JP2009187924A - Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery using the negative electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which has a high output characteristics at low temperature and has a high capacity, and to provide a negative electrode material for lithium ion secondary battery and a negative electrode for lithium ion secondary battery, for achieving the above battery. <P>SOLUTION: In the negative electrode material for the lithium ion secondary battery, a size Lc of crystallite analyzed by an X-ray diffraction method is 20 to 90 nm and an average surface distance (d002) is 0.3354 to 0.3370 nm, and a low crystallinity carbon layer is formed on its surface. The negative electrode for the lithium ion secondary battery is composed by having the above negative electrode material, and the lithium ion secondary battery is composed by using the above negative electrode. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the same. In particular, the present invention relates to a lithium ion secondary battery that requires low-temperature output, such as a power source for a hybrid vehicle and an electric vehicle (for vehicle use).

  Lithium ion rechargeable batteries have higher voltage and capacity than other rechargeable batteries, making them ideal as batteries for mobile devices (consumer use) such as mobile phones, personal computers, video cameras, etc. Have achieved. Consumer-use lithium ion secondary batteries are highly demanded to be usable for a long time, and research and development on cell design and materials have been performed mainly for higher capacity.

  In recent years, lithium ion secondary batteries have been actively studied for applications that require output characteristics such as power tools, electric bicycles, hybrid vehicles, and power sources for electric vehicles (for in-vehicle use). In particular, hybrid vehicles and electric vehicles are attracting a great deal of attention from the viewpoint of global environmental conservation such as reduction of exhaust gas, and it is predicted that a large market will be formed in the future for in-vehicle lithium ion secondary batteries. Since an in-vehicle lithium ion secondary battery is used for power assist during start-up and acceleration, high output characteristics are required. There is a problem that the output characteristics of the lithium ion secondary battery are remarkably lowered in a low temperature range of 0 ° C. or lower, and the in-vehicle lithium ion secondary battery is required to maintain high output characteristics even at −30 ° C.

  A lithium ion secondary battery is composed of a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, a cell packing these, and the like. As the negative electrode material, a carbon material capable of reversibly occluding and releasing lithium ions is mainly used. High-capacity lithium ion secondary batteries are required for consumer use, and high-crystalline graphite having high capacity and charge / discharge efficiency is mainly used for the negative electrode material (see, for example, Patent Documents 1 and 2). .

  However, for an in-vehicle lithium ion secondary battery that requires high output characteristics at low temperatures, the negative electrode material developed for consumer use cannot satisfy the characteristics. In order to satisfy the low-temperature output characteristics of the negative electrode material for in-vehicle use, it is necessary to develop a new negative electrode material different from the negative electrode that has been developed for consumer use.

  As a means for improving the low temperature characteristics of the negative electrode material, for example, a method of coating the surface of graphite particles with low crystalline carbon has been reported in the past. Low crystalline carbon is said to have a high lithium moving speed, and therefore, a negative electrode material coated with lithium is reported to have excellent input / output and cycle characteristics at low temperatures. However, in any report that attempts to improve low temperature characteristics by coating low crystalline carbon, the core particles are made of carbon material such as high crystalline graphite. It cannot be said to be sufficient as a negative electrode material for in-vehicle lithium ion secondary batteries (see, for example, Patent Documents 3 to 5). On the other hand, there is a report that low crystalline carbon itself is used as a negative electrode material and high output characteristics can be obtained compared to graphite, but low crystalline carbon has a problem that its capacity is remarkably small compared with graphite (for example, And Patent Document 6).

Japanese Patent Application Laid-Open No. 2005-259689 JP 2003-331835 A Japanese Patent No. 2643035 JP 2001-185147 A Japanese Patent No. 3106129 JP 2006-140138 A

This invention is made | formed in view of the above conventional trouble, and makes it a subject to achieve the following objectives. That is,
An object of the present invention is to provide a lithium ion secondary battery having high capacity and high output characteristics at a low temperature, a negative electrode material for a lithium ion secondary battery, and a negative electrode for a lithium ion secondary battery for realizing the same. is there.

As a result of intensive studies, the inventors have determined that the crystallite size of the negative electrode material having a specific interplanar spacing affects the output characteristics at low temperatures, and by controlling the crystallite size, it is high at high capacity and low temperature. It was found that the output characteristics can be compatible. Furthermore, it has been found that the low-temperature output characteristics can be further improved by forming a low crystalline carbon layer on the surface of the negative electrode material.
Specifically, the matters described in the following (1) to (4) are characterized.

(1) The crystallite size Lc analyzed by the X-ray diffraction method is 20 to 90 nm, the average interplanar spacing (d002) is 0.3354 to 0.3370 nm, and low crystalline carbon is present on the surface. A negative electrode material for a lithium ion secondary battery, comprising a layer.
(2) The negative electrode material for a lithium ion secondary battery as described in (1) above, wherein the average particle size (50% D) is 1 to 40 μm.
(3) A negative electrode for a lithium ion secondary battery comprising the negative electrode material for a lithium ion secondary battery according to (1) or (2).
(4) A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to (3).

  According to the present invention, it is possible to provide a lithium ion secondary battery having high capacity and high output characteristics at a low temperature, a negative electrode material for a lithium ion secondary battery, and a negative electrode for a lithium ion secondary battery for realizing the same. it can.

Hereinafter, the present invention will be described in detail.
<Anode material for lithium ion secondary battery>
The negative electrode material for a lithium ion secondary battery of the present invention (hereinafter sometimes simply referred to as “negative electrode material”) has a crystallite size Lc analyzed by X-ray diffraction method of 20 to 90 nm, average The value of the interplanar spacing (d002) is 0.3354 to 0.3370 nm, and the surface has a low crystalline carbon layer.
When the average interplanar spacing (d002) is close to the theoretical graphite value, a negative electrode material having a higher capacity and higher voltage than amorphous carbon can be obtained. Further, by reducing the crystallite size Lc, the number of edge surfaces capable of inserting and extracting lithium can be increased, and the lithium moving distance in the crystal structure can be shortened. It is considered that high output characteristics can be obtained even in

[Crystallite size Lc]
In the present invention, the crystallite size Lc is 20 to 90 nm, preferably 30 to 85 nm, and more preferably 40 to 80 nm. Those having a crystallite size Lc of more than 90 nm have high crystallinity and excellent capacity, but low temperature output characteristics tend to be inferior. Moreover, since the thing below 20 nm has low crystallinity, there exists a tendency for a capacity | capacitance to fall.

[Average spacing (d002)]
In the present invention, the average interplanar spacing (d002) is 0.3354 to 0.3370 nm, preferably 0.3354 to 0.3368 nm, and more preferably 0.3354 to 0.3365 nm. The value of the average interplanar spacing (d002) is 0.3354 nm, which is the theoretical value of graphite crystals, and is preferably closer to this value. Moreover, when it exceeds 0.3370 nm, crystallinity will fall, and there exists a tendency for the capacity | capacitance and the voltage at the time of charging / discharging to fall. The crystallite size Lc and the average interplanar spacing (d002) shown in the present invention are values obtained by measurement based on the Gakushin method.

  The negative electrode material for a lithium ion secondary battery of the present invention is not particularly limited as long as it satisfies the crystallite size Lc and the average interplanar spacing (d002) described above. For example, examples of the carbon material used for the negative electrode material for a lithium ion secondary battery include artificial graphite, natural graphite, low crystalline carbon, mesophase carbon, and the like. It is possible to obtain a negative electrode material for a lithium ion secondary battery of the present invention by performing heat treatment on these to adjust the crystallite size Lc and the average interplanar spacing (d002). It can also be obtained by heat treating a carbonaceous raw material such as a polymer compound or pitch.

  As a specific method for adjusting the crystallite size Lc and the average interplanar spacing (d002), for example, low crystalline carbon, poorly graphitized graphite, or a carbonaceous raw material is used at 2000 ° C. in an inert atmosphere. The heat treatment is mentioned above. Generally, graphitization of a carbon material is said to proceed from 2000 ° C. or higher. The heat treatment temperature is preferably 2000 ° C to 3000 ° C, more preferably 2200 to 3000 ° C, and further preferably 2400 ° C to 3000 ° C. When the heat treatment temperature is less than 2000 ° C., sufficient crystallite size Lc and average interplanar spacing (d002) tend not to be obtained. On the other hand, when the heat treatment temperature exceeds 3000 ° C., the crystallinity increases and the low-temperature output tends to decrease.

  Examples of the material to be heat-treated include coke for low crystalline carbon. From the viewpoint of bringing the average interplanar spacing (d002) close to the theoretical graphite value, it is preferable to use graphitizable carbon. Among cokes, it is preferable to use mosaic coke from the viewpoint of reducing the crystallite size Lc. Examples of graphite that is insufficiently graphitized include earth-like graphite among natural graphites. Natural graphite has different crystallinity and structure depending on the production area, mine, etc., and is classified into scaly, scaly, earthy graphite, etc. Among them, earthy graphite is preferred because of its low crystallinity. Examples of the carbonaceous raw material include polymer compounds such as vinyl chloride resin, phenol resin, furan resin, and polyvinyl resin, or pitch. Of these, graphitizable carbon such as vinyl chloride resin, polyvinyl resin, and pitch is preferably used from the viewpoint of bringing the average interplanar spacing (d002) close to the theoretical graphite value.

[Low crystalline carbon layer]
The negative electrode material for a lithium ion secondary battery of the present invention has a low crystalline carbon layer on the surface of the negative electrode material, and it is known that the insertion and desorption rate of lithium is improved by the structure, and the crystal described above Since the surface of the negative electrode material having a structure has a low crystalline carbon layer, the low-temperature output characteristics can be further improved. As a negative electrode material having a low crystalline carbon layer on the surface, it is preferable that the entire surface of the negative electrode material is covered with a low crystalline carbon layer, but a state in which low crystalline carbon is partially present may be used. .

  The method for forming the low crystalline carbon layer on the surface of the negative electrode material is not particularly limited, and examples thereof include a wet mixing method, a chemical vapor deposition method, and a mechanochemical method. The chemical vapor deposition method and the wet mixing method are preferable from the viewpoint that the reaction system can be controlled uniformly and the shape of the negative electrode material can be maintained. The carbon source for forming the low crystalline carbon layer is not particularly limited, but in the chemical vapor deposition method, aliphatic hydrocarbons, aromatic hydrocarbons, alicyclic hydrocarbons, etc. can be used. Ethane, propane, toluene, benzene, xylene, styrene, naphthalene, cresol, anthracene, or derivatives thereof. In the wet mixing method and the mechanochemical method, a polymer compound such as a phenol resin or a styrene resin, or a carbonizable solid material such as pitch can be processed as a solid or dissolved material. About processing temperature, it is preferable to heat-process at 800-1200 degreeC with a chemical vapor deposition method. If it is less than 800 degreeC, the production | generation rate of vapor deposition carbon will be slow, and there exists a tendency for processing time to become long. On the other hand, when the temperature exceeds 1200 ° C., the production rate is too high, and thus control of film formation tends to be difficult. Moreover, in the wet mixing method and the mechanochemical method, it is preferable to heat-process at 700-2000 degreeC. In the wet mixing method and the mechanochemical method, the carbon source is uniformly deposited in advance on the surface of the negative electrode material and fired, so that heat treatment can be performed even at a relatively high temperature. If it is less than 700 ° C., the carbon crystallinity is too low, so that the electrolytic solution decomposability tends to be high. When the temperature exceeds 2000 ° C., the carbon crystallinity becomes too high, and the output characteristics tend to deteriorate.

About the amount of the low crystalline carbon layer to be formed on the surface of the negative electrode material, in the present invention, the residual carbon rate of the carbon source is measured in advance by thermogravimetric analysis, etc. The product is the amount of carbon covered. Although there is no restriction | limiting in particular about the carbon content of a low crystalline carbon layer, 0.5 wt%-20 wt% are preferable, 1 wt%-15 wt% are more preferable, 2 wt%-10 wt% are more preferable. When the amount of carbon in the low crystalline carbon layer is less than 0.5 wt%, sufficient low temperature output characteristics tend not to be obtained. In addition, when the carbon content of the low crystalline carbon layer exceeds 20 wt%, the low temperature output characteristics are reduced due to the low specific surface area, and the capacity decreases due to the aggregation of particles or the presence of many low crystalline components. Is seen. The low crystalline carbon coated on the surface of the negative electrode material can be observed with a transmission electron microscope (TEM). For example, as shown in FIGS. 1 and 2, it can be confirmed that the low crystalline carbon layer formed on the surface of the negative electrode material is clearly different from the structure of the graphite layer of the negative electrode material. In addition, the arrow in FIG. 1 points out the surface of a negative electrode material, and the two arrows in FIG. 2 point out the one surface and the other surface in the surface and back surface of a low crystalline carbon layer, respectively.
Further, even when the R value obtained from the Raman spectrum analysis is used, the R value tends to increase before and after the layer formation treatment, so that the formed low crystalline carbon layer can be confirmed conveniently. Furthermore, since the low crystalline carbon layer formed on the surface of the negative electrode material is small compared with the amount of the core particles, it does not greatly affect the XRD analysis. Therefore, the average interplanar spacing (d002) and the crystallite size Lc can be regarded as equivalent to those of the core particles.

In order to confirm the low crystalline carbon layer using the R value obtained from the Raman spectrum analysis, in the Raman spectrum analysis using an argon laser beam having a wavelength of 514.5 nm, R = I1580 / I1350 (I1580 is the Raman spectrum) the intensity of the peak P1 in the range of 1580~1620cm ^-1, I1350 is confirmed by R value represented by 1350~1370cm intensity of the peak P2 in the range of ^-1). In the Raman spectrum measured using an argon laser beam having a wavelength of 514.5 nm, the peak P1 in the range of 1580 to 1620 cm ⁻¹ is high crystalline carbon, and the peak P2 in the range of 1350 to 1370 cm ⁻¹ is low crystalline carbon. Correspond.

  The negative electrode material for a lithium ion secondary battery of the present invention preferably has an average particle diameter (50% D) of 1 to 40 μm. About the grinding | pulverization of the negative electrode material of this invention, it is preferable to carry out about the negative electrode material before forming a low crystalline carbon layer. This is because when the low crystalline carbon layer is formed and then pulverized, the layered carbon peels off. Examples of the pulverizer include a hammer mill, a bead mill, a turbo mill, and a jet mill, but there is no particular limitation thereto.

The average particle size (50% D) of the negative electrode material for a lithium ion secondary battery of the present invention is preferably 1 to 40 μm, more preferably 1 to 30 μm, and even more preferably 1 to 20 μm. As the average particle size (50% D) decreases, the low-temperature output characteristics improve, but the specific surface area increases and the charge / discharge efficiency tends to decrease. On the other hand, when the average particle size (50% D) is increased, the low-temperature output characteristics are lowered, but the charge / discharge efficiency tends to be improved. Therefore, the average particle size (50% D) is preferably determined as appropriate from the viewpoint of battery design in consideration of the balance with charge / discharge efficiency and the like. In addition, regarding the negative electrode material of the present invention, when the average particle diameter (50% D) is 1 μm or less, there is no significant improvement in output characteristics, and it is difficult to make a slurry in terms of electrode preparation. Produce. On the other hand, when the average particle size (50% D) exceeds 40 μm, there arises a problem that output characteristics at low temperatures are remarkably deteriorated.
The particle size distribution can be measured by dispersing a sample in purified water containing a surfactant and measuring with a laser diffraction particle size distribution measuring device (SALD-3000J, manufactured by Shimadzu Corporation), and the average particle size is 50% D Is calculated as

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention is characterized by using the above-described negative electrode material for a lithium ion secondary battery of the present invention.
For example, the negative electrode material and the organic binder for the lithium ion secondary battery of the present invention described above are kneaded together with a solvent by a dispersing device such as a stirrer, a ball mill, a super sand mill, a pressure kneader to prepare a negative electrode material slurry, This can be applied to a current collector to form a negative electrode layer, or a paste-like negative electrode material slurry can be formed into a sheet shape, a pellet shape, etc. and integrated with the current collector. it can.

  The organic binder is not particularly limited. For example, styrene-butadiene copolymer, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) ) Ethylenically unsaturated carboxylic acid esters such as acrylates, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphoric acid Examples thereof include polymer compounds having a large ion conductivity such as sphazene and polyacrylonitrile. The content of the organic binder is preferably 1 to 20 parts by weight with respect to 100 parts by weight in total of the negative electrode material for a lithium ion secondary battery and the organic binder of the present invention.

  Moreover, you may add the thickener for adjusting a viscosity to the said negative electrode material slurry. As the thickener, for example, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like can be used.

  Moreover, you may mix a conductive support material with the said negative electrode material slurry. Examples of the conductive auxiliary material include carbon black, graphite, acetylene black, or an oxide or nitride that exhibits conductivity. The usage-amount of a conductive support agent should just be about 1 to 15 weight% of the negative electrode material of this invention.

  Further, the material and shape of the current collector are not particularly limited. For example, a strip-shaped one made of aluminum, copper, nickel, titanium, stainless steel or the like in a foil shape, a punched foil shape, a mesh shape, or the like. Use it. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

  The method of applying the negative electrode material slurry to the current collector is not particularly limited. For example, metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating And publicly known methods such as screen printing and the like. After the application, a rolling process using a flat plate press, a calendar roll, or the like is performed as necessary. Further, the integration of the negative electrode material slurry formed into a sheet shape, a pellet shape, and the like with the current collector can be performed by a known method such as a roll, a press, or a combination thereof.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention is characterized by using the above-described negative electrode for a lithium ion secondary battery of the present invention. For example, it can be obtained by placing the negative electrode for a lithium ion secondary battery and the positive electrode of the present invention facing each other with a separator interposed therebetween and injecting an electrolytic solution.

  The positive electrode can be obtained by forming a positive electrode layer on the current collector surface in the same manner as the negative electrode. In this case, the current collector may be a band-shaped material made of a metal or an alloy such as aluminum, titanium, or stainless steel in a foil shape, a punched foil shape, a mesh shape, or the like.

The positive electrode material used for the positive electrode layer is not particularly limited. For example, a metal compound, metal oxide, metal sulfide, or conductive polymer material that can be doped or intercalated with lithium ions may be used. Without limitation, for example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and double oxides thereof (LiCoxNiyMnzO ₂ , x + y + z = 1), lithium manganese spinel (LiMn) ₂ O _4), lithium vanadium _{compounds, V 2 O 5, V 6} O 13, VO 2, MnO 2, TiO 2, MoV 2 O 8, TiS 2, V 2 S 5, VS 2, MoS 2, MoS 3, Cr ₃ O ₈ , Cr ₂ O ₅ , olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, porous carbon, and the like can be used alone or in combination.

  As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene and polypropylene, cloth, microporous film, or a combination thereof can be used. In addition, when it is set as the structure where the positive electrode and negative electrode of a lithium ion secondary battery to produce are not in direct contact, it is not necessary to use a separator.

Examples of the electrolyte include lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, 3- Methyl sulfolane, 2,4-dimethyl sulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, Butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate A so-called organic electrolyte solution dissolved in a non-aqueous solvent of a simple substance such as ethyl acetate or a mixture of two or more components can be used.

  Although the structure of the lithium ion secondary battery of the present invention is not particularly limited, usually, a positive electrode and a negative electrode, and a separator provided as necessary, are wound into a flat spiral to form a wound electrode group, In general, these are laminated as a flat plate to form a laminated electrode plate group, or the electrode plate group is enclosed in an exterior body.

  The lithium ion secondary battery of the present invention is not particularly limited, but is used as a paper-type battery, a button-type battery, a coin-type battery, a laminated battery, a cylindrical battery, a rectangular battery, or the like.

  The above-described negative electrode material for a lithium ion secondary battery according to the present invention has been described as being used for a lithium ion secondary battery. It is also possible to apply to.

  The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to the following examples.

[Example 1]
(Preparation of negative electrode material)
Natural earth-like graphite from China was pulverized with a jet mill to an average particle size (50% D) of 4 μm. After 200 g of this pulverized powder and a solution obtained by dissolving 16 g of coal-based pitch (softening point 90 ° C.) in 300 ml of THF were stirred and mixed at a temperature of 40 ° C. for 1 hour, THF as a solvent was removed under reduced pressure, and then at 120 ° C. Completely dried to obtain a mixture. The mixture was fired at 900 ° C. for 3 hours in a firing furnace in a nitrogen atmosphere to obtain a negative electrode material in which a low crystalline carbon layer was formed on the particle surface. The obtained negative electrode material was pulverized with a mixer and then sieved with a 300 mesh sieve, and the portion under the sieve was used as the negative electrode material of this example. About the obtained negative electrode material, the following method evaluated the XRD analysis, the Raman spectrum analysis, the specific surface area measurement, and the average particle diameter (50% D) measurement.

○ XRD analysis (measurement of crystallite size Lc, average interplanar spacing (d002))
The measurement was performed with a wide-angle X-ray diffraction measurement apparatus manufactured by Rigaku Corporation, and the crystallite size Lc and average interplanar spacing (d002) were calculated based on the Gakushin method. The crystallite size Lc and average plane spacing (d002) were determined from the peak derived from (002).
○ R value measurement (Raman spectrum analysis)
The measurement was performed using a Raman spectrum measuring apparatus NRS-1000 type (excitation light: argon ion laser 514.5 nm) manufactured by JASCO Corporation. The R value is based on the entire measurement range (830 cm ^{−1 to} 1940 cm ⁻¹ ) as a baseline, the ratio of the peak height (Hg) derived from the G band to the peak height (Hd) derived from the D band, and Hd / Hg as R Value.
○ was measured using a BET specific surface area measurement Shimadzu _{N 2} adsorption-desorption measurement device ASAP-2010. The specific surface area was calculated by the BET method.
○ Measurement of average particle size (50% D) <Measuring method of average particle size>
A solution in which a negative electrode material sample is dispersed in purified water together with a surfactant is placed in a sample water tank of a laser diffraction particle size distribution analyzer (SALD-3000J, manufactured by Shimadzu Corporation) and circulated with a pump while applying ultrasonic waves. However, it was measured by a laser diffraction method. The 50% cumulative particle size (50% D) of the obtained particle size distribution was taken as the average particle size.

(Preparation of negative electrode for lithium ion secondary battery)
In preparing a negative electrode for a lithium ion secondary battery, a water-dispersed paint having a solid content of 98% by weight of a negative electrode material, 1% by weight of a styrene butadiene resin as a binder and 1% by weight of carboxymethyl cellulose as a thickener is prepared. It coated so that it might become a thickness of about 70 micrometers on 50 micrometers copper foil. The coated product was dried at 80 ° C. for 5 hours and at 120 ° C. for 3 hours. After drying, in order to evaluate as a negative electrode for a lithium ion secondary battery, a 9 mmφ circular shape was punched for discharge capacity evaluation, and a 16 mmφ circular shape was cut for -30 ° C DCR evaluation.

(Production of evaluation cell)
The evaluation cell was produced by injecting an electrolyte solution with a CR2016 coin cell facing the negative electrode and metallic lithium through a 40 μm polypropylene separator. The electrolytic solution used for the evaluation of the discharge capacity was one obtained by dissolving ethyl carbonate and methyl ethyl carbonate in a mixed solvent LiPF ₆ having a volume ratio of 3 to 7 so as to have a concentration of 1 mol / L (1M LiPF ₆ EC: MEC). = 3: 7). Further, -30 ° C. DCR was used for the measurement that the vinyl carbonate is added 1.5 wt% in the electrolyte of the claimed _{(1M LiPF 6 EC: MEC =} 3: 7+ 1.5wt% VC).

-Evaluation of discharge capacity and direct current resistance (DCR) at -30 ° C-
○ Evaluation conditions The discharge capacity evaluation was the discharge capacity of the first charge / discharge test. The evaluation condition is that the battery is charged to 0 V with a constant current of 0.2 mA (0.28 mA / cm ² ) in a 25 ° C. atmosphere, then charged to a current value of 0.028 mA with a constant voltage of 0 V, and then 0.2 mA. Discharge was performed to a voltage value of 1.5 V at a constant current of.
In the −30 ° C. DCR evaluation, after conducting a charge / discharge test under the same conditions as described above, the battery was charged so as to be in a 50% state of charge (SOC) at a constant current of 0.5 mA. The charged coin cell was placed in an atmosphere of −30 ° C., discharged at a constant current of 0.235 mA for 1 minute, then discharged at a constant current of 0.705 mA for 1 minute, and further discharged at a constant current of 1.175 mA for 1 minute. . From the above test, the difference (ΔV) between the voltage value of SOC 50% and the voltage value after 10 seconds of discharge at each current value was obtained, and the slope of the graph plotting the current value on the horizontal axis and ΔV on the vertical axis was −30 ° C. The DCR value was used. The -30 ° C DCR value is good if it is 340Ω or less.

[Example 2]
In Example 1, a negative electrode material was prepared and evaluated in the same manner as in Example 1 except that Chinese earth graphite was used after being treated at 2900 ° C. for 3 hours under an inert atmosphere.

[Example 3]
In Example 1, a negative electrode material was prepared and evaluated in the same manner as in Example 1 except that artificial graphite obtained by treating mosaic coke at 2800 ° C. for 3 hours in an inert atmosphere was used.

[Comparative Example 1]
In Example 1, a negative electrode was prepared in the same manner as in Example 1 except that artificial graphite obtained by mixing 30 wt% of silicon carbide as a graphitization catalyst with needle coke and treating at 2800 ° C. for 3 hours in an inert atmosphere was used. A material was prepared and evaluated in the same manner.

[Comparative Example 2]
In Example 1, except that Chinese spheroidized natural graphite was used, a negative electrode material was prepared and evaluated in the same manner as in Example 1.

[Comparative Example 3]
A negative electrode material was produced in the same manner as in Example 1 except that needle coke treated at 1500 ° C. for 3 hours in an inert atmosphere was used.

[Comparative Example 4]
In Example 1, a negative electrode material was produced and evaluated in the same manner as in Example 1 except that the formation process of the low crystalline carbon layer was not performed. In Comparative Example 4, the negative electrode material could not be evaluated because the water-dispersed paint at the time of electrode preparation was gelled.

[Comparative Example 5]
In Example 2, a negative electrode material was produced and evaluated in the same manner as in Example 2 except that the formation process of the low crystalline carbon layer was not performed.

[Comparative Example 6]
In Example 3, a negative electrode material was produced and evaluated in the same manner as in Example 3 except that the formation process of the low crystalline carbon layer was not performed.

  The evaluation results of the above examples and comparative examples are shown in Table 1 below. In Table 1, “ND” indicates No Data.

  From Table 1, it can be seen that the negative electrode materials for lithium ion secondary batteries of the examples have high capacity and high output characteristics in a low temperature range.

It is a drawing substitute photograph which shows the external appearance which observed the negative electrode material which does not have a low crystalline carbon layer on the surface with a transmission electron microscope (TEM). It is a drawing substitute photograph which shows the external appearance which observed the negative electrode material which has a low crystalline carbon layer on the surface with a transmission electron microscope (TEM).

Claims (4)

  The crystallite size Lc analyzed by the X-ray diffraction method is 20 to 90 nm, the average interplanar spacing (d002) is 0.3354 to 0.3370 nm, and has a low crystalline carbon layer on the surface. A negative electrode material for a lithium ion secondary battery.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein an average particle diameter (50% D) is 1 to 40 μm.   The negative electrode for lithium ion secondary batteries which has the negative electrode material for lithium ion secondary batteries of Claim 1 or 2.   The lithium ion secondary battery which uses the negative electrode for lithium ion secondary batteries of Claim 3.