Nonpolar Alkanes Modify Lithium‐Ion Solvation for Improved Lithium Deposition and Stripping

Abstract

DOI: 10.1002/aenm.201902116 batteries with higher energy densities.[5,6] However, lithium metal has been studied for over five decades and many challenges persist.[5–7] Lithium metal is highly reactive—a consequence of its low reduction potential—and continuous reactivity with the electrolyte leads to electrolyte loss and severe capacity fade.[5,6] Several approaches have been pursued to limit the side reactions that lithium may undergo with the electrolyte.[5,7] First, carbonate solvents primarily used in Li-ion batteries have been eschewed for ethers (1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME)) that show much higher Coulombic efficiencies upon lithium deposition and stripping.[7,8] Second, strategies such as the use of high salt content with the so-called “solvent-in-salt” systems have shown promise despite the high cost and high viscosity of the electrolyte solutions.[9–12] Furthermore, polymer coatings have been shown to support smooth lithium deposition.[13,14] Despite all—and in combination with—these strategies, the use of additives is paramount.[7] Additives are chosen that can preferentially react with lithium metal and passivate the surface, thereby limiting further reaction with the electrolyte and extending cycle life.[5,7,15] A plethora of additives such as inorganic salts (e.g., LiNO3), phosphates,[17] water,[18] solvents,[19] and fluorinated compounds,[20,21] among others have been explored.[7] Their working principle is as follows: these additives react with lithium metal to create a modified solid electrolyte interface (SEI) that is better able to stabilize the lithium metal surface. However, these additives continually react with any fresh lithium metal surface that is generated during cycling. Upon complete consumption of the additive, little is known about the lithium metal behavior. A recent study by Gasteiger et al.[22] provides important insight. They show that the cell Coulombic efficiency drops by over 60% upon consumption of the electrolyte additive (the widely used fluoroethylene carbonate) in Si-C composite anodes (which upon lithiation also reacts with and consumes the electrolyte like lithium metal). Increasing the additive concentration is often not a solution because of cost, decreased ionic conductivities, and possible hindrance of the desired redox kinetics. Hence, reactive additives may have limited utility because their continuous consumption will lead to a regime where the additive is no longer present and there is significant degradation in performance. Therefore, it is important to explore an alternative strategy: the development of nonreactive additives.[23,24] These nonreactive Lithium metal batteries have been plagued by the high reactivity of lithium. Reactive additives that can passivate the lithium metal surface and limit electrolyte accessibility to a fresh lithium surface have been widely explored, but can have limited utility with continuous consumption of the additive. In this work, an alternative strategy is explored. The use of nonreactive cosolvents such as nonpolar alkanes is studied and its is shown that hexane and cyclohexane addition to ether solvents (1,3-dioxolane and 1,2-dimethoxyethane) halves the nucleation and growth overpotentials for lithium deposition, increases the cell coulombic efficiency, improves the lithium deposition morphology, increases the electrolyte oxidative stability (>0.2 V), and doubles the cycle life—even when compared to a widely used fluorinated ether. The nonpolar alkanes modify the lithium-ion solvation environment and reduce the solvation free energy; hence reducing the reaction barrier for lithium deposition. Exploration of nonpolar alkanes as part of the electrolyte mixture is a promising strategy for controlling metal deposition.