Lithium ion secondary battery

Abstract

To provide a lithium ion secondary battery which can suppress drop of the charge capacity.SOLUTION: The lithium ion secondary battery uses a precipitation-dissolution reaction of metal lithium, and 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 discharging lithium ions. The negative electrode layer contains a Mg element, an In element, a Sn element, and a Li element.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 charge capacity.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> 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 contains Mg element, In element, Sn element, and Li element, and is a lithium-ion secondary battery.

[0007] <2> The lithium-ion secondary battery according to <1>, wherein the molar ratio of Sn element to In element (Sn / In) contained in the negative electrode layer is 0.5 or more and 3.8 or less.

[0008] <3> The lithium-ion secondary battery according to <1> or <2>, wherein the negative electrode layer includes a Li-In-Sn alloy layer containing a Li-In-Sn alloy and a Li-Mg alloy layer containing a Li-Mg alloy in order from the electrolyte layer side.

[0009] <4> The lithium-ion secondary battery has a negative electrode current collector on the side opposite to the electrolyte layer of the negative electrode layer. When the negative electrode layer is equally divided into two parallel to the lamination plane of the negative electrode layer, and the region on the negative electrode current collector side is defined as the first region and the region on the electrolyte layer side is defined as the second region, the content of In element and Sn element in the second region of the negative electrode layer is greater than that in the first region. The lithium-ion secondary battery according to any one of <1> to <3>.

[0010] <5> The lithium-ion secondary battery according to any one of <1> to <4>, wherein the electrolyte layer is a solid electrolyte layer containing a sulfide-based solid electrolyte.

[0011] <6> A lithium-ion secondary battery using 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 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 an In-Sn alloy layer containing an In-Sn alloy and a metallic Mg layer containing Mg single substance in order from the electrolyte layer side, and is a lithium-ion secondary battery.

Advantages of the Invention

[0012] The lithium-ion secondary battery of the present disclosure can suppress a decrease in charge capacity.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments according to the present disclosure will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of lithium-ion secondary batteries that do 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 common general knowledge in the relevant field.

[0015] 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 Mg element, In element, Sn element, and Li element, thereby providing a lithium-ion secondary battery.

[0016] 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 charge-discharge efficiency can be suppressed. In the present disclosure, when a Li-In-Sn layer is formed between the electrolyte layer and the Li-Mg 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-Mg layer or when a Li-In layer or a Li-Sn layer is used between the electrolyte layer and the Li-Mg layer, the reversible capacity of the lithium-ion secondary battery increases and the charge-discharge efficiency improves. When In and Sn are mixed, the melting point is lower than that of Sn alone, the adhesion between the electrolyte layer and the negative electrode layer is higher, and a decrease in charge-discharge efficiency can be suppressed.

[0017] 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 fully charged state of a 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%. SOC may be estimated, for example, from the open circuit voltage (OCV) of the lithium-ion secondary battery.

[0018] [Negative electrode] The negative electrode includes a negative electrode layer. The negative electrode may include a negative electrode current collector as required.

[0019] [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, in the range of 1 μm to 50 μm, or may be in the range of 5 μm to 20 μm.

[0020] [Negative electrode layer] The negative electrode layer contains Mg element, In element, Sn element, and Li element. Before the first charge of the lithium-ion secondary battery, the negative electrode layer may include an In-Sn alloy layer containing an In-Sn alloy and a metal Mg layer containing Mg single crystal 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-In-Sn alloy layer containing a Li-In-Sn alloy and a Li-Mg alloy layer containing a Li-Mg alloy in order from the electrolyte layer side.

[0021] The total molar ratio of In element and Sn element to Mg element contained in the negative electrode layer {(In + Sn) / Mg} may be 0.01 or more, and may be 0.0872 or more. If it is less than 0.01, the In element and Sn element contained in the negative electrode layer are too few and the charge capacity decreases. The molar ratio of Sn element to In element contained in the negative electrode layer (Sn / In) may be 0.5 or more and 3.8 or less.

[0022] When the negative electrode layer is equally divided into two parallel to the lamination plane of the negative electrode layer, and the region on the negative electrode current collector side is defined as the first region and the region on the electrolyte layer side is defined as the second region, the content of the In element and the Sn element in the second region of the negative electrode layer may be larger than the content of the In element and the Sn element in the first region. Parallel may be substantially parallel. 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. The comparison of the content of the two regions in the negative electrode layer may be performed, for example, by using SEM-EDX to perform element mapping with the view from the electrolyte layer to the negative electrode current collector as the observation field, and comparing the content of the target element in each region. The comparison of the content of the two regions in the negative electrode layer may be performed on a lithium-ion secondary battery in a state such as full charge after the first charge. The comparison of the content is not limited to this, and XPS, TOF-SIMS, etc. may be used in addition to SEM-EDX.

[0023] Before the first charge during the production of the lithium-ion secondary battery, the Sn composition ratio in the In-Sn alloy layer may be greater than 0 mol% and less than 100 mol%, the lower limit may be 20 mol% or more, and the upper limit may be 80 mol% or less, 70 mol% or less, or 60 mol% or less.

[0024] After the first charge of the lithium-ion secondary battery, the thickness of the Li-In-Sn layer may be greater than 0 μm and 100 μm or less, the lower limit may be 0.01 μm or more, 0.1 μm or more, and the upper limit may be 15 μm or less, 0.6 μm or less, or 0.35 μm or less.

[0025] After the first charge of the lithium-ion secondary battery, the Li composition ratio in the Li-Mg alloy layer may be greater than 0 mol% and less than 100 mol%, may be 20 mol% or more at the lower limit, and may be 98 mol% or less at the upper limit.

[0026] After the first charge of the lithium-ion secondary battery, the thickness of the Li-Mg alloy layer may be greater than 0 μm and 100 μm or less, may be 0.1 μm or more at the lower limit, and may be 40 μm or less at the upper limit. The metal Mg layer is formed on, for example, the negative electrode current collector. Examples of the film formation method include a method of pressing by placing Mg particles, 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 Mg 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 In-Sn alloy layer, in the same manner as described above, the In-Sn alloy layer may be formed on the solid electrolyte layer side or the negative electrode current collector side. Among them, it may be formed on the negative electrode current collector side. By forming it on the negative electrode current collector side, the adhesion between the In-Sn alloy layer and the metal Mg layer is improved.

[0027] The thickness of the negative electrode layer is not particularly limited, but may be 30 nm or more and 50 μm or less at full charge after the first charge of the lithium-ion secondary battery.

[0028] [Electrolyte layer] The electrolyte layer may be a liquid 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, a known solid electrolyte 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.

[0029] Examples of the sulfide solid electrolyte include a solid electrolyte containing an Li element, an M element (M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and an S element. Further, the sulfide solid electrolyte may further contain at least one of an O element and a halogen element. Examples of the sulfide solid electrolyte include 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 etc. Note that the description of "Li 2 S-P 2 S 5 " means a material formed 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 an F element, a Cl element, a Br element, and an I element, etc. The raw material composition containing the above 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-based solid electrolyte can be controlled by adjusting the content of each element in the raw materials. Further, the molar ratio and composition of each element in the sulfide-based solid electrolyte can be measured, for example, by ICP emission spectrometry.

[0030] The sulfide-based solid electrolyte may be a sulfide glass, a crystallized sulfide glass (glass ceramic), 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.

[0031] 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.

[0032] The glass ceramic can be obtained, for example, by heat-treating the sulfide glass. The heat treatment temperature may be 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 ceramic can be 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 can be mentioned. The heat treatment method is not particularly limited, and examples thereof include a method using a firing furnace.

[0033] Examples of the oxide-based solid electrolyte include substances having a garnet-type crystal structure containing, for example, Li element, La element, A element (A is at least one of Zr, Nb, Ta, and Al), and O element. Examples of the oxide-based solid electrolyte include, for example, Li 2 O-B 2 O 3 -P 2 O 5 , Li 2 O-SiO 2 , Li 2 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 (1 ≦ x ≦ 3) etc. may also be used.

[0034] 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.

[0035] In the present disclosure, unless otherwise specified, the average particle size 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 order of increasing particle size.

[0036] The solid electrolyte can be used alone or in combination of two or more. When using two or more solid electrolytes, the 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, but for example, it is 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.

[0037] The solid electrolyte layer can also contain a binder from the viewpoint of expressing plasticity, etc. Examples of such a binder can include materials exemplified as the binder used for the positive electrode layer described later. However, from the viewpoint 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.

[0038] The thickness of the solid electrolyte layer is not particularly limited and is usually 0.1 μm or more and 1 mm or less.

[0039] [Positive Electrode] The positive electrode includes a positive electrode layer. The positive electrode includes a positive electrode current collector as needed.

[0040] [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.

[0041] 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 , hetero-element substituted Li-Mn spinel, lithium titanate, lithium metal phosphate, LiCoN, Li 2 SiO 3 , and Li 4 SiO 4 etc. can be mentioned. The hetero-element 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 etc. Lithium titanate is, for example, Li 4 Ti 5 O 12 etc. Lithium metal phosphate is, for example, LiFePO 4 , LiMnPO 4 , LiCoPO 4 , and LiNiPO 4 etc. 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 it can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of the Li-ion conductive oxide include, for example, 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.

[0042] Examples of the solid electrolyte that can be exemplified are the solid electrolytes that can be contained in the above-described solid electrolyte layer. The content of the solid electrolyte in the positive electrode layer is not particularly limited, but when the total mass of the positive electrode layer is 100% by mass, it may be, for example, in the range of 1% to 80% by mass.

[0043] As the conductive material, known ones can be used, and examples include carbon materials and metal particles. Examples of the carbon material include, for example, 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.

[0044] Examples of the binder include acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), and the like. The content of the binder in the positive electrode layer is not particularly limited.

[0045] The thickness of the positive electrode layer is not particularly limited.

[0046] 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 positive electrode layer is obtained by applying the slurry for the positive electrode layer onto one surface of a support such as a positive electrode current collector and drying it. 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 onto one surface of a support such as a positive electrode current collector is not particularly limited, and examples include a doctor blade method, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a gravure coating method, and a 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.

[0047] [Positive Electrode Current Collector] As the positive electrode current collector, a known metal that can be used as a current collector of a lithium-ion secondary battery can be used. Examples of such a metal 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 electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. The form of the positive electrode current collector is not particularly limited, and various forms such as foil and mesh can be used.

[0048] The lithium-ion secondary battery includes, as necessary, an exterior body that houses a positive electrode layer, a negative electrode layer, an electrolyte layer, and the like. 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.

[0049] Examples of the shape of the lithium-ion secondary battery include a coin type, a laminate type, a cylindrical type, and a square type.

[0050] 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 uses 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). Further, the lithium-ion secondary battery may be used as a power source for moving bodies other than vehicles (for example, railways, ships, and airplanes), and may also be used as a power source for electrical products such as information processing devices.

[0051] FIG. 1 is a schematic cross-sectional view showing an example of a lithium-ion secondary battery before the first charge during the manufacture of the present disclosure. As shown in FIG. 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, an In-Sn alloy layer 40, a metal Mg layer 50, and a negative electrode current collector 60 in this order. The negative electrode layer includes an In-Sn alloy layer 40 and a metal Mg layer 50. FIG. 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 FIG. 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-In-Sn alloy layer 41, a Li-Mg alloy layer 51, and a negative electrode current collector 60 in this order. By the first charge, the In-Sn alloy layer 40 becomes the Li-In-Sn alloy layer 41, and the metal Mg layer 50 becomes the Li-Mg alloy layer 51. The negative electrode layer includes the Li-In-Sn alloy layer 41 and the Li-Mg alloy layer 51.

Example

[0052] (Examples 1 to 3) [Fabrication 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 diameter: 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.

[0053] [Fabrication of solid electrolyte layer] Butyl butyrate was used as the solvent. Particles of a sulfide-based solid electrolyte (average particle diameter: 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. After that, the pre-dried solid electrolyte slurry was fully 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-standing solid electrolyte layer.

[0054] [Negative electrode fabrication] Ni foil was used as the negative electrode current collector. A metal Mg layer with a thickness of 1.0 μm was formed on one side of the negative electrode current collector by vapor deposition to obtain a metal Mg layer / Ni foil. An In-Sn alloy layer with a thickness of 0.1 μm was formed on the metal Mg layer of the metal Mg layer / Ni foil by dual vapor deposition using an ion plating method to fabricate an In-Sn alloy layer / metal Mg layer / Ni foil. The obtained In-Sn alloy layer / metal Mg layer / Ni foil was punched out to obtain a negative electrode with a diameter of 14.5 mm. The target composition (mol%) of the In-Sn alloy was 67:33 for Example 1, 50:50 for Example 2, and 20:80 for Example 3. The actual composition (mol%) of the In-Sn alloy was 66:34 for Example 1, 53:47 for Example 2, and 21:79 for Example 3.

[0055] [Cell fabrication] Al was used as the positive electrode tab. Ni was used as the negative electrode tab. The prepared solid electrolyte layer was placed between the prepared positive electrode and the prepared negative electrode 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 accommodated 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.

[0056] (Comparative Example 1) In Comparative Example 1, a laminate cell was fabricated in the same manner as in Example 1, except that a metal Mg layer / Ni foil was used as the negative electrode.

[0057] (Comparative Example 2) In Comparative Example 2, a laminate cell was fabricated in the same manner as in Example 1, except that a metal Mg layer was not used, a metal In layer with a thickness of 0.1 μm was formed by vapor deposition on the negative electrode current collector, and a metal In layer / Ni foil was used as the negative electrode.

[0058] (Comparative Example 3) In Comparative Example 3, a laminate cell was fabricated in the same manner as in Example 1, except that a metal Mg layer was not used, a metal Sn layer with a thickness of 0.1 μm was formed by vapor deposition on the negative electrode current collector, and a metal Sn layer / Ni foil was used as the negative electrode.

[0059] (Comparative Examples 4 to 6) In Comparative Examples 4 to 6, a laminate cell was fabricated in the same manner as in Example 1, except that a metal Mg layer was not used, an In-Sn alloy layer with a thickness of 0.1 μm was formed by binary vapor deposition using an ion plating method on the fabricated solid electrolyte layer, and an In-Sn alloy layer / Ni foil was used as the negative electrode. The actual composition (mol%) of the In-Sn alloy was 62:38 for Comparative Example 4, 49:51 for Comparative Example 5, and 38:62 for Comparative Example 6.

[0060] (Comparative Example 7) In Comparative Example 7, a laminate cell was fabricated in the same manner as in Example 1, except that an In layer with a thickness of 0.1 μm was formed by sputtering on the fabricated solid electrolyte layer, and a metal In layer / metal Mg layer / Ni foil was used as the negative electrode.

[0061] (Comparative Example 8) In Comparative Example 8, a laminate cell was fabricated in the same manner as in Example 1, except that a Sn layer with a thickness of 0.1 μm was formed by sputtering on the fabricated solid electrolyte layer, and a metal Sn layer / metal Mg layer / Ni foil was used as the negative electrode.

[0062] [In-Sn / Mg molar ratio] Regarding the negative electrode layers of Examples 1 to 3 and Comparative Examples 7 to 8, the total molar ratio of In element and Sn element to Mg element contained in the negative electrode layer {(In + Sn) / Mg} was calculated. The results are shown in Table 1.

[0063] [Initial charge-discharge] Under the following conditions, the initial charge-discharge of each laminate cell prepared in Examples 1 to 3 and Comparative Examples 1 to 8 was carried out at 60°C. Constant current charging was carried out at a current density of 0.15 mA / cm until the voltage reached 4.2 V 2 , under the condition of 0.05 C rate, and then constant voltage charging was carried out until the current density reached 0.03 mA / cm 2 and 0.01 C rate. Constant current discharging was carried out at a current density of 0.15 mA / cm until the voltage reached 3.0 V 2 , under the condition of 0.05 C rate, and then constant voltage discharging was carried out until the current density reached 0.03 mA / cm 2 and 0.01 C rate.

[0064] [Resistance after initial charge-discharge] The resistance (Ω·cm 2 ) when a predetermined current was passed through each laminate cell after the initial charge-discharge of Examples 1 to 3 and Comparative Examples 1, 7 to 8 at a predetermined voltage for 1 second was measured by the AC impedance method. The results are shown in Table 1. Figure 3 is a graph showing the relationship between the Sn composition in the In-Sn alloy and the resistance of the laminate cell after the initial charge-discharge. The resistance of each laminate cell after the initial charge-discharge of Examples 1 to 3 decreased in the order of Example 3, Example 2, and Example 1. Compared with Comparative Example 7 using In alone in the second layer of the negative electrode layer and Comparative Example 8 using Sn alone in the second layer of the negative electrode layer, the resistance of the cell at the end of discharge was suppressed. The negative electrodes during the production of Examples 1 to 3 are presumed to be a mixture of multiple types of In-Sn alloys such as InSn 4 , InSn, In 3 Sn. Therefore, when Li is inserted into the negative electrode layer by charging the cell, the increase in the melting point of the In-Sn alloy derived from Sn is reduced by the alloying of Li and the In-Sn alloy, and it is presumed that the resistance of the cell is reduced.

[0065] [Charge capacity at 25°C] Under the following conditions, the charge capacities (mAh / cm 2 ) of each laminate cell after the first charge and discharge of Examples 1 to 3 and Comparative Examples 1, 7, and 8 were measured at 25°C while only changing the current density during charging for each cycle with the current density during discharging being 0.15 mA / cm 2 and at a 0.05 C rate. Constant current charging was performed at a current density of 6.0 mA / cm 2 until the voltage reached 4.2 V under the condition of a 2 C rate, and the charge capacity was measured. The results are shown in Table 1. Figure 4 is a graph showing the relationship between the current density and the charge capacity at 25°C of each laminate cell after the first charge and discharge of Examples 1 to 3 and Comparative Examples 1, 7, and 8. The charge capacity at 25°C shown in Figure 4 shows an increase in the charge capacity at each C rate in Examples 1 to 3 compared to Comparative Examples 1, 7, and 8. As shown in Table 1, each laminate cell after the first charge and discharge of Examples 1 to 3 prevents short circuits during charging at a 2 C rate because the adhesion between the electrolyte layer and the negative electrode layer is improved. Figure 5 is a graph showing the relationship between the Sn composition in the In-Sn alloy and the charge capacity at 25°C at a 2 C rate of the laminate cell after the first charge and discharge. The charge capacities of each laminate cell of Examples 1 to 3 at a 2 C rate shown in Figure 5 and Table 1 increase in the order of Example 3, Example 1, and Example 2. The reason why the In-rich composition showed a higher charge capacity than the Sn-rich composition in the Li-In-Sn alloy is considered that generally the activation energy of the lithium ion conduction path is lower in the Li-In alloy than in the Li-Sn alloy.

[0066]

Table 1

[0067] [Reversible capacity after 20 cycles] Under the following conditions, the reversible capacities (mAh / cm2 ) was measured at 25°C. Constant current charging was performed under the condition of a current density of 0.60 mA / cm until the voltage reached 4.2 V. 2 , and then constant voltage charging was performed until the current density reached 0.03 mA / cm at a 0.2C rate. 2 , and constant voltage charging was performed until the rate reached 0.01C. Constant current discharging was performed under the condition of a current density of 0.60 mA / cm until the voltage reached 3.0 V. 2 , and constant current discharging was performed at a 0.2C rate. The above charge and discharge were performed 20 cycles, and the reversible capacity (discharge capacity) after 20 cycles was measured. The results are shown in Table 2. As shown in Table 2, the difference in the reversible capacity after 20 cycles is small among Examples 1 to 3, and Example 2 has the largest value. Examples 1 to 3 show an improvement in the reversible capacity compared to Comparative Example 7 using In alone in the second layer of the negative electrode layer and Comparative Example 8 using Sn alone in the second layer of the negative electrode layer.

[0068]

Table 2

[0069] [SEM-EDX Measurement] For the cross-section of the solid electrolyte layer - negative electrode (Li-In-Sn alloy layer / Li-Mg alloy layer / Ni foil) after the first charge of the laminate cell of Example 2, SEM observation in secondary electron image and EDX mapping were performed at an applied voltage of 5 kV. Figure 6 shows (1) secondary electron image, (2) S element mapping, (3) In element mapping, (4) Sn element mapping, (5) O element mapping, (6) Mg element mapping, and (7) Ni 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. For the cross-section of the solid electrolyte layer - negative electrode (Li-In-Sn alloy layer / Li-Mg alloy layer / Ni foil) after the first discharge of the laminate cell of Example 2, SEM observation in secondary electron image and EDX mapping were performed at an applied voltage of 5 kV. FIG. 7 shows (1) a secondary electron image, (2) an S element mapping, (3) an In element mapping, (4) an Sn element mapping, (5) an O element mapping, (6) an Mg element mapping, and (7) an Ni element mapping in the SEM-EDX mapping of the solid electrolyte layer - negative electrode cross-section after the first discharge in Example 2. As shown in FIG. 6, 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 contents of In element and Sn element in the region on the electrolyte layer side are higher than those in the region on the negative electrode current collector side. As shown in FIG. 6, it can be seen that a Li-In-Sn alloy layer is formed between the solid electrolyte layer and the Li-Mg alloy layer. The Li-Mg alloy layer is widely dispersed, and it is confirmed that a large amount of Li reacts with Mg and the Li-Mg alloy layer functions as a Li reaction layer. As shown in FIG. 7, it can be seen that a Li-In-Sn alloy layer is maintained between the solid electrolyte layer and the Li-Mg alloy layer even at the end stage of discharge of the lithium-ion secondary battery. Since the shrinkage and interfacial peeling of the Li-In-Sn alloy layer occur less likely in terms of potential compared with the Li-Mg alloy layer, the Li-In-Sn alloy layer functions as an interfacial layer to maintain the solid-solid interface. Compared with the case where the Li-In-Sn layer does not exist, it can prevent the reduction decomposition reaction of the solid electrolyte layer by Li and suppress the increase of the interfacial resistance and the cell resistance. Also, as reasons why the Li-In-Sn alloy layer is more suitable as a protective layer than the Li-In alloy layer and the Li-Sn alloy layer, the following two points can be considered. The first point is that the melting point of the Li-In-Sn alloy is lower than that of the Li-In alloy and the Li-Sn alloy. At the same operating temperature, the Li-In-Sn alloy is softer than the Li-In alloy and the Li-Sn alloy. Therefore, during discharge when Li desorbs from the negative electrode layer, the adhesion between the solid electrolyte layer and the negative electrode layer, and the adhesion between the Li-In-Sn alloy layer and the Li-Mg alloy layer are improved, the resistance at the interface between the solid electrolyte layer and the negative electrode layer is reduced, and the charge rate characteristics are improved. The second point is that it is considered that the lithium ion conductivity of the Li-In-Sn alloy is higher than that of the Li-In alloy and the Li-Sn alloy.

Explanation of Symbols

[0070] 10 Positive current collector 20 Positive electrode layer 30 Electrolyte layer 40 In-Sn alloy layer 41 Li-In-Sn alloy layer 50 Metal Mg layer 51 Li-Mg alloy layer 60 Negative current collector 100 Lithium-ion secondary battery 200 Lithium-ion secondary battery

Claims

1. A lithium-ion secondary battery using a 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, and the negative electrode layer contains Mg element, In element, Sn element, and Li element.

2. The lithium-ion secondary battery according to Claim 1, wherein a molar ratio of Sn element to In element (Sn / In) contained in the negative electrode layer is 0.5 or more and 3.8 or less.

3. The lithium-ion secondary battery according to Claim 1, wherein the negative electrode layer includes, in order from the electrolyte layer side, a Li-In-Sn alloy layer containing a Li-In-Sn alloy and a Li-Mg alloy layer containing a Li-Mg alloy.

4. The lithium-ion secondary battery has a negative electrode current collector on a side opposite to the electrolyte layer of the negative electrode layer, and 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 In element and Sn element in the second region of the negative electrode layer is greater than the content of In element and Sn element in the first region of the negative electrode layer.

5. The lithium-ion secondary battery according to Claim 1, wherein the electrolyte layer is a solid electrolyte layer containing a sulfide-based solid electrolyte.

6. A lithium-ion secondary battery using a 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, and the negative electrode layer includes, in order from the electrolyte layer side, an In-Sn alloy layer containing an In-Sn alloy and a metallic Mg layer containing Mg simple substance.

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