Single‐Crystalline Biphasic Layered Cathodes for Sodium‐Ion Batteries

ABSTRACT 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 Mn 4+ /Mn 3+ 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.