Spinel-Layered Intergrowth Composite Cathodes for Sodium-Ion Batteries

The vital challenge of a layered manganese oxide cathode for sodium-ion batteries is its severe capacity degradation and sluggish ion diffusion kinetics caused by irreversible phase transitions. In response to this problem, the spinel-layered manganese-based composite with an intergrowth structure is ingeniously designed by virtue of an interesting spinel-to-layered transformation in the delithiated LiMn₂O₄ under Na⁺ insertion. This unique spinel-layered intergrowth structure is strongly confirmed by combining multiple structure analysis techniques. The layered component can provide more reversible capacity, while the spinel component is crucial for the stabilized crystal structure and accelerated ion diffusion kinetics. As an appealing cathode for sodium-ion batteries, the layered-spinel composite delivers a high reversible capacity of 180.9 mAh g^-1, excellent cycling stability, and superior rate capability with 55.7 mAh g^-1 at 12 C. Furthermore, the reaction mechanism upon Na⁺ extraction/insertion is revealed in detail by ex situ X-ray diffraction and X-ray photoelectron spectroscopy, indicating that Na⁺ ions can be accommodated by the layered structure at a low voltage and by the spinel at a high voltage. This study will provide a new idea for the rational design of an advanced cathode for sodium-ion batteries.