Silicon anode materials

1. A method of making a silicon anode material for an electrochemical cell that cycles lithium, the method comprising:

forming a plurality of precursor clusters, each precursor cluster comprising a volume of SiO x nanoparticles (where x≤2) and silicon nanoparticles, wherein for each precursor cluster a volume ratio between the silicon nanoparticles and the SiO x nanoparticles ranges from about 0.5 to about 5;

carbon coating each of the precursor clusters to form a plurality of carbon-coated SiO x clusters; and

reducing the SiO x nanoparticles in each of the carbon-coated SiO x clusters to form the silicon anode material comprising a plurality of carbon-encased silicon clusters, each carbon-encased silicon cluster comprising a volume of silicon nanoparticles encased in a carbon shell having an interior volume greater than the volume of the silicon nanoparticles.

2. The method of claim 1 , wherein the volume of silicon nanoparticles in each carbon-encased silicon cluster is less than the volume of SiO x nanoparticles in the precursor cluster from which the silicon nanoparticles are formed during the reducing.

3. The method of claim 1 , wherein forming the precursor clusters comprises spray drying a solution comprising the SiO x nanoparticles and polyethylene glycol (PEG).

4. The method of claim 3 , wherein each SiO x nanoparticle has a particle diameter ranging from about 10 nm to about 500 nm.

5. The method of claim 1 , wherein each precursor cluster comprises a volume of SiO x nanoparticles ranging from about 5% to about 75% and a volume of silicon nanoparticles ranging from about 25% to about 90%.

6. The method of claim 1 , wherein carbon coating each of the precursor clusters comprises:

coating the precursor cluster with a slurry comprising a carbon precursor and a solvent; and

heating the slurry coating to a temperature ranging from about 300° C. to about 800° C. for a time ranging from about 5 minutes to about 300 minutes.

7. The method of claim 6 , wherein the carbon precursor is select from the group consisting of: polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), an alginate, and combinations thereof; and

wherein the solvent is selected from the group consisting of: water, n-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and combinations thereof.

8. The method of claim 1 , wherein reducing the SiO x nanoparticles comprises magnesium vapor reduction or dihydrogen reduction at a temperature ranging from about 650° C. to about 1000° C.

9. The method of claim 8 , wherein each silicon nanoparticle has a particle diameter ranging from about 2 nm to about 200 nm.

10. The method of claim 1 , wherein the carbon-encased silicon cluster comprises from about 5 wt % to about 25 wt % of the silicon nanoparticles and from about 75 wt % to about 95 wt % of the carbon shell.

11. The method of claim 1 , wherein the carbon shell has a thickness ranging from about 10 nm to about 500 nm and a surface area ranging from about 5 m 2 /g to about 500 m 2 /g.

12. The method of claim 1 , wherein the silicon anode material is substantially free of the SiO x nanoparticles.

13. The method of claim 1 , wherein the silicon anode material has a packing density within the electrochemical cell ranging from about 5 vol. % to about 60 vol. %.

14. A method of making a silicon anode material for an electrochemical cell that cycles lithium, the method comprising:

forming a plurality of precursor clusters comprising a volume of SiO 2 nanoparticles and silicon nanoparticles, wherein a volume ratio between the silicon nanoparticles and the SiO 2 nanoparticles within the precursor clusters ranges from about 0.5 to about 5;

carbon coating each of the precursor clusters to form a plurality of carbon coated-coated SiO 2 -silicon clusters; and

reducing the SiO 2 nanoparticles in each of the carbon-coated SiO 2 -silicon clusters to form the silicon anode material comprising a plurality of carbon-encased silicon clusters, each carbon-encased silicon cluster comprising a volume of silicon nanoparticles encased in a carbon shell having an interior volume greater than the volume of silicon nanoparticles.

15. The method of claim 14 , wherein the volume of silicon nanoparticles in each carbon-encased silicon cluster is less than the volume of SiO 2 nanoparticles and silicon nanoparticles in the precursor cluster from which the silicon nanoparticles are formed during the reducing.

16. The method of claim 14 , wherein forming the precursor clusters comprises spray drying a solution comprising the SiO 2 nanoparticles and polyethylene glycol (PEG);

wherein carbon coating each of the precursor clusters comprises:

coating the precursor clusters with a slurry comprising a carbon precursor selected from the group consisting of: polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), an alginate, and combinations thereof and a solvent selected from the group consisting of: water, n-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and combinations thereof; and

heating the slurry coating at a temperature ranging from about 300° C. to about 800° C. for a time ranging from about 5 minutes to about 300 minutes; and

wherein reducing the SiO 2 nanoparticles comprises magnesium vapor reduction or dihydrogen reduction at a temperature ranging from about 650° C. to about 1000° C.