In Situ Optical Methods to Quantify Macroscopic Stress Developed on Electrode Materials During Lithiation

<br/>With the advent of mobile electronics, the need for durable high energy density storage devices like lithium-metal batteries has surged many folds in recent times. To transform the electronics industry, this need must be met as more rigid, bulky devices are replaced by their thin, lightweight yet high-performance counterparts. However, the mechanical stress induced due to electrochemical deposition of Li on the surface of the current collector of lithium-metal batteries still remains a major area of concern, directly affecting the durability of the cell. In this paper, two novel non-contact in-situ techniques are used to characterize mechanical stress experienced by electrodes during Li electroplating. Optically transparent electrochemical cells were prepared with a soft Li metal sheet as the reference electrode and Cu coated PET sheet as the working electrode or current collector. Lithium hexafluorophosphate solution in ethylene carbonate and diethyl carbonate (1.0 M LiPF6 in EC/DEC=50/50 v/v) is used as the electrolyte. The first characterization method involves a camera setup to monitor the bending angle during charge and discharge cycles. The second technique uses the reflection of a helium neon laser beam off the shiny PET surface. The displacement of the reflected beam is observed and further quantified to obtain the change in bending angle. Both methods are sensitive to structural changes during lithiation. Before Li deposition, the cell is first activated using a LSV (linear sweep voltammetry) method at 5mV s-1 and the ending voltage is set to 0 V. During this process, a minor amount of Cu oxide layer on the surface of Cu coated PET is gradually lithiated and the Solid-Electrolyte Interphase (SEI) layer is formed, which is ready for inducing further Li metal deposition. The change in bending angle was recorded with and without the SEI layer formation. Other parameters like charge density, charge time, number of charge-discharge cycles, and idle time between cycles were varied to observe the difference in bending. The angle change is then used to quantify stress. This type of in-situ stress measurement method is particularly useful to test the reliability of electrodes in cells by studying the physical changes of those materials and their surfaces as they interact with lithium.