JP2024124884A - Negative electrode active material particles and solid-state battery - Google Patents

Abstract

To provide a negative electrode active material particle which is suitable for manufacturing a battery of which an inner resistance is low, a discharge capacity is high, and an electrode strength is high.SOLUTION: A negative electrode active material particle of the present invention, includes: a silicon containing particle; and an alkyl group coupled to a silicon of the silicon containing particle, and the number of carbon of the alkyl group is 5 to 16.SELECTED DRAWING: Figure 1

Description

This disclosure relates to negative electrode active material particles and solid-state batteries.

In recent years, solid-state batteries that use solid electrolytes have been attracting attention. Compared to batteries that use electrolytes, solid-state batteries that use solid electrolytes are less susceptible to electrolyte decomposition caused by overcharging the battery, and have high cycle durability and energy density.

On the other hand, lithium-ion batteries, a type of secondary battery, have features such as high voltage and energy density and little memory effect, and are therefore used in a variety of fields, including automobiles and portable devices. As the use of lithium-ion batteries increases, there is a demand for further improvements in the performance of lithium-ion batteries.

One way to further improve the performance of lithium-ion batteries is to improve the negative electrode active material. The charge/discharge capacity of carbon materials, which are currently the main negative electrode active material, has reached its theoretical limit, making it difficult to further improve performance. In recent years, therefore, materials that alloy with lithium, such as silicon, have been attracting attention as they are capable of increasing charge/discharge capacity.

In addition, various studies are being conducted on silicon particles as a negative electrode active material for lithium-ion batteries.

For example, Patent Document 1 discloses that the initial coulombic efficiency can be improved by using silicon-based particles having an average particle size, specific surface area, and a ratio of oxygen content to specific surface area within specific ranges as the negative electrode active material of a lithium-ion battery.

Patent Document 2 discloses that the use of silicon particles modified with a non-dielectric layer that covers at least a portion of the surface as the negative electrode active material of a lithium ion battery improves battery characteristics such as resistivity and discharge capacity.

Patent Document 3 discloses that by using an active material that contains at least Si and satisfies 0.55≦I2/I1≦1.0 and 0.01≦I1, where I1 is the maximum peak intensity from 900 cm ⁻¹ to 950 cm ⁻¹ in an infrared spectrum and I2 is the maximum peak intensity from 1000 cm ⁻¹ to 1100 cm ⁻¹ in an infrared spectrum, as a negative electrode active material, it is possible to suppress a volume change of an electrode layer during charging and discharging.

JP 2017-112057 A JP 2018-138511 A JP 2022-077326 A

The present disclosure aims to provide negative electrode active material particles suitable for manufacturing batteries with low internal resistance, high discharge capacity, and high electrode strength.

The present inventors have discovered that the above problems can be solved by the following means:

<Aspect 1>
A silicon-containing particle having an alkyl group bonded to silicon of the silicon-containing particle, and the alkyl group has 5 to 16 carbon atoms.
Negative electrode active material particles.
<Aspect 2>
The negative electrode active material particles according to aspect 1, wherein the alkyl group has 8 to 14 carbon atoms.
Aspect 3
The negative electrode active material particles according to aspect 1 or 2, wherein the alkyl group is a linear alkyl group.
<Aspect 4>
A negative electrode active material layer is provided, and the negative electrode active material layer includes the negative electrode active material particle according to any one of aspects 1 to 3.
Solid-state battery.

The present disclosure provides negative electrode active material particles suitable for manufacturing batteries with low internal resistance, high discharge capacity, and high electrode strength.

1 is a graph showing the relationship between the number of carbon atoms in an alkyl group bonded to silicon in a silicon-containing particle and the slurry viscosity. 1 is a graph showing the relationship between the number of carbon atoms in an alkyl group bonded to silicon in a silicon-containing particle and electrode strength. 1 is a graph showing the relationship between the number of carbon atoms in an alkyl group bonded to silicon in a silicon-containing particle and internal resistance. 1 is a graph showing the relationship between the number of carbon atoms in an alkyl group bonded to silicon in a silicon-containing particle and discharge capacity.

The following describes in detail the embodiments of the present disclosure. Note that the present disclosure is not limited to the following embodiments, and can be modified in various ways within the scope of the gist of the present disclosure.

<Negative electrode active material particles>
The active material particles of the present disclosure include silicon-containing particles and an alkyl group bonded to silicon of the silicon-containing particles, the alkyl group having a carbon number of 5 to 16. Here, the bond between silicon of the silicon-containing particle and the alkyl group may be a covalent bond, particularly a covalent bond obtained by a hydrosilylation reaction between a hydrosilane group (-SiH) on the surface of the silicon-containing particle and an alkene.

The negative electrode active material particles disclosed herein can provide a solid-state battery with low internal resistance, high discharge capacity, and high electrode strength.

The reason for this is presumed to be as follows, without intending to be bound by any theory. That is, when the surface of the silicon-containing particle is oxidized, an insulating layer of silicon oxide is formed, which increases the internal resistance. Therefore, the internal resistance can be reduced by treating the silicon-containing particle with hydrogen fluoride to remove this insulating layer. On the other hand, when the amount of oxygen on the surface of the silicon-containing particle is reduced, the polarity of the surface of the silicon-containing particle is reduced, so that the affinity with the dispersion medium and binder used in forming the active material layer is reduced, and the strength of the obtained active material layer may be reduced. When the strength of the active material layer is low in this way, cracks are likely to occur in the active material layer, which is a factor in increasing the internal resistance of the battery. In response to this, it is believed that by bonding an alkyl group with a specific number of carbon atoms to the silicon on the surface of the silicon-containing particle, the affinity between the negative electrode active material particle and the dispersion medium and binder can be improved, and thus a negative electrode active material particle suitable for manufacturing a battery with low internal resistance, high discharge capacity, and high electrode strength can be provided.

This disclosure is explained in detail below.

<Negative Electrode Active Material Particles>
(Silicon-containing particles)
In the present disclosure, the silicon-containing particles are particles containing silicon that functions as a negative electrode active material, and are preferably particles containing silicon as a main component, for example, silicon particles that are substantially composed of silicon. Here, the main component of a particle is the component that has the largest proportion (mass fraction) in the particle. The silicon-containing particles may contain oxygen, lithium, silicon oxide (SiO _x (0<x<2)), Li _y SiO _x (0<x<2, 0<y), etc., in addition to silicon. The average particle size of the silicon-containing particles is, for example, 1 μm to 10 μm.

(Alkyl group)
Unless otherwise specified, the alkyl group having 5 to 16 carbon atoms includes linear, branched and cyclic monovalent saturated hydrocarbon groups.

The alkyl group preferably has 8 to 14 carbon atoms. The alkyl group is preferably a linear alkyl group.

(Application)
The negative electrode active material particles of the present disclosure can be used in both solid-state batteries and liquid batteries, and are preferably used in solid-state batteries.

The active material particles of the present disclosure are dispersed in a dispersion medium to obtain a slurry for the negative electrode active material layer, and the slurry is applied to a peeling substrate or other layer to obtain a negative electrode active material layer. This negative electrode active material layer slurry contains the negative electrode active material particles of the present disclosure, and optionally solid electrolyte particles, a conductive assistant, and a binder in a dispersion medium. The negative electrode active material layer slurry in which the active material particles of the present disclosure are dispersed in a dispersion medium can have a relatively small slurry viscosity due to the good affinity between the dispersion medium and the negative electrode active material particles.

Examples of dispersion media for the negative electrode active material layer slurry include non-polar solvents such as heptane, xylene, and toluene, and polar solvents such as tertiary amine solvents, ether solvents, thiol solvents, ketone solvents (e.g., diisobutyl ketone, etc.), and ester solvents (e.g., butyl butyrate, etc.).

<Method for producing negative electrode active material particles>
In the production of the negative electrode active material particles of the present disclosure, the silicon-containing particles are first treated with hydrogen fluoride to remove silicon oxide on the surfaces of the silicon-containing particles and simultaneously generate hydrosilane groups (-SiH) on the surfaces of the silicon-containing particles. Examples of the hydrogen fluoride treatment method include a method in which an aqueous hydrogen fluoride solution is added to the silicon-containing particles dispersed in a dispersion medium, the mixture is stirred, and the resulting suspension is subjected to solid-liquid separation and then dried.

Then, alkyl groups can be added to the hydrosilane groups (-Si-H) on the surfaces of the silicon-containing particles obtained as described above by a hydrosilylation reaction to produce the negative electrode active material particles of the present disclosure. One method for carrying out this hydrosilylation reaction is to add the hydrogen fluoride-treated silicon-containing particles and an alkene having 5 to 16 carbon atoms to a solvent, heat the mixture to reflux, separate the resulting suspension into solid and liquid, and then dry it.

Solid-state batteries
The solid-state battery of the present disclosure has a negative electrode active material layer, and the negative electrode active material layer contains the negative electrode active material particles of the present disclosure. In the present disclosure, the solid-state battery means a battery that contains a solid electrolyte as an electrolyte, and in particular may be an all-solid-state battery that uses only a solid electrolyte as the electrolyte.

The solid-state battery of the present disclosure has a positive electrode current collector layer, a positive electrode active material layer, a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector layer in this order, and the negative electrode active material layer can contain the negative electrode active material particles of the present disclosure.

(Positive Electrode Active Material Layer)
The positive electrode active material layer contains positive electrode active material particles, and optionally solid electrolyte particles, a conductive assistant, and a binder.

Examples of the positive electrode active material particles include lithium metal oxide particles containing lithium and at least one transition metal selected from manganese, cobalt, nickel, and titanium, such as lithium cobalt oxide ( _LixCoO2 ) particles, lithium nickel oxide ( _LiNO2 ) particles, _lithium manganate ( _LiMn2O4 ) particles, heteroelement-substituted spinel-type lithium manganate having a composition represented by Li1+xMyMn2-x- _yO4 (M=Al, Mg, Fe, Cr, Co, Ni, _Zn ), lithium titanate ( _LixTiOy ), lithium metal phosphate ( _LiMPO4 : M=Fe, Mn, Co, Ni), and lithium nickel cobalt manganate ( _Li1 + _xNi1/ 3Co1 _{/3Mn1 /3O2 )} particles, and combinations thereof.

Examples of solid electrolyte particles include sulfide-based amorphous solid electrolyte particles, such as Li ₂ S—SiS ₂ , LiI-Li ₂ S—SiS ₂ , LiI-Li ₂ S-P ₂ S ₅ , LiI-Li ₂ S-P ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , and Li ₂ S-P ₂ S ₅ ; sulfide-based crystalline solid electrolyte particles, such as Li ₇ P ₃ S ₁₁ , Li ₃ PS ₄ , and Li _3.25 P _0.75 S ₄ ; and combinations thereof.

Examples of conductive additives include carbon materials such as VGCF (Vapor Grown Carbon Fiber), carbon black, acetylene black (AB), Ketjen Black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF), as well as metal materials, and combinations of these.

Binders include, but are not limited to, polymer resins such as polyvinylidene fluoride (PVDF), acrylonitrile butadiene rubber (ABR), and styrene butadiene rubber (SBR), as well as combinations thereof.

(Negative Electrode Active Material Layer)
The negative electrode active material layer contains the negative electrode active material particles of the present disclosure, and optionally, solid electrolyte particles, a conductive assistant, and a binder.

For information about the solid electrolyte particles, conductive additive, and binder in the negative electrode active material layer, please refer to the description regarding the positive electrode active material layer.

(Solid electrolyte layer)
The solid electrolyte layer contains solid electrolyte particles and an optional binder. For the solid electrolyte particles and binder of the solid electrolyte layer, the description of the positive electrode active material layer can be referred to.

(Positive electrode current collector layer or negative electrode current collector layer)
Examples of the positive electrode current collector layer or the negative electrode current collector layer are not particularly limited, and include various metals such as silver, copper, gold, aluminum, nickel, iron, stainless steel, titanium, etc., and alloys thereof. From the viewpoint of chemical stability, etc., the positive electrode current collector layer is preferably an aluminum current collector layer, and the negative electrode current collector layer is preferably a nickel current collector layer.

The present disclosure will be described in more detail with reference to the following examples, but the scope of the present disclosure is not limited to these examples.

Example 1
<Preparation of Positive Electrode Active Material Layer>
The positive electrode mixture as the raw material for the positive electrode active material layer was placed in a polypropylene (PP) container, and the mixture was stirred for 10 minutes with an ultrasonic dispersing device (manufactured by SMT Corporation, model: UH-50) to prepare a slurry for the positive electrode active material layer.

The slurry for the positive electrode active material layer was applied to an aluminum foil as a positive electrode current collector layer using a blade method with an applicator to form a slurry layer for the positive electrode active material layer. This was dried at 100°C for 30 minutes on a hot plate to form a positive electrode active material layer on the positive electrode current collector layer.

The composition of the positive electrode mixture was as follows:
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (average particle size: 4 μm) as positive electrode active material particles;
butyl butyrate as a dispersion medium;
Vapor grown carbon fiber (VGCF) as a conductive additive;
- butyl butyrate solution (5% by weight) containing a polyvinylidene fluoride (PVdF)-based binder as binder;
Li ₂ S-P ₂ S ₅ -based glass ceramics containing LiI as solid electrolyte particles (average particle size 0.8 μm, approximately 380° C.).

(Preparation of solid electrolyte layer)
The electrolyte mixture as the raw material for the solid electrolyte layer was placed in a polypropylene (PP) container, and the mixture was stirred for 10 minutes with an ultrasonic dispersing device (manufactured by SMT Corporation, model: UH-50) to prepare a slurry for the solid electrolyte layer.

This solid electrolyte slurry was applied onto a stainless steel foil as a release sheet using a blade method with an applicator. This was then dried on a hot plate at 100°C for 30 minutes to obtain a solid electrolyte layer.

The electrolyte mixture had the following composition:
Li ₁₀ P ₂ GeS ₁₂ -based glass ceramics (average particle size 2.0 μm, approximately 690° C.) as solid electrolyte particles;
heptane as the dispersion medium;
Heptane solution (5% by mass) containing acrylonitrile butadiene rubber (ABR)-based binder as a binder

<Preparation of negative electrode active material layer>
(Hydrogen fluoride treatment)
Silicon particles (100 g) were added to ion-exchanged water (1900 g), and while stirring at room temperature, a 46% HF aqueous solution was dropped over 10 minutes, and the mixture was stirred at room temperature for 30 minutes. After stirring, the suspension was poured into a 1 L centrifuge bottle, and the solid and liquid were separated using a centrifuge (9000 rpm), and the supernatant was removed by decantation. Methanol (800 mL) was added to the settled solid, and the mixture was stirred, and then centrifuged again to decant the supernatant. This operation was repeated three times. The obtained solid was dried in a vacuum at 100 ° C. for 12 hours to obtain hydrogen fluoride-treated silicon particles.

(Hydrosilylation (alkylation) treatment)
The hydrogen fluoride-treated silicon particles (17 g) and 1-octene (carbon atom number: 8) (35 g) were added to toluene (288 g), and the mixture was heated under reflux at 120° C. for 20 hours to carry out a hydrosilylation reaction. After heating under reflux, the suspension was left to stand to allow the particles to settle, and the supernatant was decanted. Hexane was added to the obtained solid, which was stirred and then filtered under reduced pressure. This operation was repeated three times. The obtained solid was dried in a vacuum at 100° C. for 12 hours to obtain negative electrode active material particles having silicon particles with alkyl groups covalently bonded to the surface.

(Negative Electrode Active Material Layer)
The negative electrode mixture as the raw material for the negative electrode active material layer was placed in a polypropylene (PP) container, and the mixture was stirred for 10 minutes with an ultrasonic dispersing device (manufactured by SMT Corporation, model: UH-50) to prepare a slurry for the negative electrode active material layer.

The negative electrode active material layer slurry was applied to a nickel foil as a negative electrode current collector layer using a blade method with an applicator to form a negative electrode active material layer slurry layer. This was dried at 100°C for 30 minutes on a hot plate to form a negative electrode active material layer on the negative electrode current collector layer.

The composition of the negative electrode mixture was as follows:
- the negative electrode active material particles;
diisobutyl ketone as a dispersion medium;
- VGCF as a conductive additive;
- diisobutyl ketone solution (5% by mass) containing a styrene butadiene rubber (SBR)-based binder as a binder;
Li ₂ S-P ₂ S ₅ -based glass ceramics containing LiI as solid electrolyte particles (average particle size 0.8 μm, approximately 380° C.).

(Fabrication of all-solid-state batteries)
A solid electrolyte layer was superposed on the negative active material layer side of the laminate of the negative electrode current collector layer and the negative electrode active material layer to obtain a negative electrode laminate. A solid electrolyte layer was superposed on the positive active material layer side of the laminate of the positive electrode current collector layer and the positive electrode active material layer to obtain a positive electrode laminate. Then, each of the negative electrode laminate and the positive electrode laminate was roll-pressed at a linear pressure of 4 t/cm and 170°C, punched out into a circular shape of ¹ cm2, and the solid electrolyte layers were superposed and joined together to produce the all-solid-state battery of Example 1.

Example 2
An all-solid-state battery of Example 2 was produced in the same manner as in Example 1, except that 1-octene in the hydrosilylation was changed to 1-decene (number of carbon atoms: 10).

Example 3
An all-solid-state battery of Example 3 was produced in the same manner as in Example 1, except that 1-octene in the hydrosilylation was changed to 1-tetradecene (number of carbon atoms: 14).

Comparative Example 1
An all-solid-state battery of Comparative Example 1 was produced in the same manner as in Example 1, except that 1-octene in the hydrosilylation was changed to 1-octadecene (number of carbon atoms: 18).

Comparative Example 2
An all-solid-state battery of Comparative Example 2 was produced in the same manner as in Example 1, except that dolosilylation was not carried out.

Comparative Example 3
An all-solid-state battery of Comparative Example 3 was produced in the same manner as in Example 1, except that the hydrogen fluoride treatment and hydrosilylation were not carried out.

Comparative Example 4
An attempt was made to fabricate an all-solid-state battery in the same manner as in Example 1, except that 1-octene in the hydrosilylation was replaced with 1-butene (number of carbon atoms: 4). However, because 1-butene is a gas at around room temperature, heating under reflux was not possible, and the intended negative electrode active material particles could not be obtained.

"evaluation"
(Oxygen content measurement)
The negative electrode active material particles of the examples and comparative examples were melted with an inert gas using an EMGA-920 manufactured by HORIBA, and then measured by a non-dispersive infrared absorption method. The results are shown in Table 1.

(Measurement of hydrogen amount)
The negative electrode active material particles of the examples and comparative examples were melted with an inert gas using EMGA-921 manufactured by HORIBA, and then measured by a thermal conductivity method. The results are shown in Table 1.

(Measurement of carbon content)
The negative electrode active material particles of the examples and comparative examples were subjected to high-frequency heating and combustion in an oxygen stream using an EMIA-Expert manufactured by HORIBA, and the generated gas was analyzed and measured by an infrared absorption method. The results are shown in Table 1.

(Measurement of Slurry Rheology)
The slurries for the negative electrode active material layer of the examples and comparative examples were subjected to viscosity measurement at a shear rate of 100/s using an Anton Paar MCR302e with a φ50-1° cone plate. The results are shown in Table 1. The results of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in FIG.

(Measurement of electrode strength)
The laminates of the negative electrode current collector layer and the negative electrode active material layer obtained in the examples and comparative examples were roll pressed with a linear pressure of 0.25 t/cm, and punched out into a circular shape of ¹ cm2. Double-sided tape was attached to both the current collector side and the active material layer side of the punched-out laminate, and pressed with 50 N. Then, the test piece with the double-sided tape attached was pulled up and down in the vertical direction, and the load until peeling was measured. It was confirmed that the laminate peeled off within the negative electrode active material layer. The results are shown in Table 1. The results of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in FIG. 2.

(Measurement of internal resistance and discharge capacity of battery)
The internal resistance and discharge capacity of the all-solid-state batteries of the Examples and Comparative Examples were measured by a general method. The results are shown in Table 1. The results of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Figures 3 and 4.

As shown in Table 1, the negative electrode active material particles of the examples had a low oxygen content (O content) as a result of the hydrogen fluoride treatment, and a relatively high hydrogen content (H content) and carbon content (C content) as a result of the hydrosilylation treatment (alkylation) after the hydrogen fluoride treatment. As shown in Table 1 and Figure 1, the slurry containing the negative electrode active material particles of the examples had a low viscosity because the affinity between the silicon particles and the dispersion medium was improved by the alkylation of the silicon particles. As shown in Table 1 and Figures 2 to 4, the all-solid-state battery containing the negative electrode active material particles of the examples had high electrode strength, low internal resistance, and high discharge capacity.

As shown in Table 1, the negative electrode active material particles of Comparative Example 1 had a low oxygen content (O content) as a result of the hydrogen fluoride treatment, and a relatively high hydrogen content (H content) and carbon content (C content) as a result of the hydrosilylation treatment (alkylation) after the hydrogen fluoride treatment. As shown in Table 1 and Figure 1, the slurry containing the negative electrode active material particles of Comparative Example 1 had a low viscosity because the affinity between the silicon particles and the dispersion medium was improved by the alkylation of the silicon particles. As shown in Table 1 and Figures 2 to 4, the all-solid-state battery containing the negative electrode active material particles of Comparative Example 1 had a low electrode strength, a high internal resistance, and a low discharge capacity because the number of carbon atoms in the alkyl groups covalently bonded to the silicon on the surface of the silicon particles was too large.

As shown in Table 1, the negative electrode active material particles of Comparative Example 2 had a low oxygen content (O content) as a result of the hydrogen fluoride treatment, but because no hydrosilylation treatment was performed after the hydrogen fluoride treatment, the hydrogen content (H content) and carbon content (C content) were relatively low. As shown in Table 1 and Figure 1, the slurry containing the negative electrode active material particles of Comparative Example 2 had a high viscosity due to the low affinity between the silicon particles and the dispersion medium. As shown in Table 1 and Figures 2 to 4, the all-solid-state battery containing the negative electrode active material particles of Comparative Example 2 had low resistance and high capacity due to the absence of alkyl groups on the surface of the silicon particles, but had low electrode strength.

As shown in Table 1, the negative electrode active material particles of Comparative Example 3 were not subjected to hydrogen fluoride treatment or hydrosilylation treatment, so the oxygen content (O content) was high and the carbon content (C content) and hydrogen content (H content) were relatively low. As shown in Table 1, the slurry containing the negative electrode active material particles of Comparative Example 3 had a high surface oxygen content, which increased the polarity of the particle surface and made it prone to aggregation in a non-polar solvent such as diisobutyl ketone, so the viscosity was high. As shown in Table 1, the all-solid-state battery containing the negative electrode active material particles of Comparative Example 3 had good affinity with the dispersion medium because the silicon on the surface of the silicon particles was oxidized, and the electrode strength was relatively high, but the resistance was high and the capacity was low due to the presence of an oxide film.

Claims (4)

A silicon-containing particle having an alkyl group bonded to silicon of the silicon-containing particle, and the alkyl group has 5 to 16 carbon atoms.
Negative electrode active material particles. The negative electrode active material particles according to claim 1, wherein the alkyl group has 8 to 14 carbon atoms. The negative electrode active material particles according to claim 1 or 2, wherein the alkyl group is a linear alkyl group. The negative electrode active material layer includes the negative electrode active material particle according to claim 1 or 2.
Solid-state battery.

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