Study finds arched silicon anode structure could improve lithium-ion battery performance

According to foreign media reports, a study by the Okinawa Institute of Science and Technology Graduate University (OIST) has discovered a specific building block, constructed by nanoparticle technology, that can improve the negative electrode of lithium-ion batteries.

Powerful, portable and rechargeable, lithium-ion batteries are used in smartphones, laptops and electric cars . In 2019, their potential to transform the way we store and use electricity in the future was widely recognized as we move away from fossil fuels. Dr. Akira Yoshino, a new member of the OIST Board of Governors, also received the Nobel Prize for his work in developing lithium-ion batteries. Graphite is commonly used as the anode in lithium-ion batteries, but this carbon material has significant limitations.

(Photo credit: Okinawa Institute of Science and Technology Graduate University)

"When a battery is charged, lithium ions move from the battery's cathode through an electrolyte solution to the battery's anode. When the battery is operating, the lithium ions move back to the cathode, thus generating an electric current," explains Dr. Marta Haro, a former OIST researcher and first author of the study. "But in a graphite anode, six carbon atoms are needed to store one lithium ion, so the energy density of these batteries is very low."

At present, the scientific and industrial communities are trying to use lithium-ion batteries to power electric vehicles and space shuttles, so improving their energy density is crucial. Researchers are working hard to find new materials that can increase the number of lithium ions in the anode. Silicon is currently one of the most promising candidate materials, and each silicon atom can bind four lithium ions.

Dr. Haro said: "In a given volume, the silicon anode can store ten times the amount of charge as the graphite anode, and the energy density is a full order of magnitude higher. But the problem is that when lithium ions enter the anode, the volume becomes huge, about 400%, which causes the electrode to crack and break." The huge volume change also prevents the formation of a stable protective layer between the electrolyte and the anode. Therefore, every time it is charged, the protective layer needs to be constantly reorganized, depleting all the lithium ions, which shortens the battery life and reduces its rechargeability.

"We were trying to invent a stronger anode that could withstand pressure, which could not only store more lithium ions but also provide more charging cycles before corrosion," said researcher Dr. Grammatikopoulos. "So we used nanoparticles to build the structure." Described in a 2017 paper published in Advanced Science, the nanoparticles designed by the now-disbanded OIST unit can derive a cake-like layered structure in which each layer of silicon is sandwiched between tantalum metal nanoparticles, improving the structural integrity of the silicon anode while preventing excessive expansion.

While experimenting with different thicknesses of silicon layers to see how that affected the material's elasticity, the researchers noticed something strange. "At a certain thickness, the structural elasticity changes completely and the material becomes progressively stiffer," said OIST doctoral student Theo Bouloumis. "But as the thickness of the silicon layer increases further, the stiffness drops off rapidly. We had some ideas, but at the time, we didn't know the root cause of this change."

Finally, the researchers explained that the stiffness suddenly increases when a critical thickness is reached. Through atomic-scale microscopy and computer simulations, the researchers found that when silicon atoms are deposited on a layer of nanoparticles, they do not form a uniform film, but rather an inverted cone-shaped cylinder. As more silicon atoms are deposited, the cylinder becomes wider and wider. Eventually, the individual silicon columns touch each other to form an arched structure. "The arch is strong, just like an arch in civil engineering, and this also applies at the nanoscale," said Dr. Grammatikopoulos.

Importantly, the improvement in structural strength was accompanied by improved battery performance. When the scientists performed electrochemical tests, they found that the lithium-ion battery had a higher charge capacity and the protective layer was more stable, which means the battery provides more charge cycles. These improvements were only seen at the moment the pillars made contact, and before this moment occurred, the pillars were wiggling and therefore unable to provide structural integrity to the anode. If silicon deposition continued after the pillars made contact, a porous film with many voids would form, resulting in a soft, sponge-like structure.

The experiment revealed the arch structure in the battery and how it achieves its unique properties. This is not only an important step towards the commercialization of silicon anodes for lithium-ion batteries, but also an important development in the field of materials science and other potential applications.

Dr Grammatikopoulos said: "Arched structures are used when you need a material that is strong and able to withstand pressure, such as for biological implants or to store hydrogen. You can create exactly the material you need, just by changing the thickness of the layers. The material can be harder or softer, more flexible or less flexible, which is what makes nanostructures so unique."