Dear Misha, thank you for your interest in my work!
I'll answer you with pleasure:
1) Abbreviations "AL" and "SEI" are for "Aditional Layer - a new layer on the substrate, that helps to increase the signal from the ROI (range of interest)" and "Solid Electrolyte Interphase - specific electrochemical layer that appears due to difference of chemical potentials of electrolyte and anode ", respectively;
2) Terminology of "thick" and "thin" layers of SEI in this work was applied after we've got the result of the optimization procedure. As you clearly noticed, 200 nm - is a boundary thickness of SEI that splits two significantly different sets of optimal parameters of the EC.
With best regards, Kosiachkin Yehor