Like any other battery, a rechargeable lithium-ion battery is made of one or
more power-generating compartments called cells. Each cell has
three key components: a positive electrode (connected to the battery’s
positive terminal), a negative electrode (connected to the negative terminal),
and an electrolyte between them.

Newer batteries typically contain lithium iron phosphate (LiFePO4). The negative
electrode is generally made from graphite and the electrolyte varies
from one battery type to another.

All lithium-ion batteries typically work in the same way. When the battery is
charging up, the lithium-cobalt oxide (the positive electrode) gives up some of its
lithium ions, which move through the electrolyte to the negative, graphite
electrode and remain there. The battery absorbs and stores energy during this
process. When the battery is discharging, the lithium ions move back across the
electrolyte to the positive electrode, producing the energy that powers the
battery. In both cases, electrons flow in the opposite direction to the ions
around the outer circuit. Electrons do not flow through the electrolyte:
it’s effectively an insulating barrier.

The movement of ions (through the electrolyte) and electrons
(around the external circuit, in the opposite direction) are interconnected
processes, and if either stops so does the other. If ions stop moving through
the electrolyte because the battery completely discharges, electrons can’t move
through the outer circuit either, the battery will stop powering whatever you’re
using it for. Similarly, if you switch off whatever the battery is powering, the
flow of electrons stops and so does the flow of ions. The battery essentially
stops discharging at a high rate (it does keep on discharging,
at a very slow rate, even with the appliance disconnected).

Unlike simpler batteries, lithium-ion ones have built in electronic controllers
that regulate how they charge and discharge. They prevent the overcharging and
overheating that can cause lithium-ion batteries to explode in some circumstances.

-Taylor