The same material used in pencils (graphite) has long been a key component in today's lithium-ion batteries. As reliance on these batteries increases, however, graphite-based electrodes are due for an upgrade.
Silicon, used in computer chips and many other products, is appealing because it can hold 10 times the electrical charge per gram compared to graphite; however, silicon expands greatly when it encounters lithium and is too weak to withstand the pressure of electrode manufacturing. Researchers have developed a unique nanostructure that limits silicon's expansion while fortifying it with carbon. This work could inform new electrode material designs for other types of batteries and eventually help increase the energy capacity of lithium-ion batteries in electric cars, electronic devices, and other equipment.
A conductive and stable form of carbon, graphite is well suited to packing lithium ions into a batter’'s anode as it charges. Silicon can take on more lithium than graphite but it tends to balloon about 300 percent in volume, causing the anode to break apart. The researchers created a porous form of silicon by aggregating small silicon particles into microspheres about 8 micrometers in diameter — roughly the size of one red blood cell.
The electrode with porous silicon structure exhibits a change in thickness of less than 20 percent while accommodating twice the charge of a typical graphite anode. Unlike previous versions of porous silicon, the microspheres also exhibited extraordinary mechanical strength, thanks to carbon nanotubes that make the spheres resemble balls of yarn.
The researchers created the structure in several steps, starting with coating the carbon nanotubes with silicon oxide. Next, the nanotubes were put into an emulsion of oil and water. Then they were heated to boiling. The coated carbon nanotubes condense into spheres when the water evaporates. Then aluminum and higher heat were used to convert the silicon oxide into silicon, followed by immersion in water and acid to remove by-products. What emerges from the process is a powder composed of the tiny silicon particles on the surface of carbon nanotubes.
The porous silicon spheres’ strength was tested using the probe of an atomic force microscope. One of the nanosized yarn balls may yield slightly and lose some porosity under very high compressing force but it will not break. Anode materials must be able to handle high compression in rollers during manufacturing.
The next step is to develop more scalable and economical methods for making the silicon microspheres so that they can one day make their way into the next generation of high-performance lithium-ion batteries.