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 contributes to achieving high energy density, and a solid-state secondary battery using the said negative electrode. [Solution] A negative electrode for a solid-state secondary battery comprising a negative electrode active material layer, a protective layer, and a negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material that forms lithium or the like and a first resin material, the first resin material includes at least one resin material selected from either a hydrophobic resin material group X or a hydrophilic resin material group Y, the protective layer contains a second resin material and a carbon material, the second resin material includes at least one resin material selected from either a resin material group X not included in the negative electrode active material layer or a resin material group Y not included in the negative electrode active material layer, the protective layer is disposed between the negative electrode active material layer and the negative electrode current collector, and the negative electrode current collector contains at least a portion of aluminum or an aluminum alloy.

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. Since these can be used as active materials, the characteristics of the all-solid-state secondary battery are improved. 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 make the all-solid-state secondary battery thinner while increasing its output.

[0004] However, when increasing the energy density of solid-state secondary batteries using such technology, it is known that side reactions may occur between the lithium, lithium alloy, or lithium compound deposited on the negative electrode current collector and the negative electrode current collector during the charging process. Non-patent document 1 discloses that such side reactions occur particularly when aluminum foil or the like is used as the negative electrode current collector of an anode-free type negative electrode. Furthermore, when side reactions occur, the intercalation and release of lithium present in the solid-state secondary battery becomes substantially impossible, resulting in a decrease in the capacity and energy density of the solid-state secondary battery.

[0005] To prevent side reactions between the negative electrode current collector and lithium, lithium alloy, or lithium compound present in the negative electrode, a technique for providing a functional layer on the surface of the negative electrode current collector is being considered. Patent Document 2 discloses a technique for placing an alloying prevention layer between a negative electrode current collector made of aluminum or an aluminum alloy and a negative electrode containing a mixed powder of negative electrode active material particles and sulfide solid electrolyte particles. According to the content disclosed in Patent Document 2, the alloying prevention layer consists only of graphite. However, when an alloying prevention layer is applied to an anode-free type negative electrode, voids exist between the graphite particles constituting the alloying prevention layer because there is no material to hinder the diffusion of lithium. Therefore, there is a concern that lithium may precipitate in the voids during the charging process, and that the lithium may reach the negative electrode current collector, causing a side reaction. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-96610 [Patent Document 2] Japanese Patent Publication No. 2012-164571 [Non-patent literature]

[0007] [Non-Patent Document 1] Burak Aktekin et al., “SEI growth on Lithium metal anodes in solid-state batteries quantified with coulometric titration time analysis,” Nature Communications, October 31, 2023, No. 9, Article No. 6946 [Non-Patent Document 2] 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]

[0008] The present invention aims to provide a negative electrode for a solid-state secondary battery that contributes to achieving high energy density, and a solid-state secondary battery using the said negative electrode. [Means for solving the problem]

[0009] An embodiment of the present invention provides a negative electrode for a solid-state secondary battery comprising a negative electrode active material layer, a protective 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, and a first resin material, wherein the first resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y. The protective layer contains a second resin material and a carbon material, wherein the second resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y, which are not included in the negative electrode active material layer. Furthermore, the protective layer is disposed between the negative electrode active material layer and the negative electrode current collector, and the negative electrode current collector contains at least a portion of aluminum or an aluminum alloy. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a negative electrode for a solid-state secondary battery that contributes to achieving a high energy density, and a solid-state secondary battery using the negative electrode for the solid-state secondary battery. [Brief explanation of the drawing]

[0011] [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 the surface state of the negative electrode current collector of the solid-state secondary battery fabricated in Example 1. [Figure 4] It is a SEM image showing the surface state of the negative electrode current collector of the solid-state secondary battery fabricated in Comparative Example 1.

Mode for Carrying Out the Invention

[0012] 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. In addition, the negative electrode for a solid secondary battery may be simply referred to as the "negative electrode" for convenience.

[0013] <Solid secondary battery> First, an example of the solid secondary battery according to the present 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 protective layer 23. The protective 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. Further, the negative electrode 20 corresponds to the negative electrode for a solid secondary battery described later.

[0014] (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. Further, 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 the solid electrolyte contained in the solid electrolyte layer 30 described later Often, they don't have to be of the same species.

[0015] Examples of materials for the positive electrode current collector include indium (In), copper (Cu), 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 any 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 to 20 μm.

[0016] 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.

[0017] 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.

[0018] Examples of lithium salts of transition metal oxides having such a layered rock salt structure include, for example, LiNi x CoyAl z O2 (NCA), or LiNi x Co y MnzO2 (NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1), etc. Lithium salts of ternary transition metal oxides are exemplified.

[0019] 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.

[0020] The positive electrode active material may be covered by a coating layer. Such a coating layer is not particularly limited as long as it is known as a coating layer for the positive electrode active material of a solid secondary battery. Examples include Li2O-ZrO2.

[0021] Further, 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.

[0022] Examples of the shape of the positive electrode active material include particle shapes such as true spherical and ellipsoidal. Further, the particle size of the positive electrode active material is not particularly limited as long as it is within the range applicable to the positive electrode active material of a conventional solid secondary battery. The content of the positive electrode active material is also not particularly limited as long as it is within the range applicable to the positive electrode of a conventional solid secondary battery.

[0023] In addition to the above-described positive electrode current collector, positive electrode active material layer, and solid electrolyte, the positive electrode may be appropriately blended with, for example, a conductive aid, a binder, a filler, a dispersant, an ion conduction aid, etc.

[0024] 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.

[0025] (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 protective layer 23. The protective 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. So-called anode-free type (where lithium is deposited in the charged state) negative electrodes generally have a high energy density, but due to the high reactivity of the lithium deposited on the negative electrode current collector, as described above, a side reaction may occur between the negative electrode current collector and the lithium, which can lead to a decrease in the energy density of the battery. In the negative electrode 20 according to this embodiment, a predetermined protective layer 23, which will be described later, is provided between the negative electrode current collector 21 and the negative electrode active material layer 22. Specifically, by using resin materials exhibiting different properties in the first resin material contained in the negative electrode active material layer 22 and the second resin material contained in the protective layer 23, that is, by using resin materials in which one of the first and second resin materials exhibits hydrophilicity and the other exhibits hydrophobicity, 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 between the negative electrode active material layer 22 and the protective layer 23 during the charging process of the solid secondary battery 1, the adhesive state between the negative electrode active material layer 22 and the protective layer 23 can be easily resolved, resulting in uniform deposition of lithium, etc. As a result, the protective layer 23 can sufficiently suppress side reactions between lithium, etc. and the negative electrode current collector 21, and as a result, a negative electrode 20 that contributes to the realization of a high energy density can be provided. 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, while examples of lithium compounds include lithium carbide.

[0026] The negative electrode current collector contains at least a portion of aluminum or an aluminum alloy. The material of the negative electrode current collector may also be aluminum or an aluminum alloy. Aluminum or aluminum-containing alloys are highly valuable because they are flexible, lightweight, relatively easy to handle, and low-cost. The shape of the negative electrode current collector can be, for example, a plate or foil. The thickness of the negative electrode current collector is not particularly limited, but is preferably between 1 μm and 20 μm.

[0027] The negative electrode active material layer contains a negative electrode active material that forms lithium, etc., and in the charged state, lithium, etc. is deposited on the negative electrode, contributing to the realization of a high energy density. At that time, a predetermined protective layer, described later, is placed between the negative electrode active material layer and the negative electrode current collector, thereby suppressing the diffusion of lithium, etc. deposited on the negative electrode to the negative electrode current collector during the charging process. As a result, side reactions that may occur between the lithium, etc. deposited on the negative electrode and the negative electrode current collector are suppressed, and the decrease in high energy density can be suppressed. Hereinafter, this function will be referred to as the "current collector protection function".

[0028] Furthermore, the negative electrode active material layer contains a first resin material in addition to the negative electrode active material. Here, the first resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y. By including a resin material selected from the resin material group exhibiting either of these properties in the negative electrode active material layer, the negative electrode active material layer can be stabilized on the surface of the protective layer. The resin materials included in the first resin material may be used individually from the same resin group, or two or more may be used in combination.

[0029] The hydrophobic resin material group X is a group of resin materials that consist of hydrophobic resin materials such as polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF / HFP), and polytetrafluoroethylene (PTFE).

[0030] On the other hand, hydrophilic resin material group Y is a group of resin materials consisting of hydrophilic resin materials such as carboxymethylcellulose (CMC) or its salts, polyacrylamide (PAM) or its salts, polyethylene oxide (PEO) or its salts, polyvinylpyrrolidone (PVP) or its salts, and polyacrylic acid (PAA) or its salts. Examples of salt forms include lithium salts, sodium salts, potassium salts, and ammonium salts.

[0031] The content of the first resin material is preferably 1 to 20 parts by mass, and more preferably 3 to 15 parts by mass, based on 100 parts by mass of the total content of the materials constituting the negative electrode active material layer.

[0032] 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 in shape, 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.

[0033] 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.

[0034] Furthermore, the negative electrode active material layer may contain additives used in conventional solid-state secondary batteries, such as fillers and dispersants, as appropriate. Examples of insulating fillers include styrene-butadiene copolymer (SBR). As dispersants, known materials generally used in electrodes of solid-state secondary batteries can be used.

[0035] The thickness of the negative electrode active material layer is not particularly limited, but is preferably 1 μm to 20 μm. A thickness of 1 μm to 20 μm in the negative electrode active material layer suppresses an increase in the resistance value of the negative electrode active material layer, thereby maintaining the characteristics of the solid-state secondary battery. The thickness of the negative electrode active material layer is determined, for example, by scanning the cross-section after assembling and pressure-molding the solid-state secondary battery. It can be estimated by observing with a scanning electron microscope (SEM).

[0036] The protective layer contains a second resin material. The second resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y, which are not included in the negative electrode active material layer. In this way, by selecting the resin material included in the second resin material from a resin material group different from the first resin material, i.e., from the resin material group that is not included in the negative electrode active material layer, the adhesive force between the negative electrode active material layer and the protective layer is relaxed when lithium, etc., is deposited between the negative electrode active material layer, which contains the first resin material exhibiting one of the hydrophilic or hydrophobic properties, and the protective layer, which contains the second resin material exhibiting the other of the hydrophilic or hydrophobic properties, during the charging process, and the adhesive state of each layer is easily resolved. As a result, lithium, etc., is uniformly deposited between the negative electrode active material layer and the protective layer during the charging process, so the occurrence of short circuits can be suppressed. The resin materials included in the second resin material may be used individually from the same group of resins, or two or more may be used in combination.

[0037] Furthermore, because the diffusion coefficient of lithium ions in the second resin material is small, side reactions that may occur between lithium deposited on the negative electrode and the negative electrode current collector are suppressed, thereby suppressing the decrease in energy density. As the content of the second resin material changes, there is a trade-off relationship between the battery characteristics and the current collector protection function described above, so there is a suitable range for the content of the second resin material. When the content of the second resin material is high, the current collector protection function tends to improve, but because the second resin material exhibits electrical insulation properties, it may hinder the current when the battery is operating normally, thus impairing the battery characteristics. On the other hand, when the content of the second resin material is low, the battery characteristics tend not to be impaired, but it becomes difficult to ensure sufficient current collector protection. For this reason, the content of the second resin material is preferably 30 parts by mass or more and 99 parts by mass or less, and more preferably 60 parts by mass or more and 80 parts by mass or less, per 100 parts by mass of the total content of the materials constituting the protective layer.

[0038] Furthermore, the protective layer contains a carbon material in addition to the second resin material. The carbon material forms conductive paths necessary for normal battery operation within the protective layer. Carbon material is preferably used as a conductive filler because it reacts little with sulfides and is inexpensive. The carbon material is not particularly limited as long as it enhances the conductivity of the protective 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.

[0039] Regarding the carbon material content in the protective layer, there is a suitable range due to the trade-off relationship between battery characteristics and current collector protection function. When the carbon material content is high, battery characteristics tend not to be impaired, but the resin material content in the protective layer decreases, making it difficult to adequately ensure current collector protection function. On the other hand, when the carbon material content is low, the current collector protection function tends to improve as the resin material content increases, but there is a concern that battery characteristics may be impaired if sufficient conductive paths necessary for normal battery operation cannot be formed. Therefore, the carbon material content is preferably 1 to 70 parts by mass, and more preferably 20 to 40 parts by mass, per 100 parts by mass of the total content of the materials constituting the protective layer.

[0040] If the particle size of the carbon material contained in the protective layer is too large, coating the protective 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 ​​carbon material is 3 m². 2 It is preferable that it is 6m or more per g. 2 It is more preferable that it be 30 mm 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.

[0041] The protective layer may further contain an insulating filler. Examples of such insulating fillers include polypropylene (PP). If the content of the second resin material is less than the content of the carbon material, and the second resin material is selected from the hydrophilic resin material group, there is a concern that the protective layer may become excessively conductive, leading to unstable electrical properties. Therefore, the content of the insulating filler is 0 parts by mass or more and 69 parts by mass or less, and may be 20 parts by mass or more and 65 parts by mass or less, based on the total content of the materials constituting the protective layer per 100 parts by mass.

[0042] Since the protective layer does not contain active material, it is composed of non-active material. Therefore, the smaller the film thickness of the protective layer, the higher the volume energy density of the battery. On the other hand, as described above, the current collector protection function is a function of suppressing the diffusion of lithium or the like deposited on the negative electrode to the negative electrode current collector during the charging process, and there is a positive correlation between the current collector protection function and the thickness of the protective layer. Therefore, the thicker the protective layer, the better the current collector protection function. As the thickness of the protective layer changes, there is a trade-off relationship between the volume energy density of the battery and the current collector protection. Therefore, there is a suitable range for the thickness of the protective layer. Therefore, the thickness of the protective layer is preferably 0.5 μm or more and 10 μm or less, and more preferably 2 μm or more and 8 μm or less. Thus, by appropriately controlling the thickness of the protective layer, it is possible to achieve both the volume energy density of the battery and the current collector protection function.

[0043] (Solid electrolyte layer) As shown in FIG. 1, the solid electrolyte layer 30 is disposed 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 capable of moving ions.

[0044] 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, for example, Li2S-P2S5, Li2S-P2S5-LiX (X is, for example, 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[[ID=…]] n (m and n are positive numbers, Z is any one of Ge, Zn or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO qExamples include (where p and q are positive numbers, and M is one of P, Si, Ge, B, Al, Ga, or In). As the solid electrolyte, one material selected from these sulfide-based solid electrolyte materials may be used, or two or more materials may be used in combination.

[0045] As a solid electrolyte, it is preferable to use a sulfide solid electrolyte material containing sulfur (S), phosphorus (P), and lithium (Li) as constituent elements, and more specifically, it is preferable to use a sulfide-based solid electrolyte material containing Li2S-P2S5, and more particularly Li2S-P2S5-LiCl. When using a sulfide-based solid electrolyte material containing Li2S-P2S5, the molar ratio of Li2S to P2S5 is, for example, in the range of Li2S:P2S5 = 50:50 to 90:10. It may be selected.

[0046] The solid electrolyte may be in an amorphous state or a crystalline state. It may also be a mixture of amorphous and crystalline states.

[0047] 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.

[0048] 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 internal short circuits in the battery, it is preferably 10 μm or more.

[0049] <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.

[0050] (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.

[0051] (2) Negative electrode fabrication process First, various slurries (which may also be pastes) are prepared by adding the materials constituting the negative electrode active material layer 22 (negative electrode active material, first resin material, etc.) and the materials constituting the protective layer 23 (carbon material, second resin material, etc.) to a polar solvent or a non-polar solvent, respectively. Next, the obtained protective layer slurry is applied onto the prepared negative electrode current collector 21 and dried to obtain an intermediate laminate in which the protective layer 23 is formed on the negative electrode current collector 21. Furthermore, the negative electrode active material layer slurry is applied onto the protective 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 protective layer 23. Next, the obtained laminate is punched out to a predetermined size to produce the negative electrode 20. Furthermore, 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, comma coating, knife coating, etc.

[0052] (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.

[0053] First, the starting material is processed by the melt-quenching method or the mechanical milling method, which is used to produce sulfide-based Obtain a solid electrolyte material.

[0054] 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, and more preferably 800°C to 900°C. The reaction time is preferably 0.1 hours to 12 hours, and 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 / sec to 10000°C / sec, and more preferably 1°C / sec to 1000°C / sec.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] (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.

[0059] <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 on the back side of the negative electrode active material layer 22, that is, between the protective layer 23 and the negative electrode active material layer 22. This lithium forms a metal layer 24 containing lithium alloys and lithium compounds that were not present at the time of manufacture. During discharge, the 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 the protective layer 23 is positioned between the negative electrode active material layer 22 and the negative electrode current collector 21, uneven deposition of lithium and other materials can be suppressed. This suppresses short circuits and capacity degradation of the solid-state secondary battery 1, and consequently improves the characteristics of the solid-state secondary battery 1.

[0060] 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.

[0061] Based on the embodiments described above, the present invention relates to the following [1] to

[10] . [1] A negative electrode for a solid-state secondary battery comprising a negative electrode active material layer, a protective 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, and a first resin material. The first resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y. The protective layer contains a second resin material and a carbon material. The second resin material includes at least one resin material selected from either the hydrophobic resin material group X not included in the negative electrode active material layer or the hydrophilic resin material group Y not included in the negative electrode active material layer. The protective layer is disposed between the negative electrode active material layer and the negative electrode current collector, The negative electrode for a solid secondary battery is characterized in that the negative electrode current collector contains at least a portion of aluminum or an aluminum alloy. [2] The protective layer contains, with respect to 100 parts by mass of the total amount of materials constituting the protective layer, 30 parts by mass or more and 99 parts by mass of the second resin material and 1 part by mass or more and 70 parts by mass of the carbon material, as described in [1] above, a negative electrode for a solid secondary battery. [3] The hydrophobic resin material group X is a group of resin materials consisting of polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF / HFP), and polytetrafluoroethylene (PTFE), as described in [1] or [2] above, for a negative electrode for a solid secondary battery. [4] The hydrophilic resin material group Y is a group of resin materials consisting of carboxymethylcellulose (CMC) or a salt thereof, polyacrylamide (PAM) or a salt thereof, polyethylene oxide (PEO) or a salt thereof, polyvinylpyrrolidone (PVP) or a salt thereof, and polyacrylic acid (PAA) or a salt thereof, as described in any one of [1] to [3] above, for a negative electrode for a solid secondary battery. [5] A negative electrode for a solid secondary battery according to any one of [1] to [4] above, wherein the thickness of the protective layer is 0.5 μm or more and 10 μm or less. [6] The negative electrode for a solid secondary battery according to any one of [1] to [5] 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. [7] The negative electrode active material is a negative electrode for a solid secondary battery as described in [6] above, comprising amorphous carbon. [8] 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 [6] or [7] above, which is a mixture of the above. [9] The negative electrode for a solid secondary battery according to any one of [6] to [8] above, wherein the negative electrode active material layer contains 33 parts by mass to 95 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.

[10] 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 [1] to [9] above. [Examples]

[0062] 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 carried out at room temperature, and room temperature is defined as being within the range of 20°C ± 5°C.

[0063] [Example 1] A solid-state rechargeable battery was fabricated using the following process. The components were arranged as shown in Figure 1.

[0064] <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. This active material was coated with Li2O-ZrO2 using the method described in Non-Patent Literature 2. 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).

[0065] <Fabrication of the negative electrode> As detailed below, a 15 μm thick aluminum foil was prepared as the negative electrode current collector 21, a protective layer 23 was formed on the surface of the aluminum foil, and a negative electrode active material layer 22 was formed on the surface of the protective layer 23 that is opposite to the positive electrode 10.

[0066] (Preparation of protective layer) As a carbon material, it has a nitrogen adsorption specific surface area of ​​approximately 16 m². 2Carbon black (furnace black, hereinafter also referred to as "CB") at a concentration of / g was prepared. As a second resin material, an aqueous solution of polyacrylamide (solid content concentration 7.3 parts by mass) was prepared. As an insulating filler, an aqueous dispersion of polypropylene (hereinafter also referred to as "PP") resin fine particles (solid content concentration 25 parts by mass) was prepared. Next, 2.0 g of the carbon material and 128.4 g of the aqueous solution of polyacrylamide (hereinafter also referred to as "PAM") 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 liquid was approximately 10%.

[0067] 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 protective layer (hereinafter also referred to as "protective layer slurry"). The solid content concentration of the protective layer slurry was approximately 16%.

[0068] The protective layer slurry obtained by the method described above was applied to a 15 μm thick aluminum foil, which served as the negative electrode current collector 21, using an applicator. The aluminum foil was then dried in a constant temperature bath set to 80°C for 10 minutes to obtain a protective layer 23 formed on the aluminum foil. The thickness of the protective layer after drying was approximately 5 μm. At this time, the mass ratio of the materials of the components contained in the protective layer was PP:CB:PAM = 60:28:12 (parts by mass), as shown in Table 1, and the ratio of resin material to carbon material was 72:28 (parts by mass).

[0069] (Fabrication of the negative electrode active material layer) As a negative electrode active material, nitrogen adsorption specific surface area: approximately 52 m²2 CB with a DBP absorption of approximately 193 ml / 100g and silver particles with a particle size of 60 nm were 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 the CB and 1 g of silver particles were placed in a container, and 4 g of an N-methylpyrrolidone (NMP) solution containing 5% by mass of the first resin material (polyvinylidene fluoride (PVDF)) was added. 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 protective 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 protective 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, an aluminum 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 ratio of active material:resin material = 4:0.2 (parts by mass).

[0070] <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.

[0071] (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 included in the negative electrode 20 is in contact with the surface of the release film, and the negative electrode active material layer 22 included in the negative electrode 20 is in contact with the solid electrolyte sheet, and the arrangement is made so as to maximize the contact area between the negative electrode active material layer 22 and the solid electrolyte sheet. This arrangement is not disturbed. While doing so, another release film is placed on top and secured with polyimide tape. Furthermore, these are 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 is removed to create 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.

[0072] <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.

[0073] Furthermore, this solid-state rechargeable battery was sandwiched between two stainless steel plates, each 5.5 cm square and approximately 1 cm thick, on both sides in the stacking direction. Each of the two stainless steel plates had four holes in the same location, and the solid-state rechargeable battery was positioned inside the square formed by these four holes. In this state, one bolt was passed through each of the four holes from the outside of the two stainless steel plates. Then, a pressure of approximately 4 MPa was applied to the solid-state rechargeable battery by tightening the four bolts with nuts to press down on the two stainless steel plates from the outside.

[0074] <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 performed by placing the solid-state secondary battery in a constant temperature bath at 25°C. In the first cycle, the charge was 1.5mA (approximately 0.52mA / cm²) until the battery voltage reached 4.25V. 2 The battery was charged with a constant current of 0.26mA until the battery voltage reached 4.25V. Then, it was charged at a constant voltage of 1.5mA (approximately 0.52mA / cm²) until the battery voltage reached 2.5V. 2 Discharge was performed with a constant current of ).

[0075] In the charge-discharge test, we checked for the presence or absence of short circuits during the test. Furthermore, a discharge capacity of 150 mAh / g or more per unit mass of active material was evaluated as "OK," and a discharge capacity of 150 mAh / g or more per unit mass of active material was evaluated as "NG." The results are shown in Table 1. However, if a short circuit occurred during the charge-discharge test, the discharge capacity was not measured.

[0076] <SEM observation of lithium deposited during the charging process> After the charge-discharge test, the cross-section of the solid-state secondary battery was observed using a scanning electron microscope (SEM) to confirm the deposition of lithium. Observation and imaging were performed at a magnification of 2000x. 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.

[0077] <Evaluation of side reactions between lithium and the negative electrode current collector> After the charge-discharge test, the solid-state secondary battery was removed from the charge-discharge device, disassembled in a dry room, and only the negative electrode current collector was removed. An arbitrary surface opposite to the surface coated with the protective layer was observed using a scanning electron microscope (SEM). Observation and photography were performed at a magnification of 1000x. The results are shown in Figure 3.

[0078] <Evaluation of the uniformity of deposited lithium> During the charge-discharge test, the solid-state battery was removed from the charge-discharge device after the charge-discharge test program was stopped while the battery was fully charged. The solid-state battery was then taken to a dry room, the aluminum laminate film sealing the battery was removed, and an arbitrary surface of the negative electrode current collector, opposite to the surface where the protective layer was applied, was visually inspected.

[0079] In the visual observation described above, if the smoothness of the negative electrode current collector was clearly impaired, the lithium deposition was evaluated as non-uniform; otherwise, it was evaluated as uniform. The results are shown in Table 1.

[0080] [Example 2] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the components in the protective layer was set to PAM:CB = 70:30 and the insulating filler was omitted, and the above evaluation was performed. The results are shown in Table 1.

[0081] [Example 3] A solid-state secondary battery was fabricated in the same manner as in Example 2, except that the first resin material in the negative electrode active material layer contained 2% by mass of CMC and the insulating filler contained 3% by mass of SBR (styrene-butadiene copolymer), and the second resin material in the protective layer contained PVDF, with the mass ratio of the components in the protective layer being PVDF:CB = 80:20. The evaluation described above was then performed. The results are shown in Table 1.

[0082] [Example 4] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that an aluminum-deposited PET film was used as the material for the negative electrode current collector foil, and the above evaluation was performed. The results are shown in Table 1.

[0083] [Example 5] A solid-state secondary battery was fabricated in the same manner as in Example 2, except that PAA was used as the second resin material and carbon nanotubes (hereinafter also referred to as "CNT") as the carbon material in the protective layer, and the above evaluation was performed. The results are shown in Table 1.

[0084] [Example 6] A solid-state secondary battery was fabricated in the same manner as in Example 2, except that PVP was used as the second resin material and CNT as the carbon material in the protective layer, and the above evaluation was performed. The results are shown in Table 1.

[0085] [Example 7] A solid-state secondary battery was fabricated in the same manner as in Example 2, except that CMC was used as the second resin material and CNT as the carbon material in the protective layer, and the mass ratio of the components in the protective layer was set to CMC:CNT = 99:1. The evaluation described above was then performed. The results are shown in Table 1.

[0086] [Example 8] A solid-state secondary battery was fabricated in the same manner as in Example 3, except that the mass ratio of the components in the protective layer was set to PVDF:CB = 30:70, and the above evaluation was performed. The results are shown in Table 1.

[0087] [Comparative Example 1] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the protective layer 23 was not provided, and the above-mentioned An evaluation was conducted. The results are shown in Table 1. In addition, to evaluate the side reaction between lithium and the negative electrode current collector, the negative electrode current collector was removed, as in Figure 3, and an arbitrary surface opposite to the surface facing the negative electrode active material layer was observed using SEM. The resulting SEM image is shown in Figure 4.

[0088] [Comparative Example 2] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material layer 22 was omitted, and the above evaluation was performed. The results are shown in Table 1.

[0089] [Comparative Example 3] A solid-state secondary battery was fabricated in the same manner as in Example 2, except that PVDF was used as the first resin material in the negative electrode active material layer, and the mass ratio of the components in the protective layer was set to PVDF:CB = 8:92. The evaluation described above was then performed. The results are shown in Table 1.

[0090] [Comparative Example 4] A solid-state secondary battery was fabricated in the same manner as in Example 4, except that the negative electrode active material layer contained 2% by mass of CMC as the first resin material and 3% by mass of SBR as the insulating filler, and the above evaluation was performed. The results are shown in Table 1.

[0091] [Table 1]

[0092] As shown in Table 1, none of the Examples 1-8 exhibited short circuits during the charge-discharge test, demonstrating excellent performance. It was found that the negative electrode possesses discharge capacity and high energy density. Furthermore, as shown in Figure 2, for example, in the negative electrode for a solid secondary battery fabricated in Example 1, it was confirmed that a metal layer 24 with lithium uniformly deposited between the negative electrode active material layer 22 and the protective layer 23 was formed after the charge-discharge test. Therefore, Examples 1 to 8 demonstrate that an anode-free negative electrode technology that can use lithium (at least one selected from the group consisting of lithium, lithium alloys, and lithium compounds) as the active material of the negative electrode, along with a current collector protection function, can contribute to achieving high energy density, and that a solid secondary battery with high energy density can be provided by incorporating such a negative electrode.

[0093] Comparing Example 1 with Comparative Example 1, it was found that Comparative Example 1, which lacked a protective layer, had a smaller discharge capacity and did not possess a sufficiently high energy density. In Comparative Example 1, it is thought that a side reaction occurred between the lithium deposited on the negative electrode and the aluminum foil, which is the negative electrode current collector, during the charging process, resulting in a smaller discharge capacity and a decrease in energy density. Furthermore, comparing Figure 3 and Figure 4, it can be seen that the smoothness of the negative electrode current collector is lost in Figure 4 compared to Figure 3. From these observations, it can be seen that providing a protective layer can suppress side reactions in the negative electrode current collector containing aluminum foil.

[0094] Comparing Example 1 with Comparative Example 2, Comparative Example 2, which lacked a negative electrode active material layer containing the negative electrode active material and the first resin material, experienced a short circuit. This indicates that by providing a negative electrode active material layer, a high energy density can be achieved without the occurrence of a short circuit.

[0095] Comparing Example 1 with Comparative Example 3, a short circuit occurred in Comparative Example 3, where hydrophobic resin materials were used as the first and second resin materials, respectively. From this, it is considered that by using resin materials exhibiting different properties (one being hydrophilic and the other hydrophobic) as the first and second resin materials, the adhesion between the negative electrode active material layer and the protective layer can be easily resolved during the charging process, allowing lithium and other materials to precipitate uniformly and suppressing the occurrence of a short circuit.

[0096] In the comparison between Example 1 and Comparative Example 4, similar to the comparison between Example 1 and Comparative Example 3, a short circuit occurred in Comparative Example 4, where hydrophilic resin materials were used as the first and second resin materials, respectively. Therefore, it is considered that the same phenomenon as described above occurred in Comparative Example 4, resulting in a short circuit. [Explanation of Symbols]

[0097] 1 Solid state secondary battery 10 positive electrode 11 Positive electrode current collector 12 Cathode active material layer 20 Negative electrode 21 Negative current collector 22 Negative active material layer 23 Protective Layer 24 Metal Layers 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 protective 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, and a first resin material. The first resin material includes at least one resin material selected from either the hydrophobic resin material group X or the hydrophilic resin material group Y. The protective layer contains a second resin material and a carbon material. The second resin material includes at least one resin material selected from either the hydrophobic resin material group X not included in the negative electrode active material layer or the hydrophilic resin material group Y not included in the negative electrode active material layer. The protective layer is disposed between the negative electrode active material layer and the negative electrode current collector, The negative electrode for a solid secondary battery is characterized in that the negative electrode current collector contains at least a portion of aluminum or an aluminum alloy.

2. The negative electrode for a solid secondary battery according to claim 1, wherein the protective layer contains 30 to 99 parts by mass of the second resin material and 1 to 70 parts by mass of the carbon material, per 100 parts by mass of the total content of the materials constituting the protective layer.

3. The anode for a solid secondary battery according to claim 1, wherein the hydrophobic resin material group X comprises polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF / HFP), and polytetrafluoroethylene (PTFE).

4. The hydrophilic resin material group Y is a group of resin materials consisting of carboxymethylcellulose (CMC) or a salt thereof, polyacrylamide (PAM) or a salt thereof, polyethylene oxide (PEO) or a salt thereof, polyvinylpyrrolidone (PVP) or a salt thereof, and polyacrylic acid (PAA) or a salt thereof, as described in claim 1, for a negative electrode for a solid secondary battery.

5. The negative electrode for a solid secondary battery according to claim 1, wherein the thickness of the protective layer is 0.5 μm or more and 10 μm or less.

6. The negative electrode for a solid secondary battery according to claim 1, 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.

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

8. 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 6, which is a mixture of the above.

9. The negative electrode for a solid secondary battery according to claim 6, 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.

10. 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 9.

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