First-principles comparative study of Cr migration in O3 and O/P hybrid-phased NaCrO2

In layered Na transition-metal (TM) oxides, TM migration usually occurs at highly charged states and severely deteriorates the capacity and reversibility. Meanwhile, the formation of hybrid phases with the intergrowth of octahedral (O-type) and prismatic (P-type) Na layers also takes place at highly charged states. These hybrid phases are often more stable than simple O3 or P3 stackings. However, little is known about the mechanism and impact of TM migration in these hybrid phases. In this work, a comparative first-principles study is performed to understand the connections between structural changes and Cr migration in layered O3 and hybrid-phased NaCrO2. After Cr migration, the hybrid-phased NaCrO2 suffers from greater layer shrinkage than the O3 phase. Three factors affect the Cr migration energy Emig: the Na concentration, local 3D configurations, and 2D in-plane geometries. Low Na concentrations and certain local 3D configurations facilitate the Cr migration. The Cr migration barriers in both O3 and hybrid-phased NaCrO2 are positively correlated with the CrEmig. The Cr migration in 17 doped O3 and hybrid-phased NaCrO2 is surveyed. In these doped NaCrO2, a more uniform distribution of the Cr-O bond lengths usually suggests suppressed Cr migration. Optimal dopants for suppressing Cr migration are identified by considering Emig for both Cr and the dopant. Our comparative study on Cr migration in O3 and hybrid-phased NaCrO2 reveals the significant role of hybrid-phased structures in the development of layered cathode materials.