Effect of the Nature of Both Cation and Anion Substitution on the Structural Symmetry of Li-Rich 3d-Metal Chalcogenide Electrodes

Abstract

Li-rich layered chalcogenides have recently led to better understanding of the anionic redox process and its associated high capacity while providing ways to overcome its practical limitations of voltage fade and irreversibility. This study reports on the feasibility of triggering anionic activity in Li2TiS3, through anionic substitution (Se for S) or cationic substitution (Fe for Ti). Herein, the chalcogenide chemical space is further explored to prepare mono-substituted Li1.7Ti0.85Mn0.45Ch3 (Ch = S/Se) and doubly substituted cationic and anionic phases (Li1.7Ti0.85Fe0.45S3-zSez) which crystallize either in the O3- or O1-type structures depending upon substituents. All series show a bell-shape capacity variation as function of the transition metal (TM) substitution degree with values up to 240 mAh g−1. For specific compositions, a structural O3 to O1 phase transition is observed upon Li removal, which is not reversible upon Li re-insertion due to kinetic limitations and negatively affects long-term cycling performance. Density functional theory (DFT) calculations confirm the O3/O1 relative stability along the different series and point subtle electronic differences in the TM-doping, rationalizing the structural and electrochemical behaviors of these phases upon cycling. These findings provide further insights into the link between structural and electronic stability, which is of key importance for designing chalcogenide-based anionic redox compounds.