Teeth and bones, snail shells and bird eggs are formed via a process called biomineralization. Found across all kingdoms of life, this method of incorporating minerals like calcium or silica into hard tissues is clearly very useful in nature. The concept is so powerful that researchers are now working on applying it to the rather unnatural environment found within lithium ion batteries.
Organisms build mineralized tissues like shells and bones with the help of proteins, or peptides, which are organic molecules made by the cells of all living things. The specialized peptides involved in the formation of snail shells and other mineralized tissues are able to bind to the particular inorganic molecules needed to create that tissue and hold them in place. A snail, for example, cannot build its shell out of calcium directly but it can build a shell-shaped scaffold made of calcium-binding peptides. In this way peptides help to form the very finely structured materials that make up shells, bones and other parts of organisms.
Similarly, specialized peptides can be used to create very fine structures with fabricated materials, a process that has potential for improving lithium ion batteries, according to Evgenia Barannikova, a graduate student at the University of Maryland, Baltimore County. She presented her team’s research on bio-inspired lithium ion batteries today at the annual meeting of the Biophysical Society.
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Batteries in general comprise three main parts: two electrodes with opposite charges and an electrolyte that sits between the two. One of the electrodes is called the cathode, and it is made of two types of material that must be held in close proximity to one another in order for electrons to flow between them efficiently. One of the materials must have high electroactivity, a property that allows the lithium ions in the electrolyte to quickly insert themselves into and remove themselves from its chemical structure. This transfer of lithium drives the flow of electrons from the electrode to the external circuit. Performing this process efficiently requires that the material in the cathode is conductive as well, but electroactive materials have intrinsically low conductivity. As a result, combining electroactive and conductive materials improves the performance of the battery.
Using a technique that makes it possible to screen billions of peptides, the researchers isolated one that binds strongly to lithium magnesium nickel oxide, a highly electroactive material. For the conducting component, the researchers used a previously identified peptide that binds carbon nanotubes, a highly conductive material. By joining the two specialized peptides, it is possible to form a “nanobridge” between the two components of the cathode, giving the material structure at nanoscale. This helps to “prevent disaggregation of electroactive and conductive material, which currently results in loss of conductivity and low performance of some batteries,” Barannikova says.
Improving the performance of lithium ion batteries, which are rechargeable, has grown in importance as researchers look toward improving the power sources for electric vehicles. Tests of the new cathode thus far suggest that the nanostructured material performs better than current models for lithium ion batteries but Barannikova and her team are still working on creating an entire battery using these techniques.
Creating lithium ion batteries using materials structured at the nanoscale may have benefits beyond better performance, according to Barannikova. “Another aim is to see whether those peptides that have been identified will help us to synthesize those materials in environmentally friendly conditions,” she says. Organisms are able to assemble complex materials using peptides without industrial additives that can harm the environment, so it may be possible to avoid the need for the harsh solvents and high temperatures currently used in the cathode manufacturing process by following nature’s example.
“Biology has provided so many examples,” Barannikova says. “The solutions are there—we just need to look around.”