Lithium-ion batteries are widely used in consumer electronics such as cell phones and laptop computers, and more recently as a means of storing energy for electric vehicles. There are three important components that comprise a lithium-ion battery: the anode, the electrolyte and the cathode. The anode is usually graphite, the electrolyte is a solution of a lithium salt in an organic solvent, and the cathode is commonly made from lithium iron phosphate.

The key characteristic of a lithium-ion battery is its charging and discharging capacity — how much energy can be stored and released in a single use–recharge cycle, and the corresponding rate of energy transfer. The latter is particularly important for high-power applications such as electric vehicles. Cathodes prepared from nanostructured anisotropic materials are widely predicted to have superior properties over more traditional cathode materials, but the first step is to develop a reliable method of preparing them. Kisuk Kang, Chan Beum Park and co-workers from KAIST in Korea have now used peptide nanofibers as a template for assembling iron phosphate nanotubes1.

“We were fascinated by the way hybrid organic–inorganic materials are formed in nature, and wanted to use a similar process to prepare nanostructured materials for lithium-ion battery applications,” says Kang. The team prepared a peptide hydrogel by adding water to a solution of a dipeptide in an organic solution, which resulted in a crosslinked web of peptide fibers. Multiple acidic groups were displayed at the surface of the gel fibers. When this gel was treated with iron chloride, the metal ions were adsorbed at the surface. Subsequent treatment of these metalized fibers with phosphate caused the formation of an iron phosphate coating. The whole process is reminiscent of the way in which bones are formed by the coating of collagen fibers with calcium phosphate.

Fig. 1: False-color transmission electron microscopy image of the iron phosphate nanotubes.© 2010 Wiley-VCH

The iron phosphate shell remained intact when heated to 350 °C, but the peptide fiber was transformed into a layer of carbon such that the overall structure became a cross-linked web of iron phosphate nanotubes with an internal coating of carbon (Fig. 1). These nanotubes were found to be superior to other iron phosphate nanostructures as a cathode material. “The carbon coating is at least partly responsible, so our methodology for making the fibers is both simple and has intrinsic advantages,” explains Kang.