Electrodes, batteries

Abstract

The present invention provides an electrode that suppresses swelling even after repeated insertion and removal of lithium ions, and a battery equipped with such an electrode. [Solution] An electrode comprising at least one of germanium and silicon, and at least one metallic element with an atomic size larger than that of the germanium and silicon.

Description

Technical Field

[0001] The present invention relates to an electrode and a battery.

Background Art

[0002] With the development of the information society, there has been a demand for higher capacity of the negative electrode of lithium-ion batteries. Germanium has attracted attention as a next-generation negative electrode material because it has about five times the capacity of conventional carbon negative electrodes and can be charged and discharged at high speed (see, for example, Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when the insertion and desorption of lithium ions into and from the germanium negative electrode are repeated, the germanium negative electrode may expand and cracks may occur in the germanium negative electrode. When cracks occur in the germanium negative electrode, there is a problem that the life of the battery using this germanium negative electrode is shortened.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide an electrode that suppresses expansion even when the insertion and desorption of lithium ions are repeated, and a battery including the electrode.

Means for Solving the Problems

[0007] According to the present invention, it is possible to provide an electrode that suppresses swelling even when lithium ions are repeatedly inserted and removed, and a battery equipped with the electrode. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view showing a battery according to one embodiment of the present invention. [Figure 2] This figure shows the measurement results of the hardness of the electrodes prepared in Experimental Example 1. [Figure 3] This figure shows the measurement results of the Young's modulus of the electrodes fabricated in Experimental Example 1. [Figure 4] This figure shows the evaluation results of the charge and discharge characteristics of the battery prepared in Experimental Example 2. [Figure 5] This figure shows the evaluation results of the cycle characteristics of the battery fabricated in Experimental Example 2. [Figure 6] This figure shows the evaluation results of the initial discharge capacity of the battery fabricated in Experimental Example 2, and the evaluation results of the lifespan of the negative electrode constituting the battery fabricated in Experimental Example 2. [Figure 7] This figure shows the evaluation results of the lifespan of the electrodes fabricated in Experimental Example 3. [Modes for carrying out the invention]

[0009] Embodiments of the electrode and battery of the present invention will be described below. This embodiment is provided to give a better understanding of the spirit of the invention and does not limit the present invention unless otherwise specified.

[0010] [electrode] An electrode according to one embodiment of the present invention includes at least one of germanium (Ge) and silicon (Si), and at least one metal element with an atomic size larger than germanium and silicon. In other words, the electrode of this embodiment has a sea (= at least one of germanium and silicon) island (= metal element) structure, comprising a matrix (premium material) made of at least one of germanium and silicon, and the metal element dispersed within the matrix.

[0011] The base material may consist of germanium alone, silicon alone, or germanium and silicon. When the base material consists of germanium and silicon, the mixing ratio of germanium to silicon can be adjusted as appropriate depending on the application of the electrode.

[0012] In the electrodes of this embodiment, atomic size refers to atomic radius. The atomic radius of germanium is 0.125 nm, and the atomic radius of silicon is 0.111 nm. In the electrodes of this embodiment, it is sufficient that the atomic size of the metal element is larger than the atomic sizes of germanium and silicon. The larger the atomic size of the metal element, that is, the greater the difference between the atomic size of the metal element and the atomic sizes of germanium and silicon, the softer the electrode becomes compared to the case of germanium or silicon alone. This increases the effectiveness in suppressing crack formation in the electrode even when the electrode expands due to repeated insertion and removal of lithium ions. As a result, the lifespan of the electrode is extended.

[0013] The content of the metal element is preferably 0.6% or more and 10% or less, and more preferably 3% or more and 6% or less. When the content of the metal element is not less than the lower limit value, the mechanical strength of the electrode is significantly modulated, and the effect of extending the life of the electrode becomes remarkable. When the content of the metal element is not more than the upper limit value, while extending the life of the electrode, the size of the initial capacity can also be ensured.

[0014] The metal element is not particularly limited as long as it is a metal element having an atomic size larger than that of germanium and silicon. For example, copper (Cu), silver (Ag), nickel (Ni), tungsten (W), tantalum (Ta), ytterbium (Yb), etc. can be mentioned. The metal element may be used alone or in combination of two or more. When two or more metal elements are used, their mixing ratios can be appropriately adjusted according to the use of the electrode and the like. The atomic radius of copper is 0.128 nm, the atomic radius of silver is 0.144 nm, the atomic radius of nickel is 0.149 nm, the atomic radius of tungsten is 0.137 nm, the atomic radius of tantalum is 0.146 nm, and the atomic radius of ytterbium is 0.176 nm.

[0015] The thickness of the electrode of this embodiment is not particularly limited, but for example, it is preferably 10 nm or more and 100 μm or less, more preferably 100 nm or more and 1 μm or less, and even more preferably 200 nm or more and 1 μm or less. When the thickness of the electrode is not less than the lower limit value, it has a sufficient electrode capacity for device application.

[0016] The electrode of this embodiment may be provided on a substrate made of molybdenum, copper, titanium, tantalum, tungsten, niobium, nickel, iron, etc. The thickness of the substrate is not particularly limited, but for example, it is preferably 0.1 μm or more and 10 mm or less, more preferably 1 μm or more and 10 mm or less, and even more preferably 500 μm or more and 1000 μm or less. When the thickness of the substrate is not less than the lower limit value, it is easy to handle the negative electrode. When the thickness of the substrate is not more than the upper limit value, the metal film does not become bulky during battery fabrication.

[0017] The electrode of this embodiment contains at least one of germanium and silicon and at least one metal element having an atomic size larger than that of germanium and silicon. Therefore, the electrode becomes softer than in the case of only germanium or silicon, and even if the electrode expands due to repeated insertion and desorption of lithium ions, it is possible to suppress the occurrence of cracks in the electrode. As a result, the life of the electrode is prolonged.

[0018] Note that a pre-doping process for including lithium or lithium ions may be performed on part or all of the inside of the electrode of this embodiment. By pre-doping lithium ions into the electrode, particularly the negative electrode, reduction of the initial irreversible capacity due to formation of surface oxide and suppression of decomposition of the electrolyte associated therewith occur, enabling high-speed charge and discharge and leading to improvement of the battery characteristics.

[0019] [Method for manufacturing electrode] The electrode of this embodiment can be manufactured, for example, by sputtering using a target (first target) made of at least one of germanium and silicon and a target (second target) made of at least one metal element having an atomic size larger than that of germanium and silicon. By introducing an inert gas into the vacuum chamber, applying a voltage to the first target and the second target, ejecting metal ions from the first target and the second target, and forming a film on the substrate, the electrode can be manufactured.

[0020] [Battery] FIG. 1 is a cross-sectional view schematically showing a battery according to an embodiment of the present invention. As shown in FIG. 1, the battery 1 of this embodiment includes a positive electrode 10, a negative electrode 20 made of the electrode of the above-described embodiment, and an electrolyte 30 existing between the positive electrode 10 and the negative electrode 20.

[0021] Examples of the positive electrode 10 include an electrode made of lithium metal. Also, examples of the positive electrode 10 include those in which a current collector layer and an active material layer containing at least a positive electrode active material are laminated.

[0022] The current collector layer is preferably composed of at least one material with high conductivity. Examples of highly conductive materials include metals or alloys containing at least one metallic element from silver (Ag), palladium (Pd), gold (Au), platinum (Pt), aluminum (Al), chromium (Cr), and nickel (Ni), or nonmetals such as carbon (C). Considering both high conductivity and manufacturing cost, aluminum, nickel, or stainless steel are preferred. Furthermore, aluminum is less reactive with the positive electrode active material and electrolyte. Therefore, using aluminum in the current collector layer can reduce the internal resistance of the battery.

[0023] Examples of the shape of the current collector layer include foil, plate, mesh, nonwoven fabric, and foam. In addition, carbon or other materials may be placed on the surface of the current collector layer, or the surface may be roughened, in order to improve adhesion with the active material layer.

[0024] The active material layer contains a positive electrode active material that exchanges lithium ions and electrons. The positive electrode active material is not particularly limited as long as it is a material that can reversibly release and absorb lithium ions and transport electrons, and known positive electrode active materials applicable to the positive electrode of a lithium-ion battery can be used. For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), solid solution oxide (Li2MnO3-LiMO2 (M=Co, Ni, etc.)), lithium-manganese-nickel-cobalt oxide (LiNi x Mn y Co z Examples include composite oxides such as O2 (x+y+z=1), olivine-type lithium phosphate oxide (LiFePO4); conductive polymers such as polyaniline and polypyrrole; sulfides such as Li2S, CuS, Li-Cu-S compounds, TiS2, FeS, MoS2, and Li-Mo-S compounds; and mixtures of sulfur and carbon. The positive electrode active material may consist of one of the above materials alone or of two or more materials.

[0025] The active material layer contains an electrolyte that facilitates the exchange of lithium ions with the positive electrode active material. The electrolyte is not particularly limited as long as it has lithium-ion conductivity; generally, materials used in lithium-ion batteries can be used. Examples of electrolytes include inorganic solid electrolytes such as sulfide solid electrolyte materials, oxide solid electrolyte materials, halide solid electrolytes, and lithium-containing salts; polymer-based solid electrolytes such as polyethylene oxide; and gel-based solid electrolytes containing lithium-containing salts or lithium-ion-conductive ionic liquids. Of these, sulfide solid electrolyte materials are preferred from the viewpoint of high lithium-ion conductivity, good structural moldability by pressing, and good interfacial bonding properties. The electrolyte may consist of one of the above materials alone, or it may consist of two or more materials. The electrolyte contained in the first active material layer may be the same material as the electrolyte contained in the second active material layer or the solid electrolyte layer, or it may be a different material.

[0026] The active material layer may contain a conductive additive to improve the conductivity of the positive electrode. Conductive additives generally usable in lithium-ion batteries can be used. Examples include carbon black such as acetylene black and kecheng black; carbon fiber; vapor-processed carbon fiber; graphite powder; and carbon nanotubes. The conductive additive may consist of one of these materials alone or two or more.

[0027] Furthermore, the active material layer may include a binder that serves to bind the positive electrode active materials together and to the current collector layer.

[0028] For example, the electrolyte 30 can be a non-aqueous electrolyte known in lithium-ion batteries, electric double-layer capacitors, etc. As a non-aqueous electrolyte, a non-aqueous electrolyte solution obtained by dissolving an electrolyte salt in an organic solvent is preferred.

[0029] Organic solvents that have resistance to high voltage are preferred. Examples include polar solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrohydrafran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, or mixtures of two or more of these polar solvents.

[0030] The electrolyte salt is not particularly limited and includes, for example, lithium-containing salts such as LiClO4, LiPF6, LiBF4, LiAsF6, LiCF6, LiCF3CO2, LiPF6SO3, LiN(SO2F)2, LiN(SO2CF3)2, Li(SO2CF2CF3)2, LiN(COCF3)2, LiN(COCF2CF3)2, or mixtures of two or more of these salts.

[0031] The non-aqueous electrolyte may also be a polymer electrolyte containing an electrolyte and a polymer. Examples of polymers include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylic acid (PAA), lithium polyacrylate (PAALi), styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethylcellulose (CMC), polyacrylonitrile (PAN), polyimide (PI), polyurethane (PU), and derivatives thereof.

[0032] Electrolyte 30 may be a solid electrolyte. As for the solid electrolyte, there are no particular restrictions as long as it has lithium ion conductivity and insulating properties, and materials commonly used in lithium-ion batteries can be used. Examples include inorganic solid electrolytes such as sulfide solid electrolyte materials, oxide solid electrolyte materials, halide solid electrolytes, and lithium-containing salts, as well as polymer-based solid electrolytes such as polyethylene oxide, and gel-based electrolytes containing lithium-containing salts or lithium ion-conductive ionic liquids. Of these, sulfide solid electrolyte materials are preferred from the viewpoint of high lithium ion conductivity, good structural moldability by pressing, and good interfacial bonding properties. There are no particular restrictions on the form of the electrolyte material, but for example, it can be in the form of particulate matter.

[0033] The electrolyte 30 may contain an adhesive to provide mechanical strength and flexibility.

[0034] The electrolyte 30 may be in the form of a sheet having a porous substrate and a solid electrolyte held in the porous substrate. There are no particular restrictions on the form of the porous substrate, but examples include woven fabric, nonwoven fabric, mesh cloth, porous membrane, expanded sheet, punched sheet, etc. Of these forms, nonwoven fabric is preferred from the viewpoint of handling, which allows for a higher amount of solid electrolyte to be filled.

[0035] The porous substrate described above is preferably made of an insulating material. This improves the insulating properties of the solid electrolyte layer. Examples of insulating materials include nylon, polyester, polyethylene, polypropylene, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride, polyvinylidene chloride, polyvinyl chloride, polyurethane, vinylon, polybenzimidazole, polyimide, polyphenylene sulfite, polyetheretherketone, cellulose, acrylic resins, and other resin materials; natural fibers such as hemp, wood pulp, and cotton linters; and glass.

[0036] According to the battery of this embodiment, since the negative electrode is made up of the electrodes of the above-described embodiment, even if the negative electrode expands due to repeated insertion and removal of lithium ions during charging and discharging, it is possible to suppress the occurrence of cracks in the negative electrode. Therefore, the lifespan of the negative electrode is extended, and as a result, the lifespan of the battery is extended. [Examples]

[0037] The present invention will be described in more detail below with reference to experimental examples, but the present invention is not limited to the following experimental examples.

[0038] [Experimental Example 1] (Electrode fabrication) Using a sputtering apparatus (product name: SANYUELECTRON, manufactured by SVC-700RFII), electrodes consisting solely of germanium, or electrodes containing germanium and ytterbium, with a thickness of 0.5 μm, were fabricated on a 500 μm thick molybdenum substrate. The vacuum chamber of the sputtering apparatus was placed under an argon atmosphere. When fabricating electrodes made solely of germanium, only germanium was used as the target. When fabricating electrodes containing germanium and ytterbium, germanium and ytterbium were used as targets. The amount of ytterbium attached to the germanium sputtering target was adjusted to control the ytterbium content in the electrode. The ytterbium content was set to 0.6%, 3%, 10%, and 20%.

[0039] [evaluation] (Measurement of electrode hardness) In Experimental Example 1, we measured the hardness of two electrodes: one made solely of germanium and another containing germanium and ytterbium, with a ytterbium content of 3%. The hardness of the electrodes was measured using nanoindentation. The results are shown in Figure 2. The results shown in Figure 2 indicate that electrodes containing germanium and ytterbium are less hard than electrodes made of germanium alone.

[0040] (Measurement of Young's modulus of electrodes) In Experimental Example 1, we measured the Young's modulus of two electrodes: one made solely of germanium and another containing germanium and ytterbium, with a ytterbium content of 3%. The Young's modulus was measured using nanoindentation. The results are shown in Figure 3. The results shown in Figure 3 indicate that the electrodes containing germanium and ytterbium and the electrodes consisting solely of germanium have equivalent Young's moduli.

[0041] [Experimental Example 2] (Battery construction) Using the electrode fabricated in Experimental Example 1 as the negative electrode, a battery was constructed comprising a positive electrode, a negative electrode, and an electrolyte located between the positive and negative electrodes. A lithium metal electrode with a thickness of 500 μm was used as the positive electrode. For electrolysis, an electrolyte solution was used, which consisted of a 1:1 mixture of ethylene carbonate and diethyl carbonate mixed with 1 mol / L of LiF6.

[0042] [evaluation] (Evaluation of battery charge / discharge characteristics) The charge-discharge characteristics of a battery containing germanium and ytterbium, with electrodes having a ytterbium content of 10%, were evaluated. The charge-discharge characteristics were evaluated by first measuring the weight of the germanium negative electrode using an electronic balance, then applying a current value of 1 A / g relative to the film weight during both charging and discharging, and calculating the recorded capacity by dividing it by the weight of the germanium film. The results are shown in Figure 4. As shown in Figure 4, the discharge capacity hardly decreased up to the 50th charge-discharge cycle. After the 100th charge-discharge cycle, the discharge capacity decreased.

[0043] (Evaluation of battery cycle characteristics) The cycle characteristics of the battery fabricated in Experimental Example 2 were evaluated. The cycle characteristics were evaluated by applying a current of 1 A / g to the negative electrode during both charging and discharging, and then dividing the recorded capacitance by the weight of the negative electrode film. The results are shown in Figure 5. As shown in Figure 5, batteries equipped with electrodes containing 0.6%, 3%, and 10% ytterbium showed minimal decrease in discharge capacity. In contrast, batteries equipped with electrodes made solely of germanium, and batteries equipped with electrodes containing 20% ​​ytterbium, showed significant decrease in discharge capacity.

[0044] (Evaluation of the initial discharge capacity of a battery) The initial discharge capacity of the battery fabricated in Experimental Example 2 was evaluated. The initial discharge capacity was evaluated by measuring the charge-discharge characteristics and calculating the initial capacity by dividing it by the weight of the electrodes containing germanium and ytterbium. The results are shown in Figure 6. The results shown in Figure 6 indicate that batteries equipped with electrodes made solely of germanium, as well as batteries equipped with electrodes containing 0.6%, 3%, and 10% ytterbium, have a large initial discharge capacity.

[0045] (Evaluation of electrode lifespan) The electrode lifespan of the battery fabricated in Experimental Example 2 was evaluated. The electrode lifespan was evaluated using charge-discharge characteristics, similar to the method used in Experimental Example 2. The results are shown in Figure 6. The results shown in Figure 6 indicate that electrodes containing germanium and ytterbium have a longer lifespan than electrodes made of germanium alone.

[0046] [Experimental Example 3] (Electrode fabrication) Using a sputtering apparatus (product name: SVC-700RFII, manufactured by SANYUELECTRON), electrodes with a thickness of 0.5 μm containing germanium and metal elements with atomic sizes larger than germanium were fabricated on a 500 μm thick molybdenum substrate. The vacuum chamber of the sputtering apparatus was placed under an argon atmosphere. Germanium and metallic elements were used as targets. The amount of metal attached to the sputtering target was adjusted to control the content of 20 in the electrode to 3%. The metallic elements used were silver (Ag), palladium (Pd), gold (Au), platinum (Pt), aluminum (Al), chromium (Cr), and nickel (Ni).

[0047] [evaluation] (Evaluation of electrode lifespan) The lifespan of the electrodes fabricated in Experimental Example 3 was evaluated. The electrode lifespan was evaluated using charge-discharge characteristics, similar to the method used in Experimental Example 2. The results are shown in Figure 7. The results shown in Figure 7 indicate that the lifespan of the electrodes increases as the atomic size of the metal element increases. [Explanation of Symbols]

[0048] 1 battery 10 positive electrode 20 negative electrode 30 Electric field quality

Claims

1. An electrode comprising at least one of germanium and silicon, and at least one metallic element whose atomic size is larger than that of the germanium and silicon.

2. The electrode according to claim 1, wherein the content of the aforementioned metal element is 0.6% or more and 10% or less.

3. The electrode according to claim 1, wherein the aforementioned metallic element is copper, silver, nickel, tungsten, tantalum, or ytterbium.

4. A battery comprising a positive electrode, a negative electrode consisting of the electrode described in claim 1, and an electrolyte present between the positive electrode and the negative electrode.

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