Electrode active material, battery

Abstract

This suppresses the degradation of cycle characteristics in batteries containing Si-based active materials. [Solution] An electrode active material using a granule formed by secondary particle formation of primary particles containing Si element with a binder, wherein the organic solvent content is 950 ppm or less.

Description

[Technical Field]

[0001] This disclosure relates to electrode active materials and batteries. [Background technology]

[0002] Patent Document 1 discloses a battery that uses a granulated material in which primary silicon particles are converted into secondary particles with a binder as the negative electrode active material. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2024-017797 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In batteries containing Si-based active materials, there was a problem with reduced cycle performance.

[0005] Therefore, this disclosure aims to provide an electrode active material that can suppress the deterioration of cycle characteristics in batteries containing Si-based active materials. Furthermore, it provides a battery using this material. [Means for solving the problem]

[0006] The inventors discovered that one reason for the reduced cycle performance in batteries containing Si-based active materials is the residual organic solvent used during the synthesis of the granules. They reasoned that the solvent contained in the polymer solution during synthesis remains within the particles, degrading surrounding materials during battery operation and thus reducing battery performance. Based on this finding, the inventors completed their invention.

[0007] This application discloses an electrode active material using a granule formed by secondary particle formation of primary particles containing Si element with a binder, wherein the organic solvent content is 950 ppm or less.

[0008] The content of the organic solvent may be 1.2 ppm or more and 250 ppm or less.

[0009] The organic solvent may be at least a part of alcohols, glycols, esters, and amides.

[0010] The present application also discloses a battery including a composite layer having the above electrode active material.

[0011] The present application also discloses a all-solid-state battery including an all-solid-state electrolyte layer and a composite layer laminated on the all-solid-state electrolyte layer and having the above electrode active material.

Advantages of the Invention

[0012] According to the present disclosure, it is possible to suppress a decrease in cycle characteristics in a battery including a Si-based active material.

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a diagram for explaining the layer structure of the electrode body 11 of a solid-state battery.

Embodiments for Carrying Out the Invention

[0014] 1. Configuration of the Battery FIG. 1 shows a diagram for explaining a solid-state battery (all-solid-state battery) according to one embodiment. Here, as a typical example, an all-solid-state battery will be described. However, the present disclosure does not necessarily need to be an all-solid-state battery, and is applicable to any battery having an electrode body and an exterior body for sealing the same (for example, a solid-state battery (semi-solid-state battery) including a solid electrolyte and an electrolytic solution, or a battery using only all electrolytic solutions). FIG. 1 shows the layer structure of the electrode body 11 included in the solid-state battery. The solid-state battery is formed by sealing such an electrode body 11 in an exterior body. For example, the electrode body 11, which is generally rectangular in plan view, is enclosed in an exterior body that is also generally rectangular in plan view. At this time, the positive electrode terminal extends from the positive electrode current collector of the electrode body 11, and the negative electrode terminal extends from the negative electrode current collector, and the tips thereof are arranged so as to protrude from the exterior body. The respective components of the laminate 11 and their relationships will be described in more detail below.

[0015] The electrode body 11 has a positive electrode current collector 12, a positive electrode composite layer 13, an electrolyte layer 14, a negative electrode composite layer 15, and a negative electrode current collector 16. In this embodiment, the positive electrode current collector 12, the positive electrode composite layer 13, the electrolyte layer 14, the negative electrode composite layer 15, and the negative electrode current collector 16 are laminated in this order to form a unit element 11a, and a plurality of these unit elements 11a are laminated to form the electrode body 11 (only one unit element 11a is shown in FIG. 1). As described above, a positive electrode terminal is electrically connected to the positive electrode current collector 12 of the electrode body 11, and a negative electrode terminal is electrically connected to the negative electrode current collector 16 of the electrode body 11.

[0016] 1.1. Positive electrode current collector The positive electrode current collector 12 is laminated on the positive electrode composite layer 13 to collect current from the positive electrode composite layer 13. In this embodiment, the positive electrode current collector 12 is in the shape of a square foil in plan view and can be composed of a positive electrode current collecting foil which is a metal foil, and a conductive resin layer or a carbon layer laminated on the positive electrode current collecting foil. The carbon layer is laminated on the positive electrode composite layer 13, so that the positive electrode current collector 12 is laminated on the positive electrode composite layer 13. Examples of the material for the positive electrode current collector include stainless steel, nickel, chromium, gold, platinum, aluminum, iron, titanium, and zinc for the metal foil. Those obtained by plating or vapor-depositing nickel, chromium, carbon, etc. on these metal foils may also be used. The conductive resin layer can be composed of a resin in which a conductive material is dispersed, and the carbon layer can be composed of a material containing carbon.

[0017] 1.2. Positive electrode composite layer The positive electrode composite layer 13 has the above-described positive electrode current collector 12 laminated on one surface and the electrolyte layer 14 laminated on the other surface. In this embodiment, the positive electrode composite layer 13 is in the shape of a square sheet in plan view. The positive electrode composite layer 13 is a layer containing at least a positive electrode active material. Further, the positive electrode composite layer may contain at least one of an electrolyte, a conductive auxiliary material, and a binder, if necessary. The thickness of the positive electrode composite layer 13 is not particularly limited, but can be 1 μm or more and 100 μm or less, more preferably 30 μm or more and 100 μm or less.

[0018] [Positive electrode active material] Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include ternary systems (Li(Ni y , y , , ,

[0019] , 12 , x , 2-x-y ,

[0020] , , Co y Mn z )O2), NCA systems (Li(Ni x Co y Al z )O2), LiCoO2, LiMnO2, LiNiO2, LiVO2, Li 1+x Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and other rock salt layer-type active materials, LiMn2O4, Li4Ti5O 12 , Li(Ni 0.5 Mn 1.5 )O4 and other spinel-type active materials, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4 and other olivine-type active materials, Li 1+x Mn 2-x-y M y O4 (M is one or more selected from Al, Mg, Co, Fe, Ni, Zn) and other hetero-element-substituted Li-Mn spinel active materials, Li x TiO y can be mentioned.

[0019] A coating layer containing a Li-ion conductive oxide may be formed on the surface of the active material. This is because it can suppress the reaction between the active material and the solid electrolyte (especially the sulfide solid electrolyte). Examples of the Li-ion conductive oxide include LiNbO3, Li4Ti5O 12 , Li3PO4. The thickness of the coating layer is, for example, 1 nm or more and 30 nm or less.

[0020] The positive electrode active material can take the form of particulate matter, for example. The average particle size (D50) of the positive electrode active material is not particularly limited, but may be, for example, 10 nm or more, or 100 nm or more. On the other hand, the average particle size (D50) of the positive electrode active material may be, for example, 50 μm or less, or 20 μm or less. The average particle size (D50) can be calculated, for example, from measurements using a laser diffraction particle size analyzer or a scanning electron microscope (SEM).

[0021] [Electrolyte] In all-solid-state batteries and semi-solid-state batteries, the electrolyte includes at least a solid electrolyte, but in semi-solid-state batteries and batteries using only an electrolyte, it includes a liquid electrolyte (electrolyte).

[0022] Examples of solid electrolytes include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes, and organic polymer electrolytes such as polymer electrolytes. Examples of sulfide solid electrolytes include solid electrolytes containing Li, X (where X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and S. Furthermore, sulfide solid electrolytes may further contain at least one of O and halogen elements. Examples of halogen elements include F, Cl, Br, and I. Sulfide solid electrolytes may be glass (amorphous) or glass ceramics. Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-GeS2, and Li2S-P2S5-GeS2.

[0023] The electrolyte preferably contains a supporting salt and a solvent. Examples of supporting salts (lithium salts) for lithium-ion conductive electrolytes include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and organic lithium salts such as LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3. Examples of solvents used in the electrolyte include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and linear esters (linear carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The electrolyte preferably contains two or more solvents.

[0024] The mass ratio of the positive electrode active material to the electrolyte is preferably 85 / 15 to 30 / 70, and more preferably 80 / 20 to 50 / 50, in terms of positive electrode active material / electrolyte.

[0025] [Conductive additives / binders] Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and Ketjenblack (KB), and fibrous carbon materials such as carbon fibers, carbon nanotubes (CNTs), and carbon nanofibers (CNFs). Examples of binders include rubber-based binders and fluoride-based binders.

[0026] 1.3. Electrolyte layer The electrolyte layer 14 is a layer formed between the positive electrode composite layer and the negative electrode composite layer, and contains at least an electrolyte. The electrolyte may consist only of a solid electrolyte, or it may also contain a liquid electrolyte (electrolyte). The specific solid electrolyte and electrolyte are the same as those described for the positive electrode composite layer above. The thickness of the electrolyte layer is, for example, 0.1 μm or more and 1000 μm or less. More preferably, it is 0.1 μm or more and 300 μm or less, and even more preferably 1 μm or more and 100 μm or less.

[0027] 1.4.Negative electrode composite layer The negative electrode composite layer 15 is a layer containing at least a negative electrode active material and may contain at least one of an electrolyte, a conductive additive, and a binder. The electrolyte, conductive additive, and binder are the same as those described above for the positive electrode composite layer. The thickness of the negative electrode composite layer 15 is not particularly limited, but can be 1 μm or more and 100 μm or less, and more preferably 30 μm or more and 100 μm or less.

[0028] [Negative electrode active material] In this embodiment, the negative electrode active material (electrode active material) is a granulated body formed by secondary particle formation of primary particles containing Si element with a binder. While there are no particular limitations on the primary particles containing Si element, porous silicon particles are used as an example. Specifically, in this embodiment, the negative electrode active material is a granulated body formed by bonding multiple porous silicon particles together via a binder.

[0029] <Porous Silicon Particles> Porous silicon particles contain silicon with multiple voids. There are no particular restrictions on the morphology of the voids in porous silicon particles. Porous silicon particles may also contain nanoporous silicon. Nanoporous silicon refers to silicon with multiple pores having a pore diameter on the order of nanometers (less than 1000 nm, preferably 100 nm or less). Porous silicon particles may also contain pores with a diameter of 55 nm or less. Pores with a diameter of 55 nm or less are difficult to crush even by pressing. That is, porous silicon particles containing pores with a diameter of 55 nm or less tend to maintain their porous nature even after pressing. For example, per gram of porous silicon particles, there may be 0.21 cc or more, 0.22 cc / g or more, or 0.23 cc / g or more of pores with a diameter of 55 nm or less, or 0.30 cc / g or less, 0.28 cc / g or less, or 0.26 cc / g or less. The amount of pores with a diameter of 55 nm or less contained in porous silicon particles can be determined, for example, from the pore size distribution obtained by nitrogen gas adsorption or DFT.

[0030] Porous silicon particles may have a predetermined porosity. The porosity of the porous silicon particles may be, for example, 1% or more, 5% or more, 10% or more, or 20% or more, and may be 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, or 30% or less. The porosity can be determined, for example, by observation using a scanning electron microscope (SEM). A large number of samples is preferable, for example, 100 or more. The porosity can be the average value obtained from these samples.

[0031] However, it is not necessary to distinguish between voids in porous silicon particles, voids in the granules, and voids outside the granules. It is sufficient that the porosity of the entire negative electrode composite layer, including voids in porous silicon particles, voids in the granules, and voids outside the granules, is greater than 15%. In other words, regardless of the magnitude of the porosity of the porous silicon particles, the porosity of the granules, and the porosity outside the granules, if the porosity of the entire negative electrode composite layer is greater than 15%, it can be expected to suppress the change in the thickness of the negative electrode during charging.

[0032] The composition of porous silicon particles is not particularly limited. The proportion of Si element in all elements contained in porous silicon particles may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. In addition to Si, porous silicon particles may contain other elements such as Li. Other elements include Sn, Fe, Co, Ni, Ti, Cr, B, and P. Furthermore, porous silicon particles may contain impurities such as oxides. Porous silicon particles may be amorphous or crystalline. The crystalline phase contained in porous silicon particles is not particularly limited.

[0033] The shape and size of the porous silicon particles are not particularly limited. The average primary particle diameter of the porous silicon particles may be, for example, 30 nm or more, 50 nm or more, 100 nm or more, or 150 nm or more, and may be 10 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less. The particle diameter can be determined by observation with an electron microscope such as a SEM, for example, by the average value of the maximum Ferret diameter of each of the multiple particles. The number of samples is preferably large, for example 20 or more, but may be 50 or more, or 100 or more.

[0034] <Binder> The binder binds together multiple porous silicon particles. The type of binder is not particularly limited. The binder may be selected from, for example, butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, polyimide (PI) binders, carboxymethylcellulose (CMC) binders, polyacrylate binders, polyacrylic acid ester binders, etc. One type of binder may be used alone, or two or more types may be used in combination.

[0035] <Percentage of porous silicon particles and binder contained in the granulated material> The proportion of porous silicon particles and binder in the granules is not particularly limited, as long as it is sufficient to form granules. For example, the proportion of binder to the total of porous silicon particles and binder may be 1% by mass or more, 5% by mass or more, or 8% by mass or more, and may be 30% by mass or less, 28% by mass or less, 26% by mass or less, 24% by mass or less, or 22% by mass or less. When the proportion of binder to the total of porous silicon particles and binder is 1% by mass or more and 30% by mass or less, it is easier to secure a larger charge / discharge capacity.

[0036] <Number of porous silicon particles contained in the granules> A single granule contains multiple porous silicon particles. The number of porous silicon particles in a single granule may be, for example, 3 or more, 5 or more, 10 or more, or 50 or more, or 1000 or less. The number of porous silicon particles in a granule can be determined, for example, by images obtained from observation with an electron microscope or by elemental analysis.

[0037] <Particle size of granulated material> The granules are secondary particles formed by the aggregation of multiple porous silicon particles via a binder. The average particle diameter of the granules is not particularly limited. The average particle diameter of the granules may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and may be 20 μm or less, 15 μm or less, or 10 μm or less. The average particle diameter of the granules contained in the negative electrode composite layer can be determined by observation with an electron microscope such as a SEM, and can be determined, for example, as the average value of the maximum Ferret diameter of multiple granules. A large number of samples is preferable, for example, 20 or more, but may be 50 or more, or 100 or more. Alternatively, the average particle diameter (D50, median diameter) of the granules, which are extracted only from the negative electrode composite layer and measured using a laser diffraction particle distribution analyzer, may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and may be 20 μm or less, 15 μm or less, or 10 μm or less.

[0038] <Shape of granulated material> The negative electrode composite layer can be formed by pressing the composite material. In this process, the granules may be crushed in the pressing direction, resulting in an aspect ratio greater than a predetermined value. By pressing the granules to such an extent that they have an aspect ratio greater than a predetermined value, the contact resistance within the granules, the contact resistance between the granules, and the contact resistance between the granules and other materials are reduced, making it easier to further reduce the overall resistance of the negative electrode.

[0039] [solvent] Furthermore, while organic solvents are included in the synthesis of the granules (the synthesis method is illustrated in the examples), these organic solvents may remain in the granules. The organic solvent is preferably at least one of alcohols, glycols, esters, and amides, and is relatively polar. More specifically, examples include butyl acetate, propylene glycol (PG), 2-butanol, and dimethylformamide (DMF). These do not need to be used individually, and several of them may be included.

[0040] The amount of residual solvent (organic solvent content) in the granules is 950 ppm or less by mass ratio to the total granules. Preferably, it is 300 ppm or less, more preferably 250 ppm or less, and most preferably 100 ppm or less. This suppresses deterioration of cycle characteristics. On the other hand, the amount of residual solvent is preferably 1.2 ppm or more, more preferably 89 ppm or more. This suppresses deterioration of cycle characteristics while keeping costs such as increased production efficiency and costs required for the drying (solvent removal) process under control.

[0041] 1.5. Negative electrode current collector The negative electrode current collector 16 is laminated on the negative electrode composite layer 15 and collects current from the negative electrode composite layer 15. In this embodiment, the negative electrode current collector 16 is a rectangular foil in plan view and can be made of, for example, stainless steel, copper, nickel, carbon, and aluminum, or alloys thereof. Alternatively, these materials may be plated or vapor-deposited with nickel, chromium, or carbon.

[0042] 1.6. Positive terminal, negative terminal The positive and negative terminals are conductive materials and serve as terminals for electrically connecting each pole to the outside. One end of the positive terminal is electrically connected to the positive current collector 12, and the other end penetrates the outer casing and is exposed to the outside. One end of the negative terminal is electrically connected to the negative current collector 16, and the other end penetrates the outer casing and is exposed to the outside.

[0043] 1.7. Exterior The outer casing consists of rectangular sheet-like members in plan view, and includes, for example, a first sheet and a second sheet. The electrode body 11 is enclosed between the first sheet and the second sheet, and the outer edges of the first sheet and the outer edges of the second sheet are joined and sealed. Therefore, this outer casing is bag-shaped, and it encloses and seals the electrode body 11 inside.

[0044] The first and second sheets can be made of laminate film. Here, laminate film is a film having a metal layer and a sealant layer. Examples of metals used in the laminate film include aluminum and stainless steel, and examples of materials used in the sealant layer include thermoplastic resins such as polypropylene, polyethylene, polystyrene, or polyvinyl chloride. [Examples]

[0045] 2. Examples In the examples, the degradation of cycle characteristics was investigated by changing the amount of residual solvent (organic solvent content) in the negative electrode composite layer.

[0046] 2.1. Fabrication of the electrode body (battery) [Fabrication of the positive electrode composite layer] The polypropylene container contains butyl butyrate as an organic solvent, SBR as a binder, carbon fiber produced by a gas phase method as a conductive additive, Li2S-P2S5 glass ceramic as a sulfide solid electrolyte, and LiNi as a positive electrode active material. 0.8 Co 0.15 Mn 0.05 O2 was added and mixed using an ultrasonic homogenizer to obtain a slurry for the cathode composite layer. The obtained slurry was coated onto an Al foil using an applicator with a blade method. The coated material was dried on a hot plate at an appropriate temperature for 30 minutes.

[0047] [Fabrication of the negative electrode composite layer] To prepare the negative electrode composite layer, granules were first synthesized. A binder solution was prepared by dissolving and dispersing PVdF as a binder in an organic solvent. Si particles were then added to this binder solution to form a slurry. This slurry was compounded by spray drying to obtain granules before drying. The types of organic solvents used are shown in Table 1. The obtained granules before drying were vacuum-dried at 100°C to remove the solvent. The amount of residual organic solvent in the granules was adjusted by changing the vacuum drying time. The amount of residual organic solvent (organic solvent content) is shown in Table 1. A gas chromatograph-mass spectrometer (GC-MS) was used to measure the amount of residual organic solvent. More specifically, the instrument used was a Shimadzu GC-MS-QP. TM For the 2020 NX test, the measurement conditions were a heating rate of 5°C / min and a heating temperature of 300°C. This resulted in obtaining granulated material.

[0048] Butyl butyrate was used as the organic solvent, to which the prepared granules, SBR as a binder, carbon fibers produced by the gas phase as a conductive additive, and Li2S-P2S5-based glass ceramic as a sulfide solid electrolyte were added. After addition, the mixture was kneaded using an ultrasonic homogenizer to obtain a slurry for the negative electrode composite layer. The obtained slurry was coated onto Cu foil (manufactured by UACJ) using a blade method with an applicator and dried.

[0049] [Preparation of a solid electrolyte layer] Butyl butyrate was added as an organic solvent, SBR as a binder, and Li2S-P2S5-based glass ceramic as a sulfide solid electrolyte. After addition, the mixture was kneaded using an ultrasonic homogenizer to obtain a slurry for the solid electrolyte layer. The obtained slurry was coated onto an Al foil using an applicator with a blade method and dried.

[0050] [Fabrication of electrode bodies] Each fabricated layer was formed into strips, and the positive electrode composite layer and solid electrolyte layer were combined and heated at 165°C with a load of 50 kN / cm². 2 The Al foil was peeled off by roll pressing with the pressure of 50 kN / cm² to obtain a laminate of the positive electrode composite layer and the solid electrolyte layer. The negative electrode composite layer and the solid electrolyte layer were then combined and pressed at 25°C with a pressure of 50 kN / cm². 2A laminate of the negative electrode composite layer and solid electrolyte layer was obtained by roll pressing at a pressure and peeling off the aluminum foil. The laminate of the negative electrode composite layer and solid electrolyte layer was punched out to a diameter of 13 mm, and the laminate of the positive electrode composite layer and solid electrolyte layer was punched out to a diameter of 11.28 mm. The solid electrolyte layer was further transferred to the punched-out laminate of the negative electrode composite layer and solid electrolyte layer using a uniaxial press, and the laminates of the positive electrode composite layer and solid electrolyte layer were joined together. Finally, tabs were attached to each electrode, and the assembly was sealed in an aluminum laminate outer casing using a vacuum lamination sealer and restrained at a pressure of 5 MPa to form a battery.

[0051] 2.2. Test Method and Results The fabricated batteries were repeatedly charged and discharged at 1C (C-rate), and the change in capacity from the capacity at the first cycle to the capacity at the 50th cycle was expressed as a percentage, which is called the capacity retention rate (%). A higher capacity retention rate indicates that the degradation of cycle characteristics was suppressed. The results are shown in Table 1.

[0052] [Table 1]

[0053] As these results show, cycle characteristics deteriorate when the amount of residual organic solvent exceeds 950 ppm. On the other hand, it can also be seen that the efficiency of improving the suppression of further characteristic deterioration decreases when the amount of residual organic solvent is less than 1.2 ppm. [Explanation of Symbols]

[0054] 11... Electrode body, 12... Positive electrode current collector, 13... Positive electrode composite material layer, 14... Solid electrolyte layer, 15... Negative electrode composite material layer, 16... Negative electrode current collector

Claims

1. An electrode active material using a granule formed by secondary particle formation of primary particles containing Si element with a binder, The organic solvent content is 950 ppm or less. Electrode active material.

2. The electrode active material according to claim 1, wherein the content of the organic solvent is 1.2 ppm or more and 250 ppm or less.

3. The electrode active material according to claim 1, wherein the organic solvent is at least a portion of alcohols, glycols, esters, and amides.

4. A battery comprising a composite material layer having the electrode active material according to any one of claims 1 to 3.

5. All-solid electrolyte layer, The composite material comprises a layer laminated on the all-solid electrolyte layer and having the electrode active material according to claims 1 to 3. All-solid-state battery.

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