JP6857361B2 - Lithium copper-based composite oxide - Google Patents

Description

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

Lithium-ion secondary batteries occupy the most important position among energy storage devices, and in recent years, their applications have been 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 positive electrode 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 Electrochem. Soc. , 151 (6), A914-A9211,2004 Electrochem. Soc. , 144 (4), 1188-1194, 1997

However, since _{the positive electrode material such as LiFePO 4} described above has a polyanion unit in the structure that does not participate in the charge / discharge reaction, it is more than a simple oxide-based positive electrode material such as _{LiCoO 2 having a theoretical capacity of 274 mAh / g.} It is low, and when it is put into practical use, it is made into fine particles and compounded with carbon, so that the tap density is inevitably lowered.

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 copper-based composite oxide having a specific composition. Furthermore, it has been found that the lithium-copper 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 (1):
Li _m Cu _y X ¹ O _n
[In the composition formula (1), X ¹ represents Si or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
Lithium-copper composite oxide represented by.
Item 2.
Item 2. The lithium copper-based composite oxide according to Item 1, which has a monoclinic structure.
Item 3.
Item 2. The lithium copper-based composite oxide according to Item 1 or 2, wherein the average particle size is 0.1 to 100 μm.
Item 4.
The method for producing a lithium copper-based composite oxide according to any one of Items 1 to 3 above, which comprises a step of heating a mixture containing lithium, copper, silicon or germanium, and oxygen.
Item 5.
Item 4. The method according to Item 4, wherein the heating temperature is 600 ° C. or higher.
Item 6.
Composition formula (2):
Li _m Cu _y X ² O _n
[In the composition formula (2), X ² represents Si, Ti or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
A positive electrode active material for a lithium ion secondary battery containing a lithium copper-based composite oxide represented by.
Item 7.
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 6 above.
Item 8.
The positive electrode for a lithium ion secondary battery according to Item 7, further comprising a conductive auxiliary agent.
Item 9.
A lithium ion secondary battery including the positive electrode for the lithium ion secondary battery according to item 7 or 8 above.

Since the lithium copper-based composite oxide of the present invention can insert and desorb lithium ions, it can be used as a positive electrode active material for a lithium ion secondary battery. In particular, by using the lithium copper-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} CuSiO ₄ obtained in Example 1. FIG. _{The X-ray diffraction pattern of Li 2} CuSiO ₄ obtained when the firing temperature was set to 900 ° C. in Example 1, CuO as a raw material compound, and Li produced from _{Li 2} CO ₃ and SiO _{2 as raw material compounds.} It is a figure which shows the result of having compared with the X-ray diffraction pattern of ₂ SiO _3. It is a figure which shows the observation result by the scanning electron microscope (SEM) of _{Li 2} CuSiO ₄ obtained when the firing temperature was 900 degreeC in Example 1. FIG. It is a figure which shows the X-ray diffraction pattern of _{Li 2} CuGeO ₄ obtained in Example 2. FIG. It is a figure which shows the observation result by the scanning electron microscope (SEM) of _{Li 2} CuGeO ₄ obtained when the firing temperature was set to 900 degreeC in Example 2. FIG. It is sectional drawing of the test cell used in Examples 3 and 4. It is a figure which shows the measurement result of the open circuit potential performed in Example 3. It is a figure which shows the measurement result (C / 20 rate) of the charge / discharge characteristic performed in Example 3. FIG. It is a figure which shows the measurement result (C / 50 rate) of the charge / discharge characteristic performed in Example 3. It is a figure which shows the charge / discharge result of only carbon and PVdF. It is a figure which shows the measurement result of the open circuit potential performed in Example 4. It is a figure which shows the measurement result (C / 50 rate) of the charge / discharge characteristic performed in Example 4.

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-Copper Composite Oxide The lithium-copper composite oxide of the present invention has a composition formula (1):
Li _m Cu _y X ¹ O _n
[In the composition formula (1), X ¹ represents Si or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
It is a compound represented by. In the following, the compound may be referred to as "a compound represented by the composition formula (1)".

In the composition formula (1), X ¹ is silicon (Si) or germanium (Ge).

In the composition formula (1), y is 0.8 to 1.2, preferably 0.8 to 1.0 from the viewpoint of increasing the capacity.

In the above composition formula (1), m is 1.5 to 2.5, and 1.75 to 2.25 is preferable from the viewpoint of ease of insertion and desorption of lithium ions, as well as capacity and potential. Further, in the above general formula (1), n is 3.9 to 4.1, and 3.95 to 4.05 is obtained from the viewpoint of ease of insertion and desorption of lithium ions, as well as capacitance and potential. preferable.

Specific examples of the compound represented by the composition formula (1) include Li ₂ CuSiO ₄ and Li ₂ CuGeO ₄ . Among them, when used as a positive electrode active material for a lithium ion secondary battery, which will be described later, Li ₂ CuSiO ₄ is preferable from the viewpoint of performance (particularly, capacity improvement).

The crystal structure of the compound represented by the composition formula (1) is preferably a monoclinic structure. In particular, the compound represented by the composition formula (1) preferably has a monoclinic structure as the main phase. In the compound represented by the composition formula (1), the abundance of the crystal structure as the main phase is not particularly limited, and is 80 mol% or more based on the entire compound represented by the composition formula (1). Is preferable, and 90 mol% or more is more preferable. Therefore, the compound represented by the composition formula (1) can be a material having a single-phase crystal structure, or a material having another crystal structure as long as the effects of the present invention are not impaired. You can also do it. The crystal structure of the compound represented by the composition formula (1) can be confirmed by X-ray diffraction measurement.

The compound represented by the composition formula (1) has peaks at various positions in the X-ray diffraction pattern using CuKα rays.

For example, Li ₂ CuSiO ₄ has a diffraction angle of 2θ of 18.3 to 19.3 °, 26.3 to 27.0 °, 27.1 to 28.0 °, 28.8 to 29.6 °, 29. 9 to 30.5 °, 32.3 to 32.9 °, 35.5 to 36.7 °, 38.6 to 39.9 °, 40.8 to 42.0 °, 43.6 to 45.2 °, 45.7-46.8 °, 47.1-48.3 °, 48.5-49.8 °, 50.8-52.7 °, 53.7-55.2 °, 55.6 It is preferable to have peaks at ~ 58.2 °, 62.3-63.4 °, 63.8-65.1 °, 68.6-71.0 ° and the like.

Further, for example, Li ₂ CuGeO ₄ has a diffraction angle of 2θ of 17.9 to 19.2 °, 24.9 to 27.0 °, 31.6 to 33.4 °, and 35.0 to 39.2 °. 41.2 to 43.4 °, 49.2 to 51.5 °, 53.2 to 55.4 °, 56.9 to 58.7 °, 60 to 62.7 °, 63.7 to 65 It is preferable to have peaks at .2 °, 66.5 to 68.5 °, 69.9 to 71.7 °, 72.7 to 75.5 °, 76.9 to 78.4 ° and the like.

The average particle size of the compound represented by the composition formula (1) is not particularly limited, and is preferably 0.1 to 100 μm, preferably 0.1 to 50 μm, from the viewpoint of shortening the ^{Li + diffusion path.} More preferably. The average particle size of the compound represented by the composition formula (1) can be confirmed by a scanning electron microscope (SEM).

2. Method for Producing Lithium Copper Composite Oxide In the method for producing the compound represented by the above composition formula (1), as a raw material compound for obtaining a mixture containing lithium, copper, silicon or germanium, and oxygen, the raw material compound is used. Finally, the mixture may contain lithium, copper, silicon or germanium, and oxygen in a predetermined ratio, for example, lithium-containing compound, copper-containing compound, silicon-containing compound or germanium-containing compound, oxygen. Containing compounds and the like can be used.

The type of each compound such as a lithium-containing compound, a copper-containing compound, a silicon-containing compound, a germanium-containing compound, and an oxygen-containing compound is not particularly limited, and one type of each element of lithium, copper, silicon or germanium, and oxygen is used. It is also possible to use a mixture of four or more compounds containing the same, and a compound containing two or more elements of lithium, copper, silicon or germanium, and oxygen at the same time is used as a part of the raw material. It is also possible to mix and use less than four kinds of compounds.

As these raw material compounds, compounds containing no metal elements other than lithium, copper, silicon or germanium, and oxygen (particularly, rare metal elements) are preferable. Further, it is preferable that the elements other than each element of lithium, copper, silicon or germanium, and oxygen 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.

Lithium-containing compounds include metallic lithium (Li); lithium bromide (LiBr); lithium oxalate (Li ₂ C ₂ O ₄ ); lithium fluoride (LiF); lithium iodide (Li I); lithium sulfate (Li _2). SO ₄ ); methoxylithium (LiOCH ₃ ); _{ethoxylithium (LiOC 2} H ₅ ); lithium hydroxide (LiOH); lithium nitrate (LiNO ₃ ); lithium chloride (LiCl); lithium carbonate (Li ₂ CO ₃ ), etc. Can be mentioned.

Copper-containing compounds include metallic copper (Cu); copper oxide (CuO); copper hydroxide (Cu (OH) ₂ ); copper carbonate (CuCO ₃ ); copper oxalate (CuC ₂ O ₄ ); copper sulfate (CuSO). ₄ ); Copper chloride (CuCl ₂ ); Copper iodide (CuI); Copper acetate (Cu (CH ₃ COO) ₂ ) and the like.

Silicon-containing compounds include silicon (Si); silicon oxide (SiO ₂ ); tetraethoxysilane (SiOC ₂ H ₅ ); tetramethoxysilane (SiOCH ₃ ); silicon tetrabromide (SiBr ₄ ); silicon tetrachloride (SiCl). ₄ ) and the like.

Germanium compounds include germanium (Ge); germanium oxide (GeO ₂ ); germanium tetrachloride (GeCl ₄ ); germanium tetrabromide (GeBr ₄ ); germanium tetraiodide (GeI ₄ ); germanium tetrafluoride (GeF _4). ); Germanium disulfide (GeS ₂ ) and the like can be mentioned.

As oxygen-containing compounds, lithium hydroxide (LiOH); lithium carbonate (Li ₂ CO ₃ ); copper oxide (CuO); copper hydroxide (Cu (OH) ₂ ); copper carbonate (CuCO ₃ ); copper oxalate ( CuC ₂ O ₄ ); silicon oxide (SiO ₂ ); germanium oxide (GeO ₂ ) and the like.

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, the particles are preferably fine, and more preferably in the form of a powder having an average particle diameter of 1 μm or less (preferably about 10 to 500 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, copper, silicon or germanium, 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 the final product so as to have the composition of the compound represented by the above composition formula (1). 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 represented by the composition formula (1).

The method for preparing a mixture containing lithium, copper, silicon or germanium, 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.

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 a 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 at the time of mixing and heating is not particularly limited, and for example, an atmosphere of an inert gas such as argon or nitrogen, an atmosphere of hydrogen gas, 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, copper, silicon or germanium, and oxygen, the heating temperature is not particularly limited, and the obtained compound crystallinity and electrode represented by the composition formula (1) can be obtained. From the viewpoint of further improving the characteristics (capacity and potential), 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. preferable. The upper limit of the heating temperature is not particularly limited, and may be a temperature (for example, about 1500 ° C.) at which the compound represented by the above composition formula (1) can be easily produced. 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 The compound represented by the above composition formula (1) has the above composition and crystal structure, and therefore lithium ions can be inserted and removed. Therefore, the lithium ion secondary battery It can be used as a positive electrode active material for use. Therefore, the present invention includes a positive electrode active material for a lithium ion secondary battery containing the compound represented by the above composition formula (1).

Further, not only the compound represented by the composition formula (1), but also the composition formula (2):
Li _m Cu _y X ² O _n
[In the composition formula (2), X ² represents Si, Ti or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
Since the lithium copper-based composite oxide represented by is also capable of inserting and removing lithium ions, it can be used as a positive electrode active material for a lithium ion secondary battery. In the following, the lithium copper-based composite oxide represented by the composition formula (2) may be referred to as "a compound represented by the composition formula (2)". Therefore, the present invention includes a positive electrode active material for a lithium ion secondary battery containing the compound represented by the above composition formula (2).

In the following, the positive electrode active material for a lithium ion secondary battery containing the compound represented by the composition formula (1) and the positive electrode active material for a lithium ion secondary battery containing the compound represented by the composition formula (2). May be collectively referred to as "the positive electrode active material for the lithium ion secondary battery of the present invention".

In the composition formula (2), X ² is silicon (Si), titanium (Ti) or germanium (Ge).

In the composition formula (2), y is 0.8 to 1.2, preferably 0.8 to 1.0 from the viewpoint of increasing the capacity.

In the above composition formula (2), m is 1.5 to 2.5, and 1.75 to 2.25 is preferable from the viewpoint of ease of insertion and desorption of lithium ions, as well as capacity and potential. n is 3.9 to 4.1, and 3.95 to 4.05 is preferable from the viewpoint of ease of insertion and desorption of lithium ions, as well as capacitance and potential.

Specific examples of the compound represented by the composition formula (2) include Li ₂ CuSiO ₄ , Li ₂ CuTiO ₄ , and Li ₂ CuGeO ₄ . _{Among them, Li 2} CuSiO ₄ is preferable from the viewpoint of performance (particularly capacity improvement) when used as a positive electrode active material for a lithium ion secondary battery.

The crystal structure of the compound represented by the composition formula (2) is preferably a monoclinic structure. In particular, the compound represented by the composition formula (2) preferably has a monoclinic structure as the main phase. In the compound represented by the composition formula (2), the abundance of the crystal structure as the main phase is not particularly limited, and is 80 mol% or more based on the entire compound represented by the composition formula (2). Is preferable, and 90 mol% or more is more preferable. Therefore, the compound represented by the composition formula (2) can be a material having a single-phase crystal structure, or a material having another crystal structure as long as the effects of the present invention are not impaired. You can also do it. The crystal structure of the compound represented by the composition formula (2) can be confirmed by X-ray diffraction measurement.

The compound represented by the composition formula (2) has peaks at various positions in the X-ray diffraction pattern using CuKα rays.

For example, Li ₂ CuSiO ₄ has a diffraction angle of 2θ of 18.3 to 19.3 °, 26.3 to 27.0 °, 27.1 to 28.0 °, 28.8 to 29.6 °, 29. 9 to 30.5 °, 32.3 to 32.9 °, 35.5 to 36.7 °, 38.6 to 39.9 °, 40.8 to 42.0 °, 43.6 to 45.2 °, 45.7-46.8 °, 47.1-48.3 °, 48.5-49.8 °, 50.8-52.7 °, 53.7-55.2 °, 55.6 It is preferable to have peaks at ~ 58.2 °, 62.3-63.4 °, 63.8-65.1 °, 68.6-71.0 ° and the like.

Further, for example, Li ₂ CuGeO ₄ has a diffraction angle of 2θ of 17.9 to 19.2 °, 24.9 to 27.0 °, 31.6 to 33.4 °, and 35.0 to 39.2 °. 41.2 to 43.4 °, 49.2 to 51.5 °, 53.2 to 55.4 °, 56.9 to 58.7 °, 60 to 62.7 °, 63.7 to 65 It is preferable to have peaks at .2 °, 66.5 to 68.5 °, 69.9 to 71.7 °, 72.7 to 75.5 °, 76.9 to 78.4 ° and the like.

The average particle size of the compound represented by the composition formula (2) is not particularly limited, and is preferably 0.1 to 100 μm, preferably 0.1 to 50 μm, from the viewpoint of shortening the ^{Li + diffusion path.} Is more preferable. The average particle size of the compound represented by the composition formula (2) can be confirmed by a scanning electron microscope (SEM).

Method for producing a compound represented by the formula (2) include lithium, copper, and the X ^2, the step of heating a mixture comprising oxygen.

Lithium-containing compounds, copper-containing compounds, X ² containing compound is not particularly limited about the kind of each compound such as oxygen-containing compounds, lithium, copper, four or comprises one by one each element of X ^2, and oxygen can be used as a mixture of more compounds, also, lithium, copper, X ^2, and of oxygen, using two types or compounds containing more elements simultaneously as part of the raw materials, of less than four Compounds can also be mixed and used.

These raw material compounds, lithium, copper, X ^2, and oxygen other than metal elements (in particular, rare metal element) compound containing no is preferred. Further, lithium contained in the raw material compound, copper, X ^2, and for the elements other than the elements of oxygen, it is preferable that gradually disengaged or volatilization by heat treatment to be described later.

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

Examples of the lithium-containing compound include lithium oxalate (Li ₂ C ₂ O ₄ ); lithium hydroxide (LiOH); lithium nitrate (LiNO ₃ ); lithium chloride (LiCl); lithium carbonate (Li ₂ CO ₃ ). ..

Copper-containing compounds include metallic copper (Cu); copper oxide (CuO); copper hydroxide (Cu (OH) ₂ ); copper carbonate (CuCO ₃ ); copper oxalate (CuC ₂ O ₄ ); cupric chloride. (CuCl ₂ ); cupric sulfate (CuSO ₄ ); cupric nitrate (Cu (NO ₃ ) ₂ ); cupric sulfate (CuSO ₄ ) and the like.

Examples of the titanium-containing compound include titanium tetrachloride (TiCl ₄ ); titanium hydroxide (Ti (OH) ₂ ) and the like. Examples of the silicon-containing compound include silicon (Si); silicon oxide (SiO ₂ ) and the like.

Examples of the germanium compound include germanium (Ge); germanium oxide (GeO ₂ ) and the like.

As oxygen-containing compounds, lithium hydroxide (LiOH); lithium carbonate (Li ₂ CO ₃ ); copper oxide (CuO); copper hydroxide (Cu (OH) ₂ ); copper carbonate (CuCO ₃ ); copper oxalate ( CuC ₂ O _4); silicon oxide _(SiO 2); titanium oxide _(TiO 2); titanium hydroxide (Ti _{(OH) 2);} germanium oxide (GeO _2), and the like.

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

Further, as the raw material compound used in the method for producing the compound represented by the composition formula (2), a commercially available product may be used, or a commercially available product may 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, the particles are preferably fine, and more preferably in the form of a powder having an average particle diameter of 1 μm or less (preferably about 10 to 100 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).

Lithium, and copper, and X ^2, mixture comprising oxygen can be obtained by mixing the necessary materials of the feed compounds described above.

The mixing ratio of each raw material compound is not particularly limited, and it is preferable to mix the final product so as to have the composition of the compound represented by the above composition formula (2). 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 represented by the composition formula (2).

Lithium, and copper, and X ^2, not particularly limited as methods for preparing the mixture comprising oxygen, it is possible to employ a method of each raw material compound can be uniformly mixed. 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.

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

Lithium, and copper, and X ^2, when heating a mixture comprising oxygen, crystallinity and electrode characteristics of heating is not particularly limited as temperature, the compound represented by the obtained above composition formula (2) From the viewpoint of further improving (capacity and potential), 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 (for example, about 1500 ° C.) at which the compound represented by the above composition formula (2) can be easily produced. 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. ..

The positive electrode active material for a lithium ion secondary battery of the present invention is a compound represented by the above composition formula (1) or a compound represented by the composition formula (2) and a carbon material (for example, carbon black such as acetylene black). (Material) may form a complex. As a result, since the carbon material suppresses particle growth during firing, it becomes possible to obtain fine particle positive electrode active materials for lithium ion secondary batteries 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. is there.

The positive electrode active material for a lithium ion secondary battery of the present invention contains the compound represented by the above composition formula (1) or the compound represented by the composition formula (2). The positive electrode active material for a lithium ion secondary battery of the present invention may be composed only of the compound represented by the composition formula (1) or the compound represented by the composition formula (2) described above, or may be composed only of the compound represented by the composition formula (2) described above. In addition to the compound represented by the formula (1) or the compound represented by the composition formula (2), unavoidable impurities may be contained. 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 quality for lithium ion secondary battery and lithium ion secondary battery of the present invention are the compound represented by the above composition formula (1) or the composition formula (2). Except for using the compound represented by (1) as the positive electrode active material, the basic structure is known non-aqueous electrolyte (non-aqueous) lithium ion positive electrode for secondary batteries and non-aqueous electrolyte (non-aqueous) lithium ion. A configuration similar to that of the 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 has a structure in which a positive electrode active material containing the compound represented by the above composition formula (1) or the compound represented by the composition formula (2) is supported on a positive electrode current collector. Can be adopted. For example, a positive electrode material containing the compound represented by the composition formula (1) or the compound represented by the composition formula (2), a conductive auxiliary agent, and if necessary, a binder is applied to the positive electrode current collector. It can be manufactured by coating.

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 represented by the composition formula (1) or the compound represented by the composition formula (2) is 50 to 95% by volume (particularly 70 to 90% by volume), and the conductive additive is 2.5 to 25% by volume. It is preferable to contain% by volume (particularly 5 to 15% by volume) and 2.5 to 25% by volume (particularly 5 to 15% by volume) of the binder.

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 etc.} 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.

When the negative electrode material is supported on the negative electrode current collector, 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 positive electrode described above. 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, non-woven fabric, woven fabric, 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; and lithium inorganic salts such as lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate and lithium hexafluorophosphate. 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.

As the solvent, for example, a carbonate compound such as propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate; a lactone compound such as γ-butyrolactone and γ-valerolactone; tetrahydrofuran, 2-methyl tetrahydrofuran, diethyl. Ether compounds such as ether, diisopropyl ether, dibutyl ether, methoxymethane, glyme, dimethoxyethane, dimethoxymethane, diethoxymethane, diethoxyethane, propylene glycol dimethyl ether; acetonitrile; N, N-dimethylformamide; N-propyl-N- Examples thereof include methylpyrrolidinium bis (trifluoromethanesulfonyl) imide.

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 represented by the composition formula (1) or the compound represented by the composition formula (2) is used in such a lithium ion secondary battery of the present invention, it is used in the redox reaction (charge / discharge reaction). , Higher potential and energy density can be secured, and moreover, it is excellent in safety (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: As synthetic raw material powder _{of Li 2 CuSiO 4 , Li 2} CO ₃ (manufactured by Rare Metallic Co., Ltd .; 99.9% (3N)), CuO (manufactured by High Purity Chemical Laboratory Co., Ltd .; 99.99% (4N)) )) And settling amorphous SiO ₂ (manufactured by Kanto Chemical Co., Inc .; 3N) were used. Li ₂ CO ₃ , CuO, and SiO ₂ are weighed so that the ratio of lithium: copper: silicon (molar ratio) is 2: 1: 1, placed in a chrome steel container together with zirconia balls (15 mmΦ × 10 pieces), and acetone. Was added and pulverized and mixed at 400 rpm for 24 hours in a planetary ball mill (manufactured by Fritzch, 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 CuSiO ₄ ) was confirmed by powder X-ray diffraction (XRD). The results are shown in FIG.

Further, X-rays of _{Li 2 CuSiO 4} obtained when the firing temperature is 900 ° C., CuO as a raw material compound, and Li ₂ CO _{3 and Li 2} SiO ₃ produced from the raw material compounds Li 2 CO 3 and SiO _2. The result of comparing the diffraction patterns is shown in FIG.

An X-ray diffraction measuring device (manufactured by Rigaku Co., Ltd., trade name: RINT2200) was used for the powder X-ray diffraction (XRD) measurement, 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 the structural analysis, and JANA-2006 was used for the analysis program.

From FIG. 1, it was confirmed that when the firing temperature was 800 ° C. or higher, a plurality of major peaks were observed at least at a 2θ value of 15 to 70 ° C. Further, since the peak observed at the 2θ value of 15 to 70 ° becomes stronger as the firing temperature is higher, it was found that the higher the firing temperature is, the more preferable.

From FIG. 1, since the plurality of major peaks confirmed to have a 2θ value of 15 to 70 ° _{correspond to the single-phase Li 2} CuSiO ₄ , it can be seen that the single-phase Li ₂ CuSiO ₄ is obtained as a product. Do you get it. Further, from FIG. 2, a peak derived from CuO, which is a raw material compound, and _{Li 2} SiO ₃ produced from the _{raw material compounds Li 2} CO ₃ and SiO ₂ was not confirmed, and thus the single-phase Li _{2 was not confirmed.} It was found that CuSiO _{4 was obtained.}

_{Further, from FIG. 1, the crystals of Li 2} CuSiO ₄ obtained when the firing temperature is 900 ° C. have a diffraction angle represented by 2θ of 18.31 to 19 in the X-ray diffraction pattern by powder X-ray diffraction. .24 °, 26.39 to 26.96 °, 27.22 to 27.39 °, 28.90 to 29.59 °, 38.65 to 39.82 °, 40.88 to 41.92 °, 43 .63 to 45.12 °, 45.72 to 46.70 °, 47.21 to 48.23 °, 48.56 to 49.71 °, 50.87 to 52.69 °, 53.81 to 55. It was found to have peaks at 14 °, 55.66 to 58.17 °, 62.40 to 63.33 °, 63.88 to 65.04 °, and 68.67 to 70.90 °. From the results, the obtained Li ₂ CuSiO ₄ crystal has a monoclinic structure (space group C2 / m) and has a lattice constant of a = 6.457 to 6.484 Å and b = 3.340 to 3. It was found that the crystal had .345 Å, c = 9.504 to 11.183 Å, β = 93.65 to 121.78 °, and the unit lattice volume (V) was 205.1 to 206.0 Å ^3. ..

_{Further, Li 2} CuSiO ₄ obtained when the firing temperature was 900 ° C. was observed with a scanning electron microscope (SEM). The results are shown in FIG. In FIG. 3, the scale bar shows 11.7 μm. From FIG. 3, it was found that _{Li 2} CuSiO ₄ having a particle size of about 3 to 10 μm was obtained.

Further, when the chemical composition of the product obtained when the firing temperature was 900 ° C. was measured by the ICP-AES method (measuring device: iCAP6500, manufactured by Thermo Fisher Scientific Co., Ltd.), Li _2.08 Cu _1.05 It turned out to be Si _1.00 O _4.

Example 2: As synthetic raw material powder _{of Li 2 CuGeO 4 , Li 2} CO ₃ (manufactured by Rare Metallic Co., Ltd .; 99.9% (3N)), CuO (manufactured by High Purity Chemical Laboratory Co., Ltd .; 99.99% (4N)) )) And GeO ₂ (manufactured by Kanto Chemical Co., Inc .; 99.99% (4N)) were used. Li ₂ CO ₃ , CuO, and GeO ₂ are weighed so that the lithium: copper: germanium (molar ratio) is 2: 1: 1, placed in a chrome steel container together with zirconia balls (15 mmΦ × 10), and acetone. Was added and pulverized and mixed at 400 rpm for 24 hours in a planetary ball mill (manufactured by Fritzch, trade name: P-6). Then, after removing acetone under reduced pressure, the recovered powder was manually pellet-molded and calcined at 700 ° C., 800 ° C., or 900 ° 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 CuGeO ₄ ) was confirmed by powder X-ray diffraction (XRD) in the same manner as in Example 1. The results are shown in FIG.

From FIG. 4, it was confirmed that when the firing temperature was 700 ° C. or higher, a plurality of major peaks were observed at least at a 2θ value of 15 to 80 ° C. These peaks, since it corresponds to the _Li 2 CuGeO ₄ single-phase, _Li 2 CuGeO ₄ single-phase is found to be obtained as a product. Further, since the peak observed at the 2θ value of 15 to 80 ° becomes stronger as the firing temperature is higher, it was found that the higher the firing temperature is, the more preferable.

_{Further, from FIG. 4, the crystals of Li 2} CuSiO ₄ obtained when the firing temperature is 700 ° C. have a diffraction angle represented by 2θ of 17.94 to 19 in the X-ray diffraction pattern by powder X-ray diffraction. .15 °, 24.96 to 26.91 °, 31.65 to 33.32 °, 35.07 to 39.17 °, 41.30 to 43.39 °, 49.29 to 51.44 °, 53 .24 to 55.30 °, 56.92 to 58.63 °, 60.16 to 62.63 °, 63.79 to 65.19 °, 66.57 to 68.44 °, 69.92 to 71. It was found to have peaks at 64 °, 72.80 to 75.41 °, and 76.94 to 78.33 °. From the results, the obtained Li ₂ CuGeO ₄ crystal has a monoclinic structure (space group C2 / m) and has a lattice constant of a = 5.491-5.552 Å and b = 9.645-9. It was found that the crystal had .691 Å, c = 5.491 to 5.552 Å, β = 119.69 to 120.75 °, and the unit lattice volume (V) was 256.1 to 256.6 Å ^3. ..

_{Further, Li 2} CuGeO ₄ obtained when the firing temperature was 900 ° C. was observed with a scanning electron microscope (SEM). The results are shown in FIG. In FIG. 5, the scale bar shows 27.0 μm. From FIG. 5, it was found that _{Li 2} CuGeO ₄ having a particle size of about 1 to 50 μm was obtained.

Further, when the chemical composition of the product obtained at a firing temperature of 900 ° C. was measured by the EDX method (measuring device: JSM-7800F, manufactured by JEOL Ltd.), the mass ratio of Cu and Ge was 1: 1. It turned out to be 0.956.

Example 3: Measurement of charge / discharge characteristics of _{Li 2} CuSiO _{4 In order to perform charge / discharge measurement, Li 2} CuSiO ₄ , polyvinylidene fluoride (PVDF), and polyvinylidene fluoride (PVDF) obtained at a firing temperature of 900 ° C. in Example 1 above. The acetylene black (AB) was mixed in a mortar so that the volume ratio was 85: 7.5: 7.5, and the obtained slurry was applied onto an aluminum foil (thickness 20 μm) which was a positive electrode current collector. This was punched into a circle with a diameter of 8 mm to obtain 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 in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed 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. 6 was used. Prior to the charge / discharge test, the potential (that is, the open circuit potential) in the state where no current was applied to the electrodes was measured. The constant current charge / discharge measurement was set to C / 20 rate or C / 50 rate, current 10 mA / g, upper limit voltage 4.8 V, and lower limit voltage 1.5 V using a voltage switch, and started from charging. The charge / discharge measurement was performed with the cell placed in a constant temperature bath at 55 ° C. The measurement result of the open circuit potential is shown in FIG. 7, the measurement result of the charge / discharge characteristic at the C / 20 rate (relationship between each cycle and the discharge capacity) is shown in FIG. 8, and the measurement result of the charge / discharge characteristic at the C / 50 rate is shown in FIG. It is shown in FIG. The C rate means the current density required to charge and discharge the theoretical capacity of the electrode active material in 1 hour.

From FIG. 7, it was found that the open circuit potential of _{Li 2} CuSiO _{4 was about 3.0 V.} Further, from FIG. 8, it was found that the initial charge capacity for withdrawal at the C / 20 rate was about 110 mAh / g. The theoretical capacity is 316 mAh / g, and _{the initial charge / discharge capacity of Li 2} CuSiO ₄ corresponds to about one-third of the theoretical capacity. Further, as shown in FIG. 8, since the voltage (average operating voltage) at the intersection of the charge curve and the discharge curve was about 3.3 V, Li ₂ CuSiO ₄ was used as a high-potential and high-capacity positive electrode material. It turned out to be useful.

Further, from FIG. 9, _{it was confirmed that the initial charge capacity of Li 2} CuSiO ₄ was 220 mAh / g (capacity corresponding to about 70% of the theoretical capacity) at the C / 50 rate. From this, Li ₂ CuSiO ₄ is expected as a high-capacity material.

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

From FIG. 10, it was confirmed that the charge / discharge capacity could not be obtained with the electrode not using _{Li 2} CuSiO _4. By comparing FIG. 8 and FIG. 10, it was found that the high charge / discharge capacity shown in FIG. 8 _{was derived from Li 2} CuSiO _4.

Example 4: Measurement of charge / discharge characteristics of _{Li 2} CuGeO _{4 In order to perform charge / discharge measurement, Li 2} CuGeO ₄ , polyvinylidene fluoride (PVDF), and polyvinylidene fluoride (PVDF) obtained at a firing temperature of 900 ° C. in Example 2 above, and The acetylene black (AB) was mixed in a mortar so that the volume ratio was 85: 7.5: 7.5, and the obtained slurry was applied onto an aluminum foil (thickness 20 μm) which was a positive electrode current collector. This was punched into a circle with a diameter of 8 mm to obtain 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 in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed 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. 6 was used. Prior to the charge / discharge test, the potential (that is, the open circuit potential) in the state where no current was applied to the electrodes was measured. The constant current charge / discharge measurement was set to C / 50 rate, current 10 mA / g, upper limit voltage 4.8 V, and lower limit voltage 1.5 V using a voltage switch, and started from charging. The charge / discharge measurement was performed with the cell placed in a constant temperature bath at 55 ° C. The measurement result of the open circuit potential is shown in FIG. 11, and the measurement result of the charge / discharge characteristics (relationship between each cycle and the discharge capacity) is shown in FIG.

From FIG. 11, it was found that the open circuit potential of _{Li 2} CuGeO _{4 was about 2.7 V.} Further, from FIG. 12, it was found that the initial withdrawal capacity was about 120 mAh / g. The theoretical capacity is 250 mAh / g, and _{the initial charge / discharge capacity of Li 2} CuGeO ₄ corresponds to about half of the theoretical capacity. From the above results, _{it was found that Li 2} CuGeO ₄ is useful as a positive electrode material having a high potential and a high capacity.

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

Claims (8)

Composition formula (1):
Li _m Cu _y X ¹ O _n
[In the composition formula (1), X ¹ represents Si or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
A lithium copper-based composite oxide represented by and having a monoclinic structure. The lithium copper-based composite oxide according to claim 1, which has an average particle size of 0.1 to 100 μm. The method for producing a lithium copper-based composite oxide according to claim 1 or 2, which comprises a step of heating a mixture containing lithium, copper, silicon or germanium, and oxygen. The method according to claim 3, wherein the heating temperature is 600 ° C. or higher. Composition formula (2):
Li _m Cu _y X ² O _n
[In formula ^{(2), X} 2 represents a S i or Ge. y indicates 0.8 to 1.2. m indicates 1.5 to 2.5. n indicates 3.9 to 4.1. ]
A positive electrode active material for a lithium ion secondary battery, which is represented by and contains a lithium copper-based composite oxide having a monoclinic structure. 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 comprising the positive electrode for a lithium ion secondary battery according to claim 6 or 7.

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