Positive electrode active material layer and lithium secondary battery using same

Abstract

The present invention provides a means that can lower the resistance in a lithium secondary battery that is provided with a binder-containing positive electrode active material layer. The present invention relates to a positive electrode active material layer containing a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, wherein the content of conductive additive, per 100 parts by mass of the positive electrode active material, is 0 parts by mass or more than 0 parts by mass and not more than 1.5 parts by mass.

Description

Positive electrode active material layer and lithium secondary battery using the same

[0001] The present invention relates to a positive electrode active material layer and a lithium secondary battery using the same.

[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes as electrolytes have been actively conducted. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, all-solid-state batteries have the advantage that various problems caused by flammable organic electrolytes, such as those in conventional liquid-based batteries using non-aqueous electrolytes, do not occur in principle.

[0003] By the way, in a lithium secondary battery using a general solid electrolyte, the positive electrode has a structure in which a positive electrode active material layer is disposed on the surface of a positive electrode current collector. And the positive electrode active material layer includes, in addition to the positive electrode active material, a solid electrolyte for improving the lithium ion conductivity in the positive electrode active material layer, particles of the positive electrode active material and the solid electrolyte, and a binder for binding these particles to the positive electrode current collector.

[0004] Japanese Patent Application Laid-Open No. 2018-085310 discloses a technique of coating the surface of positive electrode active material particles with oxide particles containing a metal oxide containing Zr. By adopting such a configuration, it is said that the affinity between the positive electrode active material particles and the solid electrolyte can be improved, and the interfacial resistance of the positive electrode active material layer can be reduced.

[0005] However, as a result of the study by the present inventors, it has been found that in the above-mentioned document, when the positive electrode active material layer contains a binder, problems such as deterioration of electron conductivity and disconnection of ion conduction paths may occur due to the binder, increasing the resistance of the battery.

[0006] Therefore, an object of the present invention is to provide a means capable of reducing resistance in a lithium secondary battery including a positive electrode active material layer containing a binder.

[0007] The inventors of the present invention have conducted intensive studies to solve the above problems. As a result, by using a fibrous material as the binder to be included in the positive electrode active material layer and further controlling the content of the conductive assistant in the positive electrode active material layer to be 0 parts by mass or more and 1.5 parts by mass or less with respect to 100 parts by mass of the positive electrode active material, it has been found that the above problems can be solved, and the present invention has been completed.

[0008] That is, one aspect of the present invention relates to a positive electrode active material layer including a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, wherein the content of the conductive assistant is 0 parts by mass or more than 0 parts by mass and 1.5 parts by mass or less with respect to 100 parts by mass of the positive electrode active material.

[0009] FIG. 1 is a cross-sectional view schematically showing the overall structure of a laminated (internally parallel-connected type) all-solid-state lithium secondary battery (laminated secondary battery) according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining how to obtain the maximum Feret diameter and the minimum Feret diameter of the fibrous binder.

[0010] One aspect of the present invention is a positive electrode active material layer including a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, wherein the content of the conductive assistant is 0 parts by mass or more than 0 parts by mass and 1.5 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. According to this aspect, in a lithium secondary battery including a positive electrode active material layer containing a binder, the resistance can be reduced.

[0011] Further, there is also provided a lithium secondary battery including a positive electrode having the above positive electrode active material layer, a negative electrode current collector, a negative electrode on which lithium metal is deposited during charging, and a power generation element having a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode. Hereinafter, the lithium secondary battery according to this aspect will be described with reference to the attached drawings. The technical scope of the present invention should be determined based on the description in the claims and is not limited only to the following aspects. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted. Also, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0012] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) 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.

[0013] As shown in Figure 1, the stacked secondary battery 10a of this embodiment has a structure in which a flattened, roughly rectangular power generation element 21, in which the charge-discharge reaction actually proceeds, 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 negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are stacked. A negative electrode intermediate layer 14 is arranged adjacent to the surface of the negative electrode active material layer 13 that faces the solid electrolyte layer 17. The positive electrode has a structure in which a positive electrode active material layer 15 is arranged on the surface of the positive electrode current collector 11''. The negative electrode, solid electrolyte layer, and positive electrode are stacked in this order, with the negative electrode intermediate layer 14 and the positive electrode active material layer 15 facing each other via a solid electrolyte layer 17. As a result, adjacent positive electrodes, solid electrolyte layers, and negative electrodes constitute one single cell layer 19. Therefore, the stacked secondary 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 fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, respectively, which are electrically connected to the respective electrodes (negative electrode and positive electrode), and have a structure in which they are led out to the outside of the laminate film 29 by being sandwiched between the edges of the laminate film 29. The stacked secondary battery 10a is subjected to restraining pressure in the stacking direction of the power generation element 21 by a pressurizing member (not shown). Therefore, the volume of the power generation element 21 is kept constant.

[0014] The main components of the lithium secondary battery according to this embodiment will be described below. In this specification, "X to Y" indicating a range means "X or more and Y or less".

[0015] [Current collector] The current collector (negative current collector, positive current collector) has a function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There is no particular limitation on the material constituting the current collector. As the constituent material of the current collector, for example, metals such as aluminum, nickel, iron, stainless steel, titanium, copper, etc., and conductive resins can be adopted. There is also no particular limitation on the thickness of the current collector, but as an example, it is 10 to 100 μm.

[0016] [Positive electrode active material layer] The positive electrode active material layer essentially contains a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, and may contain a conductive aid as needed.

[0017] (Positive electrode active material) The type of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but LiCoO ５ , ０．５ , ４ , ２ , ４ ,

[0018] , ２ ,

[0017] , ４ , ２ , ４ , ２ , １．５ , ４ , １２ , ２ , ４ , ２ , ２ , ２ , ２ , , ５０ , ４ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni - Mn - Co)O ２ and other layered rock salt type active materials, LiMn ２ O ４ , LiNi ０．５ Mn １．５ [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​The average particle diameter (D) is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 30 μm, even more preferably in the range of 1 to 20 μm, and particularly preferably in the range of 5 to 10 μm. In this specification, the average particle diameter (D) is used. ５０ The value of ) can be measured by laser diffraction scattering.

[0019] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but for example, it is preferably 50 to 99% by mass, more preferably 60 to 95% by mass, and even more preferably 70 to 90% by mass, relative to the total mass of the positive electrode active material layer.

[0020] (Sulfide Solid Electrolyte) The positive electrode active material layer must contain a sulfide solid electrolyte. The sulfide solid electrolyte contained in the positive electrode active material layer is not particularly limited, and any known in the art can be used as appropriate. Since this sulfide solid electrolyte exhibits excellent lithium ion conductivity, it preferably contains Li and M elements, and the M element is a sulfide solid electrolyte containing at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and more preferably a sulfide solid electrolyte containing Li and P elements. As an example, LPS(Li ２ S-P ２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ Li ３．２ P ０．９６ S ４ and Li ３ PS ４ Examples of sulfide solid electrolytes include those listed above. These sulfide solid electrolytes are preferred because they have excellent lithium ion conductivity.

[0021] The ionic conductivity of a sulfide solid electrolyte at room temperature (25°C) (for example, the Li ionic conductivity) is, for example, 1 × 10⁻⁶. －５It is preferable that the S / cm is greater than or equal to 1 × 10 －４ A value of S / cm or higher is more preferable. The ionic conductivity of the sulfide solid electrolyte can be measured by the AC impedance method.

[0022] Examples of sulfide solid electrolyte shapes include spherical, ellipsoidal, and other particulate forms, as well as thin films. When the sulfide solid electrolyte is in particulate form, its average particle size (D ５０ The particle size is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, even more preferably 0.2 μm or more and 10 μm or less, and even more preferably 0.3 μm or more and 1.5 μm or less.

[0023] The content of sulfide solid electrolyte in the positive electrode active material layer is preferably 1 to 50% by mass, and more preferably 5 to 20% by mass, relative to the total mass of the positive electrode active material layer.

[0024] (Binder) The positive electrode active material layer includes a fibrous binder as a binder. In this specification, "fibrous binder" refers to a binder mainly composed of fibers that, in an observation image obtained by observing a cross-section of the positive electrode active material layer using a scanning electron microscope (SEM), have an aspect ratio of 10 or more and a minimum ferret diameter of 0.2 μm or less. Here, the aspect ratio is calculated by dividing the maximum ferret diameter of the binder by the minimum ferret diameter. The maximum ferret diameter is the maximum distance between two parallel lines when the contour of the binder is enclosed by those lines, and the minimum ferret diameter is the minimum distance between two parallel lines when the contour of the binder is enclosed by those lines. When a binder is "mainly composed of" the above fibers, it means that the area ratio of the above fiber portion to the total area of ​​the binder in the SEM observation image is 50% or more. A single fibrous binder may include non-fibrous portions (portions with an aspect ratio of less than 10 or a minimum ferret diameter exceeding 0.2 μm) that have an aspect ratio of 10 or more and a minimum ferret diameter of 0.2 μm or less. However, the area ratio of non-fibrous portions to the total area of ​​the fibrous binder in the SEM observation image must be less than 50%, preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less (the lower limit is 0%).

[0025] Fibrous binders include not only those composed of a single fiber, but also those with a structure in which two or more fibers are interconnected. Specific shapes of binders with a structure in which two or more fibers are interconnected include branched chain, radial, and mesh-like shapes, as well as combinations thereof. Here, we will explain how to determine the maximum and minimum ferret diameters in binders with a structure in which two or more fibers are interconnected. Figure 2 is a schematic diagram showing an example of a branched chain-like fibrous binder. The fibrous binder 30 shown in Figure 2 has a structure in which fibers X, Y, and Z are interconnected. Each dashed line represents a line connecting the centers (1 / 2 width) of the fiber widths, and points A, B, and C represent the ends of each dashed line. Note that the ends of each dashed line coincide with the ends of the fibers. Point D represents the intersection of the three dashed lines. In other words, the binder 30 shown in Figure 2 can be said to have a shape in which fiber X from point A to point D, fiber Y from point B to point D, and fiber Z from point C to point D are joined at point D. The maximum ferret diameter of fiber X in the binder 30 shown in Figure 2 is defined as the distance from point A to point D. Similarly, the maximum ferret diameter of fiber Y is the distance from point B to point D, and the maximum ferret diameter of fiber Z is the distance from point C to point D. Furthermore, the minimum ferret diameter of fiber X is the minimum distance between two parallel lines when the contour of the binder (fiber) between points A and D is enclosed by those lines. The same applies to the minimum ferret diameters of fibers Y and Z. In the binder 30 shown in Figure 2, fibers Y and Z are fibers with an aspect ratio of 10 or more and a minimum ferret diameter of 0.2 μm or less, but fiber X has an aspect ratio of less than 10. However, since the area of ​​the fiber X portion is less than 50% of the total area of ​​the binder 30, the binder shown in Figure 2 can be said to be a fibrous binder.

[0026] The type of fibrous binder is not particularly limited as long as it has the above-described shape in the positive electrode active material layer, but binders that fibrillate when shear force is applied are preferably used. Examples of such fibrillating binders include polytetrafluoroethylene (PTFE), carboxymethylcellulose, polyvinyl alcohol, and polyethylene. Among these, compounds containing the element fluorine are preferred, and polytetrafluoroethylene (PTFE) is more preferred. By using such a fibrous binder, a positive electrode active material layer with excellent ionic conductivity can be obtained. The fibrous binder may be used alone or in combination of two or more types. In this specification, the compound name of the binder may include not only the compound indicated by the compound name, but also forms in which part of the terminal or side chain is substituted (modified) with other substituents. In cases where a portion of the terminal or side chain is substituted (modified) with another substituent, the proportion of the constituent units in which the terminal or side chain is substituted (modified) with another substituent to 100 mol% of the total constituent units is preferably 10 mol% or less, and more preferably 5 mol% or less.

[0027] Furthermore, the positive electrode active material layer may contain a non-fibrous binder. In this specification, "non-fibrous binder" means a binder other than the "fibrous binder" described above. Specifically, in this specification, "non-fibrous binder" refers to a binder whose aspect ratio is less than 10 or whose minimum ferret diameter is greater than 0.2 μm in the observation image when a cross-section of the positive electrode active material layer is observed using a scanning electron microscope (SEM). The ratio of fibrous binder to non-fibrous binder contained in the positive electrode active material layer can be determined by observing a cross-section of the positive electrode active material layer using a scanning electron microscope. Examples of non-fibrous binders include difluorovinyl tetrafluoropropylene, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), ethyl cellulose, and acrylic resin.

[0028] The binder content in the positive electrode active material layer is preferably in the range of 0.01 to 10% by mass, more preferably in the range of 0.05 to 5.0% by mass, even more preferably in the range of 0.1 to 1.0% by mass, and still more preferably in the range of 0.1 to 0.8% by mass, relative to the total mass of the positive electrode active material layer. If the binder content is within the above range, a good balance between the mechanical strength and ionic conductivity of the positive electrode active material layer can be achieved. However, the mass ratio of fibrous binder to the total binder in the positive electrode active material layer is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, particularly preferably 98% by mass or more, and most preferably 100% by mass. If the mass ratio of fibrous binder is within the above range, the ionic conductivity of the positive electrode active material layer can be further improved. In one embodiment of the present invention, the content of fibrous binder in the positive electrode active material layer is preferably, for example, in the range of more than 0% by mass and 10% by mass or less, more preferably in the range of more than 0% by mass and 5.0% by mass or less, even more preferably in the range of more than 0% by mass and 1.0% by mass or less, even more preferably in the range of more than 0% by mass and 0.8% by mass or less, and particularly preferably in the range of more than 0% by mass and 0.5% by mass or less, with respect to the total mass of the positive electrode active material layer.

[0029] (Conductive Additives) The positive electrode active material layer may contain conductive additives. The conductive additives used in the positive electrode active material layer are not particularly limited and 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.). 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 carbon, and even more preferably carbon nanotubes. These conductive additives may be used individually or in combination of two or more.

[0030] The content of the conductive additive in the positive electrode active material layer is 0 parts by mass or more than 0 parts by mass and 1.5 parts by mass or less per 100 parts by mass of positive electrode active material. If the content of the conductive additive is 1.5 parts by mass or less, the resistance of the battery can be controlled well. The content of the conductive additive is, for example, 0 parts by mass or more and 1.2 parts by mass or less, 0 parts by mass or more and 1.0 parts by mass or less, preferably 0 parts by mass or more and 0.8 parts by mass or less, more preferably more than 0 parts by mass and 0.5 parts by mass or less, even more preferably 0.1 parts by mass or more and 0.5 parts by mass or less, and particularly preferably 0.2 parts by mass or more and 0.5 parts by mass or less per 100 parts by mass of positive electrode active material. In addition, the content of the conductive additive is, for example, less than 1.4% by mass of the total mass of the positive electrode active material layer, preferably 1.0% by mass or less, more preferably less than 1.0% by mass, even more preferably 0.5% by mass or less, with a lower limit of 0% by mass or more, and even more preferably 0.2% by mass or more.

[0031] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, and preferably 10 to 300 μm.

[0032] In another embodiment, the positive electrode active material layer may contain only the positive electrode active material, a sulfide solid electrolyte, a fibrous binder, and a conductive additive, or it may contain only the positive electrode active material, a sulfide solid electrolyte, and a fibrous binder. In this specification, "contains only" includes not only embodiments containing only a specific compound, but also embodiments that substantially contain no compounds other than that specific compound. "Substantially absent" means that a certain material is not intentionally added, and does not exclude cases where, for example, it is unintentionally mixed in or unintentionally present due to insufficient removal. In one embodiment, when a certain material is "absent" or "substantially absent," the content of that material may be 1000 ppm by mass or less, 100 ppm by mass or less, 10 ppm by mass or less, 1 ppm by mass or less, or 0.1 ppm by mass or less, relative to the total mass of the positive electrode active material layer.

[0033] The mechanism by which resistance can be reduced by using a fibrous binder in the positive electrode active material and controlling the content of the conductive additive in the positive electrode active material layer to between 0 and 1.5 parts by mass per 100 parts by mass of positive electrode active material is not fully understood, but the following mechanism is hypothesized. Since binders generally have low electronic conductivity, adding a binder to the positive electrode active material layer reduces electronic conductivity, thereby increasing resistance. Adding a conductive additive is an effective way to improve the electronic conductivity of the positive electrode active material layer. On the other hand, it has been found that when a conductive additive is added, it can interfere with contact between the positive electrode active material and the solid electrolyte, and between solid electrolytes themselves, resulting in a decrease in lithium ion conductivity in the positive electrode active material layer and, conversely, an increase in resistance. Furthermore, contact between the conductive additive and the positive electrode active material and / or solid electrolyte can promote the progress of undesirable side reactions, which can also increase resistance. Therefore, it is preferable that the amount of conductive additive in the positive electrode active material layer be as small as possible. Here, if the binder is non-fibrous, the binder covers most of the surface of the positive electrode active material and solid electrolyte, which significantly reduces electronic conductivity. In contrast, if the binder is fibrous, it does not cover most of the surface of the positive electrode active material and solid electrolyte, so the reduction in electronic conductivity is suppressed compared to the case of non-fibrous binder. This makes it possible to suppress the increase in resistance. Furthermore, since the reduction in electronic conductivity is suppressed when using a fibrous binder, the amount of conductive additive can be kept to a minimum, thereby suppressing the increase in resistance caused by the conductive additive as described above. Note that the above mechanism is based on speculation, and its accuracy does not affect the technical scope of the present invention.

[0034] [Negative Electrode Active Material Layer] The negative electrode active material layer contains a negative electrode active material and may optionally contain a solid electrolyte, a binder, and a conductive additive. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. In addition, a lithium-containing active material may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it contains lithium, and examples include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li and at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and is particularly preferably lithium metal or a lithium-containing alloy. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery is preferably a 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. In this case, the layer of lithium metal deposited during the charging process becomes the negative electrode active material layer. As the charging process progresses, the thickness of the negative electrode active material layer increases, and as the discharging process progresses, the thickness of the negative electrode active material layer decreases. 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 of a certain amount of lithium metal may be present during complete discharge. Furthermore, 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 between 0.1 and 1000 μm.

[0035] [Negative Electrode Intermediate Layer] When the lithium secondary battery according to this embodiment is of the lithium deposition type described above, the negative electrode intermediate layer may be arranged adjacent to the negative electrode current collector side surface of the solid electrolyte layer. It is preferable that the negative electrode intermediate layer is conductive as a whole. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２The volume resistivity of the negative electrode intermediate layer is less than or equal to Ω·cm, and more preferably less than or equal to 10Ω·cm. In this specification, the value of the volume resistivity of the negative electrode intermediate layer is the value measured using an electrode resistance measurement system (manufactured by HIOKI E.E. CORPORATION, product name: RM2610). Furthermore, lithium metal may be deposited inside the negative electrode intermediate layer during charging.

[0036] The negative electrode intermediate layer may contain a lithium-reactive material. While not particularly limited, the lithium-reactive material could be, for example, a carbon material. When a carbon material is included in the negative electrode intermediate layer, the generation and growth of dendrites can be particularly effectively suppressed.

[0037] Specific examples of carbon materials include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNTs), graphite, and hard carbon.

[0038] The carbon material content is, for example, 99% by mass or less, preferably 90% by mass or less, and more preferably 80% by mass or less, relative to the total mass of the negative electrode intermediate layer. The lower limit of the carbon material content is not particularly limited, but is, for example, 40% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more, relative to the total mass of the negative electrode intermediate layer. When the carbon material content is within the above range, the generation and growth of dendrites can be suppressed.

[0039] The negative electrode intermediate layer may contain a binder as needed. The type of binder is not particularly limited, and any known binder in the art can be used as appropriate. Examples of binders include fluororesins such as polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements) and polytetrafluoroethylene (PTFE), as well as styrene-butadiene rubber (SBR) and carboxymethylcellulose. Among these, it is preferable that the binder contains a fluororesin.

[0040] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably in the range of 1 to 20% by mass, and more preferably in the range of 5 to 15% by mass, relative to the total mass of the negative electrode intermediate layer. If the binder content is 1% by mass or more, a negative electrode intermediate layer with sufficient strength can be formed. If the binder content is 15% by mass or less, a negative electrode intermediate layer with sufficient lithium ion conductivity can be formed.

[0041] The negative electrode intermediate layer may contain additional metallic material. The metallic material is not particularly limited, but examples include In, Al, Si, Sn, Mg, Au, Ag, and Zn. The metallic content is not particularly limited, but is, for example, 0 to 35% by mass relative to the total mass of the negative electrode intermediate layer.

[0042] The thickness of the negative electrode intermediate layer is not particularly limited, but may be between 0.1 μm and less than 100 μm. When the thickness of the negative electrode intermediate layer is 0.1 μm or more, it is possible to suppress the occurrence of short circuits during charging. Furthermore, when the thickness of the negative electrode intermediate layer is 100 μm or less, it is possible to suppress an increase in the conduction resistance of lithium ions and obtain sufficient charging capacity for the secondary battery. In one embodiment, the thickness of the negative electrode intermediate layer is preferably between 1 μm and 20 μm, more preferably between 1 μm and 15 μm, and even more preferably between 1 μm and 10 μm.

[0043] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as the main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited and any known in the art can be used as appropriate, but examples include sulfide solid electrolytes and oxide solid electrolytes. Since this solid electrolyte exhibits excellent lithium ion conductivity, it is preferably a sulfide solid electrolyte containing the element S, more preferably a sulfide solid electrolyte containing the elements Li, M and S, wherein the M element is 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. As an example, LPS(Li ２S-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 ４ and Li ３ PS ４ Examples of sulfide solid electrolytes include those listed above. These sulfide solid electrolytes are preferred because they have excellent lithium ion conductivity.

[0044] The ionic conductivity of a sulfide solid electrolyte at room temperature (25°C) (for example, the Li ionic conductivity) is, for example, 1 × 10⁻⁶. －５ It is preferable that the S / cm is greater than or equal to 1 × 10 －４ A value of S / cm or higher is more preferable. The ionic conductivity of the solid electrolyte can be measured by the AC impedance method.

[0045] Examples of solid electrolyte shapes include spherical, ellipsoidal, and other particulate forms, as well as thin films. When the solid electrolyte is particulate, its average particle size (D ５０ The particle size is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less.

[0046] The solid electrolyte content in the solid electrolyte layer is preferably 50 to 100% by mass, and more preferably 90 to 100% by mass.

[0047] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, and known binders can be used as appropriate, but for example, the binder described above for the negative electrode intermediate layer can be similarly employed. The binder content in the solid electrolyte layer is not particularly limited, but for example, it is 1 to 10% by mass.

[0048] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, and preferably 10 to 100 μm.

[0049] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above-mentioned embodiment, and can be modified as appropriate based on the description of the claims.

[0050] Furthermore, the following items are also included in the scope of the present invention: Item 1: A positive electrode active material layer comprising a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, wherein the content of the conductive additive is 0 parts by mass or more than 0 parts by mass and 1.5 parts by mass or less, per 100 parts by mass of the positive electrode active material; Item 2: The positive electrode active material layer according to Item 1, wherein the content of the non-fibrous binder is 0% by mass or more than 0% by mass and less than 0.8% by mass (preferably 0% by mass or more and less than 0.1% by mass, more preferably 0%), relative to the total mass of the positive electrode active material layer; Item 3: The positive electrode active material layer according to Item 1 or Item 2, wherein the content of the conductive additive is 0 parts by mass or more than 0 parts by mass and 1.2 parts by mass or less (preferably 0 parts by mass or more and 0.8 parts by mass or less, more preferably 0 parts by mass or more and 0.5 parts by mass or less, even more preferably 0.1 parts by mass or more and 0.5 parts by mass or less), per 100 parts by mass of the positive electrode active material; Item 4: A positive electrode active material layer according to any one of items 1 to 3, comprising only the positive electrode active material, the sulfide solid electrolyte, and the fibrous binder; Item 5: A positive electrode active material layer according to any one of items 1 to 4, wherein the fibrous binder comprises a compound containing a fluorine element; Item 6: A positive electrode active material layer according to any one of items 1 to 5, wherein the fibrous binder is polytetrafluoroethylene (PTFE); Item 7: A positive electrode active material layer according to any one of items 1 to 6, wherein the content of the fibrous binder is greater than 0% by mass and less than or equal to 1.0% by mass (preferably greater than 0% by mass and less than or equal to 0.8% by mass, more preferably greater than 0% by mass and less than or equal to 0.5% by mass) with respect to the total mass of the positive electrode active material layer; Item 8: A positive electrode having a positive electrode active material layer according to any one of items 1 to 7; a negative electrode having a negative electrode current collector, on which lithium metal is deposited during charging; and a solid electrolyte layer interposed between the positive electrode and the negative electrode, containing a solid electrolyte; A lithium secondary battery equipped with a power generation element having the following properties.

[0051] Item 9: The lithium secondary battery according to claim 8, which is an all-solid-state battery.

[0052] 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, the operation of preparing the evaluation cells was carried out in a glove box with a dew point of -68°C or lower. Furthermore, the instruments and equipment used in the glove box were thoroughly dried beforehand. Punching was performed using punching machines manufactured by Nogami Giken Co., Ltd. for each size.

[0053] <Examples of evaluation cell fabrication> [Comparative example 1] (Fabrication of positive electrode) NMC composite oxide (LiNi) as positive electrode active material ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D ５０ ): 5 μm) and argyrodite-type sulfide solid electrolyte (Li) as a solid electrolyte. ６ PS ５ Cl, average particle diameter (D ５０ A 0.7 μm carbon nanotube (0.7 μm) and a conductive additive were weighed, and isobutyl isobutyrate as a solvent and difluorovinyl tetrafluoropropylene (fluorine-based binder) as a non-fibrous binder were dissolved in isobutyl isobutyrate and added. The mixture was then mixed using a rotation / revolution mixer (Awatori Rentaro AR-100, manufactured by Shinky Co., Ltd.). As a result, a slurry containing positive electrode active material:solid electrolyte:conductive additive:binder in a mass ratio of 87:11:1:1 was obtained. The obtained slurry was coated onto aluminum foil as a positive electrode current collector and dried to obtain a positive electrode with a positive electrode active material layer formed in this comparative example (basis weight of positive electrode active material layer: 30 mg / cm²). ２ ).

[0054] (Preparation of the solid electrolyte layer) Algyrodite-type sulfide solid electrolyte (Li ６ PS ５ Cl, average particle diameter (D ５０ A solid electrolyte slurry was prepared by mixing 97 parts by mass of (3 μm) and 3 parts by mass of styrene-butadiene rubber (SBR) as a binder with mesitylene as a solvent. A solid electrolyte layer was obtained by coating the solid electrolyte slurry onto stainless steel foil as a support and drying it.

[0055] (Fabrication of the negative electrode) Silver nanoparticles (average particle size (D) ５０ 21.5 parts by mass of (60 nm) and 64.5 parts by mass of carbon black were weighed (Ag:C = 1:3 (mass ratio)) and mixed. To 86 parts by mass of the resulting mixture, 14 parts by mass of polyvinylidene fluoride (PVdF) as a binder was added, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. The negative electrode intermediate layer slurry was coated onto stainless steel foil as a negative electrode current collector and dried to obtain a negative electrode with a negative electrode intermediate layer formed thereon.

[0056] (Preparation of evaluation cell) The positive electrode prepared above was punched out to 19 mm x 19 mm (corner radius = 1 mm). The solid electrolyte layer prepared above was also punched out to 30 mm x 30 mm (corner radius = 1 mm). The positive electrode was placed on top of the solid electrolyte layer so that the surface of the positive electrode active material layer was in contact with it, and hydrostatic pressing (CIP) was performed at 25°C and 700 MPa for 1 minute. After that, the stainless steel foil used as a support was removed from the solid electrolyte layer, the negative electrode prepared above was punched out to 21 mm x 21 mm (corner radius = 1 mm), and this was placed in contact with the solid electrolyte layer, and hydrostatic pressing (CIP) was performed at 80°C and 700 MPa for 1 minute. This created a laminate of positive electrode current collector / positive electrode active material layer / solid electrolyte layer / negative electrode intermediate layer / negative electrode current collector, which was then placed in an outer casing to obtain an evaluation cell.

[0057] [Example 1] (Preparation of positive electrode active material layer) NMC composite oxide (LiNi) as positive electrode active material ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D ５０ ): 5 μm) and argyrodite-type sulfide solid electrolyte (Li) as a solid electrolyte. ６ PS ５ Cl, average particle diameter (D ５０A 0.7 μm (0.7 μm) and carbon nanotubes as a conductive additive were mixed using an agate mortar. Polytetrafluoroethylene (PTFE) as a fibrous binder was added to the resulting mixed powder to obtain a mixed powder with a mass ratio of 87:11.2:1:0.8 for positive electrode active material:solid electrolyte:conductive additive:binder. The resulting mixed powder was lightly kneaded in an agate mortar, and then kneaded for 10 minutes while applying shear force. After confirming that the PTFE had fibrillated, the resulting kneaded material was formed into a sheet using a roll press, and the sheet-shaped kneaded material was folded and formed into a sheet again. By repeating the folding and forming into a sheet to reduce the gap, the material was stretched until it was finally 150 μm thick, and the positive electrode active material layer of this example was obtained (basis weight of positive electrode active material layer: 30 mg / cm²). ２ ).

[0058] The preparation of the solid electrolyte layer and the negative electrode was carried out using the same method as in Comparative Example 1 described above.

[0059] (Preparation of evaluation cell) The positive electrode active material layer prepared above was punched out to 19 mm x 19 mm (corner radius = 1 mm). The solid electrolyte layer prepared above was also punched out to 30 mm x 30 mm (corner radius = 1 mm). The positive electrode active material layer was placed on aluminum foil as the positive electrode current collector, and the solid electrolyte layer was placed on top of this. Hydrostatic pressing (CIP) was performed at 25°C and 700 MPa for 1 minute. The negative electrode prepared above was punched out to 21 mm x 21 mm (corner radius = 1 mm), and this was placed in contact with the solid electrolyte layer. Hydrostatic pressing (CIP) was performed at 80°C and 700 MPa for 1 minute. As a result, a laminate of positive electrode current collector / positive electrode active material layer / solid electrolyte layer / negative electrode intermediate layer / negative electrode current collector was prepared and placed in an outer casing to obtain an evaluation cell.

[0060] [Example 2] An evaluation cell for this example was prepared in the same manner as in Example 1, except that the amount of each component added was adjusted so that the mass ratio of positive electrode active material: solid electrolyte: conductive additive: binder was 87.3:11.2:0.7:0.8.

[0061] [Example 3] An evaluation cell for this example was prepared in the same manner as in Example 1, except that, in the above (preparation of the positive electrode active material layer), no conductive additive was added, and the amount of each component added was such that the mass ratio of positive electrode active material:solid electrolyte:binder was 88:11.2:0.8.

[0062] [Comparative Example 2] In the above (preparation of the positive electrode active material layer), the average particle size of the NMC composite oxide as the positive electrode active material was set to 7 μm, and the amount of each component added was set so that the mass ratio of positive electrode active material: solid electrolyte: conductive additive: binder was 87:10.7:1.5:0.8. The evaluation cell for this comparative example was prepared in the same manner as in Example 1.

[0063] [Example 4] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that the amount of each component added was set so that the mass ratio of positive electrode active material:solid electrolyte:conductive additive:binder was 87:11.2:1:0.8.

[0064] [Example 5] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that the amount of each component added in the above (preparation of the positive electrode active material layer) was set to a mass ratio of positive electrode active material:solid electrolyte:conductive additive:binder of 87:11.5:1:0.5.

[0065] [Example 6] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that the amount of each component added was such that the mass ratio of positive electrode active material:solid electrolyte:conductive additive:binder was 89:9.8:0.7:0.5, as described above for the preparation of the positive electrode active material layer.

[0066] [Example 7] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that the amount of each component added was adjusted so that the mass ratio of positive electrode active material:solid electrolyte:conductive additive:binder was 88.9:10.2:0.4:0.5.

[0067] [Example 8] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that the amount of each component added was such that the mass ratio of positive electrode active material: solid electrolyte: conductive additive: binder was 89:10.3:0.2:0.5.

[0068] [Example 9] An evaluation cell for this example was prepared in the same manner as in Comparative Example 2, except that, in the above (preparation of the positive electrode active material layer), no conductive additive was added, and the amount of each component added was such that the mass ratio of positive electrode active material:solid electrolyte:binder was 89:10.5:0.5.

[0069] [Comparative Example 3] In the above (preparation of the positive electrode active material layer), the average particle size of the NMC composite oxide as the positive electrode active material was set to 7 μm, styrene-butadiene rubber (SBR), a non-fibrous binder, was used as the binder, no conductive additive was added, and the amount of each component added was set to a mass ratio of positive electrode active material:solid electrolyte:binder of 89:10.2:0.8. The evaluation cell for this comparative example was prepared in the same manner as in Comparative Example 1.

[0070] <Charge / Discharge Test (Resistance Measurement)> The following charge / discharge tests were performed on the fabricated evaluation cells. The charge / discharge tests were conducted while applying a restraining pressure of 3 MPa in the stacking direction of the evaluation cells using a pressurizing member. First, the evaluation cells were charged in a constant temperature bath set to 60°C with a constant current equivalent to 0.05C. When the cell voltage reached 4.25V, the charging was switched to a constant voltage mode, and after the current value became equivalent to 0.02C, a 0.5-hour pause was performed. After that, the cells were discharged with a constant current equivalent to 0.1C, and discharge was continued until the cell voltage reached 2.5V, followed by a 0.5-hour pause. This cycle was considered one cycle, and two charge / discharge cycles were performed. The discharge capacity up to two cycles was measured, and the charge capacity was adjusted to 50% of the SOC relative to the discharge capacity of the second cycle. The resistance value was calculated from the voltage rise when charging for 10 seconds with a constant current equivalent to 0.05C, 0.1C, 0.2C, and 0.5C.

[0071] For the obtained resistance values, the resistance values ​​obtained in Examples 1 to 3 (average particle size of positive electrode active material of 5 μm) were set to 100% based on the resistance value obtained in Comparative Example 1, and for Examples 4 to 9 and Comparative Example 3 (average particle size of positive electrode active material of 7 μm), the resistance values ​​obtained in Comparative Example 2 were set to 100% based on the resistance value obtained in Comparative Example 2. The percentage of the resistance value (%) was calculated for each example and expressed as the relative resistance. A lower resistance indicates better cell characteristics. The results are shown in Table 1 below.

[0072]

[0073] As shown in Table 1, the evaluation cell of Comparative Example 1, which did not contain a fibrous binder, showed a higher resistance value compared to the evaluation cells of Examples 1 to 3. Furthermore, the evaluation cell of Comparative Example 2, in which the conductive additive content exceeded 1.5 parts by mass per 100 parts by mass of positive electrode active material, and the evaluation cell of Comparative Example 3, which did not contain a fibrous binder, also showed a higher resistance value compared to the evaluation cells of Examples 4 to 9. From these results, it was found that lithium secondary batteries having a positive electrode active material layer containing a fibrous binder and in which the conductive additive content is in the range of 0 parts by mass or more than 0 parts by mass but 1.5 parts by mass or less per 100 parts by mass of positive electrode active material, exhibit reduced resistance.

[0074] 10a Stacked secondary battery, 11' Negative electrode current collector, 11'' Positive electrode current collector, 13 Negative electrode active material layer, 14 Negative electrode intermediate 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, 30 Fibrous binder, X, Y, Z Fibers.

Claims

1. A positive electrode active material layer comprising a positive electrode active material, a sulfide solid electrolyte, and a fibrous binder, wherein the content of a conductive additive is 0 parts by mass or more than 0 parts by mass but 1.5 parts by mass or less, per 100 parts by mass of the positive electrode active material.

2. The positive electrode active material layer according to claim 1, wherein the content of the non-fibrous binder is 0% by mass or greater than 0% by mass and less than 0.8% by mass, relative to the total mass of the positive electrode active material layer.

3. The positive electrode active material layer according to claim 1, wherein the content of the conductive additive is 0 parts by mass or more than 0 parts by mass but 1.2 parts by mass or less, based on 100 parts by mass of the positive electrode active material.

4. The positive electrode active material layer according to claim 1, wherein the content of the conductive additive is 0 parts by mass or more than 0 parts by mass but 0.8 parts by mass or less, per 100 parts by mass of the positive electrode active material.

5. The positive electrode active material layer according to claim 1, comprising only the positive electrode active material, the sulfide solid electrolyte, and the fibrous binder.

6. The positive electrode active material layer according to claim 1, wherein the fibrous binder comprises a compound containing a fluorine element.

7. The positive electrode active material layer according to claim 1, wherein the fibrous binder is polytetrafluoroethylene (PTFE).

8. The positive electrode active material layer according to claim 1, wherein the content of the fibrous binder is greater than 0% by mass and less than or equal to 1.0% by mass with respect to the total mass of the positive electrode active material layer.

9. A lithium secondary battery comprising a power generation element having a positive electrode having a positive electrode active material layer as described in claim 1, a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte.

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