Visualizing Lithium Deposition and Identifying Two Types of Dendrites in Extreme-Fast-Charging Full Cells across the Entire Lifespan by Operando EPR and EPR Imaging

Abstract

Metallic lithium deposition processes in NCM811∥graphite full cells during extreme-fast charging of 4 C (fully charged within 15 min) are detected via operando electron paramagnetic resonance (EPR) and EPR imaging over hundreds of cycles to quantify lithium deposits and visualize their spatial distribution. EPR imaging shows that constant-voltage charge generates loose Li dendrites with divergent growth whereas overcharge leads to long dendrites with vertical growth, and these Li deposits accumulate at the anode edges, which could deplete the Li resource at the cathode edges. Moreover, quantitative EPR indicates that the stripping current correlates to the deposit surface areas, while the reintercalation current depends on the contact areas between plated Li and graphite. Overall, the results of EPR and EPR imaging suggest that the introduction of a relaxation period after extreme-fast charge for Li reintercalation into graphite is important to mitigate the accumulation of dead Li.