Powder, electrode and battery comprising such a powder

Abstract

Powder comprising particles comprising a matrix material and silicon-based domains dispersed in this matrix material, whereby the matrix material is carbon or a material that can be thermally decomposed to carbon, whereby either part of the silicon-based domains are present in the form of agglomerates of silicon-based domains whereby at least 98% of these agglomerates have a maximum size of 3 μm or less, or the silicon-based domains are not at all agglomerated into agglomerates.

Claims

1. A powder comprising particles, wherein the particles comprise a matrix material and silicon-based domains dispersed in the matrix material, wherein the matrix material comprises carbon or a material that can be thermally decomposed to carbon, wherein the particles comprise at least 90% by weight of said silicon based domains and said matrix material and either part of the silicon-based domains are present in the form of agglomerates of silicon-based domains and at least 98% of the agglomerates have a maximum size of 3 μm or less, or the silicon-based domains are not at all agglomerated into agglomerates.

2. The powder according to claim 1, wherein the matrix material comprises pitch or thermally decomposed pitch.

3. The powder according to claim 1, wherein the matrix material comprises hard carbon.

4. The powder according to claim 1, wherein at least 98% of the agglomerates of the silicon based domains have a maximum size of 2 μm or less.

5. The powder according to claim 1, wherein all agglomerates of the silicon based domains have a maximum size of 3 μm or less.

6. The powder according to claim 1, wherein the silicon-based domains are either free silicon-based domains that are not completely embedded in the matrix material or are fully embedded silicon-based domains that are completely surrounded by the matrix material, and wherein the percentage of free silicon-based domains is lower than or equal to 4 weight % of the total amount of Si in metallic or oxidized state in the composite powder.

7. The powder according to claim 6, wherein the percentage of free silicon-based domains is the percentage as determined by placing a sample of the powder in an alkaline solution for a specified time, determining the volume of hydrogen that has evolved after the specified time, calculating the amount of silicon needed for evolving this amount of hydrogen based on a production of two moles of hydrogen for every mole of silicon reacted and dividing this by the total amount of Si in metallic or oxidised state present in the sample.

8. The powder according to claim 1, wherein the silicon-based domains have a mass-based average diameter d50 which is less than 500 nm.

9. The powder according to claim 1, wherein the silicon-based domains are silicon-based particles.

10. The powder according to claim 1, wherein the particles of the powder contain at least 90% by weight of said silicon-based domains and said matrix material.

11. The powder according to claim 1, wherein the particles have a porosity of less than 20 volume %.

12. An electrode for an electrochemical cell comprising the powder of claim 1.

13. A battery containing the electrode of claim 12.

14. The powder according to claim 1, wherein the particles comprise only a matrix material and silicon-based domains.

15. The powder according to claim 1, wherein the matrix material is a continuous phase.

16. The powder according to claim 1, wherein the silicon content in the silicon-based domains is 80 weight percent or more.

17. The powder according to claim 1, wherein the powder has a BET value of less than 10 m.sup.2/g.

Description

EXAMPLE 1

(1) A submicron-sized silicon powder was obtained by applying a 60 kW radio frequency (RF) inductively coupled plasma (ICP), using argon as plasma gas, to which a micron-sized silicon powder precursor was injected at a rate of 220 g/h, resulting in a prevalent (i.e. in the reaction zone) temperature above 2000K. In this first process step the precursor became totally vaporized. In a second process step an argon flow was used as quench gas immediately downstream of the reaction zone in order to lower the temperature of the gas below 1600K, causing a nucleation into metallic submicron silicon powder. Finally, a passivation step was 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. The gas flow rate for both the plasma and quench gas was adjusted to obtain submicron silicon powder with an average particle diameter d.sub.50 of 80 nm and a d.sub.90 of 521 nm. In the present case 2.5 Nm.sup.3/h Ar was used for the plasma and 10 Nm.sup.3/h Ar was used as quench gas.

(2) A blend was made of 16 g of the mentioned submicron silicon powder and 32 g petroleum based pitch powder.

(3) This was heated to 450° C. under N.sub.2, so that the pitch melted, and, after a waiting period of 60 minutes, mixed for 30 minutes under high shear by means of a Cowles dissolver-type mixer operating at 1000 rpm.

(4) The mixture of submicron silicon in pitch thus obtained was cooled under N.sub.2 to room temperature and, once solidified, pulverized and sieved to give a powder with an average particle diameter d.sub.50 of 17.8 μm.

(5) A SEM microscopic evaluation was performed to determine if the silicon particles in the silicon powder were agglomerated in the resulting composite powder. No agglomerates with a size of 0.5 μm or higher were found.

(6) The oxygen content of the powder was 0.95 weight %.

(7) A SEM micrograph is shown in FIGS. 1 and 2, whereby it can be seen that the distribution of the silicon particle throughout the pitch was very homogeneous. In these pictures the white colour indicates silicon particles and the dark colour indicates pitch.

(8) Graphite (Showa Denko SCMG-AF) was added to the as-dried silicon powder/pitch blend by dry-mixing, to obtain a silicon powder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6, respectively.

(9) 10 g of the obtained mixture was fired in a quartz boat in a tube furnace continuously flushed with argon and heated to 1000° C. at a heating rate of 3° C./min. The sample was kept at 1000° C. for 2 h. The heating was turned off and the sample was allowed to cool to room temperature under argon atmosphere. The sample was removed from the quartz recipient, milled for 15 min in a coffee mill, and sieved to obtain a composite powder having an average particle diameter d.sub.50 of 13.6 μm. The oxygen content of the obtained composite powder was 0.8 weight %.

(10) A SEM analysis was done to confirm that the size of the agglomerates had not grown due to the firing step. This was confirmed: No agglomerates with a size of 0.5 μm or higher were observed. No porosity was visually observed.

(11) The specific surface area of the composite powder measured by the BET method was 1.8 m.sup.2/g

EXAMPLE 2

(12) 500 g of a submicron-sized silicon powder, obtained as in Example 1, was mixed with 1000 g of petroleum based pitch powder.

(13) In order to apply high shear, the blend was fed into a Haake process 11 extruder, equipped with a twin screw and heated to 400° C., with the screw running at a rotating speed of 150 rpm. The residence time in the extruder was 30 minutes.

(14) The obtained extrudate, with silicon well dispersed in the pitch material, was cooled down to less than 50° C. The injection port of the extruder and the container in which the extrudate was collected were shielded from ambient air by flushing with N.sub.2.

(15) A part of the obtained extrudate was pulverized in a mortar, and sieved to give a powder with an average particle diameter d.sub.50 of 15.9 μm.

(16) A SEM microscopic evaluation was performed to determine if the silicon particles in the silicon powder were agglomerated in the resulting composite powder. No agglomerates with a size of 0.5 μm or higher were found.

(17) The oxygen content of the powder was 0.98%.

(18) Graphite (Showa Denko SCMG-AF) was added to the resulting silicon powder/pitch blend by dry-mixing, to obtain a silicon powder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6, respectively.

(19) Hereafter, the obtained mixture was fired and sieved as described in Example 1.

(20) The average particle diameter d.sub.50 of the obtained powder was 14.1 μm and the oxygen content was 0.79%

(21) A SEM analysis was done to confirm that the size of the agglomerates had not grown due to the firing step. This was confirmed: No agglomerates with a size of 0.5 μm or higher were observed. No porosity was visually observed.

(22) The specific surface area of the composite powder measured by the BET method was 3.7 m.sup.2/g

COMPARATIVE EXAMPLE 1

(23) 16 g of a submicron-sized silicon powder, obtained as in Example 1, was dry-mixed with 32 g of petroleum based pitch powder.

(24) This was heated to 450° C. under N.sub.2, so that the pitch melted, and kept at this temperature for 60 minutes. No shear was applied.

(25) The mixture of submicron silicon in pitch thus obtained was cooled under N.sub.2 to room temperature and, once solidified, pulverized and sieved to give a composite powder with an average particle diameter d.sub.50 of 11.2 μm. The oxygen content of the powder was 1.21%

(26) A SEM microscopic evaluation was performed to determine if the silicon particles in the silicon powder were agglomerated in the resulting composite powder. The following results were obtained, with all results in μm:

(27) TABLE-US-00001 Maximum size d10 d50 d90 d98 d99 observed 0.7 1.8 2.9 3.6 3.8 5.0

(28) Graphite (Showa Denko SCMG-AF) was added to the resulting silicon powder/pitch blend by dry-mixing, to obtain a silicon powder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6, respectively.

(29) Hereafter, the obtained mixture was fired and sieved as described in Example 1. The average particle diameter d.sub.50 of the obtained powder was 16 μm, and the oxygen content was 0.9%

(30) The SEM microscopic evaluation of the silicon particles and agglomerates was repeated on the fired product. The following results were obtained, with all results in μm, showing that significant agglomeration of the silicon nanoparticles had occurred:

(31) TABLE-US-00002 Maximum size d10 d50 d90 d98 d99 observed 0.5 1.7 2.9 3.7 3.9 5.0

(32) As can be seen the results are similar to the results on the unfired product.

(33) SEM images showed porosity, especially between the silicon particles making up an agglomerate of silicon particles.

(34) The specific surface area measured by the BET method was 8.7 m.sup.2/g

COMPARATIVE EXAMPLE 2

(35) 16 g of a submicron-sized silicon powder, obtained as in Example 1, was mixed with 32 g of petroleum based pitch powder.

(36) Graphite (Showa Denko SCMG-AF) was added to the silicon powder/pitch blend by dry-mixing, to obtain a silicon powder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6, respectively. No melting step was applied.

(37) Hereafter, the obtained mixture was fired and sieved as described in Example 1. The average particle diameter d.sub.50 of the obtained powder was 14.3 μm, and the oxygen content was 0.9%

(38) The SEM microscopic evaluation of the silicon particles and agglomerates was repeated on the fired product. The following results were obtained, with all results in μm, showing that significant agglomeration of the silicon nanoparticles had occurred:

(39) TABLE-US-00003 Maximum size d10 d50 d90 d98 d99 observed 1.3 2.3 3.3 3.9 4.1 5.5

(40) SEM images showed porosity, especially between the silicon particles making up an agglomerate of silicon particles, but also at the interfaces between the graphite and decomposed pitch.

(41) The specific surface area of the composite powder measured by the BET method was 5.6 m.sup.2/g

(42) The electrochemical performance and free silicon level were determined on all products after firing, and is reported in table 1. The total silicon level of all these products was measured to be 10%+/−0.5%.

(43) TABLE-US-00004 d98 of silicon agglomerates d98 of silicon 1.sup.st discharge Coulombic (μm) agglomerates (μm) capacity efficiency at Product before firing after firing Free silicon (mAh/g) cycle 9 (%) Example 1 <0.5 <0.5 <0.3% 645 99.46 Example 2 <0.5 <0.5 <0.3% 646 99.51 Comparative 3.6 3.7 0.9% 610 99.32 example 1 Comparative 4.2 4.9% 590 99.15 example 2

(44) It should be noted that in the particular measurement conditions 0.3% free silicon was the detection limit. This detection limit can be reduced by the skilled person by increasing the sample size and/or by reducing the measurement limit of the evolved gas.

(45) As can be observed, the electrochemical performance of a powder is best only if both conditions are met: the absence of observable agglomerates of silicon particles and a low level of free silicon.