The batteries don't "explode" – they self-combust.
Better to have the phone out in the open (so you can see it catching fire) and throw it in a bucket of cold water – the water will absorb most of the heat and prevent the fire getting out of control (it won't extinguish the fire, which will continue in the water).
Rechargable Lithium batteries contain surprisingly limited amounts of it. And its not in there in its elemental form anyway. Lithium batteries burn because of the potential energy they have after charging.
You can drive nails through discharged Lithium Ion batteries without much happening.
Please don't do this, when rechargable lithium batteries are "discharged", they are not discharged fully, doing so would damage the battery. Charging circuits have a "cutoff" voltage where they report the battery to be empty, but there is still plenty of stored energy in the cells.
Better to have the phone out in the open (so you can see it catching fire) and throw it in a bucket of cold water – the water will absorb most of the heat and prevent the fire getting out of control (it won't extinguish the fire, which will continue in the water).