JP2011134691A - Electrode for nonaqueous electrolyte secondary battery, method for manufacturing the same, and non-aqueous secondary battery - Google Patents

Abstract

The present invention provides a method for producing an electrode for a lithium ion secondary battery that is excellent in electrolyte permeability and retention and has good charge rate characteristics at low temperatures.
The method for producing an electrode for a non-aqueous electrolyte secondary battery according to the present invention includes (1) a step of preparing a slurry containing an active material and a binder, and (2) attaching the slurry to the surface of a current collector. After drying, forming the mixture layer, (3) compressing the mixture layer, and (4) heating the mixture layer after step (3), and thermally decomposing part of the binder And forming pores in the mixture layer.
[Selection figure] None

Description

  The present invention mainly relates to an improvement in a method for producing an electrode suitable for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery.

  In recent years, with the development of portable devices such as mobile phones and notebook computers, the demand for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries used for the power source is increasing. Since lithium ion secondary batteries have high energy density and high electromotive force, they are being developed not only as a power source for portable devices but also as secondary batteries for electric vehicles and hybrid vehicles.

The non-aqueous electrolyte secondary battery includes a pair of electrodes, a separator disposed between the pair of electrodes, and a non-aqueous electrolyte. The electrode has a current collector and a mixture layer formed on the surface of the current collector. The mixture layer includes an active material and a binder. The mixture layer is generally obtained by applying a slurry in which an active material and a binder are dispersed in a dispersion medium to a current collector to form a coating film, and then drying the coating film.
As the positive electrode active material, for example, a lithium-containing composite oxide such as Li _x CoO ₂ and Li _x NiO ₂ (where x varies depending on charge / discharge of the battery) is used. A carbon material such as non-graphitizable carbon is used for the negative electrode active material.

Non-aqueous electrolyte secondary batteries are required to improve charge / discharge characteristics (charge / discharge rate characteristics) at a high current density (high rate).
As the current density increases, the amount of lithium ions that move between the positive electrode and the negative electrode per unit time increases. However, in the negative electrode, when the charging current density increases, a part of the lithium ions may not react with the negative electrode active material and may be deposited on the negative electrode surface. Lithium deposited on the negative electrode surface does not participate in the battery reaction. For this reason, the negative electrode characteristics may deteriorate, and the safety of the battery may decrease. The deterioration of the negative electrode characteristics is likely to occur in a low temperature environment.
Therefore, in terms of improving the performance of the nonaqueous electrolyte secondary battery, it is very important to improve the charge rate characteristics of the negative electrode particularly in a low temperature environment.

  For example, Patent Document 1 proposes that the negative electrode mixture layer is composed of a plurality of layers in the negative electrode in which the negative electrode mixture layer is formed on the surface of the negative electrode current collector. Specifically, the first negative electrode mixture layer is disposed on the negative electrode current collector side, and the second negative electrode mixture layer having higher charge rate characteristics than the first negative electrode mixture layer is disposed on the positive electrode side. Has been proposed. Artificial graphite is used for the negative electrode active material of the first negative electrode mixture layer, natural graphite is used for the negative electrode active material of the second negative electrode mixture layer, and (the coating amount of the first negative electrode mixture layer) / ( It is described that the coating amount of the second negative electrode mixture layer is 10/90 to 70/30.

  In Patent Document 2, for the purpose of improving the retention of the nonaqueous electrolyte of the negative electrode, in the negative electrode mixture layer formed on the surface of the negative electrode current collector, the porosity on the positive electrode side is set to the porosity on the current collector side. It is described that it is higher than the degree.

JP 2009-64574 A JP 2006-196457 A

  However, in patent document 1, in order to form a some negative mix layer, it is necessary to use two or more types of negative electrode slurries, and the manufacturing process of an electrode becomes complicated.

In the method of Patent Document 2, the porosity in the thickness direction of the mixture layer is changed using a substance for forming pores (hereinafter referred to as a pore-forming substance). A first slurry containing an active material is applied to a current collector to form a first mixture layer. Then, the 2nd slurry containing an active material and a pore formation material is applied to the 1st mixture layer, and the 2nd mixture layer is formed.
Thus, since it is necessary to form two mixture layers using two types of slurries, the electrode manufacturing process becomes complicated. Further, since a pore forming material is used, it is disadvantageous in terms of cost. Since the first mixture layer does not contain a pore-forming substance, the porosity is low and it is insufficient for improving the charge / discharge rate characteristics of the entire electrode.

  Then, this invention aims at providing the nonaqueous electrolyte secondary battery excellent in the charge / discharge rate characteristic with a simple manufacturing process.

One aspect of the method for producing an electrode for a non-aqueous electrolyte secondary battery of the present invention is:
(1) adding a dispersion medium to the active material and the binder to prepare a slurry;
(2) attaching the slurry to the surface of the current collector and drying to form a mixture layer;
(3) compressing the mixture layer;
(4) After the step (3), heating the mixture layer to thermally decompose a part of the binder to form pores in the mixture layer.

The ratio of the dispersion medium in the slurry is preferably 50 to 70% by weight.
The heating temperature in the step (4) is preferably 200 to 350 ° C.
The binder in the step (1) includes a first binder and a second binder,
The heating temperature in the step (4) is preferably higher than the thermal decomposition temperature of the first binder and lower than the thermal decomposition temperature of the second binder.

The first binder is preferably at least one selected from the group consisting of carboxymethyl cellulose, polyvinyl butyral, polyvinyl alcohol, polyethylene oxide, and polyacrylate.
The second binder is preferably at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, and styrene butadiene rubber.

Moreover, one aspect of the electrode for a nonaqueous electrolyte secondary battery of the present invention is an electrode obtained by the above production method.
That is, one aspect of the electrode for a nonaqueous electrolyte secondary battery of the present invention includes a current collector and a porous mixture layer formed on the surface of the current collector. The porosity increases continuously from the electrical side to the surface side.

  The mixture layer includes an inner layer composed of a region from the surface of the current collector to a predetermined depth, a surface layer composed of a region from the surface of the mixture layer to a predetermined depth, and the porosity A of the inner layer is 25 to 25. 40%, the porosity B of the surface layer is 40 to 60%, and the ratio of the porosity A of the inner layer to the porosity B of the surface layer: B / A is 1.05 or more and 2.40. It is preferable that:

  One aspect of the nonaqueous electrolyte secondary battery of the present invention is a nonaqueous electrolyte secondary battery comprising a pair of electrodes, a separator disposed between the pair of electrodes, and a nonaqueous electrolyte, wherein the pair of electrodes At least one of the electrodes is the electrode described above.

According to the present invention, it is possible to easily produce an electrode in which only one type of slurry is applied to the current collector, and the porosity on the surface side is larger than the porosity on the current collector side in the mixture layer. In the present invention, since it is not necessary to use a plurality of slurries, it is advantageous in terms of manufacturing cost, and the manufacturing process can be simplified. Therefore, the load of process management is reduced and the reliability for manufacturing is improved.
In the present invention, an electrode excellent in electrolyte permeability and retention can be provided at low cost. The charge / discharge rate characteristics of the electrode are improved, and in particular, the charge / discharge rate characteristics in a low temperature environment are improved.
By using the electrode of the present invention, a high-capacity nonaqueous electrolyte secondary battery excellent in charge / discharge rate characteristics in a low temperature environment can be obtained.

It is a schematic longitudinal cross-sectional view of the electrode for lithium ion secondary batteries of one Embodiment of this invention. It is a schematic longitudinal cross-sectional view of the lithium ion secondary battery of one Embodiment of this invention. It is a SEM photograph which shows the cross section of the precursor of No. 21 positive electrode in Example 3 of this invention. It is a figure which shows the mapping of the fluorine element by EPMA which shows the segregation state of PVDF of the precursor of the positive electrode of FIG.

One aspect of the method for producing an electrode for a non-aqueous electrolyte secondary battery of the present invention is:
(1) preparing a slurry containing an active material and a binder;
(2) attaching the slurry to the surface of the current collector and drying to form a mixture layer;
(3) compressing the mixture layer;
(4) After the step (3), heating the mixture layer to thermally decompose a part of the binder to form pores in the mixture layer.
Since the nonaqueous electrolyte can be held in the pores formed in the mixture layer, the charge / discharge rate characteristics of the electrode can be improved.

  In the step (2), due to the migration of the binder, the amount of the binder contained in the mixture layer can be continuously increased from the current collector side toward the surface side. Therefore, in the step (4), a part of the binder is thermally decomposed, and the ratio of the pores in the mixture layer, that is, the porosity of the mixture layer is changed from the current collector side to the surface side. It can be increased continuously as it goes. That is, in the step (4), a surface layer having a higher porosity than the current collector side can be formed on the surface side of the mixture layer.

By forming many pores near the surface of the electrode (mixture layer), the surface area of the mixture layer is increased, and the nonaqueous electrolyte can be smoothly permeated into the mixture layer. An electrode having a small charge transfer resistance of lithium ions can be obtained. That is, lithium ions easily move between the active material and the nonaqueous electrolyte. For this reason, the charge / discharge rate characteristics can be greatly improved not only in a room temperature environment but also in a low temperature environment.
Since an electrode can be produced using one type of slurry, the production process can be simplified, which is advantageous in terms of production cost.

  In step (1), a slurry is obtained by adding a dispersion medium such as water, N-methyl-2-pyrrolidone (hereinafter, NMP) to the active material and the binder.

In step (2), a slurry containing an active material and a binder is applied to a current collector to form a coating film. Thereafter, the coating film is dried (the dispersion medium is removed) to form a mixture layer, and an electrode precursor is obtained. At this time, a part of the binder existing near the current collector in the coating film is moved to the surface side of the coating film due to the migration phenomenon. Therefore, a large amount of binder can be present on the surface side of the mixture layer.
The binder migration phenomenon can be controlled by adjusting the drying speed of the coating film. The slower the drying speed, that is, the longer the time during which the state of low viscosity continues in the initial stage of drying is more likely to cause migration.

  In order to easily cause the migration phenomenon, it is preferable to prepare a slurry in which the ratio of the dispersion medium (hereinafter, ratio A) is 50 to 70% by weight in the step (1). When the ratio A is more than 70% by weight, the amount of the dispersion medium in the slurry becomes excessively large, and it may be difficult to stably apply the slurry to the current collector. When the ratio A is less than 50% by weight, the ratio of the binder in the slurry becomes excessively large, the binder becomes difficult to move during drying, and migration may be insufficient.

  The drying temperature is preferably 20 to 80 ° C, and more preferably 20 to 60 ° C. The drying time is preferably 0.5 to 2 hours, and more preferably 0.5 to 1 hour. Drying may be performed in the air or in a non-oxidizing atmosphere (for example, an inert gas atmosphere such as argon gas).

The heating in the step (4) is preferably performed in a non-oxidizing atmosphere (for example, an inert gas atmosphere such as argon gas) for a long time (for example, 5 to 10 hours).
When the heating time is excessively short (for example, less than 2 hours), the penetration rate of the nonaqueous electrolyte into the mixture layer may not be sufficiently improved. Moreover, although a detailed reason is unknown, when a mixture layer contains a nonaqueous electrolyte, a mixture layer will swell excessively, impedance will become high, and a charge / discharge rate characteristic and charge / discharge cycle characteristic will fall. There is.

  It is preferable that the heating temperature of a process (4) is 200-350 degreeC. When the heating temperature is less than 200 ° C., the heating time becomes long and the productivity may be lowered. When the heating temperature is higher than 350 ° C., there may be a problem that the adhesion strength between the mixture layer and the current collector is lowered.

As the binder, a material that is dispersed or dissolved in a dispersion medium in a slurry is preferably used.
The binder is a hydrophilic binder such as a cellulose derivative that is stable with respect to the nonaqueous electrolyte and can realize good cycle characteristics, and a parent electrolyte binder having a polyether structure excellent in lithium ion conductivity. Seeds can also be used in combination. Thereby, the area | region (surface layer) excellent in the binding property between active material particles and having high lithium ion conductivity can be formed.

As the hydrophilic binder, a cellulose derivative that does not dissolve in the non-aqueous electrolyte is used. Examples of the cellulose derivative include carboxymethyl cellulose (CMC), methyl cellulose (MC), and cellulose acetate phthalate (CAP). Among these, CMC is preferable.
In addition, a hydrophilic binder obtained by hydrophilizing a hydrophobic binder may be used. Examples of the hydrophobic binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and styrene butadiene rubber (SBR).

  Examples of the electrophilic binder include polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyacrylate (PAA), and polyethylene oxide (PEO). Among these, PVB is preferable.

  Furthermore, in order to impart flexibility to the electrode, a fluororesin that does not easily react with the nonaqueous electrolyte may be used in combination. Examples of the fluororesin include PVDF, PTFE, FEP, (tetrafluoroethylene-hexafluoropropylene copolymer), and PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer).

  The binder in step (1) includes a first binder and a second binder having different thermal decomposition temperatures, and the heating temperature in step (4) is higher than the thermal decomposition temperature of the first binder, and the thermal decomposition temperature of the second binder. Is preferably lower. In the step (4), a part of the first binder is thermally decomposed, whereby a surface layer having a higher porosity than the current collector side can be formed on the surface side of the mixture layer. By doing so, at least the second binder remains in the mixture layer, so that good binding between the active material particles in the mixture layer and between the mixture layer and the current collector is ensured. can do.

As the first binder, for example, at least one of the above hydrophilic and electrophilic binders is used. The first binder is preferably at least one selected from the group consisting of CMC, PVB, PVA, PEO, and PAA.
For example, the hydrophobic binder is used as the second binder. The second binder is preferably at least one selected from the group consisting of PVDF, PTFE, and SBR.

The amount of the first binder added is preferably 0.2 to 4.0 parts by weight per 100 parts by weight of the active material.
The amount of the second binder added is preferably 0.5 to 1.0 part by weight per 100 parts by weight of the active material.
The total addition amount of the first binder and the second binder is preferably 0.7 to 5.0 parts by weight, more preferably 1.5 to 5 parts by weight.
Examples of the method of adding the first binder and the second binder to the slurry include a method of obtaining a slurry by dispersing or dissolving the first binder and the second binder in a solvent such as NMP together with the active material.

As the slurry, after obtaining a composite material in which the active material particles are coated with one of the first binder and the second binder, the composite material and the other of the first binder and the second binder are dispersed in a dispersion medium such as NMP. It is preferable to use the prepared slurry.
When a composite material in which the surface of the active material particles is previously coated with a binder is used, the slipping property between the active material particles contained in the slurry is improved. When compressing a mixture layer, a compressive force is uniformly applied with respect to the thickness direction of a mixture layer. For this reason, in the whole mixture layer, the filling property of the active material particles and the adhesion between the active material particles are enhanced, and the adhesive strength between the active material particles is improved. Therefore, the electronic conductivity between the active material particles is increased, and the charge / discharge cycle characteristics are improved.
Conventionally, it has been difficult to improve the charge / discharge rate characteristics without reducing the density of the mixture layer. However, in the present invention, the charge / discharge rate characteristics of the electrode are improved and the density of the mixture layer is increased. And both. For example, in the case of a negative electrode including graphite as an active material, a negative electrode including a negative electrode mixture layer having excellent charge rate characteristics in a low temperature environment and a high capacity of about 1.3 to 1.7 g / cm ^{3 of} graphite density Can be obtained.
From the viewpoint that it is easy to ensure contact with the nonaqueous electrolyte on the surface of the active material particles (migration path of lithium ions), the binder covering the active material particles is preferably the first binder.

  The presence of the binder covering the active material in the composite material can be confirmed by X-ray photoelectron spectroscopy (XPS). For example, when X-ray photoelectron spectroscopic analysis is performed on graphite, the surface atomic concentration ratio of C1S and O1S is 95 to 100 atomic% for C1S and 0 to 5 atomic% for O1S. In contrast, in graphite coated with a binder, C1S is 85 to 95 atomic% and O1S is 5 to 15 atomic%. This difference is due to the presence of a functional group such as a carboxyl group, a carboxylic acid group, an ester group or a hydroxyl group contained in the binder covering the graphite surface.

Hereinafter, an embodiment of an electrode for a non-aqueous electrolyte secondary battery according to the present invention will be described with reference to FIG.
The electrode of the present invention is an electrode obtained by the production method of the present invention. As shown in FIG. 1, the electrode 40 for a non-aqueous electrolyte secondary battery of the present invention has a layered current collector 41 and a porous mixture layer 42 formed on the surface of the current collector 41. The mixture layer 42 continuously increases in porosity as it goes from the current collector side to the surface side. The mixture layer 42 is formed by using one kind of slurry (active material), and there is no interface as in the case where it is composed of a plurality of layers having different porosities and active materials. Therefore, there is no problem that the adhesiveness decreases along this interface during charging and discharging.

The mixture layer 42 has an inner layer 42a formed on the current collector 41 side, and a surface layer 42b having a higher porosity than the inner layer 42a. This improves the permeability of the nonaqueous electrolyte from the electrode surface and the nonaqueous electrolyte retention of the electrode. An electrode having low charge transfer resistance of lithium ions and excellent rate characteristics can be obtained.
Here, the inner layer 42a is a predetermined depth P1 from the surface of the current collector 41 in the mixture layer 42 (for example, a depth corresponding to 1/5 to 1/4 of the thickness of the mixture layer). The region A is shown.
The surface layer 42b is a region B from the surface of the mixture layer 42 to a predetermined depth P2 (for example, a depth corresponding to 1/5 to 1/4 of the thickness of the mixture layer) in the mixture layer 42. Point to. Regions A and B contain the same active material particles and the same binder, respectively.

The porosity of the inner layer and the surface layer is determined by the following method.
A cross-sectional image (SEM image) of the mixture layer in a region where about 400 active material particles can be observed is obtained.
At a predetermined depth P1 in the SEM image (for example, a depth corresponding to 1/5 to 1/4 of the thickness of the mixture layer), a line segment Q1 having a length X1 is drawn along the surface direction of the current collector, A total Y1 of the lengths of the portions across the holes in the line segment Q1 is obtained. Let Y1 / X1 × 100 be the porosity of the inner layer.
At a predetermined depth P2 in the SEM image (for example, a depth corresponding to 1/5 to 1/4 of the thickness of the mixture layer), a line segment Q2 having a length X2 is drawn along the surface direction of the current collector, A total Y2 of the lengths of the portions crossing the holes in the line segment Q2 is obtained. Let Y2 / X2 × 100 be the porosity of the surface layer.
The length X1 of the line segment Q1 and the length X2 of the line segment Q2 are, for example, a length (for example, 150 to 250 μm) that the active material particles cross about 10 to 20 pieces.

As for the thickness of the mixture layer 42, 20-60 micrometers is preferable.
From the viewpoint of electrode capacity, the porosity A of the inner layer 42a is preferably 25 to 40%.
From the viewpoint of rate characteristics, the porosity B of the surface layer 42b is preferably 40 to 60%.
From the viewpoint of rate characteristics and electrode capacity, the ratio of the porosity A of the inner layer 42a to the porosity B of the surface layer 42b: B / A is preferably 1.05 or more and 2.40 or less, more preferably 1.15. This is 2.40 or less.

  In the electrode 40, the binder content (total amount) in the mixture layer 42 is preferably 2.5 parts by weight or less per 100 parts by weight of the active material, and 0.5 parts by weight or more and 1.5 parts by weight per 100 parts by weight of the active material. The following is more preferable.

Hereinafter, an embodiment of a lithium ion secondary battery according to the present invention will be described with reference to FIG.
As shown in FIG. 2, the lithium ion secondary battery 1 includes a positive electrode 10 including a positive electrode active material, a negative electrode 11 including a negative electrode active material, a separator 13 disposed between the positive electrode 10 and the negative electrode 11, and a nonaqueous electrolyte. Is provided. For the negative electrode 11, the electrode 40 of the present invention is used. The negative electrode 11 includes a negative electrode current collector 22 and a negative electrode mixture layer 23 formed on one surface of the negative electrode current collector 22. The negative electrode mixture layer 23 has an inner layer 12a and a surface layer 12b. The inner layer 12a and the surface layer 12b are the inner layer 42a and the surface layer 42b, respectively.

The negative electrode mixture layer 23 includes at least a negative electrode active material capable of inserting and extracting lithium ions and a binder. Examples of the binder include the first binder and the second binder described above.
As the negative electrode active material, for example, lithium metal, lithium alloy, metal oxide, or carbon material is used. The average particle diameter (D50) of the negative electrode active material is, for example, 1 to 30 μm.
The lithium alloy preferably contains at least one selected from the group consisting of silicon, tin, aluminum, zinc, and magnesium. The proportion of elements other than lithium in the lithium alloy is preferably 3 to 50% by weight.
The metal oxide is preferably at least one of an oxide containing silicon and an oxide containing tin.

For the carbon material, for example, graphite or non-graphitizable carbon material is used. Examples of graphite include natural graphite and artificial graphite.
The average particle diameter (D50) of the carbon material is preferably 1.5 to 40 μm. When the average particle diameter (D50) of the carbon material is less than 1.5 μm, the specific surface area becomes extremely large and the decomposition of the electrolyte is promoted, so that the charge / discharge efficiency is lowered. Moreover, crystallinity falls. Therefore, the discharge capacity may be reduced. When the average particle diameter (D50) of the carbon material exceeds 40 μm, the electron conductivity is lowered and the resistance of the electrode is increased, so that the discharge characteristics may be lowered.
A metal foil is used for the negative electrode current collector 22. The negative electrode current collector 22 is preferably made of copper foil. The thickness of the negative electrode current collector 22 is preferably 5 to 30 μm.

  The positive electrode 10 includes a positive electrode current collector 20 and a positive electrode mixture layer 21 formed on the surface of the positive electrode current collector. The positive electrode mixture layer 21 includes at least a positive electrode active material capable of inserting and extracting lithium ions. The average particle diameter (D50) of the positive electrode active material is, for example, 5 to 20 μm. The positive electrode mixture layer 21 may include a conductive agent such as a binder and a carbon material as necessary.

For the positive electrode active material, for example, a lithium-containing metal oxide having a layered structure or a spinel structure is used. As the lithium-containing metal composite oxide, for example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and LiFePO ₄ are used. These may be used alone or in combination of two or more as a mixture or composite.
A metal foil is used for the positive electrode current collector. An aluminum foil is preferably used for the positive electrode current collector. The thickness of the positive electrode current collector is preferably 5 to 30 μm.

  The positive electrode 10, the negative electrode 11, and the separator 13 are accommodated in the exterior case 17. The exterior case 17 is filled with the electrolyte 2. A part of the electrolyte 2 is contained in the separator 13. The openings 17a and 17b of the outer case 17 are hermetically sealed by heat fusion through the resin material 16.

One end of the negative electrode lead 15 is connected to the negative electrode current collector 22. Thereby, the negative electrode lead 15 is electrically connected to the negative electrode 11. The other end of the negative electrode lead 15 is drawn out of the battery 1 from the opening 17 b of the outer case 17.
One end of the positive electrode lead 14 is connected to the positive electrode current collector 20. Thereby, the positive electrode lead 14 is electrically connected to the positive electrode 10. The other end of the positive electrode lead 14 is drawn out of the battery 1 from the opening 17 a of the outer case 17.

  The separator 13 is preferably an insulating microporous thin film having a predetermined ion permeability and mechanical strength. As the material for the microporous thin film, polyolefin such as polypropylene and polyethylene having excellent electrolyte resistance and hydrophobicity is preferable. The pore diameter of the separator 13 is, for example, not less than 0.01 μm and not more than 1 μm. The thickness of the separator 13 is, for example, 10 μm or more and 300 μm or less. The porosity of the separator 13 is, for example, 30% or more and 80% or less.

The nonaqueous electrolyte 2 includes, for example, a nonaqueous solvent and a lithium salt that dissolves in the nonaqueous solvent.
Non-aqueous solvents include, for example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); dimethyl carbonate (DMC), diethyl carbonate (DEC) ), Chain carbonates such as ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC); or chain chains such as 1,2-dimethoxyethane (DME) and 1,2-diethoxyethane (DEE) Ethers are used. These may be used alone or in combination of two or more. Among these, it is preferable to use a mixed solvent of a cyclic carbonate and a chain carbonate, or a mixed solvent of a cyclic carbonate, a chain carbonate, and an aliphatic carboxylic acid ester.

As the lithium salt, for example, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiCl, or a lithium imide salt is used. These may be used alone or in combination of two or more. Among these, it is particularly preferable to use LiPF ₆ . The concentration of the lithium salt in the electrolyte is not particularly limited, but the concentration of the lithium salt in the electrolyte is preferably 0.2 mol / L or more and 2 mol / L or less, and more preferably 0.5 mol / L or more and 1.5 mol / L or less.

Examples of the present invention will be described in detail below, but the present invention is not limited to these examples.
Example 1
As a negative electrode active material, 100 parts by weight of graphite powder (Mitsubishi Chemical Co., Ltd., average particle size 16 μm), first and second binders of the types and amounts shown in Table 1, and N-methyl-2- A slurry was obtained by adding 150 parts by weight of pyrrolidone (special reagent grade) (hereinafter, NMP) (step (1)).
The proportion of solid components (active material and binder) in the slurry was about 40% by weight. That is, the proportion of the dispersion medium in the slurry was about 60% by weight.

For the first binder, CMC (manufactured by Nippon Zeon Co., Ltd.), PVB (manufactured by Sekisui Chemical Co., Ltd.), PVA (manufactured by Kuraray Co., Ltd.), PEO, or PAA (manufactured by Toagosei Co., Ltd.) is used. It was.
As the second binder, PVDF (manufactured by Kureha Co., Ltd.), PTFE (manufactured by Shin-Etsu Chemical Co., Ltd.), or SBR (manufactured by Nippon Zeon Co., Ltd.) was used.

  Table 2 shows the thermal decomposition temperatures of various binders. The thermal decomposition temperatures of various binders in Table 2 were measured by DSC (differential scanning calorimetry). The thermal decomposition temperature referred to here is a temperature at which weight reduction starts when measured at a temperature elevation rate of 10 ° C./min under a nitrogen stream.

Using a doctor blade with a gap of 80 μm, these slurries were applied to the surface of a negative electrode current collector made of a copper foil having a thickness of 10 μm to form a coating film. This coating film was dried at 60 ° C. for 2 hours to form a negative electrode mixture layer (step (2)).
The negative electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (3)).
Thereafter, the negative electrode mixture layer was heated in an inert atmosphere at the temperature shown in Table 1 for 8 hours (step (4)).
In this way, a negative electrode mixture layer (density: 1.4 g / cm ³ , thickness: 50 μm) was formed on the surface of the negative electrode current collector, and No. 1 to 8 negative electrodes were obtained.

<< Comparative Example 1 >>
A slurry was obtained by adding 0.6 parts by weight of PVDF as a binder and 150 parts by weight of NMP as a dispersion medium to 100 parts by weight of graphite powder (manufactured by Mitsubishi Chemical Corporation, average particle size 16 μm) as a negative electrode active material (process) (1)).

Using a doctor blade with a gap of 80 μm, this slurry was applied to the surface of a negative electrode current collector made of copper foil to form a coating film. This coating film was dried at 80 ° C. for 2 hours to form a negative electrode mixture layer (step (2)).
The negative electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (4)).
In this way, a negative electrode mixture layer (density: 1.4 g / cm ³ , thickness: 50 μm) was formed on the surface of the negative electrode current collector to obtain a No. 9 negative electrode.

[Evaluation]
The following evaluation was performed about each obtained said negative electrode.
(1) Measurement of penetration rate of non-aqueous electrolyte Under a normal temperature (25 ° C.) environment, 0.02 ml of the non-aqueous electrolyte was dropped on the surface of the mixture layer with a micro syringe from above. At this time, the time until the non-aqueous electrolyte penetrates into the mixture layer and disappears from the mixture layer surface was measured. As the non-aqueous electrolyte, a non-aqueous electrolyte (manufactured by Mitsubishi Chemical Corporation) in which LiPF ₆ was dissolved in a mixed solvent of EC and EMC (volume ratio 1: 3) at a concentration of 1.0 mol / L was used.

(2) Measurement of porosity of inner layer and surface layer of mixture layer At depth P1 of 10 μm from the surface of the current collector in a cross-sectional image (SEM image) of the mixture layer in a region where about 400 active material particles can be observed Then, a line segment Q1 having a length X1 was drawn along the surface direction of the current collector, and the total length Y1 of the portions of the line segment Q1 that crossed the holes was obtained. Here, the length X1 of the line segment Q1 was set to 200 μm. And Y1 / X1 × 100 = Z1 was determined as the porosity of the inner layer.

A line segment having a length X2 along the surface direction of the current collector at a depth P2 of 10 μm from the surface of the mixture layer in the cross-sectional image (SEM image) of the mixture layer in a region where about 400 active material particles can be observed. Q2 was subtracted to determine the total length Y2 of the portion of the line segment Q2 that crosses the holes. Here, the length X2 of the line segment Q2 was set to 200 μm. And Y2 / X2 * 100 = Z2 was calculated | required as the porosity of a surface layer.
This operation was carried out 10 times, and the average value of the porosity Z1 of the inner layer and the porosity Z2 of the surface layer was determined.

(3) Evaluation of electrode characteristics a) Preparation of test cell The negative electrode obtained above was punched into a predetermined shape, a lead was attached, and a test cell was constructed using metallic lithium (thickness: 150 μm) as the counter electrode. As the electrolyte, a nonaqueous electrolyte in which LiPF ₆ was dissolved in a mixed solvent of EC and EMC (volume ratio 1: 3) at a concentration of 1.0 mol / L was used.

at a constant current of measurement 0.5 mA / cm ² of b) 0.5 mA / cm ² of the discharge capacity (capacity A), was occluded (charged) to 0.01V (vs.Li/Li ⁺⁾ anode in a lithium to Charged. Thereafter, the battery was discharged to 1.1 V (vs. Li / Li ⁺ ) at a constant current of 0.5 mA / cm ² , and the capacity A at this time was determined.

After discharge of c) 6mA / cm ² discharge capacity (Measurement of b the capacitive B)), was further charged to the final voltage of 0.01V with a constant current of 0.5 mA / cm ^2. Thereafter, the battery was discharged to 1.1 V (vs. Li / Li ⁺ ) at a constant current of 6 mA / cm ² , and the capacity B at this time was determined. And the discharge rate of the negative electrode was calculated | required from the following formula (A).
Discharge rate (%) = capacity B / capacity A × 100 (A)
The measurement of the capacity | capacitance A and the capacity | capacitance B was implemented in the environment of 25 degreeC and -10 degreeC.

(4) Evaluation results Table 3 shows the evaluation results.

As shown in Table 3, in the electrodes 1 to 8 of Example 1 of the present invention, the electrolyte permeation time was short, and excellent discharge rate characteristics were obtained.
On the other hand, in the electrode 9 of Comparative Example 1 in which only PVDF was used as the binder and heating was not performed, the electrolyte permeation time was as long as 108 seconds, PVDF was used as the second binder, and CMC was used as the second binder. Compared with the electrode 1 of Example 1 which implemented heating, the discharge rate characteristic fell.

Example 2
As a negative electrode active material, graphite powder (Mitsubishi Chemical Co., Ltd., average particle size 16 μm) 100 parts by weight, PVB or PEO of the type and amount shown in Table 4 as a first binder and NMP 150 parts by weight as a dispersion medium are added. Then, after stirring for 3 minutes with a thin-film shear type stirrer (manufactured by PRIMICS, FILMICS), the stirred product was dried at 80 ° C. for 6 hours. In this way, composite particles having the graphite particle surface coated with the first binder were obtained.
A second binder having the type and amount shown in Table 3 was added to the composite particles, and NMP was added as a dispersion medium in an amount of 150 parts by weight per 100 parts by weight of the graphite particles to obtain a slurry (step (1)).

Using a doctor blade with a gap of 80 μm, this slurry was applied to the surface of a negative electrode current collector made of copper foil to form a coating film. This coating film was dried at 80 ° C. for 2 hours to form a negative electrode mixture layer (step (2)).
The negative electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (3)).
Thereafter, the negative electrode mixture layer was heated in an inert atmosphere at a temperature shown in Table 3 for 8 hours (step (4)).
In this way, a negative electrode mixture layer (density: 1.4 g / cm ³ , thickness: 50 μm) was formed on the surface of the negative electrode current collector, and No. 10 to 18 negative electrodes were obtained.
Evaluation similar to the above was performed using this negative electrode. The evaluation results are shown in Table 5.

<< Comparative Example 2 >>
A slurry was obtained by adding 2 parts by weight of SBR as a second binder and 150 parts by weight of NMP as a dispersion medium to 100 parts by weight of graphite powder (manufactured by Mitsubishi Chemical Corporation, average particle size 16 μm) as a negative electrode active material (process) (1)).

Using a doctor blade with a gap of 80 μm, this slurry was applied to the surface of a negative electrode current collector made of copper foil to form a coating film. This coating film was dried at 80 ° C. for 2 hours to form a negative electrode mixture layer (step (2)).
The negative electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (4)).
In this way, a negative electrode mixture layer (density: 1.4 g / cm ³ , thickness: 50 μm) was formed on the surface of the negative electrode current collector to obtain a No. 19 negative electrode.
Evaluation similar to the above was performed using this negative electrode. The evaluation results are shown in Table 4.

In the negative electrodes of No. 10 to 18 that were heated, the discharge rate characteristics at low temperature were improved as compared with the negative electrode of No. 19 that was not heated.
In the negative electrodes No. 10 to 19, the electrolyte permeability and retention were improved by the pores formed by the thermal decomposition of the first binder in the negative electrode mixture layer.
Since the composite material in which the first binder was previously coated on the negative electrode active material particles was used, the slipperiness between the negative electrode active material particles contained in the slurry was improved. Therefore, the filling property of the negative electrode active material particles was improved when forming the negative electrode mixture layer. When the negative electrode mixture layer was compressed, a compressive force was uniformly applied in the thickness direction of the negative electrode mixture layer. Therefore, the density of the entire negative electrode mixture layer was increased, and the electronic conductivity between the negative electrode active material particles was improved.

Example 3
100 parts by weight of LiFePO ₄ powder (produced by Hanwha Chemical Co., Ltd., average particle size 12 μm) as a positive electrode active material, CMC (first binder) in the amount shown in Table 6, 5 parts by weight of acetylene black as a conductive agent, and 150 parts by weight of NMP was added as a dispersion medium, stirred for 3 minutes with a thin film shear type stirrer (manufactured by PRIMICS, FILMICS), and then dried at 80 ° C. for 6 hours. In this way, composite particles having the LiFePO ₄ particle surface coated with CMC were obtained.

To composite particles containing 100 parts by weight of the positive electrode active material, PVDF (manufactured by Shin-Etsu Chemical Co., Ltd.) (second binder) in an amount shown in Table 6 and 150 parts by weight of NMP as a dispersion medium were added to obtain a slurry.
Using a doctor blade with a gap of 80 μm, this slurry was applied to the surface of a positive electrode current collector made of an aluminum foil to form a coating film. This coating film was dried at 80 ° C. for 2 hours to form a positive electrode mixture layer (step (2)).

  The SEM photograph of the cross section of the No. 21 positive electrode precursor after the step (2) is shown in FIG. FIG. 4 shows the mapping state of the fluorine element indicating the dispersion state of the binder (PVDF) in the mixture layer. The positive electrode precursor 33 in FIGS. 3 and 4 includes a positive electrode current collector 31 and a positive electrode mixture layer 32. As shown in FIG. 4, after the step (2), the binder (PVDF) is segregated on the surface side of the positive electrode mixture layer 32 (on the side opposite to the positive electrode current collector 31). From this, it is inferred that the entire binder containing CMC and PVDF is segregated on the surface side of the positive electrode mixture layer 32.

The positive electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (3)).
Thereafter, the positive electrode mixture layer was heated in an inert atmosphere at 250 ° C. for 8 hours (step (4)).
In this way, a positive electrode mixture layer (density: 1.8 g / cm ³ , thickness: 45 μm) was formed on the surface of the positive electrode current collector to obtain Nos. 20 to 22 positive electrodes.
The battery of FIG. 2 was constructed using this positive electrode and the No. 9 negative electrode.
Specifically, the separator 13 was arrange | positioned between the positive electrode 10 and the negative electrode 11, and the electrode group was obtained. As the separator 13, a microporous membrane made of polyethylene (manufactured by Asahi Kasei Corporation) was used. One end of an aluminum positive electrode lead 14 was welded to the positive electrode 10. One end of a negative electrode lead 15 made of nickel was welded to the negative electrode 11. After the electrode group was housed in an outer case 17 made of an aluminum laminate film, a nonaqueous electrolyte was injected into the electrode group. As the non-aqueous electrolyte, a non-aqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1.0 mol / L in a mixed solvent of EC and EMC (volume ratio 1: 1) was used. The positive electrode lead 14 and the negative electrode lead 15 were led out from the openings 17 a and 17 b of the outer case 17, respectively, and the openings 17 a and 17 b of the outer case 17 were welded via the resin material 16.

For this positive electrode, the permeation rate of the electrolyte was measured in the same manner as described above.
Further, discharge rate characteristics were evaluated as battery characteristics by the following method.
[Evaluation of discharge rate characteristics]
The battery was charged at a constant current of 0.5 mA / cm ² until the battery voltage reached 4.2V. Thereafter, the battery was discharged at a constant current of 0.5 mA / cm ² until the battery voltage reached 2.5 V, and the discharge capacity A at this time was determined. Then, until the battery voltage reached 4.2 V, after charging at a constant current of 0.5 mA / cm ² until the battery voltage reached 2.5V, and discharged at a constant current of 6 mA / cm ^2, the time The discharge capacity B was determined. From these values, the discharge rate of the positive electrode was determined by the following formula (B).
Discharge rate (%) = capacity B / capacity A × 100 (B)
The measurement of the capacity | capacitance A and the capacity | capacitance B was implemented in the environment of 25 degreeC and -10 degreeC.
Table 7 shows the evaluation results.

<< Comparative Example 3 >>
100 parts by weight of LiFePO ₄ powder (manufactured by Hanwha Chemical Co., Ltd., average particle size 12 μm) as a positive electrode active material, 5 parts by weight of acetylene black as a conductive agent, and PVDF in an amount shown in Table 6 (manufactured by Shin-Etsu Chemical Co., Ltd.) And 150 parts by weight of NMP as a dispersion medium were added to obtain a slurry.
Using a doctor blade with a gap of 80 μm, this slurry was applied to the surface of a positive electrode current collector made of an aluminum foil to form a coating film. This coating film was dried at 80 ° C. for 2 hours to form a positive electrode mixture layer (step (2)).

The positive electrode mixture layer was compressed by a roll press (linear pressure: 400 kgf / cm) (step (4)).
In this way, a positive electrode mixture layer (density: 1.8 g / cm ³ , thickness: 45 μm) was formed on the surface of the positive electrode current collector to obtain a No. 23 positive electrode.
The battery of FIG. 2 was constructed using the No. 23 positive electrode and the No. 9 negative electrode, and the same evaluation as in Example 3 was performed.

In the battery of Example 3 using the positive electrodes of No. 20 to 22 that were heated, the discharge rate characteristics at a low temperature were compared with the battery of Comparative Example 3 using the positive electrode of No. 23 that was not heated. Improved.
In the positive electrodes of Nos. 20 to 22, the permeability and retention of the electrolyte were improved by the pores formed by the thermal decomposition of the first binder in the positive electrode mixture layer.
Since the composite material in which the positive electrode active material particles were previously coated with the first binder was used, the slipperiness between the positive electrode active material particles contained in the slurry was improved. Therefore, the filling property of the positive electrode active material particles was improved when forming the positive electrode mixture layer. When the positive electrode mixture layer was compressed, a compressive force was uniformly applied in the thickness direction of the positive electrode mixture layer. Therefore, the whole positive electrode mixture layer was densified, and the electronic conductivity between the active material particles was improved.

  The nonaqueous electrolyte secondary battery of the present invention is suitably used as a power source for equipment that requires low temperature characteristics.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Electrolyte 10 Positive electrode 11 Negative electrode 12a Inner layer 12b Surface layer 13 Separator 20 Positive electrode collector 21 Positive electrode mixture layer 22 Negative electrode collector 23 Negative electrode mixture layer 14 Positive electrode lead 15 Negative electrode lead 16 Resin material 17 Exterior Case 17a, 17b Opening 31 Positive electrode current collector 32 Positive electrode mixture layer 33 Positive electrode precursor 40 Electrode 41 Current collector 42 Mixture layer 42a Inner layer 42b Surface layer

Claims (10)

(1) adding a dispersion medium to the active material and the binder to prepare a slurry;
(2) attaching the slurry to the surface of the current collector and drying to form a mixture layer;
(3) compressing the mixture layer;
(4) After the step (3), heating the mixture layer, thermally decomposing a part of the binder, and forming pores in the mixture layer;
The manufacturing method of the electrode for nonaqueous electrolyte secondary batteries containing.   The method for producing an electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the heating temperature in the step (4) is 200 to 350 ° C. The binder in the step (1) includes a first binder and a second binder,
The heating temperature in the step (4) is higher than the thermal decomposition temperature of the first binder and lower than the thermal decomposition temperature of the second binder, The electrode for a nonaqueous electrolyte secondary battery according to claim 1 or 2. Production method.   The ratio of the dispersion medium in the slurry in the step (1) is 50 to 70% by weight. The method for producing an electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.   The said 1st binder is at least 1 sort (s) selected from the group which consists of carboxymethylcellulose, polyvinyl butyral, polyvinyl alcohol, polyethylene oxide, and a polyacrylate, The manufacture of the electrode for nonaqueous electrolyte secondary batteries of Claim 3 Method.   The method for producing an electrode for a non-aqueous electrolyte secondary battery according to claim 3, wherein the second binder is at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, and styrene butadiene rubber.   The electrode for nonaqueous electrolyte secondary batteries obtained by the manufacturing method in any one of Claims 1-6. A current collector, and a porous mixture layer formed on the surface of the current collector,
The mixture layer is an electrode for a nonaqueous electrolyte secondary battery in which the porosity continuously increases from the current collector side toward the surface side. The mixture layer includes an inner layer composed of a region from the surface of the current collector to a predetermined depth, a surface layer composed of a region from the surface of the mixture layer to a predetermined depth,
The porosity A of the inner layer is 25-40%,
The porosity B of the surface layer is 40-60%,
The ratio of the porosity A of the inner layer to the porosity B of the surface layer: B / A is 1.05 or more and 2.40 or less, The electrode for a nonaqueous electrolyte secondary battery according to claim 8. A non-aqueous electrolyte secondary battery comprising a pair of electrodes, a separator disposed between the pair of electrodes, and a non-aqueous electrolyte,
At least one of said pair of electrodes is an electrode in any one of Claims 7-9, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.