UC Berkeley

Advancements in manufacturing for printed electronics have brought with it a demand for printed, rechargeable batteries capable of powering these devices. This research aims to meet this demand by characterizing and optimizing components and performance of a printed, rechargeable Zn-MnO2 cell with an ionic liquid gel polymer electrolyte.

The cathode and anode are comprised of manganese dioxide (MnO2) and zinc (Zn) respectively. The ionic liquids 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]+[OTf]-) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]+[OTf]-) with the dissolved salt zinc trifluoromethanesulfonate (Zn(OTf)2) are used as the electrolyte. These nonaqueous electrolytes have been shown to enable rechargeable zinc chemistries as well as compatibility with solution-cast polymer membranes to form gel polymer electrolytes (GPEs). When combined with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) that is dissolved in n-methyl-2-pyrrolidone, a printable separator membrane capable of ionic conductance is produced. This separator membrane can then be incorporated into a layer-by-layer additive manufacturing process to produce a fully printed battery.

To produce consistent fully printed cells and to prevent catastrophic cell degradation and failure, additional variables must be considered and optimized. The printed electrode composition must be optimized for surface profile, rheology, and electric conductivity in order to enable uniform and dense separator layers. The ionic liquid electrolyte composition and gel polymer electrolyte morphology must be controlled in order to optimize mass transport and reaction kinetics. Furthermore, the mass transport and kinetic properties of Zn deposition and dissolution in ionic liquid electrolytes and gel polymer electrolytes must be characterized and understood. To characterize these aspects, battery components were produced via stencil casting, doctor blade coating, and solution casting. Electrochemical properties were characterized by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. Physical and electric properties were characterized by rheometry, profilometry, four point probe resistivity, and scanning electron microscopy. Cell performance was characterized by galvanostatic and potentiostatic cycle life testing and pulsed current discharging, and differential capacity analysis was performed for further insights.

Ball milling and mechanical sieving was found to be an effective method to improve printed electrode surface profile, reducing waviness and roughness each by a factor of 4 compared to unoptimized electrodes while retaining similar conductivity.

Zn deposition and dissolution were found to be kinetically (rather than mass transport) limited and therefore quasireversible in both ionic liquids. Zn2+ diffusivity ranged from 1.42-3.38e-9 cm2/s in [BMIM]+[OTf]- electrolytes and from 1.64-4.87e-9 cm2/s in [EMIM]+[OTf]- electrolytes, decreasing with higher concentrations of Zn(Otf)2 salt. [EMIM]+[OTf]- electrolytes were found to provide higher Zn diffusivity, ionic conductivity, and redox kinetics, but were also more prone to forming electrical shorts through the GPE and offered lower discharge capacities than [BMIM]+[OTf]- electrolytes. The polymer structure in the GPE was found to be highly dependent on processing temperature and pressure.

Fully printed cells were produced that prevented catastrophic cell failure from delamination of the cathode, resulting in at least a 20x improvement in cycle life compared to mechanically assembled cells. Average discharge capacities for fully printed cells ranged from 1.8-3.6 mAh/cm3, and cells were cycled at least 200 times without catastrophic failure. Critical cell failure mechanisms including phase transformations of MnO2 and deposition of Zn within poor GPE microstructures were identified. Finally, printed cells were discharged under a pulsed current discharge regime typical of those found in Internet of Things devices for 650 cycles without signs of degradation.

This research has successfully produced and characterized fully printed, rechargeable, Zn-MnO2 cells with an ionic liquid gel polymer electrolyte for printed electronics and the Internet of Things. The results presented herein seek to further understanding of Zn interactions with ionic liquid gel polymer electrolytes and additive manufacturing methods for printed energy storage devices.