JP2018200838A - Cathode active material layer and lithium ion secondary battery - Google Patents

Abstract

To improve cycle characteristics of a lithium ion secondary battery and capacity characteristics after storage at high temperature.SOLUTION: There is provided a cathode active material layer including a positive electrode active material containing a lithium composite oxide and a binder resin containing a first copolymer containing at least Formula 1 and a second copolymer containing at least Formula 2 or 3 at a mass ratio of 25:75 to 75:25.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material layer and a lithium ion secondary battery.

  In recent years, with the reduction in size and weight of various electronic devices, lithium ion secondary batteries have been used as power sources for these electronic devices. Such lithium ion secondary batteries are required to have higher energy density. In order to increase the energy density of a lithium ion secondary battery, for example, increasing the charging voltage of the lithium ion secondary battery has been studied.

  On the other hand, in the lithium ion secondary battery under a high potential, the oxidizing power of the positive electrode active material becomes strong. Therefore, on the surface of the positive electrode active material, side reactions that cause decomposition of the electrolyte and structural destruction of the positive electrode active material are accelerated. As a result, the cycle characteristics and storage characteristics of the lithium ion secondary battery are significantly deteriorated.

  In order to improve the characteristics of the lithium ion secondary battery, various proposals have been made for the binder resin contained in the positive electrode active material layer.

  For example, in the following Patent Document 1, a copolymer of vinylidene fluoride and tetrafluoroethylene is used for the binder resin contained in the positive electrode active material layer. Thereby, in patent document 1, the softness | flexibility of a positive electrode active material layer is ensured, densifying a positive electrode active material layer.

  In Patent Documents 2 and 3 below, a polymer having a polar functional group such as a carboxyl group, a sulfonic acid group, or a phosphoric acid group is used for the binder resin. Thereby, in the following patent documents 2 and 3, the cycle characteristics of the lithium ion secondary battery under a high voltage are improved.

Japanese Patent No. 5949915 JP 2014-235996 A JP 2014-1307775 A

  However, in the techniques disclosed in Patent Documents 1 to 3 described above, structural destruction of the positive electrode active material that occurs particularly in the positive electrode active material having strong oxidizing power is not sufficiently studied. Therefore, with the techniques disclosed in Patent Documents 1 to 3, it has been difficult to sufficiently improve the cycle characteristics of the lithium ion secondary battery and the capacity characteristics after high-temperature storage.

  The present invention has been made in view of the above problems. An object of the present invention is to provide a positive electrode active material layer and a lithium ion secondary battery capable of improving cycle characteristics and capacity characteristics after high-temperature storage.

上記化学式２および３において、Ｒ_１およびＲ_２は、互いに独立して、水素、またはメチル基であり、Ｒ_３は、水素、または炭素数１〜６の直鎖もしくは分枝アルキル基である。 In order to solve the above problems, according to one aspect of the present invention, a positive electrode active material containing a lithium composite oxide, a first copolymer containing at least the following chemical formula 1, and at least the following chemical formula 2 or 3 There is provided a positive electrode active material layer comprising a binder resin containing a second copolymer containing at a mass ratio of 25:75 to 75:25.

In the above chemical formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl group, and R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

  According to this configuration, the lithium ion secondary battery can improve cycle characteristics and capacity characteristics after high-temperature storage.

  The first copolymer may contain 50 mol% or more of a unit structure containing vinylidene fluoride as a monomer in the polymer main chain.

  According to this configuration, it is possible to facilitate the production of the positive electrode slurry by improving the solubility of the first copolymer in the organic solvent.

  The first copolymer may include a unit structure having tetrafluoroethylene as a monomer in a polymer main chain of 10 mol% or more and 40 mol% or less.

  According to this configuration, in the first copolymer, it is possible to achieve both resistance to oxidation, solubility in an organic solvent, and dispersibility of the positive electrode active material.

  The second copolymer may contain 90 mol% or more of a unit structure having acrylonitrile or methacrylonitrile as a monomer in the polymer main chain.

  According to this structure, the 2nd copolymer can suppress the side reaction in the surface of a positive electrode active material by improving the tolerance with respect to oxidation.

  The second copolymer containing at least the chemical formula 2 may contain a unit structure having acrylic acid or an acrylate ester as a monomer in the polymer main chain of 0.1 mol% or more and less than 10 mol%.

  According to this configuration, the second copolymer containing at least Chemical Formula 2 can achieve both resistance to oxidation and ease of production of the positive electrode slurry.

  The second copolymer containing at least Chemical Formula 3 may contain a unit structure having acrylamide as a monomer in the polymer main chain in an amount of 0.1 mol% or more and less than 10 mol%.

  According to this configuration, the second copolymer containing at least Chemical Formula 3 can achieve both resistance to oxidation and ease of production of the positive electrode slurry.

  The weight average molecular weights of the first copolymer and the second copolymer may be 300,000 or more, respectively.

  According to this configuration, in the lithium ion secondary battery, it is possible to further suppress the elution of the transition metal from the positive electrode active material.

The lithium composite oxide may be a compound represented by the following chemical formula 4.
_{Li a Co x M y O 2} ·· Formula 4
In the above chemical formula 4, M is selected from the group consisting of aluminum, nickel, manganese, chromium, iron, vanadium, magnesium, titanium, zirconium, niobium, molybdenum, tungsten, copper, zinc, gallium, indium, tin, lanthanum, and cerium. One or more metal elements selected,
0.20 ≦ a ≦ 1.20, 0.95 ≦ x <1.00, 0 <y ≦ 0.05, and x + y = 1.

  According to this configuration, the lithium ion secondary battery can be further increased in energy density.

  Moreover, in order to solve the said subject, according to another viewpoint of this invention, the positive electrode containing the positive electrode active material layer in any one of Claims 1-8, a negative electrode, and a nonaqueous electrolyte are provided. A lithium ion secondary battery is provided.

  According to this configuration, it is possible to obtain a lithium ion secondary battery with improved cycle characteristics and capacity characteristics after high-temperature storage.

  The end-of-charge voltage of the lithium ion secondary battery may be greater than 4.45V on a graphite basis.

  According to this configuration, the battery capacity can be increased by increasing the density of the lithium ion secondary battery.

  As described above, according to the present invention, it is possible to improve the cycle characteristics of the lithium ion secondary battery and the capacity characteristics after high-temperature storage.

It is explanatory drawing which shows typically the structure of the lithium ion secondary battery which concerns on one 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.

<1. Overview of lithium-ion secondary battery>
First, an outline of a positive electrode active material layer according to an embodiment of the present invention and a lithium ion secondary battery including the positive electrode active material layer will be described.

  In recent years, it has been studied to increase the energy density of lithium ion secondary batteries. Therefore, it has been studied to increase the end-of-charge voltage of the lithium ion secondary battery to more than 4.45 V on the basis of graphite.

  However, the present inventors have found that the oxidizing power of the positive electrode active material becomes strong under the environment of the high end-of-charge voltage as described above. The present inventors have found that due to the strong oxidizing power, the decomposition reaction of the electrolytic solution and the structural destruction of the positive electrode active material proceed, and the transition metal in the positive electrode active material is eluted into the electrolytic solution. In such a case, in the lithium ion secondary battery, cycle characteristics and capacity characteristics during high-temperature storage are rapidly deteriorated.

  As a result of intensive studies on the above-described problems and the like, the present inventors have found that two specific types of copolymers are used as the binder resin contained in the positive electrode. The present inventors have found that by using two specific types of copolymers, side reactions that cause structural destruction of the positive electrode active material can be suppressed, and elution of transition metals from the positive electrode active material can be suppressed. Therefore, the present inventors have found that side reactions on the surface of the positive electrode active material can be suppressed even in an environment with a high end-of-charge voltage. According to this, the lithium ion secondary battery can improve the cycle characteristics and the capacity characteristics during high-temperature storage.

  That is, the positive electrode active material layer according to this embodiment includes positive electrode active material particles containing a lithium composite oxide and a binder resin. The binder resin contains a first copolymer containing at least the following chemical formula 1 and a second copolymer containing at least the following chemical formula 2 or 3 in a mass ratio of 25:75 to 75:25.

In the above chemical formulas 2 and 3, R ₁ and R ₂ are independently of each other hydrogen or a methyl group. R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

  In addition, the positive electrode active material layer according to this embodiment is more effective when a lithium composite oxide containing cobalt (Co) is used as the positive electrode active material. Specifically, the positive electrode active material layer according to the present embodiment can suppress the elution of transition metal (ie, cobalt) from the lithium composite oxide.

Specifically, the lithium composite oxide containing cobalt (Co) may be a compound represented by the following chemical formula 4.
_{Li a Co x M y O 2} ··· Formula 4
In the above chemical formula 4, M is aluminum (Al), nickel (Ni), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium ( Zr), niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La), and cerium One or more metal elements selected from the group consisting of (Ce), 0.20 ≦ a ≦ 1.20, 0.95 ≦ x <1.00, 0 <y ≦ 0.05 Yes, x + y = 1.

  Furthermore, the lithium ion secondary battery including the positive electrode active material layer according to the present embodiment is more effective when the end-of-charge voltage is more than 4.45 V on the basis of graphite. Specifically, the lithium ion secondary battery including the positive electrode active material layer according to the present embodiment can suppress elution of transition metal from the positive electrode active material. According to this, the lithium ion secondary battery provided with the positive electrode active material layer according to the present embodiment can improve cycle characteristics and capacity characteristics during high-temperature storage.

<2. Configuration of lithium ion secondary battery>
Below, with reference to FIG. 1, the structure of the lithium ion secondary battery 10 which concerns on this embodiment demonstrated above in detail is demonstrated in detail.

  As shown in FIG. 1, the lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, and a separator layer 40 impregnated with an electrolytic solution. The form of the lithium ion secondary battery 10 is not particularly limited. The form of the lithium ion secondary battery 10 may be any of, for example, a cylindrical shape, a rectangular shape, a laminate type, a button type, 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 anything as long as it is a conductor. The current collector 21 may be, for example, aluminum, stainless steel, nickel-plated steel, or the like.

  The positive electrode active material layer 22 includes at least a positive electrode active material and a binder resin, and may further include a conductive additive. In addition, the ratio of the content of the positive electrode active material, the conductive additive, and the binder resin is not particularly limited. As the content ratio of these components, the content ratio applied in a general lithium ion secondary battery can be used.

The positive electrode active material contains a lithium composite oxide. The lithium composite oxide contained in the positive electrode active material may be a lithium composite oxide containing cobalt, and specifically, a compound represented by the following chemical formula 4.
_{Li a Co x M y O 2} ··· Formula 4
In the above chemical formula 4, M is aluminum (Al), nickel (Ni), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium ( Zr), niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La), and cerium One or more metal elements selected from the group consisting of (Ce), 0.20 ≦ a ≦ 1.20, 0.95 ≦ x <1.00, 0 <y ≦ 0.05 Yes, x + y = 1. Preferably, M may be one or more metal elements selected from the group consisting of Al, Mg, Ti, and Zr.

  When the positive electrode active material includes the lithium composite oxide represented by the above Chemical Formula 4, the lithium ion secondary battery may be used with a charge end voltage exceeding 4.45 V based on graphite. Thereby, since a lithium ion secondary battery can make higher energy density, it can increase battery capacity.

  However, when the positive electrode active material includes the lithium composite oxide represented by the above chemical formula 4, there is a possibility that the structure of the lithium composite oxide is destroyed by driving at a high potential. In such a case, cobalt may be eluted from the lithium composite oxide. In the positive electrode active material layer 22 according to the present embodiment, a binder resin containing two specific types of copolymers is used. Thereby, it is possible to suppress the elution of cobalt from the lithium composite oxide. According to this, the positive electrode active material layer 22 according to the present embodiment can improve the cycle characteristics of the lithium ion secondary battery 10 and the capacity characteristics during high-temperature storage.

  Needless to say, the positive electrode active material may contain one or more other positive electrode active materials in addition to the lithium composite oxide represented by the above chemical formula 4.

Specific examples of the other positive electrode active material include a transition metal oxide or a solid solution oxide containing lithium. For example, as the transition metal oxide containing lithium, Li / Ni / Co / Mn based composite oxide such as LiNi _x Co _y Mn _z O ₂ , Li / Ni based composite oxide such as LiNiO ₂ , or LiMn ₂ O Examples include Li / Mn composite oxides such as ₄ . In addition, as a solid solution oxide containing lithium, 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, 0.20 ≦ 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 and the} like can be exemplified.

  The binder resin contains a first copolymer containing at least the following chemical formula 1 and a second copolymer containing at least the following chemical formula 2 or 3.

In the above chemical formulas 2 and 3, R ₁ and R ₂ are independently of each other hydrogen or a methyl group. R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.

  In the present embodiment, the binder resin contains, as the first copolymer, a fluorine-based copolymer containing at least chemical formula 1 having a high fluorine content in the main chain and high resistance to oxidation. Thereby, binder resin can suppress the decomposition reaction of the electrolyte solution which arises on the surface of the positive electrode active material under high potential.

  However, when the binder resin contains only the fluorine-based copolymer containing at least Chemical Formula 1, the dispersibility of the positive electrode active material at the time of slurry formation is low. In addition, the binding property between the positive electrode active materials decreases. Thereby, the cycling characteristics of the lithium ion secondary battery 10 will fall.

  Therefore, the binder resin further contains an acrylonitrile-based copolymer containing at least chemical formula 2 or 3 as the second copolymer. The acrylonitrile-based copolymer has high dispersibility and binding property of the positive electrode active material. Thereby, the positive electrode active material layer 22 which concerns on this embodiment can improve the concern which arises when a binder resin contains only the fluorine-type copolymer which contains Chemical formula 1 at least. Therefore, the lithium ion secondary battery 10 can improve the cycle characteristics under a high potential and the capacity characteristics during high temperature storage.

  Moreover, binder resin contains the 1st copolymer which is a fluorine-type copolymer, and the 2nd copolymer which is an acrylonitrile-type copolymer. Thereby, binder resin can suppress elution of the transition metal (specifically cobalt) from a positive electrode active material. In addition, it is not clear about the mechanism which suppresses the elution of the transition metal from a positive electrode active material. However, it is considered that the first copolymer that is a fluorine-based copolymer and the second copolymer that is an acrylonitrile-based copolymer effectively suppress the structural destruction of the positive electrode active material. .

  The content ratio of the first copolymer containing at least Chemical Formula 1 and the second copolymer containing at least Chemical Formula 2 or 3 is 25:75 to 75:25 in mass ratio. The content ratio of the second copolymer containing at least chemical formula 2 or 3 is 35:65 to 65:35 in mass ratio, preferably in mass ratio. When the content ratio of the first copolymer including at least Chemical Formula 1 is less than the above range, the resistance of the binder resin to oxidation becomes low. This is not preferable because it becomes difficult to suppress side reactions such as decomposition of the electrolyte solution on the surface of the positive electrode active material. Moreover, when the content rate of the 2nd copolymer containing at least Chemical formula 2 or 3 is less than the said range, the dispersibility of the positive electrode active material at the time of slurry preparation and the binding property of a positive electrode active material will fall. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 deteriorate.

  Here, the first copolymer preferably contains 50 mol% or more of a unit structure containing vinylidene fluoride as a monomer in the polymer main chain. More preferably, the first copolymer contains 60 mol% or more of a unit structure containing vinylidene fluoride as a monomer in the polymer main chain. Note that the unit structure using vinylidene fluoride as a monomer is the unit structure on the left side in Chemical Formula 1. When the content of the unit structure containing vinylidene fluoride as a monomer is less than 50 mol%, the solubility of the first copolymer in an organic solvent is significantly reduced. This makes it difficult to produce a slurry, which is not preferable. Although the upper limit of content of the unit structure which uses vinylidene fluoride as a monomer is not specifically limited, For example, 80 mol% may be sufficient. The organic solvent is, for example, N-methyl-2-pyrrolidone (NMP).

  Moreover, it is preferable that a 1st copolymer contains 10 mol% or more and 40 mol% or less of unit structures which use tetrafluoroethylene as a monomer in a polymer principal chain. More preferably, the first copolymer contains a unit structure containing tetrafluoroethylene as a monomer in the polymer main chain in an amount of 15 mol% to 35 mol%. In addition, the unit structure using tetrafluoroethylene as a monomer is the right side unit structure in Chemical Formula 1. When the content of the unit structure containing tetrafluoroethylene as a monomer is less than 10 mol%, the first copolymer has low resistance to oxidation. Thereby, it is difficult to suppress side reactions on the surface of the positive electrode active material, which is not preferable. On the other hand, when the content of the unit structure containing tetrafluoroethylene as a monomer is more than 40 mol%, the solubility in an organic solvent is remarkably lowered. Moreover, the dispersibility of the positive electrode active material at the time of slurry preparation falls. The organic solvent is, for example, N-methyl-2-pyrrolidone (NMP).

  The second copolymer containing Chemical Formula 2 preferably contains 90 mol% or more of a unit structure having acrylonitrile or methacrylonitrile as a monomer in the polymer main chain. More preferably, the second copolymer contains 95 mol% or more of a unit structure containing acrylonitrile or methacrylonitrile as a monomer in the polymer main chain. Note that the unit structure using acrylonitrile or methacrylonitrile as a monomer is the unit structure on the left side in Chemical Formula 2 or 3. When the content of the unit structure containing acrylonitrile or methacrylonitrile as a monomer is less than 90 mol%, the second copolymer has low resistance to oxidation. Thereby, it is difficult to suppress side reactions on the surface of the positive electrode active material, which is not preferable. The upper limit of the content of the unit structure having acrylonitrile or methacrylonitrile as a monomer is not particularly limited. However, the upper limit of the content of the unit structure having acrylonitrile or methacrylonitrile as a monomer may be, for example, 97.5 mol%.

  Moreover, it is preferable that the 2nd copolymer containing Chemical formula 2 contains 0.1 mol% or more and less than 10 mol% of unit structures which use acrylic acid or acrylic ester as a monomer in a polymer principal chain. The second copolymer more preferably contains 1.0 mol% or more and 5.0 mol% or less in the polymer main chain of a unit structure having acrylic acid or an acrylate ester as a monomer. Note that the unit structure using acrylic acid or acrylic ester as a monomer is the unit structure on the right side in Chemical Formula 2. When the content of the unit structure having acrylic acid or acrylic acid ester as a monomer is less than 0.1%, the solubility of the second copolymer in an organic solvent (for example, NMP) decreases. This makes it difficult to produce a slurry and lowers the binding property between the positive electrode active materials, which is not preferable. On the other hand, when the content of the unit structure having acrylic acid or acrylic acid ester as a monomer is 10 mol% or more, the second copolymer has low resistance to oxidation. Thereby, it is difficult to suppress side reactions on the surface of the positive electrode active material, which is not preferable.

  On the other hand, it is preferable that the 2nd copolymer containing Chemical formula 3 contains 0.1 mol% or more and less than 10 mol% of unit structures which use acrylamide as a monomer in a polymer principal chain. As for the 2nd copolymer containing Chemical formula 3, it is more preferable that the unit structure which uses acrylamide as a monomer contains 1.0 mol% or more and 5.0 mol% or less in a polymer principal chain. The unit structure using acrylamide as a monomer is the right side unit structure in Chemical Formula 3. When the content of the unit structure containing acrylamide as a monomer is less than 0.1%, the solubility of the second copolymer in an organic solvent (for example, NMP) is reduced. This makes it difficult to produce a slurry, which is not preferable. On the other hand, when the content of the unit structure containing acrylamide as a monomer is 10 mol% or more, the second copolymer has low resistance to oxidation. Thereby, it is difficult to suppress side reactions on the surface of the positive electrode active material, which is not preferable.

  Furthermore, it is preferable that the weight average molecular weights of the first copolymer and the second copolymer are 300,000 or more, respectively. The weight average molecular weights of the first copolymer and the second copolymer are more preferably 400,000 or more, respectively. When the weight average molecular weight of the first copolymer and the second copolymer is less than 300,000, the binding property between the positive electrode active materials and the adhesion of the positive electrode active material layer 22 to the current collector Decreases. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 deteriorate. The upper limit of the weight average molecular weight of a 1st copolymer and a 2nd copolymer is not specifically limited. However, the upper limit of the weight average molecular weight of the first copolymer and the second copolymer may be, for example, 2,000,000. In addition, the weight average molecular weight of a 1st copolymer and a 2nd copolymer can be measured by a gel permeation chromatography (Gel Permeation Chromatography: GPC), for example.

  In this embodiment, the first copolymer containing at least Chemical Formula 1 and the second copolymer containing at least Chemical Formula 2 or 3 are used in the binder resin in a mass ratio of 25:75 to 75:25. Thereby, binder resin can suppress the side reaction in the surface of a positive electrode active material. According to this, the lithium ion secondary battery 10 can suppress elution of the metal from the positive electrode active material under a high potential, and can improve cycle characteristics and capacity characteristics during high-temperature storage.

  Examples of the conductive assistant include carbon black such as ketjen black and acetylene black, natural graphite, artificial graphite, carbon nanotubes, graphene, and carbon. Fibrous carbon such as carbon nanofibres, or a composite of these fibrous carbon and carbon black can be used. However, the conductive auxiliary agent can be used without being limited to the above substances as long as the conductivity of the positive electrode can be increased.

  The positive electrode active material layer 22 can be produced, for example, by producing a positive electrode slurry (slurry), applying the positive electrode slurry onto the current collector 21, and then drying and rolling. The positive electrode slurry (slurry) can be prepared by dispersing the positive electrode active material, the conductive additive, and the binder resin in an appropriate organic solvent. A suitable organic solvent is, for example, N-methyl-2-pyrrolidone. The density of the positive electrode active material layer 22 after rolling is not particularly limited as long as it is applicable to a positive electrode active material layer of a general lithium ion secondary battery.

  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, and may be, for example, copper, copper alloy, 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 conductive additive and a binder resin. In addition, the ratio of the content of the negative electrode active material, the conductive additive, and the binder is not particularly limited. As the content ratio of the negative electrode active material, the conductive additive, and the binder, the content ratio applied in a general lithium ion secondary battery can be used.

Examples of the negative electrode active material include graphite active material such as artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, or natural graphite coated with artificial graphite, fine particles of silicon (Si) or tin (Sn), silicon Fine particles of an oxide of (Si) or tin (Sn), an alloy of silicon or tin, or a titanium oxide (TiO _x ) -based compound such as Li ₄ Ti ₅ O ₁₂ can be used. Mixtures of these can also be used. Furthermore, as the negative electrode active material, for example, metallic lithium or the like can be used in addition to these.

  As the conductive aid, the same conductive aid as that used in the positive electrode active material layer 22 can be used. Moreover, styrene butadiene rubber (Styrene-Butadiene Rubber: SBR) etc. can be used for binder resin, for example.

  The negative electrode active material layer 32 can be prepared, for example, by preparing a negative electrode slurry, coating the negative electrode slurry on the current collector 31, and then drying and rolling. The negative electrode slurry can be prepared by dispersing the negative electrode active material, the conductive additive, and the binder resin in an appropriate solvent (for example, water). In addition, the density of the negative electrode active material layer 32 after rolling is not particularly limited as long as it can be applied to the negative electrode active material layer of a general lithium ion secondary battery.

  Separator layer 40 includes a separator and an electrolytic solution.

The separator is not particularly limited as long as it is used as a separator for a lithium ion secondary battery, and any separator can be used. As the separator, for example, a porous film or a nonwoven fabric exhibiting excellent high rate discharge performance is preferably used alone or in combination. In addition, at least one surface of the separator may be coated. The coating can be performed with an inorganic material such as Al ₂ O ₃ or SiO ₂ or a polymer such as polyvinylidene fluoride. The separator may contain an inorganic material such as Al ₂ O ₃ or SiO ₂ as a filler.

  As a material constituting the separator, for example, a polyolefin resin represented by polyethylene, polypropylene, or the like, a polyethylene terephthalate, or a polybutylene terephthalate, or the like is typical. Polyester resin, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl Ether (vinylidene difluoride-perfluorovinylethylene) copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer Ethylene (vinylidene difluoride-fluoroethylene) copolymer, vinylidene difluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene (vinylidene fluoride) difluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, tetrafluoroethylene hexafluoroethylene fluoride-tetrafluoroethylene Propylene (vinylidene difluoride-tetrafluoroethylene) copolymer, copolymer using vinylidene difluoride-ethylene-tetrafluoroethylene, etc. Can. In addition, the porosity in particular of a separator is not restrict | limited, The porosity which the separator of a general lithium ion secondary battery has can be applied.

  The electrolytic solution includes an electrolyte salt and a non-aqueous electrolyte.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN Inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K), such as, can be used. Examples of the electrolyte salt include LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C ₄ H ₉ ) ₄ NI, (C ₂ H ₅ ) ₄ N-maleate, (C ₂ H _{5) 4 N-benzoate, (} C 2 H 5) 4 N-phtalate, lithium stearyl sulfonate (lithium stearyl sulfate), lithium octyl sulfonate (lithium octy sulfate), organic ion salts such as lithium dodecyl benzene sulfonate (lithium dodecylbenzene sulphonate) can be used. These electrolyte salts may be used alone or in combination of two or more. Further, the concentration of the electrolyte salt is not particularly limited as long as it is a concentration applicable to a general lithium secondary battery. For example, the concentration of the electrolyte salt may be 0.5 mol / L to 2.0 mol / L.

  Examples of the non-aqueous electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, vinylene carbonate, and the like. ), Cyclic esters such as γ-butyrolactone or γ-valerolactone, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate Chain carbonates such as (ethylmethyl carbonate), chain esters such as methyl formate, methyl acetate or methyl butyrate, tetrahydrofuran or derivatives thereof , 3-dioxane (1,3-dioxane), 1,4-dioxane (1,4-dioxane), 1,2-dimethoxyethane (1,2-dioxyethane), 1,4-dibutoxyethane (1,4 -Ethers such as dibutyloxyne) or methyldiglyme, acetonitrile. Or nitriles such as benzonitrile, dioxolane or derivatives thereof, ethylene sulfide, sulfolane, sultone, or derivatives thereof alone, or Two or more of these can be mixed and used. However, the non-aqueous electrolyte is not limited to these.

  Furthermore, the electrolytic solution may contain various additives such as a negative electrode SEI (Solid Electrolyte Interface) forming agent or a surfactant.

  Examples of the various additives include succinic anhydride, lithium bisoxalate borate, lithium tetrafluoroborate, propane sultone, and propane sultone. Examples include butane sultone, propene sultone, 3-sulfolene, fluorinated allyl ether, and fluorinated acrylate. In addition, it is possible to apply content of the additive in a general lithium ion secondary battery as content of these various additives.

<3. Manufacturing method of lithium ion secondary battery>
Then, the manufacturing method of the lithium ion secondary battery 10 is demonstrated. However, the manufacturing method of the lithium ion secondary battery 10 is not limited to the following method, and other known manufacturing methods can also be used.

  The positive electrode 20 is manufactured as follows. First, materials constituting the positive electrode active material layer 22 (for example, a positive electrode active material, a conductive additive, and a binder resin) are mixed in a desired ratio. Subsequently, a positive electrode slurry is prepared by dispersing the mixture in an organic solvent (for example, N-methyl-2-pyrrolidone). Next, the positive electrode active material layer 22 is produced by applying the positive electrode slurry onto the current collector 21 and drying it. The coating method is not particularly limited. For example, a knife coater method, a gravure coater method, or the like can be used.

  Further, the positive electrode active material layer 22 is compressed to a desired density or thickness by a roll press. Thereby, the positive electrode 20 can be manufactured. Here, the density or thickness of the positive electrode active material layer 22 is not particularly limited, and may be any density or thickness that a positive electrode active material layer of a general lithium ion secondary battery has.

  The negative electrode 30 is also manufactured by the same method as the positive electrode 20. First, the material (for example, negative electrode active material, conductive support agent, and binder resin) which comprises the negative electrode active material layer 32 is mixed in a desired ratio. Subsequently, the negative electrode slurry is prepared by dispersing the mixture in a solvent (for example, water). Next, the negative electrode slurry is applied on the current collector 31 and dried to prepare the negative electrode active material layer 32. The coating method is not particularly limited. For example, a knife coater method, a gravure coater method, or the like can be used.

  Further, the negative electrode active material layer 32 is compressed to a desired density or thickness with a roll press. Thereby, the negative electrode 30 can be manufactured. Here, the density or thickness of the negative electrode active material layer 32 is not particularly limited as long as the negative electrode active material layer of a general lithium ion secondary battery has a density or thickness. In addition, when using metal lithium as the negative electrode active material layer 32, the negative electrode 30 can be manufactured by attaching a metal lithium foil to the current collector 31.

  Subsequently, an electrode structure is manufactured by sandwiching the separator between the positive electrode 20 and the negative electrode 30. The manufactured electrode structure is processed into a shape suitable for a desired shape (for example, a cylindrical shape, a square shape, a laminate shape, a button shape, etc.), and then inserted into a container of the shape. Furthermore, the separator is impregnated with the electrolytic solution by injecting the electrolytic solution containing the electrolyte salt and the non-aqueous electrolyte into the container. Thereafter, the electrode structure and the container filled with the electrolyte are sealed. Thereby, the lithium ion secondary battery 10 can be manufactured.

  Hereinafter, the lithium ion secondary battery according to the present embodiment will be specifically described with reference to Examples and Comparative Examples. In addition, the Example shown below is an example to the last, and the lithium ion secondary battery which concerns on this embodiment is not limited to the following example.

<Example 1>
A first copolymer and a second copolymer were prepared by the following method, and a lithium ion secondary battery according to Example 1 was manufactured.

(First copolymer: Synthesis of fluoropolymer A)
As the first copolymer, a copolymer (also referred to as fluorine-based polymer A) in which the molar ratio of vinylidene fluoride and tetrafluoroethylene was 70 mol%: 30 mol% was synthesized. Specifically, after putting 1 kg of pure water into a 4 L reaction vessel and performing nitrogen substitution, 800 g of octafluorocyclobutane was put into the reaction vessel, and the system was heated to 45 ° C. Next, 118 g of vinylidene fluoride and 40 g of tetrafluoroethylene were added to the reaction vessel. Thereafter, 0.9 g of a 50 mass% methanol solution of di-n-propyl peroxydicarbonate was charged into the reaction vessel. Further, stirring was continued for 8 hours while the internal pressure was kept constant while continuously supplying the mixed gas to the reaction vessel. As the mixed gas, vinylidene fluoride / tetrafluoroethylene = 70 mol% / 30 mol% was used. After stopping the heating, the reaction vessel was released until atmospheric pressure was reached, and the reaction product was washed with water and dried to synthesize a fluorinated polymer A containing at least Chemical Formula 1. When the weight average molecular weight of the synthesized fluoropolymer A was measured by GPC, it was 900,000 (in terms of polymethyl methacrylate).

(Second copolymer: Synthesis of acrylonitrile polymer A)
As the second copolymer, a copolymer (also referred to as acrylonitrile polymer A) in which the molar ratio of acrylonitrile to acrylic acid was 97.5 mol%: 2.5 mol% was synthesized. Specifically, 9.65 g of acrylonitrile and 0.35 g of acrylic acid (molar ratio of acrylonitrile to acrylic acid was 97.5: 2.5) were added to 90 g of ion-exchanged water. Thereafter, the temperature was raised to 60 ° C. in a nitrogen atmosphere, and then 1.065 g of ammonium persulfate was added to the mixture to cause precipitation polymerization of the polymer. As a result, an acrylonitrile-based polymer A containing at least Chemical Formula 2 was synthesized. When the weight average molecular weight of the synthesized acrylonitrile-based polymer A was measured by GPC, it was 500,000 (in terms of polyethylene oxide).

(Preparation of positive electrode)
LiCoO ₂ , carbon black, and a binder resin containing the first copolymer and the second copolymer were dissolved and dispersed in N-methyl-2-pyrrolidone to form a positive electrode slurry. The mixing ratio was 97.6: 1.2: 1.2 in terms of solid content mass ratio. The mixing ratio of the first copolymer and the second copolymer in the binder resin was 50:50 by mass ratio.

Subsequently, the positive electrode slurry was applied to one side of a current collector made of an aluminum foil and dried so that the slurry application amount after drying was 20.0 mg / cm ² . After drying, the current collector coated with the positive electrode slurry was rolled with a roll press so that the density of the positive electrode active material layer was 4.1 g / cm ³ , thereby producing a positive electrode.

(Preparation of negative electrode)
A negative electrode slurry was formed by dissolving and dispersing artificial graphite, an aqueous dispersion of styrene-butadiene copolymer fine particles, and a sodium salt of carboxymethyl cellulose in pure water. The mixing ratio was 97.5: 1.5: 1.0 in terms of solid content mass ratio.

Subsequently, the negative electrode slurry was applied to one side of a current collector made of copper foil and dried so that the slurry coating amount after drying was 12.5 mg / cm ² . After drying, the current collector coated with the negative electrode slurry was rolled with a roll press so that the negative electrode active material layer had a density of 1.6 g / cm ³ , thereby producing a negative electrode.

(Production of lithium ion secondary battery cells)
An aluminum lead wire was welded to the positive electrode produced above, and a nickel lead wire was welded to the negative electrode produced above. Thereafter, the positive electrode and the negative electrode were wound with each other through a polyethylene porous separator, and compressed to produce a flat electrode structure. The produced electrode structure was stored in a battery container formed of an aluminum laminate film in a state in which the lead wire was pulled out. Furthermore, an electrolytic solution in which 1 mol / L LiPF ₆ was dissolved in a carbonate solvent was poured into the battery container. Then, the lithium ion secondary battery which concerns on Example 1 was produced by sealing a battery container under reduced pressure.

<Examples 2 and 3>
The mixing ratio of the first copolymer (fluorine polymer A) and the second copolymer (acrylonitrile polymer A) in the binder resin of the positive electrode was changed as shown in Table 1. Other than that, the lithium ion secondary battery which concerns on Example 2 and 3 was produced by the method similar to Example 1. FIG.

<Examples 4 to 6>
As the second copolymer, acrylonitrile-based polymer B, which shows a synthesis method below, was used. Further, the mixing ratio of the first copolymer (fluorine polymer A) and the second copolymer (acrylonitrile polymer B) in the binder resin of the positive electrode was changed as shown in Table 1. Other than that was the method similar to Example 1, and produced the lithium ion secondary battery which concerns on Examples 4-6.

(Second copolymer: synthesis of acrylonitrile polymer B)
As the second copolymer, a copolymer (also referred to as acrylonitrile polymer B) having a molar ratio of acrylonitrile to acrylamide of 95 mol%: 5 mol% was synthesized. Specifically, 9.34 g of acrylonitrile and 0.66 g of acrylamide were added to 90 g of ion-exchanged water. After raising the temperature to 60 ° C. under a nitrogen atmosphere, 1.065 g of ammonium persulfate was added to the mixture to cause precipitation polymerization of the copolymer. Thereby, an acrylonitrile-based polymer B containing at least Chemical Formula 3 was synthesized. When the weight average molecular weight of the synthesized acrylonitrile-based polymer B was measured by GPC, it was 500,000 (in terms of polyethylene oxide).

<Comparative Example 1>
Only the first copolymer (fluorinated polymer A) was used as the binder resin for the positive electrode. Other than that was the same method as Example 1, and produced the lithium ion secondary battery which concerns on the comparative example 1. FIG.

<Comparative Example 2>
Only the second copolymer (acrylonitrile polymer A) was used as the binder resin for the positive electrode. Other than that was the same method as Example 1, and produced the lithium ion secondary battery which concerns on the comparative example 2. FIG.

<Comparative Example 3>
As the binder resin for the positive electrode, only a vinylidene fluoride homopolymer (KF7200 manufactured by Kureha Battery Materials Co., Ltd., also referred to as fluorinated polymer B) was used. Other than that was the same method as Example 1, and produced the lithium ion secondary battery which concerns on the comparative example 3. FIG.

<Comparative example 4>
As the binder resin for the positive electrode, a mixture of fluorine polymer B and acrylonitrile polymer A at a mass ratio of 50:50 was used. Other than that was the same method as Example 1, and produced the lithium ion secondary battery which concerns on the comparative example 4. FIG.

<Comparative Example 5>
As the binder resin for the positive electrode, a mixture of fluorine polymer A and fluorine polymer B at a mass ratio of 50:50 was used. Otherwise, a lithium ion secondary battery according to Comparative Example 5 was produced in the same manner as in Example 1.

<Comparative Examples 6-9>
The mixing ratio of the first copolymer (fluorine polymer A) and the second copolymer (acrylonitrile polymer A) in the binder resin of the positive electrode was changed as shown in Table 1. Other than that was produced by the method similar to Example 1, and the lithium ion secondary battery which concerns on Comparative Examples 6-9.

<Evaluation of lithium ion secondary battery>
First, the constant current charge was performed until the lithium ion secondary battery which concerns on Examples 1-6 and Comparative Examples 1-9 became 4.5V by 1 / 5CA of a design capacity within a 45 degreeC thermostat. Then, constant voltage charge was performed until it became 1/20 CA at 4.5V. Then, constant current discharge was performed until it became 3.0V at 1/5 CA. Through this step, the lithium ion secondary battery was initially charged. In addition, the discharge capacity of the initial discharge at this time was defined as the initial discharge capacity. CA represents a one hour discharge rate.

(Evaluation method of cycle characteristics)
Each lithium ion secondary battery after initial charge / discharge was charged at a constant current of 1 CA at a charge end voltage of 4.5 V and a discharge end voltage of 3.0 V at a temperature of 45 ° C. Thereafter, a cycle of charging at a constant voltage of 1/20 CA and discharging at a constant current of 1 CA was repeated 50 cycles. After 50 cycles, each lithium ion secondary battery was charged with a constant current at 1/5 CA, further charged with a constant voltage at 1/20 CA, and then discharged with a constant current at 1/5 CA to measure the discharge capacity. The capacity retention rate was calculated by dividing the measured discharge capacity after 50 cycles by the initial discharge capacity.

(Evaluation method of cobalt elution amount)
Each lithium ion secondary battery after the above cycle characteristic evaluation (that is, after 50 cycles) was disassembled. The amount of cobalt deposited on the negative electrode was measured by analyzing the solution in which the cobalt deposited on the negative electrode was dissolved by an inductively coupled plasma mass spectrometer (ICP-MS).

(Evaluation method of capacity characteristics after high temperature storage)
Each lithium ion secondary battery after the initial charge was charged at a constant current of 1/2 CA at a charge end voltage of 4.5 V and a discharge end voltage of 3.0 V at a temperature of 25 ° C. Thereafter, a cycle of charging at a constant voltage of 1/20 CA and discharging at a constant current of 1/2 CA was repeated two cycles. Thereafter, each lithium ion secondary battery was charged at a constant current of 1/2 CA to a charge end voltage of 4.5 V at a temperature of 25 ° C. Subsequently, after charging at a constant voltage of 1/20 CA, it was stored at a high temperature in a constant temperature bath at 60 ° C. for 1 week. The charge capacity at this time was defined as the charge capacity before high-temperature storage.

  After storage at a high temperature, each lithium ion secondary battery was discharged at a constant current of 1/2 CA to 3.0 V at a temperature of 25 ° C. Then, the charge capacity after high-temperature storage was measured by charging at a constant current of 1/2 CA until the end-of-charge voltage of 4.5 V, and then charging at a constant voltage of 1/20 CA. The capacity recovery rate after high temperature storage was calculated by dividing the measured charge capacity after high temperature storage by the charge capacity before high temperature storage.

(Adhesion evaluation method)
A positive electrode plate used for each lithium ion secondary battery was fixed on a stainless steel plate. An adhesive tape (cello tape No. 405 manufactured by Nichiban Co., Ltd.) having a width of 1.5 cm was attached to the surface of the fixed positive electrode plate on the positive electrode active material layer side. The adhesion of the positive electrode active material layer was evaluated by peeling off the attached adhesive tape in a 180 ° direction using a peel tester (SHIMAZU EZ-S, manufactured by Shimadzu Corporation).

(Evaluation results)
The evaluation results of the lithium ion secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 9 are summarized in Table 1.

  Referring to the results in Table 1, it can be seen that Examples 1 to 6 have a higher capacity retention rate and higher capacity recovery rate than Comparative Examples 1 to 9. This is considered because Examples 1-6 use the binder resin which contains specific 2 types of copolymers by specific mass ratio. Moreover, in Examples 1-6, since the adhesiveness of a positive electrode active material layer and a collector is 5 mN / mm or more, it turns out that the adhesiveness which is practically satisfactory as binder resin is obtained.

  Furthermore, in Examples 1-6, compared with Comparative Examples 1-3, it turns out that the amount of cobalt deposited on the negative electrode after 50 cycles is suppressed to 1000 ppm or less. Such an effect of suppressing elution of cobalt from the positive electrode active material cannot be obtained when the fluorine-based polymer A, the fluorine-based polymer B, or the acrylonitrile-based polymer A is used alone. Therefore, it is understood that this effect can be obtained only when a binder resin containing two specific types of copolymers at a specific mass ratio is used as defined in the present embodiment.

  As described above, the lithium ion secondary battery according to the present embodiment is suitably used under a high potential where the end-of-charge voltage is over 4.45 V on the basis of graphite. Specifically, the lithium ion secondary battery according to the present embodiment can improve cycle characteristics and capacity characteristics during high-temperature storage.

  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 belongs 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 21 Current collector 22 Positive electrode active material layer 30 Negative electrode 31 Current collector 32 Negative electrode active material layer 40 Separator layer

Claims (10)

A positive electrode active material containing a lithium composite oxide;
A binder resin containing a first copolymer containing at least the following chemical formula 1 and a second copolymer containing at least the following chemical formula 2 or 3 in a mass ratio of 25:75 to 75:25;
A positive electrode active material layer.

In the above chemical formulas 2 and 3, R ₁ and R ₂ are each independently hydrogen or a methyl group, and R ₃ is hydrogen or a linear or branched alkyl group having 1 to 6 carbon atoms.   2. The positive electrode active material layer according to claim 1, wherein the first copolymer includes 50 mol% or more of a unit structure containing vinylidene fluoride as a monomer in a polymer main chain.   3. The positive electrode active material layer according to claim 1, wherein the first copolymer includes a unit structure having tetrafluoroethylene as a monomer in a polymer main chain of 10 mol% to 40 mol%.   The positive electrode active material layer according to any one of claims 1 to 3, wherein the second copolymer includes 90 mol% or more of a unit structure having acrylonitrile or methacrylonitrile as a monomer in a polymer main chain.   The second copolymer including at least the chemical formula 2 includes a unit structure having acrylic acid or an acrylate ester as a monomer in a polymer main chain of 0.1 mol% or more and less than 10 mol%. The positive electrode active material layer according to any one of the above.   The second copolymer including at least the chemical formula 3 includes a unit structure having acrylamide as a monomer in a polymer main chain of 0.1 mol% or more and less than 10 mol%. The positive electrode active material layer described.   7. The positive electrode active material layer according to claim 1, wherein weight average molecular weights of the first copolymer and the second copolymer are 300,000 or more, respectively. The positive electrode active material layer according to claim 1, wherein the lithium composite oxide is a compound represented by the following chemical formula 4.
_{Li a Co x M y O 2} ·· Formula 4
In the above chemical formula 4, M is selected from the group consisting of aluminum, nickel, manganese, chromium, iron, vanadium, magnesium, titanium, zirconium, niobium, molybdenum, tungsten, copper, zinc, gallium, indium, tin, lanthanum, and cerium. One or more metal elements selected,
0.20 ≦ a ≦ 1.20, 0.95 ≦ x <1.00, 0 <y ≦ 0.05, and x + y = 1. A positive electrode comprising the positive electrode active material layer according to claim 1;
A negative electrode,
A non-aqueous electrolyte,
A lithium ion secondary battery comprising:   The lithium ion secondary battery according to claim 9, wherein the end-of-charge voltage is greater than 4.45 V on a graphite basis.

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