Surface-Mediated Solvent Decomposition in Li−Air Batteries: Impact of Peroxide and Superoxide Surface Terminations

A viable Li/O2 battery will require the development of stable electrolytes that do not continuously decompose during cell operation. Recent experiments suggest that reactions occurring at the interface between the liquid electrolyte and the solid lithium peroxide (Li2O2) discharge phase are a major contributor to these instabilities. To clarify the mechanisms associated with these reactions, a variety of atomistic simulation techniques, classical Monte Carlo, van der Waals-augmented density functional theory, ab initio molecular dynamics, and various solvation models, are used to study the initial decomposition of the common electrolyte solvent, dimethoxyethane (DME), on surfaces of Li2O2. Comparisons are made between the two predominant Li2O2 surface charge states by calculating decomposition pathways on peroxide-terminated (O2 2−) and superoxideterminated (O2 1−) facets. For both terminations, DME decomposition proceeds exothermically via a two-step process comprised of hydrogen abstraction (H-abstraction) followed by nucleophilic attack. In the first step, abstracted H dissociates a surface O2 dimer, and combines with a dissociated oxygen to form a hydroxide ion (OH−). The remaining surface oxygen then attacks the DME, resulting in a DME fragment that is strongly bound to the Li2O2 surface. DME decomposition is predicted to be more exothermic on the peroxide facet; nevertheless, the rate of DME decomposition is faster on the superoxide termination. The impact of solvation (explicit vs implicit) and an applied electric field on the reaction energetics are investigated. Our calculations suggest that surface-mediated electrolyte decomposition should out-pace liquid-phase processes such as solvent auto-oxidation by dissolved O2. ■ INTRODUCTION The high theoretical specific energy of the Li/O2 battery 1−6 makes it a promising candidate for energy storage in electric vehicles (EVs). However, several performance gaps must be overcome for these systems to become commercially viable. One of the primary issues relates to decomposition of the organic electrolyte. Decomposition processes have been associated with undesirable phenomena such as high charging overpotentials and limited cycle life. Therefore, a deeper understanding of these reactions is an important step in developing practical Li/O2 batteries. Identifying a stable electrolyte for Li/O2 batteries continues to be a challenge. Carbonates, a popular class of solvents for Liion batteries, appear to be incapable of providing long cycle life and high round-trip efficiencies in these systems. For example, it has been shown that the primary discharge/ charge reaction in a Li/O2 battery using a carbonate-based electrolyte is not reversible formation/decomposition of Li2O2, but instead involves highly stable phases such as Li2CO3 and other compounds that are generated by side reactions involving the electrolyte. More recent experiments have demonstrated an improvement with ether-based electrolytes, presumably due to their higher stability with respect to decomposition during cell operation. Despite this higher stability, several studies have found evidence that side reactions persist in these systems. McCloskey et al. quantified the yield of Li2O2 in a Li/O2 cell with a dimethoxyethane (DME) based electrolyte to be at best 91%. Based on the dependence of the Li2O2 yield and columbic efficiency on discharge rate, it was concluded that the dominant parasitic reactions were chemical reactions between the Li2O2 surfaces and the electrolyte. Specifically, the Li2O2 yield increased with discharge rate while the columbic efficiency remained close to the ideal 2 e/O2, suggesting that shortening the exposure time of Li2O2 surfaces to the electrolyte reduced the extent of side reactions. Freunberger et al. used FTIR, XRD, and NMR to characterize discharged Li/O2 batteries that used a tetraglyme-based electrolyte. By the end of the first discharge cycle, decomposition products such as Li2CO3, HCO2Li, CH3CO2Li, polyethers, CO2, and H2O were present (in addition to Li2O2). It was shown that changing either the salt, from LiPF6 to LiTFSI, or the solvent, from tetraglyme to triglyme or diglyme, did not stop the formation of side products. Indeed, the accumulation of side reaction products has been suggested to Received: January 9, 2015 Revised: March 25, 2015 Article