Cryo-Electron Tomography of Highly Deformable and Adherent Solid-Electrolyte Interphase Exoskeleton in Li-Metal Batteries with Ether-Based Electrolyte

The 3D nanocomposite structure of plated lithium (Li-Metal) and solid electrolyte interphases (SEI), including a polymer-rich surficial passivation layer (SEI exoskeleton) and inorganic SEI "fossils" buried inside amorphous Li matrix, is resolved using cryogenic transmission electron microscopy. With ether-based DOLDME-LiTFSI electrolyte, LiF and Li2O nanocrystals are formed and embedded in a thin but tough amorphous polymer in the SEI exoskeleton. The fast Li-stripping directions are along [1 over bar 10] or [121 over bar ], which produces eight exposed {111} planes at halfway charging. Full Li stripping produces completely sagging, empty SEI husks that can sustain large bending and buckling, with the smallest bending radius of curvature observed approaching tens of nanometers without apparent damage. In the 2nd round of Li plating, a thin Li-BCC sheet first nucleates at the current collector, extends to the top end of the deflated SEI husk, and then expands its thickness. The apparent zero wetting angle between Li-BCC and the SEI interior means that the heterogeneous nucleation energy barrier is zero. Due to its complete-wetting property and chemo-mechanical stability, the SEI largely prevents further reactions between the Li metal and the electrolyte, which explains the superior performance of Li-metal batteries with ether-based electrolytes. However, uneven refilling of the SEI husks results in dendrite protrusions and some new SEI formation during the 2nd plating. A strategy to form bigger SEI capsules during the initial cycle with higher energy density than the following cycles enables further enhanced Coulombic efficiency to above 99%.