Printing batteries: Total extrusion zone IN THE 1970s researchers lounging on bean bags at Xerox's Palo Alto Research Centre (PARC) churned out the inventions that would trigger the coming revolution in information technology. The computer mouse was born here, as were icons, windows, Ethernet networking and the laser printer. Today, although the bean bags are long gone and PARC was spun out as a subsidiary a decade ago, its researchers are once again experimenting with printing. This time they are hoping to produce the technologies for a 21st-century revolution in clean energy. PARC's Hardware Systems Laboratory is developing electric-vehicle lithium-ion batteries that can hold 20% more energy than traditional designs. Making a battery that can store more energy requires a larger cathode containing more lithium ions. However, the thicker the cathode, the slower the ions will move through it. This reduces the battery's power, leading to sluggish acceleration. PARC wants to evade this trade-off by constructing cathodes from two materials: one dense, and optimised for storage; the other porous, for the speedy transfer of charge. Wide storage regions would alternate with narrow conductive regions. That will enable a larger and more energy-dense battery to be constructed without sacrificing its power. This basic idea has been understood for some time. The trick is building the regions small enough (about 100 microns across for the storage medium, and ten for the conductor). The cathode of a typical electric car's battery would need tens of thousands of these interleaved fingers. Making such tiny features with precision would require photolithography, an expensive technology unsuitable for high-speed volume manufacturing of big batteries. The PARC researchers" solution, which would surely have impressed their free-thinking forebears, was inspired by striped toothpaste. In PARC's new battery, the two materials are mixed with an organic material to form pastes and fed into a print head containing tiny channels and nozzles. The print head moves over a metal foil, extruding the pastes alongside one another, to create thin stripes. Drying the substrate removes most of the organic material, leaving a solid cathode. In tests against otherwise identical batteries sporting cathodes made of but one substance, the co-extruded rechargeables could store a fifth more energy. Scott Elrod, the laboratory's boss, says that PARC is now discussing how to test the new battery with firms that might eventually manufacture it. Printing cathodes this way is just the beginning. PARC is now working with ARPA-E, an American government agency whose remit is to create advanced energy technologies, to try to print entire batteries. This will require five pastes - two each for the cathode and the anode, plus a separator. Co-extrusion also promises to improve the efficiency of solar cells by printing the silver wires that conduct current out of a cell. Silver paste is co-extruded with a material that burns off when the cell is heated. The result is a wire a mere 20 microns across, instead of 50 microns. Narrow wires cast smaller shadows, so more sunlight reaches the cell. Solar panels made this way are now in production, and Mr Elrod is already casting his eye over fuel cells, ultracapacitors (a novel way of storing electricity) and even catalytic converters as suitable candidates for manufacture by co-extrusion. Co-extrusion printing may never become a household phrase, like laser printing, but Xerox will surely be pleased that there's life in its old PARC yet.