JPH09129231A - Nonaqueous electrolytic secondary battery and manufacture of positive electrode active material therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance thermal stability in particular by improving a method for manufacturing LiNi(1-x) Cox O2 as a positive electrode active material for an aqueous electrolytic secondary battery. SOLUTION: In this method, the composite hydroxide of Ni and Co is synthesized by the coprecipitation method, mixed with a lithium compound and then thermally treated, thereby preparing a positive electrode active material. The active material so prepared has a true half-value breadth β (003) of a diffraction peak on a plane (003) with powder X-ray diffraction is between 0.01deg. and 0.10deg. A nonaqueous electrolyte secondary battery using the active material so obtained has large capacity as well as high thermal stability in particular.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement in a method for producing a positive electrode active material.

[0002]

2. Description of the Related Art In recent years, portable electronic devices and cordless devices have been rapidly developed, and there is a great demand for secondary batteries having a small size, a light weight and a high energy density as driving power supplies for these electronic devices. From this viewpoint, non-aqueous secondary batteries, especially lithium secondary batteries, are highly expected as batteries having high voltage and high energy density.

Under such circumstances, a lithium ion secondary battery using LiCoO ₂ as a positive electrode and a carbon material capable of intercalating / deintercalating lithium as a negative electrode has already been developed and commercialized. The operating potential of LiCoO ₂ is L
This is a problem when metallic lithium is used for the negative electrode because the battery voltage is high because it is as high as about 4 V with respect to i, and the intercalation reaction of Li is used by using the carbon material for the negative electrode. In addition, it has become possible to significantly solve the problems of safety and deterioration of charge / discharge efficiency due to the generation of dendrite-like lithium.

However, in terms of Co resources and costs,
And even higher energy density lithium ion secondary
From the viewpoint of battery development, LiCoO 2_TwoRichi to replace
The development of positive electrodes containing complex oxides of
O_TwoThe positive electrode active material centered on is attracting attention. Li
NiO_TwoAnd LiCoO_TwoThis kind of lichen
All of the complex oxides containing um show a potential close to 4V.
Hexagonal crystals that can be used for intercalation reactions
It is a layered compound having a crystal structure. From this perspective,
For example, Li_xNiO_Two(U.S. Pat. No. 4,302,518),
Li_yNi_2-yO _Two(Japanese Patent Laid-Open No. 2-40861), etc.
LiNiO_TwoRelated to or Li_yNi_xCo
_1-xO_Two(JP-A-63-299056) and Li_yN
i_1-xM_xO_Two(However, M is Ti, V, Mn, or Fe
Such as LiNiO_TwoPart of Ni in other transition metals
A lithium-containing composite oxide substituted with is proposed.
Other, A_xM_yN_zO_Two(However, A is an alkali metal, M is
Transition metal, N is one of Al, In and Sn)
-90863) and Li_xM_yN_zO_Two(However, M is F
At least one selected from e, Co, and Ni is N
At least one selected from Ti, V, Cr, and Mn
Seeds) (Japanese Patent Laid-Open No. 4-267053) and other types of gold
Proposals have been made to include elements of the genus at the same time. And this
Using these active material materials, a high discharge potential of 4V class
The development of energy density secondary batteries is underway.

The present inventors have synthesized various positive electrode active materials centering on LiNiO ₂ and evaluated their positive electrode characteristics. As a result, the LiNiO ₂ positive electrode has a specific discharge capacity of 180 mAh / g or more at the beginning of the cycle.
It was found that the characteristics were significantly deteriorated with the increase in the number of cycles and the capacity was deteriorated to about 65% of the initial capacity at the 50th cycle. Therefore, an attempt was made to improve the crystal structure by substituting a part of Ni with another transition metal element. As a result, LiNi _(1-X) Co _x O ₂ with Co added and substituted, has cycle characteristics,
It was found that both the discharge capacity and the discharge voltage were better than other lithium-containing composite oxides.

[0006]

SUMMARY OF THE INVENTION The present invention is directed to LiNiO.
₂ relates to a lithium-containing composite oxide positive electrode active material LiNi _(1-x) Co _x O _{2 in} which a part of Ni of ₂ is replaced by Co. As mentioned above, LiNi _(1-x) Co _x O ₂ is 170 mA.
It has a specific capacity of h / g or more and good cycle characteristics. However, there is a problem that the thermal stability of the active material decreases during charging, that is, in a state where lithium is deintercalated from the positive electrode active material. This is the same as in the case of the LiNiO ₂ positive electrode, but in general, the Ni-based lithium-containing composite oxide has a lower charging voltage than the conventional LiCoO ₂ positive electrode and reversibly deintercalates /
Since the amount of intercalated lithium is large, the stability of the crystal structure in a fully charged state tends to decrease.
Therefore, when the battery is left in a charged state at a high temperature, its thermal stability is lowered. (For example, the battery in a charged state is heated to 150 ° C. at a rate of 5 ° C./min.
When kept at 0 ° C, the battery ignites and bursts after a few minutes. The present inventors aim to stabilize the crystal structure of LiNi _(1-x) Co _x O ₂ positive electrode active material in order to solve the above-mentioned problems without impairing the reversible capacity of the positive electrode, and its starting material The manufacturing method and physical properties of the retroactive material were investigated and examined. As a result, the obtained positive electrode active material is required to be a composite oxide in which Ni and Co are completely dissolved, which means that a single layer attributed to hexagonal crystal as information obtained by powder X-ray diffraction method. It was found that it was necessary to detect only the peak of 1 and the peak of only a single phase in the charged state. As a result of investigating the production method of the active material from such a viewpoint, in the production method of mixing a nickel salt, a cobalt salt, and a lithium compound as conventionally known and performing heat treatment (for example, JP-A-63-299056), Obtaining a complete single phase solid solution is difficult. Therefore, a nickel and cobalt salt is co-precipitated in an aqueous solution in advance to obtain a composite hydroxide of Ni and Co, and then the composite hydroxide and a lithium compound are mixed and heat treated, It was found that a lithium-containing composite oxide consisting of a single phase was obtained.
Further, it was found that the molar ratio of Co to be solid-solubilized should be 5% or more with respect to Ni. However, the crystallinity of the obtained lithium-containing composite oxide is largely involved in ensuring the above-mentioned thermal stability, and the half-value width of the diffraction peak attributed to the 003 plane obtained by the powder X-ray diffraction method. Alternatively, it was found that the value of the crystallite size in the c-axis direction, which is obtained from the value of the half width, is important.

[0007]

In order to solve the above problems, the present invention is obtained by adding an alkaline solution to an aqueous mixed solution of a nickel salt and a cobalt salt to coprecipitate a hydroxide of nickel and cobalt. Lithium-containing composite oxide LiNi _(1-x) Co _x O ₂ obtained by mixing a lithium compound with the above composite hydroxide and heat-treating this mixture is used as the positive electrode active material, and the true diffraction peak of the 003 plane is obtained. Half width β (0
03) is 0.01 to 0.10 deg. It is what it was. Specifically, x in the structural formula of the lithium-containing composite oxide
Value of 0.05 to 0.30.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a positive electrode active material according to the present invention, an alkaline solution is added to a mixed solution of Ni salt and Co salt to coprecipitate Ni and Co hydroxides.
/ Co composite hydroxide is obtained. At this stage, it can be confirmed that the crystal structure is a solid solution in which a part of Ni is surely replaced by Co, and the powder X-ray diffraction has a single phase. Then, a Li compound is added to this composite hydroxide and a heat treatment is performed to generate a ternary composite oxide in which Li is solid-dissolved. The active material thus obtained (hereinafter referred to as the coprecipitation method) is completely composed of a single phase, and also maintains the single phase even in a charged state. Only one peak is detected.
On the other hand, the active materials synthesized by the conventional synthesis method are Ni and C.
It is difficult to obtain a complex oxide in which o is completely solid-solved, and a mixture of only two phases or more is obtained. Also, this two-phase region becomes more prominent in the charged state,
The presence of two or more peaks is recognized in the powder X-ray diffraction. Therefore, the crystal structure becomes unstable, resulting in a decrease in thermal stability when left at a high temperature. On the other hand, when the positive electrode active material obtained by this coprecipitation method is used,
Although the stability of the crystal structure in the charged state increases, it is not enough to secure sufficient thermal stability. It is important that the crystal structure of the synthesized active material is high. In other words, it is necessary that the full width at half maximum of the diffraction peak on the 003 plane by powder X-ray diffraction is sufficiently small, and the value of the true full width at half maximum β (003) is 0.01 to 0.10 deg. Is required, and preferably 0.03 to 0.07. When it exceeds 0.10, the crystallinity is low, and the stability of the crystal structure is lacking in the charged state, and the thermal stability cannot be secured. On the other hand, when β (003) is less than 0.01, sufficient thermal stability can be secured, but since the crystal structure is too rigid, problems such as reduction in reversible capacity and degradation in cycle characteristics occur. . Note that these half-value widths contribute to the positive electrode active material obtained by the coprecipitation method, and the positive-electrode active material obtained by the conventional synthesis method has no thermal stability at any value. It is difficult to secure sex. LiNi _(1-x) C
The half-width of the diffraction peak of the 003 plane of o _x O ₂ or the size of the crystallite is disclosed in JP-A-267539 or JP-A-275274, both of which are coprecipitation methods. The present invention does not relate to the positive electrode active material synthesized according to the present invention and is outside the scope of the present invention.

Here, the method of obtaining the value of the true full width at half maximum β (003) is generally used, but when X-ray diffraction is performed, the diffraction width of the peak is not only the diffraction width of the peak itself but also the X-ray itself. It includes such things as the diffraction width of the device and the spread of the diffraction width due to the device. Therefore, in the present invention, in order to obtain the value of the true full width at half maximum of the peak itself, 10 to 20 parts by weight of high-purity Si powder as an internal standard substance is added and mixed to obtain powder X.
A line diffraction measurement was performed. Then, the diffraction peak of the 003 plane of the obtained lithium-containing composite oxide (X using CuKα
In the line diffraction, 2θ is 18 to 20 deg. ) And Si 1
Diffraction peak of 11 planes (2θ is 28d in the same X-ray diffraction
eg. The true half-value width β (003) is obtained by finding and correcting the respective half-value widths (in the vicinity). Such a method is an application of a method known as the Gakushin method to the true half-width of a carbon material to the oxide of the present invention.

Finally obtained composite hydroxide Ni _(1-x) C
o _x (OH) ₂ and lithium-containing composite hydroxide LiNi
It is important that the value of x of _(1-x) Co _x O ₂ is 0.05 to 0.30, and preferably 0.05 to 0.20. If it is less than 0.05, the stabilization of the crystal structure is insufficient, and it is difficult to secure the thermal stability regardless of the value of the true full width at half maximum. On the other hand, when x exceeds 0.30, the thermal stability is high but the discharge capacity is reduced.

[0011]

【Example】

(Example 1) The present invention will be described in detail below with reference to examples. FIG. 1 shows a vertical cross-sectional view of the cylindrical battery used in this example. In the figure, 1 is a battery case formed by processing an organic electrolyte resistant stainless steel plate, 2 is a sealing plate provided with a safety valve, 3
Indicates insulating packing. Reference numeral 4 denotes an electrode plate group, in which the positive electrode and the negative electrode are spirally wound a plurality of times with the separator interposed therebetween and are housed in the case 1. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode plate group 4, respectively. The method for producing the positive electrode active material and the positive and negative electrode plates will be described in detail below.

First, the production method by coprecipitation of the Ni / Co composite hydroxide according to the present invention will be described. Commercially available nickel sulfate is added to water to prepare a saturated nickel sulfate aqueous solution,
Cobalt sulfate was added to this so that the molar ratio of Ni and Co was 85:15, and water was further added to prepare a saturated aqueous solution containing nickel sulfate and cobalt sulfate. Then, an alkaline solution in which sodium hydroxide was dissolved was slowly added to this aqueous solution with stirring, and precipitation (coprecipitation) of hydroxides of Ni and Co simultaneously started. After sufficient addition of an alkaline solution to complete the precipitation, the precipitate was collected by filtration and washed with water. Washing with water was repeated while confirming the pH, and after almost all the residual alkali had disappeared, it was dried in hot air at 100 ° C.

The Ni / Co composite hydroxide Ni _0.85 Co _0.15 (OH) ₂ thus obtained was in a single phase as a result of powder X-ray diffraction, and as a result of elemental analysis, Ni _0.85 Co _0.15 (OH) ₂ was found to be almost in a target ratio. Was confirmed to contain Co. In Example 1, nickel sulfate was used as a Ni source in the coprecipitation source material, and
Although cobalt sulfate was used as the o source, any salt can be used as long as it can form an aqueous solution, such as nickel nitrate as the Ni source and cobalt nitrate as the Co source. Although sodium hydroxide was used as the alkali source, other alkali solutions such as potassium hydroxide and lithium hydroxide may be used.

Next, the steps of mixing with a Li compound and heat treatment will be described. Lithium hydroxide was used as the Li compound, and the mixture was put into a ball mill and mixed sufficiently so that the total number of Ni and Co atoms in the Ni / Co composite hydroxide and the number of Li atoms were equal. This mixture was placed in an alumina crucible and heat-treated in air at various temperatures and times. Then, after being naturally cooled, it is pulverized and classified to form a positive electrode active material powder LiNi _0.85 Co _0.15 O having an average particle size of about 10 μm.
_{And 2} .

The powders of lithium-containing composite oxides (corresponding to batteries A to G, respectively) thus obtained under various heat treatment conditions were subjected to powder X-ray diffraction, and in each case, a single phase was formed. It was a thing.

Next, a method for manufacturing the positive electrode plate will be described. To 100 parts by weight of the obtained positive electrode active material, 4 parts by weight of acetylene black was added, and to this mixture was added a solution of polyvinylidene fluoride (PVDF) as a binder in a solvent of N-methylpyrrolidinone (NMP). It was kneaded into a paste. The amount of PVDF added was adjusted to 4 parts by weight with respect to 100 parts by weight of the positive electrode active material. Next, this paste is applied to both sides of an aluminum foil, dried and rolled to a thickness of 0.14 mm and a width of 37 m.
The positive electrode plate had a length of m and a length of 250 mm.

As the negative electrode, graphitized mesophase spheres (hereinafter referred to as mesophase graphite) were used. 100 parts by weight of this mesophase graphite was mixed with 3 parts by weight of styrene / butadiene rubber as a binder, and an aqueous solution of carboxymethyl cellulose was added and kneaded to form a paste.
Then, this paste is applied to both sides of the copper foil, dried and rolled to a thickness of 0.21 mm, a width of 39 mm, and a length of 280 mm.
Was used as the negative electrode plate.

Aluminum leads are attached to the positive electrode plate and nickel leads are attached to the negative electrode plate, and the leads are spirally wound through a polyethylene separator having a thickness of 0.025 mm, a width of 45 mm, and a length of 740 mm. Diameter 1
It was delivered to a battery case with a height of 4.0 mm and a height of 50 mm.

As the electrolytic solution, ethylene carbonate (EC) is used.
And ethyl methyl carbonate (EMC) at 20:80
1 mol / l of L as an electrolyte in a solvent mixed in a volume ratio of
a solution obtained by dissolving iPF ₆ was injected. Then, the battery was sealed to obtain a completed battery.

The LiNi part of Ni using conventional synthetic methods rather than (Comparative Example 1) coprecipitation was replaced with Co _{0. 85} Co
A positive electrode active material having a composition of _0.15 O ₂ was synthesized. First,
The atomic ratio of Ni: Co: Li of nickel hydroxide, cobalt hydroxide, and lithium hydroxide is 0.85: 0.15: 1.
It was weighed so as to be 0, and thoroughly mixed with a ball mill.
Then, this mixture was placed in an alumina crucible and heat-treated in air at various temperatures and times. Then, after natural cooling, pulverization and classification were performed to obtain a positive electrode active material powder having an average particle size of about 10 μm. As a result of powder X-ray diffraction of these active materials, the existence of some peaks which are considered to be an unreacted phase was confirmed. With respect to these active materials, a positive electrode plate was produced in the same manner as in Example 1, and the other conditions were the same as in Example 1 to form a battery, and batteries B to J were obtained.

A charging / discharging test was performed on these batteries A to J under the following conditions. Charged at 4.2V for 2 hours with constant voltage charging, until the battery voltage reaches 4.2V, 42
It was set to be a constant current charge of 0 mA. The discharge is a constant current discharge of 610 mA, and the discharge end voltage is 3.
0 V was applied. Such charging / discharging is performed under an environment of 20 ° C. for 10
Cycle was performed, and the test was stopped in the charged state of the 11th cycle, and a thermocouple was attached to the surface of the battery and installed in a constant temperature bath. Then, the temperature of the constant temperature bath is 5 ° C./min. The temperature was raised at the rate of and the temperature was maintained at 150 ° C. for 60 minutes. And the time it took for each battery to ignite or explode (battery temperature was 150
Table 1 shows the maximum exothermic temperature of the battery and the value of the discharge capacity of each battery when the ignition did not occur. It should be noted that 0 minute means that the battery ignited and exploded before the tank temperature reached 150 ° C.

[0022]

[Table 1]

From Table 1, many of the batteries A to G using the positive electrode active material obtained by the coprecipitation method have high thermal stability, but the true half width β (003) is relatively large. Also, the battery B has ignited, and thermal stability cannot be secured. It is considered that, although they are a single phase in the charged state, their crystallinity is low, so that their structures are somewhat unstable and their thermal stability is lowered. β (00
The value of 3) is 0.10 deg. Battery C to Battery G which are the following
Has a single phase and has high crystallinity, so that sufficient thermal stability is ensured. Among them, the value of β (003) is 0.07 deg. It can be seen that the following batteries D to G have low maximum heat generation temperatures and higher thermal stability. However, although the battery G having the smallest value of β (003) has high thermal stability, the discharge capacity of the battery is extremely small. It is considered that this is because the crystallinity of the positive electrode active material was too high, and deintercalation / intercalation of lithium was hindered, resulting in a decrease in capacity.

On the other hand, in the batteries H to J using the positive electrode active material obtained by the conventional synthesis method instead of the coprecipitation method, regardless of the value of the true half-value width β (003), they all ignite and burst. However, the thermal stability could not be ensured. This is because the obtained active material is not completely a single-phase solid solution and is partially a mixture with an unreacted phase, so that the two-phase region becomes more remarkable in the charged state, and how the crystallinity is improved. Even if it is improved, the stabilization of the structure is insufficient and it is considered that the thermal stability cannot be secured.

From the above, synthesis by the coprecipitation method is indispensable for ensuring thermal stability and obtaining a positive electrode active material having excellent positive electrode characteristics, and the true half-value width β (003) value. Is 0.01 to 0.10 deg. It turns out that it is necessary to be.

Example 2 Next, in the present coprecipitation method, Ni
The value of x in the general formula is 0 to 0.
A composite hydroxide of up to 50 was synthesized, and a predetermined amount of lithium hydroxide was mixed with these, and heat treatment was performed at 750 ° C. for 5 hours to obtain a lithium-containing composite oxide positive electrode active material having various composition ratios. It was (X = 0 means that Co was not solid-dissolved at all.) Then, a positive electrode plate was produced in the same manner as in Example 1, and a battery was constructed in the same manner as in Example 1 under all other conditions. The battery obtained was subjected to the same thermal stability test as in Example 1, and the discharge capacity values are shown in Table 2.

[0027]

[Table 2]

From Table 2, in the battery K and the battery L in which the value of x is 0.03 or less, the battery was ignited in the thermal stability test. This is because the solid solution amount of Co is small and the crystal structure is stabilized. It is probable that the thermal stability could not be ensured due to insufficient heating. On the other hand, when the value of x is 0.05 or more, the thermal stability is high in any case, but when the value of x exceeds 0.30, the decrease of the discharge capacity is remarkably unfavorable. It is considered that this is because the reversible capacity originally possessed by the Ni-based lithium-containing oxide is greatly affected by Co, and the reversible capacity decreases as the charging voltage increases.

Therefore, it is important that the solid solution amount of Co, that is, the value of x is 0.05 to 0.30, and preferably 0.05 to 0.20 from the viewpoint of discharge capacity.

Although mesophase graphite was used for the negative electrode in the present examples and comparative examples, it is needless to say that other graphite materials, cokes, carbon fibers, and other carbon materials capable of intercalating / deintercalating lithium. Both can be used. Furthermore, EC and EM are used as the solvent for the electrolyte.
Although the mixed solvent of C was used, other solvents such as carbonates such as propylene carbonate and diethyl carbonate, ethers such as 1,2-dimethoxyethane and 2-methyltetrahydrofuran, aliphatic carboxylic acids such as methyl prepionate and ethyl acetate. Any conventionally known solvent such as acid ester can be used alone or as a mixed solvent. Similarly for the electrolyte, LIBF ₄ , LiCl
Any of conventionally known materials such as O ₄ and LiCF ₃ SO ₃ can be used.

[0031]

As described above, according to the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention, Co replaces a part of Ni and completely forms a solid solution LiNi _{(1 -x)} Co _x
It is possible to obtain O ₂ (the value of x is 0.05 to 0.30), and the true half-value width β (003) of the diffraction peak of the 003 plane by the X-ray diffraction is 0.01 to By setting it to 0.10, when this is used for the positive electrode, it is possible to provide a non-aqueous electrolyte secondary battery that gives a high capacity and is excellent in thermal stability.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery according to an example of the present invention and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

Claims (5)

[Claims] 1. A mixed aqueous solution of nickel salt and cobalt salt
Add an alkaline solution to remove the nickel and cobalt hydroxides.
Composite hydroxide Ni obtained by coprecipitation _(1-x)C
o_x(OH)_TwoAnd a lithium compound are mixed, and this mixture
Li-containing composite oxide LiNi obtained by heat treatment of
_(1-x)Co_xO_TwoIs used as the positive electrode active material, and the 003 plane diffraction
True half-value width β (003) of the peak is 0.01 to 0.10.
deg. For non-aqueous electrolyte secondary batteries characterized by
Manufacturing method of positive electrode active material. 2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the value of x of the composite hydroxide and the lithium-containing composite oxide is 0.05 to 0.30. 3. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium compound is lithium hydroxide. 4. A mixed aqueous solution of nickel salt and cobalt salt
Add an alkaline solution to remove the nickel and cobalt hydroxides.
Composite hydroxide Ni obtained by coprecipitation _(1-x)C
o_x(OH)_Two(X is 0.05 to 0.30) and lithium hydroxide
By mixing with um and heat treating this mixture
Obtained lithium-containing composite oxide LiNi_(1-x)Co_xO
_Two(X is 0.05 to 0.30) as a positive electrode active material,
The true full width at half maximum β (003) of the diffraction peak on the 003 plane is 0.
01-0.10 deg. Non-hydroelectricity characterized by
A method for producing a positive electrode active material for a resolving secondary battery. 5. A positive electrode mainly composed of the positive electrode active material obtained by the manufacturing method according to any one of claims 1 to 4, a non-aqueous electrolyte, and lithium are intercalated / deintercalated. A non-aqueous electrolyte secondary battery comprising the obtained carbon material.