SEI Characterization Using XPS: Resolving Rinsing Effects through Cryogenic Implementation

Much remains to be understood about the chemical environment of the battery anode's solid electrolyte interphase (SEI) under operando conditions. The challenge partly arises because the main technique for SEI composition analysis, room temperature X-ray photoelectron spectroscopy (RT-XPS), is confounded by the severe SEI evolution that occurs through reactions and volatilization during RT-XPS. A recent technical breakthrough is our introduction of cryo-XPS for battery research, which uses instant plunge freezing to ensure operando-relevant interface preservation during XPS measurements. Yet uncertainty remains about which sample preparation protocol best reflects operando-relevant chemical speciation during ex situ XPS. Importantly, this work establishes that due to the surface-sensitive nature of XPS, sample preparation becomes crucial during characterization. Here, cryo-XPS is utilized as a convenient tool to identify the differences arising from different sample preparation protocols. We show that the controlled, rinse-dried cryo-sample (d-SEI) provides a more accurate representation of the operando-relevant SEI chemical composition. This is the preferred protocol because not only does d-SEI ensure probe depth certainty, unlike the unwashed blotted SEI (w-SEI), but it also reveals more buried species during cryo-XPS.