DOI: 10.1002/adma. 201606187 electrolyte or even a solid-state electrolyte, where the real interactions between the liquid electrolyte and active materials are completely missing. In addition, the highly energetic electron beam in a TEM can cause significant artifacts, such as melting of the lithium metal into liquid.[20] In contrast, in situ optical microscopy characterization [14, 21–25] can provide more real information of lithium dendrite growth process due to the much larger dimension of the cells and is free from the electron beam effect. However, the resolution of an optical microscope is normally too low to provide the detailed information on the initial stage of lithium dendrites nucleation as well as during the lithium plating/stripping processes. In this respect, SEM seems to be a good compromise between the cell size (similar to that used in an optical microscope) and the resolution (down to a few nanometers). However, so far, SEM characterizations for lithium batteries were mostly carried out as ex situ,[26–32] no in situ work has been reported except that from our group.[13]

The schematic design of the EC-SEM liquid cell in this work is shown in Figure 1a. It is composed of two chips: a top silicon chip with a SiNx membrane viewing window and a bottom chip made of quartz with two injection orifices. A pair of Cu current collectors and Li electrodes was patterned on the top chip with a microgap lying in the middle of the viewing window. The two chips were sealed by epoxy, with a separation of≈ 0.5 mm. The orifices were also sealed with the same epoxy after the injection of the liquid electrolyte (≈ 60 µL). A detailed description on the fabrication of the EC-SEM liquid cell can be found in the Supporting Information. During in situ electrochemical experiments, the growth and dissolution of lithium dendrites at the edges of the Li/Cu electrodes can be observed through the SiNx window by SEM (Figure 1b).