Method for producing cubic alkali manganate nanoparticles and cubic LiMnO2 nanoparticles produced therefrom

Abstract

The present invention aims to produce cubic LiMnO2 nanoparticles without requiring special synthesis conditions or treatment, and without the presence of coarse particles. The method for producing LiMnO2 includes: adding an organic solvent, manganese oxide nanoparticles, and lithium amide to a reaction vessel, heating under an inert atmosphere, and generating cubic LiMnO2 nanoparticles; and washing and recovering the generated particles. Wurtzite-type MnO nanoparticles are preferably used as the manganese oxide. Thus, LiMnO2 nanoparticles having a particle size roughly equivalent to that of the wurtzite-type MnO nanoparticles used as the Mn raw material are obtained. Furthermore, by controlling the reaction temperature, nanoparticles with a hollow structure can be formed.

Description

Technical Field

[0001] The present invention relates to alkali manganate nanoparticles represented by lithium manganate, in particular to cubic LiMnO2 nanoparticles. Background Art

[0002] Lithium manganese oxide is a material that has attracted much attention as a positive electrode material for lithium-ion batteries. Various materials with different ratios of Li, Mn, and O have been developed, including LiMn2O4, LiMnO3, and LiMnO2. Furthermore, the properties of positive electrode materials, such as initial discharge capacity and charge-discharge characteristics, also depend on their crystal structure. Therefore, various manufacturing methods and treatments have been proposed to achieve the desired crystal structure.

[0003] Regarding LiMnO2, for example, Non-Patent Document 1 discloses a method for obtaining cubic LiMnO2 nanoparticles by mechanically milling orthorhombic LiMnO2. This method first synthesizes a mixture of Li2CO3 and Mn2O3 by heating at 900°C in an inert atmosphere to produce bulk orthorhombic LiMnO2 as a precursor. The resulting precursor is then pulverized and miniaturized by mechanical milling to obtain the desired cubic nanoparticles. The resulting particles are believed to contain a wide range of sizes, ranging from a few micrometers to tens of nanometers.

[0004] Non-patent document 2 reports on the synthesis of cubic LiMnO2 using a high-pressure synthesis method. In this method, Li2O and Mn2O3 are mixed and synthesized in a gold capsule under ultrahigh-pressure and high-temperature conditions of 4.5 GPa and 1000°C. The resulting powder, a mixture of Li2O and LiMnO2, is then treated with water to ultimately recover single-phase cubic LiMnO2.

[0005] Prior art literature

[0006] Non-patent literature

[0007] Non-patent literature 1: Journal of Materials Chemistry A 2018, 6, 13943, Takahiko Sato, et al.

[0008] Non-Patent Literature 2: NEDO Report: April 2012, "Technology Development of High-Performance Power Storage Systems for Next-Generation Vehicles / Next-Generation Technology Development / Material Design of Next-Generation High-Capacity Cathode Oxide Materials Based on High-Pressure Synthesis," page 26 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] The method described in Non-Patent Document 1 requires high-temperature, long-term processing to obtain bulk LiMnO2, the starting material for obtaining cubic nanoparticles. Furthermore, high-temperature, long-term reaction is required to allow the precursors to react with each other. Furthermore, long-term mechanical milling is required for nanosizing, but mechanical milling makes it difficult to control particle size. The resulting particles contain a mixture of micron-sized particles, making it difficult to obtain particles with an average particle size of less than 100 nm.

[0011] The method described in Non-Patent Document 2 requires fewer processes than the technique in Non-Patent Document 1, but since synthesis is performed at extremely high pressure and high temperature, such as 4.5 GPa, equipment compatible with high pressure and high temperature conditions is required.

[0012] The present invention aims to provide a method for producing cubic alkali manganate nanoparticles, in particular cubic LiMnO2 nanoparticles, which does not require special synthesis conditions or treatments and does not contain coarse particles.

[0013] Means for solving problems

[0014] The present invention's method for producing alkali manganate nanoparticles, which addresses the aforementioned issues, includes the steps of: adding an organic solvent, manganese oxide nanoparticles, and lithium amide to a reaction vessel, heating the mixture under an inert atmosphere, and generating cubic alkali manganate nanoparticles; and washing and recovering the generated particles. In the present method for producing LiMnO2, wurtzite-type MnO nanoparticles are preferably used as a raw material.

[0015] The cubic LiMnO2 of the present invention is characterized in that it is produced by thermal synthesis and has an average particle size of 100 nm or less. It is also characterized in that it has a hollow structure.

[0016] Effects of the Invention

[0017] According to the present invention, alkali manganate nanoparticles, represented by cubic LiMnO2, can be provided without requiring special reaction equipment or additional processing such as mechanical grinding. The cubic LiMnO2 of the present invention has a hollow structure. Therefore, when used as a lithium battery material, the charge-discharge characteristics can be improved by adding inclusions to the hollow portion, thereby improving the lifespan of the positive electrode material and, consequently, the lithium battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a diagram showing an outline of the method for producing LiMnO2 of the present invention.

[0019] Figure 2 This is a diagram showing the X-ray diffraction (XRD) patterns of the particles obtained in Example 1 and Example 2.

[0020] Figure 3 (A) to (C) are transmission electron microscope images of the particles obtained in Examples 1 and 2 and the MnO particles used as a raw material. DETAILED DESCRIPTION

[0021] Hereinafter, an embodiment of the method for producing cubic alkali manganate of the present invention will be described. Here, a method for producing nanoparticles of LiMnO2, which is a typical alkali manganate, will be described.

[0022] The manufacturing method of the present invention is based on the synthesis based on thermal decomposition method, such as Figure 1 As shown, the process consists of the following steps: preparing a Mn raw material; placing the Mn raw material, Li raw material, and solvent into a reaction vessel; raising the temperature to a predetermined temperature of 350°C or less and reacting for a predetermined time (approximately 60 minutes); and recovering particles from the reaction solution. Each step is described in detail below.

[0023] <Process 1>

[0024] The production method of the present invention uses manganese oxide (MnO) nanoparticles and a lithium complex such as lithium amide (LiNH2) as raw materials. This novel combination of manganese oxide and lithium amide eliminates the need for high pressure, high temperatures of 1000°C, or long reaction times, and enables the production of cubic LiMnO2 through conventional thermal decomposition methods. In particular, the use of wurtzite-type nanoparticles as manganese oxide facilitates the production of cubic crystals and facilitates particle size control.

[0025] Although the stable crystal structure of MnO is usually rock salt type, it is known that MnO takes a wurtzite type crystal structure in theory. Although there are some manufacturing examples, it is not known to obtain only wurtzite type MnO nanoparticles stably by synthesis. The applicant has developed a method for producing wurtzite type MnO nanoparticles by synthesis based on thermal decomposition (Japanese Patent Application No. 2019-205644). In this method, by adding a prescribed reducing agent to the reaction system for synthesis, the formation of rock salt type MnO and layered double hydroxide as a by-product can be suppressed, and wurtzite type MnO with a particle size of less than 100 nm can be stably obtained.

[0026] Specifically, when synthesizing manganese oxide particles by thermally decomposing a manganese-containing compound, a reducing agent composed of at least one of a polyol-based material and a glycol stearate-based material is added to the reaction system as an additive, and a particle size inhibitor is preferably added. Then, after heating in a reduced pressure atmosphere (pressure below 1000 Pa, preferably below 100 Pa) at a temperature below 200°C, preferably 110°C to 150°C, to form nuclei, the temperature is raised to 225°C to 275°C under an inert gas atmosphere to allow the particles to grow. By controlling the reaction time of this particle growth process, the size of the manganese oxide particles can be controlled to, for example, about 2nm to 100nm, preferably below 30nm. The average particle size can be, for example, below 100nm, more preferably below 40nm. After the particles have grown, the temperature is further increased to about 300°C to allow the particles to mature, thereby achieving uniform size.

[0027] In the production method of the present invention, by using the wurtzite-type MnO nanoparticles (6 nm to 30 nm) prepared by the above method as the Mn raw material, cubic LiMnO2 nanoparticles with controlled particle size can be obtained.

[0028] As the Li raw material, Li complexes such as LiNH2 and LiNR1R2 (R1 and R2 are hydrogen or alkyl) are used. Among them, LiNH2 with high activity is particularly preferred. The Li source can be used in an equimolar or greater and 100-fold molar ratio relative to MnO, but it is preferably used in excess compared to the stoichiometric ratio. By using an excess amount of LiNH2 (for example, 50 equivalents or more), LiMnO2 particles can be stably obtained.

[0029] As the solvent, an oxygen-containing organic solvent that serves as an oxygen source for LiMnO 2 is used. As the oxygen-containing organic solvent, for example, ether solvents such as diphenyl ether, benzyl ether, and di-n-octyl ether can be preferably used, and diphenyl ether is particularly preferred.

[0030] <Process 2>

[0031] The reaction is carried out under an inert gas atmosphere. Therefore, after the Mn raw material and the solvent are put into the reaction container, the Li source (LiNH2) is put into the reaction container under the inert gas atmosphere, and the reaction container is placed in a pressure-resistant container and sealed.

[0032] <Process 3>

[0033] In an inert gas atmosphere such as nitrogen, heat to the reaction temperature (heater temperature is 150°C to 350°C). The heating rate is preferably a relatively slow rate, for example, about 5°C / minute. After reaching the reaction temperature, maintain at this temperature for a predetermined time. It is believed that the reaction proceeds as follows. The following describes the case where the Li source is LiNH2. In this reaction, wurtzite-type MnO nanoparticles react as the starting material (nucleus), and LiNH2 is coordinated on the surface of the particles. The atoms in the MnO particles react with Li on the surface of the nanoparticles to form LiMnO2 crystals. At this time, the wurtzite-type crystals (atomic arrangement) as a quasi-stable phase undergo a crystal structure phase transition to the MnO2 phase as a stable phase by heating. On the other hand, the LiNH2 present in the system is easily decomposed in the presence of other materials at high temperature due to its activity, becoming a source of Li. In this active state, Li is absorbed into the MnO crystal structure, and a cubic LiMnO2 phase is formed while absorbing oxygen in the solvent.

[0034] In addition, when the reaction temperature is set to a relatively high temperature (for example, about 350°C), the reaction rate is high, so solid particles can be obtained. On the other hand, at a relatively low temperature (for example, 200°C), hollow particles are obtained. This is believed to be because under conditions of low reaction temperature, the growth rate of LiMnO2 is slow. During this period, a rapid reaction easily occurs at the contact site between the active LiNH2 and MnO. The atoms in the MnO nanoparticles move toward the LiNH2 coordinated on the surface of the MnO nanoparticles for the reaction, so the interior becomes hollow and forms a hollow structure. In this way, by controlling the reaction temperature, the morphology of the generated particles can be controlled.

[0035] The reaction time is about 30 minutes to 2 hours. In the production method of the present invention, the above reaction is carried out, and the synthesis is completed in a short time. After the reaction, the reaction vessel is quenched and removed from the pressure vessel.

[0036] <Step 4>

[0037] The recovery of particles from the reaction system is carried out in the same manner as in the recovery of metal oxides after conventional synthesis, and centrifugal separation using a solvent is repeated, followed by washing and recovery.

[0038] The resulting particles are cubic LiMnO2 with an average particle size of 100nm or less. By appropriately selecting the particle size of the MnO nanoparticles, the average particle size can be reduced to 40nm or less. The shape can be solid or hollow.

[0039] The average particle size referred to here is an average value calculated by measuring the particle sizes of 200 to 2000 particles using a transmission electron microscope.

[0040] The production method of the present invention enables the production of cubic LiMnO2 in a short time under mild conditions below 350°C, which was previously only possible through extremely high-temperature, high-pressure, and long-term production methods. This allows the production of nanoparticles without requiring additional processing. Furthermore, since the particle size of the cubic LiMnO2 nanoparticles depends on the size of the MnO nanoparticles used as the starting material, the particle size of the LiMnO2 nanoparticles can be controlled, resulting in particles with an average particle size of less than 100 nm.

[0041] Furthermore, hollow LiMnO2 nanoparticles can be formed depending on the reaction conditions. The hollow structure reduces density, and by controlling particle size and thickness, further improvements in charge-discharge characteristics and weight reduction can be expected.

[0042] While the method for producing cubic LiMnO2 of the present invention has been described above, this method can also be applied to the synthesis of other alkali metal manganates such as NaMnO2. It is believed that by replacing the complex of the Li source with, for example, a Na source, it is possible to produce NaMnO2 nanoparticles and hollow structure nanoparticles thereof, which are expected to be candidates for other electrode materials.

[0043] Example

[0044] Hereinafter, examples of the production method of the present invention will be described.

[0045] <Example 1>

[0046] [Production of MnO Nanoparticles]

[0047] 10 mL of oleylamine as a solvent, 1.5 mmol of manganese stearate (st-Mn) as a manganese material, 3.0 mmol of ethylene glycol (EG) as a reducing agent, 3.6 mmol of trioctylphosphine (TOP) as a complexing agent, and 1.8 mmol of sulfur were used.

[0048] After filling the container (100 mL) with the material, it was kept at 70°C for 30 minutes under a nitrogen atmosphere, then heated and kept at 140°C for 2 hours under a reduced pressure atmosphere. The pressure at this time was about 100 Pa. Then, the temperature was raised to 250°C at a heating rate of 50°C / 5 minutes, and kept at 250°C for 2 hours under a N2 atmosphere to allow crystal growth. The temperature was then further raised and kept at 300°C for 15 minutes under a N2 atmosphere to mature the crystals.

[0049] 5 mL of hexane was added to the reaction solution after cooling, and the mixture was recovered in a centrifuge tube after stirring. Ethanol as a poor solvent was added to agglomerate the particles, and a centrifuge was used for sedimentation. After the supernatant was discarded, 5 mL of hexane was added and stirred with an oscillator for 30 minutes to disperse the particles. Ethanol was added again, and the same process was repeated once more to wash the particles, thereby obtaining MnO particles (red) with a particle size of 25 nm (particle size confirmed by TEM).

[0050] [Synthesis of cubic LiMnO2]

[0051] After adding 0.14 mmol of 25 nm MnO nanoparticles prepared as described above and 6 mL of diphenyl ether (liquid) as a solvent to a reaction container (100 mL), 10.8 mmol of LiNH2 (powder) was added to the reaction container under N2 atmosphere, and the reaction container was placed in a pressure-resistant container and sealed.

[0052] Under N2 atmosphere, the temperature was raised at a rate of 5°C / min until the heater temperature reached 350°C. After reaching 350°C, the temperature was maintained for 60 minutes to complete the reaction. Then, the reaction vessel was rapidly cooled and removed from the pressure vessel.

[0053] 40 mL of ethanol was added to the reaction solution, and the centrifugal separation operation was repeated five times. Then, the separated particles were washed with ethanol and recovered.

[0054] <Example 2>

[0055] 0.14 mmol of 25 nm MnO nanoparticles prepared in the same manner as in Example 1, 10.8 mmol of LiNH2 (powder), and 6 mL of diphenyl ether solvent were added to a reaction vessel in the same manner as in Example 1, and the reaction was carried out under an N2 atmosphere. In this example, the temperature was increased at a rate of 5°C / min to 200°C (heater temperature) and maintained at this temperature for 60 minutes. After the reaction, the mixture was rapidly cooled in the same manner as in Example 1, and then centrifuged (five times) and washed with ethanol to recover the particles.

[0056] [Evaluation of recovered particles]

[0057] The particles recovered in Examples 1 and 2 and the MnO used as the raw material were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The X-ray diffraction patterns are shown in Figure 2 TEM images are shown in Figure 3 (A)~(C). Figure 2 In the figure, the bars below the diffraction pattern represent the diffraction peaks of the reference, and the thick lines represent the diffraction peaks of cubic LiMnO2 (COD 1514037 Li 0.5Mn 0.5 O), and the thin line represents wurtzite-type MnO (COD 4117966 MnO). Figure 3 A schematic diagram of the particle size and structure analyzed from the TEM image is also shown in FIG.

[0058] like Figure 2 As shown, peaks were observed at positions consistent with those of the reference (cubic LiMnO2) in both the particles of Examples 1 and 2, confirming the formation of a cubic LiMnO2 phase. Furthermore, it was confirmed that the raw material MnO was wurtzite-type.

[0059] like Figure 3 As shown, the particle sizes of the particles obtained in Examples 1 and 2 were both less than 100 nm, and approximately 30 nm or less. Furthermore, TEM images confirmed that the average particle size in Example 1 was 19 nm. Furthermore, TEM images confirmed that a hollow structure was generated in Example 2, with an average particle size of 26 nm and a shell thickness of approximately 4 to 6 nm.

[0060] It should be noted that the particle size of the raw material MnO nanoparticles is roughly the same as that of the generated LiMnO2. This is believed to be due to the incorporation of Li into the MnO nanoparticles during the reaction while being treated at high temperature, causing the MO particles to dissolve due to the active LiNH2. To confirm this, the particle size of MO particles treated at 350°C without the addition of LiNH2 was measured. The results showed no effect of this heat treatment on the particle size.

[0061] In addition, MnO nanoparticles having a particle size different from that of the nanoparticles (25 nm) used in Example 1 were used as MnO nanoparticles, and the reaction was carried out in the same manner as in Example 1. As a result, it was confirmed that LiMnO2 having the same particle size as that of the MnO nanoparticles used as the raw material was obtained.

Claims

1. A method for producing alkali manganate nanoparticles, characterized in that: It includes: The invention relates to a process of adding an oxygen-containing organic solvent, hexagonal manganese (II) oxide nanoparticles and an alkali metal M complex into a reaction vessel, heating the reaction vessel under an inert atmosphere to generate cubic alkali metal manganate MMnO2 nanoparticles; and washing and recovering the generated particles.

2. The method for producing alkali manganate nanoparticles according to claim 1, wherein: The alkali metal M complex is lithium amide, and the cubic alkali metal manganate is cubic LiMnO2 nanoparticles.

3. The method for producing alkali manganate nanoparticles according to claim 1 or 2, wherein: The particle size of the hexagonal manganese (II) oxide nanoparticles is 30 nm or less.

4. The method for producing alkali manganate nanoparticles according to claim 1 or 2, wherein: The heating temperature in the inert atmosphere is 150°C to 350°C.

5. The method for producing alkali manganate nanoparticles according to claim 4, wherein: The heating temperature in an inert atmosphere is lower than 250° C., and hollow cubic alkali manganate is produced.

6. A cubic LiMnO2 nanoparticle produced by the production method according to claim 1, having an average particle size of 100 nm or less and a hollow structure.

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