Patent Publication Number: US-2023163299-A1

Title: Positive electrode material and battery

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a positive electrode material for battery, and to a battery. 
     2. Description of the Related Art 
     WO 2007/004590 (Patent Literature 1) discloses an all-solid-state lithium battery that includes a lithium ion conductive solid electrolyte based on a sulfide, and an active material covered with a lithium ion conductive oxide on the surface. 
     Am. Chem. Soc. 2010, Vol. 22, 3, 949-956 (Non Patent Literature 1) describes that a battery including a sulfide solid electrolyte comes to have an increased battery resistance after initial charging due to the formation of an interface layer at the interface between the positive electrode material and the sulfide solid electrolyte. 
     SUMMARY 
     One non-limiting and exemplary embodiment provides a positive electrode material that can reduce the resistance of a battery. 
     In one general aspect, the techniques disclosed here feature a positive electrode material including a positive electrode active material, a first solid electrolyte, and a coating material covering at least part of the surface of the positive electrode active material, wherein the first solid electrolyte is represented by the following compositional formula (1): Li a M b X c  . . . Formula (1) wherein in the compositional formula (1), a, b and c are each independently a positive real number, M includes calcium, yttrium, and at least one rare earth element other than yttrium, and X includes at least one selected from the group consisting of F, Cl, Br and I. 
     The positive electrode material provided by the present disclosure can reduce the resistance of a battery. 
     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 sectional view illustrating a schematic configuration of a positive electrode material according to embodiment 1; 
         FIG.  2    is a sectional view illustrating a schematic configuration of a battery according to embodiment 2; 
         FIG.  3    is a view illustrating a Nyquist diagram of a battery of Example 1 at 3.7 V; and 
         FIG.  4    is a view illustrating an O 1 s spectrum of an active material used in Example 1 according to X-ray photoelectron spectroscopy. 
     
    
    
     DETAILED DESCRIPTIONS 
     (Underlying Knowledge Forming Basis of the Present Disclosure) 
     Patent Literature 1 discloses an all-solid-state lithium battery that includes a lithium ion conductive solid electrolyte based on a sulfide, and an active material covered with a lithium ion conductive oxide on the surface. Patent Literature 1 describes that the lithium ion conductive oxide, especially lithium niobate (namely, LiNbO 3 ) eliminates or reduces the formation of a high-resistance layer between the sulfide solid electrolyte and the surface of the positive electrode active material, and consequently output characteristics are markedly enhanced. 
     Non Patent Literature 1 describes that a high-resistance layer is formed between the sulfide solid electrolyte and the surface of the positive electrode active material due to the mutual diffusion of the metal elements contained in the positive electrode active material and the elements constituting the solid electrolyte. Some elements that are mentioned as solid electrolyte constituents are P and S. That is, the formation of a high-resistance layer described above stems from the presence of P or S constituting the solid electrolyte. 
     According to the findings obtained from the inventions described in Patent Literature 1 and Non Patent Literature 1, a battery including a sulfide as a solid electrolyte should be designed so that the surface of an active material is covered with a coating material such as, for example, a lithium ion conductive oxide to sufficiently prevent the solid electrolyte from contact with the active material; otherwise, the resistance at the interface between the active material and the solid electrolyte becomes so high and the polarization is so increased that the battery ends up being substantially inoperable. Meanwhile, a battery including a S-free solid electrolyte is operable even in the absence of a lithium ion conductive oxide covering the surface of an active material. 
     However, extensive studies by the present inventors have found that covering the surface of an active material in a positive electrode material with a coating material reduces the battery resistance even when a first solid electrolyte contained in the positive electrode material is free from P and S. The reasons behind this are not clear but are probably mixed contributions of various factors such as the withstand voltage of the coating material, the oxidation resistance of a lithium ion conductive solid electrolyte used as the solid electrolyte in a battery, the reactivity between the coating material and the active material, the reactivity between the coating material and the solid electrolyte, and physical contact between the active material and the solid electrolyte. 
     When, for example, the lithium ion conductive solid electrolyte used as the solid electrolyte in a battery is a halide, the increase in positive electrode potential during charging of the battery induces the oxidation of the halogen contained in the solid electrolyte. Such oxidation of the halogen leads to the decomposition of the solid electrolyte. Furthermore, the halogen oxidation reaction generates a halogen gas, which creates gaps at the contact interface between the active material and the solid electrolyte to cause a decrease in effective reaction area. Consequently, the battery resistance is increased. By separating the active material and the solid electrolyte from each other through a coating material that covers the surface of the active material, the solid electrolyte is prevented from contact with the active material at a high potential, and consequently the oxidation of halogen will be eliminated or reduced. These are probably the reasons why the battery resistance can be reduced. 
     Halide solid electrolytes are crystals with high ionic bonding properties. Therefore, a solid electrolyte that contains a rare earth element having a relatively large ionic radius, in particular, Sc, and/or a lanthanoid element, has a large bond distance between the cation, namely, Sc and/or the lanthanoid element, and the anion, namely, the halogen. A large bond distance means that the bond between the cation and the anion is weak, that is, the halogen anion is easily released. The coating on the active material eliminates or reduces the oxidation of the halogen in the solid electrolyte and is thus more effective for reducing the battery resistance. 
     Based on the above findings, the present inventors have reached a positive electrode material of the present disclosure described below that can reduce the resistance of a battery. 
     (Summary of aspects of the present disclosure) 
     A positive electrode material according to the first aspect of the present disclosure includes: 
     a positive electrode active material; 
     a first solid electrolyte; and 
     a coating material covering at least part of the surface of the positive electrode active material, wherein 
     the first solid electrolyte is represented by the following compositional formula (1): 
       Li a MbX c  . . .   Formula (1)
 
     wherein 
     in the compositional formula (1), 
     a, b and c are each independently a positive real number, 
     M includes calcium, yttrium, and at least one rare earth element other than yttrium, and 
     X includes at least one selected from the group consisting of F, Cl, Br and I. 
     In the positive electrode material according to the first aspect, the coating material is interposed between the positive electrode active material and the first solid electrolyte that is a halide solid electrolyte. This coating material inhibits the transfer of electrons to and from the halide solid electrolyte even when the potential of the positive electrode is elevated during charging of a battery. That is, the halogen in the halide solid electrolyte is prevented from oxidation reaction, and consequently the decomposition of the first solid electrolyte as well as the generation of halogen gas are eliminated or reduced. As a result, the first solid electrolyte is prevented from degradation, and no or a small decrease is attained in the effective reaction area between the positive electrode active material and the first solid electrolyte. For these reasons, the positive electrode material according to the first aspect can reduce the resistance of a battery. Furthermore, the ion conductivity of the first solid electrolyte can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     In the second aspect of the present disclosure, for example, the positive electrode material according to the first aspect may be such that the coating material includes O. 
     The positive electrode material according to the second aspect can reduce the resistance of a battery more effectively. 
     In the third aspect of the present disclosure, for example, the positive electrode material according to the first or second aspect may be such that the coating material includes Li. 
     The positive electrode material according to the third aspect can attain a high carrier concentration, namely, a high Li concentration at the interface between the positive electrode active material and the first solid electrolyte. Thus, the positive electrode material according to the third aspect can reduce the resistance of a battery more effectively. 
     In the fourth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the third aspect may be such that the coating material includes at least one selected from the group consisting of lithium phosphate and lithium niobate. 
     Lithium phosphate and lithium niobate can increase the lithium ion conductivity of the coating material. Thus, the positive electrode material according to the fourth aspect can reduce the resistance of a battery more effectively. 
     In the fifth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the fourth aspect may be such that the mass ratio of the coating material to the positive electrode active material is greater than or equal to 0.5 mass % and less than or equal to 2.0 mass %. 
     When the amount of coating is controlled to the above range, lithium ions are smoothly transferred between the active material and the solid electrolyte, and thus the resistance of a battery can be reduced more effectively. 
     In the sixth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the fifth aspect may be such that the thickness of the coating material is greater than or equal to 2 nm and less than or equal to 20 nm. 
     When the thickness of the coating material is controlled to the above range, the energy density of a battery can be enhanced. Furthermore, the resistance of a battery can be reduced more effectively. 
     In the seventh aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the sixth aspect may be such that M includes at least one element selected from the group consisting of Gd and Sm. 
     The positive electrode material according to the seventh aspect can attain a further enhancement in ion conductivity of the first solid electrolyte. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     In the eighth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the seventh aspect may be such that X includes at least one element selected from the group consisting of F, Cl and Br. 
     In the ninth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the eighth aspect may be such that X includes at least two elements selected from the group consisting of F, Cl and Br. 
     In the tenth aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the ninth aspect may be such that X includes Cl and Br. 
     According to the above configurations, the ion conductivity of the first solid electrolyte can be further enhanced. Thus, the positive electrode material can further enhance the charge/discharge efficiency of a battery. 
     In the eleventh aspect of the present disclosure, for example, the positive electrode material according to any one of the first to the tenth aspect may be such that the positive electrode active material includes a lithium-containing transition metal oxide. 
     The positive electrode material according to the eleventh aspect can enhance the energy density of a battery. 
     A battery according to the twelfth aspect of the present disclosure includes: 
     a positive electrode including the positive electrode material according to any one of the first to the eleventh aspect; 
     a negative electrode; and 
     an electrolyte layer disposed between the positive electrode and the negative electrode. 
     The battery according to the twelfth aspect can attain a low resistance and can further achieve an enhancement in charge/discharge efficiency. 
     In the thirteenth aspect of the present disclosure, for example, the battery according to the twelfth aspect may be such that the electrolyte layer includes a sulfide solid electrolyte. 
     The battery according to the thirteenth aspect can attain a low resistance and can further achieve an enhancement in charge/discharge efficiency. 
     Hereinbelow, embodiments of the present disclosure will be described with reference to the drawings. 
     (Embodiment 1) 
       FIG.  1    is a sectional view illustrating a schematic configuration of a positive electrode material  1000  according to the embodiment  1 . The positive electrode material  1000  in the embodiment 1 includes a first solid electrolyte  100 , a positive electrode active material  110 , and a coating material  111  covering at least part of the surface of the positive electrode active material  110 . The first solid electrolyte  100  and the positive electrode active material  110  may be particles. The coating material  111  may cover the entire surface of the positive electrode active material  110 , or may partially cover the surface of the positive electrode active material  110 . That is, the coating material  111  covers at least part of the surface of the positive electrode active material  110 . In other words, the positive electrode active material  110  and the first solid electrolyte  100  share a portion in which the positive electrode active material  110  and the first solid electrolyte  100  are separated from each other by the coating material  111  and are out of contact with each other. The positive electrode active material  110  and the first solid electrolyte  100  may be partially in contact with each other. 
     The first solid electrolyte  100  is represented by the following compositional formula (1): 
       Li a M b X c  . . .   Formula (1)
 
     In the compositional formula (1), a, b and c are each independently a positive real number. M includes calcium, yttrium, and at least one rare earth element other than yttrium. X includes at least one selected from the group consisting of F, Cl, Br and I. 
     In the positive electrode material  1000  in the present embodiment, the coating material  111  is interposed between the positive electrode active material  110  and the first solid electrolyte  100  that is a halide solid electrolyte. The coating material  111  inhibits the transfer of electrons to and from the halide solid electrolyte even when the potential of the positive electrode is elevated during charging of a battery. That is, the halogen in the first solid electrolyte  100  is prevented from oxidation reaction, and consequently the decomposition of the first solid electrolyte  100  as well as the generation of halogen gas stemming from oxidation reaction are eliminated or reduced. As a result, the first solid electrolyte  100  is prevented from degradation, and no or a small decrease is attained in the effective reaction area between the positive electrode active material  110  and the first solid electrolyte  100 . For these reasons, the positive electrode material  1000  in the present embodiment can reduce the resistance of a battery. As a result, furthermore, the positive electrode material  1000  according to the embodiment 1 allows a battery to attain an enhanced charge/discharge efficiency. 
     As described hereinabove, the coating material  111  may uniformly cover the entire surface of the positive electrode active material  110 . In this case, the positive electrode active material  110  and the first solid electrolyte  100  are prevented from direct contact with each other, and the oxidation reaction of the first solid electrolyte  100  can be eliminated or reduced more reliably. Thus, charge/discharge characteristics of a battery can be further enhanced, and the increase in reaction overvoltage of a battery can be reduced. 
     The coating material  111  may cover only a portion of the surface of the positive electrode active material  110 . The particles of the positive electrode active material  110  can be in direct contact with one another through the portions exposed from the coating material  111 , and consequently the electron conductivity between the particles of the positive electrode active material  110  is enhanced. Thus, a battery may be operated at a high output. 
     For example, the coating material  111  may be a material having low electron conductivity. For example, an O-containing material may be used as the coating material  111 . Examples of the O-containing materials include oxide materials and oxide solid electrolytes. 
     Examples of the oxide materials that may be used as the coating materials  111  include SiO 2 , Al 2 O 3 , TiO 2 , B 2 O 3 , Nb 2 O 5 , WO 3  and ZrO 2 . Examples of the oxide solid electrolytes that may be used as the coating materials  111  include Li—P—O compounds such as Li 3 PO 4 , Li—Nb—O compounds such as LiNbO 3 , Li—B—O compounds such as LiBO 2  and Li 3 BO 3 , Li—Al—O compounds such as LiAlO 2 , Li—Si—O compounds such as Li 4 SiO 4 , Li—Ti—O compounds such as Li 4 Ti 5 O 12 , Li—Zr—O compounds such as Li 2 ZrO 3 , Li—Mo—O compounds such as Li 2 MoO 3 , Li—V—O compounds such as LiV 2 O 5 , Li—W—O compounds such as Li 2 WO 4 , and Li 2 SO 4 . 
     The coating material  111  may include an oxide solid electrolyte. Oxide solid electrolytes have high ion conductivity and high stability at high potentials. Thus, the charge/discharge efficiency may be further enhanced by using an oxide solid electrolyte. 
     The coating material  111  may include an oxoacid salt. When the coating material  111  includes an oxoacid salt, the resistance of a battery can be reduced more effectively. The oxoacid salt may be an oxoacid salt having a nonmetal or a semimetal as a cation. As described above, the “semimetal elements” are B, Si, Ge, As, Sb and Te. The “nonmetal elements” are N, P, S, Cl, Br and I. Specifically, these elements are elements that bond to oxygen to form oxoacids. 
     The coating material  111  may include at least one selected from the group consisting of B, N, P, S and Si. 
     The coating material  111  that includes at least one selected from the group consisting of B, N, P, S and Si can form a covering having low electron conductivity on the surface of the positive electrode active material  110 . Thus, the positive electrode material  1000  in the embodiment 1 can attain a further reduction in the probability that the first solid electrolyte  100  may be oxidized. Such elements as B, N, P, S and Si form strong covalent bonds with oxygen. Thus, electrons in the coating material  111  are delocalized, and a covering formed of such a coating material  111  exhibits low electron conductivity. That is, such a covering can block the transfer of electrons between the positive electrode active material  110  and the first solid electrolyte  100  even when the thickness of the coating material  111  on the surface of the positive electrode active material  110  is small, and thus can eliminate or reduce the oxidation reaction of the first solid electrolyte  100  more effectively. Thus, by virtue of the coating material  111  including at least one selected from the group consisting of B, N, P, S and Si, the positive electrode material  1000  can reduce the resistance of a battery more effectively and can further enhance the charge/discharge efficiency. 
     The coating material  111  may include Li. This configuration can increase the carrier concentration at the interface between the positive electrode active material  110  and the first solid electrolyte  100 , and thus can reduce the resistance of a battery more effectively. 
     When the coating material  111  includes Li, the molar ratio Li/(other cations) of lithium to other cations in the coating material  111  may be greater than or equal to 0.8 and less than or equal to 3.6, or may be greater than or equal to 1.0 and less than or equal to 3.0. When the molar ratio Li/(other cations) is greater than or equal to 0.8 and less than or equal to 3.6, the lithium ion conductivity in the coating material  111  can be increased. Thus, the resistance of a battery can be reduced more effectively. 
     The coating material  111  may include a glass-forming oxide such as phosphate or silicate. Here, the glass-forming oxide means a network-forming oxide that can form glass by itself. The element that serves as a cation in the glass-forming oxide, that is, the element called a network former may be, for example, Si, P, B, Ge or V. The coating material  111  including a glass-forming oxide can exhibit higher lithium ion conductivity in the coating material  111 . Specifically, the coating material  111  that includes a lithium compound of an oxide called a glass-forming oxide such as phosphate or silicate forms a partially amorphous covering having wide ion conductive paths. Thus, the lithium ion conductivity in the coating material  111  can be increased, and the resistance of a battery may be reduced more effectively. 
     The coating material  111  may include an intermediate oxide such as niobate. Here, the intermediate oxide means an oxide that cannot form glass by itself (that is, cannot form a glass network by itself) but can form glass or enter the glass network depending on the composition. The element that serves as a cation in the intermediate oxide may be, for example, Nb, Ti, Zn, Al or Zr. The coating material  111  that includes an intermediate oxide can exhibit higher lithium ion conductivity in the coating material  111 . Specifically, the coating material  111  that includes a lithium compound of an oxide called an intermediate oxide such as niobate forms a partially amorphous covering having wide ion conductive paths. Thus, the lithium ion conductivity in the coating material  111  can be increased, and the resistance of a battery may be reduced more effectively. 
     The coating material  111  may include at least one selected from the group consisting of lithium phosphate and lithium niobate. 
     Lithium phosphate and lithium niobate can increase the lithium ion conductivity of the coating material  111 . Thus, the positive electrode material  1000  of the present embodiment can reduce the resistance of a battery more effectively. 
     The coating material  111  may include at least one selected from the group consisting of lithium phosphate and lithium niobate as a main component, and may further include inevitable impurities or impurities such as starting materials, byproducts and decomposition products used or occurring in the preparation of the coating material  111 . Specifically, the coating material  111  may include lithium phosphate and lithium niobate in a total proportion relative to the mass of the whole of the coating material  111  of, for example, greater than or equal to 50% (greater than or equal to 50 mass %). For example, the coating material  111  may include lithium phosphate and lithium niobate in a total proportion of 100% (100 mass %) relative to the mass of the whole of the coating material  111  except inevitable impurities. 
     In the embodiment  1 , the coating material  111  may be LiNbO 3 . LiNbO 3  has higher ion conductivity and higher stability at high potentials. Thus, the charge/discharge efficiency of a battery can be further enhanced by using LiNbO 3 . 
     When a battery is charged to raise the potential of the positive electrode while the positive electrode active material  110  and the first solid electrolyte  100  are in direct contact with each other, electrons are extracted from the first solid electrolyte  100  and the first solid electrolyte  100  is oxidized. This is prevented effectively by the coating material  111  interposed between the positive electrode active material  110  and the first solid electrolyte  100 . However, the coating material  111  may sometimes also block the electronic contact between the positive electrode active material  110  and a conductive auxiliary or between particles of the positive electrode active material  110 . In such a case, the coating material  111  interrupts electronic paths from the current collector of the battery to the particles of the positive electrode active material  110 , and the isolated positive electrode active material  110  may not contribute to the charge/discharge reactions. That is, the apparent amount of the active material decreases and the reaction area is reduced to make it difficult to sufficiently reduce the resistance. An approach to this is to expose part of the surface of the positive electrode active material  110  from the coating material  111 . In this manner, the oxidation reaction of the first solid electrolyte  100  can be eliminated or reduced while ensuring the electronic paths at the same time. For this reason, part of the surface of the positive electrode active material  110  may be exposed from the coating material  111 . 
     In order to further eliminate or reduce the oxidation reaction of the first solid electrolyte  100  and to reduce the resistance of a battery more effectively, the coating material  111  may cover the surface of the positive electrode active material  110  in a ratio of the covered surface area to the total surface area of the positive electrode active material  110 , namely, in a coverage ratio of greater than or equal to 18%. To reduce the resistance of a battery more effectively, the coverage ratio may be greater than or equal to 47%, may be greater than or equal to 90%, or may be 100%. 
     The coverage ratio may be determined by separating O 1 s peaks in X-ray photoelectron spectroscopy (XPS). When, for example, Li(Ni, Co, Mn)O 2  is used as the positive electrode active material  110  and lithium phosphate is used as the coating material  111 , the coverage ratio may be determined by subtracting the O 1 s peak assigned to carbonic acid near 531 eV from the O 1 s peak near 532 eV, and dividing the area of the Ols peak assigned to the positive electrode active material and reaching the peak top at about 529 eV by the area of the peak resulting from the above subtraction. 
     When the accurate determination of the coverage ratio is difficult by the above method, the elemental amounts of metals, for example, Ni, Co and Mn in the positive electrode active material  110 , and the elemental amount of cation species, for example, P or Si in the coating material  111  may be measured by XPS, and the coverage ratio may be determined from the ratio of these amounts. 
     The coverage ratio may also be determined with respect to a backscattered electron image obtained with a scanning electron microscope (SEM), based on the difference in contrast stemming from the difference in composition between the active material and the coating material. 
     The coverage ratio may also be determined by mapping the constituent elements in the active material and the coating material using energy-dispersive X-ray spectroscopy (EDX). 
     The mass ratio of the coating material  111  to the positive electrode active material  110  may be less than or equal to 2.0 mass %, or may be less than or equal to 1.5 mass %. This configuration can increase the proportion of the positive electrode active material  110  or the first solid electrolyte  100  in the positive electrode, and thus can increase the energy density of a battery. 
     The mass ratio of the coating material  111  to the positive electrode active material  110  may be greater than or equal to 0.1 mass %, or may be greater than or equal to 0.5 mass %. This configuration can increase the proportion of the positive electrode active material  110  or the first solid electrolyte  100  in the positive electrode, and thus can increase the energy density of a battery. 
     The mass ratio of the coating material  111  to the positive electrode active material  110  may be greater than or equal to 0.5 mass % and less than or equal to 2.0 mass %. This configuration can increase the proportion of the positive electrode active material  110  or the first solid electrolyte  100  in the positive electrode, and thus can increase the energy density of a battery. 
     The mass ratio of the coating material  111  to the positive electrode active material  110  may be greater than or equal to 0.5 mass % and less than or equal to 1.5 mass %. 
     To determine the mass ratio of the coating material  111  to the positive electrode active material  110 , for example, the positive electrode may be dissolved with a solvent such as an acid to form an aqueous solution, the solution may be analyzed by inductively coupled plasma (ICP) emission spectroscopy to quantify the elements that are contained, and the amounts may be expressed as the mass ratio. In this process, the mass ratio may be determined based on the quantified amounts of elements that are contained in either the positive electrode active material  110  or the coating material  111 , assuming that the compositions are stoichiometric. When, for example, LiNiO 2  is covered with Li 3 PO 4 , the mass ratio of the coating material  111  may be determined from the quantified amounts of Ni and P assuming that LiNiO 2  and Li 3 PO 4  are each present in a stoichiometric composition. 
     In the compositional formula (1), the at least one rare earth element other than yttrium that is included in M may be elements including Sc and a lanthanoid element. For example, the lanthanoid elements are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The rare earth elements have similar chemical properties and thus are all usable as elements for constituting the halide solid electrolyte in the present embodiment. 
     In the compositional formula (1), M may include calcium, yttrium, and one rare earth element other than yttrium. M may include at least one rare earth element other than yttrium, and may include only one rare earth element other than yttrium. 
     In the compositional formula (1), M may include at least one selected from the group consisting of Gd and Sm. 
     In the compositional formula (1), M may include only one selected from the group consisting of Gd and Sm. 
     According to the above configurations, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     In the compositional formula (1), X may include at least one element selected from the group consisting of F, Cl and Br. In the compositional formula (1), X may include at least two elements selected from the group consisting of F, Cl and Br. In the compositional formula (1), X may include Cl and Br. According to these configurations, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The first solid electrolyte  100  may be represented by the following compositional formula (2): 
       Li 6-2a-3d Ca a (Y 1-b Gd b ) d Br 6-c Cl c  . . .   Formula (2)
 
     The compositional formula (2) satisfies 0&lt;a, 0&lt;b&lt;1, 0&lt;c&lt;6, and 0&lt;d&lt;1.5. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The compositional formula (2) may satisfy 0.01≤a≤0.3. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The compositional formula (2) may satisfy a≤0.2. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The compositional formula (2) may satisfy 0.1≤b≤0.9. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The compositional formula (2) may satisfy 0.8≤b&lt;1. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The compositional formula (2) may satisfy 1.0≤c≤1.2. 
     According to the above configuration, the ion conductivity of the first solid electrolyte  100  can be further enhanced. Thus, the charge/discharge efficiency of a battery can be further enhanced. 
     The first solid electrolyte  100  and the halide solid electrolyte may be free from sulfur. 
     For example, the positive electrode active material  110  is a material capable of occluding and releasing metal ions (for example, lithium ions). Examples of the positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides and transition metal oxynitrides. Examples of the lithium-containing transition metal oxides include Li(NiCoAl)O 2 , Li(NiCoMn)O 2  and LiCoO 2 . When, for example, a lithium-containing transition metal oxide is used as the positive electrode active material, the manufacturing cost of the positive electrode can be reduced and the average discharge voltage can be increased. 
     In order to increase the energy density of a battery, the positive electrode active material  110  may be lithium nickel cobalt manganese oxide. For example, the positive electrode active material  110  may be Li(NiCoMn)O 2 . 
     According to the above configuration, the energy density and the charge/discharge efficiency of a battery can be further increased. 
     The thickness of the coating material  111  may be greater than or equal to 1 nm and less than or equal to 100 nm. 
     The coating material  111  having a thickness of greater than or equal to 1 nm can prevent direct contact between the positive electrode active material  110  and the first solid electrolyte  100 , and thus can eliminate or reduce the side reaction of the first solid electrolyte  100 , thereby enhancing the charge/discharge efficiency. 
     Furthermore, the coating material  111  having a thickness of less than or equal to 100 nm is not excessively thick. Such a coating material  111  can ensure a sufficiently low internal resistance of a battery. As a result, the energy density of a battery can be increased. 
     The thickness of the coating material  111  may be greater than or equal to 2 nm and less than or equal to 20 nm. 
     The coating material  111  having a thickness of greater than or equal to 2 nm can prevent direct contact between the positive electrode active material  110  and the first solid electrolyte  100  more efficiently, and thus can eliminate or reduce the side reaction of the first solid electrolyte  100 . Thus, the charge/discharge efficiency can be enhanced more efficiently. 
     Furthermore, the coating material  111  having a thickness of less than or equal to 20 nm can ensure a lower internal resistance of a battery. As a result, the energy density of a battery can be increased. The thickness of the coating material  111  may be less than or equal to 40 nm. 
     The thickness of the coating material  111  may be measured by any technique without limitation. For example, the thickness of the coating material  111  may be determined by direct observation using a device such as a transmission electron microscope. Alternatively, the thickness may be determined by Ar while following the change in the XPS spectrum assigned to the active material while removing the coating layer with Ar sputtering. 
     The shape of the first solid electrolyte  100  in the embodiment  1  is not particularly limited and may be, for example, acicular, spherical, elliptical spherical or other shape. For example, the shape of the first solid electrolyte  100  may be particulate. 
     When, for example, the shape of the first solid electrolyte  100  in the embodiment 1 is particulate (for example, spherical), the median diameter may be less than or equal to 100 μm. If the median diameter is greater than 100 μm, the positive electrode active material  110  and the first solid electrolyte  100  may not be favorably dispersed in the positive electrode material  1000  to cause a decrease in charge/discharge characteristics. In the embodiment 1, the median diameter may be less than or equal to 10 μm. 
     The above configuration ensures that the positive electrode active material  110  and the first solid electrolyte  100  can be favorably dispersed in the positive electrode material  1000 . 
     In the embodiment 1, the median diameter of the first solid electrolyte  100  may be smaller than the median diameter of the positive electrode active material  110 . 
     The above configuration ensures that the first solid electrolyte  100  and the positive electrode active material  110  can be more favorably dispersed in the positive electrode material  1000 . 
     The median diameter of the positive electrode active material  110  may be greater than or equal to 0.1 μm and less than or equal to 100 μm. 
     When the median diameter of the positive electrode active material  110  in the positive electrode material  1000  is greater than or equal to 0.1 μm, the positive electrode active material  110  and the first solid electrolyte  100  can be favorably dispersed in the positive electrode material  1000 . As a result, charge/discharge characteristics of a battery are enhanced. When the median diameter of the positive electrode active material  110  is less than or equal to 100 μm, a sufficient rate of diffusion of lithium in the positive electrode active material  110  is ensured to allow a battery to be operated at a high output. 
     The median diameter of the positive electrode active material  110  may be larger than the median diameter of the first solid electrolyte  100 . In this case, the positive electrode active material  110  and the first solid electrolyte  100  can be favorably dispersed. 
     The median diameter means the particle size at 50% cumulative volume in the volume-based grain size distribution. The volume-based grain size distribution is measured with, for example, a laser diffraction measurement device or an image analyzer. 
     In the positive electrode material  1000  in the embodiment 1, the first solid electrolyte  100  and the coating material  111  may be in contact with each other as illustrated in  FIG.  1   . 
     The positive electrode material  1000  in the embodiment 1 may include a plurality of particulate first solid electrolytes  100  and a plurality of particulate positive electrode active materials  110 . 
     In the positive electrode material  1000  in the embodiment 1, the content of the first solid electrolyte  100  and the content of the positive electrode active material  110  may be the same as or different from each other. 
     &lt;Methods for producing the first solid electrolyte&gt; 
     For example, the first solid electrolyte in the embodiment 1 may be produced by the following method. 
     Ingredient powders are provided in a blending ratio that will give the target composition, and the powders are mixed with one another. For example, the ingredient powders are oxides, hydroxides, halides or acid halides. When, for example, Li 3 YBr 3 Cl 3  is the target product, LiBr and YCl 3  are provided in a molar ratio of 3:1. 
     The “M” and the “X” in the above-described compositional formula (1) may be determined by selecting the types of the ingredient powders. The values of “a”, “b” and “c” may be controlled by controlling the blending ratio of the ingredients, and selecting the synthesis process. 
     After being mixed together sufficiently, the ingredient powders are mixed, crushed and reacted using a mechanochemical milling method. Alternatively, the ingredient powders after being sufficiently mixed may be sintered in vacuum. 
     The first solid electrolyte may be obtained in the manner described above. 
     The structure of crystal phases (that is, the crystal structure) in the solid electrolyte may be determined by selecting the reaction method and the reaction conditions for the ingredient powders. 
     &lt;Methods for producing the positive electrode active material covered with the coating material&gt; 
     The positive electrode active material  110  covered with the coating material  111  may be produced by the following method. 
     First, a powder of the positive electrode active material  110  is provided. For example, a powder of the positive electrode active material  110  may be produced by a coprecipitation method. In the coprecipitation method, the positive electrode active material  110  may be produced by preparing a metal oxide as a precursor and heat-treating the precursor together with a lithium source. Alternatively, the positive electrode active material  110  may be easily purchased from powders with various compositions sold in the market. 
     Next, the surface of the particles of the positive electrode active material  110  is covered with the coating material  111 . The coating material  111  may be applied by any method without limitation. For example, the coating material  111  may be applied by a liquid-phase coating method or a vapor-phase coating method. 
     In the liquid-phase coating method, for example, a precursor solution of the coating material  111  is applied to the surface of the positive electrode active material  110 . When forming a coating material  111  containing lithium phosphate, the precursor solution may be a mixed solution of a solvent, lithium hydroxide and triethyl phosphate. 
     The ingredients are not particularly limited as long as the ingredients can be dissolved or dispersed in a solvent. Examples of the lithium sources include alkyllithiums such as tert-butyllithium, lithium alkoxides such as lithium methoxide, lithium ethoxide, lithium isopropoxide and lithium tert-butoxide, lithium iodide, lithium bromide, lithium chloride, lithium carbonate, lithium nitrate, lithium sulfate and metallic lithium. Examples of the phosphoric acid sources include trimethyl phosphate, tripropyl phosphate, tributyl phosphate, phosphoric acid, monoammonium dihydrogen phosphate, diammonium monohydrogen phosphate and triammonium phosphate. Ingredients including phosphoric acid and lithium may be used. 
     For example, the solvent is an alcohol such as ethanol. However, the solvent is not limited as long as the solvent can dissolve the ingredients or allows the ingredients to disperse. The solvent may be selected from various types of solvents depending on the ingredients. Examples of the solvents include methanol, propanol, isopropanol, butanol, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, tetrahydrofuran, hexane, benzene, toluene, methylene chloride, acetone and acetonitrile. 
     The amounts of lithium hydroxide and triethyl phosphate are controlled in accordance with the target composition of the coating material  111 . Where necessary, water may be added to the precursor solution. The precursor solution may be acidic or alkaline. 
     (Embodiment  2 ) 
     Hereinbelow, the embodiment 2 will be described. Overlaps with the description of the embodiment 1 will be omitted appropriately. 
       FIG.  2    is a sectional view illustrating a schematic configuration of a battery  2000  according to the embodiment 2. 
     The battery  2000  in the embodiment 2 includes a positive electrode  201 , an electrolyte layer  202  and a negative electrode  203 . 
     The positive electrode  201  includes the positive electrode material  1000  in the embodiment 1. 
     The electrolyte layer  202  is arranged between the positive electrode  201  and the negative electrode  203 . 
     With the above configuration, the battery  2000  can attain a low resistance. 
     The volume ratio “v1:100−v1” between the positive electrode active material  110  and the first solid electrolyte  100  present in the positive electrode  201  may satisfy 30≤v1≤95. Here, v1 indicates the volume proportion of the positive electrode active material  110  relative to the total volume of the positive electrode active material  110  and the first solid electrolyte  100  present in the positive electrode  201  taken as  100 . When 30≤v1 is satisfied, the battery  2000  may attain a sufficient energy density. When v1≤95 is satisfied, the battery  2000  may be operated at a high output more easily. 
     The thickness of the positive electrode  201  may be greater than or equal to 10 μm and less than or equal to 500 μm. The positive electrode  201  having a thickness of greater than or equal to 10 μm may ensure a sufficient energy density of the battery  2000 . When the thickness of the positive electrode  201  is less than or equal to 500 μm, the battery  2000  may be operated at a high output. 
     The electrolyte layer  202  is disposed between the positive electrode  201  and the negative electrode  203 . 
     The electrolyte layer  202  is a layer including an electrolyte material. For example, the electrolyte material is a solid electrolyte (a second solid electrolyte). That is, the electrolyte layer  202  may be a solid electrolyte layer. 
     The second solid electrolyte present in the electrolyte layer  202  may be the same as the first solid electrolyte in the embodiment 1. That is, the electrolyte layer  202  may include the first solid electrolyte in the embodiment 1. 
     The above configuration ensures that the battery  2000  can attain a further enhancement in charge/discharge efficiency. 
     The second solid electrolyte present in the electrolyte layer  202  may be a halide solid electrolyte differing from the first solid electrolyte in the embodiment 1. That is, the electrolyte layer  202  may include a halide solid electrolyte differing from the first solid electrolyte in the embodiment 1. 
     The above configuration ensures that the battery  2000  can attain enhancements in output density and charge/discharge efficiency. 
     The halide solid electrolyte present in the electrolyte layer  202  may include Y as a metal element. 
     The above configuration ensures that the battery can attain further enhancements in output density and charge/discharge efficiency. 
     The halide solid electrolyte present in the electrolyte layer  202  may be a material described hereinabove as the first solid electrolyte in the embodiment 1. 
     The second solid electrolyte present in the electrolyte layer  202  may be a sulfide solid electrolyte. That is, the electrolyte layer  202  may include a sulfide solid electrolyte. 
     According to the above configuration, the electrolyte layer  202  includes a sulfide solid electrolyte that has excellent reduction stability, and thus a low-potential material such as graphite or metallic lithium can be used as the negative electrode material. Thus, the battery  2000  can attain an enhancement in energy density. 
     For example, the sulfide solid electrolyte that is used may be 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 3.25 Ge 0.25 P 0.75 S 4  or Li 10 GeP 2 S 12 . For example, LiX (X: F, Cl, Br, I), Li 2 O, MO q  or Li p MO q  (M: any of P, Si, Ge, B, Al, Ga, In, Fe and Zn) (p and q: natural numbers) may be added to the above sulfide solid electrolytes. 
     The second solid electrolyte present in the electrolyte layer  202  may be an oxide solid electrolyte, a polymer solid electrolyte or a complex hydride solid electrolyte. 
     Examples of the oxide solid electrolytes that may be used include NASICON-type solid electrolytes represented by LiTi 2 (PO 4 ) 3  and element-substituted derivatives thereof, Perovskite-type solid electrolytes such as (LaLi)TiO 3  system, LISICON-type solid electrolytes represented by Li 14 ZnGe 4 O 16 , Li 4 SiO 4 , LiGeO 4  and element-substituted derivatives thereof, garnet-type solid electrolytes represented by Li 7 La 3 Zr 2 O 12  and element-substituted derivatives thereof, Li 3 N and H-substituted derivatives thereof, Li 3 PO 4  and N-substituted derivatives thereof, and glass or glass ceramic electrolytes based on a Li—B—O compound such as LiBO 2  or Li 3 BO 3  and containing such a material as Li 2 SO 4  or Li 2 CO 3 . 
     Examples of the polymer solid electrolytes that may be used include compounds formed between a polymer compound and a lithium salt. The polymer compound may have an ethylene oxide structure. By having an ethylene oxide structure, the polymer compound can contain a large amount of the lithium salt and thus can offer higher ion conductivity. Examples of the lithium salts that may be used 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 . The lithium salt that is used may be a single kind of lithium salt selected from those described above, or may be a mixture of two or more kinds of lithium salts selected from those described above. 
     Examples of the complex hydride solid electrolytes that may be used include LiBH 4 —LiI and LiBH 4 —P 2 S 5 . 
     The electrolyte layer  202  may include the second solid electrolyte as a main component. That is, the electrolyte layer  202  may include the second solid electrolyte in a weight ratio of, for example, greater than or equal to 50% (greater than or equal to 50 wt %) relative to the whole of the electrolyte layer  202 . 
     The above configuration ensures that the battery  2000  can attain further enhancements in charge/discharge characteristics. 
     For example, the electrolyte layer  202  may include the second solid electrolyte in a weight ratio of greater than or equal to 70% (greater than or equal to 70 wt %) relative to the whole of the electrolyte layer  202 . 
     The above configuration ensures that the battery  2000  can attain further enhancements in charge/discharge characteristics. 
     The electrolyte layer  202  may include the second solid electrolyte as a main component and may further include inevitable impurities, or impurities such as starting materials, byproducts and decomposition products used or occurring in the synthesis of the second solid electrolyte. 
     For example, the electrolyte layer  202  may include the second solid electrolyte in a weight ratio of 100% (100 wt %) relative to the whole of the electrolyte layer  202  except inevitable impurities. 
     The above configuration ensures that the battery can attain further enhancements in charge/discharge characteristics. 
     As described above, the electrolyte layer  202  may be composed solely of the second solid electrolyte. 
     The electrolyte layer  202  may include two or more kinds of materials described above as the second solid electrolytes. For example, the electrolyte layer  202  may include a halide solid electrolyte and a sulfide solid electrolyte. 
     The thickness of the electrolyte layer  202  may be greater than or equal to 1 μm and less than or equal to 300 μm. When the thickness of the electrolyte layer  202  is greater than or equal to 1 μm, a short-circuit is unlikely to occur between the positive electrode  201  and the negative electrode  203 . When the thickness of the electrolyte layer  202  is less than or equal to 300 μm, the battery may be operated at a high output easily. That is, appropriate control of the thickness of the electrolyte layer  202  ensures sufficient safety of the battery  2000  and allows the battery  2000  to be operated at a high output. 
     The negative electrode  203  includes a material that is capable of occluding and releasing metal ions (for example, lithium ions). For example, the negative electrode  203  includes a negative electrode active material. 
     Examples of the negative electrode active materials that may be used include metal materials, carbon materials, oxides, nitrides, tin compounds and silicon compounds. The metal materials may be elemental metals. Alternatively, the metal materials may be alloys. Examples of the metal materials include lithium metal and lithium alloys. Examples of the carbon materials include natural graphite, coke, semi-graphitized carbon, carbon fibers, spherical carbon, artificial graphite and amorphous carbon. From the point of view of capacity density, silicon (Si), tin (Sn), silicon compounds and tin compounds may be suitably used. 
     The negative electrode  203  may include a third solid electrolyte. This configuration enhances the lithium ion conductivity inside the negative electrode  203  and allows the battery to be operated at a high output. The third solid electrolyte present in the negative electrode  203  may be a material described hereinabove as an example of the second solid electrolyte in the electrolyte layer  202 . 
     The median diameter of particles of the negative electrode active material may be greater than or equal to 0.1 μm and less than or equal to 100 μm. When the median diameter of the particles of the negative electrode active material is greater than or equal to 0.1 μm, the particles of the negative electrode active material and the solid electrolyte may be more favorably dispersed in the negative electrode. Thus, the battery attains enhancements in charge/discharge characteristics. When the median diameter of the particles of the negative electrode active material is less than or equal to 100 μm, lithium can be diffused at a sufficient rate in the negative electrode active material. Thus, the battery may be operated at a high output. 
     The median diameter of the particles of the negative electrode active material may be larger than the median diameter of the third solid electrolyte. In this case, the particles of the negative electrode active material and the solid electrolyte can be favorably dispersed. 
     The volume ratio “v2:100−v2” between the particles of the negative electrode active material and the solid electrolyte present in the negative electrode  203  may satisfy 30≤v2≤95. Here, v2 indicates the volume proportion of the particles of the negative electrode active material relative to the total volume of the particles of the negative electrode active material and the solid electrolyte present in the negative electrode  203  taken as 100. When 30≤v2 is satisfied, the battery  2000  may attain a sufficient energy density. When v2≤95 is satisfied, the battery  2000  may be operated at a high output more easily. 
     The thickness of the negative electrode  203  may be greater than or equal to 10 μm and less than or equal to 500 μm. The negative electrode  203  having a thickness of greater than or equal to 10 μm may ensure a sufficient energy density of the battery  2000 . When the thickness of the negative electrode  203  is less than or equal to 500 μm, the battery  2000  may be operated at a high output more easily. 
     At least one selected from the group consisting of the positive electrode  201 , the electrolyte layer  202  and the negative electrode  203  may include a binder for the purpose of enhancing the adhesion between the particles. Binders are used to enhance binding properties of a material forming an electrode. Examples of the binders include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resins, polyamides, polyimides, polyamideimides, polyacrylonitrile, polyacrylic acid, polymethyl acrylate, polyethyl acrylate, polyhexyl acrylate, polymethacrylic acid, polymethyl methacrylate, polyethyl methacrylate, polyhexyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyethers, polyether sulfones, hexafluoropolypropylene, styrene butadiene rubbers and carboxymethylcellulose. Examples of the binders that may be used further include copolymers of two or more materials selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ethers, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid and hexadiene. Furthermore, a mixture of two or more materials selected from the above may be used as the binder. 
     At least one of the positive electrode  201  or the negative electrode  203  may include a conductive auxiliary for the purpose of enhancing electron conductivity. Examples of the conductive auxiliaries that may be used include graphites such as natural graphite and artificial graphite, carbon blacks such as acetylene black and Ketjen black, conductive fibers such as carbon fibers and metal fibers, fluorocarbons, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive polymer compounds such as polyaniline, polypyrrole and polythiophene. The use of a carbon conductive auxiliary allows for cost reduction. 
     The batteries  2000  in the embodiment 2 may be formed into various shapes such as coin, cylindrical, prismatic, sheet, button, flat and laminate. 
     For example, the battery  2000  in the embodiment 2 may be produced by providing the positive electrode material  1000  in the embodiment 1, a material for forming the electrolyte layer, and a material for forming the negative electrode, and fabricating a stack by a known method so as to arrange the positive electrode, the electrolyte layer and the negative electrode in this order. 
     EXAMPLES 
     Hereinbelow, the present disclosure will be described in greater detail with reference to Examples and Comparative Examples. 
     Example 1 
     [Preparation of Positive Electrode Active Material Covered with Coating Material on the Surface] 
     In an argon glove box having a dew point of less than or equal to −60° C., 6.3 mg of lithium hydroxide and 16.0 mg of triethyl phosphate were dissolved into an appropriate amount of super-dehydrated ethanol (manufactured by Wako Pure Chemical Industries, Ltd.) to give a coating material solution. In the coating material solution, the molar ratio of lithium to phosphorus was 3:1. 
     2 g of Li(NiCoMn)O 2  (hereinafter, written as NCM) as a positive electrode active material was provided on an agate mortar, and the coating material solution prepared was gradually added thereto while performing stirring. 
     After all the coating material solution had been added, stirring was performed until dryness was visually confirmed. 
     The resultant dry powder was placed into an alumina crucible and was heat-treated in an oxygen atmosphere at 400° C. for 3 hours. 
     The heat-treated powder was reground in an agate mortar to give a positive electrode active material of Example    1   covered with the coating material on the surface. The coating material was lithium phosphate (Li 3 PO 4 ). 
     [Preparation of First Solid Electrolyte] 
     In an argon atmosphere having a dew point of less than or equal to −60° C., LiCl, LiBr, CaBr 2 , YCl 3  and GdCl 3  as ingredient powders were provided in a molar ratio of LiCl:LiBr:CaBr 2 :YCl 3 :GdCl 3 =1.0:1.8:0.1:0.6:0.4. These powders were ground and mixed in a mortar. Subsequently, the resultant mixture of the ingredient powders was heat-treated in an argon atmosphere using an electric furnace at 500° C. for 3 hours. The material obtained was ground using a pestle and a mortar. Thus, a powder of a first solid electrolyte was obtained. The first solid electrolyte of Example 1 is hereinafter written to as the “HSE”. 
     [Fabrication of Battery] 
     In an argon atmosphere having a dew point of less than or equal to −60° C., the positive electrode active material NCM covered with lithium phosphate as the coating material, the first solid electrolyte HSE, and vapor-grown carbon fibers (VGCF) as a conductive auxiliary were provided. The mass ratio of the NCM, the HSE and the VGCF was NCM:HSE:VGCF=80:18:2. These were mixed together in an agate mortar to give a positive electrode mixture. 
     A stack was formed in an insulating cylinder by adding a sulfide solid electrolyte, Li 6 PS 5 Cl, (80 mg), the HSE powder (20 mg) and the positive electrode mixture (18.2 mg) in this order. A pressure of 720 MPa was applied to form a positive electrode and a solid electrolyte layer. 
     Next, a Li foil was laminated onto the side of the electrolyte layer opposite from the side in contact with the positive electrode. The resultant stack was pressed at a pressure of 80 MPa. A stack composed of the positive electrode, the solid electrolyte layer and the negative electrode was thus prepared. The negative electrode was the Li foil. 
     Next, stainless steel current collectors were placed on and under the stack. Current collector leads were attached to the current collectors. Lastly, the insulating cylinder was tightly closed with insulating ferrules to isolate the inside of the cylinder from the outer atmosphere. 
     Thus, a battery of Example 1 was fabricated. 
     [Charge/Discharge Test] 
     The battery of Example 1 was placed in a thermostatic chamber at 25° C. The battery was charged at a constant current of 0.140 mA until the voltage reached 4.3 V. Next, the battery was discharged at a current of 0.140 mA until the voltage fell to 2.5 V. 
     [Measurement of resistance] 
       FIG.  3    is a view illustrating a Nyquist diagram of the battery of Example 1 at 3.7 V. The battery of Example 1 was placed in a thermostatic chamber at 25° C. Furthermore, the battery was connected to a potentiostat equipped with a frequency response analyzer. Subsequently, the battery was charged at a constant current of 0.140 mA until the voltage reached 3.7 V and was further charged at the constant voltage. The frequency dependence of resistance components was evaluated by an AC impedance method. In this process, a resistance component that appeared near 10 5 -10 2  Hz was separated by curve fitting and was obtained as the resistance at the active material-solid electrolyte interface. By the measurement, the resistance of Example 1 was estimated to be 140 ohm. 
     [Measurement of Coverage Ratio] 
       FIG.  4    is a view illustrating an XPS O 1 s spectrum of the active material used in Example 1. The O 1 s spectrum of the surface of the positive electrode active material covered with lithium phosphate was obtained by an XPS method. Al-Kα radiation was used as the XPS source. 
     A peak centered at 528 eV and a peak centered at 532 eV were observed. The peak at 528 eV is a peak assigned to M—O (Ni—O, Mn—O, Co—O) in the positive electrode active material. The peak at 532 eV is an overlap of a peak assigned to C—O in lithium carbonate as an impurity on the surface, and a peak assigned to P—O in lithium phosphate. A product obtained by heat-treating the active material that was not covered with the coating material in an oxygen atmosphere at 400° C. was analyzed, and the area of a peak that was detected near 532 eV was subtracted from the area of the peak near 532 eV assigned to the coated active material. In this manner, the influence of lithium carbonate was eliminated, and the area of the peak assigned to the covering was calculated. 
     Based on these peaks, the proportion of M—O and P—O in O 1 s was determined, and the coverage ratio was estimated. The coverage ratio of lithium phosphate on the active material used in Example 1 was estimated to be 18%. 
     Example 2 
     [Preparation of Positive Electrode Active Material Covered with Coating Material on the Surface] 
     In an argon glove box having an argon atmosphere with a dew point of less than or equal to −60° C., 14.2 mg of lithium hydroxide and 36.0 mg of triethyl phosphate were dissolved into an appropriate amount of super-dehydrated ethanol (manufactured by Wako Pure Chemical Industries, Ltd.) to give a coating material solution. 
     [Fabrication of Battery] 
     A battery was fabricated in the same manner as in Example 1, except that the amount of coating on the active material was changed. 
     [Measurement of Resistance] 
     The resistance of the battery was evaluated in the same manner as in Example 1. 
     Example 3 
     [Preparation of Positive Electrode Active Material Covered with Coating Material on the Surface] 
     In an argon glove box having an argon atmosphere with a dew point of less than or equal to −60° C., 5.95 g of ethoxylithium (manufactured by Kojundo Chemical Lab. Co., Ltd.) and 36.43 g of pentaethoxyniobium (manufactured by Kojundo Chemical Lab. Co., Ltd.) were dissolved into 500 mL of super-dehydrated ethanol (manufactured by Wako Pure Chemical Industries, Ltd.) to give a coating material solution. 
     The coating material was applied onto the positive electrode active material NCM with use of a tumbling fluidized bed granulating-coating machine (FD-MP-01E manufactured by Powrex Corporation). The amount of the positive electrode active material added, the stirring rotational speed, and the feed rate for the coating material solution were 1 kg, 400 rpm, and 6.59 g/min, respectively. 
     The powder after the treatment was placed into an alumina crucible and was taken out to an air atmosphere. 
     Next, the powder was heat-treated in the air atmosphere at 300° C. for 1 hour. 
     The powder after the heat treatment was reground in an agate mortar to give a positive electrode active material of Example 3 covered with the coating material on the surface. The coating material was lithium niobate (LiNbO 3 ). 
     [Fabrication of Battery] 
     A battery was fabricated in the same manner as in Example 1, except that the coating material for the active material was changed. 
     [Charge/Discharge Test] 
     The battery was tested in the same manner as the charge/discharge test in Example 1. 
     [Measurement of Resistance] 
     The resistance of the battery was evaluated in the same manner as in Example 1. 
     [Measurement of Coverage Ratio] 
     An O 1 s peak assigned to Nb—O in lithium niobate appears near 530 eV. Based on the value of that peak, the coverage ratio was determined in the same manner as in Example 1. The coverage ratios on the active materials used in Examples 2 and 3 were approximately 100%. 
     Example 4 
     [Preparation of First Solid Electrolyte] 
     In an argon atmosphere having a dew point of less than or equal to −60° C., LiCl, LiBr, CaBr 2 , YCl 3  and GdCl 3  as ingredient powders were provided in a molar ratio of LiCl:LiBr:CaBr 2 :YCl 3 :GdCl 3 =1.0:1.8:0.1:0.9:0.1. These powders were ground and mixed in a mortar. Subsequently, the resultant mixture of the ingredient powders was heat-treated in an argon atmosphere using an electric furnace at 500° C. for 3 hours. The material obtained was ground using a pestle and a mortar. Thus, a powder of a first solid electrolyte of Example 4 was obtained. 
     [Fabrication of Battery] 
     A battery was fabricated in the same manner as in Example 2, except that the solid electrolyte was changed. 
     [Measurement of Resistance] 
     The resistance was evaluated in the same manner as in Example 1. 
     Comparative Example 1 
     [Provision of Positive Electrode Active Material] 
     The NCM used as the positive electrode active material in Examples 1 to 4 was used as a positive electrode active material of Comparative Example 1 without being covered with any coating material on the surface. 
     [Charge/Discharge Test] 
     The battery was tested in the same manner as the charge/discharge test in Example 1. 
     [Measurement of Resistance] 
     The resistance of the battery was measured in the same manner as in Example 1. 
     [Discussion 1] 
     Table 1 describes the coating materials, the thicknesses of the coating materials, the mass ratios, the first solid electrolytes and the values of resistance of Examples 1 to 4 and Comparative Example 1. The mass ratio indicates the ratio of the mass of the coating material to the mass of the positive electrode active material. 
     In the batteries of Example 1 and Comparative Example 1, the same HSE was used as the first solid electrolyte contained in the positive electrode material. The results confirmed that even when the first solid electrolyte that was used was HSE, the resistance of the battery was reduced by coating of the positive electrode active material with the coating material. This result indicates that the resistance of the battery can be effectively reduced by coating the surface of the positive electrode active material with the coating material regardless of the variations of types of metals constituting the solid electrolyte. 
     In the batteries of Example 1 and Example 2, the same HSE was used as the first solid electrolyte contained in the positive electrode material. In the batteries of Example 1 and Example 2, the same lithium phosphate was used as the coating material for coating the positive electrode active material contained in the positive electrode material. The results confirmed that the reduction in the resistance of the battery was larger when the coating material had a higher mass ratio relative to the positive electrode active material. It is probable that the coating material having a higher mass ratio to the positive electrode active material attained an increased coverage ratio on the positive electrode active material and consequently reduced the area of direct contact between the positive electrode active material and the solid electrolyte. That is, the coating material probably eliminated or reduced the formation of a resistance layer between the active material and the solid electrolyte, and thereby reduced the resistance of the battery. 
     As indicated by the results of Example 3 and Comparative Example 1, the resistance of the battery was reduced also when lithium niobate was used as the coating material. An oxide containing lithium, in particular, a phosphate that is a glass-forming oxide, or lithium niobate that is an intermediate oxide tends to form a thin covering when used as the coating material. In this case, the material forms a highly amorphous film by heat treatment at a low temperature. Thus, it is probable that such a film did not inhibit the transfer of lithium between the active material and the solid electrolyte and eliminated or reduced the formation of a resistance layer between the active material and the solid electrolyte. 
     Example 2 and Example 4 are the same as each other in the type of the coating material and differ in the composition of the first solid electrolyte. The results indicate that the resistance of the battery was successfully reduced regardless of the proportions of elements constituting the first solid electrolyte. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Coating  
                   
                   
                   
               
               
                   
                 Coating 
                 material 
                 Mass ratio 
                   
                 Resistance 
               
               
                   
                 material 
                 thickness [nm] 
                 [mass %] 
                 First solid electrolyte 
                 [ohm] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 COMP. 
                 None 
                 — 
                 — 
                 Li 2.8 Ca 0.1 Y 0.6 Gd 0.4 Br 2 Cl 4   
                 222 
               
               
                 EX. 1 
                   
                   
                   
                   
                   
               
               
                 EX. 1 
                 Lithium 
                 4 
                 0.5 
                 Li 2.8 Ca 0.1 Y 0.6 Gd 0.4 Br 2 Cl 4   
                 140 
               
               
                   
                 phosphate 
                   
                   
                   
                   
               
               
                 EX. 2 
                 Lithium 
                 9 
                 1.1 
                 Li 2.8 Ca 0.1 Y 0.6 Gd 0.4 Br 2 Cl 4   
                 100 
               
               
                   
                 phosphate 
                   
                   
                   
                   
               
               
                 EX. 3 
                 Lithium 
                 6 
                 1.5 
                 Li 2.8 Ca 0.1 Y 0.6 Gd 0.4 Br 2 Cl 4   
                 84 
               
               
                   
                 niobate 
                   
                   
                   
                   
               
               
                 EX. 4 
                 Lithium 
                 9 
                 1.1 
                 Li 2.8 Ca 0.1 Y 0.9 Gd 0.1 Br 2 Cl 4   
                 41 
               
               
                   
                 phosphate 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Example 5 
     [Preparation of First Solid Electrolyte] 
     In an argon atmosphere having a dew point of less than or equal to −60° C., LiCl, LiBr, CaBr 2 , YCl 3  and SmCl 3  as ingredient powders were provided in a molar ratio of LiCl:LiBr:CaBr 2 :YCl 3 :SmCl 3 =1.0:1.8:0.1:0.8:0.2. These powders were ground and mixed in a mortar. Subsequently, the resultant mixture of the ingredient powders was heat-treated in an argon atmosphere using an electric furnace at 500° C. for 3 hours. The material obtained was ground using a pestle and a mortar. Thus, a powder of a first solid electrolyte of Example 5 was obtained. 
     [Fabrication of Battery] 
     A battery was fabricated in the same manner as in Example 2, except that the first solid electrolyte was changed. 
     [Measurement of Resistance] 
     The resistance of the battery was measured in the same manner as in Example 1. 
     Comparative Example 2 
     [Fabrication of Battery] 
     A battery was fabricated in the same manner as in Example 5, except that the NCM used as the positive electrode active material in Examples 1 to 5 was not covered with any coating material on the surface. 
     [Charge/Discharge Test] 
     The battery was tested in the same manner as the charge/discharge test in Example 1. 
     [Measurement of Resistance] 
     The resistance of the battery was evaluated in the same manner as in Example 1. 
     [Discussion 2] 
     Table 2 describes the coating materials, the thicknesses of the coating materials, the mass ratios, the first solid electrolytes and the values of resistance of Example 5 and Comparative Example 2. 
     In the batteries of Example 5 and Comparative Example 2, the first solid electrolyte present in the positive electrode material contained Sm as a constituent element. The results indicate that the resistance of the battery was significantly reduced by coating of the active material with lithium phosphate. It can be thus said that the advantageous effects of the present disclosure are not limited to cases where the first solid electrolyte contains Y and Gd as constituent elements, and the present disclosure is also effective when the first solid electrolyte contains Y and a rare earth element other than Y. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Coating 
                   
                   
                   
               
               
                   
                 Coating 
                 material 
                 Mass ratio 
                   
                 Resistance 
               
               
                   
                 material 
                 thickness [nm] 
                 [mass %] 
                 First solid electrolyte 
                 [ohm] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 COMP. 
                 None 
                 — 
                 — 
                 Li 2.8 Ca 0.1 Y 0.8 Sm 0.2 Br 2 Cl 4   
                 1100 
               
               
                 EX. 2 
                   
                   
                   
                   
                   
               
               
                 EX. 5 
                 Lithium 
                 9 
                 1.1 
                 Li 2.8 Ca 0.1 Y 0.8 Sm 0.2 Br 2 Cl 4   
                 500 
               
               
                   
                 phosphate 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     The battery of the present disclosure may be used as, for example, an all-solid-state lithium secondary battery.