The processes of SEI formation and of intercalation are still not well understood on the molecular level. Scanning probe techniques are capable of analysing these processes under in-situ conditions.

The SEI-formation on graphitic electrodes operated as an Li+-ion battery anode in a standard 1 M LiPF6 EC/DMC (1 : 1) electrolyte has been studied using in-situ electrochemical scanning tunnelling microscopy (EC-STM)¹. Two different modes of in situ study were applied, one, which allowed to follow topographic and crystallographic changes (solvent co-intercalation, graphite exfoliation, SEI precipitation on the HOPG basal plane) of the graphite electrode during SEI-formation, and the second, which gave an insight into the SEI precipitation on the HOPG basal plane in real time. From the in situ EC-STM studies, not only conclusions about the SEI-topography could be drawn, but also about the formation mechanism and the chemical composition, which strongly depend on the electrode potential. It was shown that above 1.0 V vs. Li/Li+ the SEI-formation is still reversible, since the molecular structure of the solvent molecules remains intact during an initial reduction step. During further reduction, the molecular structures of the solvents are destructed, which causes irreversible charge losses.

The reversible intercalation of solvated Na-ions into graphite and the concomitant formation of ternary Na-graphite intercalation compounds (GICs) is studied by EC-STM and other in-operando techniques². Linear ethylene glycol dimethyl ether homologes (“glymes”) Gx with x+1 O-atoms were used as solvents, where x is 1-4. The intercalation mechanism of Na+(Gx)y-complexes was investigated with the focus on phase transitions and diffusion rates of the Na+(Gx)y-complexes inside the graphite lattice. For the four shortest glymes (G1 to G4), it is found by XRD that an intermediate stage 2 Na-GIC (NaC48) is formed upon partial sodiation of the graphite electrode. At full sodiation a stage 1 Na-GIC (NaC18, 112 mAh g-1) is obtained for G1, G2 and G4, while the G3-system is also forming a stage 1 Na-GIC but with less Na incorporated (NaC30, 70 mAh g-1). Phase transitions of a battery electrode upon ion-intercalation are visualised by STM on the atomic scale for the first time. In addition, local diffusion rates of the intercalated species inside the electrode were determined, a unique approach to determining kinetic effects in batteries on the atomic scale. The formation of a solid electrolyte interphase (SEI) is observed in EC-STM.

• Lukas Seidl, Sladjana Martens, Jiwei Ma, Ulrich Stimming, Oliver Schneider "In-Situ Scanning Tunneling Microscopy of the SEI Formation on Graphite Electrodes for Li-Ion Batteries" Nanoscale, 2016, 8, 14004;
• Seidl, N. Bucher, E. Chu, S. Hartung, S. Martens, O. Schneider and U. Stimming, "Intercalation of solvated Na-ions into graphite", Energy Environ. Sci.,2017, 10, 1631.