JP4040381B2 - Composite graphite particles, method for producing the same, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

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【表２】

【０１１２】
【表３】

【０１１３】
【発明の効果】
本発明の複合黒鉛質粒子は、リチウムイオン二次電池用負極材料として用いたときに、高い放電容量および高い初期充放電効率（不可逆容量が小さい）、さらに優れたハイレート特性およびサイクル特性をともに得ることができる。
また、本発明の方法によれば、前記複合黒鉛質粒子を、黒鉛化時の融着などを抑制しつつ生産性良く製造することができる。
さらに、この複合黒鉛質粒子を用いる本発明のリチウムイオン二次電池用負極、およびその負極を用いる本発明のリチウムイオン二次電池は、高い放電容量を維持したまま、不可逆容量を低減することが可能であり、さらに、初期充放電効率、急速放電効率、サイクル特性を大幅に改善することができる。
【図面の簡単な説明】
【図１】 本発明の実施例および比較例で作製し、充放電特性の評価に用いた評価電池の断面図である。
【図２】 本発明で用いた剪断圧縮加工機（造粒機）の概略図である。
【符号の説明】
１ 外装カップ
２ 作用電極
３ 外装缶
４ 対極
５ セパレータ
６ 絶縁ガスケット
７ａ，７ｂ 集電体
２１ 固定ドラム
２２ ローター
２３ 黒鉛前駆体
２４ 循環機構
２５ 排出機構
２６ ブレード
２７ ステーター
２８ ジャケット[0001]
BACKGROUND OF THE INVENTION
The present invention relates to composite graphite particles having a composite structure, a method for producing the same, a negative electrode for a lithium ion secondary battery using the composite graphite particles, and a high discharge rate and initial charge / discharge efficiency using the negative electrode. The present invention relates to a lithium ion secondary battery having excellent characteristics and cycle characteristics.
[0002]
[Prior art]
In recent years, with the miniaturization or high performance of electronic devices, there is an increasing demand for higher energy density of batteries. In this situation, a lithium secondary battery using lithium for the negative electrode has attracted attention because it has the advantages of high energy density and high voltage.
In this lithium secondary battery, since lithium metal is used as it is as a negative electrode, it is known that lithium is deposited in a dendritic state during charging and the negative electrode is deteriorated, so that the charge / discharge cycle is short. Moreover, there is a possibility that lithium deposited in a dendrite shape penetrates the separator, reaches the positive electrode, and is short-circuited.
[0003]
For this reason, each material for positive and negative electrodes is composed of two kinds of intercalation compounds with different oxidation-reduction potentials, each functioning as a lithium ion support, and the non-aqueous solvent enters and exits during the charge / discharge process. Lithium ion secondary batteries are being studied.
[0004]
As this negative electrode material, it has been proposed to use a carbon material that has the ability to occlude and release lithium ions and can prevent the precipitation of lithium metal. A wide variety of structures, structures, and forms such as a graphite crystalline structure or a disordered layer structure are known as carbon materials, and the electrode performance including the operating voltage during charging / discharging varies greatly. Among them, graphite that is particularly excellent in charge / discharge characteristics and exhibits high discharge capacity and potential flatness is promising (Japanese Patent Publication No. Sho 62-23433).
[0005]
It has been reported that the more the crystalline graphite structure is developed, the more stable the intercalation compound with lithium, and the higher the discharge capacity is obtained because a large amount of lithium is inserted between the carbon network planes. (Electrochemistry and industrial physical chemistry, 61 (2), 1383 (1993), etc.). Various layer structures are formed depending on the amount of lithium inserted, and in the region where they coexist, they are flat and have a high potential close to lithium metal (J. Electrochem. Soc., Vol. 140, 9, 2490 (1993), etc.) . Therefore, when an assembled battery is used, it is possible to obtain a high output. Generally, the theoretical capacity (limit value) of a carbon anode material is an ideal graphite intercalation compound of graphite and lithium. LiC₆In this case, the discharge capacity is 372 mAh / g.
[0006]
On the other hand, in the lithium ion secondary battery using graphite as a negative electrode material, as the crystallinity of graphite increases, side reactions that do not participate in the battery reaction such as decomposition of the electrolytic solution on the graphite surface easily occur during the first charge. The irreversible capacity (= initial charge capacity-initial discharge capacity) that cannot be taken out as the amount of electricity during the charge-discharge process is markedly increased, resulting in a discharge capacity loss of tens to hundreds of mAh / g level during the initial discharge. (J. Electrochem. Soc., Vol. 117 222 (1970), etc.).
[0007]
Side reactions such as decomposition of the electrolytic solution continue until the decomposition product is deposited and grows on the graphite (carbon) surface, and the electrons are so thick that they cannot move directly from the graphite surface to the solvent. In addition, solvent molecules and lithium ions may be co-intercalated to peel off the graphite surface layer, and the newly exposed graphite surface may react with the electrolyte to increase the irreversible capacity, that is, the initial charge / discharge efficiency. Is also reported to be low (J. Electrochem. Soc., Vol. 137, 2009 (1990)).
[0008]
Such an increase in irreversible capacity (low initial charge / discharge efficiency) can be compensated for by adding a positive electrode material to the secondary battery, but the addition of excess positive electrode material is a new problem of reduced energy density. It is desirable to avoid it.
[0009]
As described above, in a lithium ion secondary battery using graphite as a negative electrode carbon material, a high discharge capacity and a low irreversible capacity are contradictory requirements, but as a solution to this, a highly crystalline graphite material (nucleus) A method has also been proposed in which the surface of the substrate is coated with a low crystalline material to form a multilayer structure. Broadly speaking,
(1) The surface of a highly crystalline graphite material serving as a nucleus is coated with low crystalline carbon using a pyrolysis gas of an organic compound such as propane or benzene (Japanese Patent Laid-Open Nos. 4-368778 and 5) 275076),
[0010]
(2) A highly crystalline graphite material as a core is coated or impregnated with a carbon material such as pitch in a liquid phase, and then fired at a temperature of about 1000 ° C. to form a carbonaceous material on the surface layer (Japanese Patent Laid-Open No. Hei 5- No. 121066, JP-A-5-217604, JP-A-6-84516, JP-A-11-54123, JP-A-2001-229924),
(3) A graphite precursor such as a graphite crystalline material or raw coke is oxidized at about 300 ° C. in a gas phase or a liquid phase in an oxidizing atmosphere (Japanese Patent Laid-Open Nos. 10-326611 and 10-218615). Gazette),
(4) A combination of (1) to (3) (Japanese Patent Laid-Open Nos. 10-214615 and 10-284080).
[0011]
However, the above methods (1) and (4) have a problem that the production process is complicated and expensive from the viewpoint of industrial production, and the method (1) is stable because it is difficult to control the coating thickness. Thus, there is a problem that high electrode performance and powder performance cannot be exhibited.
[0012]
The above method (2) is stable because it is difficult to control the homogeneity and thickness of the surface layer because the coated graphite adheres firmly when fired at about 1000 ° C., and the coating peels off during crushing. Therefore, there is a problem that high electrode performance and powder performance cannot be exhibited.
[0013]
In the method (3) above, in order to obtain a high initial charge / discharge efficiency, it is necessary to highly oxidize, but this reduces not only the surface layer but also the crystallinity of the inside (nucleus) of the crystalline graphite material. This causes a problem that the discharge capacity is reduced.
[0014]
In addition, any of the conventional methods lacks performance in response to the recent demands for high discharge capacity, high rate characteristics (rapid charge / discharge characteristics), and cycle characteristics. Therefore, in order to obtain a high discharge capacity, (5) composite graphite particles having highly crystalline graphite particles such as natural graphite as a core have been proposed (for example, JP-A-2001-89118). . However, natural graphite generally has a flat particle shape, and when a negative electrode is formed, graphite particles tend to be oriented in one direction, causing deterioration in high rate characteristics and cycle characteristics.
[0015]
The composite graphite particles of (5) above have a high irreversible capacity because the surface of the graphite highly reactive with the electrolyte is exposed. Also, highly crystalline graphite particles (nuclei) derived from coke powder are those in which the crystals in the particles are oriented in one direction, and when forming a negative electrode, the graphite particles are easily oriented in one direction, resulting in a high rate. The characteristics and cycle characteristics were deteriorated. That is, when a plurality of highly crystalline graphites (nuclei) are interposed in one composite graphite particle and the crystal orientation in the particles is randomized, A large amount of tar pitch coating is required, leading to a reduction in discharge capacity. Therefore, when the tar pitch film is thinned, the strength of the composite graphite particles is lowered, and when the negative electrode is formed, there is a problem that it easily deforms and breaks, resulting in a decrease in high rate characteristics, cycle characteristics and initial charge / discharge efficiency.
[0016]
[Problems to be solved by the invention]
Therefore, the present invention has been made in view of the above situation, and when used as a negative electrode material for a lithium ion secondary battery, high discharge capacity and high initial charge / discharge efficiency (small irreversible capacity), Further, composite graphite particles having both excellent high-rate characteristics and cycle characteristics, a method for producing with good productivity while suppressing fusion during graphitization, and for lithium ion secondary batteries using the composite graphite particles It aims at providing the negative electrode and the lithium ion secondary battery which has the said battery characteristic.
[0017]
[Means for Solving the Problems]
The present inventor fills and / or coats a mesophase precursor in which carbonaceous fine particles are dispersed in a spherical or ellipsoidal graphite granule formed from a plurality of scaly graphites, and applies a specific volatile content. After firing so as to contain, it was found that the melt viscosity is increased by crushing as necessary and heat-treating in a non-oxidizing atmosphere, and the fusion between the materials during the heat-treatment can be suppressed. Even if a slight fusion occurs at the time of firing, it is possible to suppress peeling of the coating due to crushing. And it discovered that the composite graphite particle | grains by which the thin film of the low crystalline carbonaceous material was uniformly and firmly adhere | attached on the outer surface can be manufactured. In addition, the composite graphite particles having a unique structure thus obtained have a high discharge capacity, and an increase in irreversible capacity is suppressed, so that high initial charge / discharge efficiency is exhibited, and further excellent high rate characteristics and cycle characteristics. Have Further, it has been found that the battery performance is maintained even when the negative electrode is formed at a high density.
[0018]
This is because when the mesophase precursor is fired, the crystal structure of the coating composed of the carbonaceous layer on the outer surface is moderately disturbed by the action of the carbonaceous fine particles and has a random orientation. This is thought to have contributed to the development of excellent high-rate characteristics. Further, it is considered that excellent cycle characteristics and high rate characteristics can be realized by granulating a plurality of scaly graphites at the same time as high discharge capacity is obtained by graphite having high crystallinity. Furthermore, by granulating a plurality of flaky graphite into a spherical shape, moderate voids and irregularities are formed, and this is filled and / or coated with a mesophase precursor, so that the interface between the two is strengthened by the anchor effect. It is thought that it was closely attached.
[0019]
  That is, the present invention provides a graphite granule (A) formed by aggregating a plurality of scaly graphites, and the graphite granulation on the internal voids and / or outer surface of the graphite granule (A). The carbonaceous layer (B) having lower crystallinity than the product (A) is a composite graphite particle formed by filling and / or coating, and the carbonaceous layer (B) is a carbonaceous fine particle.(C)IncludingSee
  The content of the graphite granule (A) in the composite graphite particles is 70 to 99% by mass, and the content of the carbonaceous layer (B) in the composite graphite particles is 30 to 1% by mass. The content of the carbonaceous fine particles (C) in the carbonaceous layer (B) is 0.1 to 2% by mass.A composite graphite particle is provided.
[0020]
  The composite graphite particles have an average particle size of 5 to 100 μm, and the carbonaceous fine particles (C) have an average particle size of 0.005 to 0.5 μm.BeIs preferred.
[0022]
Interplanar spacing (d) of the carbon network layer of the composite graphite particles₀₀₂) 0.3365nm or less, Raman spectrum 1360cm^-1Peak intensity (I₁₃₆₀) And 1580cm^-1Peak intensity (I₁₅₈₀) Intensity ratio (I₁₃₆₀/ I₁₅₈₀) Is preferably 0.05 or more and less than 0.40.
[0023]
  Furthermore, the present invention provides a method for producing the composite graphite particles as a spherical or ellipsoidal graphite granule (A) formed by aggregating a plurality of scaly graphites.After the production of graphite granulated product (A)A step of filling and / or coating a mesophase precursor containing carbonaceous fine particles (C) on the internal voids and / or the outer surface thereof, and the internal voids and / or the outer surface of the graphite granule (A) by the above-mentioned steps The step of firing so that the volatile content of the mesophase precursor in the filler and / or coating formed in the coating is 2% by mass or more and less than 8% by mass, and the calcined product obtained in the above step is 900 to 3200 ° C. And a method of producing a composite graphite particle, characterized by comprising a step of heat-treating with.
[0024]
The present invention also provides a negative electrode for a lithium ion secondary battery using the composite graphite particles, and a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery. provide.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the component of the composite graphite particle of the present invention and the production method thereof, the negative electrode for a lithium ion secondary battery, and the lithium ion secondary battery will be described in detail.
[0026]
The composite graphite particles of the present invention comprise a graphite granule (A) formed by aggregating a plurality of scaly graphites, and filling and / or outer surfaces of the internal voids of the graphite granule (A). The carbonaceous layer (B) to be coated and the carbonaceous fine particles contained in the carbonaceous layer (B) are the main constituent elements. The carbonaceous layer (B) may cover each of the scaly graphite, and is filled between the scaly graphites, that is, filled in the internal voids of the graphite granule (A), or of the graphite. The graphite granule (A) is covered with the carbonaceous layer (B), and the outer surface of the graphite granule (A) is covered by the carbonaceous layer (B). Spherical to ellipsoidal particles having a coated composite structure.
[0027]
<Graphite granules>
The graphite granule (A), which is a constituent element of the composite graphite particles of the present invention, is formed by granulating a plurality of scaly graphites. As scale-like graphite, natural graphite, artificial graphite, mesophase calcined carbon (bulk mesophase) made from tar pitch, coke (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.), etc. Graphitized, etc., and those granulated using a plurality of natural graphites having high crystallinity are particularly preferable. One graphite granule (A) is usually formed by collecting 2 to 100, preferably 3 to 20, scale-like graphite. Further, it is possible to spheroidize one graphite.
[0028]
A method of forming a graphite granule (A) by aggregating a plurality of these graphites is not particularly limited. For example, a method of mixing a plurality of scaly graphites in the presence of a binder of a graphite precursor, a plurality of scaly shapes Examples of the method include applying a mechanical external force to the graphite and a combination of both, and a method of granulating by applying a mechanical external force without using a binder component is particularly preferable. Examples of the device for applying the mechanical external force include a pulverizer such as a counter jet mill (manufactured by Hosokawa Micron Corporation), a current jet (manufactured by Nissin Engineering Co., Ltd.), and SARARA (manufactured by Kawasaki Heavy Industries, Ltd.). , Granulators such as GRANUREX (manufactured by Freund Sangyo Co., Ltd.), Agromaster (manufactured by Hosokawa Micron Co., Ltd.), Newgra Machine (manufactured by Seishin Enterprise Co., Ltd.), kneaders such as pressure kneaders and two rolls, rotation A shear compression processing machine such as a ball mill, a hybridization system (manufactured by Nara Machinery Co., Ltd.), mechanomicros (manufactured by Nara Machinery Co., Ltd.), or mechanofusion system (manufactured by Hosokawa Micron Corporation) can be used. .
[0029]
In the present invention, the graphite granulated product (A) is preferably granulated densely so that the porosity in the granulated product particles is 50% or less, preferably 30% or less. When the porosity exceeds 50%, the required amount of the carbonaceous layer (B) that is a filling or coating increases, and the discharge capacity decreases, or voids remain in the composite graphite particles after coating. Therefore, when the negative electrode is produced at a high density, the composite graphite particles may be destroyed.
[0030]
The average particle diameter of the graphite granulated product (A) is preferably in the range of 5 to 100 μm, more preferably 5 to 30 μm.
The graphite granule (A) preferably has a spherical to ellipsoidal shape, and the aspect ratio (ratio of the major axis direction to the minor axis direction perpendicular thereto) is preferably 3 or less. 2 or less is more preferable.
[0031]
<Carbonaceous layer (B)>
In the composite graphite particles of the present invention, the carbonaceous layer (B) fills the internal voids of the graphite granule (A) formed by aggregating a plurality of scaly graphites, and / or the graphite granule. It has a lower crystallinity than the graphite granule (A) that covers the outer surface of (A). Interplanar spacing of carbon network layer of graphite granule (A) (d₀₀₂) Is preferably 0.3362 nm or less, more preferably 0.3358 nm or less, whereas d of the carbonaceous layer (B).₀₀₂Is preferably in the range of 0.3362 to 0.3500 nm, more preferably 0.3365 to 0.3400 nm.
[0032]
Examples of the carbonaceous material forming the carbonaceous layer (B) include petroleum-based and / or coal-based heavy oil such as tar and / or pitch, phenol resin, oxygen-crosslinked petroleum pitch, ethylene heavy oil, and the like. A heat-treated polycondensation reaction may be mentioned. Among them, mesophase precursors that form mesophase by heating, particularly petroleum-based and / or coal-based heavy oils such as tar and / or pitch are preferable.
[0033]
The coating amount of carbonaceous material (total amount required for filling and coating) is such that the ratio of the graphite granulated product (A) and the carbonaceous layer (B) constituting the composite graphite particles finally obtained is (A ): (B) = 70 to 99:30 to 1% by mass is desirable. When (B) exceeds 30% by mass, the particles are easily fused after firing, and the surface portion of the carbonaceous layer (B) is peeled off during pulverization, so that the initial charge / discharge efficiency is lowered. If (B) is less than 1% by mass, it is difficult to completely coat the graphite granule (A), and the initial charge / discharge efficiency may be reduced due to insufficient coating.
[0034]
<Carbonaceous fine particles>
In the composite graphite particles of the present invention, the carbonaceous layer (B) contains carbonaceous fine particles. The carbonaceous fine particles contained in the carbonaceous layer (B) preferably have an average particle size of 0.005 to 0.5 μm from the viewpoint of obtaining an excellent negative electrode material such as initial charge / discharge efficiency and high rate characteristics. Here, the average particle diameter is a value converted into a volume of a sphere, the shape of the carbonaceous fine particles is not limited, and any of granular, spherical, chain-like, needle-like, fibrous, plate-like, scale-like, etc. Also good. When the average particle diameter is outside the specified range, the effect of the present invention is difficult to obtain.
[0035]
Examples of the carbonaceous fine particles include primary QI (quinoline insoluble matter) components present in coal-based tar / pitch such as coal fine powder, vapor phase carbon powder, and coke powder, carbon black, ketjen black, and graphitized products thereof. Examples of the above-mentioned carbonaceous material or a fired product of the carbonaceous material, and fine powders such as artificial graphite, natural graphite, graphite fiber, and carbon nanofiber are exemplified and are not particularly limited.
[0036]
  The carbonaceous fine particles in the carbonaceous layer (B) are finally 1 to 10,000 / μm in the carbonaceous layer (B) formed by heat-treating the mesophase precursor.³  It is preferable to contain in the range. 1 / μm³  If the amount is smaller than that, the effect of improving the initial charge / discharge efficiency and the high rate characteristic becomes small, and 10,000 pieces / μm.³  If it exceeds the upper limit, it may cause a reduction in discharge capacity, etc., which is not preferable. Usually, carbonaceous fine particles are 0.1 to 0.1 in the carbonaceous layer (B).2It is preferable to be contained by mass%. When the QI (quinoline insoluble component) component present in the coal-based tar / pitch is used as the carbonaceous fine particles, the carbonaceous layer (B) finally obtained0.1-2A range of mass% is preferred. In the present invention, primary QI refers to coal scrap, coke scrap and the like. Conventionally, tar / pitch with a low primary QI is desired to improve the crystallinity of the carbonaceous layer, and those having a primary QI of about 0 have been used as the precursor of the carbonaceous layer. As a reference example, the primary QI in a normal pitch is about 0 to 10%.
[0037]
<Crystallinity of composite graphite particles>
The composite graphite particles of the present invention are characterized in that the crystallinity of the carbonaceous layer (B) interposed between the outer surface and the internal voids is lower than the crystallinity of the nucleus (graphite granulated product (A)). The crystallinity of the surface layer of the graphite particles is evaluated by a Raman spectrum using an argon laser. That is, among the nine types of lattice vibration based on the graphite structure, 1580 cm corresponding to the E2g type vibration corresponding to the lattice vibration in the in-mesh plane.^-11360cm reflecting the nearby Raman spectrum and disorder of the crystal structure, mainly crystal defects in the surface layer, stacking irregularities, etc.^-1The nearby Raman spectrum is measured with a Raman spectroscopic analyzer (NR1100, manufactured by JASCO Corporation) using an argon laser having a wavelength of 514.5 nm. From the peak intensity of each Raman spectrum, the intensity ratio (R = I₁₃₆₀/ I₁₅₈₀) Is calculated, and the larger the intensity ratio, the lower the surface crystallinity. The intensity ratio R is preferably R ≧ 0.05 from the viewpoint of reducing the irreversible capacity. When R <0.05, the irreversible capacity is large and sufficient battery performance cannot be obtained. This is presumably because the crystallization of the surface layer proceeds so much that the decomposition reaction of the electrolytic solution on the surface of the composite graphite particles easily proceeds. Further, from the viewpoint of obtaining a high discharge capacity, R <0.40 is preferable. In the case of R ≧ 0.40, since the discharge capacity of the carbonaceous layer (B) is significantly reduced, it is necessary to reduce the ratio of the carbonaceous layer (B) to the composite graphite particles. In this case, the graphite granule (A) may be insufficiently filled and covered, and the initial charge / discharge efficiency may be reduced. More preferably, 0.10 ≦ R <0.30.
[0038]
On the other hand, the average crystallinity of the composite graphite particles is determined by the interplanar spacing of the carbon network layer (d₀₀₂) And the size (Lc) of the crystallite in the C-axis direction. That is, using a CuKα ray as an X-ray source and high-purity silicon as a standard substance, a (002) diffraction peak was measured for the composite graphite particles, and d was determined from the peak position and its half-value width, respectively.₀₀₂, Lc is calculated. The calculation method is in accordance with the Gakushin Law, and a specific method includes the method described in “Carbon Fiber” (Modern Editorial Company, published in March 1986), pages 733-742.
[0039]
D by the X-ray diffraction method which is an index of the degree of development of the graphite structure of the composite graphite particles of the present invention₀₀₂And Lc is d from the viewpoint of developing a high discharge capacity.₀₀₂Preferably ≦ 0.3365 nm, Lc ≧ 40 nm, d₀₀₂It is particularly preferable that ≦ 0.3362 nm and Lc ≧ 50 nm. d₀₀₂In the case of> 0.3365 nm and Lc <40 nm, the degree of development of the graphite structure is low. Therefore, when the graphite material is used as the negative electrode material of a lithium ion secondary battery, the lithium doping amount is small and the discharge capacity is high. May not be able to get.
[0040]
<Powder characteristics of composite graphite particles>
When the composite graphite particles of the present invention are used as a negative electrode material for a lithium ion secondary battery, the average particle diameter is preferably 5 to 100 μm, particularly preferably 5 to 30 μm. The average particle size of the composite graphite particles is preferably 1.0 to 1.3 times, particularly preferably 1.0 to 1.1 times the average particle size of the graphite granulated material (A) as the core. It is.
[0041]
Although the composite graphite particles of the present invention contain highly crystalline graphite particles typified by natural graphite, a shape close to a spherical shape can be obtained. The spherical or ellipsoidal shape contributes to improvement of high rate characteristics and cycle characteristics as a negative electrode material for a lithium ion secondary battery. The average aspect ratio of the graphite particles is preferably 3 or less, particularly preferably 2 or less.
[0042]
Further, the composite graphite particles of the present invention are dense particles and can express a high bulk density. A high bulk density is advantageous because it can reduce problems such as the destruction of composite graphite particles when the negative electrode is produced at a high density. Bulk density is 0.5g / cm^ThreeIt is preferable that it is above, especially 0.7 g / cm^ThreeThe above is preferable.
[0043]
The specific surface area of the composite graphite particles of the present invention can be arbitrarily designed according to the characteristics of the lithium ion secondary battery and the properties of the negative electrode mixture paste. However, 20m²If it exceeds / g, the safety of the lithium ion secondary battery may be lowered. Generally 0.3-5m²/ G, particularly 3 m²/ G or less is preferable.
[0044]
  The production of the composite graphite particles of the present invention is a spherical or ellipsoidal graphite granule (A) formed by aggregating a plurality of scaly graphites.After the production of graphite granulated product (A)A step of filling and / or coating a mesophase precursor containing carbonaceous fine particles (C) on the internal voids and / or the outer surface thereof, and the internal voids and / or the outer surface of the graphite granule (A) by the above-mentioned steps The step of firing so that the volatile content of the mesophase precursor in the filler and / or coating formed in the coating is 2% by mass or more and less than 8% by mass, and the calcined product obtained in the above step is 900 to 3200 ° C. A method of producing composite graphite particles characterized by having a heat treatment step (hereinafter referred to as “method of the present invention”).
[0045]
  In the method of the present invention, a spherical or ellipsoidal graphite granule (A) formed by aggregating a plurality of scaly graphitesAfter the production of graphite granulated product (A)The step of filling and / or coating the mesophase precursor containing the carbonaceous fine particles (C) in the internal voids and / or the outer surface of the graphite is carried out in a molten state or a solution using a solvent in the graphite structure. It can be carried out by bringing into contact with the granules (A) and impregnating and coating the inner voids and outer surfaces of the graphite granules (A). There is no limitation on the impregnation and coating method, a method of kneading the melted or dissolved mesophase precursor and the graphite granulated product (A), and after the graphite granulated product (A) is immersed and mixed in the solution of the mesophase precursor, heating and / A method of removing the solvent by reducing the pressure, a method of immersing and mixing the solution of the mesophase precursor and the graphite granulated product (A), and then removing the solvent by a spray drying method, and a mesophase precursor in the graphite granulated product (A). May be performed by any method such as a method of adjusting the coating amount of the carbonaceous material by washing with a solvent. In these methods, from the viewpoint of uniform coating, it is desirable to include an operation of immersing the graphite granule (A) in the mesophase precursor solution and subjecting it to a reduced pressure treatment.
[0046]
In the method of the present invention, the graphite granule (A) in which the internal voids and / or the outer surface is filled and / or coated with the mesophase precursor is calcined, the mesophase precursor is mesophased, and the volatile content thereof is adjusted. At this time, the mesophase precursor is calcined so that the volatile content is 2 mass% or more and less than 8 mass%.
[0047]
The firing method may be any of reduced pressure, normal pressure, and increased pressure, and is usually performed in a temperature range of 300 to 1200 ° C, preferably 350 to 600 ° C. Although the atmosphere is preferably non-oxidizing, it can be fired in a slightly oxidizing atmosphere. In addition, you may perform a baking process in multiple times. The firing time is not particularly limited, but is usually about 0.5 to 100 hours. When a solvent is used in the filling or covering step, the solvent may be removed in the main firing step.
[0048]
The calcined mesophase coating film (precursor of the carbonaceous layer (B)) is a solid having a softening point (Mettler method) of about 360 ° C. or higher and has a graphite structure grown to some extent. Examples of the index representing the degree of growth of the graphite structure include the amount of volatile components contained in the mesophase coating (precursor of the carbonaceous layer (B)). The amount of volatile components is 2% by mass or more and less than 8% by mass, preferably 3 to 6% by mass. Here, the amount of volatile matter is measured as follows based on the fixed carbon method of JIS K2425.
[0049]
Method for measuring volatile content: 1 g of a sample (graphite precursor) is weighed into a crucible and heated in an electric furnace at 430 ° C. for 30 minutes without a lid. Then, it is set as a double crucible, it heats for 30 minutes with an 800 degreeC electric furnace, a volatile matter is removed, and a weight loss rate is made into a volatile matter.
Quinoline insoluble matter (QI) can also be used as a guideline as an index representing the degree of growth of the graphite structure. It means that the graphite structure is growing as the QI approaches 100% by mass.
[0050]
Here, QI is measured by the following filtration method based on JIS K2425.
QI measurement method: Powder material (graphite precursor) is dissolved in quinoline, heated at 75 ° C. for 30 minutes, and then suction filtered while hot using a filter. The residue is washed in the order of quinoline and acetone until the filtrate becomes colorless, then dried and weighed to calculate the quinoline insoluble matter. Diatomaceous earth is used as a filter aid. As the filter, a vertical filter 1G4 defined in JIS R3503 is used.
[0051]
A mesophase coating (a precursor of the carbonaceous layer (B)) having a large amount of volatile components or a low QI exhibits meltability in the heat treatment step after crushing. Therefore, when such a mesophase coating (precursor of the carbonaceous layer (B)) is heat-treated as it is, the shape usually changes or the materials are fused. In particular, when the volatile content is 8% by mass or more, the coating film is remelted and fused by heat treatment, so that when the negative electrode is produced at a high density, the fused particles are damaged and the coating film is peeled off. It may cause a decrease in charge / discharge efficiency, which is not preferable. The QI of the mesophase film (the precursor of the carbonaceous layer (B)) is preferably more than 80% by mass.
[0052]
On the other hand, a mesophase film having a low volatile content or a high QI does not exhibit the above-described meltability (infusibilization). However, when the volatile content is less than 2% by mass, the graphitized structure of the film is highly developed. Therefore, peeling of the coating film (the carbonaceous layer (B) on the outer surface layer of the graphite granule (A) constituting the core) may occur during crushing. Therefore, in the present invention, a mesophase film having a volatile content of 2% by mass or more and less than 8% by mass, preferably 3 to 6% by mass is formed. When based on QI, it is more than 80% by mass and less than 100% by mass, preferably 90 to 99.5% by mass.
[0053]
<Crushing process>
In the method of the present invention, the mesophase precursor-coated graphite granulated material fused in the firing step is crushed. As the crushing method, various known crushing methods can be employed. For example, various commercially available pulverizers such as a roller type, an impact type, a friction type, a compression type, a stone mill type, a moving object collision type, a vortex (air flow) type, a shear type, and a vibration type can be used.
[0054]
In the method of the present invention, since no melting deformation or fusion between solids occurs during firing, the final composite graphite particles can be obtained while maintaining the shape after the firing step. For this reason, if it adjusts to the shape of the desired product before heat processing, it is not necessary to grind | pulverize and shape | mold into a desired shape after heat processing, and a process is simplified. Furthermore, since the surface crystallized low by this can be maintained as it is, the effect of the present invention can be exhibited better. For example, when the composite graphite particles of the present invention are used as a negative electrode material for a lithium ion secondary battery, the average particle size of the mesophase-coated graphite granule after pulverization is preferably in the range of 5 to 100 μm. 5 to 50 μm is more preferable. Moreover, it is desirable to crush so that it may become 1.0 to 1.3 times the average particle diameter of a graphite granulated material (A). In the case of less than 1.0 times, the graphite granulated material (A) which is a nucleus is pulverized, the low crystallized surface is not retained, and the irreversible capacity is increased. On the other hand, when it exceeds 1.3 times, a large number of fused particles are present, and when a negative electrode is produced at a high density, peeling of the coating or the like may occur, resulting in a decrease in initial charge / discharge efficiency.
The average aspect ratio of the mesophase-coated graphite granule after pulverization is preferably 3 or less, and more preferably 2 or less, like the composite graphite particles finally obtained.
[0055]
<Heat treatment>
The mesophase-coated graphite granule, which is a precursor of composite graphite particles, is filled in a container such as a crucible or molded with a resin that is burned off during heat treatment, and heated at a high temperature in a non-oxidizing atmosphere. Composite graphite particles are formed. The heat treatment temperature is 900 to 3200 ° C. The heat treatment may be performed at a heat treatment temperature (900 to 1500 ° C.) at which the film does not substantially form a graphite structure, or may be performed at a heat treatment temperature (1500 to 3200 ° C.) that forms the graphite structure. When the heat treatment temperature exceeds 3200 ° C., it is difficult to control the properties of the finally obtained composite graphite particles, which is not preferable.
[0056]
Among these, heat treatment is performed at 1800 to 3200 ° C., particularly preferably 2000 to 3000 ° C., in order to obtain excellent battery characteristics even when a negative electrode is produced at a high density. By heating to such a high temperature for 0.5 to 50 hours, preferably 2 to 20 hours, the composite graphite particles of the present invention suitably used as a negative electrode material for a lithium ion secondary battery and the like can be obtained.
[0057]
The composite graphite particles of the present invention can be diverted to applications other than the negative electrode, for example, conductive materials for fuel cell separators and graphite for refractories, taking advantage of the characteristics thereof. Therefore, the present invention further provides a negative electrode for a lithium ion secondary battery and further a lithium ion secondary battery using the negative electrode material.
[0058]
<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention usually comprises a negative electrode, a positive electrode and a non-aqueous electrolyte as main battery components, and the positive and negative electrodes are each composed of a lithium ion carrier, and the non-aqueous solvent enters and exits during the charge / discharge process. Done in
In essence, it has a battery mechanism in which lithium ions are doped into the negative electrode during charging and dedope from the negative electrode during discharging.
[0059]
The lithium ion secondary battery of the present invention is not particularly limited except that the composite graphite particles are used as the negative electrode material, and other battery components conform to the elements of a general lithium ion secondary battery.
[0060]
<Negative electrode>
The production of the negative electrode from the composite graphite particles can be carried out in accordance with a normal molding method. However, the performance of the composite graphite particles is sufficiently drawn, and the moldability to the powder is high. If it is a method which can obtain a negative electrode stable, it will not be restrict | limited at all.
[0061]
At the time of producing the negative electrode, a negative electrode mixture obtained by adding a binder to the composite graphite particles can be used. As the binder, it is desirable to use one having chemical stability and electrochemical stability with respect to the electrolyte. For example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and styrene butadiene. Rubber, carboxymethylcellulose, etc. are used. These can also be used together.
In general, the binder is preferably used in an amount of about 1 to 20% by mass in the total amount of the negative electrode mixture.
[0062]
Specifically, for example, the composite graphite particles are adjusted to an appropriate particle size by classification or the like, and mixed with a binder to prepare a negative electrode mixture. This negative electrode mixture is usually used on one side of the current collector. Or a negative mix layer can be formed by apply | coating to both surfaces.
In this case, a normal solvent can be used. The negative electrode mixture is dispersed in the solvent, made into a paste, and then applied to the current collector and dried to make the negative electrode mixture layer uniform and strong. Glued to the body. The paste can be prepared by stirring at about 300 to 3000 rpm with a wing homomixer.
[0063]
For example, the composite graphite particles and a fluorine resin powder such as polytetrafluoroethylene can be applied after being mixed and kneaded in a solvent such as isopropyl alcohol. Also, a composite graphite particle and a fluorine resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethylcellulose are mixed with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol or the like to form a slurry. It can be applied later.
[0064]
The coating thickness when the mixture of the composite graphite particles and the binder is applied to the current collector is suitably 10 to 200 μm.
Alternatively, the composite graphite particles and resin powder such as polyethylene and polyvinyl alcohol can be dry mixed and hot press molded in a mold.
After the negative electrode mixture layer is formed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased by pressure bonding such as pressurization.
[0065]
The shape of the current collector used for the negative electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the material for the current collector include copper, stainless steel, and nickel. In the case of a foil shape, the thickness of the current collector is preferably about 5 to 20 μm.
[0066]
<Positive electrode>
As the positive electrode material (positive electrode active material), a material capable of doping / dedoping a sufficient amount of lithium is preferably selected. Examples of such positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, and vanadium oxides (V₂O_Five, V₆O₁₃, V₂O_Four, V_ThreeO₈And lithium-containing compounds such as Li compounds thereof, general formula M_XMo₆S_8-y(Wherein X is a numerical value in a range of 0 ≦ X ≦ 4, Y is a numerical value in a range of 0 ≦ Y ≦ 1, M represents a metal such as a transition metal), a chevrel phase compound, activated carbon, activated carbon fiber, etc. Can be used.
[0067]
The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. Specifically, the lithium-containing transition metal oxide is LiM (1)._1-pM (2)_pO₂(Wherein P is a numerical value in the range of 0 ≦ P ≦ 1, and M (1) and M (2) are composed of at least one transition metal element) or LiM (1)_2-QM (2)_QO_Four(Wherein Q is a numerical value in a range of 0 ≦ Q ≦ 1, and M (1) and M (2) are composed of at least one transition metal element).
In the above, examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, and Sn, and preferably Co, Fe, Mn, Ti, and Cr. , V, Al and the like.
[0068]
More specifically, as the lithium-containing transition metal oxide, LiCoO₂Lip Ni_QM_1-QO₂(M is the above transition metal element excluding Ni, preferably at least one selected from Co, Fe, Mn, Ti, Cr, V, and Al, 0.05 ≦ P, 0.5 ≦ Q ≦ 1.0. Lithium composite oxide, LiNiO₂LiMnO₂, LiMn₂O_FourEtc.
[0069]
The lithium-containing transition metal oxide as described above includes, for example, Li, transition metal oxides or salts as starting materials, these starting materials are mixed according to the composition, and a temperature range of 600 ° C. to 1000 ° C. in an oxygen atmosphere. It can obtain by baking with. The starting material is not limited to oxides or salts, and can be synthesized from hydroxides or the like.
In the lithium ion secondary battery of the present invention, the above compounds may be used alone or in combination of two or more as the positive electrode active material. Further, for example, a carbon salt such as lithium carbonate can be added to the positive electrode.
[0070]
In order to form a positive electrode with such a positive electrode material, for example, a positive electrode mixture comprising a positive electrode material, a binder, and a conductive agent for imparting conductivity to the electrode is applied to both sides of the current collector. An agent layer is formed. As the binder, any of those exemplified for the negative electrode can be used. As the conductive agent, for example, a carbon material, graphite, or carbon black is used.
[0071]
The shape of the current collector is not particularly limited, and a foil shape or a net shape such as a mesh or expanded metal is used. For example, examples of the current collector include aluminum foil, stainless steel foil, and nickel foil. The thickness is preferably 10 to 40 μm.
[0072]
Also in the case of the positive electrode, in the same way as the negative electrode, the positive electrode mixture is dispersed in a solvent to form a paste, and the paste-like positive electrode mixture is applied to a current collector and dried to form a positive electrode mixture layer. Alternatively, after the positive electrode mixture layer is formed, pressure bonding such as press pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.
[0073]
In forming the above negative electrode and positive electrode, various conventionally known additives such as a conductive agent and a binder can be appropriately used.
[0074]
<Electrolyte>
As an electrolyte used in the lithium ion secondary battery of the present invention, an electrolyte salt used in a normal nonaqueous electrolytic solution can be used, for example, LiPF.₆, LiBF_Four, LiAsF₆LiClO_Four, LiB (C₆H_Five)_Four, LiCl, LiBr, LiCF_ThreeSO_Three, LiCH_ThreeSO_Three, LiN (CF_ThreeSO₂)₂, LiC (CF_ThreeSO₂)_Three, LiN (CF_ThreeCH₂OSO₂)₂, LiN (CF_ThreeCF₂OSO₂)₂, LiN (HCF₂CF₂CH₂OSO₂)₂, LiN ((CF_Three)₂CHOSO₂)₂, LiB [C₆H_Three(CF_Three)₂]_FourLiAlCl_Four, LiSiF₆Lithium salt etc. can be used. In particular, LiPF₆, LiBF_FourIs preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the electrolytic solution is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 3.0 mol / L.
[0075]
The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery or a polymer gel electrolyte battery.
[0076]
In the case of a liquid non-aqueous electrolyte solution, as a solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, trimethylborate, tetramethylsilicate, Nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxy Aprotic organic solvents such as sazolidone, ethylene glycol and dimethyl sulfite can be used.
[0077]
When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, a matrix polymer gelled with a plasticizer (non-aqueous electrolyte) is included. Ether polymers such as polyethylene oxide and its crosslinked products, polymethacrylates, polyacrylates, fluorine polymers such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers, etc., alone or in combination Can be used.
Among these, it is desirable to use a fluorine-based polymer such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of oxidation-reduction stability.
[0078]
As the electrolyte salt and non-aqueous solvent constituting the plasticizer contained in these polymer solid electrolyte and polymer gel electrolyte, any of those described above can be used. In the case of a gel electrolyte, the concentration of the electrolyte salt in the non-aqueous electrolyte that is a plasticizer is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 2.0 mol / L.
[0079]
The method for producing such a solid electrolyte is not particularly limited. For example, a polymer compound that forms a matrix, a lithium salt and a solvent are mixed, heated and melted, a polymer compound and a lithium salt in an appropriate organic solvent. And the method of evaporating the organic solvent after the solvent is dissolved, and the polymerizing monomer, lithium salt and solvent that are the raw materials for the polymer electrolyte are mixed, and then irradiated with ultraviolet rays, electron beams, molecular beams, or the like for polymerization. Examples thereof include a method for producing a polymer electrolyte.
Moreover, 10 to 90 mass% is preferable, and, as for the addition ratio of the solvent in the said solid electrolyte, More preferably, it is 30 to 80 mass%. When the content is 10 to 90% by mass, the electrical conductivity is high, the mechanical strength is high, and film formation is easy.
[0080]
A separator can also be used in the lithium ion secondary battery of the present invention.
Although a separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. are mentioned. In particular, a synthetic resin microporous membrane is preferably used. Among these, a polyolefin microporous membrane is preferable in terms of thickness, membrane strength, and membrane resistance. Specifically, it is a microporous membrane made of polyethylene and polypropylene, or a microporous membrane that combines these.
[0081]
In the lithium ion secondary battery of the present invention, the gel electrolyte can be used because the initial charge / discharge efficiency has been improved.
The gel electrolyte secondary battery is configured by laminating a negative electrode containing the composite graphite material of the present invention, a positive electrode, and a gel electrolyte in the order of, for example, a negative electrode, a gel electrolyte, and a positive electrode, and accommodating them in a battery exterior material. Is done. In addition to this, a gel electrolyte may be further disposed outside the negative electrode and the positive electrode. In such a gel electrolyte secondary battery using the composite graphite material of the present invention for the negative electrode, the gel electrolyte contains propylene carbonate, and the graphite material powder has a particle size small enough to reduce the impedance sufficiently. Even when used, the irreversible capacity can be kept small. Therefore, a large discharge capacity is obtained and a high initial charge / discharge efficiency is obtained.
[0082]
Furthermore, the structure of the lithium ion secondary battery of the present invention is arbitrary, and the shape and form thereof are not particularly limited, and can be arbitrarily selected from a cylindrical shape, a square shape, a coin shape, a button shape, and the like. Can do. In order to obtain a sealed non-aqueous electrolyte battery with higher safety, it is desirable to have a means for detecting an increase in the internal pressure of the battery and shutting off the current when an abnormality such as overcharge occurs. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, a structure enclosed in a laminate film can also be used.
[0083]
【Example】
EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.
[0084]
Example 1
<Preparation of graphite granule (A)>
Granule treatment of scaly highly crystalline natural graphite particles having an average particle size of 30 μm and a volatile content of 0.7% using a shear compression processing machine (manufactured by Nara Machinery Co., Ltd., hybridization system) shown in FIG. Went. That is, by processing at a rotor peripheral speed of 60 m / s under a condition of a processing time of 3 minutes, mechanical properties such as impact force, compressive force, frictional force, and shear force are mainly applied to the natural graphite particles charged into the apparatus. External force was repeatedly applied. As a result, natural graphite particles were granulated, and dense spherical to ellipsoidal graphite granules (A) were formed. The obtained graphite granule (A) has an average particle diameter of 20 μm, an aspect ratio of 1.7, and crystallinity by the X-ray wide angle diffraction method is d.₀₀₂= 0.3355 nm, Lc = 86 nm. In addition, when the cross section of the obtained graphite granulated material (A) was grind | polished and the porosity (area ratio) in particle | grains was measured using the scanning electron microscope, it was about 15%.
[0085]
<Manufacture of composite graphite particles>
Coal tar pitch containing approximately 40% by mass of volatile matter (Kawasaki Steel Co., Ltd., PK-QL, primary QI = 0% by mass) is added to 44 parts by mass of tar middle oil 76 parts by mass, and ketjen black ( 0.15 parts by mass (average particle size: 0.03 μm) (about 0.6% by mass, about 1870 / μm with respect to the finally obtained carbonaceous layer (B))^ThreeThe coal tar pitch solution was produced by dispersing and mixing at a ratio of (equivalent).
[0086]
The graphite granule (A) is charged in a mass ratio of 100 parts by mass with respect to 120 parts by mass of the coal tar pitch solution in a stirrer, immersed at 150 ° C. for 1 hour, stirred and then gradually heated and depressurized. Stirring was continued after 3 hours until the internal pressure reached 50 mmHg and 200 ° C., and tar oil as a solvent was removed. The obtained mesophase precursor-coated graphite was filled in a steel container. In a firing furnace equipped with a volatile gas combustion treatment device, the mesophase precursor-coated graphite is fired at 470 ° C. for 20 hours under an inert gas flow, with respect to 100 parts by mass of the charged graphite granule (A). A fired product having a mass of 126 parts by mass was obtained. In the fired product, a carbonaceous layer (B) is filled in an internal space between a plurality of scaly graphite particles forming the graphite granule (A) and / or carbon is formed on the outer surface of the graphite granule (A). The coated graphite precursor particles in a form in which the porous layer was coated were in a slightly fused state.
[0087]
Here, for comparison, the coal tar pitch solution was baked in the same manner as described above without adding the graphite granule, and when the mass before calcination was 120 parts by mass, the ratio of 25 parts by mass after calcination It became the mass of. From this result, the mass ratio of the carbonaceous layer / graphite granule in the coated graphite particles was calculated to be 20/80. Moreover, the volatile matter content of the entire fired product was 1.4% by mass. From this, when the volatile content of the graphite granule (A) after firing is 0.7 mass% and the volatile content of the carbonaceous layer is x, 0.7 × 0.8 + x × 0.2 = 1.4 Therefore, the volatile content of the carbonaceous layer is calculated to be about 4.2% by mass. On the other hand, the measured value of the volatile content of the single fired product of the coal tar pitch solution obtained for comparison is 4.3% by mass, and from this, the calculated value of the carbonaceous layer ratio is generally correct. Was supported.
[0088]
The obtained fired product was pulverized by an impact pulverizer to obtain a composite graphite particle precursor having an average particle diameter of 22 μm and an aspect ratio of 1.7.
Subsequently, the composite graphite particle precursor was filled into a graphite crucible, and coke breeze was filled around the crucible, followed by heat treatment at 2800 ° C. for 5 hours. The composite graphite particles obtained by graphitization were not fused or deformed, and the particle shape was maintained. The obtained composite graphite particles had an average particle diameter of 21 μm, an aspect ratio of 1.7, and a specific surface area of 0.8 m.²/ G, bulk density is 1.00 g / cm^ThreeMet. The crystallinity by the X-ray wide angle diffraction method is d₀₀₂= 0.3358 nm, Lc = 85 nm, R value by Raman spectroscopy was 0.12.
[0089]
Using the graphite particles, a working electrode (negative electrode) was produced according to the following method.
<Preparation of negative electrode mixture paste>
90% by mass of the composite graphite particles obtained above and 10% by mass of polyvinylidene fluoride as a binder were mixed using N-methylpyrrolidone as a solvent and stirred at 2000 rpm for 30 minutes using a homomixer. An organic solvent-based negative electrode mixture paste was prepared.
[0090]
<Manufacture of working electrode (negative electrode)>
The negative electrode mixture paste was applied on a copper foil (current collector 7b shown in FIG. 1) with a uniform thickness, and the solvent was evaporated at 90 ° C. in vacuum to dry the paste. Next, the negative electrode mixture applied on the copper foil is pressed by a roller press, and further punched into a circular shape with a diameter of 15.5 mm, thereby comprising a negative electrode mixture layer in close contact with the current collector. The working electrode (negative electrode) 2 shown was produced.
[0091]
<Manufacture of counter electrode>
A lithium metal foil is pressed onto a nickel net and punched into a cylindrical shape with a diameter of 15.5 mm, and a current collector made of nickel net (7a in FIG. 1) and a counter electrode made of a lithium metal foil in close contact with the current collector (4 in FIG. 1) was produced.
[0092]
<Electrolyte>
LiClO in a mixed solvent of 10 mol% propylene carbonate, 50 mol% ethylene carbonate and 40 mol% diethyl carbonate_Four1 mol / dm^ThreeA non-aqueous electrolyte was prepared by dissolving at a concentration of
The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body to prepare a separator (5 in FIG. 1) impregnated with the electrolytic solution.
[0093]
<Production of evaluation battery>
A button-type secondary battery shown in FIG. 1 was manufactured as an evaluation battery.
In this button-type secondary battery, the outer cup 1 and the outer can 3 have a sealed structure that is caulked with an insulating gasket 6 at the peripheral edge thereof, and in order from the inner surface of the outer can 3 to the nickel, Current collector 7a made of net, disk-shaped counter electrode 4 made of lithium foil, separator 5 impregnated with electrolyte solution, disk-shaped working electrode (negative electrode) 2 made of negative electrode mixture, and current collector 7b made of copper foil Constitutes a battery system in which are stacked.
[0094]
In this evaluation battery, a separator 5 impregnated with an electrolyte solution is stacked between a working electrode 2 in close contact with a current collector 7b and a counter electrode 4 in close contact with the current collector 7a, and then the working electrode 2 is attached. In the exterior cup 1, the counter electrode 4 is accommodated in the exterior can 3, the exterior cup 1 and the exterior can 3 are combined, and the periphery of the exterior cup 1 and the exterior can 3 is caulked and sealed with an insulating gasket 6. Manufactured.
[0095]
This evaluation battery is a battery composed of a working electrode (negative electrode) 2 containing a graphite material that can be used as a negative electrode active material in a real battery, and a counter electrode 4 made of a lithium metal foil.
[0096]
The evaluation battery manufactured as described above was subjected to the following charge / discharge test at a temperature of 25 ° C.
<Charge / discharge test>
Constant current charging is performed until the circuit voltage reaches 0 mV at a current value of 0.9 mA, switching to constant voltage charging when the circuit voltage reaches 0 mV, and further charging is continued until the current value reaches 20 μA. Paused for a minute.
[0097]
Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 2.5V. At this time, the charge capacity and the discharge capacity were obtained from the energization amount in the first cycle, and the initial charge / discharge efficiency was calculated from the following equation.
Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity) × 100
In this test, the process of doping lithium ions into the carbon material was charged, and the process of dedoping from the graphite material was discharge.
[0098]
Next, in the second cycle, after charging in the same manner as in the first cycle, constant current discharge was performed at a current value of 18 mA until the circuit voltage reached 2.5V. At this time, the rapid discharge efficiency was obtained from the discharge capacity in the first cycle according to the following equation, and the high rate characteristics were evaluated.
Rapid discharge efficiency (%) = (second cycle discharge capacity / first cycle discharge capacity) × 100
[0099]
In addition to these evaluations, charging and discharging were repeated 10 times under the same conditions as in the first cycle, and the cycle characteristics were evaluated according to the following formula.
Cycle characteristics (%) = (discharge capacity of 10th cycle / discharge capacity of 1st cycle) × 100
[0100]
Furthermore, these charge / discharge tests were conducted by changing the electrode density of the negative electrode. Table 1 shows the measured values of discharge capacity (mAh / g), initial charge / discharge efficiency (%), rapid discharge efficiency (%), and cycle characteristics (%) per 1 g of composite graphite particles.
As shown in Table 1, the lithium ion secondary battery using the composite graphite particles of Example 1 (example of the present invention) as the working electrode (corresponding to the negative electrode of the actual battery) has a theoretical capacity of graphite (372 mAh / g). And a high initial charge / discharge efficiency (that is, a small irreversible capacity). Furthermore, it is excellent in rapid charge / discharge efficiency and cycle characteristics. In particular, even when the electrode density is set high, excellent rapid charge / discharge efficiency and cycle characteristics are exhibited.
[0101]
(Example2)
  Except that the types of carbonaceous fine particles are changed to those shown in Table 1, graphite particles are produced in the same manner as in Example 1 and a charge / discharge test using an evaluation battery is performed to measure discharge capacity, initial charge / discharge efficiency, and rapidity. The charge / discharge efficiency and cycle characteristics were evaluated.
[0102]
(Comparative Example 1)
The graphite granule (A) of Example 1 was not subjected to filling and coating with a carbonaceous material, and an evaluation battery was produced using the graphite granule (A) alone as a negative electrode material. A test was conducted. The results are shown in Table 1.
From the results shown in Table 1, the initial charge / discharge efficiency is remarkably small when a graphite granule without the carbonaceous layer (B), which is a feature of the composite graphite particles of the present invention, is used as the negative electrode material. It was. Moreover, the rapid charge / discharge efficiency and cycle characteristics were low and a tendency to deteriorate was recognized.
[0103]
(Comparative Example 2)
Except not containing carbonaceous fine particles in the carbonaceous layer (B) of Example 2, composite graphite particles similar to Example 2 were produced, and evaluation of crystallinity and evaluation of battery characteristics by a charge / discharge test were performed. . The results are shown in Table 1.
As shown in Table 1, when the carbonaceous layer does not contain carbonaceous fine particles, the initial charge / discharge efficiency is slightly small, and the rapid discharge efficiency and cycle characteristics are low.
[0108]
(Comparative Example 3)
Graphite particles were produced in the same manner as in Example 1 except that scaly natural graphite (average particle size 30 μm, thickness 5 μm) was used as it was without being granulated, and evaluation of crystallinity and charge / discharge by evaluation battery A test was conducted.
[0109]
(Comparative Example 4)
In Example 4, graphite particles were produced in the same manner as in Example 4 except that the graphite granulated material was not used, and the crystallinity was evaluated and the charge / discharge test using an evaluation battery was performed.
[0110]
[Table 1]

[0111]
[Table 2]

[0112]
[Table 3]

[0113]
【The invention's effect】
When used as a negative electrode material for a lithium ion secondary battery, the composite graphite particles of the present invention have both high discharge capacity, high initial charge / discharge efficiency (small irreversible capacity), and excellent high rate characteristics and cycle characteristics. be able to.
Moreover, according to the method of the present invention, the composite graphite particles can be produced with high productivity while suppressing fusion during graphitization.
Furthermore, the negative electrode for a lithium ion secondary battery of the present invention using the composite graphite particles and the lithium ion secondary battery of the present invention using the negative electrode can reduce the irreversible capacity while maintaining a high discharge capacity. In addition, initial charge / discharge efficiency, rapid discharge efficiency, and cycle characteristics can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an evaluation battery produced in Examples and Comparative Examples of the present invention and used for evaluation of charge / discharge characteristics.
FIG. 2 is a schematic view of a shear compression machine (granulator) used in the present invention.
[Explanation of symbols]
1 exterior cup
2 Working electrode
3 Exterior can
4 Counter electrode
5 Separator
6 Insulation gasket
7a, 7b Current collector
21 Fixed drum
22 Rotor
23 Graphite precursor
24 Circulation mechanism
25 Discharge mechanism
26 blade
27 Stator
28 jacket

Claims (6)

A graphite granule (A) formed by aggregating a plurality of scaly graphites, and an internal void and / or an outer surface of the graphite granule (A), than the graphite granule (A). low crystallinity carbonaceous layer (B) is a composite graphite particle formed by filling and / or coating, the carbonaceous layer (B) is seen containing a carbonaceous particulate (C),
The content of the graphite granule (A) in the composite graphite particles is 70 to 99% by mass, and the content of the carbonaceous layer (B) in the composite graphite particles is 30 to 1% by mass. The composite graphite particles , wherein the content of the carbonaceous fine particles (C) in the carbonaceous layer (B) is 0.1 to 2% by mass . The average particle diameter of the composite graphite particles is 5 to 100 [mu] m, the composite graphite according to claim 1, wherein the average particle size of the carbonaceous particles (C) is characterized in that it is a 0.005~0.5μm Particle. The intensity ratio of the spacing of the carbon net plane layer of the composite graphite particles _{(d 002)} is 0.3365nm or less, a peak intensity of 1360cm peak intensity of ^-1 _{(I 1360)} and 1580 cm ^-1 of the Raman spectrum _{(I 1580)} The composite graphite particles according to claim 1 or 2, wherein (I ₁₃₆₀ / I ₁₅₈₀ ) is 0.05 or more and less than 0.40. After producing a spherical or ellipsoidal graphite granule (A) formed by aggregating a plurality of scaly graphites, the graphite granule (A) obtained has an internal void and / or outer surface. Filling and / or coating a mesophase precursor containing carbonaceous fine particles (C);
The volatile content of the mesophase precursor in the filler and / or coating formed in the internal voids and / or the outer surface of the graphite granulated product (A) by the above-mentioned process is 2% by mass or more and less than 8% by mass. A firing step;
And a step of heat-treating the fired product obtained in the step at 900 to 3200 ° C. The negative electrode for a lithium ion secondary battery, characterized by using the composite graphite particles according to any one of claims 1-3. A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 5 .

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