Resolving Current-Dependent Regimes of Electroplating Mechanisms for Fast Charging Lithium Metal Anodes

Abstract

Poor fast-charge capabilities limit the usage of rechargeable Li metal anodes. Understanding the connection between charging rate, electroplating mechanism, and Li morphology could enable fast-charging solutions. Here, we develop a combined electroanalytical and nanoscale characterization approach to resolve the current-dependent regimes of Li plating mechanisms and morphology. Measurement of Li+ transport through the solid electrolyte interphase (SEI) shows that low currents induce plating at buried Li||SEI interfaces, but high currents initiate SEI-breakdown and plating at fresh Li||electrolyte interfaces. The latter pathway can induce uniform growth of {110}-faceted Li at extremely high currents, suggesting ion-transport limitations alone are insufficient to predict Li morphology. At battery relevant fast-charging rates, SEI-breakdown above a critical current density produces detrimental morphology and poor cyclability. Thus, prevention of both SEI-breakdown and slow ion-transport in the electrolyte is essential. This mechanistic insight can inform further electrolyte engineering and customization of fast-charging protocols for Li metal batteries.

Correlating Kinetics to Cyclability Reveals Thermodynamic Origin of Lithium Anode Morphology in Liquid Electrolytes

“…6,9,23,24 The interfacial resistance (R SEI ) parameterizes the kinetics of this pathway and is easily measurable with EIS of symmetric Li||Li cells. 23,24 Second is electroplating at fresh Li−electrolyte interfaces, which can happen when the SEI fractures during fast volume expansion of electroplated Li. 23,24 For this pathway, interfacial charge transfer at fresh Li−electrolyte interfaces is the key electroplating reaction (Figure 1G).…”

“…23,24 Second is electroplating at fresh Li−electrolyte interfaces, which can happen when the SEI fractures during fast volume expansion of electroplated Li. 23,24 For this pathway, interfacial charge transfer at fresh Li−electrolyte interfaces is the key electroplating reaction (Figure 1G). Recently, our group developed a transient cyclic voltammetry method using ultramicroelectrodes to measure charge-transfer kinetics at fresh Li−electrolyte interfaces.…”

“…Li/Li+ positively and increase the surface energy via eq 2, complementing the benefits of F-or anion-rich SEI for preventing SEI fracture 24 and slowing SEI growth. 9 Molecular Factors Influencing Charge-Transfer Kinetics.…”

“…The Z -fit EIS fitting software from EC-Lab was used to fit the spectrum to an equivalent circuit verified to describe the SEI (Figure S1). Only data with a frequency >20 Hz was included for the fit …”

“…One reason for these discrepancies is that the reactivity of Li is often neglected in kinetic analysis. Most electrolytes react with Li and form a nanometers-thick and ionically conductive compact SEI that directly contacts Li. , Depending on charging rate, electroplating can occur at either buried Li–SEI interfaces or fresh Li–electrolyte interfaces when the SEI fractures, with each pathway having distinct electroplating kinetics. − Resolving the kinetics of these two pathways across each class of electrolytes is thus key to establish the relationship between kinetics and Li morphology.…”