Abstract
Li metal batteries have attracted much attention as the next-generation rechargeable batteries owing to the exceptionally high theoretical capacity (3860 mAh g⁻¹) and the lowest electrode potential of Li metal electrode. Molten Li salt solvate electrolytes or highly concentrated electrolytes (HCEs) are an emerging class of electrolytes having a similarity to ionic liquids (ILs) in high ionic nature, and indeed some of which are found to behave like an IL.1 This type of the electrolytes have attracted considerable attention as prospective candidates for electrolyte materials in future Li metal batteries owing to their favorable properties both in the bulk and at the electrochemical interface. In this study, linear ether-based molten Li salt solvates were investigated for potential effects of weakly coordinating linear ethers on transport properties and battery performance. We focused on the electrolytes using lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or lithium bis(fluorosulfonyl)amide (LiFSA) as the salt and linear chain monoethers such as methylpropyl ether (MPE), n-butylmethyl ether (BME), and ethyl propyl ether (EPE) as the solvent for lithium-sulfur and lithium metal batteries. Owing to the low lithium polysulfide solubility, low viscosity, relatively high ionic conductivity, high Li ion transference number, high reductive stability, and low specific gravity, all of which are favorable for achieving high-energy density batteries, linear ether-based molten Li salt solvates were found to be effective electrolytes. The correlation between Li ion solvation and ionic transport properties of the linear ethers with different alkyl chain length were further studied to optimize the electrolyte compositions. The chemical structure of the ethers had a strong impact on the Li ion coordination structure and ionic conductivity: the monomethyl ether such as MPE and BME showed more-pronounced Li ion coordination and higher ionic conductivity whereas steric hinderance of longer alkyl chains in ethers with longer alkyl chain length such as ethyl propyl ether (EPE) resulted in lower solvation number, enhanced ion pairing and lower ionic conductivity. The improved Li ion transport property of the linear ether-based electrolytes led to the better rate performance. Furthermore, a pouch-type cell demonstrated an energy density exceeding 300 Wh kg-1 under lean electrolyte conditions.2 Shigenobu, T. Sudoh, J. Murai, K. Dokko, M. Watanabe, K. Ueno, Chem. Rec., 2023, 23, e202200301.Ishikawa, S. Haga, K. Shigenobu, T. Sudoh, S. Tsuzuki, W. Shinoda, K. Dokko, M. Watanabe, K. Ueno, Faraday Discuss., 2024, Accepted Manuscript. DOI: 10.1039/D4FD00024B
Published Version
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