Lithium secondary battery

Abstract

To provide a lithium secondary battery which has excellent production yield, and includes a sintered body capable of being produced stably and efficiently.SOLUTION: A lithium secondary battery comprises a laminate composed of a sintered body in which a positive electrode layer 12 and a negative electrode layer 16 are alternately laminated through a separator 20. In an interface region of the positive electrode layer and the separator, a relation of c1 / c2≤0.9 is satisfied, where c1 (mol%) is a concentration of a transition metal element constituting the positive electrode layer at a location 5 μm away from an end face of the positive electrode layer in the separator, and c2 (mol%) is a concentration of the transition metal element at a location 5 μm away from a principal face of the positive electrode layer in the separator. In an interface region of the negative electrode layer and the separator, a relation of c3 / c4≤0.9 is satisfied, where c3 (mol%) is a concentration of a transition metal element constituting the negative electrode layer at a location 5 μm away from an end of the negative electrode layer in the separator, and c4 (mol%) is a concentration of the transition metal element at a location 5 μm away from a principal face of the negative electrode layer in the separator.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to lithium secondary batteries. [Background technology]

[0002] Known lithium secondary batteries include a positive electrode layer made of a sintered lithium composite oxide, a negative electrode layer made of a sintered titanium-containing material, and a ceramic separator disposed between the positive and negative electrode layers. For example, Patent Document 1 discloses a lithium secondary battery that includes a positive electrode layer, a ceramic separator, and a negative electrode layer bonded together to form an integrated sintered plate, and that is impregnated with an electrolyte. The separator included in the lithium secondary battery disclosed in Patent Document 1 is a ceramic separator made of MgO and glass.

[0003] Patent Document 2 discloses an all-solid-state battery having a stack in which multiple positive electrode layers and multiple negative electrode layers are alternately stacked with solid electrolyte layers interposed therebetween. The stack disclosed in Patent Document 2 has a buffer layer within the solid electrolyte layer. The buffer layer is a layer formed by combining a metal portion and a void portion. The buffer layer may be provided within the solid electrolyte layer that is the outermost layer of the stack, or within a solid electrolyte layer located in the middle of the stack. Furthermore, the buffer layer may be provided within a side margin layer that is provided adjacent to and on the outer periphery of the positive electrode layer or the negative electrode layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 221144 [Patent Document 2] Patent Publication No. 2021-27044 Summary of the Invention [Problem to be solved by the invention]

[0005] In lithium secondary batteries, it is desired to be able to stably and efficiently produce sintered bodies that constitute electrodes.

[0006] Therefore, one of the objects of the invention according to the present disclosure is to provide a lithium secondary battery including a sintered body that can be produced stably and efficiently with a good production yield. [Means for solving the problem]

[0007] A lithium secondary battery according to the present disclosure includes a sintered laminate including multiple positive electrode layers, multiple negative electrode layers, and a separator, in which the positive electrode layers and the negative electrode layers are alternately stacked with the separator interposed therebetween. At an interface between the positive electrode layer and the separator, a concentration c1 (mol %) of a transition metal element constituting the positive electrode at a position in the separator 5 μm away from an end face of the positive electrode layer and a concentration c2 (mol %) of the transition metal element constituting the positive electrode at a position in the separator 5 μm away from a main surface of the positive electrode layer satisfy the relationship c1 / c2≦0.9. In the interfacial region between the negative electrode layer and the separator, a concentration c3 (mol %) of the transition metal element constituting the negative electrode at a position 5 μm away from an end face of the negative electrode layer in the separator and a concentration c4 (mol %) of the transition metal element constituting the negative electrode at a position 5 μm away from a main surface of the negative electrode layer in the separator satisfy the relationship c3 / c4≦0.9. [Effects of the Invention]

[0008] According to the above lithium secondary battery, a lithium secondary battery including a sintered body that can be produced stably and efficiently with a good production yield is provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional perspective view showing a laminate included in a lithium secondary battery according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram showing a cross section of a laminate included in a lithium secondary battery according to the present disclosure. [Figure 3] FIG. 3 is a schematic diagram showing part of the process for producing a laminate included in a lithium secondary battery according to the present disclosure. [Figure 4] FIG. 4 is a schematic diagram showing part of the process for producing a laminate included in a lithium secondary battery according to the present disclosure. [Figure 5] FIG. 5 is a schematic diagram showing a state in which a current collector is added to the laminate shown in FIG. [Figure 6] FIG. 6 is a schematic cross-sectional view showing a lithium secondary battery according to the present disclosure. [Figure 7] FIG. 7 is an SEM image showing measurement positions for composition analysis in a laminate included in a lithium secondary battery according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Outline of the embodiment] First, embodiments of the present disclosure will be described. A lithium secondary battery according to the present disclosure includes a sintered laminate including multiple positive electrode layers, multiple negative electrode layers, and a separator, in which the positive electrode layers and the negative electrode layers are alternately stacked with the separator interposed therebetween. In an interface region between the positive electrode layer and the separator, a concentration c1 (mol %) of a transition metal element constituting the positive electrode at a position 5 μm away from an end face of the positive electrode layer in the separator and a concentration c2 (mol %) of a transition metal element constituting the positive electrode at a position 5 μm away from a main surface of the positive electrode layer in the separator satisfy the relationship c1 / c2≦0.9. In the interfacial region between the negative electrode layer and the separator, a concentration c3 (mol %) of the transition metal element constituting the negative electrode at a position 5 μm away from an end face of the negative electrode layer in the separator and a concentration c4 (mol %) of the transition metal element constituting the negative electrode at a position 5 μm away from a main surface of the negative electrode layer in the separator satisfy the relationship c3 / c4≦0.9.

[0011] Conventionally, lithium secondary batteries have been known that include a laminate including multiple positive electrode layers and multiple negative electrode layers, with multiple cells configured within one electrode (for example, Patent Document 2). The all-solid-state battery described in Patent Document 2 includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, and further has a buffer layer made up of a metal portion and a void portion within the solid electrolyte layer. Patent Document 2 also describes that the provision of the buffer layer suppresses the occurrence of cracks that accompany charge and discharge.

[0012] As the applications of lithium secondary batteries expand, expectations are growing for integrated sintered electrodes in which a positive electrode, a negative electrode, and a separator are sintered together. However, it is difficult to stably manufacture, with a high yield, stacked integrated sintered electrodes in which multiple components with different compositions are repeatedly stacked. One factor that reduces yield is delamination in the laminate. To address this issue, the inventors discovered that in electrodes with a stacked structure, delamination tends to occur in the interface region between the positive electrode layer or negative electrode layer and the surrounding separator. They also found that delamination is suppressed when the elemental composition of the separator differs between the interface region on the main surface side of the positive electrode layer or negative electrode layer and the interface region on the end surface side. More specifically, it was found that peeling of the laminate is effectively suppressed when the concentration of the transition metal element constituting the positive electrode (or negative electrode) at a position 5 μm away from the end face of the positive electrode layer (or negative electrode layer) in the separator and the concentration of the same metal element at a position 5 μm away from the main surface of the positive electrode layer (or negative electrode layer) satisfy a specific relationship.

[0013] Specifically, in the lithium secondary battery according to the present disclosure, in the interface region between the positive electrode layer and the separator, the concentration c1 (mol %) of the transition metal element constituting the positive electrode at a position 5 μm away from the end face of the positive electrode layer in the separator and the concentration c2 (mol %) of the metal element at a position 5 μm away from the main surface of the positive electrode layer in the separator satisfy the relationship c1 / c2≦0.9. Furthermore, in the interface region between the negative electrode layer and the separator, the concentration c3 (mol %) of the transition metal element constituting the negative electrode at a position 5 μm away from the end face of the negative electrode layer in the separator and the concentration c4 (mol %) of the metal element at a position 5 μm away from the main surface of the negative electrode layer in the separator satisfy the relationship c3 / c4≦0.9. When this configuration is satisfied, the occurrence of delamination in the laminate is reduced, and lithium secondary batteries can be produced with high yield. Without being bound by any particular theory, it is believed that the migration of elements from the positive electrode layer or negative electrode layer to the separator indicates the degree of bonding between the positive electrode layer or negative electrode layer and the separator. In other words, the more elements that migrate from the positive electrode layer (negative electrode layer) to the separator, the greater the degree of bonding between the positive electrode layer (negative electrode layer) and the separator. It has been found that when the degree of bonding is relatively strong on the main surface side and relatively weak on the edge surface side, peeling after firing is less likely to occur. Without being bound by any particular theory, it is believed that the stronger the bonding between the separator and positive electrode (negative electrode) on the main surface side, which has a larger area, and the weaker the bonding between the separator and positive electrode (negative electrode) on the edge surface side, which has a smaller area, the stronger the bonding between the separator and positive electrode (negative electrode) on the edge surface side, which has a smaller area, and the stronger the bonding between the separator and positive electrode (negative electrode) on the edge surface side, which absorbs the strain that occurs between the separator and positive electrode (negative electrode), making peeling less likely to occur.

[0014] At least one of the transition metal elements constituting the positive electrode may be Co, and at least one of the transition metal elements constituting the negative electrode may be Ti. With this configuration, a lithium secondary battery can be produced more stably using materials commonly used for positive and negative electrodes.

[0015] The thickness of the separator disposed between the positive electrode layer and the negative electrode layer may be 5 to 60 μm. When the thickness of the separator is in this range, the separator reliably separates the positive electrode layer from the negative electrode layer, and the dimension (thickness) of the laminate in the stacking direction can be reduced, resulting in a compact laminate.

[0016] In the laminate, the positive electrode layers, the negative electrode layers, and the separator may be an integrally formed sintered body. By using the integral sintered body, it is possible to obtain advantages such as excellent handling properties as an electrode and production at reasonable cost.

[0017] In the lithium secondary battery, the number of cells formed by the positive electrode layer and the negative electrode layer facing each other with the separator interposed therebetween may be 3 to 200. When the number of cells is within this range, a lithium secondary battery having a practical configuration and function as a lithium secondary battery and capable of being manufactured by a rational process can be obtained.

[0018] [Specific example of embodiment] Next, specific embodiments of the lithium secondary battery of the present disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and their description will not be repeated.

[0019] (lithium secondary battery) First, an overview of a lithium secondary battery according to the present disclosure will be described. Fig. 6 is a schematic cross-sectional view showing the structure of a lithium secondary battery 10, which is one embodiment according to the present disclosure. In Fig. 6, like components are indicated by like hatching, and some reference numerals are omitted. This also applies to other figures. With reference to Fig. 6, the X-axis direction is the width direction of the laminate 1, and the Z-axis direction is the stacking direction or thickness direction of the laminate 1.

[0020] Referring to FIG. 6, a lithium secondary battery 10 includes an electrode 5 housed inside an exterior case 24. The electrode 5 includes a laminate 1 in which a plurality of positive electrode layers 12, a plurality of negative electrode layers 16, and a separator 20 are stacked. The positive electrode layers 12 and the negative electrode layers 16 are alternately stacked in the stacking direction. The separator 20 is interposed between the positive electrode layers 12 and the negative electrode layers 16. The separator 20 separates the positive electrode layers 12 and the negative electrode layers 16 from each other. The positive electrode layers 12 are made of, for example, a sintered body containing lithium cobalt oxide. The negative electrode layers 16 are made of, for example, a sintered body containing titanium. The separator 20 is made of ceramic.

[0021] An enclosed space is formed inside the exterior body 24. The electrodes 5 and the electrolyte solution 22 are accommodated in this enclosed space. In the lithium secondary battery 10, the electrolyte solution 22 is sealed inside the exterior body 24. The positive electrode layer 12, the negative electrode layer 16, and the separator 20 are also impregnated with the electrolyte solution 22.

[0022] The positive electrode layer 12, the separator 20, and the negative electrode layer 16 are collectively one integral sintered body. That is, the positive electrode layer 12, the separator 20, and the negative electrode layer 16 are bonded to one another. In this specification, the term "integral sintered body" means that the components constituting the sintered body are connected and bonded to one another without relying on a bonding method other than sintering (for example, an adhesive).

[0023] The exterior body 24 may be selected appropriately depending on the type of lithium secondary battery 10. For example, when the lithium secondary battery 10 is in the form of a coin-shaped battery as shown in FIG. 6, the exterior body 24 typically includes a positive electrode can 24a, a negative electrode can 24b, and a gasket 24c, and the positive electrode can 24a and the negative electrode can 24b are crimped together via the gasket 24c to form a sealed space. The positive electrode can 24a and the negative electrode can 24b may be made of a metal such as stainless steel, but are not particularly limited thereto. The gasket 24c may be an annular member made of an insulating resin such as polypropylene, polytetrafluoroethylene, or PFA resin, but are not particularly limited thereto.

[0024] Although the lithium secondary battery 10 shown in FIG. 6 is in the form of a coin battery, the form of the lithium secondary battery according to the present disclosure is not limited to a coin battery. For example, other forms, such as a thin secondary battery including a chip-type secondary battery or a pouch-type secondary battery, may also be used. When the lithium secondary battery is a chip-type battery that can be built into a card, the exterior body is preferably a resin substrate, and the battery elements (i.e., the positive electrode layer 12, the negative electrode layer 16, the separator 20, and the electrolyte 22) are preferably embedded within the resin substrate. For example, when the lithium secondary battery is a pouch-type secondary battery, the battery elements may be sandwiched between a pair of resin films. The pair of resin films may be bonded together with an adhesive. Furthermore, the pair of resin films may be heat-sealed by a heat press. Furthermore, a separator made of a solid electrolyte may be used as the separator, and the separator may not contain an electrolyte.

[0025] Referring to FIG. 6, the lithium secondary battery 10 includes a positive electrode current collector 14 extending from the side surface to the bottom surface of the electrode 5. The lithium secondary battery 10 also includes a negative electrode current collector 18 extending from the side surface to the top surface of the electrode 5. The positive electrode current collector 14 and the negative electrode current collector 18 may be metal foils such as copper foil or aluminum foil. The positive electrode current collector 14 is preferably disposed between the positive electrode layer 12 and an outer casing 24 (e.g., a positive electrode can 24a). The negative electrode current collector 18 is preferably disposed between the negative electrode layer 16 and an outer casing 24 (e.g., a negative electrode can 24b). A positive electrode-side carbon layer (not shown) is preferably provided between the positive electrode layer 12 and the positive electrode current collector 14 to reduce contact resistance. Similarly, a negative electrode-side carbon layer (not shown) is preferably provided between the negative electrode layer 16 and the negative electrode current collector 18 to reduce contact resistance. The positive electrode carbon layer and the negative electrode carbon layer are preferably both made of conductive carbon, and can be formed, for example, by applying a conductive carbon paste to the surface of a metal foil used as a current collector.

[0026] (Laminate) A laminate included in a lithium secondary battery according to the present disclosure will be described. FIG. 1 is a schematic cross-sectional perspective view showing a laminate 1 included in a lithium secondary battery according to the present disclosure. Referring to FIG. 1, the laminate 1 is a laminate formed by stacking multiple layers. The laminate 1 has a rectangular parallelepiped shape, the outer shape of which is defined by a width W, a depth D, and a thickness T. Note that the term "rectangular parallelepiped" as used herein does not refer only to a rectangular parallelepiped in the mathematically precise sense, but also includes three-dimensional structures having shapes similar to rectangular parallelepipeds for design and manufacturing reasons. In the laminate 1, the direction parallel to the X-axis shown in FIG. 1 is referred to as the width direction of the laminate, the direction parallel to the Y-axis is referred to as the depth direction of the laminate, and the direction parallel to the Z-axis is referred to as the stacking direction or thickness direction of the laminate. In this specification, the surfaces of the laminate 1 on which all the stacked layers are exposed (the surfaces shown in cross section in FIG. 1) are referred to as the front and back sides. The front and back sides are surfaces parallel to the XZ plane. Furthermore, the surfaces of the laminate 1 on which the stacked structure is exposed, extending between the front and back sides and along the depth direction, are referred to as side sides.

[0027] Referring to Fig. 1, both the top and bottom layers of the laminate 1 are composed of a separator 20. In the laminate 1, a positive electrode layer 12 and a negative electrode layer 16 that face each other via the separator 20 form one cell. Five cells are formed in the laminate 1 of Fig. 1. The number of cells in the laminate included in the lithium secondary battery according to the present disclosure is not limited as long as the effects of the invention are achieved, but the laminate may include, for example, 3 to 200 cells.

[0028] The laminate 1 is formed by alternately stacking multiple positive electrode layers 12 and multiple negative electrode layers 16. The positive electrode layers 12 and negative electrode layers 16 constituting the laminate 1 are each rectangular plate-shaped. The widths of the positive electrode layers 12 and negative electrode layers 16 are both smaller than the width W of the laminate 1. The negative electrode layer 16 includes a current collector layer 19 on one of its main surfaces or inside in the thickness direction. The positive electrode layers 12 and negative electrode layers 16 are each exposed on only one of the side surfaces of the laminate 1. Specifically, each of the multiple positive electrode layers 12 is exposed on the first side surface s1 of the laminate 1, but not on the second side surface s2. The positive electrode layer 12 extends from the side surface s1 to the middle of the width direction of the laminate 1, with the end surface 12e being its end in the width direction. Furthermore, each of the multiple negative electrode layers 16 is exposed on the second side surface s2 of the laminate 1, but not on the first side surface s1. The negative electrode layer 16 extends from the side surface s2 to the middle of the laminate 1 in the width direction, and the end surface 16e is the end in the width direction.

[0029] A separator 20 is interposed between the positive electrode layer 12 and the negative electrode layer 16. The separator 20 includes a first region 21, a second region 22, and a third region 23. The first region 21 extends across the entire width W of the laminate 1 and is interposed between the positive electrode layer 12 and the negative electrode layer 16 in the thickness direction of the laminate 1, separating the positive electrode layer 12 from the negative electrode layer 16. The second region 22 is a region located on a side of the positive electrode layer 12 in the width direction. The second region 22 separates the positive electrode layer 12 from the negative electrode current collector 18 (FIG. 6). The thickness of the second region 22 is approximately equal to the thickness of the positive electrode layer 12. The third region 23 is a region located on a side of the negative electrode layer 16 in the width direction. The third region 23 separates the negative electrode layer 16 from the positive electrode current collector 14 (FIG. 6). The thickness of the third region 23 is approximately equal to the thickness of the negative electrode layer 16. Note that the first region 21, the second region 22, and the third region 23 are regions defined for the sake of convenience of explanation, and the separator 20 may be an integral, continuous structure.

[0030] The positive electrode layer 12 and the separator 20 are exposed on the first side surface s1 of the laminate 1, but the negative electrode layer 16 is not exposed. Similarly, the negative electrode layer 16 including the current collector layer 19 and the separator 20 are exposed on the second side surface s2 of the laminate 1, but the positive electrode layer 12 is not exposed. According to these configurations, by arranging the positive electrode current collector 14 (FIG. 6) on the first side surface s1 and the negative electrode current collector 18 (FIG. 6) on the second side surface s2, an electrode that efficiently extracts electricity from a small lithium secondary battery can be configured.

[0031] 2 is a schematic diagram showing a cross section of the laminate 1. Referring to FIG. 2, a first region 21 of the separator 20 contacts the main surface 12a of the positive electrode layer 12 and the main surface 16a of the negative electrode layer 16. A second region 22 of the separator 20 contacts the end surface 12e of the positive electrode layer 12. An interface region Ac between the separator 20 and the positive electrode layer 12 includes a first portion b1, which is the interface region between the separator 20 and the end surface 12e of the positive electrode layer 12, and a second portion b2, which is the interface region between the separator 20 and the main surface 12a of the positive electrode layer 12. A third region 23 of the separator 20 contacts the end surface 16e of the negative electrode layer 16. The interface region Aa between the separator 20 and the negative electrode layer 16 includes a third portion b3 which is the interface region between the separator 20 and the end face 16e of the negative electrode layer 16, and a fourth portion b4 which is the interface region between the separator 20 and the main surface 16a of the negative electrode layer 16.

[0032] In the lithium secondary battery according to the present disclosure, there is a difference in composition between the first portion b1 and the second portion b2 in the interface region Ac between the positive electrode layer 12 and the separator 20, and there is a difference in composition between the third portion b3 and the fourth portion b4 in the interface region Aa between the negative electrode layer 16 and the separator 20. Before providing a specific description, the configuration of each layer will be described.

[0033] (positive electrode layer) The positive electrode layer 12 is composed of a sintered body containing lithium cobalt oxide. The positive electrode layer 12 can be made to contain no binder or conductive assistant. Specific examples of lithium cobalt oxide include, for example, LiCoO2 (hereinafter may be abbreviated as LCO). As the plate-shaped LCO sintered body, those disclosed in, for example, Japanese Patent No. 5587052 and International Publication No. 2017 / 146088 can be used. The positive electrode layer 12 preferably is an oriented positive electrode layer that includes a plurality of primary particles composed of lithium cobalt oxide, and the plurality of primary particles are oriented at an average orientation angle of more than 0° and 30° or less with respect to the plane of the positive electrode layer. Examples of the structure, composition, and specific method of such an oriented positive electrode layer include those disclosed in Patent Document 1 (International Publication No. 2019 / 221144).

[0034] In addition to LCO, examples of lithium cobalt oxide constituting the primary particles in the positive electrode layer 12 include, for example, Li x NiCoO2 (lithium nickel cobalt oxide), Li x CoNiMnO2 (lithium cobalt nickel manganese oxide), Li x CoMnO2 (lithium cobalt manganese oxide), etc. Further, in addition to lithium cobalt oxide, other lithium composite oxides may be included. Examples of the lithium composite oxide include, for example, Li x MO2 (where 0.05 < x < 1.10, M is at least one kind of transition metal, and M typically includes one or more of Co, Ni, and Mn).

[0035] When the positive electrode layer 12 is composed of a plate-shaped sintered body containing LCO, the transition metal element among the elements constituting the positive electrode layer is Co. Further, when the positive electrode layer 12 is composed of a sintered body containing Li x NiCoO2 (lithium nickel cobalt oxide), the transition metal elements among the elements constituting the positive electrode layer are Ni and Co. Further, when the positive electrode layer 12 is Li xWhen the positive electrode layer is composed of a sintered body containing CoNiMnO2 (cobalt-nickel-manganese lithium oxide), the transition metal elements constituting the positive electrode layer are Ni, Co, and Mn. This also applies to positive electrodes other than lithium cobalt oxide-based positive electrodes. For example, when the positive electrode is composed of LiFePO4 (lithium iron phosphate), the transition metal element constituting the positive electrode layer is Fe. The transition metal element constituting the positive electrode layer may also be a transition metal element such as V (vanadium). The lithium secondary battery according to the present disclosure focuses on the content of the transition metal elements constituting the positive electrode layer at a specific position in the separator. When multiple types of transition metal elements are contained, the sum of their contents is taken as the transition metal element content. When multiple types of transition metal elements are contained, the transition metal elements contained at a certain percentage or more (e.g., 0.1% or more) may be selected as the "transition metal elements constituting the positive electrode layer."

[0036] The average particle size of the multiple primary particles constituting the positive electrode layer 12 is preferably 5 μm or more. Specifically, the average particle size of the primary particles used to calculate the average orientation angle is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 12 μm or more.

[0037] The positive electrode layer 12 may contain pores. When the sintered body contains pores, particularly open pores, the electrolyte can penetrate into the sintered body when the sintered body is incorporated into a battery as a positive electrode layer, thereby improving lithium ion conductivity. The porosity of the positive electrode layer 12 is preferably 20 to 60%, more preferably 25 to 55%, even more preferably 30 to 50%, and particularly preferably 30 to 45%. The porosity of the sintered body can be measured according to a known method.

[0038] The average pore diameter of the positive electrode layers 12 is preferably 0.1 to 10.0 μm, more preferably 0.2 to 5.0 μm, and even more preferably 0.25 to 3.0 μm. Within this range, localized stress concentration at large pores is suppressed, and stress within the sintered body is more easily released uniformly. Furthermore, the pores allow the electrolyte to penetrate inside, thereby more effectively improving lithium ion conductivity.

[0039] The thickness of the positive electrode layer 12 in the laminate 1 is not particularly limited, but is preferably 2 to 200 μm, more preferably 5 to 120 μm, and even more preferably 10 to 80 μm. Within such a range, the electronic resistance is suppressed, and the migration resistance of Li ions contained in the electrolyte is also suppressed, resulting in the advantage of being able to reduce the battery resistance.

[0040] (separator) The separator 20 is made of a ceramic microporous film. The separator 20 contains magnesia (MgO). Specifically, the separator 20 can be made of magnesia (MgO) and glass. In the separator 20, the MgO and glass exist in the form of particles bonded to each other by sintering. The ceramic contained in the separator 20 may include Al2O3, ZrO2, SiC, Si3N4, AlN, etc., in addition to MgO and glass.

[0041] The glass contained in the separator 20 preferably contains 25 wt% or more of SiO2, more preferably 30 to 95 wt%, even more preferably 40 to 90 wt%, and particularly preferably 50 to 80 wt%. The glass content in the separator 20 is preferably 3 to 70 wt%, more preferably 5 to 50 wt%, even more preferably 10 to 40 wt%, and particularly preferably 15 to 30 wt%, based on the total weight of the separator 20. Within this range, both high yield and excellent charge-discharge cycle characteristics can be effectively achieved. The glass component is preferably added to the separator 20 by adding glass frit to the separator raw material powder. The glass frit preferably contains one or more of Al2O3, B2O3, and BaO as a component other than SiO2.

[0042] The thickness of the separator 20 in the laminate 1 is not particularly limited, but for example, the thickness of the first region 21 of the separator 20 (the region between the positive electrode layer 12 and the negative electrode layer 16) is preferably 5 to 60 μm, and more preferably 10 to 30 μm. The second region 22 and the third region 23 of the separator 20 can have the same thickness as the positive electrode layer 12 and the negative electrode layer 16, respectively. The porosity of the separator 20 is also not particularly limited, but can be, for example, about 30 to 70%, and preferably about 40 to 60%.

[0043] (negative electrode layer) The negative electrode layer 16 is, for example, a plate-shaped sintered body containing a titanium-containing composition. The negative electrode layer 16 may contain no binder or conductive additive. The titanium-containing sintered body is lithium titanate Li4Ti5O 12 It is preferable that the battery contains Li4Ti5O (hereinafter referred to as LTO) or niobium titanium composite oxide Nb2TiO7, and more preferably contains LTO. Although LTO is known to typically have a spinel structure, it can also adopt other structures during charge and discharge. For example, LTO can be Li4Ti5O 12 (spinel structure) and Li7Ti5O 12The reaction proceeds in the coexistence of two phases, a spinel phase (rock salt structure) and a spinel phase (rock salt structure). Therefore, LTO is not limited to a spinel structure. LTO may be partially substituted with other elements. Examples of other elements include Nb, Ta, W, Al, and Mg. An LTO sintered body can be produced, for example, according to the method described in JP 2015-185337 A.

[0044] When the negative electrode layer 16 is formed of a sintered body containing LTO, the transition metal element among the elements constituting the negative electrode layer is Ti. When the negative electrode layer 16 is formed of a sintered body containing NbTiO, the transition metal elements among the elements constituting the negative electrode layer are Nb and Ti. The lithium secondary battery according to the present disclosure focuses on the content of the transition metal element contained in the negative electrode layer at a specific position in the separator. When multiple types of transition metal elements are contained, the sum of these transition metal elements is taken as the content of the transition metal element. When multiple types of transition metal elements are contained, the transition metal element contained at a certain percentage or more (e.g., 0.1% or more) may be selected as the "transition metal element constituting the negative electrode layer."

[0045] The negative electrode layer 16 has a structure in which many primary particles are bonded together. These primary particles are preferably composed of LTO or Nb2TiO7. The negative electrode layer 16 may be configured as an integrated sintered body together with the positive electrode layer 12 and the separator 20. Alternatively, the negative electrode layer 16 may be configured as a separate sintered body from the integrated sintered body of the positive electrode layer 12 and the separator 20, and then combined.

[0046] The thickness of the negative electrode layer 16 in the laminate 1 is not particularly limited, but is preferably 1 to 150 μm, more preferably 2 to 120 μm, and even more preferably 5 to 80 μm. The primary particle size, which is the average particle size of the multiple primary particles that make up the negative electrode layer 16, is preferably 1.2 μm or less, more preferably 0.02 to 1.2 μm, and even more preferably 0.05 to 0.7 μm.

[0047] The negative electrode layer 16 preferably contains pores. By containing pores, particularly open pores, when the negative electrode layer is incorporated into a battery, the electrolyte can penetrate into the inside, thereby improving lithium ion conductivity. The porosity of the negative electrode layer 16 is preferably 20 to 60%, more preferably 30 to 55%, and even more preferably 35 to 50%. The average pore diameter of the negative electrode layer 16 is preferably 0.08 to 5.0 μm, more preferably 0.1 to 3.0 μm, and even more preferably 0.12 to 1.5 μm.

[0048] In the laminate 1, the negative electrode layer 16 may include a current collector layer 19. The current collector layer 19 may be provided inside the negative electrode layer 16 in the thickness direction. Alternatively, the current collector layer 19 may be formed so as to be exposed on one of the main surfaces of the negative electrode layer 16. The current collector layer 19 may be made of a material with excellent conductivity. The current collector layer 19 may be made of, for example, gold, silver, platinum, palladium, aluminum, copper, nickel, or the like. By including the current collector layer 19, the internal resistance of the laminate, particularly in the negative electrode, can be reduced.

[0049] (Interfacial region composition) Referring to FIG. 2, in the lithium secondary battery according to the present disclosure, in the interface region Ac between the positive electrode layer 12 and the separator 20, there is a difference in composition between a first portion b1, which is the interface region on the end face 12e side of the positive electrode layer 12, and a second portion b2, which is the interface region on the main surface 12a side of the positive electrode layer 12. Specifically, there is a difference in element composition between a position L1 on the separator 20 that is 5 μm away from the end face 12e of the positive electrode layer 12 in the width direction and a position L2 on the separator 20 that is 5 μm away from the main surface 12a of the positive electrode layer 12 in the thickness direction. In particular, the concentrations of transition metal elements constituting the positive electrode layer 12 are compared. The concentration c1 (mol %) of the transition metal element constituting the positive electrode layer 12 at position L1 is compared with the concentration c2 (mol %) of the same element at position L2. In this case, the relationship c1 / c2≦0.9 is satisfied. The ratio c1 / c2 is preferably 0.1 or more and 0.9 or less, and more preferably 0.3 or more and 0.9 or less. In other words, the separator 20 closer to the main surface 12a of the positive electrode layer 12 contains a larger amount of the transition metal elements that constitute the positive electrode layer 12 than the portion near the end surface 12e.

[0050] Similarly, in the interface region Aa between the negative electrode layer 16 and the separator 20, there is a difference in composition between the third portion b3, which is the interface region on the end face 16e side of the negative electrode layer 16, and the fourth portion b4, which is the interface region on the main surface 16a side of the negative electrode layer 16. Specifically, there is a difference between the elemental composition at position L3, which is 5 μm away from the end face 16e of the negative electrode layer 16 of the separator 20 in the width direction, and the elemental composition at position L4, which is 5 μm away from the main surface 16a of the negative electrode layer 16 of the separator 20 in the thickness direction. In particular, the concentrations of transition metal elements constituting the negative electrode layer 16 are compared. The concentration c3 (mol%) of the transition metal element constituting the negative electrode layer 16 at position L3 is compared with the concentration c4 (mol%) of the same element at position L4. In this case, the relationship c3 / c4≦0.9 is satisfied. c3 / c4 is preferably 0.1 or more and 0.9 or less, and more preferably 0.3 or more and 0.9 or less. That is, the separator near the main surface 16a of the negative electrode layer 16 contains a larger amount of the elements that make up the negative electrode layer 16 than the separator near the end surface 16e.

[0051] Note that Figure 2 is a schematic diagram, and in an actual laminate, each layer of the positive electrode layer, negative electrode layer, and separator may have a bend or distortion. In other words, the main surfaces and end surfaces of the positive electrode layer and negative electrode layer are not necessarily aligned in a straight line (see, for example, Figure 7). In this case, the most protruding position on the main surface can be used as a reference, and a position 5 μm away from that reference position in the thickness direction can be defined as a "position 5 μm away from the main surface." Alternatively, the most protruding position on the end surface can be used as a reference, and a position 5 μm away from that position in the width direction can be defined as a "position 5 μm away from the end surface." Furthermore, if multiple positive electrode layers or negative electrode layers are present, the c1 / c2 to c3 / c4 values ​​for the multiple positive electrode (negative electrode) layers can be determined, and the average values ​​can be used as the c1 / c2 to c3 / c4 values ​​of the laminate.

[0052] The elemental composition can be confirmed by known elemental analysis methods such as EDX analysis using an SEM. The total of all elements constituting the detection area is taken as 100 mol%, and the proportion (mol%) of the transition metal elements constituting the positive electrode layer or negative electrode layer is calculated.

[0053] (Manufacturing method) An example of a method for manufacturing a laminate included in a lithium secondary battery according to the present disclosure will be outlined below. Fig. 3 shows a schematic diagram of the manufacturing process of the laminate, from the step of stacking the sheets that constitute the laminate to the step of cutting the stacked sheets.

[0054] Referring to FIG. 3(1), the positive electrode green sheet 112, the negative electrode green sheet 116, and the separator green sheet 120, which are the materials constituting the laminate, are each prepared separately. Typically, the green sheets can be prepared by first preparing a slurry containing the raw materials constituting each layer, and then forming the prepared slurry into a sheet on a resin film. The negative electrode green sheet 116 may have a current collector layer 119 formed on one of its main surfaces. Each sheet is cut to a predetermined width and stacked in order to form a predetermined layer structure. Note that while the layer structure is simply shown in the example of FIG. 3, a unit U including the negative electrode green sheet 116, the separator green sheet 120, the positive electrode green sheet 112, and the separator green sheet 120 may be repeatedly stacked to form a multi-layer laminate.

[0055] Referring to FIG. 3(1), when stacking, each green sheet may be used alone in the thickness direction, or two or more sheets of the same type may be continuously stacked in the thickness direction. For example, to form the negative electrode layer 16, two negative electrode green sheets 116 each having a current collector layer 119 on one side may be stacked. When two or more sheets of the same type are stacked in the thickness direction, the stacked sheets are integrated during the sintering stage, resulting in a single layer in the sintered body. When two negative electrode green sheets 116 each having a current collector layer 119 are stacked, it is preferable to stack them so that the current collector layers 119 are in contact with each other.

[0056] Referring to FIG. 3(3), the green sheet laminate 101 is pressure-bonded. Specifically, the green sheets included in the green sheet laminate 101 can be pressure-bonded to each other by pressing. The pressing direction can be pressing in the thickness direction (Z-axis direction) of the green sheet laminate 101 or pressing in the width direction (X-axis direction). Pressing in multiple directions can also be combined, such as pressing in the thickness direction and width direction. Among these, pressing in the thickness direction of the green sheet laminate 101 is preferred. It is also preferred to press only in the thickness direction and not in the width direction. The pressing method can be, for example, cold isostatic pressing (CIP), hot isostatic pressing (WIP), hydrostatic pressing, or the like, and is not particularly limited. Pressing may be performed while heating.

[0057] Next, referring to FIG. 3(4), the green sheet laminate 101 is cut. The green sheet laminate 101 is cut on both sides to a predetermined width and cut to obtain a laminate having a predetermined depth. The stacking configuration and cutting locations can be determined according to the desired configuration of the sintered body (overall dimensions, width and thickness of each layer). In FIG. 3(4), the cutting locations are indicated by thick lines.

[0058] FIG. 4 shows the green sheet laminate 101 after cutting. The pressed and cut green sheet laminate 101 is degreased and fired to obtain a laminate that is a laminated and sintered body. The degreasing and firing can be performed under known conditions and by known methods. The thickness and width of each layer in the obtained laminated and sintered body can be confirmed, for example, by polishing the laminated and sintered body with a cross-section polisher and observing the obtained cross section with an SEM.

[0059] Next, current collectors are attached to both side surfaces of the laminated integrated sintered body. Referring to FIG. 5, a positive electrode current collector 14 is attached to the side surface of the laminate 1 where the positive electrode layer 12 is exposed, and a negative electrode current collector 18 is attached to the side surface where the negative electrode layer 16 is exposed. A conductive material, such as aluminum foil or copper foil, can be used for the positive electrode current collector 14 and the negative electrode current collector 18. The positive electrode current collector 14 is attached to cover one entire side surface of the laminate 1 and can be configured to extend onto the lower surface of the laminate 1. The negative electrode current collector 18 is attached to cover the other entire side surface of the laminate 1 and can be configured to extend onto the upper surface of the laminate 1. The positive electrode layer 12 and the positive electrode current collector 14, and the negative electrode layer 16 and the negative electrode current collector 18 can be bonded together using a conductive adhesive. For example, a conductive carbon paste can be used as the conductive adhesive. The thickness of the conductive adhesive layer is not particularly limited as long as it exhibits its effect as an adhesive layer and does not impede the effects of the invention, but it can be, for example, about 1 to 500 μm.

[0060] The electrode obtained by the above-described manufacturing method is housed inside an exterior body according to known methods and conditions, and an electrolyte solution is sealed inside to obtain a lithium secondary battery.

[0061] The width, depth, and height of the laminate, which is a laminated integral sintered body, can be appropriately selected according to the desired shape of the lithium secondary battery and are not particularly limited. For example, when forming a coin-type battery, the laminate can have a width of about 3 to 18 mm, a depth of about 3 to 18 mm, and a height of about 0.3 to 5 mm. 3 to 200 cells can be formed in this laminate.

[0062] (electrolyte) Referring to FIG. 6, the lithium secondary battery 10 may include an electrolyte solution 22. The electrolyte solution 22 is not particularly limited, and any electrolyte solution known for use in lithium secondary batteries may be used. For example, the solvent may be one or a combination of two or more selected from ethylene carbonate (EC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and γ-butyrolactone (GBL). The electrolyte dissolved in the solvent may be, for example, a lithium salt compound such as lithium hexafluorophosphate (LiPF6) or lithium fluoroborate (LiBF4). The electrolyte solution 22 may further include at least one additive selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), and lithium difluoro(oxalato)borate (LiDFOB).

[0063] The concentration of the electrolyte in the electrolytic solution 22 is preferably 0.5 to 2 mol / L, more preferably 0.6 to 1.9 mol / L, even more preferably 0.7 to 1.7 mol / L, and particularly preferably 0.8 to 1.5 mol / L.

[0064] Furthermore, a solid electrolyte or a polymer electrolyte can be used as the electrolyte in addition to the electrolytic solution 22. In this case, similar to the case of the electrolytic solution 22, it is preferable that the electrolyte is impregnated at least into the pores of the separator 20. The impregnation method is not particularly limited, but examples include a method of melting the electrolyte and infiltrating it into the pores of the separator 20, and a method of pressing a powder compact of the electrolyte against the separator 20.

[0065] [Example] The lithium secondary battery of the present disclosure will be described in more detail below with reference to examples and comparative examples. [Example 1] A lithium secondary battery was fabricated according to the methods described in the following 1 to 7. The obtained lithium secondary battery was evaluated according to the methods described in 8 to 10.

[0066] 1. Preparation of the laminate Green sheets for each layer constituting the laminate were prepared under the conditions and methods of (1) to (3). In (1) to (3), the viscosity of the slurry was measured using a Brookfield LVT viscometer. The doctor blade method was used to cast the slurry onto a PET film.

[0067] (1) Preparation of LCO green sheet (positive electrode green sheet) Co3O4 powder (manufactured by Seido Chemical Industry Co., Ltd.) and Li2CO3 powder (manufactured by Honjo Chemical Co., Ltd.) were weighed so that the molar ratio of Li / Co was 1.01, and then mixed. The mixture was kept at 780 °C for 5 hours. The obtained powder was milled in a pot mill to a volumetric standard of D 50 The powder was crushed to a particle size of 0.4 μm to obtain a powder consisting of plate-shaped LCO particles. One hundred parts by weight of the resulting LCO powder was mixed with 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 8 parts by weight of a binder (polyvinyl butyral: product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), 2 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co., Ltd.), and 4.5 parts by weight of a dispersant (product name Rheodor SP-O30, manufactured by Kao Corporation). The resulting mixture was stirred under reduced pressure to degas and the viscosity was adjusted to 4000 cP to prepare an LCO slurry. The prepared slurry was formed into a sheet on a PET film to form an LCO green sheet. The thickness of the positive electrode layer after firing was adjusted to 24 μm.

[0068] (2) Preparation of LTO green sheet (negative electrode green sheet) LTO powder (volume basis D 50100 parts by weight of a 0.06 μm particle size (Sigma-Aldrich Japan LLC), 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 20 parts by weight of a binder (polyvinyl butyral: product number BM-2, Sekisui Chemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, Kurogane Kasei Co., Ltd.), and 2 parts by weight of a dispersant (Rheodor SP-O30, Kao Corporation) were mixed. The resulting anode raw material mixture was stirred under reduced pressure to degas and the viscosity was adjusted to 4000 cP to prepare an LTO slurry. The prepared slurry was formed into a sheet on a PET film to form an LTO green sheet. The thickness of the anode layer after firing was adjusted to 10 μm.

[0069] (2´) Formation of the current collector layer Au paste (manufactured by Tanaka Kikinzoku Co., Ltd., product name: GB-2706) was printed on one side of the LTO green sheet prepared in (2) using a printer, so that the thickness of the printed layer would be 0.2 μm after firing.

[0070] (3) Preparation of separator green sheets Magnesium carbonate powder (manufactured by Konoshima Chemical Co., Ltd.) was heat-treated at 900°C for 5 hours to obtain MgO powder. The obtained MgO powder and glass frit (manufactured by Nippon Frit Co., Ltd., CK0199) were mixed in a weight ratio of 7:3. The obtained mixed powder (volume basis D 50100 parts by weight of powder (particle size 0.4 μm), 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 30 parts by weight of a binder (polyvinyl butyral: product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), 6 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co., Ltd.), and 2 parts by weight of a dispersant (product name Rheodor SP-O30, manufactured by Kao Corporation) were mixed. The resulting raw material mixture was stirred under reduced pressure to degas and the viscosity was adjusted to 4000 cP to prepare a slurry. The prepared slurry was formed into a sheet on a PET film to form a separator green sheet. After firing, the thickness of the separator layer located between the positive electrode layer and the negative electrode layer was adjusted to 25 μm. The separators located on the sides of the positive electrode layer or the negative electrode layer were adjusted to the same thickness as the positive electrode layer or the negative electrode layer.

[0071] 2.Cutting the sheet The green sheets obtained in 1 were cut into pieces for lamination.

[0072] 3.Lamination, pressing and firing Various green sheets were laminated as shown in Figure 3. When two LTO green sheets were stacked, they were laminated so that the current collector layers were in contact with each other. Each sheet was repeatedly stacked in the order shown in Figure 3 so that the number of cells formed in the laminate was 19 (note that Figure 3 only shows a portion of the repetition). The resulting laminate was subjected to CIP (cold isostatic pressing) at 100 kgf / cm 2 The green sheets were pressed together with a pressure of 0.05 to obtain an unsintered green sheet laminate. The green sheets were pressed in the thickness direction. Next, as shown in Figure 3, the unsintered green sheet laminate was cut with a Thomson blade. The cutting was performed so that the laminate was 5 mm in both the width and depth directions. After cutting, the green sheet laminate was heated from room temperature to 600°C and degreased for 5 hours, then heated to 800°C and held there for 10 minutes, after which it was cooled. In this way, a laminated sintered body was obtained.

[0073] 4. Preparation of Conductive Carbon Paste A binder (CMC: MAC350HC, manufactured by Nippon Paper Industries Co., Ltd.) was weighed out to 1.2 wt% in pure water and dissolved using a stirrer to obtain a 1.2 wt% CMC solution. A carbon dispersion (product number: BPW-229, manufactured by Nippon Graphite Co., Ltd.) and a dispersant solution (product number LB-300, manufactured by Showa Denko K.K.) were prepared. Next, the carbon dispersion, dispersant solution, and 1.2 wt% CMC solution were weighed out to a ratio of 0.22:0.29:1, and these were mixed using a planetary mixer to prepare a conductive carbon paste.

[0074] 5. The exposed positive electrode surface of the laminated sintered body is bonded to the aluminum foil with conductive carbon paste. The conductive carbon paste obtained in step 4 was screen-printed onto aluminum foil serving as a positive electrode current collector. The laminated, integrated sintered body obtained in step 3 was placed so that the exposed positive electrode surface was adhered within the undried print pattern (the area where the conductive carbon paste was applied), and after lightly pressing with a finger, it was vacuum-dried at 50°C for 60 minutes. In this way, the exposed positive electrode surface of the laminated, integrated sintered body and the positive electrode current collector were adhered via a conductive carbon adhesive layer. The thickness of the conductive carbon adhesive layer was 30 μm.

[0075] 6. The exposed negative electrode surface of the laminated sintered body is bonded to the aluminum foil with conductive carbon paste. In the same manner as in 5., an aluminum foil serving as a negative electrode current collector was adhered to the negative electrode exposed surface of the laminated integral sintered body via a conductive carbon adhesive layer.

[0076] 7. Fabrication of Lithium Secondary Batteries The positive electrode current collector, the sintered laminate, and the negative electrode current collector were stacked in this order between the positive and negative electrode cans that would form the battery case. After filling the battery with electrolyte, the positive and negative electrode cans were sealed by crimping them together with a gasket. A coin-cell lithium secondary battery with a diameter of 20 mm and a thickness of 1.6 mm was thus fabricated. The electrolyte used was a solution of LiPF6 dissolved in an organic solvent containing propylene carbonate (PC) and gamma-butyrolactone (GBL) in a 1:3 volume ratio, with a concentration of 1.5 mol / L.

[0077] 8. Evaluation 1: Measurement and composition analysis of laminated sintered body (1) Measurement of the outer width of the laminated sintered body The outer thickness (T) of the laminated integral sintered body was measured using a one-shot 3D shape measuring machine (VR3000, manufactured by Keyence Corporation). (2) Measurement of each layer in a laminated sintered body The laminated sintered body was polished with a cross-section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd.), and the obtained cross section was observed with an SEM (JSM-IT-500, manufactured by JEOL Ltd.). (3) Composition analysis of separator The composition of each of the portions L1 to L4 shown in FIG. 2 was analyzed by EDX line analysis using a SEM-EDX device (manufactured by JEOL Ltd., JSM6390LA). L1 is a position on the separator that is 5 μm away from the end face of the positive electrode layer in the width direction. L2 is a position in the separator that is 5 μm away from the main surface of the positive electrode layer in the thickness direction. L3 is a position on the separator that is 5 μm away from the end face of the negative electrode layer in the width direction. L4 is a position in the separator that is 5 μm away from the main surface of the negative electrode layer in the thickness direction. The concentration of Co, a transition metal element constituting the positive electrode layer, was calculated for L1 and L2, and the Co concentration (mol%) in L1 was compared with the Co concentration (mol%) in L2. The concentration of Ti, a transition metal element constituting the negative electrode layer, was calculated for L3 and L4, and the Ti concentration (mol%) in L3 was compared with the Ti concentration (mol%) in L4. FIG. 7 shows the positions of L1 and L2 in the sample of Example 1.

[0078] 9. Evaluation of peeling yield Ten laminated sintered bodies were produced, and each sample was visually inspected. Samples with no visible peeling were deemed to have passed, while samples with even one visible peeling were deemed to have failed. Regardless of the size or number of peelings, all samples with visible peeling were deemed to have failed. The peeling yield was calculated using the following formula. Peeling yield (%) = (number of acceptable samples / 10) x 100

[0079] 10. Battery performance evaluation (0.2C discharge capacity evaluation) The battery capacity was measured at 25°C using a battery containing the obtained laminated integral sintered body. Charging was performed at a constant current of 0.2 C until the voltage reached 2.7 V. Discharging was performed at a constant current of 0.2 C until the voltage reached 1.5 V. A second charge / discharge cycle was performed under the same conditions as the first cycle, and the discharge capacity of this second cycle was taken as the 0.2 C discharge capacity.

[0080] [Example 2] A laminated integrated sintered body was produced in the same manner as in Example 1, except that the peak temperature of the firing step in 3 was set to 830°C. A lithium secondary battery was produced using the laminated integrated sintered body. Measurements and composition analysis of the laminated integrated sintered body were carried out in the same manner as in Example 1, and yield and battery performance evaluations were carried out.

[0081] [Comparative Example 1] A laminated sintered body was produced in the same manner as in Example 1, except that the green sheet laminate was pressed in the thickness direction of the laminate, then in a direction perpendicular to the lamination (the width direction of the laminate), and again in the thickness direction of the laminate. A lithium secondary battery was produced using the laminated sintered body. Measurements and composition analysis of the laminated sintered body were carried out in the same manner as in Example 1, and the yield was evaluated.

[0082] Comparative Example 2 A laminated sintered body was produced in the same manner as in Example 2, except that the green sheet laminate was pressed in the thickness direction of the laminate, then in a direction perpendicular to the lamination (the width direction of the laminate), and again in the thickness direction of the laminate. A lithium secondary battery was produced using the laminated sintered body. Measurements and composition analysis of the laminated sintered body were carried out in the same manner as in Example 2, and the yield was evaluated.

[0083] [Evaluation results] The laminated sintered bodies of Examples 1 and 2 and Comparative Examples 1 and 2 are summarized in Table 1.

[0084] [Table 1]

[0085] As an example of the results of composition analysis, the results of composition analysis of L1 and L2 of the laminated integral sintered body of Example 1 are shown in [Table 2].

[0086] [Table 2]

[0087] The transition metal element constituting the positive electrode in the laminated integral sintered body of Example 1 was Co, and the Co concentrations in L1 and L2 were compared. Similarly, measurements were taken for L3 and L4, and the Ti concentrations in L3 and L4 were compared.

[0088] The evaluation results of the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 are summarized in Table 3.

[0089] [Table 3]

[0090] As shown in Table 3, in Examples 1 and 2, there was a difference of 10% or more in the Co or Ti concentration between the end surfaces (L1, L3) and the main surfaces (L2, L4) of the positive and negative electrode layers. The laminates of Examples 1 and 2 had a peeling yield of 100%, and no peeling was observed. Furthermore, the 0.2C discharge capacity of lithium secondary batteries using these laminates was 1.0 mAh, confirming sufficient discharge capacity. In contrast, in Comparative Examples 1 and 2, where there was no difference in the Co or Ti concentration between the end surfaces (L1, L3) and the main surfaces (L2, L4) of the positive and negative electrode layers, peeling occurred in the sintered laminate after fabrication, resulting in a yield of 0%. Conventionally, it has been considered desirable for laminates constituting electrodes to have a uniform composition. In contrast, the present disclosure has discovered an unexpected configuration in which non-uniform compositions between the main surfaces and end surfaces of the positive and negative electrode layers are preferable.

[0091] It should be understood that the embodiments disclosed herein are illustrative in all respects and are not limiting in any respect. The scope of the present disclosure is defined not by the above description but by the claims, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0092] 1 laminate, 5 electrode, 10 lithium secondary battery, 12 positive electrode layer, 16 negative electrode layer, 14 positive electrode current collector, 18 negative electrode current collector, 20 separator, 22 electrolyte, 24 outer casing, 24a positive electrode can, 24b negative electrode can, 24c gasket, 101 green sheet laminate, 112 positive electrode green sheet, 116 negative electrode green sheet, 120 separator green sheet.

Claims

1. a plurality of positive electrode layers, a plurality of negative electrode layers, and a separator; a lithium secondary battery including a sintered laminate in which the positive electrode layers and the negative electrode layers are alternately laminated with the separator interposed therebetween, the plurality of positive electrode layers, the plurality of negative electrode layers, and the separator are an integrally formed sintered body, In the interface region between the positive electrode layer and the separator, a concentration c1 (mol %) of the transition metal element constituting the positive electrode layer at a position in the separator 5 μm away from the end surface of the positive electrode layer; The concentration c2 (mol %) of the transition metal element constituting the positive electrode layer at a position in the separator 5 μm away from the main surface of the positive electrode layer is The relationship c1 / c2≦0.9 is satisfied, In the interface region between the negative electrode layer and the separator, a concentration c3 (mol %) of the transition metal element constituting the negative electrode layer at a position in the separator 5 μm away from the end surface of the negative electrode layer; the concentration c4 (mol %) of the transition metal element constituting the negative electrode layer at a position in the separator 5 μm away from the main surface of the negative electrode layer; The relationship c3 / c4≦0.9 is satisfied. Lithium secondary battery.

2. At least one of the transition metal elements constituting the positive electrode layer is Co, At least one of the transition metal elements constituting the negative electrode layer is Ti. The lithium secondary battery according to claim 1 .

3. 3. The lithium secondary battery according to claim 1, wherein the separator disposed between the positive electrode layer and the negative electrode layer has a thickness of 5 to 60 μm.

4. In the lithium secondary battery, the number of cells formed by the positive electrode layer and the negative electrode layer facing each other via the separator is 3 to 200. The lithium secondary battery according to any one of claims 1 to 3.

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