Cross-Sectional In Situ Optical Microscopy with Simultaneous Electrochemical Measurements for Lithium-Ion Full Cells

A new in situ optical microscopy set-up is introduced which allows direct observation of cross-sections of Li-ion full cells in combination with simultaneous recording of electrochemical data. The method is validated by comparison of electrochemical data from coin full cells. Color changes give insights into processes on the electrode and particle level, such as lithiation behavior and electrode thickness changes. Our observations allow the evaluation of (i) the speed of lithiation fronts for LiC₁₂ and LiC₆ through anode coatings, (ii) estimation of apparent diffusion coefficients from analysis of the color distribution in single graphite particles, as well as (iii) electrical de-contacting and re-contacting of single graphite particles in connection with (iv) electrode thickness changes. Furthermore, our direct observations from the inside of full cells give indirect insights into aging phenomena such as Li plating and SEI growth.