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

Abstract

Sodium layered oxides always suffer from sluggish kinetics and deleterious phase transformations at deep-desodiation state (<i>i.e</i>., >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 Na_<i>x</i>TmO₂ cathodes. It is found that the electrons surrounding the oxygen of the TmO₆ octahedron are rearranged by the introduction of MnO₆ and TiO₆ octahedra in Na-deficient O3-type Na_0.83Li_0.1Ni_0.25Co_0.2Mn_0.15Ti_0.15Sn_0.15O_2-δ (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 <i>ex situ</i> synchrotron X-ray absorption spectra and <i>in situ</i> 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^-1, 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.