JP3257350B2 - Non-aqueous electrolyte secondary battery and method for producing its positive electrode active material - Google Patents

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 and cordless electronic devices have been rapidly advanced, and there is a strong demand for a small, lightweight, and high energy density secondary battery as a power supply for driving such electronic devices. From such a viewpoint, non-aqueous secondary batteries, particularly 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 working potential of LiCoO ₂ is L
This is a problem when lithium metal 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 using a carbon material for the negative electrode. It has become possible to largely solve the problems of safety and reduction of charge / discharge efficiency due to generation of dendritic lithium.

However, from the viewpoint of Co resources and cost, and further from the viewpoint of developing a lithium ion secondary battery having a higher energy density, the development of a lithium-containing metal oxide cathode in place of LiCoO ₂ has been progressing.
Positive electrode active materials centering on O ₂ have been attracting attention. Li
Such lithium-containing metal oxides such as NiO ₂ and LiCoO ₂ are both layered compounds having a potential close to 4 V and having a hexagonal crystal structure capable of utilizing an intercalation reaction. From this perspective,
For example, Li _x NiO ₂ (US Pat. No. 4,302,518),
LiYO ₂ , such as Li _y Ni _{2 -y} O ₂ (Japanese Patent Application Laid-Open No. 2-40861), or Li _y Ni _x Co
_1-x O ₂ (JP-A-63-299056) and Li _y N
There has been proposed a lithium-containing metal oxide in which a part of Ni of LiNiO ₂ such as i _1-x M _x O ₂ (where M is any of Ti, V, Mn, and Fe) is substituted with another transition metal. I have.
_{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 F
e, at least one selected from Co, Ni, and N
(At least one selected from Ti, V, Cr, and Mn) (Japanese Patent Application Laid-Open No. 4-267053). Development of a high energy density secondary battery having a discharge potential of 4 V class using these active material materials has been promoted.

[0005] Further, the particle shape of the positive electrode active material has also been reported, and it is already known to use a single crystal aggregate of spherical, substantially spherical or ellipsoidal primary particles. Characteristics have not yet been obtained.

[0006]

SUMMARY OF THE INVENTION The present invention relates to LiNiO ₂
The present invention relates to a lithium-containing metal oxide positive electrode active material in which a part of Ni is replaced by another metal. LiNiO ₂ is L
A secondary battery having an operating potential close to 4 V with respect to i and having a high energy density can be realized when used as a positive electrode active material. However, in the discharge characteristics, a specific capacity of 160 mAh / g or more can be obtained at the beginning of the cycle. There was a problem that it could not be obtained. In order to solve such a problem, there have been proposed lithium-containing metal oxides in which a part of Ni is replaced with another metal as described above, and those containing simultaneously various kinds of metal elements. In particular, the better the cycle characteristics were, the more lithium-containing metal oxides whose crystal structure was composed of a single phase Ni in which a part of Ni was surely replaced by another metal were shown.

However, most of LiNiO ₂ in which a part of Ni is replaced by another metal has improved cycle characteristics, but has a tendency to reduce discharge capacity, and also has a reduced discharge voltage, resulting in high voltage and high energy. The result is a reduction in the density feature. Among these substitution metals, those in which part of Ni is substituted by Co or Al have cycle characteristics,
Both the discharge capacity and the discharge voltage were better than other lithium-containing metal oxides. Here, for example, Li
This is a method for synthesizing Ni _(1-X) Co _X O ₂ , in which a predetermined amount of a Co compound such as cobalt hydroxide is added to a Li compound such as lithium hydroxide and a Ni compound such as nickel hydroxide to perform a heat treatment. Was common. However, in such a method, it is difficult to obtain a single-phase composite oxide in which Co is completely dissolved and a part of Ni is substituted, and some unreacted phases remain. In addition, in the reaction process, the lithium ion of the lithium compound melted by heat is inserted into the interlayer portion of the layered structure in which the nickel ion, the cobalt ion, and the hydroxide ion are bonded, so that LiNi is inserted.
_(1-X) Co _X O ₂ is generated. The thus obtained lithium-containing metal oxide does not grow until the single crystal grains have a layered structure in which the single crystal grains are regularly stacked, and single crystal grains having an extremely fine layered structure grow in various directions. . Then, by the aggregation of these fine single crystal grains, irregular secondary particles having various shapes are generated. Therefore, in such a positive electrode active material particle, the layer surface where lithium is intercalated / deintercalated is not exposed toward the outside of the particle, so that the capacity characteristics and the discharge voltage characteristics particularly at the time of high-rate charge / discharge are poor. It is enough.

The present invention has been made to solve such a problem, and completely replaces a part of Ni with another metal to provide a single-phase lithium-containing metal oxide, and has a lithium intercalating / intercalating method. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a deintercalating reaction that proceeds smoothly, having a high capacity and excellent high-rate charge / discharge characteristics, and a method for producing a positive electrode active material thereof.

[0009]

In order to solve the above problems, the present invention provides an active material having a general formula LiNi _(1-X) M _X O
₂ (M = Co or Al) is a lithium-containing metal oxide, and a large number of fine crystal
A positive electrode having substantially spherical or ellipsoidal secondary particles is used, and Ni is used as a method for producing the active material.
A lithium compound is mixed with a composite hydroxide obtained by adding an alkali solution to a mixed aqueous solution of a salt and a Co salt or an Al salt to co-precipitate a hydroxide of Ni and Co or Al, and then heat treating the mixture. It is obtained by doing.

More specifically, the average particle size of the secondary particles of the positive electrode active material is 2 to 20 μm, and the value of x in the above general formula is 0.0
The range is 5 to 0.30. In addition, the heat treatment temperature of the mixture is set to 600 ° C to 800 ° C.

[0011]

In the method for producing a cathode active material according to the present invention, an alkaline solution is added to a mixed solution of a Ni salt and a Co salt or an Al salt to coprecipitate a hydroxide of Ni and Co or a hydroxide of Ni and Al. / Co or Ni / Al composite hydroxide is obtained. At this stage, the crystal structure changes part of Ni to Co.
Alternatively, it is a solid solution in which Al is surely substituted, and it can be confirmed by powder X-ray diffraction that it is a single phase. Then, by adding a Li compound to the composite hydroxide and performing a heat treatment, a ternary composite metal oxide containing Li as a solid solution can be generated. Further, in the production method according to the present invention, a single crystal grain of the generated lithium-containing composite metal oxide has a layered structure in which the single crystal grains are regularly stacked. Or it becomes ellipsoidal. As a result, the layered structure portion of each crystal in which lithium is intercalated / deintercalated is exposed toward the outside of the secondary particles, and particularly, high-rate charge / discharge characteristics are improved.

In the present invention, the positive electrode active material has a general formula LiN
The value of x in _{i (1-X) M X} O 2 , it is important that it is 0.05 to 0.30, significant reduction in particular cycle characteristics without effect can be obtained in the substitution of less than 0.05 . Conversely, 0.
If it exceeds 30, the crystal structure is distorted and the capacity as an active material is reduced. In addition, Ni / Co or Ni / A
The heat treatment temperature of the composite hydroxide and the lithium compound is 60
The temperature is preferably from 0 ° C to 800 ° C, more preferably from 700 ° C to 750 ° C. When the temperature is lower than 600 ° C., the termination of the reaction is insufficient, so that a complete single-phase lithium-containing metal oxide cannot be obtained, and no effect can be obtained. On the other hand, 800
If the temperature exceeds ℃, the crystal structure will be distorted and the capacity will be reduced.

[0013]

【Example】

(Example 1) Hereinafter, the present invention will be described in detail with reference to examples. FIG. 1 shows a longitudinal sectional view of the cylindrical battery of the present invention. In the figure, reference numeral 1 denotes a battery case formed by processing a stainless steel plate having resistance to organic electrolyte, 2 denotes a sealing plate provided with a safety valve, and 3 denotes an insulating packing. Reference numeral 4 denotes an electrode plate group, in which a positive electrode and a negative electrode are spirally wound a plurality of times through a separator, and a case 1 is provided.
Is housed inside. A positive electrode lead 5 is drawn from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn 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. Hereinafter, the method for producing the positive electrode active material and the positive and negative electrode plates will be described in detail.

First, a method for producing a Ni / Co composite hydroxide by coprecipitation according to the present invention will be described. A commercially available nickel sulfate aqueous solution was prepared by adding commercially available nickel sulfate to water,
A predetermined amount of cobalt sulfate (to obtain a target Ni / Co ratio) was added thereto, and water was further added to prepare a saturated aqueous solution containing nickel sulfate and cobalt sulfate. Then, when an alkaline solution in which sodium hydroxide was dissolved was slowly added to the aqueous solution with stirring, precipitation (coprecipitation) of hydroxides of Ni and Co started simultaneously. After the precipitation was completed by sufficiently adding an alkaline solution, the precipitate was collected by filtration and washed with water. Washing was repeated while checking the pH, and after the remaining alkali was almost completely eliminated, the resultant was dried in hot air at 100 ° C.

The Ni / Co composite hydroxide thus obtained was found to be very close to a single phase as a result of powder X-ray diffraction.
Was confirmed to contain. In the first embodiment, nickel sulfate was used as a Ni source and cobalt sulfate was used as a Co source as a raw material for coprecipitation.
Any salt that can produce an aqueous solution such as cobalt nitrate as a Co source can be used. 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 charged into a ball mill and mixed well so that the sum of the number of Ni and Co atoms of the Ni / Co composite hydroxide was equal to the number of Li atoms. This mixture was placed in a crucible made of alumina and heat-treated at 700 ° C. for 10 hours in air. Then, after natural cooling, pulverization and classification are performed to obtain an average particle size of 10
A positive electrode active material powder of μm was obtained. The size of the fine crystal grains corresponding to the primary particles was 0.2 μm to 1.5 μm.

In this way, composite hydroxides having different Ni / Co ratios are prepared, and a positive electrode active material is synthesized.
_(1-X) Lithium-containing metal oxide powder having an x value of Co _x O ₂ from 0 to 0.50 (corresponding to batteries A to H, respectively)
I got As a result of powder X-ray diffraction, in each case, a single-phase product was obtained for both the composite hydroxide and the lithium-containing metal oxide. Note that x = 0 means that the synthesis was performed without using a salt of a Co source.

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

As the negative electrode, a mesophase small sphere graphitized (hereinafter referred to as mesophase graphite) was 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 carboxymethyl cellulose solution was added and kneaded to form a paste.
And apply this paste on both sides of the copper foil, after drying,
Rolled to 0.21mm thick, 39mm wide, 280m long
m negative electrode plate.

A lead made of aluminum is attached to the positive electrode plate, and a nickel lead is attached to the negative electrode plate. 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 4.0 mm, 50 mm high battery case. The electrolyte was prepared by dissolving 1 mol / l of LiPF ₆ as an electrolyte in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 20:80. Then, the battery was sealed to obtain a completed battery.

Comparative Example 1 Instead of the coprecipitation method, Li in which a part of Ni was replaced with Co using the conventional synthesis method shown below.
A positive electrode active material having a composition of Ni _0.8 Co _0.20 O ₂ was synthesized. First, nickel hydroxide, cobalt hydroxide and lithium hydroxide are mixed at an atomic ratio of Ni: Co: Li of 0.8: 0.
The weight was weighed so as to be 2: 1.0 and sufficiently mixed by a ball mill. This mixture was put into an alumina crucible and heat-treated at 700 ° C. for 10 hours in air.
After natural cooling, pulverization and classification are performed, and the average particle size is about 10 μm.
m of the positive electrode active material powder. As a result of powder X-ray diffraction of the active material, the presence of a peak which was considered to be partially an unreacted phase was confirmed. With respect to this active material, a positive electrode plate was prepared in the same manner as in (Example 1), and a battery was formed in the same manner as in (Example 1) except for all the other conditions.

The batteries A to I were subjected to a charge / discharge cycle test under the following conditions. Charging was performed at a constant voltage of 4.2 V for 2 hours, and a constant current charge of 420 mA was set until the battery voltage reached 4.2 V. The discharge was performed at a constant current of 610 mA, and the discharge end voltage was set to 3.0 V. A cycle test of such charge / discharge was performed in an environment of 20 ° C., and the discharge capacity at the 5th cycle was set as the initial capacity, and the point at which the discharge capacity deteriorated to 300 mAh was set as the end of cycle life, and the value of the number of cycles was defined as (Table) This is shown in 1).

[0023]

[Table 1] As shown in Table 1, the smaller the x value, that is, the smaller the amount of substitution by Co, the larger the initial capacity tends to be. In the case of the battery A containing no Co, a battery with a high capacity of 650 mAh could be obtained. However, the cycle characteristics are extremely poor, and the capacity is reduced to less than half of the initial capacity in 70 cycles. Although the initial capacity is slightly reduced by gradually increasing the amount of Co substitution by the coprecipitation method, the cycle characteristics are remarkably improved, and a cycle life of 500 cycles or more is provided when the x value is in the range of 0.05 to 0.30. You can see that. Among them, when x = 0.20, the cycle characteristics were the best. On the other hand, the battery I in which the cathode active material was synthesized by the conventional synthesis method without using the coprecipitation method had an initial capacity of 5
It is as small as 20 mAh and the cycle characteristics are not good.
This is considered to be due to the fact that the synthesized positive electrode active material is not a complete single phase but a mixture with an unreacted phase. However, it is considered that this also contributes to the shape of the active material particles. FIGS. 2A and 2B show the particle structure diagrams of the positive electrode active material powders used in Battery E and Battery I, respectively, as taken by a scanning electron microscope (SEM). Although the positive electrode active material of the battery E synthesized using the coprecipitation method has a substantially spherical shape,
The positive electrode active material of Battery I synthesized by a conventional synthesis method is amorphous. That is, in the battery E, the spherical shape means that the layered structure portion where lithium can be intercalated / deintercalated is exposed toward the outside of the secondary particles, and the battery E has a high discharge rate of 610 mA. Has good capacity characteristics. Note that such spherical secondary particles are formed at the time when the composite hydroxide of Ni and Co is obtained, and the particle shape is substantially maintained even after the Li compound is added and heat treatment is performed. I have. On the other hand, battery I
It is considered that the ratio of the layered structure portion exposed to the outside of the secondary particles decreases in the irregular secondary particles as described above, and the capacity characteristics due to high-rate discharge decrease.

From the above, it is assumed that x in the positive electrode active material LiNi _(1-X) Co _X O ₂ synthesized by the coprecipitation method according to the present invention.
Is in the range of 0.05 to 0.30 in both battery capacity and cycle characteristics. Example 2 The general formula LiN was obtained by the coprecipitation method according to the present invention.
i for _{(1-X) Co X O} 2 (x = 0.20) of those were examined for the heat treatment temperature. Heat treatment temperature is 55
0 ° C, 600 ° C, 700 ° C, 750 ° C, 800 ° C, 85
The positive electrode active material was synthesized by changing the temperature to 0 ° C. and 900 ° C., respectively, and a positive electrode plate and a battery were formed using these active materials in the same manner as in (Example 1). These batteries were subjected to a charge / discharge cycle test in the same manner as in (Example 1).

Table 2 shows the initial capacity and the number of cycles at the end of life of each battery.

[0026]

[Table 2] Those having a heat treatment temperature of 600 ° C. to 800 ° C. have good initial capacity and cycle characteristics, and particularly 700 ° C. to 75 ° C.
It can be seen that the one at 0 ° C. is optimal in cycle characteristics. Battery J with a heat treatment temperature of 550 ° C. has an initial capacity,
Both of the cycle characteristics are inadequate. This is because at 550 ° C., the completeness of the crystal structure is low and a complete single-phase active material cannot be obtained, and lithium intercalation / deintercalation cannot be sufficiently performed. It is thought to be due to.
On the other hand, in the batteries O and P having the heat treatment temperature of 850 ° C. or more, both the initial capacity and the cycle characteristics tend to decrease. This is considered to be due to a decrease in capacity and a decrease in cycle characteristics due to distortion of the crystal structure of the product due to the heat treatment temperature being too high.

From the above, it is apparent that the heat treatment temperature for synthesizing the positive electrode active material is required to be 600 ° C. to 800 ° C., more preferably 700 ° C. to 750 ° C. (Example 3) LiNi _0.8 Co _0.2 O synthesized in (Example 1)
_{In 2} , the average particle size of the secondary particles of the active material was examined. The average particle size of the secondary particles is 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, and 25 μm by classification after pulverization.
Active material powders of m and 30 μm were prepared, and a positive electrode plate and a battery were respectively formed in the same manner as in Example 1 to obtain batteries Q to W. Then, a charge / discharge cycle test was performed in the same manner as in Example 1. However, for the battery W, the particle size was too large, so that it was impossible to produce a positive electrode plate, and the battery could not be configured.

Table 3 shows the initial capacity and the number of cycles at the end of life of these batteries.

[0029]

[Table 3] It can be seen that the batteries R, S, T, and U having an average particle size in the range of 2 μm to 20 μm have good initial capacity and cycle characteristics. Particularly, those having a thickness of 5 μm to 10 μm are preferable because of high capacity. Battery Q having an average particle size of 1 μm has good cycle characteristics, but has a large decrease in initial capacity. On the other hand, the battery V having a thickness of 25 μm has a relatively large initial capacity but has a remarkable decrease in cycle characteristics. Therefore,
The average particle size of the secondary particles of the positive electrode active material powder is 2 μm to 20 μm.
m, preferably 5 μm to 10 μm
m.

In the present embodiment and the comparative example, M = Co in the general formula LiNi _(1-X) M _X O ₂ of the positive electrode active material, but almost the same effect can be obtained when M = Al. Was. In this example and the comparative example, the mesophase graphite was used for the negative electrode. Of course, any other graphite material, such as cokes and carbon fibers, can be used as long as they can intercalate / deintercalate lithium. It is possible. Further, a mixed solvent of EC and EMC was used as a solvent for the electrolytic solution, but carbonates such as propylene carbonate and diethyl carbonate as other solvents,
Conventionally known solvents such as ethers such as 1,2-dimethoxyethane and 2-methyltetrahydrofuran, and aliphatic carboxylic acid esters such as methyl propionate and ethyl acetate can be used alone or as a mixed solvent. Similarly, for the electrolyte, LIBF ₄ , LiClO ₄ ,
Any conventionally known materials such as 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 or Al
Is a single-phase LiNi that completely dissolves a part of Ni
It is possible to synthesize _(1-X) M _X O ₂ , which has a high capacity and a high energy density by using this as a positive electrode,
A non-aqueous electrolyte secondary battery having excellent cycle characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery of the present invention.

2A is a diagram showing a particle structure of a positive electrode active material powder used in a battery E of the present invention. FIG. 2B is a diagram showing a particle structure of a positive electrode active material powder used in a comparative battery I.

[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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-294364 (JP, A) JP-A 8-222220 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 H01M 10/40 C01G 53/00

Claims (5)

(57) [Claims] 1. The general formula LiNi _(1-x) M _x O ₂ (M = Co
Or the value of x is in the range of 0.05 to 0.30). The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, the method comprising: the alkaline solution was added to a mixed aqueous solution of nickel salt, a higher yield Ru engineering composite hydroxide co-precipitated hydroxide of any of the hydroxide and nickel of cobalt and aluminum, the composite hydroxide Adding a lithium compound to the product and mixing the mixture, followed by heat treatment of the mixture. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery. 2. The positive electrode active material forms secondary particles.
The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 , wherein the secondary particles have an average particle diameter in a range of 2 to 20 µm. 3. The composite hydroxide has a molar ratio of either cobalt or aluminum to nickel of 0.30:
The method for producing a positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is in the range of 0.70 to 0.05 to 0.95. 4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the temperature of the heat treatment is from 600 ° C. to 800 ° C. 5. A compound of the general formula LiNi _(1-X) M _X O ₂ (M = Co
Or the value of x is in the range of 0.05 to 0.30), and the single crystal grains coprecipitate a hydroxide of either cobalt or aluminum with a hydroxide of nickel. A lithium compound is added to the obtained composite hydroxide, and this mixture is heat-treated. The single crystal grains are aggregated to form spherical, nearly spherical or ellipsoidal secondary particles. Non-aqueous electrolyte secondary including a positive electrode using a lithium-containing composite oxide as an active material, a negative electrode using a carbon material capable of intercalating and de-intercalating lithium, and a non-aqueous electrolyte battery.