Patent Publication Number: US-2022223920-A1

Title: Electrode and secondary battery including the same

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-003294, filed on 13 Jan. 2021, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an electrode and a secondary battery including the same. 
     Related Art 
     Conventionally, lithium ion secondary batteries have been widely used as secondary batteries having a high energy density. In the case of a solid-state battery where the electrolyte is solid, the battery has a cell structure in which a solid electrolyte is present between a positive electrode and a negative electrode. A plurality of the cells are stacked on one another to construct a solid lithium ion secondary battery. 
     In the case of a solid-state battery, sufficient adhesion is required between an electrode material mixture containing a positive electrode active material or a negative electrode active material and a solid electrolyte from the viewpoint of maintaining the ionic conductivity of lithium ions or the like. If the adhesion decreases due to repeated expansion and contraction during charging and discharging, electrodeposition of lithium occurs and ionic conductivity decreases. 
     In this regard, for example, Patent Document 1 discloses a structure in which both sides of a solid electrolyte layer having a dense structure are sandwiched between porous solid electrolytes, and pores of the solid electrolyte are filled with an electrode material mixture, and thereby the electrode material mixture and the solid electrolyte are integrated. 
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-226666 
     SUMMARY OF THE INVENTION 
     However, even in Patent Document 1, the porous solid electrolyte is a so-called green sheet, and the adhesion between the electrode material mixture and the solid electrolyte is insufficient, and further improvement is required. 
     In response to the above issue, it is an object of the present invention to improve the adhesion between an electrode material mixture and a solid electrolyte, and thereby suppress electrodeposition of lithium. 
     The inventors have found that the above issue can be solved by stacking an electrode material mixture layer and a solid electrolyte layer in a planar shape in pores of a metal porous body, and have completed the present invention. That is, the present invention provides the following. 
     (1) A first aspect of the present invention relates to an electrode, including a planar electrode current collector including a metal porous body, 
     an electrode material mixture layer including an electrode material mixture that fills pores of the metal porous body, and
 
a solid electrolyte layer including a solid electrolyte that fills pores of the metal porous body. The electrode material mixture layer and the solid electrolyte layer are stacked in a planar shape in the pores of the metal porous body.
 
     According to the first aspect, by stacking the electrode material mixture layer and the solid electrolyte layer in a planar shape in the pores of the metal porous body, it is possible to follow volume changes during charging and discharging, and thereby suppress electrodeposition of lithium. 
     (2) In a second aspect of the present invention according to the first aspect, the electrode further includes a tab that extends from an end of the metal porous body. In plan view, at least an end edge of the solid electrolyte layer in a direction of the tab is located beyond an end edge of the electrode material mixture layer in the direction of the tab. 
     According to the invention of the second aspect, it is possible to effectively prevent short circuits between the positive and negative electrodes and breakage of the tab as a current collector. 
     (3) A third aspect of the present invention relates to an electrode obtained by combining two of the electrodes according to the first or second aspect having the same polarity. The electrode material mixture layers of the electrodes are configured to be joined together so as to face each other. 
     According to the invention of the third aspect, the energy density can be improved by joining together a pair of identical electrodes. 
     (4) A fourth aspect of the present invention relates to a secondary battery, including the electrode according to the first or second aspect including a positive electrode including a positive electrode material mixture as the electrode material mixture, and 
     the electrode according to the first or second aspect including a negative electrode including a negative electrode material mixture as the electrode material mixture. The solid electrolyte layers of the positive electrode and the negative electrode are joined together so as to face each other. 
     According to the invention of the fourth aspect, it is possible to provide a secondary battery that achieves the effect of the first or second aspect. 
     (5) In a fifth aspect of the present invention according to the fourth aspect, a second solid electrolyte layer is disposed between the solid electrolyte layers. 
     According to the invention of the fifth aspect, it is possible to improve the effect of preventing short circuits between the positive and negative electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of a secondary battery including an electrode of the present invention; 
         FIG. 2A  is a process diagram showing an example of the method for manufacturing the electrode of the present invention; 
         FIG. 2B  is a process diagram showing an example of the method for manufacturing the electrode of the present invention; 
         FIG. 2C  is a process diagram showing an example of the method for manufacturing the electrode of the present invention; 
         FIG. 3A  is a process diagram showing another example of the method for manufacturing the electrode of the present invention; 
         FIG. 3B  is a process diagram showing another example of the method for manufacturing the electrode of the present invention; and 
         FIG. 3C  is a process diagram showing another example of the method for manufacturing the electrode of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described with reference to the drawings. The present invention is not limited to the following embodiments. In the following embodiments, a solid-state lithium ion battery will be used as an example, but the present invention can be applied to batteries other than lithium ion batteries. 
     First Embodiment 
     &lt;Overall Structure of Lithium Ion Secondary Battery&gt; 
     As shown in  FIG. 1 , a lithium ion secondary battery  100  according to the present embodiment is a solid-state battery, and is an electrode stack in which a positive electrode  10  and a negative electrode  20  are alternately arranged with a solid electrolyte layer  30  provided therebetween. A positive electrode tab  11  and a negative electrode tab  21  each extends from an end of the current collector of each electrode of the electrode stack.  FIG. 1  shows the state before tab convergence, and the convergence portion is omitted. 
     The respective components will be described below. 
     &lt;Positive Electrode and Negative Electrode&gt; 
     In this embodiment, the positive electrode  10  and the negative electrode  20  each include a current collector including a metal porous body having pores that are continuous with each other (communicating pores). 
     The pores of each current collector are filled with an electrode material mixture (positive electrode material mixture or negative electrode material mixture) containing an electrode active material, which is a region that is filled with the electrode material mixture. Conversely, the positive electrode tab  11  and the negative electrode tab  21  are regions that are not respectively filled with the electrode material mixtures. 
     (Current Collector) 
     The current collector includes a metal porous body having pores that are continuous with each other. Having pores that are continuous with each other allows the pores to be filled with a positive electrode material mixture or a negative electrode material mixture containing an electrode active material, thereby increasing the amount of the electrode active material per unit area of the electrode layer. The form of the metal porous body is not limited as long as it has pores that are continuous with each other. Examples of the form of the metal porous body include a foam metal having pores by foaming, a metal mesh, an expanded metal, a punching metal, and a metal nonwoven fabric. 
     The metal used in the metal porous body is not limited as long as it has electric conductivity. Examples thereof include nickel, aluminum, stainless steel, titanium, copper, and silver. Among these, as the current collector constituting the positive electrode, a foamed aluminum, foamed nickel, and foamed stainless steel are preferable. As the current collector constituting the negative electrode, a foamed copper and foamed stainless steel are preferable. 
     By using the current collector including the metal porous body, the amount of the active material per unit area of the electrode can be increased, and as a result, the volumetric energy density of the lithium ion secondary battery can be improved. In addition, since the positive electrode material mixture and the negative electrode material mixture are easily fixed, it is not necessary to thicken a coating slurry for forming the electrode material mixture layer when the electrode material mixture layer is thickened, unlike a conventional electrode including a metal foil as a current collector. Accordingly, it is possible to reduce a binder such as an organic polymer compound that has been necessary for thickening. Therefore, the capacity per unit area of the electrode can be increased, and a higher capacity of the lithium ion secondary battery can be achieved. 
     (Electrode Material Mixture) 
     The positive electrode material mixture and the negative electrode material mixture are respectively disposed in the pores formed within the current collectors. The positive electrode material mixture and the negative electrode material mixture respectively contain a positive electrode active material and a negative electrode active material as an essential component. 
     (Electrode Active Material) 
     The positive electrode active material is not limited as long as it can occlude and release lithium ions. Examples thereof include LiCoO 2 , Li(Ni 5/10 Co 2/10 Mn 3/10 )O 2 , Li(Ni 6/10 Co 2/10 Mn 2/10 )O 2 , Li(Ni 8/10 Co 1/10 Mn 1/10 )O 2 , Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 , Li(Ni 1/6 Co 4/6 Mn 1/6 )O 2 , Li(Ni 1/3 Co 1/3 Mn 1/2 )O 2 , Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 , LiCoO 4 , LiMn 2 O 4 , LiNiO 2 , LiFePO 4 , lithium sulfide, and sulfur. 
     The negative electrode active material is not limited as long as it can occlude and release lithium ions. Examples thereof include metallic lithium, lithium alloys, metal oxides, metal sulfides, metal nitrides, Si, SiO, and carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon. 
     (Other Components) 
     The electrode material mixture may optionally include components other than an electrode active material and ionic conductive particles. The other components are not limited, and can be any components that can be used in fabricating a lithium ion secondary battery. Examples thereof include a conductivity aid and a binder. The conductivity aid of the positive electrode is, for example, acetylene black, and the binder of the positive electrode is, for example, polyvinylidene fluoride. Examples of the binder of the negative electrode include sodium carboxyl methyl cellulose, styrene-butadiene rubber, and sodium polyacrylate. 
     (Method for Manufacturing Positive Electrode and Negative Electrode) 
     The positive electrode  10  and the negative electrode  20  are each obtained by filling pores that are continuous with each other of a metal porous body as a current collector with an electrode material mixture. First, an electrode active material and, if necessary, a binder and a conductivity aid, are uniformly mixed by a conventionally known method, and thus an electrode material mixture composition adjusted to a predetermined viscosity, preferably in the form of a paste, is obtained. 
     Subsequently, pores of a metal porous body, which is a current collector, are filled with the above electrode material mixture composition as an electrode material mixture. The method of filling the current collector with the electrode material mixture is not limited, and is, for example, a method of filling the pores of the current collector with a slurry containing the electrode material mixture by applying pressure using a plunger-type die coater. As an alternative, the interior of the metal porous body may be impregnated with an ion conductor layer by a dipping method. 
     A solid electrolyte layer  17  with which pores of a metal porous body  15  are filled, which is described later, can be formed by the same method. 
     &lt;Solid Electrolyte Layer&gt; 
     As shown in  FIG. 1 , in the present invention, a second solid electrolyte layer  30  may be formed between the positive electrode  10  and the negative electrode  20 . The same material can be used for the solid electrolyte layer  17  with which pores of the metal porous body  15  are filled, which is described later. 
     The solid electrolyte constituting the second solid electrolyte layer  30  is not limited, and is, for example, a sulfide solid electrolyte material, an oxide solid electrolyte material, a nitride solid electrolyte material, or a halide solid electrolyte material. Examples of the sulfide solid electrolyte material include LPS halogens (Cl, Br, and I), Li 2 S—P 2 S 5 , and Li 2 S—P 2 S 5 —LiI for lithium ion batteries. The above-described “Li 2 S—P 2 S 5 ” refers to a sulfide solid electrolyte material including a raw material composition containing Li 2 S and P 2 S 5 , and the same applies to the “Li 2 S—P 2 S 5 —LiI”. Examples of the oxide solid electrolyte material include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides for lithium ion batteries. Examples of the NASICON-type oxides include oxides containing Li, Al, Ti, P, and O (e.g., Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 ). Examples of the garnet-type oxides include oxides containing Li, La, Zr, and O (e.g., Li 2 La 3 Zr 2 O 12 ). Examples of the perovskite-type oxides include oxides containing Li, La, Ti, and O (e.g., LiLaTiO 3 ). 
     &lt;Structure of Electrode&gt; 
     First Embodiment 
     An embodiment of the electrode, which is a feature of the present invention, will be specifically described using  FIG. 2 .  FIG. 2  is a process diagram showing an example of the method for manufacturing the electrode of the present invention. The case of a positive electrode  10  is shown below as an example. The same can be applied to a negative electrode  20 . 
       FIG. 2A  is an XZ cross-sectional view of a positive electrode  10   a , which is one of the positive electrodes  10  in  FIG. 1 . The positive electrode  10   a  includes a planar electrode current collector including a metal porous body  15 , an electrode material mixture layer (positive electrode material mixture layer)  16  including an electrode material mixture that fills pores of the metal porous body  15 , and a solid electrolyte layer  17  including a solid electrolyte that fills pores of the metal porous body  15 . The electrode material mixture layer  16  and the solid electrolyte layer  17  are stacked in a planar shape in the pores of the metal porous body  15 . In  FIG. 1 , as shown in  FIG. 2A , the electrode material mixture layer  16  is formed above and the solid electrolyte layer  17  is formed below. 
     The term “planar” in the present invention means that the metal porous body  15  is a planar body having an XY plane in  FIG. 1  and a predetermined thickness in a Z direction. The term “stacked in a planar shape” means that the electrode material mixture layer  16  and the solid electrolyte layer  17  are stacked one above the other (in the Z direction) in the pores of the metal porous body  15 . 
     The positive electrode  10   a  can be obtained, for example, by coating the electrode material mixture layer  16  and the solid electrolyte layer  17  with a predetermined viscosity from the front and back sides of the metal porous body  15 , respectively, i.e., applying them separately on the upper and lower sides. By filling pores of the metal porous body  15  having a network structure with each of the layers, it is possible to obtain an electrode that can follow volume changes during charging and discharging using the elasticity of the metal porous body  15 , and thereby suppress electrodeposition of lithium. In addition, since the metal porous body  15  serves as a matrix, the adhesion between the electrode material mixture layer  16  and the solid electrolyte layer  17  can be maintained. 
     As shown in  FIG. 2A , in the cross-sectional view, an end edge  17   a  of the solid electrolyte layer  17  in the direction of a tab is at a position extending beyond an end edge  16   a  of the electrode material mixture layer  16  in the direction of the tab. In other words, in plan view, at least the end edge  17   a  of the solid electrolyte layer in the direction of the tab is located beyond the end edge  16   a  of the electrode material mixture layer in the direction of the tab. This effectively prevents short circuits between the positive and negative electrodes and breakage of the tab as a current collector. As shown in  FIG. 2A , the end edge  17   a  only needs to extend beyond the position of the end edge  16   a , and for example, the end edge  17   a  may be configured to cover the end edge  16   a.    
     Herein, in the present invention, the positive electrode  10   a  in  FIG. 2A  may be used as a positive electrode as is. However, in this embodiment, as shown in  FIG. 2B , the electrode  10   a  and an electrode  10   b  that are identical to each other are joined together by pressing or the like so that the electrode material mixture layers  16  face each other, to construct a positive electrode  10   c . The structure of joining together a pair of identical electrodes can improve the energy density, which is preferable. In addition, on the joining faces, the metal porous bodies  15  are intertwined with each other and joined together, so that the adhesion between the joining faces can be maintained firmly. 
     Finally, as shown in  FIG. 2C , the second solid electrolyte layer  30 , the positive electrode  10   c , the second solid electrolyte layer  30 , a negative electrode  20   c , and the second solid electrolyte layer  30  are stacked on one another. In this manner, the lithium ion secondary battery  100  in  FIG. 1  can be obtained by stacking the positive electrode and the negative electrode via the second solid electrolyte layer  30  that is independent and separate. In the negative electrode  20   c , as in the positive electrode  10   c , an electrode material mixture layer (negative electrode material mixture)  26  and a solid electrolyte layer  27  (identical to the solid electrolyte layer  17 ) are stacked one above the other in pores of a metal porous body  25 . 
     In the present invention, the second solid electrolyte layer  30  is not necessarily required, but from the viewpoint of preventing short circuits between the positive and negative electrodes, it is preferable to arrange the second solid electrolyte layer  30 . 
     Second Embodiment 
       FIG. 3  shows another embodiment of the present invention. In this embodiment, obtaining the positive electrode  10   a  in  FIG. 3A  is the same as in the first embodiment described above. In  FIG. 3B , a solid electrolyte layer  30   a  is formed on the solid electrolyte layer  17  by coating to manufacture an electrode  10   d , which is the positive electrode/solid electrolyte stack. Subsequently, as shown in  FIG. 3C , electrodes  10   d  that are identical to each other are joined together by pressing or the like so that the electrode material mixture layers  16  face each other, to construct a positive electrode  10   e . A negative electrode  20   e  is manufactured in the same manner, and finally, the positive electrode  10   e  and the negative electrode  20   e  are stacked. The lithium ion secondary battery  100  in  FIG. 1  can also be obtained by this method. 
     In this case, only one of the opposing solid electrolyte layers  30   a  in  FIG. 3C  may be formed. 
     Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be modified as appropriate. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               10  positive electrode 
               10   a  positive electrode 
               10   b  positive electrode 
               10   c  positive electrode 
               10   e  positive electrode 
               11  positive electrode tab 
               15  metal porous body 
               16  electrode material mixture layer (positive electrode material mixture layer) 
               16   a  end edge 
               17  solid electrolyte layer 
               17   a  end edge 
               20  negative electrode 
               20   c  negative electrode 
               20   e  negative electrode 
               21  negative electrode tab 
               26  electrode material mixture layer (negative electrode material mixture layer) 
               26   a  end edge 
               27  solid electrolyte layer 
               27   a  end edge 
               30  second solid electrolyte layer 
               100  lithium ion secondary battery