JP2000188096A - Positive paste composition for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance cycle characteristics at high temperature without decreasing initial battery capacity by dispersing a positive active material capable of storing and releasing lithium, a conductive material, and a binder for binding them in a solvent, and blending an organic acid in the dispersion in a specific ratio. SOLUTION: An organic acid blended by 0.3-5.0 wt.% based on the total weight is soluble in a solvent, and it is for example, organic carboxylic acid such as acetic acid or butyric acid, or organic sulfonic acid such as dodecyl sulfonic acid. When composite oxide of lithium and a transition metal is used as the positive active material, elution of the transition metal into an electrolyte is retarded. Blending of 30 wt.% or more positive active material, 2 wt.% of more conductive material such as carbon black, 2 wt.% or more binder such as fluorine-containing resin or thermoplastic resin, and 30-60 wt.% solvent such as n-methyl-2-pyrolidone is preferable, battery performance, coating uniformity, and surface smoothness are increased. A positive paste composition is uniformly kneaded and dispersed with a ball mill for example.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode paste composition for a lithium secondary battery, and more particularly, to a positive electrode suitable for a positive electrode material of a lithium secondary battery, comprising an active material capable of inserting and extracting lithium as a main component. The present invention relates to a paste composition.

[0002]

2. Description of the Related Art Conventionally, this type of lithium secondary battery has a high energy density, and has been put to practical use and widely used in the fields of information-related equipment and communication equipment with the downsizing of personal computers and mobile phones. It has led to. On the other hand, due to resource issues and environmental issues, electric vehicles and hybrid electric vehicles are being developed in the field of automobiles.
The use of lithium secondary batteries for power supplies mounted on these vehicles is being studied.

By the way, this lithium secondary battery generally includes:
A positive electrode and a negative electrode in which a positive electrode layer and a negative electrode layer containing an active material capable of inserting and extracting lithium are formed on the current collector surface,
It comprises a separator sandwiched between a positive electrode and a negative electrode, and a non-aqueous electrolyte. In the case of a lithium secondary battery for an electric vehicle, the temperature inside the vehicle used easily exceeds 30 ° C. under direct sunlight, and especially when the outside air temperature is 30 ° C.
If the temperature is higher than 50 ° C., the temperature may exceed 50 ° C., and it is required to have excellent high-temperature cycle characteristics at 50 ° C. or higher.

[0004]

However, conventional lithium secondary batteries are often used indoors, and are required to have excellent cycle characteristics at room temperature of 20 ° C. Therefore, at high temperatures above 50 ° C,
Battery performance was significantly reduced, and there was a problem in cycle characteristics. In other words, the current electrode paste composition, particularly a lithium secondary battery using the positive electrode paste composition, has insufficient cycle characteristics at high temperatures.

The reason that the conventional lithium secondary battery is inferior in cycle characteristics at high temperatures is that a composite oxide of lithium and a transition metal is used as a positive electrode active material.
In a state of use at a high temperature, it is considered that the transition metal dissolves into the electrolytic solution, so that lithium ions that repeat intercalation and deintercalation cannot return to the positive electrode active material, thereby deteriorating the electrode performance. .

Therefore, the present inventor has proposed that if the transition metal contained in the positive electrode active material is suppressed from being eluted into the electrolyte and the transition metal is left in the positive electrode active material, the intercalation / deintercalation is reduced. We thought that the lithium ions that repeat the callation could return to the positive electrode active material and improve the cycle characteristics.

As a result of diligent efforts, the present inventors have found that the incorporation of an organic acid in the composition of the positive electrode paste suppresses the transition metal from being eluted into the electrolytic solution, leading to the present invention. is there

An object of the present invention is to provide a positive electrode paste composition for a lithium secondary battery which has not only high energy density but also excellent cycle characteristics at high temperatures.

[0009]

In order to solve this problem, a positive electrode paste composition for a lithium secondary battery according to the present invention comprises a positive electrode active material capable of inserting and extracting lithium, a conductive material, and both materials. The gist is that a binder to be bound is dispersed in a solvent, and 0.3 to 5.0% by weight of an organic acid is added to the total weight.

As the positive electrode active material, a composite oxide of lithium and a transition metal can be used. As a specific example, a composite oxide of lithium and a transition metal is LiCoO 2
₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like. Whichever type is used as the active material, it is preferable to use a fine particle powder, and the particle diameter is 1 to 30 μm.
It is preferable to use those in the range.

The conductive material is not particularly limited as long as it can be mixed with an appropriate amount of the positive electrode active material to impart conductivity to the positive electrode layer. For example, carbon black, acetylene black,
One or more of carbonaceous powders such as graphite can be used as a mixture. In addition, a metal powder that is stable at the electrode potential used can also be used.

As the binder, an organic binder resin is generally used. The binder resin plays a role of binding the particles of the positive electrode active material and the conductive material, and needs to be stable to a non-aqueous electrolyte, etc., and has weather resistance, chemical resistance, heat resistance, and flame retardancy. Good properties are required. For example, fluorine-containing resins such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and thermoplastic resins such as polypropylene and polyethylene can be used. Also, styrene butadiene rubber latex,
It is also possible to use a synthetic rubber latex type adhesive such as carboxy-modified styrene butadiene rubber latex.

The solvent plays a role of uniformly dispersing the cathode active material, the conductive material and the binder resin, and also plays a role of adjusting the viscosity of the cathode paste composition obtained by mixing them. As long as the binder resin can be dissolved and can be easily dried, it can be appropriately selected. Specific examples include n-methyl-2-pyrrolidone and dimethylformamide.

The organic acid blended therein plays a role of binding the transition metal, and is not particularly limited as long as it is soluble in a solvent. For example, organic carboxylic acids such as acetic acid, butyric acid, and stearic acid, and organic sulfonic acids such as dodecylsulfonic acid can be used. And the compounding ratio of the organic acid is 0.3% in order to obtain sufficient cycle characteristics at a high temperature.
It is desirable that the content be not less than wt%. 5.0 wt% of organic acid
If it exceeds, problems arise in battery performance such as a decrease in battery capacity.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. <Configuration and Preparation Method of Positive Electrode Paste Composition> As described above, the positive electrode paste composition of the present invention includes an active material capable of inserting and extracting lithium, a conductive material, a binder resin,
It is composed of an organic acid and a solvent. Then, in manufacturing the same, a conductive material, a binder resin, and an organic acid are mixed with the positive electrode active material, and a solvent is added and dispersed to produce a so-called paste-like positive electrode mixture.

At this time, the mixing ratio of the positive electrode active material is desirably 30% by weight or more in order to obtain a sufficient battery capacity, when the whole of the prepared positive electrode paste composition is 100% by weight. Also, the compounding ratio of the conductive substance is desirably 2% by weight or more in order to obtain sufficient conductivity.
The mixing ratio of the binder resin is desirably 2 wt% or more in order to obtain a sufficient binding property. Further, as described above, the compounding ratio of the organic acid is desirably 0.3 wt% or more in order to obtain sufficient cycle characteristics at a high temperature.

However, when the conductive substance exceeds 20 wt%,
Binder resin exceeds 10wt%, organic acid 5wt%
If it exceeds the limit, a problem occurs in battery performance such as a decrease in battery capacity. Further, the mixing ratio of the solvent is desirably 30 wt% or more, in order to uniformly disperse the composition or to prevent a problem in uniformity and surface smoothness in coating. %, The viscosity becomes low, and problems such as sagging during coating occur.

In the positive electrode paste composition, it is necessary that the above-mentioned respective component substances are sufficiently and uniformly kneaded and dispersed in order to ensure good battery performance. Therefore, the kneading and dispersing step is desirably performed using a stirrer having rotating blades, a ball mill, a medium stirring mill, or the like.

<Preparation Method of Positive Electrode> The positive electrode of the lithium secondary battery is prepared by applying the positive electrode paste composition prepared and prepared as described above to the surface of the current collector to form a positive electrode layer. A metal foil such as aluminum is used for a current collector serving as a substrate to be coated. As a device used for coating, it is convenient to use a coater-type coating machine which can apply and dry the electrode paste composition continuously to the belt-shaped current collector.

Since the composition to be applied has a relatively high viscosity, it is preferable to adopt an application method such as a comma coat, a squeeze coat, a die coat, a lip coat, or the like in the coating section of the coating machine. The coating thickness of the positive electrode paste composition is
Any value between 50 and 500 μm can be used.

The drying section of the coating machine is a continuous furnace in the case of a coater system, and various types of drying such as hot air and infrared rays can be employed for drying. The drying temperature is preferably 80 ° C. or higher. If the temperature is lower than 80 ° C., the drying becomes insufficient, the solvent may remain in the positive electrode layer, and the battery performance may be reduced. However, care must be taken because if the drying temperature is too high, drying proceeds only on the surface and uniform drying cannot be expected. Pressing after drying to increase the density of the electrode layer is also effective in increasing the energy density of the battery.

<Structure of Lithium Secondary Battery> A lithium secondary battery using the electrode manufactured as described above will be described. A lithium secondary battery generally includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and a non-aqueous electrolyte. Hereinafter, components except for the above-described positive electrode will be sequentially described.

The negative electrode is composed of metallic lithium, a lithium compound,
Although a lithium alloy or the like can be used, there is a problem that dendrite is deposited due to repetition of charge and discharge. Therefore, instead of these, a powdery carbon material capable of inserting and extracting lithium is preferably used as the negative electrode active material. When a carbon material is used as the negative electrode active material, the negative electrode is prepared by mixing a binder resin with the carbon material, adding an appropriate solvent as necessary, and forming a negative electrode paste composition, like the positive electrode, made of copper foil or the like. Is applied to the surface of the negative electrode current collector and dried to form a negative electrode layer.

Examples of the carbon material usable as the negative electrode active material include natural graphite, artificial graphite, coke, carbon black, vapor-grown carbon, carbon fiber, a material obtained by carbonizing an organic polymer compound, or a heat-treated material thereof. And mixed materials. In producing the negative electrode, the same method as the positive electrode or the same method as the method for kneading and dispersing the binder resin for binding the negative electrode active material, the solvent, the negative electrode paste composition, and the coating and drying method can be used.

The separator sandwiched between the positive electrode and the negative electrode has a function of separating the positive electrode and the negative electrode, holding an electrolyte, and passing lithium ions. For this separator, a porous film such as polyethylene or polypropylene, a nonwoven fabric or a woven fabric can be used.
It is preferable that the thickness of the separator be about 10 to 200 μm.

The non-aqueous electrolyte is a non-aqueous electrolyte which is stable with respect to the above-mentioned positive electrode active material and negative electrode active material, and is capable of performing movement for performing an electrochemical reaction with the positive electrode active material and the negative electrode active material. Any water substance can be used. Usually, a lithium salt as an electrolyte is used by dissolving it in an organic solvent. Salts that can be used for the electrolyte are
_{Specifically, LiPF 6, LiA S F 6} , LiSb
F ₆ , LiBF ₄ , LiClO ₄ , LiI, LiBr,
LiCl, LiAlCl, LiHF ₂ , LiSCN, L
ISO ₃ CF _2, and the like. Of these,
LiPF ₆ , LiBF ₄ and LiClO ₄ are preferred.

The solvent for dissolving the electrolyte can be arbitrarily selected, but an organic solvent having a relatively high dielectric constant is preferably used. For example, ethylene carbonate, cyclic carbonates such as propylene carbonate, dimethyl carbonate, acyclic carbonates such as ethyl methyl carbonate, tetrahydrofuran, glimes such as 2-methyltetrahydrofuran, lactones such as γ-butyl lactone, sulfur such as sulfolane. One or more solvents such as compounds, nitriles such as acetonitrile and the like can be mentioned. Among these, in particular, one or more mixed solvents selected from cyclic carbonates such as ethylene carbonate and propylene carbonate, and non-cyclic carbonates such as dimethyl carbonate and ethyl methyl carbonate are preferably used.

Further, the above non-aqueous electrolyte is impregnated with a polymer such as polyethylene oxide, polypropylene oxide, a crosslinked isocyanate of polyethylene oxide, phenylene oxide, or a phenylene sulfide polymer as a solid electrolyte in place of the above non-aqueous electrolyte. Organic solid electrolyte, Li ₃ N, LiBCl ₄ , Li ₄ SiO
₄ , an inorganic solid electrolyte of lithium glass such as Li ₃ BO ₃ can also be used.

As the shape of the lithium secondary battery configured as described above, various shapes such as a cylindrical type, a box type, a paper type, and a card type can be used. In any case, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead or the like extending from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal. The electrode assembly is sealed together with the non-aqueous electrolyte in a battery case to complete a lithium secondary battery.

Based on the above embodiment, as an example, a positive electrode paste composition was actually prepared. Separately, a cathode paste composition outside the range specified in the present invention was prepared as a comparative example. Positive electrodes were produced using the positive electrode paste compositions of the examples and comparative examples, and cylindrical cell type lithium batteries were actually produced. Then, by comparing the cycle characteristics and the battery capacity at a high temperature, it was confirmed that the positive electrode paste composition of the present invention was effective. Hereinafter, positive electrode paste compositions of Examples and Comparative Examples, production of batteries, and comparison results of experimental data will be described in order.

Example 1 The positive electrode paste composition of Example 1 based on the above-described embodiment of the present invention comprises lithium nickelate as a lithium-based positive electrode active material, acetylene black as a conductive material, and binder resin as a binder resin. Polyvinylidene fluoride resin (hereinafter abbreviated as PVDF)
Acetic acid was used as an organic acid, and n-methyl-2-pyrrolidone (hereinafter abbreviated as “NMP”) was used as a solvent.
The respective mixing ratios in the positive electrode paste composition were 55.1 wt% of lithium nickelate, 3.2 wt% of acetylene black, 3.2 wt% of PVDF, 0.5 wt% of acetic acid, N
MP was 38.0 wt%.

For the kneading and dispersion of the positive electrode paste composition, a dispersing machine (Ultraviscomil: manufactured by IMEX) was used. This dispersing machine has a capacity of 2.0 liters, has five stirring disks, and is charged with 1.4 liters of zirconia beads having a diameter of 2 mm. The stirring disk was rotated at a rotation speed of 1000 rpm, and the positive electrode paste composition was continuously processed at 200 ml / min, kneaded and dispersed to obtain a positive electrode paste.

<Example 2> The above-described present invention is based on the embodiment. The positive electrode paste composition of Example 2 is the same as Example 1 except that stearic acid is used as an organic acid. In the same manner, the mixing ratio of each component is 52.9% by weight of lithium nickelate, 3.1% by weight of acetylene black, 3.1% by weight of PVDF, and 3.1% by weight of stearic acid.
wt% and 37.8 wt% NMP.

<Comparative Example 1> Except that no organic acid was blended, each component material and preparation conditions were the same as in Example 1, and the blending ratio of each component was 55.5 wt% of lithium nickelate and acetylene black. 3 wt%, PVDF 3.3 wt%, NM
P was 37.9 wt%.

<Comparative Example 2> Except for the compounding ratio of the organic acid, each component material and the production conditions were the same as in Example 2, and the compounding ratio of each component material was 50.3% by weight of lithium nickelate and acetylene black 2 9.9 wt%, PVDF 2.9 wt%, stearic acid 5.8 wt%, NMP 38.1 wt%.

The positive electrode paste thus prepared was applied to a thickness of 2
After coating on one side with a comma coater on a 0 μm aluminum foil, the coating was dried at 80 ° C. for 5 minutes. After drying, the backside was similarly coated and dried to obtain a positive electrode sheet having both surfaces coated with a positive electrode material. Next, roll pressing was performed at a linear pressure of 1000 kgf / cm to make the film thickness 100 μm, and then cut into a predetermined shape to produce a positive electrode.

In the negative electrode paste composition, mesocarbon microbeads were used as a negative electrode active material, PVDF was used as a binder resin, and NMP was used as a solvent. The mixing ratio in the negative electrode paste composition was as follows: mesocarbon microbeads 45.0 wt%, PVDF 5.0 wt%, N
MP was 50.0% by weight. The negative electrode paste was prepared by kneading and dispersing in the same manner as in the positive electrode paste composition. The prepared negative electrode paste was applied on one side using a comma coater on a copper foil having a thickness of 20 μm, and then dried at 80 ° C. for 5 minutes. After drying, the back side was also coated and dried, and the positive electrode material was applied to both sides. A negative electrode sheet was obtained. Next, when the linear pressure is 1000 kgf
/ Cm and roll cut to a film thickness of 100 μm, and then cut into a predetermined shape to produce a negative electrode.

Next, a thickness of 20 mm was formed between the obtained positive electrode and negative electrode.
A separator made of polyethylene having a thickness of μm was sandwiched and spirally wound to form a cylindrical cell, which was then mounted on a battery can filled with a non-aqueous electrolyte. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ as a supporting salt at a concentration of 1 mol / l in a mixed solution of equal volumes of ethylene carbonate and diethyl carbonate was used.

<Comparative Results> Examples 1 and 2, Comparative Example 1
The cycle characteristics at high temperature and the initial discharge capacity of each cylindrical cell produced using the positive electrode paste compositions of Examples 1 and 2 were evaluated and compared. The evaluation condition is a charging current of 50
A constant current charge / discharge with a discharge current of 0 mA and a discharge current of 500 mA is performed at a voltage of 3.0 to 4.1 V at 60 ° C. evaluated. Table 1 shows the results of the initial discharge capacity and capacity retention of each cylindrical cell.

[0040]

[Table 1]

As shown in Table 1 above, the battery produced using the positive electrode paste composition of Comparative Example 1 had a low discharge capacity retention rate of 82% after 100 cycles. This is because the organic acid was not blended in the positive electrode paste composition of Comparative Example 1, and the capacity of the battery was reduced.
The battery prepared using the positive electrode paste composition of Comparative Example 2 had a low initial discharge capacity of 1130 mAh. This is because the organic acid was 5 wt% in the positive electrode paste composition of Comparative Example 2.
% Of 5.8 wt%, which is considered to have decreased the capacity of the battery itself.

On the other hand, batteries prepared using the positive electrode paste compositions of Examples 1 and 2 based on the embodiment of the present invention have improved cycle characteristics at high temperatures due to the addition of the organic acid. No decrease in the initial discharge capacity was observed. From these results, it was demonstrated that the positive electrode paste composition of the present invention was a positive electrode paste composition having excellent high-temperature cycle characteristics.

The fact that such a result was obtained is as follows.
The substance reaction mechanism is considered as follows. That is, by adding an appropriate amount of an organic acid to the positive electrode paste composition, the surface of the composite oxide of lithium, which is a positive electrode active material, and a transition metal (Ni, Mn, Co, etc.) in the composition is covered with the organic acid. Therefore, conventionally, high temperature (about 60 ° C)
At the operating ambient temperature, the transition metal elutes into the electrolyte and the lithium ions that intercalate and deintercalate gradually cannot return to the positive electrode active material due to the charge / discharge reaction of the battery, reducing the charge / discharge capacity. According to the present invention, according to the present invention, even at a high use environment temperature, the transition metal can be eluted into the electrolytic solution by binding to an organic acid, for example, COO ^- ion, at the time of lithium ion deintercalation. This is considered to have been avoided and a high charge / discharge capacity could be maintained forever.

The present invention is not limited to the above-described embodiment.
It is not intended to be limited to various ones without departing from the spirit of the present invention.
Modifications are possible. For example, in the above embodiment, the positive electrode active material
Although lithium nickelate was used for the quality,
LiCoO₂, LiMn₂O ₄Various lithium secondary batteries such as
Applies to ponds. The organic acids are also acetic acid and
It is not limited to aric acid, and various organic acids are applied.
It is clear from the utility of the organic acid of the present invention. Further guidance
The conductive materials, organic binders and the like are also limited to those of the embodiments.
Of course, it is not.

[0045]

According to the present invention, there is provided a positive electrode paste composition for forming a positive electrode of a lithium secondary battery, wherein an organic acid is contained in an amount of 0.3 to 5% by weight based on 100% by weight of the entire positive electrode paste composition.
By blending, a battery produced using this positive electrode paste composition can be prepared without lowering the battery capacity at the initial stage.
It has become possible to obtain excellent cycle characteristics at high temperatures. Accordingly, the present invention opens up the possibility of using a lithium secondary battery for applications that require use in outdoor and high-temperature environments, such as a secondary battery for an electric vehicle.

Continued on the front page F term (reference) 5H003 AA02 AA04 BB01 BB11 BB12 BB14 BD04 5H014 AA02 EE01 HH01

Claims (1)

[Claims] A positive electrode active material capable of inserting and extracting lithium, a conductive material, and a binder for binding both materials are dispersed in a solvent, and 0.3 to 5.0% by weight of the total weight of an organic acid is dispersed. And a positive electrode paste composition for a lithium secondary battery.