Positive electrode for magnesium ion secondary battery and magnesium ion secondary battery

Abstract

To provide a positive electrode for a magnesium ion secondary battery and a magnesium ion secondary battery that can suppress the occurrence of internal short circuits while increasing capacity.SOLUTION: A positive electrode for a magnesium ion secondary battery according to the present invention is a positive electrode for a magnesium ion secondary battery containing disulfide (MeS2: Me is a metal) as a positive electrode active material, and the average primary particle diameter X of the disulfide is 0.3 μm≤X≤1.0 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a magnesium ion secondary battery and a magnesium ion secondary battery. [Background technology]

[0002] In recent years, environmental issues have become a serious problem. Therefore, natural energy power generation is desired from the perspective of environmental impact. However, natural energy supply is unstable, and once generated, electrical energy needs to be stored. Therefore, there is a demand for high-capacity energy storage devices that can store such large amounts of electrical energy.

[0003] Currently, lithium-ion secondary batteries and magnesium-ion secondary batteries are known as power storage devices (see, for example, Patent Documents 1 and 2). Among these, magnesium-ion secondary batteries have attracted attention as batteries that are expected to have high energy density because their carriers are divalent magnesium ions. Furthermore, magnesium metal is more stable in air than lithium metal and has a theoretical capacity per volume that is approximately 1.5 times that of lithium metal, so there is a demand for the practical application of metal secondary batteries that use magnesium metal as the anode.

[0004] Furthermore, metal secondary batteries using lithium metal have the problem of internal short circuits caused by the deposition of lithium metal in the form of dendrites.On the other hand, metal secondary batteries using magnesium metal as the anode have a low possibility of internal short circuits because the deposited magnesium metal is spherical, and are thought to be superior to other metal secondary batteries in terms of safety and cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 013328 [Patent Document 2] Patent No. 6989263 Summary of the Invention [Problem to be solved by the invention]

[0006] Disulfide (MeS2: Me is a metal) is used as the positive electrode active material for magnesium-ion secondary batteries. Using disulfide as a positive electrode active material is expected to increase capacity by utilizing conversion reactions and the redox contribution of the contained metal's tetravalent cations. However, if the disulfide particle size is large, the specific surface area decreases, hindering the movement of magnesium ions, resulting in increased polarization and a lack of battery capacity. On the other hand, if the disulfide particle size is small, the specific surface area increases, causing the electrolyte to decompose, resulting in current concentration in some areas of the magnesium metal negative electrode during charging, causing the deposited magnesium to segregate and causing an internal short circuit.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a positive electrode for a magnesium ion secondary battery and a magnesium ion secondary battery that can suppress the occurrence of internal short circuits while increasing capacity. [Means for solving the problem]

[0008] In order to solve the above problems, the positive electrode for a magnesium ion secondary battery according to the present invention is, in a first aspect, a positive electrode for a magnesium ion secondary battery containing a disulfide (MeS2: Me is a metal) as a positive electrode active material, and is characterized in that the average primary particle diameter X of the disulfide is 0.3 μm≦X≦1.0 μm.

[0009] When used as the positive electrode for the magnesium ion secondary battery, the optimal particle size of the positive electrode active material suppresses polarization, enabling a high capacity to be achieved, and the magnesium that is electrodeposited during charging does not segregate, making it possible to suppress internal short circuits.

[0010] In addition to the first aspect, the positive electrode for a magnesium ion secondary battery according to the present invention is characterized in that, as a second aspect, the Me is a transition metal.

[0011] In addition, as a third aspect, a magnesium ion secondary battery according to the present invention includes a positive electrode, a negative electrode containing a magnesium metal material, a separator, and a non-aqueous electrolyte, wherein the positive electrode contains disulfide (MeS2: Me is a metal) as a positive electrode active material, and the average primary particle diameter X of the disulfide is 0.3 μm≦X≦1.0 μm.

[0012] In addition to the third aspect, the magnesium ion secondary battery according to the present invention is characterized in that, as a fourth aspect, lithium borohydride (LiBH4) is added to the non-aqueous electrolyte solution as an additive. [Effects of the Invention]

[0013] According to the present invention, in a magnesium ion secondary battery, it is possible to increase the capacity while suppressing the occurrence of an internal short circuit. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is an exploded perspective view illustrating the configuration of a magnesium ion secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing charge / discharge curves according to Example 1. As shown in FIG. [Figure 3] FIG. 3 is a diagram showing charge / discharge curves according to Example 2. As shown in FIG. [Figure 4] FIG. 4 is a diagram showing charge / discharge curves according to Example 3. [Figure 5] FIG. 5 is a diagram showing charge / discharge curves according to Example 4. [Figure 6] FIG. 6 is a diagram showing charge / discharge curves according to Comparative Example 1. As shown in FIG. [Figure 7] FIG. 7 is a diagram showing charge / discharge curves according to Comparative Example 2. As shown in FIG. [Figure 8]FIG. 8 is a diagram showing charge / discharge curves according to Comparative Example 3. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] A first embodiment of the present invention will be described below, but the present invention is not limited to the following description. Various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

[0016] The magnesium (Mg) ion secondary battery of the present invention includes a positive electrode, a separator, a negative electrode, an electrolyte, and an exterior body 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.

[0017] (Embodiment) 1 is an exploded perspective view illustrating the configuration of a magnesium ion secondary battery according to one embodiment of the present invention. Magnesium ion secondary battery 1 includes case 110, leaf spring 111, positive electrode current collector 112, positive electrode composite layer 113, separator 114, negative electrode 115, gasket 116, and cap 117. Positive electrode current collector 112 and positive electrode composite layer 113 form positive electrode 118. The battery is not limited to this configuration, and a configuration including multiple positive electrodes and negative electrodes is also preferably used, and the battery can also be used as a magnesium ion secondary battery in a laminate pouch.

[0018] In magnesium ion secondary battery 1, case 110 and cap 117 are fixed together by caulking or the like, and the interior is filled with a non-aqueous electrolyte. Magnesium ion secondary battery 1 is liquid-tightly sealed by case 110, gasket 116, and cap 117. Furthermore, positive electrode current collector 112, positive electrode composite layer 113, separator 114, and negative electrode 115 are biased toward cap 117 by leaf spring 111. This keeps the components in close contact with each other.

[0019] (positive electrode) According to one embodiment of the present invention, there is provided a positive electrode (positive electrode 118) for a magnesium ion secondary battery comprising at least a positive electrode current collector and a positive electrode mixture layer provided on one or both sides of the positive electrode current collector.

[0020] Although there are no particular limitations on the material that constitutes the positive electrode current collector 112, it is preferable to use a metal. Specific examples include copper, aluminum, nickel, stainless steel, titanium, and other alloys. Among these, aluminum is preferable from the viewpoint of electronic conductivity and battery operating potential.

[0021] Examples of the binder used in the positive electrode composite layer 113 include 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, and the like, or a mixture thereof.

[0022] The positive electrode active material is a disulfide (MeS2: Me is a metal). In this embodiment, the average primary particle diameter X of the disulfide is 0.3 μm≦X≦1.0 μm. Using a positive electrode active material that satisfies this particle diameter range can suppress the occurrence of internal short circuits and increase capacity. In this case, if the average primary particle diameter X is less than 0.3 μm, the specific surface area increases, and the discharge capacity increases. However, the increased specific surface area also accelerates decomposition of the nonaqueous electrolyte. As a result, during charging, current concentrates in some areas on the negative electrode, causing the electrodeposited magnesium to segregate, potentially causing an internal short circuit. On the other hand, if the average primary particle diameter X is greater than 1.0 μm, the particle diameter becomes relatively large and the specific surface area becomes small, which may hinder the movement of magnesium ions, increasing polarization and potentially reducing battery capacity.

[0023] The average primary particle size was determined by averaging 10 randomly selected primary particles during electrode observation using a scanning electron microscope (SEM). The average primary particle size can be adjusted by purchasing a cathode active material that matches the required average primary particle size, or by ball milling. The faster the rotation speed and the longer the time, the finer the grinding. For example, ball milling FeS2 with an average primary particle size of 10 μm at 300 rpm for 3 hours yields an active material with an average primary particle size of 0.5 μm; ball milling FeS2 with an average primary particle size of 10 μm at 300 rpm for 4 hours yields an active material with an average primary particle size of 0.4 μm; and ball milling FeS2 with an average primary particle size of 10 μm at 300 rpm for 5 hours yields an active material with an average primary particle size of 0.3 μm.

[0024] Furthermore, it is preferable to use a transition metal for Me. Among them, Fe, Co, and Ni are preferable. By using Fe, Co, or Ni as the transition metal, the redox effect of the cation can be obtained.

[0025] In addition, the positive electrode active material may contain, for example, one of conductive carbon powder such as graphite or carbon black, carbon nanotubes, carbon nanofibers, graphene, or a mixture thereof to improve electronic conductivity.

[0026] (Negative electrode) According to one embodiment of the present invention, there is provided a negative electrode (negative electrode 115) for a magnesium ion secondary battery, which includes at least a negative electrode current collector and a magnesium layer made of a magnesium metal material provided on one or both sides of the negative electrode current collector. Alternatively, the negative electrode 115 may not include a negative electrode current collector.

[0027] Although there are no particular limitations on the material constituting the negative electrode current collector, it is preferable to use a metal. Specific examples include copper, aluminum, nickel, stainless steel, titanium, and other alloys. Among these, copper is preferred from the viewpoints of electronic conductivity and battery operating potential.

[0028] Examples of magnesium metal materials include pure magnesium metal and magnesium alloys containing other elements.

[0029] (Non-aqueous electrolyte) Examples of magnesium salts contained in the non-aqueous electrolyte include, but are not limited to, 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.

[0030] The non-aqueous solvent contained in the non-aqueous electrolyte is not particularly limited, and examples thereof include one or a mixed solvent of two or more 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 (triglyme), tetraglyme dimethyl ether (tetraglyme), sulfolane, and the like.

[0031] It is desirable to add lithium borohydride (LiBH4) as an additive to the nonaqueous electrolyte. When LiBH4 is used as an additive, it acts as a reducing agent, removing the passive film on the surface of the negative electrode made of magnesium metal, thereby improving the dissolution and deposition activity of the negative electrode. Furthermore, the ratio of magnesium salt to LiBH4 added is preferably 4:3. Adding too much LiBH4 increases the LiBH4 concentration in the electrolyte, which may reduce the activity of the magnesium dissolution and deposition reaction. Adding too little LiBH4 may not sufficiently remove the passive film on the negative electrode surface, which may reduce the activity of the magnesium dissolution and deposition reaction. However, because LiBH4 is a reducing agent, it promotes metal corrosion, making high-voltage operation difficult. However, since the disulfide used as the positive electrode active material can operate at a low potential, by combining it with the above additive, the overvoltage caused by the magnesium metal negative electrode can be suppressed, and a higher capacity can be expected.

[0032] (separator) Examples of the separator include a porous sheet separator made of polymer or fiber, and a nonwoven fabric separator.

[0033] In this embodiment, magnesium ion secondary battery 1 is prepared by arranging leaf spring 111, positive electrode current collector 112, positive electrode composite layer 113, separator 114, negative electrode 115, and gasket 116 in this order, sandwiching them between case 110 and cap 117, filling the battery with a nonaqueous electrolyte, and then fastening case 110 and cap 117 together by crimping or the like to form a liquid-tight seal, followed by an activation process. [Example]

[0034] The present invention will be described in more detail below by way of examples, but the present invention is not limited to the following embodiments.

[0035] Example 1 In Example 1, the disulfides listed in Table 1 were used as the positive electrode active material. The slurry was prepared by mixing the positive electrode active material, conductive additive (acetylene black), and binder (PVdF) in a ratio of 8:1:1, and the mixture was applied to carbon paper used as a current collector foil and dried at 80°C for 12 hours. AZ31, an alloy containing 3% aluminum and 1% zinc, was used as the magnesium metal negative electrode. A coin cell with a stainless steel exterior was fabricated using an electrolyte prepared by adding LiBH4 to Mg(TFSA)2 / G2.

[0036] <Constant current test> A constant current test was performed using the coin cell. The measurement conditions were a current density of 10 mA / g, a voltage range of 0 to 2.0 V, and a constant current charge / discharge test was performed for three cycles. The measurement temperature was room temperature (25°C). A glass filter was used as the separator. Figure 2 shows the charge / discharge curve.

[0037] <Measurement of primary particle size> The primary particle diameter was determined as the average of 10 primary particles randomly selected when observing the electrode with a scanning electron microscope (SEM). The type and average primary particle size (μm) of the disulfide in Example 1 are shown in Table 1. [Table 1]

[0038] Example 2 In Example 2, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 3.

[0039] Example 3 In Example 3, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 4.

[0040] Example 4 In Example 4, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 5.

[0041] (Comparative Example 1) In Comparative Example 1, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 6.

[0042] (Comparative Example 2) In Comparative Example 2, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 7.

[0043] (Comparative Example 3) In Comparative Example 3, the positive electrode active material used was a disulfide having an average secondary particle diameter shown in Table 1. The configuration other than the positive electrode active material was the same as in Example 1. The test was performed under the same conditions as in Example 1. The charge / discharge curves are shown in Figure 8.

[0044] As shown in Figures 2 to 5, in Examples 1 to 4, by using a cathode active material in which the average primary particle diameter X of the disulfide was within the range of 0.3 μm≦X≦1.0 μm, a reversible capacity of approximately 200 mAh / g was obtained, and furthermore, up to three cycles of reversible charge / discharge were possible. On the other hand, as shown in Figure 6, in Comparative Example 1, in which the average primary particle diameter of the disulfide was greater than 1.0 μm, the large primary particle diameter of the cathode active material resulted in large polarization, and reversible capacity was not obtained. Furthermore, as shown in Figures 7 and 8, in Comparative Examples 2 and 3, in which the average primary particle diameter of the disulfide was less than 0.3 μm, the small primary particle diameter of the cathode active material resulted in a voltage drop due to an internal short circuit of the electrodeposited magnesium during the first charge cycle. From these results, by using a cathode active material in which the average primary particle diameter X of the disulfide was within the range of 0.3 μm≦X≦1.0 μm, a magnesium ion secondary battery positive electrode and a magnesium ion secondary battery can be provided that have high capacity while suppressing the occurrence of internal short circuits. [Explanation of symbols]

[0045] 1. Magnesium-ion 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. Disulfide (MeS) was used as the positive electrode active material. ２ : Me is a metal), The average primary particle diameter X of the disulfide is 0.3 μm≦X≦1.0 μm, A positive electrode for a magnesium ion secondary battery.

2. The Me is a transition metal.

2. The positive electrode for a magnesium ion secondary battery according to claim 1 .

3. A positive electrode and a negative electrode including a magnesium metal material; A separator; a nonaqueous electrolyte; Equipped with The positive electrode is Disulfide (MeS) was used as the positive electrode active material. ２ : Me is a metal), The average primary particle diameter X of the disulfide is 0.3 μm≦X≦1.0 μm, A magnesium ion secondary battery characterized by:

4. The non-aqueous electrolyte contains lithium borohydride (LiBH ４ ) is added, 4. The magnesium ion secondary battery according to claim 3.

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