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

Abstract

The 3D nanocomposite structure of plated lithium (LiMetal) 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¯10] or [121¯], 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 LiBCC 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 LiBCC 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%.

Oxide Cathodes: Functions, Instabilities, Self Healing, and Degradation Mitigations

“…The interesting bilayer character of the cathode passivation is overlooked in the literature. Recall that on the anode side, SEIs, which are also heterogeneous multiphase composites, form as the passivation layer. − While there are still unknowns for SEIs, the established theoretical picture states that a good SEI layer should be thin and stable upon cycling and it should be primarily a Li + conductor but an electronic insulator, so that the lithiation process takes place at the interface between SEIs and the anode. The blockage of electron transport to the electrolyte suppresses the cathodic reduction of the electrolyte and the side reactions between the lithiated anode and the electrolyte.…”

Dynamic Investigation of Battery Materials via Advanced Visualization: From Particle, Electrode to Cell Level

“…The discrepancy of nanostructure and chemical formation in SEI observed by different groups might attribute to their different sample preparation, test conditions, active materials, liquid electrolyte systems, working conditions, and so on, [84][85][86][87][88] which suggest that SEI is unstable, fragile, and complex, thus need an in-depth understanding on its evolution.…”

Atomistic insights into the morphology of deposited Li

“…2G). 67 According to the above-mentioned viewpoint of SEI lm, there is still much debate on the microscopic details, which deserves to be further discussed in depth in future research. Herein, we cannot make arbitrary judgments on which standpoint is correct, but it can at least be said that the exible nature of SEI lm plays a key role in regulating the volume change during the repeated Li deposition and dissolution process.…”

“…Fortunately, in 2017, cryo-TEM which originated from the area of biological materials was groundbreakingly transplanted into materials science and has made some impressive progress in the characterization of battery materials, especially deposited Li metal. 11,37,50,52,67,102,103 For the deposited Li, preparing a pristine and clean cryo-TEM sample without any contamination or potential damage is of vital importance. To this end, Li can be directly deposited onto a Cu TEM grid with lacey carbon in a coin cell (Fig.…”

“…Fortunately, in 2017, cryo-TEM which originated from the area of biological materials was groundbreakingly transplanted into materials science and has made some impressive progress in the characterization of battery materials, especially deposited Li metal. 11,37,50,52,67,102,103…”