JP2001006681A - Negative electrode active material for lithium secondary battery and manufacture of the active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for lithium secondary battery equipped with a negative electrode having a very small internal resistance by using Ag particulates of good electric conductivity as an aid agent for electric conductivity of carbon particles and generating a special dispersion form of Ag particulates relative to the carbon particles. SOLUTION: This negative electrode active material is formed from a number of carbon particles having particles sizes from several to several tens microns and a number of Ag particulates having particle sizes of sub-micron order attached to the surface of the carbon particles, wherein part of the Ag particulates form a cluster and are attached to the carbon particles while the remainder are dispersed and attached to the carbon particles.

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 negative electrode active material used for a lithium secondary battery utilizing a release phenomenon and a method of manufacturing the same, and more particularly to a negative electrode active material capable of forming a lithium secondary battery having a small internal resistance and a method of manufacturing the same.

[0002]

[Prior Art] With the miniaturization of personal computers, mobile phones, etc.,
In the fields of information-related equipment, communication equipment, and the like, lithium secondary batteries utilizing the occlusion and release phenomena of lithium have already been put to practical use and have come into widespread use because of their high energy density. On the other hand, due to environmental issues and resource issues, expectations for electric vehicles have increased, and in the automotive field,
Attempts to use lithium batteries as power supplies for electric vehicles have also begun.

In particular, in the case of a large-capacity secondary battery used for an electric vehicle power supply, an electric power storage system, and the like, high output is expected in addition to high energy density. In order to obtain a secondary battery having a high output density, it is necessary to keep the internal resistance of the battery low. Factors that increase the internal resistance of the battery exist in various places such as the inside of the positive electrode and the negative electrode, the current collector from the electrode to the external terminal, and it is necessary to reduce each of them.

A negative electrode of a lithium secondary battery using a carbon material capable of occluding and releasing lithium as an active material is formed by binding a powdery carbon material with a binder. Even in a lithium secondary battery using such a carbon negative electrode, there is an internal resistance caused by the negative electrode. In the carbon negative electrode, a non-electroconductive film containing lithium carbonate or the like as a main component is formed on the surface thereof during charging and discharging, and the growth of the film makes it difficult to collect current between active material particles. This is the main cause of the increase in battery internal resistance caused by the carbon anode.

Hitherto, for example, the following technology has been proposed as a technology for improving the negative electrode active material for the purpose of reducing the internal resistance caused by the negative electrode of a lithium secondary battery. (1) shown in JP-A-8-273702 Patent Publication, the particles made of carbon material, hydrazine, using a reducing agent such as NaBH _4, techniques of depositing the fine metal particles in a uniform dispersed form, (2) As disclosed in JP-A-8-45548 and the like,
This is a technique using a mixture of particles made of a carbon material and metal particles having a particle diameter of several microns as a negative electrode active material.

[0006]

However, the above (1)
In the technique described above, the fine metal particles dispersed and adhered contribute to the current collection between the active material particles, but do not reach the point where a conductive path is formed inside the negative electrode, and increase the conductivity of the negative electrode itself. It has not been improved. Moreover, hydrazine, NaBH ₄ as a reducing agent, reducing power is very strong, since the toxicity is high, which is intended to leave a safety problem during the active material production.

In the technique (2), since the metal particles to be mixed have a large particle diameter, the number of contact points with the carbon particles is small, and current cannot be efficiently collected from the entire negative electrode. Further, in order to form a sufficient conductive path, it is necessary to mix a considerable amount, and there is a problem in terms of energy density, and further, it is intended to reduce the contact resistance between carbon particles. Did not.

[0008] In view of the above-mentioned problems of the prior art, the present invention provides a conductive agent for carbonaceous particles, such as Ag having good conductivity.
Fine particles are used, and A
It is an object of the present invention to provide a negative electrode active material capable of forming a negative electrode having extremely low internal resistance by making the dispersion mode of the g fine particles special. Another object of the present invention is to provide a method for producing the negative electrode active material of the present invention at low cost, safely and easily.

[0009]

The negative electrode active material for a lithium secondary battery of the present invention comprises a myriad of carbonaceous particles having a particle size of several to several tens of microns, and a submicron particle attached to the surface of the carbonaceous particles. A large number of Ag fine particles having a particle diameter of the order, a part of the Ag fine particles form a cluster and adhere to the carbonaceous particles, and the rest of the Ag fine particles are dispersed to form the carbonaceous particles. Characterized by being attached to

That is, the negative electrode active material of the present invention contains carbonaceous particles serving as a matrix, and Ag fine particles adhering to the surface thereof in two forms: an aggregated form called a cluster and a uniformly dispersed form. It is composed of FIGS. 1 and 2 show scanning electron microscope photographs (SEM photographs) of the negative electrode active material as an example of the negative electrode active material of the present invention. In addition,
The SEM photograph of FIG. 2 is an enlarged view of the center of the SEM photograph of FIG.

In the SEM photograph of FIG. 1, the spherical black particles are carbonaceous particles (artificial spherical graphite in this example), and the dotted white particles are dispersed in a state close to a single particle.
gParticles that look white with a certain area
Ag fine particles in the form of clusters. In the photograph shown in FIG. 2, the state of the form aggregated in a cluster is clearly seen.

In the negative electrode active material for a lithium secondary battery according to the present invention, the Ag fine particles uniformly dispersed in a state close to a single one assist the electric contact between the carbonaceous particles and reduce the electric resistance due to the contact between the carbonaceous particles. The function of reducing the size is shown, and the portion of the cluster-shaped Ag fine particles forms a conductive path inside the negative electrode in combination with the Ag fine particles dispersed in a state almost singly. Due to the presence of the Ag fine particles having such two dispersion forms, the negative electrode using the negative electrode active material of the present invention has a small conductive resistance inside the negative electrode. Therefore, a lithium secondary battery including the negative electrode as a component can have a low internal resistance, a large discharge capacity, and excellent output characteristics.

Further, the method for producing a negative electrode active material of the present invention comprises:
The production method for producing the negative electrode active material of the present invention, a dispersion solution preparation step of preparing a dispersion solution in which the powder of the carbonaceous particles is dispersed in a mixed solution of an aqueous solution of silver ammonium nitrate and an alcohol, A step of adding an aqueous formaldehyde solution to the dispersion solution to reduce and precipitate the Ag fine particles on the surface of the carbonaceous particles.

That is, according to the production method of the present invention, it is possible to control the rate at which Ag is precipitated from a solution to a suitable rate by a reaction called a so-called silver mirror reaction. By proceeding at the same time, it is possible to easily and inexpensively produce the negative electrode active material in the form of Ag fine particles in which a part forms a cluster as described above. In the present production method, a relatively mild reaction is used without using a reducing agent having a strong reducing ability such as hydrazine and NaBH ₄ used in the prior art, so that the safety of the production method itself is also high. .

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the negative electrode active material of the present invention, an embodiment of the production method of the present invention, which is a method for producing the negative electrode active material, will be described. An embodiment of a secondary battery will be described.

<Negative Electrode Active Material> The negative electrode active material of the present invention has a myriad of carbonaceous particles having a particle size of several to several tens of microns and a submicron order particle size attached to the surface of the carbonaceous particles. And countless Ag fine particles.

The carbonaceous particles need only be capable of inserting and extracting lithium (lithium ions), and there is no particular limitation on the substance used as the material. Examples of the carbonaceous substance that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon, and an organic compound fired body such as a phenol resin, and easily graphitizable carbon such as coke. .

Of these, natural and artificial graphites have the advantage of being capable of forming a lithium secondary battery having a large capacity (high energy density) and good power characteristics because of its high true density and excellent conductivity. There is. When a lithium secondary battery utilizing this advantage is manufactured, it is desirable that the graphite used has high crystallinity, the (002) plane spacing d ₀₀₂ is 3.4 ° or less, and the crystallite thickness in the c-axis direction. It is preferable to use one having Lc of 1000 ° or more. In addition, artificial graphite is, for example, 2800 ° C.
It can be manufactured by heat treatment at the above high temperature. In this case, the easily graphitizable carbon as a raw material includes coke and optically anisotropic small spheres (mesocarbon microbeads: MCMB) obtained in the process of heating pitches at about 400 ° C. .

When artificial graphite is used, it is desirable to use the graphitized mesophase microspheres (graphitized MCMB). This graphitized MCMB is characterized by having a spherical shape, has a small specific surface area, minimizes decomposition of the electrolytic solution, and can contribute to improvement of the packing density. Therefore, if the graphitized MCMB is used for carbonaceous particles, a secondary battery having better cycle characteristics and higher energy density can be configured. In addition, since the crystallites are arranged in a lamellar shape and the crystallite end faces are exposed on the surface of the particles, if the graphitized MCMB is used for the carbonaceous particles, the absorption and release of lithium during charging / discharging is smooth and the output is higher A battery having excellent characteristics can be formed.

[0021] Graphitizable carbon is generally obtained from tar pitch obtained from petroleum or coal, and includes coke,
MCMB, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, and the like. An organic compound fired body such as a phenol resin can also be used. Since graphitizable carbon is an inexpensive carbon material, if it is used as carbonaceous particles, a lithium secondary battery excellent in cost can be constituted. Among them, coke has the advantages of lower cost and relatively large capacity. In view of this, it is desirable to use coke. When coke is used, it is preferable to use one having a (002) plane spacing d ₀₀₂ of 3.4 ° or more and a crystallite thickness Lc in the c-axis direction of 30 ° or less.

[0021] The non-graphitizable carbon is a so-called hard carbon, and is a carbon material having a structure close to amorphous such as glassy carbon. Generally, it is a material obtained by carbonizing a thermosetting resin, and does not develop a graphite structure even when the heat treatment temperature is increased. The non-graphitizable carbon has the advantages of high safety and relatively low cost. In view of this, it is desirable to use the non-graphitizable carbon for the carbonaceous particles. Specifically, for example, a phenol resin fired body, a polyacrylonitrile-based carbon fiber, pseudo isotropic carbon, a furfuryl alcohol resin fired body, or the like can be used. More preferably, the (002) plane spacing d ₀₀₂ is 3.6 ° or more and the crystallite thickness Lc in the c-axis direction is 100 ° or less.

As the carbonaceous particles, one of the above-mentioned graphite, graphitizable carbon, non-graphitizable carbon, etc. may be used alone, or a mixture of two or more types may be used. it can. As an embodiment of mixing two or more, for example,
While securing safety during overcharge, limit lithium absorbed and released to the positive electrode to suppress the collapse of the crystal structure of the lithium composite oxide, which is the positive electrode active material, and improve cycle characteristics. For example, a case where graphite is mixed with a non-graphitizable carbon material such as non-graphitizable carbon and graphitizable carbon can be exemplified. When a mixture of such graphite and a non-graphitized carbonaceous material is used as the material of the carbonaceous particles, the mixture ratio of the two is determined by the balance between the cycle characteristics and the discharge capacity. May be determined in view of the characteristics of the above.

The particle size of the carbonaceous particles is several to several tens of microns. The more desirable range is 5μ
It is preferable to be composed of carbonaceous particles having a particle size of m to 80 μm. A more desirable range is that the average particle size is 15 μm to 50 μm.
μm, and preferably 80% or more of all the particles are within this range. If the particle size of the carbonaceous particles is too large, the specific surface area will be too small, and it will not be possible to efficiently store and release lithium into the carbonaceous particles. This is because the energy density becomes small. Conversely, if the particle size of the carbonaceous particles is too small, the amount of the binder in forming the negative electrode must be increased, which impairs the conductivity of the negative electrode.

Ag fine particles are used as fine particles to be attached to the surface of the carbonaceous particles. This is because Ag has excellent electrical conductivity, is electrochemically stable, Pt,
The reason is that it is inexpensive as compared with a noble metal such as Au.

The Ag fine particles have a submicron-order particle size. If a more desirable range is represented by a specific numerical value, it is preferable to attach fine particles having a particle size of 0.1 μm to 0.5 μm. If the particle size of the Ag fine particles is too large, the weight of Ag required for providing sufficient electron conductivity increases, and the energy density decreases. Conversely, if the particle size is too small, electron conductivity is provided. Therefore, it is difficult to form a network of Ag particles.

The Ag fine particles are classified into those that adhere to the carbonaceous particles in a form that is uniformly dispersed in a state close to the single state, and those that adhere to the carbonaceous particles in a form that forms clusters. Can be. As described above, the Ag fine particles uniformly dispersed in a state close to the single state act to reduce the electric resistance due to the contact between the carbonaceous particles, and the part of the cluster-like Ag fine particles is in a state close to the single state. It is considered that a conductive path is formed inside the negative electrode together with the Ag fine particles dispersed in the negative electrode.

The size of the formed cluster is such that several to several hundred Ag fine particles are aggregated, but more preferably several to several hundred Ag fine particles are aggregated. By expressing the range of more desirable cluster sizes in different expressions, 1
The size is preferably such that it covers 5% to 50% of the surface area of each carbonaceous particle. If the cluster is too small, it is slightly inferior in forming an efficient conductive path.If it is too large, the ratio of covering the carbonaceous particles increases, which may hinder the absorption and release of lithium into the carbonaceous particles. Because there is a nature.

The ratio of the carbonaceous particles to which the Ag fine particles are aggregated and attached in clusters, that is, the number of carbonaceous particles to which the clusters are attached is 0.1% of the total number of carbonaceous particles.
More preferably, it is a quantity corresponding to 10% to 10%.
If the amount is less than 0.1%, the formation of an efficient conductive path is slightly inferior. This is because there is a possibility that the negative electrode active material may be impaired, and the cost of the negative electrode active material becomes poor.

The ratio of the Ag fine particles to the surface of the carbonaceous particles (adhesion ratio) also affects the characteristics and the like as the negative electrode active material. If the adhesion ratio is defined by the ratio of the total weight of the carbonaceous particles to the total weight of the Ag fine particles, it is desirable that the ratio of carbonaceous particles: Ag fine particles = 99.5: 0.5 to 95: 5. When the total amount of Ag fine particles is too small, the performance is inferior in that the internal resistance of the negative electrode is reduced when the negative electrode is formed, and when the total amount of Ag fine particles is too large, the internal resistance is reduced. This is because the cost of the negative electrode active material is too high despite the effect being saturated.

<Method for Producing Negative Electrode Active Material> The method for producing the above-mentioned negative electrode active material is not particularly limited. Examples of a method for producing this negative electrode active material inexpensively, simply, and safely include: The method for producing the negative electrode active material of the present invention will be described.

The method for producing a negative electrode active material of the present invention utilizes the silver mirror reaction as described above, and is a dispersion solution in which the carbonaceous particle powder is dispersed in a mixed solution of an aqueous solution of silver ammonium nitrate and an alcohol. And a precipitation step of adding an aqueous formaldehyde solution to the dispersion solution to reduce and precipitate Ag fine particles on the surface of the carbonaceous particles.

The aqueous solution of silver ammonium nitrate used in the step of preparing the dispersion solution refers to an aqueous solution of silver nitrate in which, when ammonia water is added, an off-white precipitate of silver oxide is formed, and ammonia water is added until the precipitate disappears. More specifically, the dispersion solution preparation step is to immerse a predetermined amount of the powder composed of the carbonaceous particles in an alcohol, further add an appropriate amount of water and stir the mixture, and further add a predetermined concentration of an aqueous solution of silver nitrate and an appropriate amount of ammonia. Water may be added, and the mixture may be heated to a predetermined temperature while stirring. In addition, ethanol, propanol, butanol and the like can be used as the alcohol.

Since the weight ratio of the carbonaceous particles to Ag in the silver nitrate solution is related to the adhesion ratio of the Ag fine particles to the carbonaceous particles in the negative electrode active material, in the dispersion solution preparation step, the carbonaceous particles and The weight ratio to Ag in the silver nitrate solution is 9
It is desirable to set it in the range of 9: 1 to 90:10.

The precipitation step is carried out by dropping an appropriate amount of an aqueous solution of formaldehyde having a predetermined concentration at a predetermined rate while maintaining the temperature of the prepared dispersion solution. Formaldehyde was selected from various reducing agents because the reaction rate of Ag deposition can be made an appropriate rate. Further, the concentration of the silver nitrate solution,
The solution temperature, the concentration of the aqueous formaldehyde solution and the dropping rate during the precipitation reaction are determined by the amount of A
Since it affects the particle size and the form of adhesion of the g fine particles, it is preferable that the particle size and the form of adhesion are appropriately determined according to the form of adhesion of the Ag fine particles in the anode active material to be obtained.

In the precipitation step, the powder composed of the carbonaceous particles to which the Ag fine particles are adhered is taken out of the solution, filtered, and dispersed in water repeatedly, and dried to obtain a negative electrode active material.
The method of adjusting the particle size of the carbonaceous particles as the negative electrode active material is not particularly limited, and may be performed by a known classification method. Further, the particle size adjustment may be performed before the dispersion solution preparation step or after the precipitation step is completed.

<Lithium Secondary Battery Using Negative Electrode Active Material of the Present Invention> A lithium secondary battery using the negative electrode active material of the present invention comprises a negative electrode using the negative electrode active material and a lithium transition metal composite oxide. A positive electrode serving as a positive electrode active material and a non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent are configured as main components.

The negative electrode was prepared by mixing the above-mentioned negative electrode active material with a binder, adding an appropriate solvent thereto, and forming a paste into a paste.
It can be formed by coating and drying on the surface of a current collector such as a copper foil and compressing to increase the electrode density.

The binder plays a role of binding the active material particles, and may be a fluorinated resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. As a solvent for dispersing the active material and the binder, an organic solvent such as N-methyl-2-pyrrolidone can be used.

The positive electrode is prepared by mixing a conductive material and a binder with a positive electrode active material composed of a lithium transition metal composite oxide and adding an appropriate solvent to form a paste-like positive electrode mixture. It can be formed by coating and drying the surface of the electric body and compressing it to increase the electrode density.

As a lithium transition metal composite oxide serving as a positive electrode active material, Lithium transition metal
A lithium composite oxide powder such as CoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ can be used. Among them, LiCoO ₂ having a layered rock salt structure has a high raw material cost, but is easy to synthesize and handle, and is a positive electrode active material that can constitute a battery having good cycle characteristics and the like.
On the other hand, LiMnO ₂ having a layered rock salt structure and LiMn ₂ O ₄ having a spinel structure have low raw material costs, and are used in applications where a large amount of active materials are used, such as electric vehicles and secondary batteries used in power storage systems. , Which is advantageous.

The conductive material is used to ensure the electrical conductivity of the positive electrode, and it is preferable to use one or a mixture of two or more powdered carbon materials such as carbon black, acetylene black, and graphite. it can. As in the case of the negative electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the binder, and N-methyl- as a solvent for dispersing the active material, the conductive material, and the binder is used. An organic solvent such as 2-pyrrolidone can be used.

When a lithium secondary battery is formed, a separator is interposed between the positive electrode and the negative electrode for the purpose of separating the positive electrode and the negative electrode and holding an electrolyte. As this separator, a thin microporous film such as polyethylene or polypropylene can be used.

The non-aqueous electrolyte is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent and forms lithium ions in the electrolyte. Examples of usable lithium salts include LiBF ₄ , LiPF ₆ , LiClO ₄ , and LiCF ₃
SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN
(C ₂ F ₅ SO ₂ ) _{2 and the} like. Each of these lithium salts may be used alone, or two or more of these lithium salts may be used in combination.

As the organic solvent for dissolving the lithium salt, an aprotic organic solvent is used. For example, a mixed solvent of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether or chain ether can be used. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate.Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.Examples of the cyclic ester include gamma butyrolactone and gamma valero. Examples of lactones and cyclic ethers include tetrahydrofuran and 2-methyltetrahydrofuran, and examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether.

The lithium secondary battery of the present invention constituted as described above can have various shapes such as a cylindrical type and a laminated type. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead extends 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 electrode body is impregnated with a non-aqueous electrolyte and sealed in a battery case to complete a lithium secondary battery.

[0046]

EXAMPLE As an example, the negative electrode active material of the present invention based on the above embodiment was manufactured by the manufacturing method of the present invention based on the above embodiment, and a lithium secondary battery was manufactured using the same. As a comparative example, a secondary battery was manufactured using a negative electrode active material manufactured by changing the reducing agent used in the deposition step and a negative electrode active material to which no Ag fine particles were attached. Then, the characteristics of these secondary batteries were evaluated.

Example 1 As a carbonaceous material, graphitized mesophase microspheres (MCMB25-
28: Osaka Gas Chemical) (hereinafter simply referred to as “MCMB”)
) Was produced by the following method.

First, MCMB was added to 150 cc of ethanol.
Was immersed in 150 g of the powder, and 350 cc of ion-exchanged water was added thereto and stirred. 11.8cc of 1M silver nitrate solution and 10c of 25wt% ammonia water
c) and heated in a water bath with stirring to prepare a dispersion solution having a solution temperature of 50 ° C. Incidentally, the amount of Ag in the silver nitrate solution depends on the amount of Ag that precipitates with MCMB particles.
5 wt% when the total weight with the fine particles is 100 wt%
% Corresponding to the weight in%.

Next, 20 cc of a 36% strength aqueous formaldehyde solution was dropped into the dispersion at a rate of about 2 cc / min using a tube pump to precipitate Ag. From the solution, filtration and dispersion in water were repeated twice to sufficiently rinse, and then the powder collected by filtration again was collected in 1 solution.
After drying at 00 ° C. for 12 hours, the mixture was classified with a sieve to have a particle size of 53 μm or less to obtain a negative electrode active material.

The negative electrode active material was subjected to a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX).
As a result, it was confirmed that Ag was attached to the surface of the MCMB particles. The SEM photograph at this time is the photograph shown in FIGS. 1 and 2 described above. Ag particles are mixed with MCMB particles that are uniformly dispersed and adhered in a state close to a single particle, and a cluster is formed so as to partially cover the surface thereof, whereby MCMB particles to which the Ag particles are adhered can be confirmed. From a detailed analysis of the SEM photograph, it was confirmed that the number of MCMB particles partially covered by the clusters was equivalent to about 2% of the total number of MCMB particles, and one cluster contained several to several hundred Ag fine particles. Was formed. In this case, Ag in the finally obtained negative electrode active material was found to be 2.5 wt% by composition analysis.

The negative electrode of the lithium secondary battery was manufactured as follows. First, 5 parts by weight of polyvinylidene fluoride (PVDF) was blended as a binder with 95 parts by weight of the negative electrode active material, and an appropriate amount of N-methyl-2-pyrrolidone (NM) was added.
P) was added and kneaded with a vacuum kneader to obtain a paste-like negative electrode mixture. This negative electrode mixture was applied to both surfaces of a negative electrode current collector made of copper foil, dried, pressed and cut to obtain an electrode area of 56 mm.
A sheet-shaped negative electrode of × 500 mm was produced.

The positive electrode was manufactured as follows. Positive electrode active
The substance has the composition formula Li_1.1Mn_1.9O _FourSpine represented by
A lithium manganese composite oxide having a metal structure was used. This positive electrode
86 parts by weight of active material and 10 parts by weight of artificial graphite as conductive material
Parts, 4 parts by weight of PVDF as a binder, and an appropriate amount of N
-Methyl-2-pyrrolidone (NMP)
The mixture was kneaded with a kneader to obtain a paste-like positive electrode mixture. This positive electrode
Apply the mixture to both sides of the aluminum foil negative electrode current collector and dry it.
After drying, press and cut, electrode area 54mm x 450m
m of the sheet-shaped positive electrode was produced.

The above positive electrode and negative electrode were wound through a polyethylene separator to form a roll-shaped electrode body. This electrode assembly was subjected to current collection, housed in a 18650 type battery can having a diameter of 18 mmφ and a length of 65 m together with the non-aqueous electrolyte, and sealed to complete a cylindrical secondary battery. In addition,
As the non-aqueous electrolyte, one obtained by dissolving LiPF ₆ at a concentration of 1 M as an electrolyte in a mixed solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 3: 7 was used.

<Embodiment 2> Compared with Embodiment 1 above, Ag
This is a secondary battery using negative electrode active materials having different composite ratios of fine particles (weight ratio of all Ag fine particles to all MCMB particles). In the production of the negative electrode active material of Example 1, the amount of 1M silver nitrate solution was 11.8 cc, the amount of 25 wt% aqueous ammonia was 20 cc, and the amount of 36% aqueous solution of formaldehyde was 40 cc. A negative electrode active material having a composite ratio of fine particles of 10 wt% was prepared.

MCM partially covered by clusters
The number of B particles corresponds to about 4% of the total number of MCMB particles, and it can be confirmed that the number of B particles is larger than that of Example 1, and one cluster is composed of several to several hundred Ag fine particles. It was also confirmed that the size was not much different from that of Example 1. As a result of the composition analysis, Ag:
It was found that 5 wt% was contained. The other components of the lithium secondary battery excluding the negative electrode active material are the same as those in the first embodiment.

Comparative Example 1 In the production of the negative electrode active material in the case of Example 1, the deposition step was performed at room temperature without heating the dispersion solution. Ag on the inner wall of the container containing the solution
Was deposited, but it was confirmed that the Ag fine particles hardly adhered to the MCMB particles.

<Comparative Example 2> In the production of the negative electrode active material in the case of Example 1, a 0.56M concentration aqueous glucose solution 100c was used instead of a 36% concentration formaldehyde aqueous solution.
A negative electrode active material was produced using c as a reducing agent. As in the case of Comparative Example 1, although Ag was precipitated on the inner wall of the container containing the solution, it was confirmed that the Ag fine particles hardly adhered to the MCMB particles.

Comparative Example 3 In the production of the negative electrode active material in the case of Example 1, 100 cc of a 0.5 M NaBH ₄ aqueous solution was used instead of a 36% formaldehyde aqueous solution.
Was used as a reducing agent to produce a negative electrode active material. From the SEM observation, the presence of MCMB particles partially covered with cluster-like Ag fine particles was not observed in the obtained negative electrode active material, and the density of AMC was slightly higher than that in Example 1;
It was confirmed that the g fine particles were uniformly dispersed in a state close to a single particle. Using this negative electrode active material, a lithium secondary battery was fabricated in the same manner as in Example 1 except for the other components.

<Comparative Example 4> M used in Example 1
A lithium secondary battery was manufactured using a negative electrode active material consisting only of CMB particles. The other components of the lithium secondary battery excluding the negative electrode active material are the same as those in the first embodiment.

<Characteristic evaluation of the manufactured lithium secondary battery>
Charge / discharge tests were performed on the lithium secondary batteries of Example 1, Example 2, Comparative Example 3, and Comparative Example 4, and the discharge capacity and DC internal resistance of the batteries were measured. In addition, 1kHz
Was also measured.

The charge / discharge test was performed by a constant current charge / discharge method in a range of an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V. For each secondary battery, set the current density during charging and discharging to 0.2
The test was performed under two conditions of 5 mA / cm ² and 1 mA / cm ² . The DC internal resistance of the battery was determined by dividing the difference between the average charging voltage during charging and discharging and the average discharging voltage by twice the current value during charging and discharging.

Table 1 below shows the discharge capacity per unit weight of the positive electrode active material, the DC internal resistance value, and the AC internal resistance value at 1 kHz of each secondary battery under each charge / discharge current density condition.

[0063]

[Table 1] As is clear from Table 1, the secondary batteries of Examples 1 and 2 using the negative electrode active material of the present invention were different from the secondary batteries of Comparative Example 4 using the negative electrode active material containing no Ag fine particles. It can be confirmed that the battery is a secondary battery having a large discharge capacity and a small internal resistance of the battery. In addition, when compared with the secondary battery of Comparative Example 3 using the negative electrode active material in which Ag fine particles did not form a cluster, the secondary batteries of Example 1 and Example 2 using the negative electrode active material of the present invention It can be seen that the discharge capacity is large and the DC internal resistance of the battery is small in charging and discharging at a high current density, that is, at a high output. This can be said to be a result that well shows that the Ag fine particles adhering to the formation of clusters inside the negative electrode form an effective conductive path.

[0064]

The negative electrode active material of the present invention is obtained by adhering two types of Ag fine particles, one in which the particles are uniformly dispersed in a state close to a single substance and the other in which clusters are formed, on the surface of the carbonaceous particles. Accordingly, the present negative electrode active material has a low contact resistance between carbonaceous particles, and becomes an active material capable of forming a negative electrode having an effective conductive path therein. As a result, a lithium secondary battery using the present negative electrode active material has a low internal resistance and a large discharge capacity.

The method for producing a negative electrode active material of the present invention comprises the steps of:
Ag fine particles are deposited and adhered to the surface of carbonaceous particles using formaldehyde, a reducing agent that progresses the reaction relatively slowly. According to the production method of the present invention, the negative electrode active material of the present invention to which the Ag fine particles adhere in the above two forms can be produced inexpensively, simply, and safely.

[Brief description of the drawings]

FIG. 1 shows an SEM of an example of the negative electrode active material of the present invention.
A photograph is shown.

FIG. 2 is an enlarged view of a central portion of the SEM photograph shown in FIG.

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[Procedure amendment]

[Submission date] June 24, 1999 (1999.6.2
4)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Figure 2

[Correction method] Change

[Correction contents]

FIG. 2

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H003 AA01 AA04 BA03 BA07 BB01 BB02 BB14 BC01 BC05 BD02 BD04 5H014 AA02 BB06 BB11 CC01 EE05 EE08 HH01

Claims (5)

[Claims] 1. An innumerable carbonaceous particle having a particle diameter of several to several tens of microns, and an innumerable number of Ag fine particles having a particle diameter of a submicron order adhered to the surface of the carbonaceous particle, A part of the Ag fine particles forms a cluster and adheres to the carbonaceous particles, and the remaining part of the Ag fine particles is dispersed and adheres to the carbonaceous particles. Active material. 2. The method according to claim 1, wherein the cluster includes several to several hundred Ag.
The negative electrode active material for a lithium secondary battery according to claim 1, wherein the fine particles are formed by aggregation. 3. The lithium secondary battery according to claim 1, wherein the number of the carbonaceous particles to which the clusters are attached is a number corresponding to 0.1 to 10% of the total number of the carbonaceous particles. Negative electrode active material for secondary batteries. 4. The method according to claim 1, wherein a ratio of a total weight of the carbonaceous particles to a total weight of the Ag fine particles is 99.5: 0.5 to 95: 5. Negative electrode active material for lithium secondary batteries. 5. An innumerable carbonaceous particle having a particle diameter of several to several tens of microns, and innumerable Ag fine particles having a particle diameter of a submicron order adhered to the surface of the carbonaceous particle, A method for producing a negative electrode active material for a lithium secondary battery in which a part of Ag fine particles form clusters and adhere to the carbonaceous particles, and the remaining Ag fine particles are dispersed and adhere to the carbonaceous particles. A dispersion solution preparation step of preparing a dispersion solution in which the powder of the carbonaceous particles is dispersed in a mixed solution of an aqueous solution of silver ammonium nitrate and an alcohol, and adding an aqueous formaldehyde solution to the dispersion solution;
a step of reducing and precipitating g fine particles on the surface of the carbonaceous particles. A method for producing a negative electrode active material for a lithium secondary battery.