KR101924804B1 - Porous carbon product and use thereof - Google Patents

Abstract

A known method of preparing a porous carbon product comprises the steps of providing a template of an inorganic template material comprising interconnected pores, providing a precursor material for carbon, infiltrating the pores of the template with the precursor material, Carbonizing the precursor material, and removing the template to form a porous carbon product. Starting from this, in order to provide a method capable of cost-effective production of a porous carbon structure with a thick wall thickness, it is proposed in accordance with the present invention to provide precursor material particles and mold particles of a fusible material, It is proposed to heat the powder mixture before or during carbonization according to step (d) in such a way that the melt of the precursor material penetrates into the pores of the mold particles.

Description

POROUS CARBON PRODUCT AND USE THEREOF FIELD OF THE INVENTION [0001]

(A) providing a mold comprising an inorganic mold material comprising interconnected pores; (b) providing a precursor material for carbon; (c) infiltrating the pores of the template with the precursor material; (d) carbonizing the precursor material; (e) removing the template to form a porous carbon product.

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

Monolithic shaped bodies of carbon are used for electrodes for example fuel cells, supercapacitors and electric capacitors (secondary cells), and as adsorbents for liquids and gases, as storage media for gases, As graphite applications or as carrier materials in catalytic processes and as materials in mechanical or medical engineering.

The use of electrodes of a rechargeable lithium cell requires an electrode material that can reversibly intercalate and deintercalate lithium at low charge losses. At the same time, it aims at a possible short charging time and a high charging capacity of the battery. To this end, maximum porosity (permeability) is required simultaneously with the surface of the electrode material as small as possible. The electrode material with a large surface exhibits a relatively high charge loss, which appears to be substantially irreversible loss itself during the initial insertion of lithium.

DE 29 46 688 A1 discloses a process for the production of porous carbon using disposable preforms of porous material (so-called molds). Here, the precursor material of carbon is immersed in the pores of the mold of the inorganic mold material having a surface area of at least 1 m ² / g. SiO ₂ gel, porous glass, alumina or other refractory oxides of porous are mentioned as a suitable molding material for the mold. The mold material has a porosity of at least 40% and a mean pore size of 3 nm to 2 [mu] m.

Mixtures of polymerizable organic materials such as phenol and hexamine or phenol-formaldehyde resols are recommended as precursor materials for carbon. This mixture is introduced into the pores of the mold as a liquid or as a gas and is polymerized. After polymerization and subsequent carbonization, the template's inorganic template material is removed, for example, by dissolution in NaOH or in hydrofluoric acid.

This produces a particle- or flake-like carbon product suitable as a starting material for the preparation of electrodes for Li batteries, in principle, having a pore structure corresponding roughly to the material distribution of the mold.

The high and fast charging capacity is determined by the easy access to the inner surfaces. In this connection, so-called "hierarchical porosity" has been found to be advantageous. Large surfaces can be provided by pores in the nanometer range. In order to improve access to these pores, they are ideally connected via a continuous macroporous transport system.

Monolithic carbon products having such a hierarchical pore structure of macropores and mesopores are described in US 2005/0169829 Al. Made of silica beads having a diameter of from 800 nm to 10 μm and a polymerizable material so as to obtain a porous silica gel which is dried and completely polymerized after removal of excess liquid by polymerization to make a hierarchical pore structure The dispersion is heated in a mold to produce a SiO ₂ template.

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

DE 29 46 688 A1 US 2005/0169829 A1

Typically graphitizable carbon precursors for penetration are not soluble at high concentrations and contain a certain amount of insoluble constituents. For example, the solubility of the mesophase pitch in THF is less than 10% by volume such that after evaporation of the solvent, more than 90% of the initially filled pore volume remains in the unfilled state. The volume of the residual coating of the carbon precursor material is not significant, but is further reduced by subsequent carbonization.

Conversely, alternative carbon precursors, such as sugars, in the form of carbohydrates, such as sugars, exhibit high solubility, but the remaining sugar after evaporation of the solvent is also present in the initial mass of the carbonization process About 50% is lost.

Thus, subsequent penetration of the carbonization furnace generally results in a deposited carbon layer of only a small thickness. Therefore, in order to achieve the technically useful wall thickness of the porous carbon structure, many such penetration and carbonization processes have to be carried out alternately. However, these multiple processes increase manufacturing costs, which can lead to nonuniformity, for example, due to progressive clogging of the permeation channel.

It is an object of the present invention to provide a method which also permits a cost-effective production of a porous carbon structure with also a thick wall thickness.

Furthermore, it is an object of the present invention to indicate a suitable use of the carbon product in accordance with the present invention.

With respect to the method, in accordance with the invention, this object, starting from a method of the type described above, comprises providing precursor material particles and mold particles of a fusible material, forming a powder mixture from the particles, Is achieved by heating the powder mixture before or during carbonization according to step (d) in such a way that the melt of the precursor material penetrates into the pores of the powder.

In the process according to the invention, the precursor material of the carbon is heated in contact with the mold and softened or melted in this process to penetrate into the pores of the mold. The solvent for the carbon precursor may be omitted.

However, even in the case of good wettability of the mold material, this "direct infiltration " of the mold with the liquefied precursor material will produce the desired success if the mold is present as a monolith. Without certain precautions, an irregular occupation within the pores as well as an excessively small penetration depth for the molten precursor material will be obtained. To solve this problem, pre-produced powders from both the porous mold material and the precursor material are provided according to the present invention, the powders are uniformly mixed with one another, and the uniform powder mixture is such that the particles of the precursor material melt And heated.

This melt can penetrate directly into adjacent mold particles. The homogeneous powder mixture always maintains intimate contact of the melt precursor material with the mold particles so that a uniform distribution and occupancy is achieved over the entire pore volume of the infiltrating mold material. The high temperature prevailing during the melting of the precursor material contributes to better wettability of the surfaces of the temperature, so that even in the case of a single penetration, a high degree of filling of the pore volume is obtained in advance.

The carbonization of the precursor material occurs simultaneously with or after the infiltration of the pores of the mold particles. As the use of the solvent is dispersed, the shrinkage of the precursor material is solely due to the decomposition and evaporation process during carbonization. From this point of view, the degree of shrinkage depends only on the carbon content of the precursor material.

The inorganic mold material acts merely as a mechanically and thermally stable framework to deposit and carcine the carbon precursor material. For example, after removal by chemical dissolution, the resulting carbon product is substantially free of mold material.

If the mold particles are more finely divided, they will penetrate more quickly, more efficiently and more uniformly under otherwise identical process conditions. The mold particles are prepared, for example, by grinding porous bodies from a mold material, or by pulverizing the layers from a mold material, by pressing the powder from the mold material, or by a sol-gel process or a granulation process. A small and ideally monodisperse particle size distribution, for example achieved by sieving, is advantageous in the process according to the invention.

The powder of precursor material can also be obtained by grinding, milling, or atomizing the melt.

After the two powders are uniformly mixed with one another, the powder mixture is heated to such an extent that the precursor material melts and penetrates into the pores of the mold powder in a strongly wetting manner. Here, the precursor material may be carbonized simultaneously or subsequently.

After carbonization, the carbonized precursor material and the mold material are lumps that are intimately mixed with each other. The mold material will be removed from the agglomerate by etching so that the carbon skeleton from the carbonized precursor material will remain.

The provision of the mold particles is advantageous in that the feedstock material is converted into the mold material particles by hydrolysis or freeze decomposition and the particles deposit on the deposit surface while forming a soot body from the mold material And it is particularly advantageous to include a soot deposition method in which the soot body is segmented into the template particles.

In a modified process according to the invention, the formation of the mold comprises a soot deposition process. In this process, the liquid or gaseous starting material is subjected to a chemical reaction (hydrolysis or freeze decomposition) and deposited from the gaseous phase as a solid component on the immersion surface. The reaction zone is, for example, a burner flame or electric arc (plasma). Synthetic quartz glass, tin oxide titanium nitride and other synthetic materials are produced on an industrial scale, with the help of plasma or CVD deposition methods such as those named OVD, VAD, MCVD, PCVD or FCVD methods.

Here, in the quality of the immersed molding material for the production of the mold, it is essential that the molding material is present on the immersion surface, which may be, for example, a vessel, a mandrel, a plate or a filter as porous soot. As a result, the temperature of the deposition surface is kept low enough to prevent dense sintering of the deposited casting material. The soaked body, which is thermally enhanced thereby but porous, is obtained as an intermediate product.

Compared with the manufacturing method through the "sol-gel path ", the soot deposition method is an inexpensive method which allows the production of molds on an industrial scale in a cost-effective manner.

In the soot bodies obtained in this manner, it has been found that they particularly advantageously exhibit anisotropic mass distributions with a hierarchical pore structure due to the manufacturing process. The reason is that gaseous deposition creates primary particles of the mold material in the reaction zone with a particle size in the nanometer range that agglomerates in their way to the deposition surface and forms spherical aggregates in the form of agglomerates or aggrogates; These will also be referred to hereinafter as "secondary particles ". Particularly small cavities and pores, i.e. so-called mid-pores, are present in the nanometer range, within the primary particles and within the secondary particles, i.e. between the primary particles, while larger cavities or pores are present in the individual secondary particles Respectively.

The mold particles obtained by grinding or polishing from these also exhibit a hierarchical structure with a predetermined oligomodal pore size distribution in the mold material.

In the soot deposition method, the mold material can also be prepared in the form of black powder which is further processed into the mold particles subsequently in granulation, pressing, slurry or firing process. Granules or flakes should be referred to as intermediate products.

The layer of the mold material produced by the soot deposition can be segmented with little effort, thereby producing mold particles with platelet- or flake-like morphology.

Such mold particles, which are distinguished by a non-spherical shape, are particularly advantageous for use in the process according to the invention.

The reason for this is that particles having a spherical shape, that is, particles having the shape of a ball or a roughly spherical shape, have a smaller surface area than their volume. In contrast, particles having a non-spherical morphology exhibit a greater ratio of surface to volume, which simplifies and level the penetration into the precursor material.

From this point of view, mold particles in the form of platelet- or rod-shaped particles having a structure ratio of at least 5, preferably at least 10, have been found to be particularly advantageous.

Here, "structure ratio" is understood as a ratio of the maximum structural width of the particles to its thickness. Consequently, a structural ratio of at least 5 means that the maximum structural width of the particles is at least five times greater than its thickness. These particles have a substantially platelet- or rod-shaped morphology and are distinguished by two substantially parallel-extending large surfaces by the relatively rapid formation of molten precursor material because of the relatively small thickness of the volume to be filled .

As the thickness of the mold particles becomes smaller, the penetration using the melt precursor material is further simplified and becomes more uniform. From this point of view, it has been found advantageous that the mold particles have a mean thickness of between 10 μm and 500 μm, preferably between 20 μm and 100 μm, particularly preferably less than 50 μm.

Mold particles with a thickness of less than 10 μm have small mechanical strength and deteriorate the formation of noticeable hierarchical pore structures. At thicknesses of greater than 500 [mu] m, it becomes increasingly difficult to ensure uniform penetration into the melt precursor material.

Uniform mixing of the particles from the mold material and the precursor material is facilitated when the precursor material particles are made spherically and have a median particle size of less than 50 [mu] m, preferably less than 20 [mu] m.

Due to the spherical formation of the particles, the mixing of the non-spherical particles from the mold material is improved. It is also supported when the particles from the precursor material are slightly smaller than the particles of the precursor material. However, particle sizes of less than 1 μm tend to be dusty and are undesirable.

The degree of filling of the pores is set by the mixing ratio of the precursor material and the molding material. Preferably, the precursor material particles and the mold particles are intermixed at a volume ratio of 0.05 to 1.6, preferably 0.1 to 0.8.

0.05, the inner surface of the mold material is covered with one layer of only a small thickness so as to obtain a sponge-like web of carbon only. Therefore, smaller mixing ratios are undesirable. In contrast, at a mixing ratio of 1.6, a substantially filled pore structure is obtained depending on the initial porosity of the casting material.

Preferably, the mold material is SiO _2.

Synthetic < _{RTI ID} = 0.0 > SiO2 < / RTI > can be produced at relatively low cost on an industrial scale by soot deposition methods using inexpensive starting materials. SiO ₂ Molds withstand high temperatures during calcination. The upper limit of the temperature is previously determined by means of SiO ₂ and the beginning of the reaction of carbon to SiC (at about 1000 ℃). Removal of the mold material in the form of synthetic SiO ₂ according to method step (e) takes place by chemical dissociation.

Preferably, the pitch is suitable as the carbon precursor material.

Pitch, in particular "mesophase pitch" is a carbonaceous material having an ordered liquid crystal structure. After carbonization, the pitch melting that penetrates into the pores of the carbon structure results in a graphite-like deposit of the carbon that forms the shell of the core / shell composite, thereby allowing the microstructure of the carbon structure Clog the pores.

Alternatively, a carbohydrate is used as the carbon precursor material.

Carbohydrates, particularly sugars, such as saccharose, fructose or glucose, are non-graphitic carbon precursor materials.

Preferably, the carbon product is separated into finely divided carbon of the porous particles.

In the process according to the invention, a carbon product is obtained which normally has a monolith or a platelet- or flake-like morphology and can be easily separated into smaller particles. The particles obtained after separation have a hierarchical pore structure due to the structure of the mold and are further processed into shaped bodies or layers by, for example, standard paste or slurry methods.

With respect to the use of the carbon product, the above-mentioned object is achieved in accordance with the present invention in which the porous carbon product according to the present invention is used to produce an electrode for a rechargeable lithium battery.

Electrodes for rechargeable lithium batteries include both electrodes composed of carbon layers from a single material and composite electrodes composed of multiple materials.

Hereinafter, the present invention will be described in more detail with reference to the embodiments and drawings. The details are as follows.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of an apparatus for producing a SiO ₂ soot body.
Figure 2 shows a SEM image of a first embodiment of a porous carbon product obtained according to the method of the present invention having a hierarchical pore structure.
Figure 3 shows a SEM image of a second embodiment of a porous carbon product obtained according to the method of the present invention having a hierarchical pore structure.
Figure 4 shows a diagram of a thermogravimetric analysis during heating of a pitch-infiltrated mold in an oxygen-containing atmosphere.

The apparatus shown in Fig. 1 is provided for manufacturing a SiO ₂ soot body. A plurality of flame hydrolysis burners (2) arranged in series are arranged along the carrier tube (1) of aluminum oxide. The flame hydrolysis burner 2 is connected to the carrier tube 1 between two turning points which are stationary with respect to the longitudinal axis 4, as schematically indicated by the arrows 5 and 6, And is mounted on a joint burner block 3 that can reciprocate parallel to and move in a direction perpendicular thereto. The burner 2 is made of quartz glass and their distance to each other is 15 cm.

The flame hydrolysis burner 2 is arranged so that its main propagation direction with respect to the burner flame 7 is perpendicular to the longitudinal axis 4 of the carrier tube 1. With the aid of the flame hydrolysis burner 2, the SiO ₂ particles are oriented such that the porous SiO ₂ blank 8 is rotated about its longitudinal axis 4 so that it is built up as a layer- On the surface of the cylinder jacket of the carrier tube (1). The 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 burner gases and SiCl ₄ as feedstock for the formation of SiO ₂ particles, respectively. Here, the burner block 3 is reciprocated on the scale of two burner distances (i.e., 30 cm). During the deposition process, an intermediate temperature of about 1200 캜 is set on the blank surface 9.

After completion of the immersion process, a tube of porous SiO ₂ soot (soot tube) having a length of 3 m, an outer diameter of 400 mm and an inner diameter of 50 mm is obtained. The temperature during the formation of the soot body is kept relatively low (based on a density of 2.21 g / cm ³ of quartz glass) such that the SiO ₂ soot material has a low relative median density of 22%.

Preliminary examination

(1) In the first test, the mesophase pitch was heated to 300 DEG C in a container in nitrogen, resulting in a viscous pitch coarse. A monolithic sample of SiO ₂ soot bodies was immersed in the pitch bath and removed again after 30 minutes. It has been found that the melt pitch penetrates into the soot body only to a thickness of less than 1 mm.

(2) Then, the temperature of the pitch bath was raised to 400 캜. At this temperature the mesophase pitch is still viscous. Significant increase in degree of penetration in the soot body was not achieved. At a temperature of about 500 ° C, the pitch significantly begins to coke and evaporate.

1st Example

A sample of the soot body was polished. Since the soot body is built up as a layer-layer, the layers, one of which is located at the top of the other, show a tendency to be ground in the presence of high mechanical forces, so that a platelet or flake having a thickness in the range of 20 [ -Like particles were obtained. Particle size portions with side lengths from 500 [mu] m to 1,000 [mu] m were separated by sieving for further processing. The ratio of the maximum structural width (median) to the median thickness is about 20.

The mesophase pitch was polished and sieved to produce a pitch powder essentially consisting of spherical particles having a particle size of from 5 μm to 20 μm.

The pitch powder and soot body particles were homogeneously mixed with each other at a volume ratio of 1.6: 1, and the particle mixture was heated to a temperature of 300 캜. The viscous pitch surrounds the small SiO ₂ soot body particles and penetrates into the pores. The ratio of pitch-to-soot-body particle volume is chosen so that the pitch fills the pores so that the significant free pore volume no longer remains, where it is almost completely consumed.

After a 30 minute infiltration period, the temperature is raised to 700 ° C, which causes carbonization of the pitch. The porous composite mass is formed from non-spherical porous SiO ₂ particles, which occupy both the exterior and interior (i. E., The inner walls of the pores) as a layer of graphitizable carbon.

The SiO ₂ soot body particles into which the composite mass is introduced into the hydrofluoric acid bath are subsequently removed. Porous carbon having a structure that substantially represents a negative copy of the original SiO ₂ soot body particles (hereinafter also referred to herein as an "inverse template") after the SiO ₂ particles have been etched away Of the pre-product is obtained. The reverse mold is distinguished by a hierarchical pore structure that extends through a number of relatively large pore channels (macropores) through a finely fissured surface structure.

The reverse mold is purged, dried, segmented and decomposed into carbon flakes. The SEM image according to FIG. 2 shows the carbon structure obtained as described above with consistent pore sizes and cavity sizes of various sizes. Larger sized cavities extend through the finely cracked surface in the manner of the channels. Measurements of a specific internal surface area according to the BET method result in measurements of about 25 m ² / g.

Second Example

As described with reference to Example 1, it was prepared in the SiO ₂ soot body particles and particles of a mesophase pitch. The pitch powder and the soot body particles were uniformly mixed with each other at a volume ratio of 0.4: 1, and the particle mixture was heated to a temperature of 300 캜. The viscous pitch surrounds small SiO ₂ soot body particles and penetrates into the pores. The ratio of pitch to soot-body particles was chosen such that the pitch could not completely fill the pores.

After infiltration and carbonization, as described with reference to Example 1, the porous composite mass is occupied externally and partially occupied by the non-spherical porous SiO ₂ soot body particles as a layer of graphitizable carbon. The SiO ₂ soot body particles are then removed by etching in hydrofluoric acid to produce a pre-product of the porous carbon whose structure is derived from the initial soot body particles and structured as a thin web with thin walls, However, a large number of relatively large pore channels elongate through otherwise slightly crushed surface structures.

Carbon products are easily decomposed into carbon flakes. The SEM image according to Fig. 3 shows its structure. The size of the coherent pores and cavities extends through the finely cracked surface in the manner of the channel. Measurements of a specific internal surface area according to the BET method yielded measurements of about 50 m ² / g.

Figure 4 presents the results of a thermogravimetric analysis (according to DIN 51005 and DIN 51006) during the processing of a sample of pitch-impregnated soot-body particle agglomerates according to Example 1 before carbonization. The sample is heated in pure argon, where the weight loss is measured. The weight loss? G (%) based on the initial weight is plotted on the y-axis, and the process temperature T (占 폚) is plotted on the x-axis.

Therefore, starting from a temperature of about 300 ° C, a first weight loss is observed which can be attributed to the combustion of the activated carbon centers and subsequent carbonization. To a temperature of about 600 ° C, the weight loss is 4.4%, followed by a saturation degree corresponding to the weight of the pure carbon layer.

The carbon flakes obtained according to the process of the present invention are composed of porous carbon having a hierarchical structure. They are particularly suitable for producing electrode layers of rechargeable lithium batteries, and are particularly suitable for composite electrodes.

Claims (17)

(a) providing a template of an inorganic mold material comprising interconnected pores;
(b) providing a precursor material for carbon;
(c) infiltrating the pores of the template with the precursor material;
(d) carbonizing the precursor material;
(e) removing the template to form a porous carbon product,
Providing precursor material particles and mold particles of a fusible material, forming a powder mixture from the particles,
Heating the powder mixture before or during carbonization according to step (d) in such a way that the melt of the precursor material penetrates into the pores of the mold particles,
Wherein providing the mold particles comprises:
Wherein the feedstock material is converted into the mold material particles by hydrolysis or freeze decomposition and the particles are deposited on the deposit surface while forming a soot body from the mold material, Lt; RTI ID = 0.0 > 1, < / RTI > wherein the porous carbon product is segmented into particles.
The method according to claim 1,
Characterized in that the mold particles have a non-spherical shape.
3. The method of claim 2,
Characterized in that the mold particles are platelet- or rod-shaped with a structure ratio of at least 5. < Desc / Clms Page number 13 >
3. The method of claim 2,
Characterized in that the mold particles are platelet- or rod-shaped with a structural ratio of at least 10.
5. The method according to any one of claims 1 to 4,
Characterized in that the template particles have a mean thickness of 10 [mu] m to 500 [mu] m.
5. The method according to any one of claims 1 to 4,
Characterized in that the template particles have an intermediate thickness of 20 [mu] m to 100 [mu] m.
5. The method according to any one of claims 1 to 4,
Wherein the template particles have an intermediate thickness of less than 500 [mu] m.
5. The method according to any one of claims 1 to 4,
Wherein the precursor material particles are made spherically and have a median particle size of less than 50 [mu] m.
5. The method according to any one of claims 1 to 4,
Wherein the precursor material particles are made spherically and have a median particle size of less than 20 [mu] m.
5. The method according to any one of claims 1 to 4,
Wherein the precursor material particles and the template particles are intermixed at a volume ratio of 0.05 to 1.6.
5. The method according to any one of claims 1 to 4,
Wherein the precursor material particles and the template particles are intermixed in a volume ratio of 0.1 to 0.8.
5. The method according to any one of claims 1 to 4,
Wherein said molding material is characterized in that the SiO _2.
5. The method according to any one of claims 1 to 4,
Wherein a pitch is used as the precursor material material for the carbon.
5. The method according to any one of claims 1 to 4,
Wherein the carbohydrate is used as the precursor material for the carbon.
5. The method according to any one of claims 1 to 4,
Characterized in that the carbon product is separated into finely divided carbon of porous particles.

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