JP7707984B2 - Composite particles, positive electrode, all-solid-state battery, and method for producing composite particles - Google Patents

Description

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

Patent Document 1 (JP 2016-39062 A) discloses a cathode composite material and a sulfide all-solid-state battery having a cathode active material made of lithium nickel manganese oxide with a layered structure and a glass electrolyte that coats the surface of the cathode active material.

JP 2016-39062 A

It has been proposed to form a coating film on the surface of positive electrode active material particles. For example, in sulfide-based all-solid-state batteries, the coating film is expected to reduce resistance by preventing direct contact between the sulfide solid electrolyte and the positive electrode active material particles. However, there is room for improvement in reducing resistance.

The objective of this disclosure is therefore to reduce 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 particles include positive electrode active material particles and a coating film. The positive electrode active material particles include a lithium-containing composite oxide having a layered rock salt structure. The coating film covers at least a portion of the surface of the positive electrode active material particles. The coating film includes a phosphorus compound.

The composite particles satisfy the relationship of the following formula (1).
C _Li /C _P ≦2.5 (1)
In the above formula (1),
C _Li represents the elemental concentration of lithium (Li) determined from the peak area of the Li1s spectrum measured by X-ray photoelectron spectroscopy. C _P represents the elemental concentration of phosphorus (P) determined from the peak area of the P2p spectrum measured by X-ray photoelectron spectroscopy.

The composite particles generate 0.1 mass % or more of carbon dioxide (CO ₂ ) when heated from room temperature to 600° C. at a rate of 10° C./min, as determined by temperature programmed desorption-mass spectrometry (TPD-MS).

_CO2 originates from lithium carbonate ( _Li2CO3 ). _Li2CO3 is an impurity formed when lithium _hydroxide (LiOH), which is a reaction product of Li present on the surface of _the positive electrode active material particles and adsorbed moisture, further reacts with _CO2 in the atmosphere. Such impurities are in close contact with the surface of the positive electrode active material particles and cannot be easily removed.

On the other hand, when forming a coating film, the adhesion between the positive electrode active material particles and the coating film is important. If the adhesion is weak, for example when the positive electrode active material particles and the solid electrolyte are kneaded in a solvent, the shear force of the kneading can cause the coating film to peel off, resulting in a decrease in battery performance.

According to the new findings of the present disclosure, it is expected that the smaller the composition ratio of Li in the coating film, the stronger the bond between the coating film and the impurities ( _Li2CO3 and LiOH) present on the surface of the _positive electrode active material particles, that is, the greater the adhesion between the positive electrode active material particles and the coating film, and the lower the resistance.

[2] The lithium-containing composite oxide having a layered rock salt structure is represented by the following formula (2):
Li _a Ni _x Co _y Me _1-x-y O ₂ (2)
In the above formula (2),
Me includes at least one selected from the group consisting of Mn and Al;
a may satisfy the relationship 0.90≦a≦1.20.

x may satisfy the relationship 0.30≦x≦0.90.
The value of y may satisfy the relationship 0.10≦y≦0.40.

[3] x may satisfy the relationship 0.50≦x≦0.90.
In the above formula (2), a material in which x is 0.5 or more is also called a high nickel material. High nickel materials can have high capacity and high output.

[4] The positive electrode contains the composite particles described in any one of [1] to [3] above and a sulfide solid electrolyte.

[5] The all-solid-state battery includes the positive electrode described in [4] above.
[6] A method for producing composite particles includes the following (a) and (b):

(a) A mixture is prepared by mixing a coating liquid with positive electrode active material particles.
(b) drying the mixture to produce composite particles.

The coating liquid includes a solute and a solvent.
The coating liquid adhered to the surface of the positive electrode active material particles is dried to form a coating film. The coating film described in the above item [1] can be formed by using the coating liquid described in the above item [6].

[7] The solute may contain a phosphate compound.
The coating liquid may satisfy, for example, the relationship of the following formula (3).

0≦n _Li /n _P <1.1 (3)
In the above formula (3),
n _Li represents the molar concentration of lithium in the coating solution, and n _P represents the molar concentration of phosphorus in the coating solution.

[8] (b) above may include forming composite particles, for example, by a spray drying method.

FIG. 1 is a conceptual diagram showing a composite particle in this embodiment. FIG. 2 is a conceptual diagram showing the all-solid-state battery according to the present embodiment. FIG. 3 is a schematic flow chart of the method for producing composite particles in this embodiment.

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.

<Definitions of terms>
The words "comprising,""including,""having," 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 proceed one after the other.

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.

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 small particle size side reaches 50% in the volume-based particle size distribution. D50 can be measured by a laser diffraction method. For example, a laser diffraction particle size distribution measuring device manufactured by Shimadzu Corporation (product name SALD-7500) (or an equivalent product) may be used.

<TPD-MS measurement>
The amount of _CO2 generated can be measured by the following procedure. A TPD-MS device is prepared. For example, a TPD-MS device manufactured by Microtrack Bell, Inc., "Product name: BELCAT II" (or an equivalent product) may be used. A sample powder consisting of 5 mg of composite particles is prepared. The sample powder is placed in a container, and heated from room temperature to 600°C at a heating rate of 10°C/min under a helium (He) gas flow of 50 ml/min. The amount of _CO2 generated (m/z=44) is quantified by a mass spectrometer and integrated to obtain the amount of CO2.

<XPS Measurement>
(Composition ratio of particle surface)
C _Li and C _P in the above formula (1) can be 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 device. 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 peak area (integral value) of the Li1s spectrum is converted into the elemental concentration of _Li (C _Li ). The peak area of the P2p spectrum is converted into the elemental concentration of P (C _P ). The composition ratio (C _Li /C _P ) of the particle surface is obtained by dividing C Li by C _P.

(Coverage rate)
The coverage is also measured by XPS. By analyzing the above measurement data, the ratio of each element is obtained from the peak areas of C1s, O1s, P2p, Ni2p3, Co2p3, and Me2p3.

The coverage rate is calculated by the following formula (4).
θ=P/(P+Ni+Co+Me)×100 (4)
In the above formula (4), θ represents the coverage (%). P, nickel (Ni), Co (cobalt), and Me represent the ratio of each element. "Me2p3" and Me in the above formula (4) may be composed of manganese (Mn) or aluminum (Al), or may be composed of Mn and Al. When Me contains Mn and Al, the sum of the composition ratios of each element 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 (4) can be transformed into the following formula (4').

θ=P/(P+Ni+Co+Mn)×100 (4')
In the above formula (4'), Mn represents the element ratio of Mn determined from the peak area of Mn2p3.

<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 five fields of view for each of the ten composite particles. The arithmetic average of the film thicknesses at a total of 50 points is regarded as the film thickness.

《ICP measurement》
(Composition ratio of positive electrode active material particles)
The a, x, and y in the above formula (2) can be measured by the following procedure. A standard solution is prepared by diluting 0.01 g of positive electrode active material particles with pure water. An inductively coupled plasma atomic emission spectroscopy (ICP-AES) device is prepared. For example, an ICP-AES device manufactured by Shimadzu Corporation, product name ICPE-9000 (or an equivalent product) may be used. The emission intensity of the standard solution is measured by the ICP-AES device. A calibration curve is created from the emission intensity of the standard solution. The mole fractions of Li, Ni, Co, and Me contained in the positive electrode active material particles are determined from the emission intensity of the sample solution and the calibration curve.

(P adhesion amount)
The mass fraction of P contained in the composite particles (also referred to as "P attachment amount" or "P content") can be measured by the following procedure. A mixed acid is prepared by mixing hydrochloric acid, nitric acid, and sulfuric acid. The mixing ratio is "hydrochloric acid/nitric acid/sulfuric acid=2/3/1 (molar ratio)". A solution is prepared by dissolving the composite particles in the mixed acid. A sample solution is prepared by diluting 0.01 g of the solution to 100 mL with pure water. An aqueous solution of P (1000 ppm, 10000 ppm) is prepared. A standard solution is prepared by diluting 0.01 g of the aqueous solution with pure water. An ICP-AES device is prepared. For example, an ICP-AES device "Product name ICPE-9800" (or an equivalent product) manufactured by Shimadzu Corporation may be used. The emission intensity of the standard solution is measured by the ICP-AES device. A calibration curve is created from the emission intensity of the standard solution. The mass fraction of P contained in the composite particles is determined from the emission intensity of the sample liquid and the calibration curve.

(Mass concentration of Li, P, and Na in the coating solution)
The mass concentrations of Li, P, and Na in the coating liquid are measured by the following procedure. 100 ml of sample liquid is prepared by diluting 0.01 g of the coating liquid with pure water. Aqueous solutions of Li, P, and Na (1000 ppm, 10000 ppm) are prepared. A standard solution is prepared by diluting 0.01 g of the aqueous solution with pure water. An ICP-AES device is prepared. The emission intensity of the standard solution is measured by the ICP-AES device. A calibration curve is created from the emission intensity of the standard solution. The emission intensity of the sample liquid (diluted solution of the coating liquid) is measured by the ICP-AES device. The mass concentrations of Li, P, and Na in the coating liquid are determined from the emission intensity of the sample liquid and the calibration curve. The mass concentrations of Li and P are then converted to molar concentrations. The molar concentration of Li (n _Li ) is divided by the molar concentration of P (n _P ) to obtain the molar ratio (n _Li /n _P ).

<Composite particles>
1 is a conceptual diagram showing a composite particle in this embodiment. The composite particle 5 may be referred to as, for example, a "coated cathode active material". The composite particle 5 includes a cathode active material particle 1 and a coating film 2. The composite particle 5 may form, for example, an aggregate. That is, one composite particle 5 may include two or more cathode active material particles 1. The composite particle 5 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.

The coating film 2 is a shell of the composite particle 5. The coating film 2 covers at least a portion of the surface of the positive electrode active material particle 1. The coating film 2 contains a phosphorus compound. The inclusion of the phosphorus compound in the coating film 2 is expected to reduce resistance.

The phosphorus compound may contain, for example, Li, oxygen (O), carbon (C), etc. P may be contained in the composite particle 5 at a mass fraction of, for example, 0.2 to 10%.

In the composite particle 5, the composition ratio (C _Li /C _P ) of the particle surface is 2.5 or less (see formula (1) above). When the composition ratio (C _Li /C _P ) is 2.5 or less, the resistance can be significantly reduced. The composition ratio (C _Li /C _P ) may be, for example, 2.12 or less, 1.96 or less, 1.89 or less, or 1.73 or less. The composition ratio (C _Li /C _P ) may be zero. The composition ratio (C _Li /C _P ) may be, for example, 0.1 or more, 0.5 or more, or 1.0 or more. The composition ratio (C _Li /C _P ) may be, for example, 1.73 to 2.5.

The coverage may be, for example, 80% or more, 85% or more, or 90% or more.

The coating film 2 may have a thickness of, for example, 5 to 100 nm, 5 to 50 nm, 10 to 30 nm, or 20 to 30 nm.

The positive electrode active material particle 1 is the core of the composite particle 5. The positive electrode active material particle 1 may be a secondary particle (aggregation of primary particles). The positive electrode active material particle 1 (secondary particle) 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.

The positive electrode active material particle 1 may contain any component. The positive electrode active material particle 1 contains a lithium-containing composite oxide having a layered rock salt structure. The crystal structure of the positive electrode active material particle 1 can be identified, for example, by X-ray diffraction (XRD) or the like. The lithium-containing composite oxide is represented, for example, by the following formula (2).

Li _a Ni _x Co _y Me _1-x-y O ₂ (2)
In the above formula (2), Me includes at least one selected from the group consisting of Mn and Al, a may satisfy the relationship of 0.90≦a≦1.20, x may satisfy the relationship of 0.30≦x≦0.90, and y may satisfy the relationship of 0.10≦ y ≦0.40. a preferably satisfies the relationship of 0.95≦a≦1.10. x preferably satisfies the relationship of 0.50≦x≦0.90. When the value of x is within this range, the specific capacity tends to increase. The lithium- _{containing composite} oxide _{may be} , for example, _Li1.10Ni1 / _{3Co1 /3Mn1 / 3O2} , _{Li1.10Ni0.60Co0.20Mn0.20O2} , _{Li1.10Ni0.82Co0.15Al0.03O2 , and the like .}

The composite particle 5 has a CO ₂ generation amount of 0.1 mass% or more when the temperature is raised from room temperature to 600 ° C. at 10 ° C. / min by TPD-MS. The CO ₂ measured by TPD-MS is derived from Li ₂ CO ₃ present on the surface. When the amount of CO ₂ generated when measured under the above conditions is 0.1 mass% or more, it is considered that the adhesion between the impurities (Li ₂ CO ₃ and LiOH) present on the surface of the positive electrode active material particle 1 and the coating film is increased, and the resistance is reduced. In addition, if the amount of CO ₂ generated when measured under the above conditions increases, Li is eluted from the surface of the positive electrode active material particle 1 and inactivated, so that the battery capacity may decrease, and the impurities are deposited on the surface of the positive electrode active material particle 1, hindering the uniform formation of the coating film, so that the resistance may increase. From these viewpoints, the amount of CO ₂ generated when measured under the above conditions is preferably 1.0 mass% or less, more preferably 0.8 mass% or less.

The amount of _CO2 generated can be adjusted by the time for which the lithium-containing composite oxide is left to stand in the air after preparation. The time for standing in the air may be, for example, 5 minutes or more, 30 minutes or more, 60 minutes or more, or 120 minutes or more. However, if the lithium-containing composite oxide is left to stand for a long period of time, the amount of _CO2 generated may increase too much, so the time may be, for example, 300 minutes or less, 180 minutes or less, or 150 minutes or less.

<All-solid battery>
2 is a conceptual diagram showing an all-solid-state battery in this embodiment. The all-solid-state battery 100 may include, for example, an exterior body (not shown). The exterior body may be, for example, a pouch made of an aluminum laminate film. The exterior body may house a power generating element 50. The power generating element 50 includes a positive electrode 10, a separator layer 30, and a negative electrode 20. That is, the all-solid-state battery 100 includes a positive electrode 10, a separator layer 30, and a negative electrode 20.

《Positive electrode》
The positive electrode 10 is layered. The positive electrode 10 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, aluminum (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 30. 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 10 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, for example, Li, P, and sulfur (S). The sulfide solid electrolyte may further include, for example, 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 or argyrodite. 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 .}

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 electrode》
The negative electrode 20 is a counter electrode of the positive electrode 10. The negative electrode 20 is layered. The negative electrode 20 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 30. The negative electrode active material layer includes a negative electrode mixture. The negative electrode mixture includes negative electrode active material particles 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, silicon oxide [SiO _x (0<x<2)], and Li ₄ Ti ₅ O ₁₂ .

<Separator layer>
The separator layer 30 is interposed between the positive electrode 10 and the negative electrode 20. The separator layer 30 separates the positive electrode 10 from the negative electrode 20. The separator layer 30 includes a sulfide solid electrolyte. The separator layer 30 may further include a binder. The sulfide solid electrolyte may be the same or different between the separator layer 30 and the positive electrode composite. The sulfide solid electrolyte may be the same or different between the separator layer 30 and the negative electrode composite.

<Method of Manufacturing Composite Particles>
3 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 a mixture" and "(b) production of composite particles". This production method may further include, for example, "(c) heat treatment" or the like.

(a) Preparation of the mixture
The present manufacturing method includes preparing a mixture by mixing a coating liquid with positive electrode active material particles. The details of the positive electrode active material particles are as described above. The mixture may be, for example, a suspension or a wet powder. For example, a suspension may be formed by dispersing positive electrode active material particles (powder) in a coating liquid. For example, a wet powder may be formed by spraying a coating liquid into a powder. In the present manufacturing method, any mixing device, granulating device, etc. may be used.

The coating liquid contains a solute and a solvent. The solute contains a film material (raw material for the coating film). The coating liquid may further contain, for example, suspended matter (insoluble components), precipitates, etc.

The solute may include, for example, a phosphoric acid compound. This allows the solute to include P. The phosphoric acid compound may be, 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 phosphoric acid compound may be, for example, at least one selected from the group consisting of metaphosphoric acid and polyphosphoric acid. Metaphosphoric acid and polyphosphoric acid may have a longer molecular chain than other phosphoric acid compounds. It is believed that the phosphoric acid compound has a longer molecular chain, which makes it easier to generate a coating film having continuity. For example, an improvement in coverage is expected due to the coating film having continuity.

The solute may further contain sodium (Na). Dissolving Na in the coating liquid may improve the stability of the phosphate compound. The concentration (mass concentration) of Na in the coating liquid may be, for example, 0 to 1%. The concentration of Na may be, for example, 0.6% or less, or 0.5% or less. The concentration of Na may be, for example, 0.5 to 0.6%.

The solute may further include a lithium compound. The solute may include, for example, lithium hydroxide, lithium carbonate, lithium nitrate, etc. The molar ratio of Li to P (n _Li /n _P ) may be, for example, less than 1.1 (see formula (3) above). When the molar ratio (n _Li /n _P ) is less than 1.1, it is expected that the composition ratio (C _Li /C _P ) of the particle surface is reduced. The molar ratio (n _Li /n _P ) may be, for example, 0.75 or less, 0.30 or less, or zero. The molar ratio (n _Li /n _P ) may be, for example, 0 to 0.30, or 0.30 to 0.75.

(b) Production of Composite Particles
The present manufacturing method includes producing composite particles by drying the mixture. The coating liquid attached to the surface of the positive electrode active material particles is dried to produce a coating film. In the present manufacturing method, any drying method can be used.

For example, the composite particles may be formed 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 coating liquid. For example, the droplets are dried by hot air to form the composite particles. The use of the spray-drying method is expected to improve the coverage, for example.

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 composite particles may be produced by a tumbling fluidized bed coating apparatus. In the tumbling fluidized bed coating apparatus, "(a) preparation of the mixture" and "(b) production of the composite particles" may be carried out simultaneously.

(c) Heat Treatment
The present manufacturing method may include subjecting the composite particles to a heat treatment. The coating film may be fixed by the heat treatment. The heat treatment may also be referred to as "calcination." 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 present embodiment will be described below using examples, but the present embodiment is not limited to these.

<Production of Positive Electrode Active Material Particles>
Positive electrode active material particles A to D, which are lithium-containing composite oxides having a layered rock salt structure, were produced as follows.

《Cathode active material particles A》
Nickel (II) sulfate hexahydrate ( _NiSO4.6H2O ), cobalt (II) _sulfate heptahydrate ( _CoSO4.7H2O ), and manganese (II) sulfate pentahydrate ( _MnSO4.5H2O ) were dissolved in pure water to obtain a raw material aqueous solution. The molar ratio of Ni, Co, and Mn in the raw material aqueous solution was 1: ₁ : ₁ , and the total molar concentration of Ni, Co, and Mn in the raw material aqueous solution was 1.8 mol/L.

1 L of 10 g/L ammonia aqueous solution was prepared in a reaction vessel. 1 L of the raw material aqueous solution was dripped into the reaction vessel at a rate of 5.2 mL/min while adjusting the pH to within 11.20±0.2 using an aqueous sodium hydroxide solution to form a precipitate and obtain a precursor. From the start to the end of the precipitation reaction, the aqueous ammonia solution was appropriately added so that the ammonia concentration of the reaction solution was 10 g/L.

The precursor was mixed with _Li2CO3 so that the molar ratio of Li to Ni, Co, and Mn was 1.10, and _the mixture was fired at 800°C for 5 hours in an oxygen atmosphere. The mixture was then left to stand for 2 hours in an air atmosphere to obtain positive electrode active material particles A. The composition of the positive electrode active material particles A was measured by the above-mentioned procedure. The results are shown in Table 1 below.

《Cathode active material particles B to D》
Positive electrode active material particles B were obtained in the same manner as positive electrode active material particles A, except that the molar ratio of Ni, Co, and Mn in the raw material aqueous solution was 3:1:1. Positive electrode active material particles C were obtained in the same manner as positive electrode active material particles A, except that aluminum sulfate (Al ₂ (SO ₄ ) ₃ ) was used instead of manganese (II) sulfate pentahydrate, and the molar ratio of Ni, Co, and Al was 82:15:3. Positive electrode active material particles D were obtained in the same manner as positive electrode active material particles A, except that the particles were not allowed to stand under air. The compositions of positive electrode active material particles B to D were measured by the above-mentioned procedure. The results are shown in Table 1 below.

<Manufacturing of all-solid-state batteries>
Composite particles, positive electrodes, and all-solid-state batteries according to Nos. 1 to 7 were produced as follows. Hereinafter, for example, "composite particles according to No. 1" may be abbreviated as "No. 1."

"No. 1"
(Coating liquid)
A coating liquid 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. Furthermore, lithium hydroxide monohydrate (LiOH.H ₂ O) was dissolved in the coating liquid so that the molar ratio (n _Li /n _P ) was 0.30.

(positive electrode)
Positive electrode active material particles A were prepared. A suspension was prepared by dispersing 50 parts by mass of the powder of the positive electrode active material particles in 53.7 parts by mass of the coating liquid. A spray dryer manufactured by BUCHI, product name Mini Spray Dryer B-290, 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. The composition ratio (C _Li /C _P ) of the particle surface and the amount of CO ₂ generated (mass%) were measured by the above-mentioned procedure. The results are shown in Table 1 below. Note that No. Similarly, for No. 2 to No. 7, the composition ratio (C _Li /C _P ) on the particle surface and the amount of CO ₂ generated (mass %) were measured.

The following materials were prepared:
Sulfide solid electrolyte: _Li2S - _P2S5 - _based glass ceramics containing LiI (D50: 0.8 μm)
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 per 100 parts by mass of the composite particles. The amount of the binder was 0.7 parts by mass per 100 parts by mass of the composite particles. The positive electrode slurry was sufficiently stirred by an ultrasonic homogenizer "Type UH-50" manufactured by SMT Corporation. 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 .

(Negative electrode)
The same materials as those for the positive electrode were prepared as the sulfide solid electrolyte, conductive material, binder, and dispersion medium. A Primix Corporation stirrer (product name: Filmix, model: 30-L type) was prepared as the stirrer. The sulfide solid electrolyte, conductive material, binder, and dispersion medium were put into the stirring vessel of the stirrer. The materials in the stirring vessel were stirred at a rotation speed of 20,000 rpm for 30 minutes.

Li ₄ Ti ₅ O ₁₂ (D50: 1.0 μm) was prepared as the negative electrode active material particles, and Cu foil was prepared as the negative electrode current collector. The negative electrode active material particles were added to the stirring vessel. Stirring was performed at 15,000 rpm for 60 minutes. The mixing ratio of the negative electrode active material particles and the sulfide solid electrolyte was "composite particles/sulfide solid electrolyte = 7/3 (volume ratio)". The amount of the conductive material was 1 part by mass per 100 parts by mass of the composite particles. The amount of the binder was 2 parts by mass per 100 parts by mass of the composite particles. After the negative electrode active material particles were added, the material in the stirring vessel was stirred at a rotation speed of 15,000 rpm for 60 minutes to prepare a negative electrode slurry. The negative electrode slurry was applied to the surface of the negative electrode current collector to form a coating film. The coating film was dried at 100 ° C. for 30 minutes by a hot plate. In this way, a negative electrode blank was produced. A disk-shaped negative electrode was cut out from the negative electrode blank. The area of the negative electrode was 1 ^cm2 .

(Separator Layer)
As the sulfide solid electrolyte, _Li2S - _{P2S5 -} based glass ceramics (D50: 2.5 μm) containing LiI was prepared. As a mold for pressing, a cylindrical ceramic with an inner diameter cross-sectional area of 1 ^cm2 was prepared. 64.8 mg of the sulfide solid electrolyte was placed in the mold, smoothed, and then pressed at a pressure of 1 ton/ ^cm2 to compact the material, thereby obtaining a separator layer.

(All-solid-state battery)
In the mold, a positive electrode was placed on one side of the separator layer, and a negative electrode was placed on the other side. The negative electrode, separator layer, and positive electrode were pressed together for 1 minute under a pressure of 6 ton/ ^cm2 . A stainless steel rod was inserted into the positive electrode and the negative electrode, and restrained at 1 ton/ ^cm2 to form a power generating element. A pouch made of an aluminum laminate film was prepared as a housing. The battery element was sealed in the housing. This formed an all-solid-state battery.

"No. 2"
Positive electrode active material particles A were prepared. 10.8 parts by mass of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in 166 parts by mass of ion-exchanged water to prepare a coating solution. No lithium hydroxide monohydrate was added to the coating solution. After this, composite particles, positive electrodes, and all-solid-state batteries were manufactured in the same manner as in No. 1.

"No. 3"
Positive electrode active material particles A were prepared. 10.8 parts by mass of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in 166 parts by mass of ion-exchanged water to prepare a coating liquid. Furthermore, lithium hydroxide monohydrate was dissolved in the coating liquid so that the molar ratio (n _Li /n _P ) was 0.75. From this point on, composite particles, positive electrodes, and all-solid-state batteries were manufactured in the same manner as in No. 1.

"No. 4"
Positive electrode active material particles B were prepared. Composite particles, a positive electrode, and an all-solid-state battery were manufactured in the same manner as in No. 1, except that positive electrode active material particles B were used.

"No. 5"
Positive electrode active material particles C were prepared. Composite particles, a positive electrode, and an all-solid-state battery were manufactured in the same manner as in No. 1, except that the positive electrode active material particles C were used.

"No. 6"
Positive electrode active material particles D were prepared. Composite particles, a positive electrode, and an all-solid-state battery were manufactured in the same manner as in No. 1, except that positive electrode active material particles D were used.

"No. 7"
Positive electrode active material particles A were prepared. 10.8 parts by mass of metaphosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in 166 parts by mass of ion-exchanged water to prepare a coating liquid. Furthermore, lithium nitrate (LiNO ₃ ) was dissolved in the coating liquid so that the molar ratio (n _Li /n _P ) was 2.00. Thereafter, composite particles, positive electrodes, and all-solid-state batteries were manufactured in the same manner as in No. 1.

<Evaluation>
The capacity of the evaluation battery was confirmed by constant current-constant voltage charging and constant current discharging. The charge/discharge time rate was 1/3C. "C" is the symbol for the time rate. At a time rate of 1C, the full charge capacity of the battery is discharged in 1 hour.

The state of charge (SOC) of the evaluation battery was adjusted to 50% by a time rate of 1/3C. After adjusting the SOC, an AC impedance measurement was performed. The amplitude was 10 mV. The frequency range was 0.1 to 10 ⁶ Hz. This resulted in a Cole-Cole plot. An arc was fitted to the Cole-Cole plot. The distance between the two intersections of the fitted arc and the real axis was determined. This distance was considered to be the "interface resistance". The interface resistance of the all-solid-state battery No. 1 was set as the reference (1.0) and the interface resistance of each all-solid-state battery was relativized and evaluated. The results are shown in Table 1 below.

<Results>
Nos. 1 to 5, in which the amount of CO ₂ generated from the composite particles measured under the above conditions was 0.1 mass % or more and the composition ratio (C _Li /C _P ) of the particle surface was 2.5 or less, showed a significant reduction in interface resistance.

No. 6, in which the amount of _CO2 generated from the composite particles measured under the above conditions was less than 0.1 mass%, showed a significant increase in interface resistance. No. 7, in which the composition ratio (C _Li /C _P ) of the particle surface was greater than 2.5, showed a significant increase in interface resistance.

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.

1 Positive electrode active material particles, 2 Coating film, 5 Composite particles, 10 Positive electrode, 20 Negative electrode, 30 Separator layer, 50 Power generating element, 100 All-solid-state battery.

Claims (7)

The positive electrode active material particles and the coating film are included,
the positive electrode active material particles contain a lithium-containing composite oxide having a layered rock salt structure,
the coating film covers at least a part of the surface of the positive electrode active material particle,
The coating film contains a phosphorus compound,
The following formula (1):
C _Li /C _P ≦2.5 (1)
Fulfilling the relationship,
In the above formula (1),
C _Li represents the elemental concentration of lithium determined from the peak area of the Li1s spectrum measured by X-ray photoelectron spectroscopy,
C _P represents the elemental concentration of phosphorus determined from the peak area of the P2p spectrum measured by X-ray photoelectron spectroscopy;
The amount of carbon dioxide generated when the temperature was increased from room temperature to 600° C. at a rate of 10° C./min by thermal generated gas mass spectrometry was 0.1 mass% or more;
The lithium-containing composite oxide having a layered rock salt structure has the following formula (2):
Li _a Ni _x Co _y Me _1-x-y O ₂ (2)
In the above formula (2),
Me includes at least one selected from the group consisting of Mn and Al;
a satisfies the relationship 0.90≦a≦1.20,
x satisfies the relationship 0.30≦x≦0.90,
Composite particles, wherein y satisfies the relationship 0.10≦ y ≦0.40. The composite particles according to claim 1, in which the amount of carbon dioxide generated when the temperature is increased from room temperature to 600°C at a rate of 10°C/min is 1.0 mass% or less, as determined by thermal gas generation mass spectrometry. The composite particle according to claim 1, wherein x satisfies the relationship 0.50≦x≦0.90. A positive electrode comprising the composite particles according to any one of claims 1 to 3 and a sulfide solid electrolyte. An all-solid-state battery comprising the positive electrode according to claim 4. (a) preparing a mixture by mixing a coating liquid with positive electrode active material particles; and (b) producing composite particles by drying the mixture.
Including,
the positive electrode active material particles contain a lithium-containing composite oxide having a layered rock salt structure,
The lithium-containing composite oxide having a layered rock salt structure has the following formula (2):
Li _a Ni _x Co _y Me _1-x-y O ₂ (2)
In the above formula (2),
Me includes at least one selected from the group consisting of Mn and Al;
a satisfies the relationship 0.90≦a≦1.20,
x satisfies the relationship 0.30≦x≦0.90,
y satisfies the relationship of 0.10≦ y ≦0.40,
The coating liquid includes a solute and a solvent,
The solute comprises a phosphate compound;
The coating liquid has the following formula (3):
0≦n _Li /n _P <1.1 (3)
Fulfilling the relationship,
In the above formula (3),
n _Li represents the molar concentration of lithium in the coating liquid,
A method for producing composite particles, wherein n _P represents the molar concentration of phosphorus in the coating liquid. The method for producing composite particles according to claim 6, wherein (b) includes forming the composite particles by a spray drying method.