UC Berkeley

The prevalence of wireless device networks for the Internet of Things (IoT) depends on the development of integrated energy storage. Today, IoT devices are powered either through a wired connection, which greatly inhibits their broad distribution, or by coin cell batteries, which constrain device size, weight, cost, and form factor. New approaches to energy storage design such as printing-based fabrication techniques are well suited to simplify device integration. Using additive, high-throughput processing methods, batteries can be printed to accommodate millimeter-scale nodes and seamlessly integrated into semiconductor packaging flows. While previous studies have focused on developing printable electrode, separator, and current collector inks, none have emphasized scaling printed battery size and power to accommodate IoT system requirements. This dissertation addresses several challenges associated with designing millimeter-scale energy storage for IoT technologies, utilizing printing techniques to create batteries that could enable scalable wireless electronic networks. A comprehensive approach is taken to achieve battery scaling and integration, highlighting the interconnected impact of processing, structure, properties, and performance of printed battery materials.

Printed Zn-based batteries are explored given their compatibility with IoT performance requirements, offering high energy densities, high discharge rate capabilities, and steady discharge potentials. Zn battery chemistries are also inherently air-stable, require modest packaging, and use low-cost, earth-abundant materials, making them excellent candidates for additive integration with wireless electronics. Printed Zn-Ag2O and Zn-air batteries are presented in this work, implementing vertical cell architectures to minimize battery footprints and thick electrode films to maximize cell capacity. In both chemistries, printed battery performance is evaluated as a function of materials processing and design, with a focus on minimizing battery sizes and processing temperatures. By co-optimizing each cell component, new performance benchmarks are established for printed batteries, achieving areal capacities above 10 mAh cm-2 and power densities well above 10 mW cm-2 with millimeter-scale areas and processing temperatures below 200 °C.

Additionally, this dissertation utilizes operando characterization techniques to further study the impact of materials processing and design on printed battery performance. Operando techniques offer an emerging set of tools to investigate transient, non-equilibrium materials such as electrochemical interfaces. In this work, operando methods enable the direct observation of electrochemical reactions in printed Zn batteries, which guides battery development to improve performance and mitigate degradation. First, oxygen reduction is measured in Zn-air batteries to determine the influence of electrode composition and processing temperatures on cathodic efficiency. Second, hydrogen evolution and oxide formation are examined in Zn-air batteries to identify design and operation parameters that affect anodic corrosion. Overall, these techniques aim to complement ex-situ analysis of printed batteries and elucidate relationships between physical or chemical changes and improvements in battery performance.

Finally, this work discusses system level integration strategies for printed Zn batteries and IoT devices. Proof of concept packaging and interconnect designs are demonstrated using printing techniques for millimeter-scale and commercially available sensor nodes. Printed Zn-Ag2O batteries are also simulated under IoT operation modes to determine performance benchmarks in an integrated system with energy harvesting. Moreover, printed Zn-Ag2O battery arrays are explored to support high voltage applications.