A well-designed P2-Na0.67Mn0.85Al0.05Zn0.1O2 cathode for superior sodium-ion batteries

Abstract

A P2-Na_0.67MnO₂ (NMO) layered cathode has the merits of high capacity and easy preparation. Nevertheless, the severe phase transition (P2–O2) and inferior electronic/ionic conductivities seriously limit its practical application. Herein, a synergistic doping with Al³⁺ and Zn²⁺ into Mn-sites is employed to prepare a series of P2-Na_0.67Mn_1−x−yZn_xAl_yO₂ cathodes. In situ XRD and in situ EIS measurements profoundly verify the suppressed phase transition, enhanced structural reversibility and even faster charge transfer during the charge–discharge cycle, respectively. Moreover, DFT calculations confirm the much stronger metallicity of the NMO3 electrode than that of NMO, derived from the more likely transition capability of mobile free electrons in the vicinity of the Fermi level. As a result, a more stable structure and stronger conductivity are obtained. The optimum P2-Na_0.67Mn_0.85Al_0.05Zn_0.1O₂ (NMO3) can display more superior electrochemical performance, including working capacity (181.85 mA h g⁻¹@0.1C), cycling stability (83%@93.87 mA h g⁻¹@600 cycles@5C) in half-cells and energy density (334.5 W h kg⁻¹) in NMO3//hard carbon full-cells, thereby showing extraordinary application potential in advanced sodium-ion batteries.