Ni-driven monoclinic → P2 → O3 phase evolution in NaMn_1-x Ni _x O₂ cathodes: structural origin, sodium storage and hard carbon full cells

In this study, NaMn_1-x Ni _x O₂ (x = 0-0.5) compositions were synthesized by a solid-state method, and their structural evolution and electrochemical performance for sodium-ion batteries were systematically investigated. X-ray diffraction studies reveal that increasing Ni content induces a structural transition from the monoclinic (C2/m) phase to the hexagonal P2-type (P6₃/mmc) and eventually the rhombohedral (R3̄m, O3-type) phase. ICP-MS analysis reveals Na/TM molar ratios of 0.839-0.905, showing that Na loss during calcination contributes to the stabilization of the P2 phase at intermediate Ni contents. While the C2/m and P6₃/mmc phases coexist in the x = 0.1 and 0.2 samples, a pure P6₃/mmc phase is observed in x = 0.3. As the Ni substitution is increased further, the R3̄m phase forms together with a minor NiO impurity. Density functional theory (DFT) calculations indicate that the monoclinic phase is thermodynamically favored at low Ni contents, whereas the rhombohedral phase becomes increasingly competitive as the Ni content increases, in line with the experimental trend. Structural analysis shows that upon Ni substitution the number of Mn³⁺ ions is reduced, which removes the cooperatively Jahn-Teller distorted monoclinic phase, although XPS shows that Jahn-Teller active Ni³⁺ is present at x = 0.5 without inducing a cooperative distortion, and oxygen lattice ordering is improved. These structural modifications indicate a stabilized layered framework. Electrochemical measurements show that appropriate Ni substitution improves capacity retention and rate capability where the x = 0.5 sample exhibits the best cycling stability. Finally, the optimized cathode was utilized in a full-cell configuration with a hard carbon anode where the P/N ratio and anode presodiation are tailored as a proof of concept. The low initial coulombic efficiency (49.34-56.08%) shows that the Na inventory of the full cell still requires further optimization.