Six years after Richard Feynman gave his famous lecture on nanotechnology, "There's plenty of room at the bottom", Twentieth Century Fox released a science fiction film called Fantastic Voyage in which a miniature spaceship just 1 µm in size travels inside a human body and removes a blood clot, all while evading the body's immune system. Given that a typical human cell is about 10 µm across and the latest computer chips have feature sizes of just 45 nm, making such nanorobots is no longer out of the question. However, the biggest challenge will be how to power these autonomous nanomachines.

Lithium-ion batteries consist of two electrodes, the anode (negative) and the cathode (positive), separated by an electrolyte – a conducting material through which charged ions can move easily. During cell discharge, the positively charged lithium ions travel across the electrolyte to the cathode and so produce an electric current. When the batteries are recharged, an external current forces the ions to move in the opposite direction so that they can be stored at the anode.

Expanding anodes

Tin oxide is ideal for making the anode in lithium-ion batteries because it has a high energy density. However, upon charging, the material expands, which leads to cracking and reduced electrical conductivity, and eventually battery failure – a problem that is particularly serious in practical nanobatteries.

To investigate this expansion in detail, Jianyu Huang and colleagues made a working prototype of a nanobattery that comprises a single nanowire anode made of tin dioxide that can be charged and discharged. The battery also contains a specially designed ionic liquid electrolyte that can withstand the high vacuum of a transmission electron microscope (TEM) and a bulk lithium cobalt oxide cathode. The researchers loaded the device into the TEM to see exactly what happened when they applied –4 V against the lithium anode.

The team observed that when the tin dioxide nanowire was charged up, it swelled, twisted and then elongated (see video). These changes come about because of a process called lithiation, common to all lithium-ion batteries. This is when ions, extracted from the cathode during charging squeeze into the anode. By observing lithiation as it happens, Huang and colleagues say that their work might help design more advanced nanobatteries by understanding how the electrode accommodates volume changes associated with this process. "This will help us understand why a battery fails following cyclic charging and discharging," says team member Chongmin Wang.

Surprising behaviour

The fact that the anode nanowire elongates to nearly twice its size came as a surprise, says Huang. Normally, the wire should expand in the radial direction rather than along its length. The nanowire twisting during charging and discharging is also unexpected and astonishing, he adds. "Such behaviour must be taken into account if we want to design and build standalone nanowire batteries, because it is electrical shorting as a result of these transformations that leads to battery failure."

The researchers didn't stop there; they also recorded how the microstructure of the battery evolved during charging. They observed that it changed from being an ordered, crystalline solid to being disordered and amorphous. "We saw that a high density of mobile dislocations nucleate and are absorbed at the chemical reaction (or 'Medusa') front with the dislocation cloud serving as a precursor to solid-state amorphization," explained Huang. Electrochemical solid-state amorphization is a poorly understood process by which a crystalline material changes to an amorphous material. Amorphization will degrade a device, and controlling it is crucial for how well batteries perform and how long they last.

"While we ran short of demonstrating a fully packaged nanobattery, we believe we have made an important step towards an important goal in nanotechnology – building a single-nanowire battery consisting of a nanowire anode and cathode, and nanoscale electrolyte and packaging," Huang said.

The research, which was reported in Science (330 1515), will ultimately help create batteries with high energy densities, high power density and long cycle lifetimes.