Researchers create transparent and flexible lithium-ion battery

US researchers have invented a transparent lithium-ion battery that could lead to see-through electronic devices.

Making batteries transparent would allow scientists to more easily study what happens inside them, but they could also have a commercial use as several companies have already designed partially see-through gadgets such as mobile phones.

Researchers at Stanford University, California, used a clear silicon-based polymer as the basis for the battery and, as some of the key active materials could not be made transparent, they made the opaque components smaller than the human eye could see.

‘If something is smaller than 50 microns, your eyes will feel like it is transparent,’ said graduate student Yuan Yang, author of a paper on the research published in the Proceedings of the National Academy of Sciences.

Together with Yi Cui, an associate professor at Stanford’s SLAC National Accelerator Laboratory, Yang created a mesh-like framework for the battery electrodes with each ‘line’ in the grid measuring around 35 microns in width.

The transparent material used was a slightly rubbery compound known as polydimethylsiloxane (PDMS).

‘PDMS is pretty cheap, and already being used in plastic surgery and contact lenses,’ said Yang. ‘But it is not conductive, so we had to deposit metals onto it to make it conductive.’

To do this, the PDMS was poured into silicon moulds to create grid-patterned trenches and a metal film was evaporated over the trenches to create a conductive layer.

A liquid slurry solution containing minuscule, nano-sized active electrode materials was then dropped into the trenches.

Another key aspect of the process was developing a transparent substance to separate the electrodes, for which the researchers modified an existing gel electrolyte.

The battery appears transparent as long as the gridlines in the mesh electrodes are aligned. Light transmittance tests showed the battery was 62 per cent transparent in visible light and 60 per cent even with three full cells stacked on top of each other.

The finished device is also flexible and can be kept relatively cheap, but is only half as powerful as comparably sized lithium-ion counterparts.

‘Its cost could be similar to those of regular batteries,’ said Cui. ‘Especially if we use low-cost metals, such as current collectors, there is no reason this cannot be cheap.’