Lithium secondary battery

Abstract

To provide means, in a lithium secondary battery of lithium deposition type using a sulfur-containing component and including a copper-containing negative electrode current collector, which is capable of suppressing progress of a reaction between the negative electrode current collector and sulfur and deterioration of battery performance caused by the reaction progress.SOLUTION: In a lithium secondary battery of lithium deposition type including a copper-containing negative electrode current collector, in which a positive electrode active substance contains a sulfur element or a solid electrolyte layer contains sulfide solid electrolyte, a permeation suppression layer that suppresses permeation of a sulfur-containing compound is provided on at least a part of a surface of an exposed portion of the negative electrode current collector, on a side of the solid electrolyte layer, so as to be adjacent to the solid electrolyte layer during complete discharge.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery. [Background technology]

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is hoping that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and there has been active development of non-aqueous electrolyte secondary batteries, such as secondary batteries for driving motors, which hold the key to putting these vehicles into practical use.

[0003] Secondary batteries for driving motors are required to have extremely high output characteristics and high energy compared to consumer lithium-ion secondary batteries used in mobile phones, laptops, etc. Therefore, lithium-ion secondary batteries, which have the highest theoretical energy of all practical batteries, have attracted attention and are currently being rapidly developed.

[0004] Currently widely used lithium-ion secondary batteries use flammable organic electrolytes, and these liquid-based lithium-ion secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

[0005] In recent years, research and development of 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 solids. Therefore, all-solid-state lithium secondary batteries are essentially free from the various problems inherent in conventional liquid-based lithium-ion secondary batteries, which are often associated with flammable organic electrolytes. Furthermore, 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. One type of all-solid-state lithium secondary battery is the so-called lithium deposition type, in which lithium metal is deposited on the negative electrode current collector during charging. During charging, lithium metal is deposited between the solid electrolyte layer and the negative electrode current collector.

[0006] However, in all-solid-state batteries containing sulfur, such as those using sulfide solid electrolytes, the negative electrode current collector made of copper reacts with sulfur to produce copper sulfide, resulting in a decrease in battery performance. To address this issue, a protective coating made of carbon, nickel, chromium, or the like was conventionally formed on the surface of the negative electrode current collector, but this solution also posed another problem: an increase in battery resistance and a decrease in charge / discharge efficiency.

[0007] Patent Document 1 discloses a technique for preventing sulfidation of a copper negative electrode current collector without forming a protective coating. The technique involves forming a repair layer containing insulating fine particles in contact with the surface of the negative electrode current collector in a region of the outer periphery of the negative electrode current collector, excluding the negative electrode connection portion where the negative electrode terminal is connected, in an all-solid-state battery equipped with a solid electrolyte layer containing a sulfide solid electrolyte. The repair layer contains insulating fine particles, and the average particle size of the insulating fine particles is smaller than both the average particle size of the negative electrode active material and the average particle size of the sulfide solid electrolyte. According to the disclosure of Patent Document 1, the region where the repair layer is formed is closest to the active material layer, even within the outer periphery of the negative electrode current collector, and therefore is prone to stress concentration when the active material layer expands and contracts. Furthermore, by forming the repair layer in this stress concentration region, cracks caused by stress concentration are repaired by the fine particles in the repair layer, preventing exposure of the highly active negative electrode current collector interior. As a result, the technique is said to prevent the progression of the sulfidation reaction (the production of copper sulfide) due to the reaction between the negative electrode current collector and sulfur. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2020-87710 Summary of the Invention [Problem to be solved by the invention]

[0009] While investigating lithium-precipitation-type all-solid-state lithium secondary batteries equipped with a copper negative electrode current collector and a solid electrolyte layer containing a sulfide solid electrolyte, the inventors discovered that metallic lithium dendrites form at the boundary between the negative electrode active material layer (metallic lithium deposition layer) and the copper negative electrode current collector, causing short circuits. The formation of these dendrites is believed to be due to the following mechanism: Copper derived from the negative electrode current collector reacts with sulfur derived from the solid electrolyte at the boundary between the negative electrode active material layer and the negative electrode current collector to produce copper sulfide. As a result, the interfacial resistance between the negative electrode active material layer and the solid electrolyte layer increases at the location where copper sulfide is formed, causing variations in electrical resistance at the edge of the negative electrode active material layer. As a result, current concentrates in areas with low electrical resistance, resulting in the formation of metallic lithium dendrites at those locations.

[0010] Therefore, the present inventors attempted to apply the technology disclosed in Patent Document 1 to a lithium deposition-type all-solid-state lithium secondary battery equipped with a copper negative electrode current collector and a solid electrolyte layer containing a sulfide solid electrolyte. However, the inventors' investigations revealed that applying the technology disclosed in Patent Document 1 directly to a lithium deposition-type all-solid-state lithium secondary battery would not prevent a decline in battery performance. For example, in a lithium deposition-type all-solid-state battery, since the negative electrode active material layer is not present during full discharge, a repair part may be placed on the solid electrolyte layer. In this case, if the negative electrode active material layer (metallic lithium layer) is deposited during the charging process, the negative electrode current collector is exposed, making it impossible to prevent the progression of sulfurization due to reaction with sulfur. Furthermore, there is a problem in that the repair layer cannot respond to the stress generated by the deposition and dissolution of the negative electrode active material layer (metallic lithium layer) during charging and discharging, resulting in cracks in the repair layer, and insulating fine particles contained in the repair layer falling off and contaminating the inside of the cell. Furthermore, when the negative electrode active material layer (metallic lithium layer) precipitates during charging, the fine particles contained in the repair part may penetrate into the negative electrode active material layer (metallic lithium layer), causing a short circuit or a decrease in battery performance.

[0011] Therefore, an object of the present invention is to provide a means for suppressing the progress of a reaction between the negative electrode current collector and sulfur and the resulting deterioration in battery performance in a lithium deposition type lithium secondary battery that uses a sulfur-containing component and is provided with a copper-containing negative electrode current collector. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a lithium deposition-type lithium secondary battery having a positive electrode active material containing elemental sulfur or a solid electrolyte layer containing a sulfide solid electrolyte and a negative electrode current collector containing copper, the above-mentioned problems can be solved by providing a permeation-suppressing layer that suppresses permeation of sulfur-containing compounds on at least a partial surface of an exposed portion of the negative electrode current collector on the solid electrolyte layer side, so as to be adjacent to the solid electrolyte layer during full discharge, and have completed the present invention.

[0013] That is, one aspect of the present invention relates to a lithium secondary battery including a power generating element having a positive electrode including a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions and disposed on the surface of a positive electrode current collector, a negative electrode including a copper-containing negative electrode current collector in which lithium metal is deposited on the negative electrode current collector during charging, and a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode. The lithium secondary battery is characterized in that the positive electrode active material contains elemental sulfur or the solid electrolyte layer contains a sulfide solid electrolyte, and a permeation suppressing layer that suppresses permeation of sulfur-containing compounds is provided on at least a portion of the surface of an exposed portion of the negative electrode current collector facing the solid electrolyte layer, so as to be adjacent to the solid electrolyte layer during full discharge. [Effects of the Invention]

[0014] According to the present invention, in a lithium deposition type lithium secondary battery using a sulfur-containing component and having a copper-containing negative electrode current collector, the progress of the reaction between the negative electrode current collector and sulfur and the resulting deterioration of battery performance can be suppressed. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention. [Figure 2] 2 is an enlarged cross-sectional view of a unit cell layer of the stacked secondary battery according to the embodiment shown in FIG. 1 when fully charged. [Figure 3] 2 is an enlarged cross-sectional view of a unit cell layer of the stacked secondary battery according to the embodiment shown in FIG. 1 when fully discharged. [Figure 4] FIG. 3 is a plan view of the unit cell layer shown in FIG. 2 as viewed from above. [Figure 5] 1 is a perspective view of a stacked secondary battery according to one embodiment of the present invention; [Figure 6] FIG. 6 is a side view seen from a direction A shown in FIG. 5. [Figure 7] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of an all-solid-state lithium secondary battery (stacked secondary battery) according to another embodiment of the present invention when fully charged. [Figure 8] FIG. 8 is a plan view of the unit cell layer shown in FIG. 7 as seen from above. [Figure 9] FIG. 10 is an enlarged cross-sectional view of a unit cell layer of an all-solid-state lithium secondary battery (stacked secondary battery) according to yet another embodiment of the present invention when fully discharged. [Figure 10] 10 is an enlarged cross-sectional view of a unit cell layer of the all-solid-state lithium secondary battery (laminated secondary battery) according to the embodiment shown in FIG. 9 when fully charged. [Figure 11] 1 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0016] One embodiment of the present invention is a lithium secondary battery comprising: a power generating element including: a positive electrode including a positive electrode active material layer, which contains a positive electrode active material capable of absorbing and releasing lithium ions, disposed on the surface of a positive electrode current collector; a negative electrode including a negative electrode current collector containing copper, in which lithium metal is deposited on the negative electrode current collector during charging; and a solid electrolyte layer, which contains a solid electrolyte, interposed between the positive electrode and the negative electrode; wherein the positive electrode active material contains elemental sulfur or the solid electrolyte layer contains a sulfide solid electrolyte; and a permeation-suppressing layer, which suppresses permeation of sulfur-containing compounds, is provided on at least a portion of a surface of an exposed portion of the negative electrode current collector facing the solid electrolyte layer, so as to be adjacent to the solid electrolyte layer during full discharge.

[0017] The present embodiment will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the claims and is not limited to the following embodiment. Note that the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0018] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. The stacked-type secondary battery 10a shown in FIG. 1 has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior. Note that FIG. 1 shows a cross-section of the stacked-type secondary battery in a fully charged state, and thus a negative electrode active material layer (metallic lithium layer) 13 made of metallic lithium is present between the negative electrode current collector 11' and the solid electrolyte layer 17. Furthermore, a pressure member (not shown) applies a restraining pressure to the stacked-type secondary battery 10a in the stacking direction of the power generating element 21. This maintains a constant volume of the power generating element 21.

[0019] As shown in FIG. 1, a power generating element 21 of a stacked secondary battery 10a of this embodiment has a configuration in which a negative electrode in which a negative electrode active material layer 13 containing lithium metal is disposed on both sides of a negative electrode current collector 11′, a solid electrolyte layer 17 containing a sulfide solid electrolyte, and a positive electrode in which a positive electrode active material layer 15 containing a lithium transition metal composite oxide is disposed on both sides of a positive electrode current collector 11″ are laminated. Specifically, the negative electrode, solid electrolyte layer, and positive electrode are laminated in this order such that one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent negative electrode, solid electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0020] A portion of the negative electrode current collector 11' extends to the outside of the power generating element 21 to form a negative electrode tab 11a', which functions as a current extracting portion for extracting the current collected by the negative electrode current collector 11' to the outside of the battery during charging. Multiple negative electrode tabs 11a' are stacked on top of each other, and a negative electrode current collector plate 25 is connected to the exposed surface of the uppermost negative electrode tab 11a', which is sandwiched between the ends of the laminate film 29 and led to the outside of the laminate film 29. Similarly, a portion of the positive electrode current collector 11" extends to the outside of the power generating element 21 to form a positive electrode tab 11a", which functions as a current extracting portion for extracting the current collected by the positive electrode current collector 11" to the outside of the battery during discharging. The multiple positive electrode tabs 11 a″ are stacked on top of each other, and a positive electrode current collector plate 27 is connected to the exposed surface of the lowest positive electrode tab 11 a″, and is led out of the laminate film 29 by being sandwiched between the ends of the laminate film 29.

[0021] In the embodiment shown in FIG. 1, a resin layer 31 made of polytetrafluoroethylene (PTFE) is provided adjacent to the negative electrode active material layer 13 on the exposed surface of the solid electrolyte layer 17 side of the portion serving as the negative electrode tab 11a' (current extraction portion) of each negative electrode current collector 11'. The resin layer 31 functions as a permeation suppression layer that suppresses the permeation of sulfur-containing compounds. In the embodiment shown in FIG. 1, the resin layer 31 is bonded to the surface of the negative electrode current collector 11'. By bonding the resin layer 31 to the surface of the negative electrode current collector 11' in this manner, permeation of sulfur-containing compounds from the interface therebetween can be reliably prevented, which is preferable.

[0022] In the above description, an embodiment of the lithium secondary battery according to the present invention has been described using a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery as an example. However, the type of lithium secondary battery to which the present invention is applicable is not particularly limited, and the present invention can also be applied to a bipolar-type lithium secondary battery.

[0023] 2 is an enlarged cross-sectional view of a cell layer 19 of the stacked secondary battery according to the embodiment shown in FIG. 1 when fully charged. FIG. 3 is an enlarged cross-sectional view of the cell layer 19 of the stacked secondary battery according to the embodiment shown in FIG. 1 when fully discharged. As shown in FIG. 2, the cell layer 19 constituting the stacked secondary battery 10a according to this embodiment has a positive electrode composed of a positive electrode current collector 11″ and a positive electrode active material layer 15 disposed on the surface thereof. A solid electrolyte layer 17 containing a solid electrolyte is disposed on the surface of the positive electrode active material layer 15 opposite the positive electrode current collector 11″. Furthermore, a negative electrode current collector 11′ is disposed on the side of the solid electrolyte layer 17 opposite the positive electrode active material layer 15. During the fully charged state shown in FIG. 2, metallic lithium is deposited on the surface of the negative electrode current collector 11′ facing the solid electrolyte layer 17 at a position facing the positive electrode active material layer 15. As a result, a negative electrode active material layer 13 made of a metallic lithium layer is disposed between the solid electrolyte layer 17 and the negative electrode current collector 11′. 2, a resin layer 31 is disposed adjacent to the negative electrode active material layer 13 on the surface of the exposed portion of the negative electrode current collector 11′, which is the negative electrode tab 11a′ and faces the solid electrolyte layer 17. When the power generating element 21 is viewed from above, the outer circumferential edge of the positive electrode active material layer 15 and the outer circumferential edge of the solid electrolyte layer 17 coincide with each other. As a result, the outer circumferential edge of the negative electrode active material layer 13 also coincides with the outer circumferential edge of the solid electrolyte layer 17. Therefore, as shown in FIG. 3, during full discharge, the resin layer 31 is disposed adjacent to the solid electrolyte layer 17.

[0024] 4 is a plan view of the unit cell layer 19 shown in FIG. 2 as viewed from above. As shown in FIG. 4, in this embodiment, a portion of one side of the rectangular negative electrode current collector 11′ is extended to form a negative electrode tab 11a′ (current extraction portion), which is connected to a negative electrode current collector plate 25. Furthermore, on the surface of an exposed portion (region A shown in FIG. 4) on the solid electrolyte layer 17 side of the portion of the negative electrode current collector 11′ that forms the negative electrode tab 11a′, a resin layer (permeation suppression layer) 31 is provided adjacent to the negative electrode active material layer 13 over the entire exposed portion (region A) in the direction of the arrow shown in FIG. 4. Furthermore, when the unit cell layer 19 (i.e., the power generating element 21) is viewed from above, the end of the resin layer (permeation suppression layer) 31 on the side adjacent to the solid electrolyte layer 17 coincides with the outer peripheral edge of the positive electrode active material layer 15.

[0025] Fig. 5 is a perspective view of a stacked secondary battery according to one embodiment of the present invention, and Fig. 6 is a side view seen from direction A shown in Fig. 5.

[0026] As shown in FIGS. 5 and 6, the stacked secondary battery 100 according to this embodiment includes a stacked secondary battery 10a sealed in the laminate film 29 shown in FIG. 1, two metal plates 200 sandwiching the power generating element 21 sealed in the laminate film 29, and bolts 300 and nuts 400 as fastening members. The fastening members (bolts 300 and nuts 400) function to secure the stacked secondary battery 10a sealed in the laminate film 29 between the metal plates 200. As a result, the metal plates 200 and the fastening members (bolts 300 and nuts 400) function as pressure members that pressurize (restrain) the stacked secondary battery 10a in the stacking direction. The pressure members are not particularly limited as long as they can pressurize the stacked secondary battery 10a in the stacking direction. A combination of a plate made of a rigid material, such as the metal plate 200, and the fastening members described above is typically used as the pressure members. Furthermore, the fastening members are not limited to the bolts 300 and nuts 400, and may also include tension plates or the like that fix the ends of the metal plates 200 so as to restrain the stacked secondary batteries 10a in the stacking direction.

[0027] The lower limit of the load (restraint pressure in the stacking direction of the power generating elements) applied to the stacked secondary battery 10a is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the restraint pressure in the stacking direction of the power generating elements 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.

[0028] The main components of the above-described stacked secondary battery 10a will be described below.

[0029] [Positive electrode current collector] The positive electrode current collector is a conductive member that functions as a flow path for electrons that are released from the positive electrode toward an external load or flow from a power source toward the positive electrode as the battery reaction (charge / discharge reaction) progresses. There are no particular limitations on the material that constitutes the positive electrode current collector. Examples of materials that can be used for the positive electrode current collector include metals and conductive resins.

[0030] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, clad materials of nickel and aluminum, clad materials of copper and aluminum, etc. may also be used. In addition, metal surfaces coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, etc.

[0031] The latter conductive resin may be a resin in which a conductive filler is added to a non-conductive polymer material.

[0032] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector include at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector.

[0033] There is no particular limitation on the thickness of the positive electrode current collector, but an example is 10 to 100 μm.

[0034] [Cathode active material layer] The positive electrode constituting the lithium secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11″ as shown in FIG. 1.

[0035] 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 one 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. Examples of such a positive electrode active material include layered rock salt active materials such as LiCoO2, LiMnO2, LiNiO2, and Li(Ni-Mn-Co)O2; LiMn2O4; and LiNi 0.5 Mn 1.5 Examples of oxide active materials include spinel-type active materials such as LiFePO4 and LiMnPO4, olivine-type active materials such as LiFeSiO4 and LiMnSiO4, and Si-containing active materials such as LiFeSiO4 and LiMnSiO4. 12 , LiVO2.

[0036] Furthermore, the positive electrode active material may contain elemental sulfur. Examples of positive electrode active materials containing elemental sulfur include, but are not limited to, elemental sulfur (S), as well as particles or thin films of organic or inorganic sulfur compounds. Any material can be used as long as it utilizes the oxidation-reduction reaction of sulfur to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprenes, rubeanic acid (dithiooxamide), and polycarbon sulfides, as typified by the compounds described in International Publication No. 2010 / 044437. Among these, disulfide compounds, sulfur-modified polyacrylonitriles, and rubeanic acid are preferred, with sulfur-modified polyacrylonitrile being particularly preferred. Disulfide compounds containing dithiobiurea derivatives, thiourea groups, thioisocyanates, or thioamide groups are more preferred. Here, sulfur-modified polyacrylonitrile is a modified polyacrylonitrile containing sulfur atoms, obtained by mixing sulfur powder with polyacrylonitrile and heating the mixture under an inert gas or under reduced pressure. Its estimated structure is, for example, as shown in Chem. Mater. 2011, 23, 5024-5028, in which polyacrylonitrile is ring-closed to form a polycyclic ring, and at least a part of S is bonded to C. The compound described in this document has a peak at 1330 cm in the Raman spectrum. -1 and 1560cm -1 There is a strong peak signal near 307 cm -1 , 379cm -1 , 472cm -1 , 929cm -1A peak is present around 0.05%. On the other hand, inorganic sulfur compounds are preferred due to their excellent stability. Specific examples include elemental sulfur (S), Li2S, TiS2, TiS3, TiS4, NiS, NiS2, CuS, FeS2, MoS2, and MoS3. Among these, S, Li2S, S-carbon composite, TiS2, TiS3, TiS4, FeS2, and MoS2 are preferred, with elemental sulfur (S), Li2S, TiS2, and FeS2 being more preferred. From the viewpoint of high capacity, elemental sulfur (S) or Li2S is particularly preferred. Note that, as elemental sulfur (S), α-sulfur, β-sulfur, or γ-sulfur having an S8 structure can be used. During discharge, these elemental sulfur (S) absorb lithium ions and exist in the positive electrode active material layer in the form of lithium (poly)sulfides.

[0037] 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.

[0038] In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to this embodiment contains, from the viewpoint of output characteristics, a layered rock salt type active material containing lithium and cobalt (e.g., Li(Ni-Mn-Co)O2) or a positive electrode active material containing sulfur as the positive electrode active material.

[0039] 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 (D50) 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. In this specification, the average particle size (D50) of the positive electrode active material can be measured by a laser diffraction scattering method.

[0040] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but for example, it is preferably in the range of 30 to 99% by mass, more preferably in the range of 40 to 90% by mass, and even more preferably in the range of 45 to 80% by mass.

[0041] In the lithium secondary battery according to this embodiment, the positive electrode active material layer 15 preferably further contains a solid electrolyte. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes.

[0042] In a preferred embodiment of the lithium secondary battery according to this embodiment, from the viewpoint that the solid electrolyte exhibits excellent lithium ion conductivity and can better follow the volume change of the electrode active material accompanying charge and discharge, 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 is at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl and I, and even more preferably a sulfide solid electrolyte containing S element, Li element and P element.

[0043] 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 Li4P2S%7 skeleton include 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 10GeP2S 12 , or Li6PS5X (where X is Cl, Br, or I). The term "Li2S-P2S5" refers to a sulfide solid electrolyte obtained using a raw material composition containing Li2S and P2S5, and the same applies to other terms. Among these, sulfide solid electrolytes are preferably LPS (Li2S-P2S5), Li6PS5X (where X is Cl, Br, or I), Li7P3S, etc., from the viewpoint that they have high ionic conductivity and a low bulk modulus and can therefore follow the volume change of the electrode active material during charge and discharge. 11 , Li 3.2 P 0.96 S and Li3PS4.

[0044] 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 %.

[0045] (Conductive additives and binders) The positive electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the positive electrode active material and the solid electrolyte.

[0046] The thickness of the positive electrode active material 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 40 to 100 μm, for example.

[0047] [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, and therefore a detailed description thereof will be omitted here.

[0048] 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.

[0049] The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte.

[0050] The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 10 to 40 μm, for example.

[0051] [Negative electrode current collector] The negative electrode current collector is a conductive member that functions as a flow path for electrons that are released from the negative electrode toward the power source as the battery reaction (charge / discharge reaction) progresses, or that flow from an external load toward the negative electrode. In the lithium secondary battery according to this embodiment, the negative electrode current collector essentially contains copper. The negative electrode current collector may be made of copper alone or an alloy of copper and another metal. Furthermore, the negative electrode current collector may be made of a conductive resin obtained by adding a conductive filler containing copper to a non-conductive polymer material. There are no particular limitations on the thickness of the negative electrode current collector, but an example is 10 to 100 μm.

[0052] [Negative electrode active material layer] The lithium secondary battery according to the present embodiment is a so-called lithium deposition type in which metallic lithium is deposited on the negative electrode current collector during charging. The layer of metallic lithium deposited on the negative electrode current collector during charging is the negative electrode active material layer of the lithium secondary battery according to the present embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. Although the negative electrode active material layer need not be present during full discharge, in some cases, a negative electrode active material layer containing a certain amount of metallic lithium may be present during full discharge. Even if a certain amount of negative electrode active material layer (metallic lithium layer) is present during full discharge, it is within the technical scope of the present invention as long as the permeation suppression layer is disposed adjacent to the solid electrolyte layer. The thickness of the negative electrode active material layer (metallic lithium layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0053] [Permeation suppression layer] The lithium secondary battery according to this embodiment is also characterized in that, as shown in FIG. 3 , a permeation-suppressing layer is provided on at least a portion of the surface of the exposed portion of the negative electrode current collector on the solid electrolyte layer side so as to be adjacent to the solid electrolyte layer during full discharge. This permeation-suppressing layer is a layer that suppresses the permeation of sulfur-containing compounds. Therefore, by providing the permeation-suppressing layer, the formation of copper sulfide at the boundary between the negative electrode active material layer and the negative electrode current collector is suppressed. As a result, an increase in interface resistance, current concentration, and the formation of metallic lithium dendrites, which are caused by the formation of copper sulfide, are suppressed, thereby providing the advantage of suppressing the occurrence of internal short circuits and a decrease in battery performance.

[0054] There are no particular limitations on the constituent material of the permeation suppression layer, and various materials capable of exhibiting the above-mentioned functions can be used. The constituent material of the permeation suppression layer is preferably a material having gas barrier properties. In the lithium secondary battery according to this embodiment, the constituent material of the permeation suppression layer preferably has a gas permeability to oxygen (O2) of 10 cm 3 / m 2 ·24h·atm, 1mm or less, more preferably 1cm 3 / m2 24h atm, 1mm or less, more preferably 0.5cm 3 / m 2 When the gas permeability of the constituent material of the permeation suppressing layer is within these ranges, the permeation of gaseous sulfur-containing compounds such as hydrogen sulfide is particularly suppressed effectively.

[0055] The material constituting the permeation suppression layer is preferably a material having electronic insulation properties. In the lithium secondary battery according to this embodiment, the material constituting the permeation suppression layer preferably has a volume resistivity of 1×10 5 Ω·cm or more, and more preferably 1×10 6 Ω·cm or more, and more preferably 1×10 7 When the volume resistivity of the constituent material of the permeation-suppressing layer is within this range, sufficient electronic insulation is ensured in the permeation-suppressing layer, and the negative electrode active material layer (metallic lithium layer) can be deposited uniformly on the surface of the negative electrode current collector during charging.

[0056] Examples of materials constituting the permeation suppressing layer having the above-described properties include resin materials. Examples of such resin materials include thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene (PTFE), polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene, etc. Examples of the fluororesin include polyvinyl fluoride (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF); vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber); and epoxy resins.Among these, fluorine-containing resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF) are preferred, with polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) being particularly preferred.

[0057] There are no particular restrictions on the size of the permeation suppression layer, but the thickness of the permeation suppression layer (the size in the stacking direction of the power generating element) is preferably smaller than the thickness of the solid electrolyte layer. Specifically, the thickness of the permeation suppression layer is preferably 20 μm or less, more preferably 10 μm or less. There are also no particular restrictions on the width of the permeation suppression layer exposed outside the solid electrolyte layer, but if the width is too large, the effect of suppressing the permeation of sulfur-containing compounds will saturate. Therefore, the width is preferably 10 mm or less, more preferably 8 mm or less, even more preferably 6 mm or less, particularly preferably 4 mm or less, and most preferably 2 mm or less.

[0058] In the all-solid-state lithium secondary battery according to the above-described embodiment, as shown in FIG. 4 , a permeation suppressing layer is provided on the surface of the exposed portion (region A shown in FIG. 4 ) on the solid electrolyte layer side of the negative electrode current collector, over the entire exposed portion (region A) in the direction of the arrow shown in FIG. 4 , so as to be adjacent to the negative electrode active material layer. When the permeation suppressing layer is provided over the entire periphery of the exposed portion on the solid electrolyte layer side of the negative electrode current collector, localized copper sulfide generation and current concentration can be sufficiently suppressed, and dendrite generation and the resulting deterioration of battery performance can be sufficiently suppressed. However, even if the permeation suppressing layer is not formed over the entire periphery of the exposed portion on the solid electrolyte layer side of the negative electrode current collector, the effects of the present invention can be achieved as long as the permeation suppressing layer is formed on at least a portion of the exposed portion on the solid electrolyte layer side of the negative electrode current collector. Here, embodiments shown in FIGS. 7 and 8 are examples of other examples in which a permeation suppressing layer is provided over the entire periphery of the exposed portion on the solid electrolyte layer side of the negative electrode current collector, as shown in FIG. 4 above. FIG. 7 is an enlarged cross-sectional view of a unit cell layer of an all-solid-state lithium secondary battery (stacked secondary battery) according to another embodiment of the present invention, during full discharge. 8 is a plan view of the cell layer shown in FIG. 7 as viewed from above. In the all-solid-state lithium secondary battery according to the embodiment shown in FIGS. 7 and 8, the outer peripheral edge of the negative electrode current collector 11′ is formed to be slightly larger than the outer peripheral edges of the positive electrode active material layer 15 and the solid electrolyte layer 17. As a result, an exposed portion exists along the entire periphery of the outer peripheral edge of the negative electrode current collector 11′. A resin layer 31 is formed as a permeation-suppressing layer along the entire periphery of the exposed portion.

[0059] In the all-solid-state lithium secondary battery of the embodiment shown in FIG. 2 (fully charged) and FIG. 3 (fully discharged), when the cell layer 19 (power-generating element 21) is viewed from above, the end of the resin layer 31 (permeation-suppressing layer) adjacent to the solid electrolyte layer 17 coincides with the outer circumferential edge of the positive electrode active material layer 15. This configuration eliminates voids between the negative electrode active material layer 13, formed by the deposition of a metallic lithium layer at a position facing the positive electrode active material layer 15, and the resin layer 31 (permeation-suppressing layer). This effectively suppresses localized copper sulfide generation and current concentration, thereby preventing dendrite generation and the resulting degradation of battery performance. However, from the perspective of more reliably preventing localized copper sulfide generation and current concentration, it is preferable to position the resin layer (permeation-suppressing layer) so that the end of the permeation-suppressing layer adjacent to the solid electrolyte layer is located more inward than the outer circumferential edge of the positive electrode active material layer, as shown in FIG. 9 (fully discharged). In this case, it is more preferable that the thickness of the resin layer (permeation-suppressing layer) be equal to or less than the thickness of the solid electrolyte layer. By adopting such a configuration, it is possible to more reliably prevent the occurrence of voids between the resin layer 31 (permeation suppression layer) and the negative electrode active material layer (metallic lithium layer) 13 during full charge, as shown in FIG. 10, and to more reliably exhibit the effects of the present invention.

[0060] The method for forming the permeation suppressing layer described above on the surface of the exposed portion of the negative electrode current collector on the solid electrolyte layer side is not particularly limited, and for example, a method can be used in which a slurry in which the constituent material of the permeation suppressing layer made of the resin material described above is dispersed in an appropriate solvent is applied to the portion of the surface of the negative electrode current collector where the permeation suppressing layer is to be disposed, and the solvent is then dried. Note that, in some cases, the permeation suppressing layer may be formed by other methods.

[0061] When forming a permeation suppression layer using the coating process described above, if a relatively polar solvent is used, trace amounts of solvent remaining in the permeation suppression layer may react with the solid electrolyte, resulting in a decrease in battery performance. Therefore, when forming a permeation suppression layer using a coating process, it is preferable to use a solvent with a relatively low polarity. Examples of such low-polarity solvents include solvents with a solubility parameter (SP value) of 10 or less. The solubility parameter (SP value) is a parameter that indicates the dissolution behavior of a substance, as defined by the regular solution theory introduced by Hildebrand, and serves as a measure of the solubility of a two-component solution. Regular solution theory assumes that the only forces acting between the solvent and solute are intermolecular forces, so the solubility parameter is used as a measure of intermolecular forces. While actual solutions are not necessarily regular solutions, it is known that the smaller the difference in the SP values ​​of the two components, the greater the solubility. In this specification, the SP value used refers to values ​​found in various literature. Examples of solvents with a solubility parameter (SP value) of 10 or less include trimethylbenzene (SP value = 8.8), dimethylbenzene (SP value = 8.8), and butyl acetate (SP value = 8.5). For these reasons, the constituent material of the permeation-suppressing layer is preferably a material that can be dispersed (dissolved) in a solvent with a solubility parameter (SP value) of 10 or less, more preferably a resin material that can be dispersed (dissolved) in the solvent, and particularly preferably a fluorine-containing resin that can be dispersed (dissolved) in the solvent.

[0062] The copper sulfide generation phenomenon, which is the problem to be solved by the present invention, requires the use of a copper negative electrode current collector and the presence of a sulfur component in the power generating element. In the all-solid-state lithium secondary battery (laminated secondary battery) according to the embodiment shown in FIG. 1 and elsewhere, the solid electrolyte layer 17 functions as a sulfur source by containing a sulfide solid electrolyte. However, the sulfur source is not limited to this case. Even if the solid electrolyte layer does not contain a sulfur component, the case where the positive electrode active material contains elemental sulfur and functions as a sulfur source is also within the technical scope of the present invention. Examples of positive electrode active materials containing elemental sulfur are as described above.

[0063] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metallic 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 collector plates. 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 plate 25 and the positive electrode current collector plate 27 may be made of the same material or different materials.

[0064] [Battery exterior] As the battery exterior, a known metal can case can be used, or 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 to these. 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 because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0065] FIG. 11 is a perspective view showing the appearance of a stacked secondary battery according to one embodiment of the present invention. As shown in FIG. 11, a flat stacked secondary battery 50 has a rectangular, flat shape, and a positive electrode current collector 58 and a negative electrode current collector 59 for extracting power are drawn out from both sides thereof. A power generating element 57 is wrapped in a battery outer casing (laminate film 52) of the stacked secondary battery 50, and the periphery thereof is heat-sealed, and the power generating element 57 is sealed with the positive electrode current collector 58 and the negative electrode current collector 59 drawn out to the outside. Here, the power generating element 57 corresponds to the power generating element 21 of the stacked secondary battery 10a shown in FIG. 1 described above. The power generating element 57 is formed by stacking a plurality of unit cell layers (unit cells) 19, each of which is composed of a positive electrode (positive electrode current collector 11″ and positive electrode active material layer 15), a solid electrolyte layer 17, and a negative electrode (negative electrode current collector 11′ (and negative electrode active material layer 13)).

[0066] The lithium secondary battery according to this embodiment is not limited to a flat shape. A wound lithium secondary battery may be cylindrical, or may be a cylindrical battery modified to have a flat rectangular shape, and is not particularly limited. The cylindrical battery may be casing a laminate film or a conventional cylindrical can (metal can), and is not particularly limited. Preferably, the power generating element is casing a laminate film. This configuration can achieve weight reduction. Furthermore, there is no particular limitation on the removal of the current collectors 58 and 59 shown in FIG. 11 . The positive and negative current collectors 58 and 59 may be pulled out from the same side, or the positive and negative current collectors 58 and 59 may each be divided into multiple pieces and removed from each side; the battery is not limited to the configuration shown in FIG. 11 . In addition, in a wound type lithium secondary battery, the terminals may be formed using, for example, a cylindrical can (metal can) instead of a current collector plate.

[0067] Although the above description has been given taking the case where the secondary battery according to the present embodiment is an all-solid-state lithium secondary battery as an example, 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 is 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, as the liquid electrolyte (electrolytic solution), a solution in the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent is used. 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 appropriately adjusted.

[0068] [Battery pack] A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

[0069] A small, detachable battery pack can be formed by connecting multiple batteries in series or in parallel. Furthermore, a large-capacity, high-output battery pack suitable for vehicle drive power sources and auxiliary power sources, which require high volumetric energy density and high volumetric power density, can be formed by further connecting multiple batteries in series or in parallel. The number of batteries to be connected to form a battery pack and the number of stacked small batteries to form a large-capacity battery pack can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which it will be installed.

[0070] [vehicle] The lithium secondary battery according to the present embodiment has a high energy density per volume. In vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles, higher capacity and larger size are required compared to applications in electrical and portable electronic devices. Therefore, the lithium secondary battery according to the present embodiment can be suitably used as a power source for vehicles, for example, as a power source for driving a vehicle or an auxiliary power source.

[0071] Specifically, a battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. Since the present invention can provide a high-capacity battery with excellent output characteristics, installing such a battery can enable the construction of plug-in hybrid electric vehicles with long EV driving distances and electric vehicles with long driving distances per charge. For example, a battery or a battery pack formed by combining a plurality of such batteries can be used in hybrid vehicles, fuel cell vehicles, and electric vehicles (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) to provide vehicles with long driving distances. However, the applications are not limited to automobiles, and the battery pack can also be used as a power source for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like. [Example]

[0072] Through the following experiments, it was confirmed that in an all-solid-state battery using a copper negative electrode current collector, the presence of a sulfur source generates copper sulfide at the edge between the negative electrode current collector and the solid electrolyte layer, resulting in a decrease in battery performance.

[0073] [Experimental Example 1] (Preparation of positive electrode) First, as the constituent material of the positive electrode active material layer, the positive electrode active material NMC composite oxide (composition = LiNi 0.8 Mn 0.1 Co 0.1O2), an argyrodite-type solid electrolyte (Li6PS5Cl), which is a lithium-ion conductive halogen-containing sulfide solid electrolyte, acetylene black as a conductive additive, and styrene-butadiene rubber (SBR) as a binder were prepared.

[0074] Of these, the NMC composite oxide, solid electrolyte, and conductive additive were mixed and stirred in a glove box. Styrene-butadiene rubber (SBR) was added to 100 parts by mass of the resulting mixed powder, and mesitylene was added as a solvent to prepare a positive electrode active material slurry. The positive electrode active material slurry prepared above was then applied to one surface of aluminum foil serving as a positive electrode current collector, and roll-pressed to form a dense positive electrode active material layer (80 μm thick), thereby producing a positive electrode. The mass ratio of positive electrode active material: sulfide solid electrolyte: conductive additive: binder was 83.8:10.8:2.7:2.7.

[0075] (Fabrication of solid electrolyte layer) The sulfide solid electrolyte prepared above and a binder (SBR) were mixed in a mass ratio of 95:5, and an appropriate amount of mesitylene was added as a solvent and mixed to prepare a solid electrolyte slurry.

[0076] The solid electrolyte slurry prepared above was applied onto a stainless steel (SUS) foil serving as a negative electrode current collector, and the solvent was evaporated to form a solid electrolyte layer (thickness: 80 μm).

[0077] (Preparation of test cell) The solid electrolyte layer prepared above was transferred onto the exposed surface of the positive electrode prepared above by cold isostatic pressing (CIP).

[0078] Finally, an aluminum positive electrode tab and a nickel negative electrode tab were bonded to the aluminum foil serving as the positive electrode current collector and the SUS foil serving as the negative electrode current collector, respectively, using an ultrasonic welder. The resulting stack was placed inside an aluminum laminate film and vacuum-sealed to produce a test cell for this experimental example.

[0079] [Experimental Example 2] A test cell for this experimental example was produced in the same manner as in the above-described experimental example 1, except that copper foil was used as the negative electrode current collector instead of stainless steel (SUS) foil.

[0080] [Evaluation of capacitance characteristics] The capacity characteristics (charge and discharge efficiency) were evaluated for the test cells prepared above in Experimental Examples 1 and 2. The capacity characteristics were evaluated in a constant temperature bath at 60°C.

[0081] The charge / discharge conditions for measuring the charge / discharge efficiency were constant current constant voltage (CCCV) charging at 0.05C up to 4.2V (0.01C cutoff), followed by constant current (CC) discharging at 0.05C down to 2.5V.

[0082] The capacity value per mass of the positive electrode active material (mAh / g) was calculated from the charge / discharge capacity value obtained as a result of the above charge / discharge and the mass of the positive electrode active material contained in the positive electrode. The charge / discharge efficiency was calculated as the ratio of the discharge capacity during discharge to the charge capacity during charging. As a result, the charge / discharge efficiency of the test cell of Experimental Example 2 (negative electrode current collector: copper foil) was 75, with the charge / discharge efficiency of the test cell of Experimental Example 1 (negative electrode current collector: SUS foil) being taken as 100. Furthermore, when both test cells were disassembled and visually observed, it was confirmed that copper sulfide was formed at the edge between the negative electrode current collector and the solid electrolyte layer in the test cell of Experimental Example 2. On the other hand, such a phenomenon was not confirmed in the test cell of Experimental Example 1. These results confirmed that in all-solid-state batteries using a copper negative electrode current collector, the presence of a sulfur source causes copper sulfide to be formed at the edge between the negative electrode current collector and the solid electrolyte layer, resulting in a decrease in battery performance. [Explanation of symbols]

[0083] 10a, 50, 100 stacked secondary battery, 11' negative electrode current collector, 11'a negative tab, 11” positive electrode current collector, 11"a positive tab, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 cell layer, 21, 57 Power generation elements, 25, 59 Negative current collector plate, 27, 58 Positive current collector plate, 29, 52 Laminated film, 31 Resin layer (permeation suppression layer), 200 metal plate, 300 volts, 400 Nuts.

Claims

1. a positive electrode including a positive electrode active material layer disposed on a surface of a positive electrode current collector, the positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions; a negative electrode having a negative electrode current collector containing copper, on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; a power generating element having the positive electrode active material contains elemental sulfur or the solid electrolyte layer contains a sulfide solid electrolyte; When fully discharged, no negative electrode active material layer exists between the negative electrode current collector and the solid electrolyte layer, a permeation suppressing layer that suppresses permeation of sulfur-containing compounds is provided on at least a portion of a surface of an exposed portion of the negative electrode current collector on the solid electrolyte layer side so as to be adjacent to the solid electrolyte layer during full discharge, and the thickness of the permeation suppressing layer is smaller than the thickness of the solid electrolyte layer.

2. The lithium secondary battery according to claim 1 , wherein the permeation-suppressing layer is provided over the entire periphery of the exposed portion.

3. 3. The lithium secondary battery according to claim 1, wherein, in a plan view of the power generating element, an end of the permeation suppression layer adjacent to the solid electrolyte layer coincides with an outer peripheral edge of the positive electrode active material layer.

4. 3. The lithium secondary battery according to claim 1, wherein, in a plan view of the power-generating element, an end of the permeation-suppressing layer adjacent to the solid electrolyte layer is located more inward than an outer peripheral end of the positive electrode active material layer, and the thickness of the permeation-suppressing layer is equal to or less than a thickness of the solid electrolyte layer.

5. 5. The lithium secondary battery according to claim 1, wherein the permeation suppression layer is bonded to a surface of the negative electrode current collector.

6. 6. The lithium secondary battery according to claim 1, wherein the permeation suppression layer is made of a material having gas barrier properties.

7. 7. The lithium secondary battery according to claim 1, wherein the permeation suppression layer is made of a resin material having electronic insulation properties.

8. 8. The lithium secondary battery according to claim 1, wherein the permeation suppression layer is made of a material that can be dispersed in a solvent having a solubility parameter (SP value) of 10 or less.

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