JP2023031096A - secondary battery - Google Patents

Abstract

To improve the durability of a secondary battery using a silicon negative electrode active material.SOLUTION: A secondary battery 100 has a negative electrode 10, an electrolyte, and a positive electrode 30. The negative electrode 10 includes Si particles as a negative electrode active material. The Si particles have a clathrate structure. An average particle size of the Si particles is 0.7-8.9 μm.SELECTED DRAWING: Figure 1

Description

The present application discloses a secondary battery using a silicon-based negative electrode active material.

A battery using a silicon-based negative electrode active material is known. For example, Patent Document 1 discloses a lithium ion secondary battery using silicon oxide as a negative electrode active material. Further, Patent Document 2 discloses a lithium ion secondary battery using silicon clathrate II as a negative electrode active material.

JP 2020-113547 A Japanese Patent Application Laid-Open No. 2021-034279

Batteries using silicon-based negative electrode active materials have room for improvement in durability.

As one means for solving the above problems, the present application provides
A secondary battery, comprising a negative electrode, an electrolytic solution, and a positive electrode,
The negative electrode contains Si particles as a negative electrode active material,
The Si particles have a clathrate structure,
The Si particles have an average particle size of 0.7 μm or more and 8.9 μm or less,
A secondary battery is disclosed.

In ^the secondary battery ^of the present _disclosure , _the clathrate structure has a ratio I ₃₂₅ / I ₂₀₅ may be in the range of 1.03 to 1.21.

In the secondary battery of the present disclosure, the Si particles may have an average particle size of 2.8 μm or more and 8.9 μm or less.

The secondary battery of the present disclosure has excellent durability.

4 schematically shows the configuration of a secondary battery;

A secondary battery of the present disclosure has a negative electrode, an electrolytic solution, and a positive electrode. Here, the negative electrode contains Si particles as a negative electrode active material, the Si particles have a clathrate structure, and the Si particles have an average particle size of 0.7 μm or more and 8.9 μm or less. FIG. 1 schematically shows the configuration of a secondary battery 100 according to one embodiment. As shown in FIG. 1 , secondary battery 100 has negative electrode 10 , separator 20 and positive electrode 30 . Each of the negative electrode 10, the separator 20, and the positive electrode 30 may be in contact with or impregnated with an electrolytic solution (not shown).

1. Negative Electrode The negative electrode 10 contains Si particles as a negative electrode active material. As shown in FIG. 1, the negative electrode 10 may include a negative electrode active material layer 11 and a negative electrode current collector 12. In this case, the negative electrode active material layer 11 contains Si particles as the negative electrode active material. obtain.

1.1 Negative Electrode Active Material Layer The negative electrode active material layer 11 contains Si particles as a negative electrode active material, and may optionally contain a conductive aid, a binder, and the like. The content of each component in the negative electrode active material layer 11 is not particularly limited, and may be appropriately determined according to the intended performance of the battery. For example, when the entire negative electrode active material layer 11 (total solid content) is 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, or 100% by mass or less. Or it may be 90% by mass or less. The thickness of the negative electrode active material layer 11 is also not particularly limited, and may be, for example, 1 μm or more, 10 μm or more, or 30 μm or more, or 1 mm or less, 500 μm or less, or 100 μm or less.

1.1.1 Negative Electrode Active Material Si particles as the negative electrode active material have a clathrate structure. Since the Si particles have a clathrate structure, expansion and contraction of the Si particles due to charging and discharging of the battery are relatively small, and durability performance of the battery is likely to be improved. Whether or not the Si particles have a clathrate structure can be easily determined from the Raman spectrum or the like. In the present application, _the ratio I ₃₂₅ /I ₂₀₅ between the maximum peak intensity I 325 at 325 ± 10 cm ^-1 measured by Raman spectroscopy and the maximum peak intensity I 205 at 205 ± 10 cm ^-1 is _1.03 or more and 1.21. is within the range, the Si particles are considered to have a clathrate structure.

Si particles as the negative electrode active material have an average particle size of 0.7 μm or more and 8.9 μm or less. If the average particle size of the Si particles is too large, it becomes difficult to maintain the clathrate structure after charging and discharging the battery. This is presumed to be due to the following mechanism. That is, in a secondary battery, the reaction proceeds rapidly on the Si surface in contact with the electrolyte. It is thought that if the Si particles are too large, the specific surface area becomes small, the degree of reaction concentration increases, and the clathrate structure tends to collapse. According to the findings of the present inventors, such a problem can be easily avoided when the average particle size of the Si particles is 8.9 μm or less. The lower limit of the average particle size of Si particles is 0.7 μm or more, and may be 2.8 μm or more. The average particle size of the Si particles is the particle size (median size) at 50% of the integrated value in the volume-based particle size distribution determined by the laser diffraction/scattering method.

The negative electrode active material layer 11 may contain other negative electrode active materials in addition to the above Si particles as the negative electrode active material. For example, the negative electrode active material layer 11 may include silicon-based active materials such as Si, Si alloys, and silicon oxide other than clathrate silicon as other negative electrode active materials; carbon-based active materials such as graphite and hard carbon; and lithium titanate. Various oxide-based active materials; at least one selected from metal lithium, lithium alloys, and the like may be included.

1.1.2 Conductive Auxiliary Conductive agents include vapor-grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), carbon nanofiber (CNF), and the like. carbon materials; metallic materials such as nickel, aluminum, and stainless steel; The conductive aid may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive aid may be used alone, or two or more types may be used in combination.

1.1.3 Binder Binders include, for example, polyimide (PI)-based binders, butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate-butadiene rubber (ABR)-based binders, styrene-butadiene rubber (SBR)-based binders, Binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, and the like are included. In particular, when the negative electrode active material layer 11 contains polyimide as a binder, expansion and contraction of Si particles are more likely to be suppressed. Polyimide may be obtained by heating polyamic acid to cause an imidization reaction (dehydration/cyclization). The polyamic acid may be either aromatic or aliphatic, but preferably has an aromatic ring from the viewpoint of being a high-strength binder when polyimide is formed. Only one binder may be used alone, or two or more binders may be used in combination.

1.2 Negative Electrode Current Collector As the negative electrode current collector 12 , any one commonly used as a negative electrode current collector for secondary batteries can be employed. Further, the negative electrode current collector 12 may be in the form of foil, plate, mesh, punching metal, foam, or the like. The negative electrode current collector 12 may be a metal foil or metal mesh, or may be a carbon sheet. In particular, metal foil is excellent in handleability and the like. The negative electrode current collector 12 may consist of a plurality of foils or sheets. Examples of metals forming the negative electrode current collector 12 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, the negative electrode current collector 12 may contain at least one metal selected from Cu, Ni, and stainless steel from the viewpoint of ensuring reduction resistance and being difficult to alloy with lithium. The negative electrode current collector 12 may have some kind of coating layer on its surface for the purpose of adjusting the resistance. Further, the negative electrode current collector 12 may be a metal foil or a base material plated or vapor-deposited with the above metal. Moreover, when the negative electrode current collector 12 is composed of a plurality of metal foils, some layer may be provided between the plurality of metal foils. The thickness of the negative electrode current collector 12 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

2. Separator As shown in FIG. 1 , a separator 20 may be arranged between the negative electrode 10 and the positive electrode 30 . The separator 20 has a function of preventing contact between the negative electrode 10 and the positive electrode 30 and holding an electrolytic solution, which will be described later, to form an electrolyte layer. The separator may be a separator commonly used in secondary batteries, and examples thereof include those made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator may have a single-layer structure or a multi-layer structure. Examples of the separator having a multilayer structure include a separator having a two-layer structure of PE/PP, or a separator having a three-layer structure of PP/PE/PP or PE/PP/PE. The separator may be made of nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric. The thickness of the separator is not particularly limited, and may be, for example, 5 μm or more and 1 mm or less.

3. Positive Electrode The positive electrode 30 contains a positive electrode active material. As shown in FIG. 1, the positive electrode 30 may include a positive electrode active material layer 31 and a positive electrode current collector 32. In this case, the positive electrode active material layer 31 may contain the positive electrode active material.

3.1 Positive Electrode Active Material Layer The positive electrode active material layer 31 contains a positive electrode active material, and optionally may further contain a conductive aid, a binder, and the like. The content of each component in the positive electrode active material layer 31 is not particularly limited, and may be appropriately determined according to the intended performance of the battery. For example, when the entire positive electrode active material layer 31 (total solid content) is 100% by mass, the content of the positive electrode active material may be 50% by mass or more, 60% by mass or more, or 70% by mass or more, or 100% by mass or less. Or it may be 90% by mass or less. The thickness of the positive electrode active material layer 31 is not particularly limited either.

3.1.1 Positive Electrode Active Material As the positive electrode active material, a known positive electrode active material for secondary batteries may be used. Among known active materials, a material exhibiting a higher potential (charge/discharge potential) at which predetermined ions are occluded and released than the Si-based negative electrode active material can be used as the positive electrode active material. For example, when constructing a lithium ion battery, various kinds of lithium cobalt oxide, lithium nickel oxide, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , lithium manganate, spinel-based lithium compounds, etc. are used as the positive electrode active material. A lithium-containing composite oxide may also be used. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination. The positive electrode active material may be, for example, particulate, and its size is not particularly limited. The particles of the positive electrode active material may be solid particles or hollow particles. The particles of the positive electrode active material may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle size of the particles of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

3.1.2 Conductive Auxiliary Agent and Binder The conductive auxiliary agent and binder may be appropriately selected from those exemplified as those that can be contained in the negative electrode active material layer 11 . Each of the conductive aids and binders may be used alone, or two or more thereof may be used in combination.

3.2 Positive Electrode Current Collector As the positive electrode current collector 32, any of those commonly used as positive electrode current collectors for secondary batteries can be employed. Also, the positive electrode current collector 32 may be in the form of foil, plate, mesh, punching metal, foam, or the like. The positive electrode current collector 32 may be made of metal foil or metal mesh. In particular, metal foil is excellent in handleability and the like. The positive electrode current collector 32 may consist of a plurality of sheets of foil. Examples of metals forming the positive electrode current collector 32 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring oxidation resistance, etc., the positive electrode current collector 32 may contain Al. The positive electrode current collector 32 may have some kind of coating layer on its surface for the purpose of adjusting the resistance. Further, the positive electrode current collector 32 may be a metal foil or a base material plated or vapor-deposited with the above metal. Moreover, when the positive electrode current collector 32 is composed of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the positive electrode current collector 32 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

4. Electrolyte The electrolyte may contain, for example, lithium ions as carrier ions. The electrolytic solution may be an aqueous electrolytic solution or a non-aqueous electrolytic solution, but the performance of the non-aqueous electrolytic solution is particularly high. The composition of the electrolytic solution may be the same as that known as the composition of the electrolytic solution for secondary batteries. For example, as the electrolytic solution, a solution obtained by dissolving lithium salt in a carbonate-based solvent at a predetermined concentration can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and the like. Examples of lithium salts include LiPF ₆ and the like.

5. Other Configurations Secondary battery 100 may have each of the above-described configurations housed inside an exterior body. As the exterior body, any one known as an exterior body for a battery can be adopted. Also, a plurality of secondary batteries 100 may be arbitrarily electrically connected and arbitrarily stacked to form an assembled battery. In this case, the assembled battery may be housed inside a known battery case. The secondary battery 100 may also have other obvious configurations such as necessary terminals. Examples of the shape of the secondary battery 100 include a coin shape, a laminate shape, a cylindrical shape, a rectangular shape, and the like. As described above, the secondary battery 100 may be a lithium ion secondary battery, or may be any other secondary battery. However, in the case of a lithium ion secondary battery in particular, a higher effect is expected. can.

The secondary battery 100 can be manufactured by applying a known method. For example, it can be manufactured as follows. However, the method for manufacturing secondary battery 100 is not limited to the following method.
(1) A slurry for negative electrode layer is obtained by dispersing a negative electrode active material and the like constituting a negative electrode active material layer in a solvent. The solvent used in this case is not particularly limited, and water and various organic solvents can be used, and N-methylpyrrolidone (NMP) may be used. A negative electrode layer slurry is applied to the surface of a negative electrode current collector using a doctor blade or the like, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, thereby forming a negative electrode.
(2) A positive electrode layer slurry is obtained by dispersing the positive electrode active material and the like constituting the positive electrode active material layer in a solvent. The solvent used in this case is not particularly limited, and water and various organic solvents can be used, and N-methylpyrrolidone (NMP) may be used. The positive electrode layer slurry is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector to form a positive electrode.
(3) A laminate is obtained by sandwiching a separator between a negative electrode and a positive electrode, and having a negative electrode current collector, a negative electrode active material layer, a separator, a positive electrode active material layer, and a positive electrode current collector in this order. Other members such as terminals are attached to the laminate as necessary.
(4) The laminate is housed in a battery case, the battery case is filled with an electrolytic solution, the laminate is immersed in the electrolytic solution, and the laminate and the electrolytic solution are sealed in the battery case. Use the following battery. At the stage (3) above, the negative electrode active material layer, the separator, and the positive electrode active material layer may be impregnated with an electrolytic solution.

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

1. Preparation of Silicon Negative Electrode Active Material Crystalline silicon and clathrate silicon were prepared as silicon negative electrode active materials.

1.1 Preparation of Crystalline Silicon Si particles (Kojundo Chemical Co., Ltd., 5 μm) were pulverized using a planetary ball mill (Classic Line, manufactured by Fritsch) to obtain crystalline silicon powders having various average particle sizes. The crystalline silicon did not have a clathrate structure.

1.2 Preparation of Clathrate Silicon Si particles and Na particles were mixed at a molar ratio of 1:1 and heated at 700° C. to synthesize NaSi. After that, Na was removed by heating at 340°C. Furthermore, after further removing sodium by heating at 430° C., atomization was performed using a planetary ball mill to obtain powders having various average particle sizes. All of the obtained powders had a clathrate structure (clathrate silicon).

2. Preparation of positive electrode In a PP container, NMP as a solvent, a 5 wt% butyl butyrate solution of a PVDF-based binder, and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (average particle size 6 μm) as a positive electrode active material. and VGCF as a conductive additive were added and stirred for 10 minutes with an Awatori Mixer (ARE-310 manufactured by Thinky) to obtain a positive electrode slurry. The positive electrode slurry was applied onto an Al foil (manufactured by Showa Denko KK) by a blade method using an applicator. The coated electrode was dried on a hot plate at 80° C. for 60 minutes.

3. Preparation of negative electrode In a PP container, NMP as a solvent, polyamic acid (U varnish A, manufactured by Ube Industries) as a polyimide as a binder, a silicon active material as a negative electrode active material, and VGCF as a conductive aid and KB were added and stirred for 10 minutes with an Awatori Mixer (ARE-310 manufactured by Thinky Co.) to obtain a negative electrode slurry. An applicator was used to coat a Cu foil (manufactured by Furukawa Electric Co., Ltd.) by a blade method. The coated electrode was dried on a hot plate at 80° C. for 60 minutes.

4. Production of Battery The produced positive electrode was punched out with a diameter of 14 mm, while the negative electrode was punched out with a diameter of 16 mm. After that, the negative electrode was baked at 350° C. for 2 hours in an Ar atmosphere to cause the imidization reaction of the polyamic acid. Each electrode was moved to a glove box, and 1 ml of electrolytic solution (1.2 M LiPF ₆ solution; solvent was FEC:EC:EMC:DMC=1:2:4:3vol%) was placed on the active material layer of the negative electrode. After dripping, a separator (made of PP) was laminated, 1 ml of the electrolytic solution was dripped again, a positive electrode was laminated, and a coin cell was produced using an automatic coin cell crimping machine (manufactured by Hosen Co., Ltd.).

4.1 Comparative Example 1
In the coin cell, crystalline silicon having no clathrate structure was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 2.6 μm.

4.2 Comparative Example 2
In the coin cell, crystalline silicon having no clathrate structure was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 0.7 μm.

4.3 Comparative Example 3
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 25.0 μm.

4.4 Comparative Example 4
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 11.9 μm.

4.5 Example 1
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 2.8 μm.

4.6 Example 2
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 4.6 μm.

4.7 Example 3
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 0.7 μm.

4.8 Example 4
In the coin cell, clathrate silicon was used as the silicon negative electrode active material. The average particle size of the silicon negative electrode active material was 8.9 μm.

5. Charge-Discharge Cycle Test After the prepared coin cell was charged at a constant current of 0.2 mA to 4.2 V, the battery capacity when discharged to 2.5 V at 0.2 mA was used as the initial capacity, and the constant current was charged to 4.2 V at 2 mA. A charge/discharge test in which the battery was charged and then discharged at a constant current of 2 mA was repeated 50 times. After that, after constant current charging to 4.2 V at 0.2 mA, the battery capacity was measured when discharged to 2.5 V at 0.2 mA, and the ratio to the initial capacity was calculated. confirmed. Using the durability performance of Comparative Example 1 as a standard (100%), the durability performances of Comparative Examples 2 to 4 and Examples 1 to 4 were compared and evaluated.

In addition, the negative electrode after the initial charging and discharging was taken out and measured by Raman spectroscopy (wavelength: 532 nm). The obtained Raman spectrum was analyzed to calculate the ratio I ₃₂₅ /I ₂₀₅ between the maximum peak intensity I ₃₂₅ at 325±10 cm ⁻¹ and the maximum peak intensity I ₂₀₅ at 205±10 cm ⁻¹ . When the ratio I ₃₂₅ /I ₂₀₅ is in the range of 1.03 to 1.21, it can be said that the Si particles have a clathrate structure.

6. Evaluation Results Evaluation results are shown in Table 1 below.

The results shown in Table 1 reveal the following. First, from a comparison between Comparative Examples 1 and 2 and Examples 1 to 4, the use of clathrate silicon as the negative electrode active material is more durable than the use of crystalline silicon having no clathrate structure. It can be seen that the volatility increases. This is probably because clathrate silicon expands and contracts less than crystalline silicon due to charging and discharging of a battery.

Further, from a comparison between Comparative Examples 3 and 4 and Examples 1 to 4, even if clathrate silicon is used as the negative electrode active material, if the particle size is too large, the durability of the secondary battery is reduced. I understand. This is presumed to be due to the following mechanism. That is, in a secondary battery, the reaction proceeds rapidly on the Si surface in contact with the electrolyte. It is thought that if the Si particles are too large, the specific surface area becomes small, the degree of reaction concentration increases, and the clathrate structure tends to collapse. That is, in Comparative Examples 3 and 4, even if the clathrate structure was maintained after the initial charge and discharge, the clathrate structure gradually collapsed as the charge and discharge were repeated, and finally the capacity of the negative electrode significantly decreased. It is considered that

In the above examples, examples using specific positive electrodes and electrolytic solutions were shown, but the types of positive electrodes and electrolytic solutions are not particularly limited in the technique of the present disclosure. In addition, in the above examples, the negative electrode uses a specific current collector, binder, or conductive aid. However, in the technology of the present disclosure, the configuration of the negative electrode other than the negative electrode active material is not particularly limited. Based on the above presumed mechanism, it is considered that the durability as a secondary battery is improved by using a specific negative electrode active material in any configuration.

From the above results, it can be said that the secondary batteries having the following configurations (1) to (4) have excellent durability.
(1) The battery is a so-called electrolyte-based battery having a negative electrode, an electrolyte, and a positive electrode.
(2) The negative electrode contains Si particles as a negative electrode active material.
(3) Si particles have a clathrate structure.
(4) The Si particles have an average particle size of 0.7 μm or more and 8.9 μm or less.

10 negative electrode 11 negative electrode active material layer 12 negative electrode current collector 20 separator 30 positive electrode 31 positive electrode active material layer 32 positive electrode current collector 100 secondary battery

Claims (3)

A secondary battery, comprising a negative electrode, an electrolytic solution, and a positive electrode,
The negative electrode contains Si particles as a negative electrode active material,
The Si particles have a clathrate structure,
The Si particles have an average particle size of 0.7 μm or more and 8.9 μm or less,
secondary battery. The clathrate structure _{has a} ratio I ³²⁵ / _{I 205} between the maximum peak intensity I 325 at 325±10 cm ⁻¹ and the maximum peak intensity I ₂₀₅ at 205±10 cm −1 measured by Raman spectroscopy is _1.03 or more 1 is within the range of .21,
The secondary battery according to claim 1. The Si particles have an average particle size of 2.8 μm or more and 8.9 μm or less,
The secondary battery according to claim 1 or 2.

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