Low-Temperature-Processing of Printed Cathodes for Aqueous Metal-Air Batteries

Abstract

Developing low cost energy storage for integrated electronic applications depends on the ability to increase energy densities while reducing materials and manufacturing costs. Metal-air batteries are an emerging solution for next-generation energy storage given their superior theoretical energy storage capacity and use of low cost, earth abundant, and lightweight anode materials such as Zn, Al, and Mg. While metal-air batteries offer significant promise, inefficiencies at the air cathode dominate cell performance and limit practical energy densities. Recent studies in metal-air batteries have introduced new electrocatalysts to improve cathodic efficiency, but often rely on elevated temperatures and prolonged processing times to achieve suitable performance. Thus, rapid synthesis of air cathodes at low temperatures remains a significant challenge for mass production of metal-air batteries. In this work, we establish a printed air cathode architecture that can be fabricated below 100 °C, broadening the potential applications for metal-air batteries by lowering overall manufacturing costs and improving process compatibility for integrated systems. Furthermore, we investigate the importance of binder, solvent, and catalyst interactions and their influence on air cathode performance in a primary Zn-air battery. Additive manufacturing is used to demonstrate high throughput deposition and high accuracy patterning of metal-air battery materials. Stencil printing is chosen as a proof of concept technique to assemble the air cathode and Zn-air battery. Analogous to screen printing, stencil printing is capable of achieving thick active layers (10s-100s µm) for high capacity electrodes and is compatible with a variety of substrates including silicon wafers and flexible plastics. Cathode inks using polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) were compared to study the effects of the binder and associated solvent on the oxygen reduction reaction (ORR) as a function of processing temperature. Using a commercially available carbon powder with a platinum catalyst, cathodic half-cells were constructed to measure cell impedance and ORR overpotential as a function of binder material and annealing temperature. Thermogravimetric analysis (TGA) and x-ray photoelectron spectroscopy (XPS) were used to identify compositional changes, in both the cathode inks and printed films, related to solvent evaporation and binder degradation during annealing. Moreover, in operando pressure decay and differential electrochemical mass spectroscopy (DEMS) measurements were conducted to determine ORR efficiencies for the printed cathodes. After characterizing the printed air cathodes, printed Zn-air full-cells were fabricated to demonstrate the dependence of cell performance on binder type and annealing temperature. Higher operating voltages at a given current density were observed in cells with the PTFE binder compared to the PVDF binder, with the PTFE cells maintaining operating voltages above 1 V at current densities up to 24 mA cm-2. As a result, the PTFE cells exhibited peak power densities between 25-30 mW cm-2 at annealing temperatures as low as 80 °C, compared to 12 mW cm-2 at 350 °C for the PVDF cells. PTFE cells also displayed internal resistances less than 50 Ω with areal capacities between 2-5 mAh cm-2 with platinum concentrations as low as 5 wt%. This work represents the first demonstration of a low temperature printed air cathode for metal-air batteries. Our printed cathodes are broadly applicable to other aqueous metal-air battery systems and present a novel solution to fabricating low cost, high energy density batteries. The cathode inks are also compatible with several high throughput patterning techniques, including screen printing and slot-die coating, and could help achieve greater penetration of energy storage in emerging applications such as wearable and flexible electronics.