Patent Publication Number: US-2021184253-A1

Title: Solid electrolyte and battery using same

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a solid electrolyte and a battery including the solid electrolyte. 
     2. Description of the Related Art 
     U.S. Patent Application Publication No. 2016/0293946 discloses an all-solid-state battery containing a lithium sulfide having an argyrodite crystal structure. Japanese Unexamined Patent Application Publication No. 2011-154900 discloses an all-solid-state battery having a solid electrolyte containing sulfide glass and sulfide crystal. 
     SUMMARY 
     One non-limiting and exemplary embodiment provides a solid electrolyte having a high ionic conductivity. 
     In one general aspect, the techniques disclosed here feature a solid electrolyte including first particles consisted of a first solid electrolyte material, and second particles consisted of a second solid electrolyte material. The first solid electrolyte material has a higher ionic conductivity than the second solid electrolyte material, and the second solid electrolyte material has a lower Young&#39;s modulus than the first solid electrolyte material. 
     The present disclosure provides a solid electrolyte having a high ionic conductivity. A battery including the solid electrolyte has a high energy density. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a solid electrolyte  1000  according to a first embodiment; 
         FIG. 2  is a cross-sectional view of a battery  2000  according to a second embodiment; and 
         FIG. 3  is a cross-sectional view of a battery  3000  according to a modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described below with reference to the drawings. 
     First Embodiment 
     A solid electrolyte according to a first embodiment contains first particles consisted of a first solid electrolyte material and second particles consisted of a second solid electrolyte material. The first solid electrolyte material has a higher ionic conductivity than the second solid electrolyte material. The second solid electrolyte material has a lower Young&#39;s modulus than the first solid electrolyte material. 
     In the first embodiment, the solid electrolyte having a high ionic conductivity is densely formed without structural defects. The solid electrolyte according to the first embodiment has a high effective ionic conductivity. A battery including the solid electrolyte according to the first embodiment has a high energy density. The effective ionic conductivity of the solid electrolyte as used herein refers to the ionic conductivity of the solid electrolyte in actual use. An example of the effective ionic conductivity refers to the ionic conductivity of the solid electrolyte contained in the battery. 
     In general, application of high pressure to solid electrolyte particles having a high Young&#39;s modulus and a high ionic conductivity tends to cause delamination between the particles because of the strain generated by the pressure and density distribution. In contrast, the solid electrolyte according to the first embodiment undergoes less delamination. The solid electrolyte according to the first embodiment thus has a high ionic conductivity (e.g., high effective ionic conductivity). Moreover, a battery including the solid electrolyte according to the first embodiment further has a high energy density. 
     In general, the ionic conductivity of the solid electrolyte formed of a green compact is measured with the solid electrolyte under high pressure in a mold. However, in general, a battery including a solid electrolyte is being released from high pressure. 
     Upon release of the solid electrolyte from pressure (e.g., upon exposure of the solid electrolyte to atmospheric pressure), uneven pressure distribution and spring-back strain causes structural defects, such as delamination, in the solid electrolyte. It is noted that, as a result, the ionic conductivity of the solid electrolyte measured under high pressure may greatly differ from the ionic conductivity of the solid electrolyte measured under atmospheric pressure. 
     The solid electrolyte according to the first embodiment may further contain a particle boundary layer consisted of a third solid electrolyte material. When the solid electrolyte according to the first embodiment contains a particle boundary layer, the particle boundary layer may have a thickness smaller than the particle size of the first particles and the particle size of the second particles. The third solid electrolyte material may have a Young&#39;s modulus lower than or equal to the Young&#39;s modulus of the second solid electrolyte material. The third solid electrolyte material may have a lower Young&#39;s modulus than the second solid electrolyte material. The presence of the particle boundary layer in the solid electrolyte according to the first embodiment further prevents or reduces delamination. 
       FIG. 1  is a schematic view of a solid electrolyte  1000  according to the first embodiment. The solid electrolyte according to the first embodiment will be described below with reference to  FIG. 1 . 
     The solid electrolyte  1000  contains first particles  101 , second particles  102 , and a particle boundary layer  103 . The solid electrolyte  1000  may contain no particle boundary layer  103 . 
     The first particles  101  are consisted of a first solid electrolyte material. The second particles  102  are consisted of a second solid electrolyte material. The first solid electrolyte material has a higher ionic conductivity than the second solid electrolyte material. The second solid electrolyte material has a lower Young&#39;s modulus than the first solid electrolyte material. 
     The particle boundary layer  103  may be present between the first particles  101  and the second particles  102 . 
     The particle boundary layer  103  may be present between two adjacent first particles  101 . Similarly, the particle boundary layer  103  may be present between two adjacent second particles  102 . 
     The particle boundary layer  103  is consisted of a third solid electrolyte material. 
     The first particles  101  are connected to each other with the second particles  102  and the particle boundary layer  103  therebetween. The second particles  102  have a lower Young&#39;s modulus than the first particles  101 . The particle boundary layer  103  also preferably has a lower Young&#39;s modulus than the first particles  101 . The third solid electrolyte material preferably has a Young&#39;s modulus lower than or equal to the Young&#39;s modulus of the second solid electrolyte material. 
     The presence of the particle boundary layer  103  as described above prevents or reduces structural defects, such as delamination, even when the solid electrolyte  1000  is formed by compacting particles through the application of high pressure. As a result, the solid electrolyte has a high effective ionic conductivity. The mechanism by which the particle boundary layer  103  prevents or reduces structural defects, such as delamination, will be described below in detail. 
     Method for Manufacturing Solid Electrolyte  1000   
     An example method for manufacturing the solid electrolyte  1000  will be described below. 
     First, a powder of the first particles  101  and a powder of the second particles  102  are mixed to prepare a mixed powder. 
     As demonstrated in Example described below, the powder of the second particles  102  may contain a component for forming the particle boundary layer  103 . The powder of the second particles  102  is, for example, a glass powder containing a sulfide containing lithium sulfide and phosphorus sulfide. The glass powder contains a crystalline component and an amorphous component. The second particles  102  and the particle boundary layer  103  each contain a crystalline component and an amorphous component. 
     Next, the mixed powder is pressed and formed into the solid electrolyte  1000 . Hereinafter, such a manufacturing method is referred to as a “compaction process”. 
     Since the second particles  102  have a lower Young&#39;s modulus than the first particles  101 , the second particles  102  deform more easily than the first particles  101 . Thus, the second particles  102  deform under pressure such that the second particles  102  fit to the shape of gaps between the first particles  101 . As a result, the gaps are filled with the second particles  102 . The dense solid electrolyte  1000  is produced accordingly. 
     Furthermore, the gaps in the solid electrolyte  1000  are made smaller by filling the gaps between the first particles  101  with the second particles  102 . This configuration improves the ionic conductivity. 
     The term “particles” as used herein without distinguishing between the first particles  101  and the second particles  102  refers to the first particles  101  and the second particles  102 . A gap between two adjacent particles is filled with the particle boundary layer  103 . The surface of each particle is thus in contact with the particle boundary layer  103 . The particle boundary layer  103  improves electrical connection between two adjacent particles. In other words, if the particle boundary layer  103  is absent, part of the surface of one particle has not only contact portions in direct contact with the surfaces of adjacent particles but also non-contact portions out of contact with the surfaces of other particles. When the gaps are filled with the particle boundary layer  103 , the particle boundary layer  103  is in contact with the non-contact portions. Accordingly, the non-contact portions are in indirect contact with the surfaces of other particles with the particle boundary layer  103  therebetween. The particle boundary layer  103  thus improves electrical connection between two adjacent particles. It is noted that the first particles  101 , the second particles  102 , and the particle boundary layer  103  are all consisted of a solid electrolyte material. 
     The solid electrolyte  1000  thus has a microstructure. In the microstructure, the soft second particles  102  are present between the hard first particles  101  having a high Young&#39;s modulus such that the gaps between the first particles  101  are filled with the second particles  102 . 
     The particle boundary layer  103  may be present between the first particles  101  such that the gaps between the first particles  101  are filled with the particle boundary layer  103 . Similarly, the particle boundary layer  103  may be present between the second particles  102  such that the gaps between the second particles  102  are filled with the particle boundary layer  103 . 
     The stress is generated by spring back and uneven pressure distribution after pressure release, but the stress is absorbed by the ion-conductive soft structure (i.e., the structure composed of the second particles  102  and the particle boundary layer  103 ). Such compaction of the particles under high pressure to form the solid electrolyte  1000  prevents or reduces structural defects, such as delamination, in the solid electrolyte  1000 . As a result, the solid electrolyte  1000  has a high ionic conductivity, a high density, and a high effective ionic conductivity. Furthermore, a battery including the solid electrolyte  1000  having a high effective ionic conductivity has a high energy density. Again, the microstructure may contain no particle boundary layer  103 . 
     As described above, the first particles  101 , the second particles  102 , and the particle boundary layer  103  are all consisted of a solid electrolyte material. The particle boundary layer  103  has a thickness smaller than the size of the first particles  101  and the size of the second particles  102 . The thickness of the particle boundary layer  103  is, for example, 1/10 or less of the particle size of the first particles  101  and 1/10 or less of the particle size of the second particles  102 . The particle boundary layer  103  may have a thickness of, for example, 2 nanometers or more and 30 nanometers or less. 
     The third solid electrolyte material may have a lower ionic conductivity than the first solid electrolyte material and the second solid electrolyte material. Since the third solid electrolyte material is very thin as described above, the third solid electrolyte material does not have a great effect on the ionic conductivity of the entire solid electrolyte  1000 . 
     Examples of the solid electrolyte materials used for the first particles  101 , the second particles  102 , and the particle boundary layer  103  include known solid electrolyte materials used for batteries. Needless to say, the solid electrolyte materials conduct metal ions, such as Li ions or Mg ions. 
     Examples of the solid electrolyte materials include sulfides, oxides, and halides. 
     Examples of sulfides include Li 2 S—P 2 S 5 , Li 2 S—SiS 2 , Li 2 S—B 2 S 3 , Li 2 S—GeS 2 , Li 2 S—SiS 2 —LiI, Li 2 S—SiS 2 -LiaPO 4 , Li 2 S—Ge 2 S 2 , Li 2 S—GeS 2 —P 2 S 5 , and Li 2 SGeS 2 —ZnS. 
     Examples of halides include compounds having Li, M′, and X′. M′ is at least one element selected from the group consisting of metal elements other than Li and metalloid elements. X′ is at least one element selected from the group consisting of F, Cl, Br, and I. The “metal elements” refer to all elements (except hydrogen) included in group 1 elements to group 12 elements in the periodic table and all elements (except B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se) included in group 13 elements to group 16 elements in the periodic table. The “metalloid elements” refer to B, Si, Ge, As, Sb, and Te. For example, M′ may include Y (=yttrium). Examples of halides containing Y include Li3YCl6 and Li3YBr6. 
     Examples of oxides include oxides mainly containing a Li—Al—(Ge,Ti)—P—O material or a garnet material, such as Li 7 La 3 Zr 2 O 12 . 
     Examples of the solid electrolyte materials further include lithium-containing metal oxides, lithium-containing metal nitrides, lithium phosphate (i.e., Li 3 PO 4 ), and lithium-containing transition metal oxides. 
     Examples of lithium-containing metal oxides include Li 2 —SiO 2  and Li 2 —SiO 2 —P 2 O 5 . 
     Examples of lithium-containing metal nitrides include Li 2.9 PO 3.3 N 0.46 , called LIPON. 
     Examples of lithium-containing transition metal oxides include lithium titanium oxide. 
     The solid electrolyte materials used for the first particles  101  and the second particles  102  are selected from the above solid electrolyte materials so as to have a Young&#39;s modulus and an ionic conductivity as described above. 
     The solid electrolyte material used for the particle boundary layer  103  may also be selected from the above solid electrolyte materials so as to have a Young&#39;s modulus and an ionic conductivity as described above. 
     The solid electrolyte  1000  may contain a binder in addition to the solid electrolyte materials. Examples of the binder include polyethylene oxide and polyvinylidene fluoride. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the first solid electrolyte material may contain at least one selected from the group consisting of a sulfide, an oxide, and a halide. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the first solid electrolyte material may contain an argyrodite sulfide. The argyrodite sulfide has an inherently high ionic conductivity. 
     Examples of the argyrodite sulfide include sulfides having an argyrodite crystal structure and represented by composition formula Li α PS β Cl γ  (where 5.5≤α≤6.5, 4.5≤β≤5.5, and 0.5≤γ≤1.5). 
     Examples of the composition of the argyrodite sulfide represented by composition formula Li α PS β Cl γ  include Li 6 PS 5 Cl. Li 6 PS 5 Cl has an inherently high ionic conductivity in the form of green compact. Li 6 PS 5 Cl may have an ionic conductivity of, for example, 2 mS/cm or more and 3 mS/cm or less at room temperature (e.g., 25 degrees Celsius) under pressure in a mold. Li6PS5Cl has a Young&#39;s modulus of about 0.2 GPa. The presence of Li 6 PS 5 Cl in the first solid electrolyte material can increase the ionic conductivity of the solid electrolyte  1000 . 
     To increase the ionic conductivity of the solid electrolyte  1000 , the second solid electrolyte material may contain at least one selected from the group consisting of a sulfide, an oxide, and a halide. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the second solid electrolyte material may contain an LPS sulfide. The LPS sulfide refers to a sulfide containing lithium sulfide and phosphorus sulfide. 
     Examples of lithium sulfide contained in the LPS sulfide include Li p S (where 1.5≤p≤2.5). Examples of phosphorus sulfide contained in the LPS sulfide include P q S 5  (where 1.5≤p≤2.5). In other words, the LPS sulfide may be Li p S—P q S 5 . To increase the ionic conductivity of the solid electrolyte  1000 , Li p S—P q S 5  may be, for example, Li 2 S—P 2 S 5 . 
     To increase the ionic conductivity of the solid electrolyte  1000 , the second solid electrolyte material may contain a glass sulfide containing a triclinic crystal as a main component. 
     The second solid electrolyte material may be an LPS sulfide glass electrolyte containing a triclinic crystal as a main component. Examples of the glass electrolyte include Li 2 S—P 2 S 5  (Li 2 S:P 2 S 5 =70:30 (molar ratio). Li 2 S—P 2 S 5  (Li 2 S:P 2 S 5 =70:30 (molar ratio) in the form of green compact has an ionic conductivity of about 0.7 mS/cm and a Young&#39;s modulus of about 0.09 GPa at room temperature (e.g., 25 degrees Celsius). When Li 6 PS 5 Cl is used as the first solid electrolyte material, Li 2 S—P 2 S 5  (Li 2 S:P 2 S 5 =70:30 (molar ratio)) may be used as a suitable second solid electrolyte material. 
     The particle boundary layer  103  functions as a connection layer that connects particles to each other between a first particle  101  and a second particle  102  that are adjacent to each other, between two adjacent first particles  101 , and between two adjacent second particles  102 . When the solid electrolyte  1000  is formed by the compaction process, the Young&#39;s modulus of the third solid electrolyte material is lower than or equal to the Young&#39;s modulus of the second particles  102 . The third solid electrolyte material may have a smaller particle size than the first particles  101  and the second particles  102 . 
     The third solid electrolyte material may have lower crystallinity than the second solid electrolyte material. When the third solid electrolyte material has lower crystallinity than the second solid electrolyte material, the particle boundary layer  103  effectively functions as a connection layer that connects particles to each other. Such compaction of the particles under high pressure to form the solid electrolyte  1000  prevents or reduces structural defects, such as delamination. As a result, the solid electrolyte has a high effective ionic conductivity. 
     The third solid electrolyte material may be amorphous. When the third solid electrolyte material is amorphous, the particle boundary layer  103  effectively functions as a connection layer that connects particles to each other. Such compaction of the particles under high pressure to form the solid electrolyte  1000  prevents or reduces structural defects, such as delamination. As a result, the solid electrolyte has a high effective ionic conductivity. 
     The third solid electrolyte material may belong to the same material group as the second solid electrolyte material. When the third solid electrolyte material belongs to the same material group as the second solid electrolyte material, the solid electrolyte  1000  has a stable structure from the viewpoint of connection and thermal expansion of the particles. Furthermore, the second particles  102  have substantially the same coefficient of thermal expansion as the particle boundary layer  103 , and the solid electrolyte  1000  thus has high resistance to thermal shock and thermal cycle. 
     Examples of the third solid electrolyte material include (i) amorphous LPS glass and (ii) LPS glass having lower crystallinity than the second particles  102  and smaller particle size than the second particles  102 . 
     The Young&#39;s modulus and ionic conductivity of glass electrolytes, such as LPS glass, change with temperature or history of heat treating. The crystallinity of glass electrolytes is thus appropriately controlled by adjusting heat treating conditions, and such glass electrolytes are used as the first to third solid electrolyte materials. The crystallinity of the powders of the solid electrolyte materials can be evaluated using the profile and full width at half maximum of X-ray diffraction. 
     The first particles  101  and the second particles  102  have a size larger than the thickness (typically, several tens nanometers or less) of the particle boundary layer  103 . The first particles  101  and the second particles  102  may have an average particle size of about 0.1 micrometers or more and 10 micrometers or less. The average particle size refers to particle size D50 (cumulative 50% particle size) determined from the volume particle size distribution measured by using a laser diffraction particle size analyzer. 
     The microstructure of the solid electrolyte  1000  according to the first embodiment may be observed with a high-resolution transmission electron microscope (hereinafter, referred to as a “TEM”). Using the TEM, the crystals in the microstructures, like lattice patterns, of the first particles  101 , the second particles  102 , and the particle boundary layer  103  are observed. 
     In general, the same or similar chemical compositions have higher Young&#39;s modulus and higher ionic conductivity as the crystallinity increases. 
     The elemental analysis of the first particles  101 , the second particles  102 , and the particle boundary layer  103  can be conducted by using energy dispersive X-ray spectroscopy (hereinafter, referred to as “EDS”) or an electron probe microanalyzer (hereinafter, EPMA). 
     The particles in the microstructure of the solid electrolyte  1000  according to the first embodiment can be evaluated by using direct probe analysis, such as a microprobe system. The ionic conductivity of the surfaces of the particles can be evaluated similarly. 
     The hardness (i.e., Young&#39;s modulus) of the particles in the solid electrolyte  1000  according to the first embodiment is evaluated from the deformation of the shape of the particles in the microstructure through TEM observation. For examples, the magnitude relationship of Young&#39;s modulus is determined in the order of undeformed particles, particles deformed by pressure, and the material forming the particle boundary layer  103  without maintaining the shape under pressure, from highest to lowest. The magnitude relationship of the Young&#39;s modulus of the particles is determined accordingly. 
     The magnitude relationship between the Young&#39;s modulus of the first particles  101  and the Young&#39;s modulus of the second particles  102  is specified through microstructure observation using the TEM as described above. Similarly, the magnitude relationship between the Young&#39;s modulus of the particle boundary layer  103  and the Young&#39;s modulus of the first particles  101  and the second particles  102  is also specified through microstructure observation using the TEM. 
     When it is difficult to specify the magnitude relationship of Young&#39;s modulus through microstructure observation using the TEM, or when it is necessary to determine the Young&#39;s modulus of the particles and the particle boundary layer  103 , the following alternative methods can be used. 
     To measure an inherent Young&#39;s modulus, it is necessary to use a sample without structural defects. A portion without structural defects is selected as a sample, or a sample is processed into a sample without structural defects. Next, the Young&#39;s modulus of the sample is measured. Alternatively, for example, particles having a size of about several tens of micrometers and having no structural defects are used alone as a sample, and the Young&#39;s modulus of the particles is measured. 
     The relative relationship can be compared on the basis of displacement characteristics against pressure in response to insertion of a probe. 
     The relative softness of the particles also can also be estimated from the ratio (i.e., compressibility) of displacement to pressure in response to application of the pressure to the particles in a mold. 
     The second particles  102  may have a smaller particles size than the first particles  101 . When the second particles  102  have a smaller particle size than the first particles  101 , the second particles  102  deform easily. The gap between two adjacent first particles  101  is thus easily filled with a second particle  102 . As a result, the solid electrolyte  1000  has a high ionic conductivity. 
     The particle boundary layer  103  may have a thickness of 10 nanometers or less. When the particle boundary layer  103  has a thickness of 10 nanometers or less, the connection strength between the particles is highly stable like microstructures of ordinary ceramics. This configuration further prevents or reduces delamination of the particles, and the solid electrolyte  1000  has a high ionic conductivity. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the following formula may be satisfied. 
       0.05≤( vp 2+ vgb )/( vp 1+ vp 2+ vgb )≤0.7
 
     where vp1 represents the volume of the first particles  101 , vp2 represents the volume of the second particles  102 , and vgb represents the volume of the particle boundary layer  103 . 
     To increase the ionic conductivity of the solid electrolyte  1000 , the value of (vp2+vgb)/(vp1+vp2+vgb) may be 0.05 or more and 0.50 or less. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the value of (vp2+vgb)/(vp1+vp2+vgb) may be 0.05 or more and 0.30 or less. 
     To increase the ionic conductivity of the solid electrolyte  1000 , the value of (vgb)/(vp2+vgb) may be 0.05 or more and 0.15 or less. 
     Hereinafter, example methods for manufacturing the solid electrolyte according to the first embodiment will be described in detail. 
     First, a manufacturing method where the second particles  102  and the particle boundary layer  103  are consisted of materials belonging to the same material group (i.e., the second solid electrolyte material and the third solid electrolyte material belong to the same material group) will be described. 
     A mixture of a powder of the first solid electrolyte material and a powder of the soft solid electrolyte material is pressed to form a solid electrolyte containing a microstructure containing the second particles  102  and the particle boundary layer  103 . The soft solid electrolyte material has a lower Young&#39;s modulus than the first solid electrolyte material. Furthermore, the powder of the soft solid electrolyte material has crystallinity distribution. The crystallinity distribution will be described below. 
     The second solid electrolyte material (i.e., the soft solid electrolyte material) having crystallinity distribution is finally formed into the second solid electrolyte material and the third solid electrolyte material respectively constituting the second particles  102  and the particle boundary layer  103 . 
     The powder of the soft solid electrolyte material having crystallinity distribution is roughly divided into a powder component having low crystallinity and the remaining component. 
     The powder component having low crystallinity is distributed on the particle surfaces of the first particles  101  at the initial stage of pressing so as to connect the particles to each other. The powder component having low crystallinity forms the particle boundary layer  103  accordingly. 
     The remaining component does not form the particle boundary layer  103 . The remaining component finally forms the second particles  102  while having ionic conductivity in the microstructure. 
     This method stably forms the solid electrolyte  1000  having less structural defects and a high ionic conductivity. 
     When the second particles  102  and the particle boundary layer  103  are consisted of materials belonging to the same material group, the productivity is high, and the solid electrolyte tends to have a stable structure from the viewpoint of connection and thermal expansion of the second particles  102  and the particle boundary layer  103 . In this case, the first particles  101  have higher crystallinity than the second particles  102 , and the second particles  102  have crystallinity higher than or equal to that of the particle boundary layer  103 . Since the ionic conductivity generally increases with increasing crystallinity, the first particles  101  have higher ionic conductivity than the second particles, and the second particles  102  have ionic conductivity higher than or equal to that of the particle boundary layer  103 . 
     Since the second particles have substantially the same coefficient of thermal expansion as the particle boundary layer  103 , the solid electrolyte  1000  has high resistance to thermal shock and thermal cycle. 
     When the second particles  102  and the particle boundary layer  103  are consisted of materials belonging to the same material group, the solid electrolyte  1000  may be produced by pressing a mixture of a powder of the first solid electrolyte material, a powder of the second solid electrolyte material, and a powder of the third solid electrolyte material. 
     Next, a manufacturing method where the second particles  102  and the particle boundary layer  103  are consisted of different materials will be described. 
     The solid electrolyte  1000  is produced by pressing a mixture of a powder of the first solid electrolyte material, a powder of the second solid electrolyte material, and a powder of the third solid electrolyte material. 
     The solid electrolyte  1000  may be formed by using composite particles prepared by coating the surfaces of the first particles  101  with at least one selected from the group consisting of the second particles  102  and the particle boundary layer  103 . 
     The coating conditions can be checked by estimating the degree of compounding on the basis of the results of SEM observation or on the basis of a change in specific surface area observed by the BET method. 
     Second Embodiment 
     An energy storage device according to a second embodiment includes the solid electrolyte according to the first embodiment. The same matters as described in the first embodiment are appropriately omitted in the second embodiment. 
     A battery according to the second embodiment includes a positive electrode, a negative electrode, and an electrolyte layer between the positive electrode and the negative electrode. At least one selected from the group consisting of the positive electrode, the negative electrode, and the electrolyte layer contains the solid electrolyte according to the first embodiment. 
       FIG. 2  is a cross-sectional view of a battery  2000  according to the second embodiment. Referring to  FIG. 2 , the battery  2000  according to the second embodiment includes a positive electrode  201 , a negative electrode  203 , and an electrolyte layer  202 . The positive electrode  201  includes positive electrode active material particles  204  and the solid electrolyte  1000  according to the first embodiment. The electrolyte layer  202  is disposed between the positive electrode  201  and the negative electrode  203 . The electrolyte layer  202  is in contact with both the positive electrode  201  and the negative electrode  203 . The electrolyte layer  202  includes the solid electrolyte  1000  according to the first embodiment. The negative electrode  203  includes negative electrode active material particles  205  and the solid electrolyte  1000  according to the first embodiment. The battery  2000  is, for example, an all-solid lithium secondary battery. Since the battery  2000  according to the second embodiment includes the solid electrolyte  1000  described in the first embodiment, the battery  2000  has a high energy density. Since the solid electrolyte  1000  according to the first embodiment undergoes less delamination, the electrolyte layer  202  may be thin. The energy density of the battery is further improved by using the solid electrolyte  1000  according to the first embodiment in the electrolyte layer  202 . 
     In the second embodiment, the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202  may each contain the solid electrolyte  1000 . The electrolyte layer  202  may contain the solid electrolyte  1000  according to the first embodiment. Since the electrolyte layer  202  contains the largest amount of electrolyte material among the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 , the use of the solid electrolyte  1000  according to the first embodiment in the electrolyte layer  202  effectively improves the energy density. As long as at least one selected from the group consisting of the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202  contains the solid electrolyte  1000 , the battery has a high energy density. The positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202  may each further contain a solid electrolyte other than the solid electrolyte  1000  according to the first embodiment. 
     The positive electrode  201  contains a positive electrode active material, that is, a material into and from which metal ions can be intercalated and deintercalated. Examples of metal ions include lithium ion. The positive electrode  201  contains, for example, a positive electrode active material (e.g., positive electrode active material particles  204 ). The positive electrode  201  may contain the solid electrolyte  1000 . 
     Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-free transition metal oxides, transition metal fluorides, polyanion materials, fluorinated polyanion materials, transition metal sulfides, transition metal oxyfluorides, transition metal oxysulfides, and transition metal oxynitrides. The use of a lithium-containing transition metal oxide as a positive electrode active material reduces the costs for manufacturing the battery  2000  and increases the average discharge voltage of the battery  2000 . 
     The positive electrode  201  may contain, as a positive electrode active material, at least one selected from Li(NiCoAl)O 2  and LiCoO 2 . These transition metal oxides may be used to increase the energy density of the battery  2000 . 
     In the positive electrode  201 , the percentage of the volume vc 1  of the positive electrode active material particles  204  to the sum of the volume vc 1  of the positive electrode active material particles  204  and the volume vc 2  of the solid electrolyte  1000  is, for example, 30% or more and 95% or less. In other words, the volume ratio represented by formula vc 1 /(vc 1 +vc 2 ) may be 0.3 or more and 0.95 or less. The percentage of the volume vc 2  of the solid electrolyte  1000  to the sum of the volume vc 1  of the positive electrode active material particles  204  and the volume vc 2  of the solid electrolyte  1000  is, for example, 5% or more and 70% or less. In other words, the volume ratio represented by formula vc 2 /(vc 1 +vc 2 ) may be 0.05 or more and 0.70 or less. Appropriate control of the amount of the positive electrode active material particles  204  and the amount of the solid electrolyte  1000  allows the battery  2000  to have a sufficient energy density and operate with high output power. 
     The positive electrode  201  may have a thickness of 10 micrometers or more and 500 micrometers or less. Appropriate control of the thickness of the positive electrode  201  allows the battery  2000  to have a sufficient energy density and operate with high output power. 
     As described above, the electrolyte layer  202  may contain the solid electrolyte  1000  according to the first embodiment. The electrolyte layer  202  may contain not only the solid electrolyte  1000  according to the first embodiment but also a solid electrolyte other than the solid electrolyte according to the first embodiment. 
     Hereinafter, the solid electrolyte  1000  according to the first embodiment is referred to as a first solid electrolyte. The solid electrolyte other than the solid electrolyte according to the first embodiment is referred to as a second solid electrolyte. 
     When the electrolyte layer  202  contains not only the first solid electrolyte but also the second solid electrolyte, the first solid electrolyte and the second solid electrolyte may be uniformly dispersed in the electrolyte layer  202 . The second solid electrolyte may have a different composition from the first solid electrolyte. The second solid electrolyte may have a different structure from the first solid electrolyte. 
     The electrolyte layer  202  may have a thickness of 1 micrometer or more and 500 micrometers or less. Appropriate control of the thickness of the electrolyte layer  202  can assuredly prevent short-circuiting between the positive electrode  201  and the negative electrode  203  and enables high-output operation of the battery  2000 . 
     The negative electrode  203  contains a negative electrode active material, that is, a material into and from which metal ions can be intercalated and deintercalated. Examples of metal ions include lithium ion. The negative electrode  203  contains, for example, a negative electrode active material (e.g., negative electrode active material particles  205 ). The negative electrode  203  may contain the solid electrolyte  1000 . 
     Examples of the negative electrode active material include metal materials, carbon materials, oxides, nitrides, tin compounds, and silicon compounds. The metal materials may be single metals or alloys. Examples of the metal materials include lithium metal and lithium alloys. Examples of the carbon materials include natural graphite, coke, partially graphitized carbon, carbon fiber, spherical carbon, artificial graphite, and amorphous carbon. From the viewpoint of capacity density, the negative electrode active material may be preferably at least one selected from the group consisting of silicon (i.e., Si), tin (i.e., Sn), silicon compounds, and tin compounds. 
     In the negative electrode  203 , the percentage of the volume va 1  of the negative electrode active material particles  205  to the sum of the volume va 1  of the negative electrode active material particles  205  and the volume va 2  of the solid electrolyte  1000  is, for example, 30% or more and 95% or less. In other words, the volume ratio represented by formula va 1 /(va 1 +va 2 ) may be 0.3 or more and 0.95 or less. The percentage of the volume va 2  of the solid electrolyte  1000  to the sum of the volume va 1  of the negative electrode active material particles  205  and the volume va 2  of the solid electrolyte  1000  is, for example, 5% or more and 70% or less. In other words, the volume ratio represented by formula va 2 /(va 1 +va 2 ) may be 0.05 or more and 0.70 or less. Appropriate control of the amount of the negative electrode active material particles  205  and the amount of the solid electrolyte  1000  allows the battery  2000  to have a sufficient energy density and operate with high output power. 
     The negative electrode  203  may have a thickness of 10 micrometers or more and 500 micrometers or less. Appropriate control of the thickness of the negative electrode  203  allows the battery  2000  to have a sufficient energy density and operate with high output power. 
     The second solid electrolyte may be a sulfide solid electrolyte. The sulfide solid electrolyte may be contained in the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 . Examples of the sulfide solid electrolyte material include Li 2 S—P 2 S 5 , Li 2 S—SiS 2 , Li 2 SB 2 S 3 , Li 2 S—GeS 2 , Li 3.25 Ge 0.25 P 0.75 S 4 , and Li 10 GeP 2 S 12 . To the sulfide solid electrolyte material, LiX (X is F, Cl, Br, or I), Li 2 O, MO q , or Li p MO q  (M is P, Si, Ge, B, Al, Ga, In, Fe, or Zn, p is a natural number, and q is a natural number) may be added. The sulfide solid electrolyte material improves the ionic conductivity of the solid electrolyte  1000 . 
     The second solid electrolyte may be an oxide solid electrolyte. The oxide solid electrolyte may be contained in the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 . The oxide solid electrolyte material improves the ionic conductivity of the solid electrolyte  1000 . 
     Examples of the oxide solid electrolyte include: (i) NASICON solid electrolytes, such as LiTi 2 (PO 4 ) 3  and element-substituted products thereof; (ii) (LaLi)TiO 3 -based perovskite solid electrolytes; (iii) LISICON solid electrolytes, such as Li 14 ZnGe 4 O 16 , Li 4 SiO 4 , LiGeO 4 , and element-substituted products thereof; (iv) garnet solid electrolytes, such as Li 7 La 3 Zr 2 O 12  and element-substituted products thereof; (v) Li 3 N and H-substituted products thereof; and (vi) Li 3 PO 4  and N-substituted products thereof. 
     The second solid electrolyte may be a halogenated solid electrolyte. The halogenated solid electrolyte may be contained in the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 . The halogenated solid electrolyte material improves the ionic conductivity of the solid electrolyte  1000 . 
     Examples of the halogenated solid electrolyte include Li 2 MgX′ 4 , Li 2 FeX′ 4 , Li(Al,Ga,In)X′ 4 , Li 3 (Al,Ga,In)X′ 6 , LiOX′, and LiX′, where X′ is at least one element selected from the group consisting of F, Cl, Br, and I. Examples of the halogenated solid electrolyte include Li 3 InBr 6 , Li 3 InCl 6 , Li 2 FeCl 4 , Li 2 CrCl 4 , Li 3 OCl, and LiI. 
     The second solid electrolyte may be a complex hydride solid electrolyte. The complex hydride solid electrolyte may be contained in the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 . The complex hydride solid electrolyte material improves the ionic conductivity of the solid electrolyte  1000 . 
     Examples of the complex hydride solid electrolyte include LiBH 4 —LiI and LiBH 4 —P 2 S 5 . 
     The second solid electrolyte may be an organic polymer solid electrolyte. The organic polymer solid electrolyte may be contained in the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202 . The organic polymer solid electrolyte material improves the ionic conductivity of the solid electrolyte  1000 . 
     Examples of the organic polymer solid electrolyte include compounds composed of a polymer compound and a lithium salt. The polymer compound may have an ethylene oxide structure. The polymer compound having an ethylene oxide structure can contain more lithium salt and thus can further improve ionic conductivity. Examples of the lithium salt include LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiSO 3 CF 3 , LiN(SO 2 CF 3 ) 2 , LiN(SO 2 C 2 F 5 ) 2 , LiN(SO 2 CF 3 )(SO 2 C 4 F 9 ), and LiC(SO 2 CF 3 ) 3 . One of these lithium salts may be used alone. Alternatively, two or more of these lithium salts may be used as a mixture. 
     At least one selected from the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202  may contain a non-aqueous electrolyte solution, a gel electrolyte, or an ionic liquid for the purpose of facilitating exchange of lithium ions and improving the output characteristics of the battery  2000 . 
     The non-aqueous electrolyte solution contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. 
     Examples of the non-aqueous solvent include cyclic carbonate solvents, chain carbonate solvents, cyclic ether solvents, chain ether solvents, cyclic ester solvents, chain ester solvents, and fluorinated solvents. 
     Examples of the cyclic carbonate solvents include ethylene carbonate, propylene carbonate, and butylene carbonate. 
     Examples of the chain carbonate solvents include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. 
     Examples of the cyclic ether solvents include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. 
     Examples of the chain ether solvents include 1,2-dimethoxyethane and 1,2-diethoxyethane. 
     Examples of the cyclic ester solvents include γ-butyrolactone. 
     Examples of the chain ester solvents include methyl acetate. 
     Examples of the fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate. 
     One of these non-aqueous solvents may be used alone, or two or more of these non-aqueous solvents may be used as a mixture. 
     Examples of the lithium salt include LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiSO 3 CF 3 , LiN(SO 2 CF 3 ) 2 , LiN(SO 2 C 2 F 5 ) 2 , LiN(SO 2 CF 3 )(SO 2 C 4 F 9 ), and LiC(SO 2 CF 3 ) 3 . 
     One of these lithium salts may be used alone, or two or more of these lithium salts may be used as a mixture. 
     The lithium salt may have a concentration of 0.5 mol/L or more and 2 mol/L or less. 
     An example of the gel electrolyte is a polymer material impregnated with a non-aqueous electrolyte solution. Examples of the polymer material include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate. Other examples of the polymer material include polymers having an ethylene oxide bond. 
     Examples of the cation contained in the ionic liquid include: (i) chain aliphatic quaternary ammonium salt cations, such as tetraalkylammonium; (ii) chain aliphatic quaternary phosphonium salt cations, such as tetraalkylphosphonium; (iii) alicyclic ammoniums, such as pyrrolidinium, morpholinium; imidazolinium, tetrahydropyrimidinium, piperazinium, and piperidinium; and (iv) nitrogen-containing heterocyclic aromatic cations, such as pyridinium and imidazolium. 
     Examples of the cation forming the ionic liquid include PF 6   − , BF 4   − , SbF 5   − , AsF 6   − , SO 3 CF 3   − , N(SO 2 CF 3 ) 2   − , N(SO 2 C 2 F 5 ) 2   − , N(SO 2 CF 3 )(SO 2 C 4 F 9 ) − , and C(SO 2 CF 3 ) 3   − . 
     The ionic liquid may contain a lithium salt. 
     At least one selected from the positive electrode  201 , the negative electrode  203 , and the electrolyte layer  202  may contain a binder for the purpose of improving the adhesion between the particles. 
     Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, poly(acrylic acid), poly(methyl acrylate), poly(ethyl acrylate), poly(hexyl acrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), poly(vinyl acetate), polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethylcellulose. 
     Copolymers may also be used as a binder. Examples of such a binder include copolymers of two or more materials selected from the group consisting of tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. 
     A mixture of two or more selected from the above materials may be used as a binder. 
     At least one selected from the positive electrode  201  and the negative electrode  203  may contain a conductive assistant for the purpose of improving electronic conductivity. 
     Examples of the conductive assistant include: (i) graphites, such as natural graphite and artificial graphite; (ii) carbon blacks, such as acetylene black and Ketjenblack; (iii) conductive fibers, such as carbon fibers and metal fibers; (iv) fluorinated carbon; (v) metal powders, such as aluminum powder; (vi) conductive whiskers, such as zinc oxide whisker and potassium titanate whisker; (vii) conductive metal oxides, such as titanium oxide; and (viii) conductive polymer compounds, such as polyaniline, polypyrrole, and polythiophene. 
     The shape of the conductive assistant is not limited. Examples of the shape of the conductive assistant include needle shape, scale shape, spherical shape, and ellipsoidal shape. The conductive assistant may be in the form of particles. 
     The positive electrode active material particles  204  and the negative electrode active material particles  205  may be coated with a coating material for the purpose of reducing surface resistance. The surfaces of the positive electrode active material particles  204  may be only partially coated with a coating material. Alternatively, the surfaces of the positive electrode active material particles  204  may be entirely coated with a coating material. Similarly, the surfaces of the negative electrode active material particle  205  may be only partially coated with a coating material. Alternatively, the surfaces of the negative electrode active material particle  205  may be entirely coated with a coating material. 
     Examples of the coating material include solid electrolytes, such as sulfide solid electrolytes, oxide solid electrolytes, halogenated solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes. The coating material may be an oxide solid electrolyte. The oxide solid electrolyte has good high-potential stability. The use of an oxide solid electrolyte as a coating material improves the charge/discharge efficiency of the battery  2000 . 
     Examples of the oxide solid electrolyte that can be used as a coating material include: (i) Li—Nb—O compounds, such as LiNbO 3 ; (ii) Li—B—O compounds, such as LiBO 2  and Li 3 BO 3 ; (iii) Li—Al—O compounds, such as LiAlO 2 ; (iv) Li—Si—O compounds, such as Li 4 SiO 4 ; (v) Li 2 SO 4 ; (vi) Li—Ti—O compounds, such as Li 4 Ti 5 O1 2 ; (vii) Li—Zr—O compounds, such as Li 2 ZrO 3 ; (viii) Li—Mo—O compounds, such as Li 2 MoO 3 ; (ix) Li-V-O compounds, such as LV 2 O 5 ; and (x) Li—W—O compounds, such as Li 2 WO 4 . 
       FIG. 3  is a cross-sectional view of a battery  3000  according to a modification of the second embodiment. Referring to  FIG. 3 , a battery  3000  includes a positive electrode including a positive electrode active material layer  301  and a current collector  303 ; a negative electrode including a negative electrode active material layer  302  and a current collector  303 ; and an electrolyte layer  304  between the positive electrode and the negative electrode. In the positive electrode, the positive electrode active material layer  301  is disposed on the current collector  303 . In the negative electrode, the negative electrode active material layer  302  is disposed on the current collector  303 . The electrolyte layer  304  contains the solid electrolyte according to the first embodiment. 
     The positive electrode active material layer  301  contains the positive electrode active material described above. The negative electrode active material layer  302  contains the negative electrode active material described above. The current collectors  303  are consisted of any material having electrical conductivity. The current collectors  303  are formed of, for example, a metal film, such as copper foil. 
     Like the battery  2000 , the battery  3000  has a high energy density since the battery  3000  includes the solid electrolyte according to the first embodiment. 
     Example 
     Solid Electrolyte 
     In Example, the solid electrolyte  1000  shown in  FIG. 1  was manufactured. 
     An argyrodite sulfide was used as the first solid electrolyte material. Specifically, a powder of sulfide solid electrolyte Li 6 PS 5 Cl having an argyrodite structure (available from Ampcera Inc.) was used. 
     As a material containing the second solid electrolyte material and the third solid electrolyte material, a glass powder of Li 2 S—P 2 S 5  (Li 2 S:P 2 S 5 =70:30 (molar ratio) was used. Hereinafter, the glass powder is referred to as an “LPS glass powder”. 
     The LPS glass powder was annealed at 200 degrees Celsius. The annealed LPS glass powder contained a triclinic crystal as a main component. The annealed LPS glass powder had a wide crystallinity distribution from crystalline to amorphous. The annealed LPS glass powder had an average particle size of 5 micrometers. 
     The annealed LPS glass powder was classified by using an ultrasonic vibration sieve with a micromesh having a maximum aperture of 8 micrometers. The volume ratio of the LPS glass powder that had passed through the micromesh to the entire LPS glass powder was about 0.1. The LPS glass powder that had passed through the micromesh contained many particles that did not undergo necking. 
     The LPS glass powder that had passed through the micromesh was subjected to X-ray diffraction analysis. Since no clear peak was observed in the X-ray diffraction pattern, the LPS glass powder that had passed through the micromesh was determined to contain many particles that did not undergo sintering and crystallization. In other words, it was determined that the powder that had passed through the micromesh might be used as a material of the particle boundary layer  103 . 
     The result of classification reveals that the component of the second particles  102 , serving as a crystalline component, accounted for 90 vol %, and the component of the particle boundary layer  103 , serving as an amorphous component, accounted for 10 vol %. 
     The weight of the powder of sulfide solid electrolyte Li 6 PS 5 Cl having an argyrodite structure, and the weight of the annealed LPS glass powder were measured so as to have the proportions shown in Table 1. Next, these powders were mixed well with each other in an agate mortar under dry conditions for about 30 minutes to provide a mixed powder in which these powders were uniformly dispersed. 
     Next, the mixed powder was placed in a mold. A pressure of 800 MPa was applied to the mixed powder by using a uniaxial pressing machine at a temperature of 120 degrees Celsius for 10 minutes to provide a solid electrolyte formed of a disc-shaped green compact sample. The solid electrolyte was taken out of the mold. The solid electrolyte was produced accordingly. 
     Next, the effective ionic conductivity of the obtained solid electrolyte was measured in the following manner. As described in the embodiments, the ionic conductivity of the solid electrolyte measured under high pressure may greatly differ from the ionic conductivity of the solid electrolyte measured under atmospheric pressure. As described below, the ionic conductivity of the solid electrolyte contained in the battery was measured as an effective ionic conductivity in Example. 
     A first indium foil and a second indium foil were placed on the upper surface and the lower surface of the solid electrolyte, respectively. The first indium foil and the second indium foil each had a thickness of 50 micrometers. Next, a pressure was applied between the upper surface and the lower surface of the solid electrolyte through the first indium foil and the second indium foil. The first indium foil and the second indium foil were accordingly attached to the upper surface and the lower surface of the solid electrolyte, respectively. Finally, the solid electrolyte was released from pressure. A battery including the first indium foil, the solid electrolyte, and the second indium foil was produced accordingly. 
     Subsequently, the battery was placed in a thermostatic chamber maintained at 25° C.±1° C. 
     The atmosphere was present in the thermostatic chamber. The pressure inside the thermostatic chamber was atmospheric pressure. 
     While the battery was placed in the thermostatic chamber, the impedance of the solid electrolyte was measured by using an impedance measuring system (product name: 12608W available from Solartron Analytical) at frequencies from 110 Hz to 10 MHz through the first indium foil and the second indium foil, and the effective ionic conductivity of the solid electrolyte contained in the battery was calculated. 
     The effective ionic conductivity of the solid electrolytes according to Sample 1 to Sample 7 was shown in Table 2. 
     Secondary Battery 
     A method for manufacturing a secondary battery will be described below. In Example, the secondary battery  3000  shown in  FIG. 3  was manufactured. 
     First, a positive electrode paste, a negative electrode paste, and an electrolyte paste were prepared. 
     Positive Electrode Paste 
     The positive electrode paste contained a solid electrolyte material and a positive electrode active material. 
     The solid electrolyte material was a crystalline glass powder of argyrodite sulfide solid electrolyte Li 6 PS 5 Cl. The crystalline glass powder had an average particle size of 2 micrometers. The sulfide solid electrolyte Li 6 PS 5 Cl had an ionic conductivity of about 2 mS/cm to 3 mS/cm. 
     The positive electrode active material was a powder of layered LiNiCoAl composite oxide represented by chemical formula LiNi 0.8 Co 0.15 Al 0.05 O 2 . The layered LiNiCoAl composite oxide had an average particle size of about 5 micrometers. 
     The positive electrode paste was prepared in the following manner. 
     The solid electrolyte material and the positive electrode active material were mixed with each other and next uniformly dispersed in each other to provide a mixture. Next, hydrogenated styrene-based thermoplastic elastomer (hereinafter referred to as “SEBS”) and tetralin were added to the mixture, and the mixture was then mixed well using a planetary centrifugal mixer at room temperature (i.e., 25 degrees Celsius) at a rotation speed of 1600 rpm for about 20 minutes to produce a positive electrode paste. SEBS and tetralin were used as an organic binder and a solvent, respectively. 
     Negative Electrode Paste 
     The negative electrode paste also contained a solid electrolyte material and a negative electrode active material. 
     Like the positive electrode paste, the solid electrolyte material was a crystalline glass powder of argyrodite sulfide solid electrolyte Li 6 PS 5 Cl. The crystalline glass powder had an average particle size of 2 micrometers. 
     The negative electrode active material was a natural graphite powder having an average particle size of about 10 micrometers. 
     The negative electrode paste was prepared in the same manner as that for the positive electrode paste. 
     Electrolyte Paste 
     To the solid electrolyte according to Sample 1 to Sample 7, SEBS and tetralin were added. Next, the mixture was mixed well using a planetary centrifugal mixer at room temperature (i.e., 25 degrees Celsius) at a rotation speed of 1600 rpm for about 20 minutes to produce an electrolyte paste. SEBS and tetralin were used as an organic binder and a solvent, respectively. 
     The positive electrode paste was applied to copper foil having a thickness of about 20 micrometers by screen printing. The applied positive electrode paste was dried under vacuum at about 100 degrees Celsius for 1 hour to form the positive electrode active material layer  301  on the copper foil. The copper foil functioned as the current collector  303 . The positive electrode active material layer  301  had a thickness of about 60 micrometers. 
     Next, the electrolyte paste was applied to the positive electrode active material layer  301  by using a metal mask and a squeegee. The applied electrolyte paste had a thickness of about 100 micrometers. Next, the electrolyte paste was dried under vacuum at about 100° C. for 1 hour. A positive electrode having the electrolyte layer  304  on its surface was produced accordingly. 
     The surface of the electrolyte layer  304  was observed with an optical microscope at a magnification of 50 times. As a result, no cracking was observed on the surface of the electrolyte layer  304 . 
     The copper foil having the electrolyte layer  304  was carefully handled to keep the electrolyte layer  304  from impacts and deformation which cause structural defects inside the electrolyte layer  304 . 
     The negative electrode paste was applied to copper foil having a thickness of about 20 micrometers by screen printing. The applied negative electrode paste was dried under vacuum at about 100° C. for 1 hour to form the negative electrode active material layer  302  on the copper foil. The copper foil functioned as the current collector  303 . The negative electrode active material layer  302  had a thickness of about 80 micrometers. 
     Next, a negative electrode having the electrolyte layer  304  on its surface was produced in the same manner as that for the positive electrode. No cracking was also observed on the surface of the electrolyte layer  304  of the negative electrode. 
     Next, the positive electrode and the negative electrode were stacked on top of each other such that the surface of the electrolyte layer  304  of the positive electrode comes into contact with the surface of the electrolyte layer  304  of the negative electrode. A multilayer body was produced accordingly. An elastic sheet was stacked on each of the upper surface and the lower surface of the multilayer body. Each elastic sheet had an elastic modulus of about 5×10 6  Pa and a thickness of 70 micrometers. Next, the multilayer body was placed in a mold. The multilayer body was pressed in the mold at a pressure of 800 MPa for 100 seconds while being heated to 50° C. The multilayer body was taken out of the mold, and two elastic sheets were carefully removed so as not to damage the multilayer body. A secondary battery having a rectangular parallelepiped shape was produced accordingly. 
     A terminal electrode was attached to each of the surfaces of the current collectors  303  (i.e., copper foil) of the secondary battery such that a lead terminal was bonded to each of the surfaces of the current collectors  303  by using an Ag-based high-conductive adhesive. 
     The charging/discharging characteristics (i.e., charge capacity, discharge capacity, and charge/discharge efficiency) of the secondary battery were evaluated. In other words, the charge capacity, discharge capacity, and charge/discharge efficiency in the first measurement were measured at 0.05 C at room temperature (i.e., about 25 degrees Celsius). 
     Furthermore, the cross section of the secondary battery was observed. Specifically, the secondary battery was cut by using a Thomson blade so that the cross section of a central portion of the secondary battery was exposed. The exposed entire cross section was observed with an optical microscope at a magnification of 50 times to determine whether the inner structure of the secondary battery had structural defects, such as delamination. Five secondary batteries for each sample were used for cross-section observation. The results of the cross-section observation are shown in Table 2. In the column of “structural defects” in Table 2, the number of secondary batteries having structural defects observed in the cross section among five secondary batteries is shown as the numerator. Needless to say, the denominator is 5. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Solid Electrolyte 
               
            
           
           
               
               
               
            
               
                   
                   
                 Components (i.e., Second 
               
               
                   
                 Component (i.e., First 
                 and Third Electrolyte Materials) 
               
               
                 Sample 
                 Solid Electrolyte Material) 
                 of Second Particles and Particle 
               
               
                 Number 
                 of First Particles (vol %) 
                 Boundary Layer (vol %) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 100 
                 0 
               
               
                 2 
                 95 
                 5 
               
               
                 3 
                 90 
                 10 
               
               
                 4 
                 70 
                 30 
               
               
                 5 
                 50 
                 50 
               
               
                 6 
                 30 
                 70 
               
               
                 7 
                 0 
                 100 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Battery 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Structural 
               
               
                   
                   
                   
                   
                   
                 Defects 
               
               
                   
                   
                   
                   
                   
                 (Number of 
               
               
                   
                 Effective 
                   
                   
                 Charge/ 
                 Batteries 
               
               
                   
                 Ionic 
                 Charge 
                 Discharge 
                 Discharge 
                 Having 
               
               
                 Sample 
                 Conductivity 
                 Capacity 
                 Capacity 
                 Efficiency 
                 Structural 
               
               
                 Number 
                 [mS/cm] 
                 [mAh/g] 
                 mAh/g] 
                 [%] 
                 Defects/5) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 0.28 
                 148 
                 101 
                 68.2 
                 5/5 
               
               
                 2 
                 4.36 
                 209 
                 180 
                 86.1 
                 0/5 
               
               
                 3 
                 4.59 
                 210 
                 182 
                 86.7 
                 0/5 
               
               
                 4 
                 4.68 
                 212 
                 185 
                 87.3 
                 0/5 
               
               
                 5 
                 2.21 
                 209 
                 177 
                 84.7 
                 0/5 
               
               
                 6 
                 1.98 
                 205 
                 170 
                 82.9 
                 0/5 
               
               
                 7 
                 1.75 
                 195 
                 152 
                 77.9 
                 0/5 
               
               
                   
               
            
           
         
       
     
     The comparison of Sample 2 to Sample 6 with Sample 1 and Sample 7 shows that the addition of the LPS glass powder to the component (i.e., argyrodite sulfide) of the first particles allows batteries to have no structural defects and have a high effective ionic conductivity of 1.98 mS/cm or more. 
     Since the batteries have no structural defects, the inventors believe that delamination is suppressed during pressing. It is noted that the LPS glass powder forms the second particles  102  and the particle boundary layer  103  (second particles  102 : particle boundary layer  103 =90 vol %:10 vol %). 
     The secondary batteries according to Sample 2 to Sample 6 have a charge capacity of more than 200 mAh/g. Furthermore, the secondary batteries according to Sample 2 to Sample 6 have a higher charge/discharge efficiency (i.e., a charge/discharge efficiency of about 80% or more) than the secondary batteries according to Sample 1 and Sample 7. 
     The secondary batteries according to Sample 2 to Sample 6 had a charge capacity of more than 200 mAh/g and a charge/discharge efficiency of 80% or more. 
     The results of Sample 2 to Sample 3 together with the results of Sample 4 to Sample 6 show that, even when the LPS glass powder accounted for a volume ratio of 30% or more, the secondary batteries had a high ionic conductivity of 1.98 mS/cm or more and had no structural defects. 
     The comparison of Sample 2 to Sample 4 with Sample 5 and Sample 6 shows that, when the LPS glass powder accounts for a volume ratio of less than 50% (preferably 5% or more and less than 50%, more preferably 5% or more and 30% or less), the secondary batteries have a high ionic conductivity of about 4 mS/cm or more. 
     The results of Sample 7 show that, when the solid electrolyte contains the LPS glass powder but does not contain the argyrodite sulfide, the secondary battery has no structural defects but the solid electrolyte has a low effective ionic conductivity and a low charge/discharge efficiency. This is because of the absence of the argyrodite sulfide in the secondary battery according to Sample 7. It is noted that the argyrodite sulfide functions as an ion conduction path. 
     The results of Sample 1 show that, when the solid electrolyte contains the argyrodite sulfide but does not contain the LPS glass powder, the solid electrolyte has a low effective ionic conductivity and a low charge/discharge efficiency. This is because of the absence of the LPS glass powder in the secondary battery according to Sample 1. 
     The solid electrolyte according to the present disclosure may be used for a secondary battery. The secondary battery may be used for electronic devices and automobiles.