Li-ion batteries work by transferring lithium ions between a cathode and an anode through a liquid electrolyte. The capacity of a battery is determined by the number of ions it can contain and how quickly those ions can be exchanged. Silicon is an ideal material for anodes because it allows lithium ions to pass in and out of the electrode very quickly. But the expansion and contraction of silicon particles as the ions enter and leave degrades the silicon in short order, ruining the battery. Instead, nearly all li-ion batteries contain graphite anodes that can withstand repeated charge cycles.