Inhibition of Aluminum Corrosion with the Addition of the Tris(pentafluoroethyl)trifluorophosphate Anion to a Sulfonylamide-Based Ionic Liquid for Sodium-Ion Batteries

Abstract

Ionic liquids (ILs) based on sulfonylamide-type anions have gained widespread utility as electrolytes for secondary batteries. Although sulfonylamide-based IL electrolytes are known to form a stable passivation layer that prevents Al corrosion, the Al electrode in the Na[FSA]-[C₂C₁im][FSA] ([FSA] = bis(fluorosulfonyl)amide and [C₂C₁im] = 1-ethyl-3-methylimidazolium) IL, is found to be afflicted by pitting corrosion at potentials above 4 V vs Na⁺/Na during electrochemical measurement at 90 °C. Therefore, this study investigates the suppressive effect of [FAP]⁻ (FAP = tris(pentafluoroethyl)trifluorophosphate) on the Al corrosion behavior of the IL electrolyte. Here, the inhibited corrosion of the Al electrode is confirmed through a series of cyclic voltammetry measurements, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Charge-discharge tests performed using a Na₃V₂(PO₄)₂F₃ positive electrode demonstrates that the addition of [FAP]⁻ into the IL enhances cycling performance at the intermediate temperature of 90 °C.