If what everyone else said hasn't already put the wind up you, this portion of an article below details why lithium fires are so freaking bad.

Once lithium battery fires take hold, they are notoriously difficult to put out, especially if you don’t know what you’re doing. There have been several reports of fire departments being unable to extinguish burning EV batteries, which—amongst other things—led to Tesla issuing specific guidance to firefighters as to how to deal with such fires. There are several reasons for the severity of lithium battery fires. For a start, they burn extremely hot, and have a nasty habit of spontaneously reigniting when you think you’ve extinguished them. They also burn for a long time. If there’s any elemental lithium present, it will react with moisture in the air to produce lithium hydroxide and hydrogen gas. Hydrogen gas is flammable. Very flammable. Lithium-ion batteries contain little or no elemental lithium, but any that is present—it can form on the anode during the charging process—can present a very unwelcome surprise. The nature of the electrolyte used can also be a problem—some high-specific energy lithium-ion batteries use a flammable electrolyte that contains lithium hexafluorophosphate, which can decompose into the thoroughly unpleasant hydrofluoric acid during a fire. Lithium ion fires also produce a variety of other gases, including oxygen, which means that simply smothering the fire doesn’t work, because the fire essentially generates its own fuel. Several of these gases are flammable, and can ignite explosively, while oxygen, not flammable itself, causes other materials in the battery to burn much hotter and more rapidly. Both carbon monoxide and carbon dioxide are also produced, which can present breathing hazards to firefighters. Generally, the best thing to do with a lithium fire is to stay a long way away from it and let it burn itself out. Of course, if it happens to be, say, the car or e-bike in your garage that’s on fire, this approach may prove unsatisfactory.

Or, say you put a fucked up battery into your phone, then your phone into your pocket... Well, you might be golden if you also decided to start carrying lithium ion gel with you everywhere you go, but barring that, you'll make a pretty charred shish kabob in short order.

You're not even supposed to charge your phone near combustibles like sheets and clothings, ideally, or overcharge your batteries (to avoid overheating).