Dual-Manipulation on P2-Na0.67Ni0.33Mn0.67O2 Layered Cathode toward Sodium-Ion Full Cell with Record Operating Voltage Beyond 3.5 V

P2-type Na0.67Ni0.33Mn0.67O2, a typical layered transition metal oxide, has been extensively studied as future practical sodium ion batteries cathode due to the merit of high energy density. However, its inferior cyclability and poor rate capability have severely hindered its practical applications. Herein, a stable P2-Na0.67Ni0.23Mg0.1Mn0.67O2 layered cathode with simultaneously achieving magnesium doping and hierarchical one-dimensional nanostructure composed of nanoplate subunits assembly is reported. In-situ X-ray diffraction measurement and diffusion kinetics analysis reveal that Mg ion doping restrains the P2-O2 phase transition and reduces the activation energy of interfacial charge transfer. In addition, the hierarchical nanostructure is shown to possess robust structure, which raises the capacity retention from 74.8% to 92.2% over 150 cycles when compared with its bulk counterpart. Owing to the combined advantages, this unique material exhibits extraordinary electrochemical performance with a high capacity retention of 90.9% over 1000 cycles at 5C in half cell. More importantly, the full cell could achieve the highest average operating voltage of 3.56 V and outstanding energy density of 249.9 Wh kg−1 compared with previously reported state-of-the-art values based on layered oxide cathodes. This work may open up a new opportunity for developing high energy SIBs with practicability.