CN113036111A

CN113036111A - Negative electrode active material and battery

Info

Publication number: CN113036111A
Application number: CN202011300535.2A
Authority: CN
Inventors: 早稻田哲也; 若杉悟志; 吉田淳; 足立真辉
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2019-12-09
Filing date: 2020-11-19
Publication date: 2021-06-25
Anticipated expiration: 2040-11-19
Also published as: JP2021093343A; KR20210072700A; CN113036111B; JP7276218B2

Abstract

A main object of the present disclosure is to provide a negative electrode active material having a small volume change due to charge and discharge. The present disclosure solves the above problems by providing a negative electrode active material including primary particles having a Si phase, a MSi phase (M is a transition metal element) that is a metal silicide phase, and first voids.

Description

Negative electrode active material and battery

Technical Field

The present disclosure relates to a negative electrode active material and a battery.

Background

In recent years, development of batteries has been widely carried out. For example, the automotive industry is advancing the development of batteries for electric vehicles or hybrid vehicles. In addition, Si particles are known as an active material used for a battery.

For example, patent document 1 discloses porous silicon particles having voids as a negative electrode active material. Patent document 2 discloses particles in which an Si phase is dispersed in a metal silicide phase as a negative electrode active material. Patent document 3 discloses a composite particle containing a particle composed of an Si phase and a particle composed of a metal silicide phase as a negative electrode active material.

Documents of the prior art

Patent document 1 Japanese patent laid-open publication No. 2013-203626

Patent document 2 Japanese patent laid-open publication No. 2015-095301

Patent document 3 Japanese patent laid-open publication No. 2013-235682

Disclosure of Invention

The Si particles have a large theoretical capacity and are effective for increasing the energy density of the battery. On the other hand, Si particles change in volume greatly during charge and discharge.

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 having a small volume change due to charge and discharge.

In order to solve the above problem, the present disclosure provides a negative electrode active material including primary particles having a Si phase, a MSi phase (M is a transition metal element) that is a metal silicide phase, and first voids.

According to the present disclosure, the primary particles have the Si phase, the MSi phase that is the metal silicide phase, and the first voids, and therefore, a negative electrode active material with a small volume change due to charge and discharge can be formed.

In the above publication, the ratio of the transition metal element to the total of the Si element and the transition metal element contained in the primary particles may be 2 mol% or more and less than 50 mol%.

In the above disclosure, the proportion of the first voids in the primary particles may be 3% or more.

In the above disclosure, the transition metal element may be at least one of W, Mo, Cr, V, Nb, Fe, Ti, Zr, Hf and Os.

In the above disclosure, the transition metal element may be at least one of Cr, Ti, Zr, Hf and Os.

In the above disclosure, the negative electrode active material may be a secondary particle in which a plurality of the primary particles are aggregated and which has a second void.

In addition, the present disclosure provides a battery including a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer, wherein the negative electrode active material layer contains the negative electrode active material.

According to the present disclosure, since the negative electrode active material layer contains the negative electrode active material, a battery in which an increase in binding pressure due to charge and discharge is suppressed can be formed.

In the above disclosure, the negative electrode active material layer may contain the negative electrode active material, and β/α may be 0.02 to 0.5, where α (μm) is a thickness of the negative electrode active material layer in a stacking direction and β (μm) is an average particle diameter of the secondary particles.

According to the present disclosure, since the thickness of the negative electrode active material layer in the stacking direction and the average particle diameter of the secondary particles are in a predetermined relationship, a battery in which an increase in the binding pressure due to charge and discharge is further suppressed can be formed.

In the present disclosure, an effect of obtaining a negative electrode active material with a small volume change due to charge and discharge can be exhibited.

Drawings

Fig. 1 is a schematic cross-sectional view showing an example of the battery of the present disclosure.

Fig. 2 is a cross-sectional SEM image of the negative electrode active material (primary particles) obtained in example 4.

Fig. 3 is a cross-sectional SEM image of the negative electrode active material (primary particles) obtained in comparative example 4.

Fig. 4 is a graph showing mechanical strength obtained by the Phase-Field method.

Description of the reference numerals

1 … positive electrode active material layer

2 … electrolyte layer

3 … negative electrode active material layer

4 … positive electrode collector

5 … negative electrode current collector

6 … Battery case

10 … battery

Detailed Description

Hereinafter, the negative electrode active material and the battery of the present disclosure are described in detail.

A. Negative electrode active material

The anode active material of the present disclosure includes primary particles having a Si phase, a metal silicide phase, namely, an MSi phase (M is a transition metal element), and first voids.

The battery is required to have high energy density. Therefore, it has been studied to use an Si material having excellent capacity characteristics as an active material. However, when a Si material having a diamond crystal structure is used in an all-solid battery, cracks are generated in the electrode during repeated charge and discharge, and the battery life may be shortened. In addition, since a large stress is applied to the binding member of the battery in the charged state, a binding structure having high rigidity is required, and the binding jig may be increased in size. As a result, the energy density of the entire battery may be reduced. Therefore, various studies have been made to suppress the expansion of the Si material due to charge and discharge.

For example, patent document 1 discloses Si particles having voids inside, and patent document 3 discloses composite particles having voids between primary particles. However, these voids may be crushed under the pressing pressure at the time of manufacturing the electrode, and there is room for improvement in suppressing the expansion of Si particles. Patent document 2 discloses that particles of an Si phase are dispersed in a metal silicide phase, but there is a possibility that cracks may occur at the interface between the metal silicide phase and the Si phase during charging, and there is room for improvement in suppressing expansion of Si particles.

On the other hand, the primary particles of the anode active material of the present disclosure have a Si phase, a metal silicide phase, i.e., an MSi phase, and first voids. Thus, even if a press is applied during the production of the electrode, the metal silicide phase functions as a pillar, and the first voids can be maintained. As a result, expansion of the Si particles can be suppressed. In general, when the primary particles have voids (first voids), the resistance tends to increase. However, since the metal silicide phase has electron conductivity, even if the first voids are present, the increase in resistance can be suppressed.

1. Primary particles

The primary particle of the present disclosure has a Si phase, a metal silicide phase, i.e., an MSi phase, and first voids.

(1) Phase of Si

The primary particles in the present disclosure have a Si phase. The Si phase is a phase of Si simple substance.

The area ratio (%) of the Si phase in the primary particles may be, for example, 30% or more, 40% or more, or 50% or more. On the other hand, the area ratio of the Si phase may be 90% or less, 80% or less, or 70% or less, for example. The area ratio of the Si phase may be smaller than, the same as, or larger than that of the MSi phase described later, but from the viewpoint of capacity, it is preferable that the area ratio of the Si phase is larger.

The area ratio of the Si phase can be determined by SEM (scanning electron microscope) observation, for example. The cross section of the negative electrode active material was observed by SEM to obtain a photograph of the particles. From the obtained photograph, the Si phase was strictly differentiated by image analysis software to determine the area. Then, the area ratio (%) was calculated according to the following formula. The number of samples is preferably large, for example, 20 or more, 30 or more, 50 or more, or 100 or more.

Area ratio (%) ═ 100 × (Si phase area)/(primary particle area)

(2) MSi phase

The primary particles in the present disclosure have an MSi phase. The MSi phase is a metal silicide phase, and M is a transition metal element described later. The ratio of M (transition metal element) and Si (Si element) in the MSi phase is not particularly limited as long as the ratio is a ratio at which metal silicide can be formed.

The transition metal element M in the MSi phase is preferably at least one of W, Mo, Cr, V, Nb, Fe, Ti, Zr, Hf, and Os. Among these elements, at least one of Cr, Ti, Zr, Hf and Os is particularly preferable. The transition metal element M in the MSi phase may be only one element described above, or may be two or more elements.

The proportion of the transition metal element M is, for example, 2 mol% or more, 5 mol% or more, 10 mol% or more, or 20 mol% or more based on the total of the Si element and the transition metal element M contained in the primary particles. On the other hand, the proportion of the transition metal element M is, for example, less than 50 mol%, may be 40 mol% or less, and may be 30 mol% or less. The "Si element contained in the primary particles" includes both the Si element in the Si phase and the Si element in the MSi phase. The proportion of the transition metal element M can be confirmed by, for example, high-frequency Inductively Coupled Plasma (ICP) emission spectroscopy.

The area ratio (%) of the MSi phase in the primary particles is, for example, 5% or more, may be 10% or more, or may be 20% or more. On the other hand, the area ratio of the MSi phase is, for example, 60% or less, or may be 50% or less, or may be 40% or less. The method of calculating the area ratio of the MSi phase can be calculated by the following calculation formula, as in the case of the area ratio of the Si phase.

Area ratio (%) ═ 100 × (MSi phase area)/(primary particle area)

(3) First gap

The primary particles in the present disclosure have a first void. The proportion of the first voids (first void ratio) is, for example, 3% or more, may be 5% or more, may be 10% or more, and may be 20% or more. On the other hand, the proportion of the first voids is, for example, 60% or less, may be 50% or less, may be 40% or less, and may be 30% or less. The proportion of the first voids can be determined by the following calculation formula in the same manner as the area ratio of the Si phase.

First porosity (%) < 100 × (first void area)/(primary particle area)

The first void area in the above formula may be, for example, 1nm²Above, it may be 5nm²Above, it may be 10nm²Above, it may be 100nm²The above. On the other hand, the first void area in the above formula may be, for example, 500nm²The particle size may be 300nm²The following. The first void area may be calculated using image analysis software as described above.

In addition, the negative electrode active material of the present disclosure preferably maintains the above-described first porosity in an electrode and a battery. Since this can suppress swelling also in the electrode and the battery.

The position of the first void in the primary particle is not particularly limited, and may be in the vicinity of the surface of the primary particle or in the vicinity of the center of the primary particle.

(4) Primary particles

The composition of the primary particles of the present disclosure is not particularly limited if it has the above elements, phases, and first voids.

The phase of the primary particle may be present as one continuous phase or a plurality of discontinuous phases, but the latter is preferable. Since this can suppress local expansion of the anode active material. When the Si phase, the MSi phase, and the first voids are present as a plurality of discontinuous phases, the cross section of the primary particles can be observed as a pattern in which the Si phase, the MSi phase, and the first voids are dispersed, for example, in a speckled pattern. In the relationship between the Si phase and the MSi phase, the cross section of the primary particle can be observed as a sea-island structure in which the Si phase is a sea and the metal silicide phase (MSi phase) is an island. In the case of the sea-island structure, the number of MSi phases (islands) in the primary particle may be 10 or more, 50 or more, or 100 or more. On the other hand, the number of islands is, for example, 500 or less. Further, the area of the island may be, for example, 1nm²Above, it may be 5nm²Above, it may be 10nm²Above, it may be 100nm²The above. On the other hand, the area of the island is, for example, 500nm²The particle size may be 300nm²The following. The first void area may be calculated, for example, using image analysis software as described above.

The primary particles may have only the Si phase, the MSi phase, and the first voids, or may have another phase. Examples of the other phase include a Li phase. The proportion (area ratio) of the other phase in the primary particles may be, for example, 5% or less, 3% or less, or 1% or less.

The average primary particle diameter of the negative electrode active material is, for example, 50nm or more, may be 100nm or more, or may be 150nm or more. On the other hand, the average primary particle size of the negative electrode active material is, for example, 3000nm or less, 1500nm or less, or 1000nm or less. The average primary particle diameter of the negative electrode active material can be determined by observation with an SEM, for example. The number of samples is preferably large, and is, for example, 20 or more, 50 or more, or 100 or more. The average primary particle size of the negative electrode active material can be appropriately adjusted by, for example, appropriately changing the production conditions of the negative electrode active material and/or performing classification treatment.

2. Secondary particles

The negative electrode active material of the present disclosure may be a secondary particle in which a plurality of the primary particles are aggregated and which has a second void. When the anode active material is the secondary particle, the expansion of the anode active material layer and the battery can be further suppressed by the void (first void) of the primary particle and the second void of the secondary particle. The second voids are voids excluding voids of the primary particles (first voids) among voids inside the secondary particles observed when the cross-sectional view of the secondary particles is taken.

The proportion of the second voids (second porosity) 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 second porosity is, for example, 60% or less, may be 50% or less, may be 40% or less, and may be 30% or less. The second porosity can be obtained by the following calculation formula in the same manner as the porosity of the primary particles (first porosity).

Second porosity (%) < 100 × (second void area)/(secondary particle area)

In addition, the average particle diameter (β) of the secondary particles is preferably a value such that β/α satisfies a predetermined range in relation to the thickness (α) of the negative electrode active material layer in the stacking direction, which will be described later. β/α is, for example, 0.02 or more, and may be 0.1 or more, or may be 0.2 or more. On the other hand, β/α is, for example, 0.5 or less, and may be 0.4 or less, or may be 0.3 or less. Specific ranges for the average particle diameter of the secondary particles will be described later.

3. Negative electrode active material

The negative electrode active material of the present disclosure includes the above-described primary particles. The negative electrode active material may be the primary particles described above, or may be secondary particles in which the primary particles are aggregated. In addition, the structure may be a tertiary particle structure having a plurality of domains of secondary particles.

The negative electrode active material of the present disclosure may have a particle shape, for example. The average secondary particle diameter (β) of the negative electrode active material of the present disclosure is, for example, 1 μm or more, 2 μm or more, 5 μm or more, or 7 μm or more. On the other hand, the average secondary particle size of the negative electrode active material is, for example, 60 μm or less, may be 40 μm or less, and is particularly preferably 20 μm or less. The measurement and adjustment of the average secondary particle diameter are the same as those of the primary particles described above.

The negative active material of the present disclosure is generally used for a battery. The battery is described in detail in "b battery" described later.

The method for producing the negative electrode active material of the present disclosure is not particularly limited, and examples thereof include a method comprising: a preparation step of preparing a precursor containing an Si element, a transition metal element, and an Li element; and a Li extraction step of extracting Li element from the precursor to obtain the negative electrode active material.

The preparation step is a step of preparing a precursor containing an Si element, a transition metal element, and an Li element. The precursor may be purchased or may be prepared by itself. In the case of self-preparation, for example, a method of mixing an Si element source and a transition metal element source to prepare an Si alloy, and adding and mixing an Li element source to the Si alloy is exemplified. The amounts of the Si element and the transition metal element contained in the precursor are not particularly limited as long as the negative electrode active material of the present disclosure can be obtained, and may be appropriately adjusted. The amount of Li element is not particularly limited. The transition metal element is the above-mentioned transition metal element M.

The Li extraction step is a step of extracting Li element from the precursor to form a first void, thereby obtaining the negative electrode active material. Examples of the method for extracting Li include a method in which a Li extraction solvent such as ethanol or acetic acid is allowed to react with a precursor for a predetermined time.

Through the above-described steps, a negative electrode active material as a primary particle can be produced. The porosity of the primary particles can be adjusted by changing the amount of Li element in the precursor in the preparation step, the type of Li extraction solvent in the Li extraction step, and the reaction time between the Li extraction solvent and the precursor, for example. For example, the porosity can be increased by increasing the amount of Li element in the precursor.

In addition, the present disclosure can provide a method for producing a negative electrode active material in which a plurality of the primary particles are aggregated and secondary particles having second voids are produced, the method including: a preparation step of preparing a micelle solution containing water, a surfactant, a template material, and a polymerization initiator; a polymerization reaction step of adding the primary particles to the micelle solution to perform a polymerization reaction; and a heat treatment step of heat-treating the micelle solution after the polymerization reaction. By such a method, a negative electrode active material having second voids inside the secondary particles can be produced. The negative electrode active material of the secondary particles may be produced by adding a solvent such as butyl butyrate to a powder material containing the primary particles and a binder, and granulating the mixture.

B. Battery with a battery cell

Fig. 1 is a schematic cross-sectional view showing an example of a battery according to the present disclosure. The battery 10 shown in fig. 1 has: a positive electrode active material layer 1, a negative electrode active material layer 2, an electrolyte layer 3 formed between the positive electrode active material layer 1 and the negative electrode active material layer 2, a positive electrode current collector 4 for collecting current from the positive electrode active material layer 1, a negative electrode current collector 5 for collecting current from the negative electrode active material layer 2, and a battery case 6 housing these components. In the present disclosure, the anode active material layer 2 contains the anode active material described above.

According to the present disclosure, a battery in which the negative electrode active material layer contains the negative electrode active material described above can be formed, and the increase in binding pressure due to charge and discharge can be suppressed.

1. Negative electrode active material layer

The negative electrode active material layer is a layer containing at least the negative electrode active material. The negative electrode active material is the same as that described in the above "a.

The negative electrode active material layer may contain only the above-described negative electrode active material as a negative electrode active material, or may further contain another negative electrode active material. In the latter case, the proportion of the negative electrode active material in the entire negative electrode active material may be, for example, 50 wt% or more, 70 wt% or more, or 90 wt% or more.

The proportion of the negative electrode active material in the negative electrode active material layer is, for example, 20 wt% or more, 30 wt% or more, or 40 wt% or more. On the other hand, the proportion of the negative electrode active material is, for example, 80 wt% or less, may be 70 wt% or less, and may be 60 wt% or less.

In addition, the anode active material layer may further contain at least one of a solid electrolyte, a conductive material, and a binder as necessary. The type of the solid electrolyte is described in detail in "electrolyte layer 3" described later. The proportion of the solid electrolyte in the negative electrode active material layer is, for example, 1 wt% or more, may be 10 wt% or more, and may be 20 wt% or more. On the other hand, the proportion of the solid electrolyte in the negative electrode active material layer is, for example, 60 wt% or less, and may be 50 wt% or less.

As the conductive material, carbon materials and metal particles can be exemplified. Specific examples of the carbon material include particulate carbon materials such as Acetylene Black (AB) and Ketjen Black (KB), and fibrous carbon materials such as carbon fibers, Carbon Nanotubes (CNT), Carbon Nanofibers (CNF), and Vapor Grown Carbon Fibers (VGCF). Examples of the metal particles include Ni, Cu, Fe, and SUS. The proportion of the conductive material in the anode active material layer is, for example, 1 wt% or more, and may be 5 wt% or more. On the other hand, the proportion of the conductive material is, for example, 30 wt% or less, and may be 20 wt% or less.

Examples of the binder include rubber binders such as butadiene rubber, hydrogenated butadiene rubber, Styrene Butadiene Rubber (SBR), hydrogenated styrene butadiene rubber, nitrile butadiene rubber, hydrogenated nitro butadiene rubber, and ethylene propylene rubber, fluorinated binders such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene, and fluororubber, polyolefin thermoplastic resins such as polyethylene, polypropylene, and polystyrene, imide resins such as polyimide and polyamideimide, amide resins such as polyamide, acrylate resins such as polymethacrylate and polyethylacrylate, and methacrylic resins such as polymethyl methacrylate and polyethyl methacrylate. The proportion of the binder in the anode active material layer is, for example, 1 wt% or more and 30 wt% or less.

The thickness of the negative electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less. In the case where the negative electrode active material contains the secondary particles in the negative electrode active material layer, the thickness (α) of the negative electrode active material layer is preferably a value at which β/α satisfies a predetermined numerical range.

2. Positive electrode active material layer

The positive electrode active material layer is a layer containing at least a positive electrode active material. In addition, the positive electrode active material layer may further contain at least one of a solid electrolyte, a conductive material, and a binder as necessary.

As the positive electrode active material, an oxide active material can be exemplified. As an oxide active material used for a lithium ion battery, for example, LiCoO is exemplified₂、LiMnO₂、Li₂NiMn₃O₈、LiVO₂、LiCrO₂、LiFePO₄、LiCoPO₄、LiNiO₂、LiNi_1/3Co_1/3Mn_1/3Mn_1/3O₂And the like oxide active materials. Further, for example, LiNbO-containing materials may be formed on the surfaces of these active materials₃A coating layer of Li ion conductive oxide.

The proportion of the positive electrode active material in the positive electrode active material layer is, for example, 20 wt% or more, 30 wt% or more, or 40 wt% or more. On the other hand, the proportion of the positive electrode active material is, for example, 80 wt% or less, may be 70 wt% or less, and may be 60 wt% or less.

The types and proportions of the solid electrolyte, the conductive material, and the binder used in the positive electrode active material layer are the same as those described in the above "1. negative electrode active material layer", and therefore, the description thereof is omitted.

The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

3. Electrolyte layer

The electrolyte layer is a layer formed between the positive electrode active material layer and the negative electrode active material layer. The electrolyte constituting the electrolyte layer may be a liquid electrolyte (electrolytic solution) or a solid electrolyte, but the latter is preferable.

Typical examples of the solid electrolyte include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes; polymer electrolytes and the like.

As the sulfide solid electrolyte having lithium ion conductivity, for example, a solid electrolyte containing Li element, X element (X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and S element is exemplified. In addition, the sulfide solid electrolyte may further contain at least one of an O element and a halogen element. Examples of the halogen element include F element, Cl element, Br element, and I element.

As the sulfide solid electrolyte, for example, Li is mentioned₂S-P₂S₅、Li₂S-P₂S₅-LiI、Li₂S-P₂S₅-GeS₂、Li₂S-P₂S₅-Li₂O、Li₂S-P₂S₅-Li₂O-LiI、Li₂S-P₂S₅-LiI-LiBr、Li₂S-SiS₂、Li₂S-SiS₂-LiI、Li₂S-P₂S₅-LiBr、Li₂S-SiS₂-LiCl、Li₂S-SiS₂-B₂S₃-LiI、Li₂S-SiS₂-P₂S₅-LiI、Li₂S-B₂S₃、Li₂S-P₂S₅-Z_mS_n(wherein m and n are positive numbers, Z is any one of Ge, Zn and Ga), Li₂S-GeS₂、Li₂S-SiS₂-Li₃PO₄、Li₂S-SiS₂-Li_xMO_y(wherein x and y are positive numbers. M is any of P, Si, Ge, B, Al, Ga, In.).

Examples of the oxide solid electrolyte having lithium ion conductivity include solid electrolytes containing Li element, Y element (Y is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, and S), and O element. Specific examples thereof include Li₇La₃Zr₂O₁₂、Li_7-xLa₃(Zr_2-xNb_x)O₁₂(0≤x≤2)、Li₅La₃Nb₂O₁₂Isogarnet-type solid electrolytes; (Li, La) TiO₃、(Li,La)NbO₃、(Li,Sr)(Ta,Zr)O₃An isoperovskite type solid electrolyte; li (Al, Ti) (PO)₄)₃、Li(Al,Ga)(PO₄)₃Sodium super ion conductor (NASICON) type solid electrolytes of the like; li₃PO₄、LIPON(Li₃PO₄A compound in which a part of O is replaced with N) or other Li — P — O-based solid electrolyte; li₃BO₃、Li₃BO₃A Li-B-O solid electrolyte such as a compound in which a part of O in the electrolyte is replaced with C.

The binder is the same as that described in "1. negative electrode active material layer", and therefore, the description thereof is omitted.

4. Other constructions

The battery of the present disclosure has at least the negative electrode active material layer, the positive electrode active material layer, and the electrolyte layer described above. In general, the battery includes a positive electrode current collector for collecting the positive electrode active material layer and a negative electrode current collector for collecting the negative electrode active material layer. As the material of the positive electrode collector, SUS, aluminum, nickel, iron, titanium, and carbon may, for example, be mentioned. On the other hand, as the material of the anode current collector, for example, SUS, copper, nickel, and carbon may be cited.

In addition, the battery of the present disclosure may further have a binding jig that applies a binding pressure to the positive electrode active material layer, the electrolyte layer, and the negative electrode active material layer in a thickness direction. As the binding jig, a known jig can be used. The binding pressure is, for example, 0.1MPa or more, and may be 1MPa or more, or 5MPa or more. On the other hand, the binding pressure is, for example, 100MPa or less, and may be 50MPa or less, and may be 20MPa or less.

5. Battery with a battery cell

The battery of the present disclosure is preferably a lithium ion battery. The battery of the present disclosure may be a liquid battery or an all-solid battery, but the latter is preferable. Because it can better enjoy the effect of the present disclosure of suppressing the bound pressure rise due to charge and discharge.

In addition, the battery of the present disclosure may be either a primary battery or a secondary battery, with the secondary battery being preferred. Since it can be repeatedly charged and discharged, it is useful as a vehicle-mounted battery, for example. The secondary battery also includes use of the secondary battery as a primary battery (use for the purpose of only primary charging).

The battery of the present disclosure may be a single cell or a stacked battery. The laminated battery may be a unipolar laminated battery (parallel laminated battery) or a bipolar laminated battery (series laminated battery). Examples of the shape of the battery include a coin shape, a laminate shape, a cylindrical shape, and a square shape.

The present disclosure is not limited to the above embodiments. The above-described embodiments are examples, and have substantially the same technical idea as described in the scope of the claims of the present disclosure, and all of the embodiments that exhibit the same effects are included in the technical scope of the present disclosure.

Examples

[ example 1]

(Synthesis of negative electrode active Material (Primary particle))

(1) Preparation of metal silicide

2-5 mm bulk Si (high purity chemical) and 2-5 mm bulk transition metal (Cr) are mixed and melted by argon arc to form ingots. The ratio of the Si element and the Cr element is set to (100-X) Si — (X) M in table 1 below, where X is 2. The ingot was crushed in a tungsten mortar and then crushed in a ball mill to prepare Si alloy powder (metal silicide).

(2) Production of porous metal silicide (negative electrode active material)

0.60g of metallic Li (made of Bencheng metal) was added to 0.65g of Si contained in the prepared Si alloy powder. Then, the mixture was mixed in an Ar atmosphere with an agate mortar to obtain a LiSi precursor. To 1.0g of the obtained LiSi precursor, 250ml of ethanol (manufactured by NACALII TESSQUE) at 0 ℃ was added, and the mixture was reacted in a glass reactor under an Ar atmosphere for 120 minutes. Then, the liquid and the solid reactant a were separated by suction filtration, and the solid reactant a was recovered. 50ml of acetic acid (manufactured by NACALA TESSQUE) was added to 0.5g of the recovered solid reactant A, and the mixture was reacted in a glass reactor under an atmospheric atmosphere for 60 minutes. Then, the liquid and the solid reactant B were separated by suction filtration, and the solid reactant B was recovered. The solid reactant B was vacuum-dried at 100 ℃ for 2 hours to prepare a porous metal silicide (negative electrode active material).

(production of evaluation Battery)

0.7g of 0.75Li as a solid electrolyte₂S-0.25P₂S₅0.6g of the negative electrode active material having an average particle diameter (average primary particle diameter) of 3 μm, 0.06g of VGCF as a conductive material, and 0.24g of a butyl butyrate solution containing 5 wt% of PVDF resin as a binder were added to a polypropylene container. After the container was subjected to ultrasonic treatment for 30 seconds in an ultrasonic dispersion device, it was subjected to vibration treatment for 30 minutes using a vibrator,a negative electrode mixture was obtained. The obtained negative electrode mixture was applied to a current collector (copper foil) by a doctor blade method using an applicator, and naturally dried for 60 minutes. The negative electrode precursor thus obtained was dried on a hot plate adjusted to 100 ℃ for 30 minutes to prepare a negative electrode.

0.3g of 0.75Li as a solid electrolyte₂S-0.25P₂S₅2g of LiNi as a positive electrode active material_1/3Co_1/ ₃Mn_1/3O₂0.03g of VGCF as a conductive material, and 0.3g of a butyl butyrate solution containing 5 wt% of PVDF resin as a binder were added to a polypropylene container. The container was subjected to ultrasonic treatment in an ultrasonic dispersion device for 30 seconds, and then subjected to vibration treatment for 30 minutes using a vibrator, thereby obtaining a positive electrode mixture. The obtained positive electrode mixture was applied to a current collector (aluminum foil) by a doctor blade method using an applicator, and naturally dried for 60 minutes. The positive electrode precursor thus obtained was dried on a hot plate adjusted to 100 ℃ for 30 minutes to prepare a positive electrode.

0.4g of 0.75Li having an average particle diameter of 2 μm as a solid electrolyte₂S-0.25P₂S₅And 0.05g of a heptane solution containing 5% by weight of an ABR-based resin as an adhesive was added to the polypropylene container. The container was subjected to ultrasonic treatment in an ultrasonic dispersion device for 30 seconds, and then subjected to vibration treatment for 30 minutes using a vibrator to obtain a paste. The obtained paste was applied to a substrate (aluminum foil) by a doctor blade method using an applicator, and dried on a hot plate adjusted to 100 ℃ for 30 minutes to prepare a solid electrolyte layer.

The negative electrode, the solid electrolyte layer, and the positive electrode obtained as above were contacted and laminated in this order. This laminate was subjected to a pressure of 200MPa at 130 ℃ for 3 minutes to prepare a battery for evaluation (all-solid-state battery).

[ examples 2 to 6]

The amount of Cr was changed so that the ratio of the transition metal element in the negative electrode active material became the value of table 1. The Li amount in the above "(2) preparation of porous metal silicide (negative electrode active material)" was adjusted to the value of the first porosity in table 1. Otherwise, a battery for evaluation was produced in the same manner as in example 1.

Comparative example 1

A battery for evaluation was produced in the same manner as in example 1, except that Si particles were used as a negative electrode active material.

Comparative example 2

A battery for evaluation was produced in the same manner as in example 1, except that "(1) the production of the metal silicide" was not performed and only "(2) the production of the porous metal silicide (negative electrode active material)" was performed in example 1.

Comparative example 3

A battery for evaluation was produced in the same manner as in example 1, except that "(2) the production of the porous metal silicide (negative electrode active material)" was not performed and only "(1) the production of the metal silicide" was performed in example 1.

Comparative example 4

A negative electrode active material is prepared by the method of patent document 2 (Japanese patent laid-open publication No. 2015-095301). A battery for evaluation was produced in the same manner as in example 1, except that this negative electrode active material was used. The transition metal element contained in the metal silicide phase is the same as in example 1.

Comparative examples 5 and 6

The amount of Cr was changed so that the proportion of the transition metal element in the negative electrode active material became 50 mol%. The Li amount in the above "(2) preparation of porous metal silicide (negative electrode active material)" was adjusted to the value of the first porosity in table 1. Otherwise, a battery for evaluation was produced in the same manner as in example 1.

[ evaluation 1]

(first porosity)

Cross-sectional SEM images of the negative electrode active materials (negative electrode active materials before the production of the evaluation batteries) produced in examples 1 to 6 and comparative examples 1 to 6 were obtained. Based on the obtained SEM image, the first porosity was calculated by the following calculation formula. The respective void ratios are shown in table 1. Fig. 2 and 3 show primary particle images of example 4 and comparative example 4.

First porosity (%) < 100 × (first void area)/(primary particle area)

As shown in fig. 2, it was confirmed that the negative electrode active material (example 4) of the present disclosure had an Si phase, an MSi phase, and first voids inside the primary particles. As shown in fig. 2, the Si phase, the MSi phase, and the first voids are present as a plurality of discontinuous phases, respectively, and the cross section of the negative electrode active material is in a striped pattern. On the other hand, as shown in fig. 3, in comparative example 4, no first voids were observed at all.

(bound pressure rise)

Each of the evaluation batteries produced in examples 1 to 6 and comparative examples 1 to 6 was bound at a predetermined pressure, and the batteries were charged at a constant current of 0.1C to a predetermined voltage to perform initial charging. During the initial charging, the binding pressure of the battery was monitored, and the binding pressure in the charged state was measured. The bound pressure rise was relatively evaluated with the value of comparative example 1 being 1.00. The results are shown in Table 1.

(resistor)

The resistance at 25 ℃ and 60% SOC (State of Charge) was determined for each of the evaluation batteries produced in examples 1 to 6 and comparative examples 1 to 6. Specifically, the voltage was charged to 3.7V at 0.245mA, and then discharged at 7.35mA for 5 seconds, and the internal resistance was calculated from the voltage change. The resistance was relatively evaluated with the value of comparative example 1 set to 1.00. The results are shown in Table 1.

TABLE 1

As shown in table 1, in comparative example 2 having the first voids without the MSi phase (metal silicide phase), the increase in bound pressure was not suppressed, and the resistance was high. This is considered because the first voids are crushed in the pressing step in the production of the electrode because the metal silicide phase is not present. In comparative examples 3 and 4, which have the Si phase and the metal silicide phase and do not have the first voids, cracks are generated at the interface between the metal silicide phase and the Si phase during charging, and the increase in bound pressure cannot be suppressed.

On the other hand, in the evaluation batteries (examples 1 to 6) using the negative electrode active material (primary particles) of the present disclosure, the increase in the binding pressure was significantly suppressed. This is considered to be because the metal silicide phase functions as a pillar and the first voids can be maintained. Further, as is clear from comparative example 2, when the first voids are provided, the resistance tends to increase in general. However, since the metal silicide phase has electron conductivity, the increase in resistance can be suppressed in the evaluation batteries of examples 1 to 6. In comparative examples 5 and 6 in which the ratio of the transition metal element was 50 mol%, only the metal silicide phase was formed, and the Si phase was not formed.

As described above, it was confirmed that the negative electrode active material (primary particles) of the present disclosure has a small volume change due to charge and discharge, and therefore, a battery using the negative electrode active material can suppress an increase in the binding pressure.

[ example 7]

(Synthesis of negative electrode active Material (Secondary particle))

A micelle solution was prepared by heating a solution obtained by mixing water, a surfactant (cetyltrimethylammonium bromide), a template material (styrene), and a polymerization initiator (2, 2' -azo (2, 4-dimethylvaleronitrile)). Then, the primary particles (first porosity: 60%) prepared in example 4 and an organic solvent (octane) were added to the micelle solution, and a reaction (polymerization reaction) was performed for 3 hours. The polymerization reaction was terminated by cooling. Then, the resultant mixture was heat-treated at 500 ℃ to form secondary particles in which the primary particles were aggregated. The average particle diameter of the secondary particles was adjusted as appropriate so that the ratio (β/α) of the average particle diameter of the secondary particles to the thickness of the negative electrode active material layer in the stacking direction became the value shown in table 2. In addition, the secondary particles produced by such a method have voids (second voids) in the interior thereof.

(production of Battery)

The secondary particles were used as the negative electrode active material, and the average particle diameter of the secondary particles and the thickness of the negative electrode active material layer were adjusted so that the ratio (β/α) of the average particle diameter (β) of the secondary particles to the thickness (α) of the negative electrode active material in the stacking direction was the value shown in table 2. A battery for evaluation was produced in the same manner as in example 1 except for the above.

[ examples 8 to 11]

The average particle diameter of the secondary particles and the thickness of the negative electrode active material layer were adjusted so that β/α became the value shown in table 2. A battery for evaluation was produced in the same manner as in example 7 except for the above.

[ evaluation 2]

(bound pressure rise)

The increase in the restraining pressure of each of the evaluation batteries produced in examples 7 to 11 was measured by the same method as that of evaluation 1. The bound pressure rise was relatively evaluated with the value of example 7 being 1.00. The results are shown in Table 2.

(resistor)

The resistance of each of the evaluation batteries produced in examples 7 to 11 was calculated in the same manner as in evaluation 1. The resistance was relatively evaluated with the value of example 7 being 1.00. The results are shown in Table 2.

TABLE 2

As shown in Table 2, the larger the β/α, the smaller the bound pressure rise. This is considered to be because the average particle diameter of the secondary particles is relatively large, and thus voids between the secondary particles in the negative electrode active material layer become large, and expansion of the negative electrode active material can be absorbed. On the other hand, if β/α becomes too large, the battery resistance sharply increases. This is considered to be because the Li ion conductivity between the secondary particles is decreased because the voids between the secondary particles are relatively excessively large. From the above, it is considered that in the battery, β/α is preferably 0.02 or more and 0.5 or less in consideration of the effect of suppressing the bound pressure increase amount and the increase in the resistance.

(mechanical Strength of Metal silicide)

According to the simulation based on the Phase-Field method, the mechanical strength of Si and various metal silicides was calculated. The results are shown in FIG. 4. Further, the metal silicide with circles in the drawing is a metal silicide that is phase-separated so as not to react with Li. It is considered that if the metal silicide has a higher mechanical strength than silicon (Si), the metal silicide can sufficiently function as a pillar in the primary particle. In addition, it is considered that a metal silicide which is phase-separated and does not react with Li easily maintains a structure as a pillar.

Claims

1. A negative electrode active material includes primary particles having a Si phase, a metal silicide phase (MSi phase), and first voids, wherein M is a transition metal element.

2. The negative electrode active material according to claim 1,

the ratio of the transition metal element to the total of the Si element and the transition metal element contained in the primary particles is 2 mol% or more and less than 50 mol%.

3. The negative electrode active material according to claim 1 or 2,

the proportion of the first voids in the primary particles is 3% or more.

4. The negative electrode active material according to any one of claims 1 to 3,

the transition metal element is at least one of W, Mo, Cr, V, Nb, Fe, Ti, Zr, Hf and Os.

5. The negative electrode active material according to claim 4,

the transition metal element is at least one of Cr, Ti, Zr, Hf and Os.

6. The negative electrode active material according to any one of claims 1 to 5,

the negative electrode active material is a secondary particle in which a plurality of the primary particles are aggregated and which has a second void.

7. A battery having a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer,

the negative electrode active material layer contains the negative electrode active material according to any one of claims 1 to 6.

8. The battery pack as set forth in claim 7,

the negative electrode active material layer containing the negative electrode active material according to claim 6,

when the thickness of the negative electrode active material layer in the stacking direction is defined as α, and the average particle diameter of the secondary particles is defined as β, β/α is 0.02 to 0.5, where α and β are each a unit of μm.