Foldable water-activated reserve battery with diverse voltages.

A reserve battery is a device which is inert until its activation and generates electricity by injecting an electrolyte for the purpose of immediate use. Due to a relatively short history and the use in restricted fields, reserve batteries have not attracted attention without any technical advance such as be being flexible and foldable. In this study, we demonstrate a way of fabricating a flexible and even foldable reserve battery which is activated by various solutions. A paper electrode composed of cellulose and carbon nanotube, which is working as a cathode, was assembled with a sheet of an aluminum anode. The injection of a NaCl solution resulted in approximately 0.7 V while a KOH solution led to a much higher voltage of 1.3 V than the NaCl electrolyte. Impedance analysis unveiled that the best discharge performance was found in the reserve battery showing the smallest semicircle in impedance spectra, irrespective of electrolytes. And, folding the battery did not degrade the discharge performance, compared with an unfolded battery. Furthermore, the battery cell was even activated by seawater, resulting in about 0.7 V and a nice discharge performance. We think that our battery system can be extended to other reserve batteries requiring flexibility and foldability.