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

Sodiumlayered oxides always suffer from sluggish kinetics anddeleterious phase transformations at deep-desodiation state (i.e., >4.0 V) in O3 structure, incurring inferior ratecapabilityand grievous capacity degradation. To tackle these handicaps, here,a configurational entropy tuning protocol through manipulating thestoichiometric ratios of inactive cations is proposed to elaboratelydesign Na-deficient, O3-type Na x TmO2 cathodes. It is found that the electrons surrounding theoxygen of the TmO6 octahedron are re-arranged by theintroduction of MnO6 and TiO6 octahedra in Na-deficientO3-type Na-0.83-Li-0.1-Ni-0.25-Co-0.2-Mn-0.15-Ti-0.15-Sn-0.15-O2-& delta; (MTS15) with expanded O-Na-Oslab spacing, giving enhanced Na+ diffusion kinetics andstructural stability, as disclosed by theoretical calculations andelectrochemical measurements. Concomitantly, the entropy effect contributesto the improved reversibility of Co redox and phase-transition behaviorsbetween O3 and P3, as clearly revealed by ex situ synchrotron X-ray absorption spectra and in situ X-ray diffraction. Notably, the prepared entropy-tuned MTS15 cathodeexhibits impressive rate capability (76.7% capacity retention at 10C), cycling stability (87.2% capacity retention after 200 cycles)with a reversible capacity of 109.4 mAh g(-1), goodfull-cell performance (84.3% capacity retention after 100 cycles),and exceptional air stability. This work provides an idea for howto design high-entropy sodium layered oxides for high-power densitystorage systems.

Automated summary

A configurational entropy tuning protocol is proposed to design Na-deficient, O3-type Na x TmO2 cathodes, which can enhance the kinetics and stability of the electrodes. The entropy effect also contributes to the improved redox reversibility and phase transition behaviors. The prepared entropy-tuned cathode exhibits impressive rate capability, cycling stability, and air stability.