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 Li_1.1Al_0.04Mn_0.65Ni_0.21O₂ and excellent structural and voltage stability, and hints at the impact of short-range cation ordering and phase separation on the electrochemical performance. Neutron and X-ray diffraction results indicate that the as-synthesized compound comprises two crystallographically distinct phases -- a Li₂MnO₃ component and a Li-poor (Li_0.78Al_0.02Mn_0.67Ni_0.31O₂) 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 Li_0.78Al_0.02Mn_0.67Ni_0.31O₂ -- and highlight the presence of dilithium environments within the transition metal layer in Li₂MnO₃, with important consequences on structural stability during electrochemical cycling.