Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/05—Accumulators with non-aqueous electrolyte
  • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
  • H01M10/0561—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of inorganic materials only
  • H01M10/0562—Solid materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
  • H01M4/139—Processes of manufacture
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to a positive electrode for an all-solid-state battery.
• Solid electrolytes are materials that are primarily composed of ion conductors that are capable of ion conduction in a solid state. For this reason, all-solid-state batteries have the advantage that, in principle, they do not suffer from the various problems that arise from flammable organic electrolytes, as occurs with conventional liquid-based batteries that use non-aqueous electrolytes.
• the use of high-potential, large-capacity positive electrode materials and large-capacity negative electrode materials generally leads to a significant improvement in the output density and energy density of the battery.
• the positive electrode has a configuration in which a positive electrode active material layer is disposed on the surface of a positive electrode current collector.
• the positive electrode active material layer contains a conductive assistant to improve the electrical conductivity in the positive electrode active material layer, a solid electrolyte to improve the lithium ion conductivity in the positive electrode active material layer, and a binder to bind the particles of the positive electrode active material, conductive assistant, and solid electrolyte to each other and to bind these particles to the positive electrode current collector.
• WO 2022/050252 discloses that a polytetrafluoroethylene resin having a predetermined fine fiber structure is used as a binder contained in a mixture for an all-solid-state battery.
• the present invention aims to provide a positive electrode for an all-solid-state battery that has excellent strength and can improve the cycle durability of the battery.
• one aspect of the present invention is a positive electrode for an all-solid-state battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder including polytetrafluoroethylene, and a fibrous conductive assistant, and the positive electrode active material layer is a positive electrode for an all-solid-state battery including a composite of a film made of the polytetrafluoroethylene and the fibrous conductive assistant.
• one aspect of the present invention is a method for producing a positive electrode for an all-solid-state battery having a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, the method including a step of rolling a positive electrode mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.
• FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery, which is one embodiment of the all-solid-state battery according to the present invention.
• FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. 3(a) and 3(b) are SEM images of the samples of the positive electrode active material layer prepared in Comparative Example 2 and Example 2 after the hydrochloric acid treatment, respectively.
• One aspect of the present invention is a positive electrode for an all-solid-state battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder including polytetrafluoroethylene, and a fibrous conductive assistant, the positive electrode active material layer including a composite of a film made of polytetrafluoroethylene and the fibrous conductive assistant.
• a positive electrode with excellent strength can be obtained.
• cycle durability can be improved in an all-solid-state battery using the positive electrode for an all-solid-state battery of this aspect.
• FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention.
• FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1.
• the layered structure allows the battery to be compact and have a high capacity.
• the flat-layered non-bipolar all-solid-state lithium secondary battery shown in FIGS. 1 and 2 (hereinafter also simply referred to as a "layered secondary battery”) will be described in detail as an example.
• the internal electrical connection form (electrode structure) of the secondary battery according to this embodiment it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.
• the stacked secondary battery 10a has a flat rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides to extract power.
• the power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked secondary battery 10a, and its periphery is heat-sealed, with the power generating element 21 sealed with the negative electrode current collector 25 and the positive electrode current collector 27 extended to the outside.
• the removal of the current collectors (25, 27) shown in FIG. 1 is not limited to that shown in FIG. 1; the negative electrode current collector 25 and the positive electrode current collector 27 may be pulled out from the same side, or the negative electrode current collector 25 and the positive electrode current collector 27 may each be divided into multiple pieces and removed from each side.
• the power generating element 21 of the stacked secondary battery 10a of this embodiment has a configuration in which, during charging, a negative electrode in which a negative electrode active material layer 13 containing lithium metal is arranged on both sides of a negative electrode collector 11', a solid electrolyte layer 17, and a positive electrode in which a positive electrode active material layer 15 containing lithium transition metal composite oxide is arranged on both sides of a positive electrode collector 11" are stacked.
• the negative electrode, solid electrolyte layer, and positive electrode are stacked in this order such that one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other via the solid electrolyte layer 17.
• the stacked secondary battery 10a shown in FIG. 2 can be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.
• a negative electrode collector 25 and a positive electrode collector 27 that are electrically connected to the electrodes (negative and positive) are attached to the negative electrode collector 11' and the positive electrode collector 11", respectively, and are structured so as to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29.
• the negative electrode collector 25 and the positive electrode collector 27 may be attached to the negative electrode collector 11' and the positive electrode collector 11" of each electrode by ultrasonic welding, resistance welding, or the like, via negative electrode terminal leads and positive electrode terminal leads (not shown), respectively, as necessary.
• the power generating element 21 sealed in the laminate film 29 shown in FIG. 1 is preferably sandwiched between two plate-shaped members and further fastened using a fastening member.
• the plate-shaped member and the fastening member function as a pressure member that presses (restrains) the power generating element 21 in its stacking direction.
• the plate-shaped member include a metal plate and a resin plate.
• the fastening member include a bolt and a nut.
• the pressure member is not particularly limited as long as it is a member that can pressurize the power generating element 21 in its stacking direction.
• the pressure member a combination of a plate made of a material having rigidity like the plate-shaped member and the above-mentioned fastening member is typically used.
• tension plates that fix the ends of the plate-shaped members so as to restrain the power generating element 21 in its stacking direction may be used as the fastening members.
• the lower limit of the load applied to the power generating element 21 is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more.
• the upper limit of the restraint pressure in the stacking direction of the power generating element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.
• the current collectors (negative electrode current collector 11′ and positive electrode current collector 11′′) are conductive members that function as a flow path for electrons that are released from the positive electrode toward an external load as the battery reaction (charge/discharge reaction) progresses, or that flow from a power source toward the positive electrode.
• the material that constitutes the current collectors there are no particular limitations on the material that constitutes the current collectors, and for example, metals or conductive resins can be used.
• metals include aluminum, nickel, iron, stainless steel, titanium, and copper.
• clad materials of nickel and aluminum, and clad materials of copper and aluminum may also be used.
• a foil in which the metal surface is coated with aluminum may be used.
• aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of active materials.
• the latter conductive resins include conductive polymer materials and non-conductive polymer materials to which conductive fillers are added as necessary.
• the current collector may be a single layer structure made of a single material, or may be a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that it contains at least a conductive resin layer made of a resin having conductivity. Also, from the viewpoint of blocking the movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the above-mentioned negative electrode active material layer and positive electrode active material layer are conductive by themselves and can perform a current collecting function, it is not necessary to use a current collector as a member separate from these active material layers. In such a form, the above-mentioned negative electrode active material layer directly constitutes the negative electrode, and the above-mentioned positive electrode active material layer directly constitutes the positive electrode.
• the thickness of the current collector there are no particular limitations on the thickness of the current collector, but an example is 10 to 100 ⁇ m.
• the negative electrode active material layer 13 includes a negative electrode active material, and may include a solid electrolyte, a binder, and a conductive assistant as necessary.
• the type of the negative electrode active material is not particularly limited, but may include a carbon material, a metal oxide, and a metal active material.
• an active material containing lithium may be used as the negative electrode active material.
• Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and may include lithium metal as well as a lithium-containing alloy. Examples of the lithium-containing alloy include an alloy of Li and at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn.
• the negative electrode active material preferably includes lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably includes lithium metal or a lithium-containing alloy.
• the lithium secondary battery is preferably a so-called lithium deposition type in which lithium metal as a negative electrode active material is deposited on the negative electrode current collector during charging.
• the layer made of lithium metal deposited on the negative electrode current collector during the charging process becomes the negative electrode active material layer, so that the thickness of the negative electrode active material layer increases with the progress of the charging process, and the thickness of the negative electrode active material layer decreases with the progress of the discharging process.
• the negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be placed during full discharge.
• the thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is usually 0.1 to 1000 ⁇ m.
• the solid electrolyte layer 17 is interposed between the positive electrode active material layer and the negative electrode active material layer, and is a layer that essentially contains a solid electrolyte.
• solid electrolyte examples include sulfide solid electrolytes and oxide solid electrolytes, with sulfide solid electrolytes being preferred.
• solid electrolyte refers to a material that is mainly composed of an ion conductor capable of ion conduction in a solid, and in particular refers to a material that has a lithium ion conductivity of 1 ⁇ 10 ⁇ 5 S/cm or more at room temperature (25° C.), and this lithium ion conductivity is preferably 1 ⁇ 10 ⁇ 4 S/cm or more.
• the value of the ion conductivity can be measured by an AC impedance method.
• Examples of the sulfide solid electrolyte include LiI-Li 2 S-SiS 2 , LiI-Li 2 SP 2 O 5 , LiI-Li 3 PO 4 -P 2 S 5 , Li 2 SP 2 S 5 , LiI-Li 3 PS 4 , LiI-LiBr-Li 3 PS 4 , Li 3 PS 4 , Li 2 S-P 2 S 5 -Li 2 O, Li 2 S-P 2 S 5 -Li 2 O-LiI, Li 2 S-SiS 2 , Li 2 S-SiS 2 -LiI, Li 2 S-SiS 2 -LiBr, Li2S - SiS2 -LiCl, Li 2 S-SiS 2 -B 2 S 3 -LiI, Li 2 S-SiS 2 -P 2 S 5 -LiI, Li 2 S-B 2 S 3 , Li 2 S-P 2 S 5 -Z m S n (wherein m
• the sulfide solid electrolyte may have, for example, a Li 3 PS 4 skeleton, a Li 4 P 2 S 7 skeleton, or a Li 4 P 2 S 6 skeleton.
• sulfide solid electrolytes having a Li 3 PS 4 skeleton include LiI-Li 3 PS 4 , LiI-LiBr-Li 3 PS 4 , and Li 3 PS 4 .
• sulfide solid electrolytes having a Li 4 P 2 S 7 skeleton include Li-P-S solid electrolytes called LPS (for example, Li 7 P 3 S 11 ).
• sulfide solid electrolytes include LGPS represented by Li (4-x) Ge (1-x) P x S 4 (x satisfies 0 ⁇ x ⁇ 1).
• the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing P element, and more preferably a material containing Li 2 S-P 2 S 5 as a main component.
• the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I).
• the sulfide solid electrolyte contains Li 6 PS 5 X (wherein X is Cl, Br or I, preferably Cl).
• oxide solid electrolytes include compounds having a NASICON structure.
• examples of compounds having a NASICON structure include compounds (LAGP) represented by the general formula Li 1+x Al x Ge 2-x (PO 4 ) 3 (0 ⁇ x ⁇ 2), and compounds (LATP) represented by the general formula Li 1+x Al x Ti 2-x (PO 4 ) 3 (0 ⁇ x ⁇ 2).
• Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li 0.34 La 0.51 TiO 3 ), LiPON (e.g., Li 2.9 PO 3.3 N 0.46 ), and LiLaZrO (e.g., Li 7 La 3 Zr 2 O 12 ).
• the shape of the solid electrolyte may be, for example, particulate, such as a perfect sphere or an oval sphere, or a thin film.
• its average particle size (D50) is not particularly limited, but is preferably 40 ⁇ m or less, more preferably 20 ⁇ m or less, and even more preferably 10 ⁇ m or less.
• the average particle size (D50) is preferably 0.01 ⁇ m or more, and more preferably 0.1 ⁇ m or more.
• the solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above.
• binders include, but are not limited to, polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene (PTFE), polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, thermoplastic polymers such as styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof,
• the thickness of the solid electrolyte layer varies depending on the configuration of the intended all-solid-state battery, but is preferably within the range of 0.1 to 1000 ⁇ m, and more preferably 10 to 40 ⁇ m.
• the positive electrode active material layer includes a positive electrode active material, a solid electrolyte, a binder including polytetrafluoroethylene (PTFE), and a fibrous conductive additive.
• the positive electrode active material layer has a structure including a composite of a film made of PTFE and the fibrous conductive additive.
• the positive electrode active material is not particularly limited as long as it is a material that can release lithium ions during the charging process of the all-solid-state battery and absorb lithium ions during the discharging process.
• An example of such a positive electrode active material includes a material that contains an M1 element and an 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.
• positive electrode active material examples include layered rock salt type active materials such as LiCoO 2 , LiMnO 2 , LiNiO 2 , and Li(Ni-Mn-Co)O 2 , spinel type active materials such as LiMn 2 O 4 and LiNi 0.5 Mn 1.5 O 4, olivine type active materials such as LiFePO 4 and LiMnPO 4, and Si-containing active materials such as Li 2 FeSiO 4 and Li 2 MnSiO 4 .
• oxide active materials other than those mentioned above include Li 4 Ti 5 O 12 and LiVO 2 .
• the positive electrode active material may contain a sulfur element.
• the positive electrode active material containing a sulfur element is not particularly limited, but may be, in addition to sulfur element (S), particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and may be any material capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.
• the organic sulfur compound include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbonate sulfide, and the like.
• disulfide compounds sulfur-modified polyacrylonitrile, and rubeanic acid are preferred, and sulfur-modified polyacrylonitrile is particularly preferred.
• disulfide compound those having a dithiobiurea derivative, a thiourea group, a thioisocyanate, or a thioamide group are more preferred.
• inorganic sulfur compounds are preferred because of their excellent stability, and specifically, sulfur element (S), S-carbon composite, Li 2 S, TiS 2 , TiS 3 , TiS 4 , NiS, NiS 2 , CuS, FeS 2 , MoS 2 , MoS 3 and the like can be mentioned.
• S Li 2 S, S-carbon composite, TiS 2 , TiS 3 , TiS 4 , FeS 2 and MoS 2 are preferred, sulfur element (S), Li 2 S, TiS 2 and FeS 2 are more preferred, and sulfur element (S) or Li 2 S is particularly preferred from the viewpoint of high capacity.
• sulfur element (S) As the sulfur element (S), ⁇ -sulfur, ⁇ -sulfur, or ⁇ -sulfur having an S8 structure can be used. During discharge, these elemental sulfur (S) absorbs lithium ions and exists in the positive electrode active material layer in the form of lithium (poly)sulfides.
• two or more types of positive electrode active materials may be used in combination.
• positive electrode active materials other than those mentioned above may also be used.
• the shape of the positive electrode active material is preferably particulate.
• its average particle diameter (D50) is preferably within the range of 1 nm to 100 ⁇ m, more preferably within the range of 10 nm to 50 ⁇ m, even more preferably within the range of 100 nm to 20 ⁇ m, and particularly preferably within the range of 1 to 20 ⁇ m.
• the average particle diameter (D50) of the positive electrode active material is determined as the average value of the particle diameters of the positive electrode active material observed in several to several tens of fields of view using a scanning electron microscope (SEM) on the cross section of the positive electrode active material layer.
• the content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99 mass%, more preferably in the range of 40 to 90 mass%, and even more preferably in the range of 50 to 85 mass%.
• Solid electrolyte The specific form of the solid electrolyte contained in the positive electrode active material layer is not particularly limited, and the solid electrolytes exemplified in the section on the solid electrolyte layer and their preferred forms may be similarly adopted. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.
• the solid electrolyte content in the positive electrode active material layer is preferably within the range of 1 to 60 mass %, and more preferably within the range of 10 to 50 mass %.
• the fibrous conductive assistant can contribute to improving the conductivity (reducing resistance) in the active material layer.
• the term "fibrous conductive assistant” refers to a conductive assistant having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 ⁇ m or less in an observation image when a cross section of a positive electrode active material layer is observed using a scanning electron microscope (SEM). The aspect ratio is calculated by dividing the maximum Feret diameter by the minimum Feret diameter.
• the maximum Feret diameter is the maximum distance between two parallel straight lines when the contour of the conductive assistant is sandwiched between the two parallel straight lines
• the minimum Feret diameter is the minimum distance between the two parallel straight lines when the contour of the conductive assistant is sandwiched between the two parallel straight lines.
• the electronic conductivity of the fibrous conductive assistant is preferably 1 S/m or more, more preferably 1 ⁇ 10 2 S/m or more, even more preferably 1 ⁇ 10 4 S/m or more, and even more preferably 1 ⁇ 10 5 S/m or more.
• the upper limit of the electronic conductivity of the fibrous conductive assistant is not particularly limited, but is usually 1 ⁇ 10 7 S/m or less.
• fibrous carbon is preferred because it is lightweight and has excellent conductivity.
• type of fibrous carbon there are no particular limitations on the type of fibrous carbon as long as it has the above-mentioned shape, but examples include carbon fiber (carbon nanofiber), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Of these, carbon fiber (carbon nanofiber) and carbon nanotubes are preferred. Only one type of fibrous carbon may be used alone, or two or more types may be used in combination.
• the average fiber length of the fibrous conductive assistant contained in the positive electrode active material layer is not particularly limited, but is, for example, 5 ⁇ m or more, more preferably 7 ⁇ m or more, and even more preferably 10 ⁇ m or more. Within the above range, a stronger composite can be obtained. In addition, it is preferable because a sufficient electron conduction path can be obtained, the utilization rate of the positive electrode active material can be improved, and the capacity maintenance rate can be improved.
• the upper limit of the average fiber length is not particularly limited, but is, for example, 100 ⁇ m or less.
• the average fiber length of the fibrous conductive assistant can be obtained as the average value of the fiber lengths of the fibrous conductive assistant observed by photographing about 10 fields of view of the cross section of the positive electrode active material layer using a scanning electron microscope (SEM).
• the average fiber length of the fibrous conductive assistant is preferably greater than the average particle diameter (D50) of the positive electrode active material. This makes it possible to obtain a stronger composite. In addition, it is possible to prevent the electronic conduction path of the positive electrode active material from being broken due to the expansion and contraction of the positive electrode active material during charging, and it is possible to maintain the electronic contact between the positive electrode active materials. Therefore, the effect of the present invention can be obtained more significantly.
• the ratio of the average fiber length of the fibrous conductive assistant to the average particle diameter (D50) of the positive electrode active material is, for example, 1.1 or more, preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more.
• the upper limit of the ratio of the average fiber length of the fibrous conductive assistant to the average particle diameter (D50) of the positive electrode active material is not particularly limited, but is, for example, 100 or less.
• the content of the fibrous conductive assistant in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 5% by mass or less.
• the positive electrode active material layer may further contain a conductive assistant other than the fibrous conductive assistant (non-fibrous conductive assistant).
• a conductive assistant other than the fibrous conductive assistant non-fibrous conductive assistant.
• the particle shape of the non-fibrous conductive assistant is not particularly limited, and may be any shape such as powder, sphere, rod, needle, plate, column, irregular shape, flake, spindle shape, etc.
• the number of contact points for electronic conduction increases, so that the resistance of the positive electrode can be further reduced.
• Examples of such conductive assistants include carbon black such as acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal black.
• the proportion of the non-fibrous conductive assistant is, for example, more than 0 and less than 50 mass%, preferably 1 to 30 mass%, and more preferably 1 to 10 mass%, relative to the total mass of the conductive assistant contained in the positive electrode active material layer.
• the positive electrode active material layer contains polytetrafluoroethylene (PTFE) as a binder.
• PTFE is a polymer of tetrafluoroethylene, and can be fibrillated (fibrous) by the application of shear force due to kneading.
• the positive electrode active material, solid electrolyte, and fibrous conductive assistant are entangled and held in the fibrous structure of the fibrillated PTFE.
• polytetrafluoroethylene may include those in which the terminals or part of the side chains are substituted (modified) with other substituents.
• the proportion of structural units in which the terminals or side chains are substituted (modified) with other substituents in 100 mol % of all structural units is preferably 10 mol % or less, and more preferably 5 mol % or less.
• the positive electrode active material layer may contain a binder other than PTFE.
• the other binder may be in the same form as the binder exemplified in the solid electrolyte layer section.
• the proportion of the other binder is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 3 mass% or less, particularly preferably 1 mass% or less, and most preferably 0 mass% (i.e., no other binder is included) relative to the total mass of the binder contained in the positive electrode active material layer.
• the amount of PTFE contained in the positive electrode active material layer is not particularly limited, but is, for example, 0.1 to 5 mass%, preferably 0.1 to 3 mass%, more preferably 0.2 to 2 mass%, even more preferably 0.3 to 1.2 mass%, and even more preferably 0.4 to 1 mass%, relative to the total solid content contained in the positive electrode active material layer. If it is within the above range, it will have an excellent balance between energy density and strength.
• the positive electrode of this embodiment has a composite of a film made of polytetrafluoroethylene and a fibrous conductive assistant in the positive electrode active material layer.
• FIG. 3(a) shows an SEM image of the positive electrode active material layer of a conventional positive electrode
• FIG. 3(b) shows an SEM image of the positive electrode active material layer of the positive electrode of this embodiment.
• components other than PTFE and fibrous conductive assistant are removed in order to observe the structure of the composite.
• PTFE exists mainly in a fibrous form.
• the fibrous PTFE and the fibrous conductive assistant exist while being entangled with each other, the fibrous conductive assistant is not embedded in the PTFE, and the spatial position of the fibrous conductive assistant is not fixed by the PTFE. Also, no film-like PTFE is observed.
• FIG. 3(a) shows an SEM image of the positive electrode active material layer of a conventional positive electrode
• FIG. 3(b) shows an SEM image of the positive electrode active material layer of the positive electrode of this embodiment.
• components other than PTFE and fibrous conductive assistant are removed in order to observe the structure of the composite.
• FIG. 3(a) in
• a film-like PTFE is observed in the positive electrode active material layer of this embodiment.
• This PTFE film is formed in the in-plane direction of the positive electrode active material layer, and the film made of PTFE and the fibrous conductive assistant form a composite.
• the fibrous conductive assistant is embedded in a film made of PTFE.
• the spatial position of the fibrous conductive assistant is fixed by PTFE.
• the above composite is formed by kneading the fibrous conductive assistant into the softened PTFE and then curing it.
• the above composite has an integrated structure in which the fibrous conductive assistant is entangled while the softened PTFE is crushed.
• the PTFE contained in the positive electrode active material layer is in the form of a film to form a composite with the fibrous conductive assistant.
• a fibrous binder such as fibrillated PTFE is used.
• the positive electrode active material expands and contracts with the charge and discharge reaction.
• the positive electrode active material, solid electrolyte, and conductive assistant in the positive electrode active material layer are maintained in contact with each other.
• the positive electrode active material expands and contracts significantly, gaps are generated between the positive electrode active material and each component, and the positive electrode active material is easily isolated from the electronic conduction path and/or ionic conduction path, resulting in a decrease in the utilization rate of the positive electrode active material.
• the cycle durability of the battery may become insufficient.
• a fibrous conductive assistant is used as the conductive assistant, it is easier to secure contact between the positive electrode active materials and to maintain the electronic conduction path even if the positive electrode active material shrinks during charging, compared to when a conductive assistant in a particulate form is used.
• the electrolyte in a solid-state battery does not have fluidity like that in a liquid-based battery, it is difficult to maintain the ionic conduction path when the positive electrode active material shrinks. Therefore, even when a fibrous conductive assistant is used, sufficient improvement in cycle durability may not be obtained.
• the mechanical strength of a positive electrode active material layer produced by a dry process using fibrous PTFE as a binder is insufficient.
• the positive electrode for an all-solid-state secondary battery according to this embodiment has a composite of a film made of PTFE and a fibrous conductive assistant in the positive electrode active material layer.
• the fibrous conductive assistant becomes a filler that makes the PTFE structure stronger, and a structure with high strength can be obtained.
• the positive electrode for an all-solid-state secondary battery according to this embodiment has a high mechanical strength that allows it to exist as a free-standing film, and is excellent in handleability. In addition, such high strength allows the amount of PTFE in the positive electrode active material layer to be reduced, thereby increasing the energy density.
• the composite of the film made of PTFE and the fibrous conductive assistant can form not only an electronic conductive path, but also a high-strength ion conductive path that can follow the expansion and contraction of the positive electrode active material accompanying charging and discharging. As a result, it is believed that cycle durability can be further improved.
• At least one of the positive electrode active material and the solid electrolyte is embedded in the composite, and it is more preferable that both the positive electrode active material and the solid electrolyte are embedded in the composite. This makes it possible to more effectively ensure the electron conduction path and the ion conduction path, and to obtain the effects of the present invention more significantly.
• the manufacturing method of the positive electrode for the all-solid-state battery of this embodiment is not particularly limited as long as it can form the above-mentioned specific complex in the positive electrode active material layer, but for example, it is preferable to include a step of rolling a mixture containing a positive electrode active material, a solid electrolyte, a fibrous conductive assistant, and fibrous polytetrafluoroethylene while heating it to 40 ° C. or higher.
• the fibrous PTFE can be softened by heating it to 40 ° C. or higher. Then, by rolling it in a heated state, the softened PTFE is crushed into a membrane-like form and hardened.
• the PTFE is hardened in a state in which the fibrous conductive assistant is kneaded into the PTFE, and a structure of a complex of a membrane made of PTFE and a fibrous conductive assistant can be obtained.
• one aspect of the present invention is a method for producing a positive electrode for an all-solid-state battery having a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, the method including a step of rolling a mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.
• the manufacturing method includes (1) a mixing step of mixing a positive electrode active material, a fibrous conductive assistant, and a solid electrolyte, (2) a kneading step of kneading the mixture obtained in the mixing step with polytetrafluoroethylene (PTFE) to fibrillate the PTFE, and (3) a heating and rolling step of rolling the mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive assistant, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.
• PTFE polytetrafluoroethylene
• the positive electrode active material, the fibrous conductive assistant, and the solid electrolyte are mixed.
• the specific form of these components is as described above.
• the order of addition of each component is not particularly limited.
• the mixing means is also not particularly limited, and known methods can be appropriately referred to.
• materials other than the positive electrode active material, solid electrolyte, and fibrous conductive assistant may be added and mixed as necessary.
• other materials include non-fibrous conductive assistants.
• mixing conditions There are no particular limitations on the mixing conditions. There are no particular limitations on the mixing time, but it is, for example, 1 to 120 minutes.
• the mixing step is preferably carried out in the absence of a solvent. This makes it unnecessary to remove the solvent when forming the positive electrode active material layer. In addition, it is possible to prevent a reaction between the solid electrolyte and the solvent. In addition, it is preferable to carry out the mixing step in an inert gas atmosphere.
• a kneading step in which the mixture obtained in the mixing step and polytetrafluoroethylene (PTFE) are kneaded after the mixing step.
• PTFE can be fibrillated (fibrous) by the application of shear force due to the kneading.
• the positive electrode active material, the solid electrolyte, and the fibrous conductive assistant are entangled and held in the fibrous structure of the fibrillated PTFE.
• the timing of adding polytetrafluoroethylene (PTFE) is not particularly limited, and may be, for example, when the positive electrode active material, the solid electrolyte, and the fibrous conductive assistant are put into a mixing container in the mixing step, or may be after the mixing step and before the kneading step. Among them, it is preferable to add polytetrafluoroethylene (PTFE) after the mixing step and before the kneading step, which can suppress cutting of the fibrous conductive assistant.
• PTFE polytetrafluoroethylene
• the kneading device used for kneading is not particularly limited as long as it can apply a shear force to the PTFE, and an extruder, Banbury mixer, roller, kneader, etc. may be used as appropriate. Kneading may also be performed using a mortar.
• the above steps produce a mixture containing the positive electrode active material, solid electrolyte, fibrous conductive assistant, and fibrous PTFE.
• the mixture is preferably a powder mixture that does not substantially contain liquid components.
• the content of liquid components in the mixture is 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 0.1% by mass or less, and most preferably 0% by mass, relative to 100% by mass of the mixture.
• the hot rolling step In the hot rolling step, the mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive assistant, and the fibrous PTFE obtained in the above kneading step is rolled while being heated to 40° C. or higher.
• the rolling means is not particularly limited, and known rollers and the like can be used.
• a rolling device such as a roll press may also be used.
• the heating means is also not particularly limited, and known heating means can be appropriately adopted, but a roll press equipped with a heating mechanism can be preferably used.
• the heating temperature is below 40°C, the PTFE in the mixture does not soften sufficiently, and a composite is not formed.
• There is no particular upper limit for the heating temperature but if it is 100°C or less, it is preferable because when the mixture is sandwiched between two substrates such as SUS foil and rolled, the components in the mixture are less likely to react with the SUS foil.
• the heating temperature is 40 to 90°C, even more preferably 50 to 90°C, even more preferably 55 to 85°C, and particularly preferably 60 to 80°C.
• the powder (kneaded product) obtained by the above kneading process is sandwiched between two substrates such as SUS foil and rolled while being heated to form a sheet having a thickness of 400 to 800 ⁇ m. It is then preferable to perform multiple rolling processes so that the thickness of the sheet is gradually reduced while being heated, until the desired thickness of the positive electrode active material layer is obtained.
• the rolling process can be performed so that the thickness of the sheet is reduced by 50 to 100 ⁇ m at a time. By performing multiple rolling processes so that the thickness of the sheet is gradually reduced, it is possible to prevent the density of the positive electrode from becoming excessively high and to improve the handleability.
• This heating and rolling process can produce a sheet-shaped positive electrode active material layer having a predetermined composite structure.
• the thickness of the positive electrode active material layer varies depending on the desired configuration of the all-solid-state battery, but is preferably within the range of 0.1 to 1000 ⁇ m, and more preferably 40 to 100 ⁇ m.
• the material constituting the current collector plate (25, 27) is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a secondary battery can be used.
• a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof is preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable.
• the same material may be used for the positive electrode current collector plate 27 and the negative electrode current collector plate 25, or different materials may be used.
• the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead.
• a positive electrode lead As the material for the positive electrode and the negative electrode lead, materials used in known secondary batteries may be similarly adopted. It is preferable that the part taken out from the exterior is covered with a heat-resistant insulating heat shrink tube or the like so as not to come into contact with peripheral devices or wiring, etc., causing leakage and affecting products (e.g., automobile parts, especially electronic devices, etc.).
• Battery exterior material As the battery exterior material, a known metal can case can be used, and a bag-shaped case using a laminate film 29 containing aluminum, which can cover the power generating element as shown in Figures 1 and 2, can be used.
• the laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto.
• a laminate film is preferable from the viewpoint of being excellent in high output and cooling performance and being suitable for use in batteries for large equipment for EVs and HEVs.
• a laminate film containing aluminum is more preferable as the exterior body.
• a positive electrode for an all-solid-state battery according to claim 1 having the characteristics of claim 2 a positive electrode for an all-solid-state battery according to claim 1 or 2 having the characteristics of claim 3; a positive electrode for an all-solid-state battery according to any one of claims 1 to 3 having the characteristics of claim 4; an all-solid-state battery including a positive electrode for an all-solid-state battery according to any one of claims 1 to 4.
• PTFE polytetrafluoroethylene
• Comparative Examples 2 and 3 The positive electrode active material layers of Comparative Examples 2 and 3 were obtained in the same manner as in Comparative Example 1, except that the amounts of PTFE added were 0.8 parts by mass and 1.0 parts by mass, respectively, relative to 100 parts by mass of the mixed powder in Comparative Example 1. Here, the contents of PTFE in the positive electrode active material layer were 0.8% by mass and 1.0% by mass, respectively, relative to the total solid content contained in the positive electrode active material layer.
• Example 1 In a glove box in an argon atmosphere with a dew point of -68 ° C. or less, NMC composite oxide (LiNi 0.8 Mn 0.1 Co 0.1 O 2 , average particle size (D50): 6 ⁇ m) as a positive electrode active material, carbon nanotubes with an average fiber length of 20 ⁇ m as a conductive assistant, and an argyrodite-type sulfide solid electrolyte (Li 6 PS 5 Cl, average particle size (D50): 1 ⁇ m) as a solid electrolyte were weighed to a mass ratio of 70: 5: 25 and mixed using an agate mortar.
• NMC composite oxide LiNi 0.8 Mn 0.1 Co 0.1 O 2 , average particle size (D50): 6 ⁇ m
• carbon nanotubes with an average fiber length of 20 ⁇ m as a conductive assistant
• an argyrodite-type sulfide solid electrolyte Li 6 PS 5 Cl, average particle size (D
• PTFE polytetrafluoroethylene
• the heated and stretched sheet-like molded product was stretched in a tabletop roll press machine with a roll surface temperature of 80° C., while decreasing the gap by 50 ⁇ m in nine steps, until the thickness was finally 150 ⁇ m, to obtain a positive electrode active material layer of this example.
• Examples 2 and 3 The positive electrode active material layers of Examples 2 and 3 were obtained in the same manner as in Example 1, except that the amounts of PTFE added were 0.8 parts by mass and 1.4 parts by mass, respectively, relative to 100 parts by mass of the mixed powder in Example 1. Here, the contents of PTFE in the positive electrode active material layer were 0.8% by mass and 1.4% by mass, respectively, relative to the total solid content contained in the positive electrode active material layer.
• Example 4 The positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes having an average fiber length of 16 ⁇ m were used as the conductive assistant.
• Example 5 The positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes having an average fiber length of 25 ⁇ m were used as the conductive assistant.
• Example 6 The positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes having an average fiber length of 2 ⁇ m were used as the conductive assistant.
• Comparative Example 4 A positive electrode active material layer of this comparative example was produced in the same manner as in Comparative Example 2, except that carbon nanotubes having an average fiber length of 2 ⁇ m were used as the conductive assistant.
• the positive electrode active material layer was treated with hydrochloric acid to remove the positive electrode active material and the solid electrolyte. Specifically, the positive electrode active material layer was immersed in 35% hydrochloric acid at 25°C for 4 hours. After that, the sample was washed with pure water three times to remove the hydrochloric acid, air-dried, and then vacuum-dried at 45°C for 4 hours to obtain a sample after the hydrochloric acid treatment, and SEM observation was performed.
• Figure 3 shows SEM images of samples after hydrochloric acid treatment of the positive electrode active material layer produced in (a) Comparative Example 2 and (b) Example 2.
• Figures 3(a) and (b) are photographs observed from a direction perpendicular to the surface of the positive electrode active material layer.
• Figure 3(a) in the positive electrode active material layer produced in Comparative Example 2, a fibrous conductive assistant and fibrous PTFE are observed.
• a composite of a film made of PTFE and a fibrous conductive assistant was not observed.
• a relatively large number of spaces that appear dark in the image were observed.
• ⁇ Tensile test of positive electrode active material layer> The positive electrode active material layers prepared in each Example and Comparative Example were subjected to a tensile test in accordance with JIS K 6251:2017 to measure the breaking strength.
• the positive electrode active material layers were punched into a dumbbell shape to obtain test pieces, and a tensile test was performed at room temperature at a tensile speed of 10 mm/min using a universal testing machine manufactured by Imada Co., Ltd.
• the width and thickness of the dumbbell part of the test piece were measured to determine the breaking strength of the positive electrode active material layer. The results are shown in Table 1 below.
• the obtained dispersion was stirred at 1000 rpm for 3 minutes using a rotation and revolution mixer to prepare a slurry.
• the obtained slurry was applied onto a negative electrode current collector (made of SUS430, thickness 10 ⁇ m) so that the film thickness after drying was 40 ⁇ m, and the solvent was dried on a hot plate at 80°C for 30 minutes to obtain a laminate of a solid electrolyte layer and a negative electrode current collector.
• the thickness of the solid electrolyte layer after CIP was 20 ⁇ m.
• the following charge-discharge test was performed on the evaluation cell prepared above.
• the charge-discharge test was performed while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cell using a pressure member.
• the evaluation cell was charged at 0.01 C for 5 seconds, and then left to stand in a thermostatic chamber at 60 ° C for 5 hours, and the cell temperature was set to 60 ° C.
• the cell was charged at a constant current equivalent to 0.05 C, and when the cell voltage reached 4.2 V, the charging was switched to a constant voltage mode, and after the current value reached 0.01 C, a 0.5 hour pause was performed.
• the cell was discharged at a constant current equivalent to 0.05 C, and discharged until the cell voltage reached 2.5 V, after which a 0.5 hour pause was performed.
• This cycle was regarded as one cycle, and 10 cycles of charge-discharge were performed.
• the discharge capacity up to the 10th cycle was measured, and the percentage (%) of the discharge capacity at the 10th cycle relative to the discharge capacity at the first cycle was calculated, and the obtained value was taken as the capacity retention rate.
• Table 1 The results are shown in Table 1 below.
• the positive electrodes of Examples 1 to 6 which have a composite structure of a PTFE membrane and a fibrous conductive assistant, have higher strength than the positive electrodes of Comparative Examples 1 to 4, which do not contain the composite. It was also found that the batteries using the positive electrodes of Examples 4 to 6 have improved cycle durability compared to the case where the positive electrode of Comparative Example 4 is used. Furthermore, a comparison of Examples 4 to 6 shows that Examples 4 and 5, in which the average fiber length of the fibrous conductive assistant is greater than the average particle diameter (D50) of the positive electrode active material, have higher strength and more excellent cycle durability than Example 6.
• D50 average particle diameter
• 10a laminated battery
• 11' negative electrode current collector
• 11" positive electrode current collector
• 13 negative electrode active material layer
• 15 positive electrode active material layer
• 17 solid electrolyte layer
• 19 single cell layer
• 21 power generating element
• 25 negative electrode current collector
• 27 positive electrode current collector
• 29 laminate film.

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• 2023
  • 2023-07-26 JP JP2025535510A patent/JPWO2025022624A1/ja active Pending
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