JP2016170881A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent shear fracture of a granulation particle.SOLUTION: A nonaqueous electrolyte secondary battery includes a granulation particle having electrode active material, carboxymethyl cellulose, and smectite.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Japanese Patent Application Laid-Open No. 10-055801 (Patent Document 1) discloses a method for producing a nonaqueous electrolyte secondary battery in which an electrode mixture layer is formed via granulated particles obtained by granulating an electrode active material or the like. ing.

Japanese Patent Laid-Open No. 10-055801

  There is known a method for producing an electrode mixture layer by press-molding a granule of granulated particles containing an electrode active material or the like (also referred to as “granulated body”). For example, an electrode mixture layer such as a button-type battery is manufactured by uniaxially pressing a granular material of granulated particles in a cylindrical mold.

  A large-area electrode mixture layer can also be produced by roll-forming granulated particles. However, this method makes it difficult to reduce the thickness of the electrode mixture layer. That is, if the roll gap is narrowed to form a thin electrode mixture layer, a strong shearing load is applied to the granulated particles, and the granulated particles may be sheared and broken. When the shear fracture of the granulated particles occurs, problems such as damage to the encapsulated electrode active material or increase in the distribution of voids in the electrode mixture layer occur. As a result, the battery performance may be reduced.

  Based on the above, an object of the present invention is to suppress shear fracture of granulated particles.

  The nonaqueous electrolyte secondary battery includes granulated particles containing an electrode active material, carboxymethylcellulose, and smectite.

  Smectite exhibits thixotropic properties in which viscosity decreases and fluidity increases when a shear load is applied. By adding smectite to the granulated particles, the shear load applied to the granulated particles during molding is alleviated by the flow of inclusions. Smectite exhibits a certain degree of tackiness. However, only the smectite adhesive strength cannot maintain the structure of the granulated particles under a strong shear load. Therefore, in the above non-aqueous electrolyte secondary battery, carboxymethyl cellulose (CMC), which is a thickening material, is used in combination with smectite.

  According to the above, shear fracture of the granulated particles is suppressed.

It is a schematic sectional drawing which shows an example of a structure of the nonaqueous electrolyte secondary battery which concerns on embodiment of this invention. It is the schematic which shows an example of a structure of the electrode group which concerns on embodiment of this invention. It is the schematic which illustrates a part of manufacturing process of an electrode compound-material layer.

  Hereinafter, embodiments of the present invention (hereinafter referred to as “this embodiment”) will be described, but the present embodiment is not limited thereto.

[Nonaqueous electrolyte secondary battery]
FIG. 1 is a schematic cross-sectional view showing an example of the configuration of the nonaqueous electrolyte secondary battery according to this embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 100 includes a square case 50. The rectangular case 50 is made of, for example, an aluminum (Al) alloy. The square case 50 is provided with a positive terminal 70 and a negative terminal 72. The square case 50 contains an electrode group 80 and an electrolyte solution 81. The electrode group 80 is connected to the positive terminal 70 and the negative terminal 72. The electrolytic solution 81 penetrates into the electrode group 80.

  FIG. 2 is a schematic diagram illustrating an example of the configuration of the electrode group. The electrode group 80 is a wound electrode assembly. The electrode group 80 is formed by stacking and winding the positive electrode 10 and the negative electrode 20 with the separator 40 interposed therebetween. The positive electrode 10, the negative electrode 20, and the separator 40 are all long belt-like sheet members. The positive electrode 10 includes a positive electrode current collector core material 11 and a positive electrode composite material layer 12 (electrode composite material layer) disposed on both main surfaces of the positive electrode current collector core material 11. The negative electrode 20 includes a negative electrode current collector core material 21 and a negative electrode composite material layer 22 (electrode composite material layer) disposed on both main surfaces of the negative electrode current collector core material 21. The core material exposed portions 11a and 21a where the current collecting core material is exposed from the electrode mixture layer serve as connection portions between the positive electrode terminal 70 and the negative electrode terminal 72 (see FIG. 1).

(Electrode compound layer)
The electrode mixture layer of the present embodiment is a sheet-like molded body formed by pressure-molding a granule of granulated particles. The granulated particles contain an electrode active material, CMC and smectite.

  The electrode active material is not particularly limited. The electrode active material may be a positive electrode active material or a negative electrode active material. The positive electrode active material may be, for example, a lithium-containing metal oxide. In the present embodiment, when the electrode active material is a lithium-containing metal oxide, for example, improvement of storage characteristics can be expected by suppressing the crushing of the active material particles. The negative electrode active material may be, for example, graphite. In the present embodiment, when the electrode active material is graphite, for example, an improvement in Li acceptability can be expected by suppressing collapse and misalignment between graphite layers.

  Smectite is a kind of clay mineral and exhibits thixotropic properties. In the present embodiment, smectite and CMC are used in combination. The total of smectite and CMC is preferably 0.3 parts by mass or more and 1.5 parts by mass or less, more preferably 0.6 parts by mass or more and 1.5 parts by mass or less, with respect to 100 parts by mass of the electrode active material. It is. Within this range, the balance between the adhesion of the electrode mixture layer and the battery characteristics is good. More preferably, the sum of smectite and CMC consists of 0.3 to 0.5 parts by mass of smectite and 0.3 to 1.0 parts by mass of CMC.

  The granulated particles may contain a binder resin. However, the binder resin is not an essential component. In the present embodiment, due to the combined use of smectite and CMC, sufficient adhesive strength may be exhibited even without a binder resin. Examples of the binder resin include polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and the like.

  The granulated particles may contain a conductive material. Examples of the conductive material include carbon blacks such as acetylene black and thermal black.

[Production method of granulated particles]
The granulated particle manufacturing method includes, for example, a first granulation step and a second granulation step described below in this order. A general agitation granulator can be used as the granulator. For example, water, N-methyl-2-pyrrolidone (NMP), or the like can be used as a solvent during granulation.

  In the first granulation step, the pre-granulated particles are formed by mixing the electrode active material, CMC and the solvent. In the second granulation step, the granulated particles are formed by mixing the pre-granulated particles, smectite and the solvent. Thus, smectite can be arrange | positioned on the surface of granulated particle | grains by granulating in two steps. When smectite is localized on the surface of the granulated particles, the effect of reducing the shear load tends to increase. In the granulated particles of the present embodiment, the smectite may partially adhere to the surface of the granulated particles or may cover the entire surface of the granulated particles. The particle diameter of the granulated particles is preferably adjusted to about 0.3 to 2.0 mm, for example.

[Method for producing electrode mixture layer]
FIG. 3 is a schematic view illustrating a part of the manufacturing process of the electrode mixture layer. As shown in FIG. 3, the plurality of granulated particles 1 are supplied to the roll gap 90. The arrow in FIG. 3 has shown the rotation direction of the roll. The plurality of granulated particles 1 are consolidated by a roll gap 90 and formed into an electrode mixture layer. At this time, if a roll gap is narrowed to form a thin electrode mixture layer, a strong shear load is applied to the granulated particles. In this embodiment, since the smectite flows at this time, the shear fracture of the granulated particles is suppressed.

  As described above, the present embodiment has been described by taking a square battery as an example, but the present embodiment can naturally be applied not only to a square battery but also to a cylindrical battery and a laminated battery.

  Hereinafter, although this embodiment is described using an example, this embodiment is not limited to these.

[Preparation of non-aqueous electrolyte secondary battery]
Nonaqueous electrolyte secondary batteries (rated capacity 24 Ah) according to Samples A1 to A8, B1 to B6, and C1 were produced as follows. Here, samples A1 to A8 and B1 to B6 are examples, and sample C1 is a comparative example.

[Sample A1]
1. Preparation of negative electrode The following materials were prepared: Negative electrode active material: Graphite (d50 20 μm)
CMC: Product name “MAC500LC”, Nippon Paper Industries Co., Ltd. Smectite: Product name “Lucentite SWN-P”, Co-op Chemical Co., Ltd. Binder resin: SBR
Solvent: Water Negative electrode current collector core material: Copper foil (thickness: 14 μm).

  Here, “d50” indicates the particle size of 50% of the integrated value in the volume-based particle size distribution measured by the laser diffraction / scattering method.

1-1. First granulation step An agitation granulator “High Speed Mixer” manufactured by Earth Technica was prepared. A negative electrode active material (electrode active material), CMC, and a binder resin were charged into a stirring tank of a high speed mixer. The rotational speed of the stirring blade (agitator) was set to 300 rpm, and the rotational speed of the crushing blade (chopper) was set to 1500 rpm, and dry mixing was performed for 5 minutes. Subsequently, the solvent was put into the stirring tank so that the solid content concentration (NV) of the mixture was 80% by mass. The rotating speed of the stirring blade was set to 300 rpm, and the rotating speed of the crushing blade was set to 1200 rpm, and wet mixing was performed for 5 minutes. Thereby, the granular material of the preliminary granulated particle was obtained.

1-2. Second granulation step Smectite was added to the stirring tank, and a solvent was added so that the solid content concentration of the mixture was 71% by mass. The rotating speed of the stirring blade was set to 250 rpm and the rotating speed of the crushing blade was set to 1200 rpm, and wet mixing was performed for 5 minutes. This obtained the granule of the granulated particle. In the granular body of granulated particles, the composition (mass ratio) of each component was negative electrode active material: CMC: smectite: binder resin = 100: 0.4: 0.3: 1.0.

1-3. Electrode Composite Layer Forming Step A sheet-like negative electrode composite layer (electrode composite layer) was obtained by press-molding the granulated particles in a roll gap. Furthermore, the negative electrode composite material layer and the negative electrode current collector core material were brought into close contact with each other with another roll gap, whereby the negative electrode composite material layer was pressure-bonded to both main surfaces of the negative electrode current collector core material. Then, the negative electrode was obtained by processing the negative electrode composite material layer and the negative electrode current collector core material into predetermined dimensions. Negative electrode dimensions were as follows: Negative electrode length: 4700 mm
Negative electrode composite layer width: 100 mm
Negative electrode composite material layer thickness: 68 μm.

2. Production of positive electrode A positive electrode having the following constitution was produced. Length of positive electrode: 4500 mm
Width of positive electrode mixture layer: 94 mm
Thickness of positive electrode mixture layer: 75 μm
The thickness of the positive electrode current collector core material: 20 μm.

In this example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the positive electrode active material, and Al foil was used as the positive electrode current collector core material.

3. Production of Separator A microporous film (thickness 25 μm) in which a polyethylene (PE) layer and a polypropylene (PP) layer were laminated in the order of PP layer / PE layer / PP layer was prepared as a separator substrate.

  Alumina particles and acrylic rubber were uniformly dispersed in a solvent using a disperser “Cleamics” manufactured by M Technique. As a result, a slurry to be a heat-resistant layer was obtained. The slurry was applied onto a separator substrate using a gravure coater and dried. Thus, a separator having a heat resistant layer was obtained.

4). Preparation of Electrolytic Solution Ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of EC: DMC: EMC = 3: 4: 3. This obtained the mixed solvent. LiPF ₆ (1.0 mol / L), cyclohexylbenzene (1.0% by mass), biphenyl (1.0% by mass) and lithium bisoxalate borate (1.0% by mass) were dissolved in the mixed solvent. Thus, an electrolytic solution was obtained.

5. Assembling The positive electrode and the negative electrode were stacked with a separator in between, and wound to obtain an elliptical wound electrode group. An elliptical wound electrode group was formed into a flat shape by press molding. Thereby, a flat electrode group was obtained. A square case provided with a positive electrode terminal and a negative electrode terminal was prepared. After the positive electrode terminal and the negative electrode terminal were connected to the electrode group, the electrode group was accommodated in a rectangular case. An electrolytic solution (125 g) was injected into the square case. Thereafter, the square case was sealed. From the above, a nonaqueous electrolyte secondary battery according to Sample A1 was obtained.

[Samples A2 to A8 and B1 to B6]
As shown in Table 1, in the above “1. Production of negative electrode”, the non-aqueous solution according to samples A2 to A8 and B1 to B6 is the same as sample A1 except that the amount of each component is changed. An electrolyte secondary battery was produced.

[Sample C1]
In Sample C1, a negative electrode mixture layer was formed by coating a slurry containing a negative electrode active material or the like on a negative electrode current collector core material. The specific operation is as follows. The granulated particles of the granulated particles obtained in the above “1-2. Second granulation step” were charged into the mixing tank of the planetary mixer. Further, a solvent was added so that the solid content concentration of the mixture was 50% by mass, and the mixture was stirred for a predetermined time. This obtained the negative electrode slurry. Using a die coater, the negative electrode slurry was coated on both main surfaces of the negative electrode current collector core and dried. As a result, a negative electrode mixture layer was formed. Except for these, a nonaqueous electrolyte secondary battery according to Sample C1 was fabricated in the same manner as Sample A1. Sample C1 is a comparative example having no granulated particles.

[Evaluation of battery performance]
Each sample obtained above was evaluated as follows. In the following description, the unit “C” of the current value indicates a current value at which the rated capacity of the battery can be discharged in one hour. “CC” represents a constant current, “CV” represents a constant voltage, and “CCCV” represents a constant current-constant voltage system.

1. Measurement of initial capacity The battery was charged in a 25 ° C. environment until the battery voltage reached 4.1 V at a current value of 1 C. The battery was discharged until the battery voltage reached 3.0 V at a current value of 1 C with a 5-minute pause. Furthermore, charging and discharging under the following conditions were performed with a 5-minute pause, and the initial capacity (discharge capacity) was measured. CCCV charge: CC current (1 C), CV voltage (4.1 V), cut current (0.1 C )
CCCV discharge: CC current (1 C), CV voltage (3.0 V), cut current (0.1 C).

2. Measurement of DC resistance The SOC (State Of Charge) of the battery was adjusted to 60%. In a 25 ° C. environment, pulse discharge was performed for 10 seconds at a current value of 10 C, and the amount of voltage drop was measured. The DC resistance was calculated by dividing the voltage drop by the current value. The results are shown in Table 1. The numerical values shown in Table 1 are values obtained by averaging the measurement results of 10 batteries.

3. Measurement of capacity maintenance ratio after cycle A battery was placed in a thermostat set to 50 ° C. One thousand charge / discharge cycles were performed with one set of charge and discharge under the following conditions. After 1000 cycles, the post-cycle capacity was measured in the same manner as in “1. Measurement of initial capacity”. The post-cycle capacity retention rate was calculated by dividing the post-cycle capacity by the initial capacity. The results are shown in Table 1. The numerical values shown in Table 1 are values obtained by averaging the measurement results of five batteries. Charging conditions: CC current (2C), cut voltage (4.1V)
Discharge conditions: CC current (2C), cut voltage (3.0V).

4). Measurement of capacity retention after high temperature storage The SOC of the battery was adjusted to 100% in a 25 ° C environment. The battery was stored for 100 days in a thermostat set at 60 ° C. Thereafter, the battery was taken out from the thermostat, and the capacity after storage was measured in the same manner as in “1. Measurement of initial capacity”. The capacity retention rate after high temperature storage was calculated by dividing the capacity after storage by the initial capacity. The results are shown in Table 1. The numerical values shown in Table 1 are values obtained by averaging the measurement results of 50 batteries.

〔Results and discussion〕
As can be seen from Table 1, Samples A1 to A8 and B1 to B6 provided with granulated particles containing an electrode active material, carboxymethylcellulose and smectite showed excellent battery performance. This is probably because the shear fracture of the granulated particles was suppressed in the process of forming the granulated particles into the electrode mixture layer by roll forming.

  Samples B1 to B3 tend to have a lower capacity retention rate after high-temperature storage than other examples. Since the CMC is small and the electrode active material is not sufficiently covered with the CMC, a side reaction between the electrode active material and the electrolytic solution is likely to occur.

  Samples B4 to B6 tend to have higher direct current resistance than other examples. For this reason, the capacity retention rate after cycling and storage at high temperature is reduced. This is probably because the insertion and release reaction of Li ions in the electrode active material is inhibited due to the large amount of CMC.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Granulated particle, 10 Positive electrode, 11 Positive electrode current collection core material, 11a, 21a Core material exposed part, 12 Positive electrode compound material layer, 20 Negative electrode, 21 Negative electrode current collection core material, 22 Negative electrode compound material layer, 40 Separator, 50 Rectangular Case, 70 positive electrode terminal, 72 negative electrode terminal, 80 electrode group, 81 electrolytic solution, 90 roll gap, 100 nonaqueous electrolyte secondary battery.

Claims (1)

  A non-aqueous electrolyte secondary battery comprising granulated particles containing an electrode active material, carboxymethyl cellulose and smectite.

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