A W/Al Co-doped Na_0.44MnO₂ Cathode Material for Enhanced Sodium-Ion Storage

The Na_0.44MnO₂ cathode has attracted enormous interest owing to its low cost, low toxicity, and stable structure, but its practical application is still hindered by the limited sodium storage sites. Element doping is widely used to improve its capacity. However, cation and anion substitution could barely reach a satisfactory compromise between the structural stability and reversible capacity. Herein, we show that the above issue could be overcome via the synergetic effect of W and Al substitution. Combining the electrochemical and in situ X-ray powder diffraction (XRD) measurements, we reveal that the substitution of W effectively facilitates the tunnel-to-layered phase transformation, while the further substitution of Al eliminates the Na⁺/vacancy ordering during the extraction/insertion of Na⁺ ions, resulting in a layered Na_0.44Mn_0.94W_0.01Al_0.05O₂ (NMO-1W5Al) with negligible Na⁺/vacancy ordering upon cycling. In addition, NMO-1W5Al represents a wider Na⁺ interlayer spacing to accelerate the diffusion of Na⁺, which improves the rate performance. The high specific capacity, remarkable rate performance, and high cycling stability of NMO-1W5Al are promising for large-scale energy storage systems.