Explaining the EC–PC disparity in Li-ion batteries: how interface stiffness governs SEI formation on graphite

Abstract

In this work, we provide a new explanation of the ethylene carbonate (EC)–propylene carbonate (PC) disparity, addressing a problem that has challenged the Li-ion battery community for over three decades. We focused on the differences in physical, rather than chemical, properties between EC- and PC-based solvents to explain the differences in the interphase formation properties on the graphitic anode. By probing solid–liquid interfaces before solid electrolyte interphase (SEI) formation, we attributed the difference to the hindered kinetics of solvated Li⁺ co-intercalation into graphite at the early stage of SEI formation on the EC-graphite interface. This hindrance is due to enrichment of the interfacial region with EC molecules, which increases the local liquidus temperature of the electrolyte (T_m = 36.4 °C for EC and −48.8 °C for PC), as evidenced by the increased interfacial stiffness. The absence of such kinetic limitations for solvated Li⁺ ion intercalation in the PC electrolyte leads to a higher degree of co-intercalation of solvated molecules into graphite, resulting in severe graphite expansion upon reduction, followed by the SEI cracking along the edge plane. The following continuous co-intercalation of solvated Li⁺ closes the “co-intercalation-expansion-cracking” loop. This explains why a pure PC electrolyte cannot form a stable SEI on graphite.