Unravelling the Capacity Degradation Mechanism of Thick Electrodes in Lithium‐Carbon Dioxide Batteries via Visualization and Quantitative Techniques

Abstract Lithium‐carbon dioxide batteries (LCBs) require a thick cathode electrode to fulfill their theoretical energy density and high areal capacity (mAh cm −2 ). However, understanding the design of thick porous electrodes in LCBs is challenging because of the complexity of coupled multispecies transport. Herein, a link is established between the microscopic behaviors of thick electrodes and macroscopic electrochemical performance through a spatio‐temporal resolution technique, filling the gap in knowledge on the degradation mechanism of thick electrodes. Surprisingly, the worst utilization site with the least product deposition is in the central part of the electrode rather than the traditionally presumed separator face. The secondary structure and reaction pathway of solid products exhibit a clear tendency toward spatial growth (on the electrode surface or in the interior). Combined with quantitative modeling, a critical current density shifting the dominance is found from CO 2 to Li + ions, thereby reversing the gradient of the product distribution. Finally, a hotspot map of failure mechanisms with different operating protocols is provided, serving as a guideline for the future design of thick electrodes. This work breaks the knowledge of multi‐field coupling within porous thick electrodes and can be extended to advanced Na (Li)‐CO 2 (O 2 ) battery design.