A small spot on the concrete floor with cinderblocks to make a table / shelf for the chargers and batteries should be fine, just as long as nothing flammable is within a distance that flames could reach it if the batteries caught fire in there (not by the wall or under something). I just keep mine on a shelf in the garage during the cooler months, but I bring them inside when it’s hot.
But batteries stored directly on the floor is also bad, so you’d want something to set them on. Somehow batteries can discharge into the ground if left on a concrete floor. An old metal ammo can on a cinderblock would be good enough IMO.
Well I’ve seen it happen myself, and I was learnt of it by an old-ass electronics engineer who did calculus equations on graph paper all day to solve microwave transmitter issues, and he was more knowledgeable about electronics than any other human being I’ve ever met so I’m going to continue NOT storing batteries on the concrete floor.
You can store your batteries on the floor if you want to of course, but you’ve been warned.