JP2009238488A - Nonaqueous electrolyte secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of effectively manufacturing the nonaqueous electrolyte secondary battery using an electrode which forms a mixing agent layer on a current collector by using solvent-based slurry and raising sticking strength in the electrode and improving batter performance, and to provide its manufacturing method. <P>SOLUTION: In the nonaqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a nonaqueous electrolyte, at least one electrode of the positive electrode and the negative electrode forms a pre-coated layer 6 made of a solvent-based binding agent and particulates with average particle size of 1 μm or less, and the battery is obtained by drying the mixing agent layer 2 after forming the mixing agent layer 2 by coating solvent-based slurry including an active material, the solvent based binding agent, and a conductive agent on the pre-coated layer 6. <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 has a solvent-based binder and an average on the current collector A precoat layer composed of fine particles having a particle diameter of 1 μm or less is formed, a mixture layer is formed on the precoat layer by applying a solvent slurry containing an active material, a solvent binder and a conductive agent, and then dried. It is characterized by being an electrode obtained in this way.

  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.

Further, such movement of the solvent-based binder in the mixture layer may become remarkable when the mixture layer contains fine particles of a positive electrode active material having an average particle diameter of 1 μm or less. I found it. Examples of the fine particles of the positive electrode active material having an average particle diameter of 1 μm or less include a positive electrode active material such as the above-described LiFePO ₄ , a conductive agent 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 positive electrode active material fine particles having an average particle diameter of 1 μm or less are contained in the mixture layer 2, particles having a relatively small particle diameter in the particle size distribution of the fine particles move upward by the convection 5; It becomes unevenly distributed near the surface of the mixture layer 2. 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 decreases, the current collector 1 contains fine particles of the positive electrode active material having an average particle diameter of 1 μm or less in the mixture layer. Further, the adhesion strength of the mixture layer 2 with respect to is further reduced.

  When the fine particles of the positive electrode active material having an average particle size 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 the fine particles of the positive electrode active material 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 comprising a solvent-based binder and fine particles having an average particle diameter of 1 μm or less is formed on a current collector 1, and an active layer is formed on the precoat layer 6. A solvent-based slurry containing a substance, a solvent-based binder, and a conductive agent is applied to form the mixture layer 2, and then these are dried to produce an electrode.

  In the present invention, since the precoat layer containing 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 it 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, contact | adherence precision can be improved. In addition, since the drying temperature can be increased, the battery 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 precoat layer contains fine particles having an average particle diameter of 1 μm or less together with the solvent-based binder. By including such fine particles in the precoat layer, the adhesion to the current collector can be enhanced, and the precoat layer can be formed by uniform application.

  The content of fine particles having an average particle size of 1 μm or less in the precoat layer is preferably in the range of 0.1 to 3.0% by weight, more preferably, based on the solvent-based binder contained in the precoat layer. It is in the range of 0.5 to 1.5% by weight. If the content of fine particles having an average particle diameter of 1 μm or less in the precoat layer is too small, the precoat layer may not be applied uniformly. Also, if the content of the fine particles is too large, the slurry viscosity becomes high, and it becomes difficult to form a thin and uniform precoat layer, and since the fine particles generally have no conductivity, the current collector and the mixture layer Conductivity may be reduced.

In the present invention, the average particle size contained in the precoat layer is 1 μm or less, preferably 0.01 to 1.00 μm, more preferably 0.02 to 0.10 μm. Furthermore, the average particle size of the fine particles contained in the precoat layer is preferably 100 nm or less. Such fine particles are preferably electrochemically stable, for example, at least selected from TiO ₂ , Al ₂ O ₃ , MgO, SiO ₂ , ZnO, and SnO ₂ commercially available as nano-sized particles. One type of particle is preferred. Such fine particles in the precoat layer may be used by mixing a plurality of types.

  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 average particle size of the fine particles contained in the precoat layer is preferably smaller than the thickness of the precoat layer. If the average particle size of the fine particles contained in the precoat layer is larger than the thickness of the precoat layer, uneven coating may occur due to the surface tension of the slurry when forming the precoat layer, and adhesion to the current collector may be reduced. . The inclusion of fine particles in the precoat layer becomes more important when the thickness of the precoat layer is thin. Therefore, the thickness of the precoat layer in the present invention is preferably 0.5 μm or less, more preferably in the range of 0.05 to 0.5 μm, and still more preferably in the range of 0.05 to 0.3 μm. It is.

  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 the fine particles of the positive electrode active material having an average particle diameter of 1 μm or less, such as LiFePO ₄ , are applied, the effect of the present invention can be further exhibited.

  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, fine particles having an average particle diameter of 1 μm or less, 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. As described above, the fine particles having an average particle diameter of 1 μm or less are contained, for example, in an amount of 0.1 to 3% by weight with respect to the solvent-based binder.

  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. In the production method of the present invention, since the fine particle having an average particle diameter of 1 μm or less is contained in the solution for forming the precoat layer, coating unevenness is caused when the precoat layer is formed on the current collector. The precoat layer can be formed uniformly without causing any problems.

  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]
In a PVDF solution in which polyvinylidene fluoride (PVDF) is dissolved in N-methylpyrrolidone (NMP) so as to be 8% by weight, SiO ₂ particles (manufactured by C-I Kasei Co., Ltd.) having an average particle diameter of 25 nm are added to PVDF. It mixed so that it might become weight%. This mixed solution was applied to both sides of the aluminum foil using a 150 mesh gravure roll at a speed of 1.0 m / min, and passed through the first drying chamber (70 ° C.) and the 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 0.10 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 0.4 μm, carbon 2 wt% surface coated product) was used, and the positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder were 85: NMP was mixed as a diluent solvent at a mass ratio of 10: 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.

(Example 3)
A positive electrode was produced in the same manner as in Example 2 above, except that MgO particles having an average particle size of 31 nm (manufactured by CI Kasei Co., Ltd.) were used as the fine particles contained in the precoat layer. The produced positive electrode was designated as a positive electrode T3 of the present invention. The thickness of the precoat layer was 0.14 μm.

(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 1 except that fine particles were not added to the precoat layer. The produced positive electrode was used as a comparative positive electrode R2.

(Comparative Example 3)
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 R3.

(Comparative Example 4)
A positive electrode was produced in the same manner as in Example 2 except that fine particles were not added to the precoat layer. The produced positive electrode was used as a comparative positive electrode R4.

[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 coating uniformity]
The coating uniformity when the precoat layer forming solution was applied on the current collector was evaluated according to the following criteria.

◯: The precoat layer is uniformly formed on the current collector. ×: The precoat layer is formed in stripes on the current collector.

[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, the positive electrodes T1 to T3 of the present invention provided with the precoat layer have higher adhesion strength than the comparative positive electrodes R1 and R3 provided with no precoat layer. Further, as is clear from the comparison between the positive electrode T1 of the present invention and the comparative positive electrode R2 and the comparison between the positive electrodes T2 and T3 of the present invention and the comparative positive electrode R4, the precoat layer is formed by containing fine particles. It can be seen that the coating uniformity is excellent and the adhesion strength at the positive electrode is also improved.

  Moreover, even if it provides a precoat layer according to this invention, it turns out that a fall is hardly recognized in a negative electrode characteristic (3C / 1C).

  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. Can do. For this reason, the long-term reliability of a 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 (11)

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 comprising a solvent-based binder and fine particles having an average particle diameter of 1 μm or less on a current collector, and an active material is formed on the precoat layer. A non-aqueous electrolyte secondary battery, which is an electrode obtained by applying a solvent-based slurry containing a solvent-based binder and a conductive agent 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 0.5 micrometer or less, The nonaqueous electrolyte secondary battery of any one of Claims 1-3 characterized by the above-mentioned.   5. The nonaqueous electrolyte secondary battery according to claim 1, wherein an average particle diameter of the fine particles contained in the precoat layer is smaller than a thickness of the precoat layer.   The nonaqueous electrolyte secondary battery according to claim 1, wherein an average particle diameter of the fine particles contained in the precoat layer is 100 nm or less. The fine particles contained in the precoat layer are at least one inorganic oxide selected from TiO ₂ , Al ₂ O ₃ , MgO, SiO ₂ , ZnO, and SnO _2. The nonaqueous electrolyte secondary battery according to any one of the above.   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 includes fine particles of a positive electrode active material having an average particle diameter of 1 μm or less in the mixture layer. The non-aqueous electrolyte secondary battery according to claim 8, 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 10,
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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