JP5797783B2 - Porous carbon products and their use - Google Patents

Description

The present invention relates to a method for producing a porous carbon product, the method comprising:
(A) providing a template comprising an inorganic template material containing interconnected pores;
(B) providing a precursor of carbon;
(C) infiltrating the pores of the template with the precursor,
(D) a step of carbonizing the precursor;
(E) removing the template to form a porous carbon product.

  Furthermore, the invention relates to a suitable use of the carbon product.

  Carbon monolithic compacts are used, for example, in electrodes for fuel cells, supercapacitors and electric accumulators (secondary batteries) and as chromatographs as adsorbents for liquids and gases, as storage media for gases. It is used as a carrier material in graphic applications or catalyst processing and as a material in mechanical engineering or medical engineering.

Prior art The use for rechargeable lithium battery electrodes requires electrode materials that can be reversibly inserted and that can desorb lithium with low charge loss. At the same time, it aims at the shortest possible charging time and high charging capacity of the battery. For this, a maximum porosity (permeability) is desired with the surface of the electrode material as small as possible at the same time. An electrode material having a large surface exhibits a relatively high charge loss that manifests itself as a reversible loss upon the first insertion of lithium.

DE 29 46 688 A1 discloses a method for producing porous carbon by using a temporary preform (so-called template) of a porous material. The carbon precursor is now deposited in the template pores of the inorganic template material having a surface of at least 1 m ² / g. SiO ₂ gel, porous glass, alumina or other porous refractory oxides are listed as suitable template materials for the template. The template material has a porosity of at least 40% and an average pore size in the range of 3 nm to 2 μm.

  Polymerizable organic materials such as mixtures of phenol and hexaamine or phenol-formaldehyde resols are recommended as precursors for carbon. This mixture is introduced as liquid or gas into the pores of the template and polymerized. After polymerization and subsequent carbonization, the template inorganic template material is removed, for example, by dissolution in NaOH or in hydrofluoric acid.

  This results in a particulate or flaky carbon product that has a pore structure that roughly corresponds to the material distribution of the template and that is suitable as a starting material for the manufacture of electrodes primarily for Li batteries.

  Easy access to the inner surface is important for high and rapid charge capacity. In the present description, the so-called “hierarchical porosity” has proven advantageous. Large surfaces can be provided by pores in the nanometer range. In order to increase the accessibility to these pores, they are ideally connected by a continuous macroporous transport system.

Monolithic carbon products having such a hierarchical pore structure with macropores and mesopores are described in US 2005/0169829 A1. In order to produce a hierarchical pore structure, the SiO ₂ template is a fully polymerized porous dispersion of a dispersion of silica beads having a diameter of 800 nm to 10 μm and a polymerizable material, the polymerization being dried after removal of excess liquid. It is manufactured by heating in a mold so as to yield a quality silica gel.

The pores of the SiO ₂ template obtained in this way are subsequently impregnated with a carbon precursor, the carbon precursor is carbonized to carbon, and the SiO ₂ template is subsequently removed by dissolution in HF or NaOH. The resulting carbon product also exhibits a pore structure that approximately matches the material distribution of the template. A phenolic resin dissolved in tetrahydrofuran (THF) is used as a precursor.

TECHNICAL PROBLEMS Carbon precursor materials graphitized for normal infiltration are not soluble at high concentrations and contain insoluble composition amounts. For example, the solubility of the interlayer pitch in THF is less than 10% by volume, and after evaporation of the solvent, more than 90% of the initially filled pore volume remains unfilled. The remaining coating volume of the carbon precursor material is further but slightly reduced by subsequent carbonization.

  Conversely, alternative carbon precursors in the form of carbohydrates, such as sugars, are highly soluble, but sugars remaining after solvent evaporation lose about 50% of their initial mass in the carbonization process. As a result, a large pore volume remains unfilled here.

  Thus, subsequent infiltration with carbonization generally results in only a thin layer of deposited carbon. In order to obtain a technically useful wall thickness of a porous carbon structure, a plurality of such infiltration and carbonization processes must therefore generally be performed one after the other. Such multiple processes, however, increase manufacturing costs and they can occur non-uniformly due to, for example, gradual clogging of the osmotic channels.

  The object of the present invention is to provide a method that allows the economical production of porous carbon structures that also have a thick wall thickness.

  Furthermore, the object of the present invention is to show a suitable use of the carbon product according to the invention.

General description of the invention With regard to the method, this task starting from a method of the type is to provide precursor particles and template particles of fusible material and to produce a powder mixture from the particles, It is achieved by the method of the present invention that is heated prior to or during carbonization by method step (d) to allow the precursor material melt to penetrate into the pores of the template particles.

  In the process according to the invention, the carbon precursor is heated in contact with the template and is softened or melted in this process so that it can penetrate into the pores of the template. Solvents for the carbon precursor can be omitted.

  However, even in the case of good wettability of the template material, this “direct penetration” of the template with the liquefied precursor does not produce the desired result when the template is present as a monolith. Without special measures, it obtains an extremely small penetration depth and irregular occupancy in the pores for the molten precursor. In order to solve this problem, a powder previously produced from both a porous template material and a precursor is provided by the present invention, the powder is intimately mixed with each other, and the homogeneous powder mixture is converted into a precursor. Heat to the extent that the particles of material melt.

  This melt can penetrate directly into adjacent template particles. A homogeneous powder mixture ensures that the molten precursor is always in intimate contact with the template particles, so that a uniform distribution and occupancy is obtained over the entire pore volume of the template material to be permeated. The high temperatures during melting of common precursors result in good wettability of the template surface, resulting in a high degree of pore volume filling even in the case of a single penetration.

  The carbonization of the precursor is carried out simultaneously or following the penetration of the pores of the template particles. Since the use of solvent is omitted, the shrinkage of the precursor is simply due to decomposition and evaporation processes during carbonization. The degree of shrinkage depends in this respect on the carbon content of the precursor.

  The inorganic template material provides only as a mechanically and thermally stable framework for depositing and calcining carbon precursors. For example, after removal by chemical dissolution, the resulting carbon product is substantially free of template material.

  Finer ground template particles are faster, more efficient, and more uniform and penetrate under other identical process conditions. Template particles are produced, for example, by polishing a porous body from a template material or by crushing a layer from the template material, by pressing a powder from the template material, or by a sol-gel method or a granule method. A small, typically monodisperse particle size distribution, obtained for example by sieving, is advantageous for the process according to the invention.

  The precursor powder can also be obtained by grinding or crushing the melt, or by spraying.

  After the two powders are intimately mixed with each other, the powder mixture is heated to such an extent that the precursor is melted and penetrates very wet into the pores of the template powder. Here, the precursors may be carbonized simultaneously or later.

  After carbonization, a mass is obtained in which the carbonized precursor and template material are essentially mixed together. The template material is removed from the mass by etching, resulting in a carbon skeleton remaining from the carbonized precursor.

  Deposition of soot that converts the feedstock into template material particles by hydrolysis or pyrolysis, deposits the particles from the template material in the form of soot bodies on the deposition surface, and breaks the rods into template particles It has been found to be particularly advantageous when the process involves template particles.

  In this variant of the method according to the invention, the formation of the template comprises a soot deposition process. In this process, the liquid or gaseous starting material undergoes a chemical reaction (hydrolysis or thermal decomposition) and is deposited as a solid component from the gas phase onto the deposition surface. The reaction zone is, for example, a burner flame or an electric arc (plasma). Synthetic quartz glass, tin oxide titanium nitrate and other synthetic materials are produced on an industrial scale by such plasma or CVD deposition methods known under the name of OVD, VAD, MCVD, PCVD or FCVD methods.

  It is essential for the conditions of the deposited template material for the production of the template that the template material is present as a porous soot on the deposition surface, which can be, for example, a container, mandrel, plate or filter. This keeps the temperature of the deposition surface low so as to prevent dense sintering of the deposited template material. A “rod”, which is thermally solidified but porous, is obtained as an intermediate product.

  Compared to the “sol-gel” manufacturing method, the soot deposition process is an inexpensive method that allows the economical production of templates on an industrial scale.

  It has been found that it is particularly advantageous for the enclosure obtained in this way to exhibit an anisotropic mass distribution with a hierarchical pore structure according to the manufacturing process. The reason is that gas phase deposition results in primary particles of template material having a particle size in the nanometer range that agglomerates in this way on the deposition surface in the reaction zone, and some spherical agglomerates or aggregates on the deposition surface Are present (these are also referred to below as "secondary particles"). There are particularly small cavities and pores in the nanometer range, i.e. mesopores, in the primary particles and in the secondary particles, i.e. between the primary particles, whereas large cavities or pores are between individual secondary particles. It is formed.

  Template particles obtained from them by crushing or polishing also exhibit a hierarchical structure with a predetermined oligomeric pore size distribution in the template material.

  In the soot deposition process, the template material may be manufactured in the form of soot powder that is further processed in a granule, pressure, slurry or sintering process into template particles. Granules or flakes are mentioned as intermediate products.

  The layer of template material produced by soot deposition can be crushed with little effort, resulting in template particles having a crushed or flaky morphology.

  Such template particles characterized by a non-spherical morphology are particularly advantageous for use in the method according to the invention.

  The reason for this is that particles having a spherical morphology, ie particles having a ball-like or nearly ball-like morphology, have a small surface with respect to their volume. In contrast, particles having a non-spherical morphology exhibit a large surface to volume ratio that makes the penetration of the precursor simple and uniform.

  Template particles which are in the form of crumbs or rods having a structural ratio of at least 5, preferably at least 10, have been found to be particularly advantageous in this respect.

  “Structural ratio” is here understood as the ratio of the maximum structural width of a particle to its thickness. A structure ratio of at least 5 means that the maximum structure width of the particle is at least 5 times greater than its thickness. Such particles have a substantially flaky or rod-like shape and are characterized by two substantially parallel extended large surfaces, the penetration of the molten precursor being in the volume to be filled Since the density is relatively small, it can be carried out relatively quickly.

  As the template particle thickness decreases, the penetration with the molten precursor becomes simpler and more homogeneous. In this respect, it has been found advantageous if the template particles have an average thickness in the range of 10 to 500 μm, preferably in the range of 20 to 100 μm, in particular less than 50 μm.

  Template particles having a thickness of less than 10 μm have weak mechanical strength and exacerbate the formation of a distinct hierarchical pore structure. At thicknesses greater than 500 μm, it becomes more difficult to ensure homogeneous penetration with the molten precursor.

  Homogeneous mixing of the particles from the template material and the precursor material is easy when the precursor particles are spherical and have an average particle size of less than 50 μm, preferably less than 20 μm.

  Due to the sphere formation of the particles, the mixing with non-spherical particles from the template material is improved. This is also supported when the particles from the precursor are slightly smaller than the particles of the precursor. However, particle sizes less than 1 μm tend to be dusty and are not preferred.

  The degree of hole filling is set by the mixing ratio of the precursor and template material. Advantageously, the precursor particles and the template particles are mixed in a volume ratio in the range of 0.05 to 1.6, preferably in a volume ratio in the range of 0.1 to 0.8.

  With a mixing ratio of 0.05, the inner surface of the template material is only covered with a thinner thickness, and only sponge-like carbon is obtained. Therefore, a smaller mixing ratio is not preferable. Conversely, with a mixing ratio of 1.6, depending on the original pore volume of the template material, a substantially filled pore structure is obtained.

Advantageously, the template material is SiO _2.

Synthetic SiO ₂ can be produced on an industrial scale by a soot deposition process using relatively inexpensive starting materials and inexpensive starting materials. The SiO ₂ template withstands high temperatures during calcination. The upper temperature limit is predetermined by the initiation of the reaction of SiO ₂ and carbon with SiC (at about 1000 ° C.). Removal of the template material in the form of synthetic SiO ₂ by process step (e) is carried out by chemical dissolution.

  Pitch is advantageously suitable as a carbon precursor.

  Pitch, especially “medium phase pitch”, is a carbonaceous material with a regular liquid crystal structure. After carbonization, the pitch melt that has penetrated into the pores of the carbon structure leads to a graphite-like deposition of carbon that forms the shell of the core / shell composite, thereby avoiding clogging of cavities between the stacks of layers. Close the carbon structure micropores.

  Instead, carbohydrates are used as carbon precursors.

  Carbohydrates, particularly sugars, especially saccharose, fructose or glucose, are carbon precursors that are not graphite.

  Advantageously, the carbon product is divided into fine particles of porous particles.

  In the process according to the invention, the carbon product is usually obtained as a monolith or in the form of small pieces or flakes and is easily divided into smaller particles. The particles obtained after dispensing have a hierarchical pore structure depending on the template structure and are further processed into shaped bodies or layers, for example by standard paste or slurry methods.

  With regard to the use of carbon products, the above object is achieved by the present invention, and the porous carbon product according to the present invention is used to produce electrodes for rechargeable lithium batteries.

  Rechargeable lithium battery electrodes include both electrodes composed of carbon layers from one material and composite electrodes composed of multiple materials.

Preferred Embodiments The present invention will be described in more detail with reference to embodiments and drawings.

Diagram depicting a device for producing a SiO ₂ soot body. The figure which shows the SEM figure of the 1st embodiment of the porous carbon product obtained by the method of this invention which has hierarchical pore structure. The figure which shows the SEM figure of the 2nd embodiment of the porous carbon product obtained by the method of this invention which has hierarchical pore structure. The figure which shows the diagram of the thermogravimetric analysis during the heating of the template which penetrated pitch in oxygen-containing atmosphere.

The device shown in FIG. 1 is provided for producing a SiO ₂ enclosure. A number of flame hydrolysis burners 2 arranged in a row are arranged along an aluminum oxide carrier tube 1. The flame hydrolysis burner 2 reciprocates parallel to the axis 4 of the carrier tube 1 between two turning points which do not move with respect to the axis 4 and is movable in a direction perpendicular to them, as depicted by the arrow directions 5 and 6 Attach to the joint burner block 3. The burner 2 is made of quartz glass. Their mutual distance is 15 cm.

The flame hydrolysis burner 2 assigns the burner flame 7 a main growth direction perpendicular to the axis 4 of the carrier tube 1. Using a flame hydrolysis burner 2, SiO ₂ particles are deposited on the cylinder jacket surface of the carrier tube 1 rotating about an axis 4, and the porous SiO ₂ blank 8 is formed by a layer having an outer diameter of 400 mm. Form a layer. Individual SiO ₂ soot layers have an average thickness of about 50 μm.

The flame hydrolysis burner 2 is supplied with oxygen and hydrogen as the burner gas and SiCl ₄ as the feedstock for the formation of SiO ₂ particles, respectively. Here, the burner block 3 is reciprocated at a width of two burner distances (ie, 30 cm). An average temperature of about 1200 ° C. is set for the blank surface 9 during the deposition process.

After completion of the deposition process, a porous SiO ₂ soot tube (soot tube) is obtained with a length of 3 m, an outer diameter of 400 mm and an inner diameter of 50 mm. The temperature during the formation of the enclosure is kept relatively low, so that the SiO ₂ soot material has a low average specific gravity of 22% (for a density of quartz glass of 2.21 g / cm ³ ).

Preliminary test (1) In the first test, the interlayer pitch was heated to 300 ° C. in a nitrogen vessel, resulting in various pitch baths. A monolith sample of SiO ₂ enclosure was impregnated in the pitch bath and removed after another 30 minutes. It was found that the molten pitch penetrated only into the housing over a thickness of less than 1 mm.
(2) The pitch bath temperature was raised to 400 ° C. The interlayer pitch is still viscous at this temperature. No significant increase in the degree of penetration 5 in the rod was achieved. At a temperature of about 500 ° C., the pitch begins to coke and evaporates significantly.

First Example A housing sample was polished. Because the enclosure is stacked layer by layer, other overlying layers show a tendency to wear in the presence of high mechanical forces, resulting in non-spherical particles having a thickness in the range of 20 μm to 50 μm Small particles or flaky particles were obtained. Particle size groups having an outer length of 500 μm to 1000 μm were separated by sieving for other processing purposes. The ratio of maximum structure width (average value) and average thickness is about 20.

  Pitch powder consisting essentially of spherical particles having a particle size of 5 μm to 20 μm was produced by polishing the intermediate layer pitch and by sieving.

The pitch powder and the enclosure particles were mixed homogeneously in a volume ratio of 1.6: 1 and the particle mixture was heated to a temperature of 300 ° C. The viscous pitch covers the small SiO ₂ enclosure particles and penetrates into the pores. The ratio of the pitch and the volume of the enclosure particles is chosen so that the pitch fills the pores, so that no significant free pore volume remains and is almost completely consumed here.

After a 30 minute infiltration time, the temperature is increased to 200 ° C., thereby carbonizing the pitch. A porous composite mass is formed from non-spherical porous SiO ₂ particles filled with a layer of graphitizable carbon on the outside and inside (ie the inside wall of the pores).

Subsequently, the SiO ₂ rod particles are introduced into the fluoric acid bath as a lump of composite material. After etching the SiO ₂ particles, a porous carbon pre-product is obtained with a structure that substantially represents a negative copy of the original SiO ₂ enclosure particles. Inverse templates, in which a number of relatively large pore channels (macropores) are enlarged by other finely split surface structures, are distinguished by a hierarchical pore structure.

The reverse template is removed, dried and crushed, thereby breaking it into carbon flakes. The SEM diagram according to FIG. 2 shows a carbon structure with a number of coherent holes and cavities of various sizes. Large sized cavities are enlarged by surfaces that are finely split by channels. Measurement of a specific internal surface area according to the BET method results in a measurement of about 25 m ² / g.

Second Example SiO ₂ enclosure particles and interlayer pitch particles were prepared as described for Example 1. The pitch powder and the enclosure particles were mixed intimately with each other at a volume ratio of 0.4: 1 and the particle mixture was heated to a temperature of 300 ° C. The viscous pitch covers the small SiO ₂ enclosure particles and penetrates into the pores. The pitch and the proportion of the enclosure particles are selected so that the pitch completely fills the pores.

After infiltration and carbonization as described with respect to Example 1, the non-spherical porous SiO ₂ enclosure particles are filled with a layer of carbon that can be black carbonized on the outside and partly on the inside. A mass of composite material was obtained. Then, the SiO ₂ enclosure particles are removed by etching in hydrofluoric acid, and a thin wall derived from the original enclosure particles and enlarged by a surface structure in which a number of relatively large pore channels are split apart by other fine structures. Resulting in a porous carbon pre-product with a structure formed as a fine network.

Carbon products readily break down into carbon flakes. The SEM image according to FIG. 3 shows their structure. A large number of coherent holes and various sized cavities are enlarged by the channel due to the finely torn surface structure. A specific internal surface area measurement according to the BET method results in a measurement of about 50 m ² / g.

  FIG. 4 shows the results of a thermogravimetric analysis (according to DIN 51005 and DIN 51006) during the processing of a sample of the mass of rod particles impregnated with pitch according to Example 1 before carbonization. The sample is heated in pure argon and the mass loss is measured here. The mass loss ΔG (%) relative to the initial mass is plotted on the y-axis and the processing temperature T (° C.) is plotted on the x-axis.

  Then, starting from a temperature of about 300 ° C., the initial mass reduction due to combustion of the activated carbon center and subsequent carbonization is observed. Up to a temperature of about 600 ° C., the mass loss is 4.4% and ends with a saturation corresponding to the mass of the pure carbon layer.

  The carbon flakes obtained by the method of the present invention are composed of porous carbon having a hierarchical structure. The flakes are particularly well suited for composite electrodes, especially for producing electrode layers for rechargeable lithium batteries.

Claims (12)

Less than,
(A) providing a template of an inorganic template material comprising interconnected pores;
(B) providing a precursor for carbon;
(C) infiltrating the pores of the template with the precursor,
(D) a step of carbonizing the precursor;
(E) A method for producing a porous carbon product comprising the step of removing a template to form a porous carbon product, the method comprising providing fusible material precursor particles and template particles, A mixture is formed from the particles and the powder mixture is heated before or during carbonization according to step (d) to infiltrate the precursor melt into the pores of the template particles, providing said template particles Converting the feedstock into template material particles by hydrolysis or pyrolysis, depositing the particles on a deposition surface to form a housing from the template material, and crushing the housing into template particles. A process for producing a porous carbon product, characterized in that it comprises . The method of claim 1, wherein the template particles have a non-spherical morphology. 3. A method according to claim 2 , characterized in that the template particles are in the form of small pieces or rods having a structural ratio of at least 5 . The method according to claim 3, wherein the template particles have a structure ratio of at least 10. The template particles, and having an average thickness of the range of 10 m to 500 m, method according to any one of claims 1 to 4. The precursor particles are formed in a spherical shape, and characterized as having an average particle size of 50μm less than The method according to any one of claims 1 to 5. The precursor particles and the template particles, characterized by being engaged mixed at a volume ratio in the range of 0.05 to 1.6 A method according to any one of claims 1 to 6. 8. The method of claim 7, wherein the precursor particles and template particles are mixed at a volume ratio in the range of 0.1 to 0.8. Wherein said template material is SiO _2, the method according to any one of claims 1 to 8. 10. A method according to any one of claims 1 to 9 , characterized in that pitch is used as carbon precursor.   10. A process according to any one of claims 1 to 9, characterized in that carbohydrates are used as carbon precursors. The carbon product, characterized in that divided into fine carbon in porous particles, the method according to any one of claims 1 to 11.

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