Particulate anode materials and methods for their preparation

Abstract

Method for preparing a particulate material including particles of an element of group IVa, an oxide thereof or an alloy thereof, the method including: (a) dry grinding particles from an ingot of an element of group IVa, an oxide thereof or an alloy thereof to obtain micrometer size particles; and (b) wet grinding the micrometer particles dispersed in a solvent carrier to obtain nanometer size particles having a size between 10 to 100 nanometers, optionally a stabilizing agent is added during or after the wet grinding. Method can include further steps of (c) drying the nanometer size particles, (d) mixing the nanometer size particles with a carbon precursor; and (e) pyrolyzing the mixture, thereby forming a coat of conductive carbon on at least part of the surface of the particles. The particulate material can be used in fabrication of an anode in an electrochemical cell or electrochemical storage energy apparatus.

Claims

1. A method for preparing a particulate material comprising particles of silicon or an oxide thereof, the method comprising: (a) dry grinding particles from an ingot of silicon or an oxide thereof to obtain micrometer size particles; and (b) wet grinding the micrometer particles dispersed in a solvent carrier to obtain nanometer size particles having a size between 10 to 100 nanometers, optionally a stabilizing agent is added during or after the wet grinding; wherein the ingot comprises at least 98% of silicon or an oxide thereof.

2. A method according to claim 1, further comprising a step of (c) drying the nanometer size particles.

3. A method according to claim 2, further comprising the steps of: (d) mixing the nanometer size particles with a carbon precursor; and (e) pyrolysing the mixture, thereby forming a coat of conductive carbon on at least part of the surface of the particles.

4. A method according to claim 3, wherein the conductive carbon formed is non-powdery, and is in an amount of about 0.5-10% wt of the particulate material.

5. A method according to claim 3, wherein step (e) is performed at a temperature of about 600-800 C.

6. A method according to claim 3, wherein step (e) is performed at a rate of about 3-10 C./min.

7. A method according to claim 3, wherein step (e) is performed during a period of about 30 minutes to 2 hours.

8. A method according to claim 3, wherein step (e) is performed under inert atmosphere.

9. A method according to claim 1, wherein step (a) is performed in a bead mill, a puck and ring mill, a jet mill or a cyclone mill.

10. A method according to claim 1, wherein the carrier solvent is an organic solvent selected from the group consisting of isopropanol and cyclohexane, or is water.

11. A method according to claim 1, wherein an amount of the solvent carrier is adjusted such as to represent about 5-20% wt of the particulate material.

12. A method according to claim 1, wherein the carbon precursor is selected from the group consisting of an organic material, a cross-linkable monomer, oligomer, polymer and copolymer.

13. A method according to claim 1, wherein the carbon precursor is poly(maleic anhydride-1-alt-octadecene).

14. A method according to claim 1, wherein the stabilizing agent is a surfactant.

15. A method according to claim 1, wherein the particulate material is used as an anode in an electrochemical cell or an electrochemical storage energy apparatus selected from the group consisting of a lithium-ion battery, a silicon-air battery and a polymer battery.

16. A method for preparing a particulate material comprising particles of silicon or an oxide thereof, the method comprising: (a) dry grinding particles from an ingot of silicon or an oxide thereof to obtain micrometer size particles; (b) wet grinding the micrometer particles dispersed in a solvent carrier to obtain nanometer size particles having a size between 10 to 100 nanometers, optionally a stabilizing agent is added during or after the wet grinding; (c) mixing the nanometer size particles with a carbon precursor; and (d) pyrolysing the mixture, thereby forming a coat of conductive carbon on at least part of the surface of the particles; wherein the ingot comprises at least 98% of silicon or an oxide thereof.

17. A method according to claim 16, wherein the method comprises drying the nanometer size particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a process flow diagram illustrating the method according to the invention.

(2) FIG. 2 shows a graph of the particles size distribution obtained from laser scattering analyzer observations (LA-950V2, Horiba) of a sample of micrometer size particles according to the invention.

(3) FIG. 3 shows the evolution of the average particle size (d.sub.50) over time during dry grinding.

(4) FIG. 4 shows a graph of the particle size distribution obtained from laser scattering analyzer observations (LA-950V2, Horiba) of a further sample of micrometer size particles according to the invention.

(5) FIG. 5 shows a graph of the particle size distribution obtained from laser scattering analyzer observations (LA-950V2, Horiba) of still a further sample of micrometer size particles according to the invention.

(6) FIG. 6 shows the evolution of the average particle size (d.sub.50) over time during wet grinding.

(7) FIG. 7 shows a graph of the particle size distribution obtained from laser scattering analyzer observations (LA-950V2, Horiba) of a sample of nanometer size particles according to the invention.

(8) FIG. 8 shows the evolution of the average particle size (d.sub.50) over time during a further wet grinding.

(9) FIG. 9 shows a graph of the particle size distribution obtained from laser scattering analyzer observations (LA-950V2, Horiba) of a further sample of nanometer size particles according to the invention.

(10) FIG. 10 shows voltages profiles of the Li/Si composite anode in 1M LiPF.sub.6-EC-DEC.

DESCRIPTION OF PREFERRED EMBODIMENTS

(11) The inventors have designed a method for the preparation of a particulate material which comprises an element of group IVa, preferably silicon, an oxide thereof or an alloy thereof. The method involves dry and wet grinding steps to yield nanometer size particles. The nanometer size particles can be coated with conductive carbon. The element of group IVa can be Si. The alloy can comprise at least one of Li, Al, Mg, Fe, Ge, C, Bi, Ag, Sn, Zn, B, Ti, Sr, P and O. The material prepared by the method according to the invention is used as anode.

(12) In preferred embodiments of the invention, the element of group IVa is Si or the oxide is an Si oxide (SiO.sub.x). A process flow diagram of the method according to the invention is outlined in FIG. 1.

(13) A method for preparing a particulate Si material, wherein at least part of the surface of the particles can be coated with conductive carbon. The method comprises the following steps: (a) dry grinding Si particles from an ingot of Si or SiO.sub.x to obtain micrometer size particles; and (b) wet grinding the micrometer particles dispersed in a solvent carrier to obtain nanometer size particles having a size between 10 to 100 nanometers. Optionally, a stabilizing agent can be added during or after the wet grinding step. Such agent helps in avoiding agglomeration of the particles. Also, it allows for an effective dispersion of the particles in the solvent carrier. Suitable stabilizing agents are described for example in WO 2008/067677. They are generally commercially available and include for example surface active agents such as surfactants. As will be understood by a skilled person, any other suitable stabilizing agent can be used.

(14) The method can comprise the further step of (c) drying the nanometer size particles. Moreover, the method can comprise the further steps of: (d) mixing the nanometer size particles obtained either in step (b) or in step (c) with a carbon precursor; and (e) pyrolysing the mixture, thereby forming a coat of conductive carbon on at least part of the surface of the particles.

(15) The Si particles used in step (a) are millimeter size particles which can be obtained by the following steps: (a1) providing commercially available metallurgical grade Si; (a2) melting the Si; (a3) casting and cooling the melted Si to obtain ingots; and (a4) crushing the ingots to obtain the millimeter size Si particles. Melting of the starting material can be performed in an induction furnace using a graphite crucible. As will be understood by a skilled person any other suitable means for melting can be used in the process. Also, the melting process is performed under inert atmosphere wherein an inert gas such as for example argon, is used.

(16) The temperature of the melted Si is raised to about 1410-1650 C., preferably about 1450 C., and then it is casted in a mould and cooled to room temperature. A suitable mould type used in the process can be for example a graphite mould; however, as will be understood by a skilled person any other suitable mould type can be used. Ingots formed after cooling the melted Si are crushed into centimeter size particles, then into millimeter size particles. The crushing can be performed using a jaw crusher which can have an abrasion resistant liner, such as zirconia or tungsten carbide. Such crushing will generally yield centimeter size particles which are further ground into millimeter size particles using for example a roll crusher.

(17) In an embodiment of the invention, dry grinding of the millimeter size Si particles into micrometer particles (step (a) outlined above) can be performed for example in a jet mill, a bead mill, a puck and ring mill, or a cyclone mill. As will be understood by a skilled person, any other suitable grinding means can be used. Beads used with a bead mill can be for example 5 mm zirconia beads.

(18) The micrometer size Si particles are dispersed into a solvent carrier then subjected to grinding (wet grinding) into nanometer size particles (step (b) outlined above). This step can be performed for example in a bead mill with 0.3 mm zirconia beads. The solvent carrier can be an organic solvent. For example, the carrier solvent can be an alcohol such as a C.sub.1-C.sub.12 alcohol or water. In embodiments of the invention, the micrometer size particles were dispersed in isopropanol or furfuryl alcohol; however as will be understood by a skilled person, any other suitable solvent can be used in the process. The solvent carrier is used in an amount of about 5-20% wt, preferably about 8-15% wt, more preferably about 10% wt of the amount of Si.

(19) The nanometer size Si particles can further be mixed with a carbon precursor (step (c) outlined above). In embodiments of the invention, the particles are in wet form, i.e. still in the solvent carrier (particles obtained from step (b) outlined above). In other embodiments, the particles are in wet form (particles obtained from step (c) outlined above). The carbon precursor is intimately mixed with the Si particles in order to achieve impregnation of the particles surface such that after pyrolysis (step (e) outlined above), the conductive carbon deposited is in intimate contact with the particles.

(20) The carbon precursor can be an organic carbon precursor. Moreover, the carbon precursor can be for example a cross-linkable monomer, oligomer, polymer or copolymer, preferably poly(maleic anhydride-1-alt-octadecene). As will be understood by a skilled person, any suitable material capable of being adsorbed on the surface of the nanometer size Si particles such as to leave thereon after pyrolysis a layer of conductive carbon, can be used in the process. In embodiments of the invention, the amount of carbon precursor used can be for example about 2-10% wt, preferably about 5% wt of the amount of Si.

(21) The mixture of nanometer size Si particles and carbon precursor is subjected to pyrolysis (step (e) outlined above). This step allows for burning of the carbon precursor and deposit of a layer of conductive carbon on the surface of the nanometer size Si particles. The conductive carbon deposited is preferably non-powdery. In embodiments of the invention, pyrolysis of the mixture is performed at a temperature of about 600-800 C., preferably about 650-750 C., more preferably about 725 C. The drying rate during pyrolysis can be for example about 3-10 C./min., preferably about 6 C./min. And the drying time can be for example about 30 minutes to 2 hours, preferably about 1 hour. This step can be conducted under inert atmosphere such as for example argon atmosphere.

(22) In embodiments of the invention, a subsequent step of cooling the pyrolyzed mixture is performed. This step is conducted at a cooling rate of about 2 C./min.

(23) Micrometer size Si particles obtained in the process according to the invention, particularly in dry grinding step (a), present a mean size of 0.1-100 m. Nanometer size Si particles obtained in the process according to the invention, particularly in wet grinding step (b) present a mean size of the nanometer size particles obtained in step (b) is 10-100 nm.

(24) The invention provides according to an aspect, for a particulate Si material which is prepared by the method according to the invention and as described above. In embodiments of the invention, a mean size of the particles is about 10-100 nm, preferably about 50-90 nm, more preferably about 70 nm. Moreover, in embodiments of the invention, the material has a carbon content of about 0.5-10% wt, preferably about 2-5% wt.

(25) The invention provides according to another aspect, for an anode which is fabricated using the material according to the invention and as described above.

(26) The invention provides according to yet another aspect, for an electrochemical cell or an electrochemical storage energy apparatus which comprises the anode according to the invention and as described above.

(27) The invention provides according to a further aspect, for an electrochemical storage energy apparatus comprising the anode according to the invention and as described above. The electrochemical storage apparatus can be a lithium-ion battery, a silicium-air battery or a polymer battery.

Example 1

(28) 10 kg of commercially available metallurgical grade silicon (Si) was melted in an induction furnace using a graphite crucible under argon atmosphere. The liquid silicon was held for 10 minutes for complete homogenization at a temperature of 1450 C. and casted in a graphite mould to allow cooling to room temperature. The impurity content of the ingot obtained measured by X-ray fluorescence spectroscopy is less than 2% wt of the material.

Example 2

(29) The ingot from Example 1 was crushed into centimeter size particles using a jaw crusher (JCA-100, Makino) with an abrasion resistant zirconia liner to lower metal contamination.

Example 3

(30) The centimeter size particles from Example 2 was further ground by using a roll crusher (MRCA-1, Makino) having zirconia rolls to achieve millimeter size particles.

Example 4

(31) The millimeter size particles from Example 3 were ground on a bead mill (PV-250, Hosokawa) using 5 mm zirconia beads to achieve micrometer size particles. Laser scattering analyzer observations (LA-950V2, Horiba) show that dry milling leads to micrometer size primary particles in the range of 0.3 m-3 m (FIG. 2).

Example 5

(32) The millimeter size particles from Example 3 were ground on a puck and ring mill (Pulverisette 9, Fritsch) using tungsten carbide liner to achieve micrometer size particles (FIG. 3).

(33) Laser scattering analyzer observations (LA-950V2, Horiba) show that dry milling leads to micrometer size primary particles in the range of 0.3 m-100 m after 300 seconds (FIG. 4).

Example 6

(34) The millimeter size particles from Example 3 were ground by using a cyclone mill (150BMW, Shizuoka plant) to achieve micrometer sized powder. Laser scattering analyzer observations (LA-950V2, Horiba) show that dry milling leads to micrometer size primary particles in the range of 0.2 m-20 m after one pass (FIG. 5).

Example 7

(35) The micrometer size powder from Example 6 was dispersed in isopropyl alcohol (IPA) solution at 10% w of solid concentration in the presence of a Triton 100X surfactant agent (0.5% wt to solid) and then ground on a bead mill (SC100/32-ZZ mill, Nippon Coke) using 0.3 mm zirconia beads to achieve nanometer size particles (FIG. 6).

(36) Laser scattering analyzer observations (LA-950V2, Horiba) show that wet milling leads to nanometer size primary particles in the range of 100 nm-1000 nm after 300 minutes (FIG. 7).

Example 8

(37) The particle dispersion from Example 7 was ground on a bead mill (MSC-100-ZZ mill, Nippon Coke) using 0.03 mm zirconia beads to achieve nanometer size particles (FIG. 8).

(38) Laser scattering analyzer observations (LA-950V2, Horiba) show that wet milling leads to nanometer size primary particles in the range of 40 nm-150 nm after 695 minutes (FIG. 9).

Example 9

(39) The experiment was conducted as outlined above in Example 7 with a difference that the micrometer size powder was dispersed in cyclohexane (instead of IPA). After wet milling, the particle size was in the range of 80 nm-180 nm after 700 minutes.

Example 10

(40) In a last step, a solution of poly(maleic anhydride-1-alt-octadecene) dissolved in IPA is mixed with the Si in IPA, in a ratio of 5% wt poly(maleic anhydride-1-alt-octadecene) over Si. The mixed solution was stirred thoroughly and then dried at room temperature by blowing with dry air while stirring.

(41) The dried powder is heated to 725 C. at 6 C./min and held for 1 h at 725 C. in a rotary kiln under argon flow, and then cooled at a cooling rate of 2 C./min. After this treatment, large aggregates of carbon coated nanoparticles having a mean size of 50-200 nm are obtained. The pyrolytic carbon content is 1.4%, as measured by a C, S analyzer (LECO method). The product thus obtained is designated by CSi.

Example 11

(42) For the electrochemical evaluation, the anodes were prepared by mixing the Si powder with carbon conductor and alginate Water-soluble binder and H.sub.2O. Thereafter, the slurry was applied to the copper foil and dried at 120 C. in vacuum for 12 h. The electrochemical characterization was performed in coin-type cell with lithium metal as anode in 1M LiPF.sub.6-EC-DEC. Three anodes composite electrodes were evaluated; dry milled Si (Example 4), nano-Si in IPA (Example 8) and nano-Si in cyclohexane (Example 9). The cells were cycled between 2.5V and 10 mV. The first discharge in FIG. 10 shows that the particle size affects the voltage profile of the cell. Also, by using a different solvent as grinding media helps to reduce further the particles size. The mean voltage was gradually increased when the particle size decreased: 35 mV for Si-dry milled (0.3-3.0 m), 78 mV for nano-Si-Cyclohexane (80-180 nm) and 92 mV for nano-Si-IPA (50-150 mV). Reducing the particle size improves performance of this material and also increases slightly the discharge voltage from zero volt, which indicates that the safety of the battery is improved.

(43) Although the present invention has been described hereinabove by way of specific embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.

(44) The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.