ratio of 50:50 was made into the water-soluble organic solvent.

  On the other hand, a porous polyethylene film (thickness 13 μm, porosity 42%) was used for the substrate 41. Next, the coating solution was applied to both sides of the substrate 41 with a thickness of 2 μm. Next, the resin in the coating liquid was solidified by impregnating the base material 41 coated with the coating liquid with the coagulating liquid. This produced the separator 40a. Here, a mixture of water, DMAc, and TPG in a ratio of 50:25:25 was used as a coagulation liquid. Subsequently, water and the water-soluble organic solvent were removed from the separator 40a by washing and drying the separator 40a.

  Next, the separator 40a was sandwiched between the positive electrode 20 and the negative electrode 30 to produce an electrode structure. The electrode structure was then inserted into the test container. On the other hand, lithium hexafluorophosphate is added to a solvent in which ethylene carbonate (EC), S═O compound, ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a volume ratio of 27: 3: 50: 20. An electrolytic solution was prepared by dissolving to a concentration of 3 mol / L. Next, the electrolyte solution was injected into the test container, so that the pores in the separator 40a were impregnated with the electrolyte solution. Thereby, the lithium ion secondary battery 10 for evaluation was produced. Here, this inventor produced the multiple types of lithium ion secondary battery 10 by changing the combination of a carbon-type active material and a S = O compound as shown in Table 2.

[Cycle test]
Next, a cycle test was performed on each lithium ion secondary battery 10. Specifically, constant current / constant voltage charging was performed at 3 mA / cm ² until the battery voltage reached 4.40V, and 100 charge / discharge cycles were performed for constant current discharge until the battery voltage reached 2.75V. Then, the discharge capacity at the first cycle and the discharge capacity at the 100th cycle were measured. The initial discharge capacity of the first cycle was 200 mAh in any lithium ion secondary battery 10. The discharge capacity at the 100th cycle was as shown in Table 2 below. All the tests described above were performed in a temperature environment of 25 ° C. The measurement of the discharge capacity was performed by TOSCAT3000 Toyo System Corporation. Table 2 shows the correspondence between the combination of the carbon-based active material and the S = O compound and the discharge capacity at the 100th cycle.

  In Table 1, the G / D ratio is a value measured as follows. That is, first, Raman spectra of the carbon-based active materials 1 to 6 were obtained by Raman spectroscopy. And the area of G band and D band of a Raman spectrum was computed by the division | segmentation quadrature method. Then, the G / D ratio of the carbon-based active materials 1 to 6 was calculated by dividing the area of the G band by the area of the D band. “Ref” indicates a lithium ion secondary battery manufactured by replacing the S═O compound with EC in the above-described manufacturing method. The numerical value corresponding to the combination of the carbon-based active material and the S═O compound indicates the discharge capacity at the 100th cycle. The discharge capacity shown in bold is larger than the discharge capacity of “Ref”.

  According to Table 1, it can be seen that DMSi, DMSa, ES, and EMS all improve the cycle life when combined with carbon-based active materials 3 to 6 having a G / D ratio larger than 2. Therefore, it can be seen that these S═O compounds belong to the first quadrant of Table 1.

  It can also be seen that EiPS, EBS, and BS improve cycle life when combined with carbon-based active materials 1 and 2 having a G / D ratio of 2 or less. Therefore, it can be seen that these S═O compounds belong to the second quadrant.

[Example 2]
[Production of lithium ion secondary battery]
In order to confirm that the cycle life is improved irrespective of the mixing ratio of the silicon-based active material and the carbon-based active material, the inventor performed Example 2 described below. In Example 2, a lithium ion secondary battery 10 was produced by the same method as in Example 1 except for the following points.

(1) A solid solution of LiMnCoNiO ₂ and Li ₂ MnO ₃ was used as the positive electrode active material. A specific composition is Li _1.20 Mn _0.55 Co _0.10 Ni _0.15 O ₂ .
(2) The mass ratio of the material constituting the negative electrode 30 was 47.0% by mass of the silicon alloy, 47.0% by mass of the carbon-based active material, and 6.0% by mass of the polyacrylic acid binder. Therefore, the mass ratio of the silicon-based active material and the carbon-based active material in the negative electrode 30 is 50/50.
(3) The composition (material and volume ratio) of the solvent was changed to fluoroethylene carbonate (FEC): S═O compound: dimethyl carbonate (DMC): H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) H = 10: 5 : 45:40.

[Cycle test]
Next, a cycle test was performed on each lithium ion secondary battery 10. Specifically, constant current / constant voltage charging was performed at 3 mA / cm ² until the battery voltage reached 4.55 V, and 100 charge / discharge cycles were performed for constant current discharge until the battery voltage reached 2.50 V. Then, the discharge capacity at the first cycle and the discharge capacity at the 100th cycle were measured. The initial discharge capacity of the first cycle was 200 mAh in any lithium ion secondary battery 10. The discharge capacity at the 100th cycle was as shown in Table 3 below. All the tests described above were performed in a temperature environment of 25 ° C. The measurement of the discharge capacity was performed by TOSCAT3000 Toyo System Corporation. Table 3 shows the correspondence between the combination of the carbon-based active material and the S = O compound and the discharge capacity at the 100th cycle.

  The meaning of each parameter in Table 3 is the same as in Table 2. However, “Ref” indicates a lithium ion secondary battery manufactured by replacing the S═O compound with FEC in the above-described manufacturing method. “-” Indicates that measurement was not performed. According to Table 3, it can be seen that if the combination of the carbon-based active material and the S═O compound is appropriate, the cycle life is improved regardless of the mixing ratio of the silicon-based active material and the carbon-based active material.

[Example 3]
[Production of lithium ion secondary battery]
In order to confirm that the cycle life is improved irrespective of the mixing ratio of the silicon-based active material and the carbon-based active material, the inventor performed Example 3 described below. In Example 3, a lithium ion secondary battery 10 was produced in the same manner as in Example 1 except for the following points.

(1) A solid solution of LiMnCoNiO ₂ and Li ₂ MnO ₃ was used as the positive electrode active material. A specific composition is Li _1.20 Mn _0.55 Co _0.10 Ni _0.15 O ₂ .
(2) The mass ratio of the material constituting the negative electrode 30 was 81.8% by mass of the silicon alloy, 10.2% by mass of the carbon-based active material, and 6.0% by mass of the polyacrylic acid binder. Therefore, the mass ratio of the silicon-based active material and the carbon-based active material in the negative electrode 30 is about 80/10.
(3) The composition (material and volume ratio) of the solvent was FEC: S═O compound: DMC: H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) H = 10: 5: 45: 40.

[Cycle test]
Next, a cycle test was performed on each lithium ion secondary battery 10 by the same method as in Example 2. Table 4 shows the correspondence between the combination of the carbon-based active material and the S = O compound and the discharge capacity at the 100th cycle.

  The meaning of each parameter in Table 4 is the same as in Table 2. “Ref” indicates a lithium ion secondary battery manufactured by replacing the S═O compound with FEC in the above-described manufacturing method. According to Table 4, it can be seen that if the combination of the carbon-based active material and the S═O compound is appropriate, the cycle life is improved regardless of the mixing ratio of the silicon-based active material and the carbon-based active material.

[Example 4]
[Production of lithium ion secondary battery]
Next, in order to confirm the improvement effect of the storage capacity by the S═O compound, the present inventor performed Example 4 described below. First, the present inventor produced a lithium ion secondary battery 10 by the same method as in Example 1.

[Storage capacity evaluation test]
Next, the present inventor conducted a storage capacity evaluation test for each lithium ion secondary battery 10. Specifically, constant charge and constant voltage charge is performed at 0.3 mA / cm ² until the battery voltage reaches 4.40 V, and two charge / discharge cycles are performed to perform constant current discharge until the battery voltage reaches 2.75 V. It was. And the discharge capacity of the 2nd cycle was measured, and this was made into the initial value. The initial value was 200 mAh. Next, constant current and constant voltage charging is performed at 0.3 mA / cm ² until the battery voltage reaches 4.40 V, and the charged lithium ion secondary battery 10 is transferred to a constant temperature bath at 60 ° C. and left for 30 days. did. Next, the lithium ion secondary battery 10 was moved to a constant temperature bath at 25 ° C. and left for 12 hours. Next, the discharge capacity was measured by performing constant current discharge at 0.3 mA / cm ² until the battery voltage reached 2.75V. This was defined as the remaining capacity. The remaining capacity is an index for evaluating how much the coating film of the decomposition product of the S═O compound remains when the lithium ion secondary battery 10 after charging is stored for a long period of time. Thereafter, a constant current / constant voltage charge is performed at 0.3 mA / cm ² until the battery voltage reaches 4.40 V, and a charge / discharge cycle in which constant current discharge is performed until the battery voltage reaches 2.75 V is performed. The discharge capacity was taken as the recovery capacity.

  All the tests described above were performed in a temperature environment of 25 ° C. The measurement of the discharge capacity was performed by TOSCAT3000 Toyo System Corporation. Table 5 shows the correspondence between the combination of the carbon-based active material and S = O compound and the remaining capacity, and Table 6 shows the correspondence between the combination of the carbon-based active material and S = O compound and the recovery capacity.

  The meaning of each parameter in Tables 5 and 6 is the same as in Table 2. “-” Indicates that measurement was not performed. According to Tables 5 and 6, it can be seen that if the combination of the carbon-based active material and the S═O compound is appropriate, the storage capacity (that is, the remaining capacity and the recovery capacity) is improved.

[Example 5]
[Production of lithium ion secondary battery]
In order to confirm that the storage capacity is improved irrespective of the mixing ratio of the silicon-based active material and the carbon-based active material, the inventor performed Example 5 described below. In Example 5, a lithium ion secondary battery 10 was produced by the same method as in Example 3.

[Storage capacity evaluation test]
Next, the present inventor conducted a storage capacity evaluation test for each lithium ion secondary battery 10. Specifically, ^two charge / discharge cycles are performed in which constant current / constant voltage charging is performed at 0.3 mA / cm ² until the battery voltage reaches 4.55 V, and constant current discharge is performed until the battery voltage reaches 2.50 V. It was. And the discharge capacity of the 2nd cycle was measured, and this was made into the initial value. The initial value was 200 mAh. Next, constant current and constant voltage charging is performed at 0.3 mA / cm ² until the battery voltage reaches 4.55 V, and the charged lithium ion secondary battery 10 is transferred to a constant temperature bath at 60 ° C. and left for 30 days. did. Next, the lithium ion secondary battery 10 was moved to a constant temperature bath at 25 ° C. and left for 12 hours. Next, the discharge capacity was measured by performing constant current discharge at 0.3 mA / cm ² until the battery voltage reached 2.50V. This was defined as the remaining capacity. Thereafter, a constant current / constant voltage charge is performed at 0.3 mA / cm ² until the battery voltage reaches 4.55 V, and a charge / discharge cycle in which constant current discharge is performed until the battery voltage reaches 2.50 V is performed. The discharge capacity was taken as the recovery capacity.

  All the tests described above were performed in a temperature environment of 25 ° C. The measurement of the discharge capacity was performed by TOSCAT3000 Toyo System Corporation. Table 7 shows the correspondence between the combination of the carbon-based active material and S═O compound and the remaining capacity, and Table 8 shows the correspondence between the combination of the carbon-based active material and S═O compound and the recovery capacity.

  The meaning of each parameter in Tables 7 and 8 is the same as that in Table 2. However, “Ref” indicates a lithium ion secondary battery manufactured by replacing the S═O compound with FEC in the above-described manufacturing method. “-” Indicates that measurement was not performed. “EiPS — 10%” indicates that the volume percentage of EiPS is 10 volume%. According to Tables 7 and 8, if the combination of the carbon-based active material and the S═O compound is appropriate, the storage capacity (ie, the remaining capacity and It can be seen that the recovery capacity is improved.

  As described above, in the lithium ion secondary battery 10 according to the present embodiment, since the electrolytic solution 43 includes the S═O compound having a different structure depending on the G / D ratio of the carbon-based active material, the battery characteristics are improved.

  Specifically, in the lithium ion secondary battery 10, when the G / D ratio of the carbon-based active material is larger than 2, an S═O compound belonging to the first quadrant is used, and the G / D of the carbon-based active material is used. When the ratio is 2 or less, an S═O compound belonging to the second quadrant is used. Therefore, battery characteristics are improved.

  Further, the characteristics of the second pores 42 a formed in the porous layer 42 are different from those of the first pores 41 a formed in the base material 41. Furthermore, the electrolytic solution 43 contains hydrofluoroether. Therefore, the lithium ion secondary battery 10 can greatly improve the cycle life. That is, the porous layer 42 firmly holds the electrolyte solution in the vicinity of the electrode. The porous layer 42 prevents the separator 40a from being decomposed electrochemically. The hydrofluoroether prevents the electrolytic solution 43 from being oxidized and decomposed electrochemically. Due to these factors, it is estimated that the cycle life is greatly improved.

  Furthermore, the porous layer 42 can also be formed on both the front and back surfaces of the substrate 41. In this case, the cycle life is further improved.

  Furthermore, since the hole diameter of the second pore 42a is larger than the hole diameter of the first hole 41a, the separator 40a can be prevented from being clogged with deposits. This improves the cycle life.

  Further, the porosity of the porous layer 42, that is, the porosity of the second pore 42a is larger than the porosity of the first pore 41a, that is, the porosity of the base material 41. The clogging of the separator 40a can be prevented. This improves the cycle life.

  Further, hydrofluoroethers are 2,2,2-trifluoroethyl methyl ether, 2,2,2-trifluoroethyl difluoromethyl ether, 2,2,3,3,3-pentafluoropropyl methyl ether, 2, 2,3,3,3-pentafluoropropyldifluoromethyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2- Tetrafluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl propyl ether, 1,1,2,2-tetrafluoroethyl butyl ether, 1,1 , 2,2-Tetrafluoroethyl isobutyl ether, 1,1,2,2-tetrafluoroethyl isopenti Ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, Hexafluoroisopropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1, It is selected from the group consisting of 2,3,3,3-hexafluoropropyl ethyl ether and 2,2,3,4,4,4-hexafluorobutyl difluoromethyl ether. As shown in the examples, when the hydrofluoroether is these materials, the cycle life is greatly improved.

  Furthermore, since the electrolytic solution 43 contains 10 to 60% by volume of hydrofluoroether with respect to the total volume of the electrolytic solution 43, the cycle life is also greatly improved in this respect.

  Furthermore, since the electrolytic solution contains at least one of a chain carbonate ester and a fluoroethylene carbonate, the battery characteristics are further improved.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 20 Positive electrode 30 Negative electrode 40 Separator 41 Base material 41a 1st pore 42 Porous layer 42a 2nd pore 43 Electrolyte

Claims (5)

A negative electrode active material comprising a silicon-based active material and a carbon-based active material;
An electrolyte solution containing, as an additive, an S═O compound having a different structure depending on the G / D ratio of the carbon-based active material,
When the G / D ratio of the carbon-based active material is greater than 2, the electrolyte solution includes a first S═O compound represented by any one of the following chemical formulas 1 to 3, or the first S═ A mixture of O compounds,
When the G / D ratio of the carbon-based active material is 2 or less, the electrolytic solution is a second S═O compound represented by the following chemical formula 4 or 5, or a mixture of the second S═O compounds. seen including,
The lithium ion secondary battery , wherein the carbon-based active material is a graphite active material .

... (Chemical formula 1)

... (Chemical formula 2)

... (Chemical formula 3)
Here, R ₁ and R ₁ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₁ and R ₁ ′ may be substituted,
R ₃ and R ₃ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms and may be linked to each other, and at least a part of the hydrogen atoms constituting R ₃ and R ₃ ′ are May be replaced,
R ₄ and R ₄ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₄ and R ₄ ′ may be substituted.

... (Chemical formula 4)

... (Chemical formula 5)
Here, one of R ₅ and R ₅ ′ is a hydrocarbon group having 3 or more carbon atoms, and the other is a hydrocarbon group having 1 or 2 carbon atoms, and constitutes R ₅ and R ₅ ′. At least some of the hydrogen atoms may be substituted,
R ₂ is a hydrocarbon group having 1 or more carbon atoms, and at least a part of the hydrogen atoms constituting R ₂ may be substituted.   The lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains hydrofluoroether.   The lithium ion secondary battery according to claim 1 or 2, wherein the electrolytic solution contains at least one of a chain carbonate ester and a fluoroethylene carbonate. A step of producing a negative electrode active material by mixing a silicon-based active material and a carbon-based active material;
Selecting an S═O compound based on the G / D ratio of the carbon-based active material;
Producing an electrolyte containing a selected S═O compound as an additive,
In the step of selecting the S═O compound,
When the G / D ratio of the carbon-based active material is larger than 2, the first S═O compound represented by any one of the following chemical formulas 1 to 3 or the mixture of the first S═O compounds Selected,
When the G / D ratio of the carbon-based active material is 2 or less, a lithium ion that selects a second S═O compound represented by the following chemical formula 4 or 5 or a mixture of the second S═O compounds A method for manufacturing a secondary battery.

... (Chemical formula 1)

... (Chemical formula 2)

... (Chemical formula 3)
Here, R ₁ and R ₁ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₁ and R ₁ ′ may be substituted,
R ₃ and R ₃ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms and may be linked to each other, and at least a part of the hydrogen atoms constituting R ₃ and R ₃ ′ are May be replaced,
R ₄ and R ₄ ′ are each independently a hydrocarbon group having 1 or 2 carbon atoms, and at least a part of the hydrogen atoms constituting R ₄ and R ₄ ′ may be substituted.

... (Chemical formula 4)

... (Chemical formula 5)
Here, one of R ₅ and R ₅ ′ is a hydrocarbon group having 3 or more carbon atoms, and the other is a hydrocarbon group having 1 or 2 carbon atoms, and constitutes R ₅ and R ₅ ′. At least some of the hydrogen atoms may be substituted,
R ₂ is a hydrocarbon group having 1 or more carbon atoms, and at least a part of the hydrogen atoms constituting R ₂ may be substituted. The method for producing a lithium ion secondary battery according to claim 4 , wherein the carbon-based active material is a graphite active material.

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