Method for manufacturing positive electrode active material and magnesium storage batteries

Abstract

This positive electrode active material for a magnesium storage battery is formed from a material comprising magnesium (Mg), lithium (Li), one or more metal elements (M), and oxygen (O), and having a rock-salt structure represented by general formula (1).　(1) MgxLiyM(1-x-y)O...

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for magnesium storage batteries, and a method for manufacturing a magnesium storage battery using the same. This application claims priority based on Japanese Patent Application No. 2023-039299, filed in Japan on March 14, 2023, and Japanese Patent Application No. 2023-041425, filed in Japan on March 15, 2023, and the contents thereof are incorporated herein by reference. [Background technology]

[0002] In recent years, with the rapid proliferation of information-related devices and communication equipment such as mobile phones, the development of batteries used as their power sources has become increasingly important. Furthermore, the automotive industry and other sectors are also making progress in developing high-output, high-capacity batteries (such as lithium-ion batteries) for electric vehicles and hybrid vehicles.

[0003] On the other hand, lithium, which is used as a raw material for lithium-ion batteries, is not necessarily abundant as a resource, and its production is unevenly distributed, posing challenges to ensuring a stable supply at low cost. For this reason, magnesium batteries, which use magnesium, an abundant resource that is not concentrated in specific locations, are attracting attention as an alternative to lithium.

[0004] Patent Document 1 discloses a positive electrode active material for a magnesium-ion battery containing an olivine-type magnesium silicate compound.

[0005] Furthermore, Patent Document 2 discloses a positive electrode active material for a magnesium-ion battery containing a rhizonic acid-based compound and a perylene-based compound. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2017-208210 [Patent Document 2] Japanese Patent Publication No. 2020-027702 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the positive electrode active material for magnesium-ion batteries disclosed in Patent Document 1 has few vacancies that contribute to the insertion and removal of magnesium ions, and when applied to magnesium storage batteries, it has the problem of poor charge and discharge characteristics.

[0008] On the other hand, the positive electrode active material for magnesium-ion batteries disclosed in Patent Document 2 is an organic compound with a complex structure, such as a rhizonic acid-based compound or a perylene-based compound, which has the problems of low productivity and high cost.

[0009] This invention has been made in view of the above technical background, and aims to provide a positive electrode active material for magnesium batteries that, when applied to magnesium batteries, enables efficient insertion and removal of magnesium ions and achieves high charge-discharge characteristics. [Means for solving the problem]

[0010] The inventors have found that, as a positive electrode active material for magnesium storage batteries, they have introduced vacancies into the DRX structure of a metal oxide material having an irregular rock salt (DRX) type structure, which conventionally made reversible and effective insertion and removal of Mg ions difficult. This was achieved by pre-containing Li in the composition of the metal oxide with the DRX structure, and performing pre-charging prior to actual use as a battery through charging and discharging, thereby removing Li towards the electrolyte. They have discovered that the vacancies thus formed allow for repeated insertion and removal of Mg ions, even in a DRX structure. This invention is based on the newly discovered findings described above, and in order to solve the above problems, it provides a positive electrode active material and a method for manufacturing a magnesium battery having the following requirements.

[0011] (1) The positive electrode active material of embodiment 1 of the present invention is a positive electrode active material for a magnesium storage battery, The material is characterized by comprising magnesium (Mg), lithium (Li), one or more metallic elements (M), and oxygen (O), and having a rock salt-type structure represented by the following general formula (1). Mg x Li y M (1-x-y) O ···(1) In general equation (1), x and y satisfy x>0, y>0, and x+y<1. The following conditions must be met: y ≥ 0.198. M contains one or more elements from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, and Ir. The average valency of M, obtained by weighting the valencies according to their compositional ratios, is greater than 2.0. x and y satisfy the relationship given by the following general formula (2). 0.119 ≤ 4(x+y) 2 y(1-y)(1-y+y 2 ) ···(2)

[0014] ( 2 ) Aspects of the present invention 2 is, 1 In the positive electrode active material, M in the general formula (1) is characterized by further containing one or more elements from among Zn, Al, Si, Ga, Ge, Y, In, Sn, and Pb.

[0017] ( 3 ) Aspects of the present invention 3 The positive electrode active material of embodiment 1 is characterized in that, in the general formula (1), M consists of the elements Cr, Mn, Fe, Zn, and Mo.

[0018] ( 4 ) Aspects of the present invention 4 The method for manufacturing a magnesium storage battery is described in Embodiment 1. 3 A method for manufacturing a magnesium storage battery comprising one of the following positive electrode active materials, an electrolyte, and a negative electrode active material, characterized by having a pre-charging step in which a voltage is applied between the positive electrode active material and the negative electrode active material to diffuse the lithium contained in the positive electrode active material into the electrolyte, thereby forming pores in the positive electrode active material that allow for the insertion and removal of magnesium ions. [Effects of the Invention]

[0019] According to an aspect of the present invention, a positive electrode active material for a magnesium battery is provided that, when applied to a magnesium battery, enables efficient insertion and removal of magnesium ions and achieves high charge-discharge characteristics, and a method for manufacturing a magnesium battery having high charge-discharge characteristics is also provided. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic diagram showing the desorption of Li from the lattice during the pre-charging process of the positive electrode active material in one embodiment. [Figure 2] This is a three-dimensional diagram showing the composition of the positive electrode active material, specifically Li, Mg, and M (total amount of metal elements). [Figure 3] This is a schematic diagram showing the insertion and removal of Mg during charging and discharging of the positive electrode active material. [Figure 4] These are the XRD profile and scanning transmission electron microscope image of the synthesis state of the positive electrode active material in Example 1 of the present invention. [Figure 5] This image shows the results of EDS analysis performed using a scanning transmission electron microscope, broken down by constituent element. [Figure 6] This is a schematic diagram showing the configuration of a magnesium battery used for testing. [Figure 7] This graph shows the results of the CV test in Example 1 of the present invention. [Figure 8] This graph shows the results of the repeated charge-discharge test in Example 1 of the present invention. [Figure 9] This graph shows the comparison between charging capacity and discharging capacity in Example 1 of the present invention. [Figure 10] This graph shows the change in the composition ratio of Li and Mg in the positive electrode active material due to charging and discharging. [Figure 11] This is the XRD profile of the synthesis state of the positive electrode active material in Example 2 of the present invention. [Figure 12] This graph shows the results of the repeated charge-discharge test in Example 2 of the present invention. [Figure 13] This graph shows the comparison between charging capacity and discharging capacity in Example 2 of the present invention.

Best Mode for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show the characteristic parts enlarged for easy understanding of the characteristics, and the dimensional ratios of the respective components are not necessarily the same as the actual ones. Further, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not necessarily limited thereto, and it can be implemented by appropriately changing them without changing the effects.

[0022] [Positive electrode active material] The positive electrode active material for the magnesium storage battery of the present embodiment contains magnesium (Mg), lithium (Li), one or more metal elements (M), and oxygen (O), and is composed of a material having a rock salt type structure represented by the following general formula (1). Mg x Li y M (1-x-y) O ···(1)

[0023] In the general formula (1) above, it is preferable that y≧0.198 is satisfied. In the general formula (1) above, it is preferable that M has an average valence number obtained by weighting the valence numbers according to the composition ratio and is higher than 2.0. In the general formula (1) above, it is preferable that x and y satisfy the relationship of the following general formula (2). [[ID=z9]]0.119≦4(x+y) 2 y(1-y)(1-y+y 2 ) ···(2) It is more preferable that x and y satisfy the relationship of the following general formula. 0.12<4(x+y) 2 y(1-y)(1-y+y 2 )

[0024] In general formula (1), the metal element (M) may be one or more types. For example, it is preferable that at least one element from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, and Ir is included. Furthermore, it is more preferable that one or more elements from Zn, Al, Si, Ga, Ge, Y, In, Sn, and Pb are included along with the above metal element.

[0025] In this embodiment, it is preferable that the combination of metal elements (M) in general formula (1) includes the elements Cr, Mn, Fe, Zn, and Mo. That is, it is preferable that the metal elements (M) include all of the elements Cr, Mn, Fe, Zn, and Mo. More specifically, Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 Examples include materials with an O composition and a rock salt-type structure.

[0026] In this embodiment, the positive electrode active material, i.e., a metal oxide having a DRX structure containing Mg, the material conditions for reversibly and smoothly inserting and removing Mg ions when used in a magnesium storage battery are as follows. (1) The Li contained in the initial composition can be removed to create vacancies (the Li contained in the initial composition can be removed to create vacancies). (2) The initial composition should allow for one or two vacancy paths to percolate. (3) Percolation of one or two vacancy paths is not lost during charging and discharging.

[0027] Regarding (1) mentioned above, as shown in Figure 1, in the pre-charging process described later, Li is removed from the synthesized Li-containing positive electrode active material and diffused, for example, into the electrolyte. In other words, if Li contained in the initial composition of the positive electrode active material cannot be removed in the pre-charging process, it will not be possible to create vacancies for inserting and removing Mg ions during subsequent charging and discharging.

[0028] The proportion of Li in the initial composition of the positive electrode active material is related to the critical penetration probability P in the face-centered cubic (fcc) lattice site process. c = 0.198 or higher is sufficient. Therefore, it is preferable that y≧0.198 is satisfied in general formula (1). An example of the composition of the positive electrode active material of the above embodiment (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 In case O), the proportion of Li is 0.3, which satisfies the condition (1) described above.

[0029] Furthermore, regarding (2) mentioned above, when using a DRX structured positive electrode active material in a magnesium battery, it is necessary to allow Mg ions to move easily within the particles. In lattice probability theory, if P is the probability that a movement path exists by percolation (osmosis), then the probability that no such path exists is 1-P. By changing this probability P of the existence of a movement path, clusters of various sizes that can move continuously are formed. Then, as the probability P of the existence of a movement path increases, a phase transition occurs from a non-osmotic phase, where infinitely large clusters do not exist, to an osmotic phase, where clusters exist. The boundary between this non-osmotic phase and the osmotic phase is the critical point, and the probability P at this point is c This is the critical percolation probability. In other words, P > P c When the conditions are met, an infinite cluster is formed, and a phase transition to the osmotic phase occurs.

[0030] Furthermore, the probability of existence of the migration path P > the critical penetration probability P cWhen the conditions are met, the migration paths are infinitely interconnected within the grain, and Mg can easily diffuse using these migration paths. For example, the probability of existence P of a travel path defined as a one-vacancy path. one (P vac ,P M ,P Mg ) is shown by the following equation (3). P one =4(1-P M ) 2 P vac (1-P vac )(1-P vac +P vac 2 ) ···(3) However, P vac : Probability that a vacancy occupies a cation site, P M : Probability that a metal element other than Mg occupies the cation site, P Mg If we consider P as the probability that the element Mg occupies a cation site, then vac +P M +P Mg Assume that the formula satisfies =1 (where P is equivalent to the composition), and that Li completely detaches to form a vacancy.

[0031] This probability P of existence one Based on this, the positive electrode active material is P one ≥P c If the composition satisfies =0.119 (critical penetration probability in the fcc bonding process), Mg can easily diffuse within the site using a low activation energy transport pathway. P vac y is the composition ratio of Li, and P Mg x is the composition ratio of Mg, and P vac +P M +P Mg To satisfy =1, equation (3) becomes equation (4) below. P one =4(x+y) 2 y(1-y)(1-y+y 2 ) ···(4) Therefore, the positive electrode active material should have a composition that satisfies formula (2). Based on this, if the composition amounts of Li, Mg, and M (total amount of metal elements) in the composition of the positive electrode active material are within the range above the curve Q (the line where the critical percolation probability = 0.119) in the ternary diagram shown in FIG. 2, Mg can easily diffuse within the site using a path with a low activation energy.

[0032] Examples of the composition of the actual positive electrode active material within the range above the curve Q in the ternary diagram shown in FIG. 2 are shown below. Mg 0.35 Li 0.3 Mn 0.35 O Mg 0.35 Li 0.3 Ni 0.35 O Mg 0.4 Li 0.2 Al 0.2 Fe 0.2 O Mg 0.3 Li 0.3 Si<00 extraordinariness of the ordinary people. Mn 0.25 O Mg 0.25 Li 0.5 Sn 0.1 Mo 0.1 Ru 0.05 O Mg 0.1 Li 0.5 Ga 0.1 Ge 0.2 Pd 0.1 O Mg 0.1 Li 0.4 Cu 0.1 Ti 0.2 V 0.1 Cr 0.1 O Mg 0.3 Li 0.3 Nb 0.05 Pb<x 0.05 Y 0.05 Zn 0.25 O

[0033] Among the metal elements (M), the elements whose valence can change during charge and discharge when used in a magnesium battery and are considered "active" electrochemically are as follows. Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, Ir On the other hand, among the metal elements (M), the elements that hardly change in valence during charge and discharge and are considered electrochemically "inactive" are as follows. Zn, Al, Si, Ga, Ge, Y, In, Sn, Pb

[0034] Therefore, as can be seen from the above combination examples, the metal element M constituting the positive electrode active material of the present embodiment needs to contain at least one or more of the elements considered electrochemically "active" as described above. On the other hand, by including an element considered electrochemically "inactive", the valence of the entire metal element (M) constituting the positive electrode active material can be adjusted.

[0035] Also, regarding (3) described above, in order to ensure sufficient pores for inserting and desorbing Mg ions during charge and discharge, for example, in the positive electrode active material of an example of the composition of the present embodiment (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O), a metal element (M) whose average valence obtained by weighted averaging the valence according to the composition ratio does not become 2.0 or less is used. Specifically, in an example of the composition of the present embodiment (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O), the above-described electrochemically active metal elements (Cr, Mn, Fe, Mo) and the inactive metal element (Zn) are combined so that the minimum value of the average valence of the metal element (M) is greater than +2.6. The valence adjustment by this is shown in Table 1. In an example of the composition of the present embodiment, the average valence obtained by weighted averaging the valence of the metal element (M) according to the composition ratio can vary between +2.86 and +4.14.

[0036]

Table 1

[0037] As described above, the positive electrode active material of this embodiment allows for the repeated insertion and removal of Mg ions through charging and discharging by using a DRX structured material as the positive electrode active material of a magnesium battery. This contributes to the realization of a magnesium battery with excellent charge-discharge characteristics.

[0038] In the embodiments described above, only Mg ions are mentioned as guest cations inserted into and removed from the positive electrode active material after the pre-charging process. However, a configuration in which Li ions are also inserted into and removed as guest cations along with Mg ions is also possible.

[0039] [Method for manufacturing positive electrode active material] When manufacturing the positive electrode active material of the above-described embodiment, for example, the Pecini method (complex polymerization method) can be used. For example, Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 A method for producing a positive electrode active material having the composition of O is shown below. First, nitrates of the metal elements (Mg, Cr, Mn, Fe, Zn, Li) that constitute the positive electrode active material, ammonium molybdate, and lithium nitrate are prepared. These are then stirred with citric acid, propylene glycol, and distilled water to form a metal-citric acid complex, which is then subjected to ester polymerization to obtain a metal polymerization gel. Stirring can be carried out, for example, at 80°C to 120°C for about 4 to 5 hours.

[0040] Next, this metal polymerization gel is dried at 200°C for about 20 hours, and then heat-treated to produce a high-purity metal oxide. The heat treatment can be carried out at, for example, 400°C to 500°C for 1 to 3 hours.

[0041] The metal oxide thus obtained is then ground into a fine powder using, for example, a ball mill. The ball mill can be used to repeat the process of grinding at a rotation speed of 500 rpm for 5 minutes about 40 times. The fine powdered metal oxide is then heat-treated again. For this heat treatment, for example, heating at 600°C for about 10 hours in an argon gas atmosphere containing 5% or less hydrogen gas is sufficient. After this, the metal oxide is ground into a fine powder again using a ball mill.

[0042] Next, the aforementioned fine powder of metal oxide and sucrose (table sugar) are mixed in a weight ratio of 8:2, distilled water is added, and the mixture is stirred for about 10 minutes, followed by a carbonization heat treatment. For the carbonization heat treatment, for example, heating at 600°C for about 4 hours in an argon gas atmosphere containing 5% or less hydrogen gas is sufficient. After this, the metal oxide is uniformly stirred for about 10 minutes to obtain a positive electrode active material (Mg) as an example of the composition of this embodiment. 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) can be manufactured.

[0043] [Manufacturing method for magnesium storage batteries] When manufacturing a magnesium storage battery using the positive electrode active material of the above-described embodiment, for example, an electrolyte layer is formed inside the package (housing), and on one side of the electrolyte layer, a positive electrode active material layer containing the positive electrode active material of the above-described embodiment and a positive electrode current collector (electrode) are formed, and on the other side, a negative electrode active material layer containing the negative electrode active material and a negative electrode current collector (electrode) are formed to obtain a pre-first-charging magnesium storage battery.

[0044] The positive electrode current collector and the negative electrode current collector can be made of a conductive metal that can be used as a current collector in a battery. For example, a metallic material containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In can be used. The shape of the positive electrode current collector and the negative electrode current collector is not particularly limited and can be in various forms such as foil, mesh, or porous.

[0045] The positive electrode active material layer is the positive electrode active material of the embodiment described above (for example, Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 In addition to O), conductive materials or binders may also be included.

[0046] The electrolyte layer generally consists of an electrolyte solution and a separator. As the electrolyte solution, known electrolyte solutions used in magnesium batteries can be used, for example, a solution of Mg(AlCl2EtBu)2 dissolved in tetrahydrofuran or Mg(CB) 11 H 12 Examples include 2 / tetraglyme. Examples of separators include porous membranes such as polypropylene (PP) porous membranes, polyethylene (PE) porous membranes, and PP / PE / PP laminated porous membranes.

[0047] The negative electrode active material is not particularly limited as long as it can absorb magnesium ions during charging and release them during discharge; known materials can be used. Examples include magnesium metal, graphite, or carbon materials such as amorphous carbon.

[0048] Using the magnesium battery molded as described above, a voltage is applied between the positive electrode current collector and the negative electrode current collector (first-charging process). As a result, the lithium in the composition of the positive electrode active material, which is in its synthesized state, desorbs from the DRX structure lattice, as shown in Figure 3, and vacancies (Vac) are formed within the lattice. The desorbed lithium diffuses into the electrolyte layer, and even if charging and discharging are performed afterward, lithium will not be reinserted into the vacancies. This is because the concentration of diffused lithium in the electrolyte is extremely low (the lithium concentration in the electrolyte is approximately 0.3%).

[0049] As described above, the magnesium battery of this embodiment is formed by performing a pre-charging process. Subsequently, by repeatedly charging and discharging the resulting magnesium battery, magnesium is smoothly inserted into and removed from the voids in the positive electrode active material of the magnesium battery, as shown in Figure 3, and power can be extracted.

[0050] Although the above-described embodiment illustrates a liquid-type magnesium battery using an electrolyte, the positive electrode active material of this embodiment can be applied in exactly the same way as a constituent material for the positive electrode active material layer in a solid-type magnesium battery using a solid material as the electrolyte.

[0051] While embodiments of the present invention have been described above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the requirements of the invention. These embodiments and their variations are included in the scope and requirements of the invention, as well as in the claims and their equivalents. [Examples]

[0052] The following describes examples of verification of the present invention, but the present invention is not limited to the configurations shown in these examples. (Example 1 of verification of positive electrode active material) According to the method for producing the positive electrode active material described above, the positive electrode active material (Mg) of Example 1 of the present invention 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) was created. Figure 4 shows the XRD profile of the synthesis state of the positive electrode active material in this Example 1 of the present invention. Figure 4 indicates that the strong fundamental peaks represent an irregular salt-type structure.

[0053] Furthermore, Figure 5 shows the results of EDS analysis performed using scanning transmission electron microscopy with the positive electrode active material of Example 1 of the present invention, broken down by constituent element. As shown in Figure 5, it was confirmed that the distribution of each metal element constituting the positive electrode active material of Example 1 of the present invention is uniform at the nanometer level, both within and between particles. Table 2 also shows a comparison of the composition of the cathode active material in Example 1 of the present invention at the time of raw material preparation before synthesis and the composition after synthesis. In Table 2, "exp." indicates the composition of the final product determined by quantitative analysis after synthesis, and "cf. target" indicates the target composition of the final product. For example, it is known that some of the Li sublimates due to heat treatment. In this way, some of the elements of the raw materials are lost during the synthesis process. The composition of the raw materials at the time of raw material preparation before synthesis is almost the same as the composition of the target final product, but strictly speaking, the composition of the raw materials at the time of raw material preparation before synthesis is the composition obtained by adding the amount of elements lost during the synthesis process to the composition of the target final product.

[0054] [Table 2]

[0055] As shown in Table 2, the composition of the raw materials before synthesis of the positive electrode active material was almost identical to the composition after synthesis. This confirms that the positive electrode active material can be synthesized with the desired composition accurately and with almost no elemental loss using the method described above.

[0056] (Example 1 of magnesium battery testing) Next, we assembled the test magnesium battery shown in Figure 6. The positive electrode active material is the positive electrode active material of Example 1 of the present invention (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) was used. A magnesium ribbon was used as the negative electrode active material. As a reference electrode, a lithium foil was placed in a solution prepared by dissolving lithium bistrifluoromethanesulfonylamide (TFSA) in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME)-TFSA ionic liquid at a concentration of 0.5 M. Triglyme (G3) / Mg(TFSA)2 was used as the electrolyte. The temperature during the test was set to 90°C.

[0057] A CV test was conducted using the magnesium battery shown in Figure 6. The results are shown graphically in Figure 7. As shown in Figure 7, a relatively large anode current was observed during the first-charging process. On the other hand, during the first discharge that followed, a cathode current as large as that observed during the first-charging process was not observed. Furthermore, in subsequent charge-discharge cycles, the values ​​of the anode current and cathode current were almost symmetrical.

[0058] These results strongly suggest that the large anode current generated during the first-charging process is due to the desorption of Li ions.

[0059] Next, a constant current repetition charge-discharge test was performed using the test magnesium battery shown in Figure 6. 10.4mAg -1 Under these conditions, the cutoff voltage is 1.5~4.2V (vs. Li +The setting was changed to / Li). The result is shown in a graph in Figure 8. Figure 9 shows a graph comparing the charging capacity and the discharging capacity.

[0060] As shown in Figure 8, no significant degradation was observed after at least 20 charge-discharge cycles, confirming that the battery can be used stably as a secondary battery. Furthermore, as shown in Figure 9, the fact that the discharge capacity is smaller than the charge capacity is thought to be due to the oxidative decomposition of the electrolyte, and not due to degradation of the positive electrode active material.

[0061] Using the magnesium battery for testing shown in Figure 6, the change in the composition ratio of Li and Mg in the positive electrode active material during charging and discharging was investigated using a high-frequency inductively coupled plasma (ICP) device. The results are shown graphically in Figure 10.

[0062] As shown in Figure 10, the proportion of Li in the composition decreased sharply after the pre-charging process (1st-Ch) from the synthesized state (As-synth) of the positive electrode active material, and remained low even after subsequent charging and discharging, without returning to the composition ratio of the synthesized state. On the other hand, the proportion of Mg increased and decreased regularly during the charging and discharging cycles. This confirmed that Li was desorbed during the pre-charging process, and that vacancies were formed for subsequent insertion and desorption of Mg, and that Li was not reinserted into the lattice of the positive electrode active material.

[0063] (Verification example of positive electrode active material 2) According to the method for producing the positive electrode active material described above, the positive electrode active material of Example 2 of the present invention (Mg 0.35 Li 0.3 Mn 0.35 O) was created. Figure 11 shows the XRD profile of the synthesized state of the positive electrode active material in this Example 2 of the present invention. For reference, the XRD profiles of MgO and Li2O are also shown. As shown in Figure 11, the strong fundamental peak of the positive electrode active material in Example 2 of the present invention indicates a disordered rock salt (DRX) type structure. The peak indicated by "◇" in the profile is due to unavoidable impurities (e.g., Li2CO3).

[0064] (Example 2 of magnesium battery testing) Next, we assembled the test magnesium battery shown in Figure 6. The positive electrode active material is the positive electrode active material of Example 2 of the present invention (Mg 0.35 Li 0.3 Mn 0.35 O) was used. A magnesium ribbon was used as the negative electrode active material. As a reference electrode, a lithium foil was placed in a solution of lithium bistrifluoromethanesulfonylamide lithium (TFSA) dissolved in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME)-TFSA ionic liquid at a concentration of 0.5 M. Triglyme (G3) / Mg(TFSA)2 was used as the electrolyte. The temperature during the test was set to 90°C.

[0065] A constant current cyclic charge-discharge test was performed using the magnesium battery shown in Figure 6. (10.4 mAg) -1 Under these conditions, the cutoff voltage is 1.5~4.2V (vs. Li + The setting was changed to / Li). The result is shown in a graph in Figure 12. Figure 13 also shows a graph comparing the charging capacity and the discharging capacity.

[0066] As shown in Figure 12, although slightly inferior to the positive electrode active material of Example 1 of the present invention, no significant degradation was observed after 20 charge-discharge cycles, confirming that it can be used stably as a secondary battery. Furthermore, as shown in Figure 13, the fact that the discharge capacity is smaller than the charge capacity is thought to be due to the oxidative decomposition of the electrolyte, and not due to degradation of the positive electrode active material. [Industrial applicability]

[0067] The positive electrode active material of the present invention enables efficient insertion and removal of magnesium ions, making it possible to realize a magnesium battery with high charge-discharge characteristics. Therefore, it has industrial applicability.

Claims

1. A positive electrode active material for magnesium storage batteries, A positive electrode active material characterized by comprising magnesium (Mg), lithium (Li), one or more metallic elements (M), and oxygen (O), and having a rock salt type structure represented by the following general formula (1). MM ｘ Li ｙ M （１－ｘ－ｙ) O ・・・(1) However, in general formula (1), y ≥ 0.198 is satisfied. M contains one or more elements from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, and Ir. The average valency of M, obtained by weighting the valencies according to their composition ratios, is greater than 2.

0. x and y satisfy the relationship in general formula (2) below. 0.119≦4(x+y) 2 y(1-y)(1-y+y 2 )...(2)

2. The positive electrode active material according to claim 1, characterized in that, in the general formula (1) above, M further comprises one or more elements from among Zn, Al, Si, Ga, Ge, Y, In, Sn, and Pb.

3. The positive electrode active material according to claim 1, characterized in that, in the general formula (1) above, M consists of the elements Cr, Mn, Fe, Zn, and Mo.

4. A method for manufacturing a magnesium storage battery comprising a positive electrode active material according to claim 1 or 2, an electrolyte, and a negative electrode active material, A method for manufacturing a magnesium storage battery, characterized by having a pre-charging step in which a voltage is applied between the positive electrode active material and the negative electrode active material to diffuse lithium contained in the positive electrode active material into the electrolyte, thereby forming pores in the positive electrode active material that allow for the insertion and removal of magnesium ions.

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