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

Na₃V₂(PO₄)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 POV 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 POV bond and suppress the capacity decay. The as-prepared Na₃V_1.5Fe_0.5(PO₄)₃@C (NV_1.5Fe_0.5P@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.