Electrochemical behavior of magnesium alloys in alkali metal-tfsa ionic liquid for magnesium-battery negative electrode

An investigation was made on the reduction of the passivation of Mg negative electrode in an intermediate temperature ionic liquid, alkali metal bis(trifluoromethylsulfonyl)amide (TFSA) mixture to facilitate anodic dissolution of magnesium. The anodic dissolution of pure Mg metal electrode proceeded at substantially high potential above 2.0 V vs. Li⁺Li ^o due to passivation of the Mg surface with large hysteresis behavior in the cyclic voltammetry. In order to suppress/retard passivation of the electrode surface, several Mg alloys (ACM522, Mg-5 wt%Li, andMg2Sn/Sn) were examined as the negative electrode materials. The magnesium dissolution from ACM522 including Al, Ca, and misch metals as additive elements occurred at a higher electrode potential than that from the pure Mg metal. These metal additives seem to facilitate the formation of a corrosion-resistant surface layer, which is inadequate for the anodic dissolution of magnesium. The anodic-dissolution potential of magnesium in the Mg2Sn/Sn electrode was 1.5 V vs. Li⁺Li^o, which showed the lowest potential of that in the other Mg-alloy electrodes. Owing to the redox potential of pure Sn element higher than 1.5 V vs. Li⁺Li^o in the alkali metal-TFSA ionic liquid, Sn is electrochemically stable in the electrolyte and solely worked as the element lowering the Mg activity in the Mg-alloy electrode. Thus, lowing of Mg activity is effective on the retardation of the electrode passivation.