JP7688500B2 - Electrode and power storage device - Google Patents

Description

This specification discloses an electrode and an electricity storage device.

Conventionally, as a lithium secondary battery as an electric storage device, a battery having a negative electrode material layer and a positive electrode material layer made of an active material and a lithium ion conductive solid electrolyte, in which the mixing ratio of the active material to the solid electrolyte is continuously decreased according to the distance from the current collector, has been proposed (see, for example, Patent Document 1). This lithium secondary battery is said to be capable of realizing the thinning/large size required for the high performance/high capacity of lithium ion batteries, and to prevent the active material of the electrode material layer from migrating to the solid electrolyte layer side. In addition, as a lithium secondary battery, a battery having an all-solid-state electrode with a continuous gradient of concentration has been proposed in which the concentration of the positive electrode active material close to the positive electrode current collector is higher than the concentration of the positive electrode active material close to the solid electrolyte layer, and the concentration of the negative electrode active material close to the negative electrode current collector is higher than the concentration of the negative electrode active material close to the solid electrolyte layer (see, for example, Patent Document 2). It is said that this lithium secondary battery can improve the output characteristics of an all-solid-state battery. Also, as a lithium secondary battery, an electrode active material layer of at least one of the positive and negative electrode bodies has been proposed that has a composition distribution in which the local volume ratio of the electrode active material to the volume of the solid electrolyte material contained in a part of the electrode active material layer increases as the thickness direction of the electrode active material layer approaches from the solid electrolyte layer interface side to the current collector interface side (see, for example, Patent Document 3). It is said that this lithium secondary battery can reduce the diffusion resistance and improve the rate characteristics. Also, as a lithium secondary battery, a cathode mixture has been proposed that has a first cathode active material, a second cathode active material, and a sulfide solid electrolyte, and is characterized in that the ratio of the average particle size of the first cathode active material to the average particle size of the second cathode active material is 2.0 to 4.3 (see, for example, Patent Document 4). It is said that this lithium secondary battery can provide a cathode mixture with a high energy density per volume.

JP 2011-124028 A JP 2015-225855 A JP 2012-104270 A JP 2019-106286 A

However, in the electricity storage devices of Patent Documents 1 to 4, although the composition and concentration are adjusted to improve the battery characteristics, the increase in capacity at high output (e.g., 2C rate or higher) is still insufficient, and further improvements are required.

This disclosure has been made in consideration of these issues, and its primary objective is to provide an electrode and an electricity storage device that can further improve capacity at high output.

After extensive research to achieve the above-mentioned objective, the inventors discovered that in an electrode containing a solid electrolyte and an electrode active material in a specified range, if the rich and poor phases of the solid electrolyte are set within a specified range, the capacity at high output can be further improved, and thus completed the invention disclosed in this specification.

That is, the electrode of the present disclosure is
An electrode for use in an electricity storage device,
The electrode includes a solid electrolyte and an electrode active material,
The volume ratio A/B of the solid electrolyte A to the electrode active material B in the entire electrode is in the range of 10/90 or more and 25/75 or less,
a first phase in which a volume ratio A/B of the solid electrolyte A to the electrode active material B is in the range of 25/75 to 50/50;
a second phase in which a volume ratio A/B of the solid electrolyte A to the electrode active material B is in the range of 0/100 to 16/84;
It has the following characteristics.

The electricity storage device of the present disclosure comprises:
An electricity storage device that is an all-solid-state secondary battery,
A positive electrode and
A negative electrode;
a solid electrolyte layer interposed between the positive electrode and the negative electrode and conducting carrier ions;
At least one of the positive electrode and the negative electrode is the electrode described above.

In the electrode and the power storage device of the present disclosure, the capacity at high output can be further improved. The reason for obtaining such an effect is presumed to be as follows. For example, when the electrode contains an electrode active material and a solid electrolyte and the electrode active material has a predetermined volume ratio that is sufficiently larger than the solid electrolyte, the connection between the particles of the solid electrolyte, that is, the formation of a conduction path of the carrier ions is inhibited, and the effective ion conductivity in the electrode is significantly reduced, which may lead to a decrease in capacity at high output. In contrast, in the present disclosure, it is presumed that the decrease in the effective ion conductivity can be suppressed by separating the total amount of solid electrolyte into two phases, a first phase (rich phase) with a large amount of solid electrolyte and a second phase (poor phase) with a small amount of solid electrolyte, while keeping the total amount of solid electrolyte the same. In addition, it is presumed that this is because, with such a structure, the change in the effective ion conductivity with respect to the active material ratio is no longer linear, and the effective ion conductivity derived from the two-phase mixing rule is improved. In this way, in the present disclosure, it is presumed that the improvement in the effective ion conductivity leads to an increase in capacity at high output.

FIG. 2 is an explanatory diagram showing an example of the structure of the electricity storage device 10. FIG. 2 is an explanatory diagram showing the relationship between the amount of solid electrolyte and the Li conductive path. FIG. 2 is an image diagram of the mechanism for improving ionic conductivity by the first phase and the second phase. A model diagram of the actual electrode structure. A relationship diagram between actual measurements and calculated results of changes in positive electrode potential during charging. FIG. 1 is an explanatory diagram of a virtual prototype of a two-phase separated electrode. FIG. 1 is a diagram showing the relationship between the domain size of the first phase and the effective ionic conductivity. FIG. 1 is a relationship diagram between the charge capacity rate and the positive electrode potential during 3C charging.

(Electrodes for Electricity Storage Devices)
The electrodes of the present disclosure described in the embodiments are used in an electricity storage device. The electricity storage device may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, an alkali metal secondary battery, an alkali metal ion battery, or the like. Examples of carrier ions in the electricity storage device include alkali metal ions such as lithium ions, sodium ions, and potassium ions, and Group 2 ions such as magnesium ions, strontium ions, and calcium ions. Here, for convenience of explanation, a lithium secondary battery using lithium ions as a carrier will be described below as a main example of an electricity storage device.

The electrode of the present disclosure includes a solid electrolyte and an electrode active material. The electrode may include an electrode mixture layer including the solid electrolyte and the electrode active material, and a current collector adjacent to the electrode mixture layer. The electrode may also be a positive electrode including a positive electrode active material, or a negative electrode including a negative electrode active material. The electrode has a composition in which the volume ratio A/B of the solid electrolyte A and the electrode active material B of the entire electrode is in the range of 10/90 to 25/75. That is, when the entire solid electrolyte and the electrode active material are 100% by volume, the solid electrolyte is in the range of 10% by volume to 25% by volume, and the electrode active material is in the range of 90% by volume to 75% by volume. It is preferable that the volume ratio A/B of the entire electrode has a composition in the range of 15/85 or more. If the volume ratio A/B is in such a range, the amount of the electrode active material is relatively large, so that the capacity decrease of the electrode caused by the addition of the solid electrolyte can be further suppressed. This electrode has a two-phase separation structure with a first phase (electrolyte-rich phase) that contains a relatively large amount of solid electrolyte and a second phase (electrolyte-poor phase) that contains a relatively small amount of solid electrolyte.

In the first phase, the volume ratio A/B of the solid electrolyte A and the electrode active material B is preferably in the range of 25/75 or more and 50/50 or less. In this first phase, the volume ratio A/B is more preferably in the range of more than 25/75 and 40/60 or less, and even more preferably in the range of more than 28/72. In addition, the volume ratio A/B of the first phase may be in the range of 30/70 or more, or may be in the range of more than 30/70. That is, when the total of the solid electrolyte and the electrode active material is 100 volume%, the solid electrolyte in the first phase is preferably in the range of 25 volume% or more and 50 volume% or less, more preferably in the range of more than 25 volume% and 40 volume% or less, and even more preferably in the range of more than 28 volume%. In addition, the solid electrolyte in the first phase may be 30 volume% or more, or may be in the range of more than 30 volume%. In addition, the first phase is preferably in the range of 75 volume % or less and 50 volume % or more of the electrode active material, more preferably in the range of less than 75 volume % and 40 volume % or more, and even more preferably in the range of 72 volume % or less. In addition, the first phase may be in the range of 70 volume % or less of the electrode active material, or may be in the range of less than 70 volume %. If this electrode contains such a first phase, it is easier to ensure ionic conductivity in the electrode, which is preferable.

In the second phase, the volume ratio A/B of the solid electrolyte A to the electrode active material B is preferably in the range of 0/100 or more and 16/84 or less. In this second phase, the volume ratio A/B is more preferably in the range of 15/85 or less, and even more preferably in the range of less than 14/86. In addition, the volume ratio A/B of the second phase may be in the range of 13/87 or less, or may be in the range of less than 13/87. That is, when the total of the solid electrolyte and the electrode active material is 100 volume%, the solid electrolyte is preferably in the range of 0 volume% or more and 16 volume% or less, more preferably in the range of 15 volume% or less, and even more preferably in the range of less than 14 volume%. In addition, the solid electrolyte may be in the range of 13 volume% or less and may be in the range of less than 13 volume%. In addition, in the second phase, the electrode active material is preferably in the range of 100 volume% or less and 84 volume% or more, more preferably in the range of 85 volume% or more, and even more preferably in the range of more than 86 volume%. In addition, the second phase may be 87% by volume or more of the electrode active material, or may be in a range exceeding 87% by volume. If this electrode contains such a second phase, the relative decrease in the electrode active material is further suppressed, and it is easier to ensure the battery capacity, which is preferable.

The first phase preferably has a domain (droplet) size R that is at least twice the particle diameter d50 of the larger particles of the solid electrolyte and the electrode active material, and is in the range of 1/2 or less of the thickness T of the electrode (see FIG. 1). When the domain size R of the first phase is at least twice the particle diameter d50, the variation in ion conductivity can be further suppressed, which is preferable. Also, when the domain size R of the first phase is 1/2 or less of the thickness T of the electrode, the size of one droplet is not too large, and the occurrence of reaction unevenness in the surface can be further suppressed, which is preferable. The first phase may have a domain size in the range of 10 μm or more and 20 μm or less. The domain size R of the first phase can be determined, for example, by performing elemental analysis on the cut surface of the electrode to determine the region with a large amount of solid electrolyte, and is defined as the maximum length of this region.

It is preferable that the particle size d50 of the solid electrolyte is in a range not exceeding the particle size d50 of the electrode active material. If the particle size of the solid electrolyte is in such a range, the electrode active material is more likely to absorb and release carrier ions, which is preferable. The particle size d50 of the solid electrolyte may be, for example, in the range of 0.1 μm to 10 μm. The particle size d50 of the electrode active material may be, for example, in the range of 0.1 μm to 20 μm. The particle size of the solid electrolyte and the electrode active material is the median diameter determined by laser diffraction measurement.

In this electrode, the first and second phases preferably have a structure in which they are randomly or alternately dispersed in two phases in a plane perpendicular to the electrode thickness direction. The first phase may also be dispersed in the second phase. In this electrode, it is preferable that there are fewer layers composed of only the first phase and layers composed of only the second phase in the distribution in the electrode thickness direction and the electrode surface direction, and it is preferable that the first and second phases are mixed. In this electrode, the electrode mixture may have a region with a high solid electrolyte content (first phase) dispersed in a mottled manner in a region with a low solid electrolyte content (second phase).

The electrode preferably has a porosity in the range of 0% by volume or more and 35% by volume or less. Fewer voids result in a relatively larger amount of electrode active material, which is preferable from the standpoint of capacity and ionic conductivity. Larger voids can accommodate metals that precipitate during charging and discharging, which may be preferable from the standpoint of durability. This porosity may be 5% by volume or more, 10% by volume or more, or 30% by volume or less, or 20% by volume or less.

The thickness T of the electrode is not particularly limited, but may be set appropriately depending on the capacity of the power storage device. For example, the thickness T of the electrode may be 20 μm or more, 50 μm or more, 500 μm or less, 300 μm or less, or 200 μm or less.

The electrode may be a positive electrode containing a positive electrode active material as an electrode active material. The electrode may contain an electrode active material and a solid electrolyte, and may also contain a conductive material, a binder, and the like. The electrode may be formed, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, and the like can be used. Specifically, transition metal sulfides such as _TiS2 , _TiS3 , _MoS3 , and _FeS2 , lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0<x<1, etc., the same applies below) and Li _{(1-x) Mn2O4} , lithium cobalt composite oxides having a basic composition formula such as Li _{(1-x) CoO2 ,} lithium nickel composite oxides having a basic composition formula such as Li( _{1-x ) NiO2} , lithium _{nickel cobalt} manganese composite oxides having a basic composition formula such as _Li (1-x) _NiaCobMncO2 (a+b+c=1 ₎ , lithium vanadium composite oxides having a basic composition formula such as _LiV2O3 , and transition metal oxides having a basic composition _formula such as _V2O5 can be used. Among these, lithium transition metal composite oxides, such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiNi _1/3 Co _1/3 Mn _1/3 O _2, are preferred. The term "basic composition formula" means that other elements may be included. Alternatively, the positive electrode active material may be a carbonaceous material used in capacitors and lithium ion capacitors. Examples of carbonaceous materials include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbonaceous material preferably has a specific surface area of 1000 m ² /g or more, and more preferably 1500 m ² /g or more. When the specific surface area is 1000 m ² /g or more, the discharge capacity can be further increased. From the standpoint of ease of preparation, the specific surface area of this activated carbon is preferably 3000 m ² /g or less, and more preferably 2000 m ² /g or less.

The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance of the electrode. For example, graphite such as natural graphite (scale graphite, flake graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.), etc., or a mixture of two or more of them can be used. Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoint of electronic conductivity and coatability. The binder plays a role in connecting the active material particles and the conductive material particles, and can be, for example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine rubber, etc., or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR), etc., used alone or as a mixture of two or more. In addition, a cellulose-based aqueous binder or an aqueous dispersion of styrene butadiene rubber (SBR) can be used. As a solvent for dispersing the positive electrode active material, conductive material, and binder, for example, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. In addition, a dispersant, a thickener, and the like can be added to water, and the active material can be slurried with latex such as SBR. As a thickener, for example, polysaccharides such as carboxymethylcellulose and methylcellulose can be used alone or as a mixture of two or more kinds. Examples of the application method include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any thickness and shape can be obtained by using any of these. Examples of current collectors include aluminum, titanium, stainless steel, nickel, iron, baked carbon, conductive polymers, conductive glass, and the like, as well as aluminum or copper whose surfaces have been treated with carbon, nickel, titanium, silver, or the like for the purpose of improving adhesion, conductivity, and oxidation resistance. The surfaces of these can also be subjected to oxidation treatment. Current collectors can be in the form of foil, film, sheet, net, punched or expanded, lath, porous, foamed, or formed from fibers. The thickness of the current collector is, for example, 1 to 500 μm.

The electrode may be formed by, for example, bonding the negative electrode active material and the current collector, or may be formed by mixing the negative electrode active material, the conductive material, and the binder, adding an appropriate solvent to form a paste-like negative electrode composite, applying it to the surface of the current collector, drying it, and compressing it to increase the electrode density as necessary. Examples of the negative electrode active material include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbonaceous materials capable of absorbing and releasing lithium ions, composite oxides containing multiple elements, and conductive polymers. Examples of the carbonaceous material include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, can be charged and discharged at a high operating voltage, suppress self-discharge when a lithium salt is used as a supporting salt, and reduce irreversible capacity during charging. Examples of the composite oxide include lithium titanium composite oxide and lithium vanadium composite oxide. Of these, carbonaceous materials are preferred as the negative electrode active material from the viewpoint of safety. The conductive material, binder, solvent, etc. used in the negative electrode can be the same as those exemplified for the positive electrode. In addition to copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the negative electrode current collector can also be made of copper or the like whose surface has been treated with carbon, nickel, titanium, silver, etc. for the purpose of improving adhesion, conductivity, and reduction resistance. The surfaces of these materials can also be oxidized. The shape of the current collector can be the same as that of the positive electrode.

The electrode includes a particulate solid electrolyte. The solid electrolyte may be, for example, particles of a garnet-type oxide containing at least Li, La, and Zr. The solid electrolyte may have a basic composition of Li7.0 _+xy (La3 _-x , _Ax )(Zr2 _-y , _Ty ) _O12 , where A is one or more of Sr and Ca, T is one or more of Nb and Ta, and 0<x≦1.0 and 0<y<0.75 are satisfied. Alternatively, the solid electrolyte may be a garnet-type oxide represented by the basic composition (Li7-3z _{+xyMz )} (La3 _{- xAx} )(Zr2 _{-yTy ) O12} or (Li7-3z _+xyMz )( _{La3 - xAx} )( _{Y2- yTy} ) _O12 , where in the formula, element M is one or more of Al and Ga, element A is one or more of Sr and Ca, T is one or more of Nb and Ta, and 0≦z≦0.2, 0≦x≦0.2, and 0≦y≦2. Examples of this solid electrolyte include _Li3N , _Li14Zn ( _GeO4 ) ₄ known as LISICON, the sulfide _{Li3.25Ge0.25P0.75S4} , the perovskite type _{La0.5Li0.5TiO3} , (La2 _{/ 3Li3x} □ _{1/3-2x ) TiO3} (□: atomic vacancy), and _the NASICON type _LiTi2 ( _PO4 ) _{3 and Li1.3M0.3Ti1.7} ( _PO3 ) _{4 (} M ₌ Sc _, Al ₎ . Other examples include Li ₇ P ₃ S ₁₁ obtained from glass having a composition of 80Li ₂ S·20P ₂ S ₅ (mol %), which is a glass ceramic, and Li ₁₀ Ge ₂ PS _2, which is a sulfide-based substance having high electrical conductivity. Examples of glass-based inorganic solid electrolytes include _Li2S - _SiS2 , _Li2S -SiS2- _LiI , _Li2S - _SiS2 -Li3PO4, _Li2S -SiS2- _Li4SiO4 , _{Li2S - P2S5} , _{Li3PO4 - Li4SiO4 , Li3BO4 - Li4SiO4} , _and those _{in which SiO2} , _{GeO2 , B2O3} , _{or P2O5} is the glass- _based substance _{and Li2O} is the network modifier. In addition, examples of thio-liquid oxide solid electrolytes include _Li2S -GeS2 _- based, _Li2S -GeS2- _ZnS -based, _Li2S - _{Ga2S2 -based} , _Li2S - _{GeS2- Ga2S3 -} based, _Li2S - _{GeS2- P2S5 -} based, _Li2S -GeS2- _{SbS5 - based} , _Li2S - _GeS2 - _Al2S3 -based, _Li2S -SiS2-based, Li2S- _{P2S5 - based} , _Li2S - _Al2S3 - _based , LiS- _{SiS2 - Al2S3 -based} , and _Li2S - _SiS2 - _{P2S5 -} based solid electrolytes.

(Electrode manufacturing method)
The manufacturing method of the electrode may be a method for manufacturing the above-mentioned electrode, and may include a two-phase separation process for preparing an electrode including a first phase containing a relatively rich amount of solid electrolyte and a second phase containing less solid electrolyte than the first phase. In this process, the raw materials may be mixed biasedly so that the volume ratio A/B of the solid electrolyte A and the electrode active material B is within the range of the above-mentioned range. In this process, the raw materials to be the first phase may be dispersed in the second phase, and then pressure may be applied to form the electrode, or the raw materials to be the first phase and the raw materials to be the second phase, which are individually accumulated, may be simultaneously sprayed onto the electrode substrate. In addition, by adding a certain amount of relatively coarse electrode active material particles as electrolyte particles, a domain in which the electrolyte is sparse, that is, the second phase, can be formed in the electrode, so that the electrode targeted in the present disclosure can also be manufactured by controlling the particle size of the raw material powder used for the electrode.

(Electricity storage device)
The power storage device may include a positive electrode, a negative electrode, and an ion-conducting medium interposed between the positive electrode and the negative electrode and conducting carrier ions. In this power storage device, at least one of the positive electrode and the negative electrode may be the electrode of the present disclosure described above.

The ionically conductive medium may include an electrolyte that conducts carrier ions. Examples of the electrolyte include nonaqueous solvents. Examples of the solvent for the electrolyte include solvents for nonaqueous electrolytes. Examples of the solvent include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specifically, examples of the carbonates include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; and γ-butyl carbonate. Examples of the electrolyte include cyclic esters such as valerolactone and γ-valerolactone, chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane, nitriles such as acetonitrile and benzonitrile, furans such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. The electrolyte may be one in which a supporting salt containing ions that serve as carriers for the secondary battery is dissolved. Examples of _the supporting salt include LiPF6, _LiBF4 , _LiAsF6 , _LiCF3SO3 , _LiN ( _CF3SO2 ) ₂ , LiC( _CF3SO2 ) ₃ , LiSbF6, _LiSiF6 , _LiAlF4 , LiSCN, _LiClO4 , LiCl, _LiF , _LiBr , _LiI , and _LiAlCl4 . Among these, it is preferable to use one or more salts selected from the group consisting of inorganic salts _such as _LiPF6 , _LiBF4 , and _LiClO4 , and organic salts such as _LiCF3SO3 , _LiN ( _CF3SO2 ) ₂ , and LiC( _CF3SO2 ) ₃ in combination from _the viewpoint of electrical properties. The concentration of this supporting salt in the electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, and more preferably 0.5 mol/L or more and 2 mol/L or less.

The power storage device may be an all-solid-state secondary battery having a solid electrolyte layer that conducts carrier ions between the positive electrode and the negative electrode. The solid electrolyte layer between the positive electrode and the negative electrode may be any one or more of the above-mentioned solid electrolytes. The solid electrolyte contained in the solid electrolyte layer may be the same as or different from the solid electrolyte contained in the electrode. The solid electrolyte layer may have a substance containing boron present at the grain boundaries of the solid electrolyte particles. This substance is preferably lithium borate. Examples of lithium borate include Li ₃ BO ₃ , Li ₂ B ₄ O ₇ , and LiBO ₂ , and among these, Li ₃ BO ₃ (LBO) is more preferable. LBO is preferable because it has a lower reactivity with the solid electrolyte. The solid electrolyte layer may have an ion conductivity of 1.0×10 ⁻⁴ S/cm or more at 25° C. The ionic conductivity of the solid electrolyte layer is preferably higher, and the conductivity at 25° C. is preferably 1.0×10 ⁻⁴ (S/cm) or more, and more preferably 2.5×10 ⁻⁴ S/cm or more. This conductivity may be 1.0×10 ⁻² S/cm or less. Note that the electrical conductivity represents ionic conductivity.

1 is an explanatory diagram showing an example of the structure of an electricity storage device 10. This electricity storage device 10 has a positive electrode 12, a negative electrode 15, and a solid electrolyte layer 18. The positive electrode 12 has a positive electrode active material layer 13 and a current collector 14. The negative electrode 15 has a negative electrode active material layer 16 and a current collector 17. Either the positive electrode 12 or the negative electrode 15 is an electrode for the above-mentioned electricity storage device, has a two-phase separation type structure including a first phase 21 and a second phase 22, and may include voids 23. The first phase 21 is an electrolyte-rich phase that includes an electrode active material 24 and a solid electrolyte 25, and contains a relatively large amount of the solid electrolyte 25. This first phase 21 has a domain (droplet) structure and exists as a specific region in the second phase 22. The second phase 22 contains an electrode active material 24 and may contain a solid electrolyte 25, and is an electrolyte-poor phase containing less solid electrolyte 25 than the first phase 21. In this electrode, the first phase 21 and the second phase 22 have a structure in which they are randomly or alternately dispersed in two phases in a plane perpendicular to the electrode thickness direction. The first phase 21 may also have a structure in which they are dispersed in the second phase 22.

In the electrode and the power storage device described above, the capacity at high output can be further improved. The reason for obtaining such an effect is presumed to be as follows. FIG. 2 is an explanatory diagram showing the relationship between the amount of solid electrolyte (SE) and the Li conductive path, where FIG. 2A shows a case where there is a large amount of electrolyte, FIG. 2B shows a case where there is a small amount of electrolyte randomly, and FIG. 2C shows a case where there is a small amount of electrolyte and there is two-phase separation. FIG. 3 is an image diagram of the ion conductivity improvement mechanism of a two-phase separation type structure with a first phase and a second phase. For example, as shown in FIG. 2A, when the electrode active material and the solid electrolyte are included and the electrolyte is relatively large, the ion conductive path is maintained. However, since the electrolyte does not contribute to the charge/discharge capacity, it is preferable to have less of it. And, as shown in FIG. 2B, when the electrode active material has a volume ratio sufficiently large compared to the solid electrolyte, the connection between the particles of the solid electrolyte, that is, the formation of the conductive path of the carrier ions is inhibited, which may cause a significant decrease in the effective ion conductivity in the electrode, leading to a decrease in the capacity at high output. In contrast, in the present disclosure, as shown in FIG. 2C, the total amount of solid electrolyte is kept small, and the first phase (SE-rich phase) is separated into two phases, a first phase with a large amount of solid electrolyte (SE-rich phase) and a second phase (SE-poor phase) with a small amount of solid electrolyte, so that it is believed that the decrease in effective ionic conductivity can be suppressed. In addition, with such a structure, as shown by the dotted line in FIG. 3, the change in effective ionic conductivity with respect to the active material ratio is no longer linear, and the effective ionic conductivity derived from the two-phase rule of mixtures is improved. Thus, in the present disclosure, it is believed that the improvement in effective ionic conductivity leads to an increase in capacity at high output.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in various forms as long as they fall within the technical scope of the present disclosure.

Below, we will explain examples in which the electrodes and power storage devices disclosed herein are specifically examined.

The electrode structure was searched for using a virtually created 3D structure. First, prior to the electrode search, it was confirmed whether the experimental charging curve could be reproduced from a positive electrode structure consisting of active material particles, electrolyte particles, and conductive material virtually created on a computer. The active material and electrolyte particle size distribution was modeled to reproduce the experimental particle size distribution measurement results by laser diffraction, and the void size was modeled to reproduce the void size distribution in the SEM image. The electrode thickness T was set to 50 μm, the volume ratio A/B of electrolyte A and active material B was set to 20/80, and the void ratio was set to 33.5 volume%, and a model simulating the actual structure shown in Figure 4 was created. Figure 4 is a model diagram of the actual electrode structure. In Figure 4, the white indicates active material particles, the light gray indicates solid electrolyte (SE) particles, the fibrous conductive material, and the black indicates residual pores. The compounding ratios of each component in the created positive electrode model and the experimental compounding ratios are summarized in Table 1.

Next, the active material was set to NCM811 (LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ), and the material property parameters (mainly the Li diffusion coefficient value in the active material) used in the charging simulation were set. The charging simulation was performed using a program disclosed in a non-patent document (Solid State Ionics 2020; 344: 115079). Here, the charging simulation will be described. The effective ionic conductivity σ _e of the electrode is expressed by formula (1). Here, σ _m is the conductivity specific to the material, and θ is the volume fraction (%) of the material contributing to the conduction. τ is a tortuosity factor, and is defined as τ = (L' / L) ² using the actual conduction distance L' for the distance L connecting the start point and the end point of the conduction with a straight line. In electrodes with the same compounding ratio of the electrode constituent materials and each material, that is, electrodes with the same σ _m and θ, the relative ratio of the effective ionic conductivity can be obtained by evaluating the τ value. The τ value evaluation is performed by AC impedance calculation based on a 3D electrode image. In AC impedance calculation, the conductivity of the phase for which the τ value is to be evaluated is set to about 1/100 of the conductivity of the other phases so that a conduction current flows through the phase for which the τ value is to be evaluated, and then the resistance value of the real axis intercept on the low frequency side (i.e., the total resistance value) is compared with the resistance inherent to the material, whereby the τ value can be obtained. The main coefficients used in the simulation were the Li diffusion coefficient in the active material of 2×10 ^-9 (cm ² /s) and the reaction resistance of 80 (Ωcm ² ).
σ _e = ((θ/100)/τ)×σ _m …Formula (1)

The charging conditions were such that charging was started at 0.1C, 0.5C, and 3C from a remaining capacity SOC of 10%. A remaining capacity SOC of 100% was equivalent to 206 mAh/g. The relationship between the electrode voltage (V) and the charge capacity rate ΔSOC at the lithium metal reference potential was compared. FIG. 5 is a relationship diagram of the change in the positive electrode potential during charging between the actual measurement and the calculation result. As shown in FIG. 5, it was confirmed that the charging simulation used in this embodiment reproduced the experimental results under each charging condition of 0.1C to 3C. Using a model that confirmed that the actual measured values from the experiment could be quantitatively reproduced in this way, the effective ionic conductivity was evaluated for various electrode structures. In addition, a charging calculation was performed at a high rate (3C = 0.6 mA/cm ² ) to confirm the performance.

A two-phase separated electrode having a first phase and a second phase is shown in Figure 1. This structure can be specified by specifying (1) the volume ratio of the active material and electrolyte in the first phase and the second phase, and (2) the domain size of the first phase. The compounding ratio of the first phase and the second phase is derived from the volume ratio.

Figure 6 is an image of a virtual prototype of a two-phase separation electrode, where Figure 6A is a model including domains (droplets) of the first phase (SE-rich phase) containing a relatively large amount of solid electrolyte, and Figure 6B is a model in which particles are arranged so that the volume ratio of the active material and electrolyte is a predetermined value. The electrode structure shown in Figure 6 is a model in which the volume ratio A/B of electrolyte A and active material B in the entire electrode is 20/80, the volume ratio A/B of electrolyte A and active material B in the first phase is 31/69, the volume ratio A/B of electrolyte A and active material B in the second phase is 10/90, the porosity is 10 volume %, the conductive material is 2 volume %, the active material particle size is 4 μm, and the electrolyte particle size is 2 μm. In this way, a 3D structure was virtually prototyped, and the optimal ranges for (1) and (2) above were determined from the effective ionic conductivity of the electrode body. Next, a comparison was made of the charging capacity at a high output of 3C rate between a structure within the optimal range and a conventional structure in which the active material and solid electrolyte are randomly dispersed.

The volume ratio A/B of the solid electrolyte A and the active material B in the entire electrode including the first and second phases was set to 20/80. The domain size of the first phase was set to 12 μm. The composition ratio of the first and second phases was appropriately changed to obtain the ionic conductivity. The ionic conductivity of the conventional structure in which the solid electrolyte is present on average throughout the electrode was set to 1, and the results were normalized. Table 2 summarizes the evaluation results of the ionic conductivity of the two-phase separation type electrode when the conventional structure was normalized to 1. The ionic conductivity of 1.1 or more was evaluated as good, that of 1.3 or more was better, and that of 1.5 or more was even better. As shown in Table 2, it was inferred that the volume ratio A/B of the solid electrolyte A and the electrode active material B in the first phase is preferably in the range of 25/75 or more and 50/50 or less, preferably in the range of more than 25/75 and 40/60 or less, more preferably in the range of more than 28/72, and even more preferably in the range of 30/70 or more. It was also inferred that the volume ratio A/B of the second phase should be in the range of 0/100 to 16/84, preferably 15/85 or less, more preferably less than 14/86, and may be less than 12/88.

Figure 7 is a relationship diagram between the domain size (μm) of the first phase and the effective ionic conductivity. Here, evaluation was performed under three conditions of the first and second phases with different compositions. For compositions 1 to 3, the volume ratio A/B of the solid electrolyte A and the active material B in the entire electrode was set to 20/80. Furthermore, for compositions 1 to 3, the active material ratio in the second phase (SE-poor phase) was different, being 85 mass%, 86 mass%, and 90 mass%, respectively. The active material ratio in the first phase (SE-rich phase) of compositions 1 to 3 was the same, being set to 69 mass%. For compositions 1 to 3, evaluation was performed with domain (droplet) sizes of 8, 12, and 16 μm. As shown in Figure 7, when the domain size was small, such as less than 10 μm, the ionic conductivity tended to vary widely. The particle size d50 of the active material powder used was 5 μm, but it was presumed that the reason for this was that the number of particles contained was extremely reduced when the domain size was too small. It was presumed that if the size of one droplet is too large, such as when the domain size R exceeds 1/2 of the thickness T of the electrode, reaction unevenness within the surface is likely to occur. For this reason, it was presumed that the domain size R is preferably in the range of 1/2 or less of the electrode thickness T. In addition, since it is considered that the greater the number of particles contained in a domain, the less the variation in ion conductivity, it was presumed that the domain size R is preferably at least twice the particle size d50 of the larger particle of the active material particles and the solid electrolyte particles. In addition, since the particle size of the active material and solid electrolyte particles is preferably several μm to 10 μm, it was presumed that the domain size R of the first phase is preferably in the range of 10 μm to 20 μm. In addition, since the active material particles absorb and release ions, it is preferable that the particle size d50 of the solid electrolyte is in a range not exceeding the particle size d50 of the electrode active material.

Next, the charging capacity of a lithium secondary battery with a two-phase separated electrode as the positive electrode and a lithium secondary battery with a conventional structure was examined. Figure 8 shows the relationship between the charging capacity rate ΔSOC (%) and the positive electrode potential (V) during 3C charging. This charging capacity was evaluated using metallic Li as the counter electrode. Electrodes with compositions 4 to 6 were used to evaluate the charging capacity. Here, the active material ratio was changed to composition 4, with the first phase being 65 mass% and the second phase being 90 mass%, composition 5, with the first phase being 60 mass% and the second phase being 100 mass%, and composition 6, with the first phase and the second phase not being included and being uniform. For compositions 4 to 6, the volume ratio A/B of the solid electrolyte A and active material B in the entire electrode was set to 20/80. The above-mentioned charging conditions were such that charging was started at 3C from a remaining capacity SOC of 10%. A remaining capacity SOC of 100% was equivalent to 206 mAh/g. As shown in Figure 8, a 20% to 40% increase in charge capacity was confirmed for the two-phase separation electrode compared to the conventional electrode structure. In this way, it was found that the two-phase separation electrode can further increase capacity at a high rate of 3C charging.

It goes without saying that the electrodes and power storage devices disclosed herein are in no way limited to the above-described examples, and may be implemented in a variety of ways as long as they fall within the technical scope of the present disclosure.

This disclosure is applicable to the technical field of the battery industry.

10: power storage device, 12: positive electrode, 13: positive electrode active material layer, 14: current collector, 15: negative electrode, 16: negative electrode active material layer, 17: current collector, 18: solid electrolyte layer, 21: first phase, 22: second phase, 23: void, 24: electrode active material, 25: solid electrolyte, R: domain size, T: thickness.

Claims (6)

An electrode for use in an electricity storage device,
The electrode includes a solid electrolyte and an electrode active material,
The electrode has a volume ratio A/B of a volume A of the solid electrolyte to a volume B of the electrode active material in the entire electrode in a range of 10/90 or more and 25/75 or less,
A first phase having a volume ratio A/B in the range of 25/75 to 50/50;
A second phase having a volume ratio A/B in the range of 0/100 to 16/84,
The first phase has a domain size that is at least twice the particle size d50 of the larger particle of the solid electrolyte and the electrode active material, and is at most ½ of the thickness of the electrode;
An electrode, wherein the particle size d50 of the solid electrolyte is within a range not exceeding the particle size d50 of the electrode active material. The electrode according to claim 1, wherein the first phase and the second phase have a structure in which they are randomly or alternately dispersed in two phases in a plane perpendicular to the electrode thickness direction. The electrode according to claim 1 or 2, which satisfies any one or more of (1) to (2).
(1) The first phase has a volume ratio A/B in the range of more than 25/75 and not more than 40/60.
(2) The second phase has a volume ratio A/B of 15/85 or less. The electrode according to any one of claims 1 to 3, which satisfies any one or more of (3) to (4).
(3) The first phase has a volume ratio A/B exceeding 28/72.
(4) The second phase has a volume ratio A/B of less than 14/86. The electrode according to any one of claims 1 to 4, which satisfies any one or more of (5) to (7).
(5) The volume ratio A/B of the entire electrode is in the range of 15/85 or more.
(6) The first phase has a domain size in the range of 10 μm or more and 20 μm or less.
(7) The porosity is in the range of 0 volume % or more and 35 volume % or less. An electricity storage device that is an all-solid-state secondary battery,
A positive electrode and
A negative electrode;
a solid electrolyte layer interposed between the positive electrode and the negative electrode and conducting carrier ions;
The power storage device, wherein at least one of the positive electrode and the negative electrode is the electrode according to any one of claims 1 to 5.

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