JP2018206602A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having high energy density and excellent cycle characteristics.SOLUTION: A negative electrode for a lithium ion secondary battery includes a silicon-based negative electrode active material and a binder covering the silicone-based negative electrode active material, and 0.5≤(S-S)/(S-S) is satisfied when a specific surface area of the silicon based negative electrode active material is S, the specific surface area of the binder is S, and the specific surface area of the silicon based negative electrode active material coated on the binder is S.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery. Specifically, the present invention relates to a negative electrode for a lithium ion secondary battery having high energy density and excellent cycle characteristics, and a lithium ion secondary battery using the same.

  In recent years, attention has been focused on lithium ion secondary batteries due to their high energy density as driving power sources for electronic equipment and vehicle travel, and methods for improving cycle characteristics are being developed.

  For example, in Patent Document 1, the negative electrode active material layer contains a negative electrode active material, styrene butadiene rubber, and carboxymethyl cellulose, and the styrene butadiene rubber coverage on the particle surface of the negative electrode active material is 0.3% or more and 50% or less. A lithium ion secondary battery is disclosed. And according to the lithium ion secondary battery of patent document 1, it is supposed that it has sufficient initial output and is excellent in high temperature distribution rate durability.

JP 2013-12394 A

  However, in the lithium ion secondary battery of Patent Document 1, since a carbon-based negative electrode active material is used, a sufficient energy density cannot be obtained as compared with a silicon-based negative electrode active material. On the other hand, although the silicon-based negative electrode active material has a larger energy density than the carbon-based negative electrode active material, there is a possibility that sufficient cycle characteristics may not be obtained even with a coverage as shown in Patent Document 1.

  Further, as in Patent Document 1, since the coverage measured by observing the coverage with a scanning electron microscope (SEM) is local information, the coated binder is unevenly distributed depending on the observation location. The average coverage of the entire electrode may not be reflected.

  The present invention has been made in view of the problems of such conventional techniques. An object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery that have high energy density and excellent cycle characteristics.

The negative electrode for a lithium ion secondary battery according to an embodiment of the present invention has a specific surface area of a silicon-based negative electrode active material S _A , a specific surface area of a binder S _B , and a specific surface area of a silicon-based negative electrode active material coated with a binder S _{When C} , (S _A −S _C ) / (S _A −S _B ) is equal to or greater than a predetermined value.

  According to the present invention, a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having high energy density and excellent cycle characteristics can be provided.

It is sectional drawing which shows an example of the lithium ion secondary battery which concerns on this embodiment. In the Examples and Comparative _Examples, it is a graph showing the relationship between the _{(S A -S C) / (} S A -S B) and binder content and discharge capacity retention rate.

  Hereinafter, a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery according to the present embodiment will be described in detail with reference to the drawings. In addition, the dimension ratio of drawing is exaggerated on account of description, and may differ from an actual ratio.

  The negative electrode for a lithium ion secondary battery of this embodiment includes a silicon-based negative electrode active material and a negative electrode binder. In the following, these components will be described.

(Silicon negative electrode active material)
The silicon-based negative electrode active material can participate in a reaction that generates an electric current. Since the silicon-based negative electrode active material has a larger theoretical capacity per mass than the carbon-based negative electrode active material, the energy density can be increased.

  The silicon-based negative electrode active material may be any negative-electrode active material containing silicon, but the silicon-based negative electrode active material preferably contains 20% by mass or more of silicon. By setting the silicon content of the silicon-based negative electrode active material to 20% by mass or more, amorphous-crystal phase transition can be suppressed, and cycle characteristics of the lithium ion secondary battery can be improved.

  In the present embodiment, the silicon-based negative electrode active material includes Si, Sn, and M elements, and M is at least one element selected from the group consisting of transition elements, B, C, Mg, Al, and Zn. It is preferable. The transition element refers to an element between the Group 3 element and the Group 11 element.

  M is at least one element selected from the group consisting of B, C, Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, and Ta. More preferably. M is more preferably at least one element selected from the group consisting of C, Al, Ti, V, and Zn. Furthermore, it is most preferable that it is at least any one of Al or Ti. By including such an element in the silicon-based negative electrode active material, the cycle characteristics can be further improved while maintaining the discharge capacity.

  Note that the silicon-based negative electrode active material containing Si, Sn, and M elements may contain inevitable impurities. An inevitable impurity means what exists in a raw material or is inevitably mixed in a manufacturing process. Inevitable impurities are originally unnecessary, but they are a very small amount and do not affect the characteristics of the silicon-based negative electrode active material. The content of inevitable impurities is preferably less than 0.5% by mass, more preferably less than 0.1% by mass, and less than 0.01% by mass with respect to the entire silicon-based negative electrode active material. Is more preferable.

  The general formula of the silicon-based negative electrode active material is preferably Si—Sn—M, and more preferably Si—Sn—Ti. Here, in the general formula Si-Sn-Ti, the Sn content is 7% by mass or more and 30% by mass or less, the Ti content is more than 0% by mass and 37% by mass or less, and the balance is Si and inevitable impurities. preferable. Or in general formula Si-Sn-Ti, it is preferable that Sn content is 30 mass% or more and 51 mass% or less, Ti content exceeds 0 mass% and 35 mass% or less, and the remainder is Si and inevitable impurities. . In the general formula Si-Sn-Ti, it is more preferable that the Sn content is 7% by mass or more and 30% by mass or less, Ti is more than 7% by mass and 37% by mass or less, and the balance is Si and inevitable impurities. Alternatively, it is more preferable that the Sn content is 30% by mass or more and 51% by mass or less, the Ti content is more than 7% by mass and 35% by mass or less, and the balance is Si and inevitable impurities. Further, in the general formula Si—Sn—Ti, the Sn content is 7% by mass to 30% by mass, the Ti content is 18% by mass to 37% by mass, and the balance is Si and inevitable impurities. preferable. Or, in the general formula Si-Sn-Ti, the Sn content is 30% by mass or more and 51% by mass or less, the Ti content is more than 7% by mass and 20% by mass or less, and the balance is Si and inevitable impurities. preferable. Furthermore, in the general formula Si—Sn—Ti, the Sn content is 7% by mass or more and 21% by mass or less, the Ti content is 24% by mass or more and 37% by mass or less, and the remainder is Si and inevitable impurities. preferable. By setting the content of each element within the above range, a lithium ion secondary battery excellent in cycle characteristics can be provided.

  The average particle diameter of the silicon-based negative electrode active material is not particularly limited, but is preferably 0.1 μm to 20 μm, and more preferably 0.2 μm to 10 μm. The average particle diameter of the silicon-based negative electrode active material represents the particle diameter when the cumulative value of the particle size distribution on a volume basis is 50%, and can be measured by, for example, a laser diffraction / scattering method.

(Binder for negative electrode)
The negative electrode binder can bond silicon-based negative electrode active materials and the like. Examples of the material for forming the binder for the negative electrode include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyether nitrile (PEN), poly Acrylonitrile (PAN), polyimide (PI), polyamide (PA), polyamideimide (PAI), carboxymethylcellulose (CMC), ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF) ), Polytetrafluoroethylene (PTFE), thermoplastic resin such as polyvinyl fluoride (PVF), thermosetting resin such as epoxy resin, styrene butadiene rubber (SBR), isoprene rubber (IR), butadiene rubber (B ) Include elastomers such as. These binders for negative electrodes may be used independently and may use 2 or more types together. Among these, at least one selected from the group consisting of polyimide (PI), polyamide (PA), and polyamideimide (PAI) is preferable because of its excellent adhesion and heat resistance as a negative electrode binder.

  The tensile modulus of the negative electrode binder is preferably 2 GPa or more and 10 GPa or less. When the tensile modulus is 2 GPa or more, breakage of the binder for the negative electrode accompanying expansion of the silicon-based negative electrode active material is suppressed, so that the release of the silicon-based negative electrode active material from the negative electrode can be suppressed. It is possible to suppress a decrease in discharge capacity due to repetition of the above. That is, by setting the tensile modulus to 2 GPa or more, the cycle characteristics of the lithium ion secondary battery can be improved. When the tensile elastic modulus is 10 GPa or less, since the expansion of the silicon-based negative electrode active material due to lithium ion occlusion is not excessively suppressed, the discharge capacity of the lithium ion secondary battery can be increased.

The tensile elastic modulus can be measured, for example, according to Japanese Industrial Standard JIS K7161-1 at a test temperature of 23 ± 2 ° C. and a test speed of 1 mm / min. Specifically, it can be calculated according to an equation of E _t = (σ ₂ −σ ₁ ) / (ε ₂ −ε ₁ ). In the above formula, _Et represents the tensile modulus (Pa), σ ₁ represents the stress (Pa) at the strain ε ₁ = 0.0005, and σ ₂ represents the stress (Pa) at the strain ε ₂ = 0.0025.

The specific surface area of the silicon-based negative active material S _A, a specific surface area of the binder S _B, the specific surface area of the silicon-based negative active material coated binder when the _{S C, 0.5 ≦ (S A} -S satisfy the relation of _{C) / (S a -S B} ). In the present embodiment, by satisfying such a relationship, the silicon-based negative electrode active material can be coated in a predetermined state or more. _{Incidentally, (S A -S C) /} (S A -S B) is more preferably 0.65 or more. _The upper limit of _{(S A -S C) / (} S A -S B) is 1.

Although the silicon-based negative electrode active material has a larger energy density than the carbon-based negative electrode active material, the volume change during charge / discharge is large, and the silicon-based negative electrode active material is easily detached from the negative electrode binder. However, in the present _embodiment, by setting _{(S A -S C) / (} S A -S B) value of 0.5 or more, coating the surface of the silicon-based negative active material than in a predetermined state Can do. For this reason, since the breakage of the binder for the negative electrode accompanying the expansion of the silicon-based negative electrode active material is suppressed, it is possible to prevent the silicon-based negative electrode active material from detaching from the negative electrode and to reduce the discharge capacity due to repeated charge and discharge. Can be suppressed. That is, in this _embodiment, by setting _{(S A -S C) / (} S A -S B) value of 0.5 or more, it is possible to improve the cycle characteristics of the lithium ion secondary battery.

Here, the _{(S A -S C) / (} S A -S B) can be said to represent the coverage of the negative electrode binder to silicon-based negative active material. For example, if all of the surface of the silicon-based negative active material is coated with a binder, a same value the specific surface area S _C of the silicon-based negative active material coated on the specific surface area S _B and the binder of the silicon-based negative active material Therefore, the value of (S _A −S _C ) / (S _A −S _B ) is 1. On the other hand, if the silicon-based negative active material is not covered at all in a binder, the specific surface area S _C of the silicon-based negative active material coated on the specific surface area S _A and the binder of the silicon-based negative active material is the same value, The value of (S _A −S _C ) / (S _A −S _B ) is 0. When a part of the surface of the silicon-based negative electrode active material is covered with a binder, the value of (S _A −S _C ) / (S _A −S _B ) is a value between 0 and 1.

The specific surface area _{S A} of the silicon-based negative active material ^is preferably 6m ² / g~10m 2 / ^g, more preferably 7m ² / g~9m 2 / g. The specific surface area _{S B} of the binder ^is preferably 1m ² / g~5m 2 / ^g, more preferably 2m ² / g~4m 2 / g. The specific surface area _{S C} of the silicon-based negative active material coated binder ^is preferably 2m ² / g~5m 2 / ^g, and more preferably 3m ² / g~4m 2 / g.

The S _A, the specific surface area of S _B and S _C may be a BET specific surface area, respectively nitrogen gas was measured as an adsorption gas. With such a measuring method, the specific surface area can be easily measured. Specifically, the BET specific surface area can be measured according to JIS Z8830: 2013 (ISO 9277: 2010) (method for measuring the specific surface area of powder (solid) by gas adsorption). The BET specific surface area can be analyzed by a multipoint method by measuring nitrogen gas as an adsorbed gas by a static capacity method. Furthermore, the specific surface area S _C of the silicon-based negative active material coated on the specific surface area S _B and the binder of the binder is scraped off the one coated and heat-treated glass plate was recovered, it is used as a powdery sample it can.

  As shown in FIG. 1, the negative electrode 10 for a lithium ion secondary battery of this embodiment includes a negative electrode current collector 11 and a negative electrode active material layer 12 disposed on at least one surface of the negative electrode current collector 11. be able to.

(Negative electrode current collector 11)
The negative electrode current collector 11 is connected to a later-described negative electrode tab 65 and the like, and exchanges electrons with the outside of the lithium ion secondary battery 100. Although the material which forms the negative electrode collector 11 is not specifically limited, For example, metals, such as copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), are preferable. Among these, it is preferable to use copper (Cu) as a material for forming the negative electrode current collector 11. The thickness of the negative electrode current collector 11 is not particularly limited, but is usually about 1 μm to 100 μm.

(Negative electrode active material layer 12)
The negative electrode active material layer 12 can contain the silicon-based negative electrode active material and the negative electrode binder described above. 60 mass%-90 mass% are preferable, and, as for content of the silicon type negative electrode active material in the negative electrode active material layer 12, 70 mass%-85 mass% are more preferable. Moreover, 6 mass%-30 mass% are preferable, and, as for content of the binder for negative electrodes in the negative electrode active material layer 12, 10 mass%-25 mass% are more preferable. By setting it as the above ranges, the coverage of the silicon-type negative electrode active material by a binder can be made into a more preferable range.

  The film thickness of the negative electrode active material layer 12 is not particularly limited, but is preferably 20 μm to 80 μm, and more preferably 20 μm to 50 μm. By setting the film thickness of the negative electrode active material layer in such a range, the energy density and cycle characteristics of the lithium ion secondary battery 100 can be improved.

(Conductive aid for negative electrode)
The negative electrode active material layer 12 may further include a conductive additive for negative electrode. The conductive additive for negative electrode can effectively form an electronic network inside the negative electrode active material layer. Examples of the material for forming the negative electrode conductive additive include carbon blacks such as acetylene black, and carbon materials such as graphite and carbon fibers. These negative electrode conductive assistants may be used alone or in combination of two or more. Although content of the conductive support agent for negative electrodes in the negative electrode active material layer 12 is not specifically limited, 1 mass%-10 mass% are preferable, and 2 mass%-8 mass% are more preferable. By making content of the electroconductive auxiliary agent for negative electrodes into such a range, the electroconductivity of the negative electrode active material layer 12 can be improved.

As described above, the negative electrode for a lithium ion secondary battery of the present embodiment includes the silicon-based negative electrode active material and the binder that covers the silicon-based negative electrode active material. Furthermore, the negative electrode for a lithium ion secondary battery of this embodiment has a specific surface area of the silicon-based negative electrode active material S _A , a specific surface area of the binder S _B , and a specific surface area of the silicon-based negative electrode active material coated with the binder S _{When C} is satisfied, the relationship of 0.5 ≦ (S _A −S _C ) / (S _A −S _B ) is satisfied. Therefore, according to the negative electrode for a lithium ion secondary battery of the present embodiment, when used in a lithium ion secondary battery, the energy density is high and cycle characteristics can be improved.

[Lithium ion secondary battery]
The lithium ion secondary battery of this embodiment is equipped with the said negative electrode for lithium ion secondary batteries. Therefore, as described above, the lithium ion secondary battery of this embodiment has a high energy density and excellent cycle characteristics.

  As shown in FIG. 1, the lithium ion secondary battery 100 of this embodiment includes a positive electrode 20, a separator 30, a positive electrode tab 60, a negative electrode tab 65, an outer package 70, etc. in addition to the above-described negative electrode 10 for a lithium ion secondary battery. Can further be provided. In the embodiment of FIG. 1, the separator 30 is disposed between the positive electrode 20 and the negative electrode 10. In addition, as shown in FIG. 1, a plurality of unit cell layers 40 each including the positive electrode 20, the negative electrode 10, and the separator 30 may be stacked and electrically arranged in parallel to be the power generation element 50. Further, the lithium ion secondary battery 100 of the present embodiment is not limited to the form as shown in FIG. 1. For example, a positive electrode active material layer is disposed on one surface of the current collector, and the other current collector is disposed. A bipolar battery including a bipolar electrode in which a negative electrode active material layer is disposed on the surface may be used. Further, the present invention is not limited to the stacked lithium ion secondary battery as in the embodiment of FIG. 1, and may be a wound lithium ion secondary battery.

(Positive electrode 20)
The positive electrode 20 can include a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 can be disposed on at least one surface of the positive electrode current collector 21.

(Positive electrode current collector 21)
The positive electrode current collector 21 is connected to a positive electrode tab 60 and the like which will be described later, and exchanges electrons with the outside of the lithium ion secondary battery 100. Although the material which forms the positive electrode collector 21 is not specifically limited, Metals, such as aluminum, nickel, iron, titanium, and these alloys, are mentioned. As a material for forming the positive electrode current collector 21, the above-described single metal, an alloy in which the above-described metals are combined, a plating material in which the above-described metal combination is used, or the like can be used. Especially, it is preferable that the material which forms the positive electrode collector 21 contains aluminum from a viewpoint of electronic conductivity or battery operating potential. The thickness of the positive electrode current collector 21 is not particularly limited, but is usually about 1 μm to 100 μm.

(Positive electrode active material layer 22)
The positive electrode active material layer 22 contains, for example, a positive electrode active material, a positive electrode binder, a positive electrode conductive additive, and the like. The film thickness of the positive electrode active material layer 22 is not particularly limited, but is preferably 20 μm to 80 μm, and more preferably 20 μm to 50 μm.

(Positive electrode active material)
The positive electrode active material can participate in a reaction that generates an electric current. The content of the positive electrode active material in the positive electrode active material layer 22 is not particularly limited, but is preferably 80% by mass to 98% by mass.

Examples of the positive electrode active material include lithium-transition metal composite oxides, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. Examples of the lithium-transition metal composite oxide include LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni—Mn—Co) O ₂ , Li (Li—Ni—Mn—Co) O _2, and transitions thereof. An example in which a part of the metal is substituted with another element can be given. Examples of the lithium-transition metal phosphate compound include LiFePO ₄ . Examples of the lithium-transition metal sulfate compound include Li _x Fe ₂ (SO ₄ ) ₃ .

(Binder for positive electrode)
The positive electrode binder can bind the positive electrode active materials to each other or the positive electrode active material and the positive electrode conductive additive. Examples of the material for forming the positive electrode binder include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyether nitrile (PEN), poly Acrylonitrile (PAN), polyimide (PI), polyamide (PA), polyamideimide (PAI), carboxymethylcellulose (CMC), ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF) ), Polytetrafluoroethylene (PTFE), thermoplastic resin such as polyvinyl fluoride (PVF), thermosetting resin such as epoxy resin, styrene butadiene rubber (SBR), isoprene rubber (IR), butadiene rubber (B ) Include elastomers such as. These positive electrode binders may be used alone or in combination of two or more. Among these, at least one selected from the group consisting of polyimide (PI), polyamide (PA), and polyamideimide (PAI) is preferable because of excellent adhesion and heat resistance as a binder. Although content of the binder for positive electrodes contained in the positive electrode active material layer 22 is not specifically limited, 0.5 mass%-15 mass% are preferable, and 1 mass%-10 mass% are more preferable.

(Conductive aid for positive electrode)
The conductive additive for positive electrode can effectively form an electronic network inside the positive electrode active material layer 22 and increase the discharge capacity of the lithium ion secondary battery 100. Although content of the electroconductive support agent for positive electrodes contained in the positive electrode active material layer 22 is not specifically limited, 1 mass%-10 mass% are preferable, and 2 mass%-6 mass% are more preferable. By making content of the electroconductive auxiliary agent for positive electrodes into such a range, the electroconductivity of the positive electrode active material layer 22 can be improved.

(Separator 30)
The separator 30 can be disposed between the positive electrode 20 and the negative electrode 10. The separator 30 isolates the positive electrode 20 and the negative electrode 10 and mediates the movement of lithium ions. The thickness of the separator 30 is preferably 1 μm to 100 μm, and more preferably 5 μm to 50 μm, from the viewpoint of reducing internal resistance. The separator 30 can include a non-aqueous electrolyte. As the non-aqueous electrolyte, a gel or solid polymer electrolyte in which a lithium salt is dissolved in an ion conductive polymer and a liquid electrolyte in which a lithium salt is dissolved in an organic solvent can be held in a porous substrate layer and used.

  Examples of the ion conductive polymer used in the polymer electrolyte include polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVDF), hexafluoropropylene, polyethylene glycol (PEG), polyacrylonitrile (PAN), and polymethyl. Examples include methacrylate (PMMA) and copolymers thereof.

  Examples of the organic solvent used in the liquid electrolyte include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl. And carbonates such as carbonate (EMC) and methylpropyl carbonate (MPC).

The lithium _{salt, Li (CF 3 SO 2)} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiAsF 6, LiTaF 6, LiClO 4, compounds such as LiCF ₃ SO ₃ is Can be mentioned.

  The material for forming the porous substrate layer is not particularly limited, but it is preferable to use a thermoplastic resin such as polyethylene, polypropylene, ethylene-propylene copolymer. The porosity of the porous substrate layer is not particularly limited, but is preferably 40% to 85%. When the porosity is 40% or more, sufficient ionic conductivity can be obtained. On the other hand, when the porosity is 85% or less, the strength of the porous substrate layer can be maintained well.

(Positive electrode tab 60 and negative electrode tab 65)
The positive electrode tab 60 can electrically connect the positive electrode current collector 21 and an external device of the lithium ion secondary battery 100. The negative electrode tab 65 can electrically connect the negative electrode current collector 11 and an external device of the lithium ion secondary battery 100. The material which forms the positive electrode tab 60 and the negative electrode tab 65 is not specifically limited, For example, the at least 1 metal selected from the group which consists of aluminum, copper, titanium, nickel can be used. In addition, the material which forms the positive electrode tab 60 and the negative electrode tab 65 may be the same, or may differ.

(Exterior body 70)
The exterior body 70 can accommodate the unit cell layer 40 or the power generation element 50. Examples of the exterior body 70 include cans and those formed of a film. Moreover, the shape of the exterior body 70 is not specifically limited, It can be set as a cylindrical shape, a square shape, and a sheet | seat type. Although not particularly limited, it is preferable that the exterior body 70 is formed of a film from the viewpoint of reduction in size and weight. Among these, from the viewpoint of high output and cooling performance, the film is preferably a laminate film, and the laminate film preferably contains aluminum. Moreover, it is preferable that the lithium ion secondary battery 100 is a flat laminated lithium ion secondary battery. Such a lithium ion secondary battery can be improved in discharge capacity and heat dissipation performance, and is optimal when mounted on a vehicle. As an example of the laminate film containing aluminum, there is a three-layer laminate film of PP / aluminum / nylon.

  Although the use of the lithium ion secondary battery 100 of this embodiment is not specifically limited, as above-mentioned, an energy density is high and it is excellent in cycling characteristics. Therefore, it can be suitably used for vehicles. Specifically, the lithium ion secondary battery 100 of the present embodiment can be suitably used for a drive power source for vehicles.

[Method for producing negative electrode for lithium ion secondary battery]
The method for producing a negative electrode for a lithium ion secondary battery includes, for example, preparing a positive electrode slurry containing a negative electrode active material, applying the negative electrode slurry onto the negative electrode current collector 11, drying and pressing to form the negative electrode active material layer 12. Can be produced. The negative electrode slurry may contain a solvent in addition to the negative electrode active material, the negative electrode binder, and the negative electrode conductive additive described above.

  The solvent for the negative electrode slurry is not particularly limited, and N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, hexane, cyclohexane, water and the like can be used.

  The method for applying the negative electrode slurry to the current collector is not particularly limited, and a known method such as a spray coating method, a roll coating method, a doctor blade method, a flow coating method, a dip coating method, a screen printing method, or an ink jet method is used. be able to.

  The method for drying the negative electrode slurry is not particularly limited, and may be appropriately prepared according to the characteristics of the negative electrode slurry to be used. Moreover, the said press process is not specifically limited, For example, a calender roll, a flat plate press, etc. can be used.

  The method for increasing the coverage of the silicon-based negative electrode active material with the negative electrode binder is not particularly limited, but for example, the coverage can be improved by increasing the content of the negative electrode binder with respect to the silicon-based negative electrode active material.

[Method for producing lithium ion secondary battery]
The manufacturing method of the lithium ion secondary battery which concerns on this embodiment should just be equipped with the manufacturing method of the negative electrode for lithium ion secondary batteries mentioned above. The manufacturing method of the lithium ion secondary battery of this embodiment may include, for example, a positive electrode manufacturing process and an assembling process.

(Production process of positive electrode)
The positive electrode 20 is produced by, for example, preparing a positive electrode slurry containing a positive electrode active material, applying the positive electrode slurry onto the positive electrode current collector 21, drying, and pressing to form the positive electrode active material layer 22. Can do. The positive electrode slurry can include the positive electrode active material, the positive electrode binder, the positive electrode conductive additive and the solvent described above.

  A method for forming the positive electrode active material layer 22 by applying, drying, and pressing the positive electrode slurry onto the positive electrode current collector is not particularly limited, and the positive electrode slurry can be manufactured in the same manner as the negative electrode.

(Assembly process)
By laminating the positive electrode 20 and the negative electrode 10 produced as described above via the separator 30, the single cell layer 40 can be produced. Moreover, you may produce the electric power generation element 50 by laminating | stacking the single cell layer 40 two or more as needed. By sealing the single battery layer 40 or the power generation element 50 thus obtained in the exterior body 70, a lithium ion secondary battery can be produced.

  Hereinafter, the present embodiment will be described in more detail with reference to Examples and Comparative Examples, but the present embodiment is not limited to these.

[Example 1]
(Preparation of negative electrode)
First, the metal powder was alloyed and pulverized by a mechanical alloy method using a planetary ball mill (P-6, manufactured by Frichiu, Germany). Specifically, a metal powder prepared so as to have a mass ratio of Si: Sn: Ti = 66: 5: 29 and zirconia pulverized balls were put into a zirconia container. Then, the base which fixes the container made from zirconia was rotated at 600 rpm for 12.5 hours, and metal powder was alloyed. Then, the base was rotated at 200 rpm for 2 hours, and the alloy was pulverized. In addition, the average particle diameter of the silicon diameter negative electrode active material measured by the laser diffraction / scattering method was 2 μm.

  80% by mass of the negative electrode active material thus obtained, 5% by mass of a conductive auxiliary for negative electrode, and 15% by mass (solid content) of a binder precursor for negative electrode are dispersed in N-methylpyrrolidone and defoamed and kneaded. A negative electrode slurry was obtained by mixing in a machine (AR-100, manufactured by Thinky Co., Ltd.). In addition, acetylene black was used for the conductive support agent for negative electrodes, and polyamic acid was used for the binder precursor for negative electrodes.

  Next, the negative electrode slurry was uniformly applied on one surface of the negative electrode current collector so that the dried negative electrode active material layer had a thickness of 30 μm, and dried in vacuum for 24 hours. Thereafter, similarly, the negative electrode slurry was uniformly applied to the other surface of the negative electrode current collector so that the thickness of the dried negative electrode active material layer was 30 μm. And it was made to dry in vacuum for 24 hours, and also the negative electrode was obtained by performing drying baking at 300 degreeC in vacuum for 1 hour. The negative electrode current collector was a 12 μm thick copper foil.

(Production of battery)
Using the positive electrode of metallic lithium and the negative electrode obtained as described above, a stacked lithium ion secondary battery was produced. Specifically, a separator was disposed between the positive electrode and the negative electrode, and the positive electrode and the negative electrode were alternately laminated to produce a power generation element. The separator used was a polyolefin having a thickness of 40 μm. In this laminated body, two positive electrodes, three negative electrodes, and four separators are laminated.

A positive electrode tab and a negative electrode tab were welded to the obtained power generation element, respectively, and an electrolyte solution was injected into the exterior made of a laminate film containing aluminum with a syringe, followed by vacuum sealing to obtain a lithium ion secondary battery. Incidentally, the electrolyte concentration is such that 1 mol / L, was used by dissolving lithium hexafluorophosphate (LiPF ₆₎ in an organic solvent. As the organic solvent, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of EC: DEC = 3: 7 (volume ratio) was used.

[Example 2]
A lithium ion battery was prepared in the same manner as in Example 1 except that the composition of the negative electrode active material layer was 87.5% by mass of the negative electrode active material, 5% by mass of the conductive additive for negative electrode, and 7.5% by mass of the binder for negative electrode. Was made.

[Comparative Example 1]
A lithium ion battery was produced in the same manner as in Example 1 except that the composition of the negative electrode active material layer was 91% by mass of the negative electrode active material, 4% by mass of the conductive additive for negative electrode, and 5% by mass of the binder for negative electrode.

[Comparative Example 2]
A lithium ion battery was produced in the same manner as in Example 1 except that the composition of the negative electrode active material layer was 95% by mass of the negative electrode active material, 4% by mass of the conductive additive for negative electrode, and 1% by mass of the binder for negative electrode.

[Evaluation]
(Specific surface area S _A of silicon-based negative electrode active material)
The specific surface area S _A of the silicon-based negative active material and the BET specific surface are used as the silicon-based negative active material obtained as described above. The BET specific surface area was measured according to JIS Z8830: 2013. Specifically, the BET specific surface area was measured by a multipoint method by measuring nitrogen gas as an adsorbed gas by a static capacity method.

(Binder specific surface area S _B )
The polyamic acid was uniformly applied to a glass plate so that the thickness of the negative electrode active material layer after drying was 30 μm. Then, it was dried in vacuum for 24 hours, and further dried and fired at 300 ° C. in vacuum for 1 hour. Then, the binder formed by the heat treatment was scraped off from the glass plate and recovered in a powder form, and the BET specific surface area was measured. The BET specific surface area was measured according to JIS Z8830 as described above. Incidentally, S _B, if the material and heat treatment temperature for forming the binder are the same, a substantially same value.

(Specific surface area S _C of silicon-based negative electrode active material coated with binder)
First, an electrode slurry prepared as in the example was prepared. Next, the electrode slurry was uniformly applied to a glass plate so that the thickness of the negative electrode active material layer after drying was 30 μm. Then, it was dried in vacuum for 24 hours, and further dried and fired at 300 ° C. in vacuum for 1 hour. And the silicon type negative electrode active material coat | covered with the binder formed by heat processing was scraped off from the glass plate, and it collect | recovered in powder form, and measured the BET specific surface area. The BET specific surface area was measured according to JIS Z8830 as described above. Incidentally, a silicon-based negative active material used for the measurement of S _A, the average particle size of the silicon-based negative active material coated binder used for the measurement of S _C is substantially the same.

(Tensile modulus)
The tensile modulus of the negative electrode binder was measured according to JIS K7161-1: 2014 (ISO 527-1: 2012). The measurement was performed at a test temperature of 23 ± 2 ° C. and a test speed of 1 mm / min.

(Discharge capacity maintenance rate)
The discharge capacity retention rate was measured as follows. First, at room temperature (25 ° C.), after charging at a constant current of 0.1 C until the minimum voltage reaches 0.01 V, a charge / discharge cycle is performed at a constant current of 1.0 C until the maximum voltage reaches 2.0 V. One cycle was performed. Thereafter, 99 cycles were further charged and discharged in the same manner as in the first cycle except that the discharge rate was 0.3C. Then, in the first cycle and the 100th cycle, the discharge capacity when discharging from 2.0 V to 0.01 V is measured, and the ratio of the discharge capacity of the first cycle to the discharge capacity of the 100th cycle is defined as the discharge capacity maintenance rate. did.

  Table 1 and FIG. 2 show the evaluation results of the lithium ion batteries in Examples and Comparative Examples produced as described above.

As shown in Table 1 and FIG. 2, in the negative electrodes of Example 1 and Example 2, the value of (S _A −S _C ) / (S _A −S _B ) was 0.5 or more, so that the discharge capacity was maintained. The ratios were 98.2% and 92.3%, respectively, indicating excellent cycle characteristics. On the other hand, in the negative electrodes of Comparative Example 1 and Comparative Example 2, since the value of (S _A -S _C ) / (S _A -S _B ) is less than 0.5, the discharge capacity retention rate is 13.2% and It was 10%, and sufficient cycle characteristics were not obtained as compared with Example 1 and Example 2. That is, in the negative electrodes of Example 1 and Example 2, since the binder coverage with respect to the silicon-based negative electrode active material is a predetermined value or more, it is considered that a negative electrode having excellent cycle characteristics was obtained.

  Although the present invention has been described with reference to the examples and comparative examples, the present invention is not limited to these, and various modifications can be made within the scope of the gist of the present invention.

10 Lithium ion secondary battery negative electrode 100 Lithium ion secondary battery

Claims (4)

A silicon-based negative electrode active material;
A binder covering the silicon-based negative electrode active material;
With
The silicon-based negative active specific surface area S _A of the _material, the specific surface area S _B of the _binder, the specific surface area of the silicon-based negative active material coated on the binder when the S _C, 0.5 ≦ ( _A negative electrode for a lithium ion secondary battery satisfying the relationship of S _A -S _C ) / (S _A -S _B ). Wherein S _A, S _B and the specific surface area of S _C is a negative electrode for a lithium ion secondary battery according to claim 1 is a BET specific surface area was respectively measured nitrogen gas as an adsorption gas.   The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the binder has a tensile elastic modulus of 2 GPa or more and 10 GPa or less.   A lithium ion secondary battery provided with the negative electrode for lithium ion secondary batteries of any one of Claims 1-3.

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