I've been meaning to do the math on how much capacity used car batteries can be continuously recycled into grid storage where capacity/weight isn't a big deal but haven't gotten around to it.

My hunch is that those used batteries will single handedly store peak reusable energy for down times if electric cars grow to dominate the market.

The current automobile production is over 70 million/year. The battery capacity for a single car ranges from 30-100 kWh. The current middle class is offering 60kWh (Model3, Bolt, Leaf 2019). So if we assume only 30 kWh/car at the end of the useful battery life, we get about 2 TWh of used battery capacity every year - though that point of time is quite in the future, as batteries are expected to last at least 10 years, if not more. As a comparison, the daily energy usage of Germany is about 1.6 TWh.

Hm, I wonder where they will get enough lithium, or they are using some other technology?

In a Tesla battery pack there is only about 8 kg of lithium used - less than lead in a normal car battery. As lithium is a rather common element, it shouldn't be too difficult to mine it in the neccesary amount. Current mining capacitly is set by the comparatively low demand before electric cars.

Lithium will never be a problem. Cobalt and rare earth elements will be.

Is lithium hard to acquire? Sure, it's costly to mine, but it's not like there are shortages. Besides, it seems that Australia is a major producer of lithium.