JP2003297356A - Positive electrode black mix slurry for lithium secondary battery, positive electrode using the same, and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode mixture slurry and a positive electrode capable of accomplishing a lithium secondary battery having a good storing characteristic at high temperatures and a good cycle characteristic. <P>SOLUTION: The positive electrode mixture slurry for the lithium secondary battery is composed of a positive electrode active material, a binder, and a solvent. The positive electrode active material should contain lithium-nickel composite oxide expressed by the composition formula LiNi<SB>1-x-y-z</SB>Co<SB>x</SB>Al<SB>y</SB>Mg<SB>z</SB>O<SB>2</SB>, where the conditions should be met that 0.1≤x≤0.3 and 0.02≤y+z≤0.2, while the solvent should contain dimethyl acetoamide and either or both of N and N-dimethyl formamide. The positive electrode according to the invention is formed with this slurry of positive electrode mixture for the lithium secondary battery. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to storage of lithium
More specifically, the present invention relates to a positive electrode mixture slurry that forms a positive electrode that constitutes a lithium secondary battery.

[0002]

2. Description of the Related Art Lithium secondary batteries that utilize the absorption and desorption phenomenon of lithium have a high energy density.
With the miniaturization of mobile phones, personal computers, etc., they have become widely used in the fields of communication equipment and information related equipment. On the other hand,
Due to environmental problems and resource problems, the development of electric vehicles has been rushed in the field of automobiles, and lithium secondary batteries have been studied as a power source for these electric vehicles.

As described above, the lithium secondary battery has been demanded in a wide variety of fields, but its price is high, so that it is required to have a longer life than other secondary batteries. In order to have a long life, it is required that the so-called storage characteristics are good, for example, the capacity does not decrease and the internal resistance of the battery does not increase even when the battery is stored with a high charge rate. . In addition, it is also required that the so-called cycle characteristics are good, that is, the capacity does not decrease even if charging and discharging are repeated. In particular, an increase in the internal resistance of the battery causes a decrease in the power characteristics of the battery (the characteristics that a large output can be taken out in a short time and a large amount of electric power can be charged in a short time). It becomes important to do. In addition, since the battery reaction is activated at high temperatures and the internal resistance rises significantly, for example, when using a lithium secondary battery for applications such as power supply for electric vehicles that may be left outdoors, It is important that the storage characteristics and cycle characteristics at high temperatures are good.

At present, development of a lithium secondary battery in which a lithium transition metal composite oxide containing Ni and Co as main constituent elements is used as a positive electrode active material is underway. However, this type of lithium secondary battery has a large decrease in battery capacity and a large increase in internal resistance when stored at a high charge rate and kept at a high temperature. In addition, the capacity decreases even when charging and discharging are repeated at high temperature.

[0005]

Generally, the positive electrode of a lithium secondary battery is prepared by mixing a positive electrode active material made of powdered lithium transition metal composite oxide with a conductive material and a binder and adding a solvent to form a slurry. It is formed by applying the dried positive electrode mixture on the surface of the metal foil current collector and drying. And
Polyvinylidene fluoride or the like is mainly used as a binder, and N-methyl-2-pyrrolidone is mainly used as a solvent that dissolves polyvinylidene fluoride. N-methyl-2-pyrrolidone used as a solvent has a boiling point of about 200 ° C. Therefore, the drying after the application of the positive electrode mixture slurry in the formation of the positive electrode is usually
It is performed at a high temperature of 150 to 200 ° C.

As a result of repeated experiments by the present inventors, when the positive electrode mixture slurry was heated in the above temperature range when forming the positive electrode, a part of the solvent N-methyl-2-pyrrolidone was deteriorated. It turned out to be a solid. Then, it was found that battery characteristics such as storage characteristics in the lithium secondary battery are deteriorated by the solid matter remaining on the positive electrode.

The present invention has been made based on the above findings, and provides a positive electrode mixture slurry and a positive electrode which can realize a lithium secondary battery having good storage characteristics and cycle characteristics at high temperatures. Is an issue. Another object of the present invention is to provide a lithium secondary battery having good storage characteristics and cycle characteristics at high temperature by using the positive electrode.

[0008]

A positive electrode mixture slurry for a lithium secondary battery of the present invention is a positive electrode mixture slurry for a lithium secondary battery containing a positive electrode active material, a binder and a solvent,
The positive electrode active material, the composition formula LiNi _1-xyz Co _x Al _y
Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦
0.2) represented by the formula: lithium nickel composite oxide, wherein the solvent is dimethylacetamide and N, N-
It is characterized by containing at least one of dimethylformamide.

The present inventor has paid attention to the positive electrode active material and the solvent, and used both as the following materials to form a positive electrode mixture slurry. That is, in the positive electrode admixture slurry of the present invention, the positive active material, the composition formula LiNi _1-xyz Co _x Al _y
Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦
The lithium nickel composite oxide represented by 0.2) is included. In other words, the lithium nickel composite oxide having a basic composition of LiNiO ₂ and having a Ni site partially substituted with two or more kinds of elements such as Co and Al constitutes the positive electrode active material. . As will be described later in detail, the crystal structure of the lithium nickel composite oxide is stabilized by substituting a part of the Ni site with another element. Further, since the decomposition reaction accompanying the release of oxygen is suppressed, the thermal stability of the lithium nickel composite oxide is improved. By using a lithium nickel composite oxide having a stable crystal structure and good thermal stability as a positive electrode active material, deterioration of the positive electrode active material itself is suppressed when a lithium secondary battery is constructed.

In the positive electrode mixture slurry of the present invention, the solvent contains at least one of dimethylacetamide and N, N-dimethylformamide. Each of dimethylacetamide and N, N-dimethylformamide can easily dissolve polyvinylidene fluoride as a binder. Furthermore, even if the above two solvents are heated to a high temperature of about 200 ° C., they do not deteriorate and do not form a solid. Therefore, when the solvent contains at least one of dimethylacetamide and N, N-dimethylformamide, the positive electrode mixture slurry does not leave a dry residue of the solvent even when heated when forming the positive electrode. That is, the positive electrode mixture slurry of the present invention can be used to form a positive electrode with less impurities. Thus, by using the positive electrode mixture slurry of the present invention,
It is possible to realize a lithium secondary battery having excellent storage characteristics and cycle characteristics at high temperatures.

The positive electrode for a lithium secondary battery of the present invention is a positive electrode for a lithium secondary battery formed from a positive electrode mixture slurry containing a positive electrode active material, a binder and a solvent, wherein the positive electrode active material comprises Compositional formula LiNi _1-xyz Co _x Al _y Mg _z O
₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦ 0.2)
And a solvent containing at least one of dimethylacetamide and N, N-dimethylformamide.
That is, the positive electrode for a lithium secondary battery of the present invention is a positive electrode formed from the positive electrode mixture slurry of the present invention. By being formed using the positive electrode mixture slurry of the present invention, the positive electrode of the present invention becomes a positive electrode that can form a lithium secondary battery having excellent storage characteristics and cycle characteristics at high temperatures.

The lithium secondary battery of the present invention comprises a positive electrode formed from a positive electrode mixture slurry containing a positive electrode active material, a binder and a solvent, a negative electrode, and an electrolytic solution prepared by dissolving an electrolyte material in an organic solvent. A lithium secondary battery comprising the positive electrode active material, wherein the positive electrode active material has a composition formula of LiNi _{1-x- yz} Co _x Al _y Mg _z O
₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦ 0.2)
And a solvent containing at least one of dimethylacetamide and N, N-dimethylformamide.
That is, the lithium secondary battery of the present invention comprises the positive electrode for a lithium secondary battery of the present invention. By using the positive electrode of the present invention as described above, the lithium secondary battery of the present invention has a large discharge capacity and excellent storage characteristics and cycle characteristics.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the positive electrode mixture slurry for a lithium secondary battery of the present invention will be described, and then the positive electrode for a lithium secondary battery of the present invention formed from the positive electrode mixture slurry will be described. Further, the lithium secondary battery of the present invention constituted by using the positive electrode will be described.

<Positive Electrode Mixture Slurry> The positive electrode mixture slurry for a lithium secondary battery of the present invention is a positive electrode mixture slurry for a lithium secondary battery containing a positive electrode active material, a binder and a solvent. The active material has a composition formula of LiNi _1-xyz Co _x A.
l _y Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z
≦ 0.2), and the solvent is dimethylacetamide and N, N-.
At least one of dimethylformamide is included.

The positive electrode active material has a composition formula of LiNi _1-xyz C
o _x Al _y Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y
+ Z ≦ 0.2), which includes a lithium nickel composite oxide. The lithium nickel composite oxide is not limited to the stoichiometric composition represented by the above composition formula, but also has a non-stoichiometric composition in which some elements such as lithium and oxygen are deficient or excessive. Is also included. The present lithium nickel composite oxide is one in which a part of the Ni site is replaced by Co and at least one of Al and Mg, which have different roles. Ratio of Ni remaining without being replaced,
That is, the value of (1-x-y-z) in the composition formula is 0.
5 ≦ (1−x−y−z) ≦ 0.88. This is because when the Ni content is less than 0.5, not only the original layered rock salt structure but also a sub-phase such as a spinel structure is generated, and the battery capacity is reduced. Further, when the ratio of Ni exceeds 0.88, the substitution effect is too small to form a secondary battery having desired battery characteristics. Especially 0.7
It is preferable that ≦ (1−x−y−z) ≦ 0.8.

Co plays a role of mainly stabilizing the crystal structure of the lithium nickel composite oxide. Also, Co
Suppresses the capacity decrease due to element substitution, and
(Co, Ni) O ₂ is an all-solid-solution type, and has an advantage of minimizing deterioration of crystallinity. The stabilization of the crystal structure by Co suppresses the decrease in battery capacity particularly when stored at high temperature. In order to sufficiently exert the effect of improving the storage characteristics, the substitution ratio by Co, that is, the value of x in the composition formula is set to 0.1 ≦ x ≦ 0.3. x <
In the case of 0.1, the crystal structure is not sufficiently stabilized,
The storage characteristics of the constructed secondary battery are not good. on the other hand,
When 0.3 <x, the crystallinity of the lithium-nickel composite oxide decreases. In particular, it is more preferable that 0.1 ≦ x ≦ 0.2.

Al and Mg mainly serve to suppress the decomposition reaction of the lithium-nickel composite oxide accompanying the release of oxygen and improve the thermal stability. Further, since Al and Mg do not change their valences due to absorption and desorption of lithium, they also play a role of maintaining a stable crystal structure. It should be noted that the present lithium nickel composite oxide may contain at least one of Al and Mg. That is, any of a mode containing no Al (y = 0) Mg, a mode containing no Mg (z = 0) Al, and a mode containing both Al and Mg elements is adopted. Good. In particular, it is desirable to adopt a mode in which both elements of Al and Mg are contained from the viewpoint that the effect of improving thermal stability and the like is greater. In any of the above embodiments, the substitution ratio of Al and Mg, that is, the value of y + z in the composition formula is 0.02 ≦ y + z ≦ 0.2.
When y + z <0.02, the effect of improving thermal stability cannot be sufficiently obtained. On the other hand, if 0.2 <y + z, the battery capacity will decrease. In particular, it is preferable that 0.1 ≦ y + z ≦ 0.2.

The binder plays a role of binding the positive electrode active material materials to each other and binding the positive electrode active material material and the current collector. For example, fluororesins such as polytetrafluoroethylene, polyvinylidene fluoride and fluororubber, and thermoplastic resins such as polypropylene and polyethylene can be used. In particular, it is preferable to use polyvinylidene fluoride as a binder because it is easily soluble in a solvent such as dimethylacetamide or N, N-dimethylformamide and has high performance as a binder.

The solvent dissolves or emulsifies the above binder,
It plays a role of dispersing the positive electrode active material and the like. It is desirable to use water as the solvent from the viewpoint that solid matter is not generated by heating when forming the positive electrode. But,
It is difficult and not practical to dissolve or emulsify a binder such as polyvinylidene fluoride in water. The solvent in the positive electrode mixture slurry of the present invention contains at least one of dimethylacetamide and N, N-dimethylformamide. All of them can easily dissolve polyvinylidene fluoride and do not produce a solid matter when heated. As long as it contains one of dimethylacetamide and N, N-dimethylformamide, it may be a mixture of N-methyl-2-pyrrolidone and the like. In addition, dimethylacetamide and N, N-dimethylformamide may be used alone or as a mixture of both. The content ratio of dimethylacetamide and N, N-dimethylformamide in the solvent is not particularly limited. The content of dimethylacetamide and N, N-dimethylformamide in the solvent should be 80% by weight or more from the viewpoint of reducing the production of solid matter due to heating and constituting a lithium secondary battery having more excellent battery characteristics. Is desirable. In particular, it is more preferable that the content ratio of these is 100% by weight, that is, only dimethylacetamide or N, N-dimethylformamide is used as the solvent.

Usually, a conductive material is mixed with the positive electrode mixture slurry in order to secure the electric conductivity of the positive electrode active material layer when the positive electrode is formed. It is desirable that the positive electrode mixture slurry of the present invention also adopts a mode that contains a conductive material and the like in addition to the positive electrode active material, the binder, and the solvent described above. As the conductive material, for example, carbon substances such as carbon black, acetylene black, and graphite may be used alone or in combination of two or more thereof.

The mixing ratio of each of the above materials in the positive electrode mixture slurry of the present invention varies depending on the kind of the material used, but for example, 70 to 90 parts by weight of the positive electrode active material and 3 to 10 parts by weight of the binder are used. The conductive material may be 5 to 20 parts by weight, and a predetermined amount of solvent may be added to these. The amount of the solvent to be blended is an amount with which the positive electrode active material and the like can be uniformly dispersed, and may be appropriately determined in consideration of the viscosity that is easily applied to the current collector.

<Positive Electrode> The positive electrode for a lithium secondary battery of the present invention is formed from the positive electrode mixture slurry of the present invention described above. The positive electrode for a lithium secondary battery of the present invention has, for example, a mode in which the binder is polyvinylidene fluoride or a mode in which the content ratio of dimethylacetamide and N, N-dimethylformamide in the solvent is 80% by weight. It is desirable to adopt the above-mentioned more desirable mode in the material slurry. The method for forming the positive electrode for a lithium secondary battery of the present invention is not particularly limited. It can be formed by applying the positive electrode mixture slurry on the surface of a current collector made of a metal foil such as aluminum, drying it, and then increasing the density of the active material by pressing.

The method of applying and drying the positive electrode mixture slurry is not particularly limited. As a coating method, a method using various coater heads such as a reverse roll, comma bar, gravure, and air knife can be used. Further, for drying, various dryers such as a blower dryer, a warm air dryer, an infrared heater, a far infrared heater, etc. can be used, and they may be left standing and dried. The positive electrode mixture slurry may be applied to only one surface of the current collector, or may be applied to both surfaces of the current collector. The coating thickness on one surface is preferably 20 to 150 μm.

For example, the positive electrode mixture slurry is applied to both sides of the current collector and dried, and the positive electrode is roll-pressed to have a predetermined active material density in order to improve the energy density of the battery. The density of the positive electrode active material layer is 1.5 g / cm ³ or more.
It is preferably 0 g / cm ³ or less. If the density is smaller than these values, the positive electrode active material cannot be filled in a certain volume in an amount of the specified weight or more, so that the discharge capacity decreases. On the other hand, when the value is larger than these values, the voids of the active material layer are reduced, so that the electrolytic solution is difficult to permeate, and the positive electrode active material is not effectively used, so that the discharge capacity is also reduced.

<Lithium Secondary Battery> The lithium secondary battery of the present invention comprises a positive electrode of the present invention formed from the positive electrode mixture slurry of the present invention containing a positive electrode active material, a binder and a solvent.
It is configured to include a negative electrode and an electrolytic solution in which an electrolyte material is dissolved in an organic solvent. Except for the positive electrode, the structure is not particularly limited, and may be the structure of a known lithium secondary battery. In the lithium secondary battery of the present invention, in the constitution of the positive electrode, for example, a mode in which the binder is polyvinylidene fluoride, or the content ratio of dimethylacetamide and N, N-dimethylformamide in the solvent is 80% by weight.
It is desirable to incorporate the above-mentioned more desirable aspects such as the above aspect.

The negative electrode facing the positive electrode can be formed by sheet-forming metal lithium, a lithium alloy, or the like, or by pressing the sheet-shaped one onto a current collector net of nickel, stainless steel, or the like. However, in consideration of dendrite precipitation and the like, in order to obtain a lithium secondary battery with excellent safety, it is desirable to configure the negative electrode by using a carbon substance capable of inserting and extracting lithium as the negative electrode active material. Examples of the carbon substance that can be used include natural or artificial graphite, a fired body of an organic compound such as phenol resin, and a powdered body such as coke. Above all, it is desirable to use artificial graphite because it has high crystallinity and can form a battery with high energy density. When a carbon material is used as the negative electrode active material, the negative electrode active material is mixed with a binder, and a suitable solvent is added to form a slurry, and the negative electrode mixture is applied to the surface of a metal foil current collector such as copper. Dry to form the negative electrode. In this case, like the positive electrode,
As the binder, a fluororesin such as polyvinylidene fluoride can be used. Further, as the solvent, in addition to N-methyl-2-pyrrolidone, dimethylacetamide or N, N
-Dimethylformamide or the like can be used.

A separator is sandwiched between the positive electrode and the negative electrode. The separator holds the electrolytic solution and allows ions to pass through while separating the positive electrode and the negative electrode, and a thin microporous film such as polyethylene or polypropylene can be used.

The electrolyte solution is an electrolyte material dissolved in an organic solvent. As the organic solvent, an aprotic organic solvent, specifically, one kind of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride, or the like, or Two or more kinds of these solvents can be used. Further, as the electrolyte, LiP that produces lithium ions when dissolved in an organic solvent is used.
F ₆ , LiI, LiClO ₄ , LiAsF ₆ , LiBF _4, etc. can be used. In particular, it is desirable to use LiPF ₆ because of its high electrolyte capacity.

The lithium secondary battery composed of the above-mentioned ones can have various shapes such as a coin type, a laminated type and a cylindrical type. Regardless of which shape is adopted, the separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the positive electrode and the negative electrode are electrically connected to the positive electrode terminal and the negative electrode terminal, respectively. The battery can be completed by hermetically sealing it in a battery case together with an electrolytic solution.

<Acceptance of Other Embodiments> The embodiments of the positive electrode mixture slurry for a lithium secondary battery, the positive electrode for a lithium secondary battery, and the lithium secondary battery of the present invention have been described above. It is only one embodiment. The lithium secondary battery positive electrode mixture slurry of the present invention, the lithium secondary battery positive electrode and the lithium secondary battery, various modifications based on the knowledge of those skilled in the art, including the above embodiment,
It can be implemented in various modified forms.

[0031]

[Examples] First, a heating test was performed on various solvents to confirm the presence or absence of solids. In addition, a lithium secondary battery was manufactured using an electrode containing a solid substance generated from N-methyl-2-pyrrolidone, and a charge / discharge cycle test under high temperature was performed. Next, various positive electrode mixture slurries having different positive electrode active materials and solvents were prepared to form various positive electrodes. Then, a lithium secondary battery was produced using these positive electrodes, and the storage characteristics at high temperature were evaluated. Hereinafter, each item will be described in order.

<Heating test of solvent> N-methyl-2-pyrrolidone (hereinafter referred to as "NMP"), dimethylacetamide (hereinafter referred to as "DMA"), and N, N-dimethylformamide (which are solvents) Hereinafter, it will be referred to as "DMF") and a heating test was conducted to investigate whether or not a solid substance was formed. First, 10 of NMP (purity 99.5% or more)
ml was put in a petri dish and dried in a warm air dryer. The drying temperature was 80 to 200 ° C. After drying, it was confirmed whether or not a solid matter was formed. The results are shown in Table 1. The solid amount (wt%) in Table 1 is 10% of the total amount of NMP.
It is the amount of solid matter when it is 0 wt%.

[0033]

[Table 1]

It can be seen from Table 1 that brown solids are formed when dried at a temperature of 100 ° C. or higher. And the amount of solids produced increases with increasing temperature,
It became 3.0 wt% at 200 ° C. As a result of analyzing the solids by the infrared absorption method (IR method), it was found that all the solids were the same. Further, from the absorption spectrum of the solid matter, it is considered that NMP was hydrolyzed and decomposed into one having a carboxyl group while the N—C bond was cleaved to open the ring by heating. On the other hand, when DMA and DMF were dried in the same manner as above at a temperature of 200 ° C., no solid matter was produced.

<Charge / Discharge Cycle Test> Dry the above NMP
The electrode containing the solid matter generated as a result of
A lithium secondary battery was produced using the electrodes. The positive electrode is
LiNi, which is the positive electrode active material _0.8Co_0.15Al_0.05O₂
To 85 parts by weight of carbon black as a conductive material
0 parts by weight, 5 times polyvinylidene fluoride as a binder
Mix parts by volume, add an appropriate amount of DMA as a solvent, and add slurry.
A lee-shaped positive electrode mixture was prepared. In this positive electrode mixture slurry
1.5 g of the above solid was added. Then, the positive electrode mixture slurry
Re is coated on both sides of a 20 μm thick aluminum foil current collector.
Cloth, dry, and then roll press compress
Create a sheet-like material with a thickness of 40 μm on each side.
Made This sheet-shaped positive electrode has a size of 54 mm × 450 mm
It was cut into a size and used.

For the negative electrodes facing each other, artificial graphite was used as an active material. First, 5 parts by weight of polyvinylidene fluoride as a binder was mixed with 95 parts by weight of artificial graphite as a negative electrode active material, and an appropriate amount of DMA was added as a solvent to prepare a slurry-form negative electrode mixture. Similar to the positive electrode mixture slurry, 1.5 g of the above solid was added to the negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both sides of a copper foil current collector having a thickness of 10 μm, dried, and then compressed by a roll press to prepare a sheet-like one having a thickness of 30 μm per side of the negative electrode mixture. did. This sheet-shaped negative electrode is 56 mm
It was cut into a size of 500 mm and used.

The positive electrode and the negative electrode were wound by sandwiching a polyethylene separator having a thickness of 25 μm and a width of 58 mm between them to form a roll-shaped electrode body. Then, the electrode body was inserted into a 18650 type cylindrical battery case (outer diameter 18 mmφ, length 65 mm), an electrolytic solution was injected, and the battery case was sealed to produce a cylindrical lithium secondary battery. The electrolytic solution used was a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1.

A lithium secondary battery was prepared in the same manner as above except that the above solids were not added to the positive electrode mixture slurry and the negative electrode mixture slurry. Then, a charge / discharge cycle test was performed on these two lithium secondary batteries. First, as conditioning, at a temperature of 20 ° C., a constant current of 0.2 mA / cm ² was charged to 4.1 V, and then a constant current of 0.2 mA / cm ² was discharged to 3.0 V. went. After conditioning,
As a charge / discharge cycle test, 500 at a temperature of 60 ° C.
The cycle was charged and discharged. The charging / discharging conditions are as follows: charging is performed with a constant current having a current density of ² mA / cm ^{2 to} an upper charging voltage of 4.1 V, and discharging is performed with a constant current having a current density of ² mA / cm ^{2 to} a discharging lower limit voltage of 3.0 V. It is one cycle. Then, the discharge capacity of each of the two lithium secondary batteries was measured for each cycle, and the discharge capacity per unit weight of the positive electrode active material was obtained. Further, the capacity retention rate was calculated from the formula [capacity after cycle / initial capacity × 100] with the discharge capacity at the first cycle as the initial capacity and the discharge capacity at the 500th cycle as the capacity after the cycle. In Figure 1,
The change of the discharge capacity of each lithium secondary battery with respect to a charge / discharge cycle is shown.

As is apparent from FIG. 1, the lithium secondary battery using the electrode to which the solid substance was added had a small initial capacity, and the discharge capacity greatly decreased with repeated charging and discharging. The capacity retention rate was a low value of 55.7%. On the other hand, the lithium secondary battery using the electrode to which the solid matter was not added had a large initial capacity, the decrease in discharge capacity was small even after repeated charging and discharging, and the capacity retention rate was 77.8%.
From this result, it was confirmed that the solid matter generated as a result of heating the NMP becomes a factor that deteriorates the cycle characteristics of the lithium secondary battery.

<Evaluation of Storage Properties> (1) Various positive electrode mixture slurries having different positive electrode active materials and solvents were prepared, and various positive electrodes were formed from these positive electrode mixture slurries. Then, a lithium secondary battery was produced using each positive electrode. The lithium secondary battery is the same as the lithium secondary battery prepared by the above charge / discharge cycle test, except that the positive electrode active material and the solvent in the positive electrode mixture slurry and the solvent in the negative electrode mixture slurry are changed, respectively. It was made. Hereinafter, the manufactured lithium secondary battery will be described focusing on the positive electrode active material and the solvent in the positive electrode mixture slurry.

(A) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 11
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and DMA as a solvent. The solvent in the negative electrode mixture slurry is DM
It was set to A. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 11 secondary battery.

(B) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 12
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and DMA as a solvent. The solvent in the negative electrode mixture slurry is NM
It was set to P. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 12 secondary battery.

(C) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 13
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and NMP as a solvent. The solvent in the negative electrode mixture slurry is DM
It was set to A. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 13 secondary battery.

(D) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 14
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and DMF as a solvent. The solvent in the negative electrode mixture slurry is DM
It was set to F. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The manufactured lithium secondary battery was used as a # 14 secondary battery.

(E) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 15
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and DMF as a solvent. The solvent in the negative electrode mixture slurry is NM
It was set to P. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 16 secondary battery.

(F) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 16
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and NMP as a solvent. The solvent in the negative electrode mixture slurry is DM
It was set to F. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 16 secondary battery.

(G) LiNi _0.75 Co _0.15 Al _0.05 Mg as the positive electrode active material for the secondary battery of # 17
_A positive electrode mixture slurry was prepared using _0.05 O ₂ and NMP as a solvent. The solvent in the negative electrode mixture slurry is NM
It was set to P. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery.
The produced lithium secondary battery was used as # 17 secondary battery.

(H) LiNi _0.8 Co _0.2 O ₂ was used as the positive electrode active material for the secondary battery of # 21, and DMA was used as the solvent to prepare a positive electrode mixture slurry. The solvent in the negative electrode mixture slurry was DMA. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery. The produced lithium secondary battery was used as a # 21 secondary battery.

(G) A positive electrode mixture slurry was prepared by using LiNi _0.8 Co _0.2 O ₂ as the positive electrode active material for the # 22 secondary battery and NMP as the solvent. The solvent in the negative electrode mixture slurry was NMP. A positive electrode and a negative electrode were formed from these positive and negative electrode mixture slurry to prepare a lithium secondary battery. The manufactured lithium secondary battery was used as a # 22 secondary battery.

(2) # 11 to # 17, # 21, # 2
The storage characteristics of each of the lithium secondary batteries of No. 2 were evaluated. First, as conditioning, temperature 20 ℃
4.1 at a constant current with a current density of 0.2 mA / cm ² under
After charging to V, the battery was discharged to 3.0 V with a constant current having a current density of 0.2 mA / cm ² . After conditioning, in order to measure the initial capacity, at a temperature of 20 ° C, 1
The cycle was charged and discharged. The charging / discharging conditions are as follows: a constant current with a current density of 0.1 mA / cm ² and a charging upper limit voltage of 4.1 V.
Charging is further continued for 2 hours at a constant voltage of 4.1V, and then discharged to a discharge lower limit voltage of 3.0V with a constant current having a current density of 0.1 mA / cm ² . The discharge capacity of this charging / discharging was made into the initial capacity in 20 degreeC.

Next, in order to calculate the initial internal resistance, the input / output power was measured and the internal resistance at the time of input / output was calculated. The input / output power was measured under the following conditions. First, in a state where each lithium secondary battery was charged to 50% of the initial capacity (SOC 50%), the lithium secondary battery was discharged at a current of 1 A for 10 seconds, and the voltage at the 10th second was measured. After recharging to the state of SOC 50%, discharge it with a current of 3A for 10 seconds, and
The voltage at 0 second was measured. Further, after being charged to a state of SOC 50%, it was discharged at a current of 5 A for 10 seconds, and the voltage at the 10th second was measured. Then, the current dependency of the voltage was obtained, and the slope of the current-voltage line was used as the internal resistance at the time of output.
In addition, charging was performed in the same procedure, the voltage at each 10th second was measured, and the internal resistance at the time of input was determined from the slope of the current-voltage line. The average value of the obtained internal resistances at the time of input / output was used as the initial internal resistance.

Next, a storage test was conducted. In the storage test, charge at a constant current of 0.2 mA / cm ² at a temperature of 20 ° C. until the voltage reaches 4.1 V, and then continue charging at a constant voltage of 4.1 V for 2 hours. Thus, each of the secondary batteries was brought to a state of SOC 100%, and then stored in a constant temperature bath at 60 ° C. for 3 months. Then, after the storage, the remaining capacity and the recovery capacity were measured, the internal resistance at the time of input / output was determined in the same manner as above, and the average value was used as the internal resistance after storage.

Here, the remaining capacity was the capacity when each battery after the storage test was discharged at a temperature of 20 ° C.
The recovery capacity was the discharge capacity at the third cycle of charge and discharge of each secondary battery after measuring the remaining capacity at a temperature of 20 ° C. for three cycles. The charging / discharging conditions were as follows: charging was performed with a constant current having a current density of 0.1 mA / cm ^{2 up to} a charging upper limit voltage of 4.1 V, charging was further continued at a constant voltage of 4.1 V for 2 hours, and then the current density was 0. 1mA /
The charging / discharging for discharging to a discharge lower limit voltage of 3.0 V with a constant current of cm ² is one cycle.

Then, the formula [remaining capacity / initial capacity × 10
0] to determine the capacity remaining rate, and the formula [recovered capacity / initial capacity × 100] to determine the capacity recovery rate. Furthermore, the internal resistance increase rate was calculated from the value of the internal resistance before and after the storage test using the formula [{(internal resistance after storage / initial internal resistance) -1} × 100]. # 11- # 17, # 21, # 22
Table 2 shows the values of the initial capacity, the capacity remaining rate, the capacity recovery rate, the initial internal resistance, and the internal resistance increase rate for each secondary battery.

[0055]

[Table 2]

In Table 2, # 11, # 12, # 14,
The # 15 secondary battery corresponds to the lithium secondary battery of the present invention.
It First, LiNi was used as the positive electrode active material._0.75Co_0.15Al_0.05
Mg _0.05O₂Focus on # 11- # 17 secondary batteries using
Then, the conventional NMP is used as the solvent for the positive electrode mixture slurry.
In each of the # 13, # 16, and # 17 secondary batteries,
The partial resistance was large, and the rate of increase in internal resistance was particularly large.
In other words, storage at high temperature reduces the internal resistance of the battery.
You can see that it has risen significantly. On the other hand, the positive electrode mixture material
# 11 using DMA or DMF as solvent for Larry,
In each of the # 12, # 14, and # 15 secondary batteries, the negative electrode mixture material
Initial internal resistance is small, regardless of the type of rally solvent,
The internal resistance increase rate is also small. Also, # 13, # 16, # 1
Compared with each secondary battery of No. 7, the initial capacity is large and the remaining capacity
The survival rate and capacity recovery rate were also slightly higher. This is DM
A and DMF are solids even if heated when forming the positive electrode.
Since no substance is generated, impurities in the positive electrode are small and
It is considered that the increase in the partial resistance and the decrease in the capacitance were suppressed.
It

Next, LiNi _0.8 Co _0.2 O was added to the positive electrode active material.
Focusing on the # 21 and # 22 secondary batteries using ₂ , the increase rate of the internal resistance was extremely high in any of the secondary batteries. It should be noted that almost no difference in solvent was observed in the positive electrode mixture slurry. It is considered that this was because the positive electrode active material itself deteriorated due to storage at high temperature, so that the influence of the solvent did not clearly appear.

Further, the initial capacity, the capacity remaining rate, and the capacity recovery rate were slightly different depending on the positive electrode active material in each secondary battery. That is, LiNi _0.75 Co
_0.15 Al _0.05 Mg _0.05 O ₂ in the secondary battery of # 11 to # 17 with # 21 using LiNi _0.8 Co _{0. 2} O _2,
The initial capacity was slightly smaller than that of the # 22 secondary battery. This is LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05
When O ₂ and LiNi _0.8 Co _0.2 O ₂ are compared, it is considered that the former has a higher substitution rate of the Ni site by another element, and thus the capacity is reduced accordingly. On the other hand, the remaining capacity and the capacity recovery rate of the secondary batteries # 11 to # 17 are higher than those of the secondary batteries # 21 and # 22. It is considered that this is because the durability of the positive electrode active material at high temperature affected.

From the above, LiNi _0.75 Co _0.15 Al _0.05
Using Mg _0.05 O ₂ as the positive electrode active material, DMA or D
The lithium secondary battery of the present invention including a positive electrode formed from the positive electrode mixture slurry of the present invention using MF as a solvent,
It was confirmed that even if the secondary battery was stored in a state where the charge rate was high, an increase in internal resistance and a decrease in capacity were suppressed, and the secondary battery had good storage characteristics, particularly storage characteristics at high temperatures.

[0060]

The positive electrode mixture slurry for lithium secondary batteries of the present invention has a composition formula of LiNi _1-xyz Co _x Al _y Mg _z O ₂
And a solvent containing at least one of dimethylacetamide and N, N-dimethylformamide. Lithium nickel composite oxide, which has stable crystal structure and good thermal stability, is used as the positive electrode active material, and by using a solvent that does not form a solid even when heated, the storage characteristics at high temperatures are improved. It is a positive electrode mixture slurry that can realize a lithium secondary battery with excellent cycle characteristics. Further, the positive electrode for a lithium secondary battery of the present invention is formed from the positive electrode mixture slurry of the present invention. By using the positive electrode of the present invention, a lithium secondary battery having good storage characteristics and cycle characteristics at high temperatures can be constructed. Furthermore, the lithium secondary battery of the present invention comprises the positive electrode for a lithium secondary battery of the present invention. By using the above positive electrode, a lithium secondary battery having a large discharge capacity and excellent storage characteristics and cycle characteristics can be obtained.

[Brief description of drawings]

FIG. 1 shows changes in discharge capacity of each lithium secondary battery with respect to charge / discharge cycles.

Continued front page    (72) Inventor Kazuo Okamoto             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Yoshio Ukyo             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. F-term (reference) 5H029 AJ04 AJ05 AK03 AL07 AM03                       AM05 AM07 BJ02 BJ14 DJ09                       DJ16 DJ17 EJ11 HJ02                 5H050 AA07 AA10 BA17 CA08 CB08                       DA13 EA21 FA05 FA17 FA19                       HA02

Claims (7)

[Claims] 1. A positive electrode mixture slurry for a lithium secondary battery, comprising a positive electrode active material, a binder and a solvent, wherein the positive electrode active material has a composition formula of LiNi _1-xyz Co _x Al _y.
Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦
0.2) Lithium nickel composite oxide represented by 0.2, wherein the solvent contains at least one of dimethylacetamide and N, N-dimethylformamide. 2. The content ratio of dimethylacetamide and N, N-dimethylformamide in the solvent is 80.
The positive electrode mixture slurry for a lithium secondary battery according to claim 1, which is at least wt%. 3. The positive electrode mixture slurry for a lithium secondary battery according to claim 1, wherein the binder is polyvinylidene fluoride. 4. A positive electrode for a lithium secondary battery, which is formed from a positive electrode mixture slurry containing a positive electrode active material, a binder and a solvent, wherein the positive electrode active material has a composition formula of LiNi _1-xyz Co _x Al. _y
Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦
0.2) The lithium nickel composite oxide represented by 0.2), wherein the solvent contains at least one of dimethylacetamide and N, N-dimethylformamide. 5. A lithium secondary battery comprising a positive electrode formed from a positive electrode mixture slurry containing a positive electrode active material, a binder and a solvent, a negative electrode, and an electrolytic solution in which an electrolyte material is dissolved in an organic solvent. The composition of the positive electrode active material is LiNi _1-xyz Co _x Al _y
Mg _z O ₂ (0.1 ≦ x ≦ 0.3, 0.02 ≦ y + z ≦
0.2) represented by the lithium nickel composite oxide, wherein the solvent contains at least one of dimethylacetamide and N, N-dimethylformamide. 6. The lithium secondary battery according to claim 5, wherein the negative electrode contains artificial graphite as a negative electrode active material. 7. The lithium secondary battery according to claim 5, wherein the electrolyte material is LiPF ₆ .