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

We describe a phase-integrated P2-Na_0.7MnO₂/O3-NaMnO₂ sodium manganese oxide (Na_0.7MnO₂) cathode material for sodium-ion rechargeable batteries. By changing the crystallization cooling rate after the calcination of P2/O3-Na_0.7MnO₂, the ratio between the two different (P2-Na_0.7MnO₂ and O3-NaMnO₂) phases (and the Mn⁴⁺/Mn³⁺ ratio) in the Na_0.7MnO₂ material could be controlled. The P2/O3-Na_0.7MnO₂ 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-Na_0.7MnO₂ phase and high capacity of the O3-NaMnO₂ phase. The stability of the layered crystal structure of Na_0.7MnO₂ was also improved by increasing the portion of the P2-Na_0.7MnO₂ phase, which has a higher average oxidation state of Mn compared to that in O3-NaMnO₂; this suppressed the Jahn–Teller distortion during cycling.