Engineering High Energy Density Sodium Battery Anodes for Improved Cycling with Superconcentrated Ionic Liquid Electrolytes

Abstract

<div> <div> <div> <p>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 </p> </div> </div> </div> <div> <div> <div> <p>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. </p> </div> </div> </div>

Improving Cycle Life Through Fast Formation Using a Super-Concentrated Phosphonium Based Ionic Liquid Electrolyte for Anode-Free and Lithium Metal Batteries

“…The Au (111)|50 mol% LiFSI in P1222FSI interface was examined without applied bias and at -3.7, -8.6, and -14.4 μC/cm 2 . As shown in Figure 5a, the overall number of interfacial layers detected for the examined system was found to be around 2-3 regardless of the surface charge, similar to observations for super-concentrated NaFSI and LiFSI in pyrrolidinium FSI IL systems [46,47]. The innermost layer was defined within a distance of 0.61 nm from the Au (111)…”

“…The number of anions coordinating with one Li + is barely affected, whereas that of OFSI shows a decrease from ~ 4.5 to ~ 3.0 (Figure 5e), which also consistently responds to the change in the FSI/Li ratio. These observations were also found for LiFSI and NaFSI systems in pyrrolidinium FSI electrolytes, which implies that the trend in Li(Na)-FSI coordination changes upon applied bias, under the conditions given here for a Au metallic substrate, is more affected by salt concentration and additives than by the nature of the IL cation [46,47]. Molecular orbital energy analysis showed that these Li-FSI coordination changes affect both HOMO and LUMO levels [46], which suggests that the [Li(FSI)3] -2 unit is reductively unstable upon charging to -14.4…”

“…As shown in Figure 5c, the large interfacial coverage of [P1222] + at low negative surface charge might lead to parasitic decomposition of [P1222] + on the Li anode surface or simply blockage of sites that allow more uniform Li plating. Meanwhile, the use of higher negative polarization can eliminate these issues and bring more [LixFSIy] x-y aggregates to the surface, leading to the formation of a more favourable solid electrolyte interphase (SEI) and/or resulting in homogeneous Li deposition morphology [46]. The nature of Li-FSI coordination within the innermost layer was also examined through calculating their radial distribution function (RDF) in order to better understand the effect of electrode charging (Figure 5d and 5e).…”

“…μC/cm 2 , and this is important for active formation of an homogeneous inorganic rich-SEI layer [46].…”