Correlation between the cationic composition and anionic electrochemical activity of Li2MSeO anti-perovskites

Abstract

Li₂MChO anti-perovskites have lately attracted much attention as potential electrode materials for Li batteries owing to their high and stable specific capacity. It is rather surprising, because in terms of the structure, metal ions share the same crystallographic site. Since the redox potentials of chalcogenide anions S²⁻ or Se²⁻ and transition metal cations are close, anti-perovskites can be classified as bifunctional materials with cationic and anionic electrochemical activity. Their high chemical flexibility combined with redox-active ions make cubic anti-perovskites a rather valuable model system for tuning desired electrochemical properties. Herein, we studied six Li₂MSeO compounds with Mn, Fe, and Co and their solid solutions in Li cells and tailed their structural and electrochemical behaviour to the cationic composition. Remarkably, a booster property of Co with the highest Co³⁺/Co²⁺ potential with regard to an enhanced electrochemical performance was observed for Li₂Fe_0.5Co_0.5SeO in comparison to Li₂FeSeO, although only Fe and Se were involved in the redox process while Co remained inactive. In Li₂Fe_0.5Mn_0.5SeO, less-active Mn facilitated the oxidation of Fe and Se, positively influencing the rate capability in comparison to Li₂Co_0.5Mn_0.5SeO. Thus, we found a direct correlation between the M³⁺/M²⁺ standard redox potential and the electrochemical activity of Se²⁻.