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 K₄CO₄ 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. © 2021 Wiley-VCH GmbH.