As with most batteries you have an outer case made of metal. The use of metal is particularly important here because the battery is pressurized. This metal case has some kind of pressure-sensitive vent hole
. If the battery ever gets so hot that it risks exploding from over-pressure, this vent will release the extra pressure. The battery will probably be useless afterwards, so this is something to avoid. The vent is strictly there as a safety measure. So is the Positive Temperature Coefficient (PTC)
switch, a device that is supposed to keep the battery from overheating.
This metal case holds a long spiral comprising three thin sheets pressed together:
- A Positive electrode
- A Negative electrode
- A separator
Inside the case these sheets are submerged in an organic solvent that acts as the electrolyte. Ether is one common solvent.
The separator is a very thin sheet of microperforated plastic. As the name implies, it separates the positive and negative electrodes while allowing ions to pass through.
The positive electrode is made of Lithium cobalt oxide, or LiCoO2
. The negative electrode is made of carbon. When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiCoO2
from the carbon.
The movement of these lithium ions happens at a fairly high voltage, so each cell produces 3.7 volts. This is much higher than the 1.5 volts typical of a normal AA alkaline cell that you buy at the supermarket and helps make lithium-ion batteries more compact in small devices like cell phones. See How Batteries Work
for details on different battery chemistries.
We'll look at how to prolong the life of a lithium-ion battery and explore why they can explode next.