Investigating the Interfacial Chemistry of Organic Electrodes in Li- and Na-Ion Batteries

Organic compounds are increasingly being investigated as electrode materials for Li- or Na-ion batteries. Even though their gravimetric capacity can challenge that of their inorganic counterparts, a number of problems need further attention, not least their chemical and electrochemical stability toward the electrolyte systems. There has been speculation that several of these issues have their origin in the formation of a less stable solid electrolyte interphase (SEI) layer and its evolution during battery cycling. We here present the very first thorough characterization of the organic electrode material SEI layer using hard X-ray photoelectron spectroscopy (HAXPES), for both Li- and Na-based electrodes. Dilithium and disodium benzenediacrylates have been used for battery construction and investigated electrochemically followed by HAXPES measurements after contact with the electrolyte and after cycling. The Na-based electrodes react spontaneously with the electrolyte, and the SEI layer is dominated by inorganic species with continuous salt degradation during cycling. The Li-based electrodes display an SEI layer with primarily organic species from solvent degradation products appearing only after cycling and increasing in amount with the number of electrochemical cycles.