Lithium secondary battery

Abstract

To provide a lithium secondary battery in which a lithium alloy is used as a negative electrode active material and which is enhanced in a capacity retention rate and reduced in a resistance value.SOLUTION: A lithium secondary battery includes a negative electrode current collector layer 111, a negative electrode active material layer 112, an electrolyte layer 120, a positive electrode active material layer 131, and a positive electrode current collector layer 132, in this order. The negative electrode active material layer 112 contains lithium, a first metal element that forms, with lithium, an alloy and a second metal element that forms, with lithium, an alloy. The concentration (atomic %) of the first metal element in a surface 112a facing the electrolyte layer is higher than the concentration (atomic %) of the first metal element in a surface 112b facing the negative electrode current collector layer, and the concentration (atomic %) of the second metal element in the surface 112a facing the electrolyte layer is lower than the concentration (atomic %) of the second metal element in the surface 112b facing the negative electrode current collector layer. The first metal element is selected from tin, germanium, antimony, and bismuth.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] Lithium secondary batteries using lithium metal and / or lithium alloy as the negative electrode active material are expected to have high output voltages because of the large potential difference between the negative electrode and the positive electrode, and to have a high theoretical capacity density. Therefore, their practical application is expected, and the following lithium secondary batteries are disclosed.

[0003] For example, Patent Document 1 discloses a lithium secondary battery that utilizes the deposition-dissolution reaction of lithium metal as the reaction of the negative electrode. The negative electrode includes a negative electrode layer, and the negative electrode layer includes an alloy of the lithium metal and a different metal as the negative electrode active material. When the lithium secondary battery is fully charged, the elemental ratio of the lithium element in the alloy is 40.00 atomic% or more and 99.97 atomic% or less. According to Patent Document 1, it is said that a lithium secondary battery capable of improving the capacity retention rate can be provided.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] While lithium secondary batteries using lithium metal and / or lithium alloy as the negative electrode active material are expected to have excellent battery characteristics, in reality, the capacity retention rate is small and the resistance value is high. Therefore, there is room for improvement in lithium secondary batteries from the viewpoints of the capacity retention rate and the resistance value.

[0006] Therefore, an object of the present disclosure is to provide a lithium secondary battery that uses a lithium alloy as a negative electrode active material, has an improved capacity retention rate, and a reduced resistance value. **Means for Solving the Problems**

[0007] The present disclosure achieves the above object by the following means.

[0008] <Aspect 1> A lithium secondary battery having a negative electrode current collector layer, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer in this order, wherein the negative electrode active material layer contains lithium, a first metal element that forms an alloy with lithium, and a second metal element that forms an alloy with lithium, the concentration (atomic %) of the first metal element on the surface side of the electrolyte layer is higher than the concentration (atomic %) of the first metal element on the surface side of the negative electrode current collector layer, the concentration (atomic %) of the second metal element on the surface side of the electrolyte layer is lower than the concentration (atomic %) of the second metal element on the surface side of the negative electrode current collector layer, the first metal element is selected from tin, germanium, antimony, and bismuth, A lithium secondary battery. <Aspect 2> The lithium secondary battery according to Aspect 1, wherein the first metal element is selected from tin, germanium, and antimony. <Aspect 3> The lithium secondary battery according to Aspect 1 or 2, wherein the second metal element is at least one selected from sodium, magnesium, aluminum, silicon, calcium, zinc, gallium, germanium, strontium, rhodium, palladium, silver, barium, lead, iridium, gold, platinum, and bismuth. <Aspect 4> The lithium secondary battery according to any one of Aspects 1 to 3, including an alloy containing the lithium, the first metal element, and the second metal element. **Advantages of the Invention**

[0009] According to the present disclosure, the capacity retention rate of a lithium secondary battery using a lithium alloy as a negative electrode active material is improved, and the resistance value is reduced.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present disclosure. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] Regarding the present disclosure, "composite material" means a composition that can form a positive electrode active material layer or the like as it is or by further containing other components. Also, regarding the present disclosure, "composite material slurry" means a slurry that contains a dispersion medium in addition to the "composite material" and can form a positive electrode active material layer or the like by coating and drying.

[0013] The lithium secondary battery of the present disclosure may be a liquid battery containing an electrolytic solution as an electrolyte layer, or may be a solid battery having a solid electrolyte layer as an electrolyte layer. Note that, regarding the present disclosure, "solid battery" means a battery using at least a solid electrolyte as an electrolyte. Therefore, a solid battery may use a combination of a solid electrolyte and a liquid electrolyte as an electrolyte. Also, the lithium secondary battery of the present disclosure may be an all-solid battery, that is, a battery using only a solid electrolyte as an electrolyte.

[0014] 《Lithium Secondary Battery》 The lithium secondary battery of the present disclosure It has a negative electrode current collector layer, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer in this order. The negative electrode active material layer contains lithium, a first metal element that forms an alloy with lithium, and a second metal element that forms an alloy with lithium. The concentration (atomic %) of the first metal element on the surface side of the electrolyte layer is higher than the concentration (atomic %) of the first metal element on the surface side of the negative electrode current collector layer. The concentration (atomic %) of the second metal element on the surface side of the electrolyte layer is lower than the concentration (atomic %) of the second metal element on the surface side of the negative electrode current collector layer. The first metal element is selected from tin, germanium, antimony, and bismuth.

[0015] According to the present disclosure, the capacity retention rate of a lithium secondary battery using a lithium alloy as a negative electrode active material is improved, and the resistance value is reduced.

[0016] Although the details are not clear, it is considered that the first metal element is easily alloyed with lithium and has a high affinity with the electrolyte solution. Also, it is considered that the second metal element becomes less likely to expand even upon charging when combined with the first metal element. Without being limited to theory, it is presumed that the negative electrode active material layer 112 containing appropriately arranged first and second metal elements specifically suppresses the increase in specific surface area accompanying charge and discharge, thereby increasing the capacity retention rate. Also, by arranging a large amount of the first metal element, which is easily alloyed with lithium, on the surface 112a of the electrolyte layer side, it becomes easier for lithium to enter the negative electrode active material layer 112, thereby reducing the resistance value. Particularly in a liquid-based battery containing an electrolyte solution, the affinity between the negative electrode active material layer and the electrolyte solution becomes high, and lithium becomes easily dissolved and deposited, thereby reducing the resistance value.

[0017] FIG. 2 is a schematic diagram showing one aspect of the lithium secondary battery of the present disclosure, but is not limited to this case.

[0018] The lithium secondary battery 100 has a negative electrode current collector layer 111, a negative electrode active material layer 112, an electrolyte layer 120, a positive electrode active material layer 131, and a positive electrode current collector layer 132 in this order, and the negative electrode active material layer has an electrolyte layer side surface 112a and a negative electrode current collector layer side surface 112b. The negative electrode active material layer 112 contains lithium, a first metal element, and a second metal element. The concentration of the first metal element on the electrolyte layer side surface 112a is higher than the concentration of the first metal element on the negative electrode current collector layer side surface 112b. Also, the concentration of the second metal element on the electrolyte layer side surface 112a is lower than the concentration of the second metal element on the negative electrode current collector layer side surface 112b. The first metal element is considered to be easily alloyed with lithium and have a high affinity with the electrolyte solution. Also, it is considered that the second metal element, when combined with the first metal element, is less likely to expand even upon charging. The negative electrode active material layer 112 containing the first metal element and the second metal element appropriately arranged is presumed to specifically suppress the increase in specific surface area accompanying charge and discharge, thereby increasing the capacity retention rate. Also, by arranging a large amount of the first metal element, which is easily alloyed with lithium, on the electrolyte layer side surface, lithium easily enters the negative electrode active material layer, thereby reducing the resistance value.

[0019] <Configuration of Lithium Secondary Battery> The lithium secondary battery of the present disclosure has a negative electrode current collector layer, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer in this order.

[0020] <Negative Electrode Current Collector Layer> The material used for the negative electrode current collector layer is not particularly limited, but those commonly used as the negative electrode current collector of a lithium secondary battery can be appropriately employed. Examples of the material used for the negative electrode current collector layer include, but are not limited to, Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, or a carbon sheet. In particular, from the viewpoints of ensuring reduction resistance and being difficult to alloy with lithium, the material used for the negative electrode current collector layer may contain at least one metal selected from Cu, Ni, and stainless steel, or may be made of a carbon sheet. The negative electrode current collector layer may have some coating layer on its surface for the purpose of adjusting resistance or the like.

[0021] The shape of the negative electrode current collector layer is not particularly limited, and examples thereof include a foil shape, a plate shape, or a mesh shape. Among these, a foil shape is preferable.

[0022] The thickness of the negative electrode current collector layer is not particularly limited, and it may be 0.1 μm or more, or 1 μm or more, and may also be 1 mm or less, or 100 μm or less.

[0023] 〈Negative electrode active material layer〉 In the lithium secondary battery of the present disclosure, the negative electrode active material layer contains lithium, a first metal element that forms an alloy with lithium, and a second metal element that forms an alloy with lithium.

[0024] In the charged state, the "negative electrode active material layer" has layers of a lithium alloy of lithium and the first metal element and a lithium alloy of lithium and the second metal element as the "negative electrode active material layer". However, in the discharged state, the lithium of the lithium alloy of lithium and the first metal element and the lithium alloy of lithium and the second metal element moves to the positive electrode active material layer as lithium ions, and the "negative electrode active material layer" may not have layers of the lithium alloy of lithium and the first metal element and the lithium alloy of lithium and the second metal element, but may have layers of the first metal element and the second metal element.

[0025] The negative electrode active material layer contains at least a first metal element that alloyizes with lithium as the negative electrode active material and a second metal element that alloyizes with lithium, and may further optionally contain a conductive assistant, a binder, a solid electrolyte, etc. The negative electrode active material layer may also contain various other additives. The content of each of the negative electrode active material, conductive assistant, binder, solid electrolyte, etc. in the negative electrode active material layer may be appropriately determined according to the intended battery performance. For example, assuming the total of the negative electrode active material layer (total solid content) is 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, and may also be 100% by mass or less, or 90% by mass or less.

[0026] (Negative electrode active material) As the negative electrode active material, a first metal element that alloyizes with lithium and a second metal element that alloyizes with lithium are used. The first metal element and the second metal element contained in the negative electrode active material layer are different from each other. Here, the first metal element and the second metal element may or may not form a combination that forms an alloy.

[0027] In the present disclosure, when two metal elements are "different", it means that the types of the metal elements are different from each other, and it does not mean that the types of the metal elements are the same and only the content ratio is different when, for example, a lithium alloy is formed. Also, in the present disclosure, when two metal elements are "the same", it means that the types of the metal elements are the same as each other, and thus, it also includes the case where the types of the metal elements are the same and only the content ratio is different when, for example, a lithium alloy is formed.

[0028] (First metal element) The first metal element is selected from tin, germanium, antimony, and bismuth. The first metal element is not particularly limited, but from the viewpoints of capacity retention rate and resistance value, it is preferably selected from tin, germanium, and antimony.

[0029] In the lithium secondary battery of the present disclosure, the concentration (atomic %) of the first metal element on the surface side of the electrolyte layer is higher than the concentration (atomic %) of the first metal element on the surface side of the negative electrode current collector layer.

[0030] The concentration (atomic %) of the first metal element on the surface side of the electrolyte layer is the amount (number of atoms) of the first metal element with respect to the total amount (number of atoms) of the first metal element and the second metal element on the surface side of the electrolyte layer. Similarly, the concentration (atomic %) of the first metal element on the surface side of the negative electrode current collector layer is the amount (number of atoms) of the first metal element with respect to the total amount (number of atoms) of the first metal element and the second metal element on the surface side of the negative electrode current collector layer.

[0031] The concentration of the first metal element can be determined by scanning electron microscope (SEM) observation and elemental mapping by energy dispersive X-ray analysis (EDX) for a cross-section of the negative electrode active material layer in the discharged state or a preliminary negative electrode active material layer formed by depositing the first metal element and the second metal element on the surface of the negative electrode current collector layer described later. In the present disclosure, the "concentration of the first metal element on the surface side of the electrolyte layer" can be determined as the concentration of the first metal element in the layer closest to the electrolyte layer when the cross-section of the negative electrode active material layer or the preliminary negative electrode active material layer is divided into 30 equal parts from the surface side of the electrolyte layer to the surface side of the negative electrode current collector layer. Similarly, the "concentration of the first metal element on the surface side of the negative electrode current collector layer" can be determined as the concentration of the first metal element in the layer closest to the negative electrode current collector layer when the cross-section of the negative electrode active material layer or the preliminary negative electrode active material layer is divided into 30 equal parts from the surface side of the electrolyte layer to the surface side of the negative electrode current collector layer.

[0032] The concentration (atomic %) of the first metal element on the surface side of the electrolyte layer is not particularly limited, but may be 2.0 times or more, 2.5 times or more, 3.0 times or more, or 5.0 times or more the concentration (atomic %) of the first metal element on the surface side of the negative electrode current collector layer, and may be 1000 times or less, 100 times or less, or 10 times or less.

[0033] FIG. 1 is a schematic cross-sectional view showing one embodiment of the lithium secondary battery of the present disclosure, and is a schematic view of the negative electrode active material layer, but is not limited to this case.

[0034] The dotted line divides the cross-section of the negative electrode active material layer 112 into 30 equal parts from the electrolyte layer side surface 112a to the negative electrode current collector layer side surface 112b. In the present disclosure, the concentration of the first metal element on the electrolyte layer side surface 112a can be determined as the concentration of the first metal element in the layer 112c closest to the electrolyte layer 120. Similarly, the concentration of the first metal element on the negative electrode current collector layer side surface 112b can be determined as the concentration of the first metal element in the layer 112d closest to the negative electrode current collector layer 111. The concentration of the first metal element in the layer 112c closest to the electrolyte layer 120 is higher than the concentration of the first metal element in the layer 112d closest to the negative electrode current collector layer 111, that is, the concentration (atomic %) of the first metal element on the electrolyte layer side surface 112a is higher than the concentration (atomic %) of the first metal element on the negative electrode current collector layer side surface 112b.

[0035] (Second metal element) The second metal element is not particularly limited as long as it is a metal element that alloyizes with lithium.

[0036] The second metal element is not particularly limited, but may be at least one selected from sodium, magnesium, aluminum, silicon, calcium, zinc, gallium, germanium, calcium, strontium, rhodium, palladium, silver, barium, lead, iridium, gold, platinum, and bismuth.

[0037] In the lithium secondary battery of the present disclosure, the concentration (atomic %) of the second metal element on the electrolyte layer side surface is lower than the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface.

[0038] The concentration (atomic %) of the second metal element on the electrolyte layer side surface is the amount (number of atoms) of the second metal element with respect to the total amount (number of atoms) of the first and second metal elements on the electrolyte layer side surface. Similarly, the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface is the amount (number of atoms) of the second metal element with respect to the total amount (number of atoms) of the first and second metal elements on the negative electrode current collector layer side surface.

[0039] The concentration of the second metal element can be determined by scanning electron microscope (SEM) observation and elemental mapping by energy dispersive X-ray analysis (EDX) for a cross-section of the negative electrode active material layer in the discharged state or a preliminary negative electrode active material layer formed with the first and second metal elements on the negative electrode current collector layer described later. In the present disclosure, the "concentration of the second metal element on the electrolyte layer side surface" can be determined as the concentration of the second metal element in the layer closest to the electrolyte layer when the cross-section of the negative electrode active material layer or the preliminary negative electrode active material layer is equally divided into 30 parts from the electrolyte layer side surface to the negative electrode current collector layer side surface. Similarly, the "second metal element on the negative electrode current collector layer side surface" can be determined as the concentration of the second metal element in the layer closest to the negative electrode current collector layer when the cross-section of the negative electrode active material layer or the preliminary negative electrode active material layer is equally divided into 30 parts from the electrolyte layer side surface to the negative electrode current collector layer side surface.

[0040] The concentration (atomic %) of the second metal element on the electrolyte layer side surface is not particularly limited, but may be 0.001 times or more, 0.01 times or more, or 0.1 times or more of the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface, and may be 0.2 times or less, 0.3 times or less, 0.4 times or less, or 0.5 times or less.

[0041] In the above-described FIG. 1, in the present disclosure, the concentration of the second metal element on the electrolyte layer side surface 112a can be determined as the concentration of the second metal element in the layer 112c closest to the electrolyte layer 120. Similarly, the concentration of the second metal element on the negative electrode current collector layer side surface 112b can be determined as the concentration of the second metal element in the layer 112d closest to the negative electrode current collector layer 111. The concentration of the second metal element in the layer 112c closest to the electrolyte layer 120 is lower than the concentration of the second metal element in the layer 112d closest to the negative electrode current collector layer 111, that is, the concentration (atomic %) of the second metal element on the electrolyte layer side surface 112a is lower than the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface 112b.

[0042] The negative electrode active material layer is not particularly limited, and may include an alloy containing lithium, a first metal element, and a second metal element.

[0043] In addition, the negative electrode active material layer may contain a negative electrode active material other than lithium, a first metal element, a second metal element, and an alloy of lithium, a first metal element, and a second metal element. The negative electrode active material other than lithium, a first metal element, a second metal element, and an alloy of lithium, a first metal element, and a second metal element is not particularly limited, and examples thereof include carbon materials. Examples of the carbon material include, but are not limited to, hard carbon, soft carbon, graphite, etc.

[0044] The ratio of lithium, the first metal element, the second metal element, and the alloy of lithium, the first metal element, and the second metal element contained in the negative electrode active material layer is not particularly limited, but may be 50% by mass to 100% by mass, 60% by mass to 100% by mass, 70% by mass to 100% by mass, 80% by mass to 100% by mass, or 90% by mass to 100% by mass with respect to the negative electrode active material layer.

[0045] (Binder) The binder is not particularly limited. The binder may be, for example, a material such as polyvinylidene fluoride (PVdF), butadiene rubber (BR), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), etc., but is not limited thereto. The binder is not particularly limited, but only one kind may be used alone, or two or more kinds may be used in combination.

[0046] (Conductive aid) The conductive aid is not particularly limited. The conductive aid may be, for example, vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), carbon nanofiber (CNF), etc., but is not limited thereto. The conductive aid may be, for example, particulate or fibrous, and its size is not particularly limited. The conductive aid is not particularly limited, but only one kind may be used alone, or two or more kinds may be used in combination.

[0047] (Solid electrolyte) The material of the solid electrolyte is not particularly limited and may be, for example, a sulfide solid electrolyte, an oxide solid electrolyte, or a polymer electrolyte, etc.

[0048] Examples of sulfide solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, or argyrodite-type solid electrolytes, etc. Specific examples of sulfide solid electrolytes include Li2S-P2S5 systems (Li7P3S 11 , Li3PS4, Li8P2S9, etc.), Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-P2S5-GeS2 (Li 13 GeP3S 16 , Li 10 GeP2S 12 , etc.), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x PS 6-x Cl x , etc.; or combinations thereof can be cited, but are not limited thereto.

[0049] Examples of oxide solid electrolytes include Li7La3Zr2O 12 、Li 7-x La3Zr 1-x Nb x O 12 、Li 7-3x La3Zr2Al x O 12 、Li 3x La 2 / 3-x TiO3、Li 1+x Al x Ti 2-x (PO4)3、Li 1+x Al x Ge 2-x (PO4)3、Li3PO4, or Li 3+x PO 4-x N x (LiPON), etc., but are not limited thereto.

[0050] Sulfide solid electrolytes and oxide solid electrolytes may be glass or glass-ceramics (crystallized glass).

[0051] Examples of polymer electrolytes include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof, etc., but are not limited thereto.

[0052] The shape of the negative electrode active material is not particularly limited, but may be a common shape as the negative electrode active material of a lithium secondary battery. The negative electrode active material may be, for example, layered or sheet-like. The negative electrode active material may be one that involves lithium precipitation during charging or one that involves lithium dissolution during discharging. In this case, the negative electrode active material layer may be a layer made of a lithium alloy.

[0053] The shape of the negative electrode active material layer is not particularly limited, but may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 200 μm or less, 1150 μm or less, or 100 μm or less.

[0054] The negative electrode active material layer can be formed with reference to <Method for manufacturing a lithium secondary battery> described later.

[0055] <Electrolyte layer> <Electrolyte layer - Solid electrolyte layer> The lithium secondary battery of the present disclosure can be a solid battery, that is, can have a solid electrolyte layer as the electrolyte layer.

[0056] In addition to the solid electrolyte, the solid electrolyte layer may contain a binder or the like as necessary.

[0057] Regarding the solid electrolyte and the binder, reference can be made to the description of "<Negative electrode active material layer>" above.

[0058] The thickness of the solid electrolyte layer is not particularly limited, but for example, it may be 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0059] The solid electrolyte layer can be easily formed, for example, by dry or wet molding of an electrolyte composite material containing the above-mentioned solid electrolyte and binder or the like.

[0060] <Electrolyte layer - Separator layer> The lithium secondary battery of the present disclosure can be a liquid-based battery, that is, can have an electrolytic solution, particularly an electrolytic solution held in a separator layer, as the electrolyte layer.

[0061] (Electrolytic solution) The electrolytic solution is not particularly limited, but preferably contains a supporting salt and a solvent.

[0062] The supporting salt (lithium salt) of the electrolytic solution having lithium ion conductivity is not particularly limited, and examples thereof include inorganic lithium salts and organic lithium salts. Examples of the inorganic lithium salts include, but are not limited to, LiPF6, LiBF4, LiClO4, LiAsF6, etc. Examples of the organic lithium salts include, but are not limited to, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, LiC(CF3SO2)3, etc.

[0063] The solvent used in the electrolytic solution is not particularly limited, and examples thereof include cyclic carbonates and chain carbonates. Examples of the cyclic carbonates include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Examples of the chain carbonates include, but are not limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. The electrolytic solution is not particularly limited, and only one type may be used alone, or two or more types may be used in combination.

[0064] (Separator) The separator is not particularly limited, and a general separator for a lithium secondary battery can be appropriately adopted. As the separator, for example, non-woven fabrics such as polyolefin-based, polyamide-based, and polyimide-based can be used.

[0065] 〈Positive electrode active material layer〉 The positive electrode active material layer contains at least a positive electrode active material, and may further optionally contain a conductive assistant, a solid electrolyte, a binder, etc. The positive electrode active material layer may also contain various additives. The content of each of the positive electrode active material, conductive assistant, binder, etc. in the positive electrode active material layer may be appropriately determined according to the intended battery performance. For example, taking the whole of the positive electrode active material layer (the whole 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, 60% by mass or more, and may also be 100% by mass or less, or 90% by mass or less.

[0066] (Positive electrode active material) The material of the positive electrode active material is not particularly limited as long as it can occlude and release lithium ions. Examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMn2O4), lithium nickel cobalt manganate (NCM), LiCO 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, lithium nickel cobalt aluminate (NCA; LiNi x Co y Al z O2), Li 1+x Mn 2-x-y M y O4 (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn), such as hetero-element substituted Li-Mn spinel having a composition represented by this, but is not limited thereto.

[0067] The positive electrode active material is not particularly limited, but may have a coating layer. The coating layer is a layer containing a material having lithium ion conduction performance, low reactivity with the positive electrode active material and the solid electrolyte, and capable of maintaining the form of the coating layer without flowing even when in contact with the active material and the solid electrolyte. Specific examples of the material constituting the coating layer include, in addition to LiNbO3, Li4Ti5O 12 , Li3PO4, etc., but are not limited thereto.

[0068] The shape of the positive electrode active material is not particularly limited as long as it is a common shape for the positive electrode active material of a lithium secondary battery. The positive electrode active material may be, for example, particulate. The positive electrode active material may be primary particles or secondary particles formed by aggregation of a plurality of primary particles. The average particle diameter D of the positive electrode active material 50 may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may also be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Note that the average particle diameter D 50 is the particle diameter (median diameter) at the integrated value of 50% in the volume-based particle size distribution determined by the laser diffraction / scattering method.

[0069] Regarding the solid electrolyte, binder, and conductive assistant, reference can be made to the description of "<Negative electrode active material layer>" above.

[0070] The shape of the positive electrode active material layer is not particularly limited, but may be, for example, a sheet-shaped positive electrode active material layer having a substantially flat surface. The thickness of the positive electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0071] 〈Positive electrode current collector layer〉 The material used for the positive electrode current collector layer is not particularly limited, but a common material for the positive electrode current collector of a lithium secondary battery can be appropriately adopted. Examples of the material used for the positive electrode current collector layer include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, etc., but are not limited to this case. Further, the positive electrode current collector layer may have some coating layer on its surface for the purpose of adjusting resistance or the like. Also, the positive electrode current collector layer may be a metal foil or a substrate with the above metal plated or vapor-deposited thereon.

[0072] The shape of the positive electrode current collector layer is not particularly limited, and examples include foil shape, plate shape, or mesh shape, etc. Among these, the foil shape is preferred.

[0073] The thickness of the positive electrode current collector layer is not particularly limited, but it may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less.

[0074] The positive electrode active material layer can be manufactured by applying known methods. For example, the positive electrode active material layer can be easily formed by molding a positive electrode composite material containing the above various components in a dry or wet manner. The positive electrode active material layer may be formed together with the positive electrode current collector layer, or may be formed separately from the positive electrode current collector layer.

[0075] <Shape, etc. of lithium secondary battery> Examples of the shape of the lithium secondary battery include, but are not limited to, coin type, laminate type, cylindrical type, and square type.

[0076] <Manufacturing method of lithium secondary battery> The lithium secondary battery of the present disclosure can be manufactured by a manufacturing method including the following steps: Forming a preliminary negative electrode active material layer on the surface of the negative electrode current collector layer by vapor-depositing a first metal element and a second metal element to obtain a preliminary negative electrode laminate; Stacking the above preliminary negative electrode laminate, electrolyte layer, positive electrode active material layer holding lithium, and positive electrode current collector layer in this order to obtain a preliminary lithium secondary battery; By performing a charging operation on the above preliminary lithium secondary battery, moving lithium from the positive electrode active material layer to the preliminary negative electrode active material layer to form a negative electrode active material layer, thereby obtaining a lithium secondary battery.

[0077] (Preliminary negative electrode active material layer) The preliminary negative electrode active material layer is not particularly limited, but can be formed by vapor-depositing a first metal element and a second metal element on the surface of the negative electrode current collector layer. Although the conditions of the vapor deposition method (co-evaporation) of the first metal element and the second metal element can be adjusted to prepare the concentration of the first metal element and the concentration of the second metal element on the surface of the electrolyte layer side of the preliminary negative electrode active material layer, this is not limited to this case. Similarly, although the conditions of the vapor deposition method (co-evaporation) of the first metal element and the second metal element can be adjusted to prepare the concentration of the first metal element and the concentration of the second metal element on the surface of the negative electrode current collector layer side of the preliminary negative electrode active material layer, this is not limited to this case.

[0078] (Preliminary negative electrode laminate) The preliminary negative electrode laminate is not particularly limited, but is a laminate in which a negative electrode current collector and a preliminary negative electrode active material layer are laminated in this order.

[0079] 〈Preliminary lithium secondary battery〉 The preliminary lithium secondary battery is not particularly limited, but is a laminate in which a preliminary negative electrode laminate, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer are laminated in this order, that is, a laminate in which a negative electrode current collector, a preliminary negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer are laminated in this order.

Examples

[0080] The present disclosure will be described in more detail with reference to the examples shown below, but the scope of the present disclosure is not limited to these examples.

[0081] 《Example 1》 〈Fabrication of preliminary negative electrode laminate〉 On one side of a copper (Cu) foil serving as a preliminary negative electrode current collector, tin (Sn) as a first metal element and silver (Ag) as a second metal element were deposited by a vapor deposition method (co - evaporation) to fabricate a preliminary negative electrode laminate composed of a negative electrode current collector layer and a preliminary negative electrode active material layer. Here, on the surface side of the electrolyte layer, the first metal element became 90.1 atomic%, and the second metal element became 9.9 atomic%. And on the surface side of the negative electrode current collector layer, the conditions of co - evaporation were adjusted and film - formed so that the first metal element became 9.9 atomic% and the second metal element became 90.1 atomic%. Note that the concentration of the metal element was determined by scanning electron microscope (SEM) observation and elemental mapping by energy - dispersive X - ray analysis (EDX) for the cross - section of the preliminary negative electrode active material layer. The concentration of the first metal element on the surface side of the electrolyte layer was determined as the concentration of the first metal element in the layer closest to the electrolyte layer side when the cross - section of the preliminary negative electrode laminate was equally divided into 30 parts from the surface side of the electrolyte layer to the surface side of the negative electrode current collector layer. Similarly, the concentration of the first metal element on the surface side of the negative electrode current collector layer was determined as the concentration of the first metal element in the layer closest to the negative electrode current collector layer side when the cross - section of the preliminary negative electrode laminate was equally divided into 30 parts from the surface side of the electrolyte layer to the surface side of the negative electrode current collector layer. The second metal element was determined in the same manner as the first metal element.

[0082] 〈Fabrication of the positive electrode laminate〉 LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (84 parts by mass), acetylene black (12 parts by mass) as a conductive assistant, PVdF (4 parts by mass) as a binder, and an appropriate amount of N - methyl - 2 - pyrrolidone (NMP) as a dispersion medium were mixed to prepare a positive electrode composite slurry. Next, the obtained positive electrode composite slurry was coated on an aluminum (Al) foil serving as a positive electrode current collector and dried to fabricate a positive electrode laminate in which a positive electrode active material layer was formed on the positive electrode current collector layer.

[0083] 〈Fabrication of the preliminary lithium secondary battery〉 A preliminary negative electrode laminate and a positive electrode laminate were laminated so as to face each other with a polyolefin film (film thickness: 20 μm) as a separator, and wound in a spiral shape. Terminals were connected to the wound preliminary negative electrode laminate and positive electrode laminate, respectively, and they were housed in a battery case. 1 M LiPF6 ethylene carbonate / dimethyl carbonate (1 / 1 (volume ratio)) as an electrolytic solution was injected, sealed, and a preliminary lithium secondary battery was fabricated.

[0084] 〈Formation of Lithium Secondary Battery and Evaluation of Capacity Retention Rate〉 The preliminary lithium secondary battery was charged and discharged 200 cycles at 25°C in a constant current (current rate: 1C) mode within a cut-off voltage range of 3.3 to 4.2 V. The capacities at the first cycle and the 200th cycle were measured, and the capacity retention rate (capacity retention rate = (capacity at the 200th cycle) / (capacity at the first cycle) × 100) was calculated. The results of the capacity retention rate are shown in Table 1. The capacity retention rate in Table 1 is a relative value when the capacity retention rate of the lithium secondary battery of Comparative Example 1 is set to 1.00. Here, by performing a charging operation on the preliminary lithium secondary battery, lithium moved from the positive electrode active material layer to the preliminary negative electrode active material layer, and a negative electrode active material layer containing lithium, a first metal element, and a second metal element was formed, thereby forming a lithium secondary battery.

[0085] 〈Evaluation of Resistance Value of Lithium Secondary Battery〉 The lithium secondary battery was adjusted so that the open circuit voltage became 3.70 V. Next, at -10°C, it was discharged at a current rate of 5C for 8 seconds, the voltage drop (ΔV) was measured, and the resistance value of the lithium secondary battery (resistance value = ΔV / current value of 5C) was calculated. The results of the resistance value are shown in Table 1. The resistance value in Table 1 is a relative value when the resistance value of the lithium secondary battery of Comparative Example 1 is set to 1.00.

[0086] 《Comparative Example 1》 〈Fabrication of Preliminary Negative Electrode Laminate〉 Sn as a first metal element was deposited on one side of a Cu foil as a preliminary negative electrode current collector by a vapor deposition method to fabricate a preliminary negative electrode laminate composed of a negative electrode current collector layer and a preliminary negative electrode active material layer.

[0087] <Production of Lithium Secondary Battery, Evaluation of Capacity Retention Rate, and Evaluation of Resistance Value> Using a preliminary negative electrode laminate composed only of the first metal element, a lithium secondary battery was produced in the same manner as in Example 1. The capacity retention rate and resistance value of the lithium secondary battery were evaluated in the same manner as in Example 1. In the examples and comparative examples of this specification, the capacity retention rate and resistance value of the lithium secondary battery of Comparative Example 1 are shown as relative values with 1.00 as the reference.

[0088] <<Comparative Example 2>> <Production of Preliminary Negative Electrode Laminate> On one side of a Cu foil as a preliminary negative electrode current collector, Ag as the second metal element was deposited by a vapor deposition method to produce a preliminary negative electrode laminate composed of a negative electrode current collector layer and a preliminary negative electrode active material layer.

[0089] <<Comparative Example 3>> <Production of Preliminary Negative Electrode Laminate> On one side of a Cu foil as a preliminary negative electrode current collector, Sn as the first metal element and Ag as the second metal element were deposited by a vapor deposition method (co - vapor deposition) so that an alloy was formed to produce a preliminary negative electrode laminate composed of a negative electrode current collector layer and a preliminary negative electrode active material layer.

[0090] <<Examples 2 - 4, Comparative Example 4>> <Production of Preliminary Negative Electrode Laminate> A preliminary negative electrode laminate was produced in the same manner as in Example 1, except that the metal described in Table 1 was used as the first metal element.

[0091] <<Example 5>> <Production of Preliminary Negative Electrode Laminate> On one side of a Cu foil serving as a preliminary negative electrode current collector, tin (Sn) as a first metal element and silver (Ag) as a second metal element were deposited by a vapor deposition method (co-evaporation). Next, tin (Sn) as a first metal element and silver (Ag) as a second metal element were deposited by a vapor deposition method (co-evaporation) so that an alloy was formed. Then, tin (Sn) as a first metal element and silver (Ag) as a second metal element were deposited by a vapor deposition method (co-evaporation), and a preliminary negative electrode laminate composed of a negative electrode current collector layer and a preliminary negative electrode active material layer was produced. Here, on the surface side of the electrolyte layer, the first metal element became 90.1 atomic% and the second metal element became 9.9 atomic%, and on the surface side of the negative electrode current collector layer, the first metal element became 9.9 atomic% and the second metal element became 90.1 atomic%. The co-evaporation conditions were adjusted and film formation was carried out accordingly.

[0092] <Manufacture of Lithium Secondary Battery, Evaluation of Capacity Retention Rate, Evaluation of Resistance Value> Using the preliminary negative electrode laminates produced in Examples 2 to 5 and Comparative Examples 2 to 4, lithium secondary batteries were produced in the same manner as in Example 1. The capacity retention rates and resistance values of the lithium secondary batteries in Examples 2 to 5 and Comparative Examples 2 to 4 were evaluated in the same manner as in Example 1. The respective results were as shown in Table 1.

[0093]

Table 1

[0094] In a lithium secondary battery (Examples 1 to 5) including a negative electrode active material layer having Sn, germanium (Ge), antimony (Sb), or bismuth (Bi) as a first metal element and having Ag as a second metal element, compared with a lithium secondary battery including a negative electrode active material layer having only Sn as the first metal element or a negative electrode active material layer having only Ag as the second metal element, the capacity retention rate increased and the resistance value decreased. Among them, in a battery including a negative electrode active material layer having Sn, Ge, or Sb (Examples 1 to 3, 5) as the first metal element, the capacity retention rate and the resistance value were good. A lithium secondary battery (Example 5) further including a tin-silver alloy, which is an alloy of the first metal element and the second metal element, in the negative electrode active material layer was particularly good in terms of the capacity retention rate and the resistance value.

[0095] The first metal element arranged in a large amount on the electrolyte layer side surface is considered to be easily alloyed with lithium and to have a high affinity with the electrolyte. In addition, the second metal element arranged in a large amount on the negative electrode current collector layer side surface is considered to be less likely to expand even by charging when combined with the first metal element. The negative electrode active material layer including the first metal element and the second metal element arranged appropriately is presumed to specifically suppress the increase in the specific surface area accompanying charge and discharge, thereby increasing the capacity retention rate. Also, by arranging a large amount of the first metal element, which is easily alloyed with lithium, on the electrolyte layer side surface, lithium easily enters the negative electrode active material layer, thereby presuming that the resistance value decreases. Furthermore, it is presumed that the affinity between the negative electrode active material layer and the electrolyte increases, and lithium is easily dissolved and deposited, thereby reducing the resistance value.

[0096] 《Examples 6 to 13 (Effect of Concentration)》 〈Preparation of Preliminary Negative Electrode Laminate〉 A preliminary negative electrode laminate was prepared in the same manner as in Example 1 except that the conditions for co-evaporation were adjusted to obtain the concentrations shown in Table 2.

[0097] 〈Preparation of Lithium Secondary Battery, Evaluation of Capacity Retention Rate, Evaluation of Resistance Value〉 Using the preliminary negative electrode laminate produced in Examples 6 to 13, a lithium secondary battery was produced in the same manner as in Example 1. The capacity retention rate and resistance value of the lithium secondary batteries in Examples 6 to 13 were evaluated in the same manner as in Example 1. The respective results were as shown in Table 2.

[0098]

Table 2

[0099] When the concentration (atomic %) of the first metal element on the electrolyte layer side surface was higher than the concentration (atomic %) of the first metal element on the negative electrode current collector layer side surface, and the concentration (atomic %) of the second metal element on the electrolyte layer side surface was lower than the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface, the capacity retention rate increased and the resistance value decreased.

[0100] It is presumed that the negative electrode active material layer containing the first metal element and the second metal element appropriately arranged specifically suppressed the increase in specific surface area accompanying charge and discharge, thereby increasing the capacity retention rate. Also, by arranging a large amount of the first metal element, which is likely to be alloyed with lithium, on the electrolyte layer side surface, lithium easily enters the negative electrode active material layer, thereby presumably reducing the resistance value.

[0101] 《Examples 14 to 30 (Effect of the second metal element)》 〈Production of the negative electrode laminate〉 A preliminary negative electrode laminate was produced in the same manner as in Example 1, except that the metals listed in Table 3 were used as the second metal element.

[0102] 〈Production of the lithium secondary battery, evaluation of the capacity retention rate, evaluation of the resistance value〉 Using the preliminary negative electrode laminate produced in Examples 14 to 30, a lithium secondary battery was produced in the same manner as in Example 1. The capacity retention rate and resistance value of the lithium secondary batteries in Examples 14 to 30 were evaluated in the same manner as in Example 1. The respective results were as shown in Table 3.

[0103]

Table 3

[0104] Even when a metal other than Ag is used as the second metal element, the capacity retention rate increases and the resistance value decreases. Even when the second metal element is various types of metal elements, when combined with the first metal element, it becomes difficult to expand even by charging, specifically suppresses the increase in specific surface area accompanying charge and discharge, and thereby it is presumed that the capacity retention rate increases.

[0105] Although the preferred embodiments of the lithium secondary battery of the present disclosure have been described, those skilled in the art understand that changes can be made without departing from the scope of the claims.

Explanation of Reference Numerals

[0106] 100 Lithium secondary battery 111 Negative electrode current collector layer 112 Negative electrode active material layer 112a Electrolyte layer side surface 112b Negative electrode current collector layer side surface 112c Layer closest to the electrolyte layer 112d Layer closest to the negative electrode active material layer 120 Electrolyte layer 131 Positive electrode active material layer 132 Positive electrode current collector layer

Claims

1. It has a negative electrode current collector layer, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer in this order, The negative electrode active material layer contains lithium, a first metal element that forms an alloy with lithium, and a second metal element that forms an alloy with lithium, The concentration (atomic %) of the first metal element on the electrolyte layer side surface is higher than the concentration (atomic %) of the first metal element on the negative electrode current collector layer side surface, The concentration (atomic %) of the second metal element on the electrolyte layer side surface is lower than the concentration (atomic %) of the second metal element on the negative electrode current collector layer side surface, The first metal element is selected from tin, germanium, antimony, and bismuth, A lithium secondary battery.

2. The lithium secondary battery according to Claim 1, wherein the first metal element is selected from tin, germanium, and antimony.

3. The lithium secondary battery according to Claim 1, wherein the second metal element is at least one selected from sodium, magnesium, aluminum, silicon, calcium, zinc, gallium, germanium, strontium, rhodium, palladium, silver, barium, lead, iridium, gold, platinum, and bismuth.

4. The lithium secondary battery according to Claim 1, comprising an alloy containing the lithium, the first metal element, and the second metal element.

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