Method of forming silicon

1. A method of forming a particulate material comprising silicon, the method comprising steps of:

reducing a particulate starting material comprising silica-containing particles having an aspect ratio of at least 3:1 and a smallest dimension of less than 15 microns to form a particulate material comprising silicon; or

reducing a particulate starting material comprising silica-containing particles comprising a plurality of elongate structural elements, each elongate structural element having an aspect ratio of at least 3:1 and a smallest dimension of less than 15 microns to form a particulate material comprising silicon;

wherein the reduction takes place in the presence of a thermal moderator, and

wherein the particulate material comprising silicon has a BET value of less than 100 m 2 /g after removal of by-products of the reduction.

2. The method according to claim 1 , wherein the aspect ratio of the silica-containing particles or of the elongate structural elements is at least 5:1 and/or wherein the smallest dimension of the silica-containing particles or of the elongate structural elements is less than 10 microns.

3. The method according to claim 2 , wherein the smallest dimension of the silica-containing particles or of the elongate structural elements is less than 3 microns.

4. The method according to claim 1 , wherein substantially all silica of the starting material is reduced.

5. The method according to claim 1 , wherein the geometric surface area of the silicon material is substantially the same as the geometric surface area of the starting material and/or wherein the shape of the particles of the material comprising silicon is substantially the same as the shape of the starting material particles.

6. The method according to claim 1 , wherein silica at a surface of the starting material particles is reduced and silica at a core of the starting material particles is not reduced.

7. The method according to claim 6 , wherein the silica at the core is selectively removed.

8. The method according to claim 6 , wherein the silica at the core is selectively removed by exposure to HF.

9. The method according to claim 6 , wherein the particulate material comprising silicon formed following selective removal of silica is a flake having a hollow core, or a tube.

10. The method according to claim 6 , wherein the geometric surface area of the material comprising silicon is greater than the geometric surface area of the starting material.

11. The method according to claim 1 , wherein the starting material is n- or p-doped or wherein the starting material is undoped.

12. The method according to claim 1 , wherein the starting material particles have an open framework structure.

13. The method according to claim 1 , wherein the starting material is reduced at a reaction temperature of no more than 750° C.

14. The method according to claim 1 , wherein the starting material is reduced by exposure to liquid metal or metal vapour.

15. The method according to claim 14 , wherein the metal is magnesium or calcium.

16. The method according to claim 14 , wherein the thermal moderator is a salt, and wherein the thermal moderator is mixed with the starting material and a reducing agent or wherein a layer of thermal moderator is in contact with the starting material.

17. The method according to claim 1 , wherein the silica is reduced by exposure to a reducing composition that causes both reduction of the silica and doping of the silicon.

18. The method according to claim 1 , comprising a further step of coating at least part of the surface of the material comprising silicon.

19. The method according to claim 1 , wherein the starting material is formed by electrospinning silica or wherein the starting material is biogenic silica.

20. The method according to claim 1 , wherein the material comprising silicon has a BET value that is the same as or less than a BET value of the starting material.

21. The method according to claim 1 , wherein the material comprising silicon has a BET value of less than 50 m 2 /g.

22. The method according to claim 1 , wherein the starting material is no more than 95% silica by weight.

23. The method according to claim 1 , wherein the particulate material comprising silicon is treated to remove reaction by-products.

24. The method according to claim 23 , wherein the reaction by-products are removed by HCl treatment.

25. The method according to claim 1 , wherein the particulate material comprising silicon is etched to form silicon pillars extending from a silicon core of the particulate material.

26. The method according to claim 1 , wherein the particulate starting material comprises microcrystalline, nanocrystalline, or amorphous silica and wherein the material comprising silicon comprises microcrystalline or nanocrystalline silicon.

27. The method according to claim 1 , wherein the reducing takes place at a reaction temperature at which the starting material is reduced, and wherein the particulate starting material comprises silica with a softening temperature that is lower than the reaction temperature and a melting temperature that is higher than the reaction temperature.

28. The method according to claim 1 , wherein the starting material is heated to a reaction temperature of 650-725° C. at a heating rate of 5-15° C. per minute, held at the reaction temperature for 30-150 minutes and cooled down to room temperature at a rate of 5-15° C. per minute.

29. The method according to claim 1 , wherein the silica starting material comprises impurities selected from: (1) carbon in an amount of up to 10 wt %; and (2) Li, Na, Mg, Zn, Al, Ti, Ca, B or oxides thereof, wherein each metal oxide is present in an amount of no more than 20 wt %, and wherein the total amount of all metal oxides is no more than 45 wt %.

30. The method according to claim 1 , wherein the starting material is reduced at a reaction temperature of no more than 650° C.

31. The method according to claim 1 , comprising a further step of coating at least part of the surface of the material comprising silicon with silicon oxide or carbon.

32. The method according to claim 1 , wherein the starting material is no more than 90% silica by weight.

33. The method according to claim 1 , wherein the particulate material comprising silicon is etched by metal-assisted chemical etching to form silicon pillars extending from a silicon core of the particulate material.