Lithium metal anode is attractive due to its very negative potential and high capacity density, and however, secondary batteries with lithium metal anode are not practically used. One of the biggest obstacle against the wide spread in the market is dendrite formation at the anode during charging.[1] Many researches have been carried out where the mechanism of the dendrite formation of lithium metal were investigated in order to suppress the dendrite formation. Particularly the nano-scale and/or initial behavior of dendrite formation is of importance to elucidate the mechanism. Recently, peakforce-tapping mode AFM (atomic force microscopy) has been commercialized, where the damage to the sample surface from the AFM tip is much smaller than the damage in other conventional mode of AFM.[2] In this study, peakforce-tapping mode AFM was utilized to investigate the initial behavior of lithium electrodeposition in nano-scale in operand.

Lithium wire was cut in the electrolyte and was used as working electrode. Lithium wire was used as a counter electrode, and 0.1 M LiPF₆/PC was used as electrolyte solution. Lithium was electrodeposited on working electrode with current density of 2.5 mA cm^-2. The working electrode surface was observed by using peakforce-tapping mode AFM in operand. The height and adhesion mapping images were simultaneously obtained.

Two types of protrusions were observed in the series of height mapping images. One type of protrusions increased in size during electrodeposition, while the other type of protrusions kept their size. A series of adhesion mapping images indicate that the former type of protrusions had less adhesive surface than the latter type of protrusions. Furthermore, once the electrodeposition was stopped, the adhesion force of former type of protrusions increases and became the similar adhesion force of latter type of protrusions.[3] In PC solutions, the lithium surface is considered to be coated by surface film. The results may indicate that the surface film on the growing deposits is different from that of static-state deposits. In the case where dendrites had been suppressed, the deposits had been widely and uniformly spread and there had been no contrast at the adhesion images. The AFM technique can be used to detect the dendrite formation even at the very initial state of electrodeposition.

References:
[1] D. Aurbach et al., Solid State Ionics, 148, 405 (2002).
[2] https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/AFM/ApplicationNotes/Introduction_to_Brukers_ScanAsyst_and_PeakForce_Tapping_Atomic_Force_Microscopy_Technology_AFM_AN133.pdf.
[3] M. Kitta and H. Sano, Langmuir, 33, 1861 (2017).

Acknowledgement:
This work was partly supported by the Japan Science and Technology Agency (JST) - Advanced Low Carbon Technology Research and Development Program, “Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING)”.