Electrochemical and Thermal Evolution of P2 Na_2/3MnO₂

P2 Na_2/3MnO₂ can be used as a cathode material in sodium-ion batteries. Here, the electrochemical-temperature-dependent evolution of P2 Na_2/3MnO₂ is investigated using X-ray powder diffraction. P2 Na_2/3MnO₂ powder under a N₂ atmosphere shows evidence of the formation of a monoclinic C2/m phase, from about 450 °C. The P2 Na_2/3MnO₂ electrode sealed in a capillary undergoes a sequence of 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.