Revisiting the Ethylene Carbonate–Propylene Carbonate Mystery with Operando Characterization

The “ethylene carbonate (EC)–propylene carbonate (PC) mystery” has puzzled electrochemists for decades. Surprisingly, the minor structural difference between PC and EC, a methyl vis-à-vis a proton, prevents PC unlike EC to form a stable solid electrolyte interphase (SEI) on carbon (C), which along with the popularity of PC has impeded the development of Li-ion batteries with many years. Despite several hypotheses, the fundamental reason remains debated largely due to the lack of sufficient experimental evidence. Herein, SEI formed as a result of EC and PC reductions are analyzed by two state-of-the-art operando techniques, online electrochemical mass spectrometry and electrochemical quartz crystal microbalance with dissipation monitoring. Although both EC- and PC-based electrolytes appear to have virtually identical reaction pathways, PC is reduced much more extensively than EC and forms a much thicker SEI. However, while the SEI derived from EC remains on the electrode, PC reduction products redissolve in the electrolyte leaving the bare C electrode behind. The presented study illustrates the complex scheme of competing electro-/chemical reactions behind SEI formation and provides further scientific details needed to eventually form a consensus of the processes governing electrode/electrolyte interphases in Li-ion batteries.