TL:DR The batteries are based on Iron oxide chemistry. Form Energy is building a factory that is supposed to make the batteries for many grid energy storage facilities, such as the one mentioned in the headline.

Anyway, this sort of development sounds great. This is exactly the kind of thing we need if we are to switch to renewable energy. Li-ion batteries should be reserved for mobile applications, such as cars, laptops, phones and earbuds. There are lots of other battery chemistries available, that have a lower energy density, but they have other advantages to compensate. Those chemistries are much better suited for industrial scale applications like this.

More about the batteries

More about the factory