Complete Three-Electron Vanadium Redox in NASICON-Type Na3VSc(PO4)3 Electrode Material for Na-Ion Batteries

Abstract

Here we introduce a new NASICON-type Na 3 VSc(PO 4 ) 3 positive electrode material for Na-ion batteries demonstrating reversible (de)intercalation of 3 Na cations per formula unit within a wide voltage range with complex voltage-composition dependence. The total electrochemical capacity of the material is 170 mAh g −1 that corresponds to the complete three-electron V 2+ /V 3+ /V 4+ /V 5+ process. All the (de)sodiation stages follow a predominantly solid-solution mechanism, as shown by operando X-ray powder diffraction. The oxidation of vanadium up to +5 upon the charge of Na 3 VSc(PO 4 ) 3 to 4.5 V vs Na/Na + causes the significant transformation of the unit cell. According to ex situ Fourier-transformed infrared spectroscopy it is accompanied by the increasing distortion of the vanadium coordination environment and shortening of the vanadium-oxygen bonds. This leads to the irreversible character of the charge-discharge curve, and the initial structure can be restored after the strong overdischarge to ≈1.5 V vs Na/Na + .