Submicron sized silicon powder with low oxygen content

Abstract

A submicron sized Si based powder having an average primary particle size between 20 nm and 200 nm, wherein the powder has a surface layer comprising SiO.sub.x, with 0<x<2, the surface layer having an average thickness between 0.5 nm and 10 nm, and wherein the powder has a total oxygen content equal or less than 3% by weight at room temperature. The method for making the powder comprises a step where a Si precursor is vaporized in a gas stream at high temperature, after which the gas stream is quenched to obtain Si particles, and the Si particles are quenched at low temperature in an oxygen containing gas.

Claims

1. A Si powder having an average primary particle size between 20 nm and 200 nm, wherein the powder has a SiO.sub.x surface layer with 0<x<2, the surface layer having an average thickness between 0.5 nm and 10 nm, and wherein the powder has a total oxygen content equal to or less than 3% by weight at room temperature.

2. The Si powder of claim 1, wherein the surface layer has a thickness between 0.5 nm and 5 nm.

3. The Si powder of claim 1, having an oxidized SiO.sub.x surface layer with 1≤x<2.

4. The Si powder of claim 1, having a metallic purity of at least 98 wt % Si.

5. The Si powder of claim 1, having a total oxygen content less than 5% by weight after being aged for 1 hour at 700° C. under atmospheric conditions and in air.

6. The Si powder of claim 1, further comprising an element M selected from the group consisting of transition metals, metalloids, Group Ma elements and carbon.

7. A Li-ion battery comprising the Si powder of claim 1, as a negative electrode material.

8. A powder comprising particles, wherein the particles have an average primary particle size between 20 nm and 200 nm, wherein the particles comprise a silicon core having a surface and a layer at the surface of the silicon core, wherein the layer comprises SiO.sub.x with 0<x<2 and has an average thickness between 0.5 nm and 10 nm, and the powder has a total oxygen content equal to or less than 3% by weight at room temperature.

9. The powder of claim 8, wherein the layer at the surface of the silicon core consists of SiO.sub.x with 0<x<2.

10. The powder of claim 8, wherein the layer at the surface of the silicon core has a thickness between 0.5 nm and 5 nm.

11. The powder of claim 8, wherein the layer at the surface of the silicon core comprises SiO.sub.x with 1≤x<2.

12. The powder of claim 8, having a metallic purity of at least 98 wt % Si.

13. The powder of claim 8, having a total oxygen content less than 5% by weight after being aged for 1 hour at 700° C. under atmospheric conditions and in air.

14. The powder of claim 8, further comprising an element M selected from the group consisting of transition metals, metalloids, Group IIIa elements and carbon.

15. A Li-ion battery comprising the Si powder of claim 8, as a negative electrode material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: TEM images (low magnification (A) en high magnification (B)) showing the presence of a thin amorphous SiO.sub.x layer at the surface of Si submicron particles.

(2) FIG. 2: Delithiation curves for Si powders with oxygen level of 2.8 wt % (full line) and 25.0 wt % (dotted line), Voltage vs. Li (V) against Capacity (mAh/g).

(3) FIG. 3: Oxygen level (in wt %—left axis—full line) and BET value (m.sup.2/g—right axis—dotted line) of Si submicron powder as a function of aging temperature (° C.).

(4) FIG. 4: Oxygen level (in wt %) of Si submicron powder as a function of storage time (in number # of days) in air at room temperature.

(5) The invention may be practiced, for example, by way of the different examples described below.

Example 1

(6) A micron-sized Si powder is provided as Si precursor. A 60 kW radio frequency (RF) inductively coupled plasma (ICP) is applied, using an argon plasma with 2.5 Nm.sup.3/h argon gas. The solid silicon precursor is injected in the plasma at a rate of 220 g/h, resulting in a prevalent (i.e. in the reaction zone) temperature above 2000 K. In this first process step the Si precursor is totally vaporized followed by a nucleation into submicron sized Si powder. An argon flow of 10 Nm.sup.3/h is used as quench gas immediately downstream of the reaction zone in order to lower the temperature of the gas below 1600 K. In this way the metal nuclei will be formed. Finally, a passivation step is performed at a temperature of 100° C. during 5 minutes by adding 100 L/h of a N.sub.2/O.sub.2 mixture containing 0.15 mole % oxygen.

(7) The submicron sized Si powder has a cubic crystalline phase and a specific surface area of 40±2 m.sup.2/g (as measured by the BET technique), which corresponds to a mean primary particle size of about 60 nm. Chemical analysis shows that the oxygen content is 2.8 wt %, whilst TEM characterization shows the presence of a thin amorphous SiO.sub.x surface layer with a thickness of 1-2 nm, as is shown in FIG. 1.

(8) A paste is prepared by adding the obtained silicon powder to a 2% Na-CMC water-based solution. Subsequently acetylene black is added. The final paste, having a silicon/CMC/acetylene black ratio of 50/25/25, is finally ball milled for 30 minutes. Coatings with a thickness between 20 and 30 μm are deposited on a copper foil by doctor blade coating. The first drying of the paste was done using a conventional hot-air furnace but can also be done at room temperature or using a vacuum oven, a conveyer furnace, drying on a heated surface, drying with infra-red irradiation, drying with far infrared irradiation, drying with induction system, coating on a heated electrode, drying in a inert atmosphere. The drying method, temperature and sequence influence the stability of the paste, the internal stress and possible cracking in the dried electrode. Finally coin cell type batteries are prepared in a glove box using Li-foil as counter electrode. Battery tests are performed on the electrodes with the following conditions: cycling between 0.01 and 1.0 V at a rate of C/20, where C is defined as charging/discharging, at a rate of 3572 mAh/g per hour.

(9) Table 1 gives an overview of the capacity of the 1.sup.st delithiation step. The value in the Table is an average for 3 coin cells. A capacity of 3700 mAh/g silicon is measured, and a very low irreversible capacity of less than 8% is obtained after the first cycle (Table 1 & FIG. 2).

Counter Example CE 2

(10) A silicon powder is produced in the 60 kW radio frequency (RF) inductively coupled plasma (ICP) as described in Example 1. After quenching however a modified passivation step is applied at a temperature of 500° C. during 5 minutes, by adding 150 L/h of a N.sub.2/O.sub.2 mixture containing 0.15 mole % oxygen.

(11) The powder has a cubic crystalline phase and a specific surface area of 40±2 m.sup.2/g (as measured by the BET technique), which corresponds to a mean primary particle size of about 60 nm. Chemical analysis shows that the oxygen content is 6.8 wt %, whilst TEM characterization shows the presence of a thin amorphous SiO.sub.x surface layer with a thickness of 2-5 nm.

(12) A paste is prepared and coin cells are made and tested as described in Example 1. A delithiation capacity of 3500 mAh/g silicon is measured, and a irreversible capacity of 573 mAh/g (14%) is obtained after the first cycle (see Table 1), which is considered too high.

Counter Examples CE 3-4

(13) Two commercially available silicon samples were purchased, and oxygen contents of respectively 19.3 wt % (Counter example 3 obtained from Kaier, CN, with a BET value of 20 m.sup.2/g and an estimated average primary particle size of 130 nm) and 25 wt % (Counter Example 4 obtained from Aldrich, US, with a BET value of 34 m.sup.2/g and an estimated average primary particle size of 75 nm). The average thickness of the surface layer of Counter Example 3 is 15 nm (surface layer thickness and oxygen content are related to each other). A paste is prepared and coin cells are made and tested as described in Example 1. This results in low delithiation capacities of respectively 2800 and 1500 mAh/g silicon (see Table 1). Furthermore, high irreversible capacity values of 600 mAh/g (17%)(CExample 3) and 644 mAh/g (30%)(CExample 4) are obtained after the first cycle, which is higher than for Example 1.

(14) TABLE-US-00001 TABLE 1 Overview of coin cell testing results Delithiation First First Oxygen capacity first irreversible irreversible Example content cycle capacity capacity number (wt %) (mAh/g) (mAh/g) (%) 1 2.8 3700 305 7.6 CE 2 6.8 3500 573 14.1 CE 3 19.3 2800 600 17.6 CE 4 25.0 1500 644 30.0

(15) FIG. 2 shows the capacity (mAh/g) of the silicon in the electrodes of the coin cells of Example 1 and Counter example 4 for the first cycle.

Example 5

(16) The stability of the powder as function of time and temperature is checked in stability experiments. The powder obtained in Example 1 is annealed in air at different temperatures for 1 hour and the oxygen content of the resulting powders is measured by chemical analysis. It is illustrated in FIG. 3 that the oxygen level remains stable in air up to 700° C., after which a drastic increase up to 50 wt % oxygen takes place. In FIG. 3 the oxygen level (full line) is to the left in wt %, whilst the corresponding BET value (in m.sup.2/g—dotted line) is shown to the right, both as a function of the temperature in C.

(17) At room temperature, no significant increase of the oxygen level as a function of time is observed, as is illustrated in FIG. 4, where the oxygen level (in wt %) is shown against the time in number of days.

(18) While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.