JP7787540B2 - Method for manufacturing an electrode for an electric storage device, method for manufacturing short fibers for an electrode for an electric storage device, short fibers for an electrode for an electric storage device contained in a container, and electrode for an electric storage device - Google Patents

Description

The present invention relates to a method for manufacturing electrodes for electricity storage devices, a method for manufacturing short fibers for use in electrodes for electricity storage devices, short fibers for use in electrodes for electricity storage devices contained in a container, and electrodes for electricity storage devices.

Capacitors and secondary batteries are used in a variety of fields with the aim of reducing energy consumption and preventing global warming. In particular, the adoption of electrical energy in the automotive industry has accelerated development to further improve their performance.
Electric double layer capacitors have conventionally been used to back up memories in electronic circuits to which low voltages are applied, and have higher input/output reliability than secondary batteries.

For this reason, in recent years they have been used in power generation using natural energy sources such as solar and wind power, as well as for construction machinery, power supplies for momentary sags, and regenerative power supplies for trains. Their use in automobiles has also been considered, but the characteristics and cost did not meet the requirements, and their use in this field was not realized until recently. However, electric double-layer capacitors are now being used in electronically controlled brake systems, and their use as backup power sources for automotive electrical components, as starting energy supply for idling stop systems, brake control, power assist, and more is being considered.

An electric double layer capacitor is structured to consist of positive and negative electrodes, an electrolyte, and a separator to prevent short-circuiting between the opposing positive and negative electrodes. The electrodes are formed by applying multiple layers of a mixture of polarizable electrodes (currently mainly activated carbon), a binder to hold the activated carbon, and a conductive additive (mainly fine carbon particles) onto an aluminum foil (approximately 20 μm thick) current collector. Such an electric double layer capacitor is disclosed, for example, in Patent Document 1.

Charging of an electric double layer capacitor is achieved by electrolyte ions moving through the solution and adsorbing and desorbing onto the microporous surface of the activated carbon. An electric double layer is formed at the interface between the activated carbon powder and the electrolyte.
For reference, the particle size of normal activated carbon is, for example, about 4 to 8 μm, and the specific surface area is, for example, 1600 to 2500 m ³ /g. The electrolyte contains cations, anions, and a solvent, with tetraethylammonium salt used as the cation, tetrafluoroborate ions or the like used as the anion, and propylene carbonate, ethylene carbonate, or the like used as the solvent.

On the other hand, lithium-ion secondary batteries (lithium-ion batteries) are primarily composed of a positive electrode, a negative electrode, and a separator. The positive electrode is typically a current collector made of aluminum foil about 20 μm thick, coated to a thickness of about 100 μm with a mixture of active material powder (usually lithium cobalt oxide), a conductive additive, and a binder. The negative electrode is a current collector made of copper foil coated with a carbon material. These are separated by a separator such as polyethylene and immersed in an electrolyte to form a lithium-ion battery. Such lithium-ion batteries are disclosed, for example, in Patent Document 2.

Charging and discharging occur when lithium ions move between the positive and negative electrodes. When charging, lithium ions move from the positive electrode to the negative electrode, and charging is complete when the positive electrode runs out of lithium ions or the negative electrode can no longer store lithium ions. When discharging, the opposite occurs.

On the other hand, LiB (lithium ion battery) has been the most widely used in recent years, but the electrolyte salt (usually LiPF ₆ ) contained in the electrolyte solvent is a flammable liquid, posing a risk of fire and leakage. Furthermore, this organic electrolyte is said to cause decomposition of anions and various foreign molecules at the interface with the positive electrode, shortening the lifespan of LiB. Therefore, attempts have been made to replace the electrolyte with a solid electrolyte. Such batteries are called all-solid-state batteries because the positive electrode layer, negative electrode layer, and electrolyte layer are all solid. Although the term all-solid-state battery is not limited to the constituent materials of LiB, the term is often used to refer to all-solid-state LiB, as there is a lot of research into solidifying the electrolyte in LiBs.

The advantages of all-solid-state batteries include safety due to the flame-retardant electrolyte layer, a longer lifespan due to the fact that only lithium ions move through the electrolyte layer, making side reactions with anions and solvent molecules less likely to occur, and a wider operating temperature range due to the non-liquid electrolyte layer.

Furthermore, when trying to obtain a high-capacity, high-voltage battery using LiB, multiple individual cells must be connected together, but by using a solid electrolyte layer, it is possible to simply stack the positive electrode layer, solid electrolyte layer, and negative electrode layer in that order, which has the advantage of making it possible to produce a battery with a high energy density.

The performance of such devices is all determined by the exchange of ions and electrons. Therefore, the device's electrical capacity, charge/discharge speed, and lifespan depend on how smoothly or continuously the exchange of electrons can occur.

On the other hand, in an all-solid-state battery in which the electrolyte solution of a lithium ion battery is solidified, charging and discharging are performed by Li insertion and desorption.
For example, in Patent Document 3, a conductive elastic body is placed between the cells to suppress expansion and contraction, in Patent Document 4, a buffer layer is provided around the battery, and in Patent Document 5, various measures are taken, such as controlling the porosity.
Electrodes using a current collector made of metal fibers are also known, as disclosed in Patent Documents 6 and 7, for example.

Japanese Patent Application Laid-Open No. 2005-086113 Japanese Patent Application Laid-Open No. 2007-123156 JP 2008-311173 A JP 2015-111532 A Patent No. 5910737 Patent No. 6209706 JP 2018-106846 A

(Issues with electric double layer capacitors)
Electric double layer capacitors differ from secondary batteries, mainly lithium-ion batteries, in that they do not involve chemical reactions, have short charge storage times, and have fast current discharge times. Furthermore, in terms of energy density, electric double layer capacitors have only a few tens of Wh/L, compared to several hundred Wh/L for lithium batteries, which is an order of magnitude lower. It is because of these differences that electric double layer capacitors are being considered for use not only in energy storage but also as backup power sources for electrical equipment, as starting energy for idling stop systems, for brake control, and as power assist.

In recent years, electric double-layer capacitors have been developed for use in large-capacity power devices such as electric vehicles and energy generation systems. However, in order to efficiently transfer large amounts of energy into and out of the capacitor, there is a challenge: the capacitance must be increased and the internal resistance of the electrodes must be reduced.

(Issues with lithium-ion batteries)
On the other hand, secondary batteries, primarily lithium-ion batteries, have a relatively high energy density and can be used for long periods of time, making them suitable for use in a variety of fields, including mobile devices. In recent years, they have come to be used in automobiles, heavy machinery, energy, and other fields, and there is a demand for larger batteries to increase capacity. However, increasing battery size presents many challenges, including capacity issues, charging speed, lifespan, reliability, and manufacturing difficulties. For example, the capacity of batteries used in mobile phones is approximately 15 Wh, while the capacity of batteries used in hybrid vehicles is in the tens to hundreds of kWh range. The difference between the two is several thousand times, resulting in the aforementioned challenges.

The reaction in lithium-ion batteries is a reversible chemical reaction, and the active material expands and contracts when the electrodes are charged and discharged. This causes the active material to peel off from the current collector, degrading the charge and discharge characteristics. In other words, the battery does not always charge and discharge at the same rate, resulting in a decrease in charge and discharge capacity. Since batteries in hybrid and electric vehicles are used for many years, it is necessary to prevent the peeling between the current collector and active material in order to prevent this deterioration.

Furthermore, one of the biggest issues with lithium-ion batteries is their internal resistance. Internal resistance can be thought of as the resistance to lithium ions moving through the electrolyte between the positive and negative electrodes inside the battery, or the resistance to the exchange of electrons between materials. In other words, it is caused by the adhesion between materials, the exchange of electrons between them, and the ease with which lithium ions flow through the electrolyte.
In order to increase the size, i.e., the capacity, by applying a large amount of active material to the current collector, the capacity increases, but the resistance to lithium ion migration increases, so there is a limit to the thickness of the active material layer.
As the active material layer becomes thicker, its internal resistance slows down the charge/discharge rate. Reducing the coating thickness reduces the internal resistance, increasing the charge/discharge rate, but also reducing the capacity. Therefore, capacity is increased by stacking multiple current collectors (electrode foils) coated with active material or by increasing the area of the current collector coated with active material.

The speed of charging and discharging is also related to the amount of lithium ions generated. If more ions can be produced and moved at once, the charging and discharging speeds will be faster. Since the chemical reaction in a secondary battery occurs at the interface between the electrolyte and active material, increasing the contact area between the electrode and electrolyte will also improve the charging and discharging speeds.

Current efforts to reduce internal resistance are being made to improve additives, conductive additives, and active materials, as well as to apply fine carbon particles to the current collector beforehand. The shape of the current collector is also being improved, as mentioned above, by making it as thin as possible and by forming fine holes in the foil to increase the surface area. Similarly, research and development is being conducted on electric double layer capacitors, including improving activated carbon and additives, and increasing the contact area between the current collector and active material layer.

(Challenges of all-solid-state batteries)
On the other hand, all-solid-state batteries, in which the electrolyte of a lithium-ion battery is solidified, are charged and discharged by Li intercalation and deintercalation. During this process, the host crystal lattice repeatedly expands and contracts. If the resulting volume change is large, contact between the particles of the active material, solid electrolyte, and conductive additive is severed, reducing the amount of effective active material. In the case of all-solid-state batteries, even repeated charge and discharge at a current of about one-tenth to two-tenths of the battery capacity results in the intercalation and deintercalation of nearly one mole of Li, and the volume expansion and contraction rate reaches nearly 10%. Therefore, various methods have been proposed to suppress this expansion and contraction.

In this way, if we can reduce internal resistance in power storage devices to solve the challenge of increasing capacity, and if we can prevent the active material from peeling off from the current collector due to expansion and contraction in all-solid-state batteries, there is potential for significant improvement.

The present inventors have therefore discovered a method for further finely grinding fibers to a length of approximately a few millimeters during stirring in the production of electricity storage devices, and have also found that using this method can improve the characteristics of electricity storage devices. Furthermore, they have discovered that electricity storage devices can also be improved by using powdered metal produced by atomizing metal powder, which is produced by spraying molten metal through fine holes.

A method for manufacturing an electrode for an electricity storage device according to a first aspect of the present invention includes a fiber preparation step for obtaining aluminum or copper metal fibers; a stirring step for placing the metal fibers or the metal fibers cut with a predetermined cutting tool in a solvent and stirring the solvent to shorten the metal fibers to an average length of 0.8 mm or less; a molding step for forming a liquid or gel slurry containing the aluminum or copper short fibers produced by the stirring step, an adsorbent powder that adsorbs electrolyte ions during charging or an active material powder that chemically reacts during charging and discharging, and a binder into a predetermined shape; and a drying step for drying the slurry formed into the predetermined shape to form an electrode containing the short fibers.

A second aspect of the present invention relates to a method for producing short fibers for use in electrodes of an electricity storage device, which comprises a fiber preparation step for obtaining aluminum or copper metal fibers; a stirring step for placing the metal fibers or the metal fibers cut with a specified cutting tool in a solvent and stirring the solvent to shorten the metal fibers to an average length of 0.8 mm or less; and a shipping step for shipping the aluminum or copper short fibers produced by the stirring step in the solvent or another solvent.

The third aspect of the present invention provides short fibers for use in an electrode of an electricity storage device housed in a container, which comprises a solvent containing aluminum or copper short fibers having an average length of 0.8 mm or less, and a container that is resistant to the solvent and that contains the solvent.

The fourth aspect of the present invention relates to an electrode for an electricity storage device that includes short aluminum or copper fibers having an average length of 0.8 mm or less, and either an adsorbent powder that adsorbs electrolyte ions during charging or an active material powder that undergoes a chemical reaction during charging and discharging, and the weight ratio of the short fibers is less than 9.75% by weight.

An electrode for an electricity storage device according to a fifth aspect of the present invention is an electrode for an electricity storage device comprising a metal powder having an average particle size of 0.1 mm or less, and an adsorbent powder to which electrolyte ions are adsorbed during charging or an active material powder that undergoes a chemical reaction during charging and discharging, wherein the weight ratio of the metal powder is less than 9.75% by weight.
An electrode for an electricity storage device according to a sixth aspect of the present invention is an electrode for an electricity storage device, comprising a mixture of aluminum or copper fibers and metal powder having an average particle size of 0.1 mm or less, and an adsorbent powder to which electrolyte ions are adsorbed during charging or an active material powder that undergoes a chemical reaction during charging and discharging,
The weight ratio of the mixture is less than 9.75% by weight.

The present invention contributes to improving battery performance by using the above-mentioned short fibers and metal powder.

FIG. 2 is a cross-sectional image diagram of an electrode according to one embodiment of the present invention. FIG. 10 is a cross-sectional image diagram of another electrode according to the present embodiment. 1 is an electron microscope photograph of short fibers of the present embodiment. 1 is an electron microscope photograph of the metal powder of the present embodiment. 1 is a schematic diagram showing a method for producing a slurry according to an embodiment of the present invention. FIG. 1 is a schematic cross-sectional view of an all-solid-state battery using the electrode of the present embodiment. 1 is a graph showing the characteristics of a lithium ion battery using an electrode according to an embodiment of the present invention. 1 is a graph showing the cycle characteristics of a lithium ion battery using an electrode according to an embodiment of the present invention. 1 is a graph showing the characteristics of a lithium ion battery using the electrode of the present embodiment. 1 is a graph showing the characteristics of a lithium ion battery using an electrode according to an embodiment of the present invention.

An electrode and an electricity storage device according to an embodiment of the present invention will be described below with reference to the drawings. Note that the description will be made mainly using a lithium ion battery as the electricity storage device.
1 and 2 show electrodes (positive and negative electrodes) containing aluminum or copper short fibers A having an average diameter of 50 μm or less. Instead of the short fibers A, metal powder made of aluminum, copper, or a metal having conductivity equal to or lower than these may be mixed in.
1 and 2 are diagrams for clearly illustrating the configuration of this embodiment, and the size, thickness, length, and compounding ratio of the short fibers A, the active material powder 20, the conductive additive 30, the carbon fibers CF, etc. are different from the actual ones.

As shown in Figures 1 and 2, this electrode comprises aluminum or copper short fibers A with an average wire diameter of 50 μm or less or metal powder with an average particle size of 0.1 mm or less, active material powder 20 that is held together with the short fibers A or metal powder by binder B and undergoes a chemical reaction during charging and discharging, and optionally a conductive additive 30 held by binder B. Note that when the metal powder is used instead of the short fibers A, the metal powder will be present in place of the short fibers A in Figures 1 and 2, but because the shape of the metal powder differs from that of the short fibers A, the presence of the metal powder may differ slightly from that of the short fibers A.

[Short fiber molding]
A bundle of metal long fibers (aluminum or copper long fibers, with an average fiber diameter of 50 μm or less, preferably 25 μm or less, and more preferably 20 μm or less) produced by the coil cutting method is sandwiched between a pair of paper or plastic films, each having a thickness of, for example, 0.1 mm to 0.3 mm, and the long fiber bundle is cut to a desired length. That is, the long fiber bundle is pressed against a predetermined surface using a pressing member made of plastic, thin metal foil, paper, or other thin material, and the long fibers are cut using a cutting tool such as a cutting blade. In this embodiment, the predetermined surface is the upper surface of the plastic film, but it may also be the upper surface of a work table made of plastic, metal, or other thin material such as foil or paper. Preferably, the plastic film has a thickness of 1 mm or less. Alternatively, the long fiber bundle is placed inside a cylindrical plastic tube and cut to a desired length. The wall thickness of the plastic tube is, for example, 0.1 to 0.3 mm, preferably 0.5 mm or less. This allows for the production of metal fibers of a desired length, for example, 1 to 3 mm.

That is, the long fiber bundle is cut while it is placed inside a tubular member made of a thin material such as plastic, metal foil, or paper. The long fiber bundle may be cut together with the pressing member or the tubular member. This allows for stable and efficient cutting of the long fiber bundle. It is also preferable that the long fiber bundle be arranged relatively densely inside the tubular member. There may also be cases where the long fiber bundle is cut without using a pressing member or tubular member.

Here, in the coil cutting method, aluminum or copper foil is wound into a coil shape, and the end face of the coil is cut with a cutting tool to obtain long metal fibers (long metal fibers).
The metal fibers of this embodiment or short fibers A described below may contain small amounts of other metals such as steel, stainless steel, brass, etc.

The metal fibers produced by the coil cutting method and cut as described above have a cross section including a side corresponding to the surface of the foil and a side cut with a cutting tool. The area between the side corresponding to the surface and the cut side forms a corner, a wide V-shaped cross section, a narrow V-shaped cross section, or a shape close to a V-shaped cross section. This is expected to facilitate the short fibers A pushing through the binder B and the like in the various steps described below, making contact with the active material powder 20, other short fibers A, and the like. Alternatively, it is expected that the short fibers A will be able to easily penetrate into other short fibers A, active material powder 20, and the like.
The coil cutting method is a method for producing long fibers with a uniform shape by cutting a coiled metal foil from its end face. Therefore, the cross section of the fiber is a uniform shape, such as a square, rectangle, or trapezoid, that is the thickness of the foil and the cutting depth. Short fibers with uniform cross-sectional shape and length can improve battery performance. This method can also be applied to any metal that can be made into foil.
This method can produce short fibers of any length as long as they can be cut, but it takes time, is difficult, and increases costs.Furthermore, there is a risk of explosion due to dust when handling the fine fibers.
It is also possible to manufacture the metal fibers or short fibers A by cutting aluminum, copper, or the like using other methods such as chatter processing. It is also possible to manufacture the metal fibers by using a melt-blowing method or the like that does not involve cutting.

The metal fibers obtained by the above process are placed in a planetary centrifugal mixer (mixing device) along with two or more ceramic balls with a diameter of 10 mm and a solvent (e.g., NMP) used to form the electrode described below. The planetary centrifugal mixer is then rotated at 1,000 rpm for a predetermined time (e.g., approximately 20 minutes), stirring the solvent to obtain short fibers A with an average length of approximately 200 μm and the shape shown in Figure 3. The solvent may contain binder B described below, or it may contain binder B described below and other compounds described below. While conditions vary depending on the type of metal, Figure 3 shows an SEM photograph of aluminum short fibers A.

The average length of the short fibers A is preferably 0.8 mm or less, more preferably 0.5 mm or less, and even more preferably 0.3 mm or less.
Furthermore, as the stirring device, it is also possible to use a known stirring device that can accommodate the metal fibers and the solvent inside and can cut the metal fibers by rotating a rotating body such as a blade or inner container arranged inside.
When the metal fibers are made of copper, it is possible to produce copper short fibers A by using a stirring device with a rotor such as a blade, ceramic balls, etc., without adding the solvent.

On the other hand, Figure 4 shows an SEM photograph of aluminum metal powder formed by atomization. As shown in Figure 4, the metal powder formed by atomization has an irregular shape. Therefore, the particle size of each particle can be, for example, the maximum length of each particle, or the average of the maximum and minimum lengths. The average particle size can also be the average particle size of five randomly selected particles. Metals other than aluminum can also be powdered by atomization. Metal powder can also be produced by known methods other than atomization. The average particle size of the metal powder in Figure 4 is approximately 40 μm or 50 μm. To achieve the battery performance described below, the average particle size of the metal powder is preferably 10 μm or more, and more preferably 20 μm or more. To prevent clogging of the slurry applicator described below, the average particle size of the metal powder is preferably 80 μm or less, and more preferably 60 μm or less to more effectively prevent clogging. Metal powders within these particle size ranges are expected to be more easily dispersed in the slurry S. Furthermore, if the average particle size of the metal powder is 0.1 mm or less, it is possible to manufacture electrodes with the functions described below.

When a container containing a solvent containing short fibers A formed as described above is sold, it is possible to deliver the short fibers A to customers while preventing oxidation. This solvent may be the same solvent that is added to the agitator together with the metal fibers, or it may be another solvent. A purchaser of short fibers A in a container containing solvent can use the short fibers A together with the solvent or the short fibers A removed from the solvent to manufacture electrodes for electricity storage devices, as described below. If the solvent in the container is the same solvent that is added to the agitator together with the metal fibers, this solvent is used to manufacture electrodes, and therefore this solvent can also be used to manufacture electrodes for electricity storage devices.

[Electrode molding]
When forming the electrode shown in FIG. 1 , as shown in FIG. 5 , active material powder 20, conductive additive 30, and binder B are added to a solvent (NMP) containing short fibers A, and the mixture is stirred to form a liquid or gel-like slurry S. To prepare the slurry S, aluminum or copper short fibers A, active material powder 20, conductive additive 30, and diluted binder B are kneaded using a kneader or the like. Instead of producing short fibers A using the stirring device, active material powder 20 and binder B can be added to a solvent containing the metal fibers and stirred to cut the metal fibers into short fibers A. The dry weight ratio of active material powder 20, conductive additive 30, and binder B is sufficient as long as it is approximately the same as that of commercially available secondary batteries. Hereinafter, the weight ratio (compounding ratio) refers to the weight ratio (compounding ratio) relative to the entire electrode (excluding the electrode foil) after drying in the drying process described below.

In this embodiment, the blending ratio of binder B is 1% by weight or more and less than 6% by weight, preferably around 2-3% by weight. The blending ratio of short fiber A is preferably 1% by weight or more and less than ⁸ % by weight. A small amount of short fiber A has little effect on internal resistance, while a large amount of short fiber A actually increases internal resistance. Furthermore, exceeding this range results in a decrease in the basis weight (weight of active material powder 20 per 1 cm2 of current collector foil). The blending ratio of short fiber A is optimally 2.0% by weight or more and 5.5% by weight or less. Note that a blending ratio of short fiber A of 3.0% by weight or more and 5% by weight or less is considered most preferable, but this blending ratio varies depending on the basis weight and the amount of binder. For example, when increasing the basis weight, it is better to increase the amount of short fiber A, and it is better to increase the amount of short fiber A as the amount of binder B is reduced. When preparing the electrode shown in FIG. 2, carbon fiber CF is further added to the above to prepare slurry S. When carbon fiber CF is added, its blending ratio is preferably 1% by weight or more and less than 2% by weight. If the amount of carbon fiber CF is too small, the effect will not be apparent, and if the amount of carbon fiber CF is too large, it may cause deterioration in cycle tests.

The aluminum or copper short fibers A have an average length of less than 1 mm, and the average particle size of the aluminum powder produced by the atomization method is also 0.1 mm or less, so they are easily mixed with other ingredients in the slurry S.

Next, the slurry S is applied to a current collector foil (aluminum foil or copper foil) to a predetermined thickness, followed by pre-drying, for example, to increase viscosity. Low-temperature drying may be followed by vacuum drying. This evaporates almost all of the solvent (e.g., NMP), leaving the active material solidified via binder B. This is then press-molded to form a sheet with a positive or negative electrode placed on top of the current collector foil. This sheet is called electrode foil, and the positive and negative electrodes are called positive and negative foils, respectively. If a current collector foil is not used, the slurry S can be applied to a separator. For example, the positive electrode slurry S is applied to one thickness side of the separator, and the negative electrode slurry S is applied to the other thickness side of the separator. The positive or negative electrode slurry S may also be applied to only one side of the separator. The pre-drying dries the material to a state where the binder B is not completely hardened, making it easier to form the electrode foil into the desired shape.

Furthermore, it is also possible to form the positive or negative electrode slurry S into a sheet in a designated mold without applying it to a current collector foil or separator. In this case, the positive or negative electrode formed into a sheet is called an electrode foil.

These short fibers A and aluminum or copper metal powders preferably have a purity of 99% or more, and more preferably 99.9% or more. When the aforementioned aluminum or copper short fibers A or metal powder are used, the short fibers A or metal powder have relatively low rigidity during press molding, so they are slightly crushed by the active material 20, etc., causing deformation that bites into the active material 20.

That is, the short fibers A and the metal powder are flattened at the contacting portions, thereby bringing the short fibers A and the metal powder into closer contact with the active material powder 20, leading to a reduction in resistance to electron transfer.
In one example, a drying step is performed in which the formed electrode foil is dried by vacuum drying or the like, thereby removing moisture from the electrode and hardening the binder B, thereby completing the electrode.

[Cathode active material powder]
The positive electrode active material powder 20 may be any material that can be held by the short fibers A by the binder B or the like, or that can be held by the binder B hardened together with the short fibers A, and is preferably one with excellent cycle characteristics. Examples of the positive electrode active material powder 20 include lithium cobalt oxide (LiCoO ₂ ), lithium iron phosphate (LiFePO ₄ ), lithium manganese oxide (LiMn ₂ O ₄ ), and ternary systems containing mixtures thereof, which are used in general lithium ion batteries. It is possible to use known active materials that are used for the positive and negative electrodes of secondary batteries.

[binder]
Thermoplastic resins, polysaccharide polymer materials, etc. can be used as binder B. Examples of materials for binder B include polyacrylic resins, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), copolymers of vinylidene fluoride (VdF) and hexafluoropropylene (HFP), etc. It is possible to use known binders that are used in electrodes of secondary batteries and electric double layer capacitors.

[Conductive additive]
The conductive additive 30 may be any material that is conductive, and is preferably a material that does not undergo chemical changes due to electrolytes or solvents. Examples include graphite and carbon black. It is also possible to use known conductive additives that are used in electrodes of secondary batteries and electric double layer capacitors.

[Negative electrode active material powder]
Artificial graphite, natural graphite, and the like are used as the negative electrode active material powder 20. When these are used in the negative electrode, the operating potential on the negative electrode side is low, so when aluminum short fibers A or metal powder are used, a Li-Al reaction occurs. This causes a decrease in Li and rapid battery degradation. For this reason, aluminum short fibers A or metal powder cannot be used, and copper short fibers A or metal powder must be used. On the other hand, LTO (lithium titanate), which has been developed in recent years and has a higher reaction potential than the Li-Al reaction, can be used as the negative electrode active material powder 20. In this case, aluminum short fibers A or metal powder can be used as the negative electrode active material powder 20. It has been found that when aluminum short fibers A or metal powder are used in an amount of 1% by weight or more but less than 9.75% by weight, preferably less than 8% by weight, and more preferably 3.0% by weight or more but less than 5% by weight, the internal resistance can be significantly reduced when LTO or the like is used as the negative electrode active material powder 20.

[Applications to other devices]
The above-described electrode manufacturing method and electrode can be applied to electrodes of power storage devices such as electric double layer capacitors, secondary batteries including all-solid-state batteries, and hybrid capacitors including lithium-ion capacitors. For example, positive and negative electrodes of capacitors such as electric double layer capacitors can be produced by kneading activated carbon, conductive additive 30, and diluted binder B, instead of active material powder 20, with short fibers A or metal powder. The material used in electric double layer capacitors instead of active material powder 20 can also be considered an adsorbent powder that adsorbs electrolyte ions during charging and discharging. Well-known active material powders used in capacitors such as lithium-ion capacitors can also be used. Electrodes for all-solid-state batteries can be produced, for example, by kneading a solid electrolyte with active material powder 20, conductive additive 30, diluted binder B, and short fibers A or metal powder to form a slurry S ( FIG. 6 ). Note that electrodes such as those shown in FIGS. 1 and 2 can also be used in all-solid-state batteries.

When producing the electrode shown in FIG. 2, it is also possible to use a slurry S containing active material powder 20, conductive additive 30, binder B, and carbon fibers CF with an average thickness of 0.5 μm or less, preferably 0.3 μm or less. In this case, as shown in FIG. 2, the carbon fibers CF are arranged between the short fibers A, and a synergistic effect can be expected in the exchange of electrons between the short fibers A or metal powder and the active material powder 20.

The carbon fibers CF are in contact with the short fibers A or metal powder, the active material powder 20, the conductive additive 30, and other carbon fibers CF. In this embodiment, carbon fibers CF having an average thickness of 0.1 μm or more and less than 0.2 μm and an average length of 20 μm or more and less than 200 μm are used. In one example, the resistivity of the carbon-based conductive additive 30 is 0.1 to 0.3 Ω·cm, while the resistivity of the carbon fibers CF is, for example, 5×10 ⁻⁵ Ω·cm.

For example, even when the active material powder 20 and the short fibers A or the metal powder are not in direct contact with each other, the active material powder 20 and the short fibers A or the metal powder are electrically connected via the carbon fibers CF. Furthermore, even when the active material powder 20 and the short fibers A or the metal powder are in direct contact with each other, the connection by the carbon fibers CF further reduces the electrical resistance between the active material powder 20 and the short fibers A.
In this way, the carbon fiber CF, which has good conductivity, can reduce the resistance to electron movement between the active material powder 20 and the short fiber A or metal powder, which is advantageous in reducing the resistance of electrons moving to the input/output terminals.

[Application to coin-type secondary batteries]
The following is an example of application to an electricity storage device. Regarding application to a coin-type secondary battery, which is an electricity storage device, for example, it is possible to manufacture a coin-type battery using the short fiber A or metal powder of the present invention instead of the blown fiber or chattered fiber in JP-A-2018-106846 by using the structure described in JP-A-2018-106846.

[Application to stacked secondary batteries]
In the case of a secondary battery in which a storage unit consisting of a positive electrode, a negative electrode, and a separator is stacked in multiple layers, the electrode structure of the above embodiment can be used for only the positive electrode, only the negative electrode, or both the positive electrode and the negative electrode, as in the coin-type secondary battery.

[Application to all solid-state batteries]
The application to all-solid-state batteries will be described below.
Hereinafter, an electricity storage device (all-solid-state battery) will be described using an example of an all-solid-state lithium ion battery. Note that this embodiment can be used for all-solid-state batteries such as all-solid-state sodium ion secondary batteries, all-solid-state magnesium ion secondary batteries, and air batteries using a solid electrolyte, or all-solid-state lithium ion batteries using a Si (silicon) negative electrode that undergo significant expansion and contraction.

An example of a schematic diagram of the structure of an all-solid-state battery is shown in Figure 6. An all-solid-state battery roughly comprises a positive electrode collector 1, a positive electrode layer 2, a solid electrolyte layer 3, a negative electrode layer 4, and a negative electrode collector 5. The positive electrode layer 2 corresponds to the positive electrode mentioned above, and the negative electrode layer 4 corresponds to the negative electrode mentioned above.

(current collecting foil)
The positive electrode current collector 1 is generally a metal foil, and the metal foil can be formed of aluminum, an alloy of multiple metals, or the like. Copper foil is generally used for the negative electrode current collector 5. When the negative electrode active material powder 20 is made of lithium titanate, aluminum foil can be used for the negative electrode current collector 5. In the case of an all-solid-state battery, conductive materials such as SUS (stainless steel) and cobalt-based alloys, which have high voltage resistance and corrosion resistance, other conductive materials, conductive ceramics, and the like may also be used.

(positive electrode layer)
In order to increase discharge capacity, the material of the positive electrode active material powder 20 is preferably a substance that easily absorbs lithium ions, such as LiCoPO, _LiCoO2 , _LiMnO4 , or _LiFePO4 . A slurry S is prepared by adding, as needed, a conductive additive 30, a binder B, and a powdered solid electrolyte 14 such as an inorganic solid electrolyte or a polymer electrolyte to increase ionic conductivity, and the slurry S is then sintered or pressure-molded to form the positive electrode layer 2. In this embodiment, the positive electrode layer 2 contains short fibers A or metal powder made of aluminum, copper, or the like.

(Negative electrode layer)
As with the positive electrode layer 2, the material for the negative _electrode active material powder 20 is preferably a substance that easily absorbs ions in order to increase discharge capacity, such as _LiFePO4 , _Li4Ti5O12 , _Li4Fe4 ( _{PO4 )3 ,} SiOx, _Cu6Sn5 , _{and LiTiO4} . The negative electrode layer 4 is formed by mixing the active material powder 20, a conductive additive 30, a binder B, and _a powdered solid electrolyte 24 such as an inorganic solid electrolyte or a polymer electrolyte to increase ion conductivity to form a slurry S, and then sintering or pressure-molding the slurry S. In this embodiment, the negative electrode layer 4 contains short fibers A or metal powder made of aluminum, copper, or the like.

In the case of the positive electrode layer 2 and negative electrode layer 4 of an all-solid-state battery, the blending ratio of short fiber A is 1% by weight or more and 9.75% by weight or less, preferably 1% by weight or more and less than 8% by weight, and optimally 3% by weight or more and 6% by weight or less. Furthermore, the blending ratio of binder B is preferably 2% by weight or more and less than 5% by weight.

(Forming of positive electrode layer)
First, a liquid or gel-like slurry S is prepared containing short fibers A or metal powder, active material powder 20 that chemically reacts during charge and discharge, conductive additive 30, binder B, and solid electrolyte 14. The slurry S is prepared by kneading a mixture of aluminum or copper short fibers A or metal powder, active material powder 20, conductive additive 30, diluted binder B, and solid electrolyte 14. During charge and discharge, chemical reactions occur in which ions such as lithium ions are released from the active material powder 20 of the positive electrode into the electrolyte, and in which ions such as lithium ions are taken up by the active material powder 20.

Next, the slurry S is applied to a current collector or onto a predetermined mold or the like, and after drying at low temperature or vacuum drying, it is pressed to form an electrode sheet.
The current collector (current collecting foil) coated with the slurry S is dried at a low temperature and vacuum dried, and then the slurry S is passed between rollers to apply pressure to the slurry S, thereby forming the slurry S into a predetermined thickness according to the size of the electrode. The formed slurry S is then cut and formed into a predetermined shape (size).

The compounding ratio of the active material powder 20 and solid electrolyte 14 in the positive electrode layer 2 is preferably 80% by weight or more, more preferably 85% by weight or more, and even more preferably 90% by weight or more. Of these, the compounding ratio of the active material powder 20 is preferably 70% by weight or more, and more preferably 75% by weight or more. Meanwhile, the compounding ratio of the solid electrolyte 14 is preferably 10% by weight or more, and more preferably 15% by weight or more. The compounding ratio of short fiber A in the positive electrode layer 2 is preferably 3% by weight or more, more preferably around 5% by weight, and preferably less than 8% by weight. The remaining components are the conductive additive 30, binder B, carbon fiber CF, etc.

(Forming of negative electrode layer)
First, a liquid or gel-like slurry S is prepared containing short fibers A, active material powder 20 that chemically reacts during charging and discharging, conductive additive 30, binder B, and solid electrolyte 24. The slurry S is prepared by kneading a mixture of aluminum or copper short fibers A or metal powder, active material powder 20, conductive additive 30, diluted binder B, and solid electrolyte 24.

Next, the slurry S is applied to a current collector or onto a predetermined mold or the like, and after drying at low temperature or vacuum drying, it is pressed to form an electrode sheet.
The current collector (current collecting foil) coated with the slurry S is dried at a low temperature and vacuum dried, and then the slurry S is passed between rollers to apply pressure to the slurry S, thereby forming the slurry S into a predetermined thickness according to the size of the electrode. The formed slurry S is then cut and formed into a predetermined shape (size).

In the case of an all-solid-state capacitor that forms an electric double layer, it is also possible to use an adsorbent powder that adsorbs electrolyte ions during charging in place of the active material powder 20 in the positive or negative electrode.

The compounding ratio of the active material powder 20 and solid electrolyte 24 in the negative electrode layer 4 is preferably 80% by weight or more, more preferably 85% by weight or more, and even more preferably 90% by weight or more. Of these, the compounding ratio of the active material powder 20 is preferably 70% by weight or more, and more preferably 75% by weight or more. Meanwhile, the compounding ratio of the solid electrolyte 24 is preferably 10% by weight or more, and more preferably 15% by weight or more. The compounding ratio of short fiber A in the negative electrode layer 4 is preferably 3% by weight or more, more preferably about 5% by weight, and preferably less than 8%. The remaining components are the conductive additive 30, binder B, carbon fiber CF, etc.

(Forming of solid electrolyte layer)
A liquid or gel-like slurry is prepared by kneading a mixture containing the powdered solid electrolyte 34 and the binder B, and the slurry is then dried, pressed in a mold, pressed using a roller, or the like to obtain the solid electrolyte layer 3 having a predetermined thickness. The solid electrolyte layer 3 may be a known solid electrolyte layer. Alternatively, the solid electrolyte layer 3 may be a known gel-like solid electrolyte layer.

(Forming of electricity storage element)
The positive electrode layer 2, solid electrolyte layer 3, and negative electrode layer 4 prepared as described above are stacked and pressurized to bond the positive electrode layer 2 and the solid electrolyte layer 3, and to bond the solid electrolyte layer 3 and the negative electrode layer 4. If necessary, a positive electrode current collector 1 is stacked on the positive electrode layer 2, and a negative electrode current collector 5 is stacked on the negative electrode layer 4. This forms an electricity storage element BE consisting of the positive electrode layer 2, solid electrolyte layer 3, and negative electrode layer 4. By stacking the required number of electricity storage elements BE as shown in FIG. 6 , an all-solid-state battery is formed.

(solid electrolyte)
The solid electrolytes 14, 24, and 34 are present throughout the battery, similar to the electrolyte solution in a lithium-ion battery (LiB), and transfer ions between the positive electrode layer 2 and the negative electrode layer 4. The solid electrolytes 14, 24, and 34 may be made of the following inorganic solid electrolytes or polymer solid electrolytes.

Examples of inorganic solid electrolytes that can be used include nitrides, halides, and silicides of Li (lithium), such as _Li3N , LiI, _Li3N - _LiI - _LiOH , _LiSiO4 , _LiSoO4 - _LiI - _LiOH , _Li3PO4 -Li4SiO4, and _Li2SiS3 . Alternatively, a lithium-containing phosphate compound having a Nasicon structure, with the chemical formula _LixMy ( _PO4 ) _{3 ,} can be used. Materials in which x is 1≦x≦2 and y is 1≦y≦2, and M is composed of Al, Ti, Ge, Ga, or the like, can be used. P can also be replaced with Si or B.

As the polymer solid electrolyte, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, etc. can be used.
When the short fibers A are uniformly dispersed in the electrode of the all-solid-state battery, the short fibers A structurally suppress large deformations and the like within the electrode, and when the host crystal lattice repeatedly expands and contracts, the short fibers A prevent the active material powder 20 from losing contact with other components.

[Example]
An example will be given below in which the above-mentioned metal fibers cut to a length of 1 to 3 mm as described above are used in the positive electrode of a lithium ion battery.
The aluminum metal fibers, active material powder 20 (NMC; ternary Li(Ni _a Mn _b Co _1-a-b )O ₂ ), conductive additive 30 (AB; acetylene black), and binder B (PVDF; vinylidene prefluoride) diluted with a solvent (NMP; normal methylpyrrolidone) were used to prepare a slurry S in the weight ratio (weight ratio after drying) of active material powder 20: conductive additive 30: binder B: aluminum short fiber A of 87:5:4:4. The slurry S was then applied to an aluminum foil to prepare a positive electrode foil as described above, and a negative electrode foil not containing metal fibers such as short fiber A. A negative electrode foil containing, for example, LiPF ₆ was used as the electrolyte. A lithium ion battery was fabricated using lithium hexafluorophosphate (lithium hexafluorophosphate) (Example 1). Furthermore, a lithium ion battery was fabricated in which 1.5 wt % of carbon fiber CF was added to the slurry S described above when fabricating the positive electrode of the lithium ion battery of Example 1 (Example 2). By adding 1.5 wt % of carbon fiber CF, the weight ratio of the metal fibers, i.e., active material powder 20:conductive additive 30:binder B:aluminum, was approximately 85.8:4.9:3.9:3.9.

In addition, a lithium-ion battery was also produced in which the aluminum or copper metal fibers were not added to the slurry S described above when making the positive electrode, as in Example 1 (Comparative Example 1). By not adding the metal fibers, the weight ratio of active material powder 20:conductive additive 30:binder B was approximately 90.6:5.2:4.2.

To produce the negative electrode foils of Examples 1, 2, and Comparative Example 1, a slurry S was first prepared using active material powder 20 (natural spherical graphite), conductive additive 30 (AB; acetylene black), and binder B (PVDF; vinylidene prefluoride) diluted with a solvent (NMP; normal methylpyrrolidone) in a weight ratio of 90:5:5 (active material powder 20:conductive additive 30:binder B). The slurry S was then applied to copper foil to produce the negative electrode foils as described above.

A lithium-ion battery was also fabricated using commercially available positive and negative electrode foils (Comparative Example 2). These positive and negative electrode foils are said to have high performance among commercially available positive and negative electrode foils. The material of the positive electrode active material powder in Comparative Example 2 was the same as the material of the active material powder 20 in Examples 1, 2, and Comparative Example 1, and the material of the negative electrode active material powder in Comparative Example 2 was the same as the material of the active material powder 20 in Examples 1, 2, and Comparative Example 1.

To fabricate the lithium-ion batteries of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the positive and negative electrode foils were cut to a predetermined size, a separator made of polypropylene was sandwiched between the positive and negative electrode foils, and these were then housed in a battery container made of a laminated sheet of plastic material. The electrolyte was poured into the battery container, and the battery container was sealed, thereby fabricating the lithium-ion batteries of Example 1, Example 2, Comparative Example 1, and Comparative Example 2. Note that the sizes of the positive and negative electrode foils, the size and thickness of the separator, and the capacity of the battery container are substantially the same for Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

The average thickness of the positive electrodes (average thickness of the positive electrode layer) of Example 1, Example 2, and Comparative Example 1, excluding the thickness of the aluminum foil (collector foil), is approximately 155 μm, and the thickness of the aluminum foil of the positive electrodes of Example 1, Example 2, and Comparative Example 1 is approximately 15 μm. The average thickness of the positive electrodes (average thickness of the positive electrode layer) of Comparative Example 2, excluding the thickness of the aluminum foil (collector foil), is approximately 90 μm, and the thickness of the aluminum foil of the positive electrode of Comparative Example 2 is approximately 15 μm. The average thickness of the negative electrodes (average thickness of the negative electrode layer) of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, excluding the thickness of the copper foil (collector foil), is approximately 85 μm, and the thickness of the copper foil of the negative electrodes of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 is approximately 15 μm.

In addition, a commercially available 5000 mAh lithium ion battery was used as Comparative Example 3.
Charge and discharge tests were carried out on the lithium ion batteries of Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

FIG. 7 shows the discharge characteristics (rate characteristics) of Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3.
Because aluminum containing the above-mentioned metal fibers has low internal resistance, the active material layer of the electrode can be made two to three times thicker than usual. The active material layer is a layer containing short fibers A, active material powder 20, conductive additive 30, and binder B, and does not contain a current collecting foil. As mentioned above, the positive electrodes of Examples 1 and 2 were fabricated with a thickness approximately 1.7 times that of Comparative Example 2. As shown in Figure 7, the discharge capacity (charge capacity) per unit area of Example 1 is approximately twice that of Comparative Example 2 when the discharge current (charge current) is 2 mA/ ^cm² , and approximately 1.75 times when the discharge current (charge current) is 3 mA/ ^cm² , depending on the discharge current (charge current).

As described above, the thickness of the positive electrodes of Examples 1 and 2 is about 1.7 times that of the positive electrode of Comparative Example 2, and therefore it can be seen that the battery characteristics of Examples 1 and 2 shown in FIG. 7 are excellent.
As shown in Figure 7, the discharge capacity (charge capacity) tends to decrease as the discharge current (charge current) increases. This is a general characteristic of secondary batteries. In other words, when discharging or charging is performed rapidly, the discharge capacity or charge capacity decreases.

The positive electrode foil of the commercially available lithium-ion battery of Comparative Example 3 is formed on approximately 1800 ^cm2 of aluminum foil, with a positive electrode containing active material. Dividing 5000 mAh by 1800 ^cm2 results in approximately 2.78 mAh/ ^cm2 . As shown in Figure 7, the nominal capacity of 5000 mAh of the commercially available lithium-ion battery of Comparative Example 3 is achieved when the discharge current (charge current) is 0.2 mA/ ^cm2 . For example, when the commercially available lithium-ion battery of Comparative Example 3 is charged and discharged at 2 mA/ ^cm2 (0.72 C), it is repeatedly charged and discharged at a maximum capacity of approximately 3470 mAh. On the other hand, the lithium-ion battery of Comparative Example 2, which uses commercially available positive and negative electrode foils at the same current, has a capacity of approximately 3870 mAh. Furthermore, the lithium-ion battery of Example 1 has a capacity of approximately 7920 mAh, allowing for high-capacity charging and discharging.

The superiority of Examples 1 and 2 is also evident from a comparison with Comparative Example 1. That is, the discharge capacity (charge capacity) per unit area of Example 1 is 1.2 to 1.3 times that of Comparative Example 1 when the discharge current (charge current) is 2 mA/cm ² , and is also 1.2 to 1.3 times when the discharge current (charge current) is 3 mA/cm ^2. As mentioned above, the thicknesses of the positive electrodes of Examples 1, 2, and Comparative Example 1 are equivalent.

Using the electrode of this embodiment allows the thickness dimension of a single electrode to be increased. This configuration makes it possible to increase the discharge capacity (charge capacity) while reducing the weight and volume of the entire battery.

8 shows the cycle characteristics of the lithium-ion battery of Example 1, measured at a charge/discharge current of 2 mAh/ ^cm2 . The lithium-ion battery of Example 1 containing aluminum fibers shows a decrease in discharge capacity (charge capacity) in the early stages, but appears to settle at about 4.3 mAh/ ^cm2 .

As such, lithium-ion batteries fabricated using the aluminum or copper metal fibers described above can have approximately twice the discharge capacity (charge capacity) of lithium-ion batteries using commercially available electrode foil. Simply increasing the thickness of the positive or negative electrode, as in Comparative Example 1, does not necessarily improve performance in proportion to the increase in thickness. However, the positive or negative electrodes of this embodiment, which use the aluminum or copper metal fibers described above, can achieve performance that is proportional to or even greater than the increase in thickness. Compared to the typical electrode thickness of 90 μm for the positive electrode of Comparative Example 2, the thickness of the positive electrodes of Examples 1 and 2, excluding the aluminum foil, is 155 μm, which is 1.7 times greater than that of Comparative Example 2. In other words, by incorporating short fiber A into the electrode as described above, it is possible to achieve performance that is proportional to or greater than the increase in thickness while maintaining an electrode thickness of 120 μm or more, more preferably 150 μm or more, and even more preferably 200 μm or more.

As shown in Figure 9, the internal resistance of a battery having an electrode containing the aluminum or copper metal fibers is lower than the internal resistance of a battery having an electrode that does not contain the metal fibers. This reduction in the internal resistance of a battery having an electrode containing the metal fibers is thought to be responsible for the improved characteristics of Examples 1 and 2. Figure 9 shows the internal resistance of batteries having positive electrodes of Examples 3, 4, and 5, and the internal resistance of a battery having a positive electrode of Comparative Example 1.

In Example 3, the weight ratio of the metal fibers to the aluminum of the positive electrode in Example 1 was changed to 1.22 wt %, and in Example 4, the weight ratio of the metal fibers to the aluminum foil of the positive electrode in Example 1 was changed to 4.88 wt %. Furthermore, in Example 5, the weight ratio of the metal fibers to the aluminum foil of the positive electrode in Example 1 was changed to 9.75 wt %. The weight ratios of the active material powder 20, conductive additive 30, and binder B in Examples 3, 4, and 5 vary slightly from those in Example 1 depending on whether the metal fibers are increased or decreased.

As shown in Figures 7 and 9, when an electrode containing the above metal fibers is used as in this embodiment, charging and discharging can be performed more than twice as fast for the same discharge capacity (charge capacity). Furthermore, Example 2, in which carbon fiber CF is added, has a higher discharge capacity (charge capacity) than Example 1. Furthermore, as shown in Figure 9, Example 5, in which the above metal fibers are blended at 9.75% by weight, also has a significantly improved internal resistance compared to Comparative Example 1. Therefore, even if the blending ratio of the above metal fibers is 9.75% by weight or less, less than 9.75% by weight, or 9.5% by weight or less, it is possible to obtain the same effect as the normal electrode of Comparative Example 1.

As such, electrodes incorporating the above-mentioned metal fibers as described above can achieve properties that are proportional to or even greater than the increase in thickness. For example, if a battery is made by stacking multiple positive electrode foils and multiple negative electrode foils, and aluminum foil and copper foil are used as current collectors for each positive electrode foil and each negative electrode foil, respectively, fewer sheets of aluminum foil and copper foil are required to make a battery with the same capacity. This contributes to saving battery space, reducing battery weight, and reducing battery manufacturing costs.

The metal fibers can be produced by methods other than coil cutting, such as chatter vibration cutting, in which a cutting tool is applied to a cylindrical aluminum or copper member with a circular cross section, or milling.
It is also possible to produce the metal fibers having the above length by blowing molten metal such as aluminum into a space through fine holes.

Further Examples
An example in which the short fibers A are applied to the positive electrode of a lithium ion battery will be described below.
A slurry S was prepared using aluminum short fibers A having an average length of 0.5 mm or less, active material powder 20 (NMC; ternary Li(Ni _a Mn _b Co _1-a-b )O ₂ ), conductive additive 30 (AB; acetylene black), and binder B (PVDF; polyvinylidene fluoride) diluted with a solvent (NMP; normal methylpyrrolidone) in a weight ratio (weight ratio after drying) of active material powder 20:conductive additive 30:binder B:aluminum short fibers A of 87:5:4:4. A lithium-ion battery was then fabricated using the slurry S and a cathode foil fabricated on aluminum foil as described above. A cathode foil containing no metal fibers such as the short fibers A was also fabricated using LiPF ₆ (lithium hexafluorophosphate) as the electrolyte (Example 6).

In addition, in Example 6, the slurry S was prepared by mixing 4% by weight of the aluminum metal powder instead of 4% by weight of the short fiber A, and a lithium ion battery was also prepared using the slurry S and a positive electrode foil prepared as described above on an aluminum foil (Example 7).
In addition, instead of blending 4% by weight of short fiber A in Example 6, the slurry S was prepared by blending 4% by weight of the above-mentioned metal fiber made of aluminum cut to a length of 1 to 3 mm as described above, and a lithium ion battery was also prepared using the slurry S and a positive electrode foil fabricated as described above on an aluminum foil (Example 8).

To produce the negative electrode foils of Examples 6 to 8, a slurry S was first prepared using active material powder 20 (natural spherical graphite), conductive additive 30 (AB; acetylene black), and binder B (PVDF; polyvinylidene fluoride) diluted with a solvent (NMP; normal methylpyrrolidone) in a weight ratio of 90:5:5 (active material powder 20:conductive additive 30:binder B). This slurry S was then used to prepare negative electrode foils on copper foil as described above.

To fabricate the lithium-ion batteries of Examples 6 to 8, the positive and negative electrode foils were cut to a predetermined size, a separator made of polypropylene was sandwiched between the positive and negative electrode foils, and these were then housed in a battery container made of a laminated sheet made of a plastic material. The electrolyte was poured into the battery container, and the battery container was sealed to fabricate the lithium-ion batteries of Examples 6 to 8. Note that the size of the positive and negative electrode foils, the size and thickness of the separator, and the capacity of the battery container were substantially the same for Examples 6 to 8.

The average thickness of the positive electrodes (average thickness of the positive electrode layer) in Examples 6 to 8, excluding the thickness of the aluminum foil (collector foil), was approximately 210 to 220 μm, and the thickness of the aluminum foil in the positive electrodes in Examples 6 to 8 was approximately 15 μm. The average thickness of the negative electrodes (average thickness of the negative electrode layer) in Examples 6 to 8, excluding the thickness of the copper foil (collector foil), was approximately 110 to 120 μm, and the thickness of the copper foil in the negative electrodes in Examples 6 to 8 was approximately 15 μm. In Examples 6 to 8, the positive electrodes were quite thick at 210 to 220 μm, so the negative electrodes were also made thicker.

FIG. 10 shows the discharge characteristics (rate characteristics) of Examples 6 to 8.
The composition and composition ratio of the positive electrode of Example 8 are the same as those of Example 1. As shown in Figure 10, the discharge capacities (charge capacities) of Examples 6 and 7 are equal to or greater than the discharge capacity (charge capacity) of Example 8. In Figure 10, the discharge capacities (charge capacities) of Examples 6 and 7 are in the range of 0.5 mA/cm ² to 12.5 mA/cm ² , which is good compared to Example 8. This is because the short fibers A in the positive electrode of Example 6 are shorter than the metal fibers in the positive electrode of Example 8, and therefore the short fibers A are more easily dispersed during the production of the above-mentioned slurry S, which may have affected the results of Figure 10.

Furthermore, when the slurry S is applied to the positive electrode foil using a known application device, the slurry S of Example 6 causes less clogging in the application device than the slurry S of Example 8. This is a great advantage in mass production, and it is possible that a similar effect will be obtained in Example 7.
For example, in mass production, a mass production machine called a die coater applies the slurry S through a slit or nozzle according to the coating thickness, and problems such as clogging caused by metal fibers or uneven mixing of the metal fibers with the active material 20 can occur. However, the short staple fibers A can improve or solve these problems.
10 , the discharge capacity (charge capacity) of Examples 6 and 7 is equal to or greater than the discharge capacity (charge capacity) of Example 8, and therefore has similar characteristics. Therefore, even if the blending amounts of short fiber A and metal powder in the positive electrodes of Examples 6 and 7 are changed as in Examples 3 to 5, it is possible to achieve good discharge capacity and internal resistance similar to Examples 3 to 5. Furthermore, even if carbon fiber CF is added to the positive electrodes of Examples 6 and 7 as in Example 2, it is possible to achieve good discharge capacity and internal resistance similar to Example 2.

In Examples 1 to 8, when copper metal fibers or short fibers A are used instead of aluminum metal fibers or short fibers A to produce the negative electrode, the specific gravity of copper is three times or more that of aluminum, so the amount of copper metal fibers or short fibers A may be more than 9.75% of the amount of aluminum. However, from the perspective of ease of dispersion of copper metal fibers or short fibers A in slurry S, the amount of copper metal fibers or short fibers A is preferably less than 12% by weight, and more preferably less than 10% by weight.

If the short fibers A are made of copper, copper has a lower electrical resistivity than aluminum, so there is a high possibility that the same effects as those described above can be achieved. Taking into account the difference in specific gravity between copper and aluminum, it is preferable that the blending ratio of copper metal fibers or short fibers A be 1.5 or 2% by weight or more. However, even if the blending ratio of copper metal fibers or short fibers A is 1% by weight or more, the same effects as those described above may be achieved in some cases.

When using a metal powder made of another metal such as copper instead of aluminum powder, the same effect as described above should be achievable if the electrical resistivity of the metal is equal to or lower than that of aluminum. Taking into account the difference in specific gravity between the other metal and aluminum, the compounding ratio of the metal powder made of the other metal is preferably 1.5 or 2 wt % or more.
As is clear from the above explanation, it is possible to manufacture an electrode using a mixture of the above aluminum metal fibers and metal powder, a mixture of aluminum short fibers A and metal powder, a mixture of the above copper metal fibers and metal powder, or a mixture of copper short fibers A and metal powder in the same compounding ratio as the above aluminum or copper metal fibers or short fibers A, and the electrode will have the same effect as when the above metal fibers or short fibers A are used.

As such, lithium-ion batteries fabricated using aluminum or copper short fiber A of this embodiment can achieve superior discharge capacity (charge capacity) while increasing the electrode thickness compared to lithium-ion batteries using commercially available electrode foils. Simply increasing the thickness of the positive or negative electrode, as in Comparative Example 1, does not necessarily improve performance in proportion to the increase in thickness. However, the positive or negative electrodes of this embodiment using aluminum or copper short fiber A can achieve performance in proportion to or even greater than the increase in thickness. Compared to the typical electrode thickness of 90 μm for the positive electrode of Comparative Example 2, the thickness of the positive electrodes of Examples 1 to 5 excluding the aluminum foil thickness is 155 μm, which is 1.7 times or more thicker than Comparative Example 2. The electrodes of Examples 6 and 7, which have performance equivalent to or better than that of Examples 1 to 5, can achieve performance in proportion to or greater than the increase in thickness by incorporating short fiber A into the electrode as described above, while maintaining an electrode thickness of 120 μm or more, preferably 150 μm or more.

In Examples 6 to 8, the discharge capacity (charge capacity) does not improve in proportion to the increase in thickness as in Examples 1 to 5. In Examples 6 to 8, the positive electrode is quite thick, and it is possible that the negative electrode is not thick enough to compensate. Other factors may also be considered. For this reason, it is possible that the positive electrode thicknesses of Examples 6 to 8 may also be able to achieve characteristics that are proportional to or close to the increase in thickness.

Furthermore, even when the positive electrode is thick as in Examples 6 to 8, the presence of the metal fibers, short fibers A, or metal powder prevents or makes it difficult for the slurry S to peel off from the current collector foil when the slurry S dries. This phenomenon is thought to occur because the metal fibers, short fibers A, or metal powder uniformly dispersed in the positive electrode structurally support the thick positive electrode.
In the production of Examples 1 to 8 and Comparative Example 1, if the metal fibers, short fibers A, or metal powder were not present, the ends of the electrodes would sometimes crumble into powder or needle-like shapes when the electrodes were cut, attached, etc. In other words, dispersing the metal fibers, short fibers A, or metal powder within the electrodes as described above also prevents the electrodes from crumbling into powder or needle-like shapes.

Furthermore, lithium ion batteries manufactured using the metal powder of this embodiment, like those using short fiber A, can have superior discharge capacity (charge capacity) compared to lithium ion batteries using commercially available electrode foils.

The above disclosure is supplemented below.
(Lithium-ion battery)
The method for manufacturing an electrode for an electricity storage device according to the first aspect of the present invention comprises the steps of kneading the above-mentioned metal (mainly aluminum or copper) short fibers A or metal powder, active material powder 20, conductive additive 30, and binder B in a kneader to prepare slurry S, applying the slurry S to a predetermined metal foil (current collector foil) and forming it into a desired shape, and then drying and pressing the slurry S formed into the desired shape. The short fibers A improve clogging of the slits and nozzles with the slurry S and uneven application caused by the end fibers A during the application step, enabling a reduction in the internal resistance of the lithium-ion battery and an increase in the coating weight (thicker application), ultimately improving the characteristics of the lithium-ion battery.

In the above embodiment, when the slurry formed into a predetermined shape is dried and pressed with a rolling mill, the aluminum or copper short fibers A are connected to each other, or the short fibers A become entangled with or penetrate into the active material powder 20 and the conductive additive 30.
This configuration can significantly improve the internal resistance of the positive or negative electrode, and is useful for making the positive or negative electrode twice or three times as thick as commercially available positive or negative electrodes.

(Electric double layer capacitors and solid-state batteries)
The configurations of these devices are the same; in the case of an electric double layer capacitor, a slurry S is used in which an adsorbent powder, which is activated carbon, a conductive additive 30, and a binder B are mixed with the short fibers A, and in the case of an all-solid-state battery, a solid electrolyte is used instead of a liquid electrolyte.

REFERENCE SIGNS LIST 1 Positive electrode current collector 2 Positive electrode layer 3 Solid electrolyte layer 4 Negative electrode layer 5 Negative electrode current collector 14, 24, 34 Solid electrolyte 20 Active material powder 30 Conductive additive A Short fiber B Binder S Slurry CF Carbon fiber

Claims (14)

a fiber preparation step for obtaining aluminum or copper metal fibers;
a stirring step of placing the metal fibers or the metal fibers cut by a predetermined cutting tool in a solvent and stirring the solvent to shorten the metal fibers to an average length of 0.8 mm or less;
a molding step of molding a liquid or gel slurry containing aluminum or copper short fibers produced by the stirring step, an adsorbent powder that adsorbs electrolyte ions during charging or an active material powder that chemically reacts during charging and discharging, and a binder into a predetermined shape;
a drying step of drying the slurry formed into the predetermined shape to form an electrode containing the short fibers. The method for manufacturing an electrode for an electricity storage device described in claim 1, wherein in the fiber preparation process, the metal fibers are obtained by cutting the end faces of a coil formed by winding aluminum or copper metal foil with a cutting tool. The method for manufacturing an electrode for an electricity storage device described in claim 1 or 2, wherein, in the stirring step, the solvent is stirred while balls are placed in the solvent to promote cutting of the metal fibers. The method for manufacturing an electrode for an electricity storage device according to claim 1 or 2, wherein the stirring step stirs the solvent by rotating a blade. The method for manufacturing an electrode for an electricity storage device according to any one of claims 1 to 4, further comprising a cutting step of cutting the metal fibers while the bundle of metal fibers is pressed against a predetermined surface using a pressing member or while the bundle of metal fibers is placed in a tube, prior to the stirring step. The method for manufacturing an electrode for an electricity storage device described in claim 5, wherein the cutting step involves cutting the bundle of metal fibers together with the pressing member or the tube. A method for manufacturing an electrode for an electricity storage device according to any one of claims 1 to 6, wherein the short fibers are mixed into the slurry so that the weight ratio of the short fibers to the entire electrode after drying in the drying step is less than 9.75% by weight. a fiber preparation step for obtaining aluminum or copper metal fibers;
a stirring step of placing the metal fibers or the metal fibers cut by a predetermined cutting tool in a solvent and stirring the solvent to shorten the metal fibers to an average length of 0.8 mm or less;
and a shipping step of shipping the aluminum or copper short fibers produced by the stirring step in the solvent or another solvent. The method for producing short fibers for use in electrodes of an electricity storage device described in claim 8, wherein the fiber production process involves cutting the end faces of a coil formed by winding aluminum or copper metal foil with a cutting tool to obtain the metal fibers. The method for producing short fibers for use in electrodes of an electricity storage device described in claim 8 or 9, wherein the stirring step involves stirring the solvent while balls are placed in the solvent to promote cutting of the metal fibers. The method for producing short fibers for use in electrodes of an electricity storage device according to claim 8 or 9, wherein the stirring step stirs the solvent by rotating a blade. The method for producing short fibers for use in electrodes of an electricity storage device according to any one of claims 8 to 11, further comprising a cutting step of cutting the metal fibers while the bundle of metal fibers is pressed against a predetermined surface using a pressing member or while the bundle of metal fibers is placed in a tube, prior to the stirring step. The method for manufacturing short fibers for use in electrodes of an electricity storage device described in claim 12, wherein the cutting step involves cutting the bundle of metal fibers together with the pressing member or the tube. a stirring step of stirring aluminum or copper metal fibers in a solvent to shorten the metal fibers to an average length of 0.8 mm or less;
A method for producing a slurry for an electrode of an electricity storage device, comprising at least mixing the metal fibers, an adsorbent material powder that adsorbs electrolyte ions during charging or an active material powder that undergoes a chemical reaction during charging and discharging, and a binder to produce a liquid or gel-like slurry.