Orthorhombic Na2/3Cu0.1Mn0.9O2 cathode: Enhanced Na storage performances with the suppressed Mn–O bond anisotropy

Abstract

Orthorhombic Mn-based layered oxides (P′2) have drawn much attention as promising cathode materials for sodium ion batteries (SIBs) with high specific capacities and rich resources. However, P′2-NaxMnO2 cathode suffers from inferior rate capability and poor cycling performances, owing to the complicated phase transition during de-/sodiation process. Herein, Cu-substitution P′2-Na2/3Cu0.1Mn0.9O2 is successfully prepared to mitigate the multiple phase transition by reducing the distortion of the MnO6 octahedron. Mn–O bond anisotropy is decreased and the spacing distance of Na layer is constricted by the modulation of the CuO6 octahedron, giving enhanced electronic conduction, validated by theoretical calculations and X-ray diffraction refinement. P′2-OP4 phase transition is replaced by reversible P′2-Z phase transition with slighter volume change, leading to the lower lattice strain in both c-axis direction and a-b plane, as probed by Operando/ex situ X-ray diffraction and synchrotron X-ray absorption spectra, which strengthens the stability of lattice skeleton. Consequently, P′2-Na2/3Cu0.1Mn0.9O2 exhibits outstanding rate capabilities (102 mAh/g at 750 mA g−1) and enhanced cycle performances (91 % at 150 mA g−1 after 100 cycles). This Cu-substitution strategy provides an effective method to design the structure-stable layered oxide cathodes for SIBs.