JP6954032B2 - Electrodes for non-aqueous electrolyte secondary batteries - Google Patents

Description

The present disclosure relates to electrodes for secondary batteries.

Japanese Unexamined Patent Publication No. 2010-086717 (Patent Document 1) discloses a lithium ion secondary battery in which a plurality of slit-shaped grooves are formed in an active material layer (electrode mixture layer).

Japanese Unexamined Patent Publication No. 2010-086717

When the groove 22 is provided in the electrode mixture layer 2 provided on the surface of the electrode current collector 1 as in the electrode described in Patent Document 1, the groove 22 is not provided in the electrode mixture layer 2 (FIG. FIG. Compared with (see 6), since the path through which the ions in the electrode mixture layer 2 are conducted (particularly the path to the electrode mixture layer close to the electrode current collector) is secured by the groove 22, the ion conductivity is improved. , The discharge resistance is lowered, and the output characteristics (discharge rate) of the battery are improved.

In this way, increasing the ratio of the grooves 22 provided in the electrode mixture layer 2 improves the output characteristics of the battery. However, during the charge / discharge cycle, the electrode mixture layer 2 is easily peeled off from the electrode current collector 1 due to the expansion and contraction of the electrode mixture layer 2, so that the durability (charge / discharge cycle characteristics) of the battery is lowered. .. Further, since the amount of the electrode active material 21 in the electrode mixture layer 2 is reduced by the amount of the groove 22 formed, the battery capacity is reduced. Therefore, it has been difficult to improve the output characteristics of the battery while suppressing the decrease in the durability and the battery capacity of the battery by simply providing the groove 22 in the electrode mixture layer 2.

An object of the present disclosure is to provide an electrode for a secondary battery capable of improving the output characteristics of a battery while suppressing a decrease in battery durability and battery capacity.

The electrode for a secondary battery of the present disclosure includes an electrode current collector and an electrode mixture layer provided on at least one surface of the electrode current collector.
The electrode mixture layer has grooves provided in a honeycomb shape in a plan view seen from a direction perpendicular to the main surface of the electrode current collector.
In a plan view, when the width of the groove is t and the radius of the circle inscribed in the regular hexagon surrounded by the groove is a, the relationship of "t ≦ 0.04 × a" is satisfied and a is 0.2 mm. It is 1 mm or less.

According to the present disclosure, by providing a groove having a specific structure in the electrode mixture layer, it is possible to improve the output characteristics of the battery while suppressing a decrease in battery durability and battery capacity. The reason is considered as follows.

In order to obtain an electrode having homogeneous characteristics, it is preferable to provide grooves in the electrode mixture layer in a constant pattern. As a method of providing such a groove, a plurality of figures when the electrode mixture layer is planarly filled with a plurality of the same figures (pattern figures) in a plan view viewed from a direction perpendicular to the main surface of the electrode current collector. It is conceivable to form a groove along the boundary line of. In this case, in order to efficiently improve the ionic conductivity (output characteristics) of the electrode mixture layer while suppressing the decrease in battery capacity by reducing the ratio of grooves in the electrode mixture layer, the above is required. It is desirable that the figure of is the figure with the shortest peripheral length (perimeter) in the same area.

The figure with the shortest circumference in the same area is a circle, but the circle alone cannot fill the plane. On the other hand, the figures that can be tessellated include triangles, quadrangles, and parallel hexagons. Among these polygons, the one with the shortest circumference is a regular hexagon, which is a kind of parallel hexagons. Therefore, by using a regular hexagon as the above figure and providing grooves in a honeycomb shape along the boundary line of a plurality of regular hexagons when the electrode mixture layer is filled with the regular hexagons of a predetermined size in a plane. It is possible to improve the ionic conductivity (output characteristics) of the electrode mixture layer while suppressing the decrease in battery capacity.

Further, for example, when the above figure is a triangle or a quadrangle (see FIGS. 3 and 4), the electrode mixture layer expands and contracts due to the charge / discharge cycle, so that stress is concentrated on the corners of the triangle or the quadrangle. Therefore, cracks and peeling are likely to occur in the electrode mixture layer from the surrounding area. On the other hand, when the above figure is a regular hexagon and a groove is provided in a honeycomb shape (see FIG. 1), the angle of the corner is larger than that of a triangle or a quadrangle, so that the stress is concentrated on the corner. Is alleviated, and the occurrence of cracks and peeling of the electrode mixture layer is suppressed. As a result, the decrease in the durability (charge / discharge cycle characteristic) of the electrode is suppressed, so that the output characteristic of the battery can be improved while suppressing the decrease in the durability of the battery.

For the above reasons, the electrode mixture layer has grooves provided in a honeycomb shape (in a plan view from a direction perpendicular to the main surface of the electrode current collector), so that the durability and the battery capacity of the battery are reduced. It is considered that the output characteristics of the battery can be improved while suppressing the above.

However, when the width t of the groove provided in the electrode mixture layer becomes wide, the amount of active material in the electrode mixture layer decreases and the battery capacity decreases as the ratio of the grooves increases. Further, as the size of the regular hexagon surrounded by the groove (radius a of the inscribed circle of the regular hexagon) increases, the effect of improving the output characteristics of the battery decreases. Therefore, when the relationship of "t ≦ 0.04 × a" is satisfied and a is 0.2 mm or more and 1 mm or less, the output characteristics of the battery are suppressed while suppressing the decrease in battery durability and battery capacity. The effect of improving the above can be sufficiently obtained.

According to the present disclosure, it is possible to provide an electrode for a secondary battery capable of improving both the output characteristic and the durability (charge / discharge cycle characteristic) of the battery.

FIG. 1 is a schematic plan view showing an example of an electrode mixture layer according to the embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view of the electrode according to the embodiment of the present disclosure. FIG. 3 is a conceptual diagram for explaining the electrode mixture layer in Comparative Example 4. FIG. 4 is a conceptual diagram for explaining the electrode mixture layer in Comparative Example 5. 5 (a) to 5 (c) are schematic views showing a regular hexagon, a square, and an equilateral triangle circumscribing a circle having a radius a. FIG. 6 is a schematic cross-sectional view of an electrode having no groove in the electrode mixture layer.

Hereinafter, an example of the embodiment of the present disclosure (hereinafter, also referred to as “the present embodiment”) will be described. However, this embodiment is not limited to these. In the present specification, that is, "electrode" means at least one of "positive electrode" and "negative electrode", and "electrode mixture layer" is at least one of "positive electrode mixture layer" and "negative electrode mixture layer". The "electrode active material" indicates at least one of the "positive electrode active material" and the "negative electrode active material", and the "electrode current collector" refers to the "positive electrode current collector" and the "negative electrode current collector". Indicates at least one.

<Electrode>
With reference to FIG. 2, the electrode 10 for a secondary battery (such as a lithium ion secondary battery) includes an electrode current collector 1 and an electrode mixture layer 2 arranged on one surface of the electrode current collector 1. , Equipped with. The electrode mixture layer 2 may be provided on both the front and back surfaces of the electrode current collector 1. That is, the electrode 10 for the secondary battery includes an electrode current collector 1 and an electrode mixture layer 2 provided on at least one surface of the electrode current collector 1.

《Electrode current collector》
The electrode current collector 1 may be, for example, a copper foil, an aluminum foil, or the like. The thickness of the electrode current collector 1 may be about 5 to 30 μm.

<< Electrode mixture layer >>
The electrode mixture layer 2 contains, for example, an electrode active material 21, a binder, and the like. The electrode mixture layer 2 can be produced, for example, by applying an electrode mixture paste containing an electrode active material 21, a binder, or the like onto the electrode current collector 1 and drying the electrode mixture layer 2. The thickness of the electrode mixture layer 2 may be, for example, about 10 to 150 μm.

(Electrode active material)
The electrode active material 21 may be a negative electrode active material or a positive electrode active material. It should be noted that this embodiment is more effective when applied to a negative electrode active material (negative electrode) in which the volume fluctuates sharply during a high rate cycle. The content of the electrode active material 21 in the electrode mixture layer 2 may be, for example, about 90 to 99% by mass.

The negative electrode active material may be, for example, a carbon-based negative electrode active material such as graphite, easily graphitizable carbon, or non-graphitizable carbon, or an alloy-based negative electrode active material containing silicon (Si), tin (Sn), or the like. .. The average particle size of the negative electrode active material may be, for example, about 5 to 25 μm.

The positive electrode active material may be, for example, a lithium (Li) -containing metal oxide such as _{LiNi 1/3} Co _1/3 Mn _1/3 O _2. The average particle size of the positive electrode active material may be, for example, about 5 to 25 μm.

In the present specification, the "average particle size" is also referred to as the particle size at an integrated value of 50% ("d50" or "median diameter" in the volume-based particle size distribution measured by the laser diffraction / scattering method. ) Shall be indicated.

(Binder)
The binder may be, for example, a sodium salt of carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), Li polyacrylate or the like. The content of the binder in the negative electrode mixture layer may be, for example, about 1 to 10% by mass.

(Other ingredients)
The electrode mixture layer 2 may contain a conductive material or the like. As the conductive material, carbon blacks such as acetylene black and thermal black may be used. The content of the conductive material in the electrode mixture layer 2 is, for example, about 0 to 10% by mass.

(Honeycomb-shaped groove)
The electrode mixture layer 2 has a groove 22 provided in a honeycomb shape in a plan view seen from a direction perpendicular to the main surface of the electrode current collector 1. That is, as shown in FIG. 1, the groove 22 is provided along the boundary of a plurality of regular hexagons filled in a plane in the electrode mixture layer 2.

In the above plan view, when the width of the groove 22 is t and the radius of the circle inscribed in the regular hexagon surrounded by the groove 22 is a, the relationship of "t ≦ 0.04 × a" is satisfied and a is It is 0.2 mm or more and 1 mm or less.

When the relationship of “t ≦ 0.04 × a” is not satisfied, the battery capacity in the electrode mixture layer is reduced by the amount of the groove ratio, and the battery durability is also lowered. Further, when a is more than 1 mm, the effect of improving the output characteristics of the battery becomes small. On the other hand, when a is less than 0.2 mm, it becomes difficult to form a groove having a narrow width so as to satisfy the relationship of “t ≦ 0.04 × a”.

The depth of the groove 22 is not particularly limited, and the groove 22 may penetrate to the electrode current collector 1. The ratio of the depth of the groove 22 to the height of the electrode mixture layer 2 is preferably more than 0.8 and less than 1.0. When it is 0.8 or less, the ionic conductivity with respect to the electrode active material 21 in the vicinity of the electrode current collector 1 cannot be enhanced, and it becomes difficult to sufficiently obtain the effect of improving the output characteristics of the battery. On the other hand, when it is 1.0 (when the groove 22 penetrates to the electrode current collector 1), the electrode is formed due to the expansion and contraction of the electrode mixture layer 2 during the charge / discharge cycle, especially at the position of the groove 22. Since the mixture layer 2 is easily peeled off from the electrode current collector 1, the durability (charge / discharge cycle characteristics) of the battery is likely to be lowered.

The range of the porosity (porosity in the portion other than the groove 22) of the electrode mixture layer 2 (after pressing) may be, for example, about 25 to 45%. When the porosity is large (more than 45%), there are originally many paths through which ions are conducted and the ion conductivity is high, so that the merit of improving the output characteristics by providing the groove 22 is reduced. It is difficult in the actual manufacturing process to make the porosity of the electrode mixture layer 2 less than 25%.

The method for forming the groove 22 is not particularly limited, but for example, when the electrode mixture layer 2 is pressed by a roll press machine or the like, a protruding groove forming pattern is imparted to the roll. A method of forming the groove 22 in the electrode mixture layer 2 can be mentioned. Further, the groove 22 may be formed by physically cutting or removing a part of the electrode mixture layer 2 by a sharp blade, laser processing, or the like. Further, the groove 22 may be formed by chemically dissolving and removing a part of the electrode mixture layer 2 by an etching method or the like.

It is also conceivable to form the groove 22 at the stage of forming the electrode mixture layer 2 on the electrode current collector 1. For example, the groove 22 is formed by printing (coating the electrode mixture paste, etc.) so that the groove 22 is formed in the intended pattern by the intermittent coating method or the pattern printing method used for printers and the like. May be good.

Hereinafter, the present embodiment will be described with reference to examples, but the present embodiment is not limited thereto.

<Manufacturing of electrodes>
The electrodes (positive electrode and negative electrode) of Examples 1 to 4 and Comparative Examples 1 to 5 were manufactured as follows.

[Examples 1 to 4, Comparative Examples 2 and 3]
<< Preparation of positive electrode >>
The following materials were prepared.
Positive electrode active material: LiMn _1/3 Ni _1/3 Co _1/3 O ₂ (Average particle size: 5 μm,)
Binder: PVDF
Conductive material: Acetylene black Solvent: N-methyl-2-pyrrolidone Positive electrode current collector: Aluminum foil (thickness: 15 μm)

A positive electrode mixture paste was prepared by putting a positive electrode active material, a conductive material and a binder into a mixing tank of a mixing device, mixing them, and further adding a solvent and kneading them. The mixing ratio (mass ratio) of the solid content was positive electrode active material: conductive material: binder = 90: 8: 2.

Using a comma coater, the positive electrode mixture paste was applied to one side of the positive electrode current collector. A sheet-shaped positive electrode was manufactured by drying the applied positive electrode mixture paste to form a positive electrode mixture layer on the positive electrode current collector.

The density or porosity of the positive electrode was further adjusted by pressing with a roll press machine. At the time of this pressing, a slit groove was formed in the positive electrode mixture layer by imparting a protruding groove forming pattern to the roll.

Here, as shown in FIG. 1, the grooves are provided in a honeycomb shape along the boundaries of a plurality of regular hexagons. In each of Examples 1 to 4 and Comparative Examples 2 and 3, the size of the groove pattern of the positive electrode mixture layer (radius a of the inscribed circle of a regular hexagon) and the width t of the groove are as shown in Table 1. Changed. The depth of the grooves was set to 0.9 times the thickness of the electrode mixture layer.

<< Fabrication of negative electrode >>
The following materials were prepared.
Negative electrode active material: Artificial graphite (average particle size: 10 μm)
Thickener: CMC
Binder: SBR
Solvent: Water (ion-exchanged water)
Negative electrode current collector: Copper foil (thickness 10 μm)
Negative electrode active material (98 parts by mass), thickener (1 part by mass) and binder (1 part by mass) are added to the mixing tank of the mixing device and mixed, and a solvent is further added and kneaded to combine the negative electrodes. A material paste was prepared.

Using a comma coater, the negative electrode mixture paste was applied to one side of the negative electrode current collector. The coated negative electrode composite paste is dried, by forming the negative electrode mixture layer on the negative electrode current collector, a sheet-like negative electrode was produced.

Further, when the negative electrode was pressed in the same manner as the positive electrode, a slit groove was formed in the negative electrode mixture layer. In each of Examples 1 to 4 and Comparative Examples 2 and 3, the size of the groove pattern of the negative electrode mixture layer (radius a of the inscribed circle of a regular hexagon) and the width t of the groove are as shown in Table 1. Changed. The depth of the grooves was set to 0.9 times the thickness of the electrode mixture layer.

As described above, the electrodes (positive electrode and negative electrode) of Examples 1 to 4 and Comparative Examples 2 and 3 were manufactured.

[Comparative Example 1]
Electrodes (positive electrode and negative electrode) similar to those in Example 1 were manufactured except that neither the positive electrode mixture layer nor the negative electrode mixture layer formed a groove.

[Comparative Example 4]
As shown in FIG. 3, electrodes (positive electrode and negative electrode) similar to those in Example 1 were manufactured except that the slit groove pattern had a “lattice shape” (square shape).

[Comparative Example 5]
As shown in FIG. 4, electrodes (positive electrode and negative electrode) similar to those in Example 1 were manufactured except that the slit groove pattern was "triangular".

<Preparation of evaluation cell>
(Separator)
As a separator, a porous membrane film (thickness: 20 μm) made of polyethylene terephthalate (PET) was prepared.

(Electrolyte: non-aqueous electrolyte)
EC, DMC and DEC were mixed in a volume ratio of EC: DMC: DEC = 3: 4: 3 to obtain an aprotic solvent. Next, an electrolytic solution (non-aqueous electrolyte) was prepared by dissolving 1.0 mol / L of LiPF _{6 in the aprotic solvent.}

In the positive electrode and the negative electrode in which the grooves were formed, the electrode mixture layer at the end in the width direction was peeled off from the electrode current collector by a physical method. After that, the electrode mixture layer was peeled off, and a current collecting lead (tab) was bonded to a part of the exposed electrode current collector by ultrasonic welding. The positive electrode (the surface on the positive electrode mixture layer side) and the negative electrode (the surface on the negative electrode mixture layer side) to which the current collecting leads were joined were opposed to each other via a separator. The positive electrode, the negative electrode and the separator were impregnated with the above electrolytic solution. The positive electrode, the negative electrode and the separator were sealed with a laminated film (exterior film). In this way, batteries (evaluation cells) using the electrodes of Examples 1 to 4 and Comparative Examples 1 to 5 were produced.

<Evaluation of battery characteristics>
The batteries (evaluation cells) obtained in the above Examples and Comparative Examples are uniformly pressured (2.5 kgf / cm ² ) from the outside of the laminated film to the opposite positive electrode and negative electrode, and then the laminated film. The current collecting leads provided on the positive electrode and the negative electrode, which were taken out from the above, were connected to the evaluation terminals, and the following battery characteristics were evaluated.

<< Measurement of initial capacity >>
In the potential window (3.0 to 4.1 V), constant current / constant voltage (CC-CV) charging and constant current (CC) discharging with ^{a current density of 2.0 mA / cm 2 were repeated for 3 cycles.} The discharge capacity in the third cycle was used as the initial capacity of the prepared evaluation cell.

The measurement results of the initial capacity are shown in Table 1. The initial capacity in Table 1 is shown as a ratio (relative value) when the initial capacity in Comparative Example 1 is 100.

《Measurement of discharge resistance》
The evaluation cell in which the above initial capacity was measured was charged to a state where the SOC (charge rate) was 60%. After that, discharge was carried out sequentially for 10 seconds at a current density of less than or equal to that of the current density.
Current density: 0.2, 2.0, 4.0, 10, 20mA / cm ²
The IV resistance was calculated by dividing the amount of potential drop at each current density by the current value at the time of discharge, and used as the discharge resistance.

Table 1 shows the measurement results of the discharge resistance. The smaller the discharge resistance, the higher the output characteristics (discharge rate) of the battery. The discharge resistance in Table 1 is shown as a ratio (relative value) when the discharge resistance in Comparative Example 1 is 100.

<< Durability test: Measurement of capacity retention rate >>
The evaluation cell in which the above discharge resistance was measured was once discharged to a state of SOC 0%. The evaluation cell was subjected to a charge / discharge cycle test in an environment of 60 ° C. Specifically, in the potential window (3.0 to 4.1 V), constant current (CC) charging and constant current (CC) discharging with ^{a current density of 2.0 mA / cm 2 were repeated for 500 cycles.} The discharge capacity at the 500th cycle was measured as the capacity after 500 cycles. The capacity retention rate was calculated from the following formula.
Capacity retention rate = [capacity after 500 cycles / initial capacity] x 100 [%]

Table 1 shows the measurement results of the capacity retention rate. The larger the capacity retention rate, the higher the durability (charge / discharge cycle characteristics) of the battery.

From the results of Examples 1 to 4 and Comparative Example 1, when Comparative Example 1 in which the electrode mixture layer is not provided with a groove is used as a reference (100) in both the positive electrode and the negative electrode, the example in which the groove is provided is used as a reference. In 1 to 4, the discharge resistance is small, and it can be seen that the output characteristics of the battery are improved.

Further, from the results of Examples 1 to 4, the smaller the size of the regular hexagon (honeycomb pattern) surrounded by the groove (radius a of the inscribed circle of the regular hexagon), the smaller the discharge resistance and the higher the output characteristics of the battery. It turns out that there is a tendency to become. On the other hand, from the results of Comparative Example 2, it can be seen that as the size of the honeycomb pattern (radius a of the inscribed circle of the regular hexagon) increases, the effect of improving the output characteristics of the battery decreases.

From the results of Comparative Example 3, when the width t of the groove provided in the electrode mixture layer is widened, the effect of improving the output characteristics (effect of reducing the discharge resistance) is recognized, but the battery capacity (initial capacity) is lowered. It ends up.

From the results of Comparative Examples 4 and 5, Comparative Example 4 (see FIG. 3) in which grooves were provided in a grid pattern (square shape) and Comparative Example 5 in which grooves were provided in a triangular shape (see FIG. 3) with respect to the electrode mixture layer. In FIG. 4), the effect of improving the output characteristics (effect of reducing the discharge resistance) is recognized as compared with Comparative Example 1 in which the groove is not provided, but compared with Examples 1 to 4 in which the groove is provided in a honeycomb shape. It can be seen that the capacity retention rate is low and the durability (charge / discharge cycle characteristics) is low.

5 (a) to 5 (c) are schematic views showing a regular hexagon, a square, and an equilateral triangle circumscribing a circle having a radius a. Here, within the range of the radius a from the center of the circle (the portion surrounded by the dotted line in the figure), it is considered that there is an effect of reducing the discharge resistance by improving the ionic conductivity. However, the region 22a other than the circle (inscribed circle) of each figure has a distance from the center point of the circle, and it is considered that the effect of reducing the discharge resistance by improving the ionic conductivity is small. When the area ratio of the area 22a other than the circle to the total area of each figure is calculated (see the formula described in FIG. 5), it is 9.1% in the case of the equilateral hexagon shown in FIG. 5 (a), which is 9.1% in FIG. In the case of the square shown in (b), it is 39.6%, and in the case of the equilateral triangle shown in FIG. 5 (c), it is 21.5%. From this, it is considered that the regular hexagon (honeycomb) can be tessellated most effectively. Therefore, it is considered that when the grooves are provided in a honeycomb shape as in the present disclosure, it is possible to improve the output characteristics of the battery while suppressing the decrease in the durability and the battery capacity of the battery.

The embodiments and examples disclosed this time should be considered to be exemplary and not restrictive in all respects. The scope of the present disclosure is not described above, but is indicated by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 electrode current collector, 2 electrode mixture layer, 21 electrode active material, 22 grooves, 22a region.

Claims (1)

An electrode current collector and an electrode mixture layer provided on at least one surface of the electrode current collector are provided.
The electrode mixture layer has grooves provided in a honeycomb shape in a plan view seen from a direction perpendicular to the main surface of the electrode current collector.
In the plan view, when the width of the groove is t and the radius of the circle inscribed in the regular hexagon surrounded by the groove is a, the relationship of "t ≦ 0.04 × a" is satisfied and a is An electrode for a non-aqueous electrolyte secondary battery having a radius of 0.2 mm or more and 1 mm or less.

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