Method for producing composite powder, and for a porous composite material for electrochemical electrodes that is produced using the composite powder

Abstract

The invention relates to a method for economically producing a composite powder made of carbon and electrochemical active material. According to the invention, a melt made of a meltable carbon precursor substance having nanoparticles made of an active material distributed in the melt is provided, and said melt is divided into the composite powder, in which nanoparticles made of the active material are embedded in a matrix made of the carbon precursor substance. A porous composite material produced using the composite powder is used to produce an electrode for a secondary battery, in particular for use as an anode material. The production of the composite material comprises the following steps: providing template particles made of inorganic template material, producing a powder mixture of the composite powder and the template particles, heating the powder mixture and softening the composite powder in such a way that the composite powder penetrates the pores and is carbonated, and removing the template material to form the porous electrochemical composite material.

Claims

1. A method for producing a porous composite material for electrochemical electrodes, said method comprising: providing template particles of inorganic template material, said particles forming a pore-containing template framework of interconnected nanoparticles; producing a composite powder from a meltable carbon precursor substance and active material by: a. producing a melt from the meltable carbon precursor substance with nanoparticles of the active material distributed in the melt; and b. dividing the melt or a molten body obtained by solidification of the melt into the composite powder, wherein the nanoparticles of the active material are embedded in a matrix of the carbon precursor substance; producing a powder mixture after said producing of the melt and dividing by mixing the composite powder and the template particles, wherein the powder mixture has the composite powder and template particles intermixed therein in a volume ratio that is in a range between 0.05 and 1.6; heating the powder mixture and softening the composite powder after said mixing so as to form a softened composite such that the softened composite penetrates into the pores of the template framework and is carbonized; and removing the template framework so as to form the porous composite material.

2. The method according to claim 1, wherein said providing of the template particles comprises a soot deposition process in which a feedstock is converted by hydrolysis or pyrolysis into template material particles of a template material, and depositing said template material particles on a deposition surface so as to from a soot body of the template material, and comminuting the soot body into the template particles.

3. The method according to claim 2, wherein the template material is SiO.sub.2.

4. The method according to claim 2, wherein the template particles have a mean thickness in the range of 10 m to 500 m.

5. The method according to claim 2, wherein the template particles have a mean thickness in the range of 20 m to 100 m.

6. The method according to claim 2, wherein the template particles have a mean thickness in the range of 20 m to 50 m.

7. The method according to claim 2, and further comprising producing an anode for a rechargeable lithium battery from the porous composite material.

8. The method according to claim 1, wherein the porous composite material that is produced has nanoparticles from the active material that are embedded in a porous carbon matrix and that are accessible for an electrolyte through a channel system of open pores.

9. The method according to claim 1, and further comprising producing an anode for a rechargeable lithium battery from the composite material.

10. The method according to claim 1, wherein the dividing produces a composite powder having a mean particle size of less than 50 m.

11. The method according to claim 1, wherein the active material consists of Si or of an alloy of Si and Sn.

12. The method according to claim 1, wherein the composite powder has a weight proportion between 20% to 30% of the active material.

13. The method according to any one of claim 1, wherein said composite powder and template particles are intermixed in a volume ratio ranging between 0.1 and 0.8.

14. The method according to claim 1, wherein the meltable carbon precursor and the active material are powders, and wherein the producing the melt comprises dry mixing the powders of the carbon precursor substance and the active material so as to produce a dry mixture, and then melting of the dry mixture.

15. The method according to claim 1, wherein the dividing comprises mechanically grinding the molten body.

16. The method according to claim 1, wherein the dividing produces a composite powder having a mean particle size of less than 100 m.

17. The method according to claim 1, wherein the active material contains Si.

18. The method according to claim 1, wherein the composite powder has a weight proportion of 10% to 50% of the active material.

19. The method according to claim 1, wherein the carbon precursor substance is pitch.

Description

EMBODIMENT

(1) The invention is hereinafter explained in more detail with reference to embodiments and a drawing. In detail, in a schematic representation,

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

(3) FIGS. 2 to 5 show method steps in the production of the composite material according to the invention, in a schematic representation.

1. PRODUCTION OF COMPOSITE POWDER OF CARBON AND SILICONFIRST COMPONENT

(4) Monodisperse, substantially spherical Si nanoparticles with a particle size of about 30 nm are commercially available. The Si nanoparticles are homogeneously intermixed with finely ground powder of mesophase pitch in the weight ratio 3:1 (pitch:Si nanoparticles). The homogenized power mixture is heated to a temperature of 300 C., so that the pitch melts into a low-viscosity melt. Due to the previously generated powder homogenization the Si nanoparticles are substantially homogeneously distributed in the pitch melt. On account of their small size and the density similar to the pitch melt, the Si nanoparticles do also not settle or float. A stirring of the melt can therefore be dispensed with.

(5) After a melting period of 2 h the pitch melt cools down into a molded body in which Si nanoparticles are now present in a substantially homogeneous and finely divided form. The molded body consists of a substantially dense pitch matrix in which the Si nanoparticles are embedded.

(6) Particle sizes between 10 m and 30 m are sieved for further processing. They serve as a first component for the production of a composite material.

2. PRODUCTION OF TEMPLATE PARTICLESSECOND COMPONENT

(7) The apparatus shown in FIG. 1 serves the production of porous granulate particles of SiO.sub.2 which are used in the method of the invention as hard template particles 13. The apparatus comprises a drum 1 which is rotatable about its rotation axis 2 and which consists of a basic 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 jacket surface 1a of the drum 1 and slightly thermally densified directly into a porous SiO.sub.2 soot plate 5a.

(8) 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 the soot deposition. The burner row 3 is reciprocated in parallel with the rotation axis 2 between two stationary turning points. The flame hydrolysis burners 4 are fed with oxygen and hydrogen as combustion gases and with octamethylcyclotetrasiloxane (OMCTS) as feedstock for the formation of SiO.sub.2 particles. The size of the SiO.sub.2 primary particles produced thereby are in the nanometer range; here, several primary particles agglomerate in the burner flame 6 and are obtained in the form of more or less spherical aggregates with a specific BET surface area in the range of 25 m.sup.2/g; these form a continuous SiO.sub.2 soot layer 5 of uniform thickness on the jacket surface 1a of the drum.

(9) In the embodiment, the rotational speed of the drum 1 and the deposition rate of the flame hydrolysis burners 4 are matched such that one obtains a SiO.sub.2 soot layer 5 with a width of about 40 cm and a thickness of about 45 m (the soot layer is plotted in FIG. 1 with an exaggerated thickness for reasons of representation). The burners 4 simultaneously produce a certain pre-sintering of the soot layer 5 into a soot plate 5a by generating 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 in the left lower quadrant within the drum 1 formed as a hollow drum, and which heats the jacket surface of the drum 1 from the inside shortly after application of the soot layer 5.

(10) The porous and 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).

(11) After a little 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 jacket surface 1a of the drum.

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

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

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

(15) The template particles 13 obtained according to the method have a platelet-like or flake-like morphology and a thickness corresponding approximately to the thickness of the soot plate 5, i.e. about 45 m. They exhibit more or less planar top and bottom sides as well as lateral fracture areas with open pores.

(16) The particle size fraction with side lengths between 500 m and 1,000 m has been separated by sieving for further processing. The structure ratio A, i.e. the ratio of maximum structure width (a or b) and thickness (c) of the template particles 13, is about 20 in the embodiment.

(17) In addition to the above-described composite powder, the SiO.sub.2 template particles 13 produced thereby serve as a further component for the production of a composite material. The production thereof shall be explained in more detail hereinafter with reference to an example and FIGS. 2 to 5.

3. PRODUCTION OF A COMPOSITE MATERIAL CONSISTING OF FIRST COMPONENT AND SECOND COMPONENT

(18) Viewed under the microscope, the non-spherical, platelet-like template particles 13 are composed of a multitude of spherical aggregates of SiO.sub.2 primary particles which are interconnected, thereby forming a soot framework. A single primary particle aggregate 16 of such a type is schematically shown in FIG. 2, which consequently shows a section of a soot framework.

(19) The composite powder (as the first component) and the SiO.sub.2 primary particle aggregates 16 (as the second component) are homogeneously intermixed in a volume ratio of 1:2 (composite powder:primary particle aggregate) by means of a mixer. The mixing period is about 5 min.

(20) The particle mixture is subsequently heated to a temperature of 300 C. The low-viscosity pitch which is infiltrated with Si nanoparticles 14 covers the small SiO.sub.2 primary particle aggregates 16 and penetrates into and infiltrates the pores. The volume ratio of composite powder and primary particle aggregate is here chosen such that the pitch fills the pores to such an extent that after an infiltration period of 30 min there hardly remains a significant free pore volume.

(21) FIG. 3 schematically shows the composite obtained thereby, which consists of primary particle aggregate 16, covered by a pitch layer 15 infiltrated by Si nanoparticles 14.

(22) After the infiltration period of 30 min the temperature is raised to 700 under nitrogen and the pitch of the composite layer is reduced to carbon (carbonized). Thereafter the original pitch layer forms a graphite-like carbon layer 19 of reduced thickness. Due to shrinkage the embedded Si nanoparticles 14 are partly exposed or they get closer to the surface of the primary particle aggregate 16 or closer to the free surface, as schematically shown in FIG. 4.

(23) The carbon layer 19 has a low porosity and has a thickness of about 50 nm on average. It should here be noted that the representation of FIGS. 2 to 5 is not true to scale.

(24) After cooling one obtains a slightly porous composite mass consisting of non-spherical porous SiO.sub.2 template particles 13 that are everywhere covered with a layer of graphitizable carbon and active material.

(25) The SiO.sub.2 of the template particles 13 is subsequently removed in that the composite mass is introduced into a bath of 2-molar NaOH solution. Since the template particles consist of nanoparticles which are interconnected like a network, the NaOH solution can advance within the network structure until the whole template material is removed.

(26) FIG. 5 schematically shows the composite material with the composite structure 18 as obtained after the SiO.sub.2 primary particle aggregate has been etched away. This structure consists of a graphite-like carbon layer 19 in which nanoparticles 14 of silicon are embedded. The carbon layer 19 forms the inner wall of a cavity 17 which was originally occupied by a SiO.sub.2 nanoparticle agglomerate, i.e. by a primary particle aggregate 16. The Si nanoparticles 14 are freely accessible at least in part.

(27) The composite structure 18 extends in all spatial directions and is approximately a negative image of the mass distribution of the original SiO.sub.2 primary particle aggregate 16. It has a hierarchical pore structure and forms the composite material loaded with active material 14 within the meaning of the invention.

(28) It is important that the cavity 17 is not closed, but fluidically connected to other mesopores and macropores. It provides a free pore volume and further surface via which the Si active material fixed in the carbon matrix is accessible for an electrolyte and lithium for alloying.

(29) The composite structure 18 obtained thereby is further comminuted in case of need. This yields carbon flakes loaded with active material, in which rather large cavities pass in the form of channels through a finely rugged surface. These carbon flakes with hierarchical structure are particularly well suited for the production of electrode layers of a rechargeable battery.