All-solid secondary battery and manufacturing method thereof

Abstract

To provide an all-solid secondary battery superior in load and high-temperature characteristics, and a manufacturing method thereof.SOLUTION: An all-solid secondary battery according to the present invention comprises a positive electrode, a negative electrode and a solid electrolyte layer. The positive electrode has a mold of a positive electrode mixture containing a positive electrode active material, a conductive assistant, and a sulfide-based solid electrolyte. The positive electrode active material has, on its surface, an Nb-containing oxide layer. Of the positive electrode active material, a particle size distribution has a peak of a first frequency and a peak of a second frequency. The first frequency peak is in a range of 1-8 μm, and the second frequency peak is in a range of 15-35 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an all-solid-state secondary battery having excellent load characteristics and high-temperature characteristics, and a method for manufacturing the same. [Background technology]

[0002] In recent years, with the development of portable electronic devices such as mobile phones and laptop personal computers, and the practical application of electric vehicles, there has been a growing demand for small, lightweight secondary batteries with high capacity and high energy density.

[0003] Currently, lithium secondary batteries, particularly lithium ion secondary batteries, that can meet this demand use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO) and lithium nickel oxide (LiNiO) as the positive electrode active material, graphite or the like as the negative electrode active material, and an organic electrolyte solution containing an organic solvent and a lithium salt as the non-aqueous electrolyte.

[0004] Furthermore, with the further development of devices that use lithium-ion secondary batteries, there is a demand for lithium-ion secondary batteries with even longer life, higher capacity, and higher energy density, as well as a high demand for the reliability of these longer-life, higher-capacity, and higher-energy-density lithium-ion secondary batteries.

[0005] However, the organic electrolyte solution used in lithium-ion secondary batteries contains a flammable organic solvent, which can cause the organic electrolyte solution to generate excessive heat when an abnormality such as a short circuit occurs in the battery. In addition, with the recent trend toward higher energy density of lithium-ion secondary batteries and an increasing amount of organic solvent in the organic electrolyte solution, there is a demand for even greater reliability in lithium-ion secondary batteries.

[0006] In light of the above, all-solid-state lithium secondary batteries (all-solid-state secondary batteries) that do not use organic solvents are attracting attention. All-solid-state secondary batteries use a molded body of a solid electrolyte that does not use organic solvents instead of the conventional organic solvent-based electrolyte, and are highly safe with no risk of abnormal heat generation from the solid electrolyte.

[0007] Various improvements have been attempted in all-solid-state secondary batteries. For example, Patent Document 1 proposes mechanical milling of a mixture of a cathode active material powder having a relatively large average particle size and a cathode active material powder having a relatively small average particle size in a mixing ratio of 2:8 to 8:2 (by weight), with the particle size ratio of the two being 0.08 to 1, and an inorganic solid electrolyte. Patent Document 1 claims that this treatment results in good dispersion of the cathode active material and the inorganic solid electrolyte, increasing the contact area between the cathode active material and the inorganic solid electrolyte and resulting in a cathode composite with increased lithium ion conduction paths. It also claims that the use of this cathode composite can produce a battery with high output. Patent Document 1 also suggests using a mixture of a cathode active material with an average particle size of 5 μm and a cathode active material with an average particle size of 10 μm in a mixing ratio of 7:3 (by weight), and also discloses surface modification of the cathode active material with Ti spinel or carbon.

[0008] Furthermore, Patent Document 2 discloses a technique for a cathode composite having a first cathode active material, a second cathode active material, and a sulfide solid electrolyte, in which the ratio of the average particle size of the first cathode active material to the average particle size of the second cathode active material is set to 2.0 or more and 4.3 or less. Patent Document 2 claims that with this technique, when a cathode active material layer is formed by pressing, the first cathode active material undergoes almost no plastic deformation and does not adhere to each other, making it easier for the second cathode active material, which has a smaller average particle size, to enter the gaps that form between the particles of the first cathode active material, increasing the number of contact points between the cathode active material and other constituent materials, thereby increasing the number of Li-ion conductive paths and electron conductive paths, and enabling an improvement in the energy density per volume of the battery.

[0009] In Patent Document 2, the volume ratio of the first positive electrode active material and the second positive electrode active material is determined by, specifically, the average particle diameter (D 50 ) is 8 μm or more and 12 μm or less, and the average particle size (D 50 When the average particle diameter (D 50 ) is 6 μm or more and 15 μm or less, and the average particle size (D 50 ) is 1 μm or more and 3 μm or less, it is preferable that the ratio of the first positive electrode active material to the second positive electrode active material is in the range of 80:20 to 60:40. Furthermore, Patent Document 2 also describes that the surfaces of the first positive electrode active material and the second positive electrode active material are coated with a coating layer made of a Li-ion conductive oxide such as LiNbO3, Li3PO4, or LiPON in order to suppress reaction with the sulfide solid electrolyte. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-67499 [Patent Document 2] Japanese Patent Application Publication No. 2019-106286 Summary of the Invention [Problem to be solved by the invention]

[0011] Incidentally, the application fields of all-solid-state secondary batteries are currently expanding rapidly, and for example, applications requiring discharge at a large current value are expected. Therefore, there is a demand for improving the load characteristics to meet this demand.

[0012] In all-solid-state secondary batteries, improving their load characteristics is preferable, for example, by increasing the packing density of the positive electrode mixture compact and minimizing voids within the compact to enhance ionic conductivity. However, increasing the pressing pressure during the formation of the positive electrode mixture compact to improve the packing density can result in cracks in the positive electrode active material. When cracks occur in the positive electrode active material, the inside of the particles that are not covered with the coating layer that suppresses the reaction between the positive electrode active material and the solid electrolyte are exposed, and these exposed portions react with the solid electrolyte, causing oxidation. Such oxidation of the solid electrolyte and the resulting degradation of battery characteristics are particularly likely to occur when the battery is placed in a high-temperature environment.

[0013] For these reasons, there is a demand for the development of technology that can achieve both good load characteristics and high-temperature characteristics in all-solid-state secondary batteries.

[0014] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an all-solid-state secondary battery having excellent load characteristics and high-temperature characteristics, and a method for manufacturing the same. [Means for solving the problem]

[0015] The all-solid-state secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer, and the positive electrode has a molded body of a positive electrode mixture containing a positive electrode active material, a conductive additive, and a sulfide-based solid electrolyte, and the positive electrode active material has a Nb-containing oxide layer on its surface, and the particle size distribution of the positive electrode active material has a first frequency peak and a second frequency peak, the first frequency peak being in the range of 1 to 8 μm, and the second frequency peak being in the range of 15 to 35 μm.

[0016] Furthermore, a method for producing an all-solid-state secondary battery of the present invention includes a step of mixing a positive electrode active material, a conductive additive, and a solid electrolyte to prepare a positive electrode mixture, and a step of molding the positive electrode mixture to produce a positive electrode having a molded body of the positive electrode mixture, wherein the positive electrode active material includes large particles having a mode particle diameter of 15 to 35 μm in particle size distribution and small particles having a mode particle diameter of 1 to 8 μm in particle size distribution, and the solid electrolyte includes a sulfide-based solid electrolyte having an average particle diameter of 0.1 to 5 μm. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide an all-solid-state secondary battery having excellent load characteristics and high-temperature characteristics, and a method for manufacturing the same. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating an example of an all-solid-state secondary battery of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The all-solid-state secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer, and the positive electrode has a molded body of a positive electrode mixture containing a positive electrode active material, a conductive additive, and a sulfide-based solid electrolyte, and the positive electrode active material has a Nb-containing oxide layer on its surface.

[0020] If the positive electrode active material and the sulfide-based solid electrolyte come into direct contact within the positive electrode mixture compact, the highly reactive sulfide-based solid electrolyte may oxidize and become an insulator, resulting in a decrease in ionic conductivity within the compact. Because Nb-containing oxides have ionic conductivity, forming a Nb-containing oxide layer on the surface of the positive electrode active material can prevent direct contact between the positive electrode active material and the sulfide-based solid electrolyte without impairing ionic conductivity between them.

[0021] In addition, in the present invention, by adjusting the particle sizes of the positive electrode active material and the sulfide-based solid electrolyte, it is possible to improve the packing property while suppressing cracking of the positive electrode active material and the accompanying generation of new surfaces (new surfaces not covered with the Nb-containing oxide layer) when forming a compact of the positive electrode mixture.

[0022] In the all-solid-state secondary battery of the present invention, these effects make it possible to suppress the deterioration of each component in the positive electrode mixture and improve its load characteristics and high-temperature characteristics.

[0023] The all-solid-state secondary battery of the present invention will be described in detail below.

[0024] (positive electrode) The positive electrode of the all-solid-state secondary battery has a molded body of a positive electrode mixture containing a positive electrode active material, a conductive additive, a sulfide-based solid electrolyte, and the like. Examples of the positive electrode include a positive electrode consisting of only the molded body, and a positive electrode having a structure in which the molded body and a current collector are integrated together.

[0025] The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in conventionally known lithium ion secondary batteries, that is, an active material capable of absorbing and releasing Li ions. One type of such positive electrode active material may be used alone, or two or more types may be used in combination. However, LiCoO2, a material having the composition formula Li 1+o M 1 O2 (where -0.05≦o≦0.05, M 1 M is Co and Mg, and at least one element M selected from the group consisting of Ni, Mn, Ti, Zr and Al 1A Lithium cobalt oxides such as lithium cobalt oxides represented by the formula LiNiO2, 1+p M 2 O2 (where -0.05≦p≦0.05, M 2 is Ni and at least one element M selected from the group consisting of Co, Mn, Ti, Zr, Mg and Al 2ALithium nickel oxide such as lithium nickel oxide represented by the group consisting of: It is preferable to use the lithium cobalt oxide.

[0026] In the lithium cobalt oxide, Mg and element M 1A has the effect of increasing the stability of the lithium cobalt oxide (stability in a high voltage region and thermal stability) and suppressing the elution of Co ions. 1A It also has the effect of improving the continuous charging characteristics of the battery (the characteristic that even if the battery is continuously charged, the time period during which a minute short circuit occurs is very long).

[0027] In the lithium cobalt oxide, the amount of Mg is preferably such that the atomic ratio Mg / Co to Co is 0.002 or more, more preferably 0.005 or more, from the viewpoint of more effectively exerting the above-mentioned effects. 1A The amount of is determined in accordance with the atomic ratio M 1A / Co is preferably 0.001 or more, and more preferably 0.003 or more.

[0028] However, Mg and element M in the lithium cobalt oxide 1A If the amount of Mg is too large, the amount of Co becomes too small, and there is a risk that the effect of Mg may not be sufficiently ensured. Therefore, in the lithium cobalt oxide, the amount of Mg is preferably 0.03 or less, more preferably 0.015 or less, in terms of the atomic ratio Mg / Co to Co. In addition, in the lithium cobalt oxide, 1A The amount of Co is expressed as the atomic ratio M 1A / Co is preferably 0.03 or less, and more preferably 0.015 or less.

[0029] When the lithium cobalt oxide is used in combination with another positive electrode active material, the proportion of the lithium cobalt oxide in the total positive electrode active material is preferably 60 mass% or more. Note that, since the entire positive electrode active material may be the lithium cobalt oxide, the preferred upper limit of the content of the lithium cobalt oxide in the total positive electrode active material is 100 mass%.

[0030] The positive electrode active material has a Nb-containing oxide layer on its surface, which can be made of an Nb-containing oxide having Li-ion conductivity, such as LiNbO3.

[0031] The Nb-containing oxide layer may also contain oxides such as Li3PO4, Li3BO3, Li4SiO4, Li4GeO4, LiTiO3, and LiZrO3, or a composite compound made of two or more of these oxides and Nb-containing oxides (such as LiNbO3), together with the Nb-containing oxide.

[0032] In order to effectively suppress the reaction between the positive electrode active material and the sulfide-based solid electrolyte, the amount of the Nb-containing oxide layer is preferably adjusted so that the ratio of Nb contained in the Nb-containing oxide layer to the entire positive electrode active material including the Nb-containing oxide layer is 0.1% by mass or more. In order to prevent a decrease in capacity due to an excessively high ratio of Nb-containing oxide, the amount of the Nb-containing oxide layer is preferably adjusted so that the ratio of Nb contained in the Nb-containing oxide layer to the entire positive electrode active material including the Nb-containing oxide layer is 2.5% by mass or less.

[0033] Examples of methods for forming an Nb-containing oxide layer on the surface of a positive electrode active material include the sol-gel method, mechanofusion method, CVD method, and PVD method.

[0034] The positive electrode active material contained in the positive electrode mixture compact has a particle size distribution with a first frequency peak and a second frequency peak, with the first frequency peak in the range of 1 to 8 μm and the second frequency peak in the range of 15 to 35 μm. When a positive electrode active material with a particle size distribution having a single frequency peak is used, increasing the density of the positive electrode mixture compact requires increasing the pressing pressure, which is prone to cracking of the positive electrode active material. However, when the positive electrode active material in the positive electrode mixture compact has the particle size distribution described above and the sulfide-based solid electrolyte has an average particle size described below, small-sized particles penetrate between large-sized particles, improving the packing of the entire compact. This allows the density of the compact to be increased without excessively increasing the pressing pressure during formation. This prevents cracking of the positive electrode active material (which would otherwise result in the formation of a new surface without an Nb-containing oxide layer) while increasing the ionic conductivity of the positive electrode mixture compact. This improves the load characteristics of the all-solid-state secondary battery while suppressing a deterioration in high-temperature characteristics.

[0035] In order to adjust the particle size distribution of the positive electrode active material in the positive electrode mixture compact as described above, large-particle size particles having a mode particle size of 15 to 35 μm in the particle size distribution and small-particle size particles having a mode particle size of 1 to 8 μm in the particle size distribution may be used as the positive electrode active material in preparing the positive electrode mixture for forming the compact.

[0036] The ratio of the large particle diameter particles to the small particle diameter particles is preferably in the range of 75:25 to 95:5 by mass in order to prevent cracking of the positive electrode active material and to improve the packing properties of the entire compact.

[0037] In addition, when a cathode active material that forms secondary particles is used, primary particles may detach from the secondary particles during the formation of a compact of the cathode mixture, and it is presumed that these detached primary particles have difficulty maintaining contact with the solid electrolyte or the conductive additive, which may cause a decrease in the utilization rate of the cathode active material. Therefore, from the viewpoint of increasing the utilization rate of the cathode active material and further improving the load characteristics of the battery, it is preferable that at least one of the large-particle-size particles and the small-particle-size particles used as the cathode active material is a primary particle, and it is more preferable that both are primary particles.

[0038] The positive electrode active material composed of primary particles may be a commercially available product. Alternatively, a positive electrode active material containing a portion of secondary particles but mostly composed of primary particles may also be used.

[0039] The particle size distribution of the positive electrode active material contained in the molded body of the positive electrode mixture referred to in this specification, the positive electrode active material (large particle size particles and small particle size particles) used in preparing the positive electrode mixture, and the solid electrolyte can be measured using a particle size distribution measuring device (such as the Microtrac particle size distribution measuring device "MT3300EXII" manufactured by Microtrac-Bell Corporation) (the values ​​described in the examples below are measured using this method). The mode particle diameter of the particle size distribution is the particle diameter with the highest volume-based frequency when calculating the integrated volume from particles with small particle sizes, and the average particle diameter is the 50% diameter value (D 50 )

[0040] The content of the positive electrode active material in the positive electrode mixture is preferably 55 to 85 mass %.

[0041] The conductive additive for the positive electrode can be a carbon material such as graphite (natural graphite or artificial graphite), graphene, carbon black, vapor grown carbon fiber (VGCF), carbon nanofiber, or carbon nanotube. The content of the conductive additive in the positive electrode mixture is preferably 1 to 10 mass %.

[0042] Among carbon materials, it is preferable to use carbon particles having hydrophilic portions in order to improve the load characteristics and increase the filling property of the positive electrode mixture. The carbon particles having hydrophilic portions preferably contain hydrophilic portions at a ratio of 10% by mass or more, and preferably 12% by mass or more, while the ratio of hydrophilic portions in the carbon particles is preferably 30% by mass or less in order to prevent a decrease in electrical conductivity.

[0043] The "hydrophilic portion" of the carbon particles referred to in this specification is as follows: 0.1 g of carbon particles is added to 20 mL of an aqueous ammonia solution with a pH of 11, and the mixture is subjected to ultrasonic irradiation for 1 minute. The resulting solution is left to stand for 5 hours to precipitate the solid phase. The portion that does not precipitate and remains dispersed in the liquid phase (supernatant) corresponds to the "hydrophilic portion."

[0044] The proportion of the "hydrophilic portion" in the total amount of carbon particles is a value determined by the following method. After precipitation of the solid phase portion, the supernatant liquid is removed from the liquid, the remaining portion is dried, and the weight of the dried solid is measured. The value obtained by subtracting the obtained weight from the weight of the carbon initially added (0.1 g) is the weight of the "hydrophilic portion" dispersed in the supernatant liquid. The value obtained by dividing the weight of the "hydrophilic portion" by the weight of the carbon initially added (0.1 g) and expressing it as a percentage corresponds to the proportion of the "hydrophilic portion" in the total amount of carbon.

[0045] From the viewpoint of further improving the formability of the electrode mixture, the average particle diameter of the carbon particles having the hydrophilic portion is preferably 10 nm or more, and more preferably 20 nm or more, in terms of the average particle diameter of the primary particles. On the other hand, since it is easy to increase the proportion of the "hydrophilic portion", the average particle diameter of the primary particles is preferably 70 nm or less, and more preferably 50 nm or less.

[0046] Carbon particles such as graphite, carbon black, and carbon nanotubes, which are commonly used as conductive additives in the electrodes of batteries such as lithium-ion secondary batteries, have a hydrophilic moiety ratio of 5% by mass or less. By subjecting such carbon particles to an oxidation treatment, hydroxyl groups, carboxyl groups, ether bonds, etc. are introduced, and the conjugated double bonds of the carbon are oxidized to single bonds, and carbon-carbon bonds are partially broken, generating hydrophilic moieties, thereby obtaining carbon particles with a hydrophilic moiety ratio that satisfies the above value.

[0047] A more specific method for producing carbon particles having a hydrophilic portion ratio of 10% by mass or more includes, for example, using a porous carbon raw material (porous carbon powder, Ketjen black, porous furnace black, carbon nanofibers, carbon nanotubes, etc.), treating this with an acid (nitric acid, a nitric acid / sulfuric acid mixture, an aqueous hypochlorous acid solution, etc.), mixing this mixture with a transition metal compound (a transition metal halide, an inorganic salt of a transition metal, an organic salt of a transition metal, etc.), causing a mechanochemical reaction, heating the reaction product in a non-oxidizing atmosphere (a nitrogen atmosphere, an argon atmosphere, etc.), removing the transition metal compound and reaction products of the transition metal compound from the heated product by dissolving them with an acid, and then washing and drying the mixture.

[0048] Alternatively, carbon particles having a hydrophilic portion ratio that satisfies the above-mentioned value can be obtained by mixing the void-containing carbon raw material with the transition metal compound, heating the mixture in an oxidizing atmosphere (in an oxygen-containing atmosphere such as air), removing the transition metal compound and the reaction product of the transition metal compound from the heated product by dissolving them with an acid, and then washing and drying the mixture.

[0049] The details of the method and conditions for producing the carbon particles having a hydrophilic portion ratio of 10% by mass or more are disclosed in Patent Document 1 (WO 2015 / 133586), and the carbon particles may be produced in accordance with the description therein.

[0050] The conductive additive may contain carbon particles having a hydrophilic portion ratio of 10% by mass or more, as well as ordinary carbon particles that have not been subjected to the above treatment, such as graphite, carbon black, carbon nanotubes, vapor-grown carbon fiber (VGCF), etc. In particular, the coexistence of fibrous carbon particles such as vapor-grown carbon fiber (VGCF) or carbon nanotubes as the conductive additive can increase the conductivity of the positive electrode mixture and further improve the load characteristics.

[0051] The mixing ratio of carbon particles with a hydrophilic portion ratio of 10% by mass or more and other conductive assistant particles is preferably 10:90 to 90:10 in terms of mass ratio.

[0052] Examples of the sulfide-based solid electrolyte for the positive electrode include particles such as Li2S-P2S5, Li2S-SiS2, Li2S-P2S5-GeS2, and Li2S-B2S3-based glasses. In recent years, LGPS-based materials (Li 10 GeP2S 12 etc.) and those of the argyrodite type [such as Li6PS5Cl, Li 7-x+y PS 6-x Cl x+y (where 0.05 ≦ y ≦ 0.9, -3.0x + 1.8 ≦ y ≦ -3.0x + 5.7), Li 7-a PS 6-a Cl b Br c (where a = b + c, 0 < a ≦ 1.8, 0.1 ≦ b / c ≦ 10.0), etc.] can also be used. Among these, argyrodite-based materials with particularly high lithium ion conductivity and high chemical stability are preferably used.

[0053] From the perspective of enhancing the filling property of the formed body of the positive electrode mixture and improving the load characteristics and high-temperature characteristics of the battery in combination with adjusting the particle size distribution of the positive electrode active material as described above, the average particle diameter of the sulfide-based solid electrolyte is preferably 0.1 μm or more, more preferably 0.5 μm or more, and preferably 5 μm or less, more preferably 4 μm or less.

[0054] In addition to the sulfide-based solid electrolyte, other solid electrolytes (such as hydride-based solid electrolytes and oxide-based solid electrolytes) can also be used in the positive electrode. However, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolyte particles in the electrode for an all-solid-state battery is preferably 30 mass% or less. Note that, since all the solid electrolytes in the positive electrode may be sulfide-based solid electrolytes, the lower limit of the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolyte is 0 mass%.

[0055] Examples of hydride-based solid electrolytes include particles of LiBH4, solid solutions of LiBH4 and the following alkali metal compounds (for example, those in which the molar ratio of LiBH4 to the alkali metal compound is 1:1 to 20:1). The alkali metal compound in the solid solution may be at least one selected from the group consisting of lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbF, RbCl, etc.), cesium halides (CsI, CsBr, CsF, CsCl, etc.), lithium amide, rubidium amide, and cesium amide.

[0056] Examples of oxide-based solid electrolytes include Li7La3Zr2O 12 , LiTi(PO4)3, LiGe(PO4)3, LiLaTiO3, and other particles.

[0057] The average particle size of the solid electrolyte other than the sulfide-based solid electrolyte is preferably approximately the same as the average particle size of the sulfide-based solid electrolyte.

[0058] The content of the solid electrolyte in the positive electrode mixture is preferably 15% by mass or more, more preferably 20% by mass or more, from the viewpoint of ensuring better ionic conductivity in the positive electrode mixture compact. However, if the amount of solid electrolyte in the positive electrode mixture is too large, for example, the amount of positive electrode active material may decrease, which may result in a decrease in the battery capacity. Therefore, the content of the solid electrolyte in the positive electrode mixture is preferably 40% by mass or less, more preferably 35% by mass or less.

[0059] The positive electrode mixture may or may not contain a resin binder. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder acts as a resistance component in the positive electrode mixture, it is desirable that the amount of the resin binder be as small as possible. Therefore, it is preferable that the positive electrode mixture does not contain a resin binder, or if it does contain one, its content is 0.5 mass% or less. It is more preferable that the content of the resin binder in the positive electrode mixture is 0.3 mass% or less, and even more preferably 0 mass% (i.e., no resin binder is contained).

[0060] When a current collector is used for the positive electrode, the current collector may be made of a metal foil such as aluminum or stainless steel, a punched metal, a mesh, an expanded metal, a foamed metal, a carbon sheet, or the like.

[0061] The compact of the positive electrode mixture can be formed, for example, by compressing a positive electrode mixture prepared by mixing a positive electrode active material, a conductive additive, a solid electrolyte, and, if necessary, a binder, by pressure molding or the like.

[0062] In the case of a positive electrode having a current collector, it can be produced by bonding a molded body of the positive electrode mixture formed by the above-mentioned method to the current collector by, for example, pressing.

[0063] The thickness of the positive electrode mixture compact (in the case of a positive electrode having a current collector, the thickness of the positive electrode mixture compact per one side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the capacity of the battery. Note that, although the load characteristics of a battery are generally improved by making the positive electrode or negative electrode thinner, according to the present invention, the load characteristics can be improved even when the positive electrode mixture compact is as thick as 200 μm or more. Therefore, in the present invention, the effect is more pronounced when the thickness of the positive electrode mixture compact is, for example, 200 μm or more. Furthermore, the thickness of the positive electrode mixture compact is usually 2000 μm or less.

[0064] (Negative electrode) The negative electrode of the all-solid-state secondary battery has, for example, a molded body of a negative electrode mixture containing a negative electrode active material, a conductive additive, a solid electrolyte, and the like, and examples thereof include a negative electrode consisting of only the molded body and a negative electrode having a structure in which the molded body and a current collector are integrated together.

[0065] The negative electrode active material may be one or a mixture of two or more carbonaceous materials capable of absorbing and releasing lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fiber. Other examples of the negative electrode active material include simple substances, compounds, and alloys thereof containing elements such as Si, Sn, Ge, Bi, Sb, and In; compounds capable of charging and discharging at low voltages similar to those of lithium metal, such as lithium-containing nitrides and lithium-containing oxides; lithium metal; and lithium / aluminum alloys.

[0066] Among these, it is preferable to use lithium titanium oxide. Examples of lithium titanium oxide include those represented by the following general formula (1).

[0067] Li 1 / 3-c M 3 c Ti 5 / 3-d M 4 d ]O4(1)

[0068] In the general formula (1), M 3 is at least one element selected from the group consisting of Na, Mg, K, Ca, Sr and Ba, and M 4 is at least one element selected from the group consisting of Al, V, Cr, Fe, Co, Ni, Zn, Ym, Zr, Nb, Mo, Ta and W, and 0≦c<1 / 3, 0≦d<5 / 3.

[0069] That is, in the lithium titanium oxide represented by the general formula (1), a part of the Li site is occupied by the element M 3However, in the general formula (1), the element M 3 In the lithium titanium oxide represented by the general formula (1), Li is an element M 3 Since it does not have to be substituted with element M 3 The ratio c can be 0.

[0070] In addition, in the lithium titanium oxide represented by the general formula (1), the element M 4 is a component that enhances the electronic conductivity of lithium titanium oxide, and element M 4 When d, which represents the ratio of , satisfies 0≦d<5 / 3, the effect of improving the electronic conductivity can be satisfactorily ensured.

[0071] Only lithium titanium oxide may be used as the negative electrode active material, but when lithium titanium oxide is used in combination with other negative electrode active materials, the proportion of the negative electrode active materials other than lithium titanium oxide in the total amount of the negative electrode active materials is preferably 30 mass% or less.

[0072] The content of the negative electrode active material in the negative electrode mixture is preferably 40 to 70 mass %.

[0073] The conductive additive for the negative electrode can be a carbon material such as graphite (natural graphite or artificial graphite), graphene, carbon black, carbon nanofiber, or carbon nanotube. The content of the conductive additive in the negative electrode mixture is preferably 5 to 15 mass %.

[0074] The solid electrolyte for the negative electrode is not particularly limited as long as it has lithium ion conductivity, and for example, one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, oxide-based solid electrolytes, etc., exemplified above as usable for the positive electrode, can be used. Among the solid electrolytes exemplified above, it is more preferable to use a sulfide-based solid electrolyte because it has high lithium ion conductivity and also has the function of improving the formability of the positive electrode mixture.

[0075] The content of the solid electrolyte in the negative electrode mixture is preferably 30 to 55 mass %.

[0076] The negative electrode mixture may or may not contain a resin binder. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF). However, since the resin binder acts as a resistance component even in the negative electrode mixture, it is desirable that the amount thereof be as small as possible. Therefore, it is preferable that the negative electrode mixture does not contain a resin binder, or if it does contain one, its content is 0.5 mass% or less. It is more preferable that the content of the resin binder in the negative electrode mixture is 0.3 mass% or less, and even more preferable that it is 0 mass% (i.e., no resin binder is contained).

[0077] When a current collector is used for the negative electrode, the current collector may be made of copper or nickel foil, punched metal, mesh, expanded metal, foamed metal; carbon sheet; or the like.

[0078] The compact of the negative electrode mixture can be formed, for example, by compressing the negative electrode mixture prepared by mixing the negative electrode active material, the conductive additive, the solid electrolyte, and further a binder that is added as needed, by pressure molding or the like.

[0079] In the case of a negative electrode having a current collector, it can be produced by bonding a molded body of the negative electrode mixture formed by the above-mentioned method to the current collector by, for example, pressing.

[0080] The thickness of the negative electrode mixture compact (in the case of a negative electrode having a current collector, the thickness of the positive electrode mixture compact per one side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the capacity of the battery. Note that, although the load characteristics of a battery are generally improved by making the positive electrode or negative electrode thinner, according to the present invention, the load characteristics can be improved even when the negative electrode mixture compact is as thick as 200 μm or more. Therefore, in the present invention, when the thickness of the negative electrode mixture compact is, for example, 200 μm or more, the effect is more remarkable. Furthermore, in the present invention, when the thickness of the positive electrode mixture compact is 200 μm or more and the thickness of the negative electrode mixture compact is 200 μm or more, the effect is particularly remarkable. Furthermore, the thickness of the negative electrode mixture compact is usually 3000 μm or less.

[0081] (Solid electrolyte layer) The solid electrolyte in the solid electrolyte layer can be, for example, one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, oxide-based solid electrolytes, etc., exemplified above as those usable for the positive electrode. However, to improve battery characteristics, it is desirable to contain a sulfide-based solid electrolyte, and it is more desirable to contain a sulfide-based solid electrolyte in all of the positive electrode, negative electrode, and solid electrolyte layer.

[0082] The solid electrolyte layer may have a porous body such as a resin nonwoven fabric as a support.

[0083] The solid electrolyte layer can be formed by a method of compressing the solid electrolyte by pressure molding or the like; or a method of applying a solid electrolyte layer-forming composition prepared by dispersing the solid electrolyte in a solvent onto a substrate, a positive electrode, or a negative electrode, drying the composition, and, if necessary, performing pressure molding such as pressing.

[0084] The solvent used in the solid electrolyte layer-forming composition is preferably one that does not deteriorate the solid electrolyte. In particular, sulfide-based and hydride-based solid electrolytes undergo chemical reactions with trace amounts of water, so nonpolar aprotic solvents, such as hydrocarbon solvents like hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene, are preferred. Ultra-dehydrated solvents with a water content of 0.001% by mass (10 ppm) or less are particularly preferred. Fluorine-based solvents such as "Vertrel®" from DuPont-Mitsui Fluorochemicals, "Zeorolla®" from Zeon Corporation, and "Novec®" from Sumitomo 3M Company, as well as nonaqueous organic solvents such as dichloromethane and diethyl ether can also be used.

[0085] The thickness of the solid electrolyte layer is preferably 100 to 300 μm.

[0086] (electrode body) The positive electrode and the negative electrode can be used in a battery in the form of a laminated electrode body in which the positive electrode and the negative electrode are laminated with a solid electrolyte layer interposed therebetween, or in the form of a wound electrode body in which this laminated electrode body is wound.

[0087] When forming the electrode body, it is preferable to pressure-mold the positive electrode, negative electrode, and solid electrolyte layer in a stacked state, from the viewpoint of increasing the mechanical strength of the electrode body.

[0088] (Battery type) A cross-sectional view schematically illustrating an example of the all-solid-state secondary battery of the present invention is shown in Fig. 1. The battery 1 shown in Fig. 1 has a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 interposed between the positive electrode 10 and the negative electrode 20 enclosed in an exterior body formed by an exterior can 40, a sealing can 50, and a resin gasket 60 interposed between them.

[0089] The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to abut against the sealing can 50, thereby sealing the opening of the outer can 40 and creating an airtight structure inside the element.

[0090] The outer can and sealing can can be made of stainless steel or other materials. Materials such as polypropylene and nylon can be used for the gasket. If heat resistance is required for the battery's intended use, heat-resistant resins with melting points exceeding 240°C, such as fluororesins such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), can also be used. If the battery is intended for use in applications requiring heat resistance, a glass hermetic seal can also be used for the sealing.

[0091] The form of the all-solid-state secondary battery is not limited to one having an exterior body composed of an exterior can, a sealing can, and a gasket as shown in FIG. 1 , that is, one generally called a coin-type battery or a button-type battery, but may be, for example, one having an exterior body composed of a resin film or a metal-resin laminate film, or one having an exterior body made of metal and having a tubular (cylindrical or rectangular) exterior can with a bottom and a sealing structure that seals the opening of the can.

[0092] The all-solid-state secondary battery of the present invention can be used in the same applications as conventionally known secondary batteries, but since it has a solid electrolyte instead of an organic electrolyte solution, it has excellent heat resistance and can be preferably used in applications where it is exposed to high temperatures. [Example]

[0093] The present invention will be described in detail below based on examples, but the following examples do not limit the present invention.

[0094] Example 1 <Preparation of carbon particles with hydrophilic portions> 9 parts by mass of carbon black with an average primary particle size of 40 nm and pores of 2 nm or less, 99.6 parts by mass of Co(CH3COO)2·4H2O, and 32 parts by mass of LiOH·H2O were mixed in distilled water and stirred for 1 hour. The mixture was then filtered to obtain a mixture containing carbon black.

[0095] Next, 30 parts by mass of LiOH·HO was added to the mixture, and the mixture was heated in air at 250°C for 30 minutes using an evaporator to obtain a composite in which the lithium-cobalt compound was supported on carbon black. This composite was then placed in a 1:1:1 mixed aqueous solution of 98% concentrated sulfuric acid, 70% concentrated nitric acid, and 30% hydrochloric acid (volume ratio: 1:1:1), and ultrasonic waves were applied to dissolve the lithium-cobalt compound in the composite. The remaining solid was then filtered, washed with water, and dried.

[0096] The lithium cobalt compound was completely removed by repeating the steps of dissolving the lithium cobalt compound in the mixed aqueous solution, filtering, washing with water, and drying, to obtain carbon particles containing hydrophilic portions at a rate of 10% by mass or more.

[0097] 0.1 g of the obtained carbon particles was added to 20 ml of an aqueous ammonia solution with a pH of 11, and after irradiating with ultrasonic waves for 1 minute, the mixture was left to stand for 5 hours to precipitate the solid phase portion.

[0098] After the solid phase precipitated, the supernatant was removed and the remaining portion was dried. The weight of the dried solid was measured, and the weight of the hydrophilic portion was determined as the weight loss from the weight of the carbon particles before treatment (0.1 g). The ratio of the weight of the hydrophilic portion to the weight of the carbon particles before treatment was calculated to be 14.5% by mass.

[0099] <Preparation of positive electrode> Large particle size consisting of primary particles with a mode particle size of 26 μm in particle size distribution: LiCo 0.98 Al 0.01 Mg 0.01 O2 (positive electrode active material) and small particle diameter particles, which are also composed of primary particles and have a mode particle diameter of 6 μm in the particle size distribution: LiCo 0.98 Al 0.01 Mg0.01 O2 (positive electrode active material) and a sulfide-based solid electrolyte (Li 7.0 PS 5.4 Cl 1.2 ), carbon nanotubes ("VGCF" (trade name) manufactured by Showa Denko K.K.) (conductive additive), and the carbon particles having hydrophilic portions (conductive additive) were mixed in a mass ratio of 59.5:10.5:26.8:2.1:1.1 and thoroughly kneaded to prepare a positive electrode mixture. The particle size distribution of the positive electrode active material in which the large particle size particles and the small particle size particles were mixed had a first frequency peak and a second frequency peak, with the first frequency peak being 6 μm and the second frequency peak being 26 μm.

[0100] Next, 92 mg of the positive electrode mixture was placed in a powder molding die and pressure-molded using a press to produce a positive electrode composed of a positive electrode mixture compact. Note that both the large particle size particles and the small particle size particles of the positive electrode active material had a layer of LiNbO3 formed on their surfaces, and the proportion of Nb contained in LiNbO3 relative to the entire positive electrode active material containing LiNbO3 was 1.5 mass%.

[0101] <Formation of solid electrolyte layer> Next, a sulfide solid electrolyte (Li 2 O 3 ) having an average particle size of 0.7 μm was applied onto the positive electrode mixture compact in the powder molding die. 5.4 PS 4.4 Cl 0.8 Br 0.8 ): 16 mg was added, and pressure molding was carried out using a press to form a solid electrolyte layer on the positive electrode mixture compact.

[0102] <Preparation of negative electrode> Lithium titanate (Li4Ti5O 12The negative electrode mixture was prepared by mixing the same sulfide solid electrolyte as that used for the solid electrolyte layer and graphene (anode active material) in a mass ratio of 50:41:9 and thoroughly kneading the mixture. Next, 129 mg of the negative electrode mixture was placed on top of the solid electrolyte layer in the powder molding die, and pressure molding was performed using a press at a surface pressure of 600 MPa to form a negative electrode composed of a negative electrode mixture compact on the solid electrolyte layer, thereby producing a laminate in which the positive electrode, solid electrolyte layer, and negative electrode were stacked.

[0103] The thickness of the laminate at this time was 2250 μm, the thickness of the positive electrode mixture molded body was 730 μm, the thickness of the negative electrode mixture molded body was 1320 μm, and the thickness of the solid electrolyte layer was 200 μm. Furthermore, in the positive electrode mixture molded body, the large-diameter particles and small-diameter particles of the positive electrode active material maintained their original shapes, and it was confirmed that the particle size distribution of the positive electrode active material had a first-frequency peak at a position of approximately 6 μm and a second-frequency peak at a position of approximately 26 μm.

[0104] A stainless steel sealing can and an outer can were used as the exterior body, and a porous carbon sheet having a thickness of 0.1 mm was placed between the sealing can and the outer can and the laminated electrode body, respectively, to seal the battery, thereby producing a coin-type all-solid-state secondary battery.

[0105] Comparative Example 1 A coin-type all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the small-diameter particles having a mode particle diameter of 6 μm in the particle size distribution were replaced with large-diameter particles having a mode particle diameter of 26 μm in the particle size distribution, and the positive electrode active material in the positive electrode mixture was composed entirely of large-diameter particles.

[0106] Comparative Example 2 A coin-type all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that the large-diameter particles having a mode particle diameter of 26 μm in the particle size distribution were replaced with small-diameter particles having a mode particle diameter of 6 μm in the particle size distribution, and the positive electrode active material in the positive electrode mixture was composed entirely of small-diameter particles.

[0107] Comparative Example 3 A coin-type all-solid-state secondary battery was produced in the same manner as in Example 1, except that the small particle diameter was changed from particles having a mode particle diameter of 6 μm in the particle size distribution to particles having a mode particle diameter of 0.8 μm.

[0108] Comparative Example 4 A coin-type all-solid-state secondary battery was produced in the same manner as in Example 1, except that the large particle diameter was changed from particles having a mode particle diameter of 26 μm in the particle size distribution to particles having a mode particle diameter of 40 μm.

[0109] The all-solid-state secondary batteries of the Examples and Comparative Examples were evaluated as follows.

[0110] <Load characteristic evaluation> The produced batteries of the examples and comparative examples were subjected to constant current-constant voltage charging, which combined constant current charging at a current value of 0.2 C until the battery voltage reached 3.1 V and constant voltage charging at a voltage of 3.1 V until the current value reached 0.02 C. Furthermore, constant current discharging was performed at a current value of 0.1 C until the battery voltage reached 1.2 V, and the discharge capacity at 0.1 C was measured.

[0111] Next, the same constant current-constant voltage charging as above was performed, and then constant current discharging was performed at a current value of 0.5 C until the battery voltage reached 1.2 V, and the discharge capacity at 0.5 C was measured. The discharge capacity at 0.5 C was divided by the discharge capacity at 0.1 C to obtain a value (%), and the load characteristics of each battery were evaluated.

[0112] <High temperature characteristic evaluation> The fabricated batteries of the examples and comparative examples were subjected to constant current-constant voltage charging and constant current discharging at 0.1 C under the same conditions as those used in the load characteristic evaluation, and the discharge capacity at 0.1 C (capacity before storage) was measured. Subsequently, the same constant current-constant voltage charging as described above was performed, and each charged battery was stored in a 60°C environment for 50 days. After that, the temperature of each battery was returned to room temperature, and constant current discharging was performed at a current value of 0.1 C until the battery voltage reached 1.2 V. Furthermore, each discharged battery was subjected to constant current-constant voltage charging and constant current discharging at 0.1 C under the same conditions as those used in the load characteristic evaluation, and the discharge capacity at 0.1 C (capacity after storage) was measured. The ratio of the capacity after storage to the capacity before storage was calculated as the capacity retention rate (%), and the high-temperature characteristics (storage characteristics) of each battery were evaluated.

[0113] The results of the above evaluations are shown in Table 1.

[0114] [Table 1]

[0115] The positive electrode mixture compact of the battery of Example 1 was constructed using a positive electrode active material having a particle size distribution with a first frequency peak and a second frequency peak, the first frequency peak being in the range of 1 to 8 μm and the second frequency peak being in the range of 15 to 35 μm, and a sulfide-based solid electrolyte having an average particle size of 0.1 to 5 μm. Therefore, as shown in Table 1, the packing property of the compact could be improved without excessively increasing the pressing pressure during formation, and the load characteristics were excellent and deterioration of characteristics at high temperatures could be prevented.

[0116] On the other hand, the batteries of Comparative Examples 1 and 2, which used positive electrode active materials with particle size distributions having a single frequency peak, as well as the battery of Comparative Example 3, in which the frequency peak of small particle diameters was too small, and the battery of Comparative Example 4, in which the frequency peak of large particle diameters was too large, had poorer load characteristics due to a decrease in the packing property of the positive electrode mixture compact compared to the battery of Example 1. Furthermore, the battery of Comparative Example 2, which used only small particle diameters, and the battery of Comparative Example 3, in which the frequency peak of small particle diameters was too small, had a large specific surface area of ​​the positive electrode active material and poor high-temperature characteristics. [Explanation of symbols]

[0117] 1 All-solid-state secondary battery 10 positive electrode 20 negative electrode 30 Solid electrolyte layer 40 outer can 50 sealed cans 60 gaskets

Claims

1. An all-solid-state secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, the positive electrode has a formed body of a positive electrode mixture made of a powdery mixture containing a positive electrode active material, a conductive additive, and a sulfide-based solid electrolyte, the positive electrode active material has a Nb-containing oxide layer on a surface thereof, the particle size distribution of the positive electrode active material has a peak of a first frequency and a peak of a second frequency, An all-solid-state secondary battery, wherein the peak of the first frequency is in the range of 1 to 8 μm, and the peak of the second frequency is in the range of 15 to 35 μm.

2. 2. The all-solid-state secondary battery according to claim 1, wherein the positive electrode active material is lithium cobalt oxide.

3. 3. The all-solid-state secondary battery according to claim 1, wherein a ratio of Nb contained in the Nb-containing oxide layer to the entire cathode active material including the Nb-containing oxide layer on the surface of the cathode active material is 0.1 mass % or more and 2.5 mass % or less.

4. The all-solid-state secondary battery according to any one of claims 1 to 3, wherein the content of the positive electrode active material in the positive electrode mixture is 55 to 85 mass%.

5. The all-solid-state secondary battery according to any one of claims 1 to 4, wherein the thickness of the positive electrode mixture compact is 200 µm or more.

6. An all-solid-state secondary battery according to any one of claims 1 to 5, wherein the positive electrode mixture does not contain a resin binder or the resin binder content is 0.5 mass% or less.

7. A method for manufacturing an all-solid-state secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, comprising: a step of preparing a positive electrode mixture consisting of a powder mixture by mixing a positive electrode active material having a Nb-containing oxide layer on its surface, a conductive additive, and a solid electrolyte, and then pressure-molding the positive electrode mixture in a powder molding die to produce a positive electrode having a molded body of the positive electrode mixture; As the positive electrode active material, large-diameter particles having a mode particle diameter of 15 to 35 μm in particle size distribution and small-diameter particles having a mode particle diameter of 1 to 8 μm in particle size distribution are used, A method for producing an all-solid-state secondary battery, wherein a sulfide-based solid electrolyte having an average particle size of 0.1 to 5 μm is used as the solid electrolyte.

8. 8. The method for producing an all-solid-state secondary battery according to claim 7, wherein the large particle diameter particles and the small particle diameter particles are used in a mass ratio of 75:25 to 95:

5.

9. The method for producing an all-solid-state secondary battery according to claim 7 or 8, wherein primary particles are used for at least one of the large particle diameter particles and the small particle diameter particles.

10. A method for manufacturing an all-solid-state secondary battery according to any one of claims 7 to 9, wherein the ratio of Nb contained in the Nb-containing oxide layer to the entire positive electrode active material including the Nb-containing oxide layer on the surface of the positive electrode active material is 0.1 mass% or more and 2.5 mass% or less.

11. The method for producing an all-solid-state secondary battery according to claim 7, wherein the content of the positive electrode active material in the positive electrode mixture is 55 to 85 mass %.

12. A method for manufacturing an all-solid-state secondary battery according to any one of claims 7 to 11, wherein in preparing the positive electrode mixture, no resin binder is added, or the content of a resin binder is added so that it is 0.5 mass % or less.

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