JPH08171910A - Manufacture of positive electrode active material for lithium secondary battery - Google Patents

Abstract

PURPOSE: To make a crystal structure to be of a single phase, and thereby obtain an oxide high in crystal finishing degree by mixing coprecipitated hydrooxide which is obtained by adding alkali solution to the mixed water solution of manganese salts and nickel salts, with lithium compounds, and thereby sintering a products thus obtained at a specified temperature. CONSTITUTION: In the manufacture of a composite oxide for positive electrode active material indicated by a formula, alkali water solution is added first to the mixed water solution of manganese salt and nickel salt the amounts of which are divided by a specified volumetric ratio, so as to be coprecipitated, the product is then filtered and cleaned with water, and a manganese nickel hydrooxide is thereby obtained thereafter. It is proved by a X ray diffraction that the product is roughly of a single phase, and thereby its crystal finishing degree is high. Next, a specified amount of a lithium compound such as lithium hydrooxide and the like is mixed to the aforesaid composite hydrooxide so as to be sintered, and a composite oxide containing lithium with a crystal structure in which Li is easily moved, is thereby obtained. Furthermore, since its sintering temperature is set to be 600 to 800 deg.C, there occurs no disturbance in a crystal structure. By this constitution, the positive electrode active material can be obtained, which is high in capacity and excellent in cyclic characteristics. (in the formula, x is defined by 0.95>=x>=0.70).

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 electronic devices and cordless electronic devices have been rapidly developed, and there is a great demand for small and lightweight secondary batteries having high energy density as power sources for driving these electronic devices. From this point of view, non-aqueous secondary batteries, particularly lithium secondary batteries, are particularly expected as batteries having 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, and 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 dendrite-like Li does not deposit on the negative electrode, and the safety of the battery can be improved.

However, due to the problem of Co resources and cost, development of lithium-containing composite oxides replacing LiCoO ₂ is progressing, and LiNiO _{2 and the} like have begun to be noticed. Lithium-containing composite oxides of this kind such as LiNiO ₂ and LiCoO ₂ both show a high electric potential and are layered compounds having the same hexagonal crystal structure that can be used for the intercalation reaction, so that they are positive electrode active materials. The expectation is great as a material. From such a viewpoint, for example, Li _x NiO ₂ (US Pat. No. 4,302,518) is used.
_{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
A lithium-containing composite oxide in which a part of Ni of LiNiO ₂ , such as i _y Ni _1-x M _x O ₂ (where M is Ti, V, Mn, or Fe) is substituted with another metal, has been proposed. 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 (at least one selected from Ti, V, Cr, and Mn) (JP-A-4-267053) is also proposed. Further, development of a high energy density lithium secondary battery having a discharge potential of 4 V class using these active material materials is in progress.

[0005]

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

[0006] In response to such problems, there have been proposed lithium composite oxides in which a part of Ni is replaced with other metals as described above and those containing various metal elements at the same time. However, the one obtained by substituting a part of Ni of LiNiO ₂ with another metal has improved cycle reversibility, but tends to have a small discharge capacity and a low discharge voltage. As a result, the feature of high energy density is reduced. Of these, part of Ni is Mn
The lithium-containing composite oxides that had been substituted for had relatively good cycle reversibility, discharge capacity and discharge voltage as compared with other lithium-containing composite oxides.

Here, a part of Ni of LiNiO ₂ is replaced with Mn.
Synthesis of active materials substituted with
A method of adding a Mn compound such as manganese dioxide or manganese nitrate to a compound and a Ni compound such as nickel hydroxide and firing the mixture (hereinafter, referred to as a composite synthesis method) is generally used.
In this mixed calcination method, a calcination temperature of at least 800 ° C. or higher is required to surely replace a part of Ni with Mn.
Below this temperature, the substitution reaction was not completed as far as X-ray diffraction was observed, and a compound having a single phase and high crystal perfection could not be obtained.

However, when synthesized at a high temperature of 800 ° C. or higher, Ni and Mn enter into the site where Li should enter in the crystal, and the crystal structure is disturbed, and the cycle reversibility and discharge capacity are lowered. Thus, it was not very preferable to calcine the LiNiO ₂ -based lithium-containing composite oxide at a high temperature.

The present invention is intended to solve such a problem, in which a part of Ni is surely replaced by Mn to obtain the general formula Li.
A stable crystal field in which the crystal structure of the lithium-containing composite oxide represented by Ni _x Mn _(1-x) O ₂ is almost a single phase, the crystal perfection is high, the crystal does not collapse, and Li easily moves in the crystal. The present invention provides a manufacturing method capable of obtaining

[0010]

In order to solve the above-mentioned problems, a method for producing a positive electrode active material for a lithium secondary battery according to the present invention is a compound oxide composed of lithium, nickel and manganese, and has a general formula LiNi _x. A method for producing a positive electrode active material represented by Mn _(1-x) O _{2 in} which the x value in the formula is 0.95 ≧ x ≧ 0.70, in which a mixed aqueous solution of a manganese salt and a nickel salt is treated with an alkali. After adding a solution and coprecipitating hydroxides of manganese and nickel to obtain a composite hydroxide of manganese and nickel, the mixture is mixed with a lithium compound such as lithium hydroxide, and the mixture is heated to 600 ° C or higher and 800 ° C.
Firing is performed in the following temperature range.

[0011]

In the manufacturing method of the present invention, an alkaline solution is added to a mixed solution of a manganese salt and a nickel salt to coprecipitate manganese and a hydroxide of nickel, and thus a composite hydroxide of nickel and manganese (hereinafter referred to as Ni -Mn compound hydroxide), the crystal structure has reached a solid solution level in which Ni is partly replaced by Mn, and the crystal perfection is extremely single phase even in X-ray diffraction. It is expensive.

Then, L is added to the Ni / Mn composite hydroxide.
When an i salt is added and fired, a lithium-containing composite oxide having a crystal structure in which Li easily moves within the crystal can be obtained. Further, in the present invention, the firing temperature is 600 ° C to 800 ° C.
Therefore, there is no disorder in the crystal structure.

In order to form a mixed valence state of Ni and Mn and obtain a stable crystal structure, at least Ni
The number of substitutions of Mn with Mn is required to be 0.05 or more. But,
If the substitution number of Ni for Mn exceeds 0.30, the strain of the crystal increases, the crystal structure collapses, and the imbalance of the mixed valence state creates a situation in which Li is hard to move, and the capacity of the active material decreases. It will be noticeable.

[0014]

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

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

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

In this embodiment, nickel sulfate and manganese sulfate were used as the Ni source and the manganese source of the coprecipitation raw material, but nickel nitrate as the nickel source and manganese nitrate as the manganese source can be basically prepared as an aqueous solution. Any salt can be used. Although the sodium hydroxide aqueous solution was used as the alkaline solution, other alkaline solutions such as a potassium hydroxide aqueous solution and a lithium hydroxide aqueous solution may be used.

Next, the firing process with the Li compound will be described. Lithium hydroxide was used as the Li compound, and the Ni / Mn composite hydroxide obtained by the above coprecipitation was added so that the sum of the atomic numbers of Mn and Ni and the atomic number of Li were equal, and then pulverized with a ball mill. While thoroughly mixing, the composite was placed in an alumina crucible and baked in oxygen at 550 ° C. for 20 hours for the first step, and then at 750 ° C. for 2 hours for the second step. After firing, the mixture was slowly cooled to room temperature and pulverized to obtain a positive electrode active material powder.

Several Mn.N having different Mn / Ni ratios
As a result of an attempt to synthesize the i composite hydroxide, the x value of the general formula LiNi _x Mn _(1-x) O ₂ showing the composition of the active material was 0.7.
With the 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 0.7, although the X-ray pattern is almost a single phase, the peak intensity is weakened and the crystallinity tends to be lowered. Furthermore, when the x value was less than 0.5, the hexagonal layered structure was broken.

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 then the mixture was mixed with a solvent of N-methylpyrrolidinone (NMP) and polyvinylidene fluoride (PVDF) as a binder. ) Was dissolved to form a paste. The amount of PVDF was adjusted to 4 parts by weight with respect to 100 parts by weight of the positive electrode active material. Next, this paste was applied on one side of an aluminum foil, dried and rolled to obtain an electrode plate. FIG. 1 is a vertical sectional view of a coin-type lithium secondary battery used in an example of the present invention. In FIG. 1, the positive electrode 1 is obtained by punching the electrode plate into a disk shape and is installed inside the positive electrode case 2. The negative electrode 3 is formed by pressure-bonding metallic lithium on a stainless steel net 5, and is spot-welded inside the sealing plate 4. A polypropylene separator 6 is arranged between the positive electrode 1 and the negative electrode 3, and an electrolytic solution 7 is injected. In addition, it was sealed via a polypropylene gasket 8. The electrolyte used was one mol of lithium hexafluorophosphate (LiPF ₆ ) dissolved 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 Mn _(1-x) O ₂ is 0.1, 0.2, 0.
3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.
9, 0.95, 1.0, and these were used to produce a coin-type battery in the same manner as above. In addition, x = 1.0 is LiNiO ₂ containing no Mn. Then, a charge / discharge cycle life test was performed using these batteries. Charge / discharge conditions are 0.5 mA / c to the positive electrode at room temperature (20 ° C)
Charging and discharging were performed at a constant current of m ² , and the final charging voltage was 4.3 V and the final discharging voltage was 3.0 V.

FIG. 2 shows the result 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 50 to
80mAh / g and small cycle deterioration
At the 50th cycle, the capacity decreased to 50% of the initial capacity, and the deterioration continued thereafter.

The positive electrode active material in the range of x = 0.5 to 0.6 had an initial capacity of 110 to 120 mAh / g, but the cycle deterioration was large, and 70% of the initial capacity at the 50th cycle.
It deteriorated even after that.

The positive electrode active material in the range of x = 0.7 to 0.95 has a large initial capacity of 140 to 150 mAh / g and a small cycle deterioration.
While maintaining 0%, there was almost no decrease in capacity even after repeated cycles. However, although the positive electrode active material having x = 1.0 and containing no Mn has an initial capacity of 150 mAh / g or more, the cycle deterioration is large, and at the 50th cycle, the initial capacity is 6 mA.
It dropped to 5% and continued to deteriorate.

As is clear from the above results, the value of x in the active material LiNi _x Mn _(1-x) O ₂ synthesized using the Ni-Mn composite hydroxide prepared by coprecipitation is 0.7.
Those in the range of to 0.95 are preferable.

Next, studies were conducted to change the firing temperature for the positive electrode active material represented by the general formula LiNi _x Mn _(1-x) O ₂ having a value of x in the range of 0.70 to 0.95. The step of 550 ° C. for 20 hours, which is the first step of firing, is performed in the same manner as above, and the firing temperature is 550 ° C. and 60 for the subsequent firing.
0 ° C, 650 ° C, 700 ° C, 750 ° C, 800 ° C, 85
The temperature was 0 ° C. and 900 ° C. Then, a battery similar to the above was constructed using these positive electrode active materials, and a charge / discharge cycle test under the same conditions as the above was conducted. This result is shown in FIG.
The x value in the above formula was 0.8.

As is clear from FIG. 3, the firing temperature is 6
The active material synthesized at 00 ° C to 800 ° C has good initial capacity and cycle characteristics, and the one having a temperature of 550 ° C has insufficient initial capacity and cycleability, and 850 ° C to 900 ° C.
However, the initial capacity was slightly smaller and the cycle deterioration was larger.

From the above results, it is preferable that the baking temperature of the positive electrode active material is 600 ° C. to 800 ° C. However, at 800 ° C., cycle deterioration becomes a little larger and the initial capacity becomes a little smaller. 650 ° C to 750 ° C is preferable because the cycle deterioration is slightly increased although it is good.

In the present embodiment, the case where the x value is 0.8 has been described. As a result of conducting the same study on the firing temperature for the case where the x value is in the range of 0.70 to 0.95, The result showing the same tendency as in the case of x = 0.8 was obtained.

Using the conventional mixed synthesis method, LiNi
A positive electrode active material having a composition of _0.8 Mn _0.2 O ₂ was synthesized.
First, nickel hydroxide, lithium hydroxide, and manganese hydroxide having an atomic ratio of Ni: Mn: Li of 0.8: 0.2:
Weighed so as to be 1.0, mixed while pulverizing with a ball mill, put the mixture in an alumina crucible and baked the first stage in oxygen at 550 ° C. for 20 hours, and then 750
The second step was baked at 2 ° C for 2 hours. After firing, the product was slowly cooled to room temperature and pulverized to obtain a positive electrode active material. The X-ray diffraction pattern of this active material did not have a single phase, but had a plurality of phases. Therefore, the firing temperature for the second step is set to 6
As a result of studying changing the temperature in the range of 50 ° C. to 900 ° C., a single phase can be obtained by setting the firing temperature to 800 ° C. or higher.

FIG. 4 shows the results of the same charge / discharge cycle test as described above using the active material prepared at each of the above temperatures. The positive electrode active material having a firing temperature of 650 ° C. to 750 ° C. has a single phase. No crystal structure was obtained, the initial capacity was small, and the cycle deterioration was remarkable. At almost 50 cycles, the capacity decreased to 50% or less of the initial capacity, and the deterioration proceeded thereafter.

On the other hand, the active material synthesized at a firing temperature of 800 ° C. or higher has a single-phase crystal structure and maintains 80% of the initial capacity at the time of 50 cycles, but the capacity value is 100 mAh.
/ G or less and further decreased.

[0033]

As described above, in the method for producing a positive electrode active material for a lithium secondary battery of the present invention, an alkaline solution is added to a mixed solution of a nickel salt and a manganese salt to coprecipitate a hydroxide of nickel and manganese. By doing so, a composite hydroxide of nickel and manganese (hereinafter referred to as Ni / Mn composite oxide) is obtained, so that the crystal structure reaches a solid solution level in which a part of Ni is surely replaced by Mn, and X-ray diffraction However, it is almost a single phase and the degree of crystal perfection is extremely high. Then, when a Li salt is added to the Ni / Mn composite hydroxide and baked, a lithium-containing composite oxide having a crystal structure in which Li easily moves within 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 drawings]

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

FIG. 2 is a graph showing the relationship between the capacity of the positive electrode active material and the number of cycles when the x value 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 and the number of cycles of a positive electrode active material synthesized by a conventional manufacturing method.

[Explanation 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

Claims (2)

[Claims] 1. A lithium-containing composite oxide containing lithium, nickel and manganese, which has a general formula of LiNi _x Mn _(1-x).
The x value in the formula represented by O ₂ is 0.95 ≧ x ≧ 0.70
And a method for producing a positive electrode active material, wherein an alkali solution is added to a mixed aqueous solution of manganese salt and nickel salt to coprecipitate hydroxide of manganese and nickel to obtain a composite hydroxide of manganese and nickel. Then, a method for producing a positive electrode active material for a lithium secondary battery, which comprises mixing with a lithium compound such as lithium hydroxide and firing the mixture. 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 manganese and nickel and a lithium compound is fired at 600 ° C. or higher and 800 ° C. or lower.