Method for producing solid electrolyte-containing layer, and method for producing lithium secondary battery using said solid electrolyte-containing layer

Abstract

The purpose of the present invention is to provide a means for improving battery performance at a low temperature in a lithium secondary battery using a solid electrolyte.　The present invention provides a method for producing a solid electrolyte-containing layer, said method comprising a pressure forming step for obtaining a solid electrolyte-containing layer by pressure forming a material which contains a solid electrolyte at a pressure forming temperature of not more than 15°C.

Description

Method for manufacturing a solid electrolyte-containing layer and method for manufacturing a lithium secondary battery using the solid electrolyte-containing layer

[0001] The present invention relates to a method for manufacturing a solid electrolyte-containing layer and a method for manufacturing a lithium secondary battery using the solid electrolyte-containing layer.

[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ionic conductors capable of ion conduction in a solid state. Therefore, lithium secondary batteries using solid electrolytes do not, in principle, suffer from the various problems caused by flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery.

[0003] Generally, lithium secondary batteries using a solid electrolyte have a structure in which a power generation element, consisting of a positive electrode, a solid electrolyte layer, and a negative electrode stacked in that order, is sealed inside the battery casing. Since the solid electrolyte constituting the solid electrolyte layer is particulate, a commonly known method for producing the solid electrolyte layer is to apply pressure to a material containing the solid electrolyte to form pellets (for example, International Publication No. 2023 / 223597).

[0004] However, our own investigations have revealed that in lithium secondary batteries using solid electrolytes, battery performance may deteriorate, such as a decrease in input / output characteristics at low temperatures.

[0005] Therefore, the present invention aims to provide a means for improving the battery performance at low temperatures in a lithium secondary battery using a solid electrolyte.

[0006] The inventors of the present invention conducted diligent research to solve the above problems. As a result, they discovered that the above problems can be solved by pressurizing a material containing a solid electrolyte at a temperature of 15°C or lower in the process of manufacturing a solid electrolyte-containing layer, such as a solid electrolyte layer or an electrode active material layer (positive electrode active material layer or negative electrode active material layer), and thus completed the present invention.

[0007] In other words, one embodiment of the present invention is a method for producing a solid electrolyte-containing layer, which includes a pressure molding step of obtaining a solid electrolyte-containing layer by pressure molding a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower.

[0008] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (stacked secondary battery) according to one embodiment of the present invention. Figure 2 is a graph showing the Cole-Cole plots obtained by measuring the AC impedance of the solid electrolyte layers prepared in Example 1 and Comparative Example 1 at -20°C. Figure 3 is a graph showing the Cole-Cole plots obtained by measuring the AC impedance of the solid electrolyte layers prepared in Example 2 and Comparative Example 2 at -20°C.

[0009] One embodiment of the present invention is a method for manufacturing a solid electrolyte-containing layer, comprising a pressure molding step of obtaining a solid electrolyte-containing layer by pressure molding a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower. According to the method for manufacturing a solid electrolyte-containing layer in this embodiment, the battery performance at low temperatures can be improved in a lithium secondary battery using a solid electrolyte.

[0010] In lithium secondary batteries using solid electrolytes, a method is employed in which pressure is applied to the solid electrolyte-containing material to form pellets when manufacturing solid electrolyte-containing layers such as the solid electrolyte layer. It is believed that by pressurizing the solid electrolyte-containing material, contact between particles can be ensured, resulting in good ion conductivity. However, our research has shown that when lithium secondary batteries using solid electrolytes are used at low temperatures, the battery performance deteriorates.

[0011] The inventors of this invention conducted intensive research to solve the above problem and found that in the solid electrolyte-containing layer, the particles of the solid electrolyte contract at low temperatures, increasing grain boundary resistance and significantly reducing ion conductivity. This is one of the reasons why sufficient battery characteristics cannot be obtained at low temperatures in lithium secondary batteries using solid electrolytes. Specifically, by examining the change in unit cell volume with temperature in the solid electrolyte and positive electrode active material, it was found that the degree of particle contraction at low temperatures in the solid electrolyte is greater than that of the positive electrode active material. From this, it is thought that at low temperatures, the particles of the solid electrolyte contract, increasing the interparticle distance, which increases grain boundary resistance and leads to a significant decrease in ion conductivity.

[0012] In contrast, according to the manufacturing method of this embodiment, a solid electrolyte-containing layer is obtained by pressure molding a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower. As a result, pressure molding can be performed with the solid electrolyte particles in a contracted state, which can lead to a higher density of the solid electrolyte-containing layer after molding and a reduction in grain boundary resistance. Consequently, ion conductivity at low temperatures is improved, and battery performance at low temperatures can be enhanced.

[0013] The layer containing the solid electrolyte produced by the method of this embodiment may be, for example, a solid electrolyte layer of a lithium secondary battery, an electrode active material layer, or a combination thereof.

[0014] The embodiments of the lithium secondary battery will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following forms. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0015] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery"), which is one embodiment of the present invention. By using a stacked design, the battery can be made compact and have a high capacity. In this specification, the stacked lithium-ion secondary battery (hereinafter also simply referred to as "stacked battery") shown in Figure 1 will be used as an example to explain the details.

[0016] As shown in Figure 1, the stacked battery 10a of this embodiment has a structure in which a flattened, roughly rectangular power generation element 21, on which the charge-discharge reaction actually takes place, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generation element 21 has a structure in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which a positive electrode active material layer 15 containing positive electrode active material is arranged on both sides of a positive electrode current collector 11''. The negative electrode has a structure in which a negative electrode active material layer 13 containing negative electrode active material is arranged on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order such that one positive electrode active material layer 15 and the adjacent negative electrode active material layer 13 face each other via the solid electrolyte layer 17. As a result, adjacent positive electrodes, solid electrolyte layers, and negative electrodes This constitutes one single cell layer 19. Therefore, the stacked battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel. The negative electrode current collector 11' and the positive electrode current collector 11'' are each fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, which are electrically connected to the respective electrodes (negative and positive electrodes), and have a structure that leads out to the outside of the laminate film 29 by being sandwiched between the edges of the laminate film 29. The stacked battery 10a is subjected to restraining pressure in the stacking direction of the power generation elements 21 by a pressurizing member (not shown). Therefore, the volume of the power generation elements 21 is kept constant.

[0017] The following describes the main components of a lithium-ion secondary battery.

[0018] [Current Collector] The current collector has the function of mediating the movement of electrons from the electrode active material layer. There are no particular restrictions on the materials that make up the current collector. For example, metals or conductive resins can be used as the constituent materials of the current collector.

[0019] The current collector may be a single-layer structure made of a single material, or it may be a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a conductive resin. Furthermore, from the viewpoint of blocking the movement of lithium ions between single cell layers, a metal layer may be provided on a part of the current collector.

[0020] [Layer containing a solid electrolyte] The layer containing a solid electrolyte produced by the method of this embodiment may be any layer containing a solid electrolyte, and may be a solid electrolyte layer, a positive electrode active material layer, or a negative electrode active material layer. The above layer containing a solid electrolyte is preferably a solid electrolyte layer because a higher solid electrolyte content allows for a more pronounced effect of the present invention. The structure of the layer containing a solid electrolyte layer and its manufacturing method will be described below, using a solid electrolyte layer as an example.

[0021] [Solid Electrolyte Layer] The solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode active material layer, and contains a solid electrolyte.

[0022] The solid electrolyte is not particularly limited, and sulfide solid electrolytes, oxide solid electrolytes, halide electrolytes, etc., can be used. However, it is preferable to include a sulfide solid electrolyte or a halide solid electrolyte because it exhibits excellent lithium ion conductivity and the densification effect by pressure molding is high, and it is more preferable to include a sulfide solid electrolyte.

[0023] In this specification, "sulfide solid electrolyte" means a solid electrolyte containing the element sulfur. Because sulfide solid electrolytes have a high coefficient of linear expansion, they tend to undergo large volume changes with temperature changes. Therefore, at low temperatures, the voids between particles become larger and the interfacial resistance tends to increase, which can lead to an even more pronounced effect of the present invention. More preferably, the solid electrolyte is a sulfide solid electrolyte containing the elements Li, M, and S, wherein the M element 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 the elements S, Li, and P.

[0024] Sulfide solid electrolytes contain Li １２ , ３ , １０ , ０．７５ , ５ , ３．２５ , ２ , ３．２ , ２ , ５ , １１ , ７ , ６ , ２ , ２ , ４ , ５ , ０．２５ , ２ , ０．９６ PS ４ may have a skeleton, and Li ４ P ２ S ７ may have a skeleton, and Li ４ P ２ S ６ may have a skeleton. Li ３ PS ４ Examples of sulfide solid electrolytes having a skeleton include LiI - Li ３ PS ４ , LiI - LiBr - Li ３ PS ４ , Li ３ PS ４ etc. Also, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a skeleton include Li - P - S - based solid electrolytes called LPS. Also, as sulfide solid electrolytes, for example, Li （４－ｘ） [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​This refers to a sulfide solid electrolyte made using a raw material composition containing the above, and the same applies to other descriptions. In particular, the sulfide solid electrolyte is preferably LPS (Li) because it has high ionic conductivity and a low bulk modulus, and can therefore follow the volume change of the electrode active material accompanying charging and discharging. 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 It is selected from the group consisting of the following.

[0025] In this specification, "halide solid electrolyte" means a solid electrolyte containing a halogen element but not containing a sulfur element. Examples of halide solid electrolytes include Li ｐ M ｑ X ｒ A solid electrolyte represented by the following compositional formula is given: (where M is a metallic element, X is a halogen element, p is 1, 2, or 3, q ​​is 0 or 1, and r is one integer from 1 to 6. In this case, p, q, and r are appropriately selected so that the total charge of the compound represented by the above compositional formula is 0). The metallic element M is at least one selected from the group Al, Mg, Fe, Ga, Y, Zr, and In, but is not limited to these. The halogen element X is at least one selected from the group F, Cl, Br, and I, but is not limited to these.

[0026] Examples of solid halogen electrolytes include LiX and LiMX. ４ Li ２ MX ４ Li ２ MX ６ , or Li ３ MX ６ Examples include those having the following composition. Among them, halogen solid electrolytes are used because they allow for the creation of higher-performance lithium secondary batteries. ３ MX６ is preferable. M and X are the same as the definitions of M and X in ｐ M ｑ X ｒ respectively.

[0027] Li ３ MX ６ Examples of ３ AlF ６ are, for example, Li ３ AlCl ６ , Li ３ AlBr ６ , Li ３ AlI ６ , Li ３ GaF ６ , Li ３ GaCl ６ , Li ３ GaBr ６ , Li ３ GaI ６ , Li ３ YF ６ , Li ３ YCl ６ , Li ３ YBr ６ , Li ３ YI ６ , Li ３ InF ６ , Li ３ InCl ６ , Li ３ InBr ６ , and Li ３ InI ６ but are not limited thereto.

[0028] The ionic conductivity (for example, Li ion conductivity) of the solid electrolyte at room temperature (25 °C) is, for example, 1 × 10 －５ S / cm or more, preferably 1 × 10 －４ S / cm or more. The value of the ionic conductivity of the solid electrolyte can be measured by the AC impedance method.

[0029] Examples of the shape of the solid electrolyte include particulate ones such as true spherical and ellipsoidal. When the solid electrolyte is particulate, its average particle diameter (D ５０The particle size is not particularly limited, but is, for example, 0.1 μm or more and 20 μm or less, preferably 0.5 μm or more and 10 μm or less, more preferably 1 to 5 μm, and even more preferably 3 to 5 μm. The above range is preferable because it more effectively ensures the contact area between particles.

[0030] In this specification, the average particle diameter of solid electrolyte particles is the 50% cumulative diameter (D) based on the number of particles observed in several to tens of fields of view when observed by a scanning electron microscope (SEM) (the maximum distance between any two points on the contour line of the observed particles). ５０ ) will be adopted.

[0031] The solid electrolyte content in the solid electrolyte layer is preferably more than 50% by mass and 100% by mass or less, more preferably 80% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0032] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, but examples include the following materials.

[0033] Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), carboxymethylcellulose, polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (P Fluororesins such as CTFE, 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-PFP fluororubber), Examples include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), as well as epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethylcellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

[0034] If the solid electrolyte layer further contains a binder, the binder content in the solid electrolyte layer is not particularly limited, but is typically 1 to 10% by mass.

[0035] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually between 0.1 μm and 1000 μm, and preferably between 10 μm and 100 μm.

[0036] The density of the solid electrolyte layer is not particularly limited, but is generally 1.5–1.7 g / cm³. ３ This is preferable. Within this range, contact between the particles of the solid electrolyte is improved, and the effects of the present invention can be obtained even more significantly.

[0037] [Method for Manufacturing a Solid Electrolyte Layer] One embodiment of the present invention provides a method for manufacturing a solid electrolyte-containing layer, comprising a pressure molding step of pressurizing a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower to obtain a solid electrolyte-containing layer. In a preferred embodiment, the solid electrolyte-containing layer is a solid electrolyte layer.

[0038] The material containing the solid electrolyte described above includes a solid electrolyte and optionally a binder. The specific forms of the solid electrolyte and binder are as described above. The material is preferably in powder form.

[0039] The means for pressure molding the above material are not particularly limited, as long as the pressure molding is performed at a pressure molding temperature of 15°C or lower. Pressure molding may be uniaxial or biaxial. The pressure molding apparatus is also not particularly limited. For example, a pressure molding apparatus can be used that consists of an electronically insulated frame for holding the above material and pressure members arranged above and below to sandwich the above material. The pressure members can be made of stainless steel, for example. A hydraulic press or a mechanical press can also be used as the molding machine.

[0040] In a preferred embodiment, the pressure molding temperature is preferably 0°C or lower, more preferably -10°C or lower, even more preferably -15°C or lower, and even more preferably -20°C or lower. This allows pressure molding to be performed with the solid electrolyte particles in a more contracted state, resulting in a higher density of the solid electrolyte-containing layer after molding, and a more effective reduction in grain boundary resistance. As a result, ion conductivity at low temperatures is further improved, and battery performance at low temperatures can be further enhanced. The lower limit of the pressure molding temperature is not particularly limited, but considering the operating environment of batteries used in EVs and HEVs, it is, for example, -40°C or higher. That is, in a preferred embodiment, the pressure molding temperature is -40°C or higher and -20°C or lower.

[0041] The pressure used during the above-mentioned pressure molding is not particularly limited, but is preferably 200 to 400 MPa, and more preferably 250 to 350 MPa. When the pressure is 200 MPa or higher, the density of the solid electrolyte layer can be sufficiently increased, which can further improve ion conductivity. As a result, the effects of the present invention can be obtained even more significantly. Furthermore, when the pressure is 400 MPa or lower, it is preferable because it can suppress uneven density due to particle breakage, etc., and variations in performance are less likely to occur. The time used during the above-mentioned pressure molding is also not particularly limited, but is, for example, 0.1 to 10 minutes, and preferably 1 to 5 minutes.

[0042] Even after releasing the press and returning the solid electrolyte-containing layer manufactured by this manufacturing method to a temperature above 15°C following the pressure molding process, the reduced grain boundary resistance obtained by this manufacturing method is maintained.

[0043] However, the manufacturing method of this embodiment preferably further includes a heating step after the above-mentioned pressure molding step in which the solid electrolyte-containing layer is heated to a temperature exceeding 15°C while under pressure. This allows the particles to aggregate more, and the effects of the present invention can be obtained even more significantly. It is also preferable that the aggregated state of the particles is more effectively maintained even when the pressure is released after heating. The heating rate is not particularly limited. The final temperature of the heating step is not particularly limited, but is, for example, 20 to 30°C, and preferably 20 to 25°C. The pressure in the heating step is preferably 200 to 400 MPa, and more preferably 250 to 350 MPa. The pressure in the heating step may be the same as the pressure in the above-mentioned pressure molding step.

[0044] Preferably, the process includes a cooling step before the above-mentioned pressure molding step, in which the material containing the solid electrolyte is cooled to a temperature of 15°C or lower without pressurization. Preferably, the material is cooled to the pressure molding temperature by the above cooling step. By performing the above cooling step, the solid electrolyte layer can be densified more effectively, and thus the effects of the present invention can be obtained even more significantly. The starting temperature of the above cooling step is, for example, room temperature (20-25°C). The cooling rate is not particularly limited. In a preferred embodiment, no pressing step is performed on the material containing the solid electrolyte before the above-mentioned pressure molding step.

[0045] Furthermore, according to one embodiment of the present invention, a solid electrolyte-containing layer is provided, manufactured by the method for manufacturing a solid electrolyte-containing layer of the present invention. Since the solid electrolyte-containing layer has excellent ion conductivity at low temperatures, it can contribute to improving battery performance at low temperatures. The solid electrolyte-containing layer can be defined as being formed by press-molding a material containing a solid electrolyte at a predetermined temperature. Solid electrolytes shrink when cooled, but the amount of volume change is small compared to the size of the particles. In a solid electrolyte layer after such particles have been pressed, it is impossible or impractical to directly determine the state in which the particles are in contact with each other based on the structure or properties of the solid electrolyte layer ("impossible / impractical circumstances"). Therefore, it may be appropriate to define the solid electrolyte-containing layer as a "thing" by specifying "a solid electrolyte-containing layer formed by press-molding a material containing a solid electrolyte at a predetermined temperature."

[0046] [Positive electrode active material layer] The positive electrode active material layer contains a positive electrode active material and may optionally contain a solid electrolyte, a binder, and a conductive additive.

[0047] The positive electrode active material has the function of releasing ions such as lithium ions during charging and absorbing ions such as lithium ions during discharge. As the positive electrode active material, a metal oxide is preferred because the effects of the present invention are more pronounced. An example of the positive electrode active material is LiCoO ２ LiMnO ２ LiNiO ２ LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt type active materials such as LiMn ２ O ４ LiNi ０．５ Mn １．５ O ４ spinel-type active materials such as LiFePO ４ LiMnPO ４ Olivine-type active materials such as Li ２ FeSiO ４ Li ２ MnSiO ４Examples of metal oxides include Si-containing active materials such as the above. Other metal oxides include, for example, Li ４ Ti ５ O １２ These include... Among these, a composite oxide containing lithium and nickel (lithium-nickel composite oxide) is preferably used. When nickel-containing metal oxides such as lithium-nickel composite oxide come into contact with a solid electrolyte under a reducing atmosphere such as reduced pressure, reduction from nickel oxide to nickel is likely to occur, and therefore deactivation of the positive electrode active material is likely to progress, leading to an increase in resistance. Therefore, the effects of the present invention can be obtained even more significantly. Examples of lithium-nickel composite oxides include Li(Ni-Mn-Co)O ２ Preferably, a material in which some of these transition metals are substituted with other elements (hereinafter also simply referred to as "NMC composite oxide") can be used. The NMC composite oxide has a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co arranged in an orderly manner) atomic layers are alternately stacked with oxygen atomic layers in between. It contains one Li atom for each transition metal M atom, and the amount of Li that can be extracted is twice that of spinel-based lithium manganese oxide, meaning that the supply capacity is twice as high and it can have a high capacity.

[0048] As mentioned above, NMC composite oxides also include composite oxides in which some of the transition metal elements are substituted by other metal elements. Examples of other elements in this case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, Zn, and the like. Preferably, the elements are Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr, more preferably Ti, Zr, P, Al, Mg, and Cr, and even more preferably Ti, Zr, Al, Mg, and Cr from the viewpoint of improving cycle characteristics.

[0049] NMC composite oxides have a high theoretical discharge capacity, and therefore are preferably composed of the general formula (1): Li ａ Ni ｂ Mn ｃ Co ｄ M ｘ O ２(However, in the formula, a, b, c, d, and x satisfy 0.98 ≤ a ≤ 1.2, 0.6 ≤ b ≤ 0.9, 0 < c ≤ 0.4, 0 < d ≤ 0.4, 0 ≤ x ≤ 0.3, and b + c + d + x = 1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr.) The composition is represented by the formula where a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. Furthermore, in the all-solid-state battery according to this embodiment, the positive electrode active material is particularly preferably an NMC composite oxide (high-nickel NMC composite oxide) that satisfies 0.8 ≤ b ≤ 0.9, 0 < c ≤ 0.2, 0 < d ≤ 0.2, and 0 ≤ x ≤ 0.2 in the general formula (1) described above. It is preferable that the atomic ratio b of Ni is 0.8 or higher because it provides a better balance between capacity and life characteristics. Also, it is preferable that the atomic ratio b of Ni is 0.9 or lower because it can reduce the generation of gas associated with the electrode reaction. Therefore, it is preferable because it is less likely to cause a decrease in cycle durability due to gas generation. In this case, it is more preferable from the viewpoint of improving the balance between capacity and life characteristics that c and d in general formula (1) are 0.05 ≤ c ≤ 0.2 and 0.03 ≤ d ≤ 0.2.

[0050] The average particle size of the positive electrode active material is not particularly limited, but from the viewpoint of increasing power output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm. In this specification, the average particle size of the positive electrode active material is measured by a particle size distribution analyzer using the laser diffraction / scattering method, and the 50% cumulative diameter (D) is determined based on volume. ５０ This is the value calculated as follows:

[0051] The content of the positive electrode active material in the positive electrode active material layer is preferably 30 to 95% by mass, more preferably 40 to 85% by mass, and even more preferably 50 to 70% by mass, relative to the total mass of the positive electrode active material layer.

[0052] The solid electrolyte contained in the positive electrode active material layer may be the same as the examples and preferred forms described in the section on the solid electrolyte layer. The solid electrolyte content is preferably in the range of 1 to 60% by mass, and more preferably in the range of 10 to 50% by mass, relative to the total mass of the positive electrode active material layer.

[0053] The binder included in the positive electrode active material layer can be the same as the examples and preferred forms described in the section on the solid electrolyte layer. The binder content in the positive electrode active material layer is not particularly limited, but is, for example, 0.1 to 10% by mass.

[0054] Examples of conductive additives include, but are not limited to, metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNTs), and carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.). Furthermore, a particulate ceramic material or resin material coated with the above-mentioned metal materials by plating or the like can also be used as a conductive additive. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon; more preferably, at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon; and even more preferably, at least one carbon. These conductive additives may be used individually or in combination of two or more.

[0055] The conductive additive is preferably in the form of particulates or fibers. When the conductive additive is in particulate form, the shape of the particles is not particularly limited and may be any shape such as powder, sphere, rod, needle, plate, columnar, irregular shape, flake, spindle, etc. When the conductive additive is in particulate form, the average particle diameter is not particularly limited, but is preferably 0.01 to 10 μm. The average particle diameter of the conductive additive is measured by a particle size distribution analyzer using the laser diffraction / scattering method, and the 50% cumulative diameter (D) is determined based on volume. ５０ This is the value calculated as follows:

[0056] When the positive electrode active material layer contains a conductive additive, the content of the conductive additive in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 10% by mass, and more preferably 2 to 8% by mass.

[0057] The thickness of the positive electrode active material layer varies depending on the configuration of the lithium secondary battery, but is preferably in the range of 1 to 1000 μm, more preferably in the range of 10 to 100 μm, and even more preferably in the range of 10 to 50 μm.

[0058] [Negative electrode active material layer] The negative electrode active material layer contains negative electrode active material and may optionally contain a solid electrolyte, binder, and conductive additive.

[0059] 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 is a lithium-containing active material, 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, and is particularly preferably lithium metal.

[0060] The shape of the negative electrode active material can be particulate (spherical, fibrous), thin film, etc., but a thin film is preferred. If the negative electrode active material is particulate, its average particle diameter (D ５０ ) is, for example, in the range of 0.1 to 20 μm. In this specification, the average particle size of the negative electrode active material is measured by a particle size distribution analyzer using the laser diffraction / scattering method, and the 50% cumulative diameter (D) is based on volume. ５０ This is the value calculated as follows:

[0061] The content of the negative electrode active material in the negative electrode active material layer is preferably 40 to 100% by mass, and more preferably 40 to 85% by mass.

[0062] The solid electrolyte contained in the negative electrode active material layer may be the same as the examples and preferred forms described in the section on the solid electrolyte layer. The solid electrolyte content is preferably in the range of 1 to 60% by mass, and more preferably in the range of 10 to 50% by mass, relative to the total mass of the negative electrode active material layer.

[0063] The binder included in the negative electrode active material layer can be the same as the examples and preferred forms described in the section on the solid electrolyte layer. The binder content in the negative electrode active material layer is not particularly limited, but is, for example, 0.1 to 10% by mass.

[0064] The conductive additive included in the negative electrode active material layer may be the same as the examples and preferred forms described in the section on the positive electrode active material layer. The content of the conductive additive in the negative electrode active material layer is not particularly limited, but is, for example, 1 to 10% by mass, and more preferably 2 to 8% by mass.

[0065] The thickness of the negative electrode active material layer varies depending on the configuration of the intended all-solid-state battery, but is preferably in the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm.

[0066] Furthermore, when lithium metal or a lithium-containing alloy is used as the negative electrode active material, the all-solid-state battery may be of the so-called lithium deposition type, in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during complete discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during complete discharge. In addition, there are no particular restrictions on the thickness of the negative electrode active material layer (lithium metal layer) during complete charge, but it is usually 0.1 to 1000 μm.

[0067] The method for preparing the electrode active material layer (positive electrode active material layer or negative electrode active material layer) is not particularly limited, but if the electrode active material layer contains a solid electrolyte, the electrode active material layer can be manufactured by the method for manufacturing a layer containing a solid electrolyte according to this embodiment. That is, in a preferred embodiment of the present invention, the material containing the solid electrolyte further contains an electrode active material (positive electrode active material or negative electrode active material), and the layer containing the solid electrolyte is the electrode active material layer. The specific form of the manufacturing method is the same as that described in the section on the solid electrolyte layer above. According to the manufacturing method of this embodiment, the ionic conductivity of the electrode active material layer at low temperatures can be improved. As a result, a lithium secondary battery with excellent battery performance at low temperatures can be obtained.

[0068] <Lithium-ion secondary battery and method for manufacturing the same> One embodiment of the present invention is a lithium-ion secondary battery using a solid electrolyte-containing layer obtained by the method described above. Another embodiment of the present invention is a method for manufacturing a lithium-ion secondary battery, which includes obtaining a solid electrolyte-containing layer by the method described above. The above lithium-ion secondary battery exhibits excellent battery performance at low temperatures.

[0069] One embodiment of the present invention is a method for manufacturing a lithium secondary battery, comprising the steps of: obtaining a solid electrolyte-containing layer 1, which is a solid electrolyte layer, by the method of the present invention; obtaining a solid electrolyte-containing layer 2, which is an electrode active material layer, by the method of the present invention; and stacking the solid electrolyte-containing layer 1 and the solid electrolyte-containing layer 2 under a temperature condition of 15°C or higher and pressurizing them to obtain a laminate. According to the manufacturing method of this embodiment, a lithium secondary battery with excellent battery performance at low temperatures can be obtained. The steps of each manufacturing method of this embodiment will be described below.

[0070] [Steps to obtain Solid Electrolyte-Containing Layer 1 and Solid Electrolyte-Containing Layer 2] The steps to obtain Solid Electrolyte-Containing Layer 1 and Solid Electrolyte-Containing Layer 2 are as described above. Solid Electrolyte-Containing Layer 1, which is the solid electrolyte layer, and Solid Electrolyte-Containing Layer 2, which is the electrode active material layer, are manufactured by press-molding the materials of each layer at a press-molding temperature of 15°C or lower. This allows the materials of each layer to be compacted in a state where the solid electrolyte particles are shrunk, and each layer can be densified. As a result, the grain boundary resistance of these layers can be reduced.

[0071] [Step to obtain a laminate by laminating and pressurizing solid electrolyte-containing layer 1 and solid electrolyte-containing layer 2 under a temperature condition exceeding 15°C] In this step, solid electrolyte-containing layer 1 and solid electrolyte-containing layer 2 are laminated and pressurized under a temperature condition exceeding 15°C to obtain a laminate. By laminating and pressurizing under a temperature condition exceeding 15°C, the adhesion between solid electrolyte-containing layer 1, which is the solid electrolyte layer, and solid electrolyte-containing layer 2, which is the electrode active material layer, can be improved. In one embodiment, after the step of obtaining solid electrolyte-containing layer 1 and solid electrolyte-containing layer 2, the obtained solid electrolyte-containing layer 1 and solid electrolyte-containing layer 2 are each heated to a temperature exceeding 15°C, for example, 20 to 30°C, preferably 20 to 25°C, while under pressure. After that, the pressure is released and these layers are laminated and pressed again to bond them. The pressure at this time is not particularly limited, but for example, it is 100 to 400 MPa, preferably 100 to 300 MPa. The pressurizing time is also not particularly limited, but for example, it is 1 to 10 minutes.

[0072] The stacked battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the stacked battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0073] Although one embodiment of a lithium secondary battery has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and can be modified as appropriate based on the description of the claims.

[0074] For example, one type of battery to which the lithium secondary battery according to the present invention is applied is a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically coupled to one side of a current collector and a negative electrode active material layer electrically coupled to the opposite side of the current collector.

[0075] Furthermore, the lithium secondary battery according to this embodiment may or may not be all-solid-state. That is, the solid electrolyte layer may or may not contain a conventionally known liquid electrolyte. When the solid electrolyte layer contains a liquid electrolyte, there are no particular restrictions on the amount of liquid electrolyte that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and no leakage of the liquid electrolyte occurs.

[0076] The following items are also included in the scope of the present invention: Item 1: A method for producing a solid electrolyte-containing layer, comprising a pressure molding step of obtaining a solid electrolyte-containing layer by pressure molding a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower; Item 2: The method according to Item 1, wherein the solid electrolyte contains a sulfide solid electrolyte; Item 3: The method according to Item 1 or 2, wherein the pressure molding temperature is 0°C or lower; Item 4: The method according to any one of Items 1 to 3, wherein the pressure molding temperature is -40°C or higher and -20°C or lower; Item 5: The method according to any one of Items 1 to 4, wherein the pressure during the pressure molding is 200 to 400 MPa; Item 6: The method according to any one of Items 1 to 5, further comprising a heating step of raising the temperature of the solid electrolyte-containing layer to a temperature above 15°C while it remains under pressure after the pressure molding step; Item 7: The method according to any one of Items 1 to 6, further comprising a cooling step of cooling the material containing the solid electrolyte to a temperature of 15°C or lower without pressurizing it before the pressure molding step; Item 8; The method according to any one of items 1 to 7, wherein the solid electrolyte-containing layer is a solid electrolyte layer; Item 9; The method according to any one of items 1 to 7, wherein the material further comprises an electrode active material, and the solid electrolyte-containing layer is an electrode active material layer; Item 10; A method for manufacturing a lithium secondary battery, comprising the steps of: obtaining a solid electrolyte-containing layer 1 which is a solid electrolyte layer by the method according to item 8; obtaining a solid electrolyte-containing layer 2 which is an electrode active material layer by the method according to item 9; and stacking the solid electrolyte-containing layer 1 and the solid electrolyte-containing layer 2 under a temperature condition exceeding 15°C and pressurizing them to obtain a laminate; Item 11; A solid electrolyte-containing layer manufactured by the method according to any one of items 1 to 9.

[0077] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following examples, the instruments and devices used inside the glove box were thoroughly dried beforehand.

[0078] [Example 1] (Preparation of solid electrolyte layer) In a glove box with an argon atmosphere and a dew point of -68°C or lower, Li, which is a sulfide solid electrolyte, was heated at 25°C. ６ PS ５ 100 mg of Cl (LPSCl) was weighed and placed in a Macol tube, then clamped with a φ10 mm stainless steel pin to which a wire was attached. This was then placed in an aluminum laminate film bag without pressure, and the opening was heat-sealed. Next, the sealed Macol tube was placed in a constant temperature bath, set to -20°C, and left for 3 hours. After that, the sealed Macol tube was removed from the constant temperature bath, and immediately afterward, while still in the laminate film bag (with the sulfide solid electrolyte temperature at -20°C), it was pressed with a hydraulic press at a pressure of 300 MPa for 1 minute to obtain a solid electrolyte layer. The thickness of the solid electrolyte layer after pressing was 75-85 μm.

[0079] [Example 2] In Example 1 above, the sulfide solid electrolyte was Li ６ PS ５ Cl ０．５ Br ０．５ The solid electrolyte layer of this embodiment was obtained in the same manner as in Example 1, except that it was changed to (LPSClBr).

[0080] [Comparative Example 1] The solid electrolyte layer of this comparative example was obtained in the same manner as in Example 1, except that the procedure of placing the sealed Macol tube in a constant temperature bath and leaving it at a temperature of -20°C for 3 hours was not performed.

[0081] [Comparative Example 2] The solid electrolyte layer of this comparative example was obtained in the same manner as in Example 2, except that the procedure of placing the sealed Macol tube in a constant temperature bath and leaving it at a temperature of -20°C for 3 hours was not performed.

[0082] <Measurement of Ionic Conductivity of Solid Electrolyte Layer> For each example and comparative example, the ionic conductivity of the solid electrolyte layer fabricated by pressure molding was measured using the AC impedance method in a constant temperature bath set to -20°C while under pressure. The AC impedance measurement was performed using a Solartron electrochemical measurement system (model 1260A) under the conditions of a frequency range of 1 MHz to 100 mHz and a voltage amplitude of 10 mV.

[0083] Figure 2 is a graph showing the Cole-Cole plots obtained by measuring the AC impedance of the solid electrolyte layers prepared in Example 1 and Comparative Example 1 at -20°C. Figure 3 is a graph showing the Cole-Cole plots obtained by measuring the AC impedance of the solid electrolyte layers prepared in Example 2 and Comparative Example 2 at -20°C.

[0084] The real value of the impedance (shown by the dotted line) at the measurement point where the absolute value of the phase of the complex impedance of the arc in the Cole-Cole plot shown in Figures 2 and 3 is minimized is the resistance value R relative to the ionic conductivity of the solid electrolyte layer. ＳＥ (Ω) corresponds to the value (R ＳＥ (Ω) and the contact area S (cm²) between the solid electrolyte layer and the stainless steel pin. ２ ) and the product (R ＳＥ - S)) was considered. The above R ＳＥ Using , S, and the thickness t (cm) of the solid electrolyte layer, the ionic conductivity σ of the solid electrolyte layer can be calculated based on the following formula (1). Formula (1): σ (mS / cm) = [(R ＳＥ S (Ω・cm) ２ ) / t(cm)] －１ x 10.

[0085] The component of the arc shown in Figure 2 is thought to be predominantly due to the grain boundary resistance between solid electrolyte particles. As shown in Figure 2, the solid electrolyte layer of Example 1, which was pressure-molded at a temperature of 15°C or lower, has a grain boundary resistance of approximately 940 Ω·cm compared to Comparative Example 1, which was not pressure-molded at a temperature of 15°C or lower. ２ Approximately 460 ohms / cm ２An effect of reducing the conductivity to a certain level was achieved. Furthermore, while the ionic conductivity of the solid electrolyte layer of Comparative Example 1 at -20°C was 0.11 mS / m, the ionic conductivity of the solid electrolyte layer of Example 1 at -20°C was 0.22 mS / m, confirming that the ionic conductivity at low temperatures was improved.

[0086] Separately, AC impedance measurements were performed at 25°C on the solid electrolyte layers prepared in Example 1 and Comparative Example 1, and no significant difference in ionic conductivity was observed (not shown).

[0087] The component of the arc shown in Figure 3 is thought to be predominantly composed of grain boundary resistance between solid electrolyte particles. As shown in Figure 3, the solid electrolyte layer of Example 2, which was pressure-molded at a temperature of 15°C or lower, has a grain boundary resistance of approximately 1310 Ω·cm compared to Comparative Example 2, which was not pressure-molded at a temperature of 15°C or lower. ２ Approximately 560 Ω·cm ２ An effect of reducing the conductivity to a certain level was achieved. Furthermore, while the ionic conductivity of the solid electrolyte layer in Comparative Example 2 at -20°C was 0.07 mS / m, the ionic conductivity of the solid electrolyte layer in Example 2 at -20°C was 0.18 mS / m, confirming that the ionic conductivity at low temperatures was improved.

[0088] Separately, AC impedance measurements were performed at 25°C on the solid electrolyte layers prepared in Example 2 and Comparative Example 2, and no significant difference in ionic conductivity was observed (not shown).

[0089] 10a Stacked 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 generation element, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29 Laminate film.

Claims

1. A method for producing a solid electrolyte-containing layer, comprising a pressure molding step of obtaining a solid electrolyte-containing layer by pressure molding a material containing a solid electrolyte at a pressure molding temperature of 15°C or lower.

2. The method according to claim 1, wherein the solid electrolyte includes a sulfide solid electrolyte.

3. The method according to claim 1, wherein the pressure molding temperature is 0°C or lower.

4. The method according to claim 1, wherein the pressure molding temperature is -40°C or higher and -20°C or lower.

5. The method according to claim 1, wherein the pressure during the pressure molding is 200 to 400 MPa.

6. The method according to claim 1, further comprising a heating step of raising the temperature of the solid electrolyte-containing layer to a temperature exceeding 15°C while it remains under pressure after the pressure molding step.

7. The method according to claim 1, further comprising a cooling step of cooling a material containing a solid electrolyte to a temperature of 15°C or lower without pressurizing it, prior to the pressure molding step.

8. The method according to claim 1, wherein the solid electrolyte-containing layer is a solid electrolyte layer.

9. The method according to claim 1, wherein the material containing the solid electrolyte further contains an electrode active material, and the solid electrolyte-containing layer is an electrode active material layer.

10. A method for manufacturing a lithium secondary battery, comprising the steps of: obtaining a solid electrolyte-containing layer 1 which is a solid electrolyte layer by the method of claim 8; obtaining a solid electrolyte-containing layer 2 which is an electrode active material layer by the method of claim 9; and stacking the solid electrolyte-containing layer 1 and the solid electrolyte-containing layer 2 under a temperature condition of 15°C or higher and pressurizing them to obtain a laminate.

11. A solid electrolyte-containing layer manufactured by the method described in any one of claims 1 to 9.

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