Electrochemical and Thermal Evolution of P2 Na 2/3 MnO 2

P2 Na 2/3 MnO 2 can be used as a cathode material in sodium‐ion batteries. Here, the electrochemical‐temperature‐dependent evolution of P2 Na 2/3 MnO 2 is investigated using X‐ray powder diffraction. P2 Na 2/3 MnO 2 powder under a N 2 atmosphere shows evidence of the formation of a monoclinic C 2/ m phase, from about 450 °C. The P2 Na 2/3 MnO 2 electrode sealed in a capillary undergoes a sequence of phase transitions from the as‐prepared hexagonal P 6 3 /mmc to a secondary hexagonal P 6 3 /mmc phase followed by a transition to Mn 3 O 4 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.