Bimetallic Synergies Help the Application of Sodium Vanadyl Phosphate in Aqueous Sodium-Ion Batteries

Graphical Going uphill : The dissolution mechanism of sodium vanadium phosphate (NVP) in an aqueous electrolyte is investigated in detail. Using metal (Fe or W) doping can enhance the strength of the metal-oxygen bond to suppress partial vanadium dissolution in NVP. Furthermore, the synergy of bimetallic doping completely inhibits V dissolution to improve the cycling stability of NVP. Aqueous sodium-ion batteries (ASIB) offer many potential applications in large-scale power grids since they are inexpensive, safe, and environmentally friendly. Sodium superionic conductors (NASICON), especially carbon-coated Na 3 V 2 (PO 4 ) 3 (NVP), have attracted much attention due to the full use of their high ion migration speed. However, the poor cycle lifespan and capacity retention of NVP hinder its application in ASIB. Herein, a novel bimetal-doped Na 3 V 1.3 Fe 0.5 W 0.2 (PO 4 ) 3 (NV 1.3 Fe 0.5 W 0.2 P) cathode is designed and synthesized to achieve outstanding cycling stability (95 % of initial capacity at 50th cycle). The electrochemical behavior and charge storage mechanism of NV 1.3 Fe 0.5 W 0.2 P are systematically investigated by various in situ and ex situ characterizations. The Fe and W codoping could stabilize the NASICON framework to suppress the proton attack on the Na site in the aqueous electrolyte, thus resulting in excellent cycling stability. DFT calculations show that bimetallic doping increases the structural stability of NVP. Moreover, an ASIB fabricated using a NV 1.3 Fe 0.5 W 0.2 P cathode and a NaTi 2 (PO 4 ) 3 anode delivers 64 mAh g −1 at room temperature, 95 % capacity retention after 50 cycles (1 A g −1 ).