JP2021097017A - Composite particle for negative electrode active material - Google Patents

Abstract

To provide a composite particle for a negative electrode active material capable of providing the negative electrode active material having small volume expansion accompanied by charging in the present disclosure.SOLUTION: Provided is a composite particle for a negative electrode active material in the present disclosure. The composite particle includes: a porous Si particle; and a sublimable filler filled in a void of the porous Si particle.SELECTED DRAWING: Figure 2

Description

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

The all-solid-state battery is a battery having a solid electrolyte layer between the positive electrode layer and the negative electrode layer, and has an advantage that the safety device can be easily simplified as compared with a liquid-based battery having an electrolytic solution containing a flammable organic solvent. Has. Further, among all-solid-state batteries, all-solid-state lithium-ion batteries are attracting attention because they have a high energy density because they utilize a battery reaction involving the movement of lithium ions.

As a negative electrode active material used for the negative electrode layer of a battery, a Si-containing active material (Si-containing active material) is known. The Si-containing active material has an advantage that the theoretical capacity per volume is large, but on the other hand, the volume change due to charging and discharging is large. Patent Document 1 discloses a negative electrode active material containing a carbon-based negative electrode active material and porous SiOx particles (0 ≦ x <2), and a large number of the negative electrode active materials are provided on the surface, surface, and inside of the SiOx particles. There is a description that the volume expansion rate is reduced and the battery life is improved by including pores.

Japanese Unexamined Patent Publication No. 2018-12066

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

In order to solve the above problems, in the present disclosure, there are composite particles for a negative electrode active material, which include porous Si particles and a sublimating filler filled in the voids of the porous Si particles. Provides composite particles.

According to the present disclosure, it is possible to provide composite particles capable of imparting a negative electrode active material having a small volume expansion due to charging.

The composite particles for the negative electrode active material in the present disclosure have the effect of being able to obtain a negative electrode active material having a small expansion and contraction due to charging and discharging.

It is the schematic sectional drawing which shows an example of the composite particle in this disclosure. It is a schematic cross-sectional view explaining the effect in this disclosure. It is schematic cross-sectional view explaining the state of the slurry and the negative electrode mixture layer produced by using the conventional porous Si particles having voids.

Hereinafter, the composite particles in the present disclosure will be described in detail.

A. Composite particles The composite particles in the present disclosure are used as a negative electrode active material. FIG. 1 (a) is a schematic view 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. 1A, the composite particle 10 in the present disclosure has a porous Si particle 1 and a sublimation filler 2 filled in the void P of the porous Si particle 1.

Conventionally, attempts have been made to alleviate the volume expansion coefficient associated with charging by forming a negative electrode mixture layer using a slurry in which porous Si particles having voids and a solvent, a binder, or the like are mixed. FIG. 3 is a schematic view showing the state of the slurry 16 and the negative electrode mixture layer 17 produced by using the porous Si particles 11 having voids as an active material. When the conventional porous Si particles 11 having voids are mixed with a solvent and a binder 13, a solid electrolyte 14, a conductive material 15, etc. to produce a slurry 16, the binder 13 dissolved in the solvent is the porous Si particles 11. It invades 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 (particularly, the inner surface of the pores) of the porous Si particles 11 (FIG. 3 (b)). Therefore, the voids are compressed by the binder 13'during charging and discharging, and the effect of alleviating the volume expansion and contraction due to the voids of the porous Si particles is not sufficient.

Further, 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 it is necessary to use a large amount of binder. Further, since the solvent penetrates into the voids, there is a problem that the amount of the solvent consumed is larger than that when the solid particles are used.

On the other hand, as shown in FIG. 1A, the composite particles 10 in the present disclosure have the voids of the porous Si particles 1 filled with the sublimating filler 2, and therefore the composite particles 10 in the present disclosure. Can be prevented from entering the voids of the porous Si particles when the solvent and the binder 3 are mixed with the solvent, the binder 3 and the like to produce the slurry 6 (FIG. 2A). Therefore, after the slurry is applied and dried to form the mixture layer forming layer 7A (FIG. 2B), the sublimation filler 2 is sublimated to maintain the voids P of the porous Si particles 1. The material layer 7B can be formed (FIG. 2 (c)). Therefore, according to the composite particles in 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 impart a negative electrode active material having a small volume expansion and contraction. Therefore, the cycle characteristics of the all-solid-state battery using this are improved. In FIG. 2, 4 indicates a solid electrolyte and 5 indicates a conductive material.

Further, since the composite particles in the present disclosure are filled with the voids of the porous Si particles, they can be slurried with almost the same amount of solvent as when the solid particles are used, and the solid particles are used. It is possible to improve the binding force with almost the same amount of binder as in the case of. Therefore, the amount of solvent and binder used can be reduced.

1. 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 have a function as an active material. By using the composite particles in 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 having a small volume expansion due to charging can be imparted. As a result, the cycle characteristics of the battery can be improved.

The porous Si particles have a porosity of, for example, 3% or more, 5% or more, 10% or more, or 20% or more. On the other hand, the proportion of voids may be, for example, 60% or less, 50% or less, 40% or less, or 30% or less. The method for calculating the porosity in the porous Si particles is not particularly limited, but for example, a scanning electron microscope (SEM) image of a cross section of the porous Si particles is observed, and the porosity (%) = 100 × (porosity). It can be calculated by (area) / (particle area). The average primary particle size of the porous Si particles is, for example, 50 nm or more, 100 nm or more, or 150 nm or more. On the other hand, the average primary particle diameter 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 observation by SEM.

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 measured by the constant volume adsorption method using nitrogen gas and analyzed by the BET method.

The porous Si particles include, for example, a preparatory step of preparing a precursor containing a Si element and a Li element, and a Li extraction step of extracting the Li element from the precursor to obtain the above-mentioned porous Si particles. It can be obtained by the method.

2. Sublimation filler The sublimation filler in the present disclosure is filled in the voids of the porous Si particles. Examples of the sublimable filler include those that can be sublimated by vacuum drying (vacuum drying).

Further, the sublimable filler preferably does not react with the solvent. Further, it is preferable that the voids of the porous Si particles can be easily filled. Sulfur is preferable as the sublimation filler so as to satisfy these conditions.
Sulfur is melted by heating and can be easily filled in the voids of the porous Si particles by reducing the pressure to the vapor pressure or higher. The melting point of sulfur is 119.5 ° C., and the vapor pressure is 10 Pa (135 ° C.).

3. 3. Composite Particles The composite particles in the present disclosure are porous Si particles filled with a sublimation filler in the voids. Hereinafter, a method for producing composite particles will be described. First, the porous Si particles and the sublimation filler are mixed and heated to a temperature equal to or higher than the melting point of the sublimation filler to melt the sublimation filler. Next, while stirring the mixture, the pressure is reduced to fill the voids of the porous Si particles with the sublimation filler. Then, the mixture is cooled to room temperature in a reduced pressure state, and the filler is solidified in the voids of the porous Si particles, whereby composite particles can be obtained. The filling conditions under reduced pressure can be equal to or higher than the melting point and higher than the vapor pressure because the sublimable filler is not evaporated or sublimated while being melted.

Regarding the mixing ratio of the porous Si particles and the sublimable filler, the volume ratio of the voids: the sublimable filler in the porous Si particles is, for example, 100: 80 to 100: 120, particularly about 100: 100. The mixing ratio can be as follows.

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 In the present disclosure, it is also possible to provide a negative electrode slurry having the above-mentioned composite particles. The slurry for a negative electrode contains the composite particles in 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 the solvent 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, heptane and dimethylbutane. Examples thereof include alkanes such as methylhexane and amines such as tributylamine and allylamine.

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

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, and among them, sulfide solid electrolytes are preferable. Examples of the sulfide solid electrolyte include Li element, X element (X is at least one of P, Si, Ge, Sn, B, Al, Ga, and In), and a solid electrolyte containing S element. Can be mentioned. Further, the sulfide solid electrolyte may further contain at least one of an O element and a halogen element.

If necessary, the negative electrode slurry may contain a conductive material in order to improve electron conductivity. Examples of the conductive material include carbon nanotubes such as VGCF (gas phase carbon fiber) and carbon materials such as carbon nanofibers.

C. Method for Producing Negative Electrode Mixture Layer In the present disclosure, it is possible to provide a method for producing a negative electrode mixture layer using the above-mentioned negative electrode slurry. Specifically, a coating step of applying 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 sublimating filler from the composite particles. It is possible to provide a process and a method for producing a negative electrode mixture layer having.

In the coating step, a coating film can be formed by applying the negative electrode slurry to the surface of the negative electrode current collector, for example, using an applicator or the like. Further, in the drying step, the mixture layer forming layer is formed by drying the solvent from the coating film. The drying step is determined by the type of solvent, and can be performed by, for example, warm air drying. Further, in the filler removing step, the negative electrode mixture layer is formed by sublimating and removing the sublimable filler from the composite particles in the mixture layer forming layer.

D. All-solid-state battery In the present disclosure, it is also possible to provide an all-solid-state battery having a negative electrode mixture layer obtained by the above-mentioned method for producing a negative electrode mixture layer. 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. Further, the all-solid-state battery has a positive electrode current collector that collects electricity from the positive electrode mixture layer and a negative electrode current collector that collects electricity from the negative electrode mixture layer.

The all-solid-state battery is preferably an all-solid-state lithium battery. Further, the all-solid-state battery in the present disclosure may be a primary battery or a secondary battery, and among them, a secondary battery is preferable. This is because it can be charged and discharged repeatedly and is useful as an in-vehicle battery, for example.

The present disclosure is not limited to the above embodiment. The above embodiment is an example, and any object having substantially the same structure as the technical idea described in the claims of the present disclosure and exhibiting the same effect and effect is the present invention. Included in the technical scope of the disclosure.

(Example)
[Manufacturing 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 depressurized at 0.05 MPa for 10 minutes while stirring, and the Si voids were filled with sulfur. Finally, the mixture was cooled to room temperature in the reduced pressure state, and sulfur was solidified in the filled state to obtain the desired sulfur-filled porous Si (composite particles). The amount of sulfur was set so that the volume ratio was 1: 1 with respect to the Si voids. The 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 so that sulfur was not evaporated or sublimated while being melted.

[Preparation of positive electrode structure]
Active material _{(LiNi 1/3 Mn 1/3 Co 1/3 O} 2) and weight ratio of the sulfide solid electrolyte _{(LiI-LiO 2 -Li 2 S} -P 2 S 5) is the active material: sulfide solid electrolyte = The mixture was mixed so as to be 70:30, and this was weighed so as to be 97.0 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer. Next, the PVDF binder was weighed to 1 part by weight and the conductive auxiliary agent (vapor-grown carbon fiber, manufactured by Showa Denko KK) was weighed to 2 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer. Further, butyl butyrate was added as a solvent so that the solid content ratio was 65 wt%, and the slurry was kneaded for 1 minute using an ultrasonic homogenizer (UH-50 manufactured by SMT Co., Ltd.) to prepare a slurry for a positive electrode. .. Then, the surface of the aluminum foil is coated with a slurry for a positive electrode using an applicator, and dried with warm air at 110 ° C. for 60 minutes to form a positive electrode mixture layer, and the positive electrode current collector and the positive electrode current collector and the positive electrode mixture layer are formed. A positive electrode structure having a positive electrode mixture layer was produced.

[Preparation of negative electrode structure]
Produced sulfur filled porous Si, and the weight ratio of the sulfide solid electrolyte _{(LiI-LiO 2 -Li 2 S} -P 2 S 5) sulfur filled porous Si: mixed in a 40: sulfide solid electrolyte = 60 Then, the weight 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, the PVDF binder was weighed to 2 parts by weight and the conductive auxiliary agent (vapor-grown carbon fiber, manufactured by Showa Denko KK) was weighed to 4 parts by weight with respect to 100 parts by weight of the negative electrode mixture layer forming layer. Further, butyl butyrate was added as a solvent so that the solid content ratio was 45 wt%, and the slurry was kneaded for 1 minute using an ultrasonic homogenizer (UH-50 manufactured by SMT Co., Ltd.) to prepare a slurry for a negative electrode. .. After that, the surface of the SUS foil, which is the negative electrode current collector, is coated with a slurry for the negative electrode using an applicator, 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. Further, sulfur was removed through a process of vacuum drying at 140 ° C. for 12 hours to form a negative electrode mixture layer, and a negative electrode structure having a negative electrode current collector and a negative electrode mixture layer was produced.

[Preparation of solid electrolyte layer]
97 parts by weight of the sulfide solid electrolyte and the PVDF binder solution were added so as to have a solid content of 3 parts by weight. Further, butyl butyrate, which is a solvent, was added so that the solid content was 50 wt%, and the mixture was kneaded with an ultrasonic homogenizer (UH-50 manufactured by SMT Co., Ltd.) to obtain a slurry for a solid electrolyte layer. Obtained. A slurry for a solid electrolyte layer was applied to an aluminum foil using an applicator, and dried with warm air at 110 ° C. for 60 minutes to obtain a solid electrolyte layer.

[Battery production]
A battery was obtained by stacking the positive electrode mixture layer and the negative electrode mixture layer so as to face each other with the solid electrolyte layer from which the aluminum foil was peeled off in an inert gas, and then pressing at 4.3 tons.

(Comparative Example 1)
The negative electrode active material is changed to porous Si (unfilled with sulfur) so that the mixture of the active material and the sulfide solid electrolyte is 91.0 parts by weight and the PVDF binder is 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 Examples except that butyl butyrate was added so as to have a solid content of 30 wt% to prepare a slurry for a negative electrode.
* When the solid content ratio was set to 45 wt%, which is the same as in the example, it was set to 30 wt% because the slurry had no fluidity and could not be coated. The amount of binder was also changed to 5 wt% at 2 wt%, which was the same as in the example, because the mixture had peeled off in the drying step after coating.

(Comparative Example 2)
A battery was obtained in the same manner as in Examples 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 a slurry for the negative electrode.}

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

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

2. Cell resistance Each test cell is charged with a constant current constant voltage up to 4.2 V at a current value of 0.1 C for 20 hours under a 25 ° C environment, and is discharged with a constant voltage for 10 hours up to 3.0 V at a current value of 0.2 C. went. After repeating this charge / discharge cycle three times, the battery was charged to 4.0 at 0.2 C. As a result, a lithium ion secondary battery used for cell measurement was produced. The resistance measurement was obtained by discharging the produced battery at 3C 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. 3. Cycle test Each test cell was charged with a constant current constant voltage up to 4.2 V at a current value of 2C for 1 hour under an environment of 60 ° C., and discharged with a constant voltage up to 3.0 V at a current value of 2C for 2 hours. This charge / discharge cycle was repeated 300 times. Then, the cell resistance was measured by the method of 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 had a specific surface area close to that of the solid Si of Comparative Example 2, and could be electrodeposited under the same conditions. Therefore, the voids of the porous Si were filled with sulfur, and the binder and the solvent became voids. It is probable that it did not invade. In the examples, since the slurry solid content was high, particle sedimentation in the slurry drying step was suppressed, and a uniform mixture layer was formed, so that the peel strength was improved to the same level as when solid Si was used (Comparative Example 2). did. Further, it is considered that the reason why the increase in resistivity was suppressed in Examples as compared with Comparative Examples 1 and 2 was that the effect of alleviating the volume expansion during the cycle became more remarkable because the voids were not pressed by the binder.

1 ... Porous Si particles 2 ... Sublimation filler 3 ... Solvents and binders 4 ... Solid electrolytes 5 ... Conductive materials 6 ... Slurries 7A ... Negative electrode composite layer 7B ... Negative electrode composite layer 10 ... Composite particles

Claims (1)

Composite particles for negative electrode active material
A composite particle having a porous Si particle and a sublimation filler filled in the voids of the porous Si particle.

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