Covalency modulation enables stable Na-rich layered oxide cathodes for Na-ion batteries

Abstract

As the analogs of Li-rich materials, Na-rich transition metal layered oxides are promising cathode materials for Na-ion batteries owing to their high theoretical capacity and energy density through cumulative cationic and anionic redox. However, most of the reported Na-rich cathode materials are mainly Ru- and Ir-based layered oxides, which limits the practical application. Herein, we report a Na-rich and Ru-doped O3-type Na1.1Ni0.35Mn0.55O2 cathode to mitigate this issue. By partially substituting Mn4+ with high-electronegativity Ru4+, the structural stability and electrochemical performance of the cathode are both greatly improved. It is validated that the high covalency of Ru-O bonds could harden the structural integrity with rigid oxygen framework upon cycling, leading to enhanced O3-P3 phase transition reversibility. Ru doping also induces an enlarged interlayer spacing to boost the Na+ diffusion kinetics for improved rate capability. In addition, benefiting from the large energetic overlap between Ru 4d and O 2p states, the reinforced Ru-O covalency enables highly reversible Ru4+/Ru5+ redox accompanied with more stable oxygen redox, leading to improved specific capacity and cycling stability over cycling. Our present study provides a promising strategy for designing high-performance Na-rich layered oxide cathode materials through covalency modulation toward practical applications.

Local Construction of Mn-Based Layered Cathodes through Covalency Modulation for Sodium-Ion Batteries

“…10,41 During the charge process, no new peaks can be found in NMMO until 4.2 V (Figure 4a), suggesting a single-phase reaction during this region. With the 43,44 Notably, the characteristic peak of the Mg 2+ /Mn 4+ honeycomb ordering arrangement in NMMO disappears at the end of charge and reappears during the following discharge with a reduced magnitude. As reported previously, it is very likely that Mg 2+ migrates to the tetrahedral and octahedral sites in the Na + layers accompanied with the formation of Mg 2+ vacancies in the TM layers, which could induce the in-plane disorder of Mn 4+ with the disappearance of honeycomb ordering during the charge process.…”

“…It has been suggested that the anisotropy change of TM–O bonds is the main reason to cause the excessive glide of TMO 2 layers during the charge process. Herein, the incorporation of Ru into TM sites is competent in constructing stable TM–O lattice integrity by retuning the anisotropy change of TM–O bonds through the robust Ru–O covalent bonds, thus suppressing the notorious phase transition. , …”