Lithium metal is an ideal battery anode. However, dendrite growth and limited Coulombic efficiency during cycling have prevented its practical application in rechargeable batteries. Herein, we report that the use of highly concentrated electrolytes composed of ether solvents and the lithium bis(fluorosulfonyl)imide salt enables the high-rate cycling of a lithium metal anode at high Coulombic efficiency (up to 99.1%) without dendrite growth. With 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane as the electrolyte, a lithium|lithium cell can be cycled at 10 mA cm−2 for more than 6,000 cycles, and a copper|lithium cell can be cycled at 4 mA cm−2 for more than 1,000 cycles with an average Coulombic efficiency of 98.4%. These excellent performances can be attributed to the increased solvent coordination and increased availability of lithium ion concentration in the electrolyte. Further development of this electrolyte may enable practical applications for lithium metal anode in rechargeable batteries.

We combine direct surface force measurements with thermodynamic arguments to demonstrate that pure ionic liquids are expected to behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% at room temperature. We performed equilibrium force-distance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 4 mim][NTf 2 ]) using a surface forces apparatus with in situ electrochemical control and quantitatively modeled these measurements using the van der Waals and electrostatic double-layer forces of the Derjaguin-LandauVerwey-Overbeek theory with an additive repulsive steric (entropic) ion-surface binding force. Our results indicate that ionic liquids screen charged surfaces through the formation of both bound (Stern) and diffuse electric double layers, where the diffuse double layer is comprised of effectively dissociated ionic liquid ions. Additionally, we used the energetics of thermally dissociating ions in a dielectric medium to quantitatively predict the equilibrium for the effective dissociation reaction of [C 4 are not effectively dissociated and thus do not contribute to electrostatic screening. We also provide a general, molecular-scale framework for designing ionic liquids with significantly increased dissociated charge densities via judiciously balancing ion pair interactions with bulk dielectric properties. Our results clear up several inconsistencies that have hampered scientific progress in this important area and guide the rational design of unique, high-free-ion density ionic liquids and ionic liquid blends.Boltzmann distribution | electrostatic interaction | interfacial phenomena I onic liquids are fluids composed solely of ions (1, 2). Much of the recent scientific interest surrounding ionic liquids derives from the fact that ionic liquids have been demonstrated for numerous applications, such as safe, high-efficiency electrochemical storage devices (3, 4), self-assembly media (5), and lubrication (6). A key paradigm within ionic liquids research is that the physical properties of ionic liquids can be controlled to an unprecedented degree through the judicious design of cation-anion pairs (1-9). Thus, ionic liquids are known as "designer" solvents/materials (1). However, fully realizing this advantage requires the development of a comprehensive framework that can be used to rationalize the relationship between an ionic liquid's molecular structure and its bulk and interfacial behavior and properties, which are governed by a complex interplay of coulomb, van der Waals, dipole, hydrogen bonding, and steric interactions (10, 11).Recent work has greatly progressed a fundamental understanding of ionic liquids at charged interfaces, but there are currently inconsistencies between experiment and theory (4, 7-9, 12-23); this is particularly true for the ranges of surface-induced ordering and electrostatic screening. For example, a comparison of the values obtained from ionic conductivity and...

The electrolyte solution structure for acetonitrile (AN)-lithium salt mixtures has been examined for highly dissociated salts. Phase diagrams are reported for (AN) n -LiN(SO 2 CF 3 ) 2 (LiTFSI) and -LiPF 6 electrolytes. Single crystal structures and Raman spectroscopy have been utilized to provide information regarding the solvate species present in the solid-state and liquid phases, as well as the average solvation number variation with salt concentration. Molecular dynamics (MD) simulations of the mixtures have been correlated with the experimental data to provide additional insight into the molecular-level interactions. Quantum chemistry (QC) calculations were performed on (AN) n -Li-(anion) m clusters to validate the ability of the developed many-body polarizable force field (used for the simulations) to accurately describe cluster stability (ionic association). The combination of these techniques provides tremendous insight into the solution structure within these electrolyte mixtures.

The influence of adding the room-temperature ionic liquid ͑RTIL͒ N-methyl-N-propylpyrrolidinium bis͑trifluoromethanesulfo-nyl͒imide ͑PYR 13 TFSI͒ to P͑EO͒ 20 LiTFSI polymer electrolytes and the use of these electrolytes in solid-state Li/V 2 O 5 batteries has been investigated. P͑EO͒ 20 LiTFSI + xPYR 13 TFSI polymer electrolytes with various PYR 13 + /Li + mole fractions ͑x = 0.66, 1.08, 1.73, 1.94, 2.15, and 3.24͒ were prepared. The addition of up to a 3.24 mole fraction of the RTIL to P͑EO͒ 20 LiTFSI electrolytes, corresponding to a RTIL/PEO weight fraction of up to 1.5, resulted in freestanding and highly conductive electrolyte films reaching 10 −3 S/cm at 40°C. The electrochemical stability of PYR 13 TFSI was significantly improved by the addition of LiTFSI. Li/V 2 O 5 cells using the polymer electrolyte with PYR 13 TFSI showed excellent reversible cyclability with a capacity fading of 0.04% per cycle over several hundreds cycles at 60°C. The incorporation of the RTIL into lithium metal-polymer electrolyte batteries has resulted in a promising improvement in performance at moderate to low temperatures.Since the first report by Walden in 1914, 1 large efforts have been devoted to the investigation of room temperature ionic liquids ͑RTILs͒ as green chemistry materials, especially over the last six years. 2 RTILs that consist of organic cations and inorganic anions are attracting wide interest for applications in catalysis, fuel cells, electrochemical capacitors, and batteries 3-8 due to their nonvolatility, nonflammability, high thermal stability, and high conductivity. 9,10 Recently, a few reports have demonstrated the enhanced performance of electrochemical devices using electrolytes composed of pure RTILs 11 or RTILs doped with a suitable lithium salt ͑to supply the Li + cations required in the electrochemical reactions͒ and combined with an appropriate polymer. [12][13][14][15][16] Rechargeable lithium metal polymer electrolyte batteries ͑LMPBs͒ are now considered to be the most probable next generation of power sources for portable electronic devices and electric vehicles. The performance of LMPBs, however, is still limited by the ionic conductivity of the polymer electrolyte. Most of the polymer electrolytes reported to date do not have ionic transport properties suitable for state-of-the-art lithium batteries. Nevertheless, dry ͑molecular solvent-free͒ polymer electrolytes have been extensively investigated. Poly͑ethylene oxide͒ ͑PEO͒ based electrolytes are one of the most promising materials due to their good thermal properties and interfacial stability with the Li electrode. PEO-LiX electrolytes are hindered, however, by a low-room-temperature ionic conductivity. The addition of molecular solvents able to compete with the polymer ether oxygens for Li + cation coordination has been demonstrated as a means of attaining a high ionic conductivity at room temperature. 17,18 Unfortunately, the reactivity of such solvents in these gelled electrolytes results in a poor interfacial stability with Li metal. Addi...