JP2009230976A - Nonaqueous electrolyte secondary battery and manufacturing method for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which ensures efficient production, has high adhesion strength in an electrode, and improves battery performance in the nonaqueous secondary battery using the electrode with a mixed layer formed on a collector using a solvent slurry, and to provide the manufacturing method for the nonaqueous secondary battery. <P>SOLUTION: The nonaqueous secondary battery is provided with a positive electrode, a negative electrode and a nonaqueous electrolyte; and in this case, at least either of the positive electrode and the negative electrode is the electrode obtained by forming a pre-coat layer 6 composed of a solvent type binding agent on the collector 1 and drying the mixed layer 2 formed on the pre-coat layer 6, by applying the solvent type slurry containing an active substance, a solvent type binding agent and a conductor and then drying the layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery and a method for producing the same.

  Due to their small size and light weight, lithium ion secondary batteries are widely used as driving power sources for portable devices. In recent years, expansion to applications such as electric tools, assist bicycles, and HEVs is also expected, and the demand for lithium ion secondary batteries is increasing. However, price declines are becoming more severe year by year than volume increases, and it is the responsibility of battery manufacturers to build production systems to meet cost reductions and market demands.

  The battery production process is classified into an electrode production process, an electrode winding process, a liquid injection process, and an inspection process. In the electrode manufacturing process, it is important how to accelerate the application and drying of the slurry to the current collector. In order to speed up drying, it is preferable to increase the drying temperature. However, the slurry for producing the electrode contains a mixture of a plurality of components. In order to improve the quality of the electrode, it is necessary to produce an electrode having a uniform composition in the thickness direction of the electrode. It is. If the composition of the electrodes is not uniform, there is a risk of (1) variations in electrical resistance and (2) variations in electrode adhesion density. When the drying temperature is increased, the composition in the thickness direction of the electrode may be nonuniform.

  In the case where an electrode is produced using an organic solvent-based slurry containing a solvent-based binder, the present invention has a problem that when the drying temperature is increased, the adhesion strength of the mixture layer to the current collector is greatly reduced. They found out. As a method for improving the adhesion between the mixture layer and the current collector, in Patent Document 1, a thin film of the same component as that of the negative electrode mixture layer is formed in advance on the current collector surface to improve the adhesion. It has been proposed. In Patent Document 2 and Patent Document 3, it is proposed to form a polymer compound layer having electron conductivity between the negative electrode active material layer and the current collector.

  However, even if these conventional techniques are used, the adhesion strength with the current collector has not been sufficiently improved.

In recent years, olivine-type lithium iron phosphate LiFePO ₄ having a small active material particle size and poor conductivity has attracted attention because of its excellent safety. However, when an active material with such a small particle size is used, there arises a problem that the adhesion to the current collector is further lowered.
JP-A-11-86850 Japanese Patent Laid-Open No. 5-135759 JP-A-3-165458

  The object of the present invention is to efficiently produce a nonaqueous electrolyte secondary battery using an electrode in which a mixture layer is formed on a current collector using a solvent-based slurry, and the adhesion strength within the electrode is high. An object of the present invention is to provide a non-aqueous electrolyte secondary battery that is high and can improve battery performance and a method for manufacturing the same.

  The present invention is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein at least one of the positive electrode and the negative electrode comprises a solvent-based binder on a current collector. An electrode obtained by forming a precoat layer, applying a solvent-based slurry containing an active material, a solvent-based binder, and a conductive agent on the precoat layer to form a mixture layer, and then drying the mixture layer. It is characterized by.

  The inventors of the present invention have examined the cause of the decrease in adhesion strength when the drying temperature is increased in an electrode in which a mixture layer is formed on a current collector using an organic solvent-based slurry containing a solvent-based binder. As a result, the solvent-based binder contained in the mixture layer moves due to thermal convection generated in the mixture layer during drying and is unevenly distributed on the surface of the mixture layer. I found it.

It has also been found that such movement of the solvent-based binder in the mixture layer becomes significant when fine particles having an average particle diameter of 1 μm or less are contained in the mixture layer. Examples of such fine particles having an average particle diameter of 1 μm or less include positive electrode active materials such as the above-described LiFePO ₄ , conductive agents contained in the mixture layer, and the like.

  FIG. 2 is a cross-sectional view for explaining the movement of the solvent-based binder by convection in the mixture layer. As shown in FIG. 2A, the mixture layer 2 is provided on the current collector 1, and the solvent-based binder 3 in the mixture layer 2 is dried by applying hot air 4. The convection 5 generated in the mixture layer 2 moves upward. As a result, as shown in FIG. 2B, the solvent-based binder 3 is unevenly distributed in the vicinity of the surface of the mixture layer 2, and the amount of the binder at the interface between the current collector 1 and the mixture layer 2 is reduced. Therefore, the adhesion strength of the mixture layer 2 to the current collector 1 is reduced.

  When the mixture layer 2 contains fine particles having an average particle size of 1 μm or less, particles having a relatively small particle size in the particle size distribution of the fine particles move upward by the convection 5, and the mixture layer 2 It becomes unevenly distributed in the vicinity of the surface. Fine particles having a small particle diameter have a large surface area and move with more solvent-based binder, so that the solvent-based binder also moves along with the movement of the fine particles by convection 5 and is unevenly distributed near the surface of the mixture layer 2. To do. As a result, since the amount of the binder at the interface between the current collector 1 and the mixture layer 2 is reduced, the mixture layer for the current collector 1 when the mixture layer contains fine particles having an average particle diameter of 1 μm or less. 2 adhesion strength further decreases.

  When fine particles having an average particle diameter of 1 μm or less are contained in the solid content of the mixture layer by 2.5% by weight or more, the adhesion strength is remarkably lowered. The upper limit of the concentration in the solid content of the mixture layer of fine particles having an average particle diameter of 1 μm or less is not particularly limited, but is, for example, 95% by weight or less.

  According to the present invention, as shown in FIG. 1, a precoat layer 6 made of a solvent-based binder is formed on a current collector 1, and an active material, a solvent-based binder and a precoat layer 6 are formed on the precoat layer 6. After forming the mixture layer 2 by applying a solvent-based slurry containing a conductive agent, these are dried to produce an electrode.

  According to the present invention, since the precoat layer made of the solvent-based binder is formed between the current collector and the mixture layer, the solvent-based binder in the mixture layer is dried when the mixture layer is dried. Even if the agent moves to the vicinity of the surface of the mixture layer, the concentration of the solvent-based binder at the interface between the current collector and the mixture layer can be kept high. For this reason, adhesion strength can be raised. In addition, since the drying temperature can be increased, the electrode can be manufactured efficiently.

  In addition, since the adhesion strength of the electrode can be increased, battery performance can be improved and high reliability can be obtained. The active material has a large change in expansion and contraction during charge / discharge, and stress is easily applied to the electrode. By repeating such charge and discharge, the active material may be peeled off from the electrode. However, according to the present invention, the adhesion strength can be increased, and thus such peeling of the active material is suppressed. be able to. For this reason, battery performance can be improved and the reliability in the long term can be improved.

  In the present invention, the solvent-based binder of the precoat layer is not necessarily the same component as the solvent-based binder of the mixture layer, but is the same component as the solvent-based binder of the mixture layer. By using it for the formation of the precoat layer, the composition in the electrode can be made more uniform, and the adhesion strength can be further increased.

  The solvent-based binder used in the present invention is not particularly limited, and for example, those conventionally used as binders for electrodes of nonaqueous electrolyte secondary batteries can be used. Specific examples include polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN) and the like, modified products and derivatives thereof, copolymers containing acrylonitrile units, polyacrylic acid derivatives, and the like. .

  In the present invention, the thickness of the precoat layer is preferably 1 μm or less. When the thickness of the precoat layer exceeds 1 μm, contact between the active material in the mixture layer and the current collector becomes insufficient in the electrode obtained by drying after forming the mixture layer thereon. Electricity may not be obtained. Although the lower limit of the thickness of a precoat layer is not specifically limited, Generally it is preferable to set it as 0.05 micrometer or more. If the thickness of the precoat layer becomes too thin, the effect of the present invention that the adhesion strength is increased may not be sufficiently obtained. Accordingly, the thickness of the precoat layer is preferably in the range of 0.05 to 1 μm, and more preferably in the range of 0.05 to 0.5 μm.

  In the present invention, the method for applying the precoat layer is not particularly limited, and examples thereof include a gravure coating method. By applying using a gravure coating method, a uniform precoat layer can be formed even if the thickness is small.

According to the present invention, the electrode forming the precoat layer may be either a positive electrode or a negative electrode. As described above, when fine particles having an average particle diameter of 1 μm or less, such as LiFePO _4, are used as the positive electrode active material, the effect of the present invention can be obtained by applying the present invention when producing such a positive electrode. It can be made to show more.

  The negative electrode active material is not particularly limited, and any negative electrode active material can be used as long as it can be used as a negative electrode active material in a lithium ion secondary battery. Examples thereof include graphite (graphite), coke, tin oxide, metallic lithium, silicon, and a mixture thereof.

  Also, the positive electrode active material is not particularly limited as long as it is a positive electrode active material that can be used for a lithium ion secondary battery, and examples thereof include lithium-containing transition metal oxides such as lithium cobaltate. Specific examples other than lithium cobalt oxide include nickel such as Ni—Co—Mn based lithium composite oxide, Ni—Mn—Al based lithium composite oxide, and Ni—Co—Al based lithium composite oxide. Examples include lithium composite oxide, spinel type lithium manganate, olivine type lithium iron phosphate, and the like.

As described above, when the olivine-type lithium iron phosphate (LiFePO ₄ ) having an average particle diameter of 1 μm or less is used, the effect of the present invention is more exhibited.

  In the present invention, the solution for forming the precoat layer can be prepared from a solvent-based binder and an organic solvent. The organic solvent is not particularly limited as long as it can dissolve the solvent-based binder. For example, the organic solvent used for the slurry for forming the mixture layer can be used. The concentration of the solvent-based binder in the solution for forming the precoat layer is appropriately adjusted, but can generally be adjusted to 0.2 to 15% by weight.

  The nonaqueous electrolyte in this invention is not specifically limited, The nonaqueous electrolyte which can be used for a lithium ion secondary battery can be used.

Examples of the solute of the nonaqueous electrolyte include LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-x (C _n F _{2n + 1} ) _x (where, 1 <x <6, n = 1 or 2). These can be used individually by 1 type or in mixture of 2 or more types.

  Moreover, as a solvent of a nonaqueous electrolyte secondary battery, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) Etc. can be used. Preferably, a cyclic carbonate and a chain carbonate are used in combination.

  Although it does not specifically limit as a density | concentration of the solute in a nonaqueous electrolyte, The density | concentration of 0.8-1.8 mol / liter is mentioned.

  The production method of the present invention is a method capable of producing the nonaqueous electrolyte secondary battery of the present invention, wherein a step of forming a precoat layer on the current collector, an active material on the precoat layer, And applying a solvent-based slurry containing a solvent-based binder and a conductive agent to form a mixture layer, followed by drying.

  The method for producing a nonaqueous electrolyte secondary battery of the present invention includes a method for producing the above electrode. Therefore, the electrode obtained by the above manufacturing method has high adhesion strength in the electrode, and can improve battery performance. Moreover, since it can dry at a high drying temperature after apply | coating a solvent-type slurry, an electrode can be produced efficiently.

  Although the drying temperature at the time of drying after apply | coating a solvent type slurry is not specifically limited, The temperature of the range of 60-150 degreeC is mentioned.

  After forming the mixture layer on the precoat layer as described above, it is preferable to roll the electrode in the same manner as in a normal electrode manufacturing step. By such a rolling process, the active material in the mixture layer can be efficiently brought into contact with the current collector, and the current collecting property can be improved.

  According to the present invention, in a non-aqueous electrolyte secondary battery using an electrode in which a mixture layer is formed on a current collector using a solvent-based slurry, it can be efficiently produced, and the adhesion strength in the electrode is high. , Battery performance can be improved.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following examples, and can be appropriately modified and implemented without departing from the scope of the present invention.

Example 1
[Production of positive electrode]
A PVDF solution prepared by dissolving polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) to 8 wt% was prepared by using a 150 mesh gravure roll on both sides of an aluminum foil, and 1.0 m / The coating was applied at a rate of minutes, passed through a first drying chamber (70 ° C.) and a second drying chamber (105 ° C.), and dried to form a precoat layer. The coating amount on both sides of the precoat layer was 0.5 mg / 10 cm ² , and the thickness of the precoat layer on one side was 0.3 μm. The thickness of the precoat layer in the present invention can also be measured, for example, by coating gold on the precoat layer, performing cross-section processing by a cross section polisher method, and observing with a scanning electron microscope (SEM). .

  Next, lithium cobaltate as a positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder were mixed at a mass ratio of 95: 2.5: 2.5 using NMP as a diluent solvent. A slurry for forming a mixture layer was prepared.

  Next, the solvent-based slurry was applied on both surfaces of an aluminum foil having a precoat layer formed on both surfaces, thereby forming a mixture layer. The formed mixture layer was dried at a temperature of 120 ° C. Then, it rolled so that a packing density might be 3.60 g / ml. The obtained electrode was designated as a positive electrode T1 of the present invention.

(Example 2)
As the positive electrode active material, olivine-type lithium iron phosphate (average particle size 4 μm, carbon 2 wt% surface coated product) was used. The positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder were 85:10: NMP was mixed as a diluent solvent at a mass ratio of 5 to prepare a solvent-based slurry. A positive electrode was produced in the same manner as in Example 1, except that a mixture layer was formed using this slurry and the packing density was 1.9 g / ml. The produced positive electrode was designated as a positive electrode T2 of the present invention.

(Comparative Example 1)
A positive electrode was produced in the same manner as in Example 1 except that the slurry was applied directly on the aluminum foil to form a mixture layer without forming a precoat layer. The produced positive electrode was used as a comparative positive electrode R1.

(Comparative Example 2)
A positive electrode was produced in the same manner as in Example 2 except that the slurry was applied directly on the aluminum foil to form a mixture layer without forming a precoat layer. The produced positive electrode was used as a comparative positive electrode R2.

[Evaluation of adhesion strength of positive electrode]
The adhesion strength of each positive electrode produced as described above was measured. A positive electrode having a size of 100 mm × 25 mm is pasted on a 120 mm × 30 mm acrylic plate using a 70 mm × 20 mm double-sided tape (Nichiban NW-20, manufactured by Nichiban Co., Ltd.), and the end of the pasted electrode is attached. Using a small desktop testing machine (Nidec Sympo Co., Ltd., FGS-TV and FGP-5), pull 50 mm upwards at a constant speed (100 mm / min) in the direction of 90 degrees with the adhesive surface of the electrode. The average peel strength at the time of peeling was measured. The measurement results are shown in Table 1.

[Evaluation of charge / discharge characteristics]
Each positive electrode was cut into a size of 5 cm × 2 cm, and an aluminum tab was ultrasonically welded to obtain a positive electrode. Using this positive electrode, a three-electrode cell was prepared using lithium metal as a counter electrode and a reference electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7 so as to be 1 mol / liter was used.

  Charging / discharging was performed at a current of 1.0 C with respect to the electrode design capacity. In addition, charge performed constant current constant voltage charge (current lower limit cut 0.1C) to 4.3V on the basis of a lithium metal reference electrode, and discharge was performed up to 2.85V at 1.0C.

  A similar test was conducted with a discharge current of 3.0C, a ratio of 3.0C / 1.0C was determined, and the load characteristics (3C / 1C) are shown in Table 1.

  As shown in Table 1, this invention positive electrode T1 which provided the precoat layer has high adhesive strength compared with the comparison positive electrode R1 which does not provide the precoat layer. Moreover, this invention positive electrode T2 which used olivine type lithium iron phosphate as a positive electrode active material and provided the precoat layer has shown high adhesive strength compared with comparative positive electrode R2 which does not provide the precoat layer. In addition, in the load characteristics (3C / 1C), the positive electrode T1 of the present invention provided with the precoat layer shows the same or slightly superior load characteristics as the comparative positive electrode R1. Similarly, the positive electrode T2 of the present invention shows the same or slightly better load characteristics than the comparative positive electrode R2.

  In addition, according to the present invention, since the adhesion strength in the electrode can be increased as described above, it is possible to suppress separation and floating of the active material from the current collector due to expansion and contraction of the active material during charging and discharging. Therefore, the long-term reliability of the battery can be improved.

  Further, according to the present invention, as described above, since the electrode can be produced at a high drying temperature, the battery can be produced efficiently.

  In the above embodiments, the present invention is applied to the positive electrode, but the present invention can be applied to the negative electrode to obtain the effects of the present invention.

Sectional drawing which shows the state which formed the precoat layer on the electrical power collector according to this invention, and formed the mixture layer on the precoat layer. Sectional drawing for demonstrating the state which a binder moves to the mixture layer surface by a drying process in the conventional electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Current collector 2 ... Mixture layer 3 ... Binder 4 ... Hot air 5 ... Convection 6 ... Precoat layer

Claims (8)

A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte,
At least one of the positive electrode and the negative electrode forms a precoat layer made of a solvent-based binder on the current collector, and an active material, a solvent-based binder, and a conductive agent on the precoat layer A nonaqueous electrolyte secondary battery, which is an electrode obtained by applying a solvent-based slurry containing a mixture layer to form a mixture layer and then drying.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the solvent-based binder of the precoat layer is the same component as the solvent-based binder of the mixture layer.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the solvent-based binder is polyvinylidene fluoride.   The thickness of the said precoat layer is 1 micrometer or less, The nonaqueous electrolyte secondary battery of any one of Claims 1-3 characterized by the above-mentioned.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrode is a positive electrode.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the positive electrode contains fine particles having an average particle diameter of 1 μm or less in a solid content of the mixture layer of 2.5% by weight or more. The non-aqueous electrolyte secondary battery according to claim 5 or 6, wherein the positive electrode active material of the positive electrode is LiFePO ₄ . A method for producing the nonaqueous electrolyte secondary battery according to any one of claims 1 to 7,
Forming the precoat layer on the current collector;
A method of manufacturing a non-aqueous electrolyte secondary battery, comprising: applying the solvent-based slurry on the precoat layer to form the mixture layer, and drying the mixture layer.

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