JP2012003985A - Electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having excellent cycle characteristics.SOLUTION: An electrode for the lithium ion secondary battery comprises: an electrode active material capable of occluding and releasing a lithium ion; a carbon-based conductive auxiliary material; and a binder. The carbon-based conductive auxiliary material is an ultrafine fibrous carbon free from branch structure and having a mean fiber diameter of 10 to 900 nm.

Description

  The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the electrode for a lithium ion secondary battery.

  A lithium ion secondary battery is a type of non-aqueous electrolyte secondary battery in which lithium ions in the electrolyte are responsible for electrical conduction, using a lithium metal oxide for the positive electrode and a carbon material such as graphite for the negative electrode. What is used is a mainstream secondary battery. Lithium ion secondary batteries have a feature of high energy density among secondary batteries, and thus have a wide range of applications from small devices such as mobile phones to large devices such as electric vehicles.

  One of the problems with lithium ion secondary batteries is that battery capacity reduction (deterioration) due to repeated charge and discharge is prevented (improvement of cycle characteristics). Causes of deterioration of cycle characteristics are caused by a combination of various factors such as denaturation (deterioration) of electrode active material, electrolyte, electrolyte, etc., and increase in electrode resistance due to peeling of electrode foil and electrode active material. However, as one of the improvement methods, it has been proposed to improve the cycle characteristics by adding a fibrous carbon material into the electrode (see Patent Document 1). In this proposal, vapor grown carbon fiber is used as the fibrous carbon material. However, since vapor grown carbon fiber has a branched structure, it is difficult to increase dispersibility in the electrode. There is a problem that the carbon material in the form of agglomeration, and accordingly, the cycle characteristics are not sufficient.

JP 2007-42620 A

  An object of the present invention is to provide a lithium ion secondary battery having excellent cycle characteristics.

As a result of intensive studies in view of the above prior art, the present inventors have completed the present invention. That is, the gist of the present invention is as follows.
1. An electrode for a lithium ion secondary battery comprising an electrode active material capable of inserting and extracting lithium ions, a carbon-based conductive additive and a binder,
The electrode for a lithium ion secondary battery, wherein the carbon-based conductive additive is ultrafine fibrous carbon having an average fiber diameter of 10 to 900 nm that does not have a branched structure.
2. Above 1. A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery as described in the paragraph

  Since the electrode for lithium ion secondary batteries of the present invention has excellent cycle characteristics, a high performance lithium ion secondary battery can be provided.

2 is a scanning electron microscope image (2,000 times) of ultrafine fibrous carbon obtained by the operation of Example 1. FIG.

The electrode for a lithium ion secondary battery of the present invention includes an electrode active material capable of occluding and releasing lithium ions, a carbon-based conductive aid and a binder, and the carbon-based conductive aid has an average fiber diameter without a branched structure. It is characterized by being ultrafine fiber carbon of 10 to 900 nm.
Hereinafter, electrode active materials (positive electrode active material, negative electrode active material) capable of inserting and extracting lithium ions will be described.

[Positive electrode active material]
As the positive electrode active material contained in the lithium ion secondary battery of the present invention, any of conventionally known materials (materials capable of occluding and releasing lithium ions) known as positive electrode active materials in lithium batteries can be used. One or two or more of them can be appropriately selected and used, and among these, lithium-containing metal oxides capable of inserting and extracting lithium ions are preferable.
Examples of the lithium-containing metal oxide include a composite oxide containing lithium and at least one element selected from the group consisting of Co, Mg, Mn, Ni, Fe, Al, Mo, V, W, and Ti. Can do.

_{Specifically, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Co b V 1-b O z, Li x Co b Fe 1-b _{O 2, Li x Mn 2 O} 4, Li x Mn c Co 2-c O 4, Li x Mn c Ni 2-c O 4, Li x Mn c V 2-c O 4, Li x Mn c Fe 2- _c O ₄ (where, x = 0.02~1.2, a = 0.1~0.9 , b = 0.8~0.98, c = 1.6~1.96, z = 2 At least one selected from the group consisting of .01-2.3. Preferred lithium-containing metal _{oxides, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co a Ni 1-a O 2, Li x Mn 2 O 4, Li x Co b V 1-b There may be mentioned at least one selected from the group consisting of O _z (where x, a, b and z are the same as above). In addition, the value of x is a value before the start of charging / discharging, and increases / decreases by charging / discharging.

  The positive electrode active materials may be used alone or in combination of two or more. The average particle diameter of the positive electrode active material is 10 μm or less. When the average particle diameter exceeds 10 μm, the efficiency of the charge / discharge reaction under a large current is reduced. The average particle diameter is preferably 0.05 μm (50 nm) to 7 μm, and more preferably 1 μm to 7 μm.

[Negative electrode active material]
As the negative electrode active material contained in the lithium ion secondary battery of the present invention, one kind of conventionally known materials (materials capable of occluding and releasing lithium ions) known as negative electrode active materials in lithium-based batteries is used. Or it can select and use 2 or more types. For example, as a material capable of inserting and extracting lithium ions, a carbon material, any of Si and Sn, or an alloy or oxide containing at least one of them can be used. Among these, a carbon material is preferable.

  Typical examples of the carbon material include artificial graphite produced by heat-treating natural graphite, petroleum-based and coal-based coke, hard carbon obtained by carbonizing a resin, mesophase pitch-based carbon material, and the like. When natural graphite or artificial graphite is used, one having a (002) plane spacing d (002) of the graphite structure by powder X-ray diffraction in the range of 0.335 to 0.337 nm from the viewpoint of increasing battery capacity. preferable.

  Natural graphite refers to a graphite material that is naturally produced as ore. Natural graphite is classified into two types, scaly graphite having a high degree of crystallinity and earthy graphite having a low degree of crystallinity, depending on its appearance and properties. The scaly graphite is further divided into scaly graphite having a leaf-like appearance and scaly graphite having a lump shape. There are no particular restrictions on the origin, properties, and types of natural graphite used as the graphite material. Further, natural graphite or particles produced using natural graphite as a raw material may be subjected to heat treatment.

  Artificial graphite refers to graphite made by a wide variety of artificial techniques and a graphite material close to a perfect crystal of graphite. As a typical example, a material obtained from a tar and coke obtained from dry distillation of coal, a residue obtained by distillation of crude oil, and the like, through a firing process of about 500 to 1000 ° C. and a graphitization process of 2000 ° C. or more is used. Can be mentioned. Kish graphite obtained by reprecipitation of carbon from dissolved iron is also a kind of artificial graphite.

  Using an alloy containing at least one of Si and Sn in addition to the carbon material as the negative electrode active material has a smaller electric capacity than when using each of Si and Sn alone or using each oxide. It is effective in that it can be done. Of these, Si-based alloys are preferable.

As an Si-based alloy, an alloy of Si and at least one element selected from the group consisting of B, Mg, Ca, Ti, Fe, Co, Mo, Cr, V, W, Ni, Mn, Zn, Cu, and the like And so on. Specifically, SiB ₄ , SiB ₆ , Mg ₂ Si, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , MnSi ₂ , VSi ₂ , There may be mentioned at least one selected from the group consisting of WSi ₂ , ZnSi _{2 and the} like.

  In the present invention, as the negative electrode active material, the aforementioned materials may be used alone or in combination of two or more. The average particle diameter of the negative electrode active material is 10 μm or less. When the average particle diameter exceeds 10 μm, the efficiency of the charge / discharge reaction under a large current is reduced. The average particle diameter is preferably 0.1 to 10 μm, and more preferably 1 to 7 μm.

  Next, the ultrafine fibrous carbon having an average fiber diameter of 10 to 900 nm that does not have a branched structure included in the electrode for a lithium ion secondary battery according to the present invention will be described in detail.

<Ultrafine fibrous carbon>
-Graphitizable carbon The ultrafine fiber carbon in the present invention is preferably graphitizable carbon. Easily graphitizable carbon is a carbon raw material in which a graphite structure having a three-dimensional lamination regularity is easily generated by heat treatment at a high temperature of 2500 ° C. or higher. Also called soft carbon or soft carbon. Examples of graphitizable carbon include petroleum coke, coal pitch coke, polyvinyl chloride, and 3,5-dimethylphenol formaldehyde resin. Among these, a compound called a mesophase pitch, which can form an optically anisotropic phase (liquid crystal phase) in a molten state, or a mixture thereof is preferable because high crystallinity and high conductivity are expected. As mesophase pitch, petroleum-based mesophase pitch obtained by a method mainly consisting of hydrogenation and heat treatment of petroleum residue oil or a method mainly consisting of hydrogenation, heat treatment and solvent extraction; Coal-based mesophase pitch obtained by a method mainly comprising heat treatment or a method mainly comprising hydrogenation, heat treatment and solvent extraction; super strong acids (for example, HF, BF) using aromatic hydrocarbons such as naphthalene, alkylnaphthalene and anthracene as raw materials ₃ ) and the like, and a synthetic liquid crystal pitch obtained by polycondensation in the presence of ₃ ). Among these, the synthetic liquid crystal pitch is more preferable in that it does not contain impurities.

-Average fiber diameter The average fiber diameter of the ultrafine fiber carbon in this invention exists in the range of 10-900 nm. This average fiber diameter is a value measured from a photograph taken at a magnification of 2,000 with a field emission scanning electron microscope. The average fiber diameter of the ultrafine fibrous carbon is preferably in the range of 10 to 600 nm, more preferably in the range of 50 to 500 nm, and still more preferably in the range of 50 to 400 nm. This ultra-fine fibrous carbon having an average fiber diameter of less than 10 nm has a very low bulk density and is inferior in handleability, and when used as an ultra-fine fibrous carbon used for an electrode for a lithium ion secondary battery, This is not preferable because the strength decreases. In addition, if ultra-fine fibrous carbon having an average fiber diameter exceeding 900 nm is used for an electrode for a lithium ion secondary battery, it is not preferable because it is too thick and the gap in the electrode becomes large and the electrode density cannot be increased.
The ultrafine fibrous carbon in the present invention does not have a branched structure. Here, having no branched structure means that the ultrafine fibrous carbon does not have a granular portion bonded to other ultrafine fibrous carbon at a place other than the terminal portion, and the main axis of the ultrafine fibrous carbon. Means that the main axis of the ultrafine fibrous carbon does not have a branch-like minor axis.

-Average fiber length The average fiber length of the ultrafine fibrous carbon in the present invention is preferably in the range of 1 to 100 µm, and more preferably in the range of 1 to 50 µm. The longer the average fiber length of the ultrafine fibrous carbon, the better the conductivity in the electrode for lithium ion secondary battery, the strength of the electrode, and the electrolyte retention, but if it is too long, the fiber dispersibility in the electrode is increased. The problem of being damaged arises. Therefore, the average fiber length of the ultrafine fibrous carbon in the present invention is preferably within the above range.

-Average interplanar distance The ultrafine fibrous carbon in the present invention more preferably has an average interplanar spacing d002 of (002) planes of 0.335 to 0.340 nm measured by X-ray diffraction.

<Method for producing ultrafine fiber carbon>
In order to produce the ultrafine fiber carbon in the present invention, any technique can be used as long as it can obtain an ultrafine carbon fiber that satisfies the above-mentioned conditions. For example, at 350 ° C. and 600 s ⁻¹ . At least one thermoplastic carbon selected from the group consisting of 100 parts by mass of a thermoplastic resin having a melt viscosity of 5 to 100 Pa · s when measured, petroleum mesophase pitch, coal mesophase pitch, and synthetic liquid crystal pitch. A resin composition comprising 1 to 150 parts by mass of the precursor is molded at a temperature of 100 to 400 ° C. to produce a precursor molded body, and then the thermoplastic carbon precursor contained in the precursor molded body is stabilized and stabilized. Forming a pre-cured precursor molded body,
Further, a method is preferred in which the thermoplastic resin is removed from the stabilized precursor molded body to form a fibrous carbon precursor, and the fibrous carbon precursor is carbonized under an inert gas atmosphere to produce ultrafine fibrous carbon. It can be illustrated as a manufacturing method.

(A) a thermoplastic resin in the above preferred production method,
(B) a thermoplastic carbon precursor;
(C) a method for producing a resin composition from a thermoplastic resin and a thermoplastic carbon precursor;
(D) A method of forming a precursor molded body by molding a resin composition,
(E) a method of forming a stabilized precursor molded body by stabilizing a thermoplastic carbon precursor contained in the precursor molded body,
(F) Method for forming a fibrous carbon precursor by removing the thermoplastic resin from the stabilized precursor molded body (g) Method for producing ultrafine fibrous carbon from the fibrous carbon precursor (h) About the grinding method explain.

(A) Thermoplastic resin The thermoplastic resin used in the above preferred production method preferably has a melt viscosity of 5 to 100 Pa · s when measured at 350 ° C. and 600 s ⁻¹ . When a thermoplastic resin having a melt viscosity of less than 5 Pa · s is used, it is not preferable because it becomes difficult to form the thermoplastic carbon precursor. Further, when the melt viscosity exceeds 100 Pa · s, it is not preferable because it becomes difficult to mold a resin composition for producing ultrafine fibrous carbon. The melt viscosity of the thermoplastic resin is more preferably 7 to 100 Pa · s, and further preferably 5 to 100 Pa · s.

  In view of the fact that the thermoplastic resin used in the above preferred production method can be easily melt kneaded and melt molded with the thermoplastic carbon precursor, the glass is used when the thermoplastic resin is amorphous. When the transition point is 250 ° C. or lower and the thermoplastic resin is crystalline, the crystal melting point is preferably 300 ° C. or lower.

  The thermoplastic resin needs to be easily removed after the production of the stabilized precursor molded body. For this reason, it is preferable that it is decomposed | disassembled to 10 mass% or less of initial mass, More preferably to 5 mass% or less by hold | maintaining for 2 hours at the temperature of 450 degreeC or more and less than 600 degreeC in inert gas atmosphere. .

  An example of such a thermoplastic resin is polyolefin. Examples of the polyolefin include one or more selected from the group consisting of homopolymers, copolymers of a plurality of types of olefins, copolymers of olefins with vinyl acetate, methacrylic acid or derivatives thereof, and the like. It is more preferable that it is one or more selected from the group consisting of polyethylene, polypropylene and polymethylpentene, and polyethylene is particularly preferable.

(B) Thermoplastic carbon precursor It is preferable that the thermoplastic carbon precursor used in the preferred production method should be graphitizable carbon.
The thermoplastic carbon precursor is used in an amount of 1 to 150 parts by weight, preferably 5 to 100 parts by weight, based on 100 parts by weight of the thermoplastic resin. When the use ratio of the thermoplastic carbon precursor exceeds 150 parts by mass, a carbon precursor having a desired dispersion diameter cannot be obtained. On the other hand, when this value is less than 1 part by mass, the ultrafine fibrous carbon is obtained. This is not preferable because it causes problems such as being unable to be manufactured at low cost.

(C) Method for producing a resin composition from a thermoplastic resin and a thermoplastic carbon precursor The method for producing a resin composition from a thermoplastic resin and a thermoplastic carbon precursor is based on a method of kneading both in a molten state. Is preferred. For melt-kneading the thermoplastic resin and the thermoplastic carbon precursor, a known apparatus can be used as necessary. For example, a single-screw extruder, a twin-screw extruder, a mixing roll, a Banbury mixer, or the like is used. be able to. Of these, the same-direction twin screw extruder is preferably used for the purpose of favorably microdispersing the thermoplastic carbon precursor in the thermoplastic resin. The melt kneading temperature is preferably 100 to 400 ° C. When the melt kneading temperature is less than 100 ° C., the thermoplastic carbon precursor is not in a molten state, and micro-dispersion with the thermoplastic resin is difficult, which is not preferable. On the other hand, when the temperature exceeds 400 ° C., decomposition of the thermoplastic resin and the thermoplastic carbon precursor may proceed, which is not preferable. A more preferable range of the melt kneading temperature is 150 ° C to 350 ° C. The melt kneading time is preferably 0.5 to 20 minutes, more preferably 1 to 15 minutes. When the melt kneading time is less than 0.5 minutes, it is not preferable because micro dispersion of the thermoplastic carbon precursor is difficult. On the other hand, if it exceeds 20 minutes, the productivity of the ultrafine fibrous carbon is remarkably lowered, which is not preferable.

  In the preferable production method described above, when a resin composition is produced by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor, it is preferably melt-kneaded in a gas atmosphere having an oxygen content of less than 10% by volume. The thermoplastic carbon precursor used in the production method is thermally denatured by reacting with oxygen and becomes infusible, which may inhibit micro-dispersion in the thermoplastic resin. In order to avoid this, it is preferable to carry out melt-kneading under conditions where an inert gas is circulated and the oxygen gas content is reduced as much as possible. The oxygen gas content during melt kneading is more preferably less than 5% by volume, and further preferably less than 1% by volume.

  The resin composition obtained by the above method preferably has a dispersion diameter of the thermoplastic carbon precursor in the thermoplastic resin of 0.01 to 50 μm. In the resin composition, the thermoplastic carbon precursor forms an island phase and becomes spherical or spheroid. The dispersion diameter here means the spherical diameter of the thermoplastic carbon precursor contained in the resin composition or the major axis diameter of the spheroid. When the dispersion diameter of the thermoplastic carbon precursor in the thermoplastic resin deviates from the range of 0.01 to 50 μm, it becomes difficult to produce ultrafine fibrous carbon used for an electrode for a lithium ion secondary battery. There is. A more preferable range of the dispersion diameter of the thermoplastic carbon precursor is 0.01 to 30 μm.

  Moreover, the resin composition which consists of a thermoplastic resin and a thermoplastic carbon precursor hold | maintains this for 3 minutes at 300 degreeC, Then, the dispersion diameter in the thermoplastic resin of a thermoplastic carbon precursor will be 0.01-50 micrometers. It is preferable. Generally, when a resin composition obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor is kept in a molten state, a phenomenon in which the thermoplastic carbon precursor aggregates with time is observed. If the dispersion diameter of the thermoplastic carbon precursor exceeds 50 μm due to the aggregation of the thermoplastic carbon precursor, it may be difficult to produce ultrafine fibrous carbon used for an electrode for a lithium ion secondary battery. . The degree of agglomeration rate of the thermoplastic carbon precursor varies depending on the type of the thermoplastic resin and the thermoplastic carbon precursor to be used, but more preferably at 300 ° C. for 5 minutes, more preferably at 300 ° C. for 10 minutes or more. It is preferable to maintain a dispersion diameter in the range of 01 to 50 μm.

(D) Method of Forming Resin Composition to Form Precursor Molded Body The resin composition obtained as described above is then molded into an appropriate molded body preferably at a temperature of 100 to 400 ° C. The shape of the molded body is not particularly limited, but is preferably fibrous from the viewpoint of handling. In addition, the fibrous form said here refers to the form of fiber diameter 0.5micrometer-300 micrometers, and the length of 1 mm or more of a fiber axial direction.

  In the case of forming a fibrous precursor molded body, a method of obtaining a precursor molded body in the form of a composite fiber containing a thermoplastic carbon precursor by melt spinning the melt-kneaded resin composition from a spinneret, etc. be able to. The spinning temperature at the time of melt spinning is preferably 150 to 400 ° C, more preferably 180 to 400 ° C, and further preferably 230 to 400 ° C. The spinning take-up speed is preferably 10 to 2,000 m / min.

  If it deviates from the above range, it is not preferable because it is not possible to obtain ultrafine fibrous carbon preferable for an electrode for a lithium ion secondary battery. When melt-spinning a resin composition obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor from a spinneret, it is preferable to send the solution in a molten state and melt-spin from the spinneret. The transfer time from the melt-kneading of the thermoplastic resin and the thermoplastic carbon precursor to the spinneret is preferably within 10 minutes. In addition, a method of melt spinning the melt-kneaded resin composition by a melt blow method can be suitably employed for forming the molded body into a fibrous form.

  A method of further stretching the thermoplastic carbon precursor contained in the precursor molded body by stretching a fibrous molded body in a molten state or a softened state can also be preferably employed. This treatment is preferably performed at 100 to 400 ° C, more preferably at 150 to 380 ° C.

(E) Method of stabilizing a thermoplastic carbon precursor contained in a precursor molded body to form a stabilized precursor molded body Next, a thermoplastic carbon precursor contained in a precursor molded body obtained as described above The body is stabilized to form a stabilized precursor compact. If the thermoplastic resin removal step, which is the next step, is performed without carrying out this stabilization step, problems such as thermal decomposition or fusion of the thermoplastic carbon precursor may occur in this step. Yes, not preferred. Stabilization can be performed by a known method such as an infusibilization treatment in a gas stream of air, oxygen, ozone, nitrogen dioxide, halogen, etc., or a solution treatment of an acidic aqueous solution. Infusibilization treatment in is preferred. The gas to be used is preferably a single gas of air or oxygen or a mixed gas containing these for ease of handling. A specific method for infusibilization under a gas stream is preferably 50 to 350 ° C., more preferably 60 to 300 ° C., preferably 5 hours or less, more preferably about 0.5 to 3.5 hours. It is preferable to use a method of exposing to a predetermined gas atmosphere for a period of time.

  Due to the infusibilization, the softening point of the thermoplastic carbon precursor contained in the precursor compact is significantly increased. The softening point after the rise is preferably 400 ° C. or higher, more preferably 500 ° C. or higher, for the purpose of obtaining desired ultrafine fibrous carbon.

(F) Method of forming a fibrous carbon precursor by removing the thermoplastic resin from the stabilized precursor molded body Next, the thermoplastic resin contained therein is removed from the stabilized precursor molded body to obtain a fine fibrous form. Only the carbon precursor is separated.
In this step, it is necessary to suppress thermal decomposition of the ultrafine fibrous carbon precursor as much as possible, decompose and remove the thermoplastic resin, and separate only the fibrous carbon precursor. Examples of the method for decomposing / removing the thermoplastic resin include a method for melting / removing the thermoplastic resin with an appropriate solvent, a method for decomposing / removing the thermoplastic resin by thermal decomposition, and the like.

  Examples of the solvent that can be used in the former method include cyclohexane, hexane, toluene, xylene, decalin, and trichlorobenzene. The treatment with a solvent can be carried out preferably by dipping at 50 to 250 ° C., more preferably 60 to 210 ° C., preferably for 10 to 120 minutes, more preferably for 20 to 60 minutes.

  In the latter method, the thermoplastic resin contained in the stabilized resin composition is removed by thermal decomposition. Specifically, the thermoplastic resin contained in the stabilized resin composition is removed, and only the stabilized fibrous carbon precursor is separated to form the fibrous carbon precursor. In this step, it is necessary to suppress the thermal decomposition of the fibrous carbon precursor as much as possible, to decompose and remove the thermoplastic resin, and to separate only the fibrous carbon precursor.

  The removal of the thermoplastic resin is preferably performed under reduced pressure. By performing the process under reduced pressure, the thermoplastic resin can be efficiently removed, and in the subsequent carbonization or graphitization process of the fibrous carbon precursor, a carbon fiber with less fusion between the fibers is produced. Can do. The lower the atmospheric pressure when removing the thermoplastic resin, the better. However, it is difficult to achieve a complete vacuum, and it is preferably 0.01 to 50 kPa, more preferably 0.01 to 30 kPa, and More preferably, the pressure is 01 to 10 kPa, and particularly preferably 0.01 to 5 kPa. When removing the thermoplastic resin, a small amount of oxygen or inert gas may be present if the above atmospheric pressure is maintained. In particular, if a small amount of inert gas is present, the thermoplastic resin may melt due to thermal degradation. This is preferred because it has the advantage of suppressing wearing. The trace amount of oxygen referred to here refers to oxygen having an oxygen concentration of 30 volume ppm or less, and the inert gas atmosphere refers to a gas such as carbon dioxide, nitrogen, or argon having a volume of 20 volume ppm or less. In order to remove the thermoplastic resin, it is necessary to perform a heat treatment under reduced pressure, but it is preferable to remove the heat treatment at a temperature of 350 ° C. or higher and lower than 600 ° C. The heat treatment time is preferably 0.5 to 10 hours.

  The removal of the thermoplastic resin can also be performed in an inert gas atmosphere. When removing the thermoplastic resin under an inert gas atmosphere, it is necessary to remove the thermoplastic resin at a temperature of 350 ° C. or higher and lower than 600 ° C. In addition, the inert gas atmosphere said here refers to gas, such as a carbon dioxide, nitrogen, argon, of oxygen concentration 30 ppm or less, More preferably 20 ppm or less. As the inert gas used in this step, carbon dioxide and nitrogen can be preferably used because of cost, and nitrogen is particularly preferable. When the temperature at which the thermoplastic resin contained in the stabilizing resin composition is removed is less than 350 ° C., the thermal decomposition of the fibrous carbon precursor can be suppressed, but the thermoplastic resin cannot be sufficiently decomposed, which is not preferable. . On the other hand, if it is 600 ° C. or higher, the thermal decomposition of the thermoplastic resin can be sufficiently performed, but the thermal decomposition of the fibrous carbon precursor also occurs, and as a result, the carbon of the carbon fiber obtained from the thermoplastic carbon precursor This is not preferable because it reduces the conversion yield. The temperature for decomposing the thermoplastic resin contained in the stabilizing resin composition is preferably 380 to 550 ° C. in an inert gas atmosphere, and is preferably treated for 0.5 to 10 hours.

(G) Method for producing ultrafine fibrous carbon from a fibrous carbon precursor The fibrous carbon precursor excluding the thermoplastic resin is carbonized in an inert gas atmosphere to obtain the fibrous carbon in the present invention. be able to. This treatment can be performed by, for example, heat treatment in a graphite crucible.

  Here, before the heat treatment, it is also effective to add boron, which is a graphitization catalyst having a function of promoting the degree of graphitization, to the fibrous carbon precursor. The addition amount of the catalyst is not particularly limited. However, if the addition amount is too small, the effect is not exhibited. On the other hand, if the addition amount is too large, the resulting ultrafine fiber carbon remains as an impurity, which is not preferable. A preferable addition amount is 1,000 ppm or less, and more preferably 100 to 1,000 ppm.

  The carbonization temperature is preferably 500 to 1,500 ° C., more preferably 500 to 1,000 ° C., and even more preferably 500 to 900 ° C. as the maximum temperature. More preferably, it is 600-800 degreeC. By performing this treatment, it is possible to form a carbon structure that is useful as an ultrafine fibrous carbon used for an electrode for a lithium ion secondary battery.

  The carbonization treatment may be performed by continuously increasing the temperature and holding it at the reached temperature for a certain time, or by increasing the temperature stepwise and holding it at each reached temperature for a certain time. Good.

  The carbonization treatment time is preferably 0.1 to 24 hours, more preferably 0.2 to 10 hours, and further preferably 0.5 to 8 hours. Here, the time for holding the fibrous carbon precursor at a temperature of 2,000 ° C. or higher, preferably 2,600 ° C. or higher is preferably 0.1 to 3 hours, more preferably 0.5 to 2 hours. It's time.

Examples of the inert gas used in the carbonization of the fibrous carbon precursor include nitrogen and argon. From the viewpoint of being inexpensive, nitrogen is preferable, and there are few side reactions. Is preferably argon.
In addition, the oxygen concentration in the inert gas at the time of carbonization is preferably 20 volume ppm or less, and more preferably 10 volume ppm or less.

(H) Pulverization method Next, the pulverization method will be described. In order to produce the electrode material of the present invention, pulverization may be performed. The step of performing the pulverization method is not particularly limited, but “a method of forming a fibrous carbon precursor by removing a thermoplastic resin from a stabilized precursor molded body”, “an ultrafine from a fibrous carbon precursor”. It is preferably carried out at any or all after performing the “method for producing fibrous carbon”.

  As a pulverization method, it is preferable to apply a fine pulverizer such as a jet mill, a ball mill, or a bead mill, and classification is performed as necessary after pulverization. Wet pulverization in which activated carbon is dispersed in a dispersion medium is preferable because finely pulverized activated carbon can be easily obtained in a short time. In the case of wet pulverization, the dispersion medium is removed after pulverization. At this time, if secondary aggregation becomes significant, subsequent handling becomes very difficult. In such a case, it is preferable to perform a crushing operation using a ball mill or a jet mill after drying. In this way, it is possible to produce ultrafine fibrous carbon used as a carbon-based conductive additive for the electrode for the lithium ion secondary battery of the present invention.

  Hereinafter, a method for producing the lithium ion secondary battery of the present invention, and a separator and a non-aqueous electrolyte constituting the lithium ion secondary battery will be described.

[Method for producing electrode for lithium ion secondary battery]
The following two methods are generally used as a method for producing an electrode of a lithium ion secondary battery.
One method is a method in which activated carbon, a conductive additive, and a binder are mixed and kneaded, formed into a film by extrusion molding, rolled, stretched, and then bonded to a current collector. The other method is a method in which activated carbon, a conductive additive, a binder, and a solvent for dissolving the binder are mixed to prepare a slurry, this slurry is applied onto a current collector, and the solvent is removed, followed by pressing.

In the present invention, either method is possible, but since the latter method is suitable, the latter method will be described in detail below.
The binder can be suitably used as long as it can be formed into an electrode and has sufficient electrochemical stability. Examples of the binder include polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), synthetic butadiene rubber (SBR), a cross-linked fluoroolefin copolymer, polyimide, petroleum pitch, It is preferable to use at least one selected from the group consisting of coal pitch, phenol resin, and the like, and polyvinylidene fluoride (PVDF) is particularly preferable.

  The shape when used as a binder is not particularly limited, and may be solid or liquid (for example, emulsion), electrode manufacturing method (especially dry kneading or wet kneading), electrolyte solution, In consideration of solubility and the like, it can be appropriately selected.

  The solvent for dissolving the binder is not particularly limited as long as it dissolves the binder. Specific examples include one or more selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and the like. Or DMAc is preferable.

  Further, the addition ratio of the binder is not particularly limited, but in the case of the positive electrode, it is preferably 1 to 20% by mass with respect to the electrode material of the present invention. More preferably, it is 3 to 15% by mass. Moreover, in the case of a negative electrode, it is preferable that it is 1-20 mass% with respect to the material which has a redox capacity. More preferably, it is 5 to 15% by mass.

  When producing an electrode, since the dispersion state in a slurry is bad, it may be difficult to ensure the fluidity suitable for application | coating. In such a case, a slurrying aid may be used. Examples of the slurrying aid include one or more selected from the group consisting of polyvinylpyrrolidone, carboxymethylcellulose, polyvinyl acetate, polyvinyl alcohol and the like, and it is particularly preferable to use polyvinylpyrrolidone. It is. By adding the above-mentioned slurrying aid, sufficient fluidity can be secured even with a relatively small amount of solvent, and the dispersibility of the finely pulverized activated carbon is remarkably improved. Moreover, the generation of cracks after removal of the solvent can be reduced. The addition amount of the slurrying aid is preferably 10% by mass or less, more preferably 0.5 to 10% by mass with respect to the total of components other than the solvent in the slurry, More preferably, it is -8 mass%. On the other hand, when the amount of the slurrying aid added is more than 10% by mass, the slurry viscosity is suddenly lowered, and a dispersion failure may occur, making it difficult to produce a suitable slurry. Moreover, when this value is less than 0.5 mass%, the effect of the slurrying aid does not appear.

  The solid content concentration in the slurry (referring to the ratio of the total weight of components other than the solvent of the slurry to the total mass of the slurry) is preferably 10 to 30% by mass, more preferably 15 to 25% by mass. is there.

As the current collector for forming the electrode layer, an aluminum foil is suitable. The thickness of the current collector is preferably in the range of 10 to 50 μm.
In order to apply the slurry onto the current collector as described above, an appropriate application method such as a doctor blade can be employed. After application, the solvent is removed, for example, by treating at 60 to 100 ° C., preferably 75 to 85 ° C., preferably for 60 to 180 minutes. Then, an electrode layer can be manufactured on a current collector by pressing the coated product after removing the solvent. This press is more preferably performed under a pressure of 10 to 30 Pa, preferably 1 to 5 minutes.

  In the lithium ion secondary battery of the present invention, the thickness of the electrode layer is preferably in the range of 5 to 300 μm. When the electrode layer thickness is less than 5 μm, when trying to manufacture an arbitrary capacity cell, a large amount of separator and current collector are used, and the volume occupancy of the electrode layer in the cell decreases, resulting in an energy density. It is not preferable from the viewpoint, and the use is considerably limited. In particular, output characteristics (including low-temperature characteristics) are also important, but it becomes difficult to apply to power supply applications requiring high energy density.

  On the other hand, since it is relatively difficult to produce an electrode having an electrode thickness exceeding 300 μm due to problems of crack generation and electrode peeling, the electrode thickness is preferably about 300 μm or less from the viewpoint of stable production of the electrode. . In order to produce a more stable electrode, the electrode thickness is more preferably 200 μm or less, and for the purpose of increasing the productivity of the electrode and the output characteristics of the capacitor, a more preferable range of the electrode thickness is 10 to 10. 100 μm.

[Separator]
As the shape of the separator used in the lithium ion secondary battery of the present invention, a known shape such as a paper shape (film shape) or a porous membrane shape can be suitably employed. One or more materials selected from the group consisting of aromatic polyamide, aliphatic polyimide, polyolefin, Teflon (registered trademark), polyphenylene sulfide, and the like can be suitably used. Of these, cellulose paper, aromatic polyamide or aliphatic polyimide porous membrane is particularly preferable from the viewpoint of heat resistance and thinning. The thickness of the separator is preferably about 20 to 100 μm from the viewpoint of preventing a short circuit, but in the present invention, a separator having a thickness of about 5 to 20 μm that is sufficiently thinner than a conventional separator can be applied. When the thin separator is used, the output is improved because the internal resistance derived from the separator is reduced, and the energy density of the cell is also improved.

[Non-aqueous electrolyte]
In the lithium ion secondary battery of the present invention, a non-aqueous electrolyte is applied as the electrolyte. In general, non-aqueous electrolytes are characterized by a higher withstand voltage and higher energy density than aqueous electrolytes.

  As such a non-aqueous solvent, known ones can be suitably used. More specifically, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, γ-butyrolactone, γ- Valerolactone, acetonitrile, nitromethane, methoxyacetonitrile, nitroethane, N, N-dimethylformamide, 3-methoxypropionitrile, N-methylpyrrolidone, N, N'-dimethylimidazolidinone, dimethyl sulfoxide, sulfolane, 3-methylsulfolane , Ethyl methyl carbonate and the like can be mentioned, and one of these may be used alone, or two or more may be mixed and used. It is important that the solvent used in the electrolytic solution has an appropriate boiling point, melting point, viscosity, and relative dielectric constant. From such a viewpoint, among these, those mainly composed of propylene carbonate or γ-butyrolactone are mainly used. Preferably used.

Examples of the electrolyte used in the lithium ion secondary battery of the present invention include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , and lower aliphatic carboxylic acid. Lithium, LiCl, LiBr, LiI, chloroborane lithium, borates, imide salts, and the like can be used. Examples of borates include lithium bis (1,2-benzenediolate (2-)-O, O ') and bis (2,3-naphthalenedioleate (2-)-O, O') boric acid. Lithium, bis (2,2′-biphenyldiolate (2-)-O, O ′) lithium borate, bis (5-fluoro-2-olate-1-benzenesulfonic acid-O, O ′) lithium borate Etc. Examples of the imide salts include lithium bistrifluoromethanesulfonate imide ((CF ₃ SO ₂ ) ₂ NLi), lithium trifluoromethanesulfonate nonafluorobutanesulfonate (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) ), Lithium bispentafluoroethanesulfonate imide ((C ₂ F ₅ SO ₂ ) ₂ NLi), and the like. The concentration of the electrolyte is preferably 0.5 to 2 mol / L. One of the above electrolytes may be used alone, or two or more electrolytes may be used in combination.

As an electrolytic solution used in the lithium ion secondary battery of the present invention, an ionic liquid typified by ethylmethylimidazolium salt can also be suitably used. In this case, the electrolyte must be dissolved in the above nonaqueous solvent. There is no need to use it.
The electrical conductivity at 25 ° C. of the electrolytic solution used in the lithium ion secondary battery of the present invention is preferably 1 × 10 ⁻² S / cm or more.

  As an electrolytic solution used in the lithium ion secondary battery of the present invention, for example, an electrolyte composed of the quaternary ammonium salt is used, and at least one nonaqueous solvent selected from the group consisting of propylene carbonate, dimethyl carbonate, ethylene carbonate, and sulfolane. What was melt | dissolved in can be illustrated as a preferable thing.

<Embodiment of lithium ion secondary battery>
Embodiments of the lithium ion secondary battery of the present invention will be described below.
The cell shape of the lithium ion secondary battery of the present invention is not particularly limited, and can be implemented in any shape. Specifically, for example, cell shapes such as a button shape, a cylindrical shape, and a square shape can be given.

  Further, it is also preferable to have an internal configuration in which a plurality of pairs of positive and negative electrodes and separators are laminated. In this case, a known stack lamination type, wound type, folded lamination type, or the like can be adopted.

Examples of the exterior material of the lithium ion secondary battery of the present invention include a metal can and an aluminum laminate resin film.
The lithium ion secondary battery of the present invention can be suitably implemented in any of the above embodiments.

EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention does not receive any limitation by this.
Various measurements and analyzes in the examples were performed according to the following methods.

(1) Precursor compact, measurement of fiber diameter and fiber length of ultrafine fibrous carbon, and confirmation of shape of other carbon-based conductive aids Observation using a scanning electron microscope (S-2400, manufactured by Hitachi, Ltd.) And took photos. The average fiber diameter of ultrafine fiber carbon, etc. was measured by randomly selecting 20 locations from the obtained electron micrograph and averaging the average values of all the measurement results (n = 20). The fiber diameter was used. The average fiber length was similarly calculated.

(2) X-ray diffraction measurement of ultra-fine fibrous carbon X-ray diffraction measurement is based on RINT-2100 manufactured by Rigaku Corporation and measures the lattice spacing (d002) and crystallite size (Lc002) according to JIS R7651 method. did.

(3) Battery performance evaluation conditions Battery performance is 0.2C, 4.2V, 8 hours constant current / constant voltage charge, 0.2C, 2.75V cut-off constant current discharge at 25 ° C. Went. The discharge capacity obtained at the fifth cycle was defined as the initial capacity of this cell. Then, charging / discharging was repeated on said conditions, the capacity maintenance rate was computed, and battery performance was evaluated by the value.

[Example 1]
<Manufacture of ultra-fine fibrous carbon (carbon-based conductive additive)>
High-density polyethylene (HI-ZEX (registered trademark) 5000SR, manufactured by Prime Polymer Co., Ltd .; 350 ° C., melt viscosity 14 Pa · s at 600 s ⁻¹ ) 90 parts by mass as a thermoplastic resin, and synthetic mesofe as a thermoplastic carbon precursor -Melt and knead 10 parts by mass of pitch pitch AR / MPH (Mitsubishi Gas Chemical Co., Ltd.) with the same direction twin screw extruder ("TEM-26SS" manufactured by Toshiba Machine Co., Ltd., barrel temperature 310 ° C, under nitrogen stream). A resin composition was prepared.

  The resin composition was spun from a spinneret at 390 ° C. with a cylinder-type single-hole spinning machine to prepare a precursor molded body (a sea-island type composite fiber containing a thermoplastic carbon precursor as an island component). The fiber diameter of this precursor molded body was 300 μm. Next, the precursor molded object was hold | maintained at 215 degreeC in the air for 3 hours with the hot air dryer, and the stabilized precursor molded object was obtained.

  Next, the above-mentioned stabilized precursor compact is subjected to nitrogen replacement in a vacuum gas replacement furnace, and then the pressure is reduced to 1 kPa, and the temperature is increased to 500 ° C. at a temperature increase rate of 5 ° C./min. By holding at 500 ° C. for 1 hour, the thermoplastic resin was removed to form a fibrous carbon precursor. This fibrous carbon precursor was added to ion-exchanged water and pulverized with a mixer for 2 minutes to prepare a pre-dispersed liquid in which an ultrafine fibrous carbon precursor having a concentration of 0.1% by weight was dispersed. This preliminary dispersion was treated 10 times with a wet jet mill (Sugino Machine Co., Ltd., Starburst Lab HJP-17007, used chamber: single nozzle chamber) with a nozzle diameter of 0.17 mm and a processing pressure of 100 MPa. By repeating, a liquid in which the fibrous carbon precursor was dispersed was produced. Subsequently, the obtained solvent liquid was filtered to produce a nonwoven fabric in which the fibrous carbon precursor was dispersed.

  The nonwoven fabric in which the fibrous carbon precursor was dispersed was heated from room temperature to 2800 ° C. in 3 hours in an argon gas atmosphere to produce ultrafine fibrous carbon. The obtained ultrafine fiber carbon had an average fiber diameter of 300 nm and an average fiber length of 16 μm, and no branched structure was observed. Further, the average interplanar spacing d002 of the (002) plane measured by the X-ray diffraction method was 0.3375 nm. An electron micrograph taken here is shown in FIG.

<Manufacture of negative electrode>
2 parts by mass of ultrafine fibrous carbon (carbon-based conductive additive) produced as described above, 91 parts by mass of a negative electrode active material (scale-like graphite; manufactured by Hitachi Chemical Co., Ltd., trade name MAGD), and polyvinylidene fluoride as a binder ( A slurry was prepared by using 7 parts by mass of N-methylpyrrolidone as a solution. The prepared slurry was applied, dried, and roll pressed to prepare a negative electrode. The electrode thickness was 75 μm and the electrode density was 1.5 g / cm 3.

<Production of positive electrode>
89 parts by mass of lithium cobaltate (LiCoO ₂ , manufactured by Nippon Chemical Industry Co., Ltd.) as a positive electrode active material, 6 parts by mass of polyvinylidene fluoride as a binder, acetylene black (trade name Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, and as a solution A slurry was prepared by using N-methylpyrrolidone. The produced slurry was applied, dried, and roll pressed to produce a positive electrode. The electrode thickness was 82 μm and the electrode density was 3.0 g / cm ³ .

<Manufacture of cells>
A polyethylene porous film is used for the positive electrode, the negative electrode, and the separator prepared as described above, and is composed of a mixed solution of ethylene carbonate and ethyl methyl carbonate (3/7 mass ratio, manufactured by Kishida Chemical Co., Ltd.) containing 1 mol / L LiPF6. An electrolyte solution was injected into the cell to produce a single layer laminate cell.

<Evaluation of battery performance>
As a result of evaluating the battery performance of the lithium ion secondary battery produced by the above procedure, the capacity retention rate at the 50th time was as good as 92.2%.

[Comparative Example 1]
The electrode and the lithium ion secondary battery were operated in the same manner as in Example 1 except that vapor grown carbon fiber (VGCF, manufactured by Showa Denko) was used instead of the ultrafine fibrous carbon of Example 1. The battery performance was also evaluated by manufacturing (the thickness of the negative electrode (75 μm) and the electrode density (1.5 g / cm ³ ) were also matched to Example 1). The capacity retention rate at the 50th time was inferior at 90.7%.

  The lithium ion secondary battery electrode of the present invention and the lithium ion secondary battery using the lithium ion secondary battery electrode are suitably used in a wide variety of fields such as power supplies for various portable devices and power supplies for electric vehicles. Is possible.

Claims (2)

An electrode for a lithium ion secondary battery comprising an electrode active material capable of inserting and extracting lithium ions, a carbon-based conductive additive and a binder,
The electrode for a lithium ion secondary battery, wherein the carbon-based conductive additive is ultrafine fibrous carbon having an average fiber diameter of 10 to 900 nm that does not have a branched structure.   The lithium ion secondary battery which contains the electrode for lithium ion secondary batteries of Claim 1 as a component.