World of physics: high performance silicon anodes

They are used as car batteries or rechargeable batteries in smartphones and are constantly improving: lithium-ion batteries. A promising approach for their further development is to use silicon instead of graphite in the anode. However, since silicon anodes expand considerably when charged, battery stability suffers. Scientists have now found a solution to this problem in filigree nanostructures. They made tiny nano-arcs in the anode which were surprisingly stable. How they succeeded, report the researchers in the journal “Communications Materials”.

When charging a lithium-ion battery, lithium ions travel from the cathode to the anode and are stored there. If the anode is graphite, it takes six carbon atoms to be able to absorb a lithium ion. Unlike this, a single silicon atom can even bind four lithium ions. In theory, silicon anodes can store up to ten times more electricity in the same volume as conventional graphite anodes. But when charging, silicon anodes expand about four times and shrink again when discharging, which greatly reduces the stability of a battery.

Anode fabrication diagram

In order to still be able to use silicon for the anode, Marta Haro and her colleagues at the Okinawa Institute of Science and Technology produced delicate and at the same time stable nanostructures. Using hot steam, they first let metallic tantalum nanoparticles form a thin layer on a surface. In the next step, they deposited silicon atoms on these tiny particles. However, these did not build up evenly in an area, but rather formed tiny nanopillars. These got wider and wider at the top end until they finally touched each other: arches formed which were surprisingly stable.

The researchers used this ingenious nano-architecture to build their first lithium-ion cells. These could actually store more electricity than lithium-ion batteries with graphite anodes. At the same time, they were significantly more stable when charging and discharging over dozens of charge cycles. However, this process is not yet ready for the industrial production of lithium-ion batteries, but it paves the way for stable and efficient silicon anodes.

Winston Ferguson

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