Weakly coordinating monoether-based concentrated electrolytes: effects of frustrated Li ion coordination on ion transport and Li metal battery performance

Although Li metal batteries have attracted much attention as high energy density rechargeable batteries, poor cycle ability and safety remain great concerns for practical application. The development of suitable electrolytes is the key to improving the performance of Li metal batteries, and research on electrolytes has been vigorously pursued. In this study, weakly coordinating properties of linear monoether-based electrolytes that regulate energy level of Li ion coordination to be energetically less stable, frustrated state were exploited to improve Li ion transport via rapid Li ion exchange between the weak-coordination sites (solvent and anion). In addition to the reductive stability of the monoether solvents, formation of anion-derived inorganic-rich solid electrolyte interphase (SEI), and the improved Li ion mass transport, a significant positive shift of electrode potential of Li deposition/dissolution was responsible for high reversibility of Li metal electrode in the monoether-based electrolytes. The charge-discharge rate capabilities of Li/LiFePO₄ cells with n‑butyl methyl ether (BME)-based electrolytes were superior to those of cells with highly concentrated electrolytes (HCEs) of dimethoxyethane (DME) containing lithium bis(fluorosulfonyl)amide (LiFSA). Li/LiFePO₄ cells with BME-based electrolytes delivered a high discharge capacity of 110 mAh g⁻¹ even at a high current density of 15 mA cm⁻². This study highlights that less-stabilized Li ion coordination in the weakly coordinating electrolytes enables the enhanced Li ion transport and highly reversible deposition/dissolution of Li metal, which in turn leads to the greater charge-discharge performance of Li metal batteries at high current densities.