In rechargeable lithium-ion batteries - the most common type, used in things like laptop computers and cell phones - one of the two electrodes is graphite, a form of pure carbon consisting of sheets of carbon atoms.
Lithium ions are forced between the carbon sheets when the battery is charged and come back out again when the battery is discharged, explains Robert Hamers, a Wisconsin Distinguished Professor and chair of the UW-Madison chemistry department.
"The first few times that a battery is charged, a chemical reaction occurs on the graphite, forming a thin protective layer," Hamers says. "Over time, this layer slowly becomes thicker and eventually begins to block the flow of electrical current needed to charge the battery, degrading the performance."
A second reason the storage capacity diminishes is physical wear of the graphite itself, he adds. "There is a large volume change as the graphite expands and contracts. Over time that causes the graphite to flake and small particles to break off."
In nickel-metal-hydride batteries, the second-most common rechargeable battery, a small amount of water helps carry electrical charges between the two electrodes. Each time the battery is charged or discharged, side reactions convert some of the water to hydrogen and oxygen, which slowly leak out of the cell. When there is not enough water left, the battery dies.
