JP4666876B2 - Composite graphite material and method for producing the same, negative electrode material for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

【０１３７】
【表２】

【０１３８】
【発明の効果】
本発明の複合黒鉛質材料は、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）を得ることができる。
また、本発明の方法によれば、前記複合黒鉛質材料を、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、本発明の複合黒鉛質材料を用いるリチウムイオン二次電池用負極材、および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用いた評価用電池の構造を説明する模式断面図である。
【図２】 （ａ）は本発明で用いるメカノケミカル処理装置の作用機構を説明する図であり、（ｂ）は該装置の構成を示す概略図である。
【図３】 本発明で用いる他のメカノケミカル処理装置の概略図である。
【図４】 実施例２によって製造された複合黒鉛質材料の走査型電子顕微鏡写真である。
【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
１１ 回転ドラム
１２ 内部部材（インナーピース）
１３ 黒鉛前駆体
１４ 循環機構
１５ 排出機構
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 a composite graphite material and a production method thereof, a negative electrode material for a lithium ion secondary battery using the composite graphite material, and a lithium ion secondary using the same and having high initial charge / discharge efficiency and high discharge capacity. It relates to batteries.
[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 as a negative electrode material has attracted attention because it has advantages of high energy density and high voltage.
[0003]
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, the negative electrode is deteriorated, and the charge / discharge cycle is shortened. Moreover, there is a possibility that lithium deposited in a dendrite shape penetrates the separator, reaches the positive electrode, and is short-circuited.
[0004]
For this reason, each material for positive and negative electrodes is composed of two kinds of intercalation compounds having different oxidation-reduction potentials, each functioning as a lithium ion support, and the nonaqueous solvent is brought in and out between the layers in the charge / discharge process. Lithium ion secondary batteries are being studied.
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 graphite crystalline structure or turbulent layer structure are known for carbon materials, and the electrode performance such as operating voltage during charging / discharging is large due to the difference in structure. Different. 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 LiC between graphite and lithium. ₆ In 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]
The side reaction such as the decomposition of the electrolytic solution continues until the decomposition product is deposited and grows on the graphite (carbon) surface, and the electron has a thickness such that it cannot move directly from the graphite surface to a solvent or the like. Also, solvent molecules and lithium ions co-intercalate and the graphite surface layer peels off, and the newly exposed graphite surface reacts with the electrolyte to increase the irreversible capacity and reduce the initial charge / discharge efficiency. Has also been reported (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 2000-229924),
(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 methods (1) and (4) have a problem that the production process is complicated and expensive from the viewpoint of industrial production, and the control of the coating thickness is difficult, so that stable and high electrode performance is achieved. There is a problem that the powder performance cannot be exhibited.
In addition, the method (2) described above is stable because the coated graphite adheres firmly when fired at about 1000 ° C. and the coating peels off during crushing, making it difficult to control the homogeneity and thickness of the surface layer. Thus, there is a problem that high electrode performance and powder performance cannot be exhibited.
[0012]
In the above method (3), it is necessary to highly oxidize in order to obtain a high initial charge / discharge efficiency. There is a problem in that the discharge capacity is reduced.
[0013]
In addition, both methods have insufficient discharge capacity in response to the recent demand for higher capacity. Here, the discharge capacity of the battery greatly depends on the discharge capacity per volume of the active material (graphite) layer constituting the negative electrode. Therefore, in order to increase the discharge capacity as a battery, it is necessary to increase the discharge capacity (mAh / g) per unit weight of the active material and to fill the active material with high density. When the negative electrode is formed at a high density, in the case of the above-mentioned coated graphite particles, the coating may be peeled off, exposing the graphite surface highly reactive with the electrolyte solution, which may increase the irreversible capacity.
[0014]
[Problems to be solved by the invention]
In view of the situation as described above, the present invention provides a composite graphite material that can provide both a high discharge capacity and a high initial charge / discharge efficiency (small irreversible capacity) when used as a negative electrode material for a lithium ion secondary battery. And a method for producing the composite graphite material with high productivity while suppressing fusion during graphitization, a negative electrode for a lithium ion secondary battery using the composite graphite material, and a lithium ion secondary battery The purpose is to do.
[0015]
[Means for Solving the Problems]
The present inventor performs a mechanochemical treatment capable of simultaneously applying a shearing force and a compressive force to a graphite precursor including a highly crystalline graphite core material, and then graphitizing the precursor, thereby performing in a non-oxidizing atmosphere. Suppresses the fusion of materials during graphitization (high temperature heating), and the surface of the graphite core material made of highly crystalline graphite has a relatively low crystallinity compared to the core material. And a method for efficiently producing a composite graphite material that has a low crystalline surface layer on the outermost surface and the low crystalline surface layer does not peel from the core material. I found it.
[0016]
Further, the composite graphite material obtained by applying the above method can exhibit a higher discharge capacity when used as a negative electrode material for a lithium ion secondary battery, and is irreversible when natural graphite is used as it is. The present inventors have found that the problem of capacity increase can be solved, and have completed the present invention.
[0017]
Further, the composite graphite material including a plurality of scaly graphite particles formed into a spherical shape as a highly crystalline graphite core material has a structure in which a plurality of highly crystalline graphite particles are arranged in a random direction. Therefore, the present inventors have found that the problems of increase in irreversible capacity, high rate characteristics, and deterioration in cycle characteristics when a negative electrode is formed at a high density can be solved, and the present invention has been completed.
[0018]
That is, the present invention relates to multilayer particles composed of a graphite core material (A) and a graphite coating material (B) encapsulating the graphite core material (A) and / or the multilayer particles aggregated. The composite particles and the composite particles have a graphite surface layer (C) on the outer surface, and the crystallinity decreases in the order of (A)>(B)> (C). A composite graphite material, The Composite graphite material The spacing between the carbon mesh plane layers (d ₀₀₂ ) Is 0.3365 nm or less, the size (Lc) of the crystallite in the C-axis direction is 40 nm or more, and 1360 cm of the Raman spectrum. ^-1 Peak intensity (I ₁₃₆₀ ) And 1580cm ^-1 Peak intensity (I ₁₅₈₀ ) Intensity ratio (I ₁₃₆₀ / I ₁₅₈₀ ) Is 0.05 or more and less than 0.30 For lithium secondary battery negative electrode A composite graphite material is provided.
[0019]
The graphite core material (A) is composed of scaly graphite. Spherical or ellipsoidal granules It is preferable that
[0020]
Further, the graphite core material (A) has a surface spacing (d ₀₀₂ ) Is preferably 0.3358 nm or less.
[0022]
The present invention also provides a graphite precursor as a method for producing the composite graphite material. Raw material And a graphite core material having higher crystallinity than the graphite precursor, Heat to adjust the volatile content to 2.0 mass% or more and less than 20 mass%, After the mechanochemical treatment, it has a step of graphitization, composite A method for producing a graphite material is provided.
[0023]
Furthermore, the present invention provides a negative electrode material for a lithium ion secondary battery comprising the composite graphite material, and a lithium ion secondary battery using the negative electrode material for the lithium ion secondary battery.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the composite graphite material of the present invention, a method for producing the same, a negative electrode material for a lithium ion secondary battery, and a lithium ion secondary battery will be described in detail.
The composite graphite material of the present invention is a multilayer particle comprising a graphite core material (A) and a graphite coating material (B) encapsulating the graphite core material (A) and / or the multilayer particle. Are formed of aggregated particles, and the multilayer particles and the composite particles have a graphite surface layer (C) on the outer surface.
[0025]
In the present invention, the multilayer particle is any one of the forms shown in the following (D-1) and (D-2), and the composite particle has the following three forms ((1), (2 ), (3)), or any combination of two or more.
(1) Composite particles formed by assembling a plurality of multilayer particles (D-1) in a form in which a single graphite core material (A) is encapsulated with a graphite coating material (B)
(2) Composite particles formed by assembling multilayer particles (D-2) in a form in which two or more graphite core materials (A) are coated with a graphite coating material (B)
(3) Composite particles formed by assembling the multilayer particles (D-1) and (D-2)
[0026]
In the present invention, the graphite core material (A) is more crystalline than the graphite coating material (B). Graphite core material (A) includes tar, pitch mesophase calcined carbon (bulk mesophase), mesophase spherules, coke (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.), etc. Graphitized, artificial graphite, natural graphite, expanded graphite, graphitic carbon fiber, graphitized carbon black and the like can be mentioned, and natural graphite is particularly preferable from the viewpoint of obtaining a composite graphitic material having a high discharge capacity. Examples of the shape of the graphite core material (A) include those having various shapes such as a spherical shape, an ellipsoid shape, a scale shape, a lump shape, a plate shape, a fiber shape, and a granular shape, and are formed by processing lump graphite. May be. The average particle diameter of the graphite core material (A) is 1 to 100 μm, preferably 2 to 30 μm.
[0027]
In particular, the graphite core material (A) is preferably one in which a plurality of scaly graphites are finely granulated to a porosity of 50% by volume or less, preferably 30% by volume or less, in the composite particles. When the porosity in the composite particle exceeds 50% by volume, the required amount of the graphite precursor raw material or the graphite precursor for inclusion is increased, so that a sufficient discharge capacity cannot be obtained, The composite particles may be destroyed when the negative electrode is produced at a high density due to the remaining voids.
[0028]
One or more graphite core materials (A) may be encapsulated by the graphite coating material (B), but preferably 50 to 10,000 parts by mass with respect to 100 parts by mass of the graphite coating material (B). More preferably, the ratio is 100 to 2000 parts by mass.
[0029]
The graphite coating material (B) encapsulates the graphite core material (A) to form multilayer particles. The interior of the composite particles formed by the assembly of multilayer particles is filled or voided. May be formed so as to bond between the multilayer particles and to coat all or part of the outside of the composite particles.
Furthermore, the inside of the granulated particles having voids formed by aggregating a plurality of graphite core materials (A) is filled with a graphite coating material (B) or formed into voids to form a space between the graphite core materials (A). In addition, the outer and outer sides of the granulated particles may be covered.
[0030]
Furthermore, in the graphite material of the present invention, the multilayer particles and the composite particles have a graphite surface layer (C) on the outer surface. This graphite surface layer (C) was crystallized by being subjected to mechanochemical treatment (giving compressive force and shearing force) before graphitization treatment (graphite precursor) of the graphite coating material (B). Or may be obtained by simultaneously (continuously) crushing and mechanochemical treatment when crushing the graphite precursor of the fused graphite coating material (B). In this case, the graphite surface layer (C) forms a thin layer integrated with the graphite coating material (B), although the crystallinity is relatively lower than that of the graphite coating material (B). Although the thickness of the graphite surface layer (C) cannot be clearly defined, it is estimated that a thickness of about 0.01 to 5 μm is preferable.
[0031]
In the present invention, it is important that the graphite core material (A), the graphite coating material (B), and the graphite surface layer (C) have low crystallinity in the order of (A)>(B)> (C). is there. Thereby, while maintaining a high discharge capacity derived from the graphite core material (A), the graphite surface layer (C) can have both the characteristics of low irreversible capacity and is densely contained. Even when the negative electrode is formed at a high density by the action of the material (B), the above excellent battery characteristics are exhibited.
[0032]
In the present invention, the crystallinity of the surface of the composite graphite material (graphite surface layer (C)) 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 in-plane lattice vibration. ^-1 1360 cm reflecting the nearby Raman spectrum and disorder of the crystal structure, mainly crystal defects in the surface layer, stacking irregularities, etc. ^-1 A nearby Raman spectrum is measured by 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 = I ₁₃₆₀ / I ₁₅₈₀ ) And the higher the intensity ratio, the lower the surface crystallinity. The intensity ratio R is R ≧ 0.05 from the viewpoint of reducing the irreversible capacity. Is . When R <0.05, the irreversible capacity is large, and sufficient battery characteristics cannot be obtained. This is presumably because crystallization of the surface layer proceeds so much that the decomposition reaction of the electrolytic solution on the surface of the composite graphite material easily proceeds. R is R <0.30 from the viewpoint of minimizing the decrease in discharge capacity. Is . When R ≧ 0.30, the crystallinity may decrease excessively, or the thickness of the graphite surface layer (C) may increase, resulting in a decrease in discharge capacity.
[0033]
On the other hand, the average crystallinity of the composite graphite material is determined by the interplanar spacing of the carbon network layer (d ₀₀₂ ) 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 a graphite material. ₀₀₂ , Lc is calculated. The calculation method is in accordance with the Gakushin Law, and a specific method is described in “Carbon Fiber” (Modern Editorial Company, published in March 1986), pages 733-742.
[0034]
In the present invention, d by the X-ray diffraction method which is an index of the degree of development of the graphite structure. ₀₀₂ And Lc are d from the point that a composite graphite material exhibiting a high discharge capacity is obtained. ₀₀₂ ≦ 0.3365 nm, Lc ≧ 40 nm And , D ₀₀₂ It is particularly preferable that ≦ 0.3362 nm and Lc ≧ 50 nm. d ₀₀₂ When> 0.3365 nm and Lc <40 nm, since the degree of development of the graphite structure is low, when the composite graphite material is used as the negative electrode material of a lithium ion secondary battery, the lithium doping amount is small and the discharge is high. You may not be able to gain capacity. Further, the graphite core material (A) has a surface spacing (d ₀₀₂ ) Is preferably 0.3358 nm or less.
[0035]
The average particle diameter of the composite graphite material of the present invention is appropriately selected according to the application, the design thickness of the electrode, the adjustment of the battery characteristics, etc. The composite graphite material of the present invention is used for a lithium ion secondary battery. When used as a negative electrode material, the average particle diameter is preferably 5 to 100 μm, particularly preferably 5 to 30 μm.
[0036]
The composite graphite material of the present invention can have a nearly spherical shape when a granulated product of scaly graphite particles typified by natural graphite is used as the graphite core material (A). A spherical or ellipsoidal shape is preferable as a material for a negative electrode of a lithium ion secondary battery because it contributes to improvement of high rate characteristics and cycle characteristics. The average aspect ratio of the composite graphite material of the present invention is preferably 3 or less, and particularly preferably 2 or less.
[0037]
In this case, the composite graphite material is formed of dense particles, and can exhibit a high bulk density. A high bulk density is advantageous because it can reduce problems such as destruction of the graphite material when the negative electrode is produced at a high density. Bulk density is 0.5g / cm ^Three It is preferable that it is above, especially 0.7 g / cm ^Three The above is preferable. More preferably 1.0 g / cm ^Three That's it.
[0038]
The specific surface area of the composite graphite material of the present invention can be arbitrarily designed according to the characteristics of the lithium ion secondary battery, the properties of the negative electrode mixture paste, and the like. However, BET specific surface area is 20m ² If it exceeds / g, the safety of the lithium ion secondary battery may be lowered. Generally 0.3-5m in BET specific surface area ² / G, particularly 3 m ² / G or less is preferable. More preferably 1m ² / G or less.
[0039]
In the present invention, the production of the composite graphite material may be any method as long as it is a method capable of producing the composite graphite material having the above-mentioned form, and is not particularly limited. Provided is a method for graphitizing a graphite precursor containing a core material (A) after mechanochemical treatment (hereinafter referred to as “method of the present invention”). By this method, low crystallinity can be easily obtained in the order of graphite core material (A)> graphite coating material (B)> graphite surface layer (C). Further, an integrated composite having no interface between the graphite coating material (B) and the graphite surface layer (C) can be obtained.
[0040]
The graphite precursor used in the method of the present invention 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 graphite precursor. The amount of volatile components of the graphite precursor is 2.0% by mass or more and less than 20% by mass, preferably 4 to 15% by mass. Here, the amount of volatile matter is measured according to the following method based on the fixed carbon method of JIS K2425.
[0041]
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.
[0042]
The quinoline insoluble matter (QI) can also be used as a standard as an index indicating the degree of growth of the graphite structure. It means that the graphite structure is growing as the QI approaches 100% by mass.
[0043]
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.
[0044]
A graphite precursor having a large amount of volatile matter or a low QI exhibits meltability under graphitization conditions. Therefore, when such a graphite precursor is graphitized as it is, the shape usually changes or the materials are fused. In particular, when the volatile content is 20% by mass or more, even if the mechanochemical treatment is performed, the graphite precursor is remelted by the subsequent heat treatment, and it becomes difficult to disturb the crystal structure of only the surface layer. For these reasons, the graphite precursor used in the present invention preferably has a volatile content of less than 20% by mass and a QI of more than 50% by mass.
[0045]
On the other hand, a graphite precursor having a low volatile content or a high QI does not exhibit the above-described meltability (is infusibilized). However, if the volatile content is less than 2.0% by mass, the crystal of the surface layer is obtained by mechanochemical treatment. It becomes difficult to disturb the structure and the formation of a low crystalline surface becomes uncertain. Therefore, in the present invention, a graphite precursor having a volatile content of 2.0% by mass or more and less than 20% by mass is preferably used, and more preferably 4 to 15% by mass of a graphite precursor. When using QI, a graphite precursor of more than 50% by mass and less than 100% by mass is preferably used, and more preferably 80 to 99.5% by mass of graphite precursor.
[0046]
The graphite precursor is not particularly limited as long as it is a solid graphite material containing volatile components as described above. Preferably, at least one of petroleum-based or coal-based heavy oil such as tar and pitch is used as a starting material. And a graphite precursor produced through a polycondensation reaction of aromatic rings, for example, mesophase (bulk mesophase and the like) can be used. When tar and / or pitch is heated, aromatic components in the tar and pitch are condensed or stacked, and spherical materials called mesophase carbon microspheres are generated. When the heating is further continued, the mesophase carbon spherules coalesce to generate a bulk mesophase in which the entire region becomes a mesophase.
[0047]
The heat treatment may be performed under reduced pressure, normal pressure, or 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 heat-treated under some oxidizing atmosphere. Note that the heat treatment may be performed a plurality of times. The heat treatment time is not particularly limited, but is about 0.5 to 100 hours.
In addition, the volatile matter amount of the pitch before polycondensation reaction is about 20-40 mass%, and QI is about 0-20 mass%.
[0048]
In the method of the present invention, one or a plurality of particles serving as the graphite core material (A) may be interposed in the graphite precursor. As an example, a graphite precursor in a molten state and a graphite core material are kneaded by applying a shearing force under heating to obtain a mixed graphite precursor closely adhered to each other, and this is pulverized and adjusted to a predetermined shape A method is mentioned.
[0049]
Alternatively, a small amount of a graphite precursor in a molten state may be added to a graphite core material (A) such as natural graphite and granulated to adjust to a desired particle shape. The solution of the graphite precursor and graphite may be mixed, and the solvent may be removed by a single method or a combination of methods such as heating, decompression, and spray drying. Alternatively, a graphite precursor material (for example, coal pitch) and a graphite core material (A) may be kneaded and granulated, and then heated to cause a polycondensation reaction of an aromatic ring to obtain a graphite precursor. When using a graphite precursor raw material such as pitch, the graphite precursor raw material is dissolved in a solvent, mixed with the graphite core (A), and then a method such as heating, decompression, spray drying or the like is used alone or in combination. May be removed.
[0050]
As described above, the composite graphite material of the present invention is preferably spherical or ellipsoidal. The graphite core material (A) giving these shapes is preferably spherical or ellipsoidal and may be formed by processing massive graphite, but by granulating a plurality of scaly graphites. What was formed is more preferable. Examples of the granulation method include a method in which a mechanical external force is imparted to a plurality of scaly graphites in a dry or wet manner. As a device for applying a mechanical external force, a pulverizer such as a counter jet mill (manufactured by Hosokawa Micron Corporation), a current jet (manufactured by Nissin Engineering Co., Ltd.), SARARA (manufactured by Kawasaki Heavy Industries, Ltd.), GRANUREX (Freund Sangyo Co., Ltd.), Agromaster (Hosokawa Micron Co., Ltd.), Newgra Machine (Seisin Enterprise Co., Ltd.) and other granulators, pressure kneaders, kneaders such as two rolls, rotating ball mill, Shear compression processing machines such as a hybridization system (manufactured by Nara Machinery Co., Ltd.), mechanomicros (manufactured by Nara Machinery Co., Ltd.), and mechanofusion system (manufactured by Hosokawa Micron Corporation) can be used.
[0051]
In granulation, a binder component may be used.
As the binder component, the above graphite precursor or graphite precursor raw material can be used. The binder component may disappear during graphitization. Moreover, even if it does not form a graphite structure by graphitization, it can be used as long as the effects of the present invention are not impaired.
[0052]
Further, when petroleum-based and / or coal-based heavy oil such as tar and / or pitch is used as the graphite precursor raw material, it is preferable to further contain hydrophilic fine particles. The blending of the hydrophilic fine particles facilitates the crushing step to be described later, suppresses the peeling of the film, and contributes to further improvement of the initial charge / discharge efficiency.
[0053]
The hydrophilic fine particles are preferably those having low adhesion to lipophilic tar / pitch and mesophase produced by heating them. When it has adhesiveness, the improvement effect of crushability becomes small.
[0054]
The hydrophilic fine particles may react with carbon during the firing step or heat treatment step (graphitization), or may remain in the finally obtained graphitic particles. It is desirable that the hydrophilic fine particles and their reaction products are finally vaporized and decomposed and do not remain in the graphite particles.
[0055]
The smaller the hydrophilic fine particles, the smaller the blending amount, and the better the effects of the present invention. A preferred average diameter of the hydrophilic fine particles is 1 μm or less. If it is larger than 1 μm, it is necessary to add a large amount of hydrophilic fine particles, which may lead to deterioration of battery characteristics of the finally obtained graphite particles.
[0056]
The blending amount of the hydrophilic fine particles is preferably in the range of 0.01 to 10% by mass, particularly 0.05 to 3% by mass with respect to the graphite coating material (B) of the obtained composite graphite material. When the content is less than 0.01% by mass, the effect of improving the pulverization is small, and when the content exceeds 10% by mass, the battery characteristics of the composite graphite material finally obtained may be deteriorated. .
[0057]
Examples of the hydrophilic fine particles that satisfy the above preferred conditions include metal oxides obtained by vapor phase methods such as vapor-phase anhydrous silica, vapor-phase alumina, and vapor-phase titania, carbonaceous particles such as oxidized carbon black, and iron black. , Yellow lead, zinc yellow, yellow iron oxide, ocher, titanium yellow, red pepper, red lead, zinc white, lead white, lead sulfate, lithopone, titania, antimony oxide, alumina white, gloss white, satin white, plaster and other pigments , Kaolin clay, wax stone clay, calcined clay, aluminum silicates such as hydrous aluminum silicate composites, calcium carbonates such as chalk and chalk, calcium and magnesium carbonates such as dolomite powder, magnesite powder, basic magnesium carbonate, etc. Silicates such as magnesium carbonates, wollastonite, hydrous calcium silicate composites S, talc, magnesium silicate such as mica, quartz powder, fine powder silicic acid, diatomaceous earth, silicates such as silica powder, such as resin beads and the like.
[0058]
In the present invention, as the hydrophilic fine particles, the above-mentioned ones can be used singly or as a mixture of plural kinds thereof. Among these, anhydrous silica, titania, alumina and the like obtained by a vapor phase method that do not react with the graphite precursor raw material, the graphite precursor, and the like are particularly suitable.
[0059]
The method for mixing the hydrophilic fine particles with the graphite precursor raw material is not particularly limited, but examples include a method in which the hydrophilic fine particles are previously dispersed in a solvent, the dispersion is injected into a molten tar and / or pitch, and stirred. Is done. As the solvent, benzene, toluene, quinoline, tar middle oil, tar heavy oil, or the like can be used.
[0060]
Further, the shape of the graphite precursor containing the graphite core material (A) is not particularly limited, and may be any of granular, scaly, spherical, acicular, fibrous, etc., but spherical or elliptical Is preferred. When adjusting to a predetermined particle shape by pulverization, classification, or the like, various known methods can be employed. As the pulverization method, 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 (airflow) type, a shearing type, and a vibration type can be used.
[0061]
In the method of the present invention, no melt deformation or fusion between solids occurs at the time of graphitization, so that a composite graphite material can be obtained in the form of the raw solid graphite precursor. For this reason, if the graphite precursor containing the graphite core material (A) is provided in a desired product shape, it is not necessary to pulverize or form the graphite precursor into a desired shape after graphitization, thereby simplifying the process. Furthermore, since the surface crystallized low by this can be retained as it is, the effects of the present invention can be better achieved. For example, when the composite graphite material of the present invention is used as a negative electrode material for a lithium ion secondary battery, the graphite precursor containing the graphite core material (A) is preferably provided in a spherical shape, an ellipsoid shape, or a granular shape. . The suitable average particle diameter in this case is 5-50 micrometers, More preferably, it is 10-30 micrometers.
In addition, the graphite precursor containing a graphite core material (A) does not necessarily need to be uniform, and the amount of volatile components in the center portion and the surface portion may be different.
[0062]
<Mechanochemical treatment>
In the present invention, the mechanochemical treatment refers to a treatment in which a compressive force and a shear force are simultaneously applied to the graphite core material (A) and the graphite precursor. By mechanochemical treatment, a graphite surface layer (C) having a relatively low crystallinity relative to the graphite precursor is formed on the surface of the graphite precursor encapsulating the graphite core material (A) made of highly crystalline graphite. A composite graphite material having a multilayer structure can be obtained. Although the shearing force and compressive force are usually larger than those obtained by general stirring, these mechanical external forces are preferably applied to the surface of the graphite precursor, and the particle skeleton of the graphite core material (A) is substantially reduced. It is preferable not to destroy. When the particle skeleton of the graphite core material (A) is excessively broken, the initial charge / discharge efficiency tends to decrease. Specifically, it is preferable to suppress the reduction rate of the average particle diameter of the composite graphite material due to the addition of the mechanochemical treatment to 20% or less.
[0063]
The mechanochemical treatment is not particularly limited as long as it is an apparatus capable of simultaneously applying a compressive force and a shearing force to the graphite precursor interposed on the surface of the composite graphite material. For example, a kneader such as a pressure kneader, two rolls, rotating ball mill, hybridization system (manufactured by Nara Machinery Co., Ltd.), mechanomicros (manufactured by Nara Machinery Co., Ltd.), mechanofusion system (Hosokawa Micron Co., Ltd.) ))) Can be used.
[0064]
Among the above devices, a device that simultaneously applies a shearing force and a compressive force using a difference in rotational speed is preferable. For example, a mechano-fusion system manufactured by Hosokawa Micron Corporation showing a schematic mechanism in FIGS. 2 (a) and (b). Is preferred. That is, as shown in FIG. 2B, an apparatus having a rotating drum 11, an internal member (inner piece) 12 having a rotational speed different from that of the rotating drum 11, a circulation mechanism 14 and a discharge mechanism 15 for the graphite precursor 13. Can be used. In this apparatus, as shown in FIG. 2A, while the centrifugal force is applied to the graphite precursor 13 supplied between the rotary drum 11 and the internal member 12, the speed with the rotary drum 11 by the internal member 12 is increased. A mechanochemical treatment can be performed by repeatedly applying a compressive force and a shearing force due to the difference simultaneously.
[0065]
Further, for example, a hybridization system manufactured by Nara Machinery Co., Ltd., whose schematic mechanism is shown in FIG. 3, can be used. That is, a fixed drum 21, a rotor 22 that rotates at high speed, a circulation mechanism 24 and a discharge mechanism 25 for the graphite precursor 23, a blade 26, a stator 27, and a jacket 28 are used. The mechanochemical treatment may be performed using an apparatus that supplies the compression force and the shearing force due to the speed difference between the stationary drum 21 and the rotor 22 to the graphite precursor 23.
[0066]
The conditions of the mechanochemical treatment differ depending on the apparatus to be used and cannot be said unconditionally. However, it is preferable to set the reduction rate of the average particle diameter of the composite graphite material by the treatment to 20% or less. For example, when using an apparatus (FIG. 2) provided with a rotating drum and an internal member, a peripheral speed difference between the rotating drum and the internal member: 5 to 50 m / s, a distance between them of 1 to 100 mm, a processing time of 5 It is preferable to carry out under the condition of 60 minutes.
In the case of an apparatus (FIG. 3) having a fixed drum-high speed rotating rotor, it is preferable to carry out under conditions of a peripheral speed difference of 10 to 100 m / s between the fixed drum and the rotor and a processing time of 30 seconds to 5 minutes.
[0067]
In addition, a known conductive material, ion conductive material, surfactant, metal compound, binder in a range not impairing the effects of the present invention before, during or after the mechanochemical treatment. Various additives such as can also be used in combination.
[0068]
<High temperature heating (graphitization)>
The composite graphite material subjected to the mechanochemical treatment is adjusted to a predetermined shape, and then heated at high temperature in a non-oxidizing atmosphere using a container such as a crucible to be graphitized. The heating temperature is not particularly limited, but is preferably as high as possible from the viewpoint of increasing the degree of graphitization. Specifically, it is preferably higher than 1300 ° C, more preferably 1500 ° C or higher. The upper limit is about 3200 ° C., preferably 2800 to 3000 ° C., from the viewpoint of heat resistance of the apparatus and prevention of graphite sublimation. By heating to such a high temperature for 0.5 to 50 hours, preferably 2 to 20 hours, a high degree of graphitization with a sufficiently developed layered structure that can be suitably used as a negative electrode material for lithium ion secondary batteries, etc. The graphite material which has can be obtained.
[0069]
By the method for producing a composite graphite material of the present invention, d in the X-ray wide angle diffraction method ₀₀₂ It is possible to increase the R value in Raman spectroscopy without changing Lc and Lc. The obtained composite graphite material has a highly crystalline nucleus and a relatively low crystalline surface (graphite surface layer (C)), and the surface is in close contact with and integrated with the highly crystalline nucleus. The surface of the composite graphite material has a relatively low crystallinity and disordered structure, so that a high charge / discharge capacity is obtained and the irreversible capacity in the initial charge / discharge cycle is small ( The initial charge / discharge efficiency is high). That is, since the surface portion of the composite graphite material has a disordered structure, the irreversible capacity can be reduced while maintaining a high discharge capacity, which is extremely useful as a negative electrode material for a lithium ion secondary battery.
[0070]
The R value of the composite graphite material of the present invention is smaller than the R value of the prior art, but has an excellent irreversible capacity reduction effect. A reduction in irreversible capacity is achieved without greatly reducing the crystallinity of the graphite coating material (B) and the graphite surface layer (C), and therefore a high discharge capacity can be maintained.
[0071]
The composite graphite material of the present invention can be used for applications other than the negative electrode, for example, a conductive material for a fuel cell separator or a refractory graphite, taking advantage of its characteristics. It is suitable as a negative electrode material.
Hereinafter, a lithium ion secondary battery negative electrode using the composite graphite material of the present invention as a negative electrode material, and further a lithium ion secondary battery will be described.
[0072]
<Lithium ion secondary battery>
Lithium ion secondary batteries usually have a negative electrode, a positive electrode, and a non-aqueous electrolyte as the main battery components. The positive and negative electrodes are each composed of a lithium ion carrier, and the non-aqueous solvent enters and exits between the layers during the charge / discharge process. In essence, the battery has a battery mechanism in which lithium ions are doped into the negative electrode during charging and dedope from the negative electrode during discharging.
The lithium ion secondary battery of the present invention is not particularly limited except that the composite graphite material of the present invention is used as the negative electrode material, and other battery components conform to the elements of a general lithium ion secondary battery.
[0073]
<Negative electrode>
The production of the negative electrode comprising the composite graphite material of the present invention can be carried out in accordance with a normal molding method, but sufficiently draws out the performance of the composite graphite material and has high formability to powder, There is no limitation as long as it is a method capable of obtaining an electrochemically stable negative electrode.
[0074]
At the time of manufacturing the negative electrode, a negative electrode mixture obtained by adding a binder to the composite graphite material can be used. As the binder, it is desirable to use a material having chemical stability and electrochemical stability with respect to the electrolyte. For example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and styrene. Butadiene rubber and carboxymethyl cellulose 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.
[0075]
Specifically, the negative electrode mixture layer is prepared by mixing a composite graphite material adjusted to an appropriate particle size by classification or the like with a binder, and this negative electrode mixture is usually collected. It can be formed by applying to one or both sides of the electric body.
In this case, a normal solvent can be used. If the negative electrode mixture is dispersed in a solvent to form a paste, and then applied to the current collector and dried, the negative electrode mixture layer is uniformly and firmly formed. A negative electrode adhered to the substrate can be obtained. The paste can be prepared by stirring at about 300 to 3000 rpm with a wing homomixer.
[0076]
Further, for example, the composite graphite material of the present invention and a fluorine resin powder such as polytetrafluoroethylene may be mixed and kneaded in a solvent such as isopropyl alcohol, and then applied to form a negative electrode mixture layer. it can.
Furthermore, the composite graphite material of the present invention and a fluorine resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose are mixed with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol or the like. After forming the slurry, it can be applied to form a negative electrode mixture layer.
[0077]
The coating thickness when the negative electrode mixture comprising the mixture of the composite graphite material of the present invention and the binder is applied to the current collector is suitably 10 to 300 μm.
[0078]
The negative electrode mixture layer can also be formed by dry-mixing a composite graphite material and a resin powder such as polyethylene or polyvinyl alcohol and hot pressing in a mold.
[0079]
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.
[0080]
In the lithium ion secondary battery of the present invention, 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 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.
[0081]
<Positive electrode>
As a material for the positive electrode (positive electrode active material), it is preferable to select a material capable of doping / dedoping a sufficient amount of lithium. Examples of such positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, and vanadium oxides (V ₂ O _Five , V ₆ O ₁₃ , V ₂ O _Four , V _Three O ₈ And their lithium-containing compounds, general formula: M _x Mo ₆ S _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 represented by Can be used.
[0082]
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. The lithium-containing transition metal oxide is specifically represented by the formula: LiM (1) _1-P M (2) _P O ₂ (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 formula: LiM (1) _2-Q M (2) _Q O _Four (Wherein Q is a numerical value in the range of 0 ≦ Q ≦ 1, and M (1) and M (2) are composed of at least one transition metal element).
[0083]
In the above formula, 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, and Ti. , Cr, V, Al.
[0084]
Specific examples of lithium-containing transition metal oxides include LiCoO ₂ , Formula: Li _P Ni _Q M _1-Q O ₂ (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, LiNiO ₂ LiMnO ₂ , LiMn ₂ O _Four Etc.
[0085]
The above lithium-containing transition metal oxide is, for example, Li, an oxide or salt of a transition metal as a starting material, these starting materials are mixed according to the composition, and fired in an oxygen atmosphere at a temperature range of 600 ° C to 1000 ° C. Can be obtained. Note that the starting materials are not limited to oxides or salts, and can be synthesized from hydroxides or the like.
In the present invention, the positive electrode active material may be used alone or in combination of two or more. For example, a carbon salt such as lithium carbonate can be added to the positive electrode.
[0086]
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 production of the negative electrode can be used. As the conductive agent, for example, a carbon material, graphite, or carbon black is used.
[0087]
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.
[0088]
In the case of the positive electrode, as in the case of 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. As a result, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.
[0089]
In the present invention, when forming the negative electrode and the positive electrode as described above, conventionally known various additives such as a conductive agent and a binder can be appropriately used.
[0090]
<Electrolyte>
As the 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 , LiAsF ₆ LiClO _Four , LiB (C ₆ H _Five ) _Four , LiCl, LiBr, LiCF _Three SO _Three , LiCH _Three SO _Three , LiN (CF _Three SO ₂ ) ₂ , LiC (CF _Three SO ₂ ) _Three , LiN (CF _Three CH ₂ OSO ₂ ) ₂ , LiN (CF _Three CF ₂ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN ((CF _Three ) ₂ CHOSO ₂ ) ₂ , LiB [C ₆ H _Three (CF _Three ) ₂ ] _Four LiAlCl _Four , LiSiF ₆ Lithium salt etc. can be used. In particular, LiPF ₆ , LiBF _Four Is preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the nonaqueous electrolytic solution is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 3.0 mol / L.
[0091]
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.
[0092]
In the case of a liquid nonaqueous electrolyte solution, the solvent is 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, trimethyl borate, tetramethyl silicate, Nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2 Oxazolidone, ethylene glycol, aprotic organic solvents such as dimethyl sulfite may be used.
[0093]
When the nonaqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, a matrix polymer gelled with a plasticizer (nonaqueous electrolyte) is included. Examples of the matrix polymer include ether-based polymers such as polyethylene oxide and cross-linked products thereof, fluorine-based polymers such as polymethacrylate-based, polyacrylate-based, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Can be used alone or in combination.
Among these, it is desirable to use a fluorine-based polymer such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of redox stability.
[0094]
As the electrolyte salt and the 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 electrolyte salt concentration in the non-aqueous electrolyte as a plasticizer is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 2.0 mol / L.
[0095]
The method for producing such a solid electrolyte is not particularly limited. For example, a polymer compound that forms a matrix, a method of mixing a lithium salt and a solvent, and melting by heating, a polymer compound in a suitable organic solvent, A method of evaporating an organic solvent after dissolving a lithium salt and a solvent, and a mixture of a polymerizable monomer, a lithium salt, and a solvent that are raw materials for a polymer electrolyte, and irradiating the mixture with ultraviolet rays, electron beams, molecular beams And a method for producing a polymer electrolyte by polymerization.
[0096]
Moreover, 10 to 90 mass% is preferable, and, as for the ratio of the solvent in the said solid electrolyte, More preferably, it is 30 to 80 mass%. When it is 10 to 90% by mass, it is easy to form a solid electrolyte with high electrical conductivity and high mechanical strength.
[0097]
The lithium ion secondary battery of the present invention may have a separator.
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.
[0098]
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 a graphite material, 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. In addition to this, a gel electrolyte may be further disposed outside the negative electrode and the positive electrode. In the gel electrolyte secondary battery using such a graphite material for the negative electrode, even when propylene carbonate is contained in the gel electrolyte and the graphite material powder has a particle size small enough to reduce the impedance sufficiently The irreversible capacity can be kept small. Therefore, a large discharge capacity is obtained and a high initial charge / discharge efficiency is obtained.
[0099]
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.
[0100]
【Example】
EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples. In the following examples and comparative examples, a button-type secondary battery for evaluation of the structure shown in FIG. 1 was prepared according to the following method using the obtained composite graphite material, and a charge / discharge test was performed. The battery characteristics were evaluated.
[0101]
<Preparation of negative electrode mixture paste>
Polyvinylidene fluoride as a binder is mixed at a ratio of 10% by mass with 90% by mass of the composite graphite material using N-methylpyrrolidone as a solvent, and stirred for 30 minutes at 2000 rpm using a homomixer. A solvent-based negative electrode mixture paste was prepared.
[0102]
<Manufacture of working electrode (negative electrode)>
The negative electrode mixture paste was applied on a copper foil (current collector) with a uniform thickness, and further heated to 90 ° C. in a vacuum to volatilize the solvent and dried. Next, the negative electrode mixture applied on the copper foil was pressed by a roller press and punched into a circular shape with a diameter of 15.5 mm together with the copper foil, thereby closely contacting the current collector 7b made of copper foil. A working electrode (negative electrode) 2 comprising a negative electrode mixture layer was produced.
[0103]
<Manufacture of counter electrode>
A lithium metal foil is pressed against a nickel net and punched out on a 15.5 mm diameter cylinder, and a current collector (7a) made of nickel net and a counter electrode 4 (positive electrode) made of a lithium metal foil in close contact with the current collector Manufactured.
[0104]
<Electrolyte>
LiClO was added to a mixed solvent of 10 ol% propylene carbonate, 50 ol% ethylene carbonate and 40 ol% diethyl carbonate. _Four 1 mol / dm ^Three A non-aqueous electrolyte was prepared by dissolving at a concentration of
The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body to produce a separator 5 impregnated with the electrolytic solution.
[0105]
<Manufacture of evaluation batteries>
As an evaluation battery, as shown in FIG. 1, 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 the inner side of the outer can 3 In order, a current collector 7a made of nickel net, a disk-shaped counter electrode 4 made of lithium foil, a separator 5 impregnated with an electrolyte solution, a disk-shaped working electrode (negative electrode) 2 made of a negative electrode mixture and a copper foil. A button-type secondary battery having a structure in which the electric body 7b was laminated was manufactured.
[0106]
In the evaluation battery, the separator 5 impregnated with the electrolyte solution was sandwiched between the working electrode 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a, and then the working electrode 2 was 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.
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.
[0107]
The following charge / discharge test was performed on the evaluation battery manufactured as described above 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.
Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 2.5V. The charge capacity and 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 composite graphite material was charged, and the process of dedoping from the composite graphite material was discharge.
[0108]
Table 1 shows the measured discharge capacity (mAh / g) and initial charge / discharge efficiency (%) value (battery characteristics) per 1 g of composite graphite material powder.
As shown in Table 1, the lithium ion secondary battery using the composite graphite material of the present invention for the working electrode (corresponding to the negative electrode of the actual battery) exhibits a high discharge capacity and has a high initial charge / discharge efficiency (that is, small Irreversible capacity).
[0109]
Next, as 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, high rate characteristics (rapid discharge efficiency) were evaluated from the discharge capacity in the first cycle and the discharge capacity in the second cycle according to the following equation.
Rapid discharge efficiency (%) = (second cycle discharge capacity / first cycle discharge capacity) × 100
[0110]
In addition to these evaluation tests, 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
[0111]
In these tests, the electrode density of the negative electrode was 1.6 g / cm. ^Three And 1.8g / cm ^Three In each case, the evaluation batteries were used.
[0112]
Example 1
<Preparation of composite graphite material>
Heated at a ratio of 50 parts by mass of natural graphite (produced by Madagascar, average particle size 5 μm) with respect to 80 parts by mass of coal tar pitch (PK-QL, manufactured by Kawasaki Steel Corporation) containing about 40% by mass of volatile matter. Using a kneader, the mixture was kneaded while the coal tar pitch was melted.
[0113]
The obtained composite was coarsely pulverized and then heat-treated in a non-oxidizing atmosphere to cause polycondensation reaction of the coal tar pitch to obtain a graphite-containing graphite precursor having the following characteristics.
Volatile content: 4.2% by mass
QI (quinoline insoluble content): 96% by mass
Softening point (Mettler method): 445 ° C
The obtained graphite-containing graphite precursor contained 50 parts by mass of heat-treated coal tar pitch and 50 parts by mass of natural graphite.
[0114]
This graphite precursor was pulverized using a vortex pulverizer to prepare massive particles having an average particle diameter of 20 μm. The massive particles were put into a mechanochemical processing apparatus (manufactured by Hosokawa Micron Co., Ltd., mechanofusion system) whose structure is shown in FIGS. 2 (a) and (b), and mechanochemical processing was performed. At this time, compressive force and shearing force were repeatedly applied under conditions of a peripheral speed of the rotating drum of 20 m / s and a processing time of 10 minutes, and a distance of 5 mm between the rotating drum and the internal member. The average particle size of the graphite-containing graphite precursor after the mechanochemical treatment was 19 μm.
[0115]
Next, the mechanochemically treated graphite-containing graphite precursor was filled in a graphite crucible, and the surroundings of the crucible were filled with coke breeze and heated at 3000 ° C. for 5 hours to graphitize to obtain a composite graphite material. The obtained composite graphite material was not fused or deformed, and the particle shape was maintained. Next, an evaluation battery was manufactured using the obtained composite graphite material, and battery characteristics were evaluated. Table 1 shows the graphitization degree and the measured discharge capacity (mAh / g), initial charge / discharge efficiency (%), rapid discharge efficiency (%), and cycle characteristics (%) per 1 g of composite graphite particles. .
[0116]
As shown in Table 1, the composite graphite material (Example of the present invention) obtained in Example 1 shows a discharge capacity higher than that of Comparative Example 1, and also has an extremely high initial charge / discharge efficiency (small irreversible capacity). ). It can also be seen that the surface of the graphite is selectively crystallized.
[0117]
(Example 2)
<Preparation of graphite granules>
A mechanochemical treatment apparatus (manufactured by Nara Machinery Co., Ltd., hybridization system) shown in FIG. 3 is used to produce scaly highly crystalline natural graphite particles (made in China) with an average particle size of 30 μm and a volatile content of 0.7% It used and gave the shear compression processing. That is, by processing at a rotor peripheral speed of 60 m / s under a condition of a processing time of 3 minutes, mechanical external forces such as impact force, compressive force, and shear force are repeatedly applied to natural graphite particles introduced into the apparatus. Added. As a result, natural graphite particles were granulated to form dense spherical to ellipsoidal graphite granules. The obtained graphite granulated product has an average particle diameter of 20 μm, an aspect ratio of 1.8, and a crystallinity by d-ray wide-angle diffractometry of d. ₀₀₂ = 0.3355 nm, Lc = 86 nm.
[0118]
Moreover, when the cross section of the obtained graphite particle was grind | polished and the porosity (area ratio) in particle | grains was measured using the scanning electron microscope, it was about 15 volume%.
[0119]
<Production of composite graphite precursor>
100 parts by mass of a coal tar pitch solution was prepared by mixing 42 parts by mass of coal tar pitch (PK-QL, produced by Kawasaki Steel Co., Ltd.) containing about 40% by mass of volatile matter at a ratio of 58 parts by mass of oil in tar.
In a stirrer, 100 parts by mass of the coal tar pitch solution and 100 parts by mass of the graphite granulated product were added, and immersed and stirred at 150 ° C. for 30 minutes at an in-machine pressure of 50 mmHg, and the tar oil as a solvent was removed.
[0120]
The obtained coated graphite was filled into a steel container. In a firing furnace equipped with a volatile gas combustion treatment device, the mixture was fired at 450 ° C. for 20 hours under an inert gas flow. The fired product had a slightly fused graphite precursor.
[0121]
For comparison, the coal tar pitch solution was calcined alone at the same time. As a result, it was 25 parts by mass after calcining with respect to 100 parts by mass before calcining. It was calculated that granule = 20: 80. The volatile content of the fired product is 1.8% by mass, and the precursor part is calculated to be about 6.2 (0.7 × 0.8 + x × 0.2 = 1.8)% by mass. On the other hand, the volatile matter amount of the single fired product of the comparative coal tar pitch solution was 6.0% by mass, and it was supported that the calculated value of the ratio was almost correct.
[0122]
The fired product was crushed with an impact pulverizer. The graphite precursor obtained by pulverization had an average particle diameter of 22 μm and an aspect ratio of 1.7.
[0123]
<Mechanochemical treatment>
Subsequently, the graphite precursor was put into a mechanochemical processing apparatus (manufactured by Hosokawa Micron Co., Ltd., mechanofusion system) shown in FIGS. 3A and 3B and subjected to mechanochemical treatment. That is, compressive force and shear force were repeatedly applied under the conditions of a peripheral speed of the rotating drum of 20 m / s, a processing time of 30 minutes, and a distance of 5 mm between the rotating drum and the internal member. The average particle diameter of the graphite precursor after the mechanochemical treatment was 22 μm, and the aspect ratio was 1.7.
[0124]
<Manufacture of composite graphite material>
Next, the graphite precursor subjected to mechanochemical treatment was filled into a graphite crucible, coke breeze was filled around the crucible and heated at 3000 ° C. for 5 hours for graphitization to obtain a composite graphite material. The obtained composite graphite material was not fused or deformed, and the particle shape was maintained. The obtained composite graphite material has an average particle diameter of 22 μm, an aspect ratio of 1.7, and a specific surface area of 0.5 m. ² / G, bulk density is 1.02 g / cm ^Three Met. The crystallinity by the X-ray wide angle diffraction method is d ₀₀₂ = 0.3357 nm, Lc = 88 nm, R value by Raman spectroscopy was 0.08. FIG. 4 shows a scanning electron micrograph of the produced composite graphite material.
Using this composite graphite material, a working electrode (negative electrode) for an evaluation battery was manufactured, and battery characteristics were evaluated. The results are shown in Table 1.
[0125]
(Examples 3 to 5)
In 100 parts by mass of the coal tar pitch solution used in Example 2, 0.5 parts by mass of vapor-phase anhydrous silica fine powder (“AEROSIL 300”, Nippon Aerosil Co., Ltd., average particle diameter 7 nm) was added in advance. A composite graphite material was produced in the same manner as in Example 1 except that the volatile content of the precursor was changed. In the crushing step after firing, the load on the pulverizer was reduced, and crushing was easy. Various evaluations were performed on the obtained composite graphite material, and the results of crystallinity and battery characteristics are shown in Table 1.
[0126]
As shown in Table 1, the evaluation batteries using the composite graphite materials of Examples 2 to 5 show a high discharge capacity close to the theoretical capacity (372 mAh / g) of graphite, and have a high initial charge / discharge efficiency ( That is, it has a small irreversible capacity). In particular, the initial charge and discharge efficiencies of Examples 3 to 5 in which vapor-phase anhydrous silica fine powder is added to the graphite precursor are high. It is considered that the fused graphite precursor can be easily crushed and peeling of the graphite precursor film is suppressed. Furthermore, it has excellent rapid discharge efficiency and cycle characteristics. In particular, even when the electrode density is set high, it has excellent rapid discharge efficiency and cycle characteristics.
[0127]
(Comparative Example 1)
In Example 1, natural graphite was not used, and A graphite material was produced in the same manner as in Example 1 except that the mechanochemical treatment was not performed.
The graphitized material after graphitization was fused, and the pulverized shape could not be maintained. Therefore, the fused graphite material was again pulverized in the same manner to adjust the average particle diameter to 19 μm, and an evaluation battery was produced using the crushed graphite material. Table 1 shows the results of evaluating the degree of graphitization and battery characteristics.
[0128]
As shown in Table 1, in Comparative Example 1, which is an example of a graphite material manufactured without mechanochemical treatment, the initial charge / discharge efficiency is remarkably small (the irreversible capacity is remarkably large). In addition, compared with the comparative example 1, Example 1 has a large R value, and it turns out that the surface of a graphite material is selectively low-crystallized.
[0129]
(Comparative Example 2)
A graphite material was produced in the same manner as in Example 1 except that the same graphite precursor pulverized product as in Example 1 was not subjected to mechanochemical treatment, and an evaluation battery was produced.
The graphitized material after graphitization was slightly fused, and the pulverized shape could not be maintained. Therefore, the fused graphite material was again pulverized in the same manner to adjust to an average particle diameter of 19 μm, and an evaluation battery was produced using it. Table 1 shows the results of evaluating the degree of graphitization and battery characteristics.
[0130]
As shown in Table 1, in Comparative Example 2 where the mechanochemical treatment, which is a feature of the present invention, is not performed, the graphite surface is not low crystallized and the initial charge / discharge efficiency is extremely small (the irreversible capacity is remarkably large) .
[0131]
(Comparative Example 3)
In Example 1, the graphite precursor subjected to the mechanochemical treatment was heated not at 3000 ° C. but at 1300 ° C. for 5 hours to produce a non-graphitic material in which natural graphite was included in the non-graphite. The obtained material was not fused, and the pulverized shape was maintained. Using this, an evaluation battery was produced in the same manner as in Example 1. Table 1 shows the results of evaluating the degree of graphitization and battery characteristics.
[0132]
As shown in Table 1, in the case of Comparative Example 3 where no graphitization treatment is performed, the crystallinity of the material is low and the discharge capacity is remarkably low.
[0133]
(Comparative Example 4)
Natural graphite (Madagascar, average particle size 10 μm, average thickness 2 μm) substantially free of volatiles was mechanochemically treated in the same manner as in Example 1. The graphite after the mechanochemical treatment was not fused or deformed, and the particle shape was maintained. The average particle diameter was 9 μm, and the average thickness was 2 μm. Next, using this natural graphite, an evaluation battery was produced in the same manner as in Example 1, and the battery characteristics were evaluated. Table 1 shows the degree of graphitization and battery characteristics.
As shown in Table 1, in the case of Comparative Example 4 having no graphite coating material, the initial charge / discharge efficiency is small (the irreversible capacity is large) even when the mechanochemical treatment is performed. Also, rapid discharge efficiency and cycle characteristics are inferior. When the electrode density is increased, the graphite particles are oriented, and these characteristics are further reduced.
[0134]
(Comparative Example 5)
A battery for evaluation was produced in the same manner as in Example 1 using natural graphite substantially free from volatile matter (produced by Madagascar, average particle diameter 10 μm, average thickness 2 μm), and battery characteristics were evaluated. Table 1 shows the degree of graphitization and battery characteristics.
As shown in Table 1, in the case of Comparative Example 5 using natural graphite alone, although the discharge capacity is high, the initial charge / discharge efficiency, the rapid discharge efficiency, and the cycle characteristics are low. When the electrode density is increased, the deterioration of these characteristics is remarkable.
[0135]
(Comparative Example 6)
A graphite material was produced in the same manner as in Example 2 except that the same pulverized graphite precursor as in Example 2 was not subjected to mechanochemical treatment. Table 1 shows the results of evaluating the degree of graphitization and battery characteristics.
As shown in Table 1, in Comparative Example 6, which is an example of a graphite material manufactured without mechanochemical treatment, the initial charge / discharge efficiency is small (the irreversible capacity is large). In addition, Example 2 has a larger R value than Comparative Example 6, and it can be seen that the surface of the graphite material is selectively low crystallized.
[0136]
[Table 1]

[0137]
[Table 2]

[0138]
【The invention's effect】
When the composite graphite material of the present invention is used as a negative electrode material for a lithium ion secondary battery, a high discharge capacity and a high initial charge / discharge efficiency (small irreversible capacity) can be obtained.
Further, according to the method of the present invention, the composite graphite material can be produced with high productivity while suppressing fusion during graphitization.
Furthermore, the negative electrode material for a lithium ion secondary battery using the composite graphite material of the present invention and the lithium ion secondary battery can reduce the irreversible capacity while maintaining a high discharge capacity. Charge / discharge efficiency can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view illustrating the structure of an evaluation battery used in Examples and Comparative Examples of the present invention.
FIG. 2 (a) is a diagram for explaining an operation mechanism of a mechanochemical processing apparatus used in the present invention, and FIG. 2 (b) is a schematic diagram showing a configuration of the apparatus.
FIG. 3 is a schematic view of another mechanochemical processing apparatus used in the present invention.
4 is a scanning electron micrograph of the composite graphite material produced in Example 2. FIG.
[Explanation of symbols]
1 exterior cup
2 Working electrode
3 Exterior can
4 counter electrode
5 Separator
6 Insulation gasket
7a, 7b Current collector
11 Rotating drum
12 Internal members (inner pieces)
13 Graphite precursor
14 Circulation mechanism
15 Discharge mechanism
21 Fixed drum
22 Rotor
23 Graphite precursor
24 Circulation mechanism
25 Discharge mechanism
26 blade
27 Stator
28 jacket

Claims (7)

Multilayer particles comprising a graphite core material (A) and a graphite coating material (B) encapsulating the graphite core material (A) and / or composite particles formed by aggregation of the multilayer particles from it, said plurality layer particles and the composite particles have a graphite surface layer (C) on the outer surface, a (a)>(B)> composite graphite material crystallinity have low in the order of (C) ,
The interplanar spacing (d ₀₀₂ ) of the carbon network layer of the composite graphite material is 0.3365 nm or less, the C-axis size (Lc) of the crystallite is 40 nm or more, and the peak intensity of 1360 cm ⁻¹ of the Raman spectrum. A composite graphite for a negative electrode of a lithium secondary battery, wherein an intensity ratio (I ₁₃₆₀ / I ₁₅₈₀ ) of (I ₁₃₆₀ ) to a peak intensity (I ₁₅₈₀ ) of ₁₅₈₀ cm ⁻¹ is 0.05 or more and less than 0.30 Quality material.   The composite graphite material according to claim 1, wherein the graphite core material (A) is a spherical or ellipsoidal granulated product made of scale-like graphite. 3. The composite graphite material according to claim 1, wherein the graphite core material (A) has a carbon network layer surface spacing (d ₀₀₂ ) of 0.3358 nm or less.   It is a manufacturing method of the composite graphite material in any one of Claims 1-3, Comprising: The raw material of a graphite precursor and the graphite core material whose crystallinity is higher than this graphite precursor are mixed, and it heats. A method for producing a composite graphitic material, comprising a step of graphitizing after adjusting a volatile content to 2.0 mass% or more and less than 20 mass%, performing a mechanochemical treatment. A raw petroleum and / or coal-based heavy oil of the graphite precursor, according to claim 4, the raw material of the graphite precursor, the average diameter is characterized by adding the following hydrophilic fine particles 1μm A method for producing a composite graphite material.   The negative electrode material for lithium ion secondary batteries which consists of the composite graphite material in any one of Claims 1-3.   A lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to claim 6 as a negative electrode material.

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