Manganese Segregation in Layered Oxide Cathode by Deep Eutectic Solvent Synthesis toward High-Voltage Sodium Storage

O3-type layered oxides are promising cathodes for sodium-ion batteries with high specific capacity. Nevertheless, irreversible structure degradation and oxygen redox at high voltages impede further application toward high energy density. Here, a deep eutectic solvent (DES)-assisted synthesis strategy is developed to stabilize the layered structure by segregation of Mn sites in O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM). The DES system affords a stronger hydrogen bonding interaction, leading to a quasi-homogeneous mixing of Ni/Mn/Fe ions. Mn segregation is induced in the transition metal layer of NFM, which facilitates a gradual phase transition via O3 + P3 two-phase coexistence, rather than a sudden O3 → P3 transition with significant variation of interlayer spacing. It also promotes reversible anion redox of oxygen and reduced surface residual alkali, which confers superior cycling stability and rate performance on the DES-synthesized NFM compared to traditional sol-gel-synthesized NFM. The material achieves an initial Coulombic efficiency of 96% at 0.1 C and notable rate performance for a capacity of 115 mAh g^-1 at 10 C. Additionally, it yields a superior cyclability (80% capacity retention after 150 cycles at 0.5 C) in full cell pairing with hard carbon, validating its practical potential. This work offers a promising approach for the rational design of cathode materials with high capacity and superior stability.