High-Entropy Na-Deficient Layered Oxides for Sodium-Ion Batteries.

Sodium layered oxides always suffer from sluggish kinetics and deleterious phase transformations at deep-desodiation state (., >4.0 V) in O3 structure, incurring inferior rate capability and grievous capacity degradation. To tackle these handicaps, here, a configurational entropy tuning protocol through manipulating the stoichiometric ratios of inactive cations is proposed to elaborately design Na-deficient, O3-type NaTmO cathodes. It is found that the electrons surrounding the oxygen of the TmO octahedron are rearranged by the introduction of MnO and TiO octahedra in Na-deficient O3-type NaLiNiCoMnTiSnO (MTS15) with expanded O-Na-O slab spacing, giving enhanced Na diffusion kinetics and structural stability, as disclosed by theoretical calculations and electrochemical measurements. Concomitantly, the entropy effect contributes to the improved reversibility of Co redox and phase-transition behaviors between O3 and P3, as clearly revealed by synchrotron X-ray absorption spectra and X-ray diffraction. Notably, the prepared entropy-tuned MTS15 cathode exhibits impressive rate capability (76.7% capacity retention at 10 C), cycling stability (87.2% capacity retention after 200 cycles) with a reversible capacity of 109.4 mAh g, good full-cell performance (84.3% capacity retention after 100 cycles), and exceptional air stability. This work provides an idea for how to design high-entropy sodium layered oxides for high-power density storage systems.