Interphases enables the electrodes to operate at potentials beyond what thermodynamic stability limits of non-aqueous electrolytes would allow. The most prominent example of interphases is perhaps solid-electrolyte-interphase (SEI) on graphitic anodes in Li-ion batteries, which ensures the reversibility of Li-ion intercalation chemistries. In typical electrolytes, chemical building blocks of SEI mainly come from solvents decomposition products, and, although we often acknowledge that many aspects of SEI remains unknown, we thought we at least have establish the following foundation beyond doubt: (1) semi-carbonates from the single-electron reduction of carbonate solvents constitutes the major portion of SEI; (2) the formation of SEI is irreversible; and (3) SEI is ionic conductor and electron insulator. However, when taking a closer look at the “olde SEI” using a series of new techniques, we found some of those principles might need to be re-written.

In this work, I will summarize the newest understanding of this “least understood” component in Li-ion battery that were achieved in my own group and my collaborators.