Anode for solid-state secondary batteries and solid-state secondary batteries

Abstract

The present invention provides a negative electrode for a solid-state secondary battery that enables both high energy density and improved reliability, and a solid-state secondary battery using the said negative electrode. [Solution] A negative electrode for a solid secondary battery comprising a negative electrode active material layer, a PTC layer, and a negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material that forms at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds, the PTC layer contains a polyolefin-based thermoplastic resin, a carbon material, and a nonionic water-soluble resin, the PTC layer is disposed between the negative electrode active material layer and the negative electrode current collector, and in the charged state, at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds is deposited near the negative electrode active material layer.

Description

[Technical Field]

[0001] This invention relates to a negative electrode for a solid-state secondary battery and a solid-state secondary battery. [Background technology]

[0002] In recent years, solid-state rechargeable batteries, which use solid electrolytes, have attracted attention. To increase the energy density of such solid-state rechargeable batteries, technologies related to the negative electrode, known as anode-free or anode-less, have been proposed.

[0003] For example, Patent Document 1 discloses an all-solid-state secondary battery and a charging method thereof, in which the initial capacity of the positive electrode active material layer is greater than the initial capacity of the negative electrode active material layer. According to the contents disclosed in Patent Document 1, when such an all-solid-state secondary battery is charged beyond the charging capacity of the negative electrode active material layer, lithium ions that have moved from the positive electrode to the negative electrode are deposited as lithium, lithium alloy, or lithium compound on the back side of the negative electrode active material layer or within the negative electrode active material layer, thereby allowing lithium, lithium alloy, or lithium compound to be used as an active material, and improving the characteristics of the all-solid-state secondary battery. For example, the capacity density (capacity per unit mass) of lithium is said to be about 10 times that of graphite, which is commonly used as a negative electrode active material. Therefore, by using lithium as the negative electrode active material, it is possible to increase the output while making the all-solid-state secondary battery thinner.

[0004] However, increasing the energy density of solid-state rechargeable batteries using such technologies can make it difficult to ensure high reliability. For example, in external short-circuit tests, a large short-circuit current may be generated within the battery when it is short-circuited, causing it to overheat. As a result, there are concerns that this overheating could impair the battery's reliability.

[0005] Positive Temperature Coefficient (PTC) functionality is known as a feature that ensures high reliability of batteries. PTC functionality is a function in which electrical resistance increases when the temperature rises, and by increasing electrical resistance, it is possible to suppress short-circuit currents that may occur within the battery. Patent documents 2 to 4 disclose technologies aimed at improving the reliability of solid-state batteries using PTC functionality.

[0006] Patent Document 2 discloses a technique for placing a PTC element (hereinafter referred to as "PTC element") between a positive lead piece and a positive terminal inside an all-solid-state battery case. However, there is a concern that if the PTC element is placed far from an abnormally heat-generating part in the battery, it will take a relatively long time for the resistance to increase, and the high reliability of the battery may not be improved. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2019-96610 [Patent Document 2] Japanese Patent Application Publication No. 11-144704 [Non-patent literature]

[0008] [Non-Patent Document 1] Naoki Suzuki et al., "Synthesis and Electrochemical Properties of I4--type Li1+2xZn1-xPS4 Solid Electrolyte," Chemistry of Materials, March 9, 2018, No. 30, 2236-2244 (2018). [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention aims to provide a negative electrode for a solid secondary battery capable of achieving both high energy density and improved high reliability, and a solid secondary battery using the negative electrode for a solid secondary battery.

Means for Solving the Problems

[0010] The negative electrode for a solid secondary battery according to an embodiment of the present invention includes a negative electrode active material layer, a PTC layer, and a negative electrode current collector. The negative electrode active material layer contains a negative electrode active material that forms at least one selected from the group consisting of lithium, a lithium alloy, and a lithium compound. Further, the PTC layer contains a polyolefin-based thermoplastic resin, a carbon material, and a nonionic water-soluble resin. Furthermore, the PTC layer is disposed between the negative electrode active material layer and the negative electrode current collector, and in a charged state, at least one selected from the group consisting of lithium, a lithium alloy, and a lithium compound precipitates in the vicinity of the negative electrode active material layer.

Advantages of the Invention

[0011] According to the present invention, it is possible to provide a negative electrode for a solid secondary battery capable of achieving both high energy density and improved high reliability, and a solid secondary battery using the negative electrode for a solid secondary battery.

Brief Description of the Drawings

[0012] [Figure 1] It is a cross-sectional schematic view showing a schematic configuration of a solid secondary battery according to an embodiment of the present invention. [Figure 2] It is a SEM image showing a cross-sectional structure of the solid-state secondary battery fabricated in Example 1. [Figure 3] It is a SEM image showing a cross-sectional structure of a solid secondary battery according to another embodiment of the present invention. [Figure 4] It is a SEM image showing a cross-sectional structure of the solid-state secondary battery fabricated in Comparative Example 1.

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following figures, common components are denoted by the same reference numerals and have the same functions. Also, the "PTC layer" means a layer having the PTC (Positive Temperature Coefficient) function described above, that is, a function in which the electrical resistance increases when the temperature rises. Furthermore, for the sake of convenience, the negative electrode for a solid secondary battery may be simply referred to as the "negative electrode".

[0014] <Solid secondary battery> First, an example of a solid secondary battery according to this embodiment will be described based on FIG. 1. As shown in FIG. 1, the solid secondary battery 1 includes a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. The negative electrode 20 includes a negative electrode current collector 21, a negative electrode active material layer 22, and a PTC layer 23. The PTC layer 23 is disposed between the negative electrode current collector 21 and the negative electrode active material layer 22. The solid electrolyte layer 30 is formed between the positive electrode 10 and the negative electrode 20 and contains a solid electrolyte. The negative electrode 20 and the solid electrolyte layer 30 formed on the negative electrode 20 are collectively referred to as an electrolyte negative electrode structure 40. Also, the negative electrode 20 corresponds to the negative electrode for a solid secondary battery described later.

[0015] (Positive electrode) As shown in FIG. 1, the positive electrode 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. Also, the positive electrode active material layer 12 may further contain a solid electrolyte, and the solid electrolyte contained in the positive electrode active material layer 12 may be of the same type as or different from the solid electrolyte contained in the solid electrolyte layer 30 described later.

[0016] Examples of the material for the positive electrode current collector include indium (In), copper (Cu), magnesium Examples of metals include magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), and germanium (Ge). The positive electrode current collector may be composed of one of these metals, or it may be formed from an alloy of two or more metals. The shape of the positive electrode current collector may be, for example, a plate or foil. An undercoat layer may be applied to the positive electrode current collector. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 μm or more and 20 μm or less.

[0017] The positive electrode active material can be any positive electrode active material capable of reversibly intercalating and releasing lithium ions. Examples of such positive electrode active materials include lithium salts such as lithium cobalt oxide (hereinafter referred to as "LCO"), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminate (hereinafter referred to as "NCA"), lithium nickel cobalt manganese oxide (hereinafter referred to as "NCM"), lithium manganese oxide, and lithium iron phosphate, as well as nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, or vanadium oxide. These positive electrode active materials may be used individually or in combination of two or more.

[0018] Furthermore, the positive electrode active material is preferably formed by including a lithium salt of a transition metal oxide having a layered rock salt type structure among the lithium salts mentioned above. Here, "layered rock salt type structure" refers to a cubic rock salt type structure. <111> This structure consists of alternating, regular arrangements of oxygen and metal atomic layers in a specific direction, resulting in each atomic layer forming a two-dimensional plane. Furthermore, "cubic salt structure" refers to a type of crystalline structure called sodium chloride structure, specifically a structure in which the face-centered cubic lattices formed by the cations and anions are offset from each other by half the edge length of the unit cell.

[0019] Examples of lithium salts of transition metal oxides having such a layered rock salt-type structure include LiNi x CoyAl z O2(NCA), or LiNi xCo y Lithium salts of ternary transition metal oxides such as MnzO2(NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1) can be mentioned.

[0020] When the positive electrode active material contains a lithium salt of a ternary transition metal oxide having the above-described layered rock salt structure, the energy density and thermal stability of the solid secondary battery 1 can be improved.

[0021] The positive electrode active material may be covered with a coating layer. Such a coating layer is not particularly limited as long as it is a known coating layer for the positive electrode active material of a solid secondary battery. For example, Li2O-ZrO2 etc. can be mentioned.

[0022] Also, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity density of the solid secondary battery can be increased and the elution of metal from the positive electrode active material in the charged state can be reduced. Thereby, the solid secondary battery can improve the long-term reliability and cycle characteristics in the charged state.

[0023] Examples of the shape of the positive electrode active material include particle shapes such as true spherical and ellipsoidal. Also, the particle size of the positive electrode active material is not particularly limited and may be within the range applicable to the positive electrode active material of a conventional solid secondary battery. Incidentally, the content of the positive electrode active material is not particularly limited either and may be within the range applicable to the positive electrode of a conventional solid secondary battery.

[0024] Also, in addition to the above-described positive electrode current collector, positive electrode active material layer, and solid electrolyte, for example, a conductive auxiliary agent, a binder, a filler, a dispersant, an ion conduction auxiliary agent, etc. may be appropriately blended in the positive electrode and it may be so.

[0025] Examples of conductive additives that can be incorporated into the positive electrode include graphite, carbon black, acetylene black, Ketjenblack, carbon nanofiber, VGCF (registered trademark, a product of Resonaq), carbon nanotubes, graphene, carbon fibers, and metal powders. Examples of binders that can be incorporated into the positive electrode 10 include styrene-butadiene copolymer (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polyethylene (PE). Furthermore, known materials generally used in electrodes for solid-state secondary batteries can be used as fillers, dispersants, and ionic conductive additives that can be incorporated into the positive electrode 10.

[0026] (Negative electrode) As shown in Figure 1, the negative electrode 20 includes a negative electrode current collector 21, a negative electrode active material layer 22, and a PTC layer 23. The PTC layer 23 is disposed between the negative electrode active material layer 22 and the negative electrode current collector 21, and preferably only between the negative electrode active material layer 22 and the negative electrode current collector 21. In this embodiment, the negative electrode 20 has a predetermined PTC layer 23 between the negative electrode current collector 21 and the negative electrode active material layer 22. By providing such a predetermined PTC layer 23, when at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds (hereinafter sometimes simply referred to as "lithium, etc.") is deposited near the negative electrode active material layer 22 in the charged state of the solid secondary battery 1, the adhesion between the negative electrode active material layer 22 and the PTC layer 23 can be easily resolved, so that lithium, etc. is deposited uniformly. This prevents cracks from forming in the negative electrode active material layer 22, and as a result, it is possible to provide a negative electrode 20 that can suppress short circuits in the battery while contributing to the realization of high energy density. Furthermore, by arranging the PTC layer 23 near the power generation element of the solid secondary battery 1, the solid secondary battery 1 is given excellent PTC functionality. As a result, when the solid secondary battery 1 generates heat, the temperature rises in a relatively short time, increasing the electrical resistance and improving the reliability of the battery. By providing the solid secondary battery 1 with such a negative electrode 20, it is possible to provide a solid secondary battery 1 that can achieve both high energy density and improved reliability. Examples of lithium alloys include Li-Au alloy, Li-Pt alloy, Li-Pd alloy, Li-Si alloy, Li-Ag alloy, Li-Al alloy, Li-Bi alloy, Li-Sn alloy, Li-Mg alloy, and Li-Zn alloy, and examples of lithium compounds include lithium carbide.

[0027] Here, the vicinity of the negative electrode active material layer includes (i) the space between the negative electrode active material layer and the negative electrode current collector, (ii) within the negative electrode active material layer, and (iii) each combination of (i) and (ii). In other words, when lithium, etc. is deposited near the negative electrode active material layer in the charged state, it means that lithium, etc. is deposited in any of the patterns (i) to (iii). Furthermore, the charged state means, for example, a state in which the SOC is 30% or higher. Here, "SOC (State of Charge)" indicates the charge state of a solid-state secondary battery, where the SOC is 100% in a fully charged state and 0% in a completely discharged state.

[0028] The negative electrode current collector is preferably made of a material that does not react with lithium, that is, a material that does not form any alloys or compounds with lithium. Examples of materials that make up the negative electrode current collector include copper, stainless steel, titanium, iron, cobalt, and nickel. The negative electrode current collector may be made of any one of these metals, or it may be made of an alloy or clad material of two or more metals. The shape of the negative electrode current collector may be, for example, a plate or foil. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 μm or more and 20 μm or less.

[0029] The negative electrode active material layer contains a negative electrode active material that forms lithium, etc., and in the charged state, the negative electrode Lithium and other elements are deposited near the active material layer, contributing to the realization of a high energy density. In this case, a predetermined PTC layer, described later, is placed between the negative electrode active material layer and the negative electrode current collector, so that in the charged state, lithium and other elements are uniformly deposited between the negative electrode active material layer and the PTC layer, in the negative electrode active material layer, or both, making the battery less prone to short circuits.

[0030] The negative electrode active material preferably contains at least one selected from the group consisting of amorphous carbon, gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc, and more preferably contains amorphous carbon. Alternatively, the negative electrode active material may be a mixture of (a) amorphous carbon and (b) at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc. Examples of amorphous carbon include carbon black such as acetylene black, furnace black, and Ketjen black, and graphene. These negative electrode active materials are, for example, granular, and their primary particle size is preferably 4 μm or less, more preferably 300 nm or less. Here, the primary particle size of the negative electrode active material can be, for example, the median diameter (so-called D50) measured using a laser particle size distribution system.

[0031] When the negative electrode active material contains amorphous carbon, the negative electrode active material layer preferably contains 33 to 95 parts by mass of amorphous carbon, and more preferably 50 to 80 parts by mass of amorphous carbon, per 100 parts by mass of the total content of the materials constituting the negative electrode active material layer. By adjusting the content of amorphous carbon in the negative electrode active material, lithium and other elements can be deposited more uniformly onto the negative electrode during the charging process.

[0032] Furthermore, the negative electrode active material layer may contain additives used in conventional solid-state secondary batteries, such as fillers and dispersants, as appropriate.

[0033] The thickness of the negative electrode active material layer is not particularly limited, but it is preferably between 1 μm and 20 μm. By having a negative electrode active material layer thickness of 1 μm and 20 μm, the increase in the resistance value of the negative electrode active material layer can be suppressed, and the characteristics of the solid-state secondary battery can be maintained. The thickness of the negative electrode active material layer can be estimated, for example, by observing the cross-section of the solid-state secondary battery after assembly and pressure molding using a scanning electron microscope (SEM).

[0034] The PTC layer contains a polyolefin-based thermoplastic resin. When exposed to high temperatures, such as 100°C or higher, the polyolefin-based thermoplastic resin melts and expands in volume. This volume expansion breaks the conductive paths formed by the carbon material contained in the PTC layer, thereby enabling the PTC function. Furthermore, because the polyolefin-based thermoplastic resin is a poorly bonded resin, the adhesion between the negative electrode active material layer and the PTC layer can be relatively easily resolved, contributing to the uniform deposition of lithium and other materials near the negative electrode active material layer in the charged state.

[0035] In polyolefin-based thermoplastic resins, examples of olefin components include ethylene, propylene, isobutylene, isobutene, 1-butene, 2-butene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-hexene, 1-octene, and norvonene. Polyolefin-based thermoplastic resins are polyolefins obtained by polymerizing olefin resins, and may be polymers of a single olefin component or copolymers of two or more olefin components. Among these, polyethylene and polypropylene are preferred as polyolefin-based thermoplastic resins from the viewpoint of availability. Polyolefin-based thermoplastic resins may be used alone or in combination of two or more types.

[0036] As the content of polyolefin-based thermoplastic resin changes, there is a trade-off relationship between battery characteristics and PTC function; therefore, the content of polyolefin-based thermoplastic resin should be within a suitable range. There exists a high content of polyolefin-based thermoplastic resin, which tends to improve the PTC function, but because it is electrically insulating, it can interfere with the current during normal battery operation, potentially impairing the battery's characteristics. On the other hand, if the content of polyolefin-based thermoplastic resin is low, the battery characteristics tend not to be impaired, but it becomes difficult to ensure sufficient PTC function. Therefore, the content of polyolefin-based thermoplastic resin is preferably 40 parts by mass or more and 80 parts by mass or less, more preferably 50 parts by mass or more and 70 parts by mass or less, and even more preferably 60 parts by mass or more and 70 parts by mass or less, based on 100 parts by mass of the total content of the materials constituting the PTC layer.

[0037] Furthermore, the PTC layer contains a carbon material. The carbon material forms conductive paths necessary for normal battery operation within the PTC layer. Because carbon material reacts little with sulfides and is inexpensive, it is preferably used as a conductive filler. The carbon material is not particularly limited as long as it enhances the conductivity of the PTC layer, but specific examples include carbon black, natural graphite, artificial graphite, fibrous carbon, and nanocarbon materials, with carbon black being preferred. Examples of carbon black include furnace black, channel black, thermal black, Ketjen black, and acetylene black. Examples of fibrous carbon include carbon fiber, VGCF (registered trademark, a product of Resonaq), and carbon nanofiber. Examples of nanocarbon materials include carbon nanotubes and graphene. The carbon material may be used alone or in combination of two or more types.

[0038] Regarding the carbon material content in the PTC layer, there is a suitable range due to the trade-off relationship between battery characteristics and PTC function. When the carbon material content is high, battery characteristics tend not to be impaired, but the increased number of conductive paths in the PTC layer makes it difficult to break them, making it difficult to ensure sufficient PTC function. On the other hand, when the carbon material content is low, PTC function tends to improve, but there is a concern that battery characteristics may be impaired if sufficient conductive paths necessary for normal battery operation cannot be formed. For this reason, the carbon material content is preferably 10 parts by mass or more and 50 parts by mass or less, more preferably 15 parts by mass or more and 40 parts by mass or less, and even more preferably 20 parts by mass or more and 40 parts by mass or less, based on the total content of the materials constituting the PTC layer per 100 parts by mass.

[0039] If the particle size of the carbon material contained in the PTC layer is too large, coating the PTC layer may become difficult. There is an inverse relationship between the primary particle size of the carbon material and the BET (Brunaure-Emmett-Teller) specific surface area. The BET specific surface area of ​​the carbon material is 3 m². 2 It is preferable that it is 6m or more / g. 2 It is more preferable that it be 30m or more per g. 2 It is even more preferable that the amount is greater than or equal to / g. On the other hand, if the specific surface area of ​​the carbon material is too large, it becomes difficult to disperse the carbon material in the slurry. Therefore, the upper limit of the BET specific surface area of ​​the carbon material is 5000m². 2 It is preferable that the value be less than or equal to / g.

[0040] Furthermore, the PTC layer contains a nonionic water-soluble resin. The nonionic water-soluble resin contributes to ensuring the dispersion of carbon materials within the PTC layer. If the dispersion of carbon materials is not sufficiently ensured, the electrical resistance within the PTC layer becomes non-uniform, and lithium and other materials deposited near the negative electrode active material layer during the battery charging process may become non-uniform, potentially causing a short circuit in the battery. Also, for example, polyolefin-based thermoplastic resins are commercially available in the form of water-dispersible resin fine particles, but when mixed with non-nonionic resins such as anionic, cationic, or amphoteric resins, they may form aggregates, making it difficult to coat thin films with a thickness of 1 μm to 10 μm, for example. Therefore, a nonionic water-soluble resin is used as the water-soluble resin contained in the PTC layer.

[0041] Examples of nonionic water-soluble resins include polyacrylamide (PAM), poly(N-isopropylacrylamide) (PNIPAM), polyethylene oxide (PEO), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylate, hydroxyethylcellulose (HEC), and guar gum. These nonionic water-soluble resins may be used individually or in combination of two or more. Copolymers obtained by copolymerizing two or more monomers of nonionic water-soluble resins may also be used. Examples of copolymer types include alternating copolymers, random copolymers, block copolymers, and graft copolymers.

[0042] As the content of nonionic water-soluble resin changes, there is a trade-off relationship between battery characteristics and PTC function, and therefore there is an optimal range for the content of nonionic water-soluble resin. When the content of nonionic water-soluble resin is high, the dispersibility of the carbon material improves, and battery characteristics tend not to be impaired, but the conductive paths in the PTC layer become mechanically rigid, making it difficult to break the conductive paths and making it difficult to ensure sufficient PTC function. On the other hand, when the content of nonionic water-soluble resin is low, the PTC function tends to improve, but as mentioned above, if the dispersibility of the carbon material is not sufficiently ensured, there is a risk that the battery characteristics will be impaired. For this reason, the content of nonionic water-soluble resin is preferably 5 parts by mass or more and 30 parts by mass or less, and more preferably 5 parts by mass or more and 25 parts by mass or less, per 100 parts by mass of the total content of the materials constituting the PTC layer.

[0043] Since the PTC layer does not contain active material and is composed of inactive material, the thinner the PTC layer, the higher the energy density of the battery. On the other hand, as mentioned above, the PTC function is the function of increasing electrical resistance by raising the temperature, and there is a direct proportional relationship between electrical resistance and the thickness of the PTC layer. Therefore, the thicker the PTC layer, the better the PTC function. As the thickness of the PTC layer changes, there is a trade-off relationship between the energy density of the battery and the PTC function, so there is an optimal range for the thickness of the PTC layer. For this reason, the thickness of the PTC layer is preferably 1 μm to 10 μm, and more preferably 2 μm to 8 μm. In this way, by appropriately controlling the thickness of the PTC layer, it is possible to achieve both the energy density of the battery and the PTC function. The thickness of the PTC layer can be estimated, for example, by observing the cross-section of a solid-state secondary battery after assembly and pressure molding using a scanning electron microscope (SEM).

[0044] (solid electrolyte layer) As shown in Figure 1, the solid electrolyte layer 30 is positioned between the positive electrode 10 and the negative electrode 20 (specifically, between the positive electrode active material layer 12 and the negative electrode active material layer 22). The solid electrolyte layer 30 contains a solid electrolyte that can move ions.

[0045] The solid electrolyte is composed of, for example, a solid electrolyte material mainly composed of sulfide (hereinafter referred to as a sulfide-based solid electrolyte material). Examples of the sulfide-based solid electrolyte material include Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element such as I, Br, Cl, etc.), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (m and n are positive numbers, and Z is any one of Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, or In), etc. As the solid electrolyte, one kind of material selected from these sulfide-based solid electrolyte materials may be used, or two or more kinds of materials may be used in combination.

[0046] As the solid electrolyte, it is preferable to use a sulfide solid electrolyte material containing sulfur (S), phosphorus (P), and lithium (Li) as constituent elements. Specifically, it is more preferable to use a sulfide-based solid electrolyte material containing Li2S-P2S5, particularly Li2S-P2S5-LiCl. When using a sulfide-based solid electrolyte material containing Li2S-P2S5, the mixing molar ratio of Li2S and P2S5 may be selected, for example, in the range of Li2S:P2S5 = 50:50 to 90:10.

[0047] The solid electrolyte may be in an amorphous state, a crystalline state, or a state in which amorphous and crystalline are mixed.

[0048] The solid electrolyte layer may further contain a binder. Examples of binder materials include resins such as styrene-butadiene rubber, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and polyacrylic acid ester resins. The binder material may be the same as or different from the binder material in the positive electrode active material layer.

[0049] The thickness of the solid electrolyte layer is not particularly limited, but from the viewpoint of improving energy density, it is preferably 500 μm or less, and more preferably 100 μm or less. On the other hand, from the viewpoint of suppressing the occurrence of short circuits inside the battery, it is preferably 10 μm or more.

[0050] <Manufacturing method for solid-state rechargeable batteries> Next, a method for manufacturing the solid-state secondary battery 1 according to this embodiment will be described. The solid-state secondary battery 1 according to this embodiment can be obtained by first manufacturing a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30, and then stacking these layers.

[0051] (1) Cathode manufacturing process The process for manufacturing the positive electrode may be a dry process or a wet process. When manufactured using a dry process, the positive electrode 10 is obtained by compacting a mixture of materials constituting the positive electrode active material layer 12 into pellets or by stretching it into a sheet. When manufacturing the positive electrode 10 using these methods, the positive electrode current collector 11 may be pressed onto the manufactured pellet or sheet. When manufactured using a wet process, first, the materials constituting the positive electrode active material layer 12 (positive electrode active material, binder, etc.) are added to a non-polar solvent to produce a slurry (may also be a paste). Next, the obtained slurry is applied onto the prepared positive electrode current collector 11 and dried to obtain a laminate. Then, the obtained laminate is pressurized, for example, using hydrostatic pressure, to produce the positive electrode 10. The pressurization step may be omitted.

[0052] (2) Negative electrode fabrication process First, various slurries (which may also be pastes) are prepared by adding materials constituting the negative electrode active material layer 22 (such as negative electrode active material) and materials constituting the PTC layer 23 (such as polyolefin-based thermoplastic resin, carbon material, or nonionic water-soluble resin) to a polar solvent or a nonpolar solvent, respectively. Next, the obtained PTC layer slurry is applied onto the prepared negative electrode current collector 21 and dried to obtain an intermediate laminate in which the PTC layer 23 is formed on the negative electrode current collector 21. Furthermore, the negative electrode active material layer slurry is applied onto the PTC layer 23 of the obtained intermediate laminate and dried to obtain a laminate in which the negative electrode active material layer 22 is formed on the PTC layer 23. Next, the obtained laminate is punched out to a predetermined size to produce the negative electrode 20. The method for applying the slurry to the negative electrode current collector 21 is not particularly limited and may include, for example, screen printing, metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, die coating, and comma coating. It may be a law, a knife coating law, etc.

[0053] (3) Process for preparing the solid electrolyte layer The solid electrolyte layer 30 can be made from a solid electrolyte formed from a sulfide-based solid electrolyte material.

[0054] First, sulfide-based solid electrolyte materials are obtained by processing the starting materials using methods such as melt-quenching or mechanical milling.

[0055] For example, when using the melt-and-cool method, a sulfide-based solid electrolyte material can be produced by mixing a predetermined amount of starting materials, forming them into pellets, reacting them in a vacuum at a predetermined reaction temperature, and then rapidly cooling them. The reaction temperature of the Li2S and P2S5 mixture is preferably 400°C to 1000°C, more preferably 800°C to 900°C. The reaction time is preferably 0.1 hours to 12 hours, more preferably 1 hour to 12 hours. Furthermore, the rapid cooling temperature of the reactants is usually 10°C or lower, preferably 0°C or lower, and the rapid cooling rate is usually 1°C / second to 10000°C / second, more preferably 1°C / second to 1000°C / second.

[0056] Furthermore, when using the mechanical milling method, sulfide-based solid electrolyte materials can be produced by stirring and reacting the starting materials using a ball mill or the like. While the stirring speed and time in the mechanical milling method are not particularly limited, a faster stirring speed can increase the rate of sulfide-based solid electrolyte material production, and a longer stirring time can increase the conversion rate of the raw materials to sulfide-based solid electrolyte material.

[0057] Subsequently, the resulting mixed raw material (sulfide-based solid electrolyte material) can be heat-treated at a predetermined temperature and then pulverized to produce particulate solid electrolyte. If the solid electrolyte has a glass transition temperature, it may change from amorphous to crystalline upon heat treatment.

[0058] Next, the solid electrolyte obtained by the above method can be used to form a film using a known film formation method such as aerosol deposition, cold spray, or sputtering to produce a solid electrolyte layer 30. Alternatively, the solid electrolyte layer 30 may be produced by pressurizing individual solid electrolyte particles. Furthermore, the solid electrolyte layer 30 may be produced as a sheet by mixing the solid electrolyte with a solvent and a binder to obtain a slurry-like solid electrolyte, which is then coated and dried onto a substrate such as a PET film.

[0059] (4)Lamination process A solid-state secondary battery 1 according to this embodiment can be manufactured by arranging a positive electrode 10 and a negative electrode 20 so as to sandwich a solid electrolyte layer 30, and then pressurizing this arrangement using, for example, hydrostatic pressure. Furthermore, the solid-state secondary battery 1 according to this embodiment does not require the application of high external pressure using an end plate or the like, and can exhibit excellent discharge capacity even when the external pressure applied to the positive electrode 10, negative electrode 20, and solid electrolyte layer 30 during use is 1 MPa or less. Alternatively, an electrolyte negative electrode structure 40, which is a laminate of the negative electrode 20 and the solid electrolyte layer 30, may be manufactured, and then the positive electrode 10 may be placed on top of the electrolyte negative electrode structure 40, and the solid-state secondary battery 1 may be manufactured by pressurizing this arrangement using, for example, hydrostatic pressure.

[0060] <How to charge a solid-state rechargeable battery> Next, the charging method for the solid-state secondary battery 1 will be described. The charging method for the solid-state secondary battery 1 involves charging the solid-state secondary battery 1 based on the charging capacity of the negative electrode active material layer 22. In the initial stages of charging, lithium is absorbed into the negative electrode active material layer 22. As charging progresses, lithium is deposited near the negative electrode active material layer 22, and this lithium causes lithium alloys that were not present at the time of manufacture to form. A metal layer 24 containing a lithium compound is formed. During discharge, lithium in the negative electrode active material layer 22 and the metal layer is ionized and moves to the positive electrode 10 side. Therefore, lithium can be used as the negative electrode active material in the solid-state secondary battery 1. Furthermore, since a PTC layer 23 is arranged between the negative electrode active material layer 22 and the negative electrode current collector 21, the uneven deposition of lithium and other materials can be suppressed. This suppresses short circuits in the solid-state secondary battery 1, and consequently improves the reliability of the solid-state secondary battery 1.

[0061] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and includes all aspects included in the concept and claims of the present invention, and can be modified in various ways within the scope of the present invention.

[0062] Based on the embodiments described above, the present invention relates to the following [1] to [9]. [1] A negative electrode for a solid-state secondary battery comprising a negative electrode active material layer, a PTC layer, and a negative electrode current collector, The negative electrode active material layer contains a negative electrode active material that forms at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds. The PTC layer contains a polyolefin-based thermoplastic resin, a carbon material, and a nonionic water-soluble resin. The PTC layer is disposed between the negative electrode active material layer and the negative electrode current collector, A negative electrode for a solid-state secondary battery, characterized in that, in a charged state, at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds is deposited near the negative electrode active material layer. [2] The negative electrode for a solid secondary battery according to [1] above, wherein the PTC layer contains, per 100 parts by mass of the total material constituting the PTC layer, 40 parts by mass or more and 80 parts by mass of the polyolefin-based thermoplastic resin, 10 parts by mass or more and 50 parts by mass of the carbon material, and 5 parts by mass or more and 30 parts by mass of the nonionic water-soluble resin. [3] The negative electrode for a solid secondary battery according to [2] above, wherein the PTC layer contains, per 100 parts by mass of the total material constituting the PTC layer, 50 parts by mass or more and 70 parts by mass of the polyolefin-based thermoplastic resin, 10 parts by mass or more and 40 parts by mass of the carbon material, and 5 parts by mass or more and 25 parts by mass of the nonionic water-soluble resin. [4] The negative electrode for a solid secondary battery according to any one of [1] to [3] above, wherein the thickness of the PTC layer is 1 μm or more and 10 μm or less. [5] The negative electrode for a solid secondary battery according to any one of [1] to [4] above, characterized in that the negative electrode active material includes at least one selected from the group consisting of amorphous carbon, gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc. [6] The negative electrode active material is a negative electrode for a solid secondary battery as described in [5] above, comprising amorphous carbon. [7] The aforementioned negative electrode active material is (a) amorphous carbon and (b) at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc A negative electrode for a solid secondary battery according to [5] or [6] above, which is a mixture of the above. [8] The negative electrode active material layer contains 33 parts by mass or more and 95 parts by mass or less of amorphous carbon per 100 parts by mass of the total content of the materials constituting the negative electrode active material layer, as described in [5] to [7] above. A negative electrode for a solid-state rechargeable battery, as described in one of the following. [9] A solid-state secondary battery comprising a positive electrode, a solid electrolyte layer, and a negative electrode, The positive electrode comprises a positive electrode active material layer and a positive electrode current collector. The solid electrolyte layer is disposed between the positive electrode and the negative electrode. A solid-state secondary battery characterized in that the negative electrode is a negative electrode for a solid-state secondary battery described in any one of [1] to [8] above. [Examples]

[0063] Examples of the present invention are described below, but the present invention is not limited to these examples unless it exceeds the spirit of the invention. Unless otherwise specified, each operation is performed at room temperature, and room temperature is defined as being within the range of 20°C ± 5°C.

[0064] [Example 1] A solid-state rechargeable battery was fabricated using the following process. The components were arranged as shown in Figure 1.

[0065] <Fabrication of the positive electrode> The cathode described below was fabricated by a dry process in a dry room set to a dew point of -60°C. LiNi was used as the cathode active material. 0.8 Co 0.15 Mn 0.05O2(NCM) was prepared. Li2O-ZrO2 was coated onto this active material using the method described in Non-Patent Literature 1. Li6PS5Cl, an argyrodite-type crystal, was prepared as the solid electrolyte. Polytetrafluoroethylene (Teflon® binder manufactured by DuPont) was prepared as the binder. Carbon nanofiber (CNF) was prepared as a conductive additive. These materials were then mixed in a mass ratio of positive electrode active material:solid electrolyte:conductive additive:binder = 85:15:3:1.5, and the mixture was stretched into a sheet to produce a positive electrode sheet. This positive electrode sheet was then shaped into a square of approximately 1.7 cm and pressed onto a positive electrode current collector 11 made of 10 μm thick aluminum foil coated with a 1 μm thick undercoat layer on its surface, thereby forming a positive electrode active material layer 12 on the positive electrode current collector 11 and producing a positive electrode 10. An aluminum tab lead was welded to the positive electrode current collector 11 contained within the fabricated positive electrode 10. The initial charge capacity of the positive electrode (charge capacity in the first cycle) was approximately 15 mAh for a 4.25 V charge, and the positive electrode mass was approximately 80 mg (approximately 240 mAh / g per unit mass of active material).

[0066] <Fabrication of the negative electrode> As detailed below, a nickel foil with a thickness of 10 μm was prepared as the negative electrode current collector 21, and a PTC layer 23 was formed on the surface of the nickel foil. A negative electrode active material layer 22 was formed on the surface of the PTC layer 23, on the surface opposite to the positive electrode 10.

[0067] (Fabrication of PTC layer) As a carbon material, it has a nitrogen adsorption specific surface area of ​​approximately 16 m². 2Carbon black (furnace black, hereinafter referred to as "CB") at a concentration of / g was prepared. A polyacrylamide aqueous solution (solid content concentration 7.3 parts by mass) was prepared as a material for a nonionic water-soluble resin. In addition, a polypropylene (hereinafter also referred to as "PP") resin fine particle aqueous dispersion (solid content concentration 25 parts by mass) was prepared as a material for a polyolefin-based thermoplastic resin. Next, 2.0 g of the carbon material and 128.4 g of the polyacrylamide (hereinafter also referred to as "PAM") aqueous solution were weighed and placed in a stirring container. The stirring container was attached to a rotation / revolution mixer "ARE-310" (manufactured by THINKY Co., Ltd.) and kneaded at 2000 rpm for 10 minutes. The resulting mixture was subjected to high-pressure dispersion treatment using an ultra-high-pressure wet atomization device (manufactured by Yoshida Kogyo Co., Ltd., "NanoVator") to obtain an aqueous dispersion of furnace black. The solid content concentration of the aqueous dispersion was approximately 10%.

[0068] Next, a separate stirring container was prepared, and 14.0 g of the polypropylene resin fine particle aqueous dispersion was added and stirred at 400 rpm using a magnetic stirrer. Then, 23.1 g of the obtained furnace black aqueous dispersion was weighed out and carefully added at a rate of approximately 0.5 g per second. Next, the stirring container was attached to a rotation / revolution mixer "ARE-310" (manufactured by THINKY Co., Ltd.) and kneaded at 2000 rpm for 6 minutes to obtain a slurry-like PTC layer (hereinafter also referred to as "PTC layer slurry"). The solid content concentration of the PTC layer slurry was approximately 16%.

[0069] The PTC layer slurry obtained by the method described above was applied to a 10 μm thick nickel foil, which served as the negative electrode current collector 21, using an applicator. The nickel foil was then dried in a constant temperature bath set to 80°C for 10 minutes to obtain a PTC layer 23 formed on the nickel foil. The thickness of the PTC layer after drying was approximately 5 μm. At this time, the mass ratio of the materials of the components contained in the PTC layer was PP:CB:PAM = 60:28:12 (parts by mass), as shown in Table 1, and the ratio of carbon material to water-soluble resin was 7:3 (mass ratio).

[0070] (Fabrication of the negative electrode active material layer) As the negative electrode active material, CB (nitrogen adsorption specific surface area: approximately 52 m²) is used. 2 A mixture of CB (polycarbonate / batch) and silver particles with a particle size of 60 nm was prepared. The particle size of the silver particles can be measured using, for example, the median diameter (so-called D50) measured using a laser particle size distribution system. Next, 3 g of CB and 1 g of silver particles were placed in a container, and 4 g of N-methylpyrrolidone (NMP) solution containing 5 mass% of a binder (polyvinylidene fluoride (PVDF)) was added thereto. Furthermore, a slurry-like negative electrode active material layer (hereinafter also referred to as "negative electrode active material layer slurry") was prepared by stirring the mixed solution while gradually adding a total of 30 g of NMP. The negative electrode active material layer slurry was applied onto the PTC layer 23 using an applicator and dried in a constant temperature bath set to 80°C for 20 minutes to form a negative electrode active material layer 22 on the PTC layer 23, and the negative electrode 20 was prepared by punching out a piece approximately 2 cm square. The fabricated negative electrode was brought into a dry room set to a dew point of -60°C and vacuum-dried at 100°C for 12 hours. After vacuum drying, a nickel tab lead was welded to the negative electrode current collector 21 contained in the negative electrode 20. The mass ratio of the components contained in the negative electrode active material layer was CB:Ag:PVDF = 3:1:0.2 (parts by mass), and the active material:binder = 4:0.2 (parts by mass).

[0071] <Preparation of a solid electrolyte layer> As a solid electrolyte, Li6PS5Cl powder was prepared. To the solid electrolyte, 1 part by mass (based on solid content) of a rubber-based binder was added relative to the mass of the solid electrolyte. By stirring this mixture while adding xylene and diethylbenzene, a slurry-like solid electrolyte (hereinafter also referred to as "solid electrolyte slurry") was prepared. The solid electrolyte slurry was applied to a PET (polyethylene terephthalate) film with a release-treated surface using an applicator, dried on a hot plate at 40°C for 10 minutes, then vacuum-dried at 40°C for 12 hours, and punched out in approximately 2.1 cm squares to obtain a solid electrolyte sheet formed on the release-treated PET film. The thickness of the solid electrolyte layer after vacuum drying was approximately 65 μm.

[0072] (Fabrication of electrolyte negative electrode structure) As a support material, a 3mm thick aluminum plate with a release film attached to its surface is prepared. Next, the negative electrode 20 prepared as described above and the solid electrolyte sheet formed on the release-treated PET film are placed on top of the release film attached to the aluminum plate. At this time, the surface of the negative electrode current collector 21 contained in the negative electrode 20 is in contact with the surface of the release film, and the negative electrode active material layer 22 contained in the negative electrode 20 is in contact with the solid electrolyte sheet, so as to maximize the contact area between the negative electrode active material layer 22 and the solid electrolyte sheet. Without disturbing this arrangement, another release film is placed on top and secured with polyimide tape. Furthermore, These were vacuum-packed with aluminum laminate film, submerged in a pressurized medium, subjected to hydrostatic pressure treatment (isotropic press) at 50 MPa, and the PET film was removed to produce a solid electrolyte layer 30 formed on the negative electrode 20. Here, the negative electrode 20 and the solid electrolyte layer 30 formed on the negative electrode 20 are collectively referred to as the electrolyte negative electrode structure 40.

[0073] <Fabrication of Solid-State Rechargeable Batteries> The electrolyte negative electrode structure 40 was placed horizontally so that the negative electrode 20 was below the solid electrolyte layer 30. Next, the positive electrode 10 was placed on top of the electrolyte negative electrode structure 40 so that the surface of the positive electrode active material layer 12 was in contact with the surface of the solid electrolyte layer 30. At this time, the four sides of the electrolyte negative electrode structure 40 and the four sides of the positive electrode 10 were parallel so that the centers of the electrolyte negative electrode structure 40 and the centers of the positive electrode 10 overlapped. Then, without disturbing these arrangements, the structure was placed in an aluminum laminate film prepared as an outer casing, and the tab leads of the positive electrode 10 and negative electrode 20 were pulled out to the outside, and the solid secondary battery 1 was sealed in a vacuum to fabricate it. Furthermore, the solid secondary battery 1 was subjected to hydrostatic pressure treatment at 490 MPa for 30 minutes. By performing such hydrostatic pressure treatment, the characteristics of the solid secondary battery are greatly improved.

[0074] Furthermore, this solid-state secondary battery was sandwiched between two stainless steel plates each with a size of 5.5 cm square and a thickness of about 1 cm from both sides in the stacking direction. Four holes were opened at the same positions on each of the two stainless steel plates, and the solid-state secondary battery was arranged to fit inside the quadrilateral formed by the four holes. In this state, bolts were passed through each of the four holes so as to penetrate the two stainless steel plates from the outside of the two stainless steel plates. Then, the four bolts were tightened with nuts respectively so as to press the two stainless steel plates from the outside, thereby applying a pressure of about 4 MPa to the solid-state secondary battery.

[0075] <Charge-discharge test> The charge-discharge characteristics of the solid-state secondary battery fabricated as described above were evaluated by the following charge-discharge test. The charge-discharge test was carried out by placing the solid-state secondary battery in a constant-temperature bath at 25°C. In the first cycle, charging was performed at a constant current of 1.5 mA (about 0.52 mA / cm 2 ) until the battery voltage reached 4.25 V, and then charging was performed at a constant voltage so that the battery voltage remained at 4.25 V until the current reached 0.26 mA. Next, discharging was performed at a constant current of 1.5 mA (about 0.52 mA / cm 2 ) until the battery voltage reached 2.5 V.

[0076] During the charge-discharge test, the presence or absence of a short circuit during the charge-discharge test was confirmed. The case where no short circuit occurred was evaluated as "OK", and the case where a short circuit occurred was evaluated as "NG". The results are shown in Table 1.

[0077] <Measurement of PTC function> (Fabrication of laminate for PTC function measurement) The negative electrode 20 obtained by the method described above was placed horizontally with the negative electrode current collector 21 facing downwards. Next, a 10 μm thick Ni foil cut to a 1.7 cm square was placed on top of the negative electrode 20 so that the surface of the Ni foil was in contact with the surface of the negative electrode 20. At this time, the four sides of the negative electrode 20 and the four sides of the Ni foil were positioned so that the center of the negative electrode 20 and the center of the Ni foil overlapped, and the four sides of the Ni foil were parallel. Next, without disturbing these arrangements, the assembly was placed in an aluminum laminate film prepared as an outer casing and sealed in a vacuum. At this point, a portion of the negative electrode current collector and the Ni foil were left to protrude from the laminate film without breaking the vacuum of the battery. These protruding portions were used as terminals for PTC measurement. Next, a laminate for PTC function measurement was fabricated by hydrostatic pressure treatment at 490 MPa for 30 minutes.

[0078] (Measurement of PTC function) To raise the temperature of the resulting PTC function measurement laminate, a hot plate set to 250°C was prepared. Next, the two terminals of the PTC function measurement laminate were connected to a battery tester. Each thermocouple was connected to a HIOKI BT3562 thermocouple. To prevent short circuits caused by contact between terminals of opposite polarity or contact between terminals of opposite polarity and a conductor, these terminals were protected with insulating tape. Next, one thermocouple was attached to the front and one to the back of the PTC function measurement laminate using aluminum tape. Then, the PTC function measurement laminate was placed on a hot plate, a PTFE (polytetrafluoroethylene) sheet was placed on top, and a 10cm square metal weight weighing 700g was placed on top of that. Between 30°C and 200°C, the electrical resistance of the PTC function measurement laminate and the temperatures of the thermocouples attached to the front and back were recorded using a data logger. The average of the temperatures of the two thermocouples was used as the temperature of the PTC function measurement laminate.

[0079] The electrical resistance (Ω) of the laminate used for PTC function measurement at 200°C, measured using the method described above, was divided by the electrical resistance (Ω) of the laminate used for PTC function measurement at 30°C to determine the PTC function. The results are shown in Table 1.

[0080] <SEM observation of lithium deposited during the charging process> After the initial charge in the charge-discharge test, the cross-section of the solid-state secondary battery was observed using a scanning electron microscope (SEM) to confirm lithium deposition. Observation and imaging were performed at a magnification of 1500x. The results are shown in Figure 2. From the SEM image in Figure 2, the thickness of the negative electrode active material layer 22 was approximately 10 μm.

[0081] [Example 2] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 70:21:9 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0082] [Example 3] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 50:35:15 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0083] [Example 4] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 40:42:18 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0084] [Example 5] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the materials contained in the PTC layer was PP:CB:PAM = 70:24:6 (parts by mass) and carbon material:water-soluble resin = 8:2 (mass ratio), and the above evaluation was performed. The results are shown in Table 1.

[0085] [Example 6] A solid-state secondary battery was fabricated in the same manner as in Example 5, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 60:32:8 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0086] [Example 7] A solid-state secondary battery was fabricated in the same manner as in Example 5, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 50:40:10 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0087] [Example 8] A solid-state secondary battery was fabricated in the same manner as in Example 5, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 40:48:12 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0088] [Example 9] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the materials contained in the PTC layer was PP:CB:PAM = 70:15:15 (parts by mass) and carbon material:water-soluble resin = 5:5 (mass ratio), and the above evaluation was performed. The results are shown in Table 1.

[0089] [Example 10] A solid-state secondary battery was fabricated in the same manner as in Example 9, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 60:20:20 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0090] [Example 11] A solid-state secondary battery was fabricated in the same manner as in Example 9, except that the mass ratio of the materials contained in the PTC layer was set to PP:CB:PAM = 50:25:25 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0091] [Example 12] A solid-state secondary battery was prepared in the same manner as in Example 1, except that polyethylene oxide (hereinafter referred to as "PEO") was used as the nonionic water-soluble resin for the PTC layer, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0092] [Example 13] A solid-state secondary battery was fabricated in the same manner as in Example 12, except that the mass ratio of the components contained in the PTC layer was set to PP:CB:PEO = 50:35:15 (parts by mass), and the above evaluation was performed. The results are shown in Table 1.

[0093] [Example 14] A solid-state secondary battery was prepared in the same manner as in Example 3, except that polyvinylpyrrolidone (hereinafter referred to as "PVP") was used as the nonionic water-soluble resin for the PTC layer, and the above measurements and evaluations were performed. The results are shown in Table 1.

[0094] [Table 1]

[0095] As shown in Table 1, all of Examples 1 to 14 did not exhibit short circuits during charge-discharge tests, demonstrating excellent PTC functionality. Furthermore, as shown in Figure 2, for example, in the anode for a solid-state secondary battery fabricated in Example 1, it was confirmed that a metal layer 24 with lithium or the like uniformly deposited between the anode active material layer 22 and the PTC layer 23 was formed after the charge-discharge test. Therefore, Examples 1 to 14 demonstrate that it is possible to provide anode for a solid-state secondary battery that achieves both high energy density and improved reliability through anode-free anode technology that allows lithium or the like to be used as the active material of the anode, and excellent PTC functionality, as well as a solid-state secondary battery using such an anode.

[0096] [Example 15] The following charge-discharge tests were performed on the solid-state rechargeable battery prepared in Example 13. The charge-discharge tests were conducted by placing the solid-state rechargeable battery in a constant temperature bath at 25°C. In the first cycle, the battery voltage reached 4.25V at a rate of 60.0mA (approximately 20.8mA / cm²). 2 The battery was charged with a constant current, and then charged at a constant voltage until the battery voltage reached 4.25V and the current dropped to 0.26mA.

[0097] After the initial charge in the charge-discharge test, the cross-section of the solid-state secondary battery was observed using a scanning electron microscope (SEM) to confirm lithium deposition. Observation and photography were performed at a magnification of 500x. The results are shown in Figure 3. As shown in Figure 3, it was confirmed that lithium and other materials were uniformly deposited in the negative electrode active material layer 22. Furthermore, when checking for short circuits during the charge-discharge test, no short circuits were observed.

[0098] [Comparative Example 1] A solid-state secondary battery was prepared in the same manner as in Example 2, except that the thermoplastic resin used for the PTC layer was polyvinylidene fluoride (hereinafter referred to as "PVDF"), and a charge-discharge test was performed in the same manner as in Example 15. After the charge-discharge test, the cross-section of the solid-state secondary battery was observed using a SEM to confirm the deposition of lithium. Observation and photography were performed at a magnification of 2000x. The results are shown in Figure 4. As shown in Figure 4, in Comparative Example 1, it was confirmed that lithium and other materials were deposited non-uniformly in the negative electrode active material layer 22. In addition, when checking for the presence or absence of a short circuit during the charge-discharge test, the occurrence of a short circuit was observed. [Explanation of symbols]

[0099] 1 Solid state secondary battery 10 positive electrode 11 Positive electrode current collector 12 Cathode active material layer 20 negative electrode 21 Negative electrode current collector 22 Negative electrode active material layer 23 PTC layer 24 metal layer 30 Solid electrolyte layer 40 Electrolyte negative electrode structure

Claims

1. A negative electrode for a solid-state secondary battery comprising a negative electrode active material layer, a PTC layer, and a negative electrode current collector, The negative electrode active material layer contains a negative electrode active material that forms at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds. The PTC layer contains a polyolefin-based thermoplastic resin, a carbon material, and a nonionic water-soluble resin. The PTC layer is disposed between the negative electrode active material layer and the negative electrode current collector, A negative electrode for a solid-state secondary battery, characterized in that, in a charged state, at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds is deposited near the negative electrode active material layer.

2. The negative electrode for a solid secondary battery according to claim 1, wherein the PTC layer contains, per 100 parts by mass of the total material constituting the PTC layer, 40 parts by mass or more and 80 parts by mass of the polyolefin-based thermoplastic resin, 10 parts by mass or more and 50 parts by mass of the carbon material, and 5 parts by mass or more and 30 parts by mass of the nonionic water-soluble resin.

3. The negative electrode for a solid secondary battery according to claim 2, wherein the PTC layer contains, per 100 parts by mass of the total material constituting the PTC layer, 50 parts by mass or more and 70 parts by mass of the polyolefin-based thermoplastic resin, 15 parts by mass or more and 40 parts by mass of the carbon material, and 5 parts by mass or more and 25 parts by mass of the nonionic water-soluble resin.

4. The negative electrode for a solid secondary battery according to claim 1, wherein the thickness of the PTC layer is 1 μm or more and 10 μm or less.

5. The negative electrode for a solid secondary battery according to claim 1, wherein the negative electrode active material comprises at least one selected from the group consisting of amorphous carbon, gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc.

6. The negative electrode for a solid secondary battery according to claim 5, wherein the negative electrode active material contains amorphous carbon.

7. The aforementioned negative electrode active material is (a) amorphous carbon and (b) At least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, magnesium, and zinc The negative electrode for a solid secondary battery according to claim 5, which is a mixture of the above.

8. The negative electrode for a solid secondary battery according to claim 5, wherein the negative electrode active material layer contains 33 parts by mass or more and 95 parts by mass or less of amorphous carbon with respect to 100 parts by mass of the total content of the materials constituting the negative electrode active material layer.

9. A solid-state secondary battery comprising a positive electrode, a solid electrolyte layer, and a negative electrode, The positive electrode comprises a positive electrode active material layer and a positive electrode current collector. The solid electrolyte layer is disposed between the positive electrode and the negative electrode. A solid-state secondary battery characterized in that the negative electrode is a negative electrode for a solid-state secondary battery as described in any one of claims 1 to 8.

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