JP7622004B2 - Positive electrode, all-solid-state battery including the same, and method for producing the same - Google Patents

Description

The present invention relates to a positive electrode, an all-solid-state battery including the positive electrode, and a method for manufacturing the all-solid-state battery.

Various proposals have been made to improve the battery performance of all-solid-state batteries (e.g., Patent Documents 1 to 7, etc.). In all-solid-state batteries, the electrode body is formed from solid materials (active material particles, solid electrolyte, conductive material, etc.), so in order to improve battery performance, it is important that the solid materials are in close contact with each other.

JP 2017-103060 A JP 2015-099785 A International Publication No. 2011/010552 JP 2016-189339 A JP 2017-068929 A JP 2017-152347 A JP 2017-220339 A

Because the positive electrode active material layer is formed using positive electrode active material particles, fine gaps are formed between the positive electrode active material particles. Improving the ionic conductivity of these gaps leads to improved battery performance, but it is difficult to completely fill these gaps with solid electrolyte. Furthermore, if the entire surface of the positive electrode active material particles is coated with solid electrolyte in order to increase the contact interface between the positive electrode active material particles and the solid electrolyte, it becomes impossible to form an interface between the positive electrode active material particles and the conductive material, making it difficult to form a composite interface using the solid material that constitutes the electrode body.

The objective of the present disclosure is to provide a positive electrode capable of improving the ionic conductivity of the gaps formed between particles in an aggregate of lithium transition metal oxide particles, an all-solid-state battery including the positive electrode, and a method for manufacturing the all-solid-state battery.

[1] A positive electrode active material layer including an aggregate of lithium transition metal oxide particles and a sulfide-based solid electrolyte,
The particles have an electronic conductivity of 1.0×10 ⁻⁸ S/cm or more at a temperature of 25° C.
a first component is present in gaps having a size of 0.2 nm or more and 20 nm or less formed between the particles of the aggregate;
The first component includes a second component derived from the solid electrolyte.
[2] The positive electrode according to [1], wherein the first component has an atomic concentration of a transition metal of 3% or less and an atomic concentration of oxygen of 4% or less.
[3] The positive electrode according to [1] or [2], wherein the first component has an atomic concentration of lithium of 20% or more and 70% or less, an atomic concentration of sulfur of 5% or more and 70% or less, and an atomic concentration of phosphorus of 10% or less.
[4] The positive electrode according to any one of [1] to [3], wherein the solid electrolyte is amorphous or crystalline.
[5] The positive electrode according to any one of [1] to [4], wherein the solid electrolyte contains a halogen atom.
[6] The positive electrode according to any one of [1] to [5], wherein the particles are a layered rock salt type or spinel type positive electrode active material.
[7] The positive electrode according to any one of [1] to [6], wherein the particles have an average particle diameter D50 of 0.5 μm or less.
[8] An all-solid-state battery comprising the positive electrode according to any one of [1] to [7], a solid electrolyte layer, and a negative electrode, in this order.
[9] A method for producing an all-solid-state battery including a laminate having, in this order, a positive electrode, a solid electrolyte layer, and a negative electrode having a negative electrode active material layer, comprising:
forming a positive electrode mixture layer using a positive electrode mixture containing lithium transition metal oxide particles and a sulfide-based solid electrolyte;
forming the laminate using a mixture laminate in which the positive electrode mixture layer, the solid electrolyte layer, and a negative electrode mixture layer for forming the negative electrode active material layer are laminated,
the step of forming the laminate includes a step of performing a charge-discharge cycle in which the potential of the positive electrode mixture layer of the mixture laminate is charged until it becomes 3.5 V (vs. Li/Li ⁺ ) or more, and then discharged until it becomes 2.0 V (vs. Li/Li ⁺ ) or less;
The particles have an electronic conductivity of 1.0×10 ⁻⁸ S/cm or more at a temperature of 25° C.
[10] The method for producing an all-solid-state battery according to [9], wherein the charge-discharge cycle is performed under a temperature condition of 40° C. or higher and 100° C. or lower.
[11] The positive electrode mixture layer contains an aggregate of the particles,
The method for producing an all-solid-state battery according to [9] or [10], wherein the charge-discharge cycle is repeated until a gradient indicating diffusion of lithium ions appears in impedance of gaps formed between the particles of the aggregate.
[12] The method for producing an all-solid-state battery according to any one of [9] to [11], wherein the positive electrode mixture further contains a conductive material.

The present disclosure provides a positive electrode for an all-solid-state battery that can improve the ionic conductivity of the gaps formed between particles in an aggregate of lithium transition metal oxide particles, an all-solid-state battery including the positive electrode, and a method for manufacturing the all-solid-state battery.

1 is a HAADF image of an interface between active material particles constituting an aggregate of active material particles, observed before charge-discharge cycles. 1 is a HAADF image of an interface between active material particles constituting an aggregate of active material particles, observed after charge-discharge cycles. 1 is a Nyquist plot between a positive electrode including a positive electrode mixture layer and a reference electrode. 1 is a graph showing the relationship between temperature and the number of cycles in a charge/discharge cycle.

(Positive electrode)
The positive electrode of this embodiment has a positive electrode active material layer including an aggregate of lithium transition metal oxide particles (hereinafter also referred to as "active material particles") and a sulfide-based solid electrolyte. The active material particles have an electronic conductivity of 1.0×10 ⁻⁸ S/cm or more. Gaps of 0.2 nm to 20 nm (hereinafter also referred to as "gaps in the aggregates") are formed between the active material particles of the aggregates. A first component is present in the gaps in the aggregates, and the first component includes a second component derived from the solid electrolyte.

The gaps between the aggregates are very small, between 0.2 nm and 20 nm, as described above. On the other hand, the solid electrolyte used in all-solid-state batteries is a submicron-sized solid. Therefore, the solid electrolyte does not easily enter the gaps between the aggregates. In the positive electrode of the present disclosure, by using active material particles having an electronic conductivity within the above range, a second component derived from the solid electrolyte can be inserted into the gaps between the aggregates by the manufacturing method described below. The presence of the second component in the gaps between the aggregates makes it easier for lithium ions to diffuse within the gaps, improving the ionic conductivity of the positive electrode active material layer. This makes it possible to realize a positive electrode with a small diffusion resistance of lithium ions, which is expected to improve the battery performance of the all-solid-state battery.

The active material particles are primary particles. The average particle diameter D50 of the active material particles as primary particles is preferably 0.5 μm or less, but is not limited thereto. The average particle diameter D50 of the active material particles is, for example, 0.01 μm or more and 5 μm or less, preferably 0.05 μm or more and 1 μm or less, and more preferably 0.05 μm or more and 0.5 μm or less. The smaller the average particle diameter D50 of the active material particles, the more likely they are to aggregate. In the positive electrode of this embodiment, the second component can be present in the gaps of the aggregates, which are 0.2 nm or more and 20 nm or less. Therefore, the ion conductivity can be improved even in the positive electrode active material layer formed using active material particles with a small average particle diameter D50. The average particle diameter D50 in this specification is the particle diameter at which the cumulative frequency from the small particle diameter side in the volume-based particle size distribution is 50%, and can be measured by a laser diffraction/scattering method.

The active material particles are an active material used as a positive electrode active material, and are oxides containing lithium and transition metal elements. The active material particles are not particularly limited as long as they are lithium transition metal oxide particles that satisfy the above-mentioned electronic conductivity. The active material particles preferably contain a lithium transition metal oxide containing one or more transition metal elements selected from the group consisting of Ni, Co, and Mn. Examples of such lithium transition metal oxides include one or more selected from lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, and lithium nickel cobalt aluminum composite oxides.

The active material particles are preferably layered rock salt type or spinel type positive electrode active material. The positive electrode active material layer may contain one type of active material particles made of these active materials, or may contain two or more types. Examples of layered rock salt type positive electrode active materials include LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNiCoMnO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , etc. Examples of spinel type positive electrode active materials include LiMn ₂ O ₄ , Li(Ni _0.5 Mn _1.5 )O ₄ , etc.

The electronic conductivity of the active material particles at a temperature of 25° C. is preferably 1.0×10 ⁻⁷ S/cm or more, more preferably 1.0×10 ⁻⁶ S/cm or more, and even more preferably 1.0×10 ⁻⁵ S/cm or more, and is usually 1.0×10 ⁻¹ S/cm or less. The higher the electronic conductivity of the active material particles, the more efficiently the second component can be introduced into the gaps in the aggregates by the production method described below, which is preferable.

The electronic conductivity of the active material particles is calculated as the reciprocal of the product of the measured resistance and thickness (electronic conductivity = 1/(resistance x thickness)) by measuring the resistance and thickness of a pellet prepared for measurement using a powder resistance measurement system while changing the pressure under conditions of a temperature of 25°C. An example of the electronic conductivity of the active material particles calculated by this method is shown below.
Electronic conductivity of active material particles _made of LiNiCoMnO2: 6.4× ^10-3 S/cm
Electronic conductivity of active material particles made of _LiCoO2 : 1.5×10 ⁻⁴ S/cm
Electronic conductivity of active material particles made of LiMn ₂ O ₄ : 1.2×10 ⁻⁶ S/cm

The positive electrode active material layer includes a region formed by an aggregate of active material particles and a region formed by a solid electrolyte, and these are in a mixed state. The aggregate of active material particles is an aggregate of active material particles, which are primary particles, and is so-called secondary particles. As described above, the aggregate has gaps (gaps in the aggregate) between the active material particles of 0.2 nm or more and 20 nm or less.

The gaps in the aggregate are pore-like spaces that exist between the active material particles that make up the aggregate in the cross section of the positive electrode active material layer and are surrounded by two or more active material particles. The spaces do not include spaces formed by the solid electrolyte contained in the region formed by the solid electrolyte and the active material particles that exist in the region of the aggregate. In this specification, the gaps in the aggregate are confirmed by observing the interface of the active material particles that make up the aggregate in the cross section of the positive electrode active material layer using an electron microscope such as a transmission electron microscope (TEM). The gaps in the aggregate can also be confirmed by a method of analyzing the pore distribution, such as mercury intrusion porosimetry. The distance of the gaps in the aggregate refers to the smallest distance among the distances between the active material particles that form the gaps (active material particles that surround the gaps).

A first component is present in the gaps of the aggregates, and the first component contains a second component derived from a sulfide-based solid electrolyte. The first component refers to a component present in the gaps of the aggregates, and containing the second component means containing the same atoms as those contained in the solid electrolyte. The second component may be a decomposition product of the solid electrolyte. The first component may contain components other than the second component, but it is preferable that the first component does not contain any components derived from the active material particles, or that the content of such components is small. It is preferable that the first component that has entered the gaps of the aggregates forms a single-layer structure rather than a multi-layer structure.

Examples of atoms contained in the second component include lithium, sulfur, and phosphorus. The first component may contain oxygen, transition metals, and the like as atoms other than the second component.

The atomic concentration of the transition metal contained in the first component is preferably 3% or less, may be 2% or less, and more preferably 1% or less. The atomic concentration of oxygen contained in the first component is preferably 4% or less, may be 3% or less, and more preferably 2% or less. The atomic concentration [%] is the number of the corresponding atoms relative to the total amount of atoms contained in the first component.

The atomic concentration of lithium contained in the first component is preferably 20% or more and 70% or less, and may be 30% or more and 60% or less. The atomic concentration of sulfur contained in the first component is preferably 5% or more and 70% or less, and may be 10% or more and 60% or less. The atomic concentration of phosphorus contained in the first component is preferably 10% or less, and may be 9% or less, preferably 8% or less, and may be 1% or more. The lithium, sulfur, and phosphorus in the above contents of the first component can constitute the second component.

In this specification, the composition of the first component in the gaps between the aggregates is a value obtained by elemental analysis using energy dispersive X-ray spectroscopy (TEM-EDX) with a TEM. In elemental analysis using TEM-EDX, if the beam diameter is larger than the area to be observed, the composition in the gaps between the aggregates can be determined by subtracting the composition derived from the active material particles from the measured value.

The composition of the first component present in the gaps of the aggregates can also be determined by surface analysis such as X-ray photoelectron spectroscopy (XPS). In surface analysis using XPS, the content of lithium atoms can be analyzed in a discharged state of 2 V or less, or the valence of sulfur can be confirmed and calculated from the charge balance.

From the redox reaction that occurs during the charge/discharge cycle of the manufacturing method described below, it is believed that the first component present in the gaps between the aggregates is mostly the second component, and contains almost no components derived from the active material particles. When the first component in the gaps between the aggregates is observed using the above analysis method, it is unavoidably affected by the active material particles, and therefore it is believed that trace amounts of transition metals and oxygen derived from the active material particles are detected in the first component.

The positive electrode active material layer includes a region formed by a solid electrolyte as described above. The solid electrolyte constituting the region is a sulfide-based solid electrolyte, and includes lithium atoms and sulfur atoms, and may further include phosphorus atoms. The solid electrolyte preferably includes a halogen atom. Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms, and bromine atoms or iodine atoms are preferred. By including halogen atoms in the solid electrolyte, the ionic conductivity of the solid electrolyte is improved, and the redox reaction of the solid electrolyte is more easily caused by the manufacturing method described below.

The atomic concentration of lithium contained in the solid electrolyte is preferably 20% or more and 70% or less, and may be 30% or more and 60% or less. The atomic concentration of sulfur contained in the solid electrolyte is preferably 5% or more and 70% or less, and may be 10% or more and 60% or less. The atomic concentration of phosphorus contained in the solid electrolyte is preferably 10% or less, and may be 9% or less, preferably 8% or less, and may be 1% or more.

Examples of the solid electrolytes include Li ₂ S-P ₂ S ₅ , Li ₂ S-P ₂ S ₅ -LiI, Li ₂ S-P ₂ S ₅ -Li ₂ O, and Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -LiI, Li ₂ S-SiS ₂ -LiBr, Li ₂ S-SiS ₂ -LiCl, Li ₂ S-SiS ₂ -B ₂ S ₃ -LiI, Li ₂ S-SiS ₂ -P ₂ S ₅ -LiI, Li ₂ SB ₂ S ₃ , Li ₂ SP ₂ S ₅ -Z _m S _n (m and n are positive numbers, and Z is any one of Ge, Zn, and Ga), Li ₂ S—GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In), etc. The above description of "Li ₂ S-P ₂ S ₅ " means a sulfide-based solid electrolyte material formed using a raw material composition containing Li ₂ S and P ₂ S ₅ , The same applies to the other descriptions.

The ionic (lithium ion) conductivity of the solid electrolyte at a temperature of 25° C. may be 1.0×10 ⁻⁵ S/cm or more, or may be 1.0×10 ⁻⁴ S/cm or more. The ionic conductivity of the solid electrolyte can be calculated from the measured resistance and the thickness of the pellet by measuring the resistance of the pellet prepared for measurement by performing AC impedance measurement at a temperature of 25° C.

The solid electrolyte is preferably amorphous or crystalline. In this specification, "amorphous" means that no peak of the solid electrolyte is observed in X-ray diffraction (XRD) measurement, i.e., no periodicity as a crystal is observed, and a so-called halo pattern is observed. In this specification, "crystalline" means that a peak of the solid electrolyte is observed in XRD measurement.

The positive electrode active material layer may further contain at least one of a conductive material and a binder in addition to the active material particles and the sulfide-based solid electrolyte.

Examples of conductive materials that may be contained in the positive electrode active material layer include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black, ketjen black, carbon nanotubes (CNT), and carbon nanofibers (CNF); metals such as nickel, aluminum, and stainless steel; or combinations thereof.

Examples of binders that may be included in the positive electrode active material layer include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide (PA), polyamideimide (PAI), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile-butadiene rubber (NBR), styrene-ethylene-butylene-styrene block copolymer (SEBS), carboxymethyl cellulose (CMC), or combinations thereof.

(All-solid-state battery)
The all-solid-state battery of this embodiment has a positive electrode, a solid electrolyte layer, and a negative electrode in this order. The positive electrode may be any of those described above. In the positive electrode active material layer of the positive electrode described above, a second component derived from a sulfide-based solid electrolyte is present in the gaps of the aggregates. Therefore, lithium ions are easily diffused in the gaps of the aggregates, and the ion conductivity of the positive electrode active material layer can be improved. As a result, the all-solid-state battery of this embodiment can realize a positive electrode with a small diffusion resistance of lithium ions, and it is expected that the battery performance can be improved.

The solid electrolyte layer includes a solid electrolyte. Examples of the solid electrolyte include sulfide-based solid electrolytes, and those described above can be used. The sulfide-based solid electrolyte included in the solid electrolyte layer may be the same as or different from the sulfide-based solid electrolyte included in the positive electrode active material layer of the positive electrode, but it is preferable that they are the same.

The negative electrode may include a negative electrode active material layer. The negative electrode active material layer may include a negative electrode active material and may further include one or more of a solid electrolyte, a conductive material, and a binder. Examples of the negative electrode active material include carbon active materials such as mesocarbon microbeads, graphite, hard carbon, and soft carbon; and metal active materials such as In, Al, Si, and Sn. Examples of the solid electrolyte include the sulfide-based solid electrolyte described above. The solid electrolyte included in the negative electrode active material layer may be the same as or different from the solid electrolyte constituting the solid electrolyte layer, but is preferably the same. Examples of the conductive material and binder include the conductive material and binder that may be included in the positive electrode active material layer.

(Method of manufacturing all-solid-state battery)
The manufacturing method of the present embodiment is a manufacturing method of an all-solid-state battery including a laminate having a positive electrode, a solid electrolyte layer, and a negative electrode having a negative electrode active material layer in this order. The manufacturing method of the all-solid-state battery includes a step of forming a positive electrode mixture layer using a positive electrode mixture containing lithium transition metal oxide particles (active material particles) and a sulfide-based solid electrolyte, and a step of forming a laminate using a mixture laminate in which the positive electrode mixture layer, the solid electrolyte layer, and the negative electrode mixture layer for forming the negative electrode active material layer are laminated. The step of forming the laminate includes a step of performing a charge-discharge cycle in which the positive electrode mixture layer of the mixture laminate is charged until the potential of the positive electrode mixture layer is 3.5 V (vs. Li/Li ⁺ ) or more and discharged until the potential is 2.0 V (vs. Li/Li ⁺ ) or less. The electronic conductivity of the active material particles is 1×10 ⁻⁸ S/cm or more.

The positive electrode, solid electrolyte layer, and negative electrode active material layer may be, for example, those described above. The active material particles and sulfide-based solid electrolyte used in each step may be those described above. The electronic conductivity of the active material particles may also be within the range described above. The positive electrode mixture may further contain a conductive material. The negative electrode mixture includes a negative electrode active material, and may further contain one or both of a solid electrolyte and a conductive material. Examples of conductive materials that may be included in the positive electrode mixture and the negative electrode mixture include those described above.

The process of forming the positive electrode mixture layer can be carried out, for example, by mixing active material particles and a solid electrolyte to form a positive electrode mixture, and compressing the positive electrode mixture to form the positive electrode mixture layer. Alternatively, a positive electrode mixture containing a dispersion medium may be used, which is then coated, dried, and compressed to form the positive electrode mixture layer, or a positive electrode mixture containing a dispersion medium may be coated on a metal foil or the like, and the powder obtained by drying may be compressed to form the positive electrode mixture layer.

The content of the active material particles and solid electrolyte in the positive electrode mixture is not particularly limited. The positive electrode mixture may contain the active material particles and solid electrolyte in a volume ratio (active material particles:solid electrolyte) in the range of 1:9 to 9:1, in the range of 2:8 to 8:2, or in the range of 3:7 to 7:3.

The solid electrolyte layer and the negative electrode mixture layer can be formed in the same manner as the positive electrode mixture layer.

The composite laminate used in the process of forming the laminate can be formed by stacking the positive electrode composite layer, solid electrolyte layer, and negative electrode composite layer obtained above, and pressing them as necessary.

The positive electrode mixture layer contains active material particles having an electronic conductivity of 1×10 ⁻⁸ S/cm or more. Therefore, by performing a charge/discharge cycle, an oxidation-reduction reaction of the solid electrolyte in the positive electrode mixture layer occurs, and the sulfide-based solid electrolyte is electrolyzed on the surface of the active material particles. As a result, it is considered that the lithium and sulfur contained in the solid electrolyte react with each other and penetrate into the gaps of the aggregates along the surface of the active material particles, forming a positive electrode active material layer in which a second component derived from the solid electrolyte is present in the gaps. In contrast, when active material particles having an electrical conductivity of less than 1×10 ⁻⁸ S/cm are used, the supply of electrons from the active material particles is slowed down during charge/discharge, and the reaction of the solid electrolyte is very slow, so it is considered that the reaction of penetrating into the gaps of the aggregates as described above hardly occurs.

In the charging step of performing the charge-discharge cycle, the potential of the positive electrode mixture layer may be 3.8 V (vs. Li/Li ⁺ ) or more, 4.0 V (vs. Li/Li ⁺ ) or more, or 4.4 V (vs. Li/ ^{Li +} ) or less, or 4.2 V (vs. Li/Li ⁺ ) or less. In the discharging step of performing the charge-discharge cycle, the potential of the positive electrode mixture layer may be 1.9 V (vs. Li/Li ⁺ ) or less, 1.8 V (vs. Li/Li ⁺ ) or less, or 1.5 V (vs. Li/Li ⁺ ) or more, or 1.6 V (vs. Li/Li ⁺ ) or more. When the potential of the positive electrode mixture layer falls outside the above-mentioned range during charging and discharging in the charge-discharge cycle process, it tends to be difficult to form a positive electrode active material layer in which the second component is present in the gap.

In the charge-discharge cycle process, it is preferable to measure the impedance of the positive electrode mixture layer contained in the mixture laminate, identify the impedance of the gaps in the aggregates by a Nyquist plot between a positive electrode formed using the positive electrode mixture layer and a reference electrode, and repeat the charge-discharge cycle until a slope indicating the diffusion of ions (lithium ions) appears in this impedance. This slope was previously thought to be absent in all-solid-state batteries because it was not possible to allow solid electrolyte to enter the gaps in the aggregates. However, in batteries using a liquid electrolyte, the liquid electrolyte can permeate the gaps in the aggregates, so the above slope is confirmed.

The number of charge/discharge cycles in the charge/discharge cycle process may be 1 or more, 2 or more, 5 or more, or 10 or more. The number of charge/discharge cycles is usually 150 or less, 120 or less, 50 or less, 10 or less, or 5 or less. The number of cycles is the number of times the charge/discharge cycle is repeated, with one charge/discharge cycle in which the above-mentioned discharge is performed after the above-mentioned charging. There are no particular limitations on the state of charge (SOC) in the charge/discharge cycle process.

The charge and discharge performed in the process of forming the laminate is preferably performed under conditions of a temperature of 40°C or higher and 100°C or lower. The temperature condition is more preferably 40°C or higher and 80°C or lower, and may be 50°C or higher and 80°C or lower. By performing charge and discharge within the above temperature range, the reaction between the lithium and sulfur contained in the solid electrolyte is promoted, so that the second component is more likely to penetrate into the gaps in the aggregates. If the temperature exceeds 100°C, the active material particles and the solid electrolyte react with each other, and the positive electrode active material layer is more likely to deteriorate.

The present disclosure will be described more specifically below with reference to examples and comparative examples.
Example 1
(Preparation of Positive Electrode Mixture)
As the active material particles, particles (electronic conductivity: 6.4×10 ⁻³ S/cm, average particle diameter D50 of aggregated particles: 10 μm) of layered rock salt type active material of lithium nickel cobalt manganese composite oxide (Ni:Co:Mn=0.80:0.05:0.15 (atomic ratio)) were prepared. As the sulfide-based solid electrolyte, Li ₂ S-P ₂ S ₅ (ionic conductivity: 3.0×10 −3 S/cm) was prepared by mixing Li ₂ S and P ₂ S ₅ in a ball mill at Li ₂ S:P ₂ S ₅ = ³ :1 (mass ratio) for 5 hours. The solid electrolyte was amorphous. The active material particles, solid electrolyte, and vapor-grown carbon fiber (VGCF, manufactured by Showa Denko K.K.) as a conductive material prepared above were mixed in a volume ratio of active material particles:solid electrolyte:conductive material=10:10:1 to prepare a positive electrode mixture.

(Preparation of negative electrode mixture)
Natural graphite was prepared as the negative electrode active material. The negative electrode active material, solid electrolyte, and conductive material were mixed in a ratio of negative electrode active material:solid electrolyte:conductive material=30:20:1 (volume ratio) to prepare a negative electrode mixture. The solid electrolyte and conductive material were the same as those used in the positive electrode mixture.

(Fabrication of all-solid-state batteries)
A pellet of a compact having, in this order, a positive electrode mixture layer formed using the positive electrode mixture prepared above, a solid electrolyte layer formed using the solid electrolyte used above, and a negative electrode mixture layer formed using the negative electrode mixture prepared above was prepared. The pellet was pressed at a pressure of 400 MPa to obtain a mixture laminate. The obtained mixture laminate was used to assemble into an all-solid-state battery structure, and a charge-discharge cycle was performed in which constant current charging and discharging was performed at a current density of 0.40 to 4 mA cm ^-2 and a temperature range of 25°C to 100°C to obtain an all-solid-state battery. In the charge-discharge cycle, the potential of the positive electrode mixture layer was set so that the charge cut-off voltage was 3.5V to 4.4V (vs. Li/Li ⁺ ) and the discharge cut-off voltage was 1.0 to 2.0V (vs. Li/Li ⁺ ). After four charge-discharge cycles with a charge cutoff voltage of 4.2 V, the initial discharge capacity was 192 mAh, and the capacity retention rate calculated by dividing the discharge capacity after 100 cycles by the initial discharge capacity and multiplying the result by 100 was 97%.

(Observation of the interface of active material particles)
The interface of the active material particles constituting the aggregate of the active material particles in the positive electrode mixture layer and the positive electrode active material layer was observed by a high-angle scattering annular dark field scanning transmission microscope (HAADF-STEM). The results are shown in FIG. 1 and FIG. 2. FIG. 1 is a HAADF image of the interface of the active material particles constituting the aggregate of the active material particles observed before the charge-discharge cycle, and FIG. 2 is a HAADF image of the interface of the active material particles constituting the aggregate of the active material particles observed after the charge-discharge cycle. Comparing FIG. 1 and FIG. 2, it was confirmed that the first component entered the gap of 0.2 nm or more and 20 nm or less formed between the active material particles constituting the aggregate after the charge-discharge cycle (part surrounded by a dashed line in FIG. 2). The symbols and numbers in FIG. 1 and FIG. 2 are given for the convenience of observation.

(Analysis of composition in gaps)
The composition of the first component present in the gaps (gaps of 0.2 nm to 20 nm) of the aggregates in the positive electrode active material layer of the all-solid-state battery prepared above was quantitatively analyzed using TEM-EDX. As a result of the quantitative analysis, in all the all-solid-state batteries, the first component had a lithium atomic concentration in the range of 20% to 70%, a sulfur atomic concentration in the range of 5% to 70%, and a phosphorus atomic concentration in the range of more than 0% to 8%. In addition, in all the all-solid-state batteries, the first component had a transition metal atomic concentration of 1% or less and an oxygen atomic concentration of 3% or less. From the composition of the first component, it was found that the first component present in the gaps of the aggregates contained the second component (lithium, sulfur, phosphorus) derived from the solid electrolyte, and the component derived from the active material particles was small.

(Impedance Measurement (1))
The potential of the positive electrode mixture layer was set so that the charge cutoff voltage was 4.2 V (vs. Li/Li ⁺ ) and the discharge cutoff voltage was 1.5 V (vs. Li/Li ⁺ ), and a charge/discharge cycle was performed. The number of charge/discharge cycles and the charge rate (SOC) were changed to measure the impedance of the positive electrode mixture layer contained in the mixture laminate, and a Nyquist plot was performed between the positive electrode composed of the positive electrode mixture layer and the reference electrode. The results are shown in FIG. 3. When the impedance of the gap of the aggregate in the positive electrode mixture layer was specified, it was the part indicated by "R _ion " in FIG. 3. From FIG. 3, it was found that after two charge/discharge cycles were performed, the slope of the part indicated by "R _ion " became almost the same regardless of the number of charge/discharge cycles and the charge rate, and diffusion of lithium ions occurred in the gap of the aggregate. In FIG. 3, the portion indicated by "R _SE " represents the impedance between the solid electrolyte, the portion indicated by "R _CT " represents the impedance between the active material particles, and the portion indicated by "R _W " represents the impedance of diffusion within the active material particles.

(Impedance Measurement (2))
The charge-discharge cycle was repeated in the temperature range of 25°C to 100°C until a gradient indicating the diffusion of lithium ions was observed in the impedance of the gaps of the aggregates in the positive electrode mixture layer. The conditions of the charge-discharge cycle were set to the same conditions as those in the impedance measurement (1). The results are shown in FIG. 4. FIG. 4 is a graph showing the relationship between the temperature of the charge-discharge cycle and the number of charge-discharge cycles performed until the above-mentioned gradient was observed. From FIG. 4, it was found that the second component can be efficiently introduced into the gaps of the aggregates by performing the charge-discharge cycle under conditions of 40°C or higher and 100°C or lower. When the charge-discharge cycle was performed at a temperature of 60°C, a gradient indicating the diffusion of lithium ions was observed in the impedance of the gaps of the aggregates after four charge-discharge cycles.

Example 2
(Fabrication of all-solid-state batteries)
An all-solid-state battery was obtained in the same manner as in Example 1, except that particles of LiCoO ₂ (electronic conductivity: 1.5×10 ⁻⁴ S/cm, average particle diameter D50 of aggregated particles: 8 μm), which is a layered rock salt type active material, were used as the active material particles.

(Analysis of composition in gaps)
The composition of the first component present in the gaps (gaps of 0.2 nm or more and 20 nm or less) of the aggregates in the positive electrode active material layer of the all-solid-state battery prepared above was quantitatively analyzed using TEM-EDX. As a result of the quantitative analysis, in all the all-solid-state batteries, the first component had a lithium atomic concentration in the range of 20% or more and 70% or less, a sulfur atomic concentration in the range of 5% or more and 70% or less, and a phosphorus atomic concentration in the range of more than 0% and 10% or less. In addition, in all the all-solid-state batteries, the first component had a transition metal atomic concentration of 3% or less and an oxygen atomic concentration of 4% or less. From the composition of the first component, it was found that the first component present in the gaps of the aggregates contained the second component (lithium, sulfur, phosphorus) derived from the solid electrolyte, and the component derived from the active material particles was small.

(Impedance measurement)
The all-solid-state battery obtained above was subjected to charge-discharge cycles at a temperature of 60° C. in accordance with the procedure of impedance measurement (2) performed in Example 1, except that ^the charge cutoff voltage was set to 4.4 V (vs. Li/Li + ). After 20 charge-discharge cycles, a slope indicating the diffusion of lithium ions appeared in the impedance of the gaps in the aggregates.

Example 3
(Fabrication of all-solid-state batteries)
A mixture laminate was obtained in the same manner as in Example 1, except that particles of LiMn ₂ O ₄ (electronic conductivity: 1.2×10 ⁻⁶ S/cm, average particle diameter D50 of aggregated particles: 10 μm), which is a spinel type active material, were used as the active material particles. The obtained mixture laminate was used to assemble into an all-solid-state battery structure, and a charge-discharge cycle was performed in which constant current charging and discharging was performed at a current density of 0.40 to 2 mA cm ⁻² and a temperature range of 60° C., to obtain an all-solid-state battery. In the charge-discharge cycle, the charge cut-off voltage was set to 4.0 V (vs. Li/Li ⁺ ) and the discharge cut-off voltage was set to 1.0 to 2.0 V (vs. Li/Li ⁺ ) as the potential of the positive electrode mixture layer.

(Analysis of composition in gaps)
The composition of the first component present in the gaps (gaps of 0.2 nm to 20 nm) of the aggregates in the positive electrode active material layer of the all-solid-state battery prepared above was quantitatively analyzed using TEM-EDX. As a result of the quantitative analysis, in all the all-solid-state batteries, the first component had a lithium atomic concentration in the range of 20% to 70%, a sulfur atomic concentration in the range of 5% to 70%, and a phosphorus atomic concentration in the range of more than 0% to 5%. In addition, in all the all-solid-state batteries, the first component had a transition metal atomic concentration of 1% or less and an oxygen atomic concentration of 3% or less. From the composition of the first component, it was found that the first component present in the gaps of the aggregates contained the second component (lithium, sulfur, phosphorus) derived from the solid electrolyte, and the component derived from the active material particles was small.

(Impedance measurement)
The all-solid-state battery obtained above was subjected to charge-discharge cycles at a temperature of 60° C. in accordance with the procedure of impedance measurement (2) performed in Example 1, except that ^the charge cutoff voltage was set to 4.3 V (vs. Li/Li + ). After 60 charge-discharge cycles, a slope indicating the diffusion of lithium ions appeared in the impedance of the gaps in the aggregates.

Comparative Example 1
(Fabrication of all-solid-state batteries)
An all-solid-state battery was obtained in the same manner as in Example 1, except that particles of an olivine type active material LiFePO ₄ (electronic conductivity: 1.1×10 ⁻⁹ S/cm) were used as the active material particles.

(Impedance measurement)
The all-solid-state battery obtained above was subjected to charge-discharge cycles at a temperature of 60° C. according to the procedure of impedance measurement (2) performed in Example 1. No gradient indicating diffusion of lithium ions was observed in the impedance of the gaps in the aggregates even after 200 charge-discharge cycles. This suggests that the second component derived from the solid electrolyte does not enter the gaps in the aggregates.

Comparative Example 2
(Fabrication of all-solid-state batteries)
An all-solid-state battery was obtained in the same manner as in Example 1, except that the discharge cutoff voltage was set to 2.0 to less than 3.5 V (vs. Li/Li ⁺ ). After four charge-discharge cycles with a charge cutoff voltage of 3.0 V, the initial discharge capacity was 178 mAh, and the capacity retention rate calculated by dividing the discharge capacity after 100 cycles by the initial discharge capacity and multiplying the result by 100 was 97%. No second component derived from the solid electrolyte was generated in the gaps between the aggregates in the positive electrode active material layer of the produced all-solid-state battery.

The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by the claims, and is intended to include all modifications within the meaning and scope of the claims.

Claims (11)

a positive electrode active material layer including an aggregate that is a secondary particle formed by aggregating primary particles of a lithium transition metal oxide, and a sulfide-based solid electrolyte (excluding a positive electrode active material layer including (i) a polymer having a hydrocarbon polymer segment in a main chain, the main chain including at least one bond selected from an ester bond, an amide bond, a urethane bond, a urea bond, an imide bond, an ether bond, and a carbonate bond, (ii) a hydrogenated styrene butadiene rubber, or (iii) a butylene rubber having an amino group as a terminal functional group);
The primary particles have an electronic conductivity of 1.0×10 ⁻⁸ S/cm or more at a temperature of 25° C.
a first component is present in gaps having a size of 0.2 nm or more and 20 nm or less formed between the primary particles of the aggregate;
The first component includes a second component derived from the solid electrolyte. The positive electrode according to claim 1, wherein the first component has a transition metal atomic concentration of 3% or less and an oxygen atomic concentration of 4% or less. The positive electrode according to claim 1, wherein the first component has an atomic concentration of lithium of 20% or more and 70% or less, an atomic concentration of sulfur of 5% or more and 70% or less, and an atomic concentration of phosphorus of 10% or less. The positive electrode of claim 1, wherein the solid electrolyte is amorphous or crystalline. The positive electrode according to claim 1, wherein the solid electrolyte contains halogen atoms. The positive electrode according to claim 1, wherein the primary particles are a layered rock salt type or spinel type positive electrode active material. The positive electrode according to claim 1, wherein the average particle diameter D50 of the primary particles is 0.5 μm or less. An all-solid-state battery having a positive electrode, a solid electrolyte layer, and a negative electrode according to any one of claims 1 to 7, in this order. A method for producing an all-solid-state battery including a laminate having, in this order, a positive electrode, a solid electrolyte layer, and a negative electrode having a negative electrode active material layer, comprising:
forming a positive electrode mixture layer (excluding a positive electrode mixture layer including (i) a polymer having a hydrocarbon polymer segment in its main chain, the main chain including at least one bond selected from an ester bond, an amide bond, a urethane bond, a urea bond, an imide bond, an ether bond, and a carbonate bond, (ii) a hydrogenated styrene butadiene rubber, or (iii) a butylene rubber having an amino group as a terminal functional group) using a positive electrode mixture including an aggregate that is a secondary particle formed by aggregating primary particles of a lithium transition metal oxide and a sulfide-based solid electrolyte;
forming the laminate using a mixture laminate in which the positive electrode mixture layer, the solid electrolyte layer, and a negative electrode mixture layer for forming the negative electrode active material layer are laminated,
the step of forming the laminate includes a step of performing a charge-discharge cycle in which the potential of the positive electrode mixture layer of the mixture laminate is charged until it becomes 3.5 V (vs. Li/Li ⁺ ) or more, and then discharged until it becomes 2.0 V (vs. Li/Li ⁺ ) or less;
The step of performing the charge-discharge cycle includes repeating the charge-discharge cycle until a gradient indicating diffusion of lithium ions appears in impedance of gaps formed between the primary particles of the aggregate,
The method for producing an all-solid-state battery, wherein the primary particles have an electronic conductivity of 1.0×10 ⁻⁸ S/cm or more at a temperature of 25° C. The method for producing an all-solid-state battery according to claim 9, wherein the charge-discharge cycle is performed under a temperature condition of 40°C or higher and 100°C or lower. The method for producing an all-solid-state battery according to claim 9 , wherein the positive electrode mixture further contains a conductive material.

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