As I understand it, it is not a matter of charged vs discharged; it is a matter of “spanking new” vs “old and beat up”. Let’s recognize first that a “discharged” batt can have charge left in it but not have enough to power what it was intended to power; or, it’s being discarded because it won;t accept enough charge for the user to want to keep using it. Second, there is high power density. Third, when these batts are older and abused, there can be punctures in the membranes separating the electrodes > short circuit > VERY reactive metal > lots of batts packed together. Fourth, WHEN they have an internal damage issue, they are subject to thermal runaway and the great place to have this go on is an overheated railroad car in Houston?
Though I got to directly interact with Hg as a kid, I don’t remember playing with Li. Na was in oil in the Chem Lab.