Structural complexity in a highly reversible “anion-redox” cathode†
Abstract
Li-rich cathodes with an O2-type layer stacking offer high gravimetric capacities and fast charge–discharge rates, and are structurally more stable with respect to transition metal migration than O3-type Li-rich cathodes. However, the nature and reversibility of their charge–discharge processes remain poorly understood, in part because these materials can only be obtained through soft chemistry routes. This work provides a new structural model for a recently-reported O2-type cathode with nominal composition Li1.1Al0.04Mn0.65Ni0.21O2 and excellent structural and potential stability. Our new model hints at the impact of short-range cation ordering and phase separation on the electrochemical performance. Neutron and X-ray diffraction indicate that the as-synthesized compound comprises two crystallographically distinct phases—a Li2MnO3 component and a Li-poor (Li0.78Al0.02Mn0.67Ni0.31O2) component—most likely stacked epitaxially along the c-axis. 7Li, 17O and 27Al solid-state NMR measurements further reveal a tendency towards honeycomb ordering on the transition metal sublattice—long-range ordering in Li2MnO3 and partial, short-range ordering in Li0.78Al0.02Mn0.67Ni0.31O2—and highlight the presence of dilithium environments within the transition metal layer in Li2MnO3, with important consequences on structural stability during electrochemical cycling.
- This article is part of the themed collection: Journal of Materials Chemistry A HOT Papers