It’s a mix of different battery chemistries; various Lithium-based chemistries predominate, but there are several utility-scale batteries using iron chemistries. This will change over time as some of the heavier battery types become cheaper for stationary energy storage, and as a need for seasonal storage (instead of overnight) starts to be significant.

If you look at the graph, 0GW.

It’s unlikely that the batteries stored power for several days. Highly likely that abundance of solar is the source.

They usually use Lithium, I believe

Usually with these comments they are usually trolls