JP2000149925A - Positive active material for lithium secondary battery, its manufacture, and its use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium manganate base positive active material with high cycle characteristics and long storage characteristics even under high temperature and its use. SOLUTION: This positive active material for a lithium secondary battery contains lithium manganate of cubic system spinel structure as the main component, and has a concentration gradient layer in which the concentration of oxygen atom-substituted fluorine gradually decreases from the surface layer toward the inside of particles of the positive active material, and depth until the difference of fluorine concentration between the surface and the inside of the particle decreases to 10% of the difference between the fluorine concentration on the surface layer side interface and that of the inside of the particle is 0.5-80 nm from the surface layer side interface, atomic ratio F/Mn of the total content of fluorine to the total content of manganese in the positive active material is 0.002-0.05, and its lattice constant is 0.82405 nm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cathode material suitable for a lithium secondary battery, a method for producing the same, and a use thereof.

[0002]

2. Description of the Related Art In recent years, the performance of portable devices such as notebook personal computers and mobile phones has been remarkably advanced.
With the promotion of miniaturization, lightening and high performance, batteries used as power supplies for these devices also need to be small, lightweight, and high performance secondary batteries with little deterioration in charge / discharge cycles. The battery has been put to practical use. In addition, as environmental problems become more serious, research is being conducted on the use of lithium-ion secondary batteries as secondary batteries for electric vehicles and as secondary batteries for storing power for averaging excess power at night. I have. Regarding the positive electrode material, Li which is currently used as a positive electrode material of a commercially available lithium ion secondary battery
Since CoO ₂ uses expensive cobalt as a raw material, LiNiO ₂ or L, which is a lithium-containing transition metal oxide made from inexpensive nickel or manganese as a raw material in order to reduce manufacturing costs, is used.
Attention has been paid to iMn ₂ O ₄ , and its research is being actively promoted.

However, a conventional lithium secondary battery using a lithium-containing transition metal oxide as a positive electrode material is:
There is a problem that the capacity is reduced due to repetition of charge and discharge and the storage characteristics are significantly deteriorated at a high temperature of about 50 ° C. or higher. This is considered to be caused by decomposition of the electrolytic solution and formation of a coating film at the interface between the positive electrode, the negative electrode and the electrolytic solution, and elution of a metal element from the positive electrode material (especially in lithium manganate having a spinel structure). The phenomenon is remarkable).
It is considered that when the metal element elutes from the positive electrode, the structure in the positive electrode material changes, or the contact between the positive electrode material and the conductive agent is damaged, and the capacity decreases. It can be said that such a decrease in the characteristics of the secondary battery at a high temperature must be improved when the battery is used in a notebook personal computer that emits high heat or in an automobile exposed to the sun.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a positive electrode material for a lithium secondary battery from a cubic spinel structure having good cycle characteristics and storage stability even at a high temperature and a high capacity. To provide a lithium manganate-based positive electrode active material and a method for producing the same, wherein the lithium manganate is used as a positive electrode and has good cycle characteristics and storage characteristics even at high temperatures, and has high capacity and high output. Another object is to provide a battery.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have reached the present invention.

The present invention relates to a positive electrode active material for a lithium secondary battery mainly composed of lithium manganate having a cubic spinel structure, wherein oxygen atoms are introduced from the surface of particles of the positive electrode active material toward the inside. There is a concentration gradient layer in which the concentration of substituted fluorine decreases, and the difference in fluorine concentration between the inside of the particle and the concentration of fluorine in the particle decreases to 10% of the difference between the concentration of fluorine in the interface on the surface layer side and the concentration of fluorine in the inside of the particle. Depth has a tilted structure such that it is 0.5 to 80 nm from the surface side interface,
And a lithium secondary battery wherein the atomic ratio F / Mn between the total amount of fluorine and the total amount of manganese in the positive electrode active material is 0.002 to 0.05, and the lattice constant is 0.82405 nm or less. The present invention relates to a positive electrode active material for use.

Further, the present invention relates to a method for preparing a mixture comprising a lithium compound, a manganese compound and LiF from 500 ° C. to 800 ° C.
The method for producing a positive electrode active material for a lithium secondary battery as described above, wherein the unreacted LiF is removed by baking at a temperature of ° C. and then washing with water. Further, the present invention relates to a lithium compound, a manganese compound, B, Mg, Al, P, C
a, Ti, V, Cr, Fe, Co, Ni, Cu, Zn,
A mixture of at least one or more kinds of substitution element compounds selected from Ba, Ga, and Ta and LiF is mixed at 500 ° C to 800
The method for producing a positive electrode active material for a lithium secondary battery as described above, wherein the unreacted LiF is removed by baking at a temperature of ° C. and then washing with water. Furthermore, the present invention relates to a lithium secondary battery using the above-described positive electrode active material for a lithium secondary battery for a positive electrode.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. By using the lithium manganate having a cubic spinel-type crystal structure of the present invention as a positive electrode material, cycle characteristics and storage characteristics of a lithium secondary battery at high temperatures can be improved. This is because (1) the catalytic decomposition of the electrolytic solution by Mn is suppressed by the presence of a layer in which LiF forms a solid solution inside the spinel crystal structure at the interface between the positive electrode active material and the electrolytic solution; 2) When not covered with such a layer, the oxygen in the cathode material is dissolved as water into the electrolyte due to the attack of the acid (H ⁺ ) in the electrolyte on the cathode, and at the same time, Mn for charge compensation. (2+) dissolves into the electrolyte solution, but there is a concentration gradient layer in which the concentration of fluorine substituted for oxygen atoms in the spinel crystal structure decreases from the surface side of the particles of the positive electrode active material to the inside. Then, it is considered that the conduction of Mn ions is suppressed without obstructing the entrance and exit of Li ions, and thereby the elution of Mn is suppressed. However, the Li ion conductivity is not so large in the portion where the fluorine concentration is high, and when the oxygen atom in the spinel type crystal structure is replaced with a fluorine atom, the concentration of Mn ³⁺ decreases due to charge compensation and the concentration in the 4 V region is reduced. Since the electric capacity decreases (charging: Mn ³⁺ → Mn ⁴⁺ ), the thickness of the gradient layer, that is, the difference in fluorine concentration between the inside of the particle and the fluorine concentration in the surface side interface is different from the concentration of fluorine in the inside of the particle. Of 1
Depth to 0% is 80 nm from the surface side interface
It is preferable that the atomic ratio F / Mn between the total amount of fluorine and the total amount of manganese in the positive electrode active material is 0.05 or less. Further, when the thickness of the gradient layer is less than 0.5 nm, the thickness of the gradient layer is too small, or the atomic ratio F / Mn between the total amount of fluorine and the total amount of manganese in the positive electrode active material is 0.002.
If it is less than this, the fluorine concentration is too low and the above effects cannot be expected, which is not preferable. The lattice constant of the cubic spinel-type crystal structure of the positive electrode active material is 0.82405 nm.
Hereinafter, it is particularly preferable to be 0.82405 nm to 0.8192 nm. If the lattice constant exceeds 0.82405 nm and becomes excessively large, the bonding distance between manganese-oxygen or manganese-fluorine inside the spinel-type crystal becomes longer, so that the bonding strength between these atoms decreases, and the lattice constant decreases. Stability decreases. For this reason, when expansion and contraction are repeated by the charge / discharge cycle, the crystal structure collapses, the cycle characteristics deteriorate, and it becomes impossible to withstand repeated charge / discharge. The lattice constant is 0.8192n
If it is excessively smaller than m, the crystal lattice becomes too small, the moving speed of lithium ions decreases, and the charge / discharge characteristics deteriorate when a current having a high energy density is continuously taken out.

In the positive electrode active material for a lithium secondary battery according to the present invention, lithium, manganese, or a part of lattice sites occupied by lithium or manganese in the crystal of lithium manganate having a cubic spinel structure is B, Mg, Al, P. , Ca, T
i, V, Cr, Fe, Co, Ni, Cu, Zn, Ba,
It may be substituted with at least one or more elements selected from Ga and Ta.

The method for producing the cathode material for a lithium secondary battery according to the present invention will be described. As raw materials, a lithium compound serving as a lithium source of lithium manganate, a manganese compound serving as a manganese source and LiF, and a substitution element when substituting a part of a lattice site occupied by lithium or manganese in lithium manganate crystals. Compounds containing this element as a source.

The lithium compound serving as a lithium source is not particularly limited as long as it becomes an oxide during heat treatment, and examples thereof include lithium oxide, lithium carbonate, lithium hydroxide, lithium nitrate, lithium chloride, lithium acetate and lithium oxalate. Can be Is not particularly limited as long as it becomes oxide during the heat treatment on a manganese compound as a source of _{manganese, MnO, Mn 3 O 4,} Mn 2 O 3, MnO 2 manganese oxide such as manganese carbonate, manganese hydroxide, nitrate Manganese, manganese acetate, manganese oxalate and the like can be mentioned. The source of the substitution element (X) is a substitution element B or M
g, Al, P, Ca, Ti, V, Cr, Fe, Co, N
The compound containing i, Cu, Zn, Ba, Ga, and Ta is not particularly limited as long as it becomes an oxide during heat treatment.
Oxides, carbonates, hydroxides, nitrates, acetates, oxalates and the like containing the substitution element (X) can be mentioned.

First, these raw materials are mixed.
The total number of atoms of Li in the lithium compound of the lithium source
m_Li, Mn to m_Mn, The total number of atoms of the substitution element X
m_XThen, the composition of the mixed powder is 0.5 ≦ m_Li/
(M_Mn+ M_X) ≦ 0.7 is preferred. m_Li/
(M_Mn+ M_X) Is too small below 0.5
Indicates that the structure of the spinel becomes unstable,
When it is large, the average oxidation number of Mn becomes high, and Mn becomes³⁺
And the electric capacity in the 4V region is extremely small.
Is not preferred. Also, the amount of LiF in the mixed powder
Represents the total number of moles of LiF in the mixed powder as m_LiFThen
Child ratio m_LiF/ M_MnIs 0.02 ≦ m_LiF/ M_Mn≤2
Is preferred. m _LiF/ M_MnIs excessively smaller than 0.02
In this case, the amount of LiF is too small and the thickness of the gradient layer is thin.
It is not preferable because it becomes too much. Also, m_LiF/ M_MnIs 2
If it is too large, the amount of LiF is too large,
Is too thick, which is not preferred.

[0013] The mixing method is not particularly limited.
Any method can be used as long as it can be uniformly mixed, such as a dry method using a mixer or a ball mill or a wet mixing method using water or ethanol.

In the production of the positive electrode material for a lithium secondary battery according to the present invention, the above mixture is heated at 500 ° C. to 800 ° C. for 1 hour.
Heat treatment for more than an hour. At a temperature excessively higher than 800 ° C., the reaction between LiF and lithium manganate proceeds, and LiF
Will form a solid solution inside the crystal grains of the spinel structure,
This is not preferable because the F concentration is averaged and a gradient layer is not formed. On the other hand, if the temperature is excessively lower than 500 ° C., the growth of lithium manganate crystal particles becomes difficult to promote, and the specific surface area of the positive electrode material increases, so that the F amount per unit area becomes too small, and the F concentration on the particle surface becomes low. It is not preferable because it lowers. In a binary phase diagram of a lithium compound serving as a lithium source and LiF, in a temperature range lower than the eutectic temperature (lithium nitrate; 251 ° C, lithium hydroxide; 431 ° C, lithium carbonate: 609 ° C, lithium chloride; 501 ° C). Since LiF does not go through a molten state, it is difficult to cover the entire surface of LiF, and the reactivity is reduced, and it is difficult to dissolve LiF from the particle surface to the inside, which is not preferable. Therefore, when a lithium compound having a eutectic temperature with LiF higher than 450 ° C. is used as the lithium source, it is preferable to perform the heat treatment at a temperature higher than the eutectic temperature. In the water washing treatment after firing at 500 ° C. to 800 ° C., the solubility of LiF in water is 0.133 wt%.
(25 ° C.), and the unreacted LiF
Can be performed by using more water than the amount by which water is completely washed out, and in some cases, using hot water.
After washing with water, it is preferable to remove water by filtration and dry sufficiently.

Next, the lithium secondary battery of the present invention will be described in detail. The positive electrode of the lithium secondary battery of the present invention,
The positive electrode material for a lithium secondary battery of the present invention described above is included as an active material. The positive electrode is, specifically, the positive electrode active material for a lithium secondary battery, a conductive agent, a binder, as the conductive material, natural graphite, artificial graphite, carbonaceous materials such as cokes, and as a binder Examples thereof include thermoplastic resins such as polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, and polypropylene.

As the negative electrode of the lithium secondary battery of the present invention, a lithium metal, a lithium alloy, or a material capable of inserting and extracting lithium ions is used. Materials capable of occluding and releasing lithium ions include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, and carbon fibers.

As the electrolyte of the lithium secondary battery of the present invention, a known electrolyte selected from a non-aqueous electrolyte solution obtained by dissolving a lithium salt in an organic solvent and a solid electrolyte is used. As the lithium salt, LiCl
O ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃
One or a mixture of two or more of SO _{3 and the} like can be mentioned.

Examples of the organic solvent include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate and diethyl carbonate, ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane and tetrahydrofuran, methyl formate, methyl acetate, γ Esters such as -butyl lactone; nitriles such as acetonitrile and butyronitrile; amides such as N, N-dimethylformamide; and sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone. Are used by mixing two or more of them.

Examples of the solid electrolyte include organic solid electrolytes such as a polyethylene oxide derivative or a polymer containing the derivative, a polypropylene oxide derivative or a polymer containing the derivative, Li ₃ N, LiI, and Li ₃ N—LiI—Li.
Inorganic solid electrolytes such as OH, Li ₄ SiO ₄ and Li ₄ SiO ₄ —Li ₃ PO ₄ are exemplified. Further, a gel in which a non-aqueous electrolyte solution is held in a polymer can also be used. The shape of the lithium secondary battery of the present invention is not particularly limited, and may be any of a paper type, a coin type, a cylindrical type, a square type, and the like.

[0020]

The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the invention thereto. Examples 1 to 15 and Comparative Examples 1 to 4 LiOH · H ₂ O, a MnO ₂ substitution element X source and LiF were mixed well in a mortar at the ratios shown in Table 1 (all ratios are atomic or molar ratios), After further mixing well with a dry vibration mill,
The powder was fired in the air at the firing temperature shown in Table 1, washed with water to remove unreacted LiF, filtered, and dried to obtain a positive electrode active material powder. FIG. 1 shows the result of X-ray diffraction measurement of the positive electrode material obtained in Example 1. The obtained positive electrode active material was prepared according to JCPDS: No. L of 35-782
It was found to be a single phase having a spinel type crystal structure showing a pattern similar to that of iMn ₂ O ₄ . The positive electrode active materials of the other Examples and Comparative Examples had the same pattern. Table 2 shows the lattice constants of the spinel-type crystal structures of the positive electrode active materials of Examples and Comparative Examples.

[0021]

[Table 1] (* In Table 1, all ratios are atomic or molar.)

[0022]

[Table 2]

"Evaluation of the Thickness of the F Concentration Gradient Layer on the Surface of the Positive Electrode Active Material and Evaluation of the F Content in the Positive Electrode Active Material" of the positive electrode active material powders obtained in Examples 1 to 15 and Comparative Examples 1 to 4 On the particle surface, the concentration distribution in the depth direction of fluorine ions is examined by TOF-SIMS while sputtering an arbitrary position on the particle surface with an ion beam, and the depth at which the F ion intensity becomes 10% of the intensity at the surface layer ( The thickness of the inclined layer)
I asked. Table 3 shows the results. Note that the thickness is a thickness in terms of single crystal Si. Further, after the positive electrode active material powders obtained in Examples 1 to 15 and Comparative Examples 1 to 4 were wet-decomposed, the content of Mn was measured by ICP, and the content of F was measured by ion chromatography, and the atomic ratio F was measured. / Mn was determined. Table 3 shows the results.

[0024]

[Table 3]

[Evaluation of Charging and Discharging Characteristics] Using the positive electrode materials obtained in Examples 1 to 15 and Comparative Examples 1 to 4, a coin-type test cell was constructed as follows and charged at a high temperature (60 ° C.). The discharge characteristics were measured. 80 parts by weight of the positive electrode active material powder, 5 parts by weight of acetylene black and graphite as conductive agents and 10 parts by weight of polyvinylidene fluoride as a fluoropolymer binder were uniformly mixed in a 1-methyl-2-pyrrolidone solvent. The resultant was applied on a current collector of aluminum foil, 1-methyl-2-pyrrolidone was sufficiently dried, molded under pressure, and heat-treated to produce a disk-shaped positive electrode of about 2 cm ² . At this time, the amount of the active material finally contained in the positive electrode was adjusted to be about 20 mg. A mixture obtained by uniformly mixing 90 parts by weight of natural graphite as a negative electrode active material and 10 parts by weight of polyvinylidene fluoride as a fluorine-based polymer binder in a 1-methyl-2-pyrrolidone solvent is applied on a copper foil current collector. , 1-methyl-2-pyrrolidone was sufficiently dried, molded under pressure, and heat-treated to produce a disc-shaped negative electrode of about 2 cm ² . At this time, the amount of the active material finally contained in the negative electrode was adjusted to be about 10 mg. Using the positive electrode and the negative electrode thus obtained, LiPF ₆ was further dissolved in a nonaqueous electrolyte at a concentration of 1 M in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 2. Using this, a coin-type battery was produced. A charge / discharge cycle test at a high temperature (60 ° C.) was performed using this battery. The charging / discharging period was set to one week, and after one week, the charging / discharging was stopped, and an experiment for evaluating the amount of Mn elution described in the next section was immediately performed. Charge and discharge conditions were set as follows. The charging was performed in a constant current-constant voltage mode, the current density at the time of constant current was 0.4 mA / cm ² , the constant voltage set value was 4.2 V, and the total charging time from the start of charging was 5 hours. That is, at the start of charging / discharging, charging is performed with a constant current, and the voltage of the battery becomes 4.
When the voltage reaches 2 V (it is always shorter than 5 hours from the theoretical capacity)
To switch to the constant voltage mode, and the charging ends after 5 hours from the start of charging. The discharge was set in the 0.4 mA / cm ² constant current mode so that the discharge was terminated when the voltage of the battery reached 2.7 V. Table 4 shows the discharge capacity retention rate (%) at the 25th cycle when the discharge capacity at the first cycle is 100.

[0026]

[Table 4]

From Table 4, it can be seen that the batteries of Examples 1 to 15 have improved high-temperature (60 ° C.) cycle characteristics as compared with Comparative Examples 1 and 2, which have no gradient layer of F concentration on the surface of the positive electrode active material. I understand. On the other hand, the discharge capacity retention ratio of the battery of Comparative Example 3 was higher than that of Comparative Example 1, but slightly smaller than that of Comparative Example 2.
This is probably because the effect of improving the charge / discharge characteristics at a high temperature by the gradient layer was not sufficiently exhibited because the thickness of the F concentration gradient layer was too thin. Further, the battery of Comparative Example 4 was comparable to that of Comparative Example 2. This is presumably because the thickness of the F-concentration gradient layer was too thick, preventing the diffusion of Li ions and causing an overload.

[Evaluation of the amount of Mn eluted in the electrolyte] The battery after completion of the above-described evaluation of the charge / discharge characteristics (one-week charge / discharge) was disassembled, and Mn present in the electrolyte and precipitated on the negative electrode. Mn
Was determined by subjecting the electrolytic solution and the negative electrode to thermal decomposition in an acid to prepare a test solution, and performing Mn concentration analysis by ICP. Table 5 shows the total elution amount of Mn (μg) per 20 mg of the positive electrode active material thus determined.

[0029]

[Table 5]

As shown in Table 5, the batteries of Examples 1 to 15 exhibited a higher Mn elution amount by one week of high temperature (60 ° C.) cycle than Comparative Examples 1 and 2 having no gradient layer of F concentration on the surface of the positive electrode active material. It can be seen that it has decreased. On the other hand, the amount of Mn eluted in the battery of Comparative Example 3 was smaller than that of Comparative Example 1, but slightly larger than that of Comparative Example 2. This is considered to be because the effect of suppressing the Mn elution by the gradient layer was not sufficiently exhibited because the thickness of the gradient layer having the F concentration was too small.

[0031]

As is apparent from the above description, the positive electrode active material is mainly composed of lithium manganate having a spinel structure, and the concentration of F decreases from the surface to the inside of the particles. The depth from the surface layer to the depth of 10% of the F concentration in the surface layer is 0.1 mm from the surface layer.
It has a gradient structure of 5 to 80 nm, and the atomic ratio F / Mn of all F and all Mn of the positive electrode active material is 0.002 to 0.002.
By controlling it to be 0.05, a high output and a high energy density can be obtained as a positive electrode material for a lithium secondary battery, and a lithium manganate-based positive electrode active material having good cycle characteristics and storage characteristics even at high temperatures. A substance and a method for producing the same can be provided. A lithium secondary battery having excellent cycle characteristics and storage characteristics even at a high temperature can be provided by using this material for a positive electrode.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction diagram of a positive electrode active material obtained in Example 1.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tetsuo Yamada 5F, 1978 Kogushi, Ube City, Ube City, Yamaguchi Prefecture F-term in Ube Research Institute Ube Research Laboratories 4G048 AA04 AA05 AA06 AB05 AB08 AC06 AD06 AE05 AE06 5H029 AJ04 AJ05 AK03 AL06 AL07 AL12 AM02 AM03 AM04 AM05 AM07 AM12 AM16 CJ02 CJ08 CJ12 DJ12 DJ16 DJ17 EJ07 HJ01 HJ02 HJ04 HJ13 HJ14

Claims (5)

[Claims] In a positive electrode active material for a lithium secondary battery mainly composed of lithium manganate having a cubic spinel structure, oxygen atoms are substituted from the surface side of particles of the positive electrode active material toward the inside. There is a concentration gradient layer in which the concentration of fluorine decreases, and the depth at which the difference in fluorine concentration between the inside of the particle and the concentration of fluorine in the particle attenuates to 10% of the difference between the concentration of fluorine at the surface side interface and the concentration of fluorine inside the particle. Has an inclined structure of 0.5 to 80 nm from the surface side interface, and has an atomic ratio F / between the total fluorine amount and the total manganese amount in the positive electrode active material.
A positive electrode active material for a lithium secondary battery, wherein Mn is 0.002 to 0.05 and the lattice constant is 0.82405 nm or less. 2. In a lithium manganate crystal having a cubic spinel structure, a part of lattice sites occupied by lithium or manganese is B, Mg, Al, P, C.
a, Ti, V, Cr, Fe, Co, Ni, Cu, Zn,
2. A material which is substituted by at least one element selected from Ba, Ga, and Ta.
The positive electrode active material for a lithium secondary battery according to the above. 3. A lithium compound, a manganese compound and L
The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein after baking the mixture of iF at 500 ° C to 800 ° C, unreacted LiF is removed by a water washing treatment. 4. A lithium compound, a manganese compound, B,
Mg, Al, P, Ca, Ti, V, Cr, Fe, Co,
After baking a mixture of at least one or more kinds of substitution element compounds selected from Ni, Cu, Zn, Ba, Ga, and Ta and LiF at 500 ° C to 800 ° C, unreacted LiF is removed by washing with water. The method for producing a positive electrode active material for a lithium secondary battery according to claim 2. 5. A lithium secondary battery, wherein the positive electrode active material for a lithium secondary battery according to claim 1 is used for a positive electrode.