Composite powder for use in the negative electrode of a battery and a battery comprising such a composite powder

Abstract

A composite powder for use in the negative electrode of a battery, whereby the composite powder comprises composite particles, whereby the composite particles comprise a matrix material and silicon, whereby the composite particles have a particle size distribution having a d10 and a d90, whereby over at least part of the size range from d10 to d90 the composite particles have a size-dependent silicon content. Preferably a finer fraction of the composite powder has an average particle size D1 and a silicon content S1 and a coarser fraction of the composite powder has an average particle size D2 and a silicon content S2, whereby a size dependence factor F is defined as follows F=(S2−S1)/(D2−D1), whereby the absolute value of the size dependence factor F is at least 0.04 wt % silicon/μm.

Claims

1. A composite powder for use in a negative electrode of a battery, the composite powder comprising composite particles, which comprise a matrix material and silicon, wherein the composite particles have a volumetric particle size distribution having a d10 and a d90, wherein, over at least part of the size range from d10 to d90, the composite particles have a size-dependent silicon content, with a positive correlation between the particle size and the silicon content, wherein a fine fraction of the composite powder has an average particle size D1 and a silicon content S1 and a coarse fraction of the composite powder has an average particle size D2 and a silicon content S2, and a size dependence factor F is defined as F=(S2−S1)/(D2−D1), wherein the value of the size dependence factor F is at least 0.04 wt % silicon/μm, and wherein D1 is defined as the d50-value of the volumetric particle size distribution of the fine fraction as measured by laser diffraction and D2 is defined as the d50-value of the volumetric particle size distribution of the coarse fraction as measured by laser diffraction.

2. The composite powder according to claim 1, wherein S1 and S2 are as measured by means of X-Ray fluorescence.

3. The composite powder according to claim 1, wherein the matrix material is a carbon-based matrix material.

4. The composite powder according to claim 1, wherein the composite powder has a content of carbon in the fraction other than silicon and oxygen of at least 50 wt %.

5. The composite powder according to claim 1, wherein the value of the size dependence factor F is at least 0.15 wt % silicon/μm.

6. The composite powder according to claim 1, wherein the value of the size dependence factor F is at least 0.30 wt % silicon/μm.

7. The composite powder according to claim 1, wherein the composite powder has an average silicon content A, wherein 5.0wt %<A<60wt %.

8. The composite powder according to claim 7, wherein 7.5wt %<A<50wt %.

9. The composite powder according to claim 1, wherein the composite powder has an oxygen content and an average silicon content A expressed as wt %, wherein the oxygen content expressed in wt % is less than 33% of A.

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

11. The composite powder according to claim 1, wherein the composite powder comprises at least 90% by weight of said composite particles.

12. The composite powder according to claim 1, wherein the composite powder also contains graphite.

13. The composite powder according to claim 1, wherein the silicon is present as silicon-based particles embedded in the matrix material.

14. The composite powder according to claim 1, wherein the silicon-based particles have a chemical composition having at least 70% by weight of silicon.

15. A battery comprising the composite powder of claim 1.

Description

EXPERIMENTAL PREPARATION OF COUNTEREXAMPLES AND EXAMPLES

Counterexample 1

Not According to the Invention

(1) A silicon nano 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 circa 50 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 of 18 Nm.sup.3/h 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 N2/O2 mixture containing 1 mole % oxygen.

(2) The oxygen content of the obtained silicon Nano powder was measured and was 8.7 wt %.

(3) The particle size distribution of the silicon nano powders was determined to be: d10=43nm, d50=86 nm, d90=128 nm, d95=139 nm, d99=177 nm. The percentage of particles larger than two times d50 was 1.4%.

(4) In order to produce a composite powder, a blend was made of 16 g of the mentioned silicon nano powder and 32 g petroleum-based pitch powder.

(5) This was heated to 450° C. under N2, 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.

(6) The mixture of silicon nano powder in pitch thus obtained was cooled under N.sub.2 to room temperature and, once solidified, pulverized and sieved on a 400 mesh sieve, to produce an intermediate composite powder.

(7) 16 g of the intermediate composite powder was mixed with 24.6 g graphite for 3 hrs on a roller bench, after which the obtained mixture was passed through a mill to de-agglomerate it. At these conditions good mixing is obtained but the graphite does not become embedded in the pitch.

(8) A thermal after-treatment was given to the obtained mixture of silicon, pitch and graphite as follows: the product was put in a quartz crucible in a tube furnace, heated up at a heating rate of 3° C./min to 1000° C. and kept at that temperature for two hours and then cooled. All this was performed under argon atmosphere.

(9) The fired product was ball-milled for 1 hr at 200 rpm with alumina balls and sieved over a 40 micrometer sieve to form a final composite powder, further called composite powder CE 1.

(10) The total Si content in composite powder CE 1 was measured to be 14.7 wt % by XRF, having an experimental error of +/−0.3 wt %. This corresponds to a calculated value based on a weight loss of the pitch upon heating of circa 40 wt % and an insignificant weight loss upon heating of the other components. The particle size distribution of composite powder CE 1 was measured and is reported in table 1. The oxygen content of the composite powder CE 1 was measured to be 1.8 wt %.

(11) A sample of composite powder CE 1 was sieved over sieves of 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 25 μm, 30 μm and 40 μm. The particle size distributions and silicon and oxygen contents of the various size fractions were determined and are reported in table 1.

(12) The sieved size fractions were not further used. Further experiments were done on composite powder CE 1.

(13) TABLE-US-00001 TABLE 1 Si Content O content d10 d50 d90 (wt %) (wt %) (μm) (μm) (μm) Composite powder CE 1 14.7 1.8  5 14.2 40.8 Fraction < 8 μm 14.9 1.9  2.6  5.8  9.1 Fraction 8 μm-10 μm 14.8 1.9  3.6  7.2 11.4 Fraction 10 μm-12 μm 14.7 1.9  4.3  8.1 13.1 Fraction 12 μm-15 μm 14.7 1.8  6.0 10.5 16.2 Fraction 15 μm-20 μm 14.8 1.8  8.1 14.1 22.7 Fraction 20 μm-25 μm 14.6 1.7 11.2 16.9 28.3 Fraction 25 μm-30 μm 14.8 1.8 14.4 21.2 33.9 Fraction 30 μm-40 μm 14.6 1.7 20.1 28.5 45.3

(14) As stated, the experimental error of the measurement of the silicon content was 0.3%. It can therefore be seen that, within the experimental error margin, the silicon content of the various size fractions is the same and there is no size dependency of the silicon content, in other words the size dependence factors F and G are 0.

Example 1

According to the Invention

(15) In order to produce a composite powder according to the invention, eight separate composite powders were produced, analogously to the counterexample 1. These eight composite powders differed from composite powder CE 1 in that the silicon contents were different. This was done by adapting the ratio of the aforementioned intermediate composite powder and graphite.

(16) Composite powders with silicon contents of 8.2 wt %, 9.8 wt %, 11.4 wt %, 13.3 wt %, 14.9 wt %, 16.1 wt %, 18.2 wt % and 20.1 wt % were produced.

(17) Each of these composite powders were sieved over the sieves as mentioned in relation to CE 1, so that eight size ranges were obtained for each of the eight composite powders, so in total 64 different powders.

(18) Then, a mixture was made of several of these 64 different powders, as detailed in table 2, to obtain composite powder E 1, having the same silicon content as composite powder CE 1, so 14.7 wt % and having 1.9 wt % oxygen.

(19) TABLE-US-00002 TABLE 2 Constituent powders used for making E 1 size fraction which weight percentage which Silicon content of was used of this was used of this size constituent powder constituent powder fraction in the final mixture  8.2 wt % Si Fraction < 8 μm  3.0%  9.8 wt % Si Fraction 8 μm-10 μm  6.0% 11.4 wt % Si Fraction 10 μm-12 μm 10.0% 13.3 wt % Si Fraction 12 μm-15 μm 16.0% 14.9 wt % Si Fraction 15 μm-20 μm 26.0% 16.1 wt % Si Fraction 20 μm-25 μm 21.0% 18.2 wt % Si Fraction 25 μm-30 μm 15.0% 20.1 wt % Si Fraction 30 μm-40 μm  3.0%

(20) The particle size distributions of the composite powder E 1 and of the size fractions used are given in table 3.

(21) TABLE-US-00003 TABLE 3 Size distribution of E 1 and of the constituent powder fractions used for making it d10 d50 d90 Powder size fraction (μm) (μm) (μm)  8.2 wt % Si Fraction < 8 μm  2.8  5.9  9.2  9.8 wt % Si Fraction 8 μm-10 μm  3.8  7.0 11.5 11.4 wt % Si Fraction 10 μm-12 μm  4.5  8.3 13.1 13.3 wt % Si Fraction 12 μm-15 μm  6.2 10.5 16.1 14.9 wt % Si Fraction 15 μm-20 μm  8.1 14.2 22.9 16.1 wt % Si Fraction 20 μm-25 μm 11.0 17.8 28.1 18.2 wt % Si Fraction 25 μm-30 μm 14.7 21.2 34.2 20.1 wt % Si Fraction 30 μm-40 μm 20.3 28.7 45.8 Comμosite powder E 1 total  5.1 14.4 39.5

(22) As an example, a first size dependence factor F indicating the dependence of the silicon content on the particle size, calculated based on the coarsest and the finest fraction of E 1, can be determined to be (20.1−8.2)/(28.7−5.9)=0.52 wt % silicon/μm.

(23) As a further example, a second size dependence factor G indicating the dependence of the silicon content on the particle size, calculated based on the coarsest and the finest fraction of E 1, can be determined to be ((20.1−8.2)/14.7)/((28.7−5.9)/14.4)=0.51.

(24) Analogously, first and second size dependence factors F and G can be calculated for each combination of the other size fractions.

(25) If desired, the dependence of the silicon content on the particle size of composite powder E 1 can be calculated from the size distributions of the constituent powder fractions of composite powder E 1, under the assumption that within each individual constituent powder fraction the silicon content does not depend on the particle size. The correctness of this assumption is demonstrated by CE 1.

Counterexamples 2 and 3

Not According to the Invention

(26) Analogously to composite powder CE 1, two further counter-example composite powders were made having different silicon contents, by adapting the ratio of the aforementioned intermediate composite powder and graphite.

(27) This concerned composite powder CE 2, having properties as detailed in table 4, and composite powder CE 3, having properties as detailed in table 5.

(28) TABLE-US-00004 TABLE 4 Si Content O content d10 d50 d90 (wt %) (wt %) (μm) (μm) (μm) Composite powder CE 2 24.9 3.0  5.8 14.7 42.4 Fraction < 8 μm 25.0 3.1  2.5  6.0  9.1 Fraction 8 μm-10 μm 24.9 3.1  3.8  7.5 11.8 Fraction 10 μm-12 μm 25.1 3.0  4.4  9.1 13.7 Fraction 12 μm-15 μm 24.8 3.0  6.2 10.7 16.1 Fraction 15 μm-20 μm 24.9 3.0  7.9 14.0 22.6 Fraction 20 μm-25 μm 24.7 2.9 11.2 17.7 28.4 Fraction 25 μm-30 μm 24.8 2.9 14.6 21.0 33.7 Fraction 30 μm-40 μm 24.8 2.8 20.3 28.6 45.2

(29) TABLE-US-00005 TABLE 5 Si Content O content d10 d50 d90 (wt %) (wt %) (μm) (μm) (μm) Composite powder CE 3 35.2 5.1  5.5 14.6 41.8 Fraction < 8 μm 35.3 5.2  2.3  6.1  9.4 Fraction 8 μm-10 μm 35.1 5.2  3.6  7.6 11.7 Fraction 10 μm-12 μm 35.0 5.2  4.4  9.4 14.0 Fraction 12 μm-15 μm 35.0 5.1  6.2 10.4 15.9 Fraction 15 μm-20 μm 35.2 5.1  8.0 14.3 22.2 Fraction 20 μm-25 μm 35.2 5.0 11.5 17.0 28.6 Fraction 25 μm-30 μm 35.3 4.9 14.5 21.6 34.2 Fraction 30 μm-40 μm 35.1 4.9 20.0 28.6 45.1

Examples 2 and 3

According to the Invention

(30) Analogously to composite powder E 1, two further example composite powders were made having silicon contents matching composite powders CE 2 and CE 3.

(31) This concerned composite powder E 2, having a silicon content of 24.9 wt % and an oxygen content of 3.1 wt % and further properties as detailed in tables 6 and 7 and composite powder E 3, having a silicon content of 35.2 wt % and an oxygen content of 5.0 wt %, and further having properties as detailed in tables 8 and 9.

(32) TABLE-US-00006 TABLE 6 Constituent powders used for making E 2 size fraction which weight percentage which Silicon content of was used of this was used of this size constituent powder constituent powder fraction in the final mixture 18.6 wt % Si Fraction < 8 μm  4.0% 20.2 wt % Si Fraction 8 μm-10 μm  8.0% 22.1 wt % Si Fraction 10 μm-12 μm 12.0% 24.1 wt % Si Fraction 12 μm-15 μm 18.0% 25.2 wt % Si Fraction 15 μm-20 μm 24.0% 26.9 wt % Si Fraction 20 μm-25 μm 19.0% 29.1 wt % Si Fraction 25 μm-30 μm 13.0% 30.5 wt % Si Fraction 30 μm-40 μm  2.0%

(33) TABLE-US-00007 TABLE 7 Size distribution of E 2 and of the constituent powder fractions used for making it d10 d50 d90 Powder Size fraction (μm) (μm) (μm) 18.6 wt % Si Fraction < 8 μm  2.7  6.2  9.3 20.2 wt % Si Fraction 8 μm-10 μm  3.8  7.6 12.1 22.1 wt % Si Fraction l0 μm-12 μm  4.5  8.9 13.8 24.1 wt % Si Fraction 12 μm-15 μm  6.4 11.0 16.3 25.2 wt % Si Fraction 15 μm-20 μm  8.0 14.2 22.9 26.9 wt % Si Fraction 20 μm-25 μm 11.2 17.5 28.6 29.1 wt % Si Fraction 25 μm-30 μm 14.5 21.1 34.2 40.5 wt % Si Fraction 30 μm-40 μm 20.0 28.6 44.8 Composite Total  5.9 14.5 41.9 powder CE 2

(34) TABLE-US-00008 TABLE 8 size fraction which weight percentage which Silicon content of was used of this was used of this size constituent powder constituent powder fraction in the final mixture 27.9 wt % Si Fraction < 8 μm  4.0% 29.0 wt % Si Fraction 8 μm-10 μm  7.0% 30.9 wt % Si Fraction 10 μm-12 μm 10.0% 32.9 wt % Si Fraction 12 μm-15 μm 15.0% 35.7 wt % Si Fraction 15 μm-20 μm 26.0% 37.5 wt % Si Fraction 20 μm-25 μm 21.0% 39.8 wt % Si Fraction 25 μm-30 μm 13.0% 41.4 wt % Si Fraction 30 μm-40 μm  4.0%

(35) TABLE-US-00009 TABLE 9 Size distribution of E 3 and of the constituent powder fractions used for making it d10 d50 d90 Powder Size fraction (um) (um) (um) 27.9 wt % Si Fraction < 8 μm  2.2  5.9  9.4 29.0 wt % Si Fraction 8 μm-10 μm  3.5  7.8 11.2 30.9 wt % Si Fraction 10 μm-12 μm  4.9  9.0 14.3 32.9 wt % Si Fraction 12 μm-15 μm  6.4 10.7 16.8 35.7 wt % Si Fraction 15 μm-20 μm  8.4 14.1 22.0 37.5 wt % Si Fraction 20 μm-25 μm 12.0 17.5 28.4 39.8 wt % Si Fraction 25 μm-30 μm 14.5 21.5 33.9 41.4 wt % Si Fraction 30 μm-40 μm 20.4 28.4 44.1 Composite Total  5.4 14.4 41.3 powder E 3

(36) With respect to E 2, As an example, a first size dependence factor F indicating the dependence of the silicon content on the particle size, calculated based on the fraction 20 μm-25 μm and the fraction 10 μm-12 μm of E 2, can be determined to be (26.9−22.1)/(17.5−8.9)=0.56 wt % silicon/μm.

(37) As a further example, a second size dependence factor G indicating the dependence of the silicon content on the particle size, calculated based on same fractions of E 2, can be determined to be ((26.9−22.1)/24.9)/((17.5−8.9)/14.5)=0.33.

(38) Similarly, with respect to E 3, as an example, a first size dependence factor F indicating the dependence of the silicon content on the particle size, calculated based on the fraction 25 μm-30 μm and the fraction 12 μm-15 μm of E3, can be determined to be (39.8−32.9)/(21.5−10.7)=0.64 wt % silicon/μm.

(39) As a further example, a second size dependence factor G indicating the dependence of the silicon content on the particle size, calculated based on same fractions of E 3, can be determined to be ((39.8−32.9)/35.2)/((21.5−10.7)/14.4)=0.26

(40) Analogously, first and second size dependence factors F and G can be calculated for each combination of the other size fractions.

(41) Analysis of Counterexamples 1,2 and 3 and Examples 1,2 and 3

(42) A BET surface area was determined for all composite powders produced in counterexamples 1,2 and 3 and examples 1,2 and 3. The values are reported in table 10.

(43) TABLE-US-00010 TABLE 10 Powder BET (m.sup.2/g) CE 1 3.5 CE 2 3.8 CE 3 3.7 E l 3.6 E 2 3.8 E 3 3.9

(44) All composite powders CE 1, CE 2, CE 3. E 1, E 2 and E 3 were embedded in resin and observed by SEM. No porosity could be observed in any of these composite powders, which is equivalent to the composite powders having 0% porosity as measured by this method. As the observance of less than 5 volume % porosity by this method is difficult, the actual porosity is estimated to be between 0 and 5 volume %.

(45) The electrochemical performance of the composite powder CE 1, CE 2, CE 3. E 1, E 2 and E 3 was measured. The results are shown in table 11, below.

(46) TABLE-US-00011 TABLE 11 electrochemical performance of the composite powders CE 1, CE 2, CE 3. E 1, E 2 and E 3 Total Capacity Coulombic efficiency, Composite powder mAh/gcomposite average of cycles 5 to 50 (%) CE 1  734 99.50 E 1  736 99.76 CE 2  992 99.39 E 2  996 99.68 CE 3 1269 98.83 E 3 1266 99.22

(47) It can be seen that for all silicon contents (circa 15 wt %, circa 25wt % and circa 35 wt %), the composite powder according to the invention performs significantly better than the composite powder not according to the invention.

Counterexample 4

Not According to the Invention

(48) The same silicon nanopowder as used in counterexample 1 was used.

(49) In order to produce a composite powder, a blend was made of 15 g of the mentioned silicon nano powder and 75 g of petroleum based pitch powder.

(50) This was heated to 450° C. under N2, 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.

(51) The mixture of silicon nano powder in pitch thus obtained was cooled under N.sub.2 to room temperature and, once solidified, pulverized and sieved on a 400 mesh sieve, to produce an unfired composite powder.

(52) A thermal after-treatment was given to the unfired composite powder as follows: the unfired composite powder was put in a thin layer in quartz crucibles in a tube furnace, heated up at a heating rate of 3° C./min to 1000° C. and kept at that temperature for two hours and then cooled. All this was performed under argon atmosphere.

(53) The fired product was ball-milled for 2 hrs at 400 rpm with alumina balls and sieved over a 40 micrometer sieve to form a final composite powder, further called composite powder CE 4.

(54) The total Si content in composite powder CE 4 was measured to be 24.8 wt % by XRF, having an experimental error of +/−0.3 wt %. This corresponds to a calculated value based on a weight loss of the pitch upon heating of circa 40 wt %.

(55) The particle size distribution of composite powder CE 4 was measured and is reported in table 12. The oxygen content of the composite powder CE 4 was measured to be 3.1 wt %.

(56) A sample of composite powder CE 4 was sieved over sieves of 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 25 μm, 30 μm and 40 μm. The particle size distributions and silicon and oxygen contents of the various size fractions were determined and are reported in table 1.

(57) The sieved size fractions were not further used. Further experiments were done on composite powder CE 4.

(58) TABLE-US-00012 TABLE 12 Si Content O content d10 d50 d90 (wt %) (wt %) (μm) (μm) (μm) Composite powder CE 4 24.8 3.1  7.0 16.3 44.9 Fraction < 8 μm 24.9 3.1  3.0  5.8  9.1 Fraction 8 μm-10 μm 24.8 3.2  3.6  7.2 11.4 Fraction 10 μm-12 μm 24.6 3.0  4.5  8.1 13.9 Fraction 12 μm-15 μm 25.1 3.0  6.3 10.9 17.0 Fraction 15 μm-20 μm 24.9 3.1  8.0 14.3 21.9 Fraction 20 μm-25 μm 24.8 3.0 11.5 17.1 27.3 Fraction 25 μm-30 μm 24.8 3.1 14.8 22.2 34.5 Fraction 30 μm-40 μm 24.7 3.1 22.1 29.3 47.0

(59) The experimental error of the measurement of the silicon content was 0.3%. It can therefore be seen that, within the experimental error margin, the silicon content of the various size fractions is the same and there is no size dependency of the silicon content.

Example 4

According to the Invention

(60) In order to produce a composite powder according to the invention, eight separate composite powders were produced, analogously to the counterexample 4. These eight composite powders differed from composite powder CE 4 in that the silicon contents were different. This was done by adapting the ratio of pitch and nano silicon powder.

(61) Composite powders with silicon contents of 18.4 wt %, 19.7 wt %, 20.9 wt %, 22.8 wt %, 24.7 wt %, 26.4 wt %, 28.7 wt % and 30.6 wt % were produced.

(62) Each of these composite powders were sieved over the sieves as mentioned in relation to CE 4, so that eight size ranges were obtained for each of the eight composite powders, so in total 64 different powders were obtained.

(63) Then, a mixture was made of several of these 64 different powders, as detailed in table 13, to obtain composite powder E 4, having the same silicon content as composite powder CE 4, so 24.8 wt % and having 3.1 wt % oxygen.

(64) TABLE-US-00013 TABLE 13 Constituent powders used for making E 4 size fraction which weight percentage which Silicon content of was used of this was used of this size constituent powder constituent powder fraction in the final mixture 18.4 wt % Si Fraction < 8 μm  3.0 % 19.7 wt % Si Fraction 8-10 μm  6.0 % 20.9 wt % Si Fraction 10-12 μm 11.0 % 22.8 wt % Si Fraction 12-15 μm 15.0 % 24.7 wt % Si Fraction 15-20 μm 24.0 % 26.4 wt % Si Fraction 20-25 μm 21.0 % 28.7 wt % Si Fraction 25-30 μm 14.0 % 30.6 wt % Si Fraction 30-40 μm  6.0 %

(65) The particle size distributions of the composite powder E 4 and of the size fractions used are given in table 14.

(66) TABLE-US-00014 TABLE 14 Size distribution of E 4 and of the constituent powder fractions used for making it d10 d50 d90 Powder size fraction (μm) (μm) (μm) 18.4 wt % Si Fraction < 8 μm  3.2  6.0  9.0 19.7 wt % Si Fraction 8-10 μm  3.4  7.2 11.5 20.9 wt % Si Fraction 10-12  4.3  9.3 14.1 22.8 wt % Si Fraction 12-15  6.2 11.0 16.9 24.7 wt % Si Fraction 15-20 μm  8.1 14.3 21.8 26.4 wt % Si Fraction 20-25 μm 11.6 17.2 27.5 28.7 wt % Si Fraction 25-30 μm 14.8 22.4 34.7 30.6 wt % Si Fraction 30-40 μm 21.9 29.5 47.1 Comμosite powder E 4 total  6.9 16.1 44.8

(67) A first size dependence factor F indicating the dependence of the silicon content on the particle size, calculated based on the coarsest fraction and the fraction 8-10 μm can be determined to be (30.6−19.7)/(29.5−7.2)=0.49wt % silicon/μm.

(68) A second size dependence factor G indicating the dependence of the silicon content on the particle size, calculated based on the coarsest fraction and the fraction 8-10 μm, can be determined to be ((30.6−19.7)/24.8)/((29.5−7.2)/16.1)=0.32.

(69) Analogously, first and second size dependence factors F and G can be calculated for each combination of other fractions.

(70) Analysis of Counterexample 4 and Example 4

(71) A BET surface area was determined for the samples E 4 and CE 4. The values were 7.4 and 7.6 m.sup.2/g respectively.

(72) Composite powders CE 4 and E 4 were embedded in resin and observed by SEM. No porosity could be observed in any of these composite powders

(73) The electrochemical performance of the composite powders CE 4 and E 4 was measured. The results are shown in table 15, below.

(74) TABLE-US-00015 TABLE 15 Electrochemical performance of the composite powders CE 4 and E 4 Total Capacity Coulombic efficiency, average Composite powder mAh/g composite of cycles 5 to 50 (%) CE 4 932 99.09 E 4 935 99.34

(75) As already seen previously for the composites CE 1, CE 2, CE 3, E 1, E 2 and E 3, the composite powder E 4 according to the invention performs significantly better than the composite powder CE 4 not according to the invention.