Lithium batteries mostly burn when the devices are drawing significant current below the safe cutoff voltage. You can safely discharge a battery to zero chemic

Lithium batteries mostly burn when the devices are drawing significant current below the safe cutoff voltage. You can safely discharge a battery to zero chemical energy with a slow draw. They make discharge devices for batteries in RC cars and planes for this. Once the battery has lost enough performance you safely drain it and then dispose of it (not in the landfill because the chemicals are still toxic)- you don't want to leave it partially charged in case it gets punctured in the disposal process. Letting phone batteries drain naturally is pretty safe, because just leaving it in a low-power mode over time will cause it to self-drain at a pretty slow pace over several months. They should still be disposed of with electronics recycling so that the toxic stuff can be handled, but leaving a phone in a drawer is normally safe. Software bugs that try to turn the phone on into a high-power state right at the safety threshold are the biggest risks, or that you might try to turn it on yourself right at the safety threshold. Phone fires are typically from software bugs that fail to cut power at the safety threshold, either while a user is trying to squeeze out the last few percent of battery, or if they are in luggage and presumably jostled and buttons keep getting pushed.