Rajan Kumar1 Vivek Subramanian1

1, University of California, Berkeley, Berkeley, California, United States

The prevalence of devices used for the Internet of Things (IoT) relies heavily on the development and incorporation of integrated power sources and energy storage. Printed batteries are an emerging solution for on-device power requirements using low cost, high accuracy fabrication techniques. While several printed batteries have been previously shown, few have designed a battery that could be incorporated into an integrated device. Specifically, a printed battery with a small active electrode area (< 1 cm2) demonstrating high areal capacities (> 10 mAh cm-2) at high current densities (1-10 mA cm-2) has not been developed. This work addresses these challenges by investigating the scaling limits of a printed Zn-Ag2O battery and determining the materials and processing limitations for developing a mm2-scale battery.
Zn-Ag2O is the chosen battery chemistry given its inherent air stability, high energy density (130 Wh kg-1), and high discharge rate capability. Unlike Zn-MnO2 or Li-ion, Zn-Ag2O batteries also maintain a steady discharge voltage over a wide range of discharge rates, which is desirable for IoT applications to minimize power electronics requirements and lower overall power consumption. To assemble the battery, stencil printing was chosen based on its low cost and compatibility with various substrates including flexible plastics. Stencil printing is also better suited than other printing methods such as inkjet or gravure in order to print the thick active layers (10s-100s µm) necessary to achieve high areal capacities. Processing temperatures of each battery component were below 150°C to remain compatible with low cost, flexible substrates. Mass loading of the anode and cathode inks was optimized to maximize cell capacity while maintaining ink viscosities compatible with stencil printing. Batteries were printed with active areas between 0.01 and 0.25 cm2 and discharged at current densities between 1-20 mA cm-2. Electronic conductivity of the electrodes and ionic conductivity of the electrolyte were measured as a function of active area. In addition, electrochemical testing including impedance spectroscopy and cyclic voltammetry was performed to determine the impact of scaling on cell degradation mechanisms.
The fully printed Zn-Ag2O batteries demonstrated a steady discharge voltage of 1.45 to 1.5 V and high areal capacities of 8-12 mAh cm-2 at current densities between 1-10 mA cm-2. Self-discharge lifetimes of one month were obtained with the use of a PDMS encapsulation layer and internal resistances were typically less than 30 Ω. This work represents the first demonstration of a small, packaged, fully printed Zn-Ag2O battery with high areal capacities at high current densities, a crucial step towards realizing integrated energy storage for printed electronics systems.