You should actually read the linked article.

It's relatively sparse on technical details, but it does mention:

- Renewable hydrogen

- Compressed Air Energy Storage

- Large scale flow batteries

- Solid oxide fuel cells

Seems like hydrogen is not actually used as a storage mechanism. They convert to hydrogen on demand to power turbines.

They will be using sealed salt caverns to storage large quantities of gas cheaply.

Why are batteries not scalable?

Cost increases linearly with capacity, which is undesirable at scale. You want sublinear scaling. For example, to store a year’s worth of US electrical generation using batteries would cost approximately 500 trillion dollars.

1) Huge number, but why would you need an entire year of energy in reserve 2) If there was a desire to build that many batteries, cost would surely decrease exponentially as new mines, factories, etc. open up and competition increases.

By definition, batteries are scalable. Battery is a word that defines a method of scaling.

edit - For example; 'A battery of batteries of batteries of cells.'