Solid Electrolyte Interphase Evolution on Lithium Metal Electrodes Followed by Scanning Electrochemical Microscopy Under Realistic Battery Cycling Current Densities

Lithium metal electrodes were cycled in 1 M LiClO4 in propylene carbonate with different current densities. The local protecting properties of the solid electrolyte interphase (SEI) were probed by scanning electrochemical microscopy (SECM) in the feedback mode directly within the cell in between charging-discharging cycles. This was enabled by placing the negative electrode into an in-house micro-milled cell with a central opening in the counter electrode for inserting the microelectrode. Finite element simulation of the secondary current distribution proved that the current distribution deviates only slightly in the area of the opening provided that the SECM microelectrode is retracted during the charging-discharging cycles. The development of lithium deposits was observed by SECM and can be linked to the used charging-discharging protocol. The Li metal of protruding deposits is significantly more active for electron transfer to the mediator than the remaining parts of the surface. The developed hardware and methodology can be directly applied to other electrolytes or other battery electrodes forming protective films.