Negative electrode active material, negative electrode, and solid-state battery

Abstract

A negative electrode active material is provided that can suppress the expansion of the negative electrode active material layer during charging. [Solution] The negative electrode active material of this disclosure contains perfectly spherical porous Si particles. In the volume-based particle size distribution measured by particle size distribution measurement, the proportion of particles 0.5 μm or smaller to the total number of particles is 3.8% to 18.8%, and there is one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode active material, a negative electrode, and a solid-state battery. [Background technology]

[0002] Si-based active materials are known to have a large theoretical capacity and are effective in increasing the energy density of batteries. On the other hand, Si-based active materials are known to swell easily during charging.

[0003] Patent Document 1 discloses an all-solid-state battery. This all-solid-state battery comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer formed between the positive electrode layer and the negative electrode layer. The negative electrode layer contains a Si-based active material. The Si-based active material is a secondary particle having a plurality of primary particles. (V) V / V P The ratio is between 0.3 and 0.6. V / V P ) is defined as the volume of the secondary particle being V P The void volume of the secondary particle relative to V V This indicates the ratio. The ratio (b / a) is between 0.5 and 1.0. The ratio (b / a) represents the ratio of the length of the short side b of the primary particle to the length a of the long side b. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-13702 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The primary particles constituting the Si-based active material disclosed in Patent Document 1 are non-porous particles. While the Si-based active material disclosed in Patent Document 1 suppresses expansion during charging, there is a need for a negative electrode active material that can suppress the expansion of the negative electrode active material layer during charging.

[0006] This disclosure is made in light of the circumstances described above. One embodiment of this disclosure aims to solve the problem of providing a negative electrode active material, a negative electrode, and a solid battery that can suppress the expansion of the negative electrode active material layer during charging. [Means for solving the problem]

[0007] The following embodiments are included as means for solving the above problems. <1> Containing perfectly spherical porous Si particles, A negative electrode active material in which, in the volume-based particle size distribution measured by particle size distribution measurement, the proportion of particles 0.5 μm or smaller to the total particles is 3.8% to 18.8%, and there is one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm. <2> In the particle size distribution, the ratio of the frequency of the peak in the first particle size range to the frequency of the peak in the second particle size range is 1 / 3 or less. The particle diameter of the peak in the second particle size range is within the range of 1.0 μm to 3.0 μm. <1> The negative electrode active material described above. <3> The proportion of particles smaller than or equal to 0.5 μm relative to the total number of particles is 3.8% or more and less than 10.2%, as described above. <1> or <2> The negative electrode active material described above. <4> Having a negative electrode active material layer, The negative electrode active material layer is <1> ~ <3> A negative electrode comprising a negative electrode active material described in any one of the above and a sulfide solid electrolyte. <5> The aforementioned <4> A solid-state battery comprising a negative electrode, a positive electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode. [Effects of the Invention]

[0008] This disclosure provides a negative electrode active material, a negative electrode, and a solid-state battery that can suppress the expansion of the negative electrode active material layer during charging. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 shows the volume-based particle size distribution of the true spherical porous Si particles (negative electrode active material) of Examples 1 to 3 and the comparative example. [Figure 2] Figure 2 is a cross-sectional view of the solid-state battery according to the embodiment of the present disclosure.

Mode for Carrying Out the Invention

[0010] In the present disclosure, the numerical range indicated by using "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the present disclosure, the combination of two or more preferred embodiments is a more preferred embodiment.

[0011] (1) Negative electrode active material The negative electrode active material of the present disclosure includes true spherical porous Si particles. In the volume-based particle size distribution (hereinafter, also simply referred to as "particle size distribution") measured by particle size distribution measurement, the ratio of particles of 0.5 μm or less to all particles (hereinafter, also referred to as "small particle ratio") is 3.8% to 18.8%. One peak exists in each of the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm.

[0012] "True spherical" means that when the length of the long side of the primary particle (porous Si particle) is a and the length of the short side of the primary particle is b, the ratio of b to a (b / a) is 0.9 to 1.0. a and b are obtained by measuring the cross-sectional image of the porous Si particle. "The ratio of particles of 0.5 μm or less to all particles" indicates the ratio of the integrated value of the frequency of particles of 0.5 μm or less to the integrated value of the frequency of all particles in the particle size distribution.

[0013] Since the negative electrode active material of the present disclosure can have the above configuration, it is possible to suppress the expansion of the negative electrode active material layer during charging. This effect is presumed to be due to the following reasons, but is not limited thereto. Because the porous Si particles are perfectly spherical, the contact area between them is reduced. Therefore, it is thought that the expansion of the negative electrode active material layer during charging is effectively suppressed. Spherical porous Si particles tend to aggregate less easily than porous Si particles with a non-spherical shape. However, even spherical porous Si particles with small particle sizes (e.g., 0.5 μm or less) (hereinafter also referred to as "small particles") are prone to aggregation. If the proportion of small particles is high, many secondary particles with large particle sizes (e.g., 10 μm or more) (i.e., aggregates of small particles) tend to form in the negative electrode active material layer. In other words, the dispersibility of porous Si particles in the negative electrode active material layer is poor. As a result, the negative electrode active material layer may be prone to expansion during charging. In this disclosure, the proportion of small particles is 3.8% to 18.8%. There is one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm. The proportion of small particles being 3.8% to 18.8% indicates a low proportion of small particles. The presence of one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm indicates that not many secondary particles with large particle sizes (e.g., 10 μm or larger) (i.e., aggregates of small particles) are formed. Therefore, the dispersibility of porous Si particles in the negative electrode active material layer is excellent. As a result, the negative electrode active material layer does not expand easily during charging. The size of the voids between perfectly spherical porous Si particles tends to have a more uniform distribution than the size of the voids between non-spherical porous Si particles. When porous Si particles of a size that fits into these voids are present, aggregation of porous Si particles is considered to occur extremely easily. Therefore, it is considered important to use perfectly spherical porous Si particles whose first and second particle size ranges fall within the specified ranges in order to suppress the expansion of the negative electrode active material layer during charging. An unexpected point is that while it is expected that the expansion of the negative electrode active material layer will increase as the number of large particles increases, the expansion of the negative electrode active material layer also increased when small particles were present. Based on the above, it is presumed that the negative electrode active material of this disclosure can suppress the expansion of the negative electrode active material layer during charging.

[0014] The negative electrode active material of this disclosure comprises spherical porous Si particles. The negative electrode active material of this disclosure may further comprise non-spherical porous Si particles, or may consist solely of spherical porous Si particles.

[0015] In the particle size distribution, the proportion of particles between 0.2 μm and 10 μm to the total number of particles is not particularly limited and may be 90% to 100% or 95% to 100%. The "proportion of particles between 0.2 μm and 10 μm to the total number of particles" refers to the ratio of the integral value of the frequency of particles between 0.2 μm and 10 μm to the integral value of the frequency of all particles in the particle size distribution.

[0016] The proportion of particles smaller than 0.5 μm relative to the total number of particles (i.e., the proportion of small particles) is preferably 3.8% or more and less than 10.2%. This allows the negative electrode active material of this disclosure to further suppress the expansion of the negative electrode active material layer during charging.

[0017] In the particle size distribution, there may be one peak each in the first particle size range and the second particle size range, or there may be peaks in particle size ranges different from the first and second particle size ranges.

[0018] The particle diameter of the peak in the first particle size range is not particularly limited and may be in the range of 0.3 μm to 0.5 μm or in the range of 0.3 μm to 0.4 μm. The particle diameter of the peak in the second particle size range is not particularly limited and may be in the range of 1.0 μm to 4.0 μm, in the range of 1.0 μm to 3.0 μm or in the range of 2.0 μm to 3.0 μm.

[0019] The ratio of the frequency of the peak of the first particle size range to the frequency of the peak of the second particle size range may be less than 1.0, may be 0.9 or less, may be 0.8 or less, may be 0.4 or less, may be 1 / 3 or less, may be 0.2 or less, and may be 0.1 or more.

[0020] In the particle size distribution, the ratio of the frequency of the peak of the first particle size range to the frequency of the peak of the second particle size range is 1 / 3 or less, and the particle diameter of the peak of the second particle size range is preferably within the range of 1.0 μm to 3.0 μm. Thereby, the negative electrode active material of the present disclosure can more suppress the expansion of the negative electrode active material layer during charging.

[0021] The method for producing porous Si is not particularly limited, and any known method may be used. The method for producing porous Si is, for example, a method in which SiO X (0 < X ≦ 2) (hereinafter, also simply referred to as "SiO X ") and Mg are mixed and fired, and magnesium oxide is dissolved with an acid (for example, hydrochloric acid and nitric acid, etc.) (acid treatment) to produce porous Si. In this method, porous Si is considered to be generated by the reaction of the following formula (a). Formula (a): SiO X + XMg → XMg2O + porous Si

[0022] (2) Negative electrode The negative electrode of the present disclosure has a negative electrode active material layer. The negative electrode active material layer contains the negative electrode active material of the present disclosure and a sulfide solid electrolyte. The negative electrode may have a negative electrode current collector and a negative electrode active material layer formed on at least one main surface of the negative electrode current collector. The negative electrode active material layer contains the negative electrode active material and the sulfide solid electrolyte of the present disclosure, and may further contain at least one of a conductive material and a binder as necessary.

[0023] In the negative electrode active material layer, the volume ratio of the negative electrode active material to the total of the negative electrode active material and the sulfide solid electrolyte may be 40% to 60% by volume. The ratio of the negative electrode active material in the negative electrode active material layer is not particularly limited and may be 20% to 80% by mass.

[0024] The sulfide solid electrolyte preferably contains Li, P, and S. The sulfide solid electrolyte may further contain a halogen (for example, F, Cl, Br, I, etc.). The sulfide solid electrolyte may have a composition represented by xLiI·yLiBr·z(αLi2S·(1-α)P2S5). Here, x + y + z = 100, 0 ≦ x < 100, 0 ≦ y < 100, 0 < z ≦ 100, 0.70 ≦ α ≦ 0.80. In the negative electrode active material layer, the volume ratio of the sulfide solid electrolyte to the total of the negative electrode active material and the sulfide solid electrolyte may be 40% to 60% by volume. The ratio of the sulfide solid electrolyte in the negative electrode active material layer is not particularly limited and may be 20% to 80% by mass.

[0025] Examples of the conductive material include carbon materials, metal particles, and conductive polymers. Examples of the carbon material include acetylene black (AB), ketjen black (KB), carbon fiber, carbon nanotube (CNT), and vapor grown carbon fiber (VGCF). The volume ratio of the binder to the negative electrode active material layer may be 5% to 10% by volume.

[0026] Examples of the binder include fluoride-based binders (for example, polyvinylidene fluoride (PVDF), etc.), polyimide-based binders, and rubber-based binders. The volume ratio of the binder to the negative electrode active material layer may be 2% to 5% by volume.

[0027] The negative electrode current collector is a layer that collects current of the negative electrode active material layer. Examples of the negative electrode current collector include aluminum, SUS (Steel Use Stainless), copper, nickel, and carbon. Examples of the shape of the negative electrode current collector include a foil shape.

[0028] (3) Solid state battery The solid-state battery of this disclosure comprises a negative electrode, a positive electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode. The solid-state battery of this disclosure has at least one power generation unit. The power generation unit includes the negative electrode, the solid electrolyte layer, and the positive electrode. If the all-solid-state battery has a plurality of power generation units, the plurality of power generation units may be connected in parallel or in series.

[0029] (3.1) Solid electrolyte layer The solid electrolyte layer contains a solid electrolyte and may further contain a binder as needed. The thickness of the solid electrolyte layer may be 0.1 μm to 300 μm.

[0030] Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halogen solid electrolytes.

[0031] The sulfide solid electrolyte may be any of the examples provided for the sulfide solid electrolyte contained in the negative electrode. The sulfide solid electrolyte may contain Li and S elements. Preferably, the sulfide solid electrolyte contains at least one of P, Ge, Sn, and Si elements. The sulfide solid electrolyte may contain at least one of O and halogen elements (e.g., F, Cl, Br, and I elements). Examples of sulfide solid electrolytes contained in the solid electrolyte layer include Li2S-P2S5 and Li2S-P2S5-Z m S n (m and n are positive numbers, and Z is one of Ge, Zn, or Ga), and Li2S-SiS2-Li x MO y (x and y are positive numbers, and M is one of P, Si, Ge, B, Al, Ga, and In.) These are some examples. The description "Li2S-P2S5" indicates a material made using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions.

[0032] The solid electrolyte contained in the solid electrolyte layer may be glass, glass ceramics, or a crystalline material. Glass can be obtained by amorphous treatment of a raw material composition (for example, a mixture of Li2S and P2S5). Examples of amorphous treatment include mechanical milling. Glass ceramics can be obtained by heat treatment of glass. Crystalline materials can be obtained, for example, by solid-phase reaction treatment of a raw material composition.

[0033] The solid electrolyte content in the solid electrolyte layer may be 70% to 100% by mass, or 90% to 100% by mass.

[0034] Examples of binders that may be included in the negative electrode active material layer are the same as those exemplified above.

[0035] (3.2) Positive electrode The positive electrode includes a positive electrode active material. The positive electrode may have a positive electrode current collector and a positive electrode active material layer formed on at least one of the main surfaces of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a solid electrolyte, a conductive material, and a binder, if necessary.

[0036] Examples of positive electrode active materials include oxide active materials. Examples of oxide active materials include rock salt layered active materials (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and LiCoO2, etc.), spinel-type active materials (e.g., LiMn2O4 and Li4Ti5O 12 Examples include olivine-type active materials (e.g., LiFePO4 and LiMnPO4, etc.).

[0037] Examples of solid electrolytes include those similar to those exemplified as solid electrolytes included in the solid electrolyte layer. The proportion of solid electrolyte in the positive electrode active material layer is not particularly limited and may be 20% to 80% by mass. Examples of conductive materials include those similar to those exemplified as conductive materials that may be included in the negative electrode active material. The volume ratio of the binder to the positive electrode active material layer may be 5% to 10% by volume. Examples of binders include those similar to those exemplified as binders that may be included in the negative electrode active material. The volume ratio of the binder to the positive electrode active material layer may be 5% to 10% by volume.

[0038] The positive electrode current collector is the layer that collects current from the positive electrode active material layer. Examples of positive electrode current collectors include stainless steel (SUS), aluminum, nickel, iron, titanium, and carbon. Examples of positive electrode current collector shapes include foil.

[0039] (3.3) Exterior The solid-state battery of this disclosure typically comprises an outer casing. The outer casing houses a positive electrode, a solid electrolyte layer, and a negative electrode. The outer casing is not particularly limited and includes, for example, a laminated outer casing.

[0040] (4) Embodiment An example of a solid-state battery according to this disclosure will be described below with reference to Figure 1.

[0041] A solid-state battery 1 according to an embodiment of this disclosure comprises a power generation unit 1U and an outer casing 40. The power generation unit 1U has a negative electrode 10, a solid electrolyte layer 20, and a positive electrode 30 stacked in this order. The negative electrode 10 has a negative electrode active material layer 11 and a negative electrode current collector 12. The positive electrode 30 has a positive electrode active material layer 31 and a positive electrode current collector 32. The outer casing 40 houses the power generation unit 1U. The negative electrode active material layer 11 comprises the negative electrode active material of this disclosure and a sulfide solid electrolyte. [Examples]

[0042] The present disclosure will be described in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0043] [1] Examples and Comparative Examples [1.1] Comparative example Average particle size (D) of 0.5 μm to 1 μm 50 ) having a perfectly spherical SiO x The particles have been prepared. "Average particle size (D 50 ")" refers to the particle size (median diameter) at 50% of the integrated value in the volume-based particle size distribution determined by laser diffraction and scattering. SiOx particles and Mg were mixed, and the powder was calcined under a pressure of 100 Pa or less by vacuum. The calcination temperature was set to 750°C. The calcination time was 20 hours. It is believed that the reaction shown in equation (a) below proceeded during the calcination of the powder. Equation (a): SiOx + XMg → XMg2O + porous Si

[0044] A hydrochloric acid solution (pH=1) was prepared. The molar ratio of hydrochloric acid to the calcined powder was 10:1. The calcined powder was placed in the acid solution to dissolve the magnesium oxide in the powder (acid treatment). This yielded perfectly spherical porous Si particles (negative electrode active material).

[0045] [1.2] Example 1 Aside from changing the firing time to 10 hours, perfectly spherical porous Si particles (negative electrode active material) were obtained in the same manner as in the comparative example.

[0046] [1.3] Example 2 SiO x Particles with an average particle size of 1 μm to 2 μm (D 50 Except for changing to spherical SiOx particles having ), changing the firing temperature to 800°C, and changing the firing time to 5 hours, spherical porous Si particles (negative electrode active material) were obtained in the same manner as in the comparative example.

[0047] [1.4] Example 3 Aside from changing the firing time to 10 hours, perfectly spherical porous Si particles (negative electrode active material) were obtained in the same manner as in Example 2.

[0048] [2] Evaluation method [2.1] Volume-based particle size distribution Particle size distribution measurements were performed on porous Si particles (negative electrode active material) to determine the volume-based particle size distribution of the porous Si particles. A laser diffraction / scattering particle size distribution analyzer was used for the measurements. Table 1 shows the proportion of small particles in the volume-based particle size distribution of the porous Si particles.

[0049] [2.2] Expansion rate (%) As shown below, an evaluation cell was prepared using porous Si particles, and the expansion change rate (%) was determined.

[0050] [2.2.1] Negative electrode A negative electrode active material (porous Si particles), a sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), and an organic solvent (diisobutyl ketone) were mixed and stirred in a homogenizer to obtain a mixture. The volume ratio (porous Si particles:sulfide solid electrolyte) was 65:35. A conductive additive (VGCF) and a binder (PVDF) diluted to 5% by mass were added to the mixture and stirred in a homogenizer. This obtained a negative electrode slurry. The volume ratio of the conductive additive to the negative electrode slurry was 5% by volume. The volume ratio of the binder to the negative electrode slurry was 2% by volume. The obtained negative electrode slurry was coated onto a negative electrode current collector (nickel foil) using a cast coater and dried. This obtained a negative electrode. The negative electrode comprises a negative electrode current collector and a negative electrode active material layer formed on one of the main surfaces of the negative electrode current collector.

[0051] [2.2.2] Solid electrolyte layer A sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), a binder (PVDF) diluted to 5% by mass, and a dispersion medium (a mixture of heptane and dibutyl ether) were mixed. This yielded a slurry for the solid electrolyte layer. The solid content of the slurry for the solid electrolyte layer was 50% by mass. The solid electrolyte layer slurry was coated onto a substrate (aluminum foil) and dried. This yielded a solid electrolyte layer with a substrate. The solid electrolyte layer with a substrate comprises a substrate and a solid electrolyte layer formed on one main surface of the substrate.

[0052] [2.2.3] Positive electrode Cathode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A mixture of O2, a dispersion medium (butyl butyrate), a binder (a 5% by mass solution of PVdF-based binder with butyl butyrate), a sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), and a conductive material (VGCF) was obtained. This yielded a positive electrode slurry. The positive electrode slurry was coated onto a positive electrode current collector (aluminum foil) and dried. This yielded a positive electrode. The positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on one main surface of the positive electrode current collector.

[0053] [2.2.4] Evaluation cell The solid electrolyte layer was laminated onto the positive electrode so that it was in contact with the positive electrode active material layer, and then pressed. The substrate (aluminum foil) of the solid electrolyte layer was peeled off, and the negative electrode was laminated so that the solid electrolyte layer was in contact with the negative electrode active material layer, and then pressed. This resulted in obtaining an evaluation cell.

[0054] [2.2.5] Measurement Using a restraint jig capable of measuring restraint pressure, the evaluation cell was restrained at a specific restraint pressure (hereinafter also referred to as "restraint pressure") and charged to 4.55V (hereinafter also referred to as "full charge") by constant current-constant voltage charging at a 10-hour rate (0.1C). The restraint pressure at full charge was measured. The increase in restraint pressure at full charge relative to the restraint pressure (hereinafter also referred to as "pressure increase") was calculated. The pressure increase for Examples 1 to 4, with the pressure increase for the comparative example set to 100, was defined as the "expansion change rate (%)". A smaller expansion change rate (%) indicates less expansion of the negative electrode active material layer. The results are shown in Table 1. An acceptable expansion change rate (%) is less than 100.

[0055] [Table 1]

[0056] In Table 1, "Small particle proportion" indicates the proportion of particles smaller than 0.5 μm relative to the total particles in the particle size distribution.

[0057] In Examples 1 to 3, the negative electrode active material contained perfectly spherical porous Si particles. As shown in Table 1, the proportion of small particles ranged from 3.8% to 18.8%. As shown in Figure 1, there was one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm. Therefore, the expansion change rate in Examples 1 to 3 was less than 100%. As a result, it was found that the negative electrode active materials in Examples 1 to 3 are "negative electrode active materials that can suppress the expansion of the negative electrode active material layer during charging." [Explanation of symbols]

[0058] 1: Solid-state battery, 1U: Power generation unit, 10: Negative electrode, 11: Negative electrode active material layer, 12: Negative electrode current collector, 20: Solid electrolyte layer, 30: Positive electrode, 31: Positive electrode active material layer, 32: Positive electrode current collector, 40: Outer casing

Claims

1. Containing perfectly spherical porous Si particles, A negative electrode active material in which, in the volume-based particle size distribution measured by particle size distribution measurement, the proportion of particles 0.5 μm or smaller to the total particles is 3.8% to 18.8%, and there is one peak each in the first particle size range of 0.2 μm to 0.5 μm and the second particle size range of 0.8 μm to 4.0 μm.

2. In the particle size distribution, the ratio of the frequency of the peak in the first particle size range to the frequency of the peak in the second particle size range is 1 / 3 or less. The negative electrode active material according to claim 1, wherein the particle diameter of the peak in the second particle size range is in the range of 1.0 μm to 3.0 μm.

3. The negative electrode active material according to claim 1 or claim 2, wherein the proportion of particles 0.5 μm or smaller relative to the total number of particles is 3.8% or more and less than 10.2%.

4. Having a negative electrode active material layer, A negative electrode wherein the negative electrode active material layer comprises the negative electrode active material described in claim 1 or claim 2 and a sulfide solid electrolyte.

5. A solid-state battery comprising a negative electrode as described in claim 4, a positive electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode.

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