WO2025258604A1 - All-solid-state secondary battery - Google Patents

Abstract

Provided is a high-capacity all-solid-state secondary battery.　This all-solid-state secondary battery is characterized by: including a positive electrode, a negative electrode, and a solid-state electrolyte layer disposed between the positive electrode and the negative electrode; and an intermediate layer, containing a fibrous resin and a solid-state electrolyte, being interposed between the negative electrode and the solid-state electrolyte layer.

Description

All-solid-state secondary battery

The present invention relates to a high-capacity all-solid-state secondary battery.

Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries that use organic electrolytes, are widely used as secondary batteries. However, in recent years, as their application fields have expanded, there has been a demand for higher capacity and use in high-temperature environments, which has led to increased demands for improved safety, for example.

For these reasons, attention is being drawn to all-solid-state secondary batteries, which can ensure excellent heat resistance by using molded solid electrolytes instead of organic electrolytes that use organic solvents, which are flammable liquids.

Various studies are currently being conducted on all-solid-state secondary batteries. For example, Patent Document 1 proposes forming a solid electrolyte sheet that is strong and has excellent shape retention by retaining solid electrolyte particles inside an insulating porous substrate made of fibrous material. By forming the solid electrolyte layer of an all-solid-state secondary battery using the solid electrolyte sheet described in Patent Document 1, it becomes possible to increase the area of the solid electrolyte layer, thereby enabling the production of all-solid-state secondary batteries with higher capacity and higher energy density.

International Publication No. 2020/054081

When an all-solid-state secondary battery is charged and discharged, the volume of the electrodes changes due to the absorption and desorption reaction of Li (lithium) ions. This causes shear stress to be applied to the solid electrolyte layer, which may have a negative impact on the internal structure of the solid electrolyte layer. In particular, when a high-capacity negative electrode active material is used in the negative electrode, a large force is applied to the solid electrolyte layer during the initial charge and discharge, making it easy for cracks to form parallel to the surface within the layer. When cracks occur parallel to the surface of the solid electrolyte layer, the ionic conduction path between the positive and negative electrodes is lost, which can make it difficult to fully utilize the battery's inherent capacity.

Therefore, in all-solid-state secondary batteries, there is a need to develop technology that effectively suppresses capacity loss due to cracks occurring inside the solid electrolyte layer, which can occur due to volume changes in the negative electrode, particularly during the initial charge/discharge cycle.

The present invention was made in consideration of the above circumstances, and aims to provide a high-capacity 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 disposed between the positive electrode and the negative electrode, and is characterized in that an intermediate layer containing a fibrous resin and a solid electrolyte is interposed between the negative electrode and the solid electrolyte layer.

The present invention makes it possible to provide a high-capacity all-solid-state secondary battery.

FIG. 1 is a cross-sectional view schematically illustrating an example of an all-solid-state secondary battery of the present invention. FIG. 2 is a plan view schematically illustrating another example of the all-solid-state secondary battery of the present invention. 3 is a cross-sectional view taken along line II in FIG. 2.

The all-solid-state secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode, with an intermediate layer containing a fibrous resin and a solid electrolyte interposed between the negative electrode and the solid electrolyte layer.

In the all-solid-state secondary battery of the present invention, the fibrous resin contained in the intermediate layer interposed between the negative electrode and the solid electrolyte layer reduces the shear stress associated with volumetric changes in the negative electrode, making it less likely to propagate to the solid electrolyte layer. In the all-solid-state secondary battery of the present invention, the above-mentioned effect of the intermediate layer effectively prevents cracks from occurring inside the solid electrolyte layer due to volumetric changes in the negative electrode during the initial charge/discharge cycle, even when a high-capacity negative electrode active material is used.

While volume changes in the negative electrode during the initial charge/discharge cycle adversely affect the internal structure of the solid electrolyte layer, volume changes in the positive electrode have only a limited effect on the solid electrolyte layer. Therefore, in the battery of the present invention, the intermediate layer containing a fibrous resin that may inhibit ion conduction is positioned only between the negative electrode and the solid electrolyte layer. This allows the all-solid-state secondary battery of the present invention to be configured so that the positive electrode can be in direct contact with the solid electrolyte layer, thereby maximizing ionic conductivity between the positive electrode and the solid electrolyte layer.

In the present invention, the above-mentioned effects make it possible to more effectively utilize the inherent capacity of all-solid-state secondary batteries, thereby achieving high capacity.

The all-solid-state secondary battery of the present invention is described in detail below.

(middle class)
The intermediate layer contains a fibrous resin and a solid electrolyte.

Fiber-like resins include fibrous resins such as polyolefins such as polypropylene and polyethylene; polystyrene; aramid; polyamide-imide; polyimide; nylon; polyesters such as polyethylene terephthalate (PET); polyarylate; cellulose and modified cellulose; and fluororesins such as polytetrafluoroethylene (PTFE). One or more of these can be used. The fibrous resin may be, for example, long or short fibers processed into fibers by spinning the resins listed above, or PTFE fibrillated by applying shear force. Among these, fibrillated PTFE is preferred because it is easy to disperse the fibrous resin with relatively high uniformity within the intermediate layer and has high flexibility, allowing it to adapt to volume changes.

In the case of fibrillated PTFE, there are no particular restrictions on the diameter of the fibrous resin contained in the intermediate layer, as long as it is fibrillated. On the other hand, in the case of long or short fiber resin, the average diameter is preferably 0.01 μm or more, more preferably 0.03 μm or more, and preferably 1 μm or less, more preferably 0.5 μm or less, from the perspective of better suppressing the propagation of shear stress due to volume changes in the negative electrode to the solid electrolyte layer.

The fibrillation of PTFE in the intermediate layer can be confirmed by observing the cross section of the intermediate layer using a scanning electron microscope (SEM); the presence of fibrous components indicates that the PTFE is fibrillated. The average diameter of the long and short fiber resin can be determined by observing the cross section of the intermediate layer using an SEM, selecting 100 fibers, measuring the diameter of the thinnest part of each fiber using the scale on the SEM image, and then calculating the average diameter of the 100 fibers obtained.

The solid electrolyte in the intermediate layer is not particularly limited as long as it has Li ion conductivity; for example, sulfide-based solid electrolytes, hydride-based solid electrolytes, halide-based solid electrolytes, oxide-based solid electrolytes, etc. can be used.

Examples of sulfide-based solid electrolytes include particles of Li ₂ S—P ₂ S ₅ , Li ₂ S—SiS ₂ , Li ₂ S—P ₂ S ₅ —GeS ₂ , and Li ₂ S _{— B 2} S ₃ based glasses. In addition, thio-LISICON-type electrolytes have been attracting attention in recent years for their high Li _ion conductivity [Li 12-12a-b+c _{+ 6d - e} M ¹ ₃ + _{a -b-c-d} M ² _b M ³ _c M ⁴ _d M ⁵ _12-e X _e (where M ¹ is Si, Ge, or Sn, M ^M2 is P or V, ^M3 is Al, Ga, Y or Sb, ^M4 is Zn, Ca or Ba, ^M5 is S or either S and O, X is F, Cl, Br or I, 0≦a<3, 0≦b+c+d≦3, 0≦e≦3], argyrodite-type compounds [such as _Li6PS5Cl , represented by Li7 _{- kPS6 - kXk} (where X represents one or more halogen elements and 0.2<k<2.0), Li7 _{-f+ gPS6- fClf+g} (where 0.05≦g≦0.9, -3.0f+1.8≦g≦-3.0f+5.7), Li7 _{- hPS6- hCliBrj ]} (where h=i+j, 0<h≦1.8, 0.1≦i/j≦10.0) can also be used.

Examples of hydride-based solid electrolytes include LiBH ₄ and solid solutions of LiBH ₄ and the following alkali metal compounds (for example, those in which the molar ratio of LiBH ₄ to the alkali metal compound is 1:1 to 20:1). The alkali metal compound in the solid solution includes 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.

Examples of halide-based solid electrolytes include monoclinic LiAlCl ₄ , defective spinel or layered structure LiInBr ₄ , and monoclinic Li _6-3m Y _m X ₆ (where 0<m<2 and X=Cl or Br). Other known solid electrolytes that can be used include those described in, for example, WO 2020/070958 and WO 2020/070955.

Examples of oxide solid electrolytes include Li ₂ O—Al ₂ O ₃ —SiO ₂ —P ₂ O ₅ —TiO ₂ -based glass ceramics, Li ₂ O—Al ₂ O ₃ —SiO ₂ —P ₂ O ₅ —GeO ₂ -based glass ceramics, garnet-type Li ₇ La ₃ Zr ₂ O ₁₂ , NASICON-type Li _1+O Al _1+O Ti _2-O (PO ₄ ) ₃ , Li _1+p Al _1+p Ge _2-p (PO ₄ ) ₃ , and perovskite-type Li _3q La _2/3-q TiO ₃ .

Among these solid electrolytes, sulfide-based solid electrolytes and halide-based solid electrolytes containing chlorine (chloride-based solid electrolytes) are preferred due to their high Li-ion conductivity, with sulfide-based solid electrolytes containing Li and P being more preferred, and argyrodite-type sulfide-based solid electrolytes, which have particularly high Li-ion conductivity and are highly chemically stable, being even more preferred.

The intermediate layer can also contain a binder. Examples of binders for the intermediate layer include fluororesins such as PVDF. However, if good moldability can be ensured without using a binder, such as when the intermediate layer contains a sulfide-based solid electrolyte, then the binder need not be included.

The content of fibrous resin in the intermediate layer is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, from the perspective of better suppressing the propagation of shear stress due to volume changes in the negative electrode to the solid electrolyte layer. However, if the amount of fibrous resin in the intermediate layer is too high, for example, the amount of solid electrolyte may become too small, which could result in a decrease in the ionic conductivity of the intermediate layer. Therefore, in order to improve the ionic conductivity of the intermediate layer, the content of fibrous resin in the intermediate layer is preferably 10% by mass or less, and more preferably 8% by mass or less.

Furthermore, from the viewpoint of improving the ionic conductivity of the intermediate layer, the content of solid electrolyte in the intermediate layer is preferably 90% by mass or more, and more preferably 92% by mass or more. However, if the amount of solid electrolyte in the intermediate layer is too high, for example, the amount of fibrous resin may become too small, which may reduce the effect of suppressing cracking in the solid electrolyte layer. Therefore, in order to improve the effect of suppressing cracking in the solid electrolyte layer, the content of solid electrolyte in the intermediate layer is preferably 99.9% by mass or less, and more preferably 99.5% by mass or less.

Furthermore, the binder content in the intermediate layer is preferably 15% by mass or less, and preferably 0.5% by mass or more. On the other hand, if the intermediate layer does not require a binder from the standpoint of formability, the content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., no binder is contained).

The thickness of the intermediate layer is preferably 1 μm or more, and more preferably 10 μm or more, from the perspective of ensuring a better effect in suppressing cracking in the solid electrolyte layer. However, if the intermediate layer is made too thick, its effect will saturate and may actually lead to a reduction in the energy density of the battery, so its thickness is preferably 200 μm or less, and more preferably 150 μm or less.

The intermediate layer can be formed, for example, by pressure molding an intermediate layer-forming composition containing a fibrous resin and a solid electrolyte. Furthermore, in the case of an intermediate layer containing fibrillated PTFE as the fibrous resin, the intermediate layer (an intermediate layer-forming sheet containing a fibrous resin and a solid electrolyte) can be formed by rolling an intermediate layer-forming composition containing particulate PTFE and a solid electrolyte into a sheet, and then fibrillating the particulate PTFE within the sheet.

(Negative electrode)
Examples of the negative electrode of the all-solid-state secondary battery include a molded body (such as a pellet) obtained by molding a negative electrode mixture containing a negative electrode active material, and a structure in which a layer (negative electrode mixture layer) made of a molded negative electrode mixture is formed on a current collector.

Examples of negative electrode active materials include carbon materials such as graphite; simple substances and compounds (oxides, etc.) containing elements such as Si, Sn, Ge, Bi, Sb, and In, and alloys thereof; and compounds that can be charged and discharged at low voltages similar to those of metallic lithium, such as lithium-containing nitrides, lithium-containing oxides (lithium titanium oxides such as Li ₄ Ti ₅ O ₁₂ ), niobium composite oxides such as TiNb ₂ O ₇ , tungsten oxide, molybdenum oxide, and vanadium oxide.

As mentioned above, the all-solid-state secondary battery of the present invention can effectively suppress the occurrence of cracks in the solid electrolyte layer due to volume changes in the negative electrode, and this effect is particularly pronounced when using, among the various negative electrode active materials exemplified above, negative electrode active materials that have large capacities and large volume changes due to battery charge and discharge (e.g., simple substances, compounds, and alloys containing elements such as Si, Sn, Ge, Bi, Sb, and In; niobium composite oxides, tungsten oxide, molybdenum oxide, and vanadium oxide).

As a niobium composite oxide, an oxide having a monoclinic crystal structure and represented by the following general formula (1) is particularly preferred, as it can improve the output characteristics and charge/discharge cycle characteristics of the battery.

A _y M ¹ _α Al _x-α Nb _12-x-z M ² _z O _29-δ (1)

In the general formula (1), A is at least one element of Li and Na, ^M1 is at least one element selected from the group consisting of Fe, Mn, Zn, Cu, Ag, Mg, Ca, Sr, Ba, Co, Eu, Y, Bi, La, Ce, Nd, Sm, and Gd, ^M2 is at least one element selected from the group consisting of K, Ti, Ni, Zr, V, Mo, Ta, and W, and 0<x≦1.1, 0≦y≦24, 0≦z≦2, −1≦δ≦2, and 0<α≦0.4x.

The elements ^M1 , Fe (Fe ²⁺ ), Mn (Mn ²⁺ ), Zn (Zn ²⁺ ), Cu (Cu ⁺ , Cu ²⁺ ), Ag (Ag ⁺ ), Mg (Mg ²⁺ ), Ca (Ca ²⁺ ), Sr (Sr ²⁺ ), Ba (Ba ²⁺ ), Co (Co ²⁺ ), Eu (Eu ²⁺ , Eu ³⁺ ), Y (Y ³⁺ ), Bi (Bi ³⁺ ), La (La ³⁺ ), Ce (Ce ³⁺ , Ce ⁴⁺ ), Nd (Nd ³⁺ ), Sm (Sm ³⁺ ), and Gd (Gd ³⁺ ), have larger effective ionic radii than Nb (Nb ⁵⁺ ) and Al (Al ³⁺ ). Therefore, the niobium composite oxide represented by the general formula (1) is based on AlNb ₁₁ O ₂₉ , and a portion of the Al is substituted with the element M ¹ , so that the lattice constant of the crystal is larger than that of AlNb ₁₁ O ₂₉ , and the diffusibility of element A ions inside is increased. Therefore, in an all-solid-state secondary battery constructed using a negative electrode that uses the niobium composite oxide as the negative electrode active material, the output characteristics are improved.

Furthermore, by using the niobium composite oxide represented by the general formula (1) as the negative electrode active material, the charge-discharge cycle characteristics of the all-solid-state secondary battery can be improved compared to when, for example, AlNb ₁₁ O ₂₉ is used as the negative electrode active material. This is presumably because, by containing element M ¹ , which has an ionic radius larger than Nb (Nb ⁵⁺ ) or Al (Al ³⁺ ), the lattice constant of the oxide at a stage before element A ions, which serve as carriers, are inserted becomes larger, and the space for occluding element A ions expands, which makes it possible to suppress volumetric changes in the oxide (negative electrode active material) when element A ions are inserted and extracted during charge and discharge, making it possible to suppress electrode deterioration even when the all-solid-state secondary battery is repeatedly charged and discharged.

The niobium composite oxide represented by the general formula (1) may contain only one ^{or two or more} of Fe, Mn, Zn, Cu, Ag, Mg, Ca, Sr, Ba, Co, Eu, Y, Bi, La, Ce, Nd, Sm, and Gd as the element M 1. Among these elements M ¹ , Fe, Mn, Zn, and Cu are preferred, and Zn and Cu are more preferred, because they are particularly likely to substitute for Al due to their electron configuration and are highly effective in increasing the electronic conductivity of the niobium composite oxide.

When the niobium composite oxide represented by the general formula (1) contains, as the element ^M1 , an element having a mixed valence, such as Fe3 ⁺ /Fe2 ⁺ or Cu2 ⁺ /Cu ⁺ , many oxygen vacancies may occur (i.e., in the general formula (1), δ becomes greater than 0), which further improves the electronic conductivity and ionic conductivity of the niobium composite oxide represented by the general formula (1). Therefore, when the oxide represented by the general formula (1) contains Fe or Cu as the element ^M1 , further improvement in the output characteristics of the all-solid-state secondary battery can be expected.

The niobium composite oxide represented by the general formula (1) may contain, in addition to the element ^M1 substituting a portion of the Al, at least one element ^M2 selected from the group consisting of K, Ti, Ni, Zr, V, Mo, Ta, and W. These elements ^M2 are components that substitute for Nb without substituting a portion of the Al constituting the crystal in the niobium composite oxide represented by the general formula (1) having a monoclinic crystal structure, or that are present as a solid solution or impurity in the crystal. The niobium composite oxide represented by the general formula (1) does not need to contain the element ^M2 , and the amount z thereof may be 0. However, when the element ^M2 is contained, a z of 2 or less is acceptable because it does not affect the performance of the niobium composite oxide. The niobium composite oxide represented by the general formula (1) may also contain moisture.

In the niobium composite oxide represented by the general formula (1), Al is a component that enhances the structural stability of the niobium composite oxide, and the action of this Al improves the reversibility of the electrode active material during charge and discharge of the all-solid-state secondary battery.

In the niobium composite oxide represented by the general formula (1), as described above, a portion of Al is substituted with the element ^M1 , and the total x of the amount of Al and the amount of the element ^M1 is greater than 0 and, from the viewpoint of better exerting the above-described functions of Al and the element ^M1 , is preferably 0.8 or more. However, if the amounts of Al and the element ^M1 in the niobium composite oxide are too large, the amount of Nb in the niobium composite oxide becomes too small, and there is a risk that the functions will not be well exerted. Therefore, the total x of the amount of Al and the amount of the element ^M1 is preferably 1.1 or less, and more preferably 1.05 or less.

Furthermore, in the niobium composite oxide represented by the general formula (1), the amount α of the element ^M1 is greater than 0 and, from the viewpoint of better ensuring the effect of improving the output characteristics of an all-solid-state secondary battery, is preferably 0.05 or greater. However, if the amount of the element ^M1 in the niobium composite oxide represented by the general formula (1) is too large, the stability of the crystal structure tends to decrease, and the charge-discharge cycle characteristics of the all-solid-state secondary battery tend to deteriorate. Therefore, from the viewpoint of better charge-discharge cycle characteristics of an all-solid-state secondary battery, the amount α of the element ^M1 in the niobium composite oxide represented by the general formula (1) is preferably 0.4x or less, more specifically, more preferably 0.44 or less, and even more preferably 0.4 or less.

In the niobium composite oxide represented by the general formula (1), element A is at least one of Li and Na, and is absorbed into or desorbed from the niobium composite oxide during charging and discharging of the all-solid-state secondary battery (i.e., it functions as a carrier). The niobium composite oxide may or may not contain element A. In the case of a niobium composite oxide that does not contain element A, ions of element A are inserted into the niobium composite oxide, for example, by charging an all-solid-state secondary battery used as a negative electrode active material, or by pre-doping with ions of element A before use in an all-solid-state secondary battery, so that the niobium composite oxide contains element A.

The amount y of element A in the niobium composite oxide represented by the general formula (1) is preferably 0 or more and 24 or less.

In the niobium composite oxide represented by the general formula (1), the amount of oxygen is originally 29, similar to that _{of AlNbO29} , but may vary depending on the presence of element ^M1, etc. Specifically, in the general formula (1), δ is preferably −1 or more, preferably 2 or less, and more preferably 1.95 or less. When δ in the general formula (1) is greater than 0, oxygen deficiency occurs in the oxide. In this case, the electronic conductivity and the conductivity of element A ions of the electrode active material containing the niobium composite oxide are improved, and therefore, by using such a negative electrode active material, it is possible to further increase the energy density of the all-solid-state secondary battery.

In the niobium composite oxide represented by the general formula (1), δ relating to the amount of oxygen is determined by the amounts of the elements M ¹ , M ² , Nb, and Al that form cations, and the amount of oxygen that forms anions.

[Method for producing niobium composite oxide represented by the general formula (1)]
The method for producing the niobium composite oxide represented by the general formula (1) is not particularly limited. For example, the niobium composite oxide can be produced by synthesis using a solid-phase reaction method in which various metal oxides of constituent elements such as Nb, Al, Cu, and Zn are mixed and fired, or a reaction method in which a mixture of metal compounds prepared by coprecipitating chloride salts, nitrates, and alkoxides of the respective metals in a liquid phase is used as a precursor.

In the solid-state reaction method, firing is preferably performed at a temperature of 800°C or higher, and more preferably in the range of 900°C to 1100°C, in order to enhance the mutual diffusion of various metal ions. The firing time is not particularly limited, but can be performed for 1 to 1000 hours. If the firing temperature exceeds 1100°C, oxygen is gradually released from the sample, resulting in the formation of crystal phases other than the monoclinic crystal phase, or the composition no longer satisfying the general formula (1). Therefore, it is more preferable to hold the sample at 1100°C or higher for 10 hours or less. The cooling rate of the sample during firing is not particularly limited as long as a monoclinic crystal phase is obtained. However, to obtain a monoclinic crystal phase that is stable at this temperature, a cooling rate of 15°C/min to 60°C/min (including natural cooling) is preferred. Rapid cooling at a cooling rate of 1°C/sec to 1000°C/sec is also acceptable.

[Method for determining the composition of the niobium composite oxide represented by the general formula (1)]
The composition of the niobium composite oxide represented by the general formula (1) can be analyzed, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). When quantification using ICP-AES is difficult because the niobium composite oxide is sintered with an oxide-based solid electrolyte, making it difficult to separate the individual components, the composition can also be determined by a method that combines various elemental analysis methods such as an energy dispersive X-ray spectrometer (EDS) or a wavelength dispersive X-ray spectrometer (WDS) with an SEM or a transmission electron microscope (TEM).

[Method for confirming oxygen defects in the niobium composite oxide represented by the general formula (1)]
The occurrence of oxygen defects in the niobium composite oxides represented by the general formula (1) can be confirmed by X-ray photoelectron spectroscopy (XPS) by the presence of a mixture of peaks attributable to pentavalent Nb and peaks attributable to tetravalent Nb in these composite oxides (confirmation was performed by this method in the Examples described later). The C1s peak position of the contaminating hydrocarbon on the sample surface was set to 284.6 eV, and charging correction of the binding energy of the spectrum was performed. An XPS spectrum was obtained in the binding energy range from 202 eV to 214 eV, and the background shape was estimated by the iterative Shirley method, and the background was removed from the spectrum. For the obtained spectrum, peak fitting is performed using a Pseudo-Voigt function, and the area of the peak attributable to Nb ⁵⁺ in Nb3d3/2 (obtained at a position where the binding energy is 209.8 eV to 210.2 eV) and the area of the peak attributable to Nb ⁴⁺ (a peak is obtained at a position where the binding energy is 0.5 eV to 2 eV lower than the peak position attributable to 3d3/2 of Nb ⁵⁺ ) are determined, and the average valence of Nb is calculated. Similarly, the area of the peak attributable to Nb ⁵⁺ in Nb3d5/2 (obtained at a position where the binding energy is 206.6 eV to 207.1 eV) and the area of the peak attributable to Nb ⁴⁺ (a peak is obtained at a position where the binding energy is 0.5 eV to 2 eV lower than the peak position attributable to 3d5/2 of Nb ⁵⁺ ) are determined, and the average valence of Nb is calculated. The average value of the average valence of Nb calculated from Nb3d3/2 and the average valence of Nb calculated from Nb3d5/2 is calculated to determine the average valence of Nb contained in the active material.

Similarly to the average valence of Nb, the average valence of Cu can also be determined. An XPS spectrum is obtained in the binding energy range of 925 eV to 950 eV, and the area of the peak attributable to Cu ²⁺ of Cu2p3/2 (obtained at a binding energy position of 932.7 eV to 934.6 eV) and the area of the peak attributable to Cu ⁺ (obtained at a position 0.5 eV to 2 eV lower in binding energy than the peak position attributable to 2p3/2 of Cu ²⁺ ) are calculated to determine the average valence of Cu.

Next, the amount of various metal elements is quantified using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). 5 mg of sample is placed in a platinum crucible, and 5 ml of hydrofluoric acid and 10 ml of 50% sulfuric acid are added. A heat decomposition process is performed to obtain a mixture of concentrated sulfuric acid and fluoride salts of various metals (the hydrofluoric acid turns into white smoke and is removed). 2 ml of 30% hydrogen peroxide is added to the resulting mixture, and the sample is then diluted to 100 ml with pure water using a measuring flask. The resulting sample solution and standard solutions with known metal concentrations are measured alternately three times, and the average values are calculated for each. The amount of metal element in the sample is determined from the ratio of the signal intensity of the sample solution to that of the standard solution.

The total charge amount of the cations contained in the sample is calculated from the average valences of the Nb and Cu elements (all Al is considered to exist in the state of Al ³⁺ ) determined by XPS analysis and the contents of various metal elements determined by ICP-AES, and the content of oxygen anions (O ²⁻ ) is calculated so as to be electrically neutral with respect to the total charge amount of the cations obtained. The difference between this content and the oxygen anion content assumed to contain no oxygen defects is defined as the oxygen defect amount δ. That is, in the case of a niobium composite oxide represented by the general formula (1) that contains Cu as the element ^M1 , the difference between the oxygen content obtained by the above method and the oxygen content assumed to be composed of Al ³⁺ , Cu ²⁺ , and Nb ⁵⁺ , respectively, is the oxygen defect amount δ.

The oxygen content of a sample can also be directly quantified by placing the sample in a graphite crucible, resistively heating it in a helium stream, and detecting the carbon dioxide produced with an infrared detector.

[Method for confirming the monoclinic structure of the niobium composite oxide represented by the general formula (1)]
The crystal structure of the niobium composite oxide represented by the general formula (1) can be determined by measuring a powder X-ray diffraction (powder XRD) pattern using a Rigaku RINT2500VPC (X-rays used: CuKα rays) and comparing the pattern with a Powder Diffraction File (PDF) database or by analyzing it by the Rietveld method. When comparing crystal lattice sizes between different samples, Si powder (Rigaku, _a0 = 5.4308 Å at 298.1 K) is mixed as an internal reference when preparing the sample for powder XRD measurement, and the spectrum is corrected so that the peak attributable to X-ray diffraction of the Si (111) plane is at 2θ = 28.442 degrees. The lattice constant (d010) in the b-axis direction of the unit cell can be calculated by doubling the interplanar spacing determined from the peak attributable to diffraction of the (020) plane, assuming an X-ray wavelength of 1.5418 Å. When the niobium composite oxide represented by the general formula (1) is contained in the negative electrode as the negative electrode active material, a 1 kΩ resistor is connected to the battery, and the negative electrode of the battery is subjected to 100 hours of constant resistance discharge. The surface of the negative electrode (when the current collector and the molded body of the negative electrode mixture are bonded together, the surface opposite to the surface bonded to the current collector) is flattened and then fixed to a sample stage for powder XRD, whereby a powder XRD pattern of the electrode can be obtained.

When the niobium composite oxide represented by the general formula (1) is sintered with a crystalline sulfide-based solid electrolyte or an oxide-based solid electrolyte, a sample piece is extracted from a molded sample containing the niobium composite oxide using FIB processing, mounted on a TEM sample stage, and then thinned to a thickness of 100 nm or less. A selected area electron diffraction (SAED) pattern is obtained using a TEM, and the monoclinic crystal structure can be confirmed by analyzing this pattern.

The content of the negative electrode active material in the negative electrode mixture is preferably 10 to 99 mass%.

The negative electrode can contain a conductive additive. Examples of conductive additives include carbon materials such as carbon black (thermal black, furnace black, channel black, ketjen black, acetylene black, etc.), graphite (natural graphite, artificial graphite), graphene, and fibrous carbon (vapor-grown carbon fiber, carbon nanofiber, carbon nanotube, etc.); powders of Cu, Ni, Al, Au, and Pd, or alloys thereof, or porous bodies thereof; and one or more of these can be used. The content of the conductive additive in the negative electrode mixture is preferably 1 to 10% by mass.

The negative electrode can contain a solid electrolyte. The solid electrolyte contained in the negative electrode can be one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, halide-based solid electrolytes, and oxide-based solid electrolytes listed above as examples of solid electrolytes that can be used in the intermediate layer. Of these solid electrolytes, sulfide-based solid electrolytes are preferred due to their high Li-ion conductivity, sulfide-based solid electrolytes containing Li and P are more preferred, and argyrodite-type sulfide-based solid electrolytes, which have particularly high Li-ion conductivity and high chemical stability, are even more preferred.

The average particle size of the solid electrolyte is preferably 0.1 μm or more, and more preferably 0.2 μm or more, 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 active material and the solid electrolyte, it is preferably 10 μm or less, and more preferably 5 μm or less.

The average particle diameter of the solid electrolyte and the positive electrode active material described later in this specification refers to the 50% diameter (D ₅₀ ) in the volume-based integrated fraction when the integrated volume is determined from particles with small particle sizes using a particle size distribution measurement device (such as the Microtrac particle size distribution measurement device "HRA9320" manufactured by Nikkiso Co., Ltd.).

The solid electrolyte content in the negative electrode mixture is preferably 4 to 85 mass%.

The negative electrode can contain a binder. Specific examples include fluororesins such as PVDF. Note that, for example, when the negative electrode contains a sulfide-based solid electrolyte, if good moldability can be ensured when forming a molded body from the negative electrode mixture without using a binder, the negative electrode does not need to contain a binder.

If a binder is required in the negative electrode, the binder content in the negative electrode mixture is preferably 6% by mass or less, and preferably 0.5% by mass or more. On the other hand, if moldability can be achieved without the binder, the binder content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., no binder is contained).

The negative electrode can contain fibrous resin. When the negative electrode contains fibrous resin, the effect of suppressing the occurrence of cracks in the solid electrolyte layer caused by volume changes in the negative electrode due to charging and discharging the battery is further improved. The negative electrode can also contain fibrous carbon as a conductive additive, and it is more preferable for the negative electrode to contain fibrous carbon along with the fibrous resin. In this case, the entanglement of the fibrous resin and the fibrous carbon further improves the effect of suppressing the occurrence of cracks in the solid electrolyte layer caused by volume changes in the negative electrode due to charging and discharging the battery.

Fiber-like resins include the fibrous materials of the various resins listed above as examples that can be contained in the intermediate layer, and one or more of these can be used. The fibrous resin may be, for example, long or short fibers processed into fibers by spinning the resin, or it may be PTFE fibrillated by applying shear force. Of these, fibrillated PTFE is preferred, as it allows for the fibrous resin to be easily dispersed with relatively high uniformity within the negative electrode (inside the molded negative electrode mixture).

In the case of fibrillated PTFE, the diameter of the fibrous resin contained in the negative electrode (molded negative electrode mixture) is not particularly limited as long as it is fibrillated. On the other hand, in the case of long or short fiber resin, the average diameter is preferably 0.1 μm or more, more preferably 0.5 μm or more, and preferably 15 μm or less, more preferably 10 μm or less, from the perspective of better suppressing the propagation of shear stress due to volume changes in the negative electrode to the solid electrolyte layer.

Furthermore, from the viewpoint of better suppressing the propagation of shear stress due to volume changes in the negative electrode to the solid electrolyte layer, the average diameter of the fibrous carbon contained in the negative electrode (molded negative electrode mixture) is preferably 0.005 μm or more, more preferably 0.01 μm or more, and is preferably 1 μm or less, more preferably 0.5 μm or less.

The fibrillation of PTFE in the negative electrode (molded negative electrode mixture) can be confirmed by observing the cross section of the molded negative electrode mixture using an SEM. If fibrous components are present in areas where fluorine is detected in EDS measurement, it can be said that the PTFE is fibrillated. The average diameter of the long and short fiber resin and the average diameter of the fibrous carbon can be determined by observing the cross section of the molded negative electrode mixture using an SEM, selecting 100 fibers for each, measuring the diameter of the thinnest part of each fiber using the SEM scale, and calculating the average diameter of the 100 fibers obtained.

In order to ensure the desired effects of its use, the content of fibrous resin in the negative electrode mixture is preferably 0.1% by mass or more, and more preferably 0.4% by mass or more. However, if the amount of fibrous resin in the negative electrode is too high, there is a risk that, for example, the amount of negative electrode active material will be reduced, resulting in a low capacity. Therefore, the content of fibrous resin in the negative electrode mixture is preferably 10% by mass or less, and more preferably 5% by mass or less.

Furthermore, when fibrous carbon is contained in the negative electrode as a conductive additive, in order to ensure the desired effect of its use, the content of fibrous carbon in the negative electrode mixture is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and preferably 8% by mass or less, and more preferably 5% by mass or less. The same applies to the content when fibrous carbon is contained together with a fibrous resin.

When a current collector is used for the negative electrode, it can be made of copper or nickel foil, punched metal, mesh, expanded metal, foam metal; carbon sheet; etc.

A molded negative electrode mixture can be formed, for example, by compressing, by pressure molding, a negative electrode mixture prepared by mixing a negative electrode active material, a solid electrolyte, and optionally a conductive additive, binder, and fibrous resin. Furthermore, when fibrillated PTFE is contained in the negative electrode as a fibrous resin, a molded negative electrode mixture can be formed by rolling a negative electrode mixture that also contains particulate PTFE into a sheet, and then fibrillating the particulate PTFE within the sheet.

In the case of a negative electrode having a current collector, it can be manufactured by bonding the molded negative electrode mixture formed by the method described above to the current collector by, for example, pressing. The molded negative electrode mixture and the current collector may not be integrated, but may be housed independently within the exterior housing.

Alternatively, the negative electrode mixture may be mixed with a solvent to prepare a composition containing the negative electrode mixture, which may then be applied to a substrate such as a current collector or an intermediate layer that faces the negative electrode, dried, and then pressed to form a molded negative electrode mixture.

It is preferable to select a solvent for the negative electrode mixture-containing composition that is less likely to deteriorate the solid electrolyte. In particular, sulfide-based solid electrolytes and hydride-based solid electrolytes undergo chemical reactions with trace amounts of water, so it is preferable to use a non-polar aprotic solvent, such as hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene. It is particularly preferable to use an ultra-dehydrated solvent with a water content of 0.001% by mass (10 ppm) or less. Fluorine-based solvents such as "Vertrel (registered trademark)" manufactured by DuPont-Mitsui Fluorochemicals, "Zeorolla (registered trademark)" manufactured by Nippon Zeon Co., Ltd., and "Novec (registered trademark)" manufactured by Sumitomo 3M Company, Limited, as well as non-aqueous organic solvents such as dichloromethane and diethyl ether can also be used.

The thickness of the molded negative electrode mixture (in the case of a negative electrode having a current collector, the thickness of the molded negative electrode mixture per side of the current collector; the same applies below) is typically 100 μm or more, but from the perspective of increasing the capacity of all-solid-state secondary batteries, it is preferably 200 μm or more. Furthermore, the thickness of the molded negative electrode mixture is typically 3000 μm or less.

In the case of a negative electrode manufactured by forming a negative electrode mixture layer on a current collector using a solvent-containing negative electrode mixture composition, the thickness of the negative electrode mixture layer is preferably 10 to 1000 μm.

(positive electrode)
Examples of the positive electrode of the all-solid-state secondary battery include a molded body (such as a pellet) obtained by molding a positive electrode mixture containing a positive electrode active material, and a structure in which a layer (positive electrode mixture layer) made of a molded positive electrode mixture is formed on a current collector.

The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in known non-aqueous electrolyte secondary batteries, that is, an active material capable of absorbing and releasing Li ions. Specific examples of the positive electrode active material include spinel-type lithium manganese composite oxides _{represented by LiMrMn2 -rO4 (} wherein M is at least one element selected from the group consisting of Li, Na, K, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Zr, Fe, Co, Ni, Cu, Zn, Al, Sn, Sb, In, Nb, Ta, Mo, W, Y, Ru, and Rh, _and 0≦r≦1), _{LirMn (1-s-r) NisMtO (2-u) Fv,} a layered compound represented by LiCo 1-r M r O 2 (wherein 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, and 0.8≦r≦1.2, 0<s<0.5, 0≦t≦0.5, u+v<1, −0.1≦u≦0.2, 0≦v≦0.1); a lithium cobalt composite oxide represented by LiCo _1-r M _r O ₂ (wherein M is at least one element selected from the group consisting of Al, Mg, Ti, V, Cr, Zr, Fe, Ni, Cu, Zn, Ga, Ge, _Nb , Mo, Sn, _Sb , and Ba, and 0≦r≦ _0.5 ); Lithium nickel composite oxides represented by the formula Li 1+s M 1-r N r PO 4 F s (wherein 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≦r≦0.5); olivine-type composite oxides represented by the formula Li ₂ M _1-r N _r P ₂ O ₇ (wherein 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 , V, and Ba, and 0≦r≦0.5, 0 _≦ s _{≦ 1} ); (wherein 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, V, and Ba, and 0≦r≦0.5), and the like. Among these, only one type may be used, or two or more types may be used in combination.

From the perspective of reducing side reactions that cause battery capacity degradation and increasing electrode density, the average particle diameter of the positive electrode active material is preferably 0.1 μm or more, more preferably 0.5 μm or more, and preferably 25 μm or less, more preferably 10 μm or less. The positive electrode active material may be either primary particles or secondary particles formed by aggregation of primary particles. When the molded positive electrode mixture contains a solid electrolyte, using a positive electrode active material with an average particle diameter within the above range will ensure a large interface with the solid electrolyte, further improving the load characteristics of the battery.

If the positive electrode contains a solid electrolyte, it is preferable that the positive electrode active material have a reaction suppression layer on its surface to suppress reaction with the solid electrolyte contained in the positive electrode.

If the positive electrode active material and solid electrolyte come into direct contact within the positive electrode (molded body of positive electrode mixture), the solid electrolyte may oxidize and form a resistive layer, which could reduce ionic conductivity within the molded body. By providing a reaction inhibitor layer on the surface of the positive electrode active material that inhibits reaction with the solid electrolyte and preventing direct contact between the positive electrode active material and the solid electrolyte, it is possible to prevent a reduction in ionic conductivity within the molded body due to oxidation of the solid electrolyte.

The reaction suppression layer may be made of any material that has ion conductivity and can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of materials that can form 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, _Zr , Ta, and _W. More specifically, Nb _- containing oxides such as _LiNbO3 , _Li3PO4 , _Li3BO3 , _Li4SiO4 , _Li4GeO4 , _LiTiO3 , _LiZrO3 , and _Li2WO4 can be mentioned. The reaction suppression layer may contain only one of these oxides, or may contain two or more of them, or may even form a composite compound of multiple oxides. Among these oxides, Nb-containing oxides are preferably used, and _LiNbO3 is more preferably used.

The reaction suppression layer is preferably present on the surface in an amount of 0.1 to 2.0 parts by mass per 100 parts by mass of positive electrode active material. This range effectively suppresses the reaction between the positive electrode active material and the solid electrolyte.

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

The content of the positive electrode active material in the positive electrode mixture is preferably 20 to 95 mass%.

The positive electrode can contain a conductive additive. The conductive additives contained in the positive electrode can be the same as the various conductive additives listed above as examples that can be contained in the negative electrode. The content of the conductive additive in the positive electrode mixture is preferably 1 to 10 mass%.

The positive electrode can contain a solid electrolyte. The solid electrolyte contained in the positive electrode can be one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, halide-based solid electrolytes, and oxide-based solid electrolytes listed above as examples of solid electrolytes that can be used in the intermediate layer. Among these solid electrolytes, sulfide-based solid electrolytes are preferred due to their high Li-ion conductivity, and sulfide-based solid electrolytes containing Li and P are more preferred. Argyrodite-type sulfide-based solid electrolytes, which have particularly high Li-ion conductivity and high chemical stability, are even more preferred.

As with the negative electrode, the average particle size of the solid electrolyte in the positive electrode is preferably 0.1 μm or more, and more preferably 0.2 μm or more, from the perspective of reducing grain boundary resistance. On the other hand, from the perspective of forming a sufficient contact interface between the positive electrode active material and the solid electrolyte, it is preferably 10 μm or less, and more preferably 5 μm or less.

The solid electrolyte content in the positive electrode mixture is preferably 4 to 80 mass%.

The positive electrode can contain a binder. Specific examples include fluororesins such as PVDF. Note that, for example, when a sulfide-based solid electrolyte is contained in the positive electrode, if good moldability can be ensured when forming a molded body from the positive electrode mixture without using a binder, the positive electrode does not need to contain a binder.

If a binder is required in the positive electrode, the binder content in the positive electrode mixture is preferably 6% by mass or less, and preferably 0.5% by mass or more. On the other hand, if moldability can be achieved without the binder, the binder content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., no binder is contained).

When a current collector is used for the positive electrode, the current collector can be made of metal foil such as aluminum, nickel, or stainless steel, punched metal, mesh, expanded metal, foam metal; carbon sheet; etc.

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 optionally a binder, using pressure molding or the like.

In the case of a positive electrode having a current collector, it can be manufactured by bonding the molded positive electrode mixture formed by the method described above to the current collector by, for example, pressing. The molded positive electrode mixture and the current collector may not be integrated, but may be housed independently within the exterior housing.

Alternatively, the cathode mixture may be mixed with a solvent to prepare a cathode mixture-containing composition, which may then be applied to a substrate such as a current collector or a solid electrolyte layer that faces the cathode, dried, and then pressed to form a molded cathode mixture.

As with the solvent used in the negative electrode mixture-containing composition, it is desirable to select a solvent that is less likely to deteriorate the solid electrolyte when used in the positive electrode mixture-containing composition. It is preferable to use the various solvents listed above as examples of solvents for the negative electrode mixture-containing composition, and it is particularly preferable to use an ultra-dehydrated solvent with a moisture content of 0.001% by mass (10 ppm) or less.

The thickness of the molded positive electrode mixture (in the case of a negative electrode having a current collector, the thickness of the molded positive electrode mixture per side of the current collector; the same applies below) is typically 100 μm or more, but from the perspective of increasing the capacity of all-solid-state secondary batteries, it is preferably 200 μm or more. Furthermore, the thickness of the molded positive electrode mixture is typically 3,000 μm or less.

In the case of a positive electrode manufactured by forming a positive electrode mixture layer on a current collector using a positive electrode mixture-containing composition containing a solvent, the thickness of the positive electrode mixture layer is preferably 10 to 1000 μm.

(Solid electrolyte layer)
The solid electrolyte in the solid electrolyte layer interposed between the positive electrode and the negative electrode (the solid electrolyte layer interposed between the positive electrode and the intermediate layer) can be one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, halide-based solid electrolytes, and oxide-based solid electrolytes listed above as examples of usable electrodes. However, to improve battery characteristics, it is preferable to include a sulfide-based solid electrolyte, and it is more preferable to include an argyrodite-type sulfide-based solid electrolyte. It is even more preferable to include a sulfide-based solid electrolyte in all of the positive electrode, negative electrode, intermediate layer, and solid electrolyte layer, and it is even more preferable to include an argyrodite-type sulfide-based solid electrolyte.

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

From the perspective of improving ionic conductivity between the positive electrode and the negative electrode, the content of solid electrolyte in the solid electrolyte layer is preferably 70% by mass or more, and more preferably 80% by mass or more. Because the solid electrolyte layer may be composed solely of solid electrolyte, the upper limit of the content of solid electrolyte in the solid electrolyte layer is 100% by mass.

The solid electrolyte layer can be formed by a method such as compressing the solid electrolyte by pressure molding; or by a method in which a solid electrolyte layer-forming composition prepared by dispersing the solid electrolyte in a solvent is applied to a substrate (including a porous body that serves as a support), a positive electrode, or an intermediate layer, and then dried, followed by pressure molding such as pressing as required.

It is desirable to select a solvent that is unlikely to deteriorate the solid electrolyte when used in the solid electrolyte layer-forming composition, and it is preferable to use the same solvents as those exemplified above for the negative electrode mixture-containing composition.

The thickness of the solid electrolyte layer is preferably 10 to 500 μm.

(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, the solid electrolyte layer, the intermediate layer, and the negative electrode are laminated in this order, or in the form of a wound electrode body in which this laminated electrode body is wound.

When forming the electrode body, it is preferable to pressure mold the positive electrode, solid electrolyte layer, intermediate layer, and negative electrode in a stacked state, in order to increase the mechanical strength of the electrode body.

(Battery type)
A cross-sectional view schematically illustrating one example of an all-solid-state secondary battery of the present invention is shown in Fig. 1. The all-solid-state secondary battery 1 shown in Fig. 1 has a laminated electrode body having a positive electrode 10, a negative electrode 20, a solid electrolyte layer 30, and an intermediate layer 40 enclosed in an exterior body formed by an exterior can 50, a sealing can 60, and a resin gasket 70 interposed between them. The laminated electrode body is configured by laminating the positive electrode 10, the solid electrolyte layer 30, the intermediate layer 40, and the negative electrode 20 in this order from the bottom in the figure.

The sealing can 60 fits into the opening of the outer can 50 via a gasket 70, and the open end of the outer can 50 is tightened inward, causing the gasket 70 to abut against the sealing can 60, sealing the opening of the outer can 50 and creating a sealed structure inside the battery 1.

The positive electrode 10 has a molded positive electrode mixture 11 and a current collector 12. However, when the positive electrode 10 has the current collector 12 as shown in FIG. 1, the molded positive electrode mixture 11 and the current collector 12 may be integrated, as described above, and the molded positive electrode mixture 11 may form a positive electrode mixture layer, or the molded positive electrode mixture 11 and the current collector 12 may be independent of each other. The negative electrode 20 has a molded negative electrode mixture 21 and a current collector 22. However, when the negative electrode 20 has the current collector 22 as shown in FIG. 1, the molded negative electrode mixture 21 and the current collector 22 may be integrated, as described above, and the molded negative electrode mixture 21 may form a negative electrode mixture layer, or the molded negative electrode mixture 21 and the current collector 22 may be independent of each other.

The outer can and sealing can can be made of stainless steel or other materials. The gasket can be made of polypropylene, nylon, or other materials. If heat resistance is required for the battery's intended use, a heat-resistant resin with a melting point above 240°C can also be used. Examples of heat-resistant resins include fluororesin (such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA)), polyphenylene ether (PPE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). If the battery is used in an application requiring heat resistance, a glass hermetic seal can also be used for sealing.

Furthermore, Figures 2 and 3 show schematic diagrams illustrating another example of an 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 I-I in Figure 2.

The all-solid-state secondary battery 100 shown in Figures 2 and 3 houses an electrode assembly 200 consisting of a positive electrode, solid electrolyte layer, intermediate layer, and negative electrode within a laminate film casing 500 made of two sheets of metal laminate film, and the laminate film casing 500 is sealed at its outer periphery by heat-sealing the upper and lower metal laminate films. Note that in Figure 3, to avoid complicating the drawing, the layers that make up the laminate film casing 500 and the positive electrode, solid electrolyte layer, intermediate layer, and negative electrode that make up the electrode assembly are not shown separately.

The positive electrode of the electrode body 200 is connected to the positive electrode external terminal 300 within the battery 100, and although not shown, the negative electrode of the electrode body 200 is also connected to the negative electrode external terminal 400 within the battery 100. One end of the positive electrode external terminal 300 and the negative electrode external terminal 400 is extended outside the laminate film exterior body 500 so that they can be connected to external devices, etc.

All-solid-state secondary batteries can take various forms, including those having an exterior body composed of an outer can, a sealing can, and a gasket, as shown in Figure 1, i.e., those commonly referred to as coin-type or button-type batteries; those having an exterior body composed of a resin film or a metal-resin laminate film, as shown in Figures 2 and 3; those having an exterior body composed of a metallic, bottomed, tubular (cylindrical or rectangular) outer can and a sealing structure that seals the opening; and those having an exterior body composed of a ceramic or resin concave container and a ceramic, resin, or metal lid that seals the opening.

The present invention will be described in detail below based on examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of intermediate layer-forming sheet>
An intermediate layer-forming composition was prepared by mixing Li ₆ PS ₅ Cl (sulfide-based solid electrolyte) and PTFE particles in a mass ratio of 95:5, and this was rolled using a roll with a diameter of 100 mm and a temperature adjusted to 23° C. to obtain an intermediate layer-forming sheet with a thickness of 0.30 mm. A cross section of a portion of the obtained sheet was observed using an SEM, and it was confirmed that the PTFE was fibrillated.

<Preparation of negative electrode>
TiNb ₂ O ₇ (negative electrode active material), graphene, carbon nanotubes (length 1000 μm), PTFE particles, and Li ₆ PS ₅ Cl (sulfide-based solid electrolyte) were mixed in a mass ratio of 68: 8.8: 0.2: 2: 21 to prepare a negative electrode mixture, which was rolled using a roll with a diameter of 100 mm and a temperature adjusted to 23 ° C. to obtain a negative electrode mixture molded body with a thickness of 300 mm. A cross section of a portion of the obtained molded body was observed using SEM to confirm that the PTFE was fibrillated, and the average diameter of the carbon nanotubes was determined. The average diameter of the obtained carbon nanotubes was 1000 nm.

<Preparation of positive electrode>
A coating solution for forming a reaction suppression layer was prepared by mixing 0.86 g of lithium and 38.7 g of pentaethoxyniobium in 394 g of dehydrated ethanol. Next, the coating solution for forming a reaction suppression layer was applied to 1,000 g of positive electrode active material (LiCoO ₂ ) at a rate of 2 g per minute using a coating device using a tumbling fluidized bed. The resulting powder was heat-treated at 350°C to obtain a positive electrode material with a reaction suppression layer formed on its surface, composed of 2 parts by mass of LiNbO ₃ per 100 parts by mass of positive electrode active material.

The cathode material, vapor-grown carbon fiber (conductive additive), and _Li6PS5Cl (sulfide-based solid electrolyte) were mixed to prepare _a cathode mixture. The mixture ratio of the cathode material, conductive additive, and sulfide-based solid electrolyte was 66:4:30 by mass. This cathode mixture was placed in a powder molding die with a diameter of 9.0 mm and molded at a pressure of 1000 kgf/ ^cm2 using a press to produce a cylindrical cathode mixture molded body.

<Formation of solid electrolyte layer>
The same sulfide-based solid electrolyte as that used for the positive electrode was placed on the positive electrode mixture molded body in the powder molding die, and molding was performed using a press at a pressure of 1000 kgf/ ^cm2 , thereby forming a solid electrolyte layer on the positive electrode mixture molded body.

<Preparation of electrode body>
The intermediate layer-forming sheet was punched out to a diameter of 9.0 mm and placed on top of the solid electrolyte layer in the powder molding die, and the negative electrode mixture molded body was punched out to a diameter of 9.0 mm and placed on top of the intermediate layer-forming sheet, and a carbon-coated aluminum foil was punched out to a diameter of 9.0 mm and placed on top of that, and molding was performed using a press at a pressure of 6000 kgf/ ^cm2 , to obtain a laminated electrode body in which a positive electrode, a solid electrolyte layer, an intermediate layer, and a negative electrode were sequentially stacked.

<Assembly of all-solid-state secondary batteries>
Two sheets of flexible graphite sheet "PERMA-FOIL (product name)" (thickness: 0.1 mm, apparent density: 1.1 g/cm ³ ) manufactured by Toyo Tanso Co., Ltd. were punched to the same size as the laminated electrode body, and one of them was placed on the inner bottom surface of a stainless steel sealed can fitted with a polypropylene annular gasket. Next, the laminated electrode body was placed on top of the graphite sheet with the negative electrode facing the graphite sheet side, and another graphite sheet was placed on top of that. A stainless steel outer can was then placed over the laminated electrode body, and the open edge of the outer can was crimped inward to seal it, thereby producing a flat all-solid-state secondary battery with a diameter of approximately 9 mm, in which the graphite sheets functioning as current collectors were placed between the inner bottom surface of the sealed can and the laminated electrode body, and between the inner bottom surface of the outer can and the laminated electrode body, as shown in FIG.

Example 2
A negative electrode mixture molded body was produced in the same manner as in Example 1 except that no carbon nanotubes were used in the negative electrode and the graphene content was changed to 9.0 mass%, and a flat all-solid-state secondary battery was produced in the same manner as in Example 1 except that this negative electrode mixture molded body was used.

Example 3
A negative electrode mixture molded body was produced in the same manner as in Example 1 except that PTFE was not used, and a flat all-solid-state secondary battery was produced in the same manner as in Example 1 except that this negative electrode mixture molded body was used.

Example 4
A negative electrode mixture molded body was produced in the same manner as in Example 1, except that the negative electrode active material was changed from TiNb ₂ O ₇ to Cu _0.21 Al _0.74 Nb _11.05 O _27.89. A flat all-solid-state secondary battery was produced in the same manner as in Example 1, except that this negative electrode mixture molded body was used.

Comparative Example 1
A flat all-solid-state secondary battery was fabricated in the same manner as in Example 1, except that no intermediate layer was provided and the amount of solid electrolyte for forming the solid electrolyte layer was increased by the amount of the reduced intermediate layer.

Comparative Example 2
A flat all-solid-state secondary battery was fabricated in the same manner as in Example 2, except that no intermediate layer was provided and the amount of solid electrolyte for forming the solid electrolyte layer was increased by the amount of the reduced intermediate layer.

Comparative Example 3
A flat all-solid-state secondary battery was fabricated in the same manner as in Example 3, except that no intermediate layer was provided and the amount of solid electrolyte for forming the solid electrolyte layer was increased by the amount of the reduced intermediate layer.

The Coulombic efficiency of each flat all-solid-state secondary battery in the Examples and Comparative Examples was measured as follows. A program was created in which each battery was charged at a constant current of 0.1 C based on the positive electrode active material, and then at a constant voltage once the upper limit voltage (3.2 V) was reached, and each battery was charged using this program. Charging was stopped when the current value during constant voltage charging decreased to 0.01 C, and after measuring the open circuit voltage of each charged battery for 10 minutes, a constant current discharge at 0.1 C was performed to determine the discharge capacity. The value of the discharge capacity/charge capacity was then calculated for each battery, and this was used as the Coulombic efficiency (charge/discharge efficiency).

The measurement results, along with the presence or absence of an intermediate layer and the configuration of the negative electrode, are shown in Table 1. Note that "CNT" in Table 1 stands for carbon nanotubes.

As shown in Table 1, the flat all-solid-state secondary batteries of Examples 1 to 4, which had an intermediate layer, exhibited higher coulombic efficiency than the batteries of Comparative Examples 1 and 2, which did not have an intermediate layer. It was difficult to fabricate the flat all-solid-state secondary battery of Comparative Example 3 because the electrodes peeled off during the fabrication of the electrode laminate.

Furthermore, the batteries of Examples 1 and 4, which contained both graphene and CNT in the negative electrode, exhibited higher coulombic efficiency than the battery of Example 2, which did not contain CNT, and the battery of Example 3, which did not contain fibrous resin.

The present invention may be implemented in forms other than those described above without departing from its spirit. The embodiments disclosed in this application are merely examples, and the present invention is not limited to these embodiments. The scope of the present invention shall be interpreted in accordance with the appended claims rather than the above specification, and all modifications within the scope of the claims shall be included within the scope of the claims.

The all-solid-state secondary battery of the present invention can be used in the same applications as known secondary batteries, but as mentioned above, due to its excellent heat resistance, it can be preferably used in applications where it is exposed to high temperatures.

1, 100 All-solid-state secondary battery 10 Positive electrode 11 Molded body of positive electrode mixture (positive electrode mixture layer)
12 Current collector 20 Negative electrode 21 Molded body of negative electrode mixture (negative electrode mixture layer)
22 Current collector 30 Solid electrolyte layer 40 Intermediate layer 50 Outer can 60 Sealing can 70 Gasket 200 Electrode body 300 Positive electrode external terminal 400 Negative electrode external terminal 500 Laminate film outer case

Claims (7)

An all-solid-state secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode,
an intermediate layer containing a fibrous resin and a solid electrolyte interposed between the negative electrode and the solid electrolyte layer; The all-solid-state secondary battery described in claim 1, wherein the negative electrode contains fibrous carbon as a conductive additive and fibrous resin. The all-solid-state secondary battery according to claim 1, wherein the intermediate layer contains at least one of a sulfide-based solid electrolyte and a chloride-based solid electrolyte as the solid electrolyte. The all-solid-state secondary battery described in claim 1, wherein the intermediate layer contains 0.1 to 10 mass% of the fibrous resin and 90 to 99.9 mass% of the solid electrolyte. The all-solid-state secondary battery according to claim 1, wherein the solid electrolyte content in the solid electrolyte layer is 70 mass% or more. The all-solid-state secondary battery according to claim 1, wherein the negative electrode contains a niobium composite oxide as a negative electrode active material. The all-solid-state secondary battery according to claim 6, wherein the niobium composite oxide has a monoclinic crystal structure and satisfies the following general formula (1):
A _y M ¹ _α Al _x-α Nb _12-x-z M ² _z O _29-δ (1)
[In the general formula (1), A is at least one element of Li and Na, ^M1 is at least one element selected from the group consisting of Fe, Mn, Zn, Cu, Ag, Mg, Ca, Sr, Ba, Co, Eu, Y, Bi, La, Ce, Nd, Sm, and Gd, ^M2 is at least one element selected from the group consisting of K, Ti, Ni, Zr, V, Mo, Ta, and W, and 0<x≦1.1, 0≦y≦24, 0≦z≦2, −1≦δ≦2, and 0<α≦0.4x.]