Single-Crystalline Biphasic Layered Cathodes for Sodium-Ion Batteries

O3-type layered cathodes have attracted extensive attention due to their high reversible capacity and sufficient sodium-ion utilization. However, induced by continuous phase transitions, O3-type cathodes generally suffer from severe structural degradation and performance deterioration during the charge/discharge process. In response to this challenge, the single-crystalline O3/P2 biphasic layered cathodes (O3/P2-NNMO) were realized by a facile solvent-regulated strategy. The characterization results indicated that O3/P2-NNMO cathodes are endowed with the favorable surface chemistry environments contributed by low surface residual-sodium impurities and high Mn⁴⁺/Mn³⁺ proportion. Importantly, P2 phase was inter-grown into O3-type phase to form an interlock structure in a single-crystalline O3/P2-NNMO particle, which relieves the phase transitions to remarkably reduces lattice variation. The stable surface structure and reinforced bulk lattice cooperatively strengthen the cycling stability of cathodes. As expected, O3/P2-NNMO cathodes deliver a high reversible capacity of 117.2 mAh g^-1 at 0.1 C (14 mA g^-1), and excellent cycling stability with a capacity retention of 75% after 100 cycles at 1 C. Also, the fast Na⁺ migration pathway provided by P2 phase significantly promotes the Na⁺ diffusion kinetics and interfacial charge transfer in O3/P2-NNMO cathodes, enabling a superior rate capability. This work provides a new design idea for advanced layered cathodes for sodium-ion batteries.