From：CTECHI GROUP Limited Release time：2018-09-25
Overview：Mobile phones (as well as pads and nearly all things with a rechargeable battery like lithium polymer battery for mobile phones, commercial lithium ion battery for ebikes and Li-Ion battery for wifi router) use LiIon/LiPo. And you're right: In terms of actual incidences, lithium ion and lithium polymer are so far the safest battery chemistry to be in wide use.
Mobile phones (as well as pads and nearly all things with a rechargeable battery like lithium polymer battery for mobile phones, commercial lithium ion battery for ebikes and Li-Ion battery for wifi router) use LiIon/LiPo. And you're right: In terms of actual incidences, lithium ion and lithium polymer are so far the safest battery chemistry to be in wide use.
Ironically, lithium ion batteries have become the safest packaged battery by being the most dangerous battery chemistry.
You might be wondering what actually makes them so dangerous.
Other battery chemistries, such as lead acid or NiMH or NiCad, are not pressurized at room temperature, though extra heat does produce some internal pressure. They also have aqueous, non-flammable electrolytes. They store energy in the form of a relatively slow oxidation/reduction reaction, one whose rate of energy release is too low to, say, cause them to eject 6 foot jets of flame. Or any flame, really.
Lithium ion batteries are fundamentally different. They store energy like a spring. That's not a metaphor. Well, like two springs. Lithium ions are forced between the atoms of a covalently-bonded anode material, pushing them apart and 'stretching' the bonds, storing energy. This process is called intercalation. Upon discharge, the lithium ions move out of the active electrolyte and into the negative electrolyte. This is very much electromechanical, and both the active electrolyte and the negative electrolyte experience significant mechanical strain from this.
In fact, both the active electrolyte and the negative electrolyte alternatively increase or decrease in physical volume depending on the battery's state of charge. This change in volume is uneven however, so a fully charged lithium ion battery is actually exerting nontrivial amounts of pressure on its container or other parts of itself. Lithium ion batteries are generally under a lot of internal pressure, unlike other chemistries.
The other problem is their electrolyte is a volatile, extremely flammable solvent that will burn quite vigorously and easily.
The complex chemistry of lithium ion cells is not even completely understood, and there are a few different chemistries with different levels of reactivity and inherent danger, but the high density lipo battery can undergo thermal runaway. Basically, if they get too hot, lithium ions will begin reacting with oxygen stored as metal oxides in the cathode and release even more heat, which accelerates the reaction further.
What inevitably results is a high density lipo battery that self-ignites, sprays its highly flammable solvent electrolyte out of itself, and promptly ignites that as well, now that a fresh supply of oxygen is available. That's just bonus fire however, there is still a ton of fire from the lithium metal oxidizing with the ample store of oxygen inside.
If they get too hot that happens. If they are overcharged, they become unstable and mechanical shock can make them go off like a grenade. If they are overdischarged, some of the metal in the cathode undergoes an irreversible chemical reaction and will form metallic shunts. These shunts will be invisible, until charging expands part of the battery enough that the separating membrane is punctured by one of these shunts, creating a dead short, which of course results in fire, etc.: The lithium ion failure mode we know and love.