Lithium secondary battery

Abstract

This lithium secondary battery comprises a sintered body including a stack, said stack including a plurality of positive electrode layers, a plurality of negative electrode layers, and a separator, the positive electrode layers and the negative electrode layers being alternately stacked with the separator interposed therebetween. The sintered body includes a positive electrode connection portion that is connected to at least 2 or more of the positive electrode layers included in the stack, and contains 70–100 vol% inclusive of the positive electrode active material that constitutes the positive electrode layers.

Description

Technical Field

[0001] The present disclosure relates to a lithium secondary battery. This application claims priority based on Japanese Patent Application No. 2022-011189 filed on January 27, 2022, and incorporates by reference all the descriptions set forth in the Japanese patent application.

Background Art

[0002] In a lithium secondary battery, a positive electrode layer composed of a sintered body of a lithium composite oxide, a negative electrode layer composed of a sintered body containing titanium, and a ceramic separator disposed between the positive electrode layer and the negative electrode layer are known. For example, Patent Document 1 discloses a lithium secondary battery in which a positive electrode layer, a ceramic separator, and a negative electrode layer are formed of an integrally sintered plate in which they are bonded to each other and impregnated with an electrolytic solution. The lithium secondary battery of Patent Document 1 includes a ceramic separator composed of MgO and glass as a separator.

[0003] Patent Document 2 discloses an all-solid-state battery having a laminate in which a plurality of positive electrode layers and a plurality of negative electrode layers are alternately laminated via a solid electrolyte layer. The laminate disclosed in Patent Document 2 is characterized in that a buffer layer is provided in the solid electrolyte layer. The buffer layer may be provided in the outermost solid electrolyte layer of the laminate, or may be provided in the solid electrolyte layer located in the middle of the laminate. Further, the buffer layer may be provided in a side margin layer provided on the outer periphery thereof alongside the positive electrode layer or the negative electrode layer. The buffer layer is formed by combining a metal part and a void part.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a lithium secondary battery, it is desired to have low resistance and be stably and efficiently manufacturable.

[0006] Therefore, one object of the invention according to the present disclosure is to provide a lithium secondary battery including an electrode that has low resistance and can be stably and efficiently manufactured.

Means for Solving the Problems

[0007] The lithium secondary battery according to the present disclosure includes a plurality of positive electrode layers, a plurality of negative electrode layers, and a separator, and includes a sintered body including a laminated portion in which the positive electrode layers and the negative electrode layers are alternately laminated via the separator. The sintered body includes a positive electrode connection portion that connects to at least two or more of the positive electrode layers included in the laminated portion and contains 70% vol or more and 100% vol or less of the positive electrode active material constituting the positive electrode layer.

Advantages of the Invention

[0008] According to the above lithium secondary battery, there is provided a lithium secondary battery including a sintered body that has low resistance, has a good yield in manufacturing, and can be stably and efficiently manufactured.

Brief Description of the Drawings

[0009]

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DETAILED DESCRIPTION OF THE INVENTION

[0010] [Overview of Embodiment] First, embodiments of the present disclosure will be listed and described. The lithium secondary battery according to the present disclosure includes a plurality of positive electrode layers, a plurality of negative electrode layers, and a separator, and includes a sintered body having a laminated portion in which the positive electrode layers and the negative electrode layers are alternately laminated via the separator. The sintered body includes a positive electrode connection portion that connects to at least two or more of the positive electrode layers included in the laminated portion and contains 70% vol or more and 100% vol or less of the positive electrode active material constituting the positive electrode layer.

[0011] Conventionally, a lithium secondary battery including a laminate including a plurality of positive electrode layers and a plurality of negative electrode layers, in which a plurality of cells are formed in one electrode, is known (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 composed of a metal portion and a void portion in the solid electrolyte layer. Of the side surfaces of the laminate disclosed in Patent Document 2, a first external terminal is attached to the side surface where the positive electrode layer and the positive electrode current collector layer are exposed, and a second external terminal is attached to the side surface where the negative electrode layer and the negative electrode current collector layer are exposed. As a specific form of the external terminal, it is described that copper is baked on the side surface of the laminate, and nickel plating and tin plating are applied to the surface thereof.

[0012] A laminated electrode including a plurality of positive electrode layers and negative electrode layers has the merit of obtaining a large capacity while being small in size. On the other hand, when trying to configure the laminated electrode as a sintered body, improvement in stability in manufacturing, that is, yield, becomes an issue. On the other hand, a lithium secondary battery with lower resistance is desired. The inventors have conducted studies to reduce the resistance of the laminated electrode, and focused on the configuration of the sintered body including the laminate. Then, in the sintered body, a positive electrode connection part that connects to at least two or more of the positive electrode layers included in the laminated part is provided, and further, by making the composition of this positive electrode connection part into a specific configuration, it has been found that a laminated electrode that achieves both low resistance and stability in manufacturing can be obtained.

[0013] The lithium secondary battery having the above configuration is less likely to cause delamination between layers when manufacturing a sintered body including a laminate, and can be manufactured stably. Further, the lithium secondary battery having the above configuration has low resistance and can efficiently extract electricity from a small lithium secondary battery.

[0014] In the lithium secondary battery, the positive electrode connection part may be formed on a first side surface that is a surface where the positive electrode layer and the separator are exposed among the side surfaces of the sintered body, and is a positive electrode side edge layer that contacts an end of the positive electrode layer included in the laminated part. According to the aspect of forming a positive electrode side edge layer that focuses on the side surface of the laminate and contacts the end of the positive electrode layer exposed on the side surface of the laminate, the above effects can be surely obtained.

[0015] In the lithium secondary battery, the positive electrode connection part may be a columnar part that extends through the positive electrode layer and the separator in the stacking direction. A sintered body including a columnar part that extends through the positive electrode layer and the separator in the stacking direction can form connection parts for a large number of electrodes together, so that productivity is further improved.

[0016] In the lithium secondary battery, the positive electrode side edge layer may be sintered integrally with the stacked part. Note that being sintered integrally means that the stacked part and the side edge layer are joined to each other without passing through other joining modes (for example, an adhesive, a joining member, etc.) and form a sintered body (integrally sintered body) that is integrated. According to this configuration, the handleability as an electrode is excellent, and the electrode can be manufactured at a reasonable cost.

[0017] In the lithium secondary battery, the positive electrode side edge layer may further contain at least one metal selected from the group consisting of Au (gold), Pt (platinum), and Ir (iridium). When these metals are contained, the effect of reducing resistance can be easily obtained, and it is easy to achieve both reduction of resistance and stability in manufacturing.

[0018] In the lithium secondary battery, a current collector may be provided outside the positive electrode side edge layer. According to this configuration, a lithium secondary battery having the effects according to the present disclosure can be configured without greatly changing the design of a conventional lithium secondary battery. Therefore, a lithium secondary battery with lower resistance and excellent stability in manufacturing can be realized at a reasonable cost.

[0019] In the lithium secondary battery, on a second side surface that faces the first side surface, a negative electrode side edge layer that is in contact with an end portion of a negative electrode layer included in the laminated portion may be formed and is composed of at least one metal selected from the group consisting of Au (gold), Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), Rh (rhodium), and Cu (copper). According to this configuration, a positive electrode side edge layer with low resistance is provided on the positive electrode side, and further, a negative electrode side edge layer made of a metal such as gold is provided on the negative electrode side, whereby a lithium secondary battery with lower resistance can be provided.

[0020] In the lithium secondary battery, the positive electrode layer may be composed of a lithium composite oxide sintered body, and the negative electrode layer may be composed of a titanium-containing sintered body. The positive electrode layer composed of a lithium composite oxide sintered body and the negative electrode layer composed of a titanium-containing sintered body are known configurations, and a more stable and low-resistance lithium secondary battery can be obtained by combining with the above configuration.

[0021] [Specific Examples of Embodiments] 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 denoted by the same reference numerals, and the description thereof will not be repeated.

[0022] (Lithium Secondary Battery) First, an overview of the lithium secondary battery according to the present disclosure will be described. FIG. 1 is a schematic cross-sectional schematic view showing the structure of a lithium secondary battery 10 according to an embodiment of the present disclosure. In FIG. 1, members of the same type are shown with the same hatching, and the display of reference numerals is partially omitted. The same applies to other figures. Referring to FIG. 1, the X-axis direction is the width direction of the laminate 1, and the Z-axis direction is the lamination direction or the thickness direction of the laminate 1.

[0023] Referring to FIG. 1, in the lithium secondary battery 10, the electrode 5 is accommodated inside the exterior body 24. The electrode 5 includes a laminate 1 as a laminated portion in which a plurality of positive electrode layers 12, a plurality of negative electrode layers 16, and a separator 20 are laminated. A positive electrode side edge layer 41 as a positive electrode connection portion and a negative electrode side edge layer 42 as a negative electrode connection portion are formed in contact with each of both side surfaces of the laminate 1. The laminate 1, the positive electrode side edge layer 41, and the negative electrode side edge layer 42 constitute a sintered body 9 (FIG. 2) which is an integrally sintered body as a whole. That is, the laminate 1, the positive electrode side edge layer 41, and the negative electrode side edge layer 42 are joined to each other. In the present specification, the term "integrally sintered body" means that each member constituting the sintered body is connected and joined to each other without relying on a joining method other than sintering (for example, an adhesive or the like). A positive electrode current collector 14 and a negative electrode current collector 18 are provided in contact with each of both side surfaces of the sintered body 9. The sintered body 9, the positive electrode current collector 14, and the negative electrode current collector 18 constitute the electrode 5.

[0024] In the laminate 1, 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 layer 12 and the negative electrode layer 16. The separator 20 separates the positive electrode layer 12 and the negative electrode layer 16 from each other. The positive electrode layer 12 is made of, for example, a sintered body containing lithium cobaltate. The negative electrode layer 16 is made of, for example, a titanium-containing sintered body. The separator 20 is made of ceramic.

[0025] An airtight space is formed inside the exterior body 24. The electrode 5 and the electrolytic solution 22 are accommodated in this airtight space. In the lithium secondary battery 10, the electrolytic 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 electrolytic solution 22.

[0026] The exterior body 24 may be appropriately selected according to the type of the 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. 1, 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 caulked 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 limited thereto. The gasket 24c may be an annular member made of an insulating resin such as polypropylene, polytetrafluoroethylene, or PFA resin, and is not particularly limited.

[0027] Although the lithium secondary battery 10 shown in FIG. 1 is in the form of a coin-shaped battery, the form of the lithium secondary battery according to the present disclosure is not limited to the coin-shaped battery. For example, other forms such as a chip-type secondary battery and a thin-type secondary battery including a pouch-type secondary battery may 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 base material, and the battery elements (that is, the electrodes 5 and the electrolytic solution 22) are preferably embedded in the resin base material. 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 to each other with an adhesive. Further, the pair of resin films may be heat-sealed to each other by heat pressing. Furthermore, as the separator, a separator made of a solid electrolyte may be adopted, and a configuration that does not include an electrolytic solution may also be used.

[0028] Referring to FIG. 1, the electrode 5 of the lithium secondary battery 10 includes a positive current collector 14 that extends in contact with the sintered body 9 from the side surface to the lower surface of the sintered body 9. Further, the lithium secondary battery 10 includes a negative current collector 18 that extends in contact with the sintered body 9 from the side surface to the upper surface of the sintered body 9. The positive current collector 14 and the negative current collector 18 may be metal foils such as copper foil or aluminum foil. The positive current collector 14 is preferably disposed between the positive electrode side edge layer 41 and the exterior body 24 (e.g., the positive electrode can 24a). The negative current collector 18 is preferably disposed between the negative electrode side edge layer 42 and the exterior body 24 (e.g., the negative electrode can 24b). Further, a positive electrode side carbon layer (not shown) is preferably provided between the positive electrode side edge layer 41 and the positive current collector 14 from the viewpoint of reducing contact resistance. Similarly, a negative electrode side carbon layer (not shown) is preferably provided between the negative electrode side edge layer 42 and the negative current collector 18 from the viewpoint of reducing contact resistance. Both the positive electrode side carbon layer and the negative electrode side carbon layer are preferably composed of conductive carbon. The carbon layer can be formed, for example, by applying a conductive carbon paste to the surface of the metal foil used as the current collector.

[0029] (Sintered body) The sintered body included in the lithium secondary battery according to the present disclosure will be described. FIG. 2 is a schematic perspective view showing the sintered body 9 included in the lithium secondary battery according to the present disclosure. Referring to FIG. 2, the sintered body 9 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 in the Z-axis direction (thickness direction). The sintered body 9 also includes a positive electrode side edge layer 41 as a positive electrode connection portion formed on each of both side surfaces of the laminate 1, and a negative electrode side edge layer 42 as a negative electrode connection portion. The positive electrode side edge layer 41 is formed in contact with the first side surface s1 of the laminate 1. The first side surface s1 is a surface where the positive electrode layer 12 and the separator 20 are exposed (FIG. 3). The negative electrode side edge layer 42 is formed in contact with the second side surface s2 of the laminate 1. The second side surface s2 is a surface where the negative electrode layer 16 and the separator 20 are exposed (FIG. 3). In the example shown in FIG. 2, the sintered body 9 has a rectangular parallelepiped shape (square), but the outer shape of the sintered body is not limited thereto. For example, it may be cylindrical (round) having side surfaces, or other polygonal prism shapes.

[0030] (Stacked portion) The stacked portion included in the lithium secondary battery according to the present disclosure will be described. FIG. 3 is a schematic cross-sectional perspective view showing a laminate 1 as a stacked portion included in the lithium secondary battery according to the present disclosure. Referring to FIG. 3, the laminate 1 is a laminate in which a number of layers are stacked. The laminate 1 has a rectangular parallelepiped shape defined by its outer dimensions of width W, depth D, and thickness T. Here, the rectangular parallelepiped does not mean only a rectangular parallelepiped in the exact mathematical sense, but also includes a three-dimensional structure having a shape similar to a rectangular parallelepiped for reasons of design and manufacture. In the laminate 1, the direction parallel to the X-axis shown in FIG. 3 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, in the laminate 1, the surfaces 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 surfaces. The front and back surfaces are planes parallel to the XZ plane. Also, in the laminate 1, the surface on which the stacking structure is exposed, which extends between the front and back surfaces and extends along the depth direction, is referred to as the side surface. The side surface is a plane parallel to the YZ plane.

[0031] Referring to FIG. 3, the separator 20 is exposed on both the uppermost surface and the lowermost surface of the laminate 1. In the laminate 1, the positive electrode layer 12 and the negative electrode layer 16 facing each other through the separator 20 form one cell. Five cells are formed in the laminate 1 of FIG. 3. The number of cells in the laminate included in the lithium secondary battery according to the present disclosure is not limited as long as it has the effects of the invention, and for example, a laminate including 3 to 200 cells may be used.

[0032] The laminate 1 has a plurality of positive electrode layers 12 and a plurality of negative electrode layers 16 laminated alternately. The positive electrode layer 12 and the negative electrode layer 16 constituting the laminate 1 are each in the shape of a quadrilateral plate. The widths of both the positive electrode layer 12 and the negative electrode layer 16 are 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 layer 12 and the negative electrode layer 16 are each exposed only on one of the side surfaces of the laminate 1. Specifically, all of the plurality of positive electrode layers 12 are exposed on the first side surface s1 of the laminate 1 and not exposed on the second side surface s2. The positive electrode layer 12 extends from the side surface s1 to the middle in the width direction of the laminate 1, and the inner end surface 12e is the end in the width direction. Also, all of the plurality of negative electrode layers 16 are exposed on the second side surface s2 of the laminate 1 and not exposed on the first side surface s1. The negative electrode layer 16 extends from the side surface s2 to the middle in the width direction of the laminate 1, and the inner end surface 16e is the end in the width direction.

[0033] 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 over 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. The second region 22 is arranged in the X-axis direction with the positive electrode layer 12 and extends between the inner end surface 12e of the positive electrode layer 12 and the side surface s2. The second region 22 functions as an insulating layer that insulates between the positive electrode layer 12 and the side surface s2. The third region 23 is arranged in the X-axis direction with the negative electrode layer 16 and extends between the inner end surface 16e of the negative electrode layer 16 and the side surface s1. The third region 23 functions as an insulating layer that insulates between the negative electrode layer 16 and the side surface s1. Note that the first region 21, the second region 22, and the third region 23 are continuously connected in series without a boundary. The first region 21, the second region 22, and the third region 23 are regions partitioned for convenience of explanation, and it is preferable that the separator 20 is an integral structure that is continuously connected as a whole.

[0034] On the first side s1 of the laminate 1, the positive electrode layer 12 and the separator 20 are exposed, and the negative electrode layer 16 is not exposed. Similarly, on the second side s2 of the laminate 1, the negative electrode layer 16 including the current collector layer 19 and the separator 20 are exposed, and the positive electrode layer 12 is not exposed. In contact with the side s1, a positive electrode side edge layer 41 (FIG. 2), which is a layer containing a large amount of positive electrode active material, is provided. Similarly, in contact with the side s2, a negative electrode side edge layer 42 (FIG. 2), which is a layer of a low-resistance metal, is provided. In the lithium secondary battery according to the present disclosure, by providing the positive electrode side edge layer 41 and the negative electrode side edge layer 42 having a specific configuration, both low resistance and good manufacturing yield are achieved. Next, the configuration of each layer will be described.

[0035] (Positive electrode layer) The positive electrode layer 12 is composed of a sintered body containing lithium cobaltate. The positive electrode layer 12 can be made to contain no binder or conductive assistant. As the lithium cobaltate, specifically, for example, LiCoO2 (hereinafter, may be abbreviated as LCO) can be mentioned. As the LCO sintered body formed in a plate shape, for example, those disclosed in Japanese Patent No. 5587052 and International Publication No. 2017 / 146088 can be used. The positive electrode layer 12 preferably contains a plurality of primary particles composed of lithium cobaltate, 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 layer plane of the positive electrode layer, and is an oriented 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).

[0036] As the lithium cobaltate constituting the primary particles in the positive electrode layer 12, in addition to LCO, for example, Li x NiCoO2 (lithium nickel cobaltate), Li x CoNiMnO2 (lithium cobalt nickel manganate), Li x CoMnO2 (lithium cobalt manganate), etc. can be mentioned. Further, in addition to lithium cobaltate, other lithium composite oxides may be included. Examples of the lithium composite oxide include Li xOxides represented by MO2 (where 0.05 < x < 1.10, M is at least one transition metal, and M typically includes one or more of Co, Ni, and Mn) can be mentioned.

[0037] 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. Also, when the positive electrode layer 12 is x composed of a sintered body containing NiCoO2 (lithium nickel cobalt oxide), the transition metal elements among the elements constituting the positive electrode layer are Ni and Co. Also, when the positive electrode layer 12 is x composed of a sintered body containing CoNiMnO2 (lithium cobalt nickel manganese oxide), the transition metal elements among the elements constituting the positive electrode layer are Ni, Co, and Mn. The same applies to positive electrodes other than lithium cobalt oxide-based ones. For example, when the positive electrode is composed of LiFePO4 (lithium iron phosphate), the transition metal element among the elements constituting the positive electrode layer is Fe. Also, the transition metal element constituting the positive electrode layer may be a transition metal element such as V (vanadium).

[0038] The average particle diameter of the plurality of primary particles constituting the positive electrode layer 12 is preferably 5 μm or more. Specifically, the average particle diameter of the primary particles used for calculating the average orientation angle is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 12 μm or more.

[0039] The positive electrode layer 12 may contain pores. When the sintered body contains pores, particularly open pores, when incorporated into the battery as the positive electrode layer, the electrolyte can penetrate into the interior of the sintered body, and as a result, the lithium ion conductivity can be improved. 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 known methods.

[0040] The average pore diameter of the positive electrode layer 12 is preferably 0.1 to 10.0 μm, more preferably 0.2 to 5.0 μm, and still more preferably 0.25 to 3.0 μm. When it is within the above range, the generation of stress concentration at local large pores can be suppressed, and the stress in the sintered body is likely to be uniformly released. In addition, the improvement of lithium ion conductivity due to the internal penetration of the electrolyte by pores can be more effectively realized.

[0041] The thickness of the positive electrode layer 12 in the laminate 1 is not particularly limited, but for example, it is preferably 2 to 200 μm, more preferably 5 to 120 μm, and still more preferably 10 to 80 μm. When it is within such a range, there is an advantage that the electron resistance can be suppressed, the movement resistance of Li ions contained in the electrolyte can also be suppressed, and the battery resistance can be reduced.

[0042] (Separator) The separator 20 is composed of a microporous membrane made of ceramic. The separator 20 contains magnesia (MgO). Specifically, for example, it can be composed of magnesia (MgO) and glass. In the separator 20, MgO and glass exist in the form of particles bonded to each other by sintering. The ceramic contained in the separator 20 may contain, in addition to MgO and glass, Al2O3, ZrO2, SiC, Si3N4, AlN, etc.

[0043] The glass contained in the separator 20 preferably contains 25% by weight or more of SiO2, more preferably 30 to 95% by weight, still more preferably 40 to 90% by weight, and particularly preferably 50 to 80% by weight. The content of the glass in the separator 20 is preferably 3 to 70% by weight, more preferably 5 to 50% by weight, still more preferably 10 to 40% by weight, and particularly preferably 15 to 30% by weight based on the total weight of the separator 20. When within this range, it is possible to effectively achieve both high yield and excellent charge-discharge cycle characteristics. The addition of the glass component to the separator 20 is preferably carried out by adding glass frit to the raw material powder of the separator. The glass frit preferably contains at least one of Al2O3, B2O3, and BaO as components other than SiO2.

[0044] The thickness of the separator 20 in the laminate 1 is not particularly limited. For example, the thickness of the first region 21 (the region between the positive electrode layer 12 and the negative electrode layer 16) of the separator 20 is preferably 5 to 60 μm, more preferably 10 to 30 μm. The second region 22 and the third region 23 of the separator 20 can each have the same thickness as the positive electrode layer 12 and the negative electrode layer 16. The porosity of the separator 20 is also not particularly limited, but can be, for example, about 30 to 70%, preferably about 40 to 60%.

[0045] (Negative electrode layer) The negative electrode layer 16 is composed of, for example, a plate-shaped sintered body containing a titanium-containing composition. The negative electrode layer 16 can be made to contain no binder or conductive assistant. The titanium-containing sintered body preferably contains lithium titanate Li4Ti5O 12 (hereinafter, LTO) or niobium-titanium composite oxide Nb2TiO7, more preferably contains LTO. Although LTO is typically known to have a spinel-type structure, it can adopt other structures during charge and discharge. For example, LTO becomes Li4Ti5O 12 (spinel structure) and Li7Ti5O 12(Rock salt structure) The reaction proceeds in the two-phase coexistence. Therefore, LTO is not limited to the spinel structure. A part of LTO may be substituted with other elements. Examples of other elements include Nb, Ta, W, Al, Mg, etc. The LTO sintered body can be manufactured, for example, according to the method described in JP-A-2015-185337.

[0046] When the negative electrode layer 16 is composed of a sintered body containing LTO, the transition metal element among the elements constituting the negative electrode layer is Ti. Further, when the negative electrode layer 16 is composed of a sintered body containing Nb2TiO7, the transition metal elements among the elements constituting the negative electrode layer are Nb and Ti.

[0047] The negative electrode layer 16 has a structure in which a large number of primary particles are bonded. It is preferable that these primary particles are composed of LTO or Nb2TiO7. The negative electrode layer 16 may be formed as an integral sintered body together with the positive electrode layer 12 and the separator 20. Further, the negative electrode layer 16 may be combined after being formed as a sintered body different from the integral sintered body of the positive electrode layer 12 and the separator 20.

[0048] The thickness of the negative electrode layer 16 in the laminate 1 is not particularly limited, but is preferably, for example, 1 to 150 μm, more preferably 2 to 120 μm, and still more preferably 5 to 80 μm. The primary particle size, which is the average particle size of the plurality of primary particles constituting the negative electrode layer 16, is preferably 1.2 μm or less, more preferably 0.02 to 1.2 μm, and still more preferably 0.05 to 0.7 μm.

[0049] The negative electrode layer 16 preferably contains pores. By containing pores, particularly open pores, when incorporated into the battery as the negative electrode layer, the electrolyte can penetrate inside, and as a result, the lithium ion conductivity can be improved. The porosity of the negative electrode layer 16 is preferably 20 to 60%, more preferably 30 to 55%, and still 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 still more preferably 0.12 to 1.5 μm.

[0050] 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. Further, it may be formed to be exposed on one of the main surfaces of the negative electrode layer 16. The current collector layer 19 can be made of a material having excellent conductivity. The current collector layer 19 may be composed 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.

[0051] (Positive electrode side edge layer) The positive electrode side edge layer 41 included in the lithium secondary battery 10 according to the present disclosure contains 70% vol or more and 100% vol or less of the positive electrode active material constituting the positive electrode layer 12. When the content ratio of the positive electrode active material in the positive electrode side edge layer is 70% vol or more, it becomes possible to stably manufacture the sintered body constituting the electrode while realizing low resistance. Specifically, in the manufacture of the sintered body constituting the electrode, the occurrence of delamination between layers is small, and the sintered body can be obtained with good yield. Specifically, as the positive electrode active material, in addition to LCO exemplified in the description of the positive electrode layer 12, 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. may be mentioned.

[0052] Among the materials constituting the positive electrode side edge layer 41, other than the positive electrode active material, it is preferable to contain a metal from the viewpoint of reducing resistance. The metal that can be contained in the positive electrode side edge layer 41 is preferably at least one selected from the group consisting of Au (gold), Pt (platinum), and Ir (iridium). For example, the positive electrode side edge layer 41 preferably contains 30 vol% or less of Au. As the material constituting the positive electrode side edge layer 41, the content ratio of the positive electrode active material to the total of the positive electrode active material and the metal is preferably 70% or more, more preferably 90% or more, and even more preferably 95% or more. It is more preferable that the positive electrode side edge layer 41 is composed of the positive electrode active material and the metal.

[0053] The positive electrode side edge layer 41 is provided so as to be in contact with at least a plurality (two or more) of end faces among the end faces of the positive electrode layer 12 exposed on the side surface s1 of the laminate 1. The positive electrode side edge layer 41 is a positive electrode connection portion that connects to at least two or more of the positive electrode layers 12 included in the laminate 1. The positive electrode side edge layer 41 is preferably provided so as to be in contact with all of the end faces of the positive electrode layer 12 exposed on the side surface s1, and more preferably extends so as to cover the entire area of the side surface s1. It is considered that the resistance is reduced by the presence of the positive electrode side edge layer 41 between the positive electrode current collector 14 (FIG. 1) and the positive electrode layer 12 so as to connect the end faces of the positive electrode layer 12. Furthermore, by setting the composition of the positive electrode side edge layer to contain 70% or more of the positive electrode active material, peeling of the layer interface during the manufacturing process of the electrode is prevented, and it can be manufactured with good yield. Also, by configuring the positive electrode side edge layer 41 as a sintered body, it is considered that a lithium secondary battery excellent in cycle resistance can be obtained without elution of the side edge layer even when the charge-discharge cycle is repeated.

[0054] The thickness of the positive electrode side edge layer 41 is not particularly limited, but is preferably, for example, 2 to 500 μm, more preferably 5 to 200 μm. In addition to the above materials, the positive electrode side edge layer 41 may contain a material that does not inhibit resistance reduction and can suppress interface peeling of the sintered body. For example, in addition to the positive electrode active material, the positive electrode side edge layer 41 may contain ceramics such as oxides, silicates, phosphates, nitrides, and carbides.

[0055] (Negative electrode side edge layer) The negative electrode side edge layer 42 included in the lithium secondary battery 10 according to the present disclosure is a layer interposed between the side surface s2 of the laminate 1 and the negative electrode current collector 18. The negative electrode side edge layer 42 is provided so as to be in contact with at least a plurality (two or more) of end faces among the end faces of the negative electrode layer 16 exposed on the side surface s2 of the laminate 1. The negative electrode side edge layer 42 is a negative electrode connection part that connects to at least two or more of the negative electrode layers 16 included in the laminate 1. Preferably, the negative electrode side edge layer 42 is provided so as to be in contact with all of the end faces of the negative electrode layer 16 exposed on the side surface s2, and more preferably, it extends so as to cover the entire area of the side surface s2. It is considered that the resistance is reduced by the presence of the negative electrode side edge layer 42 between the negative electrode current collector 18 (FIG. 1) and the negative electrode layer 16 so as to connect the end faces of the negative electrode layer 16 to each other. In particular, by forming the negative electrode side edge layer 42 of a noble metal such as Au, Pt, or Ir, it is considered that a lithium secondary battery excellent in cycle resistance can be obtained without elution of the side edge layer even when the charge-discharge cycle is repeated.

[0056] The metal constituting the negative electrode side edge layer 42 is preferably at least one or a combination of two or more selected from the group consisting of Au (gold), Pt (platinum), Ir (iridium), palladium (Pd), Ag (silver), rhodium (Rh), and Cu (copper). When these metals are used, delamination between layers hardly occurs in the manufacturing process of the sintered body 9 including the negative electrode side edge layer 42, and the sintered body 9 can be obtained stably. In addition, by covering the side surface s2 of the laminate 1 with a metal material having a lower resistance than the conductive adhesive, electricity can be extracted more efficiently from the negative electrode layer 16. In the lithium secondary battery according to the present disclosure, the current collector may be directly attached to the side surface s2 via a conductive adhesive without providing the negative electrode side edge layer 42.

[0057] (Manufacturing method) The outline of the manufacturing method of the sintered body included in the lithium secondary battery according to the present disclosure will be described. FIG. 4 schematically shows a process of preparing each sheet for forming the laminate and stacking and pressing them in the manufacturing process of the sintered body.

[0058] Referring to Fig. 4(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 separately prepared. Typically, first, a slurry containing the raw materials constituting each layer is prepared, and then the prepared slurry is formed into a sheet shape on a resin film to prepare a green sheet. For the negative electrode green sheet 116, a current collector layer 119 may be formed on one of the main surfaces. Referring to Fig. 4(2), each sheet cut to a predetermined width is stacked in order so as to have a predetermined layer configuration. In the example of Fig. 4, the layer configuration is shown simply, but 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.

[0059] Referring to Fig. 4(1), at the time of stacking, each green sheet may be used alone in the thickness direction, or may be in a form in which two or more sheets of the same type are continuously stacked in the thickness direction. For example, in order 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 at the sintering stage, and thus become one layer in the sintered body. When two negative electrode green sheets 116 having current collector layers 119 are stacked, it is preferable to stack them so that the current collector layers 119 are in contact with each other.

[0060] Referring to Fig. 4(3), the green sheet laminate 101 is pressed to crimp the layers together. Specifically, the green sheets included in the green sheet laminate 101 can be crimped by pressing. It is preferable to press the green sheet laminate 101 in the thickness direction (Z-axis direction). The pressing method can be, for example, cold isostatic pressing (CIP), warm water isostatic pressing (WIP), hydrostatic pressing, etc., and is not particularly limited. The pressing may be performed while heating.

[0061] Subsequently, the green sheet laminate 101 is cut. FIG. 5 shows a part of the process for manufacturing a rectangular sintered body in which each layer is formed in a quadrilateral shape and the whole is a rectangular parallelepiped. Specifically, it schematically shows the process of cutting the green sheet laminate and arranging side-edge green sheets on both side surfaces. Referring to FIG. 5(1), the green sheet laminate 101 is cut. In FIG. 5(1), the cutting location is indicated by a thick line. First, both side surfaces of the green sheet laminate 101 are cut so as to have a predetermined width. At this time, one of the two side surfaces is cut at a position where the positive electrode layer is exposed and the negative electrode layer is not exposed, and the other of the two side surfaces is cut at a position where the negative electrode layer is exposed and the positive electrode layer is not exposed. Subsequently, it is cut in the direction along the width direction (the direction along the X-axis) so as to obtain a laminate having a predetermined depth. The form of the lamination and the setting of the cutting location may be determined according to the desired form of the sintered body (the overall dimensions, the width and thickness of each layer). As an example, it may be cut so that the width direction (the direction along the X-axis) and the depth direction (the direction along the Y-axis) are each 5 mm. As another example, it may be cut so that the distance w1 from the inner end of the positive electrode layer or the negative electrode layer to the side surface is 0.5 mm. FIG. 5(2) shows the green sheet laminate 101 after cutting.

[0062] Next, referring to FIG. 5(3), a forming material for the side-edge layer is arranged on both side surfaces of the green sheet laminate 101. For example, a paste of the material constituting the side-edge layer can be transferred to the side surface of the green sheet laminate using pad printing or stamping printing. The material constituting the side-edge layer can be prepared as a paste in advance. This paste is applied onto a base sheet such as a silicon film to produce a base sheet for the side-edge layer. The paste is transferred to the side surface of the green sheet laminate by pressing the paste-coated surface of the base sheet for the side-edge layer against the side surface of the green sheet laminate. The positive-electrode side-edge material paste 141 is transferred to the side surface of the green sheet laminate 101 on the side where the positive electrode green sheet 112 is exposed. The negative-electrode side-edge material paste 142 is transferred to the side surface of the green sheet laminate 101 on the side where the negative electrode green sheet 116 is exposed.

[0063] Next, degreasing and sintering are performed to obtain an integral sintered body 9 (Figure 2) having side edge layers on both side surfaces of the laminate. The degreasing and sintering can be carried out under known conditions and by known methods. The thickness and width of each layer in the obtained integral sintered body can be confirmed, for example, by polishing the laminated integral sintered body with a cross-section polisher and observing the obtained cross-section by SEM.

[0064] Subsequently, current collectors are attached to both side surfaces of the sintered body. Referring to Figure 1, a positive current collector 14 is attached to the positive electrode side edge layer 41 of the sintered body 9, and a negative current collector 18 is attached to the negative electrode side edge layer 42, respectively. As the positive current collector 14 and the negative current collector 18, a conductive material can be used. For example, an aluminum foil, a copper foil, or the like can be used. The positive current collector 14 can be attached so as to cover the entire positive electrode side edge layer 41 and can be further configured to extend to the lower surface of the sintered body 9. The negative current collector 18 can be attached so as to cover the entire negative electrode side edge layer 42 and can be further configured to extend to the upper surface of the sintered body 9. Between the positive electrode side edge layer 41 and the positive current collector 14, and between the negative electrode side edge layer 42 and the negative current collector 18, a conductive adhesive can be used for adhesion. As the conductive adhesive, for example, a conductive carbon paste can be used. The thickness of the conductive adhesive layer is not particularly limited as long as it exhibits the effect as an adhesive layer and does not hinder the effect of the invention, but can be, for example, about 1 to 500 μm.

[0065] The electrode obtained by the above manufacturing method can be housed inside an exterior body according to known methods and conditions, and an electrolytic solution can be sealed to obtain a lithium secondary battery.

[0066] Figure 6 shows an example of another embodiment of the sintered body included in the lithium secondary battery according to the present disclosure. Figure 6 shows a part of the process when manufacturing a sintered body formed in a round shape and having an overall cylindrical shape. As shown in Figure 6, a part of the cylinder is cut parallel to the tangent of the circle, and a shape with two opposing side surfaces is formed. This shape is referred to as "round".

[0067] Referring to Fig. 6(1), the green sheet laminate 101 obtained in Fig. 4(3) is cut out into a circular shape using a punch or the like so as to have a predetermined diameter. Next, referring to Fig. 6(2), two portions of the cylinder are cut out on a plane parallel to the depth direction of the green sheet laminate 101 (a plane parallel to the YZ plane). Referring to Fig. 6(3), among the two side surfaces exposed by the cutting, the positive electrode green sheet 112 is exposed on one side, and the negative electrode green sheet 116 is exposed on the other side. The positive electrode side edge material paste 141 and the negative electrode side edge material paste 142 are respectively disposed on these two side surfaces. The specific method can be the same as that in the case of a square shape.

[0068] Next, similar to the case of a square shape, degreasing and firing are performed to obtain a circular sintered body. Subsequently, current collectors are respectively disposed on the positive electrode side and the negative electrode side to obtain a circular electrode. Assembly is performed by a known procedure similar to the case of a square shape, and for example, a lithium secondary battery having an appearance shown in Fig. 7 can be obtained.

[0069] Figs. 8 to 10 show an example of another embodiment of the sintered body included in the lithium secondary battery according to the present disclosure. Referring to Figs. 8 and 10, the sintered body 59 has an overall cylindrical shape. The sintered body 59 includes a laminate 51, a positive electrode conduction part 541 as a positive electrode connection part, and a negative electrode conduction part 542 as a negative electrode connection part. The positive electrode conduction part 541 is a columnar part extending in the lamination direction so as to fill a via 551 which is a bottomed hole extending in the lamination direction of the laminate 51. The negative electrode conduction part 542 is a columnar part extending in the lamination direction so as to fill a via 552 which is a bottomed hole extending in the lamination direction of the laminate 51. The lamination structure in the laminate 51 is the same as that of the laminate 1, and the same components are denoted by the same reference numerals and the description thereof is omitted.

[0070] The positive electrode conduction part 541 extends through the plurality of positive electrode layers 12 and the separator 20 in the stacking direction. The positive electrode conduction part 541 is preferably provided so as to be connected to all of the positive electrode layers 12 included in the laminate 51. Further, the positive electrode conduction part 541 is arranged at a position not connected to the negative electrode layer 16. The negative electrode conduction part 542 extends through the plurality of negative electrode layers 16 and the separator 20 in the stacking direction. The negative electrode conduction part 542 is preferably provided so as to be connected to all of the negative electrode layers 16 included in the laminate 51. Further, the negative electrode conduction part 542 is arranged at a position not connected to the positive electrode layer 12.

[0071] The positive electrode conduction part 541 can be made of the same material as the positive electrode side edge layer 41 described above. That is, the positive electrode conduction part 541 contains 70% vol or more and 100% vol or less of the positive electrode active material constituting the positive electrode layer 12. Regarding other materials, it is the same as the positive electrode side edge layer 41, and the description is omitted.

[0072] The negative electrode conduction part 542 can be made of the same material as the negative electrode side edge layer 42 described above. That is, the metal constituting the negative electrode conduction part 542 is preferably at least one or a combination of two or more selected from the group consisting of Au (gold), Pt (platinum), Ir (iridium), palladium (Pd), Ag (silver), rhodium (Rh), and Cu (copper).

[0073] Figs. 9 and 10 are schematic diagrams showing a part of the process of manufacturing the sintered body 59. With reference to Figs. 9 and 10, the process of manufacturing the sintered body 59 will be described. First, the positive electrode green sheet 112, the negative electrode green sheet 116, and the separator green sheet 120, which are materials constituting the laminate, are separately prepared. Next, with reference to Fig. 9(1), a plurality of positive electrode green sheets 112 and a plurality of negative electrode green sheets 116 are laminated so as to be alternately laminated via the separator green sheet 120. With reference to Figs. 9(2) and (3), after obtaining the green sheet laminate 501, the green sheet laminate 501 is pressed to bond the layers together. With reference to Fig. 9(4), the green sheet laminate 501 obtained in Fig. 9(3) is cut out into a circular shape using a puncher or the like so as to have a predetermined diameter. Also, a separately prepared separator green sheet 120 is cut out into a circular shape so as to have a predetermined diameter.

[0074] Referring to Fig. 10(5), vias 551 and 552 are formed in the separator green sheet 120 and the green sheet laminate 501 obtained in Fig. 9(4). The via 551 in the green sheet laminate 501 penetrates the positive electrode green sheet 112 and is provided at a position not in contact with the negative electrode green sheet 116. The via 552 in the green sheet laminate 501 penetrates the negative electrode green sheet 116 and is provided at a position not in contact with the positive electrode green sheet 112. Next, referring to Fig. 10(6), separator green sheets 120 are laminated and pressure-bonded to the upper and lower surfaces of the green sheet laminate 501, respectively. At this time, the separator green sheet 120 laminated on the upper surface of the green sheet laminate is laminated so that the position of the via 551 in the separator green sheet 120 coincides with the position of the via 551 in the green sheet laminate 501. Also, the separator green sheet 120 laminated on the lower surface of the green sheet laminate is laminated so that the position of the via 552 in the separator green sheet 120 coincides with the position of the via 552 in the green sheet laminate 501. In this way, vias 551 and 552, which are bottomed holes, are formed. Next, referring to Fig. 10(7), a material that will become the positive electrode conduction portion 541 is filled into the via 551. Also, a material that will become the negative electrode conduction portion 542 is filled into the via 552. Next, degreasing and firing are performed. In this way, referring to Fig. 10(8), a sintered body 59 in which the positive electrode conduction portion 541 and the negative electrode conduction portion 542 are formed is obtained in the laminate 51.

[0075] (Electrolyte) Referring to FIG. 1, the lithium secondary battery 10 may include an electrolyte solution 22. The electrolyte solution 22 is not particularly limited, and an electrolyte solution known as an electrolyte solution in a lithium secondary battery can be used. For example, as the solvent, 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) can be used. As the electrolyte dissolved in the solvent, for example, lithium salt compounds such as lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4) can be used. The electrolyte solution 22 may further contain at least one selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), and lithium difluoro(oxalato)borate (LiDFOB) as an additive.

[0076] The concentration of the electrolyte in the electrolyte solution 22 is preferably 0.5 to 2 mol / L, more preferably 0.6 to 1.9 mol / L, still more preferably 0.7 to 1.7 mol / L, and particularly preferably 0.8 to 1.5 mol / L.

[0077] In addition, as the electrolyte, in addition to the electrolyte solution 22, a solid electrolyte or a polymer electrolyte can be used. In that case, as in the case of the electrolyte solution 22, it is preferable that at least the pores of the separator 20 are impregnated with the electrolyte. The impregnation method is not particularly limited, and examples include a method of melting the electrolyte and allowing it to penetrate into the pores of the separator 20, and a method of pressing a compact of the electrolyte against the separator 20.

[0078] [Examples and Comparative Examples] Hereinafter, the lithium secondary battery of the present disclosure will be described in more detail with reference to examples and comparative examples. [Examples 1 to 4, Comparative Examples 1 to 7] Lithium secondary batteries were fabricated according to the methods described in 1 to 10 below. The obtained lithium secondary batteries were evaluated by the methods described in Evaluations 1 to 3.

[0079] 1. Preparation of laminate (1)~(3) The green sheets of each layer constituting the laminate were prepared under the conditions and by the method described below. In (1)~(3), the viscosity of the slurry was measured using an LVT type viscometer manufactured by Brookfield. When forming the slurry on a PET film, the doctor blade method was used.

[0080] (1) Preparation of LCO green sheet (positive electrode green sheet) Co3O4 powder (manufactured by Shodo Chemical Co., Ltd.) and Li2CO3 powder (manufactured by Motoyama Chemical Co., Ltd.) were weighed so that the molar ratio of Li / Co was 1.01, and then held at 780 °C for 5 hours. The obtained powder was ground in a pot mill so that the volume-based D 50 was 0.4 μm to obtain a powder composed of LCO plate-like particles. 100 parts by weight of the obtained LCO powder, 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 Kokukin Kasei Co., Ltd.), and 4.5 parts by weight of a dispersant (product name Leodol SP-O30, manufactured by Kao Corporation) were mixed. The obtained mixture was stirred under reduced pressure to defoam 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.

[0081] (2) Preparation of LTO green sheet (negative electrode green sheet) LTO powder (volume-based D 50100 parts by weight of particles with a particle size of 0.06 μm (manufactured by Sigma-Aldrich Japan K.K.), 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, manufactured by Sekisui Chemical Co., Ltd.), 4 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 Leodol SP-O30, manufactured by Kao Corporation) were mixed. The obtained negative electrode raw material mixture was stirred and defoamed under reduced pressure, 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 negative electrode layer after firing was adjusted to 10 μm.

[0082] (2´) Formation of the current collector layer On one side of the LTO green sheet prepared in (2), an Au paste (manufactured by Tanaka Precious Metals Co., product name: GB-2706) was printed using a printing machine. The thickness of the printed layer was set to be 0.2 μm after firing.

[0083] (3) Preparation of the separator green sheet Magnesium carbonate powder (manufactured by Kojima Chemical Industry 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 Electric Glass Co., Ltd., CK0199) were mixed at a weight ratio of 7:3. The obtained mixed powder (volume-based D 50100 parts by weight of particles with a particle size of 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 Leodol SP-O30, manufactured by Kao Corporation) were mixed. The obtained raw material mixture was stirred under reduced pressure to defoam 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. The thickness of the separator layer located between the positive electrode layer and the negative electrode layer was set to 25 μm after firing. The thickness of the separator (insulating layer) located adjacent to the positive electrode layer was set to 24 μm after firing. The thickness of the separator (insulating layer) located adjacent to the negative electrode layer was set to 20 μm after firing.

[0084] 2. Cutting of the sheet The green sheets obtained in 1. were cut for laminating respectively.

[0085] 3. Lamination, pressure bonding and cutting of the laminate As shown in [Figure 4], various green sheets were laminated. When two LTO green sheets were stacked, the current collector layers were laminated so as to be in contact with each other. Each sheet was repeatedly stacked in the order shown in Figure 4 so that the number of cells formed in the laminate became 19 (note that only a part of the repetition is shown in Figure 4). The obtained laminate was subjected to 100 kgf / cm by CIP (Cold Isostatic Pressing Method). 2Pressed to bond the green sheets together to obtain an unfired green sheet laminate. In the pressing, pressure was applied in the thickness direction of the green sheet. Subsequently, the unfired green sheet laminate was cut. For Examples 1, 2, Examples 101, 102 and Comparative Examples 1 to 3, Examples 101 to 104, as shown in [Figure 5], cutting was performed using a Thomson blade so that both the width direction and the depth direction of the laminate were 5 mm, and a square laminate was obtained. For Examples 3, 4, Examples 103, 104 and Comparative Examples 5 to 7, Comparative Examples 10105 to 108, as shown in [Figure 6], cutting was performed using a hand puncher so that the laminate became a cylinder with a diameter of 16 mm, and further the side surface was cut to obtain a round laminate having a flat surface on the side surface. For Example 105, as shown in [Figure 8](7), cutting was performed using a hand puncher so that the laminate became a cylinder with a diameter of 16 mm, and further vias penetrating the positive electrode layer and vias penetrating the negative electrode layer were formed.

[0086] 4. Preparation of Side Edge Layer Paste (1) Paste for Positive Electrode Side Edge Layer First, Co3O4 powder (manufactured by Shoei Chemical Industry Co., Ltd.) and Li2CO3 powder (manufactured by Honjo Chemical Co., Ltd.) weighed so that the molar ratio of Li / Co was 1.01 were mixed, then held at 780 °C for 5 hours, and the obtained powder was volume-based D in a pot mill 50It was pulverized so that it became 0.4 μm to obtain a powder composed of LCO plate-like particles. 100 parts by weight of the obtained LCO powder, 20 parts by weight of a dispersion medium (2-ethylhexanol), 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 Leodol SP-O30, manufactured by Kao Corporation) were mixed. The obtained mixture was stirred under reduced pressure to defoam and prepare an LCO paste. Also, as an Au paste, TR-1535R paste of Tanaka Precious Metals Co., Ltd. was prepared. Next, the LCO paste and the Au paste were mixed so as to have the volume ratios shown in [Table 1], [Table 2] (Examples 1 to 4, Example 105, Comparative Examples 1 to 7). Also, as a Pt paste, TR-1535R paste of Tanaka Precious Metals Co., Ltd. was prepared. Next, the LCO paste and the Au paste were mixed so as to have the volume ratios shown in [Table 1], [Table 2] (Example 101, Comparative Examples 101 to 102). Also, Ir powder (IRE02PB, manufactured by High Purity Chemical Laboratories Co., Ltd.) was prepared as Ir, and an Ir paste was prepared in the same procedure as for preparing the LCO paste. Next, the LCO paste and the Ir paste were mixed so as to have the volume ratios shown in [Table 1], [Table 2] (Example 102, Comparative Examples 103 to 104). The mixing of the pastes was performed by putting the two pastes into a container and stirring them 500 times with a glass rod.

[0087]

Table 1

[0088]

Table 2

[0089] (2) Paste for the negative electrode side edge layer As an Au paste, TR-1535R paste of Tanaka Precious Metals Co., Ltd. was prepared.

[0090] 5. Transfer of the edge layer paste For Examples 1 to 4, Examples 101 to 104, Comparative Examples 1 to 7, and Comparative Examples 101 to 108, the paste for the positive electrode side edge layer prepared in the above 4(1) was applied to the silicon resin film so that the thickness after firing was 50 μm, and then the side surface of the green sheet laminate on the side where the positive electrode was exposed was pressed against it to transfer the paste. Also, the paste for the negative electrode side edge layer prepared in the above 4(2) was applied to the silicon resin film so that the thickness after firing was 20 μm, and then the side surface of the green sheet laminate on the side where the negative electrode was exposed was pressed against it to transfer the paste. For Example 105, among the vias formed in the laminate, the paste for the positive electrode side edge layer prepared in the above 4(1) was filled into the vias penetrating the positive electrode layer. Also, the paste for the negative electrode side edge layer prepared in the above 4(2) was filled into the vias penetrating the negative electrode layer.

[0091] 6. Debinding and Firing The green sheet laminate prepared in 5. was heated from room temperature to 600 °C and debound for 5 hours, then heated to 800 °C and fired for 10 minutes while holding, and then cooled. Thus, a laminated integrated sintered body was obtained.

[0092] Evaluation 1: Yield Evaluation In the laminated integrated sintered body, the presence or absence of peeling at the boundary between the laminate and the side edge layer was visually confirmed. According to the following formula, the ratio of the number of samples without peeling to the number of samples of the laminated integrated sintered body was calculated as the yield rate (%). Yield rate (%) = 100 × (number of samples without peeling after firing) / (number of samples prepared)

[0093] 7. Preparation of Conductive Carbon Paste Weighed binder (CMC: MAC350HC, manufactured by Nippon Paper Industries Co., Ltd.) to be 1.2 wt% with respect to pure water, dissolved it by stirring with a stirrer to obtain a 1.2 wt% CMC solution. Prepared a carbon dispersion (product number: BPW-229, manufactured by Nippon Carbon Co., Ltd.) and a dispersant solution (product number LB-300, manufactured by Showa Denko K.K.). Subsequently, weighed the carbon dispersion, the dispersant solution, and the 1.2 wt% CMC solution so that their ratio was 0.22:0.29:1, and mixed them with a planetary mixer to prepare a conductive carbon paste.

[0094] 8. Bond the positive electrode side edge layer of the laminated sintered body and the aluminum foil with the conductive carbon paste Screen-printed the conductive carbon paste obtained in 7. on the aluminum foil as the positive electrode current collector. For Examples 1 to 4, Examples 101 to 104, Comparative Examples 1 to 7, and Comparative Examples 101 to 108, place the positive electrode side edge layer of the laminated sintered body obtained in 3. so as to be within the undried printed pattern (the area where the conductive carbon paste is applied), gently press it with a finger, and then vacuum dry it at 50 °C for 60 minutes. In this way, the positive electrode side edge layer of the laminated sintered body and the positive electrode current collector were bonded via a conductive carbon adhesive layer. The thickness of the conductive carbon adhesive layer was set to 30 μm. For Example 105, the positive electrode current collector was arranged on the end face where the positive electrode conduction part was exposed among the end faces in the stacking direction of the cylindrical body. The positive electrode current collector was joined under the same conditions as in the other examples via a conductive carbon adhesive layer.

[0095] 9. Bond the negative electrode side edge layer of the laminated sintered body and the aluminum foil with the conductive carbon paste For Examples 1 to 4, Examples 101 to 104, Comparative Examples 1 to 7, and Comparative Examples 101 to 108, in the same manner as in 8., an aluminum foil as the negative electrode current collector was adhered to the negative electrode side edge layer of the laminated sintered body via a conductive carbon adhesive layer. For Example 105, a negative electrode current collector was disposed on the end face where the negative electrode conductive portion was exposed among the end faces in the stacking direction of the columnar body. The negative electrode current collector was joined under the same conditions as in other examples through a conductive carbon adhesive layer in the same manner as in other examples.

[0096] 10. Fabrication of Lithium Secondary Battery Between the positive electrode can and the negative electrode can that will constitute the battery case, the positive electrode current collector, the laminated integrally sintered body, and the negative electrode current collector were accommodated in this order from the positive electrode can toward the negative electrode can so as to be laminated, and after filling with the electrolytic solution, the positive electrode can and the negative electrode can were caulked through a gasket to be sealed. Thus, a coin cell type lithium secondary battery having a diameter of 20 mm and a thickness of 1.6 mm was fabricated. As the electrolytic solution, a solution in which LiPF6 was dissolved at a concentration of 1.5 mol / L in an organic solvent obtained by mixing propylene carbonate (PC) and γ-butyrolactone (GBL) at a volume ratio of 1:3 was used.

[0097] Evaluation 2. Evaluation of Battery Performance (0.2C Discharge Capacity Evaluation) Using the battery containing the obtained laminated integrally sintered body, the battery capacity was confirmed in an environment at 25°C. Charging was performed at a constant current of 0.2C until the voltage reached 2.7V. Discharging was performed at a constant current of 0.2C until the voltage reached 1.5V. Charge and discharge of the second cycle were performed under the same conditions as the first cycle, and the discharge capacity of the second cycle was defined as the 0.2C discharge capacity.

[0098] Evaluation 3. Evaluation of Battery Performance (Evaluation of Resistance Immediately after Discharge Start) The resistance 1 second after the start of discharge in the second cycle was measured and defined as the resistance immediately after discharge start.

[0099] [Evaluation Results] Regarding the lithium secondary batteries of Examples 1 to 4, Examples 101 to 105, and Comparative Examples 1 to 7, Comparative Examples 101 to 108, the evaluation results of the 0.2C discharge capacity and resistance value, and the yield evaluation (yield rate) in the production of the sintered body are summarized in [Table 3] and [Table 4].

[0100]

Table 3

[0101]

Table 4

[0102] As shown in [Table 3] and [Table 4], the lithium secondary batteries of Examples 1 to 4 and Examples 101 to 105 containing 70% or more of LCO, which is a positive electrode active material, in the positive electrode connection part (positive electrode side edge layer, positive electrode conduction part) had lower resistance values compared to Comparative Example 3 and Comparative Example 7 that did not have a positive electrode connection part. Further, in Comparative Example 1 and Comparative Example 5, although they had a positive electrode side edge layer, the positive electrode active material was less than 70%, and peeling occurred in the sintered body, resulting in a low yield. In contrast, all of the lithium secondary batteries of Examples 1 to 4 and Examples 101 to 105 containing 70% or more of LCO, which is a positive electrode active material, in the positive electrode connection part had a yield of 90% or more, and the yield was significantly high. Furthermore, in Comparative Example 2 and Comparative Example 6 in which the positive electrode side edge layer was composed only of Au, peeling occurred at the interface between the laminate and the side edge layer, and a sintered body for forming the secondary battery could not be obtained.

[0103] As shown in the evaluation results of [Table 3] and [Table 4], it was confirmed that by disposing a positive electrode connection part containing a positive electrode active material at a ratio of 70% or more on the positive electrode side of a laminate including a positive electrode layer, a negative electrode layer, and a separator, a lithium secondary battery with low internal resistance can be stably manufactured with a good yield.

[0104] It should be understood that the embodiments disclosed this time are illustrative in all respects and not restrictive in any way. The scope of the present disclosure is defined not by the above description but by the claims, and it is intended that all modifications within the meaning and scope equivalent to the claims be included.

Explanation of Reference Numerals

[0105] 1. 51 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 exterior body, 24a positive electrode can, 24b negative electrode can, 24c gasket, 41 positive electrode side edge layer, 42 negative electrode side edge layer, 9, 50 sintered body, 541 positive electrode conduction part, 542 negative electrode conduction part, 551, 552 via, 101, 501 green sheet laminate, 112 positive electrode green sheet, 116 negative electrode green sheet, 120 separator green sheet, 141 positive electrode side edge material paste, 142 negative electrode side edge material paste.

Claims

1. A lithium secondary battery comprising a plurality of positive electrode layers, a plurality of negative electrode layers, and a separator, and comprising a sintered body including a laminated portion in which the positive electrode layers and the negative electrode layers are alternately laminated via the separator, wherein the sintered body includes a positive electrode connection portion that connects to at least two or more of the positive electrode layers included in the laminated portion, and the positive electrode connection portion contains 70% vol or more and 100% vol or less of the positive electrode active material constituting the positive electrode layer, is composed of the positive electrode active material constituting the positive electrode layer, or is composed of the positive electrode active material and at least one metal selected from the group consisting of Au (gold), Pt (platinum), and Ir (iridium), and is sintered integrally with the laminated portion, A lithium secondary battery.

2. The positive electrode connection portion is formed on a first side surface that is a surface where the positive electrode layer and the separator are exposed among the side surfaces of the sintered body, and is a positive electrode side edge layer that contacts an end portion of the positive electrode layer included in the laminated portion, The lithium secondary battery according to claim 1.

3. The positive electrode connection portion is a columnar portion that extends through the positive electrode layer and the separator in the stacking direction, The lithium secondary battery according to claim 1.

4. A current collector is further provided outside the positive electrode side edge layer. The lithium secondary battery according to claim 2.

5. On a second side surface that is a side surface facing the first side surface, a negative electrode side edge layer is formed, which is composed of at least one metal selected from the group consisting of Au (gold), Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), Rh (rhodium), and Cu (copper) and contacts an end portion of the negative electrode layer included in the laminated portion, The lithium secondary battery according to claim 2.

6. The positive electrode layer is composed of a lithium composite oxide sintered body, and the negative electrode layer is composed of a titanium-containing sintered body, The lithium secondary battery according to any one of claims 1 to 3.

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