Direct Observation of Alternating Octahedral and Prismatic Sodium Layers in O3- Type Transition Metal Oxides

Abstract

The oxygen stacking of O3- type layered sodium transition metal oxides (O3- NaTMO2) changes dynamically upon topotactic Na extraction and reinsertion. While the phase transition from octahedral to prismatic Na coordination that occurs at intermediate desodiation by transition metal slab gliding is well understood, the structural evolution at high desodiation, crucial to achieve high reversible capacity, remains mostly uncharted. In this work, the phase transitions of O3- type layered NaTMO2 at high voltage are investigated by combining experimental and computational approaches. An OP2- type phase that consists of alternating octahedral and prismatic Na layers is directly observed by in situ X- ray diffraction and high- resolution scanning transmission electron microscopy. The origin of this peculiar phase is explained by atomic interactions involving Jahn- Teller active Fe4+ and distortion tolerant Ti4+ that stabilize the local Na environment. The path- dependent desodiation and resodiation pathways are also rationalized in this material through the different kinetics of the prismatic and octahedral layers, presenting a comprehensive picture about the structural stability of the layered materials upon Na intercalation.A novel phase transition to an OP2- type stacking sequence from an O3- layered sodium transition metal oxide (NaTi0.25Fe0.25Co0.25Mn0.25O2) is directly observed and investigated by in situ X- ray diffraction, high- resolution scanning transmission electron microscopy, and first- principles calculations. This study discusses how Jahn- Teller active Fe4+ ion promotes OP2 formation, which leads to hysteresis during resodiation.