The Anode, an integral part of a Lithium-ion cell is one of the two electrodes.
Let’s imagine that you are looking at an anode at a distance. What you will see is a metal foil, on which there is a black looking substance attached on top.
The metal foil is usually Copper, and the thin black coating on it is the Active Material. This is typically Graphite.
Graphite offers a crystalline structure in which Lithium-ion cells are able to intercalate. The need for Silicon arises as Graphite is not very energy dense. That means, the crystalline structure takes up a lot of room to intercalate with a small amount of Lithium-ions.
After many years of research, it was found that Silicon offers a better energy density and also a cost advantage to Graphite. But the downside of Silicon was that its structure is not stable.
One of the important characteristics of an active material in a cell should be that it holds its mechanical structure when Lithium-ions intercalate. Silicon in its natural form expands when Lithium-ions intercalate. This is why so far, it has not been suitable in its pure form as an active material in the anode.
Research so far has led towards using Silicon with dopants or Silicon nanostructures which aim to solve the issue of expansion during the intercalation process.
Tesla in their Battery Day presentation talked about using Silicon in its pure form. Instead of treating its expansion properties as a drawback, they have looked into using a binding material to allow the silicon to expand within bounds. With this they are able to have a higher energy density cell, lower cost, and overcome the issue of Silicon expansion.