Investigating the Mechanism of Lithium Transport at Solid Electrolyte Interphases

Reactions between carbonate electrolytes and graphite electrodes in lithium-ion storage devices produce a surface film of byproducts known as the solid electrolyte interphase (SEI). Significant progress has been made in assessing the composition and structure of these interphases; however, their impact on lithium transport during charge and discharge lacks molecular detail. Over the past decade, electrochemical impedance spectroscopy (EIS) has shown that lithium transport is limited by a combination of ion desolvation and ion conduction through the SEI, however which step is rate limiting remains unresolved. In this work, we simulate the first step in this process, i.e., ion desolvation, both into and out of two model SEI's comprised of lithium ethylene dicarbonate (LEDC) and Li2CO3 interfaced with an ethylene carbonate electrolyte. By correlating free-energy changes with solvation structure, we show that the path taken for Li+ insertion is a two-step mechanism consisting of overcoming two energy barriers to adsorption and then absorption. The largest measured barrier of the two is 59.2 kJ/mol, within the estimates obtained from EIS measurements. Ion extraction from the LEDC, however, follows a different free-energy profile determined by the flexibility of the surface groups to extend into the electrolyte. The dependence of extraction from LEDC on the nature of the surface groups, emphasized by comparison with ion extraction from the more rigid Li2CO3 surface, highlights the complex relationship between SEI composition and lithium transport.