Secondary batteries

Abstract

To reduce resistance of a secondary battery having a deposition type lithium anode to improve cycle characteristics.SOLUTION: A secondary battery of the present disclosure includes: a cathode; a solid electrolyte layer; an anode current collector; and metal lithium as an anode active material deposited between the solid electrolyte layer and the anode current collector by charging. An Mg mixture layer is present between the solid electrolyte layer and the anode current collector. The solid electrolyte layer contains a first solid electrolyte. The Mg mixture layer contains Mg and a second solid electrolyte. The Young's modulus of the second solid electrolyte is lower than the Young's modulus of the first solid electrolyte.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application discloses a secondary battery. [Background technology]

[0002] Patent Document 1 discloses an all-solid-state battery comprising a positive electrode, a solid electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the electrolyte layer and the negative electrode current collector upon charging. In Patent Document 1, a metallic Mg layer is arranged between the solid electrolyte layer and the negative electrode current collector, allowing a Li-Mg alloy to be deposited as the metallic lithium during charging. Patent Document 2 discloses a battery comprising a positive electrode, a solid electrolyte layer, a protective layer, and a negative electrode, wherein the negative electrode contains metallic lithium and the protective layer contains a predetermined Li composite oxide. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-184513 [Patent Document 2] Japanese Patent Publication No. 2020-184407 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Secondary batteries with a deposited metallic lithium anode have room for improvement in terms of resistance and cycle characteristics. [Means for solving the problem]

[0005] This application discloses several embodiments as means for solving the above-mentioned problems. <Aspect 1> A secondary battery comprising a positive electrode, a solid electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the solid electrolyte layer and the negative electrode current collector upon charging, A Mg alloy layer exists between the solid electrolyte layer and the negative electrode current collector. The solid electrolyte layer contains a first solid electrolyte. The Mg alloy layer contains Mg and a second solid electrolyte. The Young's modulus of the second solid electrolyte is lower than that of the first solid electrolyte. Secondary battery. <Aspect 2> The Young's modulus of the second solid electrolyte is 1 GPa or more and 20 GPa or less. The secondary battery of Aspect 1. <Aspect 3> The first solid electrolyte is a sulfide solid electrolyte. The secondary battery of Aspect 1 or 2. <Aspect 4> The second solid electrolyte is a complex hydride containing Li. The secondary battery of any one of Aspects 1 to 3. <Aspect 5> A protective layer exists between the Mg alloy layer and the negative electrode current collector. The protective layer contains Mg and does not contain an electrolyte. The secondary battery of any one of Aspects 1 to 4. <Aspect 6> The positive electrode contains a lithium-containing oxide as a positive electrode active material. The secondary battery of any one of Aspects 1 to 5. <Aspect 7> The negative electrode current collector contains stainless steel. The secondary battery of any one of Aspects 1 to 6.

Advantages of the Invention

[0006] The secondary battery of the present disclosure has a low resistance and excellent cycle characteristics while including a deposited metal lithium negative electrode.

Brief Description of the Drawings

[0007] [Figure 1] An example of each configuration after charging and after discharging of the secondary battery 100 is schematically shown. [Figure 2]The diagram shows a schematic example of the configuration of the secondary battery 100 after charging and after discharging. [Figure 3] This diagram outlines an example of the manufacturing process for a secondary battery 100. [Modes for carrying out the invention]

[0008] 1. Secondary battery The secondary battery according to the embodiment will be described below with reference to the drawings, but the technology of this disclosure is not limited to the following embodiments. Figure 1 shows the configuration of a secondary battery 100 according to one embodiment. As shown in Figure 1, the secondary battery 100 comprises a positive electrode 10, a solid electrolyte layer 20, a negative electrode current collector 31, and metallic lithium 32 as a negative electrode active material deposited between the solid electrolyte layer 20 and the negative electrode current collector 31 by charging. A Mg mixture layer 33 is present between the solid electrolyte layer 20 and the negative electrode current collector 31. The solid electrolyte layer 20 contains a first solid electrolyte, and the Mg mixture layer 33 contains Mg and a second solid electrolyte. The Young's modulus of the second solid electrolyte is lower than that of the first solid electrolyte.

[0009] 1.1 Positive electrode The positive electrode 10 includes at least a positive electrode active material. During charging of the secondary battery 100, lithium ions released from the positive electrode active material reach the space between the solid electrolyte layer 20 and the negative electrode current collector 31 via the solid electrolyte layer 20, receive electrons, and precipitate as metallic lithium 32. During discharge of the battery, the metallic lithium 32 between the solid electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) and returns to the positive electrode 10. The form of the positive electrode 10 may be any form known as a positive electrode for a secondary battery. For example, as shown in Figure 1, the positive electrode 10 may comprise a positive electrode current collector 11 and a positive electrode active material layer 12.

[0010] 1.1.1 Positive electrode current collector The positive electrode current collector 11 can be any material capable of functioning as a positive electrode current collector for a secondary battery. The positive electrode current collector 11 may be a metal foil or a metal mesh. In particular, metal foil offers superior handling. The positive electrode current collector 11 may consist of multiple metal foils. Examples of metals that make up the positive electrode current collector 11 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, etc. In particular, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector 11 may contain Al. The positive electrode current collector 11 may have some kind of coating layer on its surface for the purpose of adjusting resistance, etc. Also, if the positive electrode current collector 11 consists of multiple metal foils, there may be some kind of layer between the multiple metal foils. The thickness of the positive electrode current collector 11 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0011] 1.1.2 Cathode active material layer The positive electrode active material layer 12 contains a positive electrode active material and may optionally contain an electrolyte, a conductive additive, a binder, etc. Furthermore, the positive electrode active material layer 12 may contain various additives. The respective content of the positive electrode active material, electrolyte, conductive additive, binder, etc. in the positive electrode active material layer 12 can be appropriately determined according to the desired battery performance. For example, with the entire positive electrode active material layer 12 (total solid content) as 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, or it may be 100% by mass or less, or 90% by mass or less. The shape of the positive electrode active material layer 12 is not particularly limited and may, for example, be a sheet with a substantially flat surface. The thickness of the positive electrode active material layer 12 is not particularly limited and may be, for example, 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, or it may be 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less.

[0012] The positive electrode active material can be any known positive electrode active material for secondary batteries, provided that it can supply lithium to the negative electrode side during charging. For example, lithium cobalt oxide, lithium nickel oxide, LiNi1 / 3 Co 1 / 3 Mn 1 / 3 Various lithium-containing oxides such as O2, lithium manganese oxide, and spinel-based lithium compounds can be used. Alternatively, a material in which lithium is adsorbed onto sulfur can be used as the positive electrode active material. In particular, when the positive electrode 10 contains a lithium-containing oxide as the positive electrode active material, lithium ions can be appropriately supplied from the positive electrode active material to the negative electrode side during charging, and the expansion and contraction of the positive electrode active material during charging and discharging is small, making it easier to obtain high performance. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination. The positive electrode active material may be particulate, for example, and its size is not particularly limited. The particles of the positive electrode active material may be solid particles, hollow particles, or particles with voids. The particles of the positive electrode active material may be primary particles, or secondary particles formed by the aggregation of multiple primary particles. The average particle diameter (D50) of the positive electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, or it may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. In this application, the average particle diameter D50 refers to the particle diameter (median diameter) at 50% of the cumulative value in the volume-based particle size distribution determined by laser diffraction-scattering.

[0013] The surface of the positive electrode active material may be covered with a protective layer containing an ion-conducting oxide. That is, the positive electrode active material layer 12 may contain a composite comprising the above-mentioned positive electrode active material and a protective layer provided on its surface. This makes it easier to suppress reactions between the positive electrode active material and sulfides (for example, sulfide solid electrolytes described later). Examples of ion-conducting oxides that cover and protect the surface of the positive electrode active material include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, and Li4Ti5O 12Examples include Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, and Li2WO4. The coverage rate (area ratio) of the protective layer on the surface of the positive electrode active material may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more, 1 nm or more, 100 nm or less, or 20 nm or less.

[0014] The electrolyte contained in the positive electrode active material layer 12 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. In particular, when the positive electrode active material layer 12 contains a solid electrolyte (especially a sulfide solid electrolyte), an even greater effect can be expected from the technology of this disclosure.

[0015] The solid electrolyte can be any known solid electrolyte for secondary batteries. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ionic conductivity and heat resistance. Examples of inorganic solid electrolytes include oxide solid electrolytes and sulfide solid electrolytes. Examples of oxide solid electrolytes include lithium lanthanum zirconate, LiPON, and Li 1+X Al X Ge 2-X It may be one or more selected from (PO4)3, Li-SiO glass, Li-Al-SO glass, etc. The sulfide solid electrolyte may be one or more of those exemplified as the first solid electrolyte described later. Among inorganic solid electrolytes, sulfide solid electrolytes, and among them sulfide solid electrolytes containing at least Li, S, and P as constituent elements, have particularly high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be particulate, for example. The solid electrolyte may be used alone or in combination of two or more types.

[0016] The electrolyte may contain, for example, lithium ions as carrier ions. The electrolyte may also be, for example, a non-aqueous electrolyte. For example, a solution of lithium salt dissolved at a predetermined concentration in a carbonate-based solvent can be used as the electrolyte. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of lithium salts include hexafluoride phosphate.

[0017] Examples of conductive additives that may be included in the positive electrode active material layer 12 include carbon materials such as vapor-processed carbon fiber (VGCF), acetylene black (AB), Ketjenblack (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additive may be in the form of parts or fibers, and its size is not particularly limited. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0018] Examples of binders that may be included in the positive electrode active material layer 12 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, polyimide (PI) binders, and polyacrylic acid binders. A single binder may be used alone, or two or more binders may be used in combination.

[0019] 1.2 Solid electrolyte layer The solid electrolyte layer 20 contains at least a first solid electrolyte. The first solid electrolyte contained in the solid electrolyte layer 20 may be the same type as or different from the solid electrolyte that may be contained in the positive electrode active material layer 12 described above. The solid electrolyte layer 20 may further optionally contain a binder, various additives, etc. The binder contained in the solid electrolyte layer 20 may be the same type as or different from the binder that may be contained in the positive electrode active material layer 12 described above. In the solid electrolyte layer 20, only one type of the first solid electrolyte and the binder may be used individually, or two or more types may be used in combination. The solid electrolyte layer 20 does not need to be entirely made of solid material; it may contain various liquids as long as it can function properly as a secondary battery. The content of the first solid electrolyte and binder, etc. in the solid electrolyte layer 20 is not particularly limited. For example, with the entire solid electrolyte layer 20 (total solid content) being 100% by mass, the content of the first solid electrolyte may be 50% by mass or more, 60% by mass or more, or 70% by mass or more, or 100% by mass or less, or 90% by mass or less. The thickness of the solid electrolyte layer 20 is not particularly limited and may be, for example, 0.1 μm or more, or 1 μm or more, or 2 mm or less, or 1 mm or less.

[0020] In the secondary battery 100, the Young's modulus of the first solid electrolyte contained in the solid electrolyte layer 20 is higher than that of the second solid electrolyte contained in the Mg mixture layer 33 described later. That is, the first solid electrolyte is harder than the second solid electrolyte described later. The Young's modulus of the first solid electrolyte may be, for example, greater than 20 GPa. In this application, "Young's modulus" refers to the Young's modulus at 25°C. Various types of hard first solid electrolytes can be used. For example, higher performance is more likely to be achieved when the first solid electrolyte is an inorganic solid electrolyte, particularly a sulfide solid electrolyte.

[0021] The sulfide solid electrolyte may be a glass-based sulfide solid electrolyte (sulfide glass), a glass-ceramic sulfide solid electrolyte, or a crystalline sulfide solid electrolyte. The sulfide glass is amorphous. The sulfide glass preferably has a glass transition temperature (Tg). When the sulfide solid electrolyte has a crystal phase, examples of the crystal phase include a Thio-LISICON type crystal phase, a LGPS type crystal phase, and an argyrodite type crystal phase.

[0022] The sulfide solid electrolyte preferably contains, for example, Li element, X element (X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, In), and S element. The sulfide solid electrolyte may further contain at least one of O element and halogen element. The sulfide solid electrolyte preferably contains S element as the main component of the anion element.

[0023] The sulfide solid electrolyte is, for example, Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-GeS2, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-LiI-LiBr, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (where m and n are positive numbers. Z is any of Ge, Zn, Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers. M is any of P, Si, Ge, B, Al, Ga, In) and may be at least one selected therefrom.

[0024] The composition of the sulfide solid electrolyte is not particularly limited. For example, xLi2S·(100 - x)P2S5 (70 ≤ x ≤ 80), yLiI·zLiBr·(100 - y - z)(xLi2S·(1 - x)P2S5) (0.7 ≤ x ≤ 0.8, 0 ≤ y ≤ 30, 0 ≤ z ≤ 30), etc. may be mentioned. Alternatively, the sulfide solid electrolyte has a composition represented by the general formula: Li 4-x Ge 1-x P x S4 (0 < x < 1). In the above general formula, at least a part of Ge may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, at least a part of P may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of Li may be substituted with at least one of Na, K, Mg, Ca, and Zn. In the above general formula, a part of S may be substituted with a halogen (at least one of F, Cl, Br, and I). Alternatively, the sulfide solid electrolyte is Li 7-a PS 6-a X a (X is at least one of Cl, Br, and I, and a is a number of 0 or more and 2 or less). a may be 0, or may be greater than 0. In the latter case, a may be 0.1 or more, may be 0.5 or more, and may be 1 or more. Also, a may be 1.8 or less, may be 1.5 or less.

[0025] The shape of the sulfide solid electrolyte may be, for example, particulate. The average particle diameter (D50) of the sulfide solid electrolyte may be, for example, 10 nm or more and 10 μm or less. The ionic conductivity of the sulfide solid electrolyte at 25°C may be, for example, 1×10 -4 S / cm or more, or 1×10 -3 S / cm or more.

[0026] 1.3 Negative electrode current collector The negative electrode current collector 31 can be any material capable of functioning as a negative electrode current collector for a secondary battery. The negative electrode current collector 31 may be a metal foil or metal mesh, or a carbon sheet. Metal foil is particularly advantageous in terms of handling. The negative electrode current collector 31 may consist of multiple metal foils or sheets. Examples of metals that make up the negative electrode current collector 31 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring resistance to reduction and being less prone to alloying with lithium, the negative electrode current collector 31 may include at least one metal selected from Cu, Ni, and stainless steel, especially stainless steel. The negative electrode current collector 31 may have some kind of coating layer on its surface. For example, the surface of the negative electrode current collector 31 may be covered with a protective layer 34, which will be described later. Alternatively, the negative electrode current collector 31 may have a coating layer other than the protective layer 34 on its surface. Furthermore, if the negative electrode current collector 31 consists of multiple metal foils, there may be some layer between the multiple metal foils. The thickness of the negative electrode current collector 31 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0027] 1.4 Lithium Metal as a Negative Electrode Active Material The secondary battery 100 is equipped with a lithium-deposited negative electrode. Specifically, as shown in Figure 1, metallic lithium 32, which serves as the negative electrode active material, is deposited between the solid electrolyte layer 20 and the negative electrode current collector 31 during charging. In the secondary battery 100, as shown in Figure 1, it is considered that the metallic lithium 32 is deposited at least between the Mg mixture layer 33 and the negative electrode current collector 31. However, the metallic lithium 32 may be deposited between the Mg mixture layer 33 and the negative electrode current collector 31, as shown in Figure 1, or it may be deposited inside the Mg mixture layer 33, or it may be deposited between the solid electrolyte layer 20 and the Mg mixture layer 33. The metallic lithium 32 deposited between the electrolyte layer 20 and the negative electrode current collector 31 dissolves (ionizes) during discharge and is returned to the positive electrode 10.

[0028] In this application, "metallic lithium" is a concept that includes not only elemental lithium but also lithium alloys. That is, in the secondary battery 100, metallic lithium 32 may be deposited as elemental lithium or as an alloy with other metals. Examples of lithium alloys include Li-Au, Li-Mg, Li-Sn, Li-Al, Li-B, Li-C, Li-Ca, Li-Ga, Li-Ge, Li-As, Li-Se, Li-Ru, Li-Rh, Li-Pd, Li-Ag, Li-Cd, Li-In, Li-Sb, Li-Ir, Li-Pt, Li-Hg, Li-Pb, Li-Bi, Li-Zn, Li-Tl, Li-Te, and Li-At. There may be only one type of lithium alloy or two or more types. As will be described later, in the secondary battery 100, an Mg mixture layer 33 is arranged between the solid electrolyte layer 20 and the negative electrode current collector 31. Therefore, when the secondary battery 100 is charged, the metallic lithium 32 can alloy with the Mg contained in the Mg mixture layer 33 and precipitate as a Li-Mg alloy.

[0029] The amount of metallic lithium 32 deposited between the solid electrolyte layer 20 and the negative electrode current collector 31 is not particularly limited. It can be adjusted as appropriate according to the desired battery performance. However, if too much metallic lithium 32 is deposited, there is a concern that pressure concentration may occur. In this regard, as a guideline for the amount of metallic lithium 32 deposited, the charging capacity of the secondary battery 100 should be, for example, 1 mAh / cm². 2 More than 5mAh / cm 2 The following quantities are also acceptable.

[0030] According to the inventors' findings, in conventional secondary batteries equipped with a lithium deposition type negative electrode, when metallic lithium is repeatedly deposited and dissolved between the electrolyte layer and the negative electrode current collector, metallic lithium tends to deposit and dissolve unevenly. This problem is particularly likely to occur when the current load is high. In some cases, metallic lithium may grow excessively locally. Furthermore, uneven deposition and dissolution of metallic lithium increases the resistance of the secondary battery and tends to degrade its cycle characteristics. In other words, secondary batteries equipped with a lithium deposition type negative electrode have room for improvement in terms of resistance and cycle characteristics. In the secondary battery 100 of this disclosure, by arranging an Mg mixture layer 33 between the solid electrolyte layer 20 and the negative electrode current collector 31, the deposition and dissolution of metallic lithium 32 can occur more uniformly, and the resistance and cycle characteristics can be improved.

[0031] 1.5 Mg combination layer The Mg mixture layer 33 contains Mg and a second solid electrolyte. According to the inventor's new findings, when such an Mg mixture layer 33 is placed between the solid electrolyte layer 20 and the negative electrode current collector 31, lithium ions are conducted from the positive electrode 10 to the negative electrode current collector 31 via the solid electrolyte layer 20 during charging of the secondary battery 100, and the Mg and lithium in the Mg mixture layer 33 undergo an alloying reaction. Here, it is thought that the Mg mixture layer 33 forms a three-dimensional interface, improving the frequency factor of the interfacial reaction. Furthermore, it is thought that as metallic lithium 32 precipitates while alloying with the Mg in the Mg mixture layer 33, the metallic lithium 32 and the Mg mixture layer 33 adhere closely together by an anchoring effect, and the interface between metallic lithium 32 and the Mg mixture layer 33 is maintained in good condition. By maintaining a good interface between the metallic lithium 32 and the Mg mixture layer 33, interruption of conductive paths and concentration of power due to peeling of the metallic lithium 32 are suppressed, and the redissolution of the metallic lithium 32 during discharge is also made more uniform. Thus, in the secondary battery 100, by arranging the Mg mixture layer 33 between the solid electrolyte layer 20 and the negative electrode current collector 31, the input / output characteristics of lithium between the solid electrolyte layer 20 and the negative electrode current collector 31 are improved, and localized growth of metallic lithium 32 during deposition can be suppressed, thereby improving the cycle characteristics of the secondary battery 100 and reducing the resistance of the secondary battery 100.

[0032] Furthermore, according to the inventor's new findings, the second solid electrolyte contained in the Mg mixture layer 33 is softer than the first solid electrolyte contained in the solid electrolyte layer 20, which makes it easier to further reduce the resistance of the secondary battery 100 and further improve its cycle characteristics. Specifically, because the second solid electrolyte contained in the Mg mixture layer 33 is softer than the first solid electrolyte contained in the solid electrolyte layer 20, the second solid electrolyte deforms easily, and the gap between the solid electrolyte layer 20 and the negative electrode current collector 31 is easily filled by the second solid electrolyte. In addition, the deformation of the second solid electrolyte makes it easier to increase the packing density of the Mg mixture layer 33 itself. Moreover, the presence of the soft second solid electrolyte makes it easier to suppress interfacial delamination. As a result, conductive paths and ion conduction paths are better maintained between the solid electrolyte layer 20 and the negative electrode current collector 31, which is thought to further reduce the resistance of the secondary battery 100 and further improve its cycle characteristics.

[0033] As shown in Figure 1, the Mg mixture layer 33 may or may not be in contact with the solid electrolyte layer 20. Even if some intermediate layer exists between the Mg mixture layer 33 and the solid electrolyte layer 20, the effect of the Mg mixture layer 33 can still be achieved. However, a higher effect is more likely to be ensured when the Mg mixture layer 33 is in contact with the solid electrolyte layer 20.

[0034] 1.5.1 Mg In the Mg mixture layer 33, Mg may exist as Mg particles. Nuclei for metallic lithium 32 are easily and stably formed on Mg particles. Therefore, including Mg particles in the Mg mixture layer 33 makes it easier for metallic lithium 32 to precipitate more stably. Furthermore, because Mg has a wide composition range in which it can form a single phase with Li, more efficient dissolution and precipitation of lithium becomes possible.

[0035] Mg particles may be particles of pure Mg, or particles containing Mg and elements other than Mg. Examples of elements other than Mg include various metallic elements, metalloid elements, and nonmetallic elements. For example, Mg particles may be alloy particles (Mg alloy particles) containing Mg and metals other than Mg. Preferably, Mg alloy particles are alloys containing Mg as the main component (alloys in which 50 mol% or more of the total constituent elements are Mg). Mg alloy particles may contain at least one of the metals other than Mg, such as Li, Au, Al, and Ni. Mg alloy particles may or may not contain Li. Mg alloy particles may contain a β single-phase alloy of Li and Mg. Alternatively, Mg particles may be oxide particles (Mg oxide particles) containing Mg and O. Mg oxide particles may be, for example, oxide particles consisting only of Mg and O, or composite oxide particles represented as Mg-M'-O (where M' is at least one of Li, Au, Al, and Ni). When Mg oxide particles contain M', it is preferable that M' contains at least Li. M' may contain metals other than Li, or it may not contain any. In the former case, M' may be one metal other than Li, or two or more metals.

[0036] The Mg particles may be primary particles or secondary particles formed by aggregating primary particles. It is preferable that the average particle size (D50) of the Mg particles be small. It is believed that a small average particle size of the Mg particles improves the dispersibility of the Mg particles in the Mg mixture layer 33, increases the number of Li deposition sites, and allows for more uniform deposition of metallic lithium 32. The average particle size (D50) of the Mg particles may be, for example, 100 nm or more and 100 μm or less, 200 nm or more, 300 nm or more, 400 nm or more, 500 nm or more, 600 nm or more, 700 nm or more, or 800 nm or more, or 50 μm or less, 40 μm or less, 30 μm or less, 20 μm or less, 10 μm or less, 5 μm or less, or 3 μm or less. The average particle size (D50) of the Mg particles may be the same as the average particle size (D50) of the second solid electrolyte described later, or it may be larger or smaller than that.

[0037] The amount of Mg contained in the Mg mixture layer 33 is not particularly limited. From the viewpoint of increasing the precipitation site of Li as described above, the amount of Mg may be large, and from the viewpoint of improving the ionic conductivity in the Mg mixture layer 33, the amount of Mg may be small. For example, the Mg mixture layer 33 may contain 10% by mass or more and 90% by mass or less of Mg. The Mg content in the Mg mixture layer 33 may be 20% by mass or more, 30% by mass or more, or 40% by mass or more, or 80% by mass or less, 70% by mass or less, or 60% by mass or less.

[0038] 1.5.2 Second solid electrolyte The Young's modulus of the second solid electrolyte contained in the Mg mixture layer 33 is lower than that of the first solid electrolyte contained in the solid electrolyte layer 20 described above. In other words, the second solid electrolyte is softer than the first solid electrolyte described above. The Young's modulus of the second solid electrolyte may be, for example, 1 GPa or more and 20 GPa or less. It is thought that the lower the Young's modulus of the second solid electrolyte, the better the deformation performance of the second solid electrolyte, and the easier it is to exhibit the effect of eliminating the gaps described above. In this regard, the Young's modulus of the second solid electrolyte may be 18 GPa or less, 16 GPa or less, 14 GPa or less, 12 GPa or less, 10 GPa or less, 8 GPa or less, 6 GPa or less, 4 GPa or less, or 2 GPa or less. Various soft second solid electrolytes can be used in the Mg mixture layer 33.

[0039] For example, the second solid electrolyte may be a complex hydride containing Li. This complex hydride satisfies the above Young's modulus and has low reactivity with the sulfide solid electrolyte. That is, when a sulfide solid electrolyte is used as the first solid electrolyte, the reaction between the first solid electrolyte and the second solid electrolyte can be suppressed, and the durability of the battery is likely to increase. This complex hydride may be composed of a complex ion containing Li ions and H. The complex ion containing H may, for example, have an element M containing at least one of a nonmetallic element, a metalloid element, and a metallic element, and H bonded to the element M. In addition, in the complex ion containing H, the element M as the central element and the H surrounding the element M may be bonded to each other via covalent bonds. In addition, the complex ion containing H may have (M m H n ) α- It may also be represented as follows: In this case, m is any positive number, and n and α can be any positive number depending on m, the valency of element M, etc. Element M can be any nonmetallic or metallic element that can form a complex ion. For example, element M may contain at least one of B, C, and N as a nonmetallic element, or it may contain B. Also, for example, element M may contain at least one of Al, Ni, and Fe as a metallic element. In particular, when the complex ion contains B, or contains C and B, it is easier to ensure a soft and higher ionic conductivity. A specific example of a complex ion containing H is (CB9H 10 ) - , (CB 11 H 12 ) - , (B 10 H 10 ) 2- , (B 12 H 12 ) 2- (BH4) - , (NH2) - (AlH4) - , and combinations thereof are examples. In particular, (CB9H 10 ) - , (CB 11 H 12 ) -Alternatively, using a combination of these can easily ensure higher ionic conductivity. That is, the Li-containing complex hydride may also contain Li, C, B, and H.

[0040] The second solid electrolyte may be a salt having multiple types of cations and / or multiple types of anions, or it may be a molten salt obtained by melting multiple types of salts. In this case, the second solid electrolyte may contain organic cations or organic anions. Some of these salts also satisfy the above Young's modulus.

[0041] When the second solid electrolyte is a salt, the second solid electrolyte may, for example, have a first cation and a second cation, where the first cation may be at least one selected from ammonium ions, phosphonium ions, pyridinium ions, and pyrrolidinium ions, and the second cation may be a lithium ion. Alternatively, the first cation may be a tetraalkylammonium ion, and the second cation may be a lithium ion. When the second solid electrolyte has an organic cation as the first cation, the Young's modulus tends to be lower compared to when it does not have the first cation. The molar ratio of the first cation to the second cation constituting the second solid electrolyte is not particularly limited. From the viewpoint of further improving ionic conductivity, the molar ratio of the second cation to the first cation (second cation / first cation) may be between 0.05 and 19.0. The molar ratio may be 0.1 or higher, 0.2 or higher, 0.3 or higher, 0.4 or higher, 0.5 or higher, 0.6 or higher, 0.7 or higher, 0.8 or higher, 0.9 or higher, or 1.0 or higher, and may also be 10.0 or lower, 9.5 or lower, 9.0 or lower, 8.5 or lower, 8.0 or lower, 7.5 or lower, 7.0 or lower, 6.5 or lower, 6.0 or lower, 5.5 or lower, or 5.0 or lower. The cations constituting the second solid electrolyte may consist only of the first and second cations described above, or may include other cations different from the first cation. Examples of other cations include ions containing poor metal elements. Examples of poor metals include Al and Ga. The total proportion of the first and second cations to the total cations constituting the second solid electrolyte may be 50 mol% or more and 100 mol% or less, and may be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 99 mol% or more, or 100 mol%.

[0042] When the second solid electrolyte is a salt, the second solid electrolyte may have various anions, for example. For example, the second solid electrolyte may have at least one anion selected from halogen ions, halide ions, bisulfate ions, sulfonylamide ions, and complex ions containing H. Alternatively, the second solid electrolyte may have one or both of the first and second anions, where the first anion may be one or both of the halogen ion and the bisulfate ion, and the second anion may be the sulfonylamide anion. Alternatively, the second solid electrolyte may have the sulfonylamide ion. The halogen ion may be one or both of the bromide ion and the chloride ion, for example. An example of the sulfonylamide anion is the trifluoromethanesulfonylamide anion (TFSA anion, (CF3SO2)2N - ), fluorosulfonylamide anion (FSA anion, (FSO2)2N - ), fluorosulfonyl (trifluoromethanesulfonyl) amide anion (FTA anion, FSO2(CF3SO2)N - Examples include the following. The sulfonylamide anion may be a single type or a combination of two or more types. Of the above sulfonylamide anions, the TFSA anion has low polarity and particularly low reactivity with other materials. In this respect, when the second solid electrolyte contains the TFSA anion, for example, the reaction with sulfide solid electrolytes is easily suppressed. The complex ions containing H are as described above.

[0043] The amount of the second solid electrolyte contained in the Mg mixture layer 33 is not particularly limited. From the viewpoint of more easily eliminating the gap between the solid electrolyte layer 20 and the negative electrode current collector 31 and from the viewpoint of improving ionic conductivity, the amount of the second solid electrolyte may be large, and from the viewpoint of increasing the number of Li deposition starting points by increasing the amount of Mg, the amount of the second solid electrolyte may be small. For example, the Mg mixture layer 33 may contain 10% by mass or more and 90% by mass or less of the second solid electrolyte. The content of the second solid electrolyte in the Mg mixture layer 33 may be 20% by mass or more, 30% by mass or more, or 40% by mass or more, and may be 80% by mass or less, 70% by mass or less, or 60% by mass or less.

[0044] 1.5.3 Other Ingredients The Mg mixture layer 33 may contain a binder as needed. This can suppress cracking and other defects in the Mg mixture layer 33. The binder can be appropriately selected from, for example, those exemplified as binders that can be included in the positive electrode active material layer described above. The binder that can be included in the Mg mixture layer 33 may be the same type as the binder that can be included in the positive electrode active material layer described above, or it may be a different type. One type of binder may be used alone, or two or more types may be used in combination.

[0045] 1.5.4 Filling rate The packing density of the Mg mixture layer 33 is not particularly limited. When the packing density of the Mg mixture layer 33 is high, the cycle characteristics of the secondary battery tend to be better. The packing density of the Mg mixture layer 33 may be, for example, 70% or more and 100% or less. The packing density may be 80% or more, 90% or more, 95% or more, or 98% or more. The packing density of the Mg mixture layer 33 can be calculated by the following method. That is, the total volume obtained by dividing the weight of each material (Mg particles, second solid electrolyte, etc.) contained in the Mg mixture layer 33 by the true density of each material is taken as the "volume of the Mg mixture layer calculated from true density," and the volume calculated from the actual dimensions of the Mg mixture layer is taken as the "actual volume of the Mg mixture layer," and the packing density (%) can be calculated from the following formula. Packing ratio (%) = (Volume of Mg mixture layer calculated from true density) / (Actual volume of Mg mixture layer) × 100

[0046] 1.5.5 Thickness, etc. The secondary battery 100 may have only one Mg mixture layer 33, or it may have two or more layers. The total thickness of the Mg mixture layer 33 may be, for example, 0.1 μm or more and 1000 μm or less. As a method for forming the Mg mixture layer 33, for example, one method is to coat a substrate with a slurry containing at least Mg particles and a second solid electrolyte. Alternatively, one method is to form a particle layer containing Mg particles, then impregnate the particle layer with an electrolyte solution in which the second solid electrolyte is dissolved, and then dry it.

[0047] 1.6 Protective layer As shown in Figure 2, in the secondary battery 100, a protective layer 34 may be present between the Mg mixture layer 33 and the negative electrode current collector 31. The protective layer 34 may contain Mg but not electrolyte. By placing the protective layer 34 between the Mg mixture layer 33 and the negative electrode current collector 31, the diffusion of Li can be further promoted. Also, when the protective layer 34 is present, the second solid electrolyte contained in the Mg mixture layer 33 does not come into direct contact with the negative electrode current collector 31, so the deposition starting point of metallic lithium 32 can be substantially limited to the Mg in the Mg mixture layer 33 and the protective layer 34. This allows for more uniform deposition of metallic lithium 32. In Figure 2, the interface between metallic lithium 32 and the protective layer 34 is shown after charging of the secondary battery 100, but the entire protective layer 34 may be alloyed with metallic lithium 32.

[0048] The protective layer 34 is preferably a layer in which Mg has the highest molar ratio among all its constituent elements. The molar ratio of Mg in the entire protective layer 34 may be, for example, 50 mol% or more and 100 mol% or less, or 70 mol% or more, 80 mol% or more, or 90 mol% or more. The protective layer 34 may be, for example, a metal thin film containing Mg (e.g., a vapor-deposited film), or a layer containing Mg particles. The metal thin film containing Mg may be composed of Mg or a Mg alloy. The Mg particles are as described above. The protective layer 34 may also be a layer containing only Mg particles.

[0049] The thickness of the protective layer 34 may be, for example, 10 nm or more and 10 μm or less. The thickness of the protective layer 34 may be 50 nm or more or 100 nm or more, and may be 5 μm or less, 3 μm or less, 1 μm or less, or 700 nm or less. The secondary battery 100 may have only one protective layer 34 or may have two or more layers. Methods for forming the protective layer 34 include forming a film on the negative electrode current collector or pressing Mg particles. As a method for forming a film on the negative electrode current collector, for example, PVD methods such as vapor deposition or sputtering, or plating methods such as electroplating or electroless plating can be used.

[0050] Furthermore, as shown in Figure 2, the Mg mixture layer 33 and the protective layer 34 may be in contact with each other. Also, the Mg mixture layer 33 and the solid electrolyte layer 20 may be in contact with each other. Also, the protective layer 34 and the negative electrode current collector 31 may be in contact with each other. Alternatively, as shown in Figure 1, the Mg mixture layer 33 and the negative electrode current collector 31 may be in contact with each other.

[0051] 1.7 Other components The secondary battery 100 only needs to have at least the above-described components, and may also have other components. The components described below are examples of other components that the secondary battery 100 may have.

[0052] 1.7.1 Exterior The secondary battery 100 may have all of the above components housed inside an outer casing. More specifically, the parts of the secondary battery 100 excluding tabs or terminals for drawing power to the outside may be housed inside the outer casing. Any known type of battery casing can be used. For example, laminate film may be used as the casing. Alternatively, multiple secondary batteries 100 may be electrically connected and stacked as desired to form a battery pack. In this case, the battery pack may be housed inside a known battery case.

[0053] 1.7.2 Sealing resin In the secondary battery 100, each of the above components may be sealed with a resin. For example, at least the sides (surfaces along the stacking direction) of each layer shown in Figure 1 may be sealed with a resin. This makes it easier to suppress the ingress of moisture into the interior of each layer. Known curable resins and thermoplastic resins can be used as the sealing resin.

[0054] 1.7.3 Restraining Members The secondary battery 100 may or may not have a restraining member to constrain each of the above components in the thickness direction. The internal resistance of the battery is easily reduced by the restraining pressure applied by the restraining member. There are no particular restrictions on the restraining pressure applied by the restraining member. Even with a small restraining pressure from the restraining member, the secondary battery 100 has low resistance and excellent cycle characteristics. In this regard, the restraining pressure applied by the restraining member may be 5 MPa or less, 3 MPa or less, or 1 MPa or less.

[0055] 2. Manufacturing method of secondary batteries The above secondary battery 100 can be manufactured, for example, as follows. That is, as shown in Figure 3, the manufacturing method of the secondary battery 100 according to one embodiment is: The surface of the solid electrolyte layer 20 containing the first solid electrolyte is coated with a Mg mixture layer 33 containing Mg and the second solid electrolyte (Figure 3(A)). Using the solid electrolyte layer 20 coated with the Mg mixture layer 33, a laminate 50 is obtained having the positive electrode 10, the solid electrolyte layer 20, the Mg mixture layer 33, and the negative electrode current collector 31 in this order (Figure 3(B)), and, The process includes charging the laminate 50 to deposit metallic lithium 32 between the solid electrolyte layer 20 and the negative electrode current collector 31 (Figure 3(C)).

[0056] 2.1 Covering As shown in Figure 3(A), in the manufacturing method according to this embodiment, the surface of the solid electrolyte layer 20 containing the first solid electrolyte is covered with a Mg mixture layer 33 containing Mg and the second solid electrolyte. The solid electrolyte layer 20 is obtained, for example, by molding an electrolyte mixture containing the first solid electrolyte. The method for covering the surface of the solid electrolyte layer 20 with the Mg mixture layer 33 is not particularly limited. For example, the Mg mixture layer 33 can be formed on the surface of the solid electrolyte layer 20 by a coating method using a solution or slurry. Alternatively, a transfer material on which the Mg mixture layer 33 is formed on a substrate may be obtained, and the Mg mixture layer 33 may be transferred from the transfer material to the surface of the solid electrolyte layer 20. The solid electrolyte layer 20 may be pre-integrated with the positive electrode active material layer 12 or the like.

[0057] 2.2 Fabrication of Laminates As shown in Figure 3(B), in the manufacturing method according to this embodiment, a laminate 50 is obtained having a positive electrode 10, an electrolyte layer 20, an Mg mixture layer 33, and a negative electrode current collector 31 in that order, using a solid electrolyte layer 20 coated with an Mg mixture layer 33 as described above. The laminate 50 can be easily obtained, for example, by molding and laminating each of the materials described above by coating, transferring, bonding, or pressing them so that the positive electrode current collector 11, positive electrode active material layer 12, solid electrolyte layer 20, Mg mixture layer 33, and negative electrode current collector 31 are laminated in that order. The protective layer 34 described above may be provided on the surface of the negative electrode current collector 31 in advance. The laminate 50 may include at least one of each of the positive electrode current collector 11, positive electrode active material layer 12, solid electrolyte layer 20, Mg mixture layer 33, and negative electrode current collector 31. In other words, the laminate 50 only needs to have at least one laminate unit consisting of the positive electrode current collector 11, the positive electrode active material layer 12, the solid electrolyte layer 20, the Mg mixture layer 33, and the negative electrode current collector 31, and may have multiple such laminate units. In this case, the multiple laminate units may be electrically connected in series, in parallel, or not electrically connected to each other.

[0058] Before or after obtaining the above-mentioned laminate 50, pressure may be applied to each layer or the laminate 50 in the thickness direction (lamination direction). For example, each layer constituting the laminate 50 may be pressed together, or the gaps between each layer constituting the laminate 50 may be eliminated to reduce the interfacial resistance. Each layer or the laminate 50 can be pressurized by known means. For example, each layer or the laminate 50 can be pressurized in the lamination direction by various pressurizing methods such as CIP, HIP, roll press, uniaxial press, and die press. The magnitude of the pressure applied to each layer or the laminate 50 in the lamination direction can be appropriately determined according to the performance of the target battery. For example, if each layer or the laminate 50 contains a sulfide solid electrolyte, the pressure may be 100 MPa or more, 150 MPa or more, 200 MPa or more, 250 MPa or more, 300 MPa or more, or 350 MPa or more, from the viewpoint of easily integrating or eliminating gaps by plastically deforming the sulfide solid electrolyte. The pressurization time and pressurization temperature for each layer or laminate 50 are not particularly limited.

[0059] 2.3 Charging As shown in Figure 3(C), in the manufacturing method according to this embodiment, the laminate 50 obtained as described above is charged to deposit metallic lithium 32 between the solid electrolyte layer 20 and the negative electrode current collector 31. Specifically, by charging the laminate 50, lithium ions are conducted from the positive electrode active material contained in the positive electrode active material layer 12 to the negative electrode current collector 31 side via the solid electrolyte layer 20, and between the solid electrolyte layer 20 and the negative electrode current collector 31, these lithium ions receive electrons and are deposited as metallic lithium 32. The charging may be the first charge after preparing the laminate 50, or it may be the second or subsequent charge. The laminate 50 can be charged in the same way as a general battery charger. That is, the positive electrode current collector 11 and the negative electrode current collector 31 of the laminate 50 can be connected to an external power source to perform the charge.

[0060] 2.4 Other processes The manufacturing method according to this embodiment may include, in addition to the steps described above, general steps for manufacturing a secondary battery. For example, steps such as housing the laminate 50 inside an outer casing such as a laminate film, or connecting current-collecting tabs to the laminate 50. Specifically, for example, current-collecting tabs may be connected to the current-collecting elements 11 and 31 of the laminate 50 (parts of the current-collecting elements 11 and 31 may be made to protrude and used as tabs), and the laminate 50 may be housed inside a laminate film as an outer casing while the tabs are pulled out to the outside of the laminate film, the laminate film may be sealed, and then the laminate 50 may be charged via the tabs outside the laminate film.

[0061] 3. Vehicles equipped with secondary batteries As described above, the secondary battery of this disclosure can uniformly deposit metallic lithium between the solid electrolyte layer and the negative electrode current collector, resulting in low resistance and excellent cycle characteristics. Such a secondary battery can be suitably used in, for example, at least one type of vehicle selected from hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). That is, the technology of this disclosure also has the aspect of a vehicle having a secondary battery, wherein the secondary battery comprises a positive electrode, an electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the electrolyte layer and the negative electrode current collector by charging, wherein a Mg mixture layer exists between the solid electrolyte layer and the negative electrode current collector, the solid electrolyte layer contains a first solid electrolyte, the Mg mixture layer contains Mg and a second solid electrolyte, and the Young's modulus of the second solid electrolyte is lower than that of the first solid electrolyte. Details of the configuration of the secondary battery are as described above. [Examples]

[0062] As described above, one embodiment of the technology of this disclosure has been explained, but the technology of this disclosure can be modified in various ways other than the above embodiment without departing from its gist. The technology of this disclosure will be described in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0063] 1. Preparation of the positive electrode mixture Ternary cathode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 800 mg of O2, 127 mg of sulfide solid electrolyte (LiBr-LiI-Li2S-P2S5), and 12 mg of VGCF as a conductive additive were dispersed in dehydrated heptane using an ultrasonic homogenizer. The mixture was then dried at 100°C for 1 hour to obtain the positive electrode mixture.

[0064] 2. Fabrication of Mg-coated SUS current collector foil (Mg-SUS foil) By depositing Mg onto the surface of a SUS foil, a Mg-coated SUS current collector foil (Mg-SUS foil) was obtained by forming a protective Mg layer on the surface of the SUS foil. The thickness of the Mg layer was 700 nm.

[0065] 3. Preparation of the Mg mixture layer 3.1 Comparative Example 1 A solution containing SBR as a binder and solvents (mesitylene, dibutyl ether) were placed in a PP container and mixed with a shaker for 3 minutes. Then, sulfide solid electrolyte particles (average particle size D50: 800 nm, Young's modulus: 23.2 GPa) and Mg particles (average particle size D50: 800 nm) were weighed in a mass ratio of 50:50 and placed in the PP container. After mixing with a shaker for 3 minutes, the mixture was mixed with an ultrasonic disperser for 30 seconds, and this was repeated twice to obtain a Mg mixture slurry. Subsequently, the Mg mixture slurry was coated onto an Al foil using an applicator with a coating gap of 25 μm. After visually confirming that the surface was dry after coating, the Mg mixture layer according to Comparative Example 1 was formed on the Al foil by drying on a 100°C hot plate for 30 minutes.

[0066] 3.2 Example 1 Instead of sulfide solid electrolyte particles, complex hydride particles ([LiCB9H 10 ] 0.7 [LiCB 11 H 12 ] 0.3A magnesium alloy layer was formed on an Al foil in the same manner as in the comparative example, except that an average particle size D50 (2 μm) and Young's modulus (1.5 GPa) were used.

[0067] 4. Creating evaluation cells 4.1 Comparative Example 1, Example 1 In a powder compaction press cell (φ11.28 mm), 101.7 mg of sulfide solid electrolyte particles (Young's modulus: 23.2 GPa) were placed in the cell, and a press pressure of 1 ton was applied while the cell was left to stand for 1 minute to obtain a first pellet consisting of a solid electrolyte. Subsequently, 31.3 mg of the above-mentioned cathode mixture was placed on one side of the first pellet, and the cell was left to stand for 1 minute under a press pressure of 6 tons to obtain a second pellet consisting of a solid electrolyte layer and a cathode mixture layer. Next, an Al foil with a Mg mixture layer formed on it was placed on the other side of the second pellet, and the Mg mixture layer was transferred to the surface of the solid electrolyte layer by pressing with 1 ton of pressure. The Al foil was then peeled off to obtain a third pellet consisting of a Mg mixture layer, a solid electrolyte layer, and a cathode mixture layer. Finally, a Mg-SUS foil (φ11.28 mm) was placed on the Mg mixture layer side of the third pellet, and it was left to stand for 1 minute under a 1 ton press pressure to obtain a fourth pellet consisting of a SUS foil (negative electrode current collector) - Mg layer (protective layer) - Mg mixture layer - solid electrolyte layer - positive electrode mixture layer. An evaluation cell was obtained by restraining this fourth pellet with a torque of 1 MPa.

[0068] 4.2 Comparative Example 2 An evaluation cell was obtained in the same manner as described above, except that the Mg mixture layer was not transferred to the second pellet. Specifically, an Mg-SUS foil (φ11.28 mm) was placed on the other side of the second pellet, and it was left to stand for 1 minute under a press pressure of 1 ton to obtain a pellet consisting of SUS foil-Mg layer-solid electrolyte layer-cathode mixture layer. This pellet was then restrained with a torque of 1 MPa to obtain an evaluation cell.

[0069] 5. Charge / Discharge Evaluation Each evaluation cell was heated to a constant temperature of 25°C or 60°C for 3 hours, then charged and discharged for 3 cycles at 0.2C, followed by a cycle test at 0.5C. Table 1 below shows the resistance and capacity retention rate of each evaluation cell. The resistance shown in Table 1 is the resistance when discharged to 60% SOC in the 3rd cycle, and the capacity retention rate is the ratio of the capacity at 10 cycles to the capacity at 1 cycles.

[0070] [Table 1]

[0071] The results shown in Table 1 indicate the following: (1) As in Comparative Example 2, if the Mg mixture layer is not placed between the solid electrolyte layer and the SUS foil, neither the battery resistance nor the cycle characteristics will be sufficient. (2) In the case where a Mg mixture layer is placed between the solid electrolyte layer and the SUS foil, as in Comparative Example 1, and a hard sulfide solid electrolyte is used in the Mg mixture layer, the cycle characteristics are improved compared to Comparative Example 2, but the improvement effect is not sufficient. In addition, Comparative Example 1 had a slightly higher battery resistance than Comparative Example 2. This is thought to be due to the DC resistance of the Mg mixture layer. In Comparative Example 1, the solid electrolyte contained in the solid electrolyte layer and the solid electrolyte contained in the Mg mixture layer were of the same type and both were hard, so a gap was created in the Mg mixture layer, and this gap could not be filled by the solid electrolyte, and as a result, the improvement effect on resistance and cycle characteristics was small. (3) As in Example 1, when a Mg mixture layer is placed between the solid electrolyte layer and the SUS foil, and a soft complex hydride solid electrolyte is used in the Mg mixture layer, the resistance and cycle characteristics are significantly improved compared to Comparative Examples 1 and 2. In Example 1, the solid electrolyte contained in the solid electrolyte layer and the solid electrolyte contained in the Mg mixture layer were of different types, and the solid electrolyte contained in the Mg mixture layer was soft. As a result, it is thought that the soft solid electrolyte deformed, eliminating the gaps in the Mg mixture layer, etc., and consequently the improvement in resistance and cycle characteristics was greatly enhanced.

[0072] In the above embodiment, a configuration in which the surface of the SUS foil (negative electrode current collector) is protected by an Mg layer (protective layer) was illustrated, but the technology of this disclosure is not limited to this. Even when the protective layer is omitted, Example 1 shows a greater improvement in resistance and cycle characteristics than Comparative Examples 1 and 2. However, it is considered that resistance and cycle characteristics are more easily improved when a protective layer is present.

[0073] Furthermore, in the above embodiments, a combination of a sulfide solid electrolyte and a complex hydride solid electrolyte was exemplified as the solid electrolyte, but the technology of this disclosure is not limited thereto. It is believed that the desired effect is achieved when the solid electrolyte contained in the Mg mixture layer (second solid electrolyte) is softer than the solid electrolyte contained in the solid electrolyte layer (first solid electrolyte), that is, when it has a lower Young's modulus. However, from the viewpoint of (1) excellent ionic conductivity is easily ensured by the sulfide solid electrolyte, (2) improvements in resistance and cycle characteristics are easily obtained due to the flexibility of the complex hydride solid electrolyte, and (3) the reactivity of the complex hydride with the sulfide solid electrolyte is small, which can suppress electrolyte degradation, it is believed that an even higher effect can be easily obtained when a sulfide solid electrolyte and a complex hydride solid electrolyte are combined in a secondary battery.

[0074] As described above, a secondary battery having the following configuration can be said to have low resistance and excellent cycle characteristics while having a deposition-type metallic lithium anode. Specifically, the secondary battery of this disclosure comprises (I) a positive electrode, a solid electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the solid electrolyte layer and the negative electrode current collector by charging, (II) a Mg mixture layer exists between the solid electrolyte layer and the negative electrode current collector, (III) the solid electrolyte layer contains a first solid electrolyte, (IV) the Mg mixture layer contains Mg and a second solid electrolyte, and (V) the Young's modulus of the second solid electrolyte is lower than that of the first solid electrolyte. [Explanation of Symbols]

[0075] 10 positive electrode 11 Positive electrode current collector 12 Cathode active material layer 20 Electrolyte layer 31 Negative electrode current collector 32. Lithium Metals 33 Mg combination layer 34 Protective layer 50-layer structure 100 Secondary battery

Claims

1. A secondary battery comprising a positive electrode, a solid electrolyte layer, a negative electrode current collector, and metallic lithium as a negative electrode active material deposited between the solid electrolyte layer and the negative electrode current collector upon charging, A magnesium alloy layer is present between the solid electrolyte layer and the negative electrode current collector. The solid electrolyte layer includes a first solid electrolyte, The Mg mixture layer comprises at least one of Mg elemental particles and Mg alloy particles and a second solid electrolyte. The Young's modulus of the second solid electrolyte is lower than that of the first solid electrolyte. The Young's modulus of the second solid electrolyte is 1 GPa or more and 20 GPa or less. Secondary battery.

2. The first solid electrolyte is a sulfide solid electrolyte. The secondary battery according to claim 1.

3. The second solid electrolyte is a complex hydride containing Li. The secondary battery according to claim 1 or 2.

4. A protective layer exists between the Mg mixture layer and the negative electrode current collector. The protective layer contains Mg and does not contain an electrolyte. The secondary battery according to claim 1 or 2.

5. The positive electrode includes a lithium-containing oxide as the positive electrode active material. The secondary battery according to claim 1 or 2.

6. The aforementioned negative electrode current collector includes stainless steel, The secondary battery according to claim 1 or 2.

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