Two start-ups, Nexeon in the United Kingdom, and Amprius, in the United States, have, by swapping graphite for silicon as an electrode material, made lithium-ion batteries that can hold double the energy they do today. The technology, which both companies say will be on the market in the next two years, means cell phones and electric vehicles could run twice as long between charges. Lithium-ion batteries have used graphite anodes for the last 20 years, but its charge storage capacity has reached its peak. Silicon is capable of storing 10 times as much charge per weight as graphite, but practical anodes have been hard to make because expansion of the material as the lithium ions move in and out of silicon can destroy it. The companies overcame this obstacle by using different forms of structured silicon, which can swell and shrink without suffering as much mechanical stress. Amprius uses an array of vertical silicon nanowires, while Nexeon’s anode material is a powder of silicon particles whose surfaces are etched to create submicrometer pillars. Amprius is partnering with consumer electronics manufacturers, with the aim of bringing their battery to market by early 2012. Nexeon plans to set up a facility to produce 250 metric tons of silicon anode material per year, and is working with top portable electronics battery makers, with a plan of having its silicon anodes in notebooks and cell phones in two years. The popularity of these nanostructured silicon anodes may eventually be determined by their cost. Their impact could also be limited by the partnering cathodes, which would need to have capacity to match that of the anode.