Add Lithium-ion batteries to the list of electronic components that can be fabricated by spray-on and, one might suppose, ink jetting techniques.
Researchers at Rice University say they have come up with a way to spray the layers of a rechargeable lithium-ion battery onto ordinary substrates. To demonstrate the idea, the researchers sprayed a small battery onto a ceramic beer stein. Of course, the gauges by which batteries are measured include energy density and number of practical recharge cycles. On those points, there remains a lot of work to do on the idea, researchers admit.
Writing in the online journal Scientific Reports, researchers Neelam Singh, Charudatta Galande, Andrea Miranda, Akshay Mathkar, Wei Gao, Arava Leela Mohana Reddy, Alexandru Vlad and Pulickel M. Ajayan say they chose Lithium Cobalt Oxide for the positive electrode and Lithium Titanium Oxide for negative negative electrode to get an effective cell voltage of about 2.5 V. Graphite anode or high voltage cathodes could be used to increase the nominal voltage, they say, but graphite based Li-ion batteries have safety concerns.
The group dispersed single-walled nanotube (SWNT) material ultrasonically in 1-methyl-2-pyrrolidone (NMP) to form viscous, highly consistent inks suitable for spray painting to produce current collectors. Paint used to form positive electrodes was made by adding a mixture of LCO, carbon and ultrafine graphite (UFG) into Polyvinylidine fluoride (PVDF) binder solution in NMP. The group says that in Li-ion polymer batteries, it is crucial to control the microporosity of the polymer separators so the battery can retain its capacity through numerous charge/discharge cycles. This took some doing. Measures taken to reduce the porosity of the separators also reduced the ionic conductivity of the battery electrolyte, so there are tradeoffs there.
To get an anode, the group used a mixture of LTO and ultra-fine carbon was dispersed in an PVDF binder solution in NMP. The negative current collector was commercially available copper conductive paint diluted with ethanol. Li-ion batteries were fabricated by sequentially spraying the component paints on desired surface using an airbrush operating at 40 psi. The paints are sprayed through a set of masks to get the desired device geometry. A heat gun or hot plate controlled the substrate temperature. Cu and Al metal tabs were attached to the negative and positive current collectors respectively. The finished cell was transferred to an argon-filled glove box and activated by soaking for at least two hours in an electrolyte mixture of ethylene carbonate and dimethyl carbonate, then laminated.
The group has filed for a patent and continues to work on perfecting the process and materials used.
The Scientific Reports paper is here: http://www.nature.com/srep/2012/120628/srep00481/full/srep00481.html
They also posted a YouTube video: http://www.youtube.com/watch?v=qJDI5cAdhys&feature=player_embedded