Figure 6: In-situ optical microscopy study of lithium dendrite formation.

(a) Schematic illustration of the quartz cell device with transparent windows and rectangle empty space inside as the cell housing, in which a sandwich structure of lithium metal, separator and stainless steel substrate was assembled. The fabricated cell was put on the stage of optical spectroscopy with transparent window facing to the lens for in-situ characterization. (b–f) A series of dark-field optical microscope images of the cell’s cross-section (b–f, optical images versus increasing lithium deposition time), revealing the growing process of lithium dendrites at 5 mA cm⁻² on the stainless steel substrate using electrolyte with the addition of both Li₂S₈ and LiNO₃. (g–k) A series of dark-field optical microscope images of the cell’s cross-section (g–k, optical images versus increasing lithium deposition time), showing the growing process of lithium dendrites using electrolyte with the addition of only LiNO₃ at the same current density as b–f. Scale bar, 100 μm.