JP7365048B2 - Method for manufacturing positive electrode of secondary battery, and secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a positive electrode of a secondary battery including a positive electrode including a porous current collector, and a secondary battery including a positive electrode including a porous current collector.

With the spread of mobile phone terminals and the development of electric vehicles that address environmental issues, there is a demand for high-capacity secondary batteries. Lithium ion secondary batteries are popular as secondary batteries.

Lithium-sulfur batteries that have sulfur as a positive electrode active material are attracting attention as secondary batteries with even higher capacity than lithium-ion secondary batteries. Sulfur has a theoretical capacity of about 1670 mAh/g, which is about 10 times higher than that of LiCoO ₂ (about 140 mAh/g), which is a typical positive electrode active material of lithium ion batteries. In addition, sulfur is inexpensive and abundant as a resource.

In order to improve the energy density of a battery, it is effective to support a larger amount of positive electrode active material on the positive electrode. JP 2018-200827A discloses a lithium-sulfur secondary battery in which nickel sulfide, which is a positive electrode active material, is produced by sulfurizing the surface of a nickel porous plate with a large surface area.

JP2018-200827A

Embodiments of the present invention aim to provide a method for manufacturing a positive electrode of a secondary battery that is easy to manufacture and has a high energy density, and a secondary battery that is easy to manufacture and has a high energy density.

A method for manufacturing a positive electrode for a secondary battery according to an embodiment of the present invention includes a step of mixing carbon particles and sulfur at a temperature equal to or higher than the melting point of sulfur to prepare a mixture, and adding a binder and a solvent to the mixture. In addition, a step of preparing a slurry, a coating step of coating a perforated plate with the slurry, a drying step of evaporating the solvent, and a compression step of pressing the perforated plate to a thickness of 75% or less. The steps from the coating step to the drying step are repeated in the above order.

In a secondary battery according to an embodiment of the present invention, a positive electrode includes a perforated plate, a positive electrode active material disposed on an inner wall surface of the perforated plate, and an electrolyte, and the positive electrode active material includes sulfur. The positive electrode contains carbon particles impregnated with carbon particles and a binder, the amount of sulfur supported on the positive electrode is 5 mg/cm ² or more, and the surface of the positive electrode has 0.2 openings/mm with an area of 100 μm ² or more. There are ² or more.

According to the embodiments of the present invention, it is possible to provide a method for manufacturing a positive electrode of a secondary battery that is easy to manufacture and has a high energy density, and a secondary battery that is easy to manufacture and has a high energy density.

FIG. 1 is a configuration diagram of a secondary battery according to an embodiment. It is a block diagram of the secondary battery of the modification of embodiment. 1 is a flowchart of a method for manufacturing a secondary battery according to an embodiment. It is a photograph of a perforated plate used in a secondary battery of an embodiment. It is a photograph of the positive electrode before compression processing used in the secondary battery of the embodiment. It is a photograph of a positive electrode used in a secondary battery of a comparative example before compression treatment. It is a photograph of the positive electrode of the secondary battery of the embodiment. It is a photograph of the positive electrode of a secondary battery of a comparative example. It is a charge-discharge characteristic of the secondary battery of embodiment.

<Battery configuration>
As shown in FIG. 1, the secondary battery 1 (hereinafter also referred to as "battery") of the present embodiment includes a positive electrode 20 including a porous plate serving as a current collector, a separator 30, and a separator 30 that absorbs and deintercalates lithium ions. and a negative electrode 40 containing a negative electrode active material. The battery 1 is enclosed in a laminate pack 50.

The positive electrode 20 includes a perforated plate, a positive electrode active material disposed on an inner wall surface of the perforated plate, and an electrolyte. Celmet (registered trademark), which is a foamed aluminum used as the perforated plate, has a porosity of 98%, a specific surface area of 8500 m ² /m ³ , a pore diameter of 0.45 mm, and a thickness of 1.0 mm (FIG. 4).

The positive electrode active material includes sulfur-impregnated carbon particles and a binder. The electrolyte fills the internal space of a porous plate whose walls are coated with a positive electrode active material.

The binders are CMC (carboxymethyl cellulose) and SBR (styrene butadiene rubber). Although the separator 30 disposed between the positive electrode 20 and the negative electrode 40 is not an essential component, the separator 30 has a function of shortening the distance between electrodes and a function of supporting an electrolyte.

As will be described later, the battery 1 is easy to manufacture, but has a high energy density because the amount of sulfur supported on the positive electrode 20 is 5 mg/cm ² or more. That is, in the battery 1, although the amount of sulfur supported on the positive electrode 20 is large, the surface of the positive electrode 20 has 0.2 or more openings/mm ² with an area of 100 μm ² or more.

Note that the secondary battery 1A of the modified example shown in FIG. 2 is a stacked battery in which a plurality of positive electrodes 20, a plurality of separators 30, and a plurality of negative electrodes 40 are alternately stacked. A plurality of battery cells each having a positive electrode 20, a separator 30, and a negative electrode 40 share the positive electrode 20 and the negative electrode 40 with adjacent battery cells. The negative electrode 40 has a core material of a copper plate having a thickness of 8 μm, and metal lithium plates having a thickness of 60 μm on both sides. Since the plurality of battery cells are connected in parallel, the voltage of battery 1A is the same as that of battery 1.

<Battery manufacturing method>
A method for manufacturing the positive electrode 20 of the battery 1 will be explained along the flowchart of FIG.

<Step S10> Mixture Preparation Step A mixture is prepared by mixing carbon particles and sulfur at a temperature equal to or higher than the melting point of sulfur.

For example, 40 wt% of Ketjenblack (KB), which is carbon particles, and 60 wt% of sulfur (S) powder are mixed and stirred at 125° C. for 12 hours. The melting point of sulfur is 113°C. The molten sulfur impregnates the surface and pores of the porous carbon particles, producing an S/KB composite.

The carbon particles are preferably at least one of porous graphite such as KB, graphene, carbon nanohorns, fullerene, and carbon nanotubes, or aggregates thereof.
The carbon particles are particularly preferably porous carbon particles or aggregates of porous carbon particles having a hollow shell structure with pores of 2 nm or more and 50 nm or less.

<Step S20> Slurry preparation process 3.5 wt% of binder and solvent are added to 96.5 wt% of the mixture (S/KB composite), and then mixed for 12 hours using a ball mill to form a slurry of the mixture. is produced.

The binder contains CMC with an average molecular weight of 250,000 and SBR with an average molecular weight of 1 million, and the weight ratio is (CMC/SBR=2/1). The binder is not particularly limited as long as it does not react with the electrolyte.

As a binder, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR) ), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), poly Examples include polyalkylene oxides such as lithium acrylate (PAALi), ethylene oxide, or ring-opening polymers of monosubstituted epoxides, or mixtures thereof.

The solvent may be an organic solvent such as N-methyl-2-pyrrolidone (NMP), but it is preferable to use water because it is easy to handle. Note that a solid electrolyte may be used instead of the electrolytic solution.

In the method for manufacturing the positive electrode 20 of this embodiment, as described later, it is important to adjust the viscosity and solid content of the slurry in the slurry production step (S20).

<Step S30> Initialization of the number of repetitions The number of repetitions N of the coating process (S40) and the drying process (S50) is set to an initial value of 1. That is, in the positive electrode manufacturing method of this embodiment, the coating step (S40) and the drying step (S50) are repeatedly performed a predetermined number of times k.

<Step S40> Coating process Slurry is coated on the surface of the porous plate. When the slurry of the mixture is dropped onto the surface of the perforated plate, the slurry is sucked into the space inside the perforated plate. That is, the slurry permeates into the pores (voids) of the porous plate due to interfacial tension. The slurry may also be spray coated. If too much slurry is applied at once, it will spread over the surface of the perforated plate.

In the manufacturing method of the embodiment, the amount of slurry coated at one time is set in advance. For example, when producing the positive electrode 20 with a sulfur loading of 15 mg/cm ² by coating six times, the slurry is adjusted so that the sulfur loading in one coating is 2.5 mg/cm ² . Ru. Note that the amount of sulfur supported by one coating is preferably 3 mg/cm ² or less.

The amount of sulfur (supported amount) is calculated from weight measurements before and after the (coating/drying) process.

The specific surface area of the perforated plate is preferably 1500 m ² /m ³ or more, particularly preferably 3000 m ² /m ³ or more. In order to obtain a large surface area, the porous plate preferably has a porosity of 50% or more, has continuous pores, and contains almost no closed pores. Note that a porous plate with a porosity of more than 98% has weak mechanical strength and is not easy to handle.

The surface area of a flat plate having a two-dimensional surface is proportional to its planar dimension. On the other hand, the surface area of a porous plate having a three-dimensional network structure is proportional to the planar dimension (outer surface area) and thickness. The surface area of the perforated plate is preferably twice or more, particularly preferably 5 times or more, the surface area of a flat plate having a two-dimensional surface with the same dimensions in plan view. Note that from a technical standpoint, the surface area of the perforated plate is 100 times or less the surface area of a flat plate having a two-dimensional surface with the same dimensions in plan view.

The specific surface area is measured, for example, by a capacitance method. The capacitance method is a measurement method that utilizes the fact that the capacitance of a sample is proportional to its surface area. Prepare multiple metal plates with known surface areas and measure the capacitance of each. Then, by creating a calibration curve of "capacitance" versus "area", the surface area of the sample is calculated by the calibration method. On the other hand, the porosity is calculated from the specific gravity and volume of the sample.

Foamed metal, metal nonwoven fabric, metal fiber aggregate, metal particle aggregate, or the like can be used for the porous plate. The porous plate is produced, for example, by coating the inner surfaces (wall surfaces) of holes in a foamed resin with a metal layer and then decomposing the foamed resin. As the foamed resin, urethane foam is preferred because it has high porosity, high uniformity of cell diameter, and excellent thermal decomposition properties. When the metal layer is nickel, a nickel plating film is applied by coating the walls of the holes in the foamed resin with carbon powder or the like to make them conductive, or by using a direct plating method. Metal layers that are difficult to form by electroplating using an aqueous solution, such as aluminum layers, are formed by electroplating using a molten salt bath.

<Step S45> Blow Step In order to more efficiently penetrate the slurry deep into the perforated plate, it is preferable to blow gas onto the surface of the perforated plate.

For example, compressed air or nitrogen gas is sprayed onto the perforated plate using a spray nozzle.

<Step S50> Drying process The drying temperature of the slurry is set lower than the boiling point of the solvent. When the solvent is water, the drying temperature is, for example, 75°C. Therefore, the positive electrode active material is arranged in a layered manner on the inner wall surface of the porous plate without the solvent evaporating rapidly. Of course, in the drying process, the temperature may be gradually increased to finally reach the boiling point of the solvent or higher. However, it is preferred that the drying temperature be maintained below the boiling point until at least 90% by weight of the solvent has evaporated.

<Step S60, Step S70> Repetition The coating step S40 to the drying step S50 are repeatedly performed until a predetermined number k is reached. The number of repetitions k is preferably 2 times or more and 50 times or less, particularly preferably 3 times or more and 10 times or less.

If the number of repetitions k is equal to or greater than the lower limit, a positive electrode with a large surface area can be obtained. Moreover, if the number of repetitions k is equal to or less than the upper limit, productivity will not decrease.

FIG. 5 shows a photograph of the positive electrode before compression produced by the present production method. For comparison, FIG. 6 shows a photograph of a positive electrode with the same amount of sulfur supported by one application.

There are a plurality of openings on the surface of the positive electrode before compression manufactured by the present manufacturing method shown in FIG. When compared with the photograph of the surface of the porous plate shown in FIG. 4, it can be seen that the positive electrode active material is applied to the walls of the holes. In contrast, there are only crack grooves on the surface of the positive electrode of the comparative example shown in FIG.

<Step S80> Compression step The positive electrode (70 mm square) shown in FIG. 5 is compressed by a press device (pressure: 90 kN). For example, a 1 mm thick positive electrode is compressed into a 0.4 mm thick positive electrode 20. The compression conditions are such that the thickness is preferably 75% or less, particularly preferably 50% or less.

When the thickness change is within the above range, the battery 1 has a high energy density per volume and can be miniaturized. Furthermore, since the positive electrode 20 contains less electrolyte, the battery 1 has a higher energy density per weight, and can be made lighter. Furthermore, since the distance between the surface of the active material and the porous plate is shortened, adhesion is improved. Furthermore, since the electron conductivity is improved, the charge/discharge characteristics (rate characteristics) at high currents are improved.

As shown in FIG. 7, the surface of the positive electrode 20 of this embodiment has a large opening. Specifically, there are 0.2 or more openings/mm ² or more with an area of 100 μm ² or more. The aperture ratio of the surface of the positive electrode 20 is 5% or more.

A positive electrode obtained by compressing the positive electrode shown in FIG. 6 under the same conditions as the positive electrode 20 is shown in FIG. There are no openings on the surface. Therefore, in the positive electrode of the comparative example shown in FIG. 8, the area that acts as a positive electrode is approximately the same as the surface area of the two-dimensional surface. On the other hand, the specific surface area of the positive electrode 20 was 100 m ² /m ³ , which was 100 times the specific surface area of the flat positive electrode 0 (1 m ² /m ³ ). If the specific surface area of the positive electrode 20 is 10 m ² /m ³ or more, the effect of improving the energy density of the battery is significant.

A battery 1 is manufactured using the manufactured positive electrode 20. For example, an electrolytic solution is dropped onto the positive electrode 20 in a glove box under an argon atmosphere, and then the separator 30 is disposed on the positive electrode 20 . After adding an appropriate amount of electrolyte to the separator 30, the separator 30 was immersed in the electrolyte by holding at 60° C. for 60 minutes.

Examples of the separator 30 include a glass fiber separator that absorbs and retains electrolyte, a porous sheet made of resin, and a nonwoven fabric. The porous sheet is made of, for example, porous resin.

Examples of the resin constituting the porous sheet include polyolefins such as polyethylene (PE) and polypropylene (PP); a laminate having a three-layer structure of PP/PE/PP, polyimide, and aramid. In particular, polyolefin-based microporous separators and glass fiber separators are preferred as porous sheets because they are chemically stable to organic solvents and can suppress reactivity with electrolytes.

In battery 1, separator 30 is a porous polypropylene sheet with a thickness of 25 μm. The thickness of the separator, which is a porous sheet, is not limited, but for use as a secondary battery for driving a vehicle motor, it is preferably a single layer or multilayer with a total thickness of 4 μm to 60 μm. Further, the inner diameter of the micropores of the separator made of a porous sheet is preferably 10 μm or less at maximum (usually about 10 nm to 100 nm), and the porosity is preferably 20% to 80%.

The electrolytic solution was prepared by dissolving 1 mol of lithium salt in a solvent consisting of 0.5 mol of G1 (monoglyme) and 0.5 mol of G3 (triglyme), and adding hydrofluoroether (HFE: HF ₂ CF ₂ CH _{2 )} as an added solvent. This is a glyme-based solvated ionic liquid diluted with 4 moles of CO-CF ₂ CF ₂ H). The lithium salt is Li-TFSI (lithium (trifluoromethanesulfonyl)imide).

As the solvent for the electrolytic solution, aprotic organic solvents commonly used for lithium secondary batteries, such as dimethoxyethane, acetonitrile, tetrahydrofuran, dimethyl sulfoxide, butyrolactone, and sulfolane, are used. A solid electrolyte may be used instead of the electrolytic solution.

The battery 1 having the structure shown in FIG. 1 was produced by disposing the negative electrode 40 on the separator 30 above the positive electrode 20, and then injecting an electrolyte and enclosing it in a laminate pack 50.

The negative electrode 40 is a 60 μm metal lithium plate having a core material of an 8 μm thick copper plate. The negative electrode may contain at least one negative electrode active material selected from the group consisting of lithium, lithium alloy, and carbon capable of intercalating and deintercalating lithium. The negative electrode active material contained in the negative electrode absorbs and desorbs lithium ions.

In addition, in the manufacturing method of 1 A of batteries, arrangement|positioning of the positive electrode 20, the separator 30, and the negative electrode 40 is performed repeatedly.

<Battery characteristics>
The characteristics evaluation results of the battery 1 of the embodiment produced by the above method are shown below. For the charge/discharge evaluation, the cutoff potential was 1.0V-3.3V (vs. Li/Li ⁺ ), the charge/discharge rate was 0.01C, and the current density was 1.52 mA/cm ² .

As shown in FIG. 9, the capacity of battery 1 after 5 cycles was 1167 mAh/g-S. Note that the above capacity is defined by the amount of sulfur supported on the positive electrode 20. The capacity of a battery is proportional to the amount of sulfur it carries. As already explained, the amount of sulfur supported on the positive electrode 20 is 15 mg/cm ² , which is more than 10 times that of the flat positive electrode (supported amount of sulfur: 0.3 mg/cm ² ) having the same dimensions in plan view. Therefore, the capacity (energy density) defined by the planar area of the positive electrode 20 of the battery 1 is 10 times or more that of a battery having a flat plate current collector.

It goes without saying that the battery 1A, which includes a positive electrode laminate including six layers of positive electrodes 20, has a capacity six times that of the battery 1. For example, in battery 1A, the amount of sulfur supported in the positive electrode laminate was 15.4 mg/cm ² , the total capacity was 5.3 Ah, the gravimetric energy density was 269 Wh/kg, and the volumetric energy density was 430 Wh/L.

In addition, in order to make the specific surface area of the positive electrode 20 10 m ² / m ³ or more, the amount of solvent may be adjusted so that the viscosity (25° C.) of the slurry is 2 mPa/s or more and 70 mPa/s or less. preferable. Further, the solid content of the slurry is preferably 10 wt% or more and 20 wt% or less.

It goes without saying that the present invention is not limited to the embodiments and modifications described above, and that various modifications, combinations, and applications can be made without departing from the spirit of the invention.

1, 1A... Secondary battery 20... Positive electrode 30... Separator 40... Negative electrode 50... Laminate pack

Claims (6)

A step of mixing carbon particles and sulfur at a temperature higher than the melting point of sulfur to produce a mixture;
Adding a binder and a solvent to the mixture to create a slurry;
a coating step of coating a porous plate with the slurry;
a drying step of evaporating the solvent;
a compression step of pressing the porous plate to a thickness of 75% or less, in the above order,
A method for manufacturing a positive electrode for a secondary battery, characterized in that the coating step to the drying step are repeatedly performed. 2. The method for manufacturing a positive electrode for a secondary battery according to claim 1, further comprising a blowing step of blowing gas onto the porous plate between the coating step and the drying step. The method for manufacturing a positive electrode for a secondary battery according to claim 1 or 2, wherein the amount of sulfur supported on the positive electrode is 5 mg/cm ² or more. The positive electrode includes a perforated plate, a positive electrode active material disposed on an inner wall surface of the perforated plate, and an electrolyte,
The positive electrode active material includes sulfur-impregnated carbon particles and a binder,
The amount of sulfur supported on the positive electrode is 5 mg/cm ² or more,
A secondary battery characterized in that the surface of the positive electrode has 0.2 or more openings/mm ² or more with an area of 100 μm ² or more. The secondary battery according to claim 4, wherein the surface of the positive electrode has an aperture ratio of 5% or more. The secondary battery according to claim 5, comprising a positive electrode laminate including a plurality of positive electrodes.

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