JP7652121B2 - Composite particle, positive electrode, all-solid-state battery, and method for producing composite particle - Google Patents

Description

The present disclosure relates to composite particles, positive electrodes, all-solid-state batteries, and methods for producing composite particles.

JP 2010-135090 A (Patent Document 1) discloses forming a reaction suppression portion on the surface of the positive electrode active material by the PLD (Pulsed Laser Deposition) method.

JP 2010-135090 A

A sulfide-based all-solid-state battery (hereinafter may be abbreviated as "all-solid-state battery") has been developed. The all-solid-state battery contains a sulfide solid electrolyte. If the sulfide solid electrolyte comes into direct contact with the positive electrode active material particles, the sulfide solid electrolyte may deteriorate. The deterioration of the sulfide solid electrolyte (ion conduction path) may increase the battery resistance. Therefore, it has been proposed to form a coating film on the surface of the positive electrode active material particles. The coating film prevents direct contact between the positive electrode active material particles and the sulfide solid electrolyte, which may reduce the deterioration of the sulfide solid electrolyte. This is expected to reduce the initial resistance.

Conventionally, it has been proposed to form a coating film by a gas phase method (e.g., PLD method, etc.). Instead of the gas phase method, it is also possible to form a coating film by a liquid phase method. That is, it is possible to form a coating film by drying a coating liquid on the surface of the positive electrode active material particles. By adopting the liquid phase method, for example, it is expected to improve mass productivity. However, the liquid phase method tends to have a small effect on reducing the initial resistance.

The purpose of this disclosure is to reduce initial resistance.

The technical configuration and effects of the present disclosure are explained below. However, the mechanism of action in this specification includes assumptions. The mechanism of action does not limit the technical scope of the present disclosure.

1. The composite particle includes a positive electrode active material particle and a coating film. The coating film covers at least a portion of the surface of the positive electrode active material particle. The coating film includes a first layer and a second layer. The first layer includes a phosphorus compound. The second layer includes a boron compound.

Conventionally, coating films have a single-layer structure. In contrast, the coating film described in "1." above has a specific multilayer structure. That is, the coating film includes a first layer and a second layer. The first layer includes a phosphorus compound, and the second layer includes a boron compound. Although the details of the mechanism are unknown, it is expected that the coating film including the first and second layers will reduce the initial resistance.

2. In the composite particle described in "1." above, the coating film may include, for example, a first laminate portion. In the first laminate portion, the first layer is interposed between the positive electrode active material particle and the second layer.

In the coating film described in "1." above, the first layer and the second layer may be stacked in any order. For example, there may be a portion in which the first layer and the second layer are stacked in the order of the first layer and the second layer.

3. In the composite particle described in "1." above, the coating film may include, for example, a second laminated portion. In the second laminated portion, the second layer is interposed between the positive electrode active material particle and the first layer.

For example, there may be a portion in which the second layer and the first layer are stacked in the order of the second layer and the first layer.

4. The composite particles described in any one of "1." to "3." above may have a coverage of, for example, 95% or more. The coverage is measured by X-ray photoelectron spectroscopy (XPS).

By having a coating film with a multi-layer structure, for example, it is expected that the coverage rate will be improved. By improving the coverage rate, for example, it is expected that the initial resistance will be reduced.

5. The positive electrode contains the composite particles described in any one of "1." to "4." above and a sulfide solid electrolyte.

6. The all-solid-state battery includes the positive electrode described in "5." above.

7. A method for producing composite particles includes the following (a) and (b):
(a) Positive electrode active material particles are prepared.
(b) A coating film is formed on the surface of the positive electrode active material particles to produce composite particles.
The above (b) includes the following (b1) and (b2).
(b1) The first coating liquid is dried to form a first layer.
(b2) The second coating liquid is dried to form a second layer.
The first layer includes a phosphorus compound, and the second layer includes a boron compound.

The manufacturing method described in "7." above can produce the composite particles described in "1." above.

8. In the method for producing composite particles described in "7." above, the coating film may be formed to include a first laminated portion. In the first laminated portion, a first layer is interposed between the positive electrode active material particles and the second layer.

The manufacturing method described in "8." above can produce the composite particles described in "2." above.

9. In the method for producing composite particles described in "7." above, the coating film may be formed to include a second laminated portion. In the second laminated portion, the second layer is interposed between the positive electrode active material particles and the first layer.

The manufacturing method described in "9." above can produce the composite particles described in "3." above.

Below, an embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "this example") are described. However, this embodiment and this example do not limit the technical scope of the present disclosure.

FIG. 1 is a conceptual diagram showing a composite particle in this embodiment. FIG. 2 is a conceptual diagram showing the second laminated structure. FIG. 3 is a conceptual diagram showing the third laminated structure. FIG. 4 is a conceptual diagram showing the fourth stacked structure. FIG. 5 is a conceptual diagram showing an all-solid-state battery in this embodiment. FIG. 6 is a schematic flow chart of the method for producing composite particles in this embodiment.

<Terms and definitions>
The words "comprise,""include,""have," and variations thereof (e.g., "consisting of") are open-ended. The open-ended form may or may not include additional elements in addition to the required elements. The words "consisting of" are closed-ended. However, the closed form does not exclude additional elements that are normally associated with the technology or that are unrelated to the technology disclosed. The words "consisting essentially of" are semi-closed. The semi-closed form allows for the addition of elements that do not substantially affect the basic and novel characteristics of the technology disclosed.

Expressions such as "may" and "may" are used in the permissive sense, meaning "there is a possibility," rather than in the obligatory sense, meaning "must."

Elements expressed in the singular form include the plural form unless otherwise specified. For example, a "particle" can mean not only "one particle" but also "an aggregate of particles (powder, powder, particle group)."

Unless otherwise specified, the order of execution of multiple steps, actions, operations, etc. included in various methods is not limited to the order described. For example, multiple steps may proceed simultaneously. For example, multiple steps may be performed in sequence.

For example, a numerical range such as "m-n%" includes the upper and lower limits unless otherwise specified. That is, "m-n%" indicates a numerical range of "m% or more and n% or less." Furthermore, "m% or more and n% or less" includes "more than m% and less than n%." Furthermore, a numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part of this specification, in a table, in a figure, etc.

All numerical values are modified by the term "about." The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values may be approximations that may vary depending on the manner in which the disclosed technology is used. All numerical values may be expressed with significant figures. Measurements may be average values of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. In general, the more measurements are made, the more reliable the average value is expected to be. Measurements may be rounded off based on the number of significant figures. Measurements may include errors associated with, for example, the detection limits of the measuring device.

Geometric terms (e.g., "parallel," "perpendicular," "orthogonal," etc.) should not be interpreted in a strict sense. For example, "parallel" may deviate slightly from the strict meaning of "parallel." Geometric terms may include, for example, tolerances, errors, etc. in design, operation, manufacturing, etc. The dimensional relationships in each figure may not match the actual dimensional relationships. In order to facilitate understanding of the disclosed technology, the dimensional relationships (length, width, thickness, etc.) in each figure may be changed. Furthermore, some configurations may be omitted.

"Upper layer" and "lower layer" refer to the relative positional relationship between two layers in the coating film. The upper layer refers to the layer of the two layers that is farther from the positive electrode active material particles. The lower layer refers to the layer of the two layers that is closer to the positive electrode active material particles.

When a compound is expressed by a stoichiometric composition formula (e.g., "LiCoO ₂ ", etc.), the stoichiometric composition formula is merely a representative example of the compound. The compound may have a non-stoichiometric composition. For example, when lithium cobalt oxide is expressed as "LiCoO ₂ ", unless otherwise specified, the lithium cobalt oxide is not limited to a composition ratio of "Li/Co/O = 1/1/2" and may contain Li, Co, and O in any composition ratio. Furthermore, doping, substitution, etc. with trace elements may also be allowed.

"D50" refers to the particle size at which the cumulative frequency from the smaller particle size side reaches 50% in the volume-based particle size distribution. D50 can be measured by a laser diffraction method.

<XPS/Coverage Measurement>
The coverage is measured by the following procedure. An XPS device is prepared. For example, an XPS device manufactured by ULVAC-PHI, Inc., product name: PHI X-tool (or an equivalent product) may be used. A sample powder consisting of composite particles is set in the XPS. Narrow scan analysis is performed with a pass energy of 224 eV. The measurement data is processed by analysis software. For example, analysis software manufactured by ULVAC-PHI, Inc., product name: MulTiPak (or an equivalent product) may be used. The measurement data is analyzed to determine the ratio (element concentration) of each element from the peak areas of C1s, O1s, P2p, B1s, M2p (or M2p _3/2 ), etc. The coverage is calculated by the following formula (1).
θ=(P+B)/(P+B+M)×100...(1)
In the above formula (1), θ represents the coverage (%), and P (phosphorus), B (boron), and M represent the ratio of each element.

M2p, M2p _3/2 , and “M” in the above formula (1) are constituent elements of the positive electrode active material particles and represent an element other than Li and oxygen (O). That is, the positive electrode active material particles may be represented by the following formula (2).
_LiMO2 ...(2)
M may be composed of one element or multiple elements. M may include, for example, at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn) and aluminum (Al). When M includes multiple elements, the sum of the composition ratios of the elements may be 1.

For example, when the positive electrode active material particles are "LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ", the above formula (1) can be transformed into the following formula (1').
θ=(P+B)/(P+B+Ni+Co+Mn)×100...(1')
In the above formula (1'), Ni represents the element ratio of nickel determined from the peak area of Ni2p3 _/2 , Co represents the element ratio of cobalt determined from the peak area of Co2p3 _/2 , and Mn represents the element ratio of manganese determined from the peak area of Mn2p3 _/2 .

<Film thickness measurement>
The film thickness (thickness of the coating film) can be measured by the following procedure. A sample is prepared by embedding composite particles in a resin material. A cross-section is processed on the sample by an ion milling device. For example, an ion milling device "Product name Arblade (registered trademark) 5000" (or an equivalent product) manufactured by Hitachi High-Technologies Corporation may be used. The cross-section of the sample is observed by a SEM (Scanning Electron Microscope). For example, an SEM device "Product name SU8030" (or an equivalent product) manufactured by Hitachi High-Technologies Corporation may be used. The film thickness is measured in 20 fields of view for each of 10 composite particles. The arithmetic average of the film thicknesses at a total of 200 points is regarded as the film thickness.

<Composite particles>
FIG. 1 is a conceptual diagram showing a composite particle in this embodiment. The composite particle 100 may be referred to as, for example, a "coated positive electrode active material". The composite particle 100 includes a positive electrode active material particle 110 and a coating film 120. The composite particle 100 may form, for example, an aggregate. That is, one composite particle 100 may include two or more positive electrode active material particles 110. The composite particle 100 may have, for example, a D50 of 1 to 50 μm, a D50 of 1 to 20 μm, or a D50 of 5 to 15 μm.

<Coating film>
The coating film 120 is a shell of the composite particle 100. The coating film 120 covers at least a part of the surface of the positive electrode active material particle 110. The coating film 120 includes a first layer 121 and a second layer 122.

<Layered structure>
For example, the coating film 120 may include a first layer 121 and a second layer 122, each of which is composed solely of the first layer 121 and the second layer 122. For example, the coating film 120 may include a plurality of first layers 121 and a plurality of second layers 122. For example, the first layers 121 and the second layers 122 may be alternately laminated. The total number of the first layers 121 and the second layers 122 may be, for example, 2 to 10 layers, or 2 to 4 layers.

As long as the coating film 120 includes the first layer 121 and the second layer 122, the coating film 120 may further include additional layers. The coating film 120 may further include, for example, a third layer and a fourth layer (not shown). The third layer and the fourth layer may have a different composition from the first layer 121 and the second layer 122.

The first layer 121 and the second layer 122 may be stacked in any order. The coating film 120 may include, for example, a first stacked structure (first stacked portion) (see FIG. 1). In the first stacked structure, the first layer 121 is interposed between the positive electrode active material particles 110 and the second layer 122. That is, the first layer 121 is the lower layer, and the second layer 122 is the upper layer.

The coating film 120 may include, for example, a second laminate structure (second laminate portion). FIG. 2 is a conceptual diagram showing the second laminate structure. In the second laminate structure, the second layer 122 is interposed between the positive electrode active material particles 110 and the first layer 121. That is, the second layer 122 is the lower layer, and the first layer 121 is the upper layer.

The first layer 121 and the second layer 122 may overlap entirely or partially. For example, the coating film 120 may partially include a single layer structure. FIG. 3 is a conceptual diagram showing a third laminated structure. In FIG. 3, as an example, the lower layer is the first layer 121 and the upper layer is the second layer 122. Of course, the lower layer may be the second layer 122 and the upper layer may be the first layer 121. For example, the coverage of the lower layer may be lower than the coverage of the upper layer. For example, a portion of the upper layer may be in contact with the surface of the positive electrode active material particle 110.

Figure 4 is a conceptual diagram showing the fourth layered structure. For example, the coverage of the lower layer may be higher than the coverage of the upper layer. For example, a portion of the lower layer may be exposed on the surface of the coating film 120.

<First Layer>
The first layer 121 includes a phosphorus compound. The phosphorus compound refers to a compound containing at least P. The first layer 121 may include, for example, a phosphate compound. The first layer 121 may include, for example, a phosphate skeleton. The presence of the phosphate skeleton can be confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). When the first layer 121 includes a phosphate skeleton, fragments such as PO ₂ ⁻ and PO ₃ ⁻ can be detected in the TOF-SIMS of the composite particle 100.

The first layer 121 may further contain Li. The phosphorus compound may be represented by, for example, the following formula (3).
Li _y1 PO _x1 …(3)
In the above formula (3), y1 and x1 are any numbers. y1 and x1 can be identified, for example, by XPS or the like. y1 may, for example, satisfy the relationship 0≦y1<2.5. By making y1 less than 2.5, a reduction in initial resistance is expected. y1 may, for example, be 1.5 or less, 1 or less, 0.5 or less, or zero.

<Second Layer>
The second layer 122 includes a boron compound. The boron compound refers to a compound containing at least B. The second layer 122 may include, for example, a boric acid compound. The second layer 122 may include, for example, a boric acid skeleton. The presence of the boric acid skeleton can be confirmed by TOF-SIMS. When the second layer 122 includes a boric acid skeleton, fragments such as BO ₂ ^- and BO ₃ ^- can be detected in the TOF-SIMS of the composite particle 100.

The second layer 122 may further contain Li. The boron compound may be represented by, for example, the following formula (4).
Li _y2 BO _x2 …(4)
In the above formula (4), y2 and x2 are any numbers. y2 and x2 can be identified, for example, by XPS or the like. y2 may, for example, satisfy the relationship 0≦y2<2.5. By making y2 less than 2.5, a reduction in initial resistance is expected. y2 may, for example, be 1.5 or less, 1 or less, 0.5 or less, or zero.

<Film Thickness>
The coating film 120 (the entire film) may have a thickness of, for example, 5 to 100 nm, may have a thickness of 5 to 50 nm, may have a thickness of 10 to 30 nm, or may have a thickness of 20 to 30 nm.

The thickness (t1) of the first layer 121 and the thickness (t2) of the second layer 122 may, for example, satisfy the relationship "t1/t2 = 1/9 to 9/1", or may satisfy the relationship "t1/t2 = 3/7 to 7/3", or may satisfy the relationship "t1/t2 = 4/6 to 6/4".

<Coverage rate>
The coverage may be, for example, 95% or more. By having a coverage of 95% or more, a reduction in initial resistance is expected. The coverage may be, for example, 95 to 100%, or 96 to 100%.

<Positive electrode active material particles>
The positive electrode active material particle 110 is a core of the composite particle 100. The positive electrode active material particle 110 may be a secondary particle (aggregation of primary particles). The positive electrode active material particle 110 (secondary particle) may have a D50 of, for example, 1 to 50 μm, may have a D50 of 1 to 20 μm, or may have a D50 of 5 to 15 μm. The primary particle may have a maximum Feret diameter of, for example, 0.1 to 3 μm.

The positive electrode active material particles 110 may contain any component. The positive electrode active material particles 110 may contain at least one selected from the group consisting of LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(NiCoMn)O ₂ , Li(NiCoAl)O ₂ , and LiFePO _4. For example, "(NiCoMn)" in "Li(NiCoMn)O ₂ " indicates that the sum of the composition ratios in parentheses is 1. As long as the sum is 1, the amount of each component is arbitrary. Li(NiCoMn) _O2 may include, for example, LiNi1 _{/ 3Co1} / _{3Mn1/ 3O2} , _{LiNi0.5Co0.2Mn0.3O2 , LiNi0.8Co0.1Mn0.1O2 , etc. Li} ( _NiCoAl ) _O2 may _include , for example _{, LiNi0.8Co0.15Al0.05O2 , etc.}

<All-solid-state battery>
5 is a conceptual diagram showing an all-solid-state battery in this embodiment. The all-solid-state battery 200 includes a power generating element 250. The all-solid-state battery 200 may include, for example, an exterior body (not shown). The exterior body may be, for example, a pouch made of a metal foil laminate film. The exterior body may house the power generating element 250. The power generating element 250 includes a positive electrode 210, a separator layer 230, and a negative electrode 220. That is, the all-solid-state battery 200 includes a positive electrode 210, a separator layer 230, and a negative electrode 220.

Positive pole
The positive electrode 210 is layered. The positive electrode 210 may include, for example, a positive electrode active material layer and a positive electrode current collector. For example, the positive electrode active material layer may be formed by applying a positive electrode mixture to the surface of the positive electrode current collector. The positive electrode current collector may include, for example, an Al foil or the like. The positive electrode current collector may have a thickness of, for example, 5 to 50 μm.

The positive electrode active material layer may have a thickness of, for example, 10 to 200 μm. The positive electrode active material layer is in close contact with the separator layer 230. The positive electrode active material layer includes a positive electrode composite material. The positive electrode composite material includes composite particles and a sulfide solid electrolyte. That is, the positive electrode 210 includes composite particles and a sulfide solid electrolyte. Details of the composite particles are as described above.

The sulfide solid electrolyte can form an ion conduction path in the positive electrode active material layer. The amount of the sulfide solid electrolyte may be, for example, 1 to 200 parts by volume, 50 to 150 parts by volume, or 50 to 100 parts by volume, relative to 100 parts by volume of the composite particles (positive electrode active material). The sulfide solid electrolyte includes sulfur (S). The sulfide solid electrolyte may include, for example, Li, P, and S. The sulfide solid electrolyte may further include, for example, oxygen (O), silicon (Si), or the like. The sulfide solid electrolyte may further include, for example, a halogen, or the like. The sulfide solid electrolyte may further include, for example, iodine (I), bromine (Br), or the like. The sulfide solid electrolyte may be, for example, a glass ceramic type or an argyrodite type. The sulfide solid electrolyte may include at least _{one selected from the group} consisting _{of , for example, LiI-LiBr-Li3PS4, Li2S-SiS2, LiI-Li2S-SiS2 , LiI - Li2S - P2S5 , LiI} - _Li2O - _{Li2S -P2S5 ,} LiI- _{Li2S -P2O5 ,} LiI- _{Li3PO4 - P2S5 ,} Li2S- _P2S5 , and _{Li3PS4 .}

For example, "LiI-LiBr-Li ₃ PS ₄ " indicates a sulfide solid electrolyte produced by mixing LiI, LiBr, and Li ₃ PS ₄ in any molar ratio. For example, the sulfide solid electrolyte may be produced by a mechanochemical method. "Li ₂ S-P ₂ S ₅ " includes Li ₃ PS _4. Li ₃ PS ₄ can be produced, for example, by mixing Li ₂ S and P ₂ S ₅ in "Li ₂ S/P ₂ S ₅ = 75/25 (molar ratio)".

The positive electrode active material layer may further include, for example, a conductive material. The conductive material may form an electronic conduction path within the positive electrode active material layer. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of the composite particles (positive electrode active material). The conductive material may include any component. The conductive material may include, for example, at least one selected from the group consisting of carbon black, vapor grown carbon fiber (VGCF), carbon nanotubes (CNT), and graphene flakes.

The positive electrode active material layer may further contain, for example, a binder. The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of the composite particles (positive electrode active material). The binder may contain any component. The binder may contain, for example, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), styrene butadiene rubber (SBR), and polytetrafluoroethylene (PTFE).

Negative pole
The negative electrode 220 is layered. The negative electrode 220 may include, for example, a negative electrode active material layer and a negative electrode current collector. For example, the negative electrode active material layer may be formed by applying a negative electrode mixture to the surface of the negative electrode current collector. The negative electrode current collector may include, for example, copper (Cu) foil, Ni foil, or the like. The negative electrode current collector may have a thickness of, for example, 5 to 50 μm.

The negative electrode active material layer may have a thickness of, for example, 10 to 200 μm. The negative electrode active material layer is in close contact with the separator layer 230. The negative electrode active material layer includes a negative electrode mixture. The negative electrode mixture includes a negative electrode active material particle and a sulfide solid electrolyte. The negative electrode mixture may further include a conductive material and a binder. The sulfide solid electrolyte may be the same or different between the negative electrode mixture and the positive electrode mixture. The negative electrode active material particles may include any component. The negative electrode active material particles may include at least one selected from the group consisting of, for example, graphite, Si, SiO _x (0<x<2), and Li ₄ Ti ₅ O ₁₂ .

<Separator layer>
The separator layer 230 is interposed between the positive electrode 210 and the negative electrode 220. The separator layer 230 separates the positive electrode 210 from the negative electrode 220. The separator layer 230 includes a sulfide solid electrolyte. The separator layer 230 may further include a binder. The sulfide solid electrolyte may be the same or different between the separator layer 230 and the positive electrode composite. The sulfide solid electrolyte may be the same or different between the separator layer 230 and the negative electrode composite.

<Method of Manufacturing Composite Particles>
6 is a schematic flow chart of the method for producing composite particles in this embodiment. Hereinafter, the "method for producing composite particles in this embodiment" may be abbreviated as "this production method". This production method includes "(a) preparation of positive electrode active material particles" and "(b) formation of a coating film". This production method may further include, for example, "(c) heat treatment" or the like.

(a) Preparation of Positive Electrode Active Material Particles
The present manufacturing method includes preparing positive electrode active material particles, the details of which are as described above.

(b) Formation of Coating Film
This manufacturing method includes manufacturing composite particles by forming a coating film on the surface of positive electrode active material particles. This manufacturing method includes "(b1) formation of a first layer" and "(b2) formation of a second layer". Here, as an example, a method of forming a first stacked structure (see FIG. 1) is described. For example, the order of "(b1) formation of a first layer" and "(b2) formation of a second layer" may be reversed to form a second stacked structure (see FIG. 2).

(b1) Formation of the first layer
The method includes forming a first layer by drying a first coating liquid. A first coating liquid is prepared. The first coating liquid includes a first solute and a first solvent. The first solute includes a material for the first layer.

The amount of solute may be, for example, 0.1 to 20 parts by mass, 1 to 15 parts by mass, or 5 to 10 parts by mass per 100 parts by mass of the solvent. The first solvent may contain any component as long as the first solute can be dissolved in it. The first solvent may contain, for example, water, alcohol, etc. The first solvent may contain, for example, ion-exchanged water, ethanol, etc.

The first solute may include, for example, a phosphoric acid compound. The first solute may include, for example, at least one selected from the group consisting of phosphoric anhydride (P ₂ O ₅ ), orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid [(HPO ₃ ) _n ], and polyphosphoric acid. The first solute may include, for example, at least one selected from the group consisting of metaphosphoric acid and polyphosphoric acid. Metaphosphoric acid and polyphosphoric acid may have a long molecular chain compared to other phosphoric acid compounds. It is considered that a film (first layer) having continuity is easily generated by the phosphoric acid compound having a long molecular chain. For example, an improvement in coverage is expected by the first layer having continuity.

The first solute may further include, for example, a lithium compound. The first solute may include, for example, lithium hydroxide, lithium carbonate, lithium nitrate, etc.

The first mixture may be formed by mixing the first coating liquid with the positive electrode active material particles. The first mixture may be, for example, a suspension or a wet powder. For example, the suspension may be formed by dispersing the positive electrode active material particles (powder) in the first coating liquid. For example, the wet powder may be formed by spraying the first coating liquid into the powder. In this manufacturing method, any mixing device, granulating device, etc. may be used.

For example, the first layer can be formed by drying the first mixture. That is, the first layer can be generated by drying the first coating liquid attached to the surface of the positive electrode active material particles. Any drying method can be used in this manufacturing method.

For example, the first coating liquid may be dried by a spray drying method. That is, the suspension is sprayed from a nozzle to form droplets. The droplets contain the positive electrode active material particles and the first coating liquid. For example, the droplets may be dried by hot air to form the first composite particles (intermediate product).

The solid content of the suspension for spray drying may be, for example, 1 to 50% or 10 to 30% by volume. The nozzle diameter may be, for example, 0.1 to 10 mm or 0.1 to 1 mm. The hot air temperature may be, for example, 100 to 200°C.

For example, the first mixture may be formed and the first coating liquid may be dried using a tumbling fluidized bed coating apparatus.

(b2) Formation of the second layer
The method includes forming a second layer by drying a second coating liquid. A second coating liquid is prepared. The second coating liquid includes a second solute and a second solvent. The second solute includes a raw material for the second layer.

The amount of solute may be, for example, 0.1 to 20 parts by mass, 1 to 10 parts by mass, or 1 to 5 parts by mass per 100 parts by mass of the solvent. The second solvent may contain any component as long as the second solute can be dissolved in it. The second solvent may contain, for example, water, alcohol, etc. The second solvent may contain, for example, ion-exchanged water, ethanol, etc.

The second solute may include, for example, a boric acid compound. The second solute may include, for example, at least one selected from the group consisting of orthoboric acid, metaboric acid, and tetraboric acid.

The second solute may further include, for example, a lithium compound. The second solute may include, for example, lithium hydroxide, lithium carbonate, lithium nitrate, etc.

The second coating liquid may be mixed with the first composite particle (intermediate product) to form a second mixture. The second mixture may be dried to form a second layer. That is, the second coating liquid attached to the surface of the first composite particle may be dried to generate a second layer. The second composite particle (final product) may be produced by generating the second layer. The second composite particle corresponds to the composite particle 100 in FIG. 1. The second coating liquid may be dried, for example, by a spray-drying method, in the same manner as the first coating liquid.

(c) Heat Treatment
The present manufacturing method may include subjecting the first composite particle (intermediate product) and the second composite particle (final product) to a heat treatment. The coating film (each layer) may be fixed by the heat treatment. The heat treatment may also be called "baking". In the present manufacturing method, any heat treatment device may be used. The heat treatment temperature may be, for example, 150 to 300°C. The heat treatment time may be, for example, 1 to 10 hours. For example, the heat treatment may be performed in air or in an inert atmosphere. The heat treatment may be performed on the first composite particle or the second composite particle, or the heat treatment may be performed on the first composite particle and the second composite particle.

<Sample Preparation>
Composite particles, positive electrodes, and all-solid-state batteries according to Nos. 1 to 6 were produced as follows. Hereinafter, for example, "composite particles according to No. 1" may be abbreviated as "No. 1".

"No. 1"
In No. 1, the positive electrode active material particles were subjected to a single coating treatment.

The first coating liquid (phosphoric acid solution) was prepared by dissolving 10.8 parts by mass of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 166 parts by mass of ion-exchanged water.

LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was prepared as the positive electrode active material particles. A suspension was prepared by dispersing 50 parts by mass of the positive electrode active material particles in 53.7 parts by mass of the first coating liquid. A spray dryer "Product name Mini Spray Dryer B-290" (hereinafter the same) manufactured by BUCHI was prepared. The suspension was supplied to the spray dryer to produce a powder of composite particles. The supply air temperature of the spray dryer was 200° C., and the supply air volume was 0.45 m ³ /min. The composite particles were heat-treated in air. The heat treatment temperature was 200° C. The heat treatment time was 5 hours. It is considered that the coating film (first layer) of No. 1 contains a phosphorus compound (PO _x1 ) [see the above formula (3)].

The following materials were prepared:
Sulfide solid electrolyte: 10LiI-15LiBr-75Li ₃ PS ₄
Conductive material: VGCF
Binder: SBR
Dispersion medium: heptane Positive electrode current collector: Al foil

The composite particles, the sulfide solid electrolyte, the conductive material, the binder, and the dispersion medium were mixed to prepare a positive electrode slurry. The mixing ratio of the composite particles and the sulfide solid electrolyte was "composite particles/sulfide solid electrolyte = 6/4 (volume ratio)". The amount of the conductive material was 3 parts by mass with respect to 100 parts by mass of the composite particles. The amount of the binder was 3 parts by mass with respect to 100 parts by mass of the composite particles. The positive electrode slurry was sufficiently stirred by an ultrasonic homogenizer. The positive electrode slurry was applied to the surface of the positive electrode current collector to form a coating film. The coating film was dried at 100 ° C. for 30 minutes by a hot plate. This produced a positive electrode raw sheet. A disk-shaped positive electrode was cut out from the positive electrode raw sheet. The area of the positive electrode was 1 cm ² .

A negative electrode and a separator layer were prepared. The negative electrode active material particles were graphite. The same type of sulfide solid electrolyte was used between the positive electrode, the separator layer, and the negative electrode. A laminate was formed by stacking the positive electrode, the separator layer, and the negative electrode in a cylindrical jig. A power generating element was formed by pressing the laminate. A terminal was connected to the power generating element to form an all-solid-state battery.

"No. 2"
In No. 2, the positive electrode active material particles were subjected to a single coating treatment.

A second coating liquid (boric acid solution) was prepared by dissolving 6.7 parts by mass of boric acid (manufactured by Nacalai Tesque, Inc.) in 166 parts by mass of ion-exchanged water. Except that the second coating liquid was used instead of the first coating liquid (phosphoric acid solution), composite particles, a positive electrode, and an all-solid-state battery were manufactured in the same manner as in No. 1. The coating film (second layer) of No. 2 is considered to contain a boron compound (BO _x2 ) [see formula (4) above].

"No. 3"
In No. 3, the positive electrode active material particles were subjected to two coating treatments using one type of coating liquid.

LiNi1 _/ 3Co1 _/ 3Mn1 _{/ 3O2} was prepared as the positive electrode active material particles. 50 parts by mass of the positive electrode active material particles powder was dispersed in 26.9 parts by mass of a first coating liquid (phosphoric acid solution) to prepare a suspension. The suspension was supplied to a spray dryer to produce a powder of the first composite particles. The supply air temperature of the spray dryer was 200°C, and the supply air volume was 0.45 ^m3 /min.

A suspension was prepared by dispersing 50 parts by mass of the powder of the first composite particles in 26.9 parts by mass of the first coating liquid (phosphoric acid solution). The suspension was supplied to a spray dryer to produce a powder of the second composite particles. The supply air temperature of the spray dryer was 200° C., and the supply air volume was 0.45 m ³ /min. The second composite particles were heat-treated in air. The heat treatment temperature was 200° C. The heat treatment time was 5 hours. After this, a positive electrode and an all-solid-state battery were manufactured in the same manner as in No. 1.

"No. 4"
In No. 4, the positive electrode active material particles were subjected to two coating treatments using one type of coating liquid.

A second composite particle, a positive electrode, and an all-solid-state battery were manufactured in the same manner as in No. 3, except that a second coating liquid (boric acid solution) was used instead of the first coating liquid (phosphoric acid solution).

"No. 5"
In No. 5, the positive electrode active material particles were subjected to two coating treatments using two different coating solutions.

LiNi1 _/ 3Co1 _/ 3Mn1 _{/ 3O2} was prepared as the positive electrode active material particles. 50 parts by mass of the positive electrode active material particles powder was dispersed in 26.9 parts by mass of the second coating liquid (boric acid solution) to prepare a suspension. The suspension was supplied to a spray dryer to produce a powder of the first composite particles. The supply air temperature of the spray dryer was 200°C, and the supply air volume was 0.45 ^m3 /min.

A suspension was prepared by dispersing 50 parts by mass of the powder of the first composite particles in 26.9 parts by mass of the first coating liquid (phosphoric acid solution). The suspension was supplied to a spray dryer to produce a powder of the second composite particles. The supply air temperature of the spray dryer was 200° C., and the supply air volume was 0.45 m ³ /min. The second composite particles were heat-treated in air. The heat treatment temperature was 200° C. The heat treatment time was 5 hours. After this, a positive electrode and an all-solid-state battery were manufactured in the same manner as in No. 1.

"No. 6"
In No. 6, the positive electrode active material particles were coated twice with two types of coating liquid. No. 6 differs from No. 5 in the order of using the coating liquids.

LiNi1 _/ 3Co1 _/ 3Mn1 _{/ 3O2} was prepared as the positive electrode active material particles. 50 parts by mass of the positive electrode active material particles powder was dispersed in 26.9 parts by mass of a first coating liquid (phosphoric acid solution) to prepare a suspension. The suspension was supplied to a spray dryer to produce a powder of the first composite particles. The supply air temperature of the spray dryer was 200°C, and the supply air volume was 0.45 ^m3 /min.

A suspension was prepared by dispersing 50 parts by mass of the powder of the first composite particles in 26.9 parts by mass of the second coating liquid (boric acid solution). The suspension was supplied to a spray dryer to produce a powder of the second composite particles. The supply air temperature of the spray dryer was 200° C., and the supply air volume was 0.45 m ³ /min. The second composite particles were heat-treated in air. The heat treatment temperature was 200° C. The heat treatment time was 5 hours. Thereafter, a positive electrode and an all-solid-state battery were manufactured in the same manner as in No. 1.

<Evaluation>
The initial resistance of the all-solid-state battery was measured. The measurement results are shown in Table 1 below. The initial resistance in Table 1 below is a relative value. The initial resistance value of No. 5 is the reference (1.0). The initial resistances of the other samples are expressed as a ratio to the reference.

<Results>
Although there is a difference in coverage between the phosphorus compound and the boron compound, the initial resistance is approximately the same (Nos. 1 and 2).

By performing the coating process twice with one type of coating solution, there is a tendency for the coverage rate to increase slightly. However, there is almost no change in the initial resistance (No. 3, 4).

By performing the coating process twice using two different types of coating liquid, the coverage rate significantly improved and the initial resistance tended to decrease (No. 5, 6). It is believed that some kind of interaction between the first layer (phosphorus compound) and the second layer (boron compound) produced a synergistic effect.

The present embodiment and the present examples are illustrative in all respects. The present embodiment and the present examples are not limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, it is contemplated from the beginning that any configuration may be extracted from the present embodiment and the present examples and that they may be combined in any desired manner.

100 Composite particle, 110 Positive electrode active material particle, 120 Coating film, 121 First layer, 122 Second layer, 200 All-solid-state battery, 210 Positive electrode, 220 Negative electrode, 230 Separator layer, 250 Power generating element.

Claims (7)

Positive electrode active material particles;
A coating film;
Including,
the coating film covers at least a part of the surface of the positive electrode active material particle,
The coating film includes a first layer and a second layer,
the first layer comprises a phosphorus compound;
the second layer includes a boron compound;
The coating film includes a second laminated portion,
In the second laminated portion, the second layer is interposed between the positive electrode active material particles and the first layer.
Composite particles. A coverage rate of 95% or more;
The coverage is measured by X-ray photoelectron spectroscopy.
The composite particle according to claim 1 . The composite particle according to claim 1 or 2 and a sulfide solid electrolyte are included.
Positive electrode. The positive electrode according to claim 3 ,
All-solid-state battery. (a) preparing positive electrode active material particles; and (b) forming a coating film on the surfaces of the positive electrode active material particles to produce composite particles.
Including,
The (b) is
(b1) drying a first coating liquid to form a first layer; and (b2) drying a second coating liquid to form a second layer.
Including,
the first layer comprises a phosphorus compound;
The second layer includes a boron compound.
A method for producing composite particles. The coating film is formed to include a first laminated portion,
In the first laminated portion, the first layer is interposed between the positive electrode active material particles and the second layer.
The method for producing composite particles according to claim 5 . The coating film is formed to include a second laminated portion,
In the second laminated portion, the second layer is interposed between the positive electrode active material particles and the first layer.
The method for producing composite particles according to claim 5 .

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