Magnesium secondary battery and method for manufacturing magnesium secondary battery

Abstract

To provide a magnesium secondary battery in which generation of internal short-circuits can be suppressed while reducing over-voltage, and a method for manufacturing the magnesium secondary battery.SOLUTION: The magnesium secondary battery includes: a positive electrode; a negative electrode having a magnesium layer made of a magnesium alloy in an eutectic state; a separator; and an electrolyte. In powder X-ray diffraction measurement using a CuKα line of the negative electrode, the diffraction peak corresponding to the (100) plane within the range of 2θ=32°±5° and the diffraction peak corresponding to the (002) plane within the range of 2θ=35°±5° are used to determine the intensity ratio Y, defined as the diffraction peak intensity ratio of (100) / (002), and Y<15 is established.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a magnesium secondary battery and a method for manufacturing a magnesium secondary battery. [Background technology]

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

[0003] Currently, lithium ion secondary batteries and magnesium secondary batteries are known as energy storage devices. Among them, magnesium secondary batteries have attracted attention as batteries that can achieve high energy density because the carrier is a divalent magnesium ion. In addition, magnesium metal is more stable in air than lithium metal and has about 1.5 times the theoretical capacity per volume, so there is a demand for practical use of magnesium metal as a metal secondary battery in the negative electrode.

[0004] However, in the hexagonal close-packed (hcp) crystal structure of magnesium, the (0001) plane, which is the bottom surface, is electrochemically inactive and does not exhibit reversible dissolution and precipitation reactions during charging and discharging, making it difficult to put magnesium secondary batteries into practical use.

[0005] Therefore, Patent Document 1 describes that by including lithium in a magnesium alloy, two phases of a body-centered cubic lattice structure and a hexagonal close-packed structure coexist. Although there is no such description in Patent Document 1, the state in which lattice structures coexist is sometimes referred to as a eutectic state. Since the body-centered cubic lattice structure has a lower atomic packing factor than the hexagonal close-packed structure, the insertion and desorption of magnesium ions into the bulk are carried out smoothly. As a result, it is considered that the charge and discharge characteristics are improved and the overvoltage during the dissolution and precipitation of magnesium is reduced.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, since the eutectic state material has different crystal structures mixed therein, on the electrode surface, some electrochemically active surfaces are exposed in the vicinity of grain boundaries where different crystal structures are adjacent to each other. If current concentration occurs at the locations where these partially active surfaces are exposed, the magnesium deposited will precipitate in a tower shape and penetrate the separator, causing an internal short circuit. This is a problem that does not occur in other magnesium metal negative electrodes and is a major drawback of the above material when compared with other magnesium metal negative electrodes.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a magnesium secondary battery and a method for manufacturing the magnesium secondary battery that can suppress the occurrence of an internal short circuit while reducing the overvoltage.

Means for Solving the Problems

[0009] In order to solve the above problems, the magnesium secondary battery according to the present invention includes a positive electrode, a negative electrode having a magnesium layer made of a eutectic magnesium alloy, a separator, and an electrolytic solution. In the powder X-ray diffraction measurement using the CuKα ray of the negative electrode, when the intensity ratio Y is defined as the diffraction peak intensity ratio of (100) / (002), 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°, 15 < Y. Note that the diffraction peak intensity ratio may also be simply referred to as the "peak intensity ratio".

[0010] When the above magnesium secondary battery is used, even when a eutectic magnesium alloy is used, it is possible to suppress internal short circuits. Therefore, it is possible to provide a magnesium secondary battery capable of suppressing the occurrence of internal short circuits and reducing overvoltage.

[0011] In addition to the first aspect, as a second aspect, the magnesium secondary battery according to the present invention is characterized in that the magnesium alloy has a crystal structure in which a hexagonal close-packed structure and a body-centered cubic structure are in a eutectic state.

[0012] In addition to the first or second aspect, as a third aspect, the magnesium secondary battery according to the present invention is characterized in that the magnesium alloy contains lithium in the range of 6% by weight or more and 10.5% by weight or less.

[0013] In addition, as a fourth aspect, the manufacturing method of the magnesium secondary battery according to the present invention is a manufacturing method of a magnesium secondary battery including a positive electrode, a negative electrode having a magnesium layer made of a magnesium alloy, a separator, and an electrolytic solution, and includes an activation step of performing charge and discharge at 5 mA / cm 2 or more.

[0014] In addition to the fourth aspect, as a fifth aspect, the manufacturing method of the magnesium secondary battery according to the present invention is characterized in that one charge-discharge cycle in which charge and discharge are each performed for 1 hour or more is performed 10 cycles or more.

[0015] In addition to the fourth or fifth aspect, the manufacturing method for a magnesium secondary battery according to the present invention is characterized as a sixth aspect in that it includes one charge / discharge cycle in which the SOC upper limit during charging is 2% or more and 10% or less.

[0016] In addition to any one of the fourth or fifth aspects, the manufacturing method for a magnesium secondary battery according to the present invention is characterized, as a seventh aspect, in that a negative electrode having a magnesium layer made of a magnesium alloy containing 3% or more by weight of other metals is used.

[0017] In addition to any one of the fourth to seventh aspects, the manufacturing method of a magnesium secondary battery according to the present invention is characterized, as an eighth aspect, in that a negative electrode having a magnesium layer made of a magnesium alloy in a eutectic state is used. Effect of the Invention

[0018] According to the present invention, in a magnesium secondary battery, it is possible to reduce overvoltage and suppress the occurrence of internal short circuits. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is an exploded perspective view illustrating the configuration of a magnesium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A first embodiment of the present invention will be described below, but the present invention is not limited to the following description. In addition, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

[0021] The magnesium secondary battery of the present invention includes a positive electrode, a separator, a negative electrode, an electrolyte, and an exterior body that contains 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.

[0022] (Embodiment) 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 form a positive electrode 118. The battery is not limited to this form, and a form including a plurality of positive electrodes and negative electrodes is also preferably used, and the battery can also be used as a magnesium secondary battery in a laminate pouch.

[0023] In magnesium secondary battery 1, case 110 and cap 117 are fixed by caulking or the like, and the inside is filled with a nonaqueous electrolyte. Magnesium secondary battery 1 is liquid-tightly sealed by case 110, gasket 116, and cap 117. In addition, 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 each member in close contact with one another.

[0024] (positive electrode) According to one embodiment of the present invention, there is provided a positive electrode for a magnesium secondary battery (positive electrode 118) including 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.

[0025] There is no particular limitation on the material constituting the positive electrode current collector 112, but 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.

[0026] 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, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, and the like, or a mixture thereof.

[0027] The positive electrode active material is MgCo 2 O 4 , MgFeSiO 4 , S, MnO 2 , Mo 6 S 8 , V 2 O 5 In order to improve electronic conductivity, the conductive carbon powder such as graphite or carbon black, carbon nanotubes, carbon nanofibers, graphene, or a mixture thereof may be included.

[0028] (Negative electrode) According to one embodiment of the present invention, there is provided a negative electrode (negative electrode 115) for a magnesium secondary battery comprising 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. Also, the negative electrode current collector may not be used.

[0029] There is no particular limitation on the material constituting the negative electrode current collector, but it is preferable to use a metal. Specific examples include copper, aluminum, nickel, stainless steel, titanium, and other alloys. Among them, copper is preferable from the viewpoint of electronic conductivity and battery operating potential.

[0030] The magnesium metal material is a magnesium metal alloy in a eutectic state. In particular, the magnesium metal material is preferably a magnesium alloy containing 3% or more by weight of other metals as a magnesium layer. This is because the magnesium metal material is more strongly affected by the method of controlling the electrode surface state described later than magnesium alone. Furthermore, the magnesium metal alloy negative electrode is preferably a magnesium metal alloy negative electrode in a eutectic state in which two phases, a hexagonal close-packed (hcp) structure and a body-centered cubic (bcc) structure, coexist. More preferably, the magnesium metal material is a magnesium metal alloy containing lithium in the range of 6% by weight to 10.5% by weight. The reason why the negative electrode according to the present embodiment is preferable is that the bcc structure has a lower atomic packing rate than the hcp structure, so that the insertion and desorption of magnesium ions into and from the bulk is performed smoothly, and as a result, the charge and discharge characteristics are improved and the overvoltage during the dissolution and precipitation of magnesium is reduced. If a negative electrode other than that of the present invention is used, it is difficult to insert and desorb magnesium ions into and from the bulk, and there is a risk that good charge and discharge characteristics cannot be obtained. In this specification, the eutectic state includes a hypoeutectic state and a hypereutectic state, and indicates that two or more types of crystal structures can be confirmed.

[0031] When magnesium metal material is in a eutectic state, in the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, in addition to the peak derived from the hcp structure, there is a peak attributable to the (220) plane derived from the bcc structure, which exists in the 2θ range of 65° to 66°.

[0032] In the present embodiment, the magnesium metal material has a diffraction peak indicating the (100) plane within the range of 2θ = 32° ± 5° and a diffraction peak indicating the (002) plane within the range of 2θ = 35° ± 5° in the X-ray diffraction measurement of the powder. When the intensity ratio Y is defined as the diffraction peak intensity ratio of (100) / (002), it is characterized in that 15 < Y. The reason why it is preferable to use the negative electrode according to the present embodiment is that when a eutectic magnesium metal alloy negative electrode is used for the negative electrode, it is possible to suppress internal short circuit. The factors causing internal short circuit are as follows. First, when a eutectic magnesium metal alloy negative electrode is used, different crystal structures are mixed. Therefore, on the electrode surface, at the vicinity of the grain boundary where different crystal structures are adjacent to each other, a part of the electrochemically active surface is exposed. For this reason, current concentration occurs at the location where a part of the active surface is exposed, and magnesium deposited there precipitates in a tower shape. As a result, the deposit penetrates the separator and causes an internal short circuit. This is a problem that does not occur in other magnesium metal negative electrodes. To solve this problem, in the X-ray diffraction measurement, a eutectic magnesium metal alloy negative electrode with a peak intensity ratio Y of the (100) / (002) plane satisfying 15 < Y is used. Here, the crystal structure of magnesium will be described. The (001) and (002) plane orientations with the orientation plane of magnesium being the bottom surface are electrochemically inactive surfaces. On the other hand, the (101) and (110) plane orientations derived from the side surface are electrochemically active surfaces. It is known that the (110) plane is more electrochemically active than the (001) plane (for example, see C. Ling et al, electrochmica Acta 76(2012), 270 - 274). The (002) plane indicating the orientation of the bottom surface of magnesium, together with the (001) plane, is also less active than the (110).

[0033] Here, the peak intensity of the (100) plane has a positive correlation with the peak intensities of the (101) and (110) planes, and represents the peak intensities of the (101) and (110) planes. Also, the intensity ratio Y is the current density (5 mA / cm 2) It is set based on the value activated at (), and in this embodiment, it is set to 15 < Y. Further, it is more preferable that 15 < Y < 30, and even more preferable that 15 < Y < 20. When using a magnesium metal alloy negative electrode with a peak intensity ratio Y of the (100) / (002) plane of 15 < Y, there will be many active surfaces on the electrode surface. Then, as the active surface of magnesium increases, the deposition of magnesium during electrodeposition is suppressed from being biased and deposited in a tower shape, and it becomes possible to suppress internal short circuits. On the other hand, when using a magnesium metal alloy negative electrode with a peak intensity ratio Y of the (100) / (002) plane of Y ≧ 30, the overvoltage may become large during the activation process and activation may not be possible.

[0034] Also, by using a eutectic magnesium metal alloy having a surface state with a peak intensity ratio Y of the (100) / (002) plane of 15 < Y, there will be many active surfaces on the electrode surface, so the movement of magnesium ions will be smoother. As a result, it is possible to reduce the overvoltage during the dissolution and deposition of magnesium.

[0035] Starting from a eutectic magnesium metal alloy negative electrode that has just been completed as an alloy, ordinary magnesium metal negative electrodes do not satisfy the state where the peak intensity ratio Y of the (100) / (002) plane is 15 < Y. Therefore, in order to control the electrode surface state like the above magnesium metal alloy negative electrode, an activation process was performed on the eutectic magnesium metal alloy negative electrode. At this time, when performing constant current charge and discharge at a low current density, for example, less than 5 mA / cm 2 Particles with many bottom surfaces ((001) plane or (002) plane), which are thermodynamically stable crystal planes, will be formed as the deposited magnesium. On the other hand, by performing constant current charge and discharge at a high current density, particles with many side surfaces ((110) plane or (100) plane), which are thermodynamically unstable, will be deposited. That is, by performing a manufacturing method including an activation process with controlled current density, it becomes possible to produce a magnesium metal alloy negative electrode with a peak intensity ratio Y of the (100) / (002) plane of 15 < Y.

[0036] The peak intensity ratio Y of the (100) / (002) plane can be adjusted, for example, by the current density during the activation step. 2 If the current density is set to above 20 mA / cm, the electrochemically active side surfaces ((110) and (100)) will be more oriented on the surface of the deposited magnesium, and the intensity ratio Y will also be large. However, if the current density is set too high, the overvoltage will increase and constant current charging and discharging will become impossible. Therefore, the current density should be set to 20 mA / cm 2 Preferably less than 15 mA / cm 2 More preferably, it is less than 10 mA / cm 2 It is even more preferable that it is less than 1000 μm. Here, for example, a magnesium alloy containing 9% by weight of lithium (e.g., LZ91) is a eutectic magnesium alloy, so that the magnesium that is electrodeposited is localized. Furthermore, when charging and discharging at a low current density, the active surface of the deposited magnesium is small, so that the magnesium is electrodeposited in a concentrated manner in a certain area and formed in a tower shape, which may penetrate the separator and cause a short circuit. Therefore, by charging and discharging at a high current density, the deposited magnesium has many active surfaces oriented and is deposited in a multifaceted manner, suppressing the tower-shaped deposition and suppressing a short circuit.

[0037] In addition, in magnesium alloys, metals that can be in a eutectic state include scandium, strontium, and the like in addition to lithium. Lithium is less expensive, so it is industrially preferable. In addition, the eutectic state of the crystal structure is a eutectic state of two or more structures selected from, for example, an hcp structure, a bcc structure, and an fcc (face-centered cubic) structure, depending on the substitute metal. The eutectic state derived from magnesium and lithium is a eutectic state of an hcp structure and a bcc structure.

[0038] (electrolyte) The magnesium salt contained in the non-aqueous electrolyte is MgCl 2 , MgBH 4 , Mg(NO 3 )2 , Mg(TFSI) 2 , Mg(SO 2 CF 3 ) 2 , Mg(BF 4 ) 2 , Mg(CF 3 SO 3 ) 2 , Mg(PF 6 ) 2 Examples of the suitable amines include, but are not limited to, one or a mixture of two or more selected from the following:

[0039] 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.

[0040] (Separator) Examples of the separator include a porous sheet separator made of polymer or fiber, and a nonwoven fabric separator.

[0041] In this embodiment, the magnesium secondary battery 1 is prepared by arranging the leaf spring 111, the positive electrode current collector 112, the positive electrode composite layer 113, the separator 114, the negative electrode 115, and the gasket 116 in this order, sandwiching them between the case 110 and the cap 117, filling them with a non-aqueous electrolyte, and then fixing the case 110 and the cap 117 by caulking or the like to provide a liquid-tight seal, after which an activation process is carried out. In this activation process, a current of 5 mA / cm 2It is recommended to charge the battery at a current density of 5mA / cm for at least one hour. 2 It is preferable to perform 10 or more cycles of discharging at a current density of 10% or more for 1 hour or more. In addition, it is preferable to perform the activation process while keeping the SOC (State of Charge) of the battery between 2% and 10% as the upper limit of the voltage during charging. The maximum SOC may be between 2% and 10% for one cycle, but it is more preferable to satisfy this condition for all cycles. This ensures that a negative electrode with the above-mentioned peak intensity ratio Y>15 can be obtained. EXAMPLES

[0042] 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.

[0043] (Level 1) <Constant current test> In level 1, the electrodes are made of magnesium alloy containing 9% by weight of lithium, the separator is made of polyethylene, and the electrolyte is Mg(TSFA). 2 LiBH to / G2(Diethyleneglycol dimethyl ether) 4 A constant current test was carried out on a symmetrical cell with a laminate exterior using an electrolyte containing an additive of 1 mA / cm. The measurement conditions were: 2 The battery was charged for one hour and discharged for one hour for 100 cycles at room temperature (25°C). <Activation process> Before performing the constant current test, the above cell was charged at 5 mA / cm as an activation step. 2 An activation process was performed by performing 10 cycles of charging at 400° C. for 1 hour and discharging at 400° C. for 1 hour. The test results for Level 1 are shown in Table 1. [Table 1]

[0044] (Level 2) The battery structure is the same as that of Level 1. The test method was performed under the same conditions as Level 1, except for the activation process. In addition, before the constant current test, a 10 mA / cm 2 The battery was charged at 1000 V for 1 hour and discharged at 1000 V for 1 hour, and the activation process was carried out for 10 cycles. The test results for Level 2 are shown in Table 1.

[0045] (Level 3) The battery structure is the same as that of Level 1. The test method was performed under the same conditions as Level 1, except for the activation process. In addition, before the constant current test, a 5 mA / cm 2 The battery was charged at 1000 V for 0.5 hours and discharged at 1000 V for 0.5 hours, and the activation process was carried out for 10 cycles. The test results for level 3 are shown in Table 1.

[0046] (Level 4) The battery structure is the same as that of Level 1. The test method was performed under the same conditions as Level 1, except for the activation process. In addition, before the constant current test, a 4 mA / cm 2 The activation process was carried out by charging at 1000 kPa for 1 hour and discharging at 1000 kPa for 1 hour. The test results for level 4 are shown in Table 1.

[0047] (Level 5) The battery structure is the same as that of Level 1. The test method was the same as Level 1, except for the activation process. In addition, before the constant current test, a 1 mA / cm 2 The battery was charged at 1000 V for 1 hour and discharged at 1000 V for 1 hour, and the activation process was carried out for 10 cycles. The test results for level 5 are shown in Table 1.

[0048] (Level 6) The battery structure is the same as that of Level 1. The test method was performed under the same conditions as Level 1, except for the activation process. In addition, before the constant current test, a 5 mA / cm 2 The battery was charged at 1000 V for 0.4 hours and discharged at 1000 V for 0.4 hours, and the activation process was carried out for 10 cycles. The test results for level 6 are shown in Table 1.

[0049] (Level 7) The battery structure is the same as in Level 1. The test method was the same as Level 1, except for the activation process. In addition, before the constant current test, a 5 mA / cm 2 The battery was charged for 2.5 hours and discharged for 2.5 hours at 1000 kPa (1000 psi) for 10 cycles. The test results for level 7 are shown in Table 1.

[0050] (Level 8) The battery structure is the same as in Level 1. The test method was the same as Level 1, except for the activation process. In addition, before the constant current test, a 20 mA / cm 2 The battery was charged at 1000 V for 1 hour and discharged at 1000 V for 1 hour, and the activation process was carried out for 10 cycles. The test results for level 8 are shown in Table 1.

[0051] (Level 9) The battery structure is the same as that of Level 1. The test method was the same as that of Level 1, except for the activation process. In addition, the activation process was not performed. The test results for Level 9 are shown in Table 1.

[0052] From Table 1, in levels 1 to 3 where Y is 15 or more and 30 or less, the overvoltage after one cycle was 0.05 V or less, which was good, and no internal short circuit was observed. 2 In levels 1 to 3 and 6, which include an activation step in which charging is performed at the above temperature, the overvoltage after one cycle was good at 0.12 V or less, and the number of cycles at the time of internal short circuit was also good at at least 52 cycles. On the other hand, in level 5, where Y is less than 15, and level 9, where no activation step was performed, the overvoltage after one cycle was 0.20 V, and the number of cycles at the time of internal short circuit was also 10 or less, which were unfavorable results. In level 7, where Y is greater than 30, the internal short circuit occurred early, and in level 8, where the activation current value was high, the activation step itself was impossible. From the above results, it is possible to provide a magnesium secondary battery that can reduce overvoltage by using a eutectic magnesium alloy, which is an electrochemically active material, for the negative electrode, and that can suppress the occurrence of internal short circuit and reduce overvoltage by controlling the surface state of the electrode. [Explanation of symbols]

[0053] 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 positive electrode and a negative electrode having a magnesium layer made of a eutectic magnesium alloy; A separator; An electrolyte; Equipped with In powder X-ray diffraction measurement of the negative electrode using CuKα rays, when the intensity ratio Y of a diffraction peak representing a (100) plane within the range of 2θ = 32 ° ± 5 ° and a diffraction peak representing a (002) plane within the range of 2θ = 35 ° ± 5 ° is defined as the diffraction peak intensity ratio of (100) / (002), 15 < Y. A magnesium secondary battery comprising:

2. The magnesium alloy has a crystal structure in which a hexagonal close-packed structure and a body-centered cubic structure are in a eutectic state.

2. The magnesium secondary battery according to claim 1 .

3. The magnesium alloy contains lithium in the range of 6% by weight to 10.5% by weight.

3. The magnesium secondary battery according to claim 2 .

4. A method for manufacturing a magnesium secondary battery including a positive electrode, a negative electrode having a magnesium layer made of a magnesium alloy, a separator, and an electrolyte, comprising: 5mA / cm ２ The activation step involves charging and discharging the material. A method for producing a magnesium secondary battery comprising the steps of:

5. 5. The method for producing a magnesium secondary battery according to claim 4, wherein one charge / discharge cycle, in which charging and discharging are each performed for one hour or more, is performed 10 or more times.

6. 5. The method for producing a magnesium secondary battery according to claim 4, comprising one charge / discharge cycle in which the upper limit of SOC during charging is between 2% and 10%.

7. 5. The method for producing a magnesium secondary battery according to claim 4, wherein a negative electrode having a magnesium layer made of a magnesium alloy containing 3% or more by weight of other metals is used.

8. 5. The method for producing a magnesium secondary battery according to claim 4, wherein a negative electrode having a magnesium layer made of a eutectic magnesium alloy is used.

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