Recent Progress on Printed Flexible Batteries: Mechanical Challenges, Printing Technologies, and Future Prospects

Advances in wireless technologies, low-power microelectronic devices, and easy access to the internet have enabled the ability to interconnect electronic objects, so that the objects can communicate with each other, make decisions, and provide users information that helps to improve their life. Towards such an ecosystem, low-cost, wearable, flexible electronics in the form of smart watches, sensors embedded in clothing, activity trackers, and health-monitoring tags would be widely adopted. Future generations of these devices would be thin, conformable to the human body, ubiquitous, and almost imperceptible to the user. Powering these devices while retaining their mechanical properties will be a challenge. Batteries are essential for powering portable electronic devices. A battery is a closed system in which energy is stored in chemical form, and it is converted to electrical energy by connecting the battery to an external load to complete the circuit, causing electric current to flow between the anode and cathode. Electronics have traditionally been designed around commercial batteries—prismatic, cylindrical, and coin cells—which are bulky, rigid, and non-flexible, making them unsuitable for powering flexible electronics. A power source for a flexible electronic device should be thin, bendable, and mechanically compliant. Flexible electronics are fabricated by patterning traditional inorganic components in ultra-thin form or by depositing solution-processed organic/inorganic semiconductors and conductive inks on flexible substrates. Due to the thinness of active layers and conducting electrodes, these devices can be flexed to low bending radii without reaching their fracture limit (strain 1.0%). To fabricate flexible batteries, all of the rigid components of a battery are replaced by flexible counter parts. This includes replacing rigid packaging with flexible pouches, the use of flexible current collectors, and designing the active layers and interfaces to the current collectors such that cracking and delamination are prevented. An electrochemical cell consists of active layers supported on conductive substrates (current collectors) to form the anode and cathode of the battery. The electrolyte provides ionic contact between the electrodes and helps to complete the redox reactions within the cell. Printing processes such as screen, stencil, and blade printing are well established and they can be used to deposit battery components by designing printable inks for the active layers, current collectors, and electrolyte. Batteries fabricated using printing processes have the advantage of low cost, flexible form factor, ease of production, and integration with electronic devices. The term “printed battery” is used to describe a battery for which at least one of the components is solution processed and deposited using a printer. The non-printed component (current collector or separator) serves as a support for the printed components. The mechanical characteristics of a printed battery depend on its architecture and design. Over the past couple of years, there has been significant progress towards using printing-based processes to fabricate power sources. Researchers have demonstrated full-cells using conTraditional printing methods offer the advantage of well-matured technology, high accuracy of depositing inks over flexible substrates at high web speeds, and low cost of fabrication. The components of a battery—the current collectors, active layers, and separator—can all be deposited using conventional printing techniques by designing suitable inks. A combination of low thickness of printed electrodes, flexible packaging, battery architecture, and material properties makes printed batteries flexible. In this paper, we will discuss material challenges and mechanical limits of flexible printed batteries. We will review several printing techniques and present examples of batteries printed using these methods. In addition, we will briefly discuss other novel non-printed compliant batteries that have unique mechanical form.