SEI layer growth in the solid state battery materials and Li mapping using STEM-EELS

In operando TEM analysis on the Solid State Battery materials (PEO+LiTFSI)

A solid-state lithium battery for in-situ TEM observation was fabricated using Li/PEO films, hot-pressed for good electrode-electrolyte contact. The TEM battery device was tested by applying a constant biasing voltage at 4V with an elevated temperature at 60°C for 12 hours. The assembly was adapted for in-operando TEM analysis, using both ion-milled and FIB-prepared lamellae placed on TEM nanochips. Electrochemical impedance spectroscopy and galvanostatic cycling revealed increasing interfacial resistance, attributed to lithium reactions with atmospheric gases. In-operando TEM experiments combined heating and electrical biasing over extended periods, generating extensive imaging data (~160 GB). The measured current decreased from the 34 nA to 14 nA, the calculated resistance of PEO+LiTFSI is at the 10⁸ ohms level at 60°C, which is in a good agreement with the report values for PEO+LiTFSI electrolyte. The growth of a thin SEI layer (150 nm) is observed and shown as the bright contrast in the STEM-HAADF images. At the interfacial region between SPE and Li electrode, small Li and Li₂O precipitates were detected from the HRTEM image after the charging process. Before the charging process, the SPE shows as amorphous structure in HRTEM image. At the negative electrode, particles grew, with Li and Li₂O identified via electron diffraction and electron energy loss spectroscopy (EELS). Electrical measurements showed stable PEO resistance and hysteresis during voltage cycling. Combining polymers and ceramics (PEO-LAO) altered ion transport despite similar overall resistance. Multivariate analysis of TEM data gave critical insights on ion distribution, dendrite formation, and electrode modifications. The project developed unique micro-TEM cell fabrication methods and data analysis strategies. New cell designs were synthesized for deeper study of ceramic particle behaviour. The project contributes to a better understand of polymer batteries developed for electric vehicles.

The work was carried out by Ling Xie, Qaphelani Ngulube, Jonas Mindemark, Daniel Brandell, Klaus Leifer at Uppsala University.