JP4802595B2 - Carbon powder suitable for negative electrode materials for non-aqueous secondary batteries - Google Patents

Description

  The present invention relates to an inexpensive carbon powder in which carbon having a low graphitization degree is attached to a part of the surface of a natural graphite powder, and particularly to a carbon powder suitable for a negative electrode material for a non-aqueous secondary battery.

  Lithium ion secondary batteries have rapidly spread as power sources for portable electronic devices and the like. A negative electrode material of a non-aqueous secondary battery represented by a lithium ion secondary battery is carbon powder. Due to the demand for higher capacity of small batteries, negative electrode materials have also been increased in capacity, and at present, artificial graphite powder with an increased graphitization degree is mainly used as the negative electrode material for non-aqueous secondary batteries. Furthermore, in order to raise the capacity | capacitance per volume, the filling property of an electrode is raised, ie, the densification of an electrode is achieved.

  On the other hand, the negative electrode material of the non-aqueous secondary battery is also required to be low in cost, and a large amount of negative electrode material is used in the battery for a large battery for automobiles and the like, and the demand is increasing. Therefore, in place of expensive artificial graphite, attempts have been made to use natural graphite that is inexpensive, has a high degree of graphitization, and has a large true specific gravity.

  However, natural graphite has a very high degree of graphitization, so it has high reactivity with the electrolyte, increases the irreversible capacity associated with the decomposition of the electrolyte, or impairs battery performance such as storage characteristics and safety. There's a problem. In particular, recently, in order to use batteries at low temperatures for automobile applications such as electric vehicles and hybrid vehicles, attempts have been made to use propylene carbonate, which is a low melting point liquid (hereinafter abbreviated as PC, melting point −49 ° C.) as an electrolyte. A graphite powder having a high graphitization degree cannot be used because it decomposes PC.

  Therefore, an attempt to suppress the reactivity with the electrolyte by using a carbon powder with a multi-layer structure in which the surface of a graphite powder with a high degree of graphitization is coated with a carbonaceous material with a low-graphite degree turbulent structure. Has been actively conducted.

  In Patent Document 1 below, a carbon precursor such as pitch is heated to a molten state, kneaded with graphite powder, and then heat-treated at a low temperature so that the surface of the graphite powder has a graphitization degree of a turbulent structure. It is described to obtain carbon powder coated with low carbon (low temperature calcined carbon).

Patent Document 2 listed below discloses a carbon powder that is coated by depositing carbon having a turbulent layer structure and having a low graphitization degree on the surface of the graphite powder by a chemical vapor deposition (CVD) method.
All of these are based on the idea of suppressing the reaction with the electrolytic solution by covering the entire surface with graphite having a low degree of graphitization using graphite powder as a core. Therefore, a large amount of carbon with a low graphitization degree is required.

  Carbon with a low degree of graphitization, such as low-temperature calcined carbon, shows a gentle potential change, and charging / discharging occurs at a higher potential than lithium for lithium. Therefore, compared with batteries using current graphite negative electrodes, The voltage drops. Therefore, under substantial use conditions, the discharge capacity is reduced and the charge / discharge efficiency is also reduced. Further, carbon having a low graphitization degree, such as low-temperature calcined carbon, has a specific gravity smaller than that of graphite and is extremely hard, so that the electrode density does not increase, so that the discharge capacity per volume is smaller than that of graphite. Furthermore, when using the pitch which melts | melts by heating, when there are many addition amounts, a liquid phase will increase at the time of a heating, and powder will aggregate after heat processing. Therefore, there is a problem that a pulverization process is newly required and the cost is increased.

Therefore, attempts have been made to suppress the amount of carbon having a low degree of graphitization such as low-temperature calcined carbon.
Patent Document 3 below discloses a method for producing a carbon material that is carbonized after graphite powder is immersed in a carbon precursor such as pitch in a liquid state and then washed with a solvent to remove excess carbon precursor. Has been. The following Patent Document 4 also describes that carbon precursors such as pitch in a liquid state and graphite powder are mixed, heated with stirring, and then carbonized.

  Both are based on the idea that the graphite powder is the core and the entire surface is covered with carbon having a low graphitization degree to suppress the reaction with the electrolytic solution. In this case, since the pitch of the carbon precursor is brought into a liquid state and brought into contact with the graphite powder, a part of the pitch is used for filling relatively large pores of graphite, and a considerably large amount of pitch is required. . If the added amount of pitch is small, the graphite powder cannot be completely coated, and the graphite surface is exposed at a thin portion of the coating, and the required PC electrolyte solution resistance cannot be obtained.

  Furthermore, as a problem common to the above Patent Documents 1 to 4, when the entire surface of the graphite powder is coated with carbon having a low graphitization degree, the contact resistance between the powders increases, and the rate characteristics deteriorate. There is a problem.

  On the other hand, Patent Document 5 below proposes a method in which pitch powder and graphite powder are simply solid-phase mixed and then heat-treated at 600 ° C. to 800 ° C. This proposal aims to obtain a material having a capacity exceeding the theoretical capacity of 372 mAh / g of graphite, so that a large amount of pitch is added. Since the electrochemical characteristics are tested using an electrolyte containing no PC, the PC electrolyte resistance is unknown. In any case, the low graphitized low-temperature calcined carbon derived from a large amount of pitch contains the problems that the battery voltage decreases and the electrode density does not increase, so that the discharge capacity per volume decreases.

Patent Document 6 proposes a material that defines the pore volume. Specifically, it is a graphite material in which V2 / V1 = 2.2 to 3.0, where V1 is a pore volume having a pore diameter of 4 to 10 nm and V2 is a pore volume having a diameter of 30 to 100 nm. It is described that when V2 / V1 is larger than this range, the charging load characteristic is lowered, and when the V2 / V1 is smaller than this range, the specific surface area becomes too large and the initial efficiency is lowered. The pore volume in this patent document is obtained by measuring the pore distribution on the adsorption side by the BJH method using the nitrogen adsorption method. This graphite material is prepared by subjecting graphite particles to surface oxidation treatment at 500 to 1500 ° C. The electrode density and PC electrolyte resistance are not described in this patent document.
Japanese Patent Laid-Open No. 08-50897 Japanese Patent Laid-Open No. 04-368778 JP 2000-58052 A JP 09-213328 A Japanese Patent Laid-Open No. 2003-1000029 JP 2003-272625 A

  The present invention, when used as a negative electrode material for a non-aqueous secondary battery, can be used in an electrolyte containing PC (propylene carbonate), and the decrease in electrode density and rate characteristics is suppressed compared to graphite powder. It is an object to provide an inexpensive carbon powder.

  According to the present invention, the above-mentioned problems can be solved by carbon powder having a specific pore volume obtained by simply mixing natural graphite powder and pulverized pitch powder, and then heat-treating.

  In a broad sense, the present invention broadly relates to a pore diameter with respect to a pore volume V1 having a pore diameter of 2 to 50 nm in a pore distribution curve obtained by analyzing a nitrogen desorption side isotherm by the BJH method (Barrett-Joyner-Halenda method). A carbon powder based on graphite, wherein V2 / V1, which is a ratio of pore volume V2 of 50 to 200 nm, is 1 or more.

In a preferred embodiment, the present invention is a carbon powder in which carbon produced by carbonization of a carbon precursor by heat treatment at a temperature lower than the graphitization temperature is attached to a part of the surface of the natural graphite powder, V2 / V1, which is the ratio of the pore volume V2 having a pore diameter of 50 to 200 nm to the pore volume V1 having a pore diameter of 2 to 50 nm in the pore distribution curve obtained by analyzing the side isotherm by the BJH method, is 1 or more. It is the carbon powder characterized by being.

  Carbon produced by heat treatment of the carbon precursor was produced by solid-phase mixing pitch powder having an average particle size of 500 μm or less and graphite powder, and then heat-treating the mixture at a temperature of 900 ° C. to 1500 ° C. in a non-oxidizing atmosphere. It is preferable. The value of V2 / V1 is preferably 1.3 or more and 2.0 or less.

  In the carbon powder of the present invention, the entire surface of the natural graphite powder that is the base material is not covered, and only a part of the surface has a carbon precursor such as pitch attached thereto at a temperature lower than the graphitization temperature. It is coated with carbon produced by carbonization by heat treatment (hereinafter, this may be referred to as low-temperature calcined carbon). Since this low-temperature calcined carbon selectively adheres to the edge surface of the powder surface, the minute pores (referred to as micropores) on the edge surface are buried with the carbon component and disappear, resulting in a significantly reduced number. The cause is not well understood, but is probably estimated as follows.

  Natural graphite powder consists of particles obtained by mechanically pulverizing graphite, and its surface includes a basal plane (a plane parallel to the cleavage plane) and an edge plane (a plane perpendicular to the cleavage plane). Edge surfaces with large irregularities are highly active and have high wettability due to melt pitch. Therefore, after the pitch powder is solid-phase mixed with the graphite powder, when the mixture is heat-treated, the pitch that is separated in small portions is melted by the heating during the heat treatment and becomes a liquid phase to contact the surrounding natural graphite powder. In this case, it is considered that the edge surface of the powder surface is selectively wetted by the molten pitch, and the micropores mainly existing on the edge surface are filled with the molten pitch. The pitch adhering to the edge surface is carbonized during the heat treatment to become low-temperature calcined carbon.

  On the other hand, relatively large pores (referred to as macropores) present mainly in the graphite powder remain as they are without being filled with the pitch even if they are simply brought into contact with the molten pitch. Thus, while the micropores are considerably disappeared, most of the macropores remain so that the ratio of the macropores to the micropores is remarkably increased as compared with the graphite powder before the treatment.

  In the present invention, the micropore is defined as a pore having a pore diameter of 2 to 50 nm, and the macropore is defined as a pore having a pore diameter of 50 to 200 nm. By making the ratio of a certain value or more, not only the resistance to PC electrolyte is remarkably improved, but also the electrode density can be increased, the battery discharge capacity is high, and the rate characteristics are also good and inexpensive. Carbon powder can be provided.

  As will be described later, pores having a pore diameter of 2 to 50 nm are academically called mesopores, but in the present invention, in the sense of contrasting with macropores having a pore diameter of 50 to 200 nm, This is called a micropore.

  The decomposition of the electrolytic solution can occur on the entire surface of the graphite powder, but particularly occurs on the edge surface. In the carbon powder of the present invention, the edge surface of the graphite powder surface is coated with low-temperature calcined carbon and selectively modified so that the micropores disappear. It is thought that (PC electrolyte property) is greatly improved. On the other hand, since the large macropores existing inside the powder are easily crushed by pressurization when forming the electrode, the electrode density is easily increased by pressurization, and the discharge capacity of the battery at a constant volume can be increased.

  Note that there are some edge surfaces on the wall surface of the macro hole, and when the pressure during electrode fabrication is weak, the electrolyte may enter the macro hole and decompose the PC electrolyte. In order to increase the electrode density when required, the electrode is produced by increasing the pressure, so that the macropores are crushed during pressurization and the edge surface existing inside the powder does not come into contact with the electrolyte. Therefore, it is considered that the PC electrolyte resistance is remarkably improved even if the macropores are not filled.

  In addition, only the edge surface of the graphite powder surface is selectively coated with low-temperature calcined carbon to be modified, and the basal surface remains exposed without being coated. Therefore, despite the fact that the surface of the graphite powder is coated with low-temperature calcined carbon, which is inferior in conductivity, an increase in the contact resistance between the powders due to the coating is avoided, and the contact resistance remains low as in the case of graphite. Degradation of characteristics can be avoided. Further, since the amount of hard low-temperature calcined carbon is small, a decrease in specific gravity due to low-temperature calcined carbon is also suppressed, and a decrease in electrode density is suppressed.

  On the other hand, when the pitch is preheated and melted and mixed with the graphite powder after being in a liquid phase state, unlike the case of solid phase mixing, the melt pitch does not exist separately but exists continuously, Since a shearing force is applied during mixing, the molten pitch is also consumed for covering the basal surface of the graphite powder surface and filling the macropores, which are carbonized after heat treatment to become low-temperature calcined carbon. Therefore, since only the above-mentioned micropores are not selectively filled and disappear, the pore volume distribution is not significantly different from the graphite powder before treatment, and compared with the above, the macropores with respect to the micropores The proportion of is much smaller. In addition, since the coating thickness is reduced, if the pitch amount is small, the coating may be partially damaged by shrinkage during carbonization. Therefore, compared with the case where the same amount of pitch is solid-phase mixed, the electrode density is less likely to increase, and the PC electrolyte solution is likely to be decomposed. Moreover, since the surface of the graphite powder is uniformly coated with low-temperature calcined carbon that is inferior in conductivity to graphite, the rate characteristics are lowered.

  In the present invention, solid-phase mixing means mixing in a state in which no liquid component is contained, that is, in a state in which any component to be mixed does not become liquid during mixing, and there is no liquid medium for promoting mixing. Means mixing.

  According to the present invention, it is possible to provide, at a low cost, carbon powder that is a negative electrode material for a non-aqueous secondary battery that has excellent PC electrolyte resistance and good rate characteristics and electrode density (and therefore battery discharge capacity). A non-aqueous secondary battery comprising a negative electrode material produced from the carbon powder of the present invention can be mounted in applications that may be used at low temperatures, such as automobiles.

  The carbon powder of the present invention has a pore volume V2 having a pore diameter of 50 to 200 nm with respect to a pore volume V1 having a pore diameter of 2 to 50 nm in a pore distribution curve obtained by analyzing a nitrogen desorption side isotherm by the BJH method. The ratio V2 / V1 is 1 or more.

  A pore having a pore diameter of 2 to 50 nm is a micropore, and a pore having a pore diameter of 50 to 200 nm is a macropore. Therefore, the carbon powder of the present invention means that the pore volume (V2) of the macropores is equal to or larger than the pore volume (V1) of the micropores.

  In the graphite powder obtained by mechanically pulverizing natural graphite powder, the value of V2 / V1 is considerably smaller than 1 because the pore volume V1 of the micropores is larger than the pore volume V2 of the macropores (example) Within the range of 0.65 to 0.85). Therefore, the carbon powder of the present invention has a feature that the ratio of the pore volume of macropores is high.

  The pores having a diameter of 2 to 50 nm are classified as mesopores academically, and the mercury intrusion method is most commonly used as a method for measuring the pore volume larger than the mesopores. However, when this method is applied to soft particles such as natural graphite, there is a problem that the particles are deformed by press-fitting heavy mercury and accurate measurement cannot be performed.

  Therefore, in the present invention, the pore volume is obtained by a method of analyzing the nitrogen desorption side isotherm by the BJH method. This method is also known to be a method suitable for measurement of pore distribution over mesopores. The pore distribution by nitrogen gas desorption-BJH method analysis can be measured simultaneously with the specific surface area by an automatic specific surface area / pore distribution measuring device commercially available from Shimadzu Corporation et al. A range of pore volumes can be determined.

  When the value of V2 / V1 is smaller than 1, as described above in the case where the pitch is previously melted and mixed with the graphite powder, not only the PC electrolyte resistance is inferior, but also the rate characteristics and electrode density (battery discharge) Capacity) also gets worse. The value of V2 / V1 is preferably 1.3 or more and 2.0 or less. When the value of V2 / V1 is 1.3 or more, the effect of improving the electrode density and PC electrolyte resistance is further increased. On the other hand, it is difficult to realize a V2 / V1 ratio larger than 2.0 by an industrially available method.

  The carbon powder of the present invention in which the value of V2 / V1 is 1 or more is based on graphite, and more specifically, natural graphite powder is produced by heat treatment of a carbon precursor on a part of its surface ( This is a carbon powder whose surface has been modified by adhering low-temperature calcined carbon). As described above, the low-temperature calcined carbon selectively adheres to the edge surface of the graphite powder surface and fills micropores mainly present on the edge surface. Therefore, the carbon powder of the present invention has a small proportion of micropores, and the ratio V2 / V1 is 1 or more.

  This carbon powder is obtained by solid-phase mixing natural graphite powder with a carbon precursor powder, preferably a pitch powder having an average particle size of 500 μm or less, and heat-treating the resulting mixture at a temperature of 900 ° C. to 1500 ° C. in a non-oxidizing atmosphere. Can be manufactured. In the following, the present invention will be described taking carbon powder produced by this method as an example.

  The base material of the carbon powder of the present invention is natural graphite powder. Compared to amorphous carbon, graphite has a lower theoretical capacity with respect to lithium, but a higher capacity per volume, and a smaller voltage range for the entry and exit of lithium ions. Indicates a higher discharge capacity than amorphous carbon.

  The graphite includes natural graphite, artificial graphite, and quiche graphite, but in order to increase the discharge capacity in a practical battery, it is advantageous that the degree of graphitization is high. Therefore, in the present invention, an inexpensive natural graphite powder having a high degree of graphitization is used for the substrate. However, if it is a small amount (30% by mass or less, preferably 10% by mass or less of the entire graphite powder), artificial graphite or quiche graphite powder can be used in combination with natural graphite powder.

  Natural graphite powder is produced by grinding natural graphite. The natural graphite powder used in the present invention is preferably spheroidized graphite powder. The spheroidized graphite powder is one in which scaly graphite particles are folded into a shape close to a sphere, and a product produced by pulverization (spheroidization pulverization) is commercially available. When the particle shape of the graphite powder is nearly spherical, electrode orientation is suppressed and cycle characteristics are improved. Further, an appropriate gap is formed in the electrode, the electrolyte solution impregnation property is improved, the electrolyte solution is uniformly wrapped, and the low temperature characteristics and rate characteristics are improved.

The natural graphite powder to be used preferably has a pore volume of 50 to 200 nm in pore diameter (that is, macropore pore volume) of 0.005 cm ³ / g or more. If the pore volume of the macropores is smaller than this, the electrode density after the surface treatment is difficult to increase.

  The average particle size of the natural graphite powder is preferably in the range of 5 to 30 μm. If the average particle size is too small, a large amount of pitch is required for surface modification. Conversely, if the average particle size is too large, irregularities are likely to occur on the electrode surface, causing a battery short circuit. In the present invention, the average particle diameter means the particle diameter D50 when the volume fraction is 50% in the cumulative particle diameter distribution.

The specific surface area of the natural graphite powder is preferably 20 m ² / g or less, more preferably 10 m ² / g or less. If the specific surface area is too large, the amount of pitch required for surface modification will increase. The specific surface area of the carbon powder of the present invention obtained by heat treatment after solid phase mixing of the pitch is greatly reduced as compared with the natural graphite powder of the base material, as will be described later.

  The carbon precursor used for the surface modification of natural graphite powder is preferably a solid that is solid at room temperature and melts by heat treatment. Therefore, various kinds of solid hydrocarbons can be used, but it is preferable to use pitch in terms of both cost and performance.

  There are two types of pitches, petroleum and coal. Pitch is used in powder form because it is solid-phase mixed with natural graphite powder. The average particle diameter of the pitch powder is preferably 500 μm or less, and more preferably 100 μm or less. If the average particle size of the pitch powder is too large, the graphite powder cannot be sufficiently surface-modified by simple solid phase mixing and subsequent heat treatment, and the PC electrolyte resistance is reduced. On the other hand, the small average particle size of the pitch powder is advantageous in that the number of contact points with the graphite powder increases, but depending on the type of pitch, there are some that aggregate when the average particle size is too small. Therefore, what is necessary is just to determine the average particle diameter of use pitch powder suitably in balance with productivity.

  Natural graphite powder and pitch powder are mixed by simple solid phase mixing. The blending amount of the pitch powder is determined so that the value of V2 / V1 becomes 1 or more when only a part of the surface of the graphite powder is coated with the low-temperature calcined carbon derived from the pitch after solid phase mixing and heat treatment. do it. If the blending amount of the pitch powder is too small or too large, it becomes impossible to obtain a carbon powder having a V2 / V1 value of 1 or more.

The blending amount of the pitch powder is 0.5 <W / S1 <2.0, where the specific surface area of the graphite powder is S1 (m ² / g) and the amount of the pitch powder with respect to 100 parts by mass of the graphite powder is W (parts by mass). It is preferable to be within the range. If the blending amount of the pitch powder is less than this range, the surface modification may be insufficient and the PC electrolyte resistance may decrease. On the other hand, when a larger amount of pitch powder than this range is blended, the amount of low-temperature calcined carbon produced increases, the electrode density cannot be increased, and the rate characteristics also deteriorate.

Solid phase mixing of natural graphite powder and pitch may be performed by simple mixing using an appropriate dry mixing apparatus (blender, mixer, etc.).
The mixture obtained by solid phase mixing is heat-treated, and the pitch is carbonized through liquefaction. As described above, the edge surface of the natural graphite powder is preferentially wetted by the liquid phase pitch during which it melts, so that the pitch adheres to the edge surface and fills the micropores existing there. Fills with low-temperature calcined carbon after heat treatment and disappears. As a result, the carbon powder of the present invention in which the pore volume V2 of the macropores is equal to or larger than the pore volume V1 of the micropores is obtained.

  The heat treatment temperature is preferably in the range of 900 to 1500 ° C. When the temperature is lower than this, the charge / discharge efficiency of the low-temperature calcined carbon portion generated by carbonization of the pitch is lowered, and thus the charge / discharge efficiency of the obtained carbon powder is lowered. Further, the conductivity of the carbide is low, and the rate characteristics and cycle characteristics become insufficient. On the other hand, when the heat treatment temperature is higher than 1500 ° C., the crystallization of carbon proceeds, the PC tends to be decomposed, and the PC electrolyte resistance is lowered. The heat treatment is performed in an inert atmosphere to avoid carbon combustion. A nitrogen atmosphere is preferable from the viewpoint of cost. The heat treatment time is usually in the range of several tens of minutes to several hundred hours although it depends on the temperature and the amount of pitch powder.

  During the heat treatment, the melted pitch fills the micropores on the surface of the graphite powder, thereby reducing the specific surface area of the graphite powder. Moreover, the specific surface area is also reduced by eliminating crystal defects on the surface of the graphite powder by heat during the heat treatment. Therefore, the specific surface area of the carbon powder obtained by the heat treatment is remarkably reduced as compared with the specific surface area of the raw natural graphite powder. On the other hand, the average particle diameter hardly changes because the coating amount by the low-temperature calcined carbon derived from the pitch is small.

The carbon powder of the present invention obtained after the heat treatment has a pore distribution in which V2 / V1, which is the ratio of the macropore pore volume V2 to the micropore pore volume V1, is 1 or more. The average particle diameter of the carbon powder is preferably in the range of 5 to 30 μm, like the raw graphite powder. If the average particle size is too small, aggregation tends to occur and coating during electrode production becomes difficult. Conversely, if the average particle size is too large, irregularities are likely to occur on the electrode surface, causing a battery short circuit. The specific surface area of the carbon powder of the present invention is preferably 4.0 m ² / g or less. If the powder has a larger specific surface area than this, a large amount of solvent is required at the time of producing the electrode, making it difficult to produce the electrode and reducing the charge / discharge efficiency.

  In the present invention, since the blending amount of the pitch is small, even if the pitch is in a molten state during the heat treatment, the graphite powder hardly aggregates or is fused. Therefore, there is no need to grind after the heat treatment, which is advantageous in terms of cost. However, in some cases, light crushing may be performed.

  Production of the negative electrode of a non-aqueous secondary battery using the carbon powder of the present invention as a negative electrode material and preparation of the secondary battery may be carried out as conventionally known. Although this point will be briefly described below, this description is only an example, and other methods and configurations are possible.

  An appropriate binder and its solvent are mixed with the carbon powder of the negative electrode material, and an appropriate conductive agent is mixed as necessary to improve conductivity, thereby forming a slurry for coating. If necessary, mixing can be performed using a homogenizer or glass beads. When this slurry is applied to a suitable current collector (rolled copper foil, copper electrodeposited copper foil, etc.) using a doctor blade method or the like, dried, and then consolidated by roll rolling or the like, a negative electrode is produced. . As described above, since the macropores are easily crushed during the consolidation, the electrode density is increased.

  As the binder, a fluorine polymer such as polyvinylidene fluoride and polytetrafluoroethylene, a resin polymer such as carboxymethyl cellulose, and a rubbery polymer such as styrene-butadiene rubber can be used. The binder solvent may be N-methylpyrrolidone, water or the like. The conductive agent is a carbon material, metal (Ni, etc.), and the carbon material at this time includes artificial graphite, natural graphite, carbon black, acetylene black, etc., and not only powder but also fibrous material may be used. .

  The battery includes a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte as its basic structure. Even in the present invention, such a configuration is not particularly limited, and the shape of the battery is not particularly limited, and may be any of a cylindrical shape, a square shape, a coin shape, a sheet shape, and the like.

Next, the function and effect of the present invention will be specifically described with reference to examples. In the examples, parts are parts by weight unless otherwise specified. The average particle diameter is the particle diameter when the volume fraction is 50%.
Example 1
As a substrate, an average particle diameter of 20 μm, a specific surface area of 5.2 m ² / g, and a pore volume having a pore diameter of 50 to 200 nm in a pore distribution curve obtained by analyzing a nitrogen desorption side isotherm by a BJH method. Spherical pulverized natural graphite powder having a ratio R of a peak intensity near 1360 cm −1 to a peak intensity near 1580 cm −1 in an argon ion laser Raman spectrum of 0.110 cm ³ / g was 0.19 was used. .

  100 parts of this natural graphite powder was mixed in a solid phase with 5 parts of a coal-based pitch powder having an average particle size of 35 μm and a softening temperature of 80 ° C. by simple mixing using a V blender. The obtained mixed powder was put into a graphite crucible and heat-treated at 1000 ° C. for 1 hour under a nitrogen stream to obtain a carbon powder.

  The pore distribution and specific surface area of the obtained carbon powder were measured by BJH method analysis of the nitrogen desorption side isotherm using a specific surface area / pore distribution measuring device Micromeritics Asap 2010 manufactured by Shimadzu Corporation. The ratio of the pore solution V2 having a pore diameter of 50 to 200 nm to the pore volume V1 having a pore diameter of 2 to 50 nm was determined. In this example, the ratio V2 / V1 was 1.60, which was a value of 1 or more as defined in the present invention. In this measurement, the sample pretreatment temperature and time were set to 200 ° C. × 3 hours.

  In order to verify the invention described in Patent Document 6, the pore distribution curve obtained on the adsorption side has a pore diameter of 30 to 100 nm with respect to a pore volume V3 having a pore diameter of 4 to 10 nm specified in Patent Document 6. The ratio (V3 / V4) of the pore volume V4 was also obtained at the same time and was 4.84.

(Example 2)
Carbon powder was obtained in the same manner as in Example 1 except that the amount of pitch powder was changed to 9 parts. The obtained carbon powder had a V2 / V1 value of 1.11, and a V4 / V3 value of 1.87.

  A sample of the carbon powder obtained in Example 2 was observed with a TEM (transmission electron microscope). A structure in which carbon with a turbulent layer structure (low-temperature-fired carbon derived from pitch with no developed crystal structure) attached to the edge surface was observed at a part of the powder surface where the edge surface and basal surface could be clearly distinguished. It was.

(Comparative Example 1)
Carbon powder was obtained in the same manner as in Example 1 except that the amount of pitch powder was changed to 13 parts. The obtained carbon powder had a V2 / V1 value of 0.62 and a V4 / V3 value of 1.89. Since the blending amount of the pitch was too large, not only the micropores but also the macropores were filled with the low-temperature calcined carbon. Therefore, the pore volume of the macropores could not be increased, and the value of V2 / V1 was more than 1. It seems that it has become smaller.

(Example 3)
As a substrate, an average particle size of 29 μm, a specific surface area of 3.5 m ² / g, and a pore volume having a pore diameter of 50 to 200 nm in a pore distribution curve obtained by analyzing a nitrogen desorption side isotherm by a BJH method. Spherical pulverized natural graphite powder having a ratio R of a peak intensity near 1360 cm −1 to a peak intensity near 1580 cm −1 in an argon ion laser Raman spectrum of 0.16 cm ³ / g is 0.16. did.

  100 parts of this graphite powder was mixed with 5 parts of a coal-based pitch having an average particle size of 35 μm and a softening temperature of 80 ° C. in a solid phase by simple mixing using a V blender. This mixed powder was put into a graphite crucible and heat-treated at 950 ° C. for 1 hour under a nitrogen stream to obtain a carbon powder. The resulting carbon powder had a V2 / V1 value of 1.40 and a V4 / V3 value of 2.15.

(Comparative Example 2)
In this example, the same natural graphite powder and pitch powder as used in Example 1 were mixed at the same ratio, except that the mixing method was not solid phase mixing, but higher than the melting temperature of the pitch, and a dilution solvent was used. The example which is liquid phase heating mixing is shown.

The average particle size is 20 μm, the specific surface area is 5.2 m ² / g, and the pore volume with a pore diameter of 50 to 200 nm in the pore distribution curve obtained by analyzing the nitrogen desorption side isotherm by the BJH method is 0.0110 cm 3 / g, 100 parts of spheroidized and ground natural graphite powder having a ratio R of the peak intensity near 1360 cm −1 to the peak intensity near 1580 cm −1 in the argon ion laser Raman spectrum is 0.19. The mixture was mixed with 5 parts of a coal-based pitch at 100 ° C. and 100 parts of pyridine as a diluent solvent, and stirred and mixed while heating to 110 ° C. Thereafter, the pressure was reduced to recover the solvent. The mixed powder obtained by this liquid phase mixing was placed in a graphite crucible and heat-treated at 1000 ° C. for 1 hour under a nitrogen stream. The obtained carbon powder had a V2 / V1 value of 0.95 and a V4 / V3 value of 2.01.

(Comparative Example 3)
Carbon powder was obtained in the same manner as in Comparative Example 2 except that the amount of pitch powder was changed to 9 parts. The carbon powder had a V2 / V1 value of 0.81 and a V4 / V3 value of 1.70.

(Comparative Example 4)
Carbon powder was obtained in the same manner as in Comparative Example 2 except that the amount of pitch powder was changed to 13 parts. The carbon powder had a V2 / V1 value of 0.59 and a V4 / V3 value of 1.62.

(Comparative Example 5)
When V2 / V1 values of the spheroidized pulverized natural graphite powder used as the base material in Examples 1-2 and Comparative Examples 1-4 were examined, the values of 0.72 and V4 / V3 were 4.05. .

(Comparative Example 6)
When the V2 / V1 value of the spheroidized pulverized natural graphite powder used as the base material in Example 3 was examined, the values of 0.80 and V4 / V3 were 4.09.

The pore volume data of these carbon powders are shown in Table 1 together with the specific surface area (nitrogen gas adsorption BET method).
[Electrode creation]
After mixing carboxymethylcellulose (CMC) powder as a binder with 97 parts of the carbon powder produced as described above, a liquid in which styrene-butadiene rubber (SBR) was dispersed in water was added and stirred to obtain a slurry. . The compounding ratio was carbon: CMC: SBR = 97: 1: 2 (mass ratio). This slurry was applied onto a 17 μm-thick Cu foil by a doctor blade method, dried, punched out to a diameter of 13 mm, and pressed with a press molding machine to obtain an electrode.

The electrode density was determined by thickness measurement and mass measurement with a micrometer (the thickness excluding the Cu foil portion by measuring the thickness and mass of the Cu foil in advance and subtracting it). For the evaluation, a pressing force required to obtain an electrode density of 1.75 g / cm ³ was used. However, when the pressure did not reach 1.75 g / cm ³ even at 150 MPa, the density of the electrode pressurized at 150 MPa was used.

[Electrode characteristics]
(1) Resistance to PC electrolyte solution Using a polyolefin separator and a Li metal foil as the counter electrode with respect to the above electrode, the electrolyte solution was ethylene carbonate (EC): propylene carbonate (PC): dimethyl carbonate (DMC) = 1: 2. A coin battery was manufactured using a solution of the supporting electrolyte LiPF ₆ dissolved at a concentration of 1 M in a mixed solvent of 1 (volume ratio). The application amount of the test electrode was 10 to 11 mg / cm ² . The electrode characteristics of the manufactured coin battery were evaluated in the following manner.

Doped with a constant current until the potential difference becomes 0 (zero) V with respect to the counter electrode at a current value of 25 mA / g (corresponding to charging), and further maintains a constant voltage until 5 μA / cm ² while maintaining 0 (zero) V. I continued to dope. Next, dedoping (corresponding to discharge) was performed at a constant current of 25 mA / g until the potential difference became 1.5V. Since the dedope capacity corresponds to the discharge capacity when used as the negative electrode, this was used as the discharge capacity. The 100 percent of the value obtained by dividing the discharge capacity by the charge capacity (dope capacity) was defined as the charge / discharge efficiency (%). All tests were performed at 23 ° C.

(2) Rate characteristics A solution obtained by dissolving the supporting electrolyte LiPF ₆ at a concentration of 1M in a mixed solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 1: 3 (volume ratio) in the electrolytic solution, and the coating amount of the test electrode. A coin battery was prepared in the same manner as (1) except that the amount was 5 to 6 mg / cm ² .

First cycle: Doped with a constant current until the potential difference becomes 0 (zero) V with respect to the counter electrode at a current value of 25 mA / g, and at a constant voltage until 5 μA / cm ² while maintaining 0 (zero) V Continued dope. Next, undoping was performed until the potential difference became 1.5 V at a constant current of 25 mA / g.

Second cycle: Doping was performed at a constant current until the potential difference became 5 mV with respect to the counter electrode at 0.05 C, and further, doping was continued at a constant voltage until 10 μA / cm ² while maintaining 5 mV. Next, undoping was performed at a constant current of 0.05 C until the potential difference became 1.5 V.

Third cycle: Doping was carried out at a constant current until the potential difference became 5 mV with respect to the counter electrode at 0.05 C, and further, doping was continued at a constant voltage until 10 μA / cm ² while maintaining 5 mV. Next, dedoping was performed at a constant current of 1.5 C until the potential difference became 1.5 V.

The ratio of the discharge capacity at the third cycle and the second cycle was determined to evaluate the rate characteristics.
The above test results are also shown in Table 1.

As can be seen from Table 1, the untreated natural graphite powders shown in Comparative Examples 5 and 6 have a very low pressing force necessary to obtain an electrode density of 1.75 g / cm ³ , and the electrode density is low. Is excellent. However, the resistance to PC electrolyte is very poor and cannot be used in an electrolyte containing PC. With the natural graphite powder, the value of V2 / V1 was considerably smaller than 1 even when spheroidized and pulverized.

In contrast, a carbon powder having a pore distribution with a V2 / V1 value of 1 or more according to the present invention can obtain an electrode density of 1.75 g / cm ^{3 with} a pressing force of 150 MPa or less. Although the electrode density is still good, the PC electrolyte resistance is remarkably improved, it can be used sufficiently in an electrolyte containing PC, and the rate characteristic (discharge capacity ratio) is also good. Therefore, for example, it can be used as a negative electrode material of a non-aqueous secondary battery that needs to withstand a low temperature like a battery for an automobile.

However, even if the same surface modification treatment is applied to the natural graphite powder, the ratio of V2 / V1 is less than 1 in Comparative Example 1 in which the amount of pitch is too much, and the raw material of Comparative Example 5 which is a raw material is not yet. It became smaller than the V2 / V1 value in the treated graphite powder. This means that many of the macropores have been filled. In this case, since the macropores are not crushed by pressurization, the electrode density is only 1.67 g / cm ³ even at 150 MPa, which is inferior in terms of electrode density, and therefore the discharge capacity of the battery is lowered. In addition, the rate characteristics were slightly deteriorated. Although not shown in the table, when the blending amount of the pitch is too small, as estimated from untreated comparative examples 5 and 6, the value of V2 / V1 is still less than 1, and in this case, the electrode density However, the PC electrolyte resistance is deteriorated.

  On the other hand, as shown in Comparative Examples 2 to 4, even when the same natural graphite powder and pitch powder are used, if the mixing method is not solid phase mixing, but the pitch is in the liquid phase and mixed, one or more V2 / V1 It is difficult to obtain a carbon powder having a ratio. This is because the pitch adheres uniformly and thinly to the surface of the natural graphite powder. As a result, in Comparative Examples 2 and 3 where the amount of pitch is small, the edge portion of the graphite powder surface is not sufficiently surface-modified, so that the PC electrolyte resistance decreases to the same level or lower as that of the untreated electrode. The density was also lower than in the case of the solid phase mixing method with the same pitch blending amount. On the other hand, in Comparative Example 4 where the amount of pitch is large, the PC electrolyte solution resistance was improved, but the electrode density was very low, probably because sufficient low-temperature calcined carbon adhered to the edge portion to modify the surface. .

For reference, when the carbon powder of Comparative Example 3 was observed with a TEM, a structure in which carbon having a disordered layer structure adhered to only the edge surface as seen in the powder of Example 2 could not be observed.
From the above, it can be seen that carbon powder satisfying the criterion that the value of V2 / V1 is 1 or more is a negative electrode material having high electrode density, excellent PC electrolyte resistance, and good rate characteristics.

  On the other hand, the value of V4 / V3 varies greatly, and although the tendency that the electrode density increases as the ratio increases, the correlation with the PC electrolyte resistance is not clear, In addition, the correspondence with the ratio of V2 / V1 is not particularly found.

  As shown in Comparative Examples 5 and 6, untreated graphite powder has a large value of V4 / V3. However, even when surface modification is performed according to the present invention, the value of V4 / V3 increases as in Example 1. And may decrease as in the second and third embodiments. In Examples 1 to 3 in which the value of V4 / V3 was large or small, the PC electrolyte resistance was improved regardless of the value of V4 / V3.

Claims (3)

A carbon powder in which carbon produced by carbonization of a carbon precursor subjected to heat treatment at a temperature lower than the graphitization temperature is attached to a part of the surface of the natural graphite powder. The nitrogen desorption side isotherm is analyzed by the BJH method. V2 / V1, which is the ratio of the pore volume V2 having a pore diameter of 50 to 200 nm to the pore volume V1 having a pore diameter of 2 to 50 nm in the pore distribution curve determined in this manner, is 1 or more. Powder. The carbon carbon precursor has generated undergoing heat treatment, after a mean particle diameter 500μm or less of the pitch powder was mixed graphite powder and solid phase, heat treating the mixture at a temperature of a non-oxidizing atmosphere at 900 ° C. to 1500 ° C. The carbon powder according to claim 1 , wherein the carbon powder is produced from pitch powder. The carbon powder according to claim 1 or 2 , wherein V2 / V1 is 1.3 or more and 2.0 or less.