Poor Stability of Li₂CO₃ in the Solid Electrolyte Interphase of a Lithium‐Metal Anode Revealed by Cryo‐Electron Microscopy

Abstract

The solid electrolyte interphase (SEI) dictates the cycling stability of lithium‐metal batteries. Here, direct atomic imaging of the SEI's phase components and their spatial arrangement is achieved, using ultralow‐dosage cryogenic transmission electron microscopy. The results show that, surprisingly, a lot of the deposited Li metal has amorphous atomic structure, likely due to carbon and oxygen impurities, and that crystalline lithium carbonate is not stable and readily decomposes when contacting the lithium metal. Lithium carbonate distributed in the outer SEI also continuously reacts with the electrolyte to produce gas, resulting in a dynamically evolving and porous SEI. Sulfur‐containing additives cause the SEI to preferentially generate Li2SO4 and overlithiated lithium sulfate and lithium oxide, which encapsulate lithium carbonate in the middle, limiting SEI thickening and enhancing battery life by a factor of ten. The spatial mapping of the SEI gradient amorphous (polymeric → inorganic → metallic) and crystalline phase components provides guidance for designing electrolyte additives.

Conformal three-dimensional interphase of Li metal anode revealed by low-dose cryoelectron microscopy

“…The Li 2 CO 3 component is unstable against the electrolyte and will decompose to form gaseous species. 21 Therefore, the addition of FEC can induce a uniformly thin SEI layer with more stable LiF inorganic crystals and minimize unstable Li 2 CO 3 component. 21 The combination of our cryogenic scanning transmission electron microscopy (cryo-STEM) and cryo-EELS results offer unprecedented insight into the chemical composition of the SEI skin layer after just the first plating cycle.…”

“…21 Therefore, the addition of FEC can induce a uniformly thin SEI layer with more stable LiF inorganic crystals and minimize unstable Li 2 CO 3 component. 21 The combination of our cryogenic scanning transmission electron microscopy (cryo-STEM) and cryo-EELS results offer unprecedented insight into the chemical composition of the SEI skin layer after just the first plating cycle. As shown by the large-scale cryo-STEM images in Figure 2A, there is a high density of bright clusters (indicated by white arrows) against the dark Li matrix, demonstrating local enrichment of impurities.…”

“…18 The SEI skin layer generally contains two crucial types of components: inorganic crystalline grains and an organic, amorphous polymeric matrix. 21 The distribution of inorganic components inside the polymeric matrix plays a critical role in dictating the ionic and electronic conductivity 22 and mechanical stability of the SEI, and thus the cyclability of the Li metal anode. The ideal SEI skin layer maintains integrity by resilient accommodations to the volume changes of the anode materials during many cycles of Li plating and stripping.…”

Cryo-EM for battery materials and interfaces: Workflow, achievements, and perspectives

“…Solid electrolyte interphases: SEI is regarded as the most important but the least understood part in batteries. There is no consensus on the composition, content, distribution, evolution of SEI as well as its correlation with the ionic conductivity and mechanical property ( Han et al., 2021a ; Ju et al., 2021a ; Liu et al., 2019b , 2021a , 2021c ; Yuan et al., 2020a ). Two typical models that are mosaic structure ( Peled et al., 1997 ) and multilayer structure ( Aurbach et al., 1994 ) were proposed and observed in different electrolytes ( Han et al., 2021c ; Li et al., 2017 , 2021 ; Liu et al., 2018e ; Zhang et al., 2020a ).…”

Probing atomic structure of beam-sensitive energy materials in their native states using cryogenic transmission electron microscopes

“…Direct atomic imaging of the SEI's phase components and their spatial arrangement is also challenging. Using ultralow-dosage cryo-TEM, our group has achieved to probe the SEI gradient amorphous and crystalline phase components of a lithium-metal anode ( Han et al., 2021 ), and more detailed structural information of the SEI microstructures in alkali metal batteries needs to be explored urgently.…”