JP2014082030A - Positive electrode active material for magnesium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new positive electrode active material for a magnesium ion battery, having high capacity density comparable to a Chevrel compound (MoS).SOLUTION: A positive electrode active material for a magnesium ion secondary battery comprises: a compound having one kind or more structural units represented by the following formula (1) (provided that a compound comprising only MoSis excluded); or a compound represented by the following formula (2). MoX...(1) (In the formula, X represents S, Se or Te, and n represents an integer satisfying 2≤n≤6.) MoX...(2) (In the formula, X represents S, Se or Te, and m represents 1 or 2.)

Description

  The present invention relates to a positive electrode active material that can be used for a magnesium ion secondary battery, and a magnesium ion secondary battery using the positive electrode active material.

  Lithium ion secondary batteries were put into practical use in the 1990s, and have since been used as batteries for portable devices such as mobile phones and laptop computers. In recent years, they have attracted attention as batteries for electric vehicles (EV). ing. A lithium ion secondary battery is a secondary battery that performs charging and discharging by moving lithium ions between a positive electrode and a negative electrode. During charging, lithium ions are released from the positive electrode and taken in between the graphite layers of the negative electrode. During discharge, lithium ions are released from the negative electrode and taken into the positive electrode active material.

  When lithium ion secondary batteries are overcharged, dendritic metallic lithium is deposited on the negative electrode side, and highly reactive metallic lithium reacts explosively when exposed to water, generating a large amount of heat. Have a potential problem. Although lithium ion secondary batteries currently on the market are equipped with a device that shuts down the battery circuit in order to avoid danger in an emergency, this problem is rarely manifested. When used for EV, the battery is inevitably increased in size, and the amount of lithium metal deposited increases accordingly, increasing the possibility of a major accident.

  In terms of the number of Clarkes representing the element abundance ratio in the natural world, the number of Clarks of lithium is 0.006%, and lithium is essentially a rare element.

  As described above, the lithium ion secondary battery has problems in terms of cost, resources, and safety when it is increased in size, and cannot be said to be a stable energy source in the future. In particular, due to the current situation where power shortages and the review of nuclear power use are being promoted, expectations for renewable energy are increasing, but in view of the fact that stable power supply is difficult with the current technology, it is excellent. Storage technology is indispensable, and the development of next-generation secondary batteries that do not depend on lithium has become an urgent task.

  One of the candidates for the next generation secondary battery is a magnesium ion battery. A magnesium ion battery is expected to have a higher capacity because divalent ions transport charges. In addition, when a carrier metal (Mg) is used for the negative electrode, since the reactivity with water is gentle, the secondary battery has advantages in terms of safety. Furthermore, magnesium has a Clark number of 1.93% and is considered to be one of the elements with higher Clark number. Therefore, a magnesium ion battery using Mg, which is abundant in resources, can be supplied in large quantities and at low cost.

However, when trying to realize a magnesium ion battery, there is a problem that magnesium ions are easily trapped in the crystal lattice because they have a high valence of two. That is, when the battery is discharged, two electrons enter the positive electrode material for one Mg ^{2+ due} to the neutrality of the electric charge, and therefore, between the Mg ²⁺ and the positive electrode material, compared to a lithium ion battery. Large electrostatic attraction. Such a large electrostatic attraction force increases the ion trap within the crystal lattice of the positive electrode material, making the crystal lattice very fragile. Therefore, it has been known that it is difficult to deinsert magnesium ions from most positive electrode active materials.

A chevrel compound (Mo ₆ S ₈ ) was developed at the beginning of 2000 as a positive electrode active material for magnesium ion batteries capable of solving such problems (Non-patent Document 1). Mo ₆ S ₈ has a hole (site) into which a metal cation can be inserted. When Mg ²⁺ is inserted, a compound represented by MgMo ₆ S ₈ is formed. In Mo ₆ S ₈ , six metal atoms form a bond to form an octahedron to form a cluster called a cluster, and sulfur atoms are arranged around this cluster so as to form a cube.
The chevrel compound (Mo ₆ S ₈ ) has a capacity of about 60 mAh / g and an electromotive force of 1.1 V, and is a commercially available lithium ion secondary battery (capacity: about 150 mAh / g, electromotive force: 3.6 V). Although it has not reached the level, when a Grignard electrolyte is used, it exhibits good cycle characteristics and can be charged and discharged for 2000 cycles or more.

Thus, in Mo ₆ S ₈ , the structure is not broken when magnesium ions are deinserted and high cycle characteristics are exhibited. The reason is that in the Mo ₆ S ₈ positive electrode active material, electrons flowing from the negative electrode to the positive electrode at the time of discharge are received by a cluster (Mo ₆ ) in which a plurality of Mo atoms are collected by metal bonds instead of a single Mo atom. This is thought to be due to the fact that the change in valence per Mo atom is reduced and the charge spreads throughout the cluster. In other words, since the Mo cluster has a negative charge in a wide region, the distortion of the structure of the positive electrode active material due to the electrostatic attraction with Mg ²⁺ is smaller than that in the case where the Mo atom alone has a negative charge (that is, local, It is believed that the ion trap is relaxed), and that Mg ²⁺ can stably repeat the deinsertion.

In the 10 years since the report of the chevrel compound, there has been no remarkable progress in this technical field, and the chevrel compound (Mo ₆ S ₈ ) is almost the only successful example in bulk crystals as a positive electrode active material for magnesium ion batteries. In response to the recent increase in demand for power storage devices, a positive electrode active material having a higher capacity density is now required.

Nature, 407, 724-727 (2000) Chemistry of Materials, 22, 860-868 (2010)

The present invention has been made in view of the situation of the prior art, and an object thereof is to provide a new positive electrode active material for a magnesium ion battery having a high capacity density comparable to a chevrel compound (Mo ₆ S ₈ ). .

The present inventors have noted that in the chevrel compound (Mo ₆ S ₈ ), the local charge change is suppressed by the cluster structure, and the ion trap at the time of Mg ²⁺ insertion is relaxed, and the chevrel compound is expanded by a more extended concept. We investigated whether a positive electrode active material suitable for a magnesium ion battery other than the above could be searched.
When the atomic orbital energy of the outermost shell of the constituent element of the positive electrode active material is close, that is, when the energy levels of the d orbit of the outermost shell of the transition metal in the positive electrode active material and the p orbital of the ligand are close. I thought that the hybridization between orbits would increase, and that electrons would easily move between the orbits of different types of atoms. In other words, since the trajectory in which electrons entering the positive electrode are accommodated during discharge increases, the influence of electrostatic attraction with magnesium ions on the crystal lattice is reduced, and stable magnesium ion desorption is possible. It came.
Based on such a basic idea, the present inventors studied to develop a material having a cluster structure and a strong pd orbital hybrid, and capable of expanding the space where electrons enter and exit during charge and discharge. As a result, the present invention has been reached.

That is, the present invention
[1] A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
A positive electrode active material for a magnesium ion secondary battery,
[2] The positive electrode active material according to [1], wherein X is Se,
[3] The positive electrode active material according to [1] or [2], wherein n is 3 in formula (1),
[4] The positive electrode active material according to [1], comprising the compound Mo ₉ Se ₁₁
[5] A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
Magnesium ion secondary battery using as a positive electrode material.
[6] A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
A magnesium ion secondary battery comprising a positive electrode comprising a positive electrode active material, a negative electrode, and a non-aqueous electrolyte, and [7] the magnesium ion secondary battery according to [6], wherein the negative electrode comprises metallic magnesium. The purpose is to provide.

  The positive electrode active material of the present invention can be inserted without being trapped by magnesium ions during discharge, and can be used as a positive electrode material for a high capacity magnesium ion battery. Thus, the positive electrode active material of the present invention makes it possible to use magnesium, which is a resource-rich element, for a high-capacity electricity storage device, and greatly contributes to the practical use of magnesium ion batteries.

It shows the crystal structure of Mo ₉ Se ₁₁ Measurement result of powder XRD of Ag _3.6 Mo ₉ Se ₁₁ Measurement result of powder XRD of Mo ₉ Se ₁₁ Schematic of beaker triode cell Discharge characteristics of positive electrode containing Mo ₉ Se ₁₁ CV measurement result of positive electrode containing Mo ₉ Se ₁₁

The present invention relates to a positive electrode active material that can be used in a magnesium ion battery.
One preferred embodiment of the present invention is a compound having one or more structural units represented by the formula (1) (except for a compound consisting of only Mo ₆ S ₈ ).
Mo _3n X _{3n + 2} (1)
A positive electrode active material for a magnesium ion secondary battery.

  In the formula (1), X represents S, Se, or Te. N is an integer of 2 ≦ n ≦ 6, and preferably an integer of 2 ≦ n ≦ 4.

Since the compound having one or more structural units represented by the formula (1) in the present invention is close in energy level between the d orbital of the outermost shell of Mo and the p orbital of the ligand, It has a quasi-cluster structure that can be expected to have an effect. This makes it possible to reduce the influence of electrostatic attraction with magnesium ions on the crystal lattice and to stably insert and remove magnesium ions.
One preferred example of the compound having the structural unit represented by the formula (1) having such a structure is Mo ₉ Se ₁₁ , and its crystal structure is shown in FIG. The left side of FIG. 1 shows a crystal structure of Mo ₉ Se _{11 as} a structural unit, and Mo ₉ Se ₁₁ has a Mo ₉ cluster as a constituent element. Further, it is considered that Mo—Se is connected by a covalent bond, and Se—Se is connected by a very weak covalent bond. When electrons enter the orbit of Mo or Se during charge / discharge, the orbits of Mo and Se are close and strongly hybridized, so the electrons are also shared with the other atom's orbit, and between the 9 Mo by the Mo cluster. It is thought that the whole molecule is negatively charged.
The right side of FIG. 1 shows an orthorhombic crystal structure (o-Mo ₉ Se ₁₁ ) composed of a plurality of Mo ₉ Se ₁₁ .

In the present invention, the positive electrode active material for a magnesium ion secondary battery is a compound having only one type of structural unit represented by the formula (1) (for example, only Mo ₉ Se ₁₁ as shown in the diagram on the right side of FIG. Or a compound composed of two or more kinds of structural units represented by the formula (1) (for example, Mo ₉ Se ₁₁ and Mo ₁₂ Se ₁₄₎ . Or a compound composed of Mo ₆ Se ₈ and Mo ₉ Se ₁₁ ).

Another preferred embodiment of the present invention is a compound represented by formula (2) Mo _3m X _3m (2)
A positive electrode active material for a magnesium ion secondary battery.

  In the formula (2), X represents S, Se or Te, and m is 1 or 2.

As a method for synthesizing a compound having one or more structural units represented by the formula (1), Mo ₉ Se ₁₁ will be described as an example.
First, it is preferable to synthesize Ag _3.6 Mo ₉ Se _{11 and} to synthesize o-Mo ₉ Se ₁₁ by removing Ag by an oxidation reaction using iodine. In the first stage reaction, a single Ag powder, Mo powder, and Se powder are uniformly mixed at a molar ratio of 3.6: 9: 11, pelletized with a pressurizer, put into a quartz tube, etc., and vacuum sealed. Tube. Thereafter, the sample is put into an electric furnace or the like, heated to about 1000 ° C. to 1100 ° C., kept for 10 to 12 hours, and then rapidly cooled.
Next, the obtained Ag _3.6 Mo ₉ Se ₁₁ is put into iodine ethanol and reacted for 10 to 12 hours while stirring at room temperature to perform extraction of Ag. Furthermore, by reacting in an aqueous potassium iodide solution for 10 to 12 hours, AgI produced during the extraction of Ag can be dissolved and removed, and single-phase Mo ₉ Se ₁₁ can be separated and obtained.

As a method for synthesizing the compound represented by the formula (2), Mo ₆ Se ₆ will be described as an example.
First, it is preferable to synthesize In ₂ Mo ₆ Se _{6 and} to synthesize Mo ₆ Se ₆ by removing In by an oxidation reaction in a hydrogen chloride flow. In the first stage reaction, single In powder, Mo powder, and Se powder are uniformly mixed at a molar ratio of 1: 3: 3, pelletized with a pressurizer, put into a quartz tube or the like, and vacuum sealed. . Thereafter, the sample is put in an electric furnace or the like, heated to about 1000 ° C. and kept for 48 hours, then heated to about 1050 ° C. and kept for 24 hours, and then cooled. Next, the obtained In ₂ Mo ₆ Se ₆ is heated to 420 ° C. in a hydrogen chloride flow and held for 24 hours. By performing the oxidation reaction in this way, In is extracted as InCl, and single-phase Mo ₆ Se ₆ can be obtained by separation.

One preferred embodiment of the present invention is a compound having one or more structural units represented by the formula (1) (excluding a compound consisting only of Mo ₆ S ₈ ).
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6, preferably 2 ≦ n ≦ 4) or represented by the following formula (2) Compound Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
Is a magnesium ion secondary battery using as a positive electrode material.

One preferred aspect of the present invention is a compound having at least one structural unit represented by the formula (1) (excluding a compound composed only of Mo ₆ S ₈ ).
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6, preferably 2 ≦ n ≦ 4) or represented by the following formula (2) Compound Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
A magnesium ion secondary battery including a positive electrode including a positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

The positive electrode in the magnesium ion secondary battery of the present invention is represented by a compound (excluding a compound composed only of Mo ₆ S ₈ ) having one or more kinds of structural units represented by the formula (1) or a formula (2). A positive electrode active material made of the above compound, but two or more of the positive electrode active materials may be included.
As described above, the positive electrode active material may be composed of a compound having only one type of structural unit represented by the formula (1) (for example, a compound composed only of Mo ₉ Se ₁₁ ). A compound composed of two or more of the structural units represented by the formula (1) (for example, a compound composed of Mo ₉ Se ₁₁ and Mo ₁₂ Se ₁₄ , a composition composed of Mo ₆ Se ₈ and Mo ₉ Se ₁₁ . Or the like. A preferred example of the positive electrode active material is a positive electrode active material made of the compound Mo ₉ Se ₁₁ .

  The positive electrode in the magnesium ion secondary battery of the present invention is, for example, mixed with the positive electrode active material and carbon black, further mixed with a binder, and then pressed and pasted onto the current collector mesh, 60 ° C. to 70 ° C. It can be prepared by vacuum drying at a temperature for a predetermined time. As the binder, polytetrafluoroethylene or the like can be used. As the mesh, an aluminum, SUS, titanium, copper, or nickel mesh can be used, but a SUS mesh is preferable.

  As the negative electrode in the magnesium ion secondary battery of the present invention, metallic magnesium can be used.

  As the non-aqueous electrolyte, a solution in which an electrolyte is dissolved in an aprotic non-aqueous solvent is used.

  Examples of the non-aqueous solvent include acetonitrile, propylene power-bonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyllactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, tetrahydrofuran and the like can be used.

Moreover, as an electrolyte dissolved in a non-aqueous solvent, for example, Mg (ClO ₄ ) ₂ , Mg (SO ₂ CF ₃ ) _2, or the like can be used. In addition, magnesium borofluoride (Mg (BF ₄ ) ₂ ), magnesium trifluoromethylsulfonate (Mg (CF ₃ SO ₃ ) ₂ ), magnesium hexafluorophosphate (Mg (PF ₆ ) ₂ ), organic magnesium aluminate salt (Mg (AlCl ₂ BuEt) ₂ ) or the like can be used.

  In the magnesium ion secondary battery of the present invention, a separator that separates the positive electrode and the negative electrode can also be used. As a separator, the well-known material normally used as a separator of the conventional nonaqueous electrolyte battery can be used, For example, polymer films, such as a polypropylene, are used.

  The magnesium ion secondary battery of the present invention is not particularly limited in shape, such as a cylindrical shape, a square shape, a coin shape, a button shape, etc., and may be variously sized such as thin and large. it can.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

[Measuring method]
(1) Powder X-ray diffraction Rigaku RINT-Utima III was used as a measuring device, the X-ray source was CuKα, the voltage was 40 kV, and the current was 40 mA, and measurement was performed in a measurement range of 10 ° to 90 °.
(2) Charging / Discharging Measurement Using a multi-channel potential stat manufactured by Solartron, UK, a constant temperature bath (model number: SU-661, product number: 92011877) manufactured by ESPEC was used as a constant temperature bath. The device is designed for measurement using a coin cell, but the measurement was performed by changing the wiring in the measurement of the beaker triode cell.
(3) Cyclic Voltometry (CV) Measurement The current flowing by sweeping the voltage at a constant rate was measured, and insertion / extraction of magnesium ions was observed. In a beaker triode cell, measurement was performed at a measurement range: 0 V to −1.6 V to 0 V (vsAg / Ag ⁺ ), a voltage sweep speed: 0.5 mV / second, and a plot speed: 2 mV / point.

[Synthesis example]
Synthesis _o-Mo 9 _{Se 11} of _o-Mo 9 _{Se 11} was synthesized by the reaction of two stages, as follows. First, Ag _3.6 Mo ₉ Se ₁₁ was synthesized, and o-Mo ₉ Se ₁₁ was synthesized by removing Ag by an oxidation reaction using iodine.
The first stage reaction consists of a single Ag powder (High Purity Chemical Laboratory Co., Ltd., product number: 315501, purity: 99.9%), Mo powder (Wako Pure Chemical Industries, Ltd., product number: 137-04802, purity: 99.9%) and Se powder (High Purity Chemical Laboratory, product number: 222538, purity: 99.9%) in a mortar at a molar ratio of 3.6: 9: 11 and mixed for 5 minutes with a pestle After mixing evenly, the mixture was pelletized with a pressurizer, placed in a quartz tube, and vacuum sealed at 0.07 atm. Thereafter, the sample was put into an electric furnace, and the temperature was increased to 1100 ° C. at a rate of 100 ° C./hour, maintained at 1100 ° C. for 12 hours, and then rapidly cooled. The sample after firing was a black powder. The sample was placed in a mortar and mixed with a pestle to make it uniform, and then powder XRD measurement was performed. The result is shown in FIG.
Next, the obtained Ag _3.6 Mo ₉ Se ₁₁ was added to 0.2 M iodine ethanol (iodine: High Purity Chemical Laboratory, purity: 99.999%; ethanol: Wako Pure Chemical Industries, Ltd., purity: 99). .5% vol%) and allowed to react for 12 hours while stirring at room temperature to extract Ag. Furthermore, the reaction is carried out in a 6M potassium iodide aqueous solution (potassium iodide: Wako Pure Chemical Industries, Ltd., product number: 168-03975, purity: 99.5%) for 12 hours. The produced AgI was dissolved and removed, and single-phase Mo ₉ Se ₁₁ was obtained by separation. The measurement result of powder XRD of the obtained sample is shown in FIG.

[Example 1]
(1) Preparation of positive electrode After 90 parts by weight of the mixture of Mo ₉ Se ₁₁ and carbon black obtained in this way was added and mixed with 10 parts by weight of polytetrafluoroethylene (PTFE) as a binder, A SUS mesh was pressed and pasted and vacuum dried at 60 ° C. for 12 hours to obtain a positive electrode containing Mo ₉ Se ₁₁ as a positive electrode active material.

(2) a positive electrode containing Mo ₉ Se ₁₁ obtained by measuring the above discharging characteristics, to measure the discharge characteristics of the magnesium ion battery electrodes was evaluated. A beaker triode cell as shown in FIG. 4 was constructed, and the discharge characteristics of the positive electrode were evaluated. The working electrode, counter electrode, reference electrode, electrolytic solution, cell, and measurement conditions are as follows.
Working electrode configuration: Mo ₉ Se ₁₁ : Carbon black: PTFE = 45: 45: 10 (weight ratio)
-Counter electrode: Mg metal-Reference electrode: 0.01 M Ag / AgNO ₃ electrode-Electrolyte: 1 M Mg (ClO ₄ ) ₂ / AN
・ Cell: Tripolar beaker cell ・ Discharge current density: 6 mAh / g
Cut-off: -1.6 to 0 V (vsAg / Ag ⁺ )

As shown in FIG. 5, discharge was performed to −1.6 V (vsAg / Ag ⁺ ), and it was found that magnesium ions were not trapped and could be inserted. The observed initial discharge capacity was as high as about 175 mAh / g.
What is the discharge potential? Begins ^{-0.7V (vsAg / Ag +),} -1.3V (vsAg / Ag +), - from the plateau region appears in two steps at ^{1.4V (vsAg / Ag +),} 2 stage It is suggested that an electrode reaction is taking place. In addition, two peaks are also observed in the CV measurement result shown in FIG. 6, indicating that the reaction is a reversible reaction.

(Industrial applicability)
As described above, the positive electrode active material of the present invention can be used as a positive electrode material for a high-capacity magnesium ion battery, and can greatly contribute to the practical use of a magnesium ion battery.

Claims (7)

A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
A positive electrode active material for a magnesium ion secondary battery comprising:   The positive electrode active material according to claim 1, wherein X is Se.   The positive electrode active material of Claim 1 or 2 whose n is 3 in Formula (1). The positive electrode active material according to claim 1, comprising the compound Mo ₉ Se ₁₁ . A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
Magnesium ion secondary battery using as a positive electrode material. A compound having one or more structural units represented by the following formula (1) (excluding a compound consisting only of Mo ₆ S ₈ )
Mo _3n X _{3n + 2} (1)
(Wherein X represents S, Se or Te, and n is an integer of 2 ≦ n ≦ 6) or a compound represented by the following formula (2) Mo _3m X _3m (2)
(Wherein X represents S, Se, or Te, and m is 1 or 2)
The magnesium ion secondary battery containing the positive electrode containing the positive electrode active material which consists of, a negative electrode, and a non-aqueous electrolyte.   The magnesium ion secondary battery according to claim 6, wherein the negative electrode is made of magnesium metal.

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