Image caption: In the first stage, the silicon film exists as a rigid but wobbly columnar structure. In the second stage, the columns touch at the top, forming a vaulted structure, which is strong due to arch action. In the third stage, further deposition of silicon atoms results in a sponge-like structure. The red dashed lines show how the silicon deforms as a force is applied.
New research conducted by the Okinawa Institute of Science and Technology Graduate University (OIST) has identified a specific building block that improves the anode in lithium-ion batteries. The unique properties of the structure, which was built using nanoparticle technology, are explained in Communications Materials.
“In graphite anodes, six atoms of carbon are needed to store one lithium ion, so the energy density of these batteries is low,” explained Dr. Marta Haro, first author of the study.
“Silicon anodes can store ten times as much charge in a given volume [as] graphite anodes—a whole order of magnitude higher in terms of energy density,” said Dr. Haro. “The problem is, as the lithium ions move into the anode, the volume change is huge, up to around 400%, which causes the electrode to fracture and break.”
The large volume change also prevents the stable formation of a protective layer that lies between the electrolyte and the anode. Every time the battery is charged, this layer therefore must continually reform, using up the limited supply of lithium ions and reducing the battery’s lifespan and rechargeability.
The video shows that as silicon atoms are deposited in the presence of nanoparticles, columns grow in the shape of an inverted cone.
“Our goal was to try and create a more robust anode capable of resisting these stresses, that can absorb as much lithium as possible and ensure as many charge cycles as possible before deteriorating,” said Dr. Grammatikopoulos, senior author of the paper. “And the approach we took was to build a structure using nanoparticles.”
Through microscopy techniques and computer simulations at the atomic level, the researchers showed that, as the silicon atoms are deposited onto the layer of nanoparticles, they don’t form an even and uniform film. Instead, they form columns in the shape of inverted cones, growing wider and wider as more silicon atoms are deposited. Eventually, the individual silicon columns touch each other, forming a vaulted structure.
When the scientists carried out electrochemical tests, they found that the battery had an increased charge capacity. The protective layer was also more stable, meaning the battery could withstand more charge cycles.
Image Credit: Schematic created by Dr. Panagiotis Grammatikopoulos, OIST Nanoparticles by Design Unit and Particle Technology Laboratory, ETH Zürich