Silicon-carbon composite powder

Abstract

A silicon-carbon composite powder having Si and C distributed throughout each particle is provided. The weight ratio of carbon to silicon on the surface of a particle (C/Si).sub.surface is greater than the weight ratio of carbon to silicon within the total particle (C/Si).sub.total. The silicon-carbon composite powder is produced by simultaneously feeding into a reactor a gaseous stream of a SiH.sub.4, Si.sub.2H.sub.6, Si.sub.3H.sub.8 and/or organosilane and a gaseous stream of at least one hydrocarbon of ethylene, ethane, propane and acetylene and reacting the streams using plasma enhanced chemical vapor deposition.

Claims

1. A silicon-carbon composite powder, wherein, for each silicon-carbon composite particle contained in the powder, a) Si and C are distributed throughout the particle and b) a weight ratio of carbon to silicon on the particle surface (C/Si).sub.surface is greater than a weight ratio of carbon to silicon within the total particle (C/Si).sub.total, wherein 10≤(C/Si).sub.surface/(C/Si).sub.total≤50, and 0.3≤(C/Si).sub.total≤1, wherein the silicon-carbon composite powder has an average diameter of 300 nm or less, and the silicon-carbon composite powder does not exhibit a core shell structure.

2. The silicon-carbon composite powder according to claim 1 wherein Si and C are amorphous.

3. The silicon-carbon composite powder according to claim 1, obtained by a process comprising: simultaneously feeding a first gaseous stream of SiH.sub.4, Si.sub.2H.sub.6, Si.sub.3H.sub.8 and/or an organosilane and a second gas sous stream of at least one hydrocarbon selected from the group consisting of ethylene, ethane, propane and acetylene into a reactor, and reacting the first and second gaseous streams in the reactor using plasma enhanced chemical vapor deposition.

4. The silicon-carbon composite powder according to claim 3, wherein the first gaseous stream is a stream of SiH.sub.4 and/or an organosilane, a ratio of the at least one hydrocarbon to SiH.sub.4 and/or the organosilane is 0.01-3, and the at least one hydrocarbon is selected from the group consisting of ethylene, ethane and propane.

5. The silicon-carbon composite powder according to claim 4, wherein the first gaseous stream is a stream of at least one organosilane selected from the group consisting of CH.sub.3SiH.sub.3, (CH.sub.3).sub.2SiH.sub.2 and (CH.sub.3).sub.3SiH.

6. A process for manufacturing the silicon-carbon composite powder according to claim 1, the process comprising: simultaneously feeding a first gaseous stream of SiH.sub.4, Si.sub.2H.sub.6, Si.sub.3H.sub.8 and/or an organosilane and a second gaseous stream of at least one hydrocarbon selected from the group consisting of ethylene, ethane, propane and acetylene into a reactor, and reacting the first and second gaseous streams in the reactor using plasma enhanced chemical vapor deposition.

7. The process according to claim 6, wherein the first gaseous stream is a stream of SiH.sub.4 and/or organosilane, and a ratio of the at least one hydrocarbon to SiH.sub.4 and/or the organosilane is 0.01-3.

8. The process according to claim 6, wherein the at least one hydrocarbon is selected from the group consisting of ethylene, ethane and propane.

9. The process according to claim 6, wherein the first gaseous stream is a stream of at least one organosilane selected from the group consisting of CH.sub.3SiH.sub.3, (CH.sub.3).sub.2SiH.sub.2 and (CH.sub.3).sub.3SiH.

10. A method for manufacturing a lithium ion battery, the method comprising: introducing the silicon-carbon composite powder according to claim 1 into the lithium ion battery.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the x-ray diffraction (XRD) patterns of the silicon powder in comparative Example 5 (A), the inventive silicon-carbon composite powder (B), and the powder in comparative Example 6 (C).

(2) FIG. 2 shows the result of an electrochemical cell study for the silicon powder according to comparative Example 5.

(3) FIG. 3 shows the result of an electrochemical cell study for the inventive silicon-carbon composite powder.

DETAILED DESCRIPTION OF THE INVENTION

(4) In this silicon-carbon composite powder, the formation of Li.sub.xSi (x≥3.75) is suppressed as evidenced by voltage versus capacity measurements. Moreover, the higher C content at the surface of this silicon-carbon composite powder is advantageous in terms of prohibiting the excessive SEI growth.

(5) The inventive silicon-carbon composite powder has the surprising property that it does not exhibit a core shell structure which can be detected by transmission electron microscope (TEM).

(6) The ratio by weight of (C/Si).sub.surface on the surface of a particle and the ratio by weight of (C/Si).sub.total both are determined by energy dispersive x-ray analysis (EDX) in a transmission electron microscope (TEM).

(7) In a preferred embodiment the ratio (C/Si).sub.surface/(C/Si).sub.total counts for 3≤(C/Si).sub.surface/(C/Si).sub.total≤130. In another preferred embodiment the ratio (C/Si).sub.surface/(C/Si).sub.total counts for 10≤(C/Si).sub.surface/(C/Si).sub.total≤50.

(8) The ratio by weight of (C/Si).sub.total preferably is 0.01≤(C/Si).sub.total≤3, more preferably 0.3≤(C/Si).sub.total≤1.

(9) In contrast to the prior art in the present inventive powder Si and C are distributed throughout the particle. No core shell structure could be detected by transmission electron microscope (TEM). In addition no silicon carbide can be detected in the inventive powder.

(10) Si and C of the inventive powder may be crystalline or amorphous. For its later use as a part of a lithium ion battery it is preferred that Si and C are amorphous.

(11) The particles of the inventive powder may be in an isolated or in an aggregated form. Preferably the average particle diameter is 300 nm or below, more preferably it is 20-100 nm.

(12) The invention further provides a process for the manufacture of the silicon-carbon composite powder in which a gaseous stream of a SiH.sub.4, Si.sub.2H.sub.6, Si.sub.3H.sub.8 and/or organosilane and a gaseous stream of at least one hydrocarbon selected from the group consisting of ethylene, ethane, propane and acetylene are simultaneously fed into a reactor wherein the streams are reacted using plasma enhanced chemical vapor deposition.

(13) The ratio of hydrocarbon to SiH.sub.4 and/or organosilane may be selected over a wide range. Preferably the ratio is 0.01-3.

(14) The organosilane may be selected from the group consisting of CH.sub.3SiH.sub.3, (CH.sub.3).sub.2SiH.sub.2, (CH.sub.3).sub.3SiH, C.sub.2H.sub.5SiH.sub.3, (C.sub.2H.sub.5).sub.2SiH.sub.2, (C.sub.2H.sub.5).sub.3SiH, (CH.sub.3).sub.4Si, (C.sub.2H.sub.5).sub.4Si, C.sub.3H.sub.7SiH.sub.3, (C.sub.3H.sub.7).sub.4Si, C.sub.4H.sub.9SiH.sub.3, (C.sub.4H.sub.9).sub.2SiH.sub.2, (C.sub.4H.sub.9).sub.3SiH, (C.sub.4H.sub.9).sub.4Si, C.sub.5H.sub.11SiH.sub.3, (C.sub.5H.sub.11).sub.2SiH.sub.2, (C.sub.5H.sub.11).sub.3SiH, (C.sub.5H.sub.11).sub.4Si, C.sub.6H.sub.13SiH.sub.3, (C.sub.6H.sub.13).sub.4Si, (C.sub.7H.sub.15)SiH.sub.3, (C.sub.7H.sub.15).sub.2SiH.sub.2, (C.sub.7H.sub.15).sub.3SiH, (C.sub.7H.sub.15).sub.4Si.

(15) CH.sub.3SiH.sub.3, (CH.sub.3).sub.2SiH.sub.2 and (CH.sub.3).sub.3SiH are most preferred.

(16) Ethylene, ethane and propane are the preferred hydrocarbons. Acetylene is a more reactive starting material and care has to be taken to avoid the formation of SiC. That is to say, either using low concentrations of acetylene or varying the setup for the plasma enhanced chemical vapor deposition.

(17) The average residence time of the reaction mixture comprising the silane and the hydrocarbon may be used to influence the average particle average diameter. Reducing the residence time in the plasma usually reduces the particle diameter. Thus a residence time of 4 s yields an average particle size of 300 nm, a shorter residence time of 1 s results in an average particle size of below 100 nm.

(18) Using residence time of 0.5 s results in obtaining a powder with a particle average diameter below 50 nm.

(19) A further subject of the invention is the use of the silicon-carbon composite powder for the manufacture of lithium ion batteries.

(20) Having generally described the invention, a further understanding can be obtained by reference to certain specific examples provided herein for purposes of illustrations only and are not intended to be limiting.

EXAMPLES

Example 1 (According to the Invention)

(21) A radio frequency (RF) non-thermal plasma was applied to the chemical vapour deposition (CVD) method to produce powder materials. The setup consists of a RF plasma source, tubular quartz reactor, pump, process gas inlets, gas exhaust and powder collection chamber. A quartz tube reactor is evacuated to a base pressure of 10-100 mbar. The plasma source frequency is 13.56 MHz and the RF power is set at 50-200 W. SiH.sub.4 and ethylene diluted in Ar are injected simultaneously via mass flow controllers. The concentration of silane in the gas mixture is 20 vol. %. The ratio CH.sub.x/SiH.sub.4 is 0.16. The gas flow rate and pressure are used to adjust the residence time in the plasma.

(22) Longer residence time of 4 s results in obtaining a powder with particle average diameter below 300 nm. Shorter residence time of 1 s results in obtaining a powder with particle average diameter below 100 nm. Using residence time of 0.5 s results in obtaining a powder primary particle average diameter below 50 nm.

(23) In the obtained silicon-carbon composite powder Si and C is distributed throughout the particle. The ratio by weight of (C/Si).sub.surface on the surface of a particle is 3.5 and the ratio by weight of (C/Si).sub.total within the total particle is 1.0 as determined by energy dispersive x-ray analysis (EDX) in a transmission electron microscope (TEM). The ratio (C/Si).sub.surface/(C/Si).sub.total in the silicon-carbon composite powder is 3.5.

(24) Examples 2-4: Further embodiments are carried out using the conditions described in Example 1. In contrast to Example 1, in examples 2-4 the type of the carbon precursor gas CH.sub.x was varied according to the Table. The variation of the silicon-carbon composite powder parameters is given in the Table.

(25) Example 5 (comparison): Further embodiment is carried out using the conditions described in Example 1. In contrast to Example 1, only SiH.sub.4 is introduced into the quartz tube reactor. No carbon precursor gas is used. The obtained powder contains silicon only. The ratio by weight of (C/Si).sub.surface on the surface of a particle and the ratio by weight of (C/Si).sub.total within the total particle is hereby zero.

(26) This difference between the silicon powder in Example 5 and the silicon-carbon composite powder in Examples 1-4 is also apparent in x-ray diffraction (XRD) patterns shown in FIG. 1 (x-axis=20 (degrees); y-axis=intensity(a.u.)). The XRD pattern of the silicon powder produced in Example 5 (A) is characterized by a sharper peak near 28.4° due to nanocrystalline Si and a broader peak (amorphous halo) due to amorphous Si. According to the quantitative phase analysis, the amount of the crystalline silicon is 20 vol. %.

(27) The intensity of the amorphous halo in the XRD pattern of the silicon-carbon composite powder (B) is reduced and slightly shifted to higher diffraction angles compared to the silicon powder (A). A sharper peak near 28.4° is due to nanocrystalline Si, which is present in the amount of 4 vol. % in the silicon-carbon composite powder (B). The reduction of the amorphous halo intensity and halo shift indicate the formation of the silicon-carbon composite powder with Si and C distributed throughout the particle, i.e. the amorphous halo arises from Si and C mixture.

(28) An electrode slurry containing silicon powder prepared according to the comparative Example 5, an aqueous solution of styrol-butadiene-rubber and carbon nanotubes was prepared in a ball mill and subsequently coated onto a copper foil and dried. Discs were punched from the powder coatings and incorporated into coin cells with Li metal counter electrodes and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate electrolyte. FIG. 2 (x-axis=capacity (mAh/g); y-axis=voltage (V)) shows the result of an electrochemical cell study for the silicon powder according to comparative Example 5. A characteristic voltage plateau forming as a result of the dilithiation of the Li.sub.xSi (x≥3.75) phase is present. In contrast, in electrodes containing silicon-carbon composite powders prepared according to Example 1-4, the formation of the detrimental for the electrode stability phase, Li.sub.xSi (x≥3.75), is effectively suppressed, as confirmed by the absence of the characteristic voltage plateau in FIG. 3. In FIGS. 2 and 3 the numbers 1, 2, 3, 20, 50 represent the 1.sup.st, 2.sup.nd . . . loading cycle. “*” in FIG. 2 represents the plateau connected to the dilithiation of the Li.sub.xSi (x≥3.75) compounds.

(29) Example 6 (comparison): Further embodiment is carried out using the conditions described in Example 1. In contrast to Example 1, acetylene is introduced into the quartz tube reactor as carbon precursor gas CH.sub.x and the ratio of CH.sub.x/SiH.sub.4 is 1. The predominant reaction product of comparative Example 6 is amorphous SiC with just a small fraction of mostly crystalline Si, which is undesirable. According to the EDX analysis in TEM there is no significant difference in the C/Si ratio (cf.Table): the ratio by weight of (C/Si).sub.surface on the surface of a particle and the ratio by weight of (C/Si).sub.total within the total particle is 1.3. The amorphous halo in the XRD pattern for the material of comparative Example 6 (C) is shifted to higher diffraction angles indicative of the formation of the amorphous SiC. This type of material is less favorable for battery applications.

(30) TABLE-US-00001 TABLE Process gas parameters and powder properties for Examples 1-6 Ex- SiH.sub.4 CH.sub.x/ (C/ (C/ (C/Si).sub.surface/ ample CH.sub.x* (vol. %) SiH.sub.4 Si).sub.total Si).sub.surface (C/Si).sub.total 1 Ethylene 20 0.16 1.0 3.5 3.5 2 Ethane 20 0.16 0.7 16.8 25 3 Propane 20 0.15 0.4 49.0 125 4 Acetylene 20 0.14 0.3 9.0 32 5 — 20 0 0 0 0 6 Acetylene 10 1 1.3 1.3 1 * carbon precursor gas

(31) Numerous modification and variations on the present invention are possible in light of above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described therein.