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

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 syn-chrotron 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 perfor-mances (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.