JP7780932B2 - Manufacturing method for all-solid-state lithium secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing an all-solid-state lithium secondary battery.

In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is pinning its hopes on reducing carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to commercializing these vehicles.

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

Currently widely used lithium secondary batteries use a flammable organic electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

Therefore, in recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ion conductors capable of ion conduction in solids. Therefore, all-solid-state lithium secondary batteries, unlike conventional liquid-based lithium secondary batteries, do not inherently encounter various problems caused by flammable organic electrolytes. In general, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the output density and energy density of batteries. For example, elemental sulfur (S ₈ ) has an extremely large theoretical capacity of approximately 1670 mAh/g and is advantageous in that it is low cost and abundant.

Meanwhile, metallic lithium, a negative electrode active material that supplies lithium ions to the positive electrode, is known as a high-capacity negative electrode material that can be used in all-solid-state batteries. However, in all-solid-state batteries that use metallic lithium as the negative electrode active material and a sulfide solid electrolyte as the solid electrolyte, the battery characteristics may deteriorate as a result of a reaction between the metallic lithium and the sulfide solid electrolyte.

In order to address these issues, Patent Document 1 proposes an all-solid-state battery in which an intermediate layer that conducts lithium ions is formed between a negative electrode active material layer using metallic lithium and a solid electrolyte layer containing a sulfide solid electrolyte. According to Patent Document 1, by forming an intermediate layer between the metallic lithium and the solid electrolyte layer, it is possible to prevent contact between the metallic lithium and the solid electrolyte, and suppress reductive decomposition of the solid electrolyte that would otherwise result from contact with the metallic lithium. As a result, a stable charge/discharge cycle can be achieved.

Japanese Patent Application Laid-Open No. 2019-125578

However, it has been found that while the technology described in Patent Document 1 improves the input/output characteristics of the battery, the placement of the intermediate layer increases the battery's volume, which can result in a decrease in energy density.

The present invention therefore aims to provide a means for improving the input/output characteristics and energy density of all-solid-state lithium secondary batteries that use metallic lithium as the negative electrode active material.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by first placing metallic lithium on the positive electrode side (between the positive electrode current collector and the positive electrode active material layer, or in the positive electrode active material layer), and then charging the battery to deposit metallic lithium on the negative electrode current collector, thereby completing the present invention.

That is, one aspect of the present invention is a method for manufacturing an all-solid-state lithium secondary battery using metallic lithium as the negative electrode active material, the method including the steps of: preparing a laminate having, on a negative electrode current collector, a solid electrolyte layer containing a solid electrolyte, a positive electrode active material layer containing a positive electrode active material, and a positive electrode current collector, in this order, the metallic lithium being disposed between the positive electrode active material layer and the positive electrode current collector or in the positive electrode active material layer; and performing a charging reaction on the laminate to precipitate the metallic lithium between the solid electrolyte layer and the negative electrode current collector, wherein the positive electrode current collector is made of a material that does not alloy with lithium, or the surface of the positive electrode current collector adjacent to the positive electrode active material layer is coated with a material that does not alloy with lithium.

The manufacturing method of the present invention makes it possible to obtain an all-solid-state lithium secondary battery using metallic lithium as the negative electrode active material, which has excellent input/output characteristics and energy density.

FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 3 is a schematic diagram showing a method for manufacturing an all-solid-state lithium secondary battery according to one embodiment of the present invention.

The following describes the above-described embodiments of the present invention with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings are exaggerated for convenience of explanation and may differ from the actual proportions. The following describes the present invention using a stacked-type (internal parallel connection) all-solid-state lithium secondary battery, which is one form of secondary battery. As described above, the solid electrolyte that constitutes an all-solid-state lithium secondary battery is a material composed primarily of an ion conductor capable of ion conduction in a solid. Therefore, all-solid-state lithium secondary 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 lithium secondary batteries. Furthermore, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally has the advantage of significantly improving the battery's output density and energy density.

Figure 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. Figure 2 is a cross-sectional view taken along line 2-2 in Figure 1. The layered structure allows the battery to be compact and have a high capacity. This specification will be described in detail using the flat-layered, non-bipolar lithium secondary battery shown in Figures 1 and 2 (hereinafter simply referred to as a "layered battery"). However, when viewed from the perspective of the internal electrical connection configuration (electrode structure) of the lithium secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

As shown in Figure 1, the stacked battery 10a has a flat, rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and its periphery is heat-sealed, sealing the power generating element 21 with the negative electrode current collector 25 and positive electrode current collector 27 extending to the outside.

The lithium secondary battery according to this embodiment is not limited to a laminated, flat shape. A wound lithium secondary battery may be cylindrical, or may be a cylindrical battery modified to have a flat, rectangular shape, and is not particularly limited. The cylindrical battery may use a laminate film or a conventional cylindrical can (metal can) as its exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This configuration can achieve weight reduction.

Furthermore, there are no particular limitations on how the current collectors (25, 27) shown in Figure 1 are removed. The negative current collector 25 and the positive current collector 27 may be pulled out from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in Figure 1. Furthermore, in a wound-type lithium secondary battery, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

As shown in Figure 2, the stacked battery 10a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where the actual charge/discharge reaction takes place, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a structure in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order so that one positive electrode active material layer 15 faces the adjacent negative electrode active material layer 13 with the solid electrolyte layer 17 interposed between them. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute a single cell layer 19. Therefore, the stacked battery 10a shown in Figure 2 can be said to have a structure in which a plurality of cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 2, the outermost positive electrode current collectors located on both outermost layers of the power generating element 21 each have a positive electrode active material layer 15 disposed on only one side, but active material layers may be provided on both sides. In other words, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. In some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and positive electrode, respectively, without using current collectors (11', 11").

The negative electrode current collector 11' and the positive electrode current collector 11" are respectively fitted with a negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the edges of the laminate film 29, which is the battery outer casing, and extend to the outside of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11" and the negative electrode current collector 11' of each electrode via a positive electrode lead and a negative electrode lead (not shown) by ultrasonic welding, resistance welding, or the like, as necessary.

The main components of the all-solid-state lithium secondary battery obtained by the method of this embodiment are described below.

[Current collector]
The current collector functions as a medium for the transfer of electrons from the electrode active material layer, and may be made of, for example, a metal or a conductive resin.

The current collector may have a single-layer structure made of a single material, or it may have a laminated structure made of an appropriate combination of layers made of these materials.

In this embodiment, the positive electrode current collector is made of a material that does not alloy with lithium, or the surface of the positive electrode current collector adjacent to the positive electrode active material layer is coated with a material that does not alloy with lithium. If the material is alloyable with lithium, the alloying may cause corrosion of the positive electrode current collector, resulting in a decrease in conductivity. Examples of materials that do not alloy with lithium include metals such as nickel, copper, and stainless steel. Examples of materials coated with a material that does not alloy with lithium include aluminum coated with the above metals and carbon-coated aluminum. Of these, stainless steel, nickel, or carbon-coated aluminum are preferred from the standpoints of corrosion resistance and high conductivity. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 1 to 1,000 μm.

The negative electrode current collector can be made of, for example, stainless steel, copper, or nickel. There are no particular restrictions on the thickness of the negative electrode current collector, but it is, for example, 1 to 1000 μm.

[Negative electrode active material layer]
In the lithium secondary battery according to the present embodiment, metallic lithium is deposited on the negative electrode current collector by a charging reaction. The layer of metallic lithium deposited on the negative electrode current collector during this charging reaction is the negative electrode active material layer of the lithium secondary battery according to the present embodiment.

The thickness of the metallic lithium layer serving as the negative electrode active material layer varies depending on the configuration of the intended secondary battery, but is, for example, within the range of 0.1 to 1000 μm, preferably 1 to 1000 μm, and more preferably 10 to 500 μm during charging.

[Solid electrolyte layer]
In the stacked battery according to the embodiment shown in FIGS. 1 and 2 , the solid electrolyte layer is interposed between the above-described positive electrode active material layer and negative electrode active material layer, and is a layer that essentially contains a solid electrolyte.

There are no particular restrictions on the specific form of the solid electrolyte contained in the solid electrolyte layer. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes, but sulfide solid electrolytes are preferred. Because sulfide solid electrolytes are prone to reductive decomposition by metallic lithium, the effects of the present invention can be more pronounced.

Examples of the sulfide solid electrolyte include LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S ₅ , LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , Li ₃ PS ₄ , Li ₂ S-P ₂ S ₅ -LiI, Li ₂ S-P ₂ S ₅ -Li ₂ O, Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -LiBr, _Li2S - _SiS2 -LiCl, Li ₂ S—SiS ₂ —B ₂ S ₃ —LiI, Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, Li ₂ S—B ₂ S ₃ , Li ₂ S—P ₂ S ₅ —Z _m S _n (where m and n are positive numbers and Z is Ge, Zn or Ga), Li ₂ S—GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (where x and y are positive numbers and M is P, Si, Ge, B, Al, Ga or In), etc. The term "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other terms.

The sulfide solid electrolyte may have, for example, a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. Examples of sulfide solid electrolytes having a Li ₃ PS ₄ skeleton include LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , and Li ₃ PS ₄ . Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P-S solid electrolytes known as LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes that may be used include LGPS, which is represented by Li _(4-x) Ge _(1-x) P _x S ₄ (where x satisfies 0<x<1). Among these, a sulfide solid electrolyte containing P is preferred, and the sulfide solid electrolyte is more preferably a material containing Li ₂ S—P ₂ S ₅ as a main component. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li ₆ PS ₅ X (wherein X is Cl, Br, or I, preferably Cl).

Furthermore, when the sulfide solid electrolyte is a Li ₂ S—P ₂ S ₅ system, the molar ratio of Li ₂ S and P ₂ S ₅ is preferably within the range of Li ₂ S:P ₂ S ₅ = 50:50 to 90:10, and particularly preferably Li ₂ S:P ₂ S ₅ = 70:30 to 80:20.

The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. Sulfide glass can be obtained, for example, by mechanical milling (ball mill, etc.) a raw material composition. Crystallized sulfide glass can be obtained, for example, by heat treating sulfide glass at a temperature equal to or higher than the crystallization temperature. The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is preferably, for example, 1×10 ⁻⁵ S/cm or more, and more preferably 1×10 ⁻⁴ S/cm or more. The ionic conductivity value of the solid electrolyte can be measured by an AC impedance method.

Examples of oxide solid electrolytes include compounds having a NASICON structure. Examples of compounds having a NASICON structure include compounds (LAGP) represented by the general formula Li1 _{+ xAlxGe2 -x} ( _PO4 ) ₃ (0≦x≦2) and compounds (LATP) represented by the general formula Li1 _{+ xAlxTi2 -x} ( _PO4 ) _{3 (} 0≦ _{x ≦2). Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li0.34La0.51TiO3), LiPON (e.g., Li2.9PO3.3N0.46 ) , and LiLaZrO (} e.g., _{Li7La3Zr2O12 )} .

In a preferred embodiment of the present invention, the content of the sulfide solid electrolyte in the solid electrolyte layer is, for example, 50% by mass or more, preferably more than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

Examples of the shape of the solid electrolyte include particulate shapes such as spherical and oval spheres, and thin film shapes. When the solid electrolyte is particulate, its average particle size ( _D50 ) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle size ( _D50 ) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

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.

The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. The binder is not particularly limited, but examples include the following materials:

Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Fluorine resins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE ...hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylide Examples of suitable fluororubbers include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), as well as epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

The thickness of the solid electrolyte layer varies depending on the configuration of the intended all-solid-state lithium secondary battery, but from the perspective of improving the volumetric energy density of the battery, it is preferably 1000 μm or less, and more preferably 700 μm or less. On the other hand, there is no particular lower limit on the thickness of the solid electrolyte layer, but it is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

[Cathode active material layer]
1 and 2, the positive electrode active material layer contains a positive electrode active material. The positive electrode active material is not particularly limited, but from the viewpoint of increasing the output density and energy density of the battery, it is preferable to use a positive electrode active material containing sulfur.

(Positive electrode active material containing sulfur)
The type of sulfur-containing positive electrode active material is not particularly limited, but includes elemental sulfur (S) and lithium sulfide (Li ₂ S), as well as particles or thin films of organic sulfur compounds or inorganic sulfur compounds. Any substance can be used as long as it utilizes the oxidation-reduction reaction of sulfur to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprenes, rubeanic acid (dithiooxamide), polycarbon sulfides, and the like, as typified by the compounds described in International Publication No. 2010/044437. Among these, disulfide compounds, sulfur-modified polyacrylonitriles, and rubeanic acid are preferred, with sulfur-modified polyacrylonitrile being particularly preferred. As disulfide compounds, dithiobiurea derivatives, those having a thiourea group, thioisocyanate, or thioamide group are more preferred. Here, sulfur-modified polyacrylonitrile is a modified polyacrylonitrile containing sulfur atoms, obtained by mixing sulfur powder and 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 portion of S is bonded to C. The compound described in this document has strong peak signals near 1330 cm ^-1 and 1560 cm ^-1 in its Raman spectrum, and further has peaks near 307 cm ^-1 , 379 cm ^-1 , 472 cm ^-1 , and 929 cm ^-1 . On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, and specific examples include elemental sulfur (S), Li ₂ S, S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , NiS, NiS ₂ , CuS, FeS ₂ , Li ₂ S, MoS ₂ , MoS ₃ , MnS, MnS ₂ , CoS, CoS ₂ , etc. Among these, elemental sulfur (S), Li ₂ S, S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , FeS ₂ and MoS ₂ are preferred, with elemental sulfur (S), Li ₂ S, TiS ₂ and FeS ₂ being more preferred, and elemental sulfur (S) being particularly preferred from the viewpoint of high capacity and discharge starting. As the elemental sulfur (S), α-sulfur, β-sulfur, or γ-sulfur having an _S8 structure can be used.

The positive electrode active material layer may contain a sulfur-free positive electrode active material. Furthermore, in addition to the sulfur-containing positive electrode active material, the positive electrode active material layer may further contain a sulfur-free positive electrode active material. Examples of sulfur-free positive electrode active materials include layered rock salt active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , and Li(Ni—Mn—Co)O ₂ ; spinel active materials such as LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ ; olivine active materials such as LiFePO ₄ and LiMnPO ₄ ; and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ . Examples of oxide active materials other than those mentioned above include V ₂ O ₅ and Li ₄ Ti ₅ O ₁₂ .

In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those listed above may also be used. However, the proportion of the content of the sulfur-containing positive electrode active material relative to the total amount (100% by mass) of the positive electrode active material is, for example, 50% by mass or more, preferably more than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

(Positive electrode material)
In a preferred embodiment of the present invention, the positive electrode active material layer preferably contains a positive electrode material in which a sulfur-containing positive electrode active material and a solid electrolyte such as a sulfide solid electrolyte are supported in the pores of a conductive material having pores.

There are no particular restrictions on the specific form of the conductive material, and conventionally known materials can be used as appropriate. From the standpoints of excellent conductivity, ease of processing, and ease of designing the desired pore distribution, the conductive material is preferably a carbon material. In particular, a (porous) carbon material having pores is more preferable because it can form more reaction regions (three-phase interfaces), but conductive materials other than carbon materials may also be used.

Examples of carbon materials with micropores include activated carbon, Ketjen Black (registered trademark; highly conductive carbon black), (oil) furnace black, channel black, acetylene black, thermal black, lamp black, and other carbon blacks; as well as carbon particles (carbon supports) made from coke, natural graphite, and artificial graphite. It is preferable that the carbon material be primarily carbon. Here, "primarily carbon" refers to containing carbon atoms as the primary component, and encompasses both materials consisting solely of carbon atoms and materials consisting essentially of carbon atoms. "Substantially consisting of carbon atoms" means that impurities of approximately 2-3% by mass or less can be tolerated.

The BET specific surface area of the pore-containing conductive material (preferably, a carbon material) is preferably 200 m ² /g or more, more preferably 500 m ² /g or more, even more preferably 800 m ² /g or more, particularly preferably 1200 m ² /g or more, and most preferably 1500 m ² /g or more. The pore volume of the pore-containing conductive material (preferably, a carbon material) is preferably 1.0 mL/g or more, more preferably 1.3 mL/g or more, and even more preferably 1.5 mL/g or more. If the BET specific surface area and pore volume of the conductive material are within these ranges, a sufficient amount of pores can be maintained, and thus a sufficient amount of positive electrode active material can be maintained. The BET specific surface area and pore volume of the conductive material can be measured by nitrogen adsorption/desorption measurement. This nitrogen adsorption/desorption measurement is performed using a BELSORP mini manufactured by Microtrac-Bell Corporation at a temperature of -196°C using a multipoint method. The BET specific surface area is determined from the adsorption isotherm in the relative pressure range of 0.01 < P/P ₀ < 0.05. The pore volume is also determined from the volume of adsorbed N ₂ at a relative pressure of 0.96.

The average pore diameter of the conductive material is not particularly limited, but is preferably 50 nm or less, and particularly preferably 30 nm or less. If the average pore diameter of the conductive material is within this range, electrons can be sufficiently supplied to the sulfur-containing positive electrode active material located far from the pore walls. The average pore diameter of the conductive material can be calculated by nitrogen adsorption/desorption measurements, similar to the calculation of the BET specific surface area and pore volume.

When the conductive material is particulate, the average particle diameter (primary particle diameter) is not particularly limited, but is preferably 0.05 to 50 μm, more preferably 0.1 to 20 μm, and even more preferably 0.5 to 10 μm. In this specification, the "particle diameter of the conductive material" refers to the longest distance L between any two points on the contour line of the conductive material. The value of the "average particle diameter of the conductive material" is calculated as the average particle diameter of particles observed within several to several tens of fields of view using an observation tool such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

In the above positive electrode material, the specific form of the sulfur-containing positive electrode active material is the same as above. The specific form of the solid electrolyte is also the same as above.

In the above-mentioned positive electrode material, the mass ratio of the conductive material to the positive electrode active material is not particularly limited, but is, for example, 1:1 to 1:10.

By using a positive electrode material with the above-described structure, a three-phase interface where the positive electrode active material, conductive material, and solid electrolyte coexist is sufficiently formed not only on the surface of the conductive material but also inside the pores, allowing the charge/discharge reaction to proceed sufficiently. As a result, the internal resistance of the battery is sufficiently reduced, and it is believed that the charge/discharge rate characteristics are significantly improved. Furthermore, capacity characteristics and cycle durability can be improved.

One example of a method for producing a cathode material having the above configuration is as follows: First, a solution is prepared by dissolving a solid electrolyte in an organic solvent, and a conductive material is dispersed therein to obtain a dispersion. Next, after removing the solvent, the mixture is heat-treated at a temperature of approximately 150 to 600°C for approximately 1 to 5 hours. This results in a conductive material in which the solid electrolyte is impregnated into the pores of the conductive material. Next, the resulting conductive material is dry-mixed with a cathode active material containing sulfur, and then heat-treated for approximately 1 to 5 hours at a temperature that melts the cathode active material. This melts the cathode active material and penetrates into the pores of the conductive material, resulting in a composite material in which the cathode active material and the solid electrolyte are disposed (filled) inside the pores of the conductive material. While the composite material obtained in this manner can be used as a cathode material as is, it is preferable to further add and mix the solid electrolyte to the composite material, and process it in an apparatus such as a ball mill as needed to obtain a cathode material.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 35 to 99 mass%, and more preferably in the range of 40 to 90 mass%. Note that this content value is calculated based on the mass of only the positive electrode active material, excluding the conductive material and solid electrolyte.

The positive electrode active material layer may further contain a conductive additive (one that does not hold the positive electrode active material or solid electrolyte inside the pores) and/or a binder. Examples of conductive additives include carbon materials with pores such as those described above, as well as carbon materials without pores, such as graphene, carbon nanotubes, carbon nanofibers, carbon nanohorns, and fullerenes.

Similarly, the positive electrode active material layer preferably contains a solid electrolyte, more preferably a sulfide solid electrolyte. Even when using a positive electrode material in which a sulfur-containing positive electrode active material and a solid electrolyte such as a sulfide solid electrolyte are supported within the pores of the porous conductive material, it is preferable to further contain a solid electrolyte in addition to the positive electrode material, and it is particularly preferable to further contain a sulfide solid electrolyte. In this case, although not particularly limited, it is preferable to adjust the positive electrode active material layer so that, for example, 10 to 100 parts by mass of the solid electrolyte and 10 to 100 parts by mass of the conductive material are contained per 100 parts by mass of the positive electrode active material. Specific and preferred forms of solid electrolytes such as sulfide solid electrolytes can be similarly adopted as those described above in the section on the solid electrolyte layer.

The thickness of the positive electrode active material layer is not particularly limited, but is, for example, in the range of 10 to 1000 μm.

[Method of manufacturing all-solid-state lithium secondary battery]
One aspect of the present invention is a method for producing an all-solid-state lithium secondary battery using metallic lithium as a negative electrode active material, the method including the steps of: preparing a laminate having, on a negative electrode current collector, a solid electrolyte layer containing a solid electrolyte, a positive electrode active material layer containing a positive electrode active material, and a positive electrode current collector in this order, the metallic lithium being disposed between the positive electrode active material layer and the positive electrode current collector or in the positive electrode active material layer; and performing a charging reaction on the laminate to precipitate the metallic lithium between the solid electrolyte layer and the negative electrode current collector, wherein the positive electrode current collector is made of a material that does not alloy with lithium, or a surface of the positive electrode current collector adjacent to the positive electrode active material layer is coated with a material that does not alloy with lithium.

Batteries using metallic lithium as the anode are expected to have high energy densities because they have a high specific capacity and a low reduction potential relative to the standard hydrogen electrode. However, batteries using metallic lithium as the anode have the problem of being prone to short-circuiting during charging. This problem tends to be particularly pronounced in batteries that use a cathode active material containing sulfur.

After investigating the causes, it was found that an inorganic coating _such as _Li2O or _Li2CO3 is formed on the surface of metallic lithium (lithium metal foil) during the battery manufacturing process. This inorganic coating reacts with the solid electrolyte, forming a resistive layer with low ionic conductivity on the surface of metallic lithium. This results in uneven current distribution on the surface of metallic lithium, the negative electrode active material, during the battery reaction. This leads to uneven deposition of lithium during charging, resulting in the formation of dendrites, making short circuits more likely. This can result in a deterioration in the input/output characteristics of the battery.

To address this issue, the aforementioned Patent Document 1 employs a method of forming an intermediate layer that conducts lithium ions on the surface of metallic lithium. By forming an intermediate layer between the metallic lithium and the solid electrolyte layer, contact between the metallic lithium and the solid electrolyte can be suppressed, and reductive decomposition of the solid electrolyte due to contact with metallic lithium can be suppressed.

As described above, the technology in Patent Document 1 prevents contact between metallic lithium and the solid electrolyte layer, thereby suppressing side reactions between the inorganic coating on the surface of metallic lithium and the solid electrolyte, making it possible to obtain a battery that can be charged and discharged without forming a resistance layer. However, the method described in Patent Document 1 requires the formation of an intermediate layer with a finite thickness between the solid electrolyte layer and metallic lithium, which is the negative electrode active material, which reduces the volumetric energy density of the battery.

In order to suppress the formation of an inorganic coating, a method is known in which lithium is vapor-deposited onto a solid electrolyte layer to form a negative electrode active material layer. However, with this method, the application of thermal energy can cause a reaction between the lithium and the solid electrolyte, which can lead to the formation of a resistive layer at the contact surface between the lithium and the solid electrolyte layer. As a result, even when this method is used, short circuits are more likely to occur, resulting in reduced input/output characteristics. Furthermore, the need to create a vacuum during manufacturing poses cost challenges.

In contrast, the method of the present invention first places metallic lithium on the positive electrode side (between the positive electrode current collector and the positive electrode active material layer, or in the positive electrode active material layer), and then charges the battery to deposit metallic lithium on the negative electrode current collector, forming a negative electrode active material layer. In other words, the battery manufacturing process does not include a step of bringing metallic lithium (lithium metal foil) into contact with the solid electrolyte layer. Therefore, the inorganic coating on the surface of the metallic lithium does not come into contact with the solid electrolyte, thereby suppressing the formation of a resistance layer. This prevents uneven lithium deposition during the battery reaction, enabling charge and discharge without short circuits. Furthermore, because there is no need to form an intermediate layer between the negative electrode active material layer and the solid electrolyte layer, there is no reduction in energy density due to the increased volume caused by forming the intermediate layer.

Figure 3 is a schematic diagram illustrating an outline of a method for manufacturing an all-solid-state lithium secondary battery according to one embodiment of the present invention. As shown in Figure 3, in the method according to this embodiment, first, a laminate 100 is prepared as a battery precursor. Specifically, an anode current collector 111', a solid electrolyte layer 117, a cathode active material layer 115, metallic lithium 113', and a cathode current collector 111" are laminated in this order to produce the laminate 100. Next, when a charging reaction is performed on this laminate, metallic lithium 113', which has a low reduction potential, is released as lithium ions and precipitates on the anode current collector 111'. This results in an all-solid-state lithium secondary battery 100a having an anode active material layer 113 made of metallic lithium between the solid electrolyte layer 117 and the anode current collector 111'.

Note that while Figure 3 shows an example of the laminate 100 in which the metallic lithium 113' is disposed between the positive electrode active material layer 115 and the positive electrode current collector 111", the metallic lithium 113' may also be disposed within the positive electrode active material layer 115 (not shown). A specific example of a configuration in which the metallic lithium 113' is disposed within the positive electrode active material layer 115 is one in which the positive electrode active material layer 115 has a two-layer structure and the metallic lithium 113' is disposed between the two positive electrode active material layers.

There are no particular limitations on the method for producing the laminate, as long as it does not include a step of contacting metallic lithium with a solid electrolyte layer (e.g., stacking the metallic lithium and solid electrolyte layer so that they are in close contact with each other). The laminate can be produced, for example, according to the following manufacturing method. First, a powder material of the solid electrolyte is pressed to form a pre-formed solid electrolyte layer. Next, a cathode mixture containing a cathode active material for forming the cathode active material layer, metallic lithium, and a cathode current collector are placed on one side of the pre-formed solid electrolyte layer in this order, and these materials are then pressed sequentially or simultaneously to form a pre-formed structure.

The metallic lithium disposed between the positive electrode active material layer and the positive electrode current collector can be, for example, a lithium metal foil having a thickness of 0.1 to 1000 μm, preferably 1 to 1000 μm, and more preferably 10 to 500 μm. The thickness of the lithium metal foil can be appropriately set depending on the desired thickness of the negative electrode active material.

Next, a negative electrode current collector is placed on the other side of the solid electrolyte layer. Then, by applying pressure again, a laminate can be obtained in which a positive electrode current collector, metallic lithium, a positive electrode active material layer, a solid electrolyte layer, and a negative electrode current collector are stacked in this order.

The laminate is preferably produced in an inert gas atmosphere with a controlled dew point. For example, it can be produced in an inert gas atmosphere with a dew point of -60°C or lower.

Next, a charging reaction is carried out on the laminate, thereby depositing the metallic lithium between the solid electrolyte layer and the negative electrode current collector. There are no particular restrictions on the method for carrying out the charging reaction, and any known method can be used as appropriate.

The conditions for the above charging reaction are not particularly limited. The conditions for the charging reaction are not particularly limited and can be set depending on the positive electrode active material used. For example, the above charging reaction is not particularly limited, but can be performed at a temperature of, for example, 15 to 35°C, by constant current charging or constant current/constant voltage charging. The voltage conditions for the charging reaction are also not particularly limited, but for example, if the charge termination voltage is set to approximately 2.8 to 3.3 V, metallic lithium on the positive electrode side can be effectively deposited on the negative electrode current collector.

Furthermore, there are no particular restrictions on the conditions for discharging after the above charging reaction has occurred. For example, discharging can be performed by constant current discharge at a temperature of 15 to 35°C. There are also no particular restrictions on the voltage conditions during discharging, but for example, the discharge termination voltage can be set to approximately 0.8 to 1.3 V.

The above charging reaction and subsequent discharging may also serve as the initial charging and discharging process for the battery.

Although not particularly limited, the all-solid-state lithium secondary battery of this embodiment preferably does not contain either lithium oxide or lithium carbonate on the surface of the solid electrolyte layer that contacts metallic lithium. This prevents lithium oxide and lithium carbonate from reacting with the solid electrolyte layer to form a resistive layer. This prevents uneven lithium deposition and short circuits during charging, resulting in a battery with excellent charge-discharge characteristics. More preferably, the battery does not contain oxides, nitrides, or _carbonates (e.g., _Li2CO3 , _Li3N , _LiO2 ) that may be generated as a result of reactions between metallic lithium and gases such as _CO2 , _N2 , and _O2 , as well as reaction products of these with the solid electrolyte. The presence of oxides and carbonates, including lithium oxide and lithium carbonate, on the surface of the solid electrolyte layer can be confirmed by, for example, determining the composition of elements present in the material using X-ray photoelectron spectroscopy (XPS, ESCA).

That is, one aspect of the present invention provides an all-solid-state lithium secondary battery having, on an anode current collector, an anode active material layer composed of metallic lithium, a solid electrolyte layer containing a solid electrolyte, a cathode active material layer containing a cathode active material, and a cathode current collector, in this order; the cathode current collector is made of a material that does not alloy with lithium, or the surface of the cathode current collector adjacent to the cathode active material layer is coated with a material that does not alloy with lithium; and the surface of the solid electrolyte layer that comes into contact with the metallic lithium does not contain either lithium oxide or lithium carbonate.

The all-solid-state lithium secondary battery of this embodiment is not particularly limited, but it is preferable that the total content of lithium oxide and lithium carbonate present in the solid electrolyte layer be 1 mass % or less relative to the total amount of the solid electrolyte layer. This prevents the lithium oxide and lithium carbonate from reacting with the solid electrolyte layer to form a resistance layer. This prevents uneven lithium deposition during charging, which can lead to short circuits, and a battery with excellent charge/discharge characteristics can be obtained.

In a preferred embodiment of the present invention, the solid electrolyte layer is composed solely of a solid electrolyte that contains neither oxygen atoms nor carbon atoms, and the surface of the solid electrolyte layer that comes into contact with metallic lithium contains neither oxides nor carbonates. With this configuration, the effects of the present invention can be achieved even more significantly.

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

[Positive electrode lead and negative electrode lead]
Although not shown, the current collectors (11′, 11″) and the current collector plates (25, 27) may be electrically connected via positive and negative electrode leads. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive and negative electrode leads. The portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, causing electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element as shown in Figures 1 and 2 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 its 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.

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

The above describes one embodiment of an all-solid-state lithium secondary battery, but the present invention is not limited to the configuration described in the above embodiment and can be modified as appropriate based on the claims.

For example, the lithium secondary battery according to the present invention can be used in a bipolar battery, which includes a bipolar electrode having a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector.

The present invention will be explained in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, all operations were performed at room temperature (25°C).

[Example 1]
(Production of test cells (all-solid-state lithium secondary batteries))
The battery was fabricated in a glove box in an argon atmosphere with a dew point of −68° C. or lower.

(Preparation of solid electrolyte-impregnated carbon)
In a glove box with an argon atmosphere having a dew point of -68°C or less, 0.500 g of sulfide solid electrolyte (Li ₆ PS ₅ Cl, manufactured by Ampcera) was added to 100 mL of ultra-dehydrated ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and thoroughly stirred to dissolve the solid electrolyte in ethanol. 1.00 g of carbon (activated carbon, MSC-30, manufactured by Kansai Thermochemical Co., Ltd.) was added to the obtained solid electrolyte ethanol solution and thoroughly stirred to thoroughly disperse the carbon in the solution. The container containing this carbon dispersion was connected to a vacuum device, and the container was reduced in pressure to 1 Pa or less using an oil rotary pump while stirring the carbon dispersion in the container with a magnetic stirrer. Because the solvent ethanol volatilizes under reduced pressure, the ethanol was removed over time, and the carbon impregnated with the solid electrolyte remained in the container. After removing the ethanol under reduced pressure in this way, the solid electrolyte-impregnated carbon was prepared by heating to 180°C under reduced pressure and heat-treating for 3 hours.

Preparation of sulfur/solid electrolyte/carbon composites by hot impregnation with sulfur
In a glove box with an argon atmosphere having a dew point of −68° C. or lower, 2.50 g of elemental sulfur (manufactured by Aldrich) was added to 0.750 g of the solid electrolyte-impregnated carbon prepared above and thoroughly mixed in an agate mortar. The mixed powder was then placed in a sealed pressure-resistant autoclave and heated at 170° C. for 3 hours to melt the sulfur and impregnate the solid electrolyte-impregnated carbon with the sulfur. This produced a sulfur/solid electrolyte/carbon composite.

(Preparation of sulfur-containing positive electrode material)
In a glove box with an argon atmosphere and a dew point of −68°C or lower, 40 g of 5 mm diameter zirconia balls, 0.130 g of the sulfur/solid electrolyte-impregnated carbon prepared above, and 0.070 g of solid electrolyte (Li ₆ PS ₅ Cl, manufactured by Ampcera) were placed in a 45 mL zirconia container and milled at 370 rpm for 6 hours in a planetary ball mill (Premium line P-7, manufactured by Fritsch GmbH). The sulfur-containing positive electrode material had a composition of sulfur:solid electrolyte:carbon=50:40:10.

(Preparation of test cell)
A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a cylindrical tube jig manufactured by Macor (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm), and 80 mg of a sulfide solid electrolyte ( _Li6PS5Cl manufactured by Ampcera) was placed from the top of the _cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and the tube was pressed using a hydraulic press at a pressure of 75 MPa for 3 minutes to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm in the cylindrical tube jig.

Next, the cylindrical convex punch inserted from above was removed, and 7.5 mg of the sulfur-containing positive electrode material prepared above was placed on one side of the solid electrolyte layer inside the cylindrical tube. The cylindrical convex punch was then inserted again from above and pressed at a pressure of 300 MPa for 3 minutes, forming a positive electrode active material layer with a diameter of 10 mm and a thickness of approximately 0.06 mm on one side of the solid electrolyte layer. Next, the cylindrical convex punch inserted from above was removed again, and a lithium foil (manufactured by Honjo Metals Co., Ltd., thickness 0.10 mm) punched to a diameter of 10 mm and a SUS430 foil (thickness 0.010 mm) punched to a diameter of 10 mm as a positive electrode current collector were placed, in that order, on top of the positive electrode active material layer. The cylindrical convex punch was then inserted again from above.

Next, the lower cylindrical convex punch was removed, and a piece of SUS foil (thickness: 0.010 mm) punched to a diameter of 10 mm was inserted into the cylindrical tube jig from the bottom, and the cylindrical convex punch was then inserted again, and the jig was then restrained in a metal casing at a pressure equivalent to 100 MPa.

In this way, a laminate was obtained in which the negative electrode current collector, solid electrolyte layer, positive electrode active material layer, lithium metal foil, and positive electrode current collector were stacked in this order.

Next, the laminate prepared above was charged, and metallic lithium was precipitated between the negative electrode current collector and the solid electrolyte layer. Specifically, the laminate was placed in a constant temperature bath set at 25°C, and after the temperature of the laminate became constant, a charge-discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) was used to perform 3.1 V constant current constant voltage (CCCV) charging at a current density of 0.1 mA/ ^cm² with a current cutoff of 0.05 mA/ ^cm² . The battery was then discharged at a rate of 0.03 C according to the battery design. In this way, an all-solid-state lithium secondary battery was obtained.

[Example 2]
An all-solid-state lithium secondary battery was produced in the same manner as in Example 1 described above, except that in the production of the test cell, the positive electrode current collector was changed from SUS430 foil to Ni foil (thickness: 0.020 mm).

[Example 3]
An all-solid-state lithium secondary battery was produced in the same manner as in Example 1 described above, except that in the production of the test cell, the positive electrode current collector was changed from SUS430 foil to carbon-coated aluminum foil (manufactured by Showa Denko K.K., carbon layer thickness: 1 μm, aluminum layer thickness: 20 μm).

[Comparative Example 1]
A sulfur-containing positive electrode material was prepared in the same manner as in Example 1 above.

A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a cylindrical tube jig manufactured by Macor (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm), and 80 mg of a sulfide solid electrolyte ( _Li6PS5Cl manufactured by Ampcera) was placed from the top of the _cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and the tube was pressed using a hydraulic press at a pressure of 75 MPa for 3 minutes to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm in the cylindrical tube jig.

Next, the cylindrical convex punch inserted from above was removed, and 7.5 mg of the sulfur-containing positive electrode material prepared above was placed on one side of the solid electrolyte layer inside the cylindrical tube. The cylindrical convex punch was then inserted again from above and pressed at a pressure of 300 MPa for 3 minutes, forming a positive electrode active material layer with a diameter of 10 mm and a thickness of approximately 0.06 mm on one side of the solid electrolyte layer. Next, the cylindrical convex punch inserted from above was removed again, and SUS430 foil (thickness 0.010 mm) punched to a diameter of 10 mm was placed on top of the positive electrode active material layer. After that, the cylindrical convex punch was inserted again from above.

Next, the lower cylindrical convex punch was removed, and a lithium metal foil (manufactured by Honjo Metals Co., Ltd., thickness 0.10 mm) punched to a diameter of 10 mm as the negative electrode active material layer and a SUS foil (thickness 0.010 mm) punched to a diameter of 10 mm as the negative electrode current collector were inserted from the bottom of the cylindrical tube jig, and the cylindrical convex punch was reinserted, and the jig was then restrained in a metal casing at a pressure equivalent to 100 MPa.

In this manner, a laminate was obtained in which the negative electrode current collector, lithium metal foil, solid electrolyte layer, positive electrode active material layer, and positive electrode current collector were stacked in this order. Charge and discharge were then performed in the same manner as in Example 1 above, and the all-solid-state lithium secondary battery of Comparative Example 1 was obtained.

[Comparative Example 2]
In the above Comparative Example 1, a laminate was fabricated by disposing a 0.10 mm thick polyethylene oxide (PEO) film between the lithium metal foil and the solid electrolyte layer. This resulted in a laminate in which a negative electrode current collector, lithium metal foil, a polyethylene oxide layer (intermediate layer), a solid electrolyte layer, a positive electrode active material layer, and a positive electrode current collector were stacked in this order. Except for the above, an all-solid-state lithium secondary battery was fabricated using the same method as in Comparative Example 1.

[Comparative Example 3]
An all-solid-state lithium secondary battery was produced in the same manner as in Example 1 described above, except that in the production of the test cell, the positive electrode current collector was changed from SUS430 foil to aluminum foil (thickness: 0.020 mm).

The configuration of the batteries produced in each example and comparative example is shown in Table 1 below. In Table 1, the "initial Li metal placement" refers to the case where lithium foil was placed between the positive electrode current collector and the solid electrolyte layer when producing the laminate, which is the battery precursor, and is represented as the "positive electrode side," and the case where it was placed between the negative electrode current collector and the solid electrolyte layer is represented as the "negative electrode side."

<<Observation of the cross section of an all-solid-state lithium secondary battery>>
When the cross sections of the all-solid-state lithium secondary batteries of Examples 1 to 3 and Comparative Example 3 described above were observed with a scanning electron microscope (SEM) after the charging treatment was completed, it was confirmed that a layer of metallic lithium was present between the negative electrode current collector and the solid electrolyte layer.

<<Analysis of the surface of the solid electrolyte layer of an all-solid-state lithium secondary battery>>
Furthermore, for the all-solid-state lithium secondary batteries of Examples 1 to 3, the solid electrolyte layers were removed after the charging process was completed, and the surface in contact with metallic lithium was measured by X-ray photoelectron spectroscopy (XPS, ESCA), confirming that no peaks derived from oxygen atoms or carbon atoms were observed. This suggests that no oxides or carbonates were formed on the surface of the solid electrolyte layer in contact with metallic lithium. On the other hand, when a similar measurement was performed on the all-solid-state lithium secondary battery of Comparative Example 1, peaks derived from oxygen atoms and carbon atoms were confirmed, indicating that oxides and carbonates were formed on the surface of the solid electrolyte layer in contact with metallic lithium.

<Test cell evaluation>
The DC resistance and energy density of the test cells prepared in each of the above examples and comparative examples were evaluated by the following methods. All of the following measurements were performed using a charge/discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C.

(Energy density measurement)
The battery was charged at a constant current/constant voltage (CCCV) of 3.1 V at a current density of 0.1 mA/ ^cm² with a current cutoff of 0.05 mA/ ^cm² . The battery was then discharged at a rate of 0.03 C according to the battery design, and the battery capacity was confirmed and the energy density was calculated. The voltage range was 1.1 to 2.5 V, the current rate during charge/discharge was 0.05 C, and the cutoff current during charge was 0.01 C. The energy densities calculated in this manner are shown in Table 1 below as relative values, with the value for Comparative Example 1 set to 100.

(Internal resistance measurement)
The capacity value per mass of the positive electrode active material (mAh/g) was calculated from the charge/discharge capacity value obtained after 10 repeated charge/discharge cycles and the mass of the positive electrode active material contained in the positive electrode. Next, a constant current discharge was performed at a current density of 0.2 mA/cm ² to a capacity of 50% (SOC 50%) of the capacity value calculated in this way (100%). After a 30-minute rest, the battery was discharged at a discharge rate of 1 C for 10 seconds. The direct current resistance (DCR) was calculated according to Ohm's law from the voltage drop and current value at that time, and used as the internal resistance value of the test cell. The results are shown in Table 1 below. The internal resistance values (DCR) shown in Table 1 are relative values when the value in Comparative Example 1 is set to 100.

The results shown in Table 1 reveal that, as in Examples 1 to 3, the method of the present invention can improve input/output characteristics without reducing energy density in all-solid-state lithium secondary batteries using metallic lithium as the negative electrode active material. In contrast, when a process of disposing a solid electrolyte layer on metallic lithium is included, as in Comparative Example 1, the internal resistance increases. This is thought to be because an inorganic coating, such as Li ₂ O or Li ₂ CO ₃ , formed on the metallic lithium reacts with the solid electrolyte to form a resistive layer. Furthermore, when an intermediate layer is disposed between the negative electrode active material layer and the solid electrolyte layer, as in Comparative Example 2, the reaction between metallic lithium and the solid electrolyte is suppressed, but the energy density of the battery decreases due to the increased volume of the intermediate layer. Furthermore, when the positive electrode current collector is made of a metal that can be alloyed with lithium, as in Comparative Example 3, the current collector corrodes upon alloying with lithium, reducing its function as a current collector, and therefore the internal resistance is thought to increase.

10a, 100a stacked battery,
11', 111' negative electrode current collector,
11”, 111” positive electrode current collector,
13, 113 negative electrode active material layer,
15, 115 positive electrode active material layer,
17, 117 solid electrolyte layer,
19 cell layer,
21 power generating element,
25 negative electrode current collector plate,
27 positive electrode current collector plate,
29 Laminating film,
100 laminate,
113' metallic lithium.

Claims (5)

A method for producing an all-solid-state lithium secondary battery using metallic lithium as a negative electrode active material, comprising:
preparing a laminate having, on an anode current collector, a solid electrolyte layer containing a solid electrolyte, a cathode active material layer containing a cathode active material, and a cathode current collector in this order, wherein the metallic lithium is disposed between the cathode active material layer and the cathode current collector or in the cathode active material layer;
and performing a charging reaction on the laminate to deposit the metallic lithium between the solid electrolyte layer and the negative electrode current collector,
the positive electrode current collector is made of a material that does not alloy with lithium, or a surface of the positive electrode current collector adjacent to the positive electrode active material layer is coated with a material that does not alloy with lithium. The manufacturing method described in claim 1, wherein the solid electrolyte is a sulfide solid electrolyte. The manufacturing method described in claim 1 or 2, wherein the positive electrode active material is a positive electrode active material containing sulfur. The manufacturing method described in any one of claims 1 to 3, wherein the positive electrode current collector is made of stainless steel, nickel, or carbon-coated aluminum. 5. The method according to claim 1, wherein the all-solid-state lithium secondary battery contains neither lithium oxide nor lithium carbonate on a surface of the solid electrolyte layer that is in contact with the metallic lithium.