All-solid-state lithium-ion secondary battery and method for producing same

Abstract

The present invention addresses the problem of providing an all-solid-state lithium-ion secondary battery that has excellent charging / discharging characteristics.　The present invention includes at least a negative electrode layer, a separator layer, and a positive electrode layer. The negative electrode layer contains a negative electrode active substance which includes graphite. The separator layer is made of an oxide solid electrolyte. A resistance reduction layer is formed between the negative electrode layer and the separator layer, and the resistance reduction layer is made of a lithium alloy. Alternatively, the present invention includes a substance with which a lithium alloy is formed as a result of a secondary battery operation in which current flows between the negative electrode layer and the positive electrode layer.

Description

All-solid-state lithium-ion secondary battery and method for manufacturing the same

[0001] The present invention relates to an all-solid-state lithium-ion secondary battery and a method for producing the same.

[0002] High-capacity secondary batteries are used in a wide variety of applications, including electric vehicles including hybrids, as energy sources for electronic devices, and as storage equipment for renewable energy sources such as solar cells. However, in order to realize a low-carbon, smart society, there is a need for secondary batteries with even larger capacities and better charge / discharge characteristics.

[0003] Lithium-ion secondary batteries have a high energy density and utilize a carbon material such as graphite for the negative electrode to store lithium ions, while the positive electrode is made of an oxide of lithium and a transition metal, which suppresses metal deposition during charging and discharging. For these reasons, lithium-ion secondary batteries are highly practical and have become the mainstream of high-capacity secondary batteries. The next generation of lithium-ion secondary batteries is expected to be all-solid-state lithium-ion secondary batteries, which will have even larger capacities and be safer and easier to handle.

[0004] Lithium-ion secondary batteries using graphite anodes include lithium-ion secondary batteries using organic electrolytes and sulfide-type all-solid-state lithium-ion secondary batteries using sulfide solid electrolytes. The organic electrolyte can penetrate the porous graphite anode layer, forming a bond with the graphite anode active material particles over a wide reaction area. Furthermore, sulfide solid electrolytes can also be easily bonded to the electrolyte particles and to the active material particles by cold-pressing a mixture of solid electrolyte particles and graphite anode active material particles.

[0005] In contrast, oxide-type all-solid-state lithium-ion secondary batteries, which have high atmospheric stability, are fabricated by high-temperature sintering to bond between solid electrolyte particles and between the solid electrolyte and active material particles, as disclosed in Patent Document 1 and Non-Patent Document 1. However, graphite, which is difficult to sinter, is difficult to sinter, so bonding between the solid electrolyte and active material particles must be achieved by other methods, such as pressure bonding, making it difficult to fabricate a low-resistance anode that can operate at room temperature. A high-resistance anode increases the internal resistance of the battery and adversely affects charge / discharge characteristics.

[0006] Patent No. 6686945 Patent No. 6748035

[0007] Martin Finsterbusch, Timo Danner, Chin-Long Tsai, Sven Uhlenbruck, Arnulf Latz, and Olivier Guillon, ACS Appl. Mater. Interfaces 2018, 10, 22329-22339. Narumi Ohta, Shin Kimura, Junichi Sakabe, Kazutaka Mitsuishi, Tsuyoshi Ohnishi, and Kazunori Takada, ACS Appl. Energy Mater. 2019, 2, 7005-7008.

[0008] An object of the present invention is to provide an all-solid-state lithium-ion secondary battery with excellent charge-discharge characteristics and a method for producing the same.

[0009] The present invention provides an all-solid-state lithium-ion secondary battery that solves the above problems. (Configuration 1) An all-solid-state lithium-ion secondary battery having at least an anode layer, a separator layer, and a cathode layer, wherein the anode layer contains an anode active material containing graphite, the separator layer is made of an oxide solid electrolyte, and a low-resistance layer is formed between the anode layer and the separator layer, the low-resistance layer being made of a lithium alloy or containing a material that forms a lithium alloy when a secondary battery is operated by passing a current between the anode layer and the cathode layer. (Configuration 2) The all-solid-state lithium-ion secondary battery according to Configuration 1, wherein the low-resistance layer contains one or more elements selected from the group consisting of gold, silver, platinum, aluminum, tin, indium, germanium, lead, zinc, antimony, magnesium, silicon, cadmium, gallium, tellurium, and bismuth. (Configuration 3) The all-solid-state lithium-ion secondary battery according to Configuration 1, wherein the low-resistance layer has a thickness of 1 to 500 nm. (Configuration 4) The all-solid-state lithium-ion secondary battery according to Configuration 1, wherein the low resistance layer comprises one or more elements selected from the group consisting of gold, silver, platinum, tin, indium, and silicon. (Configuration 5) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 4, wherein the low resistance layer is disposed in contact with the separator layer. (Configuration 6) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 5, wherein the low resistance layer is disposed in contact with the negative electrode layer. (Configuration 7) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 6, wherein the solid electrolyte contained in the separator layer has a highly ion-conductive garnet-type or garnet-like crystal structure. (Configuration 8) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 7, wherein the solid electrolyte contained in the separator layer comprises one or more elements selected from the group consisting of a composite oxide containing Li and ceramics having a garnet-type structure, a perovskite-type structure, and a lisicon-type structure. (Configuration 9) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 8, wherein the negative electrode active material is made of graphite.(Configuration 10) The all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 9, wherein the negative electrode active material comprises graphite and a material capable of absorbing less than 10% lithium ions. (Configuration 11) The positive electrode comprises a positive electrode active material, wherein the positive electrode active material is LiM. １ ｘ Mn ２－ｘ O ４ (However, M １ is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and 0.01≦x≦0.5), a spinel-type lithium manganese composite oxide represented by Li ｘ Mn （１－ｙ－ｘ） Ni ｙ M ２ ｚ O （２－ｋ） F ｌ (However, M ２ is at least one element selected from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, and is a layered compound represented by the formula: 0.8≦x≦1.2, 0<y<0.5, 0≦z≦0.5, k+l<1, −0.1≦k≦0.2, 0≦l≦0.1), LiCo １－ｘ M ３ ｘ O ２ (However, M ３ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), lithium cobalt composite oxides represented by LiNi １－ｘ M ４ ｘ O ２ (However, M ４ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), lithium nickel composite oxide, LiM ５ １－ｘ N １ ｘ P.O.４ (However, M ５ is at least one element selected from the group consisting of Fe, Mn and Co, and N １ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), an olivine-type composite oxide represented by the formula: Li ４ Ti ５ O １２ (Configuration 12) A method for producing the all-solid-state lithium-ion secondary battery according to any one of Configurations 1 to 11, wherein the low resistance layer is formed by one or more methods selected from the group consisting of a sputtering method, a vapor deposition method, a coating method, and a foil pasting method.

[0010] According to the present invention, an all-solid-state lithium-ion secondary battery having excellent charge-discharge characteristics and a method for producing the same are provided.

[0011] FIG. 1 is a structural explanatory diagram showing, in a cross-sectional view, the configuration of an all-solid-state lithium-ion secondary battery of the present invention. FIG. 2 is a structural explanatory diagram showing, in a cross-sectional view, the configuration of an all-solid-state lithium-ion half-cell (hereinafter referred to as half-cell) of an embodiment. FIG. 3 is a characteristic diagram showing the initial charge characteristics of a half-cell obtained in Example 1 (Example 1). Since the description is given from the perspective of the negative electrode of the whole battery, the reaction of inserting lithium ions into graphite, which is the test electrode active material, is referred to as the charge reaction, and the reaction of desorbing lithium ions from graphite, which is the test electrode active material, is referred to as the discharge reaction. FIG. 4 is an AC impedance spectrum diagram of the half-cell obtained in Example 1 (Example 1) after the initial charge. FIG. 5 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Example 2 (Example 2). FIG. 6 is a diagram showing an X-ray diffraction pattern of the half-cell obtained in Example 2 (Example 2) after the initial charge. FIG. 7 is a characteristic diagram showing the initial charge and discharge of the half-cell obtained in Example 3 (Example 3). FIG. 8 is a characteristic diagram showing the battery capacity versus the number of charge and discharge cycles of the half-cell obtained in Example 3 (Example 3). FIG. 9 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Example 4 (Example 4). FIG. 1 is an AC impedance spectrum diagram of the half-cell obtained in Example 4 (Example 4) after initial charge. FIG. 2 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Example 5 (Example 5). FIG. 3 is an AC impedance spectrum diagram of the half-cell obtained in Example 5 (Example 5) after initial charge. FIG. 4 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Example 6 (Example 6). FIG. 5 is an AC impedance spectrum diagram of the half-cell obtained in Example 6 (Example 6) after initial charge. FIG. 6 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Example 7 (Example 7). FIG. 7 is an AC impedance spectrum diagram of the half-cell obtained in Example 7 (Example 7) after initial charge. FIG. 8 is an AC impedance spectrum diagram of the half-cell obtained in Comparative Example 1 (Example 8) after initial charge. FIG. 9 is a characteristic diagram showing the initial charge characteristics of the half-cell obtained in Comparative Example 1 (Example 8). Since the explanation is given here from the viewpoint of the negative electrode of the whole battery, the reaction of inserting lithium ions into the graphite, which is the test electrode active material, is referred to as the charge reaction, and the reaction of desorbing lithium ions from the graphite, which is the test electrode active material, is referred to as the discharge reaction.FIG. 1 is a diagram showing an X-ray diffraction pattern of a half-cell obtained in Example 9 after initial charging. FIG. 2 is a characteristic diagram showing initial charge characteristics of a half-cell obtained in Example 10. FIG. 3 is an AC impedance spectrum diagram of a half-cell obtained in Example 10 after initial charging. FIG. 4 is a characteristic diagram showing initial charge characteristics of a half-cell obtained in Example 11. FIG. 5 is an AC impedance spectrum diagram of a half-cell obtained in Example 11 after initial charging. FIG. 6 is an AC impedance spectrum diagram of a half-cell obtained in Example 12 after initial charging. FIG. 7 is an AC impedance spectrum diagram of a half-cell obtained in Example 13 after initial charging. FIG. 8 is an AC impedance spectrum diagram of a half-cell obtained in Example 13 after initial charging. FIG. 9 is an AC impedance spectrum diagram of a half-cell obtained in Example 14 after initial charging. FIG. 10 is an AC impedance spectrum diagram of a half-cell obtained in Example 14 after initial charging. FIG. 11 is an AC impedance spectrum diagram of a half-cell obtained in Example 15 after initial charging. FIG. 1 is a characteristic diagram showing the initial charge characteristics of a half-cell obtained in Example 16, which is an embodiment; FIG. 2 is an AC impedance spectrum diagram of a half-cell obtained in Example 16 after the initial charge; FIG. 3 is a characteristic diagram showing the initial charge characteristics of a half-cell obtained in Example 17; FIG. 4 is an AC impedance spectrum diagram of a half-cell obtained in Example 17 after the initial charge; FIG. 5 is a characteristic diagram showing the initial charge characteristics of a half-cell obtained in Example 18; FIG. 6 is an AC impedance spectrum diagram of a half-cell obtained in Example 18 after the initial charge; FIG. 7 is an X-ray diffraction pattern of a half-cell obtained in Example 18 after the initial charge; FIG. 8 is a diagram showing the element distribution in the depth direction of the negative electrode layer obtained in Example 19; the horizontal axis represents sputtering time (seconds), and the vertical axis represents intensity; and FIG. 9 is a diagram showing the element distribution in the depth direction of the negative electrode layer obtained in Example 20; the horizontal axis represents sputtering time (seconds), and the vertical axis represents intensity.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the description, AB means A or more and B or less.

[0013] <Battery Structure> As shown in Fig. 1 , the all-solid-state lithium-ion secondary battery 101 of the present embodiment includes a negative electrode current collector 11, a negative electrode active material layer 12, a low-resistance layer 13, a separator layer 15, a positive electrode active material layer 16, and a positive electrode current collector 17. Here, the negative electrode current collector 11 and the negative electrode active material layer 12 form a negative electrode (negative electrode layer) 14, and the positive electrode active material layer 16 and the positive electrode current collector 17 form a positive electrode (positive electrode layer) 18. The negative electrode active material layer 12 and the negative electrode current collector 11 may not be separated, and the negative electrode active material layer 12 may also function as the negative electrode current collector 11. Furthermore, the positive electrode active material layer 16 may also be separated from the positive electrode current collector 17, and the positive electrode active material layer 16 may also function as the positive electrode current collector 17.

[0014] The negative electrode current collector 11 and the positive electrode current collector 17 (hereinafter, both of them are also referred to as collectors) are electrodes intended to mediate the movement of electrons from one surface in contact with the positive electrode active material layer 16 (the surface where the positive electrode current collector 17 is in contact with the positive electrode active material layer 16 according to FIG. 1 ) to the other surface in contact with the negative electrode active material layer 12 (the surface where the negative electrode current collector 11 is in contact with the negative electrode active material layer 12 according to FIG. 1 ), and are not particularly limited as long as they are made of conductive materials, and metals and conductive resins can be used.

[0015] Specifically, electrodes having a metal selected from the group consisting of copper (Cu), tungsten (W), aluminum (Al), nickel (Ni), iron (Fe), titanium (Ti), gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) can be used. That is, the negative electrode current collector 11 and the positive electrode current collector 17 can be electrodes made of a metal selected from this group, an alloy containing a metal selected from this group, or a compound of a metal selected from this group with carbon (C), nitrogen (N), or the like. Stainless steel or a foil with an aluminum-coated metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and the applicability of inexpensive and easy-to-handle sputtering methods.

[0016] A preferred conductive resin is a non-conductive polymer material to which a conductive filler has been added. Examples of non-conductive polymer materials include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS). Examples of the conductive filler include one or more metals selected from the group consisting of nickel (Ni), titanium (Ti), aluminum (Al), copper (Cu), platinum (Pt), chromium (Cr), iron (Fe), zinc (Zn), tin (Sn), indium (In), and antimony (Sb), alloys containing these metals, metal compounds, or one or more selected from the group consisting of acetylene black, carbon nanofibers, carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerenes.

[0017] The negative electrode active material layer 12 is preferably made of graphite or a material containing graphite as a main component, and more preferably made of a material containing graphite as a main component. The negative electrode active material layer 12 is preferably made of graphite. Graphite is excellent in terms of improving the energy density of the battery, ease of production, and cost reduction. Here, "mainly made of graphite" means that the layer contains 90% or more but less than 100% graphite by molar ratio. The material added to the graphite is a material capable of absorbing lithium ions (i.e., a material capable of absorbing ions), and examples of such materials include materials containing silicon and tin. When the negative electrode active material layer 12 is made of graphite and silicon, for example, the molar ratio may be 95% graphite and 5% silicon.

[0018] The low-resistance layer 13 is a layer made of a material containing a metal that can be alloyed with lithium (Li) (i.e., a metal that can be alloyed). For example, it is a layer made of a lithium alloy, or a layer that is alloyed with lithium by secondary battery operation (charge / discharge operation). The presence of the low-resistance layer 13 makes it possible to reduce the electrical resistance between the negative electrode active material layer 12 and the separator layer 15 (or at the interface if they are in contact). When the electrical resistance between the negative electrode active material layer 12 and the separator layer 15 is reduced, the internal resistance of the secondary battery also decreases, improving the charge / discharge characteristics and increasing the electrical capacity.

[0019] Specific examples of the material for the low resistance layer 13 include materials containing one or more elements selected from the group consisting of gold (Au), silver (Ag), platinum (Pt), aluminum (Al), tin (Sn), indium (In), germanium (Ge), lead (Pb), zinc (Zn), antimony (Sb), magnesium (Mg), silicon (Si), cadmium (Cd), gallium (Ga), tellurium (Te), and bismuth (Bi). In particular, one or more elements selected from the group consisting of gold, silver, platinum, tin, indium, and silicon are preferably used because of their stability and ease of handling. Furthermore, the shape of the material of the low resistance layer 13 (specifically, a material containing one or more elements selected from the group consisting of gold (Au), silver (Ag), platinum (Pt), aluminum (Al), tin (Sn), indium (In), germanium (Ge), lead (Pb), zinc (Zn), antimony (Sb), magnesium (Mg), silicon (Si), cadmium (Cd), gallium (Ga), tellurium (Te), and bismuth (Bi), and particularly preferably one or more elements selected from the group consisting of gold, silver, platinum, tin, indium, and silicon) may be such that it is in contact with the separator layer 15 so as to achieve the effects of the present invention. Therefore, as long as the material is in contact with the separator layer 15 so as to achieve the effects of the present invention, it is not limited to a layered structure (specifically, a structure in which particles of the material are continuously aligned and fixed to the separator layer 15, thereby forming a thin sheet-like film in contact with the separator layer). For example, the material may be discontinuously aligned and fixed to the separator layer 15, adhering to the separator layer. The low-resistance layer 13 is preferably layered. This is because a layered structure can increase the contact area between the separator layer 15 and the negative electrode layer 14 during charge / discharge operations, thereby reducing the resistance of the interface between them and improving charge / discharge characteristics. Furthermore, when the thickness of the layered structure is calculated from the weight of the material used, the thickness is preferably 1 to 500 nm. It is more preferably 5 to 300 nm, and even more preferably 10 to 260 nm. If the thickness is too thin, a continuous layered film will not form, and if it is too thick, material costs and process costs will increase. Therefore, the thickness is preferably within the above range.

[0020] The low resistance layer 13 is preferably disposed in contact with the separator layer 15, and is preferably disposed in contact with the negative electrode layer 14, particularly the negative electrode active material layer 12. This is because when the low resistance layer 13 is in contact with these layers, the resistance is further reduced, and the contact further increases the reliability and stability of the secondary battery operation.

[0021] The low-resistance layer 13 can be formed by one or more methods selected from the group consisting of sputtering, vapor deposition methods such as electron beam vapor deposition and thermal vapor deposition, coating methods, and foil lamination methods. Sputtering and vapor deposition methods are commonly used industrially, and the coating and foil lamination methods are easy to handle because they do not require a vacuum environment. Since the low-resistance layer 13 is intended to reduce resistance across the entire interface, it has high tolerance for defects such as local pinholes (in other words, it is less susceptible to the effects of such defects). For this reason, low-cost methods such as foil lamination can be used for the low-resistance layer 13.

[0022] The separator layer 15 is a layer that functions as a separator for the all-solid-state lithium ion secondary battery, and contains or is made of an oxide solid electrolyte. Examples of the oxide solid electrolyte include a highly ion-conductive garnet-type electrolyte or an electrolyte having a garnet-like crystal structure. Specific examples of the oxide solid electrolyte include Li ７ La ３ Zr ２ O １２ , Li ７－ｘ La ３ Zr ２－ｘ Ta ｘ O １２ (where x is 0 or more and 2 or less), Li ７－３ｘ La ３ Zr ２ Al ｘ O １２ (where x is 0 or more and 0.5 or less), Li ７－ｘ La ３ Zr ２－ｘ Nb ｘ O １２ (wherein x is 0 or more and 2 or less) and Li ７－ｘ－３ｙ La ３ Zr２－ｘ Ta ｘ Al ｙ O １２ (where x is 0 or more and 2 or less, and y is 0 or more and 0.5 or less). The solid electrolyte contained in the separator layer 15 can be one or more selected from the group consisting of composite oxides containing lithium (Li) and ceramics having a garnet structure, a perovskite structure, and a lisicon structure. Specific examples of such materials include lisicon, nasicon, PEO, lipon, PVDF, and Li ３ N., Li. ３ P, LiI, LiBr, LiCl, LiF, Li ０．５ TiO ３ , Li ２ S-SiS ２ -Li ３ P.O. ４ , lithium nitride, doped Li ３ N., Li. ２ S-SiS ２ -Li ３ P.O. ４ , Li １４ Zn(GeO ４ ) ４ , Li-β-alumina, Li ３．６ Si ０．６ P ０．４ O ４ , PEO-LiClO ４ , LiN(CF ３ SO ２ ) ２ / (CH ２ CH ２ O) ８ , Al, Ga, Nb, Ta, Ca, Sr, or a combination thereof. ７ La ３ Zr ２ O １２ , Li ３ BO ３ The compound may be one or more selected from the group consisting of:

[0023] The positive electrode active material layer 16 is not particularly limited as long as it is a positive electrode active material containing lithium, but a representative example is a lithium-containing composite metal oxide. １ ｘ Mn ２－ｘ O ４ (However, M １ is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and 0.01≦x≦0.5), a spinel-type lithium manganese composite oxide represented by Li ｘ Mn （１－ｙ－ｘ） Ni ｙ M ２ ｚ O （２－ｋ） F ｌ (However, M ２ is at least one element selected from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, and is a layered compound represented by the formula: 0.8≦x≦1.2, 0<y<0.5, 0≦z≦0.5, k+l<1, −0.1≦k≦0.2, 0≦l≦0.1), LiCo １－ｘ M ３ ｘ O ２ (However, M ３ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), lithium cobalt composite oxides represented by LiNi １－ｘ M ４ ｘ O ２ (However, M ４ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), lithium nickel composite oxide, LiM ５ １－ｘ N １ ｘ P.O. ４ (However, M ５is at least one element selected from the group consisting of Fe, Mn and Co, and N １ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), an olivine-type composite oxide represented by the formula: Li ４ Ti ５ O １２ The lithium titanium composite oxide may be one or more selected from the group consisting of lithium titanium composite oxides represented by the following formula:

[0024] The battery having the above structure provides an all-solid-state lithium-ion secondary battery 101 having low internal resistance and excellent charge / discharge characteristics as an all-solid-state lithium-ion secondary battery according to one embodiment of the present invention.

[0025] <Method of Manufacturing Battery> First, a solid electrolyte is prepared by sintering a solid electrolyte material using a hot press sintering method or the like, and this solid electrolyte is used as the separator layer 15. The sintering temperature can be, for example, 800°C or higher and 1200°C or lower.

[0026] Next, the low resistance layer 13 is formed on one of the main surfaces (first main surface) of the separator layer 15. As described above, the formation method can be one or more methods selected from the group consisting of sputtering, vapor deposition such as electron beam vapor deposition and thermal vapor deposition, coating methods, and foil lamination methods. In this application, the main surface of the separator layer refers to the surface that contacts the low resistance layer, the positive electrode active material layer, or the counter electrode.

[0027] Thereafter, a positive electrode current collector 17 coated with a positive electrode active material layer 16 and a negative electrode current collector 11 coated with a negative electrode active material layer 12 are fabricated. The materials described in the section on the battery structure may be used for the positive electrode active material layer 16, the positive electrode current collector 17, the negative electrode active material layer 12, and the negative electrode current collector 11. Examples of deposition methods include lamination, sputtering, vapor deposition, CVD (Chemical Vapor Deposition), spray coating, blade coating, slit die coating, and roll coating.

[0028] Thereafter, the separator layer 15 is placed on the negative electrode current collector 11 on which the negative electrode active material layer 12 is deposited, with the negative electrode active material layer 12 facing the low resistance layer 13. Furthermore, the positive electrode current collector 17 on which the positive electrode active material layer 16 is deposited is placed on a main surface of the separator layer 15 opposite to the first main surface on which the low resistance layer 13 is disposed (this main surface is referred to as the second main surface in the present application to distinguish it from the first main surface). The negative electrode current collector 11, the negative electrode active material layer 12, the low resistance layer 13, the separator layer 15, the positive electrode active material layer 16, and the positive electrode current collector 17 are bundled and pressed together, and then the resultant is housed in a housing (not shown) to produce the all-solid-state lithium-ion secondary battery 101 ( FIG. 1 ).

[0029] The above-described manufacturing method makes it possible to provide the all-solid-state lithium-ion secondary battery 101 having the above-described structure. The all-solid-state lithium-ion secondary battery 101 is characterized by low internal resistance and excellent charge / discharge characteristics.

[0030] In the examples, in order to eliminate the influence of the positive electrode used and to investigate the interfacial bonding between the graphite and the negative electrode active material, an all-solid-state lithium-ion half-cell 102 ( FIG. 2 ) was fabricated using graphite as the test electrode 22, a LiLaZrTaO sintered body as the separator layer 25, and lithium foil as the counter electrode 26, and its characteristics were evaluated. Eight types of samples were fabricated: Examples 1 to 7 and Comparative Example 1. The low-resistance layer 23 in Examples 1 to 3 consisted of gold (Au), indium (In), silver (Ag), platinum (Pt), and silicon (Si). Comparative Example 1 did not have a low-resistance layer 23. Here, the sample fabrication methods will be summarized first, followed by a summary of the characteristics. Note that, for convenience, the examples and comparative examples may be referred to simply as "examples" in this application. In this case, Examples 1 to 7 will be referred to as Examples 1 to 7, and Comparative Example 1 will be referred to as Example 8.

[0031] [Preparation of Samples] Example 1 (Example 1) (Solid Electrolyte) A Li-ion battery with a diameter of 10 mm and a thickness of 2 mm was used as the separator layer 25 (solid electrolyte). ６．６ La ３ Zr １．６ Ta ０．４ O １２ A sintered body (manufactured by Toshima Manufacturing Co., Ltd.) was used.

[0032] (Graphite Dispersion) As the graphite dispersion, a dispersion prepared by dispersing 450 mg of graphite powder (KS6, manufactured by Imerys, average diameter 4.4 μm) in 30 g of 99.5% ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used.

[0033] (Preparation step) First, a small spray coating device (AV-8 model, manufactured by Acing Technologies) was used to spray the graphite dispersion onto an unpolished SUS304 disk having a diameter of 10 mm and a thickness of 0.5 mm, with a coating amount of 1.77 mg / cm. ２ Next, a compact sputtering apparatus (SC-701MkII ADVANCE, manufactured by Sanyu Electronics Co., Ltd.) was used to sputter graphite in a weight of 0.29 mg / cm onto both main surfaces of a sintered body of solid electrolyte (diameter 10 mm, thickness 2 mm) that forms the separator layer 25. ２ A gold (Au) layer was formed on the counter electrode 26. The gold layer on the side in contact with the test electrode 22 corresponds to the low-resistance layer 23. A 50 μm-thick lithium (Li) foil (manufactured by Honjo Chemical Co., Ltd.) was then prepared as the counter electrode 26. The lithium foil was placed on one surface of the gold-coated solid electrolyte 25, and a graphite test electrode 22 was then placed on the other surface of the gold-coated solid electrolyte 25, which served as the low-resistance layer 23. The graphite test electrode 22, the solid electrolyte 25 coated on both sides with gold layers, and the counter electrode 26 (Li foil) were then combined to obtain a primary assembly. In Example 1 (Example 1), the current collector 21 was made of SUS304, which was used as the spray-coated substrate, and a new, unpolished SUS304 disk with a diameter of 10 mm and a thickness of 0.5 mm was used as the current collector 27, which was placed on one surface of the counter electrode 26 (Li foil).

[0034] (Pressing Step) The obtained primary assembly was screwed with a torque of 2.0 Nm to apply a confining pressure of 40 MPa in the stacking direction, thereby obtaining a half-cell 102 of Example 1.

[0035] Example 2 (Example 2) and Example 3 (Example 3) Half-cells 102 of Example 2 (Example 2) and Example 3 (Example 3) were obtained in the same manner as Example 1 (Example 1), except that the amount of graphite powder applied in Example 1 (Example 1) was changed as shown in Table 1.

[0036]

[0037] Example 4 (Example 4) In Example 1 (Example 1), only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.17 mg / cm ２ A half-cell 102 of Example 4 (Example 4) was obtained in the same manner as in Example 1 (Example 1), except that the low resistance layer 23 of Example 4 (Example 4) was coated with an indium (In) layer of 1000 kJ / cm 2 and the amount of graphite powder applied was changed as shown in Table 1. Therefore, the low resistance layer 23 of Example 4 (Example 4) was formed of an indium layer.

[0038] Example 5 (Example 5) In Example 1 (Example 1), only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.27 mg / cm ２ A half-cell 102 of Example 5 (Example 5) was obtained in the same manner as in Example 1 (Example 1), except that the low-resistance layer 23 of Example 5 (Example 5) was coated with a silver (Ag) layer of 1000 kJ / cm 2 and the amount of graphite powder applied was changed as shown in Table 1. Therefore, the low-resistance layer 23 of Example 5 (Example 5) was formed of a silver layer.

[0039] Example 6 (Example 6) In Example 1 (Example 1), only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.41 mg / cm ２ A half-cell 102 of Example 6 (Example 6) was obtained in the same manner as in Example 1 (Example 1), except that the low resistance layer 23 of Example 6 (Example 6) was coated with a platinum (Pt) layer of 1000 kJ / cm 2 and the amount of graphite powder applied was changed as shown in Table 1. Therefore, the low resistance layer 23 of Example 6 (Example 6) was formed of a platinum layer.

[0040] Example 7 In Example 1, only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.06 mg / cm ２ A half-cell 102 of Example 7 (Example 7) was obtained in the same manner as in Example 1 (Example 1), except that the low resistance layer 23 of Example 7 (Example 7) was coated with a silicon layer of 1000 kJ / cm 2 and the amount of graphite powder applied was changed as shown in Table 1. Therefore, the low resistance layer 23 of Example 7 (Example 7) was formed of a silicon layer.

[0041] Comparative Example 1 (Example 8) A half-cell of Comparative Example 1 (Example 8) was obtained in the same manner as in Example 1 (Example 1), except that only one surface of sintered body 25 of solid electrolyte on which counter electrode 26 was disposed was coated with a gold layer and the amount of graphite powder applied was changed as shown in Table 1. Therefore, Comparative Example 1 (Example 8) does not have low-resistance layer 23.

[0042] [Characteristic Evaluation] (Initial Charge Test) Using the half-cells obtained in Example 1 (Example 1), Example 2 (Example 2), Examples 4 to 6 (Examples 4 to 6), and Comparative Example 1 (Example 8), a CC charge test was performed at a current of 0.05 C until the voltage reached 0.01 V relative to lithium metal. The initial charge curves obtained from the test are shown in Figures 3, 5, 9, 11, 13, and 17, respectively. Furthermore, using the half-cell obtained in Example 7 (Example 7), CC charge was performed at currents of 0.0005 C and 0.005 C for 12 hours and 10 hours, respectively, followed by CC charge at a current of 0.05 C until the voltage reached 0 V relative to lithium metal, followed by a CV charge test at 0 V for 10 hours. The initial charge curve obtained from the test is shown in Figure 15. The test environment was an argon atmosphere at room temperature (approximately 25°C) under atmospheric pressure.

[0043] (Initial Charging Test Results) Table 2 shows the test results of the initial charging capacity.

[0044]

[0045] The weight of graphite contained in the test electrode 22 made of graphite incorporated in the half-cells obtained in Example 1 (Example 1), Example 2 (Example 2), and Comparative Example 1 (Example 8) was about 1 mg / cm ２ However, Example 1 (Example 1) and Example 2 (Example 2), in which test electrode 22 made of graphite was arranged on the surface of solid electrolyte 25 coated with a gold layer, which is low-resistance layer 23, showed a decrease of about 0.3 to 0.4 mAh / cm compared to Comparative Example 1 (Example 8), in which test electrode 22 made of graphite was arranged on the surface of solid electrolyte 25 that was not coated with a gold layer (no low-resistance layer 23 was arranged). ２ Although the weights of graphite contained in the test electrodes 22 made of graphite incorporated in the half-cells obtained in Example 4 (Example 4), Example 5 (Example 5), Example 6 (Example 6), and Comparative Example 1 (Example 8) were almost the same, in Examples 4 (Example 4), 5 (Example 5), and 6 (Example 6), in which the test electrodes 22 made of graphite were disposed on the surface of solid electrolytes 25 coated with an indium layer, a silver layer, and a platinum layer, respectively, the charge capacities obtained were about 0.2 to 0.3 mAh / cm compared to Comparative Example 1 (Example 8), in which the test electrodes 22 made of graphite were disposed on the surface of solid electrolytes 25 that were not coated with these layers. ２ That is, even when an indium layer, a silver layer, or a platinum layer was provided as the low resistance layer 23, the charge capacity was about 0.2 to 0.3 mAh / cm compared to when the low resistance layer 23 was not provided. ２ Furthermore, although the weight of graphite contained in test electrode 22 made of graphite incorporated in the half-cells obtained in Example 7 (Example 7) and Comparative Example 1 (Example 8) was approximately the same, in Example 7 (Example 7), in which test electrode 22 made of graphite was placed on the surface of solid electrolyte 25 coated with a silicon layer, which was low-resistance layer 23, and CC charging was performed until the voltage reached 0 V relative to lithium metal, and CV charging was further performed for 10 hours, the charge capacity was about 0.5 mAh / cm higher than in Comparative Example 1 (Example 8), in which test electrode 22 made of graphite was placed on the surface of solid electrolyte 25 not coated with a silicon layer, and CC charging was performed until the voltage reached 0 V relative to lithium metal, i.e., in which low-resistance layer 23 was not placed. ２ A large charging capacity was also obtained.

[0046] (Method for Measuring AC Impedance After Initial Charging Test) After the above-mentioned initial charging test was performed using the half-cells obtained in Example 1 (Example 1), Example 4 (Example 4), Example 5 (Example 5), Example 6 (Example 6), Example 7 (Example 7), and Comparative Example 1 (Example 8), AC impedance was measured at an applied AC voltage of 10 mV based on the natural potential in the frequency range of 10 mHz to 1 MHz. The AC impedance spectra obtained as a result of the test are shown in Figures 4, 10, 12, 14, 16, and 18, respectively. Here, the test environment was an argon atmosphere at room temperature of approximately 25°C under atmospheric pressure.

[0047] (Results of AC Impedance Measurement After Initial Charging Test) Table 3 shows the results of AC impedance measurement after the initial charging test.

[0048]

[0049] From the AC impedance spectra obtained after the initial charging of the half-cells obtained in Example 1 (Example 1) and Comparative Example 1 (Example 8), in the structure without a gold layer sandwiched therebetween, the interface resistance between the solid electrolyte separator layer 25 and the test electrode 22 made of graphite was 3188 Ω cm ２ On the other hand, in the structure in which the low resistance layer 23 made of a gold layer is sandwiched, the interface resistance is 31 Ω cm ２ It was found that the interfacial resistance was reduced to 180 Ω cm. The half-cells obtained by employing the interface structure of Example 1 (Example 1) had significantly lower interfacial resistance and internal resistance of the cell. This resulted in improved charge-discharge characteristics and improved energy density of the battery. Furthermore, from the AC impedance spectra obtained after the initial charge of the half-cells obtained in Example 4 (Example 4), Example 5 (Example 5), Example 6 (Example 6), and Example 7 (Example 7), it was found that the interfacial resistance was reduced to 180 Ω cm even in the structures in which an indium (In) layer, a silver (Ag) layer, a platinum (Pt) layer, and a silicon (Si) layer were sandwiched. ２ , 330 Ω cm ２ , 955 Ω cm ２ and 67 Ω cm ２ and the interface resistance (3188 Ω cm) of the half cell obtained in Comparative Example 1 (Example 8). ２) and it was confirmed that the interfacial resistance of the structure was also reduced in the same way as in the case of the structure sandwiching the gold layer.

[0050] (X-ray Diffraction Measurement After Initial Charging Test) After an initial charging test was performed using half-cell 102 obtained in Example 2 (Example 2), half-cell 102 was disassembled in a glove box under an argon gas atmosphere, and the surface of the graphite layer that had been in contact with current collector 21 was facing the X-ray irradiation side, and the half-cell was sealed in an X-ray diffraction atmosphere separator (manufactured by Rigaku Corporation) (argon gas atmosphere). X-ray diffraction measurement was performed using an X-ray diffractometer (Miniflex 600, manufactured by Rigaku Corporation) in a diffraction angle range of 20° to 30° (2θ). The X-ray diffraction pattern obtained as a result of the test is shown in FIG.

[0051] (X-ray diffraction measurement results after initial charge test) From the X-ray diffraction pattern after the initial charge of the half-cell 102 obtained in Example 2 (Example 2), it was found that the gold layer reacted with lithium to form an alloy Li ３ It was found that the gold layer, which is the low-resistance layer 23, was changed to Au. This indicates that the volume expansion during the alloying reaction contributes significantly to the phenomenon in which the reduction in interfacial resistance was confirmed in a structure in which the gold layer, which is the low-resistance layer 23, was sandwiched between the test electrode 22 and the separator layer 25. It was also found that the reduction in interfacial resistance allowed lithium to be inserted into the graphite contained in the graphite layer near the current collector from the stage 2 to the stage 1 charge state, even during CC charging using a current value of 0.05 C at room temperature.

[0052] (Charge-Discharge Cycle Test) Using the half-cell 102 obtained in Example 3 (Example 3), a charge-discharge test was performed at a current value of 0.05 C in a voltage range of 0.01 to 1.2 V vs. lithium metal. The initial charge-discharge curve obtained as a result of the test is shown in Figure 7, and the battery capacity versus the number of charge-discharge cycles up to 20 cycles is shown in Figure 8. Here, the test environment was an argon atmosphere at room temperature of about 25°C under atmospheric pressure.

[0053] (Charge-Discharge Cycle Test Results) From the initial charge-discharge curve of the half-cell 102 obtained in Example 3 (Example 3), it was found that no significant hysteresis was observed in the charge-discharge curve, and the initial coulombic efficiency was also found to be greater than 70%. From the second cycle onwards, high reversibility was observed, and no noticeable capacity loss was observed. This indicates that the half-cell 102 obtained using the interface structure of the present disclosure exhibits not only high energy density but also high stability.

[0054] Furthermore, as shown below, Examples 9 to 20 were added. 12 types of samples were fabricated for the added Examples, Examples 9 to 20. In the added Examples, as in the above Examples (i.e., Examples 1 to 8), in order to eliminate the influence of the positive electrode used and to investigate purely the interfacial bonding with the graphite negative electrode active material, all-solid-state lithium-ion half-cell 102 ( FIG. 2 ) was fabricated using graphite (including those mainly composed of graphite) for test electrode 22, a LiLaZrTaO sintered body for separator layer 25, and lithium foil for counter electrode 26, and its characteristics were evaluated. The low-resistance layer 23 was made of gold (Au) in Examples 9, 10, and 17 to 20, indium (In) in Examples 11 and 12, silver (Ag) in Examples 13 and 14, and platinum (Pt) in Examples 15 and 16. In Example 17, no test electrode 22 was provided, and in Example 18, a test electrode 22 made of graphite and silicon was incorporated to fabricate a half-cell, and its characteristics were evaluated. Examples 19 and 20 are negative electrodes fabricated to evaluate the element distribution in the thickness direction of the test electrode 22. In Example 19, the test electrode 22 was made of graphite, and in Example 20, it was made of graphite and silicon. Here, as in the above examples (i.e., Examples 1 to 8), the sample preparation method will be summarized first, followed by a summary of the characteristics. In this case, Example 3 (i.e., Example 3) and Example 8 (i.e., Comparative Example 1) described above will also be used as appropriate for comparison.

[0055] [Sample Preparation] Example 9 (Solid Electrolyte) A Li-ion battery with a diameter of 10 mm and a thickness of 2 mm was used as the separator layer 25 (solid electrolyte). ６．６ La ３ Zr １．６ Ta ０．４ O １２A sintered body (manufactured by Toshima Manufacturing Co., Ltd.) was used.

[0056] (Graphite Dispersion) As the graphite dispersion, a dispersion prepared by dispersing 450 mg of graphite powder (KS6, manufactured by Imerys, average diameter 4.4 μm) in 30 g of 99.5% ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used.

[0057] (Preparation step) First, a small spray coating device (AV-8 model, manufactured by Acing Technologies) was used to spray the graphite dispersion onto an unpolished SUS304 disk having a diameter of 10 mm and a thickness of 0.5 mm, with a coating amount of 1.231 mg / cm. ２ Next, a compact sputtering apparatus (SC-701MkII ADVANCE, manufactured by Sanyu Electronics Co., Ltd.) was used to sputter graphite in a weight of 0.29 mg / cm onto both main surfaces of a sintered body of solid electrolyte (diameter 10 mm, thickness 2 mm) that forms the separator layer 25. ２ A gold (Au) layer of 150 nm thick was formed on the test electrode 22. Here, the gold layer on the side in contact with the test electrode 22 corresponds to the low-resistance layer 23. Therefore, the low-resistance layer 23 in Example 9 was formed of a gold layer with a thickness of 150 nm. Thereafter, a 50 μm thick lithium (Li) foil (manufactured by Honjo Chemical Co., Ltd.) was prepared as the counter electrode 26, and the lithium foil was placed on one surface of the solid electrolyte 25 coated with a gold layer. Subsequently, a test electrode 22 made of graphite was placed on the other surface of the solid electrolyte 25 coated with a gold layer, which would become the low-resistance layer 23. Then, the test electrode 22 made of graphite, the solid electrolyte 25 coated on both sides with gold layers, and the counter electrode 26 (Li foil) were combined to obtain a primary assembly. In Example 9, the SUS304 used as the substrate for spray coating was used as the current collector 21, and a new unpolished SUS304 disk having a diameter of 10 mm and a thickness of 0.5 mm was used as the current collector 27, which was placed on one surface of the counter electrode 26 (Li foil).

[0058] (Pressing Step) The obtained primary assembly was screwed with a torque of 2.0 Nm to apply a confining pressure of 40 MPa in the stacking direction, thereby obtaining a half-cell 102 of Example 9.

[0059]

[0060] Example 10 In Example 9, the surface facing the test electrode 22 made of graphite was coated with a material having a weight of 0.05 mg / cm ２ A half-cell 102 of Example 10 was obtained in the same manner as in Example 9, except that the half-cell 102 was coated with a gold (Au) layer of 26 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the resistance-reducing layer 23 of Example 10 was formed of a gold layer with a thickness of 26 nm.

[0061]

[0062] Example 11 In Example 9, only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.08 mg / cm ２ A half-cell 102 of Example 11 was obtained in the same manner as in Example 9, except that the low resistance layer 23 of Example 11 was coated with an indium (In) layer having a thickness of 109 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the low resistance layer 23 of Example 11 was formed of an indium layer having a thickness of 109 nm.

[0063] Example 12 In Example 11, the surface of the sintered body 25 of the solid electrolyte facing the test electrode 22 made of graphite was coated with a weight of 0.01 mg / cm ２ A half-cell 102 of Example 12 was obtained in the same manner as in Example 11, except that the half-cell 102 was coated with an indium (In) layer of 14 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the low resistance layer 23 of Example 12 was formed of an indium layer with a thickness of 14 nm.

[0064] Example 13 In Example 9, only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.11 mg / cm ２ A half-cell 102 of Example 13 was obtained in the same manner as in Example 9, except that the half-cell 102 was coated with a silver (Ag) layer of 104 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the resistance-reducing layer 23 of Example 13 was formed of a silver layer with a thickness of 104 nm.

[0065] Example 14 In Example 13, the surface of the sintered body 25 of the solid electrolyte facing the test electrode 22 made of graphite was coated with a weight of 0.03 mg / cm ２ A half-cell 102 of Example 14 was obtained in the same manner as in Example 13, except that the half-cell 102 was coated with a silver (Ag) layer of 100 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the resistance-reducing layer 23 of Example 14 was formed of a silver layer with a thickness of 29 nm.

[0066] Example 15 In Example 9, only one surface of the sintered body 25 of the solid electrolyte on which the lithium foil serving as the counter electrode 26 was disposed was coated with a gold layer, and the other surface of the sintered body 25 of the solid electrolyte on which the test electrode 22 made of graphite was disposed, i.e., the surface facing the test electrode 22, was coated with a gold layer having a weight of 0.16 mg / cm ２ A half-cell 102 of Example 15 was obtained in the same manner as in Example 9, except that the low resistance layer 23 of Example 15 was coated with a platinum (Pt) layer of 75 nm and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the low resistance layer 23 of Example 15 was formed of a platinum layer with a thickness of 75 nm.

[0067] Example 16 In Example 15, the surface of the sintered body 25 of the solid electrolyte facing the test electrode 22 made of graphite was coated with a weight of 0.04 mg / cm ２ A half-cell 102 of Example 16 was obtained in the same manner as in Example 15, except that the low-resistance layer 23 of Example 16 was coated with a platinum (Pt) layer of 19 nm in thickness and the amount of graphite powder applied was changed as shown in Table 4. Therefore, the low-resistance layer 23 of Example 16 was formed of a platinum layer with a thickness of 19 nm.

[0068] Example 17 In Example 9, the weight of the gold layer of the resistance reducing layer 23 was 0.34 mg / cm ２ A half-cell 102 of Example 17 was obtained in the same manner as in Example 9, except that the electrode 22 was changed to the electrode 22, and the current collector 21 was disposed adjacent to the gold layer that was the low-resistance layer 23. Therefore, Example 17 did not have the test electrode 22, and the low-resistance layer 23 was formed of a gold layer with a thickness of 175 nm.

[0069] Example 18 In Example 9, the graphite dispersion was changed to a graphite-silicon dispersion, and spray coating was performed to obtain a coating amount of 1.37 mg / cm ２A half-cell 102 of Example 18 was obtained in the same manner as in Example 9, except that a test electrode 22 made of graphite and silicon was incorporated and the weight of the gold layer of the resistance-reducing layer 23 was changed as shown in Table 5.

[0070] (Graphite / silicon dispersion) A graphite / silicon dispersion was prepared by dispersing 450 mg of graphite powder (KS6, average diameter 4.4 μm, manufactured by Imerys) and 45 mg of 50 nm diameter silicon powder (manufactured by Alfa Aesar) in 30 g of 99.5% ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in a molar ratio of 95:5 between the dispersed graphite and silicon.

[0071] Example 19 A negative electrode (negative electrode layer) consisting of only a test electrode 22 made of graphite and a current collector 21 that was an unpolished SUS304 disk having a diameter of 10 mm and a thickness of 0.5 mm was obtained in the same manner as in Example 9, except that the amount of graphite powder applied was changed as shown in Table 4.

[0072] Example 20: The coating amount of the test electrode 22 made of graphite and silicon was 1.717 mg / cm ２ A negative electrode (negative electrode layer) consisting of only a test electrode 22 made of graphite and silicon and a current collector 21 which was an unpolished SUS304 disk having a diameter of 10 mm and a thickness of 0.5 mm was obtained in the same manner as in Example 18, except that the above-mentioned change was made.

[0073] [Characteristics Evaluation] (Initial Charging Test) Using the half-cells 102 obtained in Examples 9, 10 to 16, 18, and 8, a CC charging test was performed at a current value of 0.05 C until the voltage reached 0.01 V relative to lithium metal. Each C rate value was calculated from the amount of graphite applied to the test electrode 22. For the half-cell obtained in Example 18, the amount of graphite applied was estimated (1.243 mg / cm) assuming that the graphite and silicon dispersed in the graphite-silicon dispersion used for spray application were applied while maintaining a weight ratio of 10:1. ２The initial charge curves obtained as a result of the test are shown in Figures 19, 22, 24, 26, 28, 30, 32, and 34, respectively. Using the half-cell 102 obtained in Example 7, CC charging was performed for 12 hours and 10 hours at currents of 0.0005 C and 0.005 C, respectively, followed by CC charging at a current of 0.05 C until the voltage reached 0 V vs. lithium metal, and then a CV charge test was performed at 0 V for 10 hours. The initial charge curve obtained as a result of the test is shown in Figure 15. The test environment was an argon atmosphere at room temperature of approximately 25°C under atmospheric pressure.

[0074] (Initial Charging Test Results) Table 6 shows the test results of the initial charging capacity.

[0075]

[0076] The weight of graphite contained in the test electrode 22 made of graphite incorporated in the half-cells 102 obtained in Examples 9, 10, and 8 was about 1 mg / cm ２ However, in Examples 9 and 10, in which the test electrode 22 made of graphite was disposed on the surface of the solid electrolyte 25 coated with a gold layer, which is the low-resistance layer 23, the current dissipation was approximately 0.2 to 0.4 mAh / cm compared to Example 8, in which the test electrode 22 made of graphite was disposed on the surface of the solid electrolyte 25 that was not coated with a gold layer (no low-resistance layer 23 was disposed). ２A larger charge capacity was obtained. Furthermore, the weight of the graphite and the weight of the gold layer of the low-resistance layer 23 contained in the test electrode 22 made of graphite and silicon incorporated in the half-cell 102 obtained in Example 18 were approximately equal to the weight of the graphite and the weight of the gold layer of the low-resistance layer 23 contained in the test electrode 22 made of graphite incorporated in the half-cell obtained in Example 9, respectively. However, it was found that a charge capacity nearly double that of Example 9 was obtained in Example 18. The weights of the graphite contained in the test electrode 22 made of graphite incorporated in the half-cells 102 obtained in Examples 11 to 16 and Example 8 were also approximately equal. However, in Examples 11 to 16, in which the test electrode 22 made of graphite was disposed on the surface of the solid electrolyte 25 coated with an indium layer, a silver layer, or a platinum layer, respectively, the charge capacity was about 0.2 to 0.3 mAh / cm compared to Example 8, in which the test electrode 22 made of graphite was disposed on the surface of the solid electrolyte 25 that was not coated with these layers. ２ That is, even when an indium layer, a silver layer, or a platinum layer was provided as the low resistance layer 23, the charge capacity was about 0.2 to 0.3 mAh / cm compared to when the low resistance layer 23 was not provided. ２ Furthermore, although the weight of graphite contained in test electrode 22 made of graphite incorporated into half-cells 102 obtained in Examples 7 and 8 was approximately the same, in Example 14, in which test electrode 22 made of graphite was placed on the surface of solid electrolyte 25 coated with a silicon layer, which was low-resistance layer 23, and CC charging was performed until the voltage reached 0 V relative to lithium metal, and CV charging was further performed for 10 hours, the charge capacity was about 0.5 mAh / cm compared to Example 8, in which test electrode 22 made of graphite was placed on the surface of solid electrolyte 25 not coated with a silicon layer, and CC charging was performed until the voltage reached 0 V relative to lithium metal, i.e., in which low-resistance layer 23 was not placed. ２ A large charging capacity was also obtained.

[0077] (Method for Measuring AC Impedance After Initial Charging Test) After the initial charging test described above, the half-cells 102 obtained in Examples 9, 10 to 16, 7, 18, and 8 were used to measure AC impedance at an applied AC voltage of 10 mV relative to the natural potential in the frequency range of 10 mHz to 1 MHz. The AC impedance spectra obtained as a result of the test are shown in Figures 20, 23, 25, 27, 29, 31, 33, 35, 16, 39, and 18, respectively. The test environment was an argon atmosphere at room temperature of approximately 25°C under atmospheric pressure.

[0078] (Results of AC Impedance Measurement After Initial Charging Test) Table 7 shows the results of AC impedance measurement after the initial charging test.

[0079]

[0080] From the AC impedance spectra obtained after the initial charging of the half-cells 102 obtained in Examples 9, 10, and 8, in the structure in which the gold layer was not sandwiched, the interfacial resistance between the solid electrolyte separator layer 25 and the test electrode 22 made of graphite was 3188 Ω cm ２ On the other hand, in the structure in which the low resistance layer 23 made of a gold layer is sandwiched, the interface resistance is about 30 Ω cm ２ It was found that the interfacial resistance was reduced to about 20 to 70 Ω cm. The half-cells obtained using the interface structure of Example 9 had significantly lower interfacial resistance and internal cell resistance. This resulted in improved charge / discharge characteristics and improved battery energy density. From the AC impedance spectra obtained after the initial charge of the half-cells 102 obtained in Examples 11 to 16 and Example 7, it was found that the interfacial resistance was reduced to about 20 to 70 Ω cm even in the structures in which an indium (In) layer, a silver (Ag) layer, a platinum (Pt) layer, and a silicon (Si) layer were sandwiched. ２ and the interface resistance (3188 Ω cm) of the half cell obtained in Comparative Example 1 (Example 8). ２), it was confirmed that the interfacial resistance of the structure was also reduced similarly to the structure in which a gold layer was sandwiched. Furthermore, the low-resistance layers 23 incorporated in the half-cells 102 obtained in Examples 10, 12, 14, and 16 were formed of metal layers with thicknesses of 10 to 30 nm, and showed low interfacial resistances almost equivalent to those of Examples 9, 11, 13, and 15 in which corresponding metal layers with thicknesses of 100 to 200 nm were incorporated. Furthermore, the AC impedance spectrum obtained after the initial charge of the half-cell 102 obtained in Example 18 showed that in the structure in which the low-resistance layer 23 made of a gold layer was sandwiched, the interfacial resistance between the solid electrolyte separator layer 25 and the test electrode 22 made of graphite and silicon was 20 Ω cm, almost equivalent to that of the half-cells obtained in Examples 9 and 10. ２ It was found that this would be the case.

[0081] (X-ray Diffraction Measurement After Initial Charging Test) After initial charging tests were performed using half-cells 102 obtained in Examples 9 and 18, half-cells 102 were disassembled in a glove box under an argon gas atmosphere, and the surface of the graphite layer that had been in contact with current collector 21 was oriented toward the X-ray irradiation side, sealed in an X-ray diffraction atmosphere separator (manufactured by Rigaku Corporation) (argon gas atmosphere), and X-ray diffraction measurement was performed using an X-ray diffractometer (Miniflex 600, manufactured by Rigaku Corporation) in a diffraction angle range of 20° to 30° (2θ). The X-ray diffraction patterns obtained as a result of the test are shown in FIGS. 5 and 28, respectively.

[0082] (X-ray diffraction measurement results after initial charge test) From the X-ray diffraction pattern after the initial charge of the half-cell 102 obtained in Example 9, it was found that the gold layer reacted with lithium to form an alloy Li ３It was found that the gold layer, which is the low-resistance layer 23, was changed to Au. This indicates that the phenomenon in which the reduction in interfacial resistance was confirmed in a structure in which the gold layer was sandwiched between the test electrode 22 and the separator layer 25 is largely due to the volume expansion during the alloying reaction. Furthermore, it was found that the reduction in interfacial resistance allowed lithium to be inserted into the graphite contained in the graphite layer near the current collector up to a charging state of Stage 2, even during CC charging at room temperature using a current value of 0.05 C. From the X-ray diffraction pattern after the initial charge of the half-cell 102 obtained in Example 18, it was found that, as in Example 9, the gold layer reacted with lithium to form the alloy Li ３ It was found that the graphite contained in the graphite-silicon layer near the current collector had changed to Au. This indicates that the volume expansion during the alloying reaction also significantly contributed to the reduction in interfacial resistance observed in a structure in which the gold layer serving as the low-resistance layer 23 was sandwiched between the test electrode 22 and the separator layer 25. However, unlike Example 9, it was found that lithium had been inserted into the graphite contained in the graphite-silicon layer near the current collector up to the stage 1 charge state during CC charging using a current value of 0.05 C at room temperature. Since no peak from crystalline silicon was observed, it was found that the silicon contained in the test electrode 22 had reacted with lithium and changed to an amorphous lithium alloy. This indicates that the volume expansion during the alloying reaction significantly contributed to the deepening of the depth of charge of graphite observed in a structure in which the test electrode 22 contained a metal that alloys with lithium and changes volume.

[0083] (Measurement of element distribution in the depth direction of the negative electrode layer) For the negative electrode layer consisting of the test electrode 22 and the current collector 21 obtained in Examples 19 and 20, a glow discharge optical emission spectrometer (GD-Profiler 2, manufactured by Horiba, Ltd.) was used to measure the element distribution in the depth direction. The element distribution in the depth direction obtained as a result of the test is shown in Figures 41 and 42, respectively. Sputtering was performed using a pulse method, and argon (Ar) and oxygen (O ２ A two-component mixed gas of these two gases in a volume ratio of 95:5 was used as the sputtering gas. The sputtering area had a diameter of 4 mm.

[0084] (Measurement Results of Element Distribution in the Depth Direction of the Negative Electrode Layer) From the element distribution in the depth direction of the negative electrode layer obtained in Example 19, it can be seen that the iron (Fe) signal, which is the signal from the SUS disk of the current collector 21, is barely observed until just before the carbon (C) signal begins to drop, indicating that the test electrode 22 is a graphite particle laminate of uniform thickness. Similarly, from the element distribution in the depth direction of the negative electrode layer obtained in Example 20, it can be seen that the iron (Fe) signal, which is the signal from the SUS disk of the current collector 21, is barely observed until just before the carbon (C) signal begins to drop, indicating that the test electrode 22 is a graphite particle laminate of uniform thickness. Furthermore, the silicon (Si) signal continues to be observed at a constant intensity until the carbon signal begins to drop, indicating that silicon is uniformly distributed within the test electrode 22.

[0085] (Initial Charging Test of Low Resistance Layer) Using the half cell 102 obtained in Example 17, a current value of 23.7 μA / cm was applied until the voltage reached 0.01 V on a lithium metal basis. ２ The CC charging test was carried out at a current value of 1.274 mg / cm when 1.00 mg of graphite was applied to a current collector 21 having a diameter of 10 mm. ２ This is the same as charging the test electrode 22 made of graphite at 0.05 C. In other words, it is almost the same current value as that used in Example 9. The initial charge curve obtained as a result of the test is shown in Figure 36. Here, the test environment was an argon atmosphere at room temperature of about 25°C under atmospheric pressure.

[0086] (Results of initial charge test of low resistance layer) The half cell 102 obtained in Example 17 did not incorporate the test electrode 22 made of graphite, but the gold layer of the low resistance layer 23 reacted with lithium to form an alloy, resulting in a charge of 0.154 mAh / cm ２ A charging capacity of 1000kJ / s was obtained.

[0087] (Method for Measuring AC Impedance of Low-Resistance Layer After Initial Charging Test) After the initial charging test described above was performed using the half-cell 102 obtained in Example 17, AC impedance was measured at an applied AC voltage of 10 mV relative to the natural potential in the frequency range of 10 mHz to 1 MHz. The AC impedance spectrum obtained as a result of the test is shown in Figure 37. Here, the test environment was an argon atmosphere at room temperature of approximately 25°C under atmospheric pressure.

[0088] (Measurement Results of AC Impedance of Low Resistance Layer After Initial Charging Test) The interface resistance calculated from the measurement results of AC impedance after the initial charging test was 31 Ω cm ２ It was found that the interface resistance was approximately the same as that of the half cells obtained in Examples 9 and 10 in which a gold layer was used as the resistance-reducing layer 23 and a test electrode 22 made of graphite was incorporated.

[0089] (Initial charging capacity of graphite layer) The half-cell 102 obtained in Example 9 incorporates the test electrode 22 made of graphite that reacts with lithium and the gold layer that is the low resistance layer 23. Therefore, the obtained initial charging capacity of 0.513 mAh / cm ２ The charge capacity of the half-cell obtained in Example 17 includes the capacities of both active materials. The half-cell obtained in Example 17 incorporates only the gold layer, which is the low resistance layer 23, and its weight is approximately the same as that of Example 9. Therefore, the charge capacity of the half-cell obtained in Example 17 is 0.154 mAh / cm ２ can be considered to be the charge capacity of the gold layer, which is the low resistance layer 23, among the charge capacities of the half-cell obtained in Example 9. That is, in the half-cell 102 obtained in Example 9, the charge capacity of the test electrode 22 made of graphite was 0.359 mAh / cm ２ (=0.513mAh / cm ２ -0.154mAh / cm ２ ) and it was found that charging with a weight capacity density of 291 mAh / g was possible.

[0090] (Initial charge capacity of graphite-silicon layer) The half-cell 102 obtained in Example 18 incorporates the test electrode 22 made of graphite and silicon that react with lithium and the gold layer that is the low resistance layer 23. Therefore, the obtained initial charge capacity of 1.124 mAh / cm ２The charge capacity of the half-cell obtained in Example 17 includes the capacities of the three active materials. The half-cell obtained in Example 17 incorporates only the gold layer, which is the low resistance layer 23, and its weight is approximately the same as that of Example 9. Therefore, the charge capacity of the half-cell 102 obtained in Example 17 is 0.154 mAh / cm ２ can be considered to be the charge capacity of the gold layer, which is the low resistance layer 23, among the charge capacities of the half-cell 102 obtained in Example 18. In other words, in the half-cell 102 obtained in Example 18, the charge capacity of the test electrode 22 made of graphite and silicon was 0.970 mAh / cm ２ (=1.124mAh / cm ２ -0.154mAh / cm ２ The charge capacity is 0.525 mAh / cm2, which is calculated from the amount of silicon applied, which is estimated assuming that the graphite dispersed in the graphite-silicon dispersion liquid used for spray application was applied while maintaining a weight ratio of 10:1 between silicon and graphite. ２ is considered to be the charge capacity of the silicon of the test electrode 22, the charge capacity of the graphite of the test electrode 22 is 0.445 mAh / cm ２It was found that a weight capacity density of 359 mAh / g was achieved during charging. Compared to the weight capacity density of the test electrode 22 consisting of graphite alone in the half-cell 102 obtained in Example 9, the graphite contained in the test electrode 22, which was a mixture of graphite and silicon in a molar ratio of 95:5, obtained in the half-cell 102 obtained in Example 18, exhibited a weight capacity density closer to the theoretical capacity. This is consistent with the results of the X-ray diffraction pattern measured after the first charge, which showed that lithium had been intercalated into the graphite up to the stage 1 charge state. Measurement of the elemental distribution in the depth direction of the anode layer revealed that silicon was uniformly distributed within the test electrode 22 consisting of graphite and silicon, which was prepared by spray coating using a graphite-silicon dispersion. Furthermore, the results of the X-ray diffraction measurement after the first charge revealed that the silicon contained in the test electrode 22 reacted with lithium and converted to an amorphous silicon alloy after charging. From these facts, it can be seen that the phenomenon in which graphite in test electrode 22 containing graphite mixed with silicon exhibited a larger weight capacity density than the weight capacity density exhibited by graphite in test electrode 22 consisting of graphite alone is largely due to the volume expansion during the alloying reaction of silicon and the formation of a network within test electrode 22 due to bonding between alloy particles, as seen in Patent Document 2 and Non-Patent Document 2.

[0091] (Charge-Discharge Cycle Test) Using the half-cell 102 obtained in Example 3, a charge-discharge test was performed at a current value of 0.05 C in a voltage range of 0.01 to 1.2 V relative to lithium metal. The initial charge-discharge curve obtained as a result of the test is shown in Figure 7, and the battery capacity versus the number of charge-discharge cycles up to 20 cycles is shown in Figure 8. Here, the test environment was an argon atmosphere at room temperature of about 25°C under atmospheric pressure.

[0092] (Charge-Discharge Cycle Test Results) From the initial charge-discharge curve of the half-cell 102 obtained in Example 3, it was found that no significant hysteresis was observed in the charge-discharge curve, and the initial coulombic efficiency was also found to be greater than 70%. From the second cycle onwards, high reversibility was observed, and no noticeable capacity loss was observed. This indicates that the half-cell 102 obtained using the interface structure of the present disclosure exhibits not only high energy density but also high stability.

[0093] The present invention relates to an all-solid-state lithium-ion secondary battery, which is expected to be a next-generation secondary battery for electric vehicles including hybrids, submarines, homes, and industries due to its electric capacity, charge / discharge and large-current characteristics, miniaturization, and safety. The present invention provides an all-solid-state lithium-ion secondary battery with low internal resistance and excellent charge / discharge characteristics. Therefore, it is believed that the present invention will greatly contribute to the development of the industry.

[0094] DESCRIPTION OF SYMBOLS 11: Negative electrode current collector 12: Negative electrode active material layer (e.g., graphite) 13: Low resistance layer 14: Negative electrode (negative electrode layer) 15: Separator layer (all-solid-state electrolyte layer, solid electrolyte, sintered body) 16: Positive electrode active material layer 17: Positive electrode current collector 18: Positive electrode (positive electrode layer) 21: Current collector 22: Test electrode (e.g., graphite) 23: Low resistance layer 25: Separator layer (all-solid-state electrolyte layer, solid electrolyte, sintered body) 26: Counter electrode (lithium foil) 27: Current collector 101: All-solid-state lithium ion secondary battery 102: All-solid-state lithium ion half-cell

Claims

1. An all-solid-state lithium ion secondary battery having at least a negative electrode layer, a separator layer, and a positive electrode layer, the negative electrode layer includes a negative electrode active material having graphite, The separator layer contains a solid electrolyte, a low resistance layer is formed between the negative electrode layer and the separator layer, the low resistance layer is made of a lithium alloy or has a substance that forms a lithium alloy when a secondary battery operation is performed in which a current flows between the negative electrode layer and the positive electrode layer.

2. 2. The all-solid-state lithium ion secondary battery according to claim 1, wherein the low resistance layer contains one or more elements selected from the group consisting of gold, silver, platinum, aluminum, tin, indium, germanium, lead, zinc, antimony, magnesium, silicon, cadmium, gallium, tellurium, and bismuth.

3. The all-solid-state lithium ion secondary battery according to claim 1 , wherein the low resistance layer has a thickness of 1 to 500 nm.

4. 2. The all-solid-state lithium ion secondary battery according to claim 1, wherein the low resistance layer is made of one or more selected from the group consisting of gold, silver, platinum, tin, indium, and silicon.

5. The all-solid-state lithium ion secondary battery in accordance with claim 1 , wherein the low resistance layer is disposed in contact with the separator layer.

6. The all-solid-state lithium-ion secondary battery according to claim 1 , wherein the low resistance layer is disposed in contact with the negative electrode layer.

7. 2. The all-solid-state lithium ion secondary battery in accordance with claim 1, wherein the solid electrolyte contained in the separator layer has a highly ion-conductive garnet-type or garnet-like crystal structure.

8. 2. The all-solid-state lithium ion secondary battery according to claim 1, wherein the solid electrolyte contained in the separator layer is one or more selected from the group consisting of a composite oxide containing Li, and ceramics having a garnet type structure, a perovskite type structure, and a LISICON type structure.

9. The all-solid-state lithium-ion secondary battery in accordance with claim 1 , wherein the negative electrode active material is made of graphite.

10. 2. The all-solid-state lithium ion secondary battery of claim 1, wherein the negative electrode active material comprises graphite and a material capable of absorbing less than 10% lithium ions.

11. The positive electrode layer contains a positive electrode active material, The positive electrode active material is LiM １ ｘ Mn ２－ｘ O ４ (However, M １ is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and 0.01≦x≦0.5), ｘ Mn （１－ｙ－ｘ） Ni ｙ M ２ ｚ O （２－ｋ） F ｌ (However, M ２ is at least one element selected from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, and is a layered compound represented by 0.8≦x≦1.2, 0<y<0.5, 0≦z≦0.5, k+l<1, −0.1≦k≦0.2, 0≦l≦0.1), LiCo １－ｘ M ３ ｘ O ２ (However, M ３ LiNi is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), １－ｘ M ４ ｘ O ２ (However, M ４ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba, and 0≦x≦0.5), lithium nickel composite oxide, LiM ５ １－ｘ N １ ｘ P.O. ４ (However, M ５ is at least one element selected from the group consisting of Fe, Mn and Co, and N １ is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba, and 0≦x≦0.5), an olivine type composite oxide represented by the formula: Li ４ Ti ５ O １２ 2. The all-solid-state lithium ion secondary battery according to claim 1, comprising at least one selected from the group consisting of lithium titanium composite oxides represented by the formula:

12. 12. A method for producing the all-solid-state lithium ion secondary battery according to claim 1 , wherein the low resistance layer is formed by one or more methods selected from the group consisting of a sputtering method, a vapor deposition method, a coating method, and a foil pasting method.