High-voltage phase stabilization and air-stability enhancement in O3-type entropy-reinforced sodium layered cathodes

Extending the depth-of-charge of the O3-type layered oxide cathodes provides a feasible solution to elevate the energy density of the sodium-ion batteries (SIBs). However, detrimental/irreversible P-to-O phase transition resulting in severe structural distortion and rapid capacity decay frequently occurs during high-voltage region above the 4.0 V cutoff. To address these challenges, herein, a rational high-entropy strategy is employed to develop a Co-free Na_0.9Ni_0.3Fe_0.2Mn_0.3Ti_0.1Cu_0.05Sn_0.05O₂ prototype cathode with the extended depth-of-charge to 4.2 V cutoff. Not only the suppressed P3-to-OP2 phase transition by slab gliding at the deep state of charge but also the resistant ability to humid air is obtained. Consequently, owing to the inhibited deteriorated structural degradation upon high-voltage cycling, the high-entropy compound manifests quasi-zero strain feature (1.29% volume variation), good rate capability (101.3 mAh g⁻¹ at 10 C), long-term cycling stability (90.7% over 400 cycles at 5 C) and a high energy density of 299.3 Wh kg⁻¹ in full cell with slight capacity decay (3% after 200 cycles at 1 C). This work highlights the significance of the high-entropy strategy in stabilizing the P-to-O structural degradation at deep desodiation state and enhancing the air stability of O3-type layered cathodes for SIBs.