High-entropy doping in NASICON cathodes: Activating the V/V redox couple and inducing a reversible single solid-solution phase reaction for advanced sodium ion batteries.

NaV(PO) (NVP) with typical NASICON structure is highly regarded as one of the most appealing cathodes for sodium-ion batteries (SIBs) for their excellent structural stability; however, the poor electronic conductivity and irreversible phase transition at high voltage (∼4 V) for the material result in poor rate capabilities and significant capacity degradation during electrochemical reactions. Herein, the high-entropy doping strategy of introducing six types of metal ions into the V-sites in NaV(PO) is used to design a high-performance cathode of NaV(Mn, Ca, Mg, Al, Zr)Nb(PO) (HE-NVP) for SIBs. After the high-entropy doping, a reversible V/V redox pair is activated at high voltage of ∼4 V, which significantly contributes to the enhancement of energy density and operating voltage.