Wet powder, mixture slurry for secondary battery, electrode for secondary battery, and method for manufacturing secondary battery

Abstract

To provide a powdered active material that prevents lithium ions from leaking out of the active material.SOLUTION: A wet powder is an electrode material for a secondary battery, and is composed of an active material that is a powder containing lithium ions and a liquid in which hydrogen ions do not substantially dissociate, and does not contain a binder, with the liquid being present on the surface of the active material that is the powder.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wet powder, a mixture slurry for a secondary battery, an electrode for a secondary battery, and a method for manufacturing a secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are widely used as power sources for driving EVs because they are lightweight and have high energy density.

[0003] Electrodes constituting secondary batteries such as lithium-ion secondary batteries are generally produced by coating a mixture slurry, in which an active material, a binder, a conductive additive, etc. are dispersed in a predetermined solvent, on the surface of a current collector made of a metal foil or the like, and then drying the coating. Furthermore, various studies are being conducted on materials, manufacturing methods, etc. in order to further increase the weight and energy density of secondary batteries.

[0004] Most positive electrode active materials are metal oxides containing lithium ions, such as lithium cobalt oxide (LCO), nickel-cobalt-manganese oxide (NCM), nickel-cobalt-aluminum oxide (NCA), and lithium iron phosphate (LFP). Negative electrode active materials include graphite, lithium titanate (LTO), and silicon.

[0005] The specified solvent is water or a non-aqueous organic solvent. Polyvinylidene fluoride (PVDF), a binder often used in positive electrodes, is insoluble in water, so N-methylpyrrolidone (NMP) is used as the solvent. However, because a large amount of energy is required for drying, the use of an aqueous solvent is being considered (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-091789 [Patent Document 2] Japanese Patent Publication No. 2022-115019 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when a positive or negative electrode active material containing lithium ions is made into a mixture slurry using an aqueous solvent, the lithium ions in the active material replace the hydrogen ions in the solvent, causing the lithium ions to dissolve into the solvent, resulting in a decrease in the capacity of the electrode. Furthermore, the positive or negative electrode active material containing lithium ions is manufactured as a powder, stored, transported, and then introduced into the mixture slurry manufacturing process. However, moisture absorption during storage and transport causes the lithium ions to dissolve from the active material into the water, similarly resulting in a decrease in the capacity of the electrode.

[0008] The present invention has been made in view of the above points, and an object of the present invention is to provide a powdered active material that prevents lithium ions from leaking out of the active material. [Means for solving the problem]

[0009] In order to solve the above problems, the wet powder of the present invention is a wet powder that is an electrode material for a secondary battery, and is characterized in that it is composed of an active material that is a powder containing lithium ions and a liquid in which hydrogen ions do not substantially dissociate, does not contain a binder, and the liquid is present on the surface of the active material that is the powder.

[0010] The content of the liquid is preferably 2% by mass or more and 20% by mass or less.

[0011] The mixture slurry for a secondary battery of the present invention is characterized in that the above-mentioned wet powder and granules and a binder are dispersed in a solvent for preparing the slurry.

[0012] The solvent for preparing the slurry may contain a solvent capable of dissociating hydrogen ions.

[0013] The electrode for a secondary battery of the present invention is an electrode for a secondary battery in which a mixture containing a powdered active material containing lithium ions and a binder is coated on a current collector, and is characterized in that a liquid in which hydrogen ions do not substantially dissociate is present on the surface of the active material.

[0014] The method for manufacturing a secondary battery of the present invention is characterized by including the steps of: adding a liquid in which hydrogen ions do not substantially dissociate to a powdered active material containing lithium ions to form a wet powdered active material in which the liquid is present on the surface of the powdered active material; adding a binder and a solvent for preparing a slurry to the wet powdered active material to prepare a mixture slurry; and applying the mixture slurry onto a current collector and then drying the coating film made of the mixture slurry. [Effects of the Invention]

[0015] According to the present invention, it is possible to prevent the loss of lithium ions from the active material, which is a powder or particle containing lithium ions, before the mixture slurry for a secondary battery is applied to a current collector and dried. [Brief explanation of the drawings]

[0016] [Figure 1] This is a photograph of the powder particles that are the active material of the positive electrode taken with an SEM. DETAILED DESCRIPTION OF THE INVENTION

[0017] Before describing the embodiments, the background to the invention of the present application will be explained.

[0018] As disclosed in Patent Document 1, many studies have been conducted on the use of aqueous solvents, which have a low environmental impact in the drying process, to prepare positive electrodes. However, when an aqueous solvent is dispersed in a positive electrode active material containing lithium ions to prepare a slurry, the lithium ions in the active material replace the hydrogen ions in the solvent, causing the lithium ions to dissolve into the solvent. This has never been addressed as a problem. Furthermore, the fact that active materials containing lithium ions absorb moisture during storage and transportation, causing the lithium ions to dissolve from the active material into the water, has also never been addressed as a problem.

[0019] The present inventors have conducted various studies on the leakage of lithium ions from active materials and have arrived at the present invention.

[0020] Active materials are generally distributed as powders, but in this application, they are referred to as granular powders, as explained below. In powder engineering, a collection of solid particles is called a powder, and typically refers to a collection of particles in which interparticle interactions, such as adhesive forces, are greater than the gravity of a single particle. When such interactions are smaller than gravity, they are called granular, and when such granular materials are intentionally included, they are called granular. However, the distinction between these is not always clearly defined. Although powders are solid, they have fluid-like properties, flowing in response to external forces, and are often treated as a material form distinct from solids, liquids, and gases.

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses.

[0022] The wet powder, the mixture slurry for a secondary battery, the electrode for a secondary battery, and the method for manufacturing a secondary battery according to the present embodiment will be described.

[0023] The wet powder according to this embodiment is composed of an active material, which is a powder containing lithium ions, and a liquid in which hydrogen ions do not substantially dissociate, and does not contain a binder, with the liquid existing on the surface of the active material, which is a powder. For example, in the case of a lithium ion secondary battery, the active material containing lithium ions is LiCoO2 (lithium cobalt oxide: LCO), LiNiO2 (lithium nickel oxide: LNO), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Examples of positive electrode active materials include nickel-cobalt-manganese oxide (NCM), lithium manganese oxide (LMO), and lithium iron phosphate (LFP), while examples of negative electrode active materials include lithium titanate (LTO).

[0024] A liquid in which hydrogen ions do not substantially dissociate is a liquid in which hydrogen ions are substantially absent, and examples thereof include N-methyl-2-pyrrolidone (NMP) and ionic liquids. N-methyl-2-pyrrolidone has hydrogen in its molecules, but because this hydrogen is covalently bonded to carbon, the hydrogen ions do not dissociate. On the other hand, a certain percentage of hydrogen ions dissociate in water, and alcohols and other liquids also dissociate hydrogen ions. Note that "hydrogen ions do not substantially dissociate" means that they will not dissociate unless placed in special conditions such as by irradiation with radiation or application of high voltage.

[0025] The reason why the wet powder according to this embodiment does not contain a binder will be explained below.

[0026] Patent Document 2 discloses a wet powder composed of aggregated particles containing a plurality of electrode active material particles, a binder resin, and a solvent. This wet powder is used as a material with a higher solids content than a mixture slurry when producing an electrode, instead of using a mixture slurry. The solvent here refers to a medium that dissolves the binder resin. Therefore, the binder resin is essential, and the binder resin is contained in the solvent. The wet powder of this embodiment corresponds to the "electrode active material particles" used as a material for producing the wet powder of Patent Document 2. Furthermore, Patent Document 2 does not describe or suggest anything about the problem of the present application or a solution to it.

[0027] Note that "wet powder granules do not contain a binder" means that the wet powder granules do not have a binder inside (no binder exists), and an embodiment in which a binder exists on the outer surface of a wet powder granules composed of an active material and a liquid is an embodiment in which "wet powder granules do not contain a binder." Even if a binder that is not soluble in the liquid that constitutes the wet powder granules comes into contact with the wet powder granules, the wet powder granules will not contain a binder.

[0028] Next, regarding the liquid being present on the surface of the powder active material, the powder of NCM (nickel-cobalt-manganese oxide), which is the positive electrode active material shown in FIG. 1, contains lithium ions, and each powder is formed as an aggregate of many fine particles. Most active materials containing lithium ions are powders with similar structures. In the wet powder of this embodiment, the liquid penetrates between the fine particles and is present on the surface of the fine particles. Furthermore, since the surface of the aggregate is formed by the surface of the fine particles, the liquid is also present on the surface of the aggregate. If there is a sufficient amount of liquid, the liquid will cover the entire surface of the fine particles and the aggregate.

[0029] In the wet powder of this embodiment, the liquid content is preferably 2% by mass or more and 20% by mass or less. If the liquid content is less than 2% by mass, the liquid cannot sufficiently cover the surfaces of the particles constituting the active material powder, resulting in patches of liquid and non-liquid on the surface of the particles, making it difficult to prevent lithium ions from escaping from the active material when it comes into contact with water. Furthermore, if the liquid content exceeds 20% by mass, excess liquid will be present between the active material aggregates, resulting in a slurry-like state, making it difficult to store, transport, and fabricate electrodes as a powder. The liquid content is more preferably 18% by mass or less, and even more preferably 15% by mass or less.

[0030] The wet powder of this embodiment may be produced by a production method using an agitation granulator as described in Patent Document 2, or may be produced by spray coating, in which a powder of an active material is placed in a tank, air is blown into the bottom of the tank to cause the powder to flow up and down, and a liquid is sprayed from above using a spray device to produce a wet powder.

[0031] In this embodiment, a mixture slurry for a secondary battery is prepared using the above-described wet powder. The positive electrode mixture slurry can be prepared, for example, by mixing the above-described wet powder of NCM and, if necessary, various known conductive additives with an aqueous binder (such as carboxymethyl cellulose or styrene butadiene rubber) in an aqueous solvent. Positive electrode active materials other than NCM, such as LCO, LMO, and LFP, can also be used. When lithium titanate powder is used as the negative electrode active material, a wet powder is prepared in the same manner as NCM, and the negative electrode mixture slurry is prepared using this. Note that a non-aqueous organic solvent may be used instead of the aqueous solvent, and the binder may be changed to one suitable for the organic solvent.

[0032] The prepared mixture slurry is applied to a metal foil current collector, dried, and pressed to prepare electrode plates (positive and negative electrode plates). In this embodiment, the amount of the organic solvent that is difficult to vaporize is small, and the liquid used to prepare the slurry is aqueous. This allows for significantly less energy required for drying than when a non-aqueous solvent is used to prepare the slurry. Furthermore, when a non-aqueous organic solvent is used to prepare the slurry, it is necessary to recover the organic solvent from the gas in the drying furnace to prevent the evaporated organic solvent from being released into the atmosphere. This requires the installation of equipment and the investment of energy for recovery. However, when an aqueous liquid is used, such equipment and energy are unnecessary, thereby reducing the environmental impact and reducing running costs. Furthermore, the liquid used to wet the active material powder and granules, from which hydrogen ions do not substantially dissociate, remains on the surface of the active material contained in the electrode plate after drying.

[0033] The positive and negative electrodes prepared as described above are wound together with a separator between them to produce an electrode assembly. The negative electrode may be made of an active material other than lithium titanate, such as metallic lithium, graphite, or silicon. This electrode assembly is then housed in a battery case together with a nonaqueous electrolyte to produce a lithium-ion secondary battery.

[0034] The binder and the solvent for preparing the slurry are not limited to those mentioned above. For example, PVDF may be used as the binder, and a non-aqueous solvent (such as NMP) may be used as the solvent.

[0035] In this embodiment, even if an aqueous solvent for preparing the slurry is used, the wet powder is prepared so that a liquid that does not substantially dissociate hydrogen ions is present on the surface of the active material, thereby preventing lithium ions in the active material from leaking out of the active material due to the preparation of the slurry, which would result in a decrease in battery capacity. Furthermore, if the wet powder is prepared immediately after the active material is produced as a powder, it is possible to prevent lithium ions from absorbing moisture and leaking out of the active material into water during storage and transportation until the active material is converted into a slurry and used to prepare an electrode.

[0036] Next, the following study was carried out to confirm how much lithium ions would leak out from the active material when a mixture slurry for producing an electrode was prepared using water.

[0037] <Consideration 1> Nickel-cobalt-manganese oxide (NCM) was used as the active material. 1 g of this active material was weighed out, immersed in 400 g of water, and left for one day. This experiment was performed three times. Table 1 shows how the amounts (mol) of Li, Ni, Co, and Mn present in the active material changed before and after immersion in water. These amounts represent the average values of the three experiments.

[0038] [Table 1]

[0039] After immersion in water for one day, Li dissolved from the active material, decreasing by 12.3%. On the other hand, Ni, Co, and Mn hardly dissolved from the active material even after immersion in water for one day. Li has a smaller ionic radius of the cation than Ni, Co, and Mn, and it is thought that Li was replaced by hydrogen ions (or hydronium ions) in the water and dissolved into the water. The amount of each metal ion dissolved in water after immersion was calculated by measuring the concentration of the metal ion in the water and converting it to a molar amount.

[0040] When Li is lost by 12.3% from NCM, the active material of the positive electrode, the battery capacity of a lithium-ion secondary battery made using this active material is reduced by approximately 10% compared to when Li is not lost.

[0041] Next, the following experiment was carried out to examine how much lithium ions are lost from the active material due to moisture in the air when the powdered active material is produced, stored, and transported.

[0042] <Consideration 2> Nickel-cobalt-manganese oxide (NCM) was used as the active material. 1 g of this active material was weighed out, mixed with the same amount of water (1 g), and immersed in the water for one day. This experiment was carried out three times. Table 2 shows how the amounts (mol) of Li, Ni, Co, and Mn present in the active material changed before and after immersion in water. This amount represents the average value of the three experiments.

[0043] [Table 2]

[0044] In Study 2, the amount of water was reduced to the same amount as the active material compared to Study 1, simulating the active material absorbing moisture from the air. After immersion in the same mass of water for one day, Li was lost by 3.1% from the active material and dissolved into the water. On the other hand, Ni, Co, and Mn hardly dissolved into the water from the active material even after immersion in water for one day.

[0045] Although the degree of Li loss in Study 2 is smaller than in Study 1, much research is being done in the battery industry to increase battery capacity even if only slightly, and even a 3.1% loss of lithium is considered a significant loss.

[0046] On the other hand, we prepared a wet powder (NMP content: 5% by mass of the total) using NCM and NMP, and then conducted an experiment in which we immersed this in water in the same manner as in Studies 1 and 2, but no decrease in lithium ions was observed in either case.

[0047] In the wet powder of this embodiment, a liquid that does not substantially dissociate hydrogen ions is present on the surface of the powder active material, and the solvent prevents contact between the active material and water, thereby preventing lithium ions from leaking from the active material into the water. As a result, almost all of the lithium ions contained in the active material can be used in the battery reaction, and a high battery capacity can be maintained.

[0048] (Other embodiments) The above-described embodiments are merely examples of the present invention, and the present invention is not limited to these examples. These examples may be combined with well-known, commonly used, or publicly known technologies, or may be partially replaced. Modified inventions that would be easily conceived by a person skilled in the art are also included in the present invention.

[0049] The active material used to prepare the wet powder is not limited to the above-mentioned materials as long as it contains lithium ions.

[0050] The diameter of each powder particle or the overall diameter distribution is not particularly limited, and any diameter and diameter distribution may be used as long as the powder particles of the active material can be used to fabricate a secondary battery.

[0051] The liquid in which the hydrogen ions that make up the wet powder do not substantially dissociate is not limited to NMP. If the element to which hydrogen in the liquid molecules is bonded is oxygen (e.g., alcohol), the hydrogen ions will dissociate, but if the element to which hydrogen is bonded is, for example, only carbon, the hydrogen ions will not substantially dissociate.

Claims

1. A wet powder that is an electrode material for a secondary battery, The battery is composed of an active material that is a powder containing lithium ions and a liquid in which hydrogen ions do not substantially dissociate, and does not contain a binder; The liquid is present on the surface of the active material, which is a powder or granule.

2. 2. The moist granular material according to claim 1, wherein the liquid content is 2% by mass or more and 20% by mass or less.

3. 3. A mixture slurry for a secondary battery, comprising the wet powder or granule according to claim 1 or 2 and a binder dispersed in a solvent for preparing a slurry.

4. 4. The mixture slurry according to claim 3, wherein the slurry-preparing solvent contains a liquid capable of dissociating hydrogen ions.

5. An electrode for a secondary battery, in which a mixture including an active material that is a powder containing lithium ions and a binder is coated on a current collector, An electrode for a secondary battery, wherein a liquid in which hydrogen ions do not substantially dissociate is present on the surface of the active material.

6. a step of adding a liquid in which hydrogen ions do not substantially dissociate to a powdery active material containing lithium ions to form a wet powdery active material in which the liquid is present on the surface of the powdery active material; a step of adding a binder and a solvent for preparing a slurry to the wet powder and granules to prepare a mixture slurry; a step of applying the mixture slurry onto a current collector and then drying the coating film made of the mixture slurry; A method for manufacturing a secondary battery, comprising:

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