P2/O3 phase-integrated Na0.7MnO2 cathode materials for sodium-ion rechargeable batteries

We describe a phase-integrated P2-Na0.7MnO2/O3-NaMnO2 sodium manganese oxide (Na0.7MnO2) cathode material for sodium-ion rechargeable batteries. By changing the crystallization cooling rate after the calcination of P2/O3-Na0.7MnO2, the ratio between the two different (P2-Na0.7MnO2 and O3-NaMnO2) phases (and the Mn4+/Mn3+ ratio) in the Na0.7MnO2 material could be controlled. The P2/O3-Na0.7MnO2 sample prepared by fast cooling contains the optimum content of the two principal phases and showed excellent electrochemical performance because of the synergy between the fast kinetics of the P2-Na0.7MnO2 phase and high capacity of the O3-NaMnO2 phase. The stability of the layered crystal structure of Na0.7MnO2 was also improved by increasing the portion of the P2-Na0.7MnO2 phase, which has a higher average oxidation state of Mn compared to that in O3-NaMnO2; this suppressed the Jahn–Teller distortion during cycling.