Electrochemical and Thermal Evolution of P2 Na2/3MnO2

P2 Na2/3MnO2 can be used as a cathode material in sodium-ion batteries. Here, the electrochemical-temperature-dependent phase diagram of P2 Na2/3MnO2 is investigated using X-ray powder diffraction. The P2 Na2/3MnO2 powder under a N2 atmosphere shows evidence of the formation of a monoclinic C2/m phase, from about 450°C. The P2 Na2/3MnO2 electrode sealed in a capillary undergoes a sequence phase transitions from the as-prepared hexagonal P63/mmc to a secondary hexagonal P63/mmc phase followed by a transition to Mn3O4 and subsequently MnO. NaF also appears parallel to the formation of the secondary hexagonal phase. These transitions suggest a local reducing environment as the Mn oxidation state evolves from 3+/4+ to 2+. The samples at various states of charge show similar thermal evolution with the exception of the discharged (Na-inserted) state which features a slightly more complex evolution. Understanding the structure and thermal evolution at various states of charge and under various conditions provides insight into the stability of these potential cathode materials.