All-solid-state secondary battery and method for manufacturing the same

Abstract

To provide an anode for all-solid secondary batteries of which the resistance value is low, a manufacturing method thereof, and an all-solid secondary battery using the anode.SOLUTION: An anode of all-solid secondary batteries comprises a compact of an anode mixture containing an anode material including an anode active material and a solid electrolyte. The anode material contains a carbon material as the anode active material, a layer containing an oxide having lithium ion conductivity is formed on the surface thereof, the amount of the oxide with respect to 100 pts.mass of the carbon material is 1 pt.mass or more, and a sulfide-based solid electrolyte is contained as the solid electrolyte. An all-solid secondary battery comprises a cathode, an anode and a solid electrolyte layer, which is interposed between the cathode and the anode, and includes the anode for all-solid secondary batteries as the anode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a negative electrode for an all-solid-state secondary battery with low resistance, a method for manufacturing the same, and an all-solid-state secondary battery using the negative electrode. [Background technology]

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

[0003] Currently, lithium secondary batteries, particularly lithium-ion secondary batteries, that can meet this requirement use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) as the positive electrode active material, graphite as the negative electrode active material, and an organic electrolyte containing an organic solvent and a lithium salt as the non-aqueous electrolyte.

[0004] Furthermore, with the continued development of devices that utilize lithium-ion secondary batteries, there is a growing demand for even longer lifespans, higher capacity, and higher energy density of lithium-ion secondary batteries.

[0005] For example, regarding increasing the capacity of lithium-ion secondary batteries (non-aqueous electrolyte secondary batteries) having an organic electrolyte, Patent Document 1 proposes a technique in which the surface of the carbon material, which is the negative electrode active material, is coated with a solid electrolyte that has lithium-ion conductivity, in order to reduce the irreversible capacity of the negative electrode.

[0006] Furthermore, lithium-ion secondary batteries are required to have longer lifespans, higher capacity, and higher energy density, as well as improved reliability.

[0007] However, because the organic electrolyte used in lithium-ion secondary batteries contains organic solvents, which are flammable substances, there is a possibility that the organic electrolyte may overheat if an abnormal situation such as a short circuit occurs in the battery. Furthermore, with the recent trend towards higher energy density in lithium-ion secondary batteries and an increase in the amount of organic solvents in the organic electrolyte, there is an even greater demand for the reliability of lithium-ion secondary batteries.

[0008] In light of the above circumstances, 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 solid electrolyte that does not use organic solvents instead of conventional organic solvent-based electrolytes, and they have a high level of safety as there is no risk of abnormal heat generation from the solid electrolyte.

[0009] Furthermore, various improvements have been attempted in all-solid-state secondary batteries. For example, Patent Document 2 proposes using a coated negative electrode active material in an all-solid-state secondary battery, in which the structural defects of the negative electrode active material having a graphite structure are coated with particulate lithium niobate with an average particle size of 1.5 nm or less, thereby suppressing heat generation and reducing resistance. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] International Publication No. 2017 / 169616 [Patent Document 2] Japanese Patent Publication No. 2017-54615 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] By the way, currently, in all-solid-state secondary batteries, the applicable fields are rapidly expanding. For example, since applications that require discharging at a large current value are also considered, it is necessary to improve the load characteristics to meet this requirement. Also, as a charging method for all-solid-state secondary batteries, it is common to perform charging (constant current charging) at a constant current until the battery voltage reaches a predetermined value, and then perform charging (constant voltage charging) at a constant voltage until the current value decreases and reaches a predetermined value. However, for example, to improve the rapid charging characteristics of all-solid-state secondary batteries, it is preferable that the charging capacity during constant current charging is large. Thus, as a means to improve the load characteristics and the charging capacity (CC capacity) during constant current charging of all-solid-state secondary batteries, for example, reducing the resistance value of the negative electrode is mentioned, so the development of technology to achieve this is required.

[0012] In Patent Document 2, as described above, it is stated that by coating the structural defect part of the negative electrode active material having a graphite structure with small particulate lithium niobate, the resistance of the negative electrode active material can be reduced. However, it cannot reduce the resistance value of the negative electrode to such an extent that the above characteristics of all-solid-state secondary batteries can be sufficiently improved.

[0013] The present invention has been made in view of the above circumstances, and its object is to provide a negative electrode for an all-solid-state secondary battery having a low resistance value, a manufacturing method thereof, and an all-solid-state secondary battery using the negative electrode.

Means for Solving the Problems

[0014] The negative electrode for an all-solid-state secondary battery of the present invention has a molded body of a negative electrode mixture containing a negative electrode material containing a negative electrode active material and a solid electrolyte. The negative electrode material contains a carbon material as a negative electrode active material, and a layer containing an oxide having lithium ion conductivity is formed on the surface. The amount of the oxide with respect to 100 parts by mass of the carbon material is 1 part by mass or more, and the solid electrolyte contains a sulfide-based solid electrolyte.

[0015] The negative electrode for an all-solid-state secondary battery of the present invention can be manufactured by the manufacturing method of the present invention, which is characterized by having the following negative electrode material forming step (A) and a step (B) of forming a molded body of a negative electrode mixture using the negative electrode material obtained through the negative electrode material forming step (A) and a sulfide-based solid electrolyte.

[0016] Here, the negative electrode material forming step (A) has either: (1) a step (i-1) of attaching an oxide having lithium ion conductivity or a material for forming the oxide to the surface of a carbon material, and a step (i-2) of firing the carbon material that has undergone the step (i-1) to form a layer containing the oxide on the surface of the carbon material such that the amount of the oxide relative to 100 parts by mass of the carbon material is 1 part by mass or more; or (2) a step (ii) of kneading a carbon material and an oxide having lithium ion conductivity to form a layer containing the oxide on the surface of the carbon material such that the amount of the oxide relative to 100 parts by mass of the carbon material is 1 part by mass or more.

[0017] Further, the all-solid-state secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, and is characterized by having the negative electrode for an all-solid-state secondary battery of the present invention as the negative electrode.

Effects of the Invention

[0018] According to the present invention, it is possible to provide a negative electrode for an all-solid-state secondary battery having a low resistance value, a manufacturing method thereof, and an all-solid-state secondary battery using the negative electrode.

Brief Description of the Drawings

[0019] [Figure 1] It is a cross-sectional view schematically showing an example of the all-solid-state secondary battery of the present invention. [Figure 2] It is a plan view schematically showing another example of the all-solid-state secondary battery of the present invention. [Figure 3] It is a cross-sectional view taken along line I-I of Figure 2.

Modes for Carrying Out the Invention

[0020] <Negative electrode for all-solid-state secondary batteries> The anode for an all-solid-state secondary battery of the present invention comprises a molded body of an anode mixture containing an anode material containing an anode active material and a solid electrolyte. The anode material used is a carbon material having a layer containing an oxide having lithium ion conductivity formed on its surface, and the solid electrolyte used is a sulfide-based solid electrolyte.

[0021] Carbon materials used as negative electrode active materials are generally hydrophobic and have low affinity for sulfide-based solid electrolytes. Therefore, in negative electrodes (molded bodies of negative electrode mixtures) containing these materials, it is difficult to form a good interface between the two, making it difficult to lower the resistance value of the negative electrode.

[0022] Therefore, in this invention, a carbon material having a layer containing an oxide having lithium ion conductivity formed on its surface is used as the negative electrode material. With this negative electrode material, the affinity with sulfide-based solid electrolytes is high and a good interface can be formed. Thus, the negative electrode for the all-solid-state secondary battery of this invention has a low resistance value and high lithium ion acceptance, making it possible to increase the CC capacity of the all-solid-state secondary battery using it (i.e., the all-solid-state secondary battery of this invention) and also improve the load characteristics.

[0023] Examples of negative electrodes for all-solid-state secondary batteries include molded bodies (such as pellets) formed by molding a negative electrode mixture, and structures in which a layer made of molded negative electrode mixture bodies (negative electrode mixture layer) is formed on a current collector.

[0024] Examples of carbon materials constituting the negative electrode material include graphite (natural graphite; artificial graphite obtained by graphitizing easily graphitizable carbon such as pyrolysis carbons, mesophase carbon microbeads, and carbon fibers at 2800°C or higher; etc.), easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), pyrolysis carbons, cokes, glassy carbons, calcined organic polymer compounds, mesophase carbon microbeads, carbon fibers, activated carbon, etc., and one or more of these can be used.

[0025] The oxides that make up the layer formed on the surface of the negative electrode material are lithium ion conductive, such as lithium niobium oxide (LiNbO3, etc.) and lithium titanium oxide (LiTi5O3, etc.). 12 Examples include lithium phosphate oxide (Li3PO4, etc.), lithium boron oxide (Li3BO3, etc.), lithium tungsten oxide (Li4WO5, etc.), and lithium aluminum oxide (LiAlO2, etc.). Note that Li3PO4 is an oxide-based solid electrolyte, but other oxide-based solid electrolytes include, for example, Li7La3Zr2O 12 LiTi(PO4)3, LiGe(PO4)3, and LiLaTiO3 can also be used as constituent materials for the layer formed on the surface of the negative electrode material.

[0026] The amount of lithium-ion conductive oxide in the negative electrode material is preferably 1 part by mass or more, and more preferably 1.5 parts by mass or more, per 100 parts by mass of carbon material, from the viewpoint of ensuring a good resistance reduction effect for the negative electrode of an all-solid-state secondary battery. However, if the amount of oxide is too large, the capacity of the negative electrode active material may decrease, or the conductivity of the molded negative electrode mixture may decrease. Therefore, the amount of lithium-ion conductive oxide in the negative electrode material is preferably 20 parts by mass or less, and more preferably 10 parts by mass or less, per 100 parts by mass of carbon material.

[0027] The oxide contained in the layer formed on the surface of the negative electrode material is preferably amorphous, as this further enhances the effect of the CC capacity of the all-solid-state secondary battery in improving its load characteristics. The amorphous nature of the oxide can be confirmed, for example, by observing whether there are any peaks representing the crystal structure of the oxide when X-ray diffraction (XRD) is performed, or whether the peaks are broad.

[0028] The negative electrode mixture in the negative electrode for all-solid-state secondary batteries can contain, together with the negative electrode material, other negative electrode active materials commonly used in lithium-ion secondary batteries. However, when using other negative electrode active materials together with the negative electrode material in the negative electrode active material, it is preferable that the proportion of the negative electrode material in all the negative electrode active materials is 60% by mass or more. Note that since the negative electrode mixture in the negative electrode for all-solid-state secondary batteries may not use negative electrode active materials other than the negative electrode material, the preferable upper limit value of the proportion of the negative electrode material in all the negative electrode active materials is 100% by mass.

[0029] The content of all the negative electrode active materials including the negative electrode material in the negative electrode mixture is preferably 30 to 70% by mass.

[0030] Examples of the sulfide-based solid electrolyte in the negative electrode for all-solid-state secondary batteries 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 argyrodite-based materials [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, sulfide-based solid electrolytes containing lithium and phosphorus are preferable because of their high lithium ion conductivity, and argyrodite-based materials with particularly high lithium ion conductivity and high chemical stability are more preferable.

[0031] The average particle size of the sulfide-based solid electrolyte is preferably 0.1 μm or larger, and more preferably 0.2 μm or larger, from the viewpoint of reducing grain boundary resistance. On the other hand, from the viewpoint of forming a sufficient contact interface between the negative electrode material and the solid electrolyte, it is preferably 10 μm or smaller, and more preferably 5 μm or smaller.

[0032] Solid electrolytes as defined herein and Positive The average particle size of the highly active material is the 50% diameter value in the volume-based integrated fraction when calculating the integrated volume from the smallest particles using a particle size distribution analyzer (such as the Microtrac particle size distribution analyzer "HRA9320" manufactured by Nikkiso Co., Ltd.). 50 ) means.

[0033] For the negative electrode of an all-solid-state secondary battery, other solid electrolytes (such as hydride-based solid electrolytes and oxide-based solid electrolytes) can be used in addition to sulfide-based solid electrolytes. However, it is preferable that the proportion of solid electrolytes other than sulfide-based solid electrolytes in the negative electrode of an all-solid-state battery be 30% by mass or less of the total amount of solid electrolyte particles. Note that all solid electrolytes in the negative electrode of an all-solid-state secondary battery may be sulfide-based solid electrolytes, so the lower limit of the proportion of solid electrolytes other than sulfide-based solid electrolytes in the total amount of solid electrolyte is 0% by mass.

[0034] Examples of hydride-based solid electrolytes include LiBH4, L i Examples include solid solutions of BH4 with the following alkali metal compounds (for example, those with a molar ratio of LiBH4 to alkali metal compound of 1:1 to 20:1). Examples of alkali metal compounds in the solid solution include lithium halides (LiI, LiBr, LiF, LiCl, etc.) and rubidium halides (RbI, RbBr, Rb F Examples include at least one selected from the group consisting of , RbCl, cesium halides (CsI, CsBr, CsF, CsCl, etc.), lithium amide, rubidium amide, and cesium amide.

[0035] Examples of oxide-based solid electrolytes include Li7La3Zr2O 12 Examples include LiTi(PO4)3, LiGe(PO4)3, and LiLaTiO3.

[0036] It is preferable that the average particle size of solid electrolytes other than sulfide-based solid electrolytes be approximately the same as that of sulfide-based solid electrolytes.

[0037] The solid electrolyte content in the negative electrode mixture is preferably 4 to 70% by mass.

[0038] The negative electrode mixture may also contain conductive additives such as carbon black and graphene, as needed. When conductive additives are included in the negative electrode mixture, their content is preferably 1 to 10% by mass.

[0039] 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 resistive component in the negative electrode mixture, it is desirable to keep its amount as small as possible. Therefore, it is preferable that the negative electrode mixture does not contain a resin binder, or if it does, its content is 0.5% by mass or less. It is more preferable that the resin binder content in the negative electrode mixture be 0.3% by mass or less, and even more preferable that it be 0% by mass (i.e., no resin binder is included).

[0040] When a current collector is used in the negative electrode of an all-solid-state secondary battery, the current collector can be made of copper or nickel foil, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc.

[0041] A negative electrode for an all-solid-state secondary battery can be manufactured, for example, by a manufacturing method comprising a negative electrode material formation step (A), and a step (B) of forming a molded body of a negative electrode mixture using the negative electrode material obtained through the negative electrode material formation step (A) and a sulfide-based solid electrolyte.

[0042] The negative electrode material formation step (A) includes, for example, a step (i-1) of attaching an oxide having lithium ion conductivity or a material for forming the oxide to the surface of a carbon material, and a step (i-2) of firing the carbon material that has undergone step (i-1) to form a layer containing the oxide on the surface of the carbon material to obtain the negative electrode material.

[0043] In step (i-1), methods for attaching an oxide having lithium ion conductivity to the surface of the carbon material include applying a composition prepared by dissolving or dispersing the oxide in a solvent to the surface of the carbon material; or dry mixing the carbon material and the oxide. In addition, in step (i-1), methods for attaching a material for forming the oxide to the surface of the carbon material include applying a composition prepared by dissolving or dispersing the material in a solvent to the surface of the carbon material.

[0044] As solvents in compositions containing the oxide or materials for forming the oxide, the following can be used: alcohols such as methanol and ethanol; nonpolar aprotic solvents represented by hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene; and so on. If the oxide or materials for forming the oxide contained in the composition are highly reactive with water, it is preferable to use a dehydrated solvent (super dehydrated solvent) with a water content of 0.001% by mass (10 ppm) or less. On the other hand, if the oxide or materials for forming the oxide are not highly reactive with water, water can be used as the solvent in the composition.

[0045] There are no particular restrictions on the method of applying the composition to the carbon material, and various known application methods can be used. Similarly, there are no particular restrictions on the method of dry mixing the oxide and the carbon material, and various known mixing methods can be used.

[0046] In step (i-2), the carbon material obtained through step (i-1) is fired to form a layer containing an oxide having lithium ion conductivity on the surface of the carbon material. If, in step (i-1), a composition containing the material for forming the oxide is applied to the carbon material, then in step (i-2), the material for forming the oxide is reacted to synthesize the oxide while simultaneously forming the layer containing the oxide.

[0047] There are no particular restrictions on the firing method in step (i-2), and various known firing methods can be used. i-2 The firing temperature in step ( ) is preferably a temperature at which the oxide can form a layer in an amorphous state, specifically preferably 450°C or lower, and preferably 300°C or higher. i-2 The firing time in ) is preferably 0.5 to 3 hours.

[0048] Furthermore, the negative electrode material formation step (A) may also include a step (ii) in which a carbon material and a lithium-ion conductive oxide are kneaded together to form a layer containing the oxide on the surface of the carbon material, instead of steps (i-1) and (i-2).

[0049] In step (ii), a carbon material and a lithium-ion conductive oxide are kneaded, i.e., mixed while shearing, to form a layer containing the lithium-ion conductive oxide on the surface of the carbon material, thereby obtaining a negative electrode material. There are no particular restrictions on the method of kneading the carbon material and the oxide, as long as it is a method that can mix the two while shearing, and methods using various known apparatuses can be employed.

[0050] Next, in step (B), a molded body of the negative electrode mixture is formed using the negative electrode material obtained in the negative electrode material formation step (A) and a sulfide-based solid electrolyte.

[0051] A molded negative electrode mixture can be formed, for example, by compressing a negative electrode mixture prepared by mixing the negative electrode material, a sulfide-based solid electrolyte, and optionally added conductive additives and binders, using pressure molding or the like. If the negative electrode for an all-solid-state secondary battery is composed solely of a molded negative electrode mixture, the negative electrode for an all-solid-state secondary battery can be obtained by this step (B).

[0052] On the other hand, if the negative electrode for an all-solid-state secondary battery has a current collector, the negative electrode for an all-solid-state secondary battery can be obtained by bonding the molded negative electrode mixture obtained in step (B) to the current collector by pressing or other means.

[0053] The thickness of the molded negative electrode mixture (in the case of a negative electrode with a current collector, the thickness of the molded positive electrode mixture per side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the battery capacity. Generally, the load characteristics of a battery can be easily improved by making the positive or negative electrode thinner, but according to the present invention, it is possible to improve the load characteristics even when the molded negative electrode mixture is thick, such as 200 μm or more. Therefore, in the present invention, the effect becomes more pronounced when the thickness of the molded negative electrode mixture is, for example, 200 μm or more. In addition, the thickness of the molded negative electrode mixture is usually 3000 μm or less.

[0054] <All-solid-state secondary battery> The all-solid-state secondary battery of the present invention comprises a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, wherein the negative electrode is the negative electrode for the all-solid-state secondary battery of the present invention.

[0055] Figure 1 shows a schematic cross-sectional view of an example of the all-solid-state secondary battery of the present invention. The all-solid-state secondary battery 1 shown in Figure 1 has an outer casing formed by an outer casing 40, a sealing casing 50, and a resin gasket 60 interposed between them, within which 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 are sealed.

[0056] 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 come into contact with the sealing can 50, thereby sealing the opening of the outer can 40 and creating a sealed structure inside the battery.

[0057] Stainless steel can be used for the outer casing and sealing can. Polypropylene and nylon can be used as gasket materials, and if heat resistance is required due to the battery's application, heat-resistant resins with melting points exceeding 240°C, such as fluororesins like tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), can also be used. Furthermore, if the battery is used in an application requiring heat resistance, a glass hermetic seal can be used for sealing.

[0058] Figures 2 and 3 also show schematic diagrams illustrating other examples of the all-solid-state secondary battery of the present invention. Figure 2 is a plan view of the all-solid-state secondary battery, and Figure 3 is a cross-sectional view taken along line II of Figure 2.

[0059] The all-solid-state secondary battery 100 shown in Figures 2 and 3 houses an electrode body 200 consisting of a positive electrode, a solid electrolyte layer, and the negative electrode of the present invention within a laminate film casing 500 made of two metal laminate films. The laminate film casing 500 is sealed by heat-sealing the upper and lower metal laminate films at its outer periphery. In Figure 3, to avoid complexity in the drawing, the individual layers constituting the laminate film casing 500, as well as the positive electrode, negative electrode, and separator constituting the electrode body, are not shown separately.

[0060] The positive electrode of the electrode body 200 is connected to the positive electrode external terminal 300 inside the battery 100. Although not shown in the figure, the negative electrode of the electrode body 200 is also connected to the negative electrode external terminal 400 inside the battery 100. The positive electrode external terminal 300 and the negative electrode external terminal 400 are drawn out to the outside of the laminate film exterior body 500 at one end side so as to be connectable to external devices and the like.

[0061] (Positive electrode) The positive electrode of the all-solid-state secondary battery has, for example, a molded body of a positive electrode mixture containing a positive electrode active material, a conductive auxiliary agent, a solid electrolyte, etc., and examples thereof include a positive electrode composed only of the molded body and a positive electrode having a structure in which the molded body and the current collector are integrated.

[0062] [[ID=I0]]The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in a conventionally known lithium-ion secondary battery, that is, an active material capable of occluding and releasing Li ions. Specific examples of the positive electrode active material include LiM x Mn 2-x O4 (where M is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and 0.01 ≦ x ≦ 0.5), spinel-type lithium manganese composite oxide represented by Li x Mn (1-y-x) Ni y M z O (2-k) F l (where M is at least one element selected from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, 0.8 ≦ x ≦ 1.2, 0 < y < 0.5, 0 ≦ z ≦ 0.5, k + l < 1, -0.1 ≦ k ≦ 0.2, 0 ≦ l ≦ 0.1), layered compound represented by LiCo 1-x M x O2 (where M is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0 ≦ x ≦ 0.5), lithium cobalt composite oxide represented by LiNi1-x M x Lithium nickel composite oxide represented as O2 (where M is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0 ≤ x ≤ 0.5), LiM 1-x N x Li4Ti5O is an olivine-type composite oxide represented as PO4 (where M is at least one element selected from the group consisting of Fe, Mn, and Co, and N is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0 ≤ x ≤ 0.5). 12 Examples include lithium titanium composite oxides represented by [formula], and one of these may be used alone, or two or more may be used in combination.

[0063] The average particle size of the positive electrode active material is preferably 1 μm or more, more preferably 2 μm or more, and preferably 10 μm or less, and more preferably 8 μm or less. The positive electrode active material may be primary particles or secondary particles formed by aggregation of primary particles. Using a positive electrode active material with an average particle size within the above range allows for a larger interface with the solid electrolyte, thereby improving the load characteristics of the battery.

[0064] The positive electrode active material preferably has a reaction-inhibiting layer on its surface to suppress its reaction with the solid electrolyte.

[0065] In a molded positive electrode mixture, direct contact between the positive electrode active material and the solid electrolyte can cause the solid electrolyte to oxidize, forming a resistance layer and potentially reducing the ionic conductivity within the molded body. By providing a reaction-inhibiting layer on the surface of the positive electrode active material to suppress its reaction with the solid electrolyte, direct contact between the positive electrode active material and the solid electrolyte can be prevented, thereby suppressing the reduction in ionic conductivity within the molded body due to oxidation of the solid electrolyte.

[0066] The reaction suppression layer should be composed of a material that has ionic conductivity and can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of materials that can constitute the reaction suppression layer include oxides containing Li and at least one element selected from the group consisting of Nb, P, B, Si, Ge, Ti, and Zr, more specifically, Nb-containing oxides such as LiNbO3, Li3PO4, Li3BO3, Li4SiO4, Li4GeO4, LiTiO3, and LiZrO3. The reaction suppression layer may contain only one of these oxides, or two or more, and furthermore, multiple of these oxides may form a composite compound. Among these oxides, it is preferable to use an Nb-containing oxide, and more preferable to use LiNbO3.

[0067] The reaction-inhibiting layer is preferably present on the surface in an amount of 0.1 to 1.0 part by mass per 100 parts by mass of positive electrode active material. Within this range, the reaction between the positive electrode active material and the solid electrolyte can be effectively suppressed.

[0068] Methods for forming a reaction-inhibiting layer on the surface of the positive electrode active material include the sol-gel method, mechanofusion method, CVD method, and PVD method.

[0069] The content of the positive electrode active material in the positive electrode mixture is preferably 60 to 95% by mass.

[0070] Examples of conductive additives for the positive electrode include graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofibers, carbon nanotubes, and other carbon materials. The content of the conductive additive in the positive electrode mixture is preferably 1 to 10% by mass.

[0071] For the positive electrode solid electrolyte, one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, and oxide-based solid electrolytes exemplified earlier as suitable for use in the negative electrode can be used. To improve battery characteristics, it is desirable to include a sulfide-based solid electrolyte.

[0072] The solid electrolyte content in the positive electrode mixture is preferably 4 to 30% by mass.

[0073] 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 resistive component in the positive electrode mixture, it is desirable to keep its amount as small as possible. Therefore, it is preferable that the positive electrode mixture does not contain a resin binder, or if it does, its content is 0.5% by mass or less. It is more preferable that the resin binder content in the positive electrode mixture be 0.3% by mass or less, and even more preferable that it be 0% by mass (i.e., no resin binder is included).

[0074] When a current collector is used for the positive electrode, the current collector can be made of metal foil such as aluminum or stainless steel, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc.

[0075] A molded 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 a binder added as needed, using pressure molding or the like.

[0076] In the case of a positive electrode having a current collector, it can be manufactured by bonding a molded positive electrode mixture formed by the method described above to the current collector by pressing or other means.

[0077] The thickness of the molded positive electrode mixture (in the case of a positive electrode with a current collector, the thickness of the molded positive electrode mixture per side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the battery capacity. Furthermore, the thickness of the molded positive electrode mixture is usually 2000 μm or less.

[0078] (Solid electrolyte layer) The solid electrolyte in the solid electrolyte layer may be suitable for use as a negative electrode. and One or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, and oxide-based solid electrolytes exemplified above can be used. However, to improve battery characteristics, it is desirable to include a sulfide-based solid electrolyte, and it is even more desirable to include a sulfide-based solid electrolyte in the positive electrode, negative electrode, and solid electrolyte layer.

[0079] The solid electrolyte layer may have a porous material, such as a resin nonwoven fabric, as a support.

[0080] The solid electrolyte layer can be formed by methods such as compressing a solid electrolyte by pressure molding; or by applying a solid electrolyte layer-forming composition, prepared by dispersing a solid electrolyte in a solvent, onto a substrate, positive electrode, or negative electrode, drying it, and then performing pressure molding such as a press treatment as needed.

[0081] When selecting a solvent for a composition that forms a solid electrolyte layer, it is preferable to choose one that does not easily degrade the solid electrolyte. In particular, since sulfide-based solid electrolytes and hydride-based solid electrolytes undergo chemical reactions with even trace amounts of water, it is preferable to use nonpolar aprotic solvents such as hydrocarbon solvents like hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene. It is especially preferable to use an ultra-dehydrated solvent with a water content of 0.001% by mass (10 ppm) or less. In addition, fluorine-based solvents such as "Bartrell®" from Mitsui DuPont Fluorochemicals, "Zeolora®" from Nippon Zeon Corporation, and "Novec®" from Sumitomo 3M Corporation, as well as non-aqueous organic solvents such as dichloromethane and diethyl ether, can also be used.

[0082] The thickness of the solid electrolyte layer is preferably 100 to 300 μm.

[0083] (electrode body) The positive and negative electrodes can be used in batteries in the form of a laminated electrode body, which is formed by stacking solid electrolyte layers, or a wound electrode body, which is formed by winding this laminated electrode body.

[0084] Furthermore, when forming the electrode body, it is preferable to pressure-molde 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.

[0085] (Battery type) The form of an all-solid-state secondary battery may include an outer casing composed of an outer casing, a sealing casing, and a gasket, as shown in Figure 1, that is, the form generally referred to as a coin-type battery or button-type battery, or an outer casing composed of a resin film or a metal-resin laminate film, as shown in Figures 2 and 3. In addition, an outer casing may have a metal, bottomed cylindrical (cylindrical or rectangular) outer casing and a sealing structure that seals its opening.

[0086] The all-solid-state secondary battery of the present invention can be applied to the same uses as conventionally known secondary batteries, but because it has a solid electrolyte instead of an organic electrolyte, it has excellent heat resistance and can be preferably used in applications that are exposed to high temperatures. [Examples]

[0087] The present invention will be described in detail below based on examples. However, the following examples are not intended to limit the present invention.

[0088] Example 1 (Formation of a solid electrolyte layer) 80 mg of sulfide-based solid electrolyte (Li6PS5Cl) was placed in a powder molding die with a diameter of 10 mm, and a solid electrolyte layer was formed by pressure molding using a press machine.

[0089] (Fabrication of the negative electrode) A coating layer-forming composition was prepared by mixing 0.1 mol of lithium and 0.125 mol of titanium tetraisopropoxide in 500 mL of dehydrated ethanol. Next, the coating layer-forming composition was applied to 500 g of graphite at a rate of 2 g per minute for 180 minutes using a coating apparatus with a rolling fluidized bed. The resulting powder was calcined at 400 °C for 30 minutes to produce a lithium ion conductive oxide (LiTi5O12 A negative electrode material (1) was obtained, consisting of graphite with a coating layer containing ) formed on its surface. The amount of oxide in the negative electrode material (1) was 1.91 parts by mass per 100 parts by mass of graphite. Furthermore, it was confirmed by the above method that the oxide constituting the coating layer in the negative electrode material (1) was amorphous.

[0090] The negative electrode material (1), graphene, and the same sulfide-based solid electrolyte used in the solid electrolyte layer were mixed in a mass ratio of 45:5:50 and thoroughly kneaded to prepare the negative electrode mixture. Next, 15 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 machine to form a negative electrode consisting of a molded negative electrode mixture on top of the solid electrolyte layer.

[0091] (Formation of a multilayer electrode) As the counter electrode, a compound was used which was formed by molding Li metal and In metal into cylindrical shapes and bonding them together. This counter electrode was placed on the side of the solid electrolyte layer in the powder molding die opposite to the negative electrode, and a laminated electrode body was fabricated by pressure molding using a press machine.

[0092] (Assembly of model cells) Using the aforementioned laminated electrode body, an all-solid-state battery (model cell) with a planar structure similar to that shown in Figure 2 was fabricated. A negative electrode current collector foil (SUS foil) and a counter electrode current collector foil (SUS foil) were attached side by side to the inner surface of the aluminum laminate film that constitutes the laminate film outer casing, with a certain amount of space between them. Each current collector foil was cut to a shape that included a main body portion facing the negative electrode side surface or the counter electrode side surface of the laminated electrode body, and portions that protrude from the main body portion toward the outside of the battery as the negative electrode external terminal 400 and the counter electrode external terminal 300.

[0093] The laminated electrode body was placed on the negative electrode current collector foil of the laminated film casing, the laminated electrode body was wrapped with the laminated film casing so that the counter electrode current collector foil was positioned on the counter electrode of the laminated electrode body, and the remaining three sides of the laminated film casing were sealed by heat fusion under vacuum to obtain a model cell.

[0094] Example 2 A coating layer-forming composition was prepared by mixing 0.1 mol of lithium and 0.1 mol of niobium pentaoxide in 500 mL of dehydrated ethanol. Next, the coating layer-forming composition was applied to 500 g of graphite at a rate of 2 g / min for 184 minutes using a coating apparatus with a rolling fluidized bed. The resulting powder was calcined at 400 °C to obtain a negative electrode material (2) consisting of graphite with a coating layer containing a lithium-ion conductive oxide (LiNbO3) formed on its surface. The amount of oxide in the negative electrode material (2) was 2.55 parts by mass per 100 parts by mass of graphite. Furthermore, it was confirmed by the above method that the oxide constituting the coating layer in the negative electrode material (2) was amorphous.

[0095] A negative electrode was prepared in the same manner as in Example 1, except that negative electrode material (2) was used instead of negative electrode material (1), and a model cell was prepared in the same manner as in Example 1, except that this negative electrode was used.

[0096] Example 3 A negative electrode material (3) was prepared in the same manner as in Example 2, except that the coating layer formation composition was applied to the graphite at a rate of 2 g per minute for 132 minutes. The amount of oxide in the negative electrode material (3) was 1.83 parts by mass per 100 parts by mass of graphite. Furthermore, it was confirmed that the oxide constituting the coating layer in the negative electrode material (3) was amorphous using the method described above.

[0097] A negative electrode was prepared in the same manner as in Example 1, except that negative electrode material (3) was used instead of negative electrode material (1), and a model cell was prepared in the same manner as in Example 1, except that this negative electrode was used.

[0098] Example 4 A negative electrode material (4) was prepared in the same manner as in Example 2, except that the coating layer formation composition was applied to the graphite at a rate of 2 g per minute for 72 minutes. The amount of oxide in the negative electrode material (4) was 1 part by mass per 100 parts by mass of graphite. Furthermore, it was confirmed that the oxide constituting the coating layer in the negative electrode material (4) was amorphous using the method described above.

[0099] A negative electrode was prepared in the same manner as in Example 1, except that negative electrode material (4) was used instead of negative electrode material (1), and a model cell was prepared in the same manner as in Example 1, except that this negative electrode was used.

[0100] Example 5 20g of graphite and 1g of Li3PO4 were thoroughly kneaded to obtain a negative electrode material (5) consisting of graphite with a coating layer containing a lithium ion conductive oxide (Li3PO4) formed on its surface. The amount of oxide in the negative electrode material (5) was 5 parts by mass per 100 parts by mass of graphite.

[0101] A negative electrode was prepared in the same manner as in Example 1, except that negative electrode material (5) was used instead of negative electrode material (1), and a model cell was prepared in the same manner as in Example 1, except that this negative electrode was used.

[0102] Example 6 The coating layer-forming composition was applied to 300g of hard carbon at a rate of 4g per minute for 190 minutes. The resulting powder was calcined at 400°C for 30 minutes to produce a lithium-ion conductive oxide (LiTi5O). 12 A negative electrode material (6) was obtained, which consisted of hard carbon with a coating layer containing ) formed on its surface. The amount of oxide in the negative electrode material (6) was 6.72 parts by mass per 100 parts by mass of hard carbon.

[0103] A negative electrode was prepared in the same manner as in Example 1, except that negative electrode material (6) was used instead of negative electrode material (1), and a model cell was prepared in the same manner as in Example 1, except that this negative electrode was used.

[0104] Comparative Example 1 A negative electrode material (7) was produced in the same manner as in Example 2, except that the coating composition for forming the coating layer on graphite was applied at a rate of 2 g per minute for 36 minutes. The amount of the oxide in the negative electrode material (7) was 0.5 parts by mass with respect to 100 parts by mass of graphite. Also, in the negative electrode material (7), it was confirmed by the above method that the oxide constituting the coating layer was amorphous.

[0105] A negative electrode was produced in the same manner as in Example 1, except that the negative electrode material (7) was used instead of the negative electrode material (1), and a model cell was produced in the same manner as in Example 1, except that this negative electrode was used.

[0106] Comparative Example 2 A negative electrode was produced in the same manner as in Example 1, except that graphite without a coating layer formed on its surface was used instead of the negative electrode material (1), and a model cell was produced in the same manner as in Example 1, except that this negative electrode was used.

[0107] Comparative Example 3 A negative electrode was produced in the same manner as in Example 1, except that hard carbon without a coating layer formed on its surface was used instead of the negative electrode material (6), and a model cell was produced in the same manner as in Example 1, except that this negative electrode was used. <00004 / 14> The following evaluations were performed on the model cells having the negative electrodes of the examples and comparative examples.

[0109] <CC Capacity Evaluation> For each model cell, constant current charging was performed at a current value of 0.05C until the voltage reached 0.62V under a pressure of 1t / cm 2 ), followed by constant voltage charging at a voltage of 0.62V until the current value reached 0.01C, and then discharging at a current value of 0.05C until the voltage reached 1.88V. A series of these steps was repeated twice, and the capacity during the second constant current charging and the capacity during the second discharging (initial capacity) were determined. Then, the value obtained by dividing the capacity during the second constant current charging of each model cell by the initial capacity was expressed as a percentage to evaluate the CC capacity.<00004ZO>

[0110] <DC Resistance (DCR) Measurement> Each model cell after initial volume measurement is pressurized (1 t / cm²) at a temperature of 23°C. 2 With the cells in the same state as during the initial capacity measurement, constant current and constant voltage charging were performed. After that, the cells were discharged at a current of 0.1C until the depth of charge (SOC) reached 50%, and then left to rest for 1 hour. For each model cell thereafter, a 10-second pulse discharge was performed at a current of 0.1C, and the voltage was measured. The voltage was calculated by subtracting the voltage rise attributed to the solid electrolyte layer and the counter electrode from the voltage difference before and after the pulse discharge, and the DCR was calculated from this value.

[0111] The smaller the DCR calculated using this method, the lower the negative electrode used in the model cell, which indicates that it is possible to construct an all-solid-state secondary battery with low internal resistance, excellent load characteristics, and a large CC capacity.

[0112] The evaluation results mentioned above, along with the composition of the negative electrode material, are shown in Table 1.

[0113] [Table 1]

[0114] The model cells fabricated in Examples 1-6 used a negative electrode consisting of a molded body of a negative electrode mixture containing a layer of lithium-ion conductive oxide on the surface of a carbon material and a sulfide-based solid electrolyte, resulting in a low DCR. Therefore, by using these negative electrodes, the load characteristics and CC capacity of all-solid-state secondary batteries can be improved.

[0115] In contrast, the model cell of Comparative Example 1 used a negative electrode containing a negative electrode material with a low amount of oxide on the surface, and the model cells of Comparative Examples 2 and 3 used a negative electrode containing a carbon material without a layer containing oxide on the surface. However, the DCR in these cases was higher than that of the example. [Explanation of Symbols]

[0116] 1,100 All-solid-state secondary battery 10 positive electrode 20 negative electrode 30 Solid electrolyte layer 40 outer cans 50 sealed cans 60 Gasket 200 Electrode body 300 Positive external terminal 400 Negative external terminal 500 Laminate film outer casing

Claims

1. An all-solid-state secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, The aforementioned negative electrode has a molded body of a negative electrode mixture containing a negative electrode material containing a negative electrode active material and a solid electrolyte. The aforementioned negative electrode material contains a carbon material as a negative electrode active material, and has a layer formed on its surface that contains an oxide having lithium ion conductivity. The layer containing the lithium-ion conductive oxide includes, as the lithium-ion conductive oxide, lithium niobium oxide, lithium titanium oxide, lithium tungsten oxide, Li7La3Zr2O12, LiTi(PO4)3, LiGe(PO4)3, or LiLaTiO3. The amount of the oxide relative to 100 parts by mass of the carbon material is 1 part by mass or more. The solid electrolyte content in the aforementioned negative electrode mixture is 4 to 70% by mass. The solid electrolyte contains a sulfide-based solid electrolyte, The all-solid-state secondary battery is characterized in that the solid electrolyte layer has a thickness of 100 to 300 μm.

2. The all-solid-state secondary battery according to claim 1, wherein the negative electrode active material is hard carbon.

3. A method for manufacturing an all-solid-state secondary battery according to claim 1 or 2, The following negative electrode material formation process (A), and A method for manufacturing an all-solid-state secondary battery, characterized by comprising a step (B) of forming a molded body of a negative electrode mixture using the negative electrode material obtained through the negative electrode material formation step (A) and a sulfide-based solid electrolyte. The negative electrode material formation step (A) is: The process comprises: (i-1) attaching a lithium-ion conductive oxide or a material for forming the oxide to the surface of a carbon material; and (i-2) firing the carbon material after step (i-1) to form a layer containing the oxide on the surface of the carbon material such that the amount of the oxide is 1 part by mass or more per 100 parts by mass of the carbon material, or The process includes a step (ii) of kneading a carbon material with an oxide having lithium ion conductivity to form a layer containing the oxide on the surface of the carbon material such that the amount of the oxide is 1 part by mass or more per 100 parts by mass of the carbon material.

4. The method for manufacturing an all-solid-state secondary battery according to claim 3, wherein in the above step (i-2), firing is performed at a temperature of 450°C or lower.

Images