What will it be then? Flywheels?

It will likely have to be a combination of many different types of storage, plus decentralization/distributed storage (electric cars, home & office battery backup), plus smart grids and appliances (time-shiftable smart electric water heaters/HVACs/car chargers), renewable biofuels for peaking, etc., riding on top of a base load of hydro and geothermal. (My vote's for nuclear, but it's wildly unpopular).

They are counting on lithium-ion batteries becoming dramatically cheaper at scale due to increasing demand for electric cars.

There's an entire agency/thinktank/R&D center working on this stuff, NREL: https://www.nrel.gov/analysis/storage-futures.html (or shorter summary: https://www.nrel.gov/docs/fy22osti/81779.pdf). They do a lot of good work, but sadly no one really listens to them, either policymakers or the public :(

Nobody is counting on lithium-ion batteries becoming dramatically ("even") cheaper. Batteries will remain one of the most expensive storage methods. They will be used mainly for load leveling, and in very small-scale systems.

Nukes are not just unpopular, they are also way, way, way more expensive than favored methods.

I'm just paraphrasing NREL there. If you have a better grasp of the industry, which solutions in particular do you see?

I like gravity storage. Pumping water up a hill, or pumping air into an underwater tank to displace water. Seems that you can get quite good efficiency with these approaches.

AFAIK though only pumped water and batteries have been done at anything approaching utility scale. Anything else and you're probably looking at several decades to reach maturity, and then another several decades to roll it out.

All but chemical methods rely only on trivial physics used industrially billions of times worldwide every day. The only open question is which will have become cheapest, in each place, at the time that conditions become favorable to build it.

That time will arrive after we have brought enough renewable generating capacity online that storage would not need to be charged by burning fossil fuel.

There are many storage alternatives, each more attractive for certain circumstances. Undersea methods are particularly cheap, but synthetic fuels -- ammonia and hydrogen -- have advantages that offset higher cost.

What we can be certain of is that Energy Vault (ERGV) investors who get out last will lose badly.