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.

Lithium-ion batteries are crucial components of modern technology, found in smartphones, laptops, and electric vehicles. Traditionally, graphite is used for the anode of a lithium-ion battery, but this carbon material has major limitations. "When a battery is being charged, lithium ions are forced to move from one side of the battery – the cathode – through an electrolyte solution to the other side of the battery – the anode. Then, when a battery is being used, the lithium ions move back into the cathode and an electric current is released from the battery," explained Dr. Marta Haro, a former researcher at OIST and first author of the new research study. "But in graphite anodes, six atoms of carbon are needed to store one lithium ion, so the energy density of these batteries is low."

With science and industry currently exploring the use of lithium-ion batteries to power electric vehicles and aerospace craft, improving energy density is critical. Researchers are now searching for new materials that can increase the number of lithium ions stored in the anode. One of the most promising candidates is silicon, which can bind four lithium ions for every one silicon atom. "Silicon anodes can store 10 times as much charge in a given volume than graphite anodes – a whole order of magnitude higher in terms of energy density," said 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 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 lifespan and rechargeability of the battery.

The research’s goal is to create a more robust anode capable of resisting these stresses, which can absorb as much lithium as possible and ensure as many charge cycles as possible before deteriorating. The approach taken was to build a structure using nanoparticles.

Previously, the now-disbanded OIST Nanoparticles by Design Unit developed a cake-like layered structure, where each layer of silicon was sandwiched between tantalum metal nanoparticles. This improved the structural integrity of the silicon anode, preventing over-swelling. While experimenting with different thicknesses of the silicon layer to see how it affected the material's elastic properties, the researchers noticed something strange. "There was a point at a specific thickness of the silicon layer where the elastic properties of the structure completely changed," said Theo Bouloumis, a current Ph.D. student at OIST who was conducting this experiment. "The material became gradually stiffer but then quickly decreased in stiffness when the thickness of the silicon layer was further increased. We had some ideas, but at the time, we didn't know the fundamental reason behind why this change occurred."

New research provides an explanation for the sudden spike in stiffness at one critical thickness. 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, uniform film. Instead, they form columns in the shape of inverted cones, growing wider as more silicon atoms are deposited. Eventually, the individual silicon columns touch each other, forming a vaulted structure. The vaulted structure is strong, just like an arch is strong in civil engineering. The same concept applies, just on a nanoscale.

The increased strength of the structure coincided with enhanced battery performance. When the scientists carried out electrochemical tests, they found that the lithium-ion battery had an increased charge capacity. The protective layer was also more stable, meaning the battery could withstand more charge cycles.