JP7779295B2 - Method for manufacturing silicon clathrate electrode active material and method for manufacturing lithium ion battery - Google Patents

Description

This disclosure relates to a method for producing a silicon clathrate electrode active material and a method for producing a lithium-ion battery.

In recent years, there has been active development of batteries. For example, in the automotive industry, development of batteries for use in electric vehicles and hybrid vehicles is progressing. Silicon is also known as an electrode active material used in batteries, particularly lithium-ion batteries.

Silicon electrode active materials have a large theoretical capacity and are effective in increasing the energy density of batteries. However, they have the problem of large expansion during charging. To address this issue, it is known that using silicon clathrate electrode active materials as silicon electrode active materials can suppress expansion during charging.

For example, Patent Document 1 discloses a silicon clathrate electrode active material having a silicon clathrate type II crystalline phase and a composition of Na _x Si ₁₃₆ (1.98<x<2.54).

Japanese Patent Application Laid-Open No. 2021-158003

Silicon clathrates can be produced by removing sodium from sodium-silicon alloys.

The types of silicon clathrate crystals that can be obtained include type I silicon clathrate and type II silicon clathrate. Type II silicon clathrate in particular efficiently stores lithium in the cage structure of the crystals, resulting in minimal expansion and contraction during battery charging and discharging.

In addition, in the sodium removal process, by-products such as diamond-type silicon (d-Si) may be produced in addition to silicon clathrate. Because these by-products do not function as electrode active materials, the effect of reducing expansion and contraction during charge and discharge when using type II silicon clathrate as an electrode active material may be limited.

For these reasons, there is a need to develop a method that can suppress the production of by-products such as type I silicon clathrate and diamond-type silicon when removing sodium from sodium-silicon alloys.

The present disclosure aims to provide a method for producing a silicon clathrate electrode active material that can suppress the production of by-products such as I-type silicon clathrate and diamond-type silicon, and a method for producing a lithium-ion battery that includes producing such a silicon clathrate electrode active material.

The present inventors have found that the above problems can be solved by the following means.
<Aspect 1>
A method for producing a silicon clathrate electrode active material, comprising the steps of:
providing a sodium-silicon alloy; and contacting the sodium-silicon alloy with aluminum fluoride particles having a D10 particle size of 15.0 μm or more to remove at least a portion of the sodium from the sodium-silicon alloy.
<Aspect 2>
2. The method of claim 1, wherein the aluminum fluoride particles have a (D90 particle size - D10 particle size)/D50 particle size ratio of 1.10 or less.
<Aspect 3>
3. The method of claim 1 or 2, wherein the silicon clathrate electrode active material is for use as a negative electrode active material in a lithium ion battery.
<Aspect 4>
A method for producing a lithium ion battery, comprising: producing a silicon clathrate electrode active material by the method according to any one of aspects 1 to 3; and forming an electrode active material layer containing the silicon clathrate electrode active material.

The present disclosure provides a method for producing a silicon clathrate electrode active material that can suppress the production of by-products such as I-type silicon clathrate and diamond-type silicon, and a method for producing a lithium-ion battery that includes producing such a silicon clathrate electrode active material.

FIG. 1 is a graph showing the measurement results of particle size distribution of aluminum fluoride particles ((a): Comparative Example, (b): Example 1, (c): Example 2). FIG. 2 is a graph showing XRD patterns of reaction products containing silicon clathrates of Examples 1 and 2 and Comparative Example 1.

Embodiments of the present disclosure are described in detail below. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosure.

<<Method for producing silicon clathrate electrode active material>>
The method for producing a silicon clathrate electrode active material of the present disclosure includes providing a sodium-silicon alloy and contacting the sodium-silicon alloy with aluminum fluoride (AlF ₃ ) particles having a D10 particle size of 15.0 μm or more to remove at least a portion of the sodium from the sodium-silicon alloy.

One possible method for producing silicon clathrate electrode active material is to react a sodium silicon (NaSi) alloy with aluminum fluoride as a sodium trapping agent to partially desorb sodium from the NaSi alloy and form a clathrate.

However, this reaction can produce not only type II silicon clathrate, which has a strong expansion-inhibiting effect, but also type I silicon clathrate, which has a weak expansion-inhibiting effect, and diamond-type silicon, which has no expansion-inhibiting effect.

The present inventors have found that in the above reaction, fine _AlF3 particles react excessively quickly with the NaSi alloy, generating strong heat, thereby promoting the production of type I silicon clathrate and diamond-type silicon.

Therefore, the present inventors have investigated the particle size of _AlF3 particles and found that the generation of I-type silicon clathrate and diamond-type silicon can be suppressed by using _AlF3 particles with a small number of fine particles. Without intending to be bound by theory, it is believed that the use of such _AlF3 can suppress excessively rapid reaction and the accompanying thermal runaway, thereby suppressing the generation of I-type silicon clathrate and diamond-type silicon.

In the context of this disclosure, the term "electrode active material" can be used as either a "positive electrode active material" or a "negative electrode active material," and is particularly used as a "negative electrode active material."

<Provision of sodium silicon alloy>
The method of the present disclosure includes providing a NaSi alloy.

NaSi alloys can be prepared by reacting a silicon (Si) source with a sodium (Na) source.

Commercially available Si powder or Si powder prepared by conventional methods can be used as the Si source.

Examples of sodium sources include, but are not limited to, sodium hydride (NaH).

The molar ratio of the Si source to the Na source may be 1:0.5-1.5, 1:0.7-1.3, or 1:0.9-1.1.

One example of a method for reacting the Si source and the Na source is to dry-mix these reactant materials and then heat them. A cutter mill can be used for dry mixing. The heating temperature may be 300°C to 700°C, and the heating time may be 20 to 60 hours. Heating may also be performed in an inert gas atmosphere such as argon.

<Sodium removal>
The method of the present disclosure includes contacting the NaSi alloy with _AlF3 particles to remove at least a portion of the Na from the NaSi alloy.

The D10 particle size of the _AlF3 particles is 15 μm or more. The D10 particle size of the _AlF3 particles may be 20 μm or more, 25 μm or more, 30 μm or more, 33 μm or more, or 35 μm or more, and may be 100 μm or less, 90 μm or less, 80 μm or less, 75 μm or less, or 70 μm or less. When the D10 particle size of _{the AlF3} particles is within the above range, that is, when there are few fine _AlF3 particles, the generation of I-type silicon clathrate and diamond-type silicon can be suppressed.

The D50 particle size of the _AlF3 particles may be 30 μm or more, 35 μm or more, 40 μm or more, 45 μm or more, or 50 μm or more, and may be 120 μm or less, 110 μm or less, or 100 μm or less.

The D90 particle size of the _AlF3 particles may be 50 μm or more, 60 μm or more, 65 μm or more, 70 μm or more, or 75 μm or more, and may be 160 μm or less, 150 μm or less, or 140 μm or less.

The value of (D90 particle size - D10 particle size)/D50 particle size of the _AlF3 particles may be 1.10 or less, and may be 0.40 or more, 0.50 or more, 0.60 or more, 0.65 or more, or 0.70 or more, and may be 1.00 or less, 0.90 or less, 0.95 or less, or 0.80 or less.

The particle size and particle size distribution of the _AlF3 particles can be measured using a particle size distribution measuring device (Shimadzu SALD-7500, Shimadzu Corporation) under the conditions of refractive index (2.25-0.00i) and built-in ultrasonic irradiation time (10 seconds).

The AlF ₃ particles used in the present disclosure may be commercially available or may be AlF ₃ particles prepared by a conventional method.

The _AlF3 particles may be subjected to a particle size adjustment process so that the D10 particle size falls within the range of the present disclosure. Examples of particle size adjustment processes include, but are not limited to, sieving.

The molar ratio of NaSi alloy to _AlF3 particles may be 1:0.10-0.60, 1:0.20-0.50, or 1:0.30-0.40.

An example of a method for contacting _AlF3 particles with an NaSi alloy is to dry-mix these reactant materials and then heat them. A cutter mill can be used for the dry mixing. The heating temperature may be 200°C to 500°C, and the heating time may be 40 hours to 80 hours. Heating may also be performed in an inert gas atmosphere such as argon.

<Application>
The silicon clathrate electrode active material obtained by the method of the present disclosure can be used as a negative electrode active material for lithium ion batteries.

<Lithium-ion battery manufacturing method>
The disclosed method of manufacturing a lithium ion battery includes manufacturing a silicon clathrate electrode active material and forming an electrode active material layer containing the silicon clathrate electrode active material.

For information on the method for producing silicon clathrate electrode active material, please refer to the above description of the method for producing silicon clathrate electrode active material of the present disclosure.

The method for forming the electrode active material layer is not particularly limited, and any known method can be used. For example, when the electrode active material layer is used as a negative electrode active material layer, a slurry containing a silicon clathrate electrode active material can be applied to a negative electrode current collector and dried to obtain a negative electrode active material layer formed on the negative electrode current collector layer.

The method for forming the battery is not particularly limited, and any known method can be used. Below, we will describe a battery in which the electrode active material layer is used as the negative electrode active material layer.

The battery manufacturing method disclosed herein may include, in addition to manufacturing a silicon clathrate electrode active material and forming a negative electrode active material layer containing the silicon clathrate electrode active material, arranging a negative electrode current collector layer, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer in this order.

<Negative electrode current collector layer>
The material used for the negative electrode current collector layer is not particularly limited, and any material that can be used as a negative electrode current collector for a battery can be appropriately adopted. For example, copper, a copper alloy, and copper plated or vapor-deposited with nickel, chromium, carbon, or the like may be used, but is not limited to these.

The shape of the negative electrode current collector layer is not particularly limited, and examples include foil, plate, and mesh. Of these, foil is preferred.

<Negative electrode active material layer>
The negative electrode active material layer of the present disclosure is a layer containing a negative electrode active material, and optionally an electrolyte, a conductive additive, and a binder.

(Negative electrode active material)
The negative electrode active material comprises a silicon clathrate electrode active material of the present disclosure.

(electrolyte)
The material of the solid electrolyte is not particularly limited, and any material that can be used as a solid electrolyte for a lithium ion battery can be used. For example, the solid electrolyte may be a sulfide solid electrolyte.

Examples of sulfide solid electrolytes include, but are not limited to, sulfide amorphous solid electrolytes, sulfide crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide solid electrolytes include Li ₂ S—P ₂ S ₅ systems (Li ₇ P ₃ S ₁₁ , Li ₃ PS ₄ , Li ₈ P ₂ S ₉ , etc.), Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Li ₂ S—P ₂ S ₅ , LiI—LiBr—Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —GeS ₂ (Li ₁₃ GeP ₃ S ₁₆ , Li ₁₀ GeP ₂ S _{12 ,} etc.), LiI—Li ₂ S—P ₂ O ₅ , LiI—Li ₃ PO ₄ —P ₂ S ₅ , Li _7-x PS _6-x Cl _x, etc.; or combinations thereof.

The sulfide solid electrolyte may be glass or crystallized glass (glass ceramic).

When the negative electrode active material layer contains a solid electrolyte, the mass ratio of the silicon clathrate electrode active material to the solid electrolyte in the negative electrode active material layer (mass of silicon clathrate electrode active material: mass of solid electrolyte) is preferably 85:15 to 30:70, and more preferably 80:20 to 40:60.

The electrolyte preferably contains a supporting salt and a solvent.

Examples of supporting salts (lithium salts) for the electrolyte solution having lithium ion conductivity include inorganic lithium salts such as _LiPF6 , _LiBF4 , _LiClO4 , and _{LiAsF6 ,} and organic lithium salts _such as _LiCF3SO3 , LiN( _CF3SO2 ) _{2 ,} LiN( _C2F5SO2 ) ₂ , LiN( _FSO2 ) _{2 , and} LiC( _CF3SO2 ) ₃ .

Examples of solvents used in the electrolyte include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and chain esters (chain carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). It is preferable that the electrolyte contain two or more solvents.

(Conductive additive)
The conductive additive is not particularly limited, and may be, for example, VGCF (Vapor Grown Carbon Fiber), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), carbon nanofibers (CNF), or the like, but is not limited thereto.

(binder)
The binder is not particularly limited, and may be, for example, a material such as polyvinylidene fluoride (PVdF), butadiene rubber (BR), or styrene butadiene rubber (SBR), or a combination thereof, but is not limited to these.

The thickness of the negative electrode active material layer may be, for example, 0.1 to 1000 μm.

<Electrolyte layer>
The electrolyte layer contains at least an electrolyte. In addition to the electrolyte, the electrolyte layer may contain a binder, etc., as necessary. For the electrolyte and the binder, reference can be made to the above description of the negative electrode active material layer of the present disclosure.

The thickness of the electrolyte layer is, for example, 0.1 to 300 μm, preferably 0.1 to 100 μm.

<Cathode active material layer>
The positive electrode active material layer is a layer containing a positive electrode active material, and optionally an electrolyte, a conductive additive, a binder, and the like.

The material of the positive electrode active material is not particularly limited. For example, the positive electrode active material may be, but is not limited to, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , a heteroelement-substituted Li-Mn spinel having a composition represented by Li _1+x Mn _2-x-y MyO ₄ (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (Li _x TiO _y ), or lithium metal phosphate (LiMPO ₄ , M is one or more metals selected from Fe, Mn, Co, and Ni).

The positive electrode active material may have a coating layer. The coating layer is a layer containing a substance that has lithium ion conductivity, low reactivity with the positive electrode active material and the solid electrolyte, and can maintain the shape of the coating layer without flowing even when in contact with the active material and the solid electrolyte. Specific examples of materials constituting the coating _layer include, but _are not limited to, _LiNbO3 , _Li4Ti5O12 , _Li3PO4 , _etc.

The positive electrode active material may be, for example, particulate. The average particle size (D50) of the positive electrode active material is not particularly limited, but may be, for example, 10 nm or more, or 100 nm or more. On the other hand, the average particle size (D50) of the positive electrode active material is, for example, 50 μm or less, or may be 20 μm or less. The average particle size (D50) can be calculated, for example, from measurements using a laser diffraction particle size distribution analyzer or a scanning electron microscope (SEM).

For details about the electrolyte, conductive additive, and binder, please refer to the above description regarding the negative electrode active material layer of this disclosure.

When the positive electrode active material layer contains a solid electrolyte, the mass ratio of the positive electrode active material to the solid electrolyte in the positive electrode active material layer (mass of positive electrode active material: mass of solid electrolyte) is preferably 85:15 to 30:70, and more preferably 80:20 to 50:50.

The thickness of the positive electrode active material layer is, for example, 0.1 μm to 1000 μm, preferably 1 μm to 100 μm, and more preferably 30 μm to 100 μm.

<Positive electrode current collector layer>
The material used for the positive electrode current collector layer is not particularly limited, and any material that can be used as a positive electrode current collector for a battery can be appropriately adopted. Examples of the material include, but are not limited to, SUS, nickel, chromium, gold, platinum, aluminum, iron, titanium, zinc, and the like, as well as metals such as these plated or vapor-deposited with nickel, chromium, carbon, and the like.

The shape of the positive electrode current collector layer is not particularly limited, and examples include foil, plate, and mesh. Of these, foil is preferred.

The lithium-ion battery produced by the method of the present disclosure may be a liquid-based battery containing an electrolytic solution as the electrolyte layer, or a solid-state battery having a solid electrolyte layer as the electrolyte layer. In the context of the present disclosure, "solid-state battery" refers to a battery that uses at least a solid electrolyte as the electrolyte; therefore, a solid-state battery may use a combination of a solid electrolyte and a liquid electrolyte as the electrolyte. The solid-state battery of the present disclosure may also be an all-solid-state battery, i.e., a battery that uses only a solid electrolyte as the electrolyte.

The lithium-ion battery produced by the method disclosed herein may be a primary battery or a secondary battery.

Lithium-ion batteries can be shaped, for example, as a coin, laminate, cylindrical, or rectangular battery.

<Synthesis of silicon clathrate electrode active material>
<Alloying>
Si powder (Kojundo Kagaku, SIEPB32) was prepared as a silicon (Si) source. This Si powder and metallic lithium (Li) were weighed out at a molar ratio of Li/Si = 4.0, and the weighed Si powder and Li were mixed in a mortar in an argon atmosphere to obtain a lithium silicon (LiSi) alloy. The obtained LiSi alloy was reacted with ethanol in an argon atmosphere and treated with hydrogen fluoride (HF) to obtain a Si powder having voids inside the primary particles, i.e., a Si powder having a porous structure.

A sodium-silicon (NaSi) alloy was produced using porous Si powder and sodium hydride (NaH) as a sodium (Na) source. The NaH used was previously washed with hexane. NaH and porous Si powder were weighed out in a molar ratio of 1.05:1, and the weighed NaH and porous Si powder were mixed in a cutter mill. The resulting mixture was heated in a heating furnace under an argon atmosphere at 500°C for 40 hours to obtain a powdered NaSi alloy.

<Clathration (sodium removal)>
Example 1
The obtained NaSi alloy and aluminum fluoride ( _AlF3 ) particles as a Na trapping agent were weighed out to a molar ratio of 1:0.35, and the weighed NaSi alloy and _AlF3 were mixed using a cutter mill to obtain a reaction raw material. The obtained powdered reaction raw material was placed in a stainless steel reaction vessel and heated in a heating furnace under an argon atmosphere at 310°C for 60 hours to obtain a reaction product containing a silicon clathrate electrode active material. In Example 1, _AlF3 particles that did not pass through a sieve with a mesh size of 75 μm were used.

Example 2
A reaction product containing the silicon clathrate electrode active material of Example 2 was obtained in the same manner as in Example 1, except that _AlF3 particles that passed through a sieve with a mesh size of 75 μm but did not pass through a sieve with a mesh size of 32 μm were used in clathration (sodium removal).

(Comparative Example 1)
In the clathration (sodium removal), a reaction product containing a silicon clathrate electrode active material of Comparative Example 1 was obtained in the same manner as in Example 1, except that _AlF3 particles passed through a sieve with a mesh size of 32 μm were used.

"evaluation"
<Particle size and particle size distribution>
The particle size and particle size distribution of the _AlF3 particles were measured using a particle size distribution measuring device (Shimadzu SALD-7500, Shimadzu Corporation) under the conditions of refractive index (2.25-0.00i) and built-in ultrasonic irradiation time (10 seconds).

<XRD>
As an index for evaluating the proportion of components contained in the reaction product in each example, measurement was carried out by XRD (X-ray diffraction method).

"result"
The measurement results of the particle size distribution of _AlF3 used in each example are shown in Table 1 and Figure 1, and the XRD pattern of the reaction product of each example is shown in Figure 2.

As shown in Table 1 and FIGS. 1 and 2, the reaction products of the examples produced by the method of the present disclosure, which had a D10 particle size within the range of the present disclosure, i.e., which used _AlF3 particles with few fine particles, had small peaks in XRD indicating type I silicon clathrate and diamond-type silicon (d-Si).

Claims (4)

A method for producing a silicon clathrate electrode active material, comprising the steps of:
providing a sodium-silicon alloy; and contacting the sodium-silicon alloy with aluminum fluoride particles having a D10 particle size of 15.0 μm or more to remove at least a portion of the sodium from the sodium-silicon alloy. The method of claim 1, wherein the aluminum fluoride particles have a (D90 particle size - D10 particle size)/D50 particle size ratio of 1.10 or less. The method of claim 1, wherein the silicon clathrate electrode active material is for use as a negative electrode active material in a lithium ion battery. A method for producing a lithium ion battery, comprising: producing a silicon clathrate electrode active material by the method according to any one of claims 1 to 3; and forming an electrode active material layer containing the silicon clathrate electrode active material.