Secondary batteries

Abstract

The present invention provides a means which is capable of improving the charge capacity during high-rate charge of a secondary battery that is provided with a solid electrolyte layer. The present invention provides a secondary battery which is provided with: a positive electrode that is obtained by arranging a positive electrode active material layer on the surface of a positive electrode collector; a negative electrode that is obtained by arranging a negative electrode active material layer on the surface of a negative electrode collector; and a first solid electrolyte layer and a second solid electrolyte layer, each of which contains a solid electrolyte and a binder. With respect to this secondary battery, the positive electrode and the negative electrode are arranged so as to face each other in such a manner that the first solid electrolyte layer and the second solid electrolyte layer are sandwiched between the positive electrode active material layer and the negative electrode active material layer; the first solid electrolyte layer is arranged on the positive electrode active material layer side, while the second solid electrolyte layer is arranged on the negative electrode active material layer side; the thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer; and the ratio (A / B) of the binder concentration (A) (% by mass) relative to the total solid content in the first solid electrolyte layer to the binder concentration (B) (% by mass) relative to the total solid content in the second solid electrolyte layer satisfies 1 < A / B ≤ 4.5.

Description

[Technical Field]

[0001] This invention relates to a secondary battery using a solid electrolyte. [Background technology]

[0002] In recent years, reducing carbon dioxide emissions has become a pressing need to address global warming. The automotive industry is hoping that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will reduce carbon dioxide emissions, and development of non-aqueous electrolyte secondary batteries, such as motor drive batteries, which are key to the practical application of these vehicles, is actively underway.

[0003] Motor-driving rechargeable batteries are required to have extremely high output characteristics and high energy density compared to consumer rechargeable batteries used in mobile phones, laptops, and other devices. Therefore, lithium-ion batteries, which have the highest theoretical energy among all practical batteries, are attracting attention and are currently undergoing rapid development.

[0004] Currently, lithium-ion secondary batteries commonly used in Japan utilize flammable organic electrolytes. Such liquid-based lithium-ion secondary batteries require stricter safety measures against leakage, short circuits, and overcharging compared to other types of batteries.

[0005] Therefore, in recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors that can conduct ions in a solid state. For this reason, in all-solid-state lithium secondary batteries, various problems caused by flammable organic electrolytes, as seen in conventional liquid-based lithium secondary batteries, do not occur in principle. In addition, generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery.

[0006] Incidentally, in recent years, there has been a growing need for higher energy density in all-solid-state secondary batteries, and consequently, there is a demand for thinner solid electrolyte layers. However, thinning the solid electrolyte layer in all-solid-state lithium secondary batteries presents a problem in that it can cause short circuits. To address this problem, the technology disclosed in International Publication No. 2020 / 166165 attempts to solve it by providing a first solid electrolyte layer and a second solid electrolyte layer in the all-solid-state secondary battery. This all-solid-state secondary battery is characterized in that the thickness and organic compound content of the first solid electrolyte layer are smaller than those of the second solid electrolyte layer. [Overview of the project] [Problems that the invention aims to solve]

[0007] However, our own investigations have revealed that, in the technique described in International Publication No. 2020 / 166165, sufficient charging capacity may not be achieved when rapidly charging a secondary battery equipped with a solid electrolyte layer.

[0008] Therefore, the present invention aims to provide a means for improving the charging capacity during rapid charging in a secondary battery equipped with a solid electrolyte layer. [Means for solving the problem]

[0009] The inventors diligently studied to solve the above problems. As a result, they found that the above problems can be solved by providing two solid electrolyte layers in a secondary battery using a solid electrolyte, making the thickness of the first solid electrolyte layer located on the positive electrode side smaller than the thickness of the second solid electrolyte layer located on the negative electrode side, and further increasing the binder concentration in the first solid electrolyte layer within a specific range compared to the binder concentration in the second solid electrolyte layer, thus completing the present invention.

[0010] In other words, one embodiment of the present invention relates to a secondary battery comprising: a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector; a negative electrode having a negative electrode active material layer disposed on the surface of a negative electrode current collector; and a first solid electrolyte layer and a second solid electrolyte layer each containing a solid electrolyte and a binder, respectively. The positive electrode and the negative electrode are arranged opposite each other so as to sandwich the first solid electrolyte layer and the second solid electrolyte layer, with the first solid electrolyte layer on the positive electrode active material layer side and the second solid electrolyte layer on the negative electrode active material layer side, the thickness of the first solid electrolyte layer being smaller than the thickness of the second solid electrolyte layer, and the ratio (A / B) of the binder concentration (A) [mass%] of the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] of the total solid content of the second solid electrolyte layer being 1 [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a perspective view showing the external appearance of a flat-layered secondary battery, which is one embodiment of the present invention. [Figure 2] Figure 2 is a cross-sectional view along the line 2-2 shown in Figure 1. [Figure 3] Figure 3 is a schematic diagram showing an enlarged cross-section of the single cell layer that constitutes the power generation element of the stacked secondary battery shown in Figures 1 and 2. [Modes for carrying out the invention]

[0012] ​One embodiment of the present invention includes a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector, a negative electrode having a negative electrode active material layer disposed on the surface of a negative electrode current collector, a first solid electrolyte layer and a second solid electrolyte layer each containing a solid electrolyte and a binder. The positive electrode and the negative electrode are oppositely arranged such that the positive electrode active material layer and the negative electrode active material layer sandwich the first solid electrolyte layer and the second solid electrolyte layer. The first solid electrolyte layer is disposed on the positive electrode active material layer side, and the second solid electrolyte layer is disposed on the negative electrode active material layer side. The thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer. A ratio (A / B) of a concentration (A) [% by mass] of the binder with respect to the total solid content of the first solid electrolyte layer to a concentration (B) [% by mass] of the binder with respect to the total solid content of the second solid electrolyte layer is 1 < A / B ≤ 4.5. This is a secondary battery. According to this embodiment, in a secondary battery provided with a solid electrolyte layer, the charging capacity during rapid charging can be improved.

[0013] Hereinafter, this embodiment will be described with reference to the drawings. However, 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 embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may be different from the actual ratios. Hereinafter, the present invention will be described by taking a laminated type (internally parallel-connected type) all-solid-state lithium secondary battery, which is one embodiment of a secondary battery, as an example. In an all-solid-state lithium secondary battery, there is an advantage that problems caused by a flammable organic electrolyte solution, such as in a conventional liquid-based lithium secondary battery, do not occur in principle. Furthermore, there is also an advantage that when a high-potential and high-capacity positive electrode material and a high-capacity negative electrode material are used, a significant improvement in the output density and energy density of the battery can be achieved.

[0014] FIG. 1 is a perspective view showing the appearance of a flat laminated all-solid-state lithium secondary battery according to an embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line 2-2 shown in FIG. 1. By making it laminated, the battery can be made compact and have a high capacity. In this specification, a non-bipolar all-solid-state lithium secondary battery of the flat laminated type shown in FIGS. 1 and 2 (hereinafter, also simply referred to as "laminated secondary battery") will be described in detail as an example. However, when viewed from the electrical connection form (electrode structure) inside the secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

[0015] As shown in FIG. 1, the laminated secondary battery 10a has a rectangular flat shape, and negative electrode current collectors 25 and positive electrode current collectors 27 for extracting electric power are drawn out from both side portions thereof. The power generation element 21 is wrapped by a battery exterior material (laminate film 29) of the laminated secondary battery 10a, and the periphery thereof is heat-sealed. The power generation element 21 is sealed in a state where the negative electrode current collector 25 and the positive electrode current collector 27 are drawn out to the outside.

[0016] As shown in FIG. 2, the laminated secondary battery 10a of this embodiment has a structure in which a flat substantially rectangular power generation element 21 where the charge and discharge reaction actually proceeds is sealed inside a laminate film 29 which is a battery exterior material. Here, the power generation 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 surfaces 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 surfaces of a negative electrode current collector 11'. The solid electrolyte layer 17 is composed of two layers, a first solid electrolyte layer and a second solid electrolyte layer, as will be described later.

[0017] The negative electrode current collector 11' and the positive electrode current collector 11'' are each fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, which are electrically connected to the respective electrodes (negative and positive electrodes), and are structured to be led out to the outside of the laminate film 29 by being sandwiched between the edges of the laminate film 29. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be attached to the negative electrode current collector 11' and the positive electrode current collector 11'' of each electrode via negative electrode terminal leads and positive electrode terminal leads (not shown) as needed, by ultrasonic welding, resistance welding, or the like.

[0018] Figure 3 is a schematic diagram showing an enlarged cross-section of a single cell layer 19 that constitutes the power generation element 21 of the stacked secondary battery 10a shown in Figures 1 and 2. As shown in Figure 3, in this embodiment, in the single cell layer 19, the positive electrode active material layer 15 and the negative electrode active material layer 13 are arranged so as to sandwich the first solid electrolyte layer 17a and the second solid electrolyte layer 17b, with the first solid electrolyte layer 17a on the positive electrode active material layer 15 side and the second solid electrolyte layer 17b on the negative electrode active material layer 13 side. In other words, the single cell layer 19 has a structure in which the positive electrode current collector 11'', the positive electrode active material layer 15, the first solid electrolyte layer 17a, the second solid electrolyte layer 17b, the negative electrode active material layer 13, and the negative electrode current collector 11' are stacked in this order. Furthermore, the thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer, and the ratio (A / B) of the binder concentration (A) [mass%] of the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] of the total solid content of the second solid electrolyte layer is 1

[0019] The secondary battery according to the present invention, having the above configuration, can improve the charging capacity during rapid charging in a secondary battery equipped with a solid electrolyte layer. Although the details of why this effect is achieved are unclear, the following mechanism is considered.

[0020] ​For example, when a secondary battery such as a lithium-ion battery is charged, lithium is released from the positive electrode active material, which can cause the positive electrode active material layer to shrink. In recent years, there has been a growing need for rapid charging of secondary batteries, but it is known that rapid charging of secondary batteries reduces their charging capacity. Our investigation has revealed that this reduction in charging capacity is caused by the rapid contraction of the positive electrode active material layer when a secondary battery is rapidly charged, resulting in delamination at the interface between the positive electrode active material layer and the solid electrolyte layer.

[0021] In the secondary battery according to the present invention, the binder concentration in the first solid electrolyte layer is higher than that in the second solid electrolyte layer. Therefore, a large amount of binder is present on the surface of the first solid electrolyte layer. These binders bind to the positive electrode active material, solid electrolyte, conductive additive, and binder present on the surface of the positive electrode active material layer, thereby increasing the adhesion between the first solid electrolyte layer and the positive electrode active material layer, and suppressing delamination of the two layers. Furthermore, because the first solid electrolyte layer is thin, a decrease in ionic conductivity is suppressed. In addition, because the second solid electrolyte layer, which is provided together with the first solid electrolyte layer, is thicker, it is possible to prevent short circuits in the secondary battery caused by dendrite formation and cracking of the solid electrolyte layer. Moreover, because the binder concentration in the second solid electrolyte layer is low, high ionic conductivity can be maintained even with a thick layer. Thus, in the secondary battery according to the present invention, by improving the adhesion between the positive electrode active material layer and the solid electrolyte layer, delamination of both layers is prevented, while short circuits due to dendrite formation and the like are prevented, and the ionic conductivity of the solid electrolyte layer can also be maintained, which is thought to improve the charging capacity during rapid charging.

[0022] In this embodiment of the stacked secondary battery 10a, it is preferable that the power generation element 21 sealed in the laminate film 29 shown in Figure 1 is sandwiched between two plate-like members and further fastened with fastening members. Thus, the plate-like members and fastening members function as pressurizing members that pressurize (restrain) the power generation element 21 in the stacking direction. Examples of plate-like members include metal plates and resin plates. Examples of fastening members include bolts and nuts. However, the pressurizing members are not particularly limited as long as they can pressurize the power generation element 21 in the stacking direction. Typically, a combination of a plate made of a rigid material, such as a plate-like member, and the aforementioned fastening members is used as the pressurizing members. Furthermore, in addition to bolts and nuts, tension plates that fix the ends of the plate-like members to restrain the power generation element 21 in the stacking direction may also be used as fastening members. The lower limit of the load applied to the power generation element 21 (restraining pressure in the stacking direction of the power generation element) is, for example, 0.1 MPa or more, preferably 0.5 MPa or more, more preferably 1 MPa or more, and even more preferably 3 MPa or more. The upper limit of the restraining pressure in the stacking direction of the power generation 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.

[0023] The following describes the main components of the secondary battery according to the present invention, which were explained above using the stacked secondary battery 10a as an example.

[0024] [Current collector] The current collectors (negative electrode current collector 11' and positive electrode current collector 11") have the function of mediating the movement of electrons from the electrode active material layer. There are no particular restrictions on the materials that make up the current collectors. For example, metals or conductive resins can be used as the materials that make up the current collectors.

[0025] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, clad materials of nickel and aluminum, or copper and aluminum may also be used. Furthermore, a foil in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoint of electronic conductivity, battery operating potential, and adhesion of the active material.

[0026] Furthermore, examples of the latter conductive resins include conductive polymer materials and resins to which conductive fillers are added as needed.

[0027] The current collector may be a single-layer structure made of a single material, or it may be a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a conductive resin.

[0028] [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 examples include carbon materials, metal oxides, and metallic active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it contains lithium, and examples include 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 is particularly preferably metallic lithium or a lithium-containing alloy. When metallic lithium or a lithium-containing alloy is used as the negative electrode active material, the secondary battery according to this embodiment may be a so-called lithium-depositing type, in which metallic lithium as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during complete discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of metallic lithium may be present during complete discharge.

[0029] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 100% by mass, and more preferably in the range of 50 to 90% by mass.

[0030] The negative electrode active material layer may further contain a solid electrolyte as needed. The inclusion of a solid electrolyte in the negative electrode active material layer can improve its ionic conductivity. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, a solid electrolyte refers to a material mainly composed of an ionic conductor capable of conducting ions in a solid state, and in particular, a lithium ion conductivity of 1 × 10⁻¹⁶ at room temperature (25°C). -5 This refers to a material with a lithium ion conductivity of S / cm or higher, and this lithium ion conductivity is preferably 1 × 10⁻⁶. -4It is above S / cm. Here, the value of ionic conductivity can be measured by the AC impedance method.

[0031] From the viewpoint of exhibiting excellent lithium ion conductivity, the solid electrolyte is preferably a sulfide solid electrolyte containing S element, more preferably a sulfide solid electrolyte containing Li element, M element and S element, and the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I, and still more preferably a sulfide solid electrolyte containing S element, Li element and P element.

[0032] The sulfide solid electrolyte may have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include, for example, a Li-P-S based solid electrolyte called LPS. Also, as the sulfide solid electrolyte, for example, Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1), such as LGPS, may be used. More specifically, for example, LPS (Li2S-P2S5), Li7P3S 11 、Li 3.2 P 0.96 S、Li 3.25 Ge 0.25 P 0.75 S4、Li 10 GeP2S 12 、or Li6PS5X (where X is Cl, Br or I), etc. are mentioned. The description of "Li2S-P2S5" means a sulfide solid electrolyte formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. Among them, from the viewpoint of high ionic conductivity, the sulfide solid electrolyte is preferably LPS (Li2S-P2S5), Li6PS5X (Ardillodite type solid electrolyte, where X is Cl, Br or I), Li7P3S 11 、Li3. 2P 0.96 Selected from the group consisting of S and Li3PS4.

[0033] Examples of solid electrolyte shapes include spherical, ellipsoidal, and other particulate forms, as well as thin films. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle size (D50) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

[0034] The solid electrolyte content in the negative electrode active material layer is preferably in the range of 1 to 60% by mass, and more preferably in the range of 10 to 50% by mass.

[0035] The negative electrode active material layer may further contain at least one of a binder and a conductive additive in addition to the negative electrode active material and solid electrolyte described above. The thickness of the negative electrode active material layer varies depending on the configuration of the secondary battery, but is preferably in the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm.

[0036] [Solid electrolyte layer] In the secondary battery according to this embodiment, the solid electrolyte layer consists of a first solid electrolyte layer and a second solid electrolyte layer, each containing a solid electrolyte and a binder, respectively. In the secondary battery according to this embodiment, the first solid electrolyte layer is arranged on the positive electrode active material layer side, and the second solid electrolyte layer is arranged on the negative electrode active material layer side, the thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer, and the ratio (A / B) of the binder concentration (A) [mass%] of the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] of the total solid content of the second solid electrolyte layer is 1

[0037] ​The solid electrolytes contained in the first and second solid electrolyte layers are not particularly limited, and the solid electrolytes and their preferred forms exemplified in the section on the negative electrode active material layer can be used in the same way. In particular, the solid electrolyte contained in the solid electrolyte layer is preferably a sulfide solid electrolyte, and more preferably an argyrodite-type solid electrolyte (Li6PS5X (where X is Cl, Br, or I)) from the viewpoint of high ionic conductivity. In addition, the solid electrolyte contained in the solid electrolyte layer may also be a solid electrolyte other than the solid electrolyte exemplified in the section on the negative electrode active material layer. The first and second solid electrolyte layers may contain the same form of solid electrolyte, or they may each contain different forms of solid electrolyte. In one preferred embodiment, the first and second solid electrolyte layers contain the same form of solid electrolyte.

[0038] The content of the solid electrolyte in the first solid electrolyte layer and the second solid electrolyte layer is preferably in the range of 10 to 100% by mass, more preferably in the range of 50 to 100% by mass, and even more preferably in the range of 90 to 100% by mass, relative to the total mass of the solid electrolyte layers. The content of the solid electrolyte in the first solid electrolyte layer and the second solid electrolyte layer may be the same or different.

[0039] The binders contained in the first solid electrolyte layer and the second solid electrolyte layer are not particularly limited, but are stable in the potential range of charge-discharge operation. Furthermore, from the viewpoint of the strength of the solid electrolyte layer and adhesion to the positive electrode active material layer, they are preferably selected from the group consisting of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), acrylic resin, and polytetrafluoroethylene (PTFE), and more preferably selected from the group consisting of polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR). The binders contained in the first solid electrolyte layer and the second solid electrolyte layer may be the same or different.

[0040] As described above, the solid electrolyte and binder contained in the first solid electrolyte layer and the second solid electrolyte layer may be the same or different. However, in a secondary battery according to one embodiment, it is preferable that the solid electrolyte contained in the first solid electrolyte layer and the solid electrolyte contained in the second solid electrolyte layer are the same, and that the binder contained in the first solid electrolyte layer and the binder contained in the second solid electrolyte layer are the same. This makes it possible to more effectively improve the charging capacity when the secondary battery is rapidly charged.

[0041] The binder concentration (A) in the first solid electrolyte layer is preferably greater than 3% by mass and less than 15% by mass, more preferably 4% by mass or more and 10% by mass or less, and even more preferably 5% by mass or more and 8% by mass or less, based on the total mass of the first solid electrolyte layer. By having the binder concentration (A) of the first solid electrolyte layer within the above range, it is possible to sufficiently improve the adhesion to the positive electrode active material layer while maintaining sufficient ionic conductivity of the solid electrolyte layer. Furthermore, the binder concentration (B) in the second solid electrolyte layer is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 6% by mass or less, and even more preferably 2.5% by mass or more and 4% by mass or less, based on the total mass of the second solid electrolyte layer. By having the binder concentration (B) of the second solid electrolyte layer within the above range, it is possible to sufficiently maintain both the strength of the solid electrolyte layer and sufficient ionic conductivity.

[0042] The ratio ((A) / (B)) of the binder concentration in the first solid electrolyte layer (A) to the binder concentration in the second solid electrolyte layer (B) is 1

[0043] ​The thickness (a) of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer and is not particularly limited as long as it does not impair the performance of the target secondary battery. For example, the thickness (a) of the first solid electrolyte layer is preferably 0.1 μm or more and 5 μm or less, more preferably 0.5 μm or more and 3 μm or less, and even more preferably 1 μm or more and 2 μm or less. On the other hand, the thickness (b) of the second solid electrolyte layer is also larger than the thickness of the first solid electrolyte layer and is not particularly limited as long as it does not impair the performance of the target secondary battery. For example, the thickness (b) of the second solid electrolyte layer is preferably 3 μm or more and 50 μm or less, and more preferably 5 μm or more and 30 μm or less. By having the thicknesses of the first solid electrolyte layer and the second solid electrolyte layer within the above ranges, it is possible to maintain a high energy density of the secondary battery while preventing short circuits in the secondary battery and improving the charging capacity when rapidly charged.

[0044] The ratio (b / a) of the thickness of the second solid electrolyte layer (b) to the thickness of the first solid electrolyte layer (a) is 1

[0045] [Cathode active material layer] In the secondary battery according to this embodiment, the positive electrode active material layer is a layer containing the positive electrode active material.

[0046] The type of positive electrode active material is not particularly limited, but it can be layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, Li(Ni-Mn-Co)O2, LiMn2O4, LiNi 0.5 Mn 1.5 Examples of active materials include spinel-type active materials such as O4, olivine-type active materials such as LiFePO4 and LiMnPO4, and Si-containing active materials such as Li2FeSiO4 and Li2MnSiO4. Other oxide active materials include, for example, Li4Ti5O 12 These are some examples.

[0047] ​The positive electrode active material layer preferably contains a positive electrode active material that shrinks during charging, and in particular, the positive electrode active material preferably contains a composite oxide containing lithium and nickel elements, which further enhances the effects of the present invention. Generally, when shrinkage of the positive electrode active material occurs, the solid electrolyte layer peels off from the positive electrode active material layer, but in the secondary battery according to the present invention, peeling can be prevented by the first solid electrolyte layer being firmly adhered to the positive electrode active material layer. A positive electrode active material containing a composite oxide with lithium and nickel elements is characterized by particularly large shrinkage during charging, but even when such a positive electrode active material is used, the secondary battery according to the present invention can suppress peeling between the positive electrode active material layer and the solid electrolyte layer.

[0048] More preferably, Li(Ni-Mn-Co)O2 and those in which some of these transition metals are substituted with other elements (hereinafter also simply referred to as "NMC composite oxide") are used as positive electrode active materials. NMC composite oxide has a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co arranged in an orderly manner) atomic layers are alternately stacked with oxygen atomic layers in between. It contains one Li atom for each transition metal M atom, and the amount of Li that can be extracted is twice that of spinel-based lithium manganese oxide, meaning that the supply capacity is twice as high and it can have a high capacity. As mentioned above, NMC composite oxide also includes composite oxides in which some of the transition metal elements are substituted with other metal elements. In that case, examples of other elements include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, Zn, etc., and preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr.

[0049] More preferably, General formula (1): Li a Ni b Mn c Co d M xO2 (wherein, in the formula, a, b, c, d, and x satisfy 0.98 ≤ a ≤ 1.2, 0.6 ≤ b ≤ 0.9, 0 < c ≤ 0.4, 0 < d ≤ 0.4, 0 ≤ x ≤ 0.3, and b + c + d + x = 1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, and Sr) is used as the cathode active material of the NMC composite oxide. Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. From the perspective of a high theoretical discharge capacity, as described above, it is preferable that 0.6 ≤ b ≤ 0.9. However, from the perspective of improving the adhesion between the cathode active material layer and the solid electrolyte layer and suppressing an increase in the interfacial resistance between them, it is more preferable that b satisfies 0.6 ≤ b ≤ 0.8. Such an NMC composite oxide with a high nickel element content has a high capacity, and due to the large expansion and contraction associated with the charge-discharge reaction, the problem that the above-mentioned interfacial resistance easily increases can occur particularly significantly. Therefore, by using an NMC composite oxide with a high nickel element content as the cathode active material, the effects of the present invention can be further exerted.

[0050] Also, it is one of the preferred embodiments that a sulfur-based cathode active material is used. Examples of the sulfur-based cathode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any substance that can utilize the redox reaction of sulfur to release lithium ions during charging and occlude lithium ions during discharging may be used.

[0051] In some cases, two or more cathode active materials may be used in combination. Of course, cathode active materials other than those described above may also be used.

[0052] Examples of the shape of the cathode active material include particulate (spherical, fibrous), thin film, etc. When the cathode active material is in a particulate shape, its average particle diameter (D 50The average particle size (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 20 μm, and particularly preferably in the range of 1 to 20 μm. 50 The value of ) can be measured by laser diffraction scattering.

[0053] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but it is preferably more than 50% by mass, more preferably in the range of 50% to 95% by mass, and even more preferably in the range of 60% to 90% by mass, relative to 100% by mass of the total solids contained in the positive electrode active material layer.

[0054] In the secondary battery according to this embodiment, the positive electrode active material layer may include a solid electrolyte in addition to the positive electrode active material. The type of solid electrolyte included in the positive electrode active material layer is not particularly limited, but it is more preferable to include a sulfide solid electrolyte. The specific form and preferred form of the solid electrolyte, such as the sulfide solid electrolyte, can be the same as those described in the section on the negative electrode (negative electrode active material layer) above.

[0055] The solid electrolyte content in the positive electrode active material layer is preferably 1% to 70% by mass, more preferably 5% to 50% by mass, and even more preferably 10% to 30% by mass, based on 100% by mass of the total solid content in the positive electrode active material layer. If the solid electrolyte content in the positive electrode active material layer is within the above range, both the ionic conductivity and energy density of the positive electrode active material layer can be achieved.

[0056] The positive electrode active material layer may further contain at least one of a binder and a conductive additive in addition to the positive electrode active material and solid electrolyte described above. The thickness of the positive electrode active material layer varies depending on the configuration of the target secondary battery, as well as the configuration of the target lithium secondary battery, but is preferably in the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm.

[0057] [Middle class] A secondary battery according to one embodiment of the present invention may have an intermediate layer containing a carbon material between the negative electrode active material layer and the second solid electrolyte layer. By having such an intermediate layer, short circuits caused by the generation of dendrites and the like in the secondary battery can be suppressed, and the charging capacity during rapid charging can be more sufficiently improved.

[0058] The carbon material is not particularly limited, but examples include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNTs), graphite, and hard carbon. Among these, carbon black is preferred, and it is more preferable that it be at least one selected from the group consisting of acetylene black, Ketjenblack®, furnace black, channel black, and thermal lamp black.

[0059] The carbon material content in the intermediate layer is not particularly limited, but is preferably in the range of 50 to 100% by mass, more preferably in the range of 60 to 100% by mass, and even more preferably in the range of 70 to 100% by mass.

[0060] Furthermore, the intermediate layer may also contain nanoparticles containing, in addition to carbon materials, one or more elements selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc.

[0061] The thickness of the intermediate layer is not particularly limited, but is preferably in the range of 1 to 50 μm, more preferably in the range of 5 to 40 μm, even more preferably in the range of 10 to 30 μm, and most preferably in the range of 5 to 15 μm. If the thickness of the intermediate layer is 1 μm or more, short circuits due to dendrite generation can be further suppressed. If the thickness of the carbon-containing layer is 50 μm or less, the decrease in energy density can be suppressed.

[0062] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Preferred materials for the current collector plate include, for example, metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. From the viewpoint of lightness, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0063] [Positive lead and negative lead] Although not shown in the diagram, the current collector and the current collector plate may be electrically connected via positive and negative leads. The materials used for the positive and negative leads can be the same as those used in known lithium secondary batteries. Furthermore, the parts removed from the casing should be covered with heat-resistant, heat-shrinkable tubing or similar material to prevent leakage current from contacting peripheral equipment or wiring and affecting the product (e.g., automotive parts, especially electronic equipment). preferable.

[0064] [Battery casing material] As the battery casing material, known metal can cases can be used, or, as shown in Figures 1 and 2, a bag-shaped case made of aluminum-containing laminate film 29 that can cover the power generation elements can be used. For example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but there are no limitations to these. Laminate film is preferable from the viewpoint of high output and excellent cooling performance, and can be suitably used for batteries in large equipment for EVs and HEVs. Furthermore, since the group pressure applied to the power generation elements from the outside can be easily adjusted, an aluminum-containing laminate film is more preferable for the casing.

[0065] The secondary battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the secondary battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0066] Although one embodiment of the secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment and may be modified as appropriate based on the description of the claims. For example, although the above description used an all-solid-state secondary battery in which the electrolyte contained in the solid electrolyte layer is entirely solid as an example, the lithium secondary battery according to this embodiment does not have to be all-solid-state. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There is no particular limit on the amount of liquid electrolyte (electrolyte) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolyte) does not occur. As the liquid electrolyte (electrolyte), a solution having the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent is used. The liquid electrolyte (electrolyte) may further contain additives other than the organic solvent and lithium salt. These additives may be used individually or in combination of two or more. In addition, the amount used when additives are used in the electrolyte can be adjusted as appropriate.

[0067] The following embodiments are also included in the scope of the present invention: a secondary battery according to claim 1 having the features of claim 2; a secondary battery according to claim 1 or claim 2 having the features of claim 3; a secondary battery according to any one of claims 1 to 3 having the features of claim 4; a secondary battery according to any one of claims 1 to 4 having the features of claim 5; a secondary battery according to any one of claims 1 to 5 having the features of claim 6; a secondary battery according to any one of claims 1 to 6 having the features of claim 7; a secondary battery according to any one of claims 1 to 7 having the features of claim 8; a secondary battery according to claim 4 having the features of claim 9; a secondary battery according to any one of claims 1 to 9 having the features of claim 10; a secondary battery according to any one of claims 1 to 10 having the features of claim 11; a secondary battery according to any one of claims 1 to 11 having the features of claim 12. [Examples]

[0068] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following examples, the instruments and devices used inside the glove box were thoroughly dried beforehand.

[0069] <Example of creating evaluation cells> [Example 1] (Fabrication of the positive electrode active material layer) As a constituent material of the positive electrode active material layer, NMC composite oxide (LiNi 0. 8Mn 0.1 Co 0.1 O2 (NMC811), an argyrodite-type sulfide solid electrolyte (Li6PS5Cl) as a solid electrolyte, acetylene black as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder were prepared. In a glove box with an argon atmosphere and a dew point of -68°C or lower, the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed in a mass ratio of 79:16:3:2, and kneaded in an agate mortar to obtain a powder composition (positive electrode mixture).

[0070] Next, the powder composition (cathode mixture) obtained above was formed into a sheet with a thickness of 100 μm using a hand roller. The sheet was punched out into a square with sides of 19 mm to create a cathode active material layer.

[0071] (Preparation of the first solid electrolyte layer) In a glove box with an argon atmosphere and a dew point of -68°C or lower, 94 parts by mass of argyrodite-type sulfide solid electrolyte (Li6PS5Cl) as the solid electrolyte was mixed with a binder solution (6 parts by mass of styrene-butadiene rubber (SBR) as the binder dissolved in mesitylene as the solvent) to prepare a solid electrolyte slurry. The obtained solid electrolyte slurry was coated onto the surface of a stainless steel foil support using an applicator, dried, and then punched out into a square approximately the same size as, or slightly larger than, the positive electrode active material layer to obtain a first solid electrolyte layer with a thickness of 1 μm and a binder concentration of 6% by mass.

[0072] (Preparation of the second solid electrolyte layer) In a glove box under an argon atmosphere with a dew point of -68°C or lower, 95 parts by mass of argyrodite-type sulfide solid electrolyte (Li6PS5Cl) as the solid electrolyte was mixed with a binder solution (5 parts by mass of styrene-butadiene rubber (SBR) as the binder dissolved in mesitylene as the solvent) to prepare a solid electrolyte slurry. The obtained solid electrolyte slurry was coated onto the surface of a stainless steel foil support using an applicator, dried, and then punched out into a square approximately the same size as, or slightly larger than, the positive electrode active material layer to obtain a second solid electrolyte layer with a thickness of 30 μm and a binder concentration of 5% by mass.

[0073] (Preparation of the intermediate layer) Silver nanoparticles and carbon black were mixed in a mass ratio of 1:3 to obtain a mixture. Next, a binder solution (styrene-butadiene rubber (SBR) as a binder dissolved in mesitylene as a solvent) was added to the mixture so that it comprised 5% by mass of the total solid content, and the mixture was mixed to obtain a slurry. Subsequently, the slurry was coated onto the surface of stainless steel foil as a support using an applicator, dried, and then punched out into a square approximately the same size as, or slightly larger than, the positive electrode active material layer to obtain an intermediate layer with a thickness of 10 μm.

[0074] (Creation of evaluation cells) A positive electrode active material layer was placed on top of an aluminum foil (approximately the same size as, or slightly larger than, the positive electrode active material layer) which served as the positive electrode current collector. Then, a first solid electrolyte layer formed on the surface of a stainless steel foil was placed on top of the positive electrode active material layer, with the exposed surface of the solid electrolyte layer facing the positive electrode active material layer, and the solid electrolyte layer was transferred onto the positive electrode active material layer by cold isostatic pressing (CIP). After peeling off the stainless steel foil adjacent to the first solid electrolyte layer, a second solid electrolyte layer formed on the surface of a stainless steel foil was placed on top of the transferred first solid electrolyte layer, with the exposed surface of the second solid electrolyte layer facing the first solid electrolyte layer, and the second solid electrolyte layer was transferred onto the first solid electrolyte layer by cold isostatic pressing (CIP). Next, the stainless steel foil adjacent to the second solid electrolyte layer was peeled off, an intermediate layer was placed on top of the transferred second solid electrolyte layer, and then stainless steel foil as a negative electrode current collector was placed on top of the intermediate layer. The cells were then laminated with a laminating film and pressed using a cold isostatic press (CIP) to obtain an evaluation cell.

[0075] [Example 2] An evaluation cell for Example 2 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 5.1% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0076] [Example 3] An evaluation cell for Example 3 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 6% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0077] [Example 4] An evaluation cell for Example 4 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 9.9% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0078] [Example 5] An evaluation cell for Example 5 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 12% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0079] [Example 6] An evaluation cell for Example 6 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 9.9% by mass, the binder concentration in the second solid electrolyte layer was 3% by mass, and the thickness of the first solid electrolyte layer was 5 μm and the thickness of the second solid electrolyte layer was 25 μm.

[0080] [Comparative Example 1] An evaluation cell for Comparative Example 1 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 3% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0081] [Comparative Example 2] An evaluation cell for Comparative Example 2 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 15% by mass and the binder concentration in the second solid electrolyte layer was 3% by mass.

[0082] [Comparative Example 3] An evaluation cell for Comparative Example 3 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 9.9% by mass, the binder concentration in the second solid electrolyte layer was 3% by mass, and the thickness of both the first and second solid electrolyte layers was 15 μm.

[0083] [Comparative Example 4] An evaluation cell for Comparative Example 4 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 9.9% by mass, the binder concentration in the second solid electrolyte layer was 3% by mass, and the thickness of the first solid electrolyte layer was 25 μm and the thickness of the second solid electrolyte layer was 5 μm.

[0084] [Comparative Example 5] An evaluation cell for Comparative Example 5 was obtained using the same method as in Example 1, except that the binder concentration in the first solid electrolyte layer was 9.9% by mass, the binder concentration in the second solid electrolyte layer was 3% by mass, and the thickness of the first solid electrolyte layer was 30 μm and the thickness of the second solid electrolyte layer was 1 μm.

[0085] [Examples 7-11 and Comparative Examples 6-7] Evaluation cells for Examples 7-11 and Comparative Examples 6-7 were obtained in the same manner as in Examples 1-5 and Comparative Examples 1-2, except that the binders contained in the first and second solid electrolyte layers were polyvinylidene fluoride (PVDF).

[0086] <Observation of the adhesion surface between the first solid electrolyte layer and the positive electrode active material layer> Each evaluation cell prepared as described above was cut along the stacking direction, and the cross-sections of the first solid electrolyte layer and the positive electrode active material layer were observed using EPMA (electron beam microanalyzer). The observations confirmed that at the bonding surface between the first solid electrolyte layer and the positive electrode active material layer, the binder (SBR or PVDF) present on the surface of the first solid electrolyte layer bonded to the positive electrode active material (NMC811), solid electrolyte (Li6PS5Cl), conductive additive (acetylene black), and binder (SBR) present on the surface of the positive electrode active material layer.

[0087] <Evaluation of rapid charging characteristics> For each evaluation cell prepared in the above examples and comparative examples, positive and negative leads were connected to the positive and negative electrode current collectors, respectively, and the charging capacity was evaluated according to the following procedure. A charge / discharge test apparatus (HJ-SD8, manufactured by Hokuto Denko Co., Ltd.) was used for the measurements. The measurements were performed in a constant temperature bath set to 60°C, and a constraining pressure of 3 MPa was applied in the stacking direction of the evaluation cells using a pressurizing member.

[0088] First, for each evaluation cell in Examples 1-11 and Comparative Examples 1-7, constant current (CC) discharge was performed with a current value equivalent to 0.1C and a lower voltage limit of 0.5V. Next, in constant current / constant voltage mode, each cell was charged from 0.5V to 2.5V with a constant current of 0.1C, and the charging capacity at this time was measured. Subsequently, constant current (CC) discharge was performed with a current value equivalent to 2C and a lower voltage limit of 0.5V. Next, in constant current / constant voltage mode, each cell was charged from 0.5V to 2.5V with a constant current of 2C, and the charging capacity at this time was measured. Then, the charging capacity ratio ((charging capacity at 2C) / (charging capacity at 0.1C)), which is the value obtained by dividing the charging capacity when charging at 2C by the charging capacity when charging at 0.1C, was calculated and used as an evaluation index for rapid charging characteristics.

[0089] [Table 1]

[0090] As shown in Table 1, the evaluation cell of the example showed a higher charge capacity ratio compared to that of the comparative example. This indicates that the evaluation cell of the example maintains a high charge capacity ratio even when rapidly charged at 2C. Therefore, it has been demonstrated that the secondary battery according to the present invention improves charge capacity even when rapidly charged. [Explanation of symbols]

[0091] 10A stacked secondary battery, 11' negative electrode current collector, 11” positive electrode current collector, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 17a First solid electrolyte layer 17b Second solid electrolyte layer 19 single cell layers, 21 Power generation elements, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29. Laminating film.

Claims

1. A positive electrode having a positive electrode active material layer arranged on the surface of a positive electrode current collector, A negative electrode is formed in which a negative electrode active material layer is arranged on the surface of a negative electrode current collector, It comprises a first solid electrolyte layer and a second solid electrolyte layer, each containing a solid electrolyte and a binder, respectively. The positive electrode and the negative electrode are arranged opposite each other such that the positive electrode active material layer and the negative electrode active material layer sandwich the first solid electrolyte layer and the second solid electrolyte layer. The first solid electrolyte layer is arranged on the positive electrode active material layer side, and the second solid electrolyte layer is arranged on the negative electrode active material layer side. The thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer. A secondary battery in which the ratio (A / B) of the binder concentration (A) [mass%] relative to the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] relative to the total solid content of the second solid electrolyte layer is 1.5 ≤ A / B ≤ 3.

0.

2. A positive electrode having a positive electrode active material layer arranged on the surface of a positive electrode current collector, A negative electrode is formed in which a negative electrode active material layer is arranged on the surface of a negative electrode current collector, It comprises a first solid electrolyte layer and a second solid electrolyte layer, each containing a solid electrolyte and a binder, respectively. The positive electrode and the negative electrode are arranged opposite each other such that the positive electrode active material layer and the negative electrode active material layer sandwich the first solid electrolyte layer and the second solid electrolyte layer. The first solid electrolyte layer is arranged on the positive electrode active material layer side, and the second solid electrolyte layer is arranged on the negative electrode active material layer side. The thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer. The ratio (A / B) of the binder concentration (A) [mass%] relative to the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] relative to the total solid content of the second solid electrolyte layer is 1 < A / B ≤ 4.

5. A secondary battery in which the first solid electrolyte layer and the second solid electrolyte layer contain a solid electrolyte of the same form.

3. A positive electrode having a positive electrode active material layer arranged on the surface of a positive electrode current collector, A negative electrode is formed in which a negative electrode active material layer is arranged on the surface of a negative electrode current collector, It comprises a first solid electrolyte layer and a second solid electrolyte layer, each containing a solid electrolyte and a binder, respectively. The positive electrode and the negative electrode are arranged opposite each other such that the positive electrode active material layer and the negative electrode active material layer sandwich the first solid electrolyte layer and the second solid electrolyte layer. The first solid electrolyte layer is arranged on the positive electrode active material layer side, and the second solid electrolyte layer is arranged on the negative electrode active material layer side. The thickness of the first solid electrolyte layer is smaller than the thickness of the second solid electrolyte layer. The ratio (A / B) of the binder concentration (A) [mass%] relative to the total solid content of the first solid electrolyte layer to the binder concentration (B) [mass%] relative to the total solid content of the second solid electrolyte layer is 1 < A / B ≤ 4.

5. A secondary battery in which the first solid electrolyte layer and the second solid electrolyte layer contain solid electrolytes having the same average particle size.

4. The secondary battery according to claim 2, wherein the ratio (A / B) is 1.2 ≤ A / B ≤ 4.

0.

5. The secondary battery according to claim 1 or 2, wherein the thickness of the first solid electrolyte layer is 0.1 μm or more and 5 μm or less.

6. The secondary battery according to claim 1 or 2, wherein the positive electrode active material layer includes a positive electrode active material that shrinks during charging.

7. The secondary battery according to claim 1 or 2, wherein the binder is selected from the group consisting of polyvinylidene fluoride, styrene-butadiene rubber, acrylic resin, and polytetrafluoroethylene.

8. The secondary battery according to claim 1 or 2, wherein the concentration (A) of the binder relative to the total solid content of the first solid electrolyte layer is greater than 3% by mass and less than 15% by mass.

9. The secondary battery according to claim 1 or 2, wherein the ratio (b / a) of the thickness of the second solid electrolyte layer to the thickness (a) of the first solid electrolyte layer is 1 < b / a ≤ 50.

10. The solid electrolyte contained in the first solid electrolyte layer and the solid electrolyte contained in the second solid electrolyte layer are the same as each other. The secondary battery according to claim 1, wherein the binder contained in the first solid electrolyte layer and the binder contained in the second solid electrolyte layer are the same as each other.

11. The secondary battery according to claim 1 or 2, wherein the positive electrode active material comprises a composite oxide containing lithium and nickel elements.

12. The secondary battery according to claim 1 or 2, further comprising an intermediate layer containing a carbon material between the negative electrode active material layer and the second solid electrolyte layer.

13. The solid electrolyte contained in the first solid electrolyte layer and the solid electrolyte contained in the second solid electrolyte layer are argyrodite-type solid electrolytes (Li ６ PS ５ A secondary battery according to claim 1, wherein X (where X is Cl, Br, or I).

14. The secondary battery according to claim 1 or 2, which is an all-solid-state lithium secondary battery.

15. The secondary battery according to claim 2, wherein the ratio (A / B) is 1.5 ≤ A / B ≤ 3.

5.

16. The secondary battery according to claim 1 or 2, wherein the ratio (A / B) is 1.8 ≤ A / B ≤ 3.0.

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