JP4062169B2 - Positive electrode material for lithium secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode material for a lithium battery having a high energy density, and more particularly to a positive electrode material for a lithium secondary battery intended for vehicle use.
[0002]
[Prior art]
An attempt to obtain a battery with a high output and a high capacity for an electric vehicle or the like using lithium nickelate as a main active material is disclosed in Japanese Patent Application Laid-Open No. 2000-77072.
[0003]
Japanese Patent Application Laid-Open No. 7-37576 discloses an attempt to use lithium nickelate having plate-like secondary particles having a layered structure in which thin pieces are regularly stacked as a positive electrode material.
[0004]
However, according to the above means, it has been difficult to ensure output characteristics required for an electric vehicle, in particular, output at a low temperature.
[0005]
[Patent Document 1]
JP-A-7-37576 [0006]
[Problems to be solved by the invention]
A secondary battery used as a power source for a moving body such as an electric vehicle such as an electric vehicle or an electric motorcycle is required to have a much higher output characteristic than a consumer battery, particularly a high output at a low temperature of −30 ° C.
[0007]
The present invention intends to increase the output of a battery by increasing the battery voltage, and to solve the output characteristics particularly at a low temperature.
[0008]
[Means for Solving the Problems]
In order to increase the output at a low temperature, the most important thing is to flatten the discharge potential of the positive electrode material.
[0009]
The present invention relates to a positive electrode material for a lithium secondary battery containing an oxide containing Li and Ni, Mn, and Co, and the particles constituting the oxide are regions in which the surface Mn concentration is higher than the internal Mn concentration. The potential of the lithium secondary battery is flattened by using a positive electrode material for a lithium secondary battery characterized by the presence of.
[0010]
For example, the composition of the surface portion is LiNi _0.3 Mn _0.6 Co _0.1 O ₂ , and the composition of the inner portion is LiNi _0.5 Mn _0.3 Co _0.2 O ₂ .
[0011]
Usually, the particles are formed with a uniform composition, but in the present invention, the composition is different between the surface and the inside of the particles.
[0012]
Further, a positive electrode material for a lithium secondary battery containing at least Li and Ni, including a tetravalent element other than Mn and a trivalent element other than Co in addition to Li and Ni,
The composition formula _{Li x Ni a (Mn y M} 1-y) b (Co z M '1-z) c O 2
(0 <x <1.2, 0 <y <1, 0 <z <1, a + b + c = 1, 9b ≦ 5a + 2.7, 0 <a <1, 0 <b <1, 0 <c <1, M : Tetravalent element different from Mn, M ': Trivalent element different from Co)
The particles constituting the positive electrode material preferably have a region in which the surface Mn concentration is higher by 10% or more than the internal Mn concentration in terms of atomic ratio. .
[0013]
Furthermore, it is desirable that the thickness of the layer having a high Mn concentration is 0.1% to 10% of the diameter of the particles constituting the positive electrode material.
[0014]
Here, as the tetravalent element, typical elements such as Si, Ge, and Sn, and tetravalent transition metals Ti, V, Fe, and W are desirable. The tetravalent element M need not be limited to one type, and may be composed of a plurality of the above-described elements.
[0015]
As the trivalent element, typical elements such as Al, Ga, and In, transition metals Sc, Cr, and Mo that are trivalent, and rare earth Y, La, Ce, Eu, Gd, and Nd are preferable. The trivalent element M ′ need not be limited to one type, and may be composed of a plurality of the above elements.
[0016]
A layer having a higher Mn concentration than the inside is formed on the surface of the positive electrode material.
[0017]
A battery using a positive electrode material having a conventional layered structure has a problem that the discharge potential has a gentle slope and the flatness of the potential is poor.
[0018]
This is because, in a conventional positive electrode material, a change in the valence of Ni ions occurs during discharge, thereby determining the voltage of the battery. The change in potential corresponding to the change when the valence changes from a tetravalent Ni ⁴⁺ ion to a divalent Ni ²⁺ ion results in a gentle voltage change with poor potential flatness.
[0019]
In contrast, the Mn spinel-based positive electrode material has a flat potential, but has a problem of low capacity and poor life.
[0020]
In the present invention, a material having a Mn concentration distribution in the particle diameter direction is used as the positive electrode material. The potential change due to the valence change of the Mn ion is flat, and is effective for improving the output of the battery, particularly for improving the output at a low temperature.
[0021]
The concentration distribution of the particles can be examined by energy dispersive X-ray spectroscopy (EDX) using a transmission electron microscope (TEM), and an oxide having a structure similar to a spinel oxide or a hexagonal oxide is formed. I think.
[0022]
The essence of the present invention is that the potential at the time of discharge is governed by the substance generated on the particle surface of the positive electrode active material.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
In order to produce the positive electrode active material of this embodiment, the following is preferable.
[0024]
Starting materials are oxides, hydroxides, carbonates, sulfates and nitrates. The raw material is used in the form of powder, and this is pulverized and mixed using a mixer such as a ball mill or a vibration mill.
[0025]
For example, when synthesizing a positive electrode material of LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , the following is performed.
[0026]
Using powders of lithium carbonate (Li ₂ CO ₃ ) and manganese dioxide (MnO ₂ ) as starting materials, these are weighed and mixed so that the molar ratio of the metal elements is equal to the material to be obtained. .
[0027]
The mixed raw material powder is put into a container made of high-purity alumina and fired (first firing) using an electric furnace at an air atmosphere of 800 ° C. to 950 ° C. The calcined powder that has been gradually cooled to room temperature is again pulverized and mixed in a mixer, and second calcined at a temperature of 1000 ° C. to 1100 ° C. in an air atmosphere.
[0028]
The obtained powder is pulverized and classified to a particle size of 40 microns or less by automatic sieving to obtain a precursor raw material of the positive electrode active material.
[0029]
Next, the obtained precursor material is put into an acidic solution, for example, a nitric acid aqueous solution. When Mn nitrate is added to the aqueous solution and then a strong alkaline aqueous solution such as sodium hydroxide is gradually added, precipitation of Mn hydroxide occurs on the surface of the precursor, and a powder raw material having a high Mn concentration on the surface is obtained. It is done. A predetermined positive electrode material is obtained by heat-treating the powder raw material at a temperature of 900 to 1000 ° C. in air.
[0030]
In order to produce a lithium secondary battery for an electric vehicle using this embodiment, the following is performed.
[0031]
First, a positive electrode active material is mixed with a conductive material of carbon material powder and a binder such as polyvinylidene fluoride (PVDF) to prepare a slurry.
[0032]
The mixing ratio of the conductive material to the positive electrode active material is preferably 5 to 20% by weight. At this time, sufficient kneading is performed using a mixer equipped with stirring means such as a rotor blade so that the powder particles of the positive electrode active material are uniformly dispersed in the slurry.
[0033]
The sufficiently mixed slurry is coated on both sides of an aluminum foil having a thickness of 15 to 25 μm by, for example, a roll transfer type coating machine. After coating on both sides, a positive electrode plate is obtained by press drying. The thickness of the coating electrode mixture is desirably 20 to 100 μm.
[0034]
For the negative electrode, graphite, amorphous carbon, or a mixture thereof is used as an active material, and mixed with a binder and applied and pressed in the same manner as the positive electrode to form an electrode. The electrode mixture thickness is preferably 20 to 70 μm. In the case of the negative electrode, a copper foil having a thickness of 7 to 20 μm is used as the current collector. The mixing ratio of application is preferably 90:10, for example, by weight ratio of the negative electrode active material and the binder.
[0035]
The coating electrode is cut to a predetermined length, and a tab for drawing current is formed by spot welding or ultrasonic welding. The tab portion is made of a metal foil made of the same material as the current collector having a rectangular shape, and is installed to take out current from the electrode.
[0036]
In the mobile lithium secondary battery of this embodiment, a large current is required to flow, and thus it is necessary to provide a plurality of tabs. The tabbed electrodes are stacked with a separator made of a porous resin, such as polyethylene (PE) or polypropylene (PP), sandwiched between them to form a group of electrodes that are stored in a cylindrical container. .
[0037]
Alternatively, a bag-shaped separator may be used to store electrodes therein, and these may be sequentially stacked and stored in a rectangular container. The container is preferably made of stainless steel or aluminum.
[0038]
After the electrode group is accommodated in the battery container, an electrolytic solution is injected and sealed.
[0039]
As the electrolytic solution, it is desirable to use a solution of LiPF ₆ , LiBF ₄ , LiClO ₄ or the like as an electrolyte in a solvent such as diethyl carbonate (DEC), ethylene carbonate (EC), or propylene carbonate (PC). The electrolyte concentration is preferably between 0.7M and 1.5M. The electrolyte is injected, the battery container is sealed, and the battery is completed.
[0040]
(Example)
In this example, precursor materials LiNi _1/3 Mn _1/3 Co _1/3 O ₂ were synthesized using Li ₂ CO ₃ , MnO ₂ , and CoCO ₃ as raw materials.
[0041]
After mixing the raw material powders, they were put in a high-purity alumina container and subjected to first firing at 950 ° C. for 20 hours and second firing at 1050 ° C. for 20 hours. All firings were performed in air. 100 g of the precursor raw material thus prepared was put into a 0.5 M nitric acid aqueous solution, and 25 g of Mn nitrate was added at the same time and well stirred.
[0042]
Even when Mn nitrate was completely dissolved, the precursor was not dissolved. A 1N aqueous solution of sodium hydroxide was added dropwise to precipitate Mn nitrate, followed by filtration. The filtered powder was dried, and then again heat treated at 950 ° C. for 2 hours in air. The positive electrode active material produced by this process was pulverized and classified so that the average particle diameter D ₅₀ = 9 to 11 microns in any case.
[0043]
A schematic view of the particles of the obtained positive electrode material is shown in FIG. The particles of the positive electrode material have a portion 1 having a high Mn concentration (higher Mn concentration compared to the inside) near the surface, and a portion 2 having a low Mn concentration (lower Mn concentration compared to the surface) inside. Exists. FIG. 2 shows the result of examining the Mn concentration by observing particles having a particle size of about 10 microns with TEM-EDX. Thus, it was found that a Mn high concentration layer having a thickness of 0.5 microns was formed from the surface.
[0044]
Using the positive electrode material of the example and the precursor raw material in the example, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ as the positive electrode material, a battery using amorphous carbon for the negative electrode was produced. .
[0045]
Although there were a 6.2Ah Any battery capacity at room temperature, as shown in FIG. 3, the test at -30 ° C., is capacity and 3.5Ah towards the battery using the positive electrode material of the present invention, LiNi _{1 / 3} Compared to batteries using Mn _1/3 Co _1/3 O ₂ as the positive electrode material, the discharge capacity is higher by 1.2Ah, and the battery voltage during discharge is higher, resulting in excellent output characteristics at low temperatures. It was clearly shown that In FIG. 3, reference numeral 3 indicates a discharge curve of a battery using the positive electrode material of the present invention, and reference numeral 4 indicates a discharge curve of a battery using LiNi _1/3 Mn _1/3 Co _1/3 O _2. .
[0046]
【The invention's effect】
According to the present invention, a high-power lithium secondary battery excellent in low temperature characteristics can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of a particle structure of a positive electrode active material according to the present invention.
FIG. 2 is a diagram showing a radial Mn concentration distribution of positive electrode active material particles of the present invention.
FIG. 3 is a diagram showing a relationship between a battery discharge capacity and a battery voltage.
[Explanation of symbols]
1 ... Mn high concentration layer, 2 ... Mn low concentration layer.

Claims (5)

The particles constituting the positive electrode material for a lithium secondary battery are oxides containing Li and Ni, Mn, Co,
A positive electrode material for a lithium secondary battery, wherein a Mn concentration on a surface of the particle is higher than a Mn concentration inside the particle. The particles constituting the positive electrode material for a lithium secondary battery are oxides containing Li and Ni, Mn, Co,
A positive electrode material for a lithium secondary battery, wherein an oxide layer containing Li and Ni, Mn, Co having a Mn concentration higher than the inside of the particle is formed on the surface of the particle.   2. The positive electrode material for a lithium battery according to claim 1, wherein a region where the Mn concentration on the surface of the particle is 10% or more higher than the Mn concentration inside the particle is 10% or more. The thickness of the oxide layer containing Li and Ni, Mn, Co having a higher Mn concentration than the inside of the particles is 0.1% to 10% of the diameter of the particles. The positive electrode material for lithium batteries as described in 2.   A lithium secondary battery using the positive electrode material according to claim 1 for a positive electrode.

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