JPH09213305A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make improvement of a charge/discharge characteristic compatible with improvement of productivity, workability, in a nonaqueous electrolyte secondary cell. SOLUTION: A positive electrode 1 is formed in a constitution comprising an LiMn2 O4 particle (positive electrode active material) of 10 to 40μm grain size, 6 to 8wt.% graphite (conductive material) of grain size 1/5 to 1/10 the LiMn2 O4 particle and 3 to 5wt.% polyvinyliclene fluoride (binding agent). A positive electrode active material of relatively large grain size is thus used, so as to well hold productivity and workability, a conductive material of relatively small grain size is used on the other hand, so as to large ensure a contact area with the positive electrode active material. A negative electrode 2 may be any of a carbon material and metal Li, Li alloy, in the case of using the former one, an Li ion secondary cell is constituted, in the case of using the latter two, a metal Li secondary cell is constituted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chargeable / dischargeable non-aqueous electrolyte secondary battery used as a power source for small electronic devices, and more particularly to an improvement of a positive electrode.

[0002]

2. Description of the Related Art A non-aqueous electrolyte secondary battery using lithium (Li) as a negative electrode active material and an organic solvent in which an electrolyte is dissolved as an electrolytic solution has low self-discharge, high operating voltage, and storage characteristics (self-discharge). (Characteristics regarding decrease in discharge capacity due to storage)
It has the advantage of being excellent, and is widely used as an operating power supply or a power supply for memory backup of high-performance small electronic devices such as watches, cameras, headphones / stereos, and video tape recorders with integrated cameras. In recent years, portable information devices such as mobile phones and notebook personal computers have been remarkably spread, and along with this, performance requirements for non-aqueous electrolyte secondary batteries have become more sophisticated.

The above non-aqueous electrolyte secondary batteries are roughly classified into
Metal Li secondary battery using metal Li or Li alloy for the negative electrode, and L using layered carbon or carbon-based compound for the negative electrode
There is an i-ion secondary battery. Lithium manganate (LiMn ₂ O ₄ ) is widely used for the former positive electrode.
On the other hand, as the latter positive electrode, lithium cobalt oxide (Li
CoO ₂ ) is used, but the use of expensive Co results in higher cost of the battery. Therefore, it has been studied to use lithium nickelate (LiNiO ₂ ) having the same crystal structure as LiCoO ₂ and similar physical properties. However, LiNiO ₂ has a problem that the composition ratio of Ni tends to be larger than that of Li depending on the temperature and atmosphere at the time of preparation, and the discharge capacity rapidly decreases when charging and discharging are repeated. Against this background, research on the use of LiMn ₂ O ₄ as a positive electrode active material is being conducted for Li-ion secondary batteries as well as for metallic Li secondary batteries.

By the way, in a non-aqueous electrolyte secondary battery using LiMn ₂ O ₄ as a positive electrode active material, in order to improve the performance, (a) fine particle LiMn ₂ O _{4 having} a particle size of about 1 to 5 μm is usually used.
(B) A conductive material is added to the positive electrode active material in an amount of 10% by weight or more.

The measure (a) is intended to promote the electrode reaction by increasing the specific surface area of LiMn ₂ O _{4 having} a not so large ionic conductivity, and to smoothly store and release the ions. There is.

On the other hand, the measure (b) is to synthesize LiMn ₂ O ₄ having a relatively large particle size (several tens of μm) using manganese dioxide for primary batteries, which is easily available, and uses this as a positive electrode active material. It is adopted when it is used, and its purpose is to secure a sufficiently large contact area with a conductive material and improve the large current discharge characteristics. The particle diameter of the conductive material that is usually used is the same as the particle diameter of the positive electrode active material. If the mixing ratio of the conductive material is lower than this, the contact between LiMn ₂ O ₄ and the conductive material in the positive electrode is rapidly lost due to the inflow and outflow of Li, and the performance deteriorates even after several tens of charge / discharge cycles. Come on.

[0007]

However, each of the above countermeasures has its own problems.

First, finely divided LiMn as shown in FIG.
_{With the} measure using ₂ O ₄ , it is difficult to manufacture an electrode having excellent flexibility. That is, even if the above fine particles are applied to a metal foil (collector) in a state of being dispersed in an appropriate solvent to form a conductive coating material, a flexible positive electrode cannot be formed, and the positive electrode active material cannot be formed. Since the substance retains the property as a powder, it is not possible to densely fill the substance and produce a high-density and high-capacity positive electrode. In particular, a sheet-shaped positive electrode for a cylindrical battery or a prismatic battery needs to be wound in a roll shape together with a separator or a sheet-shaped negative electrode, and thus such lack of flexibility is a major obstacle in manufacturing the battery. Therefore, it has been proposed to directly attach the positive electrode active material onto the current collector by pressure molding instead of coating, but when a light metal such as aluminum is used as the current collector, The lack of sufficient mechanical strength makes the pressure molding method itself unsuitable for mass production. Therefore, the productivity thereof is significantly inferior to the molded product of the active material mixture such as the positive electrode of the button type battery or the coin type battery.

On the other hand, as shown in (b), in the measure of adding the conductive material to LiMn ₂ O ₄ having a relatively large particle size at a high ratio, in order to obtain satisfactory battery performance, it is necessary to mix the materials with each other for a sufficient time. It is necessary to use a kneader with high mixing efficiency. However, when a large amount of graphite or acetylene black is added as a conductive material, the kneaded product has a property closer to that of a sol / gel rather than a paste.Therefore, it is necessary to remove bubbles remaining inside during the coating or drying process. Is difficult. Further, since it is not possible to apply the kneaded product uniformly by natural dropping, it is necessary to rely on machining for forming the positive electrode, and it is difficult to ensure the uniformity.

As described above, in the conventional non-aqueous electrolyte secondary battery, improvement in charge / discharge characteristics and improvement in productivity and workability were incompatible with each other. Then, this invention solves this problem and aims at supplying the non-aqueous electrolyte secondary battery which can make charge-discharge characteristics compatible with productivity and workability.

[0011]

The non-aqueous electrolyte secondary battery of the present invention has a composition formula LiMxOy (where M is Mn, V,
It is any one kind of transition metal element selected from Ti and Fe. ) Represented by the lithium-transition metal composite oxide as a positive electrode having an active material, a negative electrode having a lithium active material,
It has a separator for separating the positive electrode and the negative electrode, and a non-aqueous electrolytic solution, and the positive electrode has a particle size of 10 to 4
One containing 0 μm LiMxOy particles, 6 to 8% by weight of a conductive material having a particle diameter of 1/5 to 1/10 of the LiMxOy particles, and a binder is used. If the particle size of the LiMxOy particles is smaller than the above range, the problems of workability and productivity as described in the section of the prior art occur, and if it is larger than the above range, the specific surface area is insufficient. There is a high possibility that the capacity may be reduced due to the above. When the particle diameter of the conductive material is smaller than the above range, handling difficulty occurs, and when it is larger than the above range, it is difficult to secure a sufficient contact area with the positive electrode active material. Become. Further, when the content of the conductive material is less than the above range, the contact area between the positive electrode active material and the conductive material is insufficient to prevent smooth absorption / release of Li ions, and If it is more than the above range, the handleability of the kneaded product is remarkably deteriorated as in the conventional case.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, basically, fine powdery active material particles that would interfere with the production of a positive electrode for a non-aqueous electrolyte secondary battery are not used, and lithium having a relatively large particle size is used. -Maintain good productivity and workability by using a transition metal composite oxide. At the same time, in order to cover the performance of the positive electrode active material, a conductive material having a smaller particle size than that used in a normal non-aqueous electrolyte secondary battery is used, and the contact between the positive electrode active material and the conductive material is used. Secure a large enough area. Since the particle sizes of the positive electrode active material and the conductive material are optimized as described above, a large amount of conductive material is added as in the conventional case (10
Therefore, the processing becomes easy.

With respect to the above LiMxOy particles, it is more desirable to consider the particle size distribution as well, and it is particularly preferable to use particles having a 50% cumulative diameter of 30 to 40 μm.

The non-aqueous electrolyte secondary battery of the present invention may be either a Li-ion secondary battery or a metallic Li secondary battery. In the former Li-ion secondary battery, a material capable of occluding / releasing Li ions is used for the negative electrode, and a specific example of this material is a coke-based or graphite-based layered carbon material. Regarding the latter metal Li secondary battery, either metal Li or Li alloy is used as the negative electrode.

In the present invention, it is also desirable to consider the particle size distribution of the conductive material as well. When a carbonaceous material is used, it is particularly preferable to select the 50% cumulative diameter in the range of 1 to 10 μm. Is.

Further, as the electrolytic solution, LiPF ₆ , Li
BF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO
_A solution in which a lithium salt such as ₃ is dissolved in an organic solvent is used. As the organic solvent, cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate, or cyclic ester compounds such as γ-butyrolactone and γ-valerolactone can be used, and further dimethyl carbonate, ethylmethyl carbonate, carbonate It is also possible to mix chain carbonates such as diethyl and dipropyl carbonate.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

<Examples 1 to 4> I. Synthesis of LiMn ₂ O ₄ Lithium hydroxide (LiOH) and manganese carbonate (Mn
CO ₃ ) is mixed so that the atomic ratio of Li and Mn is 1: 2 to obtain a raw material composition, and the raw material composition is fired at a temperature of about 700 to 900 ° C. in an air atmosphere, and pulverized and pressure treated. After that, a lithium-manganese composite oxide having a predetermined particle size and particle size distribution was prepared through classification. This composite oxide shows the position of the diffraction peak in the powder X-ray diffraction method in JCP.
It was identified as LiMn ₂ O ₄ by checking with a DS card. In Examples 1 to 3, a sample having an average particle size (50% cumulative diameter) of 30 μm was used, and in Example 4, a sample having the same particle size of 40 μm was used.

II. Preparation of Positive Electrode Mixture Paste Graphite having an average particle size (50% cumulative diameter) of 6 μm was used as a conductive material, and polyvinylidene fluoride (PVDF) was used as a binder in the positive electrode active material synthesized as described above. The mixture was mixed at the ratio shown in (1), and this mixture was dispersed in N-methyl-2-pyrrolidone as a dispersion solvent to prepare a positive electrode mixture paste.

[0020]

[Table 1]

III. Assembly of coin-type metal Li secondary battery First, the positive electrode mixture paste was used as a current collector and had a thickness of 20 μm.
m Al foil was applied and dried to prepare a positive electrode material. Since this paste has high fluidity and excellent handleability, it was possible to easily and uniformly apply the paste onto the Al foil. Further, this positive electrode material was compression molded using a roller press and then cut into a circle having a diameter of 15 mm to produce a positive electrode having a thickness of about 0.2 mm.

FIG. 1 shows a schematic cross-sectional view of a coin-type battery assembled in this embodiment. How to assemble this battery
It is as follows. First, metal Li having a thickness of 1.6 mm
The plate is punched into a circle with a diameter of 17 mm to produce the negative electrode 2,
This is pressed onto the bottom surface of the negative electrode can 4. Next, the electrolytic solution is injected into the negative electrode can 4. This electrolytic solution was prepared by adding 1 mol / mol of LiPF ₆ as an electrolyte in a mixed solvent of propylene carbonate, ethylene carbonate and dimethyl carbonate at 30:30:40 (volume%).
It was dissolved in a concentration of 1 liter. Furthermore, a thickness of about 0.05 mm, which will be the separator 3, is formed on the negative electrode 2.
The porous polypropylene film of 1) and the positive electrode 1 described above were sequentially placed, and the positive electrode can 5 was covered via the gasket 6 for sealing, and the edge portion of the positive electrode can 5 was caulked. In this way, a coin-shaped metal L having a height of 2.5 mm and a diameter of 20 mm
An i secondary battery was produced.

As a comparative example to the above-mentioned embodiment, at least each item of the average particle size of the positive electrode active material, the average particle size of the conductive material, the added amount of the conductive material, and the particle size ratio of the positive electrode active material and the conductive material. Even when one is out of the range defined by the present invention or the positive electrode active material is not the substance defined by the present invention, the coin-shaped metal Li is similar to the examples.
A secondary battery was produced. Specific numerical values and types of positive electrode active materials are as summarized in Table 1 above, and each comparative example will be briefly described here.

<Comparative Example 1> The addition amount of graphite, which is a conductive material, was made smaller than the range specified in the present invention (4w).
t%).

<Comparative Example 2> The average particle size of graphite as a conductive material is set to be larger than the range specified in the present invention (1).
5 μm).

<Comparative Example 3> Both the average particle size and the addition amount of graphite as a conductive material were set to be larger than the range specified in the present invention (15 μm, 10 wt%).

<Comparative Example 4> A positive electrode active material having an average particle size (5
0% cumulative diameter) 20 μm LiCoO ₂ . This Li
The method for preparing CoO ₂ is as follows. First, lithium carbonate (Li ₂ CO ₃ ) and cobalt carbonate (CoC
O ₃ ) is mixed so that the atomic ratio of Li and Co is 1: 1 to obtain a raw material composition, and the raw material composition is fired at a temperature of about 900 to 1000 ° C. in an air atmosphere, and pulverized and pressure treated. After that, through classification, a lithium-cobalt composite oxide having a predetermined particle size and particle size distribution was obtained. This composite oxide shows the position of the diffraction peak in the powder X-ray diffraction method in JCP.
It was identified as LiCoO ₂ by matching with a DS card.

The absolute value of the average particle size of graphite in this comparative example is within the range of the present invention, but since the particle size of the positive electrode active material is small, the particle size ratio with respect to the positive electrode active material becomes large. There is.

Comparative Example 5 As the positive electrode active material, the average particle size is much smaller than the range specified by the present invention (1 μm).
Finely powdered LiMn ₂ O ₄ was used. This finely powdered LiMn ₂
O ₄ is a finely powdered MnC having an average particle size of about 1 μm as a Mn source.
Using manganese oxide (MnO ₂ ) obtained by heat-treating O ₃ , LiOH and this MnO ₂ are mixed so that the atomic ratio of Li and Mn is 1: 2 to obtain a raw material composition. Was fired at a temperature of about 700 to 900 ° C. in the atmosphere. The position of the diffraction peak in the powder X-ray diffraction method of the obtained lithium-manganese composite oxide was determined by JCP.
It was identified as LiMn ₂ O ₄ by checking with a DS card.

The absolute value of the average particle size of graphite in this comparative example is within the range of the present invention, but since the particle size of the positive electrode active material is small, the particle size ratio with respect to the positive electrode active material is large. There is.

The addition amount of graphite is also larger than the range of the present invention.

As described above, the discharge load performance test, the discharge temperature performance test, and the charge / discharge cycle test were performed on the coin-type metal Li secondary batteries produced in the four examples and the five comparative examples. Each test method and result will be described.

Discharge load performance test This test was conducted after 10 constant current charge / discharge cycles. This is because each battery was charged in a thermostatic chamber at 25 ° C under conditions of a current density of 0.5 mA / cm ² , an upper limit voltage of 4.2 V, and a charging time of 3 hours, and then a load resistance was connected to the battery to obtain a current density of 0.5 mA / One cycle is a process of discharging under the condition of cm ² and final voltage of 2.5V. Next, each battery was charged under the conditions of a current density of 1 mA / cm ² and an upper limit voltage of 4.2 V, and then an end voltage was set to 3.0 V, and a current density was in the range of 0.25 to 5 mA / cm ² . It was discharged while changing. The discharge capacity at this time was measured in a thermostatic chamber at 25 ° C.

The results are shown in FIG. Although the discharge capacities of all the batteries are reduced at the time of discharging with a large current, the rate of the decrease is smaller in the battery of the present invention than in the battery of the comparative example. Also, Li
The battery using LiCoO ₂ having a higher conductivity than Mn ₂ O ₄ as the positive electrode active material (Comparative Example 4) shows a high discharge capacity at a small current discharge, but shows a significant capacity decrease at a large current discharge. Therefore, it can be said that the battery of the present invention is superior to the above cobalt-based battery in terms of comprehensive discharge performance in a wide current density range.

Discharge temperature performance test Next, each battery after the above-mentioned discharge load performance test was completed with 25
^3. Current density 1mA / cm ² , upper limit voltage 4.
Charged under the condition of 2V, charging time 3 hours, then -20
A constant current discharge was performed in each of the constant temperature chambers of 0 ° C., 0 ° C. and 25 ° C. under the conditions of a final voltage of 3.0 V and a current density of 0.25 mA / cm ² . The discharge capacity at this time was measured in a thermostatic chamber at 25 ° C.

The results are shown in FIG. It was found that the discharge capacities of all the batteries decreased in the low temperature region, but the battery of the present invention showed a lower rate of decrease than the battery of the comparative example, and exhibited stable performance in a wider temperature range.

Charge / Discharge Cycle Test Each of the new batteries that did not pass the above-mentioned two kinds of tests was prepared and subjected to 100 constant current charge / discharge cycles. In this case, one cycle means a constant temperature room of 25 ° C., a current density of 1 mA / cm ² , an upper limit voltage of 4.2 V, and a charging time of 3
Charging under the condition of time, current density 0.5mA / cm
^2. Combined with discharge under the condition of final voltage of 3.0V. The discharge capacity at this time was measured in a thermostatic chamber at 25 ° C. FIG. 4 shows the results. The battery having a small content of the conductive material in the positive electrode (Comparative Example 1) showed a large decrease in capacity with the increase in the number of cycles, but the battery of the present invention (Example 1)
The capacity decrease in Example 4) was small in all cases. Conventionally, metal L using LiMn ₂ O ₄ as a positive electrode active material
Although it was said that the i secondary battery was inferior in charge / discharge cycle characteristics, it was found that in the present invention, this characteristic is improved almost as compared with the case where LiCoO ₂ is used as the positive electrode active material (Comparative Example 4).

The specific embodiments of the present invention have been described above, but the present invention is not limited to these embodiments. For example, if a layered carbon material such as coke or graphite is used for the negative electrode instead of the above-mentioned negative electrode prepared by punching out a metal Li plate, Li having excellent charge / discharge characteristics is obtained.
An ion secondary battery can be constructed. Further, the shape of the battery is not limited to the coin type, and may be any of a button type, a cylindrical type and a square type. Further, as the lithium-transition metal composite oxide that is the positive electrode active material, in addition to LiMn ₂ O ₄ described above, LiVxOy, LiTixO
Good results are also obtained using y and LiFexOy.

[0039]

As is apparent from the above description, the non-aqueous electrolyte secondary battery of the present invention is excellent in charge / discharge characteristics and, at the same time, ensures high productivity and excellent workability in the manufacturing process. It is a thing. In particular, by adopting the configuration of the present invention, the positive electrode is provided with flexibility, and it becomes possible to manufacture a sheet-shaped positive electrode. Therefore, in a cylindrical or prismatic non-aqueous electrolyte secondary battery, which has been difficult in the past, It is possible to achieve both improvement of characteristics and improvement of productivity and workability.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a coin-type metal Li secondary battery.

FIG. 2 is a graph showing the results of discharge load performance tests of coin-type metal Li secondary batteries according to examples and comparative examples of the present invention.

FIG. 3 is a graph showing the results of discharge temperature performance tests of coin-type metal Li secondary batteries according to examples and comparative examples of the present invention.

FIG. 4 is a graph showing the results of charge / discharge cycle tests of coin-type metal Li secondary batteries according to examples and comparative examples of the present invention.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode 3 Separator 4 Negative electrode can 5 Positive electrode can 6 Gasket

Claims (5)

[Claims] 1. A composition formula LiMxOy (where M is M
It is any one kind of transition metal element selected from n, V, Ti and Fe. ) A non-aqueous liquid having a positive electrode using a lithium-transition metal composite oxide as an active material, a negative electrode using lithium as an active material, a separator separating the positive electrode and the negative electrode, and a non-aqueous electrolyte. An electrolyte secondary battery, wherein the positive electrode is LiMxOy particles having a particle size of 10 to 40 μm,
A non-aqueous electrolyte secondary battery comprising 6 to 8% by weight of a conductive material having a particle diameter of 1/5 to 1/10 of the LiMxOy particles and a binder. 2. The content of the binder in the positive electrode is 3 to.
The non-aqueous electrolyte secondary battery according to claim 1, which is 5% by weight. 3. The negative electrode absorbs lithium ions /
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is composed of any one of a releasable material, metallic lithium, and a lithium alloy. 4. The LiMxOy particles are LiMn ₂ O ₄ particles having a 50% cumulative diameter of 30 to 40 μm.
The non-aqueous electrolyte secondary battery according to the above. 5. The conductive material is a carbon-based material, and
The non-aqueous electrolyte secondary battery according to claim 4, wherein the 0% cumulative diameter is 1 to 10 μm.