Researchers at Kogakuin University (Tokyo, Japan) have prototyped a transparent Li-ion battery that can charge itself by sunlight.
Using materials that are already generally used in making Li-ion batteries, the researchers were able to achieve near transparency by reducing the thickness of the battery's positive and electrodes to only 80 and 90 nm, respectively. When exposed to sunlight, the battery becomes slightly tinted, reducing green light transmittance to about 30%; after discharge, transmittance rises to about 60%.
Such transparent solar charged batteries, say the researchers, hold promise for applications such as smart windows, providing automatic tinting as well as energy storage for various purposes. Other applications could be displays or packaging on consumer and other devices.
According to the researchers, the battery has an output of 3.6 V, and has been tested over 20 charge/discharge cycles. Further testing will be needed to see how well it stands up to regular use.
The new battery builds on previous research at Stanford University (Stanford, CA) where, in 2011, scientists developed a transparent Li-ion battery that was also highly flexible. For details see the paper " Transparent lithium-ion batteries ."