JP2002083588A - Manufacturing method of positive electrode mixture for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new manufacturing method for a lithium ion secondary battery, a positive electrode mixture in particular, by solving such problem in a conventional method that if the mixing time is short, the distribution of a conductive agent is not uniformed although lowering of conductivity is suppressed, and if the mixing time is long, lowering of conductivity of AB is increased, when manufacturing the positive electrode mixture. SOLUTION: This positive electrode mixture consists of a positive electrode active material and two kinds of carbon black, AS and KB of a conductive agent, and the positive electrode active material, KB and a binding agent (NMP solution containing PVDF) are mixed for 60-120 minutes, preferably for 80-100 minutes in the primary mixing. In addition, AB, the binding agent and NMP are thrown in and are mixed for 20-60 minutes, preferably for 30-40 minutes in the secondary mixing. The distribution of the conductive agent in the positive electrode mixture is uniformed, conductivity and liquid retaining capability of an electrode is enhanced, and deterioration of a cycle caused by electrolyte is suppressed, by this manufacturing method. Thereby, the lithium ion secondary battery having a long cycle life can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery, and more particularly to a method for producing a positive electrode mixture.

[0002]

2. Description of the Related Art In recent years, electronic devices have been rapidly becoming smaller and lighter.
The demand for lighter weight and higher capacity is increasing, and high energy density lithium secondary batteries are actively researched and developed.
It has been put to practical use. In a lithium secondary battery, when lithium metal or a lithium alloy is used as a negative electrode active material, an internal short circuit occurs due to deposition of lithium into a resin upon charging, and deterioration in cycle characteristics due to miniaturization of the alloy. Many problems remain for the practical use of this battery system.

On the other hand, lithium secondary batteries using a carbonaceous material as a negative electrode active material and a transition metal oxide containing Li such as LiCoO ₂ or LiMn ₂ O ₄ as a positive electrode active material have been put to practical use by various companies. . This battery system has excellent cycle characteristics because lithium does not precipitate on the negative electrode due to charging, and is currently the mainstream of lithium secondary batteries that are in practical use. In addition, the development of high-capacity, high-energy-density secondary batteries, such as those for higher energy density or electric vehicles and power storage, is also being actively pursued. And high densification are progressing.

At present, the mainstream of the positive electrode active material is LiCoO ₂ , but Co has a small amount of reserve and is expensive. Therefore, in recent years, low cost Mn with a large amount of reserve has been used in order to reduce the cost. The development of a lithium ion secondary battery using LiMn ₂ O ₄ for the positive electrode has been active.

A method for producing a positive electrode mixture for a lithium ion secondary battery has been disclosed in Japanese Patent Application Laid-Open No. 10-312811. As shown in FIG. 2, this manufacturing method uses several types of carbon powder such as carbon black and graphite as a conductive agent, simultaneously mixes and mixes a positive electrode active material and several types of conductive agents, and then performs coating and rolling. Is going. The conductive agent not only has a role of ensuring conductivity between the active material particles and with the current collector, but also has a role of holding the electrolytic solution. This plays a large role in replenishing Li ions when Li ions move along with the movement of electrons. In order to further extend the life of the cycle characteristics, a conductive agent which has high conductivity and liquid retention and suppresses decomposition of the electrolyte is required.

[0006]

However, in order to improve conductivity and enhance liquid retention, a large amount of a conductive agent may be used. However, a decrease in the active material density in the mixture lowers the battery capacity. In such a case, it takes time to dry the electrode plate, and the number of times of rolling increases.

When KB having high conductivity and liquid retention is used as the conductive agent, the specific surface area is high, so that the decomposition reaction of the electrolytic solution occurring on the surface of the conductive agent is increased, and the cycle life is shortened. Therefore, decomposition of the electrolytic solution can be suppressed by reducing the amount of KB and mixing a conductive agent (AB) having a lower specific surface area than KB.

On the other hand, if the distribution of the conductive agent in the positive electrode mixture is biased, the degradation reaction accompanying the movement of electrons and Li ions in the active material is also biased, and the cycle life is shortened. In order to make the distribution of the conductive agent uniform, it is necessary to lengthen the kneading time. However, if the kneading time is long, the conductivity of the conductive agent is reduced. In particular, since AB has a chain structure and maintains conductivity by its network, the network is destroyed by mechanical stress due to kneading, and the conductivity is significantly reduced. For this reason, when the positive electrode mixture is manufactured by the conventional method, if the kneading time is short, the decrease in the conductivity of AB can be suppressed, but the distribution of the conductive agent becomes non-uniform, and if the kneading time is long, the conductivity of AB is reduced. There is a problem that the deterioration of the property becomes large.

The present invention solves the above-mentioned conventional problems, and provides a positive electrode mixture for a lithium ion secondary battery having a high cycle life by having high conductivity and liquid retention and suppressing decomposition of an electrolytic solution. The purpose is to:

[0010]

In order to achieve the above object, the present invention provides a primary kneading step of kneading a positive electrode active material and KB, and a secondary kneading step of adding and kneading AB after the above step. And a combining step. In the primary kneading step, the conductive agent is uniformly distributed in the mixture by performing kneading of the AB and the positive electrode active material with little decrease in conductivity even after long-time kneading, so that the positive electrode plate is formed. In the secondary kneading step, AB with less decomposition of the electrolyte solution is mixed, and kneading is performed in a short time, thereby lowering the conductivity and reducing the number of cycles due to the decomposition of the electrolyte solution. Reduction can be suppressed. Due to these synergistic effects, a positive electrode mixture for a lithium ion secondary battery having a long cycle life can be obtained.

[0011]

Embodiments of the present invention will be described below with reference to FIG. A positive electrode mixture for a lithium ion secondary battery comprising a Mn oxide of a positive electrode active material, two types of carbon blacks of AB and KB as conductive agents, and a binder, and the positive electrode material is mixed with KB by primary kneading. And a binder (an n-methyl-2-pyrrolidone (NMP) solution containing polyvinylidene fluoride (PVDF)) are kneaded for 60 to 120 minutes, preferably 80 to 100 minutes. Further, AB, the binder and NMP are charged, and secondary kneading is performed for 20 to 60 minutes, preferably 30 to 40 minutes. Then, it is coated on a base made of, for example, a copper foil, dried, and rolled. By the production method according to the present invention, the distribution of the conductive agent in the positive electrode mixture is made uniform, the conductivity and the liquid retention of the electrode plate are increased, and the deterioration of the cycle due to the electrolytic solution is suppressed. The following battery can be obtained.

At this time, assuming that the added amount of KB and the added amount of AB to the mass of the positive electrode active material are x mass% and y mass%, the value of 3x + y becomes 2.5 or more and 8.0 or less, and the value of x becomes Is 0.5 to 2.0, and the value of y is 1.0 to 4.0.
This is because KB is about 3 times more than AB in the absorption of electrolyte.
Since it has twice the value, it is necessary to use the above 3x + y calculation formula to express the liquid retention properties of AB and KB.

The above KB has a specific surface area of 500 to 150.
0 m ² / g, DBP oil absorption 300 to 450 ml
/ 100 g other carbon black may be used.
Dibutyl phthalate oil absorption (DBP oil absorption) is used as an index of the characteristics of carbon black such as KB.
It is considered that the P oil absorption corresponds to the electrolyte absorption. As the binder, PVDF dissolved in NMP is used, but styrene butadiene rubber (SBR) dissolved in an aqueous carboxymethyl cellulose (CMC) solution may be used.

[0014]

Next, specific examples of the present invention will be described.

Example 1 KB (Ketjen Black International Co., Ltd.) was added in an amount of 0.3, 0.5, and 1 to the mass of LiMn ₂ O ₄ powder (Mitsui Metals Co., Ltd.).
0, 1.5, 2.0, and 2.5% by mass, respectively, and a binder (NMP solution containing 12% by mass of PVDF)
Was added so that PVDF was 2.5% by mass with respect to LiMn ₂ O ₄ , and primary kneading was performed for 90 minutes to form a dumpling.
The characteristics of KB are listed below. (1) Surface area: 1000 m ² / g (2) DBP oil absorption: 360 ml / 100 g (3) Porosity: 69.3% (4) Apparent specific gravity: 150 g / l

To this kneaded material, 0.5 (1.0, 2.0, 3.0, 4.0% by mass) of AB (manufactured by Denki Kagaku Kogyo Co., Ltd.) and PVDF of binder and NMP were added. The mixture was mixed so that the solid content was 2.0% by mass with respect to LiMn ₂ O ₄ to be 60% by mass.
It was made into a paste of 00 cps.

This paste is coated on both sides of an aluminum foil having a thickness of 0.02 mm, dried and rolled to obtain a positive electrode mixture density of 2.8.
g / cc, thickness 0.205mm, width 37mm, length 35
The positive electrode plate was 0 mm.

As the negative electrode active material, artificial graphite manufactured by Osaka Gas Co., Ltd. was used. After mixing the same binder as that of the positive electrode with respect to the weight of the artificial graphite so that PVDF was 9% by mass, NMP was added thereto, followed by kneading to form a paste. This paste is coated on a surface of a copper foil having a thickness of 0.014 mm, dried and rolled, and has a thickness of 0.132 mm and a length of 4 mm.
A 20 mm negative electrode plate was used.

A lead made of aluminum is attached to the positive electrode plate and a lead made of nickel is attached to the negative electrode plate after the mixture is peeled off, and has a thickness of 0.036 mm, a width of 40 mm and a length of 1100 m.
spirally through a separator manufactured by Nitto Denko Corporation using polypropylene and polyethylene as raw materials,
It was inserted into a battery case having a diameter of 17 mm and a height of 50 mm.

The electrolyte is ethylene carbonate (EC)
And 1.5 mol / l of LiPF ₆ dissolved in a solvent obtained by mixing dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) at a volume ratio of 30:56:14, and after injecting the solution, sealing it. The battery A of the present embodiment
And

(Comparative Example 1) With respect to the mass of LiMn ₂ O ₄ powder, only KB was 0.5, 1.0, 1.5, 2.0,
2.5 and 3.0% by mass were respectively mixed, and a binder and NMP were added thereto so that PVDF was 4.5% by mass with respect to LiMn ₂ O ₄ and the solid content was 60% by mass. 3
Perform for 0, 60, 90, 120 and 150 minutes, and have a viscosity of 20
000 cps. The procedure was performed in the same manner as in (Example 1), and a comparative battery B was produced.

Comparative Example 2 LiMn_TwoO_FourTo the mass of the powder
Then, only AB is changed to 1.0, 2.0, 3.0, 4.0,
5.0 mass% respectively, and the binder and NMP were added thereto.
PVDF is LiMn _TwoO_Four4.5% by mass, solid content
The kneading is performed so that the ratio becomes 60% by mass.
Perform for 40, 60, 80 minutes, viscosity is 20,000 cps
Made into a paste. The procedure is the same as in (Example 1)
To make a comparative battery C.

The relative (Comparative Example 3) LiMn ₂ O ₄ powder mass, the KB 0.3,0.5,1.0,1.5,2.
0, 2.5% by mass and AB at 0.5, 1.0, 2.0,
3.0 and 4.0% by mass were added and mixed at the same time, and a binder and NMP were added thereto so that PVDF was 4.5% by mass with respect to LiMn ₂ O ₄ and the solid content was 60% by mass. Kneading was performed for 30, 60, 90, and 120 minutes to obtain a paste having a viscosity of 20,000 cps. The procedure was performed in the same manner as in (Example 1), and a comparative battery D was produced.

The battery A of the present invention and the batteries B, C, and D of the comparative example were each subjected to four cycles of initial charge / discharge, stored at 20 ° C. for 7 days, and then subjected to a charge / discharge cycle life test. The charging and discharging conditions were as follows: charging at 20 ° C. performed charging at a charging voltage of 4.3 V, and limiting current at a constant current and constant voltage of 20% of the battery capacity.
A constant current discharge at a discharge end voltage of 3.0 V was performed. The point at which the capacity falls below 80% of the capacity in the first cycle is defined as the cycle life, and (Table 1), (Table 2), (Table 3) and (Table 4) show the results of the capacity and the number of cycles in the first cycle.

[0025]

[Table 1]

From the results of the comparative battery B shown in Table 1,
The amount of KB to be added is 1.0 to 2.0 based on the LiMn ₂ O ₄ powder.
5 mass% is desirable, and when it is 0.5 mass%, the conductivity of the positive electrode is low, so that the cycle characteristics are poor. In the case of 3.0% by mass, the density of the positive electrode active material in the mixture is reduced, the battery capacity is less than 650 mAh, and since KB has a high surface area, the decomposition of the electrolytic solution that occurs on the surface of the conductive agent is large, and the cycle is reduced. The number was less than 350. Kneading time is 60 to 120
For 30 minutes. Since the conductive agent was not uniformly distributed in the mixture for 30 minutes, a non-uniform electrode reaction occurred, and the number of cycles was less than 350. Although the KB is not as good as the AB, the conductivity decreases with the increase in the kneading time. Therefore, the kneading for 150 minutes reduces the number of cycles. The viscosity is 15
Less than 000 cps, the viscosity is too low during coating,
The positive electrode mixture could not be uniformly applied to the aluminum core material.

[0027]

[Table 2]

From the results of the comparative battery C shown in (Table 2),
Amount of AB is 2.0 to 4 with respect to LiMn ₂ O ₄ powder.
0 mass% was desirable, and when it was 1.0 mass%, the conductivity of the positive electrode was low, so that the cycle characteristics were poor. In the case of 5.0% by mass, the density of the positive electrode active material in the mixture was reduced, and the battery capacity was less than 650 mAh. Kneading time is 20 ~
It was desirable to be 60 minutes, and in 10 minutes, the conductive agent could not be uniformly mixed into the mixture, and the number of cycles was less than 300. AB
Has a chain-like structure, and is kept conductive by its network. If the kneading time is lengthened, the network is broken by mechanical stress, and the conductivity is reduced. Therefore, in the kneading for 80 minutes, the number of cycles was 300 or less due to a decrease in conductivity.

[0029]

[Table 3]

From the results of the comparative battery D shown in Table 3,
The amount of KB added is 2.5% by mass based on the LiMn ₂ O ₄ powder.
In the case of, the density of the positive electrode active material in the mixture was reduced, and the battery capacity was less than 650 mAh. In the case of 0.3% by mass, the number of cycles was less than 400. When the addition amount of KB is x mass% and the addition amount of AB is y mass%, 3x +
When the value of y is larger than 8.0, the battery capacity becomes 650 mAh.
It was not possible to form a smooth ink when kneaded, and it took several times longer than usual to dry the electrode plate after coating. When 3x + y was smaller than 2.5, the ratio of the amount of the conductive agent in the mixture was small, and the number of cycles was less than 400 due to a decrease in conductivity. Therefore, the value of 3x + y is 2.5
The value of x is preferably 0.5 to 2.0, and the value of y is preferably 1.0 to 4.0.

The kneading time had a maximum value of the number of cycles under any conditions for 60 to 90 minutes. This is presumably because, when KB and AB were used alone as the conductive agent, the kneading time when the cycle number reached a maximum value was 120 minutes and 40 minutes (Table 1) and (Table 2), respectively.

[0032]

[Table 4]

From Table 4, it can be seen that the battery A of the present invention is comparative batteries B, C, and D shown in Table 1, Table 2, and Table 3.
Cycle characteristics are superior to those of In the primary kneading process, a mixture of KB having high conductivity and liquid retention properties is uniformly mixed in the mixture, and then in a second kneading process, AB having a small surface area and a low bulk density is mixed in a short time to decompose the electrolytic solution. It is considered that problems such as a decrease in conductivity of AB, an increase in the number of times of rolling in production of an electrode plate, and an increase in a drying time at the time of coating could be improved, and the cycle characteristics could be improved. At this time, the value of 3x + y is 2.5
~ 8.0, x value is 0.5 ~ 2.0, y value is 1.0 ~
4.0 is desirable.

In this embodiment (Embodiment 1), PVDF
LiMn ₂ O ₄ ratio of KB Nerigoji 2.5 wt% with respect to the primary kneading time 90 minutes, PVDF of LiMn ₂ O ₄
Is 2.0 mass% at the time of AB kneading, and the secondary kneading time is 40 minutes.
0.5 to 3.5% by mass, 20 to 60 minutes for 60 to 120 minutes
In the case of minutes, it can be similarly carried out.

In this embodiment (Comparative Examples 1, 2, and 3), P
Although the ratio of VDF to LiMn ₂ O ₄ is 4.5% by mass, the same can be applied if the ratio is 3.0 to 6.0% by mass.

In this example, the solid content of the paste was set to 4%.
Although 0 mass% and the viscosity were set to 20000 cps, the same can be applied to 50 to 70 mass% and 15,000 to 25000 cps, respectively.

[0037]

As described above, according to the present invention, there is provided a primary kneading step of kneading the positive electrode active material and KB, and a secondary kneading step of adding AB and kneading after the above-mentioned step. So
A positive electrode mixture for a lithium ion secondary battery having a uniform distribution of the conductive agent, high conductivity, and high liquid retention can be obtained, and has an advantageous effect of extending cycle life.

[Brief description of the drawings]

FIG. 1 is a process diagram of a method for producing a positive electrode mixture for a lithium ion secondary battery of the present invention.

FIG. 2 is a process diagram of a conventional method for producing a positive electrode mixture for a lithium ion secondary battery.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tsuyoshi Hatanaka 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. F term (reference) 5H029 AJ05 AJ06 AK03 AL07 AM03 AM05 AM07 CJ08 CJ28 DJ08 EJ04 HJ00 HJ01 5H050 AA07 AA13 BA17 CA09 DA10 EA10 GA10 GA26 HA01 HA20

Claims (4)

[Claims] 1. A method for producing a positive electrode mixture for a lithium ion secondary battery, comprising a positive electrode active material and two kinds of carbon blacks of acetylene black (AB) and Ketjen black (KB) as conductive agents, A method for producing a positive electrode mixture for a lithium secondary battery, comprising: a primary kneading step of kneading an active material and KB; and a secondary kneading step of adding and kneading AB after the step. 2. When the amount of KB and the amount of AB relative to the weight of the positive electrode active material are x mass% and y mass%, respectively.
2. The lithium ion secondary battery according to claim 1, wherein the value of x + y is 2.5 or more and 8.0 or less, the value of x is 0.5 to 2.0, and the value of y is 1.0 to 4.0. A method for producing a positive electrode mixture. 3. The method for producing a positive electrode mixture for a lithium ion secondary battery according to claim 1, wherein the primary kneading is performed for 60 to 120 minutes and the secondary kneading is performed for 20 to 60 minutes. 4. The positive electrode active material is a Mn oxide.
The method for producing a positive electrode mixture for a lithium ion secondary battery according to the above.