JP7533428B2 - All-solid-state battery - Google Patents

Description

This disclosure relates to all-solid-state batteries.

All-solid-state batteries are batteries that have a solid electrolyte layer between a positive electrode layer and a negative electrode layer, and have the advantage that safety devices can be simplified more easily than liquid batteries that have an electrolyte solution that contains flammable organic solvents.

For example, Patent Document 1 discloses an all-solid-state battery equipped with a negative electrode containing a Si-based active material.

JP 2018-142431 A

As shown in Patent Document 1, it is known to use Si-based active materials in the negative electrodes of all-solid-state batteries. Si-based active materials have a large theoretical capacity and are effective in increasing the energy density of batteries. However, Si-based active materials undergo large volume changes during charging and discharging, which may lead to large fluctuations in the confining pressure of all-solid-state batteries.

This disclosure was made in consideration of the above-mentioned circumstances, and its main objective is to provide an all-solid-state battery in which fluctuations in confining pressure are suppressed.

In order to solve the above problems, the present disclosure provides an all-solid-state battery in which a positive electrode current collector, a positive electrode active material layer, a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector are laminated in this order, and the negative electrode active material layer contains a Si-based active material and graphite, and when the graphite is cross-graphite, the angle between the surface direction of the (002) plane of the graphite and the surface in the stacking direction of the negative electrode current collector is 45° or more and 90° or less, the proportion of the cross-graphite in the graphite is greater than 20 mass%.

According to the present disclosure, the proportion of cross-graphite is greater than a predetermined value, resulting in an all-solid-state battery with suppressed fluctuations in confining pressure.

In the above disclosure, the proportion of the cross-graphite may be 50% by mass or more.

In the above disclosure, the mass ratio of the graphite to the Si-based active material may be 0.05 or more and 0.3 or less.

In the above disclosure, when the thickness of the negative electrode active material layer is x (μm) and the average particle diameter (D ₅₀ ) of the graphite is y (μm), y/x may be 0.16 or more and 0.56 or less.

The present disclosure has the effect of providing an all-solid-state battery in which fluctuations in confining pressure are suppressed.

FIG. 1 is a schematic diagram illustrating an example of an all-solid-state battery according to the present disclosure. FIG. 2 is a diagram for explaining intersecting graphite in the present disclosure. FIG. 2 is a diagram for explaining an example of a method for intersecting graphite with a negative electrode current collector in the present disclosure. FIG. 2 is a diagram for explaining a method for measuring confining pressure fluctuation in the examples and comparative examples.

The all-solid-state battery of the present disclosure will be described in detail below. FIG. 1 is a schematic diagram showing an example of an all-solid-state battery of the present disclosure. In the all-solid-state battery 10 shown in FIG. 1, a positive electrode collector 1, a positive electrode active material layer 2, a solid electrolyte layer 3, a negative electrode active material layer 4, and a negative electrode collector 5 are laminated in this order. The negative electrode active material layer 4 contains a Si-based active material and graphite. In the negative electrode active material layer 4, the proportion of cross graphite in the graphite is greater than a predetermined value. In this specification, when the term "graphite" is simply used, it means the entire graphite contained in the negative electrode active material layer. In other words, the term "graphite" in this specification also includes "cross graphite".

Here, the cross graphite is described with reference to the drawings. FIG. 2 is a diagram for explaining the cross graphite in the present disclosure. As shown in FIG. 2(a), the cross graphite 6 is included in the negative electrode active material layer 4 and crosses the plane of the stacking direction of the negative electrode current collector 5. To explain in more detail, as shown in FIG. 2(b), the cross graphite 6 refers to graphite included in the negative electrode active material layer 4, in which the angle θ between the plane direction A of the (002) plane and the plane of the stacking direction D2 of the negative electrode current collector 5 is 45° or more and 90° or less. In addition, in this specification, the "(002) plane" refers to the layer plane (plane horizontal to the graphite layer) of the graphite having a layered structure, which is horizontal to the carbon network of the graphene sheet constituting the graphite. In addition, the "plane direction of the (002) plane" refers to the direction A perpendicular to the stacking direction D1 of each layer of graphite as shown in FIG. 2(b). On the other hand, the "surface in the stacking direction of the negative electrode current collector" refers to the main surface of the negative electrode current collector, and refers to a surface having a normal direction in the stacking direction D2 of each layer constituting the battery as shown in FIG. 2(b). In other words, the intersecting graphite in this disclosure corresponds to graphite in which the angle of the (002) plane of the graphite with respect to the main surface of the negative electrode current collector is 45° or more and 90° or less.

According to the present disclosure, since the proportion of cross-linked graphite is greater than a predetermined value, the result is an all-solid-state battery in which fluctuations in the confining pressure are suppressed. This is believed to be because the cross-linked graphite functions as pillars in the negative electrode active material layer.

1. Negative Electrode Active Material Layer The negative electrode active material layer in the present disclosure contains a Si-based active material and graphite, and the proportion of cross graphite in the graphite is greater than a predetermined value.

The negative electrode active material layer in the present disclosure contains graphite. The graphite content is, for example, 5% by mass or more, and may be 10% by mass or more, or 20% by mass or more. On the other hand, the graphite content is, for example, 40% by mass or less, and may be 30% by mass or less.

In addition, in the negative electrode active material layer, when graphite in which the angle between the surface direction of the (002) plane of the graphite and the surface in the stacking direction of the negative electrode current collector is 45° or more and 90° or less is considered as cross graphite, the ratio of the cross graphite in the graphite is greater than a predetermined value. The ratio of cross graphite (cross ratio) is greater than 20% by mass, and may be 30% by mass or more, 40% by mass or more, or 50% by mass or more. On the other hand, the cross ratio is, for example, 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less. If the ratio of cross graphite is too small, the effect of suppressing the fluctuation of the binding pressure may not be sufficiently obtained. A method for calculating the cross ratio will be described in the examples described later. A method for crossing graphite with respect to the negative electrode current collector and a method for adjusting the ratio will also be described later.

In addition, when cross graphite perpendicular to the surface of the stacking direction of the negative electrode current collector is defined as orthogonal graphite, it is preferable that the cross graphite is mainly orthogonal graphite. "Cross graphite perpendicular to the surface of the stacking direction of the negative electrode current collector" refers to cross graphite in which the angle θ between the surface direction A and the surface of the stacking direction of the negative electrode current collector as shown in FIG. 2(b) is 80° or more and 90° or less. In addition, "cross graphite is mainly orthogonal graphite" means that the ratio of orthogonal graphite to the total amount of cross graphite is 50% by mass or more. The ratio of orthogonal graphite may be 70% by mass or more, 90% by mass or more, or 100% by mass.

The average particle diameter ( _D50 ) of graphite is, for example, 5 μm or more, and may be 7 μm or more, 10 μm or more, or 20 μm or more. On the other hand, _D50 is, for example, 50 μm or less, 45 μm or less, 40 μm or less, or 30 μm or less. The average particle diameter ( _D50 ) can be determined from the results of particle size distribution measurement by a laser diffraction scattering method.

The negative electrode active material layer in the present disclosure contains a Si-based active material. The Si-based active material may be a simple substance of Si or a Si compound. Examples of the Si compound include a Si alloy and a Si oxide. The Si alloy preferably contains a Si element as a main component.
The ratio of the Si element in the Si alloy is, for example, 50 at % or more, or may be 70 at % or more, or may be 90 at % or more. An example of the Si oxide is SiO.

The content of the Si-based active material is, for example, 20% by mass or more, may be 30% by mass or more, or may be 40% by mass or more. On the other hand, the content of the Si-based active material is, for example, 80% by mass or less, may be 70% by mass or less, or may be 60% by mass or less.

In addition, in the negative electrode active material layer, it is preferable that the mass ratio of the graphite to the Si-based active material is within a predetermined range. The mass ratio is, for example, 0.05 or more, and may be 0.1 or more, or 0.15 or more. On the other hand, the mass ratio is, for example, 0.3 or less, and may be 0.25 or less, or 0.2 or less. If the mass ratio is too low, the internal resistance of the battery may increase, and if the mass ratio is too high, the compaction property of the negative electrode active material layer may decrease.

In the negative electrode active material layer, when the thickness of the negative electrode active material layer described later is x (μm) and the _D50 of the graphite is y (μm), it is preferable that y/x is in a predetermined range. y/x is, for example, 0.16 or more, and may be 0.20 or more. On the other hand, y/x is, for example, 0.56 or less, and may be 0.40 or less, or may be 0.30 or less.
If the value of y/x is too small, the internal resistance of the battery may increase, whereas if the value of y/x is too large, the compaction property of the negative electrode active material layer may decrease.

In addition, the negative electrode active material layer in this disclosure may further contain at least one of a conductive material other than graphite, a binder, and a solid electrolyte, as necessary. The negative electrode active material layer may not contain a conductive material other than graphite, particularly a carbon material. Examples of conductive materials other than graphite include conventionally known conductive materials. Examples of binders include rubber-based binders such as butylene rubber (BR) and styrene butadiene rubber (SBR), and fluoride-based binders such as polyvinylidene fluoride (PVDF). The solid electrolyte will be described in "3. Solid electrolyte layer". The thickness of the negative electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

Here, an example of a method for crossing the graphite with respect to the negative electrode current collector in the present disclosure will be described. First, in the present disclosure, a negative electrode composition containing at least a Si-based active material and graphite is prepared, and this composition is applied to the negative electrode current collector to form a coating layer. Then, a magnetic field is applied to this coating layer. The graphite can be crossed with respect to the negative electrode current collector by controlling the orientation by applying a magnetic field. The negative electrode active material layer in the present disclosure can be formed by drying the coating layer after the magnetic field is applied.

The method of applying the magnetic field will be described in more detail with reference to the drawings. FIG. 3 is a diagram for explaining an example of a method of intersecting the graphite with the negative electrode current collector in the present disclosure. First, as shown in FIG. 3(a), a pair of magnetic field generators 20 arranged opposite to each other so as to sandwich the negative electrode current collector 5 and the coating layer 7 are used to apply a magnetic field that generates magnetic lines of force parallel to the surface direction of the negative electrode current collector 5. As a result, the surface direction A of the (002) surface of the graphite as shown in FIG. 2(b) is made parallel to the surface of the stacking direction of the negative electrode current collector 5 (i.e., θ=0°). Then, as shown in FIGS. 3(b) and (c), the angle of the magnetic field generator 20 is adjusted stepwise to adjust the angle of the magnetic lines of force, and the graphite 6 is made to intersect with the negative electrode current collector 5.

As the magnetic field generator, conventionally known components such as magnets and coils can be used. In addition, the conditions such as the strength of the magnetic field and the application time are not particularly limited, and can be appropriately adjusted according to the desired cross ratio and the viscosity of the negative electrode composition. For example, the strength of the magnetic field is 0.3 T or more and 1 T or less, and the application time is 5 seconds or more and 2 minutes or less. The cross ratio can be adjusted by changing these conditions.

2. Positive electrode active material layer The positive electrode active material layer contains at least a positive electrode active material, and may contain at least one of a conductive material and a solid electrolyte as necessary. Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include rock salt layered active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , spinel active materials such as LiMn ₂ O ₄ , Li ₄ Ti ₅ O ₁₂ , and Li (Ni _0.5 Mn _1.5 ) O ₄ , and olivine active materials such as LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , and LiCoPO ₄ . The surface of the positive electrode active material may be coated with an ion-conductive oxide, such as LiNbO _3. The conductive material and the solid electrolyte are the same as those described above.

The proportion of the positive electrode active material in the positive electrode active material layer is, for example, 20% by mass or more, may be 30% by mass or more, or may be 40% by mass or more. On the other hand, the proportion of the positive electrode active material is, for example, 80% by mass or less, may be 70% by mass or less, or may be 60% by mass or less. In addition, the thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

3. Solid Electrolyte Layer The solid electrolyte layer is a layer formed between the positive electrode active material layer and the negative electrode active material layer, and is a layer containing at least a solid electrolyte. The solid electrolyte layer may contain only a solid electrolyte, or may further contain a binder.

Examples of solid electrolytes include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes; and organic polymer electrolytes such as polymer electrolytes. Among these, sulfide solid electrolytes are particularly preferred. The thickness of the solid electrolyte layer is, for example, 0.1 μm or more and 1000 μm or less.

4. Positive electrode current collector and negative electrode current collector The all-solid-state battery in the present disclosure has a positive electrode current collector that collects electrons from the positive electrode active material layer and a negative electrode current collector that collects electrons from the negative electrode active material layer. Examples of the material for the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. On the other hand, examples of the material for the negative electrode current collector include SUS, copper, nickel, and carbon. In addition, it is preferable to appropriately select the thickness, shape, etc. of the positive electrode current collector and the negative electrode current collector according to the application of the all-solid-state battery.

5. All-solid-state battery The all-solid-state battery of the present disclosure may further include a restraining tool that applies a restraining pressure to the positive electrode current collector, the positive electrode active material layer, the solid electrolyte layer, the negative electrode active material layer, and the negative electrode current collector along the thickness direction. A known tool may be used as the restraining tool. The restraining pressure is, for example, 0.1 MPa or more, and may be 1 MPa or more. On the other hand, the restraining pressure is, for example, 50 MPa or less, and may be 20 MPa or less.

The type of all-solid-state battery in this disclosure is not particularly limited, but is typically a lithium-ion battery. In addition, the all-solid-state battery in this disclosure may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because it can be repeatedly charged and discharged, and is useful, for example, as an in-vehicle battery.

The all-solid-state battery in the present disclosure may be a single cell or a stacked battery. The stacked battery may be a monopolar stacked battery (a stacked battery connected in parallel) or a bipolar stacked battery (a stacked battery connected in series). The shape of the battery may be, for example, a coin type, a laminate type, a cylindrical type, or a square type.

This disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and anything that has substantially the same configuration as the technical ideas described in the claims of this disclosure and has similar effects is included within the technical scope of this disclosure.

[Comparative Example 1]
(Formation of negative electrode active material layer)
A paste-like negative electrode composition was prepared by mixing a Si-based negative electrode active material (Si simple substance), a dispersion medium (butyl butyrate), a binder (a 5 wt% butyl butyrate solution of a PVdF-based binder), a sulfide solid electrolyte (Li ₂ S-P ₂ S ₅ -based glass ceramic containing LiBr and LiI), and a conductive material (VGCF). The Si simple substance:VGCF was weighed out to a mass ratio of 100:15. This composition was applied on a nickel foil (negative electrode current collector) and dried to form a negative electrode active material layer. Note that VGCF does not have a crystal plane equivalent to the (002) plane of graphite, so orientation control by application of a magnetic field is not possible.

(Preparation of evaluation cells)
A paste-like positive electrode composition was prepared by mixing a positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), a dispersion medium (butyl butyrate), a binder (a 5 wt % butyl butyrate solution of a PVdF-based binder), a sulfide solid electrolyte (Li ₂ S-P ₂ S ₅ -based glass ceramic containing LiBr and LiI), and a conductive material (VGCF). This composition was applied onto an aluminum foil (positive electrode current collector) and dried to form a positive electrode active material layer.

A solid electrolyte composition was prepared by mixing a dispersion medium (heptane), a binder (a 5 wt % heptane solution of _butadiene rubber), and a sulfide solid electrolyte ( _Li2S - _P2S5 -based glass ceramic containing LiBr and LiI). This composition was applied to an aluminum foil (substrate) and dried to form a solid electrolyte layer.

The solid electrolyte layer was laminated on the positive electrode active material layer so that the solid electrolyte layer was in contact with the positive electrode active material layer, and pressed. Then, the substrate (aluminum foil) of the solid electrolyte layer was peeled off, and the negative electrode active material layer was laminated on the negative electrode active material layer so that the solid electrolyte layer was in contact with the negative electrode active material layer, and pressed. In this way, an evaluation cell as shown in Figure 4 (a) and (b) was produced. The evaluation cell was produced so that the electrode area was ¹ cm2 and the capacity ratio of the negative electrode to the positive electrode was 3.

[Comparative Example 2]
An evaluation cell was fabricated in the same manner as in Comparative Example 1 except that graphite (D ₅₀ =7 μm) was used as the conductive material instead of VGCF.

[Example 1-1]
Except for forming the negative electrode active material layer as follows, an evaluation cell was prepared in the same manner as in Comparative Example 2. First, a Si-based negative electrode active material (Si simple substance), a dispersion medium (butyl butyrate), a binder (5 wt% butyl butyrate solution of PVdF-based binder), a sulfide solid electrolyte (Li ₂ S-P ₂ S ₅ -based glass ceramic containing LiBr and LiI), and a conductive material (graphite, D ₅₀ = 7 μm) were mixed to prepare a paste-like negative electrode composition. This composition was applied onto a nickel foil (negative electrode current collector) to form a coating layer. Then, a magnetic field was applied to this coating layer to cross the graphite with respect to the negative electrode current collector. The magnetic field was applied by setting the magnetic field strength to 0.495 T and gradually changing the direction of the magnetic field lines as shown in FIG. 3(a) to FIG. 3(c). Then, the coating layer was dried to form a negative electrode active material layer.

[Examples 1-2 and 1-3]
The strength of the magnetic field was changed so that the amount of crossed graphite relative to the amount of total graphite (cross ratio) became the value shown in Table 2. Except for this, an evaluation cell was prepared in the same manner as in Example 1-1.

[Examples 2-1 to 2-5]
The _D50 of the graphite was changed so that the average particle diameter ( _D50 , μm) of the graphite relative to the film thickness (μm) of the negative electrode active material layer was the value shown in Table 3. Except for this, an evaluation cell was produced in the same manner as in Example 1-1. Note that the film thickness of the negative electrode active material layer refers to the film thickness of the negative electrode active material layer after pressing.

[Examples 3-1 to 3-6]
The amount of graphite was changed so that the mass ratio of graphite to Si-based active material in the negative electrode active material layer was a value shown in Table 4. Except for this, an evaluation cell was produced in the same manner as in Example 1-1.

[evaluation]
(Cross Ratio)
The negative electrode active material layer in the evaluation cells prepared in each Example and Comparative Example was subjected to a cross-section by ion milling. The cross-section was observed with a SEM (scanning electron microscope) to calculate the cross-ratio (mass ratio of cross-graphite to total graphite). The results are shown in Tables 1 to 4. The cross-ratio was specifically determined as follows. First, the cross-sectional SEM image of the obtained negative electrode active material layer was analyzed, and the area, the ratio of the long side to the short side (aspect ratio), and the angle of the particle long side direction (plane direction of the (002) plane) with respect to the negative electrode current collector were converted into data. Then, the total area of graphite particles with the above angle of 45° or more and 90° or less was divided by the area of all graphite particles to obtain the cross-ratio (mass ratio).

(Confining pressure fluctuation)
Four of the above evaluation cells were set in a jig as shown in Fig. 4(c), and the fluctuation of the confining pressure was measured using a load cell to evaluate the suppression effect of the confining pressure fluctuation. Specifically, the confining pressure fluctuation was normalized and evaluated using the following formula. The results are shown in Tables 1 to 4.
(Confinement pressure at the end of charging (MPa) - Confinement pressure at the start of charging (MPa)) / (Number of cells x Capacity at the end of charging (mAh))

(Compactability of negative electrode active material layer)
The thickness of the negative electrode active material layer in the evaluation cell (i.e., the thickness of the negative electrode active material layer after pressing) was measured to relatively evaluate whether the negative electrode active material layer was compacted to a thickness of 45 μm (density of 1.8 g/cc). The results are shown in Tables 1 to 4.

(Battery Resistance)
DCIR measurements were performed on the evaluation cells prepared in Examples 1-1, 2-1, 3-1, and 3-2 to determine the battery resistance. The measurements were performed at 25° C. by discharging at 1.7 C for 0.1 seconds from a state of charge (SOC) of 20%. The results are shown in Tables 3 and 4.

As shown in Table 1, Comparative Example 2, in which the negative electrode active material layer was formed without applying a magnetic field, showed the same confining pressure fluctuation value as Comparative Example 1, in which VGCF was used, and it was confirmed that the fluctuation in the confining pressure could not be suppressed. In addition, the SEM image confirmed that 20 mass% of the graphite crossed the negative electrode current collector even without applying a magnetic field. On the other hand, as shown in Tables 2 to 4, the crossing ratio could be increased to more than 20 mass% by applying a magnetic field, and the fluctuation in the confining pressure was significantly suppressed.

In addition, as shown in Table 2, it was confirmed that the effect of suppressing the fluctuation of the confining pressure is greater as the cross ratio is larger. In addition, from the results of Examples 1-1, 2-1, 3-1, and 3-2 shown in Tables 3 and 4, it was confirmed that the battery resistance is reduced by increasing the ratio of the D ₅₀ of graphite to the thickness of the negative electrode active material layer (D ₅₀ /film thickness) and the mass ratio of graphite to the Si-based active material (graphite /active material). On the other hand, from the results of Examples 2-5 and 3-6, it was suggested that when the D ₅₀ /film thickness and the mass ratio of graphite (graphite /active material) are too large, the press compaction property is inferior.

Reference Signs List 1 positive electrode current collector 2 positive electrode active material layer 3 solid electrolyte layer 4 negative electrode active material layer 5 negative electrode current collector 6 cross-sectional graphite 10 all-solid-state battery

Claims (3)

An all-solid-state battery in which a positive electrode current collector, a positive electrode active material layer, a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector are laminated in this order,
the negative electrode active material layer contains a Si-based active material and graphite,
When graphite in which the angle between the plane direction of the (002) plane of the graphite and the plane in the lamination direction of the negative electrode current collector is 45° or more and 90° or less is defined as cross graphite,
The proportion of the intersecting graphite in the graphite is greater than 20% by mass,
An all-solid-state battery , wherein y/x is 0.16 or more and 0.56 or less, where x (μm) is the thickness of the negative electrode active material layer and y (μm) is the average particle diameter (D ₅₀ ) of the graphite. The all-solid-state battery according to claim 1, wherein the proportion of the intersecting graphite is 50% by mass or more. The all-solid-state battery according to claim 1 or 2, wherein the mass ratio of the graphite to the Si-based active material is 0.05 or more and 0.3 or less.