Negative electrode for magnesium secondary battery and magnesium secondary battery

Abstract

To provide a negative electrode for a magnesium secondary battery and a magnesium secondary battery that can increase capacity while suppressing the deterioration of cycle characteristics. [Solution] The negative electrode for a magnesium secondary battery according to the present invention comprises a porous metal body having a three-dimensional network structure, holding a magnesium metal material powder which is the negative electrode active material. In powder X-ray diffraction measurement using CuKα rays, when the intensity ratio of the first diffraction peak to the second diffraction peak is Y, based on a first diffraction peak showing a (100) plane within the range of 2θ = 32° ± 5° and a second diffraction peak showing a (002) plane within the range of 2θ = 35° ± 5°, the following condition is satisfied: 0.15 ≤ Y.

Description

Technical Field

[0005]

[0001] The present invention relates to a negative electrode for a magnesium secondary battery and a magnesium secondary battery.

Background Art

[0002] In recent years, environmental problems have become serious issues. Therefore, power generation using natural energy is desired from the perspective of environmental load. However, natural energy has unstable energy supply, and the generated electrical energy needs to be stored. Therefore, there is a demand for increasing the capacity of power storage devices so that such a large amount of electrical energy can be stored.

[0003] Currently, as power storage devices, lithium-ion secondary batteries and magnesium secondary batteries are known (for example, refer to Patent Document 1). Among them, magnesium secondary batteries are attracting attention as batteries that can achieve high energy density because the carrier is divalent magnesium ions. In addition, magnesium metal is more stable in air than lithium metal and has a theoretical capacity per volume that is about 1.5 times that of lithium metal. Therefore, there is a demand for practical application as a metal secondary battery using magnesium metal as the negative electrode.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in a magnesium metal negative electrode used in a magnesium secondary battery, magnesium deposited during charging segregates, causing an internal short circuit and deteriorating the cycle characteristics. Furthermore, since the electrode surface portion of magnesium itself is an inactive surface, there is a problem that the overvoltage during charge and discharge is large and sufficient capacity cannot be obtained.

[0006] The present invention has been made in view of the above problems, and aims to provide a negative electrode for a magnesium secondary battery and a magnesium secondary battery that can increase capacity while suppressing a decrease in cycle characteristics. [Means for solving the problem]

[0007] To solve the above problems, the negative electrode for a magnesium secondary battery according to the present invention, in first respect, comprises a porous metal body having a three-dimensional network structure, holding a magnesium metal material powder which is the negative electrode active material, and is characterized in that, in powder X-ray diffraction measurement using CuKα rays, when the intensity ratio of the first diffraction peak to the second diffraction peak is Y, which consists of a first diffraction peak showing a (100) plane in the range of 2θ = 32° ± 5° and a second diffraction peak showing a (002) plane in the range of 2θ = 35° ± 5°, the negative electrode satisfies 0.15 ≤ Y.

[0008] Furthermore, in addition to the first aspect, the negative electrode for a magnesium secondary battery according to the present invention is characterized in that, as a second aspect, the magnesium metal material is magnesium or a magnesium alloy.

[0009] Furthermore, in addition to the first and second aspects, the negative electrode for a magnesium secondary battery according to the present invention is characterized in that, as a third aspect, the porous metal body is formed using copper.

[0010] Furthermore, the magnesium secondary battery according to the present invention comprises, as a fourth aspect, a positive electrode, a negative electrode comprising a porous metal body having a three-dimensional network structure in which powder of a magnesium metal material, which is a negative electrode active material, is held, a separator, and a non-aqueous electrolyte, and is characterized in that, in powder X-ray diffraction measurement of the negative electrode using CuKα rays, when the intensity ratio of the first diffraction peak to the second diffraction peak is Y, based on a first diffraction peak showing a (100) plane within the range of 2θ = 32° ± 5° and a second diffraction peak showing a (002) plane within the range of 2θ = 35° ± 5°, the ratio of Y to the second diffraction peak satisfies 0.15 ≤ Y.

[0011] Furthermore, in addition to the fourth aspect, the magnesium secondary battery according to the present invention is characterized in that, as a fifth aspect, lithium borohydride (LiBH4) is added as an additive to the non-aqueous electrolyte. [Effects of the Invention]

[0012] According to the present invention, a magnesium secondary battery can be made to have a high capacity while suppressing a decrease in cycle characteristics. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is an exploded perspective view illustrating the configuration of a magnesium secondary battery according to one embodiment of the present invention. [Figure 2] Figure 2 shows the XRD measurement results for Example 1 and Comparative Examples 1-3. [Figure 3] Figure 3 shows the charge-discharge curve for Example 1. [Figure 4] Figure 4 shows the charge-discharge curve for Comparative Example 1. [Figure 5] Figure 5 shows the charge-discharge curve for Comparative Example 2. [Figure 6] Figure 6 shows the charge-discharge curve for Comparative Example 3. [Modes for carrying out the invention]

[0014] The first embodiment of the present invention will be described below, but the present invention is not limited to the following description. Furthermore, various modifications or improvements can be made to this embodiment, and such modified or improved forms may also be included in the present invention.

[0015] The magnesium (Mg) secondary battery of the present invention includes a positive electrode, a separator, a negative electrode, an electrolyte, and an outer casing that houses them. The positive electrode and the negative electrode are arranged so that their active materials face each other, and a separator is present between the positive electrode and the negative electrode.

[0016] (Embodiment) FIG. 1 is an exploded perspective view for explaining the configuration of a magnesium secondary battery according to an embodiment of the present invention. The magnesium secondary battery 1 includes a case 110, a leaf spring 111, a positive electrode current collector 112, a positive electrode composite layer 113, a separator 114, a negative electrode 115, a gasket 116, and a cap 117. The positive electrode current collector 112 and the positive electrode composite layer 113 constitute the positive electrode 118. Moreover, not limited to this form, a form including a plurality of positive electrodes and negative electrodes is also preferably used, and it can also be used as a magnesium secondary battery using a laminated pouch.

[0017] The case 110 and the cap 117 of the magnesium secondary battery 1 are fixed by caulking or the like, and a non-aqueous electrolyte is held inside. In the magnesium secondary battery 1, it is hermetically sealed by the case 110, the gasket 116, and the cap 117. Further, the positive electrode current collector 112, the positive electrode composite layer 113, the separator 114, and the negative electrode 115 are biased toward the cap 117 side by the leaf spring 111. Thereby, a state in which each member is in close contact with each other is maintained.

[0018] (Positive Electrode) According to an embodiment of the present invention, a positive electrode for a magnesium secondary battery (positive electrode 118) including at least a positive electrode current collector 112 and a positive electrode composite layer 113 provided on one or both sides of the positive electrode current collector 112 is provided.

[0019] There is no particular limitation on the material constituting the positive electrode current collector 112, but it is preferable to use a metal. Specifically, copper, aluminum, nickel, stainless steel, titanium, other alloys, etc. can be mentioned. Among them, aluminum is preferable from the viewpoints of electron conductivity and battery operating potential.

[0020] Examples of the binder used in the positive electrode composite layer 113 include one of polyethylene, polypropylene, ethylene-propylene terpolymer, butadiene rubber, styrene-butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride (PVdF), polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, etc., or a mixture thereof.

[0021] Examples of the positive electrode active material include MgCo2O4, MgFeSiO4, FeS2, S, MnO2, Mo6S8, V2O5, etc.

[0022] In addition, the positive electrode active material may contain one of conductive carbon powders such as graphite and carbon black, carbon nanotubes, carbon nanofibers, graphene, etc., or a mixture thereof in order to improve electron conductivity.

[0023] (Negative electrode) According to an embodiment of the present invention, a negative electrode for a magnesium secondary battery (negative electrode 115), which is a porous electrode in which a powder of a magnesium metal material as a negative electrode active material is held on a metal porous body (for example, a registered trademark "Sintered Metal") having a known three-dimensional network structure as a support, is provided. From the viewpoint that the metal porous body can be used as a current collector, it is preferably formed using copper (Cu), silver (Ag), or platinum (Pt).

[0024] The magnesium metal material is magnesium or a magnesium alloy having magnesium as a main component. Magnesium preferably contains 99.9% or more of magnesium, and the balance consists of unavoidable impurities.

[0025] The negative electrode 115 according to this embodiment is a porous electrode in which a magnesium metal material powder, which is the negative electrode active material, is held in a known three-dimensional network-like structure of a porous metal body. As a result, the electrodeposited magnesium metal material is contained within the electrode, which suppresses internal short circuits caused by electrodeposited magnesium, thereby improving cycle characteristics. Furthermore, by providing a porous metal body as a support, the electrode itself becomes rigid and does not disperse within the battery (the detachment of electrodeposited magnesium and negative electrode magnesium is suppressed), thus suppressing internal short circuits. In addition, by using magnesium metal material powder as the negative electrode active material, the reaction surface area between the negative electrode active material and the electrolyte increases, increasing the solubility and extraction activity of magnesium, and enabling higher capacity.

[0026] Furthermore, in this embodiment, the magnesium metal material is characterized in that, in X-ray diffraction (XRD) measurement of the powder, when the diffraction peak showing the (100) plane within the range of 2θ = 32° ± 5° and the diffraction peak showing the (002) plane within the range of 2θ = 35° ± 5°, the intensity ratio Y of the diffraction peak showing the (100) plane to the diffraction peak showing the (002) plane is set as the diffraction peak intensity ratio of (100) / (002), Y is 0.15 or more. When Y is 0.15 or more, the active surface is largely exposed on the surface. Therefore, it becomes an active electrode, overpotential is suppressed, and segregation of electrodeposited magnesium is suppressed, making it possible to suppress internal short circuits. This is because the active side surface is largely exposed on the surface by making it into a powder. It is preferable that the peak intensity ratio is 0.15 ≤ Y ≤ 0.22.

[0027] The magnesium metal material preferably has an average particle diameter of 10 μm or more and 100 μm or less. If the average particle diameter is less than 10 μm, it becomes difficult to mold the magnesium metal anode. If the average particle diameter is greater than 100 μm, it will not become porous, and the effect of suppressing overpotential may not be obtained.

[0028] Furthermore, the negative electrode 115 has a porosity of 20% to 40%. If the porosity is 20% or less, there are fewer voids, resulting in fewer spots for the magnesium to be electrodeposited, which may cause electrodeposition on the electrode surface and lead to an internal short circuit. Conversely, if the porosity is 40% or more, the voids are simply too large, making electrode formation difficult.

[0029] (Non-aqueous electrolyte) Examples of magnesium salts included in the non-aqueous electrolyte include one or a mixture of two or more selected from MgCl2, MgBH4, Mg(NO3)2, Mg(TFSI)2, Mg(SO2CF3)2, Mg(BF4)2, Mg(CF3SO3)2, Mg(PF6)2, etc., but are not necessarily limited to these.

[0030] The non-aqueous solvent contained in the non-aqueous electrolyte is not particularly limited, but examples include one or more solvents selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, benzonitrile, diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglime), tetraglime dimethyl ether (tetraglime), sulfolane, etc.

[0031] In a non-aqueous electrolyte, it is desirable to add a boride as an additive, and in particular, lithium borohydride (LiBH4) is desirable. When LiBH4 is used as an additive, it acts as a reducing agent, so the passive film on the surface of the negative electrode made of magnesium metal material is removed, and the dissolution and extraction activity of the negative electrode is improved. Furthermore, the amount added should preferably be in a ratio of magnesium salt to LiBH4 of 4:3. If the amount of LiBH4 added is too high, the LiBH4 concentration in the electrolyte will increase, which may reduce the activity of the magnesium dissolution and extraction reaction. Conversely, if the amount of LiBH4 added is too low, the passive film on the negative electrode surface may not be sufficiently removed, which may also reduce the activity of the magnesium dissolution and extraction reaction. However, since LiBH4 is a reducing agent, it has the disadvantage of causing metal corrosion and making high-potential operation difficult. However, since the disulfide used as the positive electrode active material can operate at low potentials, combining it with the above-mentioned additives can suppress the overpotential originating from the magnesium metal negative electrode, and thus increase capacity can be expected.

[0032] (Separator) Examples of separators include porous sheet separators made of polymers or fibers, and nonwoven fabric separators.

[0033] In this embodiment, the magnesium secondary battery 1 is assembled by arranging a leaf spring 111, a positive electrode current collector 112, a positive electrode composite layer 113, a separator 114, a negative electrode 115, and a gasket 116 in order, then sandwiching these with a case 110 and a cap 117 to hold the non-aqueous electrolyte, and then sealing the case 110 and cap 117 liquid-tight by crimping or the like, before performing an activation process. [Examples]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following forms.

[0035] (Example 1) In Example 1, a copper porous material (Cellmet®) was used as the negative electrode metal porous body, and AZ31, a magnesium alloy containing 3% by weight of aluminum and 1% by weight of zinc, was used as the magnesium metal material. This magnesium metal material was machined to an average particle size (particle size) of approximately 50 μm, held in the copper metal porous body using a tablet refiner, and compacted into powder at 20 MPa using a hydraulic press to produce the negative electrode. For the positive electrode, FeS2 was used as the positive electrode active material, and a slurry was mixed in the ratio of positive electrode active material:conductive additive (acetylene black):binding agent (PVdF) = 8:1:1. This slurry was applied to carbon paper used as the current collector foil and dried at 80°C for 12 hours. A SUS coin cell was then fabricated using the negative electrode, positive electrode, and an electrolyte solution of Mg(TFSA)2 / G2 with LiBH4 added as an additive for the casing.

[0036] <(100) / (002) Peak Intensity Ratio> For magnesium metal materials, the diffraction peak intensity ratio of (100) / (002) was calculated from the diffraction peaks representing the (100) plane within the range of 2θ = 32° ± 5° and the diffraction peaks representing the (002) plane within the range of 2θ = 35° ± 5° obtained by powder X-ray diffraction (XRD).

[0037] <Constant Current Test> A constant current test was performed using the fabricated coin cell. The measurement conditions were a current density of 10 mA / g and a voltage range of 0 to 2.0 V, with a 3-cycle constant current charge-discharge test. The measurement temperature was room temperature (25°C). A glass filter was used as the separator.

[0038] Table 1 shows the characteristics of the negative electrode and the test results in Example 1. Figure 2 shows the measurement results by XRD. Figure 3 shows the charge-discharge curve for Example 1. [Table 1]

[0039] (Comparative Example 1) In Comparative Example 1, a plate-shaped current collector (plate electrode) was used as the negative electrode, which was made by cutting a plate-shaped AZ31 into the shape of a coin cell. In other words, Comparative Example 1 used a plate-shaped negative electrode that did not use powdered magnesium metal material or porous metal material. The configuration other than the negative electrode was the same as in Example 1. The test method was the same as in Example 1. Comparative Example 1 had a diffraction peak intensity ratio of 0.0136 for (100) / (002). Figure 2 shows the measurement results by XRD, and Figure 4 shows the charge-discharge curve.

[0040] (Comparative Example 2) In Comparative Example 2, a magnesium metal material with a purity of 99.0% was used as the negative electrode active material, which was ground to an average particle size (particle size) of approximately 500 μm. The configuration other than the negative electrode active material was the same as in Example 1. The test method was the same as in Example 1. Comparative Example 2 had a diffraction peak intensity ratio of 0.1415 for (100) / (002). Figure 2 shows the measurement results by XRD, and Figure 5 shows the charge-discharge curve.

[0041] (Comparative Example 3) In Comparative Example 3, AZ31 was ground to an average particle size (particle size) of approximately 50 μm as the negative electrode active material, and magnesium metal material was compacted into a powder (without a support) using a tablet refiner and a hydraulic press at 20 MPa as the negative electrode. The configuration other than the negative electrode was the same as in Example 1. The test method was the same as in Example 1. Comparative Example 3 had a diffraction peak intensity ratio of 0.2129 for (100) / (002). Figure 2 shows the measurement results by XRD, and Figure 6 shows the charge-discharge curve.

[0042] As shown in Figures 3-6, in Example 1, the porous metal body held the magnesium metal material, and by setting the diffraction peak intensity ratio of (100) / (002) to 0.2118, the discharge capacity in the third cycle was 277 mA / g and the number of cycles was 14. On the other hand, Comparative Example 1, which used a plate-shaped electrode, had a discharge capacity of 179 mA / g in the third cycle and a number of cycles of 4, which were lower than those of Example 1. Furthermore, in Comparative Example 2, which had a diffraction peak intensity ratio of 0.1415, the discharge capacity in the third cycle was 264 mA / g, which was about the same as Example 1, but the number of cycles was lower at 5. In addition, in Comparative Example 3, which did not have a negative electrode support (current collector), the discharge capacity in the third cycle was high at 337 mA / g, but the number of cycles was lower at 3, which was lower than that of Example 1. Based on the above results, by using a negative electrode in which a magnesium metal material is held in a porous metal body and the diffraction peak intensity ratio of (100) / (002) is 0.15 or higher, it is possible to provide a negative electrode for a magnesium secondary battery and a magnesium secondary battery that have high capacity while suppressing the occurrence of internal short circuits. [Explanation of Symbols]

[0043] 1. Magnesium secondary battery 110 cases 111 Leaf spring 112 Positive electrode current collector 113 Positive electrode composite layer 114 Separator 115 Negative electrode 116 Gasket 117 Cap 118 Positive electrode

Claims

1. A porous metal body having a three-dimensional network structure holds a magnesium metal powder, which is the negative electrode active material. In powder X-ray diffraction measurements using CuKα rays, when the intensity ratio of the first diffraction peak to the second diffraction peak is Y, where Y is the intensity ratio of the first diffraction peak to the second diffraction peak, where Y is the intensity ratio of the first diffraction peak to the second diffraction peak, where Y satisfies 0.15 ≤ Y, A negative electrode for a magnesium secondary battery, characterized by the following features.

2. Magnesium metal materials are magnesium or magnesium alloys. The negative electrode for a magnesium secondary battery according to feature 1.

3. The aforementioned porous metal is formed using copper. The negative electrode for a magnesium secondary battery according to feature 1.

4. Positive electrode and, A negative electrode comprising a porous metal body having a three-dimensional network structure, holding a magnesium metal powder which is the negative electrode active material, Separator and, Non-aqueous electrolyte, Equipped with, In powder X-ray diffraction measurements using CuKα rays for the negative electrode, when the intensity ratio of the first diffraction peak to the second diffraction peak is Y, given that the first diffraction peak represents the (100) plane within the range of 2θ = 32° ± 5° and the second diffraction peak represents the (002) plane within the range of 2θ = 35° ± 5°, the following conditions must be met: A magnesium secondary battery characterized by the following features.

5. The aforementioned non-aqueous electrolyte contains lithium borohydride (LiBH) as an additive. ４ ) is added, The magnesium secondary battery according to feature 4.

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