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. Moreover, after high entropy doping, the single solid solution phase transition in high voltage range is induced, effectively improving the material's structural stability and exhibiting very small volume change of 1.1 % during the Na extraction/insertion. Based on the unique advantages mentioned above, the high-entropy HE-NVP exhibits a superior discharge capacity of 122.1 mAh g at a current rate of 0.5C and maintains a remarkable capacity retention of 90.1 % after 500 cycles at a rate of 5C. This work provides a useful reference for designing advanced cathodes for SIBs by regulating high entropy doping.