Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes.

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 electrolytes (for example 1:1 ionic liquid: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 favourable solid electrolyte interphase, accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning the interfacial nanostructure via salt concentration and high-voltage preconditioning.

High Current Cycling in a Superconcentrated Ionic Liquid Electrolyte to Promote Uniform Li Morphology and a Uniform LiF-Rich Solid Electrolyte Interphase.

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 high-rate 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.

Compressively Stressed Silicon Nanoclusters as an Antifracture Mechanism for High-Performance Lithium-Ion Battery Anodes.

Silicon has been considered a good candidate for replacing the commonly used carbon anodes for lithium-ion batteries (LIBs) due to its high specific capacity, which can be up to 11 times higher than that of carbon. However, the desirable advantage that silicon brings to battery performance is currently overshadowed by its stress-induced performance loss and high electronic resistivity. The induced stress arises from two sources, namely, the deposition process (i.e., residual stress) during fabrication and the volume expansion (i.e., mechanical stress) associated with the lithiation/delithiation process. Of the two, residual stress has largely been ignored, underestimated, or considered to have a negligible effect without any rigorous evidence being put forward. In this contribution, we produced silicon thin films having a wide range of residual stress and resistivity using a physical vapor deposition technique, magnetron sputtering. Three pairs of silicon thin-film anodes were utilized to study the effect of residual stress on the electrochemical and cyclability performance as anodes for LIBs. Each set consisted of a pair of films having essentially the same resistivity, density, thickness, and oxidation amount but distinctly different residual stresses. The comparison was evaluated by conducting charge/discharge cycling and cyclic voltammetry (CV) experiments. In contrast to the fixed belief within the literature, higher compressive residual-stress films showed better electrochemical and cycle performance compared to lower residual-stress films. The results, herein, present an informed understanding of the role that residual stress plays, which will help researchers improve the development of silicon-based thin-film anodes.