Transient lithium metal plating on graphite: Operando 7Li nuclear magnetic resonance investigation of a battery cell using a novel RF probe

The development of optimal fast charging protocols requires detailed information regarding lithium inventory in a battery. We built a parallel-plate resonator RF probe and a cartridge-type single layer cell of improved designs. The probe has excellent sensitivity and homogeneity of electromagnetic field, while the cartridge allows easy cell assembly, straight-forward multiplexing, and excellent cell-to-cell data reproducibility. Herein we present our findings from operando 7Li NMR measurements on a graphite//NMC622 cell. The usual/expected sequencing of LixC6 phases from dilute to concentrated observed during charging at low (<C/5) rates is replaced at high (≥1C) rates by a direct transition to, and propagation of concentrated phases throughout the graphite electrode, with no evidence for dilute phases. Concurrently, a peak attributed to lithium naphthalenide appears at ∼20 ppm. Lithium therefore accumulates on graphite surfaces, facilitating Li metal plating under fast charge conditions. While some of the deposited Li metal can intercalate into graphite after cell charging, a significant fraction accumulates during consecutive cycles, creating a barrier for Li+ transport, which decreases the amount of recoverable Li, and its intercalation rate into graphite. Thus, Li plating not only can create safety hazards through dendrite growth, but also irreversibly impairs the fast charging capability of cells.