JP2002237302A - Lithium ion secondary battery wherein natural product- originated carbon is used as negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery wherein an inexpensive and a new carbonaceous material which lithium ion is reversibly inserted into and discharged from is used as a negative electrode. SOLUTION: By using the natural product-originated carbon, especially by using bamboo Bincho charcoal as the negative electrode material, the new and inexpensive lithium ion secondary battery can be realized which is superior in a charge-discharge capacity and stability of repetitive uses, and which has the same high-performance as that of a lithium ion secondary battery using a conventional carbonaceous material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery using a carbon derived from a natural product for a negative electrode.

[0002]

2. Description of the Related Art In recent years, portable devices such as portable telephones, laptop computers, and camera-integrated VTRs have formed a large market. There is a strong demand for a lightweight, compact, and high energy density secondary battery as a power supply for these portable devices. Lithium-ion secondary batteries are superior to other secondary batteries in these respects, and active research and development are underway. Secondary batteries using lithium metal for the negative electrode have been developed for consumer use at one time for communication equipment, but when charged, dendrites (resin-like crystals) that precipitate on the negative electrode surface or There has been an important problem that the safety and battery performance of the battery are impaired due to the generation of granular lithium and the like. For this reason, Japanese Unexamined Patent Publication No.
Various measures have been taken, such as using a lithium alloy for the negative electrode as disclosed in Japanese Patent No. 286864, and examining an electrolytic solution as disclosed in Japanese Patent Application Laid-Open No. 4-56078.

On the other hand, in order to improve this, it has been found that lithium is inserted between layers in a charge / discharge process, and a carbonaceous material capable of preventing deposition of lithium metal is used as a negative electrode. Among them, as shown in JP-A-4-237949, polymerized carbide, coke, carbon fiber, coal and petroleum pitch fired material, graphitic carbon such as mesocarbon microbeads, lower crystallinity and specific gravity,
Carbonaceous materials defined by Raman spectroscopy, specific surface area and other properties have been proposed. Also, JP-A-57-2080
No. 79 discloses that graphite having the highest crystallinity is preferably used as a carbonaceous material. However, since graphite uses intercalation of lithium ions into graphite crystals as the principle of charge and discharge, 372 mA calculated from the maximum lithium-introducing compound LiC ₆ is used.
It is reported that a capacity of at least h / g cannot be obtained, whereas a carbonaceous material calcined at 950 ° C. or lower and having a very small amount of crystallized portion exhibits a capacity larger than the theoretical capacity of graphite of 372 mAh / g, It is receiving attention as a capacity increase method.

On the other hand, research and development of such a high-performance lithium-ion secondary battery at a low price have been earnestly conducted.
In particular, LiMn ₂ O ₄ as a positive electrode material has succeeded in reducing the price of the positive electrode active material to one tenth or less at a stretch compared to conventional LiCoO ₂ , and has been partly put to practical use. However, there have been almost no examples of studies on materials for negative electrodes from the viewpoint of cost. As described above, as the negative electrode material capable of reversibly removing and inserting lithium ions, graphitic carbon such as polymer carbide, coke, carbon fiber, fired coal and petroleum pitch, and mesocarbon microbeads has been proposed and used. Has been
In either case, the raw materials are expensive or the processing temperature during production is high, raising the cost. As described above, there has been a long-awaited demand for a new negative electrode material for a lithium ion secondary battery, which is an inexpensive material capable of reversibly inserting and removing lithium ions.

[0005]

SUMMARY OF THE INVENTION It is a first object of the present invention to provide a novel material for a negative electrode capable of deintercalating lithium ions. A second object of the present invention is to provide an inexpensive negative electrode material for a lithium ion secondary battery. A third object of the present invention is to provide a lithium ion secondary battery having excellent repeated use stability using such a novel material.

[0006]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have repeatedly studied mainly on a novel negative electrode material. As a result, (1) at least the positive electrode, the non-aqueous electrolyte and the current collector are electrochemically occluded with lithium ions.
A lithium ion secondary battery comprising a negative electrode using a releasable natural product-derived carbon as an active ingredient, and (2) the natural product-derived carbon is bamboo charcoal ( The inventors have found that the lithium ion secondary battery described in 1) solves the above-mentioned problems, and have completed the present invention.

That is, the present inventors have found for the first time that carbon derived from a natural product can insert and remove lithium ions, and have succeeded in using the carbon as a negative electrode for a lithium ion secondary battery. Such a carbon material is completely novel as a material for a lithium ion secondary battery. In addition, many of these materials can be obtained at a lower cost than carbon materials conventionally used for negative electrodes for lithium ion secondary batteries, and contribute to lowering the price of lithium ion secondary batteries. In addition, the present inventors have found that some of the carbons derived from natural products are not inferior in charge / discharge characteristics and repetition characteristics as compared with carbon materials that have been put into practical use, and have completed the present invention. Was.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon derived from a natural product according to the present invention is:
It is a substance whose main component is carbon obtained by carbonizing plants, fungi, animals and the like as raw materials. Specifically, in the case of plants, leaves, stems, branches, roots, petals, fruits, and seeds are used as raw materials, and cellulose and oils obtained by processing or extracting them are also used as raw materials. Fungi are all raw materials. For animals, starch and oils and fats can be raw materials. A carbon material derived from a natural product can be obtained by carbonizing these natural products. High pressure,
The heat treatment can be carried out under normal pressure or reduced pressure and under an oxygen partial pressure of 10 ⁻⁷ to 10 ⁵ Pa. The heating temperature is
Considering economic efficiency, the temperature is desirably 3200 ° C. or lower, preferably 1300 ° C. or lower.

The carbon derived from natural products thus obtained is different from carbon materials such as graphite and carbon black which have been conventionally proposed as negative electrode materials for lithium ion secondary batteries, and is a novel material.

The natural product-derived carbon obtained as described above must be pulverized and classified to have an average particle size of 5 to 100 μm, preferably 10 to 50 μm, in order to use it as a material for a negative electrode of the present invention. For this, a ball mill,
A carbon material having an appropriate particle size and particle size distribution can be obtained by using a known pulverizing device such as an attritor, a vibration mill, a planetary mill, and a jet mill and a classifier such as a sieve.

The method for manufacturing the battery of the present invention will be described below. The carbon material derived from a natural product described above is preferably used in an amount of 1 to 100 μm, more preferably an average particle size of 5 to 50 μm.
It is pulverized to a range of μm, a binder, a solvent, and the like are added to the pulverized material to form a slurry. The slurry is applied to a metal current collector substrate such as a copper foil and dried to form an electrode. Further, the electrode material can be directly formed into an electrode shape by a method such as roll forming or compression forming.

As the current collector, any known materials may be used as long as they have electron conductivity, but non-corrosive materials are particularly preferable. Examples of such a material include conductive carbon, gold, platinum, and stainless steel, and stainless steel is preferably used. The shape of the current collector is practically a foil or a porous body.

The binder usable for the above purpose includes:
Stable against electrolyte, polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide,
Resin-based polymers such as cellulose, rubber-like polymers such as styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, styrene / butadiene /
Styrene block copolymers, hydrogenated products thereof, styrene / isoprene / styrene block copolymers, thermoplastic elastomeric polymers such as hydrogenated products thereof, syndiotactic 12-polybutadiene, ethylene / vinyl acetate copolymer, propylene .Alpha.-olefins (2 to 1 carbon atoms)
2) The ion conductivity of soft resinous polymers such as copolymers, fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, and polytetrafluoroethylene / ethylene copolymers, alkali metal ions, particularly lithium ions. Polymer composition having the same.

As the above-mentioned polymer having ion conductivity,
Is polyethylene oxide, polypropylene oxide, etc.
Of polyether polymer compounds and polyether compounds
Crosslinked polymer, polyepichlorohydrin, polyphosph
Azen, polysiloxane, polyvinylpyrrolidone, poly
For high-density vinylidene carbonate, polyacrylonitrile, etc.
Lithium salt or lithium-based molecular compound
Or a mixture of alkali metal salts
High levels of propylene, ethylene carbonate, γ-butyrolactone, etc.
Using a system containing an organic compound having a high dielectric constant
Can be. Room temperature of such an ion conductive polymer composition
Preferably has an ionic conductivity of 10 ^-FiveS / cm or less
Above, more preferably 10^-3S / cm or more.

As a solvent for dispersing and coating the above-mentioned carbon material for a negative electrode, any solvent may be used.
Those capable of dissolving or dispersing the binder are preferably used. As such a solvent, an organic solvent alone such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, cyclohexanone, acetone, 2-butanone, methylisobutylketone, or a mixed solvent containing these solvents is preferred. used.

A coating liquid can be prepared by adding a binder to the above-mentioned natural material-derived carbon material, if necessary, and using the above-mentioned solvent and a known dispersing device and / or mixing device. The coating solution prepared in this manner is coated on the current collector using a known coating method such as blade coating, wire bar coating, spin coating, spray coating, dip coating, or bead coating. be able to.
In addition, an electrostatic spray deposition (ESD) method in which an electric field is applied between a heated current collector and a nozzle containing a coating liquid to simultaneously perform film formation and heat treatment is also preferably used. The liquid applied on the current collector is subjected to a heat treatment to form a negative electrode plate capable of electrochemically inserting and extracting lithium ions.

The lithium ion secondary battery is obtained by combining the negative electrode plate thus prepared, the electrolyte solution described below, and the positive electrode plate with other battery components such as a separator, a gasket, a current collector, a sealing plate, and a cell case. Constitute. The battery that can be manufactured is not particularly limited, such as a cylindrical type, a square type, a coin type, but basically, a current collector and a negative electrode material are placed on a cell floor plate, and an electrolyte and a separator are further placed thereon. A positive electrode is placed so as to face the negative electrode, and caulked together with a gasket and a sealing plate to form a secondary battery.

Non-aqueous solvents that can be used for the electrolyte include:
Propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, 1,
A single organic solvent such as 3-dioxolane or a mixture of two or more organic solvents can be used.

In these solvents, L of about 0.5 to 2.0 M is added.
iClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃
An electrolyte such as LiAsF ₆ is dissolved to form an electrolyte.
Alternatively, a solid polymer electrolyte which is a conductor of an alkali metal cation such as lithium ion can be used. The material of the positive electrode body is not particularly limited, but is preferably made of a metal chalcogen compound that can occlude and release alkali metal cations such as lithium ions during charge and discharge. Such metal chalcogen compounds include vanadium oxide, vanadium sulfide, molybdenum oxide, molybdenum sulfide, manganese oxide, chromium oxide, titanium oxide, titanium sulfide and the like. Composite oxides and composite sulfides. Preferably, Cr ₃ O
_{8, V 2 O 5, V} 5 O 13, VO 2, Cr 2 O 5, MnO
_{2, TiO 2, MoV 2 O} 8, TiS 2 V 2 S 5 Mo
S ₂ , MoS ₃ VS ₂ , Cr _0.25 V _0.75 S ₂ , Cr _0.5
V _0.5 S _{2 and the} like. LiMY ₂ (M is Co,
Transition metals Y such as Ni are chalcogen compounds such as O and S),
Oxides such as LiM ₂ Y ₄ (M is Mn and Y is O) and WO ₃ , sulfides such as CuS, Fe _0.25 V _0.75 S ₂ and Na _0.1 CrS ₂ , phosphorus and sulfur compounds such as NiPS ₃ and FePS ₃ , Selenium compounds such as VSe ₂ and NbSe ₃ can also be used. Like the negative electrode material, these are mixed with a binder and applied on a current collector to form a positive electrode plate.

The separator holding the electrolyte is generally a material having excellent liquid retention properties. For example, the separator is impregnated with the above electrolyte using a nonwoven fabric or a porous film of a polyolefin resin. Is expressed.

[0021]

Next, the present invention will be described in detail with reference to examples. However, the examples are for describing the present invention in detail, and it is refused that the present invention is not limited by these examples. Not even.

Example 1 Commercially available bamboo charcoal made of bamboo was lightly pulverized with a planetary mill and then sieved to obtain particles having a diameter of 35 μm. The particles were immersed in an electrolytic solution containing 1 mol / l of lithium carbonate and 1: 1 by volume of ethylene carbonate and propylene carbonate, and the working electrode was a platinum-rhodium filament having a diameter of 35 μm and the counter electrode was a reference electrode. Using a lithium metal foil, a current-potential curve was measured by a potential scanning method. At this time, the working electrode was measured by using a micro operation type xyz stage in contact with target carbon particles. The measurement was performed in a dry box having a dew point of −50 ° C. or less. The details of the measurement method are described in Electrochemical and Solid State Letters, Vol. 3, No. 3, page 125.

FIG. 1 shows the current-potential curve (cyclic voltammogram) thus measured. 1V to 0V
In the potential region, insertion of lithium ions based on the negative current during the base potential scanning and desorption of the lithium ions based on the positive current during the noble potential scanning were observed. It has become clear for the first time that it can be inserted and removed. The negative current in FIG. 1 can be integrated over the potential scanning time to determine the amount of lithium ion insertion charge, and the positive current can be integrated over time to determine the amount of lithium ion desorption charge. From this charge amount Q, a change in discharge capacity represented by the following equation and a charge / discharge ratio are defined.

[0024]

[Formula 1] Change in discharge capacity = Q _Nth / Q _1st

[0025]

[Formula 2] Charge / discharge ratio = Q _discharge / Q _charge

In the above equation, Q _1st represents the charge discharged after charging in the first potential scan, and Q _Nth represents the charge discharged after charging in the N-th potential scan. Also, Q _charge and Q _{discha rge} represents a charge charged in each N-th potential scan, the after-discharge electric charge. Table 1 shows the values thus measured.

Comparative Example 1 Instead of the carbon particles used in Example 1, graphitized mesocarbon microbeads (2
The current-potential curve of the particles having a diameter of 30 μm was measured in the same manner as in Example 1 except that (calcination at 800 ° C.) was used. This material has conventionally been proposed for use as a negative electrode material for a lithium ion secondary battery. As a result of the measurement,
Very good reversible cyclic voltammograms were obtained. From this, it was found that lithium ions were reversibly inserted and removed from the carbon material, and it was proved that the carbon material has been suitably used in the past as a negative electrode material for lithium ion secondary batteries. Further, the change in the discharge capacity and the charge / discharge ratio were measured in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 2 Graphitized mesocarbon microbeads (1) were used instead of the carbon particles used in Example 1.
The current-potential curve of the particles having a diameter of 30 μm was measured in the same manner as in Example 1 except that calcination at 000 ° C. was used. This material has conventionally been proposed for use as a negative electrode material for a lithium ion secondary battery. As a result of the measurement,
As in FIG. 1, a cyclic voltammogram with good reversibility was obtained. From this, it was found that lithium ions were reversibly inserted and removed from the carbon material, and it was proved that the carbon material has been suitably used in the past as a negative electrode material for lithium ion secondary batteries. Further, the change in the discharge capacity and the charge / discharge ratio were measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 2 A current-potential curve of particles having a diameter of 35 μm was measured in the same manner as in Example 1 except that Bincho charcoal was used instead of the carbon particles used in Example 1. Was. As a result of the measurement, a cyclic voltammogram having good reversibility was obtained as in FIG. 1, and it became clear for the first time that Bincho charcoal was capable of continuously inserting and removing lithium ions. Further, the change in the discharge capacity and the charge / discharge ratio were measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 3 Except that the carbon particles used in Example 1 were replaced by bamboo ceramics,
The current-potential curve of particles having a diameter of 35 μm was measured in the same manner as in Example-1. As a result of the measurement, a cyclic voltammogram with good reversibility was obtained as in FIG. 1, and it was first clarified that the bamboo ceramics could continuously insert and remove lithium ions. Further, the change in the discharge capacity and the charge / discharge ratio were measured in the same manner as in Example 1, and the results are shown in Table 1.

[0031]

[Table 1]

From Table 1, it can be seen for the first time that the inexpensive natural-derived carbon used in the examples can reversibly insert and remove lithium ions, and that it can be used for the negative electrode of lithium ion secondary batteries. . In particular, from Table 1, it can be seen that the bamboo charcoal used in Example-1 is not inferior to the material of the comparative example conventionally proposed for the negative electrode of the lithium ion secondary battery. So, next,
An actual secondary battery cell was manufactured, and a charge / discharge test of the battery was performed.

9 parts by weight of the carbon material used in Example 1, Comparative Example 1 and Comparative Example 2 were each replaced with polyvinylidene fluoride 1
6 parts by weight of N-methylpyrrolidone were added together with the parts by weight, and the mixture was kneaded well in a mortar to prepare a slurry. This was coated with a doctor blade on a copper foil current collector and vacuum-dried at 110 ° C. for 12 hours to prepare a negative electrode for a lithium ion secondary battery. The negative electrode thus obtained was sandwiched with an electrolytic solution, a cell was made to face the lithium metal electrode, and a charge / discharge test was performed. As the electrolytic solution, a solution prepared by dissolving lithium perchlorate at a rate of 1 mol / L in a solvent in which ethylene carbonate and propylene carbonate were mixed at a volume ratio of 1: 1 was used. The cell was assembled in a glove box filled with ultra-dry air. After assembling the cell, measurement was started after leaving the cell for 2 hours so that the electrolyte solution permeated the negative electrode sufficiently.

The charge / discharge test was performed by a constant current charge / discharge method. Current density of 0.1 mA / cm ² and potential difference between electrodes is 0 V
, And then undoped until the potential difference between the electrodes reached 1.5 V. The charge / discharge was repeated three cycles, and the undoping capacity was measured. Table 2 shows the measurement results.

[0035]

[Table 2]

Although the discharge capacity of the secondary battery using the carbon material of Comparative Example 1 was smaller than the theoretical capacity of graphite, it was found in Journal of Electrochemical Society 14
This is in good agreement with the reported values described in Vol. 2, page 1041. From these facts, it is considered that these batteries and their evaluation characteristics are reliable. Further, the battery using the bamboo charcoal of Example 1 has characteristics comparable to those of conventionally proposed and used carbon materials. Therefore, it is clear that a battery having performance equivalent to that of a conventional battery can be manufactured using such a novel and inexpensive carbon material.

[0037]

The present invention provides a lithium ion secondary battery using a novel negative electrode material by using a carbon derived from a natural product that has never been proposed before. Further, since such a natural product-derived carbon material can be obtained at a very low price as compared with a conventionally used negative electrode material, it provides an inexpensive lithium ion secondary battery. Furthermore, when bamboo charcoal is used as carbon derived from a natural product, a lithium ion secondary battery having excellent repeated use stability equivalent to that of conventional materials is provided.

[Brief description of the drawings]

FIG. 1 is a current-potential curve (cyclic voltammogram) showing the repetition dependence of lithium ion charge / discharge characteristics in a carbon material of Example 1 of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kaoru Dokoko, AO BA62-110, 60 Kameoka-cho, Kawauchi, Aoba-ku, Sendai, Miyagi Prefecture (72) Yumi Fujita 3--18, Kuromatsu, Izumi-ku, Sendai, Miyagi Prefecture -201 F term (reference) 5H029 AJ00 AK02 AK03 AK05 AL06 AM03 AM04 AM05 AM07 EJ01 EJ12 5H050 AA00 BA17 CA02 CA05 CA08 CA09 CA11 CB07 DA04 EA24

Claims (2)

[Claims] At least a positive electrode, a non-aqueous electrolyte,
A lithium ion secondary battery comprising a current collector and a negative electrode using, as an active ingredient, carbon derived from a natural product capable of electrochemically storing and releasing lithium ions. 2. The lithium ion secondary battery according to claim 1, wherein the carbon derived from a natural product is bamboo charcoal made of bamboo.