JP4701484B2 - Graphite powder suitable for negative electrode of secondary battery, its production method and use - Google Patents

Description

表１に示すように、酸化熱処理後のAr雰囲気中での熱処理時の昇温速度３℃／秒の場合には、得られた黒鉛粉末の表面の閉塞構造がモザイク状にならず、閉塞領域がｃ⊥方向に連続して、閉塞領域のLc⊥値が∞である表面形態となる。
【００７２】
これに対し、この熱処理の昇温速度が５℃／秒以上になると、モザイク状の閉塞構造が生成し、それに伴って高率放電特性の改善が得られた。表１から、熱処理時の昇温速度が速いほど、また保持温度が高いほど、生成したモザイク状閉塞構造の個々の閉塞領域のLc⊥値が小さくなる傾向があり、このLc⊥値が小さいほど高率放電特性が向上することがわかる。
【００７３】
【発明の効果】
本発明により、高率放電特性に優れ、かつ放電容量と充放電効率も十分に高い、リチウムイオン二次電池の負極として最適の黒鉛粉末が提供される。それにより、リチウムイオン二次電池の高性能化を図ることができる。従って、本発明は特に高率充放電が求められる用途におけるリチウムイオン二次電池の普及に貢献する技術である。
【図面の簡単な説明】
【図１】従来の黒鉛粉末の表面閉塞構造と本発明の範囲外の条件での熱処理による閉塞構造の変化を示す模式的説明図である。
【図２】本発明の黒鉛粉末の製造過程における表面閉塞構造の形成の異なる態様を示す模式的説明図である。
【図３】本発明の黒鉛粉末の表面形態を示す原子間力顕微鏡写真である。
【図４】本発明の黒鉛粉末の表面付近の断面ＴＥＭ写真である。
【図５】本発明の黒鉛粉末の表面を真上から観察した模式図であり、ａは閉塞領域の部分、ｂは間隙口の部分を示す。
【図６】本発明の黒鉛粉末と従来の黒鉛粉末について、充放電電流密度に対する放電容量の変化を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a graphite powder having an optimum surface form as a negative electrode material for a lithium ion secondary battery and a method for producing the same. The present invention also relates to a negative electrode of a lithium ion secondary battery using the graphite powder and a lithium ion secondary battery including the negative electrode. When the graphite powder of the present invention is used for a negative electrode, a lithium ion secondary battery excellent in high rate discharge characteristics (rate characteristics) can be produced.
[0002]
[Prior art]
In lithium ion secondary batteries that are rapidly spreading as small-sized secondary batteries, negative electrodes using carbon powder that can absorb Li ions as an active material are generally used. Most of these negative electrodes are manufactured from graphite powder. . This is because crystalline graphite powder has a higher discharge capacity per unit volume and excellent charge / discharge efficiency than amorphous carbon powder.
[0003]
This graphite powder can be obtained by pulverizing natural graphite, but it was obtained after carbonization by removing organic substances by heat treating a carbonaceous raw material in terms of quality stability. A powder of graphite obtained by graphitizing a carbon material by heat treatment at a higher temperature for crystallization is preferred.
[0004]
In addition to discharge capacity and charge / discharge efficiency (ratio of discharge capacity to charge capacity), characteristics considered for graphite used as the negative electrode material for lithium ion secondary batteries include high rate discharge characteristics (at low current density). There is a ratio of discharge capacity at high current density to discharge capacity). The high rate discharge characteristic is important particularly in the case of rapid charging.
[0005]
Regarding the surface form of artificial graphite powder, International Publication No. WO 98/29335 describes a closed structure formed by bonding the ends of c-plane layers of graphite crystal having a hexagonal layered crystal structure by graphitization heat treatment. It is pointed out that it forms on the surface. In the conventional artificial graphite having this surface form, the gap surface, which is a gap between adjacent closed structures, becomes the main entry / exit site for Li ions involved in charge / discharge.
[0006]
[Problems to be solved by the invention]
In the above international publication, it is proposed to increase the discharge capacity by increasing the density per unit length of the gap surface, which is the main penetration site of Li ions (that is, decreasing the pitch of the gap surface). Has been.
[0007]
This increase in the interfacial density of the graphite powder can be achieved by any of the following methods:
(1) Perform high-speed grinding or shear grinding before graphitization heat treatment to introduce atomic level irregularities (layer defects) on the powder surface, and then perform graphitization heat treatment; or
(2) The graphite powder obtained by the graphitization heat treatment is subjected to a heat treatment that can cut the surface, such as an oxidation heat treatment, to release the blockage structure of the powder surface, and then heat-treated again in an inert gas to block the graphite powder. Reshape the structure.
[0008]
In particular, the method (2) can obtain a graphite powder having a very large gap surface density. In this way, when the density of the gap surface, which is the entry site of Li ions, is increased, a graphite powder whose discharge capacity is considerably close to 372 mAh / g, which is the theoretical capacity of the graphite negative electrode, can be obtained. Moreover, the charge / discharge rate is also sufficiently high.
[0009]
However, as long as the Li ion penetration site is limited to the gap surface, the Li ion that has passed through the penetration site diffuses into the graphite powder through the c-plane layer, so the Li ion diffusion path becomes longer. . Therefore, since it takes time to enter and escape Li ions, there is room for improvement in high rate discharge characteristics. As the current density increases, a large amount of Li ions must enter and escape. However, if the diffusion path is long and it takes time to enter and escape Li ions, a large amount of Li ions enter at high current densities. This is because the diffusion cannot catch up and the discharge capacity decreases.
[0010]
The main object of the present invention is to provide a graphite powder suitable for a negative electrode material of a lithium ion secondary battery with improved high rate discharge characteristics and excellent discharge capacity and charge / discharge efficiency, and a method for producing the same. .
[0011]
[Means for Solving the Problems]
In order to investigate the influence of the heat treatment on the closed structure of graphite, the present inventors changed the heat treatment conditions in the inert gas in the method (2) described in International Publication WO 98/29335 to close the structure. The structure was reformed and the surface morphology of the resulting graphite powder was examined using atomic force microscopy. As a result, it was found that the surface morphology varies depending on the heating rate of the heat treatment.
[0012]
Specifically, the surface of the graphite powder that has been heat-treated at a rate of temperature rise of less than 5 ° C./second has a clogging structure formed by bonding ends of the c-plane layer in the c-plane layer direction, that is, It continues long in the direction perpendicular to the c-axis (c⊥ direction), and the gap surface also continues in the same direction. Therefore, a striped block structure is formed in which gap surfaces appear at substantially constant intervals.
[0013]
On the other hand, the surface of graphite powder that has been subjected to rapid heat treatment (rapid thermal annealing) at a high temperature rise rate is dominated by partial aggregation of the closed structure starting from nucleation at non-equilibrium. Therefore, the closed structure on the powder surface is not continuous in the c⊥ direction, and a mosaic (checkered) closed structure is formed. Further, the once formed mosaic blockage structure is thermally stable and irreversible, so that its surface form is maintained even during the temperature lowering process.
[0014]
The graphite powder having this novel mosaic blockage structure exhibits high discharge capacity and charge / discharge efficiency, and also has high rate discharge characteristics. This is because, as will be described in detail later, the mosaic occlusion structure has a short Li ion diffusion path and can easily diffuse, replace, and release a large amount of Li ions in a short time. It is done.
[0015]
According to the present invention, a closed region in which the ends of the graphite c-plane layers are connected and closed is scattered on the powder surface, and the length of each of the closed regions in the direction perpendicular to the graphite c-axis ( A graphite powder is provided, wherein Lc L) is 100 nm or less.
[0016]
The graphite powder of the present invention having the above surface form can be produced by a method characterized by including the following steps:
(a) a step of heat treating a carbon material obtained by carbonizing the carbonaceous material to graphitize,
(b) at least one grinding step before carbonization, between carbonization and graphitization, and / or after graphitization,
(c) a step of heat-treating the graphite powder obtained after the steps (a) and (b) under conditions capable of shaving the surface;
(d) A step of heat-treating the graphite powder obtained in step (c) in an inert gas at a temperature rising rate of 5 ° C./second or more and maintaining the temperature at 500 ° C. or more.
[0017]
The heat treatment in step (c) is preferably an oxidation heat treatment.
According to the present invention, a negative electrode for a lithium ion secondary battery provided with the graphite powder having the above surface form, and a lithium ion secondary battery provided with the negative electrode are also provided.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
First, the surface form of the graphite powder of this invention is demonstrated with reference to the schematic diagram of FIGS.
[0019]
A conventional ordinary graphite powder obtained by pulverizing and heat-treating with graphitization has a multilayer closed structure in which several layers of closed structures are laminated, as shown in a cross-sectional view in FIG. Although not shown, this multi-layer closed structure takes a surface form extending long in the c-plane layer direction (a direction perpendicular to the c-axis, that is, the c⊥ direction). The surface morphology of the graphite powder obtained after steps (a) and (b) of the production method of the present invention is also such. Such a closed structure on the powder surface can actually be confirmed by a cross-sectional SEM or TEM photograph of the graphite powder.
[0020]
In the case of the multilayer closed structure described above, the gap surface serving as an intrusion site for Li ions is, for example, in the case of the multilayer closed structure laminated in five layers, one for every 10 c-plane layers, and the remaining nine c layers. Since the end of the face layer is closed, Li ions cannot enter. Thus, since there are few Li ion penetration sites, the discharge capacity is limited and cannot approach the theoretical capacity.
[0021]
When graphite powder having such a multilayer laminated closed structure on the surface is subjected to an oxidation heat treatment in step (c) of the production method of the present invention, the surface is scraped and flattened. As a result, as shown in FIG. The occlusion structure is opened, and the end of the c-plane layer remains cut without being bonded to the other c-plane layers. Therefore, all the c-plane layer ends become gap surfaces and become Li ion intrusion sites. Although this form is advantageous for the penetration of Li ions, it is chemically unstable and the electrolytic solution easily penetrates into the inside, so that it becomes a negative electrode with a very short cycle life and is not practical.
[0022]
Thereafter, in step (d), when heat treatment is performed in an inert gas, unstable c-plane layer ends are bonded together for stabilization, and a closed structure is re-formed. Since the surface of the graphite powder is flattened, the bonding at this time reduces the number of laminated layers even if the closed structure is laminated in multiple layers. For example, as shown in FIG. 1 (c), a surface form having a closed structure in which two layers are laminated can be used. However, the actual closing structure is naturally not uniform in this way.
[0023]
However, if the heating rate of the heat treatment in step (d) is less than 5 ° C./second, end bonding occurs substantially in an equilibrium state, and as shown in FIG. The connection between the face layers extends long in one direction, and the gap surface and the closing structure form a striped pattern, that is, a so-called striped closing structure. That is, the number of c-plane layers is reduced compared to the closed structure shown in FIG. 1 (a), but it is the same in that the closed structure continues in the c⊥ direction. However, when the number of stacked layers decreases, the density of the gap surface increases and the discharge capacity increases.
[0024]
In the present invention, the heat treatment in step (d) is rapid heat treatment with a temperature increase rate of 5 ° C./second or more. As a result, the lattice vibration at the end of the c-plane layer increases momentarily, and bonding at the end of the c-plane layer occurs in a non-equilibrium state. As a result, as shown in FIGS. 2 (a) and 2 (b), the joints are distributed to the left and right, and the ends are partially connected alternately between the c-plane layers. That is, the closed structure of the end of the c-plane layer on the surface of the graphite powder does not continue long in the c⊥ direction, but breaks short, resulting in a mosaic (checkered) closed structure.
[0025]
The mosaic blockage structure appearing on the surface of the graphite powder of the present invention may be a single layer blockage structure without stacking as shown in FIG. 2 (a), or as shown in FIG. It may be a closed structure in which more than one layer is laminated, or a coexistence state of both. In general, the number of layers of the closed structure is smaller than the graphite powder obtained after the steps (a) and (b), but there is no particular limitation. The mosaic closed structure of the graphite powder of the present invention can be seen by observing the graphite powder with an atomic force microscope that can reproduce the fine unevenness of the surface.
[0026]
When the clogging structure on the graphite powder surface is striped as shown in FIG. 1 (a) or (c), the c-plane layer completely clogged by the clogging structure can be reached in any direction in the c⊥ direction. It remains an occluded structure. Therefore, in the c-axis direction (c‖ direction) perpendicular to the c-plane layer, the gap surface, which is the entry site of Li ions, appears only between specific adjacent c-plane layers every several to a dozen layers. Li ions are easy to move in the c-plane layer (c⊥ direction), and the movement in the c-axis direction (c‖ direction) perpendicular to this is more difficult because it must pass through the c-plane layer. is there. Therefore, when the gap surface appears only in a specific c-plane layer with a gap, the diffusion distance of Li ions becomes longer and the diffusion becomes more difficult.
[0027]
On the other hand, when the surface blocking structure of the graphite powder is a checkered mosaic like the present invention, for example, as shown in FIG. All adjacent c-plane layers have a portion (referred to as a gap port in the present invention) that becomes a gap surface at some position in the c-plane layer direction (c⊥ direction). All c-plane layer layers have gap holes, and Li ions can enter the c-plane layer from these gap holes and easily move in the c-plane direction, so that Li ions diffuse quickly. . As a result, even if a large amount of Li ions enter during high rate discharge, Li ions can easily diffuse into the powder, and the high rate discharge characteristics are improved.
[0028]
The graphite powder of the present invention has the surface form of the above-mentioned mosaic closed structure. In the present invention, each block portion of the mosaic block structure is referred to as a block region. Each closed region is formed by connecting and closing the end portions of the graphite c-plane layer.
[0029]
These individual closed regions vary in length in the direction perpendicular to the graphite c-axis (Lc⊥), but all are 100 nm or less. Graphite powder with a mosaic blockage structure in which the Lc value of each blockage region exceeds 100 nm is virtually impossible to manufacture. When trying to manufacture such a powder, as shown in Fig. 1 (c) In addition, the closed region becomes a surface form continuous in the c⊥ direction (Lc = ∞).
[0030]
The smaller the value of Lc⊥ in each closed region, the shorter the Li ion diffusion distance and the higher rate discharge characteristics. This value is preferably 50 nm or less, more preferably 20 nm or less, and particularly preferably 10 nm or less. The magnitude of the Lc value in the closed region depends on the heating rate and the holding temperature during the heat treatment in step (d), and tends to be smaller as these are higher.
[0031]
The width (Lc‖) in the c-axis direction of the individual closed regions of the mosaic closed structure is not particularly limited. This width depends on the number of stacked layers of the above-mentioned closed structure and is generally in the range of 0.3354 nm to 10 nm. 0.3354 nm is the minimum value of the interlayer distance d002 of graphite, and is the theoretical value of Lc‖ in the closed region of a single layer of a complete graphite crystal. In order for Lc10 to exceed 10 nm, it is necessary to have a closed structure in which more than a dozen layers are laminated, and it is practically impossible to manufacture. A preferable value of Lc‖ is 5 nm or less.
[0032]
The width (Lc‖ value) in the c-axis direction of the closed region of the mosaic closed structure that appears on the surface of the graphite powder of the present invention can be measured from a cross-sectional SEM or TEM photograph near the surface of the graphite powder. On the other hand, the length (Lc 軸 value) in the c-axis perpendicular direction of the closed region can be measured from an atomic force microscope photograph and a TEM photograph of the graphite powder surface.
[0033]
Next, the manufacturing method of the graphite powder of this invention is demonstrated.
First, in the step (a), the carbonaceous material is heat treated to carbonize (decompose organic matter), and the obtained carbon material is further heat treated at high temperature to be graphitized.
[0034]
The carbonaceous material as a raw material is not particularly limited, and may be the same as that conventionally used for producing graphite. Specific examples include coal tar pitch or petroleum pitch, mesophase microspheres generated by heat treatment thereof, bulk mesophase which is a matrix of the microspheres, and other organic substances such as organic resins. Particularly preferred carbonaceous raw materials are mesophase spherules and bulk mesophase, and bulk mesophase is particularly preferred from the viewpoint of cost and mass productivity.
[0035]
Carbonization conditions for the carbonaceous material may be selected so that elements other than carbon contained in the raw material are almost completely removed by decomposition of the material. In order to prevent oxidation (combustion) of carbon, the carbonization heat treatment is performed in an inert atmosphere or vacuum. The heat treatment temperature for carbonization is usually in the range of 800 to 1500 ° C, and particularly preferably around 1000 ° C. The heat treatment time required for carbonization is about 30 minutes to 3 hours when the temperature is 1000 ° C., although it depends on the type of raw material, heat treatment, and temperature.
[0036]
Next, the obtained carbon material is heat-treated and graphitized. Graphitization usually requires a temperature of 2500 ° C or higher. In order to lower the graphitization temperature, a small amount of an appropriate graphitization catalyst (eg, boron) may be added. In that case, the graphitization catalyst may be added before carbonization. When a graphitization catalyst is added, the graphitization temperature can be lowered to about 1500 ° C. The upper limit of the graphitization heat treatment temperature is about 3200 ° C with the current heating technology. A preferable graphitization heat treatment temperature is 2800 to 3000 ° C. when no catalyst is added.
[0037]
This heat treatment is performed until the graphitization (crystallization) is completed. This time varies depending on the presence or absence of the catalyst and the amount of treatment, but is generally 20 minutes to 10 hours. The heat treatment atmosphere is a non-oxidizing atmosphere (eg, inert gas atmosphere or vacuum). For the graphitization heat treatment, industrially, an Atchison furnace (heating by energizing surrounding carbon powder) or an LWG furnace (direct heating by heating) is used. Such an industrial firing furnace is operated in the atmosphere, but the inside of the furnace is a non-oxidizing atmosphere composed of nitrogen and carbon monoxide.
[0038]
Step (b) is a pulverizing step for obtaining a powder. This pulverization may be performed on the carbonaceous raw material before carbonization, carbon material after carbonization, and graphite after graphitization, but after graphitization, the layered structure develops and pulverizes. Since it becomes difficult, it is preferable to grind before graphitization. Moreover, it can also grind | pulverize in these 2 steps or more. However, crushing is not performed after the next step (c).
[0039]
The pulverization can be performed using a conventional pulverizer such as a hammer mill, a fine mill, an attrition mill, a ball mill, or a disk mill.
Regarding the particle size, when used as a negative electrode material for a lithium ion secondary battery, it is known that if the average particle size is too large, the packing density decreases, and those having a particle size smaller than 1 μm deteriorate the initial charge / discharge characteristics. Therefore, it is preferable that fine particles having an average particle diameter in the range of 5 to 50 μm and smaller than 1 μm do not exist.
[0040]
Steps (a) and (b) are the same as the conventional method for producing graphite powder. If the graphite powder thus obtained is pulverized before graphitization, as described above, as shown in FIG. 1 (a), a multi-layer laminated closed structure usually extends in the c⊥ direction. It has a surface morphology with a closed structure. Conventionally, the graphite powder obtained from the steps (a) and (b) has been used as it is for the negative electrode material of a lithium ion secondary battery, but the present invention is further subjected to steps (c) and (d).
[0041]
In the step (c), the graphite powder obtained through the steps (a) and (b) is produced by a graphitization heat treatment by, for example, subjecting the graphite powder to a c-plane layer by subjecting it to an oxidation heat treatment. The occluded structure is once opened and is not connected to other c-plane layers. By scraping the surface, the ends of the c-plane layer on the powder surface are relatively flat.
[0042]
The conditions for the oxidation heat treatment are not particularly limited as long as the closure structure is substantially opened by oxidation, but the heat treatment temperature is preferably about 600 to 800 ° C. Since graphite powder with a closed structure has high oxidation resistance, it is difficult to oxidize at temperatures lower than 600 ° C., and oxidation proceeds rapidly at temperatures above 800 ° C., leading to deterioration of the entire graphite powder. The oxidation heat treatment time varies depending on the temperature and the amount of treatment, but is generally 1 to 10 hours. The heat treatment atmosphere is an oxygen-containing atmosphere, and may be a pure oxygen atmosphere or a mixed gas atmosphere of oxygen and an inert gas (eg, air).
[0043]
As a result of removing the powder surface by this oxidation heat treatment, the weight of the graphite powder is reduced by about 2 to 5%. Further, the particle size of the powder is slightly reduced (eg, about 1 to 2 μm). If necessary, the pulverization conditions are set in anticipation of this particle size reduction.
[0044]
The opening of the closing structure is not limited to the oxidation heat treatment. Other methods can also be employed as long as the clogging layer structure can be obtained by scraping off the surface structure of the graphite powder to open the blocking structure. Other methods include, for example, fluorination heat treatment or hydrogenation heat treatment. The heat treatment conditions in this case may be appropriately set by experiments so that the closing structure is opened by scraping the surface.
[0045]
When the graphite powder whose surface has been scraped is heat-treated in an inert gas atmosphere in step (d), the open ends of the c-plane layer are bonded together for stabilization. An occlusion structure is formed.
[0046]
In the present invention, the heat treatment conditions for re-forming this closed structure are important. That is, this heat treatment is a rapid heat treatment with a temperature rising rate of 5 ° C./second or more and a holding temperature of 500 ° C. or more. As already explained, as a result of such rapid thermal processing, the c-plane layer ends are bonded in a non-equilibrium state, and as a result, the bonds are distributed to the left and right, and the restructured closed structure continues in the Lc⊥ direction. Without a checkered mosaic. Thereby, the improvement of the high rate discharge characteristic is obtained.
[0047]
In the mosaic blockage structure formed by this rapid heat treatment, the higher the rate of temperature rise, the smaller the length (Lc⊥) in the c-axis ⊥ direction of each blockage region of the blockage structure. The improvement effect is large and advantageous. In that sense, the temperature rising rate is preferably 10 ° C./second or more, more preferably 25 ° C./second or more, more preferably 50 ° C./second or more, and a very high temperature rising rate of 100 ° C./second or more. it can. However, even if the rate of temperature rise exceeds 200 ° C., the effect is not improved so much.
[0048]
Thus, in order to form a mosaic blockage structure on the surface of graphite powder by rapid heat treatment, it is necessary to open the blockage structure by shaving the surface of the graphite powder by oxidation heat treatment or the like. If the closed structure is not opened in advance, a mosaic closed structure cannot be formed even by rapid heat treatment.
[0049]
The holding temperature of the heat treatment needs to be 500 ° C. or higher in order to form the above mosaic closed structure. If the holding temperature is lower than this, lattice vibrations of a size necessary for bonding between the c-plane layer ends cannot be applied, and it becomes difficult to form a closed structure. When the heating rate is the same, the higher the holding temperature, the smaller the value of Lc⊥ in the individual closed regions of the formed mosaic closed structure, which is advantageous because the effect of improving the high rate discharge characteristics is increased. In that sense, the heat treatment holding temperature is preferably 700 ° C. or higher, more preferably 1000 ° C. or higher, and further preferably 1500 ° C. or higher. For example, it can be a high temperature of 2000 to 3000 ° C. or higher.
[0050]
Note that the value of the width (Lc‖) in the c-axis direction of each closed region of the mosaic closed structure is not significantly affected by the heat treatment conditions (temperature increase rate and holding temperature). The value of Lc‖ is mainly governed by the graphitization temperature.
[0051]
The heat treatment in step (d) is performed in an inert gas atmosphere. The inert gas atmosphere may be one or more of Ar, He, Ne, and the like, for example. The heat treatment time only needs to form a closed structure and varies depending on the temperature, but is generally 1 to 10 hours. For example, at 1000 ° C., about 5 hours is a guide.
[0052]
The graphite powder of the present invention having a surface form consisting of a mosaic blockage structure obtained by the step (d) is suitable as a negative electrode material for a lithium ion secondary battery, has excellent discharge capacity and charge / discharge efficiency, and A negative electrode of a lithium ion secondary battery with improved high rate discharge characteristics can be produced.
[0053]
The negative electrode of the lithium ion secondary battery can be manufactured in the same manner as before. In general, graphite powder is formed into an electrode by molding on a current collector as an electrode substrate using an appropriate binder. As the current collector, it is possible to use any metal foil (eg, copper foil such as electrolytic copper foil, rolled copper foil) that has good supportability of graphite powder and does not dissolve when decomposed when used as a negative electrode. it can.
[0054]
Molding can be carried out by an appropriate method that has been used in the past to produce electrodes from powdered active materials, and it fully draws out the negative electrode performance of graphite powder. As long as it is stable and electrochemically stable, there is no limitation. For example, a binder made of a fluororesin powder such as polytetrafluoroethylene or polyvinylidene fluoride and an organic solvent such as isopropyl alcohol are added to graphite powder and kneaded to form a paste, and this paste is screen printed on a current collector. Method: A method of adding resin powders such as polyethylene and polyvinyl alcohol to graphite powder and dry-mixing, and hot-pressing the mixture using a mold, and simultaneously thermocompression bonding to the current collector; Using the above-mentioned fluororesin powder or water-soluble binder such as carboxymethylcellulose as a binder, slurry is formed using a solvent such as N-methylpyrrolidone, dimethylformamide, water or alcohol, and this slurry is applied to a current collector. And a method of drying.
[0055]
The negative electrode produced from the graphite powder of the present invention is combined with a suitable positive electrode active material that can be used in a lithium ion secondary battery and a non-aqueous electrolyte solution in which a lithium compound is dissolved in an organic solvent to produce a lithium ion secondary battery. be able to.
[0056]
Examples of positive electrode active materials include lithium-containing transition metal oxides LiM¹ _1-xM² _xO₂Or LiM¹ _2yM² _yO_Four (In the formula, X is a numerical value in the range of 0 ≦ X ≦ 4, Y is in a range of 0 ≦ Y ≦ 1, M¹, M²Represents a transition metal, consisting of at least one of Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn), transition metal chalcogenides, vanadium oxide (V₂O_Five, V₆O₁₃, V₂O_Four, V_ThreeO₈And its lithium compounds, general formula M_xMo₆S_8-y(Wherein X is a numerical value in the range of 0 ≦ X ≦ 4, Y is a range of 0 ≦ Y ≦ 1, and M represents a metal including a transition metal) Activated carbon fiber or the like can be used.
[0057]
The organic solvent used for the non-aqueous electrolyte is not particularly limited, and examples thereof include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- and 1,2-dimethoxyethane, and 1,2-diethyl. Ethoxyethane, γ-butyrolactam, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethyl borate, silicic acid Examples thereof include one or more of tetramethyl, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene and the like.
[0058]
As the electrolyte lithium compound, an organic or inorganic lithium compound soluble in the organic solvent to be used may be used. Specific examples of suitable lithium compounds include LiClO_Four, LiBF_Four, LiPF₆, LiAsF₆, LiB (C₆H_Five), LiCl, LiBr, LiCF_ThreeSO_Three, LiCH_ThreeSO_Three1 type, or 2 or more types can be mentioned.
[0059]
【Example】
(Example 1)
Bulk mesophase pitch obtained from petroleum pitch was coarsely pulverized and carbonized by heating to 1000 ° C. for 1 hour under an argon atmosphere to obtain a carbon material. The carbon material was pulverized by an attrition mill so that about 90% by volume had a particle size of 1 to 80 μm. Next, the pulverized carbon material was heat treated at 3000 ° C. for 30 minutes in an argon atmosphere for graphitization, and further subjected to oxidation heat treatment at 700 ° C. for 3 hours in an oxygen atmosphere to scrape the graphite powder surface. Thereafter, rapid heat treatment was performed in an Ar atmosphere at a rate of temperature rise of 100 ° C., a holding temperature of 2000 ° C., and a holding time of 6 hours to obtain the graphite powder of the present invention.
[0060]
The graphite powder was sieved to 5 μm or more and 45 μm or less, and then used for production of an electrode. The average particle size of the graphite powder was 12 μm.
FIG. 3 shows an atomic force micrograph of the obtained graphite powder. From this photograph, it is recognized that the surface of the graphite powder has a mosaic surface blocking structure in which periodic blocking regions having a length of more than 0 nm to 20 nm are arranged in a bead shape in the c-axis vertical direction. That is, Lc⊥ of this mosaic-like occlusion structure is more than 0 nm to 20 nm.
[0061]
FIG. 4 shows a cross-sectional TEM photograph of graphite powder cut in the c-axis direction. From this photograph, it can be seen that a multilayer laminated block structure is formed on the powder surface, and the pitch width in the c-axis direction (that is, c‖ of the block region) is 2 to 10 nm.
[0062]
Furthermore, FIG. 5 shows the structure of the surface of the graphite powder observed from directly above. According to the present invention, as shown in FIG. 5, a closed structure is formed on the graphite surface in which a portion a is a closed region and a portion b is a gap opening, and the closed regions are scattered in a mosaic pattern.
[0063]
Using the graphite powder of the present invention having the mosaic blockage structure obtained above, an electrode was produced by the following method.
90 parts by mass of the above-mentioned graphite powder and 10 parts by mass of polyvinylidene fluoride powder were mixed in N-methylpyrrolidone as a solvent and dried to obtain a paste. The obtained paste was applied to a uniform thickness on a 20 μm thick copper foil serving as a current collector using a doctor blade, and then dried at 80 ° C. From here, a circle with a diameter of 15.2 mm (area 1.8 cm)²) Was used as a negative electrode.
[0064]
The high rate discharge characteristics of the negative electrode were evaluated by a three-pole constant current charge / discharge test using metallic lithium as a counter electrode and a reference electrode. The electrolyte is LiPF at a concentration of 1 mol / l of a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 3.₆What was dissolved was used. Five conditions of 0.85, 4.0, 8.0, 12, and 20 mA were used for the charge / discharge current. Converted to current density, 0.47, 2.2, 4.4, 6.6 and 11.1 mA / cm, respectively²become. The press pressure during electrode formation is 750 kgf / cm.²It is. After the first charge is completed at a constant current of 0.85 mA, the same current is discharged. In the second cycle, the charge / discharge capacity is measured using the current density according to the above five conditions, and the discharge capacity ratio relative to the first is calculated. did.
[0065]
For comparison, the bulk mesophase pitch was pulverized by an attrition mill so that about 90% by volume becomes 1 to 80 μm before carbonization, then heated to 700 ° C. in an Ar atmosphere for 1 hour to be carbonized, and then heated as it is. The temperature was raised to 3000 ° C., and heat treatment was performed in the same manner as in the examples in an Ar atmosphere to obtain graphitized conventional graphite powder (that is, only the steps (a) and (b) in the production method of the present invention). The high rate discharge characteristics were similarly examined for the obtained graphite powder.
[0066]
FIG. 6 shows the test results of the discharge characteristics of the graphite powder according to the present invention and the conventional graphite powder at various current densities.
As can be seen from FIG. 6, in the negative electrode using graphite powder subjected to oxidation treatment and rapid annealing treatment after graphitization according to the present invention as an active material, 95% at 0.85 mA even when the current is increased to 20 mA. The discharge capacity was high, and the high rate discharge characteristics were excellent. The discharge capacity at 20 mA was 313 mAh / g, and the charge / discharge efficiency (percentage of discharge electricity with respect to charge electricity) was 92%, both of which were sufficiently high.
[0067]
On the other hand, as shown in FIG. 6, in the negative electrode using the conventional graphite powder, a decrease in discharge capacity was observed at a current of 12 mA, and at 20 mA, the capacity decreased to 80% at 0.85 mA. The discharge capacity at this time was 272 mAh / g, and the charge / discharge efficiency was 80%.
[0068]
The reason why the graphite powder of the present invention is excellent in the high rate discharge characteristic is that the voids are expanded in a mesh shape due to the formation of the mosaic surface blocking structure by rapid heat treatment, and the Li ion penetration / diffusion path can be easily secured. It is reasonable to think.
[0069]
(Example 2)
A graphite powder was produced in the same manner as in Example 1 except that the heating rate and holding temperature of the heat treatment in the Ar atmosphere after the oxidation heat treatment were changed as shown in Table 1.
[0070]
The value of Lc‖ and Lc⊥ in the closed region formed on the surface of the obtained graphite powder and the high rate discharge characteristics of the negative electrode produced from this (the second charge / discharge current is 20 mA (= current density 11.1 mA / cm²Table 1 also shows the results obtained in the same manner as in Example 1 for the ratio of the discharge capacity in the case of) to the discharge capacity in the case of the initial 0.85 mA.
[0071]
[Table 1]

As shown in Table 1, when the heating rate during heat treatment in the Ar atmosphere after the oxidation heat treatment is 3 ° C./second, the obstructed structure of the surface of the obtained graphite powder is not mosaic, Are continuously in the c⊥ direction, resulting in a surface form in which the Lc⊥ value of the closed region is ∞.
[0072]
On the other hand, when the heating rate of the heat treatment was 5 ° C./second or more, a mosaic-like closed structure was formed, and accordingly, high-rate discharge characteristics were improved. From Table 1, the higher the heating rate during heat treatment and the higher the holding temperature, the smaller the Lc⊥ value of the individual occluded regions of the generated mosaic blockage structure tends to be smaller. It can be seen that the high rate discharge characteristics are improved.
[0073]
【The invention's effect】
The present invention provides a graphite powder that is excellent as a high-rate discharge characteristic and that has a sufficiently high discharge capacity and charge / discharge efficiency and that is optimal as a negative electrode for a lithium ion secondary battery. Thereby, high performance of the lithium ion secondary battery can be achieved. Therefore, the present invention is a technique that contributes to the popularization of lithium ion secondary batteries in applications that require high rate charge / discharge.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing a change in the closed structure of a surface of a conventional graphite powder and heat treatment under conditions outside the scope of the present invention.
FIG. 2 is a schematic explanatory view showing different aspects of the formation of a surface blocking structure in the production process of the graphite powder of the present invention.
FIG. 3 is an atomic force micrograph showing the surface morphology of the graphite powder of the present invention.
FIG. 4 is a cross-sectional TEM photograph near the surface of the graphite powder of the present invention.
FIGS. 5A and 5B are schematic views of the surface of the graphite powder of the present invention observed from directly above, in which a represents a closed region portion and b represents a gap opening portion.
FIG. 6 is a graph showing changes in discharge capacity with respect to charge / discharge current density for the graphite powder of the present invention and the conventional graphite powder.

Claims (5)

Closed regions closed by connecting ends of the graphite c-plane layers are scattered on the powder surface, and the length (Lc⊥) perpendicular to the graphite c-axis of each of the closed regions is A graphite powder for a negative electrode material of a lithium ion secondary battery , characterized by being 100 nm or less. The method for producing graphite powder for a lithium ion secondary battery negative electrode material according to claim 1, comprising the following steps.
(a) a step of heat-treating a carbon material obtained by carbonizing a carbonaceous material to graphitize, (b) at least one grinding step performed before carbonization, between carbonization and graphitization, and / or after graphitization, (c) a step of heat-treating the graphite powder obtained after steps (a) and (b) under conditions that allow the surface to be shaved, and (d) an inert treatment of the graphite powder obtained in step (c). A process of heating in a gas at a heating rate of 5 ° C./second or more and maintaining a temperature of 500 ° C. or more. The method of claim 2, wherein the heat treatment in step (c) is an oxidation heat treatment.   A negative electrode for a lithium ion secondary battery, comprising the graphite powder according to claim 1.   A lithium ion secondary battery comprising the negative electrode according to claim 4.

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