Controllable molten-salt synthesis of stable large-sized O3-type layered oxide single-crystal for sodium-ion batteries

Sodium-ion batteries are promising candidates for large-scale energy storage because of the abundance and low cost. However, cathode development remains a major bottleneck. Although O3-type layered oxides deliver high theoretical capacities, their polycrystalline character renders them prone to grain-boundary fracture upon cycling, leading to capacity degradation. By contrast, single-crystal structures offer superior structural robustness and electrochemical stability but face challenges in synthesis due to their intrinsic sensitivity to air and moisture. Herein, we report a NaBr-assisted molten-salt route to fabricate large-sized O3-type NaCu 0.1 Fe 0.42 Mn 0.48 O 2 single-crystals with large lateral size up to 27.3 μm. The cathode delivers an initial discharge capacity of 126 mAh g −1 within 2.0–4.2 V. After 100 cycles at 0.5C, the cathode retains 70% of its initial capacity. Structural characterization indicated that T-NCFM-SC largely preserved its layered framework and exhibited improved phase-transition reversibility and interfacial stability during charge-discharge cycling. This work provides a useful strategy for improving O3-type layered oxide cathodes for sodium-ion batteries.