> Can "second life" EV batteries work as grid-scale energy storage?

Yes

is it profitable? probably not.

Looking at the price for brid battery storage, and its dropping precipitously. The cost isn't as much in the batteries them selves, it packaging, placing and then controlling them.

For example if you want to have a 200Mwhr 100Mw storage site, you'll need to place it, join it to the grid, all doable. Then you need the switch gear to make it work as you want it to.

For day ahead, 30 minute trading, thats fairly simple.

For grid stabilisation, thats a bit harder, you need to be able to lead/match/lag the grid frequency by n degrees instantaneously. which is trivial at a few kw, much harder at 100Mw