Positive electrode active material layer for all-solid-state battery, and positive electrode for all-solid-state battery and all-solid-state battery using same

Abstract

The present invention provides a means capable of increasing, in an all-solid-state battery mechanical strength of a positive electrode active material layer while minimizing a decrease in volumetric energy density. A positive electrode active material layer for an all-solid-state battery is obtained by disposing an outer layer so as to cover, in at least one circumferential direction, a positive electrode active material layer body containing a positive electrode active material, a solid electrolyte, and a first binder. The outer layer contains a positive electrode active material, a solid electrolyte, and a second binder. The binders are higher in concentration than the positive electrode active material layer body.

Description

Positive electrode active material layer for all-solid-state battery, and positive electrode for all-solid-state battery and all-solid-state battery using the same

[0001] The present invention relates to a positive electrode active material layer for an all-solid-state battery, and a positive electrode for an all-solid-state battery and an all-solid-state battery using the same.

[0002] In recent years, research and development into all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] In a typical all-solid-state lithium secondary battery, an electrode has a configuration in which an electrode active material layer is disposed on the surface of a current collector, and the electrode active material layer further contains, in addition to the electrode active material, a solid electrolyte for improving lithium ion conductivity in the electrode active material layer, a binder for binding the electrode active material particles and the solid electrolyte particles to each other and to the current collector, and, if necessary, a conductive aid for improving electronic conductivity in the electrode active material layer.

[0004] Here, all-solid-state lithium secondary batteries are used under pressure in the stacking direction to bond the layers together and reduce internal resistance. Japanese Patent Application Laid-Open No. 2021-77644 discloses a technique for arranging an inactive member on the side of the positive electrode active material layer to prevent cracks from occurring in the solid electrolyte layer under this pressure. According to this document, such a configuration can suppress cracking of the solid electrolyte layer even under pressure when the battery is in use.

[0005] However, according to the study of the present inventors, even if the technology described in the above publication is adopted, the strength of the positive electrode active material layer itself is not increased, and therefore it may not be possible to prevent cracks that occur during handling of the positive electrode active material layer, and it has been found that there is still room for improvement. On the other hand, if the binder content is increased in order to increase the strength of the positive electrode active material layer, the volumetric energy density of the battery decreases.

[0006] Therefore, an object of the present invention is to provide a means for improving the mechanical strength of a positive electrode active material layer while minimizing a decrease in volumetric energy density in an all-solid-state battery.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by forming a cathode active material layer for an all-solid-state battery by arranging an outer layer containing a cathode active material, a solid electrolyte, and a second binder, and having a binder concentration higher than that of the cathode active material layer main body, so as to cover the cathode active material layer main body containing a cathode active material, a solid electrolyte, and a first binder in at least one circumferential direction, thereby completing the present invention.

[0008] That is, one embodiment of the present invention is a positive electrode active material layer for an all-solid-state battery, comprising: a positive electrode active material layer main body containing a positive electrode active material, a solid electrolyte, and a first binder; and at least one outer layer containing a positive electrode active material, a solid electrolyte, and a second binder, the outer layer having a higher binder concentration than that of the positive electrode active material layer main body, and arranged so as to cover the positive electrode active material layer main body in at least one circumferential direction.

[0009] Fig. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1. Fig. 3 is an XZ cross-sectional view of a positive electrode active material layer 15 of a laminated battery according to one embodiment of the present invention. Fig. 4 is a schematic diagram showing an example of a branched-chain fibrous binder.

[0010] One aspect of the present invention is a cathode active material layer for an all-solid-state battery, comprising: a cathode active material layer main body containing a cathode active material, a solid electrolyte, and a first binder; and at least one outer layer containing a cathode active material, a solid electrolyte, and a second binder, the outer layer having a higher binder concentration than the cathode active material layer main body and disposed so as to cover the cathode active material layer main body in at least one circumferential direction. By adopting such a configuration, it is possible to improve the mechanical strength of the cathode active material layer while minimizing a decrease in the volumetric energy density of the all-solid-state battery.

[0011] Hereinafter, with reference to the drawings, an embodiment of an all-solid-state battery employing the above-described all-solid-state battery positive electrode active material layer according to the present embodiment will be described. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiment. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios. Below, the present invention will be described using as an example a flat stacked (internal parallel connection) all-solid-state lithium secondary battery (hereinafter simply referred to as a "stacked battery"), which is one form of secondary battery. However, in terms of the internal electrical connection form (electrode structure) of the secondary battery according to the present embodiment, it can be applied to both non-bipolar (internal parallel connection) batteries and bipolar (internal series connection) batteries.

[0012] Fig. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention, and Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1.

[0013] As shown in Fig. 1, the stacked battery 10a has a flat rectangular shape, with a negative current collector 25 and a positive current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, sealing the power generating element 21 with the negative current collector 25 and the positive current collector 27 extending to the outside. Note that the current collectors (25, 27) shown in Fig. 1 may be such that the negative current collector 25 and the positive current collector 27 extend from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple plates and extended from each side.

[0014] As shown in FIG. 2 , the stacked battery 10 a of this embodiment has a structure in which a power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a flat rectangular shape with four sides. The power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are laminated. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11″. The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11′. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are laminated in this order such that one positive electrode active material layer 15 and an adjacent negative electrode active material layer 13 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, as shown in FIG. The stacked battery 10a shown in Fig. 1 can be said to have a configuration in which a plurality of single cell layers 19 are stacked and electrically connected in parallel. Furthermore, a restraining pressure (not shown) is applied to the stacked battery 10a in the stacking direction of the power-generating element 21 by a restraining member (pressure member). Therefore, the volume of the power-generating element 21 is kept constant. In the following description, the stacking direction of the power-generating element 21 will be referred to as the Z direction, and the short and long directions of the rectangular shape of the power-generating element 21, which are directions parallel to the main surfaces of each layer, will also be referred to as the X direction and the Y direction, respectively.

[0015] A negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the electrodes (positive and negative electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of a laminate film 29 that is a battery exterior material and led out of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11″ and the negative electrode current collector 11′ of the electrodes by ultrasonic welding, resistance welding, or the like, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

[0016] 3 is an XZ cross-sectional view of the positive electrode active material layer 15 of a stacked battery according to one embodiment of the present invention. The positive electrode active material layer 15 according to this embodiment has an arbitrary XZ cross-section having the configuration shown in FIG. 3 (i.e., both end faces in the longitudinal direction (Y direction) also have the configuration shown in FIG. 3). In this embodiment, the positive electrode active material layer 15 has a positive electrode active material layer main body 15a whose outer periphery (a total of four faces: two faces (main faces) parallel to the XY plane and two faces (side faces) parallel to the YZ plane) is surrounded by a first outer layer 15b. １ The first outer layer 15b covers the １ The outer periphery (two surfaces (main surfaces) parallel to the XY plane and two surfaces (side peripheral surfaces) parallel to the YZ plane, a total of four surfaces) is formed by the second outer layer 15b ２ is covering it.

[0017] The positive electrode active material layer 15 shown in FIG. 3 includes a positive electrode active material layer main body 15a and a first outer layer 15b. １ and the second outer layer 15b ２ Each of the layers contains a positive electrode active material, a solid electrolyte, a fibrous binder, and a conductive additive. The binder concentration in each of these layers is lowest in the positive electrode active material layer main body 15a, and highest in the first outer layer 15b. １ The second outer layer 15b has the next highest concentration. ２ The concentration is highest at

[0018] The stacked battery 10a according to this embodiment preferably includes a power generating element 21 sealed in a laminate film 29 as shown in FIG. 1 , and the power generating element 21 sealed in the laminate film 29 is sandwiched between two plate-like members and further fastened using a fastening member. As a result, the plate-like members and fastening members function as pressure members that pressurize (restrain) the power generating element 21 in the stacking direction. Examples of the plate-like members include metal plates and resin plates. Examples of the fastening members include bolts and nuts. However, the pressure members are not particularly limited as long as they can pressurize the power generating element 21 in the stacking direction. A typical pressure member is a combination of a plate made of a rigid material, such as the plate-like members, and the fastening members described above. Furthermore, the fastening members may be not only bolts and nuts, but also tension plates or the like that secure the ends of the plate-like members so as to restrain the power generating element 21 in the stacking direction. The lower limit of the load applied to the power generating element 21 (constraint pressure in the stacking direction of the power generating element) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the constraint pressure in the stacking direction of the power generating element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.

[0019] The main components of the stacked battery 10a will now be described.

[0020] [Positive electrode current collector] There are no particular limitations on the material that constitutes the positive electrode current collector, and conventionally known knowledge can be referenced. For example, metals and conductive resins can be used as the material for the positive electrode current collector. There are no particular limitations on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0021] [Positive Electrode Active Material Layer] The positive electrode active material layer for an all-solid-state battery according to this embodiment contains a positive electrode active material capable of absorbing and releasing lithium ions.

[0022] The positive electrode active material is not particularly limited as long as it can release lithium ions during the charging process of the secondary battery and absorb lithium ions during the discharging process. An example of such a positive electrode active material is a material containing an M1 element and an O element, where the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. An example of such a positive electrode active material is LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt 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 ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ , LiVO ２ In some cases, two or more positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used.

[0023] In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to this embodiment is made of a layered rock salt type active material containing lithium and cobalt as the positive electrode active material from the viewpoint of output characteristics (for example, Li(Ni—Mn—Co)O ２ ) is included.

[0024] The shape of the positive electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its average particle size (D ５０) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. ５０ The value of can be measured by a laser diffraction scattering method.

[0025] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99 mass %, more preferably in the range of 40 to 90 mass %, and even more preferably in the range of 45 to 80 mass %.

[0026] The positive electrode active material layer according to this embodiment further includes a solid electrolyte. Examples of the solid electrolyte include a sulfide solid electrolyte, a resin solid electrolyte, and an oxide solid electrolyte. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of conducting ions in a solid state, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). －５ S / cm or more, and this lithium ion conductivity is preferably 1×10 －４ The ionic conductivity is 100 S / cm or more. Here, the value of the ionic conductivity can be measured by an AC impedance method.

[0027] In a preferred embodiment of this aspect, the solid electrolyte is preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing a Li element, an M element, and an S element, 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, from the viewpoint of exhibiting excellent lithium ion conductivity and being able to better follow the volume change of the electrode active material that accompanies charge and discharge, and even more preferably a sulfide solid electrolyte containing an S element, a Li element, and a P element.

[0028] The sulfide solid electrolyte is Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a skeleton include Li-P-S solid electrolytes called LPS. （４－ｘ） Ge （１－ｘ） P ｘ S ４ (x satisfies 0<x<1) or the like. More specifically, for example, LPS (Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S., Li. ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０ GeP ２ S １２ , or Li ６ P.S. ５ X (wherein X is Cl, Br or I). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions. Among them, the sulfide solid electrolyte is preferably LPS (Li ２ S-P ２ S５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ is selected from the group consisting of:

[0029] The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is preferably within the range of 1 to 70 mass %, more preferably within the range of 10 to 60 mass %, and even more preferably within the range of 20 to 55 mass %.

[0030] (Binder) The positive electrode active material layer according to this embodiment further contains a binder. The type of binder is not particularly limited, and conventionally known materials can be appropriately used as binders for electrode active material layers of secondary batteries. Among them, it is preferable that the binder contains a fibrous binder. In this specification, the term "fibrous binder" refers to a binder mainly composed of fibers having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an observation image obtained by observing a cross section of the positive electrode active material layer using a scanning electron microscope (SEM). Here, the aspect ratio is calculated by dividing the maximum Feret diameter of the binder by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the outline of the binder is sandwiched between the lines, and the minimum Feret diameter is the minimum distance between the lines when the outline of the binder is sandwiched between two parallel lines. The phrase "mainly composed" of a binder means that the area ratio of the fiber portion to the total area of ​​the binder in an SEM observation image is 50% or more. A fibrous binder may include a portion other than the fiber having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less (a portion having an aspect ratio of less than 10 or a portion having a minimum Feret diameter of more than 0.2 μm). However, the area ratio of the portion other than the fiber to the total area of ​​the fibrous binder in an 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%). Fibrous binders include not only those composed of only one fiber, but also those having a configuration in which two or more fibers are connected to each other. Specific shapes of binders having a configuration in which two or more fibers are connected to each other include branched chain, radial, and mesh shapes, as well as combinations thereof. Here, we will explain how to determine the maximum and minimum Feret diameters for a binder having a structure in which two or more fibers are connected to each other. Figure 4 is a schematic diagram showing an example of a branched fibrous binder. The binder 30 shown in Figure 4 has a structure in which fiber X, fiber Y, and fiber Z are connected to each other.Each dashed line represents a line connecting the center of the fiber width (1 / 2 width), and points A, B, and C represent the ends of each dashed line. The ends of each dashed line coincide with the ends of the fibers. Point D represents the intersection of three dashed lines. That is, the binder 30 shown in FIG. 4 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 bonded at point D. The maximum Feret diameter of fiber X in the binder 30 shown in FIG. 4 is defined as the distance from point A to point D. Similarly, the maximum Feret diameter of fiber Y is the distance from point B to point D, and the maximum Feret diameter of fiber Z is the distance from point C to point D. Furthermore, the minimum Feret diameter of fiber X is the minimum distance between two parallel lines when the outline of the binder (fiber) from point A to point D is sandwiched between the lines. The same applies to the minimum Feret diameters of fiber Y and fiber Z. In the binder 30 shown in FIG. 4 , fiber Y and fiber Z have an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less, while fiber X has an aspect ratio of less than 10. However, since the area of ​​fiber X accounts for less than 50% of the total area of ​​binder 30, the binder shown in FIG. 4 can be said to be a fibrous binder. The type of fibrous binder is not particularly limited as long as it has the above-mentioned shape in the positive electrode active material layer, but binders that fibrillate upon application of shear force are preferably used. Examples of such fibrillable binders include polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyvinyl alcohol, and polyethylene, with polytetrafluoroethylene (PTFE) being more preferred. One type of fibrous binder may be used alone, or two or more types may be used in combination. In this specification, the description of the compound name for the binder not only refers to the compound indicated by that compound name, but also includes forms in which a terminal or a portion of a side chain is substituted (modified) with another substituent. In cases in which a terminal or a portion of a side chain is substituted (modified) with another substituent, the proportion of structural units in which a terminal or a side chain is substituted (modified) with another substituent relative to 100 mol % of all structural units is preferably 10 mol % or less, and more preferably 5 mol % or less.

[0031] In addition, when the positive electrode active material layer contains a binder, it may contain a binder other than a fibrous binder (non-fibrous binder). The type of non-fibrous binder is not particularly limited, but styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), ethyl cellulose, and acrylic resin are preferred, and polyvinylidene fluoride (PVDF) is more preferred. Only one type of non-fibrous binder may be used alone, or two or more types may be used in combination. As described above, the binder may include a form in which a terminal or part of a side chain is substituted (modified) with another substituent. However, in the positive electrode active material layer, the mass proportion of the fibrous binder in the entire binder 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.

[0032] (Conductive Aid) The positive electrode active material layer preferably contains a conductive aid in addition to the positive electrode active material. The conductive aid preferably has an electronic conductivity of 1 S / m or more, and more preferably has an electronic conductivity of 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the conductive additive is not particularly limited, but is usually 1×10 ７ S / m or less.

[0033] The type of conductive additive is not particularly limited, and conventionally known materials can be appropriately used as conductive additives for the electrode active material layer of a secondary battery. Examples of conductive additives include fibrous conductive additives such as fibrous carbon. In this specification, the term "fibrous conductive additive" refers to a conductive additive having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an image observed using a scanning electron microscope (SEM). The type of fibrous carbon is not particularly limited as long as it has the above-mentioned shape, but examples include carbon fiber (specifically, vapor-grown carbon fiber (VGCF), polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, activated carbon fiber, etc.), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Among these, carbon fiber is preferred. The fibrous conductive additive (preferably fibrous carbon) may be used alone or in combination of two or more types.

[0034] On the other hand, when the positive electrode active material layer contains a conductive additive, the conductive additive may be a conductive additive other than a fibrous conductive additive (non-fibrous conductive additive). The type of non-fibrous conductive additive is not particularly limited, but examples include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, and carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), but are not limited thereto. In addition, a particulate ceramic material or a resin material coated with the above-mentioned metal material by plating or the like can also be used as a conductive additive. Only one type of non-fibrous conductive additive may be used alone, or two or more types may be used in combination.

[0035] (Characteristics of the Positive Electrode Active Material Layer) As shown in FIG. 3, the positive electrode active material layer for an all-solid-state battery according to this embodiment comprises a positive electrode active material layer main body 15a and at least one outer layer (15b) having a binder concentration higher than that of the positive electrode active material layer main body 15a and disposed so as to cover the positive electrode active material layer main body 15a in at least one circumferential direction. １ , 15b ２) is characterized in that it has. In this specification, the binder contained in the positive electrode active material layer main body is referred to as the "first binder," and the binder contained in the outer layer is referred to as the "second binder" (collectively, the binders contained in each outer layer, if multiple outer layers are present). Therefore, the order of "first" and "second" itself has no meaning; these terms are merely expressions for distinguishing the location where the binder is present. The types of components that can be contained in the positive electrode active material layer main body and the outer layer (or each outer layer, if multiple outer layers are present) and their preferred forms are as described above in the section on the positive electrode active material layer. From the viewpoint of a high effect of improving the mechanical strength of the positive electrode active material layer, it is preferable that the first binder and the second binder contain a fibrous binder. That is, it is preferable that at least one outer layer and the positive electrode active material layer main body contain a fibrous binder. It is also more preferable that all outer layers and the positive electrode active material layer main body contain a fibrous binder.

[0036] As described above, the concentration of the binder (second binder) in at least one outer layer is higher than the concentration of the binder (first binder) in the positive electrode active material layer main body. In this specification, the "binder concentration" refers to the percentage [%] of the mass ratio of each binder to 100% by mass of the constituent materials of each layer. When multiple outer layers are present, it is sufficient that the binder concentration in at least one outer layer is higher than that of the positive electrode active material layer main body, and it is preferable that the binder concentration in all outer layers is higher than that of the positive electrode active material layer main body. Furthermore, when the binder concentration in all outer layers is higher than that of the positive electrode active material layer main body, it is preferable that the layer with the highest binder concentration is located in the outermost layer, and it is more preferable that each of the multiple outer layers is arranged so that the binder concentration increases in order from the inner layer side to the outermost layer. The ratio of the concentration of the binder (second binder) in the outer layer to the concentration of the binder (first binder) in the positive electrode active material layer main body is preferably 1.1 to 3.0 times, more preferably 1.3 to 2.5 times, and even more preferably 1.5 to 2.0 times. Incidentally, the presence of regions with different compositions within the active material layer, as in the positive electrode active material layer according to the present embodiment, can cause a biased reaction, resulting in degradation of the active material layer due to current density distribution and resulting current concentration. For this reason, unless the active material layer itself is manufactured as a free-standing film, as in the positive electrode active material layer according to the present embodiment, deliberately placing regions with different compositions within the active material layer is a device that would normally be avoided. In contrast, the positive electrode active material layer according to the present embodiment is premised on the active material layer itself being manufactured as a free-standing film, and aims to solve the problem specific to such cases (ensuring mechanical strength) while minimizing the adverse effects on the volumetric energy density of the battery.

[0037] Here, when the positive electrode active material layer main body has a rectangular sheet shape as shown in Figures 2 and 3, when the outer layer is arranged to cover the positive electrode active material layer main body in the circumferential direction, it is preferably arranged to cover two main surfaces (surfaces parallel to the XY plane) of the positive electrode active material layer main body. In a preferred embodiment, an outer layer is arranged on at least one main surface of the positive electrode active material layer main body in this manner, and the outer layer with the highest binder (second binder) concentration among the outer layers arranged on that main surface is located as the outermost layer. In a more preferred embodiment, the outer layers arranged on the main surfaces are arranged so that the binder (second binder) concentration increases sequentially from the inner layer side to the outermost layer. According to these embodiments, it is possible to further increase the mechanical strength of the positive electrode active material layer while minimizing the decrease in the volumetric energy density of the battery.

[0038] Furthermore, of the two pairs of opposing surfaces constituting the remaining four surfaces (side surfaces of the rectangular sheet shape), either pair of opposing surfaces may be covered by an outer layer. However, from the viewpoint of a more excellent effect of improving mechanical strength, it is more preferable that the outer layer is arranged so as to further cover the pair of opposing surfaces parallel to the longitudinal direction of the rectangle (YZ plane shown in FIG. 3). Furthermore, one preferred embodiment is also one in which the outer layer is arranged so as to further cover the pair of opposing surfaces parallel to the short side direction of the rectangle (XZ plane shown in FIG. 3) (i.e., to cover all six surfaces of the rectangular sheet shape). In a preferred embodiment in which an outer layer is arranged on at least one side surface of the positive electrode active material layer main body as in these embodiments, the layer with the highest concentration of binder (second binder) among the outer layers arranged on the side surface is located in the outermost layer. Furthermore, in a more preferred embodiment, each of the outer layers arranged on the side surface is arranged so that the concentration of binder (second binder) increases in order from the inner layer side to the outermost layer. These embodiments also make it possible to further increase the mechanical strength of the positive electrode active material layer while minimizing the decrease in the volumetric energy density of the battery.

[0039] In a preferred embodiment, the binder (second binder) is more uniformly dispersed in at least one outer layer than the binder (first binder) in the positive electrode active material layer main body. Here, the "degree of uniform dispersion of the binder" is determined based on a coefficient of variation calculated by the following method. A smaller value of this coefficient of variation indicates a more uniform dispersion of the binder.

[0040] (Method of calculating the coefficient of variation) First, a cross section of the positive electrode active material layer in the stacking direction is observed using a scanning electron microscope (SEM) to obtain an observation image that includes the entire positive electrode active material layer. Next, an energy dispersive X-ray analysis (EDX) method is used to map elements specific to the binder (for example, fluorine (F) in the case of PTFE) for the obtained observation image to detect the presence of the binder (if multiple types of binders with different specific elements are included, the presence of each binder is detected). At this time, the size of the field of view (photography magnification) of the observation image obtained by SEM is adjusted to the same magnification as the field of view of the comparison object to obtain the observation image. Then, the observation image is divided equally into 16 sections, and the area ratio obtained from the signals of the specific elements in each section is determined, and the coefficient of variation (σ / X) is calculated by dividing the standard deviation σ of the data for the 16 sections by the average value X.

[0041] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within a range of 0.1 to 1000 μm, more preferably 40 to 100 μm. From the viewpoint of the balance between the volumetric energy density and the mechanical strength of the positive electrode active material layer, the thickness of the positive electrode active material layer itself is preferably 50 to 99%, more preferably 60 to 95%, even more preferably 70 to 90%, and particularly preferably 75 to 85% of the thickness of the above-mentioned positive electrode active material layer.

[0042] (Method for Manufacturing Positive Electrode Active Material Layer) The positive electrode active material layer for an all-solid-state battery according to one embodiment of the present invention described above can be manufactured by forming a positive electrode mixture (a mixture of materials constituting the positive electrode active material layer) in the form of a dry powder composition substantially free of liquid components such as a solvent into a sheet using a roll press. In this case, for example, a positive electrode mixture for the positive electrode active material layer main body and a positive electrode mixture for the outer layer are separately prepared so as to have different binder concentrations. Then, each positive electrode mixture is formed into a sheet using sheet forming means (a hand roller or a roll press), and the positive electrode active material layer main body is wrapped with an outer layer so as to circumferentially cover the positive electrode active material layer main body. The positive electrode active material layer for an all-solid-state battery according to the embodiment described above can be manufactured by further performing a similar roll press process.

[0043] In this specification, the term "roll press" refers to a machine that includes at least a pair of cylindrical pressure rolls arranged parallel to each other with their outer peripheral surfaces facing each other and a rotation drive mechanism for the rolls, and that sandwiches the powder composition (electrode mixture) between the pair of rolls and rolls it into a sheet. The material and size of the rolls, the type of the rotation mechanism, and the arrangement of the rolls and rotation mechanism are not particularly limited. The direction in which the electrode active material layer is extruded from between the pair of rolls is not particularly limited, and may be horizontal or downward.

[0044] First, the powder composition (electrode mixture) used as a raw material will be described. This powder composition essentially contains a positive electrode active material, a solid electrolyte, and a binder (preferably a fibrous binder). The powder composition may also contain a conductive additive, if necessary.

[0045] The term "powder composition" refers to a mixture composed of solid components that does not substantially contain liquid components such as solvents. The content of liquid components in the powder composition is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, still more preferably 0.5% by mass or less, particularly preferably 0.1% by mass or less, and most preferably 0% by mass, relative to 100% by mass of the powder composition.

[0046] There are no particular limitations on the method for obtaining the powder composition; the components constituting the powder composition may be added and mixed in any order. The mixing means are also not particularly limited; methods using conventionally known mixing means such as a mortar, mixer, or mill may be appropriately employed. Here, when the binder is a fibrous binder, there are no particular limitations on the method for incorporating the fibrous binder into the powder mixture; the fibrous binder may be prepared separately and mixed with other components. Furthermore, when a fibrillizable binder, which is a precursor of the fibrous binder, is mixed with other components, shear force may be applied to the fibrillizable binder to fibrillate it, thereby producing the fibrous binder in the powder composition. Furthermore, to enhance the dispersibility of the binder in the outer layer compared to the main body of the positive electrode active material layer, fibrillation may be carried out for a longer period of time during the preparation of the powder composition for the outer layer.

[0047] Next, the powder compositions (positive electrode mixtures) for the positive electrode active material layer main body and the outer layer prepared above are formed into sheets using sheet forming means (hand rollers or roll presses) to obtain sheets for the positive electrode active material layer main body and the outer layer. In this case, the specific sheet forming means may be selected taking into consideration the thickness of each layer, etc.

[0048] The positive electrode active material layer main body is then wrapped with an outer layer sheet so as to cover the positive electrode active material layer main body in the circumferential direction, thereby obtaining a positive electrode active material layer precursor. At this time, by adjusting the size and orientation of the outer layer sheet, it is possible to control which surface of the positive electrode active material layer main body is covered by the outer layer.

[0049] Finally, the thus obtained positive electrode active material layer precursor is subjected to the same roll press treatment as above, thereby producing the positive electrode active material layer for an all-solid-state battery according to the above-described embodiment.

[0050] In the above-described manufacturing method, it is preferable to repeatedly perform the treatment using the roll press machine while gradually reducing the gap size until the non-treated material reaches the desired thickness. This treatment has the advantage that, when a fibrous binder is used as the binder, shear force can be applied to the binder during each roll press treatment, thereby fibrillating the binder. The linear pressure applied to the treated material by the roll press machine is preferably 35 to 3500 N / cm. Here, the linear pressure applied to the treated material by the roll press machine refers to the linear pressure calculated from the load measured by a load cell attached to the roll press machine and the width of the treated material after pressing. The rotation speed of the rolls of the roll press machine is not particularly limited, but is preferably 1 to 20 m / min from the viewpoint of maintaining a sufficient press holding time.

[0051] [Solid Electrolyte Layer] The solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode current collector, and contains a solid electrolyte (usually as a main component). The specific form of the solid electrolyte contained in the solid electrolyte layer is the same as that described above, so a detailed description will be omitted here. In particular, it is preferable that the solid electrolyte layer contains a sulfide solid electrolyte.

[0052] The content of the solid electrolyte in the solid electrolyte layer is, for example, preferably in the range of 10 to 100 mass %, more preferably in the range of 50 to 100 mass %, and even more preferably in the range of 90 to 100 mass %, relative to the total mass of the solid electrolyte layer.

[0053] The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte.

[0054] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 10 to 40 μm.

[0055] [Negative electrode current collector] There are no particular limitations on the material constituting the negative electrode current collector, and conventionally known knowledge can be referenced. For example, metals and conductive resins can be used as the material constituting the negative electrode current collector. There are no particular limitations on the thickness of the negative electrode current collector, but an example is 10 to 100 μm.

[0056] [Negative Electrode Active Material Layer] The negative electrode active material layer contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes 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 thereof include metallic lithium 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 metallic lithium or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably contains metallic lithium or a lithium-containing alloy. When metallic lithium or a lithium-containing alloy is used as the negative electrode active material, the all-solid-state battery may be a so-called lithium deposition type battery in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. 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 full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during full discharge.

[0057] On the other hand, when the all-solid-state battery is not of the lithium deposition type, the negative electrode active material layer preferably further contains, in addition to the negative electrode active material, a solid electrolyte, a binder (preferably a fibrous binder), and a conductive additive, similar to the above-mentioned positive electrode active material layer. Specific examples and preferred forms of these materials are as described above in the section on the positive electrode active material layer.

[0058] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 40 to 99 mass %, and more preferably within the range of 50 to 90 mass %, for example.

[0059] [Positive and negative electrode current collectors] The material constituting the current collectors (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collectors for secondary batteries can be used. Metal materials such as aluminum, carbon-coated aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as the constituent material of the current collectors. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The negative electrode current collector 25 and the positive electrode current collector 27 may be made of the same material or different materials.

[0060] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[0061] [Battery Exterior Body] As the battery exterior body, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element, as shown in FIG. 1, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0062] Although the above description has been given using an example in which the secondary battery according to the present embodiment is an all-solid-state lithium secondary battery, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount be such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur. Note that the liquid electrolyte (electrolytic solution) is a solution in the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent. The liquid electrolyte (electrolytic solution) may further contain additives other than the organic solvent and the lithium salt. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be adjusted as appropriate.

[0063] The following embodiments are also included within the scope of the present invention: a cathode active material layer according to claim 1 having the features of claim 2; a cathode active material layer according to claim 1 or 2 having the features of claim 3; a cathode active material layer according to claim 2 having the features of claim 4; a cathode active material layer according to any one of claims 1 to 4 having the features of claim 5; a cathode active material layer according to claim 5 having the features of claim 6; a cathode active material layer according to any one of claims 1 to 6 having the features of claim 7; a cathode active material layer according to claim 7 having the features of claim 8; a cathode for an all-solid-state battery according to claim 9 having the cathode active material layer according to any one of claims 1 to 8; and an all-solid-state battery having the cathode for an all-solid-state battery according to claim 9.

[0064] 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 instruments and devices used in the glove box were thoroughly dried beforehand.

[0065] <Examples of Preparation of Positive Electrode Active Material Layer> [Example 1] A positive electrode for an all-solid-state lithium secondary battery was prepared by the following method.

[0066] First, as a constituent material of the positive electrode active material layer, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 1 μm) and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ Predetermined amounts of acetylene black (Cl, average particle diameter (D50): 0.2 μm), a particulate conductive additive, and polytetrafluoroethylene (PTFE) (a fibrillizable binder (fibrous binder)) were weighed out. Specifically, the composition of the constituent materials of the positive electrode active material layer main body was positive electrode active material: solid electrolyte: conductive additive: binder = 87.0: 11.2: 1.0: 0.8 (mass ratio). On the other hand, the composition of the constituent materials of the outer layer was positive electrode active material: solid electrolyte: conductive additive: binder = 86.4: 11.2: 1.0: 1.4 (mass ratio).

[0067] The positive electrode active material, solid electrolyte, particulate conductive additive, and binder prepared above were dispersed using a resonant acoustic mixer in a glove box with an argon atmosphere at a dew point of -68°C or less. The dispersed materials were then mixed using a kneader in the glove box. The kneader's packing ratio, calculated as bulk density, was approximately 70-95%, and the materials were mixed at 20 rpm for 7-9 minutes while applying a load. The resulting mixture (dumpling-like mass) was heated on a hot plate. As the material temperature approached 60°C, it softened and its moldability improved. The material was then heated to approximately 60°C on a hot plate and further processed as follows.

[0068] First, this material was formed using a hand roller into small pieces about 1.0 to 1.5 mm thick to obtain a cathode active material layer main body. Furthermore, small pieces obtained using the same method as for the cathode active material layer main body using the constituent material of the outer layer were rolled six times using a powder rolling machine (manufactured by Ohno Roll Co., Ltd.) while narrowing the gap, to obtain a sheet with a thickness of 150 μm to obtain an outer layer sheet. The longitudinal dimensions of the cathode active material layer main body and the outer layer sheet were the same. The cathode active material layer main body obtained above was then placed on the center of the outer layer sheet obtained above, with both longitudinal ends aligned, and the exposed portion of the outer layer sheet was folded to enclose the four sides of the cathode active material layer main body except for the longitudinal end faces, to obtain a cathode active material layer precursor. The positive electrode active material layer precursor obtained above was rolled six times using a powder rolling mill while narrowing the gap, to form a sheet with a thickness of 100 μm, thereby obtaining the positive electrode active material layer of this example. The positive electrode active material layer thus obtained had a laminated structure of outer layer (thickness 10 μm) / positive electrode active material layer main body (80 μm) / outer layer (thickness 10 μm).

[0069] <Evaluation of the Strength of the Positive Electrode Active Material Layer> For each of the positive electrode active material layer main body and the outer layer sheet prepared in the above-described examples, a tensile test was performed in accordance with JIS K 6251:2017 to measure the breaking strength. Specifically, a dumbbell-shaped sample was punched out to prepare a test piece, and a tensile test was performed at room temperature using a universal testing machine manufactured by Imada Co., Ltd., at a tensile speed of 10 mm / min. The width and thickness of the dumbbell portion of the test piece were measured, and the breaking strengths of the positive electrode active material layer main body and the outer layer sheet were found to be 0.6 [MPa] and 1.1 [MPa], respectively, confirming that the outer layer sheet exhibited higher mechanical strength. In addition, the ratio of the volumetric capacities calculated from the composition and volume proportion of the positive electrode active material layer main body and the outer layer was positive electrode active material layer main body: outer layer = 1.00:0.92. From these results, the volumetric capacity of the positive electrode active material layer produced in the above-mentioned Example was 0.97 relative to the volumetric capacity of the positive electrode active material layer main body, and the tensile strength at break was 0.8 [MPa]. That is, it was shown that the positive electrode active material layer for an all-solid-state battery according to one embodiment of the present invention can ensure a volumetric energy density substantially equivalent to that of the positive electrode active material layer main body alone, while significantly improving the mechanical strength (tensile strength at break) (by about 1.33 times).

[0070] 10a: laminated battery; 11': negative electrode current collector; 11'': positive electrode current collector; 13: negative electrode active material layer; 15: positive electrode active material layer; 15a: positive electrode active material layer main body; 15b １ First outer layer, 15b ２ Second outer layer, 17 solid electrolyte layer, 19 unit cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film, 30 fibrous binder.

Claims

1. A positive electrode active material layer for an all-solid-state battery, comprising: a positive electrode active material layer main body containing a positive electrode active material, a solid electrolyte, and a first binder; and at least one outer layer containing a positive electrode active material, a solid electrolyte, and a second binder, the outer layer having a higher concentration of the binder than that of the positive electrode active material layer main body, and disposed so as to cover the positive electrode active material layer main body in at least one circumferential direction.

2. The positive electrode active material layer for an all-solid-state battery according to claim 1, wherein the first binder and the second binder include a fibrous binder.

3. The positive electrode active material layer for an all-solid-state battery according to claim 1, wherein in at least one of the outer layers, the second binder is more uniformly dispersed than the first binder in the positive electrode active material layer main body.

4. The positive electrode active material layer for an all-solid-state battery according to claim 2, wherein the fibrous binder comprises one or more selected from the group consisting of polytetrafluoroethylene, carboxymethyl cellulose, polyvinyl alcohol and polyethylene.

5. The positive electrode active material layer for an all-solid-state battery according to claim 1, wherein the positive electrode active material layer main body has a rectangular sheet shape, the outer layer is disposed on at least one main surface of the positive electrode active material layer main body, and among the outer layers disposed on the main surface, a layer having the highest concentration of the second binder is positioned as an outermost layer.

6. The positive electrode active material layer for an all-solid-state battery according to claim 5, wherein each of the outer layers arranged on the main surface is arranged so that the concentration of the second binder increases in order from the inner layer side to the outermost layer.

7. The positive electrode active material layer for an all-solid-state battery according to claim 1, wherein the positive electrode active material layer main body has a rectangular sheet shape, the outer layer is disposed on at least one side peripheral surface of the positive electrode active material layer main body, and among the outer layers disposed on the side peripheral surface, a layer having the highest concentration of the second binder is positioned as the outermost layer.

8. The positive electrode active material layer for an all-solid-state battery according to claim 7, wherein each of the outer layers arranged on the side peripheral surface is arranged so that the concentration of the second binder increases in order from the inner layer side to the outermost layer.

9. A positive electrode for an all-solid-state battery, comprising: a positive electrode current collector; and a positive electrode active material layer for an all-solid-state battery according to any one of claims 1 to 8, disposed on a surface of the positive electrode current collector.

10. An all-solid-state battery comprising the positive electrode for an all-solid-state battery according to claim 9.

Images