JP7510143B2 - Nonaqueous electrolyte secondary battery and positive electrode active material for the same - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery and a positive electrode active material for a non-aqueous electrolyte secondary battery.

Compared to lithium, magnesium has the advantages of having a high theoretical capacity density, being an abundant resource, and being very safe. For this reason, there are high hopes for the practical application of non-aqueous electrolyte secondary batteries that use magnesium. However, divalent magnesium ions have stronger interactions and are less likely to diffuse within the solid phase than monovalent lithium ions. For this reason, magnesium non-aqueous electrolyte secondary batteries have slow electrode reactions and are difficult to obtain a practical discharge capacity.

In order to improve the discharge capacity of magnesium non-aqueous electrolyte secondary batteries, the use of transition metal composite oxides that change from a spinel-type crystal structure to a rock salt-type crystal structure by inserting magnesium and from a rock salt-type crystal structure to a spinel-type crystal structure by desorbing magnesium as a positive electrode active material has been considered (Non-Patent Documents 1 and 2). Non-Patent Document 1 describes MgMn ₂ O ₄ and MgCo ₂ O ₄ as transition metal composite oxides whose crystal structures can be changed into two phases, rock salt type and spinel type. Non-Patent Document 2 describes ZnCo ₂ O ₄ and ZnFe ₂ O ₄ .

Adv. Sci. 2015,2,1500072 J. Mater. Chem. A, 2019, 7, 12225-12235

In magnesium non-aqueous electrolyte secondary batteries that use the transition metal composite oxides described in Non-Patent Documents 1 and 2 as the positive electrode active material, the crystal structure of the transition metal composite oxide changes significantly with charging and discharging, making the crystal structure of the transition metal composite oxide prone to partial destruction. As a result, there is a problem in that the capacity of the positive electrode active material decreases significantly with charging and discharging cycles, and the charging and discharging cycle characteristics are poor.

The present invention was made in consideration of the above problems, and aims to provide a non-aqueous electrolyte secondary battery with high capacity and excellent charge-discharge cycle characteristics, and a positive electrode active material that can be advantageously used for the non-aqueous electrolyte secondary battery.

The present inventors have found that a defect spinel-type oxide in which the cation site is defective is unlikely to undergo a change in crystal structure when a cation such as magnesium is inserted, and has a high capacity in a single phase of a spinel-type crystal structure. They have also confirmed that a nonaqueous electrolyte secondary battery using a defect spinel-type oxide in which the cation site is defective in a charged state as a positive electrode active material has a high capacity and excellent charge-discharge cycle characteristics, thereby completing the present invention.
That is, in order to solve the above problems, the present invention provides the following means.

(1) The nonaqueous electrolyte secondary battery according to the first aspect comprises a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a nonaqueous electrolyte, the positive electrode active material containing a defective spinel-type oxide in which the cation sites are defective in a charged state, and the negative electrode active material containing magnesium or sodium in a charged state.

(2) In the nonaqueous electrolyte secondary battery according to the above aspect, the defect spinel oxide may be configured to contain a first cation and a second cation that are different from each other.

(3) In the nonaqueous electrolyte secondary battery according to the above aspect, the first cation may be an ion of a metal in Group 2 or Group 12 of the periodic table, and the second cation may be an ion of a transition metal.

(4) In the nonaqueous electrolyte secondary battery according to the above aspect, the defect spinel oxide may be configured to contain the second cation in an amount ranging from 0.5 mol to 1.5 mol per 1 mol of the first cation.

(5) In the nonaqueous electrolyte secondary battery according to the above aspect, the defect spinel-type oxide may be a compound represented by the following general formula (1):
A _x B _y O _z ... (1)
(In the above general formula (1), A represents an ion of a metal of Group 2 or 12 of the periodic table, B represents an ion of a transition metal, and x, y, and z represent numbers satisfying 0.5≦y/x≦1.5, 2≦x+y<z, and 3≦z≦4.)

(6) The positive electrode active material for a non-aqueous electrolyte secondary battery according to the second aspect is for a non-aqueous electrolyte secondary battery in which the negative electrode active material contains magnesium or sodium in a charged state, and contains a defective spinel-type oxide in which the cation sites are defective.

The present invention makes it possible to provide a non-aqueous electrolyte secondary battery with high capacity and excellent charge/discharge cycle characteristics, and a positive electrode active material that can be advantageously used for the non-aqueous electrolyte secondary battery.

1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. FIG. 2 is an X-ray diffraction pattern of _ZnMnO3 powder obtained in Example 1. FIG. 2 is a cross-sectional view of the electrochemical cell prepared in Example 1. 1 shows the results of cyclic voltammograms measured in Example 1. 1 shows the results of a discharge curve measured in Example 1 by GITT (constant current intermittent titration method). 1 is an X-ray diffraction pattern showing a structural change of _ZnMnO3 due to insertion of magnesium ions measured in Example 1. 1 shows the charge/discharge curve results of the charge/discharge cycle characteristics measured in Example 1. 1 shows the results of the discharge capacity and coulombic efficiency of the charge-discharge cycle characteristics measured in Example 1.

The present embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show enlarged characteristic parts for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the present invention.

FIG. 1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention.
1, the nonaqueous electrolyte secondary battery 1 includes a battery body 2 and a heat retaining section 3 for maintaining the battery body 2 at an operating temperature in the medium temperature range (50 to 300° C.). The nonaqueous electrolyte secondary battery 1 can be used by being connected to various devices (21 in the figure).

The battery body 2 includes, in a sealable battery container (not shown), a positive electrode 11, a negative electrode 12, a non-aqueous electrolyte 13 interposed between the positive electrode 11 and the negative electrode 12, and a separator 14 for isolating the positive electrode 11 and the negative electrode 12 so that they do not come into contact with each other. The positive electrode 11 and the negative electrode 12 are arranged so that the surface of the positive electrode active material layer 11b of the positive electrode 11 and the surface of the negative electrode active material layer 12b of the negative electrode 12 face each other via the separator 14. As a result, the positive electrode 11 and the negative electrode 12 are arranged so that they do not come into direct contact with each other.

(Positive electrode)
The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode active material layer 11b.
The material of the positive electrode collector 11a is not particularly limited as long as it has electrical conductivity. For example, aluminum, platinum, carbon material, molybdenum, and tungsten can be used as the material of the positive electrode collector 11a. For example, the shape of the positive electrode collector 11a can be a plate, ribbon, foil, wire, etc., but is not limited to these shapes.

The positive electrode active material layer 11b contains a positive electrode active material, a conductive assistant, and a binder.
The positive electrode active material includes a defective spinel oxide in which the cation site is defective in a charged state. The spinel oxide has a crystal structure represented by the general formula AB ₂ O ₄ , in which the cation in the A site is surrounded by four oxygen ions to form a tetrahedron, and the cation in the B site is surrounded by eight oxygen ions to form an octahedron. The defective spinel oxide has a spinel crystal structure, in which the cation in the A site and/or the cation in the B site are defective. The defective spinel oxide may further have defective oxygen ions. In this embodiment, the charged state means a state in which the charging rate of the nonaqueous electrolyte secondary battery 1 is 100%.

The defect spinel oxide preferably contains a first cation and a second cation that are different from each other. The first cation is preferably an ion of a metal in Group 2 or Group 12 of the periodic table, and more preferably an ion of a metal in Group 12. Specifically, the first cation is preferably a zinc ion. The second cation is preferably an ion of a transition metal. The transition metal is preferably a metal that can be tetravalent, and particularly preferably a metal that can be tetravalent and divalent. Specifically, the second cation is preferably a manganese ion, cobalt ion, iron ion, vanadium ion, chromium ion, molybdenum ion, or titanium ion, and particularly preferably a manganese ion or cobalt ion. When the first cation is an ion of a metal in Group 2 or Group 12 of the periodic table, the defect spinel oxide may be a normal spinel oxide in which the first cation is an A-site cation, or an inverse spinel oxide in which the first cation is a B-site cation.

The defect spinel oxide preferably contains 0.5 to 1.5 moles of the second cation per mole of the first cation, and more preferably contains 0.8 to 1.3 moles of the second cation.

The defect spinel oxide is preferably a compound represented by the following general formula (1).
A _x B _y O _z ... (1)

In the above general formula (1), A represents an ion of a metal of Group 2 or Group 12 of the periodic table. That is, A represents the first cation described above. B represents an ion of a transition metal. That is, B represents the second cation described above. x, y, and z represent numbers that satisfy 0.5≦y/x≦1.5, 2≦x+y<z, and 3≦z≦4. It is particularly preferable that x, y, and z are numbers that satisfy 0.8≦y/x≦1.3, 2≦x+y<2.5, and 3≦z≦4.

The defect spinel oxide preferably has the structural formula _A1B1O3 or _{A1.33B1.33O4 , and} particularly preferably _{A1B1O3 . A1B1O3} is _a defect spinel oxide having a structure in which some _of the cations and oxygen ions at _the B site _are defective. _A1.33B1.33O4 is _a defect spinel oxide having _a structure in which some of the cations at the B site are missing and some of the cations at _the B site are replaced by cations at the A site. Specific examples of defective spinel oxides include ZnMnO ₃ , ZnCoO ₃ , MgMnO ₃ , MnMoO _{3 , CoMoO 3 , ZnMoO 3 , ZnTiO 3 , CoTiO 3 , AlVO 3 , γ - Cr 2} O ₃ , γ-Fe ₂ O ₃ , and γ-Mn ₂ O _3. Among these defective spinel oxides, ZnMnO ₃ and ZnCoO ₃ are preferred. Zn in ZnMnO ₃ and ZnCoO ₃ is likely to take a tetrahedral coordination. For this reason, ZnMnO ₃ and ZnCoO ₃ are likely to maintain the spinel crystal structure, and even if _they change to a rock salt crystal structure, they are likely to return to the spinel crystal structure.

The defect spinel oxide is preferably a fine particle powder. The primary particles of the defect spinel oxide preferably have an average particle size in the range of 2 nm to 200 nm. By having the average particle size of the primary particles in this range, the reaction rate of the defect spinel oxide is increased, improving the capacity and charge/discharge cycle characteristics of the nonaqueous electrolyte secondary battery.

The powder of fine defective spinel oxide can be produced, for example, by a method including a reverse coprecipitation step of obtaining a reverse coprecipitate by a reverse coprecipitation method, and a calcination step of calcining the obtained reverse coprecipitate to obtain a defective spinel oxide.

In the reverse coprecipitation process, first, a metal compound containing metal ions constituting the defective spinel oxide is dissolved in water to prepare a raw material aqueous solution. The raw material aqueous solution may be any solution as long as the metal components are in an ionic state, and there are no particular limitations on the anion components. As the raw material aqueous solution, a nitrate aqueous solution, a hydrochloride aqueous solution, or a sulfate aqueous solution can be used. Next, the obtained raw material aqueous solution is dropped into an alkaline buffer solution to precipitate the metal ions as a reverse coprecipitation product. The pH of the alkaline buffer solution is preferably 10 or more, more preferably 11 or more, from the viewpoint of precipitation, and is preferably 13 or less, more preferably 12 or less, from the viewpoint of ensuring a sufficient yield. As the alkaline buffer solution, for example, a sodium carbonate buffer solution, an acetate buffer solution, a phosphate buffer solution, a citrate buffer solution, a borate buffer solution, or a tartaric acid buffer solution can be used. The precipitated reverse coprecipitation product is preferably recovered by filtration or decantation, washed with water, and then dried.

In the calcination step, the reverse coprecipitate is calcined to obtain a defective spinel oxide. Calcination is carried out in an air atmosphere. There are no particular limitations on the calcination temperature or time, so long as the temperature and time are such that a defective spinel oxide is produced. The calcination temperature is generally within the range of 350°C to 650°C. The calcination time is generally within the range of 2 hours to 24 hours.

The conductive assistant contained in the positive electrode active material layer 11b may be, for example, a known conductive material used as a conductive assistant in the positive electrode active material layer of a non-aqueous solvent secondary battery, such as carbon powder or metal fine powder. Carbon black such as acetylene black or ketjen black may be used as the carbon powder.

The binder contained in the positive electrode active material layer 11b may be any that can be used at temperatures in the medium temperature range (50 to 300°C). Examples of binder materials include, but are not limited to, thermoplastic fluororesins such as polyvinylidene fluoride (PVDF), glass, and polyimide.

The contents of the positive electrode active material, conductive material, and binder in the positive electrode active material layer 11b are not particularly limited, but the content of the positive electrode active material is preferably in the range of 80% by mass to 98% by mass. The content of the conductive material is preferably in the range of 1% by mass to 19% by mass, and the content of the binder is preferably in the range of 1% by mass to 19% by mass.

(Negative electrode)
The negative electrode 12 includes a negative electrode current collector 12a and a negative electrode active material layer 12b.
The material of the negative electrode current collector 12a is not particularly limited as long as it has electrical conductivity, and various materials exemplified as the material of the positive electrode current collector 11a can be used. As in the case of the positive electrode current collector 11a, the shape of the negative electrode current collector 12a can be a plate shape, a ribbon shape, a foil shape, a wire shape, etc., but is not limited to these shapes.

The negative electrode active material layer 12b contains magnesium as the negative electrode active material. A magnesium plate or a magnesium alloy plate can be used as the negative electrode active material layer 12b. Examples of magnesium alloys include an alloy of magnesium and aluminum, an alloy of magnesium and manganese, and an alloy of magnesium and zinc.

(Non-aqueous electrolyte)
As the non-aqueous electrolyte 13, a mixture containing a magnesium salt and an ionic liquid, or a mixture containing a magnesium salt and an organic solvent can be used.
Examples of magnesium salts include magnesium bis(trifluoromethylsulfonyl)amide [Mg(TFSA) ₂ ] and magnesium bis(fluorosulfonyl)amide [Mg(FSA) ₂ ]. The magnesium salts may be used alone or in combination of two or more.

Examples of ionic liquids include cesium bis(trifluoromethylsulfonyl)amide [CsTFSA], N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide [DEMETFSA], N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide [PP13TFSA], and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)amide [P13TFSA]. One type of ionic liquid may be used alone, or two or more types may be used in combination.

As the organic solvent, glycol ethers can be used. The glycol ethers are preferably symmetric glycol ethers in which the hydroxyl groups at both ends are substituted with the same substituent. The substituent is preferably an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, or a butyl group. Examples of glycol ethers include glyme compounds such as monoglyme [ethylene glycol dimethyl ether], ethyl monoglyme [ethylene glycol diethyl ether], butyl monoglyme [ethylene glycol dibutyl ether], methyl diglyme [diethylene glycol dimethyl ether], ethyl diglyme [diethylene glycol diethyl ether], butyl diglyme [diethylene glycol dibutyl ether], methyl triglyme [triethylene glycol dimethyl ether], ethyl triglyme [triethylene glycol diethyl ether], butyl triglyme [triethylene glycol diethyl ether], methyl tetraglyme [tetraethylene glycol dimethyl ether], ethyl tetraglyme [tetraethylene glycol diethyl ether], and butyl tetraglyme [tetraethylene glycol dibutyl ether]. One type of glyme compound may be used alone, or two or more types may be used in combination.

(Separator)
Examples of the separator 14 include porous bodies such as porous films, porous glass, and glass mesh. In addition, when the positive electrode 11 and the negative electrode 12 are configured not to be in direct contact with each other, the separator 14 does not need to be provided.

(Insulation section)
The heat retaining part 3 is provided so as to cover the side of the battery body 2. The battery body 2 is provided with a heater 3a for heating the battery body 2 and a heat insulating member 3b for maintaining the temperature of the battery body 2 at the operating temperature. The heater 3a is provided inside the heat insulating member 3b. This makes it possible to suppress the loss of thermal energy of the heater 3a, and to maintain the temperature of the battery body 2 at a predetermined operating temperature with low energy consumption. The nonaqueous electrolyte secondary battery 1 can be used well at operating temperatures in the medium temperature range. The operating temperature is usually preferably 50° C. or higher, more preferably 80° C. or higher, and preferably 200° C. or lower. However, the nonaqueous electrolyte secondary battery 1 of this embodiment may be operated at room temperature. In this case, the heat retaining part 3 may not be provided.

There are no particular limitations on the shape of the battery body 2, and it can be any known shape used for non-aqueous electrolyte secondary batteries, such as coin type, button type, sheet type, cylindrical type, or square tube type.

In the non-aqueous electrolyte secondary battery 1 of this embodiment, the positive electrode active material of the positive electrode active material layer 11b contains a defective spinel-type oxide in which the cation site is defective in a charged state, and the negative electrode active material of the negative electrode active material layer 12b contains magnesium in a charged state. When magnesium ions, which are the negative electrode active material, are inserted into the defective spinel-type oxide, which is the positive electrode active material, by discharging, the magnesium ions are coordinated to the defects of the defective spinel-type oxide, and the defects disappear. Therefore, even if a relatively large amount of magnesium ions are inserted into the positive electrode active material, the crystal structure is maintained in a spinel-type crystal structure state. Furthermore, when a large amount of magnesium ions is inserted into the positive electrode active material as the discharge progresses, the crystal structure of the positive electrode active material changes from a spinel-type crystal structure to a rock salt-type crystal structure. When the crystal structure of the positive electrode active material completely changes to a rock salt-type crystal structure, the change in the crystal structure of the positive electrode active material for each charge and discharge becomes too large, and the crystal structure of the positive electrode active material becomes easily broken in parts. For this reason, in the nonaqueous electrolyte secondary battery 1 of this embodiment, it is preferable to stop discharging before the crystal structure of the positive electrode active material completely changes to a rock _{salt type} crystal structure. For example, when the general formula of the positive electrode active material is _A1B1O3 , it is preferable to discharge the positive electrode active material at the end of discharge so that it is represented by the following general formula (2).
_{MgwA1B1O3 ...} ( _{2 )}

In the above general formula (2), w is preferably a number that satisfies 0.5<w<0.9, and particularly preferably a number that satisfies 0.5<w<0.8. By setting w to the above number, it is possible to prevent the crystal structure of the positive electrode active material from completely changing to a rock salt type crystal structure.

In the nonaqueous electrolyte secondary battery 1 of this embodiment, the positive electrode active material has a spinel crystal structure having defects, so even if a relatively large amount of magnesium ions are inserted into the positive electrode active material, the crystal structure remains in a spinel crystal structure state. Therefore, the positive electrode active material has a high capacity in a single phase of a spinel crystal structure, and the crystal structure is unlikely to change despite the high capacity. In addition, in the nonaqueous electrolyte secondary battery 1 of this embodiment, the magnesium ions inserted into the positive electrode active material prevent the positive electrode active material from completely changing to a rock salt crystal structure, improving the charge/discharge cycle characteristics.

Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.
For example, in the above embodiment, the nonaqueous electrolyte secondary battery has been described as a magnesium nonaqueous electrolyte secondary battery in which the negative electrode active material contains magnesium, but the negative electrode active material of the nonaqueous electrolyte secondary battery is not limited thereto. The nonaqueous electrolyte secondary battery may be a sodium nonaqueous electrolyte secondary battery in which the negative electrode active material contains sodium.

In the case of a sodium nonaqueous electrolyte secondary battery, a mixture containing a sodium salt and an ionic liquid, or a mixture containing a sodium salt and an organic solvent can be used as the nonaqueous electrolyte. Examples of sodium salts include sodium bis(trifluoromethylsulfonyl)amide [Na(TFSA)] and sodium bis(fluorosulfonyl)amide [Na(FSA)]. A single type of sodium salt may be used alone, or two or more types may be used in combination. The ionic liquid and organic solvent are the same as those in the case of a magnesium nonaqueous electrolyte secondary battery. Sodium and sodium alloys can be used as the negative electrode active material. Examples of sodium alloys include sodium potassium alloys, sodium lithium alloys, and sodium tin alloys.

[Example 1]
(1) Preparation of _ZnMnO3 powder 50 mL of zinc nitrate aqueous solution (concentration: 0.12 mol/L) and 50 mL of manganese nitrate aqueous solution (concentration: 0.12 mol/L) were mixed to prepare a raw material aqueous solution. 100 mL of the obtained raw material aqueous solution was dropped into a sodium carbonate buffer solution (concentration: 0.35 mol/L, pH: 11) kept at 70 ° C. over 0.5 hours. After the dropwise addition, the obtained mixture was stirred at a rotation speed of 500 rpm using a stirrer for 1 hour to precipitate zinc-manganese-containing particles (reverse coprecipitate).

The zinc-manganese-containing particles were collected by filtration, washed with pure water, and then dried. The dried zinc-manganese-containing particles were placed in a crucible, and an electric furnace was placed inside, where they were fired at 500°C for 2 hours in an air atmosphere. The fired product was then allowed to cool to room temperature.

The resulting fired product was dissolved in nitric acid, and the Zn and Mn contents in the resulting solution were measured. As a result, the Zn and Mn contents in the fired product were found to be 1:1 in molar ratio.

The resulting fired product was observed using the HAADF-STEM method to measure the particle size of the primary particles. As a result, the average particle size of 100 primary particles was 10 nm.

Furthermore, the X-ray diffraction pattern of the obtained fired product was measured using Mo-Kα radiation. The results are shown in FIG. 2. In FIG. 2, (a) is the X-ray diffraction pattern of the fired product, (b) is the X-ray diffraction pattern of ZnFe ₂ O ₄ , and (c) is the X-ray diffraction pattern of MgO. Note that (b) and (c) are X-ray diffraction patterns published in the Inorganic Compound Crystal Structure Database (ICSD).
From the above results, it was confirmed that the obtained fired product was a fine _ZnMnO3 powder having a defective spinel type crystal structure.

In addition, the X-ray diffraction pattern of the sintered product obtained by sintering the zinc-manganese-containing particles in an air atmosphere at 350 ° C for 2 hours is also shown in Figure 2. From this result, it was confirmed that it is possible to obtain _ZnMnO3 powder having a defective spinel type crystal structure by sintering the zinc-manganese-containing particles at a sintering temperature of 350 ° C to 500 ° C.

(2) Preparation of electrode material The _ZnMnO3 powder obtained in (1) above, carbon black (CB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 80:10:10 (= _ZnMnO3 powder:CB:PVDF) to obtain an electrode material.

(3) Preparation of electrode for evaluation The electrode material obtained in (2) above and a solvent (NMP: N-methyl-2-pyrrolidone) were mixed in a mass ratio of 1:5 (= electrode material: NMP) to obtain an electrode material paste. The obtained electrode material paste was applied to the surface of a current collector made of platinum foil so that the coating amount as a positive electrode active material ( _ZnMnO3 powder) was 17 μg/ ^mm2 , and then dried to obtain an electrode for evaluation.

(4) Preparation of Electrochemical Cell A three-electrode electrochemical cell 50 shown in Fig. 3 was prepared in a glove box maintained in an argon gas atmosphere. The electrochemical cell 50 is composed of an evaluation electrode 51 (working electrode), a counter electrode 52, a reference electrode 53, an electrolyte 54, a container 55, and a heat retaining part 56 for maintaining the inside of the container 55 at a predetermined operating temperature.

The evaluation electrode 51 is obtained in the above (3), and includes a current collector 51a made of platinum foil and an electrode material layer 51b provided on the current collector 51a. The counter electrode 52 is made of magnesium ribbon. The reference electrode 53 includes a reference electrode body 53a made of lithium foil, a reference electrode electrolyte 53b, a glass tube portion 53c that isolates the reference electrode body 53a from the electrolyte 54, and a porous glass portion 53d that is integrally formed with the glass tube portion 53c and ensures electrical connection between the inside and outside of the glass tube portion 53c. The reference electrode body 53a is electrically connected to the evaluation electrode 51 and the counter electrode 52, but is configured not to directly contact the electrolyte 54. The container 55 includes a container body 55a and a lid portion 55b. The heat retaining portion 56 includes a heater 56a and an aluminum block 56b. The evaluation electrode 51, counter electrode 52, and reference electrode 53 of the electrochemical cell 50 are connected to a potentiostat 61 via electrode leads 61a, 61b, and 61c, respectively. The reference electrode electrolyte 53b is N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide [DEMETFSA] containing lithium bis(trifluoromethylsulfonyl)amide [LiTFSA] at a concentration of 0.5 mol/L. The electrolyte 54 is a mixture containing magnesium bis(trifluoromethylsulfonyl)amide [Mg(TFSA) ₂ ] and cesium bis(trifluoromethylsulfonyl)amide [CsTFSA] in an atomic ratio of Mg and Cs of 10:90 (=Mg:Cs).

(5) Measurement of Cyclic Voltammetry Using the electrochemical cell 50 obtained in (4) above and an electrochemical measurement device [manufactured by BioLogic, product name: SP-300], the cyclic voltammetry of the evaluation electrode 51 was measured. During the measurement, the temperature (temperature of the electrolyte) inside the container 55 of the electrochemical cell 50 was maintained at 150° C. The scanning potential range was 2.0 V to 4.5 V (vs. reference electrode), and the scanning speed was 1 mV/sec. The measurement was performed for 10 cycles. The results are shown in FIG. 4.

4, the horizontal axis is voltage, the upper horizontal axis is voltage relative to the reference electrode (lithium), and the lower horizontal axis is voltage relative to the counter electrode (magnesium), and the vertical axis is current value.
From the results of FIG. 4, it can be seen that the _ZnMnO3 powder contained in the electrode material of the evaluation electrode 51 is capable of absorbing and releasing magnesium ions and acts as a positive electrode active material.

(6) Measurement of Discharge Curve by GITT (Constant Current Intermittent Titration) Using the electrochemical cell 50 obtained in (4) above and an electrochemical measurement device [manufactured by BioLogic, product name: SP-300], the discharge curve of the evaluation electrode 51 by GITT was measured. One cycle consisted of discharging at a current density of 10 mA for 1 hour per 1 g of the positive electrode active material (ZnMnO ₃ ), followed by leaving the electrode stationary for 4 hours. The results are shown in FIG.

In the discharge curves in Fig. 5, the horizontal axis is the discharge time (excluding the resting time), and the vertical axis is the discharge voltage, with the right vertical axis showing the voltage relative to the reference electrode (lithium) and the left vertical axis showing the voltage relative to the counter electrode (magnesium).
From the results in FIG. 5, it can be seen that the slope of the discharge curve differs between the discharge time range of 24 hours (1st cycle to 23rd cycle) (Stage 1), the discharge time range of 24 hours to 34 hours (24th cycle to 33rd cycle) (Stage 2), and the discharge time range of 34 hours or more (34th cycle and beyond) (Stage 3). This is considered to be due to a change in the crystal structure of the positive electrode active material (ZnMnO ₃ ) in Stages 1 to 3.

(7) Structural change of _ZnMnO3 due to insertion of magnesium ions While maintaining the electrochemical cell 50 obtained in (4) above at 150°C, a voltage of 1.5 V was applied to the evaluation electrode 51 as a reference electrode (lithium) voltage for 18 hours to insert magnesium ions into the ZnMnO3 of the evaluation electrode 51. Thereafter, the evaluation electrode ₅₁ was removed from the electrochemical cell 50, washed with pure water, and then dried. The atomic ratio of metal elements (Mg, Zn, Mn) was measured for the evaluation electrode 51 after drying using EDX (energy dispersive X-ray fluorescence analysis). In addition, the X-ray diffraction pattern of the evaluation electrode 51 was measured using Mo-Kα rays.

Moreover, magnesium ions were inserted into the _ZnMnO3 of the evaluation electrode 51 in the same manner as above, except that the voltage applied to the evaluation electrode 51 was set to 2.0 V and 2.5 V relative to the reference electrode voltage, and the atomic ratio of the metal elements and the X-ray diffraction pattern of the evaluation electrode 51 were measured.
These results are shown in FIG. 6 together with the simulated X-ray diffraction pattern of rock-salt structure MgZnMnO ₃ (a=8.685 Å) and the simulated X-ray diffraction pattern of [Zn _0.9 ] _8a [Zn _0.27 Mn _0.73 ] _16d [O ₁ ] _32e (a=8.375 Å).

As shown in Fig. 6, the evaluation electrode 51 to which a voltage of 1.5 V was applied as the reference electrode voltage had an atomic ratio of Mg:Zn:Mn = 0.90:0.94:1, and _ZnMnO3 was completely changed to a rock salt structure. This state is considered to correspond to Stage 3 in the discharge curve of Fig. 5. The evaluation electrode 51 to which a voltage of 2.0 V was applied as the reference electrode voltage had an atomic ratio of Mg:Zn:Mn = 0.81:0.96:1, and _ZnMnO3 was partially changed from a spinel structure to a rock salt structure, and the spinel structure and the rock salt structure coexisted. This state is considered to correspond to Stage 2 in the discharge curve of Fig. 5. The evaluation electrode 51 to which a voltage of 2.5 V was applied as the reference electrode voltage had an atomic ratio of Mg:Zn:Mn=0.58:0.95:1, and _ZnMnO3 maintained a spinel structure. This state is considered to correspond to Stage 1 in the discharge curve of Fig. 5. From the above results, it was confirmed that it is desirable to charge and discharge the electrochemical cell 50 under conditions where the reference electrode voltage is 2.0 V or higher.

(8) Evaluation of Charge-Discharge Cycle Characteristics A charge-discharge cycle test was performed on the electrochemical cell 50 obtained in (4) above while maintaining the cell at 150° C. The charge-discharge cycle conditions were as follows: charge-discharge current value was 10 mA/g based on the mass of the positive electrode active material (ZnMnO ₃ ), discharge end voltage was 2.0 V based on the reference electrode, and discharge end voltage was 4.1 V based on the reference electrode.
Figure 7 shows the charge/discharge curves up to 10 cycles, and Figure 8 shows the discharge capacity and coulomb efficiency up to 43 cycles.

In Fig. 7, the horizontal axis is the capacitance based on the mass of the positive electrode active material ( _ZnMnO3 ). The vertical axis is the voltage, with the right vertical axis showing the voltage based on the reference electrode (lithium) and the left vertical axis showing the voltage based on the counter electrode (magnesium). From the graph in Fig. 7, it can be seen that the charge/discharge curve becomes stable after the third cycle.

In Fig. 8, the horizontal axis is the number of cycles. The vertical axis on the right side is the Coulombic efficiency, and the vertical axis on the left side is the discharge capacity based on the mass of the positive electrode active material ( _ZnMnO3 ). From the graph in Fig. 8, it can be seen that the Coulombic efficiency improves with each cycle up to 10 cycles, and after 10 cycles, the Coulombic efficiency stabilizes at almost 100%. It can also be seen that the discharge capacity of the positive electrode active material stabilizes at 100 mAh/g.

The above results confirmed that the electrochemical cell (non-aqueous solvent secondary battery) obtained in Example 1 has high capacity and excellent charge/discharge cycle characteristics.

REFERENCE SIGNS LIST 1 non-aqueous electrolyte secondary battery 2 battery body 3 heat retaining portion 3a heater 3b heat insulating member 11 positive electrode 11a positive electrode current collector 11b positive electrode active material layer 12 negative electrode 12a negative electrode current collector 12b negative electrode active material layer 13 non-aqueous electrolyte 14 separator 50 electrochemical cell 51 evaluation electrode 51a current collector 51b electrode material layer 52 counter electrode 53 reference electrode 53a reference electrode body 53b reference electrode electrolyte 53c glass tube portion 53d porous glass portion 54 electrolyte 55 container 55a container body 55b lid portion 56 heat retaining portion 56a heater 56b aluminum block 61a, 61b, 61c electrode lead 61 potentiostat

Claims (4)

a positive electrode having a positive electrode active material layer including a positive electrode active material, a negative electrode having a negative electrode active material layer including a negative electrode active material, and a non-aqueous electrolyte;
the positive electrode active material contains a defect spinel oxide in which cation sites are defective in a charged state,
the negative electrode active material contains magnesium or sodium in a charged state,
The defect spinel-type oxide is a compound represented by the following general formula (1):
A _x B _y O _z ... (1)
(In the above general formula (1), A represents an ion of a metal of Group 2 or 12 of the periodic table, B represents an ion of a transition metal, and x, y, and z represent numbers satisfying 0.5≦y/x≦1.5, 2≦x+y<z, and 3≦z≦4.) a positive electrode having a positive electrode active material layer including a positive electrode active material, a negative electrode having a negative electrode active material layer including a negative electrode active material, and a non-aqueous electrolyte;
the positive electrode active material contains a defect spinel oxide in which cation sites are defective in a charged state,
the negative electrode active material contains magnesium or sodium in a charged state,
The defect spinel oxide contains a first cation and a second cation which are different from each other,
the first cation is an ion of a metal from Group 2 or 12 of the Periodic Table, and the second cation is an ion of a transition metal;
the defect spinel-type oxide contains the second cation in an amount ranging from 0.5 mol to 1.5 mol per 1 mol of the first cation; The oxide contains a defect spinel type oxide in which the cation site is defective,
The defect spinel oxide contains a first cation and a second cation which are different from each other,
the first cation is an ion of a metal from Group 2 or 12 of the Periodic Table, and the second cation is an ion of a transition metal;
The defect spinel-type oxide contains the second cation in an amount ranging from 0.5 mol to 1.5 mol per 1 mol of the first cation. The oxide contains a defect spinel type oxide in which the cation site is defective,
The defect spinel-type oxide is a compound represented by the following general formula (1):
A _x B _y O _z ... (1)
(In the above general formula (1), A represents an ion of a metal of Group 2 or 12 of the periodic table, B represents an ion of a transition metal, and x, y, and z represent numbers satisfying 0.5≦y/x≦1.5, 2≦x+y<z, and 3≦z≦4.)

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