Electrode and power storage device

Abstract

Provided are electrodes (12, 16) and power storage devices (11, 26) that can reduce ion transport resistance. Each electrode comprises an active material (20, 25), oxide particles (19) present on the surface of the active material, and a compound (23) present on the surface of each particle and on the surface of the active material in the vicinity of the particle. The compound contains fluorine, phosphorus, and oxygen. More preferably, a value I2 / I1 obtained by dividing an intensity I2 at a binding energy of 686 eV by an intensity I1 at a binding energy of 685 eV in an F1s spectrum by X-ray photoelectron spectroscopy satisfies 0.70 < I2 / I1 < 1.2.

Description

Electrodes and energy storage devices

[0001] This invention relates to an electrode and energy storage device containing an active material.

[0002] In energy storage devices, the amount of electricity that can be extracted tends to decrease as the electrochemical reaction cannot keep up with the high current density during charging and discharging. To reduce the current density dependence of energy storage devices during charging and discharging, it is necessary to reduce the internal resistance of the electrodes. The internal resistance of electrodes is broadly classified into electron transfer resistance and ion transport resistance. Patent document 1 discloses prior art that reduces ion transport resistance by additives contained in the electrolyte.

[0003] Japanese Patent Publication No. 2020-145054

[0004] Prior technology has room for improvement in reducing ion transport resistance.

[0005] This invention was made to meet this requirement and aims to provide electrodes and energy storage devices that can reduce ion transport resistance.

[0006] A first embodiment for achieving this objective is an electrode comprising an active material, oxide particles present on the surface of the active material, and a compound present on the surface of the particles and on the surface of the active material near the particles, wherein the compound comprises fluorine, phosphorus, and oxygen.

[0007] The second aspect is, in the first aspect, the intensity I of the F1s spectrum obtained by X-ray photoelectron spectroscopy at a binding energy of 686 eV. ２ The strength I at a binding energy of 685 eV １ The value I obtained by dividing by ２ / I １ is 0.70 < I ２ / I １ < 1.2.

[0008] In the third embodiment, the oxide is a solid electrolyte having a garnet-type crystalline structure containing Li, La, and Zr, in the first or second embodiment.

[0009] A fourth embodiment is the third embodiment, wherein the solid electrolyte further comprises Mg and Sr.

[0010] The fifth embodiment is an energy storage device comprising an electrode according to any of the first to fourth embodiments, wherein the electrode contains an electrolyte in which lithium hexafluoride phosphate is dissolved in a non-aqueous solvent.

[0011] The present invention comprises oxide particles present on the surface of the active material, and a compound present on the surface of the particles and on the surface of the active material near the particles. Since the compound contains fluorine, phosphorus, and oxygen, the ion transport resistance during insertion and removal from the active material can be reduced. This reduces the ion transport resistance of the electrode.

[0012] This is a cross-sectional view of the energy storage device in the first embodiment. This is a schematic diagram showing a garnet-type crystal structure. This is a schematic diagram of the electrode. This is the F1s photoelectron spectrum. This is a cross-sectional view of the energy storage device in the second embodiment.

[0013] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 is a schematic cross-sectional view of the energy storage device 11 in the first embodiment. The energy storage device 11 in this embodiment is a secondary battery that uses lithium ions as mobile ions. The energy storage device 11 includes, in order, a positive electrode 12, a separator 15, and a negative electrode 16. The positive electrode 12, the separator 15, and the negative electrode 16 are housed in a case (not shown).

[0014] The positive electrode 12 consists of a current collector layer 13 and an active material layer 14 superimposed on each other. The current collector layer 13 is a conductive material. Examples of materials for the current collector layer 13 include metals selected from Ni, Ti, Fe, and Al, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0015] The active material layer 14 contains oxide particles 19 and active material 20. To lower the resistance of the active material layer 14, the active material layer 14 may contain a conductive additive 21 (see Figure 3). Examples of conductive additives 21 include carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt, and Ag.

[0016] The active materials 20 include metal oxides having transition metals, sulfur-based active materials, and organic-based active materials. Examples of the metal oxides having transition metals include metal oxides containing one or more elements selected from Mn, Co, Ni, Fe, Cr, and V and Li. The metal oxides having transition metals include LiCoO ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiMnO ２ O ４ , LiNiVO ４ , LiNi ０．５ Mn １．５ O ４ , LiNi １／３ Mn １／３ Co １／３ O ２ 0 and LiFePO ４ are exemplified.

[0017] The sulfur-based active materials include S, TiS ２ , NiS, FeS ２ , Li ２ S, MoS ３ and sulfur-carbon composites are exemplified. The organic-based active materials include radical compounds typified by 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radicalene compounds, tetracyanoquinodimethane, and phenazine oxide.

[0018] The separator 15 separates the positive electrode 12 and the negative electrode 16 from each other and electrically insulates them. The separator 15 is made of a porous body that is durable against the active materials 20 and 25 and the electrolytic solution 22 (see FIG. 3) contained in the positive electrode 12 and the negative electrode 16, and through which ions pass but which has no electron conductivity. Examples of the separator 15 include non-woven fabrics and porous membranes made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0019] The negative electrode 16 has a current collector layer 17 and an active material layer 18 laminated thereon. The current collector layer 17 is a conductive member. Examples of the material of the current collector layer 17 include metals selected from Ni, Ti, Fe, Cu, and Si, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0020] The active material layer 18 contains particles 19 and active material 25. To lower the resistance of the active material layer 18, the active material layer 18 may contain a conductive additive (not shown). Examples of conductive additives include carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt, and Ag.

[0021] The active material 25 is a carbon-based material such as porous carbon, natural graphite, artificial graphite, easily graphitizable carbon, difficult-to-graphitize carbon, and carbon fiber, Li ４ Ti ５ O １２ , Si, Si-Li alloys, compounds containing Si and O as constituent elements (hereinafter referred to as "SiO ｘ This is referred to as "(where 0.5 ≤ X ≤ 1.5)." Examples include metallic lithium, Li-Al alloy, Li-Sn alloy, Li-Si alloy, Li-Mg alloy, Li-Si alloy, and other lithium alloys, as well as In-Sb alloy and Si-Li alloy. ｘ This is an oxide of Si, amorphous SiO ２ Examples include structures in which microcrystalline or amorphous silicon is dispersed within the matrix.

[0022] The active material layers 14 and 18 may contain a binder. Examples of binders include rubbery polymers such as fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and styrene-butadiene rubber. Examples of fluorinated resins include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene / hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0023] Examples of vinylidene fluoride polymers include homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride and copolymerizable monomers. Examples of copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen-containing copolymerizable monomers. Examples of halogen-containing monomers include chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ethers. Examples of non-halogen-containing copolymerizable monomers include olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, their esters or salts; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene. One or more copolymerizable monomers polymerize with vinylidene fluoride to form a copolymer.

[0024] Particle 19 is a crystalline or amorphous oxide-based solid electrolyte, SiO ２ Al ２ O ３ , AlOOH, MgO, CaO, ZrO ２ , BaTiO ３ Examples of oxides include those listed above. Examples of solid electrolytes include those with perovskite, NASICON, LISICON, and garnet structures.

[0025] Perovskite-type solid electrolytes are oxides containing at least Li, Ti, and La, for example, La ２／３－Ｘ Li ３Ｘ TiO ３ Examples include: NASICON-type solid electrolytes are oxides containing at least Li, M (where M is one or more elements selected from Ti, Zr, and Ge), and P, for example, Li(Al,Ti) ２ (PO ４ ) ３ and Li(Al,Ge) ２ (PO ４ ) ３ Examples include LiICON-type solid electrolytes. １４ Zn (GeO ４ ) ４Examples include garnet-type solid electrolytes, such as composite oxides containing Li, La, and Zr. The crystal structure of garnet-type electrolytes is generally given by the formula C ３ A ２ B ３ O １２ It is represented as follows. Garnet-type solid electrolytes are strongly basic, have high electrochemical stability, and are resistant to reduction by metallic lithium.

[0026] Figure 2 is a schematic diagram of a garnet-type crystal structure. In the garnet-type crystal structure, C-site Sc is dodecahedral coordinated with oxygen atoms Oa, A-site Sa is octahedral coordinated with oxygen atoms Oa, and B-site Sb is tetrahedral coordinated with oxygen atoms Oa. In the garnet-type crystal structure, Li can be present in the void V, which is the location where octahedral coordination occurs with oxygen atoms Oa. The void V is, for example, the location sandwiched between B-site Sb1 and B-site Sb2. The Li present in the void V is octahedral coordinated with oxygen atoms Oa that constitute an octahedron including the tetrahedral face Fb1 forming B-site Sb1 and the tetrahedral face Fb2 forming B-site Sb2. For example, Li oxide ７ La ３ Zr ２ O １２ In this scenario, La may occupy site C Sc, Zr may occupy site A Sa, and Li may occupy site B Sb and the void V.

[0027] Garnet-type solid electrolytes are found in the CSD (Cambridge Structural Database) X-ray diffraction file No. 422259 (Li ７ La ３ Zr ２ O １２ It has an XRD pattern similar to ). In garnet-type solid electrolytes, various elements are substituted. For example, Ca, Sr, Ba, etc. are substituted at the C site, Nb, Ta, Sn, Hf, etc. are substituted at the A site, and Al, Ga, etc. are substituted at the B site. The amount of lithium changes due to elemental substitution, and the ionic conductivity changes as the arrangement, occupancy rate, and occupancy sites of lithium ions in the crystal structure change. Due to elemental substitution, the diffraction angle and intensity ratio may differ compared to No. 422259.

[0028] Garnet-type solid electrolytes preferably have a dopant substituted for at least one of a lanthanum ion occupying the C site and a zirconium ion occupying the A site, wherein the valency of the dopant is smaller than the valency of the substituted ion. This creates voids V, which can be occupied by lithium ions, thereby increasing the amount of lithium ions contained in the crystal lattice.

[0029] The particle 19 preferably contains at least one element selected from the group consisting of, for example, Ca, Sr, and Ba, and the molar ratio of each element satisfies (1) and (2) or (3) and (4) below.

[0030] (1) 1.33≦Li / (La+A)≦3 (2) 0≦A / (La+A)≦0.67 (3) 2.0≦Li / (La+A)≦2.6 (4) 0.04≦A / (La+A)≦0.17

[0031] Let's return to Figure 1 for explanation. The proportion of particles 19 appearing in the cross-sections of the active material layers 14 and 18 is preferably 0.1-5%. This is to ensure the amount of mobile ions moving without hindering the reactivity of the active materials 20 and 25. The median diameter of the equivalent circle diameter of the particles 19 appearing in the cross-sections of the active material layers 14 and 18 is preferably 0.8 μm or less, and more preferably 0.45 μm or less. This is to increase the frequency of compound 23 (described later) being formed on the surface of the particles 19.

[0032] To determine the proportion of particles 19 and the median diameter, first, scanning electron microscope (SEM) images of the particles 19 appearing on the cross-section of the energy storage device 11 (polished surface, surface obtained by irradiation with a focused ion beam (FIB), or surface obtained by ion milling) are analyzed. The proportion of particles 19 is calculated by determining the ratio of the area of ​​particles 19 to the area of ​​the cross-sectional image. The median diameter is calculated by determining the equivalent diameter of a circle from the area of ​​each particle 19, determining the volume-based particle size distribution, and calculating the equivalent diameter of a circle at which the cumulative value of the frequencies in the particle size distribution is 50%.

[0033] Figure 3 is a schematic diagram of the positive electrode 12 (active material layer 14). The power storage device 11 (see Figure 1) contains an electrolytic solution 22 in which a lithium salt is dissolved in a non-aqueous solvent. The lithium salt is a compound used for the transfer of cations between the positive electrode 12 and the negative electrode 16. The lithium salt is lithium hexafluorophosphate LiPF ６ and the like can be mentioned.

[0034] Lithium hexafluorophosphate LiPF ６ Other lithium salts other than this may be contained in the electrolytic solution 22. The anions of other lithium salts are halide ions (I － , Cl － , Br － etc.), SCN － , BF ４ － , BF ３ (CF ３ ) － , BF ３ (C ２ F ５ ) － , PF ６ － , ClO ４ － ] , SbF ６ － , N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ ) ２ － , B(C[[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​３ SO ２ O － , C ６ F ５ SO ２ O － , B(O ２ C ２ O ２ ) ２ － RCOO － Examples include (where R is an alkyl group having 1-4 carbon atoms, a phenyl group, or a naphthyl group).

[0035] The non-aqueous solvents constituting the electrolyte 22 are broadly classified into molecular solvents, which consist mostly of molecules, and ionic liquids, which consist of cations and anions. For molecular solvents, aprotic solvents are preferred to broaden the potential window of the electrolyte 22. Examples of aprotic solvents include cyclic esters, chain esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorescein solvents, and sulfone solvents. Mixtures of these are also acceptable.

[0036] Examples of cyclic esters include carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate, as well as lactones such as β-propiolactone, γ-butyrolactone, δ-valerolactone, α-pyrone, and coumarin. Examples of chain esters include carbonate esters such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of aliphatic carboxylic acid esters include methyl formate, methyl acetate, and ethyl propionate. Examples of phosphate esters include trimethyl phosphate. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0037] Examples of amides include formamide, N-methylformamide, dimethylformamide, N-methylacetamide, dimethylacetamide, N-methylpropioamide, hexamethylphosphoramide, and N-methylpyrrolidone. Examples of sulfur compounds include dimethyl sulfoxide, sulfolane, dimethylthioformamide, and N-methylthiopyrrolidone. Examples of ketones include acetone, 4-methyl-2-pentanone, and acetylacetone. Examples of ethers include tetrahydrofuran and monoglycerides. Examples of nitro compounds include nitromethane and nitrobenzene. Fluorine solvents are compounds in which hydrogen atoms of hydrocarbons are replaced with fluorine atoms, and their derivatives.

[0038] Examples of sulfone-based solvents include trimethylene sulfone, sulfolane, difluorosulfolane, dimethyl sulfolane, monofluorosulfolane, 3-methylsulfolane, ethylmethylsulfone, and ethyl isopropylsulfone. Sulfone-based solvents are preferred because they have high thermal stability.

[0039] The reaction in which lithium salts dissolve in a molecular solvent and dissociate into free ions proceeds more readily when the relative permittivity of the non-aqueous solvent is high and when solvation of ions is easy. Therefore, a non-aqueous solvent with a relatively high relative permittivity εr (εr > 20) is preferred. Examples of molecular solvents with a relative permittivity greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, and nitro compounds. It is naturally possible to mix a non-aqueous solvent with a relative permittivity greater than 20 with a non-aqueous solvent with a relative permittivity of 20 or less in order to adjust the viscosity of the non-aqueous solvent.

[0040] Ionic liquids are compounds consisting of cations and anions, and are liquids at room temperature and pressure. If the non-aqueous solvent is an ionic liquid, the flame retardancy of the electrolyte 22 can be improved. The ionic liquid is preferably one in which one or more cation species are selected from the group consisting of ammonium, imidazolium, pyrrolidinium, and piperidinium.

[0041] The anionic component of the ionic liquid is not particularly limited. The anionic component is BF ４ － , N (SO ２ F) ２－ Inorganic anions such as B(C) ６ H ５ ) ４ － CH ３ SO ３ － CF ３ SO ３ － , N (SO ２ CF ３ ) ２ － , N (SO ２ C ４ F ９ ) ２ － Examples of organic anions include the following.

[0042] The ionic liquid may also be a solvated ionic liquid. Examples of solvated ionic liquids include those obtained by dissolving a lithium salt in a sulfone-based solvent such as sulfolane or a sulfolane derivative, or in a glyme-based solvent such as tetraglyme.

[0043] The concentration of lithium salt in the electrolyte 22 is preferably 4.0 mol / kg or less. This is because if the salt concentration of the electrolyte 22 exceeds 4.0 mol / kg, the ionic conductivity tends to decrease significantly due to the increase in the viscosity of the electrolyte.

[0044] The energy storage device 11 is manufactured, for example, as follows: Particles 19, active material 20, and conductive additive 21 are mixed, and a solution of binder dissolved in a solvent is further mixed to make a slurry. After coating the current collector layer 13 with the slurry, it is dried to obtain a positive electrode sheet.

[0045] The particles 19, the active material 25, and the conductive additive 21 are mixed, and then a solution of the binder dissolved in a solvent is added to make a slurry. The slurry is applied onto the current collector layer 17 and then dried to obtain a negative electrode sheet.

[0046] A separator 15 separates the positive electrode sheet and the negative electrode sheet. The positive electrode sheet and the negative electrode sheet are wound together on a winding machine to create a cylindrical or rectangular cell. Terminals (not shown) are connected to the current collector layers 13 and 17, respectively. Lithium hexafluoride phosphate LiPF4 is then added to a container (not shown) containing the cell. ６After filling the container with an electrolyte 22 in which the material is dissolved in a non-aqueous solvent, the container is sealed to obtain an energy storage device 11 including a positive electrode 12, a separator 15, and a negative electrode 16.

[0047] Lithium hexafluoride phosphate (LiPF) ６ If water is present in the electrolyte 22 containing LiPF ６ It undergoes hydrolysis and becomes LiPO as follows ２ F ２ This is generated.

[0048] LiPF ６ +H ２ O → LiF + PF ５ +H ２ O → LiF+2HF+POF ３ POF ３ +LiF+H ２ O → LiPO ２ F ２ +2HF

[0049] LiPO is applied to the surface of particle 19 and the surface of the active material 20 in the vicinity of particle 19. ２ F ２ A coating is formed consisting of compound 23 derived from [the compound]. The vicinity of particle 19 refers to the area inside a circle with a radius of 2 μm centered on the center of gravity of particle 19 that appears in the cross-section of the active material layer 14. The center of gravity of particle 19 is the geometric center of the cross-sectional shape of particle 19. The mechanism by which the coating is formed on the surface of particle 19 and on the surface of the active material 20 in the vicinity of particle 19 is presumed to be due to electrostatic interaction, but the details are unknown.

[0050] If particle 19 is a solid electrolyte having a garnet-type crystalline structure containing Li, La, and Zr, then particle 19 is highly reactive with basic substances and water, and in addition to the above reaction, the following reaction occurs, resulting in LiPO ２ F ２ It is thought that compound 23 derived from this compound is produced.

[0051] xH ２ O+Li ７ La ３ Zr ２ O １２ → xLiOH+H ｘ Li ７－ｘ La ３ Zr ２ O１２ PF ５ +LiOH → POF ３ +LiF+HF POF ３ +LiOH → LiPO ２ F ２ +HF

[0052] LiPO ２ F ２ This is an example of compound 23. Compound 23 is Li ｘ PO ｙ F ｚ It is represented as such, where x, y, and z are integers from 0 to 6. A fluorine-containing compound 24, such as LiF, may also be present on the surface of particle 19.

[0053] These reactions generate compound 23 containing fluorine, phosphorus, and oxygen on the surface of particle 19 and on the surface of active material 20 near particle 19. Because compound 23 has relatively high ionic bonding properties, in the microscopic region near particle 19, some of the solvent molecules coordinated to lithium ions preferentially coordinate to compound 23, facilitating the desolvation of lithium ions coordinated to some of the solvent molecules. This is presumed to reduce the solvation resistance at the interface of active material 20 located near particle 19. Furthermore, it is presumed that the presence of compound 23 on the surface of active material 20 reduces the ion transport resistance during insertion and desolvation of ions into and out of the active material 20. This reduces ion transport resistance, and is presumed to reduce the internal resistance of the positive electrode 12.

[0054] If the particles 19 are a solid electrolyte having a garnet-type crystalline structure containing Li, La, and Zr, the particles 19 are electrochemically stable even at the negative electrode 16. The mechanism by which the internal resistance of the positive electrode 12 is reduced also applies to the negative electrode 16, so the particles 19 can also reduce the internal resistance of the negative electrode 16.

[0055] The elements contained in compounds 23 and 24, with the exception of Li, can be detected by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). The presence of compounds 23 and 24 can be confirmed by overlaying the SEM images of the cross-sections of the positive electrode 12 and the negative electrode 16 with the EDX detection results. In the cross-sectional SEM image, it is preferable that the area of ​​compound 23 is larger than the area of ​​compound 24. The area of ​​compound 23 corresponds to the concentration of compound 23 (the ratio to the surface of particles 19 and active material 20), and the area of ​​compound 24 corresponds to the concentration of compound 24. A higher concentration of compound 23 than that of compound 24 is advantageous for reducing desolvation resistance.

[0056] In the cross-sectional SEM image, it is preferable that the combined area of ​​compound 23 and compound 24 is larger than the area of ​​the film containing lithium hydroxide or lithium carbonate formed on the surface of particle 19 by the reaction of water, carbon dioxide, etc., with particle 19. This is advantageous because it reduces desolvation resistance and lithium ion migration resistance.

[0057] Information on compounds 23 and 24 present on the surface of particle 19 and the surface of active material 20 is obtained by X-ray photoelectron spectroscopy (XPS). X-rays are irradiated onto the interface between the active material layer 14 and the current collector layer 13, and the interface between the active material layer 14 and the separator 15, and the F1s photoelectron spectrum generated from the interface by the photoelectric effect is measured. An example of an X-ray source is AlKα rays.

[0058] After correcting the C1s peak top of the C-C bond to appear at a bond energy of 284.8 eV, the background is subtracted before peak fitting, which separates the F1s photoelectron spectrum into a specific spectrum. The background is usually subtracted using the Shirley method, but the Tougaard method may also be used. Examples of peak fitting functions include the Lorentz function, the Gauss function, the Voigt function (a convolution of the Lorentz and Gauss functions), and the pseudo-Voigt function (a sum of the two functions).

[0059] Figure 4 shows the F1s photoelectron spectrum of the active material layer 14. In Figure 4, the horizontal axis represents the binding energy and the vertical axis represents the intensity. The active material layer 18 also shows a similar F1s photoelectron spectrum to that in Figure 4. Intensity I at a binding energy of 686 eV ２ is LiPO ２ F ２ Derived from, the strength I at a binding energy of 685 eV １ It originates from LiF. Intensity I ２ Strength I １ The value I obtained by dividing by ２ / I １ is LiPO ２ F ２ It is proportional to the relative abundance of LiF. Value I ２ / I １ is 0.7 < I ２ / I １ <1.2 is preferable. LiPO ２ F ２ This is because it has lower resistance compared to LiF, thus further reducing the resistance to lithium ion movement.

[0060] A second embodiment will be described with reference to Figure 5. In the first embodiment, the separator 15 separating the positive electrode 12 and the negative electrode 16 was described as a nonwoven fabric or porous membrane made of cellulose, polypropylene, or the like. In contrast, in the second embodiment, a power storage device 26 containing an electrolyte 29 will be described as a separator 28 separating the positive electrode 12 and the negative electrode 16. In the second embodiment, the same parts as in the first embodiment are denoted by the same reference numerals and their descriptions will be omitted below.

[0061] Figure 4 is a cross-sectional view of the energy storage device 26 in the second embodiment. The energy storage device 26 includes, in order, a positive electrode 12, a separator 28, and a negative electrode 16. The separator 28 contains an electrolyte 29. The electrolyte 29 is exemplified by an ion-conductive solid or gel-like electrolyte. A composition of the electrolyte 29 and electrolyte solution 22 may be placed on the separator 28.

[0062] The electrolyte 29 includes one or more selected from sulfide-based, oxide-based, hydride-based, halide-based, and organic-based electrolytes. Sulfide-based electrolytes include crystalline thiolysicone-type, Li １０ GeP ２ S１２ Type, argyrodite type, Li ７ P ３ S １１ Type, Li ２ S-P ２ S ５ Examples of glass and glass-ceramic materials include those represented by [mention specific examples]. Examples of oxide-based electrolytes include oxides with a NASICON-type structure, oxides with a perovskite structure, and oxides with a garnet-type structure.

[0063] Hydride-based electrolytes include LiBH ４ and lithium halide compounds (LiI, LiBr, LiCl) and lithium amide (LiNH ２ Examples of solid solutions with ) are given. The solid electrolyte of the halide is Li ３ YCl ６ Examples include polyethylene oxide, polypropylene oxide, and polyacrylonitrile.

[0064] The energy storage device 26 is manufactured, for example, as follows: An electrolyte 22 and particles 19 are mixed together, and then an active material 20 is mixed with a solvent containing a binder to create a slurry. The slurry is then applied onto the current collector layer 13 and dried to obtain the active material layer 14.

[0065] A slurry for the separator is prepared by mixing the electrolyte 22 and electrolyte 29 with a solvent containing the dissolved binder. After applying the separator slurry onto the active material layer 14, it is dried to obtain a positive electrode sheet.

[0066] A slurry is made by mixing the electrolyte 22 and particles 19, adding the active material 25, and then adding a solvent containing a dissolved binder. The slurry is applied to the current collector layer 17 and then dried to obtain the active material layer 18. A separator slurry is applied to the active material layer 18 and then dried to obtain the negative electrode sheet.

[0067] After cutting the positive electrode sheet and the negative electrode sheet into predetermined shapes, the positive electrode sheet and the negative electrode sheet are stacked and pressed together so that a separator 28 is formed between the positive electrode 12 and the negative electrode 16, and a cell is manufactured. Terminals (not shown) are connected to the current collector layers 13 and 17, respectively, and the device is sealed in a container (not shown) to obtain an energy storage device 26 including the positive electrode 12, the separator 28, and the negative electrode 16.

[0068] Since the energy storage device 26 in the second embodiment includes a positive electrode 12 and a negative electrode 16 containing particles 19, the internal resistance of the positive electrode 12 and the negative electrode 16 can be reduced, similar to the energy storage device 11 in the first embodiment.

[0069] The present invention will be described in more detail by reference to examples, but the present invention is not limited to these examples.

[0070] (Particle preparation) Li ６．９５ Mg ０．１５ La ２．７５ Sr ０．２５ Zr ２．０ O １２ Li ２ CO ３ , MgO, La(OH) ３ , SrCO ３ , ZrO ２ Li was weighed. ２ CO ３ The amount of Li was increased to account for the volatilization of Li during calcination. The weighed raw materials and non-aqueous solvent were placed in a nylon pot along with zirconia balls and ground and mixed in a ball mill for 15 hours. After drying the slurry removed from the pot, it was calcined at 1100°C for 10 hours on an MgO plate. The resulting powder was ground, placed in an MgO pod, and further calcined at 1100°C for 4 hours. The resulting powder was ground in a glove box under an argon atmosphere to obtain an oxide-based solid electrolyte (hereinafter referred to as "LLZ"). The garnet-type crystal structure of LLZ was confirmed by powder X-ray diffraction.

[0071] (Example 1) LLZ was wet-milled in a non-aqueous solvent under an argon atmosphere for 3 hours. After milling, it was dried to obtain a powder (oxide particles). The median diameter of the particles, measured by a laser diffraction particle size distribution analyzer, was 0.8 μm.

[0072] LiNi ０．６ Mn ０．２ Co ０．２ O ２ The active material, particles, acetylene black (conductive additive), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed in a ratio of 93.5:0.5:3.0:3.0 (mass ratio) and mixed to obtain a slurry. The slurry was applied onto aluminum foil and then vacuum-dried to obtain the positive electrode of Example 1.

[0073] (Example 2) LLZ was wet-milled in a non-aqueous solvent under an argon atmosphere for 6 hours. After milling, it was dried to obtain a powder (oxide particles). The median diameter of the particles, measured by a laser diffraction particle size distribution analyzer, was 0.45 μm. Using these particles, the positive electrode for Example 2 was obtained in the same manner as in Example 1.

[0074] (Example 3) LLZ was wet-milled in a non-aqueous solvent under an argon atmosphere for 60 hours. After milling, it was dried to obtain powder (oxide particles). The median diameter of the particles, measured by a laser diffraction particle size distribution analyzer, was 0.2 μm. Using these particles, the positive electrode for Example 3 was obtained in the same manner as in Example 1.

[0075] (Example 4) Instead of LLZ, alumina particles with a median diameter of 0.8 μm, measured by a laser diffraction particle size distribution analyzer, were used, and the positive electrode for Example 4 was obtained in the same manner as in Example 1.

[0076] (Example 5) LLZ was wet-milled in a non-aqueous solvent under an argon atmosphere for 2 hours. After milling, it was dried to obtain powder (oxide particles). The median diameter of the particles, measured by a laser diffraction particle size distribution analyzer, was 2.0 μm. Using these particles, the positive electrode of Example 5 was obtained in the same manner as in Example 1.

[0077] (Example 6) LLZ was wet-milled in a non-aqueous solvent for 1 hour under an argon atmosphere. After milling, it was dried to obtain a powder (oxide particles). The median diameter of the particles, measured by a laser diffraction particle size distribution analyzer, was 3.0 μm. Using these particles, the positive electrode of Example 6 was obtained in the same manner as in Example 1.

[0078] (Example 7) While Examples 1-6 prepared positive electrodes, Example 7 prepared negative electrodes. Graphite (active material), oxide particles as described in Example 1, carboxymethylcellulose dissolved in pure water (thickener), and styrene-butadiene rubber dissolved in pure water (binder) were weighed in a ratio of 96.5:0.5:1.0:2.0 (mass ratio) and mixed to obtain a slurry. The slurry was applied onto copper foil and then vacuum-dried to obtain the negative electrode in Example 7.

[0079] (Comparative Example 1) Instead of LLZ, lithium carbonate particles with a median diameter of 0.8 μm, measured by a laser diffraction particle size distribution analyzer, were used, and the positive electrode for Comparative Example 1 was obtained in the same manner as in Example 1.

[0080] (Comparative Example 2) LiNi ０．６ Mn ０．２ Co ０．２ O ２ The active material, acetylene black (conductive additive), and polyvinylidene fluoride (binder) dissolved in N-methyl-2-pyrrolidone were weighed in a ratio of 94:3:3 (mass ratio) and mixed to obtain a slurry. After coating the slurry onto aluminum foil, it was vacuum-dried to obtain the positive electrode of Comparative Example 2, which does not contain particles.

[0081] (Comparative Example 3) While Comparative Examples 1 and 2 prepared positive electrodes, Comparative Example 3 prepared negative electrodes. Graphite (active material), carboxymethylcellulose dissolved in pure water (thickener), and styrene-butadiene rubber dissolved in pure water (binder) were weighed in a ratio of 97:1:2 (mass ratio) and mixed to obtain a slurry. After coating the slurry onto copper foil, it was vacuum-dried to obtain the negative electrode for Comparative Example 3.

[0082] (Preparation of symmetrical cells) Various coin-type batteries were prepared by stacking the positive electrodes, polyethylene separators, and metallic lithium (negative electrodes) in the order of Examples 1-6 and Comparative Examples 1 and 2. An electrolyte solution prepared by dissolving lithium hexafluoride phosphate (1 mol / kg) as the electrolyte in a solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio) ratio was injected into the separator.

[0083] The coin-type battery was charged with a constant current of 0.1C until its terminal voltage reached 4.2V, and then aged at 25°C for 3 days. Next, the coin-type battery was discharged with a constant current of 0.1C until its terminal voltage reached 2.5V, and then charged with a constant current of 0.1C until its state of charge (SOC) reached 50%. The coin-type battery was disassembled, and the positive electrode was removed to obtain a positive electrode with an SOC of 50%. Another positive electrode with an SOC of 50% was prepared in the same manner.

[0084] A polyethylene separator was placed between two positive electrodes with a SOC of 50%, and the same type of electrolyte used in the coin-type battery was injected into the separator to obtain the symmetrical cells in Examples 1-6 and Comparative Examples 1 and 2.

[0085] Various coin-type batteries were fabricated by stacking the negative electrodes, polyethylene separators, and metallic lithium in the order of those used in Example 7 and Comparative Example 3. An electrolyte solution, prepared by dissolving lithium hexafluoride phosphate (1 mol / kg) as the electrolyte in a solvent mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio), was injected into the separator.

[0086] The coin-type battery was charged with a constant current of 0.1C until its terminal voltage reached 0.01V, and then aged at 25°C for 3 days. Next, the coin-type battery was discharged with a constant current of 0.1C until its terminal voltage reached 1.5V, and then charged with a constant current of 0.1C until its State of Core (SOC) reached 50%. The coin-type battery was disassembled, and the negative electrode was removed to obtain a negative electrode with an SOC of 50%. Another negative electrode with an SOC of 50% was fabricated in the same manner.

[0087] A polyethylene separator was placed between two negative electrodes with a SOC of 50%, and the same type of electrolyte used in the coin-type battery was injected into the separator to obtain the symmetrical cells in Example 7 and Comparative Example 3.

[0088] (Measurement of Charge Transfer Resistance) AC impedance measurements were performed on the symmetrical cells in Examples 1-7 and Comparative Examples 1-3 while they were stored in a constant temperature bath at -15°C, and the lithium-ion transfer resistance was determined from the arc observed between 10 Hz and 0.1 Hz. Although the example shows a coin-type battery being fabricated and its charge transfer resistance measured, a pouch-type battery may also be fabricated and its charge transfer resistance measured.

[0089] (Measurement of Capacity Retention Rate) In Examples 1-6 and Comparative Examples 1 and 2, the positive electrode, polyethylene separator, and graphite (negative electrode) were stacked in that order. Then, an electrolyte solution prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio) mixture with lithium hexafluoride phosphate (1 mol / kg) dissolved as the electrolyte was injected into the separator. The cells in Examples 1-6 and Comparative Examples 1 and 2 were obtained by vacuum degassing and sealing.

[0090] In Examples 1-6 and Comparative Examples 1 and 2, each cell was charged with a constant current of 0.2C until its terminal voltage reached 4.2V, and then aged at 25°C for 7 days. Subsequently, a constant current discharge of 0.5C was performed, and one charge-discharge cycle was carried out at 0.5C. After charging with a constant current of 0.2C until the terminal voltage reached 4.2V, the discharge capacity at 6C discharge was measured at room temperature.

[0091] LiNi ０．６ Mn ０．２ Co ０．２ O ２ After stacking the positive electrode, polyethylene separator, and the negative electrodes from Example 7 and Comparative Example 3 in that order, an electrolyte solution prepared by dissolving lithium hexafluoride phosphate (1 mol / kg) as the electrolyte in a solvent mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio) was injected into the separator. The cells from Example 7 and Comparative Example 3 were obtained by vacuum degassing and sealing.

[0092] In Example 7 and Comparative Example 3, each cell was charged with a constant current of 0.2C until its terminal voltage reached 4.2V, and then aged at 25°C for 7 days. Next, it was discharged with a constant current of 0.5C, and one charge-discharge cycle was performed at 0.5C. After charging with a constant current of 0.2C until the terminal voltage reached 2.5V, the discharge capacity at 6C was measured at room temperature.

[0093] (Measurement of XPS Spectra) The cells in which the capacity retention rate was measured were disassembled, and the removed positive or negative electrode (hereinafter referred to as "electrode") was immersed in dimethyl carbonate for 5 minutes to wash it and remove the electrolyte contained in the electrode. The dimethyl carbonate contained in the washed electrode was removed by vacuum drying.

[0094] The electrodes were placed in a transfer vessel under an argon atmosphere. After introducing the positive electrode into the XPS analyzer, the surface of the active material layer was irradiated with X-rays. The XPS conditions were: X-ray: monochromatic AlKα, X-ray beam diameter: 100 μmΦ, signal acquisition angle: 45.0°, pass energy: 280.0 eV. After correcting so that the C1s peak top of the C-C bond appears at a binding energy of 284.8 eV, the intensity I of the F1s spectrum at a binding energy of 686 eV was measured. ２ The strength I at a binding energy of 685 eV １ The value I obtained by dividing by ２ / I １ They sought it.

[0095] Charge transfer resistance of electrodes in Examples 1-7 and Comparative Examples 1-3, value I ２ / I １ Table 1 shows the discharge capacities of Examples 1-6 and Comparative Example 1 at 6C discharge, with the discharge capacity of Comparative Example 2 set to 100, and the discharge capacity of Example 7 at 6C discharge, with the discharge capacity of Comparative Example 3 set to 100.

[0096]

[0097] As shown in Table 1, the positive electrodes in Examples 1-6, which contain oxide particles, exhibited lower charge transfer resistance compared to the positive electrode in Comparative Example 1, which contains carbonate particles. Furthermore, the negative electrode in Example 7, which contains oxide particles, exhibited lower charge transfer resistance compared to the negative electrode in Comparative Example 3, which does not contain oxide particles.

[0098] The charge transfer resistance of the positive electrode in Examples 5 and 6 was equivalent to that of Comparative Example 2, which did not contain particles, but the charge transfer resistance of the positive electrode in Examples 1-4 was smaller than that of Comparative Example 2. In Examples 1-4, the median diameter of the particles in the positive electrode was 0.2-0.8 μm, and furthermore, 0.70 < I ２ / I １ In contrast to <1.2, the positive electrodes in Examples 5 and 6 had a particle median diameter of 2-3 μm, and I ２ / I １ The value was 0.70 or less. The positive electrode in Examples 1-4 had small particle size and LiPO ２ F ２ It is presumed that the charge transfer resistance was reduced compared to the positive electrodes in Examples 5 and 6 due to the higher proportion of [the element] present.

[0099] The charge transfer resistance of the positive electrode in Examples 1-3 was lower than that of the positive electrode in Example 4. In Examples 1-3, the positive electrode particles were LLZ, while in Example 4, the positive electrode particles were alumina. It was confirmed that LLZ contained in the positive electrode had a greater effect in reducing charge transfer resistance compared to alumina.

[0100] The discharge capacity in 6C discharge was highly correlated with charge transfer resistance, with lower charge transfer resistance resulting in higher discharge capacity. It was confirmed that the rate characteristics of energy storage devices can be improved by reducing the charge transfer resistance of the electrodes using oxide particles, thereby reducing the internal resistance of the electrodes.

[0101] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0102] In this embodiment, the case where the mobile ion carrying the charge between the positive electrode 12 and the negative electrode 16 is a lithium ion has been described, but it is not necessarily limited to this. Other examples of mobile ions include sodium ions, potassium ions, rubidium ions, cesium ions, and calcium ions.

[0103] In the embodiment, the energy storage device 11 was described to include a positive electrode 12 with an active material layer 14 provided on one side of a current collector layer 13, and a negative electrode 16 with an active material layer 18 provided on one side of a current collector layer 17, but it is not necessarily limited to this. For example, it is certainly possible to apply each element in the embodiment to an energy storage device that has an electrode layer (so-called bipolar electrode) with an active material layer 14 and an active material layer 18 provided on both sides of the current collector layer 13. By alternately stacking bipolar electrodes and separators 15 and housing them in a case (not shown), a so-called bipolar structure energy storage device can be obtained.

[0104] In the embodiments described, the case in which the active material layers 14 and 18 contain particles 19 has been explained, but the invention is not necessarily limited to this. The energy storage device only needs to have at least one of the active material layers 14 and 18 contain particles 19.

[0105] In the embodiments, energy storage devices 11 and 26 consisting of lithium-ion batteries have been described, but the invention is not necessarily limited to these. It is clear that the electrodes of other energy storage devices may also contain particles 19. Examples of other energy storage devices include electrochemical capacitors. Examples of electrochemical capacitors include redox capacitors that utilize redox reactions and hybrid capacitors, which are asymmetric cells that combine electric double-layer capacitors with particles 19. Examples of active materials in electrochemical capacitors include carbon-based materials such as porous carbon, natural graphite, artificial graphite, easily graphitizable carbon (hard carbon), difficult-to-graphitize carbon (soft carbon), and carbon fibers.

[0106] 11, 26 Energy storage device 12 Positive electrode 16 Negative electrode 19 Particles 20, 25 Active material 22 Electrolyte 23 Compound

Claims

An electrode containing an active material, Oxide particles present on the surface of the active material, The compound comprises the surface of the particles and the surface of the active material in the vicinity of the particles, The aforementioned compound is an electrode containing fluorine, phosphorus, and oxygen. Intensity I at a binding energy of 686 eV in the F1s spectrum obtained by X-ray photoelectron spectroscopy. ２ The strength I at a binding energy of 685 eV １ The value I obtained by dividing by ２ / I １ is 0.70 < I ２ / I １ The electrode according to claim 1, wherein the electrode is 1.

2. The electrode according to claim 1, wherein the oxide is a solid electrolyte having a garnet-type crystalline structure containing Li, La, and Zr. 　 The electrode according to claim 3, wherein the solid electrolyte further comprises Mg and Sr. 　 A power storage device comprising electrodes according to any one of claims 1 to 4, The electrode is an energy storage device containing an electrolyte in which lithium hexafluoride phosphate is dissolved in a non-aqueous solvent.

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