JP2001015108A - Positive electrode active material for lithium secondary battery, its manufacture, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve energy the density per unit volume by including a lithium- manganese composite oxide and a lithium-nickel composite oxide in a same particle. SOLUTION: A lithium-manganese composite oxide may include elements except for Li, Mn, and O and for example, substitution of a part of Mn by another element can stabilize the crystal structure. Among the substituents are B, Al, Fe, Sn, Cr, Cu, Ti, Zn, Co, and Ni. A lithium-nickel composite oxide may include elements except for Li, Ni, and O and for example, substitution of a part of Ni by another element can stabilize the crystal structure. Among the substituents are B, Al, Fe, Sn, Cr, Cu, Ti, An, Co, and Mn. The ratio of the lithium-manganese composite oxide to the total amount of the both composite oxides in the particle is preferably not less than 10 mol% and not more than 90 mol%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material used for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the positive electrode active material.

[0002]

2. Description of the Related Art A lithium ion secondary battery, which functions as a battery by absorbing and releasing lithium ions between a positive electrode active material and a negative electrode active material, has a high voltage and a high energy density, and is used for mobile phones and personal computers. , Video cameras, electric vehicles and the like.
As a positive electrode active material (positive electrode material) for lithium ion secondary batteries, LiCoO ₂ , a layered composite oxide, can obtain a high voltage of 4V class and has a high energy density, so it is already widely used. Has been On the other hand, the raw material, cobalt, is scarce in resources and expensive,
Considering the possibility that demand will continue to expand significantly in the future, there is concern over the supply of raw materials and prices may rise further. Thus, recently, a positive electrode material that can replace cobalt has been desired.

[0003] LiCoO_TwoLiNiO with the same layered structure as
_TwoIs that nickel as a raw material is cheaper than cobalt
LiCoO in terms of battery performance_TwoAs high as energy
Energy density, but decomposes and generates heat when overcharged.
Problems such as the risk of the battery exploding
Have been. From this point of view,
Manganese, which is inexpensive,
Highly safe spinel-type manganese-based composite oxide Li
Mn_TwoO _FourIs considered to be used as a cathode material.
You. However, LiMn_TwoO_FourMeans that the battery capacity is LiCoO_TwoAnd Li
NiO_TwoLower than that of
Large capacity deterioration, especially at high temperatures of 50-60 ° C
There was a problem that. Of these, charge / discharge sites at high temperatures
Regarding the crystal characteristics, in order to stabilize the crystal structure, Mn
By substituting a part with one or more elements
Although the improvement effect of suppressing the capacity deterioration is recognized, the battery capacity and
Then it will be even lower.

Therefore, recently, it has been desired to improve the capacity and cycle characteristics by mixing a lithium manganese composite oxide such as LiMn ₂ O ₄ with the above-mentioned lithium cobalt composite oxide or lithium nickel composite oxide. Proposed. JP-A-5-82131 and JP-A-8-45
No. 498, the crystal structure of spinel-type lithium manganate expands during charging, while Li _1-x CoO ₂ (0
≤ x ≤ 1) and LiNi _y Co _1-y O ₂ (0.3 ≤ y ≤ 1.0) focus on the fact that the crystal structure shrinks (the reverse during discharge) and mix both components at a fixed ratio As a result, a method for improving the cycle characteristics by suppressing the volume change at the time of charging and discharging and suppressing the separation of the positive electrode active material and the conductive agent or the separation of the positive electrode active material and the current collector is proposed. Have been.

In Japanese Patent Application Laid-Open No. Hei 7-235291, a spinel type lithium manganese composite oxide is
_x Co _1-y Ni _y O ₂ (0 <x <1,0 ≦ y ≦ 1) by mixing a predetermined amount to improve the ionic conductivity of the positive electrode active material in the charged state to improve cycle characteristics Has been proposed. Further, in Japanese Patent Application Laid-Open No. Hei 10-92430, by mixing a composite oxide mainly composed of Ni with a composite oxide mainly composed of lithium and manganese having a spinel structure, the battery capacity and stability against overcharge are improved. Ways to improve it have been proposed.

Further, in Japanese Patent Application Laid-Open No. H10-112318, a lithium manganese composite oxide such as LiMn ₂ O ₄ and a lithium nickel composite oxide such as LiNiO ₂ are mixed at a predetermined ratio to form a negative electrode active material. There has been proposed a method for improving the capacity utilization efficiency by compensating for the irreversible capacity.

[0007]

However, with the demand for further size reduction of devices using batteries such as portable devices, it is necessary to further reduce the size of secondary batteries. Is not always sufficient from the viewpoint of improving the energy density per unit volume. An object of the present invention is to solve the above problems, to use a lithium manganese composite oxide and a lithium nickel composite oxide, to provide a positive electrode active material having a high energy density per unit volume, and a lithium secondary battery using the same. Is to provide.

[0008]

In order to achieve the above object,
As a result of intensive studies conducted by the present inventors, the lithium manganese composite oxide and the lithium nickel composite oxide are not simply mixed, but if both of them are present as constituent components of one particle, more energy can be obtained. The present inventors have found that a positive electrode active material having a high density can be obtained and completed the present invention. That is, the gist of the present invention is to provide a positive electrode active material for a lithium secondary battery, characterized by containing a lithium manganese composite oxide and a lithium nickel composite oxide in the same particle, and a lithium secondary battery using the same. Exists in batteries.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The lithium manganese composite oxide used in the present invention may be a spinel type lithium manganate having a basic composition LiMn ₂ O ₄ ,
Although lithium manganate having a layered structure having O ₂ may be used, spinel-type lithium manganate is preferable in view of ease of production and cycle characteristics. The lithium manganese composite oxide may contain elements other than lithium, manganese, and oxygen. For example, by substituting a part of manganese with another atom, the crystal structure can be stabilized. Such substitution elements include B,
Al, Fe, Sn, Cr, Cu, Ti, Zn, Co, N
and a metal element such as i. Of course, it can be substituted with a plurality of elements. The preferred replacement element is Al. Further, a part of the oxygen atoms can be replaced by a halogen element such as fluorine.

Further, it is possible to partially replace a manganese atom site with lithium, for example, by using a stoichiometric amount or more of lithium as a raw material, and it is preferable because the capacity can be increased. That is, some of the manganese atoms are converted to Li, B, Al, Fe, Sn, Cr, C
It can be replaced by at least one selected from the group consisting of u, Ti, Zn, Co and Ni, and is preferable in terms of cycle and capacity. Furthermore, oxygen may have a non-stoichiometric property. Such other element substitution type lithium manganese composite oxide, for example, in the case of a lithium manganese composite oxide having a spinel structure, usually

[0011]

Embedded image

(Me is a substitution element, 0 ≦ x ≦ 1.5, 0 <y
≦ 1, -0.5 ≦ δ ≦ 0.5). Here, a preferable substitution element Me is Al and / or Li. However, as long as the crystal structure can be stabilized, the types and composition ratios of the substitution elements are not limited thereto.

The lithium nickel composite oxide used in the present invention is typically a layered structure of lithium nickel oxide having a basic composition of LiNiO ₂ . The lithium nickel composite oxide may contain elements other than lithium, nickel and oxygen. For example, the crystal structure can be stabilized by replacing part of nickel with another atom. B, Al, F
Metal elements such as e, Sn, Cr, Cu, Ti, Zn, Co, and Mn can be given. Of course, it can be substituted with a plurality of elements. Preferred replacement elements are Co and / or A
l. Further, a part of the oxygen atoms can be replaced by a halogen element such as fluorine.

Further, it is also possible to partially replace the nickel atom sites with lithium by using a stoichiometric amount or more of lithium as a raw material. That is, some of the nickel atoms are converted to Li, B, Al, Fe, Sn, C
It can be replaced with at least one selected from the group consisting of r, Cu, Ti, Zn, Co and Mn, and is preferable in terms of cycle and capacity. Furthermore, oxygen may have a non-stoichiometric property. Such other element-substituted lithium-nickel composite oxide is, for example, usually a layered structure lithium-nickel composite oxide.

[0015]

Embedded image

(Me is a substitution element, 0 ≦ x ≦ 1.5, 0 <y
≦ 1, -0.5 ≦ δ ≦ 0.5). Here, a preferable substitution element Me is Al and / or Co. However, as long as the crystal structure can be stabilized, the types and composition ratios of the substitution elements are not limited thereto.

In the present invention, at least two of these
Both of the two composite oxides are present in one particle. In this case, usually, each of the lithium manganese composite oxide and the lithium nickel composite oxide forms particles (hereinafter, these particles may be referred to as “component particles”), and further forms one particle as a whole. (Hereinafter, these particles may be referred to as “granulated particles”). The cathode active material particles of the present invention are preferably produced by mixing the above two component particles in a wet manner and drying the mixture.

The medium used in the wet mixing includes:
Usually, water is used, but in order to effectively prevent a part of lithium from being eluted from the composite oxide during the wet mixing, an organic solvent in which Li is not dissolved can be used as a medium. The mixing ratio of the two composite oxides is usually 10 mol% or more, preferably 10 mol% or more, as a molar ratio of the lithium manganese composite oxide to the total amount of the lithium nickel composite oxide and the lithium manganese composite oxide. 20 mol% or more, and usually 90
% Or less, preferably 80 mol% or less. If the ratio of the lithium manganese composite oxide is too small, there is a problem in terms of safety at the time of overcharging. It becomes difficult to be.

The average particle diameter of the component particles in the slurry obtained by wet mixing is usually 10 μm or less, preferably 5 μm or less. If the particle size of the component particles is too large, the obtained granulated particles are less likely to be spherical, and it tends to be difficult to obtain good particles in terms of battery performance such as capacity. On the other hand, if the particle size is too small, the production cost becomes too large, so that it is usually 0.1 μm or more. If the particle size of each component particle is too large, it is preferable to grind them and adjust them to the above-mentioned particle size range. The grinding operation in this case is
Before mixing the two types of raw materials, the mixing may be carried out independently for each raw material, or the two types of components may be mixed and then pulverized. Incidentally, the average particle diameter referred to here is commercially available (for example,
HORIBA LA-910) can be measured using a laser diffraction / scattering particle size distribution analyzer.

The slurry obtained by wet mixing is usually subjected to a drying treatment. As a result, the above two types of composite oxides can be made into a single particle. As the drying method,
Spray drying using a spray drier (spray drier) is preferably used because subsequent disintegration operation is unnecessary and the process can be simplified. The drying temperature is usually
It is selected from the range of 50C to 300C. The final particle diameter is governed by the diameter of the sprayed droplet, the drying speed, and the like. Regarding the droplet size, the spraying method (disk dispersion type, pressurized spray nozzle type, two-fluid spray nozzle type, etc.) The conditions such as the slurry supply speed and the slurry viscosity may be appropriately set.
The average particle diameter of the component particles to be subjected to the drying treatment is usually equal to the average particle diameter after wet mixing, and the lithium manganese composite oxide and the lithium nickel composite oxide usually have an average particle diameter of 1 respectively.
It is 0 μm or less, preferably 5 μm or less. The average particle diameter of the finally obtained granulated particles is usually 1 μm or more and about 100 μm or less. If the granulated particle size is smaller than this, the fluidity of the powder tends to deteriorate,
In addition, dust is easily generated during handling. On the other hand, if the average particle diameter of the granulated particles is too large, it is difficult to obtain a positive electrode having a uniform thickness when producing the positive electrode.

The granulated particles obtained after drying are preferably substantially spherical. As for the sphericity at this time, the aspect ratio (La / Lb) is usually 1.2 or less, preferably 1.1 or less, where La is the average major axis of the single particle and Lb is the average minor axis. And The effects of the present invention tend to be insufficient. The average particle size Rb of the granulated particles is usually at least three times the larger Ra of the average particle size of the lithium manganese composite oxide and the average particle size of the lithium nickel composite oxide as component particles, and is preferably 5 times or more. If the ratio of Rb to Ra is too small, the granulated particles are unlikely to be spherical, and as a result, sufficient battery performance may not be obtained. However, if the ratio of Rb to Ra is too large, it is not easy to manufacture.
b / Ra is usually 1000 or less.

The ratio of the lithium manganese composite oxide to the total amount of the lithium manganese composite oxide and the lithium nickel composite oxide in the particles is usually at least 10 mol%, preferably at least 20 mol%, for the same reason as the ratio during the wet mixing. % Or more, and usually 90 mol% or less,
Preferably it is 80 mol% or less. In addition, the average particle diameter of the component particles in the granulated particles is also usually 10 μm each for the lithium manganese composite oxide and the lithium nickel composite oxide for the same reason as that used for the spray drying.
m, preferably 5 μm or less, and usually 0.1 μm or less.
1 μm or more.

The positive electrode active material of the present invention can be used as a positive electrode of a lithium secondary battery. The positive electrode generally includes an active material layer containing the positive electrode active material, a conductive agent, and a binder, and a current collector. The active material layer can be usually obtained by preparing a slurry containing the above constituent components, applying the slurry on a current collector, and drying. The proportion of the active material of the present invention in the active material layer is usually 10% by weight or more, preferably 30% by weight.
And more preferably 50% by weight or more, usually 9% by weight.
It is at most 9.9% by weight, preferably at most 99% by weight.
Examples of the conductive agent used for the positive electrode include natural graphite, artificial graphite, and acetylene black. The proportion of the conductive agent in the active material layer is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight.
It is usually at most 50% by weight, preferably at most 20% by weight, more preferably at most 10% by weight.

Further, as the binder used for the positive electrode,
Polyvinylidene fluoride, polytetrafluoroethylene,
Examples thereof include polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose. The proportion of the binder in the active material layer is usually at least 0.1% by weight, preferably at least 1% by weight, more preferably at least 5% by weight, usually at most 80% by weight, preferably at most 60% by weight, More preferably, it is at most 40% by weight. Further, as the slurry solvent, N-methylpyrrolidone, tetrahydrofuran, dimethylformamide and the like can be used. The thickness of the active material layer is usually about 10 to 200 μm. As a material of the current collector used for the positive electrode, a metal such as aluminum or stainless steel is generally used, and aluminum is preferable. The active material layer obtained by coating and drying is preferably compacted by a roller press or the like in order to increase the packing density of the active material.

The lithium secondary battery of the present invention usually has the above positive electrode, negative electrode and non-aqueous electrolyte. Examples of the negative electrode active material that can be used in the lithium secondary battery of the present invention include carbon-based materials such as natural graphite and pyrolytic carbon. The negative electrode active material is usually formed as an active material layer on a current collector together with a binder, similarly to the positive electrode active material. Alternatively, lithium metal itself or a lithium alloy such as a lithium aluminum alloy can be used as the negative electrode.

Examples of the non-aqueous electrolytic solution that can be used in the lithium secondary battery of the present invention include those in which various electrolytic salts are dissolved in a non-aqueous solvent. As the electrolytic salt, LiCl
O ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiBr, LiCF ₃ SO _{3 and the} like. Examples of the non-aqueous solvent include tetrahydrofuran, 1,4-dioxane, dimethylformamide, acetonitrile, benzonitrile, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and the like. Of course, the electrolytic salt and the solvent may be used alone or as a mixture of two or more. In particular, by using a non-aqueous solvent as a mixture of a plurality of solvents, various characteristics of the battery can be improved. Usually, a separator is provided between the positive electrode and the negative electrode. Examples of the separator include a polymer porous membrane such as Teflon, polyethylene, polypropylene, and polyester, a nonwoven fabric filter such as glass fiber, and a composite nonwoven fabric filter of glass fiber and polymer fiber.

[0027]

And _{Li 1.04 Mn 1.88 Al 0.12 O 4} 100g of EXAMPLE 1 the average particle diameter of 10 [mu] m, average particle size 7 [mu] m and LiNi _{0. 85} Co _0.15 O ₂ 100g of weighed into a polyethylene container of 1L, which After adding 200g of pure water to
Wet pulverization and mixing were performed with a homogenizer. The average particle size of the slurry after the homogenizer treatment was
It was 4 μm when measured using an A-910 type laser diffraction / scattering particle size distribution analyzer. When this slurry was spray-dried using a spray dryer, granulated particles containing both a spinel-type lithium manganese composite oxide and a layered structure lithium-nickel composite oxide, which were granulated in a substantially spherical shape with an average particle size of 15 μm. was gotten. The number of the granulated particles is 20 or more, 200
An SEM photograph was taken at a magnification such that it could fall within the range of less than or equal to the number of particles, and 20 particles were randomly selected from the SEM photograph to measure the major axis and minor axis. In this sample, La / Lb = 1.06 when the average value of the 20 major axes was La and the average value of the 20 minor axes was Lb.

10 g of the positive electrode active material thus obtained
And acetylene black (AB) and polyvinylidene fluoride (PVDF) in a concentration of 50 cc using N-methyl-2-pyrrolidone as a solvent so as to have a solid concentration of 40 wt%.
The sample was collected in a polyethylene container. The mixing ratio of each component at this time was positive electrode active material: AB: PVDF = 90: 5: 5 (wt%). Furthermore, zirconia beads of 1 mmφ are added to this for 20 times.
g was added, the container was sealed, and the mixture was set on a shaker and mixed for 30 minutes. The mixed positive electrode solution is applied to a thickness of 21 μm using an applicator having a clearance of 350 μm.
m, dried at 120 ° C. and punched out with a punch to obtain a 12 mmφ positive electrode pellet. The punched out pellets were subjected to a consolidation treatment at a pressure of 24 MPa for 1 minute by a hand press. The thickness of the compacted pellet was measured with a micrometer, and the volume of the positive electrode portion was calculated. After the thickness measurement, a coin-type battery was assembled to evaluate the battery. At this time, lithium metal was used for the negative electrode material, and a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio of 3: 7) was used for the electrolytic solution.
A solution in which 1 mol / l of lithium hexafluorophosphate (LiPF ₆ ) was dissolved was used. Using this battery, the discharge capacity per unit volume of the positive electrode layer when charged and discharged at a current density of 1 mA / cm ² was measured to be 360 mAh / cm ³ (Table 1).
1).

Example 2 The same lithium manganese composite oxide having an average particle diameter of 10 μm and the same lithium nickel composite oxide having an average particle diameter of 7 μm as used in Example 1 were dry-ground using a jet mill. A lithium manganese composite oxide having an average particle size of 0.5 μm and a lithium nickel composite oxide having an average particle size of 0.4 μm were obtained. 100 g of each of the jet mill pulverized products was dispensed into a 1 L polyethylene container, and 200 g of water was added to prepare a slurry. This slurry was spray-dried under the same conditions as in Example 1 and contained both a spinel-type lithium manganese composite oxide and a layered structure lithium-nickel composite oxide, which were granulated in a substantially spherical shape with an average particle diameter of 12 μm. Granulated particles were obtained. The average major axis of the obtained granulated particles (L
When the ratio between a) and the average minor axis (Lb) was measured, La /
Lb was 1.02. Using 10 g of the positive electrode active material thus obtained, a coin-type battery was prepared in the same manner as in Example 1, and the battery was evaluated. The discharge capacity at a current density of 1 mA / cm ² was 370 mAh / cm ³ (Table 1).

Example 3 The same lithium manganese composite oxide having an average particle size of 10 μm and the same lithium nickel composite oxide having an average particle size of 7 μm as those used in Example 1 were dispensed in 100 g 1L polyethylene containers. Then, 200 g of pure water was added to prepare a slurry. This slurry was spray-dried under the same conditions as in Example 1 without performing a homogenizer treatment. When the ratio of the average major axis (La) to the average minor axis (Lb) of the obtained granulated particles was measured, La / Lb = 1.31. The average particle diameter of the granulated particles was 19 μm, but was hardly spherical. The positive electrode active material thus obtained was subjected to battery evaluation in the same manner as in Example 1. As a result, the discharge capacity at a current density of 1 mA / cm ² was:
It was 340 mAh / cm ³ (Table 1).

COMPARATIVE EXAMPLE 1 Lithium-manganese composite oxide and lithium-nickel composite oxide were each 100 g each dispensed into a 1 L polyethylene container, sealed without adding water, and thoroughly dry-mixed with a shaker. Other than the above, in the same manner as in Example 1,
The positive electrode active material was manufactured and the battery was evaluated.
The discharge capacity at a current density of m ² was 310 mAh / cm ³ (Table 1).

Comparative Example 2 The same lithium manganese composite oxide having an average particle diameter of 10 μm and the same lithium nickel composite oxide having an average particle diameter of 7 μm as those used in Example 1 were dry-pulverized using a jet mill. A spinel-type lithium manganese composite oxide having an average particle size of 0.5 μm and a lithium nickel composite oxide having a layered structure of 0.4 μm were obtained. 100 g of each of the pulverized products of the jet mill was dispensed into 1-liter polyethylene containers, sealed without adding water, and sufficiently dry-mixed with a shaker. The battery evaluation of the positive electrode active material thus obtained was performed in the same manner as in Example 1. As a result, the discharge capacity at a current density of 1 mA / cm ² was 300 mAh / cm ³ (Table 1). ).

[0033]

[Table 1]

[0034]

As described above, according to the present invention, it is possible to obtain a lithium secondary battery which is inexpensive, has excellent cycle characteristics, rate characteristics, and safety, and is particularly excellent in energy density per unit capacity.

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Claims (12)

[Claims] 1. A positive electrode active material for a lithium secondary battery, comprising a lithium manganese composite oxide and a lithium nickel composite oxide in the same particle. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein a part of manganese atoms of the lithium manganese composite oxide is substituted with one or more other atoms. 3. Other atoms are Li, B, Al, Fe, S
The positive electrode active material for a lithium secondary battery according to claim 2, wherein the positive electrode active material is at least one selected from the group consisting of n, Cr, Cu, Ti, Zn, Co, and Ni. 4. The lithium secondary battery according to claim 1, wherein a part of nickel atoms of the lithium nickel composite oxide is substituted with one or more kinds of other atoms. For positive electrode active material. 5. The method according to claim 1, wherein the other atoms are Li, B, Al, Fe, S
The positive electrode active material for a lithium secondary battery according to claim 4, wherein the positive electrode active material is at least one selected from the group consisting of n, Cr, Cu, Ti, Zn, Co, and Mn. 6. The molar ratio of the lithium manganese composite oxide to the total amount of the lithium manganese composite oxide and the lithium nickel composite oxide in the particles is 10 to 10.
The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 5, which is 90%. 7. An aspect ratio (La / Lb) of a major axis (La) and a minor axis (Lb) of the particles is 1.2 or less.
7. The positive electrode active material for a lithium secondary battery according to any one of items 1 to 6. 8. The lithium manganese composite oxide and lithium nickel composite oxide in the particles each have an average particle diameter of 1
The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 7, comprising component particles having a particle size of 0 µm or less. 9. The method according to claim 1, wherein the average particle diameter of the particles is at least three times the larger of the average particle diameters of the respective component particles of the lithium manganese composite oxide and the lithium nickel composite oxide. The positive electrode active material for a lithium secondary battery according to one of the above. 10. A lithium secondary battery using the positive electrode active material according to claim 1. 11. A method for producing a positive electrode active material for a lithium secondary battery, comprising wet-mixing a lithium manganese composite oxide and a lithium nickel composite oxide, followed by spray drying. 12. The particle size of a lithium manganese composite oxide and a lithium nickel composite oxide to be subjected to spray drying is as follows:
The method for producing a positive electrode active material for a lithium secondary battery according to claim 11, wherein each is 10 μm or less.