A W/Al Co-doped Na<sub>0.44</sub>MnO<sub>2</sub> Cathode\nMaterial for Enhanced Sodium-Ion Storage

The Na<sub>0.44</sub>MnO<sub>2</sub> cathode has attracted\nenormous\ninterest owing to its low cost, low toxicity, and stable structure,\nbut its practical application is still hindered by the limited sodium\nstorage sites. Element doping is widely used to improve its capacity.\nHowever, cation and anion substitution could barely reach a satisfactory\ncompromise between the structural stability and reversible capacity.\nHerein, we show that the above issue could be overcome via the synergetic\neffect of W and Al substitution. Combining the electrochemical and\nin situ X-ray powder diffraction (XRD) measurements, we reveal that\nthe substitution of W effectively facilitates the tunnel-to-layered\nphase transformation, while the further substitution of Al eliminates\nthe Na<sup>+</sup>/vacancy ordering during the extraction/insertion\nof Na<sup>+</sup> ions, resulting in a layered Na<sub>0.44</sub>Mn<sub>0.94</sub>W<sub>0.01</sub>Al<sub>0.05</sub>O<sub>2</sub> (NMO-1W5Al)\nwith negligible Na<sup>+</sup>/vacancy ordering upon cycling. In addition,\nNMO-1W5Al represents a wider Na<sup>+</sup> interlayer spacing to\naccelerate the diffusion of Na<sup>+</sup>, which improves the rate\nperformance. The high specific capacity, remarkable rate performance,\nand high cycling stability of NMO-1W5Al are promising for large-scale\nenergy storage systems.