Lithium-ion secondary battery

Abstract

A lithium ion secondary battery in which a positive electrode layer including a positive electrode active material and a negative electrode layer including a negative electrode active material are sequentially stacked with an interlayer solid electrolyte layer therebetween, wherein, among the interlayer solid electrolyte layers, the ratio t1 ∕ t2 of the average thickness t1 of the thickest interlayer solid electrolyte layer to the average thickness t2 of the thinnest interlayer solid electrolyte layer is 1.02 ≤ t1 ∕ t2 ≤ 1.99.

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery. This application claims priority based on Japanese Patent Application No. 2020-009573, filed on January 24, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, electronics technology has made remarkable advances, with efforts being made to make portable electronic devices smaller, lighter, thinner, and more multifunctional. Accordingly, there is a strong demand for smaller, lighter, thinner, and more reliable batteries to power these electronic devices.

[0003] Lithium-ion secondary batteries, which are currently widely used as the power source for electronic devices, have traditionally used liquid electrolytes (electrolytic solutions) such as organic solvents as the electrolyte, which is the medium for ion transport. However, batteries using liquid electrolytes can suffer from electrolyte leakage due to external impacts, etc., which can lead to a decrease in battery performance, and there is a need to further improve the reliability of lithium-ion secondary batteries.

[0004] Therefore, as one measure to improve the reliability of lithium ion secondary batteries, development is underway to use a solid electrolyte instead of a liquid electrolyte as the electrolyte, and to sandwich this between electrodes and stack or roll them up.

[0005] However, solid electrolytes are known to have lower ionic conductivity than liquid electrolytes, and various studies have been conducted to improve the output characteristics of lithium-ion secondary batteries using solid electrolytes.

[0006] Patent Document 1 discloses that rate characteristics can be improved by mixing a solid electrolyte into an electrode and controlling the ratio of the solid electrolyte to the electrode active material in the thickness direction of the electrode and the porosity of the electrode.

[0007] Patent Document 2 discloses that charge / discharge efficiency can be improved by mixing a solid electrolyte into an electrode and controlling the difference between the resistivity associated with ion movement in the electrode and the resistivity associated with electron movement to between 0 kΩ cm and 100 kΩ cm.

[0008] Patent Document 3 discloses that a solid electrolyte membrane with excellent battery characteristics can be obtained by setting the standard deviation of the electrolyte membrane thickness to 5.0 μm or less. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-104270 [Patent Document 2] International Publication No. 2014 / 002858 [Patent Document 3] Japanese Patent Application Publication No. 2017-157362 Summary of the Invention [Problem to be solved by the invention]

[0010] However, as electronic devices become more multifunctional, there is a demand for lithium ion secondary batteries that have higher output characteristics when using solid electrolytes.

[0011] The present invention solves the above problems and provides a lithium ion secondary battery that has high output characteristics when a solid electrolyte is used as the electrolyte. [Means for solving the problem]

[0012] As a result of our extensive research, we have found that the output characteristics can be improved by setting the thicknesses of multiple solid electrolyte layers in the thickness direction of a lithium ion secondary battery to a specific ratio, which led to the present invention. That is, in order to solve the above problems, the following means are provided.

[0013] In the lithium ion secondary battery according to this embodiment, a positive electrode layer containing a positive electrode active material and a negative electrode layer containing a negative electrode active material are stacked in this order with a solid electrolyte layer interposed therebetween, and in the solid electrolyte layer, the ratio t1 / t2 of the average thickness t1 of the thickest solid electrolyte layer to the average thickness t2 of the thinnest solid electrolyte layer satisfies 1.02≦t1 / t2≦1.99.

[0014] This configuration makes it possible to improve the output of lithium-ion secondary batteries that use solid electrolytes. This is based on the following principle: Compared to when the average thickness of the solid electrolyte layer included in a lithium-ion secondary battery is uniform, the charge-discharge reaction in the positive electrode layer and negative electrode layer via the solid electrolyte layer with a thin average thickness proceeds more quickly, resulting in a charge imbalance between the positive electrode layer and negative electrode layer within the lithium-ion battery. This charge imbalance accelerates the charge-discharge reaction in the solid electrolyte layer with a thick average thickness.

[0015] By setting the ratio of the average thicknesses of the solid electrolyte layers within the range of the present invention, a bias in the charge between the positive electrode layer and the negative electrode layer is generated, and the occurrence of a non-uniform reaction inside the lithium ion secondary battery due to the difference in the average thicknesses of the solid electrolyte layers is suppressed, thereby improving the output characteristics.

[0016] In the lithium ion secondary battery according to the above embodiment, the standard deviation σ of the average thickness t of each solid electrolyte layer may be 0.15≦σ≦1.66 (μm).

[0017] This suppresses the occurrence of non-uniform reactions inside the lithium ion secondary battery, while generating a moderate charge bias without bias inside the lithium ion secondary battery, thereby achieving high output characteristics.

[0018] The lithium ion secondary battery according to the above embodiment may have an intermediate layer containing the positive electrode layer or the negative electrode layer and constituent elements of the solid electrolyte at least partially between the positive electrode layer or the negative electrode layer and the solid electrolyte layer.

[0019] This allows lithium ions to be exchanged effectively at the interfaces between the positive electrode layer, the negative electrode layer, and the solid electrolyte layer, thereby reducing the interfacial resistance and further promoting the occurrence of charge imbalance and the subsequent progress of charge-discharge reactions, resulting in high output characteristics.

[0020] In the lithium ion secondary battery according to the above embodiment, the average thickness T of each solid electrolyte layer may be 4.8≦T≦9.8 (μm).

[0021] This ensures sufficient insulation between the positive electrode layer and the negative electrode layer while ensuring favorable exchange of lithium ions, thereby achieving high output characteristics. [Effects of the Invention]

[0022] The present invention makes it possible to provide a lithium ion secondary battery with high output characteristics. [Brief explanation of the drawings]

[0023] [Figure 1] 1 shows a part of a cross-sectional view in the stacking direction of a lithium ion secondary battery according to an embodiment of the present invention. [Figure 2] 10 shows a partial cross-sectional view in the stacking direction of a lithium ion secondary battery according to a modified example of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for the sake of clarity. Therefore, the dimensional ratios of the components shown in the drawings may differ from the actual ones. The materials, dimensions, shapes, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. The present invention can be implemented by making appropriate changes within the scope of its effects without departing from the spirit of the invention. For example, the configurations described in different embodiments can be implemented by appropriately combining them.

[0025] First, directions will be defined. One direction on one surface of the positive electrode layer 30 (see FIG. 1) is the x direction, and the direction perpendicular to the x direction is the y direction. The x direction is, for example, the direction in which the positive electrode external electrode 60 and the negative electrode external electrode 70 sandwich the laminate 20. The x direction and the y direction are examples of in-plane directions. The z direction is a direction perpendicular to the x direction and the y direction. The z direction is an example of a stacking direction. Hereinafter, the +z direction may be expressed as "up" and the -z direction as "down". Up and down do not necessarily coincide with the direction in which gravity is applied.

[0026] (lithium-ion secondary battery) First, the lithium ion secondary battery according to this embodiment will be described.

[0027] 1, the lithium-ion secondary battery 1 includes a laminate 20 in which a positive electrode layer 30 and a negative electrode layer 40 are stacked with a solid electrolyte layer 50 interposed therebetween. The laminate 20 is sandwiched between outer layers 55 (described later) in the stacking direction, for example. The positive electrode layer 30 includes a positive electrode current collector layer 31 and a positive electrode active material layer 32. The negative electrode layer 40 includes a negative electrode current collector layer 41 and a negative electrode active material layer 42.

[0028] A margin layer 80 is formed on the same plane as the positive electrode layer 30 and the negative electrode layer 40. The laminate 20 is a hexahedron having two end faces and two side faces formed as faces parallel to the stacking direction, and a top face and a bottom face formed as faces perpendicular to the stacking direction. The positive electrode current collector layer 31 is exposed at the first end face, and the negative electrode current collector layer 42 is exposed at the second end face.

[0029] The first end face and the second end face face each other, and the first side face and the second side face face each other. As will be described later, the positive electrode current collector layer 31 and the negative electrode current collector layer 41 are also exposed to parts of the first side face and the second side face.

[0030] A positive electrode external electrode 60 electrically connected to the positive electrode current collector layer 31 is attached so as to cover the first end surface side of the laminate 20. Note that this electrical connection is achieved by connecting the positive electrode external electrode 60 to the positive electrode current collector layer 31 of the positive electrode layer 30 exposed on the first end surface, first side surface, and second side surface of the laminate 20.

[0031] A negative electrode external electrode 70 electrically connected to the negative electrode current collector layer 41 is attached so as to cover the second end surface side of the laminate 20. Note that this electrical connection is achieved by connecting the negative electrode external electrode 70 to the negative electrode current collector layer 41 of the negative electrode layer 40 exposed on the second end surface, first side surface, and second side surface of the laminate 20.

[0032] In the following explanation in the specification, either or both of the positive electrode active material and the negative electrode active material may be collectively referred to as the active material, either or both of the positive electrode active material layer 32 and the negative electrode active material layer 42 may be collectively referred to as the active material layer, either or both of the positive electrode current collector layer 31 and the negative electrode current collector layer 41 may be collectively referred to as the current collector layer, either or both of the positive electrode layer 30 and the negative electrode layer 40 may be collectively referred to as the electrode layer, the first end face and the second end face may be collectively referred to as the end face, the first side face and the second side face may be collectively referred to as the side face, and the positive electrode external electrode 60 and the negative electrode external electrode 70 may be collectively referred to as the external electrode.

[0033] The margin layer 80 of the lithium-ion secondary battery 1 of this embodiment is preferably provided when the step between the solid electrolyte layer 50 and the positive electrode layer 30 and the step between the solid electrolyte layer 50 and the negative electrode layer 40 are large in order to eliminate the step. The margin layer 80 is preferably provided on the same plane as the positive electrode layer 30 and the negative electrode layer 40. The presence of the margin layer 80 eliminates the step between the solid electrolyte layer 50 and the positive electrode layer 30 and the negative electrode layer 40, thereby increasing the density between the solid electrolyte layer 50 and the electrode layers and making it less likely for delamination or warping to occur during firing of the lithium-ion secondary battery.

[0034] (solid electrolyte layer) The solid electrolyte layer 50 of the lithium ion secondary battery 1 of this embodiment is sandwiched between the positive electrode layer 30 and the negative electrode layer 40 in the z direction. FIG. 1 illustrates an example in which the battery has three solid electrolyte layers 50a, 50b, and 50c. The solid electrolyte layer 50a is the thinnest solid electrolyte layer, and the solid electrolyte layer 50b is the thickest solid electrolyte layer. The solid electrolyte layer 50c has a thickness between the solid electrolyte layers 50a and 50b. The thickness of each solid electrolyte layer 50 is determined based on the average thickness. The ratio t1 / t2 of the average thickness t1 of the thickest solid electrolyte layer 50b to the average thickness t2 of the thinnest solid electrolyte layer 50a is 1.02≦t1 / t2≦1.99. Here, the average thickness of the solid electrolyte layer 50 is the ratio of the average thickness t1 of the single solid electrolyte layer 50. of thickness In-plane direction in average value For example, The thickness of the solid electrolyte layer 50 average in x direction value is.

[0035] This configuration makes it possible to improve the output of lithium-ion secondary batteries that use solid electrolytes. This is based on the following principle: Compared to when the average thickness of the solid electrolyte layer is uniform, the charge-discharge reaction in the positive electrode layer and negative electrode layer via a solid electrolyte layer with a thin average thickness proceeds more quickly, resulting in a charge imbalance between the positive electrode layer and negative electrode layer within the lithium-ion battery. This charge imbalance accelerates the charge-discharge reaction in the solid electrolyte layer with a thick average thickness.

[0036] By setting the ratio of the average thickness of the thinnest solid electrolyte layer 50a to the average thickness of the thickest solid electrolyte layer 50b within the range of the present invention, an imbalance in charge occurs between the positive electrode layer and the negative electrode layer, and the occurrence of non-uniform reactions inside the lithium ion secondary battery due to the difference in the average thickness of the solid electrolyte layer 50 is suppressed, thereby improving the output characteristics.

[0037] In addition, in the solid electrolyte layer 50 of the present embodiment, the ratio t1 / t2 of the average thickness t1 of the solid electrolyte layer 50b having the thickest average thickness to the average thickness t2 of the solid electrolyte layer 50a having the thinnest average thickness is preferably 1.02≦t1 / t2≦1.99.

[0038] By setting t1 / t2 within the above range, the difference in charge bias between the positive electrode layer and the negative electrode layer in the lithium ion battery becomes smaller, and the charge bias between the positive electrode layer and the negative electrode layer becomes closer in the entire lithium ion secondary battery, thereby suppressing the occurrence of non-uniform reactions inside the lithium ion secondary battery and improving the output characteristics.

[0039] The average thickness of each solid electrolyte layer in the solid electrolyte layer 50 of this embodiment can be determined by SEM observation of a cross section of the lithium ion secondary battery 1. In the cross section of the lithium ion secondary battery 1, the average value of thicknesses at five points that divide the solid electrolyte layer 50 into approximately six equal parts is defined as the average thickness of the solid electrolyte layer 50, the thickness of the solid electrolyte layer 50b with the thickest average thickness is defined as t1, and the thickness of the solid electrolyte layer 50a with the thinnest average thickness is defined as t2.

[0040] In the solid electrolyte layer 50 of this embodiment, it is preferable that the standard deviation σ of the average thickness t of all the solid electrolyte layers satisfies 0.15≦σ≦1.66 (μm).

[0041] This suppresses the occurrence of non-uniform reactions inside the lithium ion secondary battery, while generating a moderate charge bias without bias inside the lithium ion secondary battery, thereby achieving high output characteristics.

[0042] Furthermore, in the solid electrolyte layer 50 of this embodiment, it is more preferable that the standard deviation σ of the average thickness of all the solid electrolyte layers is 0.55≦σ≦1.24 (μm).

[0043] In the solid electrolyte layer 50 of this embodiment, in the lithium ion secondary battery according to the above aspect, the average thickness T of each solid electrolyte layer preferably satisfies the relationship 4.8≦T≦9.8 (μm).

[0044] This ensures sufficient insulation between the positive electrode layer and the negative electrode layer while ensuring favorable exchange of lithium ions, thereby achieving high output characteristics.

[0045] The solid electrolyte layer 50 of this embodiment is mainly composed of a solid electrolyte. Known materials can be used as the solid electrolyte, for example, lithium titanium aluminum phosphate Li 1+x Al x Ti 2-x (PO4)3(0≦x≦0.6), Lithium germanium phosphate Li 1.5 Ge 2.0 (PO4)3, Lithium aluminum germanium phosphate Li 1.5 Al 0.5 Ge 1.5 (PO4)3, Li 3+x1 Si x1 P 1-x1 O4(0.4≦x1≦0.6), Li 3.4 V 0.4 Ge 0.6 O4, lithium germanium phosphate (LiGe2(PO4)3), Li2O-V2O5-SiO2, Li2O-P2O5-B2O3, Li3PO4, Li 0.5 La 0.5 TiO3, Li 14 Zn(GeO4)4, Li7La3ZrO 12 , Li 3.6 Si 0.6 P 0.4 O4, Li3BO3-Li2SO4 glass ceramic, Li3BO3-Li2SO4-Li2CO3 glass ceramic, polyethylene oxide, etc.

[0046] The solid electrolyte of this embodiment may have a composition modified by changing the composition ratio or by substituting a different element, as long as the properties of the solid electrolyte are obtained.

[0047] The solid electrolyte layer 50 of this embodiment is made of a phosphate compound such as titanium aluminum lithium phosphate or germanium aluminum lithium phosphate, or Li 0.5 La 0.5 TiO3, Li 3.6 Si 0.6 P 0.4 It is preferable to include an oxide such as O4.

[0048] In the solid electrolyte constituting the solid electrolyte layer 50 of this embodiment, the term "main component" refers to the component that occupies the solid electrolyte layer 50 in the largest proportion.

[0049] Examples of the minor components constituting the solid electrolyte layer 50 of this embodiment include a sintering packing agent used when forming the solid electrolyte layer, and decomposition products thereof.

[0050] (Positive and negative electrode layers) There are a plurality of positive electrode layers 30 and a plurality of negative electrode layers 40 in the laminate 20. The positive electrode layers 30 and the negative electrode layers 40 are alternately stacked with solid electrolyte layers interposed therebetween.

[0051] The positive electrode layer 30 has a positive electrode current collector layer 31 and a positive electrode active material layer 32 containing a positive electrode active material. The negative electrode layer 40 has a negative electrode current collector layer 41 and a negative electrode active material layer 42 containing a negative electrode active material.

[0052] The positive electrode current collector layer 31 and the negative electrode current collector layer 41 have excellent electrical conductivity. The positive electrode current collector layer 31 and the negative electrode current collector layer 41 are made of, for example, silver, palladium, gold, platinum, aluminum, copper, or nickel. Copper does not easily react with the positive electrode active material, the negative electrode active material, or the solid electrolyte. For example, using copper for the positive electrode current collector layer 31 and the negative electrode current collector layer 41 can reduce the internal resistance of the lithium-ion secondary battery 1. The materials constituting the positive electrode current collector layer 31 and the negative electrode current collector layer 41 may be the same or different.

[0053] The positive electrode active material layer 32 is formed on one or both sides of the positive electrode current collector layer 31. The positive electrode active material layer 32 may not be present on the side of the positive electrode current collector layer 31 on which the opposing negative electrode layer 40 is not present. The negative electrode active material layer 42 is formed on one or both sides of the negative electrode current collector layer 41. The negative electrode active material layer 42 may not be present on the side of the negative electrode current collector layer 41 on which the opposing positive electrode layer 30 is not present. For example, the positive electrode layer 30 or the negative electrode layer 40 located in the uppermost layer or the lowermost layer of the laminate 20 may not have the positive electrode active material layer 32 or the negative electrode active material layer 42 on one side.

[0054] The positive electrode active material layer 32 and the negative electrode active material layer 42 contain a positive electrode active material and a negative electrode active material that donate and accept electrons. In addition, they may contain a conductive additive, an ion-conducting additive, a binder, etc. It is preferable that the positive electrode active material and the negative electrode active material be able to efficiently insert and extract lithium ions.

[0055] The positive electrode active material and the negative electrode active material may be known materials, such as transition metal oxides and transition metal composite oxides. Specific examples of the positive electrode active material and the negative electrode active material include lithium manganese composite oxide (LiMn a Ma 1-a O3 (0.8≦a≦1, Ma=Co, Ni), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), general formula: LiNi x Co y Mn z O2 (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1), lithium vanadium compound (LiV2O5), olivine-type LiMbPO4 (where Mb is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr), lithium vanadium phosphate (Li3V2(PO4)3 or LiVOPO4), Li-excess solid solution positive electrodes represented by Li2MnO3-LiMcO2 (Mc=Mn, Co, Ni), lithium titanate (Li4Ti5O 12 ), titanium oxide (TiO2)Li s Ni t Co u Al vIt is a composite metal oxide or the like represented by O2 (0.9 < s < 1.3, 0.9 < t + u + v < 1.1).

[0056] Also, as the positive electrode active material and the negative electrode active material, it is preferable to use a phosphate compound represented by olivine-type LiMbPO4 (where Mb is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr), lithium vanadium phosphate (Li3V2(PO4)3 or LiVOPO4) as the main component.

[0057] The positive electrode active material and the negative electrode active material of the present embodiment may be those whose composition is changed by changing the composition ratio or substituting different elements, as long as the characteristics as the positive electrode active material and the negative electrode active material can be obtained.

[0058] In the positive electrode active material and the negative electrode active material that constitute the positive electrode layer 30 and the negative electrode layer 40 of the present embodiment, the main component refers to the component that occupies the positive electrode active material and the negative electrode active material and has the largest composition ratio.

[0059] Examples of the conductive auxiliary agent include carbon materials such as carbon black, acetylene black, ketjen black, carbon nanotubes, graphite, graphene, activated carbon, and metal materials such as gold, silver, palladium, platinum, copper, and tin.

[0060] Examples of the ion-conducting auxiliary agent include solid electrolytes. Specifically, for example, the same materials as those used for the solid electrolyte layer 50 can be used for the solid electrolyte.

[0061] When using a solid electrolyte as the ion-conducting auxiliary agent, it is preferable that the ion-conducting auxiliary agent and the solid electrolyte used for the solid electrolyte layer 50 use the same material.

[0062] Also, when using a solid electrolyte as the ion-conducting auxiliary agent, different solid electrolytes may be used for the positive electrode active material layer 32 and the negative electrode active material layer 42, respectively.

[0063] There is no clear distinction between the active materials constituting the positive electrode active material layer 32 and the negative electrode active material layer 42, and by comparing the potentials of two types of compounds, the compound showing a more noble potential can be used as the positive electrode active material, and the compound showing a more base potential can be used as the negative electrode active material.

[0064] (Positive electrode current collector and negative electrode current collector) The positive electrode current collector layer 31 and the negative electrode current collector layer 41 of the lithium-ion secondary battery 1 of this embodiment are preferably made of materials with high electrical conductivity, such as silver, palladium, gold, platinum, aluminum, copper, or nickel. Copper is particularly preferred because it is less likely to react with oxide-based lithium ion conductors and has the effect of reducing the internal resistance of the laminated all-solid-state battery. The positive electrode current collector layer 31 and the negative electrode current collector layer 41 may be made of the same material or different materials.

[0065] The positive electrode current collector layer 31 and the negative electrode current collector layer 41 may contain a positive electrode active material and a negative electrode active material, respectively. The content ratio of the active materials contained in each current collector is not particularly limited as long as they function as current collectors. For example, the volume ratio of the positive electrode current collector / positive electrode active material or the negative electrode current collector / negative electrode active material is preferably in the range of 90 / 10 to 70 / 30.

[0066] When the positive electrode current collector layer 31 and the negative electrode current collector layer 41 contain a positive electrode active material and a negative electrode active material, respectively, the adhesion between the positive electrode current collector layer 31 and the positive electrode active material layer 32 and between the negative electrode current collector layer 41 and the negative electrode active material layer 42 is improved.

[0067] (middle class) In the lithium-ion secondary battery 1 of this embodiment, an intermediate layer 90 may be present at least partially between the positive electrode layer 30 and the solid electrolyte layer 50, or between the negative electrode layer 40 and the solid electrolyte layer 50. While Fig. 1 shows an example in which an intermediate layer 90 is present between the surface of the lowermost positive electrode layer 30 in the z direction and the solid electrolyte layer 50b, the number and positions of the intermediate layers 90 to be formed are not limited to this example.

[0068] The intermediate layer 90 of this embodiment is preferably a layer containing the constituent elements of the positive electrode layer 30 or the negative electrode layer 40 and the solid electrolyte layer 50 .

[0069] By forming the positive electrode layer 30 or the negative electrode layer 40 into a layer containing the constituent elements of the solid electrolyte layer 50, the positive electrode layer 30, the negative electrode layer 40, and the solid electrolyte layer 50 become compatible with the intermediate layer 90, thereby reducing the interfacial resistance, further accelerating the occurrence of charge imbalance and the subsequent progress of the charge / discharge reaction, and achieving high output characteristics.

[0070] (Margin layer) The margin layer 80 of the lithium ion secondary battery 1 of this embodiment is preferably provided to eliminate the step between the solid electrolyte layer 50 and the positive electrode layer 30, and the step between the solid electrolyte layer 50 and the negative electrode layer 40. The presence of such a margin layer 80 eliminates the step between the solid electrolyte layer 50 and the positive electrode layer 30 and the negative electrode layer 40, thereby increasing the density of the laminate 20 and the positive electrode layer 30 and the negative electrode layer 40, and making it less likely that delamination or warpage will occur during firing of the lithium ion secondary battery 1.

[0071] The material constituting the margin layer 80 can be the same as the solid electrolyte material used in the solid electrolyte layer 50 .

[0072] Moreover, the solid electrolyte constituting the margin layer 80 preferably has the same configuration as the solid electrolyte constituting the solid electrolyte layer 50 .

[0073] (outer layer) In the lithium ion secondary battery 1 of this embodiment, outer layers (cover layers) 55 can be provided on both main surfaces of the laminate 20 exposed in the z direction, as needed. In this embodiment, the outer layer on the upper side in the stacking direction is referred to as a first outer layer (upper outermost layer) 55A, and the outer layer on the lower side in the stacking direction is referred to as a second outer layer (lower outermost layer) 55B. The outer layer 55 can be made of the same material as the solid electrolyte layer, but is not included in the solid electrolyte layer of this embodiment.

[0074] (Method of manufacturing lithium-ion secondary batteries) The lithium-ion secondary battery 1 of this embodiment can be manufactured by the following procedure. The materials for the positive electrode current collector layer 31, the positive electrode active material layer 32, the solid electrolyte layer 50, the negative electrode current collector layer 41, the negative electrode active material layer 42, the margin layer 80, and the intermediate layer 90 are formed into a paste. The method for forming the paste is not particularly limited, but for example, a paste can be obtained by mixing powders of the materials with a vehicle. Here, the term "vehicle" refers to a liquid medium, including solvents and binders. The binder contained in the paste for forming the green sheet or printed layer is not particularly limited, but examples include polyvinyl acetal resin, cellulose resin, acrylic resin, urethane resin, vinyl acetate resin, and polyvinyl alcohol resin. The slurry can contain at least one of these resins.

[0075] The paste may also contain a plasticizer. The type of plasticizer is not particularly limited, but phthalate esters such as dioctyl phthalate and diisononyl phthalate may be used.

[0076] By this method, a paste for a positive electrode current collector layer, a paste for a positive electrode active material layer, a paste for a solid electrolyte layer, a paste for a negative electrode active material layer, a paste for a negative electrode current collector layer, a paste for a margin layer, and a paste for an intermediate layer are prepared.

[0077] The solid electrolyte layer paste prepared above is applied to a substrate such as polyethylene terephthalate (PET) to a desired thickness and dried as necessary to prepare a solid electrolyte green sheet 5. The method for preparing the solid electrolyte green sheet 5 is not particularly limited, and known methods such as a doctor blade method, die coater, comma coater, or gravure coater can be used. Next, an intermediate layer 90, a positive electrode active material layer 32, a positive electrode current collector layer 31, and a positive electrode active material layer 32 are printed and laminated in this order on the solid electrolyte green sheet 5 by screen printing to form the intermediate layer 90 and the positive electrode layer 30. Furthermore, to fill the gap between the solid electrolyte green sheet 5 and the positive electrode layer 30, a margin layer 80 is formed by screen printing in the area other than the positive electrode layer, thereby preparing a positive electrode layer unit.

[0078] The negative electrode layer unit can be produced in the same manner as the positive electrode layer unit, and the negative electrode layer 40 and margin layer 80 are formed on the solid electrolyte green sheet 5 by screen printing to produce the negative electrode layer unit.

[0079] At this time, by adjusting the coating thickness of the solid electrolyte paste, a positive electrode layer unit and a negative electrode layer unit having solid electrolyte layers with different thicknesses are produced.

[0080] The positive electrode layer units and the negative electrode layer units are alternately stacked by offsetting them so that their ends do not coincide with each other. If necessary, outer layers (cover layers) can be provided on both main surfaces of the stack exposed in the z direction. The outer layers can be made of the same material as the solid electrolyte. Hereinafter, the sheets used to provide the outer layers may be referred to as "outermost layer sheets." In this embodiment, these outer layers are not considered to be the solid electrolyte layers 50 of the stack 1.

[0081] The above manufacturing method is for producing a parallel-type lithium ion secondary battery, but a method for manufacturing a series-type lithium ion secondary battery can be achieved by stacking the positive electrode layer and the negative electrode layer so that one end of each layer coincides with one end of the other layer, i.e., without offsetting.

[0082] The produced laminated substrate can be pressed collectively using a mold press, hot isostatic press (WIP), cold isostatic press (CIP), isostatic press, or the like to enhance adhesion. Pressing is preferably performed while heating, and can be performed at, for example, 40 to 95°C. In the method for producing an all-solid-state lithium-ion secondary battery according to this embodiment, the laminated substrate may be produced by taking into consideration in advance the positions in the z direction where cutting will be performed later, and then the laminated substrate may be cut at predetermined positions in the z direction to obtain multiple desired laminates.

[0083] The produced laminated substrate can be cut into laminated bodies of unfired lithium ion secondary batteries using a dicing device.

[0084] The lithium ion secondary battery is manufactured by removing the binder and firing the laminate. The binder removal and firing can be performed in a nitrogen atmosphere at a temperature of 600°C to 1000°C. The retention time for the binder removal and firing is, for example, 0.1 to 6 hours.

[0085] Furthermore, external electrodes can be provided to efficiently extract current from the laminate 20 of the lithium-ion secondary battery 1. The external electrodes are formed by alternately connecting positive electrode layers 30 and negative electrode layers 40 in parallel, and are joined via two opposing end faces E1, E2 of the laminate and portions of two opposing side faces S1, S2. Thus, a pair of external electrodes is formed to sandwich the end faces of the laminate. Examples of methods for forming the external electrodes 12 include sputtering, screen printing, and dip coating. In the screen printing and dip coating methods, an external electrode paste containing metal powder, resin, and solvent is prepared and used to form the external electrodes 12. Next, a baking process is performed to remove the solvent, and a plating process is performed to form terminal electrodes on the surfaces of the external electrodes. In contrast, the sputtering method allows the external electrodes and terminal electrodes to be directly formed, eliminating the need for the baking and plating processes.

[0086] The lithium-ion secondary battery 1 laminate may be sealed, for example, in a coin cell to improve moisture resistance and impact resistance. The sealing method is not particularly limited, and the laminate may be sealed with a resin after firing, for example. Alternatively, the laminate may be sealed by applying or dip-coating an insulating paste such as Al2O3 around the laminate and then heat-treating the insulating paste.

[0087] In the above embodiment, a method for manufacturing a laminated all-solid-state battery including a step of forming a margin layer using a margin layer paste has been illustrated, but the method for manufacturing a lithium-ion secondary battery according to this embodiment is not limited to this example. For example, the step of forming a margin layer using a margin layer paste may be omitted. The margin layer may be formed, for example, by deformation of the solid electrolyte layer paste during the manufacturing process of the lithium-ion secondary battery.

[0088] (Variation) 2 is a cross-sectional view of a lithium ion secondary battery 1A according to a modified example. In the lithium ion secondary battery 1A, the same components as those in the lithium ion secondary battery 1 are denoted by the same reference numerals and description thereof will be omitted.

[0089] The lithium ion secondary battery 1A shown in FIG. 2 differs from the lithium ion secondary battery 1 shown in FIG.

[0090] The lithium ion secondary battery 1A according to the modified example also provides the same effects as those of the lithium ion secondary battery 1.

[0091] Specific examples of the lithium ion secondary battery according to the embodiment have been shown so far. The characteristic configurations of the embodiments may be combined with each other. [Example]

[0092] The present invention will be described in more detail below based on the above-described embodiment, using examples and comparative examples.

[0093] Example 1 (Preparation of active material powder) The active material powder used was lithium vanadium phosphate, prepared by the following method. The starting materials, Li2CO3, V2O5, and NH4H2PO4, were dispersed in pure water and then wet-mixed in a ball mill for 12 hours. After mixing, the powder was dehydrated and dried, and then calcined at 850°C for 2 hours in a nitrogen-hydrogen mixed gas. After calcination, the powder was dispersed in pure water and then wet-pulverized in a ball mill for 1 hour. After pulverization, the powder was dehydrated and dried to obtain the active material powder, lithium vanadium phosphate.

[0094] The obtained active material powder was analyzed using an X-ray diffractometer, and it was confirmed to be lithium vanadium phosphate having the same crystal structure as NASICON-type Li3V2(PO4)3.

[0095] (Preparation of Paste for Active Material Layer) The active material layer paste was prepared by adding 15 parts of ethyl cellulose as a binder and 65 parts of dihydroterpineol as a solvent to 100 parts of the obtained active material powder, mixing and dispersing the mixture.

[0096] (Preparation of Paste-01 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-01, which was prepared using the following method. The starting materials, Li2CO3, Al2O3, TiO2, and NH4H2PO4, were dispersed in pure water and then wet-mixed in a ball mill for 12 hours. After mixing, the mixture was dehydrated and dried, and the resulting powder was then calcined in air at 800°C for 2 hours. After calcination, the mixture was dispersed in pure water and then wet-pulverized in a ball mill for 8 hours. After pulverization, the mixture was dehydrated and dried to obtain solid electrolyte powder-01.

[0097] The obtained solid electrolyte powder-01 was analyzed using an X-ray diffractometer, and it was confirmed to be lithium aluminum titanium phosphate having a crystal structure similar to that of NASICON-type LiTi2(PO4)3.

[0098] Next, 100 parts of the obtained solid electrolyte powder-01 was wet-mixed in a ball mill with 100 parts of ethanol and 200 parts of toluene as a solvent, followed by further addition of 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-01.

[0099] (Production of solid electrolyte layer sheet-01) The obtained paste-01 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-01 for solid electrolyte layer with different thicknesses were produced.

[0100] (Production of outermost layer sheet-01) The obtained solid electrolyte layer paste-01 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-01.

[0101] (Preparation of paste for current collector layer) To prepare a current collector, the active material powder obtained and Cu powder were mixed at a volume ratio of 80 / 20. After mixing, 100 parts of the mixture obtained were mixed and dispersed with 10 parts of ethyl cellulose as a binder and 50 parts of dihydroterpineol as a solvent to prepare a paste for the current collector layer.

[0102] (Preparation of margin layer paste-01) Margin layer paste-01 was prepared by adding 100 parts of the obtained solid electrolyte powder-01, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-01.

[0103] (Preparation of external electrode paste) Silver powder, epoxy resin, and a solvent were mixed and dispersed to prepare a thermosetting paste for the external electrodes.

[0104] Using these pastes, lithium ion secondary batteries were fabricated as follows.

[0105] (Fabrication of electrode layer unit) A 5 μm thick active material layer was formed on an 8 μm thick solid electrolyte layer sheet-01 using screen printing and dried at 80°C for 10 minutes. Next, a 5 μm thick current collector layer was formed on top of that using screen printing in the same printing pattern and dried at 80°C for 10 minutes. A 5-10 μm thick active material layer was then formed on top of that using screen printing in the same printing pattern and dried at 80°C for 10 minutes, thereby producing an electrode layer on the solid electrolyte layer sheet-01. Next, a margin layer with a height approximately flush with the electrode layer was formed on the outer periphery of one end of the electrode layer using screen printing and dried at 80°C for 10 minutes. Next, the PET film was peeled off to obtain a sheet of electrode layer units.

[0106] Similarly, using solid electrolyte sheets-01 with different thicknesses, sheets for a plurality of electrode layer units with solid electrolyte layers of different thicknesses were obtained.

[0107] (Production of the bottom outermost unit) A 5 μm-thick current collector layer was formed on the outermost layer sheet-01 using screen printing and dried at 80°C for 10 minutes. A 5-10 μm-thick active material layer was then formed on the outermost layer sheet-01 using screen printing in the same printing pattern and dried at 80°C for 10 minutes, thereby producing an electrode layer on the outermost layer sheet-01 with an active material layer present on only one side. Next, a margin layer of approximately the same height as the electrode layer was formed on the outer periphery of one end of the electrode layer using screen printing and dried at 80°C for 10 minutes. The PET film of the outermost layer sheet-01 was then peeled off to obtain a sheet for the lower outermost layer unit.

[0108] (Fabrication of the top outermost unit) An active material layer having a thickness of 5 to 10 μm was formed on an 8 μm-thick solid electrolyte layer sheet-01 using screen printing in the same printing pattern, and then dried at 80°C for 10 minutes. A 5 μm-thick current collector layer was then formed on the solid electrolyte layer sheet-01 using screen printing, and then dried at 80°C for 10 minutes, thereby producing an electrode layer on which an active material layer was present on only one side of the solid electrolyte layer sheet-01. Next, a margin layer having a height approximately flush with the electrode layer was formed on the outer periphery of one end of the electrode layer using screen printing, and then dried at 80°C for 10 minutes. Next, an outermost layer sheet-01 was laminated on the electrode layer, and the PET films on the solid electrolyte layer sheet-01 and outermost layer sheet-01 were peeled off to obtain a sheet for the upper outermost layer unit.

[0109] (Preparation of laminate) Using the resulting multiple electrode layer units, 50 layers were stacked alternately, offsetting each other so that their edges did not coincide, to produce a laminate. Furthermore, one lower outermost unit and one lower outermost unit were stacked on each main surface of the laminate in the stacking direction, offset in the same way as the electrode layer units. Furthermore, outer layers were formed by stacking four solid electrolyte sheets on the lower outermost unit and five on the upper outermost unit as outer solid electrolyte layers. This was then thermocompressed using a mold press and cut to produce an unsintered lithium-ion secondary battery laminate. The resulting laminate was then heated in nitrogen at a rate of 200°C / hour to a sintering temperature of 750°C, held at that temperature for 2 hours, and naturally cooled to remove the binder and obtain a lithium-ion secondary battery laminate.

[0110] (External electrode formation process) The external electrode paste was applied to both end faces of the obtained lithium ion secondary battery laminate and to cover the positive and negative electrodes exposed on both sides, and was then thermally cured at 150°C for 30 minutes to form a pair of external electrodes.

[0111] A pair of external electrodes was formed on the laminate of the lithium ion secondary battery, and this was used as the evaluation cell in Example 1.

[0112] (Solid electrolyte layer thickness measurement) The thickness of the solid electrolyte layer in the lithium ion secondary battery produced in Example 1 was measured using a scanning electron microscope (SEM). In the cross section of the lithium ion secondary battery, the thickness of each of the 49 solid electrolyte layers, excluding the exterior solid electrolyte layer, in the 50-layer laminate was measured at five points, and the average value was taken as the thickness of each solid electrolyte layer.

[0113] In the lithium ion secondary battery produced in Example 1, the average thickness t1 of the thickest interlayer solid electrolyte layer was 10.70 μm, and the average thickness t2 of the thinnest interlayer solid electrolyte was 5.98 μm, so that t1 / t2 = 0. Furthermore, when the average value of the thicknesses of the solid electrolyte layers in the 49 layers was calculated as the average thickness of the solid electrolyte layer, T = 8.67 μm.

[0114] The standard deviation σ of the solid electrolyte layer in the lithium ion secondary battery produced in Example 1 was calculated from the thickness of each solid electrolyte layer obtained, and was found to be σ=1.02 μm.

[0115] (Examples 2 to 9, Comparative Examples 1 to 4) An evaluation cell was produced in the same manner as in Example 1, except that the electrode layer units used in producing the laminate were changed to change the values ​​of t1, t2, and T.

[0116] Example 10 (Preparation of Paste-02 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-02, which was prepared using the following method. The starting materials, Li2CO3, Al2O3, GeO2, and NH4H2PO4, were dispersed in pure water and then wet-mixed in a ball mill for 12 hours. After mixing, the mixture was dehydrated and dried, and the resulting powder was then calcined in air at 800°C for 2 hours. After calcination, the mixture was dispersed in pure water and wet-pulverized in a ball mill for 8 hours. After pulverization, the mixture was dehydrated and dried to obtain solid electrolyte powder-02.

[0117] The obtained solid electrolyte powder-02 was analyzed using an X-ray diffractometer, and it was confirmed to be lithium aluminum germanium phosphate having a crystal structure similar to that of NASICON-type LiGe2(PO4)3.

[0118] Next, 100 parts of the obtained solid electrolyte powder-02 was wet-mixed with 100 parts of ethanol and 200 parts of toluene as a solvent in a ball mill, followed by further adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-02.

[0119] (Production of solid electrolyte layer sheet-02) The obtained paste-02 for solid electrolyte layer was used to form a sheet using a PET film as a substrate by the doctor blade method to obtain a sheet-02 for solid electrolyte layer B. At this time, the thickness was adjusted within the range of 5 to 15 μm to produce a plurality of sheets-02 for solid electrolyte layer having different thicknesses.

[0120] (Production of outermost layer sheet-02) The obtained solid electrolyte layer paste-02 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-02.

[0121] (Preparation of margin layer paste-02) Margin layer paste-02 was prepared by adding 100 parts of the obtained solid electrolyte powder-02, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-02.

[0122] Example 10 An evaluation cell of Example 10 was produced in the same manner as in Example 1, except that solid electrolyte sheet-02, outermost layer sheet-02, and margin layer paste-02 were used.

[0123] (Examples 11 to 18, Comparative Examples 5 to 8) The evaluation cells of Examples 11 to 18 and Comparative Examples 5 to 8 were prepared in the same manner as in Example 10, except that the values ​​of t1, t2, and T were changed by changing the electrode layer units used when preparing the laminate.

[0124] (output characteristics evaluation) The output characteristics of the evaluation cells fabricated in this example and comparative example were evaluated by charging and discharging them under the following charge and discharge conditions. Charge and discharge currents are expressed as C (C) rate notation. C rate is expressed as nC (μA) (n is a number) and refers to the current at which the nominal capacity (μAh) can be charged and discharged at 1 / n (h). For example, 1C is the charge and discharge current at which the nominal capacity can be charged in 1 h, and 2C is the charge and discharge current at which the nominal capacity can be charged in 0.5 h. For example, for a lithium-ion secondary battery with a nominal capacity of 100 μAh, the current at 0.1C is 10 μA (calculation formula: 100 μA × 0.1 = 10 μA). Similarly, the current at 0.2C is 20 μA, and the current at 1C is 100 μA.

[0125] The output characteristics were evaluated under the following conditions. At room temperature, the battery was charged at a constant current of 0.2 C (CC charging) until the battery voltage reached 1.6 V, and then charged at a constant voltage of 0.05 C (CV charging). After a 5-minute rest period, the battery was discharged at a constant current of 0.2 C until the battery voltage reached 0 V (CC discharge). The resulting discharge capacity was defined as the 0.2 C discharge capacity.

[0126] The battery was then charged at room temperature at a constant current of 0.2 C (CC charging) until the battery voltage reached 1.6 V, and then at a constant voltage of 0.05 C (CV charging). After a 5-minute rest period, the battery was discharged at a constant current of 1.0 C until the battery voltage reached 0 V (CC discharge). The resulting discharge capacity was designated the 1.0 C discharge capacity.

[0127] The ratio of the 1.0 C discharge capacity to the 0.2 C discharge capacity was calculated as the output characteristic in this example by the following formula (1). Output characteristics (%) = (1.0C discharge capacity ÷ 0.2C discharge capacity) × 100 (1)

[0128] Table 1 below shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 1 to 18 and Comparative Examples 1 to 8.

[0129] [Table 1]

[0130] From the results of Examples 1 to 9 and Comparative Examples 1 to 4, it can be confirmed that excellent output characteristics can be obtained when the ratio t1 / t2 of the average thickness t2 of the thinnest solid electrolyte layer to the average thickness t1 of the thickest solid electrolyte layer is in the range of 1.02≦t1 / t2≦1.99.

[0131] (Examples 19 to 26) Except for changing the standard deviation σ of the average thickness of the solid electrolyte layer by changing the electrode layer unit used when preparing the laminate, evaluation cells for Examples 10 to 17 were prepared in the same manner as in Example 1 and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0132] (Examples 27 to 34) Except for changing the standard deviation σ of the average thickness of the solid electrolyte layer by changing the electrode layer unit used when preparing the laminate, evaluation cells for Examples 27 to 34 were prepared in the same manner as in Example 10 and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0133] [Table 2]

[0134] From the results of Examples 19 to 34, it can be confirmed that excellent output characteristics can be obtained when the standard deviation σ of the average thickness of the solid electrolyte layer is in the range of 0.15≦σ≦1.66 μm.

[0135] Example 35 (Preparation of Intermediate Layer Paste) To prepare the intermediate layer base material, the lithium vanadium phosphate powder prepared in Example 1 and the lithium titanium aluminum phosphate powder were wet mixed in a ball mill for 16 hours, followed by dehydration and drying. After drying, the resulting powder was calcined in a nitrogen-hydrogen mixed gas at 850°C for 2 hours. The calcined product was wet-pulverized in a ball mill, then dehydrated and dried to obtain an intermediate layer base material powder.

[0136] To 100 parts of the obtained intermediate layer base powder, 15 parts of ethyl cellulose as a binder and 65 parts of dihydroterpineol as a solvent were added, and the mixture was mixed and dispersed to prepare an intermediate layer paste.

[0137] An electrode layer unit was produced in the same manner as in Example 3, except that in the production of the electrode layer unit, an intermediate layer having a thickness of 2 μm was formed on a solid electrolyte sheet by screen printing using an intermediate layer paste.

[0138] Example 36 An electrode layer unit was produced in the same manner as in Example 35, except that titanium oxide (TiO2) was used as the base powder for the intermediate layer in producing the paste for the intermediate layer.

[0139] Example 37 An electrode layer unit was produced in the same manner as in Example 35, except that in producing the intermediate layer paste, aluminum oxide (Al2O3) was used as the intermediate layer base powder.

[0140] Example 38 An electrode layer unit was produced in the same manner as in Example 35, except that in producing the intermediate layer paste, zirconium oxide (ZrO2) was used as the intermediate layer base powder.

[0141] Example 39 An electrode layer unit was produced in the same manner as in Example 12, except that in producing the intermediate layer paste, zirconium oxide (ZrO2) was used as the intermediate layer base powder.

[0142] The cross section of the obtained electrode layer unit was observed using a scanning electron microscope energy dispersive X-ray spectrometer (SEM-EDS) to analyze the constituent elements contained in the intermediate layer.

[0143] Except for using the obtained electrode layer unit, evaluation cells of Examples 35 to 39 were produced in the same manner as in Example 3, and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 3.

[0144] [Table 3]

[0145] The results of Examples 35 to 38 confirmed that the presence of an intermediate layer between the solid electrolyte layer and the electrode layer improved the output characteristics. Furthermore, a comparison of Examples 38 and 39 confirmed that the output characteristics were improved not by the composition of the intermediate layer but by the elements constituting the intermediate layer.

[0146] Example 40 In the preparation of the active material paste, lithium iron phosphate (LiFePO4) was used as the active material powder to prepare the positive electrode active material paste, and lithium titanate (Li4Ti5O 12 ) was used to prepare a negative electrode active material paste.

[0147] Except for using the prepared positive electrode active material layer paste and negative electrode active material layer paste, electrode layer unit sheets were prepared in the same manner as in Example 1. The electrode layer unit prepared using the positive electrode active material layer paste was designated as the positive electrode layer unit, and the electrode layer unit prepared using the negative electrode active material layer paste was designated as the negative electrode layer unit.

[0148] An evaluation cell of Example 40 was produced in the same manner as in Example 1, except that in producing the laminate, the obtained plurality of positive electrode layer units and negative electrode layer units were alternately stacked while being offset so that one end of the positive electrode layer unit did not coincide with one end of the negative electrode layer unit.

[0149] (Examples 41 to 48, Comparative Examples 9 to 12) The evaluation cells of Examples 41 to 48 and Comparative Examples 9 to 12 were produced in the same manner as in Example 39, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when producing the laminate.

[0150] (output characteristics evaluation) The output characteristics were evaluated under the following conditions. At room temperature, the battery was charged at a constant current of 0.2 C (CC charging) until the battery voltage reached 3.0 V, and then charged at a constant voltage of 0.05 C (CV charging). After a 5-minute rest period, the battery was discharged at a constant current of 0.2 C until the battery voltage reached 1.5 V (CC discharge). The resulting discharge capacity was defined as the 0.2 C discharge capacity.

[0151] The battery was then subjected to constant current charging (CC charging) at a constant current of 0.2 C at room temperature until the battery voltage reached 3.0 V, and then constant voltage charging (CV charging) at a current value of 0.05 C. After a 5-minute rest period, the battery was discharged at a constant current of 1.0 C until the battery voltage reached 1.5 V (CC discharging). The resulting discharge capacity was designated the 1.0 C discharge capacity.

[0152] The ratio of the 1.0 C discharge capacity to the 0.2 C discharge capacity was calculated as the output characteristic in this example by the following formula (2). Output characteristics (%) = (1.0C discharge capacity ÷ 0.2C discharge capacity) × 100 (2)

[0153] Table 4 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 40 to 48 and Comparative Examples 9 to 12.

[0154] [Table 4]

[0155] The results of Examples 40 to 48 and Comparative Examples 9 to 12 confirm that even in lithium ion secondary batteries having different positive electrode active materials and negative electrode active materials, excellent output characteristics can be obtained when the ratio t1 / t2 of the average thickness t1 of the thickest solid electrolyte layer to the average thickness t2 of the thinnest solid electrolyte is in the range of 1.02≦t1 / t2≦1.99.

[0156] Example 49 (Preparation of Paste-03 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-03, which was prepared using the following method. First, Li2CO3 and SiO2 were mixed and fired at 800°C to synthesize a precursor. The resulting precursor was mixed with Li3PO4, pressed at a pressure of 34.5 MPa, and fired at 1000°C. Surface impurities were then removed by heat treatment at 400°C. After the heat treatment, the mixture was dry-pulverized in a ball mill for 8 hours to obtain solid electrolyte powder-03.

[0157] The obtained solid electrolyte powder-03 was analyzed by an X-ray diffractometer and found to be Li 3.6 Si 0.6 P 0.4 It was confirmed that the compound has a similar crystal structure to O4.

[0158] Next, 100 parts of the obtained solid electrolyte powder-03 was wet-mixed in a ball mill with 100 parts of ethanol and 200 parts of toluene as a solvent, followed by further adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-03.

[0159] (Production of solid electrolyte layer sheet-03) The obtained paste-03 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-03 for solid electrolyte layer with different thicknesses were produced.

[0160] (Production of outermost layer sheet-03) The obtained solid electrolyte layer paste-03 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-03.

[0161] (Preparation of margin layer paste-03) Margin layer paste-03 was prepared by adding 100 parts of the obtained solid electrolyte powder-03, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-03.

[0162] An evaluation cell for Example 49 was prepared in the same manner as in Example 40, except that solid electrolyte sheet-03, outermost layer sheet-03, and margin layer paste-03 were used when preparing the laminate.

[0163] (Examples 50 to 57, Comparative Examples 13 to 16) The evaluation cells of Examples 50 to 57 and Comparative Examples 13 to 16 were produced in the same manner as in Example 49, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when producing the laminate.

[0164] (output characteristics evaluation) Table 5 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 49 to 57 and Comparative Examples 13 to 16. The output characteristics were evaluated under the same evaluation conditions as in Example 40.

[0165] [Table 5]

[0166] Example 58 (Preparation of Paste-04 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-04, which was prepared using the following method. First, LiCO3, La(OH)3, and ZrO2 were dispersed in ethanol as starting materials, and then wet-mixed in a ball mill for 12 hours. After mixing and drying, the resulting powder was heat-treated at 900°C for 5 hours. After the heat treatment, the powder was dry-pulverized in a ball mill for 12 hours to obtain solid electrolyte powder-04.

[0167] The solid electrolyte powder-04 obtained was analyzed using an X-ray diffractometer and found to be Li7La3Zr2O 12 It was confirmed that the compound has the same crystal structure as that of

[0168] Next, 100 parts of the obtained solid electrolyte powder-04 was wet-mixed with 100 parts of ethanol and 200 parts of toluene as a solvent in a ball mill, followed by further adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-04.

[0169] (Production of solid electrolyte layer sheet-04) The obtained paste-04 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-04 for solid electrolyte layer with different thicknesses were produced.

[0170] (Production of outermost layer sheet-04) The obtained solid electrolyte layer paste-04 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-04.

[0171] (Preparation of margin layer paste-04) Margin layer paste-04 was prepared by adding 100 parts of the obtained solid electrolyte powder-04, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-04.

[0172] An evaluation cell for Example 58 was prepared in the same manner as in Example 40, except that solid electrolyte sheet-04, margin layer paste-04, and outermost layer sheet-04 were used when preparing the laminate.

[0173] (Examples 59 to 66, Comparative Examples 17 to 20) The evaluation cells of Examples 59 to 66 and Comparative Examples 17 to 20 were prepared in the same manner as in Example 58, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when preparing the laminate.

[0174] (output characteristics evaluation) Table 6 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 58 to 66 and Comparative Examples 17 to 20. The output characteristics were evaluated under the same evaluation conditions as in Example 40.

[0175] [Table 6]

[0176] Example 67 (Preparation of Paste-05 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-05, which was prepared using the following method. First, LiCO3, La2O3, and TiO2 were used as starting materials and dry-mixed in an agate mortar. After mixing, the resulting powder was heat-treated at 1100°C for 12 hours, and then sintered at 1250°C for 5 hours. After sintering, it was rapidly cooled to room temperature and dry-pulverized in a ball mill for 12 hours to obtain solid electrolyte powder-05.

[0177] The obtained solid electrolyte powder-05 was analyzed by an X-ray diffractometer and found to be Li 0.56 Li 0.31 It was confirmed that the compound has a crystal structure similar to that of TiO3.

[0178] Next, 100 parts of the obtained solid electrolyte powder-05 was wet-mixed with 100 parts of ethanol and 200 parts of toluene as a solvent in a ball mill, followed by further adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-05.

[0179] (Production of solid electrolyte layer sheet-05) The obtained paste-05 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-05 for solid electrolyte layer with different thicknesses were produced.

[0180] (Production of outermost layer sheet-05) The obtained solid electrolyte layer paste-05 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-05.

[0181] (Preparation of margin layer paste-05) Margin layer paste-05 was prepared by adding 100 parts of the obtained solid electrolyte powder-05, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-05.

[0182] In preparing the active material paste, lithium manganese oxide (LiMn2O4) was used as the active material powder to prepare the active material paste for the positive electrode and the active material paste for the negative electrode.

[0183] An evaluation cell of Example 67 was produced in the same manner as in Example 40, except that the obtained positive electrode active material layer paste, negative electrode active material layer paste, solid electrolyte sheet-05, outermost layer sheet-05, and margin layer paste-05 were used when producing a laminate.

[0184] (Examples 68 to 75, Comparative Examples 21 to 24) The evaluation cells of Examples 68 to 75 and Comparative Examples 21 to 24 were produced in the same manner as in Example 67, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when producing the laminate.

[0185] (output characteristics evaluation) The output characteristics were evaluated under the following conditions. At room temperature, the battery was charged at a constant current of 0.2 C (CC charging) until the battery voltage reached 2.0 V, and then charged at a constant voltage of 0.05 C (CV charging). After a 5-minute rest period, the battery was discharged at a constant current of 0.2 C until the battery voltage reached 0.5 V (CC discharge). The resulting discharge capacity was defined as the 0.2 C discharge capacity.

[0186] The battery was then subjected to constant current charging (CC charging) at a constant current of 0.2 C at room temperature until the battery voltage reached 2.0 V, and then constant voltage charging (CV charging) at a current value of 0.05 C. After a 5-minute rest period, the battery was discharged at a constant current of 1.0 C until the battery voltage reached 0.5 V (CC discharging). The resulting discharge capacity was designated the 1.0 C discharge capacity.

[0187] The ratio of the 1.0 C discharge capacity to the 0.2 C discharge capacity was calculated as the output characteristic in this example by the following formula (3). Output characteristics (%) = (1.0C discharge capacity ÷ 0.2C discharge capacity) × 100 (3)

[0188] Table 7 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 67 to 68 and Comparative Examples 21 to 24.

[0189] [Table 7]

[0190] Example 76 (Preparation of Paste-06 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-06, which was prepared using the following method. First, LiOH·H2O and H3BO3 were mixed, placed in an alumina crucible, and heat-treated in an air atmosphere at 600°C for 3 hours to obtain precursor A. Next, Li2SO4·H2O was heat-treated in an air atmosphere at 300°C for 2 hours to obtain precursor B. The obtained precursors A and B were mixed and mechanically milled in a ball mill for 100 hours to obtain solid electrolyte powder-06.

[0191] The obtained solid electrolyte powder-06 was analyzed using an X-ray diffractometer and confirmed to be a compound with the same crystal structure as Li3BO3-Li2SO4 glass ceramic.

[0192] Next, 100 parts of the obtained solid electrolyte powder-06 was wet-mixed with 100 parts of ethanol and 200 parts of toluene as a solvent in a ball mill, followed by further adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-06.

[0193] (Production of solid electrolyte layer sheet-06) The obtained paste-06 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-06 for solid electrolyte layer with different thicknesses were produced.

[0194] (Production of outermost layer sheet-06) The obtained solid electrolyte layer paste-06 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by the doctor blade method, to obtain an outermost layer sheet-06.

[0195] (Preparation of margin layer paste-06) Margin layer paste-06 was prepared by adding 100 parts of the obtained solid electrolyte powder-06, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-06.

[0196] An evaluation cell of Example 76 was prepared in the same manner as in Example 40, except that solid electrolyte sheet-06, outermost layer sheet-06, and margin layer paste-06 were used when preparing the laminate.

[0197] (Examples 77 to 84, Comparative Examples 25 to 28) The evaluation cells of Examples 77 to 84 and Comparative Examples 25 to 28 were prepared in the same manner as in Example 67, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when preparing the laminate.

[0198] (output characteristics evaluation) Table 8 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 76 to 84 and Comparative Examples 25 to 28. The output characteristics were evaluated under the same evaluation conditions as in Example 40.

[0199] [Table 8]

[0200] Example 85 (Preparation of Paste-07 for Solid Electrolyte Layer) The solid electrolyte used was solid electrolyte powder-07, which was prepared using the following method. First, LiOH·H2O and H3BO3 were mixed, placed in an alumina crucible, and heat-treated in an air atmosphere at 600°C for 3 hours to obtain precursor A. Next, Li2SO4·H2O was heat-treated in an air atmosphere at 300°C for 2 hours to obtain precursor B. The obtained precursors A and B were mixed with Li2CO3, and mechanical milling was performed in a ball mill for 100 hours to obtain solid electrolyte powder-07.

[0201] The obtained solid electrolyte powder-07 was analyzed using an X-ray diffractometer and confirmed to be a compound with the same crystal structure as Li3BO3-Li2SO4-Li2CO3 glass ceramic.

[0202] Next, 100 parts of the obtained solid electrolyte powder-07 was wet-mixed in a ball mill with 100 parts of ethanol and 200 parts of toluene as a solvent, followed by further addition of 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing to prepare solid electrolyte layer paste-07.

[0203] (Production of solid electrolyte layer sheet-07) The obtained paste-07 for solid electrolyte layer was used to form a sheet using a doctor blade method with a PET film as a substrate to obtain a sheet for solid electrolyte layer. At this time, by adjusting the thickness in the range of 5 to 15 μm, multiple sheets-07 for solid electrolyte layer with different thicknesses were produced.

[0204] (Production of outermost layer sheet-07) The obtained solid electrolyte layer paste-07 was used to form a sheet having a thickness of 30 μm using a PET film as a substrate by a doctor blade method, to obtain an outermost layer sheet-07.

[0205] (Preparation of margin layer paste-07) Margin layer paste-07 was prepared by adding 100 parts of the obtained solid electrolyte powder-06, 100 parts of ethanol and 100 parts of toluene as solvents, and wet-mixing them in a ball mill, followed by adding 16 parts of a polyvinyl butyral binder and 4.8 parts of benzyl butyl phthalate and mixing them to prepare margin layer paste-07.

[0206] An evaluation cell of Example 85 was prepared in the same manner as in Example 40, except that solid electrolyte sheet-07, outermost layer sheet-07, and margin layer paste-07 were used when preparing the laminate.

[0207] (Examples 86 to 93, Comparative Examples 29 to 32) The evaluation cells of Examples 86 to 93 and Comparative Examples 29 to 32 were produced in the same manner as in Example 85, except that the standard deviation σ of the average thickness of the solid electrolyte layer was changed by changing the positive electrode layer unit and the negative electrode unit used when producing the laminate.

[0208] (output characteristics evaluation) Table 9 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 85 to 93 and Comparative Examples 29 to 32. The output characteristics were evaluated under the same evaluation conditions as in Example 40.

[0209] [Table 9]

[0210] Example 94 (Production of solid electrolyte layer sheet-08) The solid electrolyte sheet used was solid electrolyte sheet-08, which was prepared by the following method. The preparation method involved first dissolving and mixing polyethylene oxide (PEO) with a molecular weight of 5 million and LiCF3SO3 (LiTFS) in acetonitrile in an argon-filled glove box, and then dropping the mixture onto a Teflon sheet ("Teflon" is a registered trademark). After the dropping, a sheet was formed using the Teflon sheet as a substrate by the doctor blade method, dried at room temperature for 24 hours, and then vacuum-dried at 60°C to obtain solid electrolyte layer sheet-07. The thickness was adjusted within the range of 5 to 15 μm to prepare multiple solid electrolyte layer sheets-08 with different thicknesses.

[0211] (Preparation of positive electrode sheet) As the positive electrode active material, 100 parts of LiFePO4, 10 parts of acetylene black, and 10 parts of polyvinylidene fluoride were weighed and dispersed in N-methylpyrrolidone as a solvent to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to a portion of one side of a 10 μm thick aluminum foil to a thickness of 30 μm, and the solvent was removed by drying at 100 ° C. After removing the solvent, the positive electrode slurry was similarly applied to a portion of the other side of the aluminum foil to a thickness of 30 μm, and the solvent was removed by drying at 100 ° C. An active material layer was formed on each side of the aluminum foil.

[0212] After forming active material layer regions on both sides of the aluminum foil, the foil was rolled using a roll press and then punched out using a die to form a positive electrode sheet with an electrode size of 27 mm × 30 mm, including a region where no active material layer was present.

[0213] (Preparation of negative electrode sheet) Li4Ti5O as the negative electrode active material 12100 parts of the above, 10 parts of acetylene black, and 10 parts of polyvinylidene fluoride were weighed and dispersed in N-methylpyrrolidone as a solvent to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to a portion of one side of a 10 μm thick aluminum foil to a thickness of 30 μm, and the solvent was removed by drying at 100 ° C. After removing the solvent, the positive electrode slurry was similarly applied to a portion of the other side of the aluminum foil to a thickness of 30 μm, and the solvent was removed by drying at 100 ° C., forming active material layers on both sides of the aluminum foil.

[0214] After forming active material layer regions on both sides of the aluminum foil, the foil was rolled using a roll press and then punched out using a die to produce a negative electrode sheet with an electrode size of 28 mm × 31 mm, including a region where no active material layer was present.

[0215] (Preparation of laminate) The resulting 23 positive electrode sheets and 24 negative electrode sheets were stacked with the solid electrolyte sheet-08 interposed between them and hot-pressed at 50°C to produce a laminate. Furthermore, aluminum leads were attached to the regions of the positive electrode sheet where no active material layer was present and to the regions of the negative electrode sheet where no active material layer was present, respectively, using an ultrasonic fusion machine. The laminate was then fused to an aluminum laminate film for the exterior body, and the laminate film was folded to insert the electrode body into the exterior body. A closed section was formed by heat-sealing the exterior body except for one side, and the opening was sealed by heat-sealing while reducing the pressure using a vacuum sealer, to produce the evaluation cell for Example 94.

[0216] (Examples 95 to 102, Comparative Examples 33 to 36) The evaluation cells of Examples 95 to 102 and Comparative Examples 33 to 36 were prepared in the same manner as in Example 94, except that the thickness of the solid electrolyte sheet-08 used in preparing the laminate was adjusted to change the standard deviation σ of the average thickness of the solid electrolyte layer.

[0217] (output characteristics evaluation) Table 10 shows t1, t2, T, the calculated standard deviation σ of the solid electrolyte layer, and the evaluation results of the output characteristics in Examples 94 to 102 and Comparative Examples 33 to 36. The output characteristics were evaluated under the same evaluation conditions as in Example 40.

[0218] [Table 10]

[0219] The results in Tables 5 to 9 confirm that even if the composition of the solid electrolyte powder used in the solid electrolyte sheet is changed, excellent output characteristics can be obtained by controlling the ratio t1 / t2 of the average thickness t1 of the thickest solid electrolyte layer to the average thickness t2 of the thinnest solid electrolyte. Furthermore, the results in Table 10 confirm that the output characteristics are similarly improved even when the battery configuration is different. [Industrial Applicability]

[0220] The present invention makes it possible to provide lithium ion secondary batteries with high output characteristics, which are suitable for use as power sources for portable electronic devices, electric vehicles, and home and industrial storage batteries. [Explanation of symbols]

[0221] 1. Lithium-ion secondary battery 20··Laminate 30...Positive electrode 31 Positive electrode current collector layer 32...Cathode active material layer 40...Negative electrode 41 Negative electrode current collector layer 42...Negative electrode active material layer 50...Solid electrolyte layer 60 Positive external electrode 70 Negative external electrode 80...Margin layer

Claims

1. A lithium ion secondary battery in which a positive electrode layer containing a positive electrode active material and a negative electrode layer containing a negative electrode active material are stacked in this order with a solid electrolyte layer interposed therebetween, the lithium ion secondary battery has a thinnest solid electrolyte layer at one end side of the lithium ion secondary battery in a stacking direction, and a thickest solid electrolyte layer at the other end side of the lithium ion secondary battery, a ratio t1 / t2 of an average thickness t1 of the thickest solid electrolyte layer to an average thickness t2 of the thinnest solid electrolyte layer in the average thickness t of each layer in the solid electrolyte layer is 1.02≦t1 / t2≦1.99; Lithium-ion secondary battery.

2. In the solid electrolyte layer, the standard deviation σ of the average thickness t of each layer is 0.15≦σ≦1.66 (μm). The lithium ion secondary battery according to claim 1 .

3. At least a portion between the positive electrode layer or the negative electrode layer and the solid electrolyte layer is an intermediate layer having the positive electrode layer or the negative electrode layer and a constituent element of the solid electrolyte layer; The lithium ion secondary battery according to claim 1 or 2.

4. In the solid electrolyte layer, the average thickness t of each layer is 4.8≦T≦9.8 (μm), The lithium ion secondary battery according to any one of claims 1 to 3.

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