Passivation of aluminum current collectors in non-aqueous carbonate solutions containing sodium or potassium hexafluorophosphate salts

In this study, the investigation of the electrochemical behavior and passivation process of Al metal as a current collector for non-aqueous sodium-ion and potassium-ion batteries is performed. Electrochemical polarization in dynamic mode, in situ scratch polarization in transient mode, and surface analysis using time-of-flight secondary-ion mass spectroscopy (ToF-SIMS) are applied to understand the passivation of aluminum in fluorine-containing non-aqueous alkyl carbonate solutions. Polarization toward the cathodic direction does not result in the formation of alloys between Na or K metals and aluminum even at 0 V vs. Na⁺/Na or K⁺/K, respectively. For anodic polarization, the passivation of aluminum in the electrolyte is associated with the occurrence of an anodic reaction. ToF-SIMS surface analysis reveals that the passive layer is composed of an outer AlF₃ layer on the Al₂O₃ layer. The formation of the AlF₃ layer occurs via the reaction of Al₂O₃ with HF, Al₂O₃ + 6HF → 2AlF₃ + 3H₂O, as HF is produced as a byproduct of the oxidative decomposition of NaPF₆ or KPF₆ salts at high potential. The AlF₃ passive layer is stable up to 5 V vs. Na⁺/Na or K⁺/K, with improved stability observed in the Na solution relative to that in the K electrolyte.