Bao and his group worked with Dr. Yi Cui and his colleagues on the new silicon anode. The result could accelerate the commercialization of higher energy density Li-ion batteries for electric vehicles, cell phones and other devices.

Silicon is targeted as one of the more promising anode materials for next-generation lithium-ion batteries due to its relatively low working potential, abundance in nature, and theoretical gravimetric (specific) capacity of 3,579 mAh g⁻¹ for Li₁₅Si₄ at room temperature)—almost ten times that of commercialized graphite anodes.

However, as has been often reported, silicon experiences a significant change in volume (more than 300%) during the lithiation and delithiation processes, resulting in pulverization and loss of electrical contact, as well as formation and propagation of an unstable solid electrolyte interphase (SEI) on its surface—both of which result in rapid capacity fading of the battery. (Earlier post.)

This is a problem for all electrodes in high-capacity batteries, said Hui Wu, a former Stanford postdoc who is now a faculty member at Tsinghua University in Beijing, the other principal author of the paper.

Researchers worldwide are devising approaches to counter this structural problems; researchers in Cui’s lab have tested a number of ways to keep silicon electrodes intact and improve their performance (e.g., earlier post). Some are being explored for commercial uses, but many involve exotic materials and fabrication techniques that are challenging to scale up for production.

Self-healing is very important for the survival and long lifetimes of animals and plants. We want to incorporate this feature into lithium-ion batteries so they will have a long lifetime as well.

—Chao Wang, a postdoctoral researcher at Stanford and one of two principal authors of the paper

Wang developed the self-healing polymer in the lab of Professor Bao; for the battery project he added tiny nanoparticles of carbon to the polymer so it would conduct electricity.

Left: An electron micrograph shows cracks left in a self-healing polymer coating due to swelling of its silicon electrode during charging. Right: Five hours later, the smaller cracks have healed. (C. Wang et al., Nature Chemistry) Click to enlarge.

To make the self-healing coating, the scientists deliberately weakened some of the chemical bonds within polymers. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.

The polymer-coated electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity.

Their capacity for storing energy is in the practical range now, but we would certainly like to push that. That’s still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle, but the promise is there, and from all our data it looks like it’s working.

—Yi Cui

The self-healing electrode, which is made from silicon microparticles that are widely used in the semiconductor and solar cell industry, is the first solution that seems to offer a practical road forward, Cui said. The researchers said they think this approach could work for other electrode materials as well, and they will continue to refine the technique to improve the silicon electrode’s performance and longevity.

The research team also included Zheng Chen and Matthew T. McDowell of Stanford. Cui and Bao are members of the Stanford Institute for Materials and Energy Sciences, a joint SLAC/Stanford institute. The research was funded by DOE through SLAC’s Laboratory Directed Research and Development program and by the Precourt Institute for Energy at Stanford University.

Resources

• C. Wang et al., Nature Chemistry, 17 October 2013 doi: 10.1038/nchem.1802