Electrochemical Properties of Layered NaxNix/2Mn1−x/2O2 (0.5 ≤ x ≤ 1.1) with P3 Structure as Cathode for Sodium‐Ion Batteries

The Na properties of Ni and Mn containing layered oxides of the type Na x Ni x /2 Mn 1− x /2 O 2 are explored between Na contents of 0.5≤ x ≤1.1. Charge balance is maintained by adjusting the Ni/Mn ratio. X‐ray diffraction and scanning electron microscopy are used to characterize the structure and morphology. The primary phase for all as‐synthesized materials is P3, especially at Na contents below x ≤ 0.8. Samples with a Na content of x ≥ 0.9 lead to the formation of Na and Ni secondary phases. The Na storage properties are studied in half cells with two different voltage windows between 1.5–4.0 V and 2.2–4.5 V (vs Na + /Na). Ni and Mn redox are active between 1.5 and 4.0 V accompanying three voltage plateaus at 3.7, 3.0, and 2.1 V, respectively. An additional high‐voltage plateau (>4.0 V) is observed when increasing the cutoff voltage to 4.5 V. The initial Na content has a strong influence on the discharge capacity which ranges from 90 mAh g −1 ( x = 1.1) to 210 mAh g −1 ( x = 0.6). C‐rate tests up to 2C and cycle life over 150 cycles are discussed. Overall, the composition Na 0.6 Ni 0.3 Mn 0.7 O 2 shows the most favorable properties with respect to capacity retention, rate capability, and initial Coulomb efficiency.