Battery

Abstract

To provide a battery capable of improving battery performance in a battery containing a sulfur-based active material.SOLUTION: The positive electrode active material layer contains a positive electrode active material containing an S element and a carbon nanotube, and a specific surface area of the carbon nanotube in the positive electrode active material layer is 400m2 / g or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to batteries. [Background technology]

[0002] Patent Document 1 discloses an all-solid-state battery containing S, P2S5, and vapor-grown carbon fiber as a conductive additive. Patent Document 2 describes an all-solid-state "sodium ion battery" having a sulfur positive electrode, in which the specific surface area of ​​the positive electrode conductive additive is 1000 m 2 / g. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-212615 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-079623 Summary of the Invention [Problem to be solved by the invention]

[0004] One way to improve the energy density of solid-state batteries is to use sulfur (S)-based active materials in the positive electrode. However, the use of sulfur-based active materials in the positive electrode tends to reduce the discharge capacity and cycle characteristics. To address this issue, vapor grown carbon fiber may be included as C in the positive electrode active material layer, but this has not necessarily been able to fully resolve these problems.

[0005] Therefore, an object of the present disclosure is to provide a battery that includes a sulfur-based active material and that can improve the battery performance. [Means for solving the problem]

[0006] The present application relates to a battery having an electrode assembly in which a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer are laminated, and the positive electrode active material layer contains a positive electrode active material containing an S element and carbon nanotubes, and the specific surface area of ​​the carbon nanotubes in the positive electrode active material layer is 400 m 2 / g or greater.

[0007] The specific surface area of ​​carbon nanotubes is 1000m 2 / g or more.

[0008] The positive electrode active material layer may contain a sulfur-containing compound containing an S element and a P element.

[0009] The negative electrode active material layer may contain Li metal.

[0010] The electrolyte layer may include a solid electrolyte, and the battery may be an all-solid-state battery. [Effects of the Invention]

[0011] According to the battery of the present disclosure, the battery performance, particularly the discharge capacity and cycle characteristics, of a battery containing a sulfur-based active material can be improved. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating the layer structure of a solid-state battery 10. [Figure 2] Figure 2 is a graph showing the results of the test. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1. Battery configuration FIG. 1 illustrates a diagram of a solid-state battery (all-solid-state battery) according to one embodiment. Here, an all-solid-state battery is used as a typical example, but the present disclosure is not necessarily applicable to all-solid-state batteries, and can be applied to any battery having an electrode body and an exterior body that seals it (e.g., a solid-state battery (semi-solid battery) that contains a solid electrolyte and an electrolytic solution). FIG. 1 illustrates the layer structure of an electrode body 11 included in a solid-state battery. A solid-state battery is formed by sealing such an electrode body 11 in an exterior body. For example, an electrode body 11 that is roughly rectangular in plan view is enclosed in an exterior body that is also roughly rectangular in plan view. A positive terminal extends from the positive electrode current collector of the electrode body 11, and a negative electrode terminal extends from the negative electrode current collector, with their tips protruding from the exterior body. The components of the laminate 11 and their relationships will be described in more detail below.

[0014] The electrode body 11 has a positive electrode current collector 12, a positive electrode active material layer 13, a solid electrolyte layer 14, a negative electrode active material layer 15, and a negative electrode current collector 16. In this embodiment, the positive electrode current collector 12, the positive electrode active material layer 13, the solid electrolyte layer 14, the negative electrode active material layer 15, and the negative electrode current collector 16 are stacked in this order to form a unit element 11a, and a plurality of unit elements 11a are stacked to form the electrode body 11 (only one unit element 11a is shown in FIG. 1). As described above, a positive electrode terminal is electrically connected to the positive electrode current collector 12 of the electrode body 11, and a negative electrode terminal is electrically connected to the negative electrode current collector 16 of the electrode body 11.

[0015] 1.1. Positive electrode current collector The positive electrode current collector 12 is laminated on the positive electrode active material layer 13 to collect current from the positive electrode active material layer 13. In this embodiment, the positive electrode current collector 12 is a rectangular foil in plan view, and can be composed of a positive electrode current collector foil, which is a metal foil, and a conductive resin layer or a carbon layer laminated on the positive electrode current collector foil. The carbon layer is laminated on the positive electrode active material layer 13, thereby laminating the positive electrode current collector 12 on the positive electrode active material layer 13. Examples of materials constituting the positive electrode current collector include metal foil materials such as stainless steel, aluminum, nickel, iron, and titanium, conductive resin layers made of resins in which conductive materials are dispersed, and carbon layers made of materials containing carbon.

[0016] 1.2. Positive electrode active material layer The positive electrode active material layer 13 has the positive electrode current collector 12 laminated on one surface and the solid electrolyte layer 14 laminated on the other surface. In this embodiment, the positive electrode active material layer 13 has a rectangular sheet shape in plan view.

[0017] The positive electrode active material layer 13 contains a positive electrode active material containing an S element, a conductive additive, and optionally a sulfur-containing compound containing a P element and an S element.

[0018] [Cathode active material] The positive electrode active material contains the element S. Among them, the positive electrode active material is preferably elemental sulfur. An example of elemental sulfur is S8 sulfur. S8 sulfur has three crystal forms: α sulfur (orthorhombic sulfur), β sulfur (monoclinic sulfur), and γ sulfur (monoclinic sulfur), and any of these crystal forms may be used.

[0019] When elemental sulfur is contained as the positive electrode active material, the positive electrode active material layer may or may not have peaks corresponding to elemental sulfur in XRD measurement. Typical peaks of elemental sulfur appear at 2θ = 23.05° ± 0.50°, 25.84° ± 0.50°, and 27.70° ± 0.50° in XRD measurement using CuKα radiation. These peak positions may be within ± 0.30° or ± 0.10°, respectively.

[0020] A part or all of the elemental sulfur may be dissolved in the sulfur-containing compound described later. In other words, the positive electrode active material layer may contain a solid solution of elemental sulfur and the sulfur-containing compound. Furthermore, the S element in the elemental sulfur and the S element in the sulfur-containing compound may have a chemical bond (SS bond).

[0021] [Conductive additive] The conductive additive has a function of improving the electronic conductivity in the positive electrode active material layer. It is also presumed that the conductive additive also functions as a reducing agent that reduces elemental sulfur, for example, when mechanical milling is performed on the raw material mixture. The conductive additive is preferably present in a dispersed state in the positive electrode active material layer.

[0022] In the present disclosure, the conductive additive is carbon nanotubes (CNTs). Furthermore, the carbon nanotubes have a BET specific surface area of ​​400 m in the positive electrode active material layer. 2 / g or more, and 2 / g or more is preferable. Here, the BET specific surface area is a specific surface area calculated by the BET method using the amount of monomolecular gas adsorption on the surface of a material.

[0023] Carbon nanotubes include multi-walled carbon nanotubes and single-walled carbon nanotubes, but single-walled carbon nanotubes are preferred.

[0024] By using carbon nanotubes with such a high specific surface area as a conductive additive in the positive electrode active material layer containing the element S, the electron conduction path is amplified and the Li + The self-formation of conductive paths is promoted, improving discharge capacity and cycle characteristics.

[0025] The content of carbon nanotubes in the positive electrode active material layer is not particularly limited as long as it satisfies the above-mentioned specific surface area, but is preferably 5% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more.

[0026] [Sulfur-containing compounds] The positive electrode active material layer may contain a sulfur-containing compound having a P element and an S element as the sulfur-containing compound. The sulfur-containing compound may be only a sulfur-containing compound having a P element and an S element, or may further contain a sulfur-containing compound having another element (e.g., Ge, Sn, Si, B, or Al) and an S element. In the latter case, the sulfur-containing compound preferably contains a sulfur-containing compound having a P element and an S element as the main component of the sulfur-containing compound.

[0027] The sulfur-containing compound may be substantially free of Li element. Furthermore, the sulfur-containing compound preferably serves as an ion conduction path during charge and discharge. When the sulfur-containing compound is present in the positive electrode active material layer, even if the ion conductivity of the discharge product (e.g., LiS) is low, the sulfur-containing compound ensures an ion conduction path in the positive electrode active material layer, facilitating the progress of the discharge reaction.

[0028] The sulfur-containing compound preferably contains an ortho-structure of the P element. Specifically, the ortho-structure of the P element is a PS4 structure. The sulfur-containing compound may also contain an ortho-structure of the M element (M is, for example, Ge, Sn, Si, B, or Al). Examples of the ortho-structure of the M element include a GeS4 structure, a SnS4 structure, a SiS4 structure, a BS3 structure, and an AlS3 structure. On the other hand, the sulfur-containing compound may also contain a sulfide of the P element (for example, P2S5). The sulfur-containing compound may also contain a sulfide of the M element (M x S y ) where x and y are integers that provide electrical neutrality with S depending on the type of M. Examples of sulfides (MxSy) include GeS2, SnS2, SiS2, B2S3, and Al2S3. These sulfides are, for example, residues of the starting materials.

[0029] [others] In the positive electrode active material layer, the molar ratio of P element to S element (P / S) is not particularly limited, but may be, for example, 0.03 or more, 0.06 or more, 0.09 or more, or 0.12 or more. On the other hand, the molar ratio (P / S) is, for example, 0.5 or less, 0.3 or less, or 0.27 or less. The denominator of the molar ratio (P / S) refers to the amount of all S elements contained in the positive electrode active material layer. In this embodiment, since both the positive electrode active material and the sulfur-containing compound contain S element, the amount of S element in both is the sum of both.

[0030] The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less. The basis weight of the positive electrode layer is, for example, 3 mg / cm 2 greater than 4 mg / cm 2 It may be more than 5 mg / cm 2 It may be more than that.

[0031] Furthermore, the positive electrode active material layer can be substantially free of Li element, which can prevent a decrease in capacity. "Substantially not containing Li element" means that the proportion of Li element relative to all elements contained in the positive electrode active material layer is 20 mol% or less. The proportion of Li element may be 16 mol% or less, 8 mol% or less, 4 mol% or less, or even 0 mol%.

[0032] Furthermore, the positive electrode active material layer may be substantially free of Na element. "Substantially free of Na element" means that the ratio of Na element to all elements contained in the positive electrode active material layer is 20 mol% or less. The ratio of Na element may be 16 mol% or less, 8 mol% or less, 4 mol% or less, or even 0 mol%.

[0033] 1.3.Solid electrolyte layer The solid electrolyte layer 14 is a layer formed between the positive electrode active material layer and the negative electrode active material layer. The solid electrolyte layer contains at least a solid electrolyte, and may contain a binder as needed.

[0034] Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes, and among these, sulfide solid electrolytes are preferred. The sulfide solid electrolyte preferably contains Li, A (A is at least one of P, Ge, Si, Sn, B, and Al), and S. The sulfide solid electrolyte may further contain a halogen element. Examples of halogen elements include F, Cl, Br, and I. The sulfide solid electrolyte may also contain O.

[0035] Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-GeS2, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-LiI-LiBr, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is one of P, Si, Ge, B, Al, Ga, and In.)

[0036] The solid electrolyte layer contains, for example, 50% by volume or more of the solid electrolyte, 70% by volume or more, or 90% by volume or more. The binder used in the solid electrolyte layer is the same as that described for the negative electrode layer. The thickness of the solid electrolyte layer is, for example, 0.1 μm or more and 1000 μm or less.

[0037] 1.4.Negative electrode active material layer The negative electrode active material layer 15 is a layer containing at least a negative electrode active material. The negative electrode active material preferably contains Li element. Examples of such a negative electrode active material include lithium element and lithium alloys. Examples of lithium alloys include Li-X alloys. Here, examples of X include Mg, Ag, In, Sn, Si, Ga, Au, and Pt.

[0038] 1.5. Negative electrode current collector The negative electrode current collector 16 is laminated on the negative electrode active material layer 15 to collect current from the negative electrode active material layer 15. In this embodiment, the negative electrode current collector 16 has a rectangular foil shape in a plan view, and can be made of, for example, stainless steel, copper, nickel, carbon, aluminum, or the like.

[0039] 1.6. Positive and negative terminals The positive electrode terminal and the negative electrode terminal are electrically conductive members, and serve as terminals for electrically connecting each electrode to the outside. One end of the positive electrode terminal is electrically connected to the positive electrode current collector 12, and the other end penetrates the exterior body and is exposed to the outside. One end of the negative electrode terminal is electrically connected to the negative electrode current collector 16, and the other end passes through the exterior body and is exposed to the outside.

[0040] 1.7.Exterior body The exterior body is made of a rectangular sheet-like member in a plan view, and includes, for example, a first sheet and a second sheet. The electrode assembly 11 is enclosed between the first sheet and the second sheet, and the outer peripheral edge of the first sheet and the outer peripheral edge of the second sheet are joined and sealed. Therefore, this exterior body is bag-shaped, and the electrode assembly 11 is enclosed and sealed inside.

[0041] The first and second sheets can be made of a laminate film. Here, a laminate film is a film having a metal layer and a sealant layer. Examples of metals used in the laminate film include aluminum and stainless steel, and examples of materials used in the sealant layer include thermoplastic resins such as polypropylene, polyethylene, polystyrene, and polyvinyl chloride. [Example]

[0042] 2. Working Example In the examples, tests were carried out by changing the type and specific surface area of ​​the conductive additive used in the positive electrode active material layer.

[0043] 2.1. Preparation of the electrode body [Preparation of materials for the positive electrode active material layer] The positive electrode active material (sulfur elemental, vacuum-dried at 80°C), the sulfur-containing compound (PS), and the conductive additive (carbon material, which varied for each example; see Table 1; all materials were vacuum-dried at 120°C) were weighed out in a weight ratio (mass ratio) of 42:35:23. The raw materials were kneaded in an agate mortar for 15 minutes to obtain a raw material mixture. 1.7 g of the resulting raw material mixture was placed in a planetary ball mill, and 80 g of zirconia balls with a diameter of 4 mm were added. The container was then sealed. This container was then attached to a planetary ball mill (Fritsche P7) and mechanically milled at 400 rpm for 36 hours. After mechanical milling, the mixture was dry-classified using a 38 μm sieve to obtain the material for the positive electrode active material layer.

[0044] [Preparation of electrode body] 100 mg of sulfide solid electrolyte particles (average particle size 2.0 μm) were placed in a mold with a diameter of 11.28 mm, and 1 ton / cm 2 The mixture was pressed at 1000 kJ / min to form a solid electrolyte layer. 7.6 mg of the material for the positive electrode active material layer prepared above was laminated on one side of the formed solid electrolyte layer, and 1 ton / cm 2 The mixture was pressed with a pressure of 1000 kJ / cm 3 to form a positive electrode active material layer. An aluminum foil with a diameter of 11.28 mm was laminated on the positive electrode active material layer, and the pressure was 6 ton / cm 2 The mixture was pressed with a tungsten carbide to form a positive electrode current collector layer. On the other side of the solid electrolyte layer, a Li-Mg alloy foil (10 mass% Mg) with a diameter of 11.28 mm and a thickness of 100 μm was placed, and then a roughened Ni foil was placed on top of it, and a pressure of 1 ton / cm was applied. 2 The mixture was pressed with a .mu.m. to form a negative electrode active material layer (Li source) and a negative electrode current collector layer. Finally, the resulting laminate was constrained under a constraining pressure of 2 Nm (approximately 30 MPa) to form an electrode body.

[0045] Evaluation A constant current charge / discharge test was carried out at 60°C on each electrode body. Here, 1C is 5.84mA / cm 2 is equivalent to (1) After conditioning (discharging to 1.2 V at 0.1 C and then resting for 10 minutes), the battery was charged to 3.1 V at 0.1 C, rested for 10 minutes, discharged to 1.2 V at 0.1 C, and rested for 10 minutes, for a total of three cycles. After (2), a total of 46 cycles of charging at 0.2C to 3.1V, resting for 10 minutes, discharging at 0.2C to 1.2V, and resting for 10 minutes were performed (50 cycles including conditioning and the three cycles of (1)). However, after the condition of (2), the cycles of charging at 0.1C to 3.1V, resting for 10 minutes, discharging at 0.1C to 1.2V, and resting for 10 minutes were performed for 20 and 40 cycles.

[0046] 2.3.Results The relationship between cycles and discharge capacity for each example is shown in Figure 2. Table 1 shows the type of conductive additive, BET specific surface area, discharge capacity after 3 cycles in (1) above (referred to as "initial discharge capacity" in Table 1), and discharge capacity after 50 cycles in (2) above (referred to as "final discharge capacity" in Table 1). Here, the discharge capacity is the weight ratio of the positive electrode active material layer.

[0047] [Table 1]

[0048] As can be seen from these results, in the positive electrode containing the S element, the specific surface area of ​​400 m 2 By using single-walled and multi-walled CNTs with a specific surface area of ​​1000 m / g or more, both the initial discharge capacity and the final discharge capacity could be improved compared to vapor-grown carbon fibers and CNTs. 2 It can be said that the improvement is particularly remarkable when the porosity is set to / g or more. This is because by using single-walled and multi-walled CNTs, which have a higher specific surface area than vapor-grown carbon fibers, as a conductive additive in the positive electrode active material layer, not only is the electron conduction path in the composite containing S element amplified, but also the Li + This is thought to be because the self-formation reaction of the conductive path is also promoted. As a result, the proportion of usable sulfur active material in the positive electrode active material increases, which is thought to improve both the discharge capacity and cycle characteristics. [Explanation of symbols]

[0049] 11... Electrode body, 12... Positive electrode current collector, 13... Positive electrode active material layer, 14... Solid electrolyte layer, 15... Negative electrode active material layer, 16... Negative electrode current collector

Claims

1. A battery having an electrode assembly formed by laminating a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, the positive electrode active material layer contains a positive electrode active material containing an S element and carbon nanotubes, In the positive electrode active material layer, the specific surface area of ​​the carbon nanotubes is 400 m ２ / g or more, battery.

2. The specific surface area is 1000 m ２ 10. The battery according to claim 1, wherein the SiO2 content is 100% or more.

3. The battery according to claim 1 or 2, wherein the positive electrode active material layer contains a sulfur-containing compound containing an S element and a P element.

4. The battery according to claim 1 or 2, wherein the negative electrode active material layer contains Li metal.

5. 3. The all-solid-state battery according to claim 1, wherein the electrolyte layer includes a solid electrolyte.

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