JP2002158009A - Nonaqueous electrolyte secondary cell positive electrode activator, and nonaqueous secondary cell using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a nonaqueous electrolyte secondary cell with high capacity, excellent cycle property and load property, and to provide a lithium manganate positive electrode activator with cubic spinel structure, and the secondary cell using the same. SOLUTION: The positive electrode activator is a powder including secondary grain which is an aggregation of many primary grains. The secondary cell uses the positive electrode activator powder of which, an initial charging capacity of one grain in average value of arbitrary five or more grains, is 2.0 nAh.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery (hereinafter, referred to as an active material) and a non-aqueous electrolyte secondary battery using the same (hereinafter, referred to as a secondary battery).

[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 secondary batteries that are small and lightweight and have a high energy density. As such a device, there is a lithium ion secondary battery using a lithium metal, a lithium alloy, or carbon as a negative electrode, and research and development thereof are being actively conducted.

A lithium ion secondary battery using lithium cobalt oxide (LiCoO ₂ ), which is relatively easy to synthesize, as an active material can obtain a high voltage of 4V class, and has been put to practical use as a battery having a high energy density. I have. However, since lithium cobaltate to be used is expensive, it is desired to develop an alternative active material with lower cost and higher performance.

Lithium manganate (LiMn ₂ O ₄ ) having a cubic spinel structure (hereinafter referred to as spinel type)
Is expected as a next-generation active material because it is less expensive than lithium cobalt oxide and is superior in safety in a charged state.

However, the spinel-type lithium manganese oxide has the following problems: (1) it is difficult to increase the capacity of the battery; and (2) the capacity is significantly deteriorated due to charge / discharge cycles (the cycle characteristics are poor).

(1) Higher capacity Higher capacity of a battery can be achieved by improving the packing density of the active material.
Generally, a method of increasing the particle diameter of the active material is used. However, in this method, the load characteristics are significantly deteriorated. That is, high capacity and high load characteristics were in conflict with each other.

(2) Cycle Characteristics Conventionally, it is not sufficient to improve cycle characteristics only by synthesizing spinel-type lithium manganate having no defect in the crystal structure and containing no foreign phase in X-ray diffraction analysis. It is considered. For this reason, a method has been proposed in which part of the Mn site in lithium manganate is replaced with an element such as Cr, Co, or Ni to strengthen the crystal structure.
However, in order to obtain sufficient cycle characteristics by this method, it is necessary to increase the replacement amount. However, if the replacement amount is increased, the charge / discharge capacity is greatly reduced.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a spinel-type lithium manganate active material capable of obtaining a secondary battery having a high capacity and excellent cycle characteristics and load characteristics, and a secondary battery using the same. Another object is to provide a battery.

[0009]

Means for Solving the Problems The present inventor has conducted intensive studies to solve the above-mentioned problems, and as a result, has found the following items, and has completed the present invention.

(1) When secondary particles in which a large number of primary particles are aggregated are used, due to the influence of the primary particles and the secondary particles,
(A) Higher capacity and (b) load characteristics superior to conventional secondary batteries (hereinafter referred to as conventional secondary batteries) using spinel-type lithium manganate primary particles having a similar particle diameter are achieved. obtain.

(2) There is a strong relationship between the initial charge capacity of one secondary particle and the cycle characteristics.

(3) The initial charge capacity of one secondary particle is affected by the particle diameter of the secondary particle and the presence or absence of a sintered structure in the secondary particle.

That is, the active material of the present invention (first invention)
Is a powder containing secondary particles of spinel-type lithium manganate in which a large number of primary particles are aggregated, and the initial charge capacity of one powder particle is an average value of five or more arbitrary values (hereinafter, referred to as an initial charge capacity average value). Is less than 2.0 nAh. In the first invention, the secondary particles are preferably contained by 90% or more in number, more preferably 99% or more.
Further, the powder particles are preferably at least one of a spherical shape and an elliptical spherical shape. Furthermore, the powder has an average particle diameter of 1 to 30 μm, or a sintered part in which the secondary particles have a sintered structure (hereinafter, referred to as a sintered part), and a joint in which the primary particles are joined (hereinafter, jointed) ) Is preferable because the initial charge capacity average value tends to be less than 2.0 nAh.

A secondary battery (second invention) of the present invention uses the active material of the first invention.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The active material of the present invention (first invention)
It is a powder containing secondary particles of spinel-type lithium manganate in which many primary particles are aggregated. For this reason, a secondary battery having a high capacity and excellent load characteristics than conventional secondary batteries can be obtained. High capacity secondary batteries can be obtained because
It is considered that the packing density of lithium manganate increases due to the influence of the large particle size of the powder particles. Also, the load characteristics are better than the conventional secondary battery, due to the influence of the primary particles constituting the secondary particles contained in the powder,
It is considered that the specific surface area of the secondary particles is larger than the specific surface area of the primary particles having the same particle diameter as the secondary particles.

In the first invention, the average initial charge capacity is less than 2.0 nAh. Thereby, a secondary battery having excellent cycle characteristics can be obtained. If the number of obtaining the initial charge capacity average value is less than 5, the estimation accuracy of the initial charge capacity is poor. In addition, the initial charge capacity average value is 2.0 nAh
Above, the cycle characteristics are deteriorated and the capacity is deteriorated.
This is because, due to expansion and contraction of the crystal lattice due to charge and discharge, the junction between the primary particles is distorted or cracked, etc.
It is considered that electron conductivity deteriorated.

When measuring and evaluating the charge and discharge characteristics of an active material such as initial charge capacity, cycle characteristics and load characteristics, a composite electrode obtained by mixing and molding an active material, a conductive agent and a binder is usually used as a positive electrode. Used. In this case, there is a problem that the charge and discharge characteristics are affected by the electrode manufacturing method such as the mixing ratio between the active material and the conductive agent. However, according to a report by Uchida et al. (Electrochemistry, Vol. 65, 954 (1997)), a single-particle electrochemical measurement method in which a micro-lead electrode is brought into contact with active material particles to measure electrochemical characteristics is proposed. By using
It is possible to measure and evaluate the charge / discharge characteristics of the particles themselves excluding the influence of the conductive agent and the like. Therefore, the measurement of the charge / discharge characteristics of one powder particle in the active material of the present invention is performed by the single particle electrochemical measurement method (described later).

The ratio of the secondary particles in the active material powder is preferably 90% or more, more preferably 99% or more in number. If it is less than 90%, the function and effect of the particle diameter and specific surface area of the secondary particles cannot be sufficiently obtained.

It is preferable that the powder particles have at least one of a spherical shape and an elliptical spherical shape from the viewpoints of improving the packing density and increasing the capacity.

Further, the powder has an average particle diameter of 1 to 30 μm.
m is preferable. When the average particle diameter of the powder is less than 1 μm, the effect of including the secondary particles is small, that is, the discharge capacity at the time of high load discharge is reduced. In addition, the applicability of the mixture slurry used at the time of manufacturing the electrode is deteriorated. On the other hand, 30μ
When m exceeds m, it is difficult to make the initial charge capacity average value less than 2.0 nAh, and at the same time, the applicability of the mixture slurry used at the time of manufacturing the electrode deteriorates.

Preferably, the secondary particles have a sintered part and a joint part. Thereby, a secondary battery having a high capacity and excellent cycle characteristics can be obtained. that is,
(1) Since distortion due to charge and discharge is unlikely to occur in the sintered part,
It is considered that capacity deterioration is unlikely to occur, and (2) the packing density can be increased by reducing the amount of the conductive agent in order to improve the electron conductivity between the primary particles.

Next, a method for producing the active material of the present invention will be described.

The spinel-type lithium manganate active material of the present invention can be obtained by mixing a manganese source powder containing secondary particles in which a large number of primary particles are aggregated with a lithium source powder and subjecting the resulting mixture to heat treatment.

As the manganese source, manganese oxide,
Manganese carbonate, manganese hydroxide, manganese chloride, manganese nitrate, manganese sulfate and the like can be used. As a lithium source, lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium nitrate and the like can be used.

The heat treatment of the mixture is performed in an oxidizing atmosphere at 700 to
It is desirable to carry out at 850 ° C. for 4 to 24 hours. When the heat treatment temperature is lower than 700 ° C., sintering of the primary particles hardly occurs. On the other hand, when the heat treatment temperature exceeds 850 ° C., the average particle diameter particularly exceeds 30 μm, and the primary particles are almost sintered.

[0026]

EXAMPLES [Examples] A manganese dioxide powder and a lithium hydroxide powder composed of secondary particles having a mean particle diameter of 20 μm formed by assembling a large number of primary particles into a spherical shape and an elliptical spherical shape, were prepared by changing the atomic ratio between Li and Mn. Was precisely weighed so as to be 1: 2 and mixed. The manganese dioxide powder was prepared from a commercially available product (chemically synthesized manganese dioxide manufactured by SEDEMA), and the lithium hydroxide powder was a commercially available product. Then, it was calcined at 800 ° C. for 20 hours in an oxygen stream and gradually cooled to room temperature.

With respect to the obtained fired product, (1) metallographic observation, (2) composition analysis, (3) powder X-ray diffraction, (4)
SEM observation and (5) initial charge capacity / cycle characteristic measurement were performed.

(1) Observation with a metallographic microscope The average particle size was 20 μm.

(2) Composition Analysis Composition analysis was performed using an inductively coupled plasma atomic spectrometer (ICP). And Li that matches the charge composition:
The result of Mn = 1: 2 was obtained.

(3) Powder X-ray Diffraction The powder was identified by powder X-ray diffraction using Kα radiation of Cu. As a result, only a spinel-type substance was confirmed.

(4) SEM observation Five particles were arbitrarily selected from the obtained fired product, and SEM observation was performed on each of these particles. As a result, the total number of the particles is (a) spherical and elliptical spherical secondary particles (having a joint portion) in which a large number of primary particles having a particle diameter of 0.1 to 3 μm are aggregated; Was confirmed.

(5) Measurement of initial charge capacity / cycle characteristics (a) An apparatus for measuring constant current charge / discharge characteristics of a single particle was used. A schematic diagram of this device is shown in FIG. 2 (hereinafter, reference numerals are the same as in FIG. 2).

(B) Preliminary operations before measurement are described in the following (A) to
(G) I went like.

(A) Lithium metal 1 as a counter electrode of the active material
Was placed in the measurement cell 2.

(B) The measurement cell 2 is provided with a glass filter 3 in the lower chamber 2a where the lithium metal 1 is placed and in the upper chamber 2b where it is not placed.
Partitioned.

(C) On the glass filter 3 (upper chamber 2b)
Inside), a glass separator 4 in which active material particles are dispersed
Was placed.

(D) The measuring cell 2 was set on an observation table (not shown) of the microscope 5.

(E) Equivalent mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) using 1 mol / l of LiClO ₄ as a supporting salt
Was filled in the measurement cell 2.

(F) The lithium metal 1 and the micromanipulator 8 were connected via the microcurrent potentiometer galvanostat 7.

(G) One particle 9 is arbitrarily selected from the active material particle observation image of the CCD camera mounted on the microscope 5, and the micromanipulator 8 is operated to form a micro-lead electrode 10 (diameter) made of a platinum-rhodium alloy. 25 μm)
Was pressed against particles 9 to make electrical contact.

(C) 5 arbitrarily selected from the obtained fired product
The initial charge capacity and cycle characteristics of each of the particles 9 were measured as follows.

That is, the potential of the micro lead electrode 10 is set to 4.3 Vvs. Li ⁺ / Li (hereinafter simply referred to as V)
At a constant current of 20 nA (20 nA, 4.3 V cut-off) until it reaches
A, after charging at a constant current with a 4.3V cutoff, 20n
A: A charge / discharge cycle of discharging at a constant current with a 3.0 V cutoff was repeated 50 times. Then, the initial charge capacity and n (= 1, 2, 10, 20, 30, 40, 50)
The discharge capacity at the cycle was measured, and the capacity retention at the nth cycle was calculated by the following formula (I).

Capacity retention rate at the nth cycle (%) = (discharge capacity at the nth cycle (nAh)) / (discharge capacity at the first cycle (nAh)) × 100 (I) Is 1.0 nAh and 2 n
Ah. Further, four of the particles 9 (A, B,
Table 1 shows the results obtained for C and D), and FIG. 1 shows the relationship between the capacity retention ratio (%, average value) at the nth cycle and the number of cycles n. As can be seen from FIG. 1, the capacity retention rates at the 50th cycle were all as high as 98% or more.

COMPARATIVE EXAMPLE Manganese dioxide powder (commercial product (synthetic manganese dioxide manufactured by SEDEMA)) consisting of secondary particles having a large number of primary particles forming spherical and elliptical spheres and having an average particle diameter of 35 μm was converted to manganese. Except having used as a source powder, it carried out similarly to the said Example, and obtained the baked product.

With respect to the obtained fired product, (1) metallographic observation, (2) composition analysis, (3) powder X-ray diffraction, (4)
SEM observation and (5) initial charge capacity / cycle characteristic measurement were performed in the same manner as in the above example.

(1) Observation with a metallographic microscope The average particle size was 35 μm.

(2) Composition Analysis A result of Li: Mn = 1: 2, which coincides with the charged composition, was obtained.

(3) Powder X-ray diffraction Only spinel type substances were confirmed.

(4) SEM observation The total number of the selected five particles was (a) the particle diameter was 0.1 to 3 μm.
It was confirmed that the primary particles were spherical and elliptical secondary particles in which a large number of primary particles were aggregated (having a joint portion), and that (b) a sintered portion was present.

(5) Initial Charge Capacity / Cycle Characteristics Measurement The average initial charge capacity was 3.5 nAh, which was 2 nAh or more. Further, three of the particles 9 (a, b, and c)
Are shown in Table 1, and the relationship between the capacity retention ratio (%, average value) at the nth cycle and the number of cycles n is shown in FIG. As can be seen from FIG. 1, the capacity retention rates at the 50th cycle were as low as 93% or less.

[0051]

[Table 1]

[0052]

According to the active material of the present invention, a secondary battery having a high capacity and excellent cycle characteristics and load characteristics can be obtained. Further, according to the secondary battery of the present invention, since the active material of the present invention is used, a high capacity and excellent cycle characteristics and load characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the capacity retention ratio (%) at the n-th cycle and the number of cycles n, obtained from the particles of Examples and Comparative Examples.

FIG. 2 is a schematic view of an apparatus for measuring a constant current charge / discharge characteristic of a single particle.

[Description of Signs] 1 Lithium metal 2 Measurement cell 2a Measurement cell lower chamber 2b Measurement cell upper chamber 3 Glass filter 4 Glass separator 5 Microscope 6 Non-aqueous electrolyte 7 Microcurrent potentiogalvanostat 8 Micromanipulator 9 1 particle 10 Micro Lead electrode

Continued on front page F term (reference) 5H029 AJ03 AJ05 AK03 AL06 AL12 AM03 AM04 AM05 AM07 DJ16 DJ17 HJ02 HJ05 HJ19 5H050 AA07 AA08 BA17 CA09 CB07 CB12 FA17 FA19 HA01 HA05 HA19

Claims (6)

[Claims] 1. A powder containing secondary particles of lithium manganate having a cubic spinel structure in which a large number of primary particles are aggregated, wherein the initial charge capacity of one powder particle is an average value of five or more particles. A positive electrode active material for a non-aqueous electrolyte secondary battery, which is less than 0 nAh. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the secondary particles are contained by 90% or more in number. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the powder particles are at least one of a spherical shape and an elliptical spherical shape. 4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the average particle diameter is 1 to 30 μm. 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the secondary particles have a sintered part and a joint part. 6. A non-aqueous electrolyte secondary battery using the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1.