Structurally controlled deposition of silicon onto nanowires
Provided herein are nanostructures for lithium ion battery electrodes and methods of fabrication. In some embodiments, a nanostructure template coated with a silicon coating is provided. The silicon coating may include a non-conformal, more porous layer and a conformal, denser layer on the non-conformal, more porous layer. In some embodiments, two different deposition processes, e.g., a PECVD layer to deposit the non-conformal layer and a thermal CVD process to deposit the conformal layer, are used. Anodes including the nanostructures have longer cycle lifetimes than anodes made using either a PECVD or thermal CVD method alone.
1. An anode for a lithium battery, comprising:
a substrate;
a nanowire template rooted to the substrate;
a first layer of an electrochemically active material substantially coating the nanowire template, the first layer having a first density; and
a second layer of the electrochemically active material over the first layer and any exposed nanowire template, the second layer having a density higher than the density of the first layer, wherein the first layer is non-conformal to the nanowire template.
2. The anode of claim 1 , wherein the first density is less than 2.2 g/cm 3 .
3. The anode of claim 1 , wherein the first density is less than 2.1 g/cm 3 .
4. The anode of claim 1 , wherein the second layer has a density greater than 2.25 g/cm 3 .
5. The anode of claim 1 , wherein the second layer has a density at least 0.05 g/cm 3 greater than that of the first layer.
6. The anode of claim 1 , wherein the second layer has a density at least 0.15 g/cm 3 greater than that of the first layer.
7. The anode of claim 1 , wherein the second layer is conformal to an underlying surface.
8. The anode of claim 1 , wherein the nanowire template comprises silicide nanowires.
9. The anode of claim 1 , wherein the first layer is between about 5 and 20 microns thick at its maximum diameter.
10. The anode of claim 1 , wherein the second layer is between about 5 and 500 nanometers thick.
11. The anode of claim 1 , wherein the second layer is about 5 and 100 nanometers thick.
12. The anode of claim 1 , wherein the electrochemically active material comprises germanium.
13. The anode of claim 1 , wherein the electrochemically active material comprises tin.
14. The anode of claim 1 , wherein the electrochemically active material comprises silicon.
15. The anode of claim 1 , wherein the electrochemically active material has theoretical lithiation capacity of at least 500 mAh/g.
16. The anode of claim 15 , wherein the electrochemically active material comprises germanium.
17. An anode for a lithium battery, comprising:
a substrate;
a nanowire template rooted to the substrate;
a first layer of an electrochemically active material substantially coating the nanowire template, the first layer having a first density; and
a second layer of the electrochemically active material over the first layer and any exposed nanowire template, the second layer having a density higher than the density of the first layer, wherein the second layer has a density at least 0.15 g/cm 3 greater than the density of the first layer.
18. The anode of claim 17 , wherein the first density is less than 2.2 g/cm 3 .
19. The anode of claim 17 , wherein the electrochemically active material comprises tin.
20. The anode of claim 17 , wherein the electrochemically active material silicon-containing compound.
21. An anode for a lithium battery, comprising:
a substrate;
a nanowire template rooted to the substrate;
a first layer of an electrochemically active material substantially coating the nanowire template, the first layer having a first density; and
a second layer of the electrochemically active material over the first layer and any exposed nanowire template, the second layer having a density higher than the density of the first layer, wherein the first density is less than 2.1 g/cm 3 .
22. The anode of claim 21 , wherein the second layer is conformal to the first layer.
23. The anode of claim 21 , wherein the nanowire template comprises silicide nanowires.
24. The anode of claim 21 , wherein the second layer is between about 5 and 20 microns thick at its maximum diameter.
25. The anode of claim 24 , wherein the second layer is between about 5 and 500 nanometers thick.
26. The anode of claim 21 , wherein the electrochemically active material comprises germanium.
27. The anode of claim 21 , wherein the electrochemically active material comprises tin.
28. The anode of claim 21 , wherein the electrochemically active material is a silicon-containing compound.