Revealing Structural Insights of Solid Electrolyte Interphase in High-Concentrated Non-Flammable Electrolyte for Li Metal Batteries by Cryo-TEM.

High-concentrated non-flammable electrolytes (HCNFE) in lithium metal batteries prevent thermal runaway accidents, but the microstructure of their solid electrolyte interphase (SEI) remains largely unexplored, due to the lack of direct imaging tools. Herein, cryo-HRTEM is applied to directly visualize the native state of SEI at the atomic scale. In HCNFE, SEI has a uniform laminated crystalline-amorphous structure that can prevent further reaction between the electrolyte and lithium. The inorganic SEI component, Li2 S2 O7 , is precisely identified by cryo-HRTEM. Density functional theory (DFT) calculations demonstrate that the final Li2 S2 O7 phase has suitable natural transmission channels for Li-ion diffusion and excellent ionic conductivity of 1.2 × 10-5 S cm-1 .

Revealing the Intrinsic Atomic Structure and Chemistry of Amorphous LiO2-Containing Products in Li-O2 Batteries Using Cryogenic Electron Microscopy.

Aprotic lithium-oxygen batteries (LOBs) are promising energy storage systems characterized by ultrahigh theoretical energy density. Extensive research has been devoted to this battery technology, yet the detailed operational mechanisms involved, particularly unambiguous identification of various discharge products and their specific distributions, are still unknown or are subjects of controversy. This is partly because of the intrinsic complexity of the battery chemistry but also because of the lack of atomic-level insight into the oxygen electrodes acquired via reliable techniques. In the current study, it is demonstrated that electron beam irradiation could induce crystallization of amorphous discharge products. Cryogenic conditions and a low beam dosage have to be used for reliable transmission electron microscopy (TEM) characterization. High-resolution cryo-TEM and electron energy loss spectroscopy (EELS) analysis of toroidal discharge particles unambiguously identified the discharge products as a dominating amorphous LiO2 phase with only a small amount of nanocrystalline Li2O2 islands dispersed in it. In addition, uniform mixing of carbon-containing byproducts is identified in the discharge particles with cryo-EELS, which leads to a slightly higher charging potential. The discharge products can be reversibly cycled, with no visible residue after full recharge. We believe that the amorphous superoxide dominating discharge particles can lead researchers to reconsider the chemistry of LOBs and pay special attention to exclude beam-induced artifacts in traditional TEM characterizations.

Enabling Atomic-Scale Imaging of Sensitive Potassium Metal and Related Solid Electrolyte Interphases Using Ultralow-Dose Cryo-TEM.

Potassium-based solid electrolyte interphases (SEIs) have a much smaller damage threshold than their lithium counterpart; thus, they are significantly more beam sensitive. Here, an ultralow-dose cryogenic transmission electron microscopy (cryo-TEM) technique (≈8 e Å-2 s-1  × 10 s), which enables the atomic-scale chemical imaging of the electron-beam-sensitive potassium metal and SEI in its native state, is adapted. The potassium-based SEI consists of large brackets of diverse inorganic phases (≈hundreds of nanometers) interspersed with amorphous phases, which are different from the tiny nanocrystalline inorganic phases (≈a few nanometers) formed in a lithium-based SEI. Organic phosphate-based electrolyte solvents induce the formation of a thin and stable SEI layer for enhanced cycling performance, while the carbonate ester-based electrolytes result in large quantities of metastable KHCO3 , and K4 CO4 products in the SEI, depleting the potassium reserves in the battery. The findings provide deep insights and guidance in the selection of optimum electrolytes that should be used for potassium batteries.