Phase-Pure P2-Na_0.7(1−x)[Li_xMn_1−x]O_δ as a Cathode Material for Na-Ion Batteries

Na_0.7MnO₂ is a potential high-capacity cathode material for sodium-ion batteries. Typical Na_0.7MnO₂ material synthesized with quenching after annealing shows a P2 structure with orthorhombic distortion. However, the material is not stable when stored in ambient atmosphere and shows poor cycle performance during charge/discharge. In this study, we synthesized phase-pure P2-Na_0.7(1−x)[Li_xMn_1−x]O_δ without orthorhombic distortion by adding Li into the precursor. We show that the material can be synthesized even with natural cooling, and is stable when exposed to air. Electrochemical tests and ex situ X-ray diffraction studies reveal that the phase-pure material exhibits fewer phase transitions and less structural degradation upon Na⁺ de-intercalation/intercalation, resulting in enhanced cycle performance. For example, Na_0.64[Li_0.09Mn_0.91]O_2.07 delivers an initial discharge capacity of 170.9 mAh g⁻¹ with capacity retention of 61.1 % (104.4 mAh g⁻¹) after 100 cycles. This is significantly better than the cycle performance of the distorted P2-Na_0.7MnO_δ tested under the same conditions.