Lithium ion batteries, which rely on the movement of lithium ions into and out of a pair of electrodes, are increasingly pervading our everyday lives, being found in everything from mobile phones to cars. Although the negative electrodes of most lithium ion batteries are currently made from graphite, silicon can store ten times as much charge as graphite for a given mass, and has therefore been the focus of significant research. Yi Cui from Stanford University in the USA in collaboration with colleagues from the Chinese Academy of Sciences and Peking University in Beijing, China, have now calculated how lithium atoms can be inserted into silicon electrodes.1

Fig. 1: The calculated location for a single lithium ion inserted into a silicon nanowire grown in the [110] direction with a diameter of around 1.5 nm. The blue, green, yellow and pink colours represent the lithium ion, neighbouring silicon atoms, non-neighbouring silicon atoms and hydrogen atoms, respectively.

The researchers focused on silicon nanowire electrodes, which are much less likely than particulate or bulk silicon to fracture as a result of the volume expansion and contraction caused by lithium insertion and removal. Working from first principles, they used a simulation program to calculate the energy that binds lithium atoms to silicon nanowires in various positions around the nanowire (Fig. 1), and for different types of nanowires. The team also studied the energy barriers for lithium atom diffusion.

Silicon nanowires grown in the [110] crystallographic direction were found to have the highest binding energy to lithium atoms, making them the preferred nanowires for use in battery applications. Surface sites on the nanowires were able to bind lithium atoms more strongly than sites on the interior. Furthermore, increasing the nanowire diameter increased the binding energy. This suggests a trade-off between large nanowires, which are better at binding lithium, and small nanowires, which are better at withstanding the stress of volume expansion. The authors also found that lithium atoms bound to the surface of the nanowires were best able to diffuse around the nanowire, rather than from surface to core, which is consistent with observations from previous experimental work.

“The results should help research into next-generation lithium ion batteries,” says Cui. “These calculations provide a detailed theoretical understanding of the physical mechanism of lithium insertion into silicon wires, which is necessary to achieve optimum performance.” The researchers will continue to focus on the effects of strain in the nanowires, as well as the doping of silicon electrodes with foreign atoms.