  Using cryogenic transmission electron microscopy, we revealed three
dimensional (3D) structural details of the electrochemically plated lithium
(Li) flakes and their solid electrolyte interphase (SEI), including the
composite SEI skin-layer and SEI fossil pieces buried inside the Li matrix. As
the SEI skin-layer is largely comprised of nanocrystalline LiF and Li2O in
amorphous polymeric matrix, when complete Li stripping occurs, the compromised
SEI three-dimensional framework buckles, forming nanoscale bends and wrinkles.
We showed that the flexibility and resilience of the SEI skin-layer plays a
vital role in preserving an intact SEI 3D framework after Li stripping. The
intact SEI network enables the nucleation and growth of the newly plated Li
inside the previously formed SEI network in the subsequent cycles, preventing
additional large amount of SEI formation between newly plated Li metal and the
electrolyte. In addition, cells cycled under the accurately controlled uniaxial
pressure can further enhance the repeated utilization of the SEI framework and
improve the coulombic efficiency (CE) by up to 97%, demonstrating an effective
strategy of reducing the formation of additional SEI and inactive dead Li. The
identification of such flexible and porous 3D SEI framework clarifies the
working mechanism of SEI in lithium metal anode for batteries. The insights
provided in this work will inspire researchers to design more functional
artificial 3D SEI on other metal anodes to improve rechargeable metal battery
with long cycle life.
