Perspectives on Energy Storage for Flexible Electronic Systems

If truly thin embedded and human worn flexible electronics are to become a commercial reality for wearable electronics, medical devices, and internet of things tags, effective energy storage technologies that safely and robustly match the mechanical flexibility of the overall system form factor are required. At the same time, the energy and transient power needs of functions such as wireless connectivity, information display, and high sample rate sensing must be supported. These capabilities have time-dependent power and current requirements often not captured in simple energy and capacity metrics. In this paper, a progression of energy storage approaches, challenges and learning experiences will be presented from the perspective of an energy storage technology developer. The essential requirements for energy storage for feature-driven applications in flexible electronics are addressed with the goal of finding the most compelling fit between products needs, consumer safety and the technology capabilities of different energy storage approaches. Micropower modules from supercapacitors to microbatteries and their limitations for flexible electronics will be discussed in terms of capacity, power and charge retention as the starting point. Following this discussion, limitations of lithium technologies in this flexible and thin (< 1 mm) application space are also outlined. This paper then presents a review of key requirements for energy storage for high functionality flexible electronics prototype systems and some approaches that have been explored to meet those needs. This leads to the conclusion that safe, low cost, flexible electronics energy storage requirements may be most appropriately met using intrinsically stable battery chemistry. Furthermore, such a materials approach allows for simpler lower cost processing and packaging, such as additive printing and roll to roll processing of thin and therefore more mechanically flexible cells. Examples and performance data from such a zinc polymer battery technology are given and compared to other thin and flexible battery approaches.