Mechanistic Insights into Surface Chemistry, Stability, and Passivation of Current Collectors in Water-in-Salt Electrolytes

Abstract

Aqueous lithium-ion batteries (ALIBs) operating with highly concentrated electrolytes have gained attention as safer alternatives to nonaqueous systems. Although research has primarily focused on electrodes and electrolytes for ALIBs, studies of current collectors remain limited, despite their critical impact on battery lifespan and performance. Here, we investigate the passivation on three widely used current collectors (Al, Ti, and Cu) in the hydrate-melt electrolyte (Li(TFSI)0.7(BETI)0.3 center dot 2H2O) for ALIBs. Using dynamic and transient polarization tests combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), we reveal that Al develops a stable bilayer, an inner Al-O and outer Al-F layers, above 4 V vs. Li+/Li, which persists up to-5 V. Similarly, Ti forms a double layer composed of a thin outer Ti-F layer on the inner Ti-O film after polarization at 5 V. These layers result from the reaction with HF in the electrolyte, with the HF produced through decomposition of the hydrate-melt electrolyte. In contrast, Cu experiences serious oxidation above-3.6 V, whereas it remains in an unpassivated state at lower potentials. Through this work, it is suggested that Al and Ti current collectors are suitable for high-voltage application due to their outstanding electrochemical stability at high potentials, making them promising for use in high-voltage ALIBs.