Lockheed Martin and Rice University scientists have recently discovered how to use the all-abundant silicon to make battery electrodes that could hold up to 10 times more lithium and enhance the battery’s overall capacity from 300 mAh/gram to more than 3,000 mAh/gram. This could lead to an unprecedented rise in storage capacities for electric car batteries, in a crucial moment for their development.
“The anode, or negative, side of today’s batteries is made of graphite, which works. It’s everywhere,” says Michael Wong, from Rice. “But it’s maxed out. You can’t stuff any more lithium into graphite than we already have.”
Silicon had been already used as a replacement for graphite, though. The silicon in its raw state can soak up to ten times the lithium that graphite can absorb, but after a couple of cycles it cracks, so a solution had to be found. Carpets of silicon nanowires that absorb lithium proved to be a good solution for that, but the Rice and Lockheed scientists looked for something cheaper and easier to make.
They put micron-sized pores into the surface of a silicon wafer by applying a positive and a negative electrical charge on the wafer’s sides, and then bathing it in a hydrofluoric solvent. “The hydrogen and fluoride atoms separate,” says Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering at Rice. “The fluorine attacks one side of the silicon, forming the pores. They form vertically because of the positive and negative bias.”
The straightforward etching process makes up for a reduced price, and the observed lifetime of the newly-created anode is much longer than that of the silicon nanowire batteries, invented by Yi Cui, from Stanford. Still, the researchers have to be very careful about how they put pores in silicon, since the more space is dedicated to the hole, the less material is available to store lithium.
“We are very excited about the potential of this work,” said Steven Sinsabaugh, a Lockheed Martin Fellow. “This material has the potential to significantly increase the performance of lithium-ion batteries, which are used in a wide range of commercial, military and aerospace applications.”
As a continuation of their previous work, Biswal and Wong are now working to figure out the mechanism by which silicon absorbs lithium and how and why it breaks down. “Our goal is to develop a model of the strain that silicon undergoes in cycling lithium,” Wong said. “Once we understand that, we’ll have a much better idea of how to maximize its potential.”
Increasing the lithium ion batteries’ capacity 10 times the one currently possible would greatly increase the chances electric cars have on today’s market, and would increase their acceptance in the public’s eyes.