JP3232943B2 - Manufacturing method of positive electrode active material for lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices have been rapidly advancing, and there is a high demand for a small and lightweight secondary battery having a high energy density as a drive power source for these devices. In this respect, non-aqueous secondary batteries, especially lithium secondary batteries, are expected to have high voltage and high energy density.

Under such circumstances, a battery using LiCoO ₂ as a positive electrode and a carbon material as a negative electrode has been developed. LiCo
Since the operating potential of O ₂ is as high as 4 V with respect to Li, the battery voltage becomes high. In addition, since a carbon material is used for the negative electrode and an intercalation reaction is used, this is a problem when using metallic Li for the negative electrode. The dendritic Li does not precipitate on the negative electrode, and the safety of the battery can be improved.

However, due to the problem of Co resources and the problem of cost, development of a lithium-containing composite oxide in place of LiCoO ₂ has been progressing, and attention has been paid to LiNiO _{2 and the} like. Lithium-containing composite oxides of this kind, such as LiNiO ₂ and LiCoO ₂ , each exhibit a high potential and are layered compounds having the same hexagonal crystal structure that can be used for an intercalation reaction. The expectation is great as a material. From such a viewpoint, for example, Li _X NiO ₂ (US Pat. No. 4,302,518)
_{No.), Li y Ni 2-y} O 2 ( Japanese Patent Laid-Open No. 2-40861) as according to LiNiO _2, such as, or Li _y Ni
_X Co _1-X O ₂ (JP-A-63-299056) and L
_{i y Ni 1-X M X} O 2 ( where, M is Ti, V, Mn, either Fe) is proposed lithium-containing composite oxide obtained by substituting a part of LiNiO ₂ of Ni with other metals, such as is ing.
_{Other, A x M y N Z O} 2 ( where, A is an alkali metal, M is a transition metal, N represents Al, In, one Sn) (JP 62
-90863 JP) and _{Li X M y N Z O 2} ) ( where, M is Fe, Co, at least one selected from among Ni,
A lithium-containing composite oxide such as N is at least one selected from Ti, V, Cr, and Mn (Japanese Patent Application Laid-Open No. 4-267053) has also been proposed. Using these active material materials, development of a high energy density lithium secondary battery having a 4V-class discharge transfer has been promoted.

[0005]

Among these lithium-containing composite oxides, LiNiO ₂ exhibits an operating potential of 4 V with respect to lithium, so that when used as a positive electrode active material, a secondary battery having a high energy density can be realized. . However, as the charge / discharge cycle of the battery elapses, the battery capacity deteriorates, and at the 50th cycle, drops to 65% of the initial capacity,
There was a problem that good charge / discharge cycle characteristics could not be obtained.

[0006] In order to solve such a problem, there have been proposed lithium composite oxides in which a part of Ni is replaced with another metal as described above, and those containing simultaneously various metal elements. However, LiNiO _{2 obtained} by substituting a part of Ni with another metal has improved cycle reversibility, but has a tendency to reduce discharge capacity and discharge voltage. The result was a reduction in the feature of high energy density. In these, some of Ni
In the case of substituting with, all of cycle reversibility, discharge capacity and discharge voltage were relatively good as compared with other lithium-containing composite oxides.

Here, a part of Ni of LiNiO ₂ is replaced with Co.
Synthesis of the active material substituted with
A method of adding a Co compound such as cobalt hydroxide to a compound and a Ni compound such as nickel hydroxide and firing the mixture (hereinafter, referred to as a composite synthesis method) has been common. Part of Ni is C
In order to obtain a compound having a single-phase crystal structure by substituting with o, the firing temperature is set to a temperature range of 600 ° C to 800 ° C.
The formation of a single phase depends on the firing temperature. When the firing temperature is 600 ° C. or lower, the reaction is not completed and a compound having a single phase cannot be obtained. Further, as the ratio of Ni increases, if the temperature exceeds 800 ° C., a single phase is formed, but the crystallinity decreases.

[0008] This is considered to be due to the fact that when synthesized at a high temperature of 800 ° C or higher, Ni or Co enters the site where Li should enter in the crystal, and the crystal structure is disturbed.

The present invention has been made to solve such a problem, and it is intended that a part of Ni be surely replaced with Co to obtain a general formula Li
The crystal structure of the lithium-containing composite oxide represented by Ni _X Co _(1-X) O ₂ is made into a single phase, and a stable crystal field where the crystal perfection is high, the crystal is not collapsed, and Li can easily move in the crystal. It provides a production method that can be obtained.

[0010]

Means for Solving the Problems In order to solve the above problems, a method for producing a positive electrode active material for a lithium secondary battery according to the present invention is a lithium-containing composite oxide containing lithium, nickel and cobalt, which has a general formula of LiNi _x Co _(1-x) O ₂ , wherein the x value in the formula is 0.95 ≧ x ≧ 0.50 , a method for producing a positive electrode active material, wherein a mixed aqueous solution of a cobalt salt and a nickel salt is prepared. After obtaining a composite hydroxide of cobalt and nickel by coprecipitating a hydroxide of cobalt and nickel by adding an alkali solution, the mixture is mixed with a lithium compound such as lithium hydroxide and the mixture is fired. .

[0011]

According to the production method of the present invention, an alkali solution is added to a mixed solution of a cobalt salt and a nickel salt to co-precipitate a hydroxide of cobalt and nickel, thereby obtaining a composite hydroxide of nickel and cobalt (hereinafter referred to as Ni (Co composite hydroxide), the crystal structure has reached a solid solution level in which Ni is partly replaced with Co, and a single phase is obtained by X-ray diffraction, and the crystal perfection is extremely high. It has become something.

The Ni / Co composite hydroxide has L
When the i-salt is added and calcined, a lithium-containing composite oxide having a crystal structure in which Li easily moves in the crystal can be obtained. Further, in the present invention, the sintering temperature is set at
Therefore, there is no disorder in the crystal structure.

In order to form a mixed valence state of Ni and Co to obtain a stable crystal structure, at least Ni
Is required to be 0.05 or more. But,
When the number of substitutions of Ni for Co exceeds 0.5, the crystal strain increases, the crystal structure collapses, and the mixed valence state becomes unbalanced, creating a state in which Li is difficult to move, and the capacity of the active material is reduced. It becomes remarkable.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

First, the production method of the present invention by coprecipitation of the Ni / Co composite hydroxide will be described. Nickel sulfate, a commercially available reagent, was added to water to create a saturated aqueous solution of nickel sulfate.
A predetermined amount of cobalt sulfate (according to the target Co / Ni ratio) was added thereto, and water was further added to adjust the solution to prepare a saturated aqueous solution containing nickel sulfate and cobalt sulfate. Then, an alkaline aqueous solution in which sodium hydroxide was dissolved was slowly added to this aqueous solution with stirring, and Ni
And Co hydroxide precipitation (coprecipitation) started simultaneously. After sufficiently adding an alkaline solution to confirm that the precipitation was completed, the precipitate was collected by filtration and washed with water. Washing was repeated while measuring the pH, and after it was determined that the residual alkali had almost disappeared, the sample was dried with hot air (using a hot air dryer set at 100 ° C.).

The X-ray diffraction pattern of the Ni / Co composite hydroxide thus obtained is very close to a single phase. As a result of elemental analysis, Co and Ni have almost the desired ratio.
Was included.

In this embodiment, nickel sulfate is used as the Ni source and cobalt sulfate is used as the Co source of the coprecipitating raw material. Any salt can be used. Although an aqueous solution of sodium hydroxide was used as the alkaline solution, other alkaline solutions such as an aqueous solution of potassium hydroxide and an aqueous solution of lithium hydroxide may be used.

Next, the firing step with the Li compound will be described. Lithium hydroxide is used as the Li compound, and the sum of the number of Co and Ni atoms and the number of Li atoms are equalized to the Ni / Co composite hydroxide obtained by the above coprecipitation and pulverized by a ball mill. The composite was placed in a crucible made of alumina, baked in oxygen at 550 ° C. for 20 hours, and then baked at 750 ° C. for 2 hours. After calcination, the mixture was slowly cooled to room temperature and pulverized to obtain a positive electrode active material powder.

Some Co · N with different Co / Ni ratios
As a result of an attempt to synthesize the i-composite hydroxide, the x value of the general formula LiNi _X Co _(1-X) O ₂ indicating the composition of the active material was 0.5.
As described above, an X-ray diffraction pattern of this lithium-containing composite oxide was obtained in a single phase. However, when the x value is less than 0.5, the X-ray pattern is almost a single phase, but the peak intensity tends to be weakened and the crystallinity tends to be reduced.

Then, 5 parts by weight of acetylene black was added to 100 parts by weight of the positive electrode active material and mixed well, and the mixture was mixed with a solvent of N-methylpyrrolidinone (NMP) in polyvinylidene fluoride (PVDF) as a binder. ) Was dissolved into a kneaded paste. The amount of PVDF was adjusted to be 4 parts by weight based on 100 parts by weight of the positive electrode active material. Next, this paste was applied to one side of an aluminum foil, dried and rolled to obtain an electrode plate. FIG. 1 is a longitudinal sectional view of a coin-type lithium secondary battery used in an example of the present invention. In FIG. 1, a positive electrode 1 is obtained by punching the above-mentioned electrode plate into a disk shape, and is provided inside a positive electrode case 2. The negative electrode 3 is formed by pressing metallic lithium on a stainless steel net 5 and is spot-welded to the inside of the sealing plate 4. A separator 6 made of polypropylene is arranged between the positive electrode 1 and the negative electrode 3, and an electrolytic solution 7 is injected. In addition, sealing was performed via a polypropylene gasket 8. The electrolyte used was a solution prepared by dissolving 1 mol of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

The value of x of the positive electrode active material represented by the general formula LiNi _x Co _(1-x) O ₂ is set to 0, 0.1, 0.2,
0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 0.95, and 1.0 were used, and a coin-type battery was produced using these in the same manner as described above. Note that x = 1.
0 is LiNiO ₂ containing no Co. Next, a charge / discharge cycle life test was performed using these batteries. The charge and discharge conditions are as follows: at room temperature (20 ° C.), 0.5 mA
/ Cm ^{2 at} a constant current and charge end voltage of 4.3
V, and the discharge end voltage was 3.0 V.

FIG. 2 shows the results of the charge / discharge cycle test, where x =
The positive electrode active material in the range of 0.1 to 0.4 has an initial capacity of 120.
mAh / g.

The positive electrode active material in the range of x = 0.5 to 0.95 has a large initial capacity of 140 to 150 mAh / g and a small cycle deterioration.
The capacity was maintained at 0%, and the capacity was hardly reduced even when the cycle was repeated thereafter. However, the positive electrode active material which does not contain Co at x = 1.0 has an initial capacity of 150 mAh / g or more, but the cycle deterioration is large, and the initial capacity is 6 times at the 50th cycle.
It decreased to 5%, and then deteriorated.

As is clear from the above results, the value of x in the active material LiNi _X Co _(1-x) O ₂ synthesized using the Ni—Co composite hydroxide prepared by coprecipitation is 0.5
Those having a range of 0.95 to 0.95 are preferred.

Next, for the positive electrode active material represented by the general formula LiNi _X Co _(1-X) O _{2 in} which the value of x is in the range of 0.0 to 0.95, a study was made on changing the firing temperature. The first firing step at 550 ° C. for 20 hours is performed in the same manner as described above.
° C, 650 ° C, 700 ° C, 750 ° C, 800 ° C, 850
° C and 900 ° C. A battery similar to the above was constructed using these positive electrode active materials, and a charge / discharge cycle test was performed under the same conditions as described above. FIG. 3 shows the result. The x value in the above equation was set to 0.8.

As is apparent from FIG.
The active material synthesized at a temperature between 00 ° C and 800 ° C has good initial capacity and cycle characteristics, and the one with 550 ° C has insufficient initial capacity and cycleability, and 850 ° C and 900 ° C.
Those having a small initial capacity and a large cycle deterioration.

From the above results, the firing temperature of the positive electrode active material is preferably from 600 ° C. to 800 ° C., but at 800 ° C., the cycle deterioration is slightly increased, and the initial capacity is slightly reduced. 650 ° C. to 750 ° C. is preferable since the cycle deterioration is good but the cycle deterioration is slightly increased.

In this embodiment, the case where the x value is 0.8 has been described. As a result of the same examination of the firing temperature for each of the cases where the x value is in the range of 0.50 to 0.95, A result showing the same tendency as in the case of x = 0.8 was obtained.

Using a conventional mixed synthesis method, LiNi
_0.8Co_0.2O_TwoA positive electrode active material having the following composition was synthesized.
First, nickel hydroxide, lithium hydroxide, and cobalt hydroxide
And the atomic ratio of Ni: Co: Li is 0.8: 0.2:
1.0 and crushed with a ball mill
Mix and place the mixture in an alumina crucible in oxygen
After the first firing at 550 ° C. for 20 hours, 750 ° C.
The second-stage baking was performed at 2 ° C. for 2 hours. After firing
The material was cooled and pulverized to obtain a positive electrode active material. this
The X-ray diffraction pattern of the active material became a single phase.
When a similar charge / discharge cycle test is performed,
There was a large drop in capacity. Therefore, LiNi _XCo
_(1-X)O_TwoAt 0, 0.1, 0.2, 0.
3,0.4,0.5,0.6,0.7,0.8,0.9
A positive electrode active material was synthesized as
Test. The result is shown in FIG.

As shown in FIG. 4, the value of x is 0.5 to
Even in the range of 0.9, in the cathode active material synthesized by the conventional method, the capacity decrease with the passage of the cycle was larger than that of the present invention.

[0031]

As described above, in the method for producing a cathode active material for a lithium secondary battery according to the present invention, an alkaline solution is added to a mixed solution of a nickel salt and a cobalt salt to coprecipitate a hydroxide of nickel and cobalt. Thus, a composite hydroxide of nickel and cobalt is obtained, so that the crystal structure reaches a solid solution level in which a part of Ni is surely replaced with Co.
Even in the line diffraction, it has a single phase and the crystal perfection is extremely high. When a Li salt is added to the Ni / Co composite hydroxide and fired, a lithium-containing composite oxide having a crystal structure in which Li easily moves in the crystal can be obtained, and the capacity is large and the cycle characteristics are excellent. A positive electrode active material can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a coin-type lithium secondary battery used in this example.

FIG. 2 is a diagram showing the relationship between the capacity of the positive electrode active material and the number of cycles when the value x is changed.

FIG. 3 is a diagram showing the relationship between the capacity of the positive electrode active material and the number of cycles when the firing temperature is changed.

FIG. 4 is a diagram showing the relationship between the capacity of a positive electrode active material synthesized by a conventional manufacturing method and the number of cycles.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode case 3 Negative electrode 4 Sealing plate 5 Stainless steel net 6 Separator 7 Electrolyte 8 Gasket

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-294364 (JP, A) JP-A-5-290845 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 H01M 4/04

Claims (2)

(57) [Claims] 1. A lithium-containing composite oxide containing lithium, nickel and cobalt, having the general formula LiNi _x Co
_The value of x in the expression represented by _(1-x) O ₂ is 0.95 ≧ x ≧ 0.
A method for producing a positive electrode active material, wherein a composite hydroxide of cobalt and nickel is obtained by adding an alkaline solution to a mixed aqueous solution of a cobalt salt and a nickel salt and coprecipitating a hydroxide of cobalt and nickel. And then mixing the mixture with a lithium compound such as lithium hydroxide and calcining the mixture to produce a positive electrode active material for a lithium secondary battery. 2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein a mixture of a composite hydroxide of cobalt and nickel and a lithium compound is calcined at a temperature of 600 ° C. or more and 800 ° C. or less.