Method for producing a composite structure composed of porous carbon and electrochemical active material

Abstract

In order to provide an inexpensive product composed of a porous carbon provided with electrochemical active material, said product being suitable particularly for use as a cathode or anode material for a secondary battery, a process comprising the following process steps is proposed: (a) producing a template from inorganic material by gas phase deposition, said template comprising a framework of pores and nanoparticles joined to one another, (b) coating the template framework with an electrochemical active material or a precursor thereof, (c) infiltrating the pores of the template with a precursor substance for carbon, (d) carbonizing the precursor substance to form a carbon layer, (f) removing the template.

Claims

1. A method for producing a composite structure of porous carbon and electrochemical active material, said method comprising: a) providing template particles of inorganic template material, each of said particles forming a template framework of interconnected nanoparticles, said template framework containing pores, b) coating the template framework with a coating material that is an electrochemical active material or a precursor thereof, c) infiltrating the pores with a precursor substance for carbon, d) carbonizing the precursor substance to form a carbon layer, and e) removing the template material; wherein in the infiltrating according to method step (c) the precursor substance is a precursor for graphitizable carbon; and wherein infiltrating according to method step (c) comprises a first infiltration stage and a second infiltration stage, wherein a second precursor substance is a precursor for non-graphitizable carbon and is used in the first infiltration stage, and the precursor substance for graphitizable carbon is used in the second infiltration stage.

2. A method for producing a composite structure of porous carbon and electrochemical active material, said method comprising: a) providing template particles of inorganic template material, each of said particles forming a template framework of interconnected nanoparticles, said template framework containing pores, b) coating the template framework with a coating material that is an electrochemical active material or a precursor thereof, c) infiltrating the pores with a precursor substance for carbon, d) carbonizing the precursor substance to form a carbon layer, and e) removing the template material; and wherein in the infiltrating according to method step (c), the precursor substance is pitch.

3. The method according to claim 1, wherein the coating material of method step (b) is the precursor in a liquid phase.

4. The method according to claim 1, wherein the electrochemical active material contains Sn and wherein the precursor is SnO.sub.2.

5. The method according to claim 1, wherein during coating according to method step (b) at least 50% of a pore volume of the pores of the template framework are filled with the electrochemical active material or the precursor for the electrochemical active material.

6. The method according to claim 1, wherein the template particles are spherical, interconnected nanoparticles.

7. The method according to claim 2, wherein the coating material of method step (b) is the precursor in a liquid phase.

8. The method according to claim 2, wherein the electrochemical active material contains Sn and wherein the precursor is SnO.sub.2.

9. The method according to claim 2, wherein during coating according to method step (b) at least 50% of a pore volume of the pores of the template framework are filled with the electrochemical active material or the precursor for the electrochemical active material.

10. The method according to claim 2, wherein the template particles are spherical, interconnected nanoparticles.

Description

EMBODIMENT

(1) The invention shall now be explained in more detail with reference to embodiments and a drawing. In detail, in a schematic illustration,

(2) FIG. 1 shows an apparatus for producing SiO.sub.2 granulate particles which serve as a template in the method according to the invention;

(3) FIGS. 2 to 6 show method steps in the production of the carbon product according to the invention, in a schematic illustration.

(4) The apparatus illustrated in FIG. 1 serves to produce porous granulate particles of SiO.sub.2 which are used in the method according to the invention as hard template. The apparatus comprises a drum 1 which is rotatable about its rotation axis 2 and consists of a base body of special steel which is covered with a thin layer of silicon carbide. The drum 1 has an outer diameter of 30 cm and a width of 50 cm. A layer 5 of SiO.sub.2 soot is deposited on the outside surface 1a of the drum 1 and is directly densified slightly thermally into a SiO.sub.2 porous soot plate 5a.

(5) Flame hydrolysis burners 4 of which four are arranged one after the other in a common burner row 3 in the direction of the longitudinal axis 2 of the drum are used for soot deposition. The burner row 3 is reciprocated in parallel with the rotation axis 2 between two stationary turning points. As combustion gases, oxygen and hydrogen as well as octamethylcyclotetrasiloxane (OMCTS) are supplied to the flame hydrolysis burners 4 as feedstock material for the formation of SiO.sub.2 particles. The size of the SiO.sub.2 primary particles produced thereby is in the nanometer range, wherein plural primary particles agglomerate in the burner flame 6 and are obtained in the form of more or less spherical aggregates having a specific BET surface area in the range of 25 m.sup.2/g, which form a continuous, evenly thick SiO.sub.2 soot layer 5 on the drum outside surface 1a.

(6) In the embodiment the rotational speed of the drum 1 and the deposition rate of the flame hydrolysis burners 4 are matched such that a SiO.sub.2 soot layer 5 with a width of about 40 cm and a thickness of about 45 μm is obtained (the soot layer is plotted with an exaggerated thickness in FIG. 1 for reasons of illustration). The burners 4 simultaneously induce a certain pre-sintering of the soot layer 5 into a soot plate 5a in that they produce a mean temperature of about 1200° C. on the surface of the topmost soot layer. Pre-sintering is supported by a tubular infrared radiator 14 which is arranged within the drum 1, which is formed as a hollow drum, in the left lower quadrant, and which heats the outside surface of the drum 1 shortly after application of the soot layer 5 from the inside.

(7) The porous, slightly pre-sintered soot plate 5a obtained thereby has a mean relative density of about 22% (based on the density of quartz glass with 2.21 g/m.sup.3).

(8) After slightly more than half a drum rotation the soot plate 5a passes into the sphere of action of a blower 7 by means of which a gas stream directed against the bottom side of the soot plate 5a is produced, so that the soot plate 5a lifts off from the drum outside surface 1a.

(9) The soot plate 5a is subsequently supplied via a support roll 8 to a crushing tool 9 which consists of two counter-rotating rolls 10a, 10b between which a gap is provided having the thickness of the soot plate 5a, and the surfaces of which are provided with longitudinal profiles.

(10) The soot plate 5a passing through the gap is divided by the longitudinal profiles of the rolls 10, 10a into fragments of about the same size (granulate particles 13) which are collected in a collection container 11.

(11) A partition wall 12 is provided between the drum 1 and the crushing tool 9; the partition wall 12 is provided with an opening for passing the soot plate 5a therethrough and serves to shield the soot deposition process against the impacts of the comminuting process.

(12) The granulate particles 13 obtained according to the method have a platelet- or flake-like morphology and a thickness corresponding approximately to the thickness of the soot plate 5a, i.e. about 45 μm. They exhibit more or less planar top and bottom sides as well as lateral fracture areas with open pores. Due to the described crushing operation the granulate particles 13 also have about the same size, resulting in a narrow grain size distribution. The structural ratio “A”, i.e. the ratio of maximum structure width (a or b) and thickness (c) of the granulate particles 13, is about 10 in the embodiment.

(13) The granulate particles 13 produced in this way serve as a template for the production of porous carbon flakes, as is schematically shown in FIGS. 2 to 6 and explained in more detail hereinafter with reference to said figures.

EXAMPLE 1

(14) Viewed under the microscope, the non-spherical platelet-shaped granulate particles 13 are composed of a plurality of more or less spherical agglomerates of SiO.sub.2 nanoparticles. Such a nanoparticle agglomerate 13 is schematically shown in FIG. 2.

(15) The granulate particles 13 are introduced into a solution of urea and NaSnO.sub.3. After evaporation of the solvent a film 14 of SnO.sub.2 with a mean thickness of about 50 nm, which is schematically shown in FIG. 3 and which serves as an electrochemical active material in the sense of the invention, remains on the inner and outer surfaces of the granulate.

(16) The granulate particles coated in this way are homogeneously intermixed with finely ground pitch powder in the weight ratio 1:4 (pitch:granulate particles), and the particle mixture is heated to a temperature of 300° C. The low-viscosity pitch envelopes the small SiO.sub.2 granulate particles 13 and penetrates into the pores and infiltrates the same. The weight ratio of pitch and granulate particles is here chosen such that the pitch fills the pores to such an extent that after an infiltration period of 30 min there is hardly any significant free pore volume left.

(17) FIG. 4 schematically shows the composite which is obtained thereby and consists of granulate particles 13 and which is enveloped by a SnO.sub.2 layer 14 and a pitch layer 15.

(18) The quartz glass of the granulate particles 13 is subsequently removed in that the particles are introduced into a bath of 2 molar NaOH solution. Since the granulate particles 13 consist of nanoparticles interconnected in the manner of a network, the NaOH solution can penetrate within the network structure until the whole template material is removed.

(19) FIG. 5 schematically shows the layer composite 16 which is obtained after the SiO.sub.2 granulate particles have been etched away and which consists of the SnO.sub.2 layer 14 and the pitch layer 15. The layer composite 16 is approximately a negative image of the mass distribution of the original SiO.sub.2 granulate particles 13. The layers 14; 15 surround a cavity 17.

(20) Subsequently, the composite structure is heated in nitrogen to a temperature of 700° C. and the pitch is thereby reduced (carbonized) to carbon. This also leads to a reduction of SnO.sub.2 to tin; the metal can here also be liquefied for a short period of time and contract into fine droplets 20 within the cavity 17.

(21) As outlined in FIG. 6, the composite structure 18 obtained thereby consists of a graphite-like carbon layer 19 which forms the inner wall of an active material cavity 17, with fine particles 20 of tin adhering to the carbon layer 19. The carbon layer 19 has a low porosity and it has a thickness of about 50 nm on average. In this connection it should be noted that the illustration of FIGS. 2 to 6 is not true to scale. The composite structure 18 extends in all space directions and forms a carbon product which is loaded with active material 20 and which has a hierarchical pore structure. It is important that the cavity 17 is not completely closed, but is in fluidic communication with other mesopores and macropores, so that the tin particles 20 can be subjected to a further chemical treatment, and further, electrochemically active substances, and especially lithium, can be introduced into the cavity 17.

(22) The composite structure obtained thereby is further comminuted, if necessary. This yields carbon flakes loaded with active material, in the case of which rather large cavities extend in the manner of channels through a finely rugged surface. These carbon flakes with hierarchical pore structure are particularly well suited for the production of electrode layers of a rechargeable battery. To this end they are infiltrated in a known manner with lithium which either fully or partly fills the cavities 17 formerly occupied with template particles 13. Also substances formed and released upon discharge of the battery remain physically bound in the cavities 17, whereby it is prevented that they are evenly distributed in the electrolyte and are thus no longer available for the electrochemical reaction, and it is prevented that the individual particles de-contact electrically.

EXAMPLE 2

(23) The pores of the granulate particles 13 are coated by deposition of SiH.sub.4 from the gas phase. SiH.sub.4 is decomposed due to an increased temperature, so that a silicon layer having a thickness of about 20 nm is formed on the accessible surfaces of the granulate particles 13. Silicon thereby forms the electrochemical active material in the sense of the invention.

(24) The granulate particles coated in this way are infiltrated with liquid pitch and thereby provided with a layer of pitch, as described with reference to Example 1. The composite which is obtained thereby and consists of granulate particles and is enveloped by a silicon layer and a pitch layer is subsequently heated to a temperature of 700° C. and the pitch is thereby reduced to carbon.

(25) The quartz glass of the granulate particles is subsequently removed in that the particles are introduced into a hydrofluoric acid bath. The hydrofluoric acid can here penetrate within the network structure of the granulate particles, so that the whole template material is removed within a few minutes. Since silicon shows a much lower etch rate in hydrofluoric acid than the SiO.sub.2 of the granulate particles, the silicon layer is not significantly reduced during this etching process.

(26) The composite layer which is obtained after the SiO.sub.2 granulate particles have been etched away and which consists of silicon layer and carbon layer is approximately a negative image of the mass distribution of the original SiO.sub.2 granulate particles. The layers surround an active material cavity. The composite structure extends in all space directions and forms a carbon product which is coated with active material and has a hierarchical structure.

(27) The cavities are not fully closed, but are in fluidic communication with other mesopores and macropores, so that the silicon layer can be subjected to a further chemical treatment and can particularly be alloyed with lithium.

(28) The composite structure obtained thereby is further comminuted, if necessary, and further processed into anode material for a secondary battery, as has been explained above with reference to Example 1.

EXAMPLE 3

(29) In a modification of the procedure of Example 2 a two-stage infiltration process of the pores with carbon is provided after coating of the granulate particles with active material in the form of silicon.

(30) The coated granulate particles are here first introduced into an immersion bath of an aqueous saturated solution of sucrose. The impregnated material is subsequently dried. This impregnation and drying process is repeated once. A dried sucrose layer is formed in the pores of the granulate particles and on the surfaces of the nanoparticle agglomerates previously coated with active material. It is carbonized by heating in nitrogen at 700° C. into a film of turbostratic carbon with a thickness of about 3 nm, which shows a certain microporosity and high adhesion power vis-à-vis the silicon layer due to its turbostratic structure.

(31) Subsequently, the granulate particles which were thus coated before are infiltrated and further processed, as explained with reference to Example 2.

EXAMPLE 4

(32) To produce a cathodic material for secondary batteries based on the complex lithium compound LiFePO.sub.4, nanoparticles of said compound are homogeneously mixed into liquid pitch. The pitch loaded with the active material is ground to a particle size of about 15 μm after cooling and is mixed with SiO.sub.2 granulate particles. The particle mixture is heated to a temperature of 300° C., so that the mixture of low-viscosity pitch and the active material enters into the pores of the SiO.sub.2 granulate particles, so that a coating with pitch and active material particles embedded therein is obtained. The mean size of the nanoparticles corresponds approximately to the size of the thickness of the pitch layer.

(33) The coated granulate particles obtained thereby are heated to a temperature of 700° C. in nitrogen atmosphere, and the pitch is carbonized to graphite-like carbon with a low microporosity in such a manner that micropores with a pore diameter of less than 2 nm account for less than 6% of the total pore volume of the carbon layer.

(34) During carbonization the LiFePO.sub.4 particles embedded in the coating are mainly maintained despite the reducing effect of the carbon.

(35) After the SiO.sub.2 of the granulate particles has been removed by etching in 2 molar NaOH solution, the surface of the carbon layer that has so far been occupied by the SiO.sub.2 of the granulate particles and forms the inner wall of cavities that are interconnected via former sinter necks is also accessible. The porous composite structure produced thereby is characterized in that the graphite-like carbon layer has embedded therein particles consisting of the active material LiFePO.sub.4 which adjoin or project over the surface of the carbon layer. The active material particles embedded thereby are thus easily accessible on the one hand via pores of the composite structure for the electrolyte of a secondary battery and they are on the other hand electrically contacted in a reproducible manner via the graphite-like, hardly porous carbon layer.

LIST OF REFERENCE NUMERALS

(36) Drum 1 Drum outside surface 1a Rotation axis 2 Burner row 3 Flame hydrolysis burner 4 Layer 5 Soot plate 5a Burner flame 6 Blower 7 Support roll 8 Crushing tool 9 Rolls 10a, 10b Collection container 11 Partition wall 12 Granulate particles 13 SnO.sub.2 layer 14 Pitch layer 15 Layer composite 16 Cavity 17 Composite structure 18 Carbon layer 19 Particles of tin 20 Structural ratio “A