Real-Time Observation of Li Deposition on a Li Electrode with Operand Atomic Force Microscopy and Surface Mechanical Imaging

Nanoscale investigations of Li deposition on the surface of a Li electrode are crucial to understand the initial mechanism of dendrite growth in rechargeable Li-metal batteries during charging. Here, we studied the initial Li deposition and related protrusion growth processes at the surface of the Li electrode with atomic force microscopy (AFM) in a galvanostatic experiment under operand condition. A flat Li-metal surface prepared by precision cutting a Li-metal wire in electrolyte solution (100 mM LiPF6 in propylene carbonate) was observed with peak-force-tapping mode AFM under an inert atmosphere. During the electrochemical deposition process of Li, protrusions were observed to grow selectively. An adhesion image acquired with mechanical mapping showed a specifically small contrast on the surface of growing protrusions, suggesting that the heterogeneous condition of the surface of the Li electrode affects the growth of Li dendrites. We propose that a modification of the battery cell design resulting in a uniform solid liquid interface can contribute to the homogeneous deposition of Li at the Li electrode during charging. Further, the mechanical mapping of Li surfaces with operand AFM has proven to play a significant role in the understanding of basic mechanisms of the behavior of the Li electrode.