Lithium-ion Batteries: Research

Much of the recent efforts to improve lithium-ion batteries have focused on developing anode or cathode materials that can hold more charge in a given volume, leading to higher energy densities. Numerous research groups are focusing on replacing the graphite anode with silicon, which could potentially store up to ten times the current capacity. The downside is that silicon films tend to expand when absorbing lithium ions during charging, and shrink again when releasing lithium ions during discharge, leading to pulverization and fracturing of the anode and a short life for the battery. Recently a group led by Yi Cui of Stanford University has used silicon nanowires to create an anode that does not have this weakness. Shown in Figure 3 are images of these nanowires with and without lithium ions, taken using a scanning electron microscope (SEM).

 

Figure 3: Morphology and electronic changes in Si NWs from reaction with Li. From “High-performance lithium battery anodes using silicon nanowires.” Chan et al. Nature Nanotechnolog, 3, 31 - 35 (2008).

Another idea that has attracted a considerable amount of attention is using lithium iron phosphate (LiFePO₄) for the cathode. While it has a slightly lower capacity and significantly lower conductivity compared to lithium cobalt oxide, iron phosphate is both cheaper and less chemically reactive. Yet-Ming Chiang and his colleagues at the Massachusetts Institute of Technology (MIT) have been working to change that. In 2002 they showed that by “doping” (adding impurities to) the iron phosphate, they could achieve much higher conductivities than what had previously been thought possible. And in 2004, Chiang’s team was able to use very small (less than 100 nanometers) iron phosphate particles to improve the cathode’s capacity and conductivity.

 

A ball-and-stick model of lithium iron phosphate, where the lithium atoms are blue, the iron atoms are grey, the phosphorus atoms are yellow, and the oxygen atoms are red. From “Electronically conductive phospho-olivines as lithium storage electrodes.” S Cung, J. Bloking and Y. Chiang. Nature Material, Volume 1, October 2002.

Chiang has also been involved in research on advanced assembly techniques. A team of researchers recently used viruses to assemble lithium-ion battery cathodes out of very thin gold and cobalt-oxide wires. Viruses and other biological systems are able to recognize molecules and to assemble themselves into organized patterns, which make them ideal for microscopic battery engineering. As with the silicon anodes described above, these new cathodes take advantage of nanowires’ large surface area, which provides a greater capacity for charged particles.

 

Tunneling Electron Microscope (TEM) image of virus-templated Co3O4 nanowires. “Virus-Enabled Synthesis and Assembly of Nanowires for Lithium Ion Battery Electrodes.” Nam et al, Science, 12 May 2006 Vol 312 p 886.

Other research groups are focusing on novel electrolyte materials. As mentioned earlier, today’s lithium-ion batteries lose capacity over time, largely due to chemical reactions between the electrolytes and electrodes. Mohit Singh of the startup company SEEO is developing a novel electrolyte based on polymers, which are molecules made from long chains of repeating structural units. Singh combined a structurally stable polymer with one that is good at conducting ions to create an electrolyte layer that is both thinner and less chemically reactive than those in use today. Hiroyuki Nishide of Waseda University in Tokyo is developing a totally organic, flexible battery with electrodes made of chains of organic molecules instead of metals. This could avoid problems associated with certain metals, including limited availability and waste disposal. Compared to today’s lithium-ion batteries, Nishide’s offer the potential for faster charging and discharging and longer life, in exchange for, at the moment at least, lower charge densities.

Photograph of Nishide’s flexible polymer battery. From Takeo Suga, Hiroki Ohshiro, Shuhei Sugita, Kenichi Oyaizu, and Hiroyuki Nishide, Adv. Mater. in press (adma200803073).

Schematic showing charging and discharging reactions. From Takeo Suga, Hiroki Ohshiro, Shuhei Sugita, Kenichi Oyaizu, and Hiroyuki Nishide, Adv. Mater. in press (adma200803073).

Whatever the materials of choice for electrodes and electrolytes turn out to be, one thing is clear: for the energy efficient future we all dream of, the batteries of the future, like so many promising technologies, will depend on nanoscale engineering techniques that are still being invented.