JP2021158098A - Negative electrode layer for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode layer for a lithium ion secondary battery, which enables the enhancement in battery characteristic.SOLUTION: A negative electrode layer for a lithium ion secondary battery comprises negative electrode active material particles, solid electrolyte particles and a conductive assistant. At least one of the negative electrode active material particle and the solid electrolyte particle has a fluorinated layer in its surface.SELECTED DRAWING: None

Description

The present invention relates to a negative electrode layer for a lithium ion secondary battery and a lithium ion secondary battery.

Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and notebook computers, automobiles, and the like.
In recent years, such portable electronic devices and lithium-ion secondary batteries for automobiles have been required to be smaller and lighter, and the discharge capacity per unit mass, cycle characteristics, rate characteristics, stability as a battery, etc. have been improved. Performance improvement is desired.

Conventionally, a liquid electrolyte has been used in a lithium ion secondary battery, but an all-solid lithium ion that uses a solid electrolyte as an electrolyte in a lithium ion secondary battery can be expected to improve safety and charge / discharge at high speed. Secondary batteries are attracting attention.

As the negative electrode layer for the all-solid-state lithium ion secondary battery, for example, a negative electrode layer containing a negative electrode active material, a solid electrolyte, or the like and processed into a sheet shape or the like can be used.

For example, Patent Document 1 describes a composition for forming a negative electrode active material layer in which a negative electrode active material is contained in a solid electrolyte composition, and a negative electrode active material layer formed by applying a composition to be a negative electrode material. Has been done.

Japanese Unexamined Patent Publication No. 2016-31868

However, in an all-solid-state lithium ion secondary battery using such a negative electrode layer, lithium ion conduction is more likely to be suppressed due to resistance at the interface between particles in the negative electrode layer, as compared with the case of using a liquid electrolyte. There are challenges. It is considered that this is because the contact interface between the particles contained in the negative electrode layer tends to be smaller than that in the case of using the liquid electrolyte that permeates between the particles in the negative electrode layer. Due to the resistance at the interface between the particles, there is a concern that the lithium ion conductivity of the negative electrode layer is lowered and the characteristics of the lithium ion secondary battery are deteriorated.

Therefore, an object of the present invention is to provide a negative electrode layer for a lithium ion secondary battery having improved battery characteristics. Specifically, it is an object of securing a conduction path of lithium ions between particles in the negative electrode layer and improving lithium ion conductivity.

As a result of diligent studies, the present inventors have found that the above problems can be solved by including negative electrode active material particles having a fluorinated surface or solid electrolyte particles having a fluorinated surface in the negative electrode layer. The present invention has been completed.

That is, the present invention relates to the following [1] to [6].
[1] A negative electrode layer containing negative electrode active material particles, solid electrolyte particles, and a conductive auxiliary agent.
A negative electrode layer for a lithium ion secondary battery, wherein at least one of the negative electrode active material particles and the solid electrolyte particles has a fluorinated layer on the surface.
[2] The lithium ion rechargeable battery according to [1], wherein the solid electrolyte particles have a fluorinated layer on the surface, and the fluorine content on the outermost surface of the fluorinated layer is 95% by mass or more. Negative electrode layer for next battery.
[3] The negative electrode layer for a lithium ion secondary battery according to [1] or [2], wherein the solid electrolyte particles contain at least one of a sulfide-based solid electrolyte particle and an oxide-based solid electrolyte particle.
[4] The negative electrode layer for a lithium ion secondary battery according to any one of [1] to [3], wherein the solid electrolyte particles have an average particle size of 0.01 to 30 μm.
[5] Of [1] to [4], wherein the negative electrode active material particles have a fluorinated layer on the surface, and the fluorinated layer in the negative electrode active material particles is composed of fluorinated carbon particles. The negative electrode layer for a lithium ion secondary battery according to any one.
[6] A lithium ion secondary battery including a negative electrode including the negative electrode layer according to any one of [1] to [5], a solid electrolyte layer, and a positive electrode.

According to the negative electrode layer according to the present invention, since at least one surface of the negative electrode active material particles and the solid electrolyte particles contained in the negative electrode layer is fluorinated, the ionic conductivity of the negative electrode layer can be improved, and lithium ions can be obtained. The battery characteristics of the secondary battery can be improved.

Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be arbitrarily modified and carried out without departing from the gist of the present invention. Further, "~" indicating a numerical range is used to mean that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value.

<Negative electrode layer>
The negative electrode layer according to this embodiment is used for a lithium ion secondary battery. The negative electrode layer according to the present embodiment includes negative electrode active material particles, solid electrolyte particles, and a conductive auxiliary agent, and has a layer in which at least one of the negative electrode active material particles and the solid electrolyte particles is fluorinated on the surface.

(Negative electrode active material particles)
The negative electrode active material particles may be particles containing a negative electrode active material capable of reversibly advancing the occlusion and release of lithium ions, in other words, the desorption and insertion (intercalation) of lithium ions. The negative electrode active material is not particularly limited, and a known negative electrode active material can be used.

Examples of the negative electrode active material include carbon-based materials such as graphite, hard carbon and soft carbon, metals capable of forming alloys with lithium such as aluminum, silicon and tin, and amorphous oxides such as silicon oxide and tin oxide. , And lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like. As the negative electrode active material, it is preferable to use silicon oxide or tin oxide from the viewpoint of increasing the capacity. These may be used alone or in combination of a plurality of types.

The average particle size of the negative electrode active material particles is preferably 0.01 μm or more, more preferably 0.02 μm or more, still more preferably 0.05 μm or more, from the viewpoint of ensuring lithium ion conductivity and electron conductivity in the negative electrode layer. The average particle size of the negative electrode active material particles is preferably 30 μm or less, more preferably 20 μm or less, still more preferably 10 μm or less, from the viewpoint of ease of forming the negative electrode layer.

The average particle size of the negative electrode active material particles is determined by, for example, a particle size distribution measuring device. When the negative electrode active material particles have a fluorinated layer as described later, the diameter including the thickness of the fluorinated layer is measured as the particle size. As the average particle size of the particles, a D50 average particle size (median diameter: particle size at which the cumulative frequency becomes 50%) can be usually adopted.

Specifically, the particles are sufficiently dispersed in water or an organic solvent by ultrasonic treatment, and the particles are measured by a laser diffraction / scattering type particle size distribution measuring device. A volume-based particle size distribution is obtained by obtaining a frequency distribution and a cumulative volume distribution curve, and the particle size at a point of 50% in the cumulative volume distribution curve is defined as the D50 average particle size.
Further, after forming the negative electrode layer, the negative electrode layer can be cut and the particle size of the negative electrode active material particles can be measured from the SEM observation in the cross section thereof.

In the negative electrode layer according to the present embodiment, the content of the negative electrode active material particles is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, from the viewpoint of increasing the charge / discharge capacity. The content of the negative electrode active material particles is preferably 99% by mass or less, more preferably 97% by mass or less, still more preferably 95% by mass or less, from the viewpoint of ensuring lithium ion conductivity and electron conductivity in the negative electrode layer.
In the negative electrode layer according to the present embodiment, the content of the negative electrode active material particles can be measured, for example, by the negative electrode active material mixing ratio at the time of preparing the slurry for forming the negative electrode layer before forming the negative electrode layer. Specifically, the weight of the negative electrode active material particles is measured. Regarding the content of the negative electrode active material particles after the formation of the negative electrode layer, the content of the negative electrode active material particles is determined with a certain quantitativeness by dividing the negative electrode layer and performing elemental analysis by SEM / EDX analysis from the cross section thereof. Can be measured.

(Solid electrolyte particles)
The negative electrode layer according to this embodiment contains solid electrolyte particles. In the lithium ion secondary battery using the negative electrode layer according to the present embodiment, the lithium ions stored and released by the negative electrode active material particles are contained in the negative electrode layer, the negative electrode layer, and the solid electrolyte layer via the solid electrolyte particles contained in the negative electrode layer. You can move between.

The solid electrolyte particles are not particularly limited as long as they are particles containing a solid electrolyte having lithium ion conductivity, and known solid electrolytes can be used. Examples of the solid electrolyte include sulfide-based solid electrolytes, oxide-based solid electrolytes, lithium nitride, and inorganic solid electrolytes such as lithium iodide. These solid electrolytes may be used alone or in combination of two or more. Among them, at least one of a sulfide-based solid electrolyte and an oxide-based solid electrolyte having excellent lithium ion conductivity can be preferably used from the viewpoint of lithium ion conductivity.

The sulfide-based solid electrolyte is not particularly limited as long as it is a sulfide-based electrolyte, and those containing sulfur (S) and having lithium ion conductivity can be preferably used. The sulfide-based solid electrolyte, for _example, Li ₂ S-SiS 2 system solid electrolyte. Li ₂ S-SiS ₂ system solid electrolyte generally ^has a lithium ion conductivity of ¹⁰ -4 S / cm order. Other sulfide-based solid electrolytes include P ₂ S _5- Li ₂ S series, B ₂ S _3- Li ₂ S series, B ₂ S _3- Li ₂ S series, and GeS _2- Li ₂ S series. Can be mentioned. These may be used alone or in combination of a plurality of types.

The oxide-based solid electrolyte is not particularly limited as long as it is an oxide-based solid electrolyte, and those containing oxygen (O) and having lithium ion conductivity can be preferably used. For example, perovskite-type oxide containing lithium, garnet-type oxide containing lithium, lithium phosphate (Li ₃ PO ₄ ), lithium niobate (LiNbO ₃ ), LAGP (Li _{1 + x} Al _x Ge _2-x ) having a NASICON structure ( PO ₄ ) ₃ (0 ≦ x ≦ 1)), LATP with NASICON structure (Li _{1 + x} Al _x Ti _2-x (PO ₄ ) ₃ (0 ≦ x ≦ 1)), LZP with NASICON structure (Li _{1 + 4 x} Zr _{2-) x} (PO ₄ ) ₃ (0 ≦ x ≦ 0.4, some metal elements of LZP may be replaced with another metal element, or another metal element may be doped. Another metal element. Examples thereof include Na, Sr, Ca, Mg, La, Y, Sc, Ce, In, Al, Ge, Ti, V and the like.) These may be used alone or in plurality. Seeds may be used in combination.

The perovskite-type oxide containing lithium is _{an oxide represented by ABO 3} having a perovskite-type crystal structure, and the A site is at least one selected from the group consisting of La, Sr, Ba, Na, Ca and Nd. It is preferable that the element of the seed and Li are contained, and the B site contains at least one element selected from the group consisting of Ti, Ta, Cr, Fe, Co, Ga and Nb. Specifically, as perovskite-type oxides, lithium titanate lanthanum Li _{3 x} La _{2 / 3-x} TiO ₃ (0 ≦ x ≦ 1/6, also called LLTO), lithium niobate lanthanum (Li _x La _{(1-)). x) / 3} NbO ₃ ) (0 ≦ x ≦ 1) and the like. A part of the elements constituting lithium titanate lanthanum may be replaced with another element, or another element 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, Examples thereof include Hf, Pr, Nd, Sm, Gd, Dy, Y, Eu, Tb, etc. Specifically, La _{(2/3) -x} Sr _x Li _x TiO ₃ , Li _x La _2/3 Ti _{1 -X} Al _x O _{3 and the} like can be mentioned.

Examples of the lithium-containing garnet-type oxide include Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₅ La ₃ Nb ₂ O ₁₂ , Li ₅ La ₃ Ta ₂ O ₁₂ , and Li ₆ La ₂ BaTa ₂ O _12. Can be mentioned.

The oxide-based solid electrolyte is not limited to the crystalline material, and may be an amorphous material. For example, the oxide-based solid electrolyte may be composited with Li ₄ SiO ₄ , Li ₃ PO ₄ , Li ₃ BO ₃ , SiO ₂ , B ₂ O _3, etc. as an amorphous material.

The average particle size of the solid electrolyte particles is preferably 0.01 μm or more, more preferably 0.02 μm or more, still more preferably 0.03 μm or more. The average particle size of the solid electrolyte particles is preferably 30 μm or less, more preferably 20 μm or less, still more preferably 10 μm or less. When the average particle size of the solid electrolyte particles is within the above range, the particle size of the solid electrolyte particles and the particle size of the negative electrode active material particles are well-balanced, and the solid electrolyte particles cover the periphery of the negative electrode active material particles more closely. It will be easier. This makes it easier to secure a lithium ion conduction path between the negative electrode active material particles and the solid electrolyte particles around the negative electrode active material particles. The average particle size of the solid electrolyte particles can be measured by the same method as in the case of the negative electrode active material particles described above.

In the negative electrode layer according to the present embodiment, the content of the solid electrolyte particles is preferably 1% by mass or more, more preferably 2% by mass or more, in order to impart lithium ion conductivity to the negative electrode layer and obtain an excellent charge / discharge capacity. 3, 3% by mass or more is more preferable. The content of the solid electrolyte particles is preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less, because a large amount of the negative electrode active material is used in the negative electrode layer to increase the capacity. The content of the solid electrolyte particles can be measured by the same method as in the case of the negative electrode active material particles described above.

(Fluorinated layer)
The negative electrode layer according to the present embodiment has a layer in which at least one of the negative electrode active material particles and the solid electrolyte particles is fluorinated on the surface. Here, for at least one of the above particles, all of the particles in the negative electrode layer may have a fluorinated layer, or may include particles that do not have a fluorinated layer. Further, both the negative electrode active material particles and the solid electrolyte particles may have a fluorinated layer on the surface.

Since at least one surface of the negative electrode active material particles and the solid electrolyte particles contained in the negative electrode layer is fluorinated, the resistance at the interface between these particles (hereinafter, also referred to as grain boundary resistance) can be reduced. As a result, the conduction path of lithium ions at the interface between the particles contained in the negative electrode layer can be secured, and the lithium ion conductivity of the negative electrode layer can be improved.

The reason why the grain boundary resistance can be reduced by fluorinating at least one surface of the negative electrode active material particles and the solid electrolyte particles is not clear, but it is presumed as follows.
That is, the surface of the particles is fluorinated, so that the melting point of the particles is lowered. When the material containing these particles is heat-treated to form a sheet-like negative electrode layer, the surface of the particles having the fluorinated layer is easily softened by heating to form the fluorinated layer. At the particle interface between the particles to be possessed and the particles adjacent thereto, the adhesion between the particles is improved. As a result, it is presumed that the grain boundary resistance between the particles in the negative electrode layer is reduced and the lithium ion conductivity is improved.
Alternatively, while the particle surface is stabilized by fluorination, the particle surface is polarized due to the presence of fluorine having a high electronegativity, and the particles having a fluorinated layer and surrounding particles due to the interaction with lithium ions. The grain boundary resistance at the interface with is reduced. As a result, it is possible that a good lithium ion conduction path is formed.
Further, when the negative electrode active material particles contain oxygen atoms, oxygen atoms constituting the negative electrode active material may be missing on the surface of the particles, resulting in lattice defects. At this time, if the lattice defects are filled with the fluorine atoms contained in the fluorinated layer, the binding force of the negative electrode active material on the lithium ions is weakened, and it is considered that the lithium ions are easily moved.

Further, when the above-mentioned lattice defects occur on the surface of the negative electrode active material particles, the lattice defects are filled with fluoride ions, so that the transition metal element contained in the negative electrode active material diffuses through the lattice defects. It can also be suppressed. By suppressing the diffusion of transition metal elements, the battery characteristics are less likely to deteriorate even when the lithium ion secondary battery is repeatedly charged and discharged.

Further, since at least one surface of the negative electrode active material particles and the solid electrolyte particles is fluorinated, the hydrolysis resistance of the particles and the negative electrode layer containing the particles is also improved. The reason for this is considered to be that the particle surface is stabilized by fluorination. As a result, it is possible to prevent the particles and the negative electrode layer containing the particles from reacting with moisture in the air, and the handleability of the particles themselves and the negative electrode layer is improved.

The shape of the negative electrode active material particles and the solid electrolyte particles may be primary particles, secondary particles formed by aggregating primary particles, or a combination of primary particles and secondary particles. May be good. 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 the primary particles is It may be fluorinated. From the viewpoint of the effect of the present invention, it is preferable that the negative electrode layer according to the present embodiment contains negative electrode active material particles or solid electrolyte particles in which the surface of the secondary particles is fluorinated.

As described above, by having at least one of the negative electrode active material particles and the solid electrolyte particles having a fluorinated layer on the surface, the lithium ion conductivity can be improved by forming the lithium ion conduction path in the negative electrode layer. .. As a result, the battery characteristics of the lithium ion secondary battery are improved and become good.

When the solid electrolyte particles have a fluorinated layer on the surface, the fluorinated layer preferably has a fluorine content of 95% by mass or more on the outermost surface of the fluorinated layer. Even when the negative electrode active material particles have a fluorinated layer on the surface, the fluorinated layer preferably has a fluorine content of 95% by mass or more on the outermost surface of the fluorinated layer. When the content of fluorine on the outermost surface is 95% by mass or more and the particle surface is fluorinated to a high concentration, the lithium ion conductivity is more likely to be improved, so that the desired effect in the present invention can be obtained. The content of fluorine on the outermost surface is more preferably 97% by mass or more, and the content of fluorine on the outermost surface is further preferably 98% by mass or more.

Here, the "fluorine content on the outermost surface of the fluorinated layer" refers to the fluorine obtained by excluding the particle constituents (components constituting the above-mentioned particles) at a depth of 2 nm from the surface of the fluorinated layer. The content ratio. "Excluding particle constituents" means excluding particle constituents that are diffused or transferred from the particles to the fluorinated layer.

The fluorine content on the outermost surface of the fluorinated layer is measured by the following method. That is, at a depth of 2 nm from the surface of the fluorinated layer by element mapping with a transmission electron microscope (TEM) or depth element profile analysis from the particle surface to the inside of the grain by ESCA (X-ray photoelectron spectroscopy). , Measure the content of fluorine excluding the particle constituents.

When the fluorinated layer is formed by hydrogen fluoride gas (HF gas) described later, a trace amount of hydrogen is present on the outermost surface of the fluorinated layer. At this time, the content of hydrogen on the outermost surface of the fluorinated layer is preferably 0.1 to 5% by mass, more preferably 0.1 to 3% by mass. Within the above range, the influence of a trace amount of hydrogen on the outermost surface of the fluorinated layer can be minimized.
The hydrogen content on the outermost surface of the fluorinated layer can be measured by the same method as the above-mentioned fluorine content.

The fluorine content in the entire particles having a fluorinated layer on the surface is preferably 10% by mass or less. The fluorine content on the outermost surface of the fluorinated layer is within the above-mentioned preferable range, and the fluorine content in the entire particles having the fluorinated layer on the surface is set to 10% by mass or less so that the particles are present inside the particles. The fluorine content is reduced, and the decrease in lithium ion conductivity due to deterioration of crystallinity and excess of low ion conductive components can be reduced. In addition, it is possible to suppress a decrease in conductivity due to LiF, which is an insulating property. The fluorine content in the entire particle having the fluorinated layer on the surface is more preferably 9% by mass or less, further preferably 8% by mass or less. Further, 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 0.15% by mass or more, and 0.2% by mass. The above is more preferable.

The fluorine content in the entire solid electrolyte particles or the entire negative electrode active material particles can be obtained from the results of elemental analysis by the combustion method. Alternatively, the fluorine content can also be measured by quantitatively analyzing ^{F −} released by immersing the entire fluorinated solid electrolyte particles or the entire negative electrode active material particles in water using a fluorine ion electrode.

The thickness of the fluorinated layer on the surface of the negative electrode active material particles or the surface of the solid electrolyte particles is preferably 2 nm or more, more preferably 2.25 nm or more, still more preferably 2.5 nm or more from the viewpoint of further obtaining the effects of the present invention. .. The thickness of the fluorinated layer is preferably 50 nm or less, more preferably 20 nm or less, still more preferably 10 nm or less, from the viewpoint of suppressing a decrease in conductivity.
In particular, when the particles having the fluorinated layer include 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 further 1.5 nm or more. Preferably, 2 nm or more is even more preferable. 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.
When the fluorinated layer is a layer formed by covering the particle surface with another fluorinated particle, which will be described later, the thickness of the fluorinated layer is a point in which the effect of the present invention can be further obtained. From 0.1 μm to 0.1 μm or more, and 1 μm or less is preferable.
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.

(Method of forming fluorinated layer)
The method for forming the fluorinated layer is not particularly limited, and the fluorinated layer may be, for example, a layer of negative electrode active material particles or solid electrolyte particles whose surface is fluorinated. Alternatively, it may be a layer formed by covering the particle surface of the negative electrode active material particles or the solid electrolyte particles with another fluorinated particles.

When the surface of the particles themselves has a fluorinated layer, examples of the method for forming the particles include a method of contacting the particles with a fluorinated gas. According to such a method, the surface of the particles can be selectively fluorinated to a high concentration, and an increase in the fluorine content existing inside the particles can be suppressed. That is, the formation of LiF having low ionic conductivity inside the particles can be suppressed, and thus the decrease in ionic conductivity of the negative electrode layer itself can be suppressed. Further, from the viewpoint of preventing deterioration of the material due to heat or stress in the process of forming the fluorinated layer, a method of contacting the particles with a fluorinated gas can be preferably applied.

An example of a method of contacting particles with a fluorinated gas will be described below. When the fluorinated layer is formed by contacting the particles with a fluorinated gas, the fluorine content on the surface of the particles is higher than the fluorine content inside the particles.

The fluorinated gas is a gas containing a fluorine element, and is a halogen fluoride such as fluorine gas (F ₂ gas), hydrogen fluoride gas (HF gas), ClF ₃ , and IF ₅ , BF ₃ gas, and NF. Examples thereof include ₃ gas, PF ₅ gas, SiF ₄ gas, and SF _{6 gas.} The gas containing these fluorine elements may be used alone or may be a mixed gas with an inert gas such as nitrogen gas or argon gas.

Among these, since it does not contain other elements in the sense that it reacts purely with fluorine, fluorine gas (F ₂ gas), hydrogen fluoride gas (HF gas), or a mixed gas of these and an inert gas is used. preferable. When a fluorinated layer is formed by contact between the particles and F ₂ gas or HF gas, the negative electrode active material particles or solid electrolyte particles contain only fluorine atoms or only fluorine atoms and hydrogen atoms. It can be determined that the fluorinated layer was formed by the contact with the gas.

When a mixed gas is used, the concentration of the gas containing a fluorine element is preferably 0.01% by volume or more, preferably 0.1% by volume or more, based on the total mixed gas from the viewpoint of ease of control of the reaction and economic viewpoint. Is more preferable. The concentration of the gas containing a fluorine element is preferably 50% by volume or less, more preferably 35% by volume or less, and further preferably 20% by volume or less.

The time for contacting the particles with the gas containing the fluorine element is preferably 10 seconds or more, more preferably 1 minute or more, preferably 600 minutes or less, more preferably 300 minutes or less, still more preferably 200 minutes or less. Within such a range, a fluorinated layer having a controlled concentration can be formed on the surface of the particles with high accuracy, and a negative electrode layer having excellent cycle characteristics and charge / discharge characteristics can be formed.

When the particles and the gas containing the fluorine element are brought into contact with each other, it is preferable to control the temperature in the range of 10 to 150 ° C. When it is desired to increase the fluorine concentration on the particle surface, the reactivity between the particle surface and fluorine is increased by raising the temperature, and a layer containing desired fluorine can be formed at a high concentration. This makes it possible to efficiently form a fluorinated layer.
Contact with a gas containing a fluorine element may be performed while pressurizing. The pressure is preferably 0.6 MPa (gauge pressure) or less, more preferably 0.3 MPa or less, from the viewpoint of enhancing safety and suppressing excessive fluorination.

The contact with the gas containing a fluorine element is preferably a flow type or a batch type.
In the case of the circulation type, the particles are placed in the reaction vessel in a stationary state, and the gas containing fluorine at a predetermined concentration is continuously supplied into the open type reaction vessel to separate the particles and the gas containing fluorine. The contact method is preferred.
In the case of the batch type, a method of accommodating the particles in a closed reaction vessel having a gas atmosphere containing a fluorine element having a predetermined concentration and bringing the particles into contact with the gas containing the fluorine element is preferable.

In the case of the distribution method, from the viewpoint of uniformly contacting the particles with a gas containing a fluorine element, it is also possible to use a reaction vessel provided with a fluidized bed in which the particles are placed and flowed, or a kiln such as a tube furnace. .. When a fluidized bed is provided, it is particularly preferable because the treatment time for fluorination can be shortened, excessive fluorination can be suppressed, and more uniform fluorination can be achieved.
In the case of the batch method, it is also possible to carry out while stirring and mixing the particles in order to uniformly contact the gas containing the fluorine element with the particles.

When the fluorinated layer is a layer formed by covering the surface of the particles with another fluorinated particle, the forming method thereof is, for example, a method of covering the surface of the particles with fluorinated carbon particles. Alternatively, a method of covering the surface of the particles with fluorinated oxide particles and the like can be mentioned. Among them, the method of covering with fluorinated carbon particles makes it possible to increase the fluorine concentration by using carbon particles having chemical stability and a large specific surface area, and the effect of fluorination is sufficiently obtained. It is preferable because it can be used. For example, when the negative electrode active material particles have a fluorinated layer on the surface, it is preferable that the fluorinated layer in the negative electrode active material particles is a layer made of fluorinated carbon particles for the above reason. This does not exclude the case where the solid electrolyte particles have a layer made of fluorinated carbon particles on the surface. When the fluorinated layer is formed by such a method, the fluorine content on the particle surface becomes higher than the fluorine content inside the particle.

The carbon particles are not limited to particles, and may be fibrous carbon. In either case, it is possible to produce a carbon having fluorine chemically adsorbed or chemically bonded to the surface of the carbon.
Examples of the particulate carbon include carbon black such as Ketjen black, acetylene black, thermal black, furnace black and channel black, activated carbon, graphite, _{fullerenes such as C 60} , C ₇₀ and C ₈₄ , and diamond.
Examples of the fibrous carbon include carbon fibers and carbon nanotubes.

Among the above, particulate carbon is preferable, and Ketjen black or activated carbon having a high specific surface area is more preferable from the viewpoint of sufficiently high cycle characteristics and energy density. One type of carbon may be used alone, or two or more types may be used in combination.

When the fluorinated layer is formed by covering the surface of the particles with another fluorinated particle, the thickness of the fluorinated layer is preferably 0.1 to 1 μm. The particle size of the particles is also preferably 1 μm or less.

When the fluorinated layer is formed from fluorinated carbon particles, the carbon content in the fluorinated layer can coat the surface of the negative electrode active material particles or the surface of the solid electrolyte particles without gaps, and the surface of the particles can be coated. From the necessity of suppressing an increase in interfacial resistance, 50% by mass or more is preferable. Further, in order to obtain the effect in fluorination, the carbon content is preferably 99% by mass or less, more preferably 95% by mass or less. On the other hand, the content of fluorine in the fluorinated layer is preferably 1% by mass or more, more preferably 5% by mass or more, and preferably 50% by mass or less.
The content of fluorine and carbon in the fluorinated layer is determined by elemental analysis by the combustion method.

When the other fluorinated particles in the method of covering the surface of the particles with another fluorinated particles are carbon, the fluorinated carbon is formed by mixing and contacting the carbon and a gas containing a fluorinated element. Can be manufactured.

Examples of the gas containing a fluorine element used for fluorination include fluorine gas, hydrogen fluoride gas, halogen fluoride such as _{ClF 3} , and IF _{5 , and gaseous substances such as BF 3} , NF ₃ , PF ₅ , SiF ₄ , and SF _6. Fluoride, metal fluoride such as LiF, NiF _{2 and the} like can be mentioned.

Of these, it is preferable to use gaseous fluoride from the viewpoint of ease of handling and reducing impurities contained in the obtained fluorinated carbon, and F ₂ , ClF ₃ , and NF ₃ are more preferable. , F ₂ is particularly preferable.
When gaseous fluoride is used, it may be diluted with an inert gas such as _{N 2} in order to facilitate control of the fluorination treatment.

The temperature at which carbon is fluorinated is preferably −20 ° C. or higher because fluorine enables chemical adsorption or formation of chemical bonds on the carbon surface. Further, the temperature is preferably 350 ° C. or lower in order to suppress an extreme fluorination reaction.

The thickness of the fluorinated layer on the surface of the fluorinated carbon particles is preferably 2 nm or more, more preferably 2.2 nm or more, from the viewpoint of further obtaining the effects of the present invention. Further, the thickness is preferably 50 nm or less, more preferably 20 nm or less, from the viewpoint of suppressing a decrease in conductivity. The thickness of the fluorinated layer on the surface of the fluorinated carbon particles can also be measured in the same manner as in the case of the surface of the negative electrode active material particles or the surface of the solid electrolyte particles described above.

By mixing the fluorinated carbon obtained above with the particles, particles having a fluorinated layer can be obtained.
Examples of the mixing method include a dry method and a wet method. The dry method is a method of mixing without using a dispersion medium, but the dry method does not require drying after mixing and preparation of a dispersion liquid required for the wet method. Is simple and preferable.

Equipment used for mixing in the dry method includes various dispas, ball mills, super mixers, henschel mixers, atomizers, V-type mixers, paint shakers, conical blenders, nouter mixers, SV mixers, drum mixers, shaker mixers, and professional share mixers. , Universal mixer, ribbon type mixer, ribbon mixer, container mixer and the like. When mixing on a small scale, a rotation / revolution mixer may be used.

The mixing time of the dry method is preferably 1 to 60 minutes, more preferably 1 to 30 minutes from the viewpoint of productivity. The mixing temperature is preferably 20 to 30 ° C.

(Conductive aid)
The negative electrode layer according to this embodiment contains a conductive auxiliary agent. When the negative electrode layer contains a conductive auxiliary agent, a negative electrode layer having electron conductivity suitable for a lithium ion secondary battery can be obtained. The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent for a negative electrode can be used. Examples of the conductive auxiliary agent for the negative electrode include carbon-based materials such as carbon black and acetylene black, metals such as copper, nickel, stainless steel and iron, and conductive oxides such as indium tin oxide (ITO). A carbon-based material having excellent electron conductivity and a small particle size can be preferably used as a conductive auxiliary agent because the electron conductivity can be enhanced by finely dispersing it in the negative electrode layer. These conductive auxiliaries may be used alone or in combination of two or more.

In the negative electrode layer according to the present embodiment, the content of the conductive auxiliary agent is preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 3% by mass or more, from the viewpoint of imparting conductivity in the negative electrode layer. Further, the content of the conductive auxiliary agent is preferably 20% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, from the viewpoint of increasing the capacity by using a large amount of the negative electrode active material in the negative electrode layer.

(Other additives)
In addition to the above components, the negative electrode layer according to the present embodiment may appropriately contain a binder in order to retain the structure of the negative electrode layer itself. Conventionally known binders can be used, for example, butadiene rubber (BR), acrylate butadiene rubber (ABR), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and the like. Can be used.

Further, the negative electrode layer may contain particles of a fluorinated inorganic compound or the like. By including the particles of the fluorinated inorganic compound, the electrochemical stability of the solid electrolyte particles and the stability in the air atmosphere can be improved. Examples of the particles of the inorganic compound include _{oxide particles such as TiO 2} , SiO ₂ , and ZrO _2, and nitride particles such as TiN, SiN, and ZrN. The method for fluorinating the inorganic compound is not particularly limited, and examples thereof include a method in which a fluorine-containing gas is brought into contact with the inorganic compound, a liquid phase method, and a solid phase method.

(Manufacturing method of negative electrode layer)
The method for forming the negative electrode layer according to the present embodiment is not particularly limited. For example, the components constituting the negative electrode layer described above are dispersed or dissolved in a solvent to form a slurry, which is coated in a layered form (sheet form) and dried. , The method of pressing arbitrarily can be mentioned. If necessary, heat may be applied to perform the debinder treatment. The thickness of the negative electrode layer can be easily adjusted by adjusting the coating amount of the slurry.
In addition, instead of the wet molding as described above, the negative electrode is formed by dry press-molding a material containing a component constituting the negative electrode layer on the surface of the object on which the negative electrode layer is formed, such as a negative electrode current collector and a solid electrolyte layer. Layers may be formed. Alternatively, the negative electrode layer may be formed on another substrate and transferred to the surface of the object on which the negative electrode layer is formed. From the viewpoint that a strong negative electrode layer can be industrially and stably formed on the surface of the target on which the negative electrode layer is formed, it is preferable to form the negative electrode layer on the surface of the target by wet molding using a solvent.

The thickness of the negative electrode layer is not particularly limited, and may be appropriately determined according to the performance of the target battery. The thickness of the negative electrode layer is preferably 10 μm or more, more preferably 20 μm or more, still more preferably 30 μm or more, for example, from the viewpoint of increasing the capacity. The thickness of the negative electrode layer is preferably 500 μm or less, more preferably 300 μm or less, from the viewpoint of ensuring lithium ion conductivity and electron conductivity in the thickness direction.

<Lithium-ion secondary battery>
The lithium ion secondary battery according to the present embodiment includes a negative electrode including the above-mentioned negative electrode layer, a solid electrolyte layer, and a positive electrode. Conventionally known solid electrolyte layers and positive electrodes are used. Specific examples are shown below, but the present invention is not limited thereto.

(Negative electrode)
The negative electrode contains at least the above-mentioned negative electrode layer and a negative electrode current collector.

The negative electrode current collector may be any material having conductivity, and for example, a thin metal plate (metal foil) such as copper or aluminum, a sputter material, or the like can be used. These are preferable because they are excellent in chemical stability and current collecting property.

The method for producing the negative electrode is not particularly limited, and examples thereof include a method of laminating a negative electrode layer and a negative electrode current collector previously formed by the above method or the like, and a method of forming a negative electrode layer directly on the negative electrode current collector. Be done.

(Solid electrolyte layer)
The solid electrolyte layer may contain a solid electrolyte having lithium ion conductivity, can prevent a short circuit between the positive electrode and the negative electrode, and can move lithium ions between the positive electrode and the negative electrode. The type of the solid electrolyte is not particularly limited, but a solid electrolyte similar to the solid electrolyte exemplified as the one that can be used for the solid electrolyte particles contained in the negative electrode layer described above can be preferably used. Further, the solid electrolyte layer may appropriately contain an additive such as a binder, if necessary. The shape and thickness of the solid electrolyte layer, the forming method, and the like are not particularly limited, and may be appropriately determined according to the performance of the target battery.

(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 material having conductivity, and for example, aluminum or an alloy thereof, a thin metal plate (metal foil) such as stainless steel, a sputter material, or the like can be used. These are preferable because they are excellent in chemical stability and current collecting property.

The positive electrode active material particles may be particles containing a positive electrode active material capable of reversibly advancing the occlusion and release of lithium ions, in other words, the desorption and insertion (intercalation) of lithium ions. The positive electrode active material is not particularly limited, and a known positive electrode active material can be used.
As the positive electrode active material, for example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium nickel manganese _{oxide, Li (Ni x Co y Mn} z M a) O 2 (X + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr. A composite metal oxide represented by (at least one kind), Li _a M _b (PO ₄ ) _c (1 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, where M is Fe, V, Co, Examples thereof include a polyanion olivine type positive electrode represented by (at least one selected from Mn, Ni, and VO), polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene. These may be used alone or in combination of a plurality of types.

In addition, the positive electrode may have a binder that binds the positive electrode active materials to each other and also binds the positive electrode active material and the positive electrode current collector. Conventionally known binders can be used.
Further, the positive electrode may have a known conductive auxiliary agent for the positive electrode, and the same conductive auxiliary agent for the negative electrode can be used.
In addition to the above, the positive electrode may contain solid electrolyte particles and the like in addition to the above positive electrode active material from the viewpoint of lithium ion conductivity.

Those constituting the lithium ion secondary battery such as the solid electrolyte layer, the positive electrode and the negative electrode are stored in the battery exterior. As the material of the battery exterior, conventionally known materials can be used, and specific examples thereof include nickel-plated iron, stainless steel, aluminum or alloys thereof, nickel, titanium, resin materials, and film materials.

Examples of the shape of the lithium ion secondary battery include a coin type, a sheet type (film type), a fold type, a winding type bottomed cylindrical type, a button type, and the like, which can be appropriately selected depending on the intended use.

The lithium ion secondary battery according to the present embodiment can realize good lithium ion conductivity.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[Example 1 (Example) fluorinated SiO _powder, P ₂ S 5 -Li ₂ S-based solid electrolyte powder, acetylene black mixed negative electrode layer]
(Fluorination treatment)
SiO powder Hastelloy reactor having a content volume of 0.3L as the negative electrode active material particles (manufactured Toshima Seisakusho, purity 3N) Put 6.564g of, using _{N 2} gas comprising _{F 2} gas 20 vol% Then, by performing the fluorination treatment at room temperature for 2 hours, a fluorinated SiO powder having a fluorinated layer on the surface of the secondary particles of the SiO powder is obtained.
(Fluorine content in powder)
Fluorine content in fluorinated SiO powder quantitatively analyzed using an automatic sample combustion device (Mitsubishi Chemical Analytech Co., Ltd. (Dia Instruments), AQF-100) and an ion chromatograph (Dionex, DX120) The total amount is 0.45% by mass.
(Fluorine content on the outermost surface)
As an ESCA analysis (X-ray photoelectron spectroscopy), the fluorine content on the outermost surface of the fluorinated layer of the fluorinated SiO powder measured using ESCA5500 manufactured by ULVAC-PHI is 95% by mass or more. ..
(Average particle size)
The average particle size of the fluorinated SiO powder measured using a laser diffraction / scattering type particle size distribution measuring device (manufactured by Nikkiso Co., Ltd., product name MT-3300EX) is 1 μm.

(Preparation of negative electrode layer)
The fluorinated SiO powder obtained above is used as negative electrode active material particles having a fluorinated layer on the surface to form a negative electrode layer by the following procedure.
Weigh 1.999 g of fluorinated SiO powder at room temperature, add 0.113 g of acetylene black as a conductive auxiliary agent, and use a rotation / revolution rotation mixer (THINKY, Awatori Rentaro, ARE-310). , 2000 rpm, powder mix for 2 minutes. _{Next, 0.4 g of P 2} S _{5- Li 2} S-based solid electrolyte powder (Li ₇ P ₃ S ₁₁ , manufactured by Ampcera) is added as solid electrolyte particles, and the mixture is mixed at 2000 rpm for 10 minutes. The obtained fluorinated SiO powder, P ₂ S _5- Li ₂ S-based solid electrolyte powder, and acetylene black mixed powder are molded to prepare a negative electrode layer (thickness 50 μm).

(Preparation of solid electrolyte layer)
Next, a solid electrolyte layer is prepared by the following procedure.
A solid electrolyte _{layer (thickness 30 μm) is prepared by molding using P 2} S _5- Li ₂ S-based solid electrolyte powder as the solid electrolyte material.
(Preparation of positive electrode layer)
Weigh 0.7 g _{of LiCoO 2} as the positive electrode active material _{, 0.2 g of P 2} S _5- Li ₂ S-based solid electrolyte powder as the solid electrolyte material, and 0.05 g of acetylene black as the conductive auxiliary agent. 2000 rpm, powder mix for 10 minutes. A positive electrode layer (thickness 100 μm) is formed by molding the obtained mixed powder.
(Preparation of evaluation cell for lithium ion secondary battery)
The negative electrode layer, solid electrolyte layer, and positive electrode layer prepared in the above procedure are placed in a simple stainless steel sealed cell in an argon gas glove box together with a current collecting foil (negative electrode side: copper foil 50 μm, positive electrode side: aluminum foil 100 μm) and all solid. Assemble the evaluation cell for the lithium-ion battery.

[Example 2 (Example) SiO powder, fluorinated P ₂ S _5- Li ₂ S-based solid electrolyte powder, acetylene black mixed negative electrode layer]
(Fluorination treatment)
Contents putting 3.000g of _{P 2 S} 5 -Li 2 _S-based solid electrolyte powder as a solid electrolyte particles Hastelloy reactor product 0.3 L, with _{N 2} gas containing 20 mol% of _{F 2} gas, By performing the fluorination treatment at room temperature for 2 hours, fluorinated P ₂ S _5- Li ₂ S-based solid electrolyte powder is obtained as solid electrolyte particles having a fluorinated layer on the surface.
(Fluorine content in powder)
_{Fluorinated P 2} S _5- Li ₂ S system quantitatively analyzed using an automatic sample combustion device (Mitsubishi Chemical Analytech (Dia Instruments), AQF-100) and ion chromatography (Dionex, DX120). The total fluorine content in the solid electrolyte powder is 1.2% by mass.
(Fluorine content on the outermost surface)
Fluorine on the outermost surface of the fluorinated layer of _{fluorinated P 2} S _5- Li ₂ S-based solid electrolyte powder measured using ESC A5500 manufactured by ULVAC-PHI as ESCA analysis (X-ray photoelectron spectroscopy). The content of is 95% by mass or more.
(Average particle size)
The average particle size of the powder measured using a laser diffraction / scattering type particle size distribution measuring device (manufactured by Nikkiso Co., Ltd., product name MT-3300EX) is 10 μm.

(Preparation of negative electrode layer)
The fluorinated P ₂ S _5- Li ₂ S-based solid electrolyte powder obtained above is used as solid electrolyte particles having a fluorinated layer on the surface to form a negative electrode layer by the following procedure.
Weigh 1.999 g of fluorinated P ₂ S _{5- Li 2} S-based solid electrolyte powder at room temperature, add 0.113 g of acetylene black as a conductive auxiliary agent, and rotate / revolve rotation mixer (manufactured by THINKY, Awatori). Using Neritaro, ARE-310), powder mix at 2000 rpm for 2 minutes. Next, 0.4 g of SiO powder is added as the negative electrode active material particles, and the mixture is mixed at 2000 rpm for 10 minutes. The obtained fluorinated P ₂ S _5- Li ₂ S-based solid electrolyte powder, SiO powder, and acetylene black mixed powder are molded to prepare a negative electrode layer (thickness 50 μm).
Using the same solid electrolyte layer and positive electrode layer used in Example 1 above, the cells are placed in a simple sealed cell made of stainless steel in a glove box to assemble an evaluation cell for an all-solid-state lithium-ion battery.

[Example 3 (Comparative example) SiO powder, P ₂ S _5- Li ₂ S-based solid electrolyte powder, acetylene black mixed negative electrode layer]
The SiO powder used in Example 1 is used as the negative electrode active material particles without fluorination treatment.
I.e. prepare a negative electrode layer in the same manner as Example 1 using the negative electrode active SiO powder as material particles, P _{2 S} 5 _{-Li 2 S-based} solid electrolyte powder as a solid electrolyte particles, acetylene black as a conductive aid.
Further, using the same solid electrolyte layer and positive electrode layer as in Example 1, the cells are placed in a simple sealed cell made of stainless steel in a glove box to assemble an evaluation cell for an all-solid-state lithium-ion battery.

<Battery characteristic evaluation>
The following charge / discharge characteristic evaluation is carried out using the evaluation cells for all-solid-state lithium-ion secondary batteries of Examples 1 to 3 described above.
It is charged with a constant current up to a voltage of 4.2 V at a C-rate of 0.05 C. Further, after charging is completed, a constant current discharge is performed up to a voltage of 2 V at the same C-rate of 0.05 C. Table 1 shows the configuration of the negative electrode layer of the all-solid-state battery and the evaluation results of charge / discharge characteristics in Examples 1 to 3. Charging shows the capacity per 1 g of the positive electrode active material at 4.2 V, and discharging shows the capacity at 2 V.

From the above results, it is confirmed that at least one of the negative electrode active material particles and the solid electrolyte particles in the embodiment of the present invention exhibits excellent battery characteristics by having a fluorinated layer on the surface.

Claims (6)

A negative electrode layer containing negative electrode active material particles, solid electrolyte particles, and a conductive auxiliary agent.
A negative electrode layer for a lithium ion secondary battery, wherein at least one of the negative electrode active material particles and the solid electrolyte particles has a fluorinated layer on the surface. The lithium ion secondary battery according to claim 1, wherein the solid electrolyte particles have a fluorinated layer on the surface, and the fluorine content on the outermost surface of the fluorinated layer is 95% by mass or more. Negative electrode layer. The negative electrode layer for a lithium ion secondary battery according to claim 1 or 2, wherein the solid electrolyte particles include at least one of a sulfide-based solid electrolyte particle and an oxide-based solid electrolyte particle. The negative electrode layer for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the solid electrolyte particles have an average particle size of 0.01 to 30 μm. The present invention according to any one of claims 1 to 4, wherein the negative electrode active material particles have a fluorinated layer on the surface, and the fluorinated layer in the negative electrode active material particles is composed of fluorinated carbon particles. The negative electrode layer for a lithium ion secondary battery according to the above. A lithium ion secondary battery including a negative electrode including the negative electrode layer according to any one of claims 1 to 5, a solid electrolyte layer, and a positive electrode.