The insight into promoting sodium storage mechanism of alpha-CrPO4-type NaV3(PO4)(3) anode material for sodium-ion batteries

Exploitation of reliable insertion anode material with long cyclability and proper working potential is very meaningful to promote application of sodium ion battery (SIB). Herein, a type of porous anode material based on NaV3(PO4)(3) (NVP) nanoparticles coated by robust carbon is reported for efficient Na storage with increasing capacity and decreasing potential plateau during ultra-long cycles for the first time. According to Na-NMR characterization and XRD Rietveld refinement, it is found that the Na+ ions partially occupy all the three sites with different percentage (11.5% 4b site, 24.3% 4c site and 64.2% 4e site). The systematical characterizations of in-situ and ex-situ XRD, SEM, XPS show a gradual transformation from the alpha-CrPO4-type crystal structure to an amorphous phase of NVP, which may reduce the activation barrier of Na+ ions especially those in 4e sites gradually. More Na+-ion sites are involved in the intercalation and de-intercalation processes and the voltage plateau decreased from 1.18 to 0.85 V vs. Na+/Na. Additionally, the full battery assembled by combining NVP/C anode with the typical Na3V2(PO4)(3) cathode is also demonstrated with a long cycle life. The high performance of as-prepared NVP material provide a possible choice of reliable insertion anode material for long life span SIB.