Cu-substitution P2-Na0.66Mn1-xCuxO2 sodium-ion cathode with enhanced interlayer stability

Abstract

P2-type Mn-based layered oxides are viewed as promising cathode materials for sodium ion battery by virtue of their high theoretical capacity. Considering that pure Na2/3MnO2 suffers from poor cycling performances, Cu-substitution strategy is proposed to effectively alleviate this issue. However, the structural evolution mechanism of the Cu-containing samples still remains unclear. Herein, we propose that Cu-Substitution P2-type Na0.66Mn1-xCuxO2 with enlarged lattice parameters are conducive to improving the interlayer structure stability through mitigating TMO2 slabs distortion. Proved by synchrotron XAS spectra and ex/in situ XRD, the expansion/contraction of MnO6 octahedron is dramatically reduced with the increased Cu content, showing the facilitated Na ion diffusion. Furthermore, when the ratio of Cu to Mn reaches 1:4, the phase transition from P2 to P'2 type at the end of discharge can be suppressed, resulting in the improved interlayer skeleton stability. The Cu-containing samples with stable interlayer structure exhibit high capacity retention and outstanding rate performances (a capacity of 79.9 mAh g−1 at 5 C). This Cu-substitution strategy provides a promising approach to designing highly stable cathodes.