The watthours is what gas is for. Germany's pipeline network alone, that's not including actual gas storage sites, can store three months of total energy usage.

...or at least that's the original plan, devised some 20 years ago, Fraunhofer worked it all out back then. It might be the case that banks of sodium batteries or whatnot are cheaper, but yeah lithium is probably not going to be it. Lithium's strength is energy density, both per volume and by weight, and neither is of concern for grid storage.

Imagine bridging even a short dunkelflaute of 2 days.

That's physically impossible for a place the size of Germany, much less Europe.