2, Quorum Technologies, Lewes, , United Kingdom
The next generation high-energy batteries comprise light element metals for the cathode, electrolyte, and anode (Li-S, Li-O, and solid electrolytes, respectively) and will be replacing its non-transition metals counterpart. The former materials, however, are known to be highly reactive – the materials are very sensitive to air and electron beam. Sample preparation and experimentation are challenging because it is necessary to transfer samples between milling systems and microscopes while simultaneously maintaining the material’s integrity. A critical requirement in developing these materials for tangible applications is the ability to characterize the materials in a pristine state, without environmental modifications or contamination. Furthermore, transmission electron microscopy (TEM) is a critical analytical technique for battery research and development. Following the advancement of cryo-electron microscopy (cryo-EM) in life sciences, cryo-EM has been adapted for Li battery research, which allows for the preservation of the Li battery materials’ native state during imaging at the atomic scale. This study presents robust controlled environments that protect the material during the sample preparation phase (bulk to focused ion beam [FIB] preparation) and through the multi-length scale electron microscopy characterization phase. At the micrometer scale, scanning electron microscopy (SEM) characterization will show the morphology of the solid electrolyte while the sub-angstrom scale using the TEM providing interfacial chemistry and morphology. High quality, Li ion battery TEM specimens that are free from amorphous and Ga damage will be prepared and imaged under cryo-EM conditions.
The controlled environments workflow for SEM and TEM sample preparation and microscopy characterization involved the preparation of the bulk sample using broad ion beam (BIB) Ar+ milling to remove surface oxides. A vacuum/inert gas transfer capsule protected the sample post-BIB milling. A glove box with a positive pressure environment was used to transfer the bulk sample from the vacuum/inert gas transfer capsule to a FIB transfer system, and thereafter, to a FIB system for morphology and elemental characterization and subsequently TEM specimen preparation. A TEM half grid was secured in the cartridge of a vacuum transfer specimen holder. This cartridge was inserted into the FIB by means of the FIB transfer system. A TEM specimen was prepared using standard lift-out methods and polishing steps (30 and 5 kV) in the FIB. The cartridge with the TEM specimen was moved into the FIB transfer system and then to the glove box. Subsequently, the cartridge was mounted on the vacuum transfer specimen holder within the glove box. The TEM specimen holder was then moved to a concentrated Ar+ beam milling system. Ar+ milling was performed by rastering the beam within a defined area of the TEM specimen at decreasing milling energies. Subsequently, the TEM specimen was loaded on a cryo TEM holder inside the glove box; a glove bag with inert gas protected the TEM specimen during insertion of the cryo holder in the TEM. The cryo holder was then cooled in the TEM.
Cryo-EM imaging and analysis using energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) before and after ion milling to quantify the removal of FIB-induced damage and the workflow’s ability to prevent specimen oxidation and contamination will be presented.