WO2019188751A1 - Lithium composite oxide, positive electrode active material for secondary battery, and secondary battery - Google Patents

Abstract

The purpose of the present invention is to provide: a lithium composite oxide having a reversible capacity of approximately 250 mAh/g; a positive electrode active material for secondary batteries; and a secondary battery that includes the lithium composite oxide. The present invention relates to a lithium composite oxide having a spinel structure and characterized by having a chemical composition represented by general formula (1). General formula (1): Li1+zNiaMn2-yMbCuyO4-xFx [In general formula (1), 0<a≤0.6, 0≤b≤0.2, 0≤y≤1, 0≤x≤1, and 0≤z≤1.0, where cases in which x and y are both 0 are excluded; and M is one or more species of metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu and Sn.]

Description

Lithium composite oxide, positive electrode active material for secondary battery, and secondary battery

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

The lithium composite oxide is used as a positive electrode active material for lithium secondary batteries. Lithium secondary batteries have already been put into practical use as small power sources for cellular phones and notebook computers. Furthermore, application is also attempted in medium-sized or large-sized power sources such as automobile applications and power storage applications. Thus, 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 manganate and its derivatives are attractive materials as 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 manganate.
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 the additive was reduced and a dense lithium composite oxide film with few voids was manufactured. Has been.

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 cutoff potential of 3.5 V (vs. Li ⁺ / Li). .

JP 2014-35909 A

Mariena Mancini et al, A High-Voltage and High-Capacity Li1 + xNi0.5Mn1.5O4 Cathode Material: From Synthesis To Full Lithium-IonChem 19-C18

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

The present invention has been made in view of the above circumstances, and an object thereof 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].
The first aspect of the present invention provides the lithium composite oxide described in [1].
[1] A lithium composite oxide having a spinel structure, wherein a general formula indicating a chemical composition is represented by the following formula (1).
Li _{1 + z} Ni _a Mn _{2-y Mb} Cu _y O _4-x F _x (1)
[In the general formula (1), 0 <a ≦ 0.6, 0 ≦ b ≦ 0.2, 0 ≦ y ≦ 1, 0 ≦ x ≦ 1, 0 ≦ z ≦ 1.0, where x and Except when both y are 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 of the first aspect preferably includes the following features.
[2] The lithium composite oxide has an octahedral crystal structure having a (111) plane as a main surface, and at least one vertex of the octahedral crystal structure includes a truncated surface. Lithium composite oxide.
[3] The lithium composite oxide has an octahedral crystal structure having a (111) plane as a main surface, and at least one ridge of the octahedral crystal structure includes a cut surface. [1] or [2 ] Lithium complex oxide as described in.
[4] The lithium composite oxide has an octahedral crystal structure having a (111) plane as a main surface, and at least one inclined surface of the octahedral crystal structure has a step-and-terrace structure. [1] The lithium composite oxide according to any one of [3] to [3].
[5] The lithium composite oxide according to any one of [1] to [4], wherein 0 <y ≦ 1 in the 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).
The second aspect of the present invention provides the following positive electrode active material for a secondary battery.
[8] A positive electrode active material for a secondary battery comprising the lithium composite oxide according to any one of [1] to [7].
The third aspect of the present invention provides the following secondary battery.
[9] A secondary battery comprising the secondary battery positive electrode active material 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 is 0.025 ≦ x ≦ 0.1.
[11] LiNi _0.5 Mn _1.49 Cu _0.01 O _4.0 , LiNi _0.5 Mn _1.45 Cu _0.05 O _4.0, LiNi _0.5 Mn _1.49 Cu _0.01 O _3.9 F _0.1 , LiNi _0.5 Mn _1.49 Cu _0.01 O _3.95 F _0.05, LiNi _0.5 Mn _1.49 Cu _0.01 O _3.975 F _0.025, And LiNi _0.5 Mn _1.45 Cu _0.05 O _3.95 F _0.05 , and the lithium composite according to any one of [1] to [4] Oxides.

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.

Produced in _example, of _{LiNi 0.5 Mn 1.5-y Cu y} O 4-x F x crystals is a diagram showing a Raman spectrum of each composition. It is a figure which shows a charging / discharging curve about the said crystal manufactured in the Example. It is a figure which shows the result of discharge capacity about the said crystal manufactured in the Example. It is a figure which shows initial stage discharge capacity about the said crystal manufactured in the Example. It is a figure which shows the change of the discharge capacity with respect to the frequency of charging / discharging cycles about the said crystal | crystallization manufactured in the Example. It is a figure which shows the result of a X-ray-diffraction measurement about two types of crystals manufactured in the Example. It is a figure which shows the lattice constant change with respect to the electric potential of the positive electrode which disassembled and took out the battery used in the Example. It is a scanning electron micrograph about the lithium complex oxide obtained in the Example. It is a transmission electron micrograph about the lithium complex oxide obtained in the Example. It is a schematic diagram of the three-dimensional structure of crystal particles of a substantially regular octahedral lithium composite oxide.

Below, the preferable example for implementing this invention is demonstrated in detail. The following description is given specifically for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Within the scope of the present invention, the number, quantity, material, shape, position, type, and the like can be changed, added, omitted, and / or exchanged as necessary unless otherwise limited.
<Lithium composite oxide>
Depending on the synthesis conditions, the LNMO type lithium composite oxide has a nickel / manganese ordered arrangement type (hereinafter sometimes referred to as “P4332 type”) or a nickel / manganese irregular arrangement type (hereinafter referred to as “Fd-3m”). A crystal having a space group of “type” may be generated.
The P4332 type lithium composite oxide has an advantage that it has a large capacity when used as a lithium secondary battery and can be a long-life battery. On the other hand, there is a disadvantage that electron conductivity is low.
The Fd-3m type lithium composite oxide has an advantage in that the electron conductivity can be increased when used as a lithium secondary battery. On the other hand, there is a disadvantage that it is not suitable for a battery having a small capacity and a long life.

When manufacturing lithium composite oxides useful for lithium secondary batteries with high capacity and excellent cycle characteristics, both the high capacity characteristics that are the advantages of the P4332 type and the high electron conductivity characteristics that are the advantages of the Fd-3m type Preferably it can be done.

The lithium composite oxide of the present invention is characterized in that the general formula indicating the chemical composition is represented by the formula (1). The lithium composite oxide of the present invention preferably has a spinel crystal structure. A spinel structure is one type of crystal structure belonging to a cubic system. The lithium composite oxide of this embodiment performs either one or both of control of the cation space by Cu ion substitution and control of the anion space by F ion substitution. Thereby, the ordered arrangement of nickel / manganese is stabilized, and the crystal structure can be suppressed from phase transition from cubic to tetragonal. As a result, the lithium ion storage site increases 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, in which the general formula indicating the chemical composition is represented by the following formula (1).
Li _{1 + z} Ni _a Mn _{2-y Mb} Cu _y O _4-x F _x (1)
[In the general formula (1), 0 <a ≦ 0.6, 0 ≦ b ≦ 0.2, 0 ≦ y ≦ 1, 0 ≦ x ≦ 1, 0 ≦ z ≦ 1.0, where x and Except when both y are 0; M is one or more metal elements selected from the group consisting of Ti, V, Cr, Fe, Co, Zu, and Sn. ]

The first embodiment is an embodiment that performs either one or both of control of cation space by Cu ion substitution and control of anion space by F ion substitution.
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. It may be.
The lithium composite oxide of the present embodiment preferably has an octahedral crystal structure whose main surface corresponds to the (111) plane.
The octahedral crystal structure may be a regular octahedral crystal structure or an octahedral crystal structure having strain.
In this embodiment, it is preferable that at least one vertex of the octahedral crystal structure has a truncated surface.
The vertex having a truncated surface is preferably 2 or more, and more preferably 3 or more. Moreover, all the vertices of the octahedral crystal structure may have a truncated surface, may be 7 or less, and may be 6 or less.
The truncated surface means a surface formed by cutting away the vertex of a polyhedron.
In this embodiment, it is preferable that at least one ridge of the octahedral crystal structure has a cut ridge surface.
The ridge having a cut ridge surface is preferably 2 or more, and more preferably 3 or more. Moreover, all the ridges of the octahedral crystal structure may have a cut ridge surface, may be 11 or less, and may be 10 or less.
The cut ridge surface means a surface formed by cutting a polyhedral ridge.
In the present embodiment, it is preferable that at least one inclined surface of the octahedral crystal structure has a step-and-terrace structure.
Here, the “step terrace structure” means a plurality of flat terraces arranged so as to be parallel to each other through a step along the main surface of the crystal, and a step that is a step sub that connects the terraces. This means a micro stepped structure.
In this embodiment, 2 or more are preferable and 3 or more are more preferable as an inclined surface provided with a step terrace structure. Moreover, all the inclined planes of the octahedral crystal structure may have a step / terrace structure, or may be 7 or less, or 6 or less.
In the crystal structure of the lithium composite 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. It is preferable.
The crystal structure of the lithium composite metal oxide of the present embodiment is such that at least one ridge of the octahedral crystal structure has a cut edge surface, and at least one inclined surface of the octahedral crystal structure has a step-and-terrace structure. It is preferable.
In the crystal structure of the lithium composite metal oxide according to the present embodiment, it is preferable that at least one vertex of the octahedral crystal structure has a truncated surface, and at least one ridge of the planar crystal structure has a truncated surface.
In the crystal structure of the lithium composite metal oxide of the present embodiment, at least one vertex of the octahedral crystal structure has a truncated surface, and at least one edge of the planar crystal structure has a truncated surface, and It is preferable that at least one inclined surface of the plane crystal structure has a step-and-terrace structure. The crystal structure of the lithium composite oxide of this embodiment can be confirmed by observing it using a scanning electron microscope or a transmission electron microscope.
The crystal structure of the lithium composite oxide of the present embodiment has a crystal structure having a truncated face appearing in an octahedron having a (111) plane, and a crystal structure having a ridge face appearing in an octahedron having a (111) face Or a structure having a step-and-terrace structure on at least one inclined surface of the octahedral crystal structure, a lithium ion desorption reaction during charging of a lithium secondary battery, and a lithium ion occlusion reaction during discharge Is considered to occur stably.

The second to fourth embodiments, which are further preferred examples of the first embodiment, will be described below.
<< Second Embodiment >>
The present embodiment is a lithium composite oxide having a spinel structure, in which the general formula indicating the chemical composition is represented by the following formula (1) -1.
Li _{1 + z} Ni _a Mn _{2-y Mb} 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. ]

In the second embodiment, the cation space is controlled by Cu ion substitution.
In the above formula (1) -1, the preferred range of each symbol is the same as that described 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 site is 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, in which the general formula indicating the chemical composition is represented by the following formula (1) -2.
Li _{1 + z} Ni _a Mn _{2-y Mb} 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. ]

In the third embodiment, the anion space is controlled by F ion substitution.
In the above formula (1) -2, the preferred range of each symbol is the same as that described 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 substitution. Further, by controlling the anion space, the lithium ion conduction path is changed, the lithium ion storage site is 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, in which the general formula indicating the chemical composition is 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, and 0 ≦ z ≦ 1.0. ]

The fourth embodiment is an embodiment that performs both control of the cation space by Cu ion substitution and control of the anion space by F ion substitution.
In the above formula (1) -3, the preferred range of each symbol is the same as that described in the above formula (1).
According to the present embodiment, the phase transition to tetragonal crystal can be suppressed by controlling both the cation and anion spaces and precisely designing the site containing 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 represented by the formulas (1), (1) -1 to (1). ) -3 to obtain a lithium mixture by mixing so as to obtain a composition ratio represented by −3, and a firing step of firing the lithium mixture obtained in the mixing step.

[Mixing process]
In the mixing step, a lithium compound, a precursor containing at least manganese and nickel, a precursor containing copper as necessary, 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 Ladige mixer, or the like can be used, and it is sufficient that they are sufficiently mixed so that no fine powder is generated.

-Lithium compound The lithium compound used by this embodiment can be selected as needed. Preferred examples include one or more anhydrides selected from the group consisting of lithium hydroxide, lithium chloride, lithium nitrate and lithium carbonate, and the one or more hydrates.
-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 included in different compounds. As a preferred example, the precursor containing manganese can include at least one of manganese nitrate hexahydrate, manganese sulfate pentahydrate, and manganese chloride tetrahydrate. Examples of the precursor containing nickel include at least one of nickel nitrate hexahydrate, nickel sulfate pentahydrate, and nickel chloride tetrahydrate. Precursors containing both manganese and nickel may be used. In addition, this invention is not limited only to the said example.
-Precursor containing copper The precursor containing copper which can be used by this embodiment can be selected as needed. Examples of the precursor containing copper include copper nitrate trihydrate, copper sulfate trihydrate, and copper chloride dihydrate.

・ Reaction accelerator (flux)
The reaction accelerator (flux) used in the present embodiment can be selected as necessary. _{Specifically, NaCl, KCl, RbCl, CsCl} , CaC1 2, MgCl 2, SrCl 2, LiCl, BaCl chlorides such as ₂ and _{NH 4 Cl, Na 2 CO 3} , K 2 CO 3, Rb 2 CO 3, Examples include carbonates such as Cs ₂ CO ₃ , CaCO ₃ , MgCO ₃ , SrCO _3, and BaCO ₃ , sulfates such as K ₂ SO ₄ and Na ₂ SO ₄ , fluorides such as NaF, KF, and NH ₄ F. It is done.
Among these, KCl, K ₂ CO ₃ and K ₂ SO ₄ are preferable. Two or more reaction accelerators can be used in combination. When using 2 or more types, the ratio can be selected arbitrarily.
The method, conditions, amount and timing of incorporating the reaction accelerator into the mixture are not particularly limited and can be selected as necessary. For example, a reaction accelerator may be added when a precursor containing at least manganese and nickel is mixed with a lithium compound.
The reaction accelerator may remain in the fired lithium composite oxide, or may be removed by washing, evaporation, or the like.

[Baking process]
In the firing step, a mixture of the precursor, lithium compound, and reaction accelerator, which are raw materials, is fired to obtain a bulk of lithium composite oxide that is a fired product.
Although the holding temperature in a baking process can be selected arbitrarily, as an example, it is mentioned that it is the range of 650 degreeC or more and 900 degrees C or less, for example. More preferably, it is 700 degreeC or more and 850 degrees C or less.
The holding time at the holding temperature can be arbitrarily selected, and 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, and is, for example, 50 ° C. to 400 ° C./hour.
The rate of temperature decrease 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 such as an inert gas, a neutral gas, or an oxidizing gas can be used as necessary, but an air atmosphere is preferable.
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 further subjected to a second baking step as necessary.
In the second firing step, the lithium composite oxide can be fired by mixing with one or both of a compound containing fluorine and a compound containing copper. At this time, it is also preferable to use a combination of the above fluxes. The compound containing fluorine can be arbitrarily 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 baking can be arbitrarily selected, and as an example, it is 300 ° C. or higher and 1000 ° C. or lower, 400 ° C. or higher and 900 ° C. or lower, and 500 ° C. or higher and 800 ° C. or lower.
The time for holding at the holding temperature can be arbitrarily selected, and is, for example, 0.5 hours to 20 hours, 2 hours to 15 hours.
The temperature rising rate up to the holding temperature can be arbitrarily selected, but the temperature rising rate is usually preferably 50 ° C./hour or more and 400 ° C./hour or less, and the temperature lowering rate from the holding temperature to room temperature is usually 10 ° C. It is preferably at least 400 ° C./hour. As the firing atmosphere, air, oxygen, nitrogen, argon, or a mixed gas thereof can be used. That is, an atmosphere such as an inert gas, a neutral gas, or an oxidizing gas can be used. You may change the atmosphere on the way. For example, baking may be performed in an air atmosphere, and then baking may be performed in an oxygen atmosphere.
The obtained mass of lithium composite oxide is crushed by a pulverizer such as a roll mill as necessary, washed to remove residual lithium components and reaction accelerators, and dried.
The dry powder is crushed by a roll mill or the like as necessary. Here, crushing refers to dispersing or unraveling the agglomerated 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⁺ becomes Cu ²⁺ or Cu ³⁺ . A lithium composite oxide substituted and doped with copper 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 compound containing fluorine, a lithium composite oxide in which some of the oxygen atoms are substituted with fluorine atoms can be obtained.

The obtained mass of lithium composite oxide is crushed by a pulverizer such as a roll mill as necessary, washed to remove residual lithium components and reaction accelerators, and dried.
The dry powder is crushed by a roll mill or the like as necessary. Here, crushing refers to dispersing or unraveling the agglomerated 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, 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 particle size of the primary particles can be obtained from an image obtained by a scanning electron microscope. Although the shape of the obtained lithium composite oxide can be arbitrarily selected, for example, it may be an octahedral shape, a substantially octahedral shape, or an octahedral shape or a substantially octahedral shape having a truncated face and / or a ridge face. It's okay. These oxides may be porous.

<Positive electrode active material for secondary battery>
The positive electrode for secondary battery of this embodiment is not limited except having the above-described lithium composite oxide of the present invention.
The positive electrode of this embodiment can be manufactured by first adjusting the positive electrode mixture containing the lithium composite oxide, the conductive material and the binder of the present invention.

(Conductive material)
As the conductive material included in the positive electrode of the present embodiment, a carbon material can be used. Examples of the carbon material include graphite powder, carbon black (for example, acetylene black), and a fibrous carbon material. Since carbon black is fine and has a large surface area, the addition of a small amount to the positive electrode mixture can increase the conductivity inside the positive electrode and improve the charge / discharge efficiency and output characteristics. However, if too much is added, both the binding force between the positive electrode mixture and the positive electrode current collector by the binder and the binding force inside the positive electrode mixture are lowered, which causes an increase in internal resistance. Therefore, it is preferably used in an appropriate amount.

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

When the positive electrode mixture is made into a paste, usable organic solvents include amine solvents such as N, N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; methyl acetate And 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 that the above-described lithium composite oxide of the present invention is used.
That is, the lithium secondary battery of the present invention can have the same configuration as a conventionally known lithium secondary battery except that it has the above-described positive electrode for a lithium secondary battery. 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 negative electrode active material, compounds capable of inserting and extracting lithium ions can be used alone 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., graphite, coke, organic polymer compound fired bodies, or carbon materials such as amorphous carbon In addition, ceramic materials such as silicon oxide, LiCoN, CuO, and V ₂ O ₅ can be used.
These active materials can be used not only alone but also as a mixture of two or more thereof. Of these substances, it is preferable to use a titanium-containing oxide (for example, TiO ₂ (B) which is bronze-structured titanium oxide, Li ₄ Ti ₅ O ₁₂ which is lithium titanate) 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 pressure bonding the lithium foil to 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 additive, etc. are mixed in a solvent such as water or N-methylpyrrolidone, and then made of a metal such as copper. It can be applied and formed on a current collector. The binder is preferably formed of a polymer material, and is preferably a material that is chemically and physically stable in the atmosphere in 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), Examples thereof include fluororubber.
Examples of the conductive assistant include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, carbon fiber, graphene, and amorphous carbon. Further, conductive polymer polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene and the like can be exemplified.

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

(Electrolyte)
For example, the electrolytic solution can be arbitrarily selected, and 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 ) ( An electrolytic solution in which at least one selected from C ₄ F ₉ SO ₂ ) is used as a supporting electrolyte and dissolved in an organic solvent is preferable.

The organic solvent can be arbitrarily selected, and examples thereof include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and mixtures thereof. Among them, an electrolytic solution containing a carbonate solvent is preferable because of its high stability at high temperatures. Further, 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 between the positive electrode and the negative electrode, which is a member that achieves both electrical insulation and ion conduction. When the electrolyte is liquid, the separator also serves to hold the liquid electrolyte. Examples of the separator include porous synthetic resin films, particularly porous films made of polyolefin polymers (polyethylene, polypropylene) and glass fibers, and nonwoven fabrics. Furthermore, it is preferable that the separator has a larger size than the positive electrode and the negative electrode for the purpose of ensuring the insulation between the positive electrode and the negative electrode.

The positive electrode, negative electrode, electrolyte, separator, etc. are generally housed in some case. The case is not particularly limited and can be made of a known material and form. That is, the lithium secondary battery of the present invention is not particularly limited in its shape, and can be used as batteries having various shapes such as a coin shape, a cylindrical shape, and a square shape. Further, the case of the lithium secondary battery of the present invention is not limited, and can be used as various types of batteries such as a metal or resin case that can retain its outer shape, a soft case such as a laminate pack, and the like. .

Hereinafter, the present invention will be described more specifically by way of examples. However, 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 using the LiCl—KCl flux by the following method.

-Manufacture and evaluation of copper and fluorine co-substituted lithium composite oxide First, an oxygen-deficient copper-substituted lithium composite oxide was manufactured as follows.
Lithium chloride as the lithium source of the lithium composite oxide, nickel nitrate hexahydrate as the nickel source, manganese nitrate hexahydrate as the manganese source, copper nitrate trihydrate as the copper source, Li: Ni: Mn : Cu was mixed so that the molar ratio was 1.0: 0.50: 1.49: 0.01.

As the flux, a mixed solution of lithium chloride and potassium chloride was used. The amount of flux was about 5 times by weight with respect to the solute. These were put into an alumina crucible. Place the crucible in an electric furnace and heat in an air atmosphere under the conditions of heating temperature: 15 ° C./min, 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 warm 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 oxygen-deficient copper-substituted lithium composite oxide of LiNi _0.5 Mn _1.49 Cu _0.01 O _4-δ obtained above and potassium chloride and lithium fluoride (mixing ratio = molar ratio) And 1: 1) were fired at 800 ° C. for 10 hours in an air atmosphere. The molar ratio of the oxygen-deficient copper-substituted lithium composite oxide and the mixture was 1: 1.
Further, an oxygen atmosphere, and calcined at 700 ° C. 10 hours, in the first embodiment, as shown by way of _{LiNi 0.5 Mn 1.49 Cu 0.01 O 4} -a F a (0 ≦ a ≦ 1), Copper and fluorine co-substituted lithium composite oxides were obtained. The crystal phase was identified by XRD method, and the crystal morphology was evaluated by FE-SEM. In addition, space groups were identified by Raman spectroscopy. The results are shown in FIG. In addition, for the sake of comparison, the conditions for the amount of the lithium fluoride mixture used in the production were changed to produce those having different F ratios. For comparison, a compound not containing Cu and Fe was also produced and evaluated.

<Example 2>
Production and evaluation of fluorine-substituted lithium composite oxide Lithium chloride as the lithium source of the lithium composite oxide, nickel nitrate hexahydrate as the nickel source, manganese nitrate hexahydrate as the manganese source, Li: Ni: Mn LiNi _0.5 Mn _1.45 Cu _0.05 O _{4-a Fa} (by a method similar to that in Example 1 except that the molar ratio was 1.0: 0.50: 1.5. A fluorine-substituted lithium composite oxide of 0 ≦ a ≦ 1) was obtained. The crystal phase was identified by XRD method, and the crystal morphology was evaluated by FE-SEM. In addition, space groups were identified by Raman spectroscopy. The results are shown in FIG. In Example 2, a copper source having a different ratio was produced by changing the condition of the amount of the copper source used in the production.

・ Manufacture of secondary battery The obtained active material, Denka black and PVDF were mixed at a mass ratio of 90: 5: 5%, and a porous mixture electrode having a tap density adjusted to 3.0 g / cm ³ was collected from aluminum. It was prepared on an electric foil.
An R2032 type coin cell was manufactured using EC-DMC containing lithium metal as a counter electrode, Cellguard # 2400 as a separator, and 1M LiPF ₆ as an electrolyte.

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

-Electronic state analysis Structural optimization and electronic state analysis were performed by DFT calculation. These were carried out using the calculation software Vienna Ab-initio Simulation Package (VASP).
As calculation conditions, PAW method and GGA-PBEsol + U (Ni: U = 6.0 eV [1], Mn: U = 3.9 eV [1]) were used.
Phase stability of LiNi _0.5 Mn _1.5-y Cu _y O _4-x F _x co-substituted with Cu ²⁺ and F ⁻ using 8LiNi _0.5 Mn _1.5 O ₄ supercell containing 56 atoms The desorption mechanism was computationally predicted from the evaluation of sex and the calculation of the formation energy and the electronic state analysis of analogs with different Li compositions.

FIG. 1 shows a 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 X = 0.1 composition, that is, with LiNi _0.5 Mn _1.49 Cu _0.01 O _3.9 F _0.1 .
This means a transition to the Fd-3m structure due to destabilization of the P4332 type structure.
On the other hand, the amount of F substitution such as 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 is small. In 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 in the co-substitution, the P4332 type structure is stabilized within an extremely narrow composition range even though it contains oxygen deficiency.

In the evaluation using the coin cell, after the open circuit voltage is stabilized, the current density with respect to the positive electrode is charged at 25 ° C. until the current density becomes 30 mAh / g to 4.8, and then becomes 3.5V. Discharged until. The charge / discharge test which measures the discharge capacity at this time is performed, and the obtained charge / discharge curve is shown in FIG. A bending line appeared in the vicinity of 4.0 V due to excessive F substitution of X = 0.1 composition. This result agrees with the result of destabilization of the P4332 type structure induced by excess F substitution and transition to the Fd-3m structure, which is predicted from the result of FIG. In both cases, the discharge capacity generally achieved the theoretical capacity.

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

Further, in order to clarify the effect of the space group, a charge / discharge test was performed on representative samples having different compositions and space groups in the examples under conditions different from the above. Specifically, at 25 ° C., the current density with respect to the positive electrode is set to 30 mAh / g with respect to the weight of the positive electrode active material, and the battery is charged until 4.8 and then discharged until 2.5 V is measured. A charge / discharge test was conducted. 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 effect of anions and cations and the space group significantly affect the specific capacity and cycle characteristics, with a maximum of 250 mAh / A reversible capacity of g was obtained. It was found that the number of sites that store lithium ions can be precisely designed by controlling both the cation and anion spaces.

Furthermore, 50 charge / discharge tests were repeated for the two compounds of the Examples under the same cut-off potential range condition of 4.8V-2.5V as described above. The change of the discharge capacity with respect to the number of charge / discharge cycles is summarized in FIG. The anion and cation substitution effects, and its space group, have a significant effect on the specific capacity and cycle characteristics, and are 90% or more of LiNi _0.5 Mn _1.49 Cu _0.01 O _3.9 F _0.1 crystal. A capacity retention rate was obtained.

In order to understand this result, the X-ray diffraction measurement results of the positive electrode, which was defined by charging at an arbitrary potential and disassembled and taken out of the battery, are summarized in FIG. As a result of comparing samples in which the Cu substitution amount was fixed and only the F substitution amount was different, a tetragonal diffraction line appeared in the vicinity of 2.7 V without depending on the substitution amount. Furthermore, the splitting of diffraction lines attributed to the (111) plane near 2θ = 18 ° was detected. However, the appearance of diffraction lines derived from these tetragonal crystals and the degree of splitting of the diffraction lines attributed to the (111) plane were different, and the F _0.1 crystal was smaller than the F _0.05 crystal. From this, it was found that the phase transition to tetragonal crystal accompanying the insertion of excess lithium ions can be suppressed.

Furthermore, the result of FIG. 6 was analyzed. The diffraction lines include diffraction lines belonging to both cubic and tetragonal crystals, and the results of summarizing changes in the lattice constant with respect to the potential are summarized in FIG. It can be seen that the lattice constant does not change even when the cubic phase is 3 V or less. On the other hand, the tetragonal phase changed greatly depending on the F substitution amount. The F _0.1 crystals was found that the lattice constant variation of tetragonal than _{F 0.05} crystal is reduced.

Further, the expansion of the lattice that occurs when excess lithium ions are stored in the spinel lattice was predicted from the first-principles DFT calculation for each of the a, b, and c directions. As a result, it was found that the Yarn-Teller strain directions accompanying the generation of trivalent manganese ions are dispersed in three directions by precisely designing both the anion and cation spaces. As a result, even when excess lithium ions were stored in the lattice, it could be said that the cubic region could be expanded without phase transition to tetragonal crystal.
<Example 3>
-Crystal structure of copper and fluorine co-substituted lithium composite oxide The crystal structure of the copper and fluorine co-substituted lithium composite oxide prepared by the method described above was examined by SEM and TEM.
FIG. 8A is a scanning electron micrograph of the lithium composite oxide represented by the above-described formula (1) -1 as a general formula indicating the chemical composition.
The sample of the lithium composite oxide used for the measurement is LiNi ^0.5 Mn ^1.49 Cu ^0.01 O ^3.95 F ^0.05 .
It can be seen from FIG. 8A that the lithium composite oxide particles have a substantially regular octahedral shape.
FIG. 8B is a transmission electron micrograph of the sample.
FIG. 8B shows that the lithium composite oxide particles have a complete crystal structure. The (111) crystal plane and the (100) crystal plane appeared in the TEM image.
FIG. 8C shows an example of a three-dimensional structure of crystal grains. The crystal grains in the illustrated example have a substantially regular octahedral shape including an inclined surface composed of {111} planes and a truncated top surface of {100} planes.

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 (11)

A lithium composite oxide having a spinel structure, wherein a general formula indicating a chemical composition is represented by the following formula (1).
Li _{1 + z} Ni _a Mn _{2-y Mb} Cu _y O _4-x F _x (1)
[In the general formula (1), 0 <a ≦ 0.6, 0 ≦ b ≦ 0.2, 0 ≦ y ≦ 1, 0 ≦ x ≦ 1, 0 ≦ z ≦ 1.0, where x and Except when both y are 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 has an octahedral crystal structure with a (111) plane as a main surface,
The lithium composite oxide according to claim 1, wherein at least one vertex of the octahedral crystal structure includes a truncated surface. The lithium composite oxide has an octahedral crystal structure with a (111) plane as a main surface,
3. The lithium composite oxide according to claim 1, wherein at least one edge of the octahedral crystal structure includes a cut edge surface. 4. The lithium composite oxide has an octahedral crystal structure with a (111) plane as a main surface,
The lithium composite oxide according to any one of claims 1 to 3, wherein at least one inclined surface of the octahedral crystal structure has a step-and-terrace structure. The lithium composite oxide according to any one of claims 1 to 4, wherein 0 <y ≦ 1 in the formula (1). The lithium composite oxide according to any one of claims 1 to 4, wherein 0 <x ≦ 1 in the formula (1). The lithium composite oxide according to any one of claims 1 to 4, wherein 0 <y ≦ 1 and 0 <x ≦ 1 in the formula (1). A positive electrode active material for a secondary battery comprising the lithium composite oxide according to any one of claims 1 to 7. A secondary battery comprising the positive electrode active material for a secondary battery according to claim 8. 5. The lithium composite oxide according to claim 1, wherein x is 0.025 ≦ x ≦ 0.1. LiNi _0.5 Mn _1.49 Cu _0.01 O _4.0 , LiNi _0.5 Mn _1.45 Cu _0.05 O _4.0, LiNi _0.5 Mn _1.49 Cu _0.01 O _3.9 F _0.1 , LiNi _0.5 Mn _1.49 Cu _0.01 O _3.95 F _0.05, LiNi _0.5 Mn _1.49 Cu _0.01 O _3.975 F _0.025, and LiNi ₀ The lithium composite oxide according to any one of claims 1 to 4, which is at least one compound selected from the group _{consisting of 0.5} Mn _1.45 Cu _0.05 O _3.95 F _0.05 .

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