The increasing energy demands of mobile electronic technologies such as cellular phones, laptops and electronic readers require energy storage devices with high specific capacities. In lithium-ion batteries, electrodes of vanadium oxide and silicon theoretically offer the highest performance and thus the greatest potential for improving energy storage capabilities. However, in practice, their capacity to store energy fades on repeated charging and discharging. In a collaboration between Wuhan University of Technology in China and Harvard University in the USA, Liqiang Mai, Yajie Dong and colleagues1 have now shrunk lithium-ion batteries down to nanometer dimensions in an attempt to understand and control capacity loss.

Fig. 1: Schematic representation of a device based on a single nanowire (vanadium oxide or silicon) electrode.

In a lithium-ion battery, lithium ions move out of the cathode on charging and back into the cathode on discharging. The electrodes can accommodate a considerable change in material volume upon lithium ion insertion and removal, and scaling them down to the nanometer level has been shown to improve cycling stability. To investigate how far this scaling improvement could go, Mai, Dong and their co-workers created an all-solid-state electrical energy storage device based on a single nanowire electrode. Their simple device, with conventional counter electrodes and solid electrolytes, allowed them to probe the correlation between material composition, structure, electrical transport and electrochemistry of the nanowire electrode.

Experiments with a vanadium oxide nanowire cathode revealed that the capacity was fully recoverable on charging and discharging at 100 picoamps for up to 200 seconds, but after 400 seconds the capacity began to fade and could not be recovered. These findings suggest that the absorption of too many lithium ions by the vanadium oxide electrode causes irreversible structure change.

The researchers also conducted experiments using a silicon/amorphous silicon core/shell nanowire anode. They found that the conductance of the silicon nanowire decreased monotonically as the number of cycles increased, indicating that permanent structural change had occurred. Raman spectroscopy confirmed that after the insertion and extraction of lithium ions during cycling, the crystalline silicon core had transformed into a metastable amorphous LiSi alloy.

“Our findings show that our single-nanowire energy storage device is promising for investigating the performance of battery materials and also offers opportunities for developing nanodevices to power nanosystems,” says Mai.