P2 Orthorhombic Na_0.7[Mn_1-xLi_x]O_2+y as Cathode Materials for Na-Ion Batteries

P2-type manganese-based oxide materials have received attention as promising cathode materials for sodium ion batteries because of their low cost and high capacity, but their reaction and failure mechanisms are not yet fully understood. In this study, the reaction and failure mechanisms of β-Na_0.7[Mn_1-xLi_x]O_2+y (x = 0.02, 0.04, 0.07, and 0.25), α-Na_0.7MnO_2+y, and β-Na_0.7MnO_2+z are compared to clarify the dominant factors influencing their electrochemical performances. Using a quenching process with various amounts of a Li dopant, the Mn oxidation state in β-Na_0.7[Mn_1-xLi_x]O_2+y is carefully controlled without the inclusion of impurities. Through various in situ and ex situ analyses including X-ray diffraction, X-ray absorption near-edge structure spectroscopy, and inductively coupled plasma mass spectrometry, we clarify the dependence of (i) reaction mechanisms on disordered Li distribution in the Mn layer, (ii) reversible capacities on the initial Mn oxidation state, (iii) redox potentials on the Jahn-Teller distortion, (iv) capacity fading on phase transitions during charging and discharging, and (v) electrochemical performance on Li dopant vs Mn vacancy. Finally, we demonstrate that the optimized β-Na_0.7[Mn_1-xLi_x]O_2+y (x = 0.07) exhibits excellent electrochemical performance including a high reversible capacity of ∼183 mA h g^-1 and stable cycle performance over 120 cycles.