UC Berkeley

The advent of printed and wearable electronics has generated a renewed interest in the development of new, novel form factors for high-capacity, flexible, rechargeable batteries. This research aims to enable such advancements by characterizing and optimizing the manufacturing processes and component interactions of printed, rechargeable, zinc-based batteries by utilizing an ionic liquid-based gel polymer electrolyte.

The ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][OTf]) with zinc trifluoromethanesulfonate (Zn(OTf)2) salt dissolved in it comprises the electrolyte of interest. As a non-aqueous medium, it has been shown to enable rechargeability in zinc-based battery chemistries. When the electrolyte is combined with poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP), a solid, stable, flexible electrolyte is produced. Is is through the lens of this material that the function and characterization of printed, rechargeable batteries comprised of zinc (Zn) and manganese dioxide-based (MnO2) electrodes are investigated.

With the use of this novel electrolyte, additional variables must be taken into account in order to successfully and consistently produce high performance batteries. The printed electrode surface morphology, electrolyte composition, and manufacturing environment play significant roles in affecting the performance of the battery cells and the components within. To characterize these aspects, battery components were produced via stencil printing, dispenser printing, and casting. Their properties were investigated with electrochemical impedance spectroscopy, cyclic voltammetry, laser confocal microscopy, scanning electron microscopy, Karl Fischer titration, contact angle measurements, and cycle life tests.

The gel polymer electrolyte (GPE) was found to poorly wet the rough, wavy surfaces of the printed electrodes. Within features and cavities on the surface, air bubbles were trapped that increased the interfacial impedance and prevented cell cycling. This was overcome by wetting the interfaces with the ionic liquid-based electrolyte prior to cell assembly in order to produce cyclable batteries. With the manufacturing method determined and the surface morphology characterized, several electrolyte and gel polymer electrolyte compositions were investigated. Higher concentrations of Zn(OTf)2 resulted in cells with higher discharge capacities and higher current densities during cyclic voltammetry tests indicating the importance of having adequate charge carriers within the electrolyte. The amount of environmental water absorbed by the electrolyte was also found to be a function of the quantity of dissolved Zn(OTf)2, where more salt resulted in more higher water content. Cell testing and component characterization determined that the presence of some water within the electrolyte was beneficial to enable improved cell discharge capacities. As a result, it was shown that printed, rechargeable, zinc-based batteries can be manufactured successfully in an ambient laboratory environment without strict air quality control.

The optimal component compositions, as determined from the empirical analyses and characterization experiments, enabled the production of 25 cells with 100% yield. These cells exhibited an average discharge capacity of 0.6 mAh/cm^2 with a maximum of 1.0 mAh/cm^2 over many cycles. The cells also successfully powered a commercial, off-the-shelf microcontrol unit to confirm that printed, rechargeable, zinc-based batteries are capable of being deployed with conventional electronics.

The advancements made within this research have successfully produced and characterized printed, rechargeable, zinc-based batteries with an ionic liquid-based gel polymer electrolyte for printed and flexible electronics. The results presented herein will provide a basis with which many printed systems can be better understood and developed, especially for those comprised of multiple layers.