Unravel Electrolyte-Dependent Interphase Structures in Lithium-Sulfurized Polyacrylonitrile Batteries via Cryogenic Transmission Electron Microscopy

The emerging lithium-sulfur batteries using sulfurized polyacrylonitrile cathodes feature exceptional stability and good compatibility with various electrolytes. However, the study of correlating battery performance with electrolytes based on ultrafine interphase structures is nearly blank. This work compares the solid- and cathode-electrolyte interphases (SEI and CEI) formed in five representative electrolytes, from the perspective of ultrafine structures acquired via cryogenic transmission electron microscopy and electron energy loss spectroscopy. The premature battery failure in the baseline ester- and ether-based electrolytes is related to the thick and uneven interphase layers on both the anodes and cathodes, which reflects continuous electrolyte decomposition. The use of film-forming additives brings substantial improvement in cycle stability, which is attributed to an inorganic-rich interphase that blocks electrolyte permeation and further decomposition. However, it comes along with aggregated byproducts on both SEI and CEI. Better stability is delivered in a localized high-concentration electrolyte, which aligns with the compact SEI and CEI layers with condensed inorganic species. Based on our observations and reasoning, the formation of inorganic-rich interphase involves anions or additives, which could serve as a practical guideline for rational electrolyte designing.