Method for manufacturing electrode active-material slurry for secondary battery

Abstract

The present disclosure provides a means capable of reducing the internal resistance of a secondary battery.　The present disclosure provides a method for manufacturing electrode active-material slurry for secondary batteries, the method including: incorporating a plurality of binders having different peak top molecular weights into slurry; preparing a solution containing the binder lower in peak top molecular weight among the plurality of binders together with a conductive aid and a solvent; and then mixing an electrode active material and a solid electrolyte with the solution to obtain slurry.

Description

Method for manufacturing electrode active material slurry for secondary batteries

[0001] This invention relates to a method for producing an electrode active material slurry for secondary batteries.

[0002] In recent years, research and development on all-solid-state batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors capable of ion conduction in a solid state. Therefore, all-solid-state batteries have the advantage of not, in principle, occurring in the way that conventional liquid-based batteries using non-aqueous electrolytes are caused by flammable organic electrolytes.

[0003] The positive electrode active material layer of such an all-solid-state battery is typically manufactured by preparing an electrode active material slurry containing its constituent components together with a solvent, then applying the electrode active material slurry to the surface of a current collector and drying it.

[0004] Conventionally, Japanese Patent Publication No. 2020-145034 discloses a technique for producing positive electrode active material for all-solid-state batteries, in which a conductive additive made of carbon is dispersed in a solvent to obtain a first slurry, a sulfide solid electrolyte is dispersed in the first slurry to obtain a second slurry, and then positive electrode active material is dispersed in the second slurry to obtain a positive electrode active material slurry. According to the above document, by using a slurry obtained by such a method in the production of all-solid-state batteries, a good solid-solid interface is formed in the positive electrode active material layer, and the increase in battery resistance after charge-discharge cycles is suppressed.

[0005] However, the inventors' research has revealed that even when using the method described in the above-mentioned literature, the internal resistance of the secondary battery may still be high.

[0006] Therefore, the present invention aims to provide a means that can reduce the internal resistance of a secondary battery.

[0007] The inventors diligently conducted research to solve the above problems. As a result, they discovered that the above problems can be solved by incorporating multiple binders with different peak top molecular weights into a slurry, preparing a solution containing the binder with the smaller peak top molecular weight among these multiple binders together with a conductive additive and a solvent, and then mixing the electrode active material and solid electrolyte into this solution to prepare a slurry, thus completing the present invention.

[0008] One embodiment of the present invention, thus completed, relates to a method for producing an electrode active material slurry for secondary batteries. The method for producing an electrode active material slurry for secondary batteries contains a solvent, an electrode active material, a solid electrolyte, a conductive additive, and a binder component comprising a low molecular weight binder and a high molecular weight binder having a peak top molecular weight greater than that of the low molecular weight binder (the peak top value in the polystyrene-equivalent molecular weight distribution measured using tetrahydrofuran as an eluent by gel permeation chromatography (GPC)). The method for producing an electrode active material slurry for secondary batteries is characterized by comprising preparing a solution X containing the solvent, the low molecular weight binder, and the conductive additive, and mixing the electrode active material and the solid electrolyte with the solution X to prepare a slurry.

[0009] Figure 1 is a graph showing the results of measuring the molecular weight distribution of binder solutions prepared by dissolving three types of polyvinylidene fluoride (PVdF) with different molecular weight distributions in butyl acetate, which is the solvent, in the examples described later.

[0010] One embodiment of the present invention is a method for producing an electrode active material slurry for a secondary battery, comprising a solvent, an electrode active material, a solid electrolyte, a conductive additive, and a binder component comprising a low molecular weight binder and a high molecular weight binder having a peak top molecular weight greater than that of the low molecular weight binder (the peak top value in the polystyrene-equivalent molecular weight distribution measured using tetrahydrofuran as an eluent by gel permeation chromatography (GPC)), the method comprising: preparing a solution X comprising the solvent, the low molecular weight binder, and the conductive additive (hereinafter also simply referred to as the "first preparation step"), and mixing the electrode active material and the solid electrolyte with the solution X to prepare a slurry (hereinafter also simply referred to as the "second preparation step").

[0011] When an electrode active material slurry manufactured by this method is used to produce the electrode active material layer of a secondary battery, the internal resistance of the resulting secondary battery may be reduced. The mechanism by which the above manufacturing method produces this effect is not fully understood, but the following is hypothesized. First, in the slurry manufacturing method disclosed in Japanese Patent Application Publication No. 2020-145034, powder is added sequentially in each step, so as the solid content concentration increases, the possibility of the powder agglomerating and forming clumps also increases. If clumps form, they are difficult to break up during stirring, and if clumps remain in the electrode active material layer, it will be a factor in increasing the internal resistance of the battery. Also, the use of two or more binders is not disclosed, and it is thought that the poor compatibility of the binder with the powder is also a factor in agglomeration.

[0012] In contrast, according to the slurry manufacturing method of this embodiment, in the first preparation step, the low molecular weight binder is adsorbed onto the conductive additive, thereby suppressing the aggregation of the conductive additive. Furthermore, in the second preparation step, the electrode active material and solid electrolyte are mixed afterward, allowing the conductive additive mixed with the binder to penetrate between the particles of the electrode active material and solid electrolyte. This prevents aggregation of these particles while ensuring electron conduction paths between them. In addition, the adsorption of the solvent onto the conductive additive reduces uneven wetting of the conductive additive, and it is believed that the formation of clumps due to aggregation of the conductive additive is suppressed. It is believed that these effects work together to reduce the internal resistance of a secondary battery equipped with an electrode active material layer made using an electrode active material slurry. However, this mechanism is based solely on speculation, and its accuracy does not affect the technical scope of the present invention.

[0013] Furthermore, the electrode active material slurry for secondary batteries produced by the manufacturing method according to this embodiment also essentially contains a high molecular weight binder in addition to the low molecular weight binder. Including such a high molecular weight binder has the advantage of effectively preventing cracking of the coating film during drying when the slurry is coated to form the electrode active material layer. In the slurry manufacturing method according to this embodiment, there are no particular restrictions on the timing of adding the high molecular weight binder.

[0014] The manufacturing method for this form will be described in detail below, step by step.

[0015] <First Preparation Step> The first preparation step is the step of preparing solution X, which contains a solvent, a low molecular weight binder, and a conductive additive.

[0016] The solvent is preferably an organic solvent. Suitable organic solvents include, for example, N-alkylpyrrolidones such as N-methyl-2-pyrrolidone, aprotic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and 1,3-dimethyl-2-imidazolidinone, ester solvents such as γ-butyrolactone and butyl acetate, carbonate solvents such as ethylene carbonate and propylene carbonate, alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether, or alcohol solvents such as isopropyl alcohol and mixtures thereof. Among these, N-alkylpyrrolidones, aprotic polar solvents, or ester solvents are preferred.

[0017] A binder is a material added to an electrode active material layer formed using the resulting electrode active material slurry to improve the bonding between components of the electrode active material layer and the bonding between components of the electrode active material layer and the current collector. In this process, a low molecular weight binder refers to a binder component in the final electrode active material slurry for secondary batteries that does not have the highest peak top molecular weight. Therefore, for example, if the binder components in the final electrode active material slurry for secondary batteries have two peak top molecular weights, the binder with the smaller peak top molecular weight is the low molecular weight binder, and the binder with the larger peak top molecular weight is the high molecular weight binder. Also, if the binder components in the final electrode active material slurry for secondary batteries have three peak top molecular weights, the binder with the highest peak top molecular weight is the high molecular weight binder, and the remaining two binders with the same peak top molecular weight are both low molecular weight binders. There are no particular restrictions on the peak top molecular weight of the low molecular weight binder, but it is preferably 50,000 to 700,000, more preferably 100,000 to 600,000, and even more preferably 150,000 to 500,000. The peak top molecular weight of the binder shall be measured using the method described in the Examples section below (gel permeation chromatography (GPC)).

[0018] There are no particular restrictions on the specific composition of the binder, and those conventionally used as binders for secondary batteries can be used in the same way. Examples of binders include thermoplastic polymers such as polyethylene, polypropylene, polyethylene terephthalate (PET), polyethernitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethylcellulose (CMC) and its salts, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene-propylene rubber, ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, polyvinylidene fluoride (PVdF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and ethylene-tetrafluoroethylene copolymer (ETFE). Fluororesins such as polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-P Examples include vinylidene fluoride-based fluororubbers such as FP-type fluororubbers, vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-type fluororubbers (VDF-PFP-TFE-type fluororubbers), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-type fluororubbers (VDF-PFMVE-TFE-type fluororubbers), vinylidene fluoride-chlorotrifluoroethylene-type fluororubbers (VDF-CTFE-type fluororubbers), epoxy resins, etc. Among these, polyvinylidene fluoride (PVdF) is preferably used from the viewpoint of excellent binding properties.

[0019] Conductive additives are materials added to enhance the electronic conductivity of the electrode active material layer formed using the resulting electrode active material slurry. Examples of conductive additives include fibrous conductive additives and particulate conductive additives. Examples of fibrous conductive additives include carbon fibers such as carbon nanotubes, carbon nanohorns, carbon nanofibers, carbon nanofilaments, carbon fibrils, and vapor-grown carbon fibers. Examples of particulate conductive additives are not particularly limited and include carbon powders such as acetylene black, Ketjen black (furnace black), channel black, and thermal black.

[0020] In the first preparation step, solution X is prepared by mixing the solvent, low molecular weight binder, and conductive additive as essential components. However, other components may be added in the first preparation step to obtain solution X. Examples of such other components include high molecular weight binders. The effects of the present invention can be expressed even more significantly if the solution X obtained in the first preparation step further contains high molecular weight binders. From the definition of low molecular weight binders described above, high molecular weight binders are defined as binder components in the final secondary battery electrode active material slurry whose peak top molecular weight is not the smallest. Furthermore, high molecular weight binders may contain multiple binders with different peak top molecular weights. Therefore, in the first preparation step, both low molecular weight binders and high molecular weight binders may be used as binders, in which case two or more binders having different peak top molecular weights will be added to the solvent and mixed with a conductive additive to prepare solution X. In other words, it is preferable that solution X further contains high molecular weight binders. There are no particular restrictions on the peak top molecular weight of the high molecular weight binder, but it is preferably greater than 500,000, more preferably greater than 600,000, and even more preferably greater than 700,000. There are also no particular restrictions on the upper limit of the peak top molecular weight of the high molecular weight binder, but it is, for example, 1,000,000 or less, and preferably 900,000 or less. In one preferred embodiment, the peak top molecular weight of the low molecular weight binder is 100,000 or more and 600,000 or less, and the peak top molecular weight of the high molecular weight binder is greater than 600,000. Having the peak top molecular weights of the low molecular weight binder and the high molecular weight binder in this range is preferable because it allows the functions of each binder to be fully expressed. When using the low molecular weight binder and the high molecular weight binder simultaneously, they may be purchased or prepared separately and used individually, or a product in which they are already mixed may be purchased and used.

[0021] In the first preparation step, it is preferable to prepare a solution in which the low molecular weight binder among the solid components to be used is dissolved in the solvent beforehand. That is, the preparation of solution X in the first preparation step preferably includes preparing solution Y containing the above-mentioned solvent and low molecular weight binder (and high molecular weight binder if necessary), and preparing solution X by mixing a conductive additive with solution Y. With this configuration, the solvent and low molecular weight binder can be uniformly adsorbed onto the surface of the conductive additive, and the effects of the present invention can be expressed more significantly. Here, if it is desired to include a high molecular weight binder in solution X in the first preparation step, it is preferable to dissolve the high molecular weight binder when dissolving the low molecular weight binder in the solvent beforehand. That is, as mentioned above, it is preferable for solution X to further contain a high molecular weight binder, but it is even more preferable for solution Y to further contain a high molecular weight binder. With this configuration, the high molecular weight binder can be dispersed more uniformly in the slurry, and the effect can be expressed more significantly by adding the high molecular weight binder.

[0022] On the other hand, in the first preparation step, it is preferable to prepare a solution in which the conductive additive, among the solid components used, is dispersed in the solvent beforehand. That is, the preparation of solution X in the first preparation step preferably includes mixing the conductive additive, which is dispersed in the solvent as described above, with the low molecular weight binder. Among these, the most preferred method is to prepare a solution Y in which the low molecular weight binder is dissolved in the solvent as described above, and then add and mix a solution in which the conductive additive is dispersed in the solvent to it to prepare solution X. With these methods, the conductive additive, which is in the form of secondary particles, can be loosened, the contact area between the solvent and the low molecular weight binder and the conductive additive can be improved, and in turn, uneven wetting of the conductive additive can be reduced and clumping can be prevented.

[0023] In the first preparation step, after mixing the solvent, low molecular weight binder, and conductive additive, the solvent may be further added and mixed as needed for viscosity adjustment or other purposes to obtain solution X. Furthermore, from the viewpoint of sufficiently improving the dispersibility of the final slurry, it is preferable that the solution X obtained in the first preparation step substantially does not contain the electrode active material and solid electrolyte described later. For example, the total content of electrode active material and solid electrolyte in solution X is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, even more preferably 1% by mass or less, particularly preferably 0.5% by mass or less, and most preferably 0% by mass, based on 100% by mass of the total amount of solution X. Also, from the same viewpoint as above, the total content of solvent, binder component, and conductive additive in solution X is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, even more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass, based on 100% by mass of the total amount of solution X.

[0024] Since the components of solution X in the first preparation step are more easily dissolved and dispersed in the solution compared to the electrode active material and solid electrolyte described later, conventionally known mixing methods can be appropriately selected and used when preparing solution X in the first preparation step.

[0025] <Second Preparation Step> The second preparation step is a step in which an electrode active material and a solid electrolyte are mixed with the solution X obtained in the first preparation step described above to prepare a slurry. The slurry thus prepared can be used to form the electrode active material layer of a secondary battery.

[0026] In the second preparation step, the electrode active material and solid electrolyte are prepared. When manufacturing a positive electrode active material slurry for secondary batteries, the positive electrode active material is prepared as the electrode active material. On the other hand, when manufacturing a negative electrode active material slurry for secondary batteries, the negative electrode active material is prepared as the electrode active material.

[0027] As the positive electrode active material, there is no particular limitation as long as it can release lithium ions during the charging process of the secondary battery and occlude lithium ions during the discharging process. As an example of such a positive electrode active material, those containing M1 element and O element, and the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe and P can be mentioned. As such a positive electrode active material, for example, LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ and other layered rock salt type active materials, LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ and other spinel type active materials, LiFePO ４ , LiMnPO ４ and other olivine type active materials, Li ２ FeSiO ４ , Li ２ MnSiO ４ and other Si-containing active materials, etc. can be mentioned. Further, as oxide active materials other than the above, for example, Li ４ Ti ５ O １２ , LiVO ２ can be mentioned.

[0028] Among them, the positive electrode active material, in the fully discharged state, has the following general formula (1): Li ｘ Ni ａ M ｂ N ｃ O ２(1) It is preferable to include a lithium transition metal composite oxide having a composition represented by the formula (wherein x, a, b, and c satisfy 0.8 ≤ x ≤ 1.1, a + b + c = 1, 0.80 ≤ a ≤ 1.00, 0 ≤ b ≤ 0.20, and 0 ≤ c ≤ 0.20. M is one or more elements selected from the group consisting of Mn and Co, and N is one or more elements selected from the group consisting of Al, Sn, Nb, Ti, Zr, and Mg). This positive electrode active material is a so-called high-nickel positive electrode active material, which has a high proportion of nickel atoms in its composition, and is known to exhibit significant expansion and contraction during charging and discharging. Therefore, it is preferable that the positive electrode active material slurry contains this positive electrode active material, as this will result in the positive electrode active material layer containing this positive electrode active material, and the effects of the present invention may be more pronounced.

[0029] Furthermore, the positive electrode active material may contain sulfur. The positive electrode active material containing sulfur is not particularly limited, but examples include elemental sulfur (S), particles or thin films of organic sulfur compounds or inorganic sulfur compounds. Any material that can release lithium ions during charging and absorb lithium ions during discharge by utilizing the oxidation-reduction reaction of sulfur is acceptable. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitriles represented by the compounds described in International Publication No. 2010 / 044437, sulfur-modified polyisoprene, rubeanoic acid (dithiooxamide), and polysulfurized carbon. Among these, disulfide compounds, sulfur-modified polyacrylonitriles, and rubeanoic acid are preferred, and sulfur-modified polyacrylonitrile is particularly preferred. As disulfide compounds, those having dithiobiurea derivatives, thiourea groups, thioisocyanates, or thioamide groups are more preferred. Here, sulfur-modified polyacrylonitrile is a modified polyacrylonitrile containing sulfur atoms, obtained by mixing sulfur powder and polyacrylonitrile and heating it under an inert gas or reduced pressure. Its presumed structure is, for example, shown in Chem. Mater. 2011, 23, 5024-5028, a structure in which the polyacrylonitrile is ring-closed to become polycyclic, and at least some of the sulfur atoms are bonded to carbon atoms. The compound described in this document shows a Raman spectrum at 1330 cm⁻¹. －１ and 1560cm －１ There is a strong peak signal nearby, and furthermore, 307 cm －１ , 379cm －１ , 472cm －１ 929cm －１ A peak exists in the vicinity. On the other hand, inorganic sulfur compounds are preferred because they have excellent stability, specifically, elemental sulfur (S), Li ２ S, TiS ２ TiS ３ TiS ４ NiS, NiS ２ CuS, FeS ２ MoS ２ MoS ３ These are some examples. In particular, elemental sulfur (S), Li２ S, S-Carbon Composite, TiS ２ TiS ３ TiS ４ FeS ２ and MoS ２ Preferably, elemental sulfur (S), Li ２ S, TiS ２ and FeS ２ From the viewpoint of being more preferable and having a high capacity, elemental sulfur (S) or Li ２ S is particularly preferred. Note that elemental sulfur (S) is S ８ Structural α-sulfur, β-sulfur, or γ-sulfur can be used. During discharge, these elemental sulfurs (S) intercalate lithium ions and exist in the positive electrode active material layer in the form of lithium (poly)sulfides.

[0030] In some cases, two or more positive electrode active materials may be used in combination. Of course, other positive electrode active materials may also be used.

[0031] The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it contains lithium, and examples include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and is particularly preferably lithium metal or a lithium-containing alloy.

[0032] The electrode active material is usually in the form of particles. The average particle diameter of the electrode active material (electrode active material particles) is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. Here, the "average particle diameter" of the electrode active material particles is defined as the 50% cumulative diameter (D50) measured by a laser diffraction / scattering particle size distribution device.

[0033] A solid electrolyte is a material added to enhance the ionic conductivity of the electrode active material layer formed using the resulting electrode active material slurry. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, a solid electrolyte refers to a material mainly composed of an ionic conductor capable of conducting ions in a solid state, and in particular, a lithium ion conductivity of 1 × 10⁻¹⁶ at room temperature (25°C) －５ This refers to a material with a lithium ion conductivity of S / cm or higher, and this lithium ion conductivity is preferably 1 × 10⁻⁶. －４ The value is S / cm or higher. Here, the ionic conductivity can be measured by the AC impedance method.

[0034] The solid electrolyte is preferably a sulfide solid electrolyte containing an element S, more preferably containing an element Li, an element M, and an element S, wherein the element M contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an element S, an element Li, and an element P.

[0035] Sulfide solid electrolytes are Li ３ PS ４ It may have a skeleton, Li ４ P ２ S ７ It may have a skeleton, Li ４ P ２ S ６ It may have a skeleton. ３ PS ４ Examples of sulfide solid electrolytes with a skeleton include LiI-Li ３ PS ４ , LiI-LiBr-Li ３ PS ４ Li ３ PS ４ This can be cited. Also, Li ４ P ２ S ７Examples of the sulfide solid electrolyte having a skeleton include, for example, a Li-P-S-based solid electrolyte called LPS. Also, as the sulfide solid electrolyte, for example, LPS represented by Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (where x satisfies 0 < x < 1) such as LGPS may be used. More specifically, for example, LPS (Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S, Li ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０ GeP ２ S １２ , or Li ６ PS ５ X (where X is Cl, Br, or I) and the like can be mentioned. Note that the description of "Li ２ S-P ２ S ５ " means a sulfide solid electrolyte obtained by using a raw material composition containing Li ２ S and P ２ S ５ , and the same applies to other descriptions. Among them, the sulfide solid electrolyte is preferably LPS (Li ２ S-P ２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ PS ４ selected from the group consisting of.

[0036] Examples of solid electrolyte shapes include spherical, ellipsoidal, and other particulate forms, as well as thin films. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle size (D50) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

[0037] The second preparation step involves preparing a slurry by adding and mixing the electrode active material and solid electrolyte to the solution X obtained in the first preparation step. However, other components may be added in the second preparation step to obtain solution X. Examples of such other components include a high molecular weight binder (preferably dissolved in a solvent). However, as mentioned above, it is preferable that the high molecular weight binder is added in the first preparation step and included in solution X, and more preferably included in solution Y as described above.

[0038] In the second preparation step, the electrode active material and solid electrolyte to be added may be mixed separately into solution X, but it is preferable to mix them into solution X in the form of a uniformly mixed powder composition. When the electrode active material and solid electrolyte are added to solution X separately and then mixed, it is difficult to uniformly disperse these components in the solution. However, by adding them to solution X in the form of a powder composition beforehand, the dispersibility in the solution can be further improved, which is an advantage.

[0039] In the second preparation step, after mixing the electrode active material and the solid electrolyte, a solvent may be added and mixed as needed to adjust viscosity, etc., to obtain a slurry.

[0040] In the second preparation step, the electrode active material and solid electrolyte, which are components of the slurry, do not mix well with other components such as binders and conductive additives. For this reason, when preparing the slurry in the first preparation step, it is preferable to use equipment that can perform not only mixing but also kneading operations. Examples of such equipment include kneading equipment such as the planetary stirring type mixing and kneading equipment of the "Awatori Rentaro" series (manufactured by Thinky Co., Ltd.).

[0041] According to the manufacturing method of this embodiment, a slurry of electrode active material for secondary batteries is produced through the first and second preparation steps described above. There are no particular restrictions on the content of each component contained in the electrode active material slurry produced in this manner. For example, the content of electrode active material is preferably 82 to 93% by mass, and more preferably 85 to 90% by mass, relative to 100% by mass of the total solid content of the slurry. The content of solid electrolyte is preferably 6 to 14% by mass, and more preferably 10 to 12% by mass. Furthermore, the content of conductive additive is preferably 0.5 to 2% by mass, and more preferably 0.8 to 1.5% by mass. The content of binder is preferably 0.5 to 2% by mass, and more preferably 0.7 to 1.2% by mass.

[0042] The electrode active material slurry for secondary batteries produced by the manufacturing method according to this embodiment is used to form the electrode active material layer of a secondary battery. There are no particular restrictions on the method for forming the electrode active material layer using the slurry, and conventionally known knowledge may be referenced as appropriate. Furthermore, there are no particular restrictions on the type or specific configuration of the secondary battery, and any battery capable of charging and discharging will suffice, with lithium secondary batteries such as all-solid-state lithium secondary batteries being preferred.

[0043] Furthermore, the following items are also included in the scope of the present invention: Item 1: A method for producing an electrode active material slurry for a secondary battery, comprising a solvent, an electrode active material, a solid electrolyte, a conductive additive, and a binder component comprising a low molecular weight binder and a high molecular weight binder having a peak top molecular weight greater than that of the low molecular weight binder (the peak top value in the polystyrene-equivalent molecular weight distribution measured using tetrahydrofuran as an eluent by gel permeation chromatography (GPC)), comprising: preparing a solution X comprising the solvent, the low molecular weight binder, and the conductive additive; and mixing the electrode active material and the solid electrolyte with the solution X to prepare a slurry; Item 2: The method for producing an electrode active material slurry for a secondary battery according to Item 1, wherein the preparation of the solution X comprises: preparing a solution Y comprising the solvent and the low molecular weight binder; and mixing the conductive additive with the solution Y to prepare the solution X; Item 3: A method for producing an electrode active material slurry for a secondary battery according to item 1 or 2, wherein the preparation of solution X comprises mixing the conductive additive dispersed in the solvent with the low molecular weight binder; Item 4: A method for producing an electrode active material slurry for a secondary battery according to any one of items 1 to 3, wherein the solution X further comprises the high molecular weight binder; Item 5: A method for producing an electrode active material slurry for a secondary battery according to item 2, wherein the solution Y further comprises the high molecular weight binder; Item 6: A method for producing an electrode active material slurry for a secondary battery according to any one of items 1 to 5, wherein the electrode active material and the solid electrolyte are mixed into solution X in the form of a uniformly mixed powder composition; Item 7: A method for producing an electrode active material slurry for secondary batteries according to any one of items 1 to 6, wherein the peak top molecular weight of the low molecular weight binder is 100,000 or more and 600,000 or less (preferably 150,000 to 500,000), and the peak top molecular weight of the high molecular weight binder is greater than 600,000 (preferably greater than 700,000 and 1,000,000 or less);Item 8: The method for producing an electrode active material slurry for a secondary battery according to any one of Items 1 to 7, wherein the total content of the electrode active material and the solid electrolyte in the solution X is 10% by mass or less (preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, even more preferably 0.5% by mass or less, particularly preferably 0% by mass) based on 100% by mass of the total amount of the solution X; Item 9: The method for producing an electrode active material slurry for a secondary battery according to any one of Items 1 to 8, wherein the total content of the solvent, the binder component and the conductive additive in the solution X is 90% by mass or more (preferably 95% by mass or more, more preferably 97% by mass or more, even more preferably 99% by mass or more, even more preferably 99.5% by mass or more, particularly preferably 100% by mass) based on 100% by mass of the total amount of the solution X; Item 10: The method for producing an electrode active material slurry for a secondary battery according to any one of Items 1 to 9, wherein the solvent is an N-alkylpyrrolidone, an aprotic polar solvent, or an ester solvent.

[0044] The present invention will be described in more detail below using examples and comparative examples, but it is not limited in any way to the following examples.

[0045] <<Preparation of Cathode Active Material Slurry>> [Example 1] First, three types of polyvinylidene fluoride (PVdF) with different molecular weight distributions were prepared and dissolved in butyl acetate, a solvent, to prepare a binder solution (corresponding to "Solution Y" in the present invention). The molecular weight distribution of each of these three types of PVdF was measured using the following method (gel permeation chromatography (GPC)). As a result, the peak top molecular weight values ​​in the molecular weight distribution of the three types of PVdF were 200,000, 400,000, and 700,000, respectively. Furthermore, when the average molecular weight of the above binder solution was measured using the same method, the number average molecular weight (Mn) was 270,000, the weight average molecular weight (Mw) was 440,000, and the degree of dispersion (Mw / Mn) was 1.6. Figure 1 shows a graph of the molecular weight distribution results for the above binder solution.

[0046] (Method for measuring molecular weight distribution) Instrument name: Agilent 1200 series Column: Agilent MIXED-C (double) Column temperature: 40°C Mobile phase: Tetrahydrofuran Injection volume: 100 μL Molecular weight standard: Polystyrene Detector: RI detector (Positive).

[0047] On the other hand, LiNi, which is the positive electrode active material ０．８ Mn ０．１ Co ０．１ O ２ (Average particle size (D50) = 5 μm), and Li is an argyrodite-type sulfide solid electrolyte. ６ PS ５ Cl (D50 = 1 μm) was mixed and then mixed using a spatula to prepare a powder mixture.

[0048] Furthermore, a conductive additive solution was prepared by dispersing carbon nanotubes (average chain length 20 μm), which are conductive additives, in butyl acetate, which is a solvent. Next, this conductive additive solution was added to the binder solution prepared above, and butyl acetate, which is a solvent, was further added to adjust the viscosity, and the mixture was prepared to prepare a conductive additive / binder solution (corresponding to "Solution X" of the present invention).

[0049] Subsequently, the powder mixture prepared above was added to the conductive additive / binder solution prepared above. Using a planetary stirring type mixing and kneading device "Awatori Rentaro" (ARE-310, manufactured by Thinky Co., Ltd.), the mixture was kneaded four times at 2000 rpm for 10 seconds, 20 seconds, 30 seconds, and 1 minute. Then, butyl acetate, a solvent, was added to adjust the viscosity, and the mixture was kneaded again at 2000 rpm for 2 minutes using the same mixing and kneading device to prepare the electrode active material slurry of this example. The mass ratio of solid content in the slurry was set to positive electrode active material: solid electrolyte: conductive additive: binder = 87.0:11.2:1.0:0.8.

[0050] [Comparative Example 1] A conductive additive solution was prepared by dispersing carbon nanotubes (average chain length 20 μm), which are conductive additives, in butyl acetate, which is a solvent. Next, the same powder composition as in Example 1 was added to the conductive additive solution, and the mixture was kneaded four times at 2000 rpm for 10 seconds, 20 seconds, 30 seconds, and 1 minute using a planetary stirring type mixing and kneading device "Awatori Rentaro" (ARE-310, manufactured by Thinky Co., Ltd.).

[0051] Subsequently, the same binder solution as in Example 1 was added, and the mixture was kneaded at 2000 rpm for 2 minutes using the above-mentioned mixing and kneading apparatus. Then, butyl acetate, which is a solvent, was added to adjust the viscosity, and the mixture was kneaded again at 2000 rpm for 2 minutes using the above-mentioned mixing and kneading apparatus to prepare the electrode active material slurry of this comparative example. The mass ratio of solid content in the slurry was set to positive electrode active material: solid electrolyte: conductive additive: binder = 87.0:11.2:1.0:0.8.

[0052] [Comparative Example 2] The same powder composition as in Example 1 was added to the same binder solution as in Example 1, and the mixture was kneaded four times at 2000 rpm for 10 seconds, 20 seconds, 30 seconds, and 1 minute using a planetary stirring type mixing and kneading device "Awatori Rentaro" (ARE-310, manufactured by Thinky Co., Ltd.).

[0053] Subsequently, carbon nanotubes (average chain length 20 μm), which are conductive additives, were added, and butyl acetate, which is a solvent, was further added to adjust the viscosity. Then, the mixture was kneaded for 2 minutes at 2000 rpm using the above-mentioned mixing and kneading apparatus to prepare the electrode active material slurry of this comparative example. The mass ratio of solid content in the slurry was set to positive electrode active material: solid electrolyte: conductive additive: binder = 87.0:11.2:1.0:0.8.

[0054] <<Fabrication of the positive electrode and measurement of the volume resistivity of the positive electrode active material layer>> The electrode active material slurry prepared in the above-described examples and comparative examples was coated onto the surface of an aluminum foil positive electrode current collector using a desktop coating machine, and dried on a hot plate set to 80°C to form a positive electrode active material layer on the surface of the positive electrode current collector, thereby fabricating the positive electrode. This operation was performed inside a glove box.

[0055] Next, the positive electrode prepared above was punched out into a φ10 mm circle. Then, the resistance of the positive electrode active material layer was measured using an LCR meter (Texio Technology Co., Ltd., model LCR916), and the volume resistivity [Ω・cm] was calculated from the thickness of the positive electrode active material layer. The results are shown in Table 1 below. Note that the volume resistivity values ​​listed in Table 1 are relative values ​​with the measured value of Comparative Example 2 set to 1.

[0056]

[0057] As shown in Table 1, the volume resistivity of the positive electrode active material layer formed using the manufacturing method according to one embodiment of the present invention was significantly lower than the volume resistivity of the positive electrode active material layer obtained in the comparative example. This indicates that the method for manufacturing electrode active material slurry for secondary batteries according to one embodiment of the present invention is extremely effective in reducing the internal resistance of secondary batteries.

[0058] Furthermore, in Comparative Examples 1 and 2, which did not involve the step of first mixing the conductive additive and the low molecular weight binder, the resulting positive electrode active material layer exhibited very high volume resistivity. This is thought to be due to the fact that the slurry manufacturing method employed failed to sufficiently suppress the aggregation of the conductive additive in the slurry, and therefore could not adequately secure electron conduction paths in the positive electrode active material layer.

Claims

1. A method for producing an electrode active material slurry for a secondary battery, comprising: a solvent, an electrode active material, a solid electrolyte, a conductive additive, and a binder component comprising a low molecular weight binder and a high molecular weight binder having a peak top molecular weight greater than that of the low molecular weight binder (the peak top value in the polystyrene-equivalent molecular weight distribution measured using tetrahydrofuran as an eluent by gel permeation chromatography (GPC)), the method comprising: preparing a solution X comprising the solvent, the low molecular weight binder, and the conductive additive; and mixing the electrode active material and the solid electrolyte with the solution X to prepare a slurry.

2. The method for producing an electrode active material slurry for a secondary battery according to claim 1, comprising: preparing a solution Y containing the solvent and the low molecular weight binder; and preparing the solution X by mixing the conductive additive with the solution Y.

3. The method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the preparation of the solution X comprises mixing the conductive additive dispersed in the solvent with the low molecular weight binder.

4. The method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the solution X further comprises the high molecular weight binder.

5. The method for producing an electrode active material slurry for a secondary battery according to claim 2, wherein the solution Y further comprises the high molecular weight binder.

6. A method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the electrode active material and the solid electrolyte are mixed into the solution X in the form of a uniformly mixed powder composition.

7. A method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the peak top molecular weight of the low molecular weight binder is 100,000 or more and 600,000 or less, and the peak top molecular weight of the high molecular weight binder is greater than 600,000.

8. The method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the total content of the electrode active material and the solid electrolyte in the solution X is 10% by mass or less based on 100% by mass of the total amount of the solution X.

9. The method for producing an electrode active material slurry for a secondary battery according to claim 1 or 2, wherein the total content of the solvent, the binder component, and the conductive additive in the solution X is 90% by mass or more based on 100% by mass of the total amount of the solution X.

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