JP7323940B2 - Lithium composite oxide, positive electrode active material for secondary battery, and secondary battery - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Law 58th Battery Symposium Abstracts of Lectures

TECHNICAL FIELD The present invention relates to a lithium composite oxide, a positive electrode active material for secondary batteries, and a secondary battery.
This application claims priority based on Japanese Patent Application No. 2018-066494 filed in Japan on March 30, 2018, the content of which is incorporated herein.

Lithium composite oxides are used as positive electrode active materials for lithium secondary batteries. Lithium secondary batteries have already been put to practical use as small power sources for mobile phones and notebook computers. Furthermore, attempts have been made to apply it to medium-sized or large-sized power sources such as those used in automobiles and electric power storage. With the expansion of the application range, it is an important issue to improve the performance of the lithium secondary battery.

Spinel-type Ni-substituted lithium manganates and their derivatives are attractive electrode materials for lithium-ion batteries because of their high potential, low cost, and non-toxicity. Various studies have been conducted on spinel-type Ni-substituted lithium manganese oxide.
For example, Patent Document 1 describes that in order to improve the capacity of a battery using an LNMO-type lithium composite oxide, the content of additives was reduced to produce a dense lithium composite oxide film with few pores. It is

The lithium composite oxide described in Non-Patent Document 1 has a theoretical capacity of 147 mAh/g when used as a positive electrode active material with a lower limit of cutoff potential of 3.5 V (vs. Li ⁺ /Li). .

JP 2014-35909 A

Marilena Mancini et al, A High-Voltage and High-Capacity Li1+xNi0.5Mn1.5O4 Cathode Material: From Synthesis to Full Lithium-Ion Cells ChemSusChem 2 016, 9, 1843-1849

It is known that a specific capacity of about 200 mAh/g can be obtained by extending the lower cutoff potential range to 2.5 V (vs. Li ⁺ /Li) or less. There is a problem that the crystal structure undergoes a phase transition from cubic to tetragonal.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium composite oxide having a reversible capacity of about 250 mAh/g, a positive electrode active material for a secondary battery using the same, and a secondary battery. and

The present invention includes the following [1] to [11].
A first aspect of the present invention provides the lithium composite oxide described in [1].
[1] A lithium composite oxide having a spinel structure, wherein the general formula representing the chemical composition is represented by the following formula (1).
Li _1+z Ni _a Mn _2-y M _b Cu _y O _4-x F _x (1)
[In general formula (1), 0 < a ≤ 0.6, 0 ≤ b ≤ 0.2, 0 ≤ y ≤ 1, 0 ≤ x ≤ 1, 0 ≤ z ≤ 1.0, provided that x and M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn, except when both y are 0; ]
The lithium composite oxide of the first aspect preferably includes the following features.
[2] The lithium composite oxide according to [1], which has an octahedral crystal structure with a (111) plane as a principal plane, and at least one vertex of the octahedral crystal structure has a truncated surface. Lithium composite oxide.
[3] The lithium composite oxide has an octahedral crystal structure with a (111) plane as a principal plane, and at least one edge of the octahedral crystal structure has a truncated surface, [1] or [2] ] Lithium composite oxide as described in .
[4] The lithium composite oxide has an octahedral crystal structure with a (111) plane as a principal plane, and at least one inclined plane of the octahedral crystal structure has a step-terrace structure. The lithium composite oxide according to any one of -[3].
[5] The lithium composite oxide according to any one of [1] to [4], wherein 0<y≤1 in formula (1).
[6] The lithium composite oxide according to any one of [1] to [4], wherein 0<x≦1 in the formula (1).
[7] The lithium composite oxide according to any one of [1] to [4], wherein 0<y≦1 and 0<x≦1 in the formula (1).
A second aspect of the present invention provides the following positive electrode active material for secondary batteries.
[8] A positive electrode active material for a secondary battery, comprising the lithium composite oxide according to any one of [1] to [7].
A third aspect of the present invention provides the following secondary battery.
[9] A secondary battery comprising the positive electrode active material for a secondary battery according to [8].
Furthermore, the present invention provides the following lithium composite oxide.
[10] The lithium composite oxide according to any one of [1] to [4], wherein x satisfies 0.025≦x≦0.1.
_{[ 11 ] LiNi0.5Mn1.49Cu0.01O4.0 , LiNi0.5Mn1.45Cu0.05O4.0 , LiNi0.5Mn1.49Cu0.01O _ _ _ 3.9F0.1 , LiNi0.5Mn1.49Cu0.01O3.95F0.05 , LiNi0.5Mn1.49Cu0.01O3.975F0.025 , _ _ _ _ _ _} and LiNi _0.5 Mn _1.45 Cu _0.05 O _3.95 F _0.05 , the lithium composite according to any one of [1] to [4], which is at least one compound selected from oxide.

According to the present invention, it is possible to provide a lithium composite oxide having a capacity of 250 mAh/g or more, a positive electrode active material for a secondary battery using the same, and a secondary battery.

FIG. 2 is a diagram showing Raman spectra of LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x crystals for each composition produced in Examples. FIG. 2 is a diagram showing charge-discharge curves of the crystals produced in Examples. FIG. 4 is a diagram showing the discharge capacity results of the crystals produced in Examples. FIG. 4 is a diagram showing the initial discharge capacity of the crystals produced in Examples. FIG. 4 is a diagram showing changes in discharge capacity with respect to the number of charge-discharge cycles for the crystals produced in Examples. It is a figure which shows the result of the X-ray-diffraction measurement about two types of crystal|crystallizations manufactured in the Example. FIG. 4 is a graph showing the change in lattice constant with respect to the potential of the positive electrode taken out by disassembling the battery used in the example. 1 is a scanning electron micrograph of a lithium composite oxide obtained in an example. 1 is a transmission electron micrograph of a lithium composite oxide obtained in an example. FIG. 3 is a schematic diagram of the three-dimensional structure of crystal particles of lithium composite oxide having a substantially regular octahedral shape.

Preferred examples for carrying out the present invention are described in detail below. The following description is provided specifically for better understanding of the spirit of the invention, and does not limit the invention unless otherwise specified. Within the scope of the present invention, it is also possible to change, add, omit, and/or replace the number, amount, material, shape, position, type, etc., as necessary, unless otherwise limited.
<Lithium composite oxide>
Depending on the synthesis conditions, the LNMO type lithium composite oxide may be a nickel/manganese ordered arrangement type (hereinafter sometimes referred to as "P4332 type") or a nickel/manganese disordered arrangement type (hereinafter referred to as "Fd-3m A crystal with a space group of (sometimes described as "type") is generated.
The P4332 type lithium composite oxide has the advantage that when used as a lithium secondary battery, it has a large capacity and can be used as a long-life battery. On the other hand, it has the disadvantage of low electronic conductivity.
The Fd-3m type lithium composite oxide has the advantage of increasing the electronic conductivity when used as a lithium secondary battery. On the other hand, it has the disadvantage that it is not suitable for long-life batteries due to its small capacity.

In manufacturing lithium composite oxides useful for lithium secondary batteries with high capacity and excellent cycle characteristics, we have achieved both the high capacity characteristics of the P4332 type and the high electronic conductivity characteristics of the Fd-3m type. It is preferable to be able to

The lithium composite oxide of the present invention is characterized in that the general formula representing the chemical composition is represented by formula (1). The lithium composite oxide of the present invention preferably has a spinel crystal structure. A spinel structure is a type of crystal structure belonging to the cubic system. The lithium composite oxide of the present embodiment controls either or both of cation space control by Cu ion substitution and anion space control by F ion substitution. This stabilizes the regular arrangement of nickel/manganese and suppresses the phase transition of the crystal structure from cubic to tetragonal. As a result, lithium ion storage sites are increased, and a lithium composite oxide having a capacity of 250 mAh/g or more can be obtained.

<<First embodiment>>
The present embodiment is a lithium composite oxide having a spinel structure, the lithium composite oxide having a general formula representing the chemical composition represented by the following formula (1).
Li _1+z Ni _a Mn _2-y M _b Cu _y O _4-x F _x (1)
[In general formula (1), 0 < a ≤ 0.6, 0 ≤ b ≤ 0.2, 0 ≤ y ≤ 1, 0 ≤ x ≤ 1, 0 ≤ z ≤ 1.0, provided that x and M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn, except when both y are 0; ]

The first embodiment is an embodiment in which one or both of cation space control by Cu ion replacement and anion space control by F ion replacement is performed.
In the above formula (1), a≧0.5 is preferable.
In the above formula (1), 0≤y≤0.5 is preferable.
In the above formula (1), 0.6≦z≦0.8 is preferable.
In the above formula (1), 0≦x≦0.5 is preferable.
In the above formula (1), 0≤b≤0.1 is preferable.
M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn.
In the above formula (1), a may be any of 0<a≦0.10, 0.10<a≦0.40, and 0.40<a≦0.60.
In the above formula (1), b may be any of 0≦b≦0.08, 0.08≦b≦0.13, and 0.13≦b≦0.20.
In the above formula (1), y may be any of 0≦y≦0.30, 0.30≦y≦0.60, and 0.60≦y≦1.00.
In the above formula (1), x may be any of 0≦x≦0.30, 0.30≦x≦0.60, and 0.60≦x≦1.00.
In the above formula (1), z is any of 0 ≤ z ≤ 1.0, 0.50 ≤ z ≤ 0.60, 0.60 ≤ z ≤ 0.80, and 0.80 ≤ z ≤ 1.00 may be
The lithium composite oxide of the present embodiment preferably has an octahedral crystal structure whose main plane corresponds to the (111) plane.
The octahedral crystal structure may be a regular octahedral crystal structure or a distorted octahedral crystal structure.
In this embodiment, at least one vertex of the octahedral crystal structure preferably has a truncated surface.
The number of vertices having a truncated surface is preferably two or more, more preferably three or more. Also, all the vertices of the octahedral crystal structure may have truncated faces, 7 or less, or 6 or less.
A truncated surface means a surface formed by cutting off vertices of a polyhedron.
In this embodiment, at least one edge of the octahedral crystal structure preferably has a truncated surface.
The number of ridges having truncated surfaces is preferably two or more, more preferably three or more. In addition, all edges of the octahedral crystal structure may have truncated surfaces, or 11 or less, or 10 or less.
A truncated surface means a surface formed by excising an edge of a polyhedron.
In this embodiment, at least one inclined plane of the octahedral crystal structure preferably comprises a step-and-terrace structure.
Here, the "step/terrace structure" means a structure composed of a plurality of flat terraces arranged parallel to each other via steps along the main surface of the crystal, and steps, which are step subs connecting the terraces. means a microscopic stepped structure.
In this embodiment, the number of sloped surfaces having a step-terrace structure is preferably two or more, and more preferably three or more. In addition, all inclined planes of the octahedral crystal structure may have a step-terrace structure, and may be 7 or less, or 6 or less.
In the crystal structure of the lithium mixed metal oxide of the present embodiment, at least one vertex of the octahedral crystal structure has a truncated surface, and at least one inclined surface of the octahedral crystal structure has a step-terrace structure. is preferred.
In the crystal structure of the lithium composite metal oxide of the present embodiment, at least one edge of the octahedral crystal structure has a truncated surface, and at least one inclined surface of the octahedral crystal structure has a step-terrace structure. is preferred.
In the crystal structure of the lithium mixed metal oxide of the present embodiment, it is preferable that at least one vertex of the octahedral crystal structure has a truncated surface and at least one edge of the hedron crystal structure has a truncated surface.
The crystal structure of the lithium mixed metal oxide of the present embodiment is such that at least one vertex of the octahedral crystal structure has a truncated face, at least one edge of the hedron crystal structure has a truncated face, and At least one inclined face of the hedron crystal structure preferably comprises a step-and-terrace structure. The crystal structure of the lithium composite oxide of this embodiment can be confirmed by observation using a scanning electron microscope or a transmission electron microscope.
The crystal structure of the lithium composite oxide of the present embodiment is a crystal structure having a truncated face appearing in an octahedron having a (111) face, and a crystal structure having a truncated face appearing in an octahedron having a (111) face. , or a structure having a step-terrace structure on at least one inclined plane of the octahedral crystal structure, a lithium ion desorption reaction during charging of the lithium secondary battery and a lithium ion absorption reaction during discharge. is considered to occur stably.

Second to fourth embodiments, which are more preferable examples of the first embodiment, will be described below.
<<Second embodiment>>
The present embodiment is a lithium composite oxide having a spinel structure, the lithium composite oxide having a general formula representing the chemical composition represented by the following formula (1)-1.
Li _1+z Ni _a Mn _2-y M _b Cu _y O _4-x F _x (1)-1
[In general formula (1)-1, 0<a≦0.6, 0≦b≦0.2, 0<y≦1, 0≦x≦1, and 0≦z≦1.0. M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn. ]

The second embodiment is an embodiment in which the cation space is controlled by Cu ion replacement.
In the above formula (1)-1, the preferable range of each symbol is the same as the content explained in the above formula (1).

By controlling the cation space by Cu ion substitution, the nickel/manganese ordered arrangement is stabilized, the lithium ion storage sites are increased, and a lithium composite oxide having a capacity of 250 mAh/g or more can be obtained.

<<Third Embodiment>>
The present embodiment is a lithium composite oxide having a spinel structure, the lithium composite oxide having a general formula representing the chemical composition represented by the following formula (1)-2.
Li _1+z Ni _a Mn _2-y M _b Cu _y O _4-x F _x (1)-2
[In general formula (1), 0<a≤0.6, 0≤b≤0.2, 0≤y≤1, 0<x≤1, 0≤z≤1.0. M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn. ]

The third embodiment is an embodiment in which the anion space is controlled by F ion replacement.
In the above formula (1)-2, the preferable range of each symbol is the same as the content explained in the above formula (1).

It is considered that stable Mn ³⁺ that does not contribute to the redox reaction is generated by controlling the anion space by F ion replacement. Furthermore, by controlling the anion space, the conduction path of lithium ions is changed, the storage sites for lithium ions are increased, and a lithium composite oxide having a capacity of 250 mAh/g or more can be obtained.

<<Fourth Embodiment>>
The present embodiment is a lithium composite oxide having a spinel structure, the lithium composite oxide having a general formula representing the chemical composition represented by the following formula (1)-3.
Li _1+z Ni _a Mn _2-y Cu _y O _4-x F _x (1)-3
[In general formula (1), 0<a≤0.6, 0<y≤1, 0<x≤1, 0≤z≤1.0. ]

The fourth embodiment is an embodiment in which both the control of the cation space by Cu ion replacement and the control of the anion space by F ion replacement are performed.
In formulas (1)-3 above, the preferred range of each symbol is the same as described for formula (1) above.
According to this embodiment, phase transition to a tetragonal system can be suppressed by controlling both spaces of cations and anions and precisely designing sites for accommodating excess lithium ions.

The crystal phase of the lithium composite oxide of this embodiment can be identified by the XRD method.

<Method for producing lithium composite oxide>
In the method for producing a lithium composite oxide of the present embodiment, a lithium compound, a precursor containing at least manganese and nickel, and a reaction accelerator (flux) are combined with the above formulas (1), (1)-1 to (1 )-3 to obtain a lithium mixture, and a firing step of firing the lithium mixture obtained in the mixing step.

[Mixing process]
In the mixing step, the lithium compound, a precursor containing at least manganese and nickel, optionally containing copper, and a reaction accelerator (flux) are mixed. The mixing method is not particularly limited, and a general mixer can be used. For example, a shaker mixer, a V blender, a ribbon mixer, a Julia mixer, a Loedige mixer, or the like can be used as long as the mixture is sufficiently mixed to the extent that fine powder is not generated.

- Lithium compound The lithium compound used in the present embodiment can be selected as necessary. Preferred examples include one or more anhydrides and one or more hydrates selected from the group consisting of lithium hydroxide, lithium chloride, lithium nitrate and lithium carbonate.
- Precursor containing at least manganese and nickel The precursor containing at least manganese and nickel used in the present embodiment can be selected as necessary. Manganese and nickel may be contained in different compounds. Preferred examples of the manganese-containing precursor include at least one of manganese nitrate hexahydrate, manganese sulfate pentahydrate, and manganese chloride tetrahydrate. Nickel-containing precursors can include at least one of nickel nitrate hexahydrate, nickel sulfate pentahydrate, and nickel chloride tetrahydrate. Precursors containing both manganese and nickel may be used. It should be noted that the present invention is not limited to the above examples.
Precursor Containing Copper A precursor containing copper that can be used in the present embodiment can be selected as necessary. Precursors containing copper may include copper nitrate trihydrate, copper sulfate trihydrate, and copper chloride dihydrate.

・Reaction accelerator (flux)
The reaction accelerator (flux) used in this embodiment can be selected according to need. Specifically, chlorides such as NaCl, KCl, RbCl, CsCl, CaCl ₂ , MgCl ₂ , SrCl ₂ , LiCl, BaCl ₂ and NH ₄ Cl, Na ₂ CO ₃ , K ₂ CO ₃ , Rb ₂ CO ₃ , carbonates such _{as Cs2CO3 , CaCO3} , _MgCO3 , _SrCO3 and _BaCO3 ; sulfates such as _K2SO4 , _Na2SO4 ; fluorides such as NaF _, KF, _NH4F ; be done.
Among these, KCl, K ₂ CO ₃ and K ₂ SO ₄ are preferred. Also, two or more reaction accelerators can be used in combination. When two or more kinds are used, the ratio can be arbitrarily selected.
The method, conditions, amount, and timing of adding the reaction accelerator to the mixture are not particularly limited, and can be selected as necessary. For example, a reaction accelerator may be added when the precursor containing at least manganese and nickel is mixed with the lithium compound.
The reaction accelerator may remain in the lithium composite oxide after baking, or may be removed by washing, evaporation, or the like.

[Baking process]
In the firing step, a mixture of the precursor, the lithium compound, and the reaction accelerator, which are raw materials, is fired to obtain a mass of lithium composite oxide, which is a fired product.
The holding temperature in the firing step can be arbitrarily selected, but for example, it may be in the range of 650° C. or higher and 900° C. or lower. More preferably, it is 700°C or higher and 850°C or lower.
The holding time at the holding temperature can be arbitrarily selected, but is, for example, 0.1 to 20 hours, preferably 0.5 to 8 hours.
The heating rate up to the holding temperature can be arbitrarily selected, but is, for example, 50° C. to 400° C./hour.
The rate of temperature drop from the holding temperature to room temperature can be arbitrarily selected, but is usually 10° C. to 400° C./hour.
As the firing atmosphere, air, oxygen, nitrogen, argon, or a mixed gas thereof can be used. That is, an atmosphere of an inert gas, a neutral gas, an oxidizing gas, or the like can be used as necessary, but an air atmosphere is preferred.
The lithium composite oxide obtained by firing preferably has a composition represented by the following general formula.
Li _1+z Ni _a Mn _m1 M _b O _4-δ (2)
[In general formula (1), 0<a≤0.6, 0≤b≤0.2, 0≤m1≤1, 0≤δ≤4, and 0≤z≤1.0. M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn. ]
The lithium composite oxide may be oxygen-deficient.
The lithium composite oxide obtained as described above can be subjected to a second firing step, if necessary.
In the second firing step, the lithium composite oxide can be mixed with one or both of a fluorine-containing compound and a copper-containing compound and fired. At this time, it is also preferable to use the above fluxes in combination. Any compound containing fluorine can be selected, and examples thereof include lithium fluoride, aluminum fluoride, and zinc fluoride.
The compound containing copper can be arbitrarily selected, and examples thereof include copper nitrate trihydrate, copper sulfate trihydrate, and copper chloride dihydrate.
The holding temperature in the second firing can be arbitrarily selected, but as an example, it is 300° C. or higher and 1000° C. or lower, 400° C. or higher and 900° C. or lower, or 500° C. or higher and 800° C. or lower.
The holding time at the holding temperature can be arbitrarily selected, but is, for example, 0.5 hours or more and 20 hours or less, or 2 hours or more and 15 hours or less.
The rate of temperature increase to the holding temperature can be arbitrarily selected, but the rate of temperature increase is preferably 50°C/hour or more and 400°C/hour or less, and the rate of temperature decrease from the holding temperature to room temperature is usually 10°C. / hour or more and 400° C./hour or less. As the firing atmosphere, air, oxygen, nitrogen, argon, or a mixed gas thereof can be used. That is, an atmosphere of inert gas, neutral gas, oxidizing gas, or the like can be used. You can change the atmosphere in the middle. For example, firing may be performed in an air atmosphere, and then firing may be performed in an oxygen atmosphere.
The obtained lumps of lithium composite oxide are pulverized by a pulverizer such as a roll mill, if necessary, washed to remove residual lithium components and reaction accelerators, and dried.
In addition, the dry powder is pulverized by a roll mill or the like as necessary. Here, crushing means dispersing or loosening aggregated particles.

By mixing and firing a lithium compound, a precursor containing at least manganese and nickel, a precursor containing copper, and a reaction accelerator (flux), Mn ²⁺ or Mn ³⁺ converts into Cu ²⁺ or Cu ³⁺ Substituted, copper-doped lithium composite oxides can be obtained. In addition, an oxygen-deficient lithium composite oxide can be obtained by firing under atmospheric conditions or low-oxygen conditions.
By mixing and reacting an oxygen-deficient lithium composite oxide and a fluorine-containing compound, a lithium composite oxide in which some of the oxygen atoms are substituted with fluorine atoms can be obtained.

The obtained lumps of lithium composite oxide are pulverized by a pulverizer such as a roll mill, if necessary, washed to remove residual lithium components and reaction accelerators, and dried.
In addition, the dry powder is pulverized by a roll mill or the like as necessary. Here, crushing means dispersing or loosening aggregated particles.
The particle size of the obtained lithium composite oxide particles can be arbitrarily selected. For example, the particle size of a single particle (primary particle) may be 0.1 μm or more and 2.0 μm or less, or may be 0.2 μm or more and 1.9 μm or less, and more preferably 0.3 μm or more and 1.8 μm or less. . The size of the primary particles can be obtained from images obtained by scanning electron microscopy. The shape of the obtained lithium composite oxide can be arbitrarily selected, but for example, octahedral shape, approximately octahedral shape, or octahedral shape or approximately octahedral shape having truncated surfaces and/or truncated surfaces. you can These oxides may be porous.

<Positive electrode active material for secondary battery>
The positive electrode for a secondary battery of this embodiment is not limited except that it has the lithium composite oxide of the present invention described above.
The positive electrode of this embodiment can be produced by first preparing a positive electrode mixture containing the lithium composite oxide of the present invention, a conductive material and a binder.

(Conductive material)
A carbon material can be used as the conductive material of the positive electrode of the present embodiment. Examples of carbon materials include graphite powder, carbon black (eg, acetylene black), and fibrous carbon materials. Since carbon black is fine particles and has a large surface area, by adding a small amount of carbon black to the positive electrode mixture, the conductivity inside the positive electrode can be increased, and the charge/discharge efficiency and output characteristics can be improved. However, if too much is added, both the binding force between the positive electrode mixture and the positive electrode current collector and the binding force inside the positive electrode mixture due to the binder are reduced, causing an increase in internal resistance. Therefore, it is preferably used in an appropriate amount.

(binder)
A thermoplastic resin can be used as the binder of the positive electrode of the present embodiment.
Examples of this thermoplastic resin include polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), ethylene tetrafluoride, propylene hexafluoride, fluoride Fluororesins such as vinylidene copolymers, propylene hexafluoride/vinylidene fluoride copolymers, and tetrafluoroethylene/perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene.

When the positive electrode mixture is made into a paste, organic solvents that can be used include amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether-based solvents such as tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; and ester solvents; amide solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP);

<Lithium secondary battery>
The lithium secondary battery of the present invention is not limited except for using the lithium composite oxide of the present invention described above.
That is, the lithium secondary battery of the present invention can have the same configuration as conventionally known lithium secondary batteries, except that it has the above-described positive electrode for lithium secondary batteries. The lithium secondary battery of the present invention can be configured to have a positive electrode, a negative electrode, an electrolytic solution, and other necessary members.

(negative electrode)
As the active material of the negative electrode, a compound capable of intercalating and deintercalating lithium ions can be used singly or in combination. Examples of compounds that can occlude and release lithium ions include metal materials such as lithium, alloy materials containing titanium, silicon, tin, etc., carbon materials such as graphite, coke, baked organic polymer compounds, and amorphous carbon. , and other ceramic materials such as silicon oxide, LiCoN, CuO, and V ₂ O ₅ .
These active materials can be used not only singly, but also as a mixture of two or more of them. Among these materials, titanium-containing oxides (eg, TiO ₂ (B), which is titanium oxide with a bronze structure, and Li ₄ Ti ₅ O ₁₂ , which is lithium titanate) are preferably used as the negative electrode active material.
For example, when a lithium metal foil is used as the negative electrode active material, it can be formed by pressing the lithium foil onto the surface of a current collector made of a metal such as copper.

When an alloy material or a carbon material is used as the negative electrode active material, the negative electrode active material, a binder, a conductive aid, etc. are mixed in a solvent such as water or N-methylpyrrolidone, and then a metal such as copper is added. It can be applied and formed on a current collector. The binder is desirably made of a polymer material, and is desirably a material that is chemically and physically stable in the atmosphere inside the lithium secondary battery.

For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyimide (PI), fluorine rubber and the like.
Examples of conductive aids include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, carbon fiber, graphene, amorphous carbon, and the like. Also, conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene can be exemplified.

The electrolyte is a medium that transports charge carriers such as ions between the positive electrode and the negative electrode, and is not particularly limited, but is physically, chemically, and electrically stable under the atmosphere in which the lithium ion secondary battery is used. something is desirable.

(Electrolyte)
For example, the electrolyte can be arbitrarily selected from LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ( C ₄ F ₉ SO ₂ ) as a supporting electrolyte and dissolved in an organic solvent.

Any organic solvent can be selected, and propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, etc. and mixtures thereof can be mentioned. Among them, an electrolytic solution containing a carbonate-based solvent is preferable because of its high stability at high temperatures. A solid polymer electrolyte containing the above electrolyte in a solid polymer such as polyethylene oxide, or a solid electrolyte such as ceramic or glass having lithium ion conductivity can also be used.

It is desirable to interpose a separator, which is a member that achieves both electrical insulation and ion conduction, between the positive electrode and the negative electrode. When the electrolyte is liquid, the separator also serves to retain the liquid electrolyte. Examples of separators include porous synthetic resin films, particularly porous films made of polyolefin polymers (polyethylene, polypropylene) and glass fibers, and non-woven fabrics. Furthermore, the separator preferably has a shape larger than that of the positive electrode and the negative electrode in order to ensure insulation between the positive electrode and the negative electrode.

A positive electrode, a negative electrode, an electrolyte, a separator, and the like are generally housed in some kind of case. The case is not particularly limited, and can be made of known materials and forms. That is, the lithium secondary battery of the present invention is not particularly limited in its shape, and can be used as batteries of various shapes such as coin-shaped, cylindrical and square-shaped. Also, the case of the lithium secondary battery of the present invention is not limited, and can be used as a battery in various forms such as a case made of metal or resin that can retain its outer shape, and a soft case such as a laminate pack. .

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

<Example 1>
A LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x crystal, which is a copper/fluorine co-substituted lithium composite oxide, was grown by the following method using a LiCl—KCl flux.

- Production and Evaluation of Copper and Fluorine-Co-Substituted Lithium Composite Oxide First, an oxygen-deficient copper-substituted lithium composite oxide was produced as follows.
Lithium chloride as a lithium source of the lithium composite oxide, nickel nitrate hexahydrate as a nickel source, manganese nitrate hexahydrate as a manganese source, copper nitrate trihydrate as a kappa source, Li:Ni:Mn : Cu in a molar ratio of 1.0:0.50:1.49:0.01.

A mixture of lithium chloride and potassium chloride was used as the flux. The amount of flux was about 5 times the weight of the solute. These were put into an alumina crucible. The crucible is placed in an electric furnace and heated under the following conditions: heating temperature: 15°C/minute, holding time: 10 hours, holding temperature: 700°C, cooling rate: 200°C/hour, stop temperature: 500°C. processed. After the heat treatment, the flux was removed by immersion in hot water. As a result, an oxygen-deficient copper-substituted lithium composite oxide of LiNi _0.5 Mn _1.49 Cu _0.01 O _4-δ was obtained.

Next, a mixture of the oxygen-deficient copper-substituted lithium composite oxide of LiNi _0.5 Mn _1.49 Cu _0.01 O _4-δ obtained above, potassium chloride and lithium fluoride (mixing ratio = molar ratio 1:1) was calcined at 800° C. for 10 hours in an air atmosphere. The molar ratio of the oxygen-deficient copper-substituted lithium composite oxide to the mixture was 1:1.
Furthermore, it is fired at 700 ° C. for 10 hours in an oxygen atmosphere, and in Example 1, the desired LiNi _0.5 Mn _1.49 Cu _0.01 O _4-a F _a (0 ≤ a ≤ 1) A copper/fluorine co-substituted lithium composite oxide was obtained. The crystal phase was identified by XRD method and the crystal morphology was evaluated by FE-SEM. In addition, the space group was identified by Raman spectroscopy. The results are shown in FIG. For comparison, samples with different F ratios were also manufactured by changing the amount of the lithium fluoride mixture used in the manufacturing process. For comparison, a compound containing no Cu and Fe was also produced and evaluated.

<Example 2>
・Production and evaluation of fluorine-substituted lithium composite oxide Li:Ni:Mn with lithium chloride as the lithium source of the lithium composite oxide, nickel nitrate hexahydrate as the nickel source, and manganese nitrate hexahydrate as the manganese source. LiNi _0.5 Mn _1.45 Cu _0.05 O _4-a F _a ( A fluorine-substituted lithium composite oxide satisfying 0≦a≦1 was obtained. The crystal phase was identified by XRD method and the crystal morphology was evaluated by FE-SEM. In addition, the space group was identified by Raman spectroscopy. The results are shown in FIG. In addition, in Example 2, by changing the conditions of the amount of the copper source used in the production, products with different Cu ratios were also produced.

・Production of secondary battery The obtained active material, Denka black, and PVDF were mixed at a mass ratio of 90:5:5%, and the tap density was adjusted ^to 3.0 g/cm It was made on an electric foil.
An R2032 type coin cell was fabricated using lithium metal as the counter electrode, Celgard #2400 as the separator, and EC-DMC containing 1M LiPF ₆ as the electrolyte.

・Evaluation of constant-current charge-discharge characteristics Using the above coin cell, the constant-current charge-discharge characteristics were evaluated under the conditions of a cutoff potential range of 4.8V-2.5V (vs Li + / Li) and a current density equivalent to 0.2C. . The results are shown in FIG.

・Electronic state analysis Structural optimization and electronic state analysis were performed by DFT calculation. These were performed using calculation software Vienna Ab-initio Simulation Package (VASP).
As calculation conditions, the PAW method and GGA-PBEsol+U (Ni: U = 6.0 eV[1], Mn: U = 3.9 eV[1]) were used.
Phase stability of Cu ²⁺ and F ⁻ co-substituted LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x using 8 LiNi _0.5 Mn _1.5 O ₄ supercells containing 56 atoms The de-insertion mechanism was predicted by computational science based on the evaluation of the nature of Li, the calculation of the formation energies of analogues with different Li compositions, and the analysis of electronic states.

FIG. 1 shows the Raman spectrum of each composition of the LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x crystal. As shown in FIG. 1, broadening of the Raman spectrum was observed with the composition of X=0.1, that is, with LiNi _0.5 Mn _1.49 Cu _0.01 O _3.9 F _0.1 .
This implies a transition to the Fd-3m structure due to destabilization of the P4332-type structure.
On the other hand, LiNi _0.5 Mn _1.49 Cu _0.01 O _3.95 F _0.05 and LiNi _0.5 Mn _1.49 Cu _0.01 O _3.975 F _0.025 have a small amount of F substitution. As for the composition, sharpening of the Raman spectrum attributed to the ordered arrangement of Ni/Mn was observed. Stabilization of the P4332 type structure was confirmed. From the above results, it was found that the P4332-type structure was stabilized within a very narrow compositional range in the co-substituted product, although it contained oxygen deficiency.

In the evaluation using a coin cell, after the open circuit voltage was stabilized, the current density to the positive electrode was set at 25° C. to 30 mAh/g with respect to the weight of the positive electrode active material, and the battery was charged to 4.8, and then to 3.5V. discharged up to A charge/discharge test was conducted to measure the discharge capacity at this time, and the resulting charge/discharge curve is shown in FIG. A bending line appeared near 4.0 V due to the excessive F substitution with X=0.1 composition. This result agrees with the destabilization of the P4332-type structure induced by excessive F substitution and the transition to the Fd-3m structure predicted from the results in FIG. 1 above. Moreover, the discharge capacities of all of them almost achieved the theoretical capacities.

FIG. 3 shows the discharge capacity of a representative sample of the example after 200 cycles of charge/discharge test at a current density of 150 mAh/g. It was found that the coulombic efficiency was maintained at 99.5% or more and 97% or more compared to the initial capacity.
Therefore, it can be said that there is almost no deterioration of the electrodes and the electrolyte due to oxidative decomposition of the electrolyte in the high potential region.

Furthermore, in order to clarify the effect of the space group, charge-discharge tests were performed under conditions different from those described above for typical samples having different compositions and space groups from the examples. Specifically, the current density to the positive electrode is set to 30 mAh/g with respect to the weight of the positive electrode active material at 25° C., the battery is charged to 4.8, and then discharged to 2.5 V to measure the discharge capacity. , a charge-discharge test was performed. The results of determining the initial discharge capacity are summarized in FIG.

As shown in FIG. 4, in the cut-off potential range of 4.8V-2.5V, the substitution effects of anions and cations, as well as their space groups, significantly affect the specific capacity and cycling characteristics, with a maximum of 250 mAh/ A reversible capacity of 10 g was obtained. By controlling both the cation and anion spaces, it was found that the number of lithium ion-accommodating sites could be precisely designed.

Furthermore, the two compounds of Examples were repeatedly charged and discharged 50 times under the same cut-off potential range conditions of 4.8 V to 2.5 V as above. FIG. 5 summarizes changes in discharge capacity with respect to the number of charge-discharge cycles. The substitution effect _of anions _and cations and _{their space} groups significantly affect the specific capacity and cycle _{characteristics} . A capacity retention rate was obtained.

In order to understand this result, FIG. 6 summarizes the X-ray diffraction measurement results of the positive electrode taken out from the dismantled battery with the state of charge defined at an arbitrary potential. As a result of comparing samples with a fixed amount of Cu substitution and different amounts of F substitution, a diffraction line derived from a tetragonal crystal appeared around 2.7 V regardless of the amount of substitution. Furthermore, splitting of the diffraction line attributed to the (111) plane near 2θ=18° was detected. However, the appearance of diffraction lines derived from the tetragonal crystal and the degree of splitting of diffraction lines attributed to the (111) plane were different, and the degree was smaller in the F _0.1 crystal than in the F _0.05 crystal. From this, it was found that the phase transition to a tetragonal crystal accompanying the insertion of excess lithium ions can be suppressed.

Furthermore, the results of FIG. 6 were analyzed. The diffraction lines include diffraction lines attributed to both cubic and tetragonal crystals, and FIG. 7 summarizes the results of changes in lattice constant with respect to potential. It can be seen that the lattice constant of the cubic phase does not change even at 3 V or less. On the other hand, the appearance of the tetragonal phase changed greatly depending on the amount of F substitution. It was found that the F _0.1 crystal has a smaller lattice constant change of the tetragonal crystal than the F _0.05 crystal.

In addition, the expansion of the lattice that occurs when excess lithium ions are accommodated in the spinel lattice was predicted from first-principles DFT calculations for each of the a, b, and c orientations. As a result, it was found that the Jahn-Teller strain directions associated with the generation of trivalent manganese ions can be dispersed in three directions by precisely designing both the anion and cation spaces. Thus, it can be said that the cubic crystal region could be expanded without undergoing a phase transition to a tetragonal crystal even if excess lithium ions were accommodated in the lattice.
<Example 3>
-Crystal Structure of Copper-Fluorine-Cosubstituted Lithium Composite Oxide The crystal structure of the copper-fluorine-co-substituted lithium composite oxide produced by the above-described method was examined by SEM and TEM.
FIG. 8A is a scanning electron micrograph of a lithium composite oxide whose general formula representing the chemical composition is represented by formula (1)-1 described above.
The lithium composite oxide sample used for the measurement is LiNi ^0.5 Mn ^1.49 Cu ^0.01 O ^3.95 F ^0.05 .
From FIG. 8A, it can be seen that the particles of the lithium composite oxide are substantially octahedral.
FIG. 8B is a transmission electron micrograph of the sample.
FIG. 8B shows that the lithium composite oxide particles have a perfect crystal structure. A (111) crystal face and a (100) crystal face appeared in the TEM image.
FIG. 8C shows an example of the three-dimensional structure of crystal grains. The crystal grains in the illustrated example had a substantially regular octahedral shape with inclined planes of {111} planes and truncated planes of {100} planes.

INDUSTRIAL APPLICABILITY The present invention can provide a lithium composite oxide having a capacity of 250 mAh/g or more, a positive electrode active material for a secondary battery using the same, and a secondary battery.

Claims (4)

A lithium composite oxide having a spinel structure,
LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x (0.025≦x≦0.1, 0.01≦y≦0.05),
The lithium composite oxide has an octahedral crystal structure with the (111) plane as the main plane,
at least one vertex of the octahedral crystal structure comprises a truncated surface, at least one edge of the octahedral crystal structure comprises a truncated surface, and at least one inclined surface of the octahedral crystal structure comprises a step- A lithium composite oxide having a terrace structure . A positive electrode active material for a secondary battery, comprising the lithium composite oxide according to claim 1 . A secondary battery comprising the positive electrode active material for a secondary battery according to claim 2 . LiNi0.5Mn1.49Cu0.01O3.9F0.1 , LiNi0.5Mn1.49Cu0.01O3.95F0.05 _{, LiNi0.5Mn1.49 _} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} Cu0.01O3.975F0.025 _{and LiNi0.5Mn1.45Cu0.05O3.95F0.05} . _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _{_} _ _ The described lithium composite oxide.

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