JP2013037992A - Solid electrolyte and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid electrolyte that has relatively high lithium ion conductivity and can be sintered at low temperature.SOLUTION: A solid electrolyte of the present invention includes: a crystalline lithium ion conductive substance; and Li(B,A)O(in the formula, A represents at least one or more kinds of elements selected from among C, Al, Si, Ga, Ge, In and Sn, y satisfies 0≤y<1, z represents an average valence of (B,A), and x, y and δ satisfy a relational expression of x+z=δ/2.) as a base material. The lithium ion conductive substance is preferably a garnet-type oxide represented by a basic formula: LiLaZrAO(in the formula, A represents one kind or more of elements selected from the group consisting of Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga and Ge, and x satisfies 1.4≤x<2).

Description

  The present invention relates to a solid electrolyte and a method for producing the same.

Conventionally, an all solid-state lithium secondary battery including a solid electrolyte and an active material has been proposed. An amorphous or crystalline lithium ion conductive material is known as a solid electrolyte used in such an all solid lithium secondary battery. An amorphous lithium ion conductive material has a problem in that the lithium ion conductivity is low, although the solid-solid interface between the active material and the solid electrolyte is easily adhered. On the other hand, a crystalline lithium ion conductive material has a higher lithium ion conductivity than an amorphous material. As such a crystalline lithium ion conductive material, the present inventors have the composition formula Li _{5 + X} La ₃ (Zr _X , A _2−X ) O ₁₂ (wherein A is Sc, Ti, V, Developed a garnet-type oxide represented by one or more elements selected from the group consisting of Y, Nb, Hf, Ta, Al, Si, Ga and Ge, wherein X is 1.4 ≦ X <2) (For example, refer to Patent Document 1). This garnet oxide has a very high lithium ion conductivity among crystalline lithium ion conductive materials.

JP 2010-272344 A

  However, the crystalline lithium ion conductive material has a very high sintering temperature. For example, the sintering temperature of the garnet-type oxide described in Patent Document 1 is around 1200 ° C. Therefore, an attempt is made to improve the adhesion of the solid-solid interface between the solid electrolyte and the active material by integrally sintering the pre-sintering raw material of the crystalline lithium ion conductive material that becomes the solid electrolyte and the active material. Then, the sintering temperature may be too high and the active material may be altered. Therefore, it has been desired to develop a solid electrolyte containing a crystalline lithium ion conductive material that can be sintered at a low temperature (for example, 700 ° C.) while maintaining a relatively high lithium ion conductivity. .

  The present invention has been made in view of such problems, and provides a solid electrolyte containing a crystalline lithium ion conductive material that has a relatively high lithium ion conductivity and can be sintered at a low temperature. The main purpose is to do.

As a result of diligent research to achieve the above-described object, the present inventors have found that the above-described solid electrolyte containing the garnet-type oxide and Li ₃ BO ₃ as a base material has a relatively high lithium ion conductivity, and The present inventors have found that it can be sintered at a low temperature and have completed the present invention.

That is, the solid electrolyte of the present invention comprises a crystalline lithium ion conductive material and Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ (where A is C, Al, It is at least one element of Si, Ga, Ge, In, and Sn, y satisfies 0 ≦ y <1, z is an average valence of (B _1−y , A _y ), x, z and δ satisfy the relational expression x + z = δ / 2).

This solid electrolyte has a relatively high lithium ion conductivity and can be sintered at a low temperature of about 700 ° C. Since sintering is possible at a low temperature in this way, even if the raw material and the active material before sintering of the solid electrolyte are integrally sintered, the active material is not deteriorated due to the high temperature. The solid-solid interface between the active material and the active material becomes better. The reason why such an effect is obtained is not clear, but is presumed as follows. When there is no base material represented by Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ , contact between crystalline lithium ion conductive materials becomes point contact, and lithium ion conduction. There are fewer routes. On the other hand, in the solid electrolyte of the present invention, the base material serves as a lithium ion conduction path between the crystalline lithium ion conductive materials, so it is considered that the lithium ion conductivity can be further increased. Further, since Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ as a base material melts at a low temperature, it can be considered that sintering can be performed at a low temperature comparable to the melting temperature. The “base material” means a material that fills the gap.

In the production method of the present invention, a crystalline lithium ion conductive material and a flux Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ (where A is C, Al, Si, Ga, Ge) , In, Sn is at least one element, y satisfies 0 ≦ y <1, z is an average valence of (B _1−y , A _y ), and x, z, δ is x + z Forming a raw material body including the following formula: and the above-mentioned raw material body has a melting point of the flux or higher, and the lithium ion conductive material and the flux form a compound. And a heating step of heating at a temperature equal to or lower than the heating temperature.

  According to this manufacturing method, the above-described solid electrolyte of the present invention can be manufactured relatively easily.

4 is an explanatory diagram showing an example of a structure of an all solid-state lithium secondary battery 20. FIG. 4 is an explanatory diagram showing an example of a structure of an all solid-state lithium secondary battery 20. FIG. It is explanatory drawing which shows the outline of a structure of the sample used for experiment. 3 is an XRD pattern of Comparative Example 1. 6 is an XRD pattern of Example 3. FIG. 4 is a cross-sectional SEM photograph of a sample of Example 3. 4 is a measurement result of lithium ion conductivity of the solid electrolyte of Example 3. It is a graph which shows the relationship between the volume ratio of LLZONb and lithium ion conductivity, and the relationship between the volume ratio of LLZONb and activation energy.

The solid electrolyte of the present invention includes a crystalline lithium ion conductive material and Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ as a base material (where A is C, Al, Si, It is at least one element of Ga, Ge, In, and Sn, y satisfies 0 ≦ y <1, z is an average valence of (B _1−y , A _y ), x, z, δ satisfies the relational expression x + z = δ / 2.)

As the crystalline lithium ion conductive substance, a lithium ion conductive oxide is preferable. Among these, a garnet-type lithium ion conductive oxide is preferable. The garnet-type lithium ion conductive oxide is, for example, a basic composition Li _x A ₃ M ₂ O ₁₂ (wherein A is one or more elements selected from La, Y, Mg, Ca, Sr and Ba, and M Is one or more elements of Ti, Zr, Hf, Nb, Ta, Al, Si, Ga, Ge, and Sn, where x is a number that ensures the overall charge balance. Note that x preferably satisfies x = 24− (A average valence) × 3− (M average valence) × 2. Among them, the garnet-type lithium ion conductive oxide has a basic composition Li _{5 + x} La ₃ Zr _x A _2−x O ₁₂ (where A is Sc, Ti, V, Y, Nb, Hf, Ta, It is more preferable that one or more elements selected from the group consisting of Al, Si, Ga and Ge, x is represented by 1.4 ≦ x <2). In this case, the lithium ion conductivity is increased and the activation energy is also reduced as compared with the garnet-type oxide Li ₇ La ₃ Zr ₂ O ₁₂ (that is, x = 2) not containing the element A. For example, when the element A is Nb, the lithium ion conductivity is 2.5 × 10 ⁻⁴ Scm ⁻¹ or more and the activation energy is 0.34 eV or less. If the solid electrolyte of the present invention contains such a garnet-type lithium ion conductive oxide, when used in an all-solid-state lithium ion secondary battery, lithium ions are easily conducted and the output of the battery is improved. Moreover, since the activation energy is small, that is, the rate of change in lithium ion conductivity with respect to temperature is small, the output of the battery is stabilized. Further, it is preferable that x satisfies 1.6 ≦ x ≦ 1.95 because lithium ion conductivity is higher and activation energy is lower. Furthermore, if x satisfies 1.65 ≦ x ≦ 1.9, the lithium ion conductivity is almost maximized and the activation energy is almost minimized, which is more preferable. The details of the garnet-type lithium ion conductive oxide represented by the basic composition Li _{5 + x} La ₃ Zr _x A _2−x O ₁₂ are described in, for example, Japanese Patent Application Laid-Open No. 2010-202499. Note that the garnet-type oxide is not limited to the above-described stoichiometric composition. For example, a part of the element may be deficient or excessive, or a part of the element may be replaced with another element.

As the crystalline lithium ion conductive material, various materials other than those described above can be used. For example, a nitride of Li, a halide, an oxyacid salt, and the like can be given. _{Also, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, xLi 3 PO 4 - (1-x) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2, phosphosulfurized Compound etc. are mentioned. These may be used alone or in combination.

^{_{Preform, Li + x (B 1-}} y, A y) is represented by ^z + O ^{2- _δ.} In the formula, A is at least one element selected from C, Al, Si, Ga, Ge, In, and Sn, y satisfies 0 ≦ y <1, and z is (B _1−y , A _y ). X, z, and δ satisfy the relational expression x + z = δ / 2. Here, A is preferably Al. Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ has lithium ion conductivity, but by replacing part of boron with Al, the lithium ion conductivity can be further increased. Because it can. Note that Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ may not have a stoichiometric composition, may be partially missing, may be excessive, The part may be substituted with other elements. The base material may be Li ₃ BO ₃ . Note that an active material containing lithium or a lithium ion conductive material is liable to generate impurities such as lithium carbonate or lithium hydroxide on the surface, but Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ is It is preferable to have a melting point at a temperature slightly higher than the temperature at which they are thermally decomposed. In this case, when integrally sintering the raw material before sintering of the solid electrolyte of the present invention and the active material, lithium carbonate and lithium hydroxide present at the interface between the solid electrolyte and the active material are thermally decomposed to generate impurities. There is also an advantage that can be suppressed.

This solid electrolyte preferably has a structure in which particles of a crystalline lithium ion conductive material are dispersed in a base material. Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O having lithium ion conductivity between particles of crystalline lithium ion conductive material or between crystalline lithium ion conductive material and active material ^This is because the presence of ^2- _{δ facilitates} securing a lithium ion conduction path. In particular, the base material is more preferably a material (melted product) that is solidified after being melted including particles of a crystalline lithium ion conductive material. If the base material is solidified once it is in a liquid phase, the base material enters between the particles of the crystalline lithium ion conductive material or between these particles and the active material to further increase the lithium ion conduction path. It is because it is easy to secure.

  In this solid electrolyte, it is preferable that the crystalline lithium ion conductive material and the base material do not change in quality or produce a reaction product. For example, a crystalline lithium ion conductive material and a base material have peaks of reaction products between the crystalline lithium ion conductive material and the base material in the XRD measurement using CuKα rays, and peaks of other reaction products. It is preferable that it is not confirmed. This is because it is considered that the generation of an altered layer or a third phase that reduces the lithium ion conductivity is suppressed.

  Next, the manufacturing method of the solid electrolyte of this invention is demonstrated. The method for producing a solid electrolyte of the present invention includes (1) a forming step of forming a raw material body containing a crystalline lithium ion conductive material and a flux, and (2) a heating step of heating the raw material body at a predetermined temperature. , Including.

(1) Formation process In a formation process, the raw material body containing a crystalline lithium ion conductive substance and a flux is formed. At this time, a raw material body including a crystalline lithium ion conductive material and a flux may be formed on the surface of the body to be bonded, or such a body to be bonded may not be used. In addition, when manufacturing an all-solid-state secondary battery, as a to-be-joined body, what becomes a positive electrode or a negative electrode by sintering integrally with a raw material body in the heating process mentioned later is mentioned. Below, the case where a raw material is formed in the to-be-joined body surface in the case of formation of a raw material body is demonstrated.

Here, the details of the crystalline lithium ion conductive material are the same as those described above, and thus description thereof is omitted. The flux is expressed by Li ⁺ _x (B1 _-y , _Ay ) ^{z + O2- [} _delta] . In the formula, A is at least one element selected from C, Al, Si, Ga, Ge, In, and Sn, y satisfies 0 ≦ y <1, and z is (B _1−y , A _y ). X, z, and δ satisfy the relational expression x + z = δ / 2. Here, A is preferably Al. Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ has lithium ion conductivity, but lithium ion conductivity can be further increased by substituting part of boron with Al. Because. Note that Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ _δ may not have a stoichiometric composition, may be partially missing, may be excessive, The part may be substituted with other elements. The flux may be Li ₃ BO ₃ . This flux at least melts itself in the subsequent heating step. Thereafter, at least a part is solidified to become a base material, and becomes a solid electrolyte together with a crystalline lithium ion conductive material. This flux may have a function of lowering the melting point of the active material or crystalline lithium ion conductive material to facilitate melting. In such a case, when the flux is melted and solidified, the active material and the crystalline lithium ion conductive material are also melted and solidified to directly connect the active material and the crystalline lithium ion conductive material. It is because the interface which touches can be increased more. The flux may have a function as a sintering aid. This is because, by increasing the sinterability of the active material or crystalline lithium ion conductive material, the lithium ion conductivity can be further increased.

  When forming the raw material body, a mixture of a crystalline lithium ion conductive material and a flux may be formed as a layer, or a layer of a crystalline lithium ion conductive material may be formed and then a flux layer formed. Alternatively, a layer of crystalline lithium ion conductive material may be formed after forming a flux layer. Alternatively, the crystalline lithium ion conductive material layer and the flux layer may be alternately formed. Thus, if the raw material body contains the crystalline lithium ion conductive material and the flux as a whole, it is not necessary that both are uniformly mixed.

  In forming the raw material body, a paste obtained by adding a binder or the like to a crystalline lithium ion conductive material or flux (hereinafter also referred to as a raw material paste) may be used. As the binder, cellulose binders such as ethyl cellulose, methyl cellulose, and carboxymethyl cellulose, and various binders such as butyral resins and acrylic resins can be used. Moreover, you may use organic solvents, such as a terpionel, acetone, and toluene, as a solvent. The raw material paste can be obtained by mixing a positive electrode active material, a flux, a binder, a solvent, and the like using a normal paste manufacturing method using a triroll mill, a pot mill, or the like.

  As a method of forming a raw material paste layer on the surface of the joined body, for example, a known liquid supply method such as a dispenser, dipping, spray, etc., a doctor blade method, a printing method such as screen printing, metal mask printing, etc. Can be used. Among these, screen printing is preferable because the thickness and pattern can be controlled with high accuracy. Moreover, according to metal mask printing, it is easy to form a raw material paste with a thickness, and thus shape control becomes easy.

(2) Heating step In the heating step, the raw material body formed on the surface of the joined body is heated at a temperature equal to or higher than the melting point of the flux and equal to or lower than the temperature at which the crystalline lithium ion conductive material and the flux generate a compound. . As described above, since the heating is performed at a temperature equal to or higher than the melting point of the flux, the flux becomes a liquid phase, and the adhesion between the crystalline lithium ion conductive materials can be enhanced. Further, since the crystalline lithium ion conductive material and the flux are heated at a temperature lower than the temperature at which the compound is formed, a reaction product between the crystalline lithium ion conductive material and the flux does not occur. Thereby, the production | generation of the 3rd phase which reduces lithium ion conductivity can be suppressed. Furthermore, the heating temperature is preferably a temperature at which crystalline lithium ion conductive material and flux are not altered. Furthermore, it is preferable that the temperature is such that the active material contained in the bonded body does not change in quality. Thereby, the production | generation of the 3rd phase and a deteriorated layer which reduce lithium ion conductivity can be suppressed more. Note that the temperature at which no alteration occurs may be determined based on the processing temperature at the time of synthesis. This is because it is considered that deterioration is unlikely to occur if the temperature is lower than the processing temperature during synthesis. Specifically, the heating temperature is more than 600 ° C and preferably 900 ° C or less, more preferably 650 ° C or more and 800 ° C or less, and further preferably 700 ° C or more and 750 ° C or less. The atmosphere during heating is not particularly limited, but is preferably an air atmosphere or an oxidizing atmosphere. In such an atmosphere, the elimination of oxygen from the Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ structure is suppressed, and the crystalline lithium ion conductive material is an oxide. In this case, desorption of oxygen from the crystalline lithium ion conductive material is suppressed. For this reason, the alteration of the flux and the crystalline lithium ion conductive material hardly occurs, and the reaction product hardly occurs. In addition, although the raw material body which passed through the heating process is cooled after that, it is preferable to cool rapidly at this time. The rapid cooling may be performed by allowing to cool in the air, or may be performed by quenching in liquid nitrogen. The cooling rate is preferably 1000 ° C./sec, for example, but may be faster or slower.

In such a method for producing a solid electrolyte, a crystalline lithium ion conductive material and a base material Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ (where A is C, Al, Si) , Ga, Ge, In, Sn are at least one element, y satisfies 0 ≦ y <1, z is an average valence of (B _1−y , A _y ), x, z , Δ satisfies the relational expression x + z = δ / 2.). In particular, when the raw material body is formed on the surface of the joined body, the solid electrolyte can be in close contact with the surface of the joined body, which is preferable. Here, “close contact” means not two-dimensional contact but two-dimensional or three-dimensional contact (bonding). Whether or not it is “in close contact” can be confirmed, for example, by whether or not it is in point contact when a cross section is observed at a magnification of 5000 times using a scanning electron microscope.

  Next, an all solid state secondary battery using the solid electrolyte of the present invention will be described. The all solid state secondary battery includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and the above-described solid electrolyte interposed between the positive electrode and the negative electrode and conducting lithium ions.

The positive electrode has a positive electrode active material. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like can be used. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), Li _(1-x) Mn Lithium manganese composite oxide such as ₂ O ₄ , lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , lithium nickel composite oxide such as Li _(1-x) NiO ₂ , lithium such as LiV ₂ O ₃ Vanadium composite oxides, transition metal oxides such as V ₂ O _{5, and the} like can be used. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are more preferable. Note that an oxide-based positive electrode active material is unlikely to be plastically deformed, and in many cases, it is difficult to improve adhesion with a solid electrolyte by an external pressure. For this reason, the significance of application of the present invention is high in an all solid-state lithium secondary battery using an oxide-based positive electrode active material.

  The negative electrode has a negative electrode active material. Examples of the negative electrode active material include inorganic compounds such as lithium, lithium alloys and tin compounds, carbonaceous materials capable of inserting and extracting lithium ions, and conductive polymers. Of these, lithium alloys are preferable because they can reduce the interface resistance with the solid electrolyte. As the lithium alloy, a lithium alloy containing at least one or more additive elements among Mg, Al, Si, In, Ag, and Sn is more preferable, and one containing Al or one containing In is more preferable. In particular, those containing In are preferable because even when the number of added atoms is smaller, the interface resistance between the solid electrolyte and the negative electrode can be further reduced. The negative electrode preferably contains an additive element in the range of 10% by mass to 30% by mass of the lithium alloy, and more preferably contains the additive element in a range of 15% by mass to 25% by mass. It is more preferable that the additive element contains mass%. When the additive element contained is 10% by mass or more, the interface resistance can be further reduced, and when it is 30% by mass or less, the uniformity of the lithium alloy can be further improved. In addition, since the detail about the lithium alloy used for a negative electrode is described in Unexamined-Japanese-Patent No. 2011-70939, description is abbreviate | omitted here.

  The positive electrode and the negative electrode may have a current collector. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group.

  Since the solid electrolyte is the same as described above, detailed description thereof is omitted here. The solid electrolyte is preferably in close contact with at least one of the positive electrode and the negative electrode. For example, in order for the solid electrolyte to be in close contact with the positive electrode, the positive electrode may be used as the member to be joined in the above-described solid electrolyte manufacturing method. Moreover, you may use not only the positive electrode itself but the thing which becomes a positive electrode by heating, for example, the thing containing a positive electrode active material raw material. The same applies when the solid electrolyte is in close contact with the negative electrode.

  The shape of the all solid-state lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a rectangular type. Further, a plurality of such lithium secondary batteries connected in series may be applied to a large battery used for an electric vehicle or the like.

  The structure of the all-solid-state lithium secondary battery of the present invention is not particularly limited, and examples thereof include the structures shown in FIGS. 1 includes a solid electrolyte layer 10, a positive electrode 12 formed on one side of the solid electrolyte layer 10, and a negative electrode 14 formed on the other side of the solid electrolyte layer 10. Have. Among these, the positive electrode 12 includes a positive electrode active material layer 12a (a layer containing a positive electrode active material) in contact with the solid electrolyte layer 10 and a current collector 12b in contact with the positive electrode active material layer 12a, and the negative electrode 14 includes a solid electrolyte layer. 10 includes a negative electrode active material layer 14a (a layer containing a negative electrode active material) in contact with the electrode 10 and a current collector 14b in contact with the negative electrode active material layer 14a. On the other hand, the all-solid-state lithium secondary battery 20 in FIG. 2 includes a solid electrolyte layer 10 containing a garnet-type oxide, a positive electrode 12 formed on one side of the solid electrolyte layer 10, and the other side of the solid electrolyte layer 10. And the negative electrode 14 formed through the polymer electrolyte layer 16. Among these, the positive electrode 12 includes a positive electrode active material layer 12a and a current collector 12b, and the negative electrode 14 includes a negative electrode active material layer 14a and a current collector 14b.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

Below, the example which produced the solid electrolyte of this invention concretely is demonstrated as an Example. Specifically, as shown in FIG. 3, a sample in which a solid electrolyte was formed on Au of an Au / Al ₂ O ₃ substrate was produced.

[Example 1]
(1) Synthesis of LLZONb (theoretical chemical composition: Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ ) Using Li ₂ CO ₃ , La (OH) ₃ , ZrO ₂ , Nb ₂ O ₅ as starting materials, Li _6.75 La ₃ Zr This starting material was weighed so that the stoichiometric ratio of the basic composition of _1.75 Nb _0.25 O ₁₂ was obtained, and mixed and ground in a planetary ball mill (300 rpm / zirconia ball) in ethanol for 4 hours. Next, the mixed powder of the starting material (inorganic material) was separated from the balls and ethanol, and then calcined in an air atmosphere at 950 ° C. for 10 hours in an Al ₂ O ₃ crucible. Thereafter, Li ₂ CO ₃ is added to the calcined powder so that the amount of Li is 5 atomic% with respect to the amount of Li in the inorganic material for the purpose of compensating for the loss of Li in the main firing. For the purpose of pulverizing and mixing the obtained powder, it was pulverized and mixed in a planetary ball mill (300 rpm / zirconia ball) for 6 hours in ethanol. The obtained powder was again calcined again at 950 ° C. for 5 hours under atmospheric pressure. Subsequently, the obtained powder was molded and subjected to isostatic pressing (CIP) between cold water, and then the firing temperature was set to 1150 ° C., and the main firing was performed under atmospheric conditions for 36 hours, and LLZONb bulk pellets were obtained. Obtained. CIP was performed under the conditions of 27 ° C. and 200 MPa using water as a solvent. Furthermore, the bulk pellet was pulverized in a mortar to obtain LLZONb powder. Note that LLZONb corresponds to the crystalline lithium ion conductive material in the present application.

(2) Synthesis of Li ₃ BO ₃ Li ₂ CO ₃ and B ₂ O ₃ were used as starting materials. After weighing so that the molar ratio of Li ₂ CO ₃ : B ₂ O ₃ was 3: 1, the mixture was mixed in ethanol using a planetary ball mill. After drying, Li ₃ BO ₃ powder was synthesized under conditions of 600 ° C.-10 h. Li ₃ BO ₃ corresponds to the flux in the present application (base material after heating).

(3) Production of LLZONb + Li ₃ BO ₃ Paste LLZONb powder and Li ₃ BO ₃ powder were weighed so as to have a volume ratio of 1: 3. LLZONb + Li ₃ BO ₃ paste was prepared by mixing 2 g of these powders with 2 g of binder (manufactured by Nisshin Kasei Co., Ltd., EC vehicle (mixture of ethyl cellulose and terpione)) and kneading.

(4) Preparation of substrate An Au / Al ₂ O ₃ substrate is formed by applying an Au electrode on a general Al ₂ O ₃ substrate by screen printing and baking it at 850 ° C.-0.5 h. Prepared. This substrate is used for the sake of experimentation, and is not necessarily required when producing the solid electrolyte of the present invention.

(5) on the LLZONb + Li ₃ BO ₃ paste baking above (4) was prepared in the Au / Al ₂ O ₃ substrate to the substrate, was applied LLZONb + Li ₃ BO ₃ paste prepared in the above (3) . This was put in an electric furnace in an air atmosphere heated in advance to 700 ° C., held for 1 h, baked, and then rapidly cooled. In this way, the sample of Example 1 was prepared.

[Examples 2 to 4]
A sample of Example 2 was prepared in the same manner as in Example 1 except that the volume ratio of LLZONb powder and Li ₃ BO ₃ powder was 1: 2 in (3) above. A sample of Example 3 was prepared in the same manner as in Example 1 except that the volume ratio of LLZONb powder and Li ₃ BO ₃ powder was 1: 1. A sample of Example 4 was prepared in the same manner as in Example 1 except that the volume ratio of LLZONb powder and Li ₃ BO ₃ powder was 2: 1.

[Comparative Example 1]
Same as Example 1 except that Li ₃ BO ₃ was not added in (3) and that baking in (5) was carried out for 36 hours in an electric furnace heated to 950 ° C. Thus, a sample of Comparative Example 1 was prepared.

(XRD measurement)
The XRD measurement was performed about the produced sample. XRD measurement was performed using a TTR manufactured by Rigaku Corporation. Radiation source: CuKα line, 1 step: 0.02 deg / sec, 2θ = 10-80 deg. Measurement was carried out in the range of.

(Lithium ion conductivity measurement)
Lithium ion conductivity was measured by forming lithium as an electrode on the prepared sample using an ion coater and measuring lithium ion conductivity in the thickness direction. The measurement was performed at 5 ° C. intervals in the range of 10 ° C. to 50 ° C. in a thermostatic bath.

[Experimental result]
In FIG. 4, the XRD measurement result of the comparative example 1 is shown. In Comparative Example 1, since firing was performed in a high-temperature atmosphere, Li was volatilized from the surface of LLZONb, and it was found that the garnet phase could not be maintained and became La ₂ ZrO ₇ . In addition, when only LLZONb was used, since baking could not be performed by baking at 700 ° C. for 1 h, in Comparative Example 1, baking was performed in a high temperature atmosphere (950 ° C.). From the above, it has been found that it is difficult to obtain an LLZONb thin film even if only LLZONb powder is slurried and applied and fired.

FIG. 5 shows an XRD pattern of Example 3. In Example 3, only LLZONb, Li ₃ BO ₃ , Au, and Al ₂ O ₃ peaks were observed. From this, it was found that the decomposition of LLZONb and the reaction with Li ₃ BO ₃ hardly occurred. Although not shown, in Examples 1 to 4, only LLZONb, Li ₃ BO ₃ , Au, and Al ₂ O ₃ peaks were observed.

In FIG. 6, the cross-sectional SEM image of the torn surface of Example 3 is shown. The thickness of the solid electrolyte (LLZONb + Li ₃ BO ₃ ) layer was about 200 μm. It was confirmed that there was no large unevenness in thickness, and there were no noticeable voids. Since it is essential to suppress the short circuit between the positive and negative electrodes as a function required for the solid electrolyte, it is important that there are no conspicuous voids at this firing temperature. It turns out that it satisfies. In addition, the solid electrolyte / substrate interface is closely adhered by a two-dimensional or three-dimensional (non-point contact) bonding interface, and it is understood that no chemical reaction or large element diffusion occurs at the interface during manufacturing. It was. Therefore, even if the substrate is a positive electrode or a negative electrode, it is also closely adhered by a two-dimensional or three-dimensional (non-point contact) bonding interface, and at the time of manufacturing, chemical reaction, large element diffusion, etc. It was inferred that this would not occur.

In FIG. 7, the measurement result of the lithium ion conductivity of the solid electrolyte of Example 3 is shown. From FIG. 7, it was found that in Example 3, the activation energy was 45 kJmol ⁻¹ . In addition, the activation energy was similarly calculated | required also about the other Example.

In FIG. 8, the lithium ion conductivity and activation energy in 25 degreeC of Examples 1-4 are shown. That is, the relationship between the volume ratio of LLZONb and lithium ion conductivity and the relationship between the volume ratio of LLZONb and activation energy are shown. From this result, it was found that the volume ratio of LLZONb to the total of LLZONb and Li ₃ BO ₃ is preferably in the range of 25 to 66%. In such a range, the lithium ion conductivity and activation energy are set to LiPON (lithium ion conductivity: 2 × 10 ⁻⁶ Scm ⁻¹ , activation energy: 60 kJmol ^{−) as} a solid electrolyte of a commercially available all solid state battery. ¹ ) or higher. In particular, it was found that the range of 33 to 50% is more preferable because the lithium ion conductivity is higher and the activation energy is lower. Specifically, the lithium ion conductivity at 25 ° C. is as high as ˜5.0 × 10 ⁻⁶ Scm ⁻¹ and the activation energy is as low as ˜48 kJmol ⁻¹ , which is also better than the above-mentioned LiPON. . The details of LiPON are described in, for example, US Pat. No. 5,597,660.

In addition, when as a flux, a part of B of Li ₃ BO ₃ even when used was replaced with Al or Si, which was produced in the same manner as the samples of Example 3 were evaluated, Example A result equal to or greater than 3 was obtained. From this, it was found that the flux is not limited to Li ₃ BO _3, and a part of boron may be substituted with another element. Moreover, it was speculated that other elements substituted for boron may be not only trivalent but also divalent or tetravalent.

In addition, in the Example mentioned above, it selected based on the preliminary experiment shown below about the kind of flux and baking temperature. First, as for the type of flux, an experiment was conducted using B ₂ O ₃ , Li ₂ B ₄ O ₇ , Bi ₂ O ₃ , WO ₃ , Li ₂ WO _{4 and the} like in addition to Li ₃ BO ₃ . As a result, bonding could not be performed except for those containing boron (B). Moreover, even if it contains boron, in B ₂ O ₃ and Li ₂ B ₄ O ₇ , the bonding interface was not in close contact, or the formation of the third phase was confirmed in XRD. This shows that the flux needs to be Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ δ (A, x, y, z and δ are as described above). It was. Moreover, about the calcination temperature, it baked in the temperature range to 600 to 950 degreeC. As a result, at 600 ° C., which is lower than the melting point of the flux, the bonding interface with the substrate became point contact and did not adhere. At 950 ° C., the formation of the third phase was confirmed in XRD. From this, it was found that the heating temperature is preferably more than 600 ° C. and 900 ° C. or less. In particular, it was found that the adhesiveness at the bonding interface is higher and preferable at 650 ° C. or higher and 800 ° C. or lower. From the above, it was speculated that the selection of the type of flux and the heating temperature had a great influence on the battery characteristics. Note that Li ⁺ _x (B _1−y , A _y ) ^{z +} O ²⁻ δ has lithium ion conductivity. For this reason, it was guessed that the reduction | decrease of the lithium ion conductivity between a positive electrode active material and a solid electrolyte can be suppressed more.

  DESCRIPTION OF SYMBOLS 10 Solid electrolyte layer, 11 Electrode, 12 Positive electrode, 12a Positive electrode active material layer, 12b Current collector, 14 Negative electrode, 14a Negative electrode active material layer, 14b Current collector, 16 Polymer electrolyte layer, 20 All solid-state lithium secondary battery.

  The present invention can be used for an all solid-state lithium ion secondary battery.

Claims (10)

A crystalline lithium ion conductive material;
Li ⁺ _x (B _{1 -y} , A _y ) ^{z +} O ² _-δ as a base material (wherein A is at least one element of C, Al, Si, Ga, Ge, In, and Sn) , Y satisfies 0 ≦ y <1, z is the average valence of (B _1−y , A _y ), and x, z, and δ satisfy the relational expression x + z = δ / 2).
Containing solid electrolyte.   The solid electrolyte according to claim 1, wherein the base material is solidified after being melted including particles of the positive electrode active material.   The solid electrolyte according to claim 1 or 2, wherein a peak of a reaction product between the lithium ion conductive material and the base material is not confirmed in the XRD measurement using CuKα rays. The lithium ion conductive material has a basic formula Li _{5 + x} La ₃ Zr _x A _2−x O ₁₂ (where A is Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga). And at least one element selected from the group consisting of Ge and x is a garnet-type oxide represented by 1.4 ≦ x <2) Electrolytes. The solid electrolyte according to claim 1, wherein the base material is Li ₃ BO ₃ .   6. The volume ratio of the crystalline lithium ion conductive material to the total of the crystalline lithium ion conductive material and the base material is 33 to 50% according to claim 1. Solid electrolyte. Li ⁺ _x (B1 _-y , _Ay ) ^{z + O2- [} _delta] (wherein A is at least one of C, Al, Si, Ga, Ge, In, Sn) as a crystalline lithium ion conductive material and flux 1 or more elements, y satisfies 0 ≦ y <1, z is an average valence of (B _1−y , A _y ), and x, z, δ are relational expressions of x + z = δ / 2 And forming a raw material body including:
A heating step in which the raw material body is heated at a temperature equal to or higher than the melting point of the flux and equal to or lower than a temperature at which the lithium ion conductive material and the flux form a compound;
A method for producing a solid electrolyte, comprising:   The method for producing a solid electrolyte according to claim 7, wherein in the heating step, heating is performed at a temperature of 650 ° C. or higher and 800 ° C. or lower. The lithium ion conductive material has a basic formula Li _{5 + x} La ₃ Zr _x A _2−x O ₁₂ (where A is Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga). The method for producing a solid electrolyte according to claim 7 or 8, wherein one or more elements selected from the group consisting of Ge and Ge, x is a garnet-type oxide represented by 1.4 ≦ x <2).   The said formation process WHEREIN: The volume ratio of the said crystalline lithium ion conductive material with respect to the sum total of the said crystalline lithium ion conductive material and the said flux shall be 33-50%, Any one of Claims 7-9 A method for producing a solid electrolyte according to Item.