Lithium metal anode is the key for the next-generation high-energy batteries. But lithium dendrites formed during recharge (electrodeposition at the anode) could short-circuit the battery and lead to fires and explosions. Tremendous efforts have been devoted to modify the composition of electrolytes and the surface of lithium electrodes, but mainly improved the stability of lithium under small currents and low capacities. Here, we devise a Li symmetric cell in a fine glass capillary, and the in situ observations reveal that the morphology of lithium deposit changes from a dense porous (mossy) structure to needle-like (dendritic) structure at Sand’s time, when the concentration of electrolyte decreases to zero at the surface of the electrode. Further experiments in sandwich cells of Li|Separator(Electroylte)|Li confirm that mossy lithium can be completely blocked by anodic aluminum oxide (AAO) membrane until Sand’s time, when a sudden voltage jump stimulates dendritic lithium to form, penetrate the nanopores of AAO and then short the cell, just like the behavior of transport-limited dendritic growth of copper. By creating a free compartment on each side of the AAO membrane, our sandwich cells can stably cycle at much higher current densities and capacities, e.g. at 5mA/cm² for 5mAh/cm². Our results agree well with a current-dependent critical capacity, C_Sand~ADc₀²/I, which suggests that increasing the concentration of electrolytes c₀ or lithium ion diffusivity D, and tuning the property of SEI to maintain a relatively large active surface area A of the porous lithium are beneficial to engineering rechargeable high-rate and large-capacity lithium metal anode.

Figure: Single dendrite shoots out from the mossy lithium deposits at Sand’s time, indicating a change of growth mechanism.