A critical hurdle in realizing rechargeable lithium-metal batteries is the dendritic electrodeposition of lithium during the battery charging process. Here, we investigate the onset of dendritic morphology during galvanostatic lithium electrodeposition using electrochemical techniques combined with optical microscopy. We show that lithium dendrites initiate at the time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. This observation is explained using an analytical transport model wherein the Li + concentration within the solid electrolyte interphase (SEI) near the lithium metal surface decreases gradually as the SEI thickness and its transport resistance increases with time. At the dendrite onset time (τ onset ), the Li + concentration at the lithium-SEI interface approaches zero -a condition under which surface roughness on the lithium electrode amplifies, producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for plating. Model predictions of how τ onset varies with current density and soak time are shown to be in qualitative agreement with experimental observations. Furthermore, pulsed currents are applied to mitigate Li + concentration depletion within the SEI, thereby delaying or preventing altogether the formation of lithium dendrites.

The accelerated failure of rechargeable Li-metal batteries due to dendritic Li electrodeposition particularly during charging at low temperatures is not well-understood. In this work, we investigate the effect of temperature on the initiation of Li dendrites during galvanostatic lithium electrodeposition. Using electrochemical measurements coupled with optical microscopy, we show that the dendrite onset time increases monotonically with temperature in the range 5 °C–35 °C. This observation is explained by incorporating temperature effects into an analytical transport model for Li dendrite initiation [J. Electrochem. Soc., 165, D696 (2018)], which considers solid state Li+ diffusion through a gradually thickening solid electrolyte interphase (SEI) layer. We conclude that sluggish Li+ transport at lower temperatures accelerates the depletion of Li+ at the Li-SEI interface, and this effect causes earlier initiation of dendrites at lower temperatures. Electrochemical impedance spectroscopy measurements of the temperature-dependent transport properties of the SEI, as well as plating efficiency measurements, are used to support the model.

Low-temperature molten sodium batteries comprising molten sodium anodes, a NaSICON solid-state separator, and molten halide salt catholytes offer promise as low-cost, earth-abundant energy storage technologies. The emergence of a specific, high-voltage, sodium iodide (NaI)-based catholyte chemistry has prompted the evaluation of chemical and electrochemical properties of the molten salts, particularly at critical interfaces with high-performance NaSICON separators. Herein, batteries operated at 110 °C with NaI-AlCl3-based catholytes of differing Lewis acidities were evaluated. Batteries with >50 mol % AlCl3 (acidic catholytes) experienced a linear decline in energy efficiency during cycling, whereas batteries with >50 mol % NaI (basic catholytes) maintained >95% energy efficiency for 50 cycles (>80 days) at 2.5 mA cm–2. A three-electrode cell was developed, enabling identification of the NaSICON–catholyte interface as the source of increased battery impedance. Complementary physical and chemical characterization of the NaSICON exposed to acidic and basic catholytes showed no changes in crystallinity, bulk morphology, or bulk chemical composition, but surface sensitive X-ray photoelectron spectroscopy (XPS), however, revealed subtle changes in local NaSICON surface chemistry. In addition, Raman spectroscopy indicated that stably performing basic catholytes lack the dimer species Al2Cl6I– present in acidic catholytes. Select thermodynamic and formal charge assessments suggest that preferential interactions between these acidic dimeric species and the NaSICON surface may be responsible for the observed increases in electrochemical impedance and degraded battery performance. These results indicate that maintaining a Lewis basic catholyte avoids such potentially deleterious interactions, enabling efficient and stable battery cycling.

Iodide redox reactions in molten NaI/AlCl3 are shown to generate surface-blocking films, which may limit the useful cycling rates and energy densities of molten sodium batteries below 150°C. An experimental investigation of electrode interfacial stability at 110°C reveals the source of the reaction rate limitations. Electrochemical experiments in a 3-electrode configuration confirm an increase of resistance on the electrode surface after oxidation or reduction current is passed. Using chronopotentiometry, chronoamperometry, cyclic voltammetry, and electrochemical impedance spectroscopy, the film formation is shown to depend on the electrode material (W, Mo, Ta, or glassy carbon), as well as the Lewis acidity and molar ratio of I-/I3 - in the molten salt electrolytes. These factors impact the amount of charge that can be passed at a given current density prior to developing excessive overpotential due to film formation on the electrode surface. The results presented here guide the design and use of iodide-based molten salt electrolytes and electrode materials for grid scale battery applications.

Suppression of surface roughness and dendrite growth under pulsed current (p.c.) plating is a widely reported effect for a variety of electrodeposited metals. Often, this effect is attributed to the modulation of mass transport during pulsing. In the present contribution, we use numerical simulations and scaling analysis to shed light on the transient mass transport effects near a 2D microscale pattern subjected to p.c. plating. Specifically, we compare the microscale current distribution during p.c. to that during direct current (d.c.) plating at an equivalent time-averaged plating rate. Modeling shows that the more uniform current distribution for a given time-averaged plating rate is that obtained during d.c. plating. The current distribution during p.c. plating is found to be less uniform in comparison to d.c., and the mechanistic rationale underlying this effect is explained using scaling analysis. Results reported herein have implications to the understanding of pulsed currents in applications ranging from thin-film electroplating to battery charging.