Patent Publication Number: US-2023163434-A1

Title: Solid state battery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2021/021945, filed Jun. 9, 2021, which claims priority to Japanese Patent Application No. 2020-101149, filed Jun. 10, 2020, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a solid state battery. More specifically, the present invention relates to a laminated solid state battery formed by laminating layers, each constituting a battery constituent unit. 
     BACKGROUND OF THE INVENTION 
     Conventionally, secondary batteries that can be repeatedly charged and discharged have been used for various purposes. For example, each of the secondary batteries is used as a power supply of an electronic device such as a smartphone and a notebook computer. 
     In a secondary battery, a liquid electrolyte (electrolytic solution) such as an organic solvent has been conventionally used as a medium for moving ions. However, the secondary battery using the electrolytic solution has a problem such as leakage of the electrolytic solution. Therefore, a solid state battery including a solid electrolyte instead of a liquid electrolyte has been developed. 
     However, when a small-sized solid state battery is housed in a battery housing part of a small-sized electronic device such as a mobile device, if the solid state battery is loaded with dust, current leakage may occur between terminals of different batteries. On the other hand, for example, there has been proposed a solid state battery in which a thickness of a central part of the battery is smaller than a thickness of an end part (for example, Patent Document 1).
     Patent Document 1: Japanese Patent Application Laid-Open No. 2016-1601   

     SUMMARY OF THE INVENTION 
     In recent years, minimization of components mounted on a board has progressed, and accordingly, a thickness of a mask used for solder printing is reduced. This is because a short circuit between terminals and a short circuit between adjacent components are likely to occur when a solder printing amount increases. On the other hand, in the case of a mounted solid state battery, a size of the solid state battery is required to some extent in order to secure a battery capacity to some extent. Therefore, the mounted solid state battery is likely to be the largest component among mounting components of the board. Therefore, when the solid state battery having the shape as in Patent Document 1 is mounted on a board, a large solder printing amount is required, and thus there is a problem that short-circuiting with other extremely small mounting components easily occurs. 
       FIG.  12    is a schematic sectional view for explaining this, and illustrates an example in which a solid state battery  100  and a minimal mounting component  200  are mounted on a board  90 . The solid state battery  100  includes a power storage body having a positive electrode layer  101  and a negative electrode layer  102  laminated with a solid electrolyte layer  103  interposed therebetween, the positive electrode layer  101  includes a positive electrode current collector layer  104  and a positive electrode active material layer  105 , and the negative electrode layer  102  includes a negative electrode current collector layer  106  and a negative electrode active material layer  107 . The power storage body is covered with a protective layer  115 . A terminal electrode  116  is connected to each of the positive electrode current collector layer  104  and the negative electrode current collector layer  106 . When the mounting component  200  is mounted on the board  90 , since the mounting component  200  is a minimal component, a thickness of solder layers  96  for securing electrical connection with wiring electrodes  93  and  94  can be reduced. On the other hand, in the case of the solid state battery  100 , it is necessary to increase a thickness of solder layers  95  in order to ensure electrical connection between wiring electrodes  91  and  92  and the terminal electrode  116 . Then, there is a problem that the solder printing amount increases, and a possibility that a short circuit occurs between adjacent mounting components increases. 
     Therefore, an object of the present invention is to provide a solid state battery that prevents short circuit from occurring and enables high-density mounting when the solid state battery is mounted together with a minimal component. 
     In order to solve the above problems, a solid state battery according to one aspect of the present invention includes: a battery element body including a positive electrode layer and a negative electrode layer laminated with a solid electrolyte layer interposed between the positive electrode layer and the negative electrode layer, the battery element body defining a first end surface and a second end surface facing each other, and a peripheral surface between the first end surface and the second end surface; a first external electrode on the first end surface; a second external electrode on the second end surface; and a protective layer covering the peripheral surface of the battery element body, the solid state battery defining a first principal surface and a second principal surface facing each other in a lamination direction of the positive electrode layer and the negative electrode layer, in which the first external electrode covers the first end surface and covers a side of the first end surface of the peripheral surface of the battery element body via the protective layer, the second external electrode covers the second end surface and covers a side of the second end surface of the peripheral surface of the battery element body via the protective layer, and at least one of the first principal surface and the second principal surface include a pair of a first convex part and a second convex part, each of which extend from the first external electrode to the second external electrode along a longitudinal direction connecting the first external electrode and the second external electrode and located at opposed end parts of the solid state battery in a transverse direction to the longitudinal direction. 
     Furthermore, an electronic device according to another aspect of the present invention is an electronic device including: an elongated board; and the solid state battery according to the one aspect mounted on the board, in which a longitudinal direction of the board and a longitudinal direction connecting a first external electrode and a second external electrode of the solid state battery coincide with each other. 
     According to the present invention, it is possible to provide a solid state battery capable of preventing short circuit from occurring and enabling high-density mounting when the solid state battery is mounted together with a minimal component. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG.  1    is a schematic top view illustrating an example of a structure of a solid state battery according to a first embodiment of the present invention. 
         FIG.  2    is a schematic perspective view illustrating an example of a structure of the solid state battery according to the first embodiment of the present invention. 
         FIG.  3    is a schematic longitudinal sectional view taken along line III-III′ in  FIG.  1   . 
         FIG.  4    is a schematic longitudinal sectional view taken along line IV-IV′ in  FIG.  1   . 
         FIG.  5    is a schematic diagram for explaining the effect of the solid state battery according to the first embodiment of the present invention. 
         FIG.  6    is a schematic diagram for explaining the effect of the solid state battery according to the first embodiment of the present invention. 
         FIG.  7    is a schematic diagram for explaining the effect of the solid state battery according to the first embodiment of the present invention. 
         FIG.  8    is a schematic top view illustrating an example of a structure of a solid state battery according to a second embodiment of the present invention. 
         FIG.  9    is a schematic perspective view illustrating another example of the solid state battery according to the first embodiment of the present invention. 
         FIG.  10    is a schematic perspective view illustrating another example of the solid state battery according to the first embodiment of the present invention. 
         FIG.  11    is a schematic perspective view illustrating another example of the solid state battery according to the first embodiment of the present invention. 
         FIG.  12    is a schematic sectional view illustrating an example of a conventional solid state battery. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the “solid state battery” of the present invention will be described in detail. Although the description will be made with reference to the drawings as necessary, the illustrated contents are only schematically and exemplarily illustrated for the understanding of the present invention, and the appearance, the dimensional ratio, and the like may be different from the actual ones. 
     The “solid state battery” referred to in the present invention refers to a battery whose constituent elements are composed of a solid in a broad sense, and refers to an all-solid state battery whose battery constituent elements (particularly preferably all battery constituent elements) are composed of a solid in a narrow sense. In a preferred aspect, the solid state battery in the present invention is a laminated solid state battery configured such that layers constituting a battery constituent unit are laminated with each other, and preferably, such layers may be composed of a fired body. The “solid state battery” includes not only a so-called “secondary battery” capable of repeating charging and discharging but also a “primary battery” capable of only discharging. According to a preferred aspect of the present invention, the “solid state battery” is a secondary battery. The “secondary battery” is not excessively limited by its name, and may include, for example, a power storage device and the like. 
     The term “plan view” as used in the present specification is based on a form in a case where an object is captured from an upper side or a lower side along a thickness direction based on a lamination direction of each layer constituting the solid state battery. Furthermore, the term “sectional view” as used in the present specification is based on a form (to put it briefly, a form in the case of being cut along a plane parallel to the thickness direction) when viewed from a direction substantially perpendicular to the thickness direction based on the lamination direction of each layer constituting the solid state battery. Furthermore, the term “longitudinal direction” as used in the present specification is based on a direction along a long side when the solid state battery is viewed in a plan view, and the term “transverse direction” is based on a direction along a short side when the solid state battery is viewed in a plan view. Note that, in the drawings ( FIGS.  2  to  4  and  8  to  11   ), for convenience, the lamination direction (height direction) of the solid state battery is denoted by T, the longitudinal direction (length direction) of the solid state battery is denoted by L, and the transverse direction (width direction) of the solid state battery is denoted by W. Furthermore, the term “sectional view in the transverse direction” used in the present specification refers to a section when cut along the transverse direction, and the term “sectional view in the longitudinal direction” refers to a section when cut along the longitudinal direction. Furthermore, an “up-down direction” and a “left-right direction” used directly or indirectly in the present specification correspond to an up-down direction and a left-right direction in the drawings, respectively. Unless otherwise specified, the same reference symbols or signs indicate the same members or parts or the same semantic contents. In a preferred aspect, it can be understood that a downward direction in a vertical direction (that is, a direction in which gravity acts) corresponds to a “downward direction”, and an opposite direction corresponds to an “upward direction”. 
     First Embodiment 
     A solid state battery according to the present embodiment includes: a battery element body including a positive electrode layer and a negative electrode layer laminated with a solid electrolyte layer interposed between the positive electrode layer and the negative electrode layer, the battery element body defining a first end surface and a second end surface facing each other, and a peripheral surface between the first end surface and the second end surface; a first external electrode on the first end surface; a second external electrode on the second end surface; and a protective layer covering the peripheral surface of the battery element body, the solid state battery defining a first principal surface and a second principal surface facing each other in a lamination direction of the positive electrode layer and the negative electrode layer, in which the first external electrode covers the first end surface and covers a side of the first end surface of the peripheral surface of the battery element body via the protective layer, the second external electrode covers the second end surface and covers a side of the second end surface of the peripheral surface of the battery element body via the protective layer, and at least one of the first principal surface and the second principal surface include a pair of a first convex part and a second convex part, each of which extend from the first external electrode to the second external electrode along a longitudinal direction connecting the first external electrode and the second external electrode and located at opposed end parts of the solid state battery in a transverse direction to the longitudinal direction. 
       FIG.  1    is a schematic top view illustrating an example of a structure of a solid state battery  1  according to the present embodiment,  FIG.  2    is a schematic perspective view of the solid state battery  1 ,  FIG.  3    is a schematic longitudinal sectional view taken along line III-III′ in  FIG.  1   , and  FIG.  4    is a schematic longitudinal sectional view taken along line IV-IV′ in  FIG.  1   . 
     As illustrated in  FIG.  1   , a solid state battery  1  includes a battery element body  2  having a first end surface  2   a  and a second end surface  2   b  facing each other and a peripheral surface  2   c  disposed between the first end surface  2   a  and the second end surface  2   b , a first external electrode  4  provided on the first end surface  2   a , a second external electrode  5  provided on the second end surface  2   b , and a protective layer  3  covering the peripheral surface  2   c  of the battery element body  2 . 
     The battery element body  2  has a laminate structure including at least one battery constituent unit including a positive electrode layer  21 , a negative electrode layer  22 , and a solid electrolyte layer  23  interposed therebetween along a lamination direction, and is formed in a substantially rectangular parallelepiped shape. The battery element body  2  has the first end surface  2   a  and the second end surface  2   b  facing each other, and the peripheral surface  2   c  disposed between the first end surface  2   a  and the second end surface  2   b . Note that the peripheral surface  2   c  includes a first side surface, a second side surface, a third side surface, and a fourth side surface (none of which are illustrated), the first side surface and the second side surface are positioned to face each other in a lamination direction (for example, a T direction in  FIG.  2   ) of the positive electrode layer and the negative electrode layer, and the third side surface and the fourth side surface are positioned to face each other in, for example, a W direction in  FIG.  2   . As illustrated in  FIG.  4   , an end surface of the positive electrode layer  21  is exposed on the first end surface  2   a , and an end surface of the negative electrode layer  22  is exposed on the second end surface  2   b . Then, the peripheral surface  2   c  of the battery element body  2  is covered with the protective layer  3 . Note that corners and ridges of the battery element body  2  may be chamfered. 
     The solid state battery  1  includes a first principal surface  1   a  and a second principal surface  1   b  facing each other in the lamination direction of the positive electrode layer and the negative electrode layer, and a first side surface  1   c  and a second side surface  1   d  facing each other in the width direction of the solid state battery  1 . Each of the first principal surface  1   a  and the second principal surface  1   b  is provided with a pair of first convex part  11   a  and second convex part  11   b  extending from the first external electrode  4  to the second external electrode  5  along a longitudinal direction connecting the first external electrode  4  and the second external electrode  5  and positioned at both end parts in the transverse direction. The first convex part  11   a  includes a convex part  31   a  formed on the protective layer  3 , and a convex part  4   a  of the first external electrode  4  and a convex part  5   a  of the second external electrode  5  located at both end parts in the longitudinal direction of the convex part  31   a . Furthermore, the second convex part  11   b  includes a convex part  31   b  formed on the protective layer  3 , and a convex part  4   b  of the first external electrode  4  and a convex part  5   b  of the second external electrode  5  located at both end parts in the longitudinal direction of the convex part  31   b.    
     The convex parts  31   a  and  31   b  formed on the protective layer  3  are provided at an edge part along a long side of the solid state battery  1 , and protrude from the first principal surface  1   a  or the second principal surface  1   b . Furthermore, the convex parts  4   a  and  4   b  of the first external electrode  4  and the convex parts  5   a  and  5   b  of the second external electrode  5  are provided at apex parts of the solid state battery  1 , and protrude from the first principal surface  1   a  or the second principal surface  1   b.    
     Here, the shape of the convex parts is not particularly limited, and examples thereof include a rectangular shape, an arc shape, a curved shape, a triangular shape, and the like in a sectional view in the transverse direction. Furthermore, as described later, in the transverse direction, one or more intermediate convex parts may be provided between the pair of first convex part  11   a  and second convex part  11   b  located at both end parts in the transverse direction. Note that, in the figure, T represents the height direction of the solid state battery A, L represents the length direction of the solid state battery A, and W represents the width direction of the solid state battery A. In the present embodiment, the longitudinal direction connecting the first external electrode  4  and the second external electrode  5  corresponds to the length direction of the solid state battery A, the transverse direction corresponds to the width direction of the solid state battery A, and the lamination direction of the positive electrode layer and the negative electrode layer corresponds to the height direction. 
     The first external electrode  4  covers the first end surface  2   a  of the battery element body  2 , and covers a side of the first end surface  2   a  of the peripheral surface  2   c  of the battery element body  2  via the protective layer  3  and is electrically connected to the positive electrode layer  21 . For example, as illustrated in  FIG.  4   , the first external electrode  4  can be provided so as to cover a whole circumference of the peripheral surface  2   c  of the battery element body  2  on the side of the first end surface  2   a  via the protective layer  3 , that is, so as to have a U-shaped section. Furthermore, the second external electrode  5  covers the second end surface  2   b  of the battery element body  2 , and covers a side of the second end surface  2   b  of the peripheral surface  2   c  of the battery element body  2  via the protective layer  3  and is electrically connected to the negative electrode layer  22 . For example, as illustrated in  FIG.  4   , the second external electrode  5  can be provided so as to cover a whole circumference of the peripheral surface  2   c  of the battery element body  2  on the side of the second end surface  2   b  via the protective layer  3 , that is, so as to have a U-shaped section. 
     Hereinafter, effects of the present invention will be described.  FIG.  5    is a schematic perspective view illustrating a state where the solid state battery  1  is mounted on a board  70 . Solder is printed on pads  71  and  72  of the board  70 , and the solid state battery  1  is mounted. In the present invention, since the first external electrode  4  and the second external electrode  5  of the solid state battery  1  can be brought into direct contact with the pads  71  and  72 , the solder printing amount for ensuring electrical connection between the pads  71  and  72  and the first external electrode  4  and the second external electrode  5  can be reduced. As a result, it is possible to reduce the possibility of occurrence of a short circuit between the adjacent mounting components, and high-density mounting becomes possible. Note that, when the solder printing amount is reduced, the self-alignment property may be deteriorated, but in the present invention, surface areas of the first external electrode and the second external electrode can be increased without increasing a width of the solid state battery  1 . Accordingly, since the surface tension of the molten solder can be increased, it is possible to suppress a decrease in self-alignment property. Note that the self-alignment property means that a position of the mounting component is adjusted by the surface tension of the molten solder. 
     Furthermore,  FIG.  6    is another schematic perspective view illustrating a state where the solid state battery  1  is mounted on the board  70 . When the board deflects due to an external force and warps in a deflection direction, the solid state battery  1  has the first convex part and the second convex part at both end parts in the width direction of the first principal surface and the second principal surface, so that the occurrence of the crack of the solid state battery  1  can be suppressed by these four convex parts stretching, whereby the mechanical strength of the solid state battery  1  can be improved. 
     Furthermore,  FIG.  7    is another schematic perspective view illustrating a state where the solid state battery  1  is mounted on the board  70 . When mounting is performed using a mounter, stress tends to concentrate on a specific part as indicated by an arrow, and a crack may occur in the solid state battery  1 . In such a case, generation of cracks in the solid state battery  1  can be suppressed by these four convex parts being stretched, so that the mechanical strength of the solid state battery  1  can be improved. In particular, by using a curved shape as the sectional view shape of the convex part, the concentration of the stress can be easily alleviated. The term “curved” as used in the present specification means a shape protruding in the lamination direction with respect to the first principal surface  1   a  or the second principal surface  1   b  and having rounded corners. With such a shape, the concentration of the stress can be easily alleviated appropriately. 
     Furthermore, as described later, from the viewpoint of manufacturing the solid state battery (from the viewpoint of dipping a protective layer paste), a first side surface  3   c  and a second side surface  3   d  facing each other in the transverse direction (W direction) of the solid state battery may be a curved surface (see  FIG.  3   ). The term “curved surface” as used in the present specification refers to a rounded shape that is not perpendicular(90°) to the first principal surface  1   a  or the second principal surface  1   b . With such a shape, in a case where the stress as indicated by the arrow in  FIG.  7    concentrates on a specific part, since the first side surface and the second side surface are curved surfaces, the concentration of the stress can be easily alleviated appropriately. 
     Furthermore, regarding a degree of curvature of the convex parts  11   a  and  11   b , the degree of curvature of the convex part  11   a  and the degree of curvature of the convex part  11   b  may be different. When the shape of the solid state battery is asymmetric on left and right sides as described above, the left and right sides of the solid state battery can be specified when the solid state battery is mounted, and the solid state battery can be prevented from being mounted in an incorrect direction. 
     Hereinafter, a material used for the solid state battery of the present invention will be described. 
     In the solid state battery, each layer constituting the solid state battery is formed by firing, and the positive electrode layer, the negative electrode layer, the solid electrolyte layer, and the like may constitute a sintered layer. Preferably, the positive electrode layer, the negative electrode layer, and the solid electrolyte layer are integrally fired with each other, and therefore the solid state battery laminate may constitute an integrally fired body. 
     (Positive Electrode Layer and Negative Electrode Layer) 
     The positive electrode layer is an electrode layer containing at least a positive electrode active material. The positive electrode layer may further contain a solid electrolyte. For example, the positive electrode layer may be composed of a fired body containing at least positive electrode active material particles and solid electrolyte particles. In a preferred aspect, the positive electrode layer may be composed of a fired body substantially containing only positive electrode active material particles and solid electrolyte particles. On the other hand, the negative electrode layer is an electrode layer containing at least a negative electrode active material. The negative electrode layer may further contain a solid electrolyte. For example, the negative electrode layer may be composed of a fired body containing at least negative electrode active material particles and solid electrolyte particles. In a preferred aspect, the negative electrode layer may be composed of a fired body substantially containing only negative electrode active material particles and solid electrolyte particles. 
     The positive electrode active material and the negative electrode active material are substances involved in the transfer of electrons in the solid state battery. Ions move (are conducted) between the positive electrode layer and the negative electrode layer via the solid electrolyte, and electrons are transferred, whereby charging and discharging are performed. The positive electrode layer and the negative electrode layer are preferably layers capable of occluding and releasing sodium ions or lithium ions, preferably lithium ions as ions. That is, the solid state battery is preferably an all-solid state secondary battery in which sodium ions or lithium ions move between the positive electrode layer and the negative electrode layer via the solid electrolyte to charge and discharge the battery. 
     (Positive Electrode Active Material) 
     Examples of the positive electrode active material capable of occluding and releasing lithium ions include at least one selected from the group consisting of a lithium-containing phosphate compound having a NASICON-type structure, a lithium-containing phosphate compound having an olivine-type structure, a lithium-containing layered oxide, a lithium-containing oxide having a spinel-type structure, and the like. Examples of the lithium-containing phosphate compound having a NASICON-type structure include Li 3 V 2 (PO 4 ) 3 . Examples of the lithium-containing phosphate compound having an olivine-type structure include LiFePO 4  and/or LiMnPO 4 . Examples of the lithium-containing layered oxide include LiCoO 2  and/or LiCo 1/3 Ni 1/3 Mn 1/3 O 2 . Examples of the lithium-containing oxide having a spinel-type structure include LiMn 2 O 4  and/or LiNi 0.5 Mn 1.5 O 4 . 
     Furthermore, examples of the positive electrode active material capable of occluding and releasing sodium ions include at least one selected from the group consisting of a sodium-containing phosphate compound having a NASICON-type structure, a sodium-containing phosphate compound having an olivine-type structure, a sodium-containing layered oxide, a sodium-containing oxide having a spinel-type structure, and the like. For example, in the case of a sodium-containing phosphate compound, at least one selected from the group consisting of Na 3 V 2  (PO 4 ) 3 , NaCoFe 2 (PO 4 ) 3 , Na 2 Ni 2 Fe (PO 4 ) 3 , Na 3 Fe 2  (PO 4 ) 3 , Na 2 FeP 2 O 7 , Na 4 Fe 3  (PO 4 ) 2  (P 2 O 7 ), and NaFeO 2  as a sodium-containing layered oxide can be mentioned. 
     In addition, the positive electrode active material may be, for example, an oxide, a disulfide, a chalcogenide, a conductive polymer, or the like. The oxide may be, for example, titanium oxide, vanadium oxide, manganese dioxide, or the like. The disulfide is, for example, titanium disulfide or molybdenum sulfide. The chalcogenide may be, for example, niobium selenide. The conductive polymer may be, for example, disulfide, polypyrrole, polyaniline, polythiophene, polypara-styrene, polyacetylene, or polyacene. 
     (Negative Electrode Active Material) 
     Examples of the negative electrode active material capable of occluding and releasing lithium ions include at least one selected from the group consisting of an oxide containing at least one element selected from the group consisting of Ti, Si, Sn, Cr, Fe, Nb, and Mo, a carbon material such as graphite, a graphite-lithium compound, a lithium alloy, a lithium-containing phosphate compound having a NASICON-type structure, a lithium-containing phosphate compound having an olivine-type structure, and a lithium-containing oxide having a spinel-type structure. Examples of the lithium alloy include Li—Al. Examples of the lithium-containing phosphate compound having a NASICON-type structure include Li 3 V 2 (PO 4 ) 3  and/or LiTi 2 (PO 4 ) 3 . Examples of the lithium-containing phosphate compound having an olivine-type structure include Li 3 Fe 2 (PO 4 ) 3  and/or LiCuPO 4 . Examples of the lithium-containing oxide having a spinel-type structure include Li 4 Ti 5 O 12 . 
     Furthermore, examples of the negative electrode active material capable of occluding and releasing sodium ions include at least one selected from the group consisting of a sodium-containing phosphate compound having a NASICON-type structure, a sodium-containing phosphate compound having an olivine-type structure, a sodium-containing oxide having a spinel-type structure, and the like. 
     The positive electrode layer and/or the negative electrode layer may contain a conductive material. Examples of the conductive material contained in the positive electrode layer and the negative electrode layer include at least one kind of metal materials such as silver, palladium, gold, platinum, aluminum, copper, and nickel, and carbon, and the like. 
     Moreover, the positive electrode layer and/or the negative electrode layer may contain a conductive material. Examples of the conductive material include at least one selected from the group consisting of lithium oxide, sodium oxide, potassium oxide, boron oxide, silicon oxide, bismuth oxide, and phosphorus oxide. 
     Thicknesses of the positive electrode layer and the negative electrode layer are not particularly limited, and may be, for example, 2 μm to 50 μm, particularly 5 μm to 30 μm, independently of each other. 
     (Solid Electrolyte Layer) 
     The solid electrolyte is a substance capable of conducting sodium ions or lithium ions. In particular, the solid electrolyte layer constituting a battery constituent unit in the solid state battery forms a layer capable of conducting sodium ions or lithium ions between the positive electrode layer and the negative electrode layer. Note that the solid electrolyte may be provided at least between the positive electrode layer and the negative electrode layer. That is, the solid electrolyte may also exist around the positive electrode layer and/or the negative electrode layer so as to protrude from between the positive electrode layer and the negative electrode layer. Examples of the solid electrolyte capable of conducting lithium ions include lithium-containing polyanionic compounds having a NASICON structure, oxides having a perovskite structure, oxides having a garnet-type or garnet-type similar structure, oxide glass ceramic-based lithium ion conductors, and the like. Examples of the lithium-containing polyanionic compound having a NASICON structure include Li x M y (PO 4 ) 3  (1≤x≤2, 1≤y≤2, and M is at least one selected from the group consisting of Ti, Ge, Al, Ga, and Zr.), which is a lithium-containing phosphate compound. Examples of the lithium-containing phosphate compound having a NASICON structure include Li 1.2 Al 0.2 Ti 1.8 (PO 4 ) 3 . Examples of the oxide having a perovskite structure include La 0.55 Li 0.35 TiO 3 . Examples of the oxide having a garnet-type or garnet-type similar structure include Li 7 La 3 Zr 2 O 12 . As the oxide glass ceramic-based lithium ion conductor, for example, a phosphate compound (LATP) containing lithium, aluminum, and titanium as constituent elements, and a phosphate compound (LAGP) containing lithium, aluminum, and germanium as constituent elements can be used. Furthermore, examples of the solid electrolyte capable of conducting sodium ions include a sodium-containing phosphate compound having a NASICON structure, an oxide having a perovskite structure, and an oxide having a garnet-type or garnet-type similar structure. Examples of the sodium-containing phosphate compound having a NASICON structure include Na x M y (PO 4 ) 3  (1≤x≤4, 1≤y≤2, M is at least one selected from the group consisting of Zr and Ti, Ge, Al, Ga, Fe, and the like, and a part of P may be substituted with Si, S, or the like.). 
     The solid electrolyte may contain a conductive material. The conductive material contained in the solid electrolyte may be selected from, for example, materials similar to the conductive material that can be contained in the positive electrode layer and/or the negative electrode layer. 
     A thickness of the solid electrolyte layer is not particularly limited, and may be, for example, 1 μm to 15 μm, particularly 1 μm to 5 μm. 
     (Protective Layer) 
     The protective layer is generally formed on an outermost side of the solid state battery, and is intended for electrical, physical, and/or chemical protection. The protective layer contains a ceramic powder and an inorganic binder. The ceramic contains at least one of a metal oxide, a metal nitride, and a metal carbide. Here, the metal is defined to include a semimetal. For example, the ceramic contains at least one of Al 2 O 3 (aluminum oxide: alumina), SiO 2  (silicon oxide: quartz), SiN (silicon nitride), AlN (aluminum nitride), and SiC (silicon carbide). The inorganic binder preferably contains a lithium-containing phosphate compound. The lithium-containing phosphate compound is preferably fired. The lithium-containing phosphate compound is preferably the same as the lithium-containing phosphate compound contained in the solid electrolyte layer. However, the components or compositions of the lithium-containing phosphate compound contained in an exterior material and the solid electrolyte layer may be the same or different. 
     Furthermore, it is preferable that the protective layer is formed by integrally firing the peripheral surface of the battery element body and the fired body. Here, the peripheral surface of the battery element body on which the protective layer is integrally fired is a side surface excluding an uppermost layer and a lowermost layer of the battery element body, and the first end surface and the second end surface on which the first external electrode and the second external electrode are formed. The uppermost layer and the lowermost layer of the battery element body may be the positive electrode layer or the negative electrode layer, or a connection layer joined to the protective layer may be provided. When the connection layer is joined to the protective layer, integration of the battery element body and the protective layer is facilitated. A solid electrolyte layer containing a polyanionic compound is preferably used for the connection layer. Here, examples of the solid electrolyte containing a polyanionic compound include a lithium-containing phosphate compound as a lithium ion conductor and a sodium-containing phosphate compound as a sodium ion conductor. 
     Furthermore, from the viewpoint of integral firing, the positive electrode layer, the negative electrode layer, and the solid electrolyte layer constituting the battery element body may contain at least one common element. As such an example, the positive electrode layer may contain Al 2 O 3 , SiO 2 , MgO, or the like in addition to the positive electrode active material and the solid electrolyte, the negative electrode layer may contain Al 2 O 3 , SiO 2 , MgO, or the like in addition to the negative electrode active material and the solid electrolyte, and the solid electrolyte layer may contain Al 2 O 3 , SiO 2 , MgO, or the like in addition to the solid electrolyte. 
     Furthermore, from the viewpoint of securing the water vapor barrier property and the mechanical strength, the protective layer has an average thickness of 1 μm to 500 μm, and preferably 5 μm to 100 μm. Here, as the average thickness of the protective layer, an average thickness calculated from the thicknesses of 100 points of an upper surface part, a lower surface part, and a side surface part of the protective layer is used. 
     (External Electrode) 
     A solid state battery is generally provided with a terminal (external electrode). In particular, a positive electrode terminal (corresponding to the first external electrode) and a negative electrode terminal (corresponding to the second external electrode) are provided on the first end surface and the second end surface located on opposite sides of the battery element body. More specifically, a positive electrode terminal connected to the positive electrode layer and a negative electrode terminal connected to the negative electrode layer are provided. As such a terminal, it is preferable to use a material having high conductivity. The material of the external electrode is not particularly limited, but may be at least one selected from the group consisting of silver, gold, platinum, aluminum, copper, tin, and nickel. 
     [Method of Manufacturing Solid State Battery] 
     Hereinafter, a method of manufacturing the solid state battery according to the first embodiment of the present invention will be described. 
     The solid state battery according to the first embodiment can be manufactured by combining a green sheet method using a green sheet, a printing method such as a screen printing method, and a dipping method. In one aspect, a solid electrolyte layer is formed by the green sheet method, a positive electrode layer and a negative electrode layer are formed by the screen printing, and a protective layer is provided on the peripheral surface of the laminated body by the dipping method, whereby a solid state battery can be manufactured. Note that, hereinafter, the description will be given on the premise of this aspect, but the present invention is not limited thereto, and a predetermined laminate may be formed by the green sheet method or the screen printing method. 
     (Step of Forming Unfired Laminate) 
     First, a paste of the solid electrolyte layer is applied onto a substrate (for example, a PET film). Furthermore, a paste for the positive electrode layer, a paste for the negative electrode layer, a paste for an electrode separation part, and a paste for the exterior material are prepared. 
     Each paste can be prepared by wet-mixing a predetermined constituent material of each layer appropriately selected from the group consisting of a positive electrode active material, a negative electrode active material, a conductive material, a solid electrolyte material, an insulating material, and a conductive material with an organic vehicle in which an organic material is dissolved in a solvent. The paste for the positive electrode layer contains a positive electrode active material, a conductive material, a solid electrolyte material, an organic material, and a solvent. The paste of the negative electrode layer contains a negative electrode active material, a conductive material, a solid electrolyte material, an organic material, and a solvent. The paste of the solid electrolyte layer contains a solid electrolyte material, a conductive material, an organic material, and a solvent. The paste of the electrode separation part contains an insulating material (for example, a solid electrolyte material), a conductive material, an organic material, and a solvent. The paste of the protective layer contains a glassy material, a crystalline material, an organic material, and a solvent. 
     In the wet mixing, a medium can be used, and specifically, a ball mill method, a viscomill method, or the like can be used. On the other hand, a wet mixing method without using a medium may be used, and a Sandoz mill method, a high-pressure homogenizer method, a kneader dispersion method, or the like may be used. 
     A supporting substrate is not particularly limited as long as it can support the unfired laminate, and for example, a substrate including a polymer material such as polyethylene terephthalate can be used. When the unfired laminate is subjected to the firing step while being held on the substrate, a substrate having heat resistance to a firing temperature may be used. 
     As the solid electrolyte material contained in the paste for the solid electrolyte layer, a powder composed of a lithium-containing phosphate compound having a NASICON structure, an oxide having a perovskite structure, and/or an oxide having a garnet-type or garnet-type similar structure as described above may be used. 
     As the positive electrode active material contained in the paste for the positive electrode layer, for example, at least one selected from the group consisting of a lithium-containing phosphate compound having a NASICON-type structure, a lithium-containing phosphate compound having an olivine-type structure, a lithium-containing layered oxide, a lithium-containing oxide having a spinel-type structure, and the like may be used. 
     As the negative electrode active material contained in the paste for the negative electrode layer, for example, a negative electrode active material selected from at least one selected from the group consisting of an oxide containing at least one element selected from the group consisting of Ti, Si, Sn, Cr, Fe, Nb, and Mo, a lithium-containing phosphate compound having a NASICON-type structure, a lithium-containing phosphate compound having an olivine-type structure, a lithium-containing oxide having a spinel-type structure, and the like, a material contained in the solid electrolyte paste, a conductive material, and the like may be used. 
     The organic material contained in the paste is not particularly limited, but at least one polymer material selected from the group consisting of a polyvinyl acetal resin, a cellulose resin, a polyacrylic resin, a polyurethane resin, a polyvinyl acetate resin, a polyvinyl alcohol resin, and the like can be used. The solvent is not particularly limited as long as the organic material can be dissolved, and for example, toluene and/or ethanol may be used. 
     As the conductive material, at least one selected from the group consisting of lithium oxide, sodium oxide, potassium oxide, boron oxide, silicon oxide, bismuth oxide, and phosphorus oxide may be used. 
     The applied paste is dried on a hot plate heated to 30° C. or higher and 50° C. or lower to form a solid electrolyte layer sheet having a predetermined thickness on the substrate (for example, a PET film). 
     (Laminating Step of Battery Element Body) 
     The solid electrolyte layer sheet is peeled off from the substrate. A positive electrode layer is formed on the solid electrolyte layer sheet by the screen printing, and an electrode separation part is formed around the positive electrode layer by the screen printing to prepare a positive electrode layer-integrated solid electrolyte layer sheet. Furthermore, a negative electrode layer is formed on the solid electrolyte layer sheet by the screen printing, and an electrode separation part is formed around the negative electrode layer by the screen printing to prepare a negative electrode layer-integrated solid electrolyte sheet. The positive electrode layer-integrated solid electrolyte layer sheet and the negative electrode layer-integrated solid electrolyte sheet are alternately laminated with the solid electrolyte layer interposed therebetween to obtain a battery element body in which the solid electrolyte layer is disposed as a connection layer on the uppermost layer and the lowermost layer. Subsequently, it is preferable to perform thermo-pressure bonding at a predetermined pressure (for example, about 50 to about 100 MPa) and subsequent isotropic pressing at a predetermined pressure (for example, about 150 to about 300 MPa). As described above, a predetermined battery element body can be manufactured. 
     Next, the peripheral surface of the battery element body is dipped in the paste for the protective layer to form a protective layer. First, the protective layer is formed on the first side surface and the second side surface by dipping the upper surface of the uppermost layer (corresponding to the first side surface described above) and the lower surface of the lowermost layer (corresponding to the second side surface described above) of the battery element body into the paste for the protective layer. Next, the protective layer is formed by dipping the paste for the protective layer on the third side surface and the fourth side surface of the battery element body where the end surfaces of the positive electrode layer and the negative electrode layer are not exposed, and at this time, the paste for the protective layer is dipped so as to form convex parts on the third side surface and the fourth side surface. Examples of the method of forming the convex parts on the third side surface and the fourth side surface include a method of dipping the third side surface and the fourth side surface into the paste for the protective layer a plurality of times. By dipping the paste for the protective layer on the third side surface and the fourth side surface a plurality of times in this manner, the first side surface and the second side surface become curved surfaces as illustrated in  FIG.  3   . Note that, instead of dipping a plurality of times, a method of dipping with a paste for the protective layer having high viscosity can also be used. Alternatively, the screen printing method can also be used. For example, a protective layer sheet is prepared by the same method as described in the preceding paragraph, and a paste for the protective layer is applied by the screen printing to a part on which a convex part is to be formed to form the convex part. Next, the battery element body is sandwiched and laminated such that the protective layer sheet on which the convex part is formed is positioned at the uppermost layer and the lowermost layer. 
     (Firing Step) 
     In the firing step, the unfired laminate is fired. Although it is merely an example, the firing is performed by removing the organic material in a nitrogen gas atmosphere containing oxygen gas or in the atmosphere, for example, at 500° C., and then heating the organic material in a nitrogen gas atmosphere or in the atmosphere, for example, at 550° C. to 1000° C. The firing may be performed while pressurizing the unfired laminate in the lamination direction (in some cases, the lamination direction and a direction perpendicular to the lamination direction). Note that the firing may be performed at one time after providing the protective layer on the battery element body (simultaneous firing), or may be performed after firing the battery element body, providing the protective layer, and further performing firing (sequential firing). 
     Next, an external electrode is attached to the obtained laminate. The first external electrode is provided to be electrically connectable to the positive electrode layer, and the second external electrode is provided to be electrically connectable to the negative electrode layer. Here, the first external electrode is provided so as to cover the first end surface of the battery element body and cover the side of the first end surface of the peripheral surface of the battery element body via the protective layer. This makes it possible to obtain the first external electrode having convex parts at four corners. Furthermore, the second external electrode is provided so as to cover the second end surface of the battery element body and cover the side of the second end surface of the peripheral surface of the battery element body via the protective layer. This makes it possible to obtain a second external electrode having convex parts at four corners. For example, it is preferable to form an external electrode by dipping into a metal paste or the like. The number of times of dipping is not particularly limited, but it is preferably two or more times. Furthermore, although not particularly limited, the external electrode is preferably composed of at least one selected from silver, gold, platinum, aluminum, copper, tin, and nickel. 
     Note that, in the above manufacturing method, the case where the solid state battery is a lithium ion secondary battery has been described, but a solid state battery which is a sodium ion secondary battery can also be manufactured by using a negative electrode active material or a positive electrode active material capable of occluding and releasing sodium ions and a solid electrolyte capable of conducting sodium ions. 
     Second Embodiment 
     The present embodiment relates to an electronic device including an elongated board and the solid state battery according to the first embodiment mounted on the board, in which a longitudinal direction of the board and a longitudinal direction connecting a first external electrode and a second external electrode of the solid state battery coincide with each other. 
       FIG.  8    is a schematic top view illustrating an example of the configuration of the electronic device according to the present embodiment. A solid state battery  1  is mounted on an elongated board  70 . In the present embodiment, a longitudinal direction connecting a first external electrode  4  and a second external electrode  5  of the solid state battery  1  coincides with a longitudinal direction of the elongated board  70 . In general, in the case of the elongated board  70 , the elongated board easily deflects by an external force in the longitudinal direction and easily warps, whereas the elongated board hardly deflects in the transverse direction. Since the solid state battery  1  has a first convex part and a second convex part extending in the longitudinal direction connecting the first external electrode  4  and the second external electrode  5  on a first principal surface and a second principal surface facing each other, four convex parts stretch, so that the occurrence of cracks in the solid state battery  1  can be suppressed. As a result, the mechanical strength of the solid state battery  1  can be improved. 
     The board is not particularly limited as long as it has an elongated shape, and examples thereof include a printed circuit board. The printed circuit board is generally made of paper or glass cloth as a substrate, and hardly deflects in a fiber direction (corresponding to the longitudinal direction), and hardly warps. Therefore, by using a printed circuit board as the board and matching the fiber direction with the longitudinal direction connecting the first external electrode and the second external electrode of the solid state battery, the occurrence of cracks in the solid state battery can be further suppressed. 
     The electronic device is not particularly limited as long as the electronic device mounts the solid state battery, and examples thereof include a power device, an IoT device, a wearable device, and a real-time clock (RTC). 
     Although the embodiments of the present invention have been described above, only typical examples have been illustrated. Therefore, those skilled in the art will easily understand that the present invention is not limited thereto, and various aspects are conceivable without changing the gist of the present invention. 
     For example, in the first embodiment, the solid state battery  1  in which the pair of first convex part and second convex part located at both end parts in the transverse direction are provided as the convex parts has been described, but one or more intermediate convex parts may be provided between the pair of first convex part and second convex part in the transverse direction, and the shape of the convex parts can also take a rectangular shape, an arc shape, a curved shape, a triangular shape, or the like in the sectional view in the transverse direction. 
     Each of  FIGS.  9  to  11    is a schematic perspective view illustrating an example of a solid state battery provided with one or more intermediate convex parts. A solid state battery  81  illustrated in  FIG.  9    is an example in which intermediate convex parts  12   c  are provided between a pair of first convex part  12   a  and second convex part  12   b , and the convex parts have a rectangular shape in a transverse sectional view. Here, the first convex part  12   a  includes a convex part  32   a  formed on a protective layer  32 , and a convex part  41   a  of a first external electrode  41  and a convex part  51   a  of a second external electrode  51  located at both end parts in the longitudinal direction of the convex part  32   a . Furthermore, the second convex parts  12   b  include a convex parts  32   b  formed on the protective layer  32 , and a convex parts  41   b  of the first external electrode  41  and a convex parts  51   b  of the second external electrode  51  located at both end parts in the longitudinal direction of the convex parts  32   b . Furthermore, each of the intermediate convex parts  12   c  includes a convex part  32   c  formed on the protective layer  32 , and a convex part  41   c  of the first external electrode  41  and a convex part  51   c  of the second external electrode  51  located at both end parts in the longitudinal direction of the convex part  32   c.    
     Furthermore, a solid state battery  82  illustrated in  FIG.  10    is an example in which intermediate convex parts  13   c  are provided between a pair of first convex part  13   a  and second convex part  13   b , and the convex parts have a triangular shape in a transverse sectional view. Here, the first convex part  13   a  includes a convex part  33   a  formed on a protective layer  33 , and a convex part  42   a  of a first external electrode  42  and a convex part  52   a  of a second external electrode  52  located at both end parts in the longitudinal direction of the convex part  33   a . Furthermore, the second convex part  13   b  includes a convex part  33   b  formed on the protective layer  33 , and a convex part  42   b  of the first external electrode  42  and a convex part  52   b  of the second external electrode  52  located at both end parts in the longitudinal direction of the convex parts  33   b . Furthermore, each of the intermediate convex parts  13   c  includes a convex part  33   c  formed on the protective layer  33 , and a convex part  42   c  of the first external electrode  42  and a convex part  52   c  of the second external electrode  52  located at both end parts in the longitudinal direction of the convex part  33   c.    
     Furthermore, a solid state battery  83  illustrated in  FIG.  11    is an example in which an intermediate convex part  14   c  is provided between a pair of first convex part  14   a  and second convex part  14   b , and the convex parts have a curved shape in a sectional view in a transverse direction. Here, the first convex part  14   a  includes a convex part  34   a  formed on a protective layer  34 , and a convex part  43   a  of a first external electrode  43  and a convex part  53   a  of a second external electrode  53  located at both end parts in the longitudinal direction of the convex part  34   a . Furthermore, the second convex part  14   b  includes a convex part  34   b  formed on the protective layer  34 , and a convex part  43   b  of the first external electrode  43  and a convex part  53   b  of the second external electrode  53  located at both end parts in the longitudinal direction of the convex part  34   b . Furthermore, the intermediate convex part  14   c  includes a convex part  34   c  formed on the protective layer  34 , and a convex part  43   c  of the first external electrode  43  and a convex part  53   c  of the second external electrode  53  located at both end parts in the longitudinal direction of the convex part  34   c . Note that, in  FIGS.  9  to  11   , the number of intermediate convex parts is not limited to the illustrated number, and may be one or more. 
     According to the aspects of the solid state batteries illustrated in  FIGS.  9  to  11   , by providing one or more intermediate convex parts between the pair of first convex part and second convex part, the surface areas of the first external electrode and the second external electrode can be increased without increasing the width of each of the solid state batteries. As a result, the mounting strength can be further improved, and the surface areas of the first external electrode and the second external electrode can be increased, so that the surface tension of the molten solder can be further increased, and the deterioration of the self-alignment property can be further suppressed. Furthermore, when the board deflects in a deflection direction due to the deflection of the board by the external force, the intermediate convex parts further stretch, so that the occurrence of cracks in the solid state battery can be further suppressed. As a result, the mechanical strength of the solid state battery can be further improved. 
     The solid state battery according to one embodiment of the present invention can be used in various fields where battery use or power storage is assumed. Although it is merely an example, the solid state battery of the present invention can be used in the fields of electricity, information, and communication in which electricity, electronic device, and the like in which mobile device and the like are used (for example, electric and electronic device fields or mobile device fields including mobile phones, smartphones, notebook computers and digital cameras, activity meters, arm computers, electronic papers, and small electronic machines such as RFID tags, card-type battery money, and smartwatches.), home and small industrial applications (for example, the fields of electric tools, golf carts, and home, nursing, and industrial robots), large industrial applications (for example, fields of forklift, elevator, and harbor crane), transportation system fields (field of, for example, hybrid automobiles, electric automobiles, buses, trains, power-assisted bicycles, and electric two-wheeled vehicles), power system applications (for example, fields such as various types of power generation, road conditioners, smart grids, and household power storage systems), medical applications (medical equipment fields such as earphone hearing aids), pharmaceutical applications (fields such as dosage management systems), IoT fields, space and deep sea applications (for example, fields such as a space probe and a submersible.), and the like. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
         
           
               1 : Solid state battery 
               1   a : First principal surface 
               1   b : Second principal surface 
               1   c : First side surface 
               1   d : Second side surface 
               11   a ,  12   a ,  13   a ,  14   a : First convex part 
               11   b ,  12   b ,  13   b ,  14   b : Second convex part 
               12   c ,  13   c ,  14   c : Intermediate convex part 
               2 : Battery element body 
               2   a : First end surface 
               2   b : Second end surface 
               2   c : Peripheral surface 
               3 ,  32 ,  33 ,  34 : Protective layer 
               31   a ,  32   a ,  33   a ,  34   a : Convex part of protective layer 
               31   b ,  32   b ,  33   b ,  34   b : Convex part of protective layer 
               32   c ,  33   c ,  34   c : Convex part of protective layer 
               4 ,  41 ,  42 ,  43 : First external electrode 
               4   a ,  41   a ,  42   a ,  43   a : Convex part of first external electrode 
               5 ,  51 ,  52 ,  53 : Second external electrode 
               5   a ,  51   a ,  52   a ,  53   a : Convex part of second external electrode 
               21 : Positive electrode layer 
               22 : Negative electrode layer 
               23 : Solid electrolyte layer 
               70 : Board 
               71 ,  72 : Pad