On mystery and hysteria concerning lithium-ion cells.
All that lithium batteries out there use different chemistry with execption for their anode, mostly made from carbon. The cathode is an oxide from varying metals. In case of a so called thermal runaway it is mostly the anode carbon burning, as the lithium in a lithium battery is just %. That is why stories on using water for exstinguishing burning lithium cells are so confusing.
If lithium cells go the thermal runaway, most of the exothermal energy comes from: the thermally unstable cathode is releasing it's oxygen, reacting directly with the carbon of the anode. That fire is that hot, exstinguishing with water all of a sudden creates much vapour possibly helping spreading toxic additional chemistry from the cells. However, if a cell of a huge battery pack is burning, there might be a little chance of preventing the other cells from also burning by cooling them with much water. It is not the water accelerating a huge lithium fire, just a bit.
So best is, to let the cell burn till burnt out if there is no danger of further fire. Second best to use sand as an attempt to keep the surrounding from catching fire. Third best, use even water. But keep in mind this will eventually help spreading toxic fume. In no way one is able to exstinguish the fire, as the cathode is delivering oxygen inside the overheated cell.
The most unstable chemistry available used is cobalt dioxide. A thermal runaway may start from 130 - 150°C. So one better should keep those cells well below 70°C, as using a battery will heat it up. Other commercially available chemistry is much more robust temperature-wise. That's one reason why so many different lithium cell chemistries are existing.
Discharging the lithium battery mainly prevents from huge internal currents in case of internal shorts, introduced by many ways of mistreats of the cells before. No stored energy - no short circuit current. Basically a fully discharged lithium cell won't be able to initialize the thermal runaway any more. But do this discharge with low discharge current as an external short possibly could easily help building an internal short with pre-damaged areas inside, causing the thermal runaway to start.
And don't try to recharge fully discharged lithium-ions. That may cause another process for starting a fire.