Operando Video Microscopy of Li Plating and Re-Intercalation on Graphite Anodes During Fast Charging

The rapidly growing electric vehicle market is increasing the demand for fast charging capability of high-energy-density Lithium (Li)-ion batteries. However, state-of-art Li-ion batteries with thick graphite electrodes suffer from Li plating when charged at &gt;4C rates. This undesired Li plating can lead to electrolyte consumption, internal electrical shorting, gas evolution, capacity loss, and thermal runaway. To inform mitigation strategies for Li plating under fast-charging conditions, there is a need to improve our fundamental understanding of the Li plating process.<br/>In this work, plan-view <i>operando</i> video microscopy was performed on &gt;3 mAh cm^-2 calendared graphite electrodes during and after 6C charging [1]. This technique enables time-synchronization of the graphite working electrode voltage with the morphological evolution of the electrode. During a 6C charging half cycle, this platform was applied to visualize the spatial heterogeneity of graphite SoC across the electrode surface and the nucleation and growth of the plated Li. During the subsequent open circuit voltage (OCV) rest and discharge steps, the interaction between the plated Li and graphite was also studied. Furthermore, a 3D understanding of the changes across the electrode volume was developed by performing <i>ex situ</i> cross-sectional microscopy. Finally, pouch cells were fabricated to confirm the voltage signature associated with dead Li formation in a commercially relevant cell format.<br/>The results of this study demonstrate that: 1) during fast-charging, spatial heterogeneity in the local SoC of individual graphite particles develops; 2) preferential nucleation of plated Li occurs on the graphite particles that had the fastest lithiation rate; 3) the onset of Li plating correlates with a voltage minimum of the graphite electrode; 4) after fast-charging, galvanic corrosion currents are the primary driving force for Li re-intercalation and graphite SoC re-equilibration; and 5) during OCV rest or discharge steps, electrochemical signatures can be associated with the re-intercalation of plated Li.<br/>[1] Y. Chen, K.-H. Chen, A. J. Sanchez, E. Kazyak, V. Goel, Y. Gorlin, J. Christensen, K. Thornton, N. P. Dasgupta “Operando video microscopy of Li plating and re-intercalation on graphite anodes during fast charging” <i>J. Mater Chem. A, </i>In Press (2021).