A Reactive Molecular Dynamics Simulation Study of Single-Electron Reduction Pathways
The Smith Group, at the University of Utah, has identified reaction products of the single-electron reduction of ethylene carbonate (EC), an important component in lithium-ion battery electrolytes. Reactive force field (ReaxFF) simulations were used to discover the fate of EC, which provides insight into the structure of the solid electrolyte interphase (SEI) – a thin film covering carbon anodes that limits the charge/discharge rate. Reactive force fields allow for the making/breaking of chemical bonds during classical molecular dynamics simulations. Because electronic degrees of freedom are treated in a simplified manner, these reactive molecular dynamics (RMD) simulations allow for the study of chemical reactions in much larger collections of molecules over much longer times than can be studied using ab initio methods.
Condensed (liquid electrolyte) and gas phase (isolated molecules) RMD simulations were performed on reactions involving two forms of the EC radical anion: the linear (open) form, denoted o-EC·–, and the cyclic (closed) form, denoted c-EC·–. No reactions were observed to occur in systems containing only c-EC·– at room temperature; however, c-EC·– was found to readily react with the additive vinylene carbonate to form poly(vinylene carbonate). Since ring-opening reactions have a high energy barrier, highly elevated temperatures were required to observe the formation of o-EC·–. The RMD simulations revealed that o-EC·– was rapidly consumed in reaction with c-EC·– to form the stable compound shown in Figure 1.
In the rare event that two o-EC·– radicals combined, which was simulated in the absence of c-EC·–, the resultant product was butylene dicarbonate. Ethylene dicarbonate, a commonly proposed product of o-EC·– combination reactions, was not observed to form in these RMD simulations due to a substantial energy barrier. Hence, the mechanism for formation of ethylene dicarbonate is apparently not the commonly suggested direct combination of two o-EC·– with concurrent elimination of ethylene. An improved version of the ReaxFF method that more accurately captures the energetics of c-EC·– decomposition is currently being developed and may help in determining the mechanism for formation of ethylene dicarbonate.
This study demonstrates that the c-EC·– radical, while high in energy compared to o-EC·–, is long-lived and may play important roles in the formation of the outer SEI layer through reaction with vinylene carbonate and o-EC·–. Understanding the fate of electrolyte components such as EC and the structure of the SEI through RMD simulations should facilitate intelligent selection of electrolyte components and additives, leading to the formation of stable SEIs with improved transport properties.
Other BATT projects include simulation studies on
- structure, dynamics and capacitance of the interface between electrolytes and electrodes, including graphite and LiFePO4, as a function of temperature and electrode potential
- charge transfer resistance (Li+) at electrolyte/graphite and electrolyte/LiFePO4 interfaces as a function of temperature and electrode potential
- transport properties of novel ionic-liquid and organic-liquid electrolytes