JP7618959B2 - Oxide-based solid electrolyte powder for use in lithium-ion secondary batteries, its manufacturing method, solid electrolyte layer, and lithium-ion secondary battery - Google Patents

Description

The present invention relates to an oxide-based solid electrolyte powder for use in lithium-ion secondary batteries, a method for producing the same, a solid electrolyte layer, and a lithium-ion secondary battery.

Lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones and notebook computers.
In recent years, there has been a demand for smaller and lighter lithium ion secondary batteries for such portable electronic devices and in-vehicle use, and further improvements in performance, such as discharge capacity per unit mass, cycle characteristics, rate characteristics, and stability as a battery, are desired.

Lithium-ion secondary batteries have traditionally used liquid electrolytes, but all-solid-state lithium-ion secondary batteries, which use solid electrolytes as the electrolyte, are attracting attention because of the potential for improved safety and high-speed charging and discharging.

Methods for improving the lithium ion conductivity of such lithium ion secondary batteries have been studied so far. For example, Patent Document 1 discloses a method for producing a lithium ion conductive solid electrolyte with improved lithium ion conductivity by mixing lithium niobate (LiNbO ₃ ) and a fluoride by a solid-state reaction method.

JP 2013-157090 A

In Patent Document 1, fluorine is introduced into a solid electrolyte by a solid-state reaction method in order to increase lithium ion conductivity. However, when fluorine is introduced by a solid-state reaction method, fluorine is uniformly dispersed in the solid electrolyte. As a result, LiF with low ion conductivity is formed not only on the surface of the solid electrolyte but also inside it, and there is a concern that the ion conductivity of the solid electrolyte itself will decrease. In addition, since the introduction of fluorine by the solid-state reaction method is performed by a mechanical milling method or the like, although it is simple, there is a limit to the amount of solid electrolyte that can be produced at one time, and there is also a problem of poor productivity.

The present invention aims to provide an oxide-based solid electrolyte powder for lithium-ion secondary batteries that improves battery characteristics. Specifically, the present invention aims to improve lithium ion conductivity by ensuring a conduction path for lithium ions on the surfaces of particles contained in the oxide-based solid electrolyte powder.

As a result of extensive research, the inventors discovered that the above problems can be solved by highly fluorinating the surfaces of particles contained in an oxide-based solid electrolyte powder and by controlling the fluorine content in the entire powder to within a specific range, thereby completing the present invention.

That is, the present invention relates to the following [1] to [6].
[1]
An oxide-based solid electrolyte powder for use in a lithium-ion secondary battery,
The oxide-based solid electrolyte powder includes particles having a fluorinated layer on the surface,
the fluorine content at the outermost surface of the fluorinated layer is 95% by mass or more,
The fluorine content in the entire oxide-based solid electrolyte powder is 10 mass% or less.
Oxide solid electrolyte powder.
[2]
The oxide-based solid electrolyte powder according to [1] above, wherein the particles have an average particle size of 0.1 to 100 μm.
[3]
The oxide-based solid electrolyte powder according to the above [1] or [2], wherein the fluorinated layer has a thickness of 1 to 50 nm.
[4]
A method for producing an oxide-based solid electrolyte powder including particles having a fluorinated layer on the surface thereof by contacting particles of an oxide-based solid electrolyte with a gas containing elemental fluorine, comprising the steps of:
the fluorine content at the outermost surface of the fluorinated layer is 95% by mass or more,
The fluorine content in the entire oxide-based solid electrolyte powder is 10 mass% or less.
A method for producing an oxide-based solid electrolyte powder.
[5]
A solid electrolyte layer comprising the oxide-based solid electrolyte powder according to any one of [1] to [3] above.
[6]
A lithium ion secondary battery comprising the solid electrolyte layer according to [5] above, a positive electrode, and a negative electrode.

According to the oxide-based solid electrolyte powder of the present invention, the surfaces of the particles contained in the powder are highly fluorinated, so that a conduction path for lithium ions can be secured on the particle surfaces, and the battery characteristics of a lithium ion secondary battery can be improved.
In addition, it is believed that the high concentration of fluorination on the surfaces of particles contained in the oxide-based solid electrolyte powder stabilizes the particle surfaces and improves water repellency, which results in improved hydrolysis resistance and easier handling.

The present invention will be described in detail below, but the present invention is not limited to the following embodiments, and can be modified as desired without departing from the gist of the present invention. In addition, the use of "~" to indicate a numerical range means that the numerical values before and after it are included as the lower and upper limits.

<Oxide-based solid electrolyte powder>
The oxide-based solid electrolyte powder according to the present embodiment is used in a lithium-ion secondary battery. The powder contains particles having a fluorinated layer on the surface, and the fluorine on the outermost surface of the fluorinated layer is The oxide-based solid electrolyte powder is characterized in that the content of is 95 mass % or more, and the fluorine content in the entire oxide-based solid electrolyte powder is 10 mass % or less.
In the present invention, the "fluorine content at the outermost surface of the fluorinated layer" refers to the fluorine content at a depth of 2 nm from the surface of the fluorinated layer, excluding particle constituents (components that constitute particles, which will be described later). The term "excluding particle constituents" means excluding particle constituents that are diffused or transferred from the particles to the fluorinated layer.

In the present invention, the surfaces of the particles contained in the oxide-based solid electrolyte powder are highly fluorinated, thereby reducing the resistance at the interface between the particles (hereinafter also referred to as grain boundary resistance). As a result, the conduction path of lithium ions on the surface of the particles can be secured, and lithium ion conductivity can be improved.

The reason why the grain boundary resistance can be reduced by fluorinating the surfaces of particles contained in the oxide-based solid electrolyte powder to a high concentration is not clear, but is presumed to be as follows.
That is, the particle interface formed between the surfaces of the particles contained in the solid electrolyte powder is selectively and highly fluorinated with fluorine gas, hydrogen fluoride gas, or the like, thereby lowering the melting point of the grain boundary portion. Therefore, when the solid electrolyte powder is heat-treated to form a sheet-shaped solid electrolyte layer, the surfaces of the particles contained in the solid electrolyte powder are easily dissolved, improving the adhesion between the particles. As a result, it is presumed that the grain boundary resistance is reduced and the lithium ion conductivity is improved.
Alternatively, it is possible that the particle surface is stabilized by fluorination, while the presence of fluorine, which has a high electronegativity, polarizes the particle surface, and the interaction with lithium ions reduces the grain boundary resistance at the electrolyte interface, resulting in the formation of an excellent lithium ion conduction path.

In addition, it is presumed that the hydrolysis resistance is improved because the surfaces of the particles contained in the oxide-based solid electrolyte powder are highly fluorinated, stabilizing the particle surfaces.

The oxide-based solid electrolyte powder according to the present embodiment includes oxide-based solid electrolyte particles having a fluorinated surface layer. The particles are not particularly limited as long as they are oxide-based, and those containing oxygen (O) and having lithium ion conductivity can be suitably used. For example, perovskite-type oxides containing lithium, garnet-type oxides containing lithium, lithium phosphate (Li ₃ PO ₄ ), lithium niobate (LiNbO ₃ ), LAGP with a NASICON structure (Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦1)), LATP with a NASICON structure (Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (0≦x≦1)), LZP with a NASICON structure (Li _1+4x Zr _2-x (PO ₄ ) ₃ (0≦x≦1)), etc. (0≦x≦0.4, some of the metal elements in LZP may be replaced with other metal elements, or other metal elements may be doped. Examples of other metal elements include Na, Sr, Ca, Mg, La, Y, Sc, Ce, In, Al, Ge, Ti, and V.) These may be used alone or in combination.

The perovskite oxide containing lithium is an oxide having a perovskite crystal structure and represented by ABO ₃ , and the A site preferably contains at least one element selected from the group consisting of La, Sr, Ba, Na, Ca, and Nd, and Li, and the B site preferably contains at least one element selected from the group consisting of Ti, Ta, Cr, Fe, Co, Ga, and Nb. Specifically, examples of the perovskite oxide include lithium lanthanum titanate Li _3x La _2/3-x TiO ₃ (0≦x≦1/6, also called LLTO), lithium lanthanum niobate (Li _x La _(1-x)/3 NbO ₃ ) (0≦x≦1), and the like. Note that some of the elements constituting lithium lanthanum titanate may be replaced with other elements, or other elements may be doped. Other elements include Na, K, Rb, Ag, Tl, Mg, Sr, Ca, Ba, Nb, Ta, W, Ru, Cr, Mn, Fe, Co, Al, Ga, Si, Ge, Zr, Hf, Pr, Nd, Sm, Gd, Dy, Y, Eu, _Tb , and more specifically, La _{(2/3)-xSrxLixTiO3 , LixLa2 / 3Ti1 - xAlxO3} , and the _like .

_{Examples of garnet - type oxides include Li7La3Zr2O12 , Li5La3Nb2O12 , Li5La3Ta2O12 , and Li6La2BaTa2O12 .}

The oxide-based solid electrolyte of the present invention can be applied not only to crystalline materials but also to amorphous materials, which may be composited with _Li4SiO4 , _{Li3PO4 , Li3BO3 , SiO2 , B2O3 ,} etc.

The shape of the particles contained in the oxide-based solid electrolyte powder according to this embodiment may be a primary particle, a secondary particle formed by agglomeration of primary particles, or a combination of primary particles and secondary particles. When the particle shape is a primary particle, the surface of the primary particle may be fluorinated, and when the particle shape is a secondary particle, the surface of the secondary particle formed by agglomeration of primary particles may be fluorinated. From the viewpoint of the effects of the present invention, it is preferable that the oxide-based solid electrolyte powder according to this embodiment contains secondary particles, and the surface of the secondary particles is fluorinated.
In this manner, the surfaces of the particles contained in the oxide-based solid electrolyte powder are covered with a fluorinated layer, thereby improving the lithium ion conductivity by forming a conduction path for the lithium ions, and improving the battery characteristics of the lithium ion secondary battery.

In the oxide-based solid electrolyte powder according to this embodiment, the fluorinated layer has a fluorine content of 95% by mass or more at the outermost surface. When the fluorine content of the outermost surface is 95% by mass or more, the particle surface is fluorinated at a high concentration, and lithium ion conductivity is improved. The fluorine content of the outermost surface is more preferably 97% by mass or more.
The fluorine content at the outermost surface of the fluorinated layer is measured by the following method: the fluorine content, excluding particle constituents, at a depth of 2 nm from the surface of the fluorinated layer is measured by element mapping using a transmission electron microscope (TEM) or by depth element profile analysis from the particle surface to the inside of the grain using ESCA (X-ray photoelectron spectroscopy).

When the fluorinated layer is formed using hydrogen fluoride gas (HF gas) described later, a small amount of hydrogen will be present on the outermost surface of the fluorinated layer. In this case, the hydrogen content on the outermost surface of the fluorinated layer is preferably 0.1 to 5 mass%, and more preferably 0.1 to 3 mass%. By being in the above range, the influence of the small amount of hydrogen on the outermost surface of the fluorinated layer can be minimized.
The hydrogen content at the outermost surface of the fluorinated layer can be measured in the same manner as the fluorine content described above.

The fluorine content in the entire oxide-based solid electrolyte powder according to this embodiment is 10% by mass or less. By being 10% by mass or less, the fluorine content present inside the particles is reduced, and the deterioration of crystallinity and the decrease in lithium ion conductivity caused by the excess of low ion conductive components can be reduced. In addition, the decrease in conductivity due to insulating LiF can also be suppressed. The fluorine content in the entire oxide-based solid electrolyte powder is preferably 9% by mass or less, more preferably 8% by mass or less. In addition, the total fluorine content is preferably 0.1% by mass or more in order to form a fluorinated layer on the surface of the particles, more preferably 1% by mass or more, and even more preferably 2% by mass or more.
The fluorine content can be determined from the results of elemental analysis by a combustion method as described in the Examples below.

The thickness of the fluorinated layer is preferably 1 nm or more, more preferably 1.25 nm or more, and even more preferably 1.5 nm or more, from the viewpoint of obtaining the effects of the present invention more effectively. The thickness of the fluorinated layer is preferably 50 nm or less, more preferably 20 nm or less, and even more preferably 10 nm or less, from the viewpoint of suppressing a decrease in electrical conductivity.
In particular, when the shape of the particles contained in the oxide-based solid electrolyte powder includes secondary particles, the thickness of the fluorinated layer in the secondary particles is preferably 1 nm or more, more preferably 1.25 nm or more, and even more preferably 1.5 nm or more. The thickness of the fluorinated layer is preferably 50 nm or less, more preferably 20 nm or less, and even more preferably 10 nm or less.
The thickness of the fluorinated layer can be determined by analysis such as X-ray photoelectron spectroscopy or element mapping from a particle cross section.

The average particle size of the particles having a fluorinated layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more, from the viewpoint of stable formation, and is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less, from the viewpoint of ease of lithium ion mobility.
The average particle size of the particles having a fluorinated layer on the surface includes the thickness of the fluorinated layer, and is determined by a particle size distribution analyzer. Usually, the D50 average particle size (median size: particle size at which the cumulative frequency is 50%) can be used.
Specifically, the oxide-based solid electrolyte powder is thoroughly dispersed in water by ultrasonic treatment, and the particles are measured using a laser diffraction/scattering particle size distribution measurement device. A frequency distribution and a cumulative volume distribution curve are obtained to obtain a volume-based particle size distribution, and the particle size at the 50% point on the cumulative volume distribution curve is defined as the D50 average particle size.

<Method of producing oxide-based solid electrolyte powder>
The method for producing an oxide-based solid electrolyte powder according to the present embodiment is characterized in that the oxide-based solid electrolyte powder having a fluorinated layer according to the above embodiment is produced by contacting particles of an oxide-based solid electrolyte with a gas containing elemental fluorine. That is, by contacting particles of an oxide-based solid electrolyte before fluorination with a gas capable of fluorination, a fluorinated layer can be formed on the particle surface, and the oxide-based solid electrolyte powder according to the above embodiment can be produced.

The fluorinable gas is a gas containing fluorine element, such as fluorine gas ( _F2 gas), hydrogen fluoride gas (HF gas), _BF3 gas, _NF3 gas, _PF5 gas, _SiF4 gas, _SF6 gas, etc. These fluorine element-containing gases may be used alone or may be mixed with an inert gas such as nitrogen gas or argon gas. The fluorinated layer in the present invention may contain these gas components.

Among these, fluorine gas ( _F2 gas) or hydrogen fluoride gas (HF gas) is preferred because it does not contain other elements in the sense that it reacts only with fluorine. When a fluorinated layer is formed by contact between the particles and _F2 gas or HF gas, it can be determined that the fluorinated layer is formed by contact with such gas, since only fluorine atoms or hydrogen atoms are contained in the solid electrolyte. In particular, fluorine gas is preferred from the viewpoint of being able to make the fluorine content at the outermost surface of the fluorinated layer more concentrated.

When a mixed gas is used, the concentration of the gas containing elemental fluorine is preferably 0.01% by volume or more, and more preferably 0.1% by volume or more, based on the total mixed gas from the viewpoints of ease of reaction control and economy. The concentration of the gas containing elemental fluorine is preferably 50% by volume or less, more preferably 35% by volume or less, and even more preferably 20% by volume or less.

The time for contacting the oxide-based solid electrolyte particles with the gas containing elemental fluorine is preferably 10 seconds or more, more preferably 1 minute or more, and is preferably 240 minutes or less, more preferably 150 minutes or less. By keeping the time within this range, a fluorinated layer with a controlled concentration can be formed with high precision on the surface of the particles.

It is preferable to control the temperature at which the oxide-based solid electrolyte particles are brought into contact with the gas containing elemental fluorine within the range of 10 to 150°C. If it is desired to increase the fluorine concentration on the particle surface, the temperature can be raised to increase the reactivity between the particle surface and fluorine, thereby forming a layer containing the desired fluorine at a high concentration. This allows the fluorinated layer to be formed reliably and efficiently.

Contact with the gas containing elemental fluorine may be performed under pressure, and the pressure is preferably 0.6 MPa (gauge pressure) or less, more preferably 0.3 MPa or less, from the viewpoints of increasing safety and suppressing excessive fluorination.

The contact with the gas containing elemental fluorine is preferably carried out in a flow manner or a batch manner.
In the case of the flow-through type, a method is preferred in which the particles are placed in a reaction vessel in a stationary state, and a gas containing fluorine at a predetermined concentration is continuously supplied into an open reaction vessel to bring the particles into contact with the gas containing fluorine.
In the case of the batch type, a method is preferred in which the particles are placed in a sealed reaction vessel in an atmosphere of a gas containing elemental fluorine at a prescribed concentration and the particles are brought into contact with the gas containing elemental fluorine.

When the flow-type reaction is carried out, from the viewpoint of uniformly contacting the particles with a gas containing elemental fluorine, a reaction vessel equipped with a fluidized bed in which the particles are placed and fluidized, or a kiln such as a tubular furnace, can also be used. The use of a fluidized bed is particularly preferred, since it can shorten the fluorination treatment time and suppress excessive fluorination, thereby realizing more uniform fluorination.
When the batch method is used, the particles may be stirred and mixed in order to bring the gas containing elemental fluorine into contact with the particles uniformly.

<Solid electrolyte layer>
The solid electrolyte layer according to the present embodiment contains the oxide-based solid electrolyte powder and is used in a lithium-ion secondary battery. In addition, the solid electrolyte layer may contain a binder as necessary.
The content of the solid electrolyte powder in the solid electrolyte layer is not particularly limited and may be appropriately determined depending on the performance of the target battery. For example, the content of the solid electrolyte powder is preferably 80% by mass or more, and more preferably 90% by mass or more, based on 100% by mass of the entire solid electrolyte layer.

Examples of binders that can be contained in the solid electrolyte layer include butadiene rubber (BR), acrylate butadiene rubber (ABR), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), etc. The content of the binder in the solid electrolyte layer may be the same as in the past.

The thickness of the solid electrolyte layer is not particularly limited and may be appropriately determined according to the performance of the intended battery. For example, it is preferably 10 μm or more, more preferably 15 μm or more. By making the thickness of the solid electrolyte layer 10 μm or more, the mechanical strength is increased, and it is possible to obtain a solid electrolyte layer that is resistant to stresses such as vibration and bending and has high reliability.
The thickness of the solid electrolyte layer is preferably 200 μm or less, and more preferably 100 μm or less. By making the thickness of the solid electrolyte layer as thin as 200 μm or less, it is possible to increase the ionic conductivity between the positive and negative electrodes and also to increase the energy density of the battery.

The method for forming the solid electrolyte layer is not particularly limited. For example, the components constituting the solid electrolyte layer are dispersed or dissolved in a solvent to form a slurry, which is then coated in a layer (sheet), dried, and optionally pressed to form the solid electrolyte layer. If necessary, a binder removal treatment may be performed by applying heat. The thickness of the solid electrolyte layer can be easily adjusted by adjusting the coating amount of the slurry.
Instead of the wet molding as described above, the solid electrolyte layer may be formed by dry press molding of a solid electrolyte powder or the like on the surface of the object (positive electrode, negative electrode, etc.) on which the solid electrolyte layer is to be formed. Alternatively, the solid electrolyte layer may be formed on another substrate, which is then transferred to the surface of the object on which the solid electrolyte layer is to be formed. From the viewpoint of being able to industrially stably form a strong solid electrolyte layer on the surface of the object on which the solid electrolyte layer is to be formed, it is preferable to form the solid electrolyte layer on the surface of the object by wet molding using a solvent.

<Lithium-ion secondary battery>
The lithium ion secondary battery according to this embodiment includes the above-mentioned solid electrolyte layer, a positive electrode, and a negative electrode. The positive electrode and the negative electrode may be conventionally known. Specific examples are shown below, but are not limited thereto.

(Positive electrode)
The positive electrode contains at least a positive electrode current collector and a positive electrode active material.

The positive electrode current collector may be any conductive plate material, such as a thin metal plate (metal foil) made of aluminum or an alloy thereof, stainless steel, or the like. These are preferred because they have excellent electrolyte resistance and oxidation resistance.

The positive electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions, remove and insert (intercalate) lithium ions, or dope and dedope a counter anion (e.g., PF ₆ ⁻ ) of the lithium ions, and any known positive electrode active material can be used. Examples of the positive electrode active material include lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese oxide ( _LiMnO2 ), lithium nickel manganese oxide, a composite metal oxide represented by Li(Ni _x Co _y Mn _z M _a ) _O2 (x+y+z+a=1, 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, and M is at least one selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), and a polyanion olivine type positive electrode represented by Li _a M _b ( _PO4 ) _c (1≦a≦4, 1≦b≦2, 1≦c≦3, and M is at least one selected from Fe, V, Co, Mn, Ni, and VO).

The positive electrode may contain a binder that binds the positive electrode active materials together and also binds the positive electrode active material to the positive electrode current collector. Any binder known in the art can be used.
The positive electrode may also contain a known conductive assistant for positive electrodes, examples of which include carbon-based materials such as graphite and carbon black, metals such as copper, nickel, stainless steel, and iron, and conductive oxides such as indium tin oxide (ITO).

(Negative electrode)
The negative electrode contains at least a negative electrode current collector and a negative electrode active material.

The negative electrode current collector can be any conductive plate material, for example, a thin metal plate (metal foil) of copper, aluminum, etc. These are preferred because they have excellent electrolyte resistance and oxidation resistance.

The negative electrode active material is not particularly limited, and may be any material capable of inserting and extracting lithium ions, such as lithium metal, carbon-based materials, silicon, silicon alloys, and tin.
The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions, remove and insert (intercalate) lithium ions, or dope and de-dope a counter anion (e.g., PF ₆ ^- ) of the lithium ions, and known negative electrode active materials can be used. Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, and soft carbon, metals that can form alloys with lithium such as aluminum, silicon, and tin, amorphous oxides such as silicon oxide and tin oxide, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

In addition, the negative electrode may have a binder that binds the negative electrode active materials together and also binds the negative electrode active material to the negative electrode current collector. Conventionally known binders can be used as the binder.
The negative electrode may contain a known conductive assistant for negative electrodes, and the same conductive assistant for positive electrodes as described above can be used.

The components that make up the lithium ion secondary battery, such as the solid electrolyte layer, the positive electrode, and the negative electrode, are housed in a battery exterior. The material of the battery exterior can be any conventionally known material, but specific examples include nickel-plated iron, stainless steel, aluminum or alloys thereof, nickel, titanium, resin materials, film materials, etc.

The shape of the lithium ion secondary battery can be selected appropriately according to the application, and includes coin type, sheet type (film type), folded type, wound type, bottomed cylindrical type, button type, etc.

The lithium ion secondary battery according to this embodiment can achieve good lithium ion conductivity.

The present invention will be specifically explained with the following examples, but the present invention is not limited to these.

[Example 1 (Example): Fluorinated oxide-based solid electrolyte powder]
₍ Synthesis of _{Li0.35La0.55TiO3 powder} )
3.87 g of lithium carbonate (Li ₂ CO ₃ ) and 0.27 g of lanthanum acetate (La(CH ₃ COO) ₃ ·1.5H ₂ O) were dissolved in 23.88 g of water with stirring, and 11.98 g of titanium lactate aqueous solution (Matsumoto Fine Chemical Co., Ltd., Orgatix TC-315, Ti content 8.3%) was slowly added thereto. The resulting aqueous dispersion was dried at 80°C for 24 hours and then pulverized using an agate mortar. It was then placed in an alumina container and heat-treated at 700°C for 4 hours, and passed through a 75 μm mesh to synthesize powder. The resulting powder was measured using an X-ray diffraction device (Rigaku, Smart Lab) to confirm that Li _0.35 La _0.55 TiO ₃ was formed.
3 g of the _{Li0.35La0.55TiO3 powder} synthesized above was placed in a Hastelloy reactor having an internal volume of _{0.3 L, and the Li0.35La0.55TiO3 powder} was fluorinated using _N2 gas containing 20 volume % of _F2 gas at a pressure of _2.5 KPa and a temperature of ₁₄₀ °C for 2 hours to obtain _{Li0.35La0.55TiO3 powder} having a _fluorinated layer on the particle surface.
(Fluorine content at the outermost surface)
ESCA analysis (X-ray photoelectron spectroscopy) was performed using an ESCA5500 (manufactured by ULVAC-PHI, Inc.), and the fluorine content in the outermost surface of the fluorinated layer was found to be 100% by mass.
(Fluorine content in powder)
The fluorine content was quantitatively analyzed using an automatic sample combustion apparatus (Mitsubishi Chemical Analytech (Dia Instruments), AQF-100) and ion chromatography (Dionex, DX120), and the fluorine content in the entire _{Li0.35La0.55TiO3 powder} having a _fluorinated layer was found to be 9 mass%.
(Average particle size)
The average particle size of the powder was measured using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Nikkiso Co., Ltd., product name MT-3300EX) and was found to be 3 μm.

[Example 2 (Comparative Example) Fluorine-Untreated Oxide-Based Solid Electrolyte Powder]
Unfluorinated _{Li0.35La0.55TiO3 powder} was used. The average particle size of the powder was measured to be 3 _μm using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Nikkiso Co., Ltd., product name MT-3300EX).

<Measurement of lithium ion conductivity>
For the oxide-based solid electrolyte powders of Examples 1 and 2, the lithium ion conductivity was measured by an AC impedance method.
The lithium ion conductivity was measured using a temperature variable high frequency impedance measuring device manufactured by Toyo Corporation. The measurement was performed by depositing Au on the oxide-based solid electrolyte sample described below, pressing a SUS electrode from the outside, and performing the measurement under the conditions of an applied voltage of 20 mV, a measurement temperature of 25°C, and a measurement frequency range of 100 MHz to 20 Hz. The Cole-Cole plot obtained by the AC impedance measurement was analyzed to determine the migration resistance of Li ions, and the Li ion conductivity was calculated from the area and thickness of the pellet-shaped sample used for the measurement.
Here, as the sample for measuring lithium ion conductivity, a pellet-shaped oxide-based solid electrolyte sample obtained by pressing the oxide-based solid electrolyte powders of Examples 1 and 2 at 30 kN for 2 minutes using a pressing device was used.
The results are shown in Table 1. Each measurement value is the average of two measurements.

The above results show that the lithium ion conductivity was increased in Example 1 compared to the oxide-based solid electrolyte powder without a fluorinated layer in Example 2. This evaluation result is presumably due to the fact that the grain boundary resistance was reduced by carrying out the fluorination treatment on the particle surface.

Claims (5)

An oxide-based solid electrolyte powder for use in a lithium-ion secondary battery,
The oxide-based solid electrolyte powder includes particles having a fluorinated layer on the surface,
the fluorine content at the outermost surface of the fluorinated layer is 95% by mass or more,
The fluorine content in the entire oxide-based solid electrolyte powder is 10 mass% or less,
The average particle size of the particles is 0.5 to 100 μm.
Oxide solid electrolyte powder. The oxide-based solid electrolyte powder according to claim 1, wherein the thickness of the fluorinated layer is 1 to 50 nm. A method for producing an oxide-based solid electrolyte powder including particles having a fluorinated layer on the surface thereof by contacting particles of an oxide-based solid electrolyte with a gas containing elemental fluorine, comprising the steps of:
The oxide-based solid electrolyte powder is used in a lithium-ion secondary battery,
the fluorine content at the outermost surface of the fluorinated layer is 95% by mass or more,
The fluorine content in the entire oxide-based solid electrolyte powder is 10 mass% or less,
The particles have an average particle size of 0.5 to 100 μm;
The gas containing fluorine element includes at least one selected from the group consisting of _F2 gas, HF gas, _BF3 gas, _NF3 gas, _PF5 gas, _SiF4 gas, and _SF6 gas;
A method for producing an oxide-based solid electrolyte powder. A solid electrolyte layer comprising the oxide-based solid electrolyte powder according to claim 1 or 2. A lithium ion secondary battery comprising the solid electrolyte layer according to claim 4, a positive electrode, and a negative electrode.