JP2001118567A - Positive electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium secondary battery having superior cycle characteristic configured, such that the coating ratio of a surface of a positive electrode active material of a binder and a conductive agent is not under a certain ratio by using stable positive active material. SOLUTION: An electrode for a lithium secondary battery formed by binding a positive electrode active material 1, including a lithium transition metal compound oxide and a conductive agent 2 as a binder 3, is configured so that before charging and discharging, at least 80% of a surface of the positive electrode active material 1 is coated on one side of the conductive agent 2 and the binder 3 and after exceeding 100 cycles of charging and discharging of maximum current amount, capable of charging and discharging reversibly at 60 deg.C, at least 50% of a surface of the positive electrode active material 1 is coated in one side of the conductive agent 2 and the binder 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for storing and storing lithium.
The present invention relates to a positive electrode constituting a lithium secondary battery utilizing a desorption phenomenon.

[0002]

2. Description of the Related Art With the downsizing of personal computers, video cameras, mobile phones, and the like, high-performance batteries are required, and because of their high energy density, in the fields of information-related equipment and communication equipment, Lithium secondary batteries have already been put to practical use and have come into widespread use. Rechargeable batteries are generally
Good cycle characteristics are required in which the capacity is not significantly reduced even by repeated charging and discharging, and particularly high cost lithium secondary batteries require higher cycle characteristics.

In general, a positive electrode constituting a lithium secondary battery uses a lithium-transition metal composite oxide as a positive electrode active material, and mixes the positive electrode active material with a conductive material for ensuring electron conductivity in the positive electrode. Further, a paste-like positive electrode mixture obtained by mixing a binder is applied to the surface of the positive electrode current collector in a layered form, and then dried to produce a paste. That is, the positive electrode includes the electrode mixture layer, and the positive electrode mixture layer is formed by binding the positive electrode active material and the conductive material with the binder.

[0004] Conventionally, it is assumed that the positive electrode mixture layer constituting the positive electrode has continuous fine voids, and that the electrolyte solution penetrates into these voids, so that the Li ions in the positive electrode mixture layer are removed. Attempts have been made to secure movement. For example, in the technique disclosed in Japanese Patent Application Laid-Open No. H11-31534, it has been studied to improve the cycle characteristics of a lithium secondary battery by maintaining the porosity of the positive electrode mixture layer at a certain value or more. Have been.

[0005]

However, the present inventor conducted an experiment and found that in the most common positive electrode using polyvinylidene fluoride (PVdF) or the like as a positive electrode binder, the PVdF or the like was changed by an electrolytic solution. It was clarified that swelling ensures Li ion conductivity, and it was possible to obtain the knowledge that the conventionally required voids in the positive electrode mixture layer were hardly necessary.

On the other hand, a lithium transition metal composite oxide serving as a positive electrode active material has a structure in which primary particles close to a single crystal aggregate to form secondary particles. The lithium transition metal composite oxide itself expands and contracts due to insertion and extraction of lithium into and from the lithium transition metal composite oxide during charge and discharge. As the charge and discharge are repeated, the secondary particles collapse and become finer due to the volume change. As the secondary particles are refined, the positive electrode mixture layer increases the voids. Such an increase in the voids increases the proportion of the positive electrode active material that does not come into contact with the binder or the conductive material, thereby deteriorating the electron conductivity of the positive electrode and increasing the internal resistance. We have also obtained knowledge that this may cause deterioration of the cycle characteristics of the secondary battery.

The present invention has been made on the basis of the above findings, and has a structure in which a positive electrode active material which is structurally stable so that the ratio of the binder and the conductive material covering the surface of the positive electrode active material does not fall below a certain ratio. It is an object of the present invention to provide a positive electrode for a lithium secondary battery that can form a lithium secondary battery having good cycle characteristics by using a substance.

[0008]

A positive electrode for a lithium secondary battery according to the present invention is a lithium secondary battery formed by binding a positive electrode active material containing a lithium transition metal composite oxide and a conductive material with a binder. A positive electrode for use, wherein at least 80% of the surface of the positive electrode active material is covered with at least one of the conductive material and the binder before charging and discharging, and can be reversibly charged and discharged at 60 ° C. At least 50% of the surface of the positive electrode active material is covered with at least one of the conductive material and the binder after 100 cycles of charging and discharging of a maximum amount of electricity.

FIG. 1A schematically shows a cross section of a positive electrode formed by binding a positive electrode active material and a conductive material with a binder.
The secondary particles 1 of the lithium transition metal composite oxide as the positive electrode active material are made of, for example, polyvinylidene fluoride (PV) together with the conductive material 2 made of particles of a carbon material, for example.
It is bound by a binder 3 made of dF). When a lithium secondary battery is formed using this positive electrode and charge and discharge are repeated, secondary particles 1 of lithium transition metal composite oxide can be obtained.
Is disintegrated and miniaturized by repeating volume expansion and contraction, and becomes as shown in FIG. The secondary particles 1 of the lithium transition metal composite oxide shown in FIG. 1B have a surface 1 a that is not in contact with the conductive material 2 and the binder 3 due to the miniaturization. Due to the presence of such a surface, the electron conductivity between the positive electrode active material particles is poor, and the internal resistance of the positive electrode in such a state is large.

In the case of a lithium transition metal composite oxide having a portion having a weak particle structure, it is easy to miniaturize, and the rate of increase in the portion of the surface not covered by any of the conductive material and the binder becomes extremely large. In contrast, when using a lithium transition metal composite oxide having a stable particle structure,
Since it is difficult to miniaturize, the rate of increase in the portion not covered by any of the conductive material and the binder is small, and a positive electrode that maintains good electron conductivity can be configured.

That is, in the positive electrode for a lithium secondary battery of the present invention, at least 80% of the surface is covered with at least one of the conductive material and the binder at the beginning of the production, and the positive electrode is formed even after repeated charging and discharging. At least 50 on the surface of the active material
% Is covered with at least one of the conductive material and the binder, and the use of a lithium transition metal composite oxide having a strong particle structure as the positive electrode active material suppresses an increase in the internal resistance of the positive electrode, It becomes a positive electrode that can form a lithium secondary battery having good cycle characteristics.

The percentage of the surface of the positive electrode active material that is covered by at least one of the conductive material and the binder (coverage)
Is that the positive electrode is cut with a diamond cutter or the like, and the cross section thereof is observed with a scanning electron microscope (SEM). In the cross section, at least one of the conductive material and the binder is in contact with the total contour length of the positive electrode active material particles. Defined as a percentage of the contour length. In the actual measurement, the coverage of five portions is randomly measured in one cross section, and the average value is defined as the coverage of the positive electrode.

[0013] The charge / discharge of the maximum amount of electricity that can be charged / discharged reversibly means that a positive electrode and a negative electrode having a larger capacity than the positive electrode are opposed to each other to constitute a lithium secondary battery. This means charge / discharge that reciprocates between an empty discharge state and a fully charged state in a reversibly chargeable / dischargeable range. In general, the reversible chargeable / dischargeable range changes depending on the types of the positive electrode active material and the negative electrode active material, and the range can be managed by the battery voltage of the secondary battery. Therefore, in this specification, charging / discharging reciprocating between a predetermined end-of-charge voltage and a predetermined end-of-discharge voltage is defined as the maximum amount of charge / discharge that can be reversibly charged / discharged. Incidentally, when LiCoO ₂ , LiNiO _2, or the like is used for the positive electrode active material and a carbon material is used for the negative electrode active material, the battery voltage is about 4.1 to 4.1.
The range is 3.0 V.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment of a positive electrode for a lithium secondary battery according to the present invention will be described with respect to a positive electrode active material, a conductive material, a binder and a production of the positive electrode constituting the positive electrode. Next, a lithium secondary battery using the present positive electrode will be described.

<Positive Electrode Active Material> In the positive electrode of the present invention, the positive electrode active material contains a lithium transition metal composite oxide. Only the lithium transition metal composite oxide can be used as the positive electrode active material, or a mixture of another known positive electrode active material and a lithium transition metal composite oxide can be used as the positive electrode active material.

The lithium transition metal composite oxide is not particularly limited as long as it can be used as a positive electrode active material, but it has a layered rock-salt structure lithium-cobalt composite oxide whose basic composition is LiCoO _2. To L
layered rock salt structure lithium nickel composite oxide according to iNiO _2, layered rock salt type lithium-manganese composite oxide of the basic composition and LiMnO _2, spinel-type lithium-manganese composite oxide or the like to the basic composition as LiMn ₂ O ₄ is, 4V
A positive electrode active material that can form a lithium secondary battery having a high energy density because it can form a lithium secondary battery of a high grade. In addition to those having these basic compositions, those in which the transition metal site is replaced with another element in order to improve the characteristics as a positive electrode active material, Li
What substituted the site | part with elements, such as an alkali metal, can also be used. These lithium transition metal composite oxides may be used alone as a positive electrode active material, or may be used as a mixture of two or more.

Of these, the lithium-cobalt composite oxide is easy to synthesize, is the most stable, has good cycle characteristics, and is a positive electrode active material which is the mainstream of current lithium secondary batteries. Therefore, when giving priority to cycle characteristics, it is preferable to use a lithium-cobalt composite oxide. However, Co as a constituent element is very expensive, and the cost of a lithium battery is high. On the other hand, the lithium manganese composite oxide has the constituent element M
Since n is inexpensive, the cost as a positive electrode active material is reduced. Therefore, when giving priority to the cost of the lithium secondary battery, it is desirable to use a lithium manganese composite oxide as the positive electrode active material.

[0018] The lithium nickel composite oxide has the advantage of a large capacity and is not as expensive as the lithium cobalt composite oxide in terms of cost, and is expected as a positive electrode active material to replace the lithium cobalt composite oxide.
However, the cycle change is slightly inferior because the volume change accompanying the occlusion / desorption of lithium is relatively large. However, in the present invention, as described later, the lithium transition metal composite oxide can be subjected to a reforming treatment for inserting and extracting lithium by chemical means. Since the particle structure can be strengthened by the reforming treatment, when a lithium nickel composite oxide is used, a well-balanced lithium secondary battery having a large battery capacity and excellent cycle characteristics is obtained.

When a lithium nickel composite oxide is used, a stoichiometric composition represented by the composition formula LiNiO ₂ can be used. Further, in order to improve the cycle characteristics and the like of the secondary battery, a battery in which part of the Ni site is replaced with another element can be used. Among those substituted with other elements, the composition formula LiNi _x M1 _y M2 _z O ₂ (M1 is C
at least one selected from o and Mn; M2 is Al;
At least one selected from B, Fe, Cr, and Mg; x
+ Y + z = 1; 0.5 <x <0.95; 0.01 <y <
0.4; 0.001 <z <0.2).

The LiNi _x M1 _y M2 _z O ₂ is obtained by partially substituting Ni sites with two or more kinds of elements M1 and M2 having different roles. If the substitution ratio is defined by the ratio of leaving Ni without substitution, that is, the value of x in the composition formula, 0.5 <x <0.95. If x ≦ 0.5, not only a layered rock salt structure but also a second phase such as a spinel structure is generated. If x ≧ 0.95, the substitution effect is too small. This is because a battery having the desired good cycle characteristics cannot be formed. Note that 0.
More preferably, the range is 7 <x <0.9.

The element M1 selected from Co and Mn mainly serves to stabilize the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the lithium secondary battery are more favorably maintained, and particularly, deterioration of the battery capacity due to charge / discharge at high temperature and storage at high temperature is suppressed. In order to sufficiently exert the effect of improving the cycle characteristics, the substitution ratio of M1, that is, the value of y in the composition formula, is set to 0.01 <y <0.4. y
When ≦ 0.01, the crystal structure of the formed secondary battery is not sufficiently stabilized, so that the cycle characteristics are not good.
If ≧ 0.4, the crystallinity of the lithium-nickel composite oxide is undesirably reduced. It is more preferable that 0.1 <y <0.3. Further, the replacing element M1 is Co
Is more desirable. In Co, while suppressing the capacity reduction due to the element substitution, the obtained composite oxide Li
This is because (Co, Ni) O ₂ is an all-solid solution type and has an advantage of minimizing a decrease in crystallinity.

The element M2 selected from Al, B, Fe, Cr and Mg mainly serves to suppress the decomposition reaction of the active material due to the release of oxygen and to improve the thermal stability. Due to this role, the substitution ratio of M2, that is, the value of z in the composition formula, is set to 0.001 <z <0.2. z ≦
In the case of 0.001, a sufficient effect on safety cannot be obtained, and in the case of z ≧ 0.2, the capacity of the positive electrode decreases, which is not preferable. In addition, 0.01 <z <0.
It is more preferably set to 1. Further, the substituting element M2
It is more preferable to use Al. This is because Al has an advantage of minimizing a decrease in capacity while improving thermal stability.

The method for producing the lithium transition metal composite oxide itself is not particularly limited. For example, the composition formula Li
In order to produce a layered rock salt structure lithium nickel composite oxide represented by Ni _x Co _y Al _z O ₂ , LiOH
A so-called solid-phase method in which H ₂ O, Ni (OH) ₂ , Co ₃ O ₄ , and Al (OH) ₃ are each mixed in a predetermined amount and fired in an oxygen stream at a temperature of about 850 ° C. for a time of about 20 hours. Therefore, this can be synthesized. According to the liquid phase method, a predetermined amount of nitrate of each metal element is dissolved in ion-exchanged water, and this is sprayed and dried, and the precursor is dried under a predetermined temperature and atmosphere (for example, at 850 ° C. in an oxygen stream). Can be synthesized by firing.

The lithium transition metal composite used as the positive electrode active material in the positive electrode for a lithium secondary battery of the present invention is in the form of a powder, and the particles of the powder are formed by agglomeration of primary particles close to a single crystal and secondary particles. Has a particle structure. Then, as described above, at least 80% of the surface of the secondary particles is covered with at least one of the conductive material and the binder at the beginning of the production of the positive electrode, and after the predetermined charge / discharge is repeated. And at least 50% of the surface is covered with at least one of a conductive material and a binder.
That is, it has a particle structure such that the secondary particles do not easily collapse and become finer with charge and discharge.

Such a lithium transition metal composite oxide having a strong particle structure is not particularly limited in its production method, but can be produced by performing a modification treatment as described below.

This reforming treatment is a treatment of inserting and extracting lithium by chemical means. In the lithium transition metal composite oxide, as described above, the secondary particles are collapsed by the volume change and become fine. Then, the collapse occurs from a weak portion of the secondary particle. Therefore,
In this reforming process, before forming the positive electrode, by repeating the process of inserting and extracting lithium in advance, the collapse generated from the weak part is completed to some extent, and the lithium transition metal having a relatively stable particle structure This is a process performed for the purpose of obtaining a composite oxide. Note that the chemical means is a means excluding the electrochemical means, and is a means for inserting and extracting lithium by a process different from actual charging and discharging.

More specifically, a step of immersing the lithium transition metal composite oxide in an aqueous solution of an acid such as sulfuric acid, nitric acid or hydrochloric acid to desorb lithium from the lithium transition metal composite oxide, and after the step, Immersing and refluxing a solution of a lithium compound such as lithium iodide in an organic solvent such as acetonitrile to absorb lithium in the lithium transition metal composite oxide, and repeating these steps. This is the process to be performed.

The acid used in the step of removing lithium is as follows:
Because it does not adversely affect the subsequent battery characteristics,
Sulfuric acid is desirable, and the concentration of the aqueous solution is desirably about 0.5 to 3M. Further, the desorption step is preferably performed for about 30 to 120 minutes while sufficiently stirring the aqueous solution. After the lithium is desorbed, it is desirable to perform suction filtration and sufficiently remove moisture by vacuum drying.

As the lithium compound used in the step of inserting lithium, lithium iodide is desirable because chemical Li can be efficiently inserted, and the solvent has an appropriate boiling point and is less dangerous. , Acetonitrile is preferred. The concentration of the lithium compound is
It is desirable to be about 1 to 6M. The reflux with the lithium compound solution is preferably performed for about 2 to 10 hours, and after occlusion of lithium, it is preferable to perform suction filtration with a sufficient amount of a solvent and perform vacuum drying at room temperature for about 2 to 24 hours. Note that it is desirable that the insertion and extraction of lithium be repeated about 2 to 10 times.

Such a reforming treatment of the lithium transition metal composite oxide in which lithium is absorbed and desorbed by a chemical means is different from an electrochemical treatment by repeating charge and discharge, and is different from an electrochemical treatment before forming a positive electrode. This is a process that can be easily performed on the lithium transition metal composite oxide. According to this process, the collapse of the secondary particles generated from the weakened part is caused to occur before forming the positive electrode as an active material. It is possible to produce a lithium transition metal composite oxide in which the collapse of the secondary particles is suppressed even by charging and discharging after the composition.

When used as a positive electrode active material, it is desirable to use a lithium transition metal composite oxide having a powder particle diameter, that is, a secondary particle diameter of about 5 to 30 μm in average particle diameter. When the thickness is less than 5 μm, a large amount of conductive material is required, and disadvantages such as an excessive increase in the viscosity of the positive electrode mixture paste occur. When the thickness exceeds 30 μm, it becomes difficult to produce an electrode having an appropriate electrode mixture layer thickness. is there.

<Manufacture of Conductive Material, Binder, and Positive Electrode> The conductive material constituting the positive electrode is for ensuring the electronic conductivity of the positive electrode, and is a powder of carbon material such as carbon black, acetylene black, and graphite. One or a mixture of two or more of the bodies can be used.

The binder of the positive electrode plays a role of binding the active material particles and the conductive material particles containing the lithium transition metal composite oxide. In the case of the positive electrode of the present invention, the nonaqueous electrolyte constituting the battery is used. It must be one that swells with the liquid. As a material that swells with the nonaqueous electrolyte, a fluorine-containing resin such as polyvinylidene fluoride (PVdF), an SBR latex, or a fluorine rubber can be used. Among them, it is desirable to use PVdF from the viewpoint of adhesiveness, solubility in a solvent, and nonflammability. Note that as a dispersion medium (solvent) for dissolving the binder and dispersing the active material particles and the conductive material particles, an organic solvent such as N-methyl-2-pyrrolidone can be used.

The production of the positive electrode is not particularly limited, and may be performed by a known method. As an example, first, the positive electrode active material and the conductive material are mixed, dispersed in the binder, and the solvent is added as necessary to prepare a paste-like positive electrode mixture. Preparation of the positive electrode mixture, so that the binder sufficiently covers the active material particle surface,
For example, it is desirable to sufficiently knead using a ball mill or the like. The mixing ratio of the positive electrode active material, the conductive material and the binder is such that the conductive material is 5 parts by weight when the positive electrode active material is 100 parts by weight.
It is desirable that the content be in the range of １５15 parts by weight and the binder in the range of 4〜１０10 parts by weight.

Next, this positive electrode mixture is applied to the surface of a positive electrode current collector such as an aluminum foil by a coater or the like, and dried. Thereafter, if necessary, a compression process for increasing the density of the positive electrode mixture may be performed by a roll press or the like. The positive electrode thus manufactured is a sheet-like electrode in which a positive electrode mixture is formed in a layer on the surface of a positive electrode current collector. The sheet-shaped positive electrode is cut into appropriate dimensions according to the size, shape, etc. of the lithium secondary battery to be created, and a current-collecting lead, etc., is used for current collection from the positive electrode to the outside. It is completed through processes such as attaching it to the positive electrode.

<Lithium Secondary Battery> A lithium secondary battery using the above-described positive electrode of the present invention mainly comprises, in addition to the positive electrode, an opposing negative electrode, a separator sandwiched between the positive and negative electrodes, a non-aqueous electrolyte, and the like. It is configured as a component. One embodiment will be briefly described.

The negative electrode can be constituted by using metal lithium, lithium alloy or the like as a negative electrode active material. However, when such metal lithium or the like is used for the negative electrode, there is a possibility that dendrite is deposited on the surface of the negative electrode due to repeated charging and discharging, and there is a concern about the safety of the secondary battery. Therefore,
In consideration of the safety of the lithium secondary battery, it is desirable to use a carbon material capable of inserting and extracting lithium as the negative electrode active material.

The carbon materials that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon,
Further, there may be mentioned a fired body of an organic compound such as a phenol resin, and a powdered body of easily graphitizable carbon such as coke. The carbon material used as the negative electrode active material has respective advantages, and may be selected according to the characteristics of the lithium secondary battery to be manufactured. One type of carbon material can be used alone, or two or more types can be used in combination.

When a carbon material is used as the negative electrode active material, the negative electrode is obtained by mixing a binder into a powder of the carbon material and adding an appropriate solvent as needed to obtain a paste-like negative electrode mixture. Like the positive electrode, the negative electrode mixture is applied to the surface of a current collector made of metal foil such as copper, dried, and then, if necessary, formed by increasing the density of the negative electrode mixture by pressing or the like. As a binder,
Like the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride or the like can be used, and as a solvent, an organic solvent such as N-methyl-2-pyrrolidone can be used.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene or the like can be used. The non-aqueous electrolyte is a solution in which a lithium salt as an electrolyte is dissolved in an organic solvent.As the organic solvent, an aprotic organic solvent, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran, dioxolan,
One kind of methylene chloride or a mixture of two or more kinds thereof can be used. The electrolyte to be dissolved is LiI, LiClO ₄ , LiAsF ₆ , LiBF ₄ ,
Lithium salts such as LiPF ₆ and LiN (CF ₃ SO ₂ ) ₂ can be used.

The above-described lithium secondary battery is a main component of the lithium secondary battery. The lithium secondary battery can have various shapes such as a cylindrical type, a stacked type, a coin type, and a card type. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and current is collected from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that lead to the outside. The electrodes are connected using a lead or the like, and the electrode body is sealed in a battery case together with the non-aqueous electrolyte to complete a lithium battery.

[0042]

EXAMPLE A positive electrode of the present invention using a modified lithium transition metal composite oxide based on the above embodiment and a positive electrode using a non-modified lithium transition metal composite oxide were produced. Then, lithium secondary batteries using the respective positive electrodes were constructed, and the superiority of the positive electrode for a lithium secondary battery of the present invention was confirmed by comparing the cycle characteristics of the lithium secondary batteries.

Example A layered rock-salt structure lithium nickel composite oxide represented by a composition formula LiNi _0.8 Co _0.15 Al _0.05 O ₂ synthesized by a liquid phase method was subjected to a modification treatment.
The reforming treatment was performed as follows. 100 g of this lithium nickel composite oxide was immersed in 10 L (liter) of 1N sulfuric acid, stirred for 30 minutes, and then suction-filtered to remove water by vacuum drying. Next, 148 g of lithium iodide (LiI) was added to the lithium nickel composite oxide.
Was dissolved in 1 L of acetonitrile in which was dissolved, refluxed for 2 hours, and suction-filtered together with a large amount of acetonitrile, followed by vacuum drying at room temperature for 20 hours. Thus, the reforming process was completed by repeating the cycle of chemically inserting and extracting Li ten times.

The modified lithium nickel composite oxide was used as a positive electrode active material, and acetylene black was used as a conductive material with respect to 100 parts by weight of the positive electrode active material.
8 parts by weight, 5.9 polyvinylidene fluoride as a binder
Parts by weight, further added about 70 parts by weight of N-methyl-2-pyrrolidone as a solvent, and kneaded them sufficiently to prepare a paste-like positive electrode mixture. Then, this paste-like positive electrode mixture is applied to both sides of a 20-μm-thick aluminum foil current collector, dried, and then pressed and cut to obtain a width of 54 mm and a length of 450 mm (positive electrode mixture coating length). 4
00 mm) in the form of a sheet. In this positive electrode, the basis weight of the positive electrode active material was 7.0 per unit area on one side.
mg / cm ^2, and pressed so that the density of the positive electrode mixture layer finally becomes 2.5 g / cm ³ .

As the negative electrode, 70 parts by weight of graphitized mesophase microspheres (MCMB25-28: manufactured by Osaka Gas Chemicals) and coke (MBC: manufactured by Mitsubishi Chemical) 30 as negative electrode active materials were used.
It was constituted by using a mixture of the above-mentioned components and parts by weight. First, with respect to 100 parts by weight of the negative electrode active material, 5.3 parts by weight of polyvinylidene fluoride was mixed as a binder, and about 65 parts by weight of N-methyl-2-pyrrolidone was added as a solvent.
These were sufficiently kneaded to prepare a paste-like negative electrode mixture. Next, this paste-like negative electrode mixture was applied to a thickness of 10
It was applied to both sides of a copper foil current collector having a thickness of μm, dried, and then pressed and cut to produce a sheet-shaped negative electrode having a width of 56 mm and a length of 520 mm (a coating length of the positive electrode mixture of 500 mm). The negative electrode was coated so that the basis weight of the negative electrode active material was 5.0 mg / cm ² per unit area on one side, and the density of the negative electrode mixture layer was finally 1.3 g / cm ^3. Pressed to.

The above positive electrode and negative electrode were placed between them with a thickness of 25
A polyethylene separator having a width of 58 μm and a width of 58 mm was sandwiched therebetween and wound into a roll to form an electrode body. In the positive electrode mixture uncoated portion of the positive electrode and the negative electrode mixture uncoated portion of the negative electrode,
Before the winding, each current collecting lead is bonded in advance. In addition, the positive electrode and the negative electrode are wound so that the part where the negative electrode mixture layer of the negative electrode is always opposed to the part where the positive electrode mixture layer is formed on both surfaces of the positive electrode via a separator. .

A polyethylene insulating plate is arranged on both end surfaces of the roll-shaped electrode body, inserted into a Ni-plated iron battery case, and a negative electrode current collecting lead is placed on the bottom of the battery case. The positive electrode current collecting lead was joined to a cap for sealing the battery case.

Next, a non-aqueous electrolyte obtained by dissolving LiPF ₆ at a concentration of 1 M as an electrolyte in a mixed organic solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 3: 7 was poured into the battery case. Pressurization and depressurization were repeated to impregnate the electrode body. Then, the excess nonaqueous electrolyte was discharged, and the cap was caulked to the opening of the battery case, thereby closing the battery case and completing the assembly of the lithium secondary battery. The amount of the nonaqueous electrolyte injected was about 4 g.

After the assembled lithium secondary battery is allowed to stand at room temperature for about one day,
Charging and discharging for conditioning were performed. The conditions for this charge and discharge are as follows: a current of 100 mA, a charge end voltage of 4.1 V;
, And then discharge at a constant current of 100 mA up to a discharge end voltage of 3.0 V. By completing the conditioning, the manufacture of the lithium secondary battery was completed. In addition, a plurality of lithium secondary batteries were manufactured, and these secondary batteries were used as secondary batteries of Examples.

<Comparative Example> A lithium nickel composite oxide synthesized by a liquid phase method in the same manner as in the above-described embodiment, but without being subjected to the above-mentioned reforming treatment, the as-synthesized one was used as a positive electrode active material. A positive electrode was produced using the same. Then, using this positive electrode, a lithium secondary battery was completed. Other configurations except for the positive electrode active material are the same as those of the embodiment. The completed lithium secondary battery was used as a secondary battery of a comparative example. In addition, a plurality of secondary batteries of the comparative example were also manufactured.

<Charge / Discharge Cycle Test> The charge / discharge cycle test was performed on the secondary batteries of the above Examples and Comparative Examples under a high temperature environment of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the lithium secondary battery. Was done. In the charge / discharge cycle test, first, these secondary batteries were placed in a thermostat kept at 60 ° C., left for 2 hours, and then charge / discharge was started. The conditions of the charge / discharge cycle test were as follows: constant current charging was performed with a current of 972 mA up to a charging end voltage of 4.1 V;
A cycle in which a constant current discharge is performed to a discharge end voltage of 3.0 V with a current of 72 mA is defined as one cycle.
It was performed up to 00 cycles.

The discharge capacity per unit weight of the positive electrode active material in the first cycle was measured, and this was taken as the initial discharge capacity.
The discharge capacity per unit weight of the positive electrode active material at the 0th cycle was measured, and this was defined as the discharge capacity after 1000 cycles.
Next, the capacity deterioration rate per cycle was determined from these values. Further, the average charge voltage and average discharge voltage at the 1000th cycle were measured, and the average charge / discharge voltage difference was obtained, which was used as a guideline representing the magnitude of the polarization of the lithium secondary battery, that is, the magnitude of the internal resistance. If the average charge / discharge voltage difference is large, the secondary battery has a large polarization and a large internal resistance. If the average charge / discharge voltage difference is small, the secondary battery has a small polarization and a small internal resistance. Table 1 below shows the initial discharge capacity and the initial discharge capacity of the secondary batteries of the examples and comparative examples.
The discharge capacity after 00 cycles, the capacity deterioration rate per cycle, and the average charge / discharge voltage difference after 1000 cycles are shown.

[0053]

[Table 1]

<Observation of Cross Section of Positive Electrode Mixture Layer> The charge / discharge test was stopped after 100 cycles, the battery was opened, the positive electrode was taken out, and the positive electrode was replaced with diamond. It cut | disconnected with the cutter and the cross section of the positive electrode mixture layer was observed with SEM. Then, at five locations extracted at random, the ratio of the portion where the positive electrode active material particles are covered by at least one of the conductive material and the binder is measured, and these are averaged to obtain 100 cycles of the positive electrode. The surface coverage was determined later. In addition, the cross section observation of the positive electrode mixture layer was also performed on the positive electrode which was not charged and discharged, and the surface coverage obtained by the observation was defined as the initial surface coverage.

FIG. 2A shows a cross-sectional photograph of the initial positive electrode mixture layer in the positive electrode used in the secondary battery of the example, and FIG. 2B shows a cross-sectional photograph of the positive electrode mixture layer after 100 cycles. Also,
FIG. 2C shows a cross-sectional photograph of the initial positive electrode mixture layer of the positive electrode used in the secondary battery of the comparative example, and FIG. 2D shows a cross-sectional photograph of the positive electrode mixture layer after 100 cycles. Also,
Table 2 below shows the initial surface coverage and the surface coverage after 100 cycles of the positive electrodes used in the secondary batteries of Examples and Comparative Examples.

[0056]

[Table 2]

<Evaluation> In FIGS. 2A to 2D, white portions are particles of the positive electrode active material, and black portions around them are the conductive material and the binder. In FIGS. 2A and 2C, at least one of the conductive material and the binder is in contact with most of the contour of the positive electrode active material particles, and has a high surface coverage. You can see that. 2 (b) and FIG.
In (d), the active material particles are disintegrated (cracked) and become finer, and a portion of the outline of the active material particles where neither the conductive material nor the binder is in contact increases. I understand. In FIG. 2D made of the lithium nickel composite oxide that has not been subjected to the modification treatment, the active material particles are further refined, and a portion where neither the conductive material nor the binder is in contact is considerably large. It turns out that it occupies a part.

Further, the surface was covered with at least one of the conductive material and the binder as a surface coverage, and the active material particles were subjected to the modification treatment so that the covering state is clear from Table 2. In the positive electrode of Comparative Example using no lithium nickel composite oxide as the positive electrode active material, the coverage of the active material particle surface was extremely low by 25%. In contrast,
It can be seen that the positive electrode of the example using the modified lithium nickel composite oxide as the positive electrode active material maintains a high coverage of 84%.

Table 1 showing the cycle characteristics of the secondary battery
From the secondary battery of the comparative example, the secondary battery of the example is superior in any of the discharge capacity after 1000 cycles, the capacity deterioration rate per cycle, and the average charge / discharge voltage difference after 1000 cycles. It can be seen that the lithium secondary battery has good cycle characteristics. It is considered that this is because the coverage of the positive electrode active material with the conductive material or the binder can be kept high, so that the electron conductivity in the positive electrode is ensured, and as a result, a decrease in the discharge capacity is suppressed.

When these results are combined, at least 80% of the surface of the positive electrode active material is charged with at least one of the conductive material and the binder before charging / discharging, using a positive electrode active material in which the particle size is suppressed. After 100 cycles of charging and discharging of the maximum amount of electricity that is covered and reversibly chargeable and dischargeable at 60 ° C., at least 5
When a positive electrode in which 0% is covered with at least one of the conductive material and the binder is used, it can be estimated that the cycle characteristics of the lithium secondary battery are practically satisfactory. In addition, it can be confirmed that the above-described reforming treatment for absorbing and desorbing lithium by chemical means is effective in miniaturizing the lithium transition metal composite oxide serving as the positive electrode active material.

[0061]

According to the present invention, a positive electrode for a lithium secondary battery formed by binding a positive electrode active material containing a lithium transition metal composite oxide and a conductive material with a binder is used to reduce the size of positive electrode active material particles. Before charging / discharging, at least 80% of the surface of the positive electrode active material is covered with at least one of the conductive material and the binder, and the maximum amount of electricity that can be charged / discharged reversibly at 60 ° C. After 100 cycles of charge and discharge, at least 50% of the surface of the positive electrode active material is covered with at least one of the conductive material and the binder. A lithium secondary battery using the positive electrode having such a structure is a secondary battery having good cycle characteristics.

[Brief description of the drawings]

FIG. 1 conceptually shows a cross section of a positive electrode formed by binding a positive electrode active material and a conductive material with a binder.

FIG. 2 shows cross-sectional photographs of the positive electrode mixture layers at the initial stage and after 100 cycles in the positive electrodes used in the secondary batteries of Examples and Comparative Examples.

[Explanation of symbols]

 1: Positive electrode active material particles (secondary particles of lithium transition metal composite oxide) 1a: Surface not in contact with conductive material and binder 2: Conductive material 3: Binder

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[Procedure amendment]

[Submission date] December 20, 1999 (1999.12.
20)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Figure 2

[Correction method] Change

[Correction contents]

FIG. 2

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hideyuki Nakano 41-d, Cho-ku, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture F-term in Toyota Central R & D Laboratories Co., Ltd. 5H003 AA04 BA07 BB02 BB04 BB05 BB11 BB14 BC05 BD01 BD03 5H014 AA02 AA04 BB11 EE05 EE10 HH01 HH04 HH08 5H029 AJ05 AK03 AL06 AL07 AL08 AL12 AM03 AM04 AM05 AM07 CJ15 DJ08 HJ02 HJ14 HJ19

Claims (3)

[Claims] 1. A positive electrode for a lithium secondary battery formed by binding a positive electrode active material containing a lithium transition metal composite oxide and a conductive material with a binder, wherein the positive electrode active material is charged and discharged. At least 80% of the surface
Is covered with at least one of the conductive material and the binder, and after 60 cycles of charging and discharging of the maximum amount of electricity that can be reversibly charged and discharged at 60 ° C., the surface of the positive electrode active material is A positive electrode for a lithium secondary battery, wherein at least 50% of the positive electrode is covered with at least one of the conductive material and the binder. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the lithium transition metal composite oxide has been subjected to a reforming treatment for inserting and extracting lithium by chemical means. 3. The lithium transition metal composite oxide according to claim 1, wherein
Formula LiNi_xM1_yM2 _zO_Two(M1 is selected from Co and Mn
M2 is Al, B, Fe, Cr, M
g + x + y + z = 1;
5 <x <0.95; 0.01 <y <0.4; 0.001
<Z <0.2) lithium nickel composite oxide
The lithium secondary battery according to claim 1 or 2, wherein
Pond positive electrode.