JP6927579B2 - Lithium iron manganese-based composite oxide - Google Patents

Description

The present invention relates to a lithium iron-manganese-based composite oxide.

Lithium-ion secondary batteries occupy the most important position among energy storage devices, and in recent years, their applications are expanding, such as automobile batteries for plug-in hybrids.

Regarding the positive electrode of a lithium ion secondary battery, positive electrode _{active materials such as LiCoO 2} , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ are currently the mainstream (Non-Patent Documents 1 and 2). However, the positive electrode material containing these positive electrode active materials is expensive because it contains a large amount of rare metals such as cobalt and nickel, and also has strong flammability, which causes a heat generation accident and the like. ..

Therefore, at present, as a cathode active material that can solve such a problem, iron, which is an element abundant in nature, is used, and an iron-based poly (oxo) whose flammability is significantly suppressed by a strong polyanic acid skeleton. ) Anionic materials, especially LiFePO _4, are attracting attention (Non-Patent Document 3).

Solid State Inics, 3-4, 171-174, 1981 J. Electrochem. Soc. , 151 (6), A914-A9211,2004 J. Electrochem. Soc. , 144 (4), 1188-1194, 1997

However, LiFePO ₄ has a low charge / discharge capacity because it can reversibly insert and ^{desorb only 1 Li +} per molecule, ^{and desorption and insertion of 1 Li +} or more is required to achieve a high charge / discharge capacity. A possible positive electrode active material is required.

The present invention has been made in view of such a current situation, and an object of the present invention is to provide a novel compound useful as a positive electrode active material for a lithium ion secondary battery.

The present inventors have made extensive studies to solve the above-mentioned problems of the present invention. As a result, we succeeded in synthesizing a lithium iron-manganese-based composite oxide having a specific composition. Furthermore, it has been found that the lithium iron-manganese-based composite oxide is capable of inserting and removing lithium ions and exhibits a theoretical charge / discharge capacity high enough to be used as a positive electrode active material for a lithium ion secondary battery. The present inventors have completed the present invention by repeating further research based on these findings.

That is, the present invention typically includes the subjects described in the following sections.
Item 1.
Composition formula:
Li _{1 + m} Fe _x Mn _2-x O ₄
[In the formula, m indicates 0 <m ≦ 2. x indicates 0 <x ≦ 1. ]
Lithium iron-manganese-based composite oxide represented by.
Item 2.
Item 2. The lithium iron-manganese-based composite oxide according to Item 1, which has a tetragonal structure or a cubic structure.
Item 3.
Item 2. The lithium iron-manganese-based composite oxide according to Item 1 or 2, which has a rock salt type structure.
Item 4.
The lithium iron-manganese-based composite oxide according to any one of Items 1 to 3 above, which has an average particle size of 0.01 to 50 μm.
Item 5.
The method for producing a lithium iron-manganese-based composite oxide according to any one of Items 1 to 4, which comprises a step of heating a mixture containing lithium, iron, manganese, and oxygen.
Item 6.
Item 5. The method according to Item 5, wherein the heating temperature is 600 ° C. or higher.
Item 7.
A positive electrode active material for a lithium ion secondary battery containing the lithium iron-manganese-based composite oxide according to any one of Items 1 to 4 above.
Item 8.
A positive electrode for a lithium ion secondary battery, which comprises the positive electrode active material for a lithium ion secondary battery according to item 7 above.
Item 9.
The positive electrode for a lithium ion secondary battery according to item 8 above, further comprising a conductive auxiliary agent.
Item 10.
A lithium ion secondary battery including the positive electrode for the lithium ion secondary battery according to the above item 8 or 9.

Since the lithium iron-manganese-based composite oxide of the present invention can desorb and insert one or more lithium ions, it can be used as a positive electrode active material for a lithium ion secondary battery. In particular, by using the lithium iron-manganese-based composite oxide of the present invention as the positive electrode active material, a lithium ion secondary battery exhibiting a high charge / discharge capacity can be obtained.

It is a figure which shows the X-ray diffraction pattern of _{Li 2} FemnO ₄ obtained in Example 1. FIG. _{The X-ray diffraction pattern of Li 2} FemnO ₄ obtained when the firing temperature was set to 800 ° C. in Example 1 and other lithium manganese-based composite oxides (Li ₂ NimnO ₄ , Li ₂ ComnO 4, and Li _{2 MnO 4).} It is a figure which shows the result of having compared with the X-ray diffraction pattern of ₂ O _4). It is a figure which shows the observation result by the scanning electron microscope (SEM) of _{Li 2} FemnO ₄ obtained when the firing temperature was set to 800 degreeC in Example 1. FIG. It is sectional drawing of the test cell used in Example 2. FIG. It is a figure which shows the measurement result (0.05C rate; 55 degreeC) of the charge / discharge characteristic performed in Example 2. It is a figure which shows the measurement result (0.05C rate; 25 degreeC) of the charge / discharge characteristic performed in Example 2. It is a figure which shows the measurement result (0.1C rate) of the charge / discharge characteristic performed in Example 2. FIG. The measurement result of the charge / discharge characteristics of the electrode of only carbon and PVDF performed in Example 2 is shown. It is a figure which shows the measurement result of the rate characteristic performed in Example 2. It is a figure which shows the measurement result (0.05C rate) of the charge / discharge characteristic performed in Example 2. It should be noted that this is the result of starting from discharge. It is a figure which shows the X-ray diffraction pattern of _{LiFeMnO 4} obtained in the comparative example 1. FIG. It is a figure which shows the observation result by the scanning electron microscope (SEM) of _{LiFeMnO 4} obtained when the firing temperature was set to 800 degreeC in Comparative Example 1. The measurement result (0.05C rate; 55 ° C.) of the initial charge / discharge characteristics performed in Comparative Example 2 is shown. The measurement result (0.05C rate; 55 ° C.) of the charge / discharge characteristics in the higher-order cycle performed in Comparative Example 2 is shown.

Hereinafter, the present invention will be described in detail. In the present specification, when a numerical range is indicated, the numerical range includes the numerical values at both ends.

1. 1. Lithium iron-manganese-based composite oxide The lithium iron-manganese-based composite oxide of the present invention has a composition formula:
Li _{1 + m} Fe _x Mn _2-x O ₄
[In the formula, m indicates 0 <m ≦ 2. x indicates 0 <x ≦ 1. ]
It is a compound represented by. In the following, the compound may be referred to as "the compound of the present invention".

In the above composition formula, m is 0 <m ≦ 2, and from the viewpoint of ease of insertion and desorption of lithium ions, capacity and potential, 0 <m ≦ 1.5 is preferable, and 0.5. ≦ m ≦ 1.5 is more preferable, and 0.75 ≦ m ≦ 1.25 is even more preferable. x is 0 <x ≦ 1, and 0.25 ≦ x ≦ 1 is preferable, and 0.5 ≦ x ≦ 1 is preferable from the viewpoint of ease of insertion and desorption of lithium ions, as well as capacitance and potential. More preferably, 0.75 ≦ x ≦ 1 is even more preferable.

Specific examples of the compound of the present invention include Li ₂ FemnO ₄ .

The crystal structure of the compound of the present invention is preferably a tetragonal structure or a cubic structure, and more preferably a tetragonal structure. In particular, the compound of the present invention preferably has a tetragonal structure or a cubic structure as the main phase, and more preferably has a tetragonal structure as the main phase. In the compound of the present invention, the abundance of the crystal structure as the main phase is not particularly limited, and is preferably 80 mol% or more, more preferably 90 mol% or more, based on the entire compound of the present invention. Therefore, the compound of the present invention can be a material having a single-phase crystal structure, or can be a material having another crystal structure as long as the effects of the present invention are not impaired. The crystal structure of the compound of the present invention can be confirmed by X-ray diffraction measurement. When the compound of the present invention has a tetragonal structure, it preferably has a rock salt structure.

The compound of the present invention has peaks at various positions in an X-ray diffraction pattern using CuKα rays. For example, it is preferable that the diffraction angle 2θ has peaks at 18 to 20 °, 37 to 39 °, 43 to 45 °, 61 to 68 °, 70 to 77 °, 78 to 82 ° and the like. Among them, it is preferable to have the highest peak at 43 to 45 ° and the second highest peak at 61 to 68 °.

The average particle size of the compound of the present invention is not particularly limited, and is preferably 0.01 to 50 μm, more preferably 0.1 to 50 μm, and 0.5 to 25 μm from the viewpoint of improving performance. Is even more preferable. The average particle size of the compound of the present invention can be confirmed by a scanning electron microscope (SEM).

2. Method for Producing Lithium Iron-Manganese-based Composite Oxide The method for producing a compound of the present invention includes a step of heating a mixture containing lithium, iron, manganese, and oxygen. In the following, the method for producing the compound of the present invention may be referred to as "the method for producing the present invention".

In the production method of the present invention, as a raw material compound for obtaining a mixture containing lithium, iron, manganese, and oxygen, lithium, iron, manganese, and oxygen are finally predetermined in the mixture. It may be contained in a ratio, and for example, a lithium-containing compound, an iron-containing compound, a manganese-containing compound, an oxygen-containing compound, or the like can be used.

The types of each compound such as lithium-containing compounds, iron-containing compounds, manganese-containing compounds, and oxygen-containing compounds are not particularly limited, and four or more types containing one element each of lithium, iron, manganese, and oxygen are used. Can be used as a mixture of the above compounds, or a compound containing two or more elements of lithium, iron, manganese and oxygen at the same time is used as a part of the raw material, and less than four kinds of compounds are mixed. Can also be used.

As these raw material compounds, compounds containing no metal elements other than lithium, iron, manganese, and oxygen (particularly, rare metal elements) are preferable. Further, it is preferable that the elements other than the lithium, iron, manganese, and oxygen elements contained in the raw material compound are separated or volatilized by the heat treatment described later.

Specific examples of such a raw material compound include the following compounds.

Examples of the lithium-containing compound include metallic lithium (Li); lithium oxide (Li ₂ O); lithium bromide (LiBr); lithium fluoride (LiF); lithium iodide (Li I); lithium oxalate (Li ₂ C ₂ O). ₄ ); Lithium hydroxide (LiOH); Lithium nitrate (LiNO ₃ ); Lithium chloride (LiCl); Lithium carbonate (Li ₂ CO ₃ ) and the like.

Examples of the iron-containing compound include metallic iron (Fe); iron oxides such as iron (II) (FeO) and iron oxide (III) (Fe ₂ O ₃ ); iron bromide (II) (FeBr ₂ ) and chloride. Iron (II) (FeCl ₂ ); Iron hydroxides such as iron (II) hydroxide (Fe (OH) ₂ ), iron (III) hydroxide (Fe (OH) ₃ ); iron (II) carbonate (FeCO) ₃ ), iron carbonates such as iron (III) carbonate (Fe ₂ (CO ₃ ) ₃ ); iron (II) oxalate (FeC ₂ O ₄ ) and the like.

Examples of the manganese-containing compound include metallic manganese (Mn); _{manganese oxides such as manganese oxide (II) (MnO) and manganese oxide (IV) (MnO 2} ); manganese hydroxide (II) (Mn (OH) ₂ ), Manganese hydroxides such as manganese hydroxide (IV) (Mn (OH) ₄ ); manganese carbonates such as manganese (II) carbonate (MnCO ₃ ); manganese oxalate (II) (MnC ₂ O ₄ ) and the like. Be done.

Oxygen-containing compounds include lithium hydroxide (LiOH); lithium carbonate (Li ₂ CO ₃ ); iron oxides such as iron (II) (FeO) and iron (III) oxide (Fe ₂ O ₃ ); hydroxide. Iron hydroxides such as iron (II) (Fe (OH) ₂ ), iron hydroxide (III) (Fe (OH) ₃ ); iron (II) carbonate (FeCO ₃ ), iron (III) carbonate (Fe ₂₎ Iron carbonates such as (CO ₃ ) ₃ ); manganese oxides such as iron (II) oxalate (FeC ₂ O ₄ ); manganese oxide (II) (MnO), manganese oxide (IV) (MnO ₂ ); water Manganese hydroxides such as manganese oxide (II) (Mn (OH) ₂ ) and manganese hydroxide (IV) (Mn (OH) ₄ ); manganese carbonates such as manganese (II) carbonate (MnCO ₃ ); oxalic acid Examples thereof include manganese (II) (MnC ₂ O ₄ ).

In addition, hydrate can also be used as these raw material compounds.

Further, as the raw material compound used in the production method of the present invention, a commercially available product can be used, or it can be appropriately synthesized and used. The synthesis method for synthesizing each raw material compound is not particularly limited, and can be carried out according to a known method.

The shape of these raw material compounds is not particularly limited. From the viewpoint of ease of handling and the like, it is preferably in the form of powder. From the viewpoint of reactivity, it is preferable that the particles are fine, and it is more preferable that the particles are in the form of a powder having an average particle diameter of 1 μm or less (preferably about 10 to 200 nm, particularly preferably about 60 to 80 nm). .. The average particle size of the raw material compound can be measured by a scanning electron microscope (SEM).

A mixture containing lithium, iron, manganese, and oxygen can be obtained by mixing a necessary material among the above-mentioned raw material compounds.

The mixing ratio of each raw material compound is not particularly limited, and it is preferable to mix them so as to have the composition of the compound of the present invention which is the final product. The mixing ratio of the raw material compound is preferably such that the ratio of each element contained in the raw material compound is the same as the ratio of each element in the produced compound of the present invention.

The method for preparing a mixture containing lithium, iron, manganese, and oxygen is not particularly limited, and a method capable of uniformly mixing each raw material compound can be adopted. For example, mortar mixing, mechanical milling treatment, coprecipitation method, a method of dispersing each raw material compound in a solvent and then mixing, a method of dispersing each raw material compound in a solvent at once and mixing, etc. can be adopted. .. Among these, a mixture can be obtained by a simple method by adopting the mortar mixing method, and a uniform mixture can be obtained by adopting the coprecipitation method.

Further, when the mechanical milling process is performed as the mixing means, for example, a ball mill, a vibration mill, a turbo mill, a disc mill or the like can be used as the mechanical milling device, and the ball mill is particularly preferable. Further, when performing the mechanical milling treatment, it is preferable to perform mixing and heating at the same time.

The atmosphere during mixing and heating is not particularly limited as long as it is an inert atmosphere, and for example, an inert gas atmosphere such as argon or nitrogen, a hydrogen gas atmosphere, or the like can be adopted. Further, mixing and heating may be performed under reduced pressure such as vacuum.

When heating a mixture containing lithium, iron, manganese, and oxygen, the heating temperature is not particularly limited, and the crystallinity and electrode characteristics (capacity and potential) of the obtained compound of the present invention are further improved. From the viewpoint of increasing the temperature, the temperature is preferably 600 ° C. or higher, more preferably 700 ° C. or higher, further preferably 800 ° C. or higher, and particularly preferably 900 ° C. or higher. The upper limit of the heating temperature is not particularly limited, and may be a temperature at which the compound of the present invention can be easily produced (for example, about 1500 ° C.). In other words, the heating temperature is preferably 600 to 1500 ° C, more preferably 700 to 1500 ° C, further preferably 800 to 1500 ° C, and particularly preferably 900 to 1500 ° C. ..

3. 3. Positive Electrode Active Material for Lithium Ion Secondary Battery Since the compound of the present invention has the above-mentioned composition and crystal structure, lithium ions can be inserted and removed, so that it should be used as a positive electrode active material for lithium ion secondary battery. Can be done. Therefore, the present invention includes the above-mentioned positive electrode active material for a lithium ion secondary battery containing the compound of the present invention. In the following, the positive electrode active material for a lithium ion secondary battery containing the compound of the present invention may be referred to as "the positive electrode active material of the present invention".

In the positive electrode active material of the present invention, the compound of the present invention described above and a carbon material (for example, a material such as carbon black such as acetylene black) may form a composite. As a result, since the carbon material suppresses particle growth during firing, it becomes possible to obtain a fine particle positive electrode active material for a lithium ion secondary battery having excellent electrode characteristics. In this case, the content of the carbon material is preferably 1 to 30% by mass, more preferably 3 to 20% by mass, and particularly preferably 5 to 15% by mass in the positive electrode active material for the lithium ion secondary battery of the present invention. be.

The positive electrode active material of the present invention contains the above-mentioned compound of the present invention. The positive electrode active material of the present invention may be composed only of the above-mentioned compound of the present invention, or may contain unavoidable impurities in addition to the compound of the present invention. Examples of such unavoidable impurities include the above-mentioned raw material compounds. The content of unavoidable impurities is 10 mol% or less, preferably 5 mol% or less, and more preferably 2 mol% or less as long as the effect of the present invention is not impaired.

4. Positive electrode for lithium ion secondary battery and lithium ion secondary battery The positive electrode for lithium ion secondary battery and lithium ion secondary battery of the present invention are basically basic except that the above-mentioned compound of the present invention is used as a positive electrode active material. As the structure, a positive electrode for a known non-aqueous electrolyte (non-aqueous) lithium ion secondary battery and a structure similar to that of a non-aqueous electrolyte (non-aqueous) lithium ion secondary battery can be adopted. For example, the positive electrode, the negative electrode, and the separator can be arranged in the battery container so that the positive electrode and the negative electrode are separated from each other by the separator. Then, the lithium ion secondary battery of the present invention can be manufactured by filling the battery container with a non-aqueous electrolytic solution and then sealing the battery container. The lithium ion secondary battery of the present invention may be a lithium secondary battery. In the present specification, the "lithium ion secondary battery" means a secondary battery using lithium ions as carrier ions, and the "lithium secondary battery" is a secondary battery using a lithium metal or a lithium alloy as a negative electrode active material. Means a battery.

The positive electrode for a lithium ion secondary battery of the present invention can adopt a structure in which the positive electrode active material containing the compound of the present invention described above is supported on a positive electrode current collector. For example, it can be produced by applying the above-mentioned compound of the present invention, a conductive auxiliary agent, and a positive electrode material containing a binder, if necessary, to a positive electrode current collector.

As the conductive auxiliary agent, for example, carbon materials such as acetylene black, ketjen black, carbon nanotubes, vapor-phase carbon fibers, carbon nanofibers, graphite, and cokes can be used. The shape of the conductive auxiliary agent is not particularly limited, and for example, a powder or the like can be adopted.

As the binder, for example, a fluororesin such as polyvinylidene fluoride resin or polytetrafluoroethylene can be used.

The content of various components in the positive electrode material is not particularly limited and can be appropriately determined from a wide range. For example, the compound of the present invention described above is 50 to 95% by volume (particularly 70 to 90% by volume), the conductive auxiliary agent is 2.5 to 25% by volume (particularly 5 to 15% by volume), and the binder is used. It is preferably contained in an amount of 2.5 to 25% by volume (particularly, 5 to 15% by volume).

Examples of the material constituting the positive electrode current collector include aluminum, platinum, molybdenum, and stainless steel. Examples of the shape of the positive electrode current collector include a porous body, a foil, a plate, and a mesh made of fibers.

The amount of the positive electrode material applied to the positive electrode current collector is not particularly limited, and is preferably determined as appropriate according to the application of the lithium ion secondary battery and the like.

Examples of the negative electrode active material constituting the negative electrode include lithium metal; silicon; silicon-containing Krathrate compound; lithium alloy; M ¹ M ² ₂ O ₄ (M ¹ : Co, Ni, Mn, Sn, etc., M ² : Mn, etc. Three-dimensional or quaternary oxide represented by Fe, Zn, etc.); M ³ ₃ O ₄ (M ³ : Fe, Co, Ni, Mn, etc.), M ⁴ ₂ O ₃ (M ⁴ : Fe, Co, Ni) , Mn, etc.), MnV ₂ O ₆ , M ⁵ O ₂ (M ⁵ : Sn, Ti, etc.), M ⁶ O (M ⁶ : Fe, Co, Ni, Mn, Sn, Cu, etc.) Oxides; graphite, hard carbon, soft carbon, graphene; carbon materials mentioned above; organic systems such as _{Li 2} C ₆ H ₄ O ₄ , Li ₂ C ₈ H ₄ O ₄ , Li ₂ C ₁₆ H ₈ O _{4 and the like.} Examples include compounds.

Examples of lithium alloys include alloys containing lithium and aluminum as constituent elements, alloys containing lithium and zinc as constituent elements, alloys containing lithium and lead as constituent elements, alloys containing lithium and manganese as constituent elements, lithium and bismuth. Alloys containing lithium and nickel as constituent elements, alloys containing lithium and antimony as constituent elements, alloys containing lithium and tin as constituent elements, alloys containing lithium and indium as constituent elements; metals (scandium) , titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, MXene based alloy containing as constituent elements carbon and ^tantalum), _M 7 x BC ₃ alloy ^{(M 7: Sc, Ti,} V, Cr, Zr, Nb, Mo, Hf, Ta, etc.) and the like are quaternary layered carbonized or nitrided compounds.

The negative electrode may be composed of a negative electrode active material, and a negative electrode material containing a negative electrode active material, a conductive auxiliary agent, and, if necessary, a binder shall be supported on the negative electrode current collector. You can also. When adopting a configuration in which the negative electrode material is supported on the negative electrode current collector, a negative electrode mixture containing a negative electrode active material, a conductive auxiliary agent, and, if necessary, a binder is applied to the negative electrode current collector. Can be manufactured.

When the negative electrode is composed of a negative electrode active material, the above negative electrode active material has a shape suitable for the electrode (plate shape, etc.).
Can be obtained by molding into.

Further, when the structure in which the negative electrode material is supported on the negative electrode current collector is adopted, the types of the conductive auxiliary agent and the binder, and the contents of the negative electrode active material, the conductive auxiliary agent and the binder are those of the above-mentioned positive electrode. Can be applied. Examples of the material constituting the negative electrode current collector include aluminum, copper, nickel, stainless steel and the like. Examples of the shape of the negative electrode current collector include a porous body, a foil, a plate, and a mesh made of fibers. The amount of the negative electrode material applied to the negative electrode current collector is preferably determined as appropriate according to the application of the lithium ion secondary battery and the like.

The separator is not limited as long as it is made of a material capable of separating the positive electrode and the negative electrode in the battery and holding the electrolytic solution to ensure the ionic conductivity between the positive electrode and the negative electrode. For example, it is made of a polyolefin resin such as polyethylene, polypropylene, polyimide, polyvinyl alcohol, terminal amination polyethylene oxide; a fluororesin such as polytetrafluoroethylene; an acrylic resin; nylon; an aromatic aramid; an inorganic glass; and ceramics, and is porous. Materials in the form of a film, a non-woven fabric, a woven cloth, etc. can be used.

The non-aqueous electrolytic solution is preferably an electrolytic solution containing lithium ions. Examples of such an electrolytic solution include a solution of a lithium salt, an ionic liquid composed of an inorganic material containing lithium, and the like.

Examples of the lithium salt include lithium halide such as lithium chloride, lithium bromide and lithium iodide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluorophosphate and the like. Compounds: Lithium organic salt compounds such as bis (trifluoromethylsulfonyl) imidelithium, bis (pafluoroethanesulfonyl) imidelithium, lithium benzoate, lithium salicylate, lithium phthalate, lithium acetate, lithium propionate, glignal reagent, etc. Can be mentioned.

Examples of the solvent include carbonate compounds such as propylene carbonate, ethylene carbonate, dimethol carbonate, ethyl methyl carbonate and diethyl carbonate; lactone compounds such as γ-butyrolactone and γ-valerolactone; tetrahydrofuran, 2-methyl tetrahydrofuran and diethyl. Ether compounds such as ether, diisopropyl ether, dibutyl ether, methoxymethane, glyme, dimethoxyethane, dimethoquimethane, dietoxymethane, dietietan, propylene glycol dimethyl ether; acetonitrile; N, N-dimethylformamide; N-propyl-N-methylpyrrolidinium bis. (Trifluoromethanesulfonyl) imide and the like can be mentioned.

Further, a solid electrolyte can be used instead of the non-aqueous electrolyte solution. Examples of the solid electrolyte include lithium ion conductors such as _{Li 10} GeP ₂ S ₁₂ , Li ₇ P ₃ S ₁₁ , Li ₇ La ₃ Zr ₂ O ₁₂ , La _0.51 Li _0.34 TiO _{2.94, and the like.} Enumerated.

Since the compound of the present invention is used in such a lithium ion secondary battery of the present invention, it is possible to secure a higher potential and energy density in the redox reaction (charge / discharge reaction), and it is safe. Excellent in sex (polyanion skeleton) and practicality. Therefore, the lithium ion secondary battery of the present invention can be suitably used for, for example, a device that requires miniaturization and high performance.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

Example _1: A synthetic material powder of _{Li 2 FeMnO 4, Li 2 CO} 3 ( manufactured by Rare Metallic _{Co.; 99.9% (3N)),} Fe 2 C 2 O 4 · 2H 2 O ( manufactured by Junsei Chemical Co., Ltd .; 99.9% (3N)) and MnO ₂ (manufactured by Rare Metallic; 99.99% (4N)) were used. _{Li 2 CO 3, Fe 2 C} 2 O 4 · 2H 2 O, and lithium MnO _2: Iron Manganese (molar ratio) of 2: 1 were weighed to be 1, the zirconia balls (10 15 mm ×) In a chrome steel container, acetone was added, and the mixture was pulverized and mixed at 400 rpm for 24 hours in a planetary ball mill (manufactured by Frisch, trade name: P-6). Then, after removing acetone under reduced pressure, the recovered powder was manually pellet-molded and calcined at 600 ° C., 700 ° C., 800 ° C., 900 ° C., or 1000 ° C. for 1 hour under an argon stream. At this time, the heating rate was set to 400 ° C./h. The cooling rate was 100 ° C./h up to 300 ° C., and thereafter the mixture was allowed to cool to room temperature by natural cooling. Each of the obtained products (Li ₂ FemnO ₄ ) was confirmed by powder X-ray diffraction (XRD). The results are shown in FIG.

For the powder X-ray diffraction (XRD) measurement, an X-ray diffraction measuring device (manufactured by Rigaku Co., Ltd., trade name: RINT2200) was used, and CuKα monochromated with a monochromator was used as the X-ray source. As the measurement conditions, data was collected with the tube voltage set to 5 kV and the tube current set to 300 mA. At this time, the scanning speed was set so that the intensity was about 10,000 counts. In addition, the sample used for the measurement was sufficiently pulverized so that the particles became uniform. Rietveld analysis was performed for structural analysis, and JANA-2006 was used for the analysis program.

From FIG. 1, it was confirmed that when the firing temperature was 600 ° C. or higher, a plurality of major peaks were observed at least at a 2θ value of 30 to 65 °. These peaks, since it corresponds to the _Li 2 FeMnO ₄ single-phase, _Li 2 FeMnO ₄ single-phase is found to be obtained as a product. Further, since the peak observed at the 2θ value of 35 to 65 ° becomes stronger as the firing temperature is higher, it was found that the higher the firing temperature is, the more preferable.

_{Further, the crystals of Li 2} FemnO ₄ obtained from FIG. 1 have diffraction angles represented by 2θ of 18 to 20 °, 37 to 39 °, and 43 to 45 ° in the X-ray diffraction pattern by powder X-ray diffraction. , 61-68 °, 70-77 °, and 78-82 °. From the results, the obtained Li ₂ FemnO ₄ crystal has a tetragonal structure (space group P4 / nbm) and has a lattice constant of a = b = 3.596 to 3.610 Å and c = 14.366 to. It was found to be a crystal having 14.498 Å, α = β = γ = 90 °, and a unit lattice volume (V) of 187.2 to 187.5 Å ^3. Further, since c / a = 3.9950, _{it was found that the obtained Li 2} FemnO ₄ crystal had a rock salt type structure.

_{Further, Li 2} FemnO ₄ obtained when the firing temperature was set to 800 ° C., and other known lithium manganese-based composite oxides (Li ₂ NimnO ₄ , Li ₂ ComnO ₄ , and Li ₂ Mn ₂ O ₄ ). The result of comparing the X-ray diffraction patterns of is shown in FIG. In addition, Li ₂ NiMnO ₄ , Li ₂ ComnO ₄ , and Li ₂ Mn ₂ O ₄ are samples synthesized by the same method as described above except that the raw material compound is changed. In addition, Li ₂ FemnO ₄ , Li ₂ NimnO ₄ , Li ₂ ComnO ₄ , Li ₂ Mn ₂ O ₄ , and other iron-manganese-based composite oxides (K ₂ FemnO ₄ , Na ₂ FemnO ₄ , MgFeMnO ₄ , LiFeMnO _4). ) Is shown in Table 1 below.

From FIG. 2, it can be seen that Li ₂ FemnO ₄ and other lithium manganese-based composite oxides (Li ₂ NimnO ₄ , Li ₂ ComnO ₄ , and Li ₂ Mn ₂ O ₄ ) show completely different X-ray diffraction patterns. confirmed.

_{Further, Li 2} FemnO ₄ obtained when the firing temperature was set to 800 ° C. was observed with a scanning electron microscope (SEM). The results are shown in FIG. In FIG. 3, the scale bar shows 7.69 μm. From FIG. 3, it was found that _{Li 2} FemnO ₄ having a particle size of about 1 to 20 μm was obtained.

Example 2: Measurement of _{charge / discharge characteristics Li 2} FeMnO ₄ , polyvinylidene fluoride (PVDF), and acetylene black (AB) obtained in Example 1 above when the firing temperature is 800 ° C. Is mixed in a mortar so that the volume ratio is 85: 7.5: 7.5, and the obtained slurry is applied onto an aluminum foil (thickness 20 μm) which is a positive electrode current collector, and this is applied to a diameter of 8 mm. It was punched into a circle to form a positive electrode. Further, in order to prevent the sample from peeling off from the positive electrode current collector, the sample was pressure-bonded at 30 to 40 mPa.

Metallic lithium punched out at 14 mmφ was used for the negative electrode, and two porous films (trade name: celgard 2500) punched out at 18 mmφ were used for the separator. The electrolytic solution is an electrolytic solution (manufactured by Kishida Chemical Co., Ltd.) in which _{LiPF 6} is dissolved as a supporting electrolyte at a concentration of 1 mol / dm ³ in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 2. used. The battery was manufactured in Globebooks under an argon atmosphere because of the use of metallic lithium and the fact that when water is mixed in the electrolytic solution, it causes an increase in resistance increment. As the cell, a CR2032 type coin cell shown in FIG. 4 was used. The constant current charge / discharge characteristics were measured at a 0.05 C rate or 0.1 C rate using a voltage switcher, set to a current of 10 mA / g, an upper limit voltage of 4.8 V, and a lower limit voltage of 1.5 V, and started from charging. .. The charge / discharge measurement was carried out in a state where the cell was placed in a constant temperature bath at 55 ° C. or at room temperature (25 ° C.). The measurement results of charge / discharge characteristics at 0.05 C rate (55 ° C) (relationship between each cycle and discharge capacity) are shown in FIG. 5, and the measurement results of charge / discharge characteristics at 0.05 C rate (25 ° C) (each cycle). (Relationship between and discharge capacity) is shown in FIG. 6, and the measurement result (relationship between each cycle and discharge capacity) of charge / discharge characteristics at a 0.1 C rate (55 ° C.) is shown in FIG. The C rate refers to the current density required to charge and discharge the theoretical capacity of the electrode active material in one hour.

As shown in FIG. 5, the initial charge capacity is 280 mAh / g (about 1.9 ^{Li +} min) at a _{0.05 C rate (55 ° C.), and the theoretical capacity of Li 2} FemnO ₄ is about 285 mAh / g. It was found that a value very close to the theoretical capacity was obtained. The average operating voltage is 2.8 V, and the charge / discharge capacity that can be reversibly drawn out is about 250 mAh / g (about 1.8 Li ⁺ min). This is an energy density corresponding to 700 Wh / kg, and Li ₂ FemnO ₄ is highly expected as a positive electrode active material having a high capacity and a high energy density. Further, from FIG. 6, it was confirmed that at room temperature, only a capacity of 1 ^{Li + min was reversibly detached and inserted.} From the results, _{it was found that Li 2} FemnO ₄ can maximize its performance by raising the operating temperature. Furthermore, from FIG. 7, it was found that good cycle characteristics were obtained for the initial discharge and higher-order cycles at the 0.1 C rate. It was also found that the withdrawable capacity at the 0.1 C rate was about 200 mAh / g.

Further, _{a positive electrode was prepared in the same manner as above except that Li 2} FemnO ₄ was not used, and a charge / discharge test was conducted at a C / 20 rate (55 ° C.) under the same conditions as above. The results are shown in FIG.

From FIG. 8, it was confirmed that the charge / discharge capacity could not be obtained with the electrode not using _{Li 2} FemnO _4. By comparing FIG. 5 and FIG. 8, it was found that the high charge / discharge capacity shown in FIG. 5 _{was derived from Li 2} FemnO _4.

Furthermore, the rate characteristics of _{Li 2} FemnO _{4 were examined.} Specifically, after charging at a 0.05 C rate, the initial discharge capacity at a 0.05 C rate, a 0.1 C rate, or a 0.2 C rate was measured. The results are shown in FIG.

From FIG. 9, the volume obtained at the 0.1 C rate was 225 mAh / g, and the volume obtained at the 0.2 C rate was 100 mAh / g. From these results, it was found that _{Li 2} FemnO _{4 exhibited good rate characteristics.}

Further, the current charge / discharge characteristics were measured at a rate of 0.05 C in the same manner as described above except that the discharge was started. The results are shown in FIG.

From FIG. 10, the capacity that can be drawn out _{from Li 2 FemnO 4} after the initial discharge was about 360 mAh / g. From the results, _{it was found that when Li 2} FemnO ₄ was discharged for the first time, that is, when lithium was further inserted, a higher capacity could be obtained. Therefore, Li ₂ FemnO ₄ is highly expected as a high-capacity positive electrode active material.

Comparative Example 1: _{Li 2} CO ₃ (manufactured by Rare Metallic Co., Ltd .; 99.9% (3N)), Fe ₂ O ₃ (Junsei Chemical Co., Ltd., 99.9% (3N)), and MnO as synthetic raw material powders of _{LiFeMnO 4. 2} (rare metallic, 99.99% (4N)) was used. Li ₂ CO ₃ , Fe ₂ O ₃ and _{Mn O 2} were weighed so that the ratio of lithium: iron: manganese (molar ratio) was 1: 1: 1 and mixed in an agate mortar for about 30 minutes to obtain a raw material mixture. Then, the raw material mixture was placed in a chrome steel container together with zirconia balls (15 mmΦ × 10 pieces), acetone was added, and the mixture was pulverized and mixed at 400 rpm for 6 hours in a planetary ball mill (Fritsch; P-6). Then, after distilling off acetone under reduced pressure, the recovered powder was pellet-molded at 40 MPa and in air at 550 ° C, 600 ° C, 650 ° C, 700 ° C, 750 ° C, 800 ° C, 850 ° C, or 900 ° C. It was baked for 3 hours. After that, it was allowed to cool to room temperature by natural cooling. Each of the obtained products was confirmed by powder X-ray diffraction (XRD) in the same manner as in Example 1. The results are shown in FIG.

From FIG. 11, the obtained _{crystal of LiFeMnO 4} has a cubic crystal (space group Fd-3m) and has a lattice constant of a = b = c = 8.286 to 8.306 Å, α = β = γ = 90. It was found to be a crystal having a unit lattice volume (V) of 568.9 to 573.0 Å ^3. Further, since c / a = 1, _{it was found that the obtained LiFeMnO 4} crystal had a spinel-type structure.

_{Further, LiFeMnO 4} obtained when the firing temperature was set to 800 ° C. was observed with a scanning electron microscope (SEM). The results are shown in FIG. In FIG. 12, the scale bar shows 1.53 μm. From FIG. 12, it was found that _{LiFeMnO 4} having a particle size of about 0.3 to 3 μm was obtained.

Comparative Example 2: Measurement of _{charge / discharge characteristics In order to perform charge / discharge measurement, LiFeMnO 4} , polyvinylidene fluoride (PVDF), and acetylene black (AB) obtained in Comparative Example 1 at a firing temperature of 800 ° C. are in volume. Mix in a mortar so that the ratio is 85: 7.5: 7.5, and apply the obtained slurry on an aluminum foil (thickness 20 μm) that is a positive electrode current collector, and apply this to a circle with a diameter of 8 mm. It was punched and used as a positive electrode. Further, in order to prevent the sample from peeling off from the positive electrode current collector, the sample was pressure-bonded at 30 to 40 mPa.

Metallic lithium punched out at 14 mmφ was used for the negative electrode, and two porous films (trade name: celgard 2500) punched out at 18 mmφ were used for the separator. The electrolytic solution is an electrolytic solution (manufactured by Kishida Chemical Co., Ltd.) in which _{LiPF 6} is dissolved as a supporting electrolyte at a concentration of 1 mol / dm ³ in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 2. used. The battery was manufactured in Globebooks under an argon atmosphere because of the use of metallic lithium and the fact that when water is mixed in the electrolytic solution, it causes an increase in resistance increment. As the cell, a CR2032 type coin cell shown in FIG. 4 was used. The constant current charge / discharge characteristics were measured at a rate of 0.05 C using a voltage switcher, set to a current of 10 mA / g, an upper limit voltage of 4.8 V, and a lower limit voltage of 1.5 V, and started from charging. The charge / discharge measurement was performed with the cell placed in a constant temperature bath at 55 ° C. The measurement results (relationship between each cycle and discharge capacity) of the initial charge / discharge characteristics at the 0.05 C rate (55 ° C) are shown in FIG. 13, and the charge / discharge characteristics in the higher cycle at the 0.05 C rate (55 ° C) are shown. The measurement result (relationship between each cycle and the discharge capacity) is shown in FIG. The C rate refers to the current density required to charge and discharge the theoretical capacity of the electrode active material in one hour.

As shown in FIG. 13, it was confirmed that the initial charge / discharge capacity _{of the spinel-type LiFeMnO 4 at a 0.05 C rate was 130 mAh / g.} Compared with the rock salt type Li ₂ FemnO ₄ obtained in Example 1, it was found that the initial charge / discharge capacity of the spinel type LiFeMnO ₄ was significantly inferior to that _{of the rock salt type Li 2} FemnO _4.

Furthermore, it was found from FIG. 14 that although the spinel-type LiFemnO ₄ has relatively stable operating characteristics, the extractable capacity is significantly inferior to that of the rock-salt type rock-salt type Li ₂ FemnO _4.

1 Lithium-ion secondary battery 2 Negative electrode terminal 3 Negative electrode 4 Separator impregnated with electrolyte 5 Insulation packing 6 Positive electrode 7 Positive electrode can

Claims (8)

Composition formula:
Li _{1 + m} Fe _x Mn _2-x O ₄
[In the formula, m indicates 0.5 ≦ m ≦ 2. x indicates 0 <x ≦ 1. ]
A lithium iron-manganese-based composite oxide represented by and having a tetragonal salt-type structure. The lithium iron-manganese-based composite oxide according to claim 1, which has an average particle size of 0.01 to 50 μm. The step of heating a mixture containing lithium, iron, manganese, and oxygen is included, and the ratio of lithium, iron, and manganese in the mixture is lithium: iron: manganese, 1 + m (0.5 ≦ m ≦ 2). ): The method for producing a lithium iron-manganese-based composite oxide according to claim 1 or 2, wherein x (0 <x ≦ 1): 2-x (0 <x ≦ 1). The method according to claim 3, wherein the heating temperature is 600 ° C. or higher. A positive electrode active material for a lithium ion secondary battery containing the lithium iron-manganese-based composite oxide according to claim 1 or 2. A positive electrode for a lithium ion secondary battery, which comprises the positive electrode active material for a lithium ion secondary battery according to claim 5. The positive electrode for a lithium ion secondary battery according to claim 6, further comprising a conductive auxiliary agent. A lithium ion secondary battery including the positive electrode for a lithium ion secondary battery according to claim 6 or 7.

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