Thermally and Electrochemically Driven Topotactical Transformations in Sodium Layered Oxides Na_xVO₂

Phase diagrams and structural transformations in the complex Na x VO 2 system have been studied using electrochemical (de)intercalation and in situ and operando high resolution synchrotron powder diffraction. Starting from O′3-Na 1/2 VO 2 obtained by sodium electrochemical deintercalation of O3-NaVO 2, the structural details of irreversible and reversible thermally driven transformations to P′3 and P3 type structures are presented. Subsequently, these P′3-Na x VO 2 phases provide a platform for operando studies exploring the Na x VO 2 phase diagram as a function of sodium electrochemical (de)intercalation. In this system, three single phase domains have been found: a line phase P′3-Na 1/2 VO 2, one solid solution for 0.53 ≤ x ≤ 0.55 characterized by an incommensurate modulated structure, and a second solid solution for 0.63 ≤ x ≤ 0.65 with a defective structure resulting from a random stack of O′3 and P′3 layers. With further sodium intercalation ( x > 0.65), the structure irreversibly transforms to the starting parent phase O3-NaVO 2 . This work reveals new details about the diverse structural polymorphs found in sodium layered oxides used as electrode battery materials and the transitional pathways between them as a function of temperature and composition.