Thermally and Electrochemically Driven Topotactical Transformations in Sodium Layered Oxides Na<sub><i>x</i></sub>VO<sub>2</sub>

Phase diagrams and structural transformations in the complex Na<sub><i>x</i></sub>VO<sub>2</sub> system have been studied using electrochemical (de)­intercalation and in situ and operando high resolution synchrotron powder diffraction. Starting from O′3-Na<sub>1/2</sub>VO<sub>2</sub> obtained by sodium electrochemical deintercalation of O3-NaVO<sub>2</sub>, the structural details of irreversible and reversible thermally driven transformations to P′3 and P3 type structures are presented. Subsequently, these P′3-Na<sub><i>x</i></sub>VO<sub>2</sub> phases provide a platform for operando studies exploring the Na<sub><i>x</i></sub>VO<sub>2</sub> 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<sub>1/2</sub>VO<sub>2</sub>, one solid solution for 0.53 ≤ <i>x</i> ≤ 0.55 characterized by an incommensurate modulated structure, and a second solid solution for 0.63 ≤ <i>x</i> ≤ 0.65 with a defective structure resulting from a random stack of O′3 and P′3 layers. With further sodium intercalation (<i>x</i> > 0.65), the structure irreversibly transforms to the starting parent phase O3-NaVO<sub>2</sub>. 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.