Unveiling Mechanical Stress in Lithium-Metal Batteries for Flexible Electronics: A Novel Approach with Optical Techniques and Artificial Interfaces

The widespread use of electronics has created a growing demand for high-energy-density Lithium-metal batteries in recent times. This demand has significantly increased as more wearable devices are being designed for applications like soft robotics, health monitoring, and movement tracking, all of which require flexible batteries. The longevity of these batteries relies on their ability to repeatedly bend and flex without damage. One crucial concern is the internal mechanical stress on electrodes caused by lithiation. Dendrite formation during lithium plating not only heightens this stress but also raises safety issues and reduces battery capacity. One solution is to deposit an artificial solid electrolyte interphase on the current collector before lithiation, ensuring a dendrite-free Li layer. This paper introduces novel optical reliability methods using camera and laser to gauge the mechanical stress on flexible electrodes. These methods are simpler and more efficient, allowing for stress measurement using a camera instead of relying on complex systems like the multibeam optical stress sensor (MOSS). Within a transparent electrochemical cell, we use Cu-coated polyethylene terephthalate (PET) as the working electrode and a Li metal sheet as the reference. The electrolyte comprises a solution of lithium hexafluorophosphate in ethylene carbonate and diethyl carbonate (1.0 M LiPF6 in EC/DEC=50/50 v/v). Due to its lower Young's modulus, PET is the chosen substrate, making it highly sensitive to stress. The in-situ observation of the bending angle during lithiation under varied charge and discharge cycles provides insights into stress development.