JP2001135312A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which can keep electron con ductivity in a low potential region, to make smooth discharging response of lithium manganate oxide such as lithium manganate, threrby to improve the discharging capacity. SOLUTION: Lithium manganate oxide shown in a general formula: LixMn2-yMyO4 (where M is at least one element selected among Co, Ni, Fe, Mg, Cr, Ba, Ag, Nb, and Al, and x and y are respectively 0<x<=2, 0<=y<2) is compounded with 0.01 to 3.0 wt.% of poly-pyrrole for making a main active material contained in a positive electrode which is carried by a collector body for forming a positive electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, energy and environmental issues have
Lithium secondary batteries with high energy density
Lithium manganate (L
iMn _TwoO_Four) Is promising. Lithium manganate
Is a powder with very low conductivity, such as carbon powder.
N-methylpyrrolidone (hereinafter referred to as "conduction aid")
Below, referred to as NMP).
The slurry is applied on a current collector plate and dried.
By doing so, a positive electrode is manufactured. In this state
Is that the electronic conductivity of lithium manganate is
It is supplemented and carried on the current collector plate by the binder.

In the positive electrode having the above composition, the transfer of electrons in the charge / discharge reaction occurs only by the conductive auxiliary particles attached to the surface of the lithium manganate particles. However, the conductive auxiliary particles come into point contact with lithium manganate. In order to obtain sufficient conductivity, it is necessary to add a relatively large amount of a conductive assistant. Since the conductive auxiliary is redox-inactive, it does not contribute to the charge / discharge capacity of the positive electrode, and the addition of a large amount of the conductive auxiliary lowers the discharge capacity of the entire positive electrode.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a lithium secondary battery having improved discharge capacity by securing electron conductivity in a low potential region and smoothing a discharge reaction of a lithium manganese-based oxide such as lithium manganate. It is intended to provide a secondary battery.

[0005]

In order to solve the above-mentioned problems, a lithium manganese-based oxide particle such as lithium manganate is coated with a uniform film having conductivity, and a surface contact with these oxide particles is achieved. Improving the conductivity by taking it is mentioned.

[0006] From such a viewpoint, the present inventors have studied a method of adding a trace amount of reduced polyaniline, which is a kind of conductive polymer, to a positive electrode slurry. Reduced polyaniline is soluble in N-methylpyrrolidone (hereinafter abbreviated as NMP) and has relatively high conductivity in the range of about 3-4 V vs Li. The physical structure of the positive electrode layer using the reduced polyaniline has a structure in which the polyaniline covers the lithium manganese-based oxide particles, and the internal resistance of the cell can be reduced as compared with a positive electrode in which the reduced polyaniline is not added. It was confirmed that the application of reduced polyaniline had an effect of improving electron conductivity between lithium manganese-based oxide particles.

[0007] It has also been confirmed that the addition of a small amount of reduced polyaniline to the slurry prevents the slurry from gelling and increases the uniformity and stability over time.

[0008] However, the conductivity of the polyaniline depends on its redox state. The conductivity of polyaniline drops sharply in a potential region of 3 V or less, and becomes an insulator. On the other hand, as shown in FIG. 1, the discharge reaction of a lithium manganese-based oxide, for example, lithium manganate has discharge regions around 4 V and around 3 V, respectively.

Therefore, in the above-described positive electrode composition using reduced polyaniline, the electron conductivity of polyaniline coating the surface of lithium manganese-based oxide particles such as lithium manganate sharply drops below 3 V, and the internal resistance increases. , There is a problem that it is difficult to take out the discharge capacity of lithium manganate at 3 V or less due to an increase in overvoltage.

When reduced polyaniline is used, since the dense polyaniline layer covers the surface of the lithium manganese oxide powder, the ion diffusion into the lithium manganese oxide powder accompanying the charge / discharge reaction is inhibited. It was expected from the analysis results of the charge and discharge curves.

From these facts, the present inventors have found that among various conductive polymers, polypyrrole ensures electron conductivity even in a potential region of 3 V or less, and a lithium manganese-based oxide such as lithium manganate. It has been found that the discharge reaction can be smoothed and the discharge capacity can be improved.

That is, the lithium secondary battery according to the present invention has the general formula Li _x Mn _{2- y My} O ₄ (where M is Co, N
i, at least one element selected from Fe, Mg, Cr, Ba, Ag, Nb and Al, x and y are 0 <x ≦ 2, 0
≤ y <2) A positive electrode layer having a main active material in which polypyrrole was compounded in a range of 0.01 to 3.0% by weight with a lithium manganese-based oxide represented by the formula: It is characterized by having a positive electrode.

[0013] In the lithium secondary battery according to the present invention,
The main active material is obtained by dispersing and attaching polypyrrole powder polymerized by a chemical polymerization method to the surface of a lithium manganese oxide powder represented by the general formula, or oxidative polymerization of pyrrole monomer using the lithium manganese oxide as an oxidizing agent. Then, the surface of the lithium manganese-based oxide is preferably coated with polypyrrole, or is preferably formed by either one.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a lithium secondary battery according to the present invention will be described in detail.

This lithium secondary battery has a general formula Li _{x
Mn 2-y M y O 4} ( provided that, M is Co, Ni, Fe, M
g, Cr, Ba, Ag, Nb, and at least one element selected from the group consisting of Al, x and y are 0 <x ≦ 2 and 0 ≦ y <2). It has a structure including a positive electrode in which a positive electrode layer having a main active material compounded in a range of 0.01 to 3.0% by weight is supported on a current collector.

The lithium manganese-based oxides can be used alone or in the form of a mixture.

As the method for compounding the polypyrrole, the following two methods can be employed.

(1) A polypyrrole composite lithium manganese oxide is produced by a powder mixing method in which the lithium manganese oxide powder and the polypyrrole powder are stirred and mixed.

Specifically, the lithium manganese oxide powder and the polypyrrole powder are mixed and stirred at a high speed to adhere the polypyrrole particles to the surface of the lithium manganese oxide powder. A similar effect can be obtained by mixing the both using a mortar.

The synthesis of the polypyrrole is not particularly limited as long as it is a method of oxidizing and polymerizing pyrrole monomer regardless of whether it is aqueous or non-aqueous. For example, a method of oxidizing and polymerizing pyrrole monomer in hydrochloric acid using ammonium persulfate as an oxidizing agent is used. .

(2) Pyrrole is oxidized and polymerized in an appropriate solution using the lithium manganese oxide as an oxidizing agent to modify polypyrrole on the surface of the lithium manganese oxide. Manufacturing things.

Specifically, the surface of the lithium manganese oxide is modified with polypyrrole by using the lithium manganese oxide directly as an oxidizing agent in the oxidation of pyrrole.

In the methods (1) and (2), the amount of polypyrrole greatly affects the charge / discharge performance of the positive electrode. That is, as the addition amount of polypyrrole increases, the effect of improving the electron conductivity on the surface of the lithium manganese-based oxide increases, and the discharge capacity also increases. However, when the amount of polypyrrole is excessively increased, the density of the positive electrode layer is reduced due to the bulkiness of polypyrrole, and the electron conductivity is rather reduced. In addition, polypyrrole has lower stability at high potential than polyaniline and is easily decomposed.

From the above, it is preferable that the amount of the polypyrrole be 0.01 to 3.0% by weight based on the amount of the lithium manganese-based oxide in the positive electrode. A more preferable addition amount of the polypyrrole is 0.4 to 0.6% by weight based on the amount of the lithium manganese-based oxide in the positive electrode.

In particular, in the surface modification method of polypyrrole, a monomer is oxidized by a lithium manganese-based oxide. Therefore, if the ratio of the charged amount of pyrrole to the amount of lithium manganese-based oxide becomes excessively high, the slurry due to the bulkiness of polypyrrole to be formed is increased. Preparation becomes difficult, and the composition ratio of Li, Mn, and O constituting the lithium manganese-based oxide may vary. This is true even if the acid concentration becomes excessively high. For this reason, the surface modification condition of polypyrrole on the lithium manganese-based oxide is preferably set to an acid concentration of 0.1 to 0.5 mol / L, more preferably 0.2 mol / L. The ratio of lithium manganese oxide / pyrrole at the time of preparation was 2
00/1 to 50/1, more preferably 100/1 to 50
/ 1 is desirable.

In the lithium secondary battery according to the present invention,
The reduction polyaniline may be further added to the positive electrode layer in an amount of 0.4% by weight or more based on the total amount. However,
The upper limit of the addition of the reduced polyaniline is preferably 5.0% by weight.

[0027]

The present invention will be described in detail below with reference to specific examples and comparative examples.

(Example 1: Example of composite positive electrode in which polypyrrole powder is mixed with lithium manganate) <Synthesis of polypyrrole powder> First, 300 m hydrochloric acid having a concentration of 1.33 mol / L was placed in a 500 mL four-necked flask.
L and 20.13 g (0.3 mol) of pyrrole were added, mixed and stirred to prepare a polymerization solution.

Also, 30.66 g of ammonium persulfate
(0.14 mol) was dissolved in 53.4 mL of pure water to prepare an oxidizing agent aqueous solution.

Next, the polymerization solution was cooled to 0 ° C. with a cooling water circulation type chiller while stirring. After confirming that the temperature of the polymerization solution reached 0 ° C., the oxidizing agent aqueous solution was added dropwise. The time required for dripping the entire amount of the oxidizing agent aqueous solution was about 3 hours. Thereafter, the mixture was stirred at the same temperature for 21 hours to allow the polymerization to proceed.

After completion of the polymerization, the content of the polymerization was poured into 2 L of acetone and allowed to stand all day and night to precipitate the product, which was then filtered through a glass filter. Subsequently, it was washed three times with acetone, three times with water, and three times with acetone, and dried under reduced pressure at 60 ° C. for 5 hours. The obtained product was ground in an agate mortar.

<Preparation of Composite Positive Electrode> First, a predetermined amount of lithium manganate (LiMn ₂ O ₄ ; LM) powder, polypyrrole powder, and a conductive auxiliary (Denka Black ^R , carbon powder,
(Hereinafter referred to as DB) was stirred for 30 seconds by a rotary blade type small pulverizer to obtain a powder mixture.

Then, the above powder mixture was charged while stirring a N-methylpyrrolidone solution in which a predetermined amount of polyaniline and a binder (polyvinylidene fluoride, hereinafter referred to as PVDF) were dissolved by a homomixer, to prepare a positive electrode slurry. did. At this time, the rotation speed of the stirring blade was about 3500 rpm, and the stirring time was about 10 minutes.

The positive electrode composition (expressed as a percentage by weight) of each batch is shown in Table 1 below. A slurry without polypyrrole was also prepared for comparison. In each case, the powder (L
M, DB, polyaniline, polypyrrole) concentration is 60%
It is.

[0035]

[Table 1]

The amount of powder after drying each of the slurries is 25 to
It was applied on an Al foil as a current collector having a thickness of 20 μm to be 27 mg / cm ^2, and dried at 80 ° C. for 1 hour. Subsequently, the density of the positive electrode layer on the current collector was 2.1 g / c.
It was pressurized by a roll press so as to obtain m ³ . The density of the positive electrode layer was calculated from the weight of the positive electrode layer and its thickness.

<Preparation of Test Cell> Each of the above positive electrodes was 16 m in diameter.
m, a glass wool, a separator, a glass wool, and a lithium foil (thickness: 0.2 mm) are laminated in a coin-type test cell (based on 18650 size). A test cell was prepared. As the electrolytic solution, a solution prepared by dissolving lithium tetrafluoroborate (LiBF ₄ ) at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1: 2) was used.

The open-end potential difference of the fabricated cell is about 3.5 V
It was confirmed that there was no short circuit in the cell.

<Charge / Discharge Test> The test cells prepared in the above steps were evaluated by a charge / discharge test. The test conditions were set as follows based on the amount of lithium manganate in each cell.

Charge; constant at 27.40 mA / g per lithium manganate weight up to an upper limit of 4.2 V, and thereafter maintained at 4.2 V until a 5% current (1.370 mA / g) is reached; discharge; manganese to a lower limit of 2.2 V 27.40 mA / g constant per lithium oxide weight Table 2 below shows the results of the charge / discharge test.

[0041]

[Table 2]

As is clear from Table 2, 0.45 wt% and 1.78 w% of polypyrrole were added to the total amount of the positive electrode powder.
t% added (0.55 wt% based on lithium manganate)
It can be seen that the discharge capacity of the cells of Examples 2 and 3 in which polypyrrole was not added increased as compared with the cells of Example 1 (comparative example) in which polypyrrole was not added.

In particular, in the cell of Example 2, 174.8
Ah / kg showed a high discharge capacity.

When the effect of improving the discharge capacity in the 4V region and the effect of improving the discharge capacity in the 3V region are compared, the effect of improving the cells of Examples 2 and 3 is increased.

From the above results, the effect of improving the discharge capacity by adding polypyrrole was confirmed.

FIG. 2 shows a charging current curve after reaching 4.2 in the charging process. The vertical axis in FIG. 2 is 4.2V.
The value is normalized by dividing by the current before reaching.

From FIG. 2, it is presumed that the cells of Examples 2 to 4 to which polypyrrole was added had a faster decay of the charging current than the cell of Example 1, and that the ion diffusion during charging was improved.

Example 2 Example of Composite Positive Electrode with Polypyrrole Modified on Lithium Manganese Surface <Polypyrrole Polymerization on Lithium Manganese Surface> First, a concentration of 0.2 mol was added to a 110 mL sample tube.
Lithium manganate (LM) 4 in 100 mL of 1 / L hydrochloric acid
Then, 0 g and 0.4 g or 0.8 g of pyrrole were mixed, and the mixture was stirred at room temperature for 24 hours using a mix rotor to allow polymerization to proceed.

After completion of the polymerization, the content was filtered through a membrane filter to precipitate the content.
It was washed three times with water and three times with acetone, and dried under reduced pressure at 60 ° C. for 5 hours. The obtained product was ground in an agate mortar.

<Preparation of Composite Positive Electrode> An N-methylpyrrolidone solution in which a predetermined amount of polyaniline and a binder (polyvinylidene fluoride, hereinafter referred to as PVDF) are dissolved is stirred with a homomixer while lithium manganate modified with the polypyrrole is stirred. The powder was charged to prepare a positive electrode slurry.

The composition of the positive electrode of each batch (expressed as a percentage by weight) is shown in Table 3 below.

[0052]

[Table 3]

The amount of powder after drying each of the slurries is 25 to
It was applied on a 20 μm-thick current collector Al foil so as to have a concentration of 27 mg / cm 2 and dried at 80 ° C. for 1 hour. Subsequently, the density of the positive electrode layer on the current collector was 2.1 g / c.
It was pressurized by a roll press so as to obtain m ³ . The density of the positive electrode layer was calculated from the weight of the positive electrode layer and its thickness.

Next, a charge / discharge test was carried out under the same test conditions as in Example 1 for each of the test cells prepared as in Example 1 using the positive electrode. The test results are shown in Table 4 below. Table 4 also shows Example 1 as a comparative example described above.

[0055]

[Table 4]

As is apparent from Table 4, the cells of Examples 5 and 6 provided with the positive electrode containing the LM powder having polypyrrole surface-modified have a discharge capacity as compared with the cells of Example 1 (Comparative Example) to which polypyrrole was not added. It can be seen that it increases.

FIG. 2 shows a charging current curve after reaching 4.2 in the charging process. The vertical axis in FIG. 2 is 4.2V.
The value is normalized by dividing by the current before reaching.

As is clear from FIG. 2, the cells of Examples 5 and 6 using the LM powder modified with polypyrrole on the surface had a faster decay of the charging current than the cell of Example 1, which is a comparative example, and the cells at the time of charging were different. It is assumed that the ion diffusivity is improved.

[0059]

As described above in detail, according to the present invention, the conductivity of a positive electrode active material of a lithium manganese oxide such as lithium manganate in a potential region of 3 V or less is improved, and the discharge capacity is improved. It is possible to provide a lithium secondary battery that contributes to increasing the capacity and reducing the size of a battery system as a power supply for driving an electric vehicle and a power storage device.

[Brief description of the drawings]

FIG. 1 is a diagram showing a discharge curve of lithium manganate.

FIG. 2 shows a process of charging each cell of the first and second embodiments.
The figure which shows the charging current curve after reaching 2.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsutomu Hashimoto 5-717-1, Fukahori-cho, Nagasaki-city, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd. No. 1 Mitsubishi Heavy Industries, Ltd. Nagasaki Research Laboratory F-term (reference)

Claims (3)

[Claims] 1. The general formula Li _x Mn _{2- y My} O ₄ (provided that:
M is Co, Ni, Fe, Mg, Cr, Ba, Ag, N
at least one element selected from b and Al, x and y represent 0 <x ≦ 2 and 0 ≦ y <2).
A lithium secondary battery comprising: a positive electrode in which a current collector supports a positive electrode layer having a main active material compounded in a weight percent range. 2. The method according to claim 1, wherein the main active material is obtained by dispersing and attaching polypyrrole powder polymerized by a chemical polymerization method to a surface of the lithium manganese-based oxide powder represented by the general formula.
The lithium secondary battery according to claim 1, wherein the lithium manganese-based oxide is oxidized and polymerized using the lithium manganese-based oxide as an oxidizing agent to coat the surface of the lithium manganese-based oxide with polypyrrole. . 3. The lithium secondary battery according to claim 1, wherein the reduced polyaniline is further added to the positive electrode layer in an amount of 0.4% by weight or more based on the total amount.