Non-uniform metal deposition and dendrite formation in high density energy storage devices reduces the efficiency, safety, and life of batteries with metal anodes. Superconcentrated ionic liquid (IL) electrolytes (e.g. 1:1 IL:alkali ion) coupled with anode preconditioning at more negative potentials can completely mitigate these issues, and therefore revolutionize high density energy storage devices. However, the mechanisms by which very high salt concentration and preconditioning potential enable uniform metal deposition and prevent dendrite formation at the metal anode during cycling are poorly understood, and therefore not optimized. Here, we use atomic-force microscopy and molecular dynamics simulations to unravel the influence of these factors on the interface chemistry in a sodium electrolyte, demonstrating how a molten salt like structure at the electrode surface results in dendrite free metal cycling at higher rates. Such a structure will support the formation of a more favorable solid electrolyte interphase (SEI) accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning interfacial nanostructure via salt concentration and high voltage preconditioning. File list (2) download file view on ChemRxiv supplementary.pdf (5.03 MiB) download file view on ChemRxiv Manuscript-V1.pdf (2.06 MiB)

We explore a novel ether aided superconcentrated ionic liquid electrolyte; a combination of ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) and ether solvent, 1,2 dimethoxy ethane (DME) with 3.2 mol/kg LiFSI salt,...

Cycling stability at high capacities and water-tolerance are two key properties for the operation of high-capacity lithium (Li) metal–air batteries.

High-energy-density systems with fast charging rates and suppressed dendrite growth are critical for the implementation of efficient and safe next-generation advanced battery technologies such as those based on Li metal. However, there are few studies that investigate reliable cycling of Li metal electrodes under highrate conditions. Here, by employing a superconcentrated ionic liquid (IL) electrolyte, we highlight the effect of Li salt concentration and applied current density on the resulting Li deposit morphology and solid electrolyte interphase (SEI) characteristics, demonstrating exceptional deposition/dissolution rates and efficiency in these systems. Operation at higher current densities enhanced the cycling efficiency, e.g., from 64 ± 3% at 1 mA cm −2 up to 96 ± 1% at 20 mA cm −2 (overpotential <±0.2 V), while resulting in lower electrode resistance and dendrite-free Li morphology. A maximum current density of 50 mA cm −2 resulted in 88 ± 3% cycling efficiency, displaying tolerance for high overpotentials at the Ni working electrode (0.5 V). X-ray photoelectron microscopy (XPS), time-of-flight secondary-ion mass spectroscopy (ToF-SIMS), and scanning electron microscopy (SEM) surface measurements revealed that the formation of a stable SEI, rich in LiF and deficient in organic carbon species, coupled with nondendritic and compact Li morphologies enabled enhanced cycling efficiency at higher currents. Reduced dendrite formation at high current is further highlighted by the use of a highly porous separator in coin cell cycling (1 mAh cm −2 at 50 °C), sustaining 500 cycles at 10 mA cm −2 .