JP6977491B2 - Negative electrode material for lithium ion secondary batteries, and lithium ion secondary batteries using them. - Google Patents

Description

The techniques disclosed herein relate to graphite-based carbon negative electrode materials for lithium ion secondary batteries.

Lithium-ion secondary batteries are lighter and have higher capacity than conventional secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries, and lead batteries. Therefore, portable electronic devices such as mobile phones and notebook computers can be used. It has been put into practical use as a power source for driving.

Furthermore, in recent years, with the increase in the capacity of lithium-ion secondary batteries, it is being expanded to households and electric vehicles.

In particular, when assuming a lithium-ion secondary battery for an in-vehicle use such as an electric vehicle, low-temperature performance and high-temperature life performance are also important because they are used in various environments. Further, considering the convenience of a gasoline-powered vehicle, high input / output characteristics are required for quick charging performance that can be charged in a short time and for instantaneous acceleration and regeneration.

A carbon or graphite-based carbon material capable of intercalating lithium ions is generally used for the negative electrode of the lithium ion secondary battery.

^{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 the battery is charged and discharged. Unlike the positive electrode active material that originally contains Li as a crystal, a carbon material that accepts Li for the first time when it operates as a battery causes a side reaction in the Li insertion process, so that side reaction consumes Li, which is the initial stage. It is known to reduce efficiency.

On the other hand, the lower the resistance of the negative electrode active material / electrolytic solution interface, the lower the energy required for Li to enter the negative electrode active material, and the better the input / output characteristics tend to be. Further, if 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 be improved. Further, heat generation due to resistance leads to deterioration of the electrolytic solution and deteriorates the cycle characteristics. Therefore, by using a negative electrode active material having a low resistance at the negative electrode active material / electrolytic 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 into the entire negative electrode active material, and the easier it is for Li to diffuse, the shorter the time it takes for Li to fill the entire negative electrode active material. Further improvement of input / output characteristics can be expected by using a negative electrode active material that easily diffuses.

So far, graphitized carbon materials (hard carbon) and amorphous carbon materials have been known as materials having excellent 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 made of carbonic graphite particles in which graphite particles are coated with carbonaceous material has been proposed. More specifically, from carbonaceous coated graphite particles in which a graphite material is coated with a carbon material such as pitch and then fired at a temperature of 700 to 1500 ° C. in an inert gas atmosphere to form a carbonaceous substance on the surface layer. The pore volume of 1 nm or less determined by the HK method was 0.0010 to 0.0020 cm ³ / g based on the adsorption isotherm of carbonaceous coated graphite particles by the nitrogen method, and 1 was determined by the BJH method. A negative electrode material for a lithium ion secondary battery having a pore volume of about 100 nm of 0.020 to 0.040 cm ^{3 / g is disclosed.} It is described that the use of such a negative electrode material can improve high rate characteristics without impairing capacity, initial charge / discharge efficiency, and cycle characteristics (see, for example, Patent Document 2).

Patent No. 4877568 Japanese Unexamined Patent Publication No. 2014-170724

However, the graphitized 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. Therefore, it is known that the initial efficiency is very poor. In addition, hard carbon and amorphous carbon materials are hard because layered graphite crystals are not developed, and at the same time, the true density is low because the distance between graphite mesh surfaces is not narrowed, and per volume as a negative electrode. Since the energy density is also low, it is not suitable for in-vehicle applications where the filling density is increased to increase the capacity.

Further, the negative electrode material for the lithium ion secondary battery described in Patent Document 2 has a high rate characteristic of about 60 to 65.7% in 1C and has a sufficient quick charging characteristic. It is hard to say that there is.

Therefore, in view of the above-mentioned problems, the present invention is a graphite-based carbon negative electrode capable of obtaining a lithium ion secondary battery having significantly excellent input / output characteristics, particularly input characteristics (quick charging characteristics) of the lithium ion secondary battery. The purpose is to provide the material.

In order to achieve the above object, the negative electrode material for a lithium ion secondary battery according to the present invention is made of an amorphous carbon-coated graphite material in which the surface of graphitic particles is coated with amorphous carbon, and has a brightness of L ^*. 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 / electrolytic solution interface.

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

The negative electrode material for the lithium ion secondary battery according to the present embodiment, and the method for manufacturing the negative electrode material for the lithium ion secondary battery and the lithium ion secondary battery using the same will be described below. It should be noted that the following is an example of the embodiment, and the constituent materials, the shapes of the constituent materials or members, the conditions for processing and heat treatment, and the like can be appropriately changed as long as the gist of the present invention is not deviated.

The negative electrode material for the lithium ion secondary battery of the present 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 has a lightness L ^* value of 35. It is characterized in that it is 0.0 or less and the tap density is 0.3 g / mL or more and 0.75 g / mL or less.

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 this embodiment is 35.0 or less. ^{In general, carbon materials such as carbon black and graphite become blacker (that is, the brightness L *} value becomes smaller) as the number of edges (so-called “edges”) of the graphite mesh surface of the carbon material that serves as the entrance / exit of Li increases. Is said to be. Therefore, the ^{amorphous carbon-coated graphite material having an L *} value of 35.0 or less has many edges of the carbon material that serves as the entrance and exit of Li, so that Li is efficiently inserted and removed from the contact surface with the electrolytic solution. The negative electrode active material / electrolytic solution interface resistance, that is, the AC impedance is lowered. By using a low-resistance amorphous carbon-coated graphite material having a low AC impedance as a negative electrode material for a lithium ion secondary battery, the energy required for inserting Li is reduced, so that Li can be used even in a low temperature environment. Can be inserted and removed, and a large amount of Li can be inserted and removed at one time while suppressing deterioration of the electrolytic solution due to resistance. Therefore, it is possible to improve not only the input / output characteristics of the lithium ion secondary battery but also the low temperature characteristics and the cycle characteristics.

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

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 Hoover type marler to form a paste, and clear lacquer is added to this paste. It is a color index obtained in accordance with JIS Z 8729 for the sheets that have been kneaded and applied.

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 does not easily increase when used as a negative electrode material for 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 electrolytic solution becomes small when used as a negative electrode material for a lithium ion secondary battery because there are few voids between particles, and Li becomes The number of edges of the carbon material that can be directly inserted and removed is reduced. Therefore, the carbon material cannot accept a large amount of Li at one time, and the negative electrode active material / electrolytic solution interface resistance, that is, the AC impedance becomes high, so that the input / output characteristics deteriorate, which is not preferable.

That is, in the present embodiment, the electrode and the electrolytic solution are obtained 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. Since a sufficient contact area between the active material and the electrolytic solution can be secured at the interface with the lithium ion secondary battery, it is possible to improve the input / output characteristics of the lithium ion secondary battery.

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. More preferably, it is mL or less.

Further, in the amorphous carbon-coated graphite material of the present embodiment, it is preferable that the crystallite size La calculated from the X-ray wide-angle diffraction line is 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 for a lithium ion secondary battery, the Li insertion distance to the core inside the graphite particles becomes long, and Li is inserted during charging and discharging. This is because the diffusion rate of desorption becomes worse and the input / output characteristics deteriorate.

The crystallite size La is preferably 300 nm or less, more preferably 250 nm or less.

Further, the absorption amount of dibutyl phthalate (DBP) in the amorphous carbon-coated graphite material of the present embodiment is preferably 57 mL / 100 g or more and 120 mL / 100 g or less. The amount of DBP absorbed is considered to reflect the large contact area with the electrolytic solution on the surface of the carbon material. Therefore, when the amount of DBP absorbed is less than 56 ml / 100 g, the contact area with the electrolytic solution is small, so that the number of entrances and exits of Li is small, and the input / output characteristics are deteriorated when used as an active material of a lithium ion secondary battery. Therefore, it is not preferable. Further, when the amount of DBP absorbed exceeds 120 mL / 100 g, a large amount of solvent is required to disaggregate the particles, and the dispersibility is lowered, which is not preferable.

The amount of DBP absorbed 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.

Further, the “dibutyl phthalate (DBP) absorption amount” referred to here means a value measured in accordance with JIS K 6217-4.

The average particle size of the amorphous carbon-coated graphite material of the present embodiment (hereinafter, also referred to as “D50”) is preferably 1 μm or more and 25 μm or less. This is because 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 graphite particles as a base material to less than 1 μm, but the average of the graphite particles is Since enormous energy is required to pulverize the particle size to less than 1 μm, the cost increases and it is industrially disadvantageous. Further, when the average particle size exceeds 25 μm, the Li insertion distance to the core inside the graphite particles becomes long, and the diffusion rate of Li insertion / desorption during charging / discharging deteriorates, so that the input / output characteristics deteriorate, which is preferable. Because there is no such thing. The average particle size is preferably 2 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

Further, the "average particle size (D50)" referred to here means a 50% particle size in the volume-based cumulative particle size distribution by a laser diffraction type particle size distribution meter.

The 10% particle size (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 or more and 10 μm or less. This is preferable because when D10 is less than 0.5 μm, a side reaction with the electrolytic solution is likely to occur when used as a negative electrode material for a lithium ion secondary battery, and Li in the battery cell is consumed as an irreversible capacity. No. Further, when D10 exceeds 10 μm, the particle size distribution becomes sharp and the density of the active material as the negative electrode material of the lithium ion secondary battery is difficult to increase, which is not preferable.

The D10 is preferably 0.75 μm or more and 7.5 μm or less, and more preferably 1 μm or more and 5 μm or less.

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 the lithium ion secondary battery, so the number of entrances and exits of Li is reduced, and the input / output characteristics are reduced. This is because it is not preferable. Further, when the BET specific surface area ^{exceeds 10 m 2} / g, a side reaction with the electrolytic solution is likely to occur when used as a negative electrode material for a lithium ion secondary battery, and Li in the battery cell is consumed as an irreversible capacity. This is because it is not preferable.

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 saturation c ^* value of 0.60 or less. This is because carbon materials absorb light in the visible light region with a long wavelength as the π bond resonates widely, so colors are more likely to appear in those with a wide π bond resonance range (large crystallite size). , The π bond with a narrow resonable range (small crystallite size) is difficult to develop color.

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

Further, the “saturation c ^* value” referred to here is a value measured in accordance with JIS Z 8729, similarly to the above-mentioned brightness L ^{* value.}

The looseness density of 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 bulk density is less than 0.10 g / mL, the active material density does not easily increase when used as a negative electrode material for lithium-ion secondary batteries, and the particles are damaged when compressed to increase the density. It is not preferable because the effect of the amorphous carbon coating cannot be exhibited. When the looseness density exceeds 0.50 g / mL, when used as a negative electrode material for a lithium ion secondary battery, the contact area between the active material and the electrolytic solution becomes small due to the small number of voids between the particles. The number of edges of the carbon material from which Li can be directly inserted and removed is reduced. As a result, the carbon material cannot accept a large amount of Li at one time, the negative electrode active material / electrolytic solution interface resistance, that is, the AC impedance becomes high, and the input / output characteristics deteriorate, which is not preferable.

The looseness 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.

Further, the "looseness density" referred to here is a value measured by the ratio of the weight of the powder sample in the untapped (loose) state to the volume of the powder containing the factor of the interparticle void volume.

The AC impedance of the amorphous carbon-coated graphite material of the present embodiment is preferably a value of 360 Ω or less obtained by the evaluation method described later. The lower the AC impedance value, the lower the negative electrode active material / electrolyte interfacial resistance and the better the input / output characteristics. It is more preferably 340 Ω or less, and further preferably 320 Ω or less.

<Lithium-ion secondary battery>
Next, the lithium ion secondary battery according to this embodiment will be described. FIG. 1 is a diagram 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 has a negative electrode 11, a negative electrode current collector 12, a positive electrode 13, a positive electrode current collector 14, and a negative electrode 11 and a positive electrode 13. It is provided with a separator 15 interposed therebetween and an exterior 16 made of an aluminum laminated film or the like.

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

Since the lithium ion secondary battery according to the present embodiment has a negative electrode material made of the above-mentioned amorphous carbon-coated graphite material, it has input / output characteristics, particularly excellent input characteristics (quick charging 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 appropriately changed.

<Manufacturing method of amorphous carbon-coated graphite material>
Next, a method for producing the amorphous carbon-coated graphite material according to the present embodiment will be described.

First, the graphitic particles used as the base material and the precursor of the amorphous carbon coating layer (hereinafter, also referred to as “amorphous carbon precursor”) are mixed.

As the graphitic particles used as the base material, those having a crystallite size La of 400 nm or less are preferably used. This is because an amorphous carbon-coated graphite material using graphite particles having a crystallite size La of more than 400 nm as a base material is not preferable because the input / output characteristics deteriorate when used as a negative electrode material for a lithium ion secondary battery. be.

The average particle size (D50G) of the graphitic 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 graphite particles as the base material to less than 1 μm requires enormous energy to crush the graphite particles, which increases the cost and also increases the average particle size. If it exceeds 20 μm, the Li insertion distance to the core inside the graphite particles becomes long, and the diffusion rate of Li insertion / desorption during charging / discharging deteriorates, which is not preferable because the input / output characteristics deteriorate.

Here, examples of the amorphous carbon precursor include coal-based raw material oils, petroleum-based raw material oils, and resin-derived organic compounds. Of these, pitches and cracked heavy oil are preferable, binder pitch and ethylene bottom oil can be preferably used, and binder pitch is particularly preferable.

In particular, when pitches are used as amorphous carbon precursors, those having a softening point of 50 ° C. or higher and lower than 300 ° C., fixed carbon of 50% or higher, volatile content of 15% or higher and 60% or lower, and ash content of less than 5% are used. It is preferable to use it. If the softening point is less than 50 ° C., the pitch is softened when mixed with the graphitic particles, and the handling property is deteriorated, which is not preferable. Further, if the amount of fixed carbon is less than 50%, the amount of precursor required for coating the desired amount of amorphous carbon increases, which is not preferable. Further, if the volatile content is less than 15%, the amount of volatile gas and pyrolysis generated gas generated during the heat treatment is small, which is not preferable. Further, when the ash content exceeds 5%, impurities contained in the amorphous carbon coating layer may cause a side reaction, which is not preferable.

The "softening point temperature" referred to here is a value measured in accordance with JIS K 2425.

When the precursor of the amorphous carbon coating layer is solid at room temperature, the average particle size (D50P) is preferably larger than that of graphitic particles, and preferably 20 μm or more and 50 μm or less. If D50 is less than 20 μm, a large amount of volatile components are generated in a short time during heat treatment, and as a result, it is difficult to retain the volatile gas and its thermal decomposition product gas in the system, which is not preferable. Further, if D50 exceeds 50 μm, amorphous carbon may segregate, which is not preferable.

The ratio (D50P / D50G) of the average particle size (D50P) of the amorphous carbon precursor and the average particle size (D50G) of the graphitic particles is preferably 2 or more, more preferably 3 or more. .. When D50P / D50G is less than 2, the precursor particles become liquid in a short time and then thermally decompose to solid carbon, and can be diffused only in a narrow range. Therefore, if the amount of precursor used is small, the surface of the graphite particles can only be covered in an "island" shape, and if the amount of precursor is large to make the coating uniform, the energy density is lower than that of graphite. It is not preferable because the proportion of the amorphous carbon component increases. Further, by setting D50P / D50G in this range, it is possible to slow down the diffusion and thermal decomposition rate of the precursor in the heat treatment step described later and prevent segregation of the amorphous carbon coating layer on the graphitic particles. can.

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

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

Examples of the apparatus used for mixing include a vertical revolution type mixer (manufactured by Seishin Enterprise Co., Ltd.), and examples of the mixing conditions may be mixing at a revolution of 5 rpm and a rotation of 100 rpm.

The mixture of graphitic particles and amorphous carbon precursor is then carbonized. The method of carbonization is not particularly limited, and examples thereof include a method of heat treatment in an atmosphere of an inert gas such as nitrogen or argon with a maximum ultimate temperature of 900 ° C. or higher and 1500 ° C. or lower and a holding time of 10 hours or lower at the maximum ultimate temperature. Be done. When the maximum temperature reached is lower than 900 ° C., the irreversible capacity increases due to the low molecular weight hydrocarbons and functional groups remaining in the amorphous carbon coating layer, which is not preferable. Further, at a temperature exceeding 1500 ° C., crystallization of carbon on the surface proceeds too much, so that the crystallite size increases and the edge portion decreases. Therefore, when used as a negative electrode material for a lithium ion secondary battery, the active material / electrolyte interfacial resistance increases and the input / output characteristics deteriorate, which is not preferable.

Further, in order to achieve the object of the present invention, it is important to slow down 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-weight volatile components are released as gas and fluidized, and when the temperature rises further, the viscosity increases with thermal decomposition and thermal polymerization, and finally only the residual carbon content is solid. Remains as carbon. Then, 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, and for that purpose, the gas is not generated all at once but gradually generated over a long period of time. Therefore, it is preferable that the flow rate of the inert gas is as low as possible. Further, for the same reason as described above, it is preferable that the rate of temperature rise is slow. Specifically, the heating rate up to 650 ° C. is set to 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. Hereinafter, the setting is preferably 9 cm / min or less, more preferably 8 cm / min or less.

Further, regarding the treatment after the heat treatment, there is no problem as long as it is a light crushing treatment for eliminating the agglutination state, but the treatment of applying a strong force to the powder so as to generate a new crushed surface is amorphous. It is not preferable because the effect of coating with quality carbon is diminished.

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

The amorphous carbon-coated graphite material in the present embodiment is used as a negative electrode material for a lithium ion secondary battery for various devices or a negative electrode material for a lithium ion capacitor, and is used as a hybrid electric vehicle (HEV) or a plug. It is particularly preferably used as a negative electrode material for 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 described in more detail 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 20% by weight of Si standard sample is mixed with the sample powder as an internal standard, and an X-ray diffractometer (manufactured by Bruker AXS Co., Ltd., trade name: NEW D8 ADVANCE) is used. Was measured. Then, the crystallite size La of the graphitic particles was calculated by performing analysis using Carbon Analyzer Version 5.0A based on the methods described in the documents (1) to (4) below.
(1) Hiroyuki Fujimoto, Carbon No. 206 (2003), pp. 1-6 (2) Michio Inagaki, Carbon No. 36 (1963), pp. 25-34 (3) N.M. Iwashita, C.I. R. Park, H. et al. Fujimoto, M.D. Shiraishi, M.D. Inagaki, Carbon 42 (2004), pp. 701-714 (4) Japan Society for the Promotion of Science 117 Committee, Carbon No. 221 (2006), pp. 52-60

(B) Measurement of average particle size D50, D50G, D50P and 10% particle size D10 Wet measurement unit of laser diffraction scattering type particle size distribution measuring device (manufactured by Seishin Enterprise Co., Ltd., trade name: SK Laser Micronsizer LMS-2000e) The measurement was performed at. In the measurement, the sample was previously wetted with a surfactant and then dispersed in pure water for evaluation.

(C) the lightness ^{L *} value, measured lightness ^{L *} value and chroma ^{c *} value of chroma ^{c *} value, first, a paste by kneading a sample 0.25g and castor oil 0.5ml Hoover muller, 6.5 g of clear lacquer was added to this paste, kneaded and made into a paint, and applied on cast-coated paper using a 127 μm (5 mil) applicator to prepare a sheet. Next, the produced sheet was measured using a color difference meter (manufactured by Konica Minolta Sensing Co., Ltd., trade name: CR? 300), and the color index (L ^* value,) was specified in accordance with JIS Z 8729. It is shown by c ^{* value).}

(D) Measurement of loose bulk density A funnel was attached to the funnel stand, a funnel with an opening of 0.5 mm was placed on the funnel, and the receiver was correctly placed on the receiver stand. Next, 1 scoop of sample was placed on the flue, the sample passed through the flue was received by the receiver, and this operation was repeated until the processed sample was piled up in the receiver. Next, the mountain part was scraped off with a spatula, and then the weight of the contents of the receiver was measured. Then, the bulk density was calculated by the following mathematical formula (1). The receiver used was one with a volume of 30 mL.

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

(E) Measurement of tap density Pass about 10 g of a sample through a 32 mesh flue, and weigh 10.0 g of the sample (5.0 g when the loose bulk density is 0.4 g / ml or less) through the flue with a precision balance. It was taken and gently placed in a graduated cylinder with a volume of 25 ml. Next, set the measuring cylinder containing the sample in a tap tester (manufactured by Kuramochi Kagaku Kikai Seisakusho Co., Ltd., trade name: KRS-406), tap it 600 times, and then increase the volume of the sample in the measuring cylinder to 0.1 ml. Measured by reading with the accuracy of. Then, the bulk density was calculated by the following mathematical formula (2).

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

(F) Measurement of dibutyl phthalate (DBP) absorption amount The measurement was performed in accordance with JIS K6217-4.

(G) Electrode fabrication and evaluation test for AC impedance evaluation AC impedance was evaluated using symmetric cells with a pair of negative electrodes of the same composition.

(Preparation of negative electrode sheet)
First, 97% by weight of the sample, 1.5% by weight of a 1.5% carboxymethyl cellulose aqueous solution (CMC, manufactured by Daicel Co., Ltd.), and 1.5% by weight of a 51.7% styrene butadiene rubber (SBR) aqueous solution are added. , The slurry kneaded with a rotation / revolution mixer (manufactured by Shinky Co., Ltd.) was applied onto a high-purity copper foil using an automatic coating device (manufactured by Tester Sangyo Co., Ltd.). Next, after the coated sheet was dried, it was pressed at a linear pressure of 0.07 to 0.10 kN / mm using a roll press (manufactured by Hosen Co., Ltd.). ^{Next, after pressing, 12 electrodes having a basis weight of 35 mg / 1.77 cm 2} (Φ15) were punched using an electrode punching machine Φ15 (manufactured by Hosen Co., Ltd.), and these were punched in a vacuum dryer at 120 ° C. 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 prepared between a coin cell cap and a case and a metallic lithium foil are laminated via a separator (polypropylene, microporous film (Cellguard 2400)). rice field. Next, in this laminate, a mixture of an electrolytic solution (EC (ethylene carbonate) and DMC (dimethyl carbonate) at a ratio of 1: 2 was used as a solvent, and LiPF ₆ was dissolved as an electrolyte at a concentration of 1 mol / L. An appropriate amount was added and crimped using an automatic coin caulking machine (manufactured by Hosen Co., Ltd.) to obtain a pretreatment half cell. In addition, 12 cells for each level of this pretreatment half cell were prepared in Example and Comparative Example.

(Manufacturing of negative electrode for AC impedance evaluation)
The prepared half cell for pretreatment was set in a charging / discharging device, and Li was inserted into and detached from the negative electrode at 25 ° C. For charging, constant current charging (CC charging) was performed at 0.2 C to 10 mV, and charging was completed when the current decreased to 0.05 C. The discharge was a constant current discharge (CC discharge) at 0.2 C and cut off at 1.5 V.

Then, this charging / discharging was repeated 6 times, and charging was stopped at the 7th time when the battery was charged to a potential where the charging depth was 50% at 0.2 C. Next, the pretreatment half cell was disassembled under a dry argon atmosphere having a dew point of −80 ° C. or lower, the negative electrode was taken out, and the negative electrode was washed with dimethyl carbonate to obtain a negative electrode sheet for AC impedance evaluation. Six negative electrode sheets were prepared for each level of Examples and Comparative Examples.

(Making a symmetric cell)
First, two negative electrodes for AC impedance evaluation prepared from the same negative electrode sheet were prepared and laminated in a coin cell via a separator. Next, in this laminate, a mixture of an electrolytic solution (EC (ethylene carbonate) and DMC (dimethyl carbonate) at a ratio of 1: 2 was used as a solvent, and LiPF ₆ was dissolved as an electrolyte at a concentration of 1 mol / L. A symmetric cell was prepared by adding and caulking. In addition, three symmetric cells were prepared for each level of Examples and Comparative Examples.

(AC impedance evaluation)
The measurement was performed by 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., the measurement frequency was set to 0.01 Hz to 1,000,000 Hz, and then the measurement was started. Then, after the measurement was completed, the resistance value of 0.1 Hz was read and evaluated.

-Preparation of carbon materials according to Examples and Comparative Examples-
Table 1 shows the crystallite sizes La, D10 and D50G of the graphitic particles used as the base material in the following Examples and Comparative Examples.

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

<Example 1>
The graphite particles A and the precursor b of the amorphous carbon coating layer were mixed at room temperature so 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. At this time, the amorphous carbon precursor D50P / graphitic particles D50G was 5.2.

Using the atmosphere-compatible small roller Hers Kirn (manufactured by Noritake Company Limited Co., Ltd.), the mixed powder obtained above was used to increase the flow rate of nitrogen gas to 6.2 cm / min and the temperature inside the furnace to 650 ° C at 180 ° C. The temperature was set to / hr, the temperature was raised so that the heating rate from 650 ° C to 1000 ° C was 350 ° C / hr, and the holding time at 1000 ° C was set to 4 hours for carbonization.

Next, the amorphous carbon-coated graphite material of Example 1 was obtained by classifying with a sieve having 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 graphite particles A and B and the precursors a, b, c and d of the amorphous carbon coating layer are contained in the ratio of the amorphous carbon coating layer to 100 parts by weight of each graphite particle. However, the particles are mixed at room temperature so that they are 5 parts by weight, 10 parts by weight, and 20 parts by weight, respectively. Similar to Example 1 described above, the temperature was raised so that the temperature rising rate from 650 ° C. to 1000 ° C. was 350 ° C./hr, and the temperature was raised at 1000 ° C. The carbonization treatment was performed with the holding time set to 4 hours.

Next, the amorphous carbon-coated graphite material of Examples 2 to 8 was obtained by classifying with a sieve having an opening of 45 μm. 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.

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

(Electrode fabrication and evaluation test for input / output characteristic evaluation)
The input / output characteristics were evaluated using a coin cell having a metal Li as a counter electrode.

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

(Charging / discharging processing before input / output characteristic evaluation)
The pretreatment half cell prepared in the above-mentioned (g) AC impedance evaluation electrode preparation and evaluation test was set in a charging / discharging device, and Li was inserted into and detached from the negative electrode at 25 ° C. For charging, constant current charging (CC charging) is performed up to 5 mV at 0.1 C, charging is completed when the current is attenuated to 0.01 C, and constant current discharge (CC discharge) is performed at 0.1 C for discharging. It was cut off at 5V. Then, this charging / discharging was repeated 6 times, and the evaluation was performed by changing the charging rate or the discharging rate from the 7th time.

(Evaluation of high rate charging characteristics)
For charging, constant current charging (CC charging) is performed up to 5 mV at 0.2C, 0.5C, 1C, 2C, and 3C, and charging is completed when the current decreases to 0.01C, and discharge is fixed at 0.2C. 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 0.2 C.

(Evaluation of high-rate discharge characteristics)
Charging is performed at 0.2C with constant current charging (CC charging) up to 5mV, charging is completed when the current decreases to 0.01C, and discharging is 0.2C, 0.5C, 1C, 2C, 3C, 4C, respectively. A constant current discharge (CC discharge) was performed at 1.5V, and the cutoff was performed 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.

As a result of measuring Example 2 by the above method, the charge capacity retention rate showed a high value of 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>
The unprocessed graphite particles A and C are used as they are as the graphite material, and the atmosphere-compatible small roller Herskilln (manufactured by Noritake Company Limited) is used to flow the nitrogen gas at a flow rate of 6.2 cm / min and the temperature inside the furnace to 650. The temperature is raised so that the heating rate to ° C. is 180 ° C./hr and the heating 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. gone.

Next, the graphite materials of Comparative Examples 1 and 2 were obtained by classifying with a sieve having an opening of 45 μm. 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 B and C and the precursors b and e of the amorphous carbon coating layer is 5% by weight, respectively. Mix at room temperature so that the parts and parts are 10 parts by weight, and use a small roller harskirun (manufactured by Noritake Company Limited Co., Ltd.) for atmosphere, the flow velocity of nitrogen gas is 3.2 cm / min, and the temperature in the furnace is up to 650 ° C. The temperature was raised so that the rate of temperature rise was 150 ° C./hr and the rate of temperature rise from 650 ° C. to 1000 ° C. was 350 ° C./hr, and the holding time at 1000 ° C. was set to 4 hours for carbonization.

Next, the amorphous carbon-coated graphite materials of Comparative Examples 3 to 5 were obtained by classifying with a sieve having an opening of 45 μm. Table 3 shows the production conditions of Comparative Examples 3 to 5, and Table 4 shows the characteristics of the obtained amorphous carbon-coated graphite material.

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 It was proved that the value of AC impedance was lower than that of Comparative Examples 1 to 5 and that low resistance could be realized.

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

The negative electrode material for the lithium ion secondary battery of the present embodiment can be effectively used for, for example, a stationary lithium ion battery for an electric vehicle, a house, or the like.

10 Lithium-ion secondary battery 11 Negative electrode 12 Negative electrode current collector 13 Positive electrode 14 Positive electrode current collector 15 Separator 16 Exterior

Claims (5)

A negative electrode material for a lithium ion secondary battery made of an amorphous carbon-coated graphite material in which the surface of graphitic particles is coated with amorphous carbon, and has a lightness L ^* value of 35.0 or less and a tap density. A negative electrode material for a lithium ion secondary battery, characterized in that the amount is 0.3 g / mL or more and 0.75 g / mL or less. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the crystallite size La calculated from the 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 the absorption amount of dibutylphthalate (DBP) 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 the average particle size (D50) is 1 μm or more and 25 μm or less. A lithium ion secondary battery, wherein the negative electrode material according to any one of claims 1 to 4 is used as the negative electrode material for the lithium ion secondary battery.

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