A battery converts chemical energy into electrical energy by making a chemical reaction happen in a way that forces electrons through an external circuit.
The basic arrangement
Two electrodes made of different materials, separated by an electrolyte that lets ions pass but not electrons.
A reaction wants to happen between the two materials. The electrolyte blocks the direct path for electrons, so they have to travel through your circuit instead – which is where they do work.
Ions move through the electrolyte to balance the charge. When the reaction is exhausted, the battery is flat.
What makes lithium-ion different
Nothing is consumed. Lithium ions move from one electrode to the other and back, sliding into the structure of each. Charging pushes them one way, discharging lets them return.
That reversibility is why they recharge hundreds of times, and lithium’s low weight is why they store so much energy per kilogram.
Why they degrade
The shuttling is not perfectly clean. Each cycle causes small structural changes and forms deposits that consume lithium. Capacity falls gradually and permanently.
Heat accelerates all of it, which is why temperature matters more than any other factor.
Why they catch fire
The electrolyte is flammable and the reaction releases heat. If a cell is damaged, overcharged or manufactured with a defect, an internal short can produce heat faster than it dissipates, which produces more heat. The runaway is self-sustaining and hard to extinguish.
Solid electrolytes would remove the flammable component, which is the main reason solid-state batteries are pursued.