**4.1. Sample preparation and transfer**

For images of optimal quality, we take common carbon‐coated Au TEM grids (200 mesh, 3.05 mm diameter) directly as a cathode. Gold works as a current collector that is inert in the cathode electrochemical window, while carbon provides an ideal substrate for transmission microscopy. However, also a few micrograms of super P/PVdF slurry are deposited on the TEM grids to provide a roughened surface closer to that present in practical electrodes.

Electrochemical treatments are performed using homemade cell based on the Giessen battery design [13], resulting in Swagelok‐like battery arrangement. After the electrochemical treatment the cell is opened in an Ar‐filled glove box and the grid washed with DME and hexane to fully remove the electrolyte. The grid is then transferred in the microscope in cryogenic condition (*T* < 110 K) under N2 vapor to minimize atmospheric contamination. The samples are kept at cryogenic temperature and under high vacuum during the measurements.
