Electroactive materials for metal-ion batteries

1. A method for preparing a particulate material consisting of a plurality of composite particles that comprise a plurality of silicon nanoparticles dispersed within a conductive pyrolytic carbon matrix, the method comprising the steps of:

(a) milling a silicon starting material in the presence of a non-aqueous solvent to obtain a dispersion of silicon-containing nanoparticles having a D 50 particle diameter in the range of 30 to 500 nm in the solvent;

(b) contacting the dispersion of silicon nanoparticles in the solvent with a pyrolytic carbon precursor comprising one or more compounds comprising at least one oxygen or nitrogen atom;

(c) removing the solvent to provide silicon nanoparticles coated with the pyrolytic carbon precursor;

(d) heating the coated silicon nanoparticles to a temperature in a range of 100 to 400° C. for a period of time before step (e); and

(e) pyrolysing the coated silicon nanoparticles at a pyrolysis temperature in a range of from 600 to 1200° C. to form said plurality of composite particles that comprise a plurality of silicon nanoparticles dispersed within a conductive pyrolytic carbon matrix.

2. A method according to claim 1 , wherein the particulate material prepared by the method consists of a plurality of composite particles, wherein the composite particles comprise a plurality of silicon nanoparticles dispersed within a conductive carbon matrix, wherein:

the silicon nanoparticles comprise a nanoparticle core and a nanoparticle surface, wherein the nanoparticle surface comprises a compound of oxygen or a compound of nitrogen or a mixture thereof disposed between the nanoparticle core and the conductive carbon matrix;

the particulate material comprises 40 to 65 wt % silicon;

the particulate material comprises at least 6 wt % and less than 20 wt % oxygen;

a weight ratio of a total amount of oxygen and nitrogen to silicon in the particulate material is in a range of from 0.1 to 0.45; and

the weight ratio of carbon to silicon in the particulate material is in a range of from 0.1 to 1.

3. A method according to claim 1 , wherein the temperature to which the coated silicon nanoparticles are heated in step (d) is in a range of 200 to 400° C.

4. A method according to claim 1 , wherein the period of time for which the coated silicon nanoparticles are heated in step (d) is from 5 minutes to 10 hours.

5. A method according to claim 1 , wherein step (d) is carried out in presence of oxygen gas.

6. A method according to claim 1 , wherein step (d) is carried out in presence of air.

7. A method according to claim 1 , wherein the pyrolytic carbon precursor comprises a carbon-containing compound comprising one or more electrophilic functional groups.

8. A method according to claim 7 , wherein the pyrolytic carbon precursor is polyvinylpyrrolidone (PVP) or a copolymer of vinylpyrrolidone with one or more other ethylenically unsaturated monomers.

9. A method according to claim 7 , wherein step (d) comprises crosslinking the silicon nanoparticles and the pyrolytic precursor by a reaction between nucleophilic functional groups on a surface of the silicon nanoparticles and the one or more electrophilic functional groups of the pyrolytic carbon precursor.

10. A method according to claim 9 , wherein the pyrolytic carbon precursor is polyvinylpyrrolidone (PVP) or a copolymer of vinylpyrrolidone with one or more other ethylenically unsaturated monomers.

11. A method according to claim 7 , wherein step (d) comprises crosslinking the silicon nanoparticles and the pyrolytic carbon precursor under conditions such that a temperature increase in a reaction mixture comprising the silicon nanoparticles and the pyrolytic carbon precursor is controlled to no more than 5° C./min and a maximum temperature of the reaction mixture is maintained below 270° C. for the duration of the crosslinking reaction.

12. A method according to claim 7 , wherein step (d) comprises mixing or agitating the coated silicon nanoparticles so as to ensure a homogenous reaction temperature during the crosslinking step.

13. A method according to claim 12 , wherein the pyrolytic carbon precursor is polyvinylpyrrolidone (PVP) or a copolymer of vinylpyrrolidone with one or more other ethylenically unsaturated monomers.

14. A method according to claim 7 , wherein step (d) further comprises maintaining the coated silicon nanoparticles at a temperature in the range of from 100 to 400° C. for a period of time after completion of the crosslinking reaction.

15. A method according to claim 14 , wherein the pyrolytic carbon precursor is polyvinylpyrrolidone (PVP) or a copolymer of vinylpyrrolidone with one or more other ethylenically unsaturated monomers.

16. A method according to claim 1 , wherein the solvent is selected from the group consisting of alcohols and ketones.

17. A method according to claim 1 , wherein the solvent is isopropyl alcohol.

18. A method according to claim 1 , wherein the solvent is removed in step (c) by rotary evaporation or by spray drying.

19. A method according to claim 1 , further comprising the step of (f) reducing a size of the composite particles from step (e).

20. A method according to claim 1 , further comprising the step of (f) sieving the composite particles from step (e).

21. A method according to claim 1 , further comprising the step of (f) coating the composite particles from step (e) with a carbon coating.

22. A method according to claim 1 , wherein the particulate material comprises 40 to 65 wt % silicon.

23. A method according to claim 1 , wherein a weight ratio of a total amount of oxygen and nitrogen to silicon in the particulate material is in a range of from 0.1 to 0.45.

24. A method according to claim 1 , wherein a weight ratio of a total amount of oxygen and nitrogen to silicon in the particulate material is at least 0.2.

25. A method according to claim 1 , wherein step (d) comprises heating the coated silicon nanoparticles at a temperature in the range of 100° C. to below 200° C. for a first period of time and then heating the coated silicon nanoparticles at temperature in the range of 200 to 400° C. for a second period of time.

26. A method according to claim 25 , wherein the second period of time is from 5 minutes to 10 hours.