Challenging Prevalent Solid Electrolyte Interphase (SEI) Models: An Atom Probe Tomography Study on a Commercial Graphite Electrode.

Lithium-ion batteries (LIBs) are the dominating energy storage technology for electric vehicles and portable electronic devices. Since the resources of raw materials for LIBs are limited and recycling technologies for LIBs are still under development, improvements in the long-term stability of LIBs are of paramount importance and, in addition, would lead to a reduction in the levelized cost of storage (LCOS). A crucial limiting factor is the aging of the solid electrolyte interphase (SEI) on the active material particles in the anode. Here, we demonstrate the potential of atom probe tomography for elucidating the complex mosaic-type structure of the SEI in a graphite composite anode. Our 3D reconstruction shows unseen details and reveals the existence of an apolar organic microphase pervading the SEI over its entire thickness. This finding is in stark contrast to the prevalent two-layer SEI model, in which organic compounds are the dominating species only in the outer SEI layer being in contact with the liquid electrolyte. The observed spatial arrangement of the apolar organic microphase promises a better understanding of the passivation capability of the SEI, which is necessary to expand the battery lifetime.