Lithium ion secondary battery

Abstract

To provide a lithium ion secondary battery which can prevent drop of a reversible capacitance.SOLUTION: The lithium ion secondary battery uses a precipitation-dissolution reaction of metal lithium. The lithium ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer between the positive layer and the negative electrode. The positive electrode layer contains a positive electrode active material capable of occluding and discharging lithium ions. The negative electrode layer contains a Ag element, a Sn element, and a Li element. The mol ratio (Sn / Ag) of the Sn element to the Ag element in the negative electrode layer is in the range of 0.09 to 0.17, both inclusive.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a lithium-ion secondary battery.

Background Art

[0002] Regarding lithium-ion secondary batteries including a metal layer in the negative electrode as disclosed in Patent Document 1, various technologies have been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, since the resistance of the negative electrode layer increases at the end of discharge of the lithium-ion secondary battery, the reversible capacity of the lithium-ion secondary battery decreases with repeated charge and discharge.

[0005] The present disclosure has been made in view of the above circumstances, and the main object thereof is to provide a lithium-ion secondary battery capable of suppressing a decrease in reversible capacity.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> A lithium-ion secondary battery using a deposition-dissolution reaction of metallic lithium, The lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer between the positive electrode layer and the negative electrode layer. The positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions. The negative electrode layer contains an Ag element, a Sn element, and a Li element. A lithium-ion secondary battery in which a molar ratio (Sn / Ag) of the Sn element to the Ag element contained in the negative electrode layer is 0.09 or more and 0.17 or less.

[0007] <2> The lithium-ion secondary battery has a negative electrode current collector on a side opposite to the electrolyte layer of the negative electrode layer. When the negative electrode layer is equally divided into two in parallel to a laminated surface of the negative electrode layer, and a region on the negative electrode current collector side is defined as a first region and a region on the electrolyte layer side is defined as a second region, in the lithium-ion secondary battery according to <1>, the content of the Sn element in the second region is larger than the content of the Sn element in the first region in the negative electrode layer.

[0008] <3> In the lithium-ion secondary battery according to <1> or <2>, when the battery is fully charged, a Li composition ratio of a Li-Ag alloy formed in the negative electrode layer is 94 mol% or more and 97 mol% or less.

[0009] <4> The electrolyte layer is a solid electrolyte layer containing a sulfide-based solid electrolyte, and the lithium-ion secondary battery according to any one of <1> to <3>.

[0010] <5> A lithium-ion secondary battery using a precipitation-dissolution reaction of metallic lithium, The lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer between the positive electrode layer and the negative electrode layer. The positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions. The negative electrode layer includes, in order from the electrolyte layer side, a metallic Sn layer containing Sn simple substance and a metallic Ag layer containing Ag simple substance. A lithium-ion secondary battery in which a molar ratio (Sn / Ag) of the Sn element to the Ag element contained in the negative electrode layer is 0.09 or more and 0.17 or less.

Advantages of the Invention

[0011] The lithium-ion secondary battery of the present disclosure can suppress a decrease in reversible capacity.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, the general configuration and manufacturing process of a lithium-ion secondary battery that does not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and the common general knowledge in the relevant field.

[0014] In the present disclosure, there is provided a lithium-ion secondary battery that utilizes the deposition-dissolution reaction of metallic lithium, wherein the lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions. The negative electrode layer contains Ag element, Sn element, and Li element. There is provided a lithium-ion secondary battery in which the molar ratio (Sn / Ag) of the Sn element to the Ag element contained in the negative electrode layer is 0.09 or more and 0.17 or less.

[0015] In the present disclosure, an increase in the resistance of the negative electrode layer at the end of discharge can be suppressed, and a decrease in the charge-discharge efficiency can be suppressed. Since the melting point of Sn is low, the adhesion between the electrolyte layer and the negative electrode layer is increased, and a decrease in the charge-discharge efficiency can be suppressed. In the present disclosure, when a Li-Sn layer is formed between the electrolyte layer and the Li-Ag layer during Li insertion into the negative electrode layer, peeling of the negative electrode layer from the electrolyte layer is suppressed. Compared with the case where there is no Li-Ag layer or the Li-Ag layer is used, at the end of discharge, the resistance of the lithium-ion secondary battery is reduced, the reversible capacity is increased, and the charge-discharge efficiency is improved. When Sn is mixed, the melting point becomes lower than that of Ag alone, the adhesion between the electrolyte layer and the negative electrode layer is increased, and peeling of the negative electrode layer from the electrolyte layer can be suppressed.

[0016] The lithium-ion secondary battery of the present disclosure utilizes the deposition-dissolution reaction of metallic lithium. The lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer. In the present disclosure, the negative electrode means one including the negative electrode layer. In the present disclosure, the full charge of the lithium-ion secondary battery means the state when the state of charge (SOC) value of the lithium-ion secondary battery is 100%. SOC indicates the ratio of the charge capacity to the full charge capacity of the battery, and the full charge capacity is SOC 100%. The SOC may be estimated, for example, from the open circuit voltage (OCV) of a lithium-ion secondary battery.

[0017] [Negative electrode] The negative electrode includes a negative electrode layer. The negative electrode may include a negative electrode current collector as needed.

[0018] [Negative electrode current collector] The material of the negative electrode current collector may be a material that does not alloy with Li, and examples thereof include SUS, copper, nickel, etc. Examples of the form of the negative electrode current collector include foil shape and plate shape. The planar shape of the negative electrode current collector is not particularly limited, and examples thereof include circular shape, elliptical shape, rectangular shape, and any polygonal shape. Also, the thickness of the negative electrode current collector varies depending on the shape, but may be, for example, within the range of 1 μm to 50 μm, or may be within the range of 5 μm to 20 μm.

[0019] [Negative electrode layer] The negative electrode layer contains Ag element, Sn element, and Li element. Before the first charge of the lithium-ion secondary battery, the negative electrode layer may include a metal Sn layer containing Sn single substance and a metal Ag layer containing Ag single substance in order from the electrolyte layer side. After the first charge of the lithium-ion secondary battery, the negative electrode layer may include a Li-Sn alloy layer containing Li-Sn alloy and a Li-Ag alloy layer containing Li-Ag alloy in order from the electrolyte layer side. The negative electrode layer may not contain an Ag-C composite. That is, the carbon content rate of the negative electrode layer may be 0 mass%.

[0020] The molar ratio (Sn / Ag) of Sn element to Ag element contained in the negative electrode layer may be 0.09 or more and 0.17 or less.

[0021] When the negative electrode layer is equally divided into two parallel to the lamination plane of the negative electrode layer, with the region on the negative electrode current collector side as the first region and the region on the electrolyte layer side as the second region, the content of Sn element in the second region of the negative electrode layer may be greater than that in the first region. The parallel may be substantially parallel. The substantially parallel may be within the range of 0° to 10°. The lamination direction of the negative electrode layer is the thickness direction of the negative electrode layer. For the comparison of the contents of the two regions in the negative electrode layer, for example, using SEM-EDX, elemental mapping may be performed with the view from the electrolyte layer to the negative electrode current collector as the observation field, and the contents of the target elements in each region may be compared. The comparison of the contents of the two regions in the negative electrode layer may be performed for a lithium ion secondary battery in a state such as full charge after the first charge. The comparison of the contents is not limited to this, and in addition to SEM-EDX, XPS, TOF-SIMS, etc. may also be used.

[0022] The Li composition ratio of the Li-Ag alloy formed in the negative electrode layer may be greater than 0 mol% and less than 100 mol%, and may also be 30 mol% or more and 99 mol% or less. At the time of full charge of the lithium ion secondary battery, the Li composition ratio of the Li-Ag alloy formed in the negative electrode layer may be 94 mol% or more and 97 mol% or less. In the Li-Ag alloy layer, the average particle diameter of the Li-Ag alloy particles may be greater than 0 μm and 5 μm or less. In the negative electrode layer, the volume occupied by the Li-Ag alloy may be greater than 0 vol% and less than 100 vol%, and may also be 5 vol% or more and 80 vol% or less.

[0023] After the first charge of the lithium ion secondary battery, the thickness of the Li-Sn layer may be greater than 0 μm and 100 μm or less, the lower limit may be 0.01 μm or more, may be 0.1 μm or more, the upper limit may be 15 μm or less, may be 0.7 μm or less, and may be 0.35 μm or less.

[0024] After the first charge of the lithium-ion secondary battery, the thickness of the Li-Ag alloy layer may be greater than 0 μm and not more than 100 μm, may be not less than 0.1 μm, and may be not more than 40 μm. The metal Ag layer is formed on, for example, the negative electrode current collector. Examples of the film formation method include a method of pressing with Ag particles placed, a vapor deposition method, a sputtering method, a PVD method, and an electrolytic plating method. Among them, a vapor deposition method or a sputtering method may be used. The adhesion between the metal Ag layer and the negative electrode current collector can be improved, and an increase in the resistance of the negative electrode can be suppressed. Regarding the metal Sn layer as well, in the same manner as described above, the metal Sn layer may be formed on the negative electrode current collector side or the solid electrolyte layer side. Among them, it may be formed on the solid electrolyte layer side. By forming it on the solid electrolyte side, the adhesion between the metal Sn layer and the solid electrolyte layer becomes high.

[0025] The thickness of the negative electrode layer is not particularly limited, but may be not less than 30 nm and not more than 50 μm at full charge after the first charge of the lithium-ion secondary battery.

[0026] [Electrolyte layer] The electrolyte layer may be a liquid-based electrolyte layer using an electrolytic solution as the electrolyte, or may be a solid electrolyte layer using a solid electrolyte as the electrolyte. As the electrolytic solution, a conventionally known electrolytic solution used for lithium-ion secondary batteries can be used. The solid electrolyte layer contains at least a solid electrolyte. As the solid electrolyte to be contained in the solid electrolyte layer, known solid electrolytes that can be used for solid batteries can be appropriately used, and examples include oxide-based solid electrolytes and sulfide-based solid electrolytes. In order to suppress peeling of the negative electrode layer from the solid electrolyte layer, a relatively soft sulfide-based solid electrolyte may be used as the solid electrolyte.

[0027] Examples of sulfide solid electrolytes include solid electrolytes containing Li element, M element (M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, In), and S element. Further, the sulfide solid electrolyte may further contain at least one of O element and halogen element. Examples of sulfide solid electrolytes include, for example, Li 2 S-P 2 S 5 , Li 2 S-SiS 2 , LiX-Li 2 S-SiS 2 , LiX-Li 2 S-P 2 S 5 , LiX-Li 2 O-Li 2 S-P 2 S 5 , LiX-Li 2 S-P 2 O 5 , LiX-Li 3 PO 4 -P 2 S 5 , and Li 3 PS 4 and the like. Note that the description of "Li 2 S-P 2 S 5 " means a material obtained using a raw material composition containing Li 2 S and P 2 S 5 , and the same applies to other descriptions. Further, "X" in the above LiX represents a halogen element. Examples of the halogen element include F element, Cl element, Br element, and I element. The raw material composition containing LiX may contain one or more kinds of LiX. When two or more kinds of LiX are contained, the mixing ratio of the two or more kinds is not particularly limited. The molar ratio of each element in the sulfide solid electrolyte can be controlled by adjusting the content of each element in the raw material. Further, the molar ratio and composition of each element in the sulfide solid electrolyte can be measured, for example, by ICP emission spectrometry.

[0028] The sulfide-based solid electrolyte may be a sulfide glass, a crystallized sulfide glass (glass-ceramics), or a crystalline material obtained by a solid-phase reaction treatment on a raw material composition. The crystalline state of the sulfide-based solid electrolyte can be confirmed, for example, by performing powder X-ray diffraction measurement using CuKα rays on the sulfide-based solid electrolyte.

[0029] The sulfide glass can be obtained by subjecting a raw material composition (for example, a mixture of Li 2 S and P 2 S 5 to an amorphous treatment. Examples of the amorphous treatment include mechanical milling.

[0030] The glass-ceramics can be obtained, for example, by heat-treating a sulfide glass. The heat treatment temperature may be a temperature higher than the crystallization temperature (Tc) observed by thermal analysis measurement of the sulfide glass, and is usually 195°C or higher. On the other hand, the upper limit of the heat treatment temperature is not particularly limited. The crystallization temperature (Tc) of the sulfide glass can be measured by differential thermal analysis (DTA). The heat treatment time is not particularly limited as long as the desired crystallinity of the glass-ceramics is obtained, but is, for example, in the range of 1 minute to 24 hours, and among them, the range of 1 minute to 10 hours is mentioned. The method of heat treatment is not particularly limited, and examples thereof include a method using a firing furnace.

[0031] Examples of the oxide-based solid electrolyte include substances having a garnet-type crystal structure containing an Li element, a La element, an A element (A is at least one of Zr, Nb, Ta, and Al), and an O element. Examples of the oxide-based solid electrolyte include Li 2 O-B 2 O 3 -P 2 O 5 、Li 2 O-SiO 2 、Li2 O-B 2 O 3 , Li 1.3 Al 0.3 Ti 0.7 (PO 4 ) 3 , Li 5 La 3 Ta 2 O 12 , Li 7 La 3 Zr 2 O 12 , Li 6 BaLa 2 Ta 2 O 12 , Li 3.6 Si 0.6 P 0.4 O 4 , Li 4 SiO 4 , Li 3 PO 4 , and Li 3+x PO 4-x N x It may be, for example, (1 ≦ x ≦ 3).

[0032] From the viewpoint of good handleability, the shape of the solid electrolyte may be particulate. Also, the average particle diameter (D50) of the particles of the solid electrolyte is not particularly limited, but the lower limit may be 0.5 μm or more, and the upper limit may be 2 μm or less.

[0033] In the present disclosure, unless otherwise specified, the average particle diameter of the particles is the value of the volume-based median diameter (D50) measured by laser diffraction / scattering particle size distribution measurement. Also, in the present disclosure, the median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of the particles becomes half (50%) of the total volume when the particles are arranged in ascending order of particle size.

[0034] The solid electrolyte can be used alone or in combination of two or more. Also, when using two or more solid electrolytes, two or more solid electrolytes may be mixed, or layers of two or more solid electrolytes may be formed respectively to form a multilayer structure. The proportion of the solid electrolyte in the solid electrolyte layer is not particularly limited. For example, it may be 50% by mass or more, and may be in the range of 60% by mass or more and 100% by mass or less, may be in the range of 70% by mass or more and 100% by mass or less, or may be 100% by mass.

[0035] From the perspective of expressing plasticity, etc., the solid electrolyte layer can also contain a binder. Examples of such a binder include materials exemplified as the binder used in the positive electrode layer described later. However, from the perspective of facilitating high output and forming a solid electrolyte layer having a solid electrolyte that prevents excessive aggregation of the solid electrolyte and is uniformly dispersed, the binder contained in the solid electrolyte layer may be 5% by mass or less.

[0036] The thickness of the solid electrolyte layer is not particularly limited and is usually 0.1 μm or more and 1 mm or less.

[0037] [Positive Electrode] The positive electrode includes a positive electrode layer. The positive electrode includes a positive electrode current collector as needed.

[0038] [Positive Electrode Layer] The positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions, and may contain a solid electrolyte, a conductive material, a binder, etc. as optional components.

[0039] The positive electrode active material may contain Li element before the first charge of the lithium ion secondary battery. The positive electrode active material is, for example, lithium nickel cobalt aluminum oxide (NCA), LiCoO 2 , LiNi x Co 1-x O 2 (0 < x < 1), LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 , LiMnO 2 , LiMn 2 O 4 , LiNiO 2 , LiVO 2, Heteroatom-substituted Li-Mn spinel, lithium titanate, metal lithium phosphate, LiCoN, Li 2 SiO 3 , and Li 4 SiO 4 and the like can be mentioned. Heteroatom-substituted Li-Mn spinel is, for example, LiMn 1.5 Ni 0.5 O 4 , LiMn 1.5 Al 0.5 O 4 , LiMn 1.5 Mg 0.5 O 4 , LiMn 1.5 Co 0.5 O 4 , LiMn 1.5 Fe 0.5 O 4 , and LiMn 1.5 Zn 0.5 O 4 and the like. Lithium titanate is, for example, Li 4 Ti 5 O 12 and the like. Metal lithium phosphate is, for example, LiFePO 4 , LiMnPO 4 , LiCoPO 4 , and LiNiPO 4 and the like. The shape of the positive electrode active material is not particularly limited, but it may be particulate (positive electrode active material particles). A coating layer containing a Li-ion conductive oxide may be formed on the surface of the positive electrode active material. This is because the reaction between the positive electrode active material and the solid electrolyte can be suppressed. Examples of the Li-ion conductive oxide include LiNbO 3 , Li 4 Ti 5 O 12 , and Li 3 PO 4 and the like. The thickness of the coating layer is, for example, 0.1 nm or more, and may be 1 nm or more. On the other hand, the thickness of the coating layer is, for example, 100 nm or less, and may be 20 nm or less. The coverage rate of the coating layer on the surface of the positive electrode active material is, for example, 70% or more, and may be 90% or more.

[0040] Examples of the solid electrolyte include those that can be contained in the solid electrolyte layer described above. The content of the solid electrolyte in the positive electrode layer is not particularly limited, but may be, for example, in the range of 1% by mass to 80% by mass when the total mass of the positive electrode layer is 100% by mass.

[0041] Known materials can be used as the conductive material, and examples thereof include carbon materials and metal particles. Examples of the carbon material include acetylene black (AB), furnace black, VGCF, carbon nanotubes, and carbon nanofibers. Among them, from the viewpoint of electron conductivity, it may be at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers. Examples of the metal particles include particles such as Ni, Cu, Fe, and SUS. The content of the conductive material in the positive electrode layer is not particularly limited.

[0042] Examples of the binder include acrylonitrile-butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SBR). The content of the binder in the positive electrode layer is not particularly limited.

[0043] The thickness of the positive electrode layer is not particularly limited.

[0044] The positive electrode layer can be formed by a conventionally known method. For example, a positive electrode active material and, if necessary, other components are put into a solvent and stirred to prepare a slurry for the positive electrode layer. The slurry for the positive electrode layer is applied onto one surface of a support such as a positive electrode current collector and dried to obtain the positive electrode layer. Examples of the solvent include butyl acetate, butyl butyrate, heptane, and N-methyl-2-pyrrolidone. The method of applying the slurry for the positive electrode layer on one surface of a support such as a positive current collector is not particularly limited, and examples thereof include the doctor blade method, the metal mask printing method, the electrostatic coating method, the dip coating method, the spray coating method, the roll coating method, the gravure coating method, and the screen printing method. As the support, those having self-supportability can be appropriately selected and used, and there is no particular limitation. For example, metal foils such as Cu and Al can be used.

[0045] [Positive current collector] As the positive current collector, known metals that can be used as the current collector of a lithium-ion secondary battery can be used. Examples of such metals include metal materials containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In. Examples of the positive current collector include SUS, aluminum, nickel, iron, titanium, and carbon. The form of the positive current collector is not particularly limited, and various forms such as foil-like and mesh-like can be adopted.

[0046] The lithium-ion secondary battery may be provided with an exterior body that houses a positive electrode layer, a negative electrode layer, an electrolyte layer, etc., as necessary. The material of the exterior body is not particularly limited as long as it is stable to the electrolyte, and examples thereof include resins such as polypropylene, polyethylene, and acrylic resin.

[0047] Examples of the shape of the lithium-ion secondary battery include coin type, laminate type, cylindrical type, and square type.

[0048] The lithium-ion secondary battery may be a liquid-based lithium-ion secondary battery using an electrolytic solution as an electrolyte, or may be a solid lithium-ion secondary battery using a solid electrolyte as an electrolyte. Examples of the applications of the lithium-ion secondary battery include power sources for vehicles such as hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), battery electric vehicles (BEV), gasoline vehicles, and diesel vehicles. Among them, it may be used as a driving power source for a hybrid vehicle (HEV), a plug-in hybrid vehicle (PHEV), or a battery electric vehicle (BEV). In addition, the lithium-ion secondary battery may be used as a power source for moving bodies other than vehicles (e.g., railways, ships, airplanes), and may also be used as a power source for electrical products such as information processing devices.

[0049] Figure 1 is a schematic cross-sectional view showing an example of a lithium-ion secondary battery before the first charge during the manufacturing of the present disclosure. As shown in Figure 1, the lithium-ion secondary battery 100 before the first charge includes a positive electrode current collector 10, a positive electrode layer 20, an electrolyte layer 30, a metal Sn layer 40, a metal Ag layer 50, and a negative electrode current collector 60 in this order. The negative electrode layer includes a metal Sn alloy layer 40 and a metal Ag layer 50. Figure 2 is a schematic cross-sectional view showing an example of a lithium-ion secondary battery at full charge after the first charge of the present disclosure. As shown in Figure 2, the lithium-ion secondary battery 200 after the first charge includes a positive electrode current collector 10, a positive electrode layer 20, an electrolyte layer 30, a Li-Sn alloy layer 41, a Li-Ag alloy layer 51, and a negative electrode current collector 60 in this order. By the first charge, the metal Sn layer 40 becomes the Li-Sn alloy layer 41, and the metal Ag layer 50 becomes the Li-Ag alloy layer 51. The negative electrode layer includes the Li-Sn alloy layer 41 and the Li-Ag alloy layer 51. The negative electrode layer may be in a state of a single-layer Li-Sn-Ag alloy layer in which the Li-Sn alloy layer 41 and the Li-Ag alloy layer 51 are integrated.

Examples

[0050] (Examples 1 to 2) [Manufacture of positive electrode] Butyl butyrate was used as the solvent. NCA was used as the positive electrode active material. Particles of a sulfide-based solid electrolyte (average particle size: 2.0 μm) were used as the solid electrolyte. Al foil was used as the positive electrode current collector. The positive electrode active material, the solid electrolyte, the binder, and the conductive assistant were mixed in the following mass composition ratio in a solvent to prepare a positive electrode slurry. Mass composition ratio: positive electrode active material:solid electrolyte:binder:conductive assistant = 84.7:13.4:0.6:1.27 The prepared positive electrode slurry was coated on the Al foil with a coating gap of 225 μm. Then, the coated positive electrode slurry was pre-dried at 60 °C for 3 hours. Then, the pre-dried positive electrode slurry was dried at 165 °C for 1 hour. As a result, a positive electrode composite coated foil with a basis weight of 18.7 mg / cm 2 and a design capacity of 3.0 mAh / cm 2 was obtained. The obtained positive electrode composite coated foil was punched out to obtain a positive electrode with a diameter of 11.28 mm.

[0051] [Fabrication of Solid Electrolyte Layer] Butyl butyrate was used as the solvent. Particles of a sulfide-based solid electrolyte (average particle size: 2.0 μm) were used as the solid electrolyte. The solid electrolyte and the binder were mixed in the following mass composition ratio in a solvent to prepare a solid electrolyte slurry. Mass composition ratio: solid electrolyte:binder = 92.6:7.4 The prepared solid electrolyte slurry was coated on a release film with a coating gap of 325 μm. Then, the coated solid electrolyte slurry was pre-dried at room temperature for 3 hours. Then, the pre-dried solid electrolyte slurry was dried at 165 °C for 1 hour. The dried solid electrolyte coated foil was punched out to obtain two disks with a diameter of 14.5 mm. The solid electrolyte coated surfaces of the two disks were overlapped and pressed at 7 t. After pressing, the release films of the two disks were peeled off to obtain a self-supporting solid electrolyte layer.

[0052] [Fabrication of Negative Electrode] Ni foil was used as the negative electrode current collector. On one side of the negative electrode current collector, a metal Ag layer with a thickness of 0.368 μm in Example 1 and 0.735 μm in Example 2 was formed by sputtering to obtain a metal Ag layer / Ni foil. The obtained metal Ag layer / Ni foil was punched out to obtain a metal Ag layer / Ni foil with a diameter of 14.5 mm. A metal Sn layer with a thickness of 0.1 μm was formed on the solid electrolyte layer by sputtering to prepare a solid electrolyte layer / metal Sn layer. The obtained solid electrolyte layer / metal Sn layer and the metal Ag layer / Ni foil were stacked in this order to obtain a negative electrode (metal Sn layer / metal Ag layer / Ni foil) on the solid electrolyte layer.

[0053] [Cell fabrication] Al was used as the positive electrode tab. Ni was used as the negative electrode tab. The fabricated positive electrode, the fabricated solid electrolyte layer, and the fabricated negative electrode were arranged in this order to obtain a laminate. A positive electrode tab was attached to the positive electrode, and a negative electrode tab was attached to the negative electrode. Then, the laminate was housed in a laminate film, and the inside of the laminate film was evacuated to seal the laminate. The sealed laminate was isostatically pressed at 392 MPa using CIP (cold isostatic pressing) to fabricate a laminate cell (sometimes referred to as a cell). A spring was inserted to use a constant-pressure jig so that the restraint pressure would be constant regardless of the volume change of the laminate cell, and the fabricated laminate cell was restrained at 1 MPa.

[0054] (Comparative Examples 1 - 2) A laminate cell was fabricated in the same manner as in Example 1, except that a metal Sn layer was not formed on the solid electrolyte layer, and a metal Ag layer / Ni foil obtained by forming a metal Ag layer with a thickness of 0.368 μm in Comparative Example 1 and 0.735 μm in Comparative Example 2 by sputtering on one side of the negative electrode current collector was used as the negative electrode.

[0055] [Sn / Ag molar ratio] For the negative electrode layers of Examples 1 - 2, the molar ratio of Sn element to Ag element (Sn / Ag) contained in the negative electrode layer was calculated. The results are shown in Table 1.

[0056] [Initial charge and discharge] Under the following conditions, the first charge-discharge cycles of each of the laminate cells fabricated in Examples 1 to 2 and Comparative Examples 1 to 2 were carried out at 60°C. Constant current charging was performed under the condition of a current density of 0.15 mA / cm until the voltage reached 4.2 V. 2 Then, constant voltage charging was carried out until the current density reached 0.03 mA / cm at a 0.05 C rate. 2 Constant current discharging was performed under the condition of a current density of 0.15 mA / cm until the voltage reached 3.0 V. 2 Then, constant voltage discharging was carried out until the current density reached 0.03 mA / cm at a 0.05 C rate. 2

[0057] [Resistance after the first charge-discharge cycle] The resistance (Ω·cm) when a predetermined current was passed through each of the laminate cells after the first charge-discharge cycle of Examples 1 to 2 and Comparative Examples 1 to 2 for 1 second at a predetermined voltage was measured by the AC impedance method. The results are shown in Table 1. 2 ) Figure 3 is a graph showing the relationship between the Li composition in the Li-Ag alloy layer and the resistance of the laminate cell after the first charge-discharge cycle. The resistances of the laminate cells after the first charge-discharge cycle of Examples 1 to 2 decreased in the order of Example 2 and Example 1. Compared with Comparative Examples 1 to 2 using elemental Ag as the negative electrode layer, the resistance of the cell at the end of discharge was reduced.

[0058] [Reversible capacity at 25°C after the first charge-discharge cycle at 60°C and reversible capacity at 25°C after 50 cycles] Under the following conditions, the reversible capacities at 25°C (mAh / cm) of each of the laminate cells after the first charge-discharge cycle of Examples 1 to 2 and Comparative Examples 1 to 2 and the reversible capacities at 25°C after 50 cycles (mAh / cm) were measured at 25°C. 2 ) 2 ) Constant current charging was performed under the condition of a current density of 0.15 mA / cm until the voltage reached 4.2 V. 2 Then, constant voltage charging was carried out until the current density reached 0.03 mA / cm at a 0.05 C rate. 2 Thereafter, constant current discharging was performed under the condition of a current density of 0.15 mA / cm until the voltage reached 3.0 V. 2 ​​​、The constant current discharge was carried out under the condition of 0.05 C rate, and the reversible capacity (discharge capacity) was measured. The above charge and discharge were performed 50 cycles, and the reversible capacity after 50 cycles was measured. The results are shown in Table 1. Figure 4 is a graph showing the relationship between the reversible capacity at 25 °C of each laminate cell after charge and discharge cycles in Examples 1 to 2 and Comparative Examples 1 to 2. The 25 °C reversible capacity shown in Figure 4 has a higher retention rate of the reversible capacity even when the number of charge and discharge cycles increases in Examples 1 to 2 compared to Comparative Examples 1 to 2. The 25 °C reversible capacity after the first charge and discharge and the reversible capacity after 50 cycles of each laminate cell of Examples 1 to 2 shown in Table 1 increase in the order of Example 2 and Example 1.

[0059]

Table 1

[0060] [SEM-EDX measurement] Regarding the cross-section of the solid electrolyte layer - negative electrode (Li-Sn alloy layer / Li-Ag alloy layer / Ni foil) of the laminate cell of Example 2 after the first charge, SEM observation and EDX mapping were performed with a secondary electron image at an applied voltage of 5 kV. Figure 5 shows (1) secondary electron image, (2) S element mapping, (3) Ag element mapping, (4) Sn element mapping, (5) Ni element mapping, and (6) O element mapping in the SEM-EDX mapping of the cross-section of the solid electrolyte layer - negative electrode after the first charge of Example 2. Regarding the cross-section of the solid electrolyte layer - negative electrode (Li-Sn alloy layer / Li-Ag alloy layer / Ni foil) of the laminate cell of Example 2 after the first discharge, SEM observation and EDX mapping were performed with a secondary electron image at an applied voltage of 5 kV. Figure 6 shows (1) secondary electron image, (2) S element mapping, (3) Ag element mapping, (4) Sn element mapping, (5) Ni element mapping, and (6) O element mapping in the SEM-EDX mapping of the cross-section of the solid electrolyte layer - negative electrode after the first discharge of Example 2. As shown in FIG. 5, when the negative electrode layer is divided into two equal parts in the region on the negative electrode current collector side and the region on the electrolyte layer side, it can be seen that the content of Sn element in the region on the electrolyte layer side is larger than the content of Sn element in the region on the negative electrode current collector side. As shown in FIG. 5, it can be seen that after the first charge of the lithium ion secondary battery, a Li-Sn alloy layer is formed between the solid electrolyte layer and the Li-Ag alloy layer. The Li-Ag alloy layer is dispersed over a wide range, and it is confirmed that a large amount of Li reacts with Ag and the Li-Ag alloy layer functions as a Li reaction layer. As shown in FIG. 6, at the end of discharge of the lithium ion secondary battery, it can be seen that the Li-Sn alloy layer is maintained between the solid electrolyte layer and the Li-Ag alloy layer even at the end of discharge of the lithium ion secondary battery. Thereby, the reduction decomposition reaction of the solid electrolyte layer by Li can be prevented, and an increase in the interfacial resistance and the cell resistance can be suppressed. Also, the adhesion between the solid electrolyte layer and the negative electrode layer during discharge when Li desorbs from the negative electrode layer, and the adhesion between the Li-Sn alloy layer and the Li-Ag alloy layer are improved, the resistance at the interface between the solid electrolyte layer and the negative electrode layer is reduced, the reversible capacity is increased, and it is considered that a decrease in the reversible capacity accompanying repeated charge and discharge is also suppressed.

Explanation of Symbols

[0061] 10 Positive electrode current collector 20 Positive electrode layer 30 Electrolyte layer 40 Metal Sn layer 41 Li-Sn alloy layer 50 Metal Ag layer 51 Li-Ag alloy layer 60 Negative electrode current collector 100 Lithium ion secondary battery 200 Lithium ion secondary battery

Claims

1. A lithium-ion secondary battery using a precipitation-dissolution reaction of metallic lithium, wherein the lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer between the positive electrode layer and the negative electrode layer, the positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions, the negative electrode layer contains Ag element, Sn element, and Li element, and a molar ratio (Sn / Ag) of the Sn element to the Ag element contained in the negative electrode layer is 0.09 or more and 0.17 or less.

2. The lithium-ion secondary battery has a negative electrode current collector on a side opposite to the electrolyte layer of the negative electrode layer, when the negative electrode layer is equally divided into two in parallel to the lamination plane of the negative electrode layer, and a region on the negative electrode current collector side is defined as a first region and a region on the electrolyte layer side is defined as a second region, the content of the Sn element in the second region of the negative electrode layer is larger than the content of the Sn element in the first region. The lithium-ion secondary battery according to Claim 1.

3. The lithium-ion secondary battery according to Claim 1 or 2, wherein at full charge of the lithium-ion secondary battery, a Li composition ratio of a Li-Ag alloy formed in the negative electrode layer is 94 mol% or more and 97 mol% or less.

4. The electrolyte layer is a solid electrolyte layer containing a sulfide-based solid electrolyte. The lithium-ion secondary battery according to Claim 3.

5. A lithium-ion secondary battery using a precipitation-dissolution reaction of metallic lithium, wherein the lithium-ion secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer between the positive electrode layer and the negative electrode layer, the positive electrode layer contains a positive electrode active material capable of occluding and releasing lithium ions, the negative electrode layer includes a metallic Sn layer containing Sn simple substance and a metallic Ag layer containing Ag simple substance in order from the electrolyte layer side, and a molar ratio (Sn / Ag) of the Sn element to the Ag element contained in the negative electrode layer is 0.09 or more and 0.17 or less.

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