JP7147619B2 - Method for producing negative electrode active material - Google Patents

Description

The present disclosure relates to a method for producing a negative electrode active material with good discharge capacity.

2. Description of the Related Art In recent years, with the rapid spread of information-related equipment and communication equipment such as personal computers, video cameras, and mobile phones, the development of batteries (for example, lithium batteries) that are excellent as power sources for these devices has been emphasized. In fields other than information-related devices and communication-related devices, for example, in the automotive industry, development of lithium batteries and the like used in electric vehicles and hybrid vehicles is underway.

Si-based active materials are known as negative electrode active materials used in lithium batteries. Patent Document 1 discloses a method for producing porous silicon in which an oxide film is removed by hydrofluoric acid treatment. Patent Document 2 discloses a non-aqueous electrolyte secondary battery using a negative electrode active material composed of Si and an alloy.

JP 2013-008487 A JP 2006-286313 A

In order to improve the performance of all-solid-state batteries, negative electrode active materials with good discharge capacity are desired.

The present disclosure has been made in view of the above circumstances, and a main object thereof is to provide a negative electrode active material with good discharge capacity.

In order to achieve the above objects, the present disclosure provides a method for producing a negative electrode active material used in an all-solid-state battery, wherein a Si-based active material having an oxide film on its surface is heated at 150 ° C. in a hydrogen gas-containing atmosphere. Provided is a method for producing a negative electrode active material, comprising a heat treatment step of removing the oxide film by performing heat treatment at 600° C. or less for 1 hour or more and 24 hours or less.

According to the present disclosure, since the oxide film can be removed by heat treatment in a hydrogen gas-containing atmosphere, a negative electrode active material with good discharge capacity can be produced.

The method for producing a negative electrode active material according to the present disclosure has the effect of being able to produce a negative electrode active material with good lithium ion conductivity.

FIG. 2 is a flow chart showing an example of a method for producing a negative electrode active material in the present disclosure; 1 shows the results of infrared spectroscopic measurement of negative electrode active materials produced in Example 1, Comparative Example 1, and Reference Example. 4 shows the results of infrared spectroscopic measurement of negative electrode active materials produced in Example 1 and Comparative Examples 1 and 2. FIG. 4 shows the results of X-ray diffraction measurement of negative electrode active materials produced in Example 1 and Comparative Examples 1 and 2. FIG. It is the result of the rate performance of all-solid-state batteries produced in Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example. 1 shows the results of impedance measurement of all-solid-state batteries produced in Examples 1 to 3 and Comparative Examples 1 and 2. FIG.

A method for producing a negative electrode active material according to the present disclosure will be described below.
FIG. 1 is a flow chart showing an example of a method for producing a negative electrode active material according to the present disclosure. As shown in FIG. 1, the present disclosure includes a heat treatment step of heat-treating a Si-based active material having an oxide film on its surface in an atmosphere containing hydrogen gas.

According to the present disclosure, since the oxide film can be removed by heat treatment in a hydrogen gas-containing atmosphere, a negative electrode active material with good discharge capacity can be produced. In addition, according to the present disclosure, the crystallinity of silicon can be maintained even after heat treatment. Therefore, when the negative electrode active material of the present disclosure is used in an all-solid-state battery, rate performance such as load characteristics can be improved.

In all-solid-state batteries, the reactivity of the contact interface between the active material and the solid electrolyte is important. This is because the contact area between the active material and the solid electrolyte is small in an all-solid-state battery, so if the reactivity at the contact interface is good, the resistance can be reduced and the rate performance can be improved. Especially in an all-solid-state battery, unlike a liquid-based battery, a solid electrolyte interface (SEI) is not formed, so the surface state of the active material is important for the reactivity of the contact interface. Si-based active materials are known as negative electrode active materials used in all-solid-state batteries. A Si-based active material has a large theoretical capacity and is effective in increasing the energy density of a battery, but an oxide film is likely to be formed on the surface. Since this oxide film is a passive film, the presence of the oxide film may act as a barrier when lithium ions move. As a result, the increased resistance may reduce the discharge capacity of the active material. In the present disclosure, since the oxide film can be removed by heat treatment in an atmosphere containing hydrogen gas, a negative electrode active material with good discharge capacity can be produced.
Hereinafter, the method for manufacturing the negative electrode active material in the present disclosure will be described in detail.

1. Heat Treatment Step In the heat treatment step of the present disclosure, a Si-based active material having an oxide film on its surface is subjected to heat treatment at 150° C. or more and 600° C. or less for 1 hour or more and 24 hours or less in an atmosphere containing hydrogen gas to remove the oxide film. This is the step of removing. The Si-based active material may be produced by oneself or purchased from another party.

The Si-based active material is preferably an active material that can be alloyed with Li. Si-based active materials include, for example, simple Si and Si alloys. The Si alloy preferably contains Si element as a main component. The proportion of Si element in the Si alloy may be, for example, 50 mol % or more, 70 mol % or more, or 90 mol % or more.

The shape of the Si-based active material can be, for example, particulate, thin film, or the like. When the Si-based active material is particulate, the average particle size (D ₅₀ ) of the Si-based active material is, for example, preferably 1 nm or more and 100 μm or less, more preferably 10 nm or more and 30 μm or less, and 1 μm or more. 10 μm or less is particularly preferable. The average particle size can be determined, for example, based on image analysis using a SEM (scanning electron microscope). The number of samples is preferably large, for example 100 or more.

The oxide film is usually a film formed on the surface of the Si-based active material by natural oxidation, such as a film of SiO ₂ or the like. The thickness of the oxide film is, for example, 10 nm or more, and may be 100 nm or more. On the other hand, the thickness of the oxide film is, for example, 1000 nm or less.

The heat treatment step in the present disclosure is a step of performing heat treatment on the above-described Si-based active material in a hydrogen gas-containing atmosphere at 150° C. or more and 600° C. or less for 1 hour or more and 24 hours or less to remove the oxide film.

The atmosphere containing hydrogen gas may be an atmosphere containing only hydrogen gas, or an atmosphere containing a mixed gas of hydrogen gas and an inert gas such as argon or helium. In the case of a mixed gas, it is preferable to use hydrogen gas as the main component. The ratio of hydrogen gas in the mixed gas may be, for example, 50 vol% or more, 70 vol% or more, or 90 vol% or more. The oxide film can be removed by the reducing action using such gas.

The heat treatment temperature is 150° C. or higher, for example, 200° C. or higher, 250° C. or higher, 300° C. or higher, or 400° C. or higher. On the other hand, the heat treatment temperature is 600° C. or lower, and may be, for example, 500° C. or lower. If the heat treatment temperature is too low, the oxide film may not be sufficiently removed. On the other hand, if the heat treatment temperature is too high, the crystallinity of the Si-based active material may change.

The heat treatment time is 1 hour or longer, for example, 3 hours or longer, 6 hours or longer, or 12 hours or longer. On the other hand, the heat treatment time is 24 hours or less, and may be, for example, 20 hours or less, or may be 16 hours or less. If the heat treatment time is too short, the oxide film may not be sufficiently removed. On the other hand, if the heat treatment time is too long, the production efficiency may deteriorate.

The flow rate of hydrogen gas or mixed gas is not particularly limited, but may be, for example, 0.1 L/min or more, 0.25 L/min or more, or 0.3 L/min or more. On the other hand, the flow rate of hydrogen gas is, for example, 0.5 L/min or less, and may be 0.4 L/min or less.

The rate of temperature increase is not particularly limited, but is, for example, 1° C./min or higher, may be 3° C./min or higher, or may be 5° C./min or higher. On the other hand, the heating rate is, for example, 30° C./min or less, may be 20° C./min or less, or may be 10° C./min or less.

The heat treatment in the present disclosure can be performed, for example, by placing the Si-based active material in a chamber, introducing hydrogen gas, raising the temperature to a set temperature, and then holding the temperature for a predetermined time.

2. Negative Electrode Active Material In the negative electrode active material manufactured according to the present disclosure, the oxide film on the surface of the Si-based active material is removed by the heat treatment described above.

The presence or absence of an oxide film can be confirmed, for example, from peaks obtained by infrared spectroscopy. A broad peak is confirmed at 3100 cm ⁻¹ to 3500 cm ⁻¹ in the Si-based active material on which an oxide film is formed. On the other hand, the above broad peak is not observed in the Si-based active material from which the oxide film has been removed.

In addition, the negative electrode active material produced according to the present disclosure maintains its crystallinity even after the heat treatment described above. The crystallinity can be confirmed, for example, by the half width of the peak near 2θ=47.2 of the (220) plane obtained by X-ray diffraction measurement. Also, the crystallinity can be confirmed by a change in the peak around 1200 to 1000 cm ⁻¹ derived from the Si—O skeleton obtained by infrared spectroscopy, for example.

As described above, the negative electrode active material according to the present disclosure has a good discharge capacity because the oxide film is removed. In addition, the negative electrode active material in the present disclosure maintains the crystallinity of silicon even after heat treatment. Therefore, when used in an all-solid-state battery described later, the rate performance can be improved.

Further, in the present disclosure, in a method for manufacturing a negative electrode active material used in an all-solid-state battery, a Si-based active material having an oxide film on its surface is heated at 150° C. or higher and 600° C. or lower in a reduced pressure atmosphere for 1 hour or longer24. It is also possible to provide a method for producing a negative electrode active material, which includes a heat treatment step of removing the oxide film by performing a heat treatment for less than one hour. The reduced-pressure atmosphere is preferably, for example, a vacuum atmosphere. The vacuum atmosphere is preferably 100 Pa or less, for example. Moreover, the heat treatment temperature is preferably 200° C. or higher and 300° C. or lower, for example.

Further, in the present disclosure, in a method for producing a negative electrode active material used in an all-solid-state battery, a Si-based active material having an oxide film on its surface is heated at 150 ° C. or higher and 600 ° C. or lower for 1 hour in an inert gas atmosphere. It is also possible to provide a method for producing a negative electrode active material, which includes a heat treatment step of performing heat treatment for 24 hours or less and removing the oxide film. Examples of the inert gas atmosphere include an Ar gas atmosphere, an _N2 gas atmosphere, and an _H2 gas atmosphere. The heat treatment temperature is preferably 200° C. or higher and 300° C. or lower, for example.

3. All-solid-state battery The negative electrode active material in the present disclosure is used in an all-solid-state battery. An all-solid-state battery usually has a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer formed between the positive electrode active material and the negative electrode active material layer. In the present disclosure, the all-solid-state battery is preferably a lithium battery. Moreover, as a lithium battery, at least one of the positive electrode active material layer and the solid electrolyte layer preferably contains a sulfide solid electrolyte.

Examples of sulfide solid electrolytes include solid electrolytes containing Li element, X element (X is at least one of P, Si, Ge, Sn, B, Al, Ga, and In), and S element. mentioned. Moreover, the sulfide solid electrolyte may further contain at least one of the O element and the halogen element. Examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₅ -Li ₃ PO ₄ , LiI-P ₂ S ₅ -Li ₃ PO ₄ , LiI-Li ₂ S- _B2S3 , _{Li2SP2S5 - LiI} , _{Li2SP2S5 - LiI - LiBr , Li2SP2S5 - Li2O , Li2SP2S5} —Li ₂ O—LiI, Li ₂ SP ₂ O ₅ , LiI—Li ₂ SP ₂ O ₅ , Li ₂ S—SiS ₂ , Li ₂ S—SiS ₂ —LiI, Li ₂ S—SiS ₂ — LiI—LiBr, Li ₂ S—SiS ₂ —LiBr, Li ₂ S—SiS ₂ —LiCl, Li ₂ S—SiS ₂ —B ₂ S ₃ —LiI, Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, Li ₂ S—B ₂ S ₃ , Li ₂ SP ₂ S ₅ —Z _m S _n (where m and n are positive numbers and Z is one of Ge, Zn and Ga), Li ₂ S —GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (where x and y are positive numbers, M is P, Si, Ge, B, Al , Ga, or In.).

The sulfide solid electrolyte may be glass, glass ceramics, or crystal. Crystal phases contained in sulfide solid electrolytes include, for example, thiolysicone-type crystal phases, LGPS-type crystal phases, and aldirodite-type crystal phases.

Note that the present disclosure is not limited to the above embodiments. The above embodiment is an example, and any device that has substantially the same configuration as the technical idea described in the claims of the present disclosure and achieves the same effect is the present invention. It is included in the technical scope of the disclosure.

[Example 1]
(Preparation of negative electrode active material)
Si powder (D ₅₀ =8 μm) was added into the chamber, and hydrogen gas (ZERO-U (6N) manufactured by Sumitomo Seika Chemicals) was introduced at a flow rate of 0.25 L/min. The chamber was then heated to 600°C at 3°C/min. After reaching 600° C., heat treatment was performed by holding for 24 hours. Thus, a negative electrode active material was produced.

(Preparation of evaluation cell)
The prepared negative electrode active material, a sulfide solid electrolyte, a 5% by weight solution of PVdF-based binder in butyl butyrate, and a conductive aid (VGCF) were combined into a negative electrode active material: solid electrolyte: binder: conductive aid = 55.0. :40.0:0.6:4.4, and mixed with a butyl butyrate solution to prepare a negative electrode slurry. The negative electrode slurry was applied to a negative electrode current collector (Cu foil) and dried to prepare a negative electrode in which a negative electrode active material layer was formed on the negative electrode current collector. As the sulfide solid electrolyte, sulfide solid electrolytes containing halogen such as I and Br and having a P ₂ S ₄ ^- structure, a PS ₄ ^3- structure, and a P ₂ S ₆ ^4- structure were used. The ionic conductivity of the solid electrolyte at 25° C. was 4.5×10 ⁻³ S/cm.

A positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), a sulfide solid electrolyte, a 5% by weight solution of a PVdF-based binder in butyl butyrate, and a conductive agent (VGCF) were mixed into a positive electrode active material. Materials:solid electrolyte:binder:conduction aid=84.7:12.6:0.6:2.1 weight ratio, and mixed with butyl butyrate solution to prepare positive electrode slurry. The positive electrode slurry was applied to a positive electrode current collector (Al foil) and dried to prepare a positive electrode in which a positive electrode active material layer was formed on the positive electrode current collector. The sulfide solid electrolyte is the same as above.

The positive electrode active material layer of the positive electrode and the negative electrode active material layer of the negative electrode were opposed to each other via a solid electrolyte layer containing a solid electrolyte:binder at a weight ratio of 99.4:0.6, and a linear pressure of 5 tom / cm was applied. was pressed to prepare an evaluation cell.

[Example 2]
An all-solid battery was produced in the same manner as in Example 1, except that the processing temperature was changed to 200° C. in the production of the negative electrode active material.

[Example 3]
An all-solid-state battery was produced in the same manner as in Example 1, except that the processing temperature was changed to 400° C. in the production of the negative electrode active material.

[Comparative Example 1]
An evaluation cell was produced in the same manner as in Example 1, except that heat treatment was not performed in the production of the negative electrode active material.

[Comparative Example 2]
An all-solid-state battery was produced in the same manner as in Example 1, except that the treatment temperature was changed to 1200° C. in the production of the negative electrode active material.

[Reference example]
The Si powder was subjected to heat treatment at 250° C. for 3 hours under vacuum drying to prepare a negative electrode active material, and an evaluation cell was prepared in the same manner as in Example 1.

[evaluation]
(Infrared spectroscopic measurement and X-ray diffraction measurement)
Infrared spectroscopy was performed on the negative electrode active materials produced in Example 1, Comparative Examples 1 and 2, and Reference Example. In addition, X-ray diffraction (XRD) measurement was also performed for the negative electrode active materials produced in Example 1 and Comparative Examples 1 and 2. This was used to evaluate removal of the oxide film and crystallinity. The results are shown in FIGS. 2-4.

As shown in FIG. 2, in Example 1 and Reference Example, the broad peak at 3100 cm ⁻¹ to 3500 cm ⁻¹ confirmed in Comparative Example 1 disappeared. As a result, it was confirmed that the oxide film was removed from the negative electrode active materials of Example 1 and Reference Example.

As shown in FIG. 3, in Comparative Example 1 (untreated) and Example ¹ (600 ° C.), a peak of 1200 to 1000 cm derived from the Si—O skeleton was similarly obtained, but Comparative Example 2 (1200° C.), a change was observed in the above peak. Further, as shown in FIG. 4, the peaks seen over 2θ=47.2 to 47.4 were similar between Comparative Example 1 (untreated) and Example 1 (600° C.), but Comparative Example 2 (1200° C.) was sharp. That is, in Comparative Example 2, the half width was narrower than in Comparative Example 1 and Example 1. From this, it was confirmed that the method for producing a negative electrode active material according to the present disclosure does not affect the crystallinity of the obtained negative electrode active material.

(Battery performance evaluation)
Rate performance was evaluated for each evaluation cell produced in Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example. Specifically, it was evaluated as follows. First, at 25° C., the battery was charged at a constant current of 0.02 C to an upper limit voltage of 4.0 V, then discharged at a constant current of 0.02 C to a lower limit voltage of 2.5 V, and the initial discharge capacity was evaluated. Then, each battery was charged and discharged at a constant current of 2.0 C, and the discharge capacity was measured. The rate performance of the battery was evaluated by dividing the discharge capacity at 2C by the discharge capacity at 0.2C (2C/0.2C). The results are shown in FIG. In addition, impedance measurement was performed on each evaluation cell produced in Examples 1 to 3 and Comparative Examples 1 and 2 to evaluate the Li ion conductivity. The results are shown in FIG.

As shown in FIG. 5, it was confirmed that the batteries of Examples 1 to 3 and Reference Example had improved rate performance compared to the batteries of Comparative Examples 1 and 2.
Moreover, as shown in FIG. 6, it was confirmed that the battery of Example 1 exhibited ion conductivity equivalent to that of Comparative Example 1. Also, it was confirmed that the batteries of Examples 2 and 3 exhibit better ionic conductivity than those of Comparative Examples 1 and 2.

Claims (1)

A method for producing a negative electrode active material used in an all-solid-state battery,
a heat treatment step of subjecting a Si-based active material having an oxide film on its surface to a heat treatment at 150° C. or higher and 600° C. or lower for 1 hour or more and 24 hours or less in an atmosphere containing hydrogen gas to remove the oxide film ;
The Si-based active material is Si simple substance,
The method for producing a negative electrode active material , wherein the atmosphere containing hydrogen gas is an atmosphere containing only hydrogen gas .

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