. The electrolyte layer 202 contains a second solid electrolyte 113.

More specifically, the electrolyte layer 202 includes a first electrolyte layer 202a and a second electrolyte layer 202b. The first electrolyte layer 202a contains the second solid electrolyte 113. The second electrolyte layer 202b is located between the positive electrode 201 and the first electrolyte layer 202a and contains a third solid electrolyte 114.

The first electrolyte layer 202a may have a thickness of 100 nm or more and 100 μm or less. When the thickness of the first electrolyte layer 202a is 100 nm or more, the occurrence of a short circuit between the positive electrode and the negative electrode can be reduced more effectively. When the thickness of the first electrolyte layer 202a is 100 μm or less, high power operation can be achieved.

The second electrolyte layer 202b may have a thickness of 1 nm or more and 100 μm or less. When the thickness of the second electrolyte layer 202b is 1 nm or more, the oxidation of the first electrolyte layer 202a can be reliably inhibited. When the thickness of the second electrolyte layer 202b is 100 μm or less, high power operation can be achieved.

A halide solid electrolyte may be used as the first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114. Examples of the halide solid electrolyte include Li₃YBr₆, Li₃YBr₃Cl₃, Li₃YBr₂Cl₄, Li₃YCl₆, and compounds obtained by substituting one or some cations in these compounds with Zr.

The first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114 may be halide solid electrolytes having compositions that differ from each other. That is, the molar ratio of bromine in the second solid electrolyte 113 may be greater than the molar ratio of bromine in the first solid electrolyte 112.

This can further improve the charge and discharge characteristics of the battery 2000.

The first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114 may each be a mixture of solid electrolytes having compositions that differ from each other. For example, a mixture of Li₃YBr₆and LiCl or a mixture of Li₃YCl₃Br₃and Li₃YCl₆may be used.

This can further improve the charge and discharge characteristics of the battery 2000. The first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114 may be free from sulfur.

The shape of each of the first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114 is not limited to particular shapes, and may be, for example, an acicular shape, a spherical shape, an ellipsoidal shape, or a scale-like shape. For example, the first solid electrolyte 112 may be in the form of particles. Similarly, the second solid electrolyte 113 may be in the form of particles. The third solid electrolyte 114 may be in the form of particles.

When the first solid electrolyte 112 is in the form of particles (e.g., spherical), the median diameter of the group of particles of the first solid electrolyte 112 may be 100 μm or less. When the median diameter is 100 μm or less, the positive electrode active material 111 and the first solid electrolyte 112 can form a favorable dispersion state in the positive electrode 201. This improves the charge and discharge characteristics of the battery 2000. The median diameter of the group of particles of the first solid electrolyte 112 may be 10 μm or less.

This allows the positive electrode active material 111 and the first solid electrolyte 112 to form a favorable dispersion state in the positive electrode 201.

The term “median diameter” as used in the present specification means a particle diameter when the cumulative volume in the volumetric particle size distribution is equal to 50%. The volumetric particle size distribution is measured using a laser diffraction measurement device or an image analysis device, for example.

The positive electrode active material 111 contains a material having properties of occluding and releasing metal ions (e.g., lithium ions). Examples of the positive electrode active material 111 include lithium-containing transition metal oxides, transition metal fluorides, polyanion materials, fluorinated polyanion materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, when a lithium-containing transition metal oxide is used as the positive electrode active material 111, the battery can be produced at a low production cost and can have a high average discharge voltage.

The positive electrode active material 111 may contain Li and at least one element selected from the group consisting of Mn, Co, Ni, and Al. Examples of such a material include Li(NiCoAl)O₂, Li(NiCoMn)O₂, and LiCoO₂.

The positive electrode active material 111 may contain one active material or a plurality of active materials having compositions that differ from each other.

In the present embodiment, the positive electrode active material 111 may be Li(NiCoMn)O₂.

This can further improve the energy density and the charge and discharge efficiency of the battery 2000.

The positive electrode active material 111 may be in the form of particles, for example. The shape of the particles of the positive electrode active material 111 is not limited to particular shapes. The shape of the particles of the positive electrode active material 111 may be an acicular shape, a spherical shape, an ellipsoidal shape, or a scale-like shape.

The median diameter of the particles of the first solid electrolyte 112 may be smaller than the median diameter of the particles of the positive electrode active material 111.

This allows the first solid electrolyte 112 and the positive electrode active material 111 to form a more favorable dispersion state in the positive electrode 201.

The particles of the positive electrode active material 111 may have a median diameter of 0.1 μm or more and 100 μm or less.

When the median diameter of the particles of the positive electrode active material 111 is 0.1 μm or more, the positive electrode active material 111 and the first solid electrolyte 112 can form a favorable dispersion state in the positive electrode 201. As a result, the charge and discharge characteristics of the battery 2000 are improved.

When the median diameter of the particles of the positive electrode active material 111 is 100 μm or less, the lithium diffusion in the particles of the positive electrode active material 111 becomes faster. This enables high power operation of the battery 2000.

The median diameter of the particles of the positive electrode active material 111 may be larger than the median diameter of the particles of the first solid electrolyte 112. This allows the positive electrode active material 111 and the first solid electrolyte 112 to form a favorable dispersion state.

The positive electrode 201 may contain a plurality of particles of the first solid electrolyte 112 and a plurality of particles of the positive electrode active material 111.

The content of the first solid electrolyte 112 and the content of the positive electrode active material 111 in the positive electrode 201 may be the same or different from each other.

Regarding the volume ratio “v1:100−v1” between the positive electrode active material 111 and the first solid electrolyte 112 each contained in the positive electrode 201, 30≤v1≤95 may be satisfied. When 30≤v1 is satisfied, the battery 2000 can have a sufficient energy density. When v1≤95 is satisfied, high power operation becomes possible.

The positive electrode 201 may have a thickness of 10 μm or more and 500 μm or less. When the thickness of the positive electrode 201 is 10 μm or more, the battery 2000 can have a sufficient energy density. When the thickness of the positive electrode 201 is 500 μm or less, high power operation becomes possible.

The electrolyte layer 202 is a layer containing an electrolyte. The electrolyte is, for example, a solid electrolyte. In other words, the electrolyte layer 202 may be a solid electrolyte layer.

The electrolyte layer 202 may contain, as a solid electrolyte, at least one selected from the group consisting of a halide solid electrolyte, a sulfide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, and a complex hydride solid electrolyte.

As a halide solid electrolyte, any of the materials described above as specific examples of the first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114 may be used.

The above configuration can further improve the power density and the charge and discharge characteristics of the battery 2000.

The electrolyte layer 202 may contain a halide solid electrolyte having a composition that differs from those of the first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114. That is, the electrolyte layer 202 may contain a halide solid electrolyte having a composition that differs from that of the first solid electrolyte 112.

The electrolyte layer 202 may include, as a third electrolyte layer, another electrolyte layer provided between the first electrolyte layer 202a and the negative electrode 203. Another electrolyte layer contains a fourth solid electrolyte. Examples of the fourth solid electrolyte include the materials described above as examples of the material that can be used as the first solid electrolyte 112, the second solid electrolyte 113, and the third solid electrolyte 114.

As the first solid electrolyte 112, one of solid electrolytes to be described below as examples may be used alone, or two or more of them may be used in combination. The same applies to the second solid electrolyte 113 and the third solid electrolyte 114.

The halide solid electrolyte is represented by, for example, the following composition formula (4). In the composition formula (4), α, β, and γ are each independently a value greater than 0. M includes at least one element selected from the group consisting of a metalloid element and a metal element other than Li. X includes at least one selected from the group consisting of F, Cl, Br, and I.

Li_αM_βX_γ (4)

Metalloid elements include B, Si, Ge, As, Sb, and Te. Metal elements include: all elements included in Groups 1 to 12 of the periodic table, except for hydrogen; and all elements included in Groups 13 to 16 of the periodic table, except for B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se. Metal elements are a group of elements that can be cations when they form an inorganic compound with a halogen or with a halogen compound.

As the halide solid electrolyte, Li₃YX₆, Li₂MgX₄, Li₂FeX₄, Li(Al, Ga, In)X₄, Li₃(Al, Ga, In)X₆, or the like can be used.

The above configuration can improve the power density of the battery 2000. The above configuration can also improve the thermal stability of the battery 2000 and can inhibit the generation of harmful gas such as hydrogen sulfide.

In the present disclosure, when an element in a formula is expressed as, e.g., “(Al, Ga, In)”, it indicates at least one element selected from the group of elements in parentheses. That is, “(Al, Ga, In)” means the same as “at least one selected from the group consisting of Al, Ga, and In”. The same applies to other elements. The halide solid electrolyte exhibits excellent ionic conductivity.

In the composition formula (4), M may include Y (=yttrium).

In the composition formula (4), 2.5≤α≤3, 1≤β≤1.1, and γ=6 may be satisfied.

In the composition formula (4), X may include at least one selected from the group consisting of Cl and Br.

The above composition can further improve the ionic conductance of the solid electrolyte. This improves the power density of the battery.

A halide solid electrolyte containing Y may be a compound represented by the following composition formula (5).

Li_aM_bY_cX₆ (5)

In the composition formula (5), a+mb+3c=6 and c>0 are satisfied. In the composition formula (5), M includes at least one element selected from the group consisting of a metalloid element and a metal element other than Li and Y. m is the valence of M. X includes at least one selected from the group consisting of F, Cl, Br, and I. M includes at least one selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb. Specific examples of the halide solid electrolyte containing Y include Li₃YF₆, Li₃YCl₆, Li₃YBr₆, Li₃Yl₆, Li₃YBrCl₅, Li₃YBr₃Cl₃, Li₃YBr₅Cl, Li₃YBr₅I, Li₃YBr₃I₃, Li₃YBrI₅, Li₃YClI₅, Li₃YCl₃I₃, Li₃YCl₅I, Li₃YBr₂Cl₂I₂, Li₃YBrCl₄I, Li_2.7Y_1.1Cl₆, Li_2.5Y_0.5Zr_0.5Cl₆, and Li_2.5Y_0.3Zr_0.7Cl₆.

The above configuration can further improve the power density of the battery 2000.

Examples of the sulfide solid electrolytes include Li₂S—P₂S₅, Li₂S—SiS₂, Li₂S—B₂S₃, Li₂S—GeS₂, Li_3.25Ge_0.25P_0.75S₄, and Li₁₀GeP₂S₁₂. LiX, Li₂O, MO_q, Li_pMO_q, or the like may be added thereto. The element X in “LiX” is at least one element selected from the group consisting of F, Cl, Br, and I. The element M in “MO_q” and “LipMO_q” is at least one element selected from the group consisting of P, Si, Ge, B, Al, Ga, In, Fe, and Zn. p and q in “MO_q” and “LipMO_q” are each independently a natural number.

The battery 2000 configured as above includes a sulfide solid electrolyte with excellent reduction stability and thus can use a low-potential negative electrode material such as graphite or metallic lithium. Accordingly, the energy density of the battery 2000 can be improved.

Examples of the oxide solid electrolytes include: NASICON solid electrolytes typified by LiTi₂(PO₄)₃and element-substituted products thereof; (LaLi)TiO₃-based perovskite solid electrolytes; LISICON solid electrolytes typified by Li₁₄ZnGe₄O₁₆, Li₄SiO₄, and LiGeO₄and element-substituted products thereof; garnet solid electrolytes typified by Li₇La₃Zr₂O₁₂and element-substituted products thereof; Li₃N and H-substituted products thereof; Li₃PO₄and N-substituted products thereof; and glass and glass ceramics obtained by adding a material such as Li₂SO₄or Li₂CO₃to a base material containing a Li—B—O compound such as LiBO₂or Li₃BO₃.

Examples of the polymer solid electrolytes include compounds composed of a polymer compound and a lithium salt. The polymer compound may have an ethylene oxide structure. Since the polymer compound with the ethylene oxide structure can contain a large amount of lithium salt, this can further improve the ionic conductance. Examples of the lithium salt include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₆)₂, LiN(SO₂CF₃)(SO₂C₄F₃), and LiC(SO₂CF₃)₃. As the lithium salt, one lithium salt selected from them may be used alone, or a mixture of two or more lithium salts selected from them may be used.

Examples of the complex hydride solid electrolytes include LiBH₄—LiI and LiBH₄—P₂S₅.

The above composition can improve the power density of the battery 2000.

The electrolyte layer 202 may contain a solid electrolyte as a main component. In other words, the electrolyte layer 202 may contain 50% or more of the solid electrolyte by weight (i.e., 50 wt % or more of the solid electrolyte) with respect to the total weight of the electrolyte layer 202.

This can further improve the charge and discharge characteristics of the battery 2000.

The electrolyte layer 202 may contain 70% or more of the solid electrolyte by weight (i.e., 70 wt % or more of the solid electrolyte) with respect to the total weight of the electrolyte layer 202.

This can still further improve the charge and discharge characteristics of the battery 2000.

In addition to the solid electrolyte as the main component, the electrolyte layer 202 may further contain inevitable impurities, or starting materials used for synthesis of the third solid electrolyte 114, by-products, degradation products, and the like.

The electrolyte layer 202 may contain 100% of the solid electrolyte by weight (i.e., 100 wt % of the solid electrolyte) with respect to the total weight of the electrolyte layer 202, except for inevitable impurities.

This can still further improve the charge and discharge characteristics of the battery 2000.

As described above, the electrolyte layer 202 may consist of a solid electrolyte.

The electrolyte layer 202 may contain only one solid electrolyte selected from the group of solid electrolytes described above, or may contain two or more solid electrolytes selected from the group of solid electrolytes described above. The plurality of solid electrolytes have compositions that differ from each other. For example, the electrolyte layer 202 may contain a halide solid electrolyte and a sulfide solid electrolyte.

The electrolyte layer 202 may have a thickness of 1 μm or more and 300 μm or less. When the thickness of the electrolyte layer 202 is 1 μm or more, the positive electrode 201 and the negative electrode 203 can be more reliably separated from each other. When the thickness of the first electrolyte layer 202 is 300 μm or less, high power operation can be achieved.

The negative electrode 203 contains a material having properties of occluding and releasing metal ions (e.g., lithium ions). The negative electrode 203 contains a negative electrode active material, for example.

As the negative electrode active material, a metal material, a carbon material, an oxide, a nitride, a tin compound, a silicon compound, or the like can be used. The metal material may be a single metal. Alternatively, the metal material may be an alloy. Examples of the metal material include lithium metal and lithium alloys. Examples of the carbon material include natural graphite, coke, carbon undergoing graphitization, carbon fibers, spherical carbon, artificial graphite, and amorphous carbon. From the viewpoint of capacity density, silicon (Si), tin (Sn), a silicon compound, or a tin compound may be used.

The negative electrode 203 may contain a solid electrolyte. The above configuration improves the lithium ion conductivity inside the negative electrode 203, whereby high power operation becomes possible. As the solid electrolyte, the materials described above may be used.

The particles of the negative electrode active material may have a median diameter of 0.1 μm or more and 100 μm or less. When the median diameter of the particles of the negative electrode active material is 0.1 μm or more, the negative electrode active material and the solid electrolyte can form a favorable dispersion state in the negative electrode 203. This improves the charge and discharge characteristics of the battery 2000. When the median diameter of the particles of the negative electrode active material is 100 μm or less, the lithium diffusion in the particles of the negative electrode active material becomes faster. This enables high power operation of the battery 2000.

The median diameter of the particles of the negative electrode active material may be larger than the median diameter of particles of the solid electrolyte contained in the negative electrode 203. This allows the negative electrode active material and the solid electrolyte to form a favorable dispersion state.

When the volume ratio between the negative electrode active material and the solid electrolyte in the negative electrode 203 is expressed as “v2:100−v2”, the volume ratio v2 of the negative electrode active material may satisfy 30≤v2≤95. When 30≤v2 is satisfied, the battery 2000 can have a sufficient energy density. When v2≤95 is satisfied, high power operation becomes possible.

The negative electrode 203 may have a thickness of 10 μm or more and 500 μm or less. When the thickness of the negative electrode 203 is 10 μm or more, the battery 2000 can have a sufficient energy density. When the thickness of the negative electrode 203 is 500 μm or less, high power operation becomes possible.

At least one of the positive electrode 201, the electrolyte layer 202, and the negative electrode 203 may contain a binder to improve the adhesion between particles. The binder is used to improve the binding properties of the materials constituting the electrodes. Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamide-imide, polyacrylonitrile, polyacrylic acid, poly(acrylic acid methyl ester), poly(acrylic acid ethyl ester), poly(acrylic acid hexyl ester), polymethacrylic acid, poly(methacrylic acid methyl ester), poly(methacrylic acid ethyl ester), poly(methacrylic acid hexyl ester), polyvinyl acetate, polyvinylpyrrolidone, polyether, polyether sulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. Also, the binder may be a copolymer of two or more materials selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. The binder may be a mixture of two or more materials selected from them.

At least one of the positive electrode 201 and the negative electrode 203 may contain a conductive additive to improve the electronic conductivity. Examples of the conductive additive include: graphite 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; carbon fluoride; metal powders such as aluminum powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; and conductive polymer compounds such as polyaniline, polypyrrole, and polythiophene. Cost reduction can be achieved by using a carbon conductive additive.

The battery 2000 of Embodiment 2 can be embodied as batteries with various shapes, including a coin battery, a cylindrical battery, a prismatic battery, a sheet-like battery, a button battery, a flat-type battery, and a stacked battery.

MODIFICATION

FIG. 1B is a cross-sectional view schematically showing the structure of a battery 3000 according to a modification. The battery 3000 includes a positive electrode 201, a negative electrode 203, and an electrolyte layer 202. The electrolyte layer 202 corresponds to the first electrolyte layer 202a described in Embodiment 2. That is, the battery 3000 has the same structure as the battery 2000, except that it does not include the second electrolyte layer 202b. Unless technically incompatible, the above descriptions regarding the battery 2000 also apply to the battery 3000.

EXAMPLES

The present disclosure will be described in detail below with reference to an example and a comparative example.

(Production of Evaluation Battery 1)

In an argon atmosphere with a dew point of −60° C. or less, LiBr and YBr₃were prepared as raw material powders at a molar ratio of LiBr:YBr₃=3:1. Subsequently, the resulting mixture of the raw material powders was subjected to a milling process at a rotational speed of 600 rpm for 25 hours using a planetary ball mill (Type P-7, manufactured by Fritsch GmbH). Li₃YBr₆powder was thus obtained. Hereinafter, this solid electrolyte is referred to as “LYB”.

In an argon atmosphere with a dew point of −60° C. or less, LYB and acetylene black were prepared at a mass ratio of 93:7. They were mixed together in an agate mortar to produce a mixed material.

80 mg of a sulfide solid electrolyte, 20 mg of LYB, and 5 mg of the above-described mixed material were stacked in this order in an insulating outer cylinder. As the sulfide solid electrolyte, Li₆PS₅Cl powder was used. A pressure of 720 MPa was applied to the thus-obtained stack.

Next, as a negative electrode, an In—Li foil was stacked on the layer of the sulfide solid electrolyte. A pressure of 80 MPa was applied to the stack composed of the mixed material, the electrolyte layer, and the negative electrode.

Next, current collectors made of stainless steel were placed on the top and the bottom of the stack. The current collectors were provided with current collector leads. Finally, the insulating outer cylinder was sealed with an insulating ferrule to shield the interior of the insulating outer cylinder from the outside atmosphere.

An evaluation battery 1 for evaluating the oxidation potential of Br was thus obtained. The stacking structure of the evaluation battery 1 was as follows: (LYB+acetylene black)/LYB/sulfide solid electrolyte layer/In—Li.

(Production of Evaluation Battery 2)

In an argon atmosphere with a dew point of −60° C. or less, LiCl, YCl₃, and YBr₃were prepared as raw material powders at a molar ratio of LiCl, YCl₃, and YBr₃=3.000:0.333:0.666. They were pulverized and mixed together in a mortar. Then, the thus-obtained mixture of the raw material powders was fired in an argon atmosphere at 500° C. for 3 hours using an electric furnace. The thus-obtained material was pulverized using a pestle and a mortar. Li₃YBr₂Cl₄powder was thus obtained. Hereinafter, this solid electrolyte is referred to as “LYBC”.

An evaluation battery 2 was produced in the same manner as the evaluation battery 1, except that LYBC was used. The stacking structure of the evaluation battery 2 was as follows: (LYBC+acetylene black)/LYBC/sulfide solid electrolyte layer/In—Li.

(Production of Evaluation Battery 3)

In an argon atmosphere with a dew point of −60° C. or less, LiCl and YCl₃were prepared as raw material powders at a molar ratio of LiCl:YCl₃=2.7:1.1. Subsequently, the resulting mixture of the raw material powders was subjected to a milling process at a rotational speed of 600 rpm for 25 hours using a planetary ball mill (Type P-7, manufactured by Fritsch GmbH). Li_2.7Y_1.1Cl₆powder was thus obtained. Hereinafter, this solid electrolyte is referred to as “LYC”.

(Production of Evaluation Battery 3)

An evaluation battery 3 was produced in the same manner as the evaluation battery 1, except that LYC was used. The stacking structure of the evaluation battery 3 was as follows: (LYC+acetylene black)/LYC/sulfide solid electrolyte layer/In—Li.

(LSV Measurement)

Each of the evaluation batteries was subjected to linear sweep voltammetry (LSV) measurement. First, the battery was placed in a thermostatic chamber set at 25° C. The evaluation battery was connected to a potentio-galvanostat to perform LSV measurement. The LSV measurement was performed with the sweep rate set to 10 mV/s. The scanning range was set from the open circuit voltage (OCV) to 4.0 V vs. In—Li. In the LSV measurement, the current responses obtained when the potential was swept from the OCV to 4.0 V were plotted.

FIG. 2 is a graph showing the results of the LSV measurement. In LSV measurement, when the swept potential reaches a certain potential, a solid electrolyte is oxidized at that potential, thereby causing a current to flow. As can be seen from FIG. 2, LYB had an oxidation potential of 2.9 V vs. In—Li. LYBC had an oxidation potential of 3.1 V vs. In—Li. LYC had an oxidation potential of 3.5 V vs. In—Li.

In the case of the solid electrolyte containing Br, such as LYB or LYBC, the oxidation current rose up in the vicinity of 2.9 V to 3.1 V, and this revealed that the oxidation potential of Br in the solid electrolyte is in this range. The oxidation potential vs. Li of Br in the solid electrolyte can be calculated by adding 0.6 V (the potential of In—Li alloy vs. Li) to the potential of LYB (2.9 V). That is, according to the results obtained using the evaluation battery 1 (LYB), the oxidation potential of Br in the solid electrolyte versus Li potential was 3.5 V.

LYC had an oxidation potential of 3.5 V vs. In—Li. The oxidation potential of 3.5 V is equivalent to the potential vs. Li of 4.1 V. That is, it is considered that, in the case of the solid electrolyte containing Cl, long-time exposure to a potential of 4.1 V or more causes oxidization of Cl in the solid electrolyte, leading to collapse of the structure of the solid electrolyte.

From these results, it was found that a suitable potential for allowing debromination of a solid electrolyte containing Br to proceed is in the range of more than 3.5 V and less than 4.1 V relative to the potential of Li.

In this measurement, the point at which the current value reached 0.05 mA was regarded as the oxidation potential of each of the solid electrolytes. The rise of an oxidation current varies depending on the temperature at which measurement is performed, conditions under which an evaluation battery is produced, and the like. Therefore, the oxidation potential determined in this measurement is considered to have an error margin of about ±0.2 V.

LYB had an oxidation potential of 2.9 V vs. In—Li. LYBC had an oxidation potential of 3.1 V vs. In—Li. These facts indicate that, even if a solid electrolyte contains Br, there may be a difference of about 0.2 V in the oxidation potential depending on the composition of the solid electrolyte.

[Production of Secondary Battery]

Example 1

In an argon atmosphere with a dew point of −60° C. or less, LiCl, YCl₃, and YBr₃were prepared as raw material powders at a molar ratio of LiCl, YCl₃, and YBr₃=3.000:0.333:0.666. They were pulverized and mixed together in a mortar. Then, the thus-obtained mixture of the raw material powders was fired in an argon atmosphere at 500° C. for 3 hours using an electric furnace. The thus-obtained material was pulverized using a pestle and a mortar. Li₃YBr₂Cl₄powder was thus obtained.

In an argon atmosphere with a dew point of −60° C. or less, Li(Ni, Co, Mn)O₂(hereinafter referred to as “NCM”) as a positive electrode active material, LYBC as a solid electrolyte, and vapor grown carbon fibers (VGCF, manufactured by Showa Denko K.K.) as a conductive additive were prepared at a mass ratio of 71:27:2. They were mixed together in an agate mortar to produce a positive electrode mixture.

80 mg of a sulfide solid electrolyte, 20 mg of LYBC, and 19.5 mg of the above-described positive electrode mixture were stacked in this order in an insulating outer cylinder. As the sulfide solid electrolyte, Li₆PS₅Cl powder was used. A pressure of 720 MPa was applied to the thus-obtained stack, whereby a positive electrode and an electrolyte layer were obtained.

Next, a Li foil was placed on a surface of the electrolyte layer opposite to a surface in contact with the positive electrode. A pressure of 80 MPa was applied to the positive electrode, the electrolyte layer, and the Li foil, whereby a stack composed of the positive electrode, the electrolyte layer, and a negative electrode was produced.

Next, current collectors made of stainless steel were placed on the top and the bottom of the stack. The current collectors were provided with current collector leads. Finally, the insulating outer cylinder was sealed with an insulating ferrule to shield the interior of the insulating outer cylinder from the outside atmosphere.

Next, a current of 0.140 mA was applied to the stack until the voltage reached 3.65 V. Then, the stack was charged with a constant voltage of 3.65 V, and the charging was stopped when the current value decayed to 0.028 mA. Thereafter, the stack was discharged with a constant current until the voltage reached 2.5 V. A series of these processes correspond to a debromination treatment. The debromination treatment includes charging the stack with a constant voltage that is equal to or higher than the voltage at which the potential of the positive electrode is equal to the oxidation potential of bromine contained in the solid electrolyte. More specifically, the debromination treatment includes charging the stack with a constant current to a voltage at which the potential of the positive electrode is equal to the oxidation potential of bromine contained in the solid electrolyte and charging the stack with a constant voltage that is equal to or higher than the voltage at which the potential of the positive electrode is equal to the oxidation potential of bromine contained in the solid electrolyte. The voltage in the constant voltage charging may be equal to or higher than the voltage at which the potential of the positive electrode is equal to the oxidation potential of the bromine contained in the solid electrolyte and lower than the battery voltage at which the potential of the positive electrode is equal to the oxidation potential of chlorine.

A secondary battery of the example was thus produced.

Comparative Example 1

A secondary battery was produced in the same manner as in the example, except that the debromination treatment was not performed.

[Composition Analysis]

The secondary battery of the example was disassembled, and a power generating element, which includes the positive electrode, the electrolyte, and the negative electrode, was taken out from the insulating outer cylinder. The power generating element was cut in the thickness direction, and the cross-section was then smoothed by argon ion milling. Thereafter, the composition near the interface between the positive electrode and the electrolyte layer was analyzed using a field emission scanning electron microscope-energy dispersive X-ray analyzer (FE-SEM-EDX) to determine c1, c2, c3, d1, d2, d3, e1, e2, and e3 in the composition formulae (1), (2), and (3).

FIG. 4 is a SEM image showing a cross-section of the positive electrode and electrolyte layer having been subjected to the debromination treatment.