JP7318517B2 - Composite particles for negative electrode active material - Google Patents

Description

The present disclosure relates to composite particles for negative electrode active materials.

All-solid-state batteries are batteries that have a solid electrolyte layer between the positive electrode layer and the negative electrode layer. Compared to liquid-based batteries, which have an electrolyte containing a flammable organic solvent, the advantage is that it is easier to simplify the safety device. have In addition, among all-solid-state batteries, all-solid-state lithium-ion batteries are attracting attention because of their high energy density due to the use of battery reactions involving movement of lithium ions.

An active material containing Si (Si-containing active material) is known as a negative electrode active material used in the negative electrode layer of a battery. The Si-containing active material has the advantage of having a large theoretical capacity per volume, but on the other hand, it undergoes a large change in volume due to charging and discharging. Patent Document 1 discloses a negative electrode active material comprising a carbon-based negative electrode active material and porous SiOx particles (0≦x<2), and a large number of It is described that the inclusion of pores reduces the volume expansion rate and improves the life of the battery.

JP 2018-120866 A

In the negative electrode active material containing porous SiOx particles of Patent Document 1, the binder enters the pores (voids) during slurry production, and the binder remains in the pores even after the solvent is dried to obtain the negative electrode layer. The effect of mitigating the volume expansion rate associated with is not sufficient.
The present disclosure has been made in view of the above circumstances, and a main object of the present disclosure is to provide composite particles for a negative electrode active material that can obtain a negative electrode active material with small volume expansion due to charging.

In order to solve the above problems, the present disclosure provides composite particles for a negative electrode active material, comprising porous Si particles and a sublimable filler filled in the voids of the porous Si particles. A composite particle is provided.

ADVANTAGE OF THE INVENTION According to this indication, the composite particle which can provide the negative electrode active material with a small volume expansion accompanying charge can be provided.

The composite particles for a negative electrode active material according to the present disclosure have the effect of being able to obtain a negative electrode active material that undergoes little expansion and contraction during charging and discharging.

1 is a schematic cross-sectional view showing an example of composite particles in the present disclosure; FIG. It is a schematic sectional view explaining the effect in this indication. FIG. 3 is a schematic cross-sectional view illustrating the state of a slurry and a negative electrode mixture layer produced using conventional porous Si particles having voids.

The composite particles in the present disclosure will be described in detail below.

A. Composite Particles Composite particles in the present disclosure are used as a negative electrode active material. FIG. 1(a) is a schematic diagram showing an example of composite particles in the present disclosure, and FIG. 1(b) is a schematic diagram of a single porous Si particle used for the composite particles in the present disclosure. As shown in FIG. 1(b), the porous Si particle 1 has a plurality of voids (pores) P. As shown in FIG. Composite particles 10 in the present disclosure have porous Si particles 1 and sublimable fillers 2 filled in voids P of porous Si particles 1, as shown in FIG. 1(a).

Conventionally, attempts have been made to reduce the volume expansion rate associated with charging by forming a negative electrode mixture layer using a slurry obtained by mixing porous Si particles having voids, a solvent, a binder, and the like. FIG. 3 is a schematic diagram showing the state of a slurry 16 and a negative electrode mixture layer 17 produced using porous Si particles 11 having voids as an active material. Conventional porous Si particles 11 having voids are mixed with solvent and binder 13, solid electrolyte 14, conductive material 15, etc. to produce slurry 16, and binder 13 dissolved in the solvent dissolves porous Si particles 11. It penetrates into the void (Fig. 3(a)). In the negative electrode mixture layer 17 formed by applying the slurry and drying the solvent, the binder 13' remains in the voids (in particular, the inner surface of the pores) of the porous Si particles 11 (FIG. 3(b)). Therefore, the voids are pressed by the binder 13' during charging and discharging, and the effect of alleviating volume expansion and contraction due to the voids of the porous Si particles is not sufficient.

Moreover, when a slurry is produced using porous Si particles having voids as an active material, the specific surface area of the porous Si particles is large, and a large amount of binder must be used. Furthermore, since the solvent penetrates into the voids, there is a problem that the amount of solvent consumed is large compared to the case where solid particles are used.

On the other hand, as shown in FIG. 1A, the composite particles 10 according to the present disclosure are those in which the voids of the porous Si particles 1 are filled with the sublimation filler 2, so the composite particles 10 according to the present disclosure is mixed with the solvent, the binder 3, etc. to produce the slurry 6, the solvent and the binder 3 can be prevented from entering the pores of the porous Si particles (Fig. 2(a)). Therefore, after the slurry is applied and dried to form the mixture layer forming layer 7A (FIG. 2(b)), the sublimation filler 2 is sublimated to maintain the voids P of the porous Si particles 1. A material layer 7B can be formed (FIG. 2(c)). Therefore, according to the composite particles of the present disclosure, it is possible to prevent the voids of the porous Si particles from being pressed by the binder during charging and discharging, and it is possible to provide a negative electrode active material with small volume expansion and contraction. Therefore, an all-solid-state battery using this has improved cycle characteristics. In FIG. 2, 4 indicates a solid electrolyte and 5 indicates a conductive material.

Furthermore, since the composite particles in the present disclosure are filled with voids in the porous Si particles, they can be slurried with almost the same amount of solvent as when solid particles are used, and solid particles can be used. The binding force can be improved with approximately the same amount of binder. Therefore, the amount of solvent and binder used can be reduced.

1. Porous Si Particles Porous Si particles are primary particles, have a plurality of voids at least on the surface, usually on the surface and inside, and function as an active material. By using the composite particles of the present disclosure, it is possible to form a negative electrode mixture layer containing porous Si particles in which voids are maintained, so that a negative electrode active material with small volume expansion due to charging can be applied. As a result, it is possible to improve the cycle characteristics of the battery.

Porosity of the porous Si particles is, for example, 3% or more, may be 5% or more, may be 10% or more, or may be 20% or more. On the other hand, the ratio of voids is, for example, 60% or less, may be 50% or less, may be 40% or less, or may be 30% or less. The method for calculating the porosity in the porous Si particles is not particularly limited. area)/(particle area). The average primary particle size of the porous Si particles is, for example, 50 nm or more, may be 100 nm or more, or may be 150 nm or more. On the other hand, the average primary particle size of the porous Si particles is, for example, 3000 nm or less, may be 1500 nm or less, or may be 1000 nm or less. The average primary particle size can be determined, for example, by SEM observation.

The specific surface area of the porous Si particles is not particularly limited, but may be 20 m ² /g or more and 100 m ² /g or less. The specific surface area of the porous Si particles can be, for example, the BET specific surface area obtained by analyzing the surface area measured by the constant volume adsorption method using nitrogen gas by the BET method.

The porous Si particles have, for example, a preparation step of preparing a precursor containing Si element and Li element, and a Li extraction step of extracting Li element from the precursor to obtain the porous Si particles described above. method.

2. Sublimable Filler The sublimable filler in the present disclosure is filled in the voids of the porous Si particles. Examples of sublimable fillers include those that can be sublimated by drying under reduced pressure (vacuum drying).

Also, the sublimable filler preferably does not react with the solvent. Furthermore, it is preferable that the voids of the porous Si particles can be easily filled. Sulfur is preferable as a sublimable filler that satisfies these conditions.
Sulfur is melted by heating and can be easily filled into the pores of the porous Si particles by reducing the pressure to a level higher than the vapor pressure. Note that sulfur has a melting point of 119.5°C and a vapor pressure of 10 Pa (135°C).

3. Composite Particles The composite particles in the present disclosure are porous Si particles filled with a sublimable filler in the voids. A method for producing composite particles will be described below. First, porous Si particles and a sublimable filler are mixed and heated to the melting point of the sublimable filler or higher to melt the sublimable filler. Next, while stirring the mixture, the pressure is reduced to fill the sublimation filler into the voids of the porous Si particles. Then, the mixture is cooled to room temperature while the pressure is reduced to solidify the filler in the voids of the porous Si particles, thereby obtaining composite particles. Filling conditions under reduced pressure can be the melting point or higher and the vapor pressure or higher in order to melt the sublimable filler without evaporating or sublimating it.

The mixing ratio of the porous Si particles and the sublimable filler is such that the volume ratio of voids in the porous Si particles to the sublimable filler is, for example, 100:80 to 100:120, particularly about 100:100. A mixing ratio such as

The specific surface area of the composite particles is not particularly limited, but can be smaller than the specific surface area of the porous Si particles, and may be, for example, 10 m ² /g or more and 80 m ² /g or less.

B. Negative Electrode Slurry The present disclosure can also provide a negative electrode slurry having the composite particles described above. The negative electrode slurry contains the composite particles of the present disclosure, a solvent, a binder, and, if necessary, a solid electrolyte, a conductive material, and the like. The composite particles are as described above.

Examples of solvents include esters such as butyl butyrate, dibutyl ether and ethyl acetate; ketones such as diisobutyl ketone (DIBK), methyl ketone and methyl propyl ketone; aromatic hydrocarbons such as xylene, benzene and toluene; Examples include alkanes such as methylhexane, and amines such as tributylamine and allylamine.

The ratio of the solvent in the negative electrode slurry is, for example, 60 parts by weight or more, and may be 70 parts by weight or more, when the solid component of the negative electrode slurry is 100 parts by weight. On the other hand, the ratio of the solvent in the slurry is, for example, 120 parts by weight or less, and may be 110 parts by weight or less, when the solid component of the slurry is 100 parts by weight.

Examples of binders include rubber-based binders and fluoride-based binders. The content of the binder is preferably 1% by mass or more and 5% by mass or less based on 100% by mass of the total solid content contained in the negative electrode slurry.

The negative electrode slurry may contain a solid electrolyte, if necessary. Examples of the solid electrolyte include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes, among which sulfide solid electrolytes are preferred. 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.

The negative electrode slurry may optionally contain a conductive material to improve electronic conductivity. Examples of the conductive material include carbon materials such as carbon nanotubes such as VGCF (vapor grown carbon fiber) and carbon nanofibers.

C. Method for Producing Negative Electrode Mixed Material Layer In the present disclosure, it is possible to provide a method for producing a negative electrode mixed material layer using the negative electrode slurry described above. Specifically, a coating step of coating the negative electrode slurry to form a coating film, a drying step of removing the solvent from the coating film, and a filler removal of removing the sublimable filler from the composite particles. It is possible to provide a method for manufacturing a negative electrode mixture layer comprising the steps of:

In the coating step, the coating film can be formed by coating the negative electrode slurry on the surface of the negative electrode current collector using an applicator or the like, for example. In the drying step, the mixture layer forming layer is formed by drying the solvent from the coating film. The drying process is determined by the type of solvent, but can be carried out, for example, by hot air drying. Further, in the filler removing step, the sublimable filler is sublimated and removed from the composite particles in the mixture layer forming layer, thereby forming the negative electrode mixture layer.

D. All-solid-state battery In the present disclosure, it is also possible to provide an all-solid-state battery having the negative electrode mixture layer obtained by the method for manufacturing the negative electrode mixture layer described above. An all-solid-state battery usually has a positive electrode mixture layer, a negative electrode mixture layer, and a solid electrolyte layer formed between the positive electrode mixture layer and the negative electrode mixture layer. Furthermore, the all-solid-state battery has a positive electrode current collector that collects current for the positive electrode mixture layer and a negative electrode current collector that collects current for the negative electrode mixture layer.

The all-solid battery is preferably an all-solid lithium battery. In addition, the all-solid-state battery in the present disclosure may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because they can be repeatedly charged and discharged, and are useful, for example, as batteries for vehicles.

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 produces the same effect is the present invention. It is included in the technical scope of the disclosure.

(Example)
[Production of Composite Particles]
Porous Si (porosity 50%, 28 m ² /g) and sulfur were mixed at a weight ratio of 53:47 and heated to 125° C. to melt the sulfur. Next, the mixture was evacuated at 0.05 MPa for 10 minutes while stirring to fill the Si voids with sulfur. Finally, the mixture was cooled to room temperature while the pressure was reduced, and sulfur was solidified in the filling state to obtain the desired sulfur-filled porous Si (composite particles). The amount of sulfur was adjusted to a volume ratio of 1:1 with respect to the Si voids. Filling conditions under reduced pressure were set to a melting point (119.5° C.) or higher and a vapor pressure (10 Pa at 135° C.) or higher in order to melt sulfur without evaporating or sublimating it.

[Fabrication of positive electrode structure]
The weight ratio of the active material (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) and the sulfide solid electrolyte (LiI—LiO ₂ —Li ₂ SP ₂ S ₅ ) is active material:sulfide solid electrolyte= They were mixed at a ratio of 70:30 and weighed to 97.0 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer. Next, 1 part by weight of the PVDF binder and 2 parts by weight of the conductive aid (vapor growth carbon fiber, manufactured by Showa Denko KK) were weighed with respect to 100 parts by weight of the positive electrode mixture layer. Furthermore, butyl butyrate was added as a solvent so that the solid content was 65 wt%, and kneaded for 1 minute using an ultrasonic homogenizer (manufactured by SMT Co., Ltd. UH-50) to prepare a positive electrode slurry. . After that, the positive electrode slurry is applied to the surface of the aluminum foil using an applicator, and dried with hot air at 110 ° C. for 60 minutes to form a positive electrode mixture layer. A positive electrode structure having a positive electrode mixture layer was produced.

[Preparation of negative electrode structure]
The prepared sulfur-filled porous Si and the sulfide solid electrolyte (LiI—LiO ₂ —Li ₂ SP ₂ S ₅ ) were mixed so that the weight ratio was sulfur-filled porous Si:sulfide solid electrolyte=60:40. Then, it was weighed so as to be 94.0 parts by weight with respect to 100 parts by weight of the negative electrode mixture layer forming layer. Next, 2 parts by weight of the PVDF binder and 4 parts by weight of the conductive aid (vapor growth carbon fiber, manufactured by Showa Denko KK) were weighed with respect to 100 parts by weight of the negative electrode mixture layer forming layer. Furthermore, butyl butyrate was added as a solvent so that the solid content rate was 45 wt%, and kneaded for 1 minute using an ultrasonic homogenizer (manufactured by SMT Co., Ltd. UH-50) to prepare a negative electrode slurry. . After that, an applicator is used to apply the slurry for the negative electrode to the surface of the SUS foil, which is the negative electrode current collector, and the solvent is removed through a process of drying with warm air at 110° C. for 60 minutes to remove the negative electrode. A material layer forming layer was formed. Furthermore, sulfur was removed through a process of vacuum drying at 140° C. for 12 hours to form a negative electrode mixture layer, thereby producing a negative electrode structure having a negative electrode current collector and a negative electrode mixture layer.

[Preparation of Solid Electrolyte Layer]
97 parts by weight of a sulfide solid electrolyte and a PVDF binder solution were added so that the solid content was 3 parts by weight. Furthermore, butyl butyrate, which is a solvent, is added so that the solid content becomes 50 wt %, and the mixture is kneaded using an ultrasonic homogenizer (UH-50 manufactured by SMT Co., Ltd.) to obtain a solid electrolyte layer slurry. Obtained. An applicator was used to apply the solid electrolyte layer slurry to an aluminum foil, followed by hot air drying at 110° C. for 60 minutes to obtain a solid electrolyte layer.

[Production of battery]
In an inert gas, the solid electrolyte layer from which the aluminum foil was removed was sandwiched between the positive electrode mixture layer and the negative electrode mixture layer so that they faced each other, and then pressed at 4.3 t to obtain a battery.

(Comparative example 1)
The negative electrode active material was changed to porous Si (unfilled with sulfur), and the mixture of the active material and the sulfide solid electrolyte was 91.0 parts by weight and the PVDF binder was 5 parts by weight with respect to 100 parts by weight of the negative electrode mixture layer. A battery was obtained in the same manner as in Example except that butyl butyrate was added so that the solid content was 30 wt % to prepare a negative electrode slurry.
*When the solid content rate was set to 45 wt%, which was the same as in the example, the slurry did not have fluidity and could not be coated, so it was set to 30 wt%. The amount of binder was also changed to 5 wt% because the composite material peeled off during the drying process after coating at 2 wt%, which was the same as in the example.

(Comparative example 2)
A battery was obtained in the same manner as in Example except that the negative electrode active material was changed to solid Si particles (porous base material, particle size: 1 μm, 11 m ² /g) to prepare the negative electrode slurry.

(Evaluation method)
Evaluation methods used for evaluation in Examples and Comparative Examples are shown below. Table 1 shows the results.

1. Peel Strength An adhesive tape was attached to the obtained negative electrode structure, and the peel strength was measured by a 90° peel test. The results are shown as a ratio when Comparative Example 1 is set to 100.

2. Cell resistance Each test cell was subjected to constant current and constant voltage charging to 4.2 V at a current value of 0.1 C for 20 hours in a 25°C environment, and constant voltage discharging to 3.0 V at a current value of 0.2 C for 10 hours. gone. After repeating this charge/discharge cycle three times, the battery was charged at 0.2C to 4.0. In this way, a lithium ion secondary battery used in cell measurements was produced. The resistance measurement was obtained by discharging the prepared battery at 3 C for 10 seconds and dividing the voltage change by the current value (initial resistance). The results are shown as a ratio when Comparative Example 1 is set to 100.

3. Cycle Test Each test cell was subjected to constant-current and constant-voltage charging to 4.2 V at a current value of 2 C for 1 hour in a 60° C. environment, and constant-voltage discharging to 3.0 V at a current value of 2 C to 3.0 V for 2 hours. This charge/discharge cycle was repeated 300 times. After that, the cell resistance was measured by the evaluation method 2 (post-cycle resistance), and the resistance increase rate before and after the cycle was calculated.

The sulfur-filled porous Si of the example obtained a specific surface area close to that of the solid Si of Comparative Example 2, and could be formed into an electrode under the same conditions. presumably did not invade. In the example, since the slurry solid content is high, sedimentation of particles in the slurry drying process is suppressed, and a uniform composite layer is formed, so the peel strength is improved to the same level as when solid Si is used (Comparative Example 2). bottom. In addition, the reason why the increase in resistivity was suppressed in the example more than in the comparative examples 1 and 2 is considered to be that the voids are not pressed by the binder, so that the effect of mitigating the volume expansion during the cycle becomes more pronounced.

DESCRIPTION OF SYMBOLS 1... Porous Si particle 2... Sublimation filler 3... Solvent and binder 4... Solid electrolyte 5... Conductive material 6... Slurry 7A... Negative electrode mixture layer formation layer 7B... Negative electrode mixture layer 10... Composite particle

Claims (1)

A composite particle for a negative electrode active material contained in a negative electrode mixture layer of an all-solid lithium secondary battery ,
Porous Si particles and a sublimation filler filled in the voids of the porous Si particles ,
the sublimable filler is sulfur,
The composite particles, wherein the sublimation filler is 80 volume parts or more and 120 volume parts or less when the pores of the porous Si particles are 100 volume parts.