JP2001135313A - Positive electrode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery using positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a nonaqueous electrolyte secondary battery with excellent cycle characteristic and high capacity, and to provide a nonaqueous electrolyte secondary battery using the positive electrode active material. SOLUTION: In a positive electrode active material for a nonaqueous electrolyte secondary battery using lithium cobalt oxide shown in a formula of LiCoO2, the lithium cobalt has a diameter of ferer of 0.1 to 4 μm in a projection figure through a SEM monitoring, and the positive electrode active material is formed of sphere or oval sphere secondary particles formed of primary particles of small crystals whose mean granular diameter is not more than 2 μm. The granular diameter of the secondary particle is from 1 to 40 μm. At least a part of the small crystals constructing the secondary particle is bonded by sintering. A tap concentration of the lithium cobalt oxide is not less than 2.2 g/cm3. The invention also provides a nonaqueous electrolyte secondary battery whose construction element is the positive electrode active material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery using lithium metal, a lithium alloy or carbon as a negative electrode, and more particularly to a non-aqueous electrolyte secondary battery having improved cycle characteristics. The present invention relates to a positive electrode active material for use.

[0002]

2. Description of the Related Art In recent years, with the spread of portable telephones and notebook personal computers, there has been a strong demand for the development of a small, lightweight, and high energy density secondary battery. As such a device, there is a lithium ion secondary battery using lithium metal, a lithium alloy or carbon as a negative electrode, and research and development are being actively conducted. A lithium-ion secondary battery using a lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, as a positive electrode active material is expected to be a battery having a high energy density because a high voltage of 4 V class can be obtained, and is practically used. Has been

In response to recent demands for higher capacity and higher current, it has become necessary to take measures such as increasing the packing density of a positive electrode active material in a battery. Therefore, specifically, a method such as reducing the amount of a conductive agent such as carbon mixed with the positive electrode active material has been adopted, and further attempts have been made to increase the particle size of the positive electrode active material and improve the packing density. Since the surface area involved in lithium ion insertion / desorption reactions decreases,
The discharge capacity of the positive electrode active material is reduced. On the other hand, if the particle size is reduced to increase the surface area involved in the insertion / desorption reaction, the carbon of the conductive agent becomes insufficient, and the discharge capacity also decreases. There was a problem.

As a method for solving the above-mentioned problem, an active material for a non-aqueous electrolyte secondary battery has been proposed which comprises a large number of small crystals of lithium cobaltate aggregated to form spherical or elliptical secondary particles. (Abstracts of
9th International Meetin
go on Lithium Batteries, Po
ster II Thur 56, 1998). When the positive electrode active material is used, the packing density of the positive electrode active material is certainly increased, and the energy density of the battery is improved.However, when the charge / discharge cycle is repeated, the voltage drop at the end of discharge increases, and the capacity decreases. And the cause had not been elucidated.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a positive electrode active material for a non-aqueous electrolyte secondary battery having excellent cycle characteristics and a high capacity, and a non-aqueous electrolyte secondary battery using the same. Things.

[0006]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that there is a deep relationship between the particle size of primary particles and their uniformity and cycle characteristics. The invention has been completed.

That is, the positive electrode active material for a nonaqueous electrolyte secondary battery according to the first embodiment of the present invention is a positive electrode active material for a nonaqueous electrolyte secondary battery using lithium cobaltate represented by the formula LiCoO _2. In the above, the lithium cobaltate has a Feret diameter of a projected figure of 0.1 to 4 μm by SEM observation.
m, and comprises spherical or elliptical spherical secondary particles in which a large number of primary particles of small crystals having an average particle diameter of 2 μm or less are collected. The diameter is in the range of 1 to 40 μm, and at least a part of the small crystals constituting the secondary particles are joined by sintering, and the tap density of the lithium cobaltate is 2.2 g / cm ³ or more. It is characterized by being.

[0008] A nonaqueous electrolyte secondary battery according to a second embodiment of the present invention is characterized in that the positive electrode active material according to the first embodiment is used as a constituent element.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention, the Feret diameter of a projected figure by SEM (scanning electron microscope) observation is 0.1 to 4 μm.
By using lithium cobaltate composed of spherical or elliptical secondary particles in which a large number of small crystal primary particles having a mean particle size of 2 μm or less are in the range of
Cycle characteristics can be improved. Note that the term “ferret diameter of a projected figure” used in this specification means the maximum length of the projected figure (particularly, the length in the longest direction when an ellipsoidal sphere is measured), and the dimension is set to 0.
The reason for setting the range to 1 to 4 μm is that when the thickness is less than 0.1 μm, the discharge capacity decreases, while when it exceeds 4 μm, the capacity deterioration due to the cycle and the voltage drop at the end of discharge increase. Further, the reason why the average particle diameter of the primary particles is 2 μm or less is that if the average particle diameter exceeds 2 μm, it is difficult to make the Feret diameter in the range of 0.1 to 4 μm.

The present inventor believes that the non-uniformity of the change in expansion and contraction of the primary particles due to charge and discharge has a great effect on the cycle characteristics, and has determined the particle size distribution of the primary particles as described above. Was. Lithium cobaltate is charged and discharged with C
Kotsuki et al. Report that the shaft expands and contracts by about 3% (J
ownal of the Electrochem
icalSociety, 141, 2972 (199)
4)) has been done. Therefore, in the case of secondary particles containing coarse particles and non-uniform primary particles, the amount of change in expansion and contraction varies greatly depending on the primary particles, so that the strain inside the secondary particles increases, and finally the joints of the primary particles crack. It is considered that as a result, no electrical contact is obtained, resulting in deterioration of cycle characteristics.

[0011] Next, the reason why the secondary particles are spherical or elliptical in shape is that if they have an irregular shape other than these, a tap density of 2.2 g / cm ³ or more cannot be obtained, and excellent cycle characteristics can be obtained. This is because characteristics cannot be obtained. The reason for setting the particle diameter of the secondary particles to 1 to 40 μm is that if the particle diameter is less than 1 μm, the discharge capacity at the time of high-load discharge decreases, whereas if it exceeds 40 μm, the applicability of the mixture slurry during electrode production deteriorates. is there. Further, it is preferable that the positive electrode active material is formed of secondary particles in which at least a part of the small crystals constituting the secondary particles are joined by sintering. The conductivity can be improved, and the packing density can be improved by reducing the required amount of the conductive agent.

Next, a positive electrode active material for a non-aqueous electrolyte secondary battery
A method of manufacturing the device will be described. Cobalt of the above properties
Lithium oxide is composed of many primary particles of 0.2-0.8μm
Kobal consisting of secondary particles in the range of 1 to 40 μm
Heat source, and a lithium salt.
It is obtained by doing. And under the above conditions
Heat treatment of the mixture at about 900 ° C. in an oxidizing atmosphere.
It is preferable to perform this process for up to 12 hours.
If heat treatment at a temperature of 800 to 1000 ° C for 4 to 12 hours
The effect of the period can also be achieved. Further, the cobalt source
Cobalt tetroxide (Co₃O₄), Oxyhydroxy
And cobalt source (CoOOH)
Lithium carbonate (Li ₂CO₃), Lithium hydroxide
(LiOH), lithium nitrate (LiNO)₃)
Can be

In general, when evaluating the charge / discharge characteristics of a positive electrode active material, a composite electrode obtained by mixing and molding the active material, a conductive agent, and a binder is used as a positive electrode. In this case, there is a problem that the charge / discharge characteristics are affected by the mixing ratio of the active material and the conductive agent and the electrode manufacturing method. And Uchida et al.'S report (Electrochemistry, vol.
65, 954 (1997)), by using a single particle electrochemical measurement method in which a microlead electrode is brought into contact with active material particles to measure electrochemical characteristics, the influence of a conductive agent or the like is eliminated. The original charge / discharge characteristics can be evaluated. In confirming the effect of the present invention,
Along with the evaluation using a conventional composite electrode, a single particle cyclic voltammogram (cyclic voltammogram) was used.
ram) measurement to evaluate the charge / discharge characteristics of the active material particles.

[0014]

EXAMPLES Examples of the present invention will be described below in detail along with comparative examples. Example 1 Cobalt oxyhydroxide (CoOOH) and lithium carbonate composed of spherical or elliptical secondary particles having a particle size in the range of 1 to 40 μm in which a large number of primary particles of 0.2 to 0.8 μm are aggregated (Li ₂ CO ₃ ) was precisely weighed so that the ratio of Li to Co became 1: 1 and the aqueous solution of polyvinyl alcohol (PVA) was reduced to about 1.4 parts by weight of PVA with respect to 100 parts by weight of powder. While adding as much as possible, the mixture was mixed and granulated by a mixing granulator equipped with a stainless steel stirring blade and an agitator. Then, the mixture granulated to a size of 3 to 5 mm was dried at 120 ° C. for 5 hours, and then fired at 900 ° C. for 10 hours in an oxygen stream.

A composition analysis was performed on the obtained lithium cobaltate using an inductively coupled plasma atomic spectrometer (ICP). As a result, the results were consistent with the charged compositions. In addition, in the identification of the generated phase by powder X-ray diffraction using Cu Kα ray, JCPDS file No. 16
The phases other than the No.-427 No. LiCoO ₂ include Li ₂ C
No trace was observed except for the traces of O ₃ and Co ₃ O ₄ phases. 32 μm of the obtained lithium cobaltate
As a result of conducting SEM observation after sizing with a sieve having an aperture of, the Feret diameter of the projected figure is in the range of 0.4 to 3.5 μm,
And a large number of small crystals having an average particle size of 1.2 μm
It was confirmed that the particles consisted of spherical or elliptical secondary particles in the range of 30 μm. Further, when observation was performed at a magnification of 15,000 times with SEM, it was confirmed that the primary particles constituting the secondary particles were partially joined to each other by sintering. Further, a predetermined amount (50 g) of lithium cobalt oxide sized as described above was put into a measuring cylinder having a capacity of 100 cm ^{3, and} the measuring cylinder was repeatedly dropped 200 times on a rubber plate having a hardness of 60 to 80 from a height of 50 cm and tapped. . When the sample volume in the graduated cylinder after the tap was read and the tap density was calculated, the tap density of lithium cobaltate was 2.55 g / cm ³ .

Next, a battery was assembled using the obtained lithium cobaltate as an active material, and the charge / discharge capacity was measured.
The positive electrode active material of lithium cobaltate, acetylene black and polyvinylidene fluoride resin were mixed at a weight ratio of 90: 5: 5, and N-methylpyrrolidone (NMP) was added to form a paste. This was applied to an aluminum foil having a thickness of 20 μm so that the weight of the active material after drying was 0.05 g / cm ² , vacuum-dried at 120 ° C., and then diameter 1c.
m to form a positive electrode pellet. As shown in FIG. 1, the positive electrode pellet 5 and the negative electrode had a diameter of 16 mm.
A lithium metal pellet 2 having a diameter of 1 mm and a thickness of 1 mm was used, and a mixed solution of ethylene carbonate (EC) and diethylene carbonate (DEC) using 1 mol / liter of LiClO ₄ as a supporting salt was used as an electrolytic solution. The separator 3 is made of a polyethylene porous film having a thickness of 25 μm.
Then, a 2032 type coin battery was assembled in a glove box in which the dew point was controlled at −80 ° C. in an argon atmosphere. In FIG. 1, 1 is a negative electrode can and 6 is a positive electrode can.

The secondary battery thus manufactured is
After leaving it for about an hour and the open circuit voltage (OCV) stabilizes,
After charging to a cut-off voltage of 4.3 V at a current density of 0.5 mA / cm ² with respect to the positive electrode, the battery was left in an open circuit for one hour,
The battery was discharged to 3.0 V at a current density of 0.5 mA / cm ² .
The obtained discharge capacity at 3.5 V cutoff in the first cycle is shown in Table 1 below. In addition, when the charge / discharge cycle is repeated under the conditions of the current density shown in Table 2 below, the discharge capacity of the 3.5 V cutoff at the 11th cycle and the discharge capacity retention rate obtained by the following equation 1 are shown in the following table. 1 is also shown.

[0018]

[Formula 1] Capacity retention rate (%) = (discharge capacity at 11th cycle) / (discharge capacity at 1st cycle) × 100

Further, the cyclic voltammogram of the single particles was measured to evaluate the original charge / discharge characteristics of the positive electrode active material excluding the influence of the conductive agent and the like. FIG. 2 shows a schematic diagram of the cell and the apparatus used for the measurement. The active material particles 1 are placed on the glass filter 15 of the measuring cell partitioned into upper and lower two chambers.
The glass separator in which No. 1 was dispersed was placed, and the cell was placed on an observation table of a microscope. In the cell, ethylene carbonate (EC) and diethyl carbonate (DEC) using 1 mol / l of LiClO ₄ as a supporting salt as an electrolyte 16 are provided.
, And lithium metal 17 was used for the counter electrode and the reference electrode. While observing the particles through the image of the CCD camera mounted on the microscope 12, the micromanipulator 14 was operated, and a micro lead electrode (diameter 25 μm) 13 made of a platinum-rhodium alloy was pressed against the particles to make electrical contact. . Next, the potential of the micro lead electrode was swept by the minute current potentiostat 18 to measure a change in current. As the measurement particles 11, spherical or elliptical spherical particles having a particle diameter of 20 to 25 μm of secondary particles were selected. The potential scanning range of the micro lead electrode 13 is 3.0 to 4.3 Vvs. Li ⁺ / Li, scanning speed is 10
mV / sec. FIG. 3 shows the obtained cyclic voltammograms at the first cycle and the fifteenth cycle.

Comparative Example 1 Example 1 was repeated except that the raw materials cobalt oxyhydroxide (CoOOH) and lithium carbonate (Li ₂ CO ₃ ) were mixed and granulated, and then calcined at 950 ° C. for 10 hours in an oxygen stream. In the same manner as in the above, lithium cobaltate was obtained. As a result of performing SEM observation on the obtained lithium cobaltate, the Feret diameter of the projected figure was 0.5 to 19 μm.
And a large number of small crystal primary particles having an average particle size of 3.3 μm are aggregated in the range of 1 to 30 μm, and it was confirmed that the particles consisted of spherical or elliptical secondary particles. It was also confirmed that the primary particles constituting the secondary particles were partially joined to each other by sintering.
Using the obtained lithium cobaltate as the positive electrode active material, a coin-type battery as shown in FIG. 1 was produced in the same manner as in Example 1, and this was measured in the first cycle by the same measurement method as in Example 1. The discharge capacity at 3.5 V cutoff at the 11th cycle and the discharge capacity retention at the 11th cycle are also shown in Table 1 below.

[Comparative Example 2] Cobalt oxyhydroxide and lithium carbonate as raw materials were mixed and granulated, and then mixed in an oxygen stream.
Lithium cobaltate was obtained in the same manner as in Example 1 except that the firing was performed at 0 ° C. for 10 hours. As a result of performing SEM observation on the obtained lithium cobaltate, the Feret diameter of the projected figure was in the range of 1 to 15 μm, and the average particle diameter was 5 μm.
It was confirmed that the particles consisted of spherical or elliptical secondary particles in the range of 1 to 40 μm in which a large number of primary particles of the small crystal were aggregated. It was also confirmed that the primary particles constituting the secondary particles were partially joined to each other by sintering. Using the obtained lithium cobaltate as the positive electrode active material, a coin-type battery as shown in FIG. 1 was produced in the same manner as in Example 1, and this was measured in the first cycle by the same measurement method as in Example 1. And 3. of the eleventh cycle.
The discharge capacity at the 5 V cutoff and the discharge capacity retention rate at the 11th cycle are also shown in Table 1 below. Further, a cyclic voltammogram of a single particle of the positive electrode active material was measured in the same manner as in Example 1.
The voltammogram at the cycle is shown in FIG.

[0022]

[Table 1]

[0023]

[Table 2]

As is apparent from Example 1, the Feret diameter is in the range of 0.1 to 4 μm, and the average particle diameter is 2 μm.
1 to 40μ in which many primary particles of the following small crystals are aggregated
It was found that when the positive electrode active material according to the present invention composed of spherical or elliptical secondary particles in the range of m was used, the cycle characteristics were extremely excellent. In addition, the initial discharge capacity of the battery shown in Table 1 was good, the capacity deterioration was small even after repeated charge / discharge cycles, and the single-particle cyclic voltammogram shown in FIG.
In the fifth cycle, the waveform was maintained almost in the same manner as in the first cycle, showing high cycle characteristics. On the other hand, when the positive electrode active material including coarse primary particles having a particle size of 4 μm or more shown in Comparative Examples 1 and 2 and secondary particles having a non-uniform primary particle size is used, As the cycle characteristics deteriorated and the charge / discharge cycle of the battery was repeated as shown in Table 1, the discharge capacity and the discharge capacity maintenance ratio were significantly reduced.

Further, in the cyclic voltammogram of a single particle shown in FIG. 4, when the cycle was repeated, the waveform was disturbed, and the peak current value was significantly reduced. And when the measurement was performed while observing with a metallographic microscope in detail, secondary particles that collapsed during repeated cycles were observed.

[0026]

As described above, by using the lithium cobalt oxide according to the present invention as a positive electrode active material of a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery capable of exhibiting excellent cycle characteristics is manufactured. It becomes possible.

[Brief description of the drawings]

FIG. 1 shows a graph of 20 using the positive electrode active material obtained according to the present invention.
It is a partially cutaway perspective view of a 32 type coin battery.

FIG. 2 is a schematic diagram of an apparatus for measuring a cyclic voltammogram of a single secondary particle.

FIG. 3 is a cyclic voltammogram of single secondary particles of lithium cobaltate obtained in Example 1.

FIG. 4 is a cyclic voltammogram of a single secondary particle of lithium cobaltate obtained in Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Negative electrode can 2 Lithium metal pellet 3 Separator 4 Gasket 5 Positive electrode pellet 6 Positive electrode can 11 Single positive electrode active material particle 12 Microscope 13 Micro lead electrode 14 Micromanipulator 15 Glass filter 16 Electrolyte solution 17 Lithium metal 18 Micro current potentiostat

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazunori Matsumoto 3-18-5, China, Ichikawa, Chiba Sumitomo Metal Mining Co., Ltd. Central Research Laboratory F-term (reference) 5H003 AA04 BA01 BB05 BC01 BC06 BD02 BD05 5H029 AJ05 AK03 AL06 AL12 AM03 AM04 AM05 AM07 BJ03 BJ16 CJ02 DJ16 DJ17 HJ02 HJ05 HJ08

Claims (5)

[Claims] 1. A positive electrode active material for a non-aqueous electrolyte secondary battery using lithium cobaltate represented by the formula LiCoO ₂ , wherein the lithium cobaltate has a Feret diameter of 0.1 to 4 μm in a projected figure by SEM observation. And a spherical or elliptical secondary particle in which a large number of primary particles of small crystals having an average particle diameter of 2 μm or less are aggregated. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the secondary particles have a particle size in a range of 1 to 40 μm. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein at least a part of the small crystals constituting the secondary particles are joined by sintering. material. 4. The lithium cobalt oxide has a tap density of 2.2 g / cm ³ or more.
The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3. 5. A non-aqueous electrolyte secondary battery comprising the positive electrode active material according to claim 1 as a constituent element.