P2-layered Na0.5Li0.07Mn0.61Co0.16Ni0.16O2 cathode boosted Na-storage properties via rational sub-group element doping

Abstract

P2-layered metal oxide cathodes exhibit great promise for use in sodium ion batteries due to their unique two-dimensional tunnel structure, high energy density and high redox potential, etc. However, the inferior structural stability and irreversible phase change of P2-layered cathodes inhibit their development. The best effective strategies to improve their structural stability and sodium storage properties are via the optimization of the P2-layered tunnel structure and morphology by adjusting the Na⁺ contents, lithium substitution and rational element doping. Herein, the first sub-group element (Cu²⁺, Ag⁺ and Au⁺) doped Na_0.5Li_0.07Mn_0.61Co_0.16Ni_0.16O₂ cathodes were successfully prepared and systematically studied using operando X-ray powder diffraction (XRD), Rietveld refinement with corresponding Fourier electron cloud maps and the galvanostatic intermittent titration technique (GITT). These data indicate that the lattice parameters (a/b, c and V), energy barrier, and Na⁺ diffraction coefficient of sub-group element-doped Na_0.5Li_0.07Mn_0.61Co_0.16Ni_0.16O₂ cathodes are gradually improved in line with an increase in the sub-group element radii, which facilitate the kinetics of Na⁺ migration due to the synergistic effect between the valence and radius. Additionally, the structural stability and sodium storage mechanism of the optimized hybrid Na_0.5Li_0.07Mn_0.6Co_0.16Ni_0.16Au_0.01O₂ electrode was revealed via operando XRD.