Lithium-ion cells exploit various chemistries, but virtually all rechargeable varieties, including those in cameras and in cell phones, use lithium cobalt oxide in the cathode and graphite in the anode. Although this formulation is "inherently somewhat unsafe," according to Gerbrand Ceder, professor of materials science at the Massachusetts Institute of Technology, careful manufacturing and built-in safety devices have limited accidents to a handful. Still, Ceder explains, "battery makers have been pushing the state of charge" in a given cell because of electronics makers' demands for longer running time, so "there is now less margin for error." By stuffing more ions into the package, manufacturers have quadrupled energy capacity since commercial introduction in 1991.
Several large recalls of lithium-ion batteries used in notebook computers have raised questions about how these power packs could overheat enough to erupt in flames. Equally valid is the question of why accidents don't happen more often, given that very few occur among hundreds of millions of batteries sold annually.