Investigating Capacity Fade Mechanisms in Dual-Ion Mg-MClₓ Batteries

Abstract Mg batteries are a promising alternative to Li-based chemistries due to the high abundance, low cost, and high volumetric capacity of Mg relative to Li. Mg is also less prone to dendritic plating morphologies, promising safer operation. Mg plating and stripping is highly efficient in chloride-containing electrolytes; however, chloride is incompatible with many candidate cathode materials. In this work, we capitalize on the positive effect of chloride by using transition metal chloride cathodes with a focus on low cost, Earth-abundant metals. Both soluble and sparingly soluble chlorides show capacity fade upon cycling. Active material dissolution and subsequent crossover to the Mg anode are the primary drivers of capacity fade in highly soluble metal chloride cathodes. We hypothesize that incomplete conversion and chemical reduction by the Grignard-based electrolyte are major promoters of capacity fade in sparingly soluble metal chlorides. Modifications to the electrolyte can improve capacity retention, suggesting that future work in this system may yield low cost, high retention Mg-MClₓ batteries.