Positive electrode active material and method for producing magnesium storage battery

Abstract

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

Description

TECHNICAL FIELD

[0001] The present application relates to a positive electrode active material for a magnesium secondary battery and a manufacturing method of a magnesium secondary battery using the same.

[0002] This application claims priority based on Japanese Patent Application No. 2023-039299 filed on March 14, 2023, and Japanese Patent Application No. 2023-041425 filed on March 15, 2023, the contents of which are incorporated herein by reference. BACKGROUND

[0003] With the rapid spread of information-related devices such as portable telephones, communication devices, and the like in recent years, development of batteries used as power sources therefor has been valued. Further, in the automobile industry and the like, development of high-output and high-capacity batteries (for example, lithium secondary batteries) for electric vehicles or hybrid vehicles is also being conducted.

[0004] On the other hand, lithium used as a raw material for lithium secondary batteries is not necessarily said to be a resource-rich, and further, the distribution of production sites is uneven, and thus there are problems in stable supply at low cost. Therefore, as an alternative to lithium, magnesium secondary batteries using magnesium, which is uniformly distributed in production sites and is a resource-rich, are attracting attention.

[0005] Patent Document 1 discloses a positive electrode active material for a magnesium ion battery containing an olivine-type magnesium silicate compound.

[0006] Further, in Patent Document 2, a positive electrode active material for a magnesium ion battery containing a rosolic acid-based compound and a perylene-based compound is disclosed.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT DOCUMENTS

[0009] Patent Document 1: Japanese Patent Application Publication No. 2017-208210

[0010] Patent Document 2: Japanese Patent Application Publication No. 2020-027702 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] However, the positive electrode active material for a magnesium ion battery disclosed in Patent Document 1 has the following problem: the number of vacancies that contribute to the intercalation / deintercalation of magnesium ions is small, and when applied to a magnesium secondary battery, the charge / discharge characteristics are low.

[0013] On the other hand, the positive electrode active material for a magnesium ion battery disclosed in Patent Document 2 is a complex-structured organic compound such as a rosolic acid-based compound or a perylene-based compound, and has the problems of low productivity and high cost.

[0014] The present application has been achieved in view of such technical background, and aims to provide a positive electrode active material for a magnesium secondary battery, which enables efficient intercalation / deintercalation of magnesium ions and realizes high charge / discharge characteristics when applied to a magnesium secondary battery.

[0015] Solution to the problem

[0016] The present inventors have newly found that, as a positive electrode active material for a magnesium secondary battery, for a metal oxide material having a disordered rock salt (DRX) type structure, which has conventionally been difficult to reversibly and efficiently intercalate / deintercalate Mg ions, Li is previously contained in the composition of the metal oxide of the DRX structure, pre-charging is performed before use as a battery by actual charge / discharge, Li is deintercalated toward an electrolyte, and thus vacancies are introduced in the DRX structure. By the vacancies thus formed, even the DRX structure can repeatedly intercalate / deintercalate Mg ions.

[0017] The present application has been achieved based on the above-mentioned new findings, and in order to solve the above-mentioned problems, a positive electrode active material and a manufacturing method of a magnesium secondary battery having the following elements are provided.

[0018] (1) The positive electrode active material of Solution 1 of the present application is characterized by being a positive electrode active material for a magnesium secondary battery, wherein magnesium (Mg), lithium (Li), one or two or more metal elements (M), and oxygen (O) are contained, and a material having a rock salt type structure represented by General Formula (1) below is constituted.

[0019] Mg x Li y M (1-x-y) O (1).

[0020] In General Formula (1), x, y satisfy x > 0, y > 0, and x + y < 1.

[0021] (2) The positive electrode active material of Solution 2 of the present application is characterized in that, in the positive electrode active material of Solution 1, y satisfies y ≥ 0.198 in General Formula (1).

[0022] (3) The positive electrode active material of Solution 3 of the present application is characterized in that, in the positive electrode active material of Solution 1 or 2, M contains any one or two or more elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, Ir in General Formula (1).

[0023] (4) The positive electrode active material of Solution 4 of the present application is characterized in that, in the positive electrode active material of Solution 3, M further contains any one or two or more elements selected from the group consisting of Zn, Al, Si, Ga, Ge, Y, In, Sn, Pb in General Formula (1).

[0024] (5) The positive electrode active material according to the item 5 of the present application in any one of the items 1 to 4, characterized in that, in the general formula (1), the average valence number of M, which is weighted-averaged by the composition ratio, is higher than 2.0.

[0025] (6) The positive electrode active material according to the item 6 of the present application in any one of the items 1 to 5, characterized in that, in the general formula (1), x and y satisfy the following relation of the general formula (2).

[0026] 0.119 ≤ 4(x+y) 2 y(1-y)(1-y+y 2 ) (2)

[0027] (7) The positive electrode active material according to the item 7 of the present application in any one of the items 1 to 6, characterized in that, in the general formula (1), M is composed of each of Cr, Mn, Fe, Zn, and Mo.

[0028] (8) The manufacturing method of the magnesium storage battery according to the item 8 of the present application, characterized in that the magnesium storage battery includes the positive electrode active material according to any one of the items 1 to 7, an electrolyte, and a negative electrode active material, and the manufacturing method of the magnesium storage battery has a pre-charge process of applying a voltage 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, and forming a site in the positive electrode active material into which magnesium ions can be intercalated / deintercalated.

[0029] Effects of the Invention

[0030] According to the items of the present application, a positive electrode active material for a magnesium storage battery, which can efficiently perform intercalation / deintercalation of magnesium ions when applied to the magnesium storage battery, and can achieve high charge / discharge characteristics, and a manufacturing method of a magnesium storage battery having high charge / discharge characteristics are provided. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a schematic view showing a case of deintercalation of Li in a crystal lattice in the pre-charge process of the positive electrode active material of one embodiment.

[0032] Figure 2 is a ternary diagram showing the composition of Li, Mg, and M (total amount of metal elements) as the composition of the positive electrode active material.

[0033] Figure 3 is a schematic view showing a case of intercalation / deintercalation of Mg at the time of charge / discharge of the positive electrode active material.

[0034] Figure 4 is an XRD pattern and a scanning transmission electron microscope image of the as-synthesized positive electrode active material of Example 1 of the present application.

[0035] Figure 5 is an image showing the results of EDS analysis using a scanning transmission electron microscope, shown for each constituent element.

[0036] Figure 6 is a schematic diagram showing the configuration of a magnesium secondary battery used for the test.

[0037] Figure 7 is a graph showing the results of the CV test in Example 1 of the present application.

[0038] Figure 8 is a graph showing the results of the repeated charge-discharge test in Example 1 of the present application.

[0039] Figure 9 is a graph showing the comparison of the charge capacity and the discharge capacity in Example 1 of the present application.

[0040] Figure 10 is a graph showing the change in the composition ratio of Li and Mg in the positive electrode active material caused by charge-discharge.

[0041] Figure 11 is an XRD pattern of the as-synthesized positive electrode active material of Example 2 of the present application.

[0042] Figure 12 is a graph showing the results of the repeated charge-discharge test in Example 2 of the present application.

[0043] Figure 13 is a graph showing the comparison of the charge capacity and the discharge capacity in Example 2 of the present application. DETAILED DESCRIPTION

[0044] Hereinafter, the embodiments of the present application will be described in detail with reference to the drawings. Note that in the following description, the drawings used in the explanation will sometimes be shown with a portion that is a feature enlarged for the sake of convenience in order to facilitate understanding of the feature, and the dimensional ratios and the like of the respective constituent elements are not necessarily the same as in reality. Furthermore, the materials, dimensions, and the like exemplified in the following description are one example, and the present application is not necessarily limited thereto, and can be appropriately changed and implemented within a range where the effects are not changed.

[0045] [Positive electrode active material]

[0046] The positive electrode active material for a magnesium secondary battery of the present embodiment contains magnesium (Mg), lithium (Li), one or two or more metal elements (M), and oxygen (O), and is composed of a material having a rock salt structure represented by the following general formula (1).

[0047] Mg x Li y M (1-x-y) O(1)

[0048] Further, in the above general formula (1), it is preferable that y satisfy y > 0.198.

[0049] Further, in the above general formula (1), it is preferable that the average valence number of M, which is weightedly averaged with respect to the composition ratio, be higher than 2.0.

[0050] Further, in the above general formula (1), x and y preferably satisfy the following relationship of general formula (2).

[0051] 0.119 < 4(x+y) 2 y(1-y)(1-y+y 2 ) (2)

[0052] x and y more preferably satisfy the following relationship of general formula.

[0053] 0.12 < 4(x+y) 2 y(1-y)(1-y+y 2 )

[0054] In general formula (1), the metal element (M) can include one or two or more, for example, it is preferable to include at least any one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, Ir. Also, it is more preferable to include any one or two or more of Zn, Al, Si, Ga, Ge, Y, In, Sn, Pb together with the above metal element.

[0055] In the present embodiment, as a combination example of the metal element (M) in general formula (1), it is preferable to include each of Cr, Mn, Fe, Zn, Mo. That is, the metal element (M) preferably includes all of Cr, Mn, Fe, Zn, Mo.

[0056] More specifically, there can be mentioned a material having a composition of Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O and a rock salt type structure.

[0057] In the positive electrode active material of the present embodiment, that is, the metal oxide having a DRX structure including Mg, when used for a magnesium secondary battery, the conditions of the material for allowing Mg ions to be reversibly and smoothly intercalated / deintercalated are as described below.

[0058] (1) capable of deintercalating Li contained in the initial composition for generating a vacancy (capable of generating a vacancy by removing Li contained in the initial composition).

[0059] (2) is an initial composition like one or two-vacancy path percolation.

[0060] (3) does not lose one or two-vacancy path percolation even in charge and discharge.

[0061] Regarding the above (1), as shown in the following first-charging process, Li is deintercalated from the synthesized Li-containing positive electrode active material and diffused into the electrolyte, for example. That is, if Li contained in the initial composition of the positive electrode active material cannot be deintercalated in the first-charging process, a vacancy for intercalation / deintercalation of Mg ions cannot be generated in the subsequent charge and discharge. Figure 1

[0062] The proportion of Li contained in the initial composition of the positive electrode active material can be set to a critical percolation probability P c = 0.198 or more in the site percolation process of a face-centered cubic lattice (fcc). Therefore, in General Formula (1), it is preferable to satisfy y ≥ 0.198. In one example of the composition of the positive electrode active material 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) described above, the proportion of Li is 0.3, satisfying the condition of the above (1).

[0063] Further, regarding the above (2), when a positive electrode active material of a DRX structure is used in a magnesium secondary battery, it is necessary to make Mg ions easily move within the particles, but in the probability theory on a lattice, when the probability of the existence of a movement path generated by percolation (percolation) is set to P, the probability of the non-existence of the movement path is 1-P. If the probability P of the existence of the movement path is changed, various sizes of clusters that can continuously move are formed. Also, if the probability P of the existence of the movement path is increased, a phase change from a non-percolation phase that is a phase in which an infinite cluster does not exist to a percolation phase that is a phase in which a cluster exists occurs. The boundary from the non-percolation phase to the percolation phase is a critical point, and the probability P at this time is a critical percolation probability. That is, when P c > P c , an infinite cluster is formed, and a phase change to the percolation phase occurs.

[0064] Also, when the probability P of the existence of the movement path satisfies P c > P, the movement path is infinitely connected within the particle, and Mg can easily diffuse using the movement path.

[0065] For example, the existence probability P of a movement path defined as a one-vacancy path one (P vac , P M , P Mg ) is represented by the following equation (3).

[0066] P one = 4(1-P M ) 2 P vac (1-P vac ) (1-P vac +P vac 2 ) (3)

[0067] wherein, if P vac : a probability that a vacancy occupies a cation site, P M : a probability that a metal element other than Mg occupies a cation site, and P Mg : a probability that a Mg element occupies a cation site, then P vac + P M + P Mg = 1 (P is assumed to be equivalent to a composition), and it is assumed that Li is completely deintercalated to form a vacancy.

[0068] Based on such an existence probability P one , if the positive electrode active material is made to have a composition satisfying P one ≥ P c = 0.119 (a critical penetration probability in an fcc combination process), then Mg can easily diffuse within a site using a movement path having a low activation energy.

[0069] P vac is a composition ratio y of Li, and P Mg is a composition ratio x of Mg, and since P vac + P M + P Mg = 1 is satisfied, the equation (3) becomes the following equation (4).

[0070] P one = 4(x+y) 2 y(1-y)(1-y+y 2 ) (4)

[0071] Therefore, it is good for the positive electrode active material to have a composition satisfying the equation (2).

[0072] Based on this, if the composition amounts of Li, Mg, and M (total amount of metal elements) as a composition of the positive electrode active material are y : x : M = 0.1 : 0.1 : 0.8 or more, then Mg can easily diffuse within a site using a movement path having a low activation energy.Figure 2 The curve Q (line where critical permeation probability = 0.119) in the ternary diagram shown is a combination of the range on the upper side, and then Mg easily diffuses within the site using a path with low activation energy.

[0073] The following shows the composition of the actual positive electrode active material that becomes the ratio of Mg to the sum of the metal elements (M) of 0.1 or more and less than 0.2. Figure 2 The curve Q in the ternary diagram shown is a combination of the range on the upper side of the actual positive electrode active material.

[0074] Mg 0.35 Li 0.3 Mn 0.35 O

[0075] Mg 0.35 Li 0.3 Ni 0.35 O

[0076] Mg 0.4 Li 0.2 Al 0.2 Fe 0.2 O

[0077] Mg 0.3 Li 0.3 Si 0.15 Mn 0.25 O

[0078] Mg 0.25 Li 0.5 Sn 0.1 Mo 0.1 Ru 0.05 O

[0079] Mg 0.1 Li 0.5 Ga 0.1 Ge 0.2 Pd 0.1 O

[0080] Mg 0.1 Li 0.4 Cu 0.1 Ti 0.2 V 0.1 Cr 0.1 O

[0081] Mg 0.3 Li 0.3 Nb 0.05 Pb 0.05 Y 0.05 Zn 0.25 O

[0082] Note that the elements in the metal elements (M) in which the valence number changes at the time of charge and discharge when used in a magnesium secondary battery and are considered to be "active" electrochemically are as described below.

[0083] Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, Ir

[0084] On the other hand, among the metal elements (M), the valence number hardly changes during charge and discharge and is considered to be electrochemically “inactive” as follows.

[0085] Zn, Al, Si, Ga, Ge, Y, In, Sn, Pb

[0086] Therefore, as can be seen from the above combination examples, the metal element M constituting the positive electrode active material of this embodiment needs to contain at least one of the above-mentioned elements that are considered to be electrochemically "active". On the other hand, by including an element that is considered to be electrochemically "inert", the valence of the entire metal element (M) constituting the positive electrode active material can be adjusted.

[0087] In addition, regarding the above (3), in order to ensure sufficient vacancies for Mg ion insertion / extraction during charge and discharge, for example, in an example of the composition of this embodiment (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) in the positive electrode active material, a metal element (M) having an average valence of not less than 2.0 after weighted averaging of the valences in the composition ratio is used.

[0088] Specifically, an example of the composition of this embodiment (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0. 10), the electrochemically active metal elements (Cr, Mn, Fe, Mo) and the inert metal element (Zn) are combined so that the lowest value of the average valence of the metal element (M) is greater than +2.6. The adjustment of the valences thus achieved is shown in Table 1. In one example of the composition of this embodiment, the average valence of the metal element (M) obtained by weighted average of the valences according to the composition ratio can vary between +2.86 and +4.14.

[0089] [Table 1]

[0090]

[0091] As described above, the positive electrode active material according to the present embodiment enables the Mg ions to be repeatedly intercalated / deintercalated by charging and discharging by using a material having a DRX structure as the positive electrode active material of a magnesium secondary battery. Thus, it is possible to realize a magnesium secondary battery having excellent charge and discharge characteristics.

[0092] Note that, in the above-described embodiment, only the Mg ions are exemplified as the guest cations to be intercalated / deintercalated with respect to the positive electrode active material after the pre-charging process is performed, but it is also possible to intercalate / deintercalate the Li ions together with the Mg ions as the guest cations.

[0093] [Method for manufacturing positive electrode active material]

[0094] In manufacturing the positive electrode active material of the above-described embodiment, for example, the Pechini method (complex polymerization method) can be used.

[0095] As one example, a method for manufacturing a positive electrode active material having a composition of Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O is shown below.

[0096] First, nitrate salts of the metal elements (Mg, Cr, Mn, Fe, Zn, Li) constituting the positive electrode active material, ammonium molybdate, and lithium nitrate are prepared, and a metal citric acid complex is formed by stirring them together with citric acid, propylene glycol, and distilled water, and ester polymerization is performed to obtain a metal polymer gel. The stirring is performed, for example, for about 4 to 5 hours at 80 to 120°C.

[0097] Next, the metal polymer gel is dried at 200°C for about 20 hours, and then heat treatment is performed to produce a high-purity metal oxide. The heat treatment is performed, for example, for about 1 to 3 hours at 400 to 500°C.

[0098] The metal oxide thus obtained is pulverized into a fine powder using a ball mill, for example. The ball mill is used, for example, to repeat a pulverization process for about 5 minutes at a rotation speed of 500 rpm for about 40 times. Then, the metal oxide in the fine powder state is subjected to heat treatment again. In the heat treatment at this time, for example, heating is performed at 600°C for about 10 hours in an argon atmosphere containing 5% or less of hydrogen. After that, the metal oxide is pulverized into a fine powder using a ball mill again.

[0099] After that, the fine powder of the metal oxide and sucrose are mixed in a weight ratio of 8:2 and distilled water is added, and mixed for about 10 minutes, and then carbonization heat treatment is performed. In the carbonization heat treatment, for example, heating is performed at 600°C for about 4 hours in an argon atmosphere containing 5% or less of hydrogen. After that, by uniformly stirring the metal oxide for about 10 minutes, a positive electrode active material (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) of one example of the composition of the present embodiment can be produced.

[0100] [Method for manufacturing magnesium secondary battery]

[0101] In the case of manufacturing a magnesium secondary battery using the positive electrode active material of the above-described embodiment, for example, in a package (case), an electrolyte layer is formed, and on one side thereof, 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 across the electrolyte layer, and on the other side thereof, a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector (electrode) are formed, to obtain a molded body of a magnesium secondary battery (pre-first-charging).

[0102] As the positive electrode current collector and the negative electrode current collector, a conductive metal that can be used as a current collector of a battery can be used. For example, a metal material containing one or two or more elements selected from the group formed by Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, In can be exemplified. The shape of the positive electrode current collector and the negative electrode current collector is not particularly limited, and can be various shapes such as a foil shape, a mesh shape, a porous material, and the like.

[0103] The positive electrode active material layer can contain, in addition to the positive electrode active material of the above-described embodiment (for example, Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O), a conductive material, a binder.

[0104] The electrolyte layer generally has an electrolyte solution and a separator. As the electrolyte solution, a publicly known electrolyte solution for a magnesium secondary battery can be used, and as one example, a substance obtained by dissolving Mg(AlCl2EtBu)2in tetrahydrofuran, Mg(CB 11 H 12)2 / tetraethylene glycol dimethyl ether, etc. As the separator, for example, a porous film such as a polypropylene (PP) porous film, a polyethylene (PE) porous film, a PP / PE / PP laminated porous film, etc. can be mentioned.

[0105] As the negative electrode active material, there is no particular limitation as long as it can occlude magnesium ions at the time of charging and release the magnesium ions at the time of discharging, and a publicly known material can be used. For example, a magnesium metal, a carbon material such as graphite or amorphous carbon, etc. can be mentioned.

[0106] A voltage is applied between the positive electrode current collector and the negative electrode current collector using the formed body of the magnesium secondary battery as described above (first-charging). Thereby, as shown in FIG. 1, lithium in the composition is deintercalated from the lattice of the DRX structure, and a vacancy (Vac) is formed in the lattice in the positive electrode active material which is kept in the state after the synthesis. The deintercalated lithium diffuses to the electrolyte layer, and thereafter, even if charging and discharging are performed, lithium is not occluded in the vacancy again. This is because the concentration of the diffused lithium in the electrolyte is extremely low (the lithium concentration in the electrolyte is about 0.3%). Figure 3 As described above, by performing the first-charging, the magnesium secondary battery of the present embodiment is formed.

[0107] Thereafter, by repeatedly performing charging and discharging using the obtained magnesium secondary battery, as shown in FIG. 2, magnesium is smoothly occluded / deoccluded with respect to the vacancy in the positive electrode active material of the magnesium secondary battery, and electric power can be extracted.

[0108] Figure 3 Note that, in the above-described embodiment, a liquid system magnesium secondary battery using an electrolyte is exemplified, but the positive electrode active material of the present embodiment can be applied as a constituent material of the positive electrode active material layer completely equally even in a solid system magnesium secondary battery using a solid material as an electrolyte.

[0109] Note that, in the above-described embodiment, a liquid system magnesium secondary battery using an electrolyte is exemplified, but the positive electrode active material of the present embodiment can be applied as a constituent material of the positive electrode active material layer completely equally even in a solid system magnesium secondary battery using a solid material as an electrolyte.

[0110] The above-described embodiments are described as examples, and are not intended to limit the scope of the invention. The embodiments can be implemented in various other ways, and various omissions, substitutions, and changes can be made within the scope of the essential conditions of the invention. The embodiments, modifications thereof, and the like are included in the scope of the invention, requirements, and the like, and are included in the scope of the invention and equivalents thereof recited in the claims.

[0111] Example

[0112] The following describes verification examples of the present invention, but the present invention is not limited to the configurations shown in the verification examples.

[0113] (Verification Example 1 of Positive Electrode Active Material)

[0114] ​According to the above-described manufacturing method of the positive electrode active material, the positive electrode active material (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) of Example 1 of the present application was produced.

[0115] The XRD pattern of the as-synthesized positive electrode active material of Example 1 of the present application is shown in Figure 4 .

[0116] According to Figure 4 , the strong fundamental peak indicates an irregular rock-salt structure.

[0117] Further, the results of EDS analysis by scanning transmission electron microscopy using the positive electrode active material of Example 1 of the present application are shown for each of the constituent elements in Figure 5 .

[0118] According to Figure 5 , it was confirmed that the distribution of each of the metal elements constituting the positive electrode active material of Example 1 of the present application was on the nanometer scale and uniform within and between particles.

[0119] Further, the results of comparison of the composition at the time of raw material feeding before synthesis and the composition after synthesis of the positive electrode active material of Example 1 of the present application are shown in Table 2. In Table 2, "exp." indicates the composition of the final product determined by quantitative analysis after synthesis, and "cf. target" indicates the composition of the final product targeted. For example, it is known that a part of Li sublimates by heat treatment. In this way, a part of the elements of the raw material is lost during synthesis. The composition at the time of raw material feeding before synthesis and the composition of the final product targeted are approximately the same, but strictly speaking, the composition at the time of raw material feeding before synthesis is the composition obtained by adding the amount of the elements lost during synthesis to the composition of the final product targeted.

[0120] [Table 2]

[0121]

[0122] According to the results shown in Table 2, it was confirmed that the composition at the time of raw material feeding before synthesis and the composition after synthesis of the positive electrode active material were approximately the same, and that by the above-described manufacturing method of the positive electrode active material, the positive electrode active material of the intended composition could be accurately synthesized with almost no loss of elements.

[0123] (Verification Example 1 of Magnesium Storage Battery)

[0124] Next, a test magnesium storage battery shown in Figure 6 was assembled.

[0125] As the positive active material, the positive active material of Example 1 of the present application (Mg 0.35 Li 0.3 Cr 0.1 Mn 0.05 Fe 0.05 Zn 0.05 Mo 0.1 O) produced according to the above-mentioned production method of positive active material was used.

[0126] As the negative active material, a magnesium strip was used.

[0127] As the reference electrode, an electrode obtained by adding a lithium foil to a solution obtained by dissolving lithium bis(trifluoromethanesulfonyl)amide (TFSA) in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME)-TFSA ionic liquid at a concentration of 0.5 M was used.

[0128] As the electrolyte, triglyme dimethyl ether (G3) / Mg(TFSA)2was used.

[0129] The temperature at the time of the test was set to 90°C.

[0130] Using the test magnesium secondary battery shown in Figure 6 , a CV test was performed. The results thereof are shown in a graph in Figure 7 .

[0131] According to the results shown in Figure 7 , in the first-charging, a large anode current was observed. On the other hand, in the first discharge thereafter, a cathode current corresponding to the anode current in the first-charging was not observed. Further, in the charge-discharge after the second time, the values of the anode current and the cathode current were almost symmetrical.

[0132] These results strongly suggest that the large anode current generated in the first-charging is due to the deintercalation of Li ions.

[0133] Next, using the test magnesium secondary battery shown in Figure 6 , a repeated charge-discharge test under a constant current was performed. The conditions were 10.4 mAg -1 , and the cut-off voltage was set to 1.5 to 4.2 V (vs. Li + / Li). The results thereof are shown in a graph in Figure 8 . Further, a comparison of the charge capacity and the discharge capacity is shown in a graph in Figure 9 .

[0134] According to the results shown in Figure 8 , in the first-charging, a large anode current was observed. On the other hand, in the first discharge thereafter, a cathode current corresponding to the anode current in the first-charging was not observed. Further, in the charge-discharge after the second time, the values of the anode current and the cathode current were almost symmetrical.As a result, it was confirmed that no significant degradation was found in at least 20 cycles of charge and discharge, and the secondary battery could be used stably. Further, according to Figure 9 As a result, it was considered that the discharge capacity was smaller than the charge capacity due to the influence of oxidative decomposition of the electrolyte, rather than degradation of the positive electrode active material.

[0135] Using Figure 6 The test magnesium secondary battery shown was used to investigate the change in the composition ratio of Li and Mg in the positive electrode active material caused by charge and discharge using a high-frequency inductively coupled plasma (ICP) device. The results thereof are shown in a graph in Figure 10 .

[0136] According to Figure 10 As a result, the composition ratio of Li started from the as-synthesized state of the positive electrode active material (As-synth), and sharply decreased when the pre-charge process was performed (1st-Ch), and even after the charge and discharge, it did not return to the composition ratio of the as-synthesized state, but remained constant in a state of being lower. On the other hand, the composition ratio of Mg regularly increased and decreased due to the cycle of charge and discharge. From this, it was confirmed that Li was deintercalated in the pre-charge process, and thereafter, a site for Mg intercalation / deintercalation was formed, and Li was not intercalated again into the crystal lattice of the positive electrode active material.

[0137] (Verification Example 2 of Positive Electrode Active Material)

[0138] According to the manufacturing method of the positive electrode active material described above, the positive electrode active material of Example 2 of the present application (Mg 0.35 Li 0.3 Mn 0.35 O) was produced.

[0139] The XRD pattern of the as-synthesized state of the positive electrode active material of Example 2 of the present application is shown in Figure 11 . Note that, as a reference value, the XRD patterns of MgO and Li2O are also shown.

[0140] According to Figure 11 , the positive electrode active material of Example 2 of the present application showed a strong intensity fundamental peak of an irregular rock salt (DRX) type structure. Note that, the peak indicated by "O" in the pattern was generated by inevitable impurities (for example, Li2CO3).

[0141] (Verification Example 2 of Magnesium Secondary Battery)

[0142] Next, the test magnesium secondary battery shown was assembled. Figure 6

[0143] As the positive electrode active material, the positive electrode active material of Example 2 of the present application (Mg 0.35 Li​0.3 Mn 0.35 O).

[0144] As the negative electrode active material, magnesium rods were used.

[0145] As a reference electrode, a lithium foil was added to a solution of lithium bistrifluoromethanesulfonamide (TFSA) dissolved in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME)-TFSA ionic liquid at a concentration of 0.5 M. As the electrolyte, triethylene glycol dimethyl ether (G3) / Mg(TFSA)2 was used.

[0146] The temperature during the test was set to 90°C.

[0147] use Figure 6 The test magnesium battery shown was used to conduct repeated charge and discharge tests at a constant current. -1 Under the condition of , the cut-off voltage is set to 1.5~4.2V(vs.Li + / Li). The results are shown in a graph. Figure 12 .also, Figure 13 The comparison between charge capacity and discharge capacity is shown in a graph.

[0148] according to Figure 12 The results shown in FIG. 1 confirm that although the positive electrode active material is slightly inferior to the positive electrode active material of Example 1 of the present invention, no significant degradation is found in 20 charge and discharge cycles, and the battery can be used stably as a secondary battery. Figure 13 The results shown indicate that the discharge capacity is smaller than the charge capacity due to the influence of oxidative decomposition of the electrolyte, rather than deterioration of the positive electrode active material.

[0149] Industrial applicability

[0150] The positive electrode active material of the present invention can realize a magnesium storage battery capable of efficiently intercalating and deintercalating magnesium ions and achieving high charge and discharge characteristics. Therefore, the positive electrode active material has industrial applicability.

Claims

1. A positive electrode active material, characterized in that is a positive electrode active material for a magnesium secondary battery, wherein contains magnesium Mg, lithium Li, one or two or more metal elements M, and oxygen O, and is composed of a material having a rock salt structure represented by the following general formula (1), Mg x Li y M (1-x-y) O(1).

2. The positive electrode active material according to claim 1, wherein in the general formula (1), y satisfies y ≥ 0.

198.

3. The positive electrode active material according to claim 1 or 2, wherein in the general formula (1), M contains any one or two or more elements of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, W, Ir.

4. The positive electrode active material according to claim 3, wherein in the general formula (1), M further contains any one or two or more elements of Zn, Al, Si, Ga, Ge, Y, In, Sn, Pb.

5. The positive electrode active material according to claim 1 or 2, wherein in the general formula (1), an average valence number of M, which is weighted averaged by a composition ratio, is higher than 2.

0.

6. The positive electrode active material according to claim 1 or 2, wherein in the general formula (1), x and y satisfy a relationship of the following general formula (2), 0.119 ≤ 4(x+y) 2 y(1-y) (1-y+y 2 ) (2).

7. The positive electrode active material according to claim 1 or 2, wherein in the general formula (1), M is composed of each element of Cr, Mn, Fe, Zn, Mo.

8. A method of manufacturing a magnesium secondary battery, characterized by, the magnesium secondary battery includes the positive electrode active material according to claim 1 or 2, an electrolyte, and a negative electrode active material, the manufacturing method of the magnesium secondary battery includes a pre-charging step of applying a voltage 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, and forming a site in the positive electrode active material into which magnesium ions can be intercalated and deintercalated.

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