Recent technological advances in microelectromechanical systems fabrication, low-power microprocessors and data analytics have led to the development of new consumer products like smart trackers, smart clothing, portable medical devices, integrated artificial skins etc., where there is a need for flexible and wearable electronics. These low-cost and connected consumer devices form a new eco-system usually referred to as the ‘Internet of Things’ or IoT, which are expected to drastically change our lives. The rapid growth of the flexible electronics market to serve IoT has also dictated the development of flexible batteries as power sources, where the characteristics of these batteries are fundamentally different to conventional thick batteries. The batteries for IoT applications need to be low cost, extremely safe, have high energy density, have small footprint, be flexible and perform when stretched to unique form factors. Lithium (Li)-ion batteries have some of these characteristics, however, there are still concerns around its safety and flammability. Zinc (Zn)-anode batteries are a practical alternative because it does meet all the stringent requirements. Its high capacity (areal and volumetric) also makes it highly power and energy dense volumetrically and gravimetrically.

In general, products with superior quality or limited resources are typically expensive. That is the challenge being faced now in the battery world. Can this "and" problem be solved, i.e., can a battery with abundant and cheap materials and with good electrochemical performance be manufactured? This work aims to answer this question by tackling challenges of having both high energy density and safety features in thin film batteries for wearable devices and IoT applications. In the market, the most used types of thin film batteries are Li-ion and Zn- manganese dioxide (MnO₂), where the latter is mainly used in primary (single discharge) applications. Currently in the market, thin film Li-ion batteries that are less than 1.5 mm thin can achieve an energy density up to 378 Wh/L. Thus, they can be utilized as very powerful and efficient energy resources for a number of IoT applications. However, given the importance of safety and landfill compatibility, the aqueous solid-state electrolyte-based Zn-anode battery chemistry is considered a practical alternative solution for wearable devices and a good amount of IoT applications. In the first part of this presentation, a comprehensive literature survey of the commercially available flexible batteries is discussed. Given the advantages of Zn- silver (Ag) battery, such as high energy density, abundant raw materials, non-flammable components and safety, it is considered as a practical power source for wearable devices and IoT applications.

To tackle the challenges of fabricating batteries with robust mechanical elasticity and the same time not compromising electrochemical performance, environmentally benign and easy to be handled elastomer materials have been explored in this work to endow the stretchability and flexibility to the battery. In the second part, work on developing a thin film aqueous solid-state electrolyte-based flexible and stretchable Zn-Ag battery with high throughput and low manufacturing cost is presented. This battery started with a 267 Wh/L energy density and a low internal impedance compared to Li-ion coin cell when using the same Bluetooth pulse discharge profile. Moreover, uninterrupted battery performance is shown after repeated mechanical deformation tests, such as bending, twisting, and stretching. Overall, pairing the superior electrochemical and mechanical performance, the thin film flexible and stretchable Zn|Ag battery is proven to be well-suited to power various wearable and flexible electronics reliably and sustainably.

Since there are elastomer materials incorporated in the thin film flexible battery to endow the flexibility and stretchability to the battery, more percentage of dead materials is in the battery which leads to a relatively lower energy density compared to conventional battery with calendared electrodes. Hence, a more rigid but still bendable Zn-Ag battery is made for wearable devices with bendable batteries. This battery has a higher energy density of more than 400 Wh/L, and good electrochemical performance after repeated bending. More mechanical deformation evaluations are taking place, such as bending stiffness, tensile strain, and yield stress.

In the last part of the talk, a future road map of answering the aforementioned question will be discussed, such as increasing battery energy density by reducing non-active material loading and increasing active materials utilization, extend battery rechargeability, improve rate capability, increasing battery shelf life and exploring cheaper cathode materials.