Researchers at Rice University (Houston) under Pulickel Ajayan, a professor of mechanical engineering and materials science, have created the smallest lithium-ion battery yet. As reported in Nano Letters, the scientists have designed a lithium-ion energy-storage device into a single nanowire, paving the way for a miniaturized rechargeable source to power a new generation of nanoelectronic products. Miniaturized medical devices could also possibly benefit from this new technology.

The researchers have tested two versions of their battery/supercapacitor hybrid. The first--a sandwich with nickel/tin anode, polyethylene oxide (PEO) electrolyte, and polyaniline (PANI) cathode layers--was built as proof that lithium ions will move efficiently through the anode to the electrolyte and then to the supercapacitor-like cathode, which stores the ions in bulk and gives the device the ability to charge and discharge quickly. The second version packs the same capabilities into a single nanowire. The researchers built centimeter-scale arrays containing thousands of nanowire devices, each about 150 nm wide.

The scientists' process, according to Ajayan, tucks the cathode inside the nanowires by using PEO as a gel-like electrolyte that stores lithium ions and also serves as an electrical insulator between nanowires in an array. After much trial and error, they decided to use PANI, an easily synthesized polymer, for the cathode. Drop-coating the widened alumina pores with PEO coats the insides, encases the anodes, and leaves tubes at the top into which PANI cathodes can also be drop-coated. An aluminum current collector placed on top of the array completes the circuit.

Measuring about 50 µm tall, the experimental batteries can be as long and wide as the templates allow, making them scalable. In addition, they exhibit good capacity. However, because the nanowire devices drop off after about 20 cycles, the researchers are fine-tuning the materials to increase their ability to repeatedly charge and discharge.