Sodium is pretty much everywhere and it’s often a waste product of other activities. So in terms of damage through mining, sodium should be much better. Recycling sodium will likely never be worth it, so there’s a danger that toxic compounds will end up in the environment. Lithium, it’s much more likely to be recycled because it’s more expensive. But that again has an environmental impact. In short, there is no definitive answer. It all depends on how well mining, production and disposal are regulated and supervised.