I Make My Pi with Silicon Nanowires

As this is the first Pi Day (3.14 ~ March 14) of Tree Town Chemistry's life, I felt that I would be remiss if I were to go through it without putting up a celebratory post. But what to write about?

Like any good chemist, I turned to my periodic table for answers. Element 3 is lithium. Element 14? Silicon.

Happy Pi Day, everyone. Let's make some batteries.

One Way Your Phone Battery Destroys Itself*

The lithium (Li) cycle for
the + electrode in a Li-ion
battery.

One of the classic problems in lithium ion batteries - the kinds of batteries currently powering your phone or laptop - is the instability of the compounds that make it up. The electrode materials in a lithium ion battery must be able to suck lithium ions out of the electrolyte while charge is flowing in one direction, and then spit them back out when the polarity of the battery is reversed (charging versus discharging). In a perfect substance, the process is totally reversible.

However, in most substances, the size of one crystal unit grows substantially during the lithiation process (see the cartoon at left). This causes the individual particles of the material to swell. The strain induced on the particles, either from the growth process itself or from bumping into neighboring particles, mechanically crushes the battery material. As a result, pieces of the battery material fall off of the electrode and are rendered useless.

This presents a huge problem for chemists and materials scientists. There are two main ways to look at it. One could imagine screening hundreds of compounds, looking for ones where the percent expansion during lithiation is as small as possible, thereby minimizing the strain. Another solution to the problem is to study exactly how these compounds expand, and then try to shape them on the nanoscale in such a way that the strain forces are minimized.

Silicon Solutions
Silicon is most famous for its applications in solar energy technologies, but it has also been identified as a lithium-ion battery anode (negative electrode) with record capacity for storing electrical energy. However, silicon also undergoes huge expansion once it accepts lithium ions - on the order of 400%. That fact makes it inapplicable to traditionally-structured lithium-ion electrodes.

A 2007 study led by Professor Yi Cui at Stanford University found a way around silicon's volume expansion. Professor Cui's research team developed a method of synthesizing electrodes composed of ordered silicon nanowires. The electrode structure is not unlike your toothbrush; each bristle is a silicon wire that is directly connected to the base of the electrode, called the current collector.

The unique electrode structure brings about two major improvements. First of all, the electrical conductivity of the entire electrode is improved, since all of the silicon is directly connected to the current collector. Secondly, the silicon wires have plenty of room to expand in length and in width without clashing against one another. Cui's team observed that their silicon nanowire electrodes exhibit record-breaking storage capacity and are stable over several charge/discharge cycles.

The result made an impressive impact in the battery community and is slated for limited commercialization soon. This paper is a great example of how materials chemists can solve problems by addressing concerns that fall outside of the chemical identity of the material. In this case, changing the shape of the material particles took an unimpressive substance and made it into something industrially relevant. And who knows? If these batteries are successful commercially, we might all be celebrating Pi Day with lithium and silicon in the future.

*Disclaimer: The pulverization issue is, of course, not the only method by which lithium ion batteries fail. There are many physical and chemical changes that take place within a battery cell as it is charged and discharged repeatedly, and each material presents its own special problems. Additionally, the anode is only one half of the battery, so there is still a lot of good research going on solving similar problems for cathodes as well.

If you're interested in reading more about nanowire electrodes, U of M researchers published a similar result using germanium nanowires in 2012. This research comes from the Maldonado group and the first author was Junsi Gu.