One of the batteries I was using for this truck went bad (charger only picks up 5 cells and I can't see any connection probs with the balancing lead). So I had a look around to see how to dispose of it. It seems they are safe for landfill but should be discharged first to prevent accidents. Quite a bit of older info says to submerge in a bucket of water (correction salt water) for a couple of days to discharge. Other info I have found though says this isn't fool proof and sometimes not all the cells discharge due to electrolysis erosion. Digging a bit further, instead of using the water it's suggested to use a resistive element to fully discharge the battery. Below is my rig to do that, just a couple of 12 volt car light bulbs.
Apparently the battery can puff even if slowly discharged and the gases are flammable so the cells should be punctured after the discharge. Nail taped to a broom handle? Hopefully mine doesn't puff.