Magnesium secondary battery

Abstract

To provide a magnesium secondary battery that can improve energy density while suppressing overvoltage. [Solution] The magnesium secondary battery according to the present invention comprises 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 the negative electrode active material, is held, a separator, and a non-aqueous electrolyte. The non-aqueous electrolyte is obtained by dissolving a magnesium salt in an organic solvent, and the magnesium metal material is a magnesium alloy having multiple different crystal structures in a mixed phase.

Description

Technical Field

[0001] The present invention relates to a magnesium secondary battery.

Background Art

[0002] In recent years, environmental problems have become serious issues. Therefore, power generation from 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 energy storage devices so that such a large amount of electrical energy can be stored.

[0003] Currently, as energy storage devices, lithium-ion secondary batteries and magnesium secondary batteries are known (for example, see Patent Document 1). Among them, magnesium secondary batteries have attracted 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 about 1.5 times that of lithium metal. Therefore, practical application is required for metal secondary batteries 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] Incidentally, the magnesium metal negative electrode used in magnesium secondary batteries becomes inactive when a passive film forms on the electrode surface, increasing the overvoltage. In this case, adding lithium borohydride (LiBH4) to the electrolyte removes the passive film and suppresses the overvoltage. However, adding LiBH4 can corrode the metal components of the battery (e.g., aluminum or stainless steel), potentially making it difficult to operate at high potentials and preventing improvements in energy density.

[0006] The present invention has been made in view of the above problems, and aims to provide a magnesium secondary battery that can improve energy density while suppressing overvoltage. [Means for solving the problem]

[0007] To solve the above problems, the magnesium secondary battery according to the present invention comprises, in first view, 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, wherein the non-aqueous electrolyte is obtained by dissolving a magnesium salt in an organic solvent, and the magnesium metal material is a magnesium alloy having multiple different crystal structures in a mixed phase.

[0008] Furthermore, in addition to the first aspect, the magnesium secondary battery according to the present invention is characterized in that the organic solvent is a glim solvent, as a second aspect.

[0009] Furthermore, in addition to the first and second aspects, the magnesium secondary battery according to the present invention is characterized in that, as a third aspect, the magnesium salt is Mg(TFSA)2.

[0010] Furthermore, in addition to the first to third aspects, the magnesium secondary battery according to the present invention is characterized in that, as a fourth aspect, the magnesium metal material is LZ91.

[0011] Furthermore, in addition to the first to fourth aspects, the magnesium secondary battery according to the present invention is characterized in that, as a fifth aspect, the non-aqueous electrolyte does not contain LiBH4. [Effects of the Invention]

[0012] According to the present invention, it is possible to improve energy density while suppressing overvoltage. [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 charge-discharge curve for Example 1. [Figure 3] Figure 3 shows the charge-discharge curve for Comparative Example 1. [Figure 4] Figure 4 shows the charge-discharge curve for Comparative Example 2. [Figure 5] Figure 5 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 material layer 113, a separator 114, a negative electrode 115, a gasket 116, and a cap 117. Note that the positive electrode current collector 112 and the positive electrode composite material layer 113 constitute a positive electrode 118. Further, 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 material 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, there is provided a positive electrode (positive electrode 118) for a magnesium secondary battery including at least a positive electrode current collector 112 and a positive electrode composite material layer 113 provided on one or both sides of the positive electrode current collector 112.

[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 used in the positive electrode composite layer 113 include MgCo2O4, MgFeSiO4, FeS2, S, MnO2, Mo6S8, V2O5, etc.

[0022] In addition, the positive electrode composite layer 113 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 the 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 having a three-dimensional network structure (for example, a sintered metal fiber filter (registered trademark)) 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] As the magnesium metal material, a magnesium alloy mainly composed of magnesium is used. This magnesium alloy may be in a eutectic state in which a plurality of different crystal structures are in a mixed phase. For example, a magnesium-lithium alloy containing lithium (Li) can be mentioned, and it is preferably contained in an amount of 6 wt% or more and 10.5 wt% or less of lithium. In this specification, the term "eutectic state" includes hypoeutectic and hypereutectic states, and refers to a state in which two or more crystal structures can be observed. It is preferable that the crystal structure is one in which a hexagonal close-packed structure and a body-centered cubic lattice structure are in a eutectic state.

[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 metal porous body having a three-dimensional network structure. 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 metal porous 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] 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.

[0027] 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 magnesium to electrodeposit 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.

[0028] (Non-aqueous electrolyte) Non-aqueous electrolytes are magnesium ion-containing non-aqueous electrolytes obtained by dissolving a magnesium salt in a solvent made of grime.

[0029] Grime is, for example, CH3O(CH2CH2O) n It is an alkoxide derivative represented by CH3. Here, n is not particularly limited and is an integer selected as needed. Also, n is preferably 6 or less, but is not limited thereto. For example, it is at least one selected from the group consisting of 1,2-dimethoxyethane, diethylene glycol dimethyl ether (DMDG), triethylene glycol dimethyl ether (DMTG), tetraethylene glycol dimethyl ether (DMTeG), and formaldehyde diethyl acetal. Hereafter, DMDG will be referred to as G2, DMTG as G3, and DMTeG as G4.

[0030] The magnesium salt is, for example, at least one selected from the group consisting of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSA)2), magnesium chloride (MgCl2), magnesium bromide (MgBr2), magnesium iodide (MgI2), magnesium perchlorate (Mg(ClO4)2), magnesium tetrafluoroborate (Mg(BF4)2), magnesium hexafluorophosphate (Mg(PF6)2), magnesium hexafluoroarsenate (Mg(AsF6)2), magnesium perfluoroalkylsulfonate, and magnesium perfluoroalkylsulfonylimide. Among these magnesium salts, Mg(TFSA)2 is particularly preferred.

[0031] In this embodiment, the electrolyte does not contain additives such as borohydride. Here, borohydride is, for example, LiBH4, NaBH4, KBH4, Mg(BH4)2, Ca(BH4)2, Be(BH4)2, or Al(BH4)3.

[0032] As for the grime, if the electrolyte contains the above-mentioned additives, preferably anhydrous G2, G3, or G4 is used.

[0033] Here, the electrolyte preferably contains a magnesium complex having an active four-coordinate structure in which magnesium is coordinated with glyme. Here, the active four-coordinate structure refers to a particularly active four-coordinate complex. Specifically, a four-coordinate complex in which magnesium is coordinated with ethyl isopropyl sulfone is not an active four-coordinate structure; rather, a four-coordinate complex that is as active as a four-coordinate complex in which magnesium is coordinated with glyme is considered an active four-coordinate structure.

[0034] (Separator) Examples of separators include porous sheet separators made of polymers or fibers, and nonwoven fabric separators.

[0035] 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]

[0036] 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.

[0037] (Example 1) In Example 1, a porous copper material (Cellmet®) was used as the negative electrode metal porous body, and LZ91, a magnesium alloy containing 9% lithium and 1% zinc by weight, was used as the magnesium metal material. This magnesium metal material was ground into a powder with an average particle size (particle size) of approximately 50 μm in a glove box under an inert atmosphere, held in place by a tablet refiner on the porous copper metal, and compacted 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 mixture was applied to carbon paper used as the current collector foil and dried at 80°C for 12 hours. A coin cell made of SUS was fabricated using the negative electrode, positive electrode, and 0.4 M Mg(TFSA)2 / G2 as the electrolyte.

[0038] <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.

[0039] Table 1 shows the characteristics and test results of the negative electrode in Example 1. Figure 2 shows the charge-discharge curve for Example 1. In the judgment shown in Table 1, "OK" indicates that reversible charging and discharging is possible, "○" indicates that both conditions are met, including the ability to raise the upper limit potential to 4.0V, and "×" indicates that at least one of these conditions is not met. Note that the product containing LiBH4 as an additive (Comparative Example 3) could not raise the operating potential to 4.0V. [Table 1]

[0040] (Comparative Example 1) In Comparative Example 1, the negative electrode was made by machining AZ31, a magnesium alloy containing 3% by weight of aluminum and 1% by weight of zinc, in a glove box to an average particle size (particle size) of approximately 50 μm, and then compacting it using a hydraulic press at 20 MPa. The configuration other than the negative electrode was the same as in Example 1. The test was conducted under the same conditions as in Example 1. Figure 3 shows the charge-discharge curve for Comparative Example 1.

[0041] (Comparative Example 2) In Comparative Example 2, a plate-shaped current collector (plate electrode) was used as the negative electrode, which was made by cutting a plate-shaped LZ91 into the shape of a coin cell. In other words, Comparative Example 2 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. Figure 4 shows the charge and discharge curve for Comparative Example 2.

[0042] (Comparative Example 3) Comparative Example 3 is the same as in Example 1, except that it uses an electrolyte solution prepared by adding LiBH4 as an additive to 0.4 M Mg(TFSA)2 / G2. In the electrolyte, the molar ratio of TFSA:LiBH4 was set to TFSA:LiBH4 = 4:3. Figure 5 shows the charge-discharge curve for Comparative Example 3.

[0043] Figures 2-5 show the charge-discharge curves for Example 1 and Comparative Examples 1-3. In Example 1, good results were obtained by using a porous metal to hold LZ91 and a 0.4M Mg(TFSA)2 / G2 electrolyte. This suggests that even without LiBH4 in the electrolyte, the porous metal holding LZ91 enables reversible charging and discharging, resulting in a more active negative electrode material and enabling an operating potential of 2.0V or higher. In contrast, Comparative Example 1 used a porous metal body holding AZ31, and Comparative Example 2 used a metal plate of LZ91. Both exhibited lower activity than Example 1, resulting in increased overpotential and inability to perform reversible charging and discharging in electrolyte systems without LiBH4. Furthermore, while reversible charging and discharging was possible in Comparative Example 3 by including LiBH4 as an additive, including LiBH4 in the electrolyte resulted in an operating potential of less than 2.0V, making it difficult to improve energy density. Based on these results, it can be said that by using a negative electrode in which LZ91 is held in a porous metal body and the electrolyte is 0.4M Mg(TFSA)2 / G2, it is possible to provide a negative electrode for magnesium secondary batteries and a magnesium secondary battery with improved energy density while suppressing overvoltage. [Explanation of symbols]

[0044] 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. 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, The aforementioned non-aqueous electrolyte is prepared by dissolving a magnesium salt in an organic solvent. The aforementioned magnesium metal material is a magnesium alloy in which multiple crystal structures with different characteristics are mixed together. A magnesium secondary battery characterized by the following features.

2. The aforementioned organic solvent is a glyme solvent. The magnesium secondary battery according to feature 1.

3. The aforementioned magnesium salt is Mg(TFSA) ２ That is, The magnesium secondary battery according to feature 1.

4. The magnesium metal material is LZ91. The magnesium secondary battery according to feature 1.

5. The aforementioned non-aqueous electrolyte is LiBH ４ It does not contain The magnesium secondary battery according to feature 1.

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