Secondary battery and method for producing negative electrode collector for secondary batteries

Abstract

The present invention provides a means of suppressing an increase in the surface resistance value of a negative electrode collector in a secondary battery that comprises a negative electrode collector containing copper or an alloy thereof and that includes elemental sulfur in a power generation element thereof. Provided is a secondary battery comprising a positive electrode which has a positive electrode active material layer that contains a positive electrode active material, a negative electrode in which a negative electrode active material layer that contains a negative electrode active material is disposed on a surface of a negative electrode collector that contains copper or an alloy thereof, and a solid electrolyte layer which is between the positive electrode and the negative electrode and which contains a solid electrolyte, and being providing with power generation element which contains elemental sulfur in a part thereof, wherein a sulfide resistance layer containing copper sulfate is disposed on the solid electrolyte layer-side surface of the negative electrode collector.

Description

Secondary battery and method for manufacturing negative electrode current collector for secondary battery

[0001] The present invention relates to a secondary battery and a method for producing a negative electrode current collector for a secondary battery.

[0002] In recent years, research and development on all-solid-state batteries using oxide- or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ionic conductors that can conduct ions in a solid state. Therefore, all-solid-state batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes, as occurs in conventional liquid-based batteries that use nonaqueous electrolytes.

[0003] In all-solid-state batteries, as in conventional secondary batteries, copper or copper alloys are often used as the material for the negative electrode current collector. However, when the solid electrolyte contains a sulfide solid electrolyte containing sulfur, the copper or copper alloy constituting the negative electrode current collector corrodes upon contact with sulfur, resulting in a problem of reduced battery performance.

[0004] In order to solve this problem, International Publication No. 2014 / 156638 (corresponding to the specification of U.S. Patent Application Publication No. 2016 / 0197351) proposes a technology in which a layer made of copper sulfide (CuS) (copper sulfide layer) is provided as a sulfidation-resistant layer on the surface of the negative electrode current collector on which the negative electrode active material layer is formed. According to this document, concerns about corrosion of the negative electrode current collector are resolved, and it is possible to provide an all-solid-state secondary battery that is excellent in safety, stability, and reliability.

[0005] According to the investigations of the present inventors, it has been found that providing a copper sulfide layer as proposed in the above-mentioned literature on the surface of the negative electrode current collector on which the negative electrode active material layer is formed results in a significant increase in the surface resistance of the negative electrode current collector, which may in turn lead to an increase in the internal resistance of the battery.

[0006] Therefore, an object of the present invention is to provide a means for suppressing an increase in the surface resistance value of a negative electrode current collector in a secondary battery having a negative electrode current collector containing copper or an alloy thereof and containing elemental sulfur in the power generating element.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a secondary battery having a negative electrode current collector containing copper or an alloy thereof and containing elemental sulfur in a power generating element, the above-mentioned problems can be solved by providing a sulfuration-resistant layer containing copper sulfate on the surface of the negative electrode current collector facing the solid electrolyte layer, thereby completing the present invention.

[0008] That is, one aspect of the present invention relates to a secondary battery including a power generating element having a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material disposed on the surface of a negative electrode current collector containing copper or an alloy thereof, and a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode. The secondary battery is characterized in that the power generating element contains elemental sulfur in some portion, and the negative electrode current collector has a sulfuration-resistant layer containing copper sulfate on the surface facing the solid electrolyte layer.

[0009] Fig. 1 is a cross-sectional view schematically showing 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. Fig. 2 is a cross-sectional view showing the copper sulfate (CuSO ) constituting the negative electrode current collectors produced in Production Examples 1 to 3, obtained by XPS analysis after the sulfurization test in the Examples section described later. ４ 3 is a graph plotting the abundance (atomic %) of sulfur (S) element in the negative electrode current collector produced in Production Example 1 and in the copper foil not having a sulfuration-resistant layer formed thereon, in the depth direction from the surface, obtained by XPS analysis after the sulfuration test in the Examples section described later.

[0010] Secondary Battery One aspect of the present invention is a secondary battery including a power generating element having a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material disposed on the surface of a negative electrode current collector containing copper or an alloy thereof, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein the power generating element contains elemental sulfur at some location, and the negative electrode current collector has a sulfuration-resistant layer containing copper sulfate on the surface facing the solid electrolyte layer. The secondary battery according to this aspect suppresses an increase in the surface resistance of the negative electrode current collector. As a result, an increase in the internal resistance of the secondary battery according to this aspect can also be effectively suppressed.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate explanations will be omitted. Furthermore, the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0012] 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 also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. FIG. 1 shows a cross section of the stacked-type secondary battery during charging. 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. Here, the power-generating element 21 has a configuration in which a negative electrode, a solid electrolyte layer 17 containing a sulfide solid electrolyte, and a positive electrode are stacked.

[0013] The negative electrode has a laminated structure of a negative electrode current collector 11' made of copper foil and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11'. In addition, a negative electrode intermediate layer 14 is disposed so as to be in contact with the negative electrode active material layer 13 and the solid electrolyte layer 17. Here, the negative electrode current collector 11' made of copper foil is provided with a sulfurization-resistant layer (not shown) containing copper sulfate over the entire surface on the solid electrolyte layer 17 side (i.e., the negative electrode intermediate layer 14 side).

[0014] The positive electrode has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11". As a result, the negative electrode current collector 11', the negative electrode active material layer 13, the negative electrode intermediate layer 14, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11" constitute one unit cell layer 19. Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0015] A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and extend to the outside of the laminate film 29. A restraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0016] In the embodiment shown in FIG. 1 , the solid electrolyte layer 17 contains a sulfide solid electrolyte. As such, in the secondary battery according to this embodiment, the power generating element contains elemental sulfur somewhere. This elemental sulfur may be liberated for some reason and come into contact with the negative electrode current collector, or may be a compound containing sulfur (e.g., hydrogen sulfide) that is produced for some reason and comes into contact with the negative electrode current collector. In addition to the above, examples of the power generating element containing elemental sulfur include a positive electrode active material layer, a negative electrode active material layer, or a negative electrode intermediate layer that contains a sulfide solid electrolyte, and a positive electrode active material layer that contains a positive electrode active material containing elemental sulfur.

[0017] In any of these embodiments, a negative electrode current collector containing copper or an alloy thereof may be sulfurized by contact with elemental sulfur, resulting in an increase in surface resistance. However, from the viewpoint of improving the ionic conductivity of the solid electrolyte layer and keeping the internal resistance of the battery low, it is preferable that the solid electrolyte layer contains a sulfide solid electrolyte.

[0018] The main components of the secondary battery according to this embodiment will be described below.

[0019] [Current Collectors] The current collectors (negative electrode current collector, positive electrode current collector) have the function of mediating the transfer of electrons from the electrode active material layers (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the thickness of the current collectors, but an example is 10 to 100 μm.

[0020] (Positive electrode current collector) In this embodiment, the material constituting the positive electrode current collector is not particularly limited, and for example, metals such as aluminum, nickel, iron, stainless steel, titanium, copper, etc., or conductive resins can be used. Among these, it is preferable that the positive electrode current collector be made of aluminum.

[0021] (Negative electrode current collector) In this embodiment, the material constituting the negative electrode current collector essentially contains copper or an alloy thereof. As a negative electrode current collector made of simple copper, electrolytic copper foil or rolled tough pitch copper foil can be used. Furthermore, as a negative electrode current collector made of a copper alloy, a solid solution or precipitation-strengthened dilute alloy, such as a rolled Cu—Sn-based, Cu—Fe-based, Cu—Zr-based, Cu—Cr-based, or Corson-based alloy, can be used.

[0022] In this embodiment, as described above, the negative electrode current collector essentially contains copper or an alloy thereof. The negative electrode current collector has a surface on the solid electrolyte layer side that is coated with copper sulfate (CuSO ). ４ According to the investigations of the present inventors, it has been found that the provision of such a sulfurization-resistant layer containing copper sulfate makes it possible to significantly reduce the surface resistance of the negative electrode current collector compared to a case in which no sulfurization-resistant layer is provided.

[0023] The sulfurization-resistant layer located on the surface of the negative electrode current collector facing the solid electrolyte is not particularly limited in its specific composition as long as it contains copper sulfate. However, from the viewpoint of keeping the surface resistance of the negative electrode current collector sufficiently low, the proportion of copper sulfate in the total amount (100 mol %) of the constituent components of the sulfurization-resistant layer is preferably 21 mol % or more, more preferably 22 to 30 mol %, and even more preferably 23 to 25 mol %.

[0024] The sulfur-resistant layer may further contain components other than copper sulfate. Examples of components other than copper sulfate include copper oxide (CuO) in addition to the base material (copper (Cu) or copper alloy). Preferably, the sulfur-resistant layer further contains copper oxide. The content of copper oxide in the sulfur-resistant layer is not particularly limited. However, from the viewpoint of sufficiently reducing the surface resistance of the negative electrode current collector, the proportion of copper oxide in the total 100 mol% of the constituent components of the sulfur-resistant layer is preferably 10 mol% or more, more preferably 15 to 30 mol%, and even more preferably 20 to 28 mol%. Similarly, from the viewpoint of sufficiently reducing the surface resistance of the negative electrode current collector, the total proportion of copper sulfate and copper oxide in the total 100 mol% of the constituent components of the sulfur-resistant layer is preferably 36 to 90 mol%, more preferably 40 to 70 mol%, and even more preferably 45 to 55 mol%. The sulfur-resistant layer may contain copper sulfide, but the lower the content, the better from the viewpoint of suppressing an increase in the surface resistance value of the negative electrode current collector. From this viewpoint, specifically, the proportion of copper sulfide in 100 mol% of the total amount of the constituent components of the sulfur-resistant layer is preferably 35 mol% or less, more preferably 26 mol% or less, and even more preferably 19 mol% or less. There is no particular restriction on the lower limit of the copper sulfide content, but it is usually 3 mol% or more.

[0025] The content of each component in the sulfurization-resistant layer is measured using the method described in the Examples section below. Even when the negative electrode current collector contains a copper alloy, the preferred content ratios of each component described above can be similarly employed.

[0026] The sulfuration-resistant layer may be provided only on a portion of the surface of the negative electrode current collector facing the solid electrolyte layer, but is preferably provided on the entire surface of the negative electrode current collector facing the solid electrolyte layer. By adopting such a configuration, the effects of the present invention can be more significantly exhibited.

[0027] [Negative Electrode Active Material Layer] The negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably contains lithium metal or a lithium-containing alloy. The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 40 to 100% by mass, and more preferably within the range of 50 to 90% by mass.

[0028] The negative electrode active material layer may further contain a solid electrolyte as needed. By including a solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of ion conduction in a solid, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). －５ S / cm or more, and this lithium ion conductivity is preferably 1×10 －４ The ionic conductivity is 200 S / cm or more. Here, the value of the ionic conductivity can be measured by an AC impedance method.

[0029] From the viewpoint of exhibiting excellent lithium ion conductivity and being able to better follow the volume change of the electrode active material that accompanies charge and discharge, the solid electrolyte is preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing a Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, a Li element, and a P element.

[0030] The sulfide solid electrolyte is Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a skeleton include Li-P-S solid electrolytes called LPS. （４－ｘ） Ge （１－ｘ） P ｘ S ４ (x satisfies 0<x<1) or the like. More specifically, for example, LPS (Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S., Li. ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０ GeP ２ S １２ , or Li ６ P.S. ５ X (wherein X is Cl, Br or I). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５The same applies to other descriptions. Among them, the sulfide solid electrolyte is preferably LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ is selected from the group consisting of:

[0031] Examples of the shape of the solid electrolyte include particulate shapes such as spherical and oval spheres, and thin films. When the solid electrolyte is particulate, its average particle diameter (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 diameter (D50) is preferably 0.01 μm or more, and more preferably 0.1 μm or more. The content of the solid electrolyte in the negative electrode active material layer is, for example, preferably in the range of 1 to 60 mass%, and more preferably in the range of 10 to 50 mass%.

[0032] In addition, when lithium metal or a lithium-containing alloy is used as the negative electrode active material, the stacked secondary battery according to this embodiment may be a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in such a configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer may not be present during full discharge, in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during full discharge.

[0033] The negative electrode active material layer may further contain at least one of a binder and a conductive additive in addition to the above-described negative electrode active material and solid electrolyte. The thickness of the negative electrode active material layer varies depending on the configuration of the intended secondary battery, but is preferably within a range of, for example, 0.1 to 1000 μm, and more preferably 40 to 100 μm.

[0034] [Negative Electrode Intermediate Layer] When the secondary battery according to this embodiment is a lithium deposition type in which lithium metal or a lithium-containing alloy is deposited as a negative electrode active material on the negative electrode current collector 11′ during charging, it is preferable to include a negative electrode intermediate layer. The negative electrode intermediate layer is a layer interposed between the negative electrode active material layer and the solid electrolyte layer and contains a lithium-reactive material. Examples of the lithium-reactive material include a material capable of absorbing and releasing lithium ions during charging and a metal capable of alloying with lithium during charging.

[0035] The material capable of absorbing and releasing lithium ions is not particularly limited, but a carbon material is preferred. Specific examples of the carbon material include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNT), graphite, hard carbon, etc. Among these, carbon black is preferred, and at least one selected from the group consisting of acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal lamp black is more preferred.

[0036] Examples of metals that can be alloyed with lithium include In, Al, Si, Sn, Mg, Au, Ag, and Zn. Among these, In, Si, Sn, and Ag are preferred, and Ag is more preferred.

[0037] The lithium-reactive material may be used alone or in combination of two or more. A preferred embodiment of using two or more materials in combination is a combination of a material capable of absorbing and desorbing lithium ions and a metal capable of alloying with lithium. This ensures sufficient strength and lithium ion conductivity of the negative electrode intermediate layer. More specifically, it is preferable to use nanoparticles made of In, Si, Sn, or Ag in combination with carbon black, and it is more preferable to use nanoparticles made of Ag in combination with carbon black. When using a material capable of absorbing and desorbing lithium ions in combination with a metal capable of alloying with lithium, the blending ratio (mass ratio) of the material capable of absorbing and desorbing lithium ions to the metal capable of alloying with lithium is not particularly limited, but is preferably 10:1 to 1:1, more preferably 5:1 to 2:1.

[0038] The content of the lithium-reactive material in the negative electrode intermediate layer (when two or more materials are used in combination, this refers to the total content of those materials; the same applies hereinafter) is not particularly limited, but is preferably in the range of 50 to 100 mass %, more preferably in the range of 70 to 100 mass %, even more preferably in the range of 85 to 100 mass %, and particularly preferably in the range of 90 to 99 mass %.

[0039] The negative electrode intermediate layer may be composed solely of a lithium reactive material as long as a freestanding film can be produced using only the lithium reactive material, but may also contain a binder as necessary. The type of binder is not particularly limited, and binders known in the art can be appropriately used. Examples include polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethyl cellulose.

[0040] The binder content in the negative electrode intermediate layer is not particularly limited, but is preferably in the range of 1 to 15% by mass, and more preferably in the range of 5 to 10% by mass. If the binder content is 1% by mass or more, a negative electrode intermediate layer with sufficient strength can be formed. If the binder content is 15% by mass or less, a negative electrode intermediate layer with sufficient lithium ion conductivity can be formed.

[0041] The thickness of the negative electrode intermediate layer is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 40 μm, and even more preferably 10 to 30 μm. When the thickness of the negative electrode intermediate layer is 1 μm or more, the function of the negative electrode intermediate layer can be fully exhibited. When the thickness of the negative electrode intermediate layer is 50 μm or less, a decrease in energy density can be suppressed.

[0042] [Solid Electrolyte Layer] The solid electrolyte layer 17 is interposed between the positive electrode active material layer 15 and the negative electrode active material layer 13, and contains a solid electrolyte (usually as a main component). There are no particular restrictions on the specific form of the solid electrolyte contained in the solid electrolyte layer, and the solid electrolytes exemplified in the section on the negative electrode active material layer and their preferred forms can be similarly employed. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.

[0043] 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.

[0044] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. The thickness of the solid electrolyte layer varies depending on the configuration of the intended secondary battery, but is preferably within a range of, for example, 0.1 to 1000 μm, and more preferably 10 to 100 μm.

[0045] [Positive Electrode Active Material Layer] The positive electrode active material layer 15 contains a positive electrode active material. The positive electrode active material is not particularly limited as long as it is a material that 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 that contains an M1 element and an O element, and the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. An example of such a positive electrode active material is LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ , LiVO ２ Examples include:

[0046] 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 sulfur compounds or inorganic sulfur compounds. Any material may 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), polycarbon sulfides, and the like, as exemplified 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. As disulfide compounds, dithiobiurea derivatives, those having a thiourea group, a thioisocyanate, or a thioamide group 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 Raman spectrum of 1330 cm －１ and 1560 cm －１ There is a strong peak signal near 307 cm －１ , 379 cm －１ , 472 cm －１ , 929 cm －１ On the other hand, inorganic sulfur compounds are preferred because of their excellent stability. ２ S, TiS ２ , TiS ３ , TiS ４ , NiS, NiS ２ , CuS, FeS ２ , MoS ２ , MoS ３ Among them, S, Li ２S, S-carbon composite, TiS ２ , TiS ３ , TiS ４ , FeS ２ and MoS ２ are preferred, and elemental sulfur (S), Li ２ S, TiS ２ and FeS ２ is more preferable, and from the viewpoint of high capacity, elemental sulfur (S) or Li ２ S is particularly preferred. As the elemental sulfur (S), S ８ The sulfur element (S) can be α-sulfur, β-sulfur, or γ-sulfur having the structure: During discharge, the sulfur element (S) absorbs lithium ions and exists in the positive electrode active material layer in the form of lithium (poly)sulfides.

[0047] 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.

[0048] In a preferred embodiment, the positive electrode active material layer 15 constituting the secondary battery according to this embodiment is made of a layered rock salt type active material containing lithium and cobalt as the positive electrode active material from the viewpoint of output characteristics (for example, Li(Ni—Mn—Co)O ２ ) or a sulfur-containing positive electrode active material.

[0049] Examples of the shape of the positive electrode active material include particulate (spherical, fibrous), thin film, and the like. When the positive electrode active material is particulate, its average particle diameter 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, "particle diameter" refers to the maximum distance between any two points on the outline of a particle observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). In addition, the value of "average particle diameter" refers to the arithmetic mean value of the "particle diameter".

[0050] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably more than 50% by mass, more preferably in the range of more than 50% by mass to 95% by mass or less, and even more preferably in the range of 60% by mass to 90% by mass, relative to 100% by mass of the total solid content contained in the positive electrode active material layer.

[0051] The positive electrode active material layer preferably further contains a solid electrolyte. By including the solid electrolyte in the positive electrode active material layer, the ionic conductivity of the positive electrode active material layer can be improved. The specific form of the solid electrolyte contained in the positive electrode active material layer is not particularly limited, and the solid electrolytes and their preferred forms exemplified in the negative electrode active material layer section can be similarly employed. The content of the solid electrolyte in the positive electrode active material layer is preferably 1% by mass or more and 70% by mass or less, more preferably 5% by mass or more and 50% by mass or less, and even more preferably 10% by mass or more and 30% by mass or less, relative to 100% by mass of the total solid content contained in the positive electrode active material layer. When the content of the solid electrolyte in the positive electrode active material layer is within the above range, the ionic conductivity and energy density of the positive electrode active material layer can be both achieved.

[0052] The positive electrode active material layer preferably contains a conductive additive and a binder in addition to the positive electrode active material and solid electrolyte described above. Examples of the conductive additive include a fibrous conductive additive and / or a particulate conductive additive, and more preferably contains both of these.

[0053] In this specification, the term "fibrous conductive additive" refers to a conductive additive having a fibrous shape. The term "fibrous" encompasses, for example, elongated shapes such as columnar shapes, and is not particularly limited to linear or curved shapes. Furthermore, the term "fibrous conductive additive" may refer to a hollow tubular shape as long as it has a fibrous shape. More specifically, the "fibrous conductive additive" refers to a conductive additive having an aspect ratio (fiber length / fiber diameter) of 10 to 1,000 in an image obtained by observing the cross section of a positive electrode active material layer using a scanning electron microscope (SEM). The average fiber length of the fibrous conductive additive is preferably 10 μm or more. The average fiber length of the fibrous conductive additive is preferably 10 μm to 100 μm, more preferably 10 μm to 50 μm, even more preferably 10 μm to 40 μm, and particularly preferably 15 μm to 40 μm. By setting the average fiber length of the fibrous conductive additive within this range, the conductivity between the positive electrode active materials can be improved, and cycle durability can be further improved. The fiber length of the fibrous conductive additive can be the average value of several to several dozen fiber lengths measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). The average fiber diameter of the fibrous conductive additive is preferably 1 to 300 nm, more preferably 1 to 50 nm. The average fiber diameter of the fibrous conductive additive can be the average value of several to several dozen fiber diameters measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). Examples of fibrous conductive additives that can be used include carbon fibers such as carbon nanotubes, carbon nanohorns, carbon nanofibers, carbon nanofilaments, carbon fibrils, and vapor-grown carbon fibers. Fibrous conductive additives may be used alone or in combination.

[0054] On the other hand, the term "particulate conductive additive" refers to a conductive additive having a particle-like shape. The term "particulate" encompasses shapes such as spherical, hemispherical, ellipsoidal, short chain, scale, cylindrical, and polygonal prism, and may be linear or curved. Furthermore, the term "particulate conductive additive" refers to a conductive additive having a particle-like shape, and may be hollow inside. More specifically, the term "particulate conductive additive" refers to a conductive additive other than the above-mentioned "fibrous conductive additive," which has an aspect ratio (major axis / minor axis) of less than 10 in an image obtained by observing the cross section of a positive electrode active material layer using a scanning electron microscope (SEM).

[0055] The average primary particle diameter of the particulate conductive additive contained in the positive electrode active material layer is not particularly limited, but is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm, and particularly preferably 30 nm to 50 nm. The average secondary particle diameter of the particulate conductive additive contained in the positive electrode active material layer is not particularly limited, but is preferably 0.1 μm to 100 μm, and more preferably 0.5 μm to 30 μm. Within the above ranges, the effects of the present invention can be more significantly achieved. The average primary particle diameter of the particulate conductive additive can be the average particle diameter of several to several tens of primary particles measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). Similarly, the average secondary particle diameter can be the average particle diameter of several to several tens of secondary particles measured using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). Furthermore, the average aspect ratio of the particulate conductive additive contained in the positive electrode active material layer is preferably closer to 1, and is preferably 1 to 5, and more preferably 1 to 2. The average aspect ratio of the particulate conductive additive can be the average value of the aspect ratios of several to several tens of particulate conductive additives measured using a scanning electron microscope (SEM) or the like. The particulate conductive additive is not particularly limited, and examples thereof include carbon powders such as carbon black, such as acetylene black, ketjen black (furnace black), channel black, and thermal black. Among these, acetylene black, ketjen black (furnace black), channel black, and thermal black are preferably used from the viewpoint of being able to better follow the volume change of the positive electrode active material associated with charge and discharge. Only one type of particulate conductive additive may be used alone, or two or more types may be used in combination.

[0056] The binder has the function of maintaining the structure of the electrode active material layer by binding together the components contained in the electrode active material layer. Examples of the binder include, but are not limited to, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyimide (PI), styrene-butadiene rubber (SBR), and carboxymethyl cellulose. Of these, it is preferable to use PTFE.

[0057] The content of the binder in the positive electrode active material layer is not particularly limited, but is preferably in the range of 1 to 15 mass %, more preferably in the range of 1 to 10 mass %, and even more preferably in the range of 1 to 5 mass %, relative to 100 mass % of the total solid content in the positive electrode active material layer. By having the binder content in the above range, the strength of the positive electrode active material layer can be made more sufficient, and further, the ionic conductivity characteristics and electronic conductivity of the positive electrode active material layer can also be made sufficient.

[0058] The thickness of the positive electrode active material layer varies depending on the intended configuration of the secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm.

[0059] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element as shown in FIG. 1 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0060] The secondary battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability. Therefore, the stacked secondary battery according to the present embodiment is suitable for use as a power source for driving EVs and HEVs.

[0061] 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 can be modified as appropriate based on the claims.

[0062] For example, the 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 preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0063] The liquid electrolyte (electrolytic solution) that can be used has a form in which a lithium salt is dissolved in an organic solvent. Examples of the organic solvent that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0064] The lithium salt is Li(FSO ２ ) ２ N(lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ２ F ５ SO ２ ) ２ N, LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３Among them, the lithium salt is preferably Li(FSO ２ ) ２ N(LiFSI).

[0065] The liquid electrolyte (electrolytic solution) may further contain additives other than the above-mentioned components. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of the additive include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be adjusted as appropriate.

[0066] <<Method for manufacturing negative electrode current collector for secondary battery>> The present inventors have also found a method for efficiently manufacturing a negative electrode current collector having the above-described sulfurization-resistant layer on at least one surface. That is, according to another aspect of the present invention, a method for manufacturing a negative electrode current collector for secondary battery can also be provided.

[0067] The method for producing a negative electrode current collector for a secondary battery according to this embodiment includes placing a negative electrode current collector containing copper or an alloy thereof and a sulfur-containing substance in an oxygen-containing atmosphere with a dew point of −60 to 30° C. for 10 to 48 hours at a temperature of 50 to 80° C. to form a sulfuration-resistant layer containing copper sulfate on the surface of the negative electrode current collector. The negative electrode current collector produced by this method is suitable for use in a secondary battery in which a power generating element contains sulfur somewhere in its component parts.

[0068] With this configuration, sulfur element liberated from the sulfur-containing substance is deposited on at least one surface of the negative electrode current collector, and a layer containing copper sulfate is formed by the reaction with copper. When the negative electrode current collector manufactured in this way is applied to a secondary battery, this copper sulfate-containing layer functions as a sulfuration-resistant layer, and the surface of the negative electrode current collector is protected from hydrogen sulfide (H ２ S) can be prevented from being sulfurized by reaction with .

[0069] Specific examples of the sulfur-containing substance are not particularly limited, and include the above-mentioned sulfide solid electrolyte and cathode active material containing elemental sulfur, etc. However, other substances may also be used as long as they are capable of forming a sulfuration-resistant layer by the above-mentioned mechanism.

[0070] There are no particular restrictions on the material or size of the container that defines the atmosphere in which the negative electrode current collector containing copper or its alloy and the substance containing sulfur are placed, and any container that is not corroded or deformed by the copper (alloy) or the substance containing sulfur can be suitably used.

[0071] The atmosphere in which the negative electrode current collector containing copper or an alloy thereof and the sulfur-containing substance are placed is not particularly limited as long as it contains molecular oxygen, and an air atmosphere is typically used. The atmospheric conditions are as described above, but the temperature is preferably 50 to 70°C, more preferably 55 to 65°C. From the viewpoint of reducing the surface resistance of the negative electrode current collector, the dew point of the atmosphere is preferably −60 to 30°C, more preferably −55 to 25°C, and even more preferably −50 to 20°C.

[0072] The thickness of the sulfurization-resistant layer formed on the surface of the negative electrode current collector by the above-described process is not particularly limited, but is usually 20 nm or less, preferably less than 10 nm. On the other hand, the lower limit of this thickness is also not particularly limited, and is usually 1 nm or more.

[0073] The following embodiments are also included within the scope of the present invention: the secondary battery according to claim 1 having the features of claim 2; the secondary battery according to claim 1 or 2 having the features of claim 3; the secondary battery according to claim 3 having the features of claim 4; the secondary battery according to any one of claims 1 to 4 having the features of claim 5; the secondary battery according to claim 5 having the features of claim 6; the secondary battery according to claim 6 having the features of claim 7; the secondary battery according to any one of claims 1 to 7 having the features of claim 8; the secondary battery according to any one of claims 1 to 8 having the features of claim 9; the secondary battery according to any one of claims 1 to 9 having the features of claim 10; and the manufacturing method according to claim 11 having the features of claim 12.

[0074] The present invention will be described in more detail below with reference to examples, although the technical scope of the present invention is not limited to the following examples.

[0075] <Preparation of Negative Electrode Current Collector Having Sulfuration-Resistant Layer> [Preparation Example 1] A copper foil (thickness: 10 μm) was prepared as a negative electrode current collector. This copper foil was spread out and coated with an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ The copper foil was placed in the same container together with copper sulfate (Cl) and sealed in an air atmosphere. This state was maintained at 60°C for 2 days (48 hours), forming a sulfuration-resistant layer containing copper sulfate on the exposed surface of the copper foil. During the above procedure, the dew point inside the container was maintained at -78°C. In this manner, the negative electrode current collector of this example was produced. The thickness of the sulfuration-resistant layer was measured and found to be less than 10 nm (the same applies below).

[0076] [Preparation Example 2] A negative electrode current collector of this preparation example was prepared using the same method as in Preparation Example 1 described above, except that the dew point inside the container was maintained at 18°C ​​during the operation.

[0077] [Preparation Example 3] A negative electrode current collector of this preparation example was prepared using the same method as in Preparation Example 1 described above, except that the dew point inside the container was maintained at -46°C during the operation.

[0078] <Evaluation of sulfuration resistance of negative electrode current collector having sulfuration-resistant layer> The sulfuration resistance of the negative electrode current collectors prepared in Preparation Examples 1 to 3 described above was evaluated using the following sulfuration test.

[0079] Specifically, the negative electrode current collector and copper foil without a sulfurization-resistant layer prepared as described above were each placed in a hydrogen sulfide gas atmosphere with a dew point of −46°C at 60°C and exposed to hydrogen sulfide gas. Then, using a low-resistance resistivity meter (Loresta-GXII MCP-T710, manufactured by Nitto Seiko Analytech Co., Ltd.) equipped with a four-terminal, four-needle PSP probe, the surface resistance of the sulfurization-resistant layer (copper sulfate-containing layer) located on the surface of the negative electrode current collector after the sulfurization test was measured by the fixed constant method. The results are shown in Table 1 below. The surface resistance of a new copper foil without a sulfurization-resistant layer before the sulfurization test was 0.0007 Ω.

[0080] Furthermore, the composition of the surface of the negative electrode current collector (the surface on which the sulfuration-resistant layer was formed) after the sulfurization test and the depth of the region containing copper sulfide from the surface were measured using the following X-ray photoelectron spectroscopy (XPS). The results are shown in Table 1 below.

[0081] [Method for measuring copper sulfide depth (XPS)] Device name: X-ray photoelectron spectrometer (VersaProbe III, manufactured by ULVAC-PHI, Inc.) X-ray source: Monochromated-Al-Kα ray (1486.6 eV) 50 W Photoelectron take-off angle: 45° Measurement area: 200 μmφ Argon ion sputtering conditions Acceleration voltage: 4 kV Sputtering rate: 9.6 nm / min (SiO ２ Conversion value).

[0082] <Examples of Preparation of Evaluation Cells> [Example 1] An evaluation cell (lithium deposition type all-solid-state lithium secondary battery) was prepared by the following method. Note that, in the following, the battery preparation operation was carried out in a glove box with a dew point of −68° C. or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0083] (Preparation of Positive Electrode) NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ ), carbon fiber as a conductive additive, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ Cl) were weighed out to a mass ratio of 85:15:5. These were mixed using an agate mortar and then further stirred and mixed using a planetary ball mill. 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder was added to 100 parts by mass of the obtained mixed powder and mixed. The obtained mixture was layered on aluminum foil as a positive electrode current collector and pressed to obtain a positive electrode having a positive electrode active material layer (thickness 100 μm) on the surface of the positive electrode current collector.

[0084] (Preparation of solid electrolyte layer) Argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene butadiene rubber (SBR) as a binder to 100 parts by mass of ethylenediaminetetraacetic acid (ETA) (average particle diameter (D50): 0.8 μm) and adding mesitylene as a solvent to the mixture. This solid electrolyte slurry was applied to the surface of a stainless steel foil as a support and dried to obtain a solid electrolyte layer (thickness: 40 μm).

[0085] (Preparation of Negative Electrode Intermediate Layer) 23.25 parts by mass of silver nanoparticles (average particle size (D50): 60 nm) and 69.75 parts by mass of carbon black were weighed (Ag:C = 1:3 (mass ratio)) and mixed. 7 parts by mass of polyvinylidene fluoride (PVDF) as a binder was added to 93 parts by mass of the obtained mixture, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry. This negative electrode intermediate layer slurry was applied to the surface of the sulfurization-resistant layer side of the negative electrode current collector prepared in Preparation Example 1 described above, and dried to form a negative electrode intermediate layer (basis weight 0.5 mg / cm ２ ) was obtained.

[0086] (Preparation of Evaluation Cell) A positive electrode active material layer formed on the surface of an aluminum foil (positive electrode current collector) and a solid electrolyte layer formed on the surface of a stainless steel foil were stacked so that the exposed surface of the positive electrode active material layer and the exposed surface of the solid electrolyte layer faced each other, and pressed by cold isostatic pressing (CIP) at 700 MPa for 1 minute (first press step). This transferred the solid electrolyte layer to the exposed surface of the positive electrode active material layer. After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the solid electrolyte layer and the negative electrode intermediate layer formed on the surface of the negative electrode current collector were stacked so that the exposed surface of the solid electrolyte layer and the exposed surface of the negative electrode intermediate layer faced each other, and pressed by cold isostatic pressing (CIP) at 500 MPa for 1 minute (second press step). This transferred the negative electrode intermediate layer to the exposed surface of the solid electrolyte layer. Then, an aluminum positive electrode tab and a nickel negative electrode tab were joined to the aluminum foil (positive electrode current collector) and copper foil (negative electrode current collector), respectively, using an ultrasonic welder. The laminate thus obtained was placed inside an aluminum laminate film and vacuum sealed to obtain an evaluation cell (lithium deposition type all-solid-state lithium secondary battery) of this example.

[0087] [Example 2] An evaluation cell of this example (lithium deposition-type all-solid-state lithium secondary battery) was obtained using the same method as in Example 1 described above, except that the negative electrode current collector prepared in Preparation Example 2 described above was used as the negative electrode current collector.

[0088] [Example 3] An evaluation cell of this example (lithium deposition-type all-solid-state lithium secondary battery) was obtained using the same method as in Example 1 described above, except that the negative electrode current collector prepared in Preparation Example 3 described above was used as the negative electrode current collector.

[0089] Comparative Example 1 An evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) for this comparative example was obtained using the same method as in Example 1 described above, except that copper foil without a sulfuration-resistant layer was used as is as the negative electrode current collector.

[0090]

[0091] FIG. 2 shows the copper sulfate (CuSO ) constituting the negative electrode current collectors produced in Production Examples 1 to 3, obtained by XPS analysis after the sulfurization test. ４3 is a graph showing the molar ratios of elemental sulfur (S) in the negative electrode current collector prepared in Preparation Example 1 and the copper foil not having a sulfuration-resistant layer, plotted in the depth direction from the surface, as similarly obtained by XPS analysis after the sulfuration test.

[0092] As shown in Table 1, Figures 2 and 3, in the secondary battery according to the present invention, the provision of a sulfuration-resistant layer containing copper sulfate on the surface of the negative electrode current collector facing the solid electrolyte layer effectively suppresses copper sulfurization in the sulfuration test, and as a result, it is possible to sufficiently prevent an increase in the surface resistance of the negative electrode current collector. Therefore, it can be said that the secondary battery according to the present invention has a significant effect of suppressing an increase in the internal resistance of the battery.

[0093] REFERENCE SIGNS LIST 10a laminated secondary battery, 11' negative electrode current collector, 11" positive electrode current collector, 13 negative electrode active material layer, 14 negative electrode intermediate layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 single cell layer, 21 power generating element, 25 negative electrode current collector, 27 positive electrode current collector, 29 laminate film.

Claims

1. A secondary battery comprising a power generating element having: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode active material layer containing a negative electrode active material disposed on the surface of a negative electrode current collector containing copper or an alloy thereof; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, wherein the power generating element contains elemental sulfur at some site, and the negative electrode current collector has a sulfuration-resistant layer containing copper sulfate on the surface facing the solid electrolyte layer.

2. The secondary battery according to claim 1, wherein the solid electrolyte comprises a sulfide solid electrolyte.

3. The secondary battery according to claim 1, wherein the proportion of copper sulfate in the total amount (100 mol %) of the constituent components of the sulfuration-resistant layer is 21 mol % or more.

4. The secondary battery according to claim 3, wherein the proportion of copper sulfate in the total amount (100 mol %) of the constituent components of the sulfuration-resistant layer is 23 to 25 mol %.

5. The secondary battery according to any one of claims 1 to 4, wherein the sulfuration-resistant layer further contains copper oxide.

6. The secondary battery according to claim 5, wherein the proportion of said copper oxide in the total amount (100 mol %) of the constituent components of said sulfuration-resistant layer is 15 to 30 mol %.

7. The secondary battery according to claim 6, wherein the total amount of copper sulfate and copper oxide in the sulfuration-resistant layer is 36 to 90 mol % based on 100 mol % of the total amount of components.

8. The secondary battery according to claim 1, wherein the proportion of copper sulfide in the total amount (100 mol %) of the constituent components of said sulfur-resistant layer is 35 mol % or less.

9. The secondary battery according to claim 1 or 2, wherein the sulfuration-resistant layer is provided on the entire surface of the negative electrode current collector on the side of the solid electrolyte layer.

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

11. A method for producing a negative electrode current collector for a secondary battery, comprising placing a negative electrode current collector containing copper or an alloy thereof and a sulfur-containing substance in an oxygen-containing atmosphere with a dew point of -60 to 30°C for 10 to 48 hours at a temperature of 50 to 80°C, thereby forming a sulfuration-resistant layer containing copper sulfate on the surface of the negative electrode current collector.

12. The method for producing a negative electrode current collector for a secondary battery according to claim 11, wherein the power generating element constituting the secondary battery contains elemental sulfur in any part thereof.

13. A negative electrode current collector for use in a secondary battery having a power generating element containing elemental sulfur in any location, the negative electrode current collector comprising: a positive electrode having a positive electrode active material layer containing a positive electrode active material; a negative electrode having a negative electrode active material layer containing a negative electrode active material disposed on the surface of a negative electrode current collector; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, the negative electrode current collector comprising: a sulfur-resistant layer containing copper or an alloy thereof and copper sulfate; and the sulfur-resistant layer disposed on the solid electrolyte layer side.

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