JP2019087519A - Negative electrode material for lithium ion secondary battery, and lithium ion secondary battery using the same - Google Patents

Abstract

To provide a graphite carbon negative electrode material excellent in input and output characteristics of a lithium ion secondary battery, particularly remarkably excellent in input characteristics (rapid charge characteristics).SOLUTION: The negative electrode material for lithium ion secondary battery is comprised of amorphous carbon coated graphite material with a graphite particle surface coated with amorphous carbon and has a brightness Lvalue of at most 35.0 and a tap density of 0.3 g/mL or more and 0.75 g/mL or less.SELECTED DRAWING: None

Description

  The technology disclosed herein relates to a graphitic carbon negative electrode material for lithium ion secondary batteries.

  Lithium ion secondary batteries are lighter in weight and have higher capacity than nickel cadmium batteries, nickel hydrogen batteries, and lead batteries, which are conventional secondary batteries, and so portable electronic devices such as mobile phones, notebook computers, etc. It is put to practical use as a driving power source for

  Furthermore, in recent years, along with the increase in capacity of lithium ion secondary batteries, they are being developed for home use, electric cars and the like.

  In particular, when a lithium ion secondary battery for vehicles such as electric vehicles is assumed, low temperature performance and high temperature life performance are also important in order to be used under various environments. In addition, in consideration of convenience similar to that of gasoline vehicles, high input / output characteristics are required in order to perform quick charge performance that can be charged in a short time and to accelerate and regenerate instantaneously.

  In the negative electrode of a lithium ion secondary battery, a carbon or graphitic carbon material which can intercalate lithium ions is generally used.

In a lithium ion secondary battery, lithium ions (Li ⁺ ) move back and forth between the positive electrode active material and the negative electrode active material as charge and discharge occur. Unlike a positive electrode active material that originally contains Li as a crystal, in a carbon material that only accepts Li when it is operated as a battery, a side reaction occurs in the Li insertion process, so Li is consumed by the side reaction and the initial stage It is known that the efficiency is reduced.

  On the other hand, the lower the resistance of the negative electrode active material / electrolyte interface, the lower the energy required for Li to enter the negative electrode active material, and the better the input / output characteristics. In addition, when the resistance of the negative electrode active material / electrolyte interface is low, Li can be easily inserted even in a low temperature environment, so that the low temperature characteristics can also be improved. Further, heat generation due to resistance leads to deterioration of the electrolyte solution and the like, which deteriorates the cycle characteristics. Therefore, by using a negative electrode active material having a low resistance of the negative electrode active material / electrolyte solution interface, it is possible to improve the cycle characteristics.

  On the other hand, the Li diffusion rate reflects the ease of passage of the Li diffusion path to the entire inside of the negative electrode active material, and the more easily Li diffuses, the shorter the time until Li fills the entire negative electrode active material. Further improvement in input / output characteristics can be expected by using the negative electrode active material which is easily diffused.

  So far, non-graphitizable carbon materials (hard carbon) and amorphous carbon materials have been known as materials excellent in input / output characteristics (see, for example, Patent Document 1).

Further, from the viewpoint of achieving both a high discharge capacity and a low irreversible capacity, a negative electrode material for a lithium ion secondary battery comprising carbonaceous graphite particles in which graphite particles are coated with a carbonaceous material has been proposed. More specifically, after a carbon material such as pitch is coated on a graphite material, it is fired at a temperature of 700 to 1500 ° C. in an inert gas atmosphere to form a carbonaceous material on the surface layer from the carbonaceous coated graphite particles The pore volume of 1 nm or less determined by the HK method is 0.0010 to 0.0020 cm ³ / g based on the adsorption isotherm by nitrogen of the carbonaceous coated graphite particles, and 1 determined by the BJH method A negative electrode material for a lithium ion secondary battery is disclosed, which has a pore volume of ̃100 nm of 0.020 to 0.040 cm ³ / g. And, it is described that high rate characteristics can be improved without impairing capacity, initial charge / discharge efficiency and cycle characteristics by using such a negative electrode material (see, for example, Patent Document 2).

Patent No. 4877568 JP, 2014-170724, A

  However, the non-graphitizable carbon material (hard carbon) and the amorphous carbon material described in Patent Document 1 have many surface functional groups that cause a side reaction with the electrolytic solution, and Li is consumed by the side reaction. It is known that the initial efficiency is very bad. In addition, hard carbon and amorphous carbon materials are hard because layered graphite crystals are not developed, and the true density is low because the distance between the graphite networks is not narrowed at the same time. Since the energy density is also low, it is not suitable for applications such as in-vehicle use for achieving high capacity by increasing the packing density.

  Further, the negative electrode material for a lithium ion secondary battery described in Patent Document 2 has a high rate characteristic of about 60.1 to 65.7% of charging characteristics at 1 C, and has sufficient rapid charging characteristics. It is hard to say that

  Therefore, in view of the above problems, the present invention is a graphite-based carbon negative electrode capable of obtaining a lithium ion secondary battery having remarkably excellent input / output characteristics of a lithium ion secondary battery, in particular, input characteristics (rapid charge characteristics). Intended to provide materials.

In order to achieve the above object, the negative electrode material for a lithium ion secondary battery according to the present invention comprises an amorphous carbon-coated graphite material in which the surface of graphitic particles is coated with amorphous carbon, and the lightness L ^* It is characterized in that the value is 35.0 or less and the tap density is 0.3 g / mL or more and 0.75 g / mL or less.

  According to the negative electrode material for a lithium ion secondary battery of the present invention, the input / output characteristics of the lithium ion secondary battery can be improved by reducing the resistance of the negative electrode active material / electrolyte interface.

It is a figure which shows an example of the lithium ion secondary battery provided with the negative electrode using the amorphous carbon coating graphite material which concerns on embodiment of this invention.

  A negative electrode material for a lithium ion secondary battery according to the present embodiment, a lithium ion secondary battery using the same, and a method for producing a negative electrode material for a lithium ion secondary battery will be described below. In addition, what is demonstrated below is an example of embodiment, and the shape of a constituent material, a constituent material or a member, the conditions of processing or heat processing, etc. can be suitably changed in the range which does not deviate from the meaning of the present invention.

The negative electrode material for a lithium ion secondary battery according to this embodiment is an amorphous carbon-coated graphite material in which the surface of graphitic particles used as a base material is coated with amorphous carbon, and the lightness L ^* value is 35 It is characterized in that it is not more than 0 and the tap density is not less than 0.3 g / mL and not more than 0.75 g / mL.

  The amorphous carbon-coated graphite material of the present embodiment will be described in detail below.

<Amorphous carbon-coated graphite material>
The lightness L ^* value of the amorphous carbon-coated graphite material of the present embodiment is 35.0 or less. In general, carbon materials such as carbon black and graphite become blacker as the end (so-called "edge portion") of the graphite network surface of the carbon material which becomes an entrance and exit of Li (that is, the lightness L ^* value becomes smaller) It is said. Therefore, in the case of an amorphous carbon-coated graphite material having an L ^* value of 35.0 or less, the edge portion of the carbon material to be an inlet / outlet of Li increases, so that Li is efficiently inserted and released from the contact surface with the electrolyte. As a result, the negative electrode active material / electrolyte interface resistance, that is, the alternating current impedance becomes low. And, by using an amorphous carbon-coated graphite material of low resistance with low AC impedance as a negative electrode material of a lithium ion secondary battery, the energy required for inserting Li becomes low, so even under a low temperature environment Li Thus, it becomes possible to insert and release a large amount of Li at a time while suppressing deterioration of the electrolyte solution due to resistance. Therefore, not only the input / output characteristics of the lithium ion secondary battery but also the low temperature characteristics and the cycle characteristics can be improved.

From the viewpoint of further reducing the resistance of the negative electrode active material / electrolyte interface, the lightness L ^* value is preferably 34.6 or less, more preferably 34.2 or less.

Moreover, the "brightness L ^* value" referred to here is a value measured in accordance with JIS Z 8729, and the sample and castor oil are kneaded with a Huber-type Marler to form a paste, and a clear lacquer is added to this paste It is the coloration index calculated | required based on JISZ 8729 about the sheet | seat which knead | mixed and apply | coated.

  Further, the tap density of the amorphous carbon-coated graphite material of the present embodiment is 0.3 g / mL or more and 0.75 g / mL or less. This is because when the tap density is less than 0.3 g / mL, the active material density hardly increases when used as a negative electrode material of a lithium ion secondary battery, and the energy density per volume decreases, which is not preferable. . When the tap density exceeds 0.75 g / mL, the contact area between the active material and the electrolyte decreases when used as a negative electrode material of a lithium ion secondary battery because there are few gaps between particles, and Li The edge portion of the carbon material which can be directly inserted and desorbed is reduced. Accordingly, a large amount of Li can not be received at one time, and the negative electrode active material / electrolyte interface resistance, that is, the alternating current impedance becomes high, which is not preferable because the input / output characteristics deteriorate.

  That is, in the present embodiment, by using an amorphous carbon-coated graphite material having a tap density of 0.3 g / mL or more and 0.75 g / mL or less as a negative electrode material of a lithium ion secondary battery, an electrode and an electrolytic solution Since the contact area between the active material and the electrolyte can be sufficiently secured at the interface with the above, the input / output characteristics of the lithium ion secondary battery can be improved.

  From the viewpoint of further reducing the resistance of the negative electrode active material / electrolyte interface, the tap density is preferably 0.35 g / mL or more and 0.7 g / mL or less, and 0.4 g / mL or more and 0.65 g / mL. mL or less is more preferable.

  Further, in the amorphous carbon-coated graphite material of the present embodiment, the crystallite size La calculated from the X-ray wide-angle diffraction line is preferably 400 nm or less. This is because when an amorphous carbon-coated graphite material having a La of more than 400 nm is used as a negative electrode material of a lithium ion secondary battery, the insertion distance of Li to the inner core of the graphite particle becomes long, and the Li insertion during charge and discharge This is because the diffusion rate of desorption is degraded, and the input / output characteristics are degraded.

  In addition, 300 nm or less is preferable and, as for crystallite size La, 250 nm or less is more preferable.

  Moreover, it is preferable that the dibutyl phthalate (DBP) absorption amount in the amorphous carbon coating graphite material of this embodiment is 57 mL / 100 g or more and 120 mL / 100 g or less. The DBP absorption is considered to reflect the large contact area with the electrolytic solution on the surface of the carbon material. Therefore, when the DBP absorption amount is less than 56 ml / 100 g, the contact area with the electrolytic solution is small, so the number of inlets and outlets of Li decreases, and the input / output characteristics are deteriorated when used as an active material of a lithium ion secondary battery Not desirable for In addition, when the DBP absorption amount exceeds 120 mL / 100 g, a large amount of solvent is required to dissolve the aggregation of the particles, and the dispersibility is reduced, which is not preferable.

  The DBP absorption amount is preferably 59 mL / 100 g or more and 110 mL / 100 g or less, and more preferably 61 mL / 100 g or more and 100 mL / 100 g or less.

  Moreover, the "dibutyl phthalate (DBP) absorption amount" said here says the value measured based on JISK6217-4.

  The average particle diameter (hereinafter also referred to as “D50”) of the amorphous carbon-coated graphite material of the present embodiment is preferably 1 μm to 25 μm. In order to obtain an amorphous carbon-coated graphite material having an average particle size of 1 μm or more, it is necessary to adjust the average particle size of the graphitic particles to be a base material to less than 1 μm. In order to grind | pulverize particle diameter to less than 1 micrometer, enormous energy is needed, cost increases, and it becomes industrially disadvantageous. Also, if the average particle size exceeds 25 μm, the insertion distance of Li to the core of the inside of the graphite particle becomes long, and the diffusion rate of insertion and desorption of Li during charge and discharge becomes worse, so the input / output characteristics are preferably lowered. It is because there is not. The average particle diameter is preferably 2 μm to 20 μm, and more preferably 3 μm to 15 μm.

  Moreover, the "average particle diameter (D50)" said here means the thing of 50% particle diameter in volume-based cumulative particle size distribution by a laser diffraction type particle size distribution analyzer.

  The 10% particle diameter (hereinafter also referred to as “D10”) in the volume-based cumulative particle size distribution of the amorphous carbon-coated graphite material of the present embodiment is preferably 0.5 μm to 10 μm. This is preferable because when D10 is less than 0.5 μm, a side reaction with the electrolyte is likely to occur when used as a negative electrode material of a lithium ion secondary battery, and Li in the battery cell is consumed as an irreversible capacity. Absent. If D10 exceeds 10 μm, the particle size distribution becomes sharp, and the active material density as a negative electrode material of a lithium ion secondary battery is not likely to be high, which is not preferable.

  In addition, as for D10, 0.75 micrometer or more and 7.5 micrometers or less are preferable, and 1 micrometer or more and 5 micrometers or less are more preferable.

The BET specific surface area of the amorphous carbon-coated graphite material of the present embodiment is preferably 1.0 m ² / g or more and 10 m ² / g or less. This is because when the BET specific surface area is less than 1.0 m ² / g, the contact area with the electrolytic solution is small when used as the negative electrode material of a lithium ion secondary battery, so the number of Li entrances and outlets decreases, and the input / output characteristics Is not desirable. In addition, when the BET specific surface area exceeds 10 m ² / g, side reactions with the electrolytic solution are likely to occur when used as a negative electrode material of a lithium ion secondary battery, and Li in the battery cell is consumed as irreversible capacity. Because it is not desirable.

The BET specific surface area is preferably 1.5 m ² / g or more and 9 m ² / g or less, and more preferably 2.0 m ² / g or more and 8 m ² / g or less.

The amorphous carbon-coated graphite material of the present embodiment preferably has a chroma c ^* value of 0.60 or less. This is because carbon materials absorb light in the long-wavelength visible light region as the π-bonds resonate widely, so that the resonance possible range of the π-bonds (the crystallite size is large) tends to exhibit color The reason is that it is difficult to express color when the resonance range of the π bond is narrow (the crystallite size is small).

The saturation c ^* value is preferably 0.50 or less, more preferably 0.40 or less.

Further, the “saturation c ^* value” mentioned here is a value measured in accordance with JIS Z 8729, as in the case of the lightness L ^* value described above.

  The loose bulk density in the amorphous carbon-coated graphite material of the present embodiment is preferably 0.10 g / mL or more and 0.50 g / mL or less. This is because when the loose bulk density is less than 0.10 g / mL, it is difficult to increase the active material density when used as a negative electrode material of a lithium ion secondary battery, and the particles are broken when compressed to increase the density. It is not preferable because the effect of the amorphous carbon coating can not be exhibited. When the loose bulk density exceeds 0.50 g / mL, when used as a negative electrode material of a lithium ion secondary battery, the contact area between the active material and the electrolyte decreases because the number of voids between particles is small. The edge portion of the carbon material to which Li can be inserted and released directly is reduced. As a result, the carbon material can not accept a large amount of Li at one time, and the negative electrode active material / electrolyte interface resistance, that is, the alternating current impedance becomes high, and the input / output characteristics deteriorate, which is not preferable.

  The loose bulk density is preferably 0.15 g / mL or more and 0.45 g / mL or less, and more preferably 0.20 g / mL or more and 0.40 g / mL or less.

  The term "loose bulk density" as used herein is a value measured by the ratio of the weight of the powder sample in the non-tapped (loose) state to the volume of the powder containing the factor of the interparticle void volume.

  The alternating current impedance of the amorphous carbon-coated graphite material of the present embodiment is preferably such that the value obtained by the evaluation method described later is 360 Ω or less. As the value of the alternating current impedance is lower, the negative electrode active material / electrolyte interface resistance is lower, and the input / output characteristics are improved. More preferably, it is 340 ohms or less, More preferably, it is 320 ohms or less.

<Lithium ion secondary battery>
Next, the lithium ion secondary battery according to the present embodiment will be described. FIG. 1 is a view showing an example of a lithium ion secondary battery provided with a negative electrode using the carbon material of the present embodiment.

  As shown in FIG. 1, the lithium ion secondary battery 10 according to the present embodiment includes the negative electrode 11, the negative electrode current collector 12, the positive electrode 13, the positive electrode current collector 14, and between the negative electrode 11 and the positive electrode 13. And an exterior 16 made of an aluminum laminate film or the like.

  As the negative electrode 11, one using the amorphous carbon-coated graphite material of the present embodiment as a negative electrode material is used. In addition, general shapes can be applied to the shapes and constituent materials of members other than the negative electrode 11 such as the negative electrode current collector 12, the positive electrode 13, the positive electrode current collector 14, the separator 15 and the package 16.

  Since the lithium ion secondary battery according to the present embodiment has the negative electrode material made of the above-mentioned amorphous carbon-coated graphite material, the input / output characteristics, particularly, extremely excellent input characteristics (rapid charge characteristics) Can be realized.

  This is an example of a lithium ion secondary battery, and the shape, the number of electrodes, the size, and the like of each member may be changed as appropriate.

<Method of Manufacturing Amorphous Carbon-Coated Graphite Material>
Next, a method of manufacturing the amorphous carbon-coated graphite material according to the present embodiment will be described.

  First, graphite particles used as a base material and a precursor of an amorphous carbon coating layer (hereinafter also referred to as "amorphous carbon precursor") are mixed.

  As a graphitic particle used as a base material, it is preferable to use that whose crystallite size La is 400 nm or less. Amorphous carbon-coated graphite material using graphite particles having a crystallite size La of more than 400 nm as a base material is undesirable because it degrades input / output characteristics when used as a negative electrode material of a lithium ion secondary battery. is there.

  The average particle size (D50G) of the graphite particles used as the base material is preferably 1 μm or more and 20 μm or less. This is because adjusting the average particle size of the graphitic particles as the base material to less than 1 μm requires enormous energy to pulverize the graphitic particles, so the cost increases and the average particle size When it exceeds 20 μm, the insertion distance of Li to the core of the inside of the graphite particle becomes long, and the diffusion rate of insertion and desorption of Li during charge and discharge becomes worse, which is not preferable because the input / output characteristics deteriorate.

  Here, examples of the amorphous carbon precursor include coal-based feedstock oils, petroleum-based feedstock oils, and organic compounds derived from resins. Among these, pitches and decomposed heavy oil are preferable, a binder pitch and an ethylene bottom oil can be suitably used, and it is particularly preferable to use a binder pitch.

  In particular, when pitches are used as an amorphous carbon precursor, those having a softening point of 50 ° C. to less than 300 ° C., fixed carbon of 50% or more, volatile matter of 15% to 60%, and ash content of less than 5% It is preferred to use. When the softening point is less than 50 ° C., it is not preferable because the pitch is softened when mixed with the graphite particles and the handling property is deteriorated. Moreover, it is not preferable that the fixed carbon content is less than 50% because the amount of the precursor required to coat the desired amount of amorphous carbon increases. In addition, if the volatile content is less than 15%, the amounts of the volatile gas and the thermal decomposition product gas generated during the heat treatment decrease, which is not preferable. Further, when the ash content exceeds 5%, it is not preferable because impurities contained in the amorphous carbon coating layer may cause a side reaction.

  In addition, the "softening point temperature" said here is a value measured based on JISK 2425.

  When the precursor of the amorphous carbon coating layer is solid at normal temperature, the average particle size (D50P) is preferably larger than that of the graphite particles, and is preferably 20 μm to 50 μm. When D50 is less than 20 μm, a large amount of volatile component is generated in a short time during heat treatment, and as a result, it is difficult to keep volatile gas and its pyrolysis product gas in the system, which is not preferable. Moreover, when D50 exceeds 50 micrometers, since amorphous carbon may segregate, it is not preferable.

  The ratio (D50P / D50G) of the average particle size (D50P) of the amorphous carbon precursor to the average particle size (D50G) of the graphitic particles is preferably 2 or more, more preferably 3 or more. . If D50P / D50G is less than 2, the precursor particles become liquid in a short time and then pyrolyze to solidify and solidify only in a narrow range. Therefore, if the amount of precursor used is small, the surface of the graphitic particles can only be covered "island-like", and if the amount of precursor is increased to make the coating uniform, the energy density is lower than that of graphite It is not preferable because the proportion occupied by the amorphous carbon component increases. In addition, by setting D50P / D50G in this range, the diffusion and thermal decomposition rates of the precursor in the heat treatment process described later can be reduced to prevent segregation of the amorphous carbon coating layer on the graphitic particles. it can.

  The ratio of the amorphous carbon coating layer to the graphite particles is preferably 1 to 20 parts by weight, and more preferably 2 to 15 parts by weight. When the ratio of the amorphous carbon coating layer to the graphite particles exceeds 20 parts by weight, the DBP absorption amount of the obtained amorphous carbon-coated graphite material becomes high, which is not preferable.

  Then, when the precursor of the amorphous carbon coating layer is solid, heat is applied to the amorphous carbon precursor to generate tackiness derived from the volatile component, and the powder is fused to deteriorate the handling property. Therefore, the mixing is preferably performed at a temperature lower by at least 30 ° C. than the softening point temperature of the precursor, and particularly preferably performed at room temperature.

  Further, as an apparatus used for mixing, for example, a vertical-type revolution-type mixer (manufactured by Seishin Enterprise Co., Ltd.) and the like may be mentioned. As mixing conditions, for example, mixing may be performed at 5 rpm and 100 rpm.

  The mixture of graphitic particles and amorphous carbon precursor is then carbonized. Although the method of carbonization is not particularly limited, for example, a method of heat treatment under a maximum reach temperature of 900 ° C. or more and 1500 ° C. or less and a retention time at the maximum reach temperature of 10 hours or less under an inert gas atmosphere such as nitrogen or argon is mentioned Be If the highest temperature reaches below 900 ° C., it is not preferable because the irreversible capacity increases due to low molecular weight hydrocarbons and functional groups remaining in the amorphous carbon coating layer. At temperatures exceeding 1500 ° C., crystallization of carbon on the surface proceeds too much, so the crystallite size increases and the edge decreases. Therefore, when it is used as a negative electrode material of a lithium ion secondary battery, the active material / electrolytic solution interface resistance becomes high, and the input / output characteristics deteriorate, which is not preferable.

  Also, in order to achieve the object of the present invention, it is important to slow the rate of softening and thermal decomposition of the amorphous carbon precursor before carbonization. When heat is applied to the amorphous carbon precursor, low molecular volatiles are fluidized as gas escapes, and when the temperature is further increased, the viscosity increases with pyrolysis and thermal polymerization, and finally only the residual carbon is solid. Remain as carbon. And, in order to deposit the volatile gas and the pyrolysis gas, it is necessary to keep the gas in the system for a long time. For that purpose, the gas is not generated at once but generated for a long time gradually Because it is desirable, the inert gas flow rate is preferably as low as possible. In addition, for the same reason as described above, it is preferable that the temperature rise rate be slow. Specifically, the temperature raising rate up to 650 ° C. is 200 ° C./hr or less, preferably 180 ° C./hr or less, more preferably 160 ° C./hr or less, and the flow rate of the inert gas introduced into the system is 10 cm / min. The pressure is set to preferably 9 cm / min or less, more preferably 8 cm / min or less.

  Also, with regard to the treatment after heat treatment, there is no problem as long as it is a mild crushing treatment to eliminate the aggregation state, but the treatment of adding a strong force to the powder that a new crushed surface comes out is non-crystalline This is not preferable because the effect of the quality carbon coating is diminished.

  By the above method, the amorphous carbon-coated graphite material in the present embodiment can be obtained.

  The amorphous carbon-coated graphite material in this embodiment is used as a negative electrode material of lithium ion secondary batteries for various devices, or a negative electrode material of lithium ion capacitors, and is used as a hybrid electric vehicle (HEV) or a plug It is particularly preferably used as a negative electrode material of a battery such as a lithium ion secondary battery of an in-hybrid electric vehicle (PHEV), which has a high charge / discharge rate and requires a large capacity.

  Hereinafter, the invention according to the present application will be more specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

-Explanation of measurement method-
(A) Measurement of crystallite size La A sample powder is mixed with 20% by weight of a Si standard sample as an internal standard, and an X-ray diffractometer (manufactured by Bruker AXS Co., Ltd., trade name: NEW D8 ADVANCE) is used Measured. Then, based on the methods described in the following documents (1) to (4), the analysis was performed using Carbon Analyzer Version 5.0A to calculate the crystallite size La of the graphite particles.
(1) Hiroyuki Fujimoto, Carbon No. 206 (2003), pp. 1-6 (2) Michio Inagaki, carbon number. 36 (1963), pages 25-34 (3) N.I. Iwashita, C.I. R. Park, H .; Fujimoto, M. Shiraishi, M. Inagaki, Carbon 42 (2004), pp. 701-714 (4) Japan Society for the Promotion of Science 117th Committee, Carbon No. 221 (2006), pages 52-60.

(B) Measurement of average particle diameter D50, D50G, D50P and 10% particle diameter D10 Wet measurement unit of laser diffraction scattering type particle size distribution measuring instrument (manufactured by Seishin Co., Ltd., trade name: SK laser micronizer LMS-2000e) The measurement was carried out at In addition, in measurement, after wetting the sample with a surfactant in advance, it was dispersed in pure water and evaluated.

(C) Measurement of lightness L ^* value, chroma c ^* value The lightness L ^* value and chroma c ^* value are first prepared by kneading 0.25 g of a sample and 0.5 ml of castor oil with a Huber-type muller, To this paste, 6.5 g of clear lacquer was added, kneaded, and made into a coating to prepare a sheet coated on cast coated paper using a 127 μm (5 mil) applicator. Next, the prepared sheet is measured using a color difference meter (manufactured by Konica Minolta Sensing, Inc., trade name: CR? 300), and the coloration index (L ^* value, according to JIS Z 8729). c ^* value).

(D) Measurement of loose bulk density A funnel was attached to the funnel base, a 0.5 mm wide screen was placed on the funnel, and the receiver was correctly placed on the receiver base. Next, one sample of the sample was placed on a sieve, the sample passed through the sieve was received in the receiver, and this operation was repeated until the processed sample was piled up on the receiver. Then, using a spatula, the mountain portion was scraped off and then the weight of the contents of the receiver was measured. And loose bulk density was computed by the following numerical formula (1). The receiver used had a volume of 30 mL.

(1)
Loose bulk density (g / ml) = Weight of processed sample in receiver (g) ÷ 30 (1)

(E) Measurement of tap density About 10 g of a sample is passed through a 32 mesh sieve, and 10.0 g of the sample passed through the sieve (5.0 g if the loose bulk density is 0.4 g / ml or less) is weighed with an upper balance Take it and put it into a measuring cylinder of 25 ml in volume. Next, the measuring cylinder containing the sample is set in a tap tester (manufactured by Kuramochi Scientific Instruments Mfg. Co., Ltd., trade name: KRS-406), and after tapping 600 times, the sample volume in the measuring cylinder is 0.1 ml. It was measured by reading with the accuracy of And loose bulk density was computed by the following numerical formula (2).

(2)
Tap density (g / ml) = sample weight (g) ÷ sample volume (ml) (2)

(F) Measurement of dibutyl phthalate (DBP) absorption amount It measured based on JISK6217-4.

(G) Preparation of electrode for evaluation of alternating current impedance and evaluation test Evaluation of alternating current impedance was performed using a symmetrical cell in which negative electrodes of the same composition were paired.

(Production of negative electrode sheet)
First, 97% by weight of sample, 1.5% by weight of 1.5% aqueous solution of carboxymethylcellulose (CMC, manufactured by Daicel Corporation), and 1.5% by weight of aqueous solution of 51.7% of styrene butadiene rubber (SBR) are added. A slurry kneaded with a rotation / revolution mixer (manufactured by Shinky Co., Ltd.) was applied onto a high purity copper foil using an automatic coating apparatus (manufactured by Tester Sangyo Co., Ltd.). Next, the coated sheet was dried, and then pressed at a linear pressure of 0.07 to 0.10 kN / mm using a roll press (manufactured by Takasen Co., Ltd.). Next, after pressing, 12 sheets of an electrode having a basis weight of 35 mg / 1.77 cm ² (1515) are punched out using an electrode punching machine 1515 (manufactured by Takasen Co., Ltd.), and this is subjected to 120 ° C. in a vacuum dryer. Drying was performed for 12 hours.

(Preparation of half cell for pretreatment)
First, in a dry argon atmosphere with a dew point of −80 ° C. or less, a negative electrode sheet produced between a coin cell cap and a case and metallic lithium foil are laminated via a separator (made of polypropylene, microporous film (Celgard 2400)) The Next, in this laminate, a mixture of an electrolyte (EC (ethylene carbonate) and DMC (dimethyl carbonate) in a ratio of 1: 2 is used as a solvent, and LiPF ₆ as an electrolyte is dissolved in this at a concentration of 1 mol / L. An appropriate amount of the solution was added and caulking was performed using an automatic coin caulking machine (manufactured by Takasen Co., Ltd.) to obtain a pretreatment half cell. In addition, 12 cells of this pretreatment half cell were prepared for each one level of the example and the comparative example.

(Production of negative electrode for AC impedance evaluation)
The prepared half cell for pretreatment was set in a charge / discharge device, and Li insertion / extraction to / from the negative electrode was performed at 25 ° C. The charge was constant current charge (CC charge) to 10 mV at 0.2 C, and when the current was attenuated to 0.05 C, the charge was completed. The discharge was constant current discharge (CC discharge) at 0.2 C and cut off at 1.5 V.

  And this charge / discharge was repeated 6 times, and charge was stopped at the time of charging to the electric potential which becomes 50% of the charge depth by 0.2C at the 7th time. Next, the half cell for pretreatment was disassembled in a dry argon atmosphere with a dew point of −80 ° C. or less, the negative electrode was taken out, and washed with dimethyl carbonate to obtain a negative electrode sheet for AC impedance evaluation. In addition, six sheets of this negative electrode sheet were prepared for each one level of the example and the comparative example.

(Preparation of symmetrical cell)
First, two negative electrodes for AC impedance evaluation manufactured from the same negative electrode sheet were prepared, and were laminated via a separator in a coin cell. Next, in this laminate, a mixture of an electrolyte (EC (ethylene carbonate) and DMC (dimethyl carbonate) in a ratio of 1: 2 is used as a solvent, and LiPF ₆ as an electrolyte is dissolved in this at a concentration of 1 mol / L. The symmetrical cell was produced by adding and caulking. Three symmetrical cells were prepared for each level of the example and the comparative example.

(AC impedance evaluation)
The measurement was performed using an AC impedance measuring device (power supply; 1470E, FRA; 1255B) (manufactured by Solartron). More specifically, the symmetrical cell was kept warm for 60 minutes in a constant temperature bath adjusted to -10 ° C., and the measurement frequency was set to 0.01 Hz to 1,000,000 Hz, and then the measurement was started. After the measurement, the resistance value of 0.1 Hz was read and evaluated.

-Preparation of carbon materials according to examples and comparative examples-
The crystallite sizes La, D10 and D50G of the graphite particles used as the base material in the following Examples and Comparative Examples are shown in Table 1.

  Binder having a softening point of 80 ° C. (precursor a), 110 ° C. (precursors b, e), 250 ° C. as a precursor of the amorphous carbon coating layer in the following Examples 1 to 8 and Comparative Examples 1 to 5 Pitch (precursor c) and ethylene bottom oil (EBO, precursor d) were used. The D50P, softening point, fixed carbon (including precursor d), volatile matter (including precursor d), and ash of each of the precursors a to c and e are shown in Table 2.

Example 1
Graphite particles A and the precursor b of the amorphous carbon coating layer were mixed at room temperature such that the ratio of the amorphous carbon coating layer to 100 parts by weight of the graphite particles A was 2.5 parts by weight. The amorphous carbon precursor D50P / graphite particles D50G at this time were 5.2.

  The mixed powder obtained above is subjected to a nitrogen gas flow rate of 6.2 cm / min using an atmosphere-compatible small roller hearth kiln (manufactured by Noritake Co., Ltd.), and the temperature rising rate to a furnace temperature of 650 ° C. is 180 ° C. The temperature was raised so that the temperature rising rate from 650 ° C. to 1000 ° C. was 350 ° C./hr, and the carbonization treatment was performed with the holding time at 1000 ° C. set to 4 hours.

  Next, the amorphous carbon-coated graphite material of Example 1 was obtained by classification using a sieve with an opening of 45 μm. The production conditions at this time are shown in Table 3, and the characteristics of the obtained amorphous carbon-coated graphite material are shown in Table 4.

Examples 2 to 8
As shown in Table 3, the ratio of the amorphous carbon coating layer to 100 parts by weight of each graphite particle of the graphite particles A and B and the precursors a, b, c and d of the amorphous carbon coating layer The mixture was mixed at room temperature to 5 parts by weight, 10 parts by weight, and 20 parts by weight, respectively, and nitrogen gas flow rate and furnace temperature 650 were obtained using an atmosphere-compatible small roller hearth kiln (manufactured by Noritake Company Limited). The temperature is raised so that the temperature rising rate from 650 ° C. to 1000 ° C. becomes 350 ° C./hr as in the above-mentioned Example 1 except that the temperature rising rate to ° C. is variously changed, and the temperature at 1000 ° C. The carbonization was performed with the holding time set to 4 hours.

  Next, the resultant was classified with a sieve with an opening of 45 μm to obtain amorphous carbon-coated graphite materials of Examples 2 to 8. The production conditions of Examples 2 to 8 are shown in Table 3, and the characteristics of the obtained amorphous carbon-coated graphite material are shown in Table 4.

  In addition, a coin-type lithium ion secondary battery was produced using the amorphous carbon-coated graphite material of Example 2, and battery evaluation was performed. The preparation conditions, the evaluation method, and the evaluation results are shown below.

(Production of electrode for evaluation of input / output characteristics and evaluation test)
The evaluation of the input / output characteristics was performed using a coin cell in which metal Li was used as a counter electrode.

  Further, as the negative electrode sheet for evaluation of input / output characteristics and the half cell for pretreatment, the above-mentioned half cell for pretreatment for AC impedance evaluation was used.

(Charge / discharge treatment before input / output characteristics evaluation)
The pretreatment half cell prepared in the above-mentioned (g) Preparation of electrode for AC impedance evaluation and evaluation test was set in a charge / discharge device, and Li insertion / desorption to / from the negative electrode was performed at 25 ° C. The charge is constant current charge (CC charge) to 5 mV at 0.1 C, and when the current decays to 0.01 C, the charge is completed, and the discharge is constant current discharge (CC discharge) at 0.1 C; Cut off at 5V. And this charge / discharge was repeated 6 times, and evaluation was performed changing the charge rate or the discharge rate from the 7th time.

(Evaluation of high rate charge characteristics)
Charge is constant current charge (CC charge) up to 5mV at 0.2C, 0.5C, 1C, 2C, 3C respectively, and when the current decays to 0.01C, it is considered that the charge is completed and discharge is constant at 0.2C. A current discharge (CC discharge) was performed and cut off at 1.5 V. The charge capacity retention rate was calculated by dividing the charge capacity at each of these rates by the charge capacity at 0.2C.

(Evaluation of high rate discharge characteristics)
Charging is constant current charging (CC charging) to 5mV at 0.2C, and when the current decays to 0.01C, charging is completed, and discharging is 0.2C, 0.5C, 1C, 2C, 2C, 3C, 4C, respectively. Constant current discharge (CC discharge) was performed and cut off at 1.5V. The discharge capacity retention rate was calculated by dividing the discharge capacity at each of these rates by the discharge capacity at 0.2C.

  And as a result of measuring Example 2 by the above-mentioned method, charge capacity maintenance factor showed a high value with 97% at 0.5C, 93% at 1C, 85% at 2C, and 83% at 3C. The discharge capacity retention rate was 96% at 0.5C, 92% at 1C, 83% at 2C, 82% at 3C, 79% at 4C, and 78% at 5C.

Comparative Examples 1 and 2
Raw graphite particles A and C are used as a graphite material as they are, and using a small-sized roller hearth kiln (manufactured by Noritake Co., Ltd.) for the atmosphere, nitrogen gas flow rate 6.2 cm / min, furnace temperature 650 The temperature rise rate to 180 ° C is 180 ° C / hr, the temperature rise rate from 650 ° C to 1000 ° C is 350 ° C / hr, and the holding time at 1000 ° C is set to 4 hours for carbonization treatment went.

  Next, the graphite material of Comparative Examples 1 and 2 was obtained by classifying with a sieve of 45 μm openings. The production conditions of Comparative Examples 1 and 2 are shown in Table 3, and the characteristics of the obtained graphite material are shown in Table 4.

<Comparative Examples 3 to 5>
As shown in Table 3, the ratio of the amorphous carbon coating layer to 100 parts by weight of the graphite particles is 5 weight for each of the graphite particles B and C and the precursors b and e for the amorphous carbon coating layer. Mixed at room temperature to be 10 parts by weight and 10 parts by weight, using an atmosphere-compatible small roller hearth kiln (manufactured by Noritake Co., Ltd.), at a nitrogen gas flow rate of 3.2 cm / min, up to a furnace temperature of 650.degree. The temperature was raised such that the temperature rising rate was 150 ° C./hr and the temperature rising rate from 650 ° C. to 1000 ° C. was 350 ° C./hr, and the carbonization treatment was performed by setting the holding time at 1000 ° C. to 4 hours.

  Next, by classifying with a sieve of 45 μm openings, an amorphous carbon-coated graphite material of Comparative Examples 3 to 5 was obtained. The production conditions of Comparative Examples 3 to 5 are shown in Table 3, and the characteristics of the obtained amorphous carbon-coated graphite material are shown in Table 4.

As shown in Table 4, the amorphous carbon-coated graphite materials of Examples 1 to 8 having a lightness L ^* value of 35.0 or less and a tap density of 0.3 g / mL or more and 0.75 g / mL or less As compared with Comparative Examples 1 to 5, it was proved that the value of the AC impedance is lower, and the reduction of resistance can be realized.

In particular, in Examples 3 to 6 in which the lightness L ^* value is 34.0 or less, it can be seen that the value of the AC impedance is extremely low, and a reduction in resistance can be realized.

  The negative electrode material for a lithium ion secondary battery of the present embodiment can be effectively used, for example, for stationary lithium ion batteries for electric vehicles, homes, and the like.

DESCRIPTION OF SYMBOLS 10 lithium ion secondary battery 11 negative electrode 12 negative electrode collector 13 positive electrode 14 positive electrode collector 15 separator 16 exterior

Claims (5)

An anode material for a lithium ion secondary battery comprising an amorphous carbon-coated graphite material in which amorphous carbon is coated on the surface of a graphite particle, the lightness L ^* value is 35.0 or less, and the tap density is Is 0.3 g / mL or more and 0.75 g / mL or less, and an anode material for a lithium ion secondary battery.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a crystallite size La calculated from an X-ray wide-angle diffraction line is 400 nm or less.   The negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein a dibutyl phthalate (DBP) absorption amount is 57 mL / 100 g or more and 120 mL / 100 g or less.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein an average particle diameter (D50) is 1 μm or more and 25 μm or less.   The negative electrode material according to any one of claims 1 to 4 is used as a negative electrode material for a lithium ion secondary battery.

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