Unravelling capacity fading mechanisms in sodium vanadyl phosphate for aqueous sodium-ion batteries

Na 3 V 2 (PO 4 ) 3 (NVP), which is known as a sodium superionic conductor (NASICON), has been successfully developed as an excellent cathode material for sodium-ion batteries (SIBs). However, the capacity of NVP quickly fades when used in an aqueous electrolyte. Herein, the charge storage and capacity attenuation mechanisms of carbon-coated NVP (NVP@C) were carefully investigated by systematic material characterization and density functional theory (DFT) calculations. According to the results, protons in the aqueous electrolyte diffuse into the surface of NVP@C to occupy the sodium site and attack the nearby phosphates during the charge–discharge cycles, leading to the deformation and breakage of the P O V bond. The distorted phosphates on the surface of NVP@C gradually dissolve into the electrolyte, causing a decrease in capacity. To stabilize the phosphates on the surface of NVP, DFT calculations suggest that iron doping of NVP can effectively relieve the deformation of the P O V bond and suppress the capacity decay. The as-prepared Na 3 V 1.5 Fe 0.5 (PO 4 ) 3 @C (NV 1.5 Fe 0.5 P@C) has a capacity retention of 95% in the first ten cycles, while NVP@C retains only 55% of the initial capacity in the same number of cycles.