Nonequimolar Eight-Component Design Enables a Medium-Entropy O3/P2 Biphasic Layered Oxide for High-Performance Sodium-Ion Batteries

O3- and P2-type layered oxides are promising cathode candidates for sodium-ion batteries (SIBs). However, their practical application is severely limited by inherent single-phase drawbacks, barely alleviated via elemental doping. Herein, we report an O3/P2 biphasic layered oxide cathode, Na 0.9 Ni 0.2 Cu 0.05 Zn 0.005 Mg 0.02 Fe 0.2 Mn 0.5 Ti 0.02 Sn 0.005 O 2 (denoted as Na 0.9 NiCuZnMgFeMnTiSn) fabricated via an eight-component nonequimolar medium-entropy strategy. Distinguished from conventional high-entropy configurations relying on equimolar elemental ratios, our nonequimolar design employs electrochemically active elements (Ni, Mn, Fe, and Cu) as the dominant constituents to stabilize the crystal structure and deliver high reversible capacity. The O3 phase (contributing higher capacity) acts as the dominant component in the biphasic structure, with an optimized O3/P2 phase ratio of 81.1:18.9. Minor inactive elements (Zn, Mg, Ti, and Sn) and the secondary P2 phase synergistically promote solid-solution reaction behavior, suppress detrimental phase transitions, and accelerate Na + diffusion kinetics. Consequently, the Na 0.9 NiCuZnMgFeMnTiSn cathode delivers a reversible capacity of 143.4 mAh g −1 at 10 mA g −1 , together with remarkable cycling stability (86.0 mAh g −1 retained after 100 cycles at 200 mA g −1 ) and superior rate capability (81.8 mAh g −1 at 300 mA g −1 ). This work provides a feasible multielement medium-entropy strategy for developing stable, high-performance layered oxide cathodes for advanced SIBs.